A bi-porous graphite felt electrode with enhanced surface area and catalytic activity for vanadium redox flow batteries

In this work, a bi-porous graphite felt electrode is prepared by a simple yet effective catalytic etching method for vanadium redox reflow batteries (VRFBs). The primary pores, ∼100 μm in size and formed by voids between interconnected carbon fibers, act as the macroscopic pathways for electrolyte flow, while the secondary pores, ∼200 nm in size and formed onto carbon fibers, increase the active surfaces for electrochemical reactions. The Brunauer-Emmett-Teller results show that the specific surface area of bi-porous graphite felt is 17.73 m² g⁻¹, which is 7 times larger than that of the original graphite felt. The cyclic voltammetry and electrochemical impedance spectroscopy tests demonstrate the higher peak currents, smaller peak potential separations and lower charge transfer resistances of the bi-porous graphite felt than the original graphite felt. Battery tests show that the VRFB with the bi-porous graphite felt electrode achieves an energy efficiency of 87.02% and an electrolyte utilization of 84.07% at the current density of 200 mA cm⁻², which are 17.90% and 38.91% higher than that with the original graphite felt electrodes. More importantly, the battery can be operated at the high current densities of 300 and 400 mA cm⁻² with the energy efficiencies of 82.47% and 77.69%, among the highest performance in the open literature. All these superior results demonstrate that the bi-porous graphite felt prepared in this work offers a promise to replace conventional mono-scale porous graphite felt electrodes for VRFBs.