Why electrode orientation and carbon felt heterogeneity can influence the performance of flow batteries

The orientation of carbon felt electrodes in redox flow batteries is found to influence battery performance due to the differences in electrode activity between each face of the carbon felt. Polarisation curves were measured between 1 and 150 mA cm−2 using the positive vanadium reaction in a flowing half-cell configuration. Galvanostatic discharge measurements revealed that one face of the carbon felt sample resulted in significantly lower overpotentials indicating that this face had a greater redox activity. Thermal treatment of the carbon felt in air at 400 °C decreased the activity differences between each face of the felt, suggesting that one side of the carbon felt may have been previously exposed to heat during manufacturing. XPS analysis revealed a correlation between both the sp2/sp3 ratio and pyridinic nitrogen groups and the electrode activity, suggesting that these are descriptors for electrode activity. These findings highlight that commercial carbon felts can exhibit surprisingly large face-to-face variability which should be considered when assembling a flow battery system. Furthermore, it is shown that refining the thermal treatment of the felt during manufacturing can not only introduce relevant active sites, but also improve the overall uniformity of the electrode.