Anodes attributes/requirements

This chapter first considers the complex mix of attributes required of SOFC anodes, including matching of thermal expansion coefficients, chemical compatibility with the electrolyte and the interconnect, porous structure to allow gas permeation, and corrosion resistance to the fuel and impurities therein. Then the nickel cermet anode is described in detail, especially its fabrication processes. Steady-state anode reactions of hydrogen and carbon monoxide are analysed, followed by a description of transient effects. Finally, behaviour under current load and operation on different fuels are discussed. The details of the anode reactions and polarisations are described in Chapter 9. 

For these reasons alloying elements appear in all the commercial anodes, and very careful quality control is required to keep disadvantageous tramp elements (notably iron and copper) below defined threshold levels. Many anode failures can be attributed to poor production quality control. A guide to minimum quality standards has been produced ... 

The passivating effect is attributed to the formation of an insoluble oxide film on the silicon. The chemical attack by the hot alkaline solution on the silicon does not resume immediately after the anodic bias is removed. Several minutes are required before the passive layer breaks down and chemical attack resumes. A brief cathodic treatment, however, reactivates the silicon immediately. 

These studies indicate that the selectivity of a-aminoalkyl radical formation is Me > Et /-Pr, which is the opposite of that expected on the basis of radical stability. For example, in diisopropylmethylamine, methyl adducts are formed exclusively. Similar selectivity has been observed for oxidation of unsymmetrical amines by ferricyanide [43, 123] and in anodic processes [124]. This selectivity has been attributed to requirement of overlap between the half-vacant nitrogen p orbital and the a-CH orbital of the a-carbon. From the Newman projections below it can be seen that the conformation necessary for methyl deprotonation (a) is lower in energy than that for the isopropyl deprotonation (b) [5, 122]. 

The principles involved in expressing in this way are evidently quite analogous to those that define Brpnsted s a factor (cf. Ref. 2). However, one matter requires special mention in the case of electrode reactions, the free-energy change of the initial state of a cathodic electrochemical reaction or of the final state of an anodic one associated with a change of potential has, in previous work, been almost entirely attributed to the change the Fermi level of electrons ... 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

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