Iron-silver oxide cells

Probably the best-known battery system using an iron anode is the nickd/iron battery. It should be written (—) Fe/KOH/NiO(OH) (-t), and has its merits as a heavy duty accumulator [28]. By far less famous and much more recent are the applications of iron anodes in (rechargeable) iron/air cells [(—) Fe/K0H/02 (-t)] [29, 30] and in iron/silver oxide batteries [(—) Fe/KOH(- -LiOH)/AgO (-t)] [31, 32]. 

Margalith et al. (1966) demonstrated that T. ferrooxidans grown on iron could oxidize sulfur but the rate of oxidation was lower than with iron however, sulfur-grown cells of T. ferrooxidans could also oxidize sulfite, dithionate, thiosulfate, tetrathionate and sulfide but not thiocyanate (Silver, 1970). Details of the specific metabolic reactions for sulfur oxidation by T. ferrooxidans have been outlined in an earlier publication (Lundgren et al., 1974). 

Zinc/silver oxide button cells are designed as anode limited the cell has 5 to 10% more cathode capacity than anode capacity. If the cell were cathode limited, a zinc-nickel or zinc-iron couple could form between the anode and the cathode can resulting in the generation of hydrogen. 

The separator system and the solubility of the active materials play critical roles in determining the wet and cycle lives of the silver-based cells. The separator must have a low electrolytic resistance for discharges at high rates, yet it must have high resistance to chemical oxidation hy the silver species as well as low permeability to colloidal silver, zinc, cadmium, or iron. 

These have not achieved the hoped for commercial success because of swelling of cells due to expansion of iron disulphide and a much lower rate capability than that achieved in alkaline electrolyte-based systems such as silver oxide zinc. 

At the impure Cu anode, copper is oxidized along with more easily oxidized metallic impurities such as zinc and iron. Less easily oxidized impurities such as silver, gold, and platinum fall to the bottom of the cell as anode mud, which is reprocessed to recover the precious metals. At the pure Cu cathode, Cu2+ ions are reduced to pure copper metal, but the less easily reduced metal ions (Zn2+, Fe2+, and so forth) remain in the solution. 

Electrolytic recovery of iron from pickling solutions using tungsten carbide gas diffusion anodes was studied by Streng et al. [23]. Replacement of Tainton anodes (99% lead and 1% silver) with a gas diffusion tungsten carbide anode led to a 1.5-1.7 V decrease in the cell voltage and a 50% reduction in energy consumption. Unwanted oxidation of ferrous ions to ferric ions did not occur on these gas diffusion anodes, since the cell was operated at a lower potential. 

Coulometric determinations can be carried out in which no physical separation occurs but simply a quantitative change in oxidation state. For example, MacNevin and Baker determined iron and arsenic by anodic oxidation of iron(II) to iron(III) and arsenic(III) to arsenic(V). The reduction of titanium(IV) to titanium(Hl) and the reverse oxidation have been used for the analysis of titanium alloys. Conversely, the output current from a cell made from a silver-gauze cathode and a lead anode with potassium hydroxide electrolyte can be used to measure low concentrations of oxygen in inert gases. ... 

INCO produced electrolytic nickel at its refinery in Port Colborne, Ontario, Canada. The production started in 1926. The anodes were made by reducing nickel oxide with coke, and the anodes contained about 93.5% Ni, 4% Cu, and 1% Co. The sulfur content was low, about 0.6%. The approximate composition of the purified electrolyte was 60 g L-1 Ni2+, 95 g L-1 S042-, 35 g L-1 Na+, 55 g L-1 Cl , and 16 g L 1 boric acid, and the temperature was 60 °C. The current density of the process was 16 A/sq.ft (approximately 170 A m-2) and the cell voltage was about 1.6 V. At the normal cell operating voltage, the principal impurities - iron, cobalt, lead, arsenic, and copper - dissolved into the solution with nickel. Silver, gold, the PGMs, sulfur, selenium, and tellurium fell to the bottom of the cell as an insoluble slime. The produced cathodes... 

Reactive metals in the copper anode, such as iron and zinc, are also oxidized at the anode and enter the solution as Fe and Zn + ions. They are not reduced at the cathode, however. The less electropositive metals, such as gold and silver, are not oxidized at the anode. Eventually, as the copper anode dissolves, these metals fall to the bottom of the cell. Thus, the net result of this electrolysis process is the transfer of copper from the anode to the cathode. Copper prepared this way has a purity greater than 99.5 percent (Figure 20.7). 

A similar approach was used to recover silver from spent photographic bleach-fixer solutions.11 As shown in Figure 8.10, the iron, which was used to oxidize the silver, remains in the bleach-fixer solution, because it forms a complex with EDTA. The silver permeates the cation-exchange membranes to the iron-free concentrate stream from whence it is reclaimed by electrodeposition in a separate cell. 

By now, you may be thinking that spontaneous electrochemical processes are always beneficial, but consider the problem of corrosion, the natural redox process that oxidizes metals to their oxides and sulfides. In chemical teims, coiTOsion is the reverse of isolating a metal from its oxide or sulfide ore in electrochemical terms, the process shares many similarities with the operation of a voltaic cell. Damage from corrosion to cars, ships, buildings, and bridges runs into tens of billions of dollars annually, so it is a major problem in much of the world. We focus here on the corrosion of iron, but many other metals, such as copper and silver, also conode. 

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