Effect of Lattice Strain on Surface Chemistry, Oxygen Non-Stoichiometry and Oxygen Reduction Reactivity of Nd2NiO4+δ Thin Films

Lattice strain effects have received significant interest recently for altering the kinetics of oxygen ion diffusion and surface reactions in functional oxides used as electrodes and electrolytes of solid oxide fuel cells (SOFCs). For example, tensile strained perovskite oxide thin films, as exemplified by (La,Sr)CoO_3, were shown to clearly exhibit enhanced charge transfer properties and oxygen reduction reactivity as cathode material. Ruddlesden-Popper (RP) phases, with the chemical formula A_n+1M_nO_3n+1, have drawn broad interest as a competitive candidate for new cathode materials. The oxygen diffusion in these materials is highly anisotropic: diffusivity along the a-b plane is much higher than those along c-axis. While the oxygen exchange and diffusion in perovskite oxides is mediated by oxygen vacancies and is isotropic, RP phase oxides preferentially incorporate and transport oxygen interstitials along the a-b plane. Because of this structural anisotropy, the strain response of RP oxides in oxygen exchange and diffusion is expected also to be also anisotropic, and likely stronger along the c-axis compared to the strain response of perovskite materials.

We systematically investigated the effect of lattice strain on surface chemistry, non-stoichiometry and oxygen surface exchange kinetics of Nd₂NiO₄ (NNO) epitaxial thin films as a model RP structure, using both experimental and computational methods. NNO thin films with both (100) orientation (fast diffusion path exposed) and (001) orientation (fast diffusion path buried), and both tensile and compressive strain states were fabricated. In situ high-temperature X-ray diffraction (HTXRD) and in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) data is combined to assess the variation of oxygen non-stoichiometry and surface cation chemistry during annealing at high temperature. The strain along the c-axis, rather than the crystal orientation, was found to primarily determine the oxygen non-stoichiometry and Ni cation oxidation state of the NNO oxide thin films. Moreover, the NNO films with compressive strain along the c-axis are more reducible in low oxygen partial pressure compared to the tensile strained films. Nd segregation and surface morphology changes during annealing are more evident for the films on the NNO films with compressive strain along the c-axis. We attribute this to the elastic energy minimization along the rock-salt layers that is compressively stressed. Based on these observations, faster oxygen exchange kinetics is expected on the (100) oriented films with tensile strain along the c-axis. Investigations to determine and compare the oxygen exchange kinetics directly and quantitatively have been done using both electrical conductivity relaxation (ECR) and electrochemical impedance spectroscopy (EIS). ECR and EIS results proved faster surface exchange for (100) samples compared to (001) samples, also that tensile strain along c axis accelerated the oxygen surface exchange. On the other hand, we also utilized ab initio density functional theory (DFT) to study the effect of strain along c axis on oxygen interstitial formation and oxygen diffusivity in NNO, which also showed that tensile strain along c axis facilitates interstitial formation and oxygen transport along the a-b plane. In this work, the strain response of RP phase represented by the model Nd₂NiO_4+δ composition was found to be anisotropic, and the strain along the c-axis was found to be a key parameter for altering the non-stoichiometry, surface chemistry and oxygen reduction reactivity. These results have important implications to surface oxygen exchange kinetics on SOFC cathodes, and enable a comparison of the strain response of different classes of oxides.