Rational Design of Ir-M Nanoalloy for PEMFC Cathode Application: Combined Computational and Experimental Study

Abstract

Despite recent efforts on replacing a noble Pt to less expensive catalysts for improving oxygen reduction reaction (ORR) for PEMFC (polymer electrolyte membrane fuel cell) application, the performance and stability of a noble Pt catalyst remains superior. In this presentation, we have systematically designed the novel Ir-based ORR catalyst for the PEMFC application via the combined density functional theory (DFT) and experimental approach.First, we computationally screened the potential M element and near-surface composition in Ir3M nanoalloy (M = 3d, 4d, 5d metals) for the purpose of enhancing the activity and durability of catalyst in ORR by considering Gibbs free energy change for alloy formation process, oxygen binding energy and segregation energy of inside M element toward surface layer under vacuum and oxygen environment. Our DFT-based screening identified the pure Ir monolayer on the top of Ir3­Cr, Ir3­V, Ir3­Re, and Ir3­Tc alloy cores as the promising candidates for improving ORR activity and durability. Next, a pure Ir monolayer on the top of Ir3­Cr core (which was expected to show the most enhanced ORR activity among the computationally-screened candidates) was experimentally prepared via physical vapor deposition method (PVD) and electrochemically evaluated for confirming our DFT prediction. We successfully synthesized a 3nm Ir-covered (Ir skinlayer) Ir3­Cr nanoparticle whose surface lattice distance was contracted by 1.03% compared to the pure Ir case. The specific activity (at 0.7 V vs RHE) of a Ir skinlayer/Ir3Cr catalyst with the very high durability (showing only 0.05 % decrease from the initial activity after 3000 potential cycles) was 12.3 times higher than a Ir catalyst. The detail mechanism on the enhanced activity in Ir-M alloy was also examined. Our theoretical and experimental study highlight the general methodology for designing metal-based alloy catalysts for the application to ORR in PEMFC and such method can be further extended to the development of the next-level electrocatalyst for energy conversion.