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