Effects of surface coatings on the determination of Dchem and kchem in La2NiO4+δ by conductivity relaxation

In this work, we utilize surface modification to accurately determine oxygen diffusivity and surface exchange in La2NiO4+delta by transient conductivity. To achieve 'instantaneous' gas exchange in the reaction cell, a total pressure change was adopted instead of exchange of gas mixtures at 1 atm. Conductivity relaxation measurements were performed in the temperature range of 600-900 degrees C and at oxygen partial pressures of 0.2, 1.0 and 1.9 atm. Due to the large uncertainties associated with deriving oxygen chemical diffusion and surface coefficients simultaneously, we limited the relaxation to pure bulk diffusion control by coating nano-grained La2NiO4+delta particles on the sample surface. After determining D-chem, k(chem) was derived by fitting the relaxation data of the uncoated sample. The transient experiments via both oxidation (a step change of pO(2) from 0.2 to 1.0 atm) and reduction (1.9 to 1.0 atm) give consistent results of D-chem and k(chem) showing Arrhenius-type behaviour with activation energies of similar to 90 kJ/mol and similar to 150 kJ/mol, respectively. (C) 2011 Elsevier B.V. All rights reserved.