Electrochemical Characterization on SDC/Na<SUB>2</SUB>CO<SUB>3</SUB> Nanocomposite Electrolyte for Low Temperature Solid Oxide Fuel Cells
Descripción
Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol. 11, 5413–5417, 2011
Electrochemical Characterization on SDC/Na2CO3 Nanocomposite Electrolyte for Low Temperature Solid Oxide Fuel Cells Zhan Gao1 2 ∗ , Rizwan Raza1 , Bin Zhu1 , and Zongqiang Mao2 1
Department of Energy Technology, Royal Institute of Technology (KTH), S-10044, Stockholm, Sweden 2 Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
Our previous work has demonstrated that novel core–shell SDC/Na2 CO3 nanocomposite electrolyte possesses great potential for the development of low temperature (300–600 C) solid oxide fuel Delivered by Ingenta to: 2 CO3 electrochemical properties cells. This work further characterizes the nanocomposite SDC/Na Hong Kong Polytechnic University and conduction mechanism. The microstructure of the nanocomposite sintered at different temper: 158.132.127.134 atures was analyzed through scanningIPelectron microscope (SEM) and X-ray diffraction (XRD). The electrical and electrochemical properties Significant conductivity enhancement was Sat, 21 were Maystudied. 2011 05:21:12 observed in the H2 atmosphere compared with that of air atmosphere. The ratiocination of proton conduction rather than electronic conduction has been proposed consequently based on the observation of fuel cell performance. The fuel cell performance with peak power density of 375 mW cm−2 at 550 C has been achieved. A.C. impedance for the fuel cell under open circuit voltage (OCV) conditions illustrates the electrode polarization process is predominant in rate determination.
Keywords: Low Temperature Solid Oxide Fuel Cells, Nanocomposite, Proton Conduction, A. C.
1. INTRODUCTION During the late decade, the growing interest in developing advanced materials using the nanotechnology has been stimulated to develop high performance solid oxide fuel cells (SOFCs), an electrochemical energy generation device with high efficiency, environmental friendly and fuel flexibility. Reducing the operation temperature from typical ∼1000 C to low temperature range of 300–600 C is critical to develop economically competitive SOFCs which can be beneficial from long-term stability, materials selection and fabrication technology. Doped ceria with high-ionic conductivities (0.01–0.1 Scm−1 at 500–700 C)1 compared with yttria stabilized zirconia (YSZ) (0.1 Scm−1 at 1000 C) have been considered as solid electrolyte materials to ensure affordable power density in reducing temperature range. The main obstacle for the application of doped ceria is the possibility of reduction of Ce(IV) to Ce(III) associated with electronic conduction and mechanical integrity.2 Moreover, the doped ceria can not supply satisfied conductivity and performance at low temperatures, say bellow 500 C. Exploitation of new solid electrolyte to fill the gap of low temperature, 300–500 C ∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 6
range is still a challenge for the development of the SOFCs. Recently, ceria-based two-phase composites have been successfully developed as functional electrolytes3–7 and excellent conductivity (0.1 Scm−1 at 500 C) and fuel cell performance (1.1 Wcm−2 at 600 C)6 are created. Construction of nanocomposite electrolyte with superionic conduction by NANOCOFC approach (Nanocomposites for advanced fuel cell technology) is an effective strategy for the development of next generation low temperature SOFCs.8 Our previous work has successfully demonstrated core–shell nanocomposite materials consist of SDC core and amorphous Na2 CO3 shell. Excellent conductivity (0.1 Scm−1 above 300 C) and fuel cell performance (0.8 Wcm−2 at 550 C) have been achieved.9 In this work, the nanocomposite electrolyte is further investigated by electrochemical method and the conduction mechanism of the nanocomposite will be discussed.
2. EXPERIMENTAL DETAILS The preparation of nanocomposite electrolyte has been described in detail previously.9 Briefly, stoichiometric Ce(NO3 )3 · 6H2 O (Sigma-Aldrich) and Sm(NO3 )3 · 6H2 O
1533-4880/2011/11/5413/005
doi:10.1166/jnn.2011.3777
5413
RESEARCH ARTICLE
Impedance, Conductivity.
RESEARCH ARTICLE
Electrochemical Characterization on SDC/Na2 CO3 Nanocomposite Electrolyte for Low Temperature SOFCs
Gao et al.
(Sigma-Aldrich) solution was precipitated by the diluted (a) Na2 CO3 solution according to the molar ratio of metal ion to carbonate ion in 1:2. The precipitation was filtered by suction filtration and dried in the oven at 50 C for 12 h. After sintering the dried precipitation at 600 C, 700 C, 800 C, respectively, for 2 h, the resultant was collected and grounded thoroughly. The fuel cell was fabricated combining the tape casting and hot press procedure. The anode was consisted of NiO (50 vol.%) mixed with the nanocomposite electrolyte (50 vol.%) and the cathode was based on mixture of lithiated NiO (50 vol.%) and the nanocomposite electrolyte (50 vol.%). Thin film anode, nanocomposite electrolyte, and cathode was prepared by the tape casting technol(b) ogy and then was assembled together in a stainless steel mold followed by a press of 250 MPa under 550 C for 30 mins. Silver paste was adopted as current collector. Fuel cell was fabricated as 20 mm in diameter with an active area of 2.14 cm2 . The fuel cell characterizations, Delivered by Ingenta to: I–V (current–voltage) and I–P (current–power) were Polytechnic comHong Kong University pleted at temperatures between 350 C and 550 Dry IP : C. 158.132.127.134 hydrogen and air were used as the fuel and Sat,the21oxidant, May 2011 05:21:12 respectively. Both gas flow rates were controlled between 80–200 mLmin−1 under 1 atm pressure. The phase structure was detected by the XRD (D8 ADVANCE, Bruker AXS Corp. German). The morphology and microstructure was detected by the Hitachi S-5500 (c) scanning electron microscope (SEM). The electrochemical impedance spectroscopy measurement was performed in the frequency range from 0.1 Hz to 100 kHz with amplitude of 10 mV.
3. RESULTS AND DISCUSSION Figure 1 shows the SEM images of the SDC/Na2 CO3 nanocomposite sintered at different temperatures. Obviously, the sintering temperature has a significant effect on the grain growth. The size increases steadily with the improvement of sintering temperature. The grain size is within the range of 10–30 nm for the sample sintered at 600 C. The grains are enlarged to a range of 10–50 nm with a dominated (>90%) grain size of 20–30 nm when the sintering temperature increases to 700 C. The further increase of the sintering temperature to 800 C leads to the enhancement of the grain size to the range of 50–500 nm. Some grains with extraordinarily large size are observed. We can conclude the SDC particles are connected from the SEM images. X-ray diffraction patterns of the SDC/Na2 CO3 nanocomposite sintered at different temperatures is presented in Figure 2. Only the diffraction pattern of SDC with a cubic fluorite structure can be observed, which is in good accordance with the JCPDS file 34-394. No peaks of Na2 CO3 can be seen, suggesting it exists as amorphous state. The average crystallite size of the SDC can 5414
Fig. 1. SEM micrographs for the as prepared SDC/Na2 CO3 nanocomposite after being annealed at (a) 600 C for 2 h, (b) 700 C for 2 h, (c) 800 C for 2 h.
be obtained from the Scherrer equation D = 09/ cos where is the wavelength of the X-ray, is the diffraction angle. = 2m − 2s 1/2 , where m is the measured half height width of the (111) crystal face, s is the that of the silicon sample.10 The average crystallite size of SDC sintering at 600 C, 700 C, 800 C is 14 nm, 28 nm, 37 nm respectively. The variation tendency in grain sizes is in good accordance with the SEM results. The lattice parameter is a = 5.4479 for the sample being sintered at 700 C. The cubic ceria cell unit has been expanded compared with the ceria due to dopant of Sm ion. It indicates the solid solution obey Vegard’s rule. The sample J. Nanosci. Nanotechnol. 11, 5413–5417, 2011
Gao et al.
Electrochemical Characterization on SDC/Na2 CO3 Nanocomposite Electrolyte for Low Temperature SOFCs
contribute to the oxygen conductivities as well as the fuel cell performance. As shown in Figure 3, the conductivity b) 700 °C of the composite is obviously enhanced in the H2 atmoc) 800 °C sphere. The conductivity enhancement may be assumed to contribution from proton conduction in the composc ite. Proton probably conducts via two approaches. One mechanism is bulk proton conduction based on the paddle wheel mechanism in which proton directly coordinates with CO32− . Another is the interface proton conduction b mechanism, H+ derived from the dissociation and oxidation in the anode compartment loosely attaches to CO32− to form transitional state proton carrier HCO3− which a could transfer along the interface from anode to cathode.11 One may argue that the conductivity enhancement may 10 20 30 40 50 60 70 be ascribed to the electronic conduction caused by the 2θ/° reduction reaction of Ce(IV) to Ce(III) which is normal Fig. 2. X-ray diffraction patterns of the SDC/Na2 CO3 nanocomposite case in the pure doped ceria materials. Our approach is after being sintered at (a) 600 C for 2 h, (b) 700 C for 2 h, (c) 800 C to develop two-phase materials where another phase can for 2 h. Delivered by Ingenta extract theto: electronic conduction caused by ceria-phase in Hong Kong Polytechnic University reduced atmosphere. It is true from the fuel cell measuresintered at 700 C is selected for the fuel cell performance IP : 158.132.127.134 ments; the OCVs are all higher than 1.0 V above 400 C, characteristics based on the consideration of stability of Sat, 21 May 2011 05:21:12 from Figure 4, indicating the electronic conduction being the nanocomposite. negligible. It is well recognized that interface regions between Figure 4 presents the fuel cell performance based on the the SDC and the Na2 CO3 play an important role in the nanocomposite SDC/Na2 CO3 electrolyte. A power density conductivity. The interface provides highway for superiof 375 mWcm−2 is yielded at 550 C. The OCVs are onic conduction.8 It is presumably regarded that cationic higher than 1.0 V when the temperature exceeds 400 C. defect concentrations in the interface region are signifiEven at 350 C, the OCV reaches 0.982 V. It is well recogcantly higher than that of the bulk phase. In the air atmo+ nized that the OCV is difficult to exceed 0.9 V in the opersphere, highly mobile Na from the Na2 CO3 with loose ation process of fuel cells based on the pure doped ceria link and coordination at elevated temperatures interacts with the O2− to facilitate the oxygen ion mobility via interdue to the electronic conduction caused by the reduction facial mechanis.8 11 12 At certain high temperatures, there reaction of Ce(IV) to Ce(III) under the anodic atmosphere. may exist a thermal equilibrium of CO32− to O2− + CO2 . These notably high OCVs indicate the electronic conducThis mechanism can enhance both oxygen ion concentration is efficiently prohibited by the introduction of the tion and mobility. The carbonate in the nanocomposite can Na2 CO3 and the formation of the SDC/Na2 CO3 nanocomposite. These good performances show a good potential t (°C) for development of low temperature SOFCs. Intensity (a.u.)
a) 600 °C
450
400
3.2
350
Air/Air H2/Air
3.0
350 °C 400 °C 450 °C 500 °C 550 °C
1.0 0.9
2.8
0.8 2.6
Voltage (V)
Ln (σΤ)
400
1.1
2.4 2.2
300
0.7
250
0.6
200
0.5 150
0.4 0.3
2.0
350
100
0.2 50
0.1
1.8 1.2
1.3
1.4
1.5
1.6
1000/T (K–1) Fig. 3. Conductivity of nanocomposite derived from A.C. impedance under different atmosphere.
J. Nanosci. Nanotechnol. 11, 5413–5417, 2011
Power density (mW cm–2)
500
0
0.0 0
200
400
600
800
1000
1200
Current density (mA cm–2) Fig. 4. I–V and I–P characteristics of fuel cell.
5415
RESEARCH ARTICLE
550
Electrochemical Characterization on SDC/Na2 CO3 Nanocomposite Electrolyte for Low Temperature SOFCs
Gao et al.
Fig. 6. A.C. impedance for the fuel cell under OCV and 100 mA cm−2 at 550 C. Inset: Equivalent circuit used to model the data.
RESEARCH ARTICLE
Fig. 5. A.C. impedance for the fuel cell under OCV (solid line is the modeling result). Inset: Equivalent circuit used to model the data.
Table II. Modeling result for the A.C. impedance spectra under different conditions (corrected for the active area of 2.14 cm2 ).
A.C. impedance spectra of the fuel cell operated under Rohm (cm2 ) Rct (cm2 ) Rt (cm2 ) the OCV is illustrated in Figure 5. The results presented in 550 C OCV 0.616 0.748 1.364 Delivered to: Table I are obtained by modeling of the impedance spectraby Ingenta 0.617 0.546 1.163 550 C under 100 mAcm−2 Hong Kong Polytechnic University with a standard equivalent circuit configuration using the : 158.132.127.134 Zview software. The inductance L may be asIP a result of Sat, 21 Figure 6 presents the A.C. impedance spectra for the the stainless tube of the measurement device. TheMay ohm2011 05:21:12 fuel cell under different conditions at 550 C. Before the resistance, Rohm , predominately includes the electrolyte current polarization is applied, the impedance spectrum resistance. The electrochemical resistance, Rct , is probais characterized by a depressed arc with over electrode bly associated with the electrode process combined cathresistance of 1.364 cm2 . As shown in Table II, the ode and anode reaction. Q1 is the constant phase element, impedance spectrum reduces significantly with the current mainly caused by the interfaces between anode together passage and finally reaches 1.163 cm2 . Regardless of with cathode and electrolyte. As we can see from Table I, the nearly constant of the ohm resistance, the change Rohm deceases with the increase of temperature which of total resistance is mainly resulted from the variability is attributed to the increase of the composite conducof the electrode resistance, which also provided certificativity with temperature. The electrode polarization resistion for the rate determination of the electrode process. tance decreases dramatically from 4.04 cm2 at 350 C The modeling results indicate that the electrochemical to 0.748 cm2 at 550 C. It is likely that the interface resistance, Rct , significantly reduced under current density of the electrode and the electrolyte could be activated at of 100 mAcm−2 . It demonstrates that the current flow not elevated temperatures. The polarization resistance of the only enhances the oxygen reduction reaction of cathode electrode, e.g., 2.12 cm2 is much higher than that of the but also electrochemically activates the anode, resulting in electrolyte, 0.917 cm2 at 400 C. It can be addressed that the different mechanism of the electrode reactions. the electrode polarization process is predominant in rate determination. This indicates the fuel cell performance is mainly limited by the electrolyte and electrode interfaces. 4. CONCLUSIONS Thus, further development of compatible electrodes with The SDC/Na2 CO3 nanocomposite electrolyte has been high catalyst functions that can significantly reduce the investigated by electrochemical methods. The sintering electrode polarization losses are highly required to develop temperature has a significant effect on the grain growth. high performance low temperatures fuel cells. The conductivity enhancement has been observed when the nanocomposite is exposed to H2 atmosphere, which Table I. The ohm resistances (Rohm ), polarization resistances (Rct ) and can be attributed to proton conduction. The fuel cell pertotal resistances (Rt ) for the fuel cell (corrected for the active area of formance with peak power density of 375 mWcm−2 at 2 2.14 cm ). 550 C and OCVs higher than 1.0 V above 400 C Rohm (cm2 ) Rct (cm2 ) Rt (cm2 ) has been achieved. A. C. impedance for the fuel cell under OCV conditions illustrated the electrode polar350 C 1.184 4.04 5.224 ization process rather than ohmic process (mainly con0.917 2.12 3.037 400 C 450 C 0.843 1.668 2.511 tributed by the resistance of electrolyte) is predominant in 0.668 1.219 1.887 500 C rate determination. These results indicate that the major 0.616 0.748 1.364 550 C improvement efforts should be contributed to develop the 5416
J. Nanosci. Nanotechnol. 11, 5413–5417, 2011
Gao et al.
Electrochemical Characterization on SDC/Na2 CO3 Nanocomposite Electrolyte for Low Temperature SOFCs
compatible electrodes with high catalyst functions are highly required for achieving further high performances at low temperatures. The SDC/Na2 CO3 nanocomposite electrolyte provides the enormous potential of choice for development of high performance low temperature solid oxide fuel cells. Acknowledgments: The authors gratefully acknowledge the EC FP6 NANOCOFC project (Contract no. 032308), National Basic Research Program of China (973 Program, Grant No. 2007CB209705), National Natural Science Foundation of China (Grant No. 50902083) and China Scholarship Council.
References and Notes 1. B. C. H. Steele, Solid State Ionics 129, 95 (2000).
2. J. M. Ralph, A. C. Schoeler, and M. Krumpelt, J. Mater. Sci. 36, 1161 (2001). 3. B. Zhu, J. Power Sources 114, 1 (2003). 4. B. Zhu, X. T. Yang, J. Xu, Z. G. Zhu, S. J. Ji, M. T. Sun, and J. C. Sun, J. Power Sources 118, 47 (2003). 5. J. B. Huang, L. Z. Yang, R. F. Gao, Z. Q. Mao, and C. Wang, Electrochem. Commun. 8, 785 (2006). 6. J. B. Huang, Z. Q. Mao, Z. X. Liu, and C. Wang, Electrochem. Commun. 9, 2601 (2007). 7. J. B. Huang, Z. Q. Mao, Z. X. Liu, and C. Wang, J. Power Sources 175, 238 (2008). 8. B. Zhu, X. R. Liu, Z. G. Zhu, and R. Ljungberg, Int. J. Hydrog. Energy 33, 3385 (2008). 9. X. D. Wang, Y. Ma, R. Raza, M. Muhammed, and B. Zhu, Electrochem. Commun. 10, 1617 (2008). 10. K. Singh, S. A. Acharya, and S. S. Bhoga, Ionics 13, 429 (2007). 11. B. Zhu, S. Li, and B.-E. Mellander, Electrochem. Commun. 10, 302 (2008). 12. B. Zhu and B. E. Mellander, Solid State Ionics 77, 244 (1995).
Delivered by Ingenta Received:to: 18 August 2009. Accepted: 26 February 2010. Hong Kong Polytechnic University IP : 158.132.127.134 Sat, 21 May 2011 05:21:12
RESEARCH ARTICLE
J. Nanosci. Nanotechnol. 11, 5413–5417, 2011
5417
Lihat lebih banyak...
Comentarios