Insights into Hydrogen Spillover via Heterointerface Engineering and Spin State Density Modulation in CoO-Co₃O₄/rGO@NiP Hybrids for Alkaline Hydrogen Evolution

Heterointerface and defect engineering, coupled with electronic structure modulation, are powerful strategies for designing advanced electrocatalysts for sustainable energy. Herein, we integrate CoO-Co 3 O 4 /rGO onto a NiP matrix, leveraging their synergistic properties to enhance alkaline HER performance. The presence of heterointerfaces, defects, oxygen vacancies, and synergistic interactions within CoO-Co 3 O 4 /rGO@NiP modulates the overall electronic spin state density and enhances the net electronic conductivity among the electron-donating channels. The present in situ synthesis strategy reduces GO to rGO and deposits CoO-Co 3 O 4 on rGO layers, preventing rGO restacking and maximizing the exposure of the active sites in mixed-valence metal oxides. During the multistep HER process, the electron-enriched Co 2+ at CoO sites facilitates H + adsorption, and the high-valence Co 3+ in Co 3 O 4 promotes H 2 O activation and dissociation. The HER activity of the hybrid coating has a low overpotential (η 10 = 106.2 mV), low Tafel slope (107.9 mV dec –1 ), diminished R CT value, and long-term durability in 1.0 M NaOH electrolyte. This stems from the oxygen vacancies as well as Jahn–Teller-active e g orbitals with high spin state density at the heterointerface, enabling the fast charge transfer kinetics of HER on CoO-Co 3 O 4 /rGO@NiP.