Enhanced proton and electron reservoir abilities of polyoxometalate grafted on graphene for high-performance hydrogen evolution

Abstract

A beneficial “microenvironment effect” on the efficiency of confined electrocatalysts is predicted by theory. However, examples of its experimental confirmation are scarce for catalysts based on polyoxometalates for the hydrogen evolution reaction (HER). For this purpose, the cyclic 48-tungsto-8-phosphate [H₇P₈W₄₈O₁₈₄]³³⁻ (P₈W₄₈) was fixed in a 3D configuration on reduced graphene oxide sheets (rGO) to boost its HER activity. The HRTEM imaging and the solid state ³¹P NMR spectrum of P₈W₄₈/rGO reveal a strong interaction between individual P₈W₄₈ and transparent rGO sheets. The calculation of the interaction between P₈W₄₈ and graphene (G) sheets is difficult to perform within a reasonable period of time because of the large size and very high overall negative charge of P₈W₄₈. However, as P₈W₄₈ is symmetrical, a quarter of its structure [H₂P₂W₁₂O₄₈]¹²⁻ (P₂W₁₂) was extracted as a DFT calculation model. As P₈W₄₈ in P₈W₄₈/rGO is neutral, due to surrounding counter cations, the calculation model P₂W₁₂ is neutral with protons considering the affordable computational time. The adsorption energy for P₂W₁₂ on G (−1.55 eV) and the charge transfer between P₂W₁₂ and G (0.66 |e|) indicate that a strong interaction between P₂W₁₂ and G sheets exists. Kinetic studies show that the P₈W₄₈/rGO hybrids display excellent HER activity in acid, further confirmed by reproducible generation of hydrogen with quantitative faradaic yield and a high turnover frequency (11 s⁻¹ at 295 mV overpotential) for a noble metal-free electrocatalyst. Importantly, the overpotentials required for the HER compare well with those of the commercial Pt/C (20 wt% Pt), which indicates that P₈W₄₈/rGO is a promising cheap HER electrocatalyst. We demonstrate here the most convincing experimental evidence of the “microenvironment effect” on HER electrocatalysis by a polyoxometalate.