Open circuit voltage of an all-vanadium redox flow battery as a function of the state of charge obtained from UV-Vis spectroscopy†

A unique feature of redox flow batteries (RFBs) is that their open circuit voltage (OCV) depends strongly on the state of charge (SOC). In the present work, this relation is investigated experimentally for the all-vanadium RFB (AVRFB), which uses vanadium ions of different oxidation states as redox pairs in both half-cells. In contrast to several literature studies, which use OCV measurements to deduce the SOC via the Nernst equation, we propose a method based on UV-Vis spectroscopy for SOC estimation, thereby enabling completely independent SOC and OCV measurements. Moreover, rather than relying on data at a single wavelength this UV-Vis method uses the entire absorption spectrum to obtain more robust values for the SOC. The obtained SOC-OCV data agree reasonably well with literature values and reveal a significant influence of the thermodynamic non-ideality of the solutions on the OCV as described by the Nernst equation.