An Examination of Voltage Losses for the Hydrogen Oxidation and Reduction Reactions in an Alkaline Membrane Fuel Cell

Abstract

Recent advances in highly conductive and base stable anion exchange membrane chemistries have enabled the widespread fabrication of alkaline membrane fuel cells (AMFC).(1) A significant limitation to AMFC commercialization, however, is the sluggish hydrogen oxidation reaction (HOR) kinetics of current anode catalysts. Much work has been performed to understand the fundamental limitations of HOR in an alkaline environment(2, 3) and to study the HOR in an alkaline membrane electrode assembly (MEA)(4). It is of significant interest to the AMFC community to continue studying the HOR reaction to understand the in-situsources of voltage losses, and to develop a diagnostic tool to test the kinetics of new/alternative catalysts in an alkaline MEA. In this study, a series of MEAs were fabricated where the membrane (Tokuyama A201) and binder polymer (Tokuyama AS-4) are held constant, but where the catalyst content and composition are varied. A hydrogen pump technique was utilized where an external current source drives HOR on one electrode of the MEA and hydrogen evolution (HER) on the other. It should be noted that there is no traditional reference electrode in this system, as each electrode reaction contributes a significant overpotential. The polarization curves acquired from the hydrogen pump are analyzed for sources of voltage losses including mass transport, ohmic, and kinetic overpotentials. The causes of such losses will be discussed and presented with insight gained from modeling and experimental work. Exchange current densities of HOR and HER for both platinum on carbon and platinum ruthenium on carbon will also be presented, where all data was taken in an in-situalkaline MEA. The potential for hydrogen pump as a diagnostic method for future anode catalyst testing will also be included. Figure 1. HOR and HER overpotentials for Pt/C electrodes between 0.119 and 0.8 mg/cm2 loading as a function of current density per Pt site, with the corresponding Butler-Volmer model fit 1. J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. Xu and L. Zhuang, Energy & Environmental Science(2014). 2. J. Durst, A. Siebel, C. Simon, F. Hasche, J. Herranz and H. A. Gasteiger, Energy & Environmental Science, 7, 2255 (2014). 3. Y. Wang, G. Wang, G. Li, B. Huang, J. Pan, Q. Liu, J. Han, L. Xiao, J. Lu and L. Zhuang, Energy & Environmental Science, 8, 177 (2015). 4. M. D. Woodroof, J. A. Wittkopf, S. Gu and Y. S. Yan, Electrochemistry Communications, 61, 57 (2015). Figure 1