Carbon-doped ionic carbon nitrides for solar hydrogen production and direct electrochemical storage of solar energy

Ionic carbon nitride, potassium poly(heptazine imide) (KPHI), with carbon atoms replacing nitrogen atoms in the π-conjugated aromatic ring, is synthesized via potassium thiocyanate ionothermal processing of supramolecular complexes of melamine, cyanuric acid, and 2,5,6-triaminopyrimidine (TAP). The carbon-rich TAP in the supramolecular complex serves as both a precursor for carbon doping and a modulator of ionothermal reactivity to adjust the molecular structures of produced KPHIs. The carbon-doped KPHIs exhibit enhanced photocatalytic and dark photocatalytic activities for hydrogen evolution reactions compared to undoped KPHIs. Additionally, they demonstrate the capacity for direct electrochemical solar energy storage, with the storage performance correlating positively with the carbon doping concentration. Optical characterization reveals that carbon-doped KPHIs feature a broad light-harvesting range, prolonged photocarrier lifetime, and a high density of electron storage sites. These photophysical properties collectively contribute to the superior photocatalytic and photoelectrochemical performance of carbon-doped KPHIs. This study emphasizes the carbon-doped KPHI as a potential material for solar energy conversion and storage.