Quantifying the Impact of Oxidative Treatments on Electrode Overpotentials in the All-Vanadium Redox Flow Battery

Despite the commercialization of flow batteries, little is known about how much electrode treatment methods affect individual electrode overpotential contributions. Thermal oxidation is one of the most common electrode treatment methods in literature, which has increased the energy efficiencies of vanadium redox flow batteries (VRFBs) by 10 to 20 % depending on their operating current density. However, it is unclear how much electrode overpotential remains after these treatments, which is critical to identifying viable pathways for further improvement. Herein, we demonstrate how membrane-based reference electrodes provide an opportunity to examine individual electrode overpotentials during operation to gain deeper insights into their role in battery performance. Without oxidative treatments, negative electrode overpotential contributions range from 150 to 250 mV depending on the operating current density, overshadowing positive electrode contributions. Use of oxidative treatment reduced negative electrode contributions by nearly 50 % percent from their initial values but marginally increased positive electrode overpotential values. Treating the negative electrode while leaving the positive electrode untreated resulted in the best performance observed but still had 150 to 300 mV of electrode overpotentials remaining, suggesting that additional electrode improvements can still provide significant gains in energy efficiency.