Abstract
<jats:p>Transition-metal borides are promising oxygen evolution reaction (OER) catalysts with proven performance in alkaline conditions. Despite over a decade of available research, the role of boron in improving the activity and stability is still debated. In this work, we uncover a previously unrecognized mechanism in a multi-metallic boron-based catalyst (Co-Fe-Mo-B), whereby boron migrates to the catalyst surface during synthesis, forming a protective boron oxide shell that stabilizes the metallic core. During OER, this sacrificial shell dissolves while increasing surface area and exposing the preserved metal sites that transform into OER-active oxyhydroxides. Using advanced electron microscopy, X-ray absorption spectroscopy and operando Raman spectroscopy, complemented by density functional theory calculations, we directly track and rationalize this process from bulk to surface (during synthesis) and from pre-catalyst to active-catalyst state (during OER). DFT further reveals a thermodynamic preference for randomized Co/Fe distribution in the resulting mixed oxyhydroxide and shows that the OER overpotential decreases with increasing Fe content of the local reaction site. Direct experimental evidence is provided to concretely establish the precise role of boron in metal borides that has remained unclear until now. The optimized catalyst delivers high performance in an anion-exchange membrane electrolyzer, validated across different laboratories, with remarkable durability (>840 h). Our findings resolve the long-standing ambiguity around boron’s function in metal borides, presenting a design criterion for leveraging sacrificial non-metallic anions in OER catalysts.</jats:p>