Effects of surface coatings on the determination of Dchem and kchem in La 2NiO 4+δ by conductivity relaxation

Abstract

In this work, we utilize surface modification to accurately determine oxygen diffusivity and surface exchange in La 2NiO 4 + 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 La 2NiO 4 + δ 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 ~ 90 kJ/mol and ~ 150 kJ/mol, respectively.