(Invited) Proton Transport Properties of Yb-Doped BaZrO3 and Its Application to High Efficiency Proton Ceramic Fuel Cell
Authors: Okuyama Yuji , Kuroha Tomohiro , Kosuke Yamauchi , Mikami Yuichi , Shimada Hiroyuki , Yamaguchi Yuki , Mizutani Yasunobu , Fujishiro Yoshinobu 
Published: December 2020

Barium zirconate have high chemical stability with CO2 comparison with barium cerate. Acceptor doped barium zirconate show high proton conductivity [1]. The proton conductivity of yttrium doped barium zirconate is 1×10-2 Scm-1 at 600 ℃ and it is attracted as the electrolyte of fuel cell and water vapor electrolysis cell. The fuel cell using proton conducting oxide was called to PCFC (proton ceramic fuel cell) and work at the intermediate temperature. PCFC is expected to be the energy conversion device in future because of high efficiency [2] and low production cost [3]. Generally, nickel is used as anode of PCFC. The anode overpotential of nickel is lower than that of the other metals. However, the nickels dissolve to the proton conducting oxide and possibly increase the ohmic resistance of electrolyte. Moreover, the nickel and the electrolyte possibly form the blocking layer such as the complex oxide. Although the proton conductivity of yttrium doped barium zirconate is highest in the zirconate-type proton conductor, BaY2NiO5 is formed between the nickel and the yttrium doped barium zirconate [4] and it might work as the blocking layer of proton.

In this study, the reaction product between NiO and BaZr0.8
M
0.2O3-δ (M=Sc, In, Yb, Y, Gd) were confirmed by X-ray diffraction analysis. In order to clarify the NiO dissolution effect on the proton transport properties of BaZr0.8
M
0.2O3-δ (M=Sc, In, Lu, Yb, Y), the partial conductivity of the proton and the hole for BaZr0.8
M
0.2O3-δ (M=Sc, In, Lu, Yb, Y) with NiO were measured at temperature range of 200-600 ℃ by impedance analysis. The current efficiency on the polarization properties, then was examined for the fuel cell system using Acceptor-doped barium zirconate with NiO as electrolyte. Moreover, we investigated the effect of dopants on the ease of cell fabrication. The conventional ceramic tape-casting and firing process for fabricating laminated ceramic electronic devices was used, which is important for practical use. The power generation performance of planer cells fabricated by this process was also measured.

The X-ray diffraction patterns of 0.4 mol% NiO doped BaZr0.8
M
0.2O3-δ (M= Sc, In, Yb, Y, Gd) were measured for the the samples as-sintered and annealed under 1%H2 at 873K. The peak of the NiO or Ni phase except the barium zircanate phase was observed for 0.4 mol% NiO doped BaZr0.8
M
0.2O3-δ (M= Sc, In, Yb). On the other hand, the BaM
2NiO5 phase was observed for M=Y, Gd. When the ionic radius of dopant is in excess of the size of Yb, the BaM
2NiO5 was formed as the reaction product between NiO and BaZr0.8
M
0.2O3-δ.

The proton conductivity of the series of BaZr0.8
M
0.2O3-δ containing no NiO increased in the order of In<Sc<Lu<Yb<Y. On the other hand, there was no distinct difference of the hole conductivity for dopant. PCFC using BZM20 (M=Lu, Yb, Tm, Y) has an energy efficiency of more than 0.8 at 873 K. The proton conductivity decreased and the hole conductivity did not change for all series of BaZr0.8
M
0.2O3-δ due to NiO dissolution in BaZr0.8
M
0.2O3-δ. Therefore, proton transport number and the efficiency of the fuel cell decrease by addition to NiO.

Considering the above, the ytterbium-doped barium zirconate did not form a complex oxide with NiO and might have high proton conductivity. The performance of PCFC using BZYb20 as electrolyte was measured. The maximum power density values at 600 and 700 °C were 0.50 and 0.70 Wcm−2 for the planar cells using BZYb20, respectively. Electrochemical impedance spectroscopy of the cell showed that the ohmic resistance was 0.28 Ωcm2 at 600 °C, which was approximately three times larger than the resistance value calculated using the bulk conductivity of BZYb20 and the electrolyte thickness and was in agreement with the resistance value of BZYb20 with NiO.

[1] K.D. Kreuer et al., Solid State Ionics 145(2001)295-306.

[2] Y. Matsuzaki et al., Scientific Reports, 5:12640(2015)1-10.

[3] A. Doubois et al., ECS Transaction 78(1) (2017)1963-1972.

[4] J. Tong et al., J. Mater. Chem. , 20(2010)6333.