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half reaction occurring at the negative half-cell. The Nernst potentials for the two reduction reactions ((2.20) and (2.21) ) that take place in each half cell of the VRFB can be represented as follows: Positive half cell Negative half cell (2.20) (2.21) Where R= 8.314472 J K-1mol-1 is the Universal Gas Constant, F= 96485 C mol-1 is Faraday’s constant, T is the temperature [K], [V] are the concentrations of the different vanadium species in solution, [H+] is the hydrogen-ion concentration in the positive half-cell electrolyte, and E0+ and E0- are the standard half-cell potentials for the positive and negative half-cell reactions, respectively. 2.7.2 Operating Cell Voltage In practical systems, the actual cell potential during discharge is lower than the theoretical cell potential due to the internal resistance of the cell (internal loss). This internal loss is caused due to factors such as activation polarization, concentration polarization and ohmic potential drop (caused by current flow through resistive components). The overall cell potential can be described at any moment in time during the operation of the cell as follows: (2.22) Where E+ is the positive half-cell potential during charge (oxidation of V4+ to V5+ at the positive electrode), E- is the negative half-cell potential during charge (reduction of V3+ to V2+ at the negative electrode), η- is the activation overpotential for negative half-cell reactions [V], η+ is the concentration overpotential at the positive half-cell reactions [V], i is the current density [A cm-2], and RCell is the ohmic resistance of the cell [Ω cm2]. During charging, the cell voltage is higher than the theoretical cell potential as a result of all of the losses that impact on the overall voltage and energy efficiency of the battery during charge-discharge cycling. It is therefore desirable to reduce all of the voltage losses so as to maximize energy efficiency. Ohmic losses can be minimized by selecting membrane and electrode materials with low electrical resistance, by reducing the internal spacing and by minimizing contact resistances. Activation overvoltage losses can be reduced by using electrode materials with 37PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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