Decorating sulfur and nitrogen co-doped graphene quantum dots on graphite felt as high-performance electrodes for vanadium redox flow batteries

This paper reports on decorating sulfur (S)/nitrogen (N) co-doped graphene quantum dots (S/N co-doped GQDs) on graphite felt as high-performance electrodes for vanadium redox flow batteries (VRFBs). The S/N co-doped GQDs are synthesized through an efficient infrared-assisted pyrolysis of glucose, urea, and ammonia sulfate at 280 degrees C. The S/N co-doped GQDs, having an average diameter of 6.2 nm, contain high oxidation, amidation, and sulfuration levels (45.8 (O/C), 22.7 (N/C), and 7.8 at.% (S/C), respectively). Electrochemical impedance spectroscopy is utilized to assess the overpotential distributions for VRFBs equipped with untreated GF and S/N co-doped GQDs/GF. With the aid of S/N co-doped GQDs, the catalytic activity, equivalent series resistance, durability, and voltage efficiency are substantially improved. The improved performance is attributed to the synergistic effect of GQDs containing O functionalities, lattice N atoms, and S dopants, facilitating surface catalytic activity and accelerating charge transfer across the anode/anolyte interface for the vanadium redox couples (V (II)/V(III)). Accordingly, hierarchical S/N co-doped GQD/GF electrode paves the pathway for engineering the electrodes' nano-structure with improved catalytic activity and enhanced durability for redox flow batteries.