Adaptive hexagonal metal-oxynitride monolayers for oxygen reduction catalysis

Transition metal nitrides (TMNs) have recently attracted increasing attention as a robust alternative to platinum electrocatalysts in alkaline oxygen reduction reactions (ORR). However, the fundamental understanding on the catalytic nature of the TMNs remains elusive, impeding the further catalyst design and optimization. Here, using ZrN as a model catalyst, we demonstrate that the unexpected catalytic activity of TMNs originates from the self-adaptive behavior of surface-reconstructed oxynitride monolayers under ORR conditions. Our first-principles calculations reveal that oxygen adsorption triggers a square-to-hexagonal symmetry transition on the ZrN surface, stabilizing a hexagonal ZrNO monolayer. At quarter-hydroxyl coverage, this reconstruction generates semi-elliptical cavities that confine the highly active Zr sites. Crucially, the flexible Zr-N-Zr linkages connecting these Zr sites and the underlying ZrN substrate undergo dynamic bond-length variations during ORR, which precisely regulate oxygen intermediate adsorption and significantly enhance catalytic activity. Experimental characterization aligns well with these theoretical predictions. The as-designed ZrNO monolayer catalyst delivers a 0.882 V half-wave potential for ORR and enables zinc-air batteries with 240 mW cm⁻² peak power density—metrics that exceed state-of-the-art Pt/C. This study provides atomic-level insights into the nature of TMNs catalytic monolayers, paving the way for stable and active catalyst engineering in next-generation energy technologies.