Degradation of all vanadium redox flow batteries (VRFB) investigated by electrochemical impedance and X ray photoelectron spectroscopy: Part 2 electrochemical degradation
Abstract Electrochemical degradation (ED) of carbon felt electrodes was investigated by cycling of a flow through all vanadium redox flow battery (VRFB) and conducting half cell measurements with two reference electrodes inside the test bench. ED was detected using half cell and full cell electrochemical impedance spectroscopy (EIS) at different states of charge (SOC) Reversing the polarity of the battery to recover cell performance was performed with little success. Renewing the electrolyte after a certain amount of cycles restored the capacity of the battery. X ray photoelectron spectroscopy (XPS) reveals that the amount of surface functional increases by more than a factor of 3 for the negative side as well as for the positive side. Scanning electron microscope (SEM) images show a peeling of the fiber surface after cycling the felts, which leads to a loss of electrochemically active surface area (ECSA) Long term cycling shows that ED has a stronger impact on the negative half cell [V(II)/V(III) than the positive half cell [V(IV)/V(V) and that the negative half cell is the rate determining half cell for the VRFB.