Redox flow technology
The term redox is a short word made up of "red" (for reduction) and "ox" (for oxidation). It describes a chemical reaction in which electrons are given off (oxidation) and taken up by a reaction partner (reduction). During this process, electrical energy is released or chemically bound.
Research on this began in the middle of the 20th century. In 1986, Maria Skyllas-Kazacos and her team at the University of New South Wales patented the first electrochemical energy storage device, the vanadium redox flow battery.
The technology is based on the storage of electrical energy resulting from the potential difference when two redox pairs react with each other. The main difference to other battery technologies - apart from the energy density - is that the reaction partners of the storage medium are in dissolved form as electrolytes in tanks. In addition to vanadium-sulfuric acid as the electrolyte, there are now a number of differently structured electrolytes on a metallic and organic basis.
Design and operation of a redox flow battery
A redox flow battery consists of 5 main components:
| Number | Main component |
|---|---|
| 1 | Anolyte tank |
| 2 | Catholyte tank |
| 3 | Electrochemical converters (stack) |
| 4 | Fluid system incl. pumps, sensor technology and actuator |
| 5 | Power electronics incl. grid connection |
Each redox flow battery uses two tanks in which the electrolytes are stored. One tank contains the negatively charged electrolyte (catholyte), the other tank contains the positively charged electrolyte (anolyte).
The stack is constructed from a defi ned number of individual battery cells, with each half-cell divided by an ion-exchange membrane. The battery cells are each separated by a fluid-tight, conductive bipolar plate. The actual electrochemical reaction takes place in the graphite felts of the half cells, through which the respective positive or negative electrolyte flows. To charge or discharge the battery, the electrolyte is pumped through the stack. As soon as an electrical load is applied to the stack, the electrochemical reaction begins: the current flows and the charge is removed from the electrolyte. To recharge the electrolytes, an external voltage is applied so that the reaction is reversed.
The energy density of the battery depends on the respective electrolyte. In the most commercially widespread variant of vanadium redox flow technology, it is 15 to 20 Wh/liter in practice.