Elucidating the reaction pathway of crystalline multi-metal borides for highly efficient oxygen-evolving electrocatalysts

Abstract

Understanding the fundamental principle of catalytic performance and the mechanism of multimetal-based electrocatalysts is essential for the rational design of advanced renewable energy systems. Here, highly crystalline MMMoB₄ (M = Fe, Co) compounds with controllable compositions of multiple active metal atoms and polyacene-type boron networks were synthesized delicately by a one-step high-pressure technique to explore electrocatalytic selectivity and activity. CoFeMoB₄ and Co₂MoB₄ are revealed to be highly active and durable oxygen evolution reaction (OER) electrocatalysts under alkaline conditions. The mutually promotive activation of metals with amorphous clusters and ultra-small grains on the surface are responsible for the enhanced activity of CoFeMoB₄. More specifically, Co and Fe coupling in CoFeMoB₄ facilitates surface reconstruction into active Co hydroxide and Fe oxyhydroxide, in contrast to Co oxyhydroxide in Co₂MoB₄ and Fe oxides in Fe₂MoB₄. Dissolving Mo may provide potential space for adsorbing hydroxyl, and the optimized electronic structure with boron is mainly responsible for the long-term durability. In contrast, Mo atoms are responsible for hydrogen evolution reaction (HER) properties, and the optimized d-band center and density of states at the Fermi level make Co₂MoB₄ a superior HER catalyst. Our findings provide insight into distinguishing the catalytic pathway of multi-metal borides with improved OER activity and different roles of Mo and Co/Fe in the HER and OER.