- Wider research context / theoretical framework
Doped perovskite-type oxides have a large concentration of functional point defects which cause their catalytic activity and oxygen ion and electron conductivity. This is essential for electrochemical and catalytic applications. So far, models and studies focus on oxygen vacancies and electronic defects, while much less is known about the concentration and mobility of cationic point defects. Here, we want to shed more light on cationic defects and how they affect the stability and tendency to form surface segregations.Once we understand how the bulk cation defect chemistry influences cation segregations, we will be able to optimise the reactivity and stability of perovskite-type (electro) catalysts, which may lead to better performing solid oxide electrolysis cells for H2O and CO2 electrolysis.
- Hypotheses/research questions /objectives
How do oxygen vacancies and surface chemistry depend on the A:B cation ratio in highly doped perovskiteslike (La,Sr)1±aFeO3-d? Can highly active surface terminations be thermodynamically stable by controlling the bulk cation defect chemistry and chemical potentials? Why is cation diffusion so fast during exsolutions of B-site metals? Which model can explain both the A-site cation segregation at high p(O2) and B-site metal exsolutions at low p(O2)?
We aim to employ a broad range of analytical and electrochemical methods, primarily on thin-film model electrodes on oxide ion conducting electrolytes. These include electrochemical impedance spectroscopic measurements of electron and ion conductivity and surface oxygen exchange rates. Further, we investigate bulk defects by thermogravimetry, coulometric titration and oxygen and cation tracer diffusion with SIMS analysis and XAS studies. We will get detailed information on surface defects from operando ambient pressure XPS measurements on electrochemical cells and by in-situ impedance characterisation within the pulsed laser deposition (PLD) chamber and Auger electron microscopy during the electrochemically triggered formation of metallic nanoparticles.
- Level of originality/innovation
Although research of perovskite-type materials for application as catalysts or solid oxide cell electrodes is widespread, and cation segregation is a well-known phenomenon, there is relatively little information on the thermodynamic driving forces and diffusion mechanisms for the formation of segregations. We aim to close this gap by investigating the link between bulk cation defects – especially A and B-site vacancy defects – of perovskite-type oxides and segregation of A-and B-site cations to the surface, which determines the catalytic activity of the material. Finally, we aim to control the bulk cation defect chemistry of perovskitetype electrodes to thermodynamically stabilise highly active surface terminations.
