Batteries are devices that store chemical energy and generate electricity by a
reduction-oxidation (redox) reaction: i.e. a transformation of matter by electrons
transfer. VRB differ from conventional batteries in two ways: 1) the reaction occurs
between two electrolytes, rather than between an electrolyte and an electrode, therefore
no electro-deposition or loss in electro active substances takes place when the battery is
repeatedly cycled. 2) The electrolytes are stored in external tanks and circulated through the
stack (see Fig. 1). The electrochemical reactions occur at the VRB core: the cells. These cells
are always composed of a bipolar or end plate - carbon felt - membrane - carbon felt - bipolar or end
plates; they are then piled up to form a stack as illustrated in Fig. 1.
In the VRB, two simultaneous reactions occur on both sides of the membrane as illustrated in
Fig. 2. During the discharge, electrons are removed from the anolyte and transferred through
the external circuit to the catholyte. The flow of electrons is reversed during the charge, the
reduction is now taking place in the anolyte and the oxidation in the catholyte.