A general electrochemical formalism for vanadium redox flow batteries

Recent advancements in Vanadium Redox Flow Batteries (VRFBs) assert that their performance degradation and lack of charge retention is generally ascribed to the crossover of Vanadium species of the electrolytes through the Ion Exchange Membrane (IEM). In this report, a comprehensive electrochemical formalism is proposed to shed light on the role played by the equilibria and redox processes of the active Vanadium complexes in electrolytes in the modulation of the electrical behavior of an operating VRFB. This target is achieved by adopting the strategy to consider: a) the Vanadium species with the same oxidation state as a single ensemble of species; b) the redox processes as bridging events between the different ensembles of Vanadium species; and c) the correlations existing between chemical processes and the electrochemical status of a VRFB both in terms of potential and of self-discharge features. The developed formalism reveals the role played by the complex Vanadium coordination chemistry on the electrochemical performance of a VRFB. In detail, under Open Circuit Voltage (OCV) studies, VOH²⁺/V²⁺ and H₂VO₄⁻/VO²⁺ redox couples contribute negatively to the capacity retention of the device. During the charging process, VOH²⁺/V²⁺ and H₂VO₄⁻/VO²⁺ equilibria concur potentially for a decrease of the overall current density by up to 22% and 4.7%, respectively. Taken all together, the analyzed equilibria and the redox processes here proposed are not negligible in modulating the possible electrochemical loss of performance of a VRFB. Thus, these outcomes offer a different scenario to the widely accepted theory which considers to the crossover of active species through the ion-conducting membranes as the only one phenomenon responsible for the performance drop of these devices. Therefore, results here reported: (a) open the door for a systematic understanding of unforeseen phenomena which are potentially responsible for modulating the electrochemical performance loss of a VRFB; and (b) are a quantitative example of how to rationalize the behavior of a RFB, which is based on a complex chemistry. Finally, results shed light on a new scenario of phenomena that is necessary to master, in order to manage the operation of a VRFB and to improve its long-term stability and charge retention ability, particularly under rest conditions.