Synthesis, assembling and validation of solid oxide fuel cell units

Résumé

The main objectives of this thesis have been: the control of the structure and microstructure of the SOFC components (electrodes and electrolyte); and the electrochemical characterization of the manufactured materials. For this purpose, several techniques have been used: tape casting, replication with molds or 3D printing. Among the large number of material¿s characterization techniques applied in this work; special interest has been devoted to those related with the rheology for the slurries characterization and to those related with Electrochemistry, specifically on Electrochemical Impedance Spectroscopy (EIS), for the electrical characterization. La0.6Sr0.4Co0.2Fe0.8O3-¿ (LSCF) with perovskite structure is very interesting because they exhibit high oxygen permeability at elevated temperatures. Manufacture of commercial LSCF by aqueous colloidal processing has been carried out. The surface behavior of LSCF as a function of pH and the effect of a polyelectrolyte (Duramax D3005) on the stability are studied using zeta potential technique. Concentrated suspensions were prepared with a solid content as high as 35 vol.%. The optimum dispersing conditions were determined by means of rheological measurements for obtaining stable and fluid slurry for tape casting technique. Yttria (8 mol%) stabilized zirconia (YSZ) has widely been used as electrolyte in solid oxide fuel cells (SOFC). In order to obtain fluid slips, rheological studies of aqueous suspensions of three different commercial YSZ powders dispersed with a polyacrylic acid-based dispersant agent have been performed. Their viscosity was optimized by controlling the dispersant concentration, pH and homogenization time using an ultrasound probe. An electrical study of the sintered tapes prepared under strict control of the rheology was done by electrochemical impedance spectroscopy. An innovative design, alternative to the conventional metal supported fuel cells (MSC) is proposed. This new design permits the reduction of ~65% of the metallic supporting material, hence a decrease of the cost of any MSC assembled in this configuration and it offers the opportunity of produce at mass-scale in a cost-effective way. Furthermore, the way of preparing the microstructured MSC with a metal layer of 150-200 µm, allows us to prepare any type of patterning and thickness. This new design of SOFC comprises a 200 µm layer of a honeycomb-metallic framework with hexagonal cells which supports a layer of electrolyte and can be used as current collector. Each hexagonal cavity is further functionalized with a thin 5-10 µm of Ni-YSZ anode, in direct contact with YSZ electrolyte. Finally, a cold sealing through an electrical resistance welding process is possible because it is used interconnect material on one side of each SOFCs. 3D printing technique as a new tool for controlling the microstructure of the materials was studied. Microstructured organic-based molds have been designed for the deposition of YSZ and crofer slurries. The design of SOFC 3D prototypes for being fully 3D printed with ceramic powders and photopolymers is proposed and some successfully proofs have been performed.