On the quantification of coulombic efficiency for vanadium redox flow batteries: Cutoff voltages vs. state-of-charge limits

The most common practice in determining the coulombic efficiency (CE) of vanadium redox flow batteries (VRFBs) is to charge and discharge the VRFB to designated cutoff voltages and then use the corresponding charging and discharging times to compute the CE. A major shortcoming of this technique is that it does not account for the change in the system's capacity during cycling due the crossover of vanadium ions. Rather than using cutoff voltages, an alternative approach is to use state-of-charge (SOC) limits since any change in the system's capacity can be captured by tracking the SOC of the device. In this study, these two techniques were investigated to assess their effectiveness in quantifying the CE in a manner that accurately captures the physical processes governing the device operation. A performance model was utilized to simulate the VRFB operation under different currents and the CE of the device was computed using both techniques. Results indicate that CEs computed using SOC limits were consistently lower than those computed using cutoff voltages, and they were consistent with the change in the chemical state of the electrolytes during cyclic operation. (C) 2013 Elsevier B.V. All rights reserved.