Effect of Vanadium (IV) Sulfate Particles Present in the Posolyte on the Anodic Current of a Vanadium Redox Flow Battery

Abstract

Vanadium redox flow batteries (VRFB) are one of the most promising technologies for large scale energy conversion and storage (Cunha et al. 2014). Since it uses the same element in its four oxidation states (V(II)/V(III) and V(IV)/V(V) couples), its major advantage is the minimization of the cross-contamination problem observed in other RFBs. In addition, the fact that the electrolyte has an infinite regenerative capacity (Wang et al. 2015), makes this type of battery extremely rewarding on the economical point of view.However, their commercialization is hindered, among other reasons, by the limited quantity of stored energy (~ 40 kWh/m3), due to the low solubility of the vanadium salts (< 2 M), strongly influenced by the sulfuric acid concentration, generally used as supporting electrolyte: higher H2SO4 concentrations stabilize the V(V) species but decrease the solubility of the other salts (V(II), V(III) and V(IV)). Operating temperature also enhances the solubility of vanadium salts, inducing thereby an opposite behavior for the V(V) (solubility decreases with increasing temperatures) compared to the other valences. Therefore, the precipitation of vanadium species is considered to be an important problem for which a number of solutions has been proposed such as the use of a mixed H2SO4/HCl supporting electrolyte (Vijayakumar et al. 2013) and precipitation inhibitors (Skyllas-Kazacos et al. 2016). However, the understanding of the impact of the vanadium particles suspended in the electrolyte on the current was not yet addressed.Hence, the present work deals with the study of the impact of the presence of vanadium (IV) particles (VOSO4.5H2O powder) on the limiting current of the oxidation of V(IV) to V(V), using a classical three electrodes cell in which the working electrode is a rotating cylinder made of graphite. In addition, in order to insure an homogeneous distribution of the particles in the cell (avoid sedimentation), an additional stirring system, consisting of a cross-shaped stirrer is introduced at the bottom of the cell. Various kinds of measurements were achieved using a saturated V(IV) solution at 10 °C, in 3 M sulfuric acid: first, the effect of solid particles of VOSO4.5H2O powder was examined and then, comparatively the same experiments were performed using inert particles (glass spheres) having similar physical properties as the vanadium particles. In addition, the coupled effects of both the stirring provided from the rotating electrode and the stirring assured by the ‘cross-shaped stirrer’, on the magnitude of the current was also studied.The results show that the anodic limiting current of the V(IV) decreases, as the mass concentration of the solid particles increases. Higher apparent viscosity of the resulting mixture directly impacts the diffusion coefficient of the dissolved VO2+ species and limits the mass transfer. Moreover, the results do not show any positive effect of the presence of the solid particles (V(IV) or glass spheres) on the thickness of the diffusion layer (enhancement of the transfer of the dissolved VO2+ by some collisions between the solid particles and the electrode surface). Besides the study of the coupled effects of both stirring rates (electrode + additional stirrer) leads to some curious results on the evolution of the magnitude of the current, e.g. a ‘usual’ power law dependence until a complete independence, probably caused by a compensation of both motions in certain regimes. All these results were discussed.