Identifying Beneficial and Adverse Co ⁴⁺ Species in Cobalt‐Based Oxygen Evolution Catalysts via Precursor Polymorphism Engineering

The oxygen evolution reaction (OER) is a critical bottleneck in water electrolysis for hydrogen production, necessitating catalysts that optimize both efficiency and cost. Cobalt-based materials offer a viable alternative to noble metals, but their development is complicated by uncertainties regarding the role of Co⁴⁺ species formed during operation. Conflicting studies debate whether CoO₂ acts as the active phase or if Co⁴⁺ species may suppress reactivity. To address this, the Co⁴⁺ impact on OER activity is systematically investigated by tailoring the reconstruction of polymorphic cobalt oxysulfide. Through thermal annealing, crystallinity and local coordination are controlled to selectively stabilize γ-CoO₂ and β-CoO₂ phases under OER conditions, respectively. Structural analysis reveals that H₂O/OH^- intercalation drives lattice expansion, favoring γ-CoO₂ formation, while rigid Co─O bonds limit flexibility, yielding β-CoO₂. Mechanistic studies show γ-CoO₂ promotes superoxide (Co─O─O─Co) intermediates via the oxygen pathway mechanism (OPM), whereas β-CoO₂ follows the conventional adsorbate evolution mechanism (AEM). As a result, γ-CoO₂ exhibits superior catalytic performance, with lower overpotentials and enhanced long-term stability. These insights highlight the pivotal role of Co⁴⁺ micro-environments in OER performance, offering a rational framework for optimizing transition-metal catalysts.