By assuming an ohmic behavior of the relevant interfacial kinetics, a simple equivalent circuit was used to identify experimentally accessible parameters which could control the O flux through various technological devices. In particular, the O surface exchange coefficient which could be determined by means of isotopic exchange measurements was proportional to a characteristic electrode current density which governed the electrode resistance in solid-state electrochemical systems. In the case of ceramic ion-conducting membranes, a characteristic membrane thickness at which the change-over from bulk to surface control occurred was shown to be equal to D/k, where D was the O self-diffusion coefficient in the oxide material and k was the O surface exchange coefficient. Attention was also paid to correlations between D and k. For example, it was noted that the D/k ratio often had a value of about 0.01cm for most AO2 fluorite and ABO3 perovskite oxide materials. This implied that the preparation of membranes which were less than 100 thick would not be advantageous unless the value of k could be increased. However, the mechanisms which were responsible for correlations between D and k remained unclear.

B.C.H.Steele: Solid State Ionics, 1995, 75, 157-65