Cobalt oxide diffusion coefficient

A number of metals, such as copper, cobalt and h on, form a number of oxide layers during oxidation in air. Providing that interfacial thermodynamic equilibrium exists at the boundaries between the various oxide layers, the relative thicknesses of the oxides will depend on die relative diffusion coefficients of the mobile species as well as the oxygen potential gradients across each oxide layer. The flux of ions and electrons is given by Einstein s mobility equation for each diffusing species in each layer... 

From this it is evident that the diffusivity of La2Ni04+d was competitive with current mixed conducting perovskites with diffusion coefficients of the order of 10 ernes. Evidently the identification of a new mixed conducting ceramic warranted further optimization and hence substitution of both A and B sites was investigated [4,5,7,8,14,18]. Introducing a divalent cation to either the A or B site was expected to reduce the hyperstoichiometry and it was therefore unsurprising to find that the presence of Sr lowered the oxygen content and, as the excess oxide species were viewed as the mobile species, the diffusivity [7]. The effects of a number of dopants are illustrated in Table 1. Most notably the incorporation of cobalt was shown to enhance the low temperature diffusivity of these materials [5, 18] and these materials may prove to be attractive candidates for lower temperature cathode materials (<750 °C). 

The diffusion coefficient of radioactive Co tracers in a single crystal of cobalt oxide, CoO, is given in the Table 7.7(c). Estimate the activation energy for diffusion. 

A more complete test of the Wagner mechanism and treatment has been carried out by Fueki and Wagner and by Mrowec and Przybylski who derived values for the diffusion coefficient of nickel in NiO and cobalt in CoO from measurements of the parabolic oxidation rate constant. Since the parabolic rate constant can be expressed in the form of Equation (3.48),... 

Small Cr contents increase the rate of reaction, but at 20% Cr, the reaction rate starts to decrease and exhibits a minimum value at 25%-30% Cr (Figure 20.62). The minimum value depends on the pressure. More chromium is needed to stabilize a protective film since the diffusion coefficient of chromium in cobalt is lower than for chromium in nickel. However, since the adhesion strength of the film on Co-Cr alloys is poorer than on Ni-Cr alloys despite the identical oxidation rate of the Cr-containing Co alloys with Cr203 protective film, the practical oxidation resistance is lower. Other alloying elements, as Figure 20.62 shows, have little influence on scale resistance. 

To provide electronic coupling between the incorporated CoP(pyH)44+ counterions and the graphite electrode surface it was necessary to add to the coatings suitable redox mediators wiUi much larger diffusion coefficients. In Figive IB is shown the cyclic voltammetric response obtained from a Nafion coating which contained Os(bpy)32+ (bpy = 2,2-bipyridine) and Ru(NH3)e3+ as well as CoP(pyH)44+. The pair of current peaks near —0.25 V arises fix)m the Ru(NH3)63+/2+ couple and the pair near 0.6 V from the Os(bpy)33+/2+ coimle. This pair of mediators was chosen because they both have reasonable diflusional rates in Nafion and their formal potentials lie on either side of the formal potential of the CoP(pyH)4 couple (—O.IV) so that the cobalt center of the porphyrin could be repetitively cycled between its oxidized and reduced states by me mediator counterions which diffused to and from the electrode surface where they were oxidized or reduced. 

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