Nanocomposites for Advanced Fuel Cell Technology

Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 11, 8873–8879, 2011

Nanocomposites for Advanced Fuel Cell Technology Bin Zhu Department of Energy Technology, Royal Institute of Technology (KTH), S-10044, Stockholm, Sweden NANOCOFC (Nanocomposites for advanced fuel cell technology) is a research platform/network established based on the FP6 EC-China project www.nanocofc.org. This paper reviews major achievements on two-phase nanocomposites for advanced low temperature (300–600 C) solid oxide fuel cells (SOFCs), where the ceria-salt and ceria-oxide composites are common. A typical functional nanocomposite structure is a core–shell type, in which the ceria forms a core and the salt or another oxide form the shell layer. Both of them are in the nano-scale and the functional components. The high resolution TEM analysis has proven a clear interface in the ceria-based two-phase nanocomposites. Such interface and interfacial function has resulted in superionic conby Ingenta to:to that of conventional SOFC YSZ ductivity, above 0.1 S/cm at aroundDelivered 300 C, being comparable Osaka Daigaku (Osaka at 1000 C. Against conventional material design from theUniversity) structure the advanced nanocomposites IP :the 41.0.65.146 are designed by non-structure factors, i.e., interfaces, and by creating interfacial functionalities Tue,These 13 Dec 14:00:52 between the two constituent phases. new2011 functional materials show indeed a breakthrough in the SOFC materials with great potential.

Keywords: Nanocomposites, Two-Phase, Superionic Conduction, Interface.

Solid oxide fuel cells (SOFCs) are one of the most promising fuel cell (FC) technologies. But the challenges that face SOFCs are critical and materials are the key issue associated with high temperatures.1 2 This prevents its commercialization. For over half a century extensive SOFC R&D based on the YSZ (yttrium stabilised zirconia) has been undertaken. An electrolyte conductivity of 0.1 S/cm is a basic requirement for FCs. The YSZ reaches 0.1 S/cm at around 1000 C, thus causing the high temperature (HT) SOFC technology. Designing and development of functional materials at lower temperatures are a critical challenge2 and therefore there is a world tendency to develop SOFCs for low temperatures (LT). Many efforts are done by using thin-film technologies on YSZ to reduce the operational temperature.3–6 But a thin film electrolyte can not guarantee a long SOFC life because FC operation involves mass transport processes which can affect the electrolyte property in one or another aspect, thus causing serious degradation. Recent research discovered that the YSZ in nanometer thin films, the point defects and the dislocations may form the ionic conduction highways.5 However, in the singlephase materials (SPM) like YSZ, the point defects and dislocations make it difficult to form the long-path continuous ionic highways. Instead, more localised highly mobile ionic domains may be formed. J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 10

A more effective way is to develop new materials with improved performance, e.g., superionic conduction (SIC) at LT, say 0.1 S/cm at 300–600 C. Such materials have been developed based on the ceria two-phase material (TPM) composites.7–10 The 300–600 C FCs provide an effective way for designing and developing new functionalities and advanced materials by using nanotechnology to apply new architectures of TPM nanocomposites that yield high conductivities resulted from nanomaterials blocks and two-phase interfaces. Interfacial phase and effect in nanocomposite electrolytes lead to SIC at LT, thus high performance LTSOFCs. The interfacial mechanism opens a new scientific disciplinary with great potentials to design and develop advanced functionalities and materials and explores new opportunities. In order to develop the low temperature ceramic fuel cells we started to investigate the Li2 SO4 and its oxide composites, e.g., Li2 SO4 –Al2 O3 in early 90s.11–13 The FC power density output was improved in the composites by one order of magnitude at 0.1 Wcm−2 at 600 C with several hundreds of mAcm−2 current output. It was discovered that proton conductivity is dominating in the Li2 SO4 based electrolytes for FC functions.11 12 A wide range of the oxyacid salts, e.g., sulphate, phosphate and nitrate were examined. These materials had more or less proton conduction thus resulted in certain FC performances.14–16 Since 1996, we discovered a new type composite material, e.g., thin film ceria–alumina and ceria–silicate

1533-4880/2011/11/8873/007

doi:10.1166/jnn.2011.3501

8873

RESEARCH ARTICLE

1. INTRODUCTION

Nanocomposites for Advanced Fuel Cell Technology

Fig. 1. Advanced TPM design, interfacial mechanism, i.e., interfaces constructed by introducing the 2nd phase as ion conducting highways, versus conventional ceria SPM structural bulk mechanism.

2.2. Interfacial Sueprionic Conduction (SIC)

Unlike the conventional SIC in the single-phase bulk it takes place from low conductive phase transferred to the SIC one accompanying a phase structure change. In the two-phase Ingenta to: systems the SIC occurs at the interfacial regions between Osaka Daigaku (Osaka University)the two constituent phases. It is thus determined by the interfaces, i.e., the change in the interIP : 41.0.65.146 facial properties without involving individual phase strucTue, 13 Dec 2011 14:00:52 2. NANOCOFC APPROACH/METHODOLOGY tural changes, see Figure 2, the SIC phenomena inside 2.1. New Material Design and Functionality the SPM structure and at the TPM composite’s interDevelopments for LTSOFC Electrolytes face. Tillman first reported such SIC phenomenon in the composite materials.24 The principle of the new material design and functionalThe SIC in the TPM composites is determined by the ity developments has been made from conventional design interfaces, i.e., a process breaks symmetry of matrix by and functionality within the structure of single-phase mateintroducing a structural discontinuity.25 In contrast with the rial (SPM) to the “non-structure,” or interfaces between bulk, where electroneutrality must be obeyed, at the interconstituent phases in the TPM. For example, conventional face a narrow charged zone, the so-called space-charge SOFC material function is created by ion-doping techzone is tolerable and thermodynamically necessary.26 The nique to create oxygen vacancy in the structure of zircodefect concentrations in the space-charge zone are much nia or ceria, e.g., YSZ or SDC (Samarium doped ceria). higher than that in the bulk, which accounts for higher The oxygen ion conduction is attributed to structural oxyionic diffusivity and mobility than bulk. One efficient gen defects, so called bulk mechanism. This structuremethod to enhance the space-charge effect is to decdetermining property has several disadvantages, e.g., high orate the grain boundaries with surface-active secondionic activation energy needs to create the ion mobility and phase materials, which is also called heterogeneous low oxygen ion concentration and mobility with the structural constraints. Therefore, high temperature is a condition to activate sufficient ionic mobility, e.g., 1000 C for Str II YSZ. New NANOCOFC approach focuses on the inter0.1 faces, from a SPM structure, e.g., YSZ or SDC to the TPM, to develop the desired properties at the interfaces SPM 0.01 PII PI TPM between the two constituent phases by nano-engineering the TPM systems. Figure 1 shows the design principle of 1E-3 the TPM functionality from interfaces versus conventional SPM structural bulk mechanism. In the TPM design the 1E-4 Str I mobile ions, e.g., O2− have been created already on the particle, e.g., a ceria fluorite structure, surfaces by using 1E-5 an appropriate 2nd phase, e.g., carbonate. The functionality of O2− conduction is thus more preferential through the 1.0 1.2 1.4 1.6 1.8 interfaces via oxygen ions/vacancies than that through the 1000/T (K–1) bulk fluorite structure via oxygen vacancy. This is further Fig. 2. SIC phenomena in both SPM structure and TPM composites. described in Section 2.3. σ (cm–1)

RESEARCH ARTICLE

two-phase materials (TPMs) with unusually high electrical properties and good chemical stability, which may be used for advanced fuel cell applications.17–19 The good properties for the TPM nanocomposite thin films were suggested due to not only the composite effect but also the nano- and two-dimensional structure of the thin film natures.19 Most extensive innovations have been recently devoted to develop composites based on the ceria-salts and ceriaoxide composite electrolytes.20–23 The ceria was used more commonly as an ion doped type, e.g., Gd–CeO2 (GDC), Sm–CeO2 (SDC) and Y–CeO2 (YDC). The ceria (ion doped ceria) is well known as good oxygen ion conductors, about 10−2 Scm−1 at 600 C. These composite electrolytes have demonstrated a new generation fuel cell technology.8 Especially, nanocomposites for advanced fuel cell technology (NANOCOFC) has been developed within the EC (Turkey)-China FP6 NANOCOFC network for low temperature, 300–600 C, SOFCs, www.nanocofc.org. Delivered by

Zhu

8874

J. Nanosci. Nanotechnol. 11, 8873–8879, 2011

Zhu


doping process.27 The ceria-based composite electrolyte with remarkable ionic conductivity is victorious case of SIC transitions excited by heterogeneous doping process, where interface supplies high conductivity pathways for ionic transportation and conduction. In nanosized materials, calculations show that the mean free volume is significantly larger at boundaries, which enhances the mobility of ions at the boundary core. The enhanced boundary diffusion leads to significant enhancement of effective diffusion coefficients of materials with a high boundary density. In the case of ionic materials, this boundary mechanism or interfacial mechanism in the TPM nancomposites can thus make the SIC in the nano-regime.


8875

RESEARCH ARTICLE

known transport equations in the presence of electric field to provide useful information about ion transport in materials. We’ve taken as a case study on the ceria-carbonate two-phase system to discuss interfacial ion interaction, electric field and corresponding oxygen ion activation energy.28 The calculations showed that the oxygen ion activation energy in such interface is less than 0.2 eV (to meet well the SIC request) compared to around 1.0 eV via the classical bulk effect in the single-phase ceria. Further theoretical calculations and studies will be carried out on the interfacial SIC and ionic transport mechanisms in the ceria TPMs. There have been extensive studies and calculations on the ceria SPM materials by employing the Quantum Mechanics “First principle” with focus on the structure and bulk mechanism.29 30 2.3. Ionic Conduction Theories and The efforts on ceria surface defects’ calculations furInterfacial Mechanisms ther to TPMs are needed. The interfacial SIC and ionic The ionic conduction is described by Arrhenius equation: transport mechanisms in the ceria-based TPMs may be T = 0 exp−Ea /kT . The advanced material architecinvestigated by extending calculations from the Quantum Delivered by Ingenta to:“First principle” previously focusing on the tures emphasis on how to maximize pre-exponentional Mechanics OsakaofDaigaku University) parameter 0 that depends on concentration ions, n, (Osaka crystal structures to the ceria surface defects, in order IPionic : 41.0.65.146 ion jump-step distance, d; and how to minimize the further to design functionalities at interfaces and develop 2− 13 Dec 2011 14:00:52 migration activation energy, Ea . Classical OTue, conductivmore advanced TPM nanocomposites. ity is realized by ion-doping to create oxygen vacancies in a SPM bulk, e.g., YSZ. There are number of limita2.5. Nanocomposites and Interfacial Functionalities tions: (i) due to a strong interaction between vacancies It is important for development of synthesis methand cations resulting in high Ea values in the order of ods (e.g., sol–gel technique, co-precipitation, combustion, 1.0 eV, thus it requires a high temperature for activation of microwave heating, multi-layer growing, nano-colloid stathe mobile ions; (ii) 0 is limited by the crystal unit cell bilization, combinatorial and high-throughput synthesis at least in two aspects, (a) n is limited by the degree of and testing, thermal spray, nano-coatings, etc.) to create doping resulting in a low concentration of extrinsically crethe new materials with controlled microstructure, porosated vacancies; (b) d limited in a unit cell, i.e., of the order ity and bulk and surface/interface composition with new of some Angstrom. These mean that the structural bulk physical and chemical properties. Thus the new architecmechanism has strong constraints resulting in high activatures and desired functionalities may be developed. We tion energy, low oxygen ion concentration and mobility, have developed template-free synthesis of ceria nanowires and high temperature. and demonstrated the utility of the wires in a solid oxide The energy to form defects on the surface is signififuel cell.31 cantly lower than that inside the bulk structure. The interIn addition, the conventional doping technique to creface has, in principle, no bulk structural limit to create high ate oxygen vacancy (O2− conductivity) within the strucconcentration of mobile ions, and can thus be very disorture, i.e., bulk mechanism/methodology, is now used in the dered. This implies that the interfaces have the capacity nanocomposite case mainly as the particle surface modifier to contain higher mobile ion concentration and also long to build the nano-environment. It can thus make funcrange transport channels than that of the bulk effect occurtionalized interfaces upon using controlled surface reacring within a crystal (e.g., YSZ) unite cell. In contrast to tions in both heterogeneous and homogeneous. Many new the classical theory and bulk effect in the SPM, the interfananocomposites and functionalized interfaces or interfacial cial mechanism and TPM approach are a new principle for functionalities will be developed for successful LT, 300– designing and developing the LTSOFCs by constructing 600 C SOFCs. the interfaces as ionic highways leading to the interfacial SIC. 2.6. A Brief Description of Experimental 2.4. Theoretical Approaches X-ray powder diffraction (XRD) patterns were recorded In theoretical part, the aim is to understand the ionic on a Philips X’pert pro super Diffractormeter with Cu conduction mechanisms and ion transport in nanocomK radiation ( = 15418 Å) in the 2 range of 10–90 . posites/interfaces and provide advanced material designs. A Zesiss Ultra 55 scanning electron microscope (SEM) A valid alternative of the macro-scale model exploits the was used to examine the morphology and particle size


of the products. A high resolution transmission electron microscopy (JEOL 3000F TEM) with a beam energy of 300 KeV. Elemental maps were collected using Gatan energy-filter TEM imaging technique. The electrical properties were studied using a.c. impedance analysis. The measurements were conducted using a computerised HP 4192A LF Impedance Analyzer. The fuel cells were constructed by a hot pressing procedure. The hot-pressing technique involved loading a mold with the powders of anode successively followed by the electrolyte and finally the cathode, all being pressed in one step to form a complete fuel cell assemble for measurements. The fuel cells were fabricated in 13 mm in diameter (with a 0.7 cm2 active area) having aorund 1.0 mm in thickness to consist of an approximate 0.4 mm thick electrolyte layer, and 0.3 mm thick electrodes for anode and cathode layers, respectively.

Zhu (a)

(b)

Delivered by Ingenta to: 3. CASE STUDIES ON FUNCTIONAL NANOCOMPOSITES FOR Osaka Daigaku (Osaka University) IP : 41.0.65.146 ADVANCED LTSOFCs

RESEARCH ARTICLE

Tue, 13 Dec 2011 14:00:52

Functional nanocomposites can be classified into two categories: (i) ceria-salt composites, where various ion doped ceria, e.g., Ca2+ , Y3+ , Sm3+ and Gd3+ are often used, and the salts, chlorides, sulphates, carbonates are common; (ii) ceria-oxide composites e.g., CeO2 –Al2 O3 , CeO2 –SiO2 , and SDC-LiZn-oxide have been discovered. Typical nanocomposite structures are often in (i) a core–shell type, typically, SDC-carbonate or ceria-oxide systems; (ii) TPM nano-films, typically, CeO2 –Al2 O3 , CeO2 –SiO2 and CeO2 – Li2 SO4 thin films; (iii) a nano-fibre or nanowire type: e.g., nanowire SDC-carbonate.31 These nanocomposites are constructed based on a host ceria (SDC) phase of the nano-particles or the nanowires. The functionalities have been created between the constituent ceria and carbonate or another oxide phase in the TPM systems. It is interesting to note that the composites can possess very different properties from normal size, say m level to nano-level. Figure 3 shows SEM results for the morphology of as-prepared non-nanocomposite LiNaCO3 SDC, particle size in several m level, Figure 3(a), and nanocomposite Na2 CO3 @SDC, particles’ size are less than 100 nm, Figure 3(b). The images reveal very different shapes between these two types of the samples. TEM result with further details of nanostructure for the Na2 CO3 @SDC nanocomposite is shown in Figure 4. The large contrast difference between the inner and the outer indicates that the nanocomposite has a core (SDC)-shell (Na2 CO3 structure. It is clearly shown that SDC nanoparticles are surrounded by a uniform Na2 CO3 thin layer of 4∼6 nm. Also this Na2 CO3 shell is amorphous. The TEM analysis has proven a clear interface in the ceriacarbonate two-phase nanocomposites between the SDC and carbonate. 8876

Fig. 3. SEM images of (a) the non-nanocomposite, LiNaCO3 -SDC and (b) Na2 CO3 @SDC nanocomposite samples.

Thermal analyses were also carried out on these samples. The results are shown in Figure 5. It can be seen from Figure 5 that (a) the LiNaCO3 -SDC composites shows an endothermic peak at 507 C due to the LiNaCO3 melting. While for the nanocomposite NaCO3 -SDC, (b), a thermal effect is observed at much lower temperature between 200 and 300 C. This effect does not correspond to any individual phases. As a fact in this measured temperature region there is no any phase transition or melting effect in individual phase, SDC or Na2 CO3 (melting point at about 850 C). On the other hand, this transition has a

Fig. 4. TEM result of core–shelled Na2 CO3 @SDC nanocomposite sample.


Zhu

Nanocomposites for Advanced Fuel Cell Technology (a)

10

0.24

700 600 500

200

T(°C)

Type I: SDC-LiNaCO3

0.1

Heat flow (a.u.)

300

Type II: Na2CO3-SDC nanocomposite

1

0.22

400

0.20 0.01

σ (S·cm–1)

0.18 0.16 0.14

1E-4

0.12

1E-5

0.10

1E-6

0.08

1E-7

0.06 200

300

400

500

600

700

SDC

1E-3

1E-8 1.0

Temperature(°C)

1.5

2.0

2.5

1000/T (K–1)

(b)

Fig. 6. Temperature dependences of conductivity for the m size LiNaCO3 -SDC composite and Na2 CO3 @SDC nanocomposite sample with comparison to pure SDC.

Delivered by Ingenta SDC andto: LiZn-oxide, i.e., a composite consists of two Osaka Daigaku (Osaka University) crystalline phases. IP : 41.0.65.146 The core–shell structure of SDC coated with LithiumTue, 13 Dec 2011 Zinc14:00:52 oxide is viewed by HRTEM in Figure 7, in which

strong glass transition nature, which agrees to the amorphous core–shell region as observed in TEM, Figure 4. The results told us that in the nano-level, new interfacial structure may be formed and it does not belong to any individual constituent phases in the composites. The conductivity studies further discovered very interesting ion conduction phenomena for normal composites and nanocomposites. Figure 6 shows temperature dependences of ion conductivity for these samples. The LiNaCO3 -SDC shows that the conductivity takes a sharp leap of the SIC at around 500 C due to the carbonate melting. While for the Na2 CO3 -SDC nanocomposite, the SIC leap has experienced a temperature region of 200–300 C from 10−4 S/cm to above 0.1 S/cm, as shown in Figure 6. This behavior agrees to above thermal analyzing result, Figure 5(b). Accompanying the disorder process of the Na2 CO3 to form the amorphous shell structure, Figure 4, the SIC may thus occur at the Na2 CO3 @SDC nanocomposite interface region since both SDC and Na2 CO3 are not conductive at 200–300 C. Another typical nanocomposite is the ceria-based twooxide system, e.g., LiZn-oxide and SDC composites. XRD analysis showed that unlike the Na2 CO3 @SDC, the LiZn-oxide@SDC exhibits a crystalline structure for both J. Nanosci. Nanotechnol. 11, 8873–8879, 2011

5 nm

Fig. 7. TEM result of the LiZn-oxide coated SDC nanocomposite sample.

8877

RESEARCH ARTICLE

Fig. 5. Thermal analyses on (a) LiNaCO3 -SDC non-nanocomposite and (b) Na2 CO3 -SDC nanocomposite samples.

the SDC core is marked by a dashed circle and surrounded by Zinc oxide. It is seen they both show the almost same orientation. However, Moiré fringes are also apparently visible in the core center, which indicates that there is a slight rotation between SDC and LiZnoxide. This indicates that LiZn-oxide has been successfully coated onto the nanoparticles. The element mapping displays clearly a homogenous distribution of two phase elements/components. From dark-field TEM image it can be concluded that the LiZn-oxide coating layer is about 15– 20 nm in thickness.

RESEARCH ARTICLE


Zhu

The conductivity measurements displayed also a rapid which are fundamental requirements to circumvent the present state of the art where the technological exploitaincrease of the conductivity at above 300 C, where the tion of these materials is apparently moving faster than the conductivity reaches above 0.1 S/cm in the same conducunderstanding of their performance. So far it has become tivity level as that of the Na2 CO3 @SDC nancomposite. clear that the deliberate introduction of interface,33 i.e., These conductivties are as good as those of the conven fabrication of composite electrolyte, is a powerful method tional SOFC electrolytes, YSZ at 1000 C and SDC at on modifying the electrochemical properties of materials 800 C, respectively. for a great deal of energy-related applications, like Li-ion The current work presents new scientific principle and batteries, fuel cells, electrochemical sensors, permeators, experimental results based on new ideas: (i) to build etc. Therefore, composite electrolyte is expected to have a TPM’s architectures and properties by coating so as to substantial role in future materials R&D for SOFCs. replace the conventional SPMs; (ii) to design and develop new functional properties, e.g., SIC by constructing interfaces between two constituent phases, coating and coated 4. CONCLUSIONS phases, as ionic highways. It can be seen from both core–shelled Na2 CO3 @SDC and LiZn-oxide coated SDC The material and functionality development is based on nanocomposites that above 300 C the SIC transition two-phase material (TPM) composite architecture in nanooccurs and the conductivity reaches above 0.1 S/cm. This scale to create an interfacial superionic conduction (SIC) conductivity can not be resulted from the bulk conduction in the interfaces between the constituent phases. There mechanism from the constituent SDC and Na2 CO3 or SDC is a clearto: indication on interface structures, usually they Delivered by Ingenta and LiZn-oxide phases. Several orders of conductivity for show a core–shell Osaka Daigaku (Osaka University) structure, a nano-layer of the 2nd phase, the coated SDC higher than those of individual SDC and either carbonate or metal oxide coated on the host ceriaIP : 41.0.65.146 Na2 CO3 or LiZn-oxide indicate a strong proof for an internanoparticles. Tue, 13 Dec 2011 14:00:52 facial effect and also interfacial SIC behaviour. The TPM approach can be a powerful way to develop Such interfacial mechanism has been also discovered multifunctional new materials with many advantages over recently in “Colossal Ionic Conductivity at Interfaces of the conventional single-phase materials (SPMs). It has creEpitaxial ZrO2 :Y2 O3 /SrTiO3 Heterostructures.” 32 In this ated a great freedom to develop a wide range of funccase the ionic conductivity could be greatly improved by tional materials, not limited by the structures, as long as combining layers of the standard electrolyte materials with the selected two phases can construct functional inter10-nanometer-thick layers of strontium titanate. A large faces with desired functions. The functional TPMs offer number of oxygen vacancies—places could be formed new solutions for the FC problems/challenges leading to within the crystalline structures of the materials where a new generation FC, i.e., NANOCOFC that will greatly these two materials meet due to the differences in the accelerate FC R&D and commercialization processes. crystal structures of the materials. Thus allow the oxygen ions to move through the material interfaces, improving Acknowledgment: The EC FP6 NMP NANOCOFC the conductivity of the materials at low temperatures. project, the Swedish Innovation System (VINNOVA) and As a fact in our early work, extremely high conductivthe Swedish Energy Agency (STEM) are acknowledged ity was also observed for the CeO2 –SiO2 and CeO2 –Al2 O3 for financial supports. The author would also like to thank thin films.17–19 The author proposed the interfacial conthe NANOCOFC consortium, www.nanocofc.org. ductivity mechanism and nano-effect to explain the great conductivity enhancement.19 Indeed the interfacial mechanism plays a key role to determine the conductivity of the References and Notes nanocomposite materials. 1. B. C. H. Steele and A. Heinzel, Nature 414, 345 (2001). Based on high ionic conductivity LiZn-oxide shelled 2. J. B. Goodenough, Nature 404, 821 (2000). SDC nanocomposite electrolyte the FCs deliver high 3. P. Charpentier, P. Fragnaud, D. M. Schleich, and E. Gehain, Solid power outputs. The maximum power density of 0.8 W/cm2 State Ionics 135, 373 (2000). 4. Z. P. Shao and M. H. Sossina, Nature 49, 170 (2004). and 0.9 W/cm2 have been achieved at 520 C and 550 C, 5. H. Huang, M. Nakamura, P. Su, R. Fashing, Y. Sato, and F. B. Prinz, respectively. The Na2 CO3 @SDC used as electrolytes have J. Electrochem. Soc. 154, B20 (2007). 2 demonstrated even higher performances, e.g., 1.0 W/cm 6. Y.-I. Park, S. P. Chen, C. S. Won, Y. Saito, and B. P. Fritz, J. Elecat 480 C. The FC can be operated at temperature as low trochem. Soc. 153, A431 (2006). as 300 C or even lower. 7. B. Zhu, Inter. Energy. Res. 30, 895 (2006). 8. J. Huang, Z. Mao, Z. Liu, and C. Wang, Electrochem. Commun. The 300 C SIC with the same conductivity level as 9, 2601 (2007). YSZ at 1000 C and advanced TPMs are indeed a break9. X. Wang, Y. Ma, R. Raza, M. Muhammed, and B. Zhu, Electrochem. through in the long SOFC R&D history. There is, however, Commun. 10, 1617 (2008). a lack of extensive studies on the interfacial ionic trans10. B. Zhu, X. R. Liu, Z. G. Zhu, W. Zhu, and R. Ljungberg, Inter. J. port numbers/highways and ionic conduction mechanisms Hydrogen Energy 33, 3385 (2008). 8878


Zhu


11. B. Heed, B. Zhu, B.-E. Mellander, and A. Lunden, Solid State Ionics 46, 121 (1991). 12. A Lunden, B. E. Mellander, and B. Zhu, Acta Chem. Scand. 45, 981 (1991). 13. B. Zhu and B.-E. Mellander, Solid Oxide Fuel Cells-III, edited by S. C. Singhal and H. Iwahara, The Electrochemical Society, Pennington (1993), p. 156. 14. B. Zhu and B.-E. Mellander, J. Power Sources 52, 289 (1994). 15. B. Zhu and B.-E. Mellander, Ferroelectrics 167, 1 (1995). 16. B. Zhu, Solid State Ionics 125, 397 (1999). 17. B. Zhu, Solid State Ionics 119, 305 (1999). 18. B. Zhu, X.-G. Luo, and C.-R. Xia, Mater. Res. Bulletin 34, 1507 (1999). 19. B. Zhu, C. R. Xia, X. G. Luo, and G. Niklasson, Thin Solid Films 385, 209 (2001). 20. B. Zhu and B.-E. Mellander, Solid Oxide Fuel Cells-VI, edited by S. C. Singhal and M. Dokiya, The Electrochemical Society, Inc., Pennington (1999), p. 244. 21. B. Zhu, J. Power Sources 93, 82 (2001).

22. 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). 23. B. Zhu, J. Power Sources 114, 1 (2003). 24. T. Schober, Electrochem. & Solid State Letts. 8, A199 (2005). 25. J. Maier, Nat. Mater. 4, 805 (2005). 26. J. Jamnik, J. Maier, and S. Pejovnik, Solid State Ionics 75, 51 (1995). 27. J. Lee, S. Adams, and J. Maier, J. Electrochem. Soc. 147, 2407 (2000). 28. B. Zhu, S. Li, and B.-E. Mellander, Electrochem. Commun. 10, 302 (2007). 29. D. A. Andersson, S. I. Simak, N. V. Skorodumova, I. A. Abrikosov, and B. Johansson, Proc. Natl. Acad. Sci. USA 103, 3518 (2006). 30. D. A. Andersson, S. I. Simak, N. V. Skorodumova, I. A. Abrikosov, and B. Johansson, Phys. Rev. B 90, 031909 (2007). 31. Y, Ma, X. D. Wang, S. H. Li, M. S. Toprak, B. Zhu, and M. Muhammed, Adv. Mat. 22, 1640 (2010). 32. J. Garcia-Barriocanal, A. Rivera-Calzada, M. Varela, Z. Sefrioui, E. Iborra, C. Leon, S. J. Pennycook, and J. Santamaria, Science 321, 676 (2008). 33. J. Maier, J. Electrochem. Soc. 134, 1524 (1987).

Received: Delivered by Ingenta to: 22 February 2010. Accepted: 15 April 2010. Osaka Daigaku (Osaka University) IP : 41.0.65.146 Tue, 13 Dec 2011 14:00:52

RESEARCH ARTICLE


8879