MOF-derived nitrogen, sulfur, cobalt, and copper co-doped graphite felt for high-efficiency vanadium redox flow battery electrodes

The low electrocatalytic activity of pristine graphite felt (GF) electrodes towards V(II)/V(III) and V(IV)/V(V) redox couples poses a significant challenge in vanadium redox flow batteries (VRFBs). Here, Metal-organic frameworks (MOFs) containing Cu, Co, N, and S are proposed as precursors for the construction of metal and nonmetal co-doped GF electrodes, which exhibit enhanced catalytic activity compared to pristine GF electrodes. The synergistic effect of the metal and nonmetal components results in the N,S/Cu,Co@GF electrode exhibiting increased hydrophilicity, electrochemical reactivity, and reversibility. The modified GF electrode enabled the VRFB to achieve an energy efficiency of 76.2 % at a current density of 200 mA/cm², representing a 10.9 % improvement over the pristine GF. Even at a higher current density of 300 mA/cm², the energy efficiency remained at 65.7 %. Furthermore, the N,S/Cu,Co@GF electrodes demonstrated desirable long-term stability over 350 consecutive charge/discharge cycles at a current density of 200 mA/cm². Density functional theory further elucidates the potential catalytic mechanism of metal and nonmetal co-doping in vanadium redox reactions. The findings demonstrate that MOF-derived metal and nonmetal co-doping is an effective strategy for developing high-efficiency VRFB electrodes.