Abstract
<jats:p>Conventional theoretical models of heterogeneous electrocatalysis typically describe reaction mechanisms exclusively along the ground-state Born–Oppenheimer potential-energy surface (PES). However, this approximation is often insufficient for transition-metal-based catalysts with multiple coexisting spin states, where electronic excitations are energetically accessible at room temperature, and a weak spin–orbit coupling (SOC) hinders the reaction kinetics on the ground-state PES. In this work, we present a comprehensive spin-resolved reaction network for the oxygen evolution reaction (OER) on a β-NiOOH (001) surface by combining hybrid density functional theory (DFT) embedded-cluster calculations with lattice kinetic Monte Carlo (kMC) simulations. We demonstrate that strictly restricting the reaction dynamics to spin-allowed processes under adiabatic spin conservation creates a severe kinetic bottleneck due to spin-trapping in long-lived intermediates. Crucially, incorporating nonadiabatic intersystem-crossing (ISC) transitions driven by weak SOC systematically dismantles these bottlenecks. At room temperature, thermal activation drives transitions into higher-energy electronic configurations. Once populated, these excited states activate a dense network of alternative, purely spin-allowed reaction pathways featuring substantially smaller thermodynamic barriers, which successfully drops the overpotential for the multilevel system to a calculated value of 0.58 V. Our findings establish the critical role of nonadiabatic spin-flip transitions and demonstrate that accounting for multi-spin pathways is essential for the realistic quantitative evaluation and rational design of highly efficient transition-metal-based electrocatalysts.</jats:p>