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

• Published in: Energy & environmental science 2016-03, Vol.9 (3), p.112-123
• Main Authors: Liu, Rongji, Zhang, Guangjin, Cao, Hongbin, Zhang, Suojiang, Xie, Yongbing, Haider, Ali, Kortz, Ulrich, Chen, Banghao, Dalal, Naresh S, Zhao, Yongsheng, Zhi, Linjie, Wu, Cai-Xia, Yan, Li-Kai, Su, Zhongmin, Keita, Bineta
• Format: Article
• Language: English
• Subjects: Electrocatalysis; Electrocatalysts; Graphene; Hydrogen evolution; Mathematical models; Platinum; Polyoxometallates; Three dimensional
• ISSN: 1754-5692; 1754-5706
• DOI: 10.1039/c5ee03503a

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 [H 7 P 8 W 48 O 184 ] 33− (P 8 W 48 ) was fixed in a 3D configuration on reduced graphene oxide sheets (rGO) to boost its HER activity. The HRTEM imaging and the solid state 31 P NMR spectrum of P 8 W 48 /rGO reveal a strong interaction between individual P 8 W 48 and transparent rGO sheets. The calculation of the interaction between P 8 W 48 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 P 8 W 48 . However, as P 8 W 48 is symmetrical, a quarter of its structure [H 2 P 2 W 12 O 48 ] 12− (P 2 W 12 ) was extracted as a DFT calculation model. As P 8 W 48 in P 8 W 48 /rGO is neutral, due to surrounding counter cations, the calculation model P 2 W 12 is neutral with protons considering the affordable computational time. The adsorption energy for P 2 W 12 on G (−1.55 eV) and the charge transfer between P 2 W 12 and G (0.66 |e|) indicate that a strong interaction between P 2 W 12 and G sheets exists. Kinetic studies show that the P 8 W 48 /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 P 8 W 48 /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. A 3D configuration of high-quality graphene sheets and a polyoxometalate [H 7 P 8 W 48 O 184 ] 33− showing excellent hydrogen evolution activity at extremely low overpotential.