Crystal Structure and Electrical Properties of Transition Metal-doped (Mn,Co)3O4 Spinels

Chromia forming alloys are widely used as the interconnect for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, their high temperature oxidation and volatile Cr species migration can cause cell performance degradation. To solve these problems, many efforts have been made such as alloy composition optimization, surface modification, and applying ceramic coatings. This work has focused on the latter method using the (Mn,Co)3O4 based system as the coating candidate. In our study, a Cr-containing spinel reaction layer formed between the (Mn,Co)3O4 coating and the Cr2O3 scale on the alloy surface during long-term exposure, and Fe and Ti doping in (Mn,Co)3O4 were found reduced growth rate of the reaction layer. Since doping can affect the coating performance and electrical conductivity is an important property for SOFC components, further work is needed to investigate the effect of transition metal (TM) additions on the electrical property of (Mn,Co)3O4 spinel. Several TMdoped (Mn,Co)3O4 spinel oxides were prepared to identify a potential interconnect coating that would block chromium migrations, but also have sufficient electrical conductivity. The (Mn,Co,TM)3O4 (TM=Fe, Ni, Cu) spinel oxides were synthesized by solid state reaction. Their phase structures were determined by x-ray powder diffraction at room temperature, and their electrical conductivities were measured from 500 to 900C in air by a four-probe d.c. method. The conductivities of spinel samples increase with increasing temperature in a linear relationship between ) log( T σ and 1000/T, which suggests that conduction occurs by a small polaron hopping mechanism. The transition metal additions affect both the crystal structure and the electrical conductivity of (Mn,Co)3O4 spinel oxide. The conductivities of chromite spinels are much lower than those of (Mn,Co)3O4 , so the electrical conductivities of (Mn,Co,TM,Cr)3O4 spinel oxides, which may form in the reaction layer and thus affect the area specific resistance of the cell, will also be presented.