Identifying Beneficial and Adverse Co⁴⁺ 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 CoO 2 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‐CoO 2 and β‐CoO 2 phases under OER conditions, respectively. Structural analysis reveals that H 2 O/OH − intercalation drives lattice expansion, favoring γ‐CoO 2 formation, while rigid Co─O bonds limit flexibility, yielding β‐CoO 2 . Mechanistic studies show γ‐CoO 2 promotes superoxide (Co─O─O─Co) intermediates via the oxygen pathway mechanism (OPM), whereas β‐CoO 2 follows the conventional adsorbate evolution mechanism (AEM). As a result, γ‐CoO 2 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.