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Novel structured mixed ionic and electronic conducting cathodes of solid oxide fuel cells
Solid State Ionics 176 (2005) 1351 – 1357 www.elsevier.com/locate/ssi
Novel structured mixed ionic and electronic conducting cathodes of solid oxide fuel cells S.P. Jiang*, W. Wang School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Ave., Singapore 639798, Singapore Received 16 April 2004; received in revised form 30 December 2004; accepted 30 March 2005
Abstract A novel structured mixed ionic and electronic conducting cathode was developed based on the (LaSr)MnO3 (LSM) electronic conducting structure impregnated with Gd-doped CeO2 (GDC) ionic conducting phase. The ion impregnation of oxygen ion conducting GDC phase significantly enhances the electrocatalytic activity of the LSM electrodes for the O2 reduction reactions and eliminates the activation effects of the cathodic polarization associated with pure LSM. With impregnation of 5.8 mg cm 2 GDC in the LSM electrode coating, the electrode showed much lower electrode polarization resistance (R E) as compared to those of LSM/ Y2O3 – ZrO2 and LSM/GDC composite cathodes and the performance was comparable with those of mixed ionic and electronic conducting oxides such as (LaSr)(CoFe)O3 and (GdSr)CoO3. This is most likely due to the unique combination of the high electronic conducting LSM porous structure with high ionic conducting nano-sized GDC phase. The results demonstrated that high oxygen ion conductivity can be developed into conventional and dominant electronic conducting LSM materials by the simple ion impregnation method. D 2005 Elsevier B.V. All rights reserved. Keywords: LSM electrodes; Gd-doped ceria; Ion impregnation; MIEC; O2 reduction
1. Introduction Air pollution is a major global problem threatening modern civilization which is partly attributed to an ever increasing consumption of fossil fuels. The solid oxide fuel cell (SOFC) is an all solid energy conversion device with high efficiency and very low greenhouse emission. Thus the use of SOFC technologies can significantly reduce the production of the greenhouse gases and improve our environment. However, the cost of materials and fabrication processes for the SOFC system is still considered too high, thus inhibiting the commercial viability of this clean and environmental friendly technology. One solution is to lower the operating temperature of SOFC from the traditional 1000 -C to 600 – 700 -C. The low SOFC operating
* Corresponding author. Tel.: +65 6790 5010; fax: +65 6791 1859. E-mail address: [email protected] (S.P. Jiang). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.03.011
temperature can significantly widen the selection of materials and enable the use of much less expensive materials for the SOFC components such as electrodes, interconnect and manifolds [1– 3]. Additional benefits of reduced operating temperature include the significantly improved stability of materials and reliability of the SOFC system. The increased material selectivity and flexibility may also reduce fabrication costs. However, the overall electrochemical performance of a SOFC system will decrease with a reduced operating temperature. This is mainly due to the increased polarization resistances for the electrode reactions and the decreased electrolyte conductivity at low temperatures. The resistive contribution of the electrolyte can be reduced by decreasing the thickness of the electrolyte (i.e., 10 – 30 Am) on anodesupported structure [4 – 6]. However, reducing the electrolyte thickness while maintaining the mechanical and structure integrity of the thin film cells is a challenge for processing technologies. Due to the substantial reduction in
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the electrolyte thickness, the overall performance loss is increasingly dominated by the polarization losses of the electrode reactions of H2 oxidation on the anode and O2 reduction on the cathode. In comparison with the relatively fast H2 oxidation reactions, polarization losses for the O2 reduction process are particularly high due to the relatively high activation energy of O2 reduction and relatively slow reaction rates [7– 9]. Thus, development of cathode materials with high electrocatalytic activity and high stability is critical for the intermediate temperature SOFC or ITSOFC operating at 600 – 700 -C. Sr-doped LaMnO3 (LSM) has been extensively investigated and developed as a cathode material for high temperature SOFC due to its relatively high electrochemical activity for the O2 reduction reaction and good stability and compatibility with Y2O3 – ZrO2 (YSZ) electrolyte at SOFC operation temperatures [10]. Unfortunately, the oxygen ion conductivity of LSM materials is extremely low and its oxygen trace diffusion coefficient is in the range of 10 15 –10 16 cm2 s 1 at 700 -C [11,12]. The negligible oxygen transport ability would limit the O2 reduction reactions at or near the triple phase boundaries (tpb) between electrode, electrolyte and O2 gas. This poses practical limitations and restrictions on the application of LSM-based systems as potential cathodes for ITSOFC. The electrocatalytic activity of LSM can be improved by adding oxygen ion conducting phases such as YSZ and Gddoped CeO2 (GDC) to LSM electrodes [13 – 17]. Murray and Barnett [14,16] studied the electrode performance of LSM/YSZ and LSM/GDC composite cathodes and the lowest electrode polarization resistance was 0.49 V cm2 at 750 -C for the reaction on the LSM/GDC composite cathodes. This is considerably lower than 3.5 V cm2 measured on the pure LSM cathode at the same temperature. Another approach is to use mixed ionic and electronic conducting (MIEC) oxides such as (La,Sr)CoO3 (LSC), (La,Sr)(Co,Fe)O3 (LSCF) and (SmSr)CoO3 (SSC) oxides as cathodes [2,4,18]. However, MIEC materials based on cobaltites react easily with YSZ electrolyte to form resistive La2Zr2O7 and SrZrO3 phases at the cathode/YSZ electrolyte interface [19]. Thus, a thin protective layer such as doped CeO2 will be needed to inhibit the formation of resistive phases between YSZ electrolyte and cobaltite-based cathodes [20]. The introduction of additional layer increases the complexity of the cell structure and the fabrication cost. Other electrolytes such as doped CeO2 are chemically inert to cobaltite-based MIEC materials and also have higher oxygen ion conductivity than that of YSZ electrolytes below 700 -C. However, the doped CeO2 develops significant electronic conductivity under fuel reducing conditions and it expands on reduction [21]. This reduces the open circuit potential and decreases the cell efficiency. We have shown recently that ion impregnation is an effective method to improve the electrode electrochemical properties of Ni and Ni/YSZ cermet anodes [22,23]. By impregnation of electrocatalytic oxide particles such as Sm-
doped CeO2 (SDC) into the cermet structure, overpotential (g) for the H2 oxidation reaction was 72 mV at 250 mA cm 2 and 800 -C, which is close to that measured at 1000 -C for the reaction on the anode without impregnation. Watanabe et al. [24] used impregnation method to incorporate metal catalysts such as Pt and Ru to Sm-doped CeO2 anodes and Sr-doped LaMnO3 cathodes, leading to the significant reduction of the polarization losses of the electrodes. Recent results showed that impregnation of small amount of Gd-doped CeO2 (GDC) into LSM cathodes is also effective in the reduction of the electrode polarization resistance for O2 reduction reactions [25]. In this report, the effect of the GDC-impregnation on the microstructure and electrode performance of LSM cathodes was studied at different temperatures. The results show that high and effective oxygen ion conductivity can be developed into conventional and dominant electronic conducting LSM materials by the simple ion impregnation method and GDC-impregnated LSM electrode is a promising cathode for SOFC operating at reduced temperatures.
2. Experimental Electrolyte substrates were prepared from 8 mol% Y2O3doped ZrO2 (TZ8Y, Tosoh, Japan) by die press, followed by firing at 1500 -C for 4 h in air. The TZ8Y substrates was ¨19 mm in diameter and 1.0 mm thick. La0.72Sr0.18MnO3 (LSM) powder was synthesized by a co-precipitation technique and sintered at 900 -C. XRD showed a single perovskite phase of the powder. LSM electrode coating was applied to TZ8Y electrolytes by screen printing, that were subsequently sintered at 1150 -C for 2 h in air. Impregnation solution of 3 M 10 mol% Gd(NO3)3 + 90 mol% Ce(NO3)3 (Gd0.1Ce0.9(NO3)x ) was prepared from Aldrich chemicals of Gd(NO3)3I6H2O (99.9%) and Ce(NO3)3I6H2O (99.9%). Ion impregnation was carried out by placing a drop of the solution on top of the coating which infiltrated the cathode layer by capillary action. The sample was then heat treated at 850 -C in air for 1 h before the testing. At 850 -C the nitrate salt solutions were decomposed, forming Gd0.1Ce0.9O2 (GDC) oxide phase [22]. The impregnated oxide loading was estimated from the weight of the electrode coating before and after the impregnation treatment. The impregnated GDC loading in LSM was 0.8 mg cm 2 after one 3 M Gd0.1Ce0.9(NO3)x impregnation treatment and 5.8 mg cm 2 after six consecutive 3 M Gd0.1Ce0.9(NO3)x impregnation treatments. The typical deviation of the GDC loading by the wet impregnation was T 0.16 mg [22]. The porosity of LSM coating was ¨37% by SEM image analysis [26]. Pt paste was painted on to the opposite side of the working electrode to make the counter and reference electrodes [18]. The electrode performance of LSM with and without ion impregnation was measured in air at different temperatures, using galvanostatic polarization and
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electrochemical impedance spectroscopy (EIS). EIS was performed at open circuit, using a Solartron 1260 frequency response analyzer and a Solartron 1287 electrochemical interface. The impedance was measured in the frequency range of 0.1 Hz to 100 kHz with signal amplitude of 10 mV. The electrode interface (polarization) resistance (R E) was directly measured from the differences of low and high frequency intercepts on the impedance curves. The effect of cathodic current/polarization treatment time on the initial electrode behavior was studied under a current density of 200 mA cm 2 at 700 -C in air. The microstructure of the electrode was examined by a scanning electron microscopy (SEM, Leica S360).
3. Results and discussion Fig. 1 shows the initial impedance responses of a pure LSM electrode, a 0.8 mg cm 2 GDC-impregnated LSM electrode and a 5.8 mg cm 2 GDC-impregnated LSM electrode under a cathodic current density of 200 mA cm 2 at 700 -C in air. Before passing cathodic current, the
Fig. 1. Initial impedance responses of (a) LSM electrode, (b) 0.8 mg cm 2 GDC-impregnated LSM electrode, (c) 5.8 mg cm 2 GDC-impregnated LSM electrode as a function of cathodic current treatment time at 200 mA cm 2 and 700 -C in air.
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initial electrode behavior of the pure LSM was characterized by a very large impedance arc and the electrode polarization resistance (R E) was 72.8 V cm2. After 5 and 120 min of cathodic current treatment, R E was reduced to 48 V cm2 and 11.7 V cm2, respectively. The rapid decrease of R E (i.e., the size of the impedance arc) for the O2 reduction reactions on the freshly prepared LSM electrodes is related to the significant activation effect of the cathodic polarization treatment on the LSM electrodes [27]. Such activation effect has been attributed to the migration and removal of passivation species such as SrO2 and MnO2 enriched on the LSM electrode surface, the morphology change of the LSM electrodes and the formation of oxygen vacancies under cathodic polarization or current treatments [28 –30]. After impregnation treatment of 3 M Gd0.1Ce0.9(NO3)x nitrite solution, the LSM electrode behaved very differently. The change in the R E of the GDC-impregnated LSM electrode for the O2 reduction was considerably smaller with the cathodic current passage. For example, for the LSM electrode impregnated with 0.8 mg cm 2 GDC, R E was 7.5 V cm2 at 700 -C after 120 min of cathodic current treatment. This is very close to the initial value of 9.5 V cm2 before the cathodic current treatment (Fig. 1b). Similarly for the reaction on the 5.8 mg cm 2 GDC-impregnated LSM electrode, there was little activation effect of cathodic current treatment (Fig. 1c). Activation effect of the cathodic current passage is much less pronounced on the LSM electrodes after the ion impregnation with 3 M Gd0.1Ce0.9(NO3)x solutions. Fig. 2 shows the effect of the impregnated GDC loading on the electrode polarization resistance and activation energy of the O2 reduction reaction on the impregnated LSM electrodes. Electrode polarization resistance of the impregnated LSM electrode decreased significantly with increased GDC loading and the reduction in R E with the GDC loading follows a logarithmic function (Fig. 2a). With impregnated GDC loading of 5.8 mg cm 2, R E was 0.21 V cm2 at 700 -C, which is 56 times smaller than that of the pure LSM cathode at the same temperature. The deposition of very fine oxygen conducting (Gd,Ce)O2 particles due to the impregnation treatment of Gd0.1Ce0.9(NO3)x solutions on the LSM surface and at the LSM/YSZ interface region would effectively enhance the triple phase boundaries for the O2 reduction, leading to the significant reduction of the electrode polarization resistance of the LSM electrodes. This would be similar to the effect of YSZ and GDC phases in the LSM/YSZ and LSM/GDC composite electrodes on the promotion of the O2 reduction reaction rate [13 – 17]. On the other hand, the activation energy for O2 reduction on pure LSM and on GDC-impregnated LSM cathodes was approximately the same (Fig. 2b). For O2 reduction reaction on 3 M Gd0.1Ce0.9(NO3)x impregnated LSM cathodes, the activation energy was in the range of 143 to 151 kJ mol 1, very close to 153 kJ mol 1 observed on the pure LSM electrode.
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Fig. 2. (a) Effect of the impregnated GDC loading on the electrode polarization resistance of LSM electrodes at different temperatures and (b) activation energy of the O2 reduction reactions on GDC-impregnated LSM electrodes.
Fig. 3 shows the polarization performance of the pure and 5.8 mg cm 2 GDC-impregnated LSM electrodes measured at different temperatures in air. For the purpose of comparison, the performance of the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) electrode on GDC electrolyte measured at 700 -C in air [18] was also given in the figure. Pure LSM shows reasonable performance at high temperatures. For example at 900 -C, g for the O2 reduction at 200 mA cm 2 was 120 mV. However, as the temperature decreased to 700 -C g increased rapidly to 800 mV at the same current. This indicates that LSM electrode is not suitable cathode for the ITSOFC. However, with the GDC-impregnation treatment, the polarization performance of the LSM cathodes was significantly enhanced. At 700 -C and under a current density of 150 mA cm 2, g was 33 mV for the O2 reduction on the 5.8 mg cm 2 GDC-impregnated LSM, smaller than 105 mV obtained on the pure LSM electrode at 900 -C under the same current density. This indicates that the operational temperature for the O2 reduction reaction on the
5.8 mg cm 2 GDC-impregnated LSM is reduced by more than 200 -C as compared to that on the pure LSM electrode. Moreover, the polarization performance of the 5.8 mg cm 2 GDC-impregnated LSM was the same as that of the LSCF electrode at 700 -C [18]. The high polarization performance of the GDC-impregnated LSM electrodes clearly demonstrates the usefulness of the impregnation method in the enhancing performance of conventional LSM electrodes. Fig. 4 shows the SEM pictures of fractured cross-sections of LSM electrodes with and without GDC-impregnation after the fuel cell testing. For pure LSM electrode, LSM grains were in the range of 0.7 to 1.2 Am (Fig. 4a). After ion impregnation with 3 M Gd0.1Ce0.9(NO3)x , very fine particles were formed around LSM particles and the particle size was in the range of 100 – 200 nm (Fig. 4b). The nanosized GDC particles were deposited on the LSM surface and at the electrode and electrolyte interface region. However, the distribution of the nano-sized GDC particles appears to be discrete and does not form a continuous network at low GDC loading. With the impregnated GDC loading of 5.8 mg cm 2, the pores in the original LSM electrode were almost filled by the nano-sized GDC particles, forming a rather dense structure (Fig. 4c). The SEM image analysis showed that the porosity of the 5.8 mg cm 2 GDCimpregnated LSM electrode was less than 1%. A continuous and porous network of GDC phase was most likely formed at such high GDC loadings. The very low R E and g of the 5.8 mg cm 2 GDC-impregnated LSM electrode indicates the high electrocatalytic activity of GDC-impregnated LSM electrodes despite the dense microstructure. Fig. 5 compares the electrode polarization resistance of GDC-impregnated LSM electrodes in this study with those of the pure LSM, LSM/YSZ, LSM/GDC and selected MIEC cathodes at different temperatures reported in the literature. In general, electrode polarization resistance (R E) was
Fig. 3. Polarization curves of the pure and 5.8 mg cm 2 GDC-impregnated LSM electrodes measured at different temperatures in air. Performance of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) on GDC electrolyte measured at 700 -C in air was taken from [18].
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(a)
2µm (b)
GDC
2µm
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of LSCF and Gd0.8Sr0.2CoO3 (GSC) electrodes at temperature range of 600 –800 -C. LSCF is known to be a very good mixed ionic and electronic conductor with high electrocatalytic activity for the O2 reduction reactions [18]. The oxygen trace diffusion coefficient is ¨10 8 cm2 s 1 at 700 -C [11], which are orders of magnitude higher than that of LSM materials [11,12]. Ralph et al. [2] compared the electrode polarization resistance of different MIEC electrodes including LSCF, La0.8Sr0.2CoO3, Sm0.8Sr0.2CoO3 and Gd0.8Sr0.2CoO3. Gd0.8Sr0.2CoO3 electrode showed the lowest electrode polarization resistance at 700 -C [2]. The 5.8 mg cm 2 GDC-impregnated LSM electrodes reported in this study also show better performance than those of reported functionally graded cathodes. Xia et al. [32] studied the graded cathodes and the lowest electrode polarization resistance of 1.5 V cm2 at 700 -C was reported on the cathodes consisted of four layers of LSM/GDC, LSM/LSCF/GDC, LSCF/GDC and LSCF. Similar electrode polarization resistance (¨3.48 V cm2 at 700 -C) was also reported on a functionally graded cathode consisting of five prints of LSM/YSZ composite electrodes and five prints of La0.8Sr0.2CoO3 electrodes [33]. While the detailed mechanism of the effect of the GDC phase in the impregnated LSM structure is yet to be
(c)
GDC
YSZ electrolyte
2µm
Fig. 4. SEM pictures of fractured cross sections of (a) pure LSM electrode, (b) 0.8 mg cm 2 GDC-impregnated LSM and (c) 5.8 mg cm 2 GDCimpregnated LSM electrode after fuel cell testing.
directly measured from the impedance curves under open circuit in air. For O2 reduction reaction on pure LSM electrodes, R E varied greatly from 9 to 54 V cm2 at 700 -C [16,31]. With the addition of 50% YSZ phase in the LSM electrode, R E of the LSM/YSZ composite cathode was reduced to 2.5 V cm2 at 700 -C [14]. R E was further reduced to 1.1 V cm2 at 700 -C by substituting YSZ with high ionic conducting GDC phase in the LSM/GDC composite electrode [16]. However, for O2 reduction on the 5.8 mg cm 2 GDC-impregnated LSM electrode, R E was 0.21 V cm2 at 700 -C, significantly lower than that of both LSM/ YSZ and LSM/GDC composite cathodes [13 – 17]. Most significantly, the electrode polarization performance of the 5.8 mg cm 2 GDC-impregnated LSM is comparable to that
Fig. 5. A comparison of the electrode polarization resistance (R E) of GDC impregnated LSM electrodes in this study with those of the pure LSM, LSM/YSZ, LSM/GDC and selected MIEC electrodes reported in the literature. R E was measured by EIS in air and without specification, cathodes were tested on YSZ electrolyte. Lines are for the guide only and numbers are references cited.
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confirmed, the observed high performance of the GDCimpregnated LSM electrodes and the distribution and formation of nano-sized GDC particles in the porous LSM porous structure demonstrates that impregnation of nano-sized GDC particles has significant electrocatalytic effect on the activities of the LSM electrodes for the O2 reduction reactions in addition to the effective extension of the three phase boundary area. Oxygen ion conductivity of LSM is negligibly small (5.9 10 7 S cm 1 at 900 -C) [11]. On the other hand, CeO2-based oxides are known for their ability to store, release and transport oxygen under SOFC operating conditions due to their high ionic conductivity [21]. The oxygen ion conductivity of Sm0.2Ce0.8O2 was 0.1 S cm 1 at 800 -C [34] and for Gd-doped CeO 2 the oxygen ion conductivity was reported to be as high as 0.29 S cm 1 at 900 -C [35]. This indicates that nano-sized (Gd,Ce)O2 particles deposited in the LSM porous structure function effectively as oxygen conducting path, extending the reaction sites from the electrode/electrolyte interface region to the bulk of the LSM electrode. On the other hand, it is well known that doped CeO2 has a high surface exchange rate for oxygen. In the case of low GDC loading (e.g., 0.8 mg cm 2), the deposition of the nano-sized GDC particles appears to be isolated (Fig. 4b). However, the R E was significantly reduced despite the low GDC loading of 0.8 mg cm 2. This indicates that the existence of the nanosized GDC particles on the LSM surface would enhance the oxygen exchange rate. The comparable performance of the GDC-impregnated LSM cathode with those of MIEC electrodes is probably related to the high electrocatalytic effect and the particularly high surface areas of the impregnated nanosized ionic conducting GDC phase. Due to the formation of nano-sized GDC particles inside the porous LSM network, the surface areas of the impregnated ionic conducting phase would be much higher than that of conventional LSM and MIEC electrodes. In addition, the high surface area and electrocatalytic materials filled the pores at the critically important electrode/electrolyte interface, significantly boosting the three phase boundary areas. The dramatic improvement in the electrode performance of the GDC-impregnated LSM electrodes clearly indicates the importance of the both microstructure and mixed conductivity/electrocatalytic activity to the performance of cathodes. The results also demonstrated the feasibility of the impregnation methods in the development of cathodes with combined advantages of high electrocatalytic activity of oxygen ionic conducting phase such as GDC and the high stability and compatibility of LSM materials. This also opens an effective route to incorporate the high ionic and electronic conducting materials such as LSCF, GSC, SSC and high oxygen ion conducting oxides [2,18,36,37] into structurally stable LSM for the development of low temperature SOFC cathodes.
4. Conclusions The process and the effect of GDC ion impregnation on the electrocatalytic activity of LSM electrodes have been studied. With 5.8 mg cm 2 GDC-impregnation, the performance of impregnated LSM electrodes was significantly higher than that of LSM/YSZ composite cathodes and is compatible to those of MIEC electrodes such as LSCF and GSC. This shows that high oxygen ion conductivity can be developed into conventional and dominant electronic conducting LSM materials by the simple ion impregnation method. The advantages of the ion impregnation methods are the low temperature process and easy control of the oxygen ion conducting phase without the adverse effect of the interfacial reactions between the YSZ electrolyte and MIEC type cathodes such as cobaltites in conventional ceramic processing fabrication methods. Nevertheless, it should be pointed out here that the ion impregnation introduces additional processing and sintering steps. This is particular the case if high oxide loading is required. Future work is required to optimize the ion impregnation process and to address the long-term stability of the LSM cathodes based on the impregnated nano-sized oxide phase. References [1] S.C. Singhal, Solid State Ionics 135 (2000) 305. [2] J.M. Ralph, A.C. Schoeler, M. Krumpelt, J. Mater. Sci. 36 (2001) 1161. [3] B.C.H. Steele, Nature 414 (2001) 345. [4] C. Xia, F. Chen, M. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. [5] C. Wang, W.L. Worrell, S. Park, J.M. Vohs, R.J. Gorte, J. Electrochem. Soc. 148 (2001) A864. [6] Y.J. Leng, S.H. Chan, K.A. Khor, S.P. Jiang, P. Cheang, J. Power Sources 117 (2003) 26. [7] S.P. Jiang, S.P.S. Badwal, Solid State Ionics 123 (1999) 209. [8] S.P. Jiang, J.G. Love, Y. Ramprakash, J. Power Sources 110 (2002) 201. [9] J. Fleig, Annu. Rev. Mater. Res. 33 (2003) 361. [10] S.P. Jiang, J. Power Sources 124 (2003) 390. [11] S. Carter, A. Selcuk, R.J. Chater, J. Kajda, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53 – 56 (1992) 597. [12] R.A. De Souza, J.A. Kilner, J.F. Walker, Mater. Lett. 43 (2000) 43. [13] J.-D. Kim, G.-D. Kim, J.-W. Moon, Y.-i. Park, W.-H. Lee, K. Kobayashi, M. Nagai, C.-E. Kim, Solid State Ionics 143 (2001) 379. [14] E.P. Murray, T. Tsai, S.A. Barnett, Solid State Ionics 110 (1998) 235. [15] J.H. Choi, J.H. Jang, S.M. Oh, Electrochim. Acta 46 (2001) 867. [16] E.P. Murray, S.A. Barnett, Solid State Ionics 143 (2001) 265. [17] Y.J. Leng, S.H. Chan, K.A. Khor, S.P. Jiang, J. Appl. Electrochem. 34 (2004) 409. [18] S.P. Jiang, Solid State Ionics 146 (2002) 1. [19] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [20] H. Uchida, S.-I. Arisaka, M. Watanabe, Electrochem. Solid-State Lett. 2 (1999) 428. [21] M. Mogensen, N.M. Sammes, G.A. Tompsett, Solid State Ionics 129 (2000) 63. [22] S.P. Jiang, Y.Y. Duan, J.G. Love, J. Electrochem. Soc. 149 (2002) A1175. [23] S.P. Jiang, S. Zhang, Y.D. Zhen, A.P. Koh, Electrochem. Solid-State Lett. 7 (2004) A282.
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[32] C. Xia, W. Rauch, W. Wellborn, M. Liu, Electrochem. Solid-State Lett. 5 (2002) A217. [33] T. Hart, N.P. Brandon, M.J. Day, J.E. Shemilt, J. Mater. Sci. 36 (2001) 1077. [34] K. Eguchi, J. Alloys Compd. 250 (1997) 486. [35] D.K. Hohnke, Solid State Ionics 5 (1981) 531. [36] M. Koyama, C.-J. Wen, T. Masuyama, J. Otomo, H. Fukunaga, K. Yamada, K. Eguchi, H. Takahashi, J. Electrochem. Soc. 148 (2001) A795. [37] S.J. Skinner, J.A. Kilner, Mater. Today (2003 March) 30.
Novel structured mixed ionic and electronic conducting cathodes of solid oxide fuel cells S.P. Jiang*, W. Wang School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Ave., Singapore 639798, Singapore Received 16 April 2004; received in revised form 30 December 2004; accepted 30 March 2005
Abstract A novel structured mixed ionic and electronic conducting cathode was developed based on the (LaSr)MnO3 (LSM) electronic conducting structure impregnated with Gd-doped CeO2 (GDC) ionic conducting phase. The ion impregnation of oxygen ion conducting GDC phase significantly enhances the electrocatalytic activity of the LSM electrodes for the O2 reduction reactions and eliminates the activation effects of the cathodic polarization associated with pure LSM. With impregnation of 5.8 mg cm 2 GDC in the LSM electrode coating, the electrode showed much lower electrode polarization resistance (R E) as compared to those of LSM/ Y2O3 – ZrO2 and LSM/GDC composite cathodes and the performance was comparable with those of mixed ionic and electronic conducting oxides such as (LaSr)(CoFe)O3 and (GdSr)CoO3. This is most likely due to the unique combination of the high electronic conducting LSM porous structure with high ionic conducting nano-sized GDC phase. The results demonstrated that high oxygen ion conductivity can be developed into conventional and dominant electronic conducting LSM materials by the simple ion impregnation method. D 2005 Elsevier B.V. All rights reserved. Keywords: LSM electrodes; Gd-doped ceria; Ion impregnation; MIEC; O2 reduction
1. Introduction Air pollution is a major global problem threatening modern civilization which is partly attributed to an ever increasing consumption of fossil fuels. The solid oxide fuel cell (SOFC) is an all solid energy conversion device with high efficiency and very low greenhouse emission. Thus the use of SOFC technologies can significantly reduce the production of the greenhouse gases and improve our environment. However, the cost of materials and fabrication processes for the SOFC system is still considered too high, thus inhibiting the commercial viability of this clean and environmental friendly technology. One solution is to lower the operating temperature of SOFC from the traditional 1000 -C to 600 – 700 -C. The low SOFC operating
* Corresponding author. Tel.: +65 6790 5010; fax: +65 6791 1859. E-mail address: [email protected] (S.P. Jiang). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.03.011
temperature can significantly widen the selection of materials and enable the use of much less expensive materials for the SOFC components such as electrodes, interconnect and manifolds [1– 3]. Additional benefits of reduced operating temperature include the significantly improved stability of materials and reliability of the SOFC system. The increased material selectivity and flexibility may also reduce fabrication costs. However, the overall electrochemical performance of a SOFC system will decrease with a reduced operating temperature. This is mainly due to the increased polarization resistances for the electrode reactions and the decreased electrolyte conductivity at low temperatures. The resistive contribution of the electrolyte can be reduced by decreasing the thickness of the electrolyte (i.e., 10 – 30 Am) on anodesupported structure [4 – 6]. However, reducing the electrolyte thickness while maintaining the mechanical and structure integrity of the thin film cells is a challenge for processing technologies. Due to the substantial reduction in
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the electrolyte thickness, the overall performance loss is increasingly dominated by the polarization losses of the electrode reactions of H2 oxidation on the anode and O2 reduction on the cathode. In comparison with the relatively fast H2 oxidation reactions, polarization losses for the O2 reduction process are particularly high due to the relatively high activation energy of O2 reduction and relatively slow reaction rates [7– 9]. Thus, development of cathode materials with high electrocatalytic activity and high stability is critical for the intermediate temperature SOFC or ITSOFC operating at 600 – 700 -C. Sr-doped LaMnO3 (LSM) has been extensively investigated and developed as a cathode material for high temperature SOFC due to its relatively high electrochemical activity for the O2 reduction reaction and good stability and compatibility with Y2O3 – ZrO2 (YSZ) electrolyte at SOFC operation temperatures [10]. Unfortunately, the oxygen ion conductivity of LSM materials is extremely low and its oxygen trace diffusion coefficient is in the range of 10 15 –10 16 cm2 s 1 at 700 -C [11,12]. The negligible oxygen transport ability would limit the O2 reduction reactions at or near the triple phase boundaries (tpb) between electrode, electrolyte and O2 gas. This poses practical limitations and restrictions on the application of LSM-based systems as potential cathodes for ITSOFC. The electrocatalytic activity of LSM can be improved by adding oxygen ion conducting phases such as YSZ and Gddoped CeO2 (GDC) to LSM electrodes [13 – 17]. Murray and Barnett [14,16] studied the electrode performance of LSM/YSZ and LSM/GDC composite cathodes and the lowest electrode polarization resistance was 0.49 V cm2 at 750 -C for the reaction on the LSM/GDC composite cathodes. This is considerably lower than 3.5 V cm2 measured on the pure LSM cathode at the same temperature. Another approach is to use mixed ionic and electronic conducting (MIEC) oxides such as (La,Sr)CoO3 (LSC), (La,Sr)(Co,Fe)O3 (LSCF) and (SmSr)CoO3 (SSC) oxides as cathodes [2,4,18]. However, MIEC materials based on cobaltites react easily with YSZ electrolyte to form resistive La2Zr2O7 and SrZrO3 phases at the cathode/YSZ electrolyte interface [19]. Thus, a thin protective layer such as doped CeO2 will be needed to inhibit the formation of resistive phases between YSZ electrolyte and cobaltite-based cathodes [20]. The introduction of additional layer increases the complexity of the cell structure and the fabrication cost. Other electrolytes such as doped CeO2 are chemically inert to cobaltite-based MIEC materials and also have higher oxygen ion conductivity than that of YSZ electrolytes below 700 -C. However, the doped CeO2 develops significant electronic conductivity under fuel reducing conditions and it expands on reduction [21]. This reduces the open circuit potential and decreases the cell efficiency. We have shown recently that ion impregnation is an effective method to improve the electrode electrochemical properties of Ni and Ni/YSZ cermet anodes [22,23]. By impregnation of electrocatalytic oxide particles such as Sm-
doped CeO2 (SDC) into the cermet structure, overpotential (g) for the H2 oxidation reaction was 72 mV at 250 mA cm 2 and 800 -C, which is close to that measured at 1000 -C for the reaction on the anode without impregnation. Watanabe et al. [24] used impregnation method to incorporate metal catalysts such as Pt and Ru to Sm-doped CeO2 anodes and Sr-doped LaMnO3 cathodes, leading to the significant reduction of the polarization losses of the electrodes. Recent results showed that impregnation of small amount of Gd-doped CeO2 (GDC) into LSM cathodes is also effective in the reduction of the electrode polarization resistance for O2 reduction reactions [25]. In this report, the effect of the GDC-impregnation on the microstructure and electrode performance of LSM cathodes was studied at different temperatures. The results show that high and effective oxygen ion conductivity can be developed into conventional and dominant electronic conducting LSM materials by the simple ion impregnation method and GDC-impregnated LSM electrode is a promising cathode for SOFC operating at reduced temperatures.
2. Experimental Electrolyte substrates were prepared from 8 mol% Y2O3doped ZrO2 (TZ8Y, Tosoh, Japan) by die press, followed by firing at 1500 -C for 4 h in air. The TZ8Y substrates was ¨19 mm in diameter and 1.0 mm thick. La0.72Sr0.18MnO3 (LSM) powder was synthesized by a co-precipitation technique and sintered at 900 -C. XRD showed a single perovskite phase of the powder. LSM electrode coating was applied to TZ8Y electrolytes by screen printing, that were subsequently sintered at 1150 -C for 2 h in air. Impregnation solution of 3 M 10 mol% Gd(NO3)3 + 90 mol% Ce(NO3)3 (Gd0.1Ce0.9(NO3)x ) was prepared from Aldrich chemicals of Gd(NO3)3I6H2O (99.9%) and Ce(NO3)3I6H2O (99.9%). Ion impregnation was carried out by placing a drop of the solution on top of the coating which infiltrated the cathode layer by capillary action. The sample was then heat treated at 850 -C in air for 1 h before the testing. At 850 -C the nitrate salt solutions were decomposed, forming Gd0.1Ce0.9O2 (GDC) oxide phase [22]. The impregnated oxide loading was estimated from the weight of the electrode coating before and after the impregnation treatment. The impregnated GDC loading in LSM was 0.8 mg cm 2 after one 3 M Gd0.1Ce0.9(NO3)x impregnation treatment and 5.8 mg cm 2 after six consecutive 3 M Gd0.1Ce0.9(NO3)x impregnation treatments. The typical deviation of the GDC loading by the wet impregnation was T 0.16 mg [22]. The porosity of LSM coating was ¨37% by SEM image analysis [26]. Pt paste was painted on to the opposite side of the working electrode to make the counter and reference electrodes [18]. The electrode performance of LSM with and without ion impregnation was measured in air at different temperatures, using galvanostatic polarization and
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electrochemical impedance spectroscopy (EIS). EIS was performed at open circuit, using a Solartron 1260 frequency response analyzer and a Solartron 1287 electrochemical interface. The impedance was measured in the frequency range of 0.1 Hz to 100 kHz with signal amplitude of 10 mV. The electrode interface (polarization) resistance (R E) was directly measured from the differences of low and high frequency intercepts on the impedance curves. The effect of cathodic current/polarization treatment time on the initial electrode behavior was studied under a current density of 200 mA cm 2 at 700 -C in air. The microstructure of the electrode was examined by a scanning electron microscopy (SEM, Leica S360).
3. Results and discussion Fig. 1 shows the initial impedance responses of a pure LSM electrode, a 0.8 mg cm 2 GDC-impregnated LSM electrode and a 5.8 mg cm 2 GDC-impregnated LSM electrode under a cathodic current density of 200 mA cm 2 at 700 -C in air. Before passing cathodic current, the
Fig. 1. Initial impedance responses of (a) LSM electrode, (b) 0.8 mg cm 2 GDC-impregnated LSM electrode, (c) 5.8 mg cm 2 GDC-impregnated LSM electrode as a function of cathodic current treatment time at 200 mA cm 2 and 700 -C in air.
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initial electrode behavior of the pure LSM was characterized by a very large impedance arc and the electrode polarization resistance (R E) was 72.8 V cm2. After 5 and 120 min of cathodic current treatment, R E was reduced to 48 V cm2 and 11.7 V cm2, respectively. The rapid decrease of R E (i.e., the size of the impedance arc) for the O2 reduction reactions on the freshly prepared LSM electrodes is related to the significant activation effect of the cathodic polarization treatment on the LSM electrodes [27]. Such activation effect has been attributed to the migration and removal of passivation species such as SrO2 and MnO2 enriched on the LSM electrode surface, the morphology change of the LSM electrodes and the formation of oxygen vacancies under cathodic polarization or current treatments [28 –30]. After impregnation treatment of 3 M Gd0.1Ce0.9(NO3)x nitrite solution, the LSM electrode behaved very differently. The change in the R E of the GDC-impregnated LSM electrode for the O2 reduction was considerably smaller with the cathodic current passage. For example, for the LSM electrode impregnated with 0.8 mg cm 2 GDC, R E was 7.5 V cm2 at 700 -C after 120 min of cathodic current treatment. This is very close to the initial value of 9.5 V cm2 before the cathodic current treatment (Fig. 1b). Similarly for the reaction on the 5.8 mg cm 2 GDC-impregnated LSM electrode, there was little activation effect of cathodic current treatment (Fig. 1c). Activation effect of the cathodic current passage is much less pronounced on the LSM electrodes after the ion impregnation with 3 M Gd0.1Ce0.9(NO3)x solutions. Fig. 2 shows the effect of the impregnated GDC loading on the electrode polarization resistance and activation energy of the O2 reduction reaction on the impregnated LSM electrodes. Electrode polarization resistance of the impregnated LSM electrode decreased significantly with increased GDC loading and the reduction in R E with the GDC loading follows a logarithmic function (Fig. 2a). With impregnated GDC loading of 5.8 mg cm 2, R E was 0.21 V cm2 at 700 -C, which is 56 times smaller than that of the pure LSM cathode at the same temperature. The deposition of very fine oxygen conducting (Gd,Ce)O2 particles due to the impregnation treatment of Gd0.1Ce0.9(NO3)x solutions on the LSM surface and at the LSM/YSZ interface region would effectively enhance the triple phase boundaries for the O2 reduction, leading to the significant reduction of the electrode polarization resistance of the LSM electrodes. This would be similar to the effect of YSZ and GDC phases in the LSM/YSZ and LSM/GDC composite electrodes on the promotion of the O2 reduction reaction rate [13 – 17]. On the other hand, the activation energy for O2 reduction on pure LSM and on GDC-impregnated LSM cathodes was approximately the same (Fig. 2b). For O2 reduction reaction on 3 M Gd0.1Ce0.9(NO3)x impregnated LSM cathodes, the activation energy was in the range of 143 to 151 kJ mol 1, very close to 153 kJ mol 1 observed on the pure LSM electrode.
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Fig. 2. (a) Effect of the impregnated GDC loading on the electrode polarization resistance of LSM electrodes at different temperatures and (b) activation energy of the O2 reduction reactions on GDC-impregnated LSM electrodes.
Fig. 3 shows the polarization performance of the pure and 5.8 mg cm 2 GDC-impregnated LSM electrodes measured at different temperatures in air. For the purpose of comparison, the performance of the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) electrode on GDC electrolyte measured at 700 -C in air [18] was also given in the figure. Pure LSM shows reasonable performance at high temperatures. For example at 900 -C, g for the O2 reduction at 200 mA cm 2 was 120 mV. However, as the temperature decreased to 700 -C g increased rapidly to 800 mV at the same current. This indicates that LSM electrode is not suitable cathode for the ITSOFC. However, with the GDC-impregnation treatment, the polarization performance of the LSM cathodes was significantly enhanced. At 700 -C and under a current density of 150 mA cm 2, g was 33 mV for the O2 reduction on the 5.8 mg cm 2 GDC-impregnated LSM, smaller than 105 mV obtained on the pure LSM electrode at 900 -C under the same current density. This indicates that the operational temperature for the O2 reduction reaction on the
5.8 mg cm 2 GDC-impregnated LSM is reduced by more than 200 -C as compared to that on the pure LSM electrode. Moreover, the polarization performance of the 5.8 mg cm 2 GDC-impregnated LSM was the same as that of the LSCF electrode at 700 -C [18]. The high polarization performance of the GDC-impregnated LSM electrodes clearly demonstrates the usefulness of the impregnation method in the enhancing performance of conventional LSM electrodes. Fig. 4 shows the SEM pictures of fractured cross-sections of LSM electrodes with and without GDC-impregnation after the fuel cell testing. For pure LSM electrode, LSM grains were in the range of 0.7 to 1.2 Am (Fig. 4a). After ion impregnation with 3 M Gd0.1Ce0.9(NO3)x , very fine particles were formed around LSM particles and the particle size was in the range of 100 – 200 nm (Fig. 4b). The nanosized GDC particles were deposited on the LSM surface and at the electrode and electrolyte interface region. However, the distribution of the nano-sized GDC particles appears to be discrete and does not form a continuous network at low GDC loading. With the impregnated GDC loading of 5.8 mg cm 2, the pores in the original LSM electrode were almost filled by the nano-sized GDC particles, forming a rather dense structure (Fig. 4c). The SEM image analysis showed that the porosity of the 5.8 mg cm 2 GDCimpregnated LSM electrode was less than 1%. A continuous and porous network of GDC phase was most likely formed at such high GDC loadings. The very low R E and g of the 5.8 mg cm 2 GDC-impregnated LSM electrode indicates the high electrocatalytic activity of GDC-impregnated LSM electrodes despite the dense microstructure. Fig. 5 compares the electrode polarization resistance of GDC-impregnated LSM electrodes in this study with those of the pure LSM, LSM/YSZ, LSM/GDC and selected MIEC cathodes at different temperatures reported in the literature. In general, electrode polarization resistance (R E) was
Fig. 3. Polarization curves of the pure and 5.8 mg cm 2 GDC-impregnated LSM electrodes measured at different temperatures in air. Performance of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) on GDC electrolyte measured at 700 -C in air was taken from [18].
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(a)
2µm (b)
GDC
2µm
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of LSCF and Gd0.8Sr0.2CoO3 (GSC) electrodes at temperature range of 600 –800 -C. LSCF is known to be a very good mixed ionic and electronic conductor with high electrocatalytic activity for the O2 reduction reactions [18]. The oxygen trace diffusion coefficient is ¨10 8 cm2 s 1 at 700 -C [11], which are orders of magnitude higher than that of LSM materials [11,12]. Ralph et al. [2] compared the electrode polarization resistance of different MIEC electrodes including LSCF, La0.8Sr0.2CoO3, Sm0.8Sr0.2CoO3 and Gd0.8Sr0.2CoO3. Gd0.8Sr0.2CoO3 electrode showed the lowest electrode polarization resistance at 700 -C [2]. The 5.8 mg cm 2 GDC-impregnated LSM electrodes reported in this study also show better performance than those of reported functionally graded cathodes. Xia et al. [32] studied the graded cathodes and the lowest electrode polarization resistance of 1.5 V cm2 at 700 -C was reported on the cathodes consisted of four layers of LSM/GDC, LSM/LSCF/GDC, LSCF/GDC and LSCF. Similar electrode polarization resistance (¨3.48 V cm2 at 700 -C) was also reported on a functionally graded cathode consisting of five prints of LSM/YSZ composite electrodes and five prints of La0.8Sr0.2CoO3 electrodes [33]. While the detailed mechanism of the effect of the GDC phase in the impregnated LSM structure is yet to be
(c)
GDC
YSZ electrolyte
2µm
Fig. 4. SEM pictures of fractured cross sections of (a) pure LSM electrode, (b) 0.8 mg cm 2 GDC-impregnated LSM and (c) 5.8 mg cm 2 GDCimpregnated LSM electrode after fuel cell testing.
directly measured from the impedance curves under open circuit in air. For O2 reduction reaction on pure LSM electrodes, R E varied greatly from 9 to 54 V cm2 at 700 -C [16,31]. With the addition of 50% YSZ phase in the LSM electrode, R E of the LSM/YSZ composite cathode was reduced to 2.5 V cm2 at 700 -C [14]. R E was further reduced to 1.1 V cm2 at 700 -C by substituting YSZ with high ionic conducting GDC phase in the LSM/GDC composite electrode [16]. However, for O2 reduction on the 5.8 mg cm 2 GDC-impregnated LSM electrode, R E was 0.21 V cm2 at 700 -C, significantly lower than that of both LSM/ YSZ and LSM/GDC composite cathodes [13 – 17]. Most significantly, the electrode polarization performance of the 5.8 mg cm 2 GDC-impregnated LSM is comparable to that
Fig. 5. A comparison of the electrode polarization resistance (R E) of GDC impregnated LSM electrodes in this study with those of the pure LSM, LSM/YSZ, LSM/GDC and selected MIEC electrodes reported in the literature. R E was measured by EIS in air and without specification, cathodes were tested on YSZ electrolyte. Lines are for the guide only and numbers are references cited.
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confirmed, the observed high performance of the GDCimpregnated LSM electrodes and the distribution and formation of nano-sized GDC particles in the porous LSM porous structure demonstrates that impregnation of nano-sized GDC particles has significant electrocatalytic effect on the activities of the LSM electrodes for the O2 reduction reactions in addition to the effective extension of the three phase boundary area. Oxygen ion conductivity of LSM is negligibly small (5.9 10 7 S cm 1 at 900 -C) [11]. On the other hand, CeO2-based oxides are known for their ability to store, release and transport oxygen under SOFC operating conditions due to their high ionic conductivity [21]. The oxygen ion conductivity of Sm0.2Ce0.8O2 was 0.1 S cm 1 at 800 -C [34] and for Gd-doped CeO 2 the oxygen ion conductivity was reported to be as high as 0.29 S cm 1 at 900 -C [35]. This indicates that nano-sized (Gd,Ce)O2 particles deposited in the LSM porous structure function effectively as oxygen conducting path, extending the reaction sites from the electrode/electrolyte interface region to the bulk of the LSM electrode. On the other hand, it is well known that doped CeO2 has a high surface exchange rate for oxygen. In the case of low GDC loading (e.g., 0.8 mg cm 2), the deposition of the nano-sized GDC particles appears to be isolated (Fig. 4b). However, the R E was significantly reduced despite the low GDC loading of 0.8 mg cm 2. This indicates that the existence of the nanosized GDC particles on the LSM surface would enhance the oxygen exchange rate. The comparable performance of the GDC-impregnated LSM cathode with those of MIEC electrodes is probably related to the high electrocatalytic effect and the particularly high surface areas of the impregnated nanosized ionic conducting GDC phase. Due to the formation of nano-sized GDC particles inside the porous LSM network, the surface areas of the impregnated ionic conducting phase would be much higher than that of conventional LSM and MIEC electrodes. In addition, the high surface area and electrocatalytic materials filled the pores at the critically important electrode/electrolyte interface, significantly boosting the three phase boundary areas. The dramatic improvement in the electrode performance of the GDC-impregnated LSM electrodes clearly indicates the importance of the both microstructure and mixed conductivity/electrocatalytic activity to the performance of cathodes. The results also demonstrated the feasibility of the impregnation methods in the development of cathodes with combined advantages of high electrocatalytic activity of oxygen ionic conducting phase such as GDC and the high stability and compatibility of LSM materials. This also opens an effective route to incorporate the high ionic and electronic conducting materials such as LSCF, GSC, SSC and high oxygen ion conducting oxides [2,18,36,37] into structurally stable LSM for the development of low temperature SOFC cathodes.
4. Conclusions The process and the effect of GDC ion impregnation on the electrocatalytic activity of LSM electrodes have been studied. With 5.8 mg cm 2 GDC-impregnation, the performance of impregnated LSM electrodes was significantly higher than that of LSM/YSZ composite cathodes and is compatible to those of MIEC electrodes such as LSCF and GSC. This shows that high oxygen ion conductivity can be developed into conventional and dominant electronic conducting LSM materials by the simple ion impregnation method. The advantages of the ion impregnation methods are the low temperature process and easy control of the oxygen ion conducting phase without the adverse effect of the interfacial reactions between the YSZ electrolyte and MIEC type cathodes such as cobaltites in conventional ceramic processing fabrication methods. Nevertheless, it should be pointed out here that the ion impregnation introduces additional processing and sintering steps. This is particular the case if high oxide loading is required. Future work is required to optimize the ion impregnation process and to address the long-term stability of the LSM cathodes based on the impregnated nano-sized oxide phase. References [1] S.C. Singhal, Solid State Ionics 135 (2000) 305. [2] J.M. Ralph, A.C. Schoeler, M. Krumpelt, J. Mater. Sci. 36 (2001) 1161. [3] B.C.H. Steele, Nature 414 (2001) 345. [4] C. Xia, F. Chen, M. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. [5] C. Wang, W.L. Worrell, S. Park, J.M. Vohs, R.J. Gorte, J. Electrochem. Soc. 148 (2001) A864. [6] Y.J. Leng, S.H. Chan, K.A. Khor, S.P. Jiang, P. Cheang, J. Power Sources 117 (2003) 26. [7] S.P. Jiang, S.P.S. Badwal, Solid State Ionics 123 (1999) 209. [8] S.P. Jiang, J.G. Love, Y. Ramprakash, J. Power Sources 110 (2002) 201. [9] J. Fleig, Annu. Rev. Mater. Res. 33 (2003) 361. [10] S.P. Jiang, J. Power Sources 124 (2003) 390. [11] S. Carter, A. Selcuk, R.J. Chater, J. Kajda, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53 – 56 (1992) 597. [12] R.A. De Souza, J.A. Kilner, J.F. Walker, Mater. Lett. 43 (2000) 43. [13] J.-D. Kim, G.-D. Kim, J.-W. Moon, Y.-i. Park, W.-H. Lee, K. Kobayashi, M. Nagai, C.-E. Kim, Solid State Ionics 143 (2001) 379. [14] E.P. Murray, T. Tsai, S.A. Barnett, Solid State Ionics 110 (1998) 235. [15] J.H. Choi, J.H. Jang, S.M. Oh, Electrochim. Acta 46 (2001) 867. [16] E.P. Murray, S.A. Barnett, Solid State Ionics 143 (2001) 265. [17] Y.J. Leng, S.H. Chan, K.A. Khor, S.P. Jiang, J. Appl. Electrochem. 34 (2004) 409. [18] S.P. Jiang, Solid State Ionics 146 (2002) 1. [19] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [20] H. Uchida, S.-I. Arisaka, M. Watanabe, Electrochem. Solid-State Lett. 2 (1999) 428. [21] M. Mogensen, N.M. Sammes, G.A. Tompsett, Solid State Ionics 129 (2000) 63. [22] S.P. Jiang, Y.Y. Duan, J.G. Love, J. Electrochem. Soc. 149 (2002) A1175. [23] S.P. Jiang, S. Zhang, Y.D. Zhen, A.P. Koh, Electrochem. Solid-State Lett. 7 (2004) A282.
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