AN INVESTIGATION OF PBI/PA MEMBRANES FOR APPLICATION IN PUMP

CELLS FOR THE PURIFICATION AND PRESSURIZATION OF HYDROGEN

By

TYLER J PETEK

Submitted in partial fulfillment of the requirements

For the degree of Master of Science

Thesis Advisor: Dr. Robert F. Savinell

Department of Chemical Engineering

CASE WESTERN RESERVE UNIVERSITY

January 2012

CASE WESTERN RESERVE UNIVERSITY

SHOOL OF GRADUATE STUDIES

We hereby approve the thesis of

Tyler J Petek

Candidate for the Master of Science degree.

Robert F. Savinell

(Chair of the committee)

Jesse Wainright

(Committee Member)

Chung-Chiun Liu

(Committee Member)

06 October 2011

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Table of Contents List of Tables ...... iv List of Figures ...... v Acknowledgements ...... xiii List of Abbreviations ...... xiv Nomenclature ...... xvi Abstract ...... xx Chapter 1: Introduction ...... 1 1.1 Current U.S. Energy Situation: “A Hydrocarbon Economy” ...... 1 1.1.1 Where Our Energy Comes From ...... 1 1.1.2 Environmental Impacts of a Hydrocarbon Economy ...... 2 1.2 A Hydrogen Economy ...... 4 1.2.1 Hydrogen as a fuel ...... 4 1.2.2 Results of a Hydrogen Economy ...... 8 1.2.3 Technologies for Producing Hydrogen ...... 10 1.3 Technical Challenges to Sustaining a Hydrogen Economy ...... 14 1.3.1 Cost of Hydrogen Production ...... 15 1.3.2 Hydrogen Storage ...... 15 1.3.3 Hydrogen Distribution ...... 16 1.3.4 End-Use Hydrogen Requirements ...... 16 1.3.5 Hydrogen Purification Techniques ...... 18

1.4 How H2 pump cells could help to create a hydrogen economy ...... 22 Chapter 2: Literature Review ...... 24 2.1 Current Hydrogen Pump Cell Systems ...... 24 2.1.1 Basic function and motivation of hydrogen pump systems ...... 24 2.1.2 Traditional hydrogen pump cells ...... 25 2.2 PBI/PA as a conductive hydrogen selective membrane ...... 30 2.2.1 Conductivity of phosphoric acid ...... 31 2.2.2 Doping PBI with phosphoric acid ...... 33 2.2.3 PBI/PA conductivity and tensile strength ...... 35 2.2.4 PBI/PA tolerance of impurities ...... 37 2.3 PBI/PA hydrogen pump cells...... 38 2.4 Characterization and durability limits of PBI/PA Pump Cell ...... 40

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Chapter 3: Experimental Methodology ...... 42 3.1 Theoretical Considerations ...... 42 3.1.1 Faraday’s Law ...... 42 3.1.2 Nernst Potential ...... 43 3.1.3 Overpotentials associated with the polarization curves ...... 44 3.1.4 Impedance Spectroscopy ...... 49 3.2 Laboratory Material and Equipment ...... 56 3.2.1 BASF PBI MEAs ...... 56 3.2.2 Fuel Cell Hardware and Assembly ...... 56 3.2.3 Fuel Cell Technology’s Fuel Cell Test Stand ...... 58 3.2.4 Solartron ...... 60 3.2.5 Oscilloscope ...... 61 3.2.6 Gas Supplies...... 61 3.3 Experimental Techniques ...... 61 3.3.1 Polarization Curves ...... 62 3.3.2 Pressure Tests...... 63 3.3.3 Impedance Spectroscopy Scans ...... 64 3.3.4 Long Term Constant-Current Tests ...... 66 Chapter 4: Results and Analysis ...... 67 4.1 Characterization of a pump cell with a BASF PBI/PA MEA ...... 67 4.1.1 Temperature effects on hydrogen pump performance without feed impurities67 4.1.2 Ability of pump cell to purify hydrogen rich stream ...... 69 4.1.3 Modeling of pump cell polarization effects at 150°C ...... 72 4.1.4 Ability of a pump cell to pressurize hydrogen product ...... 87 4.1.5 Performance durability and limits ...... 91 4.2 Application analysis of a pump cell with a BASF PBI/PA MEA...... 102 4.2.1 Energy estimates for purifying and pressurizing hydrogen ...... 102 4.2.2 Comparison of the pump cell with conventional technologies ...... 105 Chapter 5: Understanding proton transport in a Phosphoric Acid Electrolyte ...... 108 5.1 Phosphoric acid equilibrium ...... 108 5.2 Proton transport in a phosphoric acid electrolyte ...... 108 5.3 Developing a simplified model ...... 110 5.4 Description of possible initial and boundary conditions...... 113 5.5 Qualitative description and ramifications ...... 117

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Chapter 6: Conclusions and Future Recommendations ...... 119

6.1 Ability of a PBI/PA pump cell to purify and pressurize H2 streams ...... 119 6.2 Durability limits of a PBI/PA pump cell...... 120 6.3 Recommendations for future work ...... 121 Appendix A – BASF PBI/PA MEA 6 Life ...... 123 Appendix B – BASF PBI/PA MEA 8 Life ...... 125 Bibliography ...... 130

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List of Tables

Table 1.1: A summary of a select number of fuel cell types [13] Page 7

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List of Figures

Figure 2.1: Schematic of hydrogen pumping in a solid polymer electrolyte cell. [66] Page 24

Figure 2.2: Polarization curves of hydrogen pump cells with different MEAs operated with zero pressure differential across the cell. [67] Page 26

Figure 2.3: Nitrogen content of purified hydrogen vs current density for a cell operated at 25°C and Δp of 170 psi. The feed stream was 10% H2 in balance N2. [66] Page 27

Figure 2.4: The diffusion of hydrogen across membranes of different thicknesses (back diffusion). [67] Page 28

Figure 2.5: Polarization characteristics of pump cells at 25°C at varying pressures and membrane thickness. [66] Page 29

Figure 2.6: Polarization curves of a single pump cell and a single fuel cell with the resulting operating polarization curve. [69] Page 30

Figure 2.7: The conductivity of diluted phosphoric acid at different temperatures. [71] Page 32

Figure 2.8: The repeat unit of Poly(2,2`-(m-phenylene)-5-5`-bibenzimidazole) [73,74] Page 33

Figure 2.9: Doping level of PBI/PA membranes as a function of the doping phosphoric acid concentration at room temperature. [74] Page 35

Figure 2.10: The conductivity of PBI/PA membranes as a function of the doping level at 25°C (○) and150°C (□). The relative humidity was 80-85%. [77] Page 35

Figure 2.11: The tensile strength of PBI/PA membranes as a function of the doping level at 25°C (○) and150°C (□). [77] Page 36

Figure 2.12: The conductivity of H3PO4 and PBI/PA membranes at various doping levels at 20% RH. [76] Page 37

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Figure 2.13: The conductivity of PBI/PA membranes and Nafion® 117 as a function of relative humidity. The doping level and temperature are indicated in the figure. [74] Page 37

Figure 2.14: Polarization curves of a PBI-based PEMFC with pure hydrogen and hydrogen containing CO at 150°C. [77] Page 38

Figure 2.15: Polarization curves obtained for a hydrogen pump cell with PBI/PA membrane at 160°C and 1.2 stoichiometric flow rates. The solid squares represent un- humidified tests. Open circles represent tests with 3% RH. The cross hairs are for humidified Nafion® membrane at 70°C for comparison. [80] Page 39

Figure 3.1a: Overpotential contributions to pump cell with iL >> i. Page 49

Figure 3.1b: Overpotential contributions to pump cell with iL ≈ i. Page 49

Figure 3.2: The electrical analogue, or equivalent circuit, for a simple chemical reaction. Page 52

Figure 3.3: The anticipated equivalent circuit of a simple pump cell. Page 53

Figure 3.4: A Nyquist plot for an RC-circuit with one charge transfer loop. Page 54

Figure 3.5: Impedance vs frequency Bode plot for an RC-circuit with one charge transfer loop. Page 54

Figure 3.6: Phase angle vs frequency Bode plot for an RC-circuit with one charge transfer loop. Page 54

Figure 3.7: A Nyquist plot for an RC-circuit with two charge transfer loops. Page 55

Figure 3.8: Impedance vs frequency Bode plot for an RC-circuit with two charge transfer loops. Page 55

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Figure 3.9: Phase angle vs frequency Bode plot for an RC-circuit with two charge transfer loops. Page 55

Figure 3.10: Equivalent circuit for a pump cell with inductive effects considered. Page 55

Figure 3.11: Nyquist plot for a cell with two distinct electrodes and impedance effects Page 56

Figure 3.12: Impedance vs frequency Bode plot for a cell with two distinct electrodes and impedance effects Page 56

Figure 3.13: Phase angle vs frequency Bode plot for a cell with two distinct electrodes and impedance effects Page 56

Figure 3.14: An exploded view of the cell hardware. The Kapton heating pads are attached on the outside of the endplates. Page 59

Figure 3.15: A simple electrical schematic of the test stand and cell. Page 61

Figure 3.16: A schematic of the cell connections during impedance spectroscopy. Page 66

Figure 4.1: Polarization curves with on a pump cell with BASF PBI/PA MEA with pure hydrogen at 5% RH on both sides of the cell at varying temperatures. Page 68

Figure 4.2: The overall cell resistance, the slope of the polarization curve, of a pump cell with a BASF PBI/PA MEA vs. the cell operating temperature as described in figure 4.1. Page 69

Figure 4.3: The conductivity of phosphoric acid (100 wt%) versus temperature. [71] Page 70

Figure 4.4: Polarization curves with on a pump cell with BASF PBI/PA MEA with 70% H2 and 30% N2 on the anode and pure H2 on the cathode at varying temperatures. Both sides of the cell operated at 5% RH. Page 71

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Figure 4.5: The overall cell resistance, the slope of the polarization curve, of a pump cell with a BASF PBI/PA MEA vs. the cell operating temperature as described in figure 4.4. Page 71

Figure 4.6: Polarization curves with on a pump cell with BASF PBI/PA MEA with simulated reformate on the anode and pure H2 on the cathode at varying temperatures. Both sides of the cell operated at 5% RH. Page 72

Figure 4.7: A comparison of the slopes of the linear regions of the polarization curves under different anodic conditions. In all cases, the cathode has pure hydrogen and both anode and cathode operated at 5% RH. Page 73

Figure 4.8: Polarization curves on a pump cell with a BASF PBI/PA MEA at 150°C and 5% RH with the cathode under pure hydrogen. Page 74

Figure 4.9: Polarization curve on a pump cell with a BASF PBI/PA MEA with pure H2 at 5% RH on both sides. Page 74

Figure 4.10: Nyquist plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 at 5% RH on both sides. Page 75

Figure 4.11: Impedance vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 at 5% RH on both sides. Page 75

Figure 4.12: Phase angle vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 at 5% RH on both sides. Page 75

Figure 4.13: Nyquist plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 on one side and 5% H2 in balance argon on the other. Both sides are at 5% RH. Page 79

Figure 4.14: Impedance vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 at 5% RH on both sides. Both sides are at 5% RH. Page 79

Figure 4.15: Phase angle vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 at 5% RH on both sides. Both sides are at 5% RH. Page 79

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Figure 4.16: Electrical analogue of a cell with a distributed electrode. This analogue is referred to as a ‘ladder model’. Page 80

Figure 4.17: Nyquist plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 on one side and 70% H2 in balance nitrogen on the other. Both sides are at 5% RH. Page 81

Figure 4.18: Impedance vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 on one side and 70% H2 on the other side. Both sides are at 5% RH. Page 81

Figure 4.19: Phase angle vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 on one side and 70% H2 on the other side. Both sides are at 5% RH. Page 81

Figure 4.20: Polarization curve on a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH with 70% H2 and 30% N2 on the anode and pure H2 on the cathode. Page 83

Figure 4.21: Nyquist plots of pump cell with BASF PBI/PA MEA at 150°C and 5% RH with 5% H2 and simulated reformate on the anode with pure hydrogen on the cathode. Page 85

Figure 4.22: Impedance vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 on one side and 5% H2 on the other side as well as simulated reformate on one side and pure hydrogen on the other. All tests are at 5% RH. Page 85

Figure 4.23: Phase angle vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 on one side and 5% H2 on the other side as well as simulated reformate on one side and pure hydrogen on the other. All tests are at 5% RH. Page 85

Figure 4.24: Polarization curve on pump cell with BASF PBI/PA MEA at 150°C with simulated reform on the anode and pure hydrogen on the cathode at 5% RH with model. Page 88

Figure 4.25: Polarization curves on a pump cell with BASF PBI/PA MEA at 150°C and 5% RH with pure hydrogen on both sides at varying applied cathodic pressures. Page 89

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Figure 4.26: Polarization curves on a pump cell with BASF PBI/PA MEA at 150°C and 5% RH with simulated reformate on the anode and pure hydrogen on the cathode at varying applied cathodic pressures. Page 90

Figure 4.27: The model of the pump cell as developed by equation (4.12) operating at 150°C with gas humidification of 5% RH. The anode is under simulated reformate at atmospheric pressure and the cathode has pure hydrogen at varying pressures. Page 91

Figure 4.28: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant applied current density over 30 hours. The cell is at 150°C with 100% H2 on both sides at 5% RH for all the tests. Page 93

Figure 4.29: Polarization curves on the pump cell between the long term tests described in figure 4.28. Page 93

Figure 4.30: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant applied current density over 22 hours. The cell is at 150°C with 100% H2 on both sides at 5% RH for all the tests. All of these tests have the cathode on the right side of the cell. Page 95

Figure 4.31: Polarization curves on the pump cell between the long term tests described in figure 4.30. All of these tests have the cathode on the right side of the cell. Page 95

Figure 4.32: A simple schematic of the proposed mechanisms for proto transport across a PBI/PA membrane. Page 96

Figure 4.33: A simple schematic of the Grotthuss mechanism for proton transport in phosphoric acid. Page 96

Figure 4.34: Nyquist plots of pump cell with 100% hydrogen on both sides after each long term tests. All of the tests were at 150°C and 5% RH. Page 97

Figure 4.35: Impedance vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure hydrogen on both sides after each long term tests. All of the tests were at 150°C and 5% RH. Page 97

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Figure 4.36: Phase angle vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with pure hydrogen on both sides after each long term tests. All of the tests were at 150°C and 5% RH. Page 97

Figure 4.37: Nyquist plots of pump cell with 100% hydrogen on both sides after each long term tests. All of the tests were at 150°C and 5% RH. Page98

Figure 4.38: Impedance vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with 100% hydrogen on both sides after each long term tests. All of the tests were at 150°C and 5% RH. Page 98

Figure 4.39: Phase angle vs frequency Bode plot of pump cell with BASF PBI/PA MEA at 150°C with 100% hydrogen on both sides after each long term tests. All of the tests were at 150°C and 5% RH. Page 98

Figure 4.40: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant 2 applied current density of 0.20 A/cm over 22 hours. The cell is at 150°C with 100% H2 on the cathode and simulated reformate on the anode. The cathode is the right side of the cell. Page 100

Figure 4.41: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant 2 applied current density of 0.20 A/cm over 22 hours. The cell is at 150°C with 100% H2 on the cathode and simulated reformate on the anode. The cathode is the left side of the cell. Page 101

Figure 4.42: Nyquist plot of pump cell with simulated reformate on one side before and after each long term tests. All of the tests were at 150°C and 5% RH. Page 102

Figure 4.43: Impedance vs frequency Bode plot of pump cell with simulated reformate on one side before and after each long term tests. All of the tests were at 150°C and 5% RH. Page 102

Figure 4.44: Phase angle vs frequency Bode plot of pump cell with simulated reformate on one side before and after each long term tests. All of the tests were at 150°C and 5% RH. Page 102

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Figure 4.45: Polarization curves conducted on cell with simulated reformate on the anode and pure hydrogen on the cathode before and after the long term tests under the same conditions. The cell was at 150°C and 5% RH. Page 102

Figure 4.46: The applied power necessary to produce pure hydrogen from simulated reformate by a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH at atmospheric pressure. Page 104

Figure 4.47: The rate of hydrogen produced from simulated reformate by a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH at atmospheric pressure. Page 104

Figure 4.48: The amount of hydrogen produced per applied Watt-hour from simulated reformate by a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH at atmospheric pressure. Page 105

Figure 4.49: The specific work and productivity of different pressure swing adsorption cycles with a feed composition of 50% product. [82] Page 107

Figure 4.50: The specific work and productivity of different pressure swing adsorption cycles with a feed composition of 90% product. [82] Page 107

Figure 5.1: A schematic of the proposed mechanisms for proton transport across a phosphoric acid electrolyte. Page110

Figure 5.2: A schematic of the Grotthuss mechanism for proton transport in phosphoric acid. Page 111

Figure 5.3: An example of the concentration distribution across the phosphoric acid electrolyte. Page 116

Figure 5.4: An example of the concentration distribution across the phosphoric acid electrolyte. Page 116

Figure 5.5: The schematic of the anticipated applied potential field across a pure phosphoric acid electrolyte. Page 117

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Acknowledgements

I would first like to thank my advisor, Professor Robert F. Savinell. From my introduction into this project all the way through the writing of this thesis, Professor

Savinell has always guided and pushed me to not only grow as an engineer but to become a better scientist.

Professor Jesse Wainright also deserves my utmost gratitude. He always took the time to discuss whatever question I had, whether it was about the theory behind something I was working on or simply why I couldn’t get something in lab to work.

A huge thanks must be given Mirko Antloga. He was always there to teach me how to assemble or wire anything in the lab (or to help turn off all the alarms!). Thank you so much for always being there to offer a word of encouragement and to help get the lab working.

I would be remiss to not thank Martin Hansen. Though we did not meet until the research for this project was near completion, Martin was a great lab mate, no matter how short a time he was here, from just keeping me company to teaching me Danish. He was a great help in the thesis writing process by returning each draft I sent him so very quickly with such detailed suggestions.

Lastly, I would like to extend my gratitude to the Chemical Engineering Department as a whole. The faculty, staff, and fellow students have always been such fun and have taken such good care of me over the last four years. I have been very blessed to get to know and spend so much time with all of you.

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List of Abbreviations

AC - alternating current

CCP - current collector plate

CFCs - chlorofluorocarbons

CH4 - methane

CO - carbon monoxide

CO2 - carbon dioxide

DC - direct current

DMAc - dimethylacetamide

EIA - U.S. Energy Information Administration

FFP - flow field plate

GWP - global warming potential

H2 - hydrogen

H2-ICEs - hydrogen internal combustion engines

HCs - hydrocarbons

HFCs - hydrofluorocarbons

ICEs - internal combustion engines

MEA - membrane electrode assembly

N2 - nitrogen

N2O - nitrous oxide

NOx - nitrous oxides

PA - phosphoric acid

PA/PBI RU - doping level, phosphoric acid groups per PBI repeat unit

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PBI - polybenzimidazole

PBI/PA - polybenzimidazole imbibed with phosphoric acid

PEMFCs - proton exchange, or polymer electrolyte, membrane fuel cell

PFCs - perfluorocarbons

PROX-CO - preferential oxidation of carbon monoxide

PSA - pressure swing adsorption

Pt - platinum

PTFE - polytetrafluoroethylene

RH - relative humidity sccm - standard cubic centimeters per minute

SF6 - sulfur hexafluoride

SMR - steam methanol reformer

SMR-WGS - steam methanol reformer used with a water gas shift reactor

TFA - trifluoroacetic acid

WGS - water gas shift

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Nomenclature

[ ] = concentration of species [ ] or [ ]

= electrode area 50 [ ]

= concentration of species j [ ] or [ ]

= concentration gradient of species j [ ] or [ ]

= capacitance of a capacitor [ ]

̅ = specific capacitance [ ]

= double layer capacitance [ ]

= diffusion coefficient of species j [ ]

= standard potential [ ]

= Nernst potential [ ]

= Faraday’s Constant 96,485 [ ] or [ ]

= enthalpy of reaction [ ]

= current density [ ]

= exchange current density [ ]

= anodic exchange current density [ ]

= cathodic exchange current density [ ]

= overall cell exchange current density [ ]

= exchange current density under condition i [ ]

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= limiting current density [ ]

= current density across a resistor [ ]

= current [ ]

= phasor current [ ]

= phasor current across a capacitor [ ]

= phasor current across an inductor [ ]

= phasor current across a resistor [ ]

= imaginary number, √

= equilibrium constant for equation i varies

= inductance of an inductor [ ]

̅ = specific inductance of an inductor [ ]

- = moles e per mole H2 2 [ ]

= moles of species j [ ]

= flow rate of species j [ ]

= flux of species j [ ]

= partial pressure at the anode [ ]

= partial pressure at the cathode [ ]

= power [ ]

= molar gas constant 8.314 [ ] or [ ]

= overall cell resistance [ ]

̃ = overall cell resistance for an area [ ]

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= ohmic resistance due to counter diffusion [ ]

= ohmic resistance due to contacts [ ]

= charge transfer resistance [ ]

= ohmic resistance due to electrodes [ ]

= ohmic resistance due to flow field plate [ ]

= ohmic resistance due to gas diffusion layer [ ]

= ionic resistances in an electrolyte [ ]

̃ = ionic resistances in an electrolyte for an area [ ]

= ohmic resistance due to membrane [ ]

̃ = resistance due to membrane for an area [ ]

= resistance of a resistor [ ]

̃ = resistance of a resistor for an area [ ]

= standard cm3 per minute [ ]

= time [ ]

= temperature [ ]

= ionic mobility of species j [ ] or [ ]

= solution velocity [ ]

= potential across a resistor [ ]

= phasor voltage [ ]

= phasor potential of a capacitor [ ]

= phasor potential of an inductor [ ]

= phasor voltage across a resistor [ ]

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= total potential across cell [ ]

( ) = time dependent variable

= concentration of added acid or base [ ] or [ ]

= time independent element, phasor

= fraction of feed that is desired product

= impedance [ ]

= impedance of a capacitor [ ]

= impedance of a resistor [ ]

= charge density of species j [ ]

= overall transfer coefficient

= anodic transfer coefficient

= cathodic transfer coefficient

= activation overpotential [ ]

= concentration overpotential [ ]

= ohmic overpotential [ ]

= total polarization overpotential [ ]

= partial pressure of H2 in condition i [ ]

= ionic conductivity of species j [ ] or [ ]

= applied potential field [ ]

= potential gradient across cell [ ]

= frequency [ ]

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An Investigation of PBI/PA Membranes for Application in Pump Cells for the Purification and Pressurization of Hydrogen

Abstract by TYLER J PETEK About 94 million tons of hydrogen is consumed annually as a chemical feedstock worldwide. Hydrogen is also a promising energy carrier. As both a chemical feedstock as well as a renewable energy carrier, very pure hydrogen is required. Electrochemical hydrogen pump cells operating at 120°C - 180°C with a polybenzimidazole membrane imbibed with phosphoric acid were investigated for the electrochemical purification and pressurization of hydrogen. In this device, dilute or impure hydrogen gas is oxidized at an anode, the proton product crosses the membrane to the cathode where they are reduced to form hydrogen gas. It was found that the pump cell could purify simulated reformate containing 3% CO. A cell voltage of 0.145 V, or 0.03 W/cm2, was required to operate the cell at 150°C and 0.2 A/cm2. This corresponds to 7.7 watt-hours per mole of hydrogen pumped which is 1,000 – 10,000 times smaller than conventional purification techniques such as pressure swing adsorption. It was also found that the electrochemical pump cells could purify and pressurize the simulated reformate to at least 20 psi above the inlet pressure with only one 50 cm2 lab prototype cell. With cell pressure differentials above 5 psi, there was an undetermined amount of hydrogen back diffusion across the membrane.

The ability of the pump cell to operate continuously over 20 hours was also investigated.

Operating at current densities above 0.4 A/cm2 resulted in a decrease in the membrane conductivity. The membrane conductivity could be recovered to some degree by reversing the electrodes (i.e. the anode became the cathode and vice versa). These effects are thought to be due to the phosphoric acid building up in the electrodes, reducing electrode performance and facilitating phosphoric acid loss from the membrane.

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Chapter 1: Introduction

1.1 Current U.S. Energy Situation: “A Hydrocarbon Economy”

The world consumed about 475 quadrillion (1015) Btu of energy in 2007. [1] While the

United States makes up about 4.6% of the world’s population [2], we consume about

21% and produce about 15% of the world’s energy. [1] With historically high energy prices, economic turmoil, and increasing concern for national security and the environment, the U.S. energy markets have been drawing increasing attention. As it is put in the EIA’s Annual Energy Outlook 2010, “Over the next few years, the key factors influencing U.S. energy markets will be the pace of the economic recovery, any lasting impacts on capital-intensive energy projects from the turmoil in financial markets, and the potential enactment of legislation related to energy and the environment.” [3]

The U.S. energy market has steadily increased to consume 96.4 quadrillion Btu in 2009 with over 81% being produced from fossil fuels and renewable energies and nuclear electric power each producing about 9%. Petroleum itself accounts for over 36% of our energy. [1] These numbers aptly demonstrate how dependent the United States is on hydrocarbons for energy. This is why our current economy is called “a hydrocarbon economy.” As we have all felt from the gas pump to the grocery store, this is not a stable economy and is subject to the influence of many factors such as politics, weather, and world economic turmoil.

1.1.1 Where Our Energy Comes From

During the middle part of the 1990s, the United States began importing more petroleum products than it was producing. Throughout the last half century, the lead supplier of

Tyler Petek | 2 petroleum into the United States changed seven times. It was not until almost 2000 that

Canada became and had stayed the highest supplier of petroleum. In 2009, Canada and

Mexico accounted for approximately 32% of the total U.S. petroleum imports. This foreign dependence has caused fluctuations in fuel prices of 30% and several “energy shocks” since the 1970s. [4] Canada is currently the top supplier of petroleum to the

United States with 2.5 million barrels imported per day. Following Canada are Mexico,

Venezuela, Saudi Arabia, Nigeria, and Iraq. [1]

Not only does the foreign petroleum market currently experience fluctuations but the

Hirsch Report, published in February of 2005, predicts that the world will experience peak oil production within the next 20 years. [5] A peaking of world oil production would cause prices and market volatility unlike anything yet experienced. A domestic alternative to our increasing dependence on foreign hydrocarbons would help to produce a safer and more stable economy with a more consistent energy source.

1.1.2 Environmental Impacts of a Hydrocarbon Economy

The major emissions from burning hydrocarbon based fuels in motor vehicles include carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOX), hydrocarbons

(HCS), lead, and particulates. Burning one gallon of gasoline will cause about 8.8 kg of

CO2 emissions. [6]

From 1990 through 2008, total emissions in United States rose about 15%, almost linearly, to about 7.05 billion metric tons carbon dioxide equivalent. Over 80% of the emissions are due to carbon dioxide. The rest of the emissions include gases such as methane, nitrous oxide, HFCs, PFCs, and SF6. [1] The carbon dioxide equivalence of a

Tyler Petek | 3 gas is found by multiplying the amount of the gas by its global warming potential (GWP).

For example, methane’s global warming potential is 25 for a 100 year span. Thus, 1 metric ton of methane is equal to 25 metric tons of carbon dioxide. [7] There have been a number of international agreements that aim to eliminate or limit the amount of harmful environmental factors that are being released. Two of the more notable agreements are the Montreal Protocol [8] and the Kyoto Protocol. [9]

The Montreal Protocol specifically targets chlorofluorocarbons (CFCs), halons, other fully halogenated CFCs, carbon tetrachloride, methyl chloroform, hydrochlorofluorocarbons, and other emissions harmful to the ozone layer. The hope was that by holding nations accountable to reducing their emissions of these compounds the ozone would recover by 2050. [8]

The Kyoto Protocol acts almost as an addition to the Montreal Protocol. It is a similar document set forth by the United Nations to hold nations accountable to reduce certain emissions. The Kyoto Protocol targets ‘greenhouse gases’, specifically carbon dioxide

(CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). Each nation specifies what percentage they will commit to reduce each emission from the base year of the document.

Depending upon the nation, this base year ranged from 1990 to 1995. [9]

Over 100 countries have acknowledged a global warming limit of a 2°C change from the pre-industrial levels. Meinshausen predicts that this 2°C limit is only achievable if the

CO2 equivalence in the atmosphere is kept to 400 ppm. Even at these suggested levels, he calls a 2°C limit only ‘likely’ achievable. [10]

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Due to the volatility of the foreign hydrocarbon market and the dangerous environmental effects of relying so heavily on a hydrocarbon economy, a domestic and more environmentally friendly energy alternative is required to ensure a safer and more stable economy with a more consistent energy source.

1.2 A Hydrogen Economy

The term “hydrogen economy” was first used during the 1970s energy crisis to describe an energy infrastructure based on hydrogen produced from non-fossil primary energy sources. [11] There is no universal definition of a hydrogen economy yet it remains the long-term goal of many nations. [12]

1.2.1 Hydrogen as a fuel

Hydrogen is the most abundant element in our universe. However, it does not naturally occur in its elemental form on earth. Therefore, it is an energy carrier, not an energy source. [13] Some energy is required to produce hydrogen, but it can be produced from domestic resources that are clean, diverse, and abundant. [14]

It is estimated that the world produced about 45 million tons of hydrogen in 2006. [15] In

2008, approximately 9 million tons of hydrogen was produced in the United States. [14]

On a world scale, 49% of the hydrogen produced went to ammonia manufacturing, 37% for petroleum refining, 8% for methanol production, and 6% for other miscellaneous small volume applications. [16] There are currently two main methods to convert hydrogen to useable energy, either by combustion or by electrochemical processes.

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Combustion

An advantage of hydrogen is that it is the lightest compound and has a very high energy to mass ratio. Hydrogen stores approximately 2.6 times more energy per unit mass than gasoline. However, due to the very low density of hydrogen at most temperatures, it needs about four times the volume of gasoline to store the same amount of energy. [17]

Despite the volume necessary, hydrogen has remained a fuel of significant interest.

Compared to natural gas and gasoline vapor hydrogen is much more flexible as a fuel with a larger range of flammability. Also, a stoichiometric hydrogen-air mixture has a miniscule ignition energy required, about 0.02 mJ. This is over ten times lower than the ignition energy required by gasoline and almost 15 times lower than that of natural gas.

[18] The combustion of hydrogen produces 241.82 kJ per mole of hydrogen at standard temperature and pressure. [19]

Hydrogen’s combustible potential can be used in internal combustion engines, commonly referred to as H2-ICEs. Hydrogen driven internal combustion engines operate under the same basic principles as regular gasoline driven ICEs: a fuel undergoes combustion and that applies a force directly against a component of the engine. When this component, usually a piston or turbine, moves the energy is converted into mechanical energy.

[20,21] It is proposed that these H2/Hybrid-ICE vehicles could be used to introduce hydrogen into the transportation market because the hybrid systems can use current hydrocarbon infrastructure. [22]

Besides the efforts made for domestic transportation using hydrogen, the U.S. government has funded hydrogen as a fuel for quite some time. For decades, NASA has

Tyler Petek | 6 relied upon combustion of hydrogen as a rocket fuel. The Centaur, Viking, Voyager, and

Apollo programs as well as other space shuttle vehicles have used hydrogen as the main fuel to deliver crew and cargo to space. The rocket engines of each shuttle flight burn about 500,000 gallons of liquid hydrogen and another 239,000 gallons are depleted by storage boil off and transfer operations. Besides rocket propellant, hydrogen has also been used to supply transportation, electrical power, and breathable oxygen for crew members. [23] In a similar fashion, the U.S. Department of Defense has been using hydrogen to power unmanned rockets. [13]

Electrochemical Conversion

Fuel cells are electrochemical devices that convert the electrochemical energy of hydrogen into electricity. One common example is the proton exchange membrane fuel cell. The electrochemical potential is converted into useful electricity by treating the fuel as described in equation (1.1) through equation (1.3).

Anode Reaction: (1.1)

Cathodic Reaction: (1.2)

Net Reaction: (1.3)

As seen in these equations, there is a cathode and an anode. The hydrogen is introduced on the anode where it is oxidized. This produces two protons and two electrons per mole of hydrogen gas. The electrons flow through an external load where the energy is useful while the protons travel through the electrolyte. The protons and electrons come together on the cathode where they react with oxygen according to equation (1.2) to form water.

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The net standard potential produced from this process is 1.23 V versus a standard hydrogen electrode.

This electrochemical process is quiet and produces no emissions besides water. Also, electrochemical reactions generate energy more efficiently than combustion reactions.

Current fuel cell efficiencies are in the 40% to 50% range. When combined heat and power applications are used, some systems boast efficiencies as high as 80%. [13]

Fuel cells are typically characterized by three things: their electrolyte, operating temperature range, and sensitivity to feed contaminants. Table 1.1 shows a summary of just a few types of fuel cells.

Table 1.1: A summary of a select number of fuel cell types [13]

Operating Sensitivity to Fuel Cell Electrolyte Temperature (°C) Hydrogen Purity

Proton Solid organic polymer: polyperfluorosulfonic acid or 60-100 High < 10 ppm CO Exchange Polybenzimidazole with 120-200 < few % CO Membrane phosphoric acid

Aqueous solution of potassium High sensitivity to Alkaline 90-100 hydroxide soaked in a matrix carbon dioxide Phosphoric Liquid phosphoric acid soaked 175-200 Sensitive to CO Acid in a matrix Low sensitivity to Liquid solution of lithium, CO, Hydrogen/carbon Molten monoxide mixtures sodium and/or potassium 600-1000 Carbonate can be used. CO2 is carbonates, soaked in a matrix required

Low sensitivity to Solid zirconium oxide to CO, Hydrogen/carbon Solid Oxide which a small amount of yttria 600-1000 dioxide/methane is added mixtures can be used

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1.2.2 Results of a Hydrogen Economy

“Hydrogen has the potential to solve two major energy challenges that confront America today: reducing dependence on petroleum imports and reducing pollution and greenhouse gas emissions.” [13] As this quote illustrates, the environmental effects and stability are the characteristics that will shape the future of the energy market.

Environmental

Hydrogen is potentially a zero-emission fuel. The only byproducts of the combustion of hydrogen and oxygen are water and heat. When hydrogen is combusted with air, as equation (1.4) shows, a certain amount of nitrous oxides will be emitted. Nitrous oxides are formed when the nitrogen in the air reacts with the oxygen at the high temperatures of hydrogen combustion. However, by controlling the ratio of gas feeds, the amount of nitrous oxide emissions can be reduced to a few parts per million. [21] This is as little as

1/200 that of diesel engines. [24]

( ) ( ) ( ) ( ) ( ) ( ) (1.4)

Fuel cells are the desired end-use of hydrogen in a hydrogen economy. They are the most efficient method of energy conversion using hydrogen as a fuel. Fuel cells can be operated purely on hydrogen and oxygen coming into the electrochemical cell without the combustion effects of the H2-ICEs. Therefore, there are no emissions besides water and heat. When operating fuel cells on hydrogen and air, the only additional component in the product stream is nitrogen. If an economy can be developed based purely on hydrogen as a fuel source the net harmful environmental effect could effectively be reduced to zero.

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Political and Economic Stability

Hydrogen can be produced from a variety of very diverse sources including water, natural gas, coal, or even biological materials. With the vast number of sources from which hydrogen can be derived, hydrogen could be produced domestically and there would no longer be a reliance on foreign fuels.

Another benefit to being able to produce hydrogen from many resources is that hydrogen can be produced anywhere instead of being confined to locals such as refineries near oil reserves. The advantage this brings is the ability to have a distributed energy system. The energy source could be produced near the end-use allowing for a much more stable energy market. Geo-politics, weather, and other miscellaneous factors would no longer have an effect on the energy a consumer would be using at the end point. It is estimated that our current energy transmission and distribution system, one which has localized power plants that distribute energy over long distances, lost upwards of 7.2% of the distributed energy in 1995; about 60% was from the lines and 40% from the transformers.

[25] By producing energy near end-use location, these loses could be reduced to almost negligible amounts.

Safety

Hydrogen, despite all of its potential benefits, has gained a somewhat negative reputation as presented in the media and entertainment. As with any fuel source there are safety concerns that come with using hydrogen. However, these safety concerns are simply different, not necessarily worse, than fuels such as gasoline and natural gas.

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Again, hydrogen is the lightest compound. Because it is so light, it diffuses very rapidly in air: approximately 3.8 times faster than natural gas. Also, hydrogen rises at a speed of about 45 miles per hour (20 m/s). This is six times faster than natural gas. Unless hydrogen is trapped, it quickly disperses to non-flammable levels. Any gas besides oxygen can cause asphyxiation, but again, because of how quickly it disperses, it is difficult for asphyxiation from hydrogen to occur. [18]

If the hydrogen is caught or released in such a way that it is ignited, it produces a flame that has very low radiant heat. This is due to the absence of carbons in the flame and the production of heat absorbing water. Because of this, the risk of secondary fires is not as high as with conventional fuels. [18] This is best demonstrated by Dr. Swain of Miami

University. [26] He made a leak in the fuel line of a gasoline car and caused a leak in the hydrogen storage of a hydrogen car. The flame on the hydrogen car went straight up and burned cleanly. After about 90 seconds, all of the hydrogen had burned or dispersed away and the temperature inside the rear window of the car did not exceed 67°F. The gasoline car, on the other hand, was engulfed in flames after about 60 seconds and the car was completely destroyed. [26]

1.2.3 Technologies for Producing Hydrogen

Of the 45 million tons of hydrogen produced annually worldwide, it is approximated that about 48% is produced from reforming natural gas, 30% is produced from petroleum, and

18% is produced from gasification of coal. Only 4% is currently produced from water electrolysis. [17]

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Fossil Fuels

Steam reforming of natural gas is currently the most technically mature method of hydrogen production. [27] Steam reforming is a process that catalytically reforms the methane in natural gas using high temperature steam to produce hydrogen syngas. This syngas is composed mostly of CO and hydrogen. [28-30] This process can be applied to municipal organic waste, waste oils, sewage sludge, paper mill sludge, black liquor, refuse derived fuel, and organic waste. [28] The most common use of steam reforming is to reform the methane in natural gas to hydrogen, commonly referred to as SMR. [28]

SMR accounts for about 95% of the hydrogen production in the United States. [29]

In steam methane reforming systems, sulfur, if any, is first removed from the natural gas with zinc oxide or activated carbon. [30] Then high temperature steam, 500°C-950°C and

360 psi-435 psi, is passed through tubes typically filled with a nickel catalyst where the steam and methane undergoes the endothermic reforming reactions. [28,30-32] The reforming reaction is described in equation (1.5). [28,30-33]

( ) ( ) → ( ) ( ) (1.5)

Equation (1.6) happens simultaneously to the reforming reaction but the reforming reaction dominates. [33]

( ) ( ) → ( ) ( ) (1.6)

In this process, excess steam is usually supplied to keep the reforming catalyst from coking. The exit temperature form steam methane reforming is usually about 760°C. [33]

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If heavier hydrocarbons are used, such as ethane, propane, or butane, the reactions are similar and are described in equations (1.7) and (1.8). [33]

( ) ( ) ( ) → ( ) ( )

(1.7)

( ) ( ) ( ) → ( ) ( )

(1.8)

The syngas produced from SMR is typically about 70-75% H2, 2-8% CH4, 7-10% CO, and 6-14% CO2 on a dry basis. [27,34]

Biomass

Biomass contributes approximately 14% of the world’s primary energy, thus making it the fourth largest source of energy in the world. [28] Biomass typically contains about

6% hydrogen by weight. [28,31] Hydrogen can be produced from biomass through many techniques. These techniques include pyrolysis, anaerobic digestion, fermentation, metabolic processing, high-pressure supercritical conversion, gasification, and microbial fuel cells. [30,35]

Pyrolysis of biomass is a thermochemical process where the biomass degrades in the absence of air. This is usually done at elevated temperatures and pressures. Conventional pyrolysis occurs at low to intermediate temperatures. This process has slow heating rates and long residence times. This is preferable for biomass with higher charcoal content.

[31] Fast, or flash, pyrolysis has higher heating rates. For tars, the temperature is typically between 400°C - 500°C. For fast pyrolysis of gases, temperatures usually range from

600°C to 650°C. When pyrolysis occurs between 325°C and 575°C, the formation is

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mainly through acetic acid. This process forms hydrogen, CO, CO2, and a little methane.

Pyrolysis through propionic acid occurs around 625°C-825°C and forms hydrogen, CO,

CO2, and a little ethylene. The ethylene then reacts with the hydrogen to form ethane.

[31] Pyrolysis of corncob has a yield of about 22% at 750°C [30] and wheat straw has a

46% yield. [31] The typical product is 20% H2, 20% CO, 10% CO2, 5% CH4, and 45%

N2. [31]

Water

Hydrogen can be produced from water via either electrolysis or thermolysis. Regardless of the technology, the overall reaction remains the same.

(1.9)

Electrolysis is the technique of splitting water into hydrogen and oxygen by passing an electric current through water. [29,36,37] In an electrolyzer, oxygen is produced at the anode and hydrogen is produced at the cathode. All electrolyzers have to have a membrane separating the hydrogen and the oxygen. [37] The electricity used for electrolysis can come from any source: from fossil fuels, wind, solar, geothermal, hydroelectric, or nuclear. The two most common types of electrolyzers are the alkaline electrolyzer and the solid polymer electrolyte electrolyzer, or polymer exchange membrane (PEM) electrolyzer. [37]

There are over 115 unique thermochemical cycles to produce hydrogen and oxygen from water. [38] The single-step thermochemical dissociation of water is very simple and still follows equation (1.9). All that is required is a high-temperature source and to keep the hydrogen and oxygen separate. [30,39] For this process to be solely thermally driven, the

Tyler Petek | 14 temperature must be greater than 2500°C. [30] The thermal energy required to drive this reaction can be supplied from nuclear reactors, fossil fuels, or a variety of other sources.

At temperatures around 2000°C, only 4% of the water molecules dissociate according to equation (1.9). [36]

Besides using nuclear reactors to supply the thermal energy or electricity to drive themolysis or electrolysis of water, respectively, nuclear reactors can be used to drive the thermochemical dissociation of water. The two most common thermochemical cycles are sulfur-iodine and copper-chloride decomposition cycles. The efficiencies of these processes can range from 42-55%, which is promising when compared to the 35% efficiency of current electrolysis technology. [40,41]

1.3 Technical Challenges to Sustaining a Hydrogen Economy

Most of the hydrogen used in the world today comes from fossil fuels. These processes generate just as much if not more greenhouse gas emissions than traditional fossil fuel uses. [17] It is for this environmental reason that steam reforming of fossil fuels, including SMR, and partial oxidation of hydrocarbons are not long term solutions. For example, 11 tons of CO2 are produced for each ton of H2 produced from SMR. [36]

Hydrogen from oil refineries are just a bridge to the future hydrogen economy. [38] Only hydrogen produced from renewables, such as water from nuclear, solar, wind, geothermal, or hydroelectric, can fully produce a sustainable hydrogen economy. [27]

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1.3.1 Cost of Hydrogen Production

The U.S. Department of Energy has put out a number of targets for the final cost of hydrogen as a fuel to the consumer. The goal can be expressed as $2000 per ton of hydrogen [40] or as $2.00-$3.00 per gallon of gasoline equivalent. [42]

Current renewable energy resources are unreliable for continuous production due to the price of the feed source. [17] In SMR, the natural gas feedstock accounts for 52-68% of the final hydrogen price for larger plants and about 40% for smaller plants. [43] This means that changes in the price of natural gas will cause price drastic changes in the produced hydrogen price. Also, CO2 sequestering from the SMR process adds about 25-

30% of the cost. [12] Despite these cost hindrances, SMR remains the most economical and mature technology available. [44,45]

1.3.2 Hydrogen Storage

One of the most critical technical barriers for the hydrogen systems is hydrogen storage.

The typical performance criteria for hydrogen storage devices are gravimetric and volumetric densities. The ideal hydrogen storage device has a high volumetric and gravimetric hydrogen capacity. It also has fast absorption kinetics. The device would operate near room temperature and ambient pressure while still being made of low cost and low weight materials. [46] The most common current technologies for hydrogen storage options are compressed gas, liquid hydrogen storage and metal hydrides.

[47,48,49]

Compressed hydrogen gas is currently the best storage technology that is on the market as well as the most economical. [50] The advantages of compressed hydrogen gas are that it

Tyler Petek | 16 is reliable, easy to use, affordable, and has an indefinite storage time. [17] The major drawback to compressed hydrogen gas is the volumetric density. At about 5000 psi, hydrogen has a density of 23.5 g/L. This means that 6 kg is about 67.5 gallons; and this is just the gas alone. Also, compressing hydrogen gas to 5000 psi uses about 8.5% of the energy of the gas. [51]

1.3.3 Hydrogen Distribution

There is currently next to no hydrogen infrastructure in place in the United States. Today, there is less than 1200 miles of compressed hydrogen pipelines. [52] There are currently two types of hydrogen delivery systems under interest: hydrogen transmission and distribution. Hydrogen transmission model has the hydrogen production occurring at a central plant and then being transmitted to a single point. In a hydrogen distribution model, central hydrogen production plants distribute hydrogen to a network of refueling stations. [17] These would be driven by compressed pipelines, cryogenic tankers by road or rail, and tube trailers with compressed gas. [48,52,53]

1.3.4 End-Use Hydrogen Requirements

The two current technologies that use hydrogen as a fuel source that could drive our economy are hydrogen internal combustion engines (H2-ICE) and fuel cells.

Electrochemical conversion of hydrogen in fuel cells has significantly higher efficiencies than the combustion of hydrogen in H2-ICEs. The “end-game” hydrogen conversion device for a sustainable hydrogen economy will be a fuel cell.

Proton exchange membrane fuel cells (PEMFCs) show the most promise for transportation and small device applications. PEMFCs have low operating temperatures,

Tyler Petek | 17 quick start-up times, and solid membrane electrode assemblies that can withstand higher pressure operation. [54] These fuel cells follow equations (1.2) through (1.4). Because of this, PEMFCs generally perform best on pure hydrogen. [55]

Currently, the only economical technology for the production of hydrogen is the reforming of fossil fuels. Fossil fuels, however, are not a renewable resource. The most economical and mature renewable source of hydrogen currently available is from biomass. As previously discussed, the product gas from these systems contains a significant amount of impurities. Reforming of fossil fuels produces hydrogen with about

7-10% CO and 6-14% CO2 on a dry basis and hydrogen from biomass could contain as much as 20% CO and 10% CO2. These impurities can cause large polarization losses that reduce efficiency and power output in PEMFCs. [56] Carbon monoxide preferentially occupies the active sites on the platinum catalyst, thus allowing less hydrogen to react to produce energy. [54]

Conventional proton exchange membrane fuel cells have a perfluorosulfonic acid polymer membrane with platinum loaded electrodes. Fuel cells with these membrane electrode assemblies (MEAs) operate between 20°C and 80°C. [55] At these temperatures, CO concentrations as low as 2-100 ppm [54] or 10-20 ppm [55] have been reported to cause significant poisoning effects. Carbon dioxide is usually thought to just have dilution effects, lowering the Nernst-potential. [55] However, at higher concentrations of CO2, around 25%, and lower CO concentrations, around 20 ppm, CO2 exhibits more than just the dilution effect. It is believed that through the reverse water gas shift reaction, CO2 can result in a CO content that exceeds 10 ppm. [54,57]

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One method to reduce the effects of CO and CO2 poisoning is to raise the temperature.

This reduces the energy required to remove the impurities from the catalyst. Li et al. tested this with PEMFCs that used polybenzimidazole (PBI) membranes that are imbibed with phosphoric acid (PA). They found that PBI/PA MEAs in PEMFCs can tolerate 0.1%

CO at 125 °C up to 0.2 A/cm2 and 3% CO at 200°C up to 0.8A/cm2. They defined

‘tolerate’ as less than 10 mV polarization losses. [55] While 3% CO is significantly better than 25 ppm (0.0025%), conventionally produced hydrogen still has higher impurity content.

Despite their tolerance to impurities, PBI/PA fuel cells have a few obstacles yet to be overcome before becoming ideal end-use hydrogen devices. One of the major problems associated with phosphoric acid doped polybenzimidazole membrane fuel cells is the leaching of phosphoric acid. When submersed in water, it was found that the majority of the acid was leached in the first ten minutes. As equation (1.4) shows, the cathode of a polymer electrolyte fuel cell will always produce water. Therefore, leaching of the doped acid from PBI/PA membranes will always be an issue, especially at higher current densities and lower temperatures when water condenses. [58]

1.3.5 Hydrogen Purification Techniques

There are currently four main technologies for the purification of hydrogen to levels acceptable for fuel cells: preferential oxidation of CO (PROX-CO), water gas shift

(WGS), hydrogen selective membranes, and pressure swing adsorption (PSA). All of these are currently employed in industrial and commercial settings, usually in combination with each other.

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Preferential Oxidation of Carbon Monoxide

Equations (1.10) and (1.11) govern PROX-CO. [59,60]

( ) ( ) ( ) (1.10)

( ) ( ) ( ) (1.11)

Equation (1.10) is the desired reaction to reduce the amount of CO in the gas. Equation

(1.11) is highly undesired because it consumes the hydrogen, which is the desired fuel.

For PROX-CO to be viable, a highly active, stable, and selective catalyst is needed to drive equation (1.10) while limiting equation (1.11). Conventional catalysts are made of noble metals, such as gold, or metal alloys, such as CuO/Ce. This process is most efficient in the temperature range of 80-177°C. There are two main disadvantages to

PROX-CO. The process is highly exothermic and therefore needs to be cooled. Also,

PROX-CO is cost prohibitive for many applications due to the expensive catalysts used.

[59]

Water Gas Shift

One of the most common methods in industry to reduce the amount of carbon monoxide in a feed stream is with steam by utilizing the water gas shift (WGS) reaction. This is described by equation (1.12). [61,62]

( ) ( ) ( ) ( ) (1.12)

This reaction is exothermic and occurs at a range of temperatures depending upon the catalyst used. [63] With copper-zinc oxide catalysts, the WGS occurs between 190°C and

250°C. With promoted iron oxide catalysts, the temperature is 250°C - 450°C. Copper- zinc oxides can be modified with iron oxides to operate at temperatures of 275°C -

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350°C. Cobalt and molybdenum sulfides can also be used because they are sulfur tolerant. [61] At low temperatures, the equilibrium is driven towards the right side of equation (1.12). The higher the temperature, the more the equilibrium is shifter back to the left of equation (1.12).

Today, WGS reactors usually have a high temperature reactor and then a low temperature reactor in series. These two stage adiabatic reactors are commonly used to treat SMR effluent streams. The SMR-WGS systems produce gas streams that are typically 70-80%

H2, 15-25% CO2, 1-4% CO, and 3-7% CH4 on a dry basis. [60,63,64]

Pressure Swing Adsorption

When under pressure, gases can be forced to adsorb on solid surfaces. This is the main concept behind pressure swing adsorption (PSA). At higher pressures, more gas is adsorbed. When the pressure is reduced, the gas is released. Because different gases are attracted to different solid surfaces with different strengths, PSA can be used to selectively purify a gas stream. [59]

The feed gas to a PSA system is usually at 20°C – 40°C and at 100 psi – 400 psi. [63]

The feed will typically pass through three different adsorbent layers. The first is usually a guard bed composed of alumina or silica gel that is used to absorb water. The second is activated carbon that adsorbs CH4, CO, CO2, and trace amounts of sulfur. The third is composed of zeolites that improve CO and N2 adsorption. [64] A one column, 10 step

PSA system can produce 99.981% hydrogen with 36 ppm CO. [65] While this is much better than PROX-CO and WGS, it is not good enough for low temperature PEMFCs.

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Industry typically employs 4-12 adsorbers to get the required purity of hydrogen. These systems have a hydrogen recovery of 70-85%. The rest of the hydrogen comes off as a waste gas that can be used to supply the heat for the endothermic reactions involved in

PSA. [63] While these systems are good for large scale industrial settings, they are too big for any transportation applications that would enable on-board reforming. [59,65]

Selective Membrane Purifiers

Selective membrane purifies are mostly driven by the fact that hydrogen has a permeation that is ten times that of other gases in SMR-WGS products. Each of these types of membranes selectively diffuses hydrogen across it so that a pure hydrogen stream can be produced. There are three categories of hydrogen selective membranes: polymeric, metallic, and zeolite. Polymeric membranes have low energy consumption, are cost effective at low volumes, and do not have significant pressure drops. However, typical polymeric membranes have low mechanical strength and have significant swelling/compacting effects. Metallic membranes have excellent permeance and mechanical durability. The disadvantage to metallic membranes is that at low temperatures they experience hydrogen embrittlement. By making membranes out of alloys, embrittlement can be deterred, but these alloy materials are quite expensive.

Zeolite is another common membrane used for purifying hydrogen. Zeolite is a polycrystalline thin film supported on a strong and rigid porous substrate. Zeolites have good temperature stability and solvent resistance but they are not selective enough to reach the necessary purification. Also, they tend to be more expensive than other membrane options. [59,64]

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1.4 How H2 pump cells could help to create a hydrogen economy

It is thought via steam reforming with natural gas, hydrogen can be economically produced and this will drive the demand for hydrogen and the transition to a hydrogen economy. The reforming of hydrocarbons, however, produces just as much harmful emissions as direct combustion. It may be necessary to first increase greenhouse gas emissions in order to transition into a hydrogen economy. Once a hydrocarbon economy is established, the technology can catch up to meet the demand with a renewable and environmentally friendly source.

The current steam methane reactor (SMR) technology does not produce hydrogen at purity levels that can operated in fuel cells efficiently. Water gas shift (WGS) reactors have been used to reduce the levels of CO, the most detrimental impurity, to 1-4%.

However, the levels of CO are still not low enough to make a hydrogen economy sustainable.

Proton exchange membrane made of Polybenzimidazole (PBI) imbibed with phosphoric acid (PA) have been shown to be operational with hydrogen feed streams containing up to 3% CO. These PBI/PA membrane electrode assemblies (MEAs) could be used as hydrogen selective membranes that could purify the hydrogen rich streams produced from SMR-WGS reactors.

Due to the current technical issues associated with operating PBI/PA membranes in fuel cells long-term, namely leaching of acid in the presence of water, these fuel cell systems are not robust enough to serve in a hydrogen economy. By using PBI/PA membranes as

Tyler Petek | 23 hydrogen selective membranes to purify hydrogen, systems in which there is no water produced, then pure hydrogen can be delivered to conventional fuel cells.

The first objective of this thesis is to investigate the application of PBI/PA membranes to hydrogen pump cells to purify the effluent gas from SMR-WGS reactors. There would be many advantages to utilizing these PBI/PA MEAs. They would be smaller than PSA systems and produce purer hydrogen with less energy consumption. This should enable on-board reforming for transportation application of fuel cells.

The second objective of this thesis is to better understand the transport processes in the

PBI/PA system and the effect those processes have on operational limits such as current density, temperature, and length of operation. Obtaining a better understanding of how the PBI/PA membrane functions as a proton transport medium would help to better design the cell or operation cycles to enable longer operation under more cost effective conditions.

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Chapter 2: Literature Review

2.1 Current Hydrogen Pump Cell Systems

2.1.1 Basic function and motivation of hydrogen pump systems

Proton selective membranes for electrochemical hydrogen recovery systems have been studied since the late 1970’s. The pioneering work for electrochemical pump cells for purifying and pressurizing hydrogen was done by Sedlak et al. in 1980. [66] In this work,

Sedlak describes a cell where a stream containing hydrogen is supplied to an anode. The hydrogen is oxidized and the protons are transported to the cathode through a solid polymer electrolyte. At the cathode, the protons are reduced and a pure hydrogen stream is produced. This system is shown schematically in figure 2.1. In this system, the protons are driven through the membrane by the applied potential difference between the two electrodes and the electrons are driven through the electrically conductive elements from the anode to the cathode. The principle equations for the operation of a pump cell are investigated in detail in chapter 3.

Electrical Device

H2 - - e e - e e- H+

H2 H2 H2

H+

H2

Anode Membrane Cathode Figure 2.1: Schematic of hydrogen pumping in a solid polymer electrolyte cell.

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The solid polymer proton exchange electrolyte systems were first developed for fuel cell membranes. The electrodes in the pump cell systems were originally high levels of platinum, ~ 4 mg Pt per cm2, deposited on the membrane. [66] These were the same

MEAs used in proton exchange membrane fuel cells.

The motivation for these electrochemical hydrogen pump cell systems originally came from the need to separate hydrogen from other gases, such as nitrogen, that exist from the thermochemical production of hydrogen from water. Also, as fuel cells developed throughout the end of the twentieth century, pump cells were investigated for recycling unused hydrogen. [66,67,68,69]

2.1.2 Traditional hydrogen pump cells

As with fuel cells, the most common pump cell membrane is a perfluorocarbon sulfonate ion exchange membrane, commercially produced by DuPont as Nafion®. The proton transport across Nafion® is very dependent upon the amount of water present in the cell.

The membrane must be hydrated in order to have any appreciable proton conductivity.

Because of this, the pump cells operated with Nafion® membranes must be operated between the freezing and boiling points of water. The most common operating temperature range of these cells is between 25°C and 80°C.

The most common representation of a pump cell’s operation is through polarization curves, as shown in figure 2.2. These show the cell voltage over a range of current densities. Ströbel et al. show the polarization curves of hydrogen pump cells operated with no pressure differential across the cell with different MEAs. [67] Each of these

MEAs have ion exchange membranes based on perfluorosulfonic acid.

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Figure 2.2: Polarization curves of hydrogen pump cells with different MEAs operated with zero pressure differential across the cell. [67]

The E-TEK MEAs had fairly thick membranes. These MEAs tend to have large resistances and transportation limitations resulting in lower limiting currents. The Gore

MEA had a thin membrane that showed lower resistances but were not stable at higher pressure operation. The ZSW MEAs had membranes based upon Nafion® that showed lower resistances and tested well at higher pressure operations. [67]

The polarization curves presented in figure 2.2 do well to show the energy requirements of the cell at an operating current. However, the composition of the product is unreported.

Besides the electrochemical transport of protons, there is some diffusion of other constituents across the membrane. For feed streams of 10% hydrogen in nitrogen in a pump cell operated at 25°C with a pressure differential across the cell of 170 psi, Sedlak et al. show the percent nitrogen in the product stream as a function of the applied current density. [66] This is shown in figure 2.3 and is indicative of nitrogen diffusing across the membrane. They found about 0.2% of the product stream at low current densities to be nitrogen. The nitrogen content of the product decreased exponentially with increasing

Tyler Petek | 27 current density. Faraday’s law describes the flow of hydrogen to be linearly dependent on the current density. Therefore, the exponential decrease in nitrogen content is due to more than just an increasing amount of hydrogen in the product.

Figure 2.3: Nitrogen content of purified hydrogen vs current density for a cell operated at 25°C and Δp of 170 psi. The feed stream was 10% H2 in balance N2. [66]

Sedlak et al. have calculated the hydrogen diffusion losses for the conditions of the cell in figure 2.3. They have shown that the diffusion of hydrogen and nitrogen across the cell follow Arrhenius relationship with temperature. As the temperature increases, the diffusion of nitrogen and the back diffusion of hydrogen decrease. The back diffusion of hydrogen has been associated with 1 – 20 mA/cm2 over a temperature range of 5 - 80°C at pressure differentials of 25 psi. [66]

The diffusion of hydrogen and other gases across the membrane has been shown in the literature to occur and to cause product contamination as well as increased energy costs.

In an attempt to better investigate the diffusion across the membranes, Ströbel et al. calculated the diffusion of hydrogen across membranes of different thicknesses. [67] The

Tyler Petek | 28 error bars in figure 2.4 are due to the swelling of the membrane at different membrane humidification.

Figure 2.4: The diffusion of hydrogen across membranes of different thicknesses (back diffusion). [67]

Thus far, the literature has shown that thicker membranes have higher ionic resistances and lower diffusion rates. The higher ionic resistances of thicker membranes result in higher cell voltages. The optimal membrane thickness will have to take the effects of both diffusion and resistance into account. Operating temperature has shown to have a significant effect on the constituent diffusion across the membrane. Higher temperatures yield lower cross-over, therefore, purer products.

Besides operating temperature, the cell can also be operated at different pressure gradients across the cell. This is advantageous because hydrogen at higher pressures yields more efficient fuel cell operation and hydrogen storage. Rohland et al. have investigated the back diffusion of hydrogen at both 24°C and 70°C at different pressure gradients across the cell. [68] The pressure gradient is the difference between the pressure

Tyler Petek | 29 of the cathode and the anode. At higher cell pressure gradients, an increase in the hydrogen back diffusion is observed.

Sedlak et al. have measured polarization curves at different cell pressure gradients and the results are shown in figure 2.5. [66] These experiments show an increase in the required cell voltage with an increase in the output pressure.

◊ ⟶ p p i, cm memb ne ○ ⟶ p p i, cm memb ne □ ⟶ p p i, cm memb ne ⟶ p p i, cm memb ne

Figure 2.5: Polarization characteristics of pump cells at 25°C at varying pressures and membrane thickness. [66]

Barbir et al. have done an analysis on the pump cell technology in light of current fuel cell advancements. [69] Figure 2.6 shows the polarization curves of a pump cell and fuel cell with the resulting operating polarization curve. Barbir et al. investigated the effects of adding a pump cell to a ten fuel cell stack. Using different placement of the pump cell

Tyler Petek | 30 in the stack and different feed flow schemes, they were able to increase the fuel utilization to almost 100%.

Figure 2.6: Polarization curves of a single pump cell and a single fuel cell with the resulting operating polarization curve. [69]

2.2 PBI/PA as a conductive hydrogen selective membrane

The literature discussed thus far has characterized the operation of hydrogen pressurization and purification via a hydrogen pump cell. As with fuel cells, the standard membrane is based on perfluorosulfonic acid. This type of membrane is limited to the temperature range of 25°C to 80°C because the sulfonic acid ionomers depends on the presence of water to solvate the protons in order to be proton conductive. [70] The typical catalyst used in pump cell MEAs, as in fuel cells, is platinum. At these operating temperatures certain constituents, such as CO, preferentially adsorb to the platinum over hydrogen, causing the cell potential to increase significantly. These poisoning effects are seen at CO levels as low as a few ppm.

Due to the electrode poisoning effects, pump cells with perfluorosulfonic based membranes can only work with feed streams composed of hydrogen and inert diluents

Tyler Petek | 31 such as nitrogen. Because of this, they can only be used to purify thermolysis effluents or pressurize already purified hydrogen streams. As discussed in chapter 1, a pump cell that can produce purified and pressurized hydrogen from the product of steam methane reformers coupled with water gas shift reactors, streams containing about 3% CO, would be most useful.

It has been found that operating at higher temperatures allows the platinum catalyst to tolerate higher levels of CO before significant poisoning effects are observed. At temperatures between 120°C and 180°C, platinum catalysts can tolerate as high as 4%

CO in the feed stream. [73,74] The perfluorosulfonic acid based membranes loose significant conductivity at these temperatures. Therefore, a proton selective ion exchange membrane that can operate at these temperatures would allow for a pump cell that could operate with the reformed methane gas product.

2.2.1 Conductivity of phosphoric acid

Phosphoric acid has been used as a proton selective electrolyte in phosphoric acid fuel cells for years. Chin et al. have measured the conductivity of phosphoric acid under a variety of conditions. [71] They found the conductivity of pure (100 wt%) phosphoric acid to increase with a near linear relationship with temperature.

Chin et al., amongst other things, also investigated the effect of the dilution of phosphoric acid. Figure 2.7 shows how the wt% of phosphoric acid affects the electrolyte conductivity at various temperatures. As the figure shows, the maximum conductivity at each temperature exists at some concentration of phosphoric acid that is less than 100%.

This maximum conductivity shifted towards higher concentration phosphoric acid at

Tyler Petek | 32 higher temperatures. Chin et al. suggest that this is due to the increasing contribution of the Grotthuss mechanism in addition to the vehicular transport. These transport mechanisms will be discussed in more detail in chapters 4 and 5. As the concentration of phosphoric acid increases beyond the maximum conductivity, the freedom of the phosphoric acid groups to rotate is limited, causing a decrease in the Grotthuss mechanism contribution. [71]

Figure 2.7: The conductivity of diluted phosphoric acid at different temperatures. [71]

Phosphoric acid has been shown to be a very suitable electrolyte for hydrogen pumps.

However, cells with liquid electrolytes are not nearly as mechanically robust as cells with solid electrolytes. A membrane that could be imbibed with phosphoric acid at these temperatures should give a pump cell the mechanical durability of the solid electrolyte

Tyler Petek | 33 with the conductivity of phosphoric acid that could tolerate higher levels of poisoning constituents than traditional pump cells.

2.2.2 Doping PBI with phosphoric acid

Savinell et al. first investigated phosphoric acid imbibed in Nafion® in 1994. [70] They found that the membranes were conductive up to 200°C. However, the conductivity decreased significantly with increasing temperature. At 180°C under a N2/H2O environment, the membrane had a conductivity of only 0.05 S/cm. A membrane that is more stable in the temperature range of 120°C – 180°C is desired for higher conductivity.

Wainright et al. were the first to investigate doping polybenzimidazole (PBI) with phosphoric acid in 1995. [72] Since then, polybenzimidazole imbibed with phosphoric acid (PBI/PA) has gained significant interest, especially for fuel cell application. [71-79]

Polybenzimidazole is a long chain polymer known for its good oxidative and thermal stability. PBI fibers, for example, are the main material used in the production of firefighter suits. [72] The PBI chain most commonly imbibed with phosphoric acid is

Poly(2,2`-(m-phenylene)-5-5`-bibenzimidazole). [73,74] The repeat unit of this polymer is shown in figure 2.8.

Figure 2.8: The repeat unit of Poly(2,2`-(m-phenylene)-5-5`-bibenzimidazole) [73,74]

Tyler Petek | 34

There are two main methods for imbibing PBI membranes with phosphoric acid. The first method is by casting a PBI membrane from solvent, such as DMAc or TFA, and then immersing the membrane in phosphoric acid. Different processes have been proposed for doping by immersion. This doping step can use phosphoric acid up at concentration up to

15 molar. The immersion time can be anywhere from 15 hours to one week. [72-79] The other method presented for doping PBI membranes with phosphoric acid is to directly cast the membrane from a PBI and phosphoric acid solution in a co-solvent. [75,76] From the different processes, doping levels of 1 – 16 moles of PA per mole of PBI repeat unit

(PA/PBI RU) have been obtained. [73]

Figure 2.9: Doping level of PBI/PA membranes as a function of the doping phosphoric acid concentration at room temperature. [74]

Maximum protonation of polybenzimidazole is reached at a doping level of two H3PO4 per PBI repeat unit. The protonation of PBI by PA occurs when the phosphoric acid group adds hydrogen onto the available nitrogen in the imidazole group. At doping levels above two PA/PBI RU leads to excess phosphoric acid in the membrane. As the doping level increases and the amount of free phosphoric acid in the membrane increases, the

Tyler Petek | 35 conductivity mechanisms become more like that of a pure phosphoric acid electrolyte.

[76] Figure 2.9 shows the doping level of phosphoric acid in PBI membranes as a function of the acid concentration. [74]

2.2.3 PBI/PA conductivity and tensile strength

The conductivity of PBI/PA membranes is a function of doping level, temperature, and relative humidity of the environment. The conductivity of PBI/PA membranes has been found to increase linearly with increasing doping levels at 25°C as well as 150°C. Li et al. have shown this in figure 2.10.

Figure 2.10: The conductivity of PBI/PA membranes as a function of the doping level at 25°C (○) and150°C (□). The relative humidity was 80-85%. [77] The benefit of increased conductivity at higher doping levels comes with the lower membrane tensile strength. The membrane tensile strength is shown to decrease exponentially with increasing doping levels by Li et al. in figure 2.11. A higher tensile strength would allow the membrane to operate at higher cell pressure gradients, allowing higher pressure product streams.

Tyler Petek | 36

Figure 2.11: The tensile strength of PBI/PA membranes as a function of the doping level at 25°C (○) and150°C (□). [77]

Ma et al. investigated the effect of different operating temperatures on the conductivity of

PBI/PA membranes. [76] They compared membranes of different doping levels over the temperature range of 20°C – 180°C. The conductivity of the membranes increases with increasing temperature. The conductivity of the PBI/PA membranes was found to be an order of magnitude smaller than pure phosphoric acid.

Figure 2.12: The conductivity of H3PO4 and PBI/PA membranes at various doping levels at 20% RH. [76]

Tyler Petek | 37

Figure 2.13: The conductivity of PBI/PA membranes and Nafion® 117 as a function of relative humidity. The doping level and temperature are indicated in the figure. [74]

Lastly, the conductivity of the PBI/PA membranes is a function of relative humidity.

Figure 2.13 shows the effects of relative humidity on the conductivity of the membranes.

[74] The conductivity of PBI/PA membranes has a linear dependence on the temperature over the temperature range of 80°C - 200°C. This is a much less severe dependence than

Nafion® membranes. Also, the conductivity of PBI/PA membranes is on the same order of magnitude as the Nafion® membranes but at much lower relative humidity.

2.2.4 PBI/PA tolerance of impurities

The benefit to membrane that can operate with high conductivity at temperatures above

100°C is that a higher concentration of CO, or other poisoning constituents, can be tolerated before saturating the platinum catalyst and then blocking the catalyst from oxidizing hydrogen. The ability of the cell to tolerate CO has been investigated by comparing polarization curves on fuel cells which employ MEAs using PBI/PA membranes. Li et al. investigated the CO poisoning effects on PBI/PA based MEAs with platinum catalysts in fuel cells. [77] Figure 2.14 shows polarization curves at 150°C with

Tyler Petek | 38 varying CO content in the anodic feed stream. The polarization curve showed decreasing cell power output with increasing CO content. An anodic feed of hydrogen with 3% CO had a voltage loss of about 100 mV at 1 A/cm2.

Figure 2.14: Polarization curves of a PBI-based PEMFC with pure hydrogen and hydrogen containing CO at 150°C. [77]

2.3 PBI/PA hydrogen pump cells

There has been little work published to date on hydrogen pump cells at high-temperatures using PBI/PA membranes. The only paper found on electrochemical hydrogen pumping with a PBI/PA membrane is by Perry et al. [80] Perry et al. investigated hydrogen pumping using a PBI/PA membrane over 120°C - 160°C and with pure hydrogen, methanol reformate, and natural gas reformates. Figure 2.15 shows operating polarization curves for a PBI/PA-based hydrogen pump cell.

It was found that the pump cell operated with very high current efficiency. The current efficiency of these cells was greater than 83% from 0 to 2 A/cm2. More specifically, above 0.4 A/cm2, current efficiencies greater than 90% were observed. [80]

Tyler Petek | 39

Perry et al. also investigated the long term operation of the pump cell with PBI/PA membrane. They found the voltage that was required to operate the cell at 0.2 A/cm2 and

1.2 stoichiometric flows at 160°C to be relatively constant at 22 mV for over 2000 hours of operation. This was confirmed with polarization curves. [80] No long term tests were performed at current densities above 0.2 A/cm2.

Figure 2.15: Polarization curves obtained for a hydrogen pump cell with PBI/PA membrane at 160°C and 1.2 stoichiometric flow rates. The solid squares represent un- humidified tests. Open circles represent tests with 3% RH. The cross hairs are for humidified Nafion® membrane at 70°C for comparison. [80]

The ability of these pump cells to purify reformate streams was also investigated by Perry et al. Two reformates were used: natural gas reformate (35.8% H2, 0.19% CO, 11.9%

CO2, and balance N2) and methanol reformate (1.03% CO, 29.8% CO2, and balance H2).

The effects of both of these feed streams was investigated between 0 – 0.4 A/cm2. The current efficiency fell linearly with increasing current density to about 90% at 0.4 A/cm2 at 160°C for both reformates. The effects of the poisoning constituents in the reformate

Tyler Petek | 40 streams were found to occur almost immediately and to be reversible with enough time under pure hydrogen. The product gas from the pump cell operated with the natural gas

2 reformates feed contained 11±1 ppm CO and 0.37±0.09% CO2 at 0.4 A/cm and 13±3

2 ppm CO and 0.19±0.02% CO2 at 0.8 A/cm . The small amount of CO and CO2 in the product is attributed to the diffusion of the constituents across the membrane. These tests were done at 160°C and no humidification. [80]

2.4 Characterization and durability limits of PBI/PA Pump Cell

The literature reported in detail investigations of the operational characteristics of a polybenzimidazole membrane imbibed with phosphoric acid as a highly conductive proton selective membrane at temperatures up to 200°C. However, most of this work was related to proton exchange membrane fuel cells and very little work has been done to characterize hydrogen pump cells. Hydrogen pump cells are worth investigating for their ability to purify and pressurize hydrogen rich feed streams in a simple, quiet, and very efficient manner.

It has been shown that PBI/PA membranes in hydrogen pump cells can purify feed streams from natural gas and methanol reformates to hydrogen rich streams containing less than 15 ppm CO and less than 0.4% CO2. The simulated natural gas and methanol reformates used in those studies contained only 0.19% CO and 1.03% CO, respectively.

It was reported in literature, however, that steam methanol reformates coupled with water gas shift reactors, the most common method for hydrogen production, produces hydrogen streams containing about 3% CO.

Tyler Petek | 41

There are still a number of aspects of operating a pump cell that have yet to be investigated. The pump cell with a PBI/PA membrane has been shown to be able to produce a sufficiently pure hydrogen stream. However, the reformates investigated had less than 3% CO, the amount reported of typical SMR-WGS products. Also, the long term operation of these pump cells was not investigated at current densities above 0.2

A/cm2. Lastly, the ability of the pump cells with PBI/PA membranes to pressurize hydrogen feed has not yet been investigated.

This thesis investigates the ability of a pump cell with a PBI/PA membrane to purify and pressurize the product of a steam methanol reformer coupled with a water gas shift reactor (70% H2, 3% CO, 20% CO2, and 7% CH4). By characterizing the operation of the pump cell with the simulated SMR-WGS feed, pure hydrogen, and 70% H2 with inert diluents, the different transport and poisoning processes were investigated.

This thesis also investigates the long term operation of pump cell. It is anticipated that long term operation of the pump cell at current densities above 0.4 A/cm2 will lead to performance degradation. If such performance degradation is observed, the transport mechanism in the PBI/PA membrane will be investigated.

Tyler Petek | 42

Chapter 3: Experimental Methodology

3.1 Theoretical Considerations

3.1.1 Faraday’s Law

Faraday’s law is used to relate the current associated with the rate an ionic species reacts.

If hydrogen is the only species being oxidized at the anode and produced at the cathode, the total current would have to be associated solely with the rate of hydrogen. This is described by equation (3.1).

(3.1)

In this equation, I is the current in amperes, Ae is the area of the electrode, n is the number of moles of electrons per mole of hydrogen, F is Faraday’s constant and equals

96,485 coulombs per mole of electrons, N is the flux of hydrogen, and t is time in seconds. The left side of equation (3.1), the current divided by the area, is defined as the current density, i.

The electrode area of the pump cells used was 50 cm2. By rearranging equation (3.1), the flow rate, flux times area, of hydrogen required is directly proportional to the applied current density.

[ ]

[ ] [ , ]

By using the density of hydrogen at standard temperature and pressure, 22.4 standard liters per mole, the flow rate in standard cubic centimeters per minute (sccm) can be obtained as a function of current density in A/cm2.

Tyler Petek | 43

[ ] [ ] [ ] [ ]

For every ampere applied to the cell per square centimeter, 384.24 sccm of hydrogen must be supplied at the anode. This flow rate is the stoichiometric amount of hydrogen required to support a given current density. Under real conditions with transport hindrances, more hydrogen than the stoichiometric value has to be supplied. It was found that operating at a minimum stoichiometric ratio of 2 was sufficient for the cells used.

This means that twice the stoichiometric amount of hydrogen was supplied to the cell at any given current density.

3.1.2 Nernst Potential

Nernst’s equation, used to express the equilibrium potential, is described as follows:

n (3.2)

The equilibrium or Nernst potential, ENernst, is the thermodynamic standard potential of the reaction, E°, minus a term associated with the species activities at the electrodes. The standard potential of a hydrogen pump cell is zero volts because hydrogen is the only species being oxidized and reduced. The second term in this equation is the change in potential due to different activities on the electrodes. Since the hydrogen is present as a gas on the electrodes, the ratio of the activities is approximated as the ratio of the partial pressures. The proton activity is assumed to be the same at both electrodes since they are at the same temperature with the same electrolyte. The temperature is given in Kelvin and

the molar gas constant, R, is .

Tyler Petek | 44

3.1.3 Overpotentials associated with the polarization curves

To drive a reaction away from the thermodynamic equilibrium, to produce a current, requires a certain potential cost. The polarization overpotentials are the potential costs associated with driving a cell away from thermodynamic equilibrium. Therefore, the total voltage across a cell is the sum of the thermodynamically required potential plus all of these overpotentials, descrbed in equation (3.3).

It should be noted that the convention for voltage in this paper is taken as positive potential going out of the cell. For instance, a fuel cell generates energy. Therefore, the potential supplied by a fuel cell would be positive. Because a hydrogen pump cell requires energy to operate, the potential across the cell is referred to as being negative.

(3.3)

The total voltage difference, ΔVtot, is measured in volts, and is the sum of the Nernst

potential and the sum of the polarization overpotentials, given as .

There are three types of overpotentials: activation, concentration, and ohmic. The total polarization overpotential is a sum of these three.

(3.4)

The activation and concentration overpotential occur at both electrodes while the ohmic overpotential is between the electrodes. For completeness, the total polarization overpotential should have separate anodic and cathodic activation and concentration overpotential terms.

(3.5)

Tyler Petek | 45

Activation Overpotential

The activation overpotential is the potential losses associated with the rate at which the reactions occur at the electrode. This overpotential will occur whenever a reaction is taking place and depends on the rate of the reaction. The activation overpotential is described by the Butler-Volmer equation.

[ ] (3.6)

In equation (3.6), R is the molar gas constant, F is Faraday’s constant, and T is the temperature in Kelvin. The transfer coefficients, αA and αC, are for the anode and cathode, respectively. The transfer coefficients describe the fraction of the activation overpotential associated with the forward and backward reaction at each electrode. The sum of the transfer coefficients is typically on the order of magnitude of the number of electrons per mole of reactant. In this equation, the anodic current is taken as positive and the cathodic is negative.

The exchange current density, i0, is related to the currents associated with the dynamic equilibrium at the electrodes. The dynamic equilibrium of the reaction of the cell at the electrode involves the feed, hydrogen in the case of a pump cell, moving from hydrogen to protons and back again. It is this exchanging of charge in the dynamic equilibrium that gives rise to the exchange current density and why the activation overpotential is sometimes referred to as the charge transfer overpotential.

By rearranging the Butler-Volmer equation, equation (3.5), the activation overpotential can be described as a function of the current density. As the equation shows, it exhibits a logarithmic dependence on the current density. For lower overpotentials, the activation

Tyler Petek | 46 overpotential has a more linear dependence on current density and can be approximated through equation (3.7).

(3.7)

Concentration Overpotential

The concentration overpotential is the potential cost due to the depletion of charge carriers at the electrode. According to Faraday’s law, equation (3.1), the applied current is proportional to the flux of charge carriers. As the applied current increases the flux of charge carriers necessary increases. The electrodes have finite area and therefore a maximum possible flux of charge carriers exists. Therefore, a limiting current density exists, iL. This limiting current is related to mass transport limitations. As the applied current increases and the flux of charge carriers necessary increases the potential cost to drive this flux increases. This is the concentration overpotential and is described in equation (3.8). This describes the concentration overpotential at the anode or the cathode.

n [ ] (3.8)

If the limiting current is much larger than the applied current, the concentration overpotential goes to zero. The concentration overpotential becomes larger as the applied current density approaches the limiting current.

Ohmic Overpotentials

The ohmic overpotentials are the potential costs due to the effects of resistances. These resistances are proportional to current density and typically follow Ohm’s law.

∑ ̃ (3.9)

Tyler Petek | 47

The sum of the cell resistances includes all of the hardware resistances, such as contact resistances of leads and the current collector plates, as well as material resistances, such as the resistance of the membrane, the electrodes, the gas diffusion layers, and the flow field plates.

∑ (3.10a)

∑ ̃ ∑ (3.10b)

Of all of these resistances, the membrane, or electrolyte, resistance is the most significant and is expected to be orders of magnitude greater than the rest of the attributed resistances. This was verified further in this research.

Overall Cell Polarization

By combining equations (3.3) through equations (3.10), the following equations can be derived for the polarization effects of the cell.

n n [ ]

̃ n [ ] ∑

(3.11a)

̃ n n [ ] ∑ (3.11b)

Figures 3.1a and 3.1b show typical overpotential contributions to polarization curves and the overall polarization curve. Both figures show the activation overpotential according to the linear approximation. In a pump cell, the potential losses are much lower than other electrochemical cells, such as fuel cells, so this approximation should be valid. Figure

3.1a shows the concentration overpotential with the limiting current much greater than the applied current range, iL >> i. It can be seen that the concentration overpotential with

Tyler Petek | 48 this approximation is very small over the entire applied current range. Figure 3.1b shows the concentration overpotential assuming that the limiting current is on the same order of magnitude as the applied current density, iL ≈ i. With this approximation, the concentration overpotential follows a logarithmic relationship to the applied current

density and dominates the overpotential as the limiting current is approached.

Activation - Activation - linear linear

Conc iL>i Conc iL ~ i Overotential(volts) Overotential(volts) Ohmic Ohmic

Total Total Current Density (A/cm2) Current Density (A/cm2)

Figure 3.1a: Overpotential contributions Figure 3.1b: Overpotential contributions with iL >> i. with iL ≈ i.

In both figures 3.1a and 3.1b, the maximum overpotential is the Nernst potential. In a hydrogen pump cell with both sides at atmospheric pressure, the Nernst potential is zero volts. As the current density increases, the overpotential becomes more negative. As the figures show, the majority of the overpotentials in this example is dominated by the ohmic overpotential. Because the hydrogen reduction-oxidation reaction is very reversible, the activation overpotential’s contribution is relatively small. As previously discussed, the concentration overpotential only becomes significant at current densities approaching the limiting current.

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3.1.4 Impedance Spectroscopy

Polarization curves are steady-state tests; they represent the steady-state potential associated with an applied steady-state, or DC, current density. Impedance spectroscopy examines the cell’s response to sinusoidal perturbations around a set DC Current on the polarization curve. By applying a sinusoidal current, or an AC current, to a cell and measuring the potential response at different frequencies, information regarding the electrical analogue of the cell can be extracted.

A phasor element is the time-independent component of a function. This is described in equation (3.12).

( ) (3.12)

In this equation, X is the phasor element of the cosinusoidal time-dependent variable, x(t).

The frequency is given as ω, j is the square-root of -1, and t is time. The impedance of a circuit element is the ratio of the phasor voltage to the phasor current across the element.

(3.13)

Impedance of Resistors

The relationship between current density and voltage for a resistor is given in equation

(3.14).

̃ (3.14)

Applying equation (3.12) to (3.14) gives the phasor form of a resistor’s potential and applied current as:

( ) (3.15a)

( ) (3.15b)

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Equation (3.14) can be rewritten as:

(3.16)

Equation (3.16) simplifies to:

(3.17)

As equation (3.17) shows, the impedance of a resistor, equation (3.18), is constant with time and frequency and is solely composed of real impedance, meaning the imaginary component of the impedance is always zero.

(3.18)

Impedance of Capacitors

The relationship between current density and voltage of a capacitor is given in equation

(3.19).

( ) ( ) ̅ (3.19)

In this equation, C is the capacitance of the capacitor. Following the same logic presented for the resistor, the relationship between the phasor voltage and current density reduces to equation (3.20).

(3.20)

Following equation (3.20), the impedance of a capacitor is written as:

(3.21)

As equation (3.21) describes, the impedance of a capacitor is solely imaginary and negative.

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Impedance of Inductors

The relationship between current density and voltage of a capacitor is given in equation

(3.22).

( ) ( ) ̅ (3.22)

In this equation, ̅ is the inductance of the inductor. Following the same logic presented for the resistor, the relationship between the phasor voltage and current density reduces to equation (3.23).

(3.23)

Following equation (3.23), the impedance of an inductor is written as:

(3.24)

As equation (3.24) describes, the impedance of an inductor is solely imaginary and positive.

Impedance Spectroscopy of an Electrochemical Reaction

A simple electrochemical reaction can be described by the electrical analogue shown in figure 3.2. Re represents the electrolyte resistance. RCT is the charge transfer resistance and Cdl is the double layer capacitance. A double layer region is formed at interfaces between two phases that have dipoles and charges. There is an electrical resistance associated with the transfer of charge and the difference of potential across the double layer results in capacitive effects.

Re Cdl

Rct

Figure 3.2: The electrical analogue, or equivalent circuit, for a simple chemical reaction. Element Freedom Value Error Error % Re Fixed(X) 0 N/A N/A Cdl Fixed(X) 0 N/A N/A Rct Fixed(X) 0 N/A N/A Data File: Circuit Model File: Mode: Run Simulation / Freq. Range (0.001 - 1000000) Maximum Iterations: 100 Optimization Iterations: 0 Type of Fitting: Complex Type of Weighting: Calc-Modulus Tyler Petek | 52

In a pump cell, there is a reaction occurring on the anode and the cathode of the pump cell; the oxidation and reduction, respectively, of hydrogen. Therefore, the equivalent circuit of a simple pump cell is as shown in figure 3.3.

Cdl-a Rmem Cdl-c

Rct-a Rct-c

Figure 3.3: The anticipated equivalent circuit of a simple pump cell. Element Freedom Value Error Error % Cdl-a Fixed(X) 0 N/A N/A This circuit contains Rct-a the double layerFixed(X) capacitance 0and the chargeN/A transfer resistanceN/A of Rmem Fixed(X) 0 N/A N/A both the anode and theCdl-c cathode. Also,Fixed(X) this equivalent0 circuit hasN/A the resistanceN/A of the Rct-c Fixed(X) 0 N/A N/A membrane, which is the electrolyte. Data File: Circuit Model File: Impedance spectroscopyMode: is typically displayed in twoRun types Simulation of plots, / theFreq. Nyquist Range and (0.001 the - 1000000) Maximum Iterations: 100 Bode plots. The NyquistOptimization plot displays Iterations: the real 0 impedance of a circuit versus the Type of Fitting: Complex Type of Weighting: Calc-Modulus imaginary impedance of a circuit over a frequency range. A Bode plot refers to two plots.

One of these plots is the real impedance of the cell versus the frequency. The second

Bode plot is the phase angle versus frequency. Figure 3.4 through figure 3.6 shows the typical Nyquist and Bode plots for an RC-circuit shown in figure 3.2.

There are a number of pieces of information that can be extracted from impedance spectroscopy. In the Nyquist plot, figure 3.4, the lower x-intercept, the high frequency intercept of the real axis, is the impedance, typically in ohms, of the membrane. If figure

3.4 the membrane resistance is about 0.01 ohms. The total charge transfer resistance is found by the diameter of the semi-circle. In this example, the total charge transfer resistance is about 0.0025 ohms. The lower real impedance occurs at high frequencies and the higher real impedance occurs at lower frequencies.

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The charge transfer resistance, the diameter of the charge transfer loop, is the resistance associated with the activation overpotential.

(3.25)

A peak in the phase angle vs frequency Bode plot, figure 3.6, is the characteristic frequency of a charge transfer loop. This is the maximum phase angle due to charge transfer. The change in the impedance of with respect to frequency, figure 3.5, is due to the effects of the capacitor at high and low frequencies. At high frequencies, the capacitor

FitResult actor acts as a short while at low frequencies it acts as a break.

-0.03

Capacitive Loop

-0.02 Z''

FitResult

-0.01 102

Rmem |Z| RCT 0 0 0.01 0.02 0.03 0.04

Z' 101 10-3 10-2 10-1 100 101 102 103 Figure 3.4: A Nyquist plot for an RC-circuit with one chargeFrequency transfer (Hz) loop. FitResult

102 -20

-15

-10

|Z| theta -5 RCT

101 0 -3 -2 -1 0 1 2 3 10-3 10-2 10-1 100 101 102 103 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz) Figure 3.5: Impedance vs frequency Figure 3.6: Phase angle vs frequency -20

-15

In a-10 real cell with both an anode and a cathode, there exists the charge transfer resistance theta -5 and capacitance of both electrodes, as figure 3.3 describes. If the charge transfer effects 0 10-3 10-2 10-1 100 101 102 103 Frequency (Hz)

Tyler Petek | 54 are different enough, there will be two separate capacitive loops on the Nyquist plot and two peaks in the Bode plot. This is shown in figure 3.7 through figure 3.9.

If the two charge transfer effects of both electrodes are the same, the capacitive loops will overlap, resulting in impedance spectroscopy more along the lines of figures 3.4 through

3.6. This overlap occurs because the capacitive effects have the same characteristic

frequency. The analogue effect is the capacitorsFitResult behave as shorts at the same frequencies.

-0.15

-0.10 Capacitive

Loops Z''

FitResult

-0.05 100

Rmem 10-1 |Z|

RCT-1 RCT-2 0 0 0.05 10-2 0.10 0.15 10-3 10-2 10-1 100 101 102 103 104 105 106 Z' Frequency (Hz) Figure 3.7: A NyquistFitResult plot for an RC-circuit with two charge transfer loops.

100 -40

-30

10-1 -20

|Z| theta

-10 RCT-2 RCT-1

10-2 0 -3 -2 -1 0 1 2 3 4 5 6 10-3 10-2 10-1 100 101 102 103 104 105 106 10 10 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

Figure 3.8: Impedance vs frequency Figure 3.9: Phase angle vs frequency -40 -30 L Cdl-a Rmem Cdl-c

-20 theta -10 Rct-a Rct-c

0 10-3 10-2 10-1 100 101 102 103 104 105 106 Figure 3.10: EquivalentFrequency (Hz) circuit for a pump cell with inductive effects considered. Element Freedom Value Error Error % L Free(+) 7E-09 N/A N/A Cdl-a Fixed(X) 0.01 N/A N/A Rct-a Free(+) 0.025 N/A N/A Rmem Fixed(X) 0 N/A N/A Cdl-c Free(±) 8 N/A N/A Rct-c Free(±) 0.1 N/A N/A

Data File: Circuit Model File: Mode: Run Simulation / Freq. Range (0.001 - 1000000) Maximum Iterations: 100 Optimization Iterations: 0 Type of Fitting: Complex Type of Weighting: Calc-Modulus Tyler Petek | 55

FitResult

-0.10 Capacitive Loops

-0.05 Z'' Rmem

RCT-1 RCT-2 0 FitResult

0 Inductive 10 Effects

10-1 |Z| 0.05 0 0.05 0.10 0.15

Z' 10-2 10-3 10-2 10-1 100 101 10 2 103 104 105 106 Figure 3.11: Nyquist plot for a cell with two distinct electrodesFrequency and impedance (Hz) effects FitResult

100 -50 Inductive RCT-2 R 0 CT-1 Effects

10-1 |Z| theta 50 Inductive Effects 10-2 100 10-3 10-2 10-1 100 101 102 103 104 105 106 10-3 10-2 10-1 100 101 102 103 104 105 106 Frequency (Hz) Frequency (Hz) Figure 3.12: Impedance vs frequency Figure 3.13: Phase angle vs frequency -50

So far,0 only the resistance and capacitive effects of the membrane and electrode reactions

theta 50 have been considered. As a result, all of the impedance measurements have had solely 100 10-3 10-2 10-1 100 101 102 103 104 105 106 negative imaginaryFrequency impedance. (Hz) If there are inductive effects due to the mass of the hardware, such as long wires or large current collector plates, flow field plates or electrodes, then there would be positive imaginary impedance. The equivalent circuit with impedance effects is shown in figure 3.10 and the Nyquist and Bode plots are shown in figure 3.11 through 3.13.

Tyler Petek | 56

3.2 Laboratory Material and Equipment

3.2.1 BASF PBI MEAs

The membranes electrode assemblies (MEAs) used in all the pump cells discussed in this thesis were supplied by BASF: The Chemical Company and are classified as Celtec-P-

1000-MEAs.

These MEAs have Polybenzimidazole (PBI) membranes that were imbibed with phosphoric acid (PA). The membranes were about 72 ± 2 µm in thickness and about 64 cm2 in total area with each side about 8 cm. This includes the non-active sealing area.

The amount of phosphoric acid imbibed in the membrane is unknown.

The electrodes are platinum on a carbon felt diffusion layer. The amount of platinum loading is unknown. The geomtetric area of the electrode in each of the tested MEAs is

50 cm2 with each side about 7.07 cm in length. The MEA is symmetrical; both the electrodes are the same. The carbon felt diffusion layer and the catalyst layer together are about 390 µm thick. This makes the entire MEA about 850 µm thick.

It was per an agreement with BASF that chemical characterization of the electrode and acid content not be conducted.

3.2.2 Fuel Cell Hardware and Assembly

The pump cell was assembled using standard fuel cell hardware manufactured by

Precision Flow Technologies (Saugerties, NY, USA). The endplates are made of stainless steel and have gas inlets and outputs through pipe threaded fittings. A piece of PTFE, 777

± 2 µm thick, is used to electrically insulate the endplates from the rest of the cell. The

Tyler Petek | 57 current collector plates (CCPs) are a pair of gold-plated plates. A pair of graphite blocks with a machined serpentine flow-pattern was used for gas distribution to the diffusion layer of the MEA. These are called flow field plates, or FFPs. A piece of grafoil was put between the graphite flow field plates and the current collector plates to ensure good electrical connectivity. In order to make sure proper compression of the MEA was obtained gaskets were placed between the MEA and the graphite flow field plates. These gaskets were supplied by BASF and were made for high temperature (120°C-200°C) operation. The gaskets were 335 ± 5 µm thick. The gasket thickness corresponds to about

85% of the thickness of the gas diffusion layer. The gasket sits between the graphite flow field plates and the membrane, sitting around the gas diffusion layer of the MEA. Kapton heating pads were attached to the outside of the endplates. There is a bore for inserting a thermocouple into the graphite flow field plates. The cell hardware is symmetrical; both sides of the MEA are the same hardware assembled in the same order. To assist in cell temperature control, if necessary, an insulating jacket is available for use. An exploded view of the cell hardware is shown in figure 3.14.

A number of precautions are taken to ensure no leaks are present and proper cell assembly occurs. Through each component of the cell hardware and the membrane outside of the active area run two centering pins; one above the active area and one below the active area. These centering pins are removed before cell operation. Also, between each gas input and output of the flow field plates and endplates are Viton O-rings, eight in total. There are holes cut in the insulating PTFE, current collector plates, and grafoil layer for these O-rings and gas flows.

Tyler Petek | 58

Eight bolts hold the cell together. There are two bolts evenly spaced on each side.

Between both the bolt head and the endplate and the nut and the endplate sit two spring washers. These spring washers have a concavity and are put together so that they concave away from each other. These allow for more uniform compression of the cell when tightening the bolts. Once the cell is assembled it is compressed to 100 inch-pounds. This is done in using a torque wrench and tightening the nuts on the bolts in increments of 10 inch-pounds. The bolts are tightened in a star pattern to ensure even compression of the membrane and diffusion layers.

Bolt holes Centering pin holes Gas flow holes

Endplate CCP Grafoil Gasket MEA FFP Viton O-rings Insulator

Figure 3.14: An exploded view of the cell hardware. The Kapton heating pads are attached on the outside of the endplates.

3.2.3 Fuel Cell Technology’s Fuel Cell Test Stand

The pump cell operation was controlled using a Fuel Cell Technologies’ (Albuquerque,

NM, USA) Fuel Cell Test Stand. The test stand was controlled through a LabVIEW program supplied by Fuel Cell Technologies. This system can control the type of gas, gas flow rate, gas temperature, gas humidification, cell temperature, applied voltage/current, and back pressure of the gas lines.

Tyler Petek | 59

The test stand can control either hydrogen or nitrogen on the anode and either air, oxygen, or nitrogen on the cathode. In order to control the gas flow rates, two MKS flow controllers are used on each the anode and the cathode; one for low flow rates and one for high flow rates. The LabVIEW program automatically calculates the flow controller output for standard cubic centimeters per minute (SCCM) flow rates hydrogen and nitrogen on the anode and for oxygen, air, or nitrogen on the cathode. When operating the cell as a hydrogen pump cell, hydrogen is fed to both sides of the cell. The program cannot appropriately calculate the flow of hydrogen on the cathode. In order to properly control hydrogen flow on the cathode, the controllers were set to air and a factor of 1.021 was applied. The actual flow of hydrogen through the MKS flow controller set for air is

1.021 times that of the readout. If the desired flow is 1000 SCCM H2 on the cathode, the set point should be 979 SCCM of air.

The gas can either be passed through a humidification bottle or bypass the humidification bottle and go straight to the cell. The humidification bottle is an insulated type 316 stainless steel bottle with internal Nafion® tubing submersed in water. The test stand can control the temperature of the humidification bottle, the lines going from the test stand into the cell, and the temperature of the cell itself. By controlling the temperature of the humidification bottle in relation to the temperature of the cell, the relative humidity of the gas as it is in the cell can be controlled. For instance, if the cell is operated at 150°C and

5% RH (relative humidity) is desired, the temperature of the humidification bottle should be set to 64°C.The line going from the humidification bottle into the cell are heated to ensure that no water is condensed before the gas reaches the cell. Also, the output lines

Tyler Petek | 60 are insulated to ensure that the gas does not cool down to rapidly upon exit from the cell.

This is to keep any water from condensing in the cell.

The test stand is equipped with an Agilent Technologies Systems DC Electronic Load

N3300A with an Agilent N3306A electronic load. The test stand is also equipped with a

TDK Lambda HWS 600-5/HD 5 volt constant potential power supply. The Agilent electronic load can control a load between 0-60 volts and 0 – 120 amps with a maximum power of 600 watts. Because the cell is operated as a hydrogen pump cell it requires potential to be put into the cell. This is unlike a fuel cell, which produces energy.

Therefore, the 5 volt constant potential power supply is necessary to ensure that the potential across the electronic load is within its control contour. A simple electrical schematic is shown in figure 3.15.

5 VDC DC Pump Cell + – Power + + Electronic Supply Load – – High Current Leads Shunt

Figure 3.15: A simple electrical schematic of the test stand and cell.

The pressure of the gas in either side of the cell can be controlled using a manual back pressure controller that the gas flows through on the output of the cell. The anode has a gauge pressure limit of 60 psig and the cathode has a gauge pressure limit of 100 psig.

3.2.4 Solartron

The instrument used for impedance spectroscopy was an SI 1280B made by Solartron

Instruments, a division of Solartron Group Ltd (Farnborough, Hampshire, UK). The

Tyler Petek | 61

Solartron instrument had a ±14.5 V, ±2 A, and 1 mHz to 20 kHz operating range. The software used for controlling and for data treatment of the impedance measurements are

Zplot and Zview, respectively. Both of these programs are from Scribner Associates

(Southern Pines, NC, USA).

3.2.5 Oscilloscope

The oscilloscope used in conjunction with the solartron was a Tektronix TDS 410A two channel digital oscilloscope. This oscilloscope has a 200 MHz maximum analog bandwidth and a 100 Megasample per second maximum digitizing rate.

3.2.6 Gas Supplies

The gas supplies used in these experiments were supplied by Airgas. The hydrogen, nitrogen, and simulated reformate gases were high purity grade and in compressed cylinders around 2000 psig. The simulated reformate gas used was a specialty mixture that was 70% H2, 3% CO, 20% CO2, and 7% CH4. This simulate gas mixture should be close to realistic product gas from a coupled steam methane reformer and water gas shift reformer (SMR-WGS). All of the gases were kept in an external tank farm and brought into the lab via copper piping and Teflon tubing.

3.3 Experimental Techniques

Except where otherwise noted, when the cell was not undergoing one of the following experimental techniques it was kept at 150°C and 5% RH (humidification bottles at

64°C) with 125 SCCM of pure hydrogen on both sides of the cell at open-circuit.

Tyler Petek | 62

3.3.1 Polarization Curves

Before the polarization curve was taken, the cell was set to the operation temperature and

5% RH for at least one hour after these conditions were reached. If pure hydrogen was to be fed on both sides, then the flows were set to 150 SCCM of hydrogen on both sides.

For tests conducted with 70% hydrogen and 30% nitrogen or the simulated reformate, the gas flows were 250 SCCM of the mixture on the anode and 250 SCCM of hydrogen on the cathode. Pure hydrogen was always supplied on the cathode to keep the partial pressure of hydrogen on the cathode defined. After one hour at the described conditions, the polarization curve measurement was conducted. If the polarization curve was following a long-term constant-current test than the polarization curve was taken as soon as the long-term test was finished.

To conduct a polarization curve on the cell, the applied current was controlled and the responding voltage was monitored. The voltage measurements were taken from the two current collector plates at the anode and the cathode of the cell. Every five seconds the voltage, current density, temperatures, and flow rates were recorded. The cell was left at a constant current until an apparent steady-state was reached for at least one minute. The current density was stepped from open-circuit through 1.0 A/cm2. For tests with pure hydrogen on both electrodes or with inert diluents on the anode, the current density was stepped at intervals of 1.0 A/cm2. Once the cell was stable at 1.0 A/cm2, the cell voltage was stepped down to open-circuit. Only two data points (ten seconds) were allowed at each point. If the cell, or the electronic load, at any point could not maintain an applied current density then the cell was set to open-circuit and the test was stopped.

Tyler Petek | 63

For tests with the simulated reformate on the anode, the current density was first stepped from open circuit through 0.01 A/cm2 at intervals of 0.002 A/cm2. Then the current density was stepped from 0.2 A/cm2 to 1.0 A/cm2 or until the electronic load could not maintain an applied current density at intervals of 0.02 A/cm2. When this occurred, the cell was set to open circuit.

The polarization curves with simulated reformate on the anode is not linear due to poisoning effects of CO. The current density interval at low current densities is kept very small so that a linear approximation can be obtained.

Originally, the flow rates were maintained at about two times the stoichiometric flow rates at each applied current density with a minimum flow rate of 150 SCCM of pure hydrogen or 250 SCCM of 70% hydrogen. These flows were more than sufficient to maintain the appropriate current densities. However, at lower flow rates, uncharacteristic error was seen in the voltage response. It was determined that this was due to water condensing in the vent tubes and back into the cell. At the higher flow rates, the gas flow was sufficient to carry all of the water out and away from the cell. Increasing the flow rate eliminated the variation of voltage and much more reproducible data was obtained.

Therefore, 700 SCCM of pure hydrogen and 1000 SCCM of 70% hydrogen were used for the entire polarization curve.

3.3.2 Pressure Tests

The pressure of the each side of the cell can be controlled via a manual back pressure controller on the test stand. Polarization curves were conducted with some applied back pressure on one side of the cell and atmospheric pressure on the other.

Tyler Petek | 64

As with normal polarization curves, the temperature and flows of the cell were set and the cell was allowed to stay there for at least one hour. Then the pressure on the cathode was slowly stepped to the desired pressure to make sure that there was no pressure spike.

Once the desired pressure conditions were obtained, polarization curves were conducted as described in section 3.2.1.

These back pressure controllers were manually controlled so they were adjusted as the test went on to keep the pressure as constant as possible. Since they were subject to human error, the pressure was kept to ± 1 psig. The pressures on the cathode tested between atmospheric and 20 psig at 5 psig intervals.

3.3.3 Impedance Spectroscopy Scans

Impedance measurements were taken with each polarization curve using the Solartron electrochemical measurement unit. If the polarization curve was conducted with pure hydrogen on both sides, then impedance measurements were taken with pure hydrogen on both sides as well as with 5% hydrogen in balance argon on the anode. The charge transfer resistance on the impedance scans with pure hydrogen was smaller than the error in the measurement. However, the electrolyte resistance could still be determined from the impedance scans on pure hydrogen. When 5% hydrogen in balance argon was supplied on the anode, the charge transfer resistance is increased by a factor of twenty.

This value is measurable by the scans. With 5% hydrogen in balance argon on the anode the working electrode and reference electrode 2 are on the anode current collector plate.

The counter electrode and reference electrode 1 are on the cathode current collector plate.

In general, when any mixture of hydrogen and other gases are applied to the cell, the

Tyler Petek | 65 working electrode and reference electrode 2 are on the anode current collector plate while the counter electrode and reference electrode 1 are on the cathode current collector plate.

A schematic of the connections made during impedance spectroscopy is shown in figure

3.16.

Reference 2 Reference 1 Working Counter Electrode Electrode

Figure 3.16: A schematic of the cell connections during impedance spectroscopy.

During all of the impedance scans, the lines for the working and the counter electrode are wound together and the lines for the two reference electrodes are wound together. Then, these two bundles are wound together. All of the lines remained wound until approximately two inches from the connections to the current collector plates to minimize the inductance effects. This was done in an attempt to minimize the inductive effects due to the lines.

The AC current amplitude was controlled for these impedance measurements. The polarization curves under pure hydrogen are linear to 1.0 A/cm2 (50 A) so an AC current amplitude of 1000 mA was used. When hydrogen mixtures were used, smaller amplitudes were sometimes required. With each polarization curve, the oscilloscope was used to monitor the sinusoidal responses to ensure that the impedance scans were taken

Tyler Petek | 66 completely under linear kinetics. The oscilloscope data was collected at three frequencies, one above 1000 Hz, one at a few hundred Hz, and one around 2 Hz. All of the impedance scans were conducted with the current amplitudes centered around open- circuit.

For most of the impedance scans the frequency used ranged from 2000 Hz to 0.1 Hz with

8 data points taken per decade. For some of the scans done with the simulated reformate, the upper frequency limit was increased to 7000 Hz.

3.3.4 Long Term Constant-Current Tests

After the cell was brought to temperature with 5% RH on both sides, the cell was set to the constant-current density and two times stoichiometric flows. During the test, data was collected every five seconds. These long term tests were first conducted over 30 hours.

However, after it was shown that the cell did not have significant changes between 20 hours and 30 hours, the tests were shortened to 22 hours. After each long term test, polarization curves and impedance scans were conducted.

Tyler Petek | 67

Chapter 4: Results and Analysis

4.1 Characterization of a pump cell with a BASF PBI/PA MEA

As mentioned in chapter 3, the convention for potential in this paper is taken as positive potential going out of the cell. Therefore, because a hydrogen pump cell requires energy to operate, the potential across the cell is referred to as being negative.

4.1.1 Temperature effects on hydrogen pump performance without feed impurities

The standard temperature operating range for fuel cells with Polybenzimidazole membranes imbibed with phosphoric acid (PBI/PA) is 120°C to 180°C. Figure 4.1 shows the polarization curves conducted on a pump cell with a BASF PBI/PA MEA at different temperatures with pure hydrogen on both sides of the cell. In all of these tests, the humidity was kept at 5% RH.

0.00 -0.02

-0.04 -0.06 -0.08 -0.10 -0.12 120 C -0.14 140 C -0.16 150 C Cell Voltage(volts) Cell -0.18 160 C -0.20 180 C -0.22 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.1: Polarization curves with on a pump cell with BASF PBI/PA MEA with pure hydrogen at 5% RH on both sides of the cell at varying temperatures.

Over the tested operating temperature range, the polarization curves are linear at least up to 1.0 A/cm2. As equation (3.11) describes, this is indicative of the limiting current at

Tyler Petek | 68 these operating conditions being significantly greater than 1.0 A/cm2. This means that the

cell has negligible concentration overpotentials.

)

2 0.18

·cm 0.17 Ω 0.16 0.15 0.14 0.13 0.12 0.11

Effective Cell resistance ( resistance Cell Effective 0.10 100 110 120 130 140 150 160 170 180 190 200 Cell Temperature (°C)

Figure 4.2: The overall cell resistance, the slope of the polarization curve, of a pump cell with a BASF PBI/PA MEA vs. the cell operating temperature as described in figure 4.1.

The slope of these polarization curves can be thought of as the effective overall cell resistance. Figure 4.2 shows a near-linear relationship between the cell temperature and the cell resistance. The higher the cell temperature, within the temperature range tested, the lower the resistance of the cell. This near-linear relationship makes sense because the conductivity of phosphoric acid also has a near-linear relationship over this temperature range as shown in figure 4.3.

The overall cell resistance of the cell includes all of the overpotential effects. The membrane resistance, and therefore the membrane conductivity, will be looked at in more detail later in this chapter.

Tyler Petek | 69

0.60

) 0.55

1 - 0.50

0.45

0.40

Conductivity (S·cm Conductivity 0.35

0.30 100 110 120 130 140 150 160 170 180 190 200 Temperature ( )

Figure 4.3: The conductivity of phosphoric acid (100 wt%) versus temperature. [71]

4.1.2 Ability of pump cell to purify hydrogen rich stream

In an attempt to investigate the ability of pump cells utilizing BASF PBI/PA MEAs to purify SMR-WGS product streams to pure hydrogen, the pump cells were characterized with a simulated reformate gas mixture of 70% H2, 3% CO, 20% CO2, and 7% CH4. It is anticipated that dilution effects as well as catalyst poisoning effects will be seen in the operation of the pump cells under this simulated reformate feed. In an attempt to separate these effects, the cell was first investigated using a feed stream of 70% H2 and 30% N2 to first investigate just the dilution effects.

As with pure hydrogen on both sides of the cell, the polarization curves under an anodic feed of 70% hydrogen with inert diluents shows purely linear effects. This again indicates that the limiting current density under these operating conditions is significantly greater than 1.0 A/cm2, causing the cell to experience negligible concentration overpotentials.

Tyler Petek | 70

0.00 -0.02

-0.04 -0.06 120C -0.08 130C -0.10 140C -0.12 -0.14 150C -0.16 160C Cell Voltage(volts) Cell -0.18 170C -0.20 180C -0.22 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.4: Polarization curves with on a pump cell with BASF PBI/PA MEA with 70% H2 and 30% N2 on the anode and pure H2 on the cathode at varying temperatures. Both

sides of the cell operated at 5% RH.

) 0.18 2

·cm 0.17 Ω 0.16 0.15 0.14 0.13 0.12 0.11

0.10 Effective Cell Resistance ( Resistance Cell Effective 100 110 120 130 140 150 160 170 180 190 200 Cell Temperature (°C)

Figure 4.5: The overall cell resistance, the slope of the polarization curve, of a pump cell with a BASF PBI/PA MEA vs. the cell operating temperature as described in figure 4.4.

Figure 4.5 shows the slope of the polarization curves, or the effective overall cell resistance, plotted against the operating temperature of the cell. The cell still exhibits lower resistance at higher temperatures, but the overall cell resistance is not linear with respect to the operating temperature.

Tyler Petek | 71

In order to fully characterize the ability of a cell with a BASF PBI/PA MEA to purify a hydrogen rich stream from SMR-WGS processes, a simulated reformate was used. This reformate is 70% H2, 3% CO, 20% CO2, and 7% CH4. This should accurately simulate the product of a steam methanol reformer coupled with a water gas shift reactor.

Polarization curves of a cell with a BASF PBI/PA MEA operating as a pump cell with the simulated reformate on the anode and pure hydrogen on the cathode over the operational temperature range are shown in figure 4.6. The current was increased from open circuit until the test stand could no longer control the potential or current across the cell. More data points were taken in these tests so that the different regions of the curves could be appropriately characterized.

0.00 -0.02 120C -0.04 130C -0.06 -0.08 140C -0.10 150C -0.12 -0.14 160C

-0.16 Cell Voltage(votls) Cell 170C -0.18 180C -0.20 -0.22 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.6: Polarization curves with on a pump cell with BASF PBI/PA MEA with simulated reformate on the anode and pure H2 on the cathode at varying temperatures. Both sides of the cell operated at 5% RH.

At low current densities, below 0.1 A/cm2, the the polarization curves exhibit a linear behavior. By taking the slope of these linear portions, the effective overall resistance of the cell can be found. Figure 4.7 compares the overall cell resistance of the three different anodic scenarios.

Tyler Petek | 72

1.2

Pure H2 on both sides cm2) ∙ 1.0 Ω 70% H2 and 30% N2 on anode 0.8 Sim Reform on anode 0.6 0.4 0.2 0.0

Effective Cell Resistance ( Resistance Cell Effective 100 110 120 130 140 150 160 170 180 190 200 Cell Temperature (°C)

Figure 4.7: A comparison of the slopes of the linear regions of the polarization curves under different anodic conditions. In all cases, the cathode has pure hydrogen and both anode and cathode operated at 5% RH.

The pump cell operated with simulated reformate on the anode has significantly greater overall cell resistance compared to the case with pure hydrogen or hydrogen with inert diluents on the anode. Also, the polarization curves are no longer linear and approach some sort of limiting current densities within the tested current density range. Because the polarization curves with 70% H2 in inert diluents, the same concentration as the simulated reformate, do not exhibit a limiting current, these results point towards the poisoning effects of carbon monoxide.

4.1.3 Modeling of pump cell polarization effects at 150°C

Figure 4.8 shows polarization curves with different anodic conditions at 150°C with 5%

RH. The different curves show the different effects of the anodic conditions.

With pure hydrogen on both sides, the polarization curve is linear because the limiting current is significantly greater than the operating current density range. The non-linear concentration overpotential is negligible.

Tyler Petek | 73

0.00 -0.02

-0.04 -0.06 -0.08 -0.10 -0.12 100% H2 both -0.14 -0.16 5% H2 on anode Cell Voltgae(volts) Cell -0.18 70% H2 on anode -0.20 Sim reform on anode -0.22 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.8: Polarization curves on a pump cell with a BASF PBI/PA MEA at 150°C and 5% RH with the cathode under pure hydrogen.

Polarization curves were conducted with the cathode on the left side and then again with the cathode on the right side of the cell. Both were at 150°C with pure hydrogen at atmospheric pressure and 5% RH on both sides. The results are shown in figure 4.9. The polarization curves with the cathode and anode switched were very similar, indicating that the cells are quite symmetrical. The average slope of the polarization curve is -0.101

± 0.002 Ω∙cm2.

0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12 -0.14 100% H2 both - cathode on the right -0.16 Cell Voltage(volts) Cell -0.18 100% H2 both - cathode on the left -0.20 -0.22 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.9: Polarization curve on a pump cell with a BASF PBI/PA MEA with pure H2 at 5% RH on both sides.

Tyler Petek | 74

Figure 4.10 shows the Nyquist plot for the pump cell with pure hydrogen on both sides of the cell at 150°C and 5% RH with 1000 mA, or 20 mA/cm2, current amplitude around open circuit. From this Nyquist plot the 50 cm2 PBI/PA membrane resistance at 150°C is estimated to be 0.00173 ± 0.00005 Ω, or about 0.0865 Ω∙cm2. Because there are no capacitive loops, the charge transfer resistances are assumed to be much smaller than the membrane resistance. The Bode plots, figure 4.11 and figure 4.12, agree with the approximation of the charge transfer resistances being significantly smaller than the membrane resistance. This is expected because the kinetics of hydrogen reduction and oxidation on platinum are very fast.

The Nyquist and Bode plots show significant impedance effects in these cells. By applying an equivalent circuit model consisting of an inductor and a resistor with a known membrane resistance of 0.0865 Ω∙cm2 the impedance of the cell has been estimated at 2.5 x 10-7 ± 0.1 x 10-7 Henry. MEA8-1-Impedance-100_H2_both-woking_left-150C-06_22_11.z

-0.001

0

0.001

0.002 Z'' (ohms)Z''

0.003

0.004 0.001 0.002 0.003 0.004 0.005 0.006 Z' (ohms)

Figure 4.10: Nyquist plot of pump cell with BASF PBI/PA MEA at 150°C with pure H2 at 5% RH on both sides.

MEA8-2-Impedance-100_H2_both-working_right-150C-06_22_11.z

10-2 |Z|

10-3 -1 0 1 2 3 4 10 10 10 Tyler10 Petek10 | 1075 Frequency (Hz) MEA8-2-Impedance-100_H2_both-working_right-150C-06_22_11.z

-2 10 -25

0

|Z| 25 theta 50

-3 10 75 -1 0 1 2 3 4 10 10 10 10 10 10 10-1 100 101 102 103 104 Frequency (Hz) Frequency (Hz) Figure 4.11: Bode plot of cell at 150°C Figure 4.12: Bode plot of cell at 150°C -25 with 100% H2 on both sides with 100% H2 on both sides

0 The overall cell voltage, equation (3.11), for the pump cell operated with pure hydrogen

25 theta at 50 atmospheric pressure on both sides can be simplified to equation (4.1). This 75 10-1 100 101 102 103 104 simplification is underFrequency the (Hz) assumption that the limiting current is significantly greater than the operating current density and that the membrane resistance dominates the cell resistance.

̃ (4.1)

̃ (4.2)

Because the cell has been shown to be very symmetrical, figure 4.9, the exchange current densities of each electrode are approximately equal. This allows equation (4.1) to be written as equation (4.2). As discussed in chapter 3, the impedance spectroscopy will only show one charge transfer loop if the charge transfer resistance and double layer capacitance of the anode and cathode are about the same. This approximation assumes that the overall exchange current density of the cell is equal to one half that of each electrode.

(4.3)

Equation (4.2) should be able to accurately model the cell voltage as shown in figure 4.8.

The charge transfer resistance is quite small, as expected, making it difficult to obtain an accurate value for the exchange current density from impedance spectroscopy. Therefore,

Tyler Petek | 76 equation (4.2) was fitted to the data in figure 4.8 and the exchange current density was calculated from the model. The cell exchange current density was calculated to be about

0.45 A/cm2. This corresponds, by equation (3.25), to an overall charge transfer resistance of about 8 mΩ. A charge transfer resistance of 8 mΩ should have been seen as a charge transfer loop in figure 4.9 but none was observed. This discrepancy between tests, especially since they were conducted with different apparatuses, points towards a calibration error between the polarization and impedance systems.

By equation (4.3), an overall cell charge transfer resistance of 0.45 A/cm2 corresponds to an anodic and cathodic exchange current density of 0.90 A/cm2 each. Again, equation

(4.3) is only valid if the charge transfer resistance and double layer capacitance of the anode and cathode are the same. This assumption should be valid because the electrodes are the same and are under the same conditions, and the hydrogen reaction is symmetrical.

In an attempt to better characterize the exchange current density, impedance spectroscopy was conducted with 5% hydrogen in balance argon on one side of the cell. By applying

5% hydrogen in an inert diluent the charge transfer resistance of that side by should be increased by a factor of 20. The impedance spectroscopy for this experiment is shown in figure 4.13 through figure 4.15.

These impedance plots do not quite follow the examples developed in chapter 3. The capacitive loop in the Nyquist plot is not a perfect semicircle. Also, the positive imaginary impedance region occurs at varying real impedance. The depression of the capacitive loop and slope of the inductive effects indicates that the electrode cannot be

Tyler Petek | 77 modeled as planar, meaning that the reaction occurs throughout the three-dimensional electrode instead of all at one plane. This electrode is referred to as having distributed effects. The electrical analogue of a cell with a distributed electrode is shown in figure

4.16.

It should be noted that the high frequency impedance approaches the same real impedance as the cell with pure hydrogen on both sides. This means that the membrane resistance is relatively constant. In order to bound the approximations of the exchange current density made by this impedance spectroscopy, the charge transfer resistance will be approximated as the diameter of the semi-circular charge transfer loop and then again as the difference between the low frequency and high frequency impedance. This should bound the approximations of the exchange current density. The diameter of the semi- circular region of the Nyquist plot of the pump cell with 5% hydrogen in balance argon on one side was found to be 0.0026 ± 0.0003 Ω. The total difference between the high- frequency impedance and the low-frequency imaginary intercept was found to be 0.0034

± 0.0003 Ω.

The ratio of the exchange current densities of an electrode under two different hydrogen partial pressures is equal to the ratio of the inverse of the hydrogen partial pressures and is expressed in equation (4.4). This equation assumes that the temperature and relative humidity remain constant.

(4.4)

Equation (4.4) indicates that the exchange current density under 5% hydrogen should be

20 times smaller than the exchange current density under pure hydrogen. Therefore,

Tyler Petek | 78 according to equation (3.25), the charge transfer resistance under 5% hydrogen should be

20 times greater than the measured value for the charge transfer resistance under pure

hydrogen. MEA8-7-Impedance-5_H2_right-working_right-150C-rescan2-06_22_11.z

-0.002

-0.001

0 Z'' (ohms)

0.001

MEA8-7-Impedance-5_H2_right-working_right-150C-rescan2-06_22_11.z

0.002 10-2

0.003

0.001 0.002 0.003 0.004 0.005 0.006 |Z| (ohms) |Z| Z' (ohms) 10-3 Figure 4.13: Nyquist plot of pump cell with BASF10-1 PBI/PA100 MEA101 at 150°C102 with10 3pure H1024

MEA8-7-Impedance-5_H2_right-working_right-150C-rescan2-06_22_11.zon one side and 5% H2 in balance argon on theFrequency other. (Hz)

-2 10 -25

0

25 theta

|Z| (ohms) |Z| 50

-3 10 75 -1 0 1 2 3 4 10 10 10 10 10 10 10-1 100 101 102 103 104 Frequency (Hz) Frequency (Hz) Figure 4.14: Bode plot of cell at 150°C Figure 4.15: Bode plot of cell at 150°C -25 with 5% H2 on one side with 5% H2 on one side

0 With a charge transfer resistance of 0.0026 Ω under 5% hydrogen the corresponding

25 theta charge50 transfer resistance under pure hydrogen should be about 0.00013 Ω. This charge 75 -1 0 1 2 3 4 10 10 10 10 10 10 2 transfer resistance Frequencycorresponds (Hz) to an exchange current density of 2.8 A/cm . This would only be the exchange current density for one electrode since the diluted hydrogen stream was only applied to one electrode.

Tyler Petek | 79

With a charge transfer resistance of 0.0034 Ω under 5% hydrogen the corresponding charge transfer resistance under pure hydrogen should be about 0.00017 Ω. This charge transfer resistance corresponds to an exchange current density of 2.1 A/cm2. Again, this would only be the exchange current density for one electrode since the diluted hydrogen stream was only applied to one electrode.

Looking at figure 4.10, a charge transfer resistance of 0.0013 – 0.0017 Ω appears to better agree with the impedance spectroscopy done on the cell with 100% hydrogen on both sides. The exchange current density of one electrode under pure hydrogen is

2 approximated from the 5% H2 in inert diluents to be 2.1 – 2.8 A/cm .

L hardw are Rmem Cdl

Rct

Rionic Cdl

Rct

Rionic Cdl

Rct

Figure 4.16: Electrical analogue of a cell with a distributed electrode.

As the anodicElement feed is dilutedFreedom with a smallValue amount of inertError diluents, 70%Error hydrogen % in 30% L hardware Fixed(X) 0 N/A N/A nitrogen,Rmem the polarizationFixed(X) curve remains0 linear but hasN/A a more negativeN/A open circuit Cdl Fixed(X) 0 N/A N/A potential.Rct It is still linearFixed(X) because the limiting0 current isN/A still significantlyN/A greater than the Rionic Fixed(X) 0 N/A N/A operatingCdl current densityFixed(X) range. The more0 negative openN/A circuit potentialN/A is due to the Rct Fixed(X) 0 N/A N/A change inRionic anodic partialFixed(X) pressure of hydrogen,0 thus N/A a change in theN/A Nernst potential. Cdl Fixed(X) 0 N/A N/A Theoretically,Rct the slopeFixed(X) of the polarization0 curve underN/A small levelsN/A of inert diluents

Data File: should be parallel to the curve with pure hydrogen on both sides. There should be no Circuit Model File: C:\Users\Tyler\SAI\ZModels\exampleDX19.m dl Mode: Run Simulation / Freq. Range (0.001 - 1000000) Maximum Iterations: 100 Optimization Iterations: 0 Type of Fitting: Complex Type of Weighting: Calc-Modulus Tyler Petek | 80 change in the membrane resistance as long as the temperature and relative humidity remain constant. mea8-80-impedance-70_h2_right-working_right-150c-07_22_11x.z

-0.0001

0

0.0001 Z'' (ohms)

mea8-80-impedance-70_h2_right-working_right-150c-07_22_11x.z 0.0002 10-2

0.0003

0.0020 0.0021 0.0022 0.0023 0.0024 |Z| (ohms) |Z| Z' (ohms) 10-3 Figure 4.17: Nyquist plot of pump cell with BASF10-1 PBI/PA100 MEA101 at 150°C102 with10 3pure H1024 on one side and 70% H2 in balance nitrogen on Frequencythe other. (Hz) mea8-80-impedance-70_h2_right-working_right-150c-07_22_11x.z

10-2 -2.5

0

2.5 theta

|Z| (ohms) |Z| 5.0

10-3 7.5 10-1 100 101 102 103 104 10-1 100 101 102 103 104 Frequency (Hz) Frequency (Hz) Figure 4.18: Bode plot of cell at 150°C Figure 4.19: Bode plot of cell at 150°C -2.5 with 70% H2 on one side with 70% H2 on one side

0 The impedance spectroscopy conducted on the pump cell with pure hydrogen on the

2.5 theta cathode5.0 and 70% hydrogen in balance nitrogen on the anode is shown in figure 4.17 7.5 10-1 100 101 102 103 104 through figure 4.19.Frequency Again, (Hz) distributed effects are seen in both the Nyquist and Bode plots. In order to bound the approximations of the exchange current density, the charge transfer resistance will be approximated as the diameter of the semi-circle as well as the difference between the low frequency and high frequency impedance. The charge transfer

Tyler Petek | 81 resistance was found to be about 5 x 10-5 Ω from the semi-circle and about 4.3 x 10-4 Ω from the total difference. Because this is the charge transfer resistance with 70% hydrogen on the anode, according to equation (4.4), this should be 70% of the charge transfer resistance under pure hydrogen. The charge transfer resistance under pure hydrogen would be 3.5 x 10-5 Ω and 3 x 10-4 Ω from the semi-circle and the total difference, respectively. From these resistances, the exchange current density of the electrodes under pure hydrogen is approximated to be between 1.2 A/cm2 and 10.4

A/cm2. Again, this would only be for the exchange current density for one electrode since the diluted hydrogen stream was only applied to one electrode.

The exchange current density for each electrode under pure hydrogen has been approximated as 0.90 A/cm2, 2.1 – 2.8 A/cm2, and 1.2 – 10.4 A/cm2 from experiments with anodic conditions of pure hydrogen, 5% hydrogen, and 70% hydrogen respectively.

There is significant discrepancy between these values, but they are still on the same order of magnitude. For the equations developed further in this chapter, the exchange current density of each electrode under pure hydrogen at 5% RH and 150°C will be approximated as 3 A/cm2.

The theoretical potential of the a pump cell operating with pure hydrogen on the cathode and 70% hydrogen in inert on the anode at atmospheric pressure and 5% RH is expressed in equation (4.5).

n ̃ (4.5)

The membrane resistance is unchanged and is 0.00173 ± 0.00005 Ω, or 0.0865± 0.0025

Ω∙cm2. Equation (4.4) again operates under the assumption that the limiting current is

Tyler Petek | 82 significantly greater than the operating current and that the ohmic resistance of the cell is dominated by the membrane.

Since the anode is under pure hydrogen and the cathode is under 70% hydrogen in inert diluents, equation (4.4) needs to be applied to equation (4.5) to appropriately estimate the exchange current densities.

n ̃ (4.6)

Applying the appropriate values to equation (4.6) yields the theoretical cell voltage as a function of current density to be equation (4.7).

(4.7)

This representation of the voltages as expressed in equation (4.7) is shown against the experimental data in figure 4.20. The model predicts smaller overpotentials than observed during the polarization curve.

0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12 Experimental Results -0.14 -0.16 Equation 4.7

Cell Potential (volts) Potential Cell -0.18 Linear Fit -0.20 -0.22 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.20: Polarization curve on a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH with 70% H2 and 30% N2 on the anode and pure H2 on the cathode.

Tyler Petek | 83

Figure 4.17 shows significant distributive effects. As figure 4.16 shows, there are two resistances associated with a distributed electrode: the charge transfer resistance as well as ionic resistances.

n ̃ ̃ (4.8)

The ionic resistance, calculated from the difference in slopes of equation (4.7) and the linear fit of the data, was found to be 0.00049 Ω, or 0.0243 Ω∙cm2. The overall cell voltage with pure hydrogen on the cathode and 70% hydrogen in inert is given as a function of current density in equation (4.9).

(4.9)

The polarization curves with simulated reformate on the anode are not linear. The effects of the concentration overpotential due to mass transport are apparent. Therefore, the total potential across the cell must include the effects of the Nernst potential, ohmic overpotential, activation overpotential, concentration overpotential, and the ionic overpotential.

n n [ ] ∑ ̃ ̃

(4.10)

Through impedance spectroscopy, figure 4.21 through figure 4.23, it was found that the membrane resistance at 150°C under pumping conditions with simulated reformate on the anode and pure hydrogen on the cathode was 0.00192 ± 0.00003 Ω, or about 0.096

Ω∙cm2. This is about the same as the membrane resistance found from the 5% hydrogen tests. This indicates that the feed composition does not affect the membrane resistance.

Tyler Petek | 84

-0.002

-0.001

0 Z'' (ohms)

0.001 5% H2 at 150C and 5% RH Simulated Reformate at 150C and 5% RH

0.002 10-2

0.003 |Z| (ohms) |Z| 0.004 0.001 0.002 0.003 0.00410-3 0.005 0.006 0.007 -1 0 1 2 3 4 Z' (ohms)10 10 10 10 10 10 Figure 4.21: Nyquist plots of pump cell with BASF PBI/PA MEAFrequency at 150°C (Hz) and 5% RH.

10-2 -25 5% H2 at 150C and 5% RH Simulated Reformate at 150C and 5% RH 0

25

5% H2 at 150C and 5% RH theta Simulated Reformate at 150C and 5% RH

|Z| (ohms) |Z| 50

10-3 75 10-1 100 101 102 103 104 10-1 100 101 102 103 104 Frequency (Hz) Frequency (Hz) Figure 4.22: Bode plots of cell at 150°C Figure 4.23: Bode plots of cell at 150°C

-25

0 The capacitive loop in the Nyquist plot is not a perfect semicircle. Again, the impedance

25 theta spectroscopy50 indicates that the electrode under simulated reformate has distributed 75 10-1 100 101 102 103 104 effects. Frequency (Hz)

The charge transfer loop of the cell under simulated reformate has a diameter on the same order of magnitude as the charge transfer loop under 5% hydrogen. The approximation of an exchange current density of 3 A/cm2 under pure hydrogen at 150°C and 5% RH should still be valid. Also, the approximation of the ionic resistance of 0.00049 Ω, or

0.0243 Ω*cm2, should still be valid.

Tyler Petek | 85

The standard potential, , should still be zero. The Nernst potential should be dictated by the partial pressures due to dilution. As with the 70% hydrogen in inert, the Nernst potential should be -0.0065 V.

2 As discussed, the exchange current density, i0, is still approximated as 3 A/cm at the cathode. At the anode, the exchange current density can be calculated from equation (4.4) assuming the exchange current density is 3 A/cm2 under pure hydrogen. The anodic exchange current density under the simulated reformate is estimated to be about 4.3

A/cm2.

The polarization curves with the anode under simulated reformate have been observed to reach a maximum applied current density. The polarization curves under pure hydrogen and 70% hydrogen on the anode were linear over the applied current density range.

Again, this is indicative of a limiting current that is significantly higher than 1.0 A/cm2.

Therefore, the maximum limiting current observed under the simulated reformate cannot be a diffusion driven limiting current.

Carbon monoxide is known to poison the platinum catalysts in fuel cells as shown in figure 2.14. Fuel cells, however, are operated with much larger anodic potentials. The anodic potential of a fuel cell is large enough to oxidize the carbon monoxide. This is why an increase in current density is seen as very low voltages. These poisoning effects are particularly apparent when the anode was under 15% CO.

Simply put, carbon monoxide poisons the cell by blocking the catalyst sites so that hydrogen cannot be oxidized. The maximum current observed in the pump cell tests with

Tyler Petek | 86 simulated reformate is a limiting current due to an increasing catalyst coverage of carbon monoxide. The term ‘limiting current’ is used to describe a maximum current due to diffusion limitations. ‘Adsorption hindered current will be used to describe the increase in potential due to a decrease in the number of active catalyst sites available to the hydrogen due to carbon monoxide poisoning.

Because the sudden increase in potential, or the adsorption hindered current, is attributed to the poisoning effects of carbon monoxide, the concentration overpotential term cannot be used to approximate these effects. There is currently no appropriate model for the adsorption hindered effects.

A quick look at how poorly the concentration overpotential approximates the adsorption hindered effects, equation (4.11) is plotted against the experimental data. At 150°C, the adsorption limiting current is estimated to approach 0.28 ± 0.02 A/cm2. This is used for the limiting current.

n [ ] (4.11)

Figure 4.24 shows this model against the experimental data. As the figure shows, the overpotential approximations presented in equation (4.11) are not adequate to model the polarization curve of the pump cell operated with simulated reformate containing catalyst poisoning constituents. It is obvious that the concentration overpotential equation does a poor job of approximating the catalyst poisoning effects of CO.

Tyler Petek | 87

0.00 -0.02

-0.04 -0.06 -0.08 -0.10 -0.12 -0.14 Model from equation 4.11

-0.16 Cell Voltage(volts) Cell -0.18 Simulated reformate on anode -0.20 -0.22 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Current Density (A/cm2)

Figure 4.24: Polarization curve on pump cell with BASF PBI/PA MEA at 150°C with simulated reform on the anode and pure hydrogen on the cathode at 5% RH with model.

A summary of the cell voltage approximated from the cell overpotentials and Nernst equation is shown in equation (4.12). This equation does a good job of predicting the cell voltage under pure hydrogen or with hydrogen in inert diluents. However, equation (4.12) does not do a good job of approximating the polarization losses due to catalyst poisoning.

n n [ ]

∑ ̃ ̃ (4.12)

4.1.4 Ability of a pump cell to pressurize hydrogen product

The ability to pressurize hydrogen efficiently would be very beneficial to making hydrogen a cost effective fuel since fuel cells operate more efficiently at higher feed pressures. Polarization curves were conducted on the pump cell with the pure hydrogen product pressurized by a manual back pressure controller. Again, in an attempt to separate the effects of inert diluents from the poisoning effects of carbon monoxide, different anodic conditions were investigated.

Tyler Petek | 88

0.00 -0.02

-0.04 -0.06 -0.08 -0.10 -0.12 100% H2 both - Atmospheric both -0.14 -0.16 100% H2 both - 5 psig on cathode Cell Voltage(volts) Cell -0.18 100% H2 both - 10 psig on cathode -0.20 -0.22 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.25: Polarization curves on a pump cell with BASF PBI/PA MEA at 150°C and 5% RH with pure hydrogen on both sides at varying applied cathodic pressures.

Figure 4.25 shows the polarization curves on a pump cell with pure hydrogen at 5% RH on both sides at 150°C with varying applied cathodic pressures. The polarization curves are linear, signifying that the limiting current is significantly greater than the operating current density. Also, the polarization curves at the different pressures are parallel and with different open circuit potentials. This is anticipated by equation (4.12). The only difference between the different tests is the applied cathodic pressure, so the only change is in the Nernst potential.

Figure 4.26 shows the polarization curves on a pump cell with a BASF PBI/PA MEA with simulated reformate. All of the tests were conducted at a cell temperature of 150°C and at gas humidification of 5% RH. In all of the trials, the anode was under the simulated reformate at atmospheric pressure. Also, the cathode had pure hydrogen flown on it in order to keep the hydrogen partial pressure well defined. The only difference between the experimental trials is the applied backpressure on the cathode.

Tyler Petek | 89

If, due to the pressure differential across the membrane, a hole was made in the membrane, the only effect expected to occur is the cross-over of hydrogen from the cathode to the anode. This would cause the simulated reformate on the anode to become more hydrogen rich, resulting in lower overpotentials. In order to check whether or not a hole was made in the membrane, the open circuit voltage of the cell was measured under

5% hydrogen in balance argon. If the voltage at open-circuit was that predicted by equation (4.12), it was determined that the membrane remained intact and without holes.

The pressure differentials shown in figure 4.26 were shown not to cause holes to be formed in the membrane.

0.00 Sim reform on anode - atmospheric both -0.02 Sim reform on anode - 5 psig on cathode -0.04 Sim reform on anode - 10 psig on cathode

-0.06 Sim reform on anode - 15 psig on cathode -0.08 Sim reform on anode - 20 psig on cathode -0.10 -0.12 -0.14

Cell Voltage(volts) Cell -0.16 -0.18 -0.20 -0.22 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Current Density (A/cm2)

Figure 4.26: Polarization curves on a pump cell with BASF PBI/PA MEA at 150°C and 5% RH with simulated reformate on the anode and pure hydrogen on the cathode at varying applied cathodic pressures.

At current densities below 0.10 A/cm2, the trends of the graph agree with those predicted by equation (4.12) as shown in figure 4.27. The Nernst potential increases with the increasing cathodic pressure and the slopes of the curves are parallel at low current

Tyler Petek | 90 densities. The model predicts the curves approaching the same current density and remaining about parallel until the adsorption hindered current limit is reached.

At higher current densities, above 0.10 A/cm2, the experimentally observed trends no longer agree with those predicted by the model. At higher current densities, as the applied pressure on the cathode increases the cell voltage actually decreases. Also, when the cell was operated with greater applied pressure on the cathode the adsorption hindered current limit is observed to increase.

0.00

-0.02

-0.04 0 psig on cathode -0.06 5 psig on cathode 10 psig on cathode Cell Voltage (volts) Voltage Cell -0.08 15 psig on cathode 20 psig on cathode -0.10 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Current Density (A/cm2)

Figure 4.27: The model of the pump cell as developed by equation (4.12) operating at 150°C with gas humidification of 5% RH. The anode is under simulated reformate at atmospheric pressure and the cathode has pure hydrogen at varying pressures.

One possible explanation for the observed trends is a significant increase in hydrogen crossover under the greater cell pressure differential. As the pressure of the pure hydrogen on the cathode increases more of the hydrogen is driven across the membrane to the anode. This hydrogen crossover makes the simulated reformate more hydrogen rich. This lessens the poisoning effects of carbon dioxide on the anode and would raise the limiting current as well as lower the Nernst and activation overpotentials. The

Tyler Petek | 91 hydrogen crossover effects should be observed to increase with higher current density because more of the hydrogen is being forced across the cell. Sedlak et al. have found that hydrogen being driven to diffuse across the membrane by a cell pressure gradient can attribute up to about 0.05 A/cm2 at higher pressure gradients. [66]

While the cell with high hydrogen crossover may appear to have higher coulombic and voltaic efficiency it is not actually performing better. The cell operates at a lower voltage because the hydrogen that is being produced on the cathode is going back to the anode and electrochemically going back to the anode. The same hydrogen is using current to be oxidized and reduced twice. The actual faradaic efficiency is actually lower; it takes more energy to produce the same amount of hydrogen than in a pump cell with holes in the membrane. In order to appropriately characterize the efficiency of the pump to purify and pressurize hydrogen reformate streams, the hydrogen recovery should be compared to the voltaic efficiency. This requires monitoring the amount of hydrogen produced.

4.1.5 Performance durability and limits

In an attempt to characterize how a pump cell with a BASF PBI/PA MEA handles operating over a long period of time, the cell voltage was monitored over 30 hour time periods at constant temperature, humidification, and applied current density. A data point was taken every five seconds. This was done at multiple current densities between open- circuit and 1.0 A/cm2 at 150°C and 5% RH. The results are shown in figure 4.28. These tests were done with pure hydrogen on both sides of the cell in an attempt to separate the effects due to the catalyst being poisoned from the rest of the effects of the membrane.

Also, polarization curves were taken before and after each long term test.

Tyler Petek | 92

0.00

-0.02

-0.04 0.2 A/cm2 -0.06 0.4 A/cm2 -0.08 0.6 A/cm2 0.8 A/cm2

-0.10 1.0 A/cm2 Cell Voltage (volts) Voltage Cell -0.12 -0.14 0 10 20 30 40 Time (hours)

Figure 4.28: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant applied current density over 30 hours. The cell is at 150°C with 100% H2 on both sides at 5% RH for all the tests. 0.00

-0.02

-0.04 -0.06 After Assembly After 0.2 A/cm2 -0.08 After 0.4 A/cm2 -0.10 After 0.6 A/cm2

Cell Voltage (volts) Voltage Cell After 0.8 A/cm2 -0.12 After 1.0 A/cm2 -0.14 0.0 0.2 0.4 0.6 0.8 1.0 Current Density (A/cm2)

Figure 4.29: Polarization curves on the pump cell between the long term tests described in figure 4.28.

During the experiments, data was collected every 5 seconds. In order to smooth the data, every point in figure 4.28 is the average over 55 seconds. With time, the cell voltage at a constant current density does in fact become more negative. It is hard to see in figure

4.28, however, by looking at the polarization curves conducted between each tests, the

Tyler Petek | 93 change in the slope of the polarization curves, which is related to the overall cell resistance, can be seen more easily.

After the pump cell was operated at 0.2 A/cm2 for 30 hours, the polarization curve does not show any notable increase in the polarization overpotentials. However, as the current density is increased, the slope of the polarization curve increases. This indicates that operating at higher current densities for a period of time causes an increase in the overpotentials of the cell.

In order to better see the effects of operating the cell for a length of time at higher current densities, on a different cell the long term tests in figure 4.30 were conducted. From figure 4.28 it was decided that 22 hour long tests were sufficient to reach equilibrium.

The data presented in figure 4.28 and figure 4.30 were performed on pump cells with two different PBI/PA MEAs. Figure 4.28 shows long term data with 0.1 A/cm2 between each test. Figure 4.30 shows the change more drastically because there is a greater change in the membrane resistance from the 0.4 A/cm2 test to the 1.0 A/cm2 test.

Again, data was collected every 5 seconds during the experiments. In order to smooth the data, every point in figure 4.30 is the average over 55 seconds. With time, the cell voltage at a constant current density does in fact become more negative. This is much easier to see in the 1.0 A/cm2 test in this set of experiments. Figure 4.31 shows the polarization curves between each test. Again, with time spent at increasing constant current density, the cell polarization overpotentials increase.

Tyler Petek | 94

0.00 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06 0.2 A/cm2 -0.07 -0.08 0.4 A/cm2

Cell Voltage (volts) Voltage Cell -0.09 1.0 A/cm2 -0.10 -0.11 -0.12 0 5 10 15 20 25 Time (hours) Figure 4.30: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant applied current density over 22 hours. The cell is at 150°C with 100% H2 on both sides at 5% RH for all the tests. All of these tests have the cathode on the right side of the cell.

0.00 -0.01 -0.02

-0.03 -0.04 -0.05 -0.06 After Cell Assembly -0.07 -0.08 After 0.2 A/cm2 -0.09 After 0.4 A/cm2 Cell Voltage (volts) Voltage Cell -0.10 -0.11 After 1.0 A/cm2 -0.12 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 2 Current Density (A/cm ) Figure 4.31: Polarization curves on the pump cell between the long term tests described in figure 4.30. All of these tests have the cathode on the right side of the cell.

The observed increase in polarization overpotentials with operating time is thought to be due to the migration of phosphoric acid in the membrane to the electrodes and potentially out of the cell. Figure 4.32 shows the anticipated mechanisms for proton transport across a PBI/PA MEA.

Tyler Petek | 95

Anode Electrolyte Cathode H3PO4 ½ H2  ½ H2 + + + + H + H3PO4 H4PO4 H4PO4 H3PO4 + H

+ + + + + + H H + H3PO4 H3PO4(H ) H3PO4(H ) H3PO4 + H H

------+ e H2PO4 ⃪ H2PO4 ⃪ H2PO4 ⃪ H2PO4 ⃪ H2PO4 + H e-  H3PO4

Figure 4.32: A simple schematic of the proposed mechanisms for proton transport across a PBI/PA membrane.

In figure 4.32, there are two types of transport mechanisms proposed; the Grotthuss and the vehicular transport mechanisms. The top and bottom mechanisms shown in figure

4.32 are vehicular transport mechanisms. The vehicular mechanism works by a charge carrier actually moving across the membrane. Because a polybenzimidazole membrane can be imbibed with more than the stoichiometric amount of phosphoric acid, there are mobile acid group in the bulk membrane. These acid groups are able to be mobile and carry a charge across the membrane. Both the top and bottom mechanisms must be present in order to maintain electroneutrality. The Grotthuss mechanism is where a proton is transferred from one phosphoric acid group to the next by a ‘hopping’ mechanism as displayed in figure 4.33. The mechanisms for proton transport will be discussed in much more detail in the next chapter.

+ HO HO HO H H+

O P OH O P OH O P OH

HO HO HO Figure 4.33: A simple schematic of the Grotthuss mechanism for proton transport in phosphoric acid.

Tyler Petek | 96

It is thought that from the vehicular mechanism, the phosphoric acid groups moving across the cell, phosphoric acid is being built up at the electrode and possibly leaving the membrane. In order to investigate this further, long term tests were conducted on the cell at constant current density with the cathode first on the right side of the cell and then again with the cathode on the left side of the cell at each current density. The impedance spectroscopy after each of these tests is shown in figure 4.34 through figure 4.39.

Impedance spectroscopy was done at each interval with the cell under 100% hydrogen on both sides as well as 5% hydrogen in argon on one side and pure hydrogen on the other.

-0.001

0

Z'' (ohms) After cell assembly After 0.2 A/cm2 with cathode on right After 0.2 A/cm2 with cathode on left 0.001 After 0.4 A/cm2 with cathode on right After 0.4 A/cm2 with cathode on left After 1.0 A/cm2 with cathode on right After cell assembly After 1.0 A/cm2 with cathode on left After 0.2 A/cm2 with cathode on right After 0.2 A/cm2 with cathode on left After 0.4 A/cm2 with cathode on right 0.002 After 0.4 A/cm2 with cathode on left After 1.0 A/cm2 with cathode on right After 1.0 A/cm2 with cathode on left

0.003 10-2

0.004

0.001 0.002 0.003 0.004 0.005 0.006 |Z| (ohms) |Z| After cell assembly After 0.2 A/cm2 with cathode on right Z' (ohms) After 0.2 A/cm2 with cathode on left 10-3 Figure 4.34: Nyquist plotAfter 0.4 of A/cm2 pump with cathode cell on withright 100%10 -1hydrogen100 on 10both1 sides102 after10 each3 long104 After 0.4 A/cm2 with cathode on left term Aftertests. 1.0 A/cm2 All with of cathode the on tests right were at 150°C and 5%Frequency RH. (Hz) After 1.0 A/cm2 with cathode on left

10-2 -25

0

25 theta

|Z| (ohms) |Z| 50

10-3 75 10-1 100 101 102 103 104 10-1 100 101 102 103 104 Frequency (Hz) Frequency (Hz) Figure 4.35: Bode plot of cell at 150°C Figure 4.36: Bode plot of cell at 150°C -25 with 100% H2 on both sides with 100% H2 on both sides

0

25 theta 50

75 10-1 100 101 102 103 104 Frequency (Hz) Tyler Petek | 97

-0.002

-0.001

0

Z'' (ohms) After cell assembly After cell assembly After 0.2 A/cm2 with cathode on right After 0.2 A/cm2 with cathode on right After 0.2 A/cm2 with cathode on left After 0.2 A/cm2 with cathode on left After 0.4 A/cm2 with cathode on right 0.001 After 0.4 A/cm2 with cathode on right After 0.4 A/cm2 with cathodeAfter on 0.4 left A/cm2 with cathode on left After 1.0 A/cm2 with cathodeAfter on 1.0 right A/cm2 with cathode on right After 1.0 A/cm2 with cathodeAfter on 1.0 left A/cm2 with cathode on left

0.002 10-2

0.003

0.001 0.002 0.003 0.004 0.005 0.006 |Z| (ohms) |Z| After cell assembly Z' (ohms) After 0.2 A/cm2 with cathode on right 10-3 After 0.2 A/cm2 with cathode on left -1 0 1 2 3 4 Figure 4.37: AfterNyquist 0.4 A/cm2 with plot cathode of on pump right cell with 5%10 hydrogen10 in inert10 on one10 side after10 each10 After 0.4 A/cm2 with cathode on left Frequency (Hz) Afterlong 1.0 A/cm2 term with cathode tests. on right All of the tests were at 150°C and 5% RH. After 1.0 A/cm2 with cathode on left

10-2 -25

0

25 theta

|Z| (ohms) |Z| 50

10-3 75 -1 0 1 2 3 4 10-1 100 101 102 103 104 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz) Figure 4.38: Bode plot of cell at 150°C Figure 4.39: Bode plot of cell at 150°C -25with 5% H2 in inert on one side with 5% H2 in inert on one side

0 The Nyquist plot under 100% hydrogen on both sides and 5% hydrogen on one side

25 theta shows50 an increase in membrane resistance from the cell assembly to after the long term 75 10-1 100 101 102 103 104 2 test at 0.2 A/cm withFrequency the cathode (Hz) on the right side of the cell from 0.00171 Ω to 0.00182

Ω, or 0.0855 Ω∙cm2 to 0.0910 Ω∙cm2. Switching the cathode to the left side of the cell and then running the cell for another 22 hours at 0.2 A/cm2 did not show a significant increase in the membrane resistance.

After the long term test at 0.4 A/cm2 with the cathode on the right side of the cell, there was a significant increase in the membrane resistance to about 0.00198 Ω, or 0.099

Tyler Petek | 98

Ω∙cm2. By switching the cathode to the left side of the cell and running the pump cell at

0.4 A/cm2 for another 22 hours has been shown to decrease the membrane resistance to about 0.00189 Ω, 0.0945 Ω∙cm2.

As was seen with running the pump cell at 0.4 A/cm2 for 22 hours, the membrane resistance increased after running at 1.0 A/cm2, this time to 0.00211 Ω, or 0.1055 Ω∙cm2.

By putting the cathode on the other side of the pump cell, the membrane resistance was decreased to 0.00192 Ω, or 0.0960 Ω∙cm2. This is close to what the membrane resistance was after the cathode was switched after the test at 0.4 A/cm2.

The trend of increasing membrane resistance after operating at long times agrees with the hypothesis that the free phosphoric acid groups in the membrane are moving to the outside of the membrane, even into the electrode. However, being able to bring the membrane resistance back down to lower resistances suggests that the acid groups can be directed back into the membrane.

Because the membrane resistance never went back to the resistance measured after cell assembly, before any long term tests, there must have been some irreversible effects. One possibility is that the acid is actually leaching out of the cell and leaving with the effluent gases.

In order to fully investigate the ability of a pump cell with a BASF PBI/PA MEA to purify a hydrogen rich stream long term tests were conducted with simulated reformate on the anode. Again, in order to keep a well-defined hydrogen partial pressure on the cathode, pure hydrogen was supplied to the cathode.

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From figure 4.6 it is seen that test stand cannot control the cell voltage above a current density of 0.24 A/cm2. Therefore, the long term tests were only conducted at 0.20 A/cm2.

Figure 4.40 shows the cell potential at a constant 0.20 A/cm2 over 22 hours with the cathode on the right side of the cell. Again, the cell is at 150°C and 5% RH. The decay of the cell voltage is apparent.

0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12 -0.14 -0.16

Cell Voltage (volts) Voltage Cell -0.18 -0.20 -0.22 0 5 10 15 20 25 Time (hours)

Figure 4.40: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant 2 applied current density of 0.20 A/cm over 22 hours. The cell is at 150°C with 100% H2 on the cathode and simulated reformate on the anode. The cathode is the right side of the cell.

Figure 4.41 shows the cell voltage over 22 hours at a constant 0.20 A/cm2 with the cathode on the left side of the cell. This test was started about 30 minutes after the long term test with the cathode on the right side of the cell was concluded.

The initial decrease in cell potential, in the time less than 1 hour, in the test conducted with the cathode on the left side of the cell is most likely due to the anode, now the right side of the cell, being poisoned by the carbon monoxide in the simulated reformate. The increase of cell potential after this time is most likely due to the carbon monoxide leaving the cathode, now the left side of the cell. It makes sense that the cathode is cleaned of

Tyler Petek | 100 carbon monoxide much slower than the anode is poisoned. The cell potential actually returns to the initial potential at the beginning of the long term tests with simulated reformate on the anode.

0.00 -0.02

-0.04 -0.06 -0.08 -0.10 -0.12 -0.14 -0.16 Cell Cell Voltage (volts) -0.18 -0.20 -0.22 0 5 10 15 20 25 Time (hours)

Figure 4.41: The cell voltage of a pump cell with a BASF PBI/PA MEA at a constant 2 applied current density of 0.20 A/cm over 22 hours. The cell is at 150°C with 100% H2 on the cathode and simulated reformate on the anode. The cathode is the left side of the cell.

Figure 4.42 through figure 4.44 show the impedance spectroscopy before and after the two long term tests. As the figures show, the membrane resistance and charge transfer loops do not show significant changes in the MEA between the tests.

Figure 4.45 shows the polarization curves before and after each long term test with simulated reformate on the anode at 150°C. Twenty-two hours at 0.2 A/cm2 with simulated reformate on the anode did not change the membrane resistance or the polarization curves significantly. This agrees with what was seen in the long term tests with pure hydrogen on both sides. The change in cell voltage was most likely due to the poisoning effects on the electrode. After the cell was reversed and the long term test run again, the polarization curve actually shows lower cell overpotentials.

Tyler Petek | 101

-0.003

-0.002

-0.001

0

Z'' (ohms) Before long term tests After 0.2 A/cm2 with cathode on right After 0.2 A/cm210-2 with cathode on left 0.001

0.002 Before long term tests After 0.2 A/cm2 with cathode on right

0.001 0.002 0.003 (ohms) |Z| 0.004 0.005 0.006 After 0.2 A/cm2 with cathode on left Z' (ohms) 10-3 Figure 4.42: Nyquist plot of pump cell with simulated10-1 reformate100 on10 one1 side10 2before 10and3 after10 4 each long term tests. All of the tests were at 150°C andFrequency 5% RH. (Hz)

10-2 -25

0

Before long term tests theta After 0.2 A/cm2 with cathode on right 25 |Z| (ohms) |Z| After 0.2 A/cm2 with cathode on left

10-3 50 -1 0 1 2 3 4 10-1 100 101 102 103 104 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz) Figure 4.43: Bode plot of cell at 150°C Figure 4.44: Bode plot of cell at 150°C

-25with simulated reformate on one side with simulated reformate on one side

0 0.00

theta 25 -0.02 -0.04 50 10-1 100 -0.06101 102 103 104 -0.08Frequency (Hz) -0.10 -0.12 -0.14 -0.16 Before long term tests Cell Voltage (volts) Voltage Cell -0.18 After 0.2 A/cm2 with cathode on right -0.20 After 0.2 A/cm2 with cathode on left -0.22 0 0.1 0.2 0.3 2 Current Density (A/cm ) Figure 4.45: Polarization curves conducted on cell with simulated reformate on the anode and pure hydrogen on the cathode before and after the long term tests under the same conditions. The cell was at 150°C and 5% RH.

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4.2 Application analysis of a pump cell with a BASF PBI/PA MEA

4.2.1 Energy estimates for purifying and pressurizing hydrogen

The energy required to operate the cell is defined in equation (4.13).

(4.13)

From the polarization curve for simulated reformate on the anode and pure hydrogen on the cathode at 150°C, shown in figure 4.6, the energy required to purify a simulated reformate stream over one hour in a pump cell with a BASF PBI/PA MEA is shown in figure 4.46. The energy required to produce pure hydrogen from the simulated reformate increases exponentially with an increase in the applied current density.

From Faraday’s law, equation (3.1), the amount of hydrogen produced at a given current density over a given time can be determined. The relationship between current density and the production rate of hydrogen produced is given in equation (4.14).

(4.14)

Figure 4.47 shows the rate of hydrogen produced from simulated reformate by a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH.

By combining equation (4.13) and equation (4.14), the amount of hydrogen produced per kilowatt-hour of energy applied to the pump cell can be determined.

(4.15)

Keep in mind, the cell voltage is related to the applied current density. This relationship is given in equation (4.12) and is shown experimentally in the polarization curves. Figure

4.48 shows the amount of hydrogen produced per watt-hour applied to the cell as a

Tyler Petek | 103 function of current density for a cell operated with various anodic conditions at 150°C and 5% RH.

2.5

2.0

hours) - 1.5

1.0

0.5 Applied Energy Energy Applied (Watt

0.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Current Density (A/cm2)

Figure 4.46: The applied power necessary to produce pure hydrogen from simulated reformate by a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH at atmospheric pressure.

0.25

0.20

0.15

0.10

producedperhour

2

0.05 molH

0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Current Density (A/cm2)

Figure 4.47: The rate of hydrogen produced from simulated reformate by a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH at atmospheric pressure.

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Moles of hydrogen pumped per hour 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 12 100% H2 10 30% H2

pumped 5% H2

2 8 Sim Reform

6

4

hr per mole of H of mole per hr -

2 Watt 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Current Density (A/cm2)

Figure 4.48: The energy required to pump H2 from various anodic conditions by a pump cell with a BASF PBI/PA MEA operated at 150°C and 5% RH at atmospheric pressure.

As figures 4.48 shows, it is more energy efficient to operate the pump cell at lower hydrogen production rates. The pump cell with a BASF PBI/PA MEA at 150°C can produce hydrogen at flow rates up to 0.23 moles of hydrogen per hour from simulated reformate. The pump cell can operate at about 0 – 10.5 watt-hours per mole of hydrogen pumped.

Equation (4.15) can be applied to the polarization curve for operating a pump cell that produces hydrogen at an increased pressure, as in figure 4.26, but as discussed, it is unsure how much of the hydrogen is actually being produced in a useful stream and how much is diffusing back across the cell. A cross-over analysis of hydrogen at the specified cell pressure gradient must be conducted in order to have an accurate energy cost for pressurized hydrogen production by a pump cell.

Tyler Petek | 105

4.2.2 Comparison of the pump cell with conventional technologies

Pressure swing adsorption is currently the most commonly used process to purify hydrogen rich streams. Kearns et al. have developed a mathematical model for pressure swing adsorption cycles. [82] Figure 4.49 and figure 4.50 show their productivity and specific work of different modeled cycles. The feed composition, or the fraction of the feed that is desired product, is given as yF.

The product flow rate used for these simulations was 0.001 kmol per second. Also, a recovery of 100% was assumed. Therefore, the product flow rate can be calculated by equation (4.16).

(4.16)

In order to compare the energy requirements of pressure swing adsorption with those of the pump cell using a BASF PBI/PA MEA, the feed composition and flow rate were used to extract how much hydrogen is produced per watt-hour using pressure swing adsorption. This relationship is described in equation (4.17).

( ⁄ ) (4.17)

By applying the range of specific energy, kJ per kmol of feed, the range of product produced per kilowatt-hour can be determined. From figure 4.49, the amount of energy, watt-hours, required to pump hydrogen of different cycles of pressure swing adsorption with a feed composition of 0.5 ranges from 10,000 – 80,000 watt-hours per mole of hydrogen pumped.

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Figure 4.49: The specific work and productivity of different pressure swing adsorption cycles with a feed composition of 50% product. [82]

Figure 4.50: The specific work and productivity of different pressure swing adsorption cycles with a feed composition of 90% product. [82]

Tyler Petek | 107

From figure 4.50, the amount of energy, watt-hours, required to pump hydrogen of different cycles of pressure swing adsorption with a feed composition of 0.5 ranges from

5,500 – 30,000 watt-hours per mole of hydrogen pumped.

The range for pressure swing adsorption with the feed composition being 70% product lies somewhere in the middle. The range presented in figure 4.48 for the pump cell, 0 –

10.5 , is much smaller than the possible range predicted in figure 4.49 or figure

4.50 for PSA, 5,500 – 80,000 . The pump cell is up to 8,000 times more energy

efficient.

The purification of hydrogen from the simulated reformate stream can be accomplished with less energy than is required in pressure swing adsorption by using a BASF PBI/PA

MEA in a pump cell. However, PSA produces hydrogen at rates on the order of 1 mole per second while the pump cell operates around 0.1 moles per hour. In order to do a true cost comparison, the capital cost of each system must be considered in the calculations.

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Chapter 5: Understanding proton transport in a Phosphoric Acid Electrolyte

5.1 Phosphoric acid equilibrium

+ - Munson found that phosphoric acid dissociates into four species: H4PO4 , H2PO4 ,

2- + H2P2O7 , and H3O . [83] The equilibrium relationships are described in equation (5.1a) and equation (5.1b).

(5.1a)

(5.1b)

If given enough time to equilibrate, the equilibrium constants for equation (5.1a) and equation (5.1b) are given in equation (5.2a) and equation (5.2b), respectively.

[ ][ ] (5.2a)

[ ][ ] (5.2b)

+ The equilibrium conditions assume that hydronium, H3O , and pyrophosphate ions,

, are in the same concentration. The two equilibrium conditions can be related by equation (5.3). In this equation, X is the concentration of an added acid or base. This value is negative for an acid or positive for a base.

[ ] [ ] √ [ ] (5.3)

Munson reports that these equilibrium ratios have a weak temperature dependence. The equilibrium values reported here were used for all analysis in this chapter.

5.2 Proton transport in a phosphoric acid electrolyte

For the sake of simplicity, the analysis conducted here assumes that K2 goes to zero,

+ - meaning the only species present in the bulk phosphoric acid are H4PO4 and H2PO4 .

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Figure 5.1 shows a schematic of the anticipated mechanisms for proton transport across a phosphoric acid electrolyte. In a hydrogen pump cell, hydrogen should be the only species oxidized at the anode and protons should be the only species reduced at the cathode.

Anode Electrolyte Cathode H3PO4 ½ H2  ½ H2 + + + + H + H3PO4 H4PO4 H4PO4 H3PO4 + H

+ + + + + + H H + H3PO4 H3PO4(H ) H3PO4(H ) H3PO4 + H H

------+ e H2PO4 ⃪ H2PO4 ⃪ H2PO4 ⃪ H2PO4 ⃪ H2PO4 + H e-  H3PO4

Figure 5.1: A schematic of the proposed mechanisms for proton transport across a phosphoric acid electrolyte.

In figure 5.1, there are two types of transport mechanisms proposed; the Grotthuss and the vehicular transport mechanisms. The top and bottom mechanisms shown in figure 5.1 are vehicular transport mechanisms. The vehicular mechanism works by a charge carrier actually moving through the electrolyte. Both the top and bottom mechanisms must be present in order to maintain electroneutrality. The charge carriers in this case are the

+ - dissociated phosphoric acid species, H4PO4 and H2PO4 .

The conductivity of phosphoric acid is seemingly quite high to be solely attributed to the vehicular mechanism, especially when the high viscosity is taken into consideration. The high conductivity is due in part to the presence of a protonic Grotthus chain conduction mechanism. [83] The Grotthuss mechanism is where a proton is transferred from one phosphoric acid group to the next by a ‘hopping’ mechanism as displayed in figure 5.2.

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+ HO HO HO H H+

O P OH O P OH O P OH

HO HO HO Figure 5.2: A schematic of the Grotthuss mechanism for proton transport in phosphoric acid.

Figure 5.1 shows that the proton transport across a phosphoric acid electrolyte can cause a build-up of phosphoric acid at the electrode interface due to the vehicular mechanisms.

This build-up of acid at the electrode could either diffuse back into the bulk, driven by the equilibrium, or leach out of the cell. In chapter 4, there were experimental observations that indicate a decrease in the membrane conductivity that could be due to the loss of acid. This is one possible explanation to the acid loss. The rest of this chapter will investigate the mechanism of proton transport across a phosphoric acid electrolyte in an attempt to better understand how the acid behaves in a PBI membrane imbibed with phosphoric acid.

5.3 Developing a simplified model

The governing equation for ionic flux, assuming dilute solution theory applies, is given in equation (5.4a). The equilibrium concentration expected of each ion, about 0.37 m, indicate that this should be a valid assumption.

(5.4a)

In this equation the subscript j indicates that it is for a specific ionic species. Also, N is the ionic flux, D is the diffusion coefficient, u is the ionic mobility, z is the number of moles of electrons per mole of ionic species, F is Faraday’s constant, c is the concentration, is the potential gradient, and v is the bulk solution velocity. The first

Tyler Petek | 111 term of equation (5.4a) is the flux due to diffusion, the second term is due to ionic migration and the third is due to convection.

In the system presented in figure 5.1, the phosphoric electrolyte is stagnant. This means that there is no solution velocity and the convection term goes to zero. The ionic flux for the system under consideration can now be written as equation (5.4b).

(5.4b)

The flux of both phosphoric acid groups must be taken into effect as well as the Grotthuss proton. During the development of this model the subscripts “H+”, “+”, and “–” will be

+ - used to indicate the Grotthuss proton, H4PO4 , and H2PO4 , respectively. Equation (5.5a) through equation (5.5c) shows the ionic flux for each species.

(5.5a)

(5.5b)

(5.5c)

There is no concentration term in the flux of the Grotthuss proton because the concentration of this proton is always zero. As figure 5.2 shows, whenever a proton moves to a phosphoric acid molecule, another proton is displaced. Thus, no concentration of protons is built up in the bulk. The concentration of the ionic migration term, , is the concentration of Grotthuss carriers, in this system that is the phosphoric acid groups.

Again, there is no concentration gradient of these species.

The change of the concentration of the ionic species with time is defined as the negative gradient of the ionic flux. This is described in equation (5.6).

(5.6)

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For each species, equation (5.6) can be written as the following.

( ) (5.7a)

( ) (5.7b)

( ) (5.7c)

The change in the concentration of the Grotthuss charge carrier species does not change across the cell so the gradient goes to zero.

At any point in the cell electroneutrality must be satisfied. Electroneutrality specifies that there must not be an imbalance of charge species. This can be expressed as the sum of all charge species’ concentration times charge, as described in equation (5.8a).

∑ (5.8a)

Because there is no concentration of Grotthuss protons, the only two contributing species are the phosphoric groups.

(5.8b)

+ Because the charge density of H4PO4 is equal to positive one and the charge density of

- H2PO4 is equal to negative one, the concentrations of both of these species must always be equal. Therefore, the change in their concentration with time must also be equal.

(5.9)

⁄ (5.10a)

(5.10b)

+ - Because the concentration of H4PO4 and H2PO4 must always be equal, the term c+/- will be used for either of their concentrations. By applying equation (5.7b) and (5.7c) to

Tyler Petek | 113 equation (5.10b), the following equation can be derived for the gradient of the concentration and potential flux.

( ) ( ) (5.11)

Applying equation (5.11) to either equation (5.7b) or (5.7c) allows for the change in concentration with time of either species to be written not as a function of the potential gradient.

⁄ (5.12)

If the ionic mobilities and diffusion coefficiens of both species are the same, equation

(5.12) can be rewritten as the following.

⁄ (5.13a)

⁄ (5.13b)

⁄ ⁄ ⁄ ⁄ ⁄ ⁄ ⁄ (5.13c) ⁄ ⁄ ⁄

⁄ (5.14) ⁄ ⁄

In order to solve equation (5.14) for the system of phosphoric acid electrolyte between two parallel plates as described in figure 5.1, an initial condition and two boundary conditions must be used.

5.4 Description of possible initial and boundary conditions

The system is assumed to have existed long enough that it is at chemical equilibrium before a potential field is applied. When time equals zero will be defined when the

+ - potential field is applied. Up until time zero, the concentration of H4PO4 and H2PO4

must equal .

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The first boundary condition will be defined by the definition of current density.

∑ (5.15)

In order to simplify the equation, the ionic conductivity of each species will be defined as follows.

(5.16)

Again, if the ionic mobility and concentration are equal and the charge are equal and

+ - opposite, than the ionic conductivity of H4PO4 and H2PO4 must be equal. The term

⁄ will be used to define their ionic conductivity.

By applying the ionic flux and this definition of ionic conductivity, the total current density can be expanded.

[ ⁄ ] (5.17)

Equation (5.17) is the definition of the total current density in the electrode because it includes all three charge carrying species. Across the electrodes, at position and

, the only species that carries the charge is protons. Therefore, the total proton flux across the electrodes can be defined by equation (5.18).

(5.18)

The flux of the protons at the electrodes is defined by the applied current density. The total ionic flux at the electrode, the proton flux, should equal the sum of the flux of the charge carriers in the electrolyte.

∑ (5.19)

By combining equations (5.5a-c) and equations (5.17) through (5.19), the total ionic flux through the electrolyte can be defined.

Tyler Petek | 115

⁄ ⁄ (5.20)

The second boundary condition will be that at half-way between the plates, at , the

change in concentration of ion with position will be zero.

(5.21)

This boundary condition implies that there exists some point of inflection at the center or

+ - cell symmetry. With the vehicular mechanism, H4PO4 and H2PO4 move across the cell but both, due to either chemical equilibrium or the transport of H+ to the electrodes, become or come from phorsphoric acid, H3PO4. Both species should be either accumulating or depleting on the anode and cathode. This is described in figure 5.3 and figure 5.4. The actual distribution can be either or some combination of these figures.

+ - However, the distribution of H4PO4 and H2PO4 must be the same to satisfy electroneutrality.

Cj Cj

x → x →

Figure 5.3: Example concentration Figure 5.4: Example concentration distribution across the electrolyte distribution across the electrolyte

Tyler Petek | 116

A summary of the model thus far is presented below.

Equation: ⁄ ⁄ ⁄

I.C.: ⁄

B.C. 1: ⁄

B.C. 2: [ ⁄ ⁄ ⁄ ]

To keep things simple, the potential field will be defined as being linear. This is described in figure 5.5 and equation (5.22).

휙

x = 0 x → x = L

Figure 5.5: The schematic of the anticipated applied potential field.

( ) (5.22)

From equation (5.22), the gradient of the potential field is apparent.

( ) (5.23)

The potential, , is not a constant but is a function of current density , temperature, and time. The following partial derivatives of the potential were determined from the experimental data presented in chapter 4 for the pump cell with pure hydrogen on both sides of the cell. The subscripts of the derivate denote what conditions were held

Tyler Petek | 117 constant. For the cases indicated, the temperature was 150°C and the current density was

1.0 A/cm2. When time is indicated as being constant, it means that the data from the polarization curves were used where the cell did not remain at the conditions for any appreciable time.

( , , )

[ ] , ⁄

[ ] ,

[ ] ,

( , , )

5.5 Qualitative description and ramifications

This chapter investigated the proton conductivity of a phosphoric acid electrolyte. There were two proposed modes of proton transport across the electrolyte, transport either by the vehicular mechanism or by the Grotthuss mechanism. With the Grotthuss mechanism, there is no phosphoric acid movement or proton concentration gradient. With the vehicular mechanism, however, the ions are moving across the cell. Due to the transport of protons and equilibrium of the phosphoric acid, the ions become or come from phosphoric acid at the electrodes. It is proposed that this could cause a build-up of phosphoric acid at the electrodes. If the applied current is pushed high enough for long enough periods of time, this build-up of phosphoric acid at the electrodes could be significant enough to cause the phosphoric acid to leave the electrolyte.

Tyler Petek | 118

This investigation into pure phosphoric electrolyte should allow for a better understanding of how the free phosphoric acid behaves in the polybenzimidazole membranes that are imbibed with phosphoric acid at doping levels much greater than two phosphoric acid groups per PBI repeat unit.

The potential for phosphoric acid to accumulate and potentially leach out of the membrane could very well be the cause of the decrease in PBI/PA conductivity as the membrane is operated at current densities above 0.4 A/cm2 for long periods of time.

Tyler Petek | 119

Chapter 6: Conclusions and Future Recommendations

6.1 Ability of a PBI/PA pump cell to purify and pressurize H2 streams

In this work, MEAs with polybenzimidazole membranes that were imbibed with phosphoric acid were supplied by BASF to be used in electrochemical hydrogen pump cells. It was found that these membranes could efficiently operate in a pump cell over the temperature range of 120°C - 180°C. A pump cell with a BASF PBI/PA MEA was shown to be able to purify a simulated hydrogen reformate stream, from SMR-WGS processes, at 150°C by applying 0.145 V applied to the cell at 0.2 A/cm2. By operating the cell at

180°C the simulated reformate could be purified at current densities as high 0.6 A/cm2 with about 0.215V applied to the cell. This simulated reformate stream was composed of

70% H2, 3% CO, 20% CO2, and 7% CH4. These experiments were done on a test with an electrode area of 50 cm2 and 5% RH on both sides of the membrane.

At 150°C, the pump cell’s ability to purify hydrogen reformate was tested over the applied current density range of 0 – 0.25 A/cm2. The cell voltage increased significantly and was no longer controllable at current densities above 0.25 A/cm2. It is thought that this ‘adsorption hindered current limit’ is due to the preferential adsorption of CO on the catalyst and is not due to the diffusion limits of the MEA.

For the cell operated at 150°C, the amount of hydrogen produced per kilowatt-hour decreases significantly with increasing production rate. At 150°C and hydrogen production rates of 0 – 0.23 moles of hydrogen per hour (0 – 0.25 A/cm2), the 50 cm2 lab scale pump cell operated at 0.5 – 10.5 watt-hours per mole of hydrogen pumped. This is about 10,000 times more energy efficient than pressure swing adsorption systems.

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However, PSA systems typically produce hydrogen at flow rates of about 3,600 moles per hour. Therefore, pump cell scale-up and capital cost must be considered for an overall economic comparison.

The pump cell with a BASF PBI/PA MEA was shown not only to be able to purify a hydrogen reformate stream but also to pressurize the pure hydrogen product. This work showed that one 50 cm2 lab pump cell could pressurize a pure hydrogen product up to 20 psi above the feed stream. It is expected that the cell can operate at even higher pressure differentials. At a hydrogen production at a pressure 20 psig higher than the feed, the cell could operate up to 0.35 A/cm2 at 150°C with an applied cell voltage of about 0.21 V.

An energy cost analysis was not conducted on the ability of the pump cell to pressurize the hydrogen product stream. It is expected that there is a significant amount of back diffusion of hydrogen in the pump cell operated at a significant pressure differential. In our experiments, the actual hydrogen production rate could not be measured directly, making the energy cost analysis difficult without further work.

6.2 Durability limits of a PBI/PA pump cell

The pump cell was controlled at 150°C with 5% RH was operated at constant current densities varying between 0.2 and 1.0 A/cm2 for pure hydrogen on both sides for over 30 hours. At lower current densities, around 0.2 A/cm2, no noticeable increase in required cell voltage was measured over 30 hours. However, at current densities at or above 0.4

A/cm2, the cell voltage required to operate the cell increased. The need for an increased voltage was a result of increasing membrane resistance.

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From a theoretical investigation of the mechanism of proton conduction through a phosphoric acid electrolyte, it is thought that operation of the PBI/PA based pump cell for long periods of time results in a decrease in the membrane conductivity because of phosphoric acid leaving the membrane.

After a long term test was conducted at constant current density and the increase in membrane resistance was verified, the cell was reversed. This means that the current was driven across the cell in the opposite direction, making what was the anode now the cathode. After 30 hours with the cell reversed, the polarization and impedance spectroscopy show that the cell and membrane resistance decreased to values close to what they were before the first long term test.

The increase in membrane resistance after operation at high current densities for long periods of time points towards an effective acid migration in the membrane to the electrodes. The ability to recover this conductivity by reversing the direction of current in the cell indicates that the majority of the acid is not leaving the cell and can be driven back through the cell. Because, after a number of long term tests, the membrane conductivity did not fully rise back to the value before the long term tests were conducted, some permanent effect does occur. The source of this permanent loss is unclear.

6.3 Recommendations for future work

This thesis research has characterized the operation of an electrochemical hydrogen pump cell using a BASF PBI/PA MEA with pure hydrogen, hydrogen with inert diluents, and hydrogen reformates on the anode. The ability of the pump cell to purify and pressurize

Tyler Petek | 122 the pump cell was characterized. It was observed that the mass transport limiting current for these cells is significantly greater than 1 A/cm2. However, when operating with 3%

CO in the hydrogen feed on the anode, the cell has an adsorption hindered current limit less than 0.65 A/cm2 in the temperature range of 120°C - 180°C. When developing a model of the polarization effects of the cell under these conditions, the approximation for concentration overpotential did a very poor job at predicting the polarization effects. A more in-depth theoretical work to examine the effects of CO could be done to better understand how the cell might behave under simulated reformate streams.

Another issue that arose during the energy analysis of the ability of the pump cell to pressurize the simulated hydrogen reformate stream. It is expected that significant back diffusion occurs at pressure differentials greater than 5 psi. By measuring the actual amount of hydrogen produced by the pump cell and comparing it to the theoretical amount predicted by Faraday’s law, the amount of back diffusion can be estimated and an accurate energy assessment of the performance of the pump cell can be obtained.

The effects of long term operation of the pump cell were investigated. It was postulated that the decreasing conductivity of the membrane was due to phosphoric acid moving in the membrane and building up at the electrodes or even possibly leaching out of the

MEA. An analysis of the acid distribution in the membrane after long term operation and careful chemical analysis to determine the acid content of the exhaust water, would help to determine exactly how much, if any, phosphoric acid is being lost from the membrane.

By better understanding where the phosphoric acid is in the membrane, a more detailed investigation into the actual proton transport mechanisms in the electrolyte can be conducted.

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Appendix A – BASF PBI/PA MEA 6 Life

Test # Date Time Slope (Ω cm2) Res (mΩ) Avg R (mΩ) Int (V) Data Type 6-May-11 11:00 AM Cell was assembled and left to come to operating conditions 1 9-May-11 9:00 AM -0.0907 -1.81 -1.80 -0.0053 Average -0.0897 -1.79 -0.0056 Mode Stopped at 0.5 A/cm2 due to hydrogen alarms 2 9-May -11 2:45 PM -0.0930 -1.86 -1.86 -0.0046 Average -0.0931 -1.86 -0.0045 Mode 3 9-May -11 3:30 PM 30 hour test at 0.2 A/cm2 4 11-May-11 12:45 AM -0.0942 -1.88 -1.88 -0.0048 Average -0.0937 -1.87 -0.0050 Mode Stopped at 0.9 A/cm2 due to hydrogen alarms 5 11-May -11 10:00 AM -0.0966 -1.93 -1.88 -0.0043 Average -0.0910 -1.82 -0.0056 Mode Stopped at 0.3 A/cm2 due to hydrogen alarms 6 11-May -11 3:10 PM -0.0936 -1.87 -1.87 -0.0041 Average -0.0934 -1.87 -0.0043 Mode 7 11-May -11 4:10 PM 30 hour test at 0.4 A/cm2 During the beginning, the H2 generators were overdrawn on and shut off. Mass transport viewed. Tanks turned on and test restarted 8 12-May -11 11:00 PM -0.0966 -1.93 -1.94 -0.0046 Average -0.0972 -1.94 -0.0041 Mode 9 13-May -11 11:00 AM -0.0976 -1.95 -1.95 -0.0045 Average -0.0973 -1.95 -0.0046 Mode 10 13-May -11 11:30 AM 30 hour test at 0.6 A/cm2 11 14-May-11 5:30 PM -0.1003 -2.01 -2.01 -0.0039 Average -0.1009 -2.02 -0.0036 Mode 12 15-May -11 10:00 PM -0.1011 -2.02 -2.02 -0.0044 Average -0.1007 -2.01 -0.0047 Mode 13 16-May -11 12:15 PM -0.1017 -2.03 -2.03 -0.0048 Average -0.1013 -2.03 -0.0051 Mode 14 18-May -11 2:30 PM Impedance scan with 100% H2 on both sides 15 18-May-11 3:20 PM Impedance scan with 100% H2 on both sides 16 18-May-11 3:30 PM Impedance scan with 100% H2 on both sides 17 18-May-11 4:20 PM Impedance scan with 5% H2 in argon on 'cathode' 18 18-May-11 4:35 PM Impedance scan with 5% H2 in argon on 'cathode' 19 18-May-11 5:00 PM Impedance scan with 5% H2 in argon on 'anode' 20 18-May-11 5:15 PM Impedance scan with 5% H2 in argon on 'anode' 21 18-May-11 5:45 PM Impedance scan with 5% H2 in argon on 'anode' 22 18-May-11 6:20 PM -0.1017 -2.03 -2.03 -0.0051 Average -0.1014 -2.03 -0.0050 Mode 23 18-May -11 6:50 PM 30 hour test at 0.8 A/cm2 24 20-May-11 2:15 AM -0.1055 -2.11 -2.10 -0.0043 Average -0.1046 -2.09 -0.0050 Mode 25 20-May -11 2:55 AM Impedance scan with 100% H2 on both sides 26 20-May-11 3:11 AM Impedance scan with 5% H2 in argon on 'anode'

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Test # Date Time Slope (Ω cm2) Res (mΩ) Avg R (mΩ) Int (V) Data Type 27 20-May-11 3:26 AM Impedance scan with 5% H2 in argon on 'cathode' 28 20-May-11 3:35 AM Impedance scan with 5% H2 in argon on 'cathode' 29 20-May-11 3:49 AM Impedance scan with 5% H2 in argon on 'anode' 30 20-May-11 12:10 PM 30 hour test at 0.8 A/cm2 31 21-May-11 8:15 PM -0.1070 -2.14 -2.14 -0.0040 Average -0.1066 -2.13 -0.0043 Mode 32 21-May -11 8:40 PM Impedance scan with 100% H2 on both sides 33 21-May-11 8:53 PM Impedance scan with 5% H2 in argon on 'cathode' 34 21-May-11 9:01 PM Impedance scan with 5% H2 in argon on 'cathode' 35 21-May-11 9:14 PM Impedance scan with 5% H2 in argon on 'anode' 36 21-May-11 9:22 PM Impedance scan with 5% H2 in argon on 'anode' 37 21-May-11 9:30 PM 11 hour test at 0.8 A/cm2 38 22-May-11 9:00 AM -0.1051 -2.10 -2.11 -0.0045 Average -0.1054 -2.11 -0.0044 Mode 39 22-May -11 9:23 AM Impedance scan with 100% H2 on both sides 40 22-May-11 9:46 AM Impedance scan with 5% H2 in argon on 'anode' 41 22-May-11 9:38 AM Impedance scan with 5% H2 in argon on 'cathode' 25-May-11 7:30 AM There was a power outage in the lab The cell was without flows for 2.5 hours. 42 25-May -11 10:30 AM -0.1069 -2.14 -2.14 -0.0045 Average -0.1073 -2.15 -0.0043 Mode 43 25-May -11 11:00 AM Impedance scan with 100% H2 on both sides 44 25-May-11 1:45 PM Impedance scan with 5% H2 in argon on 'anode' 45 25-May-11 2:00 PM Impedance scan with 5% H2 in argon on 'cathode' 46 25-May-11 2:18 PM Impedance scan with 100% H2 on both sides 47 25-May-11 2:30 PM -0.1048 -2.10 -2.10 -0.0048 Average -0.1049 -2.10 -0.0046 Mode 48 25-May -11 3:00 PM 1 hour test at 1.0 A/cm2 Power outage caused sensors to bug out and flows were shut off I got flows back up and let sit at OC 49 28-May -11 2:50 PM Not really linear. And for some reason the flows are funny at the end of them 50 28-May -11 3:40 PM Impedance scan with 100% H2 on both sides at 500 mL/min 51 28-May-11 3:50 PM Impedance scan with 100% H2 on both sides at 1000 mL/min 52 28-May-11 4:00 PM Impedance scan with 100% H2 on both sides at 500 mL/min 53 28-May-11 4:05 PM Impedance scan with 100% H2 on both sides at 1000 mL/min 54 28-May-11 4:11 PM Impedance scan with 100% H2 on both sides at 1500 mL/min 55 28-May-11 2:30 PM -0.1106 -2.21 -2.21 -0.0062 Average -0.1099 -2.20 -0.0068 Mode 30 hour test at 1.0 A/cm2 56 29-May -11 9:30 PM -0.1128 -2.26 -2.25 -0.0060 Average -0.1125 -2.25 -0.0063 Mode

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Appendix B – BASF PBI/PA MEA 8 Life

Test # Date Time Slope (Ω cm2) Res (mΩ) Avg R (mΩ) Int (V) Data Type 1 22-Jun-11 11:08 AM Impedance scan with 100% Hydrogen on both sides. WE on right 2 22-Jun-11 11:22 AM Impedance scan with 100% Hydrogen on both sides. WE on left 3 22-Jun-11 11:36 AM Impedance scan with 5% Hydrogen on right 4 22-Jun-11 11:44 AM Impedance scan with 5% Hydrogen on right 5 22-Jun-11 11:58 AM Impedance scan with 5% Hydrogen on left 6 22-Jun-11 12:06 AM Impedance scan with 5% Hydrogen on left 7 22-Jun-11 12:21 PM Impedance scan with 5% Hydrogen on right 8 22-Jun-11 12:35 PM -0.0925 -1.850 -1.853 -0.0030 Avg Inc. Cathode on right -0.0928 -1.856 -0.0038 Decreasing 9 22-Jun-11 1:00 PM -0.0947 -1.894 -1.908 -0.0029 Avg Inc. Cathode on left -0.0961 -1.922 -0.0029 Decreasing 10 22-Jun-11 1:30 PM 22 hr test at 0.2 A/cm2 with the cathode on the right 11 23-Jun-11 12:01 PM Impedance scan with 100% Hydrogen on both sides. WE on right 12 23-Jun-11 12:10 PM Impedance scan with 100% Hydrogen on both sides. WE on left 13 23-Jun-11 12:23 PM Impedance scan with 5% Hydrogen on left 14 23-Jun-11 12:47 PM Impedance scan with 5% Hydrogen on right 15 23-Jun-11 12:55 PM Impedance scan with 5% Hydrogen on right 16 23-Jun-11 1:15 PM -0.0957 -1.914 -1.909 -0.0029 Avg Inc. Cathode on right -0.0952 -1.904 -0.0038 Decreasing 17 23-Jun-11 1:40 PM -0.0991 -1.982 -2.001 -0.0026 Avg Inc. Cathode on left -0.101 -2.020 -0.0030 Decreasing 18 23-Jun-11 2:15 PM 22 hr test at 0.2 A/cm2 with the cathode on the left side 19 24-Jun-11 12:53 PM -0.1024 -2.048 -2.038 -0.0018 Avg Inc. Cathode on left -0.1014 -2.028 -0.0028 Decreasing 20 24-Jun-11 1:18 PM -0.0955 -1.910 -1.922 -0.0032 Avg Inc. Cathode on right -0.0967 -1.934 -0.0030 Decreasing 21 24-Jun-11 1:38 PM Impedance scan with 100% Hydrogen on both sides. WE on right 22 24-Jun-11 1:57 PM Impedance scan with 100% Hydrogen on both sides. WE on left 23 24-Jun-11 2:18 PM Impedance scan with 5% Hydrogen on left 24 24-Jun-11 2:29 PM Impedance scan with 5% Hydrogen on left 25 24-Jun-11 2:45 PM Impedance scan with 5% Hydrogen on right 26 24-Jun-11 3:03 PM Impedance scan with 5% Hydrogen on right 27 26-Jun-11 7:54 PM -0.0984 -1.968 -1.959 -0.0033 Avg Inc. Cathode on right -0.0975 -1.950 -0.0040 Decreasing 28 26-Jun-11 8:23 PM -0.1012 -2.024 -2.037 -0.0023 Avg Inc. Cathode on left -0.1025 -2.050 0.0027 Decreasing 29 26-Jun-11 8:48 PM Impedance scan with 100% Hydrogen on both sides. WE on right 30 26-Jun-11 9:01 PM Impedance scan with 100% Hydrogen on both sides. WE on left 31 26-Jun-11 9:21 PM Impedance scan with 5% Hydrogen on left 32 26-Jun-11 9:42 PM Impedance scan with 5% Hydrogen on right

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Test # Date Time Slope (Ω cm2) Res (mΩ) Avg R (mΩ) Int (V) Data Type 33 26-Jun-11 10:10 PM 22 hr test at 0.4 A/cm2 with the positive electrode on the right side 34 27-Jun-11 8:29 PM -0.1014 -2.028 -2.032 -0.0044 Avg Inc. Cathode on right -0.1018 -2.036 -0.0040 Decreasing 35 27-Jun-11 8:29 PM -0.1055 -2.110 -2.118 -0.0028 Avg Inc. Cathode on left -0.1063 -2.126 -0.003 Decreasing 36 27-Jun-11 9:10 PM Impedance scan with 100% Hydrogen on both sides. WE on right 37 27-Jun-11 9:44 PM Impedance scan with 100% Hydrogen on both sides. WE on left 38 27-Jun-11 10:04 PM Impedance scan with 5% Hydrogen on left 39 27-Jun-11 10:25 PM Impedance scan with 5% Hydrogen on left 40 27-Jun-11 10:46 PM Impedance scan with 5% Hydrogen on right 41 27-Jun-11 10:55 PM Impedance scan with 5% Hydrogen on right 42 28-Jun-11 11:08 AM 22 hr test at 0.4 A/cm2 with the positive electrode on the left side 43 29-Jun-11 9:13 AM -0.1046 -2.092 -2.092 -0.0024 Avg Inc. Cathode on left -0.1046 -2.092 -0.003 Decreasing 44 29-Jun-11 9:35 AM -0.0979 -1.958 -1.974 -0.0038 Avg Inc. Cathode on right -0.0995 -1.990 -0.004 Decreasing 45 29-Jun-11 10:04 AM Impedance scan with 100% Hydrogen on both sides. WE on right 46 29-Jun-11 10:13 AM Impedance scan with 100% Hydrogen on both sides. WE on left 47 29-Jun-11 10:28 AM Impedance scan with 5% Hydrogen on left 48 29-Jun-11 10:44 AM Impedance scan with 5% Hydrogen on right 49 29-Jun-11 1:28 PM 34 hr test at 1.0 A/cm2 with the positive electrode on the right side 50 30-Jun-11 9:49 PM -0.1043 -2.086 -2.089 -0.0046 Avg Inc. Cathode on right -0.1046 -2.092 -0.005 Decreasing 51 30-Jun-11 10:27 PM -0.1085 -2.170 -2.170 -0.0023 Avg Inc. Cathode on left -0.1085 -2.170 -0.004 Decreasing 52 30-Jun-11 10:51 PM Impedance scan with 100% Hydrogen on both sides. WE on right 53 30-Jun-11 11:01 PM Impedance scan with 100% Hydrogen on both sides. WE on left 54 30-Jun-11 11:28 PM Impedance scan with 5% Hydrogen on left 55 30-Jun-11 11:50 PM Impedance scan with 5% Hydrogen on right 56 1-Jul-11 12:15 AM 34 hr test at 1.0 A/cm2 with the positive electrode on the left side 57 2-Jul-11 9:22 AM -0.1034 -2.068 -2.114 -0.0024 Avg Inc. Cathode on left -0.1080 -2.160 -0.003 Decreasing 58 2-Jul-11 9:48 AM -0.0989 -1.978 -1.995 -0.0040 Avg Inc. Cathode on right -0.1006 -2.012 -0.004 Decreasing 59 2-Jul-11 10:10 AM Impedance scan with 100% Hydrogen on both sides. WE on right 60 2-Jul-11 10:19 AM Impedance scan with 100% Hydrogen on both sides. WE on left 61 2-Jul-11 10:38 AM Impedance scan with 5% Hydrogen on left 62 2-Jul-11 10:57 AM Impedance scan with 5% Hydrogen on right

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Test # Date Time Slope (Ω cm2) Res (mΩ) Avg R (mΩ) Int (V) Data Type 63 2-Jul-11 5:26 PM -0.2342 -4.684 -4.684 -0.0032 Avg Inc. 5% H2 both, cath on right Mass transport effects seen significantly after 0.3 A/cm2 64 2-Jul-11 5:47 PM -0.1260 -2.520 -2.497 0.0435 Avg Inc. 5% H2 right, cath on right -0.1237 -2.474 0.0416 Decreasing 65 2-Jul-11 6:11 PM -0.1790 -3.580 -3.580 -0.0518 Avg Inc. 5% H2 right, cath on left Mass transport effects seen significantly after 0.3 A/cm2 66 2-Jul-11 6:29 PM -0.0981 -1.962 -1.961 -0.0040 Avg Inc. Cathode on right -0.0980 -1.960 -0.0055 Decreasing 67 2-Jul-11 6:48 PM -0.1044 -2.088 -2.099 -0.0022 Avg Inc. Cathode on left -0.1055 -2.110 -0.0033 Decreasing 68 2-Jul-11 8:25 PM -0.0960 -1.920 -1.920 0.0039 Avg Inc. 5 psig left, cath on right 69 2-Jul-11 9:11 PM -0.1024 -2.048 -2.048 -0.0096 Avg Inc. 5 psig left, cath on left 70 2-Jul-11 9:45 PM -0.0989 -1.978 -1.978 -0.0158 Avg Inc. 10 psig left, cath on left 71 20-Jul-11 6:10 PM -0.1727 -3.454 -3.452 -0.0067 Avg Inc. Cath on left-70%H2-120C -0.1725 -3.450 -0.0092 Decreasing 72 20-Jul-11 6:31 PM Impedance scan with 70% Hydrogen/WE on right at 120 C. separate 73 20-Jul-11 6:42 PM Impedance scan with 70% Hydrogen on right. WE on right at 120 C. 74 21-Jul-11 9:46 AM -0.1520 -3.040 -3.029 -0.0067 Avg Inc. Cath on left-70%H2-130C -0.1509 -3.018 -0.0095 Decreasing 75 21-Jul-11 10:10 AM Impedance scan with 70% Hydrogen/WE on right at 130 C. separate 76 21-Jul-11 10:21 AM Impedance scan with 70% Hydrogen on right. WE on right at 130 C. 77 22-Jul-11 12:41 PM -0.1308 -2.616 -2.618 -0.0084 Avg Inc. Cath on left-70%H2-140C -0.1310 -2.620 -0.0096 Decreasing 78 22-Jul-11 1:08 AM Impedance scan with 70% Hydrogen on right. WE on right at 140 C. 79 22-Jul-11 8:35 AM -0.1211 -2.422 -2.444 -0.0080 Avg Inc. Cath on left-70%H2-150C -0.1233 -2.466 -0.0093 Decreasing 80 22-Jul-11 8:56 AM Impedance scan with 70% Hydrogen on right. WE on right at 150 C. 81 22-Jul-11 5:23 PM -0.1118 -2.236 -2.231 -0.0084 Avg Inc. Cath on left-70%H2-160C -0.1113 -2.226 -0.0097 Decreasing 82 22-Jul-11 5:40 PM Impedance scan with 70% Hydrogen on right. WE on right at 160 C. 83 24-Jul-11 12:41 PM -0.1123 -2.246 -2.231 -0.0077 Avg Inc. Cath on left-70%H2-170C -0.1108 -2.216 -0.0098 Decreasing 84 24-Jul-11 1:10 PM Impedance scan with 70% Hydrogen/WE on right at 170 C. 85 24-Jul-11 1:19 PM Impedance scan with 70% Hydrogen/WE on right at 170 C. 86 24-Jul-11 10:41 PM -0.1041 -2.082 -2.120 -0.0083 Avg Inc. Cath on left-70%H2-180C -0.1079 -2.158 -0.0069 Decreasing

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Test # Date Time Slope (Ω cm2) Res (mΩ) Avg R (mΩ) Int (V) Data Type 87 24-Jul-11 11:40 PM Impedance scan with 70% Hydrogen/WE on right at 180 C. 88 26-Jul-11 11:26 AM -0.1784 -3.568 -3.568 -0.0071 Avg Inc. Cath on left-simreform-180C N/A 89 26-Jul-11 11:41 AM Impedance scan with sim refrom/WE on right at 180 C. 90 26-Jul-11 3:50 PM -0.2207 -4.414 -4.414 -0.0085 Avg Inc. Cath on left-simreform-170C N/A 91 26-Jul-11 4:00 PM Impedance scan with sim refrom/WE on right at 170 C. 92 26-Jul-11 5:57 PM -0.2630 -5.260 -5.260 -0.0090 Avg Inc. Cath on left-simreform-160C N/A 93 26-Jul-11 6:08 PM Impedance scan with sim refrom/WE on right at 160 C. 94 26-Jul-11 10:25 PM -0.4931 -9.862 -9.862 -0.0096 Avg Inc. Cath on left-simreform-150C N/A 95 26-Jul-11 10:36 PM Impedance scan with sim refrom/WE on right at 150 C. 96 26-Jul-11 10:44 PM Impedance scan with sim refrom/WE on right at 150 C. 97 27-Jul-11 10:47 AM -0.4500 -9.000 -9.000 -0.0090 Avg Inc. Cath on left-simreform-140C N/A 98 27-Jul-11 11:15 AM Impedance scan with sim refrom/WE on right at 140 C. 99 27-Jul-11 3:50 PM -0.5791 -11.582 -11.582 -0.0094 Avg Inc. Cath on left-simreform-130C N/A 100 27-Jul-11 4:30 PM Impedance scan with sim refrom/WE on right at 130 C. 101 27-Jul-11 9:17 PM -1.0461 -20.922 -20.922 -0.0093 Avg Inc. Cath on left-simreform-120C N/A 102 27-Jul-11 10:30 PM Impedance scan with sim refrom/WE on right at 120 C. 103 28-Jul-11 11:46 AM -0.1654 -3.308 -3.308 -0.0071 Avg Inc. Cath on left-simreform-180C N/A 104 28-Jul-11 12:40 PM Impedance scan with sim refrom/WE on right at 180 C. 105 28-Jul-11 4:46 PM -0.2000 -4.000 -4.000 -0.0076 Avg Inc. Cath on left-simreform-170C N/A 106 28-Jul-11 5:36 PM Impedance scan with sim refrom/WE on right at 170 C. 107 28-Jul-11 8:44 PM -0.2590 -5.180 -5.180 -0.0085 Avg Inc. Cath on left-simreform-160C N/A 108 28-Jul-11 10:26 PM Impedance scan with sim refrom/WE on right at 160 C. 109 29-Jul-11 12:02 PM -0.3097 -6.194 -6.194 -0.0093 Avg Inc. Cath on left-simreform-150C N/A 110 29-Jul-11 12:52 PM Impedance scan with sim refrom/WE on right at 150 C. 111 29-Jul-11 5:06 PM -0.4183 -8.366 -8.366 -0.0091 Avg Inc. Cath on left-simreform-140C N/A 112 29-Jul-11 5:42 PM Impedance scan with sim refrom/WE on right at 140 C. 113 30-Jul-11 5:54 PM -0.2955 -5.910 -5.910 -0.0173 Avg Inc. Cath L-sr R-5psig L-130C N/A

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Test # Date Time Slope (Ω cm2) Res (mΩ) Avg R (mΩ) Int (V) Data Type 114 28-Jul-11 7:56 PM Imp scan with sim ref/WE on right 5psig left at 150 C. 115 17-Aug-11 12:56 PM -0.2541 -5.082 -5.082 -0.0210 Avg Inc. Cath L-sim R-10psig L-150C N/A 116 17-Aug-11 1:55 PM -0.2614 -5.228 -5.228 -0.0238 Avg Inc. Cath L-sim R-15psig L-150C N/A 117 17-Aug-11 3:00 PM -0.2575 -5.150 -5.150 -0.0258 Avg Inc. Cath L-sim R-20psig L-150C N/A 118 19-Aug-11 12:26 PM Open circuit potential was -0.055 V 119 19-Aug-11 12:40 PM -0.3137 -6.274 -6.274 -0.0091 Avg Inc. Cath L-sim R-150C N/A 19-Aug-11 12:40 PM 22 hr test at 0.2 A/cm2 with sr - R and cath - L at 150C 120 20-Aug-11 10:01 AM -0.3339 -6.678 -6.678 -0.0096 Avg Inc. Cath L-sim R-150C N/A 121 20-Aug-11 10:13 AM Impedance scan with sim refrom/WE on right at 150 C. 122 20-Aug-11 10:33 AM 22 hr test at 0.2 A/cm2 with sr - L and cath - R at 150C 123 21-Aug-11 10:51 AM -0.3747 -7.494 -7.494 -0.0092 Avg Inc. Cath L-sim R-150C N/A 124 21-Aug-11 11:11 AM Impedance scan with sim refrom/WE on left at 150 C.

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