Tunable CoFe-based active sites on 3D heteroatom doped graphene aerogel electrocatalysts via annealing gas regulation for efficient water splitting

Most electrocatalysts are only efficiently active for either the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) due to their intrinsic catalytic activity. It is necessary to combine these electrocatalysts for overall water splitting. Unfortunately, these catalysts are usually synthesized by using completely different routes and reactant precursors, which increase the cost of manufacture with tedious procedures. Herein, we demonstrate a versatile gas-regulated strategy for the preparation of two kinds of graphene-based electrocatalysts for the HER and OER, respectively, by using the same procedure with the same precursor. Specifically, the hydrothermal treatment of graphene oxide solution in the presence of thiourea and desirable metal ions leads to the formation of a three-dimensional reduced graphene oxide (rGO) hydrogel. The subsequent pyrolysis is efficient in regulating the active sites for the OER and HER, respectively, by adjusting the gas atmosphere. The N-2-regulated formation of CoFeO@N/S-rGO (CoFe2O4 encapsulated 3D N, S-doped rGO aerogel) results in good catalytic performance for the OER while the formation of CoFe@N-rGO (amorphous bimetallic CoFe encapsulated 3D N-doped rGO aerogel) in NH3 results in good HER catalytic performance. The combination of CoFe@N-rGO with CoFeO@N/S-rGO for overall water splitting in alkaline solution exhibits good catalytic activity to achieve a current density of 10 mA cm(-2) at a low potential of 1.63 V with outstanding stability for 100 h, outperforming most of the state-of-the-art catalysts. Therefore, our gas-regulated strategy provides a promising approach to synthesize efficient electrocatalysts for overall water splitting.