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(La,Sr)MnO3–(Y,Bi)2O3 composite cathodes for intermediate-temperature solid oxide fuel cells
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(La,Sr)MnO3e(Y,Bi)2O3 composite cathodes for intermediate-temperature solid oxide fuel cells Liuer Wu a,b, Zhiyi Jiang a,b, Shaorong Wang a,c, Changrong Xia a,b,* a
CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, China b Department of Materials Science and Engineering, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, China c Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
article info
abstract
Article history:
Yttria-stabilized bismuth oxides (YSB) are cooperated to (La,Sr)MnO3 (LSM) to form
Received 22 October 2012
composite cathodes for intermediate-temperature solid oxide fuel cells. The composite
Received in revised form
electrodes are fabricated with screen-printing technique and characterized using electro-
20 November 2012
chemical impedance spectroscopy. The interfacial polarization resistances (Rp) of the LSMe
Accepted 22 November 2012
YSB electrodes on yttria-stabilized zirconia (YSZ), samaria-doped ceria (SDC), and YSB
Available online 3 January 2013
electrolytes are analyzed regarding the electrode composition and operating temperature.
Keywords:
When YSZ is used as the electrolyte, the lowest Rp is 0.14 U cm2 at 700 C, which is only
Solid oxide fuel cells
1.8% of that for a pure LSM electrode, 5.6% of that reported for LSMeYSZ composites, and
Rp decreases with the increase of YSB content up to 80 wt.% in the LSMeYSB composite.
Composite cathode
13.2% of that for reported LSMeGDC (gadolinia-doped ceria) electrodes, demonstrating that
Yttria-stabilized bismuth oxides
YSB is very effective to enhance the performance of LSM-based cathodes. The electrode
(La,Sr)MnO3
performance is also affected by the electrolyte substrate. LSM electrodes without any YSB exhibit obviously different performance on YSZ, SDC and YSB electrolytes. However, when YSB is cooperated, Rp on different electrolytes tends to become equivalent, especially for electrodes with high YSB content. Further analysis shows that their electrochemical performance is contributed dominantly from the electrode bulk whereas the contribution from the electrode/electrolyte interface is negligible, suggesting weak electrolyte effect on the performance of LSMeYSB composite electrodes. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
(La,Sr)MnO3 perovskite is the classical cathode material for high-temperature solid oxide fuel cells (SOFCs) because of its high electrical conductivity, high thermal and chemical stability, and relatively good compatibility with
yttria-stabilized zirconia (YSZ) and doped ceria (DCO) electrolytes. Unfortunately, its low ionic conductivity and high activation energy for electrochemical reduction of oxygen result in very poor cathodic performance in the intermediate temperature (600e800 C) region [1,2]. One approach to improve its electrocatalytic activity is to add an ionically
* Corresponding author. CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, China. Tel.: þ86 551 3607475; fax: þ86 551 3601592. E-mail address: [email protected] (C. Xia). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.111
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conducting second phase such as YSZ and DCO to form a composite electrode [3e8]. For the pure LSM cathode, triple phase boundaries (TPBs), where oxygen reduction occurs, are located only at the electrode/electrolyte physical interface. The composite cathode provides a high density of TPBs by expanding the reaction sites into the electrode bulk at distance up to tens of micrometers from the electrode/electrolyte interface since oxygen ion conducting phases have the ability to transport O2, leading to improved electrode performance. In general, the performance depends on the ionic conductivity of the second phase; high conductivity results in high electrode activity. For example, Murray et al. have reported that the interfacial polarization resistance (Rp) at 700 C has decreased from 7.82 U cm2 to 2.49 U cm2 when YSZ is used as the second phase by mechanical mixing, and further to 1.06 U cm2 when gadolinia-doped ceria (GDC) is applied [6]. The LSMeGDC composites have shown better performance than the LSMeYSZ systems mainly due to the higher ionic conductivity of GDC compared with YSZ. Among the fluoritestructured oxygen ion conductors including YSZ and GDC, stabilized bismuth oxides have the highest ionic conductivities. However, these oxides have not been investigated as the ionic conducting components for LSM-based composites. It is noted that composites consisting of yttria-stabilized bismuth oxides (YSB) and Ag have been reported as the cathodes for YSZ-based SOFCs [9]. In addition, YSB nanoparticles are impregnated onto LSM backbones to form nano-structured electrodes for YSZ and samaria-doped ceria (SDC) electrolytes [10,11]. These cathodes have shown relatively high electrochemical performance, suggesting that it is possible to enhance LSM performance by cooperating bismuth oxides to form composite cathodes. In this work, YSB is added to form LSMeYSB composites. Their electrochemical performance is investigated on various electrolytes including YSZ, SDC, and YSB. The performance is further analyzed regarding different electrolytes to show the electrolyte effect on the electrode performance.
2.
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form carbonate precipitants, which were subsequently collected, dried, and calcined at 600 C in air for 2 h to yield cubic structure SDC powders. The phase structure of all the synthesized powders was confirmed by X-ray diffraction (XRD, D/Maxra) equipped with Cu Ka radiation. YSZ powders were from Farmeiya Advanced Materials Co., China. Symmetric cells with LSM/YSB electrodes on YSZ, SDC, and YSB electrolytes were prepared for electrochemical measurement. YSZ powders were cold pressed into discs and sintered at 1500 C for 5 h to form YSZ pellets with 13 mm in diameter and 0.7 mm in thickness. The SDC electrolytes were prepared using the procedure similar to YSZ and were sintered at 1400 C for 5 h. The YSB electrolytes were also prepared by the pressingefiring technique, which were sintered at 900 C for 5 h. LSMeYSB cathodes were prepared using screen-printing technique [4]. LSMeYSB slurry was prepared by mechanically mixing LSM and YSB powders with organic additives such as terpilenol and ground with agate mortar. The mixed LSMeYSB powders contained 20, 30, 40, 50, 60, 80, and 90 wt.% YSB. A pure LSM slurry was also prepared. Each slurry was then screen-printed onto both sides of the electrolyte substrates followed by drying and firing at 850 C for 2 h. This firing temperature resulted in the lowest interfacial polarization resistance among various temperatures including 800, 850, and 900 C. The electrodes were named to show their oxide composition, for example, an LSMe20YSB electrode consisted of 80 wt.% LSM and 20 wt.% YSB. The electrode performance was determined with electrochemical impedance spectroscopy (EIS). The measurement was conducted in ambient air under open circle conditions over a temperature range of 500e750 C in the frequency range of 3 MHz to 0.1 Hz with an AC signal amplitude of 10 mV using a Zahner Zennium electrochemical station. ZSimpWin (PerkinElmer Instruments) software was used to analyze the impedance to obtain equivalent circuits. Microscopic features were characterized by scanning electron microscopy (SEM, JSM-6700F).
3.
Results and discussion
3.1.
Microstructure characterization
Experimental
The materials involved in this work include (La0.85Sr0.15)0.9MnO3d (LSM), Y0.5Bi1.5O3 (YSB), Sm0.2Ce0.8O1.9 (SDC), and (Y2O3)0.08(ZrO2)0.92 (YSZ). LSM and YSB powders were prepared using glycine-nitrate method as previously reported [10] while SDC powder was synthesized using carbonate co-precipitation route [12]. All the starting chemicals were of analytical grade and from Sinopharm Chemical Reagent Co., Ltd. LSM powders were synthesized using La(NO3)3, Sr(NO3)2, and Mn(NO3)2 as the starting chemicals. The glycine-nitrate combusted powders were heated at 900 C for 2 h to form perovskite structure LSM. YSB powders were prepared with Y(NO3)3 and Bi(NO3)3 as the precursors followed by firing the combusted powder at 600 C for 2 h. SDC powders were fabricated with Ce(NO3)3 and Sm(NO3)3 precursors, which were dissolved in distilled water to form a mixed solution with a cation concentration of 0.1 mol L1. The solution was then slowly dropped into a 0.1 mol L1 ammonium carbonate solution at room temperature under mild stirring to
Fig. 1(a) shows the SEM image of the LSM electrode on YSZ electrolyte. The LSM electrode has a porous structure with thickness of about 30 mm. Because the symmetric cells are fabricated via identical procedure, the thickness of the electrode is almost the same for each electrode. Fig. 1(b) and (c) demonstrates that LSM/YSZ interface has very similar microstructure to LSM/SDC. But LSM/YSB interface is quite different; the electrode/electrolyte bonding is much stronger for LSM/YSB interface, due to the sintering temperature, 850 C, which is relatively low for the LSM/YSZ and LSM/SDC systems but high enough for LSM/YSB since YSB melts at about 960 C [13].
3.2.
LSMeYSB composite on YSZ electrolyte
An equivalent circuit is applied to evaluate impedance spectra, Fig. 2. In the circuit, L is the inductance arising from
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Fig. 1 e Cross-sectional images of (a) the LSM electrode on YSZ electrolyte, and the microstructures of the interface between the LSM electrode and (b) YSZ, (c) SDC, and (d) YSB electrolyte.
the apparatus, Ro is attributed to the resistance of the electrolyte and the lead wires, and (RCPE) in series represents the arcs presented at different frequency ranges, where R is the corresponding resistance and CPE (noted as Q) is the corresponding constant phase element. The expression for Q is Q ¼ Y0 ( jw)n, where Y0 is the admittance, w is the angular frequency, and n is an exponent (0 < n < 1). The pseudo capacitance C for each arc is calculated by C ¼ R1/n1Q1/n. The spectrum usually consists of two arcs, RHCPEH and RLCPEL corresponding to high and low frequency arc, respectively. An additional low frequency arc ðR0L ; CPE0L Þ is identified on some impedance spectra. The equivalent circuit is often used to characterize LSM-based electrodes [10]. Fig. 3 shows the Nyquist plots measured at 700 C for electrodes with various YSB loading. The electrodes are supported on YSZ electrolytes. Due to the scattering of impedance spectra, the fitting for cathodes with YSB concentration higher than 60 wt.% is not conducted. The spectrum for the LSM electrode without any YSB consists of three overlapping arcs,
Fig. 2 e Equivalent circuit used to fit the spectra of LSMbased electrodes.
Fig. 3(a). The shape is similar to the previous reported spectra for LSM, where the high frequency spectrum intercepts the real axis at approximately 45 [11,14]. The interfacial polarization resistance, Rp, which is half of the difference between the high and low frequency intercepts at the real axis, is 55.9 U cm2 at 700 C for the LSM electrode. This value is higher than those reported in the literatures, where the LSM electrodes were fabricated by sintering at much higher temperatures, about 1100 C [6,14]. It is noted that the electrode performance depends critically on its sintering temperature. For LSM-based electrode, the optimized temperature is about 1000 C [15,16], at which a much lower Rp could be achieved. In this work, the sintering temperature is set to be 850 C to avoid over coarsening of YSB, which melts at about 960 C. The high Rp probably results from the relatively low sintering temperature. However, when YSB is added, the arc decreases significantly, Fig. 3(b)e(d). At 700 C, Rp for LSMe20YSB electrode is 5.2 U cm2, and it reduces to 0.2 U cm2 for LSMe60YSB electrode, which is about 38 times smaller than that for the pure LSM electrode. Therefore, addition of YSB is very effective to increase the performance of LSM cathode. Fig. 4 shows the influence of the YSB content on the resistance and the capacitance of the high frequency arc. The high frequency resistance RH decreases with the measured temperature. It also decreases with the increase of YSB concentration. The high frequency arc is usually attributed to the O2 incorporation process from the TPB into the electrolyte or into the ionic conduction component in the composite cathodes. It is very sensitive to the TPB length [10]. For the LSM
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Fig. 3 e The Nyquist plots at 700 C for LSMeYSB electrodes with various YSB content on YSZ electrolytes. The typical frequencies (Hz) are shown for the solid symbols.
electrode, TPB is only constructed among YSZ, LSM, and the gas phase at the electrode/electrolyte physical interface. When YSB is added, additional TPB is formed among YSB, LSM, and the gas phase in the electrode bulk. The extension of TPB region provides more pathways for O2 incorporation, resulting in low RH, i.e., high O2 incorporation kinetics. Another evidence of the expanded TPB is the high frequency
capacitance CH. Electrode capacitance has been reported to be proportional to the contact area between the electronic and ionic conduction components in the composite cathodes [17]. High capacitance corresponds to high TPB length. Fig. 4(b) shows that, the capacitance increases with the increase of YSB content. It is almost not affected by the measuring temperature. It is 7e9 mF cm2 for the LSMe20YSB electrode,
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Fig. 4 e Effect of YSB content on the high frequency resistance (RH) and capacitance (CH) at various temperatures.
increases to 50e70 mF cm2 for the LSMe40YSB electrode, and further to 300e500 mF cm2 for the LSMe60YSB electrode. Similar to the high frequency arc, the low frequency arc also demonstrates decreased resistance and increased capacitance when YSB content is increased, Fig. 5. The low frequency arc is attributed to oxygen adsorption and dissociation processes on the electrode surface, which is also sensitive to the TPB length as well as the catalytic activity [18]. Reduced resistance is associated with enhanced surface process. The capacitance is often correlated with equilibrium oxygen vacancy concentration on the electrode surface where surface reaction occurs [19], i.e., TPB in the composite electrode. Large capacitance represents high TPB length. The estimated capacitance is w20 mF cm2 for the LSMe20YSB electrode, about 400 and 4000 mF cm2 for the LSMe40YSB and LSMe60YSB electrodes, respectively. Thus, adding YSB increases the TPB length. Fig. 6(a) summarizes the interfacial polarization resistance for LSMeYSB electrodes with various YSB content. Increasing YSB composition leads to a substantial decrease in Rp. The lowest Rp is obtained when YSB loading mounts up to 80 wt.%, corresponding to a volume content of about 25 vol.% for LSM. Rp for LSMe80YSB cathodes is 0.14 U cm2 at 700 C in air, which is only of 0.3% of that for the pure LSM measured under the same conditions. Further increase in YSB content to 90
Fig. 5 e Effect of YSB content on the low frequency resistance (RL) and capacitance (CL) at various temperatures.
wt.% results in a extremely high ohmic resistance (not shown) due to the decrease of the continuity of LSM phase, and hence a low electrical conductivity. For LSMeYSZ and LSMeGDC composites, the lowest Rp is usually obtained when about 50 wt.% YSZ or GDC is added [6,20]. Rp for LSMeYSZ is closely related to the connectivity of LSMeYSZ particles and decreases abruptly at the percolation limit of YSZ [21]. However, different for LSMeYSB, the lowest Rp is possibly obtained at the percolation limit of LSM. Fig. 6(b) shows the Rp comparison for LSM-based cathodes reported in the literatures with YSZ as the electrolytes [6,22e24]. Rp in this work are among the lowest values for the LSM-based electrodes, including the LSMeYSZ and LSMeGDC composites, the LSMimpregnated YSZ cathodes, and the LSM cathodes impregnated with GDC. It is noted that LSM cathodes with YSB nanoparticles have shown even lower Rp [10,11]. This is probably related to the much high ionic conductivity of YSB. The result demonstrates that adding YSB is much more effective in enhancing the electrode performance than adding YSZ and doped ceria such as GDC and SDC.
3.3.
LSM cathodes on different electrolytes
Fig. 7 shows the impedance spectra for LSM cathodes at 700 C on SDC and YSB electrolytes. The two spectra are very alike,
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and their shapes are very similar to that on YSZ, Fig. 3(a). In addition, these spectra consist of three arcs, indicating similar electrode process on different electrolytes. However, Rp is different for each electrolyte. At 700 C, Rp is 55.9 U cm2 on YSZ, 15.6 U cm2 on SDC, and only 0.43 U cm2 on YSB. Rp decreases with the increase of electrolyte conductivity, although the electrolytes are different. The conductivity is measured with the impedance spectroscopy. At 700 C, it is about 0.014 S cm1 for YSZ, 0.033 S cm1 for SDC, and 0.11 S cm1 for YSB. The result agrees well with the previous reports that Rp increases with the increase of electrolyte resistivity. For example, Hibino et al. have observed that the order of interfacial polarization resistance is the same as the electrolyte resistance, SDC < La0.9Sr0.1Gd0.8Mg0.2O3 < YSZ [25]. The result also consists with the Wang’s model, which predicts lower Rp on electrolyte with higher conductivity when oxygen vacancy diffusion step limits the cathode reaction [26], which is the case for LSM since its oxygen ion conductivity is negligible. It is should be mentioned that Rp on YSB is far lower than those on YSZ and SDC. This could be attributed to the well-developed cathode/electrolyte interface as shown in Fig. 1(d) in addition to the high ionic conductivity of YSB.
3.4.
Fig. 6 e (a) Plots of interfacial polarization resistance for LSMeYSB electrodes versus YSB content at various temperatures, (b) comparison of the resistance for various LSM-based electrodes with YSZ as the electrolytes.
LSMeYSB cathodes on different electrolytes
Fig. 8a and b show the interfacial polarization resistance for LSMeYSB composite electrodes on SDC and YSB electrolytes, respectively. YSB electrolyte is also evaluated as the electrolyte substrate though it is not a stable electrolyte for SOFCs since YSB is reducible under anode conditions. But it can be placed on the cathode side as the interlayer of bilayer-electrolyte structures [27]. Rp dependence on the electrode composition is similar for SDC, YSB as well as YSZ (Fig. 6(a)) electrolytes. Rp decreases as the YSB concentration increases in the composition range investigated in this work.
Fig. 7 e Impedance spectra at 700 C for LSM cathodes on (a) SDC and (b) YSB electrolytes.
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data shown in Figs. 6(a) and 8(a). Jiang’s model [28] shows that Rp is contributed by the bulk area specific resistance, Rv, which is associated with the process in the bulk of electrode, and the interface area specific resistance, Ri, which is associated with the electrode reaction at the physical electrode/electrolyte interface. Rv is in parallel with Ri, and thus Rp is expressed as: Rp ¼
Ri Rv Ri þ Rv
(1)
Ri reflects the charge transfer reaction associated at TPBs confined to the interface, Ri ¼
Fig. 8 e Plots of Rp for LSMeYSB electrodes at various temperatures on (a) SDC, and (b) YSB electrolytes.
Fig. 9 shows Rp comparison for LSMeYSB electrodes on the three electrolytes. Rp for pure LSM electrode is quite different; at 700 C, it is 0.43 U cm2 on YSB, 15.6 U cm2 on SDC, and 55.9 U cm2 on YSZ. The performance on SDC and YSZ approaches equivalent when YSB is cooperated. When 30 wt.% and more YSB is used, Rp on YSZ is almost equal to that on SDC. For example, Rp at 700 C for LSMe80YSB is 0.15 U cm2 on SDC electrolyte. The same performanceecontent relation could also be reached at the other temperatures based on the
(2)
where ric is the charge transfer resistivity with respect to the TPBs length, and LiTPB is the TPB length per unit area at the electrode/electrolyte physical interface. Since LSM is usually considered as a pure electronic conductor, the electrode reaction in pure LSM electrode is confined to the physical electrode/electrolyte interface. Therefore, Rp for LSM electrode without YSB is equal to Ri. Assuming that the LSM electrode has the same microstructure on YSZ and SDC electrolytes (Fig. 1(b) and (c)) since the fabrication process is controlled as identical as possible, the same LiTPB can be concluded for YSZ, and SDC electrolytes. It should be noted that LiTPB for YSB is different due to its different interface microstructure as shown in Fig. 1(d). So the difference in Ri must be attributed to ric , which is originated from not only LSM but also the electrolyte, in this case, YSZ, SDC and YSB. ric is lower for electrolyte with higher ionic conductivity. Thus, LSM shows highest Ri (i.e. Rp) on YSZ electrolyte since YSZ has the lowest conductivity. Rv is Rv ¼ Rc þ Rio þ Rel ¼
r d r d rvc þ io þ el LvTPB d 2 2
(3)
where Rc is the resistance associated with charge transfer reaction occurring in the bulk of the electrode, Rio the ionic transport resistance, Rel the electronic transport resistance, rvc the charge transfer resistivity, LvTPB the TPB length per unit volume, rio the effective ionic resistance, and rel the effective electronic resistance. Rv does not depend on the ionic conductivity of the electrolyte substrate. It depends only on electrode composition and microstructure. For the same electrode, Rv is the same although the electrolyte substrate is different. Contrast to pure LSM electrode, the composite electrode obtains smaller LiTPB due to the decrease of LSM phase at the physical electrode/electrolyte interface, larger LvTPB by expanding the TPBs into the bulk of the electrodes, and smaller rio owing to increased ionic conductivity. Therefore, with increasing YSB content, Rv decreases while Ri increases. For composite electrodes with sufficient ionic and electronic conductivities, Ri can be far larger than Rv. In this case, the overall interfacial polarization resistance Rp can be approximately represented by Rv: Rp ¼
Fig. 9 e Rp at 700 C for LSMeYSB electrodes on YSZ, SDC, and YSB electrolytes.
ric LiTPB
Ri Rv Ri Rv ¼ ¼ Rv Ri þ Rv Ri
(5)
So, for LSMeYSB electrodes with high ionic conductivity (i.e. high YSB content), Rp is almost the same for YSZ and SDC
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electrolyte. It is noted that Ri for YSB electrolyte is at least an order of magnitude smaller than that for either YSZ or SDC electrolyte due to the strong cathode/electrolyte interface, Fig. 1(d). Thus, it might not be negligible compared with Rv. Consequently, Rp for YSB is much smaller than that for SDC and YSZ.
3.5. The physical electrode/electrolyte interface and bulk polarization resistance The interfacial polarization resistance associated with the electrochemical reaction at the physical electrode/electrolyte interface can be obtained directly with the impedance spectra as shown in Figs. 3(a) and 7 for pure LSM electrodes. Ri for the composite LSMeYSB electrode cannot be directly measured, but may be estimated from Eq. (2), in which ric is a constant for the same electrolyte and thus Ri is inversely proportional to LiTPB , which is in proportion to LSM content in the LSMeYSB composite since LiTPB is the length of boundaries between LSM and electrolyte substrate. In other words, Ri is in inverse proportion to the ratio of LSM in LSMeYSB cathodes. For example, Ri for LSMe20YSB and LSMe50YSB is, respectively, 1.25 and 2 times higher than that for pure LSM cathode. Fig. 10(a) shows Ri at 700 C for LSMeYSB electrodes on YSZ and SDC electrolytes. When Ri is determined, Rv can be derived from Eq. (1):
Rv ¼
Ri Rv Ri Rp
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(6)
Fig. 10(b) shows Rv at 700 C versus YSB content. Rv is figured up from Eq. (6) combining Figs. 9 and 10(a). It is noted that Rv can be derived with the experimental data obtained with various electrolyte. Fig. 10(b) demonstrates that Rv obtained with YSZ electrolyte is very close to that with SDC. This suggests that Ri and Rv can be experimentally determined. The present results also experimentally demonstrate the validity of Jiang’s model. Comparing Fig. 10(a) and (b), it is clear that Ri increases whereas Rv decreases with YSB content. In addition, Rv is far lower than Ri, especially when YSB content is high. Therefore, when YSB is used as the ionic additive, the performance of LSM-based electrode is dominated by the reaction within electrode bulk and almost not affected by the reaction at the physical electrode/electrolyte interface.
4.
Conclusion
Bismuth based oxide is introduced as the ionic component for LSM cathode. LSMeYSB composite electrodes are successfully fabricated on YSZ, SDC, and YSB electrolytes using conventional slurry-coating technique. Addition of YSB to LSM electrodes results in a remarkable reduction of Rp. LSMeYSB electrode demonstrates Rp of 0.14 U cm2 at 700 C, which is much lower than those reported for LSM-based composites on YSZ electrolyte, likely due to the high oxygen ionic conductivity of YSB. Although Rp varies obviously for pure LSM electrodes on YSZ, SDC, and YSB electrolytes, Rp tends to become equivalent along with the increase of YSB content in LSMeYSB electrodes. For electrodes with high YSB content, Rp is dominated by Rv whereas the contribution of Ri is negligible, suggesting weak electrolyte effect on Rp. Accordingly, LSMeYSB composite electrodes are attractive for intermediatetemperature SOFCs.
Acknowledgement We thank gratefully the financial support of the Ministry of Science and Technology of China (2012CB215403) and the Ministry of Education of China (20113402110014).
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Fig. 10 e Calculated Ri (a) and Rv (b) at 700 C for LSMeYSB cathodes on YSZ and SDC electrolytes.
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[17] Mizusaki J, Tagawa H, Tsuneyoshi K, Sawata A. Reaction kinetics and microstructure of the solid oxide fuel cells air electrode La0.6Ca0.4MnO3/YSZ. J Electrochem Soc 1991;138: 1867e73. [18] Jorgensen MJ, Mogensen M. Impedance of solid oxide fuel cell LSM/YSZ composite cathodes. J Electrochem Society 2001; 148:A433e42. [19] Kawada T, Suzuki J, Sase M, Kaimai A, Yashiro K, Nigara Y, et al. Determination of oxygen vacancy concentration in a thin film of La0.6Sr0.4CoO3d by an electrochemical method. J Electrochem Soc 2002;149:E252e9. [20] Kim JD, Kim GD, Moon JW, Yi Park, Lee WH, Kobayashi K, et al. Characterization of LSM-YSZ composite electrode by ac impedance spectroscopy. Solid State Ionics 2001;143: 379e89. [21] Kim JD, Kim GD, Moon JW, Lee HW, Lee KT, Kim CE. The effect of percolation on electrochemical performance. Solid State Ionics 2000;133:67e77. [22] Leng Y, Chan S, Khor K, Jiang S. (La0.8Sr0.2)0.9MnO3eGd0.2Ce0.8O1.9 composite cathodes prepared from (Gd, Ce)(NO3)x-modified (La0.8Sr0.2)0.9MnO3 for intermediate-temperature solid oxide fuel cells. J Solid State Electrochem 2006;10:339e47. [23] Jiang SP, Wang W. Fabrication and performance of GDCimpregnated (La, Sr)MnO3 cathodes for intermediate temperature solid oxide fuel cells. J Electrochem Soc 2005; 152:A1398e408. [24] Liang F, Chen J, Jiang SP, Chi B, Pu J, Jian L. Development of nanostructured and palladium promoted (La, Sr)MnO3-based cathodes for intermediate-temperature SOFCs. Electrochem Solid-State Lett 2008;11:B213e6. [25] Hibino T, Hashimoto A, Inoue T, Tokuno J-I, Yoshida S-I, Sano M. A low-operating-temperature solid oxide fuel cell in hydrocarbon-air mixtures. Science 2000;288: 2031e3. [26] Wang YL, Zhang L, Chen FL, Xia CR. Effects of doped ceria conductivity on the performance of La0.6Sr0.4Co0.2Fe0.8O3delta cathode for solid oxide fuel cell. Int J Hydrogen Energy 2012; 37:8582e91. [27] Zhang L, Li L, Zhao F, Chen FL, Xia CR. Sm0.2Ce0.8O1.9/ Y0.25Bi0.75O1.5 bilayered electrolytes for low-temperature SOFCs with AgeY0.25Bi0.75O1.5 composite cathodes. Solid State Ionics 2011;192:557e60. [28] Jiang ZY, Xia CR, Chen FL. Efficient thickness of solid oxide fuel cell composite electrode. Chinese J Chem Phys 2010;23: 217e25.
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(La,Sr)MnO3e(Y,Bi)2O3 composite cathodes for intermediate-temperature solid oxide fuel cells Liuer Wu a,b, Zhiyi Jiang a,b, Shaorong Wang a,c, Changrong Xia a,b,* a
CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, China b Department of Materials Science and Engineering, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, China c Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
article info
abstract
Article history:
Yttria-stabilized bismuth oxides (YSB) are cooperated to (La,Sr)MnO3 (LSM) to form
Received 22 October 2012
composite cathodes for intermediate-temperature solid oxide fuel cells. The composite
Received in revised form
electrodes are fabricated with screen-printing technique and characterized using electro-
20 November 2012
chemical impedance spectroscopy. The interfacial polarization resistances (Rp) of the LSMe
Accepted 22 November 2012
YSB electrodes on yttria-stabilized zirconia (YSZ), samaria-doped ceria (SDC), and YSB
Available online 3 January 2013
electrolytes are analyzed regarding the electrode composition and operating temperature.
Keywords:
When YSZ is used as the electrolyte, the lowest Rp is 0.14 U cm2 at 700 C, which is only
Solid oxide fuel cells
1.8% of that for a pure LSM electrode, 5.6% of that reported for LSMeYSZ composites, and
Rp decreases with the increase of YSB content up to 80 wt.% in the LSMeYSB composite.
Composite cathode
13.2% of that for reported LSMeGDC (gadolinia-doped ceria) electrodes, demonstrating that
Yttria-stabilized bismuth oxides
YSB is very effective to enhance the performance of LSM-based cathodes. The electrode
(La,Sr)MnO3
performance is also affected by the electrolyte substrate. LSM electrodes without any YSB exhibit obviously different performance on YSZ, SDC and YSB electrolytes. However, when YSB is cooperated, Rp on different electrolytes tends to become equivalent, especially for electrodes with high YSB content. Further analysis shows that their electrochemical performance is contributed dominantly from the electrode bulk whereas the contribution from the electrode/electrolyte interface is negligible, suggesting weak electrolyte effect on the performance of LSMeYSB composite electrodes. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
(La,Sr)MnO3 perovskite is the classical cathode material for high-temperature solid oxide fuel cells (SOFCs) because of its high electrical conductivity, high thermal and chemical stability, and relatively good compatibility with
yttria-stabilized zirconia (YSZ) and doped ceria (DCO) electrolytes. Unfortunately, its low ionic conductivity and high activation energy for electrochemical reduction of oxygen result in very poor cathodic performance in the intermediate temperature (600e800 C) region [1,2]. One approach to improve its electrocatalytic activity is to add an ionically
* Corresponding author. CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, China. Tel.: þ86 551 3607475; fax: þ86 551 3601592. E-mail address: [email protected] (C. Xia). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.111
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conducting second phase such as YSZ and DCO to form a composite electrode [3e8]. For the pure LSM cathode, triple phase boundaries (TPBs), where oxygen reduction occurs, are located only at the electrode/electrolyte physical interface. The composite cathode provides a high density of TPBs by expanding the reaction sites into the electrode bulk at distance up to tens of micrometers from the electrode/electrolyte interface since oxygen ion conducting phases have the ability to transport O2, leading to improved electrode performance. In general, the performance depends on the ionic conductivity of the second phase; high conductivity results in high electrode activity. For example, Murray et al. have reported that the interfacial polarization resistance (Rp) at 700 C has decreased from 7.82 U cm2 to 2.49 U cm2 when YSZ is used as the second phase by mechanical mixing, and further to 1.06 U cm2 when gadolinia-doped ceria (GDC) is applied [6]. The LSMeGDC composites have shown better performance than the LSMeYSZ systems mainly due to the higher ionic conductivity of GDC compared with YSZ. Among the fluoritestructured oxygen ion conductors including YSZ and GDC, stabilized bismuth oxides have the highest ionic conductivities. However, these oxides have not been investigated as the ionic conducting components for LSM-based composites. It is noted that composites consisting of yttria-stabilized bismuth oxides (YSB) and Ag have been reported as the cathodes for YSZ-based SOFCs [9]. In addition, YSB nanoparticles are impregnated onto LSM backbones to form nano-structured electrodes for YSZ and samaria-doped ceria (SDC) electrolytes [10,11]. These cathodes have shown relatively high electrochemical performance, suggesting that it is possible to enhance LSM performance by cooperating bismuth oxides to form composite cathodes. In this work, YSB is added to form LSMeYSB composites. Their electrochemical performance is investigated on various electrolytes including YSZ, SDC, and YSB. The performance is further analyzed regarding different electrolytes to show the electrolyte effect on the electrode performance.
2.
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form carbonate precipitants, which were subsequently collected, dried, and calcined at 600 C in air for 2 h to yield cubic structure SDC powders. The phase structure of all the synthesized powders was confirmed by X-ray diffraction (XRD, D/Maxra) equipped with Cu Ka radiation. YSZ powders were from Farmeiya Advanced Materials Co., China. Symmetric cells with LSM/YSB electrodes on YSZ, SDC, and YSB electrolytes were prepared for electrochemical measurement. YSZ powders were cold pressed into discs and sintered at 1500 C for 5 h to form YSZ pellets with 13 mm in diameter and 0.7 mm in thickness. The SDC electrolytes were prepared using the procedure similar to YSZ and were sintered at 1400 C for 5 h. The YSB electrolytes were also prepared by the pressingefiring technique, which were sintered at 900 C for 5 h. LSMeYSB cathodes were prepared using screen-printing technique [4]. LSMeYSB slurry was prepared by mechanically mixing LSM and YSB powders with organic additives such as terpilenol and ground with agate mortar. The mixed LSMeYSB powders contained 20, 30, 40, 50, 60, 80, and 90 wt.% YSB. A pure LSM slurry was also prepared. Each slurry was then screen-printed onto both sides of the electrolyte substrates followed by drying and firing at 850 C for 2 h. This firing temperature resulted in the lowest interfacial polarization resistance among various temperatures including 800, 850, and 900 C. The electrodes were named to show their oxide composition, for example, an LSMe20YSB electrode consisted of 80 wt.% LSM and 20 wt.% YSB. The electrode performance was determined with electrochemical impedance spectroscopy (EIS). The measurement was conducted in ambient air under open circle conditions over a temperature range of 500e750 C in the frequency range of 3 MHz to 0.1 Hz with an AC signal amplitude of 10 mV using a Zahner Zennium electrochemical station. ZSimpWin (PerkinElmer Instruments) software was used to analyze the impedance to obtain equivalent circuits. Microscopic features were characterized by scanning electron microscopy (SEM, JSM-6700F).
3.
Results and discussion
3.1.
Microstructure characterization
Experimental
The materials involved in this work include (La0.85Sr0.15)0.9MnO3d (LSM), Y0.5Bi1.5O3 (YSB), Sm0.2Ce0.8O1.9 (SDC), and (Y2O3)0.08(ZrO2)0.92 (YSZ). LSM and YSB powders were prepared using glycine-nitrate method as previously reported [10] while SDC powder was synthesized using carbonate co-precipitation route [12]. All the starting chemicals were of analytical grade and from Sinopharm Chemical Reagent Co., Ltd. LSM powders were synthesized using La(NO3)3, Sr(NO3)2, and Mn(NO3)2 as the starting chemicals. The glycine-nitrate combusted powders were heated at 900 C for 2 h to form perovskite structure LSM. YSB powders were prepared with Y(NO3)3 and Bi(NO3)3 as the precursors followed by firing the combusted powder at 600 C for 2 h. SDC powders were fabricated with Ce(NO3)3 and Sm(NO3)3 precursors, which were dissolved in distilled water to form a mixed solution with a cation concentration of 0.1 mol L1. The solution was then slowly dropped into a 0.1 mol L1 ammonium carbonate solution at room temperature under mild stirring to
Fig. 1(a) shows the SEM image of the LSM electrode on YSZ electrolyte. The LSM electrode has a porous structure with thickness of about 30 mm. Because the symmetric cells are fabricated via identical procedure, the thickness of the electrode is almost the same for each electrode. Fig. 1(b) and (c) demonstrates that LSM/YSZ interface has very similar microstructure to LSM/SDC. But LSM/YSB interface is quite different; the electrode/electrolyte bonding is much stronger for LSM/YSB interface, due to the sintering temperature, 850 C, which is relatively low for the LSM/YSZ and LSM/SDC systems but high enough for LSM/YSB since YSB melts at about 960 C [13].
3.2.
LSMeYSB composite on YSZ electrolyte
An equivalent circuit is applied to evaluate impedance spectra, Fig. 2. In the circuit, L is the inductance arising from
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Fig. 1 e Cross-sectional images of (a) the LSM electrode on YSZ electrolyte, and the microstructures of the interface between the LSM electrode and (b) YSZ, (c) SDC, and (d) YSB electrolyte.
the apparatus, Ro is attributed to the resistance of the electrolyte and the lead wires, and (RCPE) in series represents the arcs presented at different frequency ranges, where R is the corresponding resistance and CPE (noted as Q) is the corresponding constant phase element. The expression for Q is Q ¼ Y0 ( jw)n, where Y0 is the admittance, w is the angular frequency, and n is an exponent (0 < n < 1). The pseudo capacitance C for each arc is calculated by C ¼ R1/n1Q1/n. The spectrum usually consists of two arcs, RHCPEH and RLCPEL corresponding to high and low frequency arc, respectively. An additional low frequency arc ðR0L ; CPE0L Þ is identified on some impedance spectra. The equivalent circuit is often used to characterize LSM-based electrodes [10]. Fig. 3 shows the Nyquist plots measured at 700 C for electrodes with various YSB loading. The electrodes are supported on YSZ electrolytes. Due to the scattering of impedance spectra, the fitting for cathodes with YSB concentration higher than 60 wt.% is not conducted. The spectrum for the LSM electrode without any YSB consists of three overlapping arcs,
Fig. 2 e Equivalent circuit used to fit the spectra of LSMbased electrodes.
Fig. 3(a). The shape is similar to the previous reported spectra for LSM, where the high frequency spectrum intercepts the real axis at approximately 45 [11,14]. The interfacial polarization resistance, Rp, which is half of the difference between the high and low frequency intercepts at the real axis, is 55.9 U cm2 at 700 C for the LSM electrode. This value is higher than those reported in the literatures, where the LSM electrodes were fabricated by sintering at much higher temperatures, about 1100 C [6,14]. It is noted that the electrode performance depends critically on its sintering temperature. For LSM-based electrode, the optimized temperature is about 1000 C [15,16], at which a much lower Rp could be achieved. In this work, the sintering temperature is set to be 850 C to avoid over coarsening of YSB, which melts at about 960 C. The high Rp probably results from the relatively low sintering temperature. However, when YSB is added, the arc decreases significantly, Fig. 3(b)e(d). At 700 C, Rp for LSMe20YSB electrode is 5.2 U cm2, and it reduces to 0.2 U cm2 for LSMe60YSB electrode, which is about 38 times smaller than that for the pure LSM electrode. Therefore, addition of YSB is very effective to increase the performance of LSM cathode. Fig. 4 shows the influence of the YSB content on the resistance and the capacitance of the high frequency arc. The high frequency resistance RH decreases with the measured temperature. It also decreases with the increase of YSB concentration. The high frequency arc is usually attributed to the O2 incorporation process from the TPB into the electrolyte or into the ionic conduction component in the composite cathodes. It is very sensitive to the TPB length [10]. For the LSM
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Fig. 3 e The Nyquist plots at 700 C for LSMeYSB electrodes with various YSB content on YSZ electrolytes. The typical frequencies (Hz) are shown for the solid symbols.
electrode, TPB is only constructed among YSZ, LSM, and the gas phase at the electrode/electrolyte physical interface. When YSB is added, additional TPB is formed among YSB, LSM, and the gas phase in the electrode bulk. The extension of TPB region provides more pathways for O2 incorporation, resulting in low RH, i.e., high O2 incorporation kinetics. Another evidence of the expanded TPB is the high frequency
capacitance CH. Electrode capacitance has been reported to be proportional to the contact area between the electronic and ionic conduction components in the composite cathodes [17]. High capacitance corresponds to high TPB length. Fig. 4(b) shows that, the capacitance increases with the increase of YSB content. It is almost not affected by the measuring temperature. It is 7e9 mF cm2 for the LSMe20YSB electrode,
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Fig. 4 e Effect of YSB content on the high frequency resistance (RH) and capacitance (CH) at various temperatures.
increases to 50e70 mF cm2 for the LSMe40YSB electrode, and further to 300e500 mF cm2 for the LSMe60YSB electrode. Similar to the high frequency arc, the low frequency arc also demonstrates decreased resistance and increased capacitance when YSB content is increased, Fig. 5. The low frequency arc is attributed to oxygen adsorption and dissociation processes on the electrode surface, which is also sensitive to the TPB length as well as the catalytic activity [18]. Reduced resistance is associated with enhanced surface process. The capacitance is often correlated with equilibrium oxygen vacancy concentration on the electrode surface where surface reaction occurs [19], i.e., TPB in the composite electrode. Large capacitance represents high TPB length. The estimated capacitance is w20 mF cm2 for the LSMe20YSB electrode, about 400 and 4000 mF cm2 for the LSMe40YSB and LSMe60YSB electrodes, respectively. Thus, adding YSB increases the TPB length. Fig. 6(a) summarizes the interfacial polarization resistance for LSMeYSB electrodes with various YSB content. Increasing YSB composition leads to a substantial decrease in Rp. The lowest Rp is obtained when YSB loading mounts up to 80 wt.%, corresponding to a volume content of about 25 vol.% for LSM. Rp for LSMe80YSB cathodes is 0.14 U cm2 at 700 C in air, which is only of 0.3% of that for the pure LSM measured under the same conditions. Further increase in YSB content to 90
Fig. 5 e Effect of YSB content on the low frequency resistance (RL) and capacitance (CL) at various temperatures.
wt.% results in a extremely high ohmic resistance (not shown) due to the decrease of the continuity of LSM phase, and hence a low electrical conductivity. For LSMeYSZ and LSMeGDC composites, the lowest Rp is usually obtained when about 50 wt.% YSZ or GDC is added [6,20]. Rp for LSMeYSZ is closely related to the connectivity of LSMeYSZ particles and decreases abruptly at the percolation limit of YSZ [21]. However, different for LSMeYSB, the lowest Rp is possibly obtained at the percolation limit of LSM. Fig. 6(b) shows the Rp comparison for LSM-based cathodes reported in the literatures with YSZ as the electrolytes [6,22e24]. Rp in this work are among the lowest values for the LSM-based electrodes, including the LSMeYSZ and LSMeGDC composites, the LSMimpregnated YSZ cathodes, and the LSM cathodes impregnated with GDC. It is noted that LSM cathodes with YSB nanoparticles have shown even lower Rp [10,11]. This is probably related to the much high ionic conductivity of YSB. The result demonstrates that adding YSB is much more effective in enhancing the electrode performance than adding YSZ and doped ceria such as GDC and SDC.
3.3.
LSM cathodes on different electrolytes
Fig. 7 shows the impedance spectra for LSM cathodes at 700 C on SDC and YSB electrolytes. The two spectra are very alike,
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and their shapes are very similar to that on YSZ, Fig. 3(a). In addition, these spectra consist of three arcs, indicating similar electrode process on different electrolytes. However, Rp is different for each electrolyte. At 700 C, Rp is 55.9 U cm2 on YSZ, 15.6 U cm2 on SDC, and only 0.43 U cm2 on YSB. Rp decreases with the increase of electrolyte conductivity, although the electrolytes are different. The conductivity is measured with the impedance spectroscopy. At 700 C, it is about 0.014 S cm1 for YSZ, 0.033 S cm1 for SDC, and 0.11 S cm1 for YSB. The result agrees well with the previous reports that Rp increases with the increase of electrolyte resistivity. For example, Hibino et al. have observed that the order of interfacial polarization resistance is the same as the electrolyte resistance, SDC < La0.9Sr0.1Gd0.8Mg0.2O3 < YSZ [25]. The result also consists with the Wang’s model, which predicts lower Rp on electrolyte with higher conductivity when oxygen vacancy diffusion step limits the cathode reaction [26], which is the case for LSM since its oxygen ion conductivity is negligible. It is should be mentioned that Rp on YSB is far lower than those on YSZ and SDC. This could be attributed to the well-developed cathode/electrolyte interface as shown in Fig. 1(d) in addition to the high ionic conductivity of YSB.
3.4.
Fig. 6 e (a) Plots of interfacial polarization resistance for LSMeYSB electrodes versus YSB content at various temperatures, (b) comparison of the resistance for various LSM-based electrodes with YSZ as the electrolytes.
LSMeYSB cathodes on different electrolytes
Fig. 8a and b show the interfacial polarization resistance for LSMeYSB composite electrodes on SDC and YSB electrolytes, respectively. YSB electrolyte is also evaluated as the electrolyte substrate though it is not a stable electrolyte for SOFCs since YSB is reducible under anode conditions. But it can be placed on the cathode side as the interlayer of bilayer-electrolyte structures [27]. Rp dependence on the electrode composition is similar for SDC, YSB as well as YSZ (Fig. 6(a)) electrolytes. Rp decreases as the YSB concentration increases in the composition range investigated in this work.
Fig. 7 e Impedance spectra at 700 C for LSM cathodes on (a) SDC and (b) YSB electrolytes.
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data shown in Figs. 6(a) and 8(a). Jiang’s model [28] shows that Rp is contributed by the bulk area specific resistance, Rv, which is associated with the process in the bulk of electrode, and the interface area specific resistance, Ri, which is associated with the electrode reaction at the physical electrode/electrolyte interface. Rv is in parallel with Ri, and thus Rp is expressed as: Rp ¼
Ri Rv Ri þ Rv
(1)
Ri reflects the charge transfer reaction associated at TPBs confined to the interface, Ri ¼
Fig. 8 e Plots of Rp for LSMeYSB electrodes at various temperatures on (a) SDC, and (b) YSB electrolytes.
Fig. 9 shows Rp comparison for LSMeYSB electrodes on the three electrolytes. Rp for pure LSM electrode is quite different; at 700 C, it is 0.43 U cm2 on YSB, 15.6 U cm2 on SDC, and 55.9 U cm2 on YSZ. The performance on SDC and YSZ approaches equivalent when YSB is cooperated. When 30 wt.% and more YSB is used, Rp on YSZ is almost equal to that on SDC. For example, Rp at 700 C for LSMe80YSB is 0.15 U cm2 on SDC electrolyte. The same performanceecontent relation could also be reached at the other temperatures based on the
(2)
where ric is the charge transfer resistivity with respect to the TPBs length, and LiTPB is the TPB length per unit area at the electrode/electrolyte physical interface. Since LSM is usually considered as a pure electronic conductor, the electrode reaction in pure LSM electrode is confined to the physical electrode/electrolyte interface. Therefore, Rp for LSM electrode without YSB is equal to Ri. Assuming that the LSM electrode has the same microstructure on YSZ and SDC electrolytes (Fig. 1(b) and (c)) since the fabrication process is controlled as identical as possible, the same LiTPB can be concluded for YSZ, and SDC electrolytes. It should be noted that LiTPB for YSB is different due to its different interface microstructure as shown in Fig. 1(d). So the difference in Ri must be attributed to ric , which is originated from not only LSM but also the electrolyte, in this case, YSZ, SDC and YSB. ric is lower for electrolyte with higher ionic conductivity. Thus, LSM shows highest Ri (i.e. Rp) on YSZ electrolyte since YSZ has the lowest conductivity. Rv is Rv ¼ Rc þ Rio þ Rel ¼
r d r d rvc þ io þ el LvTPB d 2 2
(3)
where Rc is the resistance associated with charge transfer reaction occurring in the bulk of the electrode, Rio the ionic transport resistance, Rel the electronic transport resistance, rvc the charge transfer resistivity, LvTPB the TPB length per unit volume, rio the effective ionic resistance, and rel the effective electronic resistance. Rv does not depend on the ionic conductivity of the electrolyte substrate. It depends only on electrode composition and microstructure. For the same electrode, Rv is the same although the electrolyte substrate is different. Contrast to pure LSM electrode, the composite electrode obtains smaller LiTPB due to the decrease of LSM phase at the physical electrode/electrolyte interface, larger LvTPB by expanding the TPBs into the bulk of the electrodes, and smaller rio owing to increased ionic conductivity. Therefore, with increasing YSB content, Rv decreases while Ri increases. For composite electrodes with sufficient ionic and electronic conductivities, Ri can be far larger than Rv. In this case, the overall interfacial polarization resistance Rp can be approximately represented by Rv: Rp ¼
Fig. 9 e Rp at 700 C for LSMeYSB electrodes on YSZ, SDC, and YSB electrolytes.
ric LiTPB
Ri Rv Ri Rv ¼ ¼ Rv Ri þ Rv Ri
(5)
So, for LSMeYSB electrodes with high ionic conductivity (i.e. high YSB content), Rp is almost the same for YSZ and SDC
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electrolyte. It is noted that Ri for YSB electrolyte is at least an order of magnitude smaller than that for either YSZ or SDC electrolyte due to the strong cathode/electrolyte interface, Fig. 1(d). Thus, it might not be negligible compared with Rv. Consequently, Rp for YSB is much smaller than that for SDC and YSZ.
3.5. The physical electrode/electrolyte interface and bulk polarization resistance The interfacial polarization resistance associated with the electrochemical reaction at the physical electrode/electrolyte interface can be obtained directly with the impedance spectra as shown in Figs. 3(a) and 7 for pure LSM electrodes. Ri for the composite LSMeYSB electrode cannot be directly measured, but may be estimated from Eq. (2), in which ric is a constant for the same electrolyte and thus Ri is inversely proportional to LiTPB , which is in proportion to LSM content in the LSMeYSB composite since LiTPB is the length of boundaries between LSM and electrolyte substrate. In other words, Ri is in inverse proportion to the ratio of LSM in LSMeYSB cathodes. For example, Ri for LSMe20YSB and LSMe50YSB is, respectively, 1.25 and 2 times higher than that for pure LSM cathode. Fig. 10(a) shows Ri at 700 C for LSMeYSB electrodes on YSZ and SDC electrolytes. When Ri is determined, Rv can be derived from Eq. (1):
Rv ¼
Ri Rv Ri Rp
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(6)
Fig. 10(b) shows Rv at 700 C versus YSB content. Rv is figured up from Eq. (6) combining Figs. 9 and 10(a). It is noted that Rv can be derived with the experimental data obtained with various electrolyte. Fig. 10(b) demonstrates that Rv obtained with YSZ electrolyte is very close to that with SDC. This suggests that Ri and Rv can be experimentally determined. The present results also experimentally demonstrate the validity of Jiang’s model. Comparing Fig. 10(a) and (b), it is clear that Ri increases whereas Rv decreases with YSB content. In addition, Rv is far lower than Ri, especially when YSB content is high. Therefore, when YSB is used as the ionic additive, the performance of LSM-based electrode is dominated by the reaction within electrode bulk and almost not affected by the reaction at the physical electrode/electrolyte interface.
4.
Conclusion
Bismuth based oxide is introduced as the ionic component for LSM cathode. LSMeYSB composite electrodes are successfully fabricated on YSZ, SDC, and YSB electrolytes using conventional slurry-coating technique. Addition of YSB to LSM electrodes results in a remarkable reduction of Rp. LSMeYSB electrode demonstrates Rp of 0.14 U cm2 at 700 C, which is much lower than those reported for LSM-based composites on YSZ electrolyte, likely due to the high oxygen ionic conductivity of YSB. Although Rp varies obviously for pure LSM electrodes on YSZ, SDC, and YSB electrolytes, Rp tends to become equivalent along with the increase of YSB content in LSMeYSB electrodes. For electrodes with high YSB content, Rp is dominated by Rv whereas the contribution of Ri is negligible, suggesting weak electrolyte effect on Rp. Accordingly, LSMeYSB composite electrodes are attractive for intermediatetemperature SOFCs.
Acknowledgement We thank gratefully the financial support of the Ministry of Science and Technology of China (2012CB215403) and the Ministry of Education of China (20113402110014).
references
Fig. 10 e Calculated Ri (a) and Rv (b) at 700 C for LSMeYSB cathodes on YSZ and SDC electrolytes.
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