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II. Assessing the levelized cost of vanadium redox flow batteries with capacity fade and rebalancing 1. Introduction Undesirable active species transport through the semi-permeable membranes separating the positive and negative electrolytes is a common mode of capacity fade in redox flow batteries (RFBs). In an ideal membrane, only supporting ions will exchange between the two electrolytes to maintain charge neutrality and balance the redox reactions in each electrolyte. Imperfect selectivity results in the passage of active species and solvent through the membrane. The crossover of active species is driven by a combination of concentration, potential, and pressure gradients between the two half-cells, which ultimately results in a concentration imbalance of charge storage materials in the negative and positive electrolytes and, consequently, a net decrease in the accessible capacity. Crossover can also lead to other chemistry-specific cross-contamination issues, such as precipitation, membrane fouling, or component degradation, which limits the combinations of half-reactions that can be used in tandem. To mitigate this effect, RFB membranes are typically designed with charge- or size-exclusion pores to impart the desired permselectivity without sacrificing conductivity. To this end, several groups are investigating new membrane materials and cell designs that minimize crossover [30–33], while others are developing models that enable a deeper understanding of thermodynamic, kinetic, and transport processes that define membrane-electrolyte interactions with RFB systems [34–37]. While significant progress has been made, crossover remains a significant issue and requires creative solutions to limit its impact on RFB performance, longevity, and, ultimately, commercial viability. The vanadium redox flow battery (VRFB) is arguably the most well-studied and widely deployed RFB system. At the time of writing, there are approximately 330 MW of VRFBs currently installed around the world with many more systems announced or under development, including a 200 MW/800 MWh plant in Dalian, China [18,38]. This system leverages a single element, vanadium, with four stable oxidation states (II, III, IV, and V) accessible within the electrochemical stability window of acidic aqueous electrolytes on carbon electrodes. The charging half-cell reactions for the VRFB are shown in Equations II-1 and II-2 below (note: reversing their direction gives the discharge reaction). 16PDF Image | Bringing Redox Flow Batteries to the Grid
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