Further insights into bifunctional mechanism in alkaline hydrogen evolution for hybridized nanocatalysts and general route toward mechanism-oriented synthesis

The classic bifunctional mechanism derived from single crystal models shows practicability for directing hydrogen evolution reaction (HER) in alkaline media for noble-metal based hybrid catalysts, however, whether it can be extensively applied to non-noble metal nano-hybrid ones remains unclear, due to lacking of effective means to identify reaction active sites and key intermediates. Here we present a representative MoSe₂/CoSe heterogeneous hollow spheres (MoSe₂/CoSe HHSs) as a nano-hybrid catalyst model and demonstrate its atomic-level identification of catalytic active sites toward alkaline HER, by virtue of advanced experimental and theoretical calculating techniques. Real-time electron paramagnetic resonance (EPR) measurements for hydroxyl reveal the promoting effect of the Co species on water dissociation, whereas X-ray absorption spectroscopy (XAS) tests of pre-and post-HER further unravel the formation of high-valence HO-Co sites and MoSe₂-H interactions after the electrolysis, both of which synergistically confirm that MoSe₂/CoSe HHSs follow the classic bifunctional mechanism. Moreover, different from the classic bifunctional mechanism in which there is no substantial interaction between the involved two components, the hybridization of MoSe₂ with CoSe also further optimizes hydrogen binding energy (HBE) of MoSe₂ revealed by density functional theory (DFT) calculations, demonstrating that both the bifunctional mechanism and HBE should be considered simultaneously when designing low-cost alternatives to noble metal catalysts. Furthermore, a general two-step Ostwald ripening strategy is also proposed for hybridized metal sulfide hollow spheres toward mechanism-oriented material design.