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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.