Lattice Relaxation and Electrochemical Performances of Cobalt-Doped Sr_0.9Ce_0.1MnO_3-δComposite Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells

Abstract

The relaxation mechanism of the octahedral distortion in Sr 0.9 Ce 0.1 Mn 1-x Co x O 3-δ (SCMCo) as a function of the cobalt content and temperature, and the electrochemical properties of the SCMCo-SDC composite cathodes were investigated. Substituted cobalt ions and manganese ions were in a mixed valence state consisting of Co 3+ /Co 2+ and Mn 4+ /Mn 3+ , respectively. The Co K-edges were shifted toward low energy by an increase in the Co content, which indicates that Co 3+ (0.55 Å) is reduced to Co 2+ (0.65 Å). The decrease in the difference in ionic radii between B-site elements relieved the octahedral distortion as confirmed by a decrease in the peak intensity in the Fourier transformed EXAFS of the Co K-edge. The lattice volume increased up to 1.4 and 1.9 vol.% with Co content and temperature, respectively. The Co-O bond angle was closer to 180 • C with an increase in Co content thereby leading to an increase in the oxygen ion conductivity. Therefore, the relaxation of the distortion by substitution of cobalt ions should affect the improvement in the oxygen mobility. The polarization resistances of the SCMCo-SDC composite cathodes at 650 • C were decreased up to four times with an increase in the cobalt content, and the power densities of the single cells (Ni-SDC/SDC/SCMCo-SDC) were also enhanced up to 177% with increasing cobalt content. However, the polarization resistance (R p ) can be increased through doping of cerium ions for structural stability and enhancement of the electrical conductivity. 4 The increase in R p is associated with a decrease in oxygen vacancy concentration and mobility, due to the strong hybridization between the Ce 4f/5d and oxygen 2p orbitals and an increase in the MnO 6 structural distortion, respectively. 5,6 We have investigated Sr 0.9 Ce 0.1 Mn 1-x Co x O 3-δ (SCMCo) to enhance the oxygen vacancy concentration and have identified the lower R p because of the increase in oxygen vacancy, which is a result of the mixed valence state of cobalt ions. 7 It is reported that BO 6 octahedral distortion can occur in most perovskite oxides (expressed as ABO 3 ) because of formation of oxygen vacancy and difference in ionic radii, which is occurred by temperature and mixed valence state changes of the transition metals. 6,8,9 Lattice changes attributed by the oxygen vacancy with elevated temperature can affect the conduction path for oxygen vacancies as a geometrical factor. 10 The difference in the ionic radii induced by the valence change of transition metals in the B-site is also another factor contributing to BO 6 distortion. We hypothesize that the difference in the ionic radius between cobalt ions and Mn ion in SCMCo has an effect on the octahedral change of BO 6 because cobalt ion has a stronger self-reduction property from Co 3+ to Co 2+ relative to other transition metal ions at elevating temperature. Therefore, the conduction properties of oxygen vacancies are a result of Co doping, in terms of the carrier mobility, based on the investigation of the effects of temperature and t...

Degradation behaviours of Fe-doped La0.8Sr0.2CoO3-δ by thermal stress

“…The resistance value at low-frequency offset of LSC increased to 3.82% after the heat treatment, while it increased to 3.31% in the LSCF system. The increase in resistance at low-frequency off set related to the oxygen diffusion at the gas/cathode interface, and surface diffusion of oxygen species [17]. LSC thus exhibited a lower polarisation resistance than LSCF, but the long-term stability against thermal stress was much higher for LSCF than for LSC.…”