New Insights into the Roles of Surface and Lattice Hydrogen in Electrocatalytic Hydrogen Oxidation

Abstract

Revealing the exact catalytic sites and reaction mechanism is crucial for the development of highly efficient hydrogen oxidation reaction (HOR) catalysts in anion exchange membrane fuel cells. The surface H is generally accepted as the reaction intermediate, and the binding energy of surface H has been proposed as an important reaction descriptor for HOR, yet the active sites and mechanism still remain inconclusive for HOR on complicated metal hydride catalysts involving both surface H and lattice H. Herein, we studied the H absorption characteristics of Pd to clearly distinguish the roles of lattice H (absorbed H, H_abs) and underpotentially deposited hydrogen (adsorbed H, H_upd) on PdH_x in HOR. The H absorption capacity and absorption/desorption kinetics were found to be size-dependent on Pd nanoparticles (NPs) ranging from 1.5 to 19.1 nm. The exchange current densities (i_0,s) of PdH_x and H_abs oxidation activities followed the same correlation with the particle sizes. After the H_upd coverage was reduced by decorating methyl violet molecules on the octahedral and cubic PdH_x model catalysts, both the HOR activities and H_abs oxidation activities were enhanced. Density functional theory calculations confirmed a nearly optimal H adsorption energy at H_abs sites after removing the monolayer of H_upd. We propose that H_abs sites are most likely the catalytic sites for HOR, and the H_abs at the subsurface may be the reaction intermediate, while H_upd acts as the “spectator” on the PdH_x system. These findings distinguish different roles of surface and lattice H in PdH_x catalysts, providing different insights into the fundamental understanding of the HOR process on metal hydrides.