A redox battery, also known as a redox flow battery, is a type of battery that converts chemical energy to electrical energy through the use of externally stored, electrically conductive chemical substances known as electrolytes. Redox refers to an oxidation-reduction reaction, which is a chemical reaction in which the atoms or molecules of one substance lose electrons, or are oxidized, and those of another gain electrons, or are reduced. This movement of charged particles creates an electrical current. This current is created when the electrolytes are pumped from the holding tanks in which they are stored into a stack where together they produce an oxidation-reduction chemical reaction that creates useful electrical current.
Redox batteries operate similarly to common household batteries. Unlike a battery used in a cell phone or alarm clock, a redox battery is not contained compactly in a single unit. Its negatively charged electrolyte is stored in one external tank, and its positively charged electrolyte is stored separately in another external tank. While these electrolytes are separated, there is no exchange of electrons between them. When electricity is required, the electrolytes are pumped into a unit known as a stack, where an oxidation-reduction reaction occurs in which electrons are exchanged between them, thereby producing an electrical current.
There are a number of advantages to using redox batteries in certain applications over other types of batteries. One major advantage is that a redox battery can store a very large amount of energy, typically from a few kilowatts to a few megawatts, depending on the amount of electrolytes available. Redox batteries can be made using non-toxic substances and do not discharge any environmentally hazardous substances in the course of their operation. They also have long lives, require little maintenance, and can be recharged relatively quickly.
On the other hand, a redox battery requires a relatively large volume of electrolytes for the amount of energy it produces, making it impractical for small-scale electricity production or for use in portable applications. The complex hardware setup of a redox battery is another disadvantage. It requires such equipment as tanks, pumps to get the electrolytes into the stack, control systems to regulate the flow of the electrolytes, and more. For these reasons, redox batteries are most commonly used in large-scale applications or remote, off-the-electrical-grid locations where large amounts of power are required.
For example, a redox battery may supply power for a remote cell phone station where power from the grid is not available and frequent maintenance would be expensive. Another application for which redox batteries are well suited is leveling out the amount of electricity available from solar or wind energy plants. Solar energy is produced when the sun shines; wind energy is produced when the wind blows. Energy production from either of these sources can drop dramatically as unfavorable changes in the weather cause undesirable drops in energy availability. A redox battery can be used to store excess electricity when it is produced and later balance out the electricity available when weather changes decrease solar or wind energy production.