Electrocatalysts silver

Silver films and Ag-CaO-SragOg cermets were chosen as the anodic electrocatalysts because of their high electrical conductivity, which is necessary for electrocatalytic operation, and also because of their high (>95%) selectivity to Cg hydrocarbons at very low (<2%) CH conversions [9]. 

But think what happens when a piece of copper is immersed in a silver nitrate solution (Fig. 7.118) and then made an electron-source electrode. The electronation of Ag+ ions to silver metal takes place on the copper, and the reddish copper surface becomes coated with a silvery color. A cross section of the electrode shows that the electrode surface has advanced toward the solution (Fig. 7.119). Silver has electro-crystallized on the copper. Thus, the copper electrode has not behaved as an electrocatalyst ithasbeen altered by electrociystallization. It is not simply an electron source. 

Rondinini, S., Mussini, P.R., Muttini, P. and Sello, G. (2001b) Silver as a powerful electrocatalyst for organic halide reduction The critical role of molecular structure. Electrochim. Acta, 46, 3245-3258. 

The electrolyte in this fuel cell is concentrated (85 wt.%) potassium hydroxide in fuel cells operated at high temperature ( 250°C) or less concentrated (35-50 wt.%) potassium hydroxide for lower temperature (<120°C) operation. The electrolyte is retained in a matrix (usually asbestos) and a wide range of electrocatalysts can be used, for example, nickel, silver, metal oxides, spinels, and noble metals. The fuel supply is limited to nonreactive constituents except for hydrogen. Carbon monoxide is a poison and the produced carbon dioxide (in the case of having carbon monoxide) will react with the potassium hydroxide to form solid potassium carbonate, thus altering the electrolyte. Even the small amount of carbon dioxide in air must be considered as a problem in the alkaline cell. 

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. 

Procedures suitable for the incorporation of chemical or biochemical substances or ion-exchange sites onto an electrode surface have been developed in various laboratories. " Such modified surfaces can then be used to perform a variety of functions. They have, for instance, been employed to preconcentrate analytes prior to voltammetric analysis and hence impove sensitivity. For example, dimethylglyoxime has been used to preconcentrate nickel and EDTA to preconcentrate silver." If this preconcentration can be achieved in an environment conducive to rapid electron transfer, then sensitivity will be enhanched even further. Alternatively electrocatalysts have been attached to electrode surfaces, and by speeding up what would otherwise be a sluggish electron transfer process, increased sensitivity has been attained. However, this approach yields no added selectivity and a large background must usually be tolerated. 

Gamburzev S, Petrov K (2002) Silver-carbon electrocatalyst for air cathodes in alkaline fuel cells. J Appl Electrochem 32(7) 805-809... 

The catalytic synthesis of acetaldehyde by partial oxidation of ethanol is actually an industrial process, the Veba-Chemie process, in which ethanol vapor—air mixture is passed over a heterogeneous silver catalyst at 500—650 °C. In fact, acetaldehyde is also a co-product in the conventional direct ethanol fuel cell, with acetic acid and CO2, because of the difficulty of C—C bond breakage even on Pt-based electrocatalysts, but the selectivity is low (Rousseau, Coutanceau, Lamy, L6ger, 2006). 

The reduction of molecular oxygen that is supplied either directly from containers or in a diluted form as air constitutes the reaction at the cathode in fuel cells. The use of air is preferable for economic reasons. Platinum metals and alloys of platinum metals are electrocatalysts for acid and alkaline electrolytes. Silver, silver alloys, nickel, carbon, and intermetallic compounds represent less expensive electrocatalysts for the oxygen electrode in alkaline solutions. In contrast to the hydrogen electrode, the overvoltage of the oxygen electrode is large at temperatures below 100 °C when a reasonable current is drawn. 

Steady-state i— U curves of the O2 reduction in 6 M KOH at 25 °C are compared for Ag, C, and Ni in Fig. 79. The curves were taken [62] by ViELSTiCH on diffusion electrodes prepared according to the techniques of JuSTi and coworkers [63]. The potential f/ ai was measured versus a saturated calomel electrode. The silver electrode shows the best performance. Carbon is a better electrocatalyst than nickel in 6 M KOH. The limiting current due to transport processes was not reached in the measurements. 

Carbon can be used [7,11,62,70—73] as an electrocatalyst for the O2 reduction in alkaline electrolytes (compare also section 5 in chapter VIII). The performance which is not so good as that of silver (see Fig. 79) appears adequate for certain purposes, for instance, in small zinc-air cells. Activation procedures [72,73] which are not of an electrochemical nature improve the performance of carbon oxygen electrodes. The performance rapidly becomes poor with decreasing pH below pH <14. In acid solution, the impregnation of carbon with platinum metals or other electrocatalysts is required. The data [73] in Table 8... 

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... 

Such dependences are also consistent with the catalytic efficacy of silver or platinum in the case of many oxygenation processes, such as the incorporation of oxygen in oxides that we have taken as our master-example. It is also striking that many good catalysts are mixed conductors this also applies to electrocatalysts which are treated in Chapter 7. This suggest that not only the concentration but also the mobility of electronic (redox centres) and ionic defects (acid-base centres, see below) is important . When this is the case, the local conditions at the reaction... 

---