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Calcium doped ceria-based materials for cost-effective intermediate temperature solid oxide fuel cells
Solid State Sciences 5 (2003) 1127–1134 www.elsevier.com/locate/ssscie
Calcium doped ceria-based materials for cost-effective intermediate temperature solid oxide fuel cells Bin Zhu a,b,c,∗ , Xiangrong Liu c , Mingtao Sun d , Shijun Ji d , Juncai Sun d a Department of Chemical Engineering & Technology, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden b Department of Thermal Science and Energy Engineering, University of Science and Technology of China (USTC), 20026, Hefei, Anhui, PR China c Goeta Technology Developer International, 171 60 Solna, Sweden d Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, PR China
Received 7 January 2003; received in revised form 2 March 2003; accepted 18 March 2003
Abstract This paper studies preparation and characterization of the calcium doping ceria (CCO) and carbonate composite materials. The material preparation was performed based on an oxalate co-precipitation. Various material characterizations were carried on the material phase structure based on XRD, TG/DSC and their fuel cell applications. The CCO materials showed a two-phase composite with very high ionic conductivity, 0.01 to 0.5 S cm−1 between 400 and 700 ◦ C. Using the CCO-composites as the electrolytes for intermediate temperature solid oxide fuel cells (ITSOFC) a high performance, e.g., 600 mW cm−2 was demonstrated at 600 ◦ C. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Ceria; Calcium doping ceria; Ceria-composite; Intermediate temperature; Solid oxide fuel cells (SOFCs)
1. Introduction In order to develop the market cost-competitive SOFCs, many studies have been oriented to reduced temperature (below 800 ◦ C) and intermediate temperature solid oxide fuel cells (ITSOFCs, 500 to 650 ◦ C), either by using thin film technology to minimise the resistance of the yttrium stabilized zirconia (YSZ) electrolyte [1–4] or by using alternative electrolyte materials, such as various ionic doped ceria etc. [5–12]. All these efforts have, however, limitations due to the deficiency of technology and the stability problem of the material. In fact, for temperatures below 650 ◦ C the SOFCs based on the thin film YSZ electrolyte cannot provide acceptable power output due to the limit of the YSZ conductivity. Commonly alternative electrolytes to replace YSZ are ceria-based oxides. These materials show the same conductivity value, 0.1 S cm−1 at 800 ◦ C, being equal to that of the YSZ at 1000 ◦ C. However, the ion conductivity (typical σ data: 5 × 10−3 S cm−1 at 600 ◦ C) is not sufficient for developing a high performance ITSOFC, because it requests * Corresponding author.
E-mail address: [email protected] (B. Zhu).
a conductivity level of 0.1 S cm−1 . In addition, in the fuel cell operation condition the reduction of Ce4+ to Ce3+ can cause certain electronic conduction resulting in a power loss. This problem becomes a technical challenge for the ceriaelectrolytes in applications [13–15]. Recent developments have been carried out on various ceria-salt composites as the electrolytes for ITSOFCs [13–16]. These materials are based on various ion doping ceria, e.g., gadolinium doping ceria (Gdx Ce1−x O2 ), GCO, samarium doping ceria (Smx Ce1−x O2 ), SCO or yttrium doping ceria (Yx Ce1−x O2 ), YCO, and other salts or compounds. The salts or compounds used in the ceriacomposites widely involved oxyacid salts (sulphates, phosphates and nitrates), halides, carbonates and hydroxides. These ceria-salt composites could, on one hand, effectively suppress electronic conduction and enhance the material stability; on the other hand, could enhance ITSOFC performances [16–19]. These new materials are expected to develop most functional and marketable ITSOFC technologies. From the point of view of cost reduction, gadolinium and samarium as the dopants (10–20 mol%) would cause higher material expense. The use of cheap dopants with acceptable electrochemical properties is more preferable. For instance, Ca2+ replaces Gd3+ or Sm3+ to form Ca2+ doped ceria
1293-2558/$ – see front matter 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00123-7
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(CCO). The CCO itself has also high conductivity compared to the GCO [20,21]. However, there is a lack of relevant CCO work in fuel cells. This study thus intends to explore practical ITSOFC technology based on the CCO-composites for applications.
2. Experimental The calcium (20 mol%) doping ceria (20CCO) was prepared by the solution route with coprecipitation. Cerium nitrate and calcium nitrate (A.R., supplied by Sigma-Aldrich) were prepared as 1 M solutions then mixed together according to the desired molar ratios. An appropriate amount of oxalate acid solution (2 M) was added to prepare the 20CCO precursor in the oxalate state. The precipitate was rinsed several times in deionized water, followed by ethanol washing for several times to remove water from the particle surfaces. The obtained precipitates were dried in an oven at 100 ◦ C over night and then ground in a mortar. The resulting powder was sintered at various temperatures for 1 hour, and relevant results are specified in the text. Based on the preparation of the ion doping ceria, we focus here on one type of the CCO–salt composite, i.e., the 20CCO was mixed by 20 wt% 66Li2CO3 :34Na2 CO3 (named as 20CCO-20NLCO), where the M2 CO3 (M = Li, Na) were purchased from A.R., Aldrich Chemical Company, Inc., USA. The mixture was well ground and heat-treated at 650 ◦ C for 0.5–1.0 h. The resulting materials were directly taken out from the furnace to room temperature and ground again thoroughly for use. The heat-treated 20CCO-20NLC powders were pressed to pellets in a cylindrical die with a diameter of 13 mm. Platinum paint (Degussa, 308A) was used as electrodes for conductivity measurements carried out by an a.c. impedance analysis. The measurements were conducted in the frequency range 5 Hz to 13 MHz using a computerised HP 4192A LF Impedance Analyzer. The temperature of the sample holder was controlled by a Eurotherm temperature controller and the sample temperature was measured with a Platinum thermocouple attached at the position of the sample. The measurements were carried out between 400 and 700 ◦ C in air atmosphere. NiO (A.R., Merck, Germany) and CCO–carbonate composites were used for the fuel cell electrodes. The silver (Leitsilber 200, Hanau, Germany) paste covered on the electrodes was used as the current collector. Stainless steel was adopted for the fuel cell device holder. The fuel cells were constructed using a composite anodesupported technique. The composite anode was made using a mixture of electrolyte, 20OCC-20NLC (60 vol.%), NiO (30 vol.%) and carbon (10 vol.%) powders. After heating the carbon was removed from the mixture to form a porous structure in the anode. The composite cathode was prepared in the similar way by using the lithiated NiO (containing 10 mol% Li) instead of NiO. The porosity of
both electrodes and the compacity of the 20CCO-20NLC electrolyte were not measured as long as they functioned for fuel cells. The anode support fuel cell was made by a hot pressing technique, where the anode (NiO-20CCO-20NLC), electrolyte (20CCO-20NLC) and cathode (lithiated NiO20CCO-20NLC) were pressed at a pressure of 30 kg cm−2 in one step at 500 ◦ C. In this construction the fuel cell assembly had the electrolyte layer ∼ 0.5 mm and the electrode layers ∼ 1–1.5 mm in thickness, respectively. The hot-pressed fuel cell assemblies were also heat-treated again at 600 ◦ C for 30 minutes. Although the NLC carbonate mixture can be melt for operation temperatures above 500 ◦ C, the 20CCO– carbonate composites containing 20 wt% of the NLC used in this work can maintain well the composite in a strong mechanical strength since it was found that all measured fuel cell samples were not deformed in the shape. On the other hand, the melt NLC may also help to prepare a dense electrolyte because it can bind strongly CCO particles together in a compact configuration. The fuel cell assemblies were mounted into the device with the following configuration: anode (H2 chamber)/electrolyte/cathode (air chamber), the cell size normally being 13 mm in diameter and 1–2 mm in thickness. The gas flows were controlled to between 50–150 ml min−1 under 1 atm pressure.
3. Results and discussions Fig. 1a shows the result for the drying 20CCO precursor. The 20CCO precursor was further studied by the DSC and TG. The results are shown in Fig. 2. It can be seen from Fig. 2 that heated at different temperatures the 20CCO precursor experiences a dehydration, decomposition of the oxalates, and crystallizes to the 20CCO. The dehydration takes place at around 100 ◦ C where the water in the oxalates is removed, accompanying a significant weight loss, about 10 wt%. Following the dehydration the continuous heating
Fig. 1. XRD patterns of 20CCO in various states: (a) drying precursor at room temperature, (b) 350 ◦ C, (c) 400 ◦ C and (d) 500 ◦ C heat-treated for 1 h.
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Fig. 2. The DSC/TGA curves of 20CCO, (a) precursor, (b) heat-treated at 500 ◦ C for 1 h.
Fig. 3. The XRD pattern for the 20CCO-20NLCO composite electrolyte. (a) Precursor at room temperature, (b) 300 ◦ C, (c) 400 ◦ C and (d) 500 ◦ C heat-treated for 1 h.
causes the ceria-oxalates decomposing, leading to a large weight loss, about 35 wt%, at the temperature region from 250 to 400 ◦ C. Then the sample remains the weight unchanged, indicating a stable crystalline structure formed, i.e., 20CCO. 400 ◦ C is sufficient to make the 20CCO crystalline structure, which has a fluorite-type structure, as shown in Fig. 1c. With increasing the temperature the 20CCO crystalline particles are growing bigger. From the XRD peaks narrowing effect shown in Fig. 1, we can calculate the crystalline particle sizes growing from 20 nm (350 ◦ C), to 60 nm (400 ◦ C) and then to 80 nm (500 ◦ C), respectively. The 20CCO-20NLC composite prepared through the above procedure should be two-phase composite materials. However, the XRD patterns shown in Fig. 3 only show the same diffractions as Fig. 1 that are caused by the 20CCO. The nature of the microstructure of this 20CCO and 20NLC composite is not yet clear at this stage, further studies are needed. On the other hand, no other new XRD diffractions were observed in Fig. 3. This means there is neither a chemical reaction nor any intermediate compounds between
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these two material phases, i.e., the 20CCO and the 20NLC, thus forming the so-called composite materials. Figs. 4 and 5 show the SEM morphologies of the pure 20CCO and 20CCO-20NLC powders. For the pure 20CCO powder, it is seen that the particles are made of a number of lamellas as definite orientations. The shapes of some particles are long bar or a tweezers-like bar to form a star shape. Some of the particles as a group are like framework to have a clear interface and surface/edge outlook. However, the shape of the pure 20CCO precursor did not change with heat treatment at 100–500 ◦ C although the crystalline structure of the 20CCO had changed considerably during the heat-treatment as shown in Fig. 1 XRD patterns. The 20CCO-20NLC composite powders look different from the pure 20CCO in morphology. The particles are not uniform in the sense that smaller particles probably stick to the large particles and make the particle surface and edges ambiguous. As the heating temperature increases, the small particles increase and the surface become coarser. In Fig. 5 we can see a lot of small particles covering the larger particle. This may be due to the fact that the carbonates in the 20CCO-20NLC composite changes the surface energy of 20CCO and helps with small particles forming in melting and solidifying of the salt. Such composite materials may have good performance in the fabrication and formation of dense electrolyte layer for ITSOFCs. Fig. 6 shows the temperature dependence of the conductivity obtained for the 20CCO-20NLCO from impedance analysis, and also compared to the fuel cell measurements using the 20CCO-20NLC electrolytes. Both measurements showed close data, but the impedance data showing some higher values, about 10% higher than those of the values obtained from the fuel cell measurements. It can be expected because the a.c. impedance measurements reflect the total conductivities concerning all mobile charge species including: oxygen ions (in 20CCO), maybe protons in the 20CCO20NLC similar to other ceria-carbonate composites [19], cations (Li+ and Na+ ) and carbonate anions in the carbonates. Therefore, it has been suggested both a.c. impedance analysis and fuel cell I–V characterization, i.e., the current density versus voltage for characterisation of the electrical properties, e.g., conductivity, should be employed [22]. Although the a.c. impedance measurements are a common way to acquire the material conductivity, the fuel cells are even better tool to directly characterize the electrolyte materials in-situ. Through measurements for the fuel cell I–V characteristics, the conductivity of the fuel cell electrolyte can be determined after subtraction of the influence of the electrodes and electrolyte/electrode interfaces as described early [22]. In some respect the conductivity obtained from the fuel cell measurements may more correctly depict the electrical property of the materials, especially when considering the material stability in in-situ environment because of the strong material stoichiometry dependence on the in-situ gas atmosphere, e.g., the ceria materials in the fuel reducing atmosphere can cause the reduction of Ce4+ to Ce3+ .
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Fig. 4. The SEM photographs of pure 20CCO heat-treated for 1 h at different temperature. (a) Precursor, (b) 300 ◦ C, (c) 500 ◦ C.
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Fig. 4. (Continued).
Thus we prefer to use the conductivity values obtained from the fuel cell measurements here. It shows the value in the range of 10−2 to 0.45 S cm−1 between 400 and 700 ◦ C, compared to 10−4 to 10−2 for pure CCO samples in the same temperature range. This conductivity value of the 20CCO-20NLCO is about two orders of magnitude higher than that of the pure CCO material without composing. High conductivity of the CCO–carbonate composite materials ensures a high performance for fuel cells. Fig. 7 shows the I–V characteristics for the fuel cell using the 20CCO-20NLCO composite as the electrolyte, indicating the highest power density of 600 and 740 mW cm−2 at 600 and 650 ◦ C, respectively. Fig. 8 shows the discharge performance with time for another 20CCO-20NLCO fuel cell at 600 ◦ C. This fuel cell has a slightly thicker electrolyte than that used for Fig. 7. The fuel cell performance was improved by increasing the cell voltage and current density during the initial discharge period within about one hour. It may be caused by the improvement and activation of the electrode catalyst function and interfaces between the electrodes/electrolyte. Then the polarization appears which causes gradually a cell voltage and current losses with the time. The cell was tested for more than 13 hours. An average power density output is around 450 mW cm−2 . The CCO materials are interesting and promising for the development of practical ITSOFCs for applications, but the challenge lies in the stabilization of the material against the fuel environment. Tsai et al. studied both pure yttrium doping ceria (YCO) and YCO/YSZ bilayered electrolytes to suppress the electronic conduction from the YCO ma-
terial for SOFC applications [1]. It was reported that the YCO/YSZ electrolyte SOFCs yielded 85 to 98% of the theoretical OCV, but only approximately 50% for YCO electrolyte SOFCs. They obtained the maximum power density of 210 mW cm−2 for the YCO/YSZ electrolyte SOFCs at 600 ◦ C. In our case, the fuel cell devices using pure CCO as the electrolyte generated around 0.8 V due to the electronic conduction caused by the reduction in the fuel environment, while the CCO–carbonate composite electrolyte fuel cells reach 1.0–1.1 V in the IT (400 to 650 ◦ C) region, close to 100% of the theoretical value. This indicates that the CCO composite materials are stable in the fuel environment in suppressing effectively the electronic conduction, resulting in a normal fuel cell voltage reached. In addition, the CCO-composite electrolytes have a higher conductivity than that of the CCO so they have demonstrated higher ITSOFC performances, e.g., 600 mW cm−2 at 600 ◦ C, approximately being three times higher than that of the CCO/YSZ bilayered electrolyte SOFCs, and about 6 times higher than that of the pure CCO electrolyte SOFC at the same temperature. This new type of composite ceramic has shown a good feasibility to improve the ceria electrolyte property. The route of the ceria-salt composite ceramic may be more promising than that of the protection YCO/YSZ bilayer, since the improvement from the material itself is more effective and preferable, subjecting also to a lower operation temperature. Furthermore, from an electrochemical point of view, the YCO/YSZ bilayer fabricated by two different structured YCO and YSZ materials may cause additional overpotential and polarization loss as well as mismatched thermal expan-
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Fig. 5. The SEM photographs of the 20CCO-20NLCO composite heat-treated for 1 h at different temperature. (a) Precursor, (b) 300 ◦ C, (c) 400 ◦ C, (d) 500 ◦ C.
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Fig. 5. (Continued).
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CCO properties, e.g., improved the material stability and enhanced its conductivity 10 to 100 times higher compared to that of the pure CCO without composing. The new CCO– carbonate composite, 20CCO-20NLC, has demonstrated the success in development of the new advanced ITSOFCs to match the cost issue for market applications. Acknowledgements
Fig. 6. Temperature dependence of conductivity for the 20CCO-20NLCO composite electrolyte. The data were obtained from both a.c. impedance analysis and fuel cell measurements in comparison.
This work is carried out based on the Swedish–Chinese bilateral cooperation program supported by the Swedish Research Council (VR), the VR/the Swedish International Cooperation Development Agency (Sida) and Carl Tryggers Stiftelse for Vetenskap Forskning (CTS). In Chinese side it is further support by Ministry of Communication. References
Fig. 7. The current–voltage (I–V) and current–power (I–P) characteristics for the fuel cell using the 20CCO-20NLCO composite electrolyte at various temperatures.
Fig. 8. Typical performances of the 20CCO-20NLCO electrolyte fuel cell discharged with time.
sion in the bilayer structure and the interfaces between two different material components. 4. Conclusions In this work a novel approach has been developed to prepare cost-effective calcium doping ceria (CCO)–carbonate composites, especially, the 20CCO-20NLC was studied in this work, to meet the demands for ITSOFC applications. This innovative ceria-composite approach has improved the
[1] T. Tsai, E. Perry, S. Barnet, J. Electrochem. Soc. 144 (1997) L130. [2] E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. [3] P. Charpentier, P. Fragnaud, D.M. Schleich, C. Lunot, Ionics 2 (1996) 312. [4] S. de Souza, S.J. Visco, L.C. DeJonghe, Solid State Ionics 98 (1997) 57. [5] B.C.H. Steele, Solid State Ionics 92 (1996) 86. [6] W. Bakker, C. Milliken, J. Hartvigsen, S. Elangovan, A. Khandkar, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Solid Oxide FCs-V, Electrochemical Society, Pennington, New York, 1997, p. 254. [7] C. Milliken, S. Guruswamy, A. Khandkar, J. Electrochem. Soc. 146 (1999) 872. [8] N. Maffei, A.K. Kuriakose, J. Power Sources 75 (1998) 162. [9] R. Maric, S. Ohara, T. Fukui, H. Yoshida, M. Nishimura, T. Inagaki, K. Miura, J. Electrochem. Soc. 146 (1999) 2006. [10] K. Eguchi, H. Mitsuyasu, Y. Mishima, M. Ohtaki, H. Arai, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Solid Oxide FCs-V, Electrochemical Society, Pennington, New York, 1997, p. 358. [11] R. Doshi, Von L. Richards, J.D. Carter, X.P. Wang, M. Krumpelt, J. Electrochem. Soc. 146 (1999) 1273. [12] K. Huang, M. Feng, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 3620. [13] M. Godickemeier, K. Sasaki, L.I. Grauckler, I. Riess, J. Electrochem. Soc. 144 (1997) 1635. [14] S.P.S. Badwal, F.T. Ciacchi, Solid State Ionics 121 (1999) 253–262. [15] A. Mineshige, T. Yasui, N. Ohmura, M. Kobure, S. Fujii, M. Inaba, Z. Ogumi, Oxygen chemical potential and mixed conduction in doped ceria under influence of oxygen partial pressure gradient, Solid State Ionics 152–153 (2002) 493. [16] B. Zhu, B.-E. Mellander, in: S.C. Singhal, H. Iwahara (Eds.), Solid Oxide Fuel Cells VI, Electrochemical Society, Pennington, New York, 1999, pp. 1–244. [17] B. Zhu, Q.X. Fu, C.R. Xia, D.K. Peng, G.Y. Meng, in: Z.Q. Mao, T.N. Veziroglu (Eds.), Hydrogen Energy Process XIII, Proc. of the 13th World Hydrogen Energy Conference, Beijing, China, June 12– 15, 2000, pp. 1–766. [18] B. Zhu, Functional ceria-salt-composite materials for advanced ITSOFC applications, J. Power Sources 114 (2003) 1. [19] B. Zhu, X.R. Liu, P. Zhou, X.T. Yang, Z.G. Zhu, W. Zhu, Electrochemistry Communication 3 (2001) 566. [20] H. Arai, T. Kunisaki, Y. Shimizu, T. Seiyama, Solid State Ionics 20 (1986) 41. [21] H. Yahiro, T. Ohuchi, K. Eguchi, H. Arai, J. Mater. Sci. 23 (1998) 1036. [22] B. Zhu, Electrochemistry Communication 1 (1999) 242.
Calcium doped ceria-based materials for cost-effective intermediate temperature solid oxide fuel cells Bin Zhu a,b,c,∗ , Xiangrong Liu c , Mingtao Sun d , Shijun Ji d , Juncai Sun d a Department of Chemical Engineering & Technology, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden b Department of Thermal Science and Energy Engineering, University of Science and Technology of China (USTC), 20026, Hefei, Anhui, PR China c Goeta Technology Developer International, 171 60 Solna, Sweden d Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, PR China
Received 7 January 2003; received in revised form 2 March 2003; accepted 18 March 2003
Abstract This paper studies preparation and characterization of the calcium doping ceria (CCO) and carbonate composite materials. The material preparation was performed based on an oxalate co-precipitation. Various material characterizations were carried on the material phase structure based on XRD, TG/DSC and their fuel cell applications. The CCO materials showed a two-phase composite with very high ionic conductivity, 0.01 to 0.5 S cm−1 between 400 and 700 ◦ C. Using the CCO-composites as the electrolytes for intermediate temperature solid oxide fuel cells (ITSOFC) a high performance, e.g., 600 mW cm−2 was demonstrated at 600 ◦ C. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Ceria; Calcium doping ceria; Ceria-composite; Intermediate temperature; Solid oxide fuel cells (SOFCs)
1. Introduction In order to develop the market cost-competitive SOFCs, many studies have been oriented to reduced temperature (below 800 ◦ C) and intermediate temperature solid oxide fuel cells (ITSOFCs, 500 to 650 ◦ C), either by using thin film technology to minimise the resistance of the yttrium stabilized zirconia (YSZ) electrolyte [1–4] or by using alternative electrolyte materials, such as various ionic doped ceria etc. [5–12]. All these efforts have, however, limitations due to the deficiency of technology and the stability problem of the material. In fact, for temperatures below 650 ◦ C the SOFCs based on the thin film YSZ electrolyte cannot provide acceptable power output due to the limit of the YSZ conductivity. Commonly alternative electrolytes to replace YSZ are ceria-based oxides. These materials show the same conductivity value, 0.1 S cm−1 at 800 ◦ C, being equal to that of the YSZ at 1000 ◦ C. However, the ion conductivity (typical σ data: 5 × 10−3 S cm−1 at 600 ◦ C) is not sufficient for developing a high performance ITSOFC, because it requests * Corresponding author.
E-mail address: [email protected] (B. Zhu).
a conductivity level of 0.1 S cm−1 . In addition, in the fuel cell operation condition the reduction of Ce4+ to Ce3+ can cause certain electronic conduction resulting in a power loss. This problem becomes a technical challenge for the ceriaelectrolytes in applications [13–15]. Recent developments have been carried out on various ceria-salt composites as the electrolytes for ITSOFCs [13–16]. These materials are based on various ion doping ceria, e.g., gadolinium doping ceria (Gdx Ce1−x O2 ), GCO, samarium doping ceria (Smx Ce1−x O2 ), SCO or yttrium doping ceria (Yx Ce1−x O2 ), YCO, and other salts or compounds. The salts or compounds used in the ceriacomposites widely involved oxyacid salts (sulphates, phosphates and nitrates), halides, carbonates and hydroxides. These ceria-salt composites could, on one hand, effectively suppress electronic conduction and enhance the material stability; on the other hand, could enhance ITSOFC performances [16–19]. These new materials are expected to develop most functional and marketable ITSOFC technologies. From the point of view of cost reduction, gadolinium and samarium as the dopants (10–20 mol%) would cause higher material expense. The use of cheap dopants with acceptable electrochemical properties is more preferable. For instance, Ca2+ replaces Gd3+ or Sm3+ to form Ca2+ doped ceria
1293-2558/$ – see front matter 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00123-7
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(CCO). The CCO itself has also high conductivity compared to the GCO [20,21]. However, there is a lack of relevant CCO work in fuel cells. This study thus intends to explore practical ITSOFC technology based on the CCO-composites for applications.
2. Experimental The calcium (20 mol%) doping ceria (20CCO) was prepared by the solution route with coprecipitation. Cerium nitrate and calcium nitrate (A.R., supplied by Sigma-Aldrich) were prepared as 1 M solutions then mixed together according to the desired molar ratios. An appropriate amount of oxalate acid solution (2 M) was added to prepare the 20CCO precursor in the oxalate state. The precipitate was rinsed several times in deionized water, followed by ethanol washing for several times to remove water from the particle surfaces. The obtained precipitates were dried in an oven at 100 ◦ C over night and then ground in a mortar. The resulting powder was sintered at various temperatures for 1 hour, and relevant results are specified in the text. Based on the preparation of the ion doping ceria, we focus here on one type of the CCO–salt composite, i.e., the 20CCO was mixed by 20 wt% 66Li2CO3 :34Na2 CO3 (named as 20CCO-20NLCO), where the M2 CO3 (M = Li, Na) were purchased from A.R., Aldrich Chemical Company, Inc., USA. The mixture was well ground and heat-treated at 650 ◦ C for 0.5–1.0 h. The resulting materials were directly taken out from the furnace to room temperature and ground again thoroughly for use. The heat-treated 20CCO-20NLC powders were pressed to pellets in a cylindrical die with a diameter of 13 mm. Platinum paint (Degussa, 308A) was used as electrodes for conductivity measurements carried out by an a.c. impedance analysis. The measurements were conducted in the frequency range 5 Hz to 13 MHz using a computerised HP 4192A LF Impedance Analyzer. The temperature of the sample holder was controlled by a Eurotherm temperature controller and the sample temperature was measured with a Platinum thermocouple attached at the position of the sample. The measurements were carried out between 400 and 700 ◦ C in air atmosphere. NiO (A.R., Merck, Germany) and CCO–carbonate composites were used for the fuel cell electrodes. The silver (Leitsilber 200, Hanau, Germany) paste covered on the electrodes was used as the current collector. Stainless steel was adopted for the fuel cell device holder. The fuel cells were constructed using a composite anodesupported technique. The composite anode was made using a mixture of electrolyte, 20OCC-20NLC (60 vol.%), NiO (30 vol.%) and carbon (10 vol.%) powders. After heating the carbon was removed from the mixture to form a porous structure in the anode. The composite cathode was prepared in the similar way by using the lithiated NiO (containing 10 mol% Li) instead of NiO. The porosity of
both electrodes and the compacity of the 20CCO-20NLC electrolyte were not measured as long as they functioned for fuel cells. The anode support fuel cell was made by a hot pressing technique, where the anode (NiO-20CCO-20NLC), electrolyte (20CCO-20NLC) and cathode (lithiated NiO20CCO-20NLC) were pressed at a pressure of 30 kg cm−2 in one step at 500 ◦ C. In this construction the fuel cell assembly had the electrolyte layer ∼ 0.5 mm and the electrode layers ∼ 1–1.5 mm in thickness, respectively. The hot-pressed fuel cell assemblies were also heat-treated again at 600 ◦ C for 30 minutes. Although the NLC carbonate mixture can be melt for operation temperatures above 500 ◦ C, the 20CCO– carbonate composites containing 20 wt% of the NLC used in this work can maintain well the composite in a strong mechanical strength since it was found that all measured fuel cell samples were not deformed in the shape. On the other hand, the melt NLC may also help to prepare a dense electrolyte because it can bind strongly CCO particles together in a compact configuration. The fuel cell assemblies were mounted into the device with the following configuration: anode (H2 chamber)/electrolyte/cathode (air chamber), the cell size normally being 13 mm in diameter and 1–2 mm in thickness. The gas flows were controlled to between 50–150 ml min−1 under 1 atm pressure.
3. Results and discussions Fig. 1a shows the result for the drying 20CCO precursor. The 20CCO precursor was further studied by the DSC and TG. The results are shown in Fig. 2. It can be seen from Fig. 2 that heated at different temperatures the 20CCO precursor experiences a dehydration, decomposition of the oxalates, and crystallizes to the 20CCO. The dehydration takes place at around 100 ◦ C where the water in the oxalates is removed, accompanying a significant weight loss, about 10 wt%. Following the dehydration the continuous heating
Fig. 1. XRD patterns of 20CCO in various states: (a) drying precursor at room temperature, (b) 350 ◦ C, (c) 400 ◦ C and (d) 500 ◦ C heat-treated for 1 h.
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Fig. 2. The DSC/TGA curves of 20CCO, (a) precursor, (b) heat-treated at 500 ◦ C for 1 h.
Fig. 3. The XRD pattern for the 20CCO-20NLCO composite electrolyte. (a) Precursor at room temperature, (b) 300 ◦ C, (c) 400 ◦ C and (d) 500 ◦ C heat-treated for 1 h.
causes the ceria-oxalates decomposing, leading to a large weight loss, about 35 wt%, at the temperature region from 250 to 400 ◦ C. Then the sample remains the weight unchanged, indicating a stable crystalline structure formed, i.e., 20CCO. 400 ◦ C is sufficient to make the 20CCO crystalline structure, which has a fluorite-type structure, as shown in Fig. 1c. With increasing the temperature the 20CCO crystalline particles are growing bigger. From the XRD peaks narrowing effect shown in Fig. 1, we can calculate the crystalline particle sizes growing from 20 nm (350 ◦ C), to 60 nm (400 ◦ C) and then to 80 nm (500 ◦ C), respectively. The 20CCO-20NLC composite prepared through the above procedure should be two-phase composite materials. However, the XRD patterns shown in Fig. 3 only show the same diffractions as Fig. 1 that are caused by the 20CCO. The nature of the microstructure of this 20CCO and 20NLC composite is not yet clear at this stage, further studies are needed. On the other hand, no other new XRD diffractions were observed in Fig. 3. This means there is neither a chemical reaction nor any intermediate compounds between
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these two material phases, i.e., the 20CCO and the 20NLC, thus forming the so-called composite materials. Figs. 4 and 5 show the SEM morphologies of the pure 20CCO and 20CCO-20NLC powders. For the pure 20CCO powder, it is seen that the particles are made of a number of lamellas as definite orientations. The shapes of some particles are long bar or a tweezers-like bar to form a star shape. Some of the particles as a group are like framework to have a clear interface and surface/edge outlook. However, the shape of the pure 20CCO precursor did not change with heat treatment at 100–500 ◦ C although the crystalline structure of the 20CCO had changed considerably during the heat-treatment as shown in Fig. 1 XRD patterns. The 20CCO-20NLC composite powders look different from the pure 20CCO in morphology. The particles are not uniform in the sense that smaller particles probably stick to the large particles and make the particle surface and edges ambiguous. As the heating temperature increases, the small particles increase and the surface become coarser. In Fig. 5 we can see a lot of small particles covering the larger particle. This may be due to the fact that the carbonates in the 20CCO-20NLC composite changes the surface energy of 20CCO and helps with small particles forming in melting and solidifying of the salt. Such composite materials may have good performance in the fabrication and formation of dense electrolyte layer for ITSOFCs. Fig. 6 shows the temperature dependence of the conductivity obtained for the 20CCO-20NLCO from impedance analysis, and also compared to the fuel cell measurements using the 20CCO-20NLC electrolytes. Both measurements showed close data, but the impedance data showing some higher values, about 10% higher than those of the values obtained from the fuel cell measurements. It can be expected because the a.c. impedance measurements reflect the total conductivities concerning all mobile charge species including: oxygen ions (in 20CCO), maybe protons in the 20CCO20NLC similar to other ceria-carbonate composites [19], cations (Li+ and Na+ ) and carbonate anions in the carbonates. Therefore, it has been suggested both a.c. impedance analysis and fuel cell I–V characterization, i.e., the current density versus voltage for characterisation of the electrical properties, e.g., conductivity, should be employed [22]. Although the a.c. impedance measurements are a common way to acquire the material conductivity, the fuel cells are even better tool to directly characterize the electrolyte materials in-situ. Through measurements for the fuel cell I–V characteristics, the conductivity of the fuel cell electrolyte can be determined after subtraction of the influence of the electrodes and electrolyte/electrode interfaces as described early [22]. In some respect the conductivity obtained from the fuel cell measurements may more correctly depict the electrical property of the materials, especially when considering the material stability in in-situ environment because of the strong material stoichiometry dependence on the in-situ gas atmosphere, e.g., the ceria materials in the fuel reducing atmosphere can cause the reduction of Ce4+ to Ce3+ .
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Fig. 4. The SEM photographs of pure 20CCO heat-treated for 1 h at different temperature. (a) Precursor, (b) 300 ◦ C, (c) 500 ◦ C.
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Fig. 4. (Continued).
Thus we prefer to use the conductivity values obtained from the fuel cell measurements here. It shows the value in the range of 10−2 to 0.45 S cm−1 between 400 and 700 ◦ C, compared to 10−4 to 10−2 for pure CCO samples in the same temperature range. This conductivity value of the 20CCO-20NLCO is about two orders of magnitude higher than that of the pure CCO material without composing. High conductivity of the CCO–carbonate composite materials ensures a high performance for fuel cells. Fig. 7 shows the I–V characteristics for the fuel cell using the 20CCO-20NLCO composite as the electrolyte, indicating the highest power density of 600 and 740 mW cm−2 at 600 and 650 ◦ C, respectively. Fig. 8 shows the discharge performance with time for another 20CCO-20NLCO fuel cell at 600 ◦ C. This fuel cell has a slightly thicker electrolyte than that used for Fig. 7. The fuel cell performance was improved by increasing the cell voltage and current density during the initial discharge period within about one hour. It may be caused by the improvement and activation of the electrode catalyst function and interfaces between the electrodes/electrolyte. Then the polarization appears which causes gradually a cell voltage and current losses with the time. The cell was tested for more than 13 hours. An average power density output is around 450 mW cm−2 . The CCO materials are interesting and promising for the development of practical ITSOFCs for applications, but the challenge lies in the stabilization of the material against the fuel environment. Tsai et al. studied both pure yttrium doping ceria (YCO) and YCO/YSZ bilayered electrolytes to suppress the electronic conduction from the YCO ma-
terial for SOFC applications [1]. It was reported that the YCO/YSZ electrolyte SOFCs yielded 85 to 98% of the theoretical OCV, but only approximately 50% for YCO electrolyte SOFCs. They obtained the maximum power density of 210 mW cm−2 for the YCO/YSZ electrolyte SOFCs at 600 ◦ C. In our case, the fuel cell devices using pure CCO as the electrolyte generated around 0.8 V due to the electronic conduction caused by the reduction in the fuel environment, while the CCO–carbonate composite electrolyte fuel cells reach 1.0–1.1 V in the IT (400 to 650 ◦ C) region, close to 100% of the theoretical value. This indicates that the CCO composite materials are stable in the fuel environment in suppressing effectively the electronic conduction, resulting in a normal fuel cell voltage reached. In addition, the CCO-composite electrolytes have a higher conductivity than that of the CCO so they have demonstrated higher ITSOFC performances, e.g., 600 mW cm−2 at 600 ◦ C, approximately being three times higher than that of the CCO/YSZ bilayered electrolyte SOFCs, and about 6 times higher than that of the pure CCO electrolyte SOFC at the same temperature. This new type of composite ceramic has shown a good feasibility to improve the ceria electrolyte property. The route of the ceria-salt composite ceramic may be more promising than that of the protection YCO/YSZ bilayer, since the improvement from the material itself is more effective and preferable, subjecting also to a lower operation temperature. Furthermore, from an electrochemical point of view, the YCO/YSZ bilayer fabricated by two different structured YCO and YSZ materials may cause additional overpotential and polarization loss as well as mismatched thermal expan-
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Fig. 5. The SEM photographs of the 20CCO-20NLCO composite heat-treated for 1 h at different temperature. (a) Precursor, (b) 300 ◦ C, (c) 400 ◦ C, (d) 500 ◦ C.
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Fig. 5. (Continued).
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CCO properties, e.g., improved the material stability and enhanced its conductivity 10 to 100 times higher compared to that of the pure CCO without composing. The new CCO– carbonate composite, 20CCO-20NLC, has demonstrated the success in development of the new advanced ITSOFCs to match the cost issue for market applications. Acknowledgements
Fig. 6. Temperature dependence of conductivity for the 20CCO-20NLCO composite electrolyte. The data were obtained from both a.c. impedance analysis and fuel cell measurements in comparison.
This work is carried out based on the Swedish–Chinese bilateral cooperation program supported by the Swedish Research Council (VR), the VR/the Swedish International Cooperation Development Agency (Sida) and Carl Tryggers Stiftelse for Vetenskap Forskning (CTS). In Chinese side it is further support by Ministry of Communication. References
Fig. 7. The current–voltage (I–V) and current–power (I–P) characteristics for the fuel cell using the 20CCO-20NLCO composite electrolyte at various temperatures.
Fig. 8. Typical performances of the 20CCO-20NLCO electrolyte fuel cell discharged with time.
sion in the bilayer structure and the interfaces between two different material components. 4. Conclusions In this work a novel approach has been developed to prepare cost-effective calcium doping ceria (CCO)–carbonate composites, especially, the 20CCO-20NLC was studied in this work, to meet the demands for ITSOFC applications. This innovative ceria-composite approach has improved the
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