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110 Redox - Principles and Advanced Applications Figure 3. Potentials and relative solubility of selected inorganic and organic redox couples for redox flow batteries. Dotted lines show the electrochemical stability limit of typical aqueous electrolytes. Dashed lines show the possibility to extend the stability limit for aqueous electrolytes using concentrated electrolytes [19, 21]. concentrated mixtures. The inner Helmholtz layer close to the electrode surface is mostly occupied by the [BMIm]þ cation or TFSI anion, respectively. Water decomposition is then largely inhibited. The redox potentials for hydrogen and oxygen evolution reactions are pH dependent. Individual control in the pH values of the anolyte and catholyte with a multi- membrane system leads to high cell operation voltage of about 3 V [22]. In contrast to the electrochemical stability of the redox species and solvents, chemical stability of electroactive species and cell components is also critical for long-term operation. Vanadium electrolytes form solid precipitates at a temperature above 40 or below 10C at concentrations above 1.6 M for all-vanadium redox flow batteries. Oxidizing V5þ and Ce4þ may cause degradation of membrane and the graphite electrode materials. Complexing agents are needed to store bromine, whereas phase separation (formation of water-insoluble emulsion) occurs for bromine complexes during charging for bromine-based flow batteries. Cross-contamination in bromine-polysulphide batteries may generate heat and release toxic Br2 and H2S. High rate performance of redox flow batteries means high power generation capability. Ideally, two active species at both sides of the cell are expected to have close rate constants. However, mismatches in reaction rates are often observed. For many electrode reactions with sluggish kinetics, catalysts are needed to reduce the polarization (i.e. to improve the voltage efficiencies) and to improve the reaction rate (Table 3) [23]. Catalysts are generally applied onto a porous material, which offers high contact area for electrolytes. The supporting materials should have high electrical conductivity, mechanical stability, reasonable cost and high levels of oxygen and hydrogen evolution overpotential for aqueous system. Carbon-based materials are commonly used for this purpose [24].PDF Image | Redox Flow Batteries Fundamentals and Applications
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