Sketch of cationic point defects in perovskite-type materials and how they influence changes in the…

Sketch of cationic point defects in perovskite-type materials and how they influence changes in the surface stoichiometry.

Catalytic reactions at high temperatures, which are, e.g. essential in Solid oxide fuel and electrolysis cells, typically rely on oxygen redox reactions on the surface of oxide catalysts, which are, in most cases, perovskite-type materials. Although these materials are already commercially used in fuel and electrolysis cells, there is still a need for improvements in power density and long-term stability. The surface state of the (electro)catalytically active electrodes is generally very different from a regular bulk-like termination due to the different surface energies of A and B-site cations. Due to the envisioned long-term (100 000 hours) operation at a high temperature of 600-800°C, the chemical surface state will thermodynamically equilibrate with the bulk, so a model that satisfactorily describes the real for a given bulk composition and operation conditions is needed for further targeted optimisation.

Our solution

In the FWF project “Cation Defect Chemistry and Segregation”, we want to tackle this issue at its core. By slight variation of the A:B cation ratio in the ABO₃ perovskite materials, we can tune the concentrations of cationic defects – vacancies and anti-site defects. Knowledge of bulk cation nonstoichiometry is essential because the driving force for cation segregation depends on the chemical potentials and concentrations of oxygen anions and A and B-site cations in the bulk. By thoroughly understanding the bulk cation defect chemistry, we can predict the stable surface terminations, which require knowledge-based optimization of catalytic properties and stability of perovskite-type materials.

Experimental realization and collaborations

Resolving these questions requires various analytical methods, which rely on collaboration with many partners. In the group’s labs, thermogravimetry and coulometric titration are used for bulk defect chemistry, while impedance spectroscopic measurements on model cells allow precise determination of surface catalytic properties. Insight into the surface chemical properties will require substantial collaboration with other research groups (Ambient pressure XPS measurements within ERC project TUCAS at the MU Leoben, Precise quantification of the bulk cation stoichiometry in the labs of Andreas Limbeck, Surface and trace analytics and chemometry at TU Wien), and Low-energy ion scattering at the CEST in Wr Neustadt to determine the composition of the topmost atomic layer. Moreover, the central facility Analytical Instrumentation Center encompasses new XPS and auger microscopy spectrometers with in-situ capabilities to live-track cation segregation and metal particle formation. By assembling the diverse information from these numerous experiments, we will finally deliver reliable models for tuning the surface chemistry of perovskite-type catalysts towards long-term performance.