Sodium/Potassium Poly(heptazine imide) with 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, the sodium/potassium poly(heptazine imide) (NaK-PHI) photocatalyst with the electron sink effect is synthesized in ternary eutectic salts (LiCl/NaCl/KCl) to improve the transport efficiency of charges and photocatalytic activity. 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 the 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.