Freeze Casting of Tubular Anode Supports for High-Performance Solid Oxide Fuel Cells

Abstract

Freeze casting method has received significant attention in recent years for the fabrication of porous materials with unique pore geometries. A typical freeze casting process involves freezing of a liquid suspension forming solvent crystals that grow in the direction of solidification. When these solvent crystals are removed by sublimation under a reduced pressure, a porous structure with unidirectional and gradient pores is achieved. Such a porous structure is considered ideal for solid oxide fuel cell (SOFC) electrodes because it allows for rapid gaseous diffusion through the bulk of the electrode and a large number of active sites toward the electrolyte–electrode interface for electrochemical reactions. In the present work, we developed a freeze casting and freeze drying process to fabricate tubular SOFC anode supports using an aqueous NiO–yttria-stabilized zirconia (YSZ) slurry. Freeze casting was performed in an aluminum mold that contained peripheral coils for flowing a coolant. After complete freezing of the slurry, the mold was connected to a vacuum chamber for freeze drying. With a proper control of the experimental setup and adjustments in the freeze drying profile, we were able to reduce the total drying time from several days to a few hours. The fabricated anode supports were coated with the thin layers of YSZ electrolyte and strontium-doped lanthanum manganite (LSM)-YSZ cathode to obtain the single cells. The electrochemical performance of the single cells was evaluated by the I–V polarization curves and electrochemical impedance spectra. Owing to the favorable gas transport properties of the freeze-cast anode, the concentration polarization was found to be greatly reduced and thus a high power density was achieved. Figure 1