Kinetic unmixing and kinetic decomposition of oxides in external electric fields

• Applicant: Professor Dr. Manfred Martin
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2016 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 319377662

Ternary oxides AmBnOx (or even multinary oxides with more than four cations) are being used in numerous devices that are exposed to various thermodynamic forces, such as oxygen chemical potential gradient (solid oxide fuel cells), electric potential gradient (or electric field) (electrolyzers, multi-layered ceramic capacitors), thermal gradient (thermoelectrics), stress gradient (piezoelectrics), etc. The materials range from purely ionic conductors (e.g. electro-lytes for solid oxide fuel cells, electrolyzers or lithium-ion batteries) to mixed conductors and semiconductors (e.g. interconnects and electrodes in solid oxide fuel cells and electrolyzers). When these ternary oxides are exposed to external driving forces (or put under operation) especially at high temperatures where the ions that compose the oxide are more mobile, the cations migrate, in general with different mobilities. As a consequence, concentration gradients of the cations are built up in opposite directions, i.e. the faster cation will be enriched in the direction of transport and vice versa. This phenomenon is called kinetic unmixing, and if the extent of unmixing becomes large enough, the original oxide will undergo kinetic decomposi-tion, i.e. new phases will form. In addition to these unmixing and decomposition phenomena morphological changes of the surfaces may appear. As in the first period of SPP 1959, it is the general aim of this proposal for the second period to investigate systematically the kinetic unmixing and kinetic decomposition of ternary or higher oxides in an external electric field. The main theoretical problem concerning the formal deriva-tion of the kinetic decomposition voltage of an oxide, that is exposed to an electric field using reversible electrodes was successfully solved within the first period of SPP 1959. In the sec-ond period we will generalize the previous analysis and continue the related experimental in-vestigations. In addition, we will investigate decomposition with ion-blocking electrodes using the so called Hebb-Wagner polarization technique. In summary, the exposure of a ternary or higher oxide to an applied voltage may result in a variety of kinetic unmixing and decomposition phenomena, depending on the nature of the used electrodes (both reversible; one reversible and one ion-blocking). These phenomena need fur-ther detailed investigations on a theoretical and on an experimental level as well.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1959: Manipulation of matter controlled by electric and magnetic fields: Towards novel synthesis and processing routes of inorganic materials