SiO₂-decorated graphite felt electrode by silicic acid etching for iron-chromium redox flow battery

In this paper, the SiO₂-decorated graphite felts are obtained by silicic acid etching graphite felts in hot air, and their performance as the electrodes of iron-chromium redox flow battery (ICRFB) is investigated. The surface morphology, microstructure and surface chemical composition of SiO₂-decorated graphite felts are characterized in detail through scanning electron microscopy, specific surface area analysis, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. The electrocatalytic activity of SiO₂-decorated graphite felts and the performance in ICRFBs are studied by electrochemical impedance spectroscopy, cyclic voltammetry, and single-cell charge-discharge cycling tests. Results indicate that the SiO₂ introduced by the decomposition of silicic acid can increase the effective specific surface area of the graphite felts, and concurrently facilitate the oxidation of the SiO₂-decorated graphite felts in hot air. The number of oxygen functional groups and the defect sites on the surface of the SiO₂-decorated graphite felts increase to a great degree that especially have a considerable influence on the negative reaction activity of the ICRFB. An ICRFB assembled with graphite felts etched by 50 wt% silicic acid as the electrodes has a voltage efficiency and energy efficiency of 86.27% and 79.66% at a current density of 120 mA/cm², respectively. The decay rate of capacity is only 0.46% per cycle. The cost-effective and convenient method demonstrated in this paper can coordinate the control of oxygen functional groups, surface area and electrical conductivity of graphite felts, which may provide an example for improving the electrode materials for the commercial application of ICRFBs.