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An Investigation of Pbi/Pa Membranes for Application in Pump

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An Investigation of Pbi/Pa Membranes for Application in Pump 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 Tyler Petek | i 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 Tyler Petek | ii 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 Tyler Petek | iii 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 Tyler Petek | iv List of Tables Table 1.1: A summary of a select number of fuel cell types [13] Page 7 Tyler Petek | v 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
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