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Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell
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Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell M. Ajmal Khan a,b,*, Cheng Xu a, Zhenlun Song a, Rizwan Raza b, Muhammad Ashfiq Ahmad b, Ghazanfar Abbas b, Bin Zhu c a
CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b Department of Physics, COMSATS Institute of Information Technology, Lahore, 5400, Pakistan c Department of Energy Technology, KTH, Stockholm, 10044, Sweden
article info
abstract
Article history:
The present study is focused on ceria based mixed (ionic and electronic conductor) com-
Received 14 October 2017
posite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) oxide material was prepared by solid state
Received in revised form
reaction, which can be used as anode materials for solid oxide fuel cell. The effect of Ti and
27 December 2017
Al oxides were analyzed on the NiZn-SDC composite with respect to its conductivity and
Accepted 27 January 2018
catalytic activity in hydrogen atmosphere. The average crystallite size of the composite
Available online xxx
was found to be 40e100 nm by XRD and SEM. The DC conductivity was determined by 4probe technique. The electrochemical impedance spectrum (EIS) was also examined in
Keywords:
hydrogen atmosphere within a temperature range of 350e550 C. The maximum power
Nanocomposite anode
density 370 mW/cm2 was achieved at 650 C.
Nanostructure
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Non-symmetrical fuel cell LTSOFC NiZn-SDC composite
Introduction The world is conscious for discovering the new and alternative sources of energies, due to rapid reduction of fossil fuels such as coal, oil and natural gas [1]. Fuel cells are fast growing and environment friendly alternative source of energy [2]. The fuel cell consists of three layers; anode, electrolyte and
cathode [3]. Fuel cells are classified into several families on the basis of the electrolyte materials, e.g. Solid oxide fuel cells (SOFCs), Molten carbonate fuel cells (MCFCs), Phosphoric acid fuel cells (PAFCs), Alkaline fuel cells (AFCs), and Proton exchange membrane fuel cells (PEMFCs) [4e7]. The working/ operating temperature of each cell family is different. However, the solid oxide fuel cell (SOFC) has achieved more
* Corresponding author. CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail addresses: [email protected], [email protected], [email protected], [email protected] (M. Ajmal Khan). https://doi.org/10.1016/j.ijhydene.2018.01.166 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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attention as compared to others due to its versatile characteristics (its high efficiency, multiple fuels, low operating temperature and its durability) [8,9]. Ni/YSZ cremate anodes were showed good electronic conductivity and high electrochemical performance at 800e1000 C [10]. But, low temperature is a desired feature of SOFC technology that may lead to its commercialization with the advantages of its friendly ecosystem and cost reduction [11]. Many conventional materials, e.g. yttria stabilized zirconia (YSZ), gadolinia doped ceria (GDC) and Samaria doped ceria (SDC) [12e14] have been established, which may play their role in decreasing the operating temperature. Ceria is the main functional oxide material [15] which can be used for this intention. Its composite oxides have mixed (ionic/electronic) behavior in anodic environment due to its reduction of Ce4þ into Ce3þ during the sintering process [16]. Cell performance of these anode materials depends on its structure, mixed ionic/electronic behavior, compositions, fabrication technique and conditions [17e22]. Ni-SDC [23], CeCu-oxide [24] and LiNiFe-SDC [25] composite anodes has been exhibited the good mixed conductivity and electrochemical performance 760 mWcm2 at 550 O C in hydrogen atmosphere [25]. The formation of Al2CO3 and a cation-disordered “NiO-like” in composite NACL oxide at room temperature was enhanced ion mobility and caused to increase anode activity [26]. A new SCDC-NCAL functional semiconductor-ionic composite material is alternative of electrolyte for fuel cells with better performances 617 mW/cm2 at 550 C [27]. CeeAleZn alloy has been developed in which Ce utilized to solve the passivation problem and enhanced the anode efficiency and stability [28,29]. The existence of Ce-oxide in anodic environment caused the destruction of thin film, which helped the improvement of stability and performance of the anode [30e32]. Ti-oxide is also an attractive material due to its mechanical, thermal, electrical properties [33], excellent behavior under reduced atmosphere [34], and its addition to the B-site perovskite increases both their structural stability and catalytic activity [35]. It is estimated that the low percentage of Ti4þ in the Ti-oxide lattice is being substituted by Ce4þ due to a difference in their ionic radii (80 pm versus 61 pm for Ce4þ and Ti4þ, and both have the same coordination number 6) [36], which enhanced characteristics such as thermal, mechanical and electrical of composite anode materials. Zn-oxide is a structurally stable [37] n-type semiconductor, which can be used to improve the conductivity and stability, and to decrease the polarization losses in addition to promoting catalytic activation at the anode [38]. The present study is motivate to solve the current issue of high operating conditions for SOFC. We are used here modifying approach of materials being used in order to improve properties and/or reduce operating temperatures. Mostly, modifications in the anode aim to avoid some high operating issues concerning conventionally used Ni-based materials, such as carbon deposition and sulfur poisoning, besides enhancing catalytic activity. In this study, we are modified ZnNi-SDC anode by introducing the Ti and Al oxides and see their effect on its conductivity and catalytic activity in hydrogen atmosphere. Composite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) is a mixed (ionic and electronic) conductor and
used as anode. Composite material was further characterized by XRD, SEM, TGA, EIS and fuel cell performance.
Experimental procedure Preparation of nanocomposite oxide material Ce (NO3)3$6H2O (Aldrich, USA), Sm (NO3)3$6H2O (Aldrich, USA), NiCO3 (Aldrich, USA), Zn (NO3)2$6H2O (Aldrich, USA) were utilized as initiating raw chemicals for the synthesis of NiZn-SDC powder, by the dry method, which was sintered at 600 C for 4 h. Then Al2NO3 (Aldrich, USA) and TiO2 (Aldrich, USA) were added by solid state method into NiZn-SDC composite oxide. A homogenous mixture of these Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZN-SDC) powders was prepared by using a mortar and pestle arrangement for 1 h. The grinded mixture was heated in a furnace for 10 h for sintering at 850 C and then annealed in air environs. The appropriate ratios of 70 wt % ATZN-oxide and 30 wt % SDC electrolyte were mixed to develop the Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZN-SDC) composite oxide. Finally, the Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZNSDC) composite oxide was again homogenized by using a mortar and pestle arrangement for 2 h. The dry pressing technique was employed to make the button fuel cell pellets. The cell thickness was 1 mm, with composite anode layer (0.56 mm), the electrolyte layer (0.24 mm), and cathode layer (0.20 mm). Electrolyte (SDC) was pressed between layers of the composite anode Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) and the BSCF cathode with a hydraulic press under load of 280 kg cm2. A pellet of diameter 13 mm for single cell test with an active area of 0.64 cm2 was prepared. The pressed fuel cell pellets was sintered at 600 C for 40 min. The both exterior surfaces (anode and the cathode) of the pellets were coated with a silver paste for improving the electrical contacts. The fuel cell performance was measured by using the electronic load (PLZ664WA, Kikusui). Hydrogen was employed as a fuel on the anode side and air as oxidant on the cathode side. The open circuit voltage (OCV) and current data were recorded over the temperature range 500e700 C and IeV curves were discussed. From these results, the power density was computed and current versus power densities (IeP curves) were also sketched. The H2 gas flow rate was controlled from 110 ml min1 at 1 atm pressure. Thermo gravimetric analysis (TGA) was shepherded on a Mettler Toledo TGE/SDTA 851e (Greifensee, Switzerland), and composite oxide was heated from 25 C to 1000 C at the rate of 10 C min1 in a 70 mL alumina pan. A constant flow of nitrogen (50 ml min1) was maintained to provide an inert atmosphere during the pyrolysis.
Crystal structure and microstructure analysis The phase and crystal structure analysis were recorded by Powder X-ray diffraction (PAN-Alytical X'Pert Pro MPD, Netherlands) with Cu K radiation (35 kV voltage, and a 30 mA current at room temperature with a scanning rate of 0.005). SEM analysis was performed (Hitachi High-Tech, S3400 energy used between 5 ev and 15 kV) to observe the
Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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 x x x ( 2 0 1 8 ) 1 e8
morphology of composite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZNSDC) oxide.
Electrochemical analysis Electrochemical impedance spectrum (EIS) of composite oxide was investigated at different temperatures 400e700 C in hydrogen atmosphere. Auto Versa-STAT 2273 (Princeton Applied Research, U.S.A) was implemented to measure the data over 0.1 Hz to 1 M Hz frequency range by applying 10 mV signal. The Z-view software has taken up for the curve fitting with an equivalent circuit for simulating the results of experimental data.
Conductivity measurement In order to find the conductivity, a pellet of Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) composite oxide having 3 mm thickness and 13 mm diameter was fabricated and sintered for 1 h at 700 C. The electrical contacts were realized by coating silver past on both sides of the pellet. The DC conductivity was determined with four probe method, in hydrogen atmosphere, by utilizing KD 2531 Digital Micro-ohmmeter, China. The conductivity was calculated by equation; s ¼ L=RA
(1)
where s is conductivity, L is the thickness of the pellet, R is the resistance and A is the active area of the pellet.
Results and discussions Fig. 1 presents the crystallographic structure of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide, which was
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sintered for 10 h at 850 C. The indexed pattern depicts that the composite oxide has individual phases, which were examined in detail by MDI-Jade 5 software, but no peaks of Ni and Al oxides were observed. The ceria has a cubic fluorite structure {JCPDS Card No: 34-0394 with fm-3m (225) space group}, Zn-oxide possesses a hexagonal structure {JCPDS Card No: 76-0704 with P63mc (225) space group}, Ti-oxide has a hexagonal structure {JCPDS Card No: 77-2170 with C2/m (12) space group}. Composite oxide are chemically stable at high temperature and having average nano crystallite size (60e100 nm). These multiple phases indicate the composite nature of the material, which help in improving the conductivity and electrochemical performance of the cell as already reported in the literature [39,40]. The small amount of Nioxide, Al-oxide may be doped in the Zn-oxide and SDC oxide. It has been reported into the literature that the Al contents with Zn and ceria oxides may be used to solve the passivation problem and enhanced the anode efficiency and stability [26,27], which may cause to enhance the catalytically activity, triple phase boundary (TPB) and increase the rate adsorption of H2. It is predictable that the low percentage of Ti4þ in the Ti-oxide lattice is being substituted by Ce4þ due to a difference in their ionic radii (80 pm versus 61 pm for Ce4þ and Ti4þ, and both have the same coordination number 6) [33,34]. It is also reported into the literature that Ti-oxide may be caused to enhance the electrical properties [35,36] of the material, due to this reason the electrical conductivity of the composite material is also enhanced. Fig. 2 (aec) shows the SEM results of Al0.05Ni0.1Ti.05Zn0.80SDC (ANTZ-SDC) composite oxide that possess suitable porosity with particle size of 40e100 nm. These results reveal that the composite oxide forms clusters due to the nano particle agglomeration. The SDC electrolyte particles supply oxygen ions for conduction. The aim of combining electrolyte
Fig. 1 e XRD Pattern of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide. Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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Fig. 2 e Scanning Electron Microscopy (SEM) ImageAl0.05Ni0.1Ti0.05Zn0.80-SDC (ANTZ-SDC) composite oxide. (SDC) into the (ANTZ) oxide was to increase the ionic conductivity from the anode to the inner of the solid electrolytes and to abate the interfacial resistance crossover of the anode/electrolyte interface. The small particle size of the composite materials has caused to enhance the conductivity and electrochemical performance of the cell. The TGA curve of the Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide is presented in Fig. 3. Initially, the weight loss from room temperature to 200 C can be related to the dehydration and thermal decomposition of the dried raw chemicals [41]. The residual organic matter, possibly, burns between 290 and 350 C [42]. A gradual crystallization and evaporation of the nitrates occur between 350 C to 600 C. Beyond, these temperatures it crystallizes perfectly.
The DC conductivities of composite oxides ANTZ-SDC and NiZn-SDC were measured in hydrogen atmosphere. The results are presented in the form of an Arrhenius plot as shown in Fig. 4. It can be seen that the DC conductivities as recorded at 680 C were found to be 7.64 and 6.55 Scm1 in hydrogen atmosphere for ANTZ-SDC and NiZn-SDC, respectively. Since the electrical conductivity of composites oxide was to be found 7.64 Scm1 which is 70 times higher than SDC electrolyte (0.1 Scm1). Hence, it can be used as anode or cathode as suggested by Zhu [42]. Moreover, the conductivity of composite oxide was noticed to enhance with rising temperature. This trend of the
Fig. 3 e Thermo gravimetric analysis of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide.
Fig. 4 e Arrhenius plot DC conductivities of composite oxides in H2 atmosphere.
Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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composite material reveals that it behaves like a semiconductor [43,44]. The activation energy was calculated by linear fitting technique in a hydrogen atmosphere and was found to have a value of 0.54 eV. A very small value of the activation energy shows that the chemical reaction in the fuel cell starts rapidly with the high catalytic activity [45]. Electrochemical impedance spectrum (EIS) of the Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide was measured in the hydrogen atmosphere in a temperature range of 500e680 C in Fig. 5(a). For comparison, the EIS of whole cell were also illustrated under the operating condition in Fig. 5(b). Fig. 5(a) shows their spectrum in a typical Nyquist plot with an equivalent circuit as obtained by Z-view software. The highfrequency intercept on the real axis is symbolized as the ohmic resistance (Ro), in which includes the resistances of electrodes, lead wires [43]. The low frequency intercept relates to the total resistance of the electrode [46]. The difference between the high frequency and low frequency intercepts with the real axis embodies the total polarization resistance (Rp) of the material [47]. It is noted that the ohmic resistance (Ro) of
Fig. 5 e Electrochemical Impedance Sectra, a) Composite Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) oxide in H2 atmosphere, b) Cell, Al0.05Ni0.1Ti0.05Zn0.80-SDC (ANTZ-SDC) as anode SDC electrolyte and BSCF cathode under the operating conditions.
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cell and composite anode are decreased due to the increase temperature. The ohmic resistance (Ro) of the composite anode ANTZ-SDC corresponding to the high-frequency intercept of the impedance spectra with the real axis, is 0.47, 0.39, 0.32, 0.26 and 0.198 U cm2 at 500, 550, 600, 650, and 680 C, respectively. The polarization resistance (Rp) is 0.31, 0.26, 0.24, 0.20 and 0.14 U cm2 at 500, 550, 600, 650, and 680 C, respectively. Both Ro and Rp were significantly decreased with the increase of temperature. Due to a difference in ionic radii (80 pm versus 61 pm for Ce4þ and Ti4þ, and both have the same coordination number 6) [34e36], Ti-contents were substituted in ceria lattice and improved the composite properties and this effect can be enhanced into surface modification of the composite by sintering at different temperatures [33e36]. AleZn contents into ceria has been used to reduce the resistance problem which may cause to increase anode efficiency and stability [28,30]. Ti contents also improved the catalytic activity of ceria [46,47], which caused to improve the reaction activity at triple phase boundary [34e36]. Owing to the significant ionic and electronic conduction of TiO2, it is effective to be utilized as the mixed conductor [48]. The lower polarization resistance effect and improved catalytic activity of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZSDC) composite oxide were caused to enhance the electrochemical performance and conductivity. To see the effect of increasing temperature on the spectral shape of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite, the spectra was modeled by employing a Z-view software and equivalent circuit shown in the inset of Fig. 5(a). Firstly, the impedance spectra were fitted with two RQ constant phase elements. The best fitting result suggests that the spectra are the result of a complex interplay between several competing reactions (including the reactions of electro-catalytic activation), charge transfer, ohmic resistance and mass transfer [49]. Fig. 5(a) shows the proposed circuit model, in which an inductor (L) and a resistor (Ro) are connected with two RQ constant phase elements in series. The inductance (L) effect was created due to the stainless tube of the measurement device [50]. In an H2 atmosphere, Ro denotes ohmic resistance for protons, oxygen ions and electrons due to introducing protons from H2 [51]. While the resistor R1 is associated with charge transfer reactions at the anode, which respond sensitively to a disturbance in the high frequency range. The resistor R2 is related to mass transfer at the anode, responsive to a signal in the low frequency range. Constant phase element (Q) was formed due to interfaces between the anode and the electrolyte. R2 diminished with increasing temperature and resulted in a rise of conductivity of (ANTZ-SDC) composite oxide. The magnitudes of high frequency semi-circle arches decreased with increased temperature 500e680 C, which may be associated with the modified anode surface morphology. Fuel cells were formed in order to further study the role of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide in the working of the cell. Cells were prepared with Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZN-SDC) composite oxide as anode, SDC electrolyte, and BSCF cathode and their performances were measured in terms of power density and open circuit voltage and shown in Fig. 6. The fuel cell exhibited the maximum power density of 370 mW/cm2 at 650 C. Fig. 7 was perceived the degradation behavior in electrochemical performance in first 24 h, may be owing to the electro catalytic process of the
Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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analysis indicate that their particle size falls in the range of 40e100 nm. Both Ro and Rp were significantly decreased with the increase of temperature. The ohmic resistance having the values 0.47, 0.39, 0.32, 0.26 and 0.198 U cm2 at 500, 550, 600, 650, and 680 C, respectively. The polarization resistance having the values 0.31, 0.26, 0.24, 0.20 and 0.14 U cm2 at 500, 550, 600, 650, and 680 C, respectively. Arrhenius plots of DC conductivity in the hydrogen atmosphere were drawn for estimating the Activation energy (Ea) in the temperature range (500e700 C). The Activation energy (Ea) of DC conductivity was 0.54 eV in the hydrogen atmosphere. The maximum power density of 370 mW/cm2 has been achieved at 650 C.
Acknowledgments Fig. 6 e Fuel cell performance Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) as a node using SDC electrolyte and BSCF cathode.
This work has been completed with a generous funding of Higher Education Commission, Islamabad, Pakistan under Startup Research Grant Program (SRGP), No: 21-736/SRGP/ R&D/HEC/2016 and the Swedish Agency for Innovation Systems (VINNOVA). The GETT fuel cell international AB is also acknowledged for their support in completing the work. It is also acknowledge for Surface Protection Research Group, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (CAS), Zhejiang Province Ningbo 315201, China.
references
Fig. 7 e Short term stability of the cell with Al0.05Ni0.1Ti. 05Zn0.80-SDC (ANTZ- SDC) as anode SDC electrolyte and BSCF cathode.
surface and transformation in morphology at anode/electrolyte interface. The long term stability is not performed due to lack of facilities. But still for short term results are encouraging. In future, long term stability will be accomplished.
Conclusions The present study is focused on ceria based mixed (ionic and electronic conductor) composite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) oxide materials sintered at 850 C for 10 h, which can be used as anode materials for solid oxide fuel cell. The effect of Ti and Al oxides were analyzed on the NiZn-SDC composite with respect to its conductivity and catalytic activity in hydrogen atmosphere. The Al and Ti elements were doped into NiZn-SDC composite and their XRD and SEM
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sacrificial anode for marine applications. Corrosion Sci 2008;50(8):2232e8. Sina H, Emamy M, Saremi MA, Keyvani, Mahta M, Campbell J. The influence of Ti and Zr on electrochemical properties of aluminum sacrificial anodes. Mat Sci Eng A 2006;431:263e76. Singla M, Dwivedi DD, Singh L, Chawla V. Development of aluminium based silicon carbide particulate metal matrix composite. J Min Mat Char Eng 2009;8:455e67. Kholmanov IN, Barborini E, Vinati S, Piseri P, Podesta A, Ducati C, et al. The influence of the precursor clusters on the structural and morphological evolution of nanostructured TiO2 under thermal annealing. Nanotechnology 2003;14:1168e73. Fellipe SS, Teofilo MS. Novel materials for solid oxide fuel cell technologies: a literature review. Int J Hydrogen Energy 2017;42:26020e36. Neagu D, Irvine JTS. Structure and properties of La0.4Sr0.4TiO3 ceramics for use as anode materials in solid oxide fuel cells. Chem Mater 2010;22:5042e53. Mandal SK, Das AK, Nath TK. Temperature dependence of solubility limits of transition metals (Co, Mn, Fe, and Ni) in ZnO nanoparticles. Appl Phys Lett 2006;89:1441051e3. n A. Effect of Zn Rodrı´guez JC, Marchi AJ, Borgna A, Monzo content on catalytic activity and physicochemical properties of Ni-based catalysts for selective hydrogenation of acetylene. J Catal 1997;171:268e78. Lei Z, Rong L, Shanwen T. An intermediate temperature fuel cell based on composite electrolyte of carbonate and doped barium cerate with SrFe0.7Mn0.2Mo0.1O3d cathode. Int J Hydrogen Energy 2013;38:16546e51. Lei Z, Rong L, Arno K, Shanwen T. A stable intermediate temperature fuel cell based on doped-ceria carbonate composite electrolyte and perovskite cathode. Electrochem Commun 2011;13:582e5. Shuyan L, Zhe L, Xiqiang H, Bo W, Wenhui S. Electrical and thermal properties of (Ba0.5Sr0.5) 1xSmxCo0.8Fe0.2O3d perovskite oxides. Solid State Ionics 2007;178:417e22. Sikander U, Sufian S, Salam MA. A review of hydrotalcite based catalysts for hydrogen production systems. Int J Hydrogen Energy 2017;42:19851e68. Bin Z. Functional ceria-salt-composite materials for advanced ITSOFC applications. J Power Sources 2003;114:1e9. Hee SY, Seung WC, Dokyol L, Byong HK. Synthesis and characterization of Gd1-xSrxMnO3 cathode for solid oxide fuel cells. J Power Sources 2001;93:1e7. Liang dong F, Chengyang W, Ose O, Rizwan R, Manish S, Bin Z. Mixed ion and electron conductive composites for single component fuel cells:I. Effects of composition and pellet thickness. J Power Sources 2012;217:164e9. Ghazanfar A, Rizwan R, Ashfaq M, Ashraf CM, Ajmal Khan M, Imran A, et al. Electrochemical study of nanostructured electrode for low-temperature solid oxide fuel cell (LTSOFC). Int J Energy Res 2014;38:518e23. Chia KH, Chia FT, Chi SL. Effect of Ti substitution on the electrochemical properties of LaCr1xTixO3þ0.5x (x ¼ 0e0.15). J Sol Gel Sci Technol 2016;78(2):394e402. Wanqiang L, Qian D, Fei L, Jing L, Dayong J, Limin W. Effect of Ce on electrochemical properties of the TiVNi quasicrystal material as an anode for Ni/MH batteries. Int J Hydrogen Energy 2013;38:14810e5. n P, Gonza lez-Elipe A, Caballero P. Capel F, Moure C, Dura Titanium local environment and electrical conductivity of TiO2-doped stabilized tetragonal zirconia. J Mat Sci 2000;35:345e52. Young MP, Hoo JL, Hong YB, Jin SA, Haek YK. Effect of anode thickness on impedance response of anode-supported solid oxide fuel cells. Int J Hydrogen Energy 2012;37:4394e400.
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Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell M. Ajmal Khan a,b,*, Cheng Xu a, Zhenlun Song a, Rizwan Raza b, Muhammad Ashfiq Ahmad b, Ghazanfar Abbas b, Bin Zhu c a
CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b Department of Physics, COMSATS Institute of Information Technology, Lahore, 5400, Pakistan c Department of Energy Technology, KTH, Stockholm, 10044, Sweden
article info
abstract
Article history:
The present study is focused on ceria based mixed (ionic and electronic conductor) com-
Received 14 October 2017
posite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) oxide material was prepared by solid state
Received in revised form
reaction, which can be used as anode materials for solid oxide fuel cell. The effect of Ti and
27 December 2017
Al oxides were analyzed on the NiZn-SDC composite with respect to its conductivity and
Accepted 27 January 2018
catalytic activity in hydrogen atmosphere. The average crystallite size of the composite
Available online xxx
was found to be 40e100 nm by XRD and SEM. The DC conductivity was determined by 4probe technique. The electrochemical impedance spectrum (EIS) was also examined in
Keywords:
hydrogen atmosphere within a temperature range of 350e550 C. The maximum power
Nanocomposite anode
density 370 mW/cm2 was achieved at 650 C.
Nanostructure
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Non-symmetrical fuel cell LTSOFC NiZn-SDC composite
Introduction The world is conscious for discovering the new and alternative sources of energies, due to rapid reduction of fossil fuels such as coal, oil and natural gas [1]. Fuel cells are fast growing and environment friendly alternative source of energy [2]. The fuel cell consists of three layers; anode, electrolyte and
cathode [3]. Fuel cells are classified into several families on the basis of the electrolyte materials, e.g. Solid oxide fuel cells (SOFCs), Molten carbonate fuel cells (MCFCs), Phosphoric acid fuel cells (PAFCs), Alkaline fuel cells (AFCs), and Proton exchange membrane fuel cells (PEMFCs) [4e7]. The working/ operating temperature of each cell family is different. However, the solid oxide fuel cell (SOFC) has achieved more
* Corresponding author. CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail addresses: [email protected], [email protected], [email protected], [email protected] (M. Ajmal Khan). https://doi.org/10.1016/j.ijhydene.2018.01.166 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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attention as compared to others due to its versatile characteristics (its high efficiency, multiple fuels, low operating temperature and its durability) [8,9]. Ni/YSZ cremate anodes were showed good electronic conductivity and high electrochemical performance at 800e1000 C [10]. But, low temperature is a desired feature of SOFC technology that may lead to its commercialization with the advantages of its friendly ecosystem and cost reduction [11]. Many conventional materials, e.g. yttria stabilized zirconia (YSZ), gadolinia doped ceria (GDC) and Samaria doped ceria (SDC) [12e14] have been established, which may play their role in decreasing the operating temperature. Ceria is the main functional oxide material [15] which can be used for this intention. Its composite oxides have mixed (ionic/electronic) behavior in anodic environment due to its reduction of Ce4þ into Ce3þ during the sintering process [16]. Cell performance of these anode materials depends on its structure, mixed ionic/electronic behavior, compositions, fabrication technique and conditions [17e22]. Ni-SDC [23], CeCu-oxide [24] and LiNiFe-SDC [25] composite anodes has been exhibited the good mixed conductivity and electrochemical performance 760 mWcm2 at 550 O C in hydrogen atmosphere [25]. The formation of Al2CO3 and a cation-disordered “NiO-like” in composite NACL oxide at room temperature was enhanced ion mobility and caused to increase anode activity [26]. A new SCDC-NCAL functional semiconductor-ionic composite material is alternative of electrolyte for fuel cells with better performances 617 mW/cm2 at 550 C [27]. CeeAleZn alloy has been developed in which Ce utilized to solve the passivation problem and enhanced the anode efficiency and stability [28,29]. The existence of Ce-oxide in anodic environment caused the destruction of thin film, which helped the improvement of stability and performance of the anode [30e32]. Ti-oxide is also an attractive material due to its mechanical, thermal, electrical properties [33], excellent behavior under reduced atmosphere [34], and its addition to the B-site perovskite increases both their structural stability and catalytic activity [35]. It is estimated that the low percentage of Ti4þ in the Ti-oxide lattice is being substituted by Ce4þ due to a difference in their ionic radii (80 pm versus 61 pm for Ce4þ and Ti4þ, and both have the same coordination number 6) [36], which enhanced characteristics such as thermal, mechanical and electrical of composite anode materials. Zn-oxide is a structurally stable [37] n-type semiconductor, which can be used to improve the conductivity and stability, and to decrease the polarization losses in addition to promoting catalytic activation at the anode [38]. The present study is motivate to solve the current issue of high operating conditions for SOFC. We are used here modifying approach of materials being used in order to improve properties and/or reduce operating temperatures. Mostly, modifications in the anode aim to avoid some high operating issues concerning conventionally used Ni-based materials, such as carbon deposition and sulfur poisoning, besides enhancing catalytic activity. In this study, we are modified ZnNi-SDC anode by introducing the Ti and Al oxides and see their effect on its conductivity and catalytic activity in hydrogen atmosphere. Composite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) is a mixed (ionic and electronic) conductor and
used as anode. Composite material was further characterized by XRD, SEM, TGA, EIS and fuel cell performance.
Experimental procedure Preparation of nanocomposite oxide material Ce (NO3)3$6H2O (Aldrich, USA), Sm (NO3)3$6H2O (Aldrich, USA), NiCO3 (Aldrich, USA), Zn (NO3)2$6H2O (Aldrich, USA) were utilized as initiating raw chemicals for the synthesis of NiZn-SDC powder, by the dry method, which was sintered at 600 C for 4 h. Then Al2NO3 (Aldrich, USA) and TiO2 (Aldrich, USA) were added by solid state method into NiZn-SDC composite oxide. A homogenous mixture of these Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZN-SDC) powders was prepared by using a mortar and pestle arrangement for 1 h. The grinded mixture was heated in a furnace for 10 h for sintering at 850 C and then annealed in air environs. The appropriate ratios of 70 wt % ATZN-oxide and 30 wt % SDC electrolyte were mixed to develop the Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZN-SDC) composite oxide. Finally, the Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZNSDC) composite oxide was again homogenized by using a mortar and pestle arrangement for 2 h. The dry pressing technique was employed to make the button fuel cell pellets. The cell thickness was 1 mm, with composite anode layer (0.56 mm), the electrolyte layer (0.24 mm), and cathode layer (0.20 mm). Electrolyte (SDC) was pressed between layers of the composite anode Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) and the BSCF cathode with a hydraulic press under load of 280 kg cm2. A pellet of diameter 13 mm for single cell test with an active area of 0.64 cm2 was prepared. The pressed fuel cell pellets was sintered at 600 C for 40 min. The both exterior surfaces (anode and the cathode) of the pellets were coated with a silver paste for improving the electrical contacts. The fuel cell performance was measured by using the electronic load (PLZ664WA, Kikusui). Hydrogen was employed as a fuel on the anode side and air as oxidant on the cathode side. The open circuit voltage (OCV) and current data were recorded over the temperature range 500e700 C and IeV curves were discussed. From these results, the power density was computed and current versus power densities (IeP curves) were also sketched. The H2 gas flow rate was controlled from 110 ml min1 at 1 atm pressure. Thermo gravimetric analysis (TGA) was shepherded on a Mettler Toledo TGE/SDTA 851e (Greifensee, Switzerland), and composite oxide was heated from 25 C to 1000 C at the rate of 10 C min1 in a 70 mL alumina pan. A constant flow of nitrogen (50 ml min1) was maintained to provide an inert atmosphere during the pyrolysis.
Crystal structure and microstructure analysis The phase and crystal structure analysis were recorded by Powder X-ray diffraction (PAN-Alytical X'Pert Pro MPD, Netherlands) with Cu K radiation (35 kV voltage, and a 30 mA current at room temperature with a scanning rate of 0.005). SEM analysis was performed (Hitachi High-Tech, S3400 energy used between 5 ev and 15 kV) to observe the
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morphology of composite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZNSDC) oxide.
Electrochemical analysis Electrochemical impedance spectrum (EIS) of composite oxide was investigated at different temperatures 400e700 C in hydrogen atmosphere. Auto Versa-STAT 2273 (Princeton Applied Research, U.S.A) was implemented to measure the data over 0.1 Hz to 1 M Hz frequency range by applying 10 mV signal. The Z-view software has taken up for the curve fitting with an equivalent circuit for simulating the results of experimental data.
Conductivity measurement In order to find the conductivity, a pellet of Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) composite oxide having 3 mm thickness and 13 mm diameter was fabricated and sintered for 1 h at 700 C. The electrical contacts were realized by coating silver past on both sides of the pellet. The DC conductivity was determined with four probe method, in hydrogen atmosphere, by utilizing KD 2531 Digital Micro-ohmmeter, China. The conductivity was calculated by equation; s ¼ L=RA
(1)
where s is conductivity, L is the thickness of the pellet, R is the resistance and A is the active area of the pellet.
Results and discussions Fig. 1 presents the crystallographic structure of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide, which was
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sintered for 10 h at 850 C. The indexed pattern depicts that the composite oxide has individual phases, which were examined in detail by MDI-Jade 5 software, but no peaks of Ni and Al oxides were observed. The ceria has a cubic fluorite structure {JCPDS Card No: 34-0394 with fm-3m (225) space group}, Zn-oxide possesses a hexagonal structure {JCPDS Card No: 76-0704 with P63mc (225) space group}, Ti-oxide has a hexagonal structure {JCPDS Card No: 77-2170 with C2/m (12) space group}. Composite oxide are chemically stable at high temperature and having average nano crystallite size (60e100 nm). These multiple phases indicate the composite nature of the material, which help in improving the conductivity and electrochemical performance of the cell as already reported in the literature [39,40]. The small amount of Nioxide, Al-oxide may be doped in the Zn-oxide and SDC oxide. It has been reported into the literature that the Al contents with Zn and ceria oxides may be used to solve the passivation problem and enhanced the anode efficiency and stability [26,27], which may cause to enhance the catalytically activity, triple phase boundary (TPB) and increase the rate adsorption of H2. It is predictable that the low percentage of Ti4þ in the Ti-oxide lattice is being substituted by Ce4þ due to a difference in their ionic radii (80 pm versus 61 pm for Ce4þ and Ti4þ, and both have the same coordination number 6) [33,34]. It is also reported into the literature that Ti-oxide may be caused to enhance the electrical properties [35,36] of the material, due to this reason the electrical conductivity of the composite material is also enhanced. Fig. 2 (aec) shows the SEM results of Al0.05Ni0.1Ti.05Zn0.80SDC (ANTZ-SDC) composite oxide that possess suitable porosity with particle size of 40e100 nm. These results reveal that the composite oxide forms clusters due to the nano particle agglomeration. The SDC electrolyte particles supply oxygen ions for conduction. The aim of combining electrolyte
Fig. 1 e XRD Pattern of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide. Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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Fig. 2 e Scanning Electron Microscopy (SEM) ImageAl0.05Ni0.1Ti0.05Zn0.80-SDC (ANTZ-SDC) composite oxide. (SDC) into the (ANTZ) oxide was to increase the ionic conductivity from the anode to the inner of the solid electrolytes and to abate the interfacial resistance crossover of the anode/electrolyte interface. The small particle size of the composite materials has caused to enhance the conductivity and electrochemical performance of the cell. The TGA curve of the Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide is presented in Fig. 3. Initially, the weight loss from room temperature to 200 C can be related to the dehydration and thermal decomposition of the dried raw chemicals [41]. The residual organic matter, possibly, burns between 290 and 350 C [42]. A gradual crystallization and evaporation of the nitrates occur between 350 C to 600 C. Beyond, these temperatures it crystallizes perfectly.
The DC conductivities of composite oxides ANTZ-SDC and NiZn-SDC were measured in hydrogen atmosphere. The results are presented in the form of an Arrhenius plot as shown in Fig. 4. It can be seen that the DC conductivities as recorded at 680 C were found to be 7.64 and 6.55 Scm1 in hydrogen atmosphere for ANTZ-SDC and NiZn-SDC, respectively. Since the electrical conductivity of composites oxide was to be found 7.64 Scm1 which is 70 times higher than SDC electrolyte (0.1 Scm1). Hence, it can be used as anode or cathode as suggested by Zhu [42]. Moreover, the conductivity of composite oxide was noticed to enhance with rising temperature. This trend of the
Fig. 3 e Thermo gravimetric analysis of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide.
Fig. 4 e Arrhenius plot DC conductivities of composite oxides in H2 atmosphere.
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composite material reveals that it behaves like a semiconductor [43,44]. The activation energy was calculated by linear fitting technique in a hydrogen atmosphere and was found to have a value of 0.54 eV. A very small value of the activation energy shows that the chemical reaction in the fuel cell starts rapidly with the high catalytic activity [45]. Electrochemical impedance spectrum (EIS) of the Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide was measured in the hydrogen atmosphere in a temperature range of 500e680 C in Fig. 5(a). For comparison, the EIS of whole cell were also illustrated under the operating condition in Fig. 5(b). Fig. 5(a) shows their spectrum in a typical Nyquist plot with an equivalent circuit as obtained by Z-view software. The highfrequency intercept on the real axis is symbolized as the ohmic resistance (Ro), in which includes the resistances of electrodes, lead wires [43]. The low frequency intercept relates to the total resistance of the electrode [46]. The difference between the high frequency and low frequency intercepts with the real axis embodies the total polarization resistance (Rp) of the material [47]. It is noted that the ohmic resistance (Ro) of
Fig. 5 e Electrochemical Impedance Sectra, a) Composite Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) oxide in H2 atmosphere, b) Cell, Al0.05Ni0.1Ti0.05Zn0.80-SDC (ANTZ-SDC) as anode SDC electrolyte and BSCF cathode under the operating conditions.
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cell and composite anode are decreased due to the increase temperature. The ohmic resistance (Ro) of the composite anode ANTZ-SDC corresponding to the high-frequency intercept of the impedance spectra with the real axis, is 0.47, 0.39, 0.32, 0.26 and 0.198 U cm2 at 500, 550, 600, 650, and 680 C, respectively. The polarization resistance (Rp) is 0.31, 0.26, 0.24, 0.20 and 0.14 U cm2 at 500, 550, 600, 650, and 680 C, respectively. Both Ro and Rp were significantly decreased with the increase of temperature. Due to a difference in ionic radii (80 pm versus 61 pm for Ce4þ and Ti4þ, and both have the same coordination number 6) [34e36], Ti-contents were substituted in ceria lattice and improved the composite properties and this effect can be enhanced into surface modification of the composite by sintering at different temperatures [33e36]. AleZn contents into ceria has been used to reduce the resistance problem which may cause to increase anode efficiency and stability [28,30]. Ti contents also improved the catalytic activity of ceria [46,47], which caused to improve the reaction activity at triple phase boundary [34e36]. Owing to the significant ionic and electronic conduction of TiO2, it is effective to be utilized as the mixed conductor [48]. The lower polarization resistance effect and improved catalytic activity of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZSDC) composite oxide were caused to enhance the electrochemical performance and conductivity. To see the effect of increasing temperature on the spectral shape of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite, the spectra was modeled by employing a Z-view software and equivalent circuit shown in the inset of Fig. 5(a). Firstly, the impedance spectra were fitted with two RQ constant phase elements. The best fitting result suggests that the spectra are the result of a complex interplay between several competing reactions (including the reactions of electro-catalytic activation), charge transfer, ohmic resistance and mass transfer [49]. Fig. 5(a) shows the proposed circuit model, in which an inductor (L) and a resistor (Ro) are connected with two RQ constant phase elements in series. The inductance (L) effect was created due to the stainless tube of the measurement device [50]. In an H2 atmosphere, Ro denotes ohmic resistance for protons, oxygen ions and electrons due to introducing protons from H2 [51]. While the resistor R1 is associated with charge transfer reactions at the anode, which respond sensitively to a disturbance in the high frequency range. The resistor R2 is related to mass transfer at the anode, responsive to a signal in the low frequency range. Constant phase element (Q) was formed due to interfaces between the anode and the electrolyte. R2 diminished with increasing temperature and resulted in a rise of conductivity of (ANTZ-SDC) composite oxide. The magnitudes of high frequency semi-circle arches decreased with increased temperature 500e680 C, which may be associated with the modified anode surface morphology. Fuel cells were formed in order to further study the role of Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) composite oxide in the working of the cell. Cells were prepared with Al0.05Ni0.1Ti.05Zn0.80-SDC (ATZN-SDC) composite oxide as anode, SDC electrolyte, and BSCF cathode and their performances were measured in terms of power density and open circuit voltage and shown in Fig. 6. The fuel cell exhibited the maximum power density of 370 mW/cm2 at 650 C. Fig. 7 was perceived the degradation behavior in electrochemical performance in first 24 h, may be owing to the electro catalytic process of the
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analysis indicate that their particle size falls in the range of 40e100 nm. Both Ro and Rp were significantly decreased with the increase of temperature. The ohmic resistance having the values 0.47, 0.39, 0.32, 0.26 and 0.198 U cm2 at 500, 550, 600, 650, and 680 C, respectively. The polarization resistance having the values 0.31, 0.26, 0.24, 0.20 and 0.14 U cm2 at 500, 550, 600, 650, and 680 C, respectively. Arrhenius plots of DC conductivity in the hydrogen atmosphere were drawn for estimating the Activation energy (Ea) in the temperature range (500e700 C). The Activation energy (Ea) of DC conductivity was 0.54 eV in the hydrogen atmosphere. The maximum power density of 370 mW/cm2 has been achieved at 650 C.
Acknowledgments Fig. 6 e Fuel cell performance Al0.05Ni0.1Ti.05Zn0.80-SDC (ANTZ-SDC) as a node using SDC electrolyte and BSCF cathode.
This work has been completed with a generous funding of Higher Education Commission, Islamabad, Pakistan under Startup Research Grant Program (SRGP), No: 21-736/SRGP/ R&D/HEC/2016 and the Swedish Agency for Innovation Systems (VINNOVA). The GETT fuel cell international AB is also acknowledged for their support in completing the work. It is also acknowledge for Surface Protection Research Group, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (CAS), Zhejiang Province Ningbo 315201, China.
references
Fig. 7 e Short term stability of the cell with Al0.05Ni0.1Ti. 05Zn0.80-SDC (ANTZ- SDC) as anode SDC electrolyte and BSCF cathode.
surface and transformation in morphology at anode/electrolyte interface. The long term stability is not performed due to lack of facilities. But still for short term results are encouraging. In future, long term stability will be accomplished.
Conclusions The present study is focused on ceria based mixed (ionic and electronic conductor) composite Al0.05Ni0.1Ti0.05Zn0.80-SDC (ATZN-SDC) oxide materials sintered at 850 C for 10 h, which can be used as anode materials for solid oxide fuel cell. The effect of Ti and Al oxides were analyzed on the NiZn-SDC composite with respect to its conductivity and catalytic activity in hydrogen atmosphere. The Al and Ti elements were doped into NiZn-SDC composite and their XRD and SEM
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Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166
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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 x x x ( 2 0 1 8 ) 1 e8
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Please cite this article in press as: Ajmal Khan M, et al., Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.166