In Situ 3D Electron Diffraction to Investigate Redox Reactions of Perovskite-Based Energy Materials

Promovendus/a: Daphne Vandemeulebroucke

Promotor(en): Prof. Joke Hadermann

Climate change is one of the biggest challenges for science in the twenty-first century, and the development of new technologies that lower the carbon footprint has never been more pressing. Many of these alternative energy applications involve inorganic crystalline solids exposed to redox conditions. In this research area, the development of in situ 3D electron diffraction at elevated temperatures in gas and vacuum unlocks great new potential, as it can unravel atomic structure transformations in operational conditions. That way, we can investigate the origins of e.g. high performance and degradation properties. A particularly interesting group of energy materials is based on the perovskite structure, with ABX3 as chemical formula. This project studied the influence of high temperature reducing atmospheres on the atomic structure of two perovskite-based energy materials : LaxSr2−xMnO4−???? and (Ca,Sr)(Fe,Mn)O3−????.

The first group of materials – i.e. Ruddlesden-Popper LaxSr2−xMnO4−???? – have good performance properties as symmetric solid oxide fuel cell electrodes. With 3D electron diffraction, we found new incommensurately modulated structures for La0.25Sr1.75MnO4−???? and La0.5Sr1.5MnO4−???? upon annealing in hydrogen gas, which have never been reported before. Further, we discovered differences in defect structure and ordering that can be linked to previously unexplained differences in electrical conductivity with lanthanum concentration.

Next, (Ca,Sr)(Fe,Mn)O3−???? was studied, which is an oxygen carrier in the CLOU process. This is a carbon capturing combustion technique that inherently separates expelled CO2 from air. The performance properties of CaMnO3−???? can be improved by A or B site doping with e.g. strontium or iron. Using in situ 3D electron diffraction, we discovered that this is connected to differences in ordering of oxygen vacancies for the undoped versus doped materials. Further, we could successfully solve and refine the structure of CaMnO2.75 for the first time ever.

However, at the highest temperatures during in situ experiments in gas atmospheres, a reaction of the samples with the Si3N4 heating chips gave rise to a SiO2 shell around the crystals. This caused experimental limitations and discrepancies between in situ and ex situ annealing results. Therefore, a systematic investigation was performed into the potential of a protective graphene coating.

Still, this research illustrates how a combination of various in situ and ex situ electron microscopy techniques led to new insights in the structure-property relations of redox-based energy materials.