<p>In this study, a series of Co₃O₄/C₃N₄ composites with oxygen vacancies​ was successfully fabricated by incorporating varying loadings of Co₃O₄ onto g-C₃N₄, aiming to enhance the photocatalytic performance for H₂O₂ production. Among the prepared samples, the 20 Co₃O₄/C₃N₄ composite exhibited remarkable photocatalytic activity and stability. The superior performance is primarily attributed to the introduction of oxygen vacancies, which not only facilitated the migration and separation of charge carriers but also provided additional active sites for O₂ adsorption. The optimal 20 Co₃O₄/C₃N₄ sample achieved an H₂O₂ production rate of 174.2 µmol·g^− 1·h^− 1, which is 4.8 times higher​ than that of pure g-C₃N₄ (36 µmol·g^− 1·h^− 1). Linear sweep voltammetry (LSV) tests revealed a lower onset potential and a smaller Tafel slope​ for the 20 Co₃O₄/C₃N₄ electrode, indicating reduced overpotential and accelerated reaction kinetics. Furthermore, in situ infrared spectroscopy confirmed that the two-electron oxygen reduction reaction (2e⁻ ORR)​ served as the primary pathway for H₂O₂ generation in the Co₃O₄/C₃N₄ catalytic system. This work provides deep insights into the role of oxygen vacancies in photocatalytic H₂O₂ synthesis, offering experimental evidence and theoretical references for related research, and demonstrating potential for practical applications.</p> Graphical Abstract <p>Compositing g-C₃N₄ with Co₃O₄ at optimal loadings successfully introduced oxygen vacancies and constructed a Co₃O₄/C₃N₄ heterojunction. This strategic modification led to a substantial enhancement in the charge separation efficiency, thereby boosting the photocatalytic H₂O₂ production rate from 36 µmol·g^− 1·h^− 1 (pristine g-C₃N₄) to 174.2 µmol·g^− 1·h^− 1 for the optimal composite</p> <p></p>