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At the negative electrode: At the positive electrode: Redox Flow Batteries: Fundamentals and Applications 105 http://dx.doi.org/10.5772/intechopen.68752 V2þ þVO2þ þ2Hþ ! 2V3þ þH2O ð5Þ 2V2þ þVO2þ þ4Hþ !3V3þ þ 2H2O ð6Þ V3þ þ VO2þ ! 2VO2þ ð7Þ V2þ þ2VO2þ þ2Hþ !3VO2þ þ H2O ð8Þ V3þ þ VO2þ ! 2VO2þ ð9Þ V2þ þVO2þ þ2Hþ !2V3þ þ H2O ð10Þ Side reactions such as hydrogen evolution due to water decomposition and CO2 evolution due to the oxidation of carbon-based electrode may occur during operation [3]. The battery perfor- mance is generally evaluated with three efficiencies: coulombic efficiency (CE), voltage effi- ciency (VE) and energy efficiency (EE), which are defined as following: CE 1⁄4 discharge capacity 100% ð11Þ charge capacity VE 1⁄4 average discharge voltage 100% ð12Þ average charge voltage EE 1⁄4 CEVE ð13Þ The CE reduces because of crossover of vanadium ions during cell operation and side reac- tions. The VE is related to the operation current density, ionic conductivity of membrane, electrode materials, flow rate and mass transport of electrolyte. As current density increases, the VE reduces due to the increase in polarization. 3. Types and configurations of redox flow batteries Conventional redox flow batteries have two divided electrolyte reservoirs (Figure 2a). Catholyte and anolyte are separated by a membrane, which permits ions to pass through it. The most common working ions in aqueous media, Hþ (349.8 S cm2 mol1) and OH (198.0 S cm2 mol1), have the highest limiting molar conductivity among all known cations and anions, respec- tively [4]. All-vanadium redox flow batteries, for instance, have V3þ/V2þ redox reactions on the negative side (anolyte) and VO2þ/VO2þ on the positive side (catholyte). Such battery uses the same metal ions on both sides. Crossover of metal ions through the membrane will then not cause contamination of the electrolyte. In contrast, for redox flow batteries with different metal ions such as Fe3þ/Fe2þ and Cr3þ/Cr2þ in an iron-chromium flow battery, the cross-contamination via ion penetration may cause irreversible performance loss.PDF Image | Redox Flow Batteries Fundamentals and Applications
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