The high‐temperature zirconia electrolyte fuel cell is materials‐limited in both its performance and its range of potential application. The most significant materials limitations and problems occur with the cathode. Cathode materials must satisfy four general criteria: (i) chemical, (ii) electrochemical, (iii) mechanical, and (iv) economical. In this paper, the interaction between cathode materials and cell performance is considered quantitatively for three general classes of cathodes, viz., metals, oxides with embedded current collectors, and electronically conducting oxides.

The electrical conductivity of nickel oxide has been studied at temperatures between 600° and 1350°C and partial pressures of oxygen between 1 atm and 10−4 atm. The slope of log conductivity vs reciprocal temperature plots increases when the temperature is high enough for the sample to come into thermodynamic equilibrium with the atmosphere, and the temperature at which the change in slope occurs is observed to depend on the rate of heating or cooling of the crystal. The heat of formation of nickel vacancies may be determined from the magnitude of the change in slope by a simple calculation. Nickel oxide is a p-type semiconductor [M. Verwey, M. Haaijman, H. Romeijn, and M. van Oosterhout, Philips Research Repts. 5, 173 (1950)] in which the conductivity is proportional to the concentration of Ni3+ ions in the lattice. When thermodynamic equilibrium is established with the atmosphere the conductivity is proportional to the ⅙th power of the oxygen partial pressure. The analysis of the conduction mechanism and the conduction data may be self-consistently correlated with nickel diffusion, nickel vacancy diffusion, and weight changes resulting from equilibration with different oxygen partial pressures. The following constants may be determined from these correlations: heat of formation of nickel vacancies, activation energy for nickel vacancy migration, activation energy for electron hole migration, and concentrations of nickel vacancies and Ni3+ ions as a function of temperature and oxygen pressure. Combined results give the following for the concentration of nickel vacancies: 0.11 (PO2)⅙ exp(−17 800/RT) vacancies per ion pair.

A procedure is described in this paper for distinguishing in the measured electrical properties of polycrystalline ~-alumina, the separate contributions of the grain boundaries and of the crystal, i.e., the grain interiors. This separation is brought about through the use of a model for these properties. Certain quantitative consequences of the model are developed and compared with experimental results. For most sintered ~-alumina ceramic, the electrical properties are determined more by the characteristics of the grain boundaries than by those of the interior of the grains.Polycrystalline E-alumina is used in electrochemical devices to circumvent the deleterious high anisotropy in the electrical and mechanical properties of single crystals. Use of the polycrystalline ceramic does require, however, consideration of the effects of grain boundaries. A procedure is described here for distinguishing in the measured electrical properties of the ceramic, the separate contributions of the grain boundaries and of the crystal, i.e., the interior of the grains. This separation is brought about through the use of a model for the electrical properties of p-alumina. Certain quantitative consequences of this model are developed and compared with experimental results. Previous WorkThe conductivity both of /%alumina :single crystals and of polycrystalline ceramic has already received very considerable attention (1-15). Weber and Kummer reported that E-alumina exhibits high ionic conductivity, but no electronic conductivity (1). The ionic conductivity is due to the high mobility of sodium ions in planes perpendicular to the c-axis of the hexagonal structure. However, there is no conductivity parallel to the c-axis. They reported singlecrystal specific resistivity values of 30 and 3.5 ohm-cm at room temperature and 300~ respectively. Comparable values for polycrystalline ceramic prepared from single-crystal material was 25,0 and 18 ohm-cm. They attributed the difference to interface resistivity between the crystals of the polycrystalline material. The activation energy associated with the resistivity of single crystals was given as 3.8 kcal/mole (2).Imai and Harata, on observing that the activation energy associated with conduction in sintered /~-alumina was considerably larger than for single crystals, concluded that the conductivity of ceramic is governed by grain boundary conduction (4).Jones and Miles found that the Arrhenius ~olot curved significantly below 200~ (5). Another activated process with a higher activation energy controlled the conductivity at lower temperatures. These authors speculated that this process was grain boundary contact resistance.Whittingham and Huggins measured the conductivity of a single crys,tal from --150~ to 820~ (8).They found plots of log ~T to be linear in 1/T over this entire temperature interval. The conductivity was found to be 72 ohm-cm at 25~ The activation energy was 3.79 kcal/mole. They reported the conductivity to be sensitive to the presence of moisture below 5,0~ Imai and Harata repo...

Polarization observed on applying ac and dc electric fields to samples of β alumina, sodalime glass, and single crystals of sodium chloride have been studied. It is concluded that the large capacitances shown by these materials, which are generally attributed to space-charge effects, are more likely caused by reactions at the electrodes which build up cell potentials. Electrochemical models, involving these cell potential effects, are presented and compared with previous space-charge models which have been applied to solid electrolytes.

The electrical conductivity of magnesium oxide at temperatures in the region of 1300°C is observed to depend upon the partial pressure of oxygen surrounding the sample. The conductivity increases at oxygen pressures both higher and lower than 10-5 atmospheres. At this pressure the conductivity is a minimum. This effect is increased as the iron content is increased and is almost absent in the purest samples.The conductivity is electronic rather than ionic and the number of charge carriers is controlled by the number of lattice vacancies. The dependence of conductivity on oxygen pressure may be satisfactorily explained by changes in stoichiometry and thus lattice defects in magnesium oxide. These changes in stoichiometry are larger when the magnesium oxide is contaminated with a variable valence impurity like iron than when it is pure.