A correlation of the hydrogen evolution reaction activity to the number of defects formed by the decomposition of doped phosphorus species in carbon nanotubes

Phosphorus-doped carbon materials are one of the novel carbon catalysts for the hydrogen evolution reaction (HER) that have attracted considerable attention in recent years. However, the role of C―P species played in the HER activity is still not clear. Phosphorus-doped carbon nanotubes were prepared by chemical vapor deposition and annealed at 900, 1000 and 1200 °C to remove all or part of the phosporus, resulting in four samples with different amounts of substitutional-, pyridine- and pyrrole-like P species. The correlations between the HER activity and the contents of the three species were investigated. Results showed that the content of substitutional P decreased with annealing temperature and none was retained at 1200 °C. The HER activity increased with annealing temperature and the sample annealed at 1200 °C had the highest HER activity in an acid medium with an overpotential of 0.266 V at a current density of 10 mA cm⁻². Density functional theory calculations showed that the pentagon- and nine-membered ring defects formed by the elimination of substitutional P mainly contributed to the HER activity.