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expected value in terms of volumetric energy density for an aqueous RFB: This cell transfers one electron n = 1 per molecule, anolyte and catholyte are present as c = 2 M solutions and the cell voltage takes maximum advantage of the stability window of carbon electrodes in water, The result is a battery with energy density E Any RFB chemistry that features a higher volumetric energy content, while simultaneously satisfying the other constraints (power, cost, safety, efficiency) would be desirable. Power density: The power that can be drawn from a battery is limited by the overvoltage that has to be applied to reach a certain current I. This total resistance Rtotal, often given as area-specific resistance (ASR) of the power converter, comprises the single resistances Relec, Rmem, RCT and Rmass as given in equation 7. Relec and Rmass are from the realm of power converter design and electrochemical engineering, and are therefore not discussed here [23,24,43,115]. The membrane resistance Rmem is determined by the used membrane or separator, but that is pre-determined by the used chemistry [83,157]. Ideally size-exclusion membranes [90] or fluorinated exchange membranes with high proton concentrations are used to keep Rmem low. In non-aqueous solvents or in neutral aqueous electrolytes the membrane can have a significant contribution to the ASR. One major factor is the charge transfer resistance RCT that depends on the employed redox couple. The electron transfer constant k0 which determines RCT (see equation 8) spreads over more than three orders of magnitude for different redox couples used for RFBs (see table 1). With c = 2 M L-1, n = 1 and k0 = 0.001 cm s-1, the RCT = 0.1 cm2 would contribute only minimally to the ASR for which an upper bound of 1.5 cm2 was given [156]. Therefore, the ideal RFB chemistry has an electron transfer constant of Long-time capacity stability is an obvious criterion for an RFB. In terms of the chemistry this can be subdivided into cycling stability and chemical stability. The former is diminished by side-reactions during charge and discharge, and cross-over through the membrane. Only the fraction of electrolyte volume in the power converter is subjected to it. The latter, chemical stability, concerns either individual oxidation states or redox molecules themselves. For most of these phenomena mitigation strategies can be found. For example, in a VRFB hydrogen evolution takes place [57], ion-specific cross-over through the membrane occurs [158] and V2+ is oxidized by O2. All these effects lead to an imbalance in the electrolyte, anolyte and catholyte are not at the same SOC during battery operation. Re-mixing of the Page 44 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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