Interfacial reactions associated with ceramic ion transport membranes

Abstract

Assuming ohmic behaviour for the relevant interfacial kinetics a simple equivalent circuit has been used to identify experimentally accessible parameters which may control the oxygen flux through a variety of technological devices. In particular the oxygen surface exchange coefficient (k cm s−1), which can be determined by isotopic exchange measurements is proportional to a characteristic electrode current density (jEA cm−2) which determines the electrode resistance (RE Ωcm2) in solid-state electrochemical systems. For ceramic ion-conducting membranes a characteristic membrane thickness (Lc) at which the change-over from bulk to surface control occurs is shown to be equal to D*/k where D* (cm2 s−1) is the oxygen self-diffusion coefficient in the oxide material. Attention is also drawn to correlations between D and k. It is noted, for example, that the ratio D/k often has a value around 10−2 cm (100 μm) for most AO2 fluorite and ABO3 perovskite oxide materials, which implies that fabricating membranes less than 100 μm thick will not be advantageous unless the value of k can be specifically increased. Mechanisms responsible for correlations between D and k remain obscure and should be a fruitful area for further investigations. Finally, specific examples of materials selection for ceramic fuel cell operation over a wide range of temperatures (450–;1000 °C) are briefly surveyed.