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Evaluation of La1.8−xPrxSr0.2CuO4−δ oxides as cathode materials for IT-SOFCs
Materials Chemistry and Physics xxx (2015) 1e6
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Evaluation of La1.8xPrxSr0.2CuO4d oxides as cathode materials for ITSOFCs L.M. Kolchina a, N.V. Lyskov a, b, *, P.P. Pestrikov a, S.Ya. Istomin a, G.N. Mazo a, E.V. Antipov a a b
Department of Chemistry, Moscow State University, Moscow 119991, Russia Institute of Problems of Chemical Physics RAS, Acad. Semenov av. 1, Chernogolovka, Moscow District 142432, Russia
h i g h l i g h t s
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Pr- and Sr-doped La2CuO4 (LPSCO) was estimated as a cathode material for SOFCs. Correlations between oxygen content and conductivity were discussed. Electrical conductivity of LPSCO reached 50 S/cm at 750 C in air. Deposition of LPSCO on GDC electrolyte gave the best electrochemical performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2015 Received in revised form 16 August 2015 Accepted 30 August 2015 Available online xxx
Conducting properties, thermal expansion, and chemical stability of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) related to the K2NiF4-type structure have been investigated in order to estimate their appropriateness to be used as a cathode material for solid oxide fuel cells. Electrical conductivity of La1.8xPrxSr0.2CuO4d demonstrates metallic-like behavior in the temperature range of 100e900 С at variable oxygen partial pressure (pO2 ¼ 103e1 atm), and reaches 50 S/cm at 750 C in air. The cuprates display thermal expansion coefficients (TECs) of about 14$106 K1 that are compatible with La0.8Sr0.2Ga0.85Mg0.15O3d (LSGM) and Ce0.9Gd0.1O1.95 (GDC) solid electrolytes. The electrocatalytic activity study of La1.6Pr0.2Sr0.2CuO4d electrode deposited on GDC as well as LSGM shows that the lowest polarization resistance is achieved in case of deposition of the electrode on GDC electrolyte. © 2015 Elsevier B.V. All rights reserved.
Keywords: Oxides Electrical conductivity Electrochemical properties Thermal expansion
1. Introduction Mixed ionic-electronic conductors (MIEC) attract much attention as promising cathode materials for solid oxide fuel cells (SOFCs) operating in the intermediate temperature range of
* Corresponding author. Institute of Problems of Chemical Physics RAS, Acad. Semenov av. 1, Chernogolovka, Moscow District 142432, Russia. E-mail address: [email protected] (N.V. Lyskov).
500e750 C [1e4]. Materials possessing high electronic and high oxide-ion conductivity as well as catalytic activity in the oxygen reduction reaction (ORR) allow accelerating the kinetic of the cathode reaction due to extending of the reaction area from a triple phase boundary (gas/electrode/electrolyte) to a double phase interface (gas/electrode). However, the problems caused by chemical reactions at an electrode/electrolyte interface and a TEC mismatch between an electrode and an electrolyte result in the fact that a strictly limited number of functional materials can be combined to create SOFCs [3e5]. It is widely accepted that complex
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Please cite this article in press as: L.M. Kolchina, et al., Evaluation of La1.8xPrxSr0.2CuO4d oxides as cathode materials for IT-SOFCs, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.08.059
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oxides with perovskite or perovskite-related structure can possess high mixed ionic-electronic conductivity and satisfy the main requirements for SOFC cathode materials [1e5]. Complex oxides of the A2BO4 composition, where A e a rareearth or alkali-earth cation, B e transition metal, are well known to crystallize in three different structure types called T, T*, or T0 structures depending on the transition metal environment [6]. It was previously showed [7] that the presence of rock-salt type slabs in the T-structure (commonly known as a K2NiF4-type structure) facilitates oxygen ion diffusion. Therefore, the oxides relating to the T-structure are the most attractive to be considered as fast oxideion conductors. Recently, rare-earth nikelates and cuprates with T-structure are regarded as a promising group of MIECs with potential application as the SOFC cathodes [8e16]. Among rare-earth cuprates of Ln2CuO4, where Ln ¼ LaeSm, only La2CuO4 crystallizes in the T-structure. Substitution of Sr for La in La2CuO4 allows enhancing electrical conductivity [10,12]. At the same time introduction of praseodymium into the La2CuO4 structure may simultaneously improve the catalytic activity and electrical conductivity [14] as praseodymium cuprate exhibits a high electrocatalytic activity in ORR among undoped Ln2CuO4 [15,16]. There are no studies of Pr- and Sr-doped La2CuO4 as the cathode material for SOFCs available to our knowledge. In this work we present the results of the investigations of hightemperature behavior and electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) as well as their electrochemical characterization as the SOFC cathode material. 2. Experimental The La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) samples were synthesized by conventional solid state route in air. Pre-fired La2O3, Pr6O11, SrCO3 and CuO powders used as initial reagents were mixed in stoichiometric ratio by ball-milling for 1 h in heptane. Then the obtained mixtures were pressed into pellets and sintered at 1000 C for 50 h in air. Phase purity of the samples was controlled by X-ray powder diffraction (XRD) recorded on Huber G670 Guinier diffractometer (CuKa1 radiation, Ge monochromator, image foil detector). The oxygen content was characterized by iodometric titration. Thermogravimetric analysis was performed in artificial air (20% O2(g), 80% Ar(g)) and argon from 100 С to 950 С with a heating rate of 10 /min by Netzsch STA 449C thermoanalyser. Thermal expansion coefficient measurements were carried on Netzsch 402C dilatometer operated in air (25e900 C, 10 /min). For this purpose the powders were pressed into pellets (8 mm in diameter and 5.0e5.5 mm in length) and sintered at 1000 C for 10 h in air. To study chemical stability of the La1.8xPrxSr0.2CuO4d with solid electrolytes, mixtures of La1.6Pr0.2Sr0.2CuO4d with Ce0.9Gd0.1O1.95 (GDC) and La0.8Sr0.2Ga0.85Mg0.15O3d (LSGM) electrolytes commonly used in the intermediate temperature range were prepared in the ratio 1:1 by mass and annealed at 900 C and 1000 C for 25 h in air. For DC conductivity measurements single-phase powders were pressed into pellets under pressure of 5 tons per cm2 and sintered at 1000 С for 10 h in air. Relative density was ~90%. To create current collectors Pt-paste was placed on opposing sides of a pellet. Potential electrodes were created with Pt-paste as two rings around a pellet. A distance between Pt-contacts was about 10 mm. Samples coated with Pt-paste were annealed at 900 С for 10 h in air to eliminate organic binder. Electrical conductivity measurements were performed by conventional four-point DC technique in the temperature range of 100e900 С at the oxygen partial pressures of 103e1 atm using a P-30 potentiostat/galvanostat (Elins Ltd, Russia) in cyclic voltamperometry (CVA) mode in the voltage range
from 50 mV to 50 mV at the voltage scan rate of 20 mV/s. The resulting specific resistivity was recalculated from the slopes of CVA curves taking into account a current collector area and a distance between potential electrodes. The temperature of the sample was measured by a PtePt/Rh thermocouple positioned close to the sample with an accuracy of ±1 С. To examine the electrochemical performance of an electrode/ electrolyte interface symmetric cells of electrode/electrolyte/electrode were fabricated. GDC and LSGM pellets (relative density ~95%) used as an electrolyte were prepared from Ce0.9Gd0.1O1.95 and La0.8Sr0.2Ga0.85Mg0.15O3d powders (Sigma Aldrich®), respectively, by uniaxial pressing followed by sintering at 1400 C for 4 h in air. La1.8xPrxSr0.2CuO4d powders used as an electrode were mixed with an organic binder (Heraeus V006) in the 1:1 ratio and then screen printed on a polished surface of the electrolyte pellets using of VS-Monoprint PES HT PW 77/55 (Verseidag-Tecfab GmbH) woven mesh to create working and counter electrodes. Then the pellets were dried at 150 C for 1 h and calcinated at 900 C or 1000 C for 4 h in air. The area of single electrode was ~0.25 cm2. Ptpaste was placed on the face site of GDC pellet then it was annealed and used as a reference electrode. Electrochemical characterization of the electrode/electrolyte interface was carried out by AC impedance spectroscopy. Impedance spectra were recorded using a Z-500P impedance spectrometer (Elins Ltd, Russia) over the frequency range of 500 MHze0.01 Hz at signal amplitude of 30 mV. Measurements were performed using a three-electrode technique at the OCV conditions as a function of temperature (500e800 C) in air. The sample temperature was controlled by a PtePt/Rh thermocouple placed near the sample. 3. Results and discussion XRD patterns of La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) shown in Fig. 1(a) and (b) were fully indexed in the tetragonal I-centered (I4/ mmm space group) unit cell with parameters: а ¼ 3.7636(5) Å, с ¼ 13.183(2) Å for x ¼ 0.2 and а ¼ 3.7645(3) Å, с ¼ 13.163(2) Å for x ¼ 0.4. Increase of Pr content leads to unit cell compression along the c-axis concerned with smaller Pr radius (r(Pr3þ) ¼ 1.179 Å) in comparison with La one (r(La3þ) ¼ 1.216 Å) [17]. The study of La1.6Pr0.2Sr0.2CuO4d reactivity with GDC and LSGM solid electrolytes was assessed by annealing of mixture of electrode and electrolyte powders at 1000 C for 25 h in air. XRD patterns of
Fig. 1. X-ray diffraction patterns of La1.6Pr0.2Sr0.2CuO4d (a), La1.4Pr0.4Sr0.2CuO4d (b) powders, and mixtures of La1.6Pr0.2Sr0.2CuO4d with LSGM (c) and GDC solid electrolytes annealed at 1000 C (d) and 900 C (e) for 25 h in air.
Please cite this article in press as: L.M. Kolchina, et al., Evaluation of La1.8xPrxSr0.2CuO4d oxides as cathode materials for IT-SOFCs, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.08.059
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3
the annealed mixtures are shown in Fig. 1(cee). No additional peaks indicating new phase formation were detected on the XRD pattern of the La1.6Pr0.2Sr0.2CuO4d and LSGM mixture. However, as was noted in [18], cation exchange between La2xSrxCuO4 and La0.83Sr0.17Ga0.83Mg0.17O3d phases occurs due to cation diffusion and leads to a broadening and a shift of corresponding peaks on the diffraction pattern, while no new phase was detected. In case of GDC electrolyte appearance of a new phase was detected after annealing at 1000 C (Fig. 1(d)). Additional reflections can be related to the second member of the Ruddlesden-Popper series (La,Pr,Sr)3Cu2O6d. Also peaks shift related to the phase with fluorite structure is observed. That can be explained by incorporation of rare-earth oxides into GDC structure and, consequently, formation of solid solution [19,20]. However, the XRD pattern of the La1.6Pr0.2Sr0.2CuO4d and GDC mixture annealed at 900 C reveals no peaks related to new phases (Fig. 1(e)). The position of the GDC peaks complies with those of ICDD PDF-2 (#75-161). Thus, the La1.6Pr0.2Sr0.2CuO4d is chemically compatible with GDC below 900 C. The oxygen content in the La1.8xPrxSr0.2CuO4.00±0.02 coincides with stoichiometric and corresponds to fully filled oxygen sublattice. As is noted in the [21], in case of La1.8Sr0.2CuO4d significant number of vacancies present in oxygen sublattice, whereas the oxygen sublattice completely filled in Pr- and Sr-doped samples, that indicates the possibility of increasing in the oxidation state of Pr or/and Cu. To study the thermal behavior of the samples thermogravimetric analysis was conducted. Fig. 2(a) shows how oxygen stoichiometry of the samples varies with temperature in air. Heating of
the both samples up to 600e700 C leads to oxygen content reduction due to the loss of oxygen from the crystal structure and the formation of oxygen vacancies. The last ones can promote oxygen mobility in the crystal structure. As is clear from Fig. 2(a), less change in oxygen content is observed for La1.4Pr0.4Sr0.2CuO4d. Probably, increase in praseodymium content is conducive to keeping oxygen in the crystal structure. Further increase in the temperature up to 950 C does not lead to decrease in weight of the samples. It should be noted that the oxygen loss of the samples is partially reversible, which is, probably, due to mismatch between kinetics of the reactions upon heating and cooling. Fig. 2(b) presents an example of TG curves of the La1.6Pr0.2Sr0.2CuO4d sample in air and argon. Upon heating the curves demonstrate the similar behavior, whereas upon cooling the oxygen loss was irreversible in case of argon atmosphere. According to the XRD analysis, no noticeable change in XRD pattern of the initial and after-TGA samples was detected, which clearly emphasizes the stability of the materials at high temperature. Thermal expansion of La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) was investigated in the temperature range of 100e900оС in air. Dilatometry curves given in Fig. 3 represent different temperature behavior for the studied samples. In case of La1.4Pr0.4Sr0.2CuO4d the temperature dependence of relative elongation is linear at 100e900 C and the TEC is 14.6$106 K1 whereas the TEC of La1.6Pr0.2Sr0.2CuO4d increases from 10.7$106 K1 (100e500 C) to 15.7$106 K1 (500e900 C). The observed difference presumably concerns with unequal changes in oxygen content for both samples that is in agreement with the thermal analysis data. As one can see from Fig. 3, the relative elongations of La1.6Pr0.2Sr0.2CuO4d and the solid electrolytes based on LSGM [22] and GDC [23] are close, which indicates their compatibility. Fig. 4 shows the temperature dependences of electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) under various partial oxygen pressures. The both compounds demonstrate metalliclike behavior in the studied temperature range (100e900 C) and oxygen partial pressures of pO2 ¼ 103e1 atm. Considering influence of рО2 on electrical transport properties of the studied cuprates (Fig. 4, inserts), one can conclude that conductivity does not considerably change within limits of рО2 ¼ 103e1 atm. Positive slope of logelog plots of conductivity vs. pO2 pointed to the p-type conductivity under these experimental conditions. It is worth mentioning that holes are the main charge carriers in
Fig. 2. Thermogravimetric analysis of La1.8xPrxSr0.2CuO4d samples: (a) e oxygen content in air as a function of temperature (for x ¼ 0.2 and 0.4); (b) oxygen content for x ¼ 0.2 as a function of temperature in air and argon atmosphere.
Fig. 3. Thermal expansion curves of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) compared with ones for LSGM [22] and GDC [23] solid electrolytes in the temperature range of 100e900оС in air.
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1 OxO ⇔ O2 ðgÞ þ VO$$ þ 2e0 2
(3)
Due to the thermal activation holes localized on the copper atoms can act as traps for free electrons (e0 ):
Cu$Cu þ e0 ⇔CuxCu
Fig. 4. Temperature dependencies of electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) under various partial oxygen pressures. Inserts: logelog plots of electrical conductivity as a function of partial oxygen pressure at different temperatures.
the Sr-doped lanthanum cuprates [24,25]. Holes are basically localized at copper atoms (Cu$Cu ), and the mechanism of their generation can be expressed by the following equations. First, lanthanum is substituted by strontium: La2 CuO4
0 2SrO þ CuO!2SrLa þ CuxCu þ 3OxO þ VO$$
(4)
As a result, conductivity decreases with an increase in temperature and is nearly independent on oxygen partial pressure (in the range of рО2 ¼ 103e1 atm) at constant temperature. In order to compare the influence of Sr and Pr dopants on electrical transport properties of La2CuO4, electrical conductivity as a function of temperature was plotted for various compositions (Fig. 5). The La1.6Pr0.2Sr0.2CuO4d displays rather high conductivity of ~70 S/cm at 500-900оС in air. It should be pointed out that conductivity of the Pr- and Sr-doped La2CuO4 is an order of magnitude higher than that for the undoped cuprate [12] and La1.6Pr0.4CuO4 [14] and comparable with that for La1.5Sr0.5CuO4 [12] in the high temperature range. The substitution of Pr for La in La2CuO4 or La2xSrxCuO4 leads to a decrease in conductivity in all temperature range. In case of La2xPrxCuO4 with small hole concentration the charge carriers may be localized on the Pr-atoms, which leads to a considerable reduction of conductivity [14]. In contrast the hole concentration in La1.8xPrxSr0.2CuO4d is higher in comparison with one in La2xPrxCuO4 due to Sr-doping. Consequently, relative decrease in a number of the charge carriers for the Pr-doped La2xSrxCuO4 is lower; therefore, a relative change in conductivity is small. Decrease of electrical conductivity of La1.8xPrxSr0.2CuO4d itself with Pr doping may be caused by the Anderson localization of states due to random distribution of A-cations [26]. One can notice that electrical conductivity of La1.8xPrxSr0.2CuO4d at elevated temperature (500-900оС) becomes almost independent on chemical composition and comparable to that of La1.5Sr0.5CuO4d reported in [12]. That can be explained by the different change in oxygen stoichiometry for these compounds (Fig. 2) and an association of oxygen vacancies [24]: 0 0 SrLa þ OxO ⇔ SrLa VO$$
$
00
þ Oi
00 1 Oi þ 2Cu$Cu ⇔ O2 ðgÞ þ 2CuxCu 2
(5)
(6)
(1)
0 e strontium where CuxCu e copper atom in its regular position, SrLa x atom in the lanthanum regular position, OO e oxygen atom in its regular position, VO$$ e oxygen vacancy. Then oxygen incorporates into the crystal structure from a gas phase that leads to partial or full filling of oxygen vacancies in the anion sublattice, and, as a consequence, to generation of hole charge carriers due to the charge compensation according to the following equation:
1 2CuxCu þ VO$$ þ O2 ðgÞ/2Cu$Cu þ OxO 2
(2)
The presented defect formation model can be used for a description of conducting properties of the investigated compounds. According to the results of the thermal analysis, oxygen content in La1.8xPrxSr0.2CuO4d decreases in the high-temperature range. The same effect is observed under low рО2. In consequence, generation of electron charge carries described by the following equation is possible:
Fig. 5. Electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) compared to undoped, Sr- [12], and Pr-doped [14] La2CuO4 in air.
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Probably, the intensive oxygen loss is, the more contribution of the processes (5) and (6) to electrical transport is. Therefore conductivity of La1.6Pr0.2Sr0.2CuO4d and La1.4Pr0.4Sr0.2CuO4d is equal at the high temperature. With the purpose of the electrochemical characterization of the La1.8xPrxSr0.2CuO4d electrode/electrolyte interface, the composition of x ¼ 0.2 was chosen due to its higher conductivity in the intermediate temperature range and better compatibility with solid electrolytes resulted from its lower TEC in the range of 100e500 С. Fig. 6(a) and (b) shows the typical impedance spectra for La1.6Pr0.2Sr0.2CuO4d electrodes applied on LSGM electrolyte compared to those applied on GDC in air. These spectra consist of two arcs that imply at least two distinguishable rate-determining steps of oxygen reduction process take place at the electrode. The impedance spectra were fitted with the equivalent electrical circuit shown in Fig. 6(c) over the studied temperature range. The equivalent circuit contained a resistance (Rel) in series with two parallel circuits included a resistance and a constant phase element (R1CPE1, R2-CPE2). The high-frequency intercept of the impedance arcs with real axes corresponds to the electrolyte resistance (Rel). The values of R1 and R2 resistances were determined by an extrapolation of high- and low-frequency arcs of the impedance spectrum to the real axis, respectively. Area specific resistance (ASR) was calculated as a sum of R1 and R2 resistances.
5
The Arrhenius plots of ASR for La1.6Pr0.2Sr0.2CuO4d electrode deposited on LSGM and GDC are shown in Fig. 7. As is clear from Fig. 7 lower ASR was achieved in case of GDC electrolyte (2.07 U cm2
Fig. 7. Temperature dependence of ASR for La1.6Pr0.2Sr0.2CuO4d electrode deposited on LSGM and GDC electrolytes in air.
Fig. 6. Impedance spectra of La1.6Pr0.2Sr0.2CuO4d electrode deposited on LSGM (a) and GDC (b) electrolytes at various temperatures in air; (c) equivalent electrical circuit used to fit impedance data.
Please cite this article in press as: L.M. Kolchina, et al., Evaluation of La1.8xPrxSr0.2CuO4d oxides as cathode materials for IT-SOFCs, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.08.059
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at 750 C). Probably, in case of the LSGM electrolyte the 20e30% higher ASR (2.60 U cm2 at 750 C) caused by a change in stoichiometry of the phases at the electrode/electrolyte interface due to the cation interdiffusion across the interface. Otherwise, the difference in oxide-ion conductivity of the solid electrolytes [1,27] can be the reason of the distinct ASR. Additional studies are needed to confirm this assumption. The obtained ASR is relatively high that limits successful application of La1.6Pr0.2Sr0.2CuO4d as a SOFC cathode material at the intermediate temperatures. The improving of the electrode morphology is required to reduce the ASR. Activation energy was calculated from Arrhenius plot 1/ASR vs. 1/T. The ASR temperature dependencies for the electrodes on LSGM and GDC electrolytes are symbate that confirmed by proximity of their activation energies (1.58 eV and 1.50 eV, respectively) within the defining error (±0.04 eV). Such similarity is indicative of the fact that the electrode properties have the main impact on the characteristics of the electrode/electrolyte interface.
Science and Technology (Center of electrochemical energy), and MSU-development Program up to 2020. Kolchina L.M. is grateful to HTAS for the financial support.
4. Conclusion
[10]
La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) powders with K2NiF4-type structure were synthesized and assessed for possible application as a SOFC cathode material. Electrical conductivity of La1.8xPrxSr0.2CuO4d demonstrates metallic-like behavior and varies from 70 to 40 S/cm in the temperature range of 500e900 С in air. The TECs of the cuprates are about 14$106 K1 that are compatible with those for LSGM and GDC solid electrolytes. No chemical reaction of La1.8xPrxSr0.2CuO4d with LSGM and GDC was detected after annealing at 1000 C and 900 C for 25 h, respectively. La1.6Pr0.2Sr0.2CuO4d electrodes in combination with GDC electrolyte exhibit lower polarization resistances in comparison to those deposited on LSGM electrolyte. Good thermal and chemical compatibility of the materials with the solid electrolytes as well as their high electrical conductivity provide some advantages for further optimization of the electrode morphology in order to improving electrode performance in oxygen reduction reaction.
[11]
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Acknowledgments This work was partially supported by Russian Foundation for Basic Research (grant no. 15-38-20247), Skolkovo Institute of
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Evaluation of La1.8xPrxSr0.2CuO4d oxides as cathode materials for ITSOFCs L.M. Kolchina a, N.V. Lyskov a, b, *, P.P. Pestrikov a, S.Ya. Istomin a, G.N. Mazo a, E.V. Antipov a a b
Department of Chemistry, Moscow State University, Moscow 119991, Russia Institute of Problems of Chemical Physics RAS, Acad. Semenov av. 1, Chernogolovka, Moscow District 142432, Russia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Pr- and Sr-doped La2CuO4 (LPSCO) was estimated as a cathode material for SOFCs. Correlations between oxygen content and conductivity were discussed. Electrical conductivity of LPSCO reached 50 S/cm at 750 C in air. Deposition of LPSCO on GDC electrolyte gave the best electrochemical performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2015 Received in revised form 16 August 2015 Accepted 30 August 2015 Available online xxx
Conducting properties, thermal expansion, and chemical stability of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) related to the K2NiF4-type structure have been investigated in order to estimate their appropriateness to be used as a cathode material for solid oxide fuel cells. Electrical conductivity of La1.8xPrxSr0.2CuO4d demonstrates metallic-like behavior in the temperature range of 100e900 С at variable oxygen partial pressure (pO2 ¼ 103e1 atm), and reaches 50 S/cm at 750 C in air. The cuprates display thermal expansion coefficients (TECs) of about 14$106 K1 that are compatible with La0.8Sr0.2Ga0.85Mg0.15O3d (LSGM) and Ce0.9Gd0.1O1.95 (GDC) solid electrolytes. The electrocatalytic activity study of La1.6Pr0.2Sr0.2CuO4d electrode deposited on GDC as well as LSGM shows that the lowest polarization resistance is achieved in case of deposition of the electrode on GDC electrolyte. © 2015 Elsevier B.V. All rights reserved.
Keywords: Oxides Electrical conductivity Electrochemical properties Thermal expansion
1. Introduction Mixed ionic-electronic conductors (MIEC) attract much attention as promising cathode materials for solid oxide fuel cells (SOFCs) operating in the intermediate temperature range of
* Corresponding author. Institute of Problems of Chemical Physics RAS, Acad. Semenov av. 1, Chernogolovka, Moscow District 142432, Russia. E-mail address: [email protected] (N.V. Lyskov).
500e750 C [1e4]. Materials possessing high electronic and high oxide-ion conductivity as well as catalytic activity in the oxygen reduction reaction (ORR) allow accelerating the kinetic of the cathode reaction due to extending of the reaction area from a triple phase boundary (gas/electrode/electrolyte) to a double phase interface (gas/electrode). However, the problems caused by chemical reactions at an electrode/electrolyte interface and a TEC mismatch between an electrode and an electrolyte result in the fact that a strictly limited number of functional materials can be combined to create SOFCs [3e5]. It is widely accepted that complex
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oxides with perovskite or perovskite-related structure can possess high mixed ionic-electronic conductivity and satisfy the main requirements for SOFC cathode materials [1e5]. Complex oxides of the A2BO4 composition, where A e a rareearth or alkali-earth cation, B e transition metal, are well known to crystallize in three different structure types called T, T*, or T0 structures depending on the transition metal environment [6]. It was previously showed [7] that the presence of rock-salt type slabs in the T-structure (commonly known as a K2NiF4-type structure) facilitates oxygen ion diffusion. Therefore, the oxides relating to the T-structure are the most attractive to be considered as fast oxideion conductors. Recently, rare-earth nikelates and cuprates with T-structure are regarded as a promising group of MIECs with potential application as the SOFC cathodes [8e16]. Among rare-earth cuprates of Ln2CuO4, where Ln ¼ LaeSm, only La2CuO4 crystallizes in the T-structure. Substitution of Sr for La in La2CuO4 allows enhancing electrical conductivity [10,12]. At the same time introduction of praseodymium into the La2CuO4 structure may simultaneously improve the catalytic activity and electrical conductivity [14] as praseodymium cuprate exhibits a high electrocatalytic activity in ORR among undoped Ln2CuO4 [15,16]. There are no studies of Pr- and Sr-doped La2CuO4 as the cathode material for SOFCs available to our knowledge. In this work we present the results of the investigations of hightemperature behavior and electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) as well as their electrochemical characterization as the SOFC cathode material. 2. Experimental The La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) samples were synthesized by conventional solid state route in air. Pre-fired La2O3, Pr6O11, SrCO3 and CuO powders used as initial reagents were mixed in stoichiometric ratio by ball-milling for 1 h in heptane. Then the obtained mixtures were pressed into pellets and sintered at 1000 C for 50 h in air. Phase purity of the samples was controlled by X-ray powder diffraction (XRD) recorded on Huber G670 Guinier diffractometer (CuKa1 radiation, Ge monochromator, image foil detector). The oxygen content was characterized by iodometric titration. Thermogravimetric analysis was performed in artificial air (20% O2(g), 80% Ar(g)) and argon from 100 С to 950 С with a heating rate of 10 /min by Netzsch STA 449C thermoanalyser. Thermal expansion coefficient measurements were carried on Netzsch 402C dilatometer operated in air (25e900 C, 10 /min). For this purpose the powders were pressed into pellets (8 mm in diameter and 5.0e5.5 mm in length) and sintered at 1000 C for 10 h in air. To study chemical stability of the La1.8xPrxSr0.2CuO4d with solid electrolytes, mixtures of La1.6Pr0.2Sr0.2CuO4d with Ce0.9Gd0.1O1.95 (GDC) and La0.8Sr0.2Ga0.85Mg0.15O3d (LSGM) electrolytes commonly used in the intermediate temperature range were prepared in the ratio 1:1 by mass and annealed at 900 C and 1000 C for 25 h in air. For DC conductivity measurements single-phase powders were pressed into pellets under pressure of 5 tons per cm2 and sintered at 1000 С for 10 h in air. Relative density was ~90%. To create current collectors Pt-paste was placed on opposing sides of a pellet. Potential electrodes were created with Pt-paste as two rings around a pellet. A distance between Pt-contacts was about 10 mm. Samples coated with Pt-paste were annealed at 900 С for 10 h in air to eliminate organic binder. Electrical conductivity measurements were performed by conventional four-point DC technique in the temperature range of 100e900 С at the oxygen partial pressures of 103e1 atm using a P-30 potentiostat/galvanostat (Elins Ltd, Russia) in cyclic voltamperometry (CVA) mode in the voltage range
from 50 mV to 50 mV at the voltage scan rate of 20 mV/s. The resulting specific resistivity was recalculated from the slopes of CVA curves taking into account a current collector area and a distance between potential electrodes. The temperature of the sample was measured by a PtePt/Rh thermocouple positioned close to the sample with an accuracy of ±1 С. To examine the electrochemical performance of an electrode/ electrolyte interface symmetric cells of electrode/electrolyte/electrode were fabricated. GDC and LSGM pellets (relative density ~95%) used as an electrolyte were prepared from Ce0.9Gd0.1O1.95 and La0.8Sr0.2Ga0.85Mg0.15O3d powders (Sigma Aldrich®), respectively, by uniaxial pressing followed by sintering at 1400 C for 4 h in air. La1.8xPrxSr0.2CuO4d powders used as an electrode were mixed with an organic binder (Heraeus V006) in the 1:1 ratio and then screen printed on a polished surface of the electrolyte pellets using of VS-Monoprint PES HT PW 77/55 (Verseidag-Tecfab GmbH) woven mesh to create working and counter electrodes. Then the pellets were dried at 150 C for 1 h and calcinated at 900 C or 1000 C for 4 h in air. The area of single electrode was ~0.25 cm2. Ptpaste was placed on the face site of GDC pellet then it was annealed and used as a reference electrode. Electrochemical characterization of the electrode/electrolyte interface was carried out by AC impedance spectroscopy. Impedance spectra were recorded using a Z-500P impedance spectrometer (Elins Ltd, Russia) over the frequency range of 500 MHze0.01 Hz at signal amplitude of 30 mV. Measurements were performed using a three-electrode technique at the OCV conditions as a function of temperature (500e800 C) in air. The sample temperature was controlled by a PtePt/Rh thermocouple placed near the sample. 3. Results and discussion XRD patterns of La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) shown in Fig. 1(a) and (b) were fully indexed in the tetragonal I-centered (I4/ mmm space group) unit cell with parameters: а ¼ 3.7636(5) Å, с ¼ 13.183(2) Å for x ¼ 0.2 and а ¼ 3.7645(3) Å, с ¼ 13.163(2) Å for x ¼ 0.4. Increase of Pr content leads to unit cell compression along the c-axis concerned with smaller Pr radius (r(Pr3þ) ¼ 1.179 Å) in comparison with La one (r(La3þ) ¼ 1.216 Å) [17]. The study of La1.6Pr0.2Sr0.2CuO4d reactivity with GDC and LSGM solid electrolytes was assessed by annealing of mixture of electrode and electrolyte powders at 1000 C for 25 h in air. XRD patterns of
Fig. 1. X-ray diffraction patterns of La1.6Pr0.2Sr0.2CuO4d (a), La1.4Pr0.4Sr0.2CuO4d (b) powders, and mixtures of La1.6Pr0.2Sr0.2CuO4d with LSGM (c) and GDC solid electrolytes annealed at 1000 C (d) and 900 C (e) for 25 h in air.
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3
the annealed mixtures are shown in Fig. 1(cee). No additional peaks indicating new phase formation were detected on the XRD pattern of the La1.6Pr0.2Sr0.2CuO4d and LSGM mixture. However, as was noted in [18], cation exchange between La2xSrxCuO4 and La0.83Sr0.17Ga0.83Mg0.17O3d phases occurs due to cation diffusion and leads to a broadening and a shift of corresponding peaks on the diffraction pattern, while no new phase was detected. In case of GDC electrolyte appearance of a new phase was detected after annealing at 1000 C (Fig. 1(d)). Additional reflections can be related to the second member of the Ruddlesden-Popper series (La,Pr,Sr)3Cu2O6d. Also peaks shift related to the phase with fluorite structure is observed. That can be explained by incorporation of rare-earth oxides into GDC structure and, consequently, formation of solid solution [19,20]. However, the XRD pattern of the La1.6Pr0.2Sr0.2CuO4d and GDC mixture annealed at 900 C reveals no peaks related to new phases (Fig. 1(e)). The position of the GDC peaks complies with those of ICDD PDF-2 (#75-161). Thus, the La1.6Pr0.2Sr0.2CuO4d is chemically compatible with GDC below 900 C. The oxygen content in the La1.8xPrxSr0.2CuO4.00±0.02 coincides with stoichiometric and corresponds to fully filled oxygen sublattice. As is noted in the [21], in case of La1.8Sr0.2CuO4d significant number of vacancies present in oxygen sublattice, whereas the oxygen sublattice completely filled in Pr- and Sr-doped samples, that indicates the possibility of increasing in the oxidation state of Pr or/and Cu. To study the thermal behavior of the samples thermogravimetric analysis was conducted. Fig. 2(a) shows how oxygen stoichiometry of the samples varies with temperature in air. Heating of
the both samples up to 600e700 C leads to oxygen content reduction due to the loss of oxygen from the crystal structure and the formation of oxygen vacancies. The last ones can promote oxygen mobility in the crystal structure. As is clear from Fig. 2(a), less change in oxygen content is observed for La1.4Pr0.4Sr0.2CuO4d. Probably, increase in praseodymium content is conducive to keeping oxygen in the crystal structure. Further increase in the temperature up to 950 C does not lead to decrease in weight of the samples. It should be noted that the oxygen loss of the samples is partially reversible, which is, probably, due to mismatch between kinetics of the reactions upon heating and cooling. Fig. 2(b) presents an example of TG curves of the La1.6Pr0.2Sr0.2CuO4d sample in air and argon. Upon heating the curves demonstrate the similar behavior, whereas upon cooling the oxygen loss was irreversible in case of argon atmosphere. According to the XRD analysis, no noticeable change in XRD pattern of the initial and after-TGA samples was detected, which clearly emphasizes the stability of the materials at high temperature. Thermal expansion of La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) was investigated in the temperature range of 100e900оС in air. Dilatometry curves given in Fig. 3 represent different temperature behavior for the studied samples. In case of La1.4Pr0.4Sr0.2CuO4d the temperature dependence of relative elongation is linear at 100e900 C and the TEC is 14.6$106 K1 whereas the TEC of La1.6Pr0.2Sr0.2CuO4d increases from 10.7$106 K1 (100e500 C) to 15.7$106 K1 (500e900 C). The observed difference presumably concerns with unequal changes in oxygen content for both samples that is in agreement with the thermal analysis data. As one can see from Fig. 3, the relative elongations of La1.6Pr0.2Sr0.2CuO4d and the solid electrolytes based on LSGM [22] and GDC [23] are close, which indicates their compatibility. Fig. 4 shows the temperature dependences of electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2, 0.4) under various partial oxygen pressures. The both compounds demonstrate metalliclike behavior in the studied temperature range (100e900 C) and oxygen partial pressures of pO2 ¼ 103e1 atm. Considering influence of рО2 on electrical transport properties of the studied cuprates (Fig. 4, inserts), one can conclude that conductivity does not considerably change within limits of рО2 ¼ 103e1 atm. Positive slope of logelog plots of conductivity vs. pO2 pointed to the p-type conductivity under these experimental conditions. It is worth mentioning that holes are the main charge carriers in
Fig. 2. Thermogravimetric analysis of La1.8xPrxSr0.2CuO4d samples: (a) e oxygen content in air as a function of temperature (for x ¼ 0.2 and 0.4); (b) oxygen content for x ¼ 0.2 as a function of temperature in air and argon atmosphere.
Fig. 3. Thermal expansion curves of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) compared with ones for LSGM [22] and GDC [23] solid electrolytes in the temperature range of 100e900оС in air.
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1 OxO ⇔ O2 ðgÞ þ VO$$ þ 2e0 2
(3)
Due to the thermal activation holes localized on the copper atoms can act as traps for free electrons (e0 ):
Cu$Cu þ e0 ⇔CuxCu
Fig. 4. Temperature dependencies of electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) under various partial oxygen pressures. Inserts: logelog plots of electrical conductivity as a function of partial oxygen pressure at different temperatures.
the Sr-doped lanthanum cuprates [24,25]. Holes are basically localized at copper atoms (Cu$Cu ), and the mechanism of their generation can be expressed by the following equations. First, lanthanum is substituted by strontium: La2 CuO4
0 2SrO þ CuO!2SrLa þ CuxCu þ 3OxO þ VO$$
(4)
As a result, conductivity decreases with an increase in temperature and is nearly independent on oxygen partial pressure (in the range of рО2 ¼ 103e1 atm) at constant temperature. In order to compare the influence of Sr and Pr dopants on electrical transport properties of La2CuO4, electrical conductivity as a function of temperature was plotted for various compositions (Fig. 5). The La1.6Pr0.2Sr0.2CuO4d displays rather high conductivity of ~70 S/cm at 500-900оС in air. It should be pointed out that conductivity of the Pr- and Sr-doped La2CuO4 is an order of magnitude higher than that for the undoped cuprate [12] and La1.6Pr0.4CuO4 [14] and comparable with that for La1.5Sr0.5CuO4 [12] in the high temperature range. The substitution of Pr for La in La2CuO4 or La2xSrxCuO4 leads to a decrease in conductivity in all temperature range. In case of La2xPrxCuO4 with small hole concentration the charge carriers may be localized on the Pr-atoms, which leads to a considerable reduction of conductivity [14]. In contrast the hole concentration in La1.8xPrxSr0.2CuO4d is higher in comparison with one in La2xPrxCuO4 due to Sr-doping. Consequently, relative decrease in a number of the charge carriers for the Pr-doped La2xSrxCuO4 is lower; therefore, a relative change in conductivity is small. Decrease of electrical conductivity of La1.8xPrxSr0.2CuO4d itself with Pr doping may be caused by the Anderson localization of states due to random distribution of A-cations [26]. One can notice that electrical conductivity of La1.8xPrxSr0.2CuO4d at elevated temperature (500-900оС) becomes almost independent on chemical composition and comparable to that of La1.5Sr0.5CuO4d reported in [12]. That can be explained by the different change in oxygen stoichiometry for these compounds (Fig. 2) and an association of oxygen vacancies [24]: 0 0 SrLa þ OxO ⇔ SrLa VO$$
$
00
þ Oi
00 1 Oi þ 2Cu$Cu ⇔ O2 ðgÞ þ 2CuxCu 2
(5)
(6)
(1)
0 e strontium where CuxCu e copper atom in its regular position, SrLa x atom in the lanthanum regular position, OO e oxygen atom in its regular position, VO$$ e oxygen vacancy. Then oxygen incorporates into the crystal structure from a gas phase that leads to partial or full filling of oxygen vacancies in the anion sublattice, and, as a consequence, to generation of hole charge carriers due to the charge compensation according to the following equation:
1 2CuxCu þ VO$$ þ O2 ðgÞ/2Cu$Cu þ OxO 2
(2)
The presented defect formation model can be used for a description of conducting properties of the investigated compounds. According to the results of the thermal analysis, oxygen content in La1.8xPrxSr0.2CuO4d decreases in the high-temperature range. The same effect is observed under low рО2. In consequence, generation of electron charge carries described by the following equation is possible:
Fig. 5. Electrical conductivity of La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) compared to undoped, Sr- [12], and Pr-doped [14] La2CuO4 in air.
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Probably, the intensive oxygen loss is, the more contribution of the processes (5) and (6) to electrical transport is. Therefore conductivity of La1.6Pr0.2Sr0.2CuO4d and La1.4Pr0.4Sr0.2CuO4d is equal at the high temperature. With the purpose of the electrochemical characterization of the La1.8xPrxSr0.2CuO4d electrode/electrolyte interface, the composition of x ¼ 0.2 was chosen due to its higher conductivity in the intermediate temperature range and better compatibility with solid electrolytes resulted from its lower TEC in the range of 100e500 С. Fig. 6(a) and (b) shows the typical impedance spectra for La1.6Pr0.2Sr0.2CuO4d electrodes applied on LSGM electrolyte compared to those applied on GDC in air. These spectra consist of two arcs that imply at least two distinguishable rate-determining steps of oxygen reduction process take place at the electrode. The impedance spectra were fitted with the equivalent electrical circuit shown in Fig. 6(c) over the studied temperature range. The equivalent circuit contained a resistance (Rel) in series with two parallel circuits included a resistance and a constant phase element (R1CPE1, R2-CPE2). The high-frequency intercept of the impedance arcs with real axes corresponds to the electrolyte resistance (Rel). The values of R1 and R2 resistances were determined by an extrapolation of high- and low-frequency arcs of the impedance spectrum to the real axis, respectively. Area specific resistance (ASR) was calculated as a sum of R1 and R2 resistances.
5
The Arrhenius plots of ASR for La1.6Pr0.2Sr0.2CuO4d electrode deposited on LSGM and GDC are shown in Fig. 7. As is clear from Fig. 7 lower ASR was achieved in case of GDC electrolyte (2.07 U cm2
Fig. 7. Temperature dependence of ASR for La1.6Pr0.2Sr0.2CuO4d electrode deposited on LSGM and GDC electrolytes in air.
Fig. 6. Impedance spectra of La1.6Pr0.2Sr0.2CuO4d electrode deposited on LSGM (a) and GDC (b) electrolytes at various temperatures in air; (c) equivalent electrical circuit used to fit impedance data.
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at 750 C). Probably, in case of the LSGM electrolyte the 20e30% higher ASR (2.60 U cm2 at 750 C) caused by a change in stoichiometry of the phases at the electrode/electrolyte interface due to the cation interdiffusion across the interface. Otherwise, the difference in oxide-ion conductivity of the solid electrolytes [1,27] can be the reason of the distinct ASR. Additional studies are needed to confirm this assumption. The obtained ASR is relatively high that limits successful application of La1.6Pr0.2Sr0.2CuO4d as a SOFC cathode material at the intermediate temperatures. The improving of the electrode morphology is required to reduce the ASR. Activation energy was calculated from Arrhenius plot 1/ASR vs. 1/T. The ASR temperature dependencies for the electrodes on LSGM and GDC electrolytes are symbate that confirmed by proximity of their activation energies (1.58 eV and 1.50 eV, respectively) within the defining error (±0.04 eV). Such similarity is indicative of the fact that the electrode properties have the main impact on the characteristics of the electrode/electrolyte interface.
Science and Technology (Center of electrochemical energy), and MSU-development Program up to 2020. Kolchina L.M. is grateful to HTAS for the financial support.
4. Conclusion
[10]
La1.8xPrxSr0.2CuO4d (x ¼ 0.2; 0.4) powders with K2NiF4-type structure were synthesized and assessed for possible application as a SOFC cathode material. Electrical conductivity of La1.8xPrxSr0.2CuO4d demonstrates metallic-like behavior and varies from 70 to 40 S/cm in the temperature range of 500e900 С in air. The TECs of the cuprates are about 14$106 K1 that are compatible with those for LSGM and GDC solid electrolytes. No chemical reaction of La1.8xPrxSr0.2CuO4d with LSGM and GDC was detected after annealing at 1000 C and 900 C for 25 h, respectively. La1.6Pr0.2Sr0.2CuO4d electrodes in combination with GDC electrolyte exhibit lower polarization resistances in comparison to those deposited on LSGM electrolyte. Good thermal and chemical compatibility of the materials with the solid electrolytes as well as their high electrical conductivity provide some advantages for further optimization of the electrode morphology in order to improving electrode performance in oxygen reduction reaction.
[11]
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Acknowledgments This work was partially supported by Russian Foundation for Basic Research (grant no. 15-38-20247), Skolkovo Institute of
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