Operando Imaging of Adsorbate-Charge Cooperative Reconstruction into Terminal Active Co-OH Sites.

Layered transition-metal (oxy)hydroxides are highly active oxygen evolution reaction (OER) catalysts, yet the origins of their structural dynamics and the pre-OER redox processes governing active-site formation and charge compensation remain unclear. Here we introduce an adsorbate-and-charge cooperative activation framework: reaction intermediates and local charge compensation jointly induce and stabilize catalytically competent coordination/valence motifs. Operando imaging and DFT calculations reveal that, under OER conditions, tetrahedral and undercoordinated edge Co sites convert into metastable terminal octahedral Co-OH motifs. These motifs mediate OH- adsorption and stabilize oxygen intermediates, serving as key initiation centers for subsequent oxygen evolution. Concomitant electronic restructuring increases states near the Fermi level and enhances electron transfer. Mechanistically, activation proceeds via two potential-dependent stages: an initial adsorbate- and charge-driven formation of active Co-OH-stabilized octahedral motifs, followed by higher-potential lattice oxidation that does not generate new active sites but instead constructs a charge-storage host framework, thereby functionally distinguishing active-site generation from lattice charge regulation. This framework stabilizes and electronically optimizes the active Co-OH centers. This assignment is corroborated by in situ XRD (11° → 12.5° → 13°), consistent with DFT-simulated diffraction and cross-sectional STEM (∼0.68 nm). Collectively, these results uncover the preoxidation-triggered formation of Co-OH motifs as a key origin of terminal Co activity and reveal the intrinsically reaction- and charge-coupled dynamic nature of layered transition-metal (oxy)hydroxide catalysts, providing a conceptual foundation for designing dynamically adaptive electrocatalysts.