Tailoring the Hydrogen Spillover Effect in Ni-Based Heterostructure Catalysts for Boosting the Alkaline Hydrogen Oxidation Reaction.

Abstract

Improving the catalytic efficiency of platinum group metal-free (PGM-free) catalysts for the sluggish alkaline hydrogen oxidation reaction (HOR) is crucial to the anion exchange membrane fuel cell. Recently, numerous Ni-based heterostructures have been designed based on bifunctional theory to enhance HOR activity by optimizing the binding energy of both H* and OH*; however, their activities are still far inferior to those of PGM catalysts. Indeed, the long transfer pathway for intermediates between different active sites in such heterostructures has rarely been investigated, which could be the reason for the bottleneck. Here, we design a Ni/MoO_xH_y heterostructure catalyst to promote H* migration from the Ni side to the interface for alkaline HOR via the hydrogen spillover effect. In situ X-ray absorption fine structure, Raman characterizations, H/D kinetic isotope effects, and theoretical calculations have proven facile H* migration from the Ni side to the interface, which further reacts with OH* on the MoO_xH_y surface. Besides, the hydrogen spillover effect is also beneficial for the preservation of the metallic phase of Ni during the reaction. The catalyst exhibits a high activity with J_k,m of 58.5 mA mg_Ni^-1 and j_0,s of 42 μA cm_Ni^-2, which is among the best PGM-free catalysts and is even comparable to some PGM catalysts. It also shows the highest power density (511 mW cm^-2) as a PGM-free anode when assembled into fuel cells under moderate back pressure. These findings prove that in addition to optimizing electrophilicity and oxophilicity for active sites, we could also improve the HOR activity from the transfer pathway for intermediates, which provides insight into the design of other efficient HOR catalysts.