Abstract: A 3 M V(IV) bromide solution in 3-4 M HBr or HBr/HCl mixture is added to both sides of the redox flow cell or battery. On fully charging the cell, the vanadium (IV) bromide solution is reduced to produce 3M VBr2 in the negative half-cell, while the bromide ions in the positive half-cell are oxidised to produce 1.5 M Br3— or ClBr2. On discharge, the VBr2 is oxidised to VBr3 in the negative half cell while the Br3 or ClBr2— ions are reduced to Br ions in the positive half cell. The cell comprises carbon or graphite felt bonded onto plastic or conducting plastic sheets as substrate materials and the two half cells are separated by an anion or cation exchange membrane.