Microstructure evolution of proton-conducting ceramic films fabricated by wet-chemical methods

Xiao, Yao; Waser, Rainer (Thesis advisor); Schneider, Jochen M. (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

Electrochemical energy converters using proton conducting solid oxide ceramic based electrolytes attained considerable interest as they offer a device operation in the so called intermediate temperature (IT) range (400-600 ℃). This enables the use of less expensive materials and leads to less interfacial reactions in the devices. Moreover the fuel is not diluted by the produced water vapor due to its evolution at the cathode side. Y-doped BaZrO3 (BZY) has a high proton conductivity and exceptional chemical stability under acid gas condition, and is therefore a promising electrolyte candidate for the use in protonic ceramic fuel cells (PCFC). Nevertheless issues remain due to the refractory nature of BZY, which in turn leads to smaller grains and more proton transport blocking grain boundaries. By the use of thin film BZY as electrolyte an improved functionality due to the shorter diffusion path for the protons is expected. Several thin film deposition techniques have been employed to fabricate BZY films, such as pulsed laser deposition (PLD), atomic layer deposition (ALD) and chemical solution deposition (CSD). Compared to other methods, CSD offers a number of advantages such as low costs, scalability and simplicity. In spite of these advantages challenges such as increase of grain size, control of microstructure and film orientation, and development of sufficiently thick and dense BZY films remain in order to optimize the conductivity and hence the functional response. In the present thesis, these issues were addressed by a combination of modified precursor chemistry with adjusted thermal processing, introduction of a dedicated seed layer concept, usage of auxiliary layers influencing the strain, and finally the infiltration technique. The application of several techniques such as thermal analysis coupled with mass spectrometry, X-ray diffraction (XRD), scanning electron microscopy (SEM) etc., enabled the characterization of the different stages of the CSD process. Electrochemical impedance spectroscopy (EIS) was used to determine functional features such as activation energy and conductivity.At first the crystallization behavior of an established BZY precursor system was investigated under different annealing conditions. It was found that through modification of annealing atmosphere (dry to humid), the crystallization temperature can be decreased to 500 ℃, which is considerably lower than the present records (700 ℃) for CSD-derived BZY films. Then the influence of chemically modified precursor chemical solution on the film microstructures was systematically investigated. By modifying the number of methylene groups of the applied solvents (from acetic, via propionic to butyric acid) precursor solutions with a considerably different decomposition and phase formation behavior were obtained. The butyric acid (BA) based solution shows the lowest decomposition temperature (400 ℃) and the corresponding amorphous films obtained after pyrolysis have the densest surfaces in this solution series. The resulting crystalline BZY films are dense and columnar grown with an average grain size of about 60 nm representing the best microstructure in this solvent series. Subsequently, the influence of the solution concentration was found to be critical for the orientation of the CSD-derived BZY films. With a tailored decrease of the concentration a seed layer concept was implemented, which enabled the controlled fabrication of grainy, (001), and (111) oriented films, respectively. Hence, the grain size of the (111) oriented film can reach about 200 nm, which is significantly larger than that (20-60 nm) of polycrystalline BZY films typically derived by established CSD approaches or even PLD. Both in-plane conductivity of the (001) oriented films grown on sapphire and out-of-plane conductivity of the (111) oriented films deposited on platinized silicon wafers correspond well to the range of 10-3- 10-6 S/cm which is typically found in the literature. In order to further improve the electrical properties, a novel concept using CSD-derived porous cathode and anode films as auxiliary layers underneath the BZY films was introduced. These films give the strain for a grain size increase during co-sintering. After sintering at 1200 ℃ for 4 h, BZY films with a high degree of densification were attained, accompanied with average grain sizes of 300-350 nm, which is comparable to that in BZY pellets sintered at 1600 ℃ for 24 h. For the porous NiO/BZY composite films, the in-plane conductivity is enhanced to 2*10-4 S/cm at 600 ℃, which is quite close to that (10-3 S/cm) of optimized BZY bulk materials. Finally, BZY films with a thickness of about 2 μm were obtained using a composite approach which consists of a chemical sol containing BZY particles and a BZY precursor solution. Meanwhile, the anode/electrolyte composite structure after sintering shows a denser surface than the pure BZY structure under the same annealing condition.

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