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solubility. This leads to low energy density (both per mass and per volume). Therefore, flow batteries are best suited for stationary applications. 1.3 Redox Couples There are many possible choices for a pair of redox couples. Ideal candidates will be highly soluble, chemically stable, inexpensive, and reversible. In addition, the difference in standard potential between the couples should be substantial to provide a high cell voltage. Based on standard potentials, hydrogen or oxygen gas evolution may also occur in the aqueous solutions. One possible pair of active species is the all-vanadium pair. This pair is very commonly used due to a high potential and reversibility of the solutions. As an ion, vanadium has four stable oxidation states. The more negatively charged ions V(II) and V(III) are used in the anolyte and the more positively charged states V(IV) and V(V) are used in the catholyte. In order for vanadium to exist in ionic form, a vanadium- based salt is dissolved in sulfuric acid. During cell operation, protons pass through the membrane to maintain electrostatic equilibrium. The half cell reactions are given in Table 1.1. Table 1.1: Half cell reactions of the all-vanadium couples. Anolyte charge V3+ +e− →V2+ discharge V2+ −e− →V3+ Catholyte VO2+ +H2O−e− →VO2+ +2H+ VO2+ +e− +2H+ →VO2+ +H2O Because vanadium is employed on both sides of the cell, any crossover of active species through the membrane does not permanently degrade the system. Although such crossover does reduce efficiency, it does not lead to the formation of irreversible compounds and the desired chemistry can be restored through subsequent recharging [3]. This property prolongs the usefulness of the electrolytes. Another convenient feature of the vanadium ion is that each of its four oxidation states corresponds to a 3PDF Image | PERFORMANCE EVALUATION OF A REDOX FLOW BATTERY
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