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Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
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Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials ndez-Dı´az c V. Cascos a,*, R. Martı´nez-Coronado b, J.A. Alonso a, M.T. Ferna a
Instituto de Ciencia de Materiales de Madrid, C.S.I.C, Cantoblanco, E-28049, Madrid, Spain Texas Materials Institute and Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, United States c Institut Laue Langevin, BP 156X, Grenoble, F-38042, France b
article info
abstract
Article history:
Brownmillerite-type oxides Ca2Fe2-xCoxO5-d (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4) have been
Received 10 July 2014
explored as possible cathodes for solid oxide fuel cells (SOFC). The samples have been
Received in revised form
prepared, characterized and tested as cathode materials in single solid-oxide fuel cells. As
26 November 2014
shown in a neutron powder diffraction (NPD) study at RT, for x ¼ 0 and 0.2 the compounds
Accepted 12 January 2015
crystallized in a single phase with brownmillerite-type structure (s.g. Pcmn), whereas for
Available online xxx
x ¼ 0.4, 0.6, 0.8, 1, 1.2, and 1.4 the samples crystallized in a supercell, two times the c-axis and the volume of that for a typical brownmillerite (s.g Pcmb). This superstructure consists
Keywords:
of Fe1O4 and Co1O4 tetrahedral layers containing Fe1 and Co1 atoms, which alternate with
Cathode
(Fe,Co)2O6 and (Fe,Co)3O6 octahedral layers. In an “in situ” NPD experiment of Ca2Fe0.8-
IT-SOFC
Co1.2O5-d at the working temperature of the SOFC, this compound shows the presence of a
Ca2Fe2O5
sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms that
Brownmillerite superstructure
suggest a large lability and mobility. In single test cells these cathode materials generated a
Neutron diffraction
maximum power of 412 mW/cm2 at 850 C with pure H2 as a fuel. The electrodes were supported on a 300-mm-thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3-d (LSGM). The measured thermal expansion coefficients between 300 and 850 C exhibit an excellent chemical compatibility with the electrolyte. The obtained compounds display a semiconductor-like behavior with conductivity values at the SOFCs working temperatures (650e850 C) that are sufficient to yield a good performance in IT-SOFC. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Mixed-conducting oxide materials (MIECs) as oxygendeficient perovskite-type compounds have been studied
intensively over the last years, because they have promising applications in high-temperature electrochemical devices, such as ceramic membranes for oxygen separation and partial oxidation of hydrocarbons, electrodes of solid oxide fuel cells (SOFCs) and sensors [1e4]. The perovskite-type ferrites
* Corresponding author. Tel.: þ34 91 334 9000; fax: þ34 91 372 0623. E-mail address: [email protected] (V. Cascos). http://dx.doi.org/10.1016/j.ijhydene.2015.01.067 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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derived from AFeO3d where A is an alkaline-earth cation, have a high level of oxygen ionic diffusivity [5e7]. However, these perovskites exhibit serious disadvantages like thermodynamic instability under large oxygen chemical potential gradients, poor thermomechanical properties associated with very high chemical and thermal expansion, and reactivity with CO2 and H2O [8]. So, a promising approach refers to modifications of A2Fe2O5±d brownmillerites [8e11]: they are superstructures of oxygen-deficient Fe-containing perovskites, where FeO6 octahedra alternate, in layers, with FeO4 tetrahedra. These phases have already attracted much attention, due to their ability of accommodation of large amounts of oxygen vacancies in the lattice. This in turn results in fast oxygen ionic conductivity in the material [12e15]. Recently some brownmillerite-type structure compounds have been found to exhibit relatively high ionic-electronic mixed conductivity, promising oxygen permeability, and compatible thermal expansion coefficients (TECs) with solid electrolytes [16]. The “brownmillerite” itself, Ca2Fe2O5, where the oxygen vacancies ordered along (010) planes contribute to the ionic conductivity, forming one-dimensional diffusion pathways for oxygen ion migration in the tetrahedral layers [9], possesses, however, an almost null electronic conductivity at 850 C. In order to combine a good oxygen-ion mobility and a sufficient electronic conductivity as required for MIEC oxides, we have followed the strategy of introducing Co at the Fe positions of the Ca2Fe2O5 system in Ca2Fe2-xCoxO5-d (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4), considerably extending the compositional range already explored in Ref. [17] for x ¼ 0.2 and 0.4. In the present work, we have studied its structure and properties of the whole family, showing that Ca2Fe2-xCoxO5-d (x ¼ 1.2 and 1.4) oxides can be successfully used as cathodes in SOFCs with H2 as a fuel.
Experimental section Ca2Fe2xCoxO5d (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4) polycrystalline powders were prepared by soft-chemistry procedure. Stoichiometric amounts of CaCO3, C2FeO4$2H2O, and Co(NO3)2,6H2O were solved under stirring in citric acid aqueous solutions with some drops of nitric acid. The solution was then slowly evaporated, leading to organic resins which were dried at 120 C and decomposed at 600 C for 12 h in order to eliminate the organic materials and the nitrates. The obtained precursors were then heated at 900 C and 1100 C for 12 h in air with an intermediate grinding leading to the formation of the required oxides. Initially, the structural characterization of the products was performed by XRD with a Bruker-axs D8 Advanced diffractometer (40 kV, 30 mA), controlled by a DIFFRACTPLUS software, in Bragg-Brentano reflection geometry with Cu Ka ˚ ) and a PSD (Position Sensitive Detector). radiation (l ¼ 1.5418 A A filter of nickel allows the complete removal of CuKb radiation. The data were obtained between 10 and 64 in steps of 0.05 . For the structural refinement, NPD patterns were collected at the D2B diffractometer of the Institut Laue˚ within Langevin, Grenoble, with a wavelength l ¼ 1.594 A the 2q range from 15 to 140 at 25 C for all the samples, and
additionally at 200, 400, 600 and 800 C for x ¼ 0.2 sample. For the T 200 C collection, about 2 g of the sample was contained in a quartz tube open to the atmosphere and placed in the isothermal zone of a furnace with a vanadium resistor operating under vacuum (PO2 z 1 106 Torr). For the 25 C measurement, about 2 g of sample was contained in a vanadium can; in all cases a time of 3 h was required to collect a full diffraction pattern. The NPD data were analyzed by the Rietveld method [18] with the FULLPROF program [19]. A pseudoVoigt function was chosen to generate the line shape of the diffraction peaks. The irregular background coming from the quartz container was extrapolated from points devoid of reflections. In the final run, the following parameters were refined: scale factor, background points, zero shift, half-width, pseudo-Voigt corrected for asymmetry parameters, unit-cell parameters and positional coordinates. The coherent scattering lengths of Ca, Fe, Co and O were 4.70, 9.45, 2.49 and 5.803 fm, respectively. Measurements of the thermal expansion coefficient required the use of sintered cylindrical samples (5 mm diameter x 2 mm thickness) and measurements of the conductivity required the use of sintered bars samples (3 3 10 mm3). Densification was performed by uniaxial pressing of pellets that were subsequently calcined at 1100 for 12 h; the obtained density is around 80e85% of the theoretical one. Thermal expansion of the sintered samples was carried out in a dilatometer Linseis L75HX1000, between 25 and 900 C in air atmosphere. The conductivity was measured between 25 and 850 C in air by the four-point method in barshaped pellets under DC currents between 0.1 and 0.5 A. The currents were applied and collected with a PotentiostatGalvanostat AUTOLAB PGSTAT 302 from ECO CHEMIE. Single cell tests were carried out using LSGM pellets as electrolyte, Ca2Fe2-xCoxO5-d (x ¼ 1.2, 1.4) (CFCO) as cathode material, and SrMo0.8Fe0.2O3-d (SMFO) as anode material, recently developed in our group [20]. The LSGM pellets of 20mm diameter were sintered at 1450 C for 20 h and then polished with a diamond wheel to a thickness of 300 mm La0.4Ce0.6O2d (LDC) was used as a buffer layer between the anode and the electrolyte in order to prevent the interdiffusion of ionic species. Inks of LDC, SMFO and CFCO were prepared with a binder (V-006 from Heraeus). LDC ink was screenprinted onto one side of the LSGM disk followed by a thermal treatment at 1300 C in air for 1 h. SMFO was subsequently screen printed onto the LDC layer and fired at 1100 C in air for 1 h. CFCO was finally screen printed onto the other side of the disk and fired at 1100 C in air for 1 h. The working electrode area of the cell was 0.25 cm2 (0.5 0.5 cm). Pt gauze with a small amount of Pt paste in separate dots was used as current collector at both the anodic and the cathodic sides for ensuring electrical contact. The cells were tested in a vertical tubular furnace at 800 and 850 C; the anode side was fed with pure dry H2, with a flow of 20 ml/min, whereas the cathode worked in an air flow of 100 ml/min. The fuel-cell tests were performed with an AUTOLAB 302N Potentiostat/Galvanostat by changing the voltage of the cell from the OCV (“Open current voltage”) to 0.1 V, with steps of 0.010 V, holding 10 s at each step. Current density was calculated by the recorded current flux through the effective area of the cell (0.25 cm2). Each VI (voltage-intensity) scan corresponds to one cycle; the
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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activation of the cell was followed in subsequent cycles until the full power of the single cell was reached. The activation is necessary to totally reduce the anode to the electronicconducting perovskite phase. It takes about 20 min at 800 C in the presence of pure H2. Scanning electron microscopy (SEM) images were carried out with a Hitachi Se3000N and an analyzer from Oxford Instrument, model INCAx-sight.
Results and discussion Crystallographic characterization The synthesis of pure and well crystallized powders of the Ca2Fe2-xCoxO5-d series was checked by x-ray diffraction measurements. Fig. 1 shows the XRD pattern of the prepared Ca2Fe2-xCoxO5-d powders. It was observed that Ca2Fe2-xCoxO5d (x ¼ 0, 0.2) crystallized in a single phase with brownmillerite˚ , b ¼ 14.7929(1) A ˚ and type structure with a ¼ 5.59222(7) A ˚ for x ¼ 0 and a ¼ 5.58102(6) A ˚ , b ¼ 14.7936(1) A ˚ c ¼ 5.42329(6) A ˚ , for x ¼ 0.2. The obtained unit-cell paand c ¼ 5.40758(5) A rameters are in excellent agreement with those reported in ˚ the literature (a ¼ 5.5946(1), b ¼ 14.8273(2) and c ¼ 5.4307(1) A [21]). Ca2Fe2-xCoxO5-d (x ¼ 0.4, 0.6, 0.8, 1, 1.2, 1.4) crystallized in a supercell, two times the c-axis and the volume of that for a typical brownmillerite. The unit-cell parameters were
Fig. 1 e XRD patterns with CuKa radiation for Ca2Fe2¡xCoxO5¡d family.
˚ , b ¼ 14.8027(1) A ˚ and c ¼ 11.08709(8) A ˚ for x ¼ 1. a ¼ 5.35796(4) A These values are also in perfect agreement with those re˚ and ported in the literature (a ¼ 5.3675(1), b ¼ 14.7787(3) A ˚ [22]). No impurities phases were found. c ¼ 11.1072(3) A
Table 1 e Unit-cell parameters, atomic positions, occupancies, magnetic moment, displacement factors, ˚ ) of reliability factors and selected atomic distances (A Ca2Fe2¡xCoxO5¡d (x ¼ 0.2) in the Pcmn (no. 62) space group, from NPD data at RT. Ca2Fe2xCoxO5d ˚) a (A ˚) b (A ˚) c (A ˚ 3) V (A Ca1 8d (x,y,z) x y z ˚ 2) Biso (A focc (Fe1,Co1) 4a (0, 0, 0) ˚ 2) Biso (A focc(Fe) focc(Co) Moment (mB) Fe1 (Fe2,Co2) 4c (x, 1/4, z) x z ˚ 2) Biso (A focc (Fe) focc(Co) Moment (mB) Fe2 O1 8d (x, y, z) x y z ˚ 2) Biso (A focc O2 8d (x, y, 0) x y z ˚ 2) Biso (A focc O3 4c (x, 1/4, z) x z ˚ 2) Biso (A focc Fiability factors c2 Rp(%) Rwp(%) Rexp(%) RBragg(%) R Mag(%) ˚) Distances (A Tetrahedral sites Fe1(Co1)eO1(x4) Fe1(Co1)eO2(x2) Octahedral sites Fe2(Co2)eO2(x2) Fe2(Co2)eO3(x2)
X ¼ 0.2 5.57800(2) 14.77983(4) 5.40394(2) 445.511(2) 0.02319(4) 0.10825(1) 0.48300(6) 0.639(5) 1.00 0.228(5) 0.886(1) 0.114(1) 3.57(5) 0.93556(3) 0.94544(3) 0.017(5) 0.846(1) 0.154(1) 2.72(5) 0.23947(5) 0.98462(1) 0.26174(7) 0.491(5) 0.988(1) 0.07101(3) 0.13983(1) 0.02490(5) 0.797(6) 1.00 0.87523(6) 0.59996(6) 0.268(8) 0.996(1) 4.15 3.18 4.05 1.99 2.82 3.49
1.9585(4) 2.1085(2) 1.8457(3) 1.8976(4)
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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A neutron powder diffraction (NPD) study at room temperature (RT) for the Ca2Fe2-xCoxO5-d family and high temperature (up to 800 C) for Ca2Fe0.8Co1.2O5d was useful to investigate the structural details. For Ca2Fe2-xCoxO5-d (x ¼ 0.2) the crystal structure was defined in the orthorhombic Pcmn space group (No. 62), Z ¼ 4. Ca atoms are located at 8d (x, y, z) positions, (Fe,Co)1 are distributed at random at 4a (0, 0, 0), (Fe,Co)2 distributed at random at 4c (x, ¼, z), O1 and O2 atoms are ordered at two different 8d (x, y, z) sites and O3 are placed at 4c (x, ¼,z). The occupancy factor for Fe and Co atoms and for all the oxygen atoms (O1, O2 and O3) was also refined in order to determine the stoichiometric formula; O2 occupancies were fixed to unity since they converged to values higher than 1. O1 and O2 converged to values lower than 1 thus giving a slight deviation from the full stoichiometry (Table 1). The final refined composition at 25 C is Ca2Fe1.73Co0.27O4.98. In the case of Ca2Fe2xCoxO5d (x ¼ 0.4. 0.6, 0.8, 1, 1.2 and 1.4) the crystal structural was defined in the orthorhombic Pcmb group (No. 57), Z ¼ 8. Ca1 and Ca2 atoms are placed at two different 8e (x, y, z) sites, whereas (Fe,Cob)1 and (Co,Feb)1 are located at two different 4d (x, ¼, z) positions. (Fe,Co)2 and (Fe,Co)3 are distributed at 4c (x, ½, ¾) and 4a (0, 0, 0) sites, respectively. Two non-equivalent O atoms (O1, O2) are placed at two different 4d (x, ¼, z), and (O3, O4, O5 and O6) at four different 8e (x, y, z) positions. The occupancy factors for Fe and Co atoms and for all the oxygen atoms was also refined, giving
a deviation from the full stoichiometry that increases according as the amount of iron in the sample decreases from Ca2Fe1.61Co0.39O4.99 to Ca2Fe0.55Co1.45O4.89 as shown in Table 1. This means that the introduction of Cobalt ions at the Fe sublattice is effective in creating a measurable amount of oxygen vacancies. This is essential to get a good performance of these materials as cathodes in SOFC. The cell volume obtained for Ca2Fe2-xCoxO5-d at room temperature decreases as ˚3 the amount of Co increases, shrinking from 887.96(3) A ˚ 3 (x ¼ 1.4) as shown in Table 1; the (x ¼ 0.4) to 871.99(4) A introduction of a smaller cation at the B-site causes the reduction of the unit-cell volume. A low-angle magnetic peak in the RT NPD patterns suggests that the samples are magnetically ordered above room temperature, so the magnetic structures were fully determined from NPD data collected at this temperature for Ca2Fe1.8Co0.2O5d (Pcmn group) and Ca2Fe0.8Co1.2O5d, Ca2Fe0.6Co1.4O5d (Pcmb group) and introduced into the refinement. A G-type magnetic structure perfectly fits the magnetic intensities and a propagation vector k ¼ (0,0,0) was identified in the three compounds. The moments were found to be collinear and (anti)parallel with the c axis in Ca2Fe1.8Co0.2O5d and collinear and (anti)parallel with the a axis in Ca2Fe0.6Co1.4O5d and Ca2Fe0.8Co1.2O5d. The main magnetic peaks are indexed in Fig. 2a,b,c. As the amount of cobalt is increasing, the magnetic moment decreases as
Fig. 2 e Observed (red crosses), calculated (black line) and difference (bottom line) NPD Rietveld profiles at RT for a) Ca2Fe0.6Co1.4O5-d, b)Ca2Fe0.8Co1.2O5¡d and c) Ca2Fe1.8Co0.2O5¡d. d) NPD Rietveld profiles at 800 C for Ca2Fe0.8Co1.2O5¡d. The two series of markers correspond to the positions of the allowed Bragg reflections for the crystallographic phase and the magnetic reflections. The irregular background observed in the high-temperature diagram is due to the quartz container. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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Table 2 e Unit-cell parameters, atomic positions, occupancies, magnetic moment, displacement factors, reliability factors ˚ ) of Ca2Fe2¡xCoxO5¡d (x ¼ 0.4, 0.6, 0.8, 1, 1.2, 1.4) in the Pcbm (no. 57) space group, from NPD and selected atomic distances (A data at RT. Ca2Fe2xCoxO5d ˚) a (A ˚) b (A ˚) c (A ˚ 3) V (A Ca1 8e (x, y, z) x y z ˚ 2) Biso (A focc Ca2 8e (x, y, z) x y z ˚ 2) Biso (A focc (Fe1, Co1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Fe) focc (Co) Moment (mB) (Fe1, Co1b) (Co1, Fe1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Co) focc (Fe) Moment (mB) (Co1, Fe1b) (Co2, Fe2) 4c (x, 1/2, 3/4) x ˚ 2) Biso (A focc (Co) focc (Fe) Moment (mB) (Co2, Fe2) (Co3, Fe3) 4a (0, 0, 0) ˚ 2) Biso (A focc (Co) focc (Fe) Moment (mB) (Co3, Fe3) O1 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O2 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O3 8e (x, y, z) x y z ˚ 2) Biso (A focc O4 8e (x, y, z) x y z ˚ 2) Biso (A
X ¼ 0.4
X ¼ 0.6
X ¼ 0.8
X ¼ 1.2
X ¼ 1.4
5.3906(1) 14.7745(3) 11.1490(3) 887.96(3)
5.3780(2) 14.7769(4) 11.1234(4) 883.97(5)
5.3693(1) 14.7914(3) 11.1073(3) 882.13(4)
5.3548(1) 14.7918(3) 11.0790(2) 877.53(3)
X¼1
5.3452(1) 14.8129(3) 11.0569(3) 875.49(4)
5.3301(1) 14.8379(4) 11.0258(3) 871.99(4)
0.0168(1) 0.39504(7) 0.7607(1) 0.541(5) 1.00
0.0155(2) 0.39312(8) 0.7603(1) 0.901(8) 1.00
0.0188(1) 0.39233(7) 0.76086(7) 1.025(7) 1.00
0.0193(1) 0.3911(6) 0.7612(7) 0.530(6) 1.00
0.0183(1) 0.3918(9) 0.7577(1) 0.927(7) 1.00
0.0083(1) 0.39314(7) 0.75640(9) 0.608(6) 1.00
0.4870(1) 0.60972(7) 0.5123(1) 0.541(5) 1.00
0.4881(2) 0.61027(8) 0.5111(1) 0.901(8) 1.00
0.4894(1) 0.60729(6) 0.50882(7) 1.025(7) 1.00
0.4910(1) 0.6067(6) 0.5097(7) 0.530(6) 1.00
0.4913(1) 0.6080(8) 0.5130(1) 0.927(7) 1.00
0.4887(1) 0.60956(7) 0.5137(1) 0.608(6) 1.00
0.0544(1) 0.53693(5) 0.287(3) 1.00 e 3.9(4)
0.0518(1) 0.53642(8) 0.476(6) 1.00 e 3.1(4)
0.0463(1) 0.53318(4) 0.612(4) 1.00 e 2.3(2)
0.04317(9) 0.53429(4) 0.731(5) 1.00 e 2.0(5)
0.0400(1) 0.53259(8) 0.857(8) 0.588(1) 0.412(1) 1.8(5)
0.0435(1) 0.52988(8) 0.008(6) 0.452(1) 0.548(1) 1.8(3)
0.4489(1) 0.7197(7) 0.287(3) 0.392(3) 0.608(3) 3.3(5)
0.4508(3) 0.7219(1) 0.476(6) 0.734(8) 0.266(2) 2.6(5)
0.4276(4) 0.7140(2) 0.612(4) 0.880(1) 0.120(1) 2.7(3)
0.4212(3) 0.7178(2) 0.731(5) 1.00 e 2.9(4)
0.4260(5) 0.7175(2) 0.857(8) 1.00 e 2.6(2)
0.4278(1) 0.7139(2) 0.008(6) 1.00 e 0.5(3)
0.5017(1) 0.287(3) 0.398(2) 0.602(2) 2.8(5)
0.4955(2) 0.476(6) 0.618(2) 0.382(2) 2.2(5)
0.4985(1) 0.612(4) 0.206(1) 0.794(1) 2.9(3)
0.4975(1) 0.731(5) 0.352(1) 0.648(1) 2.5(4)
0.4978(2) 0.857(8) 0.460(1) 0.540(1) 2.5(2)
0.4983(2) 0.008(6) 0.580(1) 0.420(1) 2.5(3)
0.287(3) e 1.00 3.2(5)
0.476(6) e 1.00 3.3(4)
0.612(4) 0.550(1) 0.450(1) 1.8(3)
0.731(5) 0.556(1) 0.444(1) 1.5(4)
0.857(8) 0.674(1) 0.326(1) 1.1(5)
0.008(6) 0.746(1) 0.254(1) 1.1(2)
0.0873(2) 0.69351(9) 0.65(1) 1.00
0.0905(2) 0.6939(1) 1.01(2) 1.00
0.0868(1) 0.69166(8) 0.45(2) 0.918(1)
0.0889(1) 0.69125(8) 0.674(2) 0.94(1)
0.0837(2) 0.6914(1) 0.037(2) 0.882(1)
0.0825(1) 0.69348(7) 0.159(2) 1.00
0.6057(1) 0.56335(8) 0.16(1) 1.00
0.5973(2) 0.5632(1) 0.55(2) 1.00
0.6020(1) 0.56343(7) 0.411(2) 0.95(1)
0.6013(1) 0.56374(7) 0.514(2) 0.986(1)
0.5994(2) 0.56423(8) 0.391(1) 1.00
0.6037(2) 0.56335(7) 0.474(2) 1.00
0.2532(1) 0.48317(3) 0.62476(9) 0.22(1) 0.993(1)
0.2546(3) 0.48322(4) 0.6251(1) 0.53(1) 0.988(2)
0.2531(1) 0.48374(3) 0.62452(7) 0.654(1) 1.00
0.2515(1) 0.48420(3) 0.62449(8) 0.329(9) 1.00
0.2474(2) 0.48406(3) 0.6249(1) 0.461(1) 1.00
0.2575(2) 0.48563(3) 0.6258(1) 0.356(1) 0.966(1)
0.7515(2) 0.48824(3) 0.6275(1) 0.63(1)
0.7483(3) 0.4883(5) 0.6268(1) 0.85(2)
0.7520(1) 0.48972(3) 0.62751(7) 0.771(8)
0.7504(1) 0.48977(3) 0.62730(7) 0.341(7)
0.7476(2) 0.49091(3) 0.6263(1) 0.927(1)
0.7556(2) 0.49040(3) 0.6278(1) 0.407(1)
(continued on next page)
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
Table 2 e (continued ) Ca2Fe2xCoxO5d focc O5 8e (x, y, z) x y z ˚ 2) Biso (A focc O6 8e (x, y, z) x y z ˚ 2) Biso (A focc Reliability factors c2 Rp(%) Rwp(%) Rexp(%) RBragg(%) R Mag(%) ˚) Distances (A Tetrahedral Sites Fe1(Co1b)eO1 Fe1(Co1b)eO2 Fe1(Co1b)eO5(x2) Co1(Fe1b)eO1 Co1(Fe1b)eO2 Co1(Fe1b)eO6(x2) Octahedral sites Fe2(Co2)eO3(x2) Fe2(Co2)eO4(x2) Fe2(Co2)eO6(x2) Co3(Fe3)eO3(x2) Co3(Fe3)eO4(x2) Co3(Fe3)eO5(x2)
X ¼ 0.4
X ¼ 0.6
X ¼ 0.8
X¼1
X ¼ 1.2
X ¼ 1.4
0.999(1)
0.978(2)
1.00
1.00
1.00
0.924(1)
0.0159(2) 0.36434(5) 0.46901(8) 0.93(1) 1.00
0.0135(1) 0.36013(7) 0.4697(1) 0.80(2) 1.00
0.0124(1) 0.36060(5) 0.46917(8) 1.15(2) 0.965(3)
0.0156(1) 0.36026(4) 0.46955(8) 1.131(1) 0.990(1)
0.0169(1) 0.35968(6) 0.46885(1) 0.734(1) 0.947(2)
0.0141(1) 0.36160(5) 0.47053(9) 0.230(1) 1.00
0.5105(1) 0.35732(5) 0.78466(7) 0.64(1) 1.00
0.5120(2) 0.3596(8) 0.7849(1) 1.227(2) 1.00
0.5097(1) 0.36115(5) 0.78324(8) 1.05(2) 0.995(3)
0.5142(1) 0.36081(4) 0.78299(7) 0.72(1) 0.986(1)
0.5148(1) 0.3614(6) 0.7817(1) 0.758(1) 1.00
0.5171(1) 0.36081(6) 0.7797(1) 1.71(1) 1.00
4.19 3.29 4.17 2.04 4.07 4.90
4.69 3.41 4.45 2.05 4.73 6.70
7.66 3.93 5.14 1.86 6.39 3.65
4.68 3.08 3.94 1.82 4.43 3.25
1.99 3.51 4.47 3.17 3.98 4.19
3.55 2.58 3.37 1.79 3.69 5.25
1.906(1) 1.856(1) 1.8897(9) 1.971(1) 1.937(1) 1.7745(9)
1.912(1) 1.911(1) 1.823(1) 1.963(3) 1.932(3) 1.795(1)
1.9051(9) 1.9202(9) 1.8130(7) 1.846(1) 1.907(1) 1.8657(9)
1.877(1) 1.932(1) 1.8093(8) 1.803(2) 1.960(2) 1.859(1)
1.882(2) 1.973(1) 1.795(1) 1.839(3) 1.928(4) 1.864(1)
1.925(1) 1.917(1) 1.8068(9) 1.854(2) 1.906(2) 1.859(1)
1.951(1) 1.925(1) 2.1442(7) 1.965(1) 1.961(1) 2.0357(9)
1.916(1) 1.939(1) 2.111(1) 1.968(1) 1.962(1) 2.095(1)
1.9329(8) 1.9350(8) 2.0908(6) 1.9522(8) 1.9480(7) 2.0912(6)
1.930(1) 1.925(1) 2.092(6) 1.9418(9) 1.9490(9) 2.0960(7)
1.945(1) 1.914(1) 2.086(1) 1.928(1) 1.955(1) 2.106(1)
1.889(1) 1.928(1) 2.093(1) 1.963(1) 1.924(1) 2.0805(8)
shown in Table 2. After the full refinement we achieved a good agreement between the observed and calculated NPD patterns at RT, illustrated for Ca2Fe2xCoxO5d (x ¼ 0.2, 1.2, 1.4) in Fig. 2a, b and c. Tables 1 and 2 summarize the unit-cell, atomic positions, occupancies, magnetic moments, displacement parameters and discrepancy factors after the Rietveld refinement in the Pcmn and Pcmb groups. The inset on Fig. 3c illustrates the crystal superstructure of Ca2Fe2xCoxO5d (x ¼ 1), which is two times the c-axis and the volume of that of a typical brownmillerite. In this case, a model with site ordering of Fe and Co over the two tetrahedral sites and mixing of Fe and Co over the octahedral sites was found to be the right model [22], so this superstructure consists of Fe1O4 and Co1O4 tetrahedral layers containing Fe1 and Co1 atoms, which alternate with (Fe,Co)2O6 and (Fe,Co)3O6 octahedral layers. The tetrahedral layers consist of two Fe1O4 tetrahedral groups alternating with one Co1O4 tetrahedral group where Fe1O4 and Co1O4 are sharing O1 and O2 atoms, while the octahedral layers are formed by two (Fe,Co)3O6 octahedral groups alternating with one (Fe,Co)2O6 octahedral group, which are sharing O3 and O4 atoms. Finally, tetrahedral and octahedral layers are sharing O5 and O6 atoms. Table 3 summarizes selected bond distances for the Ca2Fe2xCoxO5d series. The two octahedral sites in
Ca2Fe2xCoxO5d (x ¼ 1) show similar bond lengths, with elongation of the two out of plane bonds for both sites, (Fe,Co) ˚ and (Fe,Co)3eO5 ¼ 2.0960(7) A ˚ and four 2eO6 ¼ 2.092(6) A ˚, shorter distances in the ac plane, (Fe,Co)2eO3 ¼ 1.930(1) A ˚ , (Fe,Co)3eO3 ¼ 1.9418(9) A ˚ and (Fe,Co) (Fe,Co)2eO4 ¼ 1.925(1) A ˚ . On the other hand, in the tetrahedral 3eO4 ¼ 1.9490(9) A layers, the Fe1O4 tetrahedra are compressed along the b axis ˚ distances than with a much shorter Fe1eO5 ¼ 1.8093(8) A ˚ and Fe1eO2 ¼ 1.932(1) A ˚ bond lengths Fe1eO1 ¼ 1.877(1) A lying on the ac plane, while the Co1O4 have a much shorter ˚ distances in the ac plane than the Co1eO1 ¼ 1.803(2) A ˚ and the Co1eO6 ¼ 1.859(1) A ˚ . The elonCo1eO2 ¼ 1.960(2) A gation of out-of-plane bonds for the octahedral sites occurs in regular brownmillerite compounds as well [23,24]. Such elongation seems to be required by this structure type in order to accommodate the linkage between octahedral and tetrahedral layers. Compression of tetrahedral out-of-plane bonds can accompany the elongation of octahedra. The oxygen atoms contained in the tetrahedral and octahedral layers are all of the same type (O2) for a regular brownmillerite [25], while there are two different types of oxygens, O5 and O6 for Ca2Fe2xCoxO5d. However, the major difference between the sublattices of a typical brownmillerite and Ca2Fe2xCoxO5d is in the splitting of the O3 site, in a regular brownmillerite, into
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
7
Fig. 3 e Thermal variation of (a) a unit-cell parameter (left axis) b/2 unit-cell parameter (right axis), (b) c/2 unit-cell parameter (left axis) and volume (right axis) and c) oxygen occupancy factor, from in situ NPD data. The inset illustrates the crystal structure of the oxygen-deficient brownmillerite superstructure for x > 0.4. O1, O2, O3, O4, O5, O6 atoms (red spheres), Ca1 (orange spheres), Ca2 (yellow spheres), Fe1 (green spheres), Co1 (light blue spheres), Co2, Fe2 (pink spheres) and Co3,Fe3 (dark blue spheres). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
two sites labeled O1 and O2 in Ca2Fe2xCoxO5d. An inspection of the structure reveals that O1 and O2 are the oxygens bonded only to the tetrahedral cations with no connection to the octahedral ions. In fact, it appears that the symmetry of the octahedral layer does not deviate significantly from that of a regular brownmillerite, while the symmetry of the
tetrahedral chains is the major factor that drives the formation of a super structure. The thermal evolution of the crystal structure under the cathode conditions in a SOFC was evaluated by NPD for x ¼ 1.2. For this purpose, the sample was contained in a quartz tube opened to the air atmosphere and the NPD data were collected
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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Table 3 e Unit-cell parameters, atomic positions, occupancies, displacement factors, reliability factors and selected atomic ˚ ) of Ca2Fe2¡xCoxO5¡d (x ¼ 1.2) in the Pcbm (no. 57) space group, from NPD data at 25e800 C. distances (A Ca2Fe0.8Co1.2O5d ˚) a (A ˚) b (A ˚) c (A ˚ 3) V (A Ca1 8e (x, y, z) x y z ˚ 2) Biso (A focc Ca2 8e (x, y, z) x y z ˚ 2) Biso (A focc (Fe1, Co1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Fe) focc (Co) (Co1, Fe1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Co) focc (Fe) (Co2, Fe2) 4c (x, 1/2, 3/4) x ˚ 2) Biso (A focc (Co) focc (Fe) (Co3, Fe3) 4a (0, 0, 0) ˚ 2) Biso (A focc (Co) focc (Fe) O1 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O2 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O3 8e (x, y, z) x y z ˚ 2) Biso (A focc O4 8e (x, y, z) x y z ˚ 2) Biso (A focc O5 8e (x, y, z) x y z
25
200
400
600
800
5.3452(1) 14.8129(3) 11.0569(3) 875.49(4)
5.3459(1) 14.9151(2) 11.0463(2) 880.77(3)
5.3579(1) 15.0149(2) 11.0530(2) 889.18(3)
5.3729(1) 15.0926(3) 11.0673(2) 897.46(3)
5.3902(1) 15.1612(4) 11.0877(3) 906.11(4)
0.0183(1) 0.3918(9) 0.7577(1) 0.927(7) 1.00
0.0099(1) 0.39184(7) 0.75721(9) 1.092(6) 1.00
0.0019(2) 0.3931(7) 0.7615(1) 1.405(6) 1.00
0.0058(2) 0.39030(9) 0.7619(1) 1.961(7) 1.00
0.0019(2) 0.3885(1) 0.7639(1) 2.33(1) 1.00
0.4913(1) 0.6080(8) 0.5130(1) 0.927(7) 1.00
0.4884(1) 0.60847(7) 0.51499(9) 1.092(6) 1.00
0.4893(1) 0.61021(7) 0.5098(1) 1.405(6) 1.00
0.4931(2) 0.60843(9) 0.5061(1) 1.961(7) 1.00
0.4929(2) 0.60786(9) 0.5346(1) 2.33(1) 1.00
0.0400(1) 0.53259(8) 0.857(8) 0.588(1) 0.412(1)
0.0406(1) 0.53446(7) 0.228(6) 0.588(1) 0.412(1)
0.0374(1) 0.53310(6) 0.307(6) 0.588(1) 0.412(1)
0.0431(2) 0.53261(9) 0.885(7) 0.588(1) 0.412(1)
0.0397(2) 0.53571(8) 1.024(9) 0.588(1) 0.412(1)
0.4260(5) 0.7175(2) 0.857(8) 1.00 e
0.4333(4) 0.7185(2) 0.228(6) 1.00 e
0.4301(4) 0.7172(2) 0.302(6) 1.00 e
0.4319(5) 0.7149(2) 0.885(7) 1.00 e
0.4370(5) 0.7264(2) 1.024(9) 1.00 e
0.4978(2) 0.857(8) 0.460(1) 0.540(1)
0.4962(1) 0.228(6) 0.460(1) 0.540(1)
0.4969(2) 0.302(6) 0.460(1) 0.540(1)
0.5004(2) 0.885(7) 0.460(1) 0.540(1)
0.4923(2) 1.024(9) 0.460(1) 0.540(1)
0.857(8) 0.674(1) 0.326(1)
0.228(6) 0.674(1) 0.326(1)
0.302(6) 0.674(1) 0.326(1)
0.885(7) 0.674(1) 0.326(1)
1.024(9) 0.674(1) 0.326(1)
0.0837(2) 0.6914(1) 0.037(2) 0.882(1)
0.0858(2) 0.69137(9) 0.72(3) 0.874(1)
0.0887(2) 0.6912(1) 1.65(2) 1.00
0.0904(2) 0.6918(1) 2.14(2) 1.00
0.0980(3) 0.6912(1) 2.60(3) 1.00
0.5994(2) 0.56423(8) 0.391(1) 1.00
0.6045(2) 0.56170(9) 1.06(3) 1.00
0.6044(2) 0.5613(1) 1.19(2) 1.00
0.6082(2) 0.5611(1) 2.01(2) 1.00
0.6037(3) 0.5609(1) 2.14(2) 1.00
0.2474(2) 0.48406(3) 0.6249(1) 0.461(1) 1.00
0.2548(2) 0.48383(3) 0.6233(1) 1.07(1) 0.996(2)
0.2540(2) 0.48345(3) 0.6229(1) 1.00(1) 0.911(2)
0.2552(2) 0.48416(4) 0.6241(1) 1.21(1) 0.934(1)
0.02535(2) 0.48321(4) 0.6250(1) 1.44(2) 0.929(2)
0.7476(2) 0.49091(3) 0.6263(1) 0.927(1) 1.00
0.7566(2) 0.49126(3) 0.6254(1) 0.79(1) 0.966(1)
0.7517(2) 0.49193(3) 0.6255(1) 1.02(1) 0.961(2)
0.7549(2) 0.49145(4) 0.6265(1) 1.09(1) 0.907(1)
0.7555(2) 0.49251(5) 0.6250(1) 1.57(2) 0.924(2)
0.0169(1) 0.35968(6) 0.46885(1)
0.0162(1) 0.36106(5) 0.46947(9)
0.0153(1) 0.35795(8) 0.4676(1)
0.0193(1) 0.35952(8) 0.4724(1)
0.0125(2) 0.4741(1) 0.35883(8)
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
Table 3 e (continued ) Ca2Fe0.8Co1.2O5d 2
˚ ) Biso (A focc O6 8e (x, y, z) x y z ˚ 2) Biso (A focc Reliability factors c2 Rp(%) Rwp(%) Rexp(%) RBragg(%) ˚) Distances (A Tetraedral sites Fe1(Co1b)eO1 Fe1(Co1b)eO2 Fe1(Co1b)eO5(x2) Co1(Fe1b)eO1 Co1(Fe1b)eO2 Co1(Fe1b)eO6(x2) Octahedral sites Fe2(Co2)eO3(x2) Fe2(Co2)eO4(x2) Fe2(Co2)eO6(x2) Co3(Fe3)eO3(x2) Co3(Fe3)eO4(x2) Co3(Fe3)eO5(x2)
25
200
400
600
800
0.734(1) 0.947(2)
0.85(1) 1.00
1.02(1) 1.00
1.98(2) 1.00
2.71(2) 1.00
0.5148(1) 0.3614(6) 0.7817(1) 0.758(1) 1.00
0.5207(1) 0.35897(7) 0.7820(1) 2.12(1) 1.00
0.5228(1) 0.35933(8) 0.7787(1) 1.52(1) 1.00
0.5241(1) 0.35709(9) 0.7829(1) 2.46(2) 1.00
0.5233(2) 0.35567(9) 0.7849(1) 2.63(2) 0.962(2)
1.99 3.51 4.47 3.17 3.98
1.90 1.67 2.17 1.57 5.92
1.72 1.56 2.06 1.57 5.59
1.61 1.52 1.99 1.57 5.37
1.76 1.58 2.07 1.56 7.80
1.877(1) 1.932(1) 1.8093(8) 1.803(2) 1.960(2) 1.859(1)
1.858(1) 1.920(1) 1.829(1) 1.881(3) 1.963(3) 1.834(1)
1.873(1) 1.932(1) 1.805(1) 1.877(3) 1.956(3) 1.834(1)
1.903(1) 1.900(1) 1.814(1) 1.853(4) 1.948(4) 1.850(2)
1.877(1) 1.942(1) 1.808(1) 1.868(4) 2.044(4) 1.790(2)
1.930(1) 1.925(1) 2.092(6) 1.9418(9) 1.9490(9) 2.0960(7)
1.916(1) 1.960(1) 2.133(1) 1.944(1) 1.907(1) 2.1027(9)
1.933(1) 1.951(1) 2.144(1) 1.936(1) 1.919(1) 2.156(1)
1.933(1) 1.938(1) 2.191(1) 1.955(1) 1.926(1) 2.145(1)
1.913(1) 1.987(1) 2.224(1) 1.958(1) 1.916(1) 2.161(1)
in situ at 200, 400, 600 and 800 C. Fig 2 d illustrates the good agreement between the observed and calculated NPD patterns for this oxide at 800 C. Fig. 2b and d show no structural transition across the temperature range under study (25e800 C). Table 3 includes the results obtained from the refinements at the different temperatures for Ca2Fe0.8Co1.2O5d. The thermal evolution of the oxygen content in air was also studied by NPD. Fig. 3 illustrates the temperature dependence of the concentration of oxygen vacancies (d) (Fig. 3c) and unitcell parameters and volume (Fig. 3a,b) for Ca2Fe0.8Co1.2O5d. The oxygen content decreases when heating the sample from Ca2Fe0.8Co1.2O4.90(1) at RT to Ca2Fe0.8Co1.2O4.82(1) at 800 C. These vacancies are essential to drive the required O2 motion in a MIEC oxide.
Thermal expansion measurements The thermal expansion of each compound was measured on dense cylindrical pellets (5 mm diameter x 2 mm thickness) sintered at 1100 C for 12 h. The dilatometric analysis was carried out between 25 and 900 C for several cycles in air; the data were only recorded during the heating runs. Fig. 4 shows a regular variation of the thermal expansion along the Ca2Fe2-xCoxO5d series. However, the relative thermal expansion is not totally linear in all the temperature range under study in Ca2Fe2xCoxO5d (x ¼ 0 and 0.2). A change in the slope can be observed around 650e700 C in Ca2Fe2O5-d that some authors [21] have related to a structural transition to Icmm. The same happens with Ca2Fe1.8Co0.2O5-d around 500e525 C and it could be related to the same structural transition. The thermal
expansion coefficients obtained for the Ca2Fe2-xCoxO5-d system are included in Fig. 4. These values perfectly match with the values usually displayed by SOFC electrolytes (10e13 106 K1).
Electrical conductivity measurements and chemical compatibility The electrical conductivity was obtained in air by the dc fourprobe method from 25 to 850 C (Fig. 5) leading a semiconductor-like behavior in all the range of temperatures. The Ca2Fe1.8Co0.2O5d, with only a 10% of cobalt, shows a null conductivity in the temperature range of 25e800 C, but as the amount of cobalt is increasing, the conductivity rises up to 18 Scm1 at 850 C for x ¼ 1.2. This can be assigned to the fact that the mobility of p-type carriers of cobalt is much higher than that of iron [26]. On the other hand, there is a little decrease in the electrical conductivity when the Co content is x ¼ 1.4 with respect to x ¼ 1.2 as shown Fig. 5. This reduction of the electrical conductivity could be related with to oxygen ion vacancies in this compound, as the carriers’ motion happens via M-O-M paths in the lattice [27], and Ca2Fe0.6Co1.4O5d has a large amount of oxygen ion vacancies. Although the electrical conductivities obtained for Ca2Fe2xCoxO5d are considerably lower than those presented by other Co perovkite materials [26,28], it is enough to be used successfully as cathode materials. The chemical compatibility of Ca2FeCoO5d with the LSGM electrolyte has also been checked by firing mixtures of both powdered materials at 1100 C for 24 h; the inset of Fig. 5 shows a Rietveld analysis of the product, consisting of a mixture of both unaltered perovskite phases, indicating that
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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Fig. 4 e Thermal expansion determined by dilatometry of the Ca2Fe2¡xCoxO5¡d series.
the formation of unwanted secondary phases when the material is tested in single-cells is not favored.
Fuel-cell tests The performance of Ca2Fe2xCoxO5d as cathode material was also tested in single cells in an electrolyte-supported configuration with a 300 mm-thick LSGM electrolyte using air in contact with the cathode and dry H2 as a fuel. Fig. 6a illustrates
the cell voltage and power density as a function of current density at 800 and 850 C for the x ¼ 1.2 cathode. The maximum power densities generated by the cell are 234 and 372 mW/cm2, respectively. Fig. 6b illustrates the cell voltage and power density as a function of current density at 800 and 850 C for the x ¼ 1.4 cathode. The maximum power densities generated by the cell are 238 and 412 mW/cm2, respectively. The present performances open the possibility of considering these materials as
Fig. 5 e Dc-conductivity as a function of temperature for Ca2Fe2¡xCoxO5¡d series. The inset shows the Rietveld-refined XRD profiles of a mixture of LSGM and Ca2FeCoO5-d after a thermal treatment at 1100 C in air, showing no reaction products between both phases other than the initial reactants. The first and second series of Bragg positions correspond to Ca2FeCoO5-d and LSGM, respectively. Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
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Fig. 6 e Cell voltage (left axis) and power density (right axis) as a function of the current density for the test cell with the configuration SMFO/LDC/LSGM/CFCO (x ¼ 1.2, 1.4) in pure H2 measured at T ¼ 800 and 850 C with Pt as current collector.
cathode for SOFCs. As shown in Fig. 6a and b, a slight increase of the output power of the single cells is observed from x ¼ 1.2 to x ¼ 1.4 cathodes, correlated with the effect promoted by Co, enhancing the formation of oxygen vacancies. Although Ca2Fe0.6Co1.4O5d exhibits a slightly lower electronic conductivity values than Ca2Fe0.6Co1.2O5d at the working temperatures (800e850 C), it seems that the presence of a higher number of oxygen vacancies for x ¼ 1.4 compensates this effect, demonstrating that a suitable ionic mobility is an important requisite for MIEC oxides in SOFCs.
Scanning electron microscopy Fig. 7a and b shows the micrographs of the Ca2Fe0.8Co1.2O5d and Ca2Fe0.6Co1.4O5d cathode surfaces respectively after the single-
cell tests analyzed by scanning electron microscopy (SEM). Both figures show a similar microstructure exhibiting a good porosity, which is one of the essential requirements for optimal cathode materials as it favors the diffusion and reduction process of the oxygen throughout the bulk of the cathode.
Conclusions In this work, we have shown that Ca2Fe2xCoxO5d oxides crystallize in a brownmillerite-type structure (s.g. Pcmn) or in a brownmillerite superstructure (s.g. Pcmb). We show that the introduction of Co ions at the Fe sublattice is effective in creating a measurable extra amount of oxygen vacancies. Ca2Fe2xCoxO5d (x ¼ 1.2 and 1.4) oxides can be successfully
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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to the Institut Laue-Langevin (ILL) for making all facilities available.
references
Fig. 7 e SEM images of a) Ca2Fe0.8Co1.2O5¡d cathode surface and b) Ca2Fe0.6Co1.4O5¡d cathode surface after the single cell test.
utilized as cathode material in single SOFC cells in an electrolyte (LSGM)-supported configuration. Maximum powers of 372 and 412 mW/cm2 were obtained at 850 C with pure H2 as a fuel, respectively. An “in situ” neutron power diffraction experiment, from 25 C to 800 C, reveals that the structure of Ca2Fe0.8Co1.2O5d does not change in all the range of temperatures. The sufficiently high number of oxygen vacancies induced upon Co doping along with high isotropic displacement factors suggests a high ionic conductivity at the working temperatures. The electronic conductivity seems to be sufficient to deliver a good performance, resulting in an excellent catalyst for oxygen reduction. The thermal expansion coefficients are perfectly compatible with the electrolyte. Finally, an excellent chemical compatibility with the electrolyte LSGM during 24 h is also observed.
Acknowledgments We thank the financial support of the Spanish Ministry of Education to the project MAT2013-41099-R and we are grateful
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Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials ndez-Dı´az c V. Cascos a,*, R. Martı´nez-Coronado b, J.A. Alonso a, M.T. Ferna a
Instituto de Ciencia de Materiales de Madrid, C.S.I.C, Cantoblanco, E-28049, Madrid, Spain Texas Materials Institute and Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, United States c Institut Laue Langevin, BP 156X, Grenoble, F-38042, France b
article info
abstract
Article history:
Brownmillerite-type oxides Ca2Fe2-xCoxO5-d (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4) have been
Received 10 July 2014
explored as possible cathodes for solid oxide fuel cells (SOFC). The samples have been
Received in revised form
prepared, characterized and tested as cathode materials in single solid-oxide fuel cells. As
26 November 2014
shown in a neutron powder diffraction (NPD) study at RT, for x ¼ 0 and 0.2 the compounds
Accepted 12 January 2015
crystallized in a single phase with brownmillerite-type structure (s.g. Pcmn), whereas for
Available online xxx
x ¼ 0.4, 0.6, 0.8, 1, 1.2, and 1.4 the samples crystallized in a supercell, two times the c-axis and the volume of that for a typical brownmillerite (s.g Pcmb). This superstructure consists
Keywords:
of Fe1O4 and Co1O4 tetrahedral layers containing Fe1 and Co1 atoms, which alternate with
Cathode
(Fe,Co)2O6 and (Fe,Co)3O6 octahedral layers. In an “in situ” NPD experiment of Ca2Fe0.8-
IT-SOFC
Co1.2O5-d at the working temperature of the SOFC, this compound shows the presence of a
Ca2Fe2O5
sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms that
Brownmillerite superstructure
suggest a large lability and mobility. In single test cells these cathode materials generated a
Neutron diffraction
maximum power of 412 mW/cm2 at 850 C with pure H2 as a fuel. The electrodes were supported on a 300-mm-thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3-d (LSGM). The measured thermal expansion coefficients between 300 and 850 C exhibit an excellent chemical compatibility with the electrolyte. The obtained compounds display a semiconductor-like behavior with conductivity values at the SOFCs working temperatures (650e850 C) that are sufficient to yield a good performance in IT-SOFC. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Mixed-conducting oxide materials (MIECs) as oxygendeficient perovskite-type compounds have been studied
intensively over the last years, because they have promising applications in high-temperature electrochemical devices, such as ceramic membranes for oxygen separation and partial oxidation of hydrocarbons, electrodes of solid oxide fuel cells (SOFCs) and sensors [1e4]. The perovskite-type ferrites
* Corresponding author. Tel.: þ34 91 334 9000; fax: þ34 91 372 0623. E-mail address: [email protected] (V. Cascos). http://dx.doi.org/10.1016/j.ijhydene.2015.01.067 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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derived from AFeO3d where A is an alkaline-earth cation, have a high level of oxygen ionic diffusivity [5e7]. However, these perovskites exhibit serious disadvantages like thermodynamic instability under large oxygen chemical potential gradients, poor thermomechanical properties associated with very high chemical and thermal expansion, and reactivity with CO2 and H2O [8]. So, a promising approach refers to modifications of A2Fe2O5±d brownmillerites [8e11]: they are superstructures of oxygen-deficient Fe-containing perovskites, where FeO6 octahedra alternate, in layers, with FeO4 tetrahedra. These phases have already attracted much attention, due to their ability of accommodation of large amounts of oxygen vacancies in the lattice. This in turn results in fast oxygen ionic conductivity in the material [12e15]. Recently some brownmillerite-type structure compounds have been found to exhibit relatively high ionic-electronic mixed conductivity, promising oxygen permeability, and compatible thermal expansion coefficients (TECs) with solid electrolytes [16]. The “brownmillerite” itself, Ca2Fe2O5, where the oxygen vacancies ordered along (010) planes contribute to the ionic conductivity, forming one-dimensional diffusion pathways for oxygen ion migration in the tetrahedral layers [9], possesses, however, an almost null electronic conductivity at 850 C. In order to combine a good oxygen-ion mobility and a sufficient electronic conductivity as required for MIEC oxides, we have followed the strategy of introducing Co at the Fe positions of the Ca2Fe2O5 system in Ca2Fe2-xCoxO5-d (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4), considerably extending the compositional range already explored in Ref. [17] for x ¼ 0.2 and 0.4. In the present work, we have studied its structure and properties of the whole family, showing that Ca2Fe2-xCoxO5-d (x ¼ 1.2 and 1.4) oxides can be successfully used as cathodes in SOFCs with H2 as a fuel.
Experimental section Ca2Fe2xCoxO5d (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4) polycrystalline powders were prepared by soft-chemistry procedure. Stoichiometric amounts of CaCO3, C2FeO4$2H2O, and Co(NO3)2,6H2O were solved under stirring in citric acid aqueous solutions with some drops of nitric acid. The solution was then slowly evaporated, leading to organic resins which were dried at 120 C and decomposed at 600 C for 12 h in order to eliminate the organic materials and the nitrates. The obtained precursors were then heated at 900 C and 1100 C for 12 h in air with an intermediate grinding leading to the formation of the required oxides. Initially, the structural characterization of the products was performed by XRD with a Bruker-axs D8 Advanced diffractometer (40 kV, 30 mA), controlled by a DIFFRACTPLUS software, in Bragg-Brentano reflection geometry with Cu Ka ˚ ) and a PSD (Position Sensitive Detector). radiation (l ¼ 1.5418 A A filter of nickel allows the complete removal of CuKb radiation. The data were obtained between 10 and 64 in steps of 0.05 . For the structural refinement, NPD patterns were collected at the D2B diffractometer of the Institut Laue˚ within Langevin, Grenoble, with a wavelength l ¼ 1.594 A the 2q range from 15 to 140 at 25 C for all the samples, and
additionally at 200, 400, 600 and 800 C for x ¼ 0.2 sample. For the T 200 C collection, about 2 g of the sample was contained in a quartz tube open to the atmosphere and placed in the isothermal zone of a furnace with a vanadium resistor operating under vacuum (PO2 z 1 106 Torr). For the 25 C measurement, about 2 g of sample was contained in a vanadium can; in all cases a time of 3 h was required to collect a full diffraction pattern. The NPD data were analyzed by the Rietveld method [18] with the FULLPROF program [19]. A pseudoVoigt function was chosen to generate the line shape of the diffraction peaks. The irregular background coming from the quartz container was extrapolated from points devoid of reflections. In the final run, the following parameters were refined: scale factor, background points, zero shift, half-width, pseudo-Voigt corrected for asymmetry parameters, unit-cell parameters and positional coordinates. The coherent scattering lengths of Ca, Fe, Co and O were 4.70, 9.45, 2.49 and 5.803 fm, respectively. Measurements of the thermal expansion coefficient required the use of sintered cylindrical samples (5 mm diameter x 2 mm thickness) and measurements of the conductivity required the use of sintered bars samples (3 3 10 mm3). Densification was performed by uniaxial pressing of pellets that were subsequently calcined at 1100 for 12 h; the obtained density is around 80e85% of the theoretical one. Thermal expansion of the sintered samples was carried out in a dilatometer Linseis L75HX1000, between 25 and 900 C in air atmosphere. The conductivity was measured between 25 and 850 C in air by the four-point method in barshaped pellets under DC currents between 0.1 and 0.5 A. The currents were applied and collected with a PotentiostatGalvanostat AUTOLAB PGSTAT 302 from ECO CHEMIE. Single cell tests were carried out using LSGM pellets as electrolyte, Ca2Fe2-xCoxO5-d (x ¼ 1.2, 1.4) (CFCO) as cathode material, and SrMo0.8Fe0.2O3-d (SMFO) as anode material, recently developed in our group [20]. The LSGM pellets of 20mm diameter were sintered at 1450 C for 20 h and then polished with a diamond wheel to a thickness of 300 mm La0.4Ce0.6O2d (LDC) was used as a buffer layer between the anode and the electrolyte in order to prevent the interdiffusion of ionic species. Inks of LDC, SMFO and CFCO were prepared with a binder (V-006 from Heraeus). LDC ink was screenprinted onto one side of the LSGM disk followed by a thermal treatment at 1300 C in air for 1 h. SMFO was subsequently screen printed onto the LDC layer and fired at 1100 C in air for 1 h. CFCO was finally screen printed onto the other side of the disk and fired at 1100 C in air for 1 h. The working electrode area of the cell was 0.25 cm2 (0.5 0.5 cm). Pt gauze with a small amount of Pt paste in separate dots was used as current collector at both the anodic and the cathodic sides for ensuring electrical contact. The cells were tested in a vertical tubular furnace at 800 and 850 C; the anode side was fed with pure dry H2, with a flow of 20 ml/min, whereas the cathode worked in an air flow of 100 ml/min. The fuel-cell tests were performed with an AUTOLAB 302N Potentiostat/Galvanostat by changing the voltage of the cell from the OCV (“Open current voltage”) to 0.1 V, with steps of 0.010 V, holding 10 s at each step. Current density was calculated by the recorded current flux through the effective area of the cell (0.25 cm2). Each VI (voltage-intensity) scan corresponds to one cycle; the
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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activation of the cell was followed in subsequent cycles until the full power of the single cell was reached. The activation is necessary to totally reduce the anode to the electronicconducting perovskite phase. It takes about 20 min at 800 C in the presence of pure H2. Scanning electron microscopy (SEM) images were carried out with a Hitachi Se3000N and an analyzer from Oxford Instrument, model INCAx-sight.
Results and discussion Crystallographic characterization The synthesis of pure and well crystallized powders of the Ca2Fe2-xCoxO5-d series was checked by x-ray diffraction measurements. Fig. 1 shows the XRD pattern of the prepared Ca2Fe2-xCoxO5-d powders. It was observed that Ca2Fe2-xCoxO5d (x ¼ 0, 0.2) crystallized in a single phase with brownmillerite˚ , b ¼ 14.7929(1) A ˚ and type structure with a ¼ 5.59222(7) A ˚ for x ¼ 0 and a ¼ 5.58102(6) A ˚ , b ¼ 14.7936(1) A ˚ c ¼ 5.42329(6) A ˚ , for x ¼ 0.2. The obtained unit-cell paand c ¼ 5.40758(5) A rameters are in excellent agreement with those reported in ˚ the literature (a ¼ 5.5946(1), b ¼ 14.8273(2) and c ¼ 5.4307(1) A [21]). Ca2Fe2-xCoxO5-d (x ¼ 0.4, 0.6, 0.8, 1, 1.2, 1.4) crystallized in a supercell, two times the c-axis and the volume of that for a typical brownmillerite. The unit-cell parameters were
Fig. 1 e XRD patterns with CuKa radiation for Ca2Fe2¡xCoxO5¡d family.
˚ , b ¼ 14.8027(1) A ˚ and c ¼ 11.08709(8) A ˚ for x ¼ 1. a ¼ 5.35796(4) A These values are also in perfect agreement with those re˚ and ported in the literature (a ¼ 5.3675(1), b ¼ 14.7787(3) A ˚ [22]). No impurities phases were found. c ¼ 11.1072(3) A
Table 1 e Unit-cell parameters, atomic positions, occupancies, magnetic moment, displacement factors, ˚ ) of reliability factors and selected atomic distances (A Ca2Fe2¡xCoxO5¡d (x ¼ 0.2) in the Pcmn (no. 62) space group, from NPD data at RT. Ca2Fe2xCoxO5d ˚) a (A ˚) b (A ˚) c (A ˚ 3) V (A Ca1 8d (x,y,z) x y z ˚ 2) Biso (A focc (Fe1,Co1) 4a (0, 0, 0) ˚ 2) Biso (A focc(Fe) focc(Co) Moment (mB) Fe1 (Fe2,Co2) 4c (x, 1/4, z) x z ˚ 2) Biso (A focc (Fe) focc(Co) Moment (mB) Fe2 O1 8d (x, y, z) x y z ˚ 2) Biso (A focc O2 8d (x, y, 0) x y z ˚ 2) Biso (A focc O3 4c (x, 1/4, z) x z ˚ 2) Biso (A focc Fiability factors c2 Rp(%) Rwp(%) Rexp(%) RBragg(%) R Mag(%) ˚) Distances (A Tetrahedral sites Fe1(Co1)eO1(x4) Fe1(Co1)eO2(x2) Octahedral sites Fe2(Co2)eO2(x2) Fe2(Co2)eO3(x2)
X ¼ 0.2 5.57800(2) 14.77983(4) 5.40394(2) 445.511(2) 0.02319(4) 0.10825(1) 0.48300(6) 0.639(5) 1.00 0.228(5) 0.886(1) 0.114(1) 3.57(5) 0.93556(3) 0.94544(3) 0.017(5) 0.846(1) 0.154(1) 2.72(5) 0.23947(5) 0.98462(1) 0.26174(7) 0.491(5) 0.988(1) 0.07101(3) 0.13983(1) 0.02490(5) 0.797(6) 1.00 0.87523(6) 0.59996(6) 0.268(8) 0.996(1) 4.15 3.18 4.05 1.99 2.82 3.49
1.9585(4) 2.1085(2) 1.8457(3) 1.8976(4)
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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A neutron powder diffraction (NPD) study at room temperature (RT) for the Ca2Fe2-xCoxO5-d family and high temperature (up to 800 C) for Ca2Fe0.8Co1.2O5d was useful to investigate the structural details. For Ca2Fe2-xCoxO5-d (x ¼ 0.2) the crystal structure was defined in the orthorhombic Pcmn space group (No. 62), Z ¼ 4. Ca atoms are located at 8d (x, y, z) positions, (Fe,Co)1 are distributed at random at 4a (0, 0, 0), (Fe,Co)2 distributed at random at 4c (x, ¼, z), O1 and O2 atoms are ordered at two different 8d (x, y, z) sites and O3 are placed at 4c (x, ¼,z). The occupancy factor for Fe and Co atoms and for all the oxygen atoms (O1, O2 and O3) was also refined in order to determine the stoichiometric formula; O2 occupancies were fixed to unity since they converged to values higher than 1. O1 and O2 converged to values lower than 1 thus giving a slight deviation from the full stoichiometry (Table 1). The final refined composition at 25 C is Ca2Fe1.73Co0.27O4.98. In the case of Ca2Fe2xCoxO5d (x ¼ 0.4. 0.6, 0.8, 1, 1.2 and 1.4) the crystal structural was defined in the orthorhombic Pcmb group (No. 57), Z ¼ 8. Ca1 and Ca2 atoms are placed at two different 8e (x, y, z) sites, whereas (Fe,Cob)1 and (Co,Feb)1 are located at two different 4d (x, ¼, z) positions. (Fe,Co)2 and (Fe,Co)3 are distributed at 4c (x, ½, ¾) and 4a (0, 0, 0) sites, respectively. Two non-equivalent O atoms (O1, O2) are placed at two different 4d (x, ¼, z), and (O3, O4, O5 and O6) at four different 8e (x, y, z) positions. The occupancy factors for Fe and Co atoms and for all the oxygen atoms was also refined, giving
a deviation from the full stoichiometry that increases according as the amount of iron in the sample decreases from Ca2Fe1.61Co0.39O4.99 to Ca2Fe0.55Co1.45O4.89 as shown in Table 1. This means that the introduction of Cobalt ions at the Fe sublattice is effective in creating a measurable amount of oxygen vacancies. This is essential to get a good performance of these materials as cathodes in SOFC. The cell volume obtained for Ca2Fe2-xCoxO5-d at room temperature decreases as ˚3 the amount of Co increases, shrinking from 887.96(3) A ˚ 3 (x ¼ 1.4) as shown in Table 1; the (x ¼ 0.4) to 871.99(4) A introduction of a smaller cation at the B-site causes the reduction of the unit-cell volume. A low-angle magnetic peak in the RT NPD patterns suggests that the samples are magnetically ordered above room temperature, so the magnetic structures were fully determined from NPD data collected at this temperature for Ca2Fe1.8Co0.2O5d (Pcmn group) and Ca2Fe0.8Co1.2O5d, Ca2Fe0.6Co1.4O5d (Pcmb group) and introduced into the refinement. A G-type magnetic structure perfectly fits the magnetic intensities and a propagation vector k ¼ (0,0,0) was identified in the three compounds. The moments were found to be collinear and (anti)parallel with the c axis in Ca2Fe1.8Co0.2O5d and collinear and (anti)parallel with the a axis in Ca2Fe0.6Co1.4O5d and Ca2Fe0.8Co1.2O5d. The main magnetic peaks are indexed in Fig. 2a,b,c. As the amount of cobalt is increasing, the magnetic moment decreases as
Fig. 2 e Observed (red crosses), calculated (black line) and difference (bottom line) NPD Rietveld profiles at RT for a) Ca2Fe0.6Co1.4O5-d, b)Ca2Fe0.8Co1.2O5¡d and c) Ca2Fe1.8Co0.2O5¡d. d) NPD Rietveld profiles at 800 C for Ca2Fe0.8Co1.2O5¡d. The two series of markers correspond to the positions of the allowed Bragg reflections for the crystallographic phase and the magnetic reflections. The irregular background observed in the high-temperature diagram is due to the quartz container. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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Table 2 e Unit-cell parameters, atomic positions, occupancies, magnetic moment, displacement factors, reliability factors ˚ ) of Ca2Fe2¡xCoxO5¡d (x ¼ 0.4, 0.6, 0.8, 1, 1.2, 1.4) in the Pcbm (no. 57) space group, from NPD and selected atomic distances (A data at RT. Ca2Fe2xCoxO5d ˚) a (A ˚) b (A ˚) c (A ˚ 3) V (A Ca1 8e (x, y, z) x y z ˚ 2) Biso (A focc Ca2 8e (x, y, z) x y z ˚ 2) Biso (A focc (Fe1, Co1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Fe) focc (Co) Moment (mB) (Fe1, Co1b) (Co1, Fe1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Co) focc (Fe) Moment (mB) (Co1, Fe1b) (Co2, Fe2) 4c (x, 1/2, 3/4) x ˚ 2) Biso (A focc (Co) focc (Fe) Moment (mB) (Co2, Fe2) (Co3, Fe3) 4a (0, 0, 0) ˚ 2) Biso (A focc (Co) focc (Fe) Moment (mB) (Co3, Fe3) O1 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O2 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O3 8e (x, y, z) x y z ˚ 2) Biso (A focc O4 8e (x, y, z) x y z ˚ 2) Biso (A
X ¼ 0.4
X ¼ 0.6
X ¼ 0.8
X ¼ 1.2
X ¼ 1.4
5.3906(1) 14.7745(3) 11.1490(3) 887.96(3)
5.3780(2) 14.7769(4) 11.1234(4) 883.97(5)
5.3693(1) 14.7914(3) 11.1073(3) 882.13(4)
5.3548(1) 14.7918(3) 11.0790(2) 877.53(3)
X¼1
5.3452(1) 14.8129(3) 11.0569(3) 875.49(4)
5.3301(1) 14.8379(4) 11.0258(3) 871.99(4)
0.0168(1) 0.39504(7) 0.7607(1) 0.541(5) 1.00
0.0155(2) 0.39312(8) 0.7603(1) 0.901(8) 1.00
0.0188(1) 0.39233(7) 0.76086(7) 1.025(7) 1.00
0.0193(1) 0.3911(6) 0.7612(7) 0.530(6) 1.00
0.0183(1) 0.3918(9) 0.7577(1) 0.927(7) 1.00
0.0083(1) 0.39314(7) 0.75640(9) 0.608(6) 1.00
0.4870(1) 0.60972(7) 0.5123(1) 0.541(5) 1.00
0.4881(2) 0.61027(8) 0.5111(1) 0.901(8) 1.00
0.4894(1) 0.60729(6) 0.50882(7) 1.025(7) 1.00
0.4910(1) 0.6067(6) 0.5097(7) 0.530(6) 1.00
0.4913(1) 0.6080(8) 0.5130(1) 0.927(7) 1.00
0.4887(1) 0.60956(7) 0.5137(1) 0.608(6) 1.00
0.0544(1) 0.53693(5) 0.287(3) 1.00 e 3.9(4)
0.0518(1) 0.53642(8) 0.476(6) 1.00 e 3.1(4)
0.0463(1) 0.53318(4) 0.612(4) 1.00 e 2.3(2)
0.04317(9) 0.53429(4) 0.731(5) 1.00 e 2.0(5)
0.0400(1) 0.53259(8) 0.857(8) 0.588(1) 0.412(1) 1.8(5)
0.0435(1) 0.52988(8) 0.008(6) 0.452(1) 0.548(1) 1.8(3)
0.4489(1) 0.7197(7) 0.287(3) 0.392(3) 0.608(3) 3.3(5)
0.4508(3) 0.7219(1) 0.476(6) 0.734(8) 0.266(2) 2.6(5)
0.4276(4) 0.7140(2) 0.612(4) 0.880(1) 0.120(1) 2.7(3)
0.4212(3) 0.7178(2) 0.731(5) 1.00 e 2.9(4)
0.4260(5) 0.7175(2) 0.857(8) 1.00 e 2.6(2)
0.4278(1) 0.7139(2) 0.008(6) 1.00 e 0.5(3)
0.5017(1) 0.287(3) 0.398(2) 0.602(2) 2.8(5)
0.4955(2) 0.476(6) 0.618(2) 0.382(2) 2.2(5)
0.4985(1) 0.612(4) 0.206(1) 0.794(1) 2.9(3)
0.4975(1) 0.731(5) 0.352(1) 0.648(1) 2.5(4)
0.4978(2) 0.857(8) 0.460(1) 0.540(1) 2.5(2)
0.4983(2) 0.008(6) 0.580(1) 0.420(1) 2.5(3)
0.287(3) e 1.00 3.2(5)
0.476(6) e 1.00 3.3(4)
0.612(4) 0.550(1) 0.450(1) 1.8(3)
0.731(5) 0.556(1) 0.444(1) 1.5(4)
0.857(8) 0.674(1) 0.326(1) 1.1(5)
0.008(6) 0.746(1) 0.254(1) 1.1(2)
0.0873(2) 0.69351(9) 0.65(1) 1.00
0.0905(2) 0.6939(1) 1.01(2) 1.00
0.0868(1) 0.69166(8) 0.45(2) 0.918(1)
0.0889(1) 0.69125(8) 0.674(2) 0.94(1)
0.0837(2) 0.6914(1) 0.037(2) 0.882(1)
0.0825(1) 0.69348(7) 0.159(2) 1.00
0.6057(1) 0.56335(8) 0.16(1) 1.00
0.5973(2) 0.5632(1) 0.55(2) 1.00
0.6020(1) 0.56343(7) 0.411(2) 0.95(1)
0.6013(1) 0.56374(7) 0.514(2) 0.986(1)
0.5994(2) 0.56423(8) 0.391(1) 1.00
0.6037(2) 0.56335(7) 0.474(2) 1.00
0.2532(1) 0.48317(3) 0.62476(9) 0.22(1) 0.993(1)
0.2546(3) 0.48322(4) 0.6251(1) 0.53(1) 0.988(2)
0.2531(1) 0.48374(3) 0.62452(7) 0.654(1) 1.00
0.2515(1) 0.48420(3) 0.62449(8) 0.329(9) 1.00
0.2474(2) 0.48406(3) 0.6249(1) 0.461(1) 1.00
0.2575(2) 0.48563(3) 0.6258(1) 0.356(1) 0.966(1)
0.7515(2) 0.48824(3) 0.6275(1) 0.63(1)
0.7483(3) 0.4883(5) 0.6268(1) 0.85(2)
0.7520(1) 0.48972(3) 0.62751(7) 0.771(8)
0.7504(1) 0.48977(3) 0.62730(7) 0.341(7)
0.7476(2) 0.49091(3) 0.6263(1) 0.927(1)
0.7556(2) 0.49040(3) 0.6278(1) 0.407(1)
(continued on next page)
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
Table 2 e (continued ) Ca2Fe2xCoxO5d focc O5 8e (x, y, z) x y z ˚ 2) Biso (A focc O6 8e (x, y, z) x y z ˚ 2) Biso (A focc Reliability factors c2 Rp(%) Rwp(%) Rexp(%) RBragg(%) R Mag(%) ˚) Distances (A Tetrahedral Sites Fe1(Co1b)eO1 Fe1(Co1b)eO2 Fe1(Co1b)eO5(x2) Co1(Fe1b)eO1 Co1(Fe1b)eO2 Co1(Fe1b)eO6(x2) Octahedral sites Fe2(Co2)eO3(x2) Fe2(Co2)eO4(x2) Fe2(Co2)eO6(x2) Co3(Fe3)eO3(x2) Co3(Fe3)eO4(x2) Co3(Fe3)eO5(x2)
X ¼ 0.4
X ¼ 0.6
X ¼ 0.8
X¼1
X ¼ 1.2
X ¼ 1.4
0.999(1)
0.978(2)
1.00
1.00
1.00
0.924(1)
0.0159(2) 0.36434(5) 0.46901(8) 0.93(1) 1.00
0.0135(1) 0.36013(7) 0.4697(1) 0.80(2) 1.00
0.0124(1) 0.36060(5) 0.46917(8) 1.15(2) 0.965(3)
0.0156(1) 0.36026(4) 0.46955(8) 1.131(1) 0.990(1)
0.0169(1) 0.35968(6) 0.46885(1) 0.734(1) 0.947(2)
0.0141(1) 0.36160(5) 0.47053(9) 0.230(1) 1.00
0.5105(1) 0.35732(5) 0.78466(7) 0.64(1) 1.00
0.5120(2) 0.3596(8) 0.7849(1) 1.227(2) 1.00
0.5097(1) 0.36115(5) 0.78324(8) 1.05(2) 0.995(3)
0.5142(1) 0.36081(4) 0.78299(7) 0.72(1) 0.986(1)
0.5148(1) 0.3614(6) 0.7817(1) 0.758(1) 1.00
0.5171(1) 0.36081(6) 0.7797(1) 1.71(1) 1.00
4.19 3.29 4.17 2.04 4.07 4.90
4.69 3.41 4.45 2.05 4.73 6.70
7.66 3.93 5.14 1.86 6.39 3.65
4.68 3.08 3.94 1.82 4.43 3.25
1.99 3.51 4.47 3.17 3.98 4.19
3.55 2.58 3.37 1.79 3.69 5.25
1.906(1) 1.856(1) 1.8897(9) 1.971(1) 1.937(1) 1.7745(9)
1.912(1) 1.911(1) 1.823(1) 1.963(3) 1.932(3) 1.795(1)
1.9051(9) 1.9202(9) 1.8130(7) 1.846(1) 1.907(1) 1.8657(9)
1.877(1) 1.932(1) 1.8093(8) 1.803(2) 1.960(2) 1.859(1)
1.882(2) 1.973(1) 1.795(1) 1.839(3) 1.928(4) 1.864(1)
1.925(1) 1.917(1) 1.8068(9) 1.854(2) 1.906(2) 1.859(1)
1.951(1) 1.925(1) 2.1442(7) 1.965(1) 1.961(1) 2.0357(9)
1.916(1) 1.939(1) 2.111(1) 1.968(1) 1.962(1) 2.095(1)
1.9329(8) 1.9350(8) 2.0908(6) 1.9522(8) 1.9480(7) 2.0912(6)
1.930(1) 1.925(1) 2.092(6) 1.9418(9) 1.9490(9) 2.0960(7)
1.945(1) 1.914(1) 2.086(1) 1.928(1) 1.955(1) 2.106(1)
1.889(1) 1.928(1) 2.093(1) 1.963(1) 1.924(1) 2.0805(8)
shown in Table 2. After the full refinement we achieved a good agreement between the observed and calculated NPD patterns at RT, illustrated for Ca2Fe2xCoxO5d (x ¼ 0.2, 1.2, 1.4) in Fig. 2a, b and c. Tables 1 and 2 summarize the unit-cell, atomic positions, occupancies, magnetic moments, displacement parameters and discrepancy factors after the Rietveld refinement in the Pcmn and Pcmb groups. The inset on Fig. 3c illustrates the crystal superstructure of Ca2Fe2xCoxO5d (x ¼ 1), which is two times the c-axis and the volume of that of a typical brownmillerite. In this case, a model with site ordering of Fe and Co over the two tetrahedral sites and mixing of Fe and Co over the octahedral sites was found to be the right model [22], so this superstructure consists of Fe1O4 and Co1O4 tetrahedral layers containing Fe1 and Co1 atoms, which alternate with (Fe,Co)2O6 and (Fe,Co)3O6 octahedral layers. The tetrahedral layers consist of two Fe1O4 tetrahedral groups alternating with one Co1O4 tetrahedral group where Fe1O4 and Co1O4 are sharing O1 and O2 atoms, while the octahedral layers are formed by two (Fe,Co)3O6 octahedral groups alternating with one (Fe,Co)2O6 octahedral group, which are sharing O3 and O4 atoms. Finally, tetrahedral and octahedral layers are sharing O5 and O6 atoms. Table 3 summarizes selected bond distances for the Ca2Fe2xCoxO5d series. The two octahedral sites in
Ca2Fe2xCoxO5d (x ¼ 1) show similar bond lengths, with elongation of the two out of plane bonds for both sites, (Fe,Co) ˚ and (Fe,Co)3eO5 ¼ 2.0960(7) A ˚ and four 2eO6 ¼ 2.092(6) A ˚, shorter distances in the ac plane, (Fe,Co)2eO3 ¼ 1.930(1) A ˚ , (Fe,Co)3eO3 ¼ 1.9418(9) A ˚ and (Fe,Co) (Fe,Co)2eO4 ¼ 1.925(1) A ˚ . On the other hand, in the tetrahedral 3eO4 ¼ 1.9490(9) A layers, the Fe1O4 tetrahedra are compressed along the b axis ˚ distances than with a much shorter Fe1eO5 ¼ 1.8093(8) A ˚ and Fe1eO2 ¼ 1.932(1) A ˚ bond lengths Fe1eO1 ¼ 1.877(1) A lying on the ac plane, while the Co1O4 have a much shorter ˚ distances in the ac plane than the Co1eO1 ¼ 1.803(2) A ˚ and the Co1eO6 ¼ 1.859(1) A ˚ . The elonCo1eO2 ¼ 1.960(2) A gation of out-of-plane bonds for the octahedral sites occurs in regular brownmillerite compounds as well [23,24]. Such elongation seems to be required by this structure type in order to accommodate the linkage between octahedral and tetrahedral layers. Compression of tetrahedral out-of-plane bonds can accompany the elongation of octahedra. The oxygen atoms contained in the tetrahedral and octahedral layers are all of the same type (O2) for a regular brownmillerite [25], while there are two different types of oxygens, O5 and O6 for Ca2Fe2xCoxO5d. However, the major difference between the sublattices of a typical brownmillerite and Ca2Fe2xCoxO5d is in the splitting of the O3 site, in a regular brownmillerite, into
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
7
Fig. 3 e Thermal variation of (a) a unit-cell parameter (left axis) b/2 unit-cell parameter (right axis), (b) c/2 unit-cell parameter (left axis) and volume (right axis) and c) oxygen occupancy factor, from in situ NPD data. The inset illustrates the crystal structure of the oxygen-deficient brownmillerite superstructure for x > 0.4. O1, O2, O3, O4, O5, O6 atoms (red spheres), Ca1 (orange spheres), Ca2 (yellow spheres), Fe1 (green spheres), Co1 (light blue spheres), Co2, Fe2 (pink spheres) and Co3,Fe3 (dark blue spheres). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
two sites labeled O1 and O2 in Ca2Fe2xCoxO5d. An inspection of the structure reveals that O1 and O2 are the oxygens bonded only to the tetrahedral cations with no connection to the octahedral ions. In fact, it appears that the symmetry of the octahedral layer does not deviate significantly from that of a regular brownmillerite, while the symmetry of the
tetrahedral chains is the major factor that drives the formation of a super structure. The thermal evolution of the crystal structure under the cathode conditions in a SOFC was evaluated by NPD for x ¼ 1.2. For this purpose, the sample was contained in a quartz tube opened to the air atmosphere and the NPD data were collected
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
Table 3 e Unit-cell parameters, atomic positions, occupancies, displacement factors, reliability factors and selected atomic ˚ ) of Ca2Fe2¡xCoxO5¡d (x ¼ 1.2) in the Pcbm (no. 57) space group, from NPD data at 25e800 C. distances (A Ca2Fe0.8Co1.2O5d ˚) a (A ˚) b (A ˚) c (A ˚ 3) V (A Ca1 8e (x, y, z) x y z ˚ 2) Biso (A focc Ca2 8e (x, y, z) x y z ˚ 2) Biso (A focc (Fe1, Co1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Fe) focc (Co) (Co1, Fe1b) 4d (x, 1/4, z) x z ˚ 2) Biso (A focc (Co) focc (Fe) (Co2, Fe2) 4c (x, 1/2, 3/4) x ˚ 2) Biso (A focc (Co) focc (Fe) (Co3, Fe3) 4a (0, 0, 0) ˚ 2) Biso (A focc (Co) focc (Fe) O1 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O2 4d (x, 1/4, z) x z ˚ 2) Biso (A focc O3 8e (x, y, z) x y z ˚ 2) Biso (A focc O4 8e (x, y, z) x y z ˚ 2) Biso (A focc O5 8e (x, y, z) x y z
25
200
400
600
800
5.3452(1) 14.8129(3) 11.0569(3) 875.49(4)
5.3459(1) 14.9151(2) 11.0463(2) 880.77(3)
5.3579(1) 15.0149(2) 11.0530(2) 889.18(3)
5.3729(1) 15.0926(3) 11.0673(2) 897.46(3)
5.3902(1) 15.1612(4) 11.0877(3) 906.11(4)
0.0183(1) 0.3918(9) 0.7577(1) 0.927(7) 1.00
0.0099(1) 0.39184(7) 0.75721(9) 1.092(6) 1.00
0.0019(2) 0.3931(7) 0.7615(1) 1.405(6) 1.00
0.0058(2) 0.39030(9) 0.7619(1) 1.961(7) 1.00
0.0019(2) 0.3885(1) 0.7639(1) 2.33(1) 1.00
0.4913(1) 0.6080(8) 0.5130(1) 0.927(7) 1.00
0.4884(1) 0.60847(7) 0.51499(9) 1.092(6) 1.00
0.4893(1) 0.61021(7) 0.5098(1) 1.405(6) 1.00
0.4931(2) 0.60843(9) 0.5061(1) 1.961(7) 1.00
0.4929(2) 0.60786(9) 0.5346(1) 2.33(1) 1.00
0.0400(1) 0.53259(8) 0.857(8) 0.588(1) 0.412(1)
0.0406(1) 0.53446(7) 0.228(6) 0.588(1) 0.412(1)
0.0374(1) 0.53310(6) 0.307(6) 0.588(1) 0.412(1)
0.0431(2) 0.53261(9) 0.885(7) 0.588(1) 0.412(1)
0.0397(2) 0.53571(8) 1.024(9) 0.588(1) 0.412(1)
0.4260(5) 0.7175(2) 0.857(8) 1.00 e
0.4333(4) 0.7185(2) 0.228(6) 1.00 e
0.4301(4) 0.7172(2) 0.302(6) 1.00 e
0.4319(5) 0.7149(2) 0.885(7) 1.00 e
0.4370(5) 0.7264(2) 1.024(9) 1.00 e
0.4978(2) 0.857(8) 0.460(1) 0.540(1)
0.4962(1) 0.228(6) 0.460(1) 0.540(1)
0.4969(2) 0.302(6) 0.460(1) 0.540(1)
0.5004(2) 0.885(7) 0.460(1) 0.540(1)
0.4923(2) 1.024(9) 0.460(1) 0.540(1)
0.857(8) 0.674(1) 0.326(1)
0.228(6) 0.674(1) 0.326(1)
0.302(6) 0.674(1) 0.326(1)
0.885(7) 0.674(1) 0.326(1)
1.024(9) 0.674(1) 0.326(1)
0.0837(2) 0.6914(1) 0.037(2) 0.882(1)
0.0858(2) 0.69137(9) 0.72(3) 0.874(1)
0.0887(2) 0.6912(1) 1.65(2) 1.00
0.0904(2) 0.6918(1) 2.14(2) 1.00
0.0980(3) 0.6912(1) 2.60(3) 1.00
0.5994(2) 0.56423(8) 0.391(1) 1.00
0.6045(2) 0.56170(9) 1.06(3) 1.00
0.6044(2) 0.5613(1) 1.19(2) 1.00
0.6082(2) 0.5611(1) 2.01(2) 1.00
0.6037(3) 0.5609(1) 2.14(2) 1.00
0.2474(2) 0.48406(3) 0.6249(1) 0.461(1) 1.00
0.2548(2) 0.48383(3) 0.6233(1) 1.07(1) 0.996(2)
0.2540(2) 0.48345(3) 0.6229(1) 1.00(1) 0.911(2)
0.2552(2) 0.48416(4) 0.6241(1) 1.21(1) 0.934(1)
0.02535(2) 0.48321(4) 0.6250(1) 1.44(2) 0.929(2)
0.7476(2) 0.49091(3) 0.6263(1) 0.927(1) 1.00
0.7566(2) 0.49126(3) 0.6254(1) 0.79(1) 0.966(1)
0.7517(2) 0.49193(3) 0.6255(1) 1.02(1) 0.961(2)
0.7549(2) 0.49145(4) 0.6265(1) 1.09(1) 0.907(1)
0.7555(2) 0.49251(5) 0.6250(1) 1.57(2) 0.924(2)
0.0169(1) 0.35968(6) 0.46885(1)
0.0162(1) 0.36106(5) 0.46947(9)
0.0153(1) 0.35795(8) 0.4676(1)
0.0193(1) 0.35952(8) 0.4724(1)
0.0125(2) 0.4741(1) 0.35883(8)
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
9
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3
Table 3 e (continued ) Ca2Fe0.8Co1.2O5d 2
˚ ) Biso (A focc O6 8e (x, y, z) x y z ˚ 2) Biso (A focc Reliability factors c2 Rp(%) Rwp(%) Rexp(%) RBragg(%) ˚) Distances (A Tetraedral sites Fe1(Co1b)eO1 Fe1(Co1b)eO2 Fe1(Co1b)eO5(x2) Co1(Fe1b)eO1 Co1(Fe1b)eO2 Co1(Fe1b)eO6(x2) Octahedral sites Fe2(Co2)eO3(x2) Fe2(Co2)eO4(x2) Fe2(Co2)eO6(x2) Co3(Fe3)eO3(x2) Co3(Fe3)eO4(x2) Co3(Fe3)eO5(x2)
25
200
400
600
800
0.734(1) 0.947(2)
0.85(1) 1.00
1.02(1) 1.00
1.98(2) 1.00
2.71(2) 1.00
0.5148(1) 0.3614(6) 0.7817(1) 0.758(1) 1.00
0.5207(1) 0.35897(7) 0.7820(1) 2.12(1) 1.00
0.5228(1) 0.35933(8) 0.7787(1) 1.52(1) 1.00
0.5241(1) 0.35709(9) 0.7829(1) 2.46(2) 1.00
0.5233(2) 0.35567(9) 0.7849(1) 2.63(2) 0.962(2)
1.99 3.51 4.47 3.17 3.98
1.90 1.67 2.17 1.57 5.92
1.72 1.56 2.06 1.57 5.59
1.61 1.52 1.99 1.57 5.37
1.76 1.58 2.07 1.56 7.80
1.877(1) 1.932(1) 1.8093(8) 1.803(2) 1.960(2) 1.859(1)
1.858(1) 1.920(1) 1.829(1) 1.881(3) 1.963(3) 1.834(1)
1.873(1) 1.932(1) 1.805(1) 1.877(3) 1.956(3) 1.834(1)
1.903(1) 1.900(1) 1.814(1) 1.853(4) 1.948(4) 1.850(2)
1.877(1) 1.942(1) 1.808(1) 1.868(4) 2.044(4) 1.790(2)
1.930(1) 1.925(1) 2.092(6) 1.9418(9) 1.9490(9) 2.0960(7)
1.916(1) 1.960(1) 2.133(1) 1.944(1) 1.907(1) 2.1027(9)
1.933(1) 1.951(1) 2.144(1) 1.936(1) 1.919(1) 2.156(1)
1.933(1) 1.938(1) 2.191(1) 1.955(1) 1.926(1) 2.145(1)
1.913(1) 1.987(1) 2.224(1) 1.958(1) 1.916(1) 2.161(1)
in situ at 200, 400, 600 and 800 C. Fig 2 d illustrates the good agreement between the observed and calculated NPD patterns for this oxide at 800 C. Fig. 2b and d show no structural transition across the temperature range under study (25e800 C). Table 3 includes the results obtained from the refinements at the different temperatures for Ca2Fe0.8Co1.2O5d. The thermal evolution of the oxygen content in air was also studied by NPD. Fig. 3 illustrates the temperature dependence of the concentration of oxygen vacancies (d) (Fig. 3c) and unitcell parameters and volume (Fig. 3a,b) for Ca2Fe0.8Co1.2O5d. The oxygen content decreases when heating the sample from Ca2Fe0.8Co1.2O4.90(1) at RT to Ca2Fe0.8Co1.2O4.82(1) at 800 C. These vacancies are essential to drive the required O2 motion in a MIEC oxide.
Thermal expansion measurements The thermal expansion of each compound was measured on dense cylindrical pellets (5 mm diameter x 2 mm thickness) sintered at 1100 C for 12 h. The dilatometric analysis was carried out between 25 and 900 C for several cycles in air; the data were only recorded during the heating runs. Fig. 4 shows a regular variation of the thermal expansion along the Ca2Fe2-xCoxO5d series. However, the relative thermal expansion is not totally linear in all the temperature range under study in Ca2Fe2xCoxO5d (x ¼ 0 and 0.2). A change in the slope can be observed around 650e700 C in Ca2Fe2O5-d that some authors [21] have related to a structural transition to Icmm. The same happens with Ca2Fe1.8Co0.2O5-d around 500e525 C and it could be related to the same structural transition. The thermal
expansion coefficients obtained for the Ca2Fe2-xCoxO5-d system are included in Fig. 4. These values perfectly match with the values usually displayed by SOFC electrolytes (10e13 106 K1).
Electrical conductivity measurements and chemical compatibility The electrical conductivity was obtained in air by the dc fourprobe method from 25 to 850 C (Fig. 5) leading a semiconductor-like behavior in all the range of temperatures. The Ca2Fe1.8Co0.2O5d, with only a 10% of cobalt, shows a null conductivity in the temperature range of 25e800 C, but as the amount of cobalt is increasing, the conductivity rises up to 18 Scm1 at 850 C for x ¼ 1.2. This can be assigned to the fact that the mobility of p-type carriers of cobalt is much higher than that of iron [26]. On the other hand, there is a little decrease in the electrical conductivity when the Co content is x ¼ 1.4 with respect to x ¼ 1.2 as shown Fig. 5. This reduction of the electrical conductivity could be related with to oxygen ion vacancies in this compound, as the carriers’ motion happens via M-O-M paths in the lattice [27], and Ca2Fe0.6Co1.4O5d has a large amount of oxygen ion vacancies. Although the electrical conductivities obtained for Ca2Fe2xCoxO5d are considerably lower than those presented by other Co perovkite materials [26,28], it is enough to be used successfully as cathode materials. The chemical compatibility of Ca2FeCoO5d with the LSGM electrolyte has also been checked by firing mixtures of both powdered materials at 1100 C for 24 h; the inset of Fig. 5 shows a Rietveld analysis of the product, consisting of a mixture of both unaltered perovskite phases, indicating that
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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Fig. 4 e Thermal expansion determined by dilatometry of the Ca2Fe2¡xCoxO5¡d series.
the formation of unwanted secondary phases when the material is tested in single-cells is not favored.
Fuel-cell tests The performance of Ca2Fe2xCoxO5d as cathode material was also tested in single cells in an electrolyte-supported configuration with a 300 mm-thick LSGM electrolyte using air in contact with the cathode and dry H2 as a fuel. Fig. 6a illustrates
the cell voltage and power density as a function of current density at 800 and 850 C for the x ¼ 1.2 cathode. The maximum power densities generated by the cell are 234 and 372 mW/cm2, respectively. Fig. 6b illustrates the cell voltage and power density as a function of current density at 800 and 850 C for the x ¼ 1.4 cathode. The maximum power densities generated by the cell are 238 and 412 mW/cm2, respectively. The present performances open the possibility of considering these materials as
Fig. 5 e Dc-conductivity as a function of temperature for Ca2Fe2¡xCoxO5¡d series. The inset shows the Rietveld-refined XRD profiles of a mixture of LSGM and Ca2FeCoO5-d after a thermal treatment at 1100 C in air, showing no reaction products between both phases other than the initial reactants. The first and second series of Bragg positions correspond to Ca2FeCoO5-d and LSGM, respectively. Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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Fig. 6 e Cell voltage (left axis) and power density (right axis) as a function of the current density for the test cell with the configuration SMFO/LDC/LSGM/CFCO (x ¼ 1.2, 1.4) in pure H2 measured at T ¼ 800 and 850 C with Pt as current collector.
cathode for SOFCs. As shown in Fig. 6a and b, a slight increase of the output power of the single cells is observed from x ¼ 1.2 to x ¼ 1.4 cathodes, correlated with the effect promoted by Co, enhancing the formation of oxygen vacancies. Although Ca2Fe0.6Co1.4O5d exhibits a slightly lower electronic conductivity values than Ca2Fe0.6Co1.2O5d at the working temperatures (800e850 C), it seems that the presence of a higher number of oxygen vacancies for x ¼ 1.4 compensates this effect, demonstrating that a suitable ionic mobility is an important requisite for MIEC oxides in SOFCs.
Scanning electron microscopy Fig. 7a and b shows the micrographs of the Ca2Fe0.8Co1.2O5d and Ca2Fe0.6Co1.4O5d cathode surfaces respectively after the single-
cell tests analyzed by scanning electron microscopy (SEM). Both figures show a similar microstructure exhibiting a good porosity, which is one of the essential requirements for optimal cathode materials as it favors the diffusion and reduction process of the oxygen throughout the bulk of the cathode.
Conclusions In this work, we have shown that Ca2Fe2xCoxO5d oxides crystallize in a brownmillerite-type structure (s.g. Pcmn) or in a brownmillerite superstructure (s.g. Pcmb). We show that the introduction of Co ions at the Fe sublattice is effective in creating a measurable extra amount of oxygen vacancies. Ca2Fe2xCoxO5d (x ¼ 1.2 and 1.4) oxides can be successfully
Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067
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to the Institut Laue-Langevin (ILL) for making all facilities available.
references
Fig. 7 e SEM images of a) Ca2Fe0.8Co1.2O5¡d cathode surface and b) Ca2Fe0.6Co1.4O5¡d cathode surface after the single cell test.
utilized as cathode material in single SOFC cells in an electrolyte (LSGM)-supported configuration. Maximum powers of 372 and 412 mW/cm2 were obtained at 850 C with pure H2 as a fuel, respectively. An “in situ” neutron power diffraction experiment, from 25 C to 800 C, reveals that the structure of Ca2Fe0.8Co1.2O5d does not change in all the range of temperatures. The sufficiently high number of oxygen vacancies induced upon Co doping along with high isotropic displacement factors suggests a high ionic conductivity at the working temperatures. The electronic conductivity seems to be sufficient to deliver a good performance, resulting in an excellent catalyst for oxygen reduction. The thermal expansion coefficients are perfectly compatible with the electrolyte. Finally, an excellent chemical compatibility with the electrolyte LSGM during 24 h is also observed.
Acknowledgments We thank the financial support of the Spanish Ministry of Education to the project MAT2013-41099-R and we are grateful
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Please cite this article in press as: Cascos V, et al., Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.01.067