(Invited) Chemical Intricacies of Mixed Ionic-Electronic Conducting Perovskite Oxides

Abstract

Mixed ionic-electronic conductors (MIEC) play a critical role in electrochemical devices, such as batteries and fuel cells. Ideally, both the electrodes (anode and cathode) should support a fast transport of electrons and the working ions to realize acceptable reaction kinetics and current density/rates. Crystal chemistry and the resulting chemical bonding control the MIEC properties and the device performances ensuing from them. This presentation will illustrate the richness and complexities displayed by perovskite-based MIECs, which are employed as electrodes in solid oxide fuel cells, oxygen separation membranes, etc., by taking a few basic fundamental materials parameters/factors: By taking the ionic sizes of the A and B cations in ABO3 perovskite oxides, the presentation will show the ability to tune the electronic and ionic conductivities in MIECs. Specifically, for example, how the approach of B-O-B bond angle towards 180o by replacing the A cations with progressively increasing size can cause an increase in bandwidth, electron delocalization, and electronic conductivity, and eventually a semiconductor-to-metal transition.By taking the size difference between the A cations in the double perovskite oxides, such as ALnCo2O6- d (A = alkaline earth and Ln = lanthanide), how oxygen content (6-d) and hole concentration can be altered drastically under even identical synthesis conditions due to the preference of different coordination numbers for A and Ln ions, e.g., the preference of larger cations like Ba2+ for 12-fold coordination while smaller cations like Y3+ for 8-fold coordination.By taking the coordination number of trivalent cobalt ions as an example, the presentation will illustrate how the low-spin to high-spin transition that has plagued the cobalt-containing perovskite oxides with huge thermal expansion coefficient can be eliminated by designing materials with tetrahedral-site cobalt ions instead of octahedral-site cobalt ions.By taking the differences in the magnitude of the charge-transfer gap between the B3+/4+:3d band and the top of the O2-:2p band in ABO3 perovskite oxides, the presentation will explain how the oxygen flux in oxygen gas separation membranes can be altered due to the differences in the degree of covalence in the B-O bond.