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Cobalt-free Sr0.7Y0.3CuO2+δ as a cathode for intermediate-temperature solid oxide fuel cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 0 3 0 e1 0 3 8
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Cobalt-free Sr0.7Y0.3CuO2Dd as a cathode for intermediate-temperature solid oxide fuel cell Xifeng Ding a,b,*, Xiaojia Gao a, Jingyu Shen a, Jun Wang a, Wenliang Zhu a, Xiong Wang a, Jinguo Jiang a a
School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China b School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
article info
abstract
Article history:
Cobalt-free oxide Sr0.7Y0.3CuO2þd (SYCu) with one-dimensional structure has been inves-
Received 27 June 2013
tigated as potential cathode material for intermediate temperature solid oxide fuel cell (IT-
Received in revised form
SOFC) applications. The crystal structure, chemical compatibility, thermal expansion and
21 October 2013
electrochemical performance were examined by X-ray diffraction technique, electro-
Accepted 31 October 2013
chemical workstation and thermal dilatometer. One-dimensional structure Sr1xYxCuO2þd
Available online 2 December 2013
and Sr2xYxCuO3þd phases appeared as the main parts after calcination above 900 C. The
Keywords:
Solid oxide fuel cell
electrolyte. High electro-catalytic performance was obtained for SYCu cathode in a sym-
Cathode
metrical cell with a polarization resistance (Rp) of 0.029 U cm2 and an overpotential of
Cobalt-free oxide
4.9 mV at 100 mA/cm2 at 800 C, showing great promising use as cathode materials for IT-
Electrochemical performance
SOFCs. In addition, the polarization resistance of SYCu cathode remain constant after
copper based oxide SYCu showed a thermal expansion coefficient (TEC) about 11.1 106/ C at 25e800 C, exhibiting good physical compatibility with samarium doped cerium (SDC)
operation at 800
C for 100 h, showing excellent long-term stability at operation
temperature. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) have been attracted with great interest in recent years owing to their unique merits such as high efficiency, low emissions and excellent fuel flexibility at high operating temperatures (>1273 K) [1e3]. Considering the practical application requirements, it is desirable to lower the operating temperature of SOFCs to intermediate temperature range (600e800 C) in which condition alloys could be used as interconnects to reduce the cost [4]. However, the electrode
polarization resistance and electrolyte ohmic resistance increased dramatically and emerged as new problems at lower temperature [5,6]. The main voltage loss is caused by cathodic polarization because the oxygen reduction process at cathode side is more difficult than the hydrogen oxidation process at anode side [7]. Currently, cobalt-based perovskites, such as AxA’1xCoO3d (A ¼ Ln, Y; A0 ¼ alkalis; x 0.5) [8,9], Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) [10] and AA’Co2O5þd (A ¼ Ln; A0 ¼ alkalis) [11,12], have been widely investigated as IT-SOFC cathodes. These so-called mixed ionic and electronic conductors (MIEC) have been
* Corresponding author. School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China. Tel.: þ86 25 84313349. E-mail addresses: [email protected], [email protected] (X. Ding). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.10.161
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 0 3 0 e1 0 3 8
proved as promising candidate cathode with excellent electrocatalytic activity, in that a certain degree of oxygen ion conduction increases the effective surface area for oxygen molecule-to-ion conversion. Therefore, triple phase boundary (TPB) length for oxygen reduction reaction (ORR) is enlarged and polarization loss is reduced [13,14]. Nevertheless, large thermal expansion coefficient (i.e., physical incompatibility with electrolyte), chemical instability, high temperature volatility and high cost make cobalt-base oxides as obstacles to the development of IT-SOFC cathodes [15]. Developing cobalt-free cathodes are therefore to be a new research direction. Some interesting cuprates have been investigated so far as potential cathodes for IT-SOFCs [16e23]. Perovskite type oxide La1xSrxCuO2.5d (0.1 x 0.5) was investigated as a candidate IT-SOFC cathode material due to a large amount of oxygen defects and high electronic conductivity [16e21]. Among the perovskite-related structures, the RuddlesdenePopper (RP) series of intergrowth oxides (AO) (ABO3) n, such as La2xSrxCuO4d (n ¼ 1) and La2xSr1þxCu2O6þd (n ¼ 2) has also been drawn much attention as alternative cathode materials [22,23]. For instance, cuprates in the RP series show high electronic conductivity and good oxygen mobility. In these studies, generally a more suitable physical compatibility with doped ceria-based electrolyte and a lower sintering temperature are found besides high electronic conductivity and considerable oxygen defects. SrCuO2 was investigated due to its superconductivity [24]. It was also observed that SrCuO2 has a larger amount of oxygen defects than La1xSrxCuO2.5d [25,26]. Furthermore, it is expected that other advantages of cuprates will be retained, such as compatibility with cerium based electrolyte, excellent electrical conductivity and low cost of materials. In this study, performance of cobalt-free oxide, yttrium doped Sr0.7Y0.3CuO2þd (SYCu) was evaluated as a promising cathode for IT-SOFCs. Yttrium is used as a dopant to enhance its conductivity due to its valence and ionic radius close to Sr element.
2.
Experimental
2.1.
Preparation of SYCu samples
SYCu powder was synthesized via glycine-nitrate combustion method, where glycine was used as both complexing agents and fuels during the combustion reaction. Y2O3 (99.9%) was dissolved in excess dilute nitric acid under stirring, and the solution was kept at 95 C until the oxide dissolved completely. An excess nitrate was required to dissolve Y2O3 completely and to prevent metal ions from precipitating. Stoichiometric amounts of Sr(NO3)2 (99.5%) and Cu(NO3)2∙3H2O (99.5%) were dissolved in the foregoing solution. The molar ratio of glycine to metal ions in the final solution was 3:1. The measured pH value of final solution was 2. After that the solution was heated at 250 C on an electric heater until it spontaneously combusted. The as-formed precursor was then calcined at 800, 900, 950, 1000 and 1030 C for 2 h in air, respectively. The obtained powders were characterized by X-ray diffractometry (XRD, D/ max-Ⅲ). The precursor was subjected to thermal analysis using a differential scanning calorimetry combined with thermogravimetry (DSC/TG, STA449c). Average valence of copper ion
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for the powder calcined at 950 C was measured by iodometry process [27].
2.2.
Preparation of SDC electrolyte
The SDC powders were synthesized by carbonate coprecipitation method. Ammonium carbonate as the precipitant and cerium and samarium nitrates were used as the cation sources. The molar ratio of Ce(NO3)3∙6H2O (99.9%) and Sm(NO3)3∙6H2O (99.9%) was 4:1. The nitrate solution was dropped into the ammonium carbonate solution under mild stirring to form carbonate precipitants. Note that molar ratio of the ammonium carbonate versus cation (Ce3þ þ Sm3þ) was set to be 2.5 in order to obtain spherical particles [28]. Washed repeatedly with distilled water and ethanol, the precipitate was dried at 90 C, and then calcined at 900 C for 2 h in air. The as-synthesized powders were ground with 5 wt% PVA in a mortar, and pressed into pellets (v15 mm) under 200 MPa, then sintered at 1200 C for 6 h.
2.3.
Cell fabrication and testing
The symmetrical cells were fabricated on SDC electrolyte to measure the polarization resistance and cathodic overpotential using an electrochemical workstation (CHI604D). SYCu slurry were deposited onto both sides of the substrates via a spin coating method, followed by sintered at 900, 950, 1000 C for 3 h in air. The slurry composition is consisted of SYCu powders, ethyl cellulose and ethanol in a weight ratio of 10:1:80. The slurry was coated at a rotation speed of 3000 rpm for 40 s every time. In order to attain the electrode layer with the thickness of w20 mm, the repetitive coating was performed 20 times. Finally, the symmetrical cells were constructed with the effective cathode area of w0.3317 cm2.
2.4.
Compatibility measurement
The SYCu and SDC powders were mixed with a quality ratio of 1:1, and calcined at 950 C for 5 h to test the chemical compatibility by XRD. In addition, a bar (nominal dimension of 5 mm 5 mm 60 mm) was pressed under 200 MPa and sintered at 950 C for 6 h to measure thermal expansion coefficient (TEC) using a dilatometer (RPZ-13-10P). In order to investigate composition stability and long-term stability the SYCu powders after heat treatment at 950 C were calcined at 800 C for 100 h and 950 C for 10 h, respectively.
3.
Results and discussion
3.1.
TG/DSC analysis
The TG and DSC curves obtained with the powders via autocombustion reaction are shown in Fig. 1. The TG curve remains approximately stable until w650 C. Afterwards the remnant began to decompose and the weight of sample decreases. A sharp decrease on TG curve around 800 C could be attributed to the release of CO2 from carbonates in the raw powders, and was verified by the presence of an endothermic peak at 809 C on DSC curve. A more conspicuous
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Fig. 1 e DSC and TG curves of the SYCu powder after autocombustion measured in air at a heating temperature rate of 5 K/min.
decomposition reaction is observed until 950 C and the whole progress brings a weight loss about 11.5%. The TG curve is relatively smooth above 950 C, indicating that chemical reactions were completed.
3.2.
XRD characterization
The XRD patterns of SYCu calcined at 800e1030 C are presented in Fig. 2. Carbonates SrCO3 and Y2Cu2O5 was detected for SYCu after calcination at 800 C for 2 h (Fig. 2a). With the increasing calcination temperature, Sr1xYxCuO2þd and Sr2xYxCuO3þd phases are found after calcination at 900 C for 2 h (Fig. 2b), with a small amount of SrY2O4 and Y2Cu2O5 as secondary phases in the samples, which are considered as the obstacle of oxygen ion migration. The sketch map of the crystal structures of SrCuO2 and Sr2CuO3 is shown in Fig. 3. A similar structure is found in both compounds, namely, it is so-
Fig. 2 e XRD patterns of the powders calcined at different temperature. Symbols indicate the presence of Sr1LxYxCuO2Dd (*), Sr2LxYxCuO3Dd (C), SrY2O4 (,), Y2Cu2O5 (B), SrCO3(-), CuO(:).
called quasi-one-dimensional structure in which CueO quadrilateral is parallel to atb plane and repeats along the a axis [25]. The simplest one-dimensional structure crystal Sr2CuO3 was taken as an example. Originally, a crystal model was built as a K2NiF4 type tetragonal structure without apical oxygen. It was found that partial apical oxygen sites could be occupied and the amount of apical oxygen is adjustable as doping mechanism [26,29,30]. Excellent oxygen ion conductivity is expected due to the large amount of oxygen vacancy in this structure [31]. And the case is similar with SrCuO2. Moreover, in the case of synthesis of quasi-one-dimensional cuprates, several phases, such as Sr2CuO3, Sr14Cu24O41 and SrCuO2 are impeding the synthesis of single phase sample, i.e, a single phase is difficult to obtain [32,33]. According to those mentioned above, Sr1xYxCuO2þd and Sr2xYxCuO3þd in the compounds are regarded as useful phases for oxygen ionic conducting. Assuming that the little doping has no effect on the reference intensity ratios (RIR) values, the relative contents were calculated by Jade 6.5. The total relative content of Sr1xYxCuO2þd and Sr2xYxCuO3þd has been listed in Table 1. With the increasing calcination temperature, the relative content of useful phases decreases. The average valence of Cu in SYCu calcined at 950 C is measured by iodometry method. The copper valence is Cu1.93þ in Sr0.7Y0.3CuO2þd under ambient condition and the stoichiometry for the sample is Sr0.7Y0.3CuO2.115 at room temperature. The XRD patterns of SYCu and SDC powder mixtures after co-fired at 950 C for 5 h are shown in Fig. 4. All peaks have been labeled with SYCu and SDC patterns, respectively, and no other phases are observed, indicating that SYCu has a good chemical compatibility with SDC electrolyte.
3.3.
Impedance spectra
Fig. 5 shows the impedance spectra measured from 500 to 800 C for SYCujSDCjSYCu symmetric cells. All of the impedance spectra were ohmic resistance-corrected for the sake of comparison. The difference between low-frequency and highfrequency intercepts with real axis can be approximated to the resistance of the interfacial polarization resistance (Rp). It is observed that Rp decreases gradually as temperature
Fig. 3 e The schematic view of the crystal structure of Sr2CuO3.
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Table 1 e The relative contents of several phases formed in Sr0.7Y0.3CuO2Dd (SYCu) after calculating at different temperature. Temperature ( C) 900 950 1000 1030
Sr1xYxCuO2þd
Sr2xYxCuO3þd
SrY2O4
Y2Cu2O5
Sr1xYxCuO2þd þ Sr2xYxCuO3þd
0.596 0.108 0.265 0.184
0.28 0.666 w0 w0
0.123 0.089 0.449 0.35
w0 0.137 0.286 0.466
0.876 0.774 0.265 0.184
increases from 500 to 800 C. At 700 C the Rp is 0.11 U cm2, which meets with the requirement of IT-SOFC [34]. Gratifyingly, the Rp is 0.029 U cm2 at 800 C, about 74% lower than that of a recently reported novel cobalt based cathode Sr0.3Y0.7CoO2.65d (0.11 U cm2) [35]. The impedance spectra are evaluated by fitting impedance data with the equivalent circuit shown in Fig. 6, where Rs is the overall ohmic resistances including the electrolyte resistance, electrode resistance, lead resistance and contact resistance. Rct (high-frequency resistance) is interpreted to oxygen ion transferring from the TPB to the electrolyte, and Rd (low-frequency resistance) is related to dissociative absorption of O2 or the diffusion of neutral oxygen on surface [36]. CPE1 and CPE2 are the corresponding constant phase elements. The fitting results are shown in Table 2. Fig. 7 shows that SYCu yields an activation energy of w111.3 kJ/mol, which is close to that of Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF, 120.3 kJ/mol) [37], and much smaller than La0.8Sr0.2CoO3d (164 kJ/mol) [38]. All these indicate that SYCu has a high electro-catalytic activity for oxygen reduction reaction at intermediate temperature. Fig. 8 shows that SYCu has a higher Ea for Rct than Rd. Combined with Table 2, a ratelimiting step transition can be observed at 650 C. Originally, the resistance related to oxygen ion transferring from the TPB to the electrolyte owns an approximate double value. As the temperature increases, the Rct decreases dramatically. Between 600 and 650 C the transition occurs, and dissociative absorption of O2/the diffusion of neutral oxygen on the
surface turn into the dominated step at a temperature higher than 650 C. The Ea of Rd is 97.2 kJ/mol for SYCu, which is much smaller than that of LSCu (158 kJ/mol) [18]. The immense difference could be attributed to the higher oxygen ion motility of SYCu than that of LSCu [39]. It has also been proved that a large amount of oxygen defects exists in Sr2CuO3 [26,31]. The similar structure in SYCu provides favorable pathways for oxygen ion migration. However, further researches should be carried out to interpret the oxygen diffusion dynamics in SYCu. In this study sintering temperature has an influence on both composition and microstructure of electrode. Therefore, analysis must be carried out based on the two sides. Fig. 9 shows the impedance spectra of SYCu cathode sintered at 900e1000 C. It is observed that the sample sintered at 900 C has a higher Rp than that of two other samples, though the least useful phases are obtained. It is obvious that high-frequency arc interpreted as oxygen ion transfer from the TPB to the electrolyte or grain boundary resistance [36] make up main part of impedance arc. Thence, such a high resistance is attributed to poor adhesion between electrode and electrolyte. On the contrary, a big diameter of low-frequency is observed in the sample sintered at 1000 C. The lowfrequency arc is interpreted as the diffusion of neutral oxygen on surface and the diffusion of oxygen molecules in electrode pores. Generally, the arc representing the diffusion of oxygen molecules occurs only at a low oxygen partial pressure. In this study cathode thin film melted partly after sintering at 1000 C, and the porous microstructure was destroyed which lead to the difficult diffusion of oxygen molecules in the bulk. The resistances are fitted and shown in Fig. 10. The low-frequency resistances remain almost constant above 700 C and make up above 60% of polarization resistances. Therefore, the diffusion of oxygen molecules in the bulk is concerned as rate-determining step, which supports the above-mentioned inference. In addition, the increase of second phase in SYCu cathode may contribute to the polarization resistance as well.
3.4.
Fig. 4 e XRD patterns for SYCu, SDC and SYCu/SDC powders. The symbols stand for the different materials (*: SYCu; C: SDC).
Cathodic overpotential
Cathodic overpotential curves for SYCu as a function of current density at 800 C are shown in Fig. 11. And similar curves for La0.75Sr0.25CuO3d (LSCu) [17], Sm0.5Sr0.5CoO3d (SSC) [40], and La2NiO4þd (LN) [41] are also provided for reference. The cathodic overpotential of these electrodes increases with increasing current density. At a current density of 100 mA/ cm2, SYCu exhibits the lowest cathodic overpotential of w4.9 mV and the value is about 5.1, 12.8 and 25.5 mV lower than that of LSCu [17], SSC [40], and LN cathode at the same
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Table 2 e The fitting value of impedance spectra at various temperatures. Temp. ( C) 500 550 600 650 700 750 800
Rs(U cm2)
Rct(U cm2)
Rd(U cm2)
Rp(U cm2)
14.56 7.197 4.286 2.683 1.824 1.298 0.999
2.2280 0.9600 0.2323 0. 0785 0. 0449 0.0146 0.0139
1.1920 0.5308 0.3162 0.1800 0.0662 0. 0394 0.0151
3.4200 1.4908 0.5485 0. 2585 0.1111 0.0540 0. 0291
quite close to that of impedance value (0.029 U cm2) for SYCu at 800 C. Generally, the result confirms that SYCu is a promising cathode candidate for IT-SOFCs.
3.5.
Fig. 5 e Impedance spectra of the SYCu cathode on SDC electrolytes from 500 to 800 C.
condition [41]. The encouraging performance could be due to the existence of considerable oxygen defects as well. At low overpotential levels (20 mV), the overpotential h vs. current density I relation can be expressed by the following linear equation [42]: h¼
RT I nFi0
Thermal expansion
Close thermal expansion behavior of adjacent components is normally required to avoid cracking during thermal cycling for SOFCs. The typical physical compatibility between SYCu and SDC was investigated in term of the thermal expansion. The thermal expansion curves of SYCu and SDC samples are exhibited in Fig. 12. The calculated thermal expansion coefficient (TEC) of SYCu (11.1 106/ C) from room temperature to 800 C is much close to that of the SDC electrolyte (12.1 106/ C), and much lower than that of cobalt-based cathode, such as Ln0.6Sr0.4CoO3d (17.1e21.3 106/ C) [43] and LnBaCo2O5þd (19e21 106/ C) [44]. The lower TEC may be ascribed to that there is no spin state transition of B-site cation in copper based oxide, which is normally existed in cobalt oxides. The TEC is also smaller than that of other perovskite copper oxides, e.g. La1xSrxCuO2.5d (16.8e17.9 106/ C) [20] and La0.7Sr0.3CuO3d (13.6 106/ C) [45]. It is mentioned in iodometry measurement, the couple Cu2þ/Cuþ is discovered instead of Cu2þ/Cu3þ, differing from other copper-based materials used as SOFC cathodes. According to the theory of thermal expansion developed by Ruffa [46], the TEC value is inversely proportional to the metal-oxide bond length. Due to
(1)
where, i0 is the exchange current density, n is the number of electrons involved in the electrode reaction, F is the Faraday constant, R is the gas constant, and T is the measured temperature. The term RT/nFi0 with the unit of an ohm square centimeter (U cm2) refers to the polarization resistance of the electrode reaction at open circuit potential. The polarization resistance of the electrode reaction was derived from a linear fitting of cathodic curve within the low overpotential range. The yielded polarization resistance (0.024 U cm2) which is
Fig. 6 e Equivalent circuit of the electrode reaction.
Fig. 7 e The Arrhenius curve of the SYCu and BSCF.
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Fig. 8 e The Arrhenius curves of the Rct and Rd for SYCu and the Arrhenius curve of Rd for LSCu.
the longer ionic radii of Cu and Cu (Cu /0.54 A, Cu2þ/0.73 A, A, and all data are obtained from Shannon ionic radii Cuþ/0.77 [47]), it is not difficult to understand the smaller TEC for SYCu in this study. 2þ
3.6.
þ
3þ
Composition stability and long-term stability
To investigate the composition stability of SYCu oxides at the same calcination temperature, a sample calcined at 950 C for 10 h was characterized by XRD. The XRD patterns of SYCu oxides are shown in Fig. 13. For comparison, the XRD patterns of a sample calcined at 950 C for 2 h are also shown in Fig. 13. The angle of diffraction peaks haven’t changed and no new phase is formed after calcining for 10 h, suggesting excellent composition stability for long sintering time.
Fig. 10 e Total polarization resistance (Rp), high-frequency resistance (Rh) and low-frequency resistance (Rl) of SYCu cathode at various temperature.
Instability of cathode containing alkaline-earth elements were reported in the presence of CO2, especially BSCF [48]. It was reported that the reactivity of CO2 with BSCF occurred even in the presence of a thimbleful of CO2 at low temperature. Therefore, a sample after heat treatment at 950 C was calcined at 800 C for 100 h in air to investigate the long-term stability at operation temperature. The XRD pattern of SYCu oxides are shown in Fig. 14. For comparison, the XRD patterns of a sample calcined at 950 C for 2 h are also shown in Fig. 14. No carbonate or other new phase are observed after calcination at 800 C for about 4 days, suggesting excellent long-term stability at operation temperature in air. Correspondingly, the impedance spectra was recorded at 800 C for 100 h to verify the result obtained from XRD. Fig. 15 shows impedance spectra of the SYCu cathode measured at 800 C as a function of time. After operation at 800 C for 100 h no obvious change are observed
50
SYCu LSCu-ref[17] SSC-ref[40] LN-ref[41] fitting
Overpotential (mV)
40
30
30.4mv
20
17.7mv 10
0
10mv 4.9mv 0
50
100
150
i (A/cm2) Fig. 9 e Impedance spectra of SYCu cathode sintered at various temperatures.
Fig. 11 e Cathodic overpotential curves for SYCu, LSCu, La2NiO4Dd (LN) and SSC.
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Fig. 12 e Thermal expansion curves of SYCu and SDC measured from room temperature to 800 C. The Table of thermal expansion coefficient (TEC) is inserted. La0.6Sr0.4CoO3Ld [42] and LaBaCo2O5Dd [43] were also presented for reference.
Fig. 14 e XRD patterns of the powders calcined at 800 C for 10 h in air.
0.030
with increasing operation time, suggesting a stable electrochemical performance at operation temperature in the atmosphere.
0h 20h 40h 60h 80h 100h
0.025
2
-ImZ (Ω ·cm )
0.020
4.
Conclusion
Sr0.7Y0.3CuO2þd (SYCu) with a dominant one-dimensional structure has been synthesized by GNP method and shows novel high catalytic activity for oxygen reduction reaction. A small polarization resistance (Rp) of 0.029 U cm2 and cathodic overpotential of 4.9 mV (at a current density of 100 mA/cm2) has been achieved for SYCu cathode on SDC electrolyte at 800 C. The activated energy (Ea) for Rp is close to BSCF cathode. Furthermore, SYCu is much compatible with SDC
0.015
0.010
0.005
0.000 0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
2
ReZ ( Ω ·cm )
Fig. 15 e Impedance spectra of the SYCu cathode at 800 C as a function of time.
electrolyte both chemically and physically comparing with corresponding cobalt based cathode materials. In addition, the polarization resistance of SYCu cathode remain constant after operation at 800 C for 100 h, showing excellent longterm stability at operation temperature. It is proved that SYCu is attractive cathode material for intermediate temperature SOFCs.
Acknowledgments
Fig. 13 e XRD patterns of the powders calcined at 950 C for 2 h and 10 h.
The authors gratefully acknowledge the financial supports provided by the Natural Science Foundation of China (No. 50902069, 21001064), Natural Science Foundation of Jiangsu Province (No. BK2012806) and the Fundamental Research Funds for the Central Universities (No. 30920130111022).
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[23] Kim YN, Manthiram A. La1.85Sr1.15Cu2xCoxO6þd intergrowth oxides as cathodes for intermediate temperature solid oxide fuel cells. Electrochim Acta 2012;70:375e81. [24] Feng SP, Guo HM, Yu L. Electronic structure of kinetic energy driven curprate superconductors. Int J Mod Phy B 2008;22:3757e811. [25] Sologubenko AV, Gianno K, Ott HR. Heat transport by lattice and spin excitations in the spin-chain compounds SrCuO2 and Sr2CuO3. Phys Rev B 2001;64:054412. [26] Liu QQ, Yang H, Qin XM, Yang LX, Li FY, Yu Y, et al. Correlation of superconductivity with the ordering state at the apical oxygen layer in the Sr2CuO3þd superconductor. Physica C 2007;460-462:56e7. [27] Appelman EH, Morss LR, Kini AM. Oxygen content of superconducting perovskites, La2xSrxCuOy and YBa2Cu3Oy. Inorg Chem 1987;26:3237e9. [28] Li JG, Ikegami T, Wang YR, Mori T. Reactive ceria Nanopowders via carbonate precipitation. J Am Ceram Soc 2002;85:2376e8. [29] Jin CQ, Wu ZJ, Laffez P, Laffez P, Tamura T, Adachi S, et al. Superconductivity at 80K in (Sr,Ca)3Cu2O4þdCl2y induced by apical oxygen doping. Nature 1995;375:301e3. [30] Hiroi Z, Takano M, Azuma M, Takeda Y. A new family of copper oxide superconductors Srnþ1CunO2nþ1þd stabilized at high pressure. Nature 1993;364:315e7. [31] Neudert R, Drechsler SL, Malek J, Rosner H, Kielwein M, Hu Z, et al. Four-band extended Hubbard Hamiltonian for the onedimensional cuprate Sr2CuO3: distribution of oxygen holes and its relation to strong intersite Coulomb interaction. Phys Rev B 2000;62:10752e65. [32] Ivanova TA, Jacyna-Onyszkiewicz I, AugustyniakYablokova MA, Yablokov YuV, Shustov VA. Electron spin resonance of localized copper states in Sr1xAxCuO2 Ceramics (A ¼ Liþ, Kþ, La3þ;x 0.15). Phys Solid State 2005;47:1540e3. [33] Ingle NJC, Hammond RH, Beasley MR. Molecular beam epitaxial growth of SrCu2O3: metastable structures and the role of epitaxy. J Appl Phys 2002;91:6371e8. [34] Brandon NP, Skinner S, Steele BCH. Recent advances in materials for fuel cells. Annu Rev Mater Res 2003;33:183e213. [35] Li Y, Kim YN, Cheng JG, Alonso JA, Hu ZW, Chin YY, et al. Oxygen-deficient perovskite Sr0.7Y0.3CoO2.65d as a cathode for intermediate-temperature solid oxide fuel cells. Chem Mater 2011;23:5037e44. [36] Kim JD, Kim GD, Moon JW, Park YI, Lee WH, Kobayashi Koichi, et al. Characterization of LSMeYSZ composite electrode by ac impedance spectroscopy. Solid State Ionics 2001;143:379e89. [37] Liu QL, Khor KA, Chan SH. High-performance lowtemperature solid oxide fuel cell with novel BSCF cathode. J Power Sources 2006;161:123e8. [38] Ralph JM, Schoeler AC, Krumpelt M. Materials for lower temperature solid oxide fuel cells. J Mater Sci 2001;36:1161e72. [39] Adler SB. Factors governing oxygen reduction in solid oxide fuel cell cathodes. Chem Rev 2004;104:4791e843. [40] Fukunaga H, Koyama M, Takahashi N, Wen C, Yamada K. Reaction model of dense Sm0.5Sr0.5CoO3 as SOFC cathode. Solid State Ionics 2000;132:279e85. [41] Zhao K, Xu Q, Ping D, Chen M, Kim BH. Microstructure and electrochemical properties of porous La2NiO4þd electrodes spin-coated on Ce0.8Sm0.2O1.9 electrolyte. Ionics 2012;18:75e83. [42] Fu C, Sun K, Zhang N, Chen X, Zhou D. Electrochemical characteristics of LSCF-SDC composite cathode for intermediate temperature SOFC. Electrochim Acta 2007;52:4589e94. [43] Lee KT, Manthiram A. Comparison of Ln0.6Sr0.4CoO3d (Ln ¼ La, Pr, Nd, Sm, and Gd) as cathode materials for intermediate temperature solid oxide fuel cells. J Electrochem Soc 2006;153:794e8.
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Cobalt-free Sr0.7Y0.3CuO2Dd as a cathode for intermediate-temperature solid oxide fuel cell Xifeng Ding a,b,*, Xiaojia Gao a, Jingyu Shen a, Jun Wang a, Wenliang Zhu a, Xiong Wang a, Jinguo Jiang a a
School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China b School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
article info
abstract
Article history:
Cobalt-free oxide Sr0.7Y0.3CuO2þd (SYCu) with one-dimensional structure has been inves-
Received 27 June 2013
tigated as potential cathode material for intermediate temperature solid oxide fuel cell (IT-
Received in revised form
SOFC) applications. The crystal structure, chemical compatibility, thermal expansion and
21 October 2013
electrochemical performance were examined by X-ray diffraction technique, electro-
Accepted 31 October 2013
chemical workstation and thermal dilatometer. One-dimensional structure Sr1xYxCuO2þd
Available online 2 December 2013
and Sr2xYxCuO3þd phases appeared as the main parts after calcination above 900 C. The
Keywords:
Solid oxide fuel cell
electrolyte. High electro-catalytic performance was obtained for SYCu cathode in a sym-
Cathode
metrical cell with a polarization resistance (Rp) of 0.029 U cm2 and an overpotential of
Cobalt-free oxide
4.9 mV at 100 mA/cm2 at 800 C, showing great promising use as cathode materials for IT-
Electrochemical performance
SOFCs. In addition, the polarization resistance of SYCu cathode remain constant after
copper based oxide SYCu showed a thermal expansion coefficient (TEC) about 11.1 106/ C at 25e800 C, exhibiting good physical compatibility with samarium doped cerium (SDC)
operation at 800
C for 100 h, showing excellent long-term stability at operation
temperature. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) have been attracted with great interest in recent years owing to their unique merits such as high efficiency, low emissions and excellent fuel flexibility at high operating temperatures (>1273 K) [1e3]. Considering the practical application requirements, it is desirable to lower the operating temperature of SOFCs to intermediate temperature range (600e800 C) in which condition alloys could be used as interconnects to reduce the cost [4]. However, the electrode
polarization resistance and electrolyte ohmic resistance increased dramatically and emerged as new problems at lower temperature [5,6]. The main voltage loss is caused by cathodic polarization because the oxygen reduction process at cathode side is more difficult than the hydrogen oxidation process at anode side [7]. Currently, cobalt-based perovskites, such as AxA’1xCoO3d (A ¼ Ln, Y; A0 ¼ alkalis; x 0.5) [8,9], Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) [10] and AA’Co2O5þd (A ¼ Ln; A0 ¼ alkalis) [11,12], have been widely investigated as IT-SOFC cathodes. These so-called mixed ionic and electronic conductors (MIEC) have been
* Corresponding author. School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China. Tel.: þ86 25 84313349. E-mail addresses: [email protected], [email protected] (X. Ding). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.10.161
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 0 3 0 e1 0 3 8
proved as promising candidate cathode with excellent electrocatalytic activity, in that a certain degree of oxygen ion conduction increases the effective surface area for oxygen molecule-to-ion conversion. Therefore, triple phase boundary (TPB) length for oxygen reduction reaction (ORR) is enlarged and polarization loss is reduced [13,14]. Nevertheless, large thermal expansion coefficient (i.e., physical incompatibility with electrolyte), chemical instability, high temperature volatility and high cost make cobalt-base oxides as obstacles to the development of IT-SOFC cathodes [15]. Developing cobalt-free cathodes are therefore to be a new research direction. Some interesting cuprates have been investigated so far as potential cathodes for IT-SOFCs [16e23]. Perovskite type oxide La1xSrxCuO2.5d (0.1 x 0.5) was investigated as a candidate IT-SOFC cathode material due to a large amount of oxygen defects and high electronic conductivity [16e21]. Among the perovskite-related structures, the RuddlesdenePopper (RP) series of intergrowth oxides (AO) (ABO3) n, such as La2xSrxCuO4d (n ¼ 1) and La2xSr1þxCu2O6þd (n ¼ 2) has also been drawn much attention as alternative cathode materials [22,23]. For instance, cuprates in the RP series show high electronic conductivity and good oxygen mobility. In these studies, generally a more suitable physical compatibility with doped ceria-based electrolyte and a lower sintering temperature are found besides high electronic conductivity and considerable oxygen defects. SrCuO2 was investigated due to its superconductivity [24]. It was also observed that SrCuO2 has a larger amount of oxygen defects than La1xSrxCuO2.5d [25,26]. Furthermore, it is expected that other advantages of cuprates will be retained, such as compatibility with cerium based electrolyte, excellent electrical conductivity and low cost of materials. In this study, performance of cobalt-free oxide, yttrium doped Sr0.7Y0.3CuO2þd (SYCu) was evaluated as a promising cathode for IT-SOFCs. Yttrium is used as a dopant to enhance its conductivity due to its valence and ionic radius close to Sr element.
2.
Experimental
2.1.
Preparation of SYCu samples
SYCu powder was synthesized via glycine-nitrate combustion method, where glycine was used as both complexing agents and fuels during the combustion reaction. Y2O3 (99.9%) was dissolved in excess dilute nitric acid under stirring, and the solution was kept at 95 C until the oxide dissolved completely. An excess nitrate was required to dissolve Y2O3 completely and to prevent metal ions from precipitating. Stoichiometric amounts of Sr(NO3)2 (99.5%) and Cu(NO3)2∙3H2O (99.5%) were dissolved in the foregoing solution. The molar ratio of glycine to metal ions in the final solution was 3:1. The measured pH value of final solution was 2. After that the solution was heated at 250 C on an electric heater until it spontaneously combusted. The as-formed precursor was then calcined at 800, 900, 950, 1000 and 1030 C for 2 h in air, respectively. The obtained powders were characterized by X-ray diffractometry (XRD, D/ max-Ⅲ). The precursor was subjected to thermal analysis using a differential scanning calorimetry combined with thermogravimetry (DSC/TG, STA449c). Average valence of copper ion
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for the powder calcined at 950 C was measured by iodometry process [27].
2.2.
Preparation of SDC electrolyte
The SDC powders were synthesized by carbonate coprecipitation method. Ammonium carbonate as the precipitant and cerium and samarium nitrates were used as the cation sources. The molar ratio of Ce(NO3)3∙6H2O (99.9%) and Sm(NO3)3∙6H2O (99.9%) was 4:1. The nitrate solution was dropped into the ammonium carbonate solution under mild stirring to form carbonate precipitants. Note that molar ratio of the ammonium carbonate versus cation (Ce3þ þ Sm3þ) was set to be 2.5 in order to obtain spherical particles [28]. Washed repeatedly with distilled water and ethanol, the precipitate was dried at 90 C, and then calcined at 900 C for 2 h in air. The as-synthesized powders were ground with 5 wt% PVA in a mortar, and pressed into pellets (v15 mm) under 200 MPa, then sintered at 1200 C for 6 h.
2.3.
Cell fabrication and testing
The symmetrical cells were fabricated on SDC electrolyte to measure the polarization resistance and cathodic overpotential using an electrochemical workstation (CHI604D). SYCu slurry were deposited onto both sides of the substrates via a spin coating method, followed by sintered at 900, 950, 1000 C for 3 h in air. The slurry composition is consisted of SYCu powders, ethyl cellulose and ethanol in a weight ratio of 10:1:80. The slurry was coated at a rotation speed of 3000 rpm for 40 s every time. In order to attain the electrode layer with the thickness of w20 mm, the repetitive coating was performed 20 times. Finally, the symmetrical cells were constructed with the effective cathode area of w0.3317 cm2.
2.4.
Compatibility measurement
The SYCu and SDC powders were mixed with a quality ratio of 1:1, and calcined at 950 C for 5 h to test the chemical compatibility by XRD. In addition, a bar (nominal dimension of 5 mm 5 mm 60 mm) was pressed under 200 MPa and sintered at 950 C for 6 h to measure thermal expansion coefficient (TEC) using a dilatometer (RPZ-13-10P). In order to investigate composition stability and long-term stability the SYCu powders after heat treatment at 950 C were calcined at 800 C for 100 h and 950 C for 10 h, respectively.
3.
Results and discussion
3.1.
TG/DSC analysis
The TG and DSC curves obtained with the powders via autocombustion reaction are shown in Fig. 1. The TG curve remains approximately stable until w650 C. Afterwards the remnant began to decompose and the weight of sample decreases. A sharp decrease on TG curve around 800 C could be attributed to the release of CO2 from carbonates in the raw powders, and was verified by the presence of an endothermic peak at 809 C on DSC curve. A more conspicuous
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Fig. 1 e DSC and TG curves of the SYCu powder after autocombustion measured in air at a heating temperature rate of 5 K/min.
decomposition reaction is observed until 950 C and the whole progress brings a weight loss about 11.5%. The TG curve is relatively smooth above 950 C, indicating that chemical reactions were completed.
3.2.
XRD characterization
The XRD patterns of SYCu calcined at 800e1030 C are presented in Fig. 2. Carbonates SrCO3 and Y2Cu2O5 was detected for SYCu after calcination at 800 C for 2 h (Fig. 2a). With the increasing calcination temperature, Sr1xYxCuO2þd and Sr2xYxCuO3þd phases are found after calcination at 900 C for 2 h (Fig. 2b), with a small amount of SrY2O4 and Y2Cu2O5 as secondary phases in the samples, which are considered as the obstacle of oxygen ion migration. The sketch map of the crystal structures of SrCuO2 and Sr2CuO3 is shown in Fig. 3. A similar structure is found in both compounds, namely, it is so-
Fig. 2 e XRD patterns of the powders calcined at different temperature. Symbols indicate the presence of Sr1LxYxCuO2Dd (*), Sr2LxYxCuO3Dd (C), SrY2O4 (,), Y2Cu2O5 (B), SrCO3(-), CuO(:).
called quasi-one-dimensional structure in which CueO quadrilateral is parallel to atb plane and repeats along the a axis [25]. The simplest one-dimensional structure crystal Sr2CuO3 was taken as an example. Originally, a crystal model was built as a K2NiF4 type tetragonal structure without apical oxygen. It was found that partial apical oxygen sites could be occupied and the amount of apical oxygen is adjustable as doping mechanism [26,29,30]. Excellent oxygen ion conductivity is expected due to the large amount of oxygen vacancy in this structure [31]. And the case is similar with SrCuO2. Moreover, in the case of synthesis of quasi-one-dimensional cuprates, several phases, such as Sr2CuO3, Sr14Cu24O41 and SrCuO2 are impeding the synthesis of single phase sample, i.e, a single phase is difficult to obtain [32,33]. According to those mentioned above, Sr1xYxCuO2þd and Sr2xYxCuO3þd in the compounds are regarded as useful phases for oxygen ionic conducting. Assuming that the little doping has no effect on the reference intensity ratios (RIR) values, the relative contents were calculated by Jade 6.5. The total relative content of Sr1xYxCuO2þd and Sr2xYxCuO3þd has been listed in Table 1. With the increasing calcination temperature, the relative content of useful phases decreases. The average valence of Cu in SYCu calcined at 950 C is measured by iodometry method. The copper valence is Cu1.93þ in Sr0.7Y0.3CuO2þd under ambient condition and the stoichiometry for the sample is Sr0.7Y0.3CuO2.115 at room temperature. The XRD patterns of SYCu and SDC powder mixtures after co-fired at 950 C for 5 h are shown in Fig. 4. All peaks have been labeled with SYCu and SDC patterns, respectively, and no other phases are observed, indicating that SYCu has a good chemical compatibility with SDC electrolyte.
3.3.
Impedance spectra
Fig. 5 shows the impedance spectra measured from 500 to 800 C for SYCujSDCjSYCu symmetric cells. All of the impedance spectra were ohmic resistance-corrected for the sake of comparison. The difference between low-frequency and highfrequency intercepts with real axis can be approximated to the resistance of the interfacial polarization resistance (Rp). It is observed that Rp decreases gradually as temperature
Fig. 3 e The schematic view of the crystal structure of Sr2CuO3.
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Table 1 e The relative contents of several phases formed in Sr0.7Y0.3CuO2Dd (SYCu) after calculating at different temperature. Temperature ( C) 900 950 1000 1030
Sr1xYxCuO2þd
Sr2xYxCuO3þd
SrY2O4
Y2Cu2O5
Sr1xYxCuO2þd þ Sr2xYxCuO3þd
0.596 0.108 0.265 0.184
0.28 0.666 w0 w0
0.123 0.089 0.449 0.35
w0 0.137 0.286 0.466
0.876 0.774 0.265 0.184
increases from 500 to 800 C. At 700 C the Rp is 0.11 U cm2, which meets with the requirement of IT-SOFC [34]. Gratifyingly, the Rp is 0.029 U cm2 at 800 C, about 74% lower than that of a recently reported novel cobalt based cathode Sr0.3Y0.7CoO2.65d (0.11 U cm2) [35]. The impedance spectra are evaluated by fitting impedance data with the equivalent circuit shown in Fig. 6, where Rs is the overall ohmic resistances including the electrolyte resistance, electrode resistance, lead resistance and contact resistance. Rct (high-frequency resistance) is interpreted to oxygen ion transferring from the TPB to the electrolyte, and Rd (low-frequency resistance) is related to dissociative absorption of O2 or the diffusion of neutral oxygen on surface [36]. CPE1 and CPE2 are the corresponding constant phase elements. The fitting results are shown in Table 2. Fig. 7 shows that SYCu yields an activation energy of w111.3 kJ/mol, which is close to that of Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF, 120.3 kJ/mol) [37], and much smaller than La0.8Sr0.2CoO3d (164 kJ/mol) [38]. All these indicate that SYCu has a high electro-catalytic activity for oxygen reduction reaction at intermediate temperature. Fig. 8 shows that SYCu has a higher Ea for Rct than Rd. Combined with Table 2, a ratelimiting step transition can be observed at 650 C. Originally, the resistance related to oxygen ion transferring from the TPB to the electrolyte owns an approximate double value. As the temperature increases, the Rct decreases dramatically. Between 600 and 650 C the transition occurs, and dissociative absorption of O2/the diffusion of neutral oxygen on the
surface turn into the dominated step at a temperature higher than 650 C. The Ea of Rd is 97.2 kJ/mol for SYCu, which is much smaller than that of LSCu (158 kJ/mol) [18]. The immense difference could be attributed to the higher oxygen ion motility of SYCu than that of LSCu [39]. It has also been proved that a large amount of oxygen defects exists in Sr2CuO3 [26,31]. The similar structure in SYCu provides favorable pathways for oxygen ion migration. However, further researches should be carried out to interpret the oxygen diffusion dynamics in SYCu. In this study sintering temperature has an influence on both composition and microstructure of electrode. Therefore, analysis must be carried out based on the two sides. Fig. 9 shows the impedance spectra of SYCu cathode sintered at 900e1000 C. It is observed that the sample sintered at 900 C has a higher Rp than that of two other samples, though the least useful phases are obtained. It is obvious that high-frequency arc interpreted as oxygen ion transfer from the TPB to the electrolyte or grain boundary resistance [36] make up main part of impedance arc. Thence, such a high resistance is attributed to poor adhesion between electrode and electrolyte. On the contrary, a big diameter of low-frequency is observed in the sample sintered at 1000 C. The lowfrequency arc is interpreted as the diffusion of neutral oxygen on surface and the diffusion of oxygen molecules in electrode pores. Generally, the arc representing the diffusion of oxygen molecules occurs only at a low oxygen partial pressure. In this study cathode thin film melted partly after sintering at 1000 C, and the porous microstructure was destroyed which lead to the difficult diffusion of oxygen molecules in the bulk. The resistances are fitted and shown in Fig. 10. The low-frequency resistances remain almost constant above 700 C and make up above 60% of polarization resistances. Therefore, the diffusion of oxygen molecules in the bulk is concerned as rate-determining step, which supports the above-mentioned inference. In addition, the increase of second phase in SYCu cathode may contribute to the polarization resistance as well.
3.4.
Fig. 4 e XRD patterns for SYCu, SDC and SYCu/SDC powders. The symbols stand for the different materials (*: SYCu; C: SDC).
Cathodic overpotential
Cathodic overpotential curves for SYCu as a function of current density at 800 C are shown in Fig. 11. And similar curves for La0.75Sr0.25CuO3d (LSCu) [17], Sm0.5Sr0.5CoO3d (SSC) [40], and La2NiO4þd (LN) [41] are also provided for reference. The cathodic overpotential of these electrodes increases with increasing current density. At a current density of 100 mA/ cm2, SYCu exhibits the lowest cathodic overpotential of w4.9 mV and the value is about 5.1, 12.8 and 25.5 mV lower than that of LSCu [17], SSC [40], and LN cathode at the same
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Table 2 e The fitting value of impedance spectra at various temperatures. Temp. ( C) 500 550 600 650 700 750 800
Rs(U cm2)
Rct(U cm2)
Rd(U cm2)
Rp(U cm2)
14.56 7.197 4.286 2.683 1.824 1.298 0.999
2.2280 0.9600 0.2323 0. 0785 0. 0449 0.0146 0.0139
1.1920 0.5308 0.3162 0.1800 0.0662 0. 0394 0.0151
3.4200 1.4908 0.5485 0. 2585 0.1111 0.0540 0. 0291
quite close to that of impedance value (0.029 U cm2) for SYCu at 800 C. Generally, the result confirms that SYCu is a promising cathode candidate for IT-SOFCs.
3.5.
Fig. 5 e Impedance spectra of the SYCu cathode on SDC electrolytes from 500 to 800 C.
condition [41]. The encouraging performance could be due to the existence of considerable oxygen defects as well. At low overpotential levels (20 mV), the overpotential h vs. current density I relation can be expressed by the following linear equation [42]: h¼
RT I nFi0
Thermal expansion
Close thermal expansion behavior of adjacent components is normally required to avoid cracking during thermal cycling for SOFCs. The typical physical compatibility between SYCu and SDC was investigated in term of the thermal expansion. The thermal expansion curves of SYCu and SDC samples are exhibited in Fig. 12. The calculated thermal expansion coefficient (TEC) of SYCu (11.1 106/ C) from room temperature to 800 C is much close to that of the SDC electrolyte (12.1 106/ C), and much lower than that of cobalt-based cathode, such as Ln0.6Sr0.4CoO3d (17.1e21.3 106/ C) [43] and LnBaCo2O5þd (19e21 106/ C) [44]. The lower TEC may be ascribed to that there is no spin state transition of B-site cation in copper based oxide, which is normally existed in cobalt oxides. The TEC is also smaller than that of other perovskite copper oxides, e.g. La1xSrxCuO2.5d (16.8e17.9 106/ C) [20] and La0.7Sr0.3CuO3d (13.6 106/ C) [45]. It is mentioned in iodometry measurement, the couple Cu2þ/Cuþ is discovered instead of Cu2þ/Cu3þ, differing from other copper-based materials used as SOFC cathodes. According to the theory of thermal expansion developed by Ruffa [46], the TEC value is inversely proportional to the metal-oxide bond length. Due to
(1)
where, i0 is the exchange current density, n is the number of electrons involved in the electrode reaction, F is the Faraday constant, R is the gas constant, and T is the measured temperature. The term RT/nFi0 with the unit of an ohm square centimeter (U cm2) refers to the polarization resistance of the electrode reaction at open circuit potential. The polarization resistance of the electrode reaction was derived from a linear fitting of cathodic curve within the low overpotential range. The yielded polarization resistance (0.024 U cm2) which is
Fig. 6 e Equivalent circuit of the electrode reaction.
Fig. 7 e The Arrhenius curve of the SYCu and BSCF.
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Fig. 8 e The Arrhenius curves of the Rct and Rd for SYCu and the Arrhenius curve of Rd for LSCu.
the longer ionic radii of Cu and Cu (Cu /0.54 A, Cu2þ/0.73 A, A, and all data are obtained from Shannon ionic radii Cuþ/0.77 [47]), it is not difficult to understand the smaller TEC for SYCu in this study. 2þ
3.6.
þ
3þ
Composition stability and long-term stability
To investigate the composition stability of SYCu oxides at the same calcination temperature, a sample calcined at 950 C for 10 h was characterized by XRD. The XRD patterns of SYCu oxides are shown in Fig. 13. For comparison, the XRD patterns of a sample calcined at 950 C for 2 h are also shown in Fig. 13. The angle of diffraction peaks haven’t changed and no new phase is formed after calcining for 10 h, suggesting excellent composition stability for long sintering time.
Fig. 10 e Total polarization resistance (Rp), high-frequency resistance (Rh) and low-frequency resistance (Rl) of SYCu cathode at various temperature.
Instability of cathode containing alkaline-earth elements were reported in the presence of CO2, especially BSCF [48]. It was reported that the reactivity of CO2 with BSCF occurred even in the presence of a thimbleful of CO2 at low temperature. Therefore, a sample after heat treatment at 950 C was calcined at 800 C for 100 h in air to investigate the long-term stability at operation temperature. The XRD pattern of SYCu oxides are shown in Fig. 14. For comparison, the XRD patterns of a sample calcined at 950 C for 2 h are also shown in Fig. 14. No carbonate or other new phase are observed after calcination at 800 C for about 4 days, suggesting excellent long-term stability at operation temperature in air. Correspondingly, the impedance spectra was recorded at 800 C for 100 h to verify the result obtained from XRD. Fig. 15 shows impedance spectra of the SYCu cathode measured at 800 C as a function of time. After operation at 800 C for 100 h no obvious change are observed
50
SYCu LSCu-ref[17] SSC-ref[40] LN-ref[41] fitting
Overpotential (mV)
40
30
30.4mv
20
17.7mv 10
0
10mv 4.9mv 0
50
100
150
i (A/cm2) Fig. 9 e Impedance spectra of SYCu cathode sintered at various temperatures.
Fig. 11 e Cathodic overpotential curves for SYCu, LSCu, La2NiO4Dd (LN) and SSC.
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Fig. 12 e Thermal expansion curves of SYCu and SDC measured from room temperature to 800 C. The Table of thermal expansion coefficient (TEC) is inserted. La0.6Sr0.4CoO3Ld [42] and LaBaCo2O5Dd [43] were also presented for reference.
Fig. 14 e XRD patterns of the powders calcined at 800 C for 10 h in air.
0.030
with increasing operation time, suggesting a stable electrochemical performance at operation temperature in the atmosphere.
0h 20h 40h 60h 80h 100h
0.025
2
-ImZ (Ω ·cm )
0.020
4.
Conclusion
Sr0.7Y0.3CuO2þd (SYCu) with a dominant one-dimensional structure has been synthesized by GNP method and shows novel high catalytic activity for oxygen reduction reaction. A small polarization resistance (Rp) of 0.029 U cm2 and cathodic overpotential of 4.9 mV (at a current density of 100 mA/cm2) has been achieved for SYCu cathode on SDC electrolyte at 800 C. The activated energy (Ea) for Rp is close to BSCF cathode. Furthermore, SYCu is much compatible with SDC
0.015
0.010
0.005
0.000 0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
2
ReZ ( Ω ·cm )
Fig. 15 e Impedance spectra of the SYCu cathode at 800 C as a function of time.
electrolyte both chemically and physically comparing with corresponding cobalt based cathode materials. In addition, the polarization resistance of SYCu cathode remain constant after operation at 800 C for 100 h, showing excellent longterm stability at operation temperature. It is proved that SYCu is attractive cathode material for intermediate temperature SOFCs.
Acknowledgments
Fig. 13 e XRD patterns of the powders calcined at 950 C for 2 h and 10 h.
The authors gratefully acknowledge the financial supports provided by the Natural Science Foundation of China (No. 50902069, 21001064), Natural Science Foundation of Jiangsu Province (No. BK2012806) and the Fundamental Research Funds for the Central Universities (No. 30920130111022).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 0 3 0 e1 0 3 8
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