Abstract
Spin polarization is a promising strategy for the oxygen evolution reaction (OER) due to the spin‐dependent behaviors of intermediates, but its application in transition metal chalcogenides for anion‐exchange membrane water electrolyzers (AEMWE) remains rarely reported. Here, density functional theory calculations reveal that partial substitution of selenium (Se) by boron (B) in cobalt diselenide (CoSe
2
) induces lattice contraction strain, reducing the spin polarization degree of cobalt (Co) and triggering a high‐spin to low‐spin transition. Experimental results confirm the presence of compressive strain and enhanced catalytic performance in B‐doped CoSe
2
. Notably, the B2‐CoSe
2
sample, featuring a lattice strain of −0.27%, achieves a current density of 50 mA cm
−2
at an overpotential of only 367 mV in 1 M KOH—35 mV lower than that of pristine CoSe
2
. When applied to an AEMWE, this catalyst requires only 2.05 V to achieve a current density of 1 A cm
−2
and exhibits exceptional stability over 120 h at 25 °C in 1 M KOH electrolyte, outperforming commercial RuO
2
. Collectively, these findings demonstrate that B‐doping‐induced lattice contraction strain plays a critical role in modulating the spin polarization and spin state of Co, thereby providing a new paradigm for designing OER catalysts through lattice strain engineering.