Electrical Property of Thick Film Electrolyte for Solid Oxide Fuel Cell

Transcription

1 Journal of Metals, Materials and Minerals, Vol.18 No.2 pp.7-11, 28 Electrical Property of Thick Film Electrolyte for Solid Oxide Fuel Cell Thitimaporn DUANGMANEE 1, Suda WANNAKITTI 2, Rapeepong SUWANWARANGKUL 1 and Sumittra CHAROJROCHKUL 2* 1 Department of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology, Thammasart University (Rangsit Campus), Pathumthani National Metal and Materials Technology Center, Klong Luang, Pathumthani, 1212 Abstract Received Nov. 13, 28 Accepted Jan. 26, 29 The electrical property of thick film electrolyte for solid oxide fuel cell (SOFC) has been studied. The thick film electrolyte was Yttria Stabilized Zirconia (YSZ) in which the ZrO 2 was doped with 3, 8, and 1 mol% of Y 2 O 3 (3YSZ, 8YSZ and 1YSZ). The 8YSZ thick film electrolyte shows the highest ionic conductivity characterized using an Impedance Spectroscopy (SI) in the temperature range of C with 25 C interval. The impedance spectra consist of bulk resistance and grain boundary resistance of the electrolyte. The microstructure was investigated using Scanning Electron Microscope (SEM). The correlation between the electrical conductivity and grain boundary has been determined. Key words : Solid oxide fuel cell, Yttria stabilized zirconia, Bulk resistance, Grain boundary resistance. Introduction A fuel cell technology is interesting over recent years as an efficient electrochemical energy conversion device. It produces electricity from an external supply of fuel (on the anode side) and oxidant (on the cathode side). A Solid Oxide Fuel Cell (SOFC) is one type of fuel cells which uses a ceramic component as an electrolyte. SOFC is anticipated to be a very competitive device for electrical power generation because of its high efficiency and low emission. (1) Yttria Stabilized Zirconia (YSZ) is the most commonly used electrolyte for SOFC because of its unique properties such as high chemical and thermal stability, excellent mechanical properties and high ionic conductivity over a wide range of conditions. (2, 3, 4) YSZ is well suited for an operation of fuel cells at high temperature (9-1 C). (5, 6) However, there is a motivation to decrease the operation temperature to below 8 C. However, a decrease of the operating temperature critically requires a further development of SOFC. One approach is to reduce the thickness of YSZ electrolyte. Therefore, the ionic conductivity for low temperature applications was studied. There are various techniques to fabricate the thick film electrolyte such as dip-coating (7), tape casting. The dip-coating of sol or gel is a coating technology that has the potential to produce thin (.1-1 µm) and gas-tight ceramic layers on dense substrates. However, it is difficult to prepare gas-tight film on a porous substrate, even by using multiple layers technology. Tape casing is a low cost process for making thin and flat ceramic sheet. The process in a fabrication of a thick film electrolyte by tape casting has permitted the lowering of the operating (8, 9) temperature. Enormous amounts of effort can be found in the literature on ionic conductivity improvements for the oxide electrolyte materials. The effects of various factors, such as dopants, local structure, microdomain, impurity, sintering temperature, sintering time and processing on ionic conductivity in zirconia-based electrolyte have been discussed. The main target of this research is to study the correlation between electrical property and microstructure of YSZ thick film electrolytes which were fabricated using tape casting process. Materials and Experimental Procedures Specimens of ZrO 2 doped with 3, 8, and 1 mol% of Y 2 O 3 supplied from MEL, UK (3YSZ, 8YSZ and 1YSZ) were fabricated using a doctor blade tape casting process. A mixture of xylene: Methyl Ethyl Ketone peroxide (MEK): butylacetate was used as a solvent. Solution of Acrylic polymer (PM-685) and Dioctyl Phthalate (DOP) was used as binder and plasticizer, respectively. The thick film YSZ of each composition was sintered at 15 C in air for 2 and 4 hours, and 155 C in air for 1 and Tel. +66 () , , Fax. +66 () , t_duangmanee@hotmail.com Corresponding author: S. Charojrochkul Tel. +66 ()256465, Fax. +66 () , sumittrc@mtec.or.th

2 8 DUANGMANEE, T. et al. 2 hours with a heating and cooling rate at 18 C per hour. The thickness of the sintered tape was around 17-2 µm. c) These specimens have been characterized using a JEOL JDX-353 X-Ray diffraction technique for phase identification, a scanning electron microscopy technique (JEOL 541SEM) for microstructural analysis, and using a Solartron 126 Impedance/Gain- Phase Analyser in the frequency range of.5 Hz to 1 MHz from 275 to 6 C for AC-impedance property. The relationship between grain size and grain boundaries of the thick film electrolytes that relate to conductivity has been discussed. Results and discussion Phase Identification XRD analysis was performed to confirm the phases of thick film electrolytes. XRD patterns of 3YSZ have shown tetragonal phase structure while those of 8YSZ and 1YSZ are cubic phase structure as shown in Figure 1. a) Figure 1. XRD patterns of the specimens of 3YSZ, 8YSZ, and 1YSZ, sintered at 15 C for 2 and 4 hours, and at 155 C for 1 and 2 hours, respectively. Microstructure SEM microstructures of thick film electrolytes in Figure 2 -(d), (e)-(h), and (i)-(l) clearly show the grain characteristic of 3YSZ, 8YSZ and 1YSZ, respectively. 3YSZ, 8YSZ, and 1YSZ have shown the small, medium, and large grain size, respectively. In the same sintering temperature, there is an increase in grain size from 2 to 4 hours at 15 C and 1 to 2 hours at 155 C. Moreover, an increase of grain size is larger from sintering temperature at 15 C to 155 C for 2 hours. The grain size of thick film electrolytes is larger when the sintering temperature is higher, and sintering time is longer. The sintering temperature and sintering time significantly affect to microstructures of thick film electrolytes. b) (d)

3 Electrical Property of Thick Film Electrolyte for Solid Oxide Fuel Cell 9 (e) (f) 1 L σ = (2) R A Figure 2. SEM micrographs of thick film electrolytes of -(d) 3YSZ, (e)-(h) 8YSZ, and (i)-(l) 1YSZ sintered at 15 C for 2 and 4 hours and at 155 C for 1 and 2 hours, respectively. Electrical Properties (g) (k) (i) AC-impedance spectroscopy is widely employed to obtain information related to the electrical behavior of the bulk (grain interiors) and the grain boundary. (1) The total resistance of an electrolyte is given by Rt = Rb + Rgb (1) (h) where R t is a total resistance (Ω), R b is a bulk resistance (Ω) and R gb is a grain boundary resistance (Ω). The conductivity is defined as (l) (j) where L is the sample thickness (cm) and A is the sample area (cm 2 ). The conductivity data were plotted using an Arrhenius equation ( ) σ Ea σ = exp (3) T kt where E a is the activation energy of electrical conduction (ev), k is the Boltzman s constant (ev/k), T is the absolute temperature (K) and σ is a preexponential factor being a constant in a certain temperature range. The electrical conductivities were extrapolated and the activation energies were calculated from the slope of Arrhenius plots. The resistances from AC-impedance measurements from 275 to 6 C were plotted in Z versus Z as shown in Figure 3 for 3YSZ, 8YSZ and 1YSZ. They were sintered at 15 C for 2 and 4 hours and at 155 C for 1 and 2 hours and were measured at 35 C. The first and the second semicircles represent the bulk and the grain boundary resistances, respectively. The bulk resistances of 3YSZ are smaller than the grain boundary resistances as shown in Figure 3, as 3YSZ have small grain sizes. The effect from grain boundary is greater than that from bulk. While the microstructures have shown the grain size of 8YSZ larger than those of 3YSZ which result in impedance profile as shown in Figure 3. The effect of bulk resistance of 8YSZ is greater than that of grain boundary. The impedance profiles of 1YSZ have shown the bulk resistances being higher than the grain boundary resistances similar to 8YSZ. For the sintered specimen at 155 C for 2 hours, the bulk resistance is the highest while the grain boundary resistance is the lowest as shown in Figure ,2h

4 1 DUANGMANEE, T. et al , 2h , 2h Figure 3. Nyquist plots for 3YSZ, 8YSZ, and 1YSZ, sintered at 15 C for 2 and 4 hours, and 155 C for 1 and 2 hours and measured at 35 C. The plots of natural log of conductivity versus the reciprocal of temperatures for bulk and grain boundaries and total conductivities were shown in Figure 4-. The electrical conductivities of thick film electrolytes are greatly influenced by their microstructures. 3YSZ has the smallest grain size thus results in a high grain boundary resistance. In the opposite, 1YSZ contains the largest grain size which leads to a high bulk resistance. In this research, 8YSZ sintered at 155 C for 1 hour shows the highest total conductivity as shown in Figure 4. Figure 4. Conductivity versus 1/temperature (Kelvin) for bulk, grain boundaries, and total conductivities of 3YSZ, 8YSZ and 1YSZ, sintered at different temperature and time and measured from 275 to 6 C. Conclusions The microstructures of thick film electrolytes were significantly related to the sintering temperature and sintering time. The thick film electrolyte of 8YSZ has shown the highest ionic conductivity from an AC-impedance measurement and the best condition is from the sintering temperature at 155 C for 1 hour. The electrical properties of thick film electrolytes are greatly influenced by the microstructures especially grain size and amount of grain boundaries. Acknowledgments The authors would like to acknowledge research grants from TU research fund (28) by Thammasart University, National Metal and Materials Technology Center (MTEC) and Sirindhorn International Institute of Technology (SIIT).

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