Thermal stability of concentrated V(V) electrolytes in the vanadium redox cell

The vanadium redox battery currently employs solutions of up to 2 MV(II)/V(III) and 2 MV(IV)/V(V) as the negative and positive half-cell electrolytes. This concentration is limited by the solubility of the different vanadium ions in the temperature range of 10 to 40 degrees C. Generally, the solubility of V(II), V(III), and V(IV) increases with an increase in temperature; however, the V(V) electrolyte suffers from the effect of thermal precipitation at temperatures of 40 degrees C and above. While thermal precipitation is a serious problem in solutions of V(V) concentrations between 1.5 and 2.0 M a surprising result was observed at concentrations above 3.0 M. As the results presented here show, at higher vanadium concentrations the V(V) solution demonstrated increased stability and there was no evidence of thermal precipitation over a 30 day period at temperatures above 40 degrees C.