Effect of Heterointerface on Cathode Performance of Solid Oxide Fuel Cells: (Sm,Sr)CoO3-Based Oxide
Authors: Manriki Kohei , Muroyama Hiroki , Matsui Toshiaki , Eguchi Koichi 
Published: February 2020

Solid oxide fuel cells (SOFCs) offer many advantages such as high energy conversion efficiency and high fuel flexibility. However, their high operating temperatures (700-900 °C) sometimes accelerate the degradation of components and result in a long time to start-up and shut- down time of the systems. Thus, many researchers have tried to reduce the operating temperature of SOFCs. The lowering of operating temperatures leads to the decrease in the reaction rate over electrodes. One possible approach to overcome this issue is the development of cathode materials with high activity for oxygen reduction reaction (ORR). Nowadays, mixed ionic-electronic conductors, a series of ABO3 perovskite-type materials such as (Ln,Sr)CoO3-δ (Ln: Lanthanoid), have attracted much attention as promising cathode materials due to their higher performance in comparison to a conventional cathode of (La,Sr)MnO3+δ. The previous studies on electrode kinetics of (La,Sr)CoO3-δ cathode revealed that the rate-determining step of the electrode reaction was the oxygen exchange process over the surface [1,2]. Recently, the oxygen surface exchange property at the heterointerface between (La,Sr)CoO3-δ and (La,Sr)2CoO4-δ (K2NiF4-type structure) has been studied. In particular, Sase et al. have reported that the rate of oxygen surface exchange was much faster at the (La,Sr)CoO3-δ/(La,Sr)2CoO4-δ heterointerface than at the surface of (La,Sr)CoO3-δ [3]. Yashiro et al. demonstrated high electrochemical performance of the heterointerface-introduced (La,Sr)CoO3-δ-based composite cathode [4].

In this study, then, we focused on (Sm,Sr)CoO3-δ, which has higher performance than (La,Sr)CoO3-δ, and examined the effect of Sm0.5Sr0.5CoO3-δ(SSC113)/SmSrCoO4(SSC214) heterointerface on the cathode performance. The oxide materials with different weight ratios of SSC113 and SSC214 were synthesized by mixing starting materials of metal nitrates in the desired ratios. The resulting composite oxide was mixed with Ce0.9Gd0.1O1.95 (GDC) in a weight ratio of 70:30. The symmetric cells consist of the obtained material as electrodes and GDC as an electrolyte were fabricated. The electrode performance was evaluated in air at 550-800 °C by the electrochemical impedance spectroscopy. The composite material with SSC113:SSC214 = 80:20 wt.% exhibited higher performance than SSC113. This was attributed to the promotion of electrode reaction. The 18O isotope exchange reaction was also carried out for dense pellets of SSC-based materials. The composite pellets of SSC113 and SS214 provided higher surface exchange coefficient than SSC113. These results indicate that the introduction of (Sm,Sr)CoO3-δ
/SmSrCoO4 heterointerface is one of the promising ways to design the high performance cathode for SOFCs operative at low temperatures.

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[2]T. Kawada, K. Masuda, H. Suzuki, A. Kaimai, K. Kawamura, Y. Nigara, J. Mizusaki, H. Yugami, H. Arashi, N. Sakai, et al., Solid State Ionics, 121, E252 (2002).

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