Zinc/bromine

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). 

Zinc—bromine storage batteries (qv) are under development as load-leveling devices in electric utilities (64). Photovoltaic batteries have been made of selenium or boron doped with bromine. Graphite fibers and certain polymers can be made electrically conductive by being doped with bromine. Bromine is used in quartz—haUde light bulbs. Bromine is used to etch aluminum, copper, and semi-conductors. Bromine and its salts are known to recover gold and other precious metals from their ores. Bromine can be used to desulfurize fine coal (see Coal conversion processes). Table 5 shows estimates of the primary uses of bromine. 

An overview of the most important quaternary ammonium salts tested for possible applicability in zinc-bromine batteries is presented in Table 2. A rough classification has been applied according to the substance classes of the substituents attached to the nitrogen. 

The two basic requirements for efficient bromine storage in zinc-bromine batteries, which need to be met in order to ensure low self-discharge and more over a substantial reduction of equilibrium vapor pressure of Br2 of the polybromide phase in association with low solubillity of active bromine in the aqueous phase. As mentioned by Schnittke [4] the use of aromatic /V-substitucnts for battery applications is highly problematic due to their tendency to undergo bromination. Based on Bajpai s... 

M. Eskra, P. Eidler, R. Miles, Zinc-bromine battery development for electric vehicle applications, Proc. 24 h hit. Symp. Automotive technology and Automation, Florence, 1991. 

K. Kawahara, 4 1 Int. Zinc-bromine Battery Symp. Perth, Australia, Toyota Central R D Inc. Handout, Feb. 1987. 

R. J. Bellows First Int. Zinc-Bromine Battery Symp., Jan 1985, Final Progress Report, at... 

While the zinc/chlorine battery is preferred for utility load-leveling applications [49], the zinc/bromine system is the more promising one for electric vehicle requirements [50, 51]. 

Zinc is electrodeposited from the sodium zincate electrolyte during charge. As in the zinc/bromine battery, two separate electrolytes loops ("posilyte" and "nega-lyte") are required. The only difference is the quality of the separator The zinc/ bromine system works with a microporous foil made from sintered polymer powder, but the zinc/ferricyanide battery needs a cation exchange membrane in order to obtain acceptable coulombic efficiencies. The occasional transfer of solid sodium ferrocya-nide from the negative to the positive tank, to correct for the slow transport of complex cyanide through the membrane, is proposed [54],... 

Even though zinc-bromine batteries operate with a slightly acidic electrolyte (pH 3), they are discussed here briefly, because... 

In the other study. X-ray fluorescence spectroscopy was used to analyze trace element concentrations by observing dusts on 37 ram diameter cellulose acetate filters (20). Twenty-three elutriator and twenty-three area samples from 10 different bales of cotton were analyzed. The average fraction of total dust accounted for by the elements analyzed was 14.4% amd 7.6% for vertical elutriator and area samples, respectively. Although the variation in absolute quantity of atn element was high, the relative abundance of an element was consistent for measurements within a bale. Averaged over all the samples analyzed, calcium was the most abundant element detected (3.6%), followed by silicon (2.9%), potassium (2.7%), iron (1.1%), aluminum (1.1%), sulfur (1.0%), chlorine (0.8%) and phosphorous (0.6%). Other elements detected in smaller aunounts included titanium, manganese, nickel, copper, zinc, bromine, rubidium, strontium, barium, mercury amd lead. 

Various materials have been used as separators in zinc—bromine cells. Ideally a material is needed which allows the transport of zinc and bromide ions but does not allow the transport of aqueous bromine, polybromide ions, or complex phase structures. Ion selective membranes are more efficient at blocking transport then nonselective membranes.These membranes, however, are more expensive, less durable, and more difficult to handle then microporous membranes (e.g., Daramic membranes).The use of ion selective membranes can also produce problems with the balance of water between the positive and negative electrolyte flow loops. Thus, battery developers have only used nonselective microporous materials for the separator. 

Fig. 9.21 Schematic diagrams of zinc-bromine battery systems (a) cell with cation selective membrane (b) cell with reservoir for poly bromide and microporous separator...

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

Figure 11. Schematic of the Exxon zinc-bromine complex cell. (Courtesy of Exxon Research and Engineering Company, Dr. R. Bellows.)...

Zinc-Bromine Battery Development, Sixth Program Review, Phase III, Exxon Research and Engineering Company to DOE/SANDIA, 16-3187 (May 1984). Insulation and Enclosure Development for High Temperature EV Batteries, Final Report, Union Carbide, Linde Division to DOE, Contract No. DE-AC02-8DET25426 (May 1982). 

Nafion membranes have been used as separators in the hydrogen-chlorine cell/ " hydrogen-bromine cell, and zinc-bromine cell because of their excellent chemical inertness in these aggressive environments. The function of the separator in these cells, similar to that in a water electrolyzer, is the separation of the molecular species, such as hydrogen, chlorine, bromine, and metallic zinc, which cause self-discharge and efficiency loss when they migrate across the separator. 

---