Modeling the Environment-Dependent Kinetics of Oxygen Reduction Reaction – Effect of Relative Humidity

Abstract

For proton exchange membrane fuel cells (PEMFCs) to achieve broad commercialization, the kinetics of oxygen reduction reaction (ORR) need to be better understood to improve the efficiency with minimized use of expensive Pt-based electrocatalyst(1). The increasing demands of PEMFC-powered heavy-duty vehicles make this issue critical, especially to meet the efficiency target (ultimately 72%)(2). Previous first principle-based theoretical analyses reasonably explained catalyst-dependent activity in ideal conditions (on single crystal catalysts in HClO4 solution under room temperature)(3). To design a practical catalyst, further analysis is required to bridge the gap between the ideal and actual PEMFC conditions (typically, covered with perfluorosulfonic acids under 60 to 90 °C, and 30 to 100% RH). In this talk, we examine the effect of relative humidity (RH) on ORR activity on a platinum surface. The effect of RH on ORR activity has been a controversial topic: an experimental study suggested lower RH increases the ORR activity(4), while another study showed the opposite trend(5). By using a multiscale continuum model, we validate our hypothesis that the controversial experimental results stem from the difference in the properties of the electrode/electrolyte interface. In the mathematical calculation, we assume that the polycrystalline platinum catalysts consist of multiple single crystalline surfaces(6) and that their activity is described by the summation of the activities on the single crystalline surfaces. Based on transition state theory, the reaction rate of each elementary step is calculated from the reaction free energy, reactant concentration, and activation energy with explicit accounting of the double-layer structure and interactions. In the presentation, the calculation results will be compared with experimental solid-state data using microelectrodes, and the mechanism of RH-dependent ORR activity will be discussed. Reference I. E. L. Stephens, A. S. Bondarenko, U. Grønbjerg, J. Rossmeisl and I. Chorkendorff, Energy & Environmental Science, 5 (2012).D. A. Cullen, K. C. Neyerlin, R. K. Ahluwalia, R. Mukundan, K. L. More, R. L. Borup, A. Z. Weber, D. J. Myers and A. Kusoglu, Nature Energy, 6, 462 (2021).H. A. Hansen, V. Viswanathan and J. K. Nørskov, The Journal of Physical Chemistry C, 118, 6706 (2014).M. Shibata, M. Inaba, K. Shinozaki, K. Kodama and R. Jinnouchi, Journal of The Electrochemical Society, 169, 044507 (2022).H. Xu, Y. Song, H. R. Kunz and J. M. Fenton, Journal of The Electrochemical Society, 152 (2005).G. A. Tritsaris, J. Greeley, J. Rossmeisl and J. K. Nørskov, Catalysis Letters, 141, 909 (2011).