Binder-free carbon nano-network wrapped carbon felt with optimized heteroatom doping for vanadium redox flow batteries

Abstract

In this work, we propose to bind carbon nanofibrous networks onto a carbon felt substrate via the self-assembly process of polyaniline for vanadium redox flow batteries. The binder-free carbon nano-network wrapped carbon felt has a specific surface area of 161 m² g⁻¹, which is one hundred times higher than that of pristine carbon felt (0.4 m² g⁻¹) and thermally treated carbon felt (1.0 m² g⁻¹). Besides, the surface compositions of carbon nanofibers are also optimized by sulfur and nitrogen co-doping to selectively catalyze vanadium redox reactions with low parasitic reaction activity. It was found that the optimized N–S co-doping and the hierarchically porous structure collaboratively provide abundant active sites and an efficient mass transfer pathway, promoting the advection–diffusion–reaction process. Meanwhile, the nanofiber network also improves the interconnectivity among the micron-scale fibers of carbon felt, reducing the cell internal resistance. As a result, the present electrode exhibits an encouraging energy efficiency of 82.4% at a remarkably high current density of 320 mA cm⁻² in a vanadium redox flow battery system, as compared to that obtained by the thermally treated carbon felt (66.8%). Furthermore, the carbon nano-network wrapped carbon felt also demonstrates a superior long-term stability over 1000 cycles, showing great promise in practical flow battery applications.