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2.1.1.4. Zinc-Bromine Perhaps the most complicated of all the commercialized RFB electrolyte chemistries is Zinc- Bromine (Zn-Br). Here, metallic zinc is plated and stripped on the anode, while liquid bromine is evolved and reduced from the cathode. Like the all-Fe RFB, the Zinc-Bromine RFB can be considered a “hybrid flow battery.” Upon discharge of the RFB, the following redox reactions occur: Catholyte: Br2 +2e-2Br- (5) Anolyte: Zn0 Zn2+ + 2e- (6) The relatively high theoretical cell voltage of 1.82 V, coupled with the high electrolyte concentrations, enable an energy density of 60-70 Wh/L. This chemistry is more complicated than others in that multiple phase-change reactions occur. Whereas with most of the previous electrolyte chemistries all reagents remain in the liquid phase, here zinc is electroplated out as a solid and bromine is evolved into the liquid phase. Advanced complexing agents are added to the electrolytes to stabilize Br- and Zn-containing species and minimize the Br2 vapor pressure and Zn dendritic growth [17, 18, 19]. 2.1.2. Separators The primary role of the membrane separator is to allow the flow of electrolyte between the positive and negative compartments for charge balance, while preventing the transport of electroactive species, which leads to capacity decay. Other key attributes of the membrane are high oxidative/reductive stability, flexible mechanical properties, and low cost. The membrane separator is a critical component to flow battery performance, durability, and cost. However, a membrane that satisfies all the mentioned requirements does not exist, thus current research efforts are focused on a variety of membrane composition and morphology to optimize flow battery performance. There are two broad categories of flow battery membranes: 1) ion exchange membranes: dense film with ionic moieties that are tethered to a hydrocarbon or perfluorinated backbone, which instills hydrophilic character, and 2) porous membranes with nonionic polymer backbone and engineered pore size/density. Both types of membranes have positive attributes, but also have properties that need to be further improved, which will be discussed in Section 2.3.2 Membranes. There are two types of ion exchange membranes that are differentiated by the type of bound charge in the polymer backbone. The bound charges in the polymers develop a Donnan potential, which determines the ion selectivity of the membrane. Cation exchange membranes (CEM) have fixed negative charges with a negative Donnan potential that predominantly allows cation mobility through the membrane. The opposite is found in anion exchange membranes [20, 21]. The attached negative charge is typically a sulfonate moiety, due its stability and high ion dissociation values. However, carboxylate and phosphonate groups have also been investigated. The state-of-the-art of cation exchange membranes are perfluorinated sulfonic acid (PFSA) such as Nafion (Chemours), Aciplex (Asahi) or Flemion (AGC). These types of membranes are industrially employed in the chloro-alkali process and used in large demonstration-size acidic vanadium and Fe-Cr flow batteries due to low proton resistance and superior chemical durability. The primary downside to PFSA membranes is high cost, which accounts for up to 30-40% of the stack hardware [22, 23]. Sulfonated hydrocarbon polymers are being developed as low cost alternatives to PFSA due to inexpensive chemical feedstock and 5 Chapter 6 Redox Flow BatteriesPDF Image | REDOX FLOW BATTERIES Chapter 6
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