(52h) Stability and Catalytic Activity of Non-Stoichiometric Overlayers in Bimetallic Systems

The stability and oxygen reduction reaction (ORR) activity of cathode catalysts are key challenges that prevent large-scale adoption of proton-exchange membrane fuel cells in the transportation sector. In the past, bimetallic overlayer catalysts have been proposed as catalysts to increase ORR activity. The overlayer structures are typically considered to be a pseudomorphic structure presenting a 1:1 ratio between the underlayer metal (M) and the overlayer metal (M*). However, past experimental work with Pd@Pt core@shell nanoparticles displayed non-stoichiometric ratios between Pd and Pt atoms per layer.¹ Similarly, a combined experimental and theoretical study of Pt*/Au overlayers found that the development of a non-stoichiometric model was necessary to rationalize the experimentally observed stability and ORR activity of the catalyst.² In this talk, we apply density functional theory (DFT) calculations to systematically explore the possibility of non-stoichiometric overlayers of other late transition metals deposited on gold. We find that non-stoichiometric structures are more thermodynamically stable than their stoichiometric counterparts for every overlayer metal studied due to the release of tensile strain in the overlayer. A descriptor-based approach reveals that the non-stoichiometric structures are also more resistant to dissolution and have significant differences in reactivity when compared to their stoichiometric counterparts. These results open the door to new strategies of developing and fine-tuning electrocatalysts that are both highly active and stable under reaction conditions.

1. Wang, X., Choi, S.-I., Roling, L. T., Luo, M., Ma, C., Zhang, L., Chi, M., Liu, J., Xie, Z., Herron, J. A., Mavrikakis, M. & Xia, Y. Nat. Commun. 6, 7594 (2015).
2. Lopes, P. P., Li, D., Lv, H., Wang, C., Tripkovic, D., Zhu, Y., Schimmenti, R., Daimon, H., Kang, Y., Snyder, J., Becknell, N., More, K. L., Strmcnik, D., Markovic, N. M., Mavrikakis, M. & Stamenkovic, V. R. Nat. Mater. 19, 1207–1214 (2020).