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 that are pumped through the system on separate sides of a membrane. Ion transfer inside the cell (accompanied by current flow through an external circuit) occurs across the membrane while the liquids circulate in their respective spaces.
A flow battery may be used like a fuel cell (where new charged negolyte (a.k.a. reducer or fuel) and charged posolyte (a.k.a. oxidant) are added to the system) or like a rechargeable battery (where an electric power source drives regeneration of the reducer and oxidant).
The fundamental difference between conventional and flow batteries is that energy is stored in the electrode material in conventional batteries, while in flow batteries it is stored in the electrolyte. According to Battery Council International, this provides flow batteries with advantages for scalability and long-duration energy storage capabilities, making them ideal for stationary applications that demand consistent and reliable power.
Flow batteries have certain technical advantages over conventional rechargeable batteries with solid electroactive materials, such as independent scaling of power (determined by the size of the stack) and of energy (determined by the size of the tanks), long cycle and calendar life, and potentially lower total cost of ownership at multi-hour half cycle design times. However, flow batteries suffer from a lower cycle energy efficiency (50–80%). This drawback stems from the need to operate flow batteries at high (>= 100 mA/cm2) current densities to reduce the effect of internal crossover (through the membrane/separator) and to reduce the cost of power (size of stacks). Also, most flow batteries (Zn-Cl2, Zn-Br2 and H2-LiBrO3 are exceptions) have lower specific energy (heavier weight) than lithium-ion batteries. The heavier weight results mostly from the need to use a solvent (usually water) to maintain the redox active species in the liquid phase.
Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.43 volts. The energy capacity increased with the volume of the fluids in the tanks, and the power increases with the size of the stack.
Flow batteries can be classified using different schemes:
1) Full-flow (where all reagents are in fluid phases: gases, liquids, or liquid solutions), such as vanadium redox flow battery vs semi-flow, where one or more electroactive phases are solid, such as zinc-bromine battery.
2) Type of reagents: inorganic vs. organic and organic forms. As of 2025, only inorganic RFBs have been demonstrated on a commercial scale. Organic RFBs often suffer from poor durability.
3) By cell separator design: Membrane RFBs vs membraneless RFB.
Patent Classifications for flow batteries had not been fully developed as of 2021. Cooperative Patent Classification considers flow batteries as a subclass of regenerative fuel cell (H01M8/18), even though it is more appropriate to consider fuel cells as a subclass of flow batteries.