Coupling Cationic g-C3N4 with Anionic Metal–Oxygen Clusters for Photocatalytic H2O2 Production

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.