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

The outstanding electrocatalytic activity of transition metal ruthenium (Ru) phosphides toward hydrogen evolution reaction (HER) has received wide attentions. However, it remains unclear regarding the effect of ruthenium phosphide phase on the HER performance. Herein, a two-step method is developed to synthesize three types of ruthenium phosphides including Ru2P, RuP and RuP2 nanoparticles with similar morphology, dimension, loading density and electrochemical surface area (ECSA) on graphene nanosheets by simply controlling the phytic acid (PA) dosage as a P source and metal complexing reagent. Electrochemical tests reveal that the Ru2P/graphene showing the highest intrinsic HER activity, followed by RuP/graphene and RuP2/graphene in turn. The Ru2P/graphene affords a current density of 10 mA cm-2 at overpotential of 18 mV in acidic media. Theoretical calculation further shows that P-deficient Ru2P has lower free energy of hydrogen adsorption (ΔGH*) on the surface than other two P-rich ruthenium phosphides (RuP, RuP2), which confirms the excellent intrinsic HER activity of Ru2P and is consistent with the experiment results. The work reveals for the first time a clear trend of HER activity among three ruthenium phosphide phases.