Flow battery

A number of carbon materials are ideal for the construction of three-dimensional electrodes for metal ion removal, and commercial cells have been marketed based on beds of granular carbon [120], reticulated vitreous carbon [121], and carbon felt [122]. Details of the cell designs and examples of then-performance have been described previously [4,116,117]. 

Most commonly, the battery will be configured with a stack of bipolar cells (10 -100 cells per stack) to give a useful output voltage and with parallel flows for the electrolytes to each of the cells in the stack. Hence, the electrodes will be bipolar with a solid core from carbon, graphite, or a carbon/polymer composite and the three-dimensional elements bonded or pressed onto either side of the solid core. The composites are a blend of a chemically stable polymer and a micron-scaled carbon powder, most commonly an activated carbon Radford et al. [127] have considered the influence of the source of the carbon and the chemical and thermal treatments on the properties of such activated carbons, especially the pore size and distribution [126]. Even though reticulated vitreous carbon has been used for the three-dimensional elements [117], the predominant materials are graphite cloths or felts with a thickness of up to 5 mm, and it is clear that such layers are essential to scale the current density and thereby achieve an acceptable power density. Details of electrode performance in the more developed flow batteries are not available but, for example, Skyllas-Kazacos et al. [124] have tabulated an overview of the development of the all vanadium redox flow battery that includes the electrode materials and the chemical and thermal treatments used to enhance activity and stability. 

and Wright, P.M. (1971) in Industrial Electrochemical Processes (ed. A.T. Kuhn), Elsevier, Amsterdam, p. 

Pletcher, D. and Walsh, EC. (1990) Industrial Electrochemistry, 2nd edn. Chapman Hall, London. 

Kinoshita, K. (1988) Carbon Electrochemical and Physicochemical Properties, John Wiley Sons, Inc., New York. 

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Redox flow batteries, under development since the early 1970s, are stUl of interest primarily for utility load leveling applications (77). Such a battery is shown schematically in Figure 5. Unlike other batteries, the active materials are not contained within the battery itself but are stored in separate tanks. The reactants each flow into a half-ceU separated one from the other by a selective membrane. An oxidation and reduction electrochemical reaction occurs in each half-ceU to generate current. Examples of this technology include the iron—chromium, Fe—Cr, battery (79) and the vanadium redox cell (80). 

Other flow batteries investigated for both electric vehicle appHcation and utiUty load leveling include 2inc [7440-66-6]—[7782-50-5] Zn—Q.25 and zinc—bromine [7726-95-6]., Zn—Br2, batteries (78,81,82). 

Figure 1. Operating principle of a zinc-flow battery...

Karigl [71] defined a format diffusion coefficient for bromine transport through a poly thy lene separator of a zinc-flow battery by considering the separator a diffusion layer. A value of Dsep(Br3 ) = 2.77 10 10 m2 s l was obtained. 

The most interesting results of a study of a real 12 V/l kWh zinc-flow battery (Powercell Lda.) with a charge capacity of 92 Ah are reviewed in Figs. 6 and 7. 

Figure 6. Concentration of the complexing cations MEP1 (A) and MEM1 (0) in the complex electrolyte phase during one total charge-discharge cycle of a model zinc-flow battery. Taken from Ref. [90],...

As shown by several investigations [91], the bromine-rich polybromide phase by itself is hardly flammable and fireextinguishing properties have been reported occasionally. The formation of polybrominated dibenzo-dioxins (PBrDD) and furans (PBrDF) due to the plastic-containing housing of a zinc-flow battery cannot be totally neglected in the case of a fire, but their concentrations are far away from the tetrachloro dibenzodioxine (TCDD) toxic equivalents even in a worst-case scenario. 

Storage of bromine by formation of a polybromide phase with a lowering of the vapor pressure by more than one magnitude, to at least 10% of the value of Br2 at maximum, is the basic requirement for safe application in zinc-flow batteries [91, 92]. No information is available concerning negative health effects of the com-plexing agents MEM and MEP. 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

There are three types of zinc-flow batteries (belonging in general to the group of flow or redox batteries) which have been studied intensively two of them are similar with respect to the reactants involved, the... 

All in all, this system is more complicated than the other flow batteries and this handicap postpones wider application. 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

Another type of redox flow battery that utilizes carbon electrodes and soluble reactants involving vanadium compounds in H2S04 is under evaluation [38,39] ... 

A new type of power supply for electric cars eliminates the need for recharging. A fuel cell is a battery that produces electricity while reactants are supplied continuously from an external source. Because reactants continuously flow into the cell, a fuel cell is also known as a flow battery. Unlike the fuel supply of a more conventional battery, the fuel supply in a fuel cell is unlimited. As in the combustion of gasoline in a conventional engine, the overall reaction in a fuel cell is the oxidation of a fuel by oxygen. 

The redox flow battery (RFB) concept was first proposed by L. H. Thaller at the NASA Lewis Research Center, Cleveland, Since then, it... 

Another system under investigation is the iron/ chromium redox flow battery (Fe/Cr RFB) developed by NASA. The performance requirements of the membrane for Fe/Cr RFB are severe. The membrane must readily permit the passage of chloride ions, but should not allow any mixing of the chromium and iron ions. An anionic permselective membrane CDIL-AA5-LC-397, developed by Ionics, Inc., performed well in this system. ° It was prepared by a free radical polymerization of vinylbenzyl chloride and dimethylaminoethyl methacrylate in a 1 1 molar ratio. One major issue with the anionic membranes was its increase in resistance during the time it was exposed to a ferric chloride solution. The resistance increase termed fouling is related to the ability of the ferric ion to form ferric chloride complexes, which are not electrically repelled by the anionic membrane. An experiment by Arnold and Assink indicated that... 

Other flow batteries investigated for both electric vehicle applications and utility load leveling include zinc-chlorine. Zn-CL, and zinc-bromine, Zn-Br2, batteries. 

Heintz, A. and Ch. Illenberger. 1998. Thermodynamics of vanadium redox flow batteries Electrochemical and calorimetric investigations. Ber. Bunsenges. Phys. Chem. 102 1401-1409. 

Figure 9.10 is a plot of steepwater composition taken from successive steeps across a ten steep, continuous-flow battery in 1950 when process water used for steep acid provided a rich bacterial inoculum. Today s continuous countercurrent steep batteries have approximately the same types of changes in every property except relative... 

Zn-bromine flow and vanadium redox flow are special cases of secondary batteries. Here, liquid electrode materials are used on one (Zn-Br flow) or both sides (V redox flow) of the electrochemical cell. In contrast to regular batteries, which are typically completely closed systems, the liquid electrode materials in flow batteries are circulated and replenished from tanks (Figure 3.5.5). Therefore, the flow batteries possess large electrodes, the effective size of which is just limited by the volume of those tanks. This partly decouples energy and power capabilities of the batteries, allowing one to optimize both separately. 

Regenerative Fuel Cells or Redox Flow Batteries... 

Figure 2.1 Redox flow battery or regenerative fuel cell...

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