Green surface treatment of graphite felt using modified TEMPO mediated oxidation for use in vanadium redox flow batteries

MTMO (modified TEMPO((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) mediated oxidation) process was suggested as a green surface treatment to functionalize carboxylic acid group (COOH) on lower crystallinity carbon materials (LCCMs). With the proposed treatment, COOH contents on the surface of graphite felt (GF) was increased twice that of conventional heat-treated GF (CH–GF), resulting in high hydrophilicity without defects on the surface of LCCMs. These are attributed to the mild process feature of LCCMs using pre-activation of TEMPO to N-ammonium TEMPO, which changes hydroxyl groups to COOH without damaging carbon structure even on the LCCMs. Moreover, the MTMO process has environmental-friendly features such as a recyclable oxidizing agent, non-toxic effluents (NaCl and water), and room-temperature (25℃) aqueous process. Benefitting from these advantages, the maximum current densities toward vanadium ion redox reaction (VIRR) improved, and the charge transfer resistance for VIRR also decreased to 38.3 % of CH–GF. The energy efficiency for using MTMO–GF electrodes on both sides also improved by 119.4 % (200 mA cm⁻²) of using CH–GF during the cycling test of vanadium redox flow battery (VRFB). Furthermore, the VRFB using MTMO–GF electrodes exhibited stable operation for 500 cycles (200 mA cm⁻²), while VRFB using CH–GF failed at around 300th cycle.