- Email: [email protected]
Ce0.8Sm0.2O2−δ composite
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Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2Ld composite Huiqing Hu a, Qizhao Lin a,*, Zhigang Zhu a, Xiangrong Liu b, Bin Zhu b,c,** a
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Jinzhai Road, Hefei 230026, China b Department of Energy Technology, Royal Institute of Technology, Stockholm S-100 44, Sweden c Hubei Collaborative Innovation Center for Advanced Materials, Faculty of Physics and Electronic Technology, Hubei University, Wuhan, Hubei 430062, China
article info
abstract
Article history:
A Large-size engineering single layer fuel cell (SLFC) consisting of a nano-structured
Received 19 February 2014
Li0.4Mg0.3Zn0.3O2d/Ce0.9Sm0.1O2d (LMZSDC) composite with an active area of 25 cm2
Received in revised form
(6 cm 6 cm 0.1 cm) is successfully fabricated. The SLFC is evaluated by testing the cell
22 April 2014
durability with a time-dependent degradation using an H2 fuel and an air oxidant at 600 C for
Accepted 25 April 2014
over 120 h. A maximum power of 12.8 W (512 mW cm2) is achieved at 600 C.In the initial
Available online xxx
operation stage around 50 h, the cell’s performance decreases from 12.8 to 11.2 W; however, after this point, the performance was consistently stable, and no significant degradation is
Keywords:
observed in the current density or the cell performance. The device performed excellently at
Single layer fuel cell (SLFC)
low temperatures with a delivered power output of more than 250 mW cm2 at a temperature
Co-doped ceria
as low as 400 C. By curve fitting the X-ray photoelectron spectroscopy (XPS) results, the ratio
XPS
of Ce3þ/(Ce3þþCe4þ) before and after the long-time operation is analyzed. The ratio increased
Degradation
from 28.2% to 31.4% in the electrolyte which indicates a reduction occurs in the beginning operation that causes an initial performance loss for the device power output and OCV. Electrochemical impedance analyses indicate that the LMZSDC had a high ionic transport, and the device had quick dynamic processes and, thus, a high fuel cell performance. The LMZSDC is a new type of ionic material that has been successfully applied to SLFCs. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As an electrochemical energy conversion device, solid oxide fuel cells (SOFCs) are advantageous in several ways; they have
a high conversion efficiency, minimal pollution and zero noise compared to traditional thermal power systems [1,2]. However, conventional SOFCs require a high operation temperature (800e1000 C), are expensive, and often have
* Corresponding author. ** Corresponding author. Department of Energy Technology, Royal Institute of Technology, Stockholm S-100 44, Sweden. Tel.: þ46 (0) 87907403; fax: þ46 (0) 8204161. E-mail addresses: [email protected] (Q. Lin), [email protected] (B. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2014.04.185 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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technological challenges; therefore, their commercialization has been limited thus far [3]. Single layer fuel cell (SLFC) is a breakthrough innovation in the field of traditional SOFCs and garners attention as a promising new method for fuel cell R&D. Possessing a completely different structure, this novel device can function as traditional SOFCs but without using the electrolyte separator and anode/electrolyte/cathode complex cell construction. All the functions are integrated within one-layer, named the “Three in One” [4e8]. In conventional SOFCs, the electrolyte separator is critical for transporting ions and blocking electrons crossing over the device so as to avoid the short circuit. The electrolyte actually becomes a bottleneck for SOFC commercialization. Therefore, since this barrier is removed in the SLFC, the commercialization process is full of great potential. Instead, only one homogenous mixture layer of simple composite is used in the SLFC, which has two basic phases: the ionic conductor and the semiconductor phases, providing the percolating paths for ions and electrons, respectively [9e11]. The ionic conductors are usually Sm0.2Ce0.8O1.9 (SDC) and Ce0.9Gd0.1O1.9 (GDC). The semiconductors, which work as binary catalysts for both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction, are normally transition metal oxides, such as NiO, CuO, ZnO and FeOx [11]. SLFC’s schematic illustration has been presented in previous papers [4e6]. The working mechanism of SLFC is different from that of conventional SOFCs, which can be described via the following process [7]: Anode:
photoelectron spectroscopy (XPS), electrochemical impedance spectra (EIS) were carried out and the microstructure characteristics of the nano-structured composites were investigated. The variation in the cerium valence state of this SLFC before and after long-time operation was also examined from the large size engineering fuel cell, 6 cm 6 cm.
Experimental Synthesis of LMZSDC-LNO (Li0.4Mg0.3Zn0.3O/ Ce0.8Sm0.2O2d-Li0.4Ni0.6O2d)
1 H2 þ O2 /H2 O 2
The Li0.44Ni0.56O2 (LNO) was synthesized by a solid state reaction method. Stoichiometric amounts of Li2CO3 and NiCO3$2Ni(OH)2$4H2O in a molar ratio of Li:Ni ¼ 0.44:0.56 were mixed and then ground in a mortar. Next, this mixture was sintered at 800 C for 2 h and then completely ground in order to obtain LNO powder. The Li0.4Mg0.3Zn0.3O2d/Ce0.9Sm0.1O2d (LMZSDC) nanoparticles were obtained by using the stepwise coprecipitation method. A flowchart of this process is given in Fig. 1. Ce(NO3)3$6H2O and Sm(NO3)3$6H2O were first coprecipitated with Na2CO3 and K2CO3 with a molar ratio of (Ce3þþSm3þ):CO2þ 3 ¼ 1:2 to form a carbonate precursor, and LiNO3,Mg(NO3)2$6H2O and Zn(NO3)2$6H2O were coprecipitated with Na2CO3 and K2CO3 to form another carbonate precursor with a total molar ratio of (Liþ þ Mg2þ þ Zn2þ):CO2 3 ¼ 1:1. The Na2CO3 and K2CO3 used for co-precipitation was ina molar ratio of Na: K ¼ 5: 1. Then, these two precursors were mixed together in a molar ratio of Liþ:Mg2þ:Zn2þ:Ce3þ:Sm3þ ¼ 4:3:3:9:1, followed by a washing and drying. The fuel cells were constructed with the as-prepared LMZSDC-LNO using a weight ratio of LMZSDC:LNO ¼ 5.2:4.8. By cold-pressing shaping, the cell was sized to 6 cm 6 cm with a 0.1 cm thickness. Then, the cell was hot-pressed under 30 tons at 650 C for 120 min in order to form a dense and
Currently, SLFC development is still at an initial stage. The previous research has primarily focused on some basic materials explorations and exploration of fundamental issues, such as the cell performance, safety problems and the choosing of suitable starting materials [4e7]. Ceria-based oxides, especially the samarium doped ceria (SDC), are widely applied to SLFCs due to the high ionic conductivity at a low temperature. As one of the most important and critical issues for energy conversion devices, the performance degradation after longtime operation needs to be carefully studied. Additionally, this is strong evidence attesting to SLFC science and application. The direct detection of the cerium valence state conversion from Ce4þ to Ce3þ when using the SDC-based electrolyte and the variation of cerium valence state distribution can reflect the influence performance behavior for the conventional SOFCs [10,12]. Thus far, there has not yet been any reported research conducted on the time-dependent performance of SLFC nor any corresponding studies with XPS. In the present study, a new type of SLFC was fabricated with Li0.4Mg0.3Zn0.3O2d/Ce0.9Sm0.1O2d as the ionic conductor and Li0.44Ni0.56O2 as the semiconductor. Furthermore, the X-ray
Fig. 1 e Experimental flowchart for the synthesis of LMZSDC component produced by stepwise co-precipitation method.
H2 /2Hþ þ 2e Cathode: 1 O2 þ 2e /O2 2 Overall reaction:
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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Max-3A diffractometer with Cu-Ka radiation. The XRD spectra were recorded from 20 to 80 at a speed of 8 /min. Additionally, the samples’ morphologies were studied using a Zeiss Ultra 55 field emission scanning electron microscopy (FESEM). TEM images of the samples on a carbon-coated copper grid were taken using the JEM-2010 electron microscope that was operated at a 200 kV accelerating voltage. The cell’s electrical conductivity was measured in the air by an A.C. impedance spectroscopy method in a temperature range of 300e650 C using an electrochemical work station (IM6, ZAHNER, Germany). The measurement was performed in a frequency range of 1 Hze100 kHz with a 5 mV bias voltage. Xray electron spectroscopy (XPS) was performed on a VG ESCALAB MKII electron spectrometer with Al Ka as the x-ray source. The electrochemical performance of the fuel cells was measured at 400e600 C by a computerized instrument, in which hydrogen and air were used as the fuel and oxidant, respectively. The gas flow rate was in the range of 1000 ml/min at 1 atm pressure.
Results and discussion Phase analysis
Fig. 2 e (a) The XRD patterns of the as-prepared LMZSDC powder and (b) LMZSDC-LNO sample before and after long time operation. mechanically strong plate with an active area of 25 cm2. The copper gratings were attached as the current collector on the electrodes’ external surfaces.
Characterization and measurement of the LMZSDC-LNO oxides The crystal structure of the as-prepared composite material was analyzed by an X-ray diffraction (XRD) using a Rigaku-D/
The XRD pattern (Fig. 2a) of the as-prepared LMZSDC powder indicate the standard cubic fluorite structure of CeO2 (JCPDS 34-0394) with some second phases belonging to MgO (JCPDS 45-0946) and ZnO (JCPDS 74-0534). Furthermore, the asprepared sample is in a composite status. The half width of the peak in the XRD pattern was evidently broadened, which is also indicative of the nano-crystalline nature of the sample prepared by the co-precipitation technique. This is in accordance with the result reported by Yoon that the solubility limit of MgO in GDC was found to be approximately 0.1 mol%, and some of the MgO existed as a second phase [13,14]. There was no LiO pattern observed due to doping, which is the same as Sm3þ and part of Mg2þ and Zn2þ into ceria. We calculated the particle size to be in 20e30 nm range using the Scherrer equation. Fig. 2b exhibits the XRD patterns of the LMZSDCLNO sample before and after the long-time operation. As is clearly demonstrated, the two phases of LNO and MZSDC existed before and after the test, and no other phase was found after 120 h operation. It is evident that there was no obvious interaction between the two materials. This is similar
Fig. 3 e HR-TEM images of the individual (a) LNO and (b) LMZSDC powders and (c) the SEM picture of the cell after 120 h operation. Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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AC impedance
Fig. 4 e Impedance spectra of the cell which carried out at different temperatures and the illustration of an equivalent circuit.
to A. Mai’s results [15]in which there was a lack in the formation of additional phases after a long-time operation.
Microstructure/morphology Fig. 3 exhibits the high-resolution transmission electron microscope (HR-TEM) images of the composite samples. As shown in Fig. 3, LNO’s particle size was about 100 nm, and the LMZSDC was approximately10e25 nm, which corresponds with the result calculated from the XRD analysis. The SEM analysis of the cell’s cross-section indicated that all the constituent phase particles were homogenously distributed. Additionally, the particle sizes ranged from tens up to a few hundred nanometers, in accordance with the HR-TEM results. A porous structure with a porosity of ca. 30% for the device component was observed, which would facilitate the mass transfer of the reactants and products in the cell so as to deliver high power outputs.
Fig. 5 e Electrochemical performance of SLFC at different temperatures.
The complex AC impedance technique was used to evaluate the cell’s electrochemical performance. The cell’s electrochemical impedance spectroscopy was recorded under opencircuit at 300 C-600 C with a four-probe configuration. From Fig. 4, the spectrum exhibits a typical grain - grain boundary semicircle followed by a short tail. In general, the arcs at the high frequency impedance are ascribed to the charge transport process which would indicate that the reactions are fast kinetic processes in the fuel cell, whereas the tail in low frequency is related to the non-charge transport process, including the molecule diffusion, absorption, dissociation steps, etc. A simple model was used to simulate the Nyquist plots using ZSimpWin software for Fig. 4. In the equivalent circuit, R1 denotes the ohmic resistance that was contributed by both the oxygen ions and electrons. R2Q and R3Q denote the charge transfer and mass transfer process, respectively. According to the spectra, we may conclude a fast kinetic process in the cell, and the material once had a high catalytic activity for both H2 and O2. As the measurement temperature raised from 300 C to 600 C, the arcs in the sample’s spectra shrank. The electrical conductivity for the as-prepared sample is further calculated based upon the EIS results measured in air. The total conductivity st was determined using the following equation: st ¼ L/s Rt where L is the thickness, and s is the sample’s area. The electrical conductivity is gradually increased from 300 C to 600 C, and the total conductivity reached a maximum of 0.1 S cm1 at 600 C. The optimization of the ionic LMZSDC and electronic LNO weight ratio in the asprepared sample would produce suitable electrons and ions, and furthermore, corresponding transport and dynamic processes in the single cell would be enhanced because of the high, balanced electronic-ionic conductivity.
Cell performance The LMZSDC is a new kind of functional material for an SLFC. The function of Liþ in LMZSDC is to avoid the relatively poor densification behavior of the ceria-based solid solutions in the cell [16]. The introduction of Zn2þ and Mg2þ in LMZSDC alters the electronic states of these particles for an improved catalytic activity and improved grain boundary conductivity [17]. It has been reported that LNO is a semiconductor with a p-type conductivity and that the SDC is a typical oxygen ion conductor [4].We have carried out both EIS and fuel cell studies on the pure LMZSDC to prove its good ionic conducting property. Therefore, the LMZSDC-LNO material can satisfy the basic necessary requirement for a SLFC. Fig. 5 displays the SCFC’s electrochemical performance at various temperatures. As the temperature increased in a single cell, the OVC correspondingly increased. The cell preformed well, and a maximum power of 12.8 W (Pmax is 512 mW cm2) was achieved for a large-sized engineering cell (6 cm 6 cm 1 mm, 25 cm2 of active area) at 600 C.It should be mentioned that for a SLFC lacking an electrolyte layer, overcoming the electrolyte limit can thus be operated at very low temperature; even at 400 C, more than 250 mW cm2have still been achieved. This is a technically useful advancement with great commercialization potential.
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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Fig. 6 e The durability test result of the single cell under fixed current density at 600 C.
Cell durability The cell’s performance stability was studied at 600 C for over 120 h, and this is the first durability test for this new type of
5
device. The cell was operated for 8 h a day, cooled down to room temperature in atmosphere, and then this process was repeated for 15 days; i.e., the device has experienced 15 hotcold breakdown cycles. A comparison between the power output measured before and after the 15 cycles accumulated for 120 h is presented in Fig. 6. As is clearly demonstrated, the device had a relatively good long-term stability since the OCV and the power density of the cell experienced minimal degradation. Generally, the degradation is dominated by an increase of the cells’ polarization resistance after long-term operation [15]. In our results, no significant negative influence on the current density was observed. In an initial stage over a period of 50 h operation, the cell performance decreased from 12.8 W (512 mW cm2) to 11.2 W (450 mW cm2). Thereafter, the cell output reached a steady state with no further degradation from hour 50 to hour 120. This suggests that a proper ratio of the two materials and a proper fabrication process will optimize the cell’s performance, and the long-term stability of the cell can also be improved [16]. The OCV value only slightly degrades throughout the whole test process which could be attributed to the stability of the electronic conductivity of LMZSDC in the reduction atmospheres. During the durability test, the degradation in the initial stage for the cell’s power density and the voltage output can be partially attributed to the change of the cerium ion valence state in the LMZSDC ionic material [17]. In the nano-sized ceria, some Ce3þ ions were present in the grain boundary, which may have been formed by the reduction of Ce4þ with the electrons that were left behind when half an oxygen vacancy was created in the lattice [18e20]. Both cerium valence states co-existed, and the conversion from Ce4þ to Ce3þ occurred with the reaction of 4CeO2 / 2Ce2O3 þ O2 or Ce4þ þ e / Ce3þ. Therefore, the ratio of Ce3þ/(Ce3þ þ Ce4þ) increases under the reduction condition in the presence of H2. XPS is a powerful tool that is capable of identifying the states of the elements in bulk materials [21]. The XPS spectra for the device material before and after the durability test are shown in Fig. 7, in which the six red peaks were characterized as Ce4þ ions, and the four peaks in green were characterized as Ce3þ ions. Analyzed by curve-fitting, the values of the Ce3þ/ (Ce3þþCe4þ) ratio in the cell before and after long-term operation changed from 28.2% to 31.4%. These results reveal that the transformation of cerium valence state plays an important role in the cell performance and durability.
Conclusions
Fig. 7 e XPS curve fitting results for the device material before and after 120 h operation.
In this study, we have constructed SLFCs using a new type SLFC component consisting of ionic conductor of LMZSDC homogenously mixed with LNO as the single-component oxide layer. The LMZSDC was a composite based on codoping of part of Li/Mg/Zn and Sm into CeO2 in co-existence with some undoped MgO/ZnO. The LNO particle size is approximately 100 nm, and the LMZSDC is approximately10e25 nm. A maximum power of 12.8 W (512 mW cm2) has been achieved for a large-sized engineering cell (6 cm 6 cm 1 mm, 25 cm2 of active area) at 600 C. Furthermore, the time-dependent degradation was measured
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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for the cell at 600 C over 120 h with H2 fuel and air oxidant. No significant negative influence on the current density and OCV was observed. After an initial stage decrease, the cell performance maintains very stable, without any further degradation. Determined by XPS curve fitting, direct evidence of stabilization of the Ce3þ/(Ce3þþCe4þ) ratio before and after long-time operation was identified. The engineered SLFCs produced in this work performed well, are inexpensive, and have long-time stability. All these advantages demonstrate the great potential that a SLFC consisting of a nano-structured LMZSDC composite has for commercial application.
Acknowledgments This work was supported by National Key Basic Research Program (No. 2010CB227300) founded by MOST, National Natural Science Foundation of China (No. 51376171) and Swedish Research Council (VR, No. 621-2011-4 983), European Commission FP7 TriSOFC project (No. 303454).
references
[1] Maricle DL, Swarr TE, Karavolis S. Enhanced ceria-a lowtemperature SOFC electrolyte. Solid State Ionics 1992;52:173e82. [2] Anderson M, Yuan J, Sunde´n B. Review on modeling development for multiscale chemical reactions coupled transport phenomena in solid oxide fuel cells. Appl Energy 2010;87:1461e76. [3] Gorte RJ. Recent developments towards commercialization of solid oxide fuel cells. AIChE J 2005;51:2377e81. [4] Zhu B, Ma Y, Wang X, Raza R, Qin HY, Fan LD. A fuel cell with a single component functioning simultaneously as the electrodes and electrolyte. Electrochem Commun 2011;13:225e7. [5] Zhu B, Raza R, Abbas G, Singh M. An electrolyte-free fuel cell constructed from one homogenous layer with mixed conductivity. Adv Funct Mater 2011;21:2465e9. [6] Zhu B, Raza R, Qin HY, Fan LD. Single-component and threecomponent fuel cells. J Power Sources 2011;196:6362e5. [7] Zhu B, Raza R, Qin HY, Liu QH, Fan LD. Breakthrough fuel cell technology using ceria-based multi-functional nanocomposites. Appl Energy 2011;106:163e75.
[8] Fuel cells: three in one (research highlights). Nat Nanotechnol 2011;6:330. [9] Fan LD, Wang C, Osamudiamen O, Raza R, Singh M, Zhu B. Mixed ion and electron conductive composites for single component fuel cells: I. Effects of composition and pellet thickness. J Power Sources 2012;217:164e9. [10] Hu HQ, Lin QZ, Zhu ZG, Zhu B, Liu XR. Fabrication of electrolyte-free fuel cell with Mg0.4Zn0.6O/ Ce0.8Sm0.2O2deLi0.3Ni0.6Cu0.07Sr0.03O2d layer. J Power Sources 2014;248:577e81. [11] Zhu B, Qin HY, Raza R, Liu QH, Fan LD, Patakangas J. A singlecomponent fuel cell reactor. Int J Hydrog Energy 2011;36:8536e41. [12] Liu QH, Qin HY, Raza R, Fan LD, Li YD, Zhu B. Advanced electrolyte-free fuel cells based on functional nanocomposites of a single porous component: analysis, modeling and validation. RSC Adv 2012;2:8036e40. [13] Cho YH, Cho PS, Graeme A, Kim DK, Lee JH, Kim DY. Enhancement of grain-boundary conduction in gadoliniadoped ceria by the scavenging of highly resistive siliceous phase. Acta Mater 2007;55:4807e15. [14] Yamamura H, Takeda S, Kakinuma K. Relationship between oxide-ion conductivity and dielectric relaxation in Sm-doped CeO2. Solid State Ionics 2007;178:889e93. [15] Mai A, Becker M, Assenmacher W, Tietz F, Hathiramani D, Ivers-Tiffe´e E, et al. Time-dependent performance of mixedconducting SOFC cathodes. Solid State Ionics 2006;177:1965e8. [16] Lynch ME, Yang L, Qin W, Choi JJ, Liu M, Blinn K, et al. Enhancement of La0.6Sr0.4Co0.2Fe0.8O3d durability and surface electrocatalytic activity by La0.85Sr0.15MnO3d investigated using a new test electrode platform. Energy Environ Sci 2011;4:2249e58. [17] Zhu WZ, Deevi SC. Development of interconnect materials for solid oxide fuel cells. Mater Sci Eng A 2003;348:227e43. [18] Choudhury B, Choudhury A. Ce3þ and oxygen vacancy mediated tuning of structural and optical properties of CeO2 nanoparticles. Mater Chem Phys 2012;131:666e71. [19] Park P, Ledford J. Effect of crystallinity on the photoreduction of cerium oxide: a study of CeO2 and Ce/Al2O3 catalysts. Langmuir 1996;12:1794e9. [20] Lin TN, Lee MC, Yang RJ, Chang JC, Kao WX, Lee LS. Chemical state identification of Ce3þ/Ce4þ in the Sm0.2Ce0.8O2d electrolyte for an anode-supported solid oxide fuel cell after long-term operation. Mater Lett 2012;81:185e8. [21] Papavasiliou J, Avgouropoulos G, Ioannides T. Effect of dopants on the performance of CuOeCeO2 catalysts in methanol steam reforming. Appl Catal B Environ 2007;69:226e34.
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2Ld composite Huiqing Hu a, Qizhao Lin a,*, Zhigang Zhu a, Xiangrong Liu b, Bin Zhu b,c,** a
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Jinzhai Road, Hefei 230026, China b Department of Energy Technology, Royal Institute of Technology, Stockholm S-100 44, Sweden c Hubei Collaborative Innovation Center for Advanced Materials, Faculty of Physics and Electronic Technology, Hubei University, Wuhan, Hubei 430062, China
article info
abstract
Article history:
A Large-size engineering single layer fuel cell (SLFC) consisting of a nano-structured
Received 19 February 2014
Li0.4Mg0.3Zn0.3O2d/Ce0.9Sm0.1O2d (LMZSDC) composite with an active area of 25 cm2
Received in revised form
(6 cm 6 cm 0.1 cm) is successfully fabricated. The SLFC is evaluated by testing the cell
22 April 2014
durability with a time-dependent degradation using an H2 fuel and an air oxidant at 600 C for
Accepted 25 April 2014
over 120 h. A maximum power of 12.8 W (512 mW cm2) is achieved at 600 C.In the initial
Available online xxx
operation stage around 50 h, the cell’s performance decreases from 12.8 to 11.2 W; however, after this point, the performance was consistently stable, and no significant degradation is
Keywords:
observed in the current density or the cell performance. The device performed excellently at
Single layer fuel cell (SLFC)
low temperatures with a delivered power output of more than 250 mW cm2 at a temperature
Co-doped ceria
as low as 400 C. By curve fitting the X-ray photoelectron spectroscopy (XPS) results, the ratio
XPS
of Ce3þ/(Ce3þþCe4þ) before and after the long-time operation is analyzed. The ratio increased
Degradation
from 28.2% to 31.4% in the electrolyte which indicates a reduction occurs in the beginning operation that causes an initial performance loss for the device power output and OCV. Electrochemical impedance analyses indicate that the LMZSDC had a high ionic transport, and the device had quick dynamic processes and, thus, a high fuel cell performance. The LMZSDC is a new type of ionic material that has been successfully applied to SLFCs. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As an electrochemical energy conversion device, solid oxide fuel cells (SOFCs) are advantageous in several ways; they have
a high conversion efficiency, minimal pollution and zero noise compared to traditional thermal power systems [1,2]. However, conventional SOFCs require a high operation temperature (800e1000 C), are expensive, and often have
* Corresponding author. ** Corresponding author. Department of Energy Technology, Royal Institute of Technology, Stockholm S-100 44, Sweden. Tel.: þ46 (0) 87907403; fax: þ46 (0) 8204161. E-mail addresses: [email protected] (Q. Lin), [email protected] (B. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2014.04.185 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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technological challenges; therefore, their commercialization has been limited thus far [3]. Single layer fuel cell (SLFC) is a breakthrough innovation in the field of traditional SOFCs and garners attention as a promising new method for fuel cell R&D. Possessing a completely different structure, this novel device can function as traditional SOFCs but without using the electrolyte separator and anode/electrolyte/cathode complex cell construction. All the functions are integrated within one-layer, named the “Three in One” [4e8]. In conventional SOFCs, the electrolyte separator is critical for transporting ions and blocking electrons crossing over the device so as to avoid the short circuit. The electrolyte actually becomes a bottleneck for SOFC commercialization. Therefore, since this barrier is removed in the SLFC, the commercialization process is full of great potential. Instead, only one homogenous mixture layer of simple composite is used in the SLFC, which has two basic phases: the ionic conductor and the semiconductor phases, providing the percolating paths for ions and electrons, respectively [9e11]. The ionic conductors are usually Sm0.2Ce0.8O1.9 (SDC) and Ce0.9Gd0.1O1.9 (GDC). The semiconductors, which work as binary catalysts for both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction, are normally transition metal oxides, such as NiO, CuO, ZnO and FeOx [11]. SLFC’s schematic illustration has been presented in previous papers [4e6]. The working mechanism of SLFC is different from that of conventional SOFCs, which can be described via the following process [7]: Anode:
photoelectron spectroscopy (XPS), electrochemical impedance spectra (EIS) were carried out and the microstructure characteristics of the nano-structured composites were investigated. The variation in the cerium valence state of this SLFC before and after long-time operation was also examined from the large size engineering fuel cell, 6 cm 6 cm.
Experimental Synthesis of LMZSDC-LNO (Li0.4Mg0.3Zn0.3O/ Ce0.8Sm0.2O2d-Li0.4Ni0.6O2d)
1 H2 þ O2 /H2 O 2
The Li0.44Ni0.56O2 (LNO) was synthesized by a solid state reaction method. Stoichiometric amounts of Li2CO3 and NiCO3$2Ni(OH)2$4H2O in a molar ratio of Li:Ni ¼ 0.44:0.56 were mixed and then ground in a mortar. Next, this mixture was sintered at 800 C for 2 h and then completely ground in order to obtain LNO powder. The Li0.4Mg0.3Zn0.3O2d/Ce0.9Sm0.1O2d (LMZSDC) nanoparticles were obtained by using the stepwise coprecipitation method. A flowchart of this process is given in Fig. 1. Ce(NO3)3$6H2O and Sm(NO3)3$6H2O were first coprecipitated with Na2CO3 and K2CO3 with a molar ratio of (Ce3þþSm3þ):CO2þ 3 ¼ 1:2 to form a carbonate precursor, and LiNO3,Mg(NO3)2$6H2O and Zn(NO3)2$6H2O were coprecipitated with Na2CO3 and K2CO3 to form another carbonate precursor with a total molar ratio of (Liþ þ Mg2þ þ Zn2þ):CO2 3 ¼ 1:1. The Na2CO3 and K2CO3 used for co-precipitation was ina molar ratio of Na: K ¼ 5: 1. Then, these two precursors were mixed together in a molar ratio of Liþ:Mg2þ:Zn2þ:Ce3þ:Sm3þ ¼ 4:3:3:9:1, followed by a washing and drying. The fuel cells were constructed with the as-prepared LMZSDC-LNO using a weight ratio of LMZSDC:LNO ¼ 5.2:4.8. By cold-pressing shaping, the cell was sized to 6 cm 6 cm with a 0.1 cm thickness. Then, the cell was hot-pressed under 30 tons at 650 C for 120 min in order to form a dense and
Currently, SLFC development is still at an initial stage. The previous research has primarily focused on some basic materials explorations and exploration of fundamental issues, such as the cell performance, safety problems and the choosing of suitable starting materials [4e7]. Ceria-based oxides, especially the samarium doped ceria (SDC), are widely applied to SLFCs due to the high ionic conductivity at a low temperature. As one of the most important and critical issues for energy conversion devices, the performance degradation after longtime operation needs to be carefully studied. Additionally, this is strong evidence attesting to SLFC science and application. The direct detection of the cerium valence state conversion from Ce4þ to Ce3þ when using the SDC-based electrolyte and the variation of cerium valence state distribution can reflect the influence performance behavior for the conventional SOFCs [10,12]. Thus far, there has not yet been any reported research conducted on the time-dependent performance of SLFC nor any corresponding studies with XPS. In the present study, a new type of SLFC was fabricated with Li0.4Mg0.3Zn0.3O2d/Ce0.9Sm0.1O2d as the ionic conductor and Li0.44Ni0.56O2 as the semiconductor. Furthermore, the X-ray
Fig. 1 e Experimental flowchart for the synthesis of LMZSDC component produced by stepwise co-precipitation method.
H2 /2Hþ þ 2e Cathode: 1 O2 þ 2e /O2 2 Overall reaction:
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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Max-3A diffractometer with Cu-Ka radiation. The XRD spectra were recorded from 20 to 80 at a speed of 8 /min. Additionally, the samples’ morphologies were studied using a Zeiss Ultra 55 field emission scanning electron microscopy (FESEM). TEM images of the samples on a carbon-coated copper grid were taken using the JEM-2010 electron microscope that was operated at a 200 kV accelerating voltage. The cell’s electrical conductivity was measured in the air by an A.C. impedance spectroscopy method in a temperature range of 300e650 C using an electrochemical work station (IM6, ZAHNER, Germany). The measurement was performed in a frequency range of 1 Hze100 kHz with a 5 mV bias voltage. Xray electron spectroscopy (XPS) was performed on a VG ESCALAB MKII electron spectrometer with Al Ka as the x-ray source. The electrochemical performance of the fuel cells was measured at 400e600 C by a computerized instrument, in which hydrogen and air were used as the fuel and oxidant, respectively. The gas flow rate was in the range of 1000 ml/min at 1 atm pressure.
Results and discussion Phase analysis
Fig. 2 e (a) The XRD patterns of the as-prepared LMZSDC powder and (b) LMZSDC-LNO sample before and after long time operation. mechanically strong plate with an active area of 25 cm2. The copper gratings were attached as the current collector on the electrodes’ external surfaces.
Characterization and measurement of the LMZSDC-LNO oxides The crystal structure of the as-prepared composite material was analyzed by an X-ray diffraction (XRD) using a Rigaku-D/
The XRD pattern (Fig. 2a) of the as-prepared LMZSDC powder indicate the standard cubic fluorite structure of CeO2 (JCPDS 34-0394) with some second phases belonging to MgO (JCPDS 45-0946) and ZnO (JCPDS 74-0534). Furthermore, the asprepared sample is in a composite status. The half width of the peak in the XRD pattern was evidently broadened, which is also indicative of the nano-crystalline nature of the sample prepared by the co-precipitation technique. This is in accordance with the result reported by Yoon that the solubility limit of MgO in GDC was found to be approximately 0.1 mol%, and some of the MgO existed as a second phase [13,14]. There was no LiO pattern observed due to doping, which is the same as Sm3þ and part of Mg2þ and Zn2þ into ceria. We calculated the particle size to be in 20e30 nm range using the Scherrer equation. Fig. 2b exhibits the XRD patterns of the LMZSDCLNO sample before and after the long-time operation. As is clearly demonstrated, the two phases of LNO and MZSDC existed before and after the test, and no other phase was found after 120 h operation. It is evident that there was no obvious interaction between the two materials. This is similar
Fig. 3 e HR-TEM images of the individual (a) LNO and (b) LMZSDC powders and (c) the SEM picture of the cell after 120 h operation. Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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AC impedance
Fig. 4 e Impedance spectra of the cell which carried out at different temperatures and the illustration of an equivalent circuit.
to A. Mai’s results [15]in which there was a lack in the formation of additional phases after a long-time operation.
Microstructure/morphology Fig. 3 exhibits the high-resolution transmission electron microscope (HR-TEM) images of the composite samples. As shown in Fig. 3, LNO’s particle size was about 100 nm, and the LMZSDC was approximately10e25 nm, which corresponds with the result calculated from the XRD analysis. The SEM analysis of the cell’s cross-section indicated that all the constituent phase particles were homogenously distributed. Additionally, the particle sizes ranged from tens up to a few hundred nanometers, in accordance with the HR-TEM results. A porous structure with a porosity of ca. 30% for the device component was observed, which would facilitate the mass transfer of the reactants and products in the cell so as to deliver high power outputs.
Fig. 5 e Electrochemical performance of SLFC at different temperatures.
The complex AC impedance technique was used to evaluate the cell’s electrochemical performance. The cell’s electrochemical impedance spectroscopy was recorded under opencircuit at 300 C-600 C with a four-probe configuration. From Fig. 4, the spectrum exhibits a typical grain - grain boundary semicircle followed by a short tail. In general, the arcs at the high frequency impedance are ascribed to the charge transport process which would indicate that the reactions are fast kinetic processes in the fuel cell, whereas the tail in low frequency is related to the non-charge transport process, including the molecule diffusion, absorption, dissociation steps, etc. A simple model was used to simulate the Nyquist plots using ZSimpWin software for Fig. 4. In the equivalent circuit, R1 denotes the ohmic resistance that was contributed by both the oxygen ions and electrons. R2Q and R3Q denote the charge transfer and mass transfer process, respectively. According to the spectra, we may conclude a fast kinetic process in the cell, and the material once had a high catalytic activity for both H2 and O2. As the measurement temperature raised from 300 C to 600 C, the arcs in the sample’s spectra shrank. The electrical conductivity for the as-prepared sample is further calculated based upon the EIS results measured in air. The total conductivity st was determined using the following equation: st ¼ L/s Rt where L is the thickness, and s is the sample’s area. The electrical conductivity is gradually increased from 300 C to 600 C, and the total conductivity reached a maximum of 0.1 S cm1 at 600 C. The optimization of the ionic LMZSDC and electronic LNO weight ratio in the asprepared sample would produce suitable electrons and ions, and furthermore, corresponding transport and dynamic processes in the single cell would be enhanced because of the high, balanced electronic-ionic conductivity.
Cell performance The LMZSDC is a new kind of functional material for an SLFC. The function of Liþ in LMZSDC is to avoid the relatively poor densification behavior of the ceria-based solid solutions in the cell [16]. The introduction of Zn2þ and Mg2þ in LMZSDC alters the electronic states of these particles for an improved catalytic activity and improved grain boundary conductivity [17]. It has been reported that LNO is a semiconductor with a p-type conductivity and that the SDC is a typical oxygen ion conductor [4].We have carried out both EIS and fuel cell studies on the pure LMZSDC to prove its good ionic conducting property. Therefore, the LMZSDC-LNO material can satisfy the basic necessary requirement for a SLFC. Fig. 5 displays the SCFC’s electrochemical performance at various temperatures. As the temperature increased in a single cell, the OVC correspondingly increased. The cell preformed well, and a maximum power of 12.8 W (Pmax is 512 mW cm2) was achieved for a large-sized engineering cell (6 cm 6 cm 1 mm, 25 cm2 of active area) at 600 C.It should be mentioned that for a SLFC lacking an electrolyte layer, overcoming the electrolyte limit can thus be operated at very low temperature; even at 400 C, more than 250 mW cm2have still been achieved. This is a technically useful advancement with great commercialization potential.
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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Fig. 6 e The durability test result of the single cell under fixed current density at 600 C.
Cell durability The cell’s performance stability was studied at 600 C for over 120 h, and this is the first durability test for this new type of
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device. The cell was operated for 8 h a day, cooled down to room temperature in atmosphere, and then this process was repeated for 15 days; i.e., the device has experienced 15 hotcold breakdown cycles. A comparison between the power output measured before and after the 15 cycles accumulated for 120 h is presented in Fig. 6. As is clearly demonstrated, the device had a relatively good long-term stability since the OCV and the power density of the cell experienced minimal degradation. Generally, the degradation is dominated by an increase of the cells’ polarization resistance after long-term operation [15]. In our results, no significant negative influence on the current density was observed. In an initial stage over a period of 50 h operation, the cell performance decreased from 12.8 W (512 mW cm2) to 11.2 W (450 mW cm2). Thereafter, the cell output reached a steady state with no further degradation from hour 50 to hour 120. This suggests that a proper ratio of the two materials and a proper fabrication process will optimize the cell’s performance, and the long-term stability of the cell can also be improved [16]. The OCV value only slightly degrades throughout the whole test process which could be attributed to the stability of the electronic conductivity of LMZSDC in the reduction atmospheres. During the durability test, the degradation in the initial stage for the cell’s power density and the voltage output can be partially attributed to the change of the cerium ion valence state in the LMZSDC ionic material [17]. In the nano-sized ceria, some Ce3þ ions were present in the grain boundary, which may have been formed by the reduction of Ce4þ with the electrons that were left behind when half an oxygen vacancy was created in the lattice [18e20]. Both cerium valence states co-existed, and the conversion from Ce4þ to Ce3þ occurred with the reaction of 4CeO2 / 2Ce2O3 þ O2 or Ce4þ þ e / Ce3þ. Therefore, the ratio of Ce3þ/(Ce3þ þ Ce4þ) increases under the reduction condition in the presence of H2. XPS is a powerful tool that is capable of identifying the states of the elements in bulk materials [21]. The XPS spectra for the device material before and after the durability test are shown in Fig. 7, in which the six red peaks were characterized as Ce4þ ions, and the four peaks in green were characterized as Ce3þ ions. Analyzed by curve-fitting, the values of the Ce3þ/ (Ce3þþCe4þ) ratio in the cell before and after long-term operation changed from 28.2% to 31.4%. These results reveal that the transformation of cerium valence state plays an important role in the cell performance and durability.
Conclusions
Fig. 7 e XPS curve fitting results for the device material before and after 120 h operation.
In this study, we have constructed SLFCs using a new type SLFC component consisting of ionic conductor of LMZSDC homogenously mixed with LNO as the single-component oxide layer. The LMZSDC was a composite based on codoping of part of Li/Mg/Zn and Sm into CeO2 in co-existence with some undoped MgO/ZnO. The LNO particle size is approximately 100 nm, and the LMZSDC is approximately10e25 nm. A maximum power of 12.8 W (512 mW cm2) has been achieved for a large-sized engineering cell (6 cm 6 cm 1 mm, 25 cm2 of active area) at 600 C. Furthermore, the time-dependent degradation was measured
Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185
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for the cell at 600 C over 120 h with H2 fuel and air oxidant. No significant negative influence on the current density and OCV was observed. After an initial stage decrease, the cell performance maintains very stable, without any further degradation. Determined by XPS curve fitting, direct evidence of stabilization of the Ce3þ/(Ce3þþCe4þ) ratio before and after long-time operation was identified. The engineered SLFCs produced in this work performed well, are inexpensive, and have long-time stability. All these advantages demonstrate the great potential that a SLFC consisting of a nano-structured LMZSDC composite has for commercial application.
Acknowledgments This work was supported by National Key Basic Research Program (No. 2010CB227300) founded by MOST, National Natural Science Foundation of China (No. 51376171) and Swedish Research Council (VR, No. 621-2011-4 983), European Commission FP7 TriSOFC project (No. 303454).
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Please cite this article in press as: Hu H, et al., Time-dependent performance change of single layer fuel cell with Li0.4Mg0.3Zn0.3O/Ce0.8Sm0.2O2d composite, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.185