Identifying Beneficial and Adverse Co4+ Species in Cobalt‐Based Oxygen Evolution Catalysts via Precursor Polymorphism Engineering

This study resolves conflicting roles of Co⁴⁺ species in cobalt‐based oxygen evolution reaction (OER) catalysts by engineering precursor polymorphism. Through controlled annealing of cobalt oxysulfide, researchers selectively stabilize γ‐CoO2 (enabling oxygen pathway mechanism) and β‐CoO2 (following conventional mechanism). γ‐CoO2 demonstrates superior performance, highlighting micro‐environment design for efficient catalysis. Abstract 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. This study resolves conflicting roles of Co⁴⁺ species in cobalt-based oxygen evolution reaction (OER) catalysts by engineering precursor polymorphism. Through controlled annealing of cobalt oxysulfide, researchers selectively stabilize γ-CoO 2 (enabling oxygen pathway mechanism) and β-CoO 2 (following conventional mechanism). γ-CoO 2 demonstrates superior performance, highlighting micro-environment design for efficient catalysis. Abstract 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. Advanced Science, Volume 12, Issue 43, November 20, 2025.