Abstract
Surface oxides formed on Pt electrocatalysts at high potential (>0.8 V) suppress the oxygen reduction reaction (ORR) activity. Reduced activity due to surface oxide contributes to a noticeable amount of fuel cell efficiency loss.[1] While mathematical models were developed, , agreement with experimental results is limited to standard steady-state conditions.[2] A kinetic model is needed to explain a dynamically operating fuel cell, where the catalyst is often exposed to a range of temperature, humidity, and potential over time.Here, we conducted time-resolved measurements of ORR kinetics and oxide coverage for Pt and PtCo cathode catalysts under various temperature and relative humidity (RH). We found that the dependance of Pt oxide (PtO) coverage on the RH can be expressed by a modified Langmuir adsorption isotherm. In contrast to conventional understanding that RH-dependence of PtO coverage is due to water being the reactant, our results suggest that the dependency is due to ionomer co-adsorption on Pt surface. Analysis of the ORR activity and PtO coverage suggests two types of PtO. The time-resolved measurement allowed us to deconvolute the two PtO types and develop a kinetic model that gives improved prediction capability over a range of dynamic operation commonly observed in a real fuel cell.This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. Reference [1] J. Mitzel, Q. Zhang, P. Gazdzicki, K.A. Friedrich, J. Power Sources, 488 (2021) 229375.[2] P. Subramanian, T. A. Greszler, J. Zhang, W. Gu, and R. Makharia, Journal of The Electrochemical Society, 159 (5), B531-B540 (2012)
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