Analysis of the impact of remote oxygen plasma treatment on the surface chemistry and electrochemical properties of graphite felt electrodes for redox flow batteries†

The effects of a remote oxygen plasma (ROP) treatment on the surface of commercial graphite felts were investigated and compared against a conventional thermal treatment. In contrast to methodologies where the sample is directly exposed to the plasma, ROP allows for a high control of sample–plasma interaction, thereby avoiding extensive etching processes on the fibre surface. To assess the impact of ROP treatment time, the electrodes were subjected to three different periods (10, 60, and 600 seconds). X-ray photoelectron spectroscopy showed that the ROP treatment introduced nearly three times more surface oxygen functionalities than the thermal treatment. Raman spectroscopy measurements revealed a significant increase in amorphous carbon domains for the ROP samples. The thermal treatment favoured increases in graphitic defects and resulted in an order of magnitude larger ECSA compared to the ROP treated materials despite having lower content in oxygen functionalities. The electrochemical analysis showed enhanced charge-transfer overpotentials for GF400. The ROP samples exhibited a lower mass-transport overpotential than the thermally treated material and had similar permeabilities, which overall translated to the thermal treatment offering better performance at fast flow rates. However, at slow flow rates (∼10 mL min⁻¹), the ROP treatment for the shortest period offered comparable performance to conventional thermal treatment.