Cation Diffusion in Non-Stoichiometric (La, Sr)MnO3/CeO2 Nano-Composite Cathode

Abstract

Co-sintering of stacks or cells containing electrodes, electrolytes, and separators is essential fabrication technology to improve power density per unit volume significantly. Generally, cathode materials have low heat-resistant temperatures, and they are easily decomposed or degraded by sintering at a high temperature. (La0.8Sr0.2) MnO3 (LSM) shows relatively high resistance and preferable low-reactivity with fluorite electrolytes during sintering at a high temperature. But it shows lower cathodic properties than lanthanum strontium cobaltite or lanthanum strontium cobalt ferrite because of poor oxygen diffusion. Thus, we have investigated LSM/ceria nanocomposite to improve oxygen diffusion.Nanocomposite precursor powder containing LSM and ceria was synthesized by glycine method. The composite ratio of LSM and ceria was fixed at 9:1 (molar ratio). The dense composite compact was obtained by sintering the powder at 1200oC for 5 h in air, and fine ceria particles were homogeneously dispersed with LSM. Electrical conductivity and oxygen diffusion coefficient were estimated with these dense composites. Oxygen diffusion coefficients were estimated by electrical conductivity relaxation method. The amount of cation dissolved in ceria was investigated from XRD peak shifts. The defect of La or Sr in LSM formed by sintering with ceria was estimated with magnetic properties of LSM and LSM/ceria nanocomposite. The defect was also investigated using the relationship between activation energy and composition of a-site defect LSM or La-defect LSM.LSM showed a low oxygen diffusion coefficient, and the coefficient of LSM/ceria nanocomposite was much improved even though ceria composite ratio is 10%. However, LSM/ceria composite showed much lower conductivity than LSM since a-site cation of LSM, La or Sr, may be easily diffused into ceria. XRD peaks derived from only ceria were shifted toward lower angle for LSM/ceria nanocomposite prepared under various sintering conditions. The shift revealed that 2-5% La in LSM was diffused into ceria, assuming that only La is diffused. Diffusion of La into ceria increases in oxide ionic conductivity and the diffusion would benefit the composite's cathodic properties in terms of oxygen diffusion. However, if the diffusion would bring about La defect LSM and deviations from the optimum composition, the diffusion would be detrimental in terms of electrical conductivity.Then we investigated the effects of the diffusion on the defect structure of LSM in LSM/ceria nanocomposite by magnetic properties and electrical conductivities. Assuming that only La of LSM was diffused into ceria, the loss amount of La from LSM was estimated with magnetic phase transition temperatures for LSM and LSM/ceria nanocomposite. The temperatures were measured by SQUID. The overall loss amount of La from LSM was increased according to solid solution of La in ceria. But, the loss amount was slightly different from the solution amount to ceria.The ionic radius of Sr2+ is similar to that of La3+. Dissolution of Sr in ceria also leads to XRD peaks shift toward lower angle as well as that of La. Then, we investigate whether the loss from LSM was derived from only La (La-defect LSM) or both La and Sr ( a-site-defect LSM). La-defect and a-site-defect LSM dense ceramics with various defect concentrations were prepared by glycine method. Activation energy of these LSM ceramics was estimated from the temperature dependence of electrical conductivity measured in air. That of La-defect LSM was monotonically decreased with increasing defect concentration. A-site-defect LSM showed no significant change in activation energy within the defect concentration less than 8%. The relationship between activation energy of LSM/ceria nanocomposite and the defect concentration of LSM was measured using various sintering conditions. The relationship showed the activation energy of LSM/ceria was almost independent of the defect concentration. Therefore, Sr would simultaneously diffuse from LSM to ceria during sintering LSM/ceria nanocomposite. Electrical degradation of LSM/ceria nanocomposite would be derived from Sr defect of LSM as well as La.