Oxygen exchange measurements on perovskites as cathode materials for solid oxide fuel cells

Oxygen exchange measurements have been applied to determine simultaneously the chemical diffusion coefficient and the surface exchange coefficient of oxide perovskites. The oxygen partial pressure is changed in a step-wise manner and the relaxation of the oxide ceramics is followed as a function of time.

Instead of recording the amount of exchanged oxygen, the rate of oxygen incorporation reactions is measured directly. It is shown how the kinetic parameters can be extracted from the time dependence of the oxygen flux between the sample and the surrounding gas phase. The effect of finite switching times of the reactor on the relaxing oxygen flux is investigated theoretically.

The time constant of the reactor is shown to be negligibly small under the chosen experimental conditions. The oxygen exchange measurements have been performed on La0.6Sr0.4CoO3−δ (LSC) at 725 and 825°C as a function of the oxygen partial pressure p(O2) ranging from 10−4 to 10−3bar. The chemical diffusion coefficient is almost independent of p(O2) at constant temperature, whereas the exchange coefficient increases significantly with increasing p(O2).

Finally, the dependence of the chemical exchange coefficient on the oxygen partial pressure is interpreted in terms of different mechanisms for the surface reaction.