Silver cathodes, oxygen reduction

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

G. Silver Cathodes for Oxygen Reduction in Alkaline Electrolyte... 

Several molecules were investigated because of the applicability in a system of interest. Silver oxide was investigated by Kotz and Yeager" for the significance of the silver/silver oxide electrode as a cathode in batteries and as a catalyst for oxygen reduction. Mercaptobenzothiazole was studied" " for its role as a corrosion inhibitor for several metals. Von Raben et could follow... 

Noble metals applied as electrocatalysts for the oxygen reduction have been largely utilized because of their high electrocatalytic activity and stability. Investigations are concentrated on platinum, palladium, silver and gold. The application of noble metal catalysts is limited by two fundamental disadvantages high cost and low availability. Thus, it is important to construct cathodes with small amounts of the noble metal which are obtained, for example, by dispersed platinum on an appropriate support. 

N03 in the chamber resulting from the reactions of NO with O2 and 02 the latter is an intermediate product of electrodic reduction of oxygen in the silver cathode. Accordingly, the concentration of NO measured with a Clark probe in a biological medium will usually be one to two orders of magnitude lower than the concentration measured with a porphyrinic sensor. The highest sensitivity for NO is obtained at a potential of approximately 0.9 V. 

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]. 

PO2 can be determined by a means of a PO2 electrode (Clark electrode). In this technique, oxygen diffuses from the blood sample across a gas-permeable membrane into an electrochemical system which consists of a platinum cathode and a silver/silver chloride anode. Reduction of the oxygen occurs at the cathode, resulting in the generation of a current which can be measured, the current being directly proportional to the PO2. This is an example of an amperometric technique... 

The IBM group led by Brusic et al. [57,58] also studied the use of polyaniline derivatives for corrosion protection of copper as well as silver. The unsubstituted polyaniline, in neutral base form, provided good corrosion protection both at open-circuit potential and at high anodic potentials. The dissolution of metal (both Cu and Ag) was decreased by a factor of 100 when the metal surface was completely covered by the neutral polyaniline. However, polyaniline doped with dodecylbenzene-sulfonic acid (the conductive form of the polymer) increased the corrosion rate of Cu and Ag in water. The doped polymer in contact with the metal is spontaneously reduced at a rate faster than the oxygen reduction rate. The faster cathodic process in turn increases the overall rate of the anodic reaction, which is the dissolution of Cu and Ag, as opposed to the formation of a passive oxide layer. 

The silver cathode is inert, unless oxygen or another reducible species can diffuse to it. A semipermeable membrane through which only oxygen can diffuse surrounds the electrodes, and then the reduction reaction takes place. 

The properties of the interface at which the formation of oxide ions occurs have been of special interest [6, 7, 28—35]. While solid electrocatalysts, Pt [28, 29, 31, 32] and C [30], were studied mainly, a molten silver cathode was employed in another type of zirconia-electrolyte fuel cell developed [34,35] at the General Electric Research and Development Center in Schenectady. Since the hindrance of the electrochemical steps of the O2 reduction at the cathode surface is small [28, 32] on platinum around 1000 °C, it is hard to elucidate the reaction mechanism beyond the net reaction 1. Analysis [33] of the potential distribution curves inside Zro 9Yo 2 02.i in contact with two platinum electrodes showed at 1380°C that the electronic hole contribution to the conductivity in the bulk of the specimen depended upon as would be expected from the equilibrium of reaction 15. The partial oxygen pressure had values between 10 and 10 atm. However, if the production of oxide ions is assumed to occur at the cathode solely by reaction 15, the rate of production is much lower than the rate of loss at the anode. A cathodic reaction of the type... 

It was considered that hydrogen peroxide is formed at silver and chalcocite surfaces when oxygen is present as this compound is an intermediate in the cathodic reduction of oxygen. Clearly, then, there is interaction between the two reactions that make up the mixed potential system, namely, xanthate oxidation and oxygen reduction. Thus, care must be taken in considering the individual processes in isolation. 

Multiphase gold or palladium-based alloys never show dissolution of Au or Pd but often exhibit progressive surface ennoblement due to selective dissolution of copper or silver from the outer 2-3 atomic layers Heat treatment often decomposes multicomponent alloys into a Pd-Cu rich compound and an Ag-rich matrix with corrosion of the latter phase in deaerated artificial saliva and S -containing media . Au-Cu-rich lamellae have similarly been observed, again with preferential attack on Ag-rich phases or matrix. These effects presumably arise from the ability of the noble alloy phases to catalyse the cathodic reduction of oxygen . 

Silver and gold, which are corrosion resistant in many solutions, are rather efficient catalysts for the cathodic reduction of oxygen and certain other reactions. Some sp-metals (mercury, tin, zinc) exhibit interesting catalytic properties for the cathodic reduction of CO2. Copper might be a very interesting material for a number of electrochemical reactions, but so far has not been examined thoroughly. 

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