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Effects of 8 mol% yttria-stabilized zirconia with copper oxide on solid oxide fuel cell performance
Author's Accepted Manuscript
Effects of 8 mol % yttria-stabilized zirconia with copper oxide on solid oxide fuel cell performance Jin Goo Lee, Ok Sung Jeon, Kwang Hyun Ryu, Myeong Geun Park, Sung Hwan Min, Sang Hoon Hyun, Yong Gun Shul
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(15)00385-5 http://dx.doi.org/10.1016/j.ceramint.2015.02.144 CERI10066
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
24 December 2014 6 February 2015 26 February 2015
Cite this article as: Jin Goo Lee, Ok Sung Jeon, Kwang Hyun Ryu, Myeong Geun Park, Sung Hwan Min, Sang Hoon Hyun, Yong Gun Shul, Effects of 8 mol % yttria-stabilized zirconia with copper oxide on solid oxide fuel cell performance, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.02.144 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of 8 mol % Yttria-Stabilized Zirconia with Copper Oxide on Solid Oxide Fuel Cell Performance Jin Goo Lee§,a Ok Sung Jeon§, a Kwang Hyun Ryu,b Myeong Geun Park,a Sung Hwan Min,b Sang Hoon Hyun and Yong Gun Shul*a a
Department of Chemical and Bio-molecular Engineering, Yonsei University, 134 Shinchon-dong Seodaemun-gu Seoul 120-749, Republic of Korea. b
c
LTC Co., Ltd., Seoul 120-749, Anyang, Gyeonggi, Republic of Korea.
Department of Advanced Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku, Seoul 120-749, Republic of Korea.
Abstract Solid oxide fuel cells (SOFCs) have advanced rapidly for the last decades. Since 8 mol % yttriastabilized zirconia (8YSZ) has been considered the most conventional material for SOFC electrolyte, a number of studies have been focused on improving 8YSZ performance by changing dopants or adding a trace of metal oxide. In this study, the effect of a trace of CuO (100 ppm) on the performance of 8YSZ-based SOFCs is investigated. It is found that addition of a trace of CuO not only promotes a densification of the YSZ electrolyte by acting as a sintering aid but also increases the amount of the oxygen vacancy in the YSZ electrolyte. The ionic conductivities are about 0.0173 S cm-1 and 0.0196 S cm-1 in pristine and CuO (100 ppm)-YSZ electrolyte, respectively. The cell performance is about 0.5103 W cm-2 at 800 °C, which is about 1.5 times higher than the cell based on the pure YSZ electrolyte. The gadolinium-doped ceria (GDC)/8YSZ bilayer cell test also shows similar improvement to the single YSZ cell tests. Thus, the introduction of a trace of Cu (100 ppm) to the 8YSZ can be promising for a solid oxide fuel cell electrolyte. Keywords: yttria-stabilized zirconia, copper oxide, bilayer, electrolyte, solid oxide fuel cells *a Corresponding author - Fax: +82-02-393-6594; Tel: +82-02-2123-3554; E-mail: [email protected]
1
1. Introduction Zirconia-based electrolyte has been studied for solid oxide fuel cells (SOFCs) due to desirable stability in both reducing and oxidizing atmospheres.[1-3] Pure zirconia has a poor ionic conduction and sinterability, and the crystal structures vary with the temperature range: monoclinic at room temperature, tetragonal above 1170 °C, and cubic above 2370 °C. Doping with aliovalent cations, typically lower oxidation state cations such as Y+3, stabilizes a cubic fluorite structure of the zirconia from room temperature to its melting point.[4] 8 mol% yttria-stabilized zirconia (8YSZ) has the increased oxygen vacancy concentration, which leads to enhancing the ionic conductivity and extending oxygen partial pressure range where oxygen ion conduction can occur. In addition to the single doping in zirconia, several ternary systems doping with yttria and other rare-earth metals such as calia and scandia have been studied regarding their structures and electrical properties.[5,6] These aim to enhance the conductivity by optimizing the average size of dopant cations, to increase the stability of Sc-containing materials by codoping, or to decrease the cost of Ln3+-stabilized phases by mixing rare-earth with cheaper alkaline earth dopants.[7] When it comes to improving the sinterability of the 8YSZ, the addition of transition metals such as iron to the YSZ can be effective to improve its sinterability as well as electrical properties. Quiang Dong et al. reported the addition of 1 wt.% FeO1.5 not only increases the grain boundary conduction due to its scavenging effect on SiO2 impurity but also significantly reduces the sintering temperature of the 8 mol% YSZ samples.[8] Kiyoshi Kobayashi et al. reported that the partial conductivity of conduction electrons equilibrated at unit oxygen partial pressure increased with increasing TiO2 content, but the partial conductivity of conduction holes and band gap energy are almost independent of TiO2 content.[9] A. Kaiser also reported that highly titania-doped zirconia is more suitable for SOFC anode materials due to a good mixed conduction.[10] X. J. Huang et al. and T. S. Zhang et al. showed that various transition material oxides such as NiO, MnO2, TiO2, MnO2, Tb4O7, Nb2O5 and WO6 can be doped into yttria-stabilised zirconia (YSZ) to form solid-state solutions.[11,12] However, the effects of CuO on sinterability and electrical properties of the YSZ have been reported only in a range of the high 2
concentrations of CuO.[13-16] In this study, the effects of a trace of CuO on the performance of 8YSZ-based SOFCs was investigated, and the single-layer (YSZ) and bilayer cells (gadolinium-doped ceria (GDC)/YSZ) were tested at an intermediate temperature. Since we found the addition of 100 ppm CuO to the YSZ electrolyte is most effective for the ionic conductivity and cell performance (Figure S1 and S2) in previous experiments, the YSZ electrolyte with 100 ppm CuO is mainly discussed in this paper. 2. Experimental Powder preparation: NiO (J. T. Baker), 8 mol% YSZ (LTC), and carbon black as a pore former were ball-milled in ethanol for 24 h, and then dried at 80 °C for 12 h. The ratio of NiO to 8YSZ was 6:4 and 10 weight % of the carbon black for the NiO powder was added. The YSZ slurry was prepared by ball-milling process. 8YSZ powders were ball-milled with EFKA 4340 as a dispersant in a mixed toluene/isopropyl alcohol (IPA) for 12 h. Di-n butylphthalate (DBP), Triton-X as a plasticiser, polyvinyl butyral (PVB) as a binder were added to the suspension. Cu(NO3)26H2O was used as a precursor, and added into the suspension to have 100 ppm of CuO contents. The slurry was ball-milled for 24 h in ethanol. The La0.8Sr0.2MnO3 (LSM) powder was synthesized by conventional solid-state method. The stoichiometric amount of the metal oxides were ball-milled in ethanol for 24 h, and then dried at 60 °C for 24 h. The dried powders were calcined at 700 °C for 3 h. Single-cell fabrication: The Ni-YSZ powder was pressed at 50 MPa to fabricate disk-like anode supports with a diameter of 2.5 cm. The anode substrates were partially sintered at 1100 °C for 3 h. The YSZ slurries with and without 100 ppm CuO were coated on the Ni-YSZ anode by dip-coating method. The half-cell was fully sintered at 1400 °C for 3 h. The cathode paste was coated on the YSZ electrolyte side, and the cell was finally sintered at 1200 °C for 2 h. Characterizations: The crystal structures of the YSZ electrolyte with and without 100 ppm CuO were determined by X-ray diffraction (XRD, Rigaku, D/Max-2200 model) with Cu Kα radiation at a wavelength of 1.5406
Å after sintered at 1400 °C for 3 h. X-ray fluorescence (XRF) was 3
measured in the YSZ electrolytes to identify Cu contents in the YSZ samples, and the data corresponded to the amount of CuO in each samples. To check shrinkage of the YSZ electrolyte with and without 100 ppm CuO, the YSZ pellets with 12.7 mm of diameter, 2.5 mm of thickness, and 0.65 g of weight were prepared. Gas permeability tests were conducted on the half-cells (NiYSZ/YSZ) sintered at 1400 °C for 3 h after reduction of NiO to Ni. The relative densities of the electrolytes were measured by Archimedes principle. O1s core level of each YSZ sample was detected by X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo U. K.) with monochromated Al X-ray sources (Al Kα line: 1486.6 eV). Field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6701F model) was used to observe the cross-sectional images of the single- and bilayer cells. Individual cells (2 cm × 2 cm) with the electrode area of 1.1 cm2 were used in the cell test. Two Pt mesh were attached to each of the anodes and cathodes, and Pt paste was used only on the cathode side as a current collector. A Pyrex sealant adhered to the cell and a zirconia tube in a testing apparatus. The temperature was gradually increased to 850 °C, and then remained for 30 min to ensure complete sealing. The temperature was down to 800 °C, and then hydrogen gas was fed into the anode for 3 h to reduce nickel oxide to nickel metal. The single-layer and bi-layer cells were tested at 800 °C and 700 °C, respectively. The electrochemical measurement was conducted in the flow rates of air of 400 sccm and hydrogen of 200 sccm, respectively. The electrochemical analysis was carried out by using a FC Impedance meter (Kikusi, KFM-2030 model) and versatile multichannel potentiostat (VSP, Biologic, VMP3B-10 model). The frequency range was varied from 0.1 Hz to 1 MHz with the applied AC amplitude of 35 mV. 3. Results and Discussion Figure 1 shows XRD patterns of the YSZ electrolytes with and without 100 ppm CuO. All YSZ samples had the simple fluorite cubic structures, and no secondary phase with respect to CuO was found. This may be due to very small amount of CuO content in the YSZ electrolyte. The lattice parameter of the cubic YSZ lattice was calculated as an average value from the angular positions of the peak maxima according to the equation below: 4
a = λඥℎଶ + ݇ ଶ + ݈ଶ /2 sin ߠ where θ denotes the diffraction angle (in radians), λ represents the wavelength of the radiation and h, k and l are the Miller indices of the respective Bragg reflection. The lattice parameter of the YSZ powder with 100 ppm CuO was 5.1448 Å, which a little lower than that of the YSZ without CuO (a = 5.1522 Å). The effect of 100 ppm CuO on densification of the YSZ electrolyte is shown in Figure 2. The slight difference of the YSZ electrolytes with and without 100 ppm CuO in shrinkage appeared at sintering temperature of 1200 °C, showing 44.4 and 41.4 %, respectively. However, when both YSZ electrolytes were sintered at 1300 °C, the shrinkages of the YSZ electrolyte with and without 100 ppm CuO showed large difference with about 65.3 and 53.1 %, respectively. According to the phase diagram of CuO(Cu2O)-ZrO2, the eutectic point between CuO and ZrO2 is 1130 °C.[17] Although it cannot be applied directly to the ternary phase diagram of CuO(Cu2O)-Y2O3-ZrO2, we believed that the higher shrinkage of the YSZ with 100 ppm CuO may be related to the eutectic point between CuO(Cu2O)-Y2O3-ZrO2. At 1400 °C of the sintering temperature, shrinkage of the YSZ electrolyte with 100 ppm CuO was slightly higher than that of the pure YSZ electrolyte due to the enough temperature for sintering of the pure YSZ electrolyte. This corresponds to the result of gas permeability tests shown in Figure 3. The gas permeability of the YSZ electrolyte with 100 ppm CuO was about 10-9 mol m-1 s-1 Pa-1, which is slightly lower than that of the pure YSZ electrolyte. The relative densities of the YSZ electrolytes with 100 ppm CuO and without CuO were about 97 % and 96 %, respectively, which supports the permeability results. These demonstrate that 100 ppm CuO can promote densification of the YSZ electrolyte at relatively low temperatures. Figure 4 shows SEM images of the YSZ electrolyte without and with 100 ppm CuO when sintered at 1300 °C. The YSZ electrolyte without CuO had a number of nano-sized pores and irregular grain size in a range from 100 nm to 1 µm (Figure 4a and 4b). On the other hand, the number of nano-sized pores was reduced and relatively regular grain size appeared in the YSZ electrolyte with 100 ppm CuO compared to the YSZ electrolyte without CuO (Figure 4c and 4d). 5
Interestingly, the YSZ electrolyte without CuO had clean surfaces of the grains while unknown liquid phase existed around the grain boundaries of the YSZ electrolyte with 100 ppm CuO. We tried to check it by measuring energy dispersive X-ray spectroscopy (EDX), but it cannot be clarified. Yang et al. reported that Cu(CuO) can exist at the grain boundary as liquid-phase or melt into the grain.[18] Hence, we believed that the unknown liquid phases on the grain boundaries may be Cu(CuO). O1s core level X-ray photoelectron spectra of the YSZ electrolytes with various CuO contents after the cell test are shown in Figure 5. The spectra associated with the oxidation state of CuO cannot be detected due to very small amount of CuO contents. The O1s spectrum of the YSZ electrolytes is deconvoluted into four peaks. The O 1s peaks of the investigated ceramics at binding energies 528.4 eV were attributed to the lattice oxygen at the normal sites in the structures.[19] The lattice oxygen was reduced when CuO was added to the YSZ electrolyte, indicating more oxygen vacancies can be created by doping CuO. O1s peaks at 529.7, 531.7, and 533.2 eV are assigned to the chemisorbed oxygen. The intense peak at 529.7 eV is due to the dissociatively chemisorbed oxygen, Ox- (x<2). The peaks at 531.7 eV and 533.2 eV are related to either OH or O atoms at the surface feature in the YSZ and chemisorbed molecular oxygen, O2δ- (δ<2), respectively.[20,21] The adsorption oxygen increased in the YSZ electrolyte with CuO. It can also demonstrate that more oxygen vacancies are produced by adding a trace of CuO into the YSZ electrolyte because the adsorption oxygen can be related to the amount of oxygen deficiency.[22] The more oxygen vacancies in the YSZ lattice would lead to higher ionic conductivity. Figure 6 shows the SEM images of the single-layer YSZ cells with and without 100 ppm CuO. The electrolyte thicknesses were about 20 µm in both samples. Based on the similar YSZ electrolyte thicknesses, the cell tests were carried out at 800 °C. Figure 7 shows the I-V curves and impedance spectra of the single-layer YSZ cells with and without 100 ppm CuO. The open circuit voltage (OCV) and the slope between 0.6-0.8 V of the YSZ cells with different amount of CuO are represented in Table S1. The YSZ cells with 100 ppm CuO had the highest OCV value among 6
other cells, and the open circuit voltages were 1.192 V and 1.115 V in the YSZ cells with and without 100 ppm CuO, respectively. Considering the OCV can be related to gas leakage, the YSZ cell with 100 ppm CuO can have the slightly higher OCV due to the higher shrinkage and lower gas permeability as mentioned above. The maximum power densities were improved by adding 100 ppm CuO from 0.36 W cm-2 into 0.51 W cm-2 (Figure 7a). In general, the voltage drop comes from activation, ohmic and concentration loss in low, medium and high current ranges, respectively. The ohmic loss mainly results from the electrolyte resistance. As shown in Table S1, the YSZ cell with 100 ppm CuO had more gradual slope between 0.6-0.8 V where the ohmic loss mainly contributes to the voltage loss, which can indicate improvement of the YSZ electrolyte performance. In Figure 7b, the intercept of x-axis and impedance semicircle at high frequency can be typically interpreted as an electrolyte resistance. The electrolyte resistances were 0.12 Ω cm2 and 0.14 Ω cm2 at 800 °C in the YSZ samples with and without 100 ppm CuO, respectively. From the electrolyte resistances, the ionic conductivity of the YSZ electrolytes with 100 ppm CuO and without CuO was simply calculated. The ionic conductivities are about 0.0173 S cm-1 and 0.0196 S cm-1 in pristine and 100 ppm CuO-YSZ electrolyte, respectively. Consequently, adding 100 ppm CuO can have a positive effect on improving the YSZ-based SOFC performance. For the SOFC operation at intermediate temperatures, the GDC/YSZ (100 ppm CuO) bi-layer cells were fabricated and tested at 700 °C. Figure 8 shows cross-sectional SEM and elemental mapping image of the GDC/YSZ bi-layer cell with 100 ppm CuO. The electrolyte thickness of the YSZ and GDC were about 10 µm and 9 µm, respectively. It can be found that Ce elements were distributed only in the GDC electrolyte. Figure 9 shows the I-V curves and impedance spectra of the GDC/YSZ bi-layer cells with and without 100 ppm CuO at 700 °C. As the single-layer YSZ cell tests, the OCV was slightly higher in the bi-layer cells with 100 ppm CuO than in the cells without CuO, showing 1.01 V and 0.98 V, respectively. The peak power density of the GDC/YSZ bi-layer cell without CuO was about 0.43 W cm-2, while the bi-layer cell with 100 ppm CuO had about 0.54 W cm-2. Considering the difference between the single-layer YSZ cells with and without 100 ppm 7
CuO in the peak power densities was about 0.15 W cm-2, the difference of 0.11 W cm-2 between the bi-layer cells can be reasonable. In Figure 8b, the electrolyte resistances were about 0.29 Ω cm2 and 0.34 Ωcm2 in the bilayer cells with and without 100 ppm CuO, respectively. A diameter of the impedance semicircles can refer to the polarization resistance or electrode resistance. Both bi-layer cells were similar in the polarization resistances, summit frequencies, and even semicircle shapes. The polarization resistances were about 0.20 Ω cm2 and 0.21 Ω cm2 in the bilayer cells with and without 100 ppm CuO, respectively. These means the effects of the electrode reactions on the cell performances can be excluded in the performance improvement of the bi-layer cells with 100 ppm CuO. Consequently, addition of 100 ppm CuO to the YSZ electrolyte can be a promising method to prepare the YSZ single or GDC/YSZ bilayer electrolytes for IT-SOFCs. Conclusions The YSZ electrolyte with 100 ppm CuO exhibited better sinterability and ionic conductivity than the pure YSZ electrolyte. The YSZ electrolyte with 100 ppm CuO facilitated densification of the YSZ electrolyte below 1300 °C. The higher ionic conductivity of the YSZ electrolyte with 100 ppm CuO may be due to the increased oxygen vacancy concentration. Furthermore, the performances of the single-layer YSZ cells with 100 ppm CuO were about 1.5 times higher (0.51 W cm-2) than the cell with the pure YSZ electrolyte (0.36 Wcm-2) at 800 °C. The main reason for this would be denser YSZ electrolyte by CuO acting as a sintering aid and high ionic conductivity based on the increased oxygen vacancies. The GDC/YSZ (100 ppm CuO) bi-layer cells also showed higher cell performance at 700 °C, showing similar tendency to the single-layer YSZ cell tests. As a result, addition of 100 ppm CuO into the YSZ electrolyte is greatly effective for improvement of SOFC performance at intermediate temperatures as well as energy saving during sintering process. Acknowledgements This work was supported by Technology Innovation Program (10037616) funded by the Ministry of Knowledge Economy (MKE, Korea) § The authors contributed equally to this work. 8
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Energ. 34 (2009) 7903. 9. K. Kobayashi, S. Yamaguchi, T. Higuchi, S. Shin, Y. Iguchi, Electronic transport properties and electronic structure of TiO -doped YSZ, Solid State Ionics 135 (2000) 643.
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12. T.S. Zhang, Z.H. Du, S. Li, L.B. Kong, X.C. Song, J. Lu, J. Ma, Transitional metal-doped 8 mol% 9
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16. J. C. Ruiz-Morales, J. Canales-Vazquez, D. Marrero-Lopez, J. Pena-Martinez, A. Tarancon, J. T. S. Irvine, P. Nunez, Is YSZ stable in the presence of Cu?, J. Mater. Chem. 18 (2008) 5072. 17. E. M. Levin, C. R. Robbins, M. F. Mamurdie, Phase diagrams for ceramists, American Ceramic Society, Columbus, OH, Vol. 2 (1969) 33. 18. C. F. YANG, L. WU and T. S. WU, Effect of CuO on the sintering and dielectric characteristics of (Ba1-xSrx) (Ti0.9Zr0.1)O3 ceramics, J. Mater. Sci, 27 (1992) 6573-6578
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Figure captions Figure 1. X-ray diffraction patterns of the YSZ electrolytes with and without 100 ppm CuO Figure 2. Shrinkage of the YSZ electrolytes with and without 100 ppm CuO depending on sintering temperatures Figure 3. Gas permeability of the YSZ electrolytes with and without 100 ppm CuO Figure 4. SEM images of the YSZ electrolyte without ((a) and (b)) and with 100 ppm CuO ((c) and (d)) when sintered at 1300 °C Figure 5. O1s core level X-ray photoelectron spectra of (a) the pristine and (b)YSZ electrolytes with 100 ppm CuO Figure 6. Cross-sectional SEM images of the YSZ cell (a) without and (b) with 100 ppm CuO Figure 7. I-V curves and impedance spectra of the single-layer cells with the YSZ electrolyte and the 100 ppm Cu-YSZ electrolyte at 800 °C Figure 8. Cross-sectional SEM image and elemental mapping image of the bilayer cells with the GDC-YSZ (Cu 100 ppm) electrolyte Figure 9. I-V curves of the bilayer cells with the GDC-YSZ and with the GDC-YSZ (Cu 100 ppm) electrolyte at 700 °C
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Figures Figure 1.
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Figure 2.
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CuO 0 ppm CuO 100 ppm
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40 1150
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Figure 4.
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Effects of 8 mol % yttria-stabilized zirconia with copper oxide on solid oxide fuel cell performance Jin Goo Lee, Ok Sung Jeon, Kwang Hyun Ryu, Myeong Geun Park, Sung Hwan Min, Sang Hoon Hyun, Yong Gun Shul
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(15)00385-5 http://dx.doi.org/10.1016/j.ceramint.2015.02.144 CERI10066
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
24 December 2014 6 February 2015 26 February 2015
Cite this article as: Jin Goo Lee, Ok Sung Jeon, Kwang Hyun Ryu, Myeong Geun Park, Sung Hwan Min, Sang Hoon Hyun, Yong Gun Shul, Effects of 8 mol % yttria-stabilized zirconia with copper oxide on solid oxide fuel cell performance, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.02.144 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of 8 mol % Yttria-Stabilized Zirconia with Copper Oxide on Solid Oxide Fuel Cell Performance Jin Goo Lee§,a Ok Sung Jeon§, a Kwang Hyun Ryu,b Myeong Geun Park,a Sung Hwan Min,b Sang Hoon Hyun and Yong Gun Shul*a a
Department of Chemical and Bio-molecular Engineering, Yonsei University, 134 Shinchon-dong Seodaemun-gu Seoul 120-749, Republic of Korea. b
c
LTC Co., Ltd., Seoul 120-749, Anyang, Gyeonggi, Republic of Korea.
Department of Advanced Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku, Seoul 120-749, Republic of Korea.
Abstract Solid oxide fuel cells (SOFCs) have advanced rapidly for the last decades. Since 8 mol % yttriastabilized zirconia (8YSZ) has been considered the most conventional material for SOFC electrolyte, a number of studies have been focused on improving 8YSZ performance by changing dopants or adding a trace of metal oxide. In this study, the effect of a trace of CuO (100 ppm) on the performance of 8YSZ-based SOFCs is investigated. It is found that addition of a trace of CuO not only promotes a densification of the YSZ electrolyte by acting as a sintering aid but also increases the amount of the oxygen vacancy in the YSZ electrolyte. The ionic conductivities are about 0.0173 S cm-1 and 0.0196 S cm-1 in pristine and CuO (100 ppm)-YSZ electrolyte, respectively. The cell performance is about 0.5103 W cm-2 at 800 °C, which is about 1.5 times higher than the cell based on the pure YSZ electrolyte. The gadolinium-doped ceria (GDC)/8YSZ bilayer cell test also shows similar improvement to the single YSZ cell tests. Thus, the introduction of a trace of Cu (100 ppm) to the 8YSZ can be promising for a solid oxide fuel cell electrolyte. Keywords: yttria-stabilized zirconia, copper oxide, bilayer, electrolyte, solid oxide fuel cells *a Corresponding author - Fax: +82-02-393-6594; Tel: +82-02-2123-3554; E-mail: [email protected]
1
1. Introduction Zirconia-based electrolyte has been studied for solid oxide fuel cells (SOFCs) due to desirable stability in both reducing and oxidizing atmospheres.[1-3] Pure zirconia has a poor ionic conduction and sinterability, and the crystal structures vary with the temperature range: monoclinic at room temperature, tetragonal above 1170 °C, and cubic above 2370 °C. Doping with aliovalent cations, typically lower oxidation state cations such as Y+3, stabilizes a cubic fluorite structure of the zirconia from room temperature to its melting point.[4] 8 mol% yttria-stabilized zirconia (8YSZ) has the increased oxygen vacancy concentration, which leads to enhancing the ionic conductivity and extending oxygen partial pressure range where oxygen ion conduction can occur. In addition to the single doping in zirconia, several ternary systems doping with yttria and other rare-earth metals such as calia and scandia have been studied regarding their structures and electrical properties.[5,6] These aim to enhance the conductivity by optimizing the average size of dopant cations, to increase the stability of Sc-containing materials by codoping, or to decrease the cost of Ln3+-stabilized phases by mixing rare-earth with cheaper alkaline earth dopants.[7] When it comes to improving the sinterability of the 8YSZ, the addition of transition metals such as iron to the YSZ can be effective to improve its sinterability as well as electrical properties. Quiang Dong et al. reported the addition of 1 wt.% FeO1.5 not only increases the grain boundary conduction due to its scavenging effect on SiO2 impurity but also significantly reduces the sintering temperature of the 8 mol% YSZ samples.[8] Kiyoshi Kobayashi et al. reported that the partial conductivity of conduction electrons equilibrated at unit oxygen partial pressure increased with increasing TiO2 content, but the partial conductivity of conduction holes and band gap energy are almost independent of TiO2 content.[9] A. Kaiser also reported that highly titania-doped zirconia is more suitable for SOFC anode materials due to a good mixed conduction.[10] X. J. Huang et al. and T. S. Zhang et al. showed that various transition material oxides such as NiO, MnO2, TiO2, MnO2, Tb4O7, Nb2O5 and WO6 can be doped into yttria-stabilised zirconia (YSZ) to form solid-state solutions.[11,12] However, the effects of CuO on sinterability and electrical properties of the YSZ have been reported only in a range of the high 2
concentrations of CuO.[13-16] In this study, the effects of a trace of CuO on the performance of 8YSZ-based SOFCs was investigated, and the single-layer (YSZ) and bilayer cells (gadolinium-doped ceria (GDC)/YSZ) were tested at an intermediate temperature. Since we found the addition of 100 ppm CuO to the YSZ electrolyte is most effective for the ionic conductivity and cell performance (Figure S1 and S2) in previous experiments, the YSZ electrolyte with 100 ppm CuO is mainly discussed in this paper. 2. Experimental Powder preparation: NiO (J. T. Baker), 8 mol% YSZ (LTC), and carbon black as a pore former were ball-milled in ethanol for 24 h, and then dried at 80 °C for 12 h. The ratio of NiO to 8YSZ was 6:4 and 10 weight % of the carbon black for the NiO powder was added. The YSZ slurry was prepared by ball-milling process. 8YSZ powders were ball-milled with EFKA 4340 as a dispersant in a mixed toluene/isopropyl alcohol (IPA) for 12 h. Di-n butylphthalate (DBP), Triton-X as a plasticiser, polyvinyl butyral (PVB) as a binder were added to the suspension. Cu(NO3)26H2O was used as a precursor, and added into the suspension to have 100 ppm of CuO contents. The slurry was ball-milled for 24 h in ethanol. The La0.8Sr0.2MnO3 (LSM) powder was synthesized by conventional solid-state method. The stoichiometric amount of the metal oxides were ball-milled in ethanol for 24 h, and then dried at 60 °C for 24 h. The dried powders were calcined at 700 °C for 3 h. Single-cell fabrication: The Ni-YSZ powder was pressed at 50 MPa to fabricate disk-like anode supports with a diameter of 2.5 cm. The anode substrates were partially sintered at 1100 °C for 3 h. The YSZ slurries with and without 100 ppm CuO were coated on the Ni-YSZ anode by dip-coating method. The half-cell was fully sintered at 1400 °C for 3 h. The cathode paste was coated on the YSZ electrolyte side, and the cell was finally sintered at 1200 °C for 2 h. Characterizations: The crystal structures of the YSZ electrolyte with and without 100 ppm CuO were determined by X-ray diffraction (XRD, Rigaku, D/Max-2200 model) with Cu Kα radiation at a wavelength of 1.5406
Å after sintered at 1400 °C for 3 h. X-ray fluorescence (XRF) was 3
measured in the YSZ electrolytes to identify Cu contents in the YSZ samples, and the data corresponded to the amount of CuO in each samples. To check shrinkage of the YSZ electrolyte with and without 100 ppm CuO, the YSZ pellets with 12.7 mm of diameter, 2.5 mm of thickness, and 0.65 g of weight were prepared. Gas permeability tests were conducted on the half-cells (NiYSZ/YSZ) sintered at 1400 °C for 3 h after reduction of NiO to Ni. The relative densities of the electrolytes were measured by Archimedes principle. O1s core level of each YSZ sample was detected by X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo U. K.) with monochromated Al X-ray sources (Al Kα line: 1486.6 eV). Field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6701F model) was used to observe the cross-sectional images of the single- and bilayer cells. Individual cells (2 cm × 2 cm) with the electrode area of 1.1 cm2 were used in the cell test. Two Pt mesh were attached to each of the anodes and cathodes, and Pt paste was used only on the cathode side as a current collector. A Pyrex sealant adhered to the cell and a zirconia tube in a testing apparatus. The temperature was gradually increased to 850 °C, and then remained for 30 min to ensure complete sealing. The temperature was down to 800 °C, and then hydrogen gas was fed into the anode for 3 h to reduce nickel oxide to nickel metal. The single-layer and bi-layer cells were tested at 800 °C and 700 °C, respectively. The electrochemical measurement was conducted in the flow rates of air of 400 sccm and hydrogen of 200 sccm, respectively. The electrochemical analysis was carried out by using a FC Impedance meter (Kikusi, KFM-2030 model) and versatile multichannel potentiostat (VSP, Biologic, VMP3B-10 model). The frequency range was varied from 0.1 Hz to 1 MHz with the applied AC amplitude of 35 mV. 3. Results and Discussion Figure 1 shows XRD patterns of the YSZ electrolytes with and without 100 ppm CuO. All YSZ samples had the simple fluorite cubic structures, and no secondary phase with respect to CuO was found. This may be due to very small amount of CuO content in the YSZ electrolyte. The lattice parameter of the cubic YSZ lattice was calculated as an average value from the angular positions of the peak maxima according to the equation below: 4
a = λඥℎଶ + ݇ ଶ + ݈ଶ /2 sin ߠ where θ denotes the diffraction angle (in radians), λ represents the wavelength of the radiation and h, k and l are the Miller indices of the respective Bragg reflection. The lattice parameter of the YSZ powder with 100 ppm CuO was 5.1448 Å, which a little lower than that of the YSZ without CuO (a = 5.1522 Å). The effect of 100 ppm CuO on densification of the YSZ electrolyte is shown in Figure 2. The slight difference of the YSZ electrolytes with and without 100 ppm CuO in shrinkage appeared at sintering temperature of 1200 °C, showing 44.4 and 41.4 %, respectively. However, when both YSZ electrolytes were sintered at 1300 °C, the shrinkages of the YSZ electrolyte with and without 100 ppm CuO showed large difference with about 65.3 and 53.1 %, respectively. According to the phase diagram of CuO(Cu2O)-ZrO2, the eutectic point between CuO and ZrO2 is 1130 °C.[17] Although it cannot be applied directly to the ternary phase diagram of CuO(Cu2O)-Y2O3-ZrO2, we believed that the higher shrinkage of the YSZ with 100 ppm CuO may be related to the eutectic point between CuO(Cu2O)-Y2O3-ZrO2. At 1400 °C of the sintering temperature, shrinkage of the YSZ electrolyte with 100 ppm CuO was slightly higher than that of the pure YSZ electrolyte due to the enough temperature for sintering of the pure YSZ electrolyte. This corresponds to the result of gas permeability tests shown in Figure 3. The gas permeability of the YSZ electrolyte with 100 ppm CuO was about 10-9 mol m-1 s-1 Pa-1, which is slightly lower than that of the pure YSZ electrolyte. The relative densities of the YSZ electrolytes with 100 ppm CuO and without CuO were about 97 % and 96 %, respectively, which supports the permeability results. These demonstrate that 100 ppm CuO can promote densification of the YSZ electrolyte at relatively low temperatures. Figure 4 shows SEM images of the YSZ electrolyte without and with 100 ppm CuO when sintered at 1300 °C. The YSZ electrolyte without CuO had a number of nano-sized pores and irregular grain size in a range from 100 nm to 1 µm (Figure 4a and 4b). On the other hand, the number of nano-sized pores was reduced and relatively regular grain size appeared in the YSZ electrolyte with 100 ppm CuO compared to the YSZ electrolyte without CuO (Figure 4c and 4d). 5
Interestingly, the YSZ electrolyte without CuO had clean surfaces of the grains while unknown liquid phase existed around the grain boundaries of the YSZ electrolyte with 100 ppm CuO. We tried to check it by measuring energy dispersive X-ray spectroscopy (EDX), but it cannot be clarified. Yang et al. reported that Cu(CuO) can exist at the grain boundary as liquid-phase or melt into the grain.[18] Hence, we believed that the unknown liquid phases on the grain boundaries may be Cu(CuO). O1s core level X-ray photoelectron spectra of the YSZ electrolytes with various CuO contents after the cell test are shown in Figure 5. The spectra associated with the oxidation state of CuO cannot be detected due to very small amount of CuO contents. The O1s spectrum of the YSZ electrolytes is deconvoluted into four peaks. The O 1s peaks of the investigated ceramics at binding energies 528.4 eV were attributed to the lattice oxygen at the normal sites in the structures.[19] The lattice oxygen was reduced when CuO was added to the YSZ electrolyte, indicating more oxygen vacancies can be created by doping CuO. O1s peaks at 529.7, 531.7, and 533.2 eV are assigned to the chemisorbed oxygen. The intense peak at 529.7 eV is due to the dissociatively chemisorbed oxygen, Ox- (x<2). The peaks at 531.7 eV and 533.2 eV are related to either OH or O atoms at the surface feature in the YSZ and chemisorbed molecular oxygen, O2δ- (δ<2), respectively.[20,21] The adsorption oxygen increased in the YSZ electrolyte with CuO. It can also demonstrate that more oxygen vacancies are produced by adding a trace of CuO into the YSZ electrolyte because the adsorption oxygen can be related to the amount of oxygen deficiency.[22] The more oxygen vacancies in the YSZ lattice would lead to higher ionic conductivity. Figure 6 shows the SEM images of the single-layer YSZ cells with and without 100 ppm CuO. The electrolyte thicknesses were about 20 µm in both samples. Based on the similar YSZ electrolyte thicknesses, the cell tests were carried out at 800 °C. Figure 7 shows the I-V curves and impedance spectra of the single-layer YSZ cells with and without 100 ppm CuO. The open circuit voltage (OCV) and the slope between 0.6-0.8 V of the YSZ cells with different amount of CuO are represented in Table S1. The YSZ cells with 100 ppm CuO had the highest OCV value among 6
other cells, and the open circuit voltages were 1.192 V and 1.115 V in the YSZ cells with and without 100 ppm CuO, respectively. Considering the OCV can be related to gas leakage, the YSZ cell with 100 ppm CuO can have the slightly higher OCV due to the higher shrinkage and lower gas permeability as mentioned above. The maximum power densities were improved by adding 100 ppm CuO from 0.36 W cm-2 into 0.51 W cm-2 (Figure 7a). In general, the voltage drop comes from activation, ohmic and concentration loss in low, medium and high current ranges, respectively. The ohmic loss mainly results from the electrolyte resistance. As shown in Table S1, the YSZ cell with 100 ppm CuO had more gradual slope between 0.6-0.8 V where the ohmic loss mainly contributes to the voltage loss, which can indicate improvement of the YSZ electrolyte performance. In Figure 7b, the intercept of x-axis and impedance semicircle at high frequency can be typically interpreted as an electrolyte resistance. The electrolyte resistances were 0.12 Ω cm2 and 0.14 Ω cm2 at 800 °C in the YSZ samples with and without 100 ppm CuO, respectively. From the electrolyte resistances, the ionic conductivity of the YSZ electrolytes with 100 ppm CuO and without CuO was simply calculated. The ionic conductivities are about 0.0173 S cm-1 and 0.0196 S cm-1 in pristine and 100 ppm CuO-YSZ electrolyte, respectively. Consequently, adding 100 ppm CuO can have a positive effect on improving the YSZ-based SOFC performance. For the SOFC operation at intermediate temperatures, the GDC/YSZ (100 ppm CuO) bi-layer cells were fabricated and tested at 700 °C. Figure 8 shows cross-sectional SEM and elemental mapping image of the GDC/YSZ bi-layer cell with 100 ppm CuO. The electrolyte thickness of the YSZ and GDC were about 10 µm and 9 µm, respectively. It can be found that Ce elements were distributed only in the GDC electrolyte. Figure 9 shows the I-V curves and impedance spectra of the GDC/YSZ bi-layer cells with and without 100 ppm CuO at 700 °C. As the single-layer YSZ cell tests, the OCV was slightly higher in the bi-layer cells with 100 ppm CuO than in the cells without CuO, showing 1.01 V and 0.98 V, respectively. The peak power density of the GDC/YSZ bi-layer cell without CuO was about 0.43 W cm-2, while the bi-layer cell with 100 ppm CuO had about 0.54 W cm-2. Considering the difference between the single-layer YSZ cells with and without 100 ppm 7
CuO in the peak power densities was about 0.15 W cm-2, the difference of 0.11 W cm-2 between the bi-layer cells can be reasonable. In Figure 8b, the electrolyte resistances were about 0.29 Ω cm2 and 0.34 Ωcm2 in the bilayer cells with and without 100 ppm CuO, respectively. A diameter of the impedance semicircles can refer to the polarization resistance or electrode resistance. Both bi-layer cells were similar in the polarization resistances, summit frequencies, and even semicircle shapes. The polarization resistances were about 0.20 Ω cm2 and 0.21 Ω cm2 in the bilayer cells with and without 100 ppm CuO, respectively. These means the effects of the electrode reactions on the cell performances can be excluded in the performance improvement of the bi-layer cells with 100 ppm CuO. Consequently, addition of 100 ppm CuO to the YSZ electrolyte can be a promising method to prepare the YSZ single or GDC/YSZ bilayer electrolytes for IT-SOFCs. Conclusions The YSZ electrolyte with 100 ppm CuO exhibited better sinterability and ionic conductivity than the pure YSZ electrolyte. The YSZ electrolyte with 100 ppm CuO facilitated densification of the YSZ electrolyte below 1300 °C. The higher ionic conductivity of the YSZ electrolyte with 100 ppm CuO may be due to the increased oxygen vacancy concentration. Furthermore, the performances of the single-layer YSZ cells with 100 ppm CuO were about 1.5 times higher (0.51 W cm-2) than the cell with the pure YSZ electrolyte (0.36 Wcm-2) at 800 °C. The main reason for this would be denser YSZ electrolyte by CuO acting as a sintering aid and high ionic conductivity based on the increased oxygen vacancies. The GDC/YSZ (100 ppm CuO) bi-layer cells also showed higher cell performance at 700 °C, showing similar tendency to the single-layer YSZ cell tests. As a result, addition of 100 ppm CuO into the YSZ electrolyte is greatly effective for improvement of SOFC performance at intermediate temperatures as well as energy saving during sintering process. Acknowledgements This work was supported by Technology Innovation Program (10037616) funded by the Ministry of Knowledge Economy (MKE, Korea) § The authors contributed equally to this work. 8
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Figure captions Figure 1. X-ray diffraction patterns of the YSZ electrolytes with and without 100 ppm CuO Figure 2. Shrinkage of the YSZ electrolytes with and without 100 ppm CuO depending on sintering temperatures Figure 3. Gas permeability of the YSZ electrolytes with and without 100 ppm CuO Figure 4. SEM images of the YSZ electrolyte without ((a) and (b)) and with 100 ppm CuO ((c) and (d)) when sintered at 1300 °C Figure 5. O1s core level X-ray photoelectron spectra of (a) the pristine and (b)YSZ electrolytes with 100 ppm CuO Figure 6. Cross-sectional SEM images of the YSZ cell (a) without and (b) with 100 ppm CuO Figure 7. I-V curves and impedance spectra of the single-layer cells with the YSZ electrolyte and the 100 ppm Cu-YSZ electrolyte at 800 °C Figure 8. Cross-sectional SEM image and elemental mapping image of the bilayer cells with the GDC-YSZ (Cu 100 ppm) electrolyte Figure 9. I-V curves of the bilayer cells with the GDC-YSZ and with the GDC-YSZ (Cu 100 ppm) electrolyte at 700 °C
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