First-Principles Investigation on the BaM
2NiO5 Second Phase in Ba(Zr,M)O3 Solid Electrolyte in Proton-Conducting Solid Oxide Fuel Cell
Authors: Nakamura Kaoru , Mori Masashi , Okuyama Yuji 
Published: July 2021

BaZrO3, in which Zr sites are substituted with rare earth elements M, has been investigated for use as a solid electrolyte in Protonic Ceramic Fuel Cells (PCFC). However, co-sintered electrodes-derived Ni solution causes second phase formation of BaM
2NiO5 [1] and decrease of rare earth element solution level in BaZrO3. This problem greatly limits the operating temperature of the PCFCs and the selection of additive elements. In addition, the formation mechanism of the BaM
2NiO5 is still unclear. Even if the formation of BaM
2NiO5 is not confirmed in short-time synthesis experiments, it may precipitate during long-term operation, leading to degradation of the PCFCs. Thus, in this study, we investigated the formation tendency of BaM
2NiO5 and the precipitation mechanism in Ba(Zr,M)O3 by using first-principles calculations.
All first-principles calculations were performed using the VASP code [2] with the consideration of spin polarization. Interactions between ions and electrons were described by the Projected Augmented Wave (PAW) method [3]. Perdew–Burke–Ernzerhof revised for solids (PBEsol) was utilized to approximate exchanges and correlate interactions of electrons [4,5]. 4f electron of M elements (M = Lu, Yb, Tm and Gd) was assumed to be treated as a frozen core state. The kinetic cut-off energy for the plane wave was 500 eV. The k-point mesh for integration in the Brillouin zone was carefully selected as a Γ-point centered dense k-mesh. These calculation conditions were confirmed to be sufficient to acquire well-converged total energy, below 0.1 meV/atom. Atomic positions and cell parameters of each structure were optimized until the forces on each atom and cell converged below 1×10-4eV/Å.
Figure 1 (a) shows the schematic illustration of BaM
2NiO5 primitive cell. Formation energies of BaM
2NiO5 were evaluated by energy differences between BaM
2NiO5 and sum of the energy of each single oxide (BaO, M
2O3 and NiO). Figure 1 (b) shows the evaluated formation energies of BaM
2NiO5 (M = Sc, In, Lu, Yb, Tm, Y and Gd). BaSc2NiO5 and BaIn2NiO5 were found to be energetically unstable due to their positive formation energies. On the other hand, formation energies of BaLu2NiO5, BaYb2NiO5, BaTm2NiO5, BaY2NiO5 and BaGd2NiO5 show negative values. However, experimentally, the precipitation of BaLu2NiO5 and BaYb2NiO5 has not been confirmed [1]. This discrepancy can be attributed to the fact that the equilibrium condition including Zr was not considered in first-principles calculation. Therefore, considering the equilibrium condition of precipitation of Ba2
MNiO5 in Ba(Zr, M, Ni)O3, the experimentally confirmed threshold of precipitation is located around the formation energy of -0.4 eV/f.u. in Fig. 1(b). For the very early stage of the actual precipitation mechanism of Ba2MNiO5, it is thought that the binding energy between cations in Ba(Zr, M, Ni)O3 is involved. This point is still under investigation and will be discussed in detail on the day of the presentation.
References
[1] Y. Mikami, T. Kuroha, T. Goto, K. Yamauchi, T. Kamata, K. Takeuchi, N. Taniguchi, Y. Tsuji, and Y. Okuyama, 21th international conference on Solid State Ionics (2017).
[2] G. Kresse and J. Furthmüller, Phys. Rev. B 54 (1996) 11169..
[3] P. E. Blöchl, Phys. Rev. B50, 17953 (1994).
[4] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett.77, 3865–3868 (1996).
[5] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett.78, 1396 (1997).

Figure 1