Benchmarking Three Ruthenium Phosphide Phases for Electrocatalysis of the Hydrogen Evolution Reaction: Experimental and Theoretical Insights.

The outstanding electrocatalytic activity of ruthenium (Ru) phosphides toward the hydrogen evolution reaction (HER) has received wide attention. However, the effect of the Ru phosphide phase on the HER performance remains unclear. Herein, a two-step method was developed to synthesize nanoparticles of three types of Ru phosphides, namely, Ru2 P, RuP, and RuP2 , with similar morphology, dimensions, loading density, and electrochemical surface area on graphene nanosheets by simply controlling the dosage of phytic acid as P source. Electrochemical tests revealed that Ru2 P/graphene shows the highest intrinsic HER activity, followed by RuP/graphene and RuP2 /graphene. Ru2 P/graphene affords a current density of 10 mA cm-2 at an overpotential of 18 mV in acid media. Theoretical calculations further showed that P-deficient Ru2 P has a lower free energy of hydrogen adsorption on the surface than other two, P-rich Ru phosphides (RuP, RuP2 ), which confirms the excellent intrinsic HER activity of Ru2 P and is consistent with experiment results. The work reveals for the first time a clear trend of HER activity among three Ru phosphide phases.