Fabrication And Performances Of Micro Tubular Solid Oxide Fuel Cells

Micro tubular solid oxide fuel cells (MT-SOFCs) have millimeter or sub-millimeter grade diameters. They have attracted increasing interests due to their high volumetric power density, good mechanical properties, good thermo-cycling behavior, and quick start-up and shut-down operations. They will be applied in a variety of regions such as power sources for portable devices, uninterruptible power supplies (UPS) and dynamical power sources for automobiles. In this study, a phase inversion-sintering process has been developed to prepare the hollow fibre ceramic micro tubes with a special pore structure, from which the micro tubular solid oxide fuel cells with ideal structures and good performances were subsequently fabricated. It provides a new route to form the micro tubular solid oxide fuel cells with simplified operation and fairly low cost.First of all, the YSZ (Y2O3 stabilized ZrO2) and SDC (Sm2O3 doped CeO2) electrolyte hollow fibers were prepared by the phase inversion-sintering process. The obtained YSZ and SDC hollow fibers have a diameter of 1.1~1.4mm and wall thickness of 210~230μm. Gas-tight YSZ hollow fibers may be obtained by sintering at temperatures higher than 1500℃, and their bending strength may attain up to 355MPa when sintered at 1600℃, while the SDC hollow fibers may be sintered into gas-tight micro tubes under temperatures higher than 1450℃with the bending strength of higher than 200MPa. The electrolyte-supported micro tubular SOFCs were fabricated by dip-coating Ni/YSZ anode and LSM (La0.8Sr0.2MnO3-x) cathode (for YSZ tube) or Ni/SDC anode and BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3) cathode (for SDC tube) on the outer and inner surfaces of the YSZ or SDC electrolyte micro tubes. The YSZ and the SDC hollow fiber supported micro cells exhibit the highest output densities of 87 mW·cm-2 at 850℃and 106 mW·cm-2 at 750℃, respectively with H2/O2 as the fuel/oxidant. The low output power density is the result of the large thicknesses (210~230μm) of the electrolyte hollows and the finger-like pores conserved in the fiber walls. Therefore, the Ni containing anode-supported micro cells with an ultra thin electrolyte film may be a more favorable structure to give high output power densities.The NiO/YSZ anode micro tubes have been prepared through an improved phase inversion-sintering process. NiO/YSZ anode powders were dispersed under stirring in the PESf/NMP polymer solution with PVP as the dispersant and pore-maker to form a spinning solution. Following the same procedures as for the preparation of YSZ and SDC fibers, NiO/YSZ hollow fibers were obtained. These fibers were then reduced by H2 at 800℃to form the Ni/YSZ cermet anode micro tubes. The effects of sintering temperature on the properties of the anode fibers have been extensively investigated. The resultant anode micro tubes possess an asymmetric structure comprising of a micro porous outer layer integrated with a finger-like porous sub-layer. Furthermore, the outer surface of the micro tubes is very smooth and is favorable for coating electrolyte films. As the sintering temperature was increased from 1200°C to 1400°C, the mechanical strength and the electrical conductivity of the Ni/YSZ hollow fibers increased from 35 MPa to 178 MPa and from 30S·cm-1 to 772 S·cm-1, respectively but the porosity decreased from 64.2% to 37.0%. The optimum sintering temperature was found to be between 1350°C and 1400°C for Ni/YSZ hollow fibers applied as the anode support for micro tubular SOFCs.A dip-coating and co-sintering technique was applied to form the thin and dense YSZ electrolyte films on the outer surface of the NiO/YSZ micro tubes to form anode/electrolyte half cells. A suspension containing 15wt% of YSZ powder, 2wt% PVB binder, 5~6wt% TEA/YSZ surfactant and a small amount of DBP and lubricant PEG was firstly produced. The NiO/YSZ hollow fiber precursor was then dipped into the suspension for about 1 min so as to form a green YSZ film. Each dip-coating operation may finally produce a YSZ film of around 5μm thickness. In order to make a thin but dense enough electrolyte film, twice dip-coating operation resulting in a 10μm thickness film has proved to be the best. The one-coating resulted films (~5μm) may contain defects but three-coating resulted films (~15μm) are susceptible to crack in addition to the high electrical resistance. The coated NiO-YSZ hollow fibers were then co-sintered at 1400°C to form the dense electrolyte films that are integrated tightly with the porous anode micro tubes. On the outer surface of the anode/electrolyte half cell, a layer of porous LSM membrane was coated as the cathode so as to form a complete single micro tubular cell. The sintering temperature for the cathode integration is 1200°C, and the thickness of the final cathode is about 20μm.The resultant anode-supported micro tubular cells were finally tested at various conditions. When H2/O2, H2/Air or CH4/Air were used as the fuel/oxidant gases, the highest power density of the anode-supported cell attains up to 960, 820 or 480 mW·cm-2 at 800°C, respectively. However, carbon deposition has been observed in the use of methane as fuel, leading to the fast degradation of the cell output density.