First principles study of Ir3Ru, IrRu and IrRu3 catalysts for hydrogen oxidation reaction: Effect of surface modification and ruthenium content

Abstract

The catalysis of hydrogen oxidation reaction (HOR) at the anode is one of the most important topics in achieving high-performance proton exchange membrane fuel cells, especially in the presence of hydrogen fuel impurities and during the start-up/shutdown cycling and cell reversal. To improve catalytic HOR performance, Iridium-Ruthenium alloy (IrRu) catalysts have been explored to address the issues. For fundamental understanding of the catalytic HOR activities, this paper employs density functional theory (DFT) calculations to elucidate and interpret the surface modifications and Ru content effect on hydrogen adsorption energy of the IrRu alloys. The catalytic HOR activity trend of the alloyed IrRu catalysts is calculated to be Ir3Ru > IrRu > IrRu3, which is opposite to that experimentally observed. However, if the surface enrichment of Ir atoms on the IrRu surfaces to form core-shell type catalysts, the calculated trend becomes to IrRu3 > IrRu > Ir3Ru, which is in agreement with the experiment result. In spite of higher surface energy on IrRu3 core-shell surface, the compelling d-band downshift can attribute to the surface charge depletion, decreasing hydrogen adsorption energy, and resulting in the highest catalytic activity. For Ir3Ru core-shell catalyst, a relatively higher charge accumulation on the surface Ir is observed, which can up-shift the d-band and increase hydrogen adsorption, resulting the lowest catalytic activity. The IrRu catalyst has intermediate d-band downshift and hydrogen adsorption energy so that its catalytic activity remains between IrRu3 and Ir3Ru for both alloyed and core-shell structures.