Three-dimensional annealed WO3 nanowire/graphene foam as an electrocatalytic material for all vanadium redox flow batteries

This paper presents a three-dimensional annealed tungsten trioxide nanowire/graphene sheet (3D annealed WO3 NWs/GS) foam as an excellent and low-cost electrocatalyst. It was prepared using vanadium redox flow battery (VRFB) electrodes through an in situ self-assembly of graphene sheets prepared by mild chemical reduction, followed by freeze-drying and annealing. The 3D annealed WO3 NWs/GS foam exhibits the highest electrocatalytic activities toward V2+/V3+ and VO2+/VO2+ redox couples among all the tested samples. Charge-discharge tests further confirm that a single flow cell of a VRFB using the 3D annealed WO3 NWs/GS foam demonstrates excellent energy efficiencies of 79.49% and 83.73% at current densities of 80 mA cm(-2) and 40 mA cm(-2), respectively, which are much higher than those of cells assembled with pristine graphite felt (GF) and 3D WO3 NWs/GS foam without annealing treatment. Moreover, it shows no obvious degradation after 50 charge-discharge cycles. These results are attributed to the formation of new W-O-C bonds, confirming that the WO3 NWs are anchored strongly to the GS, which is key to facilitating the redox reactions of the vanadium redox couples. Moreover, the 3D annealed WO3 NWs/GS foam exhibits a 3D hierarchical porous structure, which can provide more surface electroactive sites to improve the electrochemical performance of VRFBs.