Conductivity of Co-Ionic Composite electrolytes based on Oxygen-Ion and Proton Conductors

Solid oxide fuel cells (SOFCs) have attracted as clean power sources because of their have high energy conversion and fuel flexibility. In particular, the operation of SOFCs at intermediate temperature (IT) has lots of advantages such as fast start-up, reduced fabrication cost, extensive materials choice, and extended durability. IT-SOFC operations can be accomplished with highly ionic conductive SOEs at the IT.

Recently, SOFCs employ oxygen-ion or proton conductors as solid oxide electrolytes (SOEs). SOFCs based on oxygen-ion conductors such as yttria-stabilized zirconia (YSZ) and rare-earth doped ceria (RDC) have extensively researched. Until now, the most widely used SOE is YSZ. The YSZ SOEs present sufficient oxygen ionic conductivity at high temperature of about 1000°C. On the other hand, RDC SOEs present high ionic conductivity of the same order of magnitude at the IT. However, RDC SOEs exhibits mixed ionic-electronic conduction behaviors due to the reduction of Ce⁴⁺ to Ce³⁺at reducing atmospheres. This causes electronic transport within the SOE, resulting in significant decreasing cell voltage. 

Compared to oxygen conductors, SOFCs based on proton conductor such as yttrium-doped barium zirconate (BZY) and yttrium-doped barium cerate (BCY) have received attention as alternative IT-SOEs, lately. These perovskite-type oxides have high proton conductivity in humid hydrogen atmospheres at IT. In addition, the activation energy for ionic conduction is much smaller than the oxide ion conductor. However, BZY is hard to be sintered with low grain boundary conductivity. And BCY show poor chemical stability in atmospheres containing H₂O and CO₂.

Hence, in this work, composite SOEs, mixed with the oxygen-ion (e.g. RDC) and proton conductors (e.g.BZY), are studied for decreasing SOFC operating temperature. The conductivity of composite SOEs can be enhanced due to highly interfacial energy between oxygen-ion and proton conductors with extra defects by heterointerfaces of two materials.

1. H. Iwahara, H. Uchida, K. Ono, and K. Ogaki, J. Electrochem. Soc., 135, 529 (1988).

2. K.D. kreuer, Solid State Ionics,125, 285 (1999).

3. F. Iguchi, T. Yamada, N. sata, T. Tsurui, H. Yugami, Solid State Ionics, 177, 2381 (2006).

4. J. Tong, D. Clark, M. Hoban, and R. O’Hayre, Solid State Ionics,181, 496 (2010).

5. W. S. Wang and A. V. Virkar, J. power source, 142, 1 (2005).

6. R. B. Cervera, Y. Oyama, S. Miyoshi, K. Kobayashi, T. Yagi and S. Yamaguchi, Solid State Ionics,179, 236 (2010).

7. W. G. Coors, Solid State Ionics,178, 481 (2007).

8. D. shima, and S. M. Haile, , Solid State Ionics,97, 443 (2007).

9. C. Peng, J. Melnik, J. Luo, A.R. Sanger, and K. T. Chuang, Solid State Ionics,181, 1372 (2010).

10. H. I. Yoo, J. I. Yeon, and J. K. Kim, Solid State Ionics,180, 1443 (2009).

* Corresponding authors: jyoung@sejong.ac.kr (J.-Y. Park)