Investigation of Transport Properties of Bzcy (BaZrxCe1-xY0.15O3-
δ, x=0, 0.2, 0.4, 0.6)
Authors: Lim Dae-Kwang , Jeon Sang-Yun , Im Ha-Ni , Song Sun-Ju 
Published: February 2020

Some perovskite-type oxides have shown high proton conductivity in wet hydrogen atmosphere. In these perovskite oxides the partial substitution of an acceptor dopant for B-site results in formation of oxygen vacancies, which lead to the formation of protonic defects by the absorption of water in wet atmosphere. Among various proton conducting perovskite-type oxides, Y-doped Barium cerate and barium zirconate are nearly isomorphic. Therefore, the basic phases of a binary solid solution where B-sites are randomly occupied by either Zr4+ or Ce4+. This is because the chemical instability of Y-doped BaCeO3-
δ in the presence of CO2-, H2O- or SO2-containing atmosphere below 973K was substantially improved by replacing Ce with Zr to form a solid solution, and the mechanical strength was also enhanced when the proton conductivity and thermal stability remained adequate over a wide range of operation conditions for the fuel cell.

To elucidate the transport properties and defect structure of a Ce/Zr-coexisting preovskite structure, BZCY(BaZrxCe1-xY0.15O3-
δ, x=0, 0.2, 0.4, 0.6) was chosen as the model system. First, we study the electrical properties of BZCY in various oxygen and water vapor atmospheres by four-probe DC measurements as a function of temperature. And then, these properties are analyzed analytically, based on the defect structure of the material. Furthermore, we use the conductivity relaxation measurement technique to monitor the monotonic/non-monotonic(two-fold) relaxation for the BZCY under various themodynamic conditions and extract the chemical diffusivities by solving Fick’s second law for each case on the basis of hydrogen- and oxygen- decoupled diffusion as proposed by Professor Yoo1-3.

Fig. 1 shows the typical relaxation profiles of the mean total conductivity upon oxidation and reduction across an identical oxygen activity window. During the redox reaction driven by the oxygen chemical potential gradient at a fixed water vapor activity, oxygen may be incorporated into the BZCY by ambipolar diffusion of V••
o and 2h• in the p-type oxidizing regime. In Fig. 2, conductivity relaxation profiles upon hydration and dehydration at a fixed pO2(≈0.21 atm) are shown as a function of water vapor activities at 1023K. The figures clearly reveal the violation of monotonic relaxation governed by the chemical diffusion of water, according to the ambipolar diffusion of oxygen vacancies and proton. As shown in the fig. 2, non-monotonic twofold relaxation behavior was clearly observed for the BCZY system.

1. H.I. Yoo et al., Phys Chem Chem Phys, 10, 974 (2008)

2. H.I. Yoo et al., Solid State Ionics, 180, 1443 (2009)

3. J.I. Yeon et al., Solid State Ionics, 181, 1323 (2010)

4. S. Hamakawa et al., Solid State Ionics, 148, 71 (2002)

5. S.-J. Song et al., Solid State Ionics, 167, 99 (2004)

6. S.M. Haile et al., Nature, 125, 271 (1999)

7. D. Pergolesi et al., Nature Materials, 9, 846 (2010)

8. H. Iwahara et al., Solid State Ionics, 125, 271 (1999)

9. Y.C. Yang et al., Sensors and Actuators B, 140, 273 (2009)

10. C. Zuo et al., Advanced Materials, 18, 3318 (2006).