• unlimited access with print and download
    $ 37 00
  • read full document, no print or download, expires after 72 hours
    $ 4 99
More info
Unlimited access including download and printing, plus availability for reading and annotating in your in your Udini library.
  • Access to this article in your Udini library for 72 hours from purchase.
  • The article will not be available for download or print.
  • Upgrade to the full version of this document at a reduced price.
  • Your trial access payment is credited when purchasing the full version.
Buy
Continue searching

Experimental and numerical study of a proton exchange membrane electrolyzer for hydrogen production

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Sachin S Deshmukh
Abstract:
Hydrogen as a fuel source has received attention from researchers globally due to its potential to replace fossil based fuels for energy production. Research is being performed on hydrogen production, storage and utilization methods to make its use economically feasible relative to current energy sources. The PEM electrolyzer is used to produce hydrogen and oxygen using water and electricity. Focus of our study is to provide a benchmark experiment and numerical model of a single cell electrolyzer that can assist in improving the current state of understanding of this system. Parametric analysis of an experimental cell was performed to understand the effect of operating parameters of electrolyzer on its performance. A CFD model was developed to model the physics of electrolyzer. The model was validated with the experiment; the information presented here can be used as a tool to improve the design from thermo-fluid aspects of the electrolyzer.

iv TABLE OF CONTENTS ABSTRACT ....................................................................................................................... iii LIST OF FIGURES ........................................................................................................... vi ACKNOWLEDGEMENTS ................................................................................................ x CHAPTER 1 ....................................................................................................................... 1 Hydrogen Economy ........................................................................................................ 1 Methods of Hydrogen Production ............................................................................... 2 Why PEM Electrolysis? .................................................................................................. 3 What is a PEM Electrolyzer? .......................................................................................... 4 Thermodynamics of a PEM Electrolyzer ........................................................................ 5 Literature Review ........................................................................................................... 8 Developing improved and cheaper membrane and catalyst alternatives .................... 8 Stable long term performance ..................................................................................... 9 Conclusions from literature review and proposed study .............................................. 10 CHAPTER 2 ..................................................................................................................... 12 Design of single cell PEM electrolyzer ........................................................................ 12 Membrane Electrode Assembly (MEA) ................................................................... 13 Porous titanium sheet ................................................................................................ 13 Titanium flow field ................................................................................................... 16 External plate and other parts.................................................................................... 16 Test facility ................................................................................................................... 18 Centrifugal pump ...................................................................................................... 20 Storage tank .............................................................................................................. 21 Deionized water system ............................................................................................ 21 Temperature controller .............................................................................................. 23 DC power supply ...................................................................................................... 24 DAQ system .................................................................................................................. 25 Campbell Scientific data logger ................................................................................ 25 Campbell Scientific multiplexer ............................................................................... 25 Laptop computer ....................................................................................................... 26 Measurement sensors .................................................................................................... 27 Thermistor ................................................................................................................. 27 Flowmeter ................................................................................................................. 33 Water resistivity sensor ............................................................................................. 35 Current sensor ........................................................................................................... 37 Measurement program and data collection ................................................................... 37 Experimental test procedure ......................................................................................... 38 CHAPTER 3 ..................................................................................................................... 41 Introduction ................................................................................................................... 41 Numerical model ........................................................................................................... 41

v Qualitative 3D CFD analysis of flow field design ........................................................ 42 CFD model for the PEM electrolyzer ........................................................................... 45 Computational domain, governing equations and boundary conditions ................... 47 Other data ...................................................................................................................... 56 CFD modeling procedure .............................................................................................. 56 CHAPTER 4 ..................................................................................................................... 58 Experimental test results ............................................................................................... 58 Experimental data analysis ........................................................................................... 65 Discussion of experimental results ............................................................................... 72 Numerical modeling results .......................................................................................... 74 3D CFD modeling results (Initial straight channel design) ...................................... 74 Other 3D CFD modeling ............................................................................................... 82 2D CFD modeling results ............................................................................................. 87 Numerical data analysis .............................................................................................. 105 Discussion of CFD results .......................................................................................... 113 CHAPTER 5 ................................................................................................................... 116 PEM electrolyzer cell .................................................................................................. 116 Test facility ................................................................................................................. 118 Parametric experimental test procedure ...................................................................... 119 CFD modeling ............................................................................................................. 119 Summary ..................................................................................................................... 121 APPENDIX A NOMENCLATURE ............................................................................... 122 APPENDIX B EQUIPMENT SPECIFICATIONS ........................................................ 123 APPENDIX C MEASUREMENT UNCERTAINITY ANALYSIS .............................. 140 REFERENCES ............................................................................................................... 142 VITA ............................................................................................................................... 147

vi LIST OF FIGURES Figure 1. Operation of a PEM electrolyzer with the reactions involved ....................... 4 Figure 2. Thermodynamic quantities for the reaction in water electrolysis .................. 7 Figure 3. MEA for electrolyzer with Pt for hydrogen and mixed Ir/Ru Oxide ........... 14 Figure 4. Fixture to make holes in the MEA for cell assembly .................................. 14 Figure 5. Porous titanium sheet for water, gases and electron transport in the cell .... 15 Figure 6. Straight channel titanium flow field for PEM electrolyzer.......................... 15 Figure 7. Assembly of the PEM electrolyzer cell for experimental analysis .............. 17 Figure 8. Final assembly of the PEM electrolyzer cell with insulation to prevent any heat loss from the cell .................................................................................. 17 Figure 9. Exploded view of PEM electrolyzer cell ..................................................... 18 Figure 10. Test facility layout for characterizing PEM electrolyzer ............................. 19 Figure 11. Mag-drive centrifugal pump used to circulate DI water in the flow circuit 20 Figure 12. Polypropylene water storage tank ................................................................ 21 Figure 13. Detailed assembly of water storage tank ..................................................... 22 Figure 14. De ionized water system for PEM water electrolyzer ................................. 22 Figure 15. Water bath used as a temperature controller for deionized water ................ 23 Figure 16. DC power supply (0-8V 0-125 A) for PEM water electrolyzer. ................. 24 Figure 17. Campbell Scientific data logger CR10X ..................................................... 25 Figure 18. Campbell Scientific multiplexer AM 16/32................................................. 26 Figure 19. Loggernet GUI for monitoring operating parameters .................................. 27 Figure 20. YSI 44032 thermistor element ..................................................................... 28 Figure 21. Thermistor attached to the stereo cable with heat shrink tubing ................. 29 Figure 22. Thermistor plumbing fitting for measuring temperature ............................. 29 Figure 23. DC half bridge used to measure the temperature using thermistor.............. 30 Figure 24. Calibration of oxygen thermistor as compared to thermistor ...................... 32 Figure 25. Calibration of hydrogen thermistor as compared to thermistor ................... 33 Figure 26. Omega FT601B flowmeter .......................................................................... 34 Figure 27. Wiring diagram for the flow meter .............................................................. 34 Figure 28. Resistivity sensor installed in its custom designed fitting ........................... 35 Figure 29. Foxboro resistivity analyzer......................................................................... 36 Figure 30. Resistivity as a function of voltage from the Foxboro analyzer .................. 36 Figure 31. Empro MLA current shunt ........................................................................... 37 Figure 32. Initial design of straight channel flow field ................................................. 42 Figure 33. Initial CFD domain of the straight channel flow field ................................. 43 Figure 34. Final design of straight channel flow field after CFD analysis ................... 44 Figure 35. Final CFD domain of the modified straight channel flow field design ....... 45 Figure 36. 2D CFD domain of PEM electrolyzer for 1/16 inch depth flow field ......... 48 Figure 37. Gambit 2D mesh for PEM electrolyzer with 1/16 inch depth ..................... 48 Figure 38. 2D CFD domain of PEM electrolyzer for 1/32 inch depth flowfield .......... 49 Figure 39. Gambit 2D mesh for PEM electrolyzer with 1/32 inch depth ..................... 49 Figure 40. Boundary conditions for 2D CFD modeling of the PEM electrolyzer ........ 55 Figure 41. Source and sink terms for multi-physics PEM electrolyzer model ............. 55 Figure 42. Change in temperature vs. time (25.8 ºC, 902 mlpm and depth 1/16th)...... 59 Figure 43. Typical flow rate vs. time obtained during test ........................................... 59

vii Figure 44. Current and voltage vs. time for test performed at 25.8 °C, 902 mlpm ....... 60 Figure 45. Current and voltage vs time for test performed at 25.8 °C, 902 mlpm ........ 60 Figure 46. Current and voltage vs time for test performed at 25.8 °C, 582 mlpm ........ 61 Figure 47. Current and voltage vs time for test performed at 25.8 °C, 582 mlpm ........ 61 Figure 48. Current and voltage vs time for test performed at 58.7 °C, 902 mlpm ........ 62 Figure 49. Current and voltage vs time for test performed at 58.7 °C, 902 mlpm ........ 62 Figure 50. Current and voltage vs time for test performed at 58.7 °C, 582 mlpm ........ 63 Figure 51. Current and voltage vs time for test performed at 58.7 °C, 582 mlpm ........ 63 Figure 52. Polarization curve during the first test after cell assembly .......................... 64 Figure 53. I-V curve for the PEM electrolyzer with flow field depth 1/16" ................. 65 Figure 54. I-V curve for the PEM electrolyzer with flow field depth 1/32" ................. 66 Figure 55. I-V curve for all parametric variations considered for this work ................ 66 Figure 56. I-V curve showing the effect of depth of flow field at high temperature .... 67 Figure 57. Change in temperature for O 2 side of cell for 1/16" depth flow field .......... 68 Figure 58. Change in temperature for H 2 side of cell for 1/16" depth flow field .......... 69 Figure 59. Change of temperature across the electrolyzer cell for 1/16" flow field ..... 70 Figure 60. Change in temperature for O 2 side of cell for 1/32" depth flow field .......... 70 Figure 61. Change in temperature for H 2 side of cell for 1/32" depth flow field .......... 71 Figure 62. Change of temperature across the electrolyzer cell for 1/32" flow field ..... 71 Figure 63. Pressure drop across the one side of PEM electrolyzer for water flow only 72 Figure 64. Velocity magnitude contours at the center along the depth of flow field .... 75 Figure 65. Pathlines for the water flow through the initial design of flow field ........... 76 Figure 66. Zoomed view of flow pattern through the flow channels ............................ 76 Figure 67. Velocity pattern for water flow through channels (Alternate view) ............ 77 Figure 68. Velocity contours for water flow through the final design of flow field ..... 78 Figure 69. Velocity inside the flow channels at the center along the length of ............ 79 Figure 70. Pathlines of water flow through the final design of flow field .................... 79 Figure 71. Zoomed view of the pathlines of water flow through channels ................... 80 Figure 72. Velocity pattern for water flow through channel (Alternate view) ............. 81 Figure 73. Effect of 3D grid size on the velocity inside the flow channels .................. 82 Figure 74. Pressure drop in electrolyzer cell obtained from 3D CFD modeling .......... 83 Figure 75. Mountain-ridge pattern on the catalyst layer after electrolysis .................... 83 Figure 76. Pattern on the porous layer face in contact with catalyst layer .................... 84 Figure 77. Effect of permeability factor (1/16” flow field) in Darcy term of momentum equation of CFD model................................................................................ 85 Figure 78. Effect of permeability factor (1/32” flow field) in Darcy term of momentum equation of CFD model................................................................................ 85 Figure 79. Cell potential (left) and pressure (right) contour of the PEM electrolyzer at 25.8°C, 582 mlpm and 1/16" flow field depth ............................................. 88 Figure 80. Temperature (left) and velocity magnitude (right) contours of the PEM electrolyzer at 25.8 °C, 582 mlpm and 1/16" flow field thickness .............. 89 Figure 81. Hydrogen (left) and oxygen (right) volume fractions of PEM electrolyzer at 25.8 °C, 582 mlpm and 1/16" flow field thickness ...................................... 90 Figure 82. Water volume fraction of PEM electrolyzer at 25.8 °C, 582 mlpm and 1/16" flow field thickness ...................................................................................... 91

viii

Figure 83. Cell potential (left) and pressure (right) contour of the PEM electrolyzer at 58.7°C, 902 mlpm and 1/16" flow field thickness ....................................... 92 Figure 84. Temperature (left) and velocity (right) contour of the PEM electrolyzer at 58.7°C, 902 mlpm and 1/16" flow field thickness ....................................... 93 Figure 85. Hydrogen (left) and oxygen (right) volume fraction contour of the PEM electrolyzer at 58.7 °C, 902 mlpm and 1/16" flow field thickness .............. 94 Figure 86. Water volume fraction contour of the PEM electrolyzer at 58.7 °C, 902 mlpm and 1/16" flow field thickness ........................................................... 95 Figure 87. Cell potential and pressure contours of the PEM electrolyzer at 25.8 °C, 582 mlpm and 1/32" flow field thickness ........................................................... 96 Figure 88. Temperature and velocity contours of the PEM electrolyzer at .................. 97 Figure 89. Hydrogen (left) and oxygen (right) volume fraction contours of the PEM electrolyzer at 25.8 °C, 582 mlpm and 1/32" flow field thickness .............. 98 Figure 90. Water volume fraction contour of the PEM electrolyzer at 25.8 °C, 582 mlpm and 1/32" flow field thickness ........................................................... 99 Figure 91. Cell potential (left) and pressure (right) contours of the PEM electrolyzer at 58.7 °C, 902 mlpm and 1/32" flow field thickness .................................... 100 Figure 92. Temperature (left) and velocity (right) contours of the PEM electrolyzer at 58.7°C, 902 mlpm and 1/32" flow field thickness ..................................... 101 Figure 93. Hydrogen (left) and oxygen (right) volume fraction contours of the PEM electrolyzer at 58.7 °C, 902 mlpm and 1/32" flow field thickness ............ 102 Figure 94. Water volume fraction contour of the PEM electrolyzer at 58.7 °C, 902 mlpm and 1/32" flow field thickness ......................................................... 103 Figure 95. Grid independency results for 2D CFD modeling (1/16" flow field) ........ 104 Figure 96. Grid independency results for 2D CFD modeling (1/16" flow field) ........ 104 Figure 97. Polarization curve for PEM electrolyzer for 1/16" depth flow field ......... 105 Figure 98. Polarization curve for PEM electrolyzer for 1/32" depth flow field ......... 106 Figure 99. Polarization curve for PEM electrolyzer from CFD simulations .............. 106 Figure 100. Change in temperature for anode side vs. current density for PEM electrolyzer with 1/16" depth flow field .................................................... 108 Figure 101. Change in temperature for cathode side vs. current density for PEM electrolyzer with 1/16" depth flow field .................................................... 108 Figure 102. Change in temperature vs. current density for PEM electrolyzer with 1/16" depth flow field .......................................................................................... 109 Figure 103. Change in temperature for anode side vs. current density for PEM electrolyzer with 1/32" depth flow field .................................................... 109 Figure 104. Change in temperature for cathode side vs. current density for PEM electrolyzer with 1/32" depth flow field .................................................... 110 Figure 105. Change in temperature vs. current density for PEM electrolyzer with 1/32" depth flow field .......................................................................................... 110 Figure 106. Volume fraction of oxygen vs. current density for PEM electrolyzer with 1/16" depth flow field ................................................................................ 111 Figure 107. Volume fraction of hydrogen vs. current density for PEM electrolyzer with 1/16" depth flow field ................................................................................ 112 Figure 108. Volume fraction of oxygen vs. current density for PEM electrolyzer with 1/32" depth flow field ................................................................................ 112

ix Figure 109. Volume fraction of oxygen vs. current density for PEM electrolyzer with 1/32" depth flow field ................................................................................ 113

x ACKNOWLEDGEMENTS I would like to acknowledge the people and organizations that have made this work possible. First of all, I will express my sincere thanks to Dr. Robert F. Boehm for his continuous support and guidance for this work as well as for my graduate level studies at UNLV. I would also like to recognize the other members of my committee; Dr. Yahia Baghzouz, Dr. Yitung Chen, Dr. Daniel P. Cook, and Dr. Jianhu Nie. Your continuous assistance with this project has been greatly appreciated. I would also like to thank Dr. Jaci Battista and Dr. Shizhi Qian for allowing me to use their lab facilities for this project. Next, I would like to express my gratitude to US Department of Energy who funded this project and without whose financial support this project would not have been possible. Additionally, my thanks to lab colleagues Chris Halford, Rick Hurt and Suresh Sadineni for their input on many of the technical aspects of the project. Finally, I would like to thank my family for their support.

1 CHAPTER 1 INTRODUCTION Hydrogen Economy

There are rising global concerns over possible alternatives to fossil fuels that can meet future energy demand. Hydrogen has received serious attention from the scientific community globally as an alternative to fossil fuels because of its following properties: • Clean burning • Easy conversion into electrical and thermal energy • Supports seasonal storage of energy • Easy transportation As the availability of renewable energy namely solar, wind, hydro-power, etc suffer from temporal variation, hydrogen can be used as energy storage and thereby assist in achieving a continuous supply of energy in either thermal or electrical form. Due to these features of hydrogen as a fuel, continuous research is ongoing globally for improving and developing novel ideas for hydrogen production, storage and utilization. One of the key areas of development particularly in hydrogen utilization is PEM fuel cells. They use hydrogen to produce continuous power from an electrochemical reaction with air in the presence of electro-catalysts and an electrolyte such as a Nafion tm

membrane. The reason that they have become so popular is that they can start quickly, one of the advantages of fuel cells that make them ideal for automobile application. In this role, they have potential for replacing engine that consumes fossil fuels as a cleaner approach to power the vehicles. Other than automobiles, fuel cells can also be used to power stationary applications like (residential, stand alone systems etc) either by

2 themselves or with a combined renewable (photovoltaic, wind, hydro turbines etc) hydrogen system, where hydrogen can be used as a fuel to smooth out the temporal availability of renewable energy sources [1]. Methods of Hydrogen Production Hydrogen can be produced from various ways which can be generally classified into three categories namely [2]: • Chemical • Electrochemical • Biological Chemical: Hydrogen production from chemical means is obtained by processes like steam reforming of methane, coal gasification, biomass and thermal cracking of methane. Electrochemical: Alkaline, PEM, solid oxide, sea water and photo electrochemical electrolysis falls under this category of hydrogen production. Biological: Fermentation of bacteria and bio photolysis fall under this category of hydrogen production. An interesting review was provided on costing and efficiency of the different hydrogen production methods. Not much information was given on the purity of hydrogen produced by each method, which is very important when considering the application to PEM fuel cells.

3 Why PEM Electrolysis?

There are several of methods to produce hydrogen as described above. Steam reforming and electrolysis (Alkaline/PEM) are well established methods with commercial products available. Whereas other methods like solar-hydrogen production, photo- electrochemical as well as biological methods are still in the early research stages. What makes PEM electrolyzer particularly important is described in this section. PEM fuel cells need high purity hydrogen, so as to prevent any poisoning of the electrodes, this cannot be attained from reforming technology which is widely used in chemical industry to produce hydrogen. High purity hydrogen can be obtained from PEM electrolyzers which can then be directly used in fuel cells. Also PEM electrolyzers are relatively simple in design and can operate at higher current density as compared to alkaline electrolyzers. Though PEM electrolyzers were first developed in the 1960’s and were commercially available from the 1970’s [3], there are very few published studies available in the literature on the design and performance optimization of PEM electrolyzers on the cell level as well as the stack basis. CFD has been widely used to study various thermo-fluid based design aspects of fuel cells but very few studies were found on modeling of PEM electrolyzers. It is strongly believed that validated CFD analysis cannot only help in understanding the physical phenomena in existing electrolyzers but also assist in their further development. This was the reason that motivated us to pursue a detailed experimental and numerical study of the PEM electrolyzer.

4 What is a PEM Electrolyzer?

A PEM electrolyzer is used to generate hydrogen and oxygen from water and electrical energy. Figure 1 shows the pictorial representation of PEM electrolyzer. PEM

Figure 1. Operation of a PEM electrolyzer with the reactions involved

is a material with selective conductivity to protons. PEM combined with an anode and a cathode forms a Membrane Electrode Assembly (MEA). The most commonly used PEM material is Nafion TM from DuPont. The electrodes are generally made from noble metals in order to withstand the strong acidity of Nafion TM . Under applied potential greater than thermodynamic potential (1.23 V) water is dissociated at the interface of anode, water and membrane into proton, electrons and oxygen. Protons moving through PEM combine with electrons that transfer through external electrical circuit at the cathode to form hydrogen.

5 Along with the PEM electrolyzer cell stack, a de-ionized water systems, storage tank, DC power supply, water pump, water temperature controller, gas separator and dryer equipment are needed to operate a PEM water electrolyzer system.

Thermodynamics of a PEM Electrolyzer

Water electrolysis is driven by a heterogeneous chemical reaction given in Fig 1. The amount of reversible work required for water electrolysis can be obtained by applying first and second law of thermodynamics for reacting systems [4]. Considering a steady state steady flow process for the electrolyzer we have,

G rev W

− = (1) The change in Gibbs energy for the water dissociation reaction assuming constant temperature and pressure can be obtained as follows.

R R R STHG

=

(2) where H R is the change in enthalpy of formation and S R is the change in entropy for the reaction. These two can be evaluated using the following expressions

OH H O H H H R H 22 2 1 2 −       += (3)

OH S O S H S R S 22 2 1 2 −       += (4) The change in enthalpy and entropy for species (H 2 , O 2 , H 2 O) can be obtained from either tabulated data such as JANAF [5] or using (assuming ideal gas and constant pressure process)

6

dTCH S H T T Sp TS ref ref ∫ += , 0 , (5)

dT T C S S S T T Sp TS ref ref ∫ += , 0 , (6) An empirical relation for specific heat at constant pressure can be expressed as a function of temperature using the following relation [6]. The constants α, β, γ, δ and ε for the given species can be found in table 1. This can be integrated and further used to evaluate change in enthalpy and entropy for the reaction.

      ++++= 432 TTTTR p C εδγβα (7) Once the change in Gibbs energy for the reaction is obtained, notice that it is negative (work done on the system is negative). Data generally used for calculations are known for formation of species, and hence the sign should be changed to positive for proper work direction for the process. General behavior of the change in enthalpy of formation, product of temperature, entropy and Gibbs energy with temperature of water (liquid) is given in Fig 2. Notice that with the energy needed to dissociate water given by DH decreases with the increasing temperature. Also the Gibbs energy and thereby the thermodynamic potential needed to dissociate decreases with the increase in temperature.

Gas α β x 10 3 γ x 10 6 δ x 10 9 ε x 10 12

H 2 O 4.070 -1.108 4.152 -2.964 0.807 O 2 3.626 -1.878 7.055 -6.764 2.156 H 2 3.057 2.677 -5.810 5.521 -1.812

Table 1. Constants for calculating specific heat at constant pressure [6]

7 200 225 250 275 300 298 308 318 328 338 348 358 Temperature (K) DH, DG (kj/mol) 45 50 55 60 T*DS (kj/mol) DH DG T*DS

Figure 2. Thermodynamic quantities for the reaction in water electrolysis

Reversible potential in electrical form can be obtained from the reversible work using F n rev W rev U ⋅ = (8) where n is the number of electrons taking part in reaction and F is the Faradays number (96485 J/mol V). Finally, the ideal thermodynamic efficiency for the electrolyzer can be defined as

H ST STH H G H Input Output

− = −

=

== 1 1 η (9) Notice that the thermodynamic efficiency will be greater than 1 for practical systems (at standard conditions 1 bar, 298.15 K, the efficiency is 1.205). The efficiency will approach 1 when t*DS/DH approaches 0 and infinity when the Gibbs energy is zero, which happens around 4400 K. Actual efficiency of the electrolyzer cell will be less than 100 % as the energy input is greater than the enthalpy of formation of hydrogen and

8 oxygen from water. This is due to over potential and other electrical losses and the actual efficiency is given by [7].

ξ η η + − = 1 ** 482.1 DCloss cell i ii V (10) where V cell is the operating cell potential which depends on the operating current density i. The typical efficiency of the industrial electrolyzer is between 65-80 % [7].

Literature Review

PEM electrolyzers were first developed after the discovery of Nafion in 1960’s. The first commercially available PEM electrolyzers were available in the 1970’s. Review of status of research and development of alkaline and PEM electrolyzer was reported [3]. Various design issues particularly related to materials were outlined. Further development of PEM electrolyzers can be broadly classified as follows. Developing improved and cheaper membrane and catalyst alternatives Development of catalyst material and preparation has been reported by many researchers globally. Different types of catalyst preparation methods have been reported which are basically of chemical, physical and electrochemical nature. Chemical methods use metal salt solution and reducing agents to obtain a layer of catalyst material on the membrane directly [8-18]. Physical methods that were reported include sol-gel method [19-28], sputtering and heat press method [29], and thermal vapor deposition [30]. The sol-gel method involves the preparation of a catalyst ink (liquid Nafion, NaOH, Glycerol and catalyst material) which is mixed thoroughly and painted on a substrate (generally Teflon) and thermally treated until the layer becomes solid. The solid layer then is heat pressed on both sides of the Nafion and the substrate is removed similar to a decal. In the

9 sputtering methods, nano sized catalyst particles are sputtered on the substrate which is heat pressed on the membrane. An electrochemical method of catalyst layer formation has been reported [31, 32]. The key advantage of the electrochemical method as compared to previously discussed methods is significantly less catalyst loading can be deposited on the membrane which is both an ionic contact with the membrane and an electronic contact with current supplier. Note that some of the methods are reported for making MEAs for fuel cells but the idea can be extended to make MEAs for electrolyzers. Stable long term performance Long-term-performance stability is very important to determine the service life of the electrolyzer. The long-term data for single cell as well as bipolar electrolyzer have been reported [26, 33, 8, 13]. Generally the data are reported in the form of potential vs. time (order of thousands of hours). A very interesting study on the tear-down analysis of two 100 kW PEM electrolyzers was performed showing an uneven thickness of the membrane and a degradation of the cathode side [34]. Increase in hydrogen generation capacity Most of the initial development work on electrolyzers was performed on laboratory level hardware. Development was also focused towards increasing the active area and number of cells in the cell stack (also called Bi-polar arrangement of cells). In the bipolar arrangement the flow to each cell is connected in parallel, and the electrical connections between cells are in series. The performance of bipolar electrolyzers was reported [33,8] in the literature.

10 It was found that there were very few studies reported on modeling of PEM electrolyzers, particularly CFD studies. A very interesting article was reported by authors asking a question on how can theory help the development of fuel cells [35]. Answering this question the authors go into various theories explaining physical phenomena from the microscopic to macroscopic level for the fuel cell. Also how the sound understanding of theory behind the fuel cells can assist in the faster development of the fuel cells for commercialization. Similar questions can be asked for the PEM electrolyzer whose process is reverse to that of the fuel cell. Authors have predicted the power required to produce high-pressure hydrogen from high-pressure electrolysis and compared it to atmospheric-pressure water electrolysis [36]. A simple model with an equivalent circuit was used to predict the performance of a PEM water electrolyzer [37]. These authors reported the voltage vs. current density as a function of temperature. A finite difference method was used to solve the charge transport equation using Butler-Volmer kinetics [38]. 2D mass, energy and charge equations were solved. Polarization curves were presented as a function of temperature and compared with experimental results [39]. A 2D CFD model with coupled mass, momentum, species and charge transport was solved in COMSOL by our group [40].

Conclusions from literature review and proposed study

Clearly, from the review presented in the previous section, that there is paucity of data on PEM electrolyzers. As a result, the thermo-fluid behavior inside the cell is not well understood. The thermo-fluid behavior inside the electrolyzer cell and its optimization is really needed to improve the performance of PEM electrolyzers.

11 Computational fluid dynamics has been used for decades to analyze thermo-fluid behavior for a variety of engineering applications. CFD has been used to model the PEM fuel cells whose physics is very relevant to PEM electrolyzer behavior. It was also found that no experimental data were reported in the literature that can be directly used for conducting CFD studies. It appears that validated CFD studies that capture the physics of the PEM electrolyzer behavior have not been reported. Based on the overall experimental and numerical studies literature survey, the focus of current study was proposed. An experimental single cell PEM electrolyzer and test facility to characterize the cell performance has been constructed. A basic CFD study was performed to obtain nearly uniform flow throughout the flow field. In this study an analysis, where, operating parameters were performed. These parameters included temperature, flow rate, voltage/current, and a geometric parameter (depth of the flow field). Using the experimental geometric data, the CFD model was developed, in the Fluent code, to solve the mass, momentum, energy, species transport (volume fraction basis) and charge transport equations. Comparisons of the model with experimental data for model validation were performed.

Full document contains 160 pages
Abstract: Hydrogen as a fuel source has received attention from researchers globally due to its potential to replace fossil based fuels for energy production. Research is being performed on hydrogen production, storage and utilization methods to make its use economically feasible relative to current energy sources. The PEM electrolyzer is used to produce hydrogen and oxygen using water and electricity. Focus of our study is to provide a benchmark experiment and numerical model of a single cell electrolyzer that can assist in improving the current state of understanding of this system. Parametric analysis of an experimental cell was performed to understand the effect of operating parameters of electrolyzer on its performance. A CFD model was developed to model the physics of electrolyzer. The model was validated with the experiment; the information presented here can be used as a tool to improve the design from thermo-fluid aspects of the electrolyzer.