Unraveling the Contrasting Dynamics of Reconstruction in Wolframite Cobalt Molybdate Polymorphs for Oxygen Evolution Reaction Electrocatalysis

Abstract

Understanding the dynamic structural evolution of active sites under operating conditions is crucial for designing high-performance electrocatalysts for the oxygen evolution reaction (OER). In this study, we explore the distinct reconstruction behaviors of two cobalt-based molybdate polymorphs, α-CoMoO₄ and β-CoMoO₄, with wolframite-type structures. α-CoMoO₄ undergoes a gradual surface reconstruction, forming amorphous cobalt oxyhydroxide, consistent with the lattice oxygen oxidation mechanism (LOM). In contrast, β-CoMoO₄ rapidly transforms into CoOOH through an acid group dissociation (AGD) mechanism involving the dissociation of MoO₄^2– groups. Real-time tracking of the phase transition by cyclic voltammetry (CV) and quick-scanning X-ray absorption fine structure (QXAFS) reveals detailed kinetic insights into these processes. Density functional theory (DFT) calculations attribute the divergent reconstruction pathways to the competition between Co–O and Mo–O bond strengths, with stronger Mo–O bonds facilitating the rapid reconstruction of β-CoMoO₄. Notably, surface-sensitive soft X-ray absorption spectroscopy (sXAS) demonstrates that β-CoMoO₄ forms a higher concentration of μ₂-OH-Co^2+/3+ active sites, resulting in its intrinsic activity being 2.1 times that of α-CoMoO₄. This work underscores the advantages of AGD-driven reconstruction for generating active sites and provides insights into the rational design of efficient OER electrocatalysts.