Oxygen reduction reactions on a mixed conducting lanthanum cobaltite, (La,Sr)CoO₃: LSC, have been widely investigated both from scientific interest and from the technological importance as a cathode of a solid oxide fuel cell. Since the reaction pathway is not simple in a practical porous electrode, model electrode approaches have been adopted by many researchers. However, the reaction mechanism and rate determining processes may not be always identical between model and porous electrodes, which can mislead the research and development.

The authors have been making efforts to improve oxygen reduction activity of LSC by utilizing the promotion effect of a co-existing Ruddlesden-Popper phase, (La,Sr)₂CoO₄ : LSC214, which was first observed at the surface of a dense ceramic LSC(1). The concept worked for a dense film electrode but not for a practical porous electrode except when the initial performance of LSC was somehow poor(2). The difference between dense and porous LSC may account for the results. The followings are what we found in the course of the research:

1. The oxygen exchange rate around on LSC was faster around the thermally precipitated LSC214 particles. (1)

2.Application of a porous LSC214 layer on a dense film LSC electrode showed increased reaction rate and decreased activation energy. (3)

3. Application of a porous LSC214 layer on a porous LSC electrode appeared to improve the performance when the base LSC layer had somehow large electrochemical resistance. (2)

4. Well-prepared LSC electrode showed the electrochemical resistance of about 0.5 ohm cm2 in air at 600˚C, and no further improvement was achieved by application of LSC214 layer on the electrode ore mixing LSC214 particles in the electrode.

5. Effective utilization length of a porous LSC electrode evaluated by the electrochemical impedance was thinner than expected from the surface reaction and bulk diffusion constants of dense ceramic LSC.

6. Surface reaction rate of a dense LSC decreased significantly with Sr segregation in the surface/sub-surface layers.

7. Strontium on the surface of LSC film appeared to be removed when it was in contact with porous LSC214 layer. Based on the above observations, the difference of dense and porous LSC electrodes will be discussed in terms of Sr segregation and in the role of LSC214.

References

(1) M. Sase, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, N. Sakai, K. Yamaji, T. Horita, H. Yokokawa, Solid State Ionics 178(35), 1843 (2008).

(2) K. Yashiro, T. Nakamura, M. Sase, F. Hermes, K. Sato, T. Kawada, J. Mizusaki, Electrochem. Solid State Lett. 12(9), B135-B137 (2009).

(3) T. Kawada, T. Tairako, K. Nagao, A. Unemoto, M. Sase, K. Yashiro, S. Hashimoto, and K. Amezawa, Proc. 16th Intern. Conf. Nanotechnology, Sendai, Japan, August 22-25, 2016