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Electrochemical performance of La2Cu1−xCoxO4 cathode materials for intermediate-temperature SOFCs
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Electrochemical performance of La2Cu1LxCoxO4 cathode materials for intermediate-temperature SOFCs Qiang Li a, Xu Zeng a, Liping Sun a, Hui Zhao a,*, Lihua Huo a, Jean-Claude Grenier b a b
Key Laboratory of Functional Inorganic Material Chemistry, Heilongjiang University, Ministry of Education Harbin 150080, PR China ICMC Bordeaux-CNRS, 87 Avenue du Dr. A. Schweitzer, Universite´ de Bordeaux 1, F-33608, Bordeaux, France
article info
abstract
Article history:
K2NiF4-type structure oxides La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) are synthesized and evaluated
Received 20 September 2011
as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The
Received in revised form
materials are characterized by XRD, SEM and electrochemical impedance spectrum (EIS),
1 November 2011
respectively. The results show that no reaction occurs between La2Cu1xCoxO4 electrode
Accepted 4 November 2011
and Ce0.9Gd0.1O1.95 (CGO) electrolyte at 1000 C. The electrode forms good contact with the
Available online 30 November 2011
electrolyte after sintering at 800 C for 4 h in air. The electrode properties of La2Cu1xCoxO4 are studied under various temperatures and oxygen partial pressures. The optimum composition of La2Cu0.8Co0.2O4 results in 0.51 U cm2 polarization resistance (Rp) at 700 C in
Keywords: Intermediate
temperature
solid
air. The rate limiting step for oxygen reduction reaction (ORR) is the charge transfer
oxide fuel cells
process. La2Cu0.8Co0.2O4 cathode exhibits the lowest overpotential of about 50 mV at
La2Cu1xCoxO4 cathode material
a current density of 48 mA cm2 at 700 C in air.
Electrochemical properties
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) that convert directly chemical energy into electrical energy through electrochemical processes have been regarded as a promising energy conversion and generation system due to their high efficiency, fuel adaptability and low pollution [1,2]. There is increasing interest in the development of intermediate temperature solid oxide fuel cells (IT-SOFCs). Reducing operation temperature lowers problems with sealing and thermal degradation, and allows the use of low-cost interconnection materials. The improvement of cathode performance is one of the most important issues for the development of IT-SOFCs. Up to now, most of the studies concerned with cathode materials for IT-SOFCs are devoted to various perovskite-type oxides, such as La1xSrxCo1-yFeyO3, Ba0.5Sr0.5CoxFe1xO3 [3,4]. Recently, some mixed ionic-electronic conducting materials
(MIECs) with K2NiF4 structure were reported [5,6]. These oxides are usually formulated as A2BO4, which can be regarded as a staking of ABO3 perovskite layers alternating with AO rock-salt layers along the c-direction [7]. These oxides have been demonstrated to be able to accommodate a significant amount of oxygen non-stoichiometry in the structure. Preliminary results are promising in terms of oxygen diffusion and surface exchange coefficients. These A2BO4 compounds exhibit relatively high oxygen-ion diffusivity, compatible thermal expansion coefficients (TECs) with solid electrolytes, and predominant p-type electronic conductivity in whole p(O2) range where the K2NiF4-type phase exists [8,9]. These results imply that K2NiF4 oxides are likely to be new cathode materials for IT-SOFCs. The interesting transport properties of these La2CuO4 derivatives are due to their capability to accommodate a wide range of oxygen non-stoichiometry that can be
* Corresponding author. E-mail address: [email protected] (H. Zhao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.014
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modified by slight changes in the composition or by oxidation-reduction treatments. In this way, several copper based systems were proposed by doping either at the La position with alkaline-earth elements (Sr or Ba, for example) [10] or at the Cu position with other transition metals, typically Co and Ni [11,12]. Seok et al. [13] reported that the electrical conductivity of Co-doped La2CuO4 increased with the increase of Co content at elevated temperature. This behavior was attributed to the ability of Co ions to localize charge carriers derived by excess oxygen and also to an effect of hole doping in the sx2 -y2 conduction band by fewer 3d electrons of Co ions (compared to copper ions). Another study [14] indicated that incorporating cobalt ions into the copper sublattice increased interstitial oxygen content, and the oxygen permeability increased. Thermal expansion coefficients of La2Cu1xCoxO4 (x ¼ 0.02e0.3) materials were found to be in the range 12.2e13.2 106 K1 at 470e1100 K, which are compatible with many solid electrolyte materials such as Ce0.9Gd0.1O1.95 (CGO), Ce0.8Sm0.2O1.95 (SDC) and so on. These results inspire us to investigate La2Cu1xCoxO4 compounds as possible cathode materials for IT-SOFCs. In this preliminary work, the electrochemical properties of Codoped La2CuO4 materials supported on CGO electrolyte and the kinetics of oxygen reduction on these electrodes are studied.
2.
Experimental
The single-phase La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3, 0.4) powders were prepared through solid state reaction according to ref. [11]. The Ce0.9Gd0.1O1.95 (CGO) powders were prepared according to ref. [15]. CGO powders were first pressed uniaxially at 220 MPa to form a pellet and then sintered at 1400 C for 10 h. The La2Cu1xCoxO4 powders were mixed with terpineol to form ink, which was subsequently painted on one side of the CGO pellet to form working electrode (WE) with area of 0.5 cm2. Platinum paste was painted on the other side of the pellet in symmetric configuration, as the counter electrode (CE). A Pt wire was used as reference electrode (RE) and put on the same side of the working electrode. The RE was normally placed 2e5 mm from the WE, ensuring that this distance was at least three times the thickness of the electrolyte. Pt gauze attached to a Pt wire, was then press-contacted to the La2Cu1xCoxO4 and Pt paste electrodes with the aid of a spring loaded ceramic cap, thus serving as the final current collector. The cell was first heated up to 400 C to eliminate organic binders, followed by sintering at 800 C for 4 h in air. The structure and phase purity of the materials were investigated by X-ray powder diffraction on a Bruker D8Advance diffractometer Cu Ka radiation. Fourier-transform infrared (FT-IR) spectroscopy of the La2Cu1xCoxO4 materials was performed on a Bruker Equinox 55 spectrophotometer in the mid-IR range from 4000 to 400 cm1 and with a resolution of 4 cm1 using KBr pellets as standards. The morphology and microstructure of the sintered electrodes were examined with Hitachi S-4800 FEG-SEM. The conductivities of cathode materials were measured with the DC 4 terminal method in air using Keithley 2400 digital source meter and AI 808P program temperature controller controlled by computer, from 300 to
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800 C. The electrochemical impedance spectroscopy (EIS) was recorded over the frequency range 0.1 Hze1 MH using AutolabPGStat30, the EIS fitting analysis was performed with the Zview software. The measurements were performed at equilibrium potential as a function of temperature (500e700 C). The dc polarization experiments were performed by the chronoamperometry method [16], which involved a potential step followed by recording the current density as a function of time. The cathode overpotential was calculated according to the following equation: hWE ¼ DUWR iRel where hWE represents the cathode overpotential, DUWR is the applied voltage between working electrode and reference electrode, i is the current flowing through the test cell and Rel is the resistance of the electrolyte obtained from the impedance spectrum.
3.
Results and discussions
Fig. 1 shows the X-ray diffraction (XRD) patterns of La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3, 0.4) powders after sintered at 1000 C for 24 h in air. The samples with x ¼ 0.1e0.3 possess a single phase with K2NiF4-type structure, which is in agreement with the literature results [14]. When the value of x increases to 0.4, the main diffraction peaks of La2Cu1xCoxO4 shift to higher values. Some additional diffraction peaks appear, indicating the formation of impurity phase La4Co3O9 (Fig. 1, inset). Therefore the nominal composite La2Cu0.6Co0.4O4 will not be further evaluated in this study. As we know that in some cases, the interfacial chemical reaction between cathode and electrolyte is undesirable for long term stability of SOFCs, which increase the interfacial polarization resistance, and thus resulting in degradation of the performance of SOFCs [17], it is necessary to investigate the possible chemical reaction between. As an example, the reactivity of La2Cu0.8Co0.2O4 with the CGO electrolyte was studied by mixing La2Cu0.8Co0.2O4 with CGO powders in 1:1 weight ratio, and then heattreating at 1000 C for 12 h in air. Fig. 2 shows the XRD patterns
Fig. 1 e XRD patterns of La2Cu1LxCoxO4 (x [ 0.1, 0.2, 0.3, 0.4) materials sintered at 1000 C for 12 h in air.
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Fig. 2 e XRD patterns of La2Cu0.8Co0.2O4 (a), CGO (b) and La2Cu0.8Co0.2O4-CGO mixtures after sintering at 1000 C for 12 h in air.
of La2Cu0.8Co0.2O4, CGO and the heat-treated La2Cu0.8Co0.2O4CGO mixture. It is observed that in the mixture, La2Cu0.8Co0.2O4 and CGO remain their structures unchanged, no new diffraction patterns are detected except those from La2Cu0.8 Co0.2O4 and CGO (Fig. 2(c)). This result indicates that La2Cu0.8Co0.2O4 has good chemical compatibility with CGO electrolyte. Fig. 3 shows the FT-IR spectrum of La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) material. The absorption peak at 516 cm1 is assigned to the retractile vibration of the A-OⅡ-B bond in the A2BO4 compound, which is characteristic absorption of A2BO4 materials in the FT-IR spectrum [18]. A strong absorption band was observed between 650 and 670 cm1, which is assigned to the bending modes of the Cu(Co)-OⅠ linkages in the basal planes [19]. The absorption peaks become stronger with the increase of Co doping content, which is consistent with
Fig. 3 e FT-IR spectra of La2Cu1LxCoxO4 (x [ 0.1, 0.2, 0.3) cathode materials.
decrease of the Jahn-Teller distortion of the CuO6 octahedra and increase of symmetry of the crystal structure. Previous study [14] has indicated that the CueO bonds along the c axis are longer than within the basal (a, b) plane because of a strong Jahn-Teller effect of Cu2þ ion in the octahedral site of CuO6 in K2NiF4 structure La2CuO4. Doping with Co ions reduces the local Jahn-Teller distortion, and hence reduces the c axis and increases the a axis. On the other hand, the crystal structure of K2NiF4 was composed of orbital with sx2 y2 of the itinerant electron state in the a-b plane and localized electron in dz2 orbital lying parallel to the c axis [20]. It is proposed that the charge localization and the increase of hole concentration in the basal plane of delocalized states by Co ions play major roles in the establishment of the electrical conduction in Co doped La2CuO4. Arrhenius plots for the electrical conductivity of La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) measured in air are shown in Fig. 4. It was found that the electrical conductivity of the La2Cu1xCoxO4 oxides gradually increases with increasing temperature, indicating the conducting behavior of semiconductors. The conductivities, s, were measured in the temperature range 300e800 C in air. The activation energies for conduction were determined from the plots, using the following expression derived for the small polaron mechanism [21]: s¼
A Ea exp kT T
where A is the preexponential factor, k the Boltzmann constant, T the absolute temperature and Ea the activation energy. The highest electrical conductivity is obtained for La2Cu0.8Co0.2O4 with a value of 28 S cm1 at 800 C. The electrical conductivity of La2Cu1xCoxO4 improved with doping of cobalt, which can be assigned to the progressive delocalization of the copper atomic as well as decreasing bandwidth in the structure. The activation energy (Ea) calculated from the linear fit is 0.31e0.42 eV, similar to the reported literature values [14]. In the experiments, we found that the sintering condition had remarkable effects on the electrode performance. Fig. 5 is
Fig. 4 e Arrhenius plots of electrical conductivity of La2Cu1LxCoxO4 (x [ 0.1, 0.2, 0.3).
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Fig. 5 e Nyquist plot of La2Cu0.8Co0.2O4 cathode sintered at different temperatures for 4 h and measured at 700 C in air.
a typical Nyquist plot of La2Cu0.8Co0.2O4 cathode on CGO electrolyte after sintering at different temperatures for 4 h and then measured at 700 C in air. From the impedance spectrum, it is observed that there are at least two arcs presented at low sintering temperature. These arcs tended to merge into one arc with the increase of sintering temperature. The intercept value of the impedance arcs with the real axis at high frequency corresponds to the resistance of the electrolyte and lead wires, while the difference between the intercept values of the diagram with the real axis at high and low frequencies is attributed to the cathode polarization resistance (Rp) of La2Cu0.8Co0.2O4 electrode. The Rp was relatively large when the sintering temperature was low (750 C). When the sintering temperature reached 800 C, Rp reduced to the lowest value. Rp increased again when the sintering temperature was up to 900 C. The effects of sintering temperature on Rp can be understood from the microstructure evolution of La2Cu0.8Co0.2O4 electrode. Poor contact was formed between the electrode/electrolyte interface at low sintering temperature (750 C), whereas high sintering temperature (900 C) led to the over-sintering phenomena of La2Cu0.8Co0.2O4 electrode, such as particle agglomeration. This effect decreased the electrode porosity and triple phase boundary (TPB) length, resulting in the increase of Rp. The similar over-sintering effect has been observed before in the other cathode materials [22]. Fig. 6 is the SEM image of La2Cu0.8Co0.2O4 electrode on CGO electrolyte after sintering at 800 C for 4 h. A fine microstructure with moderate porosity and well-necked particles has been formed. The average particle size was about 2 mm and the average thickness value of the painted cathode layer was about 10 mm, and the deviation of different electrodes does not exceed 2 mm, respectively. This kind of microstructure is benefit for the improved cathode property, as that observed from Fig. 5. Hereafter, all the studied cathodes below were sintered at 800 C for 4 h, to obtain the best sintering performance. The temperature dependence of polarization resistance (Rp) for La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) materials is presented
Fig. 6 e SEM images of the La2Cu0.8Co0.2O4 electrode sintered at 800 C (a) and the cross-section image (b).
in Fig. 7, together with the calculated activation energy. It was found that the activation energy values are around 1.29e1.53 eV for all the samples studied. La2Cu0.8Co0.2O4 gave the lowest polarization resistance among the Co doped
Fig. 7 e Arrhenius plots of the Rp for La2Cu1LxCoxO4 cathodes measured in air.
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La2Cu1xCoxO4. It is 0.51 U cm2 at 700 C in air, which is smaller than that of nickel doped La2Cu0.6Ni0.4O4 cathodes [12], but still larger than these well-known materials, such as Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) [23]. The reason that La2Cu0.8Co0.2O4 exhibits the lowest polarization resistance maybe due to the fact that this material has the highest electrical conductivity among La2Cu1xCoxO4 materials. The high electrical conductivity has been found to be highly beneficial to extend the active oxygen reduction site from the TPB to the whole exposed cathode surface, thus significantly reduce the cathode polarization resistance [24], and enhance the electrocatalytic properties of the electrode. Hereafter, all the electrode behavior studies are performed on La2Cu0.8Co0.2O4 material with CGO electrolyte. To study the oxygen reduction reaction (ORR) that occurred on the electrode, impedance spectra measurements were performed as a function of oxygen partial pressure. Fig. 8 shows typical impedance spectra of the LCC/CGO test cell measured at 700 C under various oxygen partial pressures ðPO2 Þ. From the impedance spectrum, it is observed that for all the cathodes measured under variable oxygen partial pressures, the impedance spectra can be separated into two arcs located in the high frequency zone and the low frequency zone, respectively. This suggests that the oxygen reduction reaction over the electrodes is composed of at least two different processes. In order to clarify this point, an equivalent circuit was used to fit the measurement data (Fig. 8 inset). Here Rel represents the intercept value of the impedance spectrum at high frequency side with the real axis, which corresponds to the resistance of the electrolyte and lead wires. RH and RL are the resistance corresponding to the arc1 and arc2, respectively. CPE is a constant phase element. The total polarization resistance (Rp) is the sum of RH and RL. From the fitting results, it can be found that the resistance of the low frequency arc (RL) is much larger than that of the high frequency arc (RH). This means that the RL related process is the reaction rate-limiting step. The corresponding capacitances (obtained from the relaxation frequency of the two arcs, according to the relation f ¼ 1/2pRC) are about 103 and
101 F for the high and low frequency arcs respectively, and remain unchanged with the oxygen partial pressure. These values are similar to those previously reported for the Nd2NiO4 cathode [25]. It was consistent with electrochemical reactions that occurred on electrode [26]. These results further prove that two arcs exist in the impedance spectrum, and they represent two different electrochemical processes occurred on electrode. The electrode polarization resistances (Rp) of La2Cu0.8 Co0.2O4 cathode at 600e700 C under various PO2 are shown in Fig. 9 (a and b), from which the PO2 dependence of the Rp can be calculated. Generally, Rp varies with the oxygen partial pressure according to the following equation: Rp ¼ Rop PO2
n
n ¼ 1=2; O2;ads: %2Oads: (oxygen surface diffusion)
adsorption,
dissociation
and
surface
n ¼ 1=4; Oads: þ 2e= þ VO,, %OxO (charge transfer process)
a
b
Fig. 8 e Impedance spectra of La2Cu0.8Co0.2O4 electrode at 700 C under various oxygen partial.
Fig. 9 e Dependence of RH (a) and RL (b) on oxygen partial pressure for La2Cu0.8Co0.2O4 cathode at 600e700 C.
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x ,, n ¼ 0; O2 TPB þ VO %OO
(oxygen ion transfer into the electrolyte) The value of n could give useful information about the type 1=4 of species involved in the reactions [27,28]. ThePO2 relationship was considered as the contribution of the charge transfer process on the electrode [29], which was observed before for Ba0.5Sr0.5Co0.8Fe0.2O3 and La2xSrxCuO4 cathodes [30,31]. In a previous study, thePO O2 relationship has been observed for La0.8Sr0.2Co1xMnxO3 and Sr1.5LaxMnO4 cathode materials [32,33]. It was due to the oxygen ion transfer into the electrolyte. According to Fig. 9(a and b), we observed that n value was around 0 for RH and 0.25 for RL, respectively. Considering the weak dependence of RH on oxygen partial pressure, and the mixed conducting properties of La2Cu1xCoxO4 material, we propose that the high frequency arc was caused by the oxygen ion transfer form the TPB to CGO electrolyte, similar to those reported K2NiF4-type cathode materials, such as Ln2NiO4 [34]; whereas the low frequency arc was due to the charge transfer process on the electrode. Compared with the results obtained in Fig. 9(a and b), it is also observed that RL was always larger than RH in the whole range of measurement oxygen partial pressure from 600 to 700 C. Therefore it is concluded that the charge transfer process is the major rate limiting step for La2Cu1xCoxO4 cathode. Cathodic overpotential is another important parameters to evaluate electrode performance. Fig. 10 shows cathodic polarization curves for La2Cu0.8Co0.2O4 cathode as a function of temperature. It was observed that at fixed overpotential, the current density increased with increasing temperatures, indicating a thermo-activated electrochemical reaction was improved with the temperature. The lowest polarization overpotential, 50 mV was obtained for La2Cu0.8Co0.2O4 cathode at a current density of 48 mA cm2 at 700 C in air. This overpotential value is quite lower than the reported La2Cu0.2 Ni0.8O4 cathode material [35]. At low overpotential (less than 20 mV), we can expect a linear expression [36], i ¼ i0ZFh/RT, where i is the current density, i0 the exchange current density, n the overpotential, and F, R have their normal meanings,
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respectively. From the inverse of the derivative of i against n, we can obtain the polarization resistance (Rp) for La2Cu0.8 Co0.2O4 cathode. The Rp value obtained at 700 C in air was 0.55 U cm2, which is in agreement with the result obtained from impedance measurement. As we expected, the enhanced conductivity combined with the increase of interstitial oxygen formed by Co doping in La2CuO4 materials makes low polarization resistance possible. Compared to the LSCF cathode material, the polarization resistance of La2Cu1xCoxO4 material is still high. Whereas considering the thermal expansion coefficient (TEC), La2Cu1xCoxO4 has a much similar TEC (La2Cu1xCoxO4, (12.6e13.2) 106 K1 in air [14]) compared to that of the CGO (13.1 106 K1) electrolyte, whereas the TEC value of LSCF material is quite large (La0.6Sr0.4Co0.2Fe0.8O3, 17.5 106 K1 in ari [37]). Therefore, the La2Cu1xCoxO4 material can be considered as a promising cathode candidate for IT-SOFCs, given that the electrode performances can be further improved by optimizing the microstructure of electrode, such as to form nanofiber or nanotube cathodes, and so on.
4.
Conclusions
La2Cu1xCoxO4 materials have been studied as promising cathode for IT-SOFC based on ceria electrolyte. No reaction was found between La2Cu0.8Co0.2O4 electrode and CGO electrolyte after heat-treatment at 1000 C for 12 h in air, indicating high chemical compatibility of these materials at high temperature. The rate-limiting step for oxygen reduction reaction on La2Cu0.8Co0.2O4 electrode is the charge transfer process. The polarization resistance obtained at 700 C in air is about 0.51 U cm2 for the La2Cu0.8Co0.2O4 electrode, and the lowest overpotential is 50 mV at a current density of 48 mA cm2.
Acknowledgments The Project was supported by National Natural Science Foundation of China (51072048, 51102083), Foundation of Heilongjiang Province Postdoctoral (LBH-Z09019), China Postdoctoral Science Foundation (20100471116), University of Heilongjiang (Hdtd2010-04).
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Fig. 10 e The overpotential-current density curves for La2Cu0.8Co0.2O4 cathode measured in air at various temperatures.
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intermediate-temperature SOFCs. Int J Hydrogen Energy 2010;35:9151e7. Kim YM, Kim-Lohsoontorn P, Baek SW, Bae J. Electrochemical performance of unsintered Ba0.5Sr0.5Co0.8Fe0.2O3d, La0.6Sr0.4Co0.8Fe0.2O3d, and La0.8Sr0.2MnO3d cathodes for metal-supported solid oxide fuel cells. Int J Hydrogen Energy 2011;36:3138e46. Chun WS, Rob H, Justin R. Cathode materials for solid oxide fuel cell. J Solid State Electr 2010;14:1125e44. Mauvy F, Bassat JM, Boehm E, Manaud JP, Dordor P, Grenier JC. Oxygen electrode reaction on Nd2NiO4þd cathode materials: impedance spectroscopy study. Solid State Ionics 2003;158:17e28. Irvine JTS, Sinclair DC, West AR. Electroceramics: characterization by impedance spectroscopy. Adv Mater 1990;2:132e8. Takeda Y, Kanno R, Noda M, Yamamoto O. Cathodic polarization phenomena of perovskite oxide electrodes with stabilized zirconia. J Electrochem Soc 1987;134:A2656e61. Sicbeit E, Hammouche A, Kleitz M. Impedance spectroscopy analysis of La1xSrxMnO3-yttria-stabilized zirconia electrode kinetics. Electrochim Acta 1995;40:1741e53. He F, Wu TZ, Peng RR, Xia CR. Cathode reaction models and performance analysis of Sm0.5Sr0.5CoO3d-BaCe0.8Sm0.2O3d composite cathode for solid oxide fuel cells with proton conducting electrolyte. J Power Sources 2009;194:263e8. Lin Y, Ran R, Shao ZP. Silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3d as cathodes for a proton conducting solid-oxide fuel cell. Int J Hydrogen Energy 2009;35:8281e8. Li Q, Zhao H, Huo LH, Sun LP, Cheng XL, Grenier JC. Electrode properties of Sr doped La2CuO4 as new cathode material for intermediate-temperature SOFCs. Electrochem Commun 2007;9:1508e12. Lee CB, Baek SW, Bae J. Cathodic behavior of La0.8Sr0.2Co1xMnxO3-d perovskite oxide on YSZ electrolyte for intermediate temperature-operating solid oxide fuel cells. Solid State Ionics 2008;179:1465e9. Sun LP, Li Q, Huo LH, Zhao H, Zhang GY, Lin N, et al. Synthesis and performance of Sr1.5LaxMnO4 as cathode materials for intermediate temperature solid oxide fuel cell. J Power Sources 2011;196:5835e9. Mauvy F, Lalanne C, Bassat JM, Grenier JC, Zhao H, Huo LH, et al. Electrode properties of Ln2NiO4þd (Ln ¼ La, Nd, Pr) AC impedance and DC polarization studies. J Electrochem Soc 2006;153:A1547e53. Kharton VV, Tsipis EV, Yaremchenko AA, Frade JR. Surfacelimited oxygen transport and electrode properties of La2Ni0.8Cu0.2O4þd. Solid State Ionics 2004;166:327e37. Steele BCH. Interfacial reactions associated with ceramic ion transport membranes. Solid State Ionics 1995;75:157e65. Ullmann H, Trofimenko N, Tietz F, Sto¨ver D, AhmadKhanlou A. Correlation between thermal expansion and oxide ion transport in mixed conducting perovskite-type oxides for SOFC cathodes. Solid State Ionics 2000;138: 79e90.
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Electrochemical performance of La2Cu1LxCoxO4 cathode materials for intermediate-temperature SOFCs Qiang Li a, Xu Zeng a, Liping Sun a, Hui Zhao a,*, Lihua Huo a, Jean-Claude Grenier b a b
Key Laboratory of Functional Inorganic Material Chemistry, Heilongjiang University, Ministry of Education Harbin 150080, PR China ICMC Bordeaux-CNRS, 87 Avenue du Dr. A. Schweitzer, Universite´ de Bordeaux 1, F-33608, Bordeaux, France
article info
abstract
Article history:
K2NiF4-type structure oxides La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) are synthesized and evaluated
Received 20 September 2011
as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The
Received in revised form
materials are characterized by XRD, SEM and electrochemical impedance spectrum (EIS),
1 November 2011
respectively. The results show that no reaction occurs between La2Cu1xCoxO4 electrode
Accepted 4 November 2011
and Ce0.9Gd0.1O1.95 (CGO) electrolyte at 1000 C. The electrode forms good contact with the
Available online 30 November 2011
electrolyte after sintering at 800 C for 4 h in air. The electrode properties of La2Cu1xCoxO4 are studied under various temperatures and oxygen partial pressures. The optimum composition of La2Cu0.8Co0.2O4 results in 0.51 U cm2 polarization resistance (Rp) at 700 C in
Keywords: Intermediate
temperature
solid
air. The rate limiting step for oxygen reduction reaction (ORR) is the charge transfer
oxide fuel cells
process. La2Cu0.8Co0.2O4 cathode exhibits the lowest overpotential of about 50 mV at
La2Cu1xCoxO4 cathode material
a current density of 48 mA cm2 at 700 C in air.
Electrochemical properties
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) that convert directly chemical energy into electrical energy through electrochemical processes have been regarded as a promising energy conversion and generation system due to their high efficiency, fuel adaptability and low pollution [1,2]. There is increasing interest in the development of intermediate temperature solid oxide fuel cells (IT-SOFCs). Reducing operation temperature lowers problems with sealing and thermal degradation, and allows the use of low-cost interconnection materials. The improvement of cathode performance is one of the most important issues for the development of IT-SOFCs. Up to now, most of the studies concerned with cathode materials for IT-SOFCs are devoted to various perovskite-type oxides, such as La1xSrxCo1-yFeyO3, Ba0.5Sr0.5CoxFe1xO3 [3,4]. Recently, some mixed ionic-electronic conducting materials
(MIECs) with K2NiF4 structure were reported [5,6]. These oxides are usually formulated as A2BO4, which can be regarded as a staking of ABO3 perovskite layers alternating with AO rock-salt layers along the c-direction [7]. These oxides have been demonstrated to be able to accommodate a significant amount of oxygen non-stoichiometry in the structure. Preliminary results are promising in terms of oxygen diffusion and surface exchange coefficients. These A2BO4 compounds exhibit relatively high oxygen-ion diffusivity, compatible thermal expansion coefficients (TECs) with solid electrolytes, and predominant p-type electronic conductivity in whole p(O2) range where the K2NiF4-type phase exists [8,9]. These results imply that K2NiF4 oxides are likely to be new cathode materials for IT-SOFCs. The interesting transport properties of these La2CuO4 derivatives are due to their capability to accommodate a wide range of oxygen non-stoichiometry that can be
* Corresponding author. E-mail address: [email protected] (H. Zhao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.014
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modified by slight changes in the composition or by oxidation-reduction treatments. In this way, several copper based systems were proposed by doping either at the La position with alkaline-earth elements (Sr or Ba, for example) [10] or at the Cu position with other transition metals, typically Co and Ni [11,12]. Seok et al. [13] reported that the electrical conductivity of Co-doped La2CuO4 increased with the increase of Co content at elevated temperature. This behavior was attributed to the ability of Co ions to localize charge carriers derived by excess oxygen and also to an effect of hole doping in the sx2 -y2 conduction band by fewer 3d electrons of Co ions (compared to copper ions). Another study [14] indicated that incorporating cobalt ions into the copper sublattice increased interstitial oxygen content, and the oxygen permeability increased. Thermal expansion coefficients of La2Cu1xCoxO4 (x ¼ 0.02e0.3) materials were found to be in the range 12.2e13.2 106 K1 at 470e1100 K, which are compatible with many solid electrolyte materials such as Ce0.9Gd0.1O1.95 (CGO), Ce0.8Sm0.2O1.95 (SDC) and so on. These results inspire us to investigate La2Cu1xCoxO4 compounds as possible cathode materials for IT-SOFCs. In this preliminary work, the electrochemical properties of Codoped La2CuO4 materials supported on CGO electrolyte and the kinetics of oxygen reduction on these electrodes are studied.
2.
Experimental
The single-phase La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3, 0.4) powders were prepared through solid state reaction according to ref. [11]. The Ce0.9Gd0.1O1.95 (CGO) powders were prepared according to ref. [15]. CGO powders were first pressed uniaxially at 220 MPa to form a pellet and then sintered at 1400 C for 10 h. The La2Cu1xCoxO4 powders were mixed with terpineol to form ink, which was subsequently painted on one side of the CGO pellet to form working electrode (WE) with area of 0.5 cm2. Platinum paste was painted on the other side of the pellet in symmetric configuration, as the counter electrode (CE). A Pt wire was used as reference electrode (RE) and put on the same side of the working electrode. The RE was normally placed 2e5 mm from the WE, ensuring that this distance was at least three times the thickness of the electrolyte. Pt gauze attached to a Pt wire, was then press-contacted to the La2Cu1xCoxO4 and Pt paste electrodes with the aid of a spring loaded ceramic cap, thus serving as the final current collector. The cell was first heated up to 400 C to eliminate organic binders, followed by sintering at 800 C for 4 h in air. The structure and phase purity of the materials were investigated by X-ray powder diffraction on a Bruker D8Advance diffractometer Cu Ka radiation. Fourier-transform infrared (FT-IR) spectroscopy of the La2Cu1xCoxO4 materials was performed on a Bruker Equinox 55 spectrophotometer in the mid-IR range from 4000 to 400 cm1 and with a resolution of 4 cm1 using KBr pellets as standards. The morphology and microstructure of the sintered electrodes were examined with Hitachi S-4800 FEG-SEM. The conductivities of cathode materials were measured with the DC 4 terminal method in air using Keithley 2400 digital source meter and AI 808P program temperature controller controlled by computer, from 300 to
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800 C. The electrochemical impedance spectroscopy (EIS) was recorded over the frequency range 0.1 Hze1 MH using AutolabPGStat30, the EIS fitting analysis was performed with the Zview software. The measurements were performed at equilibrium potential as a function of temperature (500e700 C). The dc polarization experiments were performed by the chronoamperometry method [16], which involved a potential step followed by recording the current density as a function of time. The cathode overpotential was calculated according to the following equation: hWE ¼ DUWR iRel where hWE represents the cathode overpotential, DUWR is the applied voltage between working electrode and reference electrode, i is the current flowing through the test cell and Rel is the resistance of the electrolyte obtained from the impedance spectrum.
3.
Results and discussions
Fig. 1 shows the X-ray diffraction (XRD) patterns of La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3, 0.4) powders after sintered at 1000 C for 24 h in air. The samples with x ¼ 0.1e0.3 possess a single phase with K2NiF4-type structure, which is in agreement with the literature results [14]. When the value of x increases to 0.4, the main diffraction peaks of La2Cu1xCoxO4 shift to higher values. Some additional diffraction peaks appear, indicating the formation of impurity phase La4Co3O9 (Fig. 1, inset). Therefore the nominal composite La2Cu0.6Co0.4O4 will not be further evaluated in this study. As we know that in some cases, the interfacial chemical reaction between cathode and electrolyte is undesirable for long term stability of SOFCs, which increase the interfacial polarization resistance, and thus resulting in degradation of the performance of SOFCs [17], it is necessary to investigate the possible chemical reaction between. As an example, the reactivity of La2Cu0.8Co0.2O4 with the CGO electrolyte was studied by mixing La2Cu0.8Co0.2O4 with CGO powders in 1:1 weight ratio, and then heattreating at 1000 C for 12 h in air. Fig. 2 shows the XRD patterns
Fig. 1 e XRD patterns of La2Cu1LxCoxO4 (x [ 0.1, 0.2, 0.3, 0.4) materials sintered at 1000 C for 12 h in air.
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Fig. 2 e XRD patterns of La2Cu0.8Co0.2O4 (a), CGO (b) and La2Cu0.8Co0.2O4-CGO mixtures after sintering at 1000 C for 12 h in air.
of La2Cu0.8Co0.2O4, CGO and the heat-treated La2Cu0.8Co0.2O4CGO mixture. It is observed that in the mixture, La2Cu0.8Co0.2O4 and CGO remain their structures unchanged, no new diffraction patterns are detected except those from La2Cu0.8 Co0.2O4 and CGO (Fig. 2(c)). This result indicates that La2Cu0.8Co0.2O4 has good chemical compatibility with CGO electrolyte. Fig. 3 shows the FT-IR spectrum of La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) material. The absorption peak at 516 cm1 is assigned to the retractile vibration of the A-OⅡ-B bond in the A2BO4 compound, which is characteristic absorption of A2BO4 materials in the FT-IR spectrum [18]. A strong absorption band was observed between 650 and 670 cm1, which is assigned to the bending modes of the Cu(Co)-OⅠ linkages in the basal planes [19]. The absorption peaks become stronger with the increase of Co doping content, which is consistent with
Fig. 3 e FT-IR spectra of La2Cu1LxCoxO4 (x [ 0.1, 0.2, 0.3) cathode materials.
decrease of the Jahn-Teller distortion of the CuO6 octahedra and increase of symmetry of the crystal structure. Previous study [14] has indicated that the CueO bonds along the c axis are longer than within the basal (a, b) plane because of a strong Jahn-Teller effect of Cu2þ ion in the octahedral site of CuO6 in K2NiF4 structure La2CuO4. Doping with Co ions reduces the local Jahn-Teller distortion, and hence reduces the c axis and increases the a axis. On the other hand, the crystal structure of K2NiF4 was composed of orbital with sx2 y2 of the itinerant electron state in the a-b plane and localized electron in dz2 orbital lying parallel to the c axis [20]. It is proposed that the charge localization and the increase of hole concentration in the basal plane of delocalized states by Co ions play major roles in the establishment of the electrical conduction in Co doped La2CuO4. Arrhenius plots for the electrical conductivity of La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) measured in air are shown in Fig. 4. It was found that the electrical conductivity of the La2Cu1xCoxO4 oxides gradually increases with increasing temperature, indicating the conducting behavior of semiconductors. The conductivities, s, were measured in the temperature range 300e800 C in air. The activation energies for conduction were determined from the plots, using the following expression derived for the small polaron mechanism [21]: s¼
A Ea exp kT T
where A is the preexponential factor, k the Boltzmann constant, T the absolute temperature and Ea the activation energy. The highest electrical conductivity is obtained for La2Cu0.8Co0.2O4 with a value of 28 S cm1 at 800 C. The electrical conductivity of La2Cu1xCoxO4 improved with doping of cobalt, which can be assigned to the progressive delocalization of the copper atomic as well as decreasing bandwidth in the structure. The activation energy (Ea) calculated from the linear fit is 0.31e0.42 eV, similar to the reported literature values [14]. In the experiments, we found that the sintering condition had remarkable effects on the electrode performance. Fig. 5 is
Fig. 4 e Arrhenius plots of electrical conductivity of La2Cu1LxCoxO4 (x [ 0.1, 0.2, 0.3).
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Fig. 5 e Nyquist plot of La2Cu0.8Co0.2O4 cathode sintered at different temperatures for 4 h and measured at 700 C in air.
a typical Nyquist plot of La2Cu0.8Co0.2O4 cathode on CGO electrolyte after sintering at different temperatures for 4 h and then measured at 700 C in air. From the impedance spectrum, it is observed that there are at least two arcs presented at low sintering temperature. These arcs tended to merge into one arc with the increase of sintering temperature. The intercept value of the impedance arcs with the real axis at high frequency corresponds to the resistance of the electrolyte and lead wires, while the difference between the intercept values of the diagram with the real axis at high and low frequencies is attributed to the cathode polarization resistance (Rp) of La2Cu0.8Co0.2O4 electrode. The Rp was relatively large when the sintering temperature was low (750 C). When the sintering temperature reached 800 C, Rp reduced to the lowest value. Rp increased again when the sintering temperature was up to 900 C. The effects of sintering temperature on Rp can be understood from the microstructure evolution of La2Cu0.8Co0.2O4 electrode. Poor contact was formed between the electrode/electrolyte interface at low sintering temperature (750 C), whereas high sintering temperature (900 C) led to the over-sintering phenomena of La2Cu0.8Co0.2O4 electrode, such as particle agglomeration. This effect decreased the electrode porosity and triple phase boundary (TPB) length, resulting in the increase of Rp. The similar over-sintering effect has been observed before in the other cathode materials [22]. Fig. 6 is the SEM image of La2Cu0.8Co0.2O4 electrode on CGO electrolyte after sintering at 800 C for 4 h. A fine microstructure with moderate porosity and well-necked particles has been formed. The average particle size was about 2 mm and the average thickness value of the painted cathode layer was about 10 mm, and the deviation of different electrodes does not exceed 2 mm, respectively. This kind of microstructure is benefit for the improved cathode property, as that observed from Fig. 5. Hereafter, all the studied cathodes below were sintered at 800 C for 4 h, to obtain the best sintering performance. The temperature dependence of polarization resistance (Rp) for La2Cu1xCoxO4 (x ¼ 0.1, 0.2, 0.3) materials is presented
Fig. 6 e SEM images of the La2Cu0.8Co0.2O4 electrode sintered at 800 C (a) and the cross-section image (b).
in Fig. 7, together with the calculated activation energy. It was found that the activation energy values are around 1.29e1.53 eV for all the samples studied. La2Cu0.8Co0.2O4 gave the lowest polarization resistance among the Co doped
Fig. 7 e Arrhenius plots of the Rp for La2Cu1LxCoxO4 cathodes measured in air.
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La2Cu1xCoxO4. It is 0.51 U cm2 at 700 C in air, which is smaller than that of nickel doped La2Cu0.6Ni0.4O4 cathodes [12], but still larger than these well-known materials, such as Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) [23]. The reason that La2Cu0.8Co0.2O4 exhibits the lowest polarization resistance maybe due to the fact that this material has the highest electrical conductivity among La2Cu1xCoxO4 materials. The high electrical conductivity has been found to be highly beneficial to extend the active oxygen reduction site from the TPB to the whole exposed cathode surface, thus significantly reduce the cathode polarization resistance [24], and enhance the electrocatalytic properties of the electrode. Hereafter, all the electrode behavior studies are performed on La2Cu0.8Co0.2O4 material with CGO electrolyte. To study the oxygen reduction reaction (ORR) that occurred on the electrode, impedance spectra measurements were performed as a function of oxygen partial pressure. Fig. 8 shows typical impedance spectra of the LCC/CGO test cell measured at 700 C under various oxygen partial pressures ðPO2 Þ. From the impedance spectrum, it is observed that for all the cathodes measured under variable oxygen partial pressures, the impedance spectra can be separated into two arcs located in the high frequency zone and the low frequency zone, respectively. This suggests that the oxygen reduction reaction over the electrodes is composed of at least two different processes. In order to clarify this point, an equivalent circuit was used to fit the measurement data (Fig. 8 inset). Here Rel represents the intercept value of the impedance spectrum at high frequency side with the real axis, which corresponds to the resistance of the electrolyte and lead wires. RH and RL are the resistance corresponding to the arc1 and arc2, respectively. CPE is a constant phase element. The total polarization resistance (Rp) is the sum of RH and RL. From the fitting results, it can be found that the resistance of the low frequency arc (RL) is much larger than that of the high frequency arc (RH). This means that the RL related process is the reaction rate-limiting step. The corresponding capacitances (obtained from the relaxation frequency of the two arcs, according to the relation f ¼ 1/2pRC) are about 103 and
101 F for the high and low frequency arcs respectively, and remain unchanged with the oxygen partial pressure. These values are similar to those previously reported for the Nd2NiO4 cathode [25]. It was consistent with electrochemical reactions that occurred on electrode [26]. These results further prove that two arcs exist in the impedance spectrum, and they represent two different electrochemical processes occurred on electrode. The electrode polarization resistances (Rp) of La2Cu0.8 Co0.2O4 cathode at 600e700 C under various PO2 are shown in Fig. 9 (a and b), from which the PO2 dependence of the Rp can be calculated. Generally, Rp varies with the oxygen partial pressure according to the following equation: Rp ¼ Rop PO2
n
n ¼ 1=2; O2;ads: %2Oads: (oxygen surface diffusion)
adsorption,
dissociation
and
surface
n ¼ 1=4; Oads: þ 2e= þ VO,, %OxO (charge transfer process)
a
b
Fig. 8 e Impedance spectra of La2Cu0.8Co0.2O4 electrode at 700 C under various oxygen partial.
Fig. 9 e Dependence of RH (a) and RL (b) on oxygen partial pressure for La2Cu0.8Co0.2O4 cathode at 600e700 C.
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x ,, n ¼ 0; O2 TPB þ VO %OO
(oxygen ion transfer into the electrolyte) The value of n could give useful information about the type 1=4 of species involved in the reactions [27,28]. ThePO2 relationship was considered as the contribution of the charge transfer process on the electrode [29], which was observed before for Ba0.5Sr0.5Co0.8Fe0.2O3 and La2xSrxCuO4 cathodes [30,31]. In a previous study, thePO O2 relationship has been observed for La0.8Sr0.2Co1xMnxO3 and Sr1.5LaxMnO4 cathode materials [32,33]. It was due to the oxygen ion transfer into the electrolyte. According to Fig. 9(a and b), we observed that n value was around 0 for RH and 0.25 for RL, respectively. Considering the weak dependence of RH on oxygen partial pressure, and the mixed conducting properties of La2Cu1xCoxO4 material, we propose that the high frequency arc was caused by the oxygen ion transfer form the TPB to CGO electrolyte, similar to those reported K2NiF4-type cathode materials, such as Ln2NiO4 [34]; whereas the low frequency arc was due to the charge transfer process on the electrode. Compared with the results obtained in Fig. 9(a and b), it is also observed that RL was always larger than RH in the whole range of measurement oxygen partial pressure from 600 to 700 C. Therefore it is concluded that the charge transfer process is the major rate limiting step for La2Cu1xCoxO4 cathode. Cathodic overpotential is another important parameters to evaluate electrode performance. Fig. 10 shows cathodic polarization curves for La2Cu0.8Co0.2O4 cathode as a function of temperature. It was observed that at fixed overpotential, the current density increased with increasing temperatures, indicating a thermo-activated electrochemical reaction was improved with the temperature. The lowest polarization overpotential, 50 mV was obtained for La2Cu0.8Co0.2O4 cathode at a current density of 48 mA cm2 at 700 C in air. This overpotential value is quite lower than the reported La2Cu0.2 Ni0.8O4 cathode material [35]. At low overpotential (less than 20 mV), we can expect a linear expression [36], i ¼ i0ZFh/RT, where i is the current density, i0 the exchange current density, n the overpotential, and F, R have their normal meanings,
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respectively. From the inverse of the derivative of i against n, we can obtain the polarization resistance (Rp) for La2Cu0.8 Co0.2O4 cathode. The Rp value obtained at 700 C in air was 0.55 U cm2, which is in agreement with the result obtained from impedance measurement. As we expected, the enhanced conductivity combined with the increase of interstitial oxygen formed by Co doping in La2CuO4 materials makes low polarization resistance possible. Compared to the LSCF cathode material, the polarization resistance of La2Cu1xCoxO4 material is still high. Whereas considering the thermal expansion coefficient (TEC), La2Cu1xCoxO4 has a much similar TEC (La2Cu1xCoxO4, (12.6e13.2) 106 K1 in air [14]) compared to that of the CGO (13.1 106 K1) electrolyte, whereas the TEC value of LSCF material is quite large (La0.6Sr0.4Co0.2Fe0.8O3, 17.5 106 K1 in ari [37]). Therefore, the La2Cu1xCoxO4 material can be considered as a promising cathode candidate for IT-SOFCs, given that the electrode performances can be further improved by optimizing the microstructure of electrode, such as to form nanofiber or nanotube cathodes, and so on.
4.
Conclusions
La2Cu1xCoxO4 materials have been studied as promising cathode for IT-SOFC based on ceria electrolyte. No reaction was found between La2Cu0.8Co0.2O4 electrode and CGO electrolyte after heat-treatment at 1000 C for 12 h in air, indicating high chemical compatibility of these materials at high temperature. The rate-limiting step for oxygen reduction reaction on La2Cu0.8Co0.2O4 electrode is the charge transfer process. The polarization resistance obtained at 700 C in air is about 0.51 U cm2 for the La2Cu0.8Co0.2O4 electrode, and the lowest overpotential is 50 mV at a current density of 48 mA cm2.
Acknowledgments The Project was supported by National Natural Science Foundation of China (51072048, 51102083), Foundation of Heilongjiang Province Postdoctoral (LBH-Z09019), China Postdoctoral Science Foundation (20100471116), University of Heilongjiang (Hdtd2010-04).
references
Fig. 10 e The overpotential-current density curves for La2Cu0.8Co0.2O4 cathode measured in air at various temperatures.
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