Defect chemistry and electrochemical characteristics of La-based perovskite materials

Dissertation, RWTH Aachen University, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2022). = Dissertation, RWTH Aachen University, 2022
Perovskite materials of the type ABO3 are generally known for their flexibility regarding changes in their defect chemistry with the help of A- or B-site doping, by which it is possible to influence their structural and catalytic properties. Changes in the defect chemistry involve both modifications in the oxygen nonstoichiometry δ and in the B-site oxidation state within the material. They are strongly dependent on the applied synthesis methods and atmospheric conditions, as well as on the operating temperatures. For the use of such demand-optimized materials, e. g. as air electrodes in solid oxide cells, significant improvements in efficiency and performance can be achieved in the field of energy storage and conversion systems. In the scope of this thesis, the lanthanum-based compositions La1−xFeO3−δ (x = 0 − 0.1) and La1−xCaxMnO3+δ (x = 0 − 0.5) were synthesized using the conventional solid state synthesis method in air and subsequently characterized. The impact of lanthanum-substoichiometry and calcium content on the respective B-site oxidation state (Fe/Mn), as well as on the oxygen nonstoichiometry was investigated. With the help of powder X-ray diffraction solely orthorhombic structures for La1−xFeO3−δ were determined. However, the increasing introduction of lanthanum-substoichiometry lead to an additional formation of the secondary phase Fe2O3 next to stoichiometric LaFeO3. This could be verified using various microscopic investigation methods. Thermogravimetric investigations on the La1−xFeO3−δ series in pure oxygen atmosphere did not reveal significant mass changes in dependence on the lanthanum content or temperature, which suggested that no significant oxygen uptake or release occured. It has been suspected that instead of lanthanum-substoichiometry rather the basic perovskite LaFeO3 and Fe2O3 are formed. LaMnO3 on the other hand showed a great structural diversity from a rhombohedral phase up to two differently distorted orthorhombic phases. With increasing calcium content, the less distorted orthorhombic modification was stabilized. Investigations of the oxygen nonstoichiometry of the La1−xCaxMnO3+δ series revealed a strong dependence on the calcium content, temperature and atmosphere. Unlike in LaFeO3, significant and reversible mass changes could be observed, which were correlated with oxygen uptake and release processes. The δ-values of the initial La1−xCaxMnO3+δ compositions were obtained from thermogravimetric measurements in reducing atmosphere. The determination of the manganese oxidation state in the La1−xCaxMnO3+δ series was performed indirectly by using the obtained thermogravimetric data and literature models and directly by X-ray absorption near edge spectroscopy experiments. In this way, the influence of intrinsic and extrinsic effects on the manganese oxidation state of the differently calcium-doped samples was revealed. Subsequent current–voltage and electrochemical impedance spectroscopy measurements in dependence on the calcium content and temperature enabled the determination of the samples’ electrochemical behavior and catalytic activity. Thus, up to temperatures of 850 °C, high calcium contents in the samples showed the highest measured currents under the influence of voltage, as well as the smallest polarisation resistances. The highest current, in combination with a small polarisation resistance, was obtained at 900 °C at medium calcium contents. It could be distinguished between a surface process and a process taking place at the triple phase boundary, and their activation energies could be determined. This allowed for drawing careful correlations between the defect chemistry and electrochemical or electrocatalytical properties observed within the sample series.
Published by RWTH Aachen University, Aachen