Identification of active sites and synergistic effect in multicomponent carbon-based Ru catalysts during electrocatalytic hydrogen evolution

Single atom catalysts (SACs) were reported to demonstrate exciting catalytic features for a number of reactions, including hydrogen evolution reaction (HER). However, the true role of these single atom sites in catalysts remains elusive, particularly for those prepared via pyrolysis, where the formation of active nanoparticle counterparts is often unavoidable. Here we report a Ru based catalyst (Ru embedded in N doped carbon spheres (Ru/NPCS)) comprising of both Ru nanoclusters and Ru single sites, who demonstrates activity exceeding Pt catalyst and mass activity among the best of the Ru based catalysts under acidic conditions. The integration of proton exchange membrane water electrolysis with Ru/NPCS as a cathode exhibited an excellent hydrogen generation activity and extraordinary stability (during 120 h of electrolysis) with a 1/48 Ru loading (16.5 µg_Ru·cm⁻²) of a commercial 20% Pt/C catalyst. Through precisely tailoring the dispersion status of the catalysts, we reveal that while ruthenium nanoclusters actively catalyze HER via Volmer–Tafel mechanism, the Ru SACs barely catalyze HER, with H* adsorption difficult to occur. Moreover, no synergy between Ru SACs and Ru cluster is revealed, meaning the Ru SACs act as a spectator rather than active species during H₂ evolution.