Enhanced oxygen surface exchange of La2NiO4+δ by means of a thin surface layer of silver

Abstract

Abstract The oxygen surface exchange kinetics of the mixed ionic–electronic conductor La 2 NiO 4+δ has been enhanced by coating the material with a 200 nm thin layer of silver. Due to the catalytic activity of silver the surface exchange coefficient ( k chem ) was increased by one order of magnitude at 600 °C. This method thus reduced the limiting role of the surface redox process permitting the determination of the chemical diffusion coefficient of oxygen ( D chem ) by means of the conductivity relaxation technique between 600 and 850 °C at oxygen partial pressures of 0.1 and 0.01 bar. D chem -values range between 2 × 10 − 5 and 2 × 10 − 4 cm 2 s − 1 with activation energies of ~ 50 kJmol − 1 . Over the course of a complete temperature cycle the silver layer was removed via gas phase transport at high temperatures which allowed the comparison of oxygen surface exchange coefficients with and without surface activation. Depth profiles of the surface of a tested sample by X-ray photoelectron spectroscopy showed no evidence of silver within the topmost 500 nm, indicating that Ag-deposition did not affect the diffusivity of the material. Additionally, the morphology of deposited silver films was studied after annealing between 500 and 800 °C by SEM.