The Oxygen Reduction Reaction on Pt and Ag Catalysts in Alkaline Media in Rrde and Half-Cell Setups

Abstract

Understanding the Oxygen Reduction Reaction (ORR) in alkaline media is essential to optimize alkaline fuel cells or metal-air batteries. The catalytic activity of various ORR catalysts has usually been evaluated via Rotating (Ring) Disk Electrode (R(R)DE) measurements. However, using an electrolyte with dissolved oxygen at low KOH concentrations measured in a microampere regime at room temperature does not reflect the real cell conditions. 1 Yet, RRDE is still a valuable tool for probing the reaction pathway of catalysts in alkaline media. Our study combined traditional RRDE with Gas Diffusion Electrodes (GDEs) half-cell measurements using Pt and Ag catalysts. The polarization curve of Pt-GDE exhibits a kink near 0.8 V vs. RHE leading to a significant potential drop. This specific feature can be explained by the RRDE results, which display an increased peroxide formation in that potential region due to the adsorption/desorption of hydroxyl adsorbates. 2 However, Ag does not exhibit such behavior since the peroxide formation is below typical operating potentials. Depending on the operating conditions, Ag can catalyze the ORR more effectively than Pt in alkaline conditions while significantly reducing production costs. Furthermore, our study presents a general approach for evaluating ORR catalysts under experimentally challenging conditions such as elevated temperature and high KOH molarity. References: (1) Ehelebe, K.; Schmitt, N.; Sievers, G.; Jensen, A. W.; Hrnjić, A.; Collantes Jiménez, P.; Kaiser, P.; Geuß, M.; Ku, Y.-P.; Jovanovič, P. ; et al. Benchmarking Fuel Cell Electrocatalysts Using Gas Diffusion Electrodes: Inter-lab Comparison and Best Practices. ACS Energy Lett. 2022 , 7 , 816–826. (2) Ramaswamy, N.; Mukerjee, S. Fundamental Mechanistic Understanding of Electrocatalysis of Oxygen Reduction on Pt and Non-Pt Surfaces: Acid versus Alkaline Media. Advances in Physical Chemistry 2012 , 2012 , 1–17. Figure 1