Oxygen exchange kinetics of La 0.4Sr 0.6FeO 3− δ by simultaneous application of conductivity relaxation and carrier gas coulometry

Abstract

Oxygen exchange measurements were applied to La 0.4Sr 0.6FeO 3− δ in order to determine the chemical diffusion coefficient and the chemical surface exchange coefficient as a function of oxygen partial pressure over the range from 10 −4 to 10 −2.7 bar at 700 and 900 °C. In addition, temperature-dependent measurements were performed between 650 and 900 °C at constant p(O 2)=10 −3.37 bar. Both electronic conductivity relaxation experiments and carrier gas coulometry were applied simultaneously and the results of the two methods were in close agreement with each other. The surface exchange coefficient was found to be proportional to p(O 2) 0.8 at 700 °C and p(O 2) 0.7 at 900 °C, respectively. The activation energy for the Arrhenius behavior of the surface exchange coefficient amounted to (46±10) kJ mol −1 at p(O 2)=10 −3.37 bar. The chemical diffusion coefficient was found to be almost independent of p(O 2) at 700 °C with values around 2.5×10 −6 cm 2 s −1. The mechanism for the surface reaction of gaseous oxygen with the solid oxide is interpreted by proposing the adsorption of negatively charged oxygen molecules onto the surface of the ceramics.