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Chapter 2. Redox Flow Batteries Each electrolyte consists of an aqueous solution of a redox-active compound and most often also a supporting electrolyte. The positive electrolyte, also called the posolyte (or catholyte), contains a material with a high redox potential, whereas the negative electro- lyte, called the negolyte (or anolyte), contains a material with a low redox potential. The difference in redox potential between the electrolytes creates a voltage difference across the cell or battery. The electrolytes are pumped from their reservoirs through the cell stack, where they are distributed over the electrodes via flow plates. To increase the battery voltage to the necessary level for commercial use, several layers of the electrode- membrane-electrode arrangement are placed in a bipolar arrangement and connected in series. The redox reactions take place at the interface between electrolyte and electrode. The exchanged electrons are transported to and from the electrodes through the con- ductive graphite flow plates and current collectors, and between the two sides of the cell through an external circuit. The transport of electrons creates a charge imbalance between positive and negative side, which is counteracted by the movement of ions through the ionically conductive membrane that separates the two sides of the cell. This working principle means that the power and energy of RFBs can be scaled inde- pendently; the energy scales with the volume of electrolyte, concentration of exchanged electrons, and the difference in redox potential between posolyte and negolyte, whereas the power scales with the active area of the cell stack. This proves to be a major ad- vantage over encapsulated secondary batteries when it comes to large-scale storage of electrical energy. As the energy-to-power ratio increases, the cost per kWh of stored energy approaches the cost of the electrolytes [38], as was illustrated in Figure 1.1. The working principle of RFBs has not changed significantly since their invention, but many different types of electrodes, membranes, and especially electrolyte chemistries have been investigated. A few of the most noteworthy RFB chemistries are presented in the following sections, divided into metal- and inorganic-based and quinone-based systems. A more detailed description of other relevant RFB components is given subsequently. 2.1 Metal- and Inorganic-Based Systems 2.1.1 Iron-Chromium The iron-chromium system was the first modern RFB system to be demonstrated, pri- marily developed by the National Aeronautics and Space Administration (NASA) in the 1970s and 1980s [39, 40]. It utilises a negolyte containing Cr2+/Cr3+ and a posolyte con- taining Fe3+/Fe2+, both dissolved in aqueous hydrochloric acid. The half-cell reactions are presented in Equations 2.1 and 2.2 with their respective standard reduction potentials. Cr2+ −↽−⇀− Cr3+ + e− | −0.41 V vs. SHE (2.1) Fe3+ + e− −↽−⇀− Fe2+ | +0.77 V vs. SHE (2.2) Several demonstration systems with powers in the range 10–60kW were produced in Japan in the 1980s [21]. However, due to the slow kinetics of the Cr2+/Cr3+ couple and cross-contamination of the electrolytes, the system was largely abandoned in favour of 6PDF Image | Organic Redox Flow Batteries 2023
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