Spectroscopic Determination of VV Concentration of Vanadium Flow Battery Catholytes
Authors: Quill Nathan , Oboroceanu Daniela , Albu Sergiu P. , Lenihan Catherine , Buckley D. Noel , Lynch Robert P. 
Published: February 2020

There is a significant need for a technique to determine the concentration of VV in vanadium flow battery (VFB) catholytes (concentrated mixtures of VIV (vanadyl; typically in the form of VO++ ions) and VV (pervanadyl; typically in the form of VO2
+ ions) in H2SO4). A large number of studies on VFB catholytes are concerned with either the energy density of the catholyte,1, 2 precipitation of VV from the catholyte3-5 or concentration changes within the catholyte6,7,10 (e.g. state-of-charge (SoC) monitoring and capacity loss) during the operation of a flow cell or battery. For any such studies to produce consistent, repeatable results, a straightforward and accurate technique for the determination of VV concentration is required.

Such a technique would also be useful for the ex-situ determination of the SoC of a VFB catholyte. The SoC of the catholyte is given by the ratio of the concentration of VV, [VV], to the total vanadium concentration C (where C = [VIV] + [VV]). As a VFB is cycled, the transfer of water and vanadium ions across the membrane separating the two half cells leads to significant (>5%) concentration changes in the electrolyte in each half-cell.11 Furthermore, differing coulombic efficiencies in each half-cell can combine with the vanadium concentration imbalance to lead to significant SoC imbalance in operational cells that have been cycled multiple times. Since it is important to track these changes to facilitate timely battery maintenance, there is a clear need for an accurate technique to determine [VV] both in pure solutions and in mixtures with VIV.

While there are a number of possible approaches to determine [VV] (e.g. reductive potentiometric titration, coulometric titration, direct spectrophotometric titration), they all require considerable care to perform accurately. A direct spectrophotometric titration presents a considerable challenge because the spectrum of VO2
+ is relatively featureless at visible wavelengths. Furthermore, the absorbance of mixtures of VIV and VV is complicated by the formation of a highly absorbing complex from the two ions.6-9

We have previously characterized the UV-vis absorption characteristics of concentrated VIV/VV mixed solutions6,7 and have shown that it is possible to determine the SoC of VFB catholytes by analysis of their spectra. In this talk, we will show how this understanding can be used to develop a novel reductive spectroscopic titration for VV that is both accurate and straightforward to perform. Variations of this technique can be used to measure either C or [VV] in a VFB catholyte at any SoC.

A model of the ideal titration will be presented (see Fig. 1), followed by a discussion of the practical difficulties involved in performing this titration with high precision. From this, two practical titration procedures will be proposed with suggestions for the appropriate level of dilution and choice of a suitable reducing agent. Results from actual titrations will then be presented at a range of VV concentrations. The precision of the technique will be demonstrated by comparing the results with alternative estimates of [VV].

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Figure 1