Abstract
Vanadium redox flow batteries (VRFBs) are a promising solution to the demand for large-scale energy storage [1, 2]. A critical component of VRFBs are the electrodes, commonly manufactured from highly porous, carbon materials [3, 4]. The reactivity of electrode materials is of great interest, with previous studies on carbon felt materials revealing a significant degree of performance variability for both pristine and treated electrode samples [5–8]. Thus, the heterogeneous nature of carbon felt needs to be further investigated to better understand performance variations in VRFB systems.
Recently, a rate constant distribution has been observed for single fibre electrodes extracted from carbon felt for the ferri/ferrocyanide redox reaction. Importantly, it was found that the distribution of rate constants from single fibre measurements was statistically different depending on which the side of the felt these fibres were extracted from. Although the spatial variation in reaction kinetics was established for the ferri/ferrocyanide redox system, it is thought that this variation may be present with other electroactive species, such as vanadium. It is well established that the properties and performance of carbon felt is enhanced during post-production thermal treatment [9–11]. Therefore, it is not implausible that uneven thermal exposure during manufacturing may also lead to variations in electrode activity.
In this study, the influence of electrode configuration on overpotential was investigated for commercially available PAN-based carbon felt using the VO2+/VO2+ redox reaction. Charge and discharge tests were carried out in a flowing half-cell, where the measured half-cell potential was IR-corrected post-run and then compared to the open circuit potential to extract the electrode overpotential. XPS, Raman spectroscopy, and BET surface area analysis are used to characterise the electrode samples so that differences in surface chemistry and structure could be determined. The electrode overpotential was found to be significantly different depending which orientation of felt in the cell, with the overpotential being much lower when the more active side of the electrode faced the membrane. Thermal treatment of the carbon felt in air at 400 °C resulted in this redox activity difference to be minimised. This suggests that one side of the carbon felt may have already possessed the same active surface groups as those which are formed during this thermal treatment, with this thermal treatment acting to equalise the inherent activity of both faces.
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