Fabrication and Characterization of Hollow Fibre Micro-tubular Solid Oxide Fuel Cells
Creators: name: Droushiotis, Nicolas D.
Publisher: name: Imperial College London
Year: 2011

description: Despite three decades of development of solid oxide fuel cells (SOFCs) since the conception of the tubular Siemens–Westinghouse design, no practical alternatives to yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (CGO) electrolytes have been established. However, there have been considerable improvements in the performance of SOFCs, decreasing their specific overall costs, by decreasing operating temperatures, understanding their reaction kinetics, increasing specific surface areas of electrode / electrolyte / reactant three-phase boundaries, establishing new fabrication techniques and employing new geometric designs. So called micro-tubular SOFCs (MT-SOFCs) are one of the most promising geometric designs, though a misnomer, as tube diameters are normally several millimetres, significantly smaller than Siemens–Westinghouse SOFCs with 22 mm tube diameters. This three-year Ph.D. project was aiming to establish the feasibility of, and develop, a novel design of SOFC, fabricated using hollow fibres (HFs) with diameters of hundreds of micrometres, thereby increasing the specific surface area of electrodes, increasing the power output per unit volume/mass, facilitating sealing at high temperatures, and decreasing costs. Collaborators used a spinneret in phase inversion process to produce HFs with non-porous, gas-tight cores and porous outer layers ca. 50-100 μm thick; suspensions of YSZ or CGO particles were used to produce the precursor micro-tubes for electrolyte-supported structures. After sintering the HFs, Ni was deposited electrolessly onto their inner surfaces to form Ni-YSZ anodes, using aqueous nickel (II) solutions and (sodium) hypophosphite (H2PO2-) as the reducing agent. With YSZ electrolyte-supported structures, lanthanum strontium manganite (LSM)-YSZ particles were then coated onto outer surfaces of the HFs to form cathodes; these cells produced only 46-400 W m-2 at 800 oC, compared with ca. 800 W m-2 at 600 oC for CGO-supported cells. Anode-supported structures were also produced using non-conductive, porous NiO-YSZ HFs as anode precursors. YSZ particles were suspended in ethanol and electrophoretically deposited (EPD) onto the external surface of NiO-YSZ HFs, requiring electric fields of ca. 22 kV m-1 between a tubular Cu cathode, placed inside the porous HF precursor, and a tubular platinised titanium mesh anode; this implied they had an effective positive charge. The YSZ-coated NiO-YSZ fibres were then co-sintered at 1500 oC. Mixed (YSZ-LSM) and pure LSM cathode layers, for creating functional layers and enhanced current collector electrodes, were deposited using a paint brush and re-sintered at 1200 oC. The resulting anode-supported HF-MT-SOFCs delivered peak power density of 2 kW m-2 at 800 oC. Collaborators then used a triple orifice spinneret in the phase inversion process to co-extrude CGO/NiO-CGO dual layer-HFs, which were then co-sintered. Dispersions of CGO-LSCF particles were then painted or sprayed onto their outer surfaces, as "graded" LSCF-CGO porous cathode precursors that were then sintered at 1200 oC. HF-MT-SOFC fabrication was completed by winding a silver wire current collector spirally round the cathode. Similar arrangements were used for collecting the current from the HF lumen (anode). The use of functional cathode layers, higher porosity anodes, improved anode and cathode current collectors, and optimizing the thickness of the electrolyte layer and operating parameters, enabled maximum power densities of ca. 25 kW m-2 at ca. 600 oC, believed to be a record for a single MT-SOFC. The effects of electrolyte thickness (100-10 μm), cell length (10-50 mm), and anode morphologies / porosities were also determined. HF-MT-SOFCs were found to be stable to reduction/oxidation and thermal cycling for up to 8 days. Finally, a novel design for stacking individual HF-MT-SOFC in series (voltage scale up) and parallel (current scale up) was studied experimentally; 3 HF-MT-SOFCs in parallel delivered ca. 0.67 W (=3.4 kW m-2) at 7.5 kA m-2, 0.45 V and 600 oC.