Batteries Redox flow batteries (RFB) represent recently the most advanced electro-chemical energy storage system that is commercialized on level tens of kilowatts to megawatts. A flow battery is charged and discharged by a reversible reduction-oxidation reaction between the two liquid electrolytes of the battery. The RFB is scalable system which can be operated in wide range of conditions. It can be integrated easily into smart-grids and it is advantageous in connection to wind farms. The batteries are further characterized by high longevity in the range of several years, easy maintenance and overall energy efficiency up to 90 %.

There are various RFB systems on the market. Among them, the superior position holds vanadium redox flow battery (VRB) that uses vanadium electrolyte on both sides. As electrolyte, sulfuric acid is used. Such setup gives advantage of avoiding contamination since the electrolytes are of the same nature. In the charged state, the positive electrode contains V5+ ions, while the negative side hosts V2+ . During discharging, the V5+ is converted into V4+ and V2+ into V3+ plus electric power is obtained. The migration of ions through separator does not cause any damage of system. There are also other RFB systems possible to realize within the redox potential window of hydrogen and oxygen evolution. All systems strike for higher energy density or lower cost of the electrolyte, such as zinc-bromine, vanadium-bromine, polysulfide bromine, zinc-cerium, iron-chromium, iron-aluminium or the class of metal air systems such as rechargeable zinc-air or vanadiumair batteries.

The fumasep® membrane is the new heart of a Redox Flow Battery. FUMATECH BWT GmbH produces both porous separators and nonporous functional anion- and cation-exchange membranes for various types of batteries. In VRFB one can find its right choice, whether aiming of high efficiency, high operation current density when high speed of charging is required or some compromise between the two modes.

Our porous membranes are characterized by lower cost and low specific area resistance but low selectivity and a potential risk of electrolyte transfer, however. Our non-porous ion-exchange membranes are advantageous as for their high coulomb efficiency and avoidance of any liquid transfer between both electrolytes. Cation-exchange membranes are primarily an impermeable, proton-conductive barrier to the electrolyte. As all of the RFB systems are based on cations of different oxidation state, a selective proton-conductive anion-exchange of high rejection towards other cations is the most optimum choice for maximum efficiency.