Molecular Engineering of Organic Photosensitizes for P-Type Dye

Molecular Engineering of Organic Photosensitizes for P-type Dye-Sensitized Solar Cells and the Immobilization of Molecular Catalyst for the Hydrogen Evolution Reaction

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Damian Richard Beauchamp

Graduate Program in Chemistry

The Ohio State University

2016

Master's Examination Committee:

Yiying Wu Ph.D., Advisor

James Cowan Ph.D.

Damian Richard Beauchamp

2016

Abstract

Solar energy has become an important component in the clean energy mix. There are several different kinds of solar cells that have been developed over decades. The focus of the first three chapters will be p-type dye-sensitized solar cells (DSSCs), which are omnipotent for obtaining high efficiency and cost effective tandem DSSCs. The efficiency of p-type DSSCs lags behind their n-type counterpart due to being less investigated. Herein, the attempts to increase performance of the p-type component via molecular engineering of organic photosensitizers is described. Through the addition of bulky hydrophobic alkyl chains performance can be enhanced, though it was found that the location of these alkyl chains is a critical factor. Additionally, by adopting a double- acceptor single-donor design, as described in chapter 3, when employing the commonly used triphenylamine donor moiety, one can simultaneously increase the molar extinction coefficient while reducing the synthetic steps yielding one the fields top performing photosensitzers.

In addition to the conversion of solar energy to electrical energy, the storage of intermittent renewable energy is important. Energy can be stored mechanically (e.g. pumped hydro, fly wheels, compressed air, etc.), electrochemically (e.g. batteries and capacitors), or in chemical bonds (e.g. hydrolysis, carbon dioxide reduction, etc.). Of these methods hydrolysis to produce hydrogen has been identified as an attractive potential method. This is because hydrogen has high specific energy, can be transported, ii and used as a fuel in fuel cells emitting only water. The problem is industry currently employs steam-methane reforming to produce hydrogen, because catalysts currently employed for hydrolysis are expensive (i.e. noble metals) and/or unstable. Therefore finding a more abundant, lower cost, and stable catalyst which can be easily processed has been of importance. Molybdenum disulfide based catalysts have been identified as a good candidate because of their low Gibbs free energy of proton absorption. The molecular variants have the highest density of catalytically active sites, but suffer from desorption from electrode surfaces. Herein a molecular molybdenum disulfide catalyst is immobilized via polymer coordination yielding a catalytic material which can be easily processed into films via a resin. This produced stable catalytic films on electrode surfaces, which show good activity toward hydrogen evolution via water reduction.

iii

Dedication

This document is dedicated to my wife, Cynda Beauchamp, who has supported and dealt

with me through my entire academic career. Kallum Beauchamp, my son, who fills my

heart with joy and love and who reminds me everyday what really matters. My parents,

Richard and Jacqueline Beauchamp, who have supported me and taught me among many

other things to be self-motivated, to have levity, and the importance of education.

Additionally, I dedicate this thesis to Matthew Heaver for whom I continue to try to make

a positive and lasting impact on the world. Last but not least, my great friend, Nathan

Stopczynski whom I have known since early childhood and have had a great deal of

wonderful adventures with, I will always cherish our enduring friendship.

Acknowledgments

I would like to thank Kevin A. Click for his support, assistance, and hard work, through our undergraduate and graduate careers, especially with the BH. Additionally, I would like to thank Xiaodi Ren and Kate Fisher for all of their hard work developing our start-up company - Kair Battery, it was a wonderful journey. I would like to acknowledge all others in my lab for their assistance and wonderful intellectual contributions. John

Bair, Director of the Center of Design and Manufacturing Excellence at The Ohio State

University, who helped me develop my natural abilities and taught me a multitude about product/system design, market positioning, cost analysis, pitching, and many other invaluable business related concepts. S. Michael Camp Ph.D., Founder and Executive

Director of The Technology Entrepreneurship and Commercialization (TEC) Institute at

The Ohio State University's Fisher School of Business, was a wonderful mentor and motivator for me as I was bitten by the 'entrepreneurial bug'. Finally, I would like to thank my advisor, Yiying Wu Ph.D., for allowing the group to follow their imaginations, be creative in their own way, pushing us to do our best work, and funding support.

Vita

May 2004 ...... Streetsboro High School

May 2012 ...... B.S. Chemistry, Kent State University

2012 to present ...... Graduate Teaching Associate, Department

of Chemistry, The Ohio State University

Publications

1. Membrane Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel

Production. Click, K., Beauchamp, D. R., Huang, Z., Chen, W., and Wu, Y., Journal of the American Chemical Society, 2016, Accepted.

2. An Aqueous Lithium-Iodine Solar Flow Battery for the Simultaneous Conversion and

Storage of Solar Energy. Yu, M., McCulloch, W. D., Beauchamp, D. R., Huang, Z.,

Ren, X., and Wu, Y., Journal of the American Chemical Society, 2015, 137, 8332 - 8335

DOI:10.1021/jacs.5b03626

3. Dye-Sensitized Indium Tin Oxide for High-Current Photocathodes. Huang, Z., He, M.,

Yu, M., Click, K., Beauchamp, D., and Wu, Y., Angewandte Chemie International

Edition, 2015, 54, 1 - 6. DOI: 10.1002/anie.201500274R1

4. Double-Acceptor as a Superior Organic Dye Design for p-Type DSSCs: High

Photocurrents and Observed Light Soaking Effect. Click, K.1, Beauchamp, D.1, Garret,

B., Haung, Z., Hadad C. M., Wu, Y., PCCP, 2014, 16, 26103 - 26111.

DOI:10.1039/c4cp04010d.

Fields of Study

Major Field: Chemistry

vii

Table of Contents

Abstract ...... ii

Dedication ...... iv

Acknowledgments...... v

Vita ...... vi

Publications ...... vi

Fields of Study ...... vii

Table of Contents ...... viii

List of Tables ...... xii

List of Figures ...... xiii

Chapter 1: Introduction into Dye-Sensitized Solar Cells ...... 1

1.1 Discovery and Current State ...... 1

1.2 Working Principle ...... 2

1.3 P-type DSSCs and Their Importance Towards Tandem DSSCs ...... 6

1.4 Aqueous DSSCs and New Horizons for Dye-Sensitized Photo-Electrodes ...... 10

Chapter 2 Effect of the Addition of Low-Lying Alkoxy Arm on Solar Cell Performance

...... 11

viii

2.1 Introduction ...... 11

2.2 Experimental ...... 13

2.2a Synthesis of novel 4-(bis(4-(5-(2,2-dicyanovinyl)thiophen-2-

yl)phenyl)amino)-2-(pentyloxy)benzoic acid (DRB1) and control dye: 4-(bis(4-(5-

(2,2dicyanovinyl)thiophen- 2-yl)phenyl)amino)benzoic acid (P1) ...... 13

2.2b Ultraviolet visible (UV-vis) absorption and emission spectra of DRB1 and P1

...... 23

2.2c Solar Cell Fabrication ...... 25

2.2d Solar Cell Testing ...... 28

2.2f Dye Loading Investigation ...... 28

2.3 Results and Discussion ...... 28

2.3a Synthetic challenges on route to DRB1 ...... 28

2.3c Solar Cell Performance and Comparison ...... 31

2.3d Effect of Dye Loading on Solar Cell Performance ...... 33

2.4 Conclusion ...... 35

Chapter 3: Double-Acceptor as a Superior Organic Dye Design for P-type DSSCs: High

Photocurrents and Observed Light Soaking Effect ...... 36

3.1 Introduction ...... 36

3.2 Experimental ...... 39

3.2a Collaborative Synthesis of Novel Photosenstizers (BH2, BH4, and BH6) ..... 39 ix

3.2b UV-vis absorption and emission photo-spectroscopy of BH2, BH4, and BH6 42

3.2c Electrochemistry of BH2, BH4, and BH6 ...... 43

3.2d Solar cell fabrication ...... 43

3.2e Solar Cell Testing ...... 44

3.2f Dye Loading Investigation ...... 44

3.2g Electrolyte Preparation ...... 45

3.2h Light Soaking Methods ...... 45

3.2i DFT calculations of BH2, BH4, and BH6 (Carried out by Ben Garrett) ...... 45

3.3 Results and Discussion ...... 46

3.3a Electronic properties of BH2, BH4, and BH6 ...... 46

3.3b Solar Cell Performance and Comparison ...... 52

3.3c DFT calculations of BH2, BH4, and BH6 (Carried out by Ben Garrett) ...... 54

3.3d Effect of Dye Loading on Solar Cell Performance ...... 55

3.3e Electrolyte Effect on Cell Performance ...... 56

3.3f Light Soaking Investigation via Impedance Spectroscopy ...... 58

3.4 Conclusion ...... 60

Chapter 4 A Scalable, Processable, and Stable Polymer Immobilized Molecular

Hydrogen Evolution Catalyst Inspired by Molybdenum Disulfide ...... 61

4.1 Introduction ...... 61

4.2 Experimental ...... 64

4.2a Synthesis and Resin Preparation ...... 64

4.2b Characterization ...... 66

4.2c Electrochemical Set-up and Characterization ...... 67

4.2d Hydrogen Detection and Performance Evaluation ...... 67

4.3 Results and Discussion ...... 69

4.3a Effect of Catalyst Loading and pH on Overpotential ...... 69

4.3b Control, Scan Rate Effect, and Loading Comparisons ...... 73

4.3c Simulated Solar Water Splitting Electrochemical Stress Test: Accelerated

Stability Test Evaluated via X-ray Photoelectron Spectroscopy (XPS) ...... 76

4.3d Kinetic Evaluation via Tafel Slope Analysis ...... 80

4.3e Simulated Long-Term Electrolyzer Stress Test - Hydrogen Detection, Faradaic

Efficiency, and Other Performance Metrics ...... 81

4.3f Advantages, Disadvantages, and Potential Strategies for Improvement ...... 84

4.4 Conclusion ...... 85

Bibliography ...... 87

List of Tables

Table 1: Different conditions attempted for deprotection of the methyl protecting group.

...... 31

Table 2: Summary of average performance parameters for DRB1 and P1...... 32

Table 3: Summary of all oxidation and reduction peaks of electrochemical measurements of dyes. [V] vs NHE. (a) Denotes irreversible oxidation/reduction peaks. (b) ΔECV calculated by Eox1 - ERed1...... 49

Table 4: Summary of optical and electrochemical properties of the dyes. [V] vs NHE. (a)

* - E0-0 taken to be intersection of absorption and normalized emission spectra. (b) (S /S ) =

o E0-0 + E red . (c) Determined from the start of absorption of the longer wavelength should absorption...... 52

Table 5: Summary of cell results, from Figure 27, using DMHII electrolyte...... 53

Table 6: Relation between dye loading and molar extinction on photocurrent...... 56

Table 7: Performance results of BH4 in the JV curve shown in Figure 29...... 58

Table 8: Summarizes the short circuit current change of the 3 cells from Figure 30...... 60

Table 9: The raw data used to construct the calibration curve of Figure 32...... 68

Table 10: Detailed hydrogen detection data used to construct Figure 44...... 83

xii

List of Figures

Figure 1: A photograph of a dye sensitized solar cell with various components labeled. .. 2

Figure 2: Generic energy diagram showing electron propagation for a n-type DSSC...... 4

Figure 3: Generic energy diagram showing hole propagation for a p-type DSSC...... 5

Figure 4: Generic energy diagram showing hole propagation for an n-type semi- conductor with a p-type dye...... 6

Figure 5: Generic JV curve depicting the short circuit current (Jsc), open circuit voltage

(Voc), and fill factor (FF)...... 7

Figure 6: Commercially available n-type dyes MK2, D149, and Z907...... 12

Figure 7: Molecular structure of DRB1 and P1...... 13

Figure 8: Williamson ether synthsis of 1 yielding 3...... 14

Figure 9: Esterification of 3 yielding 4...... 15

Figure 10: Buchwald-Hartwig coupling of 4 & 5 yielding 6...... 16

Figure 11: Iodination of 6 yielding 7...... 18

Figure 12: Methoxy deprotection of 7 yielding 8...... 19

Figure 13: Suzuki coupling of 8 & 9 yielding 10...... 21

Figure 14: Knoevenagel condensation of 10 & 11 yielding 12...... 22

Figure 15: Absorbance (red line) and emission (black line)of DRB1 (a) and P1 (b)...... 25

Figure 16: The transmittance of bare NiO films before (green and black lines) and after sensitization with DRB1 (red line) and P1 (blue line)...... 27 xiii

Figure 17: The overall synthetic path to DRB1...... 29

Figure 18: Different iodination conditions yielding a triiodinated (top) and a diiodinated

(bottom) product...... 30

Figure 19: (a) The J-V curves of three DRB1 and P1 sensitized solar cells illuminated with 100 mW/cm2 light. (b) The J-V curves of the top performing DRB1 and P1 cells from (a). (c) The J-V curves of three DRB1 and P1 sensitized solar cells in the dark. .... 33

Figure 20: A plot of dye loading dependance on dye soaking solution concentration for

DRB1 (red stars with dash-dot-dot line) and P1 (black squares with dash-dot line)...... 34

Figure 21: Molecular structures of previously reported P1 and O2 dye...... 37

Figure 22: Molecular structure of the BH dye series...... 39

Figure 23: Total synthesis of the BH dye series...... 39

Figure 24: UV-Vis of BH series dyes in methylene chloride...... 47

Figure 25: Cyclic voltammagram of 0.1mM BH series dyes in methylene chloride with a

0.1M tetra-butyl ammonium hexafluorophosphate supporting electrolyte. The scan rate was 100mV/s...... 48

Figure 26: The absorbance (solid line) and emission (dashed line) spectra for BH2, 4, and

6...... 50

Figure 27: JV curve of dyes with cells constructed with 1.0M DMHII / 0.1M iodine electrolyte in acetonitrile...... 53

Figure 28: DFT calculations with B3lyp level of theory with 6-31G basis sets...... 54

Figure 29: JV curve of BH4 with 1.0M LiI, TBAI, and DMHII and 0.1M iodine in acetonitrile...... 57

xiv

Figure 30: (a) Transport Resistance measured at Voc as a function of light soaking and time. (b) Jsc as a function of light soaking and time. (c) Voc as a function of light soaking process and time. (d) Capacitance measured at Voc for BH4 cells described in Figure 29 and Table 7 (D = DMHII; L = LiI; T = TBAI)...... 59

Figure 31: Synthetic route to MoO(S2)2PV-4-P, with the material under each of its corresponding molecular structures...... 65

Figure 32: Calibration curve showing hydrogen as a function of mean peak area of the

GC-TCD used for hydrogen detection...... 68

Figure 33: (a) FT-IR comparison of PV-4-P, MoO(O2)2PV-4-P, and MoO(S2)2PV-4-P.

(b) UV-vis of MoO(S2)2PV-4-P in DMSO at the indicated concentrations...... 70

Figure 34: (a) The TGA of MoO(S2)2PV-4-P. (b) XRD of the MoO(S2)2PV-4-P after

(TGA) after heating to 7000C ...... 71

Figure 35: NMR (400 MHz) of PV-4-P (as purchased from Sigma Aldrich) in d6-DMSO.

...... 72

6 Figure 36: NMR (400 MHz) of MoO(S2)2PV-4-P in d -DMSO...... 72

Figure 37: Linear sweep voltammetry (LSV) of the bare GC, PV-4-P on GC, and

MoOS4PV-4-P (132 μg) on GCE all in 0.5M H2SO4...... 73

Figure 38: LSV of MoO(S2)2PV-4-P in 0.5 M H2SO4 comparing scan rates of 10 and 50 mV/s at the (a) 501st and (b) 1001st scans...... 74

Figure 39: (a) LSV of MoO(S2)2PV-4-P with varying pH (scan rate = 50 mV/s). (b)

Linear sweep voltammetry (LSV) of the MoOS4PV-4-P at the different indicating loading on GCE all in 0.5M H2SO4 (scan rate = 50 mV/sec)...... 75

Figure 40: The LSV of MoO(S2)2PV-4-P (132 μg) on GCE in 0.5M H2SO4 over 1,001 cycles...... 77

Figure 41: EDX of molybdenum and sulfur of a unused film and a film after 1,001 accelerated cycle test (same parameters as described in the main text) in 0.5 M H2SO4. 78

Figure 42: (a) XPS of the non-cycled MoO(S2)2PV-4-P in the Mo 3d and S 2s BE regime.

(b) XPS of the uncycled MoO(S2)2PV-4-P in the S 2p BE regime. (c) XPS of cycled

MoO(S2)2PV-4-P in the Mo 3d and S 2s BE regime. (d) XPS of non-cycled MoO(S2)2PV-

4-P in the Mo S 2p BE regime...... 79

Figure 43: The Tafel slope of 132 μg of MoO(S2)2PV-4-P (167 nmols of active catalyst) loaded onto GCE in 0.5M H2SO4 at a 10mV/s scan rate...... 81

Figure 44: The 2 hour CA of MoOS4PV-4-P (132 μg) in 0.5 M H2SO4 and the H2 detected over the 2 hours. A photograph showing the hydrogen bubbles formed during the CA (inset)...... 82

xvi

Chapter 1: Introduction into Dye-Sensitized Solar Cells

1.1 Discovery and Current State

Dye-sensitized solar cells (DSSCs) convert light energy into electrical energy.

Since their inception in 1991 by Grätzel et. al. DSSCs have shown promise in the solar energy sector.1 The most glaring advantage is their ability to surpass the so called

Shockley-Queisser2 limit through implementing a tandem design. Tandem devices became a viable option as a result of Lindquist et. al. developing the first p-type DSSC in

1999.3 A tandem device is a device in which both the anode (n-type DSSC) and cathode

(p-type DSSC) are photo-active. The advantageous traits are not limited to the potential efficiency, but include ease of large-scale manufacture, low impurity sensitivity, and being aesthetically pleasing.

The initial invention by Michael Grätzel was via an n-type DSSC, which use an n- type semi-conductor, typically titanium dioxide (TiO2), as the electrode to which the dye sensitizer is anchored. The counter electrode is typically a piece of fluorine doped tin- oxide (FTO) glass with nano-sized platinum deposited. Between the two electrodes is an electrolyte, which facilitates electron (e-) conduction between the two electrodes.

- Typically lithium iodide (LiI)/triiodide (I3 ) is the electrolyte employed, though many others have been used. A picture of a cell with the various parts can be seen in Figure 1.

Figure 1: A photograph of a dye sensitized solar cell with various components labeled.

1.2 Working Principle

A semi-conductor is a material that has a gap between the valance band (VB) and conduction band (CB) (commonly referred to as a band gap). Wide band gap semiconductors are ideal for use in DSSCs, because such a material reduces parasitic light absorption (i.e. the light absorbed by the semi-conductor). A n-type semi-conductor has a fermi-level (Ef), the theoretical energy level at which the probability of electron occupation is 50%, near the CB. As a result the majority carriers of an n-type semi- conductor are electrons. The sensitizer which is anchored to the semi-conductor in a n- type DSSC must have a lowest unoccupied molecular orbital (LUMO) which has a

2 potential more negative than the CB of the n-type semi-conductor. This thermodynamically favorable energy alignment allows for the excited state of the photosensitizer to reduce the n-type semi-conductor. The resulting oxidized dye is reduced by

- - - the I component of the electrolyte. The redox potential of I /I3 is ~0.35 V vs. normal hydrogen electrode (NHE). This redox potential dictates that the highest occupied molecular orbital (HOMO) be more positive for the reduction of the oxidized dye to be reduced back to its ground state. The electron injected into the n-type semi-conductor travels through the external circuit to the FTO/Pt counter electrode and reduces the

- - oxidized from of the electrolyte (I3 ) to I . This process completes the circuit, thus converting the absorbed photons into electrical current. A diagram summarizing this process can be found in Figure 2.

Figure 2: Generic energy diagram showing electron propagation for a n-type DSSC.

A p-type semi-conductor has a fermi-level (Ef) near the VB. As a result the majority carrier of a p-type semi-conductor are holes (h+). The sensitizer which is anchored to the semi-conductor in a p-type DSSC must have a HOMO which has a potential more positive than the VB of the p-type semi-conductor. This thermodynamically favorable energy alignment allows for the excited state of the photosensitizer to oxidize the p-type semi-conductor. The resulting reduced dye is oxidized by

- - - the I3 component of the electrolyte. The redox potential of I /I3 is ~0.35 V vs. NHE. This redox potential dictates that the LUMO have a more negative potential than ~-0.35 V vs.

NHE for the reduced dye to be oxidized back to its ground state. The hole which was

4 initially injected into the p-type semi-conductor travels through the external circuit to the

- - FTO/Pt counter electrode and oxidizes the reduced from of the electrolyte (I ) to I3 . This process completes the circuit, thus converting the absorbed photons into electrical current. A diagram summarizing this process can be found in Figure 3.

Figure 3: Generic energy diagram showing hole propagation for a p-type DSSC.

Recently, our group showed that the traditional rule of using "p-type" dyes with their respective p-type semi-conductors can be disobeyed. If a dye has proper energy alignment with a semi-conductor it does not seem to matter whether the semi-conductor is a p- or n-type. The so called dye-controlled interfacial electron transfer was the first 5 report of its kind in the field.4 This means that a traditionally "p-type" photosensitizer can be used with a n-type semi-conductor to produce cathodic photocurrent as long as the energy alignment of the HOMO and CB is thermodynamically favorable (Figure 4). This was a significant discovery as it allows for many more non-traditional semi-conductors to be used in p-type DSSCs.

Figure 4: Generic energy diagram showing electron propagation for an n-type semi- conductor with a p-type dye.

1.3 P-type DSSCs and Their Importance Towards Tandem DSSCs

The current produced from a tandem DSSC is limited by the lowest current producing photo-electrode, which is currently the p-type component. When evaluating solar cell

6 performance the three metrics used are the open circuit voltage (Voc), short circuit current

(Jsc), maximum power (Pmax), and incident power (Pinc). From these metrics the fill factor

(FF) and efficiency (η) can be calculated using equations 3 and 4 (Figure 5).

[Eqn. 3]: Pmax = Voc · Jsc · FF

-1 [Eqn. 4]: η = Voc · Jsc · FF · Pinc · 100

Figure 5: Generic JV curve depicting the short circuit current (Jsc), open circuit voltage (Voc), and fill factor (FF).

To date the n-type DSSCs have been far more investigated compared to the p-type variant. This has resulted in efficiencies (η) as high as 13%.5 The η of p-type DSSC on the other hand is far less reaching only 0.46%.6 Several factors have been identified for limiting the overall performance of p-type DSSCs and will be discussed below.

The Voc, Jsc, and FF are the three factors that contribute to the overall efficiency determination of a DSSC. In an individual p- and n-type DSSCs, the maximum Voc obtainable is the difference between the fermi-level of the conducting oxide and the redox potential of the electrolyte. The Voc is thereby tunable by the using different electrolytes and/or different semiconductors. Indeed, an example of the former can be

7 seen in literature through use of a Co(II)/(III) based electrolyte to increase the Voc.

Which has the disadvantage of ambient air instability. The latter could be addressed through employment of p-type semiconductors other than NiO such as copper gallium oxide (CGO).8,9 Currently, the most widely used semi-conductor used to date has been

NiO. NiO has a VB more positive compared to the CB of TiO2, and can be made fairly transparent. Though NiO is not an ideal semi-conductor for this application because nickel has a d8 valancy, thus it absorbs light. Also, it is a Mott insulator resulting in a low hole mobility. Our group has attempted to address the aforementioned by developing new p-type semi-conductors in particular through copper gallium oxide (CGO)8,9 and copper scandium oxide (CSO),10 but those materials were unsuccessful for use in DSSCs and are not the focus of this thesis.

The p-type DSSC is a critical component if the full potential of this form of solar energy conversion is to be realized through a tandem solar cell device.11 Therefore, it is

8 desirable to increase the Jsc, FF, and η. The photo-current is limited by the transport resistance (charge collection efficiency) which is stunted by various recombination pathways,12 which could theoretically be circumvented via strategic dye design and/or development of higher carrier mobility semiconductors compared to the commonly used

NiO (e.g. CGO, CSO, etc.). The two major recombination pathways have been identified to be the electron of the reduced dye or electrolyte recombining with the holes in the surface of NiO.13 Herein the intelligent dye design approach was employed to address the photo-current limitations.

The low hole mobility, which results in an increased probability of hole/electron recombination at the electrolyte/semi-conductor interface, and the transparency issues of

NiO can be addressed via molecular engineering of the photo-sensitizer and are the focus of chapters 2 and 3 of this thesis. Low hole mobility translates into an increased probability that the injected hole will recombine with the electron in the dye or the electrolyte. One way to solve the problem of hole recombination with the electron in the dye is to physically separate the LUMO from the NiO surface and/or extend the excited state life time of the photo-sensitizer.14 Another strategy for reducing recombination between the hole in NiO and the reduced electrolyte is to add some form of blocking layer (i.e. hydrophobic alkyl/alkoxy chains) to the dye to physically inhibit the contact of the NiO surface and the electrolyte. These methods has proven to be useful yielding the top performing p-type DSSCs to date.11,15,16 To address the transparency issue of NiO one can design dyes with greater molar extinction coefficients for improved light harvesting.15

1.4 Aqueous DSSCs and New Horizons for Dye-Sensitized Photo-Electrodes

Since 1991 the majority of n-type and p-type DSSCs employ an organic solvent based electrolyte, even though in 1976 an aqueous electrolyte was used for a zirconium oxide dye-sensitized photocell.17 The reason for the change to organic based electrolytes was a result of the performance improvement.1 To date there have been relatively few, reports of aqueous based n-type DSSCs, when compared to organic and none for p-type

DSSCs. Some of the most notable reports incorporate surfactant additives to improve surface wetting and stability. For example O'Reagan et. al. employed "Cheno" cheno- deoxycholic acid as a co-adsorbate and Kim et. al. used Triton-X.18,19 The former example obtained an efficiency of 4.09% using dye D149 with Cheno.18 Currently the highest efficiency (5%) todate for aqueous DCCSs was achieved by Spiccia et. al. employing the MK2 dye with a small fraction of polyethylene glycol in the electrolyte.20

Using aqueous based electrolytes presents glaring advantages over the organics, if the performance where comparable. These advantages include greater heat capacity, reduced leaking due to polymer sealant dissolution, lower environmental hazards at the end of their life cycle, and reduced production costs. Beyond these advantages dye-sensitized photo-electrodes have found utility beyond dye-sensitized solar cells to so called dye- sensitized photoelectrochemical cells for solar water splitting and solar flow batteries.

Chapter 2 Effect of the Addition of Low-Lying Alkoxy Arm on Solar Cell Performance

2.1 Introduction

Preventing hole recombination through the addition of alkyl chains has been shown to be successful method for improving overall conversion efficiency. The first report in 2006 added hexylchains to the oligiothiophene pi-linker of the a photosensitizer for n-type DSSCs.21 One of the dyes in that study (MK-2) is still one the best performing organic n-type dyes today and is commercially available. Other studies then followed adding alkyl or alkoxy chains to the pi-linker,22,23 alkoxy chains to the acceptor unit,24 alkoxy chains to the donor unit,25–28 and further adding alkyl chains to both the donor moiety and down the pi-linker.29 In the years since these pioneering examples new dyes functionalized both organic (e.g. D149) and inorganic (e.g. Z907) for solid-state, aqueous electrolyte, and organic electrolyte with alkoxy and/or alkyl groups continue to be developed and/or employed (Figure 6).30–38 Through these studies it was found that when these hydrocarbon chains are added to the donor moiety promising results are obtained.25–

28 This makes sense as these units are considered electron donors, especially when bonded directly on an aromatic moiety.

Similar, but fewer approaches employing alkyl/alkoxy chains have been conducted for p-type DSSCs. The most notable dyes being those that have produced the highest performing p-type DSSCs, which all contain alkyl chains along the pi-linker between the donor and acceptor moieties.11,15,16 To date there has been no example of 11 functionalizing the donor moiety of a p-type dye with alkyl or alkoxy groups, even though the benefits of doing so have been highlighted through the n-type variant. It is the purpose of this study to explore such a strategy.

Figure 6: Commercially available n-type dyes MK2, D149, and Z907.

P1 at the time of this study was one of the best performing photo-sensitizers in the p-type DSSC field, yet contained no alkyl/alkoxy chains. Coupled with the fact it only requires four synthetic steps to obtain makes it an especially attractive dye to this day for p-type DSSCs. In an attempt to increase cell performance through reduced recombination between the hole in NiO and the electrolyte two dyes were synthesized. The control dye

(4-(bis(4-(5-(2,2-dicyanovinyl)thiophen-2-yl)phenyl)amino)benzoic acid (common name

P1)) previously synthesized by Sun et. al.39 and a second novel dye (4-(bis(4-(5-(2,2-

12 dicyanovinyl)thiophen-2-yl)phenyl)amino)-2-(pentyloxy)benzoic acid (common name

DRB1). The difference between the two dyes is the pentoxy chain meta to the carboxylic acid anchoring moiety and can be seen in Figure 7. This functional group was introduced in an attempt to shield the NiO surface from coming in contact with the electrolyte thus reducing hole/electron recombination.

Figure 7: Molecular structure of DRB1 and P1.

2.2 Experimental

2.2a Synthesis of novel 4-(bis(4-(5-(2,2-dicyanovinyl)thiophen-2-yl)phenyl)amino)-2-

(pentyloxy)benzoic acid (DRB1) and control dye: 4-(bis(4-(5-(2,2dicyanovinyl)thiophen-

2-yl)phenyl)amino)benzoic acid (P1)

Figure 8: Williamson ether synthsis of 1 yielding 3.

4-bromo-2-(pentyloxy)benzoic acid - Figure 8 - (3): To a clean dry two neck

500mL round bottom flask equipped with a stir bar was added DMSO (150 mL) followed by potassium hydroxide (KOH) flakes (25.29 g, 451.1 mmol) whilst stirring. The mixture was allowed to stir for 10 minutes prior to the application of an ice bath for 10 additional minutes. (Note: The DMSO initially colorless turned a yellowish/amber color while stirring with KOH). The ice bath was removed and 4-bromo-2-hydroxybenzoic acid (1)

(10.00 g, 460.8 mmol) was added followed directly by the addition of 1-bromopentane

(2) (12 mL, 98 mmol) via syringe at once. After 3.75 hours DMSO (50 mL) was added to the stirring solution which was a pale yellow color. The solution was allowed to stir for a total of 20.5 hours after which an ice bath was applied and water (100 mL) was added.

Then the solution was nuetralized (pH 7) with HCl (1M). Once pH 7 was reached a white solid precipitated out of solution. The precipitate was filtered, and the filtrate had more

HCl added resulting in more precipitate which was filtered again, this was repeated two more times (after which no more precipitate formed). The combined filter cakes were then washed with hexanes (~300 mL) and dried in a vacuum oven (70oC, 1.5 hours).

1 (Yeild 8.50 g, 29.6 mmol; 64%) H NMR (400 MHz, CDCL3): δ = 7. 28 (d, 1H, J = 8.08

Hz), 7.13 (d, 1H, J = 1.88 Hz), 7.04 (dd, 1H, J = 1.80, 8.00 Hz), 3.96 (t, 2H, J = 6.52 Hz),

1.67 (p, 2H, J = 6.92 Hz), 1.36 (m, 4H), 0.88 (t, 3H, J = 7.14 Hz).

Figure 9: Esterification of 3 yielding 4.

Methyl 4-bromo-2-(pentyloxy)benzoate - Figure 9 - (4): To a 500 mL 2-neck round bottom flask equipped with a condenser and stir bar was added methanol (MeOH)

(270 mL), 3 (8.50 g, 29.6 mmol), and concentrated sulfuric acid (4 mL). The solution was heated (75oC) and allowed to stir for 22 hours. At the 4.5 hour mark a small aliquot was removed (via syringe) and extracted with dimethylchloride (DCM), thin layer chromatography (TLC) showed no sign of starting material, the organic extract was then concentrated leaving a viscous cloudy oil, an NMR of the oil showed product had been formed. After the 22 hours the reaction mixture was poured into a separatory funnel that contained water (100 mL) and DCM (200 mL) and the organic phase was removed, the aqueous layer was extracted with DCM (3x100 mL), the combined organic phases were

15 washed with water (400 mL) followed by brine (250 mL), dried with MgSO4, filtered, and concentrated (rotorary evaporator). The resulting golden oil was found to be pure via

NMR, aside from a small amount of DCM. The oil was then dried in a vacuum oven over

o 1 night (60 C). (Yeild 8.03 g, 26.7 mmol; 90%). H NMR (400 MHz, CDCL3): δ = 7. 67

(d, 1H, J = 8.68 Hz), 7.11 (m, 2H), 4.02 (t, 2H, J = 6.50 Hz), 3.89 (s, 3H), 1.85 (p, 2H, J

= 6.98 Hz), 1.44 (m, 4H), 0.95 (t, 3H, J = 7.16 Hz).

Figure 10: Buchwald-Hartwig coupling of 4 & 5 yielding 6.

Methyl 4-(diphenylamino)-2-(pentyloxy)benzoate - Figure 10 - (6): To a dry 250 mL 3-neck round bottom flask equipped with a stir bar was added 4 (7.64 g, 25.4 mmol) and diphenylamine (5) (4.64 g, 27.4 mmol). The 3-neck was fitted with a bubbler, septa, and gas line through which argon was flowed for 20 minutes prior to the addition of fresh dry toluene (120 mL; dried over sodium metal with benzophenone as the indicator). The bubbler was removed and replaced with a septa under a rapid flow of argon and the system was degassed several times and the bubbler was replaced. Potassium tert-butoxide

(5.92 g), tris(dibenzylideneacetone)dipalladium(0) (Pd2dba3) (5 mol%, 1.16 g, 1.27 16 mmol), and tetra-n-butylammonium fluoride (0.74 g, 2.54 mmol) were added via removing the septa with a rapid flow of argon. The septa was replaced and the walls of the round bottom were rinsed with toluene (10 mL) the solution was degassed several

(~5) times, after which a very rapid flow of argon was allowed to flow through the system for 10 minutes. The argon flow was slowed to produce one bubble per second and the solution was allowed to stir at room temperature (r.t.) undisturbed for 2 hours after which an aliquot was removed (via syringe) and analyzed via TLC. The TLC showed a very light spot for the diphenylamine (which was in access to begin) the reaction was allowed to stir for an additional 45 minutes to ensure a complete reaction. After a total time of 2.75 hours the reaction mixture was poured into water (100 mL) and acidified to pH = 7 with 1 M HCl. The solution was then extracted with DCM (3 x 100 mL), dried with MgSO4, filtered, and concentrated (rotorary evaporator). The crude product was then purified via column chromatography (silica gel) with an initial eluent system of 2:1 hexanes:DCM, but once the diphenylamine started to elute only DCM was used as the eluent. The fractions containing pure product were combined and concentrated (rotorary evaporator) to a pure orange oil. (Yield 4.5 g, 11 mmol 46%). 1H NMR (400 MHz,

CDCL3): δ = 7.72 (d, 1H, J = 8.52 Hz), 7.32 (m, 4H), 7.14 (m, 6H), 6.55 (m, 2H), 3.87 (s,

3H), 3.82 (t, 2H, J = 6.54 Hz), 1.76 (p, 2H, J = 6.96 Hz), 1.39 (m, 4H), 0.92 (t, 3H, J =

7.18 Hz).

Figure 11: Iodination of 6 yielding 7.

Methyl 4-(bis(4-iodophenyl)amino)-2-(pentyloxy)benzoate - Figure 11 - (7): To a dry 3-neck 500 mL round bottom flask equipped with a stir bar was added 6 (4.0 g, 10 mmol), dioxane (200 mL), and zinc acetate (3.77g, 20.5mmol). An ice bath was applied and the solution was allowed to stir for 15 minutes before ICl (1.34mL, 25.7 mmol) in dioxane (60mL) was added dropwise via a dropping funnel over ~1 hour. The solution was allowed to stir and come to r.t. while monitored every hour via TLC. After 3 hours after TLC showed the reaction was complete at which time a small aliquot was removed worked up and analyzed via NMR, which confirmed the reaction to be complete. To the reaction solution was added sodium thiosulfate (1 M, 26 mL) whilst stirring this mixture was then poured into water (150 mL) followed by the addition of DCM (200 mL). The organic phase was separated and the aqueous was extracted with DCM (3 x 100 mL). The combined organic fractions were washed with aqueous sodium thiosulfate (300 mL), dried (MgSO4), concentrated (rotorary evaporator), and filtered through a silica plug

18 eluting with DCM. The eluted eluent was then concentrated to yield a crude dark brown- orange very viscous sap (6.5g). In a 250 mL single neck round bottom the brown-orange sap was re-dissolved in a minimal amount of DCM on which was carefully layered hexanes. Almost instantly small yellow needle crystals began to grow at the edges of the round bottom at the DCM:hexane interface. The flask was capped and left undisturbed for 24 hours, after which solvent was decanted and the resulting yellow needle crystals were washed several times with hexanes resulting in pure product. (Yield 6.23 g, 9.7

1 mmol; 94%). H NMR (400 MHz, CDCL3): δ = 7.71 (d, 1H, J = 8.04 Hz), 7.59 (d, 4H, J

= 8.60 Hz), 6.87 (d, 4H, J = 8.64 Hz), 6.55 (m, 2H), 3.87 (s, 3H), 3.83 (t, 2H, J = 6.49

Hz), 1.77 (p, 2H, J = 6.89 Hz), 1.40 (m, 4H), 0.93 (t, 3H, J = 7.14 Hz).

Figure 12: Methoxy deprotection of 7 yielding 8.

4-(bis(4-iodophenyl)amino)-2-(pentyloxy)benzoic acid - Figure 12 - (8): To a

100mL 3-neck round bottom flask was added protected 7 (1.20 g, 1.87 mmol), THF

(18mL), NaOH/MeOH (18mL, 2M), and a stir bar. The solution was refluxed (69oC) and after 2 hours the solution was allowed to cool to 40oC. The contents of the 3-neck round bottom flask were then concentrated to dryness (pale yellow solid) via rotorary evaporator (51oC). To the solid was added enough water to get all of the solid off the walls of the flask and into solution. Next, a stir bar was added and HCl (1M) was added whilst stirring until a pH 2 was obtained, at which point a white solid precipitated out.

(Note: Upon initial addition of water not all of the solid was completely dissolved and the water had a very pale yellow suspension. Upon acidification the water became clear and there was a distinct white precipitate.) The white solid was filtered, washed with water several times, followed by two generous washings with hexanes. The solid was collected and dried in a vacuum oven over night and confirmed to be pure via NMR. (Yield 1.17g,

1 1.87 mmol; quantitative). H NMR (400 MHz, CDCL3): δ = 8.01 (d, 1H, J = 8.72 Hz),

7.65 (d, 4H, J = 8.88 Hz), 6.90 (d, 4H, J = 8.76 Hz), 6.67 (dd, 1H, J = 2.12, 8.72 Hz),

6.55 (d, 1H J = 2.08 Hz), 4.02 (t, 2H, J = 6.50 Hz), 1.85 (p, 2H, J = 6.94 Hz), 1.40 (m,

4H), 0.94 (t, 3H, J = 7.10 Hz).

Figure 13: Suzuki coupling of 8 & 9 yielding 10.

4-(bis(4-(5-formylthiophen-2-yl)phenyl)amino)-2-(pentyloxy)benzoic acid -

Figure 13 - (10): To a dry 100 mL 3-neck round bottom flask equipped with a stir bar was added 8 (500 mg, 0.797 mmol) and thiophene (9) (274 mg, 1.75 mmol). The 3-neck was fitted with a bubbler, septa, and gas line through which argon was flowed for 20 minutes prior to the addition dioxane (32 mL). The bubbler was removed and replaced with a septa under a rapid flow of argon and the system was degassed several times and the bubbler was replaced. Potassium carbonate (5 mL, 1 M), tris(dibenzylideneacetone)dipalladium(0) (Pd2dba3) (80 mg, 0.087 mmol), and tri-tert-butylphosphonium tetrafluoroborate (80 mg, 0.28 mmol) were added by removing the septa with a rapid flow of argon. After all the reagents had been added the solution was degassed 3 times with argon, after which argon was flowed rapidly through the system for 10 minutes. The argon flow was slowed to produce one bubble per second and the solution was allowed to stir at r.t. for 24 hours. After, the reaction mixture was poured into water (100 mL) and extracted with DCM (3 x 100 mL), dried with MgSO4, filtered, and concentrated

(rotorary evaporator). The crude product was then purified via column chromatography

(silica gel). The fractions containing pure product were combined and concentrated

1 (rotorary evaporator). (Yield 424 mg, 0.712 mmol; 89%). H NMR (400 MHz, CDCL3):

δ = 9.92 (s, 2H), 8.08 (d, 1H, J = 8.72 Hz), 7.77 (d, 2H, J = 3.96 Hz), 7.67 (d, 4H, J =

8.72 Hz), 7.41 (d, 2H, J = 3.96 Hz), 7.23 (d, 4H, J = 8.72 Hz), 6.81 (dd, 1H, J = 2.04,

8.00 Hz), 6.69 (d, 1H J = 2.04 Hz), 4.06 (t, 2H, J = 6.50 Hz), 1.86 (p, 2H, J = 6.94 Hz),

1.41 (m, 4H), 0.93 (t, 3H, J = 7.10 Hz).

Figure 14: Knoevenagel condensation of 10 & 11 yielding 12.

4-(bis(4-(5-(2,2-dicyanovinyl)thiophen-2-yl)phenyl)amino)-2-(pentyloxy)benzoic acid - Figure 14 - (12): To a 100mL 3-neck round bottom flask containing a stir bar was added ethanol (EtOH) (200 mL), 10 (174 mg, 0.292 mmol), malonitrile (11) (67 mg, 1.01 mmol), and 32 drops of TEA. The flask was equipped with septa and a condenser and was degassed several times. Heat was applied with stirring and the flask was allowed to stir for 5 hours under a blanket of argon. The heat was removed and the flask was 22 allowed to cool to r.t. at which time it was poured into water (200mL) and DCM (100 mL) was added, which resulted in an emulsion. Salt was then poured into the separatory funnel and shook vigorously, upon settling the organic layer was red this layer was separated and the aqueous extracted with DCM (3 x 100 mL). The combined extracts were then dried with MgSO4, filtered, concentrated (roto. vap.), and purified via silica gel column chromatography (eluent 2% EtAc in DCM). (Yield 153 mg, 0.221 mmol; 76%).

1 H NMR (400 MHz, CDCL3): δ = 8.00 (d, 1H, J = 8.68 Hz), 7.74 (s, 2H), 7.65 (d, 2H, J

= 4.24 Hz), 7.60 (d, 4H, J = 8.72 Hz), 7.36 (d, 2H, J = 4.08 Hz), 7.14 (d, 4H, J = 8.68

Hz), 6.81 (dd, 1H, J = 2.08, 8.64 Hz), 6.64 (d, 1H J = 2.04 Hz), 3.98 (t, 2H, J = 6.48 Hz),

1.78 (p, 2H, J = 6.94 Hz), 1.31 (m, 4H), 0.83 (t, 3H, J = 7.10 Hz).

P1 dye. The synthesis of P1 has been previously reported and can be found in the literature.39

2.2b Ultraviolet visible (UV-vis) absorption and emission spectra of DRB1 and P1

In order to probe the light absorption and excited state electronics of DRB1 UV- vis and emission spectroscopy were carried out in acetonitrile (MeCN) at a concentration of 0.01 mM. The same experiments were carried out on P1 in MeCN at 0.01 mM for comparison to DRB1. The maximum absorption for DRB1 occurs at 465 nm yielding a molar extinction coefficient (ε) of 67,050 M-1 cm-1, whereas P1's absorbance was bathochromically shifted to 474 nm corresponding to a ε of 62,690 M-1 cm-1. The

23 electronic transition for P1 has been thoroughly investigated by others, which shows the

HOMO is physically located on the triphenylamine (TPA) moiety and the LUMO is located on each of the terminal dicyano acceptors. The addition of an alkoxy chain in the case of DRB1 would not dramatically change the physical location of either the HOMO or LUMO. Although, it was expected to see a change in the energy level of the HOMO of

DRB1. The 9 nm hypsochromic shift of DRB1 compared to P1 was unexpected as the addition of an electron donating group (i.e. ethers) to the HOMO of the sensitizer would theoretically shift to more negative values, while leaving the LUMO of the dye unchanged. Such a change would have theoretically shifted the absorbance maximum to lower energy/longer wavelengths due to reducing the energy difference between the

HOMO and LUMO. The maximum emission wavelengths for DRB1 and P1 were found to be 646 and 623 nm, respectively (Figure 15). The intersection of a molecules absorbance and emission spectra yields the exciton energy (E0-0), which is the energy difference betwixt the HOMO and LUMO of, in this case, a molecule. The absorption and emission intersection wavelength for DRB1 and P1 was found to be 547 and 552 nm corresponding to E0-0 of 2.27 and 2.25 eV, respectively.

The ground state reduction potential (E(S/S-)) of DRB1 and P1 was found to be -

0.63 and -0.77 V vs. NHE, respectively. The corresponding excited state reduction potential (E(S*/S-)) was calculated using equation 2 and found to be 1.64 and 1.48 V vs.

NHE for DRB1 and P1, respectively.

- - [Eqn. 2]: E(S*/S ) = E0-0 + E(S/S )

Figure 15: Absorbance (red line) and emission (black line)of DRB1 (a) and P1 (b).

It is noted that the E(S*/S-) and E(S/S-) for P1 were found to be 1.42 and -0.83 V vs. NHE in the literature, respectively. The E(S*/S-), which can be approximately interpreted as the HOMO energy level, for DRB1 is ~1.14 V more positive than the VB edge of NiO (~0.5 V vs. NHE) providing sufficient thermodynamic driving force for hole injection into the NiO VB from the excited state of DRB1.

2.2c Solar Cell Fabrication

The NiO used for all the solar cells described in this thesis was prepared from NiCl2 using a sol-gel procedure previously reported in the literature.40 The resulting green paste was doctor bladed onto flourine doped tin oxide (FTO) conductive glass and allowed to rest at r.t. covered with a Petri dish for 30 - 40 minutes. The resulting film was annealed 25 in a furnace at 400oC for 30 minutes at a 2oC/minute ramp rate and allowed to cool naturally to r.t.. The films thicknesses were measured using a profilometer (thickness range = 0.8 - 1.4 μm) and the transmittance of the film was collected.

The films were soaked in 0.1 mM dye solutions (either DRB1 or P1) with MeCN as the solvent. Transmittance of the sensitized films was taken after ~16 hours of soaking and compared to the bare NiO films prior to soaking. It can be seen in figure 16 that after soaking in the dye solution the films transmittance decreases by 35.8% where DRB1 has maximal absorbance (465 nm) indicating the film has been sensitized by DRB1. The same was done for P1, which exhibited a transmittance decrease of 36.2% at 474 nm as can be seen in Figure 16.

Figure 16: The transmittance of bare NiO films before (green and black lines) and after sensitization with DRB1 (red line) and P1 (blue line).

The counter electrode was a nano-particulate platinum deposited on FTO (with a predrilled hole for electrolyte injection) from a 25 μM H2PtCl6 in isopropanol. After deposition onto FTO glass a Petri-dish was used to cover the FTO glass and the isopropanol was allowed to evaporate over night. The dry FTO was annealed at 385oC for

20 minutes with a ramp rate of 5oC/minute. The electrolyte used was 1 M lithium iodide

(LiI) and 0.1 M iodine (I2) in MeCN. The photoelectrode and platinized counter electrode were sandwiched together around a piece of Surlyn60 and place in a 100oC oven for 5 minutes. Once cooled electrolyte was injected and the injection hole was sealed with

Surlyn60 and thin glass slide cover. 27

2.2d Solar Cell Testing

Three solar cells were built for each DRB1 and P1 and tested via linear sweep voltametry (LSV) scanning from -10 mV to 150 mV at a rate of 1 mV/sec. in both the light and the dark. The J-V curves (LSV) for both DRB1 and P1 were generated while illuminated with a 100 mW cm−2 AM 1.5 illumination generated by a solar simulator

(Small-Area Class B, Solar Simulator, PV Measurements, Inc.)

2.2f Dye Loading Investigation

Seven dye solutions for each DRB1 and P1(14 total) were prepared ranging in concentration from 2 - 30 μM. NiO films were then soaked overnight (~ 11 hours). UV- vis was performed on the solution before and after soaking the NiO films.

2.3 Results and Discussion

2.3a Synthetic challenges on route to DRB1

In short, the first step was a Williamson-ether synthesis followed by a transesterification. Next, a Buchwald-Hartwig coupling was employed to obtain the triphenylamine moiety with a subsequent iodination, then deprotection of the methyl ester

28 back to the carboxylic acid. Suzuki coupling yielded the 10 followed by a Knoevenagle condensation to the target DRB1. Overall the synthesis is 7 steps, 3 longer compared to

P1. The detailed synthesis of DRB-1 is outlined below (Figure 17).

Figure 17: The overall synthetic path to DRB1.

While synthesizing DRB1 over iodination of the TPA during the halogenantion reaction with iodide chloride (ICl) the over-iodinated species in Figure 18 was observed.

Over iodination was prevented by lowering the temperature to 0oC during the addition of the ICl, diluting the ICl in the reaction solvent (dioxane), and monitoring the reaction via

NMR. Employing this strategy quantitative conversion of the initial TPA to the diiodinated product was achieved.

Figure 18: Different iodination conditions yielding a triiodinated (top) and a diiodinated (bottom) product.

The 4-bromo-2-(pentyloxy)benzoic acid (3) was protected with a methyl group to yield methyl 4-bromo-2-(pentyloxy)benzoate (6) with H2SO4 in MeOH to prevent the coordination of the carboxylic acid to the Pt catalyst. The methyl ester was carried through the proceeding reactions until the methyl protected DRB1 was obtained. Several deprotection strategies were attempted without success as highlighted in Table 1. In order to overcome this issue the diiodinated TPA (7) was deprotected using KOH in a

MeOH/THF solvent yielding 8. This carboxylic acid containing TPA was carried through the subsequent Suzuki coupling and Knovenangle condensation without any issues.

Mass of Temperature methylester Deprotection Solvent (mL) (oC); Time Atmosphere protected Reagents (hours) DRB1 (mg) Methylene chloride NaOH in MeOH 25 23; 72 Ambient (55) (7 mL, 2 M) 25 Ethylacetate (5) LiI (14 mg) Reflux; 20 Inert (Ar) 30 Tetrahydrofuran (8) LiI (12 mg) Reflux; 20 Inert (Ar) Table 1: Different conditions attempted for deprotection of the methyl protecting group.

2.3c Solar Cell Performance and Comparison

When evaluating solar cell performance the three metrics used are the open circuit voltage (Voc), short circuit current (Jsc), maxium power (Pmax), and incident power (Pinc).

2 The Pinc for all tests was 100 mW/cm . From these metrics the fill factor (FF) and efficiency (η) can be calculated using equations 3 and 4. Table 2 summarizes the performance, which was obtained from the J-V curves of Figure 19 (a) and (b).

[Eqn. 3]: Pmax = Voc · Jsc · FF

-1 [Eqn. 4]: η = Voc · Jsc · FF · Pinc · 100

Dye Jsc (mA/cm2) Voc (mV) F.F. η (%) DRB1 1.52 ± 0.19 84 ± 10 0.33 ± 0.01 0.042 ± 0.012 P1 2.50 ± 0.25 100 ± 9 0.34 ± 0.12 0.084 ± 0.018 Table 2: Summary of average performance parameters for DRB1 and P1.

The efficiency (η) of DRB1 was surprisingly half that compared to P1. The electronics of the DRB1 were not thought to be the issue as the E(S*/S-) for P1 is 160 mV more negative compared to DRB1, meaning DRB1 had a 160 mV greater driving force for hole injection compared to P1.

The solar cells were next subjected to the same LSV experiment as above in the dark. Given the alkoxy chain was added in order to prevent recombination between the

- - NiO semi-conductor and the I /I3 electrolyte, the dark current for DRB1 should, theoretically, be lower than the that of P1. Figure 19 (c) shows that this was not the case and that P1 has a lower dark current.

2.3d Effect of Dye Loading on Solar Cell Performance

Dye loading can have dramatic effects on DSSC performance. Given the proximity of the alkoxy chain to the anchoring group (carboxylic acid) it was thought the alkoxy chain could inhibit dye loading either through "intra" or "inter" inhibition. An adsorption isotherm was used to probe this hypothesis. Knowing the ε of both DRB1 and

P1 and using Beer-Lambert's law (equation 5) the exact concentration before and after soaking could be calculated.

[Eqn. 5]: A = εbc

This difference was taken to be the amount of dye which loaded onto the NiO film. A plot of the dye adsorption (nmol/cm2) vs. initial dye soaking solution concentration (mM) can be seen in Figure 20. The dye loading of P1 was greater on average by 30 nmol/cm2.

Figure 20: A plot of dye loading dependance on dye soaking solution concentration for DRB1 (red stars with dash-dot-dot line) and P1 (black squares with dash-dot line).

2.4 Conclusion

It was initially hypothesized that DRB1 would outperform P1, but performance of

P1 was superior to that of DRB1. The reason for this was found to be the decreased dye loading observed for DRB1 when compared to P1. The dark current measurements showed that recombination due to electrolyte/semi-conductor interaction was not better for DRB1 either. Therefore it was concluded that the alkoxy chain prevented efficient dye loading due to the increased sterics near the carboxylic acid anchoring group of DRB1.

Chapter 3: Double-Acceptor as a Superior Organic Dye Design for P-type DSSCs: High Photocurrents and Observed Light Soaking Effect

3.1 Introduction

The maximum current obtainable from a tandem cell is limited by the lowest current producing photoactive electrode, which is currently the p-type component. Photo- cathodes (p-type DSSCs) have been lower efficiencies and photo-currents owing to being less investigated. Therefore, it is desirable to increase the photo-current. This photocurrent is limited by the transport resistance (charge collection efficiency) and various recombination pathways all of which can be circumvented via intelligent dye design and/or development of higher carrier mobility semiconductors compared to the commonly used NiO. Herein, intelligent dye design approach to address the photocurrent limitations.

When designing a dye for p-type DSSCs, high molar extinction coefficients are of particular importance because they reduce parasitic light absorption by NiO through use of thinner films of NiO. We envisaged that replacing a carboxylic acid anchor on PMI-

[BT]n-TPA with an additional PMI-[BT]n arm, would increase cell performance due to an increase in molar absorptivity. An example of device improvement due to such a concept can be seen in P1 dye,39 which was a top performing dye in p-type DSSCs, compared to its analog O2,41 which was synthesized two years later (molecular structure of P1 and O2 36 are provided in Figure 21). The difference between these two dyes is a single acceptor and double anchor, in the case of O2, compared to the double acceptor and single anchor architecture of P1. As a result P1 has a larger molar extinction coefficient yielding better overall device performance. Therefore, a “double acceptor” dye design is suggested as the ideal design for dyes containing the triphenylamine (TPA) donor moiety. Supporting this suggestion is the fact that synthesis of the “double acceptor” design is shorter than the single acceptor design. Thus, higher molar extinction coefficients can be achieved via less synthetic steps.

Figure 21: Molecular structures of previously reported P1 and O2 dye.

Increased device performance upon light soaking has been observed previously in the literature for both n-type and solid state DSSCs, but, to our knowledge, no such phenomena has been observed in p-type DSSCs. The light soaking effect is the phenomena whereby a solar cell device increases in performance upon being exposed to 37 illumination. Similar to reports in literature, the preliminary data herein suggests the origin of performance increase upon light soaking in our p-type systems could be attributed to lithium cations in the redox mediator.42–45 Our results show that when lithium is replaced by larger cations (i.e. TBA and DMHII) no light soaking effect is observed. This observation has been made in solid state DSSCs.45

Herein is reported photocurrent among the highest for p-type DSSCs, 7.4 mA/cm2, with new dyes (Figure 22) whose synthetic scheme is shorter than PMI-T6-

TPA.11 Relative to previous record cells the NiO films used herein were prepared in the traditional manner and electrolytes used were air stable.6 It is important to note that the dye soaking solutions used were dilute (0.01mM) and film thicknesses were between 2.0

- 2.5 µm, which is ideal when considering large scale production. Beyond that we report the first ever observed light soaking effect in p-type DSSCs, whereby performance is observed to increase to a maximum upon light soaking. The cells containing these new stable dyes show improvement for 47 hours after which their performance is shown to be stable for weeks.

Figure 22: Molecular structure of the BH dye series.

3.2 Experimental

3.2a Collaborative Synthesis of Novel Photosenstizers (BH2, BH4, and BH6)

Figure 23: Total synthesis of the BH dye series.

Tert-butyl 4-(diphenylamino)benzoate (1). To an oven dried 250 mL 3-neck round bottom flask containing a stir bar was charged with potassium tert-butoxide (4.18 g, 37.2 mmol), diphenylamine (3.00 g, 17.7 mmol), and tert-butyl 4-bromobenzoate (4.92 g, 19.2 mmol). The flask was fitted with septa, purged with argon (3x's), toluene (88 mL) was added at once via syringe, and the system was purged with argon once more. Next,

t Pd2(dba)3 (811 mg, 886 μmol) and HP( Bu)3BF4 (514 mg, 1.77 mmol) we added together at once and the system was purged with argon (3x's). The solution was allowed to stir (24 hours), after which it was poured into water (100 mL) and the organic layer was separated. The aqueous layer was extracted with methylene chloride (3 x 75 mL), the organic extracts were combined, dried over Mg(SO4)2, filtered, and concentrated in vacuo. The crude solid was purified via column chromatography using on eluent of methylene chloride/hexanes yielding white solid (5.35 g, 87%). 1HNMR (400MHz,

CDCl3): δ = 7.82 (d, 2H, J = 8.88 Hz), 7.32-7.26 (m, 4H), 7.15 -7.07 (m, 6H), 6.99 (d,

13 2H, J = 8.84 Hz), 1.57 (s, 9H). CNMR (400MHz, CDCl3): δ = 165.76, 151.76, 147.02,

130.79, 129.63, 125.72, 124.53, 124.29, 120.58, 80.51, 28.41. MS (ESI-TOF): m/z [M +

+ Na ] = 368.16

Tert-butyl 4-(bis(4-iodophenyl)amino)benzoate (2). To a 250 mL single neck round bottom flask containing a stir bar was charged with dioxane (62 mL), Zn (II) acetate (5.31 g, 28.9 mmol), and ICl (2.65 mL, 51.0 mmol, 3.10 g/mL). The flask was fitted with dropping funnel containing 1 (5.00 g, 14.5 mmol) dissolved in dioxane (24

40 mL) and the solution was added dropwise (2 hours). The resulting solution was allowed to stir for 3 hours. After which the reaction solution was poured into sodium thiosulfate

(1M, 400 mL), the organic layer was separated, and the aqueous was extracted with methylene chloride (3 x 100 mL). The organic extracts were combined, dried over

Mg(SO4)2, filtered, and concentrated in vacuo. The resulting dark grey solid was filtered over a column of silica using methylene chloride as the eluent yielding a light grey solid

1 (8.30 g, 96%). M.P.: 180 °C. HNMR (400MHz, CDCl3): δ = 7.84 (d, 2H, J = 8.84 Hz),

7.57 (d, 4H, J = 8.80), 7.00 (d, 2H, J = 8.84 Hz), 6.85 (d, 4H, J = 8.80 Hz), 1.57 (s, 9H).

13 CNMR (400MHz, CDCl3): δ = 165.43, 150.57, 146.44, 138.73, 131.01, 127.05, 126.07,

121.82, 87.67, 80.85, 28.38. MS (ESI-TOF): m/z [M + Na+] = 619.96

Tert-butyl 4-(bis(4-iodophenyl)amino)benzoate (2) was converted to tert-butyl 4-

(bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)amino)benzoate (3) by Kevin

Click via Miyaura borylation reaction and is outlined in detail, including characterization, in literature.15

2-(2,6-diisopropylphenyl)-1H-benzo[5,10]anthra[2,1,9-def]isoquinoline-1,3(2H)- dione. Was synthesized following procedures from literature.46

8-bromo-2-(2,6-diisopropylphenyl)-1H-benzo[5,10]anthra[2,1,9-def]isoquinoline

-1,3(2H)-dione. Was synthesized following procedures from literature.47

Synthesis of 4, 5, and 6. Were synthesized following procedures from literature.48

The TPA unit (3) was used by Kevin Click in a series of Suzuki-Miyaura couplings with PMI-[BT]n-I, whose oligothiophene length varied, to produce the ester protected dye (7, 8, & 9). All the coupling reactions employed to yield 7, 8, & 9 used

41 the same catalyst and solvent but the temperatures were varied to minimize de-iodination products. A simple de-protection in methylene chloride with trifluoroacetic acid afforded the final BH dyes. The detailed synthesis is summarized in literature.15 The synthetic routes to obtain BH2, 4, & 6 are shorter by 6, 4, & 2 synthetic steps respectively when compared to PMI-T6-TPA. This fact is important with respect to the large scale production, as the synthetic route for obtaining the dyes for these devices needs to be as short as possible to help keep production costs low.

General information: All synthetic reagents, solvent, and silica were purchased from either Sigma-Aldrich or Fisher Scientific with the exception of tert-Butyl 4- bromobenzoate which was purchased from Frontier Scientific. 3-hexyl-1,2- dimethylimidazolium iodide (DMHII) was purchased from Solaronix. All products were characterized by 1HNMR and 13CNMR using a Avance III 400 instrument and ESI-MS was utilized by a BrukerMicroTOF (ESI) outfitted with an Agilent 1200 LC. MALDI-

MS was obtained usinga Bruker-Daltonics UltrafleXtreme MALDI-TOF-TOF MS.

3.2b UV-vis absorption and emission photo-spectroscopy of BH2, BH4, and BH6

All solutions were 0.01 mM in methylene chloride and taken at room temperature.

UV-Vis absorption of dye solutions were obtained using a Perkin Elmer Lambda 950 instrument. Emission spectra were obtained using a Fluoromax-4 from Horiba Scientific instrument with an excitation wavelength corresponding to the maximum absorbance for each dye which can be seen in Table 1.

3.2c Electrochemistry of BH2, BH4, and BH6

Cyclic voltammetry was obtained using a Gamry potentiostat. All dye solutions were 0.1 mM in anhydrous methylene chloride with a 0.1 M tetrabutyl ammonium hexafluorophosphate supporting electrolyte which were purged with argon 20 minutes prior to scans. The scan rate for all cyclic voltammograms were 100 mV/s. The working electrode, counter electrode and reference electrode were a Pt dish, Pt mesh, and a 0.01

M Ag/AgNO3 electrode respectively. The reference electrode was calibrated using ferrocene as the internal standard (E°(Fc+/Fc) = 0.640 V vs NHE).

3.2d Solar cell fabrication

The NiO films were prepared using a modified sol-gel method.49,5051 The green sol-gel solution was then doctor bladed on to FTO glass and heated at 450C for 30mins.

Film thickness was varied by repeated cycles of doctor blading and heating. Film thickness was determined using a AlphaStep D-100 profilemeter from KLA-Tencor corporation. Pt counter electrodes with predrilled holes were prepared by thermal

o deposition of a 25 µM H2PtCl6 solution in isopropyl alcohol on FTO glass at 385 C for 20 minutes. NiO films were soaked overnight in 0.01 mM dye solutions in DMF for BH2 and BH4 and methylene chloride for BH6. The sensitized films were then washed with either DMF (BH2 and BH4 films) or methylene chloride (BH6 films), and air dried. The cell was assembled by sandwiching Surlyn60 film between the working and counter 43 electrode which was clamped and heated. Electrolyte was injected through the predrilled hole of the counter electrode under vacuo. The hole was then sealed by heating a

Surlyn60 film under a thin glass slide.

3.2e Solar Cell Testing

Solar cells were built and tested via linear sweep voltametry (LSV) scanning from

-10 mV to 150 mV at a rate of 1 mV/sec. while illuminated by a 100 mW cm−2 AM 1.5 light generated by a solar simulator (Small-Area Class B, Solar Simulator, PV

Measurements, Inc.).

3.2f Dye Loading Investigation

Nine NiO films were constructed as described above with thicknesses between

1.8-1.9 µm thick for all films. A volumetric pipette was used to measure exactly

10.00mL of each dye solution (0.01mM for all dyes, BH2 and BH4 in DMF and BH6 in methylene chloride) in which 3 films for each dye was soaked in individual 10.00 mL dye solutions. Using the known molar extinction coefficients of the dyes, concentrations of the solutions before and after soaking were determined via the UV-Vis spectrometer described above to determine amount of dye loaded onto the NiO surface.

3.2g Electrolyte Preparation

Three different electrolytes were prepared each with a different cation were prepared by to give 1.0 M electrolytes each having the same iodide anion but varying cations being lithium, tetrabutylammonium (TBA), and 3-hexyl-1,2- dimethylimidazolium (DMHII), all in MeCN with the same concentration of iodine

(0.1M).

3.2h Light Soaking Methods

Nine cells were assembled using the three BH dyes (3 cells per dye).

Chornoamperometry was carried out first by light soaking the cells at open circuit conditions. Next, J-V curves were measured followed by impedance measurements. This general sequence of testing was followed for all cells every 8 hours until the light soaking effect was no longer observed. After each test sequence, the cells were stored in a dry dark place until the next test.

3.2i DFT calculations of BH2, BH4, and BH6 (Carried out by Ben Garrett)

Gaussian 09 was used for all computational studies.52 All BH dyes were optimized using density functional theory (DFT) with a 6-31G basis set. The optimum dihedral angle was found for a simpler oligothiophene donor acceptor dye.11 This

45 dihedral angle was used as the dihedral angle between thiophenes in the BH dye series geometry optimizations. A vibrational frequency analysis was performed to confirm each

BH dye as a minimum on the potential energy surface. The alkyl-side chains along the oligothiophene was reduced from –C6H13 to –C3H7 to reduce computational expense.

3.3 Results and Discussion

3.3a Electronic properties of BH2, BH4, and BH6

The UV-vis data in methylene chloride can be seen Figure 3.3. BH2, 4, & 6 have maximum absorption at 520, 515, and 488 nm, corresponding to molar extinction coefficients of 8.4 x 104, 9.6 x 104, & 9.6 x 104 M-1cm-1, respectively. The absorption peaks at 265, 486 and 509 nm correspond mainly to the π-π* N-(2,6- diisopropylphenyl)perylene-3,4-dicarboximide (PMI) with the charge transfer absorption at the longer wavelength shoulder at 520 nm with an absorption peak rising at 375 nm corresponding to the π-π* of the oligothiophene bridge.48 This peak increases in intensity from BH2-BH6 as the length of the conjugated oligothophene double acceptor is lengthened (Figure 24).

Figure 24: UV-Vis of BH series dyes in methylene chloride.

A bathochromic shift is noticed as the oligothiophene bridge is extended. This can be attributed to a more negative shift of the highest occupied molecular orbital (HOMO) energy level due to increased conjugation with the lowest unoccupied molecular orbitals

(LUMO) energy remaining constant. This is supported by both the cyclic voltammogram

(Figure 25) and the DFT calculations which illustrate HOMO

Figure 25: Cyclic voltammagram of 0.1 mM BH series dyes in methylene chloride with a 0.1M tetra-butyl ammonium hexafluorophosphate supporting electrolyte. The scan rate was 100mV/s.

location on the TPA-oligothiophene donor. The electrochemistry of all the dyes was obtained using cyclic voltammetry in methylene chloride. The dyes show reversible first oxidation peaks corresponding to the oxidation of the donor triphenylamine and oligothiophene moiety where DFT calculations show HOMO orbital location. The first oxidation peak shifts are 1.18, 1.16, and 1.14 V vs. NHE, for BH2, BH4, and BH6 respectively (Table 3). This first oxidation shifts more negative going from BH2 to BH6.

This could be explained by increased radical cation delocalization across the conjugated oligothiophene units, lowering the energy needed to oxidize the donor moiety.

b Dye ERed2 [V] ERed1 [V] Eox1 [V] Eox2 [V] Eox3 [V] Eox4 [V] ΔECV BH2 -1.13 -0.65 1.18 1.37 1.55 1.73 1.83 BH4 -1.09a -0.64 1.16 1.48a 1.70a N/A 1.80 BH6 1.03a -0.65 1.14 1.56a N/A N/A 1.79 Table 3: Summary of all oxidation and reduction peaks of electrochemical measurements of dyes. [V] vs NHE. (a) Denotes irreversible oxidation/reduction peaks. (b) ΔECV calculated by Eox1 - ERed1.

The BH dye series show similar shape in UV-vis absorption spectra compared to the single acceptor/double anchor design of the PMI-[BT]n-TPA dye, but the BH dyes show both increased molar extinction coefficients as well as bathochromic shifts. Again, this is attributed to an increase in conjugation due to two π-linker bridges, opposed to two anchoring groups (COOH) of the PMI-[BT]n-TPA single acceptor model.

Emission spectra (Figure 26) were obtained in 0.01 mM methylene chloride solutions at room temperature with excitation wavelengths corresponding to absorption wavelength maximums as described in Table 1. The stokes shift for BH2, BH4 and BH6 were 81, 66, and 87 nm respectively. The intersection of the normalized fluorescence spectra and absorption spectra were taken to be the E0-0 of the dyes.

Figure 26: The absorbance (solid line) and emission (dashed line) spectra for BH2, 4, and 6.

Also, HOMO location of BH2 is more localized near the electron withdrawing anchor (COOH), affording a more positive oxidation potential. As the oligothiophene chain is increased, the HOMO moves away from the TPA unit where the carboxylic acid anchor resides and spreads across the oligothiophene chain hence increasing conjugation of the HOMO and increasing the distance from the carboxylic acid. The PMI-[BT]n-TPA show the same trend in that the HOMO negatively shifts with increasing conjugation of the oligothiophene bridges as expected, but the BH series dyes show a more positive

HOMO shift, possibly due to proximity of the carboxylic acid anchoring group and less delocalization of the HOMO.

A more complex irreversible oxidation of the donor moiety can be seen at more positive potentials where radical dications and trications exist, but herein the focus was on the frontier oxidations and reductions. Reversible reduction peaks corresponding to reduction of the PMI acceptor unit can be observed at -0.66, -0.64, and -0.65 V with reference to NHE for BH2, BH4 and BH6, respectively. The reduction potentials remain relatively constant as the oligothiophene chain is extended, meaning the PMI acceptor moiety is independent of the donor subunit's electronic character. This is consistent with

DFT calculations which show the LUMO is localized on the PMI moiety. The same trend was observed for the PMI-[BT]n-TPA. Electrochemical measurements are summarized in table 3.0 in the supporting information. Electrochemical and optical measurements show that the energy alignments of BH2, 4 and 6 all have a large driving force for hole injection into NiO (~0.5 V vs. NHE) and a favorable regeneration energy for the redox

- - mediator I /I3 (~0.35V vs. NHE). A summary of all the pertinent electronic information can be found in Table 4.

λ (nm)/ ε (M-1 λ E0 (V E0 (V vs E a E(S*/S−)b(V Dye abs, max em, max ox red 0-0 ΔE c cm-1) (nm) vs NHE) NHE) (eV) vs. NHE) opt. 380/64,580 , BH2 604 1.18 -0.65 2.25 1.60 2.12 520/83,700 405/89,830 , 515 BH4 581 1.16 -0.64 2.28 1.64 2.11 95,510 422/99,980 , BH6 575 1.14 -0.65 2.27 1.62 2.11 488/96,450 Table 4: Summary of optical and electrochemical properties of the dyes. [V] vs NHE. (a) * - E0-0 taken to be intersection of absorption and normalized emission spectra. (b) (S /S ) = o E0-0 + E red . (c) Determined from the start of absorption of the longer wavelength shoulder absorption.

3.3b Solar Cell Performance and Comparison

The solar cells yielding high photocurrents were fabricated using common morphology NiO (2.0 - 2.5 µm thick film) opposed to the, synthetically more cumbersome, NiO microballs (6.0 µm thick film) employed to achieve the previous high photocurrent using PMI-T6-TPA.6 The NiO films were soaked overnight in 0.01 mM dye solutions. The electrolyte used was a 1.0 M 3-hexyl-1,2-dimethylimidazolium iodide

(DMHII) with 0.1 M iodine in acetonitrile. A more detailed description of solar cell fabrication can be found in the experimental section. Figure 27 shows the J-V curves of

2 the BH series dyes. BH4 shows the best performance with a Jsc of 7.4 mA/cm , Voc of

128 mV, and a fill factor (FF) of 0.30 resulting in an overall efficiency (η) of 0.28. BH2 and BH6 had very similar performance among themselves with a Jsc of 4.3 and 4.4

2 mA/cm and a Voc of 97 and 95 mV, respectively, resulting in a fill factor of 0.31 and an efficiency of 0.13 for both BH2 and BH6. Table 5 summarizes the solar cell results.

Figure 27: JV curve of dyes with cells constructed with 1.0M DMHII / 0.1M iodine electrolyte in acetonitrile.

Dye Jsc(mA/cm2) Voc(mV) F.F. η BH2 4.3 97 0.31 0.13 BH4 7.4 128 0.30 0.28 BH6 4.4 95 0.31 0.13 Table 5: Summary of cell results, from Figure 27, using DMHII electrolyte.

3.3c DFT calculations of BH2, BH4, and BH6 (Carried out by Ben Garrett)

Structure and conformation, along with electron density distribution, of the BH series dyes were studied using density functional theory (DFT) with B3lyp level of theory with 6-31G basis sets. As shown in Figure 28, the electron density distribution of the

LUMO for the three dyes were quite similar to each other and were significantly localized on the PMI unit. The dihedral angles between thiophene units in the oligothiophene arms of the BH dyes are not symmetric. This likely accounts for the observation that the LUMO resides on only one arm of the BH dyes. The LUMO+1 orbital in all cases exhibit electron density located on the opposite arm, where the energies between LUMO and LUMO+1 are small. Additionally, the HOMO for the three dyes were quite similar, where the electron density is delocalized across the oligiothiophene arms.

Figure 28: DFT calculations with B3lyp level of theory with 6-31G basis sets.

3.3d Effect of Dye Loading on Solar Cell Performance

The increased device performance is attributed to the increased molar extinction coefficients of the BH dyes (8.4 x 104 to 9.6 x 104 M-1cm-1) compared to the single acceptor dye design used in PMI-T6-TPA (6.7 x 104 M-1cm-1). It was initially thought upon designing the BH dye series, that device performance might suffer due to low dye loading because of increased steric bulk. We found that BH2, 4 and 6 loaded 25 ± 1, 22 ±

3, 8.4 ± 6 nmol/cm2 respectively. The adsorption measurements were obtained via UV- vis measurements before and after film soaking. The observation that one anchoring group is sufficient for dye loading is further supported by the very dilute soaking solution used in this study. These results indicate one anchoring group is sufficient for good dye loading and performance.

Molar extinction coefficients cannot entirely explain the performance of the dyes due to the fact that BH4 has a significantly increased Jsc when compared to BH2 and BH6 which have similar Jsc but much different molar extinction coefficients. Extending to six thiophenes (BH6) maximizes light absorption, but caused decreased dye loading (BH2, 4 and 6 loaded 25 ± 1, 22 ± 3, 8.4 ± 6 nmol/cm2). There was found to be a balance between dye loading and the molar extinction coefficient. Evidence of this balance can be seen in

Table 6 where BH2 and BH6 have similar Jsc values but BH6's larger molar extinction coefficient, compared to BH2, is negated by the fact that dye loading is lower due to the significantly increased steric bulk.

1- -1 2 Dye λ abs, max (nm)/ ε (M cm ) nmol/cm Adsorbed Jsc BH2 380/64,580 , 520/83,700 25 ± 1 4.3 BH4 405/89,830 , 515 95,510 22 ± 3 7.4 BH6 422/99,980 , 488/96,450 8.4 ± 6 4.4 Table 6: Relation between dye loading and molar extinction on photocurrent.

Lastly, it is also possible that only two thiophenes (BH2) inadequately shields the surface of NiO. Increasing the length of the oligothiophene bridge can create more hydrophobic bulk due to the hexyl chains, which has been shown to decrease recombination between the redox mediator and semiconductor surface.21,53–57 Moreover, extending the oligothiophene length can prevent recombination by increasing the electron tunneling distance between the electron located in the excited state LUMO of the dye and vacancies on the NiO surface.8,21 Performance of the cell was maximized when the oligothiophene bridge contained four thiophenes (BH4) which indicates a balance between all of these factors was obtained.

3.3e Electrolyte Effect on Cell Performance

Next, solar cells using the best performing dye (BH4) were fabricated using 3 different 1.0 M electrolytes each having the same iodide anion but varying cations being lithium, tetrabutylammonium (TBA), and 3-hexyl-1,2-dimethylimidazolium (DMHII).

Each electrolyte had the same concentration of iodine (0.1M). The film thickness for the cells were between 2.0 and 2.2 µm thick. Figure 29 shows the J-V curve for the three

56 cells and Table 7 summarizes the results. The DMHII electrolyte cell gave similar results when compared to the prior solar cell data described above in that the Jsc was 7.2

2 mA/cm , Voc was 116 mV, and the fill factor was 0.30 with a resulting efficiency of 0.25.

The two other electrolytes containing LiI and TBA showed almost identical performance

2 with a Jsc of 6.41 and 6.46 mA/cm and a Voc of 124 and 126 mV resulting in a fill factor of 0.32 and 0.29 respectively, and efficiencies of 0.25 for both cells.

Figure 29: JV curve of BH4 with 1.0M LiI, TBAI, and DMHII and 0.1M iodine in acetonitrile.

2 Electrolyte Jsc (mA/cm ) Voc (mV) F.F. η TBAI 6.5 126 0.29 0.24 LiI 6.4 124 0.32 0.25 DMHII 7.2 116 0.30 0.25 Table 7: Performance results of BH4 in the JV curve shown in Figure 29.

3.3f Light Soaking Investigation via Impedance Spectroscopy

Another interesting result of the different electrolytes used was the observed light soaking effect for the cell in the presence of lithium cations. The lithium cation containing cell shows an increase in current as a function of time until a plateau is reached and no increase in current is observed. Nine cells were constructed with three dyes (3 cells per dye) and a detailed time/light soaking study was conducted, whereby the cells were tested approximately every 8 hours. At the beginning of each test, chronoamperometry was conducted until the current increase due to light soaking reached a consistent maximum. Then, JV data was collected followed by impedance data. Once the initial testing for the cells was complete, they were stored in the dark until the next testing. It was found that over 47 hours of testing the transport resistance continually decreased while the short circuit current increased (Figure 6a and 6b, respectively). The

Voc of BH2 and BH4 remained relatively constant but the Voc for BH6, the largest dye, increased over the light soaking treatments (Figure 6c). In addition, we observed a significant difference in the capacitance of the cells. The cell that contained lithium cations had a much larger capacitance (Figure 6d). These results were not further investigated, though it was suspected that the lithium ions were sufficiently small enough

58 to diffuse to or from the NiO surface, which mechanism was happening is not known. For n-type DSSC's the light soaking mechanism has been attributed to lithium ions diffusing

45 to the n-type semi-conductor (TiO2) surface. In the case of the p-type DSSCs it could be that the lithium ions, which initially diffuse to the NiO surface upon electrolyte injection, diffuse away from the NiO surface during device operation as holes are injected into the

NiO.

rd th Electrolyte Jsc (mA) at 3 Jsc (mA) at 100 Difference (mA) TBAI 1.796 1.802 0.006 LiI 1.771 1.794 0.023 DMHII 2.009 2.017 0.008 Table 8: Summarizes the short circuit current change of the 3 cells from Figure 30. The rd th rd th "3 " and "100 " refers to the 3 and 100 Jsc obtained from the linear sweeps of the same cells while illuminated.

3.4 Conclusion

We report three novel dyes, BH2, 4, and 6, for p-type DSCs, with BH4 yielding photocurrent up to 7.4 mA/cm2, via a shorter synthetic scheme compared to a single acceptor design. The devices containing these dyes required no exotic materials or fabrication methods and common components and simple bench top cell building procedures were used to achieve the device performance reported herein. Dilute dye solutions (0.01 mM) and film thicknesses between 2.0 - 2.5 µm were used to achieve the high photocurrents. Also, the first reported light soaking effect for p-type DSSCs was observed. To date we have evidence that the source of the light soaking effect is based on the presence of lithium cations in the electrolyte. When lithium is replaced by larger cations (i.e. TBA and DMHII), no light soaking effect is observed. The mechanism for performance increase due to the light soaking effect reported is currently under detailed investigation.

Chapter 4 A Scalable, Processable, and Stable Polymer Immobilized Molecular Hydrogen Evolution Catalyst Inspired by Molybdenum Disulfide

4.1 Introduction

Molecular mimics of molybdenum disulfide (MoS2) edge sites have been identified as promising catalysts for the hydrogen evolution reaction (HER). Increasing the density of catalytically active edge sites while simultaneously immobilizing a catalyst, which is easily processable, has been a challenge. Herein we present the first molecular molybdenum disulfide edge site mimic to be coordinated into a polymer backbone, yielding discrete catalytic sites throughout the polymer films. This large super structure affords a protective environment akin to the tertiary structure of an enzymatic protein, whereas most biomimetic molecular analogs are isolated and exposed catalytic centers. The scalable 'green' synthesis employed only earth abundant elements and the catalytic polymer resin allows for easy processing. The water insoluble polymer prevents catalyst dissolution from the electrode surface through the covalent binding of the discrete molybdenum HER catalysts. The strong interaction prevents its desorption while simultaneously increasing the density of active edge sites.

In order for a hydrogen based economy to be realized, cost effective catalysts for

60,61 HER are required. Hydrogen (H2) yields the highest specific energy of any fuel.

Currently, industry employs steam methane reforming to produce H2, but this method generates CO2 negating the potential 'green' advantage of using H2 as a fuel source. The

'greener' alternative would be to produce H2 electro- or photoelectro- catalytically. The 61 best catalyst for the HER to date is platinum, but platinum is listed as an 'endangered' element and is expensive,62 therefore a more abundant and lower cost catalyst is required.

MoS2 edge site has a Gibbs free energy of hydrogen adsorption (ΔGads(H)) just above that of platinum hence it being identified as a low cost highly active alternative to

63 63 platinum for H2 production. Since the pioneering theoretical study by Nørskov, et. al. and the identification of the MoS2 edge sites as the catalytic active moiety by

Chorkendorff, et. al.64, increasing the number of catalytically active sites of molybdenum sulfide catalysts has been identified as a challenge by both Jaramillo, et. al. and Wang, et.al,. respectively.65,66 Thus, much effort towards maximizing edge site density has been made. Examples of these efforts can be seen in the form of nanoparticles,64,67–73 nanosheets,74–80 composites,81–83 films,84–86 and molecular edge site mimics.87–91

The molecular edge site mimics inherently have highest density of catalytically active edge sites of all the examples listed above.87,90 Also, the electronics of the molecular analogs can be tuned via ligand exchange and the wet chemistry methods employed allow for the synthesis to be scaled,89,91 though the molecular analogs exhibit desorption from both electrodes and photoelectrodes.88,92 Hence the immobilization of molybdenum sulfide catalyst on a photoactive substrate for photoelectrochemical water splitting has been identified by Gray & Lewis, et. al. as a challenging yet promising direction for solar driven water splitting devices.93 Towards this aim, Grätzel, et. al. photoelectrochemically deposited molybdenum sulfide onto a copper (I) oxide photocathode from which hydrogen was evolved photocatalytically.94

Herein, a molecular MoS2 edge site mimic (MoO(S2)2L) with two bridging persulfide ligands coordinated into polyvinyl-4-pyridine (PV-4-P) is presented. By incorporating the molecular molybdenum sulfide catalyst into a polymer one can provide an immobilizing acidically stable support network while simultaneously increasing the density of active edge sites when compared to bulk MoS2. This catalytic polymer has the potential to be applied to photoelectrodes evidenced by recent work employing cobaloxime coordinated PV-4-P on a gallium phosphide (GaP) photocathode.95

Work previously reported by our group showed that pre-organizing the Mo-center in MoO(S2)2L moieties limited side reactions and improved synthetic utility. We recently described a series of Mo-S molecular catalysts based on a MoO(S2)2L2 structural motif

2- that show electrocatalytic activity for HER. The Mo-S2 moieties mimic the terminal edges in the MoS2 that are active sites for HER. Little work has been done with

96,97 MoO(S2)2L2 , but the chemistry of similar peroxo complexes, MoO(O2)2L2, is well developed, owing to their utility as oxidation catalysts for organic transformations.98–104

103,104 Accordingly, the library of known MoO(O2)2L2 is large. A general method was

2- 2- developed where peroxo (O2 ) ligands in MoO(O2)2L2 is exchanged with persulfide (S2 )

2- to afford MoO(S2)2L2. The unique S2 ligand exchange method is a powerful method to produce many interesting MoO(S2)2L2 complexes. The MoS2 edge site mimetic complexes were synthesized open to air, in two steps, from low-cost materials and aqueous solvent including MoO3, H2O2, 2,2’-bipyridine (bpy), (NH4)2S and S8. No advanced synthetic techniques or special equipment is thus necessary for their synthesis.

2- 2- In this work, we apply the peroxo (O2 ) to persulfide (S2 ) ligand exchange method to a

63 polymer based ligand moiety. We exploited this synthetic method replacing the monomeric bipyridyl ligands (L) with a pyridyl containing polymer PV-4-P. Once the

2- Mo-center was pre-organized as MoO(O2)2PV-4-P the peroxide (O2 ) was exchanged

2- with persulfide (S2 ) to afford MoO(S2)2PV-4-P.

The catalytic polymer of this study (MoO(S2)2PV-4-P) was synthesized in two steps in ambient air using wet chemistry techniques employing mild temperatures

(maximum 60oC) and only 'green' solvents (water and methanol (MeOH)).105 A processable resin was deposited onto a glassy carbon electrode (GCE) surface exhibiting an onset potential of -247±2 mV vs. reversible hydrogen electrode (RHE) at a current density of 1 mA/cm2 (from here all potentials are referenced vs. RHE).

4.2 Experimental

4.2a Synthesis and Resin Preparation

MoO(O2)2PV-4-P - was synthesized from an adapted procedure previously reported from our group. To a 30% hydrogen peroxide solution (20 mL) was added

o MoO3 (1.00 g, 6.90 mmol) and heated (60 C, 15 hours) with stirring. The solution was allowed to cool to room temperature then diluted with water (total volume 100 mL). The resulting aqueous solution of MoO(O2)2(H2O)2 (14 mL, 70 mM) was added dropwise over 10 minutes to a stirring solution of PV-4-P (200 mg, 1.01 mmol) in MeOH (56 mL).

Upon completion of the addition the mixture was allowed to stir for 1 hour after which

64 the yellow precipitate (Figure 31) was filtered and washed several times with water followed by MeOH and dried under vacuum overnight at room temperature.

Figure 31: Synthetic route to MoO(S2)2PV-4-P, with the material under each of its corresponding molecular structures.

MoO(S2)2PV-4-P - was synthesized according to a persulfide ligand exchange procedure developed in our group. To a stirring ammonium sulfide solution (5.2 mL, 40-

48% wt. in water) was added elemental sulfur (1.5 g, 47 mmol) and heated (60oC) until all the sulfur dissolved. To the resulting 60oC polysulfide solution was added

MoO(O2)2PV-4-P (200 mg, 0.53 mmol). After two hours the mixture was cooled to room temperature, filtered, washed with water followed by carbon disulfide, and dried under vacuum at r.t. for 24 hours. (All reagents where purchased from Sigma-Aldrich and used as received. All solvents were purchased from Fisher Scientific.)

To a vial containing MoO(S2)2PV-4-P (20 mg) was added MeOH (400 μL) and

DMSO (200 μL) and was sonicated for 10 minutes followed by vortexing (this sonicating process was repeated 3 times). 4 μL of the resulting solution was deposited on a glassy carbon electrode (GCE), which was previous polished using an aqueous Al2O3 65 suspension (1 μM) and a polishing cloth, and solvent was removed in a vacuum desiccator at room temperature.

4.2b Characterization

Energy-dispersive X-ray spectroscopy (EDX) data was collected using a

FEI/Philips Sirion scanning electron microscope. Thermogravimetric analysis (TGA) data was collected conducted using a TGA7 thermogravimetric analyzer (Perkin-Elmer) over a temperature range of 20oC to 750oC at a rate of 10oC/min in ambient air. X-Ray diffraction (XRD) data was collected using a Bruker D8 advanced XRD with a dwell time of 3.0 seconds and a step size of 0.010 (2θ). X-ray photoelectron spectroscopy (XPS) data was collected using a Kratos Axis Ultra XPS spectrometer with airtight chamber module. The XPS spectra were collected using monochromatic Al Kα radiation. In XPS analysis, all spectra were calibrated by referencing the binding energy of carbon-carbon bond at 284.7 eV. Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR) data was collected using an Avance III 400 instrument and ESI-MS was recorded using a

BrukerMicroTOF (ESI) outfitted with an Agilent 1200 LC. Ultraviolet - visible adsorption spectroscopy (UV-vis) data was collected using Perkin Elmer Lambda 950 instrument scanning from 250 nm to 800nm. Fourier Transform Infrared (FT-IR) data was collected using Thermo Scientific Nicolet iS5, iD7 ATR.

4.2c Electrochemical Set-up and Characterization

Acid solutions were prepared in a 125 mL Erlenmeyer flask using deionized water

(18 MΩ cm) and 12 M H2SO4 to give a final H2SO4 concentration of 0.5 M. The

Erlenmeyer flasks were then fitted with septa and degassed for one hour each by bubbling a strong flow of argon through the solutions. The reference electrode was a Ag-Ag/Cl

(saturated KCl) electrode (calibrated using potassium ferrocynanide as the internal standard (0.196 V vs. NHE)) with platinum as the counter electrode. The working electrode was a GCE with catalyst deposited. The aforementioned three electrode set-up with sulfuric acid (0.5 M) electrolyte was used for all experiments outlined below, unless otherwise explicitly stated.

4.2d Hydrogen Detection and Performance Evaluation

Hydrogen was detected using a Shimadzu gas chromatograph equipped with a thermal conductance detector using argon as the carrier gas. The instrument was calibrated by injecting known volumes of hydrogen (Figure 32 and Table 9).

Figure 32: Calibration curve showing hydrogen as a function of mean peak area of the GC-TCD used for hydrogen detection.

H2 (µmol) Mean Area SD %RSD Area 1 Area 2 Area 3 0.02 5,049 194 3.85 5,052 4,854 5,243 0.05 13,363 193 1.45 13,140 13,464 13,485 0.10 28,627 430 1.50 28,143 28,965 28,773 0.21 58,038 339 0.585 57,873 58,428 57,813 0.31 87,378 229 0.262 87,228 87,641 87,263 Table 9: The raw data used to construct the calibration curve of Figure 32.

4.3 Results and Discussion

4.3a Effect of Catalyst Loading and pH on Overpotential

Compared to the PV-4-P starting material, the Fourier Transform Infrared (FT-IR) spectrum (Figure 33a) of MoO(O2)2PV-4-P shows the appearance of both the Mo=O stretch at 940 cm-1 and the O-O stretch at 890 cm-1, which have been reported for similar bipyridine analogs.106 After treatment in the polysulfide solution, the Mo=O stretch was still present at 940 cm-1, while the O-O stretch was not observed, and the S-S stretch at

524 cm-1 was observed, providing evidence the ligand exchange was successful.107 The

UV-vis absorption spectra of MoO(S2)2PV-4-P (0.4 mM) and PV-4-P (0.4 mM) were recorded in DMSO (Figure 33b). Two shoulder peaks were observed for the

MoO(S2)2PV-4-P sample at 434 nm and 573 nm corresponding to the disulfide to molybdenum and the pyridine to molybdenum charge transfers, respectively.107 The latter peak supports the coordination of the Mo metal center to the pyridines of the PV-4-P.

Figure 33: (a) FT-IR comparison of PV-4-P, MoO(O2)2PV-4-P, and MoO(S2)2PV-4-P. (b) UV-vis of MoO(S2)2PV-4-P in DMSO at the indicated concentrations.

In order to determine the amount of catalyst present in a given sample of

MoO(S2)2PV-4-P thermogravimetric analysis (TGA) was employed. The first weight change at ~50oC was attributed to residual solvent evaporation. The subsequent two stages of decomposition at ~300oC and ~550oC resulted from combustion of the PV-4-P

108 o and oxidation of MoO(S2)2, respectively (Figure 34a). At 700 C the decomposition product was determined via X-Ray Diffraction (XRD) to be MoO3 (Figure 34b), which allows for the calculation of moles of Mo within a given sample of MoO(S2)2PV-4-P.

Given each active catalytic site (MoO(S2)2) has one molybdenum, the loading density was determined to be 1.5 μmol catalyst per 1 mg of catalytic polymer (MoO(S2)2PV-4-P), corresponding to a Mo wt% of 12. This translates to 1 uncoordinated pyridine pair for every 1 pair of coordinated pyridines.

Figure 34: (a) The TGA of MoO(S2)2PV-4-P. (b) XRD of the MoO(S2)2PV-4-P after

(TGA) after heating to 7000C

1 When the H NMR of PV-4-P was compared to MoO(S2)2PV-4-P a shift in the aromatic peaks attributed to the pyridine protons was observed indicating a change of the electron environment due to molybdenum coordination (Figure 34 and 35). These shifts were not dramatic because 75% of the pyridines remained uncoordinated. The

MoO(O2)2PV-4-P species showed no solubility in any dueterated solvent, thus no NMR spectra was obtained. It is not known what causes the triplet at ~7.2 ppm. Also, Energy-

Dispersive X-ray spectroscopy (EDX) confirms the homogenous distribution of the catalyst throughout the films via elemental mapping of molybdenum and sulfur, this will be discussed later in this chapter.

6 Figure 35: NMR (400 MHz) of PV-4-P (as purchased from Sigma Aldrich) in d -DMSO.

6 Figure 36: NMR (400 MHz) of MoO(S2)2PV-4-P in d -DMSO.

4.3b Control, Scan Rate Effect, and Loading Comparisons

To begin, three tests were conducted to confirm the MoO(S2)2PV-4-P was performing as a catalyst. Bare GCE, GCE with PV-4-P deposited, and GCE with

MoO(S2)2PV-4-P deposited were all tested via linear sweep voltametry (LSV) sweeping from 100 to -400 mV at a scan rate of 50 mV/s. As can be seen in Figure 37 both the bare

Figure 37: Linear sweep voltammetry (LSV) of the bare GC, PV-4-P on GC, and MoOS4PV-4-P (132 μg) on GCE all in 0.5M H2SO4.

GCE and the GCE - PV-4-P electrodes perform the same with minimal current increase over the potential range. On the other hand, the GCE with MoO(S2)2PV-4-P deposited shows a dramatic increase of current starting at -247±2 mV (potential required to produce a current density of 1 mA/cm2). Also, it is worth noting a bias of -321 mV was required to obtain a current density of 10 mA/cm2, which is a useful metric to report as this is the expected current for a solar water splitting device operating at 12.3% efficiency.66 The overpotential dependence on scan rate was evaluated by scanning from

400 to -400 mV at scan rates of 10 and 50 mV/s. Figure 38a and 38b confirms that there

Figure 38: LSV of MoO(S2)2PV-4-P in 0.5 M H2SO4 comparing scan rates of 10 and 50 mV/s at the (a) 501st and (b) 1001st scans.

is no appreciable effect of scan rate on the on-set/overpotential, going from 50 to 10 mV/s. Next the pH dependence was evaluated by decreasing the sulfuric acid concentration to half (0.25 M) of the original (0.50 M). A decrease in overpotential was observed as a result of the acid concentration decrease as can be seen in Figure 39a. The

74 affect of catalyst loading on the on-set potential was evaluated by loading 66, 132, and

267 μg of MoO(S2)2PV-4-P onto the GCE electrode. As can be seen in Figure 39b the on-set potential decreases going from 66 to 132 μg, but increases going from 132 to 267

μg of MoO(S2)2PV-4-P. This phenomena was attributed to the non-conductive nature of the PV-4-P polymer backbone and the fact that if the catalytic polymer layer becomes too thick electrons were not as effectively conducted from the GCE surface to the

MoO(S2)2PV-4-P electrolyte interface.

Figure 39: (a) LSV of MoO(S2)2PV-4-P with varying pH (scan rate = 50 mV/s). (b) Linear sweep voltammetry (LSV) of the MoOS4PV-4-P at the different indicating loading on GCE all in 0.5M H2SO4 (scan rate = 50 mV/sec).

4.3c Simulated Solar Water Splitting Electrochemical Stress Test: Accelerated Stability

Test Evaluated via X-ray Photoelectron Spectroscopy (XPS)

The stability of the MoO(S2)2PV-4-P film was probed using linear sweep voltammetry (LSV) scanning from 300 mV to -400 mV at a scan rate of 50 mV/s in 0.5

M H2SO4 (Figure 40). Following the first linear sweep, cyclic voltammetry was conducted over the same potential range for 125 cycles at a scan rate of 100 mV/s, after which another linear sweep was conducted using the same parameters as the first LSV scan. This process was repeated until 1,000 scans had been completed. This accelerated stability test method models the cycling a HER catalyst would endure in a solar water splitting device.90 The catalyst remains stable going from 126 to 1,000 scans with the overpotential shifting 17 mV more negative at a current density of 30 mA/cm2 (Figure

40). This slight increase in required overpotential is similar to that observed for

2- 90 [Mo3S13] going from 100 to 1,000 scans.

Figure 40: The LSV of MoO(S2)2PV-4-P (132 μg) on GCE in 0.5M H2SO4 over 1,001 cycles.

Next, the catalytic polymer resin was deposited on a fluorine doped tin oxide

(FTO) glass and subjected to the same aforementioned accelerated cycling test. EDX elemental mapping of the cycled film showed no difference in Mo and S distribution when compared to a pristine unused film (Figure 41).

Figure 41: EDX of molybdenum and sulfur of a unused film and a film after 1,001 accelerated cycle test (same parameters as described in the main text) in 0.5 M H2SO4.

The X-ray Photoelectron Spectroscopy (XPS) of a pristine unused film (Figure

42a and 42c) and a cycled film (Figure 42b and 42d) shows little change demonstrating the polymeric HER catalyst is stable in acidic conditions. All the XPS spectra were calibrated to C 1s at 284.7 eV. Only one molybdenum species could be detected (Mo

(VI)) resulting in a doublet with binding energies (BE) of 229.5 eV and 233.1 eV corresponding to the Mo 3d5/2 and 3d3/2, respectively. These BEs are comparable to other molybdenum (VI) sulfide BEs previously reported.109 Three different sulfur doublets

(2p3/2 and 2p1/2) were detected. Two of these doublets were attributed to the two electronically different sulfur environments corresponding to the sulfurs of the disulfide ligands with BEs of (i) 161.6 eV and 162.8 eV and (ii) 163.0 eV and 164.2 eV. The third doublet (iii), with BEs of 167.6 eV and 168.8 eV, was attributed to the sulfur of the S=O 78

2- 87,110 from SO4 and DMSO. The sulfur doublet (iii) of the film before use is attributed to

DMSO, which was a solvent used in the dropcasting of the MoO(S2)2PV-4-P (Figure

2- 42b). The area of these peaks increased after cycling which was attributed to SO4 of the sulfuric acid (Figure 42d). This phenomena has been reported for other heterogeneous molybdenum sulfide catalyst.87

Figure 42: (a) XPS of the non-cycled MoO(S2)2PV-4-P in the Mo 3d and S 2s BE regime. (c) XPS of the uncycled MoO(S2)2PV-4-P in the S 2p BE regime (Note: The S=O feature in the non-cycled film comes from the DMSO as stated above). (b) XPS of cycled MoO(S2)2PV-4-P in the Mo 3d and S 2s BE regime. (d) XPS of non-cycled MoO(S2)2PV- 4-P in the Mo S 2p BE regime.

4.3d Kinetic Evaluation via Tafel Slope Analysis

The slope from the Tafel plot can help, in some cases, elucidate the rate determining step of the HER reaction. For proton reduction in solution three elementary steps have been identified: Volmer, Heyrovsky, and Tafel reactions.111 The first step is the Volmer reaction yielding a Tafel slope of 120 mV/dec, whereby by an electron reduces the HER catalyst followed by proton adsorption (Rxn. 1).

+ - [Rxn. 1]: H3O + e → Hads + H2O

This is followed by either the Heyrovsky reaction giving a Tafel slope of 40 mV/dec, whereby electrochemical desorption takes place (Rxn. 2),

+ - [Rxn. 2]: Hads + H3O + e → H2 + H2O

- or - by a Tafel reaction with a Tafel slope of 30 mV/dec, whereby recombination of two Hads occurs (Rxn. 3).

[Rxn. 3]: Hads + Hads → H2

To elucidate the electron transfer kinetics of MoO(S2)2PV-4-P, the Tafel slope was obtained at a scan rate of 10 mV/s yielding a slope of 81±2 mV/dec (Figure 6.12).

Figure 43: The Tafel slope of 132 μg of MoO(S2)2PV-4-P (167 nmols of active catalyst) loaded onto GCE in 0.5M H2SO4 at a 10mV/s scan rate.

4.3e Simulated Long-Term Electrolyzer Stress Test - Hydrogen Detection, Faradaic

Efficiency, and Other Performance Metrics

To model the conditions an HER catalyst would experience in an acid based water electrolyzers chronoamperometry (CA) was conducted over 2 hours at an applied potential of -350 mV (Figure 44). Stability in acid is advantageous, because acid based water electrolyzers could be run in reverse operating as a fuel cell (i.e. generating electricity), this allows for "charging" and "discharging" from a single system for stationary energy storage, and are more compact when compared to alkaline based

81 variants.112 A 0.50 mL aliquot from the head-space (33.9 mL) of the electrochemical cell was sampled after 20 minutes, then every 10 minutes thereafter until a total time of two hours was reached, over which hydrogen was continuously produced (Figure 44). Over the two hours the current decreased slightly which is attributed to the buildup of hydrogen bubbles on the catalyst surface. During this process bubbles could be seen growing on and coming off of the catalyst surface (Figure 44 inset). The faradaic

Figure 44: The 2 hour CA of MoO(S2)2PV-4-P (132 μg) in 0.5 M H2SO4 and the H2 detected over the 2 hours. A photograph showing the hydrogen bubbles formed during the CA (inset). efficiency was found to be 100% at 20 minutes and levels off at ~75% after 60 minutes

(Table 10) this reduction in faradaic efficiency was attributed to the increased growth of 82 bubbles on the electrode surface over time and their sticking to the shaft on the electrode as can be seen in the inset of Figure 3. The hydrogen bubble growth and desorption reached a constant rate after 60 minutes, hence the constant faradaic efficiency of ~75% from there on. Hydrogen was evolved at a rate of 3.1 μmol h-1 (6.2 μmol detected over

2h). The moles of catalyst loaded onto the GCE were determined to be 198 nmol yielding a TOF of 16 h-1.

Hydrogen Hydrogen in Hydrogen Integrated detected in 33.9mL corrected for chrono- Time 0.5mL headspace withdrawn 0.5mL amperometry (min) Area (μmols) (μmols) sample (μmols) (Coulombs) 20 5855 0.0240 1.63 1.63 0.315 30 7775 0.0307 2.08 2.11 0.459 40 9406 0.0364 2.47 2.50 0.600 50 11212 0.0427 2.90 2.93 0.736 60 13106 0.0493 3.34 3.39 0.868 70 15240 0.0568 3.85 3.90 0.998 80 16932 0.0627 4.25 4.31 1.13 90 19456 0.0715 4.85 4.91 1.25 100 20575 0.0754 5.11 5.18 1.38 110 23149 0.0844 5.72 5.80 1.50 120 25101 0.0912 6.18 6.27 1.67 Table 10: Detailed hydrogen detection data used to construct Figure 44.

4.3f Advantages, Disadvantages, and Potential Strategies for Improvement

When evaluating a catalyst's performance the field considers four metrics to be of particular importance, they are the onset potential, the TOF, and stability. As stated above the onset potential can be determined to be the potential which yields current densities from 0.5 to 5 mA/cm2, as well as the 10mA/cm2 metric for potential use in a solar driven water electrolyzer. In the case of molybdenum sulfur based catalyst for HER examined to date the onset potentials at 10 mA/cm2 range from -110 to -254 mV vs. RHE. The TOF indicated the rate at which hydrogen is produced on a per catalytic site basis and ranges

-1 from 36 to 360,000 H2 s per catalyst over a range of applied potential from -100 to -350 mV vs. RHE. Evaluating stability is typically carried out through accelerated stability tests and extended periods of applying a constant potential (i.e. chronoamperometry).65,66

In the case of the catalyst reported herein the exhibits an onset potential of -321 mV vs. RHE, which is lower when compared to other catalyst reported. The TOF of

-1 MoO(S2)2PV-4-P was found to be 16 H2 s per catalyst at an applied potential of -350 mV vs. RHE. The stability this catalyst showed no sign of degradation over 1,000 cycle accelerated stability test as well as over 2 hours of chronoamperometry. The monomeric molybdenum sulfide based catalysts have the highest density of the catalytically active sulfur sites, which is important as this reduces wasted inactive material. These molecular catalyst are difficult to immobilize for extended periods, thus the major advantage of the this system is that the monomers are coordinated to a polymer from which a resin can be produced and applied to various electrode substrates.

The current system could be improved by using a conductive polymer in order to increase the electron transfer from the electrode to the individual catalytic moieties. Also, this could be accomplished through the carbonization of the catalytic polymer before or after being applied to the electrode. This would theoretically provided a nitrogen doped graphitic network through which the molybdenum sulfur catalyst would be homogeneously distributed, as can be seen from the aforementioned EDX data.

4.4 Conclusion

The MoO(S2)2PV-4-P was easily processed from resin into films, resulting in the catalyst immobilized on an electrode surface. The catalytic polymer film was stable over an 1,000 scan accelerated stability test, showed an onset potential of -247±2 mV, with a faradaic efficiency of 100% after 20 minutes. Hydrogen was evolved at a rate of 3 μmol per hour from an aqueous electrolyte (0.5 M H2SO4) with an applied bias of -350 mV vs.

RHE corresponding to a turn over frequency (TOF) of 16 h-1. The Tafel slope was found to be 81±2 mV/dec. In closing, we report the first MoS2 edge-cite mimic coordinated into polymeric backbone via a 2 step scalable wet chemistry synthetic method. The scalability, processability, and stability of this polymeric HER catalyst demonstrates a new direction for molybdenum sulfide edge site mimic catalysts. Additionally, this method allows for a theoretically high density of catalytically active edge sites. Further development is currently being done and could allow for their use in large scale water

85 electrolyzers and solar water splitting devices where catalyst immobilization and stability are important.

Bibliography

(1) O’Regan, B.; Grätzel, M. Nature 1991, 353, 737–740.

(2) Shockley, W.; Queisser, H. J. J. Appl. Phys. 1961, 32, 510.

(3) He, J.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1999, 103,

8940–8943.

(4) Huang, Z.; He, M.; Yu, M.; Click, K.; Beauchamp, D.; Wu, Y. Angew. Chemie Int.

Ed. 2015.

(5) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-

Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Nat.

Chem. 2014, 6, 242–247.

(6) Powar, S.; Wu, Q.; Weidelener, M.; Nattestad, A.; Hu, Z.; Mishra, A.; Bäuerle, P.;

Spiccia, L.; Cheng, Y.-B.; Bach, U. Energy Environ. Sci. 2012, 5, 8896.

(7) Powar, S.; Daeneke, T.; Ma, M. T.; Fu, D.; Duffy, N. W.; Götz, G.; Weidelener,

M.; Mishra, A.; Bäuerle, P.; Spiccia, L.; Bach, U. Angew. Chem. Int. Ed. Engl.

2013, 52, 602–605.

(8) Yu, M.; Natu, G.; Ji, Z.; Wu, Y. J. Phys. Chem. Lett. 2012, 3, 1074–1078.

(9) Yu, M.; Draskovic, T. I.; Wu, Y. Phys. Chem. Chem. Phys. 2014, 16, 5026–5033.

(10) Draskovic, T. I.; Yu, M.; Wu, Y. Inorg. Chem. 2015, 54, 5519–5526.

(11) Nattestad, a; Mozer, a J.; Fischer, M. K. R.; Cheng, Y.-B.; Mishra, a; Bäuerle, P.;

Bach, U. Nat. Mater. 2010, 9, 31–35.

(12) Huang, Z.; Natu, G.; Ji, Z.; Hasin, P.; Wu, Y. J. Phys. Chem. C 2011, 115, 25109–

25114. 87

(13) Huang, Z.; Natu, G.; Ji, Z.; He, M.; Yu, M.; Wu, Y. J. Phys. Chem. C 2012, 116,

26239–26246.

(14) Ji, Z.; Natu, G.; Huang, Z.; Kokhan, O.; Zhang, X.; Wu, Y. J. Phys. Chem. C 2012,

116, 16854–16863.

(15) Beauchamp, D. R.; Click, K. A.; Garrett, B. R.; Huang, Z.; Hadad, C. M.; Wu, Y.

Phys. Chem. Chem. Phys. 2014.

(16) Liu, Z.; Xiong, D.; Xu, X.; Arooj, Q.; Wang, H.; Yin, L.; Li, W.; Wu, H.; Zhao,

Z.; Chen, W.; Wang, M.; Wang, F.; Cheng, Y.-B.; He, H. ACS Appl. Mater.

Interfaces 2014, 6, 3448–3454.

(17) TSUBOMURA, H.; MATSUMURA, M.; NOMURA, Y.; AMAMIYA, T. Nature

1976, 261, 402–403.

(18) Law, C.; Moudam, O.; Villarroya-Lidon, S.; O’Regan, B. J. Mater. Chem. 2012,

22, 23387.

(19) Jung, Y.-S.; Yoo, B.; Lim, M. K.; Lee, S. Y.; Kim, K.-J. Electrochim. Acta 2009,

54, 6286–6291.

(20) Xiang, W.; Huang, F.; Cheng, Y.-B.; Bach, U.; Spiccia, L. Energy Environ. Sci.

2013, 6, 121–127.

(21) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am.

Chem. Soc. 2006, 128, 14256–14257.

(22) Choi, H.; Baik, C.; Kang, S. O.; Ko, J.; Kang, M.-S.; Nazeeruddin, M. K.; Grätzel,

M. Angew. Chemie 2008, 120, 333–336.

(23) Kim, C.; Choi, H.; Kim, S.; Baik, C.; Song, K.; Kang, M.-S.; Kang, S. O.; Ko, J. J.

Org. Chem. 2008, 73, 7072–7079.

(24) Ito, S.; Miura, H.; Uchida, S.; Takata, M.; Sumioka, K.; Liska, P.; Comte, P.;

Péchy, P.; Grätzel, M. Chem. Commun. (Camb). 2008, 5194–5196.

(25) Yum, J.-H.; Hagberg, D. P.; Moon, S.-J.; Karlsson, K. M.; Marinado, T.; Sun, L.;

Hagfeldt, A.; Nazeeruddin, M. K.; Grätzel, M. Angew. Chem. Int. Ed. Engl. 2009,

48, 1576–1580.

(26) Li, R.; Lv, X.; Shi, D.; Zhou, D.; Cheng, Y.; Zhang, G.; Wang, P. J. Phys. Chem.

C 2009, 113, 7469–7479.

(27) Jiang, X.; Marinado, T.; Gabrielsson, E.; Hagberg, D. P.; Sun, L.; Hagfeldt, A. J.

Phys. Chem. C 2010, 114, 2799–2805.

(28) Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. J.

Am. Chem. Soc. 2010, 132, 16714–16724.

(29) Zhang, X.-H.; Wang, Z.-S.; Cui, Y.; Koumura, N.; Furube, A.; Hara, K. J. Phys.

Chem. C 2009, 113, 13409–13415.

(30) Tian, H.; Jiang, X.; Yu, Z.; Kloo, L.; Hagfeldt, A.; Sun, L. Angew. Chemie 2010,

122, 7486–7489.

(31) Lin, L.-Y.; Tsai, C.-H.; Wong, K.-T.; Huang, T.-W.; Wu, C.-C.; Chou, S.-H.; Lin,

F.; Chen, S.-H.; Tsai, A.-I. J. Mater. Chem. 2011, 21, 5950.

(32) Jiang, X.; Karlsson, K. M.; Gabrielsson, E.; Johansson, E. M. J.; Quintana, M.;

Karlsson, M.; Sun, L.; Boschloo, G.; Hagfeldt, A. Adv. Funct. Mater. 2011, 21,

2944–2952.

(33) Yum, J.-H.; Holcombe, T. W.; Kim, Y.; Yoon, J.; Rakstys, K.; Nazeeruddin, M.

K.; Grätzel, M. Chem. Commun. 2012, 48, 10727.

(34) Bomben, P. G.; Borau-Garcia, J.; Berlinguette, C. P. Chem. Commun. (Camb).

2012, 48, 5599–5601.

(35) Yeh-Yung Lin, R.; Lin, H.-W.; Yen, Y.-S.; Chang, C.-H.; Chou, H.-H.; Chen, P.-

W.; Hsu, C.-Y.; Chen, Y.-C.; Lin, J. T.; Ho, K.-C. Energy Environ. Sci. 2013, 6,

2477.

(36) Lin, R. Y.-Y.; Chuang, T.-M.; Wu, F.-L.; Chen, P.-Y.; Chu, T.-C.; Ni, J.-S.; Fan,

M.-S.; Lo, Y.-H.; Ho, K.-C.; Lin, J. T. ChemSusChem 2015, 8, 105–113.

(37) Kimura, M.; Masuo, J.; Tohata, Y.; Obuchi, K.; Masaki, N.; Murakami, T. N.;

Koumura, N.; Hara, K.; Fukui, A.; Yamanaka, R.; Mori, S. Chem. - A Eur. J. 2013,

19, 1028–1034.

(38) Yeh-Yung Lin, R.; Wu, F.-L.; Chang, C.-H.; Chou, H.-H.; Chuang, T.-M.; Chu,

T.-C.; Hsu, C.-Y.; Chen, P.-W.; Ho, K.-C.; Lo, Y.-H.; Lin, J. T. J. Mater. Chem. A

2014, 2, 3092.

(39) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. J. Am. Chem.

Soc. 2008, 130, 8570–8571.

(40) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E. J. Photochem.

Photobiol. A Chem. 2008, 199, 1–7.

(41) Ji, Z.; Natu, G.; Huang, Z.; Wu, Y. Energy Environ. Sci. 2011, 4, 2818.

(42) States, U. S.; Gregg, B. A.; Chen, S.; Ferrere, S. 2003, 3019–3029.

(43) Wang, Q.; Zhang, Z.; Zakeeruddin, S. M.; Gra, M. 2008, 7084–7092.

(44) Environ, E.; Listorti, A.; Creager, C.; Sommeling, P.; Kroon, J.; Palomares, E.;

Fornelli, A.; Breen, B.; Barnes, P. R. F.; Durrant, J. R.; Regan, B. O. 2011, 3494–

3501.

(45) Yang, L.; Xu, B.; Bi, D.; Tian, H.; Boschloo, G.; Sun, L.; Hagfeldt, A.; Johansson,

E. M. J. J. Am. Chem. Soc. 2013, 135, 7378–7385.

(46) Feiler, L.; Langhals, H.; Polborn, K. Liebigs Ann. 1995, 1995, 1229–1244.

(47) Baffreau, J.; Ordronneau, L.; Leroy-Lhez, S.; Hudhomme, P. J. Org. Chem. 2008,

73, 6142–6147.

(48) Cremer, J.; Mena-Osteritz, E.; Pschierer, N. G.; Müllen, K.; Bäuerle, P. Org.

Biomol. Chem. 2005, 3, 985–995.

(49) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E. J. Photochem.

Photobiol. A Chem. 2008, 199, 1–7.

(50) Li, L.; Gibson, E. a; Qin, P.; Boschloo, G.; Gorlov, M.; Hagfeldt, A.; Sun, L. Adv.

Mater. 2010, 22, 1759–1762.

(51) He, M.; Ji, Z.; Huang, Z.; Wu, Y. J. Phys. Chem. C 2014, 140227080825006.

(52) M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. S.; M. dA. Robb, J. R.

Cheeseman, G. Scalmani, V. Barone, B. M.; G. A. Petersson, H. Nakatsuji, M.

Caricato, X. Li, H. P. H.; A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg,

M. H.; M. Ehara, K. Toyo, J. C. . and D. J. F. 2010.

(53) Nishida, J.; Masuko, T.; Cui, Y.; Hara, K.; Shibuya, H.; Ihara, M.; Hosoyama, T.;

Goto, R.; Mori, S.; Yamashita, Y. J. Phys. Chem. C 2010, 114, 17920–17925.

(54) Cui, Y.; Wu, Y.; Lu, X.; Zhang, X.; Zhou, G.; Miapeh, F. B.; Zhu, W.; Wang, Z.-

S. Chem. Mater. 2011, 23, 4394–4401.

(55) Shen, P.; Liu, Y.; Huang, X.; Zhao, B.; Xiang, N.; Fei, J.; Liu, L.; Wang, X.;

Huang, H.; Tan, S. Dye. Pigment. 2009, 83, 187–197.

(56) Kroeze, J. E.; Hirata, N.; Koops, S.; Nazeeruddin, M. K.; Schmidt-Mende, L.;

Grätzel, M.; Durrant, J. R. J. Am. Chem. Soc. 2006, 128, 16376–16383.

(57) Forneli, A.; Planells, M.; Sarmentero, M. A.; Martinez-Ferrero, E.; O’Regan, B.

C.; Ballester, P.; Palomares, E. J. Mater. Chem. 2008, 18, 1652.

(58) Kim, M.-J.; Yu, Y.-J.; Kim, J.-H.; Jung, Y.-S.; Kay, K.-Y.; Ko, S.-B.; Lee, C.-R.;

Jang, I.-H.; Kwon, Y.-U.; Park, N.-G. Dye. Pigment. 2012, 95, 134–141.

(59) Ji, Z.; Natu, G.; Huang, Z.; Wu, Y. Energy Environ. Sci. 2011, 4, 2818.

(60) Armor, J. N. Catal. Letters 2005, 101, 131–135.

(61) Turner, J. a. Science 2004, 305, 972–974.

(62) Davies, E. Chem. World 2011, 50–54.

(63) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch,

S.; Chorkendorff, I.; Nørskov, J. K. J. Am. Chem. Soc. 2005, 127, 5308–5309.

(64) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.;

Chorkendorff, I. Science 2007, 317, 100–102.

(65) Yan, Y.; Xia, B.; Xu, Z.; Wang, X. ACS Catal. 2014, 4, 1693–1705.

(66) Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F.

ACS Catal. 2014, 4, 3957–3971.

(67) Bonde, J.; Moses, P. G.; Jaramillo, T. F.; Nørskov, J. K.; Chorkendorff, I. Faraday

Discuss. 2009, 140, 219–231.

(68) Chang, Y. H.; Lin, C. Te; Chen, T. Y.; Hsu, C. L.; Lee, Y. H.; Zhang, W.; Wei, K.

H.; Li, L. J. Adv. Mater. 2013, 25, 756–760.

(69) Vrubel, H.; Merki, D.; Hu, X. Energy Environ. Sci. 2012, 5, 6136.

(70) Benck, J. D.; Chen, Z.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. ACS Catal.

2012, 2, 1916–1923.

(71) Ge, X.; Chen, L.; Zhang, L.; Wen, Y.; Hirata, A.; Chen, M. Adv. Mater. 2014, 26,

3100–3104.

(72) Tsai, C.; Abild-Pedersen, F.; Nørskov, J. K. Nano Lett. 2014, 14, 1381–1387.

(73) Najmaei, S.; Zou, X.; Er, D.; Li, J.; Jin, Z.; Gao, W.; Zhang, Q.; Park, S.; Ge, L.;

Lei, S.; Kono, J.; Shenoy, V. B.; Yakobson, B. I.; George, A.; Ajayan, P. M.; Lou,

J. Nano Lett. 2014, 14, 1354–1361.

(74) Huang, X.; Zeng, Z.; Bao, S.; Wang, M.; Qi, X.; Fan, Z.; Zhang, H. Nat. Commun.

2013, 4, 1444.

(75) Ge, P.; Scanlon, M. D.; Peljo, P.; Bian, X.; Vubrel, H.; O’Neill, A.; Coleman, J.

N.; Cantoni, M.; Hu, X.; Kontturi, K.; Liu, B.; Girault, H. H. Chem. Commun.

2012, 48, 6484.

(76) Cao, L. Nano Lett. 2014, 1–6.

(77) Yan, Y.; Xia, B.; Ge, X.; Liu, Z.; Wang, J. Y.; Wang, X. ACS Appl. Mater.

Interfaces 2013, 5, 12794–12798.

(78) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie,

Y. Adv. Mater. 2013, 25, 5807–5813.

(79) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan,

B.; Xie, Y. J. Am. Chem. Soc. 2013, 135, 17881–17888.

(80) Gao, M.-R.; Chan, M. K. Y.; Sun, Y. Nat. Commun. 2015, 6, 7493.

(81) Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S.

O. Nano Lett. 2014, 14, 1228–1233.

(82) Chen, Z.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T.

F. Nano Lett. 2011, 11, 4168–4175.

(83) Yan, Y.; Ge, X.; Liu, Z.; Wang, J.-Y.; Lee, J.-M.; Wang, X. Nanoscale 2013, 5,

7768–7771.

(84) Murugesan, S.; Akkineni, A.; Chou, B. P.; Glaz, M. S.; Vanden Bout, D. a.;

Stevenson, K. J. ACS Nano 2013, 7, 8199–8205.

(85) Wang, T.; Zhuo, J.; Du, K.; Chen, B.; Zhu, Z.; Shao, Y.; Li, M. Adv. Mater. 2014,

26, 3761–3766.

(86) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Nano Lett.

2013, 13, 1341–1347.

(87) Jaramillo, T. F.; Bonde, J.; Zhang, J.; Ooi, B.-L.; Andersson, K.; Ulstrup, J.;

Chorkendorff, I. J. Phys. Chem. C 2008, 112, 17492–17498.

(88) Kristensen, J.; Zhang, J.; Chorkendorff, I.; Ulstrup, J.; Ooi, B. L. Dalton Trans.

2006, 3985–3990.

(89) Karunadasa, H. I.; Chang, C. J.; Long, J. R. Nature 2010, 464, 1329–1333.

(90) Kibsgaard, J.; Jaramillo, T. F.; Besenbacher, F. Nat. Chem. 2014, 6, 248–253.

(91) Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J.

Science 2012, 335, 698–702.

(92) Hou, Y.; Abrams, B. L.; Vesborg, P. C. K.; Björketun, M. E.; Herbst, K.; Bech, L.;

Setti, A. M.; Damsgaard, C. D.; Pedersen, T.; Hansen, O.; Rossmeisl, J.; Dahl, S.;

Nørskov, J. K.; Chorkendorff, I. Nat. Mater. 2011, 10, 434–438.

(93) Devices, W. S. W.; Mckone, J. R.; Lewis, N. S.; Gray, H. B. Chem. Mater. 2014.

(94) Morales-Guio, C. G.; Tilley, S. D.; Vrubel, H.; Grätzel, M.; Hu, X. Nat. Commun.

2014, 5, 3059.

(95) Krawicz, A.; Yang, J.; Anzenberg, E.; Yano, J.; Sharp, I. D.; Moore, G. F. J. Am.

Chem. Soc. 2013, 135, 11861–11868.

(96) Chakrabarty, P. K.; Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1992, 31,

3573–3577.

(97) Gushchin, a. L.; Laricheva, Y. L.; Alferova, N. I.; Virovets, a. V.; Sokolov, M. N.

J. Struct. Chem. 2013, 54, 752–758.

(98) Schlemper, E. O.; Schrauzer, G. N.; Hughes, L. A. Polyhedron 1984, 3, 377–380.

(99) Chakravorti, M. C.; Ganguly, S.; Bhattacharjee, M. Polyhedron 1993, 12, 55–58.

(100) Zhu, W.; Li, H.; He, X.; Shu, H.; Yan, Y. J. Chem. Res. 2006, 2006, 774–775.

(101) Brito, J. A.; Gomez, M.; Muller, G.; Teruel, H.; Clinet, J.-C.; Dunach, E.; Maestro,

M. A. Eur. J. Inorg. Chem. 2004, 2004, 4278–4285.

(102) Carrasco, C. J.; Montilla, F.; Alvarez, E.; Mealli, C.; Manca, G.; Galindo, A.

Dalton Trans. 2014, 43, 13711–13730.

(103) Conte, V.; Floris, B. Dalton Trans. 2011, 40, 1419–1436.

(104) Dickman, M. H.; Pope, M. T. Chem. Rev. 1994, 94, 569–584.

(105) Capello, C.; Fischer, U.; Hungerbühler, K. Green Chem. 2007, 9, 927.

(106) Gamelas, C. a.; Gomes, A. C.; Bruno, S. M.; Almeida Paz, F. a.; Valente, A. a.;

Pillinger, M.; Romão, C. C.; Gonçalves, I. S. Dalt. Trans. 2012, 41, 3474.

(107) Chakrabarty, P. K.; Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1992, 2, 3573–

3577.

(108) Zeng, L.; Zheng, C.; Deng, C.; Ding, X.; Wei, M. ACS Appl. Mater. Interfaces

2013, 5, 2182–2187.

(109) Müller, A.; Jostes, R.; Jaegermann, W.; Bhattacharyya, R. Inorganica Chim. Acta

1980, 41, 259–263.

(110) Feng, Z.; Velari, S.; Cossaro, A.; Castellarin-cudia, C.; Verdini, A.; Vesselli, E.;

Dri, C.; Peressi, M.; Vita, A. De; Comelli, G. 2015.

(111) Conway, B. E.; Tilak, B. V. Electrochim. Acta 2002, 47, 3571–3594.

(112) Kotrel, S., Brauninger, S. Handbook of Heterogeneous Catalysis; WILEY-VCH

Verlag GmbH & Co.: Weinheim, 2008.