A Water-Splitting Carbon Nitride Photoelectrochemical Cell with Efficient Charge Separation and Remarkably Low Onset Potential

A simple method to grow a closely packed carbon nitride (CN) film by the crystallization of CN monomers on a conductive substrate followed by a thermal condensation is reported. The as-synthesized CN exhibits excellent performance as photoanode material in a photoelectrochemical cell. Detailed (photo)electrochemical and transient absorption measurements indicate excellent charge separation properties, high hole-extraction efficiency (up to 50 %), a long electron lifetime, and low amount of defect states below the CN conduction band. Consequently, the CN photoanode exhibits a markedly low overpotential of 0.25 V versus reversible hydrogen electrode (RHE), which is comparable with the state-of-the-art metal-based photoanodes, an impressive photocurrent density of 116 μA cm⁻² at 1.23 V versus RHE in an alkaline solution without sacrificial agent, as well as excellent stability over a wide pH range (0–13).