Highly Active Hollow Porous Carbon Spheres@Graphite Felt Composite Electrode for High Power Density Vanadium Flow Batteries

Abstract Improving power density is considered as one of the most effective methods to decrease the cost of vanadium flow batteries (VFBs). To reduce the polarization loss of VFBs, a hollow porous carbon sphere, whose inner and outer surfaces can both provide active sites catalyzing the redox reactions of vanadium redox couples, is designed and prepared by using nickel metal–organic framework (Ni‐MOF) as a template with the assistance of polyvinylpyrrolidone (PVP). This catalyst with its high specific surface area can be simply in situ loaded on the graphite felt forming a composite electrode. The composite electrode exhibits outstanding electrocatalytic activity, as the vanadium ions can participate in the reaction on both inner and outer surfaces of the hollow spheres. As a result, a VFB single cell assembled with this electrode can achieve a high energy efficiency of 82.7% at a current density of 200 mA cm −2 and a peak power density of 958 mW cm −2 , which is much higher than those of pristine graphite felts. This work provides a new idea for the development of highly active electrocatalysts for VFBs and further improves the power density of a VFB.