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Preparation and characterization of Sm0.2Ce0.8O1.9/Na2CO3 nanocomposite electrolyte for low-temperature solid oxide fuel cells Zhan Gao a, Rizwan Raza b, Bin Zhu b, Zongqiang Mao a,*, Cheng Wang a, Zhixiang Liu a a b
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China Department of Energy Technology, Royal Institute of Technology (KTH), S-10044, Stockholm, Sweden
article info
abstract
Article history:
Sm0.2Ce0.8O1.9 (SDC)/Na2CO3 nanocomposite synthesized by the co-precipitation process
Received 16 May 2010
has been investigated for the potential electrolyte application in low-temperature solid
Received in revised form
oxide fuel cells (SOFCs). The conduction mechanism of the SDC/Na2CO3 nanocomposite
25 November 2010
has been studied. The performance of 20 mW cm2 at 490 C for fuel cell using Na2CO3 as
Accepted 14 December 2010
electrolyte has been obtained and the proton conduction mechanism has been proposed.
Available online 4 February 2011
This communication demonstrates the feasibility of direct utilization of methanol in low-
Keywords:
power density of 512 mW cm2 at 550 C for fuel cell fueled by methanol has been achieved.
Solid oxide fuel cells
Thermodynamical equilibrium composition for the mixture of steam/methanol has been
Nanocomposite
calculated, and no presence of C is predicted over the entire temperature range. The long-
Conductivity
term stability test of open circuit voltage (OCV) indicates the SDC/Na2CO3 nanocomposite
Proton conduction
electrolyte can keep stable and no visual carbon deposition has been observed over the
Sodium carbonate
anode surface.
temperature SOFCs with the SDC/Na2CO3 nanocomposite electrolyte. A fairly high peak
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) are environmentally friendly energy conversion technologies that convert chemical energy directly into electrical energy with high efficiency and excellent fuel flexibility. Nowadays, there is tremendous interest in reducing the operational temperature of SOFCs from around 1000 C to the low-temperature range of 300e600 C because it can significantly improve material compatibility, reduce the cost and prolong the lifetime. However, the conventional electrolyte including yttria-stabilized zirconia (YSZ) and doped ceria (DCO) cannot provide satisfied conductivity in the lowtemperature range. Development of new electrolyte with sufficiently high conductivity (of the order 101 S cm1) is of great importance for high performance low-temperature SOFCs.
During the last decade, ceria-based composite electrolyte consisting of the ceria phase and the salt phase, e.g., SDCeM2CO3 (M ¼ Li, Na), has been explored for low-temperature SOFCs and conspicuous achievement has been made [1e3]. Recently, novel nanocomposite electrolyte of Sm0.2Ce0.8O1.9 (SDC)/Na2CO3, which yields distinguished conductivity (101 S cm1 at 300 C) and excellent performance (800 mW cm2 at 550 C) has been successfully demonstrated [4]. However, the conduction mechanism, particularly in the conductivity enhancement needs to be further investigated. In this communication, the conduction mechanism of the SDC/Na2CO3 nanocomposite electrolyte has been illustrated. The feasibility of direct utilization of methanol in lowtemperature SOFCs based on SDC/Na2CO3 nanocomposite electrolyte has been successfully demonstrated.
* Corresponding author. Tel.: þ86 10 62780537; fax: þ86 10 62792648. E-mail address:
[email protected] (Z. Mao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.061
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2.
Experimental
The SDC/Na2CO3 nanocomposite was synthesized by a wetchemical co-precipitation process [5]. Briefly, stoichiometric Ce(NO3)3$6H2O (SigmaeAldrich) and Sm(NO3)3$6H2O (SigmaeAldrich) solution was precipitated by the diluted Na2CO3 solution according to the molar ratio of metal ion to carbonate ion in 1:2. The precipitation was collected by suction filtration and dried in the oven at 50 C for 12 h. SDC/Na2CO3 nanocomposite was obtained by sintering the dried precipitation at 700 C for 2 h and followed by grinding thoroughly. SDC powder was prepared by oxalate co-precipitation process [1]. SDC pellets with 10 mm in diameter and 1 mm in thickness were prepared by uniaxially pressing of SDC powder and sintering at 1350 C for 5 h. Conductivity in air was performed using a PerkinElmer 5210 frequency response analyzer combined with EG&G PAR potentiostat/galvanostat 263A. Impedance spectra were typically taken in the frequency range from 0.1 MHz to 100 mHz with applied ac voltage amplitude of 10 mV in the open-circuit state. For comparison, the microcomposite of SDC and Na2CO3 (SDCeNa2CO3) was prepared by the solid state reaction method. The SDC and Na2CO3 was thoroughly ground and then the resultant was sintered at 700 C for 2 h. The fuel cell was fabricated by the dry press process using the SDC/Na2CO3 nanocomposite, SDCeNa2CO3 microcomposite, pure Na2CO3 as electrolyte, respectively. Lithiated NiO (LiNiO2) was prepared by the solid state reaction method [1]. Both the composite cathode and composite anode was obtained by mixing the LiNiO2 (50 vol.%) and electrolyte
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(50 vol.%). The anode, electrolyte and cathode were uniaxially pressed into a pellet of 13 mm in diameter and 1 mm in thickness at a pressure of 300 MPa and then sintered at 600 C for 30 min. The thicknesses of anode, electrolyte and cathode were 0.6 mm, 0.2 mm and 0.1 mm respectively. Silver paste was used as current collector and sealant. The fuel cells with the effective working area of 0.64 cm2 were performed between 400 and 600 C. Hydrogen and air was used as the fuel and the oxidant, respectively. Both gas flow rates were controlled between 40 and 200 mL min1 under 1 atm pressure. For the direct utilization of methanol, methanol steam and air were used as the fuel and the oxidant, respectively. The liquid methanol was vaporized before being introduced into the anode compartment, and the steam flow rate was controlled to be about 50e300 mL min1. Air flow rate was about 300 mL min1 under 1 atm pressure. For the long-term stability test of open circuit voltage (OCV), the fuel cell was fabricated by a tape casting and hot press procedure with the effective working area of 2.14 cm2. The equivolume solution of water and methanol with mole ratio of 20/9 was introduced directly into the anode compartment using a syringe pump at a controlled flow rate of 0.1 mL min1. The phase structure was detected by the X-ray diffraction (XRD, D8 ADVANCE, Bruker AXS Corp. German). The morphology and microstructure were detected by the scanning electron microscope (SEM, JEOL, JSM6301F). The d.c. conductivity of the electrolyte including SDC/Na2CO3 nanocomposite, SDCeNa2CO3 microcomposite and Na2CO3 was obtained from fuel cell IeV characteristics subtracting the influence of the electrodes and electrode/electrolyte interfaces using hydrogen as fuel [6].
Fig. 1 e SEM images of the (a) Na2CO3, (b) SDC, (c) SDC/Na2CO3 nanocomposite and (d) SDCeNa2CO3 microcomposite.
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3.
Results and discussions
3.1.
Microstructure
Fig. 1 shows the SEM images of the different electrolytes. The Na2CO3 (Fig. 1a) exhibits irregular shape with a few microns and the SDC (Fig. 1b) appears to be rod-like particles. The particle sizes are smaller than 100 nm for the SDC/Na2CO3 nanocomposite (Fig. 1c) with uniform distribution. The carbonate covers on the SDC homogenously. The particle sizes of the SDCeNa2CO3 microcomposite electrolyte (Fig. 1d) are about a few microns. Compared with the SDCeNa2CO3 microcomposite electrolyte, the SDC/Na2CO3 nanocomposite consists of much more interfacial regions between the two constituent phases due to their smaller sizes and more homogenous distributions. They provide high conductivity pathway for ionic conduction, and have the capacity to increase the interfacial mobile ion concentration than that of the bulk [7]. The XRD patterns of the different electrolytes are presented in Fig. 2. Na2CO3 displays extremely complex phase structure whereas SDC powder presents only a cubic fluorite structure. The broadening of the Bragg peaks evidences the nanocrystalline nature of the SDC/Na2CO3 nanocomposite. Compared with the SDC/Na2CO3 nanocomposite, the peak intensity of the SDCeNa2CO3 microcomposite is evidently strengthened and the half height widths of the peaks reduce sharply. All the diffraction peaks for composite samples can be indexed well to the cubic fluorite phase of SDC. No Na2CO3 phase is detected, suggesting the Na2CO3 exists as amorphous state and disperses uniformly on the surface of SDC grains.
3.2. Characterization of fuel cell using Na2CO3 as electrolyte Fig. 3 illustrates typical IeV and IeP characteristics of the asfabricated fuel cell using Na2CO3 as electrolyte. The maximum power density of 20 mW cm2 is achieved corresponding to the current density of 51 mA cm2 at 490 C. It is assumed that the
Fig. 3 e IeV and IeP characteristics of the as-fabricated fuel cell based on Na2CO3 electrolyte using hydrogen and air as fuel and oxidant, respectively.
performance is attributable solely to proton conduction in the Na2CO3 electrolyte because neither Naþ nor CO2 can 3 contribute the steady current output in the H2/air fuel cell. ion accepts the proton and Under fuel cell operation, CO2 3 becomes the protonic carrier by forming the temporal or intermediate ion species, e.g., HCO 3 . It is similar to the HSO4 suggested for proton conduction in Li2SO4. Protons move or jump between two neighboring CO2 3 ions with similar barrier configuration according to the paddle wheel mechanism [8]. Conceivably, the conductivity of Na2CO3 cannot be contributed by the CO2 with only 300 ppm in the air. The Na2CO3 exists as solid state at 490 C, consequently, CO2 3 ions are not effectively mobile to produce the current output. Even if these CO2 molecules can be totally catalyzed to CO2 3 , it can only contribute fuel cell current at mA level according to Faraday law. The OCV of fuel cell seriously deteriorates when the operational temperature exceeds 482 C, e.g., the OCV at 490 C is only 0.64 V, whereas it is 1.0 V at 440 C and 0.8 V at 460 C. M.J. Harris [9,10] reported that Na2CO3 has a second-order ferroelastic phase transition at 755 K (482 C), involving a symmetry change from hexagonal to monoclinic. It undergoes a continuous loss of long-range order in its ferroelastic transition, known as lattice melting that may be responsible for the deterioration of OCV.
3.3.
Fig. 2 e XRD patterns of the (a) Na2CO3, (b) SDC, (c) SDC/ Na2CO3 nanocomposite and (d) SDCeNa2CO3 microcomposite.
Conductivity
Fig. 4 presents the variation of the conductivity with temperature for the different electrolytes. The conductivities follow the sequence of Na2CO3 < SDC < SDCeNa2CO3 microcomposite < SDC/Na2CO3 nanocomposite in the temperature range. The fuel cell IeV characteristics is utilized to determine the conductivity of specific ions in the multi-ion system, e.g., Hþ and O2 in H2/air fuel cell. Proton conduction contributes to the conductivity of Na2CO3, while oxygen ion conduction is responsible for that of SDC. The interface between SDC and Na2CO3 where the interfacial superionic conduction takes place facilitates the oxygen ion and proton transportation, resulting in the conductivity enhancement of
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 9 8 4 e3 9 8 8
Fig. 4 e Temperature dependence of conductivities for (a) Na2CO3 (from fuel cell IeV characteristics), (b) SDC (from a.c. impedance), (c) SDC-Na2CO3 microcomposite (from IeV characteristics), (d) SDC/Na2CO3 nanocomposite (from IeV characteristics).
the SDCeNa2CO3 microcomposite electrolyte [11]. Introducing interfaces is regarded as one effective strategy to dramatically enhance the ionic conductivity in solid electrolytes, leading to the redistribution of ions in the space-charge regions [7,11]. Sata et al. [12] prepared the defined heterolayered films composed of CaF2 and BaF2 by molecular-beam epitaxy. For interfacial spacing greater than 50 nm, the ionic conductivity, which is parallel to the interfaces, increases proportionally with interface density. As shown in Fig. 4, the conductivity of the SDC/Na2CO3 nanocomposite is about one to several times higher than that of the SDCeNa2CO3 microcomposite due to larger amount of interfaces distributed homogenously among nano-particles. These results agree well with the SEM observations.
Fig. 5 e IeV and IeP characteristics of fuel cell fueled by methanol using as-prepared SDC/Na2CO3 nanocomposite electrolyte.
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Fig. 6 e Thermodynamically calculated equilibrium composition for the mixture of steam/methanol with temperature.
3.4. Methanol fueled fuel cell using SDC/Na2CO3 nanocomposite as electrolyte Fig. 5 illustrates typical IeV and IeP characteristics of fuel cell based on the SDC/Na2CO3 nanocomposite electrolyte fueled by methanol. The maximum power density of 512 mW cm2 at 550 C is achieved, which is much higher than the previous reports [13e15]. It can be attributed to the SDC/Na2CO3 nanocomposite electrolyte with high conductivity and compatible electrode with highly electrocatalytic activity. It indicates SDC/Na2CO3 nanocomposite electrolyte possesses enormous potential for the direct utilization of methanol in low-temperature SOFCs. Fig. 6 presents the thermodynamically calculated equilibrium composition for the mixture of steam/methanol as a function of temperature. Provided that the anodic compartment is a perfectly mixed isothermal reactor with homogeneous temperature and pressure, equilibrium compositions at the anode of an SOFC can be calculated according to the Gibbs free energy minimization approach [16]. The species of water (H2O), hydrogen (H2), carbon monoxide (CO), methane (CH4)
Fig. 7 e long-term stability test of OCV for the fuel cell using equivolume solution of water and methanol as fuel.
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and carbon dioxide (CO2) are predicted at equilibrium in the temperature range. The species of water (H2O) and hydrogen (H2) are dominant between 400 C and 600 C. The mole fraction of H2O and H2 is 40.55% and 38.13% at 550 C. The presence of C is not predicted over the entire temperature range, indicating the introduction of water steam completely suppresses carbon deposition. Consequently, the steam/methanol ratio of 20/9 is regarded to be enough to avoid carbon formation. Fig. 7 shows the long-term stability test of OCV for the fuel cell fabricated by tape casting combined with hot press procedure using equivolume solution of water and methanol as fuel. The OCV decreased from 0.882 V to 0.847 V after performing for 213 min. Only slight decline can be observed, indicating the SDC/Na2CO3 nanocomposite electrolyte can keep stable. No carbon deposition was observed over the anode surface after visual inspection during the operation, which is consistent with thermodynamic calculations.
4.
Conclusions
The SDC/Na2CO3 nanocomposite electrolyte has been successfully prepared by the co-precipitation process. Compared with the SDCeNa2CO3 microcomposite, significant conductivity enhancement of the SDC/Na2CO3 nanocomposite electrolyte can be ascribed to the larger amount and more homogenous interfaces between the constituent SDC and Na2CO3 phases. The proton conduction in the Na2CO3 has been proposed by investigating on the fuel cell using Na2CO3 as electrolyte. The maximum power density reaches as high as 512 mW cm2 for the fuel cell based on the SDC/ Na2CO3 nanocomposite electrolyte when operating on methanol at 550 C. The SDC/Na2CO3 nanocomposite has great potential for application as electrolyte in high performance low-temperature SOFCs operating on methanol. Thermodynamical calculation for the equilibrium composition of steam/ methanol mixture indicates no presence of C is predicted over the entire temperature range. The long-term stability test of OCV shows the SDC/Na2CO3 nanocomposite electrolyte can keep stable and no visual carbon deposition has been observed over the anode surface.
Acknowledgment This work is financially supported by the Swedish Agency for Innovation Systems (VINNOVA), National Basic Research Program of China (973 Program, Grant No. 2007CB209705) and China Scholarship Council.
references
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