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La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 thin films obtained by pulsed laser ablation: Effect of the substrate on the electrochemical behavior
Solid State Ionics 192 (2011) 584–590
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 thin films obtained by pulsed laser ablation: Effect of the substrate on the electrochemical behavior I. Ruiz de Larramendi a, S. Vivès a, N. Ortiz-Vitoriano a, J.I. Ruiz de Larramendi a, M.I. Arriortua b, T. Rojo a,⁎ a b
Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo.644, 48080 Bilbao, Spain Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo.644, 48080 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 18 January 2010 Accepted 10 February 2010 Available online 11 March 2010 Keywords: Solid oxide fuel cell Cathodes Perovskite Pulsed laser deposition Electrochemical impedance spectroscopy
a b s t r a c t Polycrystalline La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 (LSCFN) samples were prepared by the liquid mix process at 600 °C. The analysis of X-ray powder diffraction (XRD) patterns indicated that the sample was single phase and crystallized in an orthorhombic structure (Pbnm). High quality LSCFN films have been deposited on different yttria-stabilized zirconia (YSZ) substrates by conventional painting method and by pulsed laser deposition (PLD). The structure of the films was analyzed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). All films are polycrystalline with a marked texture exhibiting pyramidal grains in the surface with different size distribution. Nanostructured cathode thin films with vertically aligned nanopores (VANP) have been obtained. Electrochemical Impedance Spectroscopy (EIS) measurements of LSCFN/YSZ/LSCFN test cells were performed. Half cells deposited by painting showed higher resistance than those deposited by PLD. Experimental results indicate that the PLD deposition on (110) single crystal substrate is the best. The area specific resistance value at 850 °C is 0.21 Ωcm2 pointing out that strontium can be substituted for calcium in the material without compromising the electrochemical properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Perovskite oxides, Ln1 − xAxBO3 − δ (Ln = rare earth; A = Sr, Ca, Ba; B = Cr, Mn, Fe, Co or Ni), exhibit a variety of magnetic and electronic properties [1,2]. Some perovskites display good performance as cathode materials in high temperature solid oxide fuel cells (SOFCs), due to their mixed, electronic and ionic conductivity [3]. La1 − xSrxMnO3 (LSM) perovskites are representative electronic conductors that have been extensively used as cathodes in ZrO2-based SOFCs [4]. These are good electronic conductors, but the lack of oxide-ion vacancies and its conductivity at the working temperatures, make it necessary to use thick and porous electrodes containing an array of triple-phase boundaries where gas, electrolyte and electrode meet [5]. These LSM oxides have the disadvantage of forming the low oxide-ion conductivity product of the pyrochlore oxide La2Zr2O7 at the boundary with the electrolyte and yttria-stabilized zirconia (YSZ) under an annealing temperature of more than 1200 °C, restricting the fabrication processes of SOFCs [6]. An additional problem is its high thermal expansion coefficient, which could be solved by substituting some of the Mn ions by Ni or Fe cations. It is also known that LaNiO3 has a very high electronic conductivity at room temperature, but it is unstable above 850 °C, where it decomposes to La2NiO4 and NiO. When Sr is introduced in the A-site of the LaNiO3 perovskite and Ni is substituted by Fe, the material is
stable at high temperature exhibiting high electronic conductivity (e.g., La0.6Sr0.4Ni0.2Fe0.8O3 435 S/cm at 800 °C) [7,8]. Calcium is another effective doping-element at the A-site of ABO3 and the obtained phases are cheaper. Moreover, the calcium cation exhibits similar ionic radius to that of the La3+ which could give rise to a higher stability than the strontium substitution [9]. In this way, La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 (LSCFN in the following) seems to be a very good candidate as cathode for SOFC devices. On the other hand, pulsed laser deposition (PLD) has been recently used to obtain La–Sr–Co–O thin films to be utilized in SOFC devices reducing the size and cost of the cells [10–17]. In this work, we have prepared several La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 (LSCFN) thin films by PLD and studied the effect of the substrate on the crystallinity and porosity of the samples. LSCFN films have been deposited on single crystal and polycrystalline YSZ substrates by PLD. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrical conductivity measurements have been performed. The electrical conductivity data have also been compared with both LSM thin films obtained by PLD and test cells made painting LSCFN material on both sides of a YSZ single crystal. 2. Experimental 2.1. Sample preparation
⁎ Corresponding author. Tel.: + 34 94 6012458; fax: + 34 94 6013500. E-mail address: [email protected] (T. Rojo). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.02.012
Powders of La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ were prepared by the liquid mix process. Appropriate amounts of the nitrate salts [La
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(NO 3 ) 3 ·6H 2 O; Ca(NO 3 ) 2 ·H 2 O; Sr(NO 3 ) 2 ; Fe(NO 3 ) 3 ·9H 2 O; Ni (NO3)2·6H2O] and citric acid were dissolved in distilled water and later the suitable volume of ethylene glycol was added. The resulting solution was stirred and heated in a heating plate until the formation of a gel. After that, the gel which was already treated in a sand bath, was calcined at 600 °C in a oven for 12 h with a 1°/min rate. 2.2. Sample characterization The crystalline powder was characterized by X-ray powder diffraction data, collected using a Philips PW1710 and Philips X'Pert-MPD (Bragg–Brentano geometry) diffractometers, with CuKα radiation, and fitted using the FULLPROF program [18,19]. 2.3. Thin films preparation The obtained powders were pressed into pellets (size 13 mm diameter and 2 mm thickness) and sintered at 1200 °C for 10 h. Using these sintered pellets as targets, LSCFN films were deposited on single crystals (with different directions: (100), (110), (111)) and polycrystalline YSZ substrates by PLD. The samples were deposited using a LAMBDA PHYSIC Compex 102 KrF excimer laser (248 nm, 150 mJ/pulse) at a frequency of 15 Hz. The target was placed in a rotating target holder in a vacuum chamber with an initial pressure of 2·10− 6 mbar. The substrates were mounted on a heater and the films were deposited at a substrate temperature of 700 °C, taking into account the existing literature [10–13] and our previous experience in PLD in similar samples [20,21]. The substrate temperature is one of the main parameter affecting atomic surface mobility during the deposition process. Oxygen gas was flowed into the chamber in a constant flux during deposition, keeping a pressure of 0.3 mbar. Deposition time was 120 min. In order to compare transport properties with those corresponding to the LSCFN samples, LSM thin films have been prepared in the same deposition conditions using a commercial powder (Praxair). 2.4. Characterization of the thin films X-ray powder diffraction data were used to characterize the obtained films, using a Philips PW1710 and Philips X'Pert-MPD (Bragg–Brentano geometry) diffractometers, with CuKα radiation, and fitted with the FULLPROF program [18,19]. The microstructure of the obtained films was observed by scanning electron microscopy (SEM) using a JEOL JSM-6400 microscope at 20 kV accelerating voltage. In order to obtain SEM images from the cross section of the samples, the thin films were mechanically fractured using a diamond point.
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fitting of each semicircle in the EIS measurements were obtained by least square refinement. 3. Results and discussion 3.1. Sample characterization The X-ray patterns obtained at room temperature of the sintered LSCFN sample at different heating temperatures are plotted in Fig. 1. The analysis of the data revealed that the sample obtained at 600 °C was a pure phase. The patterns were indexed on the basis of a distorted perovskite structure with orthorhombic GdFeO3-type [22] symmetry (space group Pbnm) similar to that of LaFeO3 [23]. The effect of doping with calcium and strontium induces a structural distortion, but no structural transitions were detected in our system, as those observed in other similar phases [24]. Each unit of four units with the approximate pffiffifficell consists pffiffiffi dimensions 2ap × 2ap × 2ap , where ap is the lattice parameter of the ideal cubic unit cell. The calculated cell parameters and the volume of the unit cell are shown in Table 1. Crystallographic data of other related phases have been included for comparison. When strontium and calcium are introduced in the lanthanum position of the perovskite, the unit cell volume and the pseudo-cubic lattice constant decrease. This fact can be explained as due to the change in the volume which depends on factors such as the difference in the ionic radii of the dopant strontium and calcium cations with respect to that of lanthanum, as well as the difference between the ionic radii of the transition metals at the B-site of the perovskite. The La1 − xA′xFe0.8Ni0.2O3 (A′ = divalent metal cation) perovskite oxides are mixed hopping conductors with more than one type of transition metal ions. The introduction of a cation with lower oxidation state (Sr2+ and Ca2+) in the lanthanide (La3+) position, produces an electronic unbalance in the perovskite lattice together with formation of Fe4+ cations and/or the generation of oxygen vacancies [9,27]. Usually, the Fe-based [28] and Ni-based [29] perovskites exhibit high electronic conductivity, giving rise to mixed oxygen ionic and electrical conductivities. This suggests that a preferential electronic charge compensation of Fe3+→Fe4+ occurs in the La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ sample. In this way, the increase of the electron localization due to the decrease in the covalency of the Fe4+–O bond together with the preferential formation of Fe4+ cations
2.5. Electrochemical measurements Testing cells of LSCFN/YSZ/LSCFN were performed by deposition of La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ on the two sides of a YSZ wafer. The transport properties were measured by electrochemical impedance spectroscopy (EIS) and the results were compared with those obtained for the LSCFN/YSZ/LSCFN cells made by painting both sides of the electrolyte with a LSCFN paste. The thicknesses of the substrates and films were 0.50 mm and 3 μm, respectively. Platinum grids pressed on both sides of the sample were used as electrical contacts. Electrochemical Impedance Spectroscopy (EIS) measurements of LSCFN/YSZ/LSCFN test cells were conducted using a Solartron 1260 Impedance Analyzer. The frequency range was 10− 2 to 106 Hz with a signal amplitude of 50 mV. All these electrochemical experiments were performed at equilibrium from 850 °C to room temperature, under zero dc current intensity and under air over a cycle of heating and cooling. Impedance diagrams were analyzed and fitted using the Zview software. Resistance, capacitance and the values from the
Fig. 1. XRD patterns obtained at room temperature for La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ powder sample heated at: (a) 150 °C and (b) 600 °C (experimental — circles, fitted — line, and difference between them — lower line).
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Table 1 Crystallographic parameters of La0.6Ca0.2Sr0.2Fe0.8Ni0.2O3 − δ and other related phases. Sample
G. E.
a (Å)
b (Å)
c (Å)
V (Å3)
a′ (Å)a
τb
Ref.
La0.6Ca0.4Fe0.8Ni0.2O3 − δ La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ La0.20Pr0.40Sr0.26Ca0.14FeO3 − δ La0.6Sr0.4FeO3 − δ LaFeO3
Pnma Pbnm Pnma Pnma Pbnm
5.534(2) 5.510(5) 5.5017(5) 5.604(4) 5.557
7.755(2) 5.584(6) 7.7851(6) 7.991(8) 5.565
5.503(2) 7.786(9) 5.5047(3) 5.374(6) 7.854
236.2(1) 239.64(2) 235.77 240.66 242.90
3.894 3.913 3.892 3.918 3.931
0.9738 0.9716 0.9636 0.9650 0.9605
[25] This work [24] [26] [23]
a b
a′ : pseudo-cubic lattice constant (V/Z)1/3. τ : tolerance factor.
give rise to a decrease in the electrical conductivity as the iron amount is increased. 3.2. Characterization of the thin films The XRD diffractograms of the target, the polycrystalline YSZ substrate and the thin film deposited on polycrystalline YSZ are shown in Fig. 2. The diffraction profile and the fitted profiles of the thin film deposited at 700 °C on single crystal substrates (with different directions: (100), (110), (111)) are given in Fig. 3. The reflections corresponding to the LSCFN crystalline phase are observed in all deposited samples, together with very strong peaks due to the single crystal and polycrystalline YSZ substrates. In the films deposited on the single crystal substrates with (100), (110), (111) directions an intense peak appearing at about 29, 35 and 50°(θ), respectively, is also observed.
Fig. 2. Diffraction profiles of the La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ phase: of the target (upper side), of the polycrystalline YSZ substrate (medium side) and experimental (circles), fitted (line) and difference between them (lower line) profiles of the thin film deposited at 700 °C on the polycrystalline YSZ (lower side) substrate.
The polycrystalline growth observed in all the films when the deposition is carried out on single crystal substrates is not epitaxial which can be attributed to the difference in the substrate structure (fluorite-type, Fm3m S.G.) and the target structure (perovskite-type Pbnm S.G.). An important texture appears in the films deposited on single crystal substrates (see Fig. 3) which was not appreciated in the film deposited on the polycrystalline substrate (see Fig. 2). The texture is in the preferential direction of the single crystal substrate. The peaks of the higher relative intensity correspond to the (200) direction, which is similar to that of the target. In the single crystal substrates, the second reflection corresponds to the preferential direction of the substrate (see Table 2). This trend was also observed in the Pr0.8Sr0.2Fe0.8Ni0.2O3 − δ films deposited on YSZ single crystal [30]. In addition, the partial oxygen pressure can not only affect the oxygen
Fig. 3. Experimental (circles), fitted (line) and difference between them (lower line) profiles of the La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ thin film deposited at 700 °C on the single crystal substrates with different directions: (111) upper side, (110) middle side and (100) lower side.
I.R. de Larramendi et al. / Solid State Ionics 192 (2011) 584–590 Table 2 Classification of the diffraction peaks according to their relative intensity in the XRD patterns. Sample or substrate type
Relative intensity
LSCFN powder
(200) (112) (020) (200) (112) (020) (200) (112) (020) (200) (112) (020) (200) (112) (020)
Polycrystalline
Single crystal (100)
Single crystal (110)
Single crystal (111)
Higher (100%)
Lower (312) (204) (024) (312) (204) (024) (224) (400) (041) (220) (004) (312) (204) (024)
(220) (004)
(202) (022)
(220) (004)
(202) (022)
(312) (204) (024) (312) (204) (024) (220) (004)
(202) (022) (224) (400) (041) (224) (400) (041)
(224) (400) (041) (224) (400) (041) (220) (004)
(110) (002) (110) (002) (110) (002)
(110) (002) (202) (022)
(110) (002)
stoichiometry but also the texture and orientation of this kind of films [12]. The diffraction peaks of the thin films are weak, showing a particle size of about 70 nm according to Scherrer's formula [31]. The peaks of the films are quite wider than those of the LSCFN target, pointing out a smaller particle size and, probably, a certain stress in the film due to the difference between the substrate and film structures. The SEM images, which are taken on the surface, and the cross section of the films after the electrochemical study are shown in Fig. 4. In SEM micrographs, pyramidal grains with a diameter of 80–100 nm are found. These results are similar to those described in the literature for other perovskite oxides [32,33]. A size distribution of the grains in all samples was observed but when depositing on the polycrystalline substrate, smaller grains were detected. The cross-section images show a columnar growth of the films, although the thin film deposited on the polycrystalline substrate exhibits narrower columns, which agrees well with the bigger amount of small grain observed in the corresponding surface micrograph. Likewise, the film deposited on polycrystalline YSZ is more dense, which could difficult the gas transport across the film. It is worth mentioning the attainment in these processes of nanostructured thin films and vertically aligned nanopores (VANP) with an average pore size of 25–45 nm when
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single crystal substrates were used, favoring the oxygen permeability. The existence of these VANPs relieves: i) the internal thermal stress partially or completely, ii) the lattice strain caused by the differences in the thermal expansion coefficients and iii) the lattice mismatch between the electrode and the electrolyte [34]. It can also be observed in the images of the cross section that all the films have a thickness of about 1.5–2 µm, being slightly lower in the case of the sample deposited on the single crystals. 3.3. Electrochemical measurements The influence of the deposition method (comparing the PLD technique and the conventional painting) and the type of substrate have been studied by electrochemical impedance spectroscopy (EIS). The as-synthesized LSCFN powder was dispersed in a commercial organic vehicle (Nextech Materials) forming a paste. To achieve a homogeneous paste, a powder/vehicle ratio of 1:1 in weight was used. This paste was smeared on both surfaces of a YSZ pellet as thinly as possible with a brush. The final sintering of the cathode was performed at 1050 °C during 90 min to form porous electrodes well adhered to the surface of the YSZ electrolytes. The EIS spectra for the LSCFN/YSZ/LSCFN cells measured in air at different temperatures are shown in Fig. 5. The mobility of the species becomes higher as the temperature is increased, favoring a smaller resistance. The influence of the deposition method has been considered taking into account the total resistance of the electrodic processes contribution. The Area Specific Resistance (ASR) is deduced from the relation: ASR = Relectrode · Surface Area/2 where Relectrode is the resistance assigned to the electrode processes (see inset in Fig. 5). The Arrhenius plot of the ASR values for the half cells deposited by both painting and PLD methods is shown in Fig. 6. The obtained values are summarized in Table 3. The ASR values in the painted material are higher than those obtained in the PLD process which can be attributed to a worse adherence of the film due to the inhomogeneity of the electrode/ electrolyte interface. The activation energy values for all deposited films are similar, but higher for the PLD method. This difference can be explained considering the conduction processes across the cathode. In this way, using the painting technique, the obtained film is porous, being able to conduce using two routes: three phase boundary-3 PB (through gas/electrode/electrolyte interface) and two phase boundary-2 PB (through gas/electrode interface and across the electrode to
Fig. 4. SEM images of the surface and cross section of the films on: (a) (100), (b) (110), (c) (111) single crystals and (d) polycrystalline YSZ substrates.
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The influence of the type of substrate on the Nyquist plot of LSCFN/ YSZ/LSCFN cell measured at different temperatures, in air, is shown in Fig. 7. The appearance of three depressed semicircles indicates
Fig. 5. Impedance spectra of LSCFN/YSZ/LSCFN cells obtained by painting measured at various temperatures in air. The impedance data are plotted after electrolyte ohmic drop correction.
Fig. 6. Arrhenius plots of ASR for LSCFN electrode on polycrystalline YSZ obtained by different deposition methods.
the electrolyte). On the other hand, on having deposited by PLD, very dense films that prevent the existence of 3 PB regions were obtained [35]. The oxygen reduction at thin dense films is one-dimensional with well-controlled bulk diffusion length. In the case of porous electrodes, the reaction rate depends on the kinetics of both the surface and bulk pathways, and thus the measured activation energy may consist of contributions from both pathways.
Table 3 ASR values at different temperatures for the films deposited by PLD on (100), (110), (111) single crystal and polycrystalline YSZ substrates (s.c.: single crystal) and using conventional painting method. Temperature °C 700 750 800 850
ASR/Ωcm2 Polycrystalline
(100) s.c.
(110) s.c.
(111) s.c.
Painting
1.98 0.66 0.39 0.33
2.21 0.89 0.50 0.23
1.39 0.77 0.30 0.21
1.66 1.14 0.68 0.37
103.63 34.56 19.46 10.96
Fig. 7. Impedance diagrams obtained with LSCFN/YSZ half cells, under air, at different temperatures: (a) 300 °C and (b) 500 °C using a polycrystalline YSZ substrate and (c) 450 °C using a (110) single crystal as substrate. The numbers by data points are frequency logarithms.
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the existence of different processes at low temperatures. In all substrates, a semicircle due to the bulk of the compound appears at high frequencies, with capacitance values of 8.7 10− 12, 8.4 10− 11, 8.6 10− 11 and 6.3 10− 11 F for the (100), (110), (111) single crystal and polycrystalline substrates, respectively. These values agree with those described in the bibliography [36]. The following semicircle that is observed at not so high frequencies is due to the grain boundary of the substrate, with a capacitance of 4.5 10− 8 F. In the case of the single crystals, this contribution is obviously not observed. Finally, the semicircles at medium and lower frequencies due to the different processes taking place in the electrode (interface processes, electrode reactions ...) are found, but these contributions are better studied at high temperatures. An example of the fit obtained for both substrates at different temperatures is shown in Fig. 7. As can be seen, the equivalent circuit with the best fit is shown as inset, where R1 is the ohmic resistance, R2 is related to the YSZ bulk contribution and R3 attributed to the substrate grain boundary (only observed when the polycrystalline substrate is used). Then, R4 and R5 are the diffusion in the perovskite and dissociative adsorption on the electrode surface, respectively [14]. R4 and R5 have constant phase elements (CPE) in parallel to simulate the distribution of relaxation time in the real system. Both semicircles are overlapped, making difficult the analysis of the results obtained for each process. The Arrhenius plot of capacitances of the different contributions for two substrates (polycrystalline and (110) single crystal) is shown in Fig. 8. A study according to the methodology proposed by Schouler et al. was also carried out [37]. In this case, the thermal evolution of the relaxation frequencies, fr, and capacitances, C, is independent from the geometric characteristics of the sample, which enabled us to associate the different contributions to their respective processes. At high frequencies (HF), the different electrolyte contributions only observable at low temperatures are detected. The contributions at higher frequencies are due to the processes produced in the bulk and in the grain boundary (indicated as g.b.). The capacitance values obtained for the YSZ polycrystalline electrolyte bulk and grain boundary processes are 10− 11 and 10− 8 F, respectively. On the other hand, two different contributions at lower frequencies can be seen for the electrodic processes which are associated to the MIEC (mixed ionic and electronic conductors) behavior [38]. The oxygen reduction mechanism on porous MIEC electrodes may involve several processes such as charge transfer at the current collector/electrode interface and electrode/electrolyte interfaces, oxygen exchange at the electrode surface, bulk and surface diffusion of oxygen species and gas phase diffusion. To be able to assign every contribution with its respective semicircle, it would be necessary to perform measurements in different oxygen partial pressures, so that this study will be carried out in further works under way. The Arrhenius plots of the ASR values for LSCFN/YSZ/LSCFN half cells using different substrates such as (100), (110), (111) YSZ single crystal and polycrystalline are given in Table 3. All the films show values of ASR below to 1 Ωcm2 even at intermediate temperatures. The use of single crystal substrates gives rise to a lower ASR value due to the preferential growth that these films present favoring the gas transport across it. 4. Conclusions Polycrystalline thin films of La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 have been deposited on four different substrates: (100), (110), (111) YSZ single crystal and polycrystalline YSZ, at 700 °C. All films are polycrystalline with a marked texture, favoring the preferential direction of the substrate. The examination of the cross-section structure of the films by SEM revealed a dense ordered columnar structure, with narrower columns for the sample deposited on the polycrystalline substrate. Nanostructured cathode thin films with vertically aligned nanopores
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Fig. 8. Arrhenius plots of capacitances (C) for the LSCFN/YSZ/LSCFN cells under air and zero dc conditions (HF: high frequencies; MF: medium frequencies; LF: low frequencies) for two different substrates: (a) YSZ polycrystalline and (b) YSZ (110) single crystal.
(VANP) are obtained. On the surface, pyramidal grains with a certain size distribution are visible. The film thickness is about 2 µm, being slightly lower in the case of the single crystal substrates. The electrochemical measurements of the LSCFN material on (100), (110), (111) YSZ single crystal and polycrystalline YSZ substrates as electrolyte showed a better performance for the (110) single crystal substrate. The obtained ASR value at 850 °C of 0.21 Ω cm2 points out that strontium can be substituted for the cheaper calcium cation without compromising the electrochemical properties. The deposition technique is an important parameter which influences on the electrochemical behavior. The painted films are porous giving rise to three phase boundary (TPB) conduction where the reactive gas meets the electrode and the electrolyte phases. In the case of PLD thin films there is no direct contact between the cathode, electrolyte and gas (hence no TPB conduction exists). Therefore, in this case the oxygen reduction reaction occurs anywhere on the cathode surface, forming oxide anions which diffuse into the bulk of the electrode material towards the electrolyte, confirming the existence of mixed ionic and electronic conduction in this material.
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Acknowledgements This work has been partially financed by the Spanish CiCyT under project MAT2004-02425 and MAT2007-66737-C02-01 and by the Government of the Basque Country under project IT-312-07. I. Ruiz de Larramendi thanks the Government of the Basque Country for funding her research activities as postdoc within the Project GIC07/126-IT312-07 and N. Ortiz-Vitoriano thanks the Eusko Jaurlaritza/Gobierno Vasco for her predoctoral fellowship. References [1] A. Peña, J. Gutiérrez, J. Campo, J.M. Barandiarán, L. Lezama, I. Gil de Muro, T. Rojo, Eur. J. Inorg. Chem. 16 (2008) 2569. [2] R. Cortés-Gil, M. Hernando, M.L. Ruiz-González, E. Céspedes, C. Prieto, J.M. Alonso, M. Vallet-Regí, A. Hernando, J.M. González-Calbet, Chem. Eur. J. 14 (29) (2008) 9038. [3] Y. Takeda, Y. Sakaki, T. Ichikawa, N. Imanishi, O. Yamamoto, M. Mori, N. Mori, T. Abe, Solid State Ionics 72 (1994) 257. [4] C. Chervin, R.S. Glass, S.M. Kauzlarich, Solid State Ionics 176 (2005) 17. [5] J. Richter, P. Holtappels, T. Graule, T. Nakamura, L.J. Gauckler, Monatsch. Chem. 140 (2009) 985. [6] G. Zhu, X. Fang, C. Xia, X. Liu, Ceram. Int. 31 (1) (2005) 115. [7] R. Chiba, F. Yoshimura, Y. Sakurai, Solid State Ionics 124 (1999) 281. [8] R. Chiba, F. Yoshimura, Y. Sakurai, Solid State Ionics 152/153 (2002) 575. [9] P. Ciambelli, S. Cimino, L. Lisi, M. Faticanti, G. Minelli, I. Pettiti, P. Porta, Appl. Catal. B 33 (2001) 193. [10] X. Chen, N.J. Wu, A. Ignatiev, J. Eur. Ceram. Soc. 19 (1999) 819. [11] X. Chen, N.J. Wu, D.L. Ritums, A. Ignatiev, Thin Solid Films 342 (1999) 61. [12] X. Chen, S. Wang, Y.L. Yang, L. Smith, N.J. Wu, B.-I. Kim, S.S. Perry, A.J. Jacobson, A. Ignatiev, Solid State Ionics 146 (3,4) (2002) 405. [13] X. Chen, N.J. Wu, L. Smith, A. Ignatiev, Appl. Phys. Lett. 84 (2004) 2700. [14] N. Imanishi, T. Matsumura, Y. Sumiya, K. Yoshimura, A. Hirano, Y. Takeda, D. Mori, R. Kanno, Solid State Ionics 174 (2004) 245. [15] D. Mori, H. Oka, Y. Suzuki, N. Sonoyama, A. Yamada, R. Kanno, Y. Sumiya, N. Imanishi, Y. Takeda, Solid State Ionics 177 (2006) 535.
[16] N. Imanishi, Y. Sumiya, K. Yoshimura, T. Matsumura, A. Hirano, Y. Takeda, D. Mori, R. Kanno, Solid State Ionics 177 (2006) 749. [17] L.R. Pederson, P. Singh, X.-D. Zhou, Vacuum 80 (2006) 1066. [18] J. Rodríguez-Carvajal, Physica B 192 (1–2) (1993) 55. [19] J. Rodríguez-Carvajal, FULLPROF Program: Rietveld Pattern Matching Analysis of Powder Patterns, ILL, Grenoble, 1994. [20] I. Ruiz de Larramendi, N. Ortiz, R. López-Antón, J.I. Ruiz de Larramendi, T. Rojo, J. Power Sources 171 (2007) 747. [21] I. Ruiz de Larramendi, R. López-Antón, J.I. Ruiz de Larramendi, T. Rojo, Appl. Phys. A 93 (3) (2008) 655. [22] S. Geller, J. Chem. Phys. 24 (1956) 1236. [23] S.E. Dann, D.B. Currie, M.T. Weller, M.F. Thomas, A.D. Al-Rawwas, J. Solid State Chem. 109 (1994) 134. [24] K. Vidal, L.M. Rodríguez-Martínez, L. Ortega-San-Martin, E. Díez-Linaza, M.L. Nó, T. Rojo, A. Laresgoiti, M.I. Arriortua, Solid State Ionics 178 (2007) 1310. [25] I. Ruiz de Larramendi, N. Ortiz, J.I. Ruiz de Larramendi, T. Rojo, Electrochem. Soc. Trans. 7 (1) (2007) 1157. [26] A.N. Nadeev, S.V. Tsybulya, A.N. Shmakov, G.N. Kryukova, I.S. Yakovleva, L.A. Isupova, J. Structural Chem. 48 (6) (2007) 1105. [27] G. Pecchi, P. Reyes, R. Zamora, C. Campos, L.E. Cadús, B.P. Barbero, Catal. Today 133–135 (2008) 420. [28] V.V. Kharton, A.A. Viskupa, E.N. Naumovicha, V.N. Tikhonovicha, Mater. Res. Bull. 34 (1999) 1311. [29] K. Huang, H.Y. Lee, J.B. Goodenough, J. Electrochem. Soc. 145 (8) (1998) 3220. [30] I. Ruiz de Larramendi, R.L. Antón, J.I. Ruiz de Larramendi, S. Baliteau, F. Mauvy, J.C. Grenier, T. Rojo, J. Power Sources 169 (1) (2007) 35. [31] A.L. Patterson, Phys. Rev. 56 (1939) 978. [32] N. Scarisoreanu, F. Craciun, G. Dinescu, P. Verardi, M. Dinescu, Thin Solid Films 453–454 (2004) 399. [33] W. Wu, K.H. Wong, C.L. Choy, J. Phys, D: Appl. Phys. 32 (1999) L57. [34] J. Yoon, R. Araujo, N. Grunbaum, L. Baqué, A. Serquis, A. Caneiro, X. Zhang, H. Wang, Appl. Surf. Sci. 254 (1) (2007) 266. [35] J. Sunarso, S. Baumann, J.M. Serra, W.A. Meulenberg, S. Liu, Y.S. Lin, J.C. Diniz da Costa, J. Membr. Sci. 320 (2008) 13. [36] J.T.S. Irvine, D.C. Sinclair, A.R. West, Adv. Mater. 2 (1990) 132. [37] E.J.L. Schouler, N. Mesbahi, G. Vitter, Solid State Ionics 9 (10) (1983) 989. [38] F. Mauvy, C. Lalanne, J.M. Bassat, J.C. Grenier, H. Zhao, L. Huo, P. Stevens, J. Electrochem. Soc. 153 (8) (2006) A1547.
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 thin films obtained by pulsed laser ablation: Effect of the substrate on the electrochemical behavior I. Ruiz de Larramendi a, S. Vivès a, N. Ortiz-Vitoriano a, J.I. Ruiz de Larramendi a, M.I. Arriortua b, T. Rojo a,⁎ a b
Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo.644, 48080 Bilbao, Spain Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo.644, 48080 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 18 January 2010 Accepted 10 February 2010 Available online 11 March 2010 Keywords: Solid oxide fuel cell Cathodes Perovskite Pulsed laser deposition Electrochemical impedance spectroscopy
a b s t r a c t Polycrystalline La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 (LSCFN) samples were prepared by the liquid mix process at 600 °C. The analysis of X-ray powder diffraction (XRD) patterns indicated that the sample was single phase and crystallized in an orthorhombic structure (Pbnm). High quality LSCFN films have been deposited on different yttria-stabilized zirconia (YSZ) substrates by conventional painting method and by pulsed laser deposition (PLD). The structure of the films was analyzed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). All films are polycrystalline with a marked texture exhibiting pyramidal grains in the surface with different size distribution. Nanostructured cathode thin films with vertically aligned nanopores (VANP) have been obtained. Electrochemical Impedance Spectroscopy (EIS) measurements of LSCFN/YSZ/LSCFN test cells were performed. Half cells deposited by painting showed higher resistance than those deposited by PLD. Experimental results indicate that the PLD deposition on (110) single crystal substrate is the best. The area specific resistance value at 850 °C is 0.21 Ωcm2 pointing out that strontium can be substituted for calcium in the material without compromising the electrochemical properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Perovskite oxides, Ln1 − xAxBO3 − δ (Ln = rare earth; A = Sr, Ca, Ba; B = Cr, Mn, Fe, Co or Ni), exhibit a variety of magnetic and electronic properties [1,2]. Some perovskites display good performance as cathode materials in high temperature solid oxide fuel cells (SOFCs), due to their mixed, electronic and ionic conductivity [3]. La1 − xSrxMnO3 (LSM) perovskites are representative electronic conductors that have been extensively used as cathodes in ZrO2-based SOFCs [4]. These are good electronic conductors, but the lack of oxide-ion vacancies and its conductivity at the working temperatures, make it necessary to use thick and porous electrodes containing an array of triple-phase boundaries where gas, electrolyte and electrode meet [5]. These LSM oxides have the disadvantage of forming the low oxide-ion conductivity product of the pyrochlore oxide La2Zr2O7 at the boundary with the electrolyte and yttria-stabilized zirconia (YSZ) under an annealing temperature of more than 1200 °C, restricting the fabrication processes of SOFCs [6]. An additional problem is its high thermal expansion coefficient, which could be solved by substituting some of the Mn ions by Ni or Fe cations. It is also known that LaNiO3 has a very high electronic conductivity at room temperature, but it is unstable above 850 °C, where it decomposes to La2NiO4 and NiO. When Sr is introduced in the A-site of the LaNiO3 perovskite and Ni is substituted by Fe, the material is
stable at high temperature exhibiting high electronic conductivity (e.g., La0.6Sr0.4Ni0.2Fe0.8O3 435 S/cm at 800 °C) [7,8]. Calcium is another effective doping-element at the A-site of ABO3 and the obtained phases are cheaper. Moreover, the calcium cation exhibits similar ionic radius to that of the La3+ which could give rise to a higher stability than the strontium substitution [9]. In this way, La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 (LSCFN in the following) seems to be a very good candidate as cathode for SOFC devices. On the other hand, pulsed laser deposition (PLD) has been recently used to obtain La–Sr–Co–O thin films to be utilized in SOFC devices reducing the size and cost of the cells [10–17]. In this work, we have prepared several La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 (LSCFN) thin films by PLD and studied the effect of the substrate on the crystallinity and porosity of the samples. LSCFN films have been deposited on single crystal and polycrystalline YSZ substrates by PLD. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrical conductivity measurements have been performed. The electrical conductivity data have also been compared with both LSM thin films obtained by PLD and test cells made painting LSCFN material on both sides of a YSZ single crystal. 2. Experimental 2.1. Sample preparation
⁎ Corresponding author. Tel.: + 34 94 6012458; fax: + 34 94 6013500. E-mail address: [email protected] (T. Rojo). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.02.012
Powders of La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ were prepared by the liquid mix process. Appropriate amounts of the nitrate salts [La
I.R. de Larramendi et al. / Solid State Ionics 192 (2011) 584–590
(NO 3 ) 3 ·6H 2 O; Ca(NO 3 ) 2 ·H 2 O; Sr(NO 3 ) 2 ; Fe(NO 3 ) 3 ·9H 2 O; Ni (NO3)2·6H2O] and citric acid were dissolved in distilled water and later the suitable volume of ethylene glycol was added. The resulting solution was stirred and heated in a heating plate until the formation of a gel. After that, the gel which was already treated in a sand bath, was calcined at 600 °C in a oven for 12 h with a 1°/min rate. 2.2. Sample characterization The crystalline powder was characterized by X-ray powder diffraction data, collected using a Philips PW1710 and Philips X'Pert-MPD (Bragg–Brentano geometry) diffractometers, with CuKα radiation, and fitted using the FULLPROF program [18,19]. 2.3. Thin films preparation The obtained powders were pressed into pellets (size 13 mm diameter and 2 mm thickness) and sintered at 1200 °C for 10 h. Using these sintered pellets as targets, LSCFN films were deposited on single crystals (with different directions: (100), (110), (111)) and polycrystalline YSZ substrates by PLD. The samples were deposited using a LAMBDA PHYSIC Compex 102 KrF excimer laser (248 nm, 150 mJ/pulse) at a frequency of 15 Hz. The target was placed in a rotating target holder in a vacuum chamber with an initial pressure of 2·10− 6 mbar. The substrates were mounted on a heater and the films were deposited at a substrate temperature of 700 °C, taking into account the existing literature [10–13] and our previous experience in PLD in similar samples [20,21]. The substrate temperature is one of the main parameter affecting atomic surface mobility during the deposition process. Oxygen gas was flowed into the chamber in a constant flux during deposition, keeping a pressure of 0.3 mbar. Deposition time was 120 min. In order to compare transport properties with those corresponding to the LSCFN samples, LSM thin films have been prepared in the same deposition conditions using a commercial powder (Praxair). 2.4. Characterization of the thin films X-ray powder diffraction data were used to characterize the obtained films, using a Philips PW1710 and Philips X'Pert-MPD (Bragg–Brentano geometry) diffractometers, with CuKα radiation, and fitted with the FULLPROF program [18,19]. The microstructure of the obtained films was observed by scanning electron microscopy (SEM) using a JEOL JSM-6400 microscope at 20 kV accelerating voltage. In order to obtain SEM images from the cross section of the samples, the thin films were mechanically fractured using a diamond point.
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fitting of each semicircle in the EIS measurements were obtained by least square refinement. 3. Results and discussion 3.1. Sample characterization The X-ray patterns obtained at room temperature of the sintered LSCFN sample at different heating temperatures are plotted in Fig. 1. The analysis of the data revealed that the sample obtained at 600 °C was a pure phase. The patterns were indexed on the basis of a distorted perovskite structure with orthorhombic GdFeO3-type [22] symmetry (space group Pbnm) similar to that of LaFeO3 [23]. The effect of doping with calcium and strontium induces a structural distortion, but no structural transitions were detected in our system, as those observed in other similar phases [24]. Each unit of four units with the approximate pffiffifficell consists pffiffiffi dimensions 2ap × 2ap × 2ap , where ap is the lattice parameter of the ideal cubic unit cell. The calculated cell parameters and the volume of the unit cell are shown in Table 1. Crystallographic data of other related phases have been included for comparison. When strontium and calcium are introduced in the lanthanum position of the perovskite, the unit cell volume and the pseudo-cubic lattice constant decrease. This fact can be explained as due to the change in the volume which depends on factors such as the difference in the ionic radii of the dopant strontium and calcium cations with respect to that of lanthanum, as well as the difference between the ionic radii of the transition metals at the B-site of the perovskite. The La1 − xA′xFe0.8Ni0.2O3 (A′ = divalent metal cation) perovskite oxides are mixed hopping conductors with more than one type of transition metal ions. The introduction of a cation with lower oxidation state (Sr2+ and Ca2+) in the lanthanide (La3+) position, produces an electronic unbalance in the perovskite lattice together with formation of Fe4+ cations and/or the generation of oxygen vacancies [9,27]. Usually, the Fe-based [28] and Ni-based [29] perovskites exhibit high electronic conductivity, giving rise to mixed oxygen ionic and electrical conductivities. This suggests that a preferential electronic charge compensation of Fe3+→Fe4+ occurs in the La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ sample. In this way, the increase of the electron localization due to the decrease in the covalency of the Fe4+–O bond together with the preferential formation of Fe4+ cations
2.5. Electrochemical measurements Testing cells of LSCFN/YSZ/LSCFN were performed by deposition of La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ on the two sides of a YSZ wafer. The transport properties were measured by electrochemical impedance spectroscopy (EIS) and the results were compared with those obtained for the LSCFN/YSZ/LSCFN cells made by painting both sides of the electrolyte with a LSCFN paste. The thicknesses of the substrates and films were 0.50 mm and 3 μm, respectively. Platinum grids pressed on both sides of the sample were used as electrical contacts. Electrochemical Impedance Spectroscopy (EIS) measurements of LSCFN/YSZ/LSCFN test cells were conducted using a Solartron 1260 Impedance Analyzer. The frequency range was 10− 2 to 106 Hz with a signal amplitude of 50 mV. All these electrochemical experiments were performed at equilibrium from 850 °C to room temperature, under zero dc current intensity and under air over a cycle of heating and cooling. Impedance diagrams were analyzed and fitted using the Zview software. Resistance, capacitance and the values from the
Fig. 1. XRD patterns obtained at room temperature for La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ powder sample heated at: (a) 150 °C and (b) 600 °C (experimental — circles, fitted — line, and difference between them — lower line).
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Table 1 Crystallographic parameters of La0.6Ca0.2Sr0.2Fe0.8Ni0.2O3 − δ and other related phases. Sample
G. E.
a (Å)
b (Å)
c (Å)
V (Å3)
a′ (Å)a
τb
Ref.
La0.6Ca0.4Fe0.8Ni0.2O3 − δ La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ La0.20Pr0.40Sr0.26Ca0.14FeO3 − δ La0.6Sr0.4FeO3 − δ LaFeO3
Pnma Pbnm Pnma Pnma Pbnm
5.534(2) 5.510(5) 5.5017(5) 5.604(4) 5.557
7.755(2) 5.584(6) 7.7851(6) 7.991(8) 5.565
5.503(2) 7.786(9) 5.5047(3) 5.374(6) 7.854
236.2(1) 239.64(2) 235.77 240.66 242.90
3.894 3.913 3.892 3.918 3.931
0.9738 0.9716 0.9636 0.9650 0.9605
[25] This work [24] [26] [23]
a b
a′ : pseudo-cubic lattice constant (V/Z)1/3. τ : tolerance factor.
give rise to a decrease in the electrical conductivity as the iron amount is increased. 3.2. Characterization of the thin films The XRD diffractograms of the target, the polycrystalline YSZ substrate and the thin film deposited on polycrystalline YSZ are shown in Fig. 2. The diffraction profile and the fitted profiles of the thin film deposited at 700 °C on single crystal substrates (with different directions: (100), (110), (111)) are given in Fig. 3. The reflections corresponding to the LSCFN crystalline phase are observed in all deposited samples, together with very strong peaks due to the single crystal and polycrystalline YSZ substrates. In the films deposited on the single crystal substrates with (100), (110), (111) directions an intense peak appearing at about 29, 35 and 50°(θ), respectively, is also observed.
Fig. 2. Diffraction profiles of the La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ phase: of the target (upper side), of the polycrystalline YSZ substrate (medium side) and experimental (circles), fitted (line) and difference between them (lower line) profiles of the thin film deposited at 700 °C on the polycrystalline YSZ (lower side) substrate.
The polycrystalline growth observed in all the films when the deposition is carried out on single crystal substrates is not epitaxial which can be attributed to the difference in the substrate structure (fluorite-type, Fm3m S.G.) and the target structure (perovskite-type Pbnm S.G.). An important texture appears in the films deposited on single crystal substrates (see Fig. 3) which was not appreciated in the film deposited on the polycrystalline substrate (see Fig. 2). The texture is in the preferential direction of the single crystal substrate. The peaks of the higher relative intensity correspond to the (200) direction, which is similar to that of the target. In the single crystal substrates, the second reflection corresponds to the preferential direction of the substrate (see Table 2). This trend was also observed in the Pr0.8Sr0.2Fe0.8Ni0.2O3 − δ films deposited on YSZ single crystal [30]. In addition, the partial oxygen pressure can not only affect the oxygen
Fig. 3. Experimental (circles), fitted (line) and difference between them (lower line) profiles of the La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 − δ thin film deposited at 700 °C on the single crystal substrates with different directions: (111) upper side, (110) middle side and (100) lower side.
I.R. de Larramendi et al. / Solid State Ionics 192 (2011) 584–590 Table 2 Classification of the diffraction peaks according to their relative intensity in the XRD patterns. Sample or substrate type
Relative intensity
LSCFN powder
(200) (112) (020) (200) (112) (020) (200) (112) (020) (200) (112) (020) (200) (112) (020)
Polycrystalline
Single crystal (100)
Single crystal (110)
Single crystal (111)
Higher (100%)
Lower (312) (204) (024) (312) (204) (024) (224) (400) (041) (220) (004) (312) (204) (024)
(220) (004)
(202) (022)
(220) (004)
(202) (022)
(312) (204) (024) (312) (204) (024) (220) (004)
(202) (022) (224) (400) (041) (224) (400) (041)
(224) (400) (041) (224) (400) (041) (220) (004)
(110) (002) (110) (002) (110) (002)
(110) (002) (202) (022)
(110) (002)
stoichiometry but also the texture and orientation of this kind of films [12]. The diffraction peaks of the thin films are weak, showing a particle size of about 70 nm according to Scherrer's formula [31]. The peaks of the films are quite wider than those of the LSCFN target, pointing out a smaller particle size and, probably, a certain stress in the film due to the difference between the substrate and film structures. The SEM images, which are taken on the surface, and the cross section of the films after the electrochemical study are shown in Fig. 4. In SEM micrographs, pyramidal grains with a diameter of 80–100 nm are found. These results are similar to those described in the literature for other perovskite oxides [32,33]. A size distribution of the grains in all samples was observed but when depositing on the polycrystalline substrate, smaller grains were detected. The cross-section images show a columnar growth of the films, although the thin film deposited on the polycrystalline substrate exhibits narrower columns, which agrees well with the bigger amount of small grain observed in the corresponding surface micrograph. Likewise, the film deposited on polycrystalline YSZ is more dense, which could difficult the gas transport across the film. It is worth mentioning the attainment in these processes of nanostructured thin films and vertically aligned nanopores (VANP) with an average pore size of 25–45 nm when
587
single crystal substrates were used, favoring the oxygen permeability. The existence of these VANPs relieves: i) the internal thermal stress partially or completely, ii) the lattice strain caused by the differences in the thermal expansion coefficients and iii) the lattice mismatch between the electrode and the electrolyte [34]. It can also be observed in the images of the cross section that all the films have a thickness of about 1.5–2 µm, being slightly lower in the case of the sample deposited on the single crystals. 3.3. Electrochemical measurements The influence of the deposition method (comparing the PLD technique and the conventional painting) and the type of substrate have been studied by electrochemical impedance spectroscopy (EIS). The as-synthesized LSCFN powder was dispersed in a commercial organic vehicle (Nextech Materials) forming a paste. To achieve a homogeneous paste, a powder/vehicle ratio of 1:1 in weight was used. This paste was smeared on both surfaces of a YSZ pellet as thinly as possible with a brush. The final sintering of the cathode was performed at 1050 °C during 90 min to form porous electrodes well adhered to the surface of the YSZ electrolytes. The EIS spectra for the LSCFN/YSZ/LSCFN cells measured in air at different temperatures are shown in Fig. 5. The mobility of the species becomes higher as the temperature is increased, favoring a smaller resistance. The influence of the deposition method has been considered taking into account the total resistance of the electrodic processes contribution. The Area Specific Resistance (ASR) is deduced from the relation: ASR = Relectrode · Surface Area/2 where Relectrode is the resistance assigned to the electrode processes (see inset in Fig. 5). The Arrhenius plot of the ASR values for the half cells deposited by both painting and PLD methods is shown in Fig. 6. The obtained values are summarized in Table 3. The ASR values in the painted material are higher than those obtained in the PLD process which can be attributed to a worse adherence of the film due to the inhomogeneity of the electrode/ electrolyte interface. The activation energy values for all deposited films are similar, but higher for the PLD method. This difference can be explained considering the conduction processes across the cathode. In this way, using the painting technique, the obtained film is porous, being able to conduce using two routes: three phase boundary-3 PB (through gas/electrode/electrolyte interface) and two phase boundary-2 PB (through gas/electrode interface and across the electrode to
Fig. 4. SEM images of the surface and cross section of the films on: (a) (100), (b) (110), (c) (111) single crystals and (d) polycrystalline YSZ substrates.
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The influence of the type of substrate on the Nyquist plot of LSCFN/ YSZ/LSCFN cell measured at different temperatures, in air, is shown in Fig. 7. The appearance of three depressed semicircles indicates
Fig. 5. Impedance spectra of LSCFN/YSZ/LSCFN cells obtained by painting measured at various temperatures in air. The impedance data are plotted after electrolyte ohmic drop correction.
Fig. 6. Arrhenius plots of ASR for LSCFN electrode on polycrystalline YSZ obtained by different deposition methods.
the electrolyte). On the other hand, on having deposited by PLD, very dense films that prevent the existence of 3 PB regions were obtained [35]. The oxygen reduction at thin dense films is one-dimensional with well-controlled bulk diffusion length. In the case of porous electrodes, the reaction rate depends on the kinetics of both the surface and bulk pathways, and thus the measured activation energy may consist of contributions from both pathways.
Table 3 ASR values at different temperatures for the films deposited by PLD on (100), (110), (111) single crystal and polycrystalline YSZ substrates (s.c.: single crystal) and using conventional painting method. Temperature °C 700 750 800 850
ASR/Ωcm2 Polycrystalline
(100) s.c.
(110) s.c.
(111) s.c.
Painting
1.98 0.66 0.39 0.33
2.21 0.89 0.50 0.23
1.39 0.77 0.30 0.21
1.66 1.14 0.68 0.37
103.63 34.56 19.46 10.96
Fig. 7. Impedance diagrams obtained with LSCFN/YSZ half cells, under air, at different temperatures: (a) 300 °C and (b) 500 °C using a polycrystalline YSZ substrate and (c) 450 °C using a (110) single crystal as substrate. The numbers by data points are frequency logarithms.
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the existence of different processes at low temperatures. In all substrates, a semicircle due to the bulk of the compound appears at high frequencies, with capacitance values of 8.7 10− 12, 8.4 10− 11, 8.6 10− 11 and 6.3 10− 11 F for the (100), (110), (111) single crystal and polycrystalline substrates, respectively. These values agree with those described in the bibliography [36]. The following semicircle that is observed at not so high frequencies is due to the grain boundary of the substrate, with a capacitance of 4.5 10− 8 F. In the case of the single crystals, this contribution is obviously not observed. Finally, the semicircles at medium and lower frequencies due to the different processes taking place in the electrode (interface processes, electrode reactions ...) are found, but these contributions are better studied at high temperatures. An example of the fit obtained for both substrates at different temperatures is shown in Fig. 7. As can be seen, the equivalent circuit with the best fit is shown as inset, where R1 is the ohmic resistance, R2 is related to the YSZ bulk contribution and R3 attributed to the substrate grain boundary (only observed when the polycrystalline substrate is used). Then, R4 and R5 are the diffusion in the perovskite and dissociative adsorption on the electrode surface, respectively [14]. R4 and R5 have constant phase elements (CPE) in parallel to simulate the distribution of relaxation time in the real system. Both semicircles are overlapped, making difficult the analysis of the results obtained for each process. The Arrhenius plot of capacitances of the different contributions for two substrates (polycrystalline and (110) single crystal) is shown in Fig. 8. A study according to the methodology proposed by Schouler et al. was also carried out [37]. In this case, the thermal evolution of the relaxation frequencies, fr, and capacitances, C, is independent from the geometric characteristics of the sample, which enabled us to associate the different contributions to their respective processes. At high frequencies (HF), the different electrolyte contributions only observable at low temperatures are detected. The contributions at higher frequencies are due to the processes produced in the bulk and in the grain boundary (indicated as g.b.). The capacitance values obtained for the YSZ polycrystalline electrolyte bulk and grain boundary processes are 10− 11 and 10− 8 F, respectively. On the other hand, two different contributions at lower frequencies can be seen for the electrodic processes which are associated to the MIEC (mixed ionic and electronic conductors) behavior [38]. The oxygen reduction mechanism on porous MIEC electrodes may involve several processes such as charge transfer at the current collector/electrode interface and electrode/electrolyte interfaces, oxygen exchange at the electrode surface, bulk and surface diffusion of oxygen species and gas phase diffusion. To be able to assign every contribution with its respective semicircle, it would be necessary to perform measurements in different oxygen partial pressures, so that this study will be carried out in further works under way. The Arrhenius plots of the ASR values for LSCFN/YSZ/LSCFN half cells using different substrates such as (100), (110), (111) YSZ single crystal and polycrystalline are given in Table 3. All the films show values of ASR below to 1 Ωcm2 even at intermediate temperatures. The use of single crystal substrates gives rise to a lower ASR value due to the preferential growth that these films present favoring the gas transport across it. 4. Conclusions Polycrystalline thin films of La0.6Sr0.2Ca0.2Fe0.8Ni0.2O3 have been deposited on four different substrates: (100), (110), (111) YSZ single crystal and polycrystalline YSZ, at 700 °C. All films are polycrystalline with a marked texture, favoring the preferential direction of the substrate. The examination of the cross-section structure of the films by SEM revealed a dense ordered columnar structure, with narrower columns for the sample deposited on the polycrystalline substrate. Nanostructured cathode thin films with vertically aligned nanopores
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Fig. 8. Arrhenius plots of capacitances (C) for the LSCFN/YSZ/LSCFN cells under air and zero dc conditions (HF: high frequencies; MF: medium frequencies; LF: low frequencies) for two different substrates: (a) YSZ polycrystalline and (b) YSZ (110) single crystal.
(VANP) are obtained. On the surface, pyramidal grains with a certain size distribution are visible. The film thickness is about 2 µm, being slightly lower in the case of the single crystal substrates. The electrochemical measurements of the LSCFN material on (100), (110), (111) YSZ single crystal and polycrystalline YSZ substrates as electrolyte showed a better performance for the (110) single crystal substrate. The obtained ASR value at 850 °C of 0.21 Ω cm2 points out that strontium can be substituted for the cheaper calcium cation without compromising the electrochemical properties. The deposition technique is an important parameter which influences on the electrochemical behavior. The painted films are porous giving rise to three phase boundary (TPB) conduction where the reactive gas meets the electrode and the electrolyte phases. In the case of PLD thin films there is no direct contact between the cathode, electrolyte and gas (hence no TPB conduction exists). Therefore, in this case the oxygen reduction reaction occurs anywhere on the cathode surface, forming oxide anions which diffuse into the bulk of the electrode material towards the electrolyte, confirming the existence of mixed ionic and electronic conduction in this material.
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Acknowledgements This work has been partially financed by the Spanish CiCyT under project MAT2004-02425 and MAT2007-66737-C02-01 and by the Government of the Basque Country under project IT-312-07. I. Ruiz de Larramendi thanks the Government of the Basque Country for funding her research activities as postdoc within the Project GIC07/126-IT312-07 and N. Ortiz-Vitoriano thanks the Eusko Jaurlaritza/Gobierno Vasco for her predoctoral fellowship. References [1] A. Peña, J. Gutiérrez, J. Campo, J.M. Barandiarán, L. Lezama, I. Gil de Muro, T. Rojo, Eur. J. Inorg. Chem. 16 (2008) 2569. [2] R. Cortés-Gil, M. Hernando, M.L. Ruiz-González, E. Céspedes, C. Prieto, J.M. Alonso, M. Vallet-Regí, A. Hernando, J.M. González-Calbet, Chem. Eur. J. 14 (29) (2008) 9038. [3] Y. Takeda, Y. Sakaki, T. Ichikawa, N. Imanishi, O. Yamamoto, M. Mori, N. Mori, T. Abe, Solid State Ionics 72 (1994) 257. [4] C. Chervin, R.S. Glass, S.M. Kauzlarich, Solid State Ionics 176 (2005) 17. [5] J. Richter, P. Holtappels, T. Graule, T. Nakamura, L.J. Gauckler, Monatsch. Chem. 140 (2009) 985. [6] G. Zhu, X. Fang, C. Xia, X. Liu, Ceram. Int. 31 (1) (2005) 115. [7] R. Chiba, F. Yoshimura, Y. Sakurai, Solid State Ionics 124 (1999) 281. [8] R. Chiba, F. Yoshimura, Y. Sakurai, Solid State Ionics 152/153 (2002) 575. [9] P. Ciambelli, S. Cimino, L. Lisi, M. Faticanti, G. Minelli, I. Pettiti, P. Porta, Appl. Catal. B 33 (2001) 193. [10] X. Chen, N.J. Wu, A. Ignatiev, J. Eur. Ceram. Soc. 19 (1999) 819. [11] X. Chen, N.J. Wu, D.L. Ritums, A. Ignatiev, Thin Solid Films 342 (1999) 61. [12] X. Chen, S. Wang, Y.L. Yang, L. Smith, N.J. Wu, B.-I. Kim, S.S. Perry, A.J. Jacobson, A. Ignatiev, Solid State Ionics 146 (3,4) (2002) 405. [13] X. Chen, N.J. Wu, L. Smith, A. Ignatiev, Appl. Phys. Lett. 84 (2004) 2700. [14] N. Imanishi, T. Matsumura, Y. Sumiya, K. Yoshimura, A. Hirano, Y. Takeda, D. Mori, R. Kanno, Solid State Ionics 174 (2004) 245. [15] D. Mori, H. Oka, Y. Suzuki, N. Sonoyama, A. Yamada, R. Kanno, Y. Sumiya, N. Imanishi, Y. Takeda, Solid State Ionics 177 (2006) 535.
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