The effect of low-defected carboxylic acid functional group–rich carbon nanotube–doped electrode on the performance of aqueous vanadium redox flow battery

Modified (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-mediated oxidation (MTMO) is introduced to fabricate low-defected carboxylic acid functional group–rich carbon nanotube (TEMPO-CNT) through facile and eco-friendly chemical preparation. Due to the MTMO, the O=C-O portion (18.2%), representing the amount of active site to vanadium ion redox reaction (VIRR), reaches the nearly same with conventionally acid-treated CNT (AT-CNT, 18.9%). However, the intensity ratio of D to G band of TEMPO-CNT is measured lower value (1.14) than that of AT-CNT (1.29) in Raman spectra, showing the MTMO is the better strategy to functionalize carboxylic groups on CNT with the uniform structure and low-defected feature. Furthermore, when the TEMPO-CNT is utilized for the catalyst for VIRR, the catalytic activity increases to 2.11 (negolyte) and 2.03 (posolyte) times compared to AT-CNT, and the reversibility of VIRR is also improved. These results attribute to the 41.6% lower charge transfer resistance than AT-CNT, demonstrating that the low-defected CNT structure of TEMPO-CNT induced a facile electron transfer, resulting in the high catalytic performance. With that, the energy efficiency (EE) and discharge capacity of vanadium redox flow battery (VRFB) adopting TEMPO-CNT display 58.8% and 16.8 Ah L⁻¹ even at high current density (250 mA cm⁻²), whereas those of AT-CNT are only 52.3% and 6.8 Ah L⁻¹. Regarding long-term stability, the TEMPO-CNT and AT-CNT preserved 98.8% and 91.4% of retention rate in EE at 200 mA cm⁻² for 200 cycles, respectively, indicating that the MTMO is the promising option to fabricate the catalyst to use in the practical VRFB.