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where U is the voltage of the battery (V), I is the current (A), S is the contacting area of electrode with the separator (m2). For an RFB, the discharge voltage typically deviates from the theoretical potential due to polarization. The overpotential is not only relevant to reaction kinetics and diffusion coefficient of redox species in electrolytes, but also pertains to the ionic conductivity of separators. In addition, the discharge current density is also associated with these factors. The power density is determined by the following factors: voltage of the battery, reaction rate of redox species, and ionic conductivity of the separator. Coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) are three parameters for evaluating battery efficiency. CE is the ratio of the discharge capacity and the charge capacity. CE and EE can be calculated from the following equations (Eqs. 3 and 4): where Q and E represent the electric quantity and energy, respectively. The footnotes of discharge and charge represent the discharging and charging process, respectively. EE is the product of CE and VE (Eq. 5), which EE reflects the energy utilization. Generally, the three efficiencies are less than 100% due to the crossover of redox species, instability of redox species at varied charge states, polarization, ionic conductivity of electrolyte, and electronic conductivity of current lead. In addition, the lifetime of a battery is also important for an energy storage device. The stability of the electroactive materials at all charge states decides the lifetime of a battery. Inorganic Redox-Active Materials As the most important component in RFBs, catholytes and anolytes play a vital role in RFBs. The storage and release of energy of RFBs are accomplished by redox reactions. Redox-active inorganic materials in aqueous electrolytes are commonly used in traditional RFBs. All-Vanadium RFB All-vanadium redox flow batteries (VRBs) are the most extensively studied and promising systems because of the excellent electrochemical reversibility between different oxidation states of vanadium. VRBs were first proposed by Skyllas-Kazacos et al. in 1986 (31), and have a history of over 30 years. Up to now, VRBs can demonstrate a power/energy scale of MW/MWh, which has been commercialized in the energy storage grid. VRBs usually use VOSO4 or (VO2)2SO4 sulfate solution as the catholyte and V2(SO4)3 or VSO4 sulfate solution as the anolyte. The theoretical potential of VRFB is 1.25 V based on the following reactions: 4 Qin and Fan; Clean Energy Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2020.PDF Image | Electroactive Materials Next-Generation Redox Flow Batteries
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