Flow battery

A flow battery, or redox flow battery (after reduction–oxidation), is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange (accompanied by flow of electric current) occurs through the membrane while both liquids circulate in their own respective space. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.2 volts. A flow battery may be used like a fuel cell (where the spent fuel is extracted and new fuel is added to the system) or like a rechargeable battery (where an electric power source drives regeneration of the fuel). While it has technical advantages over conventional rechargeables, such as potentially separable liquid tanks and near unlimited longevity, current implementations are comparatively less powerful and require more sophisticated electronics. The energy capacity is a function of the electrolyte volume (amount of liquid electrolyte), and the power is a function of the surface area of the electrodes. A flow battery is a rechargeable fuel cell in which an electrolyte containing one or more dissolved electroactive elements flows through an electrochemical cell that reversibly converts chemical energy directly to electricity (electroactive elements are 'elements in solution that can take part in an electrode reaction or that can be adsorbed on the electrode'). Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also known. Flow batteries can be rapidly 'recharged' by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines) while simultaneously recovering the spent material for re-energization. Many flow batteries use carbon felt electrodes due to its low cost and adequate electrical conductivity, although these electrodes somewhat limit the charge / discharge power due to their low inherent activity towards many redox couples. In other words, a flow battery is just like an electrochemical cell, with the exception that the ionic solution (electrolyte) is not stored in the cell around the electrodes. Rather, the ionic solution is stored outside of the cell, and can be fed into the cell in order to generate electricity. The total amount of electricity that can be generated depends on the size of the storage tanks. Flow batteries are governed by the design principles established by electrochemical engineering. Various types of flow cells (batteries) have been developed, including redox, hybrid and membraneless. The fundamental difference between conventional batteries and flow cells is that energy is stored not as the electrode material in conventional batteries but as the electrolyte in flow cells. The redox (reduction–oxidation) cell is a reversible cell in which electrochemical components are dissolved in the electrolyte. Redox flow batteries are rechargeable (secondary cells). Because they employ heterogeneous electron transfer rather than solid-state diffusion or intercalation they are more appropriately called fuel cells rather than batteries. In industrial practice, fuel cells are usually, and unnecessarily, considered to be primary cells, such as the H2/O2 system. The unitized regenerative fuel cell on NASA's Helios Prototype is another reversible fuel cell. The European Patent Organisation classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18). Examples of redox flow batteries are the Vanadium redox flow battery, polysulfide bromide battery (Regenesys), and uranium redox flow battery. Redox fuel cells are less common commercially although many systems have been proposed.