Enhancement of the electrochemical activity of a commercial graphite felt for vanadium redox flow battery (VRFB), by chemical treatment with acidic solution of K2Cr2O7

Abstract

Vanadium redox flow battery Chemlcal activation I!lectroactivlty enhanoement Carbon-oxygen catalytlc sites Graphite felt wettabillty Charge-discharge electrolysls The nove! thermal-<hemical activation method is proposed to prepare the graphite felt with highly functiona lized surface and enhanced active surface area, to be used as the positive half-cell electrode for the vanadium redox flow batteries (VRFBs). The presaibed treatment consists ofboiling the felt within K 2 Cr 2 0,. solution in 5 M H:z$() 4 , at different temperatures and time durations. It is time efficient, effective and durable. The cyclic vol tammetry 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 characteri2ed by Fil R, that actas catalytic sites toward the positive half-cell redox couple reaction 01◊2 + .oV0 2 + ). Moreover, the improvement of various other properties of the graphite felt, resulted due to activation are a1so quantified, such as the surface roughness of fi bers, the wettability toward electrolyte, the adsorption affinity against the vanadium and the surface energy of the graphite felt. The improved reversibility of vQ2 + /V◊2 + redox couple reaction against graphite felt is a1so 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-&scharge cycles, shows the improvement of the voltage efficiency and faradaic yield, as well as, the consistency of the performance over three cycles ( ~ 16 h).

The Role of Oxygenic Groups and sp3 Carbon Hybridization in Activated Graphite Electrodes for Vanadium Redox Flow Batteries

“…As the solvent interactions are also important at the electrode‐electrolyte interface, the study of the water interactions with all the representative surfaces is done separately, although we expect trends in the interaction between the VO 2+ /VO 2 + redox couple and the electrode surfaces to be independent of the solvent. The adsorption energies can provide insights into the reactant species’ interactions with the electrode surface and can be related to the kinetics of the reaction [34–36] . So, in this study, adsorption energies of V IV , V V, and H 2 O are determined on the surfaces of graphite felt (GF) using Equation : …”

“…The binding of vanadium redox couples to the oxygenic groups follows exactly the same order. The conclusion could be drawn that the sp 2 /sp 3 hybridization is directly linked with the affinity of the positive half‐cell redox couple (VO 2+ /VO 2 + ) with the graphite electrode, which results in higher currents and lower overpotential [36] . The fabrication of the oxygenic groups (OH, C=O, COOH) is responsible for the sp 3 hybridization of the carbon atoms in graphite felts.…”

Effect of the presence of solid particles, on the vanadyl sulfate (VOSO4) oxidation current

“…Another parameter which can influence the answer of the electrode could be the activation of the surface area of the graphite [18] . As a matter of fact, oxygenated groups (C-O) at the surface of the graphite seem to enhance the electrocatalytic activity of the graphite.…”

The Role of Oxygenic Groups and Sp3 Carbon Hybridization in Activated Graphite Electrodes for Vanadium Redox Flow Batteries

“…As the solvent interactions are also important at the electrodeelectrolyte interface, the study of the water interactions with all the representative surfaces is done separately, although we expect trends in the interaction between the VO 2+ /VO2 + redox couple and the electrode surfaces to be independent of the solvent. The adsorption energies can provide insights into the reactant species' interactions with the electrode surface and can be related to the kinetics of the reaction [34][35][36]. So, in this study, adsorption energies of V (IV), V (V), and H2O are determined on the surfaces of graphite felt (GF) using the following equation.…”

“…The conclusion could be drawn that the sp 2 /sp 3 hybridization is directly linked with the affinity of the positive half-cell redox couple (VO 2+ /VO2 + ) with the graphite electrode, which results in higher currents and lower overpotential. [36] The fabrication of the oxygenic groups (OH, C=O, COOH) is responsible for the sp 3 hybridization of the carbon atoms in graphite felts.…”