| Summary: | 碩士 === 國立臺灣科技大學 === 材料科技研究所 === 95 === Processing and characterization of Ni-YSZ cermet anode powders replaced with Ni-YSZ nanofibers developed by impregnation method is mainly investigated in this thesis. The processing and characterization of electrospinning of YSZ fibers were also discussed.
In this experiment, YSZ nanofibers were fabricated by electrospinning process which is a low cost and plenty production method. Electrospinning was involved with many effective parameters in which viscosity of the solution and applied electric field are the absolute parameters.
The better result is achieved with PVP concentration of 11.32wt% (alcohol×25wt%), viscosity of the solution of 334cps, electric field of 20kV and YSZ fibers with diameter of 40nm. In order to combine nanofiber with SOFC anode, YSZ fibers are calcined at 550℃/2h to remove the polymer and final sintered at 1300℃/1h for good crystallinity and sinterability.
To understand the behavior of YSZ nanofibers and to check the distribution of NiO, the fibers are impregnated with Ni(NO3)2•6H2O solution of concentrations 0.34M, 0.68M and 3M. The concentration of 3M is found to be appropriate to get continue phase of NiO. Besides, the pH of the solution is also observed to effect the adhesion of Ni+. For better adhesion of Ni+, the PH of impregnating solution should be higher than IPE (isoelectric point)(4.8) of YSZ. Therefore in the present study, the PH of impregnating solution is taken as 5.4.
In the present study, anode made of Ni-YSZ nanofiber is controlled by two parameters, in which first one is changing the concentration of impregnating solution and the second one is adding 25 wt% YSZ powder to YSZ nanofibers to become [75 wt%YSZ fiber + 25 wt%YSZ powder] mixed composition. Three different compositions used to prepare anode are as follows:
(1) YSZ fiber dry impregnated by 0.68M nickel nitrate solution, denoted as F0.68M.
(2) YSZ fiber dry impregnated by 3M nickel nitrate solution, denoted as
F3M.
(3) The mixture of 75 wt% YSZ fiber + 25 wt% YSZ powder which is wet impregnated by 3M nickel nitrate solution, denoted as FP3M.
From the microstructure of NiO distribution in anode impregnated by 0.68M nickel nitrate solution, it was found that the content of NiO is too less to form the continue phase and hence can’t conduct the electrons to the outer circuit. The higher concentration of nickel nitrate solution(3M) was found to form the better continue phase. But after analyzing through AC impedance, it was found that pure YSZ fibers support F3M anode suffer to huge impedance for charge transfer. In comparison with mixed YSZ support, FP3M anode does not suffer to the above phenomenon. Combining the microstructure and impedance analysis, it reveals that the cause for huge impedance for charge transfer is due to high porosity of YSZ fibers and week adhesion between electrode and electrolyte. Moreover, the conducting path is too small at the anode/electrolyte interface and hence suffered to huge impedance for charge transfer and makes the reaction poor. In contrary to the mixed support anode, the impedance for charge transfer is less in FP3M.
Tafel curve indicates the linearity between the current passing through the anode and the activation overpotential. At 800℃, the activation overpotential sequence is found to be ηF0.68M > ηF3M ηFP3M. In the region of higher currents, we are able measure the limiting current density which is an indicator of the concentration overpotential. At the same concentration overpotential (1.23V), the reaction current of FP3M is the largest indicating that FP3M can generate larger currents.
The power densities measured for the three semi-cells at 800℃ are
F0.68M:17.6mW/cm2，F3M:23.5mW/cm2，FP3M:28.9mW/cm2. It is found that the performance of semi-cell with FP3M anode is better than other two cells, indicating the use of FP3M made of YSZ fiber by impregnation method as an anode in the fabrication of single cell with 8YSZ as electrolyte and cathode made of oxides for better performance.
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