Super-hydrophilic activated carbon chest skeleton-like structure with self-doped heteroatoms (N, S, & O) derived from spring onion root for VRFB applications

Vanadium redox flow battery (VRFB) is a most interesting rechargeable battery for grid scale energy storage application. For the extensive commercialization of VRFBs, low-cost electrode materials should be developed with higher electrochemical activity, faster heterogeneous electron transfer, and smaller voltage loss. Herein, spring onion root-derived activated carbon (SOR_AC) with self-doped heteroatoms (N, S, & O) was synthesized by using pyrolysis and chemical activation methods. The resultant activated carbon (SOR_AC_800) derived at optimized temperature (800 °C) possesses the superior physical properties such as higher specific surface area, larger pore volume, rich defective sites, and super-hydrophilic nature. Also, the self-doped heteroatoms provide additional electrochemical catalytic active sites for both VO2+/VO2+ and V2+/V3+ redox reactions. Thus, SOR_AC_800 modified electrode exhibited higher reversibility with lowest ΔEp for VO2+/VO2+ (246 mV) and V2+/V3+ (165 mV), Rct (3.73 Ω), and higher Cdl (24.70 mF cm−2). The obtained result is comparatively better than pristine graphite felt (GF), SOR_C_500 (pre‑carbonized carbon at 500 °C), and SOR_AC_X (X = 600, 700, and 900) modified GFs. It is also found that the VRFB full cell consists of SOR_AC_800/GF as both positive and negative electrode exhibited low ohmic loss, higher discharge capacity (22.79 Ah L−1 at 50 mA cm−2), CE (96.96 %), VE (78.10 %), EE (75.73 %), long-term retention of EE (99.4 %) for 200 cycles at 100 mA cm−2, and 100 % of capacity recovery rate. Finally, this present work suggested the inexpensive, abundant, and eco-friendly bio-waste derived activated carbon with exceptional physical and electrochemical properties for VRFB applications.