LiAlO<SUB>2</SUB>–LiNaCO<SUB>3</SUB> Composite Electrolyte for Solid Oxide Fuel Cells
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Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol. 11, 5402–5407, 2011
LiAlO2–LiNaCO3 Composite Electrolyte for Solid Oxide Fuel Cells Rizwan Raza1 2 ∗ , Zhan Gao1 , Tavpraneet Singh3 , Gajendra Singh4 , Song Li5 , and Bin Zhu1
RESEARCH ARTICLE
1
Department of Energy Technology, Royal Institute of Technology (KTH), 10044, Stockholm, Sweden 2 Department of Physics, COMSATS Institute of Information Technology, 54000, Lahore, Pakistan 3 Department of Chemical Engineering, Indian Institute of Technology, 110016, Delhi, India 4 DCET, Panjab University, 160 014, Chandigarh, India 5 Institute of Materials and Technology, Dalian Maritime University, 116026, Dalian, P. R. China
This paper reports a new approach to develop functional solid oxide fuel cells (SOFC) electrolytes Delivered by Ingentaapproaches to: based on nanotechnology and two-phase nanocomposite using non-oxygen ion or proton conductors, e.g., lithium aluminate-lithium sodium carbonate, with great freedom in material Hong Kong Polytechnic University design and development. BenefitedIP by :nanotechnology and nanocomposite technology, the lithium 158.132.127.134 aluminate-lithium sodium carbonate two-phase composite electrolytes can significantly enhance Fri, 20 May 2011 05:37:04 the material conductivity and fuel cell performance at low temperatures, such as 300 C – 600 C compared to non-nano scale materials. The conductivity mechanism and fuel cell functions are discussed to be benefited by the interfacial behavior between the two constituent phases in nano-scale effects, where oxygen ion and proton conductivity can be created, although there are no intrinsic mobile oxygen ions and protons. It presents a new scientific approach to design and develop fuel cell materials in breaking the structural limitations by using non-ionic conductors on the desired ions i.e., proton and oxygen ions, and creating high proton and oxygen ion conductors through interfaces and interfacial mechanism.
Keywords: Two-Phase, Low Temperature (300–600 C), Proton, Oxygen Ion Conduction. 1. INTRODUCTION Nanocomposite electrolytes (GDC-NaCO3 , SDC-NaCO3 etc.) have been recently investigated for performance in intermediate and low temperature (300 C–700 C) SOFCs, showing a new generation fuel cell technology.1–4 Much attention has been drawn to the interfacial behavior in two-phase composite electrolytes. In these new functional nanocomposite electrolytes, the ceria-carbonate twophase composite electrolyte systems have received special attention, showing a high conductivity of 10−1 Scm−1 at temperatures below 500 C, which can be compared to YSZ conductivity at 1000 C.5–8 In addition, both proton and oxygen ion transport have been reported,8–10 which is responsible for the enhanced material conductivity and electrode kinetics, and as a result, excellent fuel cell power output of 1.1 Wcm−2 at 600 C has been achieved.10 These features are significantly different from either GDC and SDC electrolytes and corresponding SOFCs, for which conductivity reaches 10−1 Scm−1 when temperature is kept above 800 C; or MCFCs, in which CO2− 3 transport is the ∗
Author to whom correspondence should be addressed.
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major process and can only function when the carbonate ions are in molten state and hence the fuel cell gives sufficient power only when the temperature is as high as 650 C. Recently, research has been carried out on nonceria-based two-phase composite materials, e.g., lithium aluminate-lithium sodium carbonate used as electrolytes for low temperature solid oxide or ceramic fuel cells, exhibiting a new approach to develop functional SOFC electrolytes based on non-oxygen ion or proton ion conductors with great freedom in material design and development.11 LiAlO2 and carbonate system is commonly used in MCFCs. LiAlO2 also has an advantage of being stable10 in the LiAlO2 –LiNaCO3 system as compared to SDC. However, research in this direction is in a nascent stage presently and needs much more investigation. Previous investigations were carried out using commercial LiAlO2 with particle size at m level, which can function only at temperatures above 600 C, usually at 650 C, to achieve good fuel cell performance,11 when the carbonate was in a completely molten state. It could leave some argument or confusion on the role of molten carbonate ions and also, in 1533-4880/2011/11/5402/006
doi:10.1166/jnn.2011.3784
Raza et al.
LiAlO2 –LiNaCO3 Composite Electrolyte for Solid Oxide Fuel Cells
A powder containing nanoparticles of -lithium aluminate (LiAlO2 ) was prepared by sol–gel method.15 Stoichiometric amounts (here a molar ratio of 1:1) of lithium nitrate (LiNO3 ) and aluminium nitrate (Al(NO3 )3 · 9H2 O) were taken and dissolved separately in de-ionized water. They were then added slowly, while stirring, to aqueous citric acid monohydrate (C6 H8 O7 · H2 O). The molar ratio of metal ions to citric acid was chosen to be 1:1.5. The solution was stirred on a hot plate at 80 C for 4 hours to form a gel. The gel was then dried overnight in an oven at 120 C. The dried gel was sintered in air at 900 C for 2 hours and then ground finely to obtain nano-LiAlO2 powder. Nanocomposite of LiAlO2 and carbonate was prepared in a much similar approach as preparing the ceria-carbonate composites.1–4 A lithium sodium carbonate (LiNaCO3 ) salt mixture was prepared by mixing thoroughly the two salts, lithium carbonate (Li2 CO3 ) and sodium carbonate (Na2 CO3 ), their molar ratio being 2:1 respectively. LiAlO2 and LiNaCO3 were mixed together in different proportions to get different composites (10%, 15%, 20%, 25%, 30% and 40%). Each mixture was ground thoroughly and sintered in air at 680 C for 1 hour. It was grounded again in order to obtain fine powder as electrolyte for fuel cell. J. Nanosci. Nanotechnol. 11, 5402–5407, 2011
3. RESULTS AND DISCUSSION It has been reported11 that the LiAlO2 synthesized by the procedure stated in 2.1 is in nano-scale. We calculated the particle size S(BET) with Brunauer-Emmit-Teller (BET) method and compare with the SEM results and observed the good agreement of the sizes of the composite electrolytes as shown in Table I. Figure 1 shows the morphology of the SEM micrographs of LiAlO2 –LiNaCO3 composite electrolyte. SEM results show that the homogenous nanocomposite structure and local melting areas are the domains in the LiAlO2 –LiNaCO3 to construct the ionic conducting paths. Homogenous and highly distributed two phase regions can promote the fast ionic conduction. It can be seen from Figure 2 that the maximum OCV for the 10%, 15%, 20%, 25%, 30%, and 40% carbonate in composite electrolyte are 1.07 V, 1.03 V, 1.02 V, 1.0 V, Table I.
Particle size calculation for 10% carbonate contaminent.
Electrolyte
S(BET)
S(SEM)
(Composition 10%) LiAlO2 –LiNaCO3
50 nm
60 nm
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molten state, the salt corrosion problem remained an issue. Field-emission scanning electron microscope This investigation looks further into previously investi(FESEM—A Zeiss Ultra 55) was used to characterize gated commercial non-nano-LiAlO2 particles. In this work the morphology, microstructure and particle size of comnanoparticles of LiAlO2 were used and the correspondposite electrolyte. The particle size was calculated using ing fuel cell studied in a range of 450 C–600 C. The Brunauer-Emmit-Teller (BET). results obtained below 500 C arouse special significance because of the melting point of LiNaCO3 being 510 C, 2.2. Fuel Cell Fabrication and also compared to an operating temperature of 650 C in Electrochemical Performance the previously investigated commercial LiAlO2 –LiNaCO3 system.11 This work further proves that the non-oxygen The anode supported fuel cell was fabricated using a uniand proton conducting materials can also be developed as axial die-press.1–4 Pure electrode, a mixture of nickel oxide ceramic fuel cell electrolytes and function with high perand copper oxide, mixed with the composite electrolyte formance. By using nanotechnology and two-phase mate(1:1.5 by weight) was used for both the anode and the rial architecture we can construct nanocomposites with cathode. The pure electrode layer on the anode side and advanced material properties and fuel cell performances silver paste on the cathode side acted as the current colsuperior to those with non-nano materials and convenlectors. For a single fuel cell pellet, 0.2 g of the mixtional single-phase materials, e.g., yttrium stabilized zircoture of electrode and the composite was used. 0.25–3 g of nia (YSZ). More importantly, our materials developed by the composite was used as the electrolyte. The pellet was using new approaches can be used to make ceramic fuel by Ingenta obtained by Delivered to:pressing these layers at 200 MPa in one step. cells with sufficient functions and having lowKong operating The circular fuel cell pellet made was 13 mm in diameter Hong Polytechnic University temperatures i.e., below 600 C. These are the IP advanced and 0.8 mm thickness with an active area of 0.64 cm2 . : 158.132.127.134 material design and development approachesFri, employed in 2011A05:37:04 stainless steel was used for carrying out the fuel cell 20 May our currently ongoing EC FP6 NMP NANOCOFC (Multimeasurements. At about 550 C the fuel and oxidant were functional Nanocomposites for advanced fuel cell techsupplied. I–V and open-circuit measurements were carried nology) project (http://www.nanocofc.org) to develop next out between 450 C and 600 C. Pure H2 was used as the generation fuel cell technology.12 fuel and compressed air as the oxidant. The Ac impedance spectrum was determined with electrochemical instrument/Potentiostat (VERASTA2273) in 2. EXPERIMENTAL DETAILS the range of 1 MHZ to 10 MHZ. The experimental curve 2.1. Synthesis of Composite Electrolyte simulated with ZSimpWin software.
LiAlO2 –LiNaCO3 Composite Electrolyte for Solid Oxide Fuel Cells
Raza et al. 0
200 400 600 800 1000 1200 1400 1600 1800
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OCV (V)
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Fig. 1.
SEM image of the prepared nano-LiAlO2 –LiNaCO3 electrolyte.
Power density (mW/cm2)
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Temprature (°C) Fig. 2. OCV-T characteristics for varying carbonate content in electrolyte.
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Maximum power density (mW cm–2)
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OCV (V)
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0
Current density mA/cm2 0.92 V, 0.81 V, consecutively at 600 C which decreases with fall in temperatures. Fig. 3. Fuel cell performance for 10% carbonate contaminent in the Figure 3 shows the I–V and I–P characteristics for the electrolyte. fuel cell using the electrolyte with 15% carbonate. The cell Delivered OCV reaches around 1.07 V and maximum power densityby Ingenta to: In University Figure 6, AC impedance was analyzed and shows 2 Hong Kong Polytechnic is 388 mW/cm at 600 C. the Nyquist plot simulated with ZSimpWin software. The IP : 158.132.127.134 Figure 4 shows the dependence of maximum power detailed discussions of the above results can be found density on carbonate composition of the Fri, fuel 20 cellMay elec-2011 05:37:04 below. trolyte. It can be seen that a maximum power density of 370 mW/cm2 was obtained for 15% carbonate containment 3.1. Fuel Cell Performance and Electrical in the composite electrolyte at 600 C. Power densities of Properties of the Nanocomposite Electrolyte 300 mW/cm2 at 550 C and 208.5 mW/cm2 at 500 C were also achieved for the same composition. All these The composite electrolyte of the LiAlO2 –carbonate is a performances are much higher than other compositions of multi-ion mobile system, which consists of M+ (M = the composite electrolyte. Li, Na), CO2− in air atmosphere, O2− and H+ in a H2 3 Figure 5 shows temperature dependence of electrical containing atmosphere, for instance a fuel cell.11 Pure H2 conductivities for the various LiAlO2 –carbonate composand air were used as the fuel and the oxidant, respectively, ite electrolytes. Following the same trends as maximum for testing of the fuel cells (FC). Samples varying from power density, at all temperatures, the highest value for 10% (w/w) carbonate to 40% (w/w) carbonate were tested. conductivity is shown by the electrolyte containing 15% It was found that with decreasing carbonate content, the carbonate composition. OCV increased. For the sample with 40% carbonate, it was
600 °C 550 °C 500 °C 450 °C
350 300 250 200 150 100 50 0 10
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Carbonate content %age Fig. 4. Comparison dependence of the max power using different LiAlO3 –LiNaCO3 composite electrolytes.
J. Nanosci. Nanotechnol. 11, 5402–5407, 2011
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LiAlO2 –LiNaCO3 Composite Electrolyte for Solid Oxide Fuel Cells
The plot for I–V characteristics shows that the decrease in OCV, below 500 C is more rapid compared to that from 0.25 600 C to 500 C. This can be explained by the formation 10% 15% of a solid soft phase. Since the melting point of LiNaCO3 20% 0.20 25% is 510 C, so below 500 C some of the carbonate solidi30% fies and some is left in the liquid state resulting in reduced 40% conductivity. 0.15 In the FC as there is only H2 and oxygen being supplied externally so it can be stated that the only mobile ions 0.10 available for transport are H+ and O2− resulting in steady current. Li+ , Na+ and CO2− 3 are also mobile but they are blocked at the electrodes, and hence can’t give a steady 0.05 current. Pure LiAlO2 is an insulator of oxide ion, indicating that 0.00 in the composite electrolyte, the major portion of the cur1.10 1.15 1.20 1.25 1.30 1.35 1.40 rent is due to H+ ion transport but minor O2− transport 1000/T (K–1) may also be present. The A.C. impedance analysis can only determine all Fig. 5. Conductivity-temperature characteristics for various Delivered by Ingenta to: contributions. At elevated temperatures, the compositions. mobile ions’ + Hong Kong Polytechnic University M and CO2− 3 in carbonate are highly mobile together with observed that the rise in the OCV was initiallyIPslow as : 158.132.127.134 O2− ions, which may even contribute higher conductivity compared to the other samples, which may beFri, explained by 2011 20 May than05:37:04 others. Bodén et al studied detailed A.C. conductivthe fact that due to increase in the carbonate content in the ities of the SDC-carbonate composite system in different electrolyte, its cover over the LiAlO2 particles increases, gas atmospheres and suggested that “no mechanism for therefore resulting in a thick reaction layer that may hinder ionic transport can be concluded.” 16 It can be understand ionic conduction, thus resulting in a low OCV. The maxias that A.C. conductivity reflects all mobile ionic species’ mum current density also showed similar behavior as that contributions and the role and conductivity contributions of OCV, reaching maximum at 15% but again decreasing at form oxygen ion and protons which concern the fuel cell 10%. It may be clarified by the fact that at 10% carbonate performances still remain unclear. content, the LiNaCO3 cover on the LiAlO2 particles is too little to create sufficient interactions between the carbonate and LiAlO2 interfaces, resulting in less ion transport, thus 3.2. Fuel Cell Processes and Functions decreasing the current density. Also, at temperatures below It should be pointed that there is an essential differ510 C as most of the carbonate has solidified so there may be crossover of the gas. ence between the MCFCs and the composite SOFCs T (°C)
600
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20 10 15.8
2k 0 39.8 k
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Z, Msd. Z, Calc. 79.4 m
2.51 501 m
15.8 k 63.1 k
–Z″, ohm
126 k 158 k 200 k
Iter #: 2 Chsq: 2.16E–04
251 k 316 k 398 k 501 k 631 k
–1
0
1
2
3
Z′, ohm Fig. 6.
AC impedance analysis of 10% carbonate containment.
J. Nanosci. Nanotechnol. 11, 5402–5407, 2011
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Model: LR(QR) Wgt: Modulus
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LiAlO2 –LiNaCO3 Composite Electrolyte for Solid Oxide Fuel Cells
Raza et al.
in this work. MCFCs are based on molten carbonate performance was observed for the sample containing 15% LiNaCO3 by weight. It exhibited a maximum power denions are mobile, and ionic electrolytes, in which CO2− 3 sity of 368 mW/cm2 at 600 C, 300 mW/cm2 at 550 C, ion process. In the comtransport is governed by CO2− 3 2− 208.5 mW/cm2 at 500 C and 135 mW/cm2 at 450 C. posite electrolyte fuel cells, it is difficult for CO3 ions It should be point out that all these performances have to move due to absence of a source ion. Instead, probeen created based on non-oxygen ion and proton conductton or oxygen ion conduction from the composite mateing materials through interfacial mechanism to contribute rial becomes the predominant process to perform the FC great fuel cell performances at low temperatures, below function. 600 C. In summary, by the combination of nano- and ceramicA model of dual conduction of H+ and O2− ions is composite technologies, the composite electrolytes usher suggested to explain the high current and power outputs. us into a new and promising direction for the development The use of air as oxidant rules out the transport of CO2− of advanced LTSOFCs. The use of LiAlO2 , an oxide ion 3 ions. Also LiAlO2 being an insulator for O2− ions points insulator, in a composite electrolyte for SOFCs demonto interesting behavior at the solid phase (LiAlO2 )-liquid strates the intricate nature of behavior at the LiAlO2 – phase (LiNaCO3 ) interface. By manipulating the intercarbonate interface. While the exact mechanism of the faces nanotech based composites, the working temperainterfacial behavior between the two phases of the comture of conventional SOFCs is tremendously reduced from posite electrolyte still remains to be studied, it nevertheless 1000 C to below 600 C, which opens new opportunities offers us an intriguing and a very promising field for furby employing nanotechnology. The development of this ther research and development of low-cost, Delivered efficient andby Ingenta to: fuel cell sets ground for further research to understanding low-temperature fuel cells. Hong Kong Polytechnic University the interfacial behavior in composite electrolytes and the HCO− IPbe: formed 158.132.127.134 3 , as an intermediate proton carrier, may use of nanoparticles in fuel cell technology. in fuel cell environment for by attaching H+ on CO2− Fri, 20 May 2011 05:37:04 3 This work presents a new scientific approach to design proton transport, as reported earlier.9 The carbonate existand develop fuel cell materials in breaking the structural ing in the composite electrolytes can indeed contribute limitations by using non-ionic conductors on the desired favourably to both oxygen and proton conductivity as well ions i.e., proton and oxygen ions and creating high proton as to fuel cell performance. and oxygen ion conductors through interfaces and interfacial mechanism. 3.3. Nano-Effects and Advanced Nanocomposite Two-Phase Material Approach Acknowledgments: This research was carried at the Royal Institute of Technology (KTH), Stockholm under The fuel cell performance, electrical properties, and their the Student Exchange programs between KTH, Stockdependence on composition of the LiAlO2 –carbonate holm and the Indian Institute of Technology, Delhi electrolytes, may help us to understand better the and Panjab University, Chandigarh, KTH-CSC (Chinese nano-effects and two-phase and interfacial conduction Scholar Council) postdoctoral program and also Higher mechanisms. Education of Pakistan (HEC). The research is also financed Zhu et al. also reported single-crystal to a polycrysby the current EC FP6 NMP NANOCOFC project (Contalline material means that grain sizes and grain boundtract No. 32308) project. aries have to be taken into account regarding their effects on the ionic conductivity. If the grains are nano-sized, the effects may be considerable since the surface region may References and Notes be comparable in volume to the bulk. Maintaining a fine 1. B. Zhu, J. Power Sources 93, 82 (2001). grained single-phase material with high surface area at 2. B. Zhu, J. New Mater. Electrochem. Syst. 4, 239 (2001). the relatively high temperatures of an operating fuel cell 3. B. Zhu, J. Power Sources 114, 1 (2003). is however not easy. Sintering and crystal growth will in 4. J. B. Huang, L. Z. Yang, R. F. Gao, Z. Q. Mao, and C. Wang, 18 fact act to decrease this area. However, if a two-phase Electrochem. Commun. 8, 785 (2006). material consisting of two ionic conductors is formed, a 5. X. Wang, Y. Ma, R. Raza, M. Muhammad, and B. Zhu, Electrochem. Commun. 10, 1617 (2008). large interface region may be maintained even at elevated 6. R. Raza, X. Wang, Y. Ma, and B. Zhu, J. Nanosci. Nanotechnol. 10, temperatures.
4. CONCLUSIONS The SOFC using nano-LiAlO2 –LiNaCO3 composite as electrolyte was successfully tested, and it exhibited good performance reflected by a good OCV, Current Density and Power Density. Among various compositions, the best 5406
1203 (2010). 7. B. Zhu, X. T. Yang, J. Xu, Z. G. Zhu, S. J. Ji, M. T. Sun, and J. C. Sun, J. Power Sources 118, 47 (2003). 8. B. Zhu, X. R. Liu, and T. Schober, Electrochem. Commun. 6, 378 (2004). 9. B. Zhu, X. R. Liu, P. Zhou, X. T. Yang, Z. G. Zhu, and W. Zhu, Electrochem. Commun. 3, 566 (2001). 10. J. B. Huang, L. Z. Yang, R. F. Gao, Z. Q. Mao, and C. Wang, Electrochem. Commun. 8, 785 (2006).
J. Nanosci. Nanotechnol. 11, 5402–5407, 2011
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LiAlO2 –LiNaCO3 Composite Electrolyte for Solid Oxide Fuel Cells
11. S. Li, X. Wang, and B. Zhu, Electrochem. Commun. 9, 2863 (2007). 12. B. Zhu, Int. J. Energy Res. 33, 1126 (2009). 13. S. Terada, K. Higaki, I. Nagashima, and Y. Ito, J. Power Sources 83, 227 (1999) . 14. R. B. Khomane, A. Agrawal, and B. D. Kulkarni, Mater. Lett. 61, 4540 (2007).
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Received: 16 October 2009. Accepted: 19 March 2010.
Delivered by Ingenta to: Hong Kong Polytechnic University IP : 158.132.127.134 Fri, 20 May 2011 05:37:04
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