Enhanced solar-to-chemical conversion of seawater to H₂O₂ via defect-rich sulphur-doped poly (heptazine imide) photocatalysts

Sustainable on-site hydrogen peroxide (H₂O₂) production from oxygen and water using visible light is an appealing method for decentralized water treatment and green oxidation chemistry. However, it often faces challenges due to weak O₂ activation and rapid charge recombination in carbon nitride photocatalysts. In this study, we report a sulphur-functionalized poly (heptazine imide) (S-PHI) made through KCl-assisted polymerization. The controlled addition of different atoms and changes to the framework improve crystallinity, stacking order, and defect chemistry. XRD and vibrational spectroscopy confirm the creation of a heptazine-imide network with strain-induced structural changes. XPS shows C–S bonding without oxidized sulphur species present. S-PHI shows improved visible-light absorption (Eg ∼ 2.64 eV; LHE ∼91% up to 440 nm), strong photoluminescence quenching, a slightly longer carrier lifetime (∼10.48 ns), a larger electrochemically active surface area (Cdl: 61.5 mF cm⁻²), lower interfacial charge-transfer resistance, and a more negative flat-band potential (−1.62 V), which supports oxygen reduction. With low-power 405 nm LED light and ethanol, S-PHI produces 16,400 μmol g⁻¹ h⁻¹ H₂O₂, increasing to 38,142 μmol g⁻¹ h⁻¹ in untreated seawater with O₂ bubbling. The apparent quantum yields reach up to 45.1%, and the SCC efficiency is 0.31%. Rotating-disk analysis (n ∼ 2.29) and scavenger tests indicate a mainly two-electron O₂ reduction pathway, with an extra ¹O₂-mediated contribution from defect states and photosensitized pathways. This work showcases defect-engineered PHI as a strong and scalable option for solar-driven H₂O₂ production in real saline environments.