Influence of Surface/Interface on the Performance of MIEC Cathode for SOFC

La_0.6Sr_0.4CoO₃ has been studied as an SOFC cathode because of its high mixed ionic electronic conductivity. The mixed conductivity makes the zone of oxygen reduction reaction extended from triple-phase boundary to two-phase boundary. The rate limiting step of the cathode reaction is oxygen reduction reaction on two-phase boundary (1). Sase et al. reported that the oxygen reduction is activated at hetero-interface of La_0.6Sr_0.4CoO₃ and La_1.5Sr_0.5CoO₄ (2). The hetero- interface effect leads to development of higher performance electrode. However, the detail mechanism is still not clear.

This study aims to elucidate the mechanism of the hetero-interface effect by the relationship between the electrode performance and the surface chemistry.

Thin-film electrodes were fabricated on the electrolyte substrate of Ce_0.9Gd_0.1O_1.95 (10GDC) by pulse laser deposition (PLD) method. The two-layer electrodes were made: the first layer was La_0.6Sr_0.4CoO₃ (LSC40); the second layer, La_1.5Sr_0.5CoO₄(LSC155), was prepared exactly onto the first layer. Additionally single layer electrodes of LSC155 and LSC40 were also formed on the same 10GDC electrolyte to evaluate the hetero- interface effect. Several samples were prepared with changing thickness of LSC155.

A micro probe apparatus equipped with a potentio-galvano stat and a frequency response analyzer was employed for the electrochemical measurement. The measured electrode could be selected by manipulating probes of the micro prober. This setup enables to measure four electrodes simultaneously without changing the sample and its thermal history. The thin-film electrodes, the porous Pt on the side surface, and the porous Pt on the opposite side of the sample were used as working, reference, and counter electrodes, respectively. The measurement conditions were at 873, 973, and 1073 K, in air.

The electrode reaction resistances of the hetero-interface electrodes were measured by an impedance measurement. It was expected that the resistance of the double layer electrodes should become larger with increase in thickness of LSC155 due to longer ionic diffusion pathway. However, the measured electrode resistances did not depend much on LSC155 thickness. Electrode resistances of most of the double-layered electrodes were smaller than those of the LSC155 single-phase electrode in spite of LSC155 thickness. This phenomenon implies the electrochemical properties of LSC155 film on LSC40 might be modified and different from those of LSC155 film on a substrate

Focusing on the LSC40 single-phase electrode, the electrode resistance increased from 873 to 973 K. It may be the influence of rearrangement of cations, which can occur over 923 K: the deposition temperature of the electrodes. The resistances of the layered electrodes were smaller than that of LSC40 single-phase electrode. Element distribution was evaluated by EDX analysis. The enrichment of strontium was observed at the surface grains of the degraded LSC40. The concentrations of lanthanum and cobalt were lower at the surface grains. This means the surface composition changed from that of bulk. Therefore, the segregation of strontium on the surface of LSC40 may degrade the electrode performance.

This research was supported by JST-PREST and JST-CREST project.

References

1. T. Kawada, K. Masuda, J. Suzuki, A. Kaimai, K. Kawamura, Y. Nigara, J. Mizusaki, H. Yugami, H. Arashi, N. Sakai, H. Yokokawa, Solid State Ionics, 271–279, 121 (1999).

2. M. Sase, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, N. Sakai, K. Yamaji, T. Horita, H. Yokokawa, Solid State Ionics, 1843–1852, 178 (2008).