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The voltage required to run charging and discharging cycles is the same as the open circuit potential from thermodynamic point of view. However, applying the open circuit voltage does not run the cycles in practical applications, as kinetics of redox reactions require more voltage for charging and loss of voltage while discharging. The difference in applied voltage and open circuit potential is called overpotential. Butler-Volmer equation is the classic one to describe the kinetics of redox reactions: [11] π=π0{exp(πΌπππΉππ )βexp(βπΌπππΉππ )},π0 =π΄exp(βπΈπ),ππ =πβπ (1.3) π π π π π π Where i, i0, Ξ±a, Ξ±c, F, R, T, n, Ea, U, V, and Ξ·s are obtained current density, exchange-current density, anodic activity coefficient, cathodic activity coefficient, Faradayβs constant, universal gas constant, temperature, number of electrons transferred, activation energy, open-circuit potential, applied voltage, and overpotential, respectively. Exchange-current density accounts for the rate of forward and reverse reactions at equilibrium, which is represented by an Arrhenius expression. Based on Butler-Volmer equation, the amount of overpotential dictates the kinetics of redox reactions. Higher temperature, lower activation energy and higher activity coefficient result to lower overpotential. Lowering overpotential is desirable due to lower power required to charge and higher power discharged from RFB cells. This improvement leads to higher voltage and energy efficiency. As depicted in Figure 1. 4, a simple VRFB system consists of endplates, current collectors, flow field plates (FF), porous electrodes, gaskets, IEM, and inlet and outlet tubing. Endplates, tightened to each other by nuts and bolts, are responsible for aligning and contain all the other components of a VRFB cell. Furthermore, they provide a platform for installing inlet and outlet tubing, which allows electrolytes to flow in and out of the cell. 6PDF Image | Analysis of Fluid Flow in Redox Flow Batteries
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