(Invited) Factors Influencing the Performance of Vanadium Flow Batteries: Electrodes and Electrolytes
Authors: Buckley D. Noel , Bourke Andrea , Oboroceanu Daniela , Quill Nathan , Lenihan Catherine , Al-Hajji Safi Maria , Miller Mallory A. , Savinell Robert F. , Wainright Jesse S. , Lynch Robert P. 
Published: December 2020

There is presently great interest in redox flow batteries1-8 (RFBs) for large-scale energy storage9 due to advantages over other electrical energy storage (EES) technologies, and research activities in this area have grown exponentially in recent years. The vanadium flow battery (VFB), also known as the vanadium redox flow battery (VRFB), is commonly regarded as one of the most promising flow batteries.2-3,10-12 Active areas of research on VFBs include electrolytes,13-16 electrodes,5-7 membranes,8 cell design and modelling,8 and performance and state-of-charge (SoC) monitoring.17-18 In this paper, we will review our work in two areas: kinetics on carbon electrodes and thermal stability of electrolytes.
The kinetics of the VII-VIII, VIII-VIV and VIV-VV redox couples have been studied for a range of different carbon materials using a variety of techniques, and it is clear that the kinetic rates depend strongly both on the type of carbon used and on the preparation of the electrode surface. We will discuss our investigations5-7 of the effects of anodic and cathodic pretreatments of carbon on electrode kinetics in the VFB. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), we showed for four different types of carbon that electrode treatments at negative potentials enhance the kinetics of VIV-VV and inhibit the kinetics of VII-VIII while electrode treatments at positive potentials inhibit the kinetics of VIV-VV and enhance the kinetics of VII-VIII. These observations may explain conflicting reports in the literature. We examined in detail the potentials required for activation and deactivation of electrodes. The results suggest that interchanging the positive and negative electrodes in a vanadium flow battery (VFB) would reduce the overpotential at the negative electrode and so improve the performance. This is supported by flow-cell experiments. Thus, periodic catholyte-anolyte interchange, or equivalent alternatives such as battery overdischarge, show promise of improving the voltage efficiency of VFBs.
We have developed13-16 a standard methodology for measuring the thermal stability of vanadium catholytes based on their induction time for precipitation of V2O3. Using this, we have investigated the thermal stability of typical vanadium flow battery (VFB) catholytes at temperatures in the range 30–70°C for VV concentrations of 1.4–2.2 mol dm-3 and sulfate concentrations of 3.6–5.4 mol dm-3. In all cases, V2O5 precipitates after an induction time, which decreases with increasing temperature. Plots of the logarithm of induction time versus the inverse of temperature (equivalent to Arrhenius plots) show excellent linearity and all have similar slopes, yielding a value of 1.791±0.020 eV for the activation energy. The logarithm of induction time also increases linearly with sulfate concentration and decreases linearly with VV concentration. The slopes of these plots give values of concentration coefficients β
S and β
V5 which quantify the effects of concentration on induction times. Combining these with the Arrhenius slope, we have constructed a model to predict the stability of sulfate-based vanadium catholytes. The model can also be used as a basis for accelerated testing of the thermal stability of electrolytes.
We have investigated a range of additives and shown that many suggested additives are ineffective. However, the Group V elements phosphorus and arsenic in the +5 oxidation state have a significant stabilizing effect. We have also quantified their effect in the temperature range 30-70°C.

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