Manufacturing optimizations for micro-tubular fuel cells by extrusion and dip coating techniques

The optimisation of micro-tubular Solid Oxide Fuel Cells (mSOFCs) was studied, including anode extrusion, electrolyte dip coating and heat treatment of the resultant half-cells. The study of NiO-YSZ anode extrusion started with an analysis of powder packing, followed by the determination of the optimum liquid content required to produce reliable roll milled viscous paste mixtures. Rheological tests were carried out using a simple die configuration to predict the extrusion pressure during the anode processing. The mechanisms, which lead to differences between the predicted and experimental extrusion pressures were explored. The pore former (corn starch) in the paste formulation was used as a microscopic indicator of phase maldistribution distribution. This redistribution gave a dense ceramic layer at the interface between the bulk paste and the die pin or mandrel. It was suggested that this redistribution was the major contributor to the varied extrusion pressure recordings as extrusion progressed. The phenomenon was thought to be a significant contributor to the experimentally measured pressure being three times of the predicted value. Extrusions with a solid load of ~70 vol% were shown to exhibit stable pressures and produce homogeneous defect-free anode tubes. A YSZ electrolyte coating method was developed, involving the determination of binder and dispersant fractions within the slurry to optimise performance, plus the control of coating thickness by adjusting processing parameters (solid weight fraction and withdrawal speed). Two-step heat treatments were employed to generate a dense electrolyte layer. Dip coated electrolyte layers with a dense sintered thickness between 20 and 30 µm were obtained at a solid mass fraction of 50 wt% in the slurry and a withdrawal rate of 80 mm/min. The heat treatment to develop a dense electrolyte structure was determined as a pre-heating of the anode tube at 1100 °C followed by a re-heating of the electrolyte coated dual structure at 1350 °C. After the addition of cathode (La\(_0\)\(_.\)\(_8\)Sr\(_0\)\(_.\)\(_2\)MnO\(_3\), LSM) and current collection (silver) components by brush coating, the fabricated full cells are electrically characterised in terms of current-voltage polarization and electrochemical impedance spectroscopy (EIS). The open circuit voltage (OCV) and peak power density were 0.82 V and 0.11 W/cm\(^2\) respectively.