Silver cathodes

Electroplating Objects can be electroplated with a metal such as silver in a method similar to that used to refine copper. The object to be silver plated is the cathode of an electrolytic cell that has a silver anode, as shown in Figure 21-21. At the cathode, silver ions present in the electrolyte solution are reduced to silver metal by electrons from an external power source. The silver forms a thin coating over the object being plated. The anode consists of a silver bar or sheet, which is oxidized to silver ions as electrons are removed by the power source. Current passing through the cell must be carefully controlled in order to get a smooth, even metal coating. 

The anode is Zn metal, the cathode silver oxide (most coimnonly as Ag20) and the electrolyte an alkaline solution (20-45% KOH or NaOH). The overall reaction is ... 

However, for technical use of AFC, the long-term behavior of AFC components is important, especially that of the electrodes. Nickel can be used for the hydrogen oxidation reaction (catalyst in the anode) and on the cathode silver can be used as catalyst (see next section), no expensive noble metal (platinum) is necessary, because the oxygen reduction reaction kinetics are more rapid in alkaline electrolytes than in acids and the alkaline electrochanical environment in AFC is less corrosive compared to acid fuel cell conditions. Both catalysts and electrolyte represents a big cost advantage. The advantages of AFC are not restricted only to the cheaper components, as shown by Giilzow [1996]. 

The most common approach for the production of high purity silver is electrolytic refining. Anodes of silver dor6 are electrolysed using a silver nitrate electrolyte and pure silver is deposited on stainless steel cathodes. Silver deposits on the cathode in a dendritic crystalline form, which readily separates from the cathode by scraping and collects in the bottom of the cell. The anode is separated by a cloth membrane and retains the anode slimes. The slimes contain gold, platinum and palladium and are further processed to recover these metals. 

The German company Siemens later modified these electrodes with skeleton metal catalysts. Small amounts of titanium were added to the anodic nickel catalysts, and nickel, bismuth, and titanium were added to the cathodic silver catalysts. Fuel cells with such electrodes and a matrix electrolyte operated at 95°C and a current density of 400 mA/cm had a working voltage of 0.8 to 0.9 V. 

A wide variation is found in the detailed design and operating parameters of Moebius-type cells but the typical range of conditions may be summarized A cell may have between four and twenty cathodes operating at 100- 500 A and from -1 5 to —2 8 V The cells arc usually operated near ambient temperature with a cathode current density of 20--40 mA cm While the cathode silver purity is dependent upon anode quality, electrolyte purity and operating conditions >99 9% wt Ag is usually obtained, with 99.99% wt being realized in some cases. 

At the cathode Silver ions receive electrons, and form a layer of... 

Cathode Silver chloride Lead chloride Cuprous iodide Cuprous thiocyanate Cuprous chloride"... 

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