INTERFACIAL REACTIONS ASSOCIATED WITH CERAMIC ION-TRANSPORT MEMBRANES

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 (j(E) A cm(-2)) which determines the electrode resistance (R(E) Omega cm(2)) in solid-state electrochemical systems. For ceramic ion-conducting membranes a characteristic membrane thickness (L(c)) at which the changeover from bulk to surface control occurs is shown to be equal to D-*/k where D-* (cm(2) 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 mu m) for most AO(2) fluorite and ABO(3) perovskite oxide materials, which implies that fabricating membranes less than 100 mu m 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 degrees C) are briefly surveyed.