PhD Tobias Huber

Identifying, Visualizing and Modifying Reaction Pathways of Oxygen Reduction on Sr-Doped Lanthanum Manganite (LSM) Model Electrodes

Abstract

Sr-doped lanthanum manganite (LSM) is the most commonly used cathode material in solid oxide fuel cells (SOFC). Nevertheless, many aspects of the oxygen reduction at LSM electrodes are not yet completely understood. Particularly important in this respect is the resistance of oxygen reduction kinetics (ORK), which often exhibits the highest loss in thin film electrolyte SOFCs. As a consequence, substantial optimization becomes possible when finding the rate limiting steps and understanding ORK of the reaction mechanisms. The aim of this thesis is to reveal, visualize, quantify and modify the reaction pathways of oxygen reduction on LSM, to identify the rate limiting steps and to find correlations between structural or chemical properties and ORK. In order to facilitate these experiments, several new measurement set-ups were designed and constructed to extend the experimental prospects, to combine existing measurement techniques, analytical methods and experimental tools.

In the present thesis, oxygen reduction kinetics of LSM on yttria stabilized zirconia (YSZ) was investigated by means of geometrically well defined, dense LSM microelectrodes and thin films on YSZ (100) single crystals. For the sake of comparison, also Pt/YSZ systems were analyzed. LSM was deposited by pulsed laser deposition (PLD), Pt by means of magnetron sputter deposition. Both types of thin films were subsequently microstructured by photolithography and chemical or ion beam etching. The electrochemical characterization of the resulting electrochemical half cells was performed by impedance spectroscopy, current-voltage measurements and 18O tracer experiments, combined with time of flight secondary ion mass spectrometry (ToF-SIMS) analysis.

Impedance measurements on differently shaped and sized macroscopic and microscopic LSM electrodes identified two parallel reaction pathways of ORK, a surface and a bulk path. By adding an oxygen blocking Pt capping layer, the LSM bulk path could partly be blocked. By varying the three phase boundary (3PB) length, the polarization resistance of the surface path was shown to scale with the 3PB length. On circular microelectrodes in the lower temperature region (lower ca. 700 °C), most of the current is carried by the 3PB length related surface path, while at high temperatures (higher ca. 700 °C) the bulk path is dominating.

It is shown experimentally, e.g. by thermovoltage measurements, impedance spectroscopy and infrared camera pictures, as well as by finite element modelling, that substantial temperature gradients may arise in asymmetrically heated microelectrode experiments. For a complete avoidance of the temperature inhomogeneity problem, a novel symmetrically heated vacuum-micro-contact set-up was built. This was essential for accurate impedance measurements on microelectrodes and also facilitated quantitative 18O tracer exchange experiments on polarized microelectrodes in controlled atmosphere.

Tracer exchange measurements on both, polarized and non-polarized polycrystalline microelectrodes, exhibit resoluble contributions from diffusion and surface exchange kinetics of grains and grain boundaries. The two parallel and interacting diffusion pathways, via grains (Db and kb) and grain boundaries (Dgb and kgb) were also successfully simulated by a 3D finite element model. These investigations showed that grain boundaries may not only facilitate fast oxygen diffusion, but also fast oxygen exchange kinetics. Variation from stoichiometric LSM to A-site non-stoichiometric LSM ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Experiments were also performed on epitaxial layers without grain boundaries, where properties (Db and kb) are close to those of the grains in polycrystalline layers. Additionally, cathodically polarized microelectrodes showed a tremendous increase of 18O concentration in the LSM films with an apparent uphill diffusion, which could be explained and simulated.

The novel measurement set-up was also used to verify a bulk path through Pt microelectrodes with an electrode polarization resistance depending on the Pt grain size in the low temperature region ( lower 450 °C). The corresponding activation energy is 0.15 eV and the rate limiting step most probably oxygen diffusion along the Pt grain boundaries.