Effects of Doping on O Ion Migration in Pr2NiO4-Based Oxides

Recent studies related to SOFC technology involve materials design on the electrodes and electrolyte layes in search of candidate materials with high O ion conductivity even at lower operating temperatures. This involves understanding the detailed mechanisms of O ion diffusivity at materials which are known to be good O ion conductors at low temperatures. From the many candidate materials, a group of perovskite materials in the K₂NiF₄ structure has been eyed as promising materials for these purpose, specifically Pr₂NiO₄-based materials. Interest in the study of Pr₂NiO₄ and Pr₂NiO₄-based materials rooted from its potential application as a mixed ionic and electronic conductor (MIEC) as a cathode material for SOFC.

In this study, first principles calculation based on density functional theory (DFT) was used to analyze the structural and electronic properties of Pr₂NiO₄ and doped Pr₂NiO₄ systems (Pr_2-yR_yNi_1-x-y-zE_xT_zO₄ (R=La, and E=Cu and T=Ga)). With the aim of determining possible Pr₂NiO₄-based candidate materials for the electrolyte layer, the effects of doping on the migration of O^-2 on an assumed promising system was analyzed.

Two types of cation dopant sites were assumed in this study: (1) a large cation dopant substituting Pr, and (2) a smaller cation dopant substituting Ni. For the first case, the effect of La dopants, with different concentrations, on the change in the structural and electronic properties of the host bulk structure was analyzed. It was observed that La dopants have a dominant effects on the change in the lattice parameter along the the c direction due to the difference in ionic radius. This is assumed to be a factor affecting the O ion conductivity within the material. For the second case, co-doping of Cu and Ga (Pr₂Ni_0.5Cu_0.25Ga_0.25O₄), and doping of Ga (Pr₂Ni_0.5Ga_0.5O₄) with different structures were analyzed. For the co-doped system, it was observed that due to nearly similar atomic radii of Cu and Ga, no cation-O atoms elongation were observed. Cu dopants are considered to increase the electronic conductivity of the system. On the other hand, Ga doping shortens the Ga-Oap distance while elongating the Ni-Oap distance, which affects O ion migration through interstisialcy. Furthermore, electronic property analysis also shows that there is a minimal state for the Ga orbitals along the Fermi level as compared with the host Ni atom. And that higher oxidation state of Ga than Ni will tend to cause increased concentration of interstitial O atoms. These results show favorable characteristics for an electrolyte material.