Investigation And Application Of Direct Carbon Solid Oxide Fuel Cells Fabricated By Tape Casting Technique

A direct carbon solid oxide fuel cell?DC-SOFC?is attracting increasing attention because of its high energy density,environmental friendliness,no need for precious metal catalysts and all-solid-state structure.Fabrication and portable application of DC-SOFCs by tape casting are studied in this thesis.Ni-YSZ anode supported DC-SOFC fabricated by tape casting and improvement of the problems between YSZ electrolyte and LSCF cathode are discussed deeply.A novel DC-SOFC stack is fabricated on an electrolyte plate with punching holes to connect singles cells in series.At the same time,the electrolyte-supported DC-SOFC is fabricated by aqueous tape casting,which provides the experimental basis for decreasing the fabrication cost of the cells and reducing the environmental pollution problem caused by organic solvent slurry.Tape casting technique is simple,low-cost,industrialized and produced largly,which is not only suitable for preparing electrolyte films,but also for electrode films.An anode-supported solid oxide fuel cell?SOFC?is fabricated by tape casting technique.Yttrium stabilized zirconia?YSZ?is used as the material of electrolyte,and a cermet of nickel and YSZ is used as the material of anode.A composite of LSCF-GDC is used as the cathode material.YSZ-GDC and GDC interlayers are prepared between the cathode and the electrolyte film.The output performance and impedance spectrum of cells are tested with 3vol.%humidified hydrogen and solid carbon with 5 wt.%Fe as fuel,respectively.It is found that the open circuit voltage?OCV?of a SOFC operated on humidified hydrogen reaches 1.0V at 800?.A maximum power density of a SOFC with the double-interlayer cathode(680mW×cm^-2)is 83%,which is larger than that of the cell without interlayers(372 mW×cm^-2).Meanwhile,when activated carbon is used as the fuel,the SOFC with the addition of double-interlayer shows an OCV of 0.95 V at 850?.Its maximum power density is 429mW×cm^-2,which is almost twice of that of a similar direct carbon SOFC without the interlayers(225 mW×cm^-2).Especially,the double-interlayer cathode increases the fuel utilization of a DC-SOFC by 33%.Although the power density of the anode-supported DC-SOFC is relatively high,the mechanical strength and stability are far less than the electrolyte-supported cells,which greatly limit the practical application value of the anode-supported DC-SOFC.In order to further enhance the feasibility of DC-SOFC in practical applications,a four-cell-in-series stack of solid oxide fuel cell is fabricated on a single yttrium-stabilized-zirconia electrolyte plate by tape casting technique.The slurry for the tape casting process is characterized and optimized through Zeta potential and viscosity measurements.The optimal slurry is used to prepare the electrolyte plate,with electrical connecting holes,for the stack.The stack is operated directly with 5 wt.%Fe-loaded activated carbon as the fuel.With a total effective area of 8.8 cm² and carbon fuel of 5 g,this direct carbon solid oxide fuel cell stack shows an open circuit voltage of 3.82 V and an output power of 2.2 W at 820?.It gives a discharging platform of over 3.0 V for 10.5 h with a constant discharging current of 200 mA at 800?.Then voltage drops to 0 V after discharging for 14 h,releasing a discharging energy of 8.6W×h.This work shows a possibility of developing DC-SOFCs for applications in portable or distributed power sources.The DC-SOFCs in the above experiments are fabricated by non-aqueous tape casting.Although the technique has a fast drying rate,good wetting performance and high green quality,its solvent is flammable and costly.The water-based solvent has the advantages of non-toxicity,nonflammability,high accessibility and low cost.However,the research on the fabrication of YSZ electrolyte by aqueous tape casting is not enough,which limits its application and development.Thus,the preparation of YSZ electrolyte-supported DC-SOFCs by aqueous tape casting is studied.The drying rate of green tape is accelerated to shorten the preparation cycle of cells by infrared heating device.At the same time,through doping 4 wt.%Al₂O₃ into YSZ and controlling the sintering temperature,not only the sintering performance of YSZ electrolyte is promoted,but also the electrochemical properties of DC-SOFC is improved significantly.The DC-SOFC with 4 wt.%Al₂O₃ and sintered at 1450? is tested with 0.5 g Fe-loaded?5 wt.%?activated carbon as the fuel.It gives an open circuit voltage of1.00 V and a peak power density of 351 mW×cm^-2 at 850?.The DC-SOFC discharges at a constant current of 30 mA for 25.3 h at 800?,giving a discharging energy of 0.82 W×h×g^-1.