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.