Studies on properties of rayon- and polyacrylonitrile-based graphite felt electrodes affecting Fe/Cr redox flow battery performance

The performances of rayon (RGF) and polyacrylonitrile (PGF) based graphite felts as electrodes are compared in the iron-chromium redox flow battery (ICRFB). The essential differences in structure between RGF and PGF are also characterized in this study. The results show that the RGF electrode displays excellent electrochemical performance for negative redox couple (Cr3+/Cr2+), resulting in the cell with higher charge efficiency and slower capacity decay. However, due to the high degree of graphitization and electrochemical activity for positive redox couple (Fe2+/Fe3+) in PGF, the cell using PGF has higher voltage efficiency and energy efficiency, as well as lower area surface resistance than the cell with RGF. The fundamental difference between RGF and PGF is the crystallites structure in radial direction and basal plane of the fiber. PGF with the core-rim structure has better graphitization stacked in radial direction of the fiber. Further, PGF has more defective carbons exposed in the basal planes, while the surface of RGF exhibits a higher concentration of oxygen functional groups. The role of catalysts in the ICRFB has also been studied by introducing bismuth (Bi) into RGF and PGF used as negative electrodes. The electrocatalytic activities of RGF and PGF before and after modification with Bi are investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Bismuth deposition on the negative electrode is found to improve the negative reaction in ICRFB.