OVERCHARGE IN THE VANADIUM REDOX BATTERY AND CHANGES IN ELECTRICAL-RESISTIVITY AND SURFACE FUNCTIONALITY OF GRAPHITE-FELT ELECTRODES

The influence of overcharge on the performance of the vanadium redox cell that employs graphite-felt electrodes is examined. The electrical, electrochemical and surface properties of the graphite-felt electrodes are investigated in order to determine the contribution and effect of each deterioration mechanism to the loss in cell performance during overcharge. The electrical resistivity of the electrodes and the cell resistance show a slight increase after overcharge. The effect of overcharge on the surface chemistry of three types of graphite-felt electrodes is investigated using X-ray photoelectron spectroscopy (XPS). The XPS analysis reveal a slight increase in the overall surface oxygen content of the graphite felts after overcharge, together with a shift to higher oxidation states that is consistent with the formation of C-OH, -C=O, -COOH and -COOR groups. The proportion of each of these groups varies between graphite-felt types. The measured change in the relative concentrations of surface oxygen functional groups on the graphite felt is believed to be responsible for the observed increase in the electrical resistivity and electrochemical activity of the electrode materials during cell overcharge.