A Study of V(IV/V) and V(II/III) Reactions on Carbon Based Electrodes
Authors: Song Chaojie , Neagu Roberto , Fatih Khalid 
Published: September 2023

The vanadium redox flow battery (VRFB) is one of the most promising energy storage technologies for large scale commercialization. Carbon based electrodes, such as carbon felts or carbon paper, are used for V(II/III) and V(IV/V) reactions. To improve the vanadium redox reaction kinetics, the carbon electrodes are usually activated by a variety of methods such as thermal treatment, heteroatom doping, catalyst incorporation etc. [1] It is generally considered that surface functional group such as C–OH, C=O play an important role in vanadium redox reaction [1-6]. Even though there are reports showing higher surface functional group content e.g. total C–O content leads to improved vanadium redox reaction kinetics [2-6], qualitative correlation of the surface property of the carbon materials and the vanadium redox reaction is not reported. There is a need for fundamental understanding of the surface properties of the carbon electrodes and their relation with the reaction kinetics, including the key parameters that determine them.
In this work, using cyclic voltammetry (CV), we studied the surface properties of a variety of carbon based electrodes (glassy carbon (GC), edge plane pyrolytic graphite, base plane pyrolitic graphite, graphite (from Pine Research, called graphite-Pine thereafter), graphite rod, and H2SO4 soaked graphite rod), and the V(II/III) and V(IV/V) reaction kinetics on these electrodes. The goal of soaking the graphite rod electrode in H2SO4 is to activate the electrode and make it comparable to the carbon felt electrode [1]. XPS was also used to analyze the functional group. The relationship between the surface properties and the reaction kinetic parameters was analyzed.
Fig. 1 compares the cyclic voltammograms (CV) of the edge plane, basal plane graphite and GC electrodes in 2 M H2SO4. Two redox couples are observed, ascribed to C=O and COOH group redox peak respectively [7]. On basal plane electrode, these two redox peaks are less significant, and an extra reduction peak is observed at 0.24 V due to the C–OH reduction. On the GC electrode, the C=O redox peak is still discernable but the COOH redox peak is almost undetectable. A capacitance region appears between 0.6 – 0.75 V vs Ag/AgCl for all these electrodes. Similar features are observed with the graphite-Pine, graphite rod and H2SO4 soaked graphite rod electrodes with significantly enhanced redox peaks and larger capacitance. The H2SO4 soaked graphite rod electrode presents the highest surface functional group density and capacitance. XPS also shows that the H2SO4 soaked graphite rod electrode has the highest total C–O content.
CV of V(IV/V) and V(II/III) were measured using these electrodes at different scan rates. Fig. 2 shows the CV of V(IV/V) reaction on these electrodes. The three graphite rod electrodes show better reversible feature than the edge plane, basal plane and GC electrodes. Diffusion coefficient, transfer coefficient and reaction rate constant were obtained from the CVs. The diffusion coefficient follows the sequence of soaked graphite rod > graphite rod > Graphite-Pine > Edge plane > GC > basal plane electrode. The transfer coefficient and reaction rate constant are also different for these electrodes. Similarly, the diffusion coefficient, transfer coefficient and reaction rate constant are obtained for the V(II/III) reaction. The diffusion coefficient and reaction rate constant for the V(II/III) reaction are smaller than that for the V(IV/V) reaction on the corresponding electrodes. For both V(IV/V) and V(II/III), a linear relationship was found for diffusion coefficient versus the logarithm of capacitance and C=O group functional group. Higher value of log(capacitance) and log(C=O density) leads to larger diffusion coefficient. However, there is no clear relationship between transfer coefficient, reaction rate constant vs capacitance or C=O density. It seems that the capacitance and C=O functional group density are the importance parameters determining the vanadium redox reaction kinetics.
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Figure 1