Electrical conductivity and chemical diffusivity of NiAl2O4 spinel under internal reforming fuel cell conditions

Abstract

NiAl₂O₄ spinel was formed by solid state reaction. Its electrical conductivity was measured in the temperature range of 680–940 °C and under various oxygen-rich environments, as well as under reducing conditions. From the temperature dependence of the conductivity, the activation energies for conduction increase for decreasing oxygen partial pressures. From the partial oxygen pressure dependence, the defect structure of the material was analysed. The conductivity change with respect to P _O2 can be attributed to singly and doubly ionized nickel vacancies. The chemical diffusivity of the oxide was determined by conductivity relaxation upon abrupt change in P _O2 in the surrounding atmosphere. The oxygen chemical diffusion coefficient is of the order of magnitude of 10⁻⁴ cm² s⁻¹.