Water transport study across commercial ion exchange membranes in the vanadium redox flow battery

The water transfer behaviour of Selemion CMV, AMV and DMV membranes (Asahi Glass, Japan) has been studied in the vanadium redox cell, as was the water transfer across Nafion 117 membrane (E.I. Du Pont, USA). The earlier water transport studies of a variety of commercial ion exchange membranes and non-ionic separators in the vanadium redox cell have shown that the net water transport through anion exchange membranes and non-ionic separators in the vanadium redox cell is from the positive half cell (+ve) to the negative half cell (-ve), while for cation exchange membranes the net water transport is in the opposite direction. In the present study, it was found that a significant amount of water is transferred across cation exchange membranes from the -ve vanadium half cell electrolyte to the +ve vanadium half cell electrolyte by the hydration shells of V2+ and V3+ ions which carry a large amount of water and can easily permeate through cation exchange membranes due to their relatively high charge numbers. The net amount of water of hydration which is transferred across anion exchange membranes from the -ve half cell electrolyte, however, is almost equal to the net amount of water of hydration which is transferred from the +ve half cell electrolyte. Thus, the net amount of water which is transferred across anion exchange membranes is in the same direction as the osmotic water transfer.