Jochen Joos

Electrooxidation of Reformate Gases at Model Anodes

This paper summarizes the experimental and modeling results concerning the electrooxidation of hydrogen and carbon monoxide, the main oxidizable compounds in reformates, at patterned nickel anodes on polycrystalline yttria stabilized zirconia electrolytes. The line specific resistance of the three phase boundary was evaluated within a wide range of gas compositions and temperature. The investigations showed that microstructural stability, impurities, accelerated degradation and reversible dynamic changes are key issues which have to be considered. Elementary kinetic models, parameterized with literature data, temperature-programmed desorption and reaction and quantum chemical calculation results were in excellent agreement with the experimental data. For the first time it could be shown that the line specific resistance values evaluated by means of patterned anodes are applicable in homogenized and space resolved models for cermet anodes.

Transient 3D FEM Impedance-Model for Mixed Conducting Cathodes

The performance of solid oxide fuel cells (SOFCs) is often determined by the polarization resistance of the electrodes. Electrochemical impedance spectroscopy (EIS) enables a deconvolution of individual electrochemical processes. In case of mixed ionic-electronic conducting (MIEC-) cathodes the impedance spectra result from the coupling of gas diffusion, surface exchange and bulk diffusion of oxygen ions. In this paper we present a three-dimensional (3D) finite element method (FEM) model which allows the transient simulation of the underlying processes in a porous cathode structure. The developed model is validated with a well established homogenized 1D model by comparing the area specific resistance and the corresponding impedance spectra. In case of a homogeneous 3D microstructure the FEM simulation results show an excellent agreement with the homogenized 1D model. Furthermore, the 3D FEM model is applied for impedance simulations of a technical MIEC cathode which microstructure was reconstructed from FIB tomography.

Directional freeze-cast hybrid-backbone meso-macroporous bodies as micromonolith catalysts for gas-to-liquid processes

Materials with spatially organized and multimodal porosities are very attractive in catalysis, as they can reconcile nano-confinement effects in micro- and mesopores with fast molecular transport in wide macropores. However, the associated large pore volumes often result in low overall thermal conductivities, and thus suboptimal heat management in reactions with a high thermal signature, usually with a deleterious impact on the catalytic performance. Here we report the directional freeze-casting assembly of bimodally meso-macroporous micromonolithic bodies with a hybrid backbone composed of intimately bound carbon nanotubes (CNTs) and ZrOx–Al2O3 nanocrystals. A honeycomb-shaped and axially oriented macroporous architecture is achieved through the use of zirconium acetate as an ice growth modulator. (S)TEM and EDX nanospectroscopy show that the nanoscale intimacy between the CNT and oxide backbone components depends on the synthesis route of the mother slurry. As revealed by X-ray tomography, coupled to quantitative image analysis, not only the macrochannel size and wall thickness, but also the extent of axial heterogeneities in macropore diameter and spatial orientation depend on the axial temperature gradient rate during casting. The structured bodies are explored as carriers for cobalt-based catalysts for the Fischer–Tropsch production of synthetic hydrocarbons from syngas, of central significance in intensified X-to-liquid processes. Hybrid CNT-Al2O3 backbone micromonolith catalysts show a high selectivity to C3–8 olefins, owing to the fast evacuation of these primary reaction products from the metal active sites through the directional macropore system. Remarkably, the high olefin selectivity is maintained up to higher operating temperatures compared to reference catalysts based on all-oxide supports, due to a higher effective thermal conductivity which inhibits the development of hotspots under industrially relevant operating conditions.

Parameter Estimation for a Reconstructed SOFC Mixed-Conducting LSCF-Cathode

The performance of a solid oxide fuel cell (SOFC) is strongly affected by electrode polarization losses, which are related to the composition and the microstructure of the porous materials. A model that can decouple the effects associated with the geometrical arrangement, shape, and size of the particles together with material distribution on one side and the material properties on the other can give a relevant improvement in the understanding of the underlying processes. A porous mixed ionic-electronic conducting (MIEC) cathode was reconstructed by focused ion beam tomography. The detailed geometry of the microstructure is used for 3D calculations of the electrochemical processes in the electrode and to calibrate a well-established reduced model obtained by averaging. We perform a model-based estimation of the parameters describing the main processes and estimate their confidence regions using the calibrated reduced model.