Stability of Catholytes in Vanadium Redox Flow Batteries and the Kinetics of Precipitation of V V

There is considerable interest in vanadium flow batteries (VFBs) for storage of electrical energy, particularly in conjunction with renewable energy sources such as wind and solar 1 . Like other flow battery systems the VFB is an electrochemical device that converts electrical energy to chemical energy which is stored in the electrolyte 2 . Typical cells have carbon felt electrodes separated by a proton exchange membrane. The catholyte and the anolyte are circulated through the electrodes from reservoirs. Both electrolytes are highly acidic, typically 3 mol dm -3 H 2 SO 4 . The active species are V 2+ /V 3+ ( i.e. V II /V III ) in the anolyte and VO 2+ /VO 2 + ( i.e. V IV /V V ) in the catholyte. The energy density of VFBs is limited by the solubility of the vanadium species in the electrolyte. On the negative side, the solubility of V 3+ and V 2+ generally increases with temperature at any given acid concentration and decreases with increasing concentration of the common anion (bisulphate). On the positive side, the solubility of the V V species, VO 2+ , increases with temperature and decreases with increasing concentration of bisulphate due to the common ion effect. The V V species in the catholyte, VO 2 + , can precipitate according to the reaction 3 2 VO 2 + + H 2 SO 4 → V 2 O 5 + 2H + . (1) This reaction is usually found to be very slow and, in practice, supersaturated solutions in sulphuric acid where the concentration of VO 2 + exceeds the thermodynamic limit set by Equation (1) can persist for very long periods of time 3 . The stability of such metastable solutions decreases, as expected, as the concentration of VO 2 + is increased. However, the stability (time to precipitation) of solutions increases with increasing concentration of bisulphate at a given concentration of VO 2 + . Although there have been several studies 3,4 of the stability of V V in the catholyte of VFBs, there is an absence of detailed kinetic studies of the precipitation process. In this paper we report a quantitative study of the kinetics of precipitation as a function of composition and temperature. In agreement with earlier studies, when a solution of V V in H 2 SO 4 was held above room temperature an induction period was observed (which depended on the solution composition and the temperature) after which precipitation occurred. Light scattering measurements were carried out to investigate the precipitation process in detail. In a typical measurement, a sample of solution w