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104 Redox - Principles and Advanced Applications safety concerns for large-scale applications, redox flow batteries show great advantages over other types of batteries such as lead-acid and lithium-ion batteries and are expected to have increasing commercial space through technological development in future. Therefore, the redox chemistry and technical fundamentals of flow batteries, which determine the technolog- ical success and market penetration, need to be well understood. 2. Classic vanadium redox flow batteries Among various flow batteries, vanadium redox flow battery is the most developed one [1]. Large commercial-scale vanadium redox flow batteries are currently in construction. The structure and charge-discharge reactions of vanadium redox flow batteries are schematically shown in Figure 1. During discharging, reduction occurs at the cathode and oxidation occurs at the anode as shown in Eqs. (1)–(3) (discharge: !, charge: ). While these redox reactions occur, proton ions diffuse across the membrane and electrons transfer through an external circuit. Negative electrode : V2þ $ V3þ þ e ð1Þ Positive electrode : VO2þ þ 2Hþ þ e $ VO2þ þ H2O ð2Þ Overall reaction : V2þ þ VO2þ þ 2Hþ $ VO2þ þ V3þ þ H2O ð3Þ The standard cell voltage for the all-vanadium redox flow batteries is 1.26 V. At a given temperature, pH value and given concentrations of vanadium species, the cell voltage can be calculated based on the Nernst equation: 2þ3þþ þ22þ E1⁄4 1:26V–RT=Fln VO V = 1⁄2VO2 1⁄2H V ð4Þ where R, T and F are the universal gas constant, absolute temperature and Faraday constant, respectively. The crossover of vanadium ions through the membrane may occur, resulting in self-discharge with the unwanted mixing of vanadium species at both sides of the cell, as following [2]: Figure 1. A schematic of a vanadium redox flow battery: (a) charge reaction and (b) discharge reaction.PDF Image | Redox Flow Batteries Fundamentals and Applications
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