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 + δ 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 °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 + δ particles on the sample surface. After determining Dchem, kchem was derived by fitting the relaxation data of the uncoated sample. The transient experiments via both oxidation (a step change of pO2 from 0.2 to 1.0 atm) and reduction (1.9 to 1.0 atm) give consistent results of Dchem and kchem showing Arrhenius-type behaviour with activation energies of ~ 90 kJ/mol and ~ 150 kJ/mol, respectively.