Impact of Surface Carbonyl- and Hydroxyl-Group Concentrations on Electrode Kinetics in an All-Vanadium Redox Flow Battery

This study investigates the effect of thermal activation of all-vanadium redox flow battery (RFB) carbon-felt electrodes on their electrode kinetics. Using X-ray photoelectron spectroscopy, thermal activation is shown to increase the content of the C-OH group, decrease the content of the C=O group, and not affect the O-C=O group, with all these surface moieties already being present in the nonactivated carbon felt. Rotating disk electrode studies were performed using custom electrodes fabricated using the carbon felt to investigate the kinetics of the V²⁺/V³⁺ and VO²⁺/VO2⁺ redox couples in H₂SO₄ and to deconvolute the impact of thermal activation on electrode kinetics. We demonstrate that V²⁺/V³⁺ kinetics is sluggish compared to VO²⁺/VO2⁺ kinetics (equilibrium rate constant (k₀) = 4.98 × 10^-8 m·s^-1 vs 8.81 × 10^-8 m·s^-1) and that thermal activation enhanced V²⁺/V³⁺ kinetics while inhibiting VO²⁺/VO2⁺ kinetics. The enhancement in V²⁺/V³⁺ kinetics was attributed to the oxygen-containing groups -C-OH added during thermal activation. Using thermally activated carbon-felt V²⁺/V³⁺ electrodes yielded an overall increase in energy efficiency (EE) from 75 ± 3.7 to 90 ± 4.5% and voltage efficiency (VE) from 76 ± 4 to 92 ± 4.6%. On the other hand, using thermally activated carbon-felt VO²⁺/VO2⁺ electrodes lowered EE from 75 ± 4 to 73 ± 3.6% and VE from 76 ± 4 to 74 ± 4%. The optimal combination of thermally activated carbon-felt V²⁺/V³⁺ electrodes and untreated carbon-felt VO²⁺/VO2⁺ electrodes resulted in the most efficient RFB configuration.