| This thesis aims at evaluating the robot-controlled direct-write microfabrication technique for creating SC-muSOFCs with coplanar electrodes, and at characterizing the fabricated cells to advance the understanding of this fuel cell technology. Direct-write microfabrication consists of the pressure-driven extrusion of the electrode material in the form of inks through micronozzles and its robotically controlled deposition on the electrolyte substrate in the desired shape. Electrode inks were synthesized from conventional SOFC electrode materials by adapting inks used for screen printing to the direct-writing technique. Coplanar microscale electrode structures of different shapes and geometries were successfully fabricated. The Newtonian flow behavior of the inks, however, caused slight variations in electrode width and interelectrode gap. The development of viscoelastic, gel-like inks improved the shape retention of the deposited electrode structures and led to enhanced control of the electrode dimensions.;Subsequently, the limits to the miniaturization of such micro fuel cells were systematically investigated using cells composed of one line per electrode. Below a critical electrode width, the establishment of a stable OCV and power output was impeded by the small active electrode surface area, which did not enable a stable oxygen partial pressure gradient between the closely-spaced microelectrodes. This miniaturization limit could be circumvented by increasing the number of electrode lines to form interdigitated electrode structures with maximized surface area and reduced ohmic resistance. These original results highlight that, despite the general consensus on the need for microscale electrode structures and model-based predictions of optimal cell performance for electrodes of several microns in width, miniaturization limits must be considered in cell design and fabrication. At the same time, these observations present an experimental confirmation of the advantages of using interdigitated microelectrode patterns as compared to a single line per electrode.;Furthermore, the effects of electrode shape, current collection method and electrode material on cell performance were characterized. A proof-of-concept of SC-muSOFCs with geometrically complex coplanar electrode patterns was provided. Additionally, for electrodes of similar dimensions, differences in electrode shape were found not to significantly affect the cell performance. Performance limitations of SC-muSOFCs with coplanar interdigitated electrodes due to long electronic conduction paths and resulting elevated ohmic resistance are known to the community, but were quantified for the first time in this thesis. A 50% loss in power was observed, when the current was collected, according to the state-of-the-art procedure, on the segment connecting the single lines of the respective electrode and not on the whole electrode surface. The use of a more conductive, nickel-rich anode did not improve current collection because of nickel instability in fuel-air mixtures. An efficient current collection method remains a critical challenge for improving the performance output of these fuel cells. Cells with different combinations of anode and cathode materials were characterized in order to investigate the effect of cell component materials on the cell functioning. The observation of chemical interaction between closely-spaced coplanar cathodes and anodes revealed the need for proper selection of cell component materials, both with respect to cell performance as well as possible interactions.;Finally, theoretical considerations of the cell performance led to the identification of several challenges for modeling the performance of SC-muSOFCs with coplanar electrodes. These challenges include an accurate modeling of the ohmic resistance to ionic and electronic conduction, details of electrode reaction kinetics under single-chamber operating conditions and the effects of gas transport in the vicinity of closely-spaced electrodes. However, a simplified electrochemistry-based model can be used as a tool for predicting the performance of cells composed of one line per electrode and establishing design guidelines for SC-muSOFCs with coplanar electrodes.;In the second part of this thesis, the fabricated cells with interdigitated electrodes were electrochemically characterized at 700°C in a methane-air mixture. The generation of open circuit voltages of ∼0.8 V and maximum power densities of a few mW/cm2 confirmed the feasibility of SC-muSOFCs with coplanar electrodes by direct-write microfabrication and their practicability for fuel cell applications in the mW-range.;Evaluating the fuel utilization and efficiency of the tested cells emphasized the potential use of SC-muSOFCs for cogeneration, as sensors and in energy harvesting applications where waste gases from industrial or automotive exhaust gas streams can be transformed into electricity. (Abstract shortened by UMI.)... |
