Understanding functional materials for energy conversion or storage devices requires profound knowledge of their surface as well as their bulk properties. The bulk can often be sufficiently described by defect formation and trapping and equilibrium concentrations for electronic as well as ionic carriers. The surface, in particular at operating conditions, is much stronger influenced by additional factors such as impurities, interactions with the gas phase or near-surface restructuring effects. In further contrast to the bulk, local charge neutrality doesn’t have to be maintained. This enables dipole formation with extremely strong implications on the concentration of charged defects, charge transfer processes and the kinetics of oxygen exchange reactions. The importance of understanding these phenomena is highlighted by the large scatter of experimentally determined oxygen surface exchange parameters in literature of common MIECs, that have the same bulk composition but different surfaces, see ToC Graphic.

In this contribution, recent experimental results of in-situ impedance spectroscopy during pulsed laser deposition (i-PLD) will be presented that show the excellent performance of several MIEC oxygen electrodes for solid oxide cells in the pristine state and their degradation of surface exchange kinetics upon exposure to acidic gases. Similar adverse effects could also be achieved by using acidic oxide overlayers of 0.05-0.5 nm nominal thickness. Most importantly, also improving the surface kinetics is possible for most common MIEC materials by depositing decoration layers of basic oxides. Despite the consistent direction of changes (basic surface = higher activity and vice versa) that was observed for several materials, the exact mechanistic effect on either activation energy, prefactor or specific defect concentrations is often unclear and will be discussed.

In addition to indirect evidence on dipole formation observable in electrochemical measurements, we also found direct evidence for dipoles from both electrons as well as ions of a MIEC. Electronic evidence are substantial changes in the work function created by sub-monolayer decoration layers. Ionic evidence was found in ToF-SIMS measurements that due to the sampling depth of 1-2 nm allowed simultaneous detection of the respective decoration layer and the host MIEC. Considerable changes in secondary ion formation due to surface-near dipole formation were observed and are discussed together with site-specific properties of decoration ions as well as the stability of a sub-nm decoration layer towards diffusion into the MIEC.