Scalable synthesis of ultrathin P,O–doped carbon composite CoP nanosheets for efficient electrocatalytic hydrogen evolution in natural seawater

Given the inexhaustible availability of seawater, producing high–purity hydrogen by using seawater is a promising pathway for green energy production. Herein, ultrathin (∼1.5 nm) P,O–doped carbon composite–CoP nanosheets (POC–CoP NSs) were synthesized via a one-step in situ pyrolysis of deep eutectic solvents without the use of surfactants or solvents. The structure wherein CoP is tightly bonded to the surface of P,O–doped carbon not only enhances the catalyst's hydrogen evolution reaction (HER) activity but also improves its stability and durability. Specifically, the POC–CoP NSs exhibited excellent HER activity in 1.0 M KOH and natural seawater. In alkaline and seawater media, a current density of 10 mA cm⁻² was achieved with overpotentials of only 63 and 127 mV, respectively, which is close to those of a commercial 20 % Pt/C catalyst (22 and 91 mV, respectively), suggesting that POC–CoP NSs are an excellent non–precious metal HER catalyst. In addition, the catalyst exhibited a low overpotential at high current densities; only 296 mV was required to reach a current density of 1000 mA⁻² while maintaining excellent stability and durability. The scalable synthesis approach using environmentally friendly deep eutectic solvents provides a promising pathway for practical applications.