Fundamental issues in modeling of mixed ionic-electronic conductors (MIECs)

The reaction at an MIEC/gas interface is electrochemical in nature when one of its charge-transfer steps influences the rate of the overall reaction. Changes in electrical states of the surfaces of an MIEC may not only enhance or hinder the rate of the reaction but also reverse the direction of the reaction (from anodic to cathodic or vice versa). Expressions similar to the Butler-Volmer equation are the most proper phenomenological description of the kinetics at an MIEC electrode, although the effect of the electrical state can be much smaller than that of the chemical state under certain conditions. Unless the kinetics are infinitely fast, the instantaneous relationship among concentrations of the species involved in a reaction is determined more by kinetics than thermodynamics. if an MIEC is assumed to be metallic in which bulk dielectric displacement cannot occur, both electric field and polarization in the bulk phase must vanish. If a huge capacitive response of an MIEC is indeed caused by space-dependent bulk polarization, displacement current density in the MIEC cannot be ignored in impedance analysis and the validity of local electroneutrality must be carefully evaluated. If local electroneutrality prevails, dielectric displacement vanishes, and the Fermi level is uniform in an MIEC, then the concentration of all defects must be uniform as must be the composition of the MIEC. It is important to adhere to these fundamental principles in order to obtain proper relations for the ionic, electronic, and oxygen fluxes and the correct influence of the interfaces on the charge-transfer and chemical reactions. (C) 1999 Published by Elsevier Science B.V. All rights reserved.