Nitrogen Containing Carbon Nanofibers As Non-Noble Metal Cathode Catalysts In

Home , Boron carbides

Nitrogen Containing Carbon Nanofibers as Non-Noble Metal Cathode Catalysts in

PEM and Direct Methanol Fuel Cells

Dissertation

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy

in the Graduate School of The Ohio State University

Elizabeth Joyce Biddinger, B.S.ChE

Graduate Program in Chemical Engineering

* * * *

The Ohio State University

2010

Dissertation Committee:

Professor Umit S. Ozkan, Advisor

Professor Jeffrey Chalmers

Professor Stuart Cooper

Elizabeth Joyce Biddinger

2010

ABSTRACT

PEM and direct methanol fuel cells (DMFC) have great potential for use as alternative fuel energy conversion devices. Before this potential can be realized, however, performance improvements must be made and material costs need to be reduced. The limiting reaction in the PEMFCs and DMFCs is the oxygen reduction reaction (ORR), which occurs at the cathode. In an attempt to improve the reaction kinetics, substantial loadings of Pt catalysts are required on the cathode. This significantly increases the overall cost of the fuel cell. Also, in DMFCs, methanol crossover from the anode allows for competing reactions at the cathode catalyst to occur, reducing the power output of the fuel cell further. As the demand for fuel cells increase,

the demand for Pt will far outpace the supply of Pt.

Replacements studied for Pt cathode catalysts include Pt alloys, other noble

metals, chalcogenides and nitrogen-containing carbons. Nitrogen-containing carbons made from simple precursors can provide an economical replacement to Pt catalysts.

Before this can be realized, improvements in the activity and selectivity of the nitrogen- containing carbons need to occur.

The work presented here involves the study of nitrogen-containing carbon nanostructures (CNx) as ORR catalysts for PEMFCs and DMFCs. Improving the ORR

catalytic performance in both activity and selectivity for CNx catalysts, while gaining a

better understanding of the catalyst materials and the way they are evaluated were the

major driving forces behind this research.

CNx catalyst performance was studied by incorporating heteroatoms beyond nitrogen and surface functional groups into the catalyst. Boron and sulfur heteroatoms were studied along with oxygen functional groups. It was found that the methods to introduce boron into the nanostructure had a large impact on the ORR performance.

Sulfur did not have an effect on the ORR performance, but was successfully used as a

CNx growth promoter in the form of thiophene during acetonitrile pyrolysis. An increase

in oxygen functional groups on the surface of CNx catalysts improved the ORR

selectivity to water formation.

The role CNx catalyst nanostructure plays in ORR activity was studied using

model nanofiber systems with both high levels of graphitic edge plane exposure and low

levels of graphitic edge plane exposure. Experiments showed that un-doped graphitic

edge planes were not the ORR active site. Incorporation of nitrogen into the graphitic

edge planes significantly improved ORR activity compared to the nitrogen-free

nanofibers.

The use of electrochemical half cell methods have been evaluated and reported

here. Rotating Ring Disk Electrode (RRDE) testing is commonly used to measure the

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ORR activity and selectivity of a catalyst. The factors affecting catalyst selectivity reporting including catalyst loading and RRDE catalyst ink aging were studied.

In addition to these studies, the performance of CNx catalysts developed in the laboratory for use as cathode catalysts in DMFCs were evaluated. It was found that CNx catalysts are both methanol tolerant and inactive towards the methanol oxidation reaction, making them favorable potential DMFC catalysts.

Throughout the studies, materials developed and evaluated were characterized using classic heterogeneous catalysis techniques to gain a better understanding of the systems being analyzed. These techniques include, X-Ray Photoelectron Spectroscopy

(XPS), Transmission Electron Microscopy (TEM), Temperature Programmed Oxidation

(TPO) and Desorption (TPD) studies and Thermogravimetric Analysis, among others.

DEDICATION

To Josh, Miracle and Merlin for all of your love and support

ACKNOWLEDGMENTS

I gratefully acknowledge the assistance and camaraderie I have received in the laboratory from the many members of the HCRG both past and present.

I am especially indebted to those who laid the ground work for this project: Paul

Matter, PhD, and his undergraduate assistants Ling Zhang, Eugenia Wang and Maria

Arias.

Thank you to those who have more recently worked on the “PEM Team:” undergraduates Doug Knapke, Jesaiah King, Katie Luthman and Hilary Marsh along with high school student Kate Baker, and of course, fellow graduate students Deepika Singh and Dieter von Deak, who ideas are bounced off of on nearly a daily basis.

This group would not be possible without the guidance and dedication of my advisor Professor Umit S. Ozkan.

I acknowledge the financial assistance provided to this project through NSF,

DOE-BES, and Ohio Department of Development Wright Center for Innovation.

Finally, a deep thanks to my family and friends, especially my husband Josh, for supporting me in my endeavors as a graduate student.

VITA

November 6, 1982 ………………. Born, Ashland, Ohio

June 2001 ………………………… H.S. Diploma, Mapleton High School, Ashland,

Ohio

Summer 2003 …………………….. Chemical Engineering Co-Op, Cooper Tire &

Rubber Company, Findlay, Ohio

Summer 2004 …………………… Chemical Engineering Co-Op, Cooper Avon Tyres,

Melksham, England

Winter 2004-Spring 2005………… Undergraduate Research Assistant, Ohio University,

Athens, Ohio

June 2005 …………………………. B.S. Chemical Engineering, Ohio University,

Athens, Ohio

2005-Present ………………………Graduate Research Associate, Ohio State University,

Columbus Ohio

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PUBLICATIONS

Book Chapter

• P.H. Matter, E.J. Biddinger, U.S. Ozkan, “Non-precious metal oxygen reduction catalysts for PEM fuel cells,” Catalysis, Spivey, J.J., Ed. The Royal Society of Chemistry, Cambridge, UK, Vol. 20 (2007), 338-361.

Invited Papers

• E.J. Biddinger, D. von Deak, U.S. Ozkan, “Nitrogen-Containing Carbon Nanostructures as Oxygen-Reduction Catalysts,” Topics in Catalysis, 52 (2009), 1566-1574.

• E.J. Biddinger, U.S. Ozkan, “Methanol Tolerance of CNx Oxygen Reduction Catalysts,” Topics in Catalysis, 46 (2007), 339-348.

Journal Articles

• E.J. Biddinger, D.S. Knapke, D. von Deak, U.S. Ozkan, “Effect of Sulfur as a Growth Promoter for CNx Nanostructures as PEM and DMFC ORR Catalysts,” Applied Catalysis B – Environmental, 96 (2010), 72-82.

• M.P. Woods, E.J. Biddinger, P.H. Matter, B. Mirkelamoglu, U.S. Ozkan, “Correlation Between Oxygen Reduction Reaction and Oxidative Dehydrogenation Activities over Nanostructured Carbon Catalysts,” Catalysis Letters, 136 (2010), 1-8.

• E.P. Bonnin, E.J. Biddinger, G.G. Botte, “Effect of catalyst on electrolysis of ammonia effluents,” Journal of Power Sources, 182 (2008), 284-290.

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• P.H. Matter, E. Wang, M. Arias, E.J. Biddinger, U.S. Ozkan, “Oxygen reduction reaction activity and surface properties of nanostructured nitrogen-containing carbon,” Journal of Molecular Catalysis A: Chemical, 264 (2007), 73-81.

• P.H. Matter, E. Wang, M. Arias, E.J. Biddinger, U.S. Ozkan, “Oxygen Reduction Reaction Catalysts Prepared from Acetonitrile Pyrolysis over Alumina-Supported Metal Particles,” Journal of Physical Chemistry B, 110 (2006), 18374-18384.

FIELDS OF STUDY

Major Field: Chemical Engineering

Area of Interest: Heterogeneous Catalysis

TABLE OF CONTENTS

ABSTRACT ...... ii DEDICATION ...... v ACKNOWLEDGMENTS ...... vi VITA ...... vii LIST OF FIGURES ...... xv LIST OF TABLES ...... xxi CHAPTER 1. Introduction...... 1 CHAPTER 2. Research Objectives ...... 10 CHAPTER 3. Literature Review of Carbon-Nitrogen ORR Catalysts and Related Materials ...... 12 3.1 Carbon-Nitrogen ORR Catalysts ...... 12 3.1.1 Non-Pyrolyzed Macrocycles as ORR Catalysts ...... 12 3.1.2 Pyrolyzed Macrocycles as ORR Catalysts ...... 14 3.1.3 ORR C-N Catalysts Derived from Simple Precursors ...... 15 3.1.3.a Materials and Preparation of C-N Catalysts Derived from Simple Precursors . 16 3.1.3.b Characteristics of C-N ORR Catalysts ...... 19 3.1.4 Active Site Debate in C-N ORR Catalysts ...... 25 3.2 Other Heteroatom-Containing and Functionalized Carbon Nanostructures ...... 30 3.2.1 Types of Carbon Nanostructure and their Growth Characteristics ...... 30 3.2.2 Nitrogen-Containing Carbon Nanostructures ...... 34 3.2.3 Sulfur-Containing Carbon Nanostructures ...... 35 3.2.4 Boron-Containing Carbon Nanostructures ...... 36 x

3.2.5 Oxygen Functional Groups on Carbon ...... 39 3.2.5.a Oxygen Functional Groups in ORR ...... 43 CHAPTER 4. Experimental Methods ...... 47 4.1 Catalyst Preparation ...... 47

4.1.1 Magnesia Supported CNx Catalysts ...... 47 4.1.2 Stacked Platelet Nanofibers ...... 51 4.1.3 Nanofibers with Basal Plane Exposure ...... 53 4.1.4 Nanofiber Acid Oxidation ...... 54 4.1.4.a Nanofiber Acid Oxidation Using Nitric and Sulfuric Acid ...... 54

4.1.4.b CNx Acid Oxidation Using Nitric Acid Only ...... 55 4.1.5 Ammonia Treatments on Nanofibers ...... 55 4.1.5.a Ammonia-Treated Stacked Platelets and Nanofibers with Basal Plane Exposure ...... 55

4.1.5.b Ammonia-Treated CNx Catalysts ...... 56

4.1.6 Ethanol-Treated CNx ...... 56 4.1.7 Other Heteroatom Incorporation into Carbon Nanofibers ...... 57

4.1.7.a Sulfur as a Growth Promoter in CNx ...... 57

4.1.7.b Boron-Incorporated CNx Catalysts ...... 58

4.1.8 Graphitic Site Blocking on CNx ...... 58 4.1.9 Commercial Catalysts ...... 60 4.2 Catalyst Characterization ...... 60 4.2.1 Nitrogen Physisorption ...... 60 4.2.2 X-Ray Photoelectron Spectroscopy (XPS) ...... 61 4.2.3 Transmission Electron Microscopy (TEM) ...... 62 4.2.4 Hydrophobicity Testing ...... 62 4.2.5 Temperature Programmed Oxidation (TPO) Experiments ...... 63 4.2.5.a Temperature Programmed Oxidation – Thermogravimetric Analysis (TPO- TGA) Experiments ...... 63 4.2.5.b Temperature Programmed Oxidation (TPO) Experiments Using an External Reactor System ...... 64

4.2.6 Temperature Programmed Desorption-Temperature Programmed Oxidation (TPD-TPO) Experiments ...... 64 4.3 Electrochemical Testing and Related Studies ...... 65 4.3.1 Rotating Ring Disk Electrode (RRDE) Testing ...... 66 4.3.2 Methanol Tolerance and Methanol Oxidation Activity Testing ...... 70 4.3.3 RRDE Studies to Examine the Effect of Catalyst Loading ...... 71 4.3.4 Catalyst Layer Thickness Determination ...... 72 4.3.5 RRDE Catalyst Ink Aging Studies ...... 72 4.3.6 Electrochemical Reduction and Oxidation of Hydrogen Peroxide ...... 73

CHAPTER 5. Effect of Sulfur as a Growth Promoter for CNx Nanostructures as PEM and DMFC ORR Catalysts ...... 75 5.1 Motivation for Studying Sulfur as a Growth Promoter ...... 75 5.2 Results & Discussion ...... 77 5.2.1 Thiophene as a Growth Promoter ...... 77 5.2.2 Catalyst Morphology ...... 78 5.2.3 Activity and Selectivity Testing ...... 81 5.2.4 Hydrophobicity Testing ...... 83 5.2.5 Surface Species Characterization ...... 84

5.2.6 Stability of Sulfur in CNx Nanostructures ...... 92 5.3 Thiophene Usage as a Growth Promoter Concluding Remarks ...... 99

CHAPTER 6. Impact of Oxygen Functional Groups on CNx ORR Catalysts ...... 102 6.1 Oxygen Functional Groups on Carbon and their Potential Role in ORR ...... 102 6.2 Results & Discussion ...... 104

6.2.1 CNx Treated with HNO3 ...... 104 6.2.2 Additional Oxygen Functional Group Observations ...... 109 6.3 Conclusions for Oxygen Functional Groups in ORR ...... 110 CHAPTER 7. Role of Graphitic Edge-Plane Exposure in Carbon Nanostructures for Oxygen Reduction Reaction ...... 112 7.1 Motivation in Studying Role of Graphitic Edge-Plane Exposure ...... 112 7.2 Results and Discussion ...... 113

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7.2.1 Synthesis and Physical Characterization ...... 113 7.2.2 Chemical Characterization ...... 121 7.2.2.a X-Ray Photoelectron Spectroscopy ...... 121 7.2.2.b Temperature-Programmed Oxidation Experiments ...... 131 7.2.3 Activity and Selectivity Testing ...... 135 7.3 Conclusions to Investigation of Role of Graphitic Edge Plane Exposure ...... 140

CHAPTER 8. Effect of Ammonia Treatment on CNx Catalysts ...... 142

8.1 Background into CNx Catalysts Treated with Ammonia ...... 142 8.2 Results & Discussion ...... 143

8.2.1 Ammonia Treatment Process Observations on CNx Materials ...... 143

8.2.2 Surface Species Analysis of Treated CNx ...... 146

8.2.3 Activity and Selectivity Observations for CNx Oxidized and Treated with Ammonia ...... 148

8.3 Conclusions to Studies on CNx Treated with Ammonia ...... 151

CHAPTER 9. Use of Boron and Phosphorus in CNx Catalysts ...... 152

9.1 Overview of Purpose of Using Boron and Phosphorus in CNx Catalysts ...... 152

9.2 Graphitic Site Blocking of CNx ...... 154

9.3 Boron Incorporation into CNx ...... 156

9.3.1 Surface Species Analysis on Boron Incorporated CNx ...... 157

9.3.2 Surface Area and Pore Volume of Boron Incorporated CNx ...... 160

9.3.3 Activity of Boron-Incorporated CNx ...... 161

9.3.4 Acid Treatment of Boron-Incorporated CNx ...... 162

9.4 Conclusions for Graphitic Site Blocking and Boron Incorporation into CNx ...... 164 CHAPTER 10. Evaluation of RRDE Testing Methods and their Impact on Reported ORR Catalyst Results ...... 166 10.1 Background on RRDE Testing ...... 166

10.2 Examination of Catalyst Loading Effects on the Selectivity of CNx and Pt/VC ORR Catalysts using RRDE ...... 168 10.2.1 Effect of Catalyst Loading on Selectivity ...... 170 10.2.2 RRDE Catalyst Layer Thickness Observations ...... 176

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10.2.3 Electroreduction of H2O2 by CNx Catalysts ...... 182 10.3 RRDE Catalyst Ink Aging Effects on Selectivity to Water Formation in ORR . 184 10.3.1 RRDE Results with Ink Aging ...... 185 10.3.2 Role of Ethanol in Ink Aging ...... 188 10.4 Summary of RRDE Method Examinations ...... 190

CHAPTER 11. Methanol Tolerance of CNx Oxygen Reduction Catalysts ...... 192 11.1 Introduction to Direct Methanol Fuel Cell Cathode Catalysts ...... 192 11.2 MOR Activity and Effect of Methanol on ORR with Pt and Pt/Ru Catalysts .... 194

11.3 MOR Activity and Effect of Methanol on ORR with CNx Catalysts ...... 198 CHAPTER 12. Conclusions...... 206 CHAPTER 13. Recommendations for Future Work ...... 209 CHAPTER 14. Glossary of Acronyms ...... 214 REFERENCES ...... 217

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LIST OF FIGURES

Figure 1. PEM fuel cell schematic. Drawing by Paul Matter...... 4

Figure 2. Typical fuel cell polarization curve displaying the types of performance losses and power density...... 6

Figure 3. Monthly average Pt and Ru prices July 1992-March 201011 ...... 8

Figure 4. Examples of macrocycles used as catalysts for ORR. As published in31...... 14

Figure 5. Types of nitrogen groups in carbon and their binding energy positions in XPS. As published in 199...... 20

31 Figure 6. FeN2-type site. As illustrated in ...... 22

Figure 7. Proposed FeN2+2 active site from Dodelet group. Illustration courtesy of Dieter von Deak...... 23

Figure 8. Examples of carbon nanostructures. As published in199 ...... 31

Figure 9. Illustration of electrical conductivity within and between graphitic planes. .... 33

Figure 10. Armchair and zigzag graphitic edge planes ...... 39

Figure 11. Oxygen functional group locations on graphite...... 40

Figure 12. Acetontrile pyrolysis treatment conditions...... 51

Figure 13. Stacked platelet nanofiber...... 52

Figure 14. Setup for Rotating Ring Disk Electrode. Figure courtesy of Dieter von Deak...... 66

Figure 15. Typical RRDE activity results. (Catalyst: CNx on 2%Fe/MgO-HCl washed). 69 xv

Figure 16. Sample comparison of ring current corrected for collection efficiency to

absolute disk current using the RRDE method (Catalyst: CNx on 2%Fe/MgO-HCl washed)...... 70

Figure 17. Weight gain during pyrolysis of catalysts made with thiophene:acetonitrile combinations (y1 axis, circles) and the BET surface area of those materials (y2 axis, triangles) as published in325...... 78

Figure 18. TEM images of a) CNx- no thiophene, b) CNx-1.8% thiophene, c) CNx-7.3% 325 thiophene, d) CSy-100% thiophene. As published in ...... 80

Figure 19. Onset of activity comparison for thiophene containing catalysts. Inset:

Activity comparison of the linear scan at 1000 rpm in oxygen saturated 0.5M H2SO4 for 325 CNx -8.9% thiophene and 100% thiophene catalysts. As published in ...... 82

Figure 20. Hydrophobicity comparison of CNx, CNx(8.9%) and CSy catalysts showing their dispersion in water. As published in325...... 84

Figure 21. XPS S 2p region for selected CNx-thiophene catalysts (labels are mol% thiophene in acetonitrile solution) deconvoluted using two doublets, b) XPS S 2p region 325 deconvoluted using three doublets for CNx-8.9%thiophene. As published in ...... 87

Figure 22. XPS N 1s region deconvoluted for select CNx-thiophene catalysts (labels are mol% thiophene in acetonitrile solution). As published in325...... 89

Figure 23. XPS O1s region deconvoluted for select CNx-thiophene catalysts (labels are mol% thiophene in acetonitrile solution). As published in325...... 91

Figure 24. Locations of possible oxygen functional groups by XPS binding energy. As published in325...... 92

Figure 25. a) Selectivity, n, (number of electrons transferred per oxygen molecule) from RRDE as a function of normalized oxygen surface content. Inset: Visual comparison of disk (absolute current) to ring (after adjustment for theoretical collection efficiency) currents at 100rpm in oxygen saturated 0.5M H2SO4 for CNx-8.9%thiophene. b) Comparison of ring to disk currents used in determination of selectivity from RRDE.

System is oxygen-saturated 0.5M H2SO4 with background currents subtracted for both ring and disk. As published in 325...... 95

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Figure 26. TPOs ran in 10%O2/He a) TG signal, b) Mass signals for 44 (CO2), 30 (x50) (NOx), and 64 (x150 for CNx-1.8%, 3.6%, 5.5% and 8.9% thiophene; x10 for 100% 325 thiophene catalyst) (SO2). As published in ...... 96

Figure 27. TPD-TPO experiment for CNx-3.6% thiophene a)TPD in He showing mass signals 18 (H2O), 44 (CO2) and 64 (SO2), b) TPO in 5%O2/He showing mass signals 30 325 (NOx), 44 (CO2) and 64 (SO2). As published in ...... 98

Figure 28. Wet-TPD-TPO experiment for CNx-3.6% thiophene a) wet-TPD in 10%H2O/He showing mass signals 44 (CO2) and 64 (SO2), b) TPO in 5%O2/He showing 325 mass signals 30 (NOx), 44 (CO2) and 64 (SO2). As published in ...... 101

Figure 29. TEM images of a) and b) CNx grown on Fe/MgO, and c) and d) CNx-HNO3. As published in199...... 105

Figure 30. O 1s XPS spectra for CNx and CNx -HNO3...... 107

Figure 31. Comparison of ORR activity for CNx and CNx-HNO3 measured using RRDE at 1000 rpm. Inset: Selectivity of CNx and CNx-HNO3...... 108

Figure 32. TEM images of stacked platelets-HCl washed. Inset to a): Illustration of graphite sheet orientation in stacked platelets. Box in a) refers to region where b) was taken. As submitted to338...... 115

Figure 33. TEM images of nanofibers with basal plane exposure after HCl washing. Insets represent the lower-magnification images of the same nanofibers. As submitted to338...... 117

Figure 34. TEM images of a) stacked platelets and b) nanofibers with basal plane exposure after acid oxidation and ammonia treatment at 900°C for 19.5h. As submitted to338...... 121

Figure 35. N 1s region of the X-ray photoelectron spectra comparing stacked platelets after oxidation and ammonia treatments. Signal intensities were magnified where noted for comparison. As submitted to338...... 124

Figure 36. O 1s region of the X-ray photoelectron spectra comparing stacked platelets after oxidation and ammonia treatments. Signal intensities were magnified where noted, for comparison. As submitted to338...... 126

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Figure 37. S 2p region of the X-ray photoelectron spectra for stacked platelets-HCl- 338 HNO3:H2SO4. As submitted to ...... 128

Figure 38. O 1s region of the X-ray photoelectron spectra comparing nanofibers with basal plane exposure after oxidation and ammonia treatments. Signal intensities were magnified where noted for comparison. As submitted to338...... 130

Figure 39. Temperature programmed oxidation profiles for stacked platelets after

oxidation and ammonia treatments. Gray is m/z=44 (CO2), pink is (m/z=30) x50 (NOx), green is m/z=64 (SO2) x500 for all treatments except “HCl-HNO3:H2SO4” which is x100. As submitted to338...... 133

Figure 40. Temperature programmed oxidation profiles for nanofibers with basal plane

exposure after oxidation and ammonia treatments. Gray is m/z=44 (CO2), pink is (m/z=30) x50 (NOx), green is m/z=64 (SO2) x500 for all treatments except “HCl- 338 HNO3:H2SO4” which is x100. As submitted to ...... 135

Figure 41. a) ORR RRDE results of stacked platelets after oxidation and ammonia treatments at 1000rpm in oxygen saturated electrolyte after background subtraction. b) Selectivity results. As submitted to338...... 137

Figure 42. ORR RRDE results of nanofibers with basal plane exposure after oxidation and ammonia treatments at 1000rpm in oxygen saturated electrolyte after background subtraction. As submitted to338...... 138

Figure 43. TEM images of broken nanofibers on CNx-conc HNO3-900°C NH3...... 145

Figure 44. TEM images of CNx-conc HNO3 treated...... 145

Figure 45. N1s spectra for ammonia treated CNx...... 148

Figure 46. RRDE comparison for ammonia and nitric acid treated CNx. (top) Activity comparison on disk, (bottom) Selectivity comparison from ring results...... 150

Figure 47. Site blocking on graphitic edges by boron and phosphorus groups. Illustration courtesy of Paul Matter...... 153

Figure 48. RRDE activity results for site-blocked CNx...... 155

Figure 49. B 1s spectra for boron incorporated CNx...... 158

Figure 50. Atomic ratios of surface elements on boron incorporated CNx...... 160 xviii

Figure 51. RRDE activity for boron-incorporated CNx ...... 162

Figure 52. B 1s spectra from XPS for CNx-B:C=0.10 washed with 5M HCl...... 163

Figure 53. ORR activity for B-CNx catalysts before and after 5M HCl treatment...... 164

Figure 54. Select catalyst disk loadings for activity testing from literature. Authors listed are first authors44,122,126,167,296,356-359,362...... 169

Figure 55. Example comparison of ring current (top) to disk current (bottom) in oxygen- saturated 0.5M H2SO4 at 1000rpm after background subtraction for a CNx catalyst with 142 μg/cm2 loading, which had the lowest selectivity to water formation out of all tests performed. Inset: Absolute current comparison of ring to disk after adjustment for theoretical collection efficiency...... 172

Figure 56. Selectivity as a function of catalyst loading on disk. Selectivity is taken at 0.5

V vs. NHE. Theoretical collection efficiency of N=0.22 was used. a) CNx catalyst, b) Pt/VC catalyst...... 174

Figure 57. SEM top view images showing the distribution of catalyst on a model aluminum rod with same geometric area as the glassy carbon electrode. a) for 142 μg/cm2 2 CNx catalyst loading and b) 1420 μg/cm CNx catalyst loading...... 178

Figure 58. Example SEM side-view images of CNx catalyst loadings on aluminum rods. a) 142 μg/cm2 loading. Thickness indicated in image is 17.8 μm. b) 426μg/cm2 loading. Thickness indicated in image is 52.1 μm. c) 1420 μg/cm2 loading. Thicknesses indicated in image are 207 μm and 424 μm...... 179

Figure 59. Average CNx catalyst thickness as a function of catalyst loading on the disk. Error bars show 1 standard deviation...... 180

Figure 60. Selectivity as a function of average CNx catalyst thickness. Selectivity values taken at 0.5V vs. NHE...... 181

Figure 61. (Top) ORR activity on CNx as a function of rotation rate in oxygen saturated electrolyte, (bottom) hydrogen peroxide reduction activity as a function of rotation rate in

6.25 mM H2O2...... 183

Figure 62. a) Ring (top) and disk (bottom) currents for RRDE tests on CNx catalysts exposed to different ink aging periods. (Measurements in oxygen-saturated 0.5M H2SO4 at 100rpm with 142 μg/cm2 of catalyst loading on the disk). (b) Resulting oxygen

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reduction reaction selectivity as a function of age of the RRDE ink used. As accepted in353...... 186

Figure 63. ORR selectivity as a function of RRDE catalyst ink age for Pt/VC catalyst with a 426 µg/cm2 loading on the disk at 100rpm. As accepted in353...... 187

Figure 64. ORR selectivity results for CNx treated with ethanol (Catalyst loading on disk: 426 μg/cm2). As accepted in353...... 190

Figure 65. Cyclic voltammetric scan for 20%Pt/VC from 1.2 to 0.2 to 1.2V vs. NHE in

0.5M H2SO4 saturated with oxygen at 0 rpm showing both 1.0M methanol and methanol- free systems...... 195

Figure 66. Reduction sweep voltammogram for 20%Pt/VC in 0.5M H2SO4 solutions at 1000 rpm showing both 1.0M methanol and methanol-free systems...... 196

Figure 67. Reduction sweep voltammogram for Pt:Ru/VC in 0.5M H2SO4 solutions at 1000 rpm showing both 1.0M methanol and methanol-free systems...... 198

Figure 68. Cyclic voltammetric scan for CNx grown on Fe/Al2O3 from 1.2 to 0.0 to 1.2V vs. NHE in 0.5M H2SO4 saturated with oxygen at 0 rpm showing both 1.0M methanol and methanol-free systems...... 200

Figure 69. Cyclic voltammetric scan for CNx grown on Ni/Al2O3 from 1.2 to 0.0 to 1.2V vs. NHE in 0.5M H2SO4 saturated with oxygen at 0 rpm showing both 1.0M methanol and methanol-free systems...... 200

Figure 70. Reduction sweep voltammogram for CNx grown on Co/SiO2 in 0.5M H2SO4 solutions at 1000 rpm showing both 1.0M methanol and methanol-free systems...... 202

Figure 71. Cyclic voltammetric scan for CNx grown on Fe/Al2O3 from 1.2 to 0.0 to 1.2V vs. NHE in 0.5M H2SO4 saturated with oxygen at 0 rpm shown both 3.0M methanol and methanol-free systems...... 204

LIST OF TABLES

Table 1. Types of fuel cells and their features ...... 2 Table 2. Surface composition of catalysts from XPS as published in325...... 85

Table 3. Surface species (atomic %) on CNx and CNx-HNO3 from XPS analysis. As published in199 ...... 107 Table 4. Mass loss during ammonia treatment. As submitted to338...... 120 Table 5. Surface species concentrations from XPS analysis on stacked platelets after oxidation and ammonia treatments. As submitted to338...... 122 Table 6. Surface species concentrations from XPS analysis on nanofibers with basal plane exposure after oxidation and ammonia treatments. As submitted to338...... 129

Table 7. Mass losses during heat treatment of CNx catalysts...... 144

Table 8. Surface species composition (atomic%) from XPS on CNx after ammonia treatments...... 147

Table 9. Surface chemical species (atomic%) composition for boron incorporated CNx catalysts ...... 159

Table 10. Surface area and pore volume for select Boron incorporated CNx ...... 161 368 Table 11. CNx catalysts tested in 1.0M Methanol+0.5M H2SO4 solution ...... 203

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CHAPTER 1. Introduction

As demands for energy increase and pollution regulations become stricter, new

fuel sources and alternative energy conversion devices are needed. The fuel cell has been

promoted as an energy conversion device that can use alternative fuels such as hydrogen

and methanol, as well as more traditional fuel sources such as methane and diesel,

depending upon the type. Fuel cells are attractive because of their varied fuel sources,

increased fuel efficiency and less polluting exhaust. Fuel cells types are divided by their

operating temperature, component materials and fuel. Some of the more common fuel

cell types and their unique features are listed in Table 1. The direct methanol fuel cell

(DMFC) is actually a sub-type of the proton exchange membrane fuel cell (PEMFC),

where very little is changed in the system beyond the fuel source and the electrolyte

thickness.

PEMFCs and DMFCs have the most promise for mobile applications such as cell phones, laptops and automobiles, because of their low operating temperature (60-80°C) and relatively fast start-up time compared to higher operating temperature fuel cells.

Their solid polymer electrolyte also aids in the manufacturability and lifetime of the fuel cell because corrosive electrolyte leaks are eliminated.

Table 1. Types of fuel cells and their features Identifying Operating Type of Fuel Cell Fuel Components Temperature Variety; hydrogen, diesel, All components Solid Oxide (SOFC) 800°C-1000°C methane, coal ceramic gas and more Molten Carbonate Carbonate salt Hydrogen 650°C (MCFC) electrolyte Liquid Phosphoric Acid (PAFC) Hydrogen phosphoric acid 150°C-200°C electrolyte Liquid alkaline Alkaline (AFC) Hydrogen 90°C-100°C electrolyte Proton Exchange Membrane or Polymer Solid polymer Hydrogen 80°C Electrolyte Membrane electrolyte (PEMFC) Direct Methanol Fuel Cell Aqueous Solid polymer 60°C (DMFC) methanol electrolyte

Today, there are limited PEMFC and DMFC systems available on the market.

Most systems seen are exploratory in nature. The first emerging market for DMFCs appears to be that of battery replacements for laptops as high energy density is required and the systems are small enough for cost to not be as significant of an issue. All of the major auto companies have fuel cell prototypes, but each one reportedly cost over $1 million to produce because of the economy of scale, raw material expenses and engineering time put into them. The city of San Francisco, as well as a few European cities, have partnered with fuel cell companies to operate PEM fuel cell buses for actual use. Pilot test facilities have also been installed for PEM fuel cell operated fork lift 2 operations in warehouses1-3. The military has made some investments in fuel cell research in hopes of reducing the battery weight carried by soldiers in the field. These examples illustrate the interest in fuel cells for many applications and in some cases, the acceptance of their use by the general public. Significant advances are still needed, however, before fuel cells can move out of such niche trial markets.

Fuel cells are electrochemical devices that allow for conversion of chemical energy directly into electrical energy by avoiding the restricting heat generating Carnot cycle. Fuel cells differ from batteries in that they are continuous systems rather than batch. A fuel cell is made up of two electrodes, an electrolyte material that separates the electrodes, and the circuit that connects the two electrodes. At the anode, fuel oxidation occurs. At the cathode, oxidant reduction occurs. The electrolyte separates the electrodes so that a short circuit does not occur and the fuel and oxidant do not mix, while still allowing for the transport of selected ions to the opposite electrode. The electrons that are not able to pass through the electrolyte are forced through an external circuit, producing useable electricity.

In a PEM fuel cell, hydrogen is the fuel fed to the anode. It is oxidized in the following reaction:

- + 4 H2  2e + 2H 0.00 V vs. NHE (HOR)

Where all potentials are at the standard temperature of 25°C. A schematic of a PEM fuel cell is shown in Figure 1. The electrons move through the circuit to the cathode, while the protons move through the ion selective membrane, commercially perfluorosulfonic

3 acid (Nafion), to the cathode to participate in the following reaction when oxygen or air is

fed to the cathode:

+ - 4 O2 + 4H + 4e  2H2O 1.23 V vs. NHE (ORR)

The overall reaction for a PEM fuel cell is:

H2 + ½ O2  H2O 1.23 V vs. NHE

PEM fuel cells operate at about 80°C. Operation much above this level is limited by the degradation of the Nafion membrane.

Figure 1. PEM fuel cell schematic. Drawing by Paul Matter.

In a DMFC, the feed to the anode is 0.5-2M aqueous methanol5. Methanol is oxidized by the reaction: 4 + - 6 CH3OH + H2O  CO2 + 6H + 6e 0.02 V vs. NHE (MOR)

The cathode reaction remains the same in the DMFC. Normally the membrane is also

Nafion. The overall DMFC reaction is:

CH3OH + 3/2O2  CO2 + 2H2O 1.21 V vs. NHE

Typical operating temperatures for DMFCs are about 60°C, because of the desire to keep methanol in the liquid state. The MOR is irreversible, meaning the deviation in potential at which the reaction begins is far from the theoretical potential. This causes power output losses (Figure 2 shows how power is a function of current and potential). The advantage of DMFCs is the convenience of liquid fuel handling, opposed to compressed hydrogen gas.

There are three main types of performance losses in fuel cells: kinetic losses, ohmic losses and mass transfer losses. Kinetic losses in a fuel cell are observed by the initial drop from the theoretical operating voltage as shown in part A in the fuel cell

polarization curve in Figure 2. Kinetic losses are typically due to deviations from the

thermodynamic potentials of the reactions. These kinetic losses are related to the

catalysts present at the anode and cathode. Ohmic losses are observed in section B of the

fuel cell operation curve. Ohmic losses are due to resistances in the fuel cell. These can

be due to membrane resistances and poor electrical conductivity in the catalyst layers.

Mass transfer losses are observed by the drop in potential in section C. Water formation

blocking the active sites of the catalyst is a main cause of mass transfer losses.

5 While research needs to be continued on all components that cause performance

losses in fuel cells, the oxygen reduction reaction (ORR) is one of the largest contributors to fuel cell performance losses in PEM and DM fuel cells. The ORR in acidic media, which both the PEMFC and DMFC are, is highly irreversible. The ORR is the limiting reaction in the PEM and direct methanol fuel cells. The best catalysts developed allow for the ORR to occur at 0.9V vs. NHE. That is an overvoltage of 0.3V. This means that the performance of a PEM fuel cell is automatically dropped by at least 0.3V before any current is ever drawn.

Figure 2. Typical fuel cell polarization curve displaying the types of performance losses and power density.

6 The commercial catalyst used in today’s PEM and DM fuel cells is Pt supported

on Vulcan carbon, a type of carbon black. Due to the high overvoltages at the cathode,

the cathode is loaded with significantly more Pt than the anode. State-of-the-art Pt loadings on the anode are 0.05 mg/cm2, while the cathode has 0.4 mg/cm2 7.

Platinum is an expensive noble metal. Using the current catalyst loadings, the Pt

costs would be prohibitively high for use in an automobile fuel cell system. For an 85

kW fuel cell system that could be used in 75 kW (100 hp) vehicle, the Pt costs alone

would be $3200-$4500 8-11 using March 2010 Pt US market prices. For most standard

automobiles today, the catalyst costs in a fuel cell system would be much higher due to

their higher power demands. The platinum market price has fluctuated widely in the past

decade with an overall upward trend, slowed by the recent global economy, as seen in

Figure 3. If this 75 kW system was priced at the peak of the market in March 2008, the

system would have cost an additional $2000.

Pt is also available in only a few regions of the world. The largest reserves of Pt

are located in South Africa and the Ural Mountain regions of Russia. Political instability

in these regions could cause disruptions in the Pt supplies for fuel cell production. The

supply is limited as well. Some estimate that with the current catalyst loadings, there

would only be enough Pt to outfit 10-20% of the cars produced annually with fuel

cells8,12.

Figure 3. Monthly average Pt and Ru prices July 1992-March 201011

Cost and availability are not the only disadvantages that Pt catalysts have. Pt is known to sinter during fuel cell operation, where the nano-sized Pt particles agglomerate into larger particles, therefore reducing the active surface area of the catalyst13. In

DMFCs, methanol crossover, where methanol leaches through the membrane to the cathode, is a problem. When methanol reaches the cathode, the Pt reacts with the methanol, taking away active sites for ORR in a competing reaction, causing a mixed potential. The mixed potential causes coulombic losses in the fuel cell. The operating potentials are required to be below 0.5V vs. NHE to have favorable ORR, which drops the efficiency of the fuel cell significantly 14,15.

8 Selectivity of the cathode catalyst is also a major concern. Oxygen can be

reduced to form either water or hydrogen peroxide. The formation of hydrogen peroxide

occurs in the undesirable side reaction:

+ - O2 + 2H + 2e ↔ H2O2 0.695 V vs. NHE

With only two electrons being transferred in this process, there is a drop in the current.

More importantly, hydrogen peroxide degrades the components of the fuel cell and can

significantly reduce the overall lifetime of the fuel cell16.

To reduce the cost, increase current, reduce the overpotential, improve selectivity,

and provide methanol resistance, many catalyst alternatives to Pt/VC have been studied.

These include Pt-alloys, chalcogenides, other noble metal transition metal catalysts, and

non-noble metal catalysts. The difficulty in finding ORR catalyst replacements to Pt for

fuel cells are increased due to the acidic and oxidizing fuel cell environment.

Among the Pt-free catalysts, the chalcogenide and non-noble metal C-N catalysts

have been the subject of most of the non-Pt ORR studies. Chalcogenide ORR catalysts

have especially been studied as DMFC ORR catalysts due to their inactivity for methanol

oxidation6,17-23. In general, the most active ORR chalcogenide catalysts are ruthenium

containing. Ruthenium containing catalysts are also subject to wide fluctuations in price

and are much higher in cost in recent years due to the increased demand for catalyst

materials. Figure 3 illustrates the price climb in both Pt and Ru since 1992. With the flexibility in precursors, C-N catalysts do not have to be subject to the high costs similar to the noble-metal markets.

9 CHAPTER 2. Research Objectives

The overall objective of this research is to study nitrogen-containing carbon nanostructures (CNx) as catalysts for the oxygen reduction reaction (ORR) in acidic media for use in PEM and direct methanol fuel cells (DMFCs). This area of research addresses two main needs:

- Improving ORR activity and selectivity. Currently, these catalysts underperform

compared to the commercial Pt-based catalysts. Improvements in current density

and overpotential are needed before these catalysts can become viable

replacements to platinum.

- Understanding the nature of activity. There is much debate in the field on what

makes these types of catalysts active. Some believe that an iron stabilized by

nitrogen supported on carbon is the source of activity. Others believe that a

transition metal is not part of the active site and that nitrogen incorporated into the

graphitic matrix of the carbon may be the cause of activity.

These two needs go hand-in-hand with each other. In order to design better performing catalysts, the active site should be well understood.

To address these needs, the research program had multiple focal points. The role

that heteroatoms and surface functional groups beyond nitrogen plays in these CNx

catalysts have been studied using sulfur and boron heteroatoms and oxygen functional groups. Nanostructure of the carbon has been suggested to play a role in ORR activity.

The role of graphitic edge plane exposure in ORR activity has been investigated. The way the electrochemical testing methods are performed can also have an impact on the results reported. Electrochemical half cell testing methods have been investigated and the role that the catalyst treatment plays in these tests has been studied. In addition to these studies, it is also important to understand what environments these CNx materials

can perform in. Testing of catalyst performance for ORR in the presence of methanol has

also been studied.

CHAPTER 3. Literature Review of Carbon-Nitrogen ORR Catalysts and Related Materials

3.1 Carbon-Nitrogen ORR Catalysts

The high costs and limited availabilities associated with noble-metal based ORR

catalysts have prompted research on catalysts that are free of noble metals. The materials

that can be studied as ORR catalysts for PEM and direct methanol fuel cells is limited

due to the acidic and oxidizing environment of the fuel cell. Obtaining materials that are

able to remain stable in this environment is difficult. Carbon-nitrogen (C-N) materials

are the most studied class of non-noble metal ORR catalysts. Three generations of these

catalysts have been studied in the literature based upon their starting materials and

treatment conditions: non-pyrolyzed macrocycles, pyrolyzed macrocycles and non-

macrocycle-based catalysts.

3.1.1 Non-Pyrolyzed Macrocycles as ORR Catalysts

Carbon-based materials have been studied as ORR catalysts for nearly five

decades. The first of such catalysts were transition-metal-containing macrocycles

12 supported on high surface area carbons24,25. Generally macrocycles are large molecules with a cyclic ring. Inside these rings, metal ions can be stabilized. For these purposes, there were metal ions stabilized by the central nitrogen groups such as shown in the examples of Figure 4. Macrocycles were selected for study as a model to the nature- inspired hemoglobin-type active site for oxygen reduction. Jasinski first reported on these materials as ORR catalysts in alkaline media using cobalt phthalocyanine in

196424,25. While the macrocycles showed substantial activity for the electroreduction of oxygen, they proved to be unstable in the fuel cell acidic environment that is required for

PEM and phosphoric acid fuel cells26. For this reason, macrocycles supported on carbon without further treatment have not been studied much in the last thirty years. Several reviews have gone in-depth into the history of these untreated macrocycles used as ORR catalysts26-30.

Figure 4. Examples of macrocycles used as catalysts for ORR. As published in31.

3.1.2 Pyrolyzed Macrocycles as ORR Catalysts

Beginning in the 1970’s, Jahkne27 and others26,28,32-36 found that heat treating the macrocycles on carbon support in an inert environment, or pyrolyzing, produced an ORR catalyst that still was active, but now had a greater stability in the fuel cell environment.

Several reviews have covered these catalysts in detail as well26,28-31,37,38. It was observed

that this pyrolysis step could be as low as 400°C and as high as 1000°C26-28,32-36,39-41, to

increase stability while maintaining activity. The majority of the materials studied

showed that treatment temperatures closer to 1000°C were more desirable for stability,

however27,41.

A variety of metal ions have been used in the pyrolyzed macrocycle catalysts.

The metal ions used in the pyrolyzed macrocycles have included iron26,28,36,39,40,42-74,

cobalt27,33,34,41,46,49,62,65-68,70,73,75-101, nickel49,67,73 and copper68,73, among others. Iron- and

14 cobalt-centered macrocycles have been studied the most. This is due to their higher

activity compared to the other metal-containing macrocycle systems. The types of

macrocycles used in these heat-treated studies include porphyrins33,39,45,52,54-

62,66,67,70,72,74,81,84-87,98,99 (including tetraphenylporphyrins (TPP) and tetramethoxyphenyl

porphyrins (TMPP)), phthalocyanines (Pc)36,40,44,49-51,53,64,75,91-97 and

dihydrodibenzotetraazaannulenes (TAA)34,76-78 most commonly, with other macrocycle

derivatives on occasion. In addition to transition-metal-containing macrocycles, non-

metal macrocycles have been pyrolyzed to form ORR catalysts34,65,76,90,102. While these

catalysts were not as active as the iron- and cobalt-containing catalysts, they still had

substantial ORR activity.

While the early work in pyrolyzed macrocycles as ORR catalysts focused on

creating an ORR active catalyst that would maintain adequate stability by studying

different macrocycles and treatment temperatures, use of pyrolyzed macrocycle catalysts

more recently has been focused in two main areas—studying of the active site and

altering the preparation methods such as carbon treatment46,49,89,103-105. The active site

discussions will be covered in a later section of this literature review.

3.1.3 ORR C-N Catalysts Derived from Simple Precursors

While the pyrolyzed macrocycles showed improved stability in acidic media and

were active for ORR, the macrocycle precursors did not prove to be an economical

solution. The cost of macrocycles is still very high. Gupta, et al. were among the first to

report using precursors other than macrocycles to get similar C-N ORR catalysts106. 15 They heat treated polyacrylonitrile supported on a carbon black mixed with an iron or

cobalt salt. This heat-treated mixture resulted in an ORR active catalyst. Since this time,

pyrolysis of a variety of simple sources of carbon, nitrogen and transition metals together

has successfully been used as ORR catalysts in acidic electrolytes. Reviews of these non-

macrocycle ORR catalysts have been published31,37,107-110, including a review from our

research group31. Recently, a cross-laboratory study was published on C-N catalysts

using multiple preparation techniques and precursors also111.

3.1.3.a Materials and Preparation of C-N Catalysts Derived from Simple Precursors

Primarily iron63,111-164 and cobalt82,111,113,144,145,151-157,165-182 have been used as the

metals in these non-macrocycle C-N ORR catalysts, but nickel140-142,144,145,151,152 and other

metals have also been used with less success. Catalysts made with iron or cobalt have

shown the most ORR activity. While those made with nickel have been less active. The

source of the iron or cobalt is usually a salt like acetate116,119,141-145, hydroxide63,114, chloride63,164 or sulfate183. These transition metal salts are normally supported on a type of carbon black. Typical metal loadings range from 50 ppm to 10wt%119,144. A maxima

in loading typically occurs, but the concentration depends upon the metal precursor,

support and heat treatment. In addition, ORR C-N catalysts have been made in the absence of a transition metal140-142,144,145,184-187. These catalysts have had measurable

ORR activity, but are frequently less active than the Fe- and Co-containing C-N catalysts.

Some have cited the ORR activity for these transition-metal free C-N ORR catalysts

being due to contamination of the support with iron or other metals26,121.

16 Many types of supports have been studied for the C-N catalysts, with the majority

being various types of carbon black. The commercial ORR platinum catalyst carbon

black support is Vulcan carbon XC-72 and has been used frequently as a support to the transition metals in C-N catalysts118,142,164,169. The Dodelet research group has done

extensive work examining the role that carbon black type plays in the ORR

performance121,123-126,129-132. Early on, they found that carbons with more amorphous

structures were more reactive to the nitrogen-containing treatments resulting in better performing catalysts121,123. More recently, they have found that supports with high

microporosity have better performance125,126. Others have examined non-carbon black

supports with some success. These supports include activated carbons135,165,186, carbon

nanotubes159,178,179,188-191 and oxides140,141,143-145,192. In our research group at Ohio State,

we have shown that a carbon-based support is not necessary as along as a carbon source

is introduced into the heat treatment step of the catalyst development140,141,143-145. In

place of the carbon support, oxide supports of alumina, silica and magnesia have been

used. These oxide supports were used so that the role of carbon black could be removed

from the study. Additionally, the oxide supports can be removed with acidic or basic

washes to make a more concentrated catalyst with higher electrical conductivity

(compared to when the oxide is still present).

When a macrocycle is not being used in the preparation of the C-N catalyst, the nitrogen source is commonly introduced in the gas phase at an elevated temperature over the transition metal supported material. Ammonia treatments over transition metal supported carbon blacks at temperatures similar to macrocycle pyrolysis temperatures have successfully produced active ORR catalysts88,118,123,125,135,137,138,159,187. The ammonia 17 concentration, duration, type of carbon black and carbon pretreatments all play a role in

the success of the resulting material as an ORR catalyst, in addition to the transition metal

used. Higher temperatures and ammonia concentrations along with increased treatment

durations tend to improve ORR activity. Carbon blacks that are more amorphous118,123

and have more microporosity124,125 also respond better to ammonia treatments. An additional carbon oxidizing pretreatment also has increased the success of ammonia

118,123 treatments on creating ORR-active catalysts . Acetonitrile (CH3CN) introduced in the gas phase at elevated temperature over transition metal supported catalysts is also commonly used creating C-N ORR catalysts63,140-145,178,193. The acetonitrile can serve as

both the carbon and nitrogen source as well. In our group, we have used acetonitrile as

the only source of nitrogen and carbon when producing nitrogen-containing carbon

140,141,143-145 nanostructured (CNx) catalysts over oxide supports .

Nitrogen-containing polymers have also been used as the carbon and nitrogen

source in C-N ORR catalysts, as related back to the early work of Gupta, et al. heat-

treating polyacrylonitrile with a transition metal over a carbon support106. Polymers used

include poly(4-vinylpyridine)172, polyaniline174, polypyrrole174, poly(3-

methylthiophene)174 and poly-o-phenylendiamine156. Typically, a metal is still mixed in

the polymer matrix as a cobalt or iron support prior to heat treatment156,172,174.

In 2006, Bashyam and Zelenay reported on the preparation of a C-N ORR catalyst from synthesizing a cobalt polypyrrole without a heat treatment step194. Unlike the non-

heat treated macrocycles studied decades ago, this catalyst showed remarkable stability in

fuel cell testing. After 100 hours of fuel cell operation, no substantial activity was lost.

18 The stability results for the new cobalt polypyrrole catalyst were considered better than

the pyrolyzed marcocycles and related heat-treated non-macrocycle C-N catalysts. ORR activity of these catalysts was also substantial and comparable to other C-N catalysts.

While Bashyam and Zelenay created what they called a “new class” of ORR C-N

catalysts, nearly all of the work published in the four years since has been on the heat- treated C-N materials prepared using both simple49,88,127-132,134-139,146,147,150,159,160,162,164,170-

172,184-186,192,195-197 and macrocycle45,46,74,83,85-87,89,100,103-105,198 precursors.

3.1.3.b Characteristics of C-N ORR Catalysts

Much of the work on C-N ORR catalysts derived from simple precursors have used similar precursors. The differences in the studies have involved investigating the treatment conditions and developing a better understanding of these C-N materials.

Three main characteristic areas have been the focus of much of the attention: surface species, microporosity and nanostructure. Much of the understanding developed is also transferable to the pyrolyzed macrocycle catalysts.

The concentrations and types of carbon, nitrogen and transition metal have been extensively examined on the C-N catalysts. To a lesser degree, oxygen has been examined. X-Ray photoelectron spectroscopy (XPS) studies are typically used for identification of surface species and surface concentrations. Carbon is the major component of all C-N ORR catalysts. It appears to be largely graphitic in nature after

treatment. Some broadening of the graphitic peak in XPS has been reported with

introduction of nitrogen114,142. Because the graphitic peak is so dominant, other carbon

species are not frequently identified. 19 In most reports, pyridinic-nitrogen, located as a substituted heteroatom on the graphitic edge plane, and quarternary-nitrogen, a substituted nitrogen deeper in the graphitic plane bonded to at least three carbons, are identified111,123,140-142,144,145,156,186.

Figure 5 is an illustration of the possible types of nitrogen located in graphitic carbons.

Other nitrogens including pyrrolic-nitrogen have also been identified in some catalysts111,123,156,186. Typical total nitrogen surface concentrations are in the 1 to 9

atomic% range111,123,140-142,144,145,156,186.

Figure 5. Types of nitrogen groups in carbon and their binding energy positions in XPS. As published in 199.

Pyridinic-N has commonly been cited as a required species for ORR activity. It is

thought that the pyridinic-N species are the groups required to stabilize iron- or cobalt-

ions118,150. Others, including our research group, have hypothesized that pyridinic-N

alone may be contributing to the ORR activity140-142,144,145,156,184. In both cases, the

20 observation was that catalysts with increased pyridinic-N content had improved ORR

activity. More recently, some have observed increases in quarternary-N content with

ORR activity88,186.

In some cases, XPS has been used in identification and quantification of iron (or

cobalt) species in the C-N ORR catalysts. The small quantities of iron used limit the

usefulness of XPS, as high resolution and very long scan times are required to even

observe any iron at the typical concentrations. Iron is thought to bond to or be stabilized

by nitrogens119,120,146,147,149,195. Some have reported a macrocycle-like center with iron being stabilized by four nitrogens120. Others have reported iron being stabilized by two

nitrogens, as shown in Figure 6119,120. Time-of-Flight Secondary Ion Mass Spectrometry

(ToF-SIMS) has also been used in identifying Fe-N type species115,120. As the iron

content increases to percent levels, metallic iron has been identified to form43. Activity has not been shown to increase with metallic iron, resulting in the observation that a maxima with iron content occurs119,120.

31 Figure 6. FeN2-type site. As illustrated in .

Oxygen functionalities have been studied less, but are commonly reported as part of XPS analysis on C-N catalysts. Generally, only overall oxygen content is reported.

From most studies, it is unclear if oxygen plays a role in ORR performance on C-N catalysts. Any carbon-based catalyst exposed to air will have some oxygen functionalities on the surface because of oxygen species adsorption at room temperature200-202. Later sections of this chapter will cover oxygen functional groups in

more depth.

Recently, the porous structure of C-N catalysts has been analyzed more closely using nitrogen physisorption techniques125,128,138,187,203,204. Depending upon the carbon

support used, heat treatment conditions and post-pyrolysis treatments, the porosity of the resulting C-N catalysts can be different. Researchers in the Dodelet group have shown that microporosity plays an important role in ORR catalyst activity125,128,138,203. Catalysts

with a significant proportion of the pores being of diameters less than 2nm have shown to

22 improve the ORR activity of their catalysts125,128,138,203. They have hypothesized that two

Fe-N2 sites are located across from each other in the micropores, forming a pseudo Fe-N4

type active site, illustrated in Figure 7.

Figure 7. Proposed FeN2+2 active site from Dodelet group. Illustration courtesy of Dieter von Deak.

Work coming out of Oakridge National Laboratories recently has shown active

ORR catalysts using nitrogen-containing mesoporous carbons187,204 that contrasts to the microporous work from Dodelet. These carbons had a large quantity of mesopores, and very little micropores187,204. The quantity of micropores decreased when carbon was grown on mesoporous silica supports204.

The carbon nanostructures produced when C-N ORR catalysts are pyrolyzed have

been discussed less frequently than many other characteristics of the C-N catalysts.

23 However, in our research group140-142,144,145 and others205 observations have been made that carbon nanostructure may play a role in creating active ORR catalysts. We have found that catalysts that are mostly composed of stacked cup nanofibers with high graphitic edge plane exposure are more active towards ORR than catalysts composed mostly of nanotubes140,141,144,145. Several other observations are paired with the

nanostructures that may merely make the nanostructure observation happenstance rather

than a direct correlation to ORR activity. The first of these is that catalysts with mostly

stacked cup nanostructures also have more pyridinic-N content, which is located on the graphitic edge plane. The second is that the catalysts with the most high-edge plane exposure nanofibers were made with iron- and cobalt-containing supports using acetonitrile pyrolysis, while those with little edge plane exposure were made with nickel- containing supports140-142,144,145. These observations leave the door open to pyridinic-N

playing a role but not the nanostructure and the metals used being more significant than

nanostructure. Further studies of this were performed and are included in a later chapter.

While the C-N ORR catalysts have shown substantial activity towards ORR in

acidic media, they still are not as high of performing as the Pt/VC commercial catalysts.

Recent strives toward improved ORR activity have been published by the Dodelet group

in Science138 and discussed by Gasteiger as a promising material206. Gasteiger’s

comments are significant, as he previously declared C-N catalysts had little chance, if

any, of becoming viable commercial catalysts8. In the Science article from the Dodelet group, they show that their most recent generation of Fe-N-C catalysts had turnover frequencies (TOF) comparable to that of Pt/VC ORR catalysts138. This can be exciting

news, but high turnover frequency of a catalyst is not enough. A catalyst with a low 24 active site density can have a high TOF but still be a low performing material overall

because of the amount of material that would be needed. Additionally, determination of

an active site concentration to predict a TOF is difficult and has even been acknowledged

by the Dodelet group128. In addition, the active site is still being debated in the literature

and will be discussed further in a later section of this chapter.

This problem of lower concentration of active sites on C-N ORR catalysts becomes a challenge when full fuel cell testing is performed. For most studies, ORR catalysts are first studied for activity and selectivity using the electrochemical half cell techniques of rotating disk electrode (RDE) and rotating ring disk electrode (RRDE).

The better catalysts are then manufactured into membrane electrode assemblies (MEA) to be tested in full fuel cells. In a half cell thin-film method such as RDE and RRDE, mass transfer effects are not significant. In a full fuel cell, mass transfer plays an important role in the operation of the fuel cell. Substantial increases in the thickness of the cathode catalyst layer can increase the mass transfer losses, thereby reducing the performance of the fuel cell.

3.1.4 Active Site Debate in C-N ORR Catalysts

While C-N ORR catalysts continue to improve, there is still more improvement

necessary for them to become viable commercial catalysts. One key to developing better

catalysts is to increase the active site density on the materials. This will require a better

understanding of the nature of the active sites on C-N ORR catalysts and the ORR

mechanism which occurs on these materials. 25 Much debate has occurred in the literature on the nature of the active site in both

the pyrolyzed macrocycles and the C-N non-macrocycle catalysts. In general, it is

thought that the active site for both these types of catalysts is the same or similar. The

argument in the literature is on what the active site looks like. Metal (Fe,Co)-nitrogen

active sites are argued by one side45,46,69,112,116,119-121,124,125,129-

132,136,138,147,149,154,155,158,159,176,194,195,207, while non-metal active

sites34,35,57,58,65,88,101,140,141,143-145,156,169,170,184,186,197,205 are argued by the other.

Researchers involved in the active site debate have studied both pyrolyzed

macrocycle- and non-macrocycle-based C-N ORR catalysts to support their arguments.

Before the early work in simple precursors began, people were already questioning what

the active site of the ORR C-N catalysts was. Some believed that the macrocycle center

remained intact after pyrolysis treatment32,66,208. Others made observations that as the

temperature of pyrolysis increased above 600°C or 700°C, the macrocycle center

decomposed28,40,41. Additionally, when transition metal-free macrocycles were pyrolyzed

on a carbon support, ORR activity was still observed34,65,76,90,102. Some believed that

transition metal contamination in the carbon black was the source of the non-metal

macrocycle ORR activity26,121, as it had previously been shown that a transition metal like

iron or cobalt could be mixed with a non-metal centered macrocycle prior to heat treatment and the resulting pyrolyzed material was ORR active65,209.

The use of non-macrocycle precursors further elevated the active site debate for

C-N ORR catalysts. While the pyrolysis treatment of a metal, nitrogen source and carbon

26 source together could result in a macrocycle-like metal-nitrogen coordination, it was

more difficult to argue this than if the material originally had a macrocycle in it.

For about the last decade, the leading voice in support of the Fe-centered active

site has come from the Dodelet research group. Early on, Dodelet and others argued that

the iron center bound to two or four nitrogens was the active site for ORR (as shown in

Figure 6)26,119,120. This theory was supported by ToF-SIMS measurements, which showed

119,120 120 the presence of FeN2 groups and FeN4 groups . The iron ions needed to be dilute

enough to not separate into metallic iron, which they observed to reduce the ORR activity

of the catalyst119. More recently, Dodelet has expanded upon this active site model to

suggest that the true active site is the combination of two FeN2 active sites in near proximity to each other in carbon micropores to form a pseudo-FeN4 site (labeled an

125,128,138,203 FeN2+2 site) similar to the intact macrocycle, as illustrated in Figure 7 . A

comparison of FeN2+2 to FeN4 revealed a much more active FeN2+2 site in acidic

media203.

Several German research groups have worked together and separately in support

of the metal-N active site45,47,69,71,72,193. 57Fe Mössbauer has been used in several occasions by these researchers to examine their C-N catalsysts. Schulenburg, et al. concluded that a 6-fold coordinated Fe3+ was part of the active site72. They suggested the

site was an FeN4-type coordinated with two additional bonds to either oxygen or

carbon72. Koslowski, et al. reported observing two active sites through Mössbauer

45 analysis; an FeN4 and an CFeN2 site . They found that the CFeN2 site degraded in the

45 presence of H2O2, suggesting that the FeN4 site was more stable .

27 A large contingent of Japanese researchers have been actively involved in the

ORR C-N active site debate. For the most part, they have reported evidence of a metal- free active site88,172,185,186. Some of the research has involved X-Ray Absorption (XAS) experiments to study the activity of nitrogen species88. Others have been involved in

using non-carbon black carbon precursors to eliminate metal contamination in their

studies185,186. Maruyama has been the most vocal in support of heme-type active sites from this contingent146,147,149,195. The work supporting the Fe-N type sites coming from

this research group have also included unconventional precursors including bovine

hemoglobin149 and amino acids147.

In our own research group at Ohio State, we have contributed to the active site

debate by creating CNx catalysts grown by pyrolyzing acetonitrile over sol-gel alumina

supports made with less than 1ppm transition metals contamination140. This catalyst

showed substantial ORR activity. After an HF wash to remove the alumina support, the

catalyst was within 200 mV of the commercial Pt/VC catalyst, indicating significant

ORR activity without the presence of a transition metal140. It was also observed that

pyrolysis over an iron-containing alumina support created catalysts with even more

activity, while pyrolysis over nickel-containing alumina had less ORR activity compared

to the transition-metal free catalyst140,141. Several trends were observed with these catalysts. It was found that ORR activity increased with pyridinic-nitrogen content and

as well as with graphitic edge plane exposure of the nanostructures140,141. These trends

were also observed when Vulcan carbon142, silica144,145 and magnesia144,145 were used as

catalyst supports. It was hypothesized that iron and cobalt acted as catalysts for the

creation of active sites during pyrolysis rather than directly participating in the ORR 28 activity. While this hypothesis may be true, the studies were unable to rule out the

possibility of two active sites being present – one that was transition metal free and one that was a metal-nitrogen site.

To illustrate how uncertain this debate on the ORR active site of C-N catalysts is, several researchers have published conflicting papers themselves. The majority of these researchers once argued for an Fe-N or Co-N type active site, but more recently have argued for a metal-free active site100,178. This small shift in no way settles the debate,

however. There are still many papers being published arguing for each side.

In addition to investigations of the active site experimentally, there have been a

small number of studies examining the C-N catalyst active site debate using computational techniques. Metal-N active sites have been modeled recently for electron donation properties by in the Anderson group210-212. Titov, et al. used DFT to model

FeNx sites on carbon nanotubes paired with EXAFS simulation calculations to show

161 activity towards FeNx sites . Metal-free active sites have also been proposed for ORR

activity 35,57,64,101,205. Preliminary modeling of carbon-nitrogen systems for electron

donation properties has been performed by several groups197,213-217, which might provide

some insight into the possible active sites for ORR on C-N type catalysts. At this point, there still is no definitive answer to what the active site in the CNx catalysts is. This

makes it more difficult to improve the catalysts as the desired active site properties are

unknown.

29 3.2 Other Heteroatom-Containing and Functionalized Carbon Nanostructures

Nanostructured carbons do not only have potential for applications as fuel cell

catalysts. Carbon nanofibers (CNFs) are known for their high surface areas and

conductive properties218,219. CNFs have recently been investigated for use as catalyst

supports218-231, in lithium ion batteries 219,232,233 and in hydrogen storage devices 234-236.

Carbon nanofibers can have many different geometries and properties. They can also be heteroatom-doped or surface-functionalized to further alter the properties of the nanofibers.

3.2.1 Types of Carbon Nanostructure and their Growth Characteristics

The properties of carbon nanofibers are dictated by the geometry of the structure, degree of edge-plane exposure, fiber diameter, fiber length, degree of graphitization, and orientation of the graphite planes. Nanostructure is dependent upon the starting materials, catalysts and growth temperatures. Examples of some carbon nanostructures are shown in Figure 8. The structures in this figure vary greatly in the edge plane exposure and orientation of the graphite planes.

Figure 8. Examples of carbon nanostructures. As published in199 .

Single-walled and multi-walled nanotubes (SWNT and MWNT, respectively) are the most widely used and well characterized nanofibers. These nanotubes are composed of graphitic sheets that are rolled into a tube. The graphitic edge plane exposure is minimal, as edge planes are only located at the ends of the tubes, which are up to microns in length219. Multiple graphite sheet layers compose the tube in MWNTs, while SWNTs

are only 1 graphite layer thick. MWNTs and SWNTs have been made by a variety of

methods including chemical vapor deposition (CVD) using natural gas and xylene237, and

toluene vapors238 as the carbon sources. The catalysts have included many metal systems

such as cobalt supported on silica materials239, and bi-metallics like cobalt-molybdenum and iron-molybdneum238. Both MWNTs and SWNTs can have metallic properties219.

SWNTs with a certain chirality and diameter can also behave as a semiconductor219. One

drawback of using MWNTs or SWNTs is that most uses require high purity levels of the

nanotube type239,240. Purification to one type of nanotube is still a challenge239,240. 31 Herringbone nanofibers, also known as fishbone nanofibers, are composed of

graphitic sheets oriented at an angle to the longitudinal axis of the nanofiber. This

orientation leaves a large amount of graphitic edge plane exposure revealed with very

little basal plane exposure. Herringbone nanofibers have been reported to be made by

decomposition of ethylene and hydrogen over a bimetallic copper-nickel catalyst232.

They have been used in demonstration for use in lithium ion batteries232, hydrogen

storage235 and as catalyst supports227.

Stacked platelet nanofibers are nanofibers with graphitic planes stacked on one

another oriented perpendicular to the longitudinal axis. Once again, they have a large

amount of graphitic edge plane exposure with a smaller amount of basal planes. Stacked

232 platelets have been made by decomposition of CO-H2 gas streams over iron and iron- copper241 catalysts. Stacked platelets have shown to be useful as platinum227 and

nickel229,230 catalyst supports because the facets of the metal can be controlled and well-

adhered to the carbon edge planes227,229,230.

Ribbon nanofibers are an analog of stacked platelet nanofibers. In ribbon

nanofibers, the stacked graphitic sheets are oriented parallel to the longitudinal axis227.

This gives the nanofiber high edge plane exposure with increased electrical conductivity compared to stacked platelets. This is because electrical conductivity is high within the graphitic sheet, but low between graphitic sheets as illustrated in Figure 9. Ribbons have

242 been made from the decomposition of CO-H2 streams over iron-based catalysts . It has

been observed that the temperature of decomposition has the greatest effect on

32 determining if a stacked platelet or a ribbon nanofiber is formed242. Ribbons were also

shown to be useful as a catalyst support227.

Figure 9. Illustration of electrical conductivity within and between graphitic planes.

Stacked cup nanofibers are nanofibers that are similar to herringbone nanofibers

except that there are hollow spaces between groups of graphitic sheets in the interior of

the nanofiber. Stacked cup nanofibers also have some edge plane exposure. They also

have been called “bamboo” nanofibers in the literature. Stacked cup nanofibers have

been made with nickel and sulfur catalysts when decomposing natural gas and xylene237.

Stacked cup nanostructures are commonly grown over iron-containing catalysts when nitrogen precursors are used191,243-246, as will be discussed later in this chapter.

Many other nanofiber types have been reported in the literature, but the ones discussed above are a good illustration of the varieties. In addition to nanofibers, carbon can also form into other non-nanofiber nanostructures. This includes the nano-onions

33 that are illustrated in Figure 8 and nanocubes that formed with acetonitrile pyrolysis over

magnesia, as observed in our own research group144,145. The body of literature on such

non-nanofiber carbon nanostructures is much smaller.

Extensive work has been done on carbon nanofibers. The discussions above are

considered a very brief overview. For more in-depth investigations into non-nanotube nanofibers, Baker and Rodriguez have published extensively in the area218,225-

230,232,234,235,241,242,247-259. Nanotube reviews are prevalent and include231,260-262.

3.2.2 Nitrogen-Containing Carbon Nanostructures

Strelko, et al. have modeled carbons doped with nitrogen, phosophorus and boron

and found them all to have significant electron donation properties216, which may be why

there are so many areas in which C-N materials are being researched.

Many researchers have worked on developing C-N nanostructures for uses

beyond ORR catalysts. A variety of preparation techniques can be used. Depending

upon the technique, the resulting C-N nanofiber properties will vary including

nanostructure, nitrogen content and type, and graphitic nature of the carbon. Nitrogen

can be introduced during carbon nanofiber growth191,243-246,263-270 or in a post treatment

step as discussed in earlier sections with ammonia treatment88,118,123,125,135,137,138,159,187. C-

N precursors that have been used during the growth of carbon nanostructures include

acetonitrile191,245,263,265,267,268, melamine243,246,264 and pyridine245,266,268-270. Ammonia also

34 can be introduced at low levels during carbon growth with hydrocarbon type

materials244,266.

Because of the wide variety of treatment conditions that can be used to obtain

nitrogen containing carbon nanostructures, only a few will be outlined here.

Kvon, et al. observed nanofibers with high degree of edge exposure while using a

system of methane/hydrogen/pyridine for carbon deposition over Ni-Cu catalysts269.

Nakajima, et al. made fibers with less edge exposure using system of acetonitrile/nitrogen for carbon fiber growth over Ni catalysts268. When iron was used as the nanofiber growth

catalyst and a carbon-nitrogen precursor was used for growth, stacked cup nanofibers

were commonly observed191,243-246,264.

3.2.3 Sulfur-Containing Carbon Nanostructures

Just as nitrogen can act as an electron donor, in the graphitic carbon structure,

sulfur has the potential to act as an electron donor. Sulfur has been used in the growth of

carbon nanostructures253,257,271-274. In these studies, sulfur has been reported as a fiber

growth-promoter. These literature reports do not indicate, however, if the presence of

sulfur alters the structure of the fibers or if sulfur is incorporated into the final product.

H2S and thiophene are the two primary sulfur precursors that have been used in the

growth of carbon nanofibers. Examples of sulfur-carbon systems that have been reported for the growth of carbon nanostructures include H2S with hydrogen/ethylene over

253 271-274 unsupported Co , thiophene with benzene/hydrogen catalyzed by ferrocene , H2S

35 257 with ethane/steam catalyzed by Fe-Ni , and H2S with natural gas/air/ammonia catalyzed

by iron pentacarbonyl (Applied Scientific as referenced in237).

Pretreatment of catalysts with sulfur before carbon fiber growth has also been reported253. Kim, et al. has studied the growth of carbon fibers by the decomposition of

253 ethylene over an H2S treated Co catalyst . They observed that low level H2S treatments

(up to 100 ppm) to the catalyst promoted growth of the carbon nanofibers and concentrations above 100 ppm acted as a poison in the growth of carbon nanofibers.

3.2.4 Boron-Containing Carbon Nanostructures

The introduction of boron into carbon nanostructures can create high electrical

conducting nanotubes275,276, ceramic nanostructures277, carbon oxidation resistant materials278-282, as well as, increase the degree of graphitization of the carbon283-285. Not

all of these properties co-exist in the nanostructures. The properties the nanostructure

possesses is dependent upon the boron-to-carbon ratio, preparation methods and

nanostructure.

It is possible to have boron-carbon materials in two possible forms – boron

carbides and boron-doped carbons. Boron is the main component in boron carbides,

while carbon is the predominant component in boron-doped carbons. Boron carbides are

277 ceramic materials with atomic compositions between B10.5C and B4C . Many B-doped

carbon studies have involved nanostructures (SWNTs286 and MWNTs275,276,284-289).

These nanotubes can be made by arc discharge276,284,285,287,288, laser ablation286, CVD

36 275,283, and partial substitution 289. Intermediate concentrations of B can cause isolated B- rich particles to form. In situ boron-introduction systems have included boron trichloride275,283,290 and diborane291. Post treatment conditions to introduce boron into

289,292,293 CNFs have included high temperature treatments of B2O3 .

When boron is introduced into nanotube growth systems, boron tends to build up

in higher concentrations at the ends of the nanotubes. Hsu, et al. reported BC3 forming at

the tips of the tubes284. These exposed ends preferentially are formed to be the zigzag

type284,285. Few armchair tipped nanotubes have been observed in the presence of

boron285. The zigzag and armchair configurations of graphite are shown in Figure 10.

The presence of boron on the zigzag tips is thought to prevent the closure of the nanotube

end284,285. This causes the nanofibers to continue to grow. Some B-doped MWNTs have been observed to have lengths ranging from ~5to 100 μm285.

Studies have shown that boron-doped MWNTs have increased electrical

conductivity compared to non-doped MWNTs275,276. Non-doped MWNTs were found to

have electrical resistances one order of magnitude276 and two orders of magnitude 275

smaller than the B-doped MWNTs. The increased conductivity may be due to the

increase in tube length and not due to any change in the electron-donation properties of

the carbon, however.

When boron is heat treated with amorphous carbons, it is known to increase the

graphitization properties of the carbon294. Increased crystallinity of carbon

nanostructures has also been observed with the introduction of boron283-285.

37 Boron and nitrogen have been doped together into carbons, as well290-293,295-298. It

has been suggested that nitrogen changes the electronic properties for SWNTs, but not

the physical dimensions of the SWNTs, while boron changes the physical dimensions296.

The methods used for B-N doped nanocarbons are very similar to those listed above for boron-only doped carbons. The boron and nitrogen atoms can substitutionally replace carbon in the graphite planes, however, this only occurs at low levels of doping (levels less than 10atomic% Boron292,295). At higher levels of doping, separate phases of BN and

C are formed. This phenomenon was observed when BC2N nanostructures via pyrolysis

290 of CH3CN-BCl3 were made over cobalt catalysts at 1000°C . These BC2N

nanostructures produced “stacked cup cone” structures with thick graphitized walls.

Using High-Resolution TEM and Electron Energy Loss Spectroscopy (EELS), it was

determined that the nanostructure layers alternated between BN and C. This phase

separation was said to be more stable than the incorporation of B and N into the C phase.

It was hypothesized that the phase separation occurs during the initial growth of the

carbon nanofibers and was not a product of diffusion once the fiber was grown290.

The introduction of B and N into carbon nanotubes was shown to increase

graphitization297. Redlich, et al. found that the degree of graphitization increased

compared to undoped samples by XRD. These doped CNTs also had a ten-fold increase

in length when compared to the non-doped CNTs 297. The effect of boron on the

nanotube tip morphology may also be the cause of the increased length in B-N doped carbon nanotubes, as Stephan, et al. reported “tube ends that were ill formed and not closed” 295.

Armchair

Zigzag

Figure 10. Armchair and zigzag graphitic edge planes

3.2.5 Oxygen Functional Groups on Carbon

Carbon commonly has oxygen functional groups located on its surface because of its exposure to air200-202. Carbon is intentionally functionalized with oxygen for many

reasons including to change their hydrophilic properties299, increase their reactivity300 and

to create anchoring sites for other functional groups or metals to be added to169,301-304.

The type of oxygen functional groups added to carbon depends upon the type of carbon

and oxidation treatment. These oxygen functional groups include carbonyls, phenols,

carboxyls, quinones, carboxylic anhydrides, lactones, lactols, hydroxyls, and ethers200,305.

39 Figure 11 illustrates the location of some of the types of oxygen functional groups that can be present on carbon.

Figure 11. Oxygen functional group locations on graphite.

Rivin has reported a carbon oxidation pathway where the type of functional group

306 progresses with treatment, ultimately oxidizing into CO2 :

O C—H [O] C—OH [O] C=O [O] ║ [O] —C—OH —H

Gaseous treatments, chemical treatments, electrochemical treatments and, plasma

treatments all can be used to oxidize carbon. Gaseous treatments to oxidize carbon

include exposure to air at room or elevated temperatures299,302,307-309, treatment in

307 308 oxygen and N2O . Chemical oxidation can occur by suspending the carbon in an oxidizing solution. The chemical species, concentration of the chemical, temperature and duration of the treatment can have a significant impact on the degree of oxidation and the functional groups added to the carbon surface118,307,309,310. Chemicals used for oxidizing

carbon include: nitric acid118,169,202,299,300,303,304,307-312, nitric acid/sulfuric acid

combinations299,302,303,307, hydrogen peroxide299,304,308,311, phosphoric acid309, potassium

permanganate299,303, hydrochloric acid313, ruthenium tetraoxide303 and acetic acid309. The

electrolyte and the charge passed through the electrolyte in the presence of the carbon

controls the degree of oxidation by electrochemical means314. Electrolytes used for

electrochemical oxidation of carbons include ammonium bicarbonate314 hydrochloric

acid313 and, potassium nitrate315. Oxidation of carbon through plasma treatments requires

quick exposure of the carbon to plasmas containing oxygen in a dry environment316,317.

With all of these treatment techniques the oxidation length, concentration of the oxidant

and temperature play a significant role in the degree of oxidation, types of functional

groups, and the structural effects to the carbon118,307,309,310,314,315.

Further treatment to the carbon after oxidation may be required. Depending upon

the method, amorphous carbon residues310 and chemical residues299 may require additional washing and treatments. Other functional groups, such as nitrogen groups

41 from the oxidation treatment with nitric acid312 and sulfur from sulfuric acid treatments318 can be added to the carbon as well.

A wide variety of carbons have been studied after oxidation. They include carbon blacks118,202,300,309, activated carbons302,304,308,311, carbon fibers309,310,313-315,319,320 and, carbon nanofibers299,303,307,309,312,321. The types of carbon also control the functional group species that are present. The defects, edge plane exposure, pore size and surface area can control the type and quantity of oxygen functional groups added to carbon surfaces in comparable treatments. Functional groups already present on the surface of the carbon also can play a role in further oxidation treatments.

No one analysis technique can completely characterize the oxygen functional groups present on carbon. Combinations of complementary techniques are needed to gain a good understanding of the functional groups present. Exposure to air may also alter the type and quantity of functional groups present on carbon200-202,314. The most common characterization techniques used for oxidized carbons are XPS, temperature programmed desorption (TPD) experiments, titration methods and infrared methods.

Other techniques that have also been used to a lesser degree in the characterization of oxygen functionalized carbons include: Electron Spin Resonance spectroscopy, ToF-

SIMS, isotopic labeling, dye adsorption, and chemical reaction and adsorption probing.

Structural changes to the carbon are commonly monitored through electron microscopy, surface area and pore volume analysis, and Raman spectroscopy.

XPS and IR techniques, including transmission FTIR and DRIFTS, can provide qualitative examinations of oxidized carbons. Sensitivity and deconvolution of the

42 spectra can add challenges to the analysis of these techniques201,313,314. Deconvolution of

the C 1s spectra to identify the carbon-oxygen species tends to be easier than

identification of the oxygen species in the O 1s spectra of XPS315 . Carbons must be heavily functionalized or very diluted for practical IR spectra to be obtained due to the black-body like absorption abilities of many carbons201,308. TPD and titration

experiments are considered more quantitative techniques for identification of oxygen

functional groups on carbon. In TPD experiments, the oxidized carbon is exposed to an

inert atmosphere (or a vacuum) and the desorption gases are monitored while the

temperature is ramping. The temperature of the desorption and gaseous species (water,

carbon monoxide or carbon dioxide) can indicate the type of functional groups

present307,308. Deconvolution methods of TPD spectra can be difficult, but there have

been several methods presented in the literature on deconvolution techniques to identify the oxygen functional groups307,308. Titration experiments, particularly Boehm

titrations200,322, can be used to detect acidic and basic functional groups on the carbon

surface310. The strength of the acidic or basic functional group and concentration can

dictate the accuracy of the titration method311. Also if the functional groups are located

in small pores on the carbon surface, the titrant may not be able to reach the species,

causing an underestimate of the surface species311.

3.2.5.a Oxygen Functional Groups in ORR

Controlling oxygen functional groups on carbon supports has aided in the preparation of platinum-based ORR catalysts for PEM fuel cells301,302. The anchoring

and particle dispersion properties that oxygen functional groups lend to carbon supports

43 have aided in the increase in ORR activity for Pt-based ORR catalysts301,302. The role of

oxygen functional groups in non-noble metal ORR catalysts has also been studied to a

smaller extent118,121,169,300,323. These studies have shown that carbon blacks containing

oxygen functional groups improve ORR activity over non-functionalized carbon

blacks118,121,169,300,323. The Popov and Dodelet groups have concluded that the role of the oxygen functional groups were not to improve ORR activity themselves, but to provide a better support medium for the active sites118,121,169,323. Nabae, et al. concluded that

oxygen functional groups were the active site and that copper added to the catalyst

created an oxygen adsorption site300.

The Popov group studied ORR catalysts composed of a cobalt ethylene diamine

chelate supported on oxidized and un-oxidized carbon blacks that were then heat treated

in inert atmospheres169,323. They also investigated un-oxidized carbon blacks, oxidized

carbon blacks and the cobalt-free chelate supported on un-oxidized and oxidized carbon blacks for comparison. It was observed that even the oxidized carbon black had a 100 mV improved activity over pristine carbon black. The addition of the nitrogen- containing chelate improved the activity even more. The cobalt catalyst on the oxidized carbon performed the best for ORR. It was found that the cobalt particle size was significantly reduced when the cobalt chelate was dispersed on an oxidized carbon black.

Popov cited the acidity of the quinone groups on the carbon surface as an effective dispersant of the basic cobalt chelate. The presence of the quinone groups was verified through the observation of the quinone/hydroquinone redox couple at 0.55V vs. NHE during cyclic voltammetry testing.

44 The Dodelet group also studied the effect of the pretreatment of carbon blacks on

their Fe-N-C ORR catalysts118. The oxidized carbons were heat treated in ammonia, then

an iron salt was added to the carbon and heat treated. It was found that the presence of

oxygen functional groups on the carbon black increased the nitrogen content of the

support after ammonia treatment. Even when the nitrogen and iron were not added to the

oxidized carbon, the oxidized carbon had superior activity to pristine carbon black. The

Fe-N-C catalysts prepared on oxidized supports showed higher ORR activity than those

prepared on pristine supports118. In a separate study of 19 different carbon supports, the

Dodelet group also found that if the catalyst did not contain any nitrogen, then the ORR

activity was proportional to oxygen content121. Once nitrogen was introduced to the

catalyst, the ORR activity was controlled by the nitrogen species121.

While both Popov and Dodelet attribute the presence of oxygen functional groups

as important in the preparation of metal-supported catalysts, Nabae, et al. believe that the oxygen functional groups play a more pivotal role during oxygen reduction300. They

studied modified carbon blacks with and without copper. The carbon blacks were

modified by mixing with HNO3 and NH4H2PO4 and then heat treating in inert atmosphere

at 823K. The carbon was oxidized to create quinone/hydroquinone oxygen functional

groups that were shown to participate in ORR. The phosphorous groups were added to

the carbon to increase the proton mobility in the electrocatalyst. While the ORR activity

improved when copper was added to the catalyst, the catalyst had significant activity

without it. The authors hypothesized copper added an oxygen adsorption site for the

catalyst and was not part of the active site. They hypothesized the quinone/hydroquinone

redox couple to be the active site for ORR on their catalyst300. 45 The Savy group also observed less sintering of the catalyst when they studied the oxygen content on carbon supports used for Co-macrocycle pyrolysis76. They found that the presence of oxygen on the carbon increased the selectivity of the catalyst to water formation. It was also observed that during high temperature pyrolysis, the cobalt was converted to cobalt oxides instead of metallic cobalt when higher amounts of oxygen were present on the carbon support76.

46 CHAPTER 4. Experimental Methods

4.1 Catalyst Preparation

Several types of nitrogen containing carbon nanostructured (CNx) catalyst materials were studied to investigate the role of nanostructure, heteroatom species and other properties. Development of CNx ORR catalysts involves the growth and treatment

of carbon nanostructures. These nanostructures are grown by flowing carbon-containing

(and possibly other heteroatom-containing) reactant gas streams over fiber-growth

catalysts at elevated temperatures (usually 600°C to 900°C). The resulting nanostructure

and chemical composition depends upon the treatment temperature, duration, reactant gas

and growth catalyst.

4.1.1 Magnesia Supported CNx Catalysts

The magnesia (MgO) supported CNx catalysts were used as a baseline for

comparison and a basic material for modification. This material was selected because of

the ability to remove the growth support (compared to Vulcan carbon142 also previously

studied in the research group) and the ease of preparation (compared to alumina140,141 and

silica144,145 catalysts also previously studied in the research group) while still maintaining

144,145 a high level of activity and selectivity . In most cases the CNx was grown over

2%Fe/MgO growth catalyst using acetonitrile pyrolysis which was then washed to

remove the exposed MgO and Fe.

Catalysts were prepared using high-surface area magnesia nanopowder from

Sigma Aldrich as the support. The MgO was left as-received or impregnated with metal

salts. Incipient wetness impregnation (IWI) technique was used for the doping of the

metal salts into the support. The IWI method was used to evenly spread the metal over the support and within the support pores. In the IWI technique, the MgO support is weighed out first. Then the total volume of the pores based upon the mass of support desired to impregnate and the measured BJH pore volume is calculated. An aqueous

solution with a total volume equal to the total volume of the pores is made with the metal

salt to give the desired metal loading on the support. The aqueous metal salt solution is

then slowly mixed with the support to evenly distribute the metal on the support material.

The metal salt precursors used for impregnation were iron (II) acetate, cobalt (II) acetate

and nickel (II) acetate. Typical metal loadings for the prepared fiber growth catalyst

were two weight percent. Impregnated supports were dried at 110°C. These impregnated

supports served as the nanofiber growth catalysts.

Acetontirle (CH3CN) pyrolysis to form the CNx catalyst was performed by

placing 2g of the MgO support as received or the dried metal-impregnated MgO support 48

into a quartz catalyst reactor boat loaded in a quartz calcination tube in a high-

temperature furnace. The calcination tube with the nanofiber growth catalyst was purged

with nitrogen (N2) at 150 mL/min for 30 minutes before any treatment was started to eliminate any oxygen in the reactor system. The acetonitrile bubbler used in the nanofiber growth was also purged with N2 to remove any oxygen in the system. The

gases from the acetonitrile purge prior to nanofiber growth were set to bypass the reactor

system so as not to introduce any acetonitrile before the desired temperature. After

purging the system, N2 was flowed through the calcination tube over the nanofiber growth catalyst while heating the furnace at 10°C/min to 900°C. Once 900°C was

reached, the N2 stream was flowed through an acetontrile bubbler at room temperature

and then to the furnace held at 900°C for two hours to grow the nitrogen-containing

carbon nanostructures (CNx). The CNx materials were cooled back to room temperature

in nitrogen (See Figure 12 of pyrolysis growth procedures for a visual depiction).

After fiber growth, approximately 1.5g of CNx grown over MgO was washed with

250 mL of 1M HCl at 60°C for 1 hour while stirring with a magnetic stirbar. This acid wash was performed to remove any exposed metals and the magnesia support itself.

After washing, the CNx was vacuum filtered with about 1L excess DI water. The CNx

was collected into a beaker by rinsing the material off of the filter paper. These final CNx

catalysts were dried in air in a 110°C oven for 24 hours.

The weight gain of CNx materials during fiber growth was calculated using the following equation:

final CN weight − adjusted initial precursor weight weight gain = x ×100 adjusted initial precursor weight

Where the final CNx weight was the mass of CNx recovered after pyrolysis and adjusted

initial precursor weight was the mass of the precursor (metal/support) loaded into the

sample boat adjusted for the mass loss found to occur during heating at 10°C/min in N2 to

900°C. For 2%Fe/MgO, the precursor was found to lose 41.8% of its mass during heating. Typical weight gains were 20-30% for CNx grown over 2wt% Fe/MgO. Mass

losses of 70-80% were typical after HCl washing.

Figure 12. Acetontrile pyrolysis treatment conditions.

4.1.2 Stacked Platelet Nanofibers

Stacked platelets were prepared to examine the role of edge plane exposure.

Stacked platelets are nanofibers with graphite sheets stacked upon one another perpendicular to the longitudinal axis of the nanofiber (see Figure 13). As prepared, they contained no nitrogen, but were later treated to contain nitrogen if desired.

Basal planes

Edge planes

Figure 13. Stacked platelet nanofiber.

To prepare the stacked platelet nanofibers, a procedure following the outline of a

patent by Rodriguez and Baker was used324. An unsupported 95:5 atomic ratio

iron:copper (Fe:Cu) nanofiber growth catalyst was prepared for the creation of stacked

platelet nanofibers. This 95:5 Fe:Cu catalyst was prepared using a co-precipitation

method. In this method, the respective nitrate salts were made into 1M solutions and

combined to form the 95:5 Fe:Cu ratio. A small amount of ammonium carbonate (0.5M)

was added to a beaker and the nitrate solution was slowly added while stirring.

Additional ammonium carbonate was added during the nitrate addition process to maintain a pH between 8.1 and 8.4. After all of the nitrate solution was added, the co-

precipitation catalyst was vacuum filtered with excess DI water and allowed to dry at

110°C. The dried 95:5 Fe:Cu catalyst was calcined at 350°C for 2 hours while exposed to air.

To grow the stacked platelets, 200mg of the calcined 95:5 Fe:Cu catalyst was loaded into a quartz sample boat in a quartz calcination tube in a high temperature furnace. The unsupported metal catalyst was reduced in 10% H2 in He at 600°C for 12 hours before stacked platelet growth. Immediately following the reduction step without cooling, the calcination tube and sample was purged with He to remove any hydrogen from the system. Stacked platelets were grown over the reduced catalyst after the purge using a 4:1 CO:H2 reactant stream with a balance of He (88 ml/min CO, 22 ml/min H2 and 21 ml/min He). This was performed at 600°C for 2 hours before being cooled under

He. About 1.3g – 1.4g of nanofibers and growth catalyst were collected after carbon deposition.

The resulting stacked platelet nanofibers were washed with 1M HCl as described in the MgO supported CNx catalyst preparation section to remove any exposed copper or iron.

4.1.3 Nanofibers with Basal Plane Exposure

As a comparison to the stacked platelet nanofibers, nanofibers with low graphitic edge-plane exposure, or high basal plane exposure, were prepared. The preparation of the nanofibers with basal plane exposure followed the same outline as the stacked platelet nanofibers. For the nanofibers with basal plane exposure, a 6:4 Fe:Cu nanofiber growth 53

catalyst was prepared and calcined. To grow the nanofibers with basal plane exposure,

100 mg of 6:4 Fe:Cu catalyst was reduced in hydrogen at 600°C before nanofiber growth

in 4:1 CO:H2, balance He stream at 675°C for 2 hours. After cooling in He, the

nanofibers with basal plane exposure were also washed in HCl as described in the

magnesia supported CNx preparation section.

4.1.4 Nanofiber Acid Oxidation

Nanofibers were oxidized with acid to add oxygen functional groups to the carbon

surface. There were two reasons that oxygen functional groups were added to the

nanofiber surface. The first was to study the role of oxygen functionalities on carbon and

nitrogen-containing carbon ORR performance. The second was to create sites for

nitrogen to incorporate into the carbon nanostructure during high temperature ammonia

treatment. Two types of acid oxidation were used; concentrated nitric acid oxidation and

concentrated nitric and sulfuric acid oxidation. It was found that the more severe

concentrated nitric and sulfuric acid combination was required prior to ammonia

treatment on highly graphitic stacked platelet nanofibers and nanofibers with basal plane

exposure. Less severe nitric acid treatments were used on CNx catalysts.

4.1.4.a Nanofiber Acid Oxidation Using Nitric and Sulfuric Acid

Nanofibers were oxidized to add oxygen functional groups to the carbon surface prior to high temperature ammonia treatment using nitric and sulfuric acid. A concentrated solution of 1:1 by volume HNO3 (69 mass%) : H2SO4 (97 mass%) was prepared as the

oxidation medium. Nanofibers were suspended in 200 ml of the concentrated acid

solution at 60°C under constant stirring using a magnetic stir bar for 3 hours, before

allowing to cool 1 hour. The nanofiber-acid suspension was slowly added to excess DI

water for dilution before vacuum filtering the nanofibers with additional excess DI water.

The collected nanofibers were dried in air at 110°C for 24 hours.

4.1.4.b CNx Acid Oxidation Using Nitric Acid Only

CNx catalysts were oxidized to add oxygen functional groups to the carbon

surface using nitric acid. A concentrated solution 69 mass% HNO3 was used as the

oxidation medium. CNx nanofibers were suspended in 250 ml of the concentrated acid solution at 60°C under constant stirring using a magnetic stir bar for 3 hours, before allowing to cool 1 hour. The nanofiber-acid solution was slowly, and carefully, vacuum filtered with excess DI water. The collected nanofibers were dried in air at 110°C for 24 hours.

4.1.5 Ammonia Treatments on Nanofibers

Ammonia treatments at elevated temperatures were performed on a variety of nanofibers and CNx catalysts to alter the nitrogen content in them.

4.1.5.a Ammonia-Treated Stacked Platelets and Nanofibers with Basal Plane Exposure

Ammonia was used to introduce nitrogen into the stacked platelet nanofibers and

nanofibers with basal plane exposure after HNO3:H2SO4 acid oxidation. Nanofibers

(approximately 0.2-0.3g) were placed in a quartz sample boat and purged with He. After

purging, 72% NH3 in He was introduced into the reactor and the furnace was ramped at

10°C/min to 600 or 900°C. Ammonia treatment durations were 4 hours at 600 or 900°C

and 19.5 hours at 900°C, before cooling in He for the acid oxidized stacked platelets.

The acid oxidized nanofibers with basal plane exposure were treated at 900°C for 19.5

hours in ammonia for comparison.

4.1.5.b Ammonia-Treated CNx Catalysts

Ammonia treatments were performed on CNx catalysts grown over 2%Fe/MgO

and HCl washed. Approximately 200 mg of the CNx catalyst was loaded into a quartz calcination boat. A stream of 38% ammonia in helium (total flowrate of 57ml/min) was flown over the CNx material while heating at 10°C/min. The temperature of the furnace

was brought to either 600°C or 900°C and held for 4 hours in the ammonia stream. The

catalyst was cooled under helium.

Additional batches of CNx catalyst were treated with ammonia after concentrated

nitric acid oxidation.

4.1.6 Ethanol-Treated CNx

To probe the solvent effect on ink aging, CNx was treated in an ethanol soak or

left as prepared to examine the effects of ethanol on the catalyst. CNx catalyst was

soaked in 100% ethanol in a ratio of 1:100 catalyst:ethanol by mass in two closed vials at

room temperature for 1 week. One uncapped vial was placed directly in a drying oven at 56

110°C to dry. This catalyst was labeled “CNx-ethanol soaked”. The contents of the other

vial was vacuum filtered with 1L excess DI water before being collected and dried at

110°C. This catalyst was labeled “CNx-ethanol soaked, DI water rinsed”.

4.1.7 Other Heteroatom Incorporation into Carbon Nanofibers

Incorporation of other heteratoms into the graphitic nanostructure can also alter the electronic structure of the CNx catalyst. Heteroatoms were incorporated into the

nanostructure both during fiber growth and through post treatment of the CNx catalysts.

4.1.7.a Sulfur as a Growth Promoter in CNx

Sulfur was used as a CNx growth promoter during acetonitrile pyrolysis. The

2%Fe/MgO fiber growth catalyst described above was used for these studies. The

acetonitrile pyrolysis technique described in the magnesia-supported CNx catalyst section

was modified by using a range of mixtures of thiophene and acetonitrile in the organic

precursor bubbler. Before treatment, the organic bubbler was purged with nitrogen for

exactly 15 minutes to keep the conditions consistent while removing any oxygen from the

system. All other conditions for the preparation remained the same. Thiophene (C4H4S)

was added to the acetonitrile bubbler at levels of 1.8, 3.6, 5.5, 7.3, 8.9, 11.5 mol% as a

growth promoter. For comparison, carbon nanostructures were also formed by

pyrolyzing 100 mol% thiophene over the same fiber growth catalysts. All catalysts in

this study are reported using the mol% thiophene in the acetonitrile solution, e.g.,

CNx(7.3) denotes CNx catalysts prepared from a 7.3% thiophene in acetonitrile mixture.

CSy refers to the carbon nanostructures grown using thiophene only.

All carbon nanostructures were washed in 2.4M HCl at 60°C, vacuum filtered

with excess deionized and distilled water and dried in an atmospheric oven at 110°C to

remove the magnesia support and any exposed iron.

4.1.7.b Boron-Incorporated CNx Catalysts

Boron was incorporated into CNx catalysts using a wet impregnation technique followed by a heat treatment. 2%Fe/MgO supported CNx catalysts were prepared and

HCl washed as described above. A solution of 6 mL DI water and an appropriate amount

of ammonium pentaborate ((NH4)2B10O16 ∙ 8H2O) was made. B:C atomic ratios equaling

0.025, 0.05, 0.10, and 0.25 were used when the CNx catalyst was assumed to be all carbon for the calculation. This solution was mixed into 200 mg of CNx while stirring in

a vial suspended in a 90°C water bath for twenty minutes. The slurry was dried at 110°C

before being heat treated. The dried B-CNx material was heat-treated at 900°C for 4

hours in N2 flowing at 150 mL/min. Materials developed using this method were

identified by the B:C atomic ratio of the ammonium pentaborate – CNx slurry before heat

treatment. For this ratio, it was assumed that the CNx catalyst was 100% carbon.

4.1.8 Graphitic Site Blocking on CNx

Park and Baker have reported on blocking the armchair and zigzag graphitic edge

planes on carbon nanofibers230. Attempts were made to determine the location of the 58

CNx active site by selective blocking of the armchair and zigzag edge planes. CNx grown on 2%Fe/MgO-HCl washed and stacked platelets following oxidation and ammonia treatments were used in this study. The zigzag graphitic edge can be blocked by boron groups while the armchair edge can be blocked by phosphorus groups.

The starting catalyst material was wet impregnated with methylphosphonic acid or ammonium pentaborate to add the blocking materials. To do this, approximately 0.25g of carbon catalyst was added to a small vial and 2 ml of DI water containing enough boron or phosphorus precursor to make 5wt% additive on the carbon was mixed with the carbon while stirring at 90°C. The solution container was rinsed into the catalyst paste with a small amount of excess DI water. The catalyst was stirred for about 45 minutes until the observable liquid was gone. The catalyst and precursor were then dried overnight in air at 110°C.

To create the materials with site blocking, the wet-impregnated material was heat- treated for three hours at 450°C in 10%O2 in 80 ml/min of flow. After heat treatment,

some of the boron or phosphorus groups should be bound to the carbon. Any residual material not bound to the carbon was removed through a wash with 1.0M HCl as described in the CNx preparation section.

As a comparison, a “blank-blocked” CNx was also propeared that went through the entire site-blocking treatment process in the absence of the methylphosphonic acid or ammonium pentaborate.

4.1.9 Commercial Catalysts

Commercial catalysts were used throughout the study as a comparison to those created in the laboratory. The most common commercial catalyst used as a comparison was 20% Pt/Vulcan Carbon (VC). This catalyst was obtained from Electrochem, Inc. and manufactured by E-Tek. From the same manufacturers, 20%Pt-10%Ru on Vulcan carbon was also used.

4.2 Catalyst Characterization

Catalysts were characterized throughout the entire process from preparation through end-of-use to gain a better understanding of the positive and negative features of the catalyst for future catalyst development. Both physical and chemical characterizations were performed on the catalysts. Characterization instrumentation used for the described work is available in the research group laboratories, the Campus

Electron Optics Facility, and the Chemistry Department’s Surface Analysis Facility.

4.2.1 Nitrogen Physisorption

Nitrogen physisorption was performed on catalyst materials to determine the BET

(Brunauer, Emmett and Teller) surface area and BJH (Barrett, Joyner and Halenda) cumulative desorption pore volume and pore size distribution. Greater surface area can

60 increase the quantity of active sites on a catalyst, though with increased surface area, electrical contact resistance is increased. The distribution of the volume of the pores also can be used in understanding of the transport of water to and from the catalyst during fuel cell operation. A Micromeritics ASAP 2010 was used for this analysis.

To perform the analysis, a quartz analysis tube was evacuated and then filled with nitrogen. The tube was weighed filled with nitrogen and then 100-200mg of catalyst was placed in the tube. The sample was evacuated and then heated at 130°C overnight while continuing to degass. The sample was removed from the system after backfill with nitrogen and weighed. The sample weight after pretreatment was used in the surface area and pore volume analysis. The analysis was performed at liquid nitrogen temperatures.

4.2.2 X-Ray Photoelectron Spectroscopy (XPS)

XPS was used to study the chemical surface species on the catalysts. A Kratos

Ultra Axis Spectrometer was used with a monochromated aluminum anode source set at

13 kV and 10 mA. Catalyst samples were prepared into pellets in stainless steel disk sample holders. They were prepared using pellets to eliminate the carbon signal coming from carbon tape that is commonly used for XPS sample preparation. A survey scan was collected with an 80 eV pass energy. Individual regions were scanned with a 20 eV pass energy and included Mg 1s, Fe 2p, O 1s, N 1s, C 1s, B 1s, Cl 2p, S 2p, and Mg 2p, depending upon the sample.

Sensitivity factors reported for the anode and position by the manufacturer were

used in the calculation of species contents in the catalysts. The spectra baselines were

determined using a Shirley-type background fitting. Spectra were fitted using

Lorentzian-Gaussian combination peaks. The concentrations of surface species above

about 1% are able to be detected by the instrument more easily. From the information

gained by XPS, the type of nitrogen present and surface composition can be determined.

Other dopant species can also be investigated by XPS.

4.2.3 Transmission Electron Microscopy (TEM)

Morphological characterization of the catalysts was done using a FEI Tecnai F20

XT TEM in imaging mode set at 200kV. Catalysts were prepared for imaging by

suspending catalyst in ethanol, sonicating and then depositing the catalyst suspension on

lacey-formvar carbon supported on 200 mesh copper TEM grids.

4.2.4 Hydrophobicity Testing

The relative hydrophobicity of the CNx catalysts was examined by suspending

1mg of catalyst in 5mL of DI water. The suspensions were sonicated for 30 minutes and then visually compared. Photographs were taken of the suspended catalysts side-by-side on a white backdrop to study the hydrophobicity.

4.2.5 Temperature Programmed Oxidation (TPO) Experiments

Temperature programmed oxidation (TPO) experiments can be used in

determining types of carbon, degree of graphitization, edge plane exposure and the nature

of chemical species present in the carbon. Depending on the degree of graphitization, the

edge plane exposure and amount of defects, the carbon will oxidize at different

temperatures. By connecting the product gas line to a residual gas analyzer, the chemical

composition of what is oxidizing can be monitored. TPO experiments can be run using

several different set-ups, including in conjunction with thermogravimetric experiments.

Several TPO experimental set-ups have been used in the study of CNx materials.

4.2.5.a Temperature Programmed Oxidation – Thermogravimetric Analysis (TPO-TGA)

Experiments

The types of carbon present in the CNx were studied through the use of

Temperature programmed oxidation (TPO) experiments ran on a thermogravimetric

analyzer (TGA). The mass loss observed during oxidation can aid in determining the

relative amounts of carbon types as well as the effectiveness of washing. Approximately

5-10 mg of the catalyst studied was placed in a Setaram TGA-DSC 111 instrument using platinum crucibles. The reactant gas was 60 ml/min 10% O2/He. Temperatures were ramped at 5°C/min to 750°C for the experiment. Blank experiments with the same temperature profile and gases were run with empty crucibles to subtract any mass changes due to buoyancy effects. The TPO-TGA experiments were also paired for product gas analysis by connecting the exit gas to a Cirrus residual mass analyzer. Mass

signals from 1-100 were scanned during these experiments to aid in the determination of

components of a particular carbon.

4.2.5.b Temperature Programmed Oxidation (TPO) Experiments Using an External

Reactor System

Temperature programmed oxidation (TPO) experiments were performed using an

external reactor system when oxidation temperatures of the materials approached 750°C,

the maximum operating temperature of the TGA, or when more than 5 to 10 mg of

material needed to be oxidized to identify a species being studied through oxidation. The

external reactor used was a fritted quartz bulb reactor that was positioned vertically in an

upright high temperature furnace. Approximately 25 mg of the catalyst to be studied was

loaded into the reactor for study. The reactant gas was 50 ml/min 10% O2/He.

Temperatures were ramped at 5°C/min to 900°C for the experiment. The product gas

was monitored using Cirrus residual mass analyzer. Mass signals from 1-100 were scanned during these experiments to aid in the determination of components of a particular carbon and the temperature at which oxidation occurred.

4.2.6 Temperature Programmed Desorption-Temperature Programmed Oxidation (TPD-

TPO) Experiments

Temperature programmed desorption (TPD) – temperature programmed oxidation

(TPO) coupled experiments were performed using a Micromeritics Autochem II system coupled to a MKS Cirrus residual gas analyzer to study the CNx catalyst surface and bulk

species, as well as the stability of the catalyst species. TPD-TPO experiments were performed with approximately 25 mg of catalyst. A helium gas stream was flowed over the catalyst bed while heating at 5°C/min to 900°C and cooling using forced convection back to room temperature for the TPD portion of the experiment to detect any desorbed species. Following the TPD, a TPO was immediately ran in 5%O2/He at 5°C/min to

750°C to characterize the catalyst material remaining after the TPD. The product gas

mass signals of the entire experiment were monitored from 1-100.

To study the effect of the presence of water on the catalyst, 10mol% water vapor

was introduced into the TPD temperature ramp to 900°C. The catalyst bed was cooled in

He and then a TPO was performed as described above for the dry TPD-TPO experiment.

The experiments involving water during the TPD are named wet TPD-TPO experiments.

4.3 Electrochemical Testing and Related Studies

Electrochemical testing methods were used on the CNx catalysts to study their

behavior as fuel cell materials using electrochemical half cell methods. Additional

phenomena studies were also performed.

4.3.1 Rotating Ring Disk Electrode (RRDE) Testing

Activity testing of the ORR catalyst was performed using the half-cell

electrochemical technique known as Rotating Ring Disk Electrode (RRDE) testing.

Figure 14 is a schematic of the set-up for RRDE testing. Inks for RRDE testing were prepared fresh for each test using a composition of 1:10:160 (by mass) catalyst: 5%

Nafion in aliphatic alcohols: ethanol. The inks were sonicated for 30 min. Three-5μL

aliquots of ink were applied to the glassy carbon disk electrode with an area of 0.1642

cm2 to coat the disk without covering the Teflon casing or the Pt ring.

Figure 14. Setup for Rotating Ring Disk Electrode. Figure courtesy of Dieter von Deak.

All tests were performed in a half cell containing a 0.5M sulfuric acid (H2SO4) electrolyte. A PAR Bistat connected to a model 636 RRDE setup was used. An

Ag/AgCl(sat KCl) reference electrode and a Pt wire counter electrode were used in the system. All RRDE tests were performed using cyclic voltammagram sweeps. For CNx catalysts, the sweeps were from 1.2V to 0.0V to 1.2V vs. Normal Hydrogen Electrode

(NHE). Materials containing Pt were cycled from 1.2V to 0.2V to 1.2V vs. NHE to avoid

H2 production, occurring at 0.0V vs. NHE.

To begin RRDE testing, the electrolyte was saturated with oxygen. A CV was run at 10 mV/s to remove any gaseous oxygen from the CNx pores and fully wet the catalyst.

The solution was then purged with Ar. In the Ar purged system, CVs at 50 mV/s were run on the disk paired with the ring set at a constant 1.2V vs. NHE until steady-state curves were obtained in each. Then a CV at 10 mV/s rotating at 100 rpm was completed with the ring set at a constant 1.2 V to obtain a background current for both the ring and disk. The solution was then saturated with oxygen. Steady-state CVs at 50 mV/s were run paired with the ring held at 1.2V. The disk (at 10 mV/s) and ring (1.2 V) were run while rotating at 100 rpm to obtain the H2O2 production rates at the ring. CVs at 10 mV/s were also run for the disk in oxygen at 0 rpm and 1000 rpm. Later studies also collected the ring data during the 1000rpm scan in oxygen.

The ORR activity, identified as the onset of activity, was quantified as the potential at which the current obtained in O2-saturated solution at 0 rpm is 10% higher

67 than the Ar background. For more active samples this separation occurs at higher values, closer to the theoretical potential of 1.2 V. Figure 15 is a set of typical reduction scans for activity testing. While the onset of activity is actually calculated using 0rpm data, as seen from this figure, the onset at all rotations will occur at the same potential.

Selectivity of the catalyst is determined by a pairing of the glassy carbon disk with the platinum ring at 100 or 1000 rpm. The ring detects H2O2 production formed from the following reaction at the ring:

+ - H2O2 → O2 + 2H + 2e 0.695 V vs.NHE

The number of electrons transferred, n, equals 4, if only H2O is formed during ORR. If only H2O2 is formed, then n equals 2. The value of n is calculated from the ring and disk current measurements by:

4I n = D  I R  I D +    N  and the selectivity to water formation is calculated by:

 I R   I D −  H O Selectivity =  N  ×100 2  I   I + R   D N 

Where ID is the disk current, IR is the ring current, and N is the ring collection efficiency.

The manufacturer reports N for this instrument as 0.20, indicating that 20% of all H2O2

68 passing closely by the ring in laminar flow at the ring potential held will be detected.

Figure 16 is a representative comparison of the ring to the disk.

In recent work, we have found that the ring may perform at its optimum levels if it is conditioned before testing and not held at a constant potential excessively. To accomplish this, the ring is no longer held at a constant potential during disk scans that are performed merely to get a consistent scan. Prior to each 10 mV/sec scan of the disk where the ring is also normally paired (as noted above), the ring is conditioned. Prior to the first Argon background collection scan, the ring was conditioned by performing 20 -

100 mV/sec CVs from -0.1V to 1.8V to -0.1V vs. NHE. Subsequent ring conditionings were run for only 10 CVs.

Figure 15. Typical RRDE activity results. (Catalyst: CNx on 2%Fe/MgO-HCl washed). 69

Figure 16. Sample comparison of ring current corrected for collection efficiency to absolute disk current using the RRDE method (Catalyst: CNx on 2%Fe/MgO-HCl washed).

4.3.2 Methanol Tolerance and Methanol Oxidation Activity Testing

The inactivity of the ORR catalysts to the oxidation of methanol and tolerance to

the presence of methanol during ORR were studied using the RRDE setup. CNx ORR

catalysts developed in the HCRG studied for methanol tolerance and inactivity included

CNx grown over the following supports: 2%Fe/Al2O3-HF washed, 2%Ni/Al2O3-HF washed, 2%Fe/SiO2-KOH-HCl washed, 2%Co/SiO2-KOH-HCl washed, 2%Ni/SiO2-

KOH-HClwashed, 2%Fe/MgO-HCl washed, 2%Co/MgO-HCl washed and 2%Ni/MgO-

HCl washed. Commercial catalysts used were 20%Pt/Vulcan Carbon (VC) and 20:10

Pt:Ru/VC both from E-Tek.

To study ORR catalysts in the presence of methanol, RRDE testing was first performed as described above. Once the methanol-free tests were completed, the half cell solution was changed to 1.0 M methanol + 0.5 M H2SO4 or 3.0 M methanol + 0.5 M

H2SO4 without disturbing the catalyst thin film on the disk. The fresh solution was purged with Ar for 30 min. Then steady-state CVs and a disk and ring background were taken as described above. The methanol solution was then saturated with oxygen. The steady-state CVs, disk and ring at 100 rpm and disk only at 0 rpm and 1000 rpm were run as described above.

4.3.3 RRDE Studies to Examine the Effect of Catalyst Loading

When comparing work done by the HCRG to other researchers, it was noticed that the catalyst loadings used in RRDE testing varied significantly. A study was completed to investigate the effect of catalyst loading using both commercial 20%Pt/VC

(E-Tek) and CNx grown over 2%Fe/MgO-HCl washed.

RRDE testing procedures were followed as described above, except that the amount of ink loaded on the glassy carbon disk varied. Ink levels studied were based upon aliquot applications. Each one-5μL aliquot is equivalent to 142 μg/cm2 loading on the glassy carbon RRDE disk when assuming 100% dispersion of the catalyst in the ink. 71

Loading levels studied include 1 aliquot (142 μg/cm2), 2 aliquots (284 μg/cm2), 3 aliquots

(426 μg/cm2), 4 aliquots (568 μg/cm2), 6 aliquots (852 μg/cm2), 8 aliquots (1136 μg/cm2) and 10 aliquots (1420 μg/cm2). Testing was randomized to eliminate any experimental

drift effects.

4.3.4 Catalyst Layer Thickness Determination

Film thickness and surface roughness of the catalyst films were studied with a

Sirion FEG Scanning Electron Microscope (SEM). In this study, aluminum rods with the

same diameters as the RRDE glassy carbon disk were used as “model” electrodes for

applying the catalyst ink. Films were prepared by depositing 5 μL aliquots of catalyst

inks onto the ends of aluminum rods in the same manner and at the same loadings as

described in the RRDE catalyst loading studies. For SEM observation, the aluminum

rods were placed horizontally so that the sides of the films were exposed directly to the

SEM detector. The rods were tilted when necessary to give a clear observation of the

film thickness and surface details. Film thicknesses were measured at a minimum of five

locations along the disk for each loading. The average and standard deviations of the

thicknesses were calculated.

4.3.5 RRDE Catalyst Ink Aging Studies

It has also been observed that the age of the RRDE ink has an effect on the

selectivity results. To study this, RRDE procedures were followed as described above, 72

with the 20%Pt/VC and CNx on 2%Fe/MgO-HCl washed catalyst inks being prepared and aged.

Catalyst inks for RRDE testing were prepared using 1:10:160 (by mass) catalyst:

5% Nafion in aliphatic alcohols: ethanol. The capped ink vial was sonicated in a low energy sonicator water bath for 30min prior to use. Catalyst ink sonicated for 30 minutes immediately following preparation was considered fresh. After the immediate use, the ink was allowed to sit capped until Day 1 use, 24 hours later. Prior to each use, the ink was sonicated for 30min to maintain good dispersion of the catalyst in the ink suspension.

Catalyst inks were used fresh and after 1, 2, 3, 4 and 11 days of aging.

Catalyst was applied to the glassy carbon disk working electrode in 5µL-aliquot increments. Catalyst loadings used on the disk were 142µg/cm2 and 426 µg/cm2

assuming 100% dispersion of the catalyst in the ink.

4.3.6 Electrochemical Reduction and Oxidation of Hydrogen Peroxide

Changes in hydrogen peroxide detected at the ring with catalyst loading may

suggest that ORR catalysts participate in a two-step mechanism where oxygen is first

electrochemically reduced to hydrogen peroxide and then further electrochemically

reduced to water. If this is the case, CNx catalysts should have some electrochemical

reduction activity towards hydrogen peroxide. A series of experiments were performed

to analyze the electrochemical activity towards hydrogen peroxide reduction and

oxidation. 73

To have a direct comparison, RRDE testing was performed as described above

2 using CNx grown over 2%Fe/MgO-HCl washed at a loading of 426 µg/cm in 100.0 mL

of fresh 0.5M H2SO4 as the electrolyte. Immediately following the RRDE measurements,

hydrogen peroxide reduction and oxidation experiments were performed following a

similar method to that reported by Jaouen and Dodelet137.

Background linear voltamogram scans were collected in argon-purged electrolyte.

The reduction scan was from +0.65V to 0.0V vs. NHE at 10 mV/sec, while the oxidation

scan was from +0.65V to +1.1V vs. NHE at 10 mV/sec. Both were collected at a rotation

of 100rpm.

After the backgrounds were collected, a known amount of 30% H2O2 was added

to the 100.0 mL of H2SO4 in the electrochemical half cell to make a concentration of

either 6.25 mM H2O2 or 1.3 mM H2O2. The volume added to the half cell was considered

minimal so that 100 mL was considered the total volume of the solution.

Following the introduction of hydrogen peroxide, reduction scans were run all

together before any oxidation scans were performed. This is to eliminate any oxygen

noise interference produced from hydrogen peroxide oxidation. Also during scans, the

electrolyte was continuously purged with argon to keep any oxygen out of the system as

well. Reduction scans were collected at the following rotation rates: 100 rpm, 500 rpm,

1000rpm, 2000 rpm and 3000rpm. Rotation rates were randomized during experiments.

Once the reduction scans were complete, oxidation scans were performed in the half cell at the same rotation rates as listed for the reduction scans.

CHAPTER 5. Effect of Sulfur as a Growth Promoter for CNx Nanostructures as PEM and DMFC ORR Catalysts

As published in Applied Catalysis B: Environmental325

5.1 Motivation for Studying Sulfur as a Growth Promoter

As the activity of C-N catalysts continues to improve, manufacturing issues will

have to be considered. If nanostructured carbons are to be used as fuel cell catalysts, then

it is likely that their yield will have to be increased using a growth promoter in the

energy-intensive manufacturing process. Sulfur has been reported as a growth promoter

in the production of carbon nanofibers253,257,271-274. While reports discuss the benefits of

low-levels of sulfur during the nanofiber growth-process, significant discussion on the presence of sulfur in the carbon nanostructures after production has not occurred. From

these reports, it is unclear if sulfur remains in the carbon nanostructure and if so, whether

it has an impact on the behavior of the nanostructure, especially in catalytic systems. The

incorporation of sulfur into the graphitic nanostructure of carbon could alter the

electronic behavior of the carbon, thereby changing the performance of carbon 75

nanostructures used in electrochemical systems, such as ORR catalysts for PEMFCs and

DMFCs. Sulfur could also become a contaminant to different components of fuel cells if it were not well adhered to the catalyst.

In this study, we have investigated the use of thiophene as a growth promoter for the production of CNx catalysts from acetonitrile pyrolysis and its impact on the catalyst

behavior for the oxygen reduction reaction in a PEM or DMFC environment. It should

be noted that, in this study, there was no effort made to optimize the catalyst formulation

or the testing protocols to get the best performance since the focus of the study is an

understanding of the nature of the sulfur species incorporated during nanofiber growth

and their impact on the ORR activity and selectivity.

The detailed preparation of CNx catalysts made with thiophene as a growth

promoter is described in the Experimental Methods Chapter. CNx catalysts were made

through acetonitrile pyrolysis at 900°C over a 2wt%Fe/MgO nanofiber growth catalyst.

Thiophene was mixed with the acetonitrile before introduction to the pyrolysis furnace in

levels of 1.8, 3.6, 5.5, 7.3, 8.9 and 11.5mol% thiophene in aceonitrile. For comparison, a

catalyst was also made using 100% thiophene as the carbon precursor. All catalysts were

washed with HCl to remove the exposed MgO and Fe.

5.2 Results & Discussion

5.2.1 Thiophene as a Growth Promoter

The addition of thiophene into the acetonitrile pyrolysis feed stream did promote

the deposition of carbon. At all thiophene growth promoter levels studied, sample weight

gain during pyrolysis was greater than acetonitrile-only pyrolysis. As shown in Figure

17, carbon deposition during pyrolysis goes through a maximum with increasing thiophene % in the feed mixture and the maximum occurs at 7.3% thiophene concentration. This shows that sulfur, in the form of thiophene, can act as a growth promoter when the feed stream contains both carbon and nitrogen in the form of acetonitrile. Previous studies using thiophene and other sulfur-containing growth promoters have been for non-heteroatom containing carbon nanostructures. Even when thiophene was used as the only carbon precursor, significant amounts of carbon deposition occurred, indicating that thiophene does not inhibit the carbon deposition process even at high levels.

Interestingly, there was an inverse relationship between the amount of carbon deposition and the BET surface area of the catalysts, also shown in Figure 17. This decrease in surface area could be caused by several reasons including the carbon nanostructure changing with growth promoter concentration or carbon depositing on itself reducing the amount of exposed surface.

5.2.2 Catalyst Morphology

To determine the impact of the use of thiophene as a growth promoter on the morphology of the CNx catalysts, TEM imaging was performed. CNx catalysts made with no sulfur growth promoters grown over 2wt%Fe/MgO produce mainly stacked cup nanofibers with varying diameters as shown in Figure 18a. These stacked cup nanofibers have graphite edge planes exposed at an acute angle to the longitudinal axis. Also, as previously reported, all CNx catalysts grown using a magnesia support contained some 78

nanocubes, regardless of the metal on the support144,145. This observation held for these

CNx catalysts as well, though there were significantly more nanofibers than nanocubes.

When the lowest level of thiophene growth promoter was used, 1.8% (Figure

18b), the morphology of the CNx catalyst still remained mostly stacked cup nanofibers.

There were more of the larger, less organized nanofibers and more nanocubes were also found compared to the thiophene-free catalyst. At the highest levels of growth promotion

(7.3% thiophene), the catalyst morphology was no longer dominated by the stacked cup nanofibers (Figure 18c). There were many more of the large, disordered nanofibers and a significant amount of nanocubes. Only a few stacked cup nanofibers were found. The change in morphology distribution along with the maximum growth promotion for the catalyst made with 7.3% thiophene may signify a change in the growth mechanism of the carbon deposition with higher thiophene contents. In comparison, the catalyst made with thiophene only (Figure 18d) was composed of only nanocubes, indicating that acetonitrile was the precursor required to form stacked cup nanofibers over 2wt%Fe/MgO fiber growth catalysts. Others have also observed changes in morphology with increased growth promoter concentration, as well, though the sulfur source and treatment conditions dictated whether changes observed were desirable or undesirable for the specific application253,271,273.

Figure 18. TEM images of a) CNx- no thiophene, b) CNx-1.8% thiophene, c) CNx-7.3% 325 thiophene, d) CSy-100% thiophene. As published in .

5.2.3 Activity and Selectivity Testing

Activity and selectivity testing using the Rotating Ring Disk Electrode (RRDE)

technique was performed on the CNx catalysts manufactured using the growth promoter to determine if the use of thiophene had an impact on the catalyst performance for ORR.

The catalyst made with acetonitrile only had an onset of activity at 760 mV vs. NHE.

All catalysts made with pyrolyzing thiophene and acetonitrile combined had onsets of

activity within a very tight range, near to that of the catalyst prepared with acetonitrile

only, as shown in Figure 19. The deviations in onset of activity measurements for these

catalysts are not considered significant enough to distinguish differences in ORR activity.

When thiophene was used as the only feedstock for the carbon growth, the ORR activity

was very low. The onset of activity for the CSy catalyst was 510 mV vs. NHE, which

would be considered a very poor catalyst for ORR. The inset to Figure 19 illustrates the significant difference with a voltammogram comparison of the CSy catalyst (no

acetonitrile) to CNx(8.9). From the activity testing, it appears that as long as the catalyst precursor contained some amount of acetonitrile, the ORR activity could remain unchanged. Selectivity calculations obtained from RRDE testing found the selectivity of

the CNx-thiophene catalysts to range from an average of 3.6 to 3.9 electrons transferred

per oxygen molecule. No trend in the selectivity of the catalysts with thiophene

precursor content was observed.

Figure 19. Onset of activity comparison for thiophene containing catalysts. Inset: Activity comparison of the linear scan at 1000 rpm in oxygen saturated 0.5M H2SO4 for 325 CNx -8.9% thiophene and 100% thiophene catalysts. As published in .

There have been studies in the literature about the reliability of the RRDE

technique for selectivity measurements30,326-330. It has been reported in the literature328-330

and demonstrated in our laboratory (presented in a later chapter on RRDE methods) that

selectivity determined by RRDE is dependent upon the catalyst loading on the electrode

disk. In this study, the catalyst loading was kept constant at 426µg/cm2.

To verify the selectivity results obtained by the RRDE technique, additional

analysis using the Koutecky-Levich (K-L) Equation was performed on some of the catalysts. K-L analysis results were comparable to those obtained by the RRDE, (n= 3.7

82 from K-L analysis as compared to a selectivity of n=3.63 from RRDE measurements). It should be noted that the Koutecky-Levich analysis is not without its own shortcomings71,331-333, but the comparison between the two methods provides additional confidence in the reported selectivity values.

Factors affecting selectivity of the CNx-thiophene catalysts will be discussed further in Section 5.2.5.

5.2.4 Hydrophobicity Testing

Relative hydrophobicity tests were performed by dispersing the catalysts in water with sonication. The use of thiophene as a growth promoter did not change the hydrophobicity/philicity properties of the catalysts as long as acetonitrile was still in the feed stream. An example of the hydrophobicity differences between catalysts made with acetonitrile and with thiophene-only is shown in Figure 20. Carbon nanostructures grown using thiophene only are considerably more hydrophobic as can be seen from the lack of dispersion in water. This observation is consistent with the lack of edge planes observed over these materials (as confirmed through TEM) and could be linked to the differences in the activity of CNx catalysts grown with or without the sulfur growth promoter and the CSy materials.

Figure 20. Hydrophobicity comparison of CNx, CNx(8.9%) and CSy catalysts showing their dispersion in water. As published in325.

5.2.5 Surface Species Characterization

X-Ray Photoelectron Spectroscopy (XPS) was used to characterize the surface

species on the catalysts produced. The S 2p region was scanned to determine if there was

any detectable sulfur, and if so, what type of sulfur species were present on the surface of

the CNx catalysts using thiophene as a growth promoter. Previous reports in the literature

focus on the growth promotion effects of sulfur, but do not extensively discuss the sulfur

that could be incorporated into the product carbon nanofibers253,257,271-274. The impact of

the presence of sulfur in the nanofiber could be significant depending upon the

application. XPS analysis showed that there was a significant amount of sulfur on the

nanostructure surfaces. Surface sulfur content for the CNx-thiophene catalysts ranged from 0.7 to 1.6 atomic%, as shown in Table 2. The catalyst prepared using 100% thiophene (CSy) had 3.6% sulfur on its surface. Figure 21a shows the S 2p region for a

representative group of the CNx-thiophene and CSy catalysts. Up to three types of sulfur

can be identified in these catalysts. The peak at 168.3 eV was attributed to sulfate

Table 2. Surface composition of catalysts from XPS as published in325. %Thiophene in 0% 1.8% 3.6% 5.5% 8.9% 11.5% 100% acetonitrile O 1s 1.9 10.0 8.6 5.4 15.4 6.1 8.0 N 1s total 9.3 7.6 7.4 8.3 7.7 7.2 0.0 Pyridinic 26.1 26.7 27.3 23.7 31.4 23.7 0.0 Quartenary 47.8 47.9 47.8 49.4 45.6 48.6 0.0 Pyridinic-O 26.1 25.4 24.9 27.0 23.0 27.8 0.0 C 1s 88.8 81.7 82.7 85.4 75.2 85.9 88.9 S 2p 0.0 0.7 1.4 1.0 1.6 0.9 3.1

species318,334. The literature has some discrepancies in the assignment of sulfur species in the 163-164 eV region. Figure 21a shows one doublet being fitted to the 163-164eV envelope. S-C species can be identified at 163.5eV78,334,335. The peak at 164.1 eV was

assigned to elemental sulfur334,335. The assignment of C-S is less certain, as some have identified C-S at 163 eV78, while others attributed 163.6 eV peak to possibly S-S or S-

C335 and 163.6 eV to only S-S334. Figure 6b is an example of when two doublet peaks are

fitted in the 163-164eV envelope. Both fits are reasonably good and possible. It is

possible that both elemental S and S-C species are present in the thiophene-CNx catalysts

with excess elemental sulfur being present on the surface of the catalyst. The sulfate 85

identified could have been elemental sulfur that, when exposed to air, reacted to form

sulfate. This is possible, as the samples are exposed to air after they are synthesized.

Interestingly, the fitting of the CNx(1.8%) catalysts required an additional lower binding energy doublet. This species has not been identified at this time, but sulfides are commonly found in this region.

X-ray photoelectron spectra of the N 1s region for the CNx-thiophene catalysts were examined to determine if the introduction of sulfur led to significant changes in the

nature or the quantity of the detected surface nitrogen species (Figure 22). CNx catalysts

grown with acetonitrile only over 2wt%Fe/MgO contained 9.3% nitrogen with the

nitrogen content being split into 26.1% pyridinic-N (located on the graphitic edge

planes), 47.8% quarternary-N (located inside the graphitic plane), and 26.1% pyridinic-

N-O, as shown in Table 2. These results are similar to what has previously been observed

145 in our research for CNx catalysts grown over Fe/MgO supports . When thiophene was

used as a growth promoter, the nitrogen level dropped to between 7 and 8 atomic%. The

breakdown of the nitrogen species changed little with the introduction of thiophene into

the pyrolysis feed stream, as shown in Table 2 and in Figure 22. This relatively high amount of nitrogen and limited change to the nitrogen species most likely contributes to why no significant change in ORR activity for these catalysts was observed. It also suggests that the presence of thiophene does not alter the mechanism for incorporation of nitrogen into the graphite structure through acetonitrile pyrolysis. The absence of any detectable nitrogen in the CSy may also explain why it was relatively inactive for ORR.

XPS analysis showed that the oxygen content increased significantly with the use

of thiophene as a growth promoter. Oxygen content went from under 2 atomic% of the

surface when no thiophene was used to more than 15% when 8.9% thiophene was

introduced into the feed stream. While some of the oxygen content can be attributed to

the sulfate species formation, the contribution from sulfate in the O 1s region does not

account for all of the oxygen increase. It appears that the presence of sulfur may promote

Figure 22. XPS N 1s region deconvoluted for select CNx-thiophene catalysts (labels are mol% thiophene in acetonitrile solution). As published in325.

the addition of oxygen species onto the carbon. Five types of oxygen were identified from the O 1s spectra, as deconvoluted on representative catalysts in Figure 23. These species can be identified as physically adsorbed oxygen at 530.1 eV 336; carbonyls and 89

quinones at 531.0 eV308,318; sulfur-oxygen compounds at 531.6 eV337; ethers, and C=O in

anhydrides or lactones at 532.3 eV308,318; and ether oxygens in anhydrides or lactones at

533.3 eV308,318. A peak fit with fewer curves fails to recognize oxygen-sulfur species and quinone species, although we have evidence of their presence from XPS (S 2p region) and electrochemistry testing, respectively. The FWHM of the peaks remains reasonable at about 1eV on all of the fits using five curves and have peak locations that match multiple literature reports well. The typical oxygen species found in graphite and their binding energy assignments are shown in Figure 24. The composition fractions changed little with sulfur content, with each component making up 15-25% of the oxygen spectra, this may also be due to the combination of oxygen species identified in the literature at each binding energy.

While no trend in selectivity with thiophene concentration or resulting sulfur content of the catalysts was observed, a trend in selectivity with oxygen surface content was observed. Selectivity of the catalysts increased with oxygen content as shown in

Figure 25a where the oxygen content was normalized for the surface area of the catalysts.

The inset to Figure 25a is a visual comparison of the ring current (corrected for collection efficiency) to the absolute disk current. A series of curves used in the calculation of the selectivity of the catalyst from RRDE testing is shown in Figure 25b. Due to the fairly even distribution of types of oxygen species on the catalyst surface, no one type of oxygen (from XPS results) was found to control the selectivity trend with oxygen functionalization. The presence of oxygen may contribute to the complete formation to

water. This was also observed on the CNx catalysts in a separate study when they were treated with nitric acid to add oxygen functional groups 199. Some researchers have

Figure 23. XPS O1s region deconvoluted for select CNx-thiophene catalysts (labels are mol% thiophene in acetonitrile solution). As published in325.

Figure 24. Locations of possible oxygen functional groups by XPS binding energy. As published in325.

hypothesized that the quinone/hydroquione groups play a role in oxygen reduction 169,300, though no clear trend in our study was observed with the quinone groups when comparing them to the selectivity of the catalysts found.

5.2.6 Stability of Sulfur in CNx Nanostructures

Once it was determined that thiophene could be used as a growth promoter for

CNx and that the ORR activity was not affected by the presence of sulfur in the final CNx product, further studies were performed to examine if sulfur species might elute under conditions that would be relevant to an actual fuel cell environment. RRDE testing does not readily reveal the ability of a catalyst to withstand the fuel cell environment for long durations without activity loss, material degradation, or leaching of the catalyst components into other parts of the fuel cell. To address the preliminary concerns of degradation and leaching, temperature programmed oxidation (TPO) and temperature

programmed desorption (TPD) experiments were performed. In addition, desorption tests

were also performed in the presence of water to mimic the humidified conditions of a fuel

cell environment better.

The temperature at which significant oxidation of carbon materials begins can be

a marker for the orderliness of the material and can imply what the oxidation resistance

over long periods of time may be for the material. In addition, the chemical species that

are evolved during oxidation indicates the chemical composition of the material. TPO

experiments were performed in a TGA-DSC instrument connected to a mass

spectrometer for product analysis. It was found that all CNx-thiophene catalysts had similar oxidation properties to CNx made without thiophene, as shown by the TG signal in Figure 26a. Significant oxidation of these catalysts began at about 485°C. While the largest mass loss occurred for the CSy catalyst at the same temperature as the CNx- thiophene catalysts, an initial mass loss of 18% was observed beginning at about 220°C.

From the mass spectrometry data collected at the outlet of the reaction, a significant SO2

peak was observed for the CSy catalyst during the 18% mass loss region (Figure 26b).

The other catalysts made with varying levels of thiophene also had SO2 evolved at this temperature, but with signal intensities more than an order of magnitude smaller than the

100% thiophene catalyst. This small amount of SO2 evolution on the CNx-thiophene catalysts did not cause significant mass loss due to the minimal amount evolved. During the oxidation period beginning at about 485°C, carbon oxidation, nitrogen oxidation and sulfur oxidation products were observed in the product gas analysis. It was observed that

CO2 evolved earlier than the NOx and SO2 during the final oxidation stage. Carbon that

was less graphitized or contained more defects may begin the oxidation process before

the more ordered carbon305. From the product analysis, it appears that the nitrogen and sulfur imbedded in the carbon came more from the latter type of carbon. Overall, the

TPO results suggest that there are two-types of sulfur in the CNx-thiophene and CSy

catalysts – sulfur that was easily evolved from the surface and sulfur that was

incorporated into the carbon matrix.

To further study the two types of sulfur observed and to evaluate the desorption

properties of the catalysts, TPD experiments were performed on the CNx-thiophene catalysts that were immediately followed by TPOs. From these TPD experiments where

the temperature was ramped in He to 900°C before cooling under the same atmosphere,

an SO2 desorption peak was observed that began at about 175°C and ended about one

hundred degrees later (an illustration of this is shown for the CNx-3.6%thiophene catalyst

experiment in Figure 27a). This peak corresponds well to the sulfur peak observed

during the TGA-TPO experiments discussed above, suggesting that the early sulfur

evolution in the TPOs was due to the desorption of adsorbed sulfur on the surface of the

catalyst. The TPO experiments that immediately followed the TPD also showed sulfur

oxidation during the carbon oxidation (Figure 27b). The information gained from the

TGA-TPO and TPD-TPO experiments support the sulfur species identified by XPS,

namely elemental sulfur, S-O and S-C. The S-O sulfur would be desorbed at lower

temperatures and the sulfur which was incorporated into the graphite matrix, i.e., S-C, was only released during the carbon oxidation.

While TPD-TPO experiments were useful in verifying the types of sulfur species present in the CNx-thiophene catalysts and the temperatures at which sulfur began to

evolve from the surface of the catalyst, the impact of the high-humidity fuel cell environment may alter these properties, especially the desorption temperatures. To address this, TPD experiments were repeated by introducing 10% water into the He feed stream during temperature ramping to 900°C (labeled “wet TPD”). Wet TPD

experiments were followed by TPO after the catalyst was cooled in He. It was found that sulfur desorption was not significantly affected by the presence of water. As shown in

Figure 28a, the desorption of sulfur began at about the same temperature as when it did in 98

dry conditions. Also, to be noted from the wet-TPD was that water began to oxidize the

CNx catalysts at about 700°C. The TPO immediately following the TPD (shown in

Figure 28b) also did not show any early sulfur desorption, indicating that all adsorbed

sulfur on the surface of the catalyst was removed during the initial heat ramp. During the

carbon oxidation stage, small quantities of SO2 were still observed. The temperature of

the sulfur desorption and carbon oxidation in both the presence of water and the presence

of oxygen are significantly higher than the operating temperatures for PEM and DMFC

of 60-80°C. This should limit the amount of sulfur desorption into the fuel cell and

carbon oxidation during operation. Pretreatments of the catalyst could also be performed

to reduce the risk of any sulfur contamination.

5.3 Thiophene Usage as a Growth Promoter Concluding Remarks

It was shown that thiophene can be used as a growth promoter in the acetonitrile

pyrolysis production of CNx catalysts for the oxygen reduction reaction. Low levels (2-

5mol%) of thiophene promote carbon deposition without significantly altering the carbon

nanostructure. Even though sulfur was incorporated into the CNx both as adsorbed

elemental sulfur and its oxides or S-C, the ORR activity was not impacted. Selectivity of the catalyst to water formation was not a function of thiophene used as a growth promoter, but rather a function of the amount of oxygen functional groups present on the

CNx surface. The use of thiophene did increase the amount of oxygen functional groups present.

If the CNx-thiophene catalysts were pretreated in an inert atmosphere at 250-

300°C, the adsorbed sulfur could be removed from the surface of the catalyst,

significantly reducing the possibility of sulfur leaching into the fuel cell.

100

101

CHAPTER 6. Impact of Oxygen Functional Groups on CNx ORR Catalysts

Published in part in Topics in Catalysis199

6.1 Oxygen Functional Groups on Carbon and their Potential Role in ORR

Several important questions remain about the CNx nanostructures and their performance as ORR catalysts. While previous studies in the research group showed a correlation between the pyridinic-N content and ORR activity140,142,145, it may still be possible to functionalize the graphite surface further through different treatments. There is a large volume of literature on surface functionalization of carbon76,118,121,169,202,299-

304,308-310,312 and it may be possible to tailor the surface composition of CNx materials through such techniques. It is also important to examine the stability of these materials through various treatments in different media.

Carbons are oxidized to change their hydrophilic properties299, increase their reactivity300 and to create anchoring sites for other functional groups or metals169,301-304.

102

A wide variety of oxygen functional groups can be found on the surface of carbons,

depending upon the type of carbon and oxidation treatments. Nitric acid treatments have

been used to oxidize carbon118,169,202,299,300,303,304,308-310,312. While nitric acid treatments on

carbon have shown an increase in oxygen functional groups, they also led to the addition

of more surface nitrogen312. Researchers investigating ORR activity for PEM and DMFC

ORR catalysts also found that oxidizing carbon-containing catalysts aided in the

improvement of activity76,118,121,169,300. Some of these researchers state that the addition

of the oxygen groups to the surface aided in cobalt or iron dispersion for the metal-N-C

catalysts being studied76,169. ORR activity was found to be proportional to the oxygen

content when nitrogen was not present in the carbon catalyst121. When nitrogen and oxygen groups were present on the catalyst surface, they found that nitrogen controlled the ORR activity121. It has also been shown that nitrogen incorporation through ammonia

treatment on carbon black is much more effective when the carbon has been oxidized

with nitric acid first118. While some researchers believe that oxygen groups contribute to

higher ORR activity through better incorporation of nitrogen and metal particles, some

have attributed the ORR activity to quinone/hydroquinone oxygen groups in catalysts

which are oxidized carbons containing copper300. In these catalysts it is believed that the

copper acts as an adsorption site for the oxygen during ORR, but the

quinone/hydroquinone site is the actual active site300.

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6.2 Results & Discussion

6.2.1 CNx Treated with HNO3

With the above findings about oxygen functional groups in mind, nitric acid treatments were studied using CNx grown over Fe/MgO that had been washed with HCl to remove the magnesia and exposed iron. These nitric acid-treated catalysts were labeled

CNx-HNO3 in this study. After three hours of treatment in concentrated nitric acid, the suspension containing the CNx and nitric acid was amber in color, suggesting partial removal of carbon tars and amorphous carbons from CNx. While some carbon was

removed, it was not a significant portion. Greater than 90% of the CNx catalyst mass was

collected through the vacuum filtration process.

Significant change to the CNx nanostructures was not observed when the pre- and

post-treated catalysts were studied with TEM. At low magnifications (as shown in Figure

29a and c), the nanostructure appeared to be consistently stacked cup nanofibers and

nanocubes for both the CNx and CNx – HNO3 samples, as expected with CNx catalysts

grown over Fe/MgO. The graphitic planes did not appear to be affected either, as shown

in the high-resolution images of Figure 29b and d.

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a) b)

c) d)

Figure 29. TEM images of a) and b) CNx grown on Fe/MgO, and c) and d) CNx-HNO3. As published in199.

While the nanostructure of the CNx remained intact, the surface species were altered with the nitric acid treatment. Table 3 lists the surface composition of both 105

catalysts as analyzed with XPS. A four-fold increase was observed in oxygen content on the surface of the CNx-HNO3 catalyst. This is consistent with other studies involving

surface oxidation with nitric acid. The O 1s region spectra did not change appreciably

with the nitric acid treatment (as shown in Figure 30), suggesting that CNx catalysts

already contain a similar distribution of oxygen species compared to CNx treated with

HNO3. The types of oxygen present in both materials and their XPS binding energy

included physisorbed oxygen (530.1 eV)336, quinones and carbonyls (531.0 eV)308,318,

C=O in anhydrides or lactones and ethers (532.3 eV)308,318 and ether oxygens in lactones

or anhydrides (533.3 eV)308,318. The nitric acid treatment only appeared to increase the quantity of oxygen on the surface. While the table indicates that the portion of the surface attributed to nitrogen decreases slightly with nitric acid treatment, the ratio of N to C remains constant. This indicates that the nitrogen content most likely did not change significantly but is reduced because of the large oxygen surface coverage. The pyridinic-

N component of the N 1s region increased from 24.1% to 28.8% with nitric acid treatment. The acid treatment may be incorporating small amounts of nitrogen into the graphitic edges from the HNO3 source, but this is still unclear.

106

Table 3. Surface species (atomic %) on CNx and CNx-HNO3 from XPS analysis. As published in199 CNx CNx-HNO3 O 1s 1.5 6.5

N 1s (total) 7.7 7.2 Pyridinic 24.1 28.8 (portion of N)

C 1s 90.8 86.3

Figure 30. O 1s XPS spectra for CNx and CNx -HNO3.

107

The combined retention of nanostructure, increase in oxygen surface content, and

increase in pyridinic-N, appeared to result in an overall increase in the catalytic

performance of the CNx-HNO3 catalyst. As shown in Figure 31, the ORR activity for

CNx-HNO3 increased both in onset of activity and current density. In addition, the selectivity, as illustrated in the inset to Figure 31, significantly improved as well. Further

treatments and characterization of the CNx-HNO3 catalyst are required to better

determine the impact of each effect on the improved activity and selectivity as well as

any synergetic effects.

Figure 31. Comparison of ORR activity for CNx and CNx-HNO3 measured using RRDE at 1000 rpm. Inset: Selectivity of CNx and CNx-HNO3. 108

6.2.2 Additional Oxygen Functional Group Observations

In addition to directly studying the oxygen functional groups, several of the studies reported in other chapters and published elsewhere have suggested that oxygen functional groups play several roles.

Oxygen functional groups can be added to nitrogen-free carbon nanostructures, such as the case with nanofibers with basal plane exposure and stacked platelet nanofibers reported in another chapter of this volume, as a processing step before nitrogen introduction. While the intent was to create locations on the graphitic nanofibers where ammonia could incorporate nitrogen at high temperature through HNO3:H2SO4

acid oxidations, a significant amount of oxygen was added. For the stacked platelet

nanofibers, 15.4 atomic% oxygen was found to be on the surface after acid oxidation.

Prior to this oxidation step only 1.5% oxygen was located on the surface. Similar

increases were observed with nanofibers with basal plane exposure treatment, except at

lower levels due to the low graphitic edge plane exposure. Acid oxidation pretreatments

proved to be important in being able to incorporate nitrogen into the carbon nanostructure

through high temperature ammonia treatment. Without the oxygen functional groups

present first, the ammonia treatment was not as successful in incorporating nitrogen.

109

Activity testing showed that addition of oxygen functional groups alone on these

nanofibers did not have a significant impact on ORR activity or selectivity.

In a separate study examining the impact of thiophene as a growth promoter in

CNx catalysts (also in a separate chapter and published in Applied Catalysis B:

Environmental325), it was found that use of thiophene increased the affinity for surface

oxygen to be present on the CNx catalysts produced. The use of thiophene in the growth

process allowed for surface oxygen to increase to 15.4% of the surface compared to only

1.9% surface coverage for CNx catalysts grown in the absence of the thiophene growth

promoter. While changes in oxygen functional group concentration was not found to

alter the ORR activity, selectivity to water formation was observed to increase with total

oxygen surface content. There was no trend in any one type of oxygen functional group

as identified by XPS fitting, suggesting that there may be multiple types of oxygen playing a role and that further investigation is needed to identify the types.

6.3 Conclusions for Oxygen Functional Groups in ORR

Preliminary observations on the role of oxygen functional groups in ORR

performance were made on several systems. It was found that HNO3 treatments to CNx

materials improved both ORR activity and selectivity. HNO3 treatments significantly

increased the oxygen surface content of the catalyst, suggesting that oxygen content had a

role in improved ORR performance. CNx materials produced with thiophene as a growth 110 promoter supported this observation that increased oxygen groups improved ORR performance. When thiophene was used as a growth promoter, the oxygen surface content increased as did the ORR selectivity of the catalyst. Oxygen functional group impact was not apparent when there was no nitrogen incorporated into the carbon nanostructure, as was the case with nanofibers with basal plane exposure and stacked platelet nanofibers.

111

CHAPTER 7. Role of Graphitic Edge-Plane Exposure in Carbon

Nanostructures for Oxygen Reduction Reaction

As submitted to Journal of Physical Chemistry C338

7.1 Motivation in Studying Role of Graphitic Edge-Plane Exposure

From work in our research group140,142,145 and others205, carbon nanostructure has

been proposed to have a role in carbon-based ORR catalyst performance. We have previously reported that nitrogen-containing carbon catalysts with higher graphitic edge plane exposure, such as stacked cups compared to nanotubes, have higher ORR activity140,142,145.

Graphitic plane orientation and edge plane exposure of nanostructures can be

controlled by altering the nanofiber growth conditions237,255,339. Changes to the growth

temperature, medium and fiber growth catalyst can significantly alter the resulting carbon

nanostructures produced255. Stacked platelets, stacked cups and herringbone nanofibers

112

are commonly used when high edge-plane exposure is desired220,229,242,340. Nanotubes are

commonly the nanofiber of choice when minimal edge plane exposure is desired242,341.

The intent of work discussed in this chapter was to study the role of carbon

nanostructure in ORR carbon-based catalysts by comparing nanofibers with high graphitic edge-plane exposure (stacked platelets) to nanofibers with very little edge-plane

exposure (nanofibers with basal plane exposure) as model structures. These nanofibers

have been grown and treated to incorporate nitrogen into the nanostructure. The

objective of this study is not to develop high performance catalyst materials, but rather

probe the role of nanostructure using these model systems.

7.2 Results and Discussion

7.2.1 Synthesis and Physical Characterization

The successful preparation and treatment of well controlled carbon nanostructure

types is not trivial and must be discussed before any conclusions can be made about their

use as model catalyst systems in ORR.

Small changes in Fe:Cu ratio and nanofiber growth temperature can change the

carbon nanostructure produced significantly. TEM analysis of the nanofibers produced

was performed to verify that stacked platelets with high edge plane exposure were

prepared. Figure 32 shows examples of the nanofibers prepared using 95:5 Fe:Cu

113 catalyst at a 600°C growth temperature after HCl washing. Nanofiber diameters varied from 50nm to 300nm, with the nanofiber shown in Figure 32a exhibiting a typical diameter. Most important for this study are the orientation and exposure of the graphitic edge planes. Under these preparation conditions, graphitic sheets were positioned perpendicular to the longitudinal axis of the nanofiber, allowing for large degrees of graphitic edge plane exposure and minimal basal plane exposure, as illustrated in the inset to Figure 32a. Figure 32b shows the graphitic edge plane exposure for these stacked platelets at a higher magnification. The perpendicular orientation of the graphitic sheets verified that stacked platelet nanofibers were produced. Additionally, all nanofibers studied with TEM using the 95:5 Fe:Cu catalyst had similar shape and perpendicular graphitic edge plane exposure.

114

a) b)

Figure 32. TEM images of stacked platelets-HCl washed. Inset to a): Illustration of graphite sheet orientation in stacked platelets. Box in a) refers to region where b) was taken. As submitted to338.

In contrast, carbon deposition over 6:4 Fe:Cu at 675°C produced a variety of

nanofibers, as shown in Figure 33. Two main types of nanofibers were observed; large

helical nanofibers (Figure 33a-b) and smaller hollow tubes (Figure 33c). The helical-type

nanofibers had diameters of 200nm to 500nm. While the hollow tubes had diameters of

30nm to 100nm. Interestingly, high resolution analysis of both types of nanofibers

produced from the 6:4 Fe:Cu growth catalyst showed long graphitic sheets parallel to the

longitudinal axis with low levels of edge plane exposure and high levels of graphitic

basal plane exposure, even though at lower magnification the nanofibers appeared very different (insets to Figure 33a-c). After the observation that both types of nanofibers grown from 6:4 Fe:Cu at 675°C had mostly graphitic basal plane exposure, these 115

nanofibers were deemed acceptable as a comparison structure to the stacked platelet

nanofibers with high edge plane exposure. These nanofibers with basal plane exposure

were preferred to a commercial multi-walled nanotube (MWNT) because the physical dimensions of the nanofibers with basal plane exposure were more similar to the stacked platelets than the very small diameter MWNTs.

116

a) b)

Figure 33. TEM images of nanofibers with basal plane exposure after HCl washing. Insets represent the lower-magnification images of the same nanofibers. As submitted to338.

It is known that the incorporation of nitrogen into graphitic materials can be difficult118. To aid in the nitrogen incorporation, mild oxidation of the carbon can be 117

performed. In concentrated acid oxidations, such as the HNO3 and H2SO4 used in this

study, defects are created in the graphitic structure providing sites for oxygen functional

groups to attach261. A constant temperature treatment at 60°C was selected as opposed to

refluxing. A nanofiber with basal plane exposure that has a high concentration of surface

defects would result in losing the objective of the study to examine the role of graphitic

edge plane exposure. While the oxidation temperature was rather mild, oxidation was obviously occurring, as seen through even simple hydrophobicity observations. When both types of nanostructures were suspended in water, they tended to not disperse well, indicating their hydrophobic nature. When the nanofibers were added to the

HNO3:H2SO4 solution, they immediately dispersed into the solution and did not settle out

even with repeated washings with water. This indicated a change to their hydrophobic

nature, indicative of addition of oxygen functional groups on the surface of the

nanofibers.

Significant differences in the carbon nanostructure were not observed with

oxidation treatment using TEM analysis (images not shown), also verifying that the

oxidation procedure was rather mild.

Ammonia treatments at elevated temperatures are known to incorporate nitrogen

into carbon matrices118,121,123,124,339. With increased graphitization of the carbon,

nitrogen-incorporation becomes more difficult124, hence the need to create sites for ammonia attack through oxidation arises. Ammonia is proposed to introduce nitrogen into carbon through a corrosion process where ammonia directly reacts with the carbon to

124 form HCN and H2 or exchanges with the oxygen functional groups on the carbon 118

surface, ultimately incorporating nitrogen into the carbon123. In this study, after a 4-hour

ammonia treatment at 600°C, little mass was lost (19%) from the stacked platelets, as

listed in Table 4 of mass losses with ammonia treatment. A 4-hour treatment duration at

900°C in ammonia also saw minimal mass loss. Interestingly, after 19.5 hours at 900°C in an ammonia-rich stream, a significant percentage of the mass (76%) was lost from the starting acid oxidized stacked platelets. This suggests that not only temperature, but also duration is important for significant ammonia corrosion on the carbon to occur. The oxidation pretreatment was also found to increase the ammonia corrosion. As a comparison, an ammonia treatment on unoxidized stacked platelets for 19.5 hours at

900°C led to a 42% mass loss only. For the oxidized nanofibers with basal plane exposure treated at 900°C for 19.5 hours in ammonia, a mass loss of 47% was also observed, suggesting that ammonia was able to successfully corrode carbon and incorporate nitrogen into the nanofibers. However, further analysis was required using

XPS and temperature programmed oxidation experiments to verify nitrogen incorporation.

119

Table 4. Mass loss during ammonia treatment. As submitted to338. Sample & Treatment Mass Loss (%)

Stacked platelets -HCl – HNO3:H2SO4– 600°C NH3 (4 hr) 19

Stacked platelets -HCl – HNO3:H2SO4 – 900°C NH3 (4 hr) 7

Stacked platelets - HCl – HNO3:H2SO4 – 900°C NH3 (19.5 hr) 76

Stacked platelets-HCl-900°C NH3 (19.5 hr) 42

Nanofibers with basal plane exposure - HCl – HNO3:H2SO4 – 900°C NH3 (19.5 hr) 47

The significant mass loss during ammonia treatment at 900°C for extended time would suggest that the carbon nanofiber structures may be attacked or altered. TEM analysis did not reveal significant changes in the carbon nanostructure as indicated by examining the graphitic planes of Figure 34.

120

a) b)

Figure 34. TEM images of a) stacked platelets and b) nanofibers with basal plane exposure after acid oxidation and ammonia treatment at 900°C for 19.5h. As submitted to338.

7.2.2 Chemical Characterization

7.2.2.a X-Ray Photoelectron Spectroscopy

Beyond the physical structure of the nanofibers used in this study, it is important

to know the chemical composition. X-Ray photoelectron spectroscopy (XPS) was used

to analyze the types of surface species on the nanofibers. Table 5 shows surface species

content for stacked platelets. As expected, the stacked platelet surface was largely

composed of graphitic carbon.

121

Table 5. Surface species concentrations from XPS analysis on stacked platelets after oxidation and ammonia treatments. As submitted to338. Treatment Species Concentration (atomic %)

Fe 2p O 1s N 1s C 1s S 2p

HCl 0.4 1.5 0.0 98.1 0.0

HCl – HNO3:H2SO4 0.0 15.4 1.6 82.1 0.9

HCl – HNO3:H2SO4– 600°C NH3 (4 hr) 0.0 1.4 3.4 95.2 0.0

HCl – HNO3:H2SO4 – 900°C NH3 (4 hr) 0.0 1.0 0.6 98.4 0.0

HCl – HNO3:H2SO4 – 900°C NH3 (19.5 hr) 0.0 1.0 0.5 98.5 0.0

XPS analysis showed no detectable surface nitrogen in the stacked platelets prior to acid oxidation and ammonia treatments. Figure 35 shows that after acid oxidation and ammonia treatments, nitrogen was present in the stacked platelets. Depending upon the treatment, the nitrogen species and content differed. After acid oxidation treatment on the stacked platelets, 1.6 atomic % nitrogen was observed on the surface of the nanofiber.

This nitrogen is residual from the HNO3 used in the oxidation treatment. An assignment of N-O species318 aligns with this assumption. The stacked platelets with the highest nitrogen content were the ones treated at 600°C in ammonia. The nitrogen spectra broadened to shift to lower binding energies with this lower temperature ammonia treatment. With increased ammonia treatment temperature and duration, the nitrogen surface content decreased and the species appeared more prevalent at lower binding energies. It is likely that after the 600°C treatment, there was a mix of nitrogen bonded to 122

the surface oxygen and nitrogen beginning to be incorporated into the graphitic lattice.

At higher temperatures, the oxygen functional groups are being removed and at the

locations where the oxygen was once attached, nitrogen is being substituted by

incorporating into the carbon in the form of pyridinic-N. Typical binding energies for the pyridinic-N are 398.0-398.9 eV121,342.

123

Oxygen content also follows the discussion of the nitrogen content for stacked

platelets. A large amount (15.4%) of surface oxygen is present immediately following

acid oxidation of the stacked platelets. The quantity of surface oxygen diminishes

significantly with ammonia treatment. There is a possibility that nitrogen is both

124

exchanging with surface oxygen (as evidenced by the increase at 600°C in nitrogen

content) and desorbing from the surface with temperature as well.

As seen from Figure 36, there was a wide distribution of oxygen species present

on the stacked platelets. O 1s oxygen functional group assignments include: physically

adsorbed oxygen (530.1 eV)336, quinones and carbonyls (531.0 eV)308,318, sulfur-oxygen

functional groups (531.6 eV)337, C=O in anhydrides or lactones and ethers (532.3 eV)308,318, ether oxygens in lactones or anhydrides (533.3 eV)308,318 and oxygen atoms in

carboxyl groups (534.2 eV)308,318. While there was not a clear trend between treatment

used and the nature of the functional groups formed, a few interesting observations were

made beyond total oxygen content. The lower binding energy species appeared to be

most resilient to ammonia attack or formed most readily when exposed to the atmosphere

after treatment. Large quantities of quinone or carbonyl groups were added with acid

oxidation and removed or masked any physisorbed oxygen found on the stacked platelets.

125

If ammonia was incorporating nitrogen into the carbon matrix through an oxygen

functional group pathway, the loss of oxygen species with ammonia treatment would be

expected. It has been proposed in the literature that ammonia exchanges with C=O 126

oxygen functionalities on carbon nanostructures to incorporate nitrogen into the

carbon123. Most of the functionalities that can be identified by XPS have a C=O feature that could participate in the nitrogen exchange with ammonia. This would also explain why the small amount of carboxylic species disappears with ammonia treatments, as did the lactone and anhydride assignment for the stacked platelets after extended duration ammonia treatment at 900°C. The acid-oxidized stacked platelets had a large quantity of

carbonyl and quinone-type groups identified, that again, decreased with ammonia

treatment. Some of these most likely desorbed from the surface with simple heating, but

a portion may have been subject to ammonia attack.

Also a product of the acid oxidation, a small amount of sulfur was detected on the

surface of the acid oxidized stacked platelets. This appeared to be a sulfate species

318,334 located on the carbon surface as seen through the S 2p1/2 assignment of 163.8 eV ,

shown in Figure 37. The absence of detectable lower binding energy sulfur species

indicates that little, if any sulfur was incorporated into the nanostructure325. With

ammonia treatment, sulfur was no longer detectable in XPS, suggesting it was also part of

an oxygen functional group either exchanged with nitrogen or desorbed from the

graphitic surface with treatment.

127

Figure 37. S 2p region of the X-ray photoelectron spectra for stacked platelets-HCl- 338 HNO3:H2SO4. As submitted to .

Comparing the surface composition of the nanofibers with basal plane exposure to

the stacked platelets, overall there were fewer surface species present on the nanofibers

with basal plane exposure after each treatment, as seen in Table 6. Acid oxidation of the

nanofibers with basal plane exposure was effective in adding oxygen surface

functionalities, however, at a lesser degree than the treatment of stacked platelets. The

absence of graphitic edges for the oxygen functional groups to readily adhere to is likely

to be the reason behind the reduced amount of surface oxygen after acid oxidation on the

nanofibers with basal plane exposure. It has been reported that, during oxygen

functionalization of nanotubes, defects in the basal planes are created for the oxygen

functional groups to adhere to261. In this study, creation of more defects was not desirable since it would make the nanofibers with basal plane exposure become more and

128 more like the high-edge-exposure stacked platelets. Some of the oxygen was required, however, to be able to incorporate nitrogen into the nanofibers with basal plane exposure through the oxygen exchange process with ammonia treatment.

Table 6. Surface species concentrations from XPS analysis on nanofibers with basal plane exposure after oxidation and ammonia treatments. As submitted to338. Treatment Species Concentration (atomic %)

Fe 2p O 1s N 1s C 1s S 2p

HCl 0.0 0.7 0.0 99.3 0.0

HCl – HNO3:H2SO4 0.0 5.5 trace 94.3 trace

HCl – HNO3:H2SO4 – 900°C NH3 (19.5 hr) 0.0 0.5 trace 99.5 0.0

There were several observations that can be made by comparing the oxygen species on the nanofibers with basal plane exposure to the stacked platelets, as seen in

Figure 38. The untreated nanofibers with basal plane exposure had less physisorbed oxygen than the stacked platelets. This may be due to the lack of edge planes for the oxygen to adhere to. With acid oxidation, the largest component was again quinones and carbonyls, similar to the spectra taken over stacked platelets. After ammonia treatment at

900°C for 19.5 hours, the nanofibers with basal plane exposure had very little oxygen left. A large component was physisorbed oxygen, which is likely due to exposure to the atmosphere after treatment. It is possible that most of the reactive oxygen was exchanged 129

during ammonia treatment. Another explanation for the reduced amount of non-

physisorbed oxygen species on the surface of the nanofibers with basal plane exposure compared to the stacked platelets, is that oxygen functional groups are likely not as well adhered to the carbon surface on the basal planes. This would allow for more of the oxygen functional groups to desorb from the surface of the carbon with heating.

After ammonia treatment of the acid-oxidized nanofibers with basal plane exposure, only a trace amount of nitrogen was observed in XPS and could not be quantified. Oxygen groups were largely evolved off after ammonia treatment as well.

Although there was no significant nitrogen incorporation, the mass losses reported above for these nanofibers still suggest that ammonia corrosion was occurring.

7.2.2.b Temperature-Programmed Oxidation Experiments

Temperature-programmed oxidation (TPO) experiments were performed to

further study the composition of the nanofibers and the stability of the carbon. The

temperature at which carbon oxidizes can be indicative of the type of carbon, degree of

graphitization and edge plane exposure.

The TPO profiles for stacked platelets after various treatments are presented in

Figure 39. The mass signals for 44 (CO2), 30 (NOx) and 64 (SO2) are shown. As seen from the figure, a shift in the carbon oxidation to higher temperatures with treatment, suggesting a change in the oxidation resistance. As treatment severity increases on the stacked platelets from simple HCl wash to high temperature ammonia treatments, the easily oxidizable carbon continues to be removed, thereby shifting the observed oxidation onset temperature from 430°C to 515°C. For the acid-oxidized stacked platelets, early evolution of nitrogen and sulfur groups is observed prior to carbon oxidation. This suggests that much of the nitrogen and sulfur groups were attached to the surface of the 131 stacked platelets rather than incorporated into the graphitic matrix, which was also observed in XPS. However, there were small amounts of nitrogen and sulfur that evolved during the carbon oxidation period suggesting that a small amount of the nitrogen and sulfur was incorporated into the graphitic matrix with acid oxidation treatment, even though they were below XPS detection limits.

132

Figure 39. Temperature programmed oxidation profiles for stacked platelets after oxidation and ammonia treatments. Gray is m/z=44 (CO2), pink is (m/z=30) x50 (NOx), green is m/z=64 (SO2) x500 for all treatments except “HCl-HNO3:H2SO4” which is x100. As submitted to338.

133

After ammonia treatments, the low temperature surface species evolution during

TPO was not observed. Larger quantities of nitrogen were observed to evolve with the

carbon oxidation, verifying the observation from XPS of a shift to more pyridinic-type

nitrogen with ammonia treatment rather than surface-attached nitrogen. A small amount of sulfur left in the graphite appeared to stay in the carbon matrix even after ammonia treatment, though in lesser quantities with increased ammonia treatment severity.

TPO experiments performed on the nanofibers with basal planes revealed similar

trends with treatment compared to the stacked platelet TPO experiments, as seen in

Figure 40. The nanofibers with basal plane exposure had a much broader carbon

oxidation envelop. This broader envelop may be due to the heterogeneity of the

nanofibers with basal plane exposure, with its two types of nano-geometries. Once again,

surface nitrogen and sulfur species were observed evolving from the nanofibers at low

temperatures after acid oxidation treatments, supporting the XPS findings that nitrogen

and sulfur are on the surface rather than incorporated into the graphitic nanostructure.

The TPO experiments were able to verify that nitrogen was present in the carbon

nanostructure. This finding shows that ammonia treatment was successful in

incorporating nitrogen into the graphitic nanostructure, even though the nitrogen content

was below quantifiable levels from XPS.

134

Figure 40. Temperature programmed oxidation profiles for nanofibers with basal plane exposure after oxidation and ammonia treatments. Gray is m/z=44 (CO2), pink is (m/z=30) x50 (NOx), green is m/z=64 (SO2) x500 for all treatments except “HCl- 338 HNO3:H2SO4” which is x100. As submitted to .

7.2.3 Activity and Selectivity Testing

The characterization results discussed above show that nanofibers with low- and

high-graphitic edge plane exposure were successfully made and post-treated to incorporate nitrogen. While nitrogen levels were low compared to nitrogen-containing 135

carbon nanostructures (CNx) studied as ORR catalysts in our research group previously140,142,145,325, they were deemed high enough to investigate further for ORR activity.

Rotating ring disk electrode (RRDE) tests were performed on the nanofibers after each treatment process to study the role of nanostructure and composition on ORR activity and selectivity. The results obtained for the stacked platelets are shown in Figure

41. Untreated stacked platelets showed minimal oxygen reduction activity, with an onset of activity at 0.444 V vs. NHE. Both acid oxidation and ammonia treatment at 600°C on the stacked platelets showed minimal ORR activity as well. Increasing the ammonia treatment temperature on the stacked platelets to 900°C began to produce a material with some ORR activity. By increasing the ammonia treatment duration at 900°C, the activity was improved by another 300mV shift towards better ORR activity compared to the untreated stacked platelets. The additional ammonia treatment duration also saw a significant increase in current density. Selectivity results were shown to trend with ORR activity as illustrated in Figure 41b. The untreated stacked platelets had poor selectivity to water formation, while the stacked platelets treated at 900°C for 19.5 hours in ammonia had a selectivity approaching 3.9 electrons transferred per oxygen molecule, equivalent to 95% selectivity to water formation.

136

137

In comparison to the stacked platelets, the nanofibers with basal plane exposure had significantly less activity under the same treatment conditions, as shown in Figure

42. Untreated nanofibers with basal plane exposure had even less ORR activity than untreated stacked platelets. Prolonged ammonia treatment at 900°C on nanofibers with basal plane exposure increased the ORR onset of activity, but only by 150 mV to a 0.565

V vs. NHE onset of activity.

Figure 42. ORR RRDE results of nanofibers with basal plane exposure after oxidation and ammonia treatments at 1000rpm in oxygen saturated electrolyte after background subtraction. As submitted to338.

138

While none of the materials studied would be considered good ORR catalysts,

there is enough activity to use them as model systems to discuss the role that

nanostructure and composition play in ORR activity.

High nitrogen contents in carbon-based catalysts are frequently cited as a

significant contributor to ORR activity. Observations with the acid-oxidized stacked platelets and the stacked platelets treated with ammonia at 600°C ammonia following acid oxidation showed that gross nitrogen content does not dictate ORR performance.

The location of the nitrogen plays an important role in the ORR activity. As the nitrogen species shifted to more pyridinic-N type, the ORR activity increased, even though the overall surface nitrogen content dropped by almost a factor of seven when comparing the stacked platelets treated in ammonia at 900°C to those treated at 600°C. Previous studies in our research group have also seen ORR activity trends with pyridinic-N

content140,142,145. From those studies, it was unclear if the pyridinic-N was directly impacting ORR activity or if it was merely acting as a marker for edge plane exposure, as it is located on the edge plane and the most active catalysts had both the highest graphitic edge plane exposure content and highest pyridinic-N content140,142,145. Since the same

starting stacked platelet nanofibers were used before and after ammonia treatment for this

study, the role that pyridinic-N and edge plane exposure can be better distinguished.

These results suggest that pyridinic-N content plays a much stronger role than edge plane

exposure. Rather, edge plane exposure provides the appropriate geometry for nitrogen

incorporation leading to an active site. Nanofibers with basal plane exposure, on the other

hand, did not possess the right geometry for the formation of ORR active sites.

139

In addition to nanostructure and nitrogen, a few observations about oxygen

content may also be made from this study. High oxygen contents on the nanofibers did

not substantially improve the ORR activity without the presence of nitrogen incorporated

into the nanostructure. A small improvement in ORR activity was observed with

increased oxygen content on the nanofibers with basal plane exposure. It is possible that

this improvement was due to the increase in graphitic defects created during the acid

oxidation process, as current density began to approach that of the acid oxidized stacked

platelets.

7.3 Conclusions to Investigation of Role of Graphitic Edge Plane Exposure

Stacked platelet nanofibers and nanofibers with basal plane exposure were

successfully grown and characterized after various treatments with acids and/or ammonia. The stacked platelets, with high graphitic edge plane exposure, and nanofibers with basal plane exposure and minimal edge plane exposure, were studied before and after ammonia treatment to probe the role that nanostructure plays in ORR activity.

While neither type of nanofiber had large quantities of nitrogen or was highly active for

ORR, they were considered useful model systems. It was found that edge plane exposure

alone does not promote ORR activity. Rather, carbon nanostructures with high edge

plane exposure provide the appropriate locations for nitrogen to incorporate into the

140 graphitic matrix. The presence of oxygen functionalities alone were also not sufficient to have a significant impact on ORR activity.

141

CHAPTER 8. Effect of Ammonia Treatment on CNx Catalysts

8.1 Background into CNx Catalysts Treated with Ammonia

Ammonia treatments at elevated temperatures over carbon have been reported to

introduce nitrogen into the carbon structure88,118,123,125,135,137,138,159,187. The carbons used

in these studies were not normally nitrogen-containing prior to ammonia treatment.

Studying the effect of ammonia treatment on carbon-nitrogen materials is of interest.

Ammonia and nitric acid treatments have were studied on CNx materials grown

over 2%Fe/MgO-HCl washed. This investigation has been undertaken to study the types

of nitrogen present in the CNx catalysts and also attempt to improve ORR activity by

increasing nitrogen content in the catalyst nanostructure.

Studies performed for this set of experiments have involved the treatment of CNx

with concentrated nitric acid at 60°C for three hours followed by filtering and drying.

These acid-oxidized CNx materials were treated with ammonia for study. The ammonia

treatments were performed at 900°C and 600°C both in 38% NH3 for 4 hours. Ammonia

treatments followed nitric acid treatment, because the literature suggests that pretreating

142

carbon materials with nitric acid allows for better incorporation of nitrogen into the

carbon structure123. Villers, et al. proposed that ammonia exchanges with C=O oxygen functionalities on carbon nanostructures to incorporate nitrogen into the carbon123.

These types of functionalities are frequently present after nitric acid oxidation treatments303,307,311.

For comparison of the materials several other catalysts have been treated for study

including CNx-HNO3 treated at 900°C in He only for 4 hours, and CNx treated at 600°C

and 900°C in ammonia without prior HNO3 treatment.

8.2 Results & Discussion

8.2.1 Ammonia Treatment Process Observations on CNx Materials

Since ammonia is known to corrode carbon, examination of the mass loss with

treatment is important. Immediate differences were observed for mass loss from the varying treatment levels, as compiled in Table 7. Ammonia treatment at 600°C did not cause a significant mass loss even when the CNx was oxidized first. While it is possible

to have mass loss with heat treatment without nitrogen incorporation, it is unlikely that

nitrogen incorporation will occur with ammonia treatment without mass loss. This lack

of mass loss after ammonia treatment at 600°C suggests that ammonia treatment at this

temperature and duration may not be sufficient for significant nitrogen incorporation.

When CNx catalysts were treated at 900°C more mass was lost compared to the 600°C 143

treatments, even when ammonia was not present during treatment. Mass loss at 900°C

from the oxidized CNx without ammonia is likely due to loss of oxygen functional groups

and attached carbon307,308. Ammonia treatment at 900°C had a more significant mass loss over the oxidized CNx. This indicates that the temperatures and pretreatment were

substantial enough to promote ammonia corrosion and oxygen exchange.

Table 7. Mass losses during heat treatment of CNx catalysts. Catalyst Mass loss

CNx-conc HNO3-900°C He 12.6%

CNx-conc HNO3-600°C NH3 7.9%

CNx-conc HNO3-900°C NH3 36.7%

CNx-600°C NH3 3.7%

CNx-900°C NH3 12.5%

When the oxidized CNx catalyst and the oxidized CNx catalyst treated at 900°C

with ammonia were analyzed using TEM imaging, it was found that there were many

more broken nanofibers in the catalyst after ammonia treatment. Figure 43 shows examples of the broken nanofiber edges in the HNO3-900°C NH3 sample, while Figure

44 are examples of the larger masses of fibers prior to ammonia treatment without

evident breakage of the nanofibers. These observations in TEM support the mass loss

finding with ammonia treatment at 900°C over oxidized CNx.

144

Figure 43. TEM images of broken nanofibers on CNx-conc HNO3-900°C NH3.

Figure 44. TEM images of CNx-conc HNO3 treated.

145

8.2.2 Surface Species Analysis of Treated CNx

Through XPS analysis, it was found that the types of nitrogen alter as the

treatment path progresses. Untreated CNx materials have N 1s spectra that can be

deconvoluted into three types of nitrogen (as seen in Figure 45): edge-plane exposed

pyridinic-N at 398.8 eV, graphitic substituted quarternary-N at 401.1 eV and a third type

which is still debated, but could be pyridinic-N-O at 402 eV342. Similar spectra have

been reported previously for CNx catalysts grown over Fe/MgO from the research

group144,145. After nitric acid treatment, only two peaks are present: 400.9 eV and 398.6

eV. These are most likely pyrrolic-N and pyridinic-N, respectively342.

A much larger oxygen component is also revealed from the O 1s spectra (as listed

in Table 8) for the CNx treated with nitric acid. This is expected, as nitric acid treatment is an oxidizing treatment that adds oxygen functional groups. Ammonia treatment causes a reduction in the amount of oxygen and the reappearance of the three nitrogen peaks seen in the untreated CNx material. The loss in oxygen content can be expected as part of

the ammonia exchange process123 and from desorption of surface oxygen species with

heat treatment307,308. After the 600°C ammonia treatment, there is an intermediate

amount of oxygen left and an increase in nitrogen content. This nitrogen could be

completely incorporated into the graphitic structure or some could be externally bound to

oxygen groups318. At this point in time, it is not clear why the nitrogen content decreases

on the CNx that had been oxidized and then ammonia treated at 900°C. One possible

reason may include that with the loss of carbon, nitrogen species are evolved from the

CNx as well. This also supports the observation of less pyridinic-nitrogen after ammonia

146 treatment, as carbon (and nitrogen) will be evolved off of the graphitic edges first.

Another is if oxygen and nitrogen groups are bound together at 600°C, then when the oxygen groups desorb, some of the nitrogen could as well.

Table 8. Surface species composition (atomic%) from XPS on CNx after ammonia treatments. CNx- CNx-

CNx- conc HNO3- Conc HNO3-

CNx conc HNO3 600°C NH3 900°C NH3 O 1s 1.5 6.5 1.5 1.3 N 1s (total) 7.7 7.2 8.9 5.7 pyridinic 24.1 40.5 34.4 26.6 C 1s 90.8 86.3 89.6 93.0

147

Figure 45. N1s spectra for ammonia treated CNx.

8.2.3 Activity and Selectivity Observations for CNx Oxidized and Treated with Ammonia

Activity and selectivity results from RRDE are shown in Figure 46. Both oxidized CNx and oxidized CNx treated at 900°C in ammonia have a promotion effect for

ORR while the oxidized CNx treated at 600°C in ammonia did not. While a decrease in

nitrogen is observed in oxidized CNx treated at 900°C in ammonia, TEM images showed

an increase in edge plane exposure from the breakage of the nanofibers treated at 900°C

in ammonia. The oxidized CNx may have improved activity because of the synergistic effects of oxygen on the surface of the catalyst. Oxygen functional groups on carbon 148

have been the focus of much study on their own200,202,303,305,307-310,314,322. It has also been

observed that increase in oxygen functionality on the surface of C-N catalysts increase

ORR activity slightly118,121,169,300,323.

The presence of additional oxygen functional groups on the CNx surface after acid

oxidation has improved selectivity according to the RRDE results. This observation has

been observed on other treated CNx catalysts reported in other chapters. Ammonia

treatments on the CNx also improved the selectivity towards water formation compared to

untreated CNx.

149

Figure 46. RRDE comparison for ammonia and nitric acid treated CNx. (top) Activity comparison on disk, (bottom) Selectivity comparison from ring results.

150

8.3 Conclusions to Studies on CNx Treated with Ammonia

CNx catalysts were oxidized with nitric acid and then treated with ammonia at either 600°C or 900°C. It was found that ammonia treatment did not increase nitrogen content with mass loss as expected. ORR activity was improved for both the oxidized catalyst and the oxidized catalyst treated at 900°C with ammonia. More extensive study of these ammonia and nitric acid treated catalysts are needed to better understand the impact of these treatments for PEM & DMFC catalysts.

151

CHAPTER 9. Use of Boron and Phosphorus in CNx Catalysts

9.1 Overview of Purpose of Using Boron and Phosphorus in CNx Catalysts

Boron and phosphorus can be used in multiple ways when studying carbon nanostructures. Boron and phosphorus groups can be used as graphitic edge site blockers. They also can become heteroatom dopants to the carbon, which can alter the electronic structure of the nanostructures among other properties.

Graphitic edge site blocking can be used to probe active site locations and bondings of other species to carbon. Park and Baker have reported on a method where a boron oxide functional group can block the zigzag graphitic edge and a phosphorus oxide functional group can block the armchair graphitic edge230. An illustration of such blocking is shown in Figure 47. Typically one graphitic edge type is blocked at a time and then the performance of the nanofiber is analyzed. When there is a significant drop in performance with site blocking, the site that was blocked can be concluded to have the active site located on it.

152

Figure 47. Site blocking on graphitic edges by boron and phosphorus groups. Illustration courtesy of Paul Matter.

Boron288,343-348 and phosphorus264,347-350 have both been doped into carbon

nanostructures for various purposes. The introduction treatments and type of nanofibers

dictate the properties that these doped nanofibers can have. Ozaki, et al. have shown that

low levels of boron doping into nitrogen-containing carbon can improve ORR activity345,346. Strelko, et al.216 and Huang, et al217. both also have shown with modeling

improved electron donation properties of boron-doped carbons. In contrast Garsuch, et

al. have shown a negative impact on ORR when carbon-nitrogen thin films were doped

with boron153. This suggests that preparation and boron concentrations and types may

have a significant impact on the ORR performance. 153

In this chapter, results at attempting to use site blocking in CNx to study ORR are

discussed, as well as attempts to dope CNx catalysts with boron for improved ORR activity.

9.2 Graphitic Site Blocking of CNx

It an attempt to determine the ORR active site location, CNx grown over

2%Fe/MgO-HCl washed was site blocked using a modified procedure by Park and

Baker230 that used phosphorus and boron groups to block the armchair and zigzag

graphitic edge planes, respectively. CNx grown over 2%Fe/MgO-HCl washed should be

an appropriate candidate for site blocking, as it has a large proportion of stacked cup

nanofibers with high edge plane exposure. To block the CNx edges, ammonium

pentaborate or methyl phosphonic acid was introduced into the CNx using a wet

impregnation technique. The resulting material was then calcined at 450°C to adhere the

boron or phosphorus functional groups to the graphitic edge. Unattached boron or

phosphorus was removed through a 1M HCl wash following calcination.

It is expected that the untreated catalyst would have the highest ORR activity and

the catalyst with the active edge blocked would have the lowest ORR activity. From

RRDE results of these site-blocked CNx catalysts, the expected observations were not

made (Figure 48). After using boron to site block the zigzag edge, the ORR catalyst

activity significantly improved compared to the untreated CNx. Site blocking with 154

phosphorus resulted in a catalyst that was less active than the untreated CNx, but

comparable to the CNx that had gone through a site blocking procedure without the active boron or phosphorus precursors (“Blank-Blocked CNx”). This suggests that site blocking itself may not have been successful, but rather, boron may be promoting ORR activity on the CNx catalyst. A second possibility is that the ammonium component of the boron

precursor did not completely evolve off of the catalyst during heat treatment and it

instead increased the nitrogen content of the catalyst. Further analysis is required to

support or disprove this second possibility. Further study on the use of boron in CNx

catalysts is discussed in the next section of this chapter.

Figure 48. RRDE activity results for site-blocked CNx. 155

9.3 Boron Incorporation into CNx

After observing ORR activity promoting results with boron site blocking attempts

on CNx, it was desired to study boron incorporation directly into CNx catalysts. Also,

incorporation of low levels of boron into C-N catalysts has been reported in the literature to have a positive effect on ORR as recently reported by Ozaki345,346. In contrast,

researchers in the Dahn group observed a loss of ORR activity with thin film sputtered

carbon-nitrogen catalysts containing boron153. Higher levels of boron can cause phase

separation instead of incorporation into the graphitic structure, so boron concentrations

and preparation methods may be key.290.

Preliminary attempts have been made to incorporate boron into the graphitic

structure of CNx catalysts grown over 2%Fe/MgO-HCl washed. Others have

incorporated boron into carbon at low levels through methods as easy as piling carbon

fiber bundles onto a mound of boron oxide and heat treating in an inert289. Preliminary

attempts involved a wet impregnation technique where DI water and ammonium

pentaborate precursor were made into a solution that was then added to CNx grown over

2%Fe/MgO-HCl washed at a temperature of 90°C while stirring. The appropriate

amount of ammonium pentaborate precursor was added to the water to make B:C atomic

ratios equaling 0.025, 0.05, 0.10, and 0.25 when the CNx catalyst was assumed to be all 156

carbon for the calculation. A catalyst sample was also prepared using wet impregnation

methods with water only to study the effect of the treatment conditions on CNx. After

drying the wet-impregnated B-CNx, it was heat treated in nitrogen at 900°C for 4 hours to

incorporate the boron into the nanostructure. It was believed that at some B:C level,

phase separation would occur, but it was unknown at which precursor loading, as reports

have shown significantly more boron has to be added than what actually is

290 incorporated . To continue the study, some of the prepared boron-CNx catalysts were treated with HCl to try and remove any excess boron not bound to carbon.

9.3.1 Surface Species Analysis on Boron Incorporated CNx

The effectiveness of incorporating boron was analyzed using XPS. It was found

that boron oxide, B-C, and possibly B-N were in the catalysts (see Figure 49). The higher

binding energy peaks are attributed to boron oxides and the two lower peaks are due to B-

C and B-N species298,346,351,352.

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Figure 49. B 1s spectra for boron incorporated CNx.

Table 9 is a compilation of the surface composition for the boron-incorporated

CNx. The boron oxide was formed either by incomplete high temperature treatment or through reactions when exposed to air after treatment. At this point, it is unknown if the

boron also has been incorporated into the graphitic structure or has phase separated to

graphite and boron nitride. A substantial amount of oxygen was also found on the

surface of the B-CNx catalysts, further verifying the boron oxide phase identification.

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When comparing the atomic ratios of the surface species, as in Figure 50, it can

be seen that the amount of boron compared to carbon increased with each increase in

boron addition. This is not clearly seen in Table 9, because of the oxygen surface content variability. This oxygen surface content variability is not easily explained and further testing is required. While the nitrogen content of the catalyst surface appeared to change significantly when tabulated, less change in nitrogen content when compared to the carbon present at the surface is observed.

After the blank treatment on CNx (B:C = 0.00), the oxygen and nitrogen surface groups are decreased. After heat treatment in inert at 900°C, it can be expected that surface oxygen and some nitrogen may desorb from the surface307,308, explaining the

drop.

Table 9. Surface chemical species (atomic%) composition for boron incorporated CNx catalysts

CNx B:C=0.00 B:C=0.025 B:C=0.05 B:C=0.10 B:C=0.25

O 1s 3.0 1.9 29.3 28.2 15.4 23.5 N 1s (total) 7.0 5.9 6.0 3.6 4.2 4.8 pyridinic 26.2 19.0 26.6 21.0 30.8 29.8 C 1s 90.0 92.2 59.6 59.5 66.9 58.7 B1s 0.00 0.00 5.0 8.8 13.5 13.0

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Figure 50. Atomic ratios of surface elements on boron incorporated CNx.

9.3.2 Surface Area and Pore Volume of Boron Incorporated CNx

Through nitrogen physisorption experiments, it was observed that the addition of

boron into CNx at high levels significantly reduced the surface area and pore volume (see

Table 10). This suggests that the boron oxide material was coating the surface of the CNx

catalyst, which is also supported through XPS studies. The blank treatment on CNx also

saw a significant increase in surface area with treatment. This also complements the

observations from XPS on this catalyst. If surface area is gained, it is possible that some

160 of the graphitic edges were attacked during heat treatment. This could cause a loss in pyridinic-nitrogen content, as well.

Table 10. Surface area and pore volume for select Boron incorporated CNx BET Surface Area BJH Desorption (m2/g) Pore Volume (cm3/g)

CNx 252.4 0.92 B:C=0.00 284.6 1.00 B:C=0.025 179.7 0.70 B:C=0.25 61.9 0.30

9.3.3 Activity of Boron-Incorporated CNx

The addition of boron at these levels and in this method did not significantly alter the onset of ORR activity for the catalyst as indicated by RRDE testing (Figure 51).

Current densities and voltammogram slopes did change with treatment. The blank treated catalyst had the biggest increase in current density. An increase in surface area could increase the current density, if the exposed species were active. The catalysts with the two highest boron levels had reduced slopes before reaching a limiting current. This may suggest a change in catalyst behavior in ORR, but it is difficult to verify this.

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Figure 51. RRDE activity for boron-incorporated CNx

9.3.4 Acid Treatment of Boron-Incorporated CNx

In an attempt to remove the boron oxide from the surface of the CNx and retain the boron-nitrogen and boron-carbon species, the CNx-B:C=0.25 and CNx-B:C=0.10 were post-treated with 5M HCl at 60°C before filtering with excess DI water. In the graphitic site blocking method, Park and Baker suggested that an acid wash would remove unattached boron230.

XPS analysis of these HCl treated B-CNx catalysts showed that the HCl treatment

was not successful in removing the boron oxide layer from the catalyst. Interestingly, the

B-C and B-N species identified previously, as seen in Figure 49, were not observed after

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HCl treatment (as seen for the CNx-B:C=0.10 catalyst in Figure 52). After HCl treatment, the boron oxide peak was the only observed peak and was strong.

Figure 52. B 1s spectra from XPS for CNx-B:C=0.10 washed with 5M HCl.

When the catalysts that were HCl post-treated were tested using RRDE, the

current density of the catalysts was observed to improve with the HCl treatment, as seen

in Figure 53. The onset of activity for these catalysts did not appreciably change. It is

not clear why the current density would increase with HCl treatment on the B-CNx 163 catalysts. Further analysis is required. Some possible reasons could include an increase in catalyst surface area and an increase in the exposure of graphitic edge planes.

Figure 53. ORR activity for B-CNx catalysts before and after 5M HCl treatment.

9.4 Conclusions for Graphitic Site Blocking and Boron Incorporation into CNx

Techniques were used to both selectively block the graphitic edges on CNx and to incorporate boron into the graphitic matrix of CNx. Results showed that graphitic site

164

blocking methods using boron and phosphorus groups did not prevent the CNx catalyst from being ORR active. Rather, after attempting to block the zigzag graphitic edge plane with boron, the ORR activity of the catalyst improved. This suggests that boron may act as a promoter to ORR activity. After this observation, CNx catalysts were doped with

boron intentionally. The methods used resulted in catalysts coated in a boron oxide film,

rather than incorporation into the graphitic matrix. ORR activity was also not

significantly improved with boron-doping attempts. A 5M HCl treatment was used on

some of the boron-doped CNx catalysts to remove some of the boron oxide layer. This treatment was successful in making a minor improvement in ORR activity, even though the boron oxide species were still very prevalent from XPS analysis. Other boron

treatment methods may need to be investigated to determine the right concentration and

treatment for improved ORR activity on CNx catalysts.

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CHAPTER 10. Evaluation of RRDE Testing Methods and their Impact on Reported

ORR Catalyst Results

In part accepted to Electrochemical and Solid-State Letters353 and in part in preparation

for submission to the Journal of the Electrochemical Society.

10.1 Background on RRDE Testing

Typical ORR catalyst research involves a path where catalysts are first screened

for ORR activity and selectivity for water formation in the rotating ring disk electrode

(RRDE) half cell environment. Then the best performing RRDE catalysts (and

sometimes a few poor ones for comparison) are manufactured into membrane electrode

assemblies (MEAs) and tested in full fuel cells.

The RRDE half cell technique is generally regarded as the best method for

simultaneously testing the ORR activity and selectivity of the catalysts directly. In this

technique, a cyclic voltammogram (CV) on a catalyst-coated disk is performed to

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monitor the activity of the catalyst, while a ring (typically Pt) surrounding the disk is held

at a constant potential where hydrogen peroxide can be detected in an acidic environment

to simulate the cathode side on the PEM or DM fuel cell. The reactions that may occur at

the disk are the desired complete reduction to water4:

- + O2 + 4e + 4H → 2H2O 1.23V vs. NHE

And the undesired incomplete reduction to hydrogen peroxide4:

- + O2 + 2e + 2H → H2O2 0.695V vs. NHE

Hydrogen peroxide is an undesired product not only because it leads to a loss in

efficiency due to the decreased number of electrons transferred per O2 molecule, but

more importantly, because of its ability to degrade the components of the fuel cell354.

A technique related to RRDE is the rotating disk electrode (RDE) technique. The

RDE system does not have a ring used for the direct determination of selectivity.

Commonly used, the Koutecky-Levich technique allows for the calculation of selectivity

to water formation from the RDE experimental data.

Just as in any experimental method, these electrochemical half cell techniques

have their limitations. Alterations to the experimental methodology have the potential to

alter the experimental results. In this chapter, the results of two investigations involving

the RRDE technique are discussed. The effect of catalyst loading on the selectivity

towards water formation has been studied, as well as the effect of RRDE catalyst ink

aging on the selectivity results.

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10.2 Examination of Catalyst Loading Effects on the Selectivity of CNx and Pt/VC

ORR Catalysts using RRDE

While RDE has been used to study ORR for quite some time, there have been

reports in the literature that the technique may not always give comparable results for

selectivity30,71,326,327. Recently, letters have also been published reporting on the

observation that the selectivity of the catalyst changed with loading level on the disk of

an RRDE setup for several ORR catalysts: carbon-nitrogen, Pt-based and chalcogenide catalysts328-330,355. Interestingly, researchers have recently reported that this observation

of selectivity changes with catalyst loading level is dependent upon the generation of

catalyst used from the same laboratory137.

A survey of the literature shows that the catalyst loadings vary widely for both

carbon-nitrogen and Pt-based catalysts using the RRDE technique. While catalyst

loadings as different as 70 μg/cm2 356 and 4370 μg/cm2 144 have been reported, majority of

the studies fall in the 100 μg/cm2 to 1100 μg/cm2 range44,125,345,357-361. An illustration of

the distribution can be seen in Figure 54.

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Figure 54. Select catalyst disk loadings for activity testing from literature. Authors listed are first authors44,122,126,167,296,356-359,362.

In this section, a study of the effect of catalyst loading on the ORR activity-

selectivity measurements using RRDE technique for CNx and Pt/VC catalysts as well as the variation of the experimentally determined collection efficiency with catalyst loading is presented. The possibility of a two-step mechanism through an H2O2 reduction step has

been probed. SEM characterization of model electrodes with different loadings is also

presented.

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10.2.1 Effect of Catalyst Loading on Selectivity

When catalysts are tested for activity and selectivity using RRDE techniques,

activity is determined from the cyclic voltammogram (CV) performed on the catalyst-

coated disk. Selectivity is determined by comparing the current produced at the catalyst-

coated disk to the current at the ring. As mentioned in the Experimental Methods

Chapter, the disk current is a result of the reduction reactions taking place at the disk,

namely formation of H2O and H2O2 from O2. The ring catalyzes the oxidation of H2O2

formed over the disk and the ring current is a result of this reaction. Since only a portion

of H2O2 formed at the disk reaches and/or is oxidized at the ring, the current measured at

the ring represents only a fraction of the total H2O2 produced. This fraction is the collection efficiency, N, of the RRDE system. For results reported and discussed in this section, the theoretical collection efficiency as reported by the manufacturer for the setup used in the study will be used. This is a value of Ntheo=0.22, implying that for the system studied, 22% of the H2O2 produced at the disk is assumed to reach the ring and be

oxidized.

An example comparison of the ring current to the disk current in an ORR half cell

test is shown in Figure 55. This is a comparison for CNx catalyst studied with 142

μg/cm2 loading on the disk in an oxygen-saturated electrolyte. A substantial amount of

hydrogen peroxide was detected at the ring, shown from the relatively large ring current.

2 The selectivity of CNx loaded at 142 μg/cm was n = 3.47, which is equivalent to 73%

selectivity to water formation. The inset to Figure 55 shows the comparison of the disk

current to the ring current after the ring current has been adjusted for the collection

170

efficiency of Ntheo=0.22. If the catalyst had been 100% selective to water formation, the

ring current would have been zero. If the catalyst formed only H2O2, then the ring

current and disk currents would have been the same after adjustment for collection

efficiency. In addition, Figure 55 can also be used to represent the ORR activity of the

catalyst from the disk. The onset of activity can be reported as 770 mV vs. NHE for the

CNx catalyst shown. As a reference, the same experiment ran on the blank glassy carbon disk electrode has been added to the Figure 55 graphs. It is easily seen that that the activity is due to the CNx catalyst and not the underlying glassy carbon. The glassy

carbon had an onset of activity at about 400 mV vs. NHE. Current was minimal in this

material, as well, because of the low surface area of the polished disk.

171

172

RRDE tests were performed on the CNx materials, by varying the catalyst loading

on the disk from 142 μg/cm2 to 1420 μg/cm2. A substantial increase in selectivity to

water with catalyst loading was observed as shown in Figure 56a. The impact of catalyst

loading on selectivity was the most dramatic at low loadings. This phenomenon has also

been recently observed by other catalyst loading studies328.

There have been reports in the literature which suggest that CNx-type catalysts participate in a two-step ORR mechanism with H2O2 being formed first and further

28,363 reacting to form H2O , while Pt-based catalysts have been hypothesized to participate

364 in a complete reduction to water without an H2O2 intermediate step . There are other

studies which suggest that H2O2 intermediates could be possible in both types of catalysts28,365. The impact of catalyst loading on commercial Pt/VC catalysts were also

studied to better understand if the observed catalyst loading effect was specific to the type

of catalyst used.

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Figure 56. Selectivity as a function of catalyst loading on disk. Selectivity is taken at 0.5 V vs. NHE. Theoretical collection efficiency of N=0.22 was used. a) CNx catalyst, b) Pt/VC catalyst.

It was observed (as in Figure 56b) that Pt catalyst selectivity also increased with

catalyst loading, albeit the trend was much less pronounced compared to CNx. The

selectivity achieved with 142 μg/cm2 of catalyst loading was significantly higher for the 174

Pt catalyst than it was for the CNx test catalysts at the same catalyst loading (n=3.78

versus n= 3.47), which would mean that there was less room for improvement of ORR

selectivity with increased loading for the Pt catalyst. At higher loading levels, the

selectivities achieved for the two catalysts were comparable. Selectivity increases with

catalyst loading was also observed by Bonakdarpour, et al., using 3M Pt nanoparticles329.

In that literature report, however, the observed selectivities were closer to n=3.0. The authors used catalyst loadings which were significantly lower than used in our study, which can explain the selectivity differences. As catalyst loading increased, they also observed marked improvement in selectivity to water formation329. In general, the Pt- based catalysts produce ORR current densities higher than that of CNx catalysts. This

may be due to a higher active site density on the Pt catalysts compared to many CNx

catalysts reported to date in the literature. Pt catalysts may have higher selectivites at the

206 same loadings as CNx because of the higher current densities . With higher active site

densities, a reaction that occurs in a two-step mechanism may have a higher probability

to reach completion within the same catalyst layer thickness. This would result in an

observation that selectivity to water formation would be higher at lower catalyst loadings

than a catalyst with fewer active sites. It is possible that when the Pt catalyst is so

dispersed that the active site density is significantly reduced that the two steps cannot

occur as readily, thereby lowering the catalyst selectivity, which would explain the

observation of n~3.0 by Bonakdarpour, et al. on Pt catalysts329. Adzic has also suggested

that a two-step process with H2O2 as an intermediate and a complete four-electron

175

transfer process without an H2O2 intermediate would be indistinguishable with RRDE if

365 the H2O2 intermediate was given sufficient time to completely react to water .

It should also be noted that, throughout this study, onset of activities of the catalysts

tested were monitored. Any variations in onset of activities with catalyst loading were

statistically insignificant, which points to the reliability of the technique in comparing

activities of different catalysts, independent of any catalyst loading

10.2.2 RRDE Catalyst Layer Thickness Observations

If collection efficiency is not a significant factor in the trend of selectivity with

catalyst loading observed in this report, electrocatalyst loading must contribute to

selectivity differences beyond fluid dynamics. If the hypothesized two-step process

occurs in ORR catalysts, additional catalyst layer thickness should improve the

probability of the second step being completed.

A closer examination of the catalyst layer thickness was performed to determine if

any trends in catalyst layer thickness to selectivity could be obtained. Aluminum rods

with the same diameter as the RRDE disk were coated with CNx catalyst at the same

loading levels and using the same ink application method as studied for selectivity and

collection efficiency measurements. The catalyst layers on the aluminum rods were then

examined with SEM to obtain approximate measurements of catalyst layer thicknesses

for each catalyst loading. While the catalyst ink may disperse slightly differently on

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aluminum than glassy carbon, the model appeared to be representative of the behavior

observed on the RRDE electrodes.

Figure 57 and Figure 58 are example SEM images obtained through this

procedure. From Figure 57 (angled views of the top of the disk), it can be seen that the

catalyst layer was able to disperse across the aluminum rod tip, further supporting the

assumption that the aluminum rods could be used as a model. The catalyst layer showed

areas of catalyst particle agglomeration at 142 μg/cm2 (Figure 57a), but still nearly

completely covered the rod tip. As the amount of catalyst increased on the aluminum rod

tip (Figure 57b), more catalyst began accumulating in the center and less at the edges.

This can be attributed to the application process where the drop from the pipet tip is

deposited at the center of the disk. Visual observations of the RRDE glassy carbon disk

after application of catalyst inks also supported the SEM images similar to the disk-top

views shown in Figure 57. Side-view SEM images allowed for measurement of the catalyst layer thickness (examples shown in Figure 58). With 142 μg/cm2 catalyst

loading, the average catalyst layer thickness was 21.1 μm and was fairly evenly

distributed (a standard deviation of ±1.8 μm). When the catalyst loading reached 1420

μg/cm2, a large, cracked mound of catalyst was observed on the rod tip. The thickness

varied significantly with an average thickness being 230 μm with a standard deviation of

±160 μm for the 1420 μg/cm2 sample. Figure 59 is a plot of the average film thickness with catalyst loading. As expected, the average and standard deviation increased with catalyst loading. Roughness of the catalyst layer could also play a role in the fluid

177 dynamics of the system. The catalyst layers were deemed rough, but were not quantified for a roughness factor.

178

17.8µm

52.1µm

424µm 207µm

179

Figure 59. Average CNx catalyst thickness as a function of catalyst loading on the disk. Error bars show 1 standard deviation.

The RRDE method was developed for an electrode disk with a surface level with the ring height366. The thicknesses observed through SEM have potential to alter the fluid dynamics of what was initially intended for the method. If the thicknesses were too large to fit the model, then the collection efficiency experiments should have been significantly different from the theoretical collection efficiency, which was not the case

(data not shown here). The use of experimental collection efficiency did not make up for

the differences in selectivity with loading. This suggests that the catalyst layer thickness

must play another role beyond RRDE fluid dynamics. Figure 60 shows the variation of

selectivity with the catalyst layer thickness. As expected, the trend is very similar to that

180 observed when selectivity was plotted against catalyst loading (Figure 56a). If a multi- step mechanism with a hydrogen peroxide intermediate is occurring at the cathode, then an increased catalyst layer thickness would improve the probability of the mechanism going to completion forming the final product of water. This was suggested by

Bonakdarpour, et al. also328. The work in this paper also suggests that as the catalyst layer is thickened, more oxygen is fully reduced to form water, thereby supporting a multi-step mechanism for both CNx and Pt type ORR catalysts.

Figure 60. Selectivity as a function of average CNx catalyst thickness. Selectivity values taken at 0.5V vs. NHE.

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10.2.3 Electroreduction of H2O2 by CNx Catalysts

Changes in hydrogen peroxide detected at the ring with catalyst loading may

suggest that ORR catalysts participate in a two-step mechanism where oxygen is first

electrochemically reduced to hydrogen peroxide and then further electrochemically

reduced to water. If this is the case, CNx catalysts should have some electrochemical

reduction activity towards hydrogen peroxide. A series of experiments were performed to analyze the electrochemical activity towards hydrogen peroxide reduction and oxidation.

Figure 61a shows a comparison of ORR activity as a function of rotation rate in

oxygen-saturated 0.5M H2SO4. Currents continue to increase with rotation rate with the

maximum current at 3000rpm being just under -6 mA/cm2. When hydrogen peroxide is

being reduced, the currents are lower, but still increase with rotation, showing there is

indeed activity towards hydrogen peroxide reduction. It should be noted, that 6.25mM

H2O2 is greater than the concentration generally accepted for saturated oxygen in 0.5M

H2SO4. The higher current in the less concentrated oxygen-saturated electrolyte means that CNx catalysts are more active towards oxygen reduction than they are towards hydrogen peroxide reduction alone. This does not mean, though that the CNx catalysts

cannot participate in a two-step process in ORR. The reduction of hydrogen peroxide to

water may be the limiting reaction. Further studies at 1.3 mM H2O2 in 0.5M H2SO4 are

needed to draw any additional conclusions about these hydrogen peroxide reduction

results.

182

Figure 61. (Top) ORR activity on CNx as a function of rotation rate in oxygen saturated electrolyte, (bottom) hydrogen peroxide reduction activity as a function of rotation rate in 6.25 mM H2O2.

183

The results of this study point to the difficulty of comparing selectivities from

separate experiments. This challenge is even bigger when selectivities of different catalysts are compared, especially when the nanostructures of the catalysts are quite different as in the case of CNx and Pt/VC catalysts. If film thickness plays such a pivotal role in determining the selectivity of ORR catalysts, it raises the question whether comparisons should be made on an equal mass loading per geometric area or they should be made on an equal film thickness basis. For practical purposes, it is much easier to perform RRDE tests on an equal mass per geometric area basis, but variations due to catalyst film thickness should be taken into consideration.

10.3 RRDE Catalyst Ink Aging Effects on Selectivity to Water Formation in ORR

Preliminary observations in our laboratory at Ohio State have shown that catalyst

loading may not be the only contributing factor to changes in oxygen reduction reaction

(ORR) selectivity measurements when using RRDE. This section will focus on the

observation that RRDE catalyst ink preparation technique can play a significant role in

the selectivities observed for both commercial Pt/VC and nitrogen-containing carbon

nanostructured (CNx) catalysts.

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10.3.1 RRDE Results with Ink Aging

In an attempt to better understand the possible variables in RRDE testing beyond catalyst loading, catalyst inks were allowed to age for up to 11 days to determine the ink aging effect on RRDE results obtained. An example comparison of the ring to the disk currents for different ink aging periods is shown in Figure 62a. As seen in the figure, there is a dramatic decrease in the ring current with ink aging coupled by a more modest increase in the disk current. The onset of activity does not appear to change with ink aging. Figure 62b shows the corresponding selectivity, n, the number of electrons transferred per oxygen molecule, calculated from the ring and disk currents. To be able to observe greater changes in selectivity due to catalyst aging, a low catalyst loading on the RRDE disk was chosen, as catalysts with lower disk loadings give lower selectivity measurements328-330,355. Over the span of 11 days, the catalyst selectivity increased from

3.46 to 3.80 electrons. Even in just the span of 24 hours, catalyst selectivity increased appreciably, indicating that aging of catalyst ink can have a significant impact even in a short period of time. When similar experiments were run at higher CNx loadings on the

RRDE disk (426µg/cm2), ink aging was again seen to improve selectivity, resulting in a change of n from 3.67 to 3.83 in 11 days. The change was less pronounced, but still significant, since the starting selectivity at Day 0 was much higher at this loading.

185

186

Similarly, the results reported in Figure 62 were also observed for commercial

Pt/VC catalysts. Figure 63 shows the selectivity increase with ink aging for a commercial Pt/VC catalyst when the catalyst loading on the RRDE disk is 426 µg/cm2.

The increase in selectivity is smaller, and requires a longer aging period in order to be observed, but nevertheless, the trend remains.

Figure 63. ORR selectivity as a function of RRDE catalyst ink age for Pt/VC catalyst with a 426 µg/cm2 loading on the disk at 100rpm. As accepted in353.

187

While the reasons behind these observations of increased selectivity and relatively

unchanged overall activity are not yet clear, possible explanations include a change in the

physical properties of the catalysts, such as particle size or surface area, due to repeated

sonication. Alternative explanations may involve changes in hydrophilicity of the

surface355,367 and/or additions of surface functional groups, the most likely functional

group candidates being the oxygen functional groups199,325.

It is not believed that loss of solvent due to heating during sonication accounts for

the increase in selectivity due to an increase in catalyst loading. A solvent heating study

found that during sonication, a 6°C increase in ink temperature is observed. In the

system used, this is equivalent to a 0.00025 mass% loss in ethanol after each sonication.

10.3.2 Role of Ethanol in Ink Aging

To study the impact of ethanol treatment without the addition of Nafion or

repeated sonications on the catalyst, a batch of CNx was soaked in excess ethanol for ten

days. After ten days, the CNx catalyst was either directly dried to remove the excess

ethanol or vacuum filtered with DI water before drying. Fresh inks were made of these

catalysts and immediately tested at a RRDE catalyst loading of 426 µg/cm2.

Once again, significant changes in ORR activity were not observed, but the

selectivity towards water formation did increase with ethanol treatment, as seen in Figure

64. This shows that exposure of CNx to ethanol has a major impact on the selectivity of the catalyst. Possible explanations include ethanol imparting oxygen functional groups 188

on the catalyst surface; increases in hydrophilicity of the catalyst which would in turn

improve the wettability of the catalyst367; and ethanol penetrating the graphitic layers of the carbon, perturbing the nanostructure227. Preliminary XPS results were inconclusive to

the role that oxygen functional groups may play. The selectivity increase was more

pronounced for the catalyst that was not rinsed with water before drying, an observation

which may be in agreement with the hydrophilicity argument. If ethanol molecules are

227 penetrating the graphite layers, as suggested by Baker, et al. , heating of the CNx in

ethanol instead of water may also increase the effect. Small changes in nanostructure,

supported by the observation that the BET surface area of the ethanol-treated catalysts

2 2 increased from 148 m /g for the untreated CNx catalyst to 182 m /g for the CNx-ethanol

2 soaked and 188 m /g for the CNx-ethanol soaked-DI water rinsed, which may also contribute to the selectivity differences.

189

Figure 64. ORR selectivity results for CNx treated with ethanol (Catalyst loading on disk: 426 μg/cm2). As accepted in353.

While further study is needed to better understand why RRDE ink aging results in improved selectivity to water formation for ORR catalysts, the observation is reproducible for both CNx and Pt/VC catalysts.

10.4 Summary of RRDE Method Examinations

An extended study on the effect catalyst loading levels on the ORR selectivity results obtained from RRDE analyses was performed using both CNx and commercial

Pt/VC catalysts. It was found that catalyst loading plays a significant role in the determination of selectivity to water formation. ORR selectivities calculated by RRDE 190

were observed to increase by as much as 0.35 electrons per oxygen molecule by

increasing the catalyst loading. This level of increase in selectivity is significant in

determining the viability of a catalyst for commercial applications. SEM images of CNx

catalyst loadings showed that catalyst thickness increased substantially with loading.

The results suggest that increased catalyst layer thickness increases the selectivity to water formation by increasing the probability that the two-step reaction mechanism to water formation can reach completion. The results discussed in this paper support the need for a uniform standard of catalyst loading or uniform catalyst film thickness be used for RRDE testing. Before any experimental results can be compared to the literature, one must take into consideration the catalyst loading levels used in the studies to be compared.

Catalyst loading on the RRDE disk was found to not be the only factor in the selectivity results reported from the method. Aging of RRDE catalyst inks for both CNx

and Pt/VC catalysts was found to increase the resulting selectivity to water formation of

the catalysts while leaving the activity relatively unchanged. Increases in selectivity were

observed to be as high as 0.35 electrons per oxygen molecule after 11 days of aging CNx

ink. Possibilities for the increase in selectivity with ink age include changes in the

physical properties of the catalyst, changes in the hydrophilicity of the catalyst surface

and changes in the surface function groups of the catalyst. Further research is required to

fully elucidate the reason for selectivity increases with RRDE ink aging.

191

CHAPTER 11. Methanol Tolerance of CNx Oxygen Reduction Catalysts

As published in Topics in Catalysis368

11.1 Introduction to Direct Methanol Fuel Cell Cathode Catalysts

While DMFCs have advantages in fuel supply and storage, methanol crossover

limits the technology currently. Methanol crossover, where the methanol fed to the

anode permeates through the membrane to the cathode side, inhibits the Pt-based cathode catalysts369 and causes a reduction in the fuel cell coulombic efficiency14. The methanol

oxidation reaction (MOR) occurs at the cathode as a parasitic reaction, reducing the open

circuit potential370 and causing a mixed potential15. There is also a possibility that the

methanol and MOR intermediates and products could poison the cathode catalyst5.

In an attempt to deal with the methanol crossover, various catalysts have been

investigated for the DMFC cathode that can be tolerant to methanol. Platinum is active to

both MOR and the oxygen reduction reaction (ORR). The orientation of the Pt crystals

192

and Pt particle size are known to determine which reaction is favored. There have been

studies that focused on developing Pt catalysts with higher selectivity for ORR371. Pt-

alloys have also been investigated for use as DMFC cathodes. Alloys studied include

those of Co6, Cr361,372, Fe6,373,374 and Ni375, among others. Undesired MOR can increase with time on the Pt-alloys if the metal leaches out of the catalyst, leaving MOR active Pt behind369. Macrocycles (both pyrolyzed and non-pyrolyzed) have demonstrated positive

results for ORR activity in DMFCs42,44,48,60,100, though the non-pyrolyzed macrocycles

had questionable stability44. Pyrolyzed macrocycles have shown inactivity for MOR in

both anodic and cathodic sweeps42, as well as in DMFC tests42,60 while having little effect

of methanol on the ORR activity. Chalcogenide ORR catalysts have also been screened

for DMFC cathode catalysts. Chalcogenides tested include compounds of Ru-Mo-Se17,18,

Ru-Mo-S19,20, Ru-Mo-Se-O17,21, Rh-Ru-S22, Re-Ru-S22, Rh-S376 and Ru-Se6,23,376. The chalcogenides tested in methanol did not demonstrate any methanol catalyst poisoning and were not active for MOR. While studied to a lesser degree, metal oxides have also shown methanol tolerance when used as ORR catalysts in methanol-containing systems,

377 with ZrO2-x being the most ORR active .

Mixed Reactant DMFCs (MRDMFCs) rely on the selectivity of both anode and

cathode catalysts for their operation6,376,378-380. MRDMFCs are desirable, because the gas

sealing requirements can be relaxed378 and the anode and cathode feeds are introduced together379. Selective cathode catalysts that have been studied for MRDMFCs include

chalcogenides and macrocycles6,376,379. Compact mixed reactant (CMR) fuel cells take

the MRDMFC concept further by removing the bipolar plates and replacing the

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membrane with a porous one378. This design reduces the size and cost of the fuel cell as

well as increases the active catalyst area378. For MRDMFCs, the tolerance and inactivity

of the cathode catalyst for methanol is even more crucial since the exposure of the

cathode catalysts to methanol is no longer limited to small quantities that crossover from

the anode. On the contrary, cathode catalysts are exposed to much higher concentrations

of methanol. Therefore, any activity towards methanol oxidation or any poisoning by

methanol over the cathodic active sites would cause much larger coulombic losses.

Some of the CNx catalysts grown over metal-impregnated alumina, silica and

magnesia supports that were previously studied in the Ozkan group for PEM fuel cell

cathodes141,143-145, were examined for their methanol tolerance during ORR. Similar to the

Pt-free macrocycles and chalcogenides, the CNx catalysts studied were found to be also inactive to MOR.

11.2 MOR Activity and Effect of Methanol on ORR with Pt and Pt/Ru Catalysts

To gauge the performance of the CNx catalysts, 20 wt% Pt/VC and Pt:Ru

(20:10)/VC were studied for their MOR activity using the RRDE half-cell set-up. The

effect of methanol on ORR activity was also examined. Figure 65 shows the cyclic

voltammetry scans for 20%Pt/VC catalyst in methanol-containing electrolyte saturated

with O2. An ORR voltammogram for the methanol-free system is also included for

comparison. As seen in the figure, a significant anodic peak is present during the 194

Figure 65. Cyclic voltammetric scan for 20%Pt/VC from 1.2 to 0.2 to 1.2V vs. NHE in 0.5M H2SO4 saturated with oxygen at 0 rpm showing both 1.0M methanol and methanol- free systems.

reduction sweep from 1.2 V to 0.2 V vs. NHE that is not present in the methanol-free solution. Another anodic peak is present during the sweep from 0.2 V to 1.2 V vs. NHE.

Both of these peaks can be attributed to the oxidation of methanol5,15. CV scans were also

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Figure 66. Reduction sweep voltammogram for 20%Pt/VC in 0.5M H2SO4 solutions at 1000 rpm showing both 1.0M methanol and methanol-free systems.

taken for the same catalyst at 1000 rpm, for methanol-O2, and O2-only systems. Figure 66 shows the reduction sweeps from 1.2 to 0.2 V for these systems. The methanol oxidation feature in methanol-only system becomes prominent between 0.9 and 0.5 V and decreases to zero at more reducing potentials. The anodic peak during the reduction scan in the methanol-O2 system can be seen to have contributions from both oxygen reduction

and methanol oxidation reactions. However, as shown by Paulus et al.15, in the area

where methanol oxidation is dominant, the current is not simply a sum of the oxygen-free

methanol oxidation current and the methanol-free oxygen reduction currents.

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For comparison, we have also tested the PtRu/VC catalysts, which are known for

MOR activity14. As presented in Figure 67, PtRu catalysts showed significant activity towards MOR. The MOR was the prevailing reaction in the mixed potential in oxygen- saturated methanol-containing solutions until about 0.55 V at 1000 rpm. ORR currents were comparable to that of Pt/VC catalysts after MOR became limited.

With the occurrence of methanol crossover to the cathode in a DMFC, the mixed potentials shown above illustrate the problem with having Pt-based cathode catalysts in

DMFCs. The parasitic MOR at the cathode is likely to lead to significant power loss and cannot be overcome until the system is operating at a potential less than 0.5 V.

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Figure 67. Reduction sweep voltammogram for Pt:Ru/VC in 0.5M H2SO4 solutions at 1000 rpm showing both 1.0M methanol and methanol-free systems.

11.3 MOR Activity and Effect of Methanol on ORR with CNx Catalysts

The methanol oxidation activity and the effect of methanol on ORR activity were

tested for CNx catalysts, using parameters similar to those used for Pt and PtRu catalysts.

Catalysts studied were previously developed and characterized in the PEM-type

142 environment. CNx grown over iron-, and nickel-impregated Vulcan carbon , alumina140,141, silica144,145 and magnesia144,145 supports, and cobalt-impregnated silica and

magnesia supports144,145 were used all after washing to remove the oxide support and exposed metals. Results for two different CNx catalysts with different nanostructures and 198

different ORR performances are presented in Figure 68 and Figure 69. The first catalyst

chosen is a highly active CNx catalyst grown over Fe/Al2O3 support (Figure 68). This

catalyst consists of mostly stacked cup structures with a high degree of edge plane

exposure and a high pyridinic-N content. The second is a catalyst prepared using a

Ni/Al2O3 substrate (Figure 69). This catalyst, which was characterized by multi-walled nanotube structures, had very low pyridinic-N content and very low ORR activity. The

RRDE voltammograms for both catalysts were obtained at 0 rpm and the results for the methanol-free O2 and the methanol-O2 systems are plotted in the same figure for comparison. Neither catalyst has any activity for methanol oxidation. There is also no effect of methanol on ORR activity since the curves are essentially the same (differences are well within the experimental error) for the methanol-free system and methanol-O2

system. These results indicate CNx catalysts have high methanol tolerance and no activity

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Figure 68. Cyclic voltammetric scan for CNx grown on Fe/Al2O3 from 1.2 to 0.0 to 1.2V vs. NHE in 0.5M H2SO4 saturated with oxygen at 0 rpm showing both 1.0M methanol and methanol-free systems.

Figure 69. Cyclic voltammetric scan for CNx grown on Ni/Al2O3 from 1.2 to 0.0 to 1.2V vs. NHE in 0.5M H2SO4 saturated with oxygen at 0 rpm showing both 1.0M methanol and methanol-free systems.

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for MOR regardless of their level of activity for ORR. When mass transfer limitations are

reduced, the catalysts still exhibit a high methanol tolerance and a complete lack of any

MOR activity. Figure 70 shows CV curves obtained for CNx catalysts grown over

Co/SiO2 substrate at 1000 rpm rotation rate. Again, there is no difference between the

ORR voltammograms with or without methanol in the system. Similar observations were

made when tests were repeated for other CNx catalysts. There was no MOR activity or

methanol poisoning effect in any of the CNx catalysts tested, implying that a change in the pyridinic-N content or nanostructure does not change the behavior towards methanol.

The ORR mechanism does not appear to change with addition of methanol, either. With addition of methanol, the calculated n values, obtained using the ring data did not change

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Figure 70. Reduction sweep voltammogram for CNx grown on Co/SiO2 in 0.5M H2SO4 solutions at 1000 rpm showing both 1.0M methanol and methanol-free systems.

(data not shown). Here ‘‘n’’ represents the number of electrons that can be harnessed per

O2 molecule reduced and is a measure of the ORR selectivity. A value of n = 4, represents 100% selectivity for complete reduction of O2 to H2O. Similar observations were also reported by Paulus et al. on Pt catalysts15. Table 11 summarizes the MOR activity and methanol tolerance experiments using the RRDE technique in 1.0 M methanol + 0.5 M H2SO4 solutions. The ORR activity and selectivity to water data that were presented in previous publications are also included in the table.

As reported in methanol ORR studies on macrocylces and chalcogenides, the active site for MOR is not present without Pt. This has been observed again with the CNx

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368 Table 11. CNx catalysts tested in 1.0M Methanol+0.5M H2SO4 solution Tolerance Onset of Selectivit Selectivity Methanol to Metal/Support Activity y to water to water Oxidation Methanol (mV vs. NHE) n (%H O ) Activity 2 Poisoning 140,141 2%Fe/Al2O3 740 3.91 98% No Yes 145 2%Fe/SiO2 780 3.95 99% No Yes 2%Fe/MgO 770 3.83 145 95% No Yes 145 2%Co/SiO2 720 3.7 92% No Yes 2%Co/MgO 720 3.7 145 92% No Yes 140,141 2%Ni/Al2O3 470 3.56 89% No Yes 145 2%Ni/SiO2 580 3.7 92% No Yes 2%Ni/MgO 670 3.3 145 82% No Yes 20%Pt/VC 890 3.92 141 98% Yes No Pt:Ru (20:10)/VC 850 3.90 98% Yes No

catalysts. It is also note-worthy that any metal remaining from the metal-doped substrates

(e.g., Fe, Co) does not appear to impart any activity for methanol oxidation.

To determine if methanol concentration had an impact on the catalysts, one of the

best performing catalysts, CNx on Fe/Al2O3, was tested with the RRDE technique in a 3.0

M methanol, 0.5M H2SO4 solution, using the same method as previously mentioned

methanol tolerance and ORR activity tests. Once again, no significant changes were

observed in ORR polarization curves with the addition of methanol as seen in Figure 71.

The absence of both methanol oxidation and catalyst poisoning at 3.0 M methanol is

significant for DMFC and Mixed Reactant Direct Methanol Fuel Cell (MRDMFC)

applications. DMFCs generally operate in the range of 0.5-2.0M methanol5. With the crossover methanol, the concentrations at the cathode are likely to be much lower. This 203

implies that these catalysts can handle methanol concentrations even much higher than

what would be encountered at the cathodes of DMFCs.

Figure 71. Cyclic voltammetric scan for CNx grown on Fe/Al2O3 from 1.2 to 0.0 to 1.2V vs. NHE in 0.5M H2SO4 saturated with oxygen at 0 rpm shown both 3.0M methanol and methanol-free systems.

The total inactivity and high tolerance of the CNx catalysts at high methanol concentrations is even more significant for MRDMFCs and make them a possible candidate as ORR catalysts. MRDMFCs operate under the same methanol concentrations as traditional DMFCs, but instead of a small fraction of the feed methanol transported 204

across the membrane, the cathode is subjected to the total inlet methanol concentration.

This increased methanol exposure to the cathode requires that the cathode catalyst not be affected by methanol and be completely inactive towards it in the system. The CNx

catalysts appear to be capable of meeting this requirement.

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CHAPTER 12. Conclusions

Modifications to the previously reported CNx catalysts were made to study the

role of nanostructure and heteroatom incorporation. This was done by modifying CNx

catalysts grown on 2%Fe/MgO with ammonia, boron and nitric acid post treatments.

Thiophene was introduced as a sulfur source during acetonitrile pyrolysis to both

incorporate sulfur into the nanostructure and promote increased growth rates for CNx

nanofibers. Stacked platelet nanofibers with high edge plane exposure and nanofibers

with basal plane exposure were made and treated with ammonia to examine the impact of

graphitic edge plane exposure on nitrogen containing carbon catalysts.

Modifications to CNx with ammonia after acid oxidation showed that treatments

at 900°C could improve ORR activity, and 600°C ammonia treatments were not

sufficient. The acid oxidation treatment with HNO3 on CNx alone was able to improve

ORR activity and selectivity. It was further found that increases in oxygen functional groups on the surface of CNx catalysts could improve ORR selectivity.

It was found that graphitic edge site blocking methods on CNx using boron

promoted ORR activity rather than blocking an active site. Further treatments to CNx to

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incorporate boron created boron oxide films on the catalysts, which did not promote ORR

activity.

Sulfur, in the form of thiophene, was found to be a successful CNx growth

promoter when introduced at low levels into acetonitrile pyrolysis over 2%Fe/MgO

nanofiber growth catalysts. While sulfur was both incorporated into the graphitic

structure and accumulated on the surface of CNx, ORR activity was not affected by its

presence. Sulfur not incorporated into the CNx nanostructure could also be removed

through mild heat treatment in inert, which may improve the viability of using a growth

promoter when creating CNx materials for use in fuel cells.

Through examination of the role of graphitic edge plane exposure with stacked

platelets and nanofibers with basal plane exposure, it was found that graphitic edge

planes undoped were not sufficient to catalyze ORR. After nitrogen was incorporated

into the graphitic edge planes, even at low concentrations, significant ORR activity was

observed. It was also found that the presence of nitrogen alone on nanofibers does not

play a significant role in ORR. Rather, nitrogen has to be incorporated into the graphitic

matrix for ORR activity to be observed.

In addition to studies on the modification of CNx and other nanofiber materials,

electrochemical studies were performed. It was found that all CNx catalysts tested were inactive towards methanol oxidation and tolerant to the presence of methanol during

ORR testing. The overall ORR activity and selectivity of the CNx catalysts did not

change in the presence of methanol, even when the poor-performing catalysts were

207 tested. The best CNx ORR catalysts had higher ORR activity than the commercial Pt/VC catalysts in the presence of methanol above 0.5V vs. NHE. Catalyst loading was found to be a significant variable in the reporting of selectivity from RRDE testing. The aging of

RRDE catalyst inks was also found to have an impact on the selectivity reported by

RRDE.

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CHAPTER 13. Recommendations for Future Work

There are still many unanswered questions about CNx materials and their use as

ORR catalysts. While there are many directions that could be followed to investigate

CNx catalysts, recommendations here are suggestions based upon the work presented in the previous chapters. Suggestions for future work can fall into three categories: investigation of the active site, role of heteroatoms and oxygen functional groups, and evaluation of CNx precursors.

As stated previously, the active site debate is ongoing in the literature. We have contributed in the past to studies that support the argument for a non-metal active site140

(which may be acting alone or in tandem with a metal-centered active site). Further investigation into the active site and the role of iron in ORR activity could include poisoning attempts to the iron materials. In traditional heterogeneous catalysis, H2S is known to poison iron readily and rapidly, even at low temperatures and concentrations.

Treatments to CNx catalysts after preparation, but prior to half cell testing, with H2S could poison any iron in the catalyst. If iron is part of the active site, then it is expected that ORR activity would be poor after H2S treatment. While in an earlier chapter, it was shown that the use of thiophene as a CNx growth promoter does not inhibit ORR activity,

209

the sulfur groups formed from that treatment may be too bulky to be located in the pores

where iron-based active sites may be located.

The CNx active site can also be investigated using the experimental technique X-

Ray Absorption Spectroscopy (XAS). Recently, other catalysts in the research group have been analyzed using XAS techniques using the Advanced Photon Source (APS) at

Argonne National Laboratories. Investigation of the iron K-edge under electrochemical operating conditions can provide some insight into the coordination and bonding of iron.

Results from these studies will not be absolute in determination of the ORR active site because Fe-O, Fe-C and Fe-N bonds are typically undistinguishable from each other.

Investigation of the nitrogen edge under operating conditions may give a more clear understanding of the active site. Examination of this edge would allow for identification of the nitrogen species such as pyridinic, quarternary and pyrrolic while under operating conditions. Bonding to iron should also be able to be identified. Any changes to species with operation would help determine the active components of the ORR catalyst. This proposed experiment would be particularly difficult, because many beamlines available are not set to operate in the lowest energy levels where nitrogen is studied. Additionally, nitrogen XAS is typically performed under vacuum because of interference of atmospheric nitrogen.

The use of boron as a CNx dopant was discussed briefly in an earlier chapter. It was found that the treatment conditions significantly altered the effect boron had on the resulting CNx ORR activity. Use of heat treatments in inert did not prove to be

satisfactory in improving ORR activity, however, attempts at using site blocking methods 210 that involved a calcination step were successful in creating ORR active catalysts. It is recommended that the site blocking method with boron be studied further. Additional treatments using this methodology have the potential to improve ORR activity.

There is significant work that can be pursued in exploring the role of oxygen functional groups in CNx catalyst activity and selectivity. It was shown in research presented in this volume that addition of oxygen functional groups can improve ORR selectivity towards water formation. In some cases, ORR activity can also be improved.

At this time, it is unclear why these results have been observed and what type of oxygen functional groups play a role in the catalyst improvements. There is still significant uncertainty in the assignment of the oxygen functional groups present on the CNx materials. Further analysis to determine the oxygen functionalities definitively is needed.

Several methods including Raman, FTIR, XPS and TPD will most likely need to be used in tandem to determine the oxygen functionalities present. After this determination is made, then studying the techniques to control the oxygen functionalities on CNx surfaces can be investigated, hopefully leading to an understanding of the role that oxygen functionalities play in ORR catalysts.

As found through the study of stacked platelets and nanofibers with basal plane exposure, carbon nanostructure can play an important role in creating active ORR CNx catalysts. There are many directions possible for further study in this area. Three areas will be suggested here.

211

While stacked platelets were found to have the needed graphitic edge plane

exposure to create ORR active sites, the electrical conductivity of stacked platelets is

poor compared to nanotubes. This is because electrical conductivity is much higher

within graphitic planes than between. The orientation of the graphitic sheets in stacked platelets is perpendicular to the longitudinal axis, meaning there will be significant resistances to electrical conductivity down the length of the nanofiber. If the analog to stacked platelets, ribbons, could be used instead, a more active ORR catalyst could be tailored to place active edge sites on the ends. Literature reports are less clear on the preparation of ribbon nanofibers, which should be able to be prepared by slightly tuning the stacked platelet nanofiber preparation conditions.

In earlier publications from the research group, it was shown that catalysts with significant quantities of stacked cup nanostructures had the highest ORR activities140,141,144,145. In these catalysts 60-70% of the nanofibers present were stacked

cups. It would be ideal to be able to produce stacked cup nanofibers with 100%

concentrations. This would require that magnesia not be used as a support, as it is known

to form nanocubes as well. Preliminary results from the group using self-assembled iron

nanoparticles on dispersed silicon wafers as the nanofiber growth catalysts during

acetonitrile pyrolysis have been able to produce 100% stacked cup nanofibers, but in

quantities less than 1 mg per batch (opposed to 300-500 mg per batch for CNx grown

over 2%Fe/MgO-HCl washed). Investigating high surface area materials to support these

iron nanoparticles during acetonitrile pyrolysis would be interesting. The support should

be able to keep the nanoparticles from agglomerating without participating in the

212 nanofiber growth process to produce undesired nanostrucutres. Mesoporous silica should be investigated as one such support. A non-traditional support like polymethylmethacrylate (PMMA) nanospheres381 could also be used. In this case, the

PMMA support could be burned off before pyrolysis, leaving shells of iron nanoparticles behind.

In addition to the suggested work above, the use of the baseline catalyst CNx grown over 2%Fe/MgO-HCl washed should be evaluated more closely. While this catalyst may have substantial ORR activity and be easy to produce, some of its other characteristics may make it a less than ideal catalyst for use in an actual fuel cell. The bulk density of the CNx grown over 2%Fe/MgO-HCl is significantly higher than CNx grown over VC, Al2O3 and SiO2, as well as, the commercial Pt/VC catalysts. This higher bulk density has the potential to increase mass transfer resistances within a fuel cell significantly. It is also possible that membrane electrode assembly (MEA) preparation could be hindered by the use of thicker catalyst layers to obtain the same activity levels as other catalysts.

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CHAPTER 14. Glossary of Acronyms

BET. Brunauer, Emmett and Teller. A method for determination of material

surface area using a multi-layer adsorption theory.

CNx. Nitrogen-Containing Carbon Nanostructure. This is the general carbon-

nitrogen catalyst studied in this dissertation.

CV. Cyclic Voltammetry or Voltammogram. An electrochemical method where

potential is controlled by scanning at a known rate and range in both cathodic and anodic

directions while monitoring current.

DMFC. Direct Methanol Fuel Cell. A type of low-temperature fuel where fuel fed is 1-3M aqueous methanol and oxidant is oxygen.

HCRG. Heterogeneous Catalysis Research Group. The research group of

Professor Umit S. Ozkan in the Department of Chemical and Biomolecular Engineering at The Ohio State University.

HOR. Hydrogen Oxidation Reaction. In a PEM fuel cell, this reaction occurs at the anode. In an acidic aqueous system at 25°C, the reaction is:

214

- + H2  2e + 2H 0.00 V vs. NHE

MEA. Membrane Electrode Assembly. The interior components of a fuel cell that combines the gas diffusion layers, catalyst layers and membrane electrolyte.

MOR. Methanol Oxidation Reaction. In a direct methanol fuel cell, this reaction occurs at the anode. In an acidic aqueous system at 25°C, the reaction is:

+ - CH3OH + H2O  CO2 + 6H + 6e 0.02 V vs. NHE

ORR. Oxygen Reduction Reaction. The reaction occurring at the cathode in both direct methanol and PEM fuel cells. In an acidic aqueous system at 25°C, the reaction is:

+ - O2 + 4H + 4e  2H2O 1.23 V vs. NHE

NHE. Normal Hydrogen Electrode. Standard reference in which electrochemical potentials are reported. It is the potential at which hydrogen oxidizes in an aqueous acidic system at 25°C.

PEMFC. Proton Exchange Membrane Fuel Cell. Also known as Polymer

Electrolyte Membrane Fuel Cell. This is a low temperature hydrogen fuel cell.

RRDE. Rotating Ring Disk Electrode. This is an electrochemical half cell testing method in which one reaction is performed on the disk electrode and its products are monitored on the ring during laminar flow. In the study of ORR catalysts, the disk reduces oxygen and the ring monitors any hydrogen peroxide products.

215

SEM. Scanning Electron Microscope. An electron microscope where electrons are backscattered from sample for detection.

TEM. Transmission Electron Microscope. An electron microscope where

electrons are transmitted through sample for detection.

TGA. Thermogravimetric Analyzer. An instrument in which temperature can be

controlled while monitoring mass changes.

TPO. Temperature Programmed Oxidation. An experiment where temperature is

controlled while flowing an oxidizing gas over a sample. The products are analyzed for

characteristic temperatures and species.

XPS. X-Ray Photoelectron Spectroscopy. A surface sensitive method in which

an X-ray beam is directed at a sample which then photoemits electrons at characteristic

binding energies. These characteristic binding energies are based upon the chemical

species and chemical bonding of the species.

XRD. X-Ray Diffraction. A method in which monochromated X-rays bombard a

sample and the reflected X-rays are detected at the scattering angle. The scattering angle at which the rays are reflected are characteristic of the chemical phases for crystalline materials.

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