Development of Protonic Ceramic Conductors for SOFC and Sensor
Authors: Taniguchi Noboru , Kuroha Tomohiro , Mikami Yuichi 
Published: February 2020

In recent years, commercialization of residential fuel cell "Ene-Farm" has begun in Japan, and SOFC using zirconia as electrolyte is also being introduced into the market. However, in order to disseminate residential and commercial cogeneration systems or mono-generation systems, a more reliable and lower cost system is required, which is considered the key to widespread use. We have been researching and developing fuel cell systems that can realize high reliability and low cost with SOFC since 1989. As an SOFC concept that realizes high reliability and low cost, we consider an SOFC that operates at 600 ° C, which can use inexpensive and highly durable metal materials, and while maintaining high efficiency of SOFC, We aimed to develop SOFC that realizes reliability and low cost. In order to construct an SOFC operating at low temperature, an electrolyte which is sufficiently functional at low temperature and which is chemically and physically stable is required. Proton conductors found by Iwahara et al in the 1980s had low ionic conductivity and no chemical stability was also investigated. However, it was expected that ionic conduction activation energy is low, ionic conduction occurs even at a relatively low temperature, and fuel utilization rate as SOFC can be improved.

Therefore, we focused on development of perovskite type proton conductors with high ion conductivity and high durability, and developed them. The target ionic conductivity was 1.0 × 10 -2 S / cm or more, and the chemical stability was aimed not to cause composition and phase change in a fuel gas atmosphere containing water vapor, hydrogen, carbon dioxide gas1-3). Results of examining the moisture resistance, carbon dioxide resistance and conductivity of samples of 348 kinds of BaCe1-xMxO3-α, SrCe1-xMxO3-α, BaZr1-x MxO3-α, CaZr1-x MxO3-α, BaZr1-x-y CexMyO3-α , Finally, it was found that BaZr 0.8 M 0.2 O 3 - α is the best proton conductor from the viewpoint of durability and conductivity, but BaZr 0.4 Ce 0 .4 In 0 .2 O 3 - α can be used as an electrolyte of a hydrogen sensor in air and in a reducing atmosphere. By using BaZr 0.4 Ce 0 .4 In 0 .2 O 3 - α as an electrolyte, platinum for the anode, gold aluminum electrode for the cathode and holes for rate limiting hydrogen diffusion on the anode side, a sensor capable of linearly detecting the hydrogen concentration in the atmosphere was constructed. The sensor was able to detect hydrogen concentration linearly from 0% to 5%4,5).

On the other hand, when used as an electrolyte of a fuel cell, it is necessary to confirm that the material of the anode and the cathode do not react with the proton conductor. Because it is made by firing cells at high temperature. We fabricated proton conductors using In, Y, Yb, etc. as substitutional elements of M and investigated the conductivity and reactivity with Ni of anode materials. At the present time, it was found that BaZr0.8 Yb0.2O3 - α, which replaced M with Yb at 20%, was the most stable and had high protonic conductivity. Also, in the cell using BaZr 0.8 M 0.2 O 3 – α proton conductor as the electrolyte, Ni cermet as the anode and LaSrMnO 3 as the cathode, the maximum output of 0.42 W / cm 2 at 600 ° C. could be obtained.

References

1) N. Taniguchi, et al., Solid State Ionics, 53-56, 998-1003, (1992) 2) N. Taniguchi, et al., J. Electrochem. Soc., 143(6), 1886-1890, (1996) 3) N. Taniguchi, et al., J. Electrochem. Soc., 145(5), 1744-1748, (1998) 4) N. Taniguchi, et al., Solid State Ionics, 145, 349-355, (2001) 5) N. Taniguchi, et al., Solid State Ionics, 176, 2979-2983, (2005)