Thickness-dependent hydrogen and oxygen evolution reaction activity of Co2+-rich surface layers on electrodeposited cobalt

In this study, hydrodynamically controlled electrodeposition tunes the near-surface structure of cobalt and reveals the role of Co 2+ -rich layer thickness in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity. Cobalt electrodes prepared under diffusion-controlled and convection-induced deposition exhibited distinct loading, morphology, crystal structure, and near-surface composition. Depth-resolved X-ray photoelectron spectroscopy and transmission electron microscopy revealed a metallic Co-rich interior surrounded by a Co 2+ -rich oxygen-containing surface region. Neither particle size nor double-layer capacitance accounted for the observed mass-activity trend. Instead, within the diffusion-controlled series at −0.35 V, HER and OER mass activities peaked at a Co 2+ -rich layer thickness of ∼35 nm and declined at ∼60 nm. Electrochemical impedance spectroscopy revealed the lowest interfacial impedance at ∼35 nm, supporting an optimal balance between active-site accessibility and charge/ion transport. These findings establish Co 2+ -rich layer thickness as a precatalyst descriptor for bifunctional water-splitting activity.