Enhancement of the electrochemical activity of a commercial graphite felt for vanadium redox flow battery (VRFB), by chemical treatment with acidic solution of K2Cr2O7
Résumé
The novel thermal–chemical activation method is proposed to prepare the graphite felt with highly functionalized surface and enhanced active surface area, to be used as the positive half-cell electrode for the vanadium redox flow batteries (VRFBs). The prescribed treatment consists of boiling the felt within K2Cr2O7 solution in 5 M H2SO4, at different temperatures and time durations. It is time efficient, effective and durable. The cyclic voltammetry analysis enable to optimize the operating conditions i.e. boiling at 140 °C for 2 h. The activation treatment creates different surface oxygen functional groups, as characterized by FTIR, that act as catalytic sites toward the positive half-cell redox couple reaction (VO2+⇌VO2+). Moreover, the improvement of various other properties of the graphite felt, resulted due to activation are also quantified, such as the surface roughness of fibers, the wettability toward electrolyte, the adsorption affinity against the vanadium and the surface energy of the graphite felt. The improved reversibility of VO2+/VO2+ redox couple reaction against graphite felt is also evaluated and confirmed by the increase of intrinsic heterogeneous electronic transfer constant by 2.2 and 5.5 folds for the oxidation and reduction reactions, respectively. The performance evaluation of electrode at stack level by charge–discharge cycles, shows the improvement of the voltage efficiency and faradaic yield, as well as, the consistency of the performance over three cycles (∼16 h).
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