| Summary: | 碩士 === 國立臺灣科技大學 === 材料科技研究所 === 97 === This research was focused on the electrospinning and cathode material, Investigation of electrospinning 8YSZ fiber and its application in SOFC composite cathode, fiber has appear several amazing characteristics such as very large surface area to volume ratio. Using fiber in the cathode to expand the effective catalysis area of cathode, and also improve dissolution ability of the electrode to oxygen. The cathode effect on polarization loss of cathode oxygen reduction reaction and electrical chemical reaction with electrochemical impedance spectroscopy. Tafel curve analysis was used to observe the performance of electrochemical reaction at the TPB’s and power density measurement was used to check the efficiency of the fuel cell.
8YSZ fibers were fabricated by electrospinning process which is a low cost production methode. Electrospinning was involved with many effective parameters. Viscosity of the solution and the applied voltage played a significant role to generate long and uniform size fibers. Electrospinning technique was adopted to produce 8YSZ fibers using PVP as solvent material. The best result is achieved with PVP concentration of 9.89 wt% and electric field of 20 kV to get smooth fibers. Fibers of average size of 70 nm were generated using electrospinning.
According to the statistics from Thermogravimetry Analysis(TGA), no weight loss above 600℃. Sintering temperature above 1000℃, 8YSZ fibers analyzed by XRD was Cubic phase. Scanning electron micrographs of the fibers sintered at various temperatures show increasing fiber size and length.
The mixture of 8YSZ fibers and La0.6Sr0.4Co0.2Fe0.8 powder(LSCF) at the ratio of 20:80 in weight percent were used to screen print on 8YSZ electrolyte. The bulks were sintered at different temperatures(1000, 1050, 1100 and 1150℃). Scanning electron micrographs of the cathode sintered at various temperatures show increasing fibers and LSCF grains size and decreasing porosity of cathode.
AC impedance analysis of electrolyte-supported half cell(LSCF-8YSZ fiber/8YSZ/Pt) , the least value of area specific resistance at sintering temperature of 1050℃, because this sintering temperature has more triple phase boundary(TPB) and the best porosity distribution. Moreover, the energy required to activate the electrochemical reaction is lower(1.11 eV) in the cathode sintered at 1050℃.
Exchange current density measured using Tafel curves and polarization curve(I vs.η curve) indicates that the electrochemical reaction is faster at sintering temperature of 1050℃.
Among the sintering temperature of LSCF-8YSZ fiber cathode, sintering temperature at 1050℃ performed the best because the cathode electrical impedance was quite low(0.77 Ωcm2) and the highest change current density(141.46 mA/cm2). Additionally the catalysis was significantly improved, due to the fastest Oxygen reduction reaction rate.
The efficiency of the single cell in converting the chemical energy in to electrical energy was estimated by measuring the power density using 40%H2+60N2(100 sccm) as fuel and air as oxidant. Using Ni-8YSZ as anode material, Ni-8YSZ/8YSZ/LSCF-8YSZ fibers single cell has shown the power density of 46.66 mW/cm2 at 800℃ which is much higher than Ni-8YSZ/8YSZ/LSCF-8YSZ powders. And to go though the single cell test with excellent ionic conductivity of the electrolyte materials Zr0.92Y0.155Mg0.005O2.0775(YMSZ) replaced 8YSZ powders of anode, Ni-YMSZ/8YSZ/LFCF-8YSZ fibers has shown higher power density of 51.26 mW/cm2 than traditional anode material.
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