Zinc powder anodes

Other alkaline primary cells couple zinc with oxides of mercury or silver and some even use atmospheric oxygen (zinc—air cell). Frequendy, zinc powder is used in the fabrication of batteries because of its high surface area. Secondary (rechargeable) cells with zinc anodes under development are the alkaline zinc—nickel oxide and zinc—chlorine (see Batteries). 

Alkaline batteries were introduced in the early 1960s they last two to five times longer than Zn-carbon cells on continuous discharge and command two or three times the price in the USA (far more in Europe and the East). Alkaline cells became a necessary invention and they succeeded as a result of the requirements of the electronic devices. The essential improvement was the change from ammonium chloride and/or zinc chloride electrolyte to alkaline (KOH) electrolyte, the steel can construction, the outside cathode, and the zinc powder (large surface) anode. A main low-cost feature is that they use pressed cathodes and do not need to follow "jellyroll"... 

Some battery-producing companies prefer purchasing pure, nonamalgamated zinc powder to apply their own proprietary corrosion protection system. The general trend is to keep the anodes of all the consumer cells mercury-free (usually indicated by a "green label) and to make them disposable with the regular household trash. The exceptions to this rule are those cells where this makes no sense, such as cells with a mercuric oxide cathode. 

The trimethylsilyloxy (TMSO) group is stable under the coupling conditions in acetonitrile (Table 12, number 6). After oxidative dimerization the TMS-ether can be mildly hydrolyzed (H+ and H2O) to the phenol or converted to a dibenzofuran. 1,2-Dialkoxybenzenes have been trimerized to triphenylenes (Table 5, numbers 7, 8). The reaction product is the triphenylene radical cation, which is reduced to the final product either by zinc powder or in a flow cell consisting of a porous anode and cathode [188]. Anodic trimerization of catechol ketals yields triphenylene ketals, which can function as a platform for receptors, for example, in an artificial caffeine receptor [190]. 

A highly active zinc powder can be generated in bulk by electrolysis of a tetraalkylammonium salt in dimethylformamide between a zinc anode and a platinum cathode [183]. This reagent has been used in stoichiometric amounts for Re-formatsky-type reactions where the tribromide 61 is used as a precursor for the isoprenyl group [184]. Zinc promoted 1,4-elimination of bromine to give the butadiene system is followed by a Reformatsky reaction of the remaining allylic bromine substituent. 

This sacrificial anode process compares favorably with Wakselman s classical Barbier procedure which requires activated zinc powder, and CF3Br under pressure, leading to lower yields with benzaldehyde (52% in DMF)21. These good results compared to those obtained from a mixture of CF3ZnBr and (CF3)2Zn allow one to illustrate the formation of a transient organozinc compound of the type CF3—Zn , which would be the really active species. This prompts the following question does the reduction of Zn2+ in Zn... 

Alkaline manganese dioxide batteries have been around since 1959. They are usually known as alkaline batteries. They are a variation of Leclanche s cells. They use an electrolyte produced manganese dioxide as cathode (to improve its purity and increase the capacity of the cell), powdered zinc as anode (to provide a larger surface area for the reaction), and a highly... 

Silver oxide cells were developed in the 1960s. These cells use silver oxide mixed with carbon (to increase the electronic conductivity of the material) as cathode, amalgamated pellet zinc powder as anode, and a solution of potassium hydroxide or sodium hydroxide with dissolved zin-cates in water as electrolyte. Permion (a radiation graft of methacrylic acid onto a polyethylene membrane) is used as separator. The cell reactions are... 

The anode active material of the alkaline Zn/Mn02 cell is zinc powder with median particle diameters of... 

In alkaline Mn02-Zn systems the same electrochemically active materials, manganese dioxide and zinc, are used as in Leclanche cells. The only difference to the latter system is the cathode sleeve construction and the highly conductive KOH electrolyte (7 M 0.55 (Ocm)-l) as already mentioned. The anode is formed of zinc powder instead of a zinc can. Electrolytic manganese dioxide is used for the cathode material. Due to its low polarization and high voltage characteristic it performs far better than chemically produced manganese dioxide or natural ore. 

The zinc passivation, which will be increased with high current densities and low temperatures, is very important for practical applications. The zinc-covering layers may be dense or porous and more or less soluble depending on accompanying ZnO precipitations from over-saturated zincate solutions. Large-surface powdered zinc does not easily passivate, while smooth surfaces show this tendency. As a result, powdered-zinc anodes operate satisfactory with small amounts of electrolyte, while zinc can anodes of Leclanche cells do not. 

During the operation of the cell the conductivity of the KOH solution changes by dissolution of Zn(OH)2- The bulk conductivity of a powdered-zinc gel anode is determined by the metallic structure. In Figure 4 the relationship between weight percent of zinc and the resistivity of an electrolyte/metal-powder paste is shown [26]. Good electronic conductivity is achieved when the mixture contains 35-70 % zinc. The usable cell capacity is exhausted as soon as the amount of metallic zinc in the anode gel decreases to about 30 % of weight [27,28]. The admixture of materials wth good surface conductivity improves the anode quality with respect to efficiency and capacity [29,30]. 

Porous zinc anodes are produced by cold-pressing of zinc powder wetted with mercury, that welds the particles together [31]. In another process the porosity is controlled by filler materials, e g. NH4CI, which can be easily removed later. Plastic binders are also used to fabricate porous zinc electrodes. 

Alkaline cells use the same zinc-manganese dioxide couple as Leclanche cells. However, the ammonium chloride electrolyte is replaced with a solution of about 30 wt% potassium hydroxide (KOH) to improve ionic conductivity. The ceU reactions are identical to those above, but the battery construction is rather different (Figure 9.7). The negative material is zinc powder, and the anode (negative terminal) is a brass pin. The positive component is a mixture of Mn02 and carbon powder that surrounds the anode. A porous cylindrical barrier separates these components. The positive terminal (cathode) is the container, which is a nickel-plated steel can. 

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