0 Understanding the Vanadium Redox Flow Batteries Christian Blanc and Alfred Rufer Laboratoire d’Electronique Industrielle, Ecole Polytechnique Federale de Lausanne Switzerland 1. Introduction Vanadium redox flow batteries (VRB) are large stationary electricity storage systems with many potential applications in a deregulated and decentralized network. Flow batteries (FB) store chemical energy and generate electricity by a redox reaction between vanadium ions dissolved in the electrolytes. FB are essentially comprised of two key elements (Fig. 1): the cell stacks, where chemical energy is converted to electricity in a reversible process, and the tanks of electrolytes where energy is stored. Electrode Electrode Tank Reservoir Anolyte Tank Reservoir Catholyte Pump Pump Cation Exchange Membrane H + + - (a) membrane carbon felt bipolar plate end plate end plate (b) Fig. 1. (a) The schematics of the vanadium redox flow battery. (b) View of the different components composing a VRB stack. The surfaces in contact with the catholyte are coloured in blue and in orange for the anolyte. The most significant feature of the FB is maybe the modularity of their power (kW) and energy (kWh) ratings which are independent of each other. In fact, the power is defined by the size and number of cells whereas the energetic capacity is set by the amount of electrolyte stored in the reservoirs. Hence, FB can be optimized for either energy and/or power delivery. Over the past 30 years, several redox couples have been investigated (Bartolozzi, 1989): zinc bromine, polysulfide bromide, cerium zinc, all vanadium, etc. Among them, VRB has the best chance to be widely adopted, thanks to its very competitive cost, its simplicity and because it contains no toxic materials. 18