| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 96 === Ni-Ce0.78Gd0.2Sr0.02O2-δ anode materials, having high ionic conductivity GDCSr fibers and fine Ni particles, sintered at different temperatures were developed for intermediate temperature solid oxide fuel cell application. Electrospinning deposition technique was adopted to produce nano sized GDCSr fibers using PVP as solvent material. Viscosity of the solution and the applied voltage played a significant role to generate long and uniform size fibers. The flow rate of 1.5 litre/h and voltage of 20 kV were maintained throughout the experiment. Fibers of average size of 100nm were generated using a collector placed at a distance of 12 cm. XRD of the GDCSr fiber sintered at different temperatures revealed in cubic phase. Scanning electron micrographs indicate that the size and length of fiber increase with increasing the sintering temperature. The existence of cubic structure and formation of uniform size fibers was confirmed by bright field image and diffraction patterns of transmission electron microscopy. The mixture of fiber and powder at weight of 75:25 was used to screen print on the GDCSr electrolyte. The anode films were prepared via a nickel wet dipping process and sintered at different temperatures. The micrograph of the anode sintered at 1200 ℃/1hr has a well defined microstructure in terms of electrolyte area covered with nickel and the triple phase boundary line between electrolyte, electrode and gas phase. Higher sintering temperature resulted in the formation agglomerates of nickel particles and crack in the films, which might be due to particle size of the initial powder.
Two important aspects for using fiber based anode, the kinetics of the hydrogen oxidation reaction and the effect of the microstructure on the electrochemical performance of the anode. Insight in these two aspects will lead to a better understanding and further improvement of the anode by changing the sintering temperature of the fiber based anode. For electrochemical characterization of the electrodes, impedance and tafel measurements are performed. Impedance measurements performed under standard conditions resulted in spectra, which when analyzed with an equivalent circuit, are built up of two semicircles and are found dominating with sintering temperature. The high frequency semicircle is ascribed to charge transfer phenomenon at the interface of anode and electrolyte and the low frequency semicircle is ascribed to the concentration polarization. The area specific resistance of around 28 ��-cm2 is observed at 550 ℃ for the anode sintered at 1200oC for 1hr. The significant reduction in overall electrode resistance in half cell with anode sintered at 1200 ℃/1hr can be attributed to an increased number of active sites. Moreover, the energy required to activate the electrochemical reaction is lower (0.86eV) in the anode sintered at 1200 ℃. The equivalent circuit constructed by fitting the Cole-Cole plots shows the deviation in the distribution of nickel particles and discontinuity of the interface between electrolyte and anode. Hydrogen oxidation reaction was estimated using tafel curves by calculating the exchange current density. Higher exchange current density of 5.2mA/cm2 was observed in the hall cell for the anode sintered at 1200oC/1hr at all working temperatures from 400 ℃ to 550 ℃. Half cells with fiber anode, bulk electrolyte and Pt cathode were developed. Half cell with anode sintering temperature of 1200 ℃/1h has shown better performance compared to other sintering temperatures, due to increase in triple phase boundary regions with long fiber anodes. The maximum power density of 10.02mW/cm2 is recorded in the voltage-current characteristic curves drawn at different temperatures. Transmission electron microscopy analysis was carried out to identify the reason for the reduce in reduction temperature (600 ℃/2hr) Electrolyte material after reduction has shown larger and higher number of dislocations compared to the electrolyte before reduction. SEM images have shown the difference in grain size. Grain size is found larger after reduction. XRD patterns reveal cubic phase in both materials. This was also confirmed from TEM diffraction pattern. The possible reason might be the addition of higher ionic radii elements such as Sr for Ce, enhances the lattice distortion and to compensate this lattice distortion dislocations are formed in the material. Moreover, the reduction of CeO2 to Ce2O3 has effected in increasing the dislocations in the material due to increase in electronic conductivity. The overall data indicates that the sintering temperature of the fiber anode has a significant effect on electrochemical performance of the half cell.
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