Sodium/potassium poly(heptazine imide) with an electron sink effect for hydrogen peroxide photosynthesis

Abstract

Artificial photosynthesis is a potential hydrogen peroxide (H₂O₂) production strategy, but the poor charge separation and transfer limit the photocatalytic efficiency. Here, a sodium/potassium poly(heptazine imide) (NaK-PHI) photocatalyst with an electron sink effect is synthesized in ternary eutectic salts (LiCl/NaCl/KCl) to improve the transport efficiency of charges and photocatalytic activity. The formation of H₂O₂ is catalyzed by NaK-PHI through the ˙O₂⁻-engaged and ¹O₂-engaged oxygen reduction reaction and four-electron water oxidation reaction pathways. Introducing a cyano group as an electron-withdrawing group enhances the local negative charge density of PHI and accelerates the separation of carriers by attracting holes. Introducing Na⁺ and K⁺ triggers the electron sink effect and photogenerated electrons are trapped on NaK-PHI, thereby suppressing the recombination of electron–hole pairs. Benefiting from the strong built-in electron field induced by cyano groups and alkali metal ions, the NaK-PHI exhibits an H₂O₂ rate of 672.5 μmol g⁻¹ h⁻¹ in pure water, outperforming most reported carbon nitride photocatalysts. NaK-PHI achieves an apparent quantum yield of 13.9% at 420 nm and a solar-to-chemical conversion efficiency of 0.68%. This strategy of utilizing an internal electric field driving force to improve the migration and transportation of photogenerated carriers provides a new method for efficient H₂O₂ photosynthesis.