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Abstract
Saturation concentrations of vanadium(V) and vanadium(IV) species were established conducting solubility studies of vanadium pentoxide and vanadyl sulfate in sulfuric acid concentrations ranging from 0 to 9M over a temperature range of 10°C-50°C. Total vanadium and total sulfur in the liquid samples at equilibrium were determined by ICP. The solubilities were found to be strongly I to the second dissociation constant of H 2 SO 4 and temperature. It was not possible to determine the solubility Of V2 O3 because of transformation Of V2 O 3 to VOSO4 in sulfuric acid solutions. However, total vanadium(III) concentrations as high as 2M were achieved with reagent grade V2 O 3 in 5M H2 SO4 by simple dissolution at 20°C. Supersaturated vanadium(V) electrolytes were prepared by electrolytic oxidation of V(IV) solution in different sulfuric acid concentrations to investigate the stability and properties of V(V) solutions over a temperature range of 20°C-50°C. The evaluation of thew V(V) solutions based on desupersaturation experiments, properties such as density, conductivity, viscosity, and cyclic voltammetry and 51 V NMR studies indicated that 3.0-3.5M V(V) solution in 5-6M total sulfate/bisulfate is a suitable composition for a high energy density vanadium redox. battery. Further increase in V(V) concentration above 3.5M, decreases the conductivity, electrochemical activity and reversibility, and increases the viscosity exponentially. It was found that the induction time for the precipitation of V 2 O 5 from V(V) solutions increases with increasing sulfuric acid concentrations because of increases in H+ ions, formation of sulfate/bisulfate complexes and dimerisation/polymerisation of VO2+ ions. The kinetic study of thermal precipitation of V(V) solutions indicated that the growth rate follows first order kinetics; under the conditions of low supersaturations and it obeys a second order rate equation at high supersaturations. Supersaturated V(V) solutions when exposed to atmosphere absorbs significant moisture which affect the properties and precipitation behaviour. A large number of additives and their blends were evaluated to inhibit the precipitation of supersaturated V(V) solutions at 40°C. A phosphate based formulation, KS11, was found to increase the induction time of 4M V(V) solution in 6M total sulfate/bisulfate by about 10 times. The formulation KS11 also exhibited encouraging results against the precipitation of supersaturated V(II) and V(IV) solutions. pitation of supersaturated V(II) and V(IV) solutions.