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(10) Activation over-potentials deriving from the reaction kinetics, mass transport over- potentials and Ohmic losses are all sources of a discrepancy between the charging and discharging voltage (see equation 6). The coulombic efficiency CE multiplied by the voltage efficiency VE yields the energy efficiency EE of a RFB which indicates how much of the energy that is supplied to the battery during charging can be extracted upon discharge. (11) RFBs have several benefits over conventional secondary batteries. For instance, external storage of the electrolyte prevents the self-discharge of the stored solutions. Further, as the electrodes are not subjected to continuous plating or intercalation reactions, electrode deterioration is minimised. When a conventional battery undergoes repeated charge/discharge cycles, the electrode materials expand and contract which results in their degradation over time[34]. Thus, lifetime can be much improved when there is no phase change at the electrodes during cycling, as in a RFB. The predicted lifetime for a RFB tends to exceed 10 years and the VRFB is rated at 10 000 cycles [35]. The life-limiting component of a RFB is typically the cell stack. Wearing of the membranes separating the electrode compartments is a source of performance degradation and for the VRFB in particular, the cell stack is expected to have a lifetime of 10 to 15 years for a RFB undergoing 1000 charge/discharge cycles per year [36]. The electrolyte storage tanks, plumbing, structural components, power electronics and controls of a RFB should have longer useful lifetimes [35], but pump replacement may also be required periodically. Crossover of active species between the half-cells due to an inefficiency of the separator is a source of capacity fade and as such, a degree of electrolyte maintenance may be necessary. However, with stack and pump replacement, VRFBs can operate for more than 20 years [36]. Hazards relating to the operation of RFBs include the use of flammable, toxic and/or corrosive electrolytes. While the flowing electrolytes aid heat dissipation and RFBs such as the VRFB classically used aqueous solutions, flammability should be a consideration if a RFB electrolyte is based on a non-aqueous solvent. Further, acidic supporting electrolytes are typically used which necessitates the use of acid-resistant components and toxicity associated with active species is of concern in the event of leakages. Page 12 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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