A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell that converts chemical energy directly to electricity. 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.[1] 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.
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Various classes of flow batteries exist including the redox (reduction-oxidation) flow battery, in which all electroactive components are dissolved in the electrolyte. If one or more electroactive component is deposited as a solid layer the system is known as a hybrid flow battery.[2] The main difference between these two types of flow battery is that the energy of the redox flow battery can be determined fully independently of the battery power, because the energy is related to the electrolyte volume (tank size) and the power to the reactor size. The hybrid flow battery, similar to a conventional battery, is limited in energy to the amount of solid material that can be accommodated within the reactor. In practical terms this means that the discharge time of a redox flow battery at full power can be varied, as required, from several minutes to many days, whereas a hybrid flow battery may be typically varied from several minutes to a few hours.
Another type of flow battery is the redox fuel cell.[3] This has a conventional flow battery reactor, which only operates to produce electricity (i.e., it is not electrically recharged). Recharge occurs by reduction of the negative electrolyte using a fuel (e.g. hydrogen) and oxidation of the positive electrolyte using an oxidant (typically oxygen or air).
Examples of redox flow batteries are the vanadium redox flow battery, polysulfide bromide battery (Regenesys), and uranium redox flow battery.[4] Hybrid flow batteries include the zinc-bromine, zinc-cerium [5] and lead-acid flow batteries. Redox fuel cells are less common commercially although many systems have been proposed.[6][7][8][9]
| Couple | Max. cell voltage (V) | Average electrode power density (W/m2) | Average fluid energy density (W·h/kg) |
|---|---|---|---|
| Iron-tin | 0.62 | <200 | |
| Iron-titanium | 0.43 | <200 | |
| Iron-chrome | 1.07 | <200 | |
| Vanadium-vanadium (sulphate) | 1.4 | ~800 | 25 |
| Vanadium-vanadium (bromide) | 50 | ||
| Sodium/bromine polysulfide | 1.54 | ~800 | |
| Zinc-bromine | 1.85 | ~1,000 | 75 |
| Lead-acid (methanesulfonate) | 1.82 | ~1,000 | |
| Zinc-cerium (methanesulfonate) | 2.43 | <1,200–2,500 |
Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of flexible layout (due to separation of the power and energy components), long cycle life (because there are no solid-solid phase changes), quick response times, no need for "equalisation" charging (the over charging of a battery to ensure all cells have an equal charge) and no harmful emissions. Some types also offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/ overdischarge.
On the negative side, flow batteries are rather complicated in comparison with standard batteries as they may require pumps, sensors, control units and secondary containment vessels. The energy densities vary considerably but are, in general, rather low compared to portable batteries, such as the Li-ion.
Fuel cells are electrochemical energy conversion devices that convert chemical energy directly to electrical energy in which a fuel and an oxidant undergo electron transfer reactions at the anode and cathode of an operating electrochemical cell respectively, separated by an ion exchange membrane. These devices are not subject to Carnot's limitations for heat engines and can ideally generate electricity as long as they are supplied with fuel and an oxidant. They differ from batteries in that the active chemical species are supplied externally, rather than stored internally as is the case in batteries. So, power and energy specifications can be scaled up independently for a fuel cell, while the energy density of a battery is limited by the amount of active material that can be stored inside it.
Under these definitions a flow battery is a special type of rechargeable battery in which the dissolution of active species in the electrolyte permits external storage of reactants, thereby allowing independent scale up of power and energy density specifications. Also, external storage of reactants avoids self-discharge that is observed in primary and secondary battery systems. Electrolyte in a fuel cell remains at all times within the reactor (in the form of an ion-exchange membrane, for example). What flows into the reactor are only the electroactive chemicals, which are non-conducting (e.g., hydrogen, methanol, oxygen, etc.) This is in contrast to a flow battery in which at least some of the electrolyte (generally the majority in weight and volume terms) flows through the reactor.
Flow batteries are also distinguished from fuel cells by the fact that the chemical reaction involved is often reversible; i.e., they are generally of the secondary battery type and so they can be recharged without replacing the electroactive material. Also, an important factor in the redox flow battery (see below for classes of batteries) is that the power and energy density of redox flow batteries are independent of each other in contrast to rechargeable secondary batteries.
To add to the confusion the European Patent Organisation classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).
Flow batteries are normally considered for relatively large (1 kW·h – 10 MW·h) stationary applications. These are for
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