Electrochemical Performance of La0.6Sr0.4TixFe1-XO3 -Based Composite Cathode Materials for Solid Oxide Fuel Cells

Introduction

SOFCs have received a lot of attention as promising electrochemical power generation devices due to their high electrical efficiency and adaptability to a variety of fuels such as hydrocarbons, ammonia, and hydrogen. The utilization of LaSrTiFeO3-δ perovskite oxide was proposed as an oxygen permeable membrane and electrode materials of SOFCs [1,2]. Durability in thermal cycling test is an important factor for applying SOFCs to power sources. The reduction of operating temperature and enhancement of thermal-cycling capability are indispensable requirement for industrial-scale commercialization of SOFCs.

 SOFCs are multilayer structure consist of an electrolyte layer inserted between anode and cathode layers. These layers should be well matched for thermal expansion coefficient (TEC) to prevent cracking and delamination during operation and cycling.However, the TEC values of cathode materials with high catalytic activity are much higher than SOFC electrolytes.TEC mismatch between electrolyte and cathode also will cause a fracture on the cathode layer owing to thermal stresses developed on heating and cooling.[3]

 In this study, In order to suppress the delamination and cracks, Cathode material consist of La_0.6Sr_0.4Ti_xFe_1-xO_3-δ (0<x<0.4) combined with La_0.6Sr_0.4Co_yFe_1-yO_3-δ(0<y<1) for SOFC was developed.

Experimental Details

NiO-Yttria-stabilized Zirconia (YSZ, 8 mol % Y₂O₃) cermet (NiO:YSZ��60:40 wt%, 600 μm thick) was used as an anode substrate, Gadolinium-doped Ceria (GDC, 10 mol % GdO₂) as electrolyte materials, and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) ,  La_0.6Sr_0.4Ti_0.3Fe_0.7O₃(LSTF) as cathode materials. YSZ (8 μm thick), GDC (3 μm thick) were prepared by screen printing on NiO-YSZ anode support and co-firing at 1350°C. Cathode films (30 μm thick) were prepared by screen printing on (NiO-YSZ / YSZ / GDC) substrate and firing at 1000°C

Thermomechanical analysis and I-V characteristics were measured by impedance spectroscopy.

Results and Disscusions

Two cathode films were prepared. The first one was prepared using LSCF, meanwhile the other one was prepared using composite of LSCF and LSTF. Figure 1 show the SEM images of both samples. It was found that LSCF cathode surface contain fracture. On the other hand, LSCF and LSTF composite cathode does not contain fracture and delamination. The TEC value of LSCF (~ 17× 10^{− 6} K^{− 1}) is larger than about that of the cell (12 ~ 13× 10^{− 6} K^{− 1}), whereas the TEC of LSTF ( 11~ 12× 10^{− 6} K^{− 1}) is low . The composite of LSCF and LSTF offer a solution for this problem indicated by the TEC value is close to the cell. As a result, cracks are suppressed. It was found that the optimum composition for cathode material is LSCF 70 wt% - LSTF 30wt%.And be in contact with the fuel gas(H₂,NH₃),crstal structure of LSCF-LSTF is stable.

Conclusions

We developed Cathode material consist of LSTF combined with LSCF. It was found that LSCF:LSTF=70:30 was optimal composition in terms of performance and durability.

By structure and composition control , to achieve a further improvement of performance and durability.

Acknowledgement

This work was supported by Council for Science, Technology and Innovation(CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier”(Funding agency:JST).

References

[1] Y. Takahashi et.al., Solid St ate Ionics, 181 (2010) 300–305.

[2] Y. Takahashi et.al., Solid State Ionics, 181 (2010) 1516–1520.

[3] K. Park et.al., International Journal of Hydrogen Energy, 35,(2010) 8670–8677