CuO/Cu(OH) ₂ Heterostructure with Sustained Dynamic Re-equilibration and High HER Activity

CuO and Cu(OH)₂ are active copper-based catalysts for alkaline hydrogen evolution, yet their direct synthesis often yields architectures that degrade under operating conditions. Here, copper sulfide (Cu₂S) is used as a chemically metastable precursor that undergoes rapid, water-driven electrochemical reconstruction into a dynamically re-equilibrating CuO/Cu(OH)₂ heterostructure under alkaline HER conditions. Combined ex situ and operando analyses reveal a sequential transformation from Cu₂S through Cu₂O and nanocrystalline CuO to microneedle-like Cu(OH)₂ domains intimately interfaced with defect-rich CuO. This reconstructed interface does not represent a terminal phase but instead operates in a bias-stabilized dynamic steady state, in which hydroxylation and dehydroxylation processes continuously regenerate the Cu(OH)₂ phase during catalysis. During the reconstruction, the intrinsic instability of Cu⁺ in aqueous environments accelerates sulfur depletion and oxygen incorporation, enabling water to act as a structural reactant that drives sustained interconversion between oxide and hydroxide domains. The resulting heterostructure exhibits enhanced intrinsic activity in HER, delivering an overpotential of 70 mV at 10 mA cm^-2, a Tafel slope of 78 mV dec^-1, and a forty-fold reduction in charge-transfer resistance relative to pristine Cu₂S. Temperature-programmed desorption and Auger electron spectroscopy demonstrate rapid and reversible exchange of surface hydroxyl species, while Raman isotope experiments (H₂O/D₂O) directly confirm continuous hydroxyl turnover within the Cu(OH)₂ lattice under cathodic bias. Long-term galvanostatic operation (∼138 h) reveals sustained activity with preserved microneedle morphology, indicating that electrochemical bias stabilizes the dynamic CuO/Cu(OH)₂ equilibrium. These findings establish Cu₂S as a model metastable precursor for generating self-regenerating oxyhydroxide interfaces and provide general mechanistic insight into the design of electrocatalysts that operate under nonequilibrium, water-mediated conditions.