With growing global emphasis on green and low-carbon development, energy-related challenges have attracted heightened scholarly and public attention. Polyoxometalates (POMs)—well-defined anionic metal–oxygen clusters—exhibit exceptional capacity for rapid, reversible multi-electron transfer while maintaining structural integrity. In this study, graphitic carbon nitride (g-C₃N₄) was protonated using hydrochloric acid to yield the cationic derivative g-C₃N₄–H⁺Cl⁻. Subsequently, three nanostructured g-C₃N₄-based composite photocatalysts—denoted as g-C₃N₄-H-MMo₆ (M = Ni, Fe, Co)—were synthesized via electrostatic assembly between the protonated support and POM anions. Comprehensive characterization was conducted to elucidate phase purity, morphology, surface properties, elemental composition, and chemical states. Under visible-light irradiation, photocatalytic H₂O₂ production performance was significantly enhanced: g-C₃N₄-H-NiMo₆ achieved a 3.91-fold increase in activity relative to pristine g-C₃N₄. Catalytic efficiency followed the trend g-C₃N₄-H-NiMo₆ > g-C₃N₄-H-FeMo₆ > g-C₃N₄-H-CoMo₆. Electrochemical tests confirmed the role of MMo₆ (M = Ni, Fe, Co). The photogenerated electrons of g-C₃N₄ are transferred to the anchored POMs and reduce dissolved O₂ to H₂O₂ (O₂ + 2H⁺ + 2e⁻ → H₂O₂), while the photogenerated holes oxidize H₂O to H₂O₂ (2H₂O + 2h⁺ → H₂O₂ + 2H⁺), achieving sacrificial-agent-free H₂O₂ photosynthesis with an overall reaction of O₂ + 2H₂O → 2H₂O₂. Collectively, electrostatic immobilization of POMs onto protonated g-C₃N₄ affords robust, high-performance photocatalysts with simultaneously improved activity, stability, and structural fidelity.