Mixed oxide cathodes

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

Transition-metal oxides and their mixtures are widely employed in numerous industrial applications, especially as cathode materials for batteries and fuel cells [1,2], Practice poses certain well-known requirements to oxide materials, first of all, to uniformity of the size distribution of particles, to homogeneity of mixed oxides, etc. To meet these demands, two broad categories of methods are now in use, vs (i) mechanical methods and (ii) chemical methods. 

Single and mixed oxides synthesised using these procedures are widely probed as cathode materials, superionics, magnetic materials, catalysts, and other multipurpose materials [4-20],... 

Although not yet optimized for C-C bond formation, these reactions demonstrated the potential for using mixed anodic oxidation - cathodic reduction sequences for initiating transformations that would be difficult to accomplish using more conventional chemical routes. It is hard to imagine putting a strong... 

Substituted nickel oxides, such as LiNii j /3ojAl/l2, are prime candidates for the cathode of advanced lithium batteries for use in large-scale systems as required for hybrid electric vehicles. On charging these mixed oxides the nickel is oxidized first to Ni + then the cobalt to Co +. SAFT has constructed cells with these substituted nickel oxides that have been cycled 1000 times at 80% depth of discharge with an energy density of 120—130 Wh/kg. ... 

Saturating the electrolyte with iron(lll) hydroxide (e.g., by addition of aqueous solutions of ferric nitrate) and simultaneously adding cobaltous salts leads to in situ formation of a mixed Fe(llI)/Co(ll)/Co(IIl) deposit, which exhibits catalytic activity comparable to that of Fe304 shown by the current voltage curve in Fig. 11. Such mixed oxidic catalyst coatings are composed of very small oxide crystals, which evidently are dissolved upon current interruption due to dissociative oxide dissolution. The transfer of dissolved metal ions to the cathode followed by cathodic deposition of the metal, however, can be completely prohibited, if the potential of the cathode due to optimal electrocatalysis of cathodic hydrogen evolution proceeds with an over-... 

For oxide electrocrystallization, the last condition is the most strenuous, sine many oxides are insulating non-stoichiometric compounds, however, are sufficient conductive. When two (or more) substances are codeposited, certain specific feature of the crystallization can be expressed for both cathodic and anodic processes on th basis of the thermodynamics of binary (or more complex) systems. If stabl multicomponent phases exist, then it is their deposition (not the deposition of mixture of simpler products) that preferentially proceeds in a certain potential regioi In such cases, intermetallic compounds are deposited in cathodic processes, and th deposition of mixed oxides takes place in anodic processes. These products ca represent both chemical compounds and solid solutions. 

Anodic and Mixed Anodic/Cathodic Voitammogroms Anodic waves as well as cathodic waves are encountered in voltammetry. An example of an anodic wave is illustrated in curve A of Figure 23-10, where the electrode reaction involves the oxidation of iron(II) to iron(III) in the presence of citrate ion. Note that by convention, anodic current is given a negative value. A limiting current is obtained at about -1-0.1 V, which is due to the half-reaction... 

III. ACTIVATION OF MIXED-CONDUCTING PEROVSKITE-TYPE OXIDE CATHODES... 

An example of the use of nanoionic materials as the cathode in a SOFC is that of nanotubes of Lao.6Sro.4Co03 and Lao.6Sro.4Coo.2Feo.8O3 [136]. These perovskite-type mixed oxides (see Chapters 9 and 12) are widely used as cathode materials, and the nanotubes were prepared by denitration, microwave irradiation and calcination at 800 °C. The shape and size of the nanotubes was controlled by the porous template used, although their application in a fuel cell was not reported. 

Fig. 4.13 Relationship of the mixed-electrode cathodic and anodic polarization curves (solid lines) to the oxidation and reduction components (dashed lines) of the individual anodic and cathodic reactions...

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