Effect of samarium (Sm3+) doping on structure and electrical conductivity of double perovskite Sr2NiMoO6 as anode material for SOFC

Journal of Alloys and Compounds 725 (2017) 1123e1129

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Effect of samarium (Sm3þ) doping on structure and electrical conductivity of double perovskite Sr2NiMoO6 as anode material for SOFC Pravin Kumar a, Sabrina Presto b, A.S.K. Sinha c, Salil Varma d, Massimo Viviani b, *, Prabhakar Singh a, ** a

Department of Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi, 221 005, India CNR-ICMATE, c/o DICCA-UNIGE, P.le Kennedy 1 e Pad. D, 16129, Genova, Italy Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, 221 005, India d Chemistry Division, Mod. Labs., Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 May 2017 Received in revised form 21 July 2017 Accepted 22 July 2017 Available online 26 July 2017

Samarium (Sm3þ) doped double perovskite Sr2-xSmxNiMoO6d (SSNM) with 0.01  x  0.05 is synthesized via citrate-nitrate auto-combustion route. X-ray analysis shows the presence of main phase with tetragonal symmetry and space group I4/m, along with a small amount of SrMoO4 phase. The amount of secondary phase is found to increase with increasing dopant concentration. Micro-structural study reveals the formation of uniform grains with grain growth inhibited by doping. XPS analysis clearly indicates the reduction of Mo6þ to Mo5þ. Oxygen vacancies are also found to increase with dopant concentration. Monotonic trends of variation of conductivity and activation energy are observed. In particular, superior conductivity with lower most activation energy is obtained for the sample with x ¼ 0.05. For this composition, impedance measurement is repeated under H2 atmosphere and a significant increase of the conductivity is observed. For air-sintered material, in addition to Mo reduction a new mechanism is proposed, based on the formation of Sr and Mo vacancies which accounts for the effect of doping on lattice parameters, phase composition and conductivity. © 2017 Elsevier B.V. All rights reserved.

Keywords: Double perovskite Electrical conductivity Combustion synthesis SOFC Anode material Defects model

1. Introduction Alternative energy sources and clean technologies are required to fulfill the environmental needs of 21st century [1]. For energy conversion applications, Solid Oxide Fuel Cells (SOFC) are one of most suitable devices having some positive characteristics as high efficiency, fuel flexibility and near zero emissions of greenhouse gases [2e4]. The main innovation with respect to state of the art SOFC is expected from the development of materials with high performance at temperatures lower than 750  C [5]. For instance, oxides with perovskite structure have shown high conductivity under reducing conditions and therefore are deeply investigated as potential substitutes of Ni in anode [5,6].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Viviani), [email protected] (P. Singh). http://dx.doi.org/10.1016/j.jallcom.2017.07.211 0925-8388/© 2017 Elsevier B.V. All rights reserved.

In this context, B-site ordered double perovskite derived materials, with general formula A2B0 B00 O6 structured by alkaline earth element (A ¼ Sr, Ca, Ba) and heterovalent transition metals (B0 ¼ Fe, Co, Ni, Cr, etc. and B00 ¼ Mo, W, etc.), are raising a significant interest as potential mixed ionic-electronic conductors under reducing atmosphere and in the temperature range of 600e700  C [7e11]. The structure of B-site ordered double perovskites A2B0 B00 O6 is represented by two types of octahedral B0 O6 and B00 O6, oriented in three dimensions of crystal lattice [12]. Pure phase materials are deeply studied for the colossal magneto resistance and multiferroic behaviour which was reported for some compositions. The presence of cations with multiple oxidation states opens the possibility to reduce the material and to increase the electronic conductivity through the formation of small polarons. Due to excellent electrical conductivity, a few double perovskite series were investigated with the substitution of isovalent and aliovalent cations at A, B0 and B00 sites [13,14]. Mo based systems show higher electrical conductivity [15], particularly those represented by Sr2MMoO6 (M ¼ Mg, Mn, Fe, Co, Ni, Zn etc.) which were

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studied in different environments (air/H2/H2-Ar/CH4) at intermediate temperature [16e19]. Therefore, several double perovskite systems are investigated as a potential candidate for anode materials for SOFC [6,20e24]. Among them, various doped materials play an important role in the improvement of novel anodes due to good compatibility with general electrolyte materials [14,25]. Fe doping in Sr2MgMoO6-d system was found to enhance the electrical conductivity with increasing doping concentration [15]. Li et al. [26] reported that Mn substitution on Mg site in Sr2Mg1xMnxMoO6-d system enhances the catalytic activity. Sr2NiMoO6d system is a potential candidate for anode material in SOFC due to its good electronic conductivity, high catalytic activity and thermal stability with other cell components [27]. The electrical conductivity of Sr2NiMoO6 has been found in range 0.1e250 Scm1 under various reducing atmospheres [28]. Wei et al. [25] investigated Sr2NiMoO6 and found stable electrical conductivity with thermal expansion coefficient matching that of LSGM electrolyte. It has been reported that some of rare-earth (La3þ, and Sm3þ) doping on Sr-site of Sr2MgMoO6 shows better electro-catalytic activity thanks to better fuel oxidation and electrochemical performance [29,30]. A few additional phases like SrMoO4 were observed in such systems, which may affect the structural and electrical properties [14,31e33]. Vasala et al. [11] reported that the impurity of SrMoO4 in Sr2MgMoO6 increases the electrical conductivity after reduction of SrMoO4 to SrMoO3. The SrMoO4 impurity was found to stabilize the evaporation of Mo during high temperature synthesis process of Sr2MgMoO6 [11]. In the present work the Sm-doped double perovskite system, Sr2xSmxNiMoO6 with x ¼ 0.01, 0.04 and 0.05, was investigated. The role of Sm on the structural behaviour of Sr2NiMoO6 was thoroughly studied by using XRD (X-ray powder diffraction), SEM (scanning electron microscopy) and XPS (X-ray photoelectron spectroscopy) and its effect on electrical conductivity was explored by electrochemical impedance spectroscopy in air and hydrogen. To the best of our knowledge, this paper forms the first report on the investigated system.

2.2. Characterizations The X-ray diffraction patterns were recorded at room temperature on grinded ceramics (Rigaku Miniflex II desktop, Cu-Ka radiation, D2q ¼ 0.02 , integration ¼ 1.7 s/point). The XPS spectra of all samples were taken on a KRATOS (Amicus model) highperformance analytical instrument utilizing Mg target under 1.0 106 Pa pressure. The morphology of fractured ceramics was recorded using ZEISS scanning electron microscope (EVO-18). Sintered pellets were silver coated and fired at 700  C for 20 min. The electrical impedance of all silver coated samples was recorded in air by two-probe method using Wayne Kerr (6500 P Series) LCR meter in the temperature range 250e600  C at an interval of 25  C and frequency range 20 Hz-1 MHz. The impedance measurements in reducing atmosphere were made with electrochemical interface (EI, mod. SOLARTRON 1286, Schlumberger) used in potentiostatic mode and connected to a Frequency Response Analyzer (FRA, mod. SOLARTRON 1250, Schlumberger), in a range of frequency between 101e105 Hz. Impedance data were collected at constant temperature after allowing enough time for equilibration. 3. Results and discussion 3.1. XRD The phase composition and crystal structure of the system Sr2were determined by powder X-ray diffraction at room temperature. Fig. 1(a)-(c) represent the X-ray diffraction patterns of the system Sr2-xSmxNiMoO6d with compositions 0.01  x  0.05. The peaks of major phase can be assigned to the

xSmxNiMoO6d

2. Experimental section 2.1. Synthesis process Polycrystalline ceramic samples Sr2-xSmxNiMoO6d with x ¼ 0.01, 0.04, and 0.05, abbreviated as SSNM01, SSNM04 and SSNM05 respectively, were synthesized using citrate-nitrate autocombustion route. Auto combustion technique is an extremely facile, time-saving and energy-efficient route for the synthesis. The potential advantages of this method over conventional methods include better homogeneity, better compositional control of stoichiometry, lower processing temperature and low cost. Stoichiometric amount of Sr(NO3)2 (99.9%), Ni(NO3)2$6H2O (99%), (NH4)6Mo7O24$4H2O (99%), Sm2O3 (99.9%) and citric acid were weighted accurately as starting materials. The Sm2O3 was dissolved into dilute nitric acid to get samarium nitrate. Further, all appropriate nitrates were fully dissolved into distilled water separately and mixed together to make homogenous solution. The pH value was adjusted to 6 by addition of ammonium hydroxide. The solution was slowly heated on a hot plate at 250  C under continuous stirring until formation of a gel, which transformed into a dried ash after self-ignition. In order to brake agglomerates, ignited powders were ground using agate mortar and pestle and calcined into alumina crucible at 850  C in air for 12 h to remove organics and obtain the oxide. After calcination, samples were ground and pressed into disk shaped pellet at optimum load of ~100 kg/cm2 and finally sintered at 1350  C for 12 h with heating rate of 4  C/ min.

Fig. 1. Rietveld refinement of X-ray diffraction pattern of different samples (a) SSNM01, (b) SSNM04 and (c) SSNM05. In this refinement, Yobs, Ycal, and Yobs-Ycal represent, respectively, the experimental data, calculated data, and the difference of experimental and calculated data.

P. Kumar et al. / Journal of Alloys and Compounds 725 (2017) 1123e1129

double perovskite tetragonal phase Sr2-xSmxNiMoO6d (s.g. I4/m) using JCPDS card no. 15-0601. An impurity phase is observed for all compositions and identified as tetragonal SrMoO4 (s.g. I41/a) using JCPDS card no. 85-0586. Based on previous literature the presence of impurity phase SrMoO4 (SMO) is common in this system [13,31,34]. The Rietveld refinement of XRD data for all SSNM-x samples is also shown in Fig. 1 (a)-(c), where the impurity phase is indicated with asterisk symbols. Peak profiles were modeled with pseudo-Voigt function and considering a six-coefficient polynomial for the background. The Rwp (R-weighted-pattern), Rp (R-pattern), and c2 values indicate that the refinement results are admissible and are reported in Table 1 together with lattice parameters and volume fraction of the SMO phase. It is important to observe that lattice parameters of Sm-doped samples do not change appreciably with Sm concentration. On the contrary, the amount of SMO increases with increasing Sm concentration.

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an increase of oxygen vacancies concentration in the lattice. Fig. 3(c) depicts the XPS spectra for Ni-2p. For all compositions the Ni-2p3/2 spectrum could be fitted with Ni2þ multiplets only, which allows to rule out the presence of Ni in the trivalent state (Ni3þ). This is in agreement with previous investigations [36,37] carried out on double perovskite systems like Sr2Fe1.4Ni0.1Mo0.5O6d and Sr2FeMo0.65Ni0.35O6d. 3.4. Electrical conductivity Fig. 4(a)e(c) show the impedance spectra measured in air at 450  C for the compositions SSNM01, SSNM04 and SSNM05 respectively. Data are represented as Nyquist plots of imaginary (Z00 ) vs. real (Z0 ) impedance and they are fitted by nonlinear least squares procedure to obtain the value of resistance. The total conductivity of the samples was calculated using the formula

s ¼ L=RS

3.2. SEM Fig. 2 (a)e(c) depict SEM images of fractured samples sintered at 1350  C. The figures clearly show that all the compositions bear fine grains of irregular shape and non-homogenous size. The average grain size of compositions SSNM01, SSNM04 and SSNM05 calculated by the linear intercept method was found to be 3.2 mm, 2.34 mm and 1.49 mm respectively. This trend can be considered as an effect of dopant segregation at the grain boundary, which inhibits the grain growth during sintering. In fact, segregation is known to be driven by the decrease in interfacial free energy that accompanies the densification process and is therefore a general feature of polycrystalline ceramics [35]. 3.3. XPS The XPS analysis was performed in order to study the oxidation state of different ions of powder samples. The XPS spectra corresponding to Mo-3d, O-1s and Ni-2p of SSNM-x samples are schematically shown in Fig. 3(a)-(c). The XPS data of all compositions were calibrated with respect to standard C-1s peak appeared at 284.6 eV. Fig. 3(a)-(c) show the complex Lorentzian peak fitting of elevated peaks. Fig. 3(a) clearly indicates that for all samples Mo-3d spectra are split into two asymmetrical peaks, corresponding to the mixed valence state of Mo5þ/Mo6þ. The peak parameters are given for each sample in Table 2. The relative amount of reduced Mo tends to increase with Sm doping. Fig. 3(b) shows the peaks of O-1s core level spectra, which appear asymmetrical for all compositions. The peaks of oxygen spectra were fitted corresponding to two kinds of oxygen varieties, viz. lattice oxygen and adsorbed oxygen. The intensity of lattice oxygen is decreasing with Sm doping, while the intensity of adsorbed oxygen is increasing. The presence of oxygen adsorbed at the surface is determined by several factors, including defects, and cannot be easily correlated to the concentration of Sm. On the contrary, lattice oxygen can be considered as direct indication of oxygen vacancies concentration. The percentage of lattice oxygen given in Table 2 shows that increasing in Sm doping causes

(1)

where s is the total electrical conductivity, L is the thickness of pellet, R is the resistance, and S is the surface area. Fig. 5 shows the Arrhenius plots (logsT vs. 1000/T) for all samples of system SSNMx. The activation energy was calculated from the slope of the above plot by applying the Arrhenius relation

s ¼ s0 =T$expðEa =kTÞ

(2)

where, s0 is pre-exponential factor, Ea is the activation energy for conduction, k is the Boltzmann constant and T is the absolute temperature. From this figure, it is clear that the conductivity increases by increasing the amount of Sm. The activation energy Ea for sample SSNM01, SSNM04 and SSNM05 was found to be 1.06 eV, 0.92 eV and 0.88 eV, respectively. Fig. 6 shows the comparison of Arrhenius plots in air and H2 for the composition SSNM05. This figure clearly indicates that conductivity is considerably increased in reducing atmosphere. Also, the activation energy is considerably affected by the reduction process, resulting in 0.32 eV. The experimental results reported above can be explained by means of defect chemistry. When introducing a heterovalent ion in the structure of a double perovskite, a charged defect is formed which needs to be compensated by other defects with opposite charge. In particular, the incorporation of Sm at Sr site corresponds to a positive defect and the electroneutrality condition requires the simultaneous formation of negative defects. Similar situations have been already described in literature and in all cases the charge compensation is modeled by the formation of electrons or, equivalently, by the reduction of cationic species [34]. However, when ceramics are treated in air the charge compensation mechanism modelled by formation of electrons is no longer valid because Mo reduction is limited by the high oxygen partial pressure. In addition, the formation of the secondary phase (SMO) is not taken into account by this model. XRD analyses suggest that all Sm atoms are incorporated in the perovskite phase, because no other secondary phase than SMO was detected. In spite

Table 1 Variation in lattice parameters, GoF parameters of Rietveld refinement and SMO volume fraction in the system SSNM-x (0.01  x  0.05). Samples

SSNM01 SSNM04 SSNM05

Lattice Parameters a ¼ b (Å)

c (Å)

Cell Volume (Å)3

5.5474(1) 5.5467(1) 5.5472(1)

7.8933(2) 7.8931(2) 7.8938(2)

242.9097 242.8397 242.9120

Rp

Rwp

c2

% of volume fraction of SrMoO4

16.0 15.4 16

13.7 12.6 12.8

4.92 3.70 3.51

3.0 3.4 4.0

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Fig. 2. Field emission scanning electron micrograph of fractured pellets of composition (a) SSNM01, (b) SSNM04 and (c) SSNM05.

of the smaller size of Sm3þ with respect to Sr2þ, the lattice constants of the SSNM phase are not affected by the amount of Sm doping, while on the contrary, the amount of SMO increases with the concentration of Sm. These results also cannot be explained by

the electronic charge compensation mechanism only. To address above mentioned limitations, the following model for defects structure in SSNM under oxidizing atmosphere is proposed:

Fig. 3. XPS spectra for compositions SSNM01, SSNM04 and SSNM05: (a) Mo-3d, (b) O-1s and (c) Ni-2p.

P. Kumar et al. / Journal of Alloys and Compounds 725 (2017) 1123e1129

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Table 2 Fitting results of Mo-3d and O-1s XPS spectra for the system SSNM-x (0.01  x  0.05). Samples

Mo6þ

Parameters

SSNM01

Binding Energy(eV) Area Binding Energy(eV) Area Binding Energy(eV) Area

SSNM04 SSNM05

Mo5þ

3d5/2

3d3/2

3d5/2

3d3/2

231.46 6427.16 231.70 4183.06 232.47 3974.97

234.69 5064.61 234.97 3768.77 235.58 3828.33

232.14 3885.51 232.45 3608.54 231.88 4355.06

235.48 4799.49 235.79 3689.52 235.23 3830.56

Mo5þ/Mo6þ

Lattice e O %

0.76

37.71

0.92

27.63

1.04

24.21

holds: 2Sr2 NiMoO6

00

000

000 2Sm2 O3 þ 3NiO þ MoO3












!4Sm$Sr þ 2VSr þ 2VMo

þ

6VO$$

þ

Mo Mo

þ

3Ni Ni

þ

12O O (3)

€ger-Vink notation is used to indicate lattice point defects where Kro and mass and charge balance are expressed for two formula units of the double perovskite structure. In Eq. (3) the incorporation of Sm at Sr site is associated to the formation of cationic and anionic vacancies. The following equilibrium among defects concentrations



h 000 i h 00 i    000 Sm$Sr þ 2 VO$$ ¼ 2 VSr þ 6 VMo

(4)

The simultaneous formation of vacancies, especially in the anionic sub-lattice can compensate the shrinkage expected by the incorporation of Sm, similarly to what happens in other doped perovskites like BaCeO3 [38,39]. Sr and Mo vacancies account for the presence of the secondary phase (SMO), which is formed at expenses of the main SSNM phase. Given the stoichiometry of SrMoO4, we can assume that:

Fig. 4. Nyquist plots for compositions: (a) SSNM01, (b) SSNM04 and (c) SSNM05 at 450  C, in air.

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Fig. 6. Arrhenius (logsT vs. 1000/T) plot for composition SSNM05 in H2 atmosphere and in air. In the inset the conductivity plot in air and H2 at 500  C.

Fig. 5. Arrhenius (logsT vs. 1000/T) plot of various samples of the system SSNM-x (0.01  x  0.05), in air.

h 00 i h 000 i 000 VSr ¼ VMo

(5)

Eq. (4) can then be rewritten as:



h 00 i 1    VO$$ ¼ 4 VSr  Sm$Sr 2

(6)

By assuming that molar concentration of Sr vacancies equals the one of SMO, Eq. (6) can be used to calculate the concentration of oxygen vacancies, as reported in Table 3. From Table 3, it is clear that oxygen vacancies increase with increasing doping, which is in agreement with XPS results (Table 2) and can account for the substantial insensitivity of lattice volume from Sm incorporation. Fig. 5 shows that by increasing Sm doping the conductivity is improved also under oxidizing conditions. The contribution to conductivity from SMO is negligible in air so that these features have to be explained by the defects structure of SSNM. Eq. (6) gives a rationale because the increase in the oxygen vacancies is expected to give higher ionic conductivity. The ionic character of conduction is also confirmed by activation energy values, which are in the range of 1 eV. Basically, activation energy for the oxide ion conduction process is in the range 0.5e1.0 eV [40]. In case of mixed ionic electronic conductivity, the electronic contribution shows much lower activation energy, in the range 0.1e0.2 eV, as reported for doped Ti and Cr perovskites [41,42]. When the material is exposed to reducing atmosphere, a different mechanism is established, i.e. the electron compensation described as: Sr2 NiMoO6

Sm2 O3 þ 6SrO þ 4NiO þ 4MoO3











!2Sm$Sr   þ 4Mo0Mo þ VO$$ þ 6SrSr þ 4Ni Ni þ 23OO þ O2

(7)

Eq. (7) includes the reduction of Mo as an effect of both Sm incorporation and formation of additional oxygen vacancies due to the low p(O2). The charge balance here is:



     Mo0Mo ¼ Sm$Sr þ 2 VO$$

(9)

Therefore, a significant increase of the electronic conductivity is expected because of the higher concentration of Mo5þ ions. Also, an increase in the ionic conductivity should take place because of the

formation of oxygen vacancies. The comparison of conductivity in air and H2, shown in Fig. 6 for sample SSNM05, is in agreement with the expected increase in conductivity and the change towards mixed ionic-electronic type conduction, as indicated by the lowering of activation energy. This result is also in agreement with previous results by Huang et al. [29] who reported high electrical conductivity (1.11 Scm1) for Sr2NiMoO6-d in reducing atmosphere at 800  C. Some contribution to conductivity can be expected in this case by the secondary phase, as reported by Vasala et al. [11] for the system Sr2MgMoO6. Under reducing conditions SrMoO4 is apparently reduced to the highly conductive SrMoO3 phase, which increases the electrical conductivity of the system. Surface analysis by XPS indicated that Mo5þ concentration increases with Sm doping even under oxidizing conditions. Some level of reduction in samples treated in air can be also deduced by activation energy which is slightly depressed when introducing Sm. This phenomenon might suggest that electron compensation is partially activated at high p(O2) but further investigations are needed to confirm such hypotheses. The conductivity result in H2 atmosphere indicates that SSNM05 composition is a potential candidate as anode material for SOFC. 4. Conclusions Double perovskite system Sr2-xSmxNiMoO6d (SSNM) with 0.01  x  0.05 was studied. Powders were successfully prepared by citrate-nitrate auto-combustion with SrMoO4 as a minor secondary phase. The amount of secondary phase was increasing with Sm doping, while lattice parameters of SSNM were not affected by concentration of Sm. Micro-structural studies revealed that for 5 mol % Sm grains have smaller size. The XPS on samples treated in air confirmed the presence of mixed valence state of Mo6þ/5þ and the formation of oxygen vacancies. The sample SSNM05 has the highest conductivity with lowest activation energy of 0.88 eV in air, which decreases to 0.32 eV in H2. A defects structure in oxidizing conditions was proposed to

Table 3 Oxygen vacancies molar concentration in the system SSNM-x (0.01  x  0.05) in oxidized state calculated with Eq. (6). Sm at. %

SMO vol. %

VSr at. %

VO at. %

1 4 5

3.0 3.4 4.0

4.12 4.74 5.54

15.99 16.94 19.65

P. Kumar et al. / Journal of Alloys and Compounds 725 (2017) 1123e1129

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