international journal of hydrogen energy 35 (2010) 2684–2688
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Improved ceria–carbonate composite electrolytes Rizwan Razaa,b,*, Xiaodi Wangc,d, Ying Mac,d, Xiangrong Liua,e, Bin Zhua,d,e a
Department of Energy Technology, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan c Division of Functional Materials, Royal Institute of Technology (KTH), 16440 Stockholm, Sweden d College of Material Science and Chemical Engineering, Harbin Engineering University, 150001, China e GETT Fuel Cells AB, Stora Nygatan 33, S-10314 Stockholm, Sweden b
article info
abstract
Article history:
It has been successfully demonstrated that the fuel cells using the ceria–carbonate
Received 21 March 2009
composite as electrolytes have achieved excellent performances of 200–1150 W/cm2 at 300–
Received in revised form
600 C. Previously it was reported these ceria–carbonate composite electrolytes have been
13 April 2009
prepared with two-step processes: step 1, prepare ion-doped ceria which was prepared
Accepted 15 April 2009
usually through the wet-chemical co-precipitation process; step 2, mixing the doped ceria
Available online 14 May 2009
with carbonates in various compositions. We first report here to prepare the SDC– carbonate composites within one-step chemical co-precipitation process, i.e. mixing
Keywords:
carbonates and preparing the SDC in the same process. The one-step process has provided
Low temperature
a number of advantages: (i) to reduce the involved preparation processes to enhance the
Ceria–carbonate
production, to make the produced materials in good quality control, more homogenous
Nanocomposites
composites microstructure; (ii) as results, these composites showed also different micro-
Interfacial mechanism
structures and electrical properties. It has significantly improved the ceria–carbonate
Superionic conduction
conductivities and cause the superionic conduction at much lower temperatures; (iii) to reduce manufacturing costs also. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The solid oxide fuel cells (SOFCs) have been developed from previous several decades based on ion-doped single-phase and single-ion conducting oxide conductors, e.g., YSZ (yttrium stabilized zirconia) and ion-doped ceria, e.g., Gd3þ and Sm3þ doped CeO2 (GDC and SDC) [1,2]. The ion-doped ceria as a major candidate to realize the LTSOFC until 500 C [1]. However, critical challenges limit seriously its applications because: (i) low conductivity, 5 103–1.0 102 Scm1 (600 C), not sufficient for high performance SOFCs which require at 0.1 Scm1 level [2–5]; (ii) the Ce4þ partially reduced
to Ce3þ in the fuel cell causing e-conduction to decrease significantly in the open circuit voltage (OCV) and thus the cell efficiency; (iii) nano-sized ceria particles enhanced econduction; (iv) poor mechanical property caused by the electronic reduction [2–5]. To develop conventional SOFCs to lower temperatures, 300–600 C, i.e. LTSOFCs, operated at a temperature lower than that of a hot plate of a typical electric kitchen-range, opens new opportunities for SOFC commercialization [6]. To significantly reduce the SOFC operational temperatures, new electrolytes are needed. In recent years, various ceria-based composites, widely involved ceria–salts and ceria–oxide
* Corresponding author at. Department of Energy Technology, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden. Tel.: þ46 87907403. E-mail address:
[email protected] (R. Raza). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.04.038
international journal of hydrogen energy 35 (2010) 2684–2688
composites have been developed [7–13]. The ceria–salts, especially ceria–carbonate composites become the focus; the R&D activities in this area have been extensively carried out. Latest result reported by Huang et al. by using such composite as electrolytes, their fuel cell performance reached 1100 mW/cm2 at 600 C [13]. Previously reported the ceria–carbonate composite electrolytes have been prepared with two-step processes: step 1, prepare ion-doped ceria (The samarium doped CeO2 is more commonly reported.) which was prepared usually by using the wet-chemical co-precipitation process; step 2, mixing the doped ceria with carbonates (usually Li, Na and K-carbonates) in various compositions [6–10]. The SDC– carbonate composite electrolytes have been reported with much better fuel cell performances than those of the SDC. In the same time they can improve the material stability [7–13]. Especially, strong activities have been carried out in recent years focusing on nanocomposite approaches in NANOCOFC (Nanocomposites for advanced fuel cell technology) – a currently ongoing EC FP6 NMP project (www.nanocofc.org) coordinated by Prof. Bin Zhu, where two-phase materials with nano-scale particles are to be used to construct and develop multifunctional materials for low temperature, 300–600 C ceramic or solid oxide fuel cells [6]. We used the NANOCOFC approach to develop ceria and carbonate nanocomposites for electrolytes. In this work, we first report to prepare the SDC–carbonate composites in a simplified preparation within one-step chemical co-precipitation process, i.e. mixing carbonates and preparing the SDC in the same process.
2.
Experimental
2.1.
Synthesis of ceria–carbonate composites
Nanostructure SDC–Na2CO3, i.e. nanocomposite electrolyte was synthesized by a co-precipitation process. In synthesis of ceria–carbonate composites following raw chemicals were used for 1.0 M solutions, Ce (NO3)3$6H2O (Sigma–Aldrich) and Sm (NO3)3$6H2O (Sigma–Aldrich). According to desired molar ratios, the solution of Sm (NO3)3$6H2O was mixed with the solution of Ce (NO3)3$6H2O. According to ‘‘metal ion: carbonate ion ¼ 1:2’’ in molar ratio, a pertinent amount of Na2CO3 solution (1.0 M) was added slowly (10 ml/min) to complete the ceria–carbonate composites within a wetchemical co-precipitation process. There was a mixture of SDC and carbonates in the same process. After this process the mixture was filtered by suction filtration method. That precipitate was dried over night in the oven at 50 C. Finally, that dried solid was crushed in a mortar with pestle, and sintered at 800 C for 2 h, and analysed the amount of the Na2CO3 in the SDC–Na2CO3 by EDX attached to the SEM. Conventional two-step preparation was also used to prepare the SDC–LiNa2CO3 composite electrolyte for comparison. The 0.5 M Ce (NO3)3$6H2O solution was mixed with the Sm (NO3)3$6H2O with 1:4 molar ratios. The ammonia (33% NH3, Sigma–Aldrich) of an appropriate amount was added in order to co-precipitate the cations, e.g., Sm3þ and Ce3þ for SDC precursor in the hydroxide state. The pH value was 10. The precipitate was sluice three times in deionized water, followed by ethanol washing several times in order to remove
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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 700 C for 2 h to obtain the SDC. After that, the SDC and LiNaCO3 powders were mixed according to suitable weight ratio, and again ground. Finally, that powder was sintered at 700 C in the furnace for 30 min and again pulverized to get two-step SDC–LiNaCO3 electrolyte. The electrode materials were prepared by using Li, Cu, and Ni carbonates through conventional method (high temperature solid state reaction) and sintered at 800 C for 4 h. The electrodes were prepared as composite type, i.e. mixture of electrode: electrolyte: active carbon in weight ratio of 10:4:1.
2.2.
Fabrication of fuel cells
The solid oxide fuel cells were fabricated by hot pressing procedure. The dry-powder-pressing technique involved loading a mold with the powders of anode successively followed by the electrolyte and finally the cathode, all being pressed in one step to form a complete fuel cell assembly for measurements. The cell structure made through this procedure consists of anode (composite with electrolyte)–electrolyte–cathode (composite with the electrolyte). The electrolyte of (i) normal SDC–LiNaCO3 electrolyte and (ii) nanocomposite SDC–Na2CO3 were used for comparisons. The above fuel cells were fabricated for 13 mm in diameter (with a 0.7 cm2 active area) for single cell test. These fuel cells had 0.8–1.0 mm of thickness consisting of an approximate 0.3–0.5 mm thick electrolyte layer and 0.3–0.5 mm thick electrodes for anode and cathode layers, respectively. The pressed fuel cell pellets were sintered at 600 C for 0.5 h. Finally, both anode and cathode surfaces were painted by silver paste as a current collects for fuel cell measurements.
2.3.
Analysis of SDC–Na2CO3 microstructure
The crystalline phase structure was analyzed by a D/Max-3A Regaku X-ray diffractometer (XRD) with Co Ka radiation, 35 kV voltage, and 30 mA current. For microstructure analysis, a Philips XL-30 scanning electron microscopy (SEM) was used. Thermal analysis was taken by differential scanning calorimetry (DSC) simultaneously carried out in DSC 2920 thermal analyzer from the American Company TA Instruments. The temperature range was measured between ambient temperature and 600 C with a heating rate of 10 C/min. The transmission electron microscope (TEM) was performed using a JEOL JEM-2100F microscope on a carbon-coated copper grid used to find the morphology including the size, shape and arrangement of the particles which make up the specimen as well as their relationship to each other on the scale of atomic diameters, the obtained sample’s crystallographic information such as the arrangement of atoms in the specimen and detection of atomic-scale defects in areas of a few nanometers in diameter.
2.4.
Fuel cell measurement
The fuel cell was measured under a variable resistance load, which adjusts the outputs of cell voltage and power. By
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international journal of hydrogen energy 35 (2010) 2684–2688
collecting data of the cell voltage and current under each resistance load, I (current)–V (voltage) or I–P (power) curve can be drawn from the collected data. These curves are called I–V or I–P characteristics. In our measurements, we used a computerized instrument (L-43) to complete the results handling processes.
3.
Results and discussion
The X-ray diffraction patterns of the SDC–Na2CO3 sintered at 800 C for 2 h were shown in Fig. 1. The peaks show that samarium atoms have doped into lattice of CeO2 crystals. However, there are no XRD reflections detected for carbonate [11]. It may suggest that the ceria–carbonates used in our electrolyte are two-phase materials where the carbonate is an amorphous phase. The XRD results may be concluded that there was neither chemical reaction nor new compound between the SDC and carbonate phases [11]. The carbonate component was amorphous and highly distributed among the SDC. The microstructure of sintered SDC–Na2CO3 was examined by scanning electron microscopy (SEM), and shown in Fig. 2. The image reveals that prepared SDC–Na2CO3 composites nanoparticle morphology is tetrahedron shaped in nano scale between 30 and 100 nm. The faces of the tetrahedron are parallel to the closed pack anion layers, i.e. {111} planes, and this is emphasized in a more extensive model of the structure to create superionic paths for ionic conduction (which is under study). The size and morphology of nanocomposite particles can be controlled by the preparation skills, sintering temperature and time as well. On the other hand, the conventional two-step prepared SDC–LiNaCO3 composites consist of much large particle size in micro meter level [12,13]. The SDC–Na2CO3 nanocomposite and conventional SDC– LiNaCO3 composite electrolytes have exhibited very different morphologies, especially, when we used the NANOCOFC approach to develop the materials. The crystallographic and more details of nanostructure of the SDC–Na2CO3 nanocomposites were investigated by transmission electron microscopy (TEM) as shown in Fig. 3.
0
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Fig. 2 – SEM images of SDC–Na2CO3.
The carbonate is coated on the SDC particle surface in a coreshell structure as observed clearly by TEM. The carbonate shell is an amorphous. This amorphous nature can facilitate ionic conduction. The details will be studied in the next step through theoretical and experimental calculations of the ionic conductivities for the SDC two-phase composites. Fig. 4, shows the results from DSC analysis. It can be seen from Fig. 4 that there is a glass transition between 150 C and 200 C for SDC but sodium carbonate does not show any thermal transition. The difference in temperature (DT ¼ T (sample) T (reference)) is observed negative, which indicates an endothermic. However, extrinsic effects due to the presence of grain boundaries or large amount of the nanoparticle surfaces can broaden the transitions, which complicate the analysis of the results. The amorphous nature may reflect increased disorder of the Na2CO3 regions/ layers on the SDC surfaces at rising temperature. Though it doesn’t involve any melting effect of the carbonate, the disorder of the carbonate (Na2CO3) can facilitate ionic transportation. Furthermore, softened amorphous Na2CO3
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shown in Fig. 4(a,b) for SDC–LiNaCO3 and SDC–Na2CO3, respectively. The open circuit voltage (OCV) is 0.98 V at 590 C for SDC–LiNaCO3 while 1.018 V at 500 C for SDC–Na2CO3 electrolytes. The improved ceria–carbonate electrolyte (one step) displays significantly higher OCV than that of the conventional one. The lower OCV in the conventional composite (two-step) electrolyte fuel cell may be due to less dense of the electrolyte which caused some gas penetration directly through the electrolyte to make electrical voltage losses. This confirms that the nanocomposite SDC–Na2CO3 electrolyte (one step) membrane is dense. We observed that the performance of fuel cell using the SDC–Na2CO3 nanocomposite electrolyte was enhanced significantly at lower temperatures, 300–500 C, even functioned well between 200 and 300 C than those using the conventional SDC–LiNaCO3 composite electrolytes. It may be explained due to an interfacial superionic conduction mechanism in the two-phase composites first reported by Schober [14], large amount of surfaces of nanoparticles ceria and interfaces in the SDCnanocomposites can significantly enhance and improve the ionic conductivity. The best performance of about 1100 mW/cm2 was reported by Huang et al. for the conventional SDC–LiNaCO3 composite electrolyte fuel cell at 600 C [12, 13]. While better fuel cell performance of 1150 mW/cm2 has been achieved at temperature 500 C in this work. The advantages of the nanocomposites are obvious. The excellent fuel cell performances achieved so far for the nanocomposite electrolytes show that ceria–carbonates nanocomposite electrolyte is appropriate for the low temperature (300–500 C, even 200–300 C) fuel cells. The stability of the fuel cell can be expected to improve due to only involving the solid carbonate and also at much lower temperatures in fuel cell operations. In the meantime it also requires developments of high catalytic electrodes we will report elsewhere.
Fig. 4 – DSC curve of SDC–Na2CO3 composite.
could overcome electronic conduction of CeO2 in anodic environment and enhance density of solid electrolyte same as other ceria–carbonate composites [7–13]. Conversely, the conventional SDC–LiNaCO3 composites with particle sizes at mm level showed much like carbonate melting effect with a sharp endothermic peak around 500 C of the LiNaCO3 melting point (510 C) [12]. The melting carbonate in the composites with a disordered or amorphous nature can thus significantly facilitate the ionic conductivity through the two-phase regions and greatly improve the fuel cell performances [6–14]. Fig. 5 shows the I–V characteristics for comparison of two fuel cells using the conventional SDC–LiNaCO3 composite and SDC–Na2CO3 nanocomposite as electrolytes. The maximum fuel cell power density reached about 1000 mW/ cm2 and 1150 mW/cm2 at temperatures 590 C and 500 C as
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Fig. 5 – I–V/I–P characteristics of fuel cells with different electrolytes: (a) SDC–LiNaCO3 and (b) SDC–Na2CO3 nanocomposite at various temperatures.
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international journal of hydrogen energy 35 (2010) 2684–2688
Conclusions
The ceria–carbonate nanocomposite has been developed with nanotechnology and composite synthesis method, i.e. the NANOCOFC approach. The improved nanocomposite electrolyte as-prepared shows a glass transition between 150 and 200 C which is significantly different from the conventional SDC–LiNaCO3 composites prepared in two steps. The SDC–Na2CO3 nanocomposite electrolyte fuel cells have achieved the excellent fuel cell performance of 1150 mW/ cm2 at 500 C and significantly improved performances at low temperatures. The fuel cell power density achieved so far is the highest. We first report to prepare the SDC–carbonate nanocomposites in a simple one-step process. It has provided a number of advantages: to simplify preparation processes for better quality control; more homogenous in nano-scale; improve significantly the ceria-carbonate conductivities and cause the superionic conduction at low temperatures. It may also be concluded that the nanostructured can be optimized by improving synthesis procedures. The improved ceria-carbonate nanocomposites may develop a new advancement and cost effective fuel cell technology for commercialization. Further studies will be continued on theoretical and experimental work to improve nanocomposites with different models and characteristics as well as high catalytic electrodes for lower temperatures.
Acknowledgement This work is supported by the EC FP6 NANOCOFC project (Contract no. 032308), Swedish Research Council (VR)/the Swedish agency for international cooperation development (Sida), Carl Tryggers Stiftelse for Vetenskap Forskning (CTS) and VINNOVA (the Swedish Agency for Innovation Systems). The HEC and CIIT, Pakistan also acknowledge for PhD scholarship.
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