Theoretical Investigation on the Stability of Second Phase Formation in Ba(Zr, M)O3-Δ By Electrode Derived Ni and Co Elements
Authors: Nakamura Kaoru , Kobayashi Shun , Matsuda Ryuma Malik , Mori Masashi , Shimada Hiroyuki , Mizutani Yasunobu , Okuyama Yuji 
Published: December 2024

Ba(Zr,M)O3-δ is a perovskite-type oxide in which B-site is partially substituted by the dopant element M. It is expected to be applied as a solid electrolyte in proton-conducting ceramic fuel cells (PCFC). In recent years, there has been concern that the diffusion of Ni from the anode and Co from the cathode e into the solid electrolyte causes second phase formation, resulting in cell degradation. For example, depending on the type and amount of dopant, highly resistive phase, BaM
2NiO5, is reported to be precipitated [1]. Thus, understanding the precipitation conditions and precipitation mechanism of the second phase is essential to control cell degradation. Our previous studies have shown that the energetic stability of the precipitates is dominant in the formation of the precipitates by the anode-derived Ni. On the other hand, concerning the formation of cathode-derived Co precipitates, a complex oxide considered to be spinel precipitates at the grain boundary.However, not enough research has been done, and there is a possibility of other compounds precipitating as well, depending on conditions.
In the present study, we referred to our previous experimental evaluation and performed first-principles calculations to theoretically examine the precipitation of the second phase associated with the solid solution of Ni and Co in Ba(Zr,M)O3. Anode-derived Ni reacts with dopant element M to form precipitates, and the formation tendency was found to be organized by the ionic radius of the dopant element M. Co from the cathode is thought to segregate with Ni at the grain boundary of the solid electrolyte, and this mechanism was investigated in terms of segregation energy and solid solution energy of Ni in spinel at the grain boundary. Furthermore, the formation energies of complex oxides involving Co for various M elements were evaluated by first-principles calculations to investigate the relative stability of the oxides under various equilibrium conditions organized by chemical potentials. The results suggest that BaCoO3 is stable in an oxidation atmosphere, and this trend was most pronounced for M=Yb. It is thought that BaCoO3 precipitation does not deplete dopants from the solid electrolyte matrix phase and is not likely to lead to degradation. At the conference, the detailed precipitation mechanism and the changes in the lattice constants of BaZrO3 due to the solid solution of different elements with respect to chemical expansion will be discussed.
Acknowledgment
This research result was obtained as a result of the commissioned work (Development of Ultra-High Efficiency Protonic Ceramic Fuel Cell Devices WP2, WP3, JPNP20003) of the New Energy and Industrial Technology Development Organization (NEDO). We would like to express our gratitude to all parties involved.
Reference
[1] T. Kuroha, Y. Niina, M. Shudo, G. Sakai, N.Matsunaga, T. Goto, K. Yamauchi, Y. Mikami, and Y. Okuyama, Optimum dopant of barium zirconate electrolyte for manufacturing of protonic ceramic fuel cells, Journal of power sources, 506 (15), 230134 (2021).