Water Affinity of Vanadium Electrolytes

Rates of water transfer across a Nafion membrane between vanadium electrolytes (VV, VIV, VIII and VII) and H2SO4 solutions of various concentrations were measured and the H2SO4 concentration corresponding to a zero-transfer rate was thus estimated. The difference in sulfate concentration between an electrolyte and the equivalent H2SO4 solution varies considerably with the oxidation state of the vanadium. The value is close to zero for VIV but has a large positive value for VV and large negative values for VIII and VII. Both the water affinity, defined in terms of the chemical potential of water, and the thermodynamic activity of water was estimated for vanadium electrolytes at 25°C from the measured values of equivalent H2SO4 concentration. The water affinity increases in the order VII ≈ VIII < VIV < VV and the activity of water in the order VV < VIV < VIII ≈ VII < H2O.