Comparing Electrode Overpotential Contributions in Vrfbs Amongst Multiple Techniques

Abstract

Redox flow batteries (RFBs) offer a potential remedy for the escalating need for energy storage amid the ongoing transition from carbon-intensive fossil energy to renewable energy sources. Yet, many RFB chemistries are shown to have low energy efficiencies which can be attributed, in part, to ineffective electrode materials. Identifying overpotential contributions from electrodes under operating conditions is a key step in understanding how to design better RFB electrodes Previously, three-electrode setups using rotating disk electrodes (RDE) were the effective technique at isolating overpotentials; however, this can only be accomplished by looking at one side of the RFB at a time. RDEs do not easily replicate the operational conditions of a power cell or the porous nature of carbon electrodes used in RFBs. Alongside the normal operation of an RFB, symmetry cells are another approach that can be utilized in quantifying the performance of electrodes. The advantage of a symmetrical cell setup allows us to monitor the overpotential contributions of two different electrodes in the same electrolyte concurrently. Membrane-based reference electrodes (MBRE), and other reference electrode integration concepts, allow us to use three-electrode techniques in operational RFB power cells. As a case study, we examined the effect of oxidative electrode treatments on the positive and negative side of the VRFB using multiple methods. While other studies have shown that an untreated negative electrode may be the key inhibitor in VRFB performance, we have quantified the overpotential at 50% state of charge to be 194 mV when 50 mA cm-2 is applied. After an oxidative treatment is applied to the negative electrode, the overpotential is reduced to 111 mV. Applying this to other techniques, we can compare the effectiveness of a non-symmetry cell setup vs. a symmetry cell setup, which displayed that oxidative treatment techniques appear to have an opposite effect on the positive electrode. A treated-positive electrode shows an overpotential of around 100 mV whereas an untreated-positive electrode only contributes 15 mV less of overpotential. These findings are further compared using common reference electrodes placed in the electrolyte tank, providing insights into the effectiveness of power cell techniques in improving RFB electrode performance.