(Invited) Increased Oxide Ion Diffusivity and Surface Exchange on Pr2NiO4 Base Oxide by Au Dispersion

Mass transport property in nano-size structure controlled materials are attracting much interest because such effects have a possibility to improve the performance of the electrochemical solid state devices such as Solid Oxide Fuel Cells (SOFCs). There are many approaches and ideas for the improvement of oxide ion conductivity in electrolyte by nano size effects. In this study, we demonstrated the 3 dimension (3D) tensile strain in Pr₂NiO₄, mixed conductor, was successfully introduced by dispersing Au particles in grain. The maximum lattice distortion estimated from X-ray diffraction measurement was 0.23 % at 2 mol% Au dispersion. Au dispersed Pr₂NiO₄ shows a large positive effect on oxide ion diffusivity and surface exchange coefficient.

   The doubled oxygen permeation flux was achieved with dispersing 2 mol% Au in PNCG at 873 K, indicating the tensile strain in PNCG lattice positively works for the oxygen permeation property. Since oxygen permeation is controlled by interstitial oxygen concentration and its mobility, further details study on effects of tensile strain on oxygen diffusivity were studied by using ¹⁸O tracer diffusion. The depth profiles of x mol% Au/PNCG (x=0, 1, 2, 3) are studied with SIMS analysis, and the estimated oxygen tracer diffusivity (D*) and the surface exchange coefficient (k*) are much increased with dispersing Au, and the enhancement in D value was well agreed with that in oxygen permeation rate.  Redox titration measurement suggests that 3D tensile strain increased amount of excess oxygen.  Interstitial oxygen in Pr₂NiO₄ is introduced in Rock salt layer of K₂NiF₄ structure, and this is origin for the fast oxygen diffusivity and high surface activity for oxygen dissociation.

  Recently, there is much attention of surface composition in oxide ionic conductors using the low-energy ion scattering (LEIS) technique to understand the surface reaction and degradation. LEIS analysis was also performed on Au dispersed Pr₂NiO₄ and it was found that surface of Pr₂NiO₄ was enriched with Pr.  However, Pr segregation was prevented by introduction of Au and so tensile strain also positively works for preventing Pr segregation.   On the other hand, surface oxygen content is also decreased by Au dispersion and so surface oxygen vacancy seems to be much enriched.  Therefore, increased surface exchange coefficient by Au dispersion could be assigned to the high concentration of oxygen vacancy on surface of Pr₂NiO₄ dispersed with Au.

  In summary, although Au dispersion was not uniform, effect of tensile strain on oxide ion mobility could be observed in Pr₂NiO₄ by Au dispersion. Furthermore, tensile strain affected the surface composition, which may enhance the k* values. If the better dispersion state with fine particles could be achieved, there is a possibility to develop the higher oxide ionic conductor with uniform strained material.