Bulk Properties of the Oxygen Reduction Catalyst SrCo_0.9Nb_0.1O_3-δ

The perovskite SrCo_0.9Nb_0.1O_3-δ (SCN) has excellent electrochemical activity toward oxygen reduction, and it is also valuable as a possible model material for other state-of-the-art perovskite catalysts based on strontium and cobalt, such as Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF). Here we report thermogravimetric, conductivity, and diffraction measurements from SCN. We find that the thermodynamic stability limits of SCN are slightly more favorable than those reported for BSCF, although both materials exhibit a slow oxidative partial decomposition under likely operating conditions. In SCN, this decomposition is thermodynamically preferred when the average formal oxidation state of cobalt is greater than ∼3.0+, but due to sluggish kinetics, metastable SCN with higher cobalt valence can be observed. The oxygen stoichiometry 3-δ varies from 2.45 to 2.70 under the conditions studied, 500-1000 °C and 10^-4-1 bar O₂, which encompass both stable and metastable behavior. The electronic conductivity is p-type and thermally activated, with a value at 600 °C in air of 250 S cm^-1, comparable to that of La_0.8Sr_0.2MnO_3-δ. The polaron migration enthalpy decreases linearly from 0.30 to 0.05 eV as 3-δ increases from 2.52 to 2.64. Thermal and chemical expansivities are also reported.