Electrochemical Properties of Co and Yb Doped BaZrO3 and Its Application to PCFC Cathodes
Authors: Okuyama Yuji , Mikami Yuichi , Nishiya Tomoshiro , Kasuga Keita , Goto Takehito , Asano Hiroshi , Yamauchi Kosuke , Kuroha Tomohiro 
Published: December 2024

Protonic ceramic fuel cells (PCFC) are expected to have high power density and high energy efficiency because they can operate at 500-700°C and do not dilute the fuel. BaZrO3 doped with Yb is a candidate electrolyte for PCFC because it has high proton conductivity and chemical stability, does not react with Ni in the anode to form complex oxides, and can be manufactured industrially [1]. Many researchers have been exploring the cathode, but a material with higher activity is needed to improve the power density and efficiency. In this study, we investigated the electrochemical and PCFC cathodic properties of BaZr0.875-x
Yb0.125Co
x
O3-δ (x=0.0-0.75), which is a Yb-doped BaZrO3 electrolyte with Co added to provide electronic conductivity.

To clarify the electrochemical properties of BaZr0.875-x
Yb0.125Co
x
O3-δ, the electrical conductivity was measured by the two-terminal AC method and the proton concentration was evaluated by TGA. The Co valence was confirmed by XPS and the coefficient of thermal expansion was evaluated by XRD. The electrical conductivity was found to increase with increasing Co content. The TGA results showed that the proton concentration decreased with Co addition, and increased with Co addition above 0.375. XPS results indicate that Co was mainly tetravalent below x=0.375, but at x=0.5, Co was both tetravalent and trivalent. The increase in proton concentration can be attributed to the increase in oxide ion vacancies due to compensation of trivalent Co. The coefficient of thermal expansion was found to be 9.8×10-6/K at x=0.5, which is close to that of BaZr0.8Yb0.2O3-δ, a candidate electrolyte for PCFC.

The anode-supported PCFCs using BaZr0.875-x
Yb0.125Co
x
O3-δ as cathode were constructed and power density was investigated, with the highest power output (545 mWcm-2 at 600°C) obtained at x=0.5. Further DRT analysis of the cell impedance showed that the electrode impedance of the anode-supported PCFC could be separated into five component. The isotope effect of the impedance was investigated by replacing the anode gas from H2O-H2 to D2O-D2, and the isotope effect was observed in the two components on the high frequency under OCV, indicating that it is attributable to the anodic reaction. On the other hand, no isotope effects were observed in the three high-frequency components under OCV, but isotope effects were observed in the highest-frequency component under bias (0.6 V). This component is considered to be the component attributed to the water vapor formation reaction and was found to have the highest resistance among the impedance components of the cathode.

Acknowledgment

This paper is based on results obtained from a project, (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

[1] T. Kuroha, Y. Niina, M. Shudo, G. Sakai, N.Matsunaga, T. Goto, K. Yamauchi, Y. Mikami, and Y. Okuyama, Journal of power sources, 506 (2021) 230134.