(Invited) Effects of Cobalt Co-Doping on Mixed Ion Conductivity in La(Sr)Ga(Fe)O3 for Oxygen Separation
Authors: Ishihara Tatsumi , Kobayashi Masahiro , Song Jun Tae , Takagaki Atsushi 
Published: December 2020

Mixed ion conductor shows oxide ion and electronic conduction simultaneously and expecting for application to oxygen separation membrane from air. Because of significant volume change depending on oxygen partial pressure, oxygen separation membrane using mixed ion conductor has disadvantage of narrow range of oxygen partial pressure resulting in the limited permeation rate. Therefore, increased oxide ion conductivity and mechanical strength have been required for mixed ion conducting membrane for oxygen separator. In our previous study, we found that Fe doped LaGaO3 shows reasonably high oxide ion conductivity and chemical stability. In particular, La0.7Sr0.3Ga0.6Fe0.4O3 (LSGF) shows largest oxide permeation rate from air to He. In this study, for further increase in oxygen permeation rate, effects of dopant to Ga site in LSGF was investigated. Application to CH4 partial oxidation was also studied.
All LSGF sample was prepared with conventional solid state reaction method using oxide as source materials. Partial substitution was performed for Ga site in LSGF and dense sample without impurity phase was confirmed by XRD measurement. For oxygen permeation measurement, La0.6Sr0.4CoO3 catalyst was coated on the surface of LSGF disk (Φ17mm diameter, 0.5mm thickness) It was found that the oxygen permeation from air to He was sensitively influenced by dopant and the oxygen permeation rate was increased as the following order, Co>Al>In>Ni>Mn at 1273 K. On the other hand, in case of Ru and Ir which is easily reduced to metallic state decreased the oxygen permeation rate significantly. Therefore, it was found that addition of 10 mol % of Co or Al is effective for increasing oxygen permeation rate of LSGF. Effects of Co in LSGF was further studied from oxygen permeation rate and the highest oxygen permeation rate was achieved at 3 mol% in Ga site of LSGC. On this optimized composition, La0.7Sr0.3Ga0.57Fe0.4Co0.03O3, shows 225 μmol min-1 cm-2 at 1273 K and this value is almost 2.5 times larger than that of LSGF. Effects of small amount of Co on electrical conductivity in LSGF were also studied. Although LSGF shows high hole conduction like log(σ/Scm-1)=0.6, the conductivity was increased by Co doping, in particular, at lower temperature. In contrast, Al doping decreases total conductivity and so, increase in oxygen permeation rate could be assigned to the increased mobility of oxide ion in LSGF.
Application of Co doped LSGF to CH4 partial oxidation was further studied. Although optimized amount of Co doped for Ga site in LSGF is 3 mol%, the membrane was broken when 3 mol% doped LSGF was used for oxygen membrane. 1% Co doped LSGF was applied for oxygen permeation membrane and it was found that amount of oxygen permeation rate was 1.5 times increased comparing with LSGF. In addition, no crack was formed in membrane. The amount of oxygen permeation was achieved 364μmol cm2 min-1 which is corresponded to 8.9ml/min cm2 at 1273K. In summary, this study reveals that doping small amount of Co to Ga site of LSGF is highly effective for increasing oxygen permeation rate without decreasing chemical stability.