Ni Coated with N-doped Graphene Layer as Active and Stable H2 Evolution Cocatalysts for Photocatalytic Overall Water Splitting

Cocatalysts are of great significance for photocatalytic overall water splitting (POWS). Ni-based cocatalysts are attractive candidates for replacing representative noble metals such as Pt, but they suffer from poor stability and low efficiency. Herein, Ni nanoparticles encapsulated in a nitrogen-doped ultrathin graphene (NC) layer are demonstrated to be an active, stable, and low-cost cocatalyst for POWS. It was determined that the H2 evolution rate over Ni@NC/SrTiO3 (STO) is about 3.2 times that of Ni/STO and is even superior to that of a typical Pt cocatalyst. Experiments and XPS studies showed that the unfavorable oxidation of Ni during the reaction is effectively suppressed through the selective coating of N-doped graphene. In addition to the improvement in the charge carrier dynamics, a kinetic study revealed that the apparent activation energy of POWS is reduced by 44% after the introduction of N. With the help of a machine learning potential, an in-depth theoretical study was carried out, and active sites with appropriate H* adsorption were statistically clarified by performing single-point energy calculations for approximately 700 sites on each model structure. The theoretical calculations demonstrated that the distribution of active sites with high spin densities is remarkably promoted due to the modification of the electronic structure of graphene by N doping. This work demonstrates the great potential of using highly stable and active Ni-based photocatalysts for POWS.