Ligand functionalization of mononuclear Pt-polyoxometalate boosts hydrogen evolution activity

Hydrogen evolution reaction (HER) for electrocatalytic water splitting to produce green hydrogen has recently emerged as a green process to solve energy and environmental issues. In this study, we have developed mononuclear Pt-polyoxometalates of [Pt₁Mo₆(OH)₆O₁₈](NH₄)₂ (PtMo₆) and [Pt₁Mo₆(OH)₃O₁₈)O₃C₅H₉]TBA₂ (PtMo₆-L, TBA: tetrabutylammonium), which are well characterized by various technologies. PtMo₆-L clusters coated onto glassy carbon show good performances in HER, achieving an overpotential of 37 mV@10 mA·cm^–2 with a Tafel slope of 42 mV·dec⁻¹, which outperforms the naked PtMo₆ cluster and is comparable to the commercial 20 wt% Pt/C. PtMo₆-L should obey Heyrovsky-Volmer mechanism that is similar with Pt/C. In contrast, PtMo₆ should adopt Tafel-Volmer mechanism. The experimental results combined with Density Functional Theory calculations corroborate that the Pt-O-H group of polyoxometalate clusters should be the active site and play an important role during the efficient HER. Moreover, mechanistic studies reveal that the more negative charge of the C₅H₉-bonded O1 in PtMo₆-L is prone to adsorb and activate the proton (H⁺), resulting in enhanced electrocatalytic HER activity. In all, these efforts push forward the HER research to study the relationships between polyoxometalate structures and electrocatalytic property at the precise atomic/molecular level.