Effect of La doping on the electrochemical activity of double perovskite oxide Sr2FeMoO6 in alkaline medium

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Journal of Alloys and Compounds 484 (2009) 555–560

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

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ffect of La doping on the electrochemical activity of double perovskite oxider2FeMoO6 in alkaline medium

. Azizi, A. Kahoul ∗, A. Aziziaboratoire d’Energétique et d’Electrochimie du Solide, Université F. Abbas de Sétif, 19000 Sétif, Algeria

r t i c l e i n f o

rticle history:eceived 16 January 2009eceived in revised form 29 April 2009ccepted 29 April 2009

a b s t r a c t

The crystalline structure, grain morphology, electrical and electrochemical properties of Sr2−xLaxFeMoO6

(x = 0, 0.25, 0.5 and 1) double perovskite has been investigated by means of X-ray powder diffraction,scanning electron micrography, electrical and electrochemical measurements. It was found that thegrain morphology, the resistivity and the electrochemical activity are strongly influenced by La doping.

vailable online 6 May 2009

eywords:ouble perovskitea dopinglectrochemical activity

While the surface area as the determining factor in the oxygen reaction rate was excluded, the electricalresistivity was found to have a great effect on the electrochemical activity of the compounds.

© 2009 Elsevier B.V. All rights reserved.

urface areaesistivity

. Introduction

Electrochemical reduction and evolution of oxygen in alkalineedium are of considerable interest [1,2]. They are most important

rocesses in many electrochemical devices such as metal–air bat-eries [3], fuel cells [4] and water electrolysers [5]. Perovskite oxidesf the general formula ABO3 are an important class of materials thatas been extensively studied as electrocatalysts not only for thexygen evolution [6] but for its reduction as well [7,8], and can thuse used simultaneously as bifunctional electrode [9]. So far, bothnsubstituted (ABO3) and substituted (A1−xA′

xBO3, AB1−yB′yO3, etc.)

ystems have been investigated, almost all of which contain rare-arth or alkali earth cations at the A sites and 3d transition metalons at the B sites.

A subclass of perovskite oxide is represented by the generalormula A2BB′O6, better known as “double perovskite”, where As an alkaline-earth atom such as Sr, Ba or Ca, and B and B′ areransition-metal atoms. It has been reported that the electronoping in Sr2FeMoO6 (SFMO), achieved via the substitution of aivalent alkaline earth Sr2+ by a trivalent La3+ ion, will reducehe mean size of the A site cations. This produces an expansion

f the mean radius of the atomic species at (B, B′) sites of theA2−xA′

x)BB′O6 oxides. A rising of the Curie temperature (Tc) forotential technological applications has been reported [10]. Thisubstitution is associated with an increased disorder in Fe/Mo sub-

∗ Corresponding author. Tel.: +213 36 92 51 33; fax: +213 36 92 51 33.E-mail address: [email protected] (A. Kahoul).

925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2009.04.143

lattice of Sr2−xLaxFeMoO6 (SLFMO) [11], leading to a reduction ofthe saturation magnetization Ms [12]. These substituted oxideshave attracted considerable scientific and technological interestin recent years due to its reported room-temperature low-fieldmagnetoresistance (LFMR) [13,14]. The latter, related to the vari-ation of electrical resistivity when a magnetic field is applied, is aconsequence of the half-metallic character of the mentioned dou-ble perovskite. Depending on the synthesis conditions, the crystalstructure of SFMO can be cubic or tetragonal [15], where there is reg-ular arrangement of corner-sharing FeO6 and MoO6 octahedra. Theperfect alternating order of Fe and Mo ions in the octahedral sitespromotes the equilibrium reaction Fe(III) + Mo(V) ⇔ Fe(II) + Mo(VI)in which the itinerant minority spin electron is shared by bothtypes of atoms. This specific arrangement of alternating differentcations can be of great interest from the electrical and electro-catalytic point of view, since the properties of perovskites asconductors and electrocatalysts are generally determined by thenature, oxidation states and relative arrangement of B-site cations[16].

The origin of the catalytic activity is not yet fully understood, butseveral hypotheses have been suggested: (i) a relation between thecatalytic activity and the density of states at the Fermi level [17]; (ii)the influence of the metal–oxygen binding energy and the � backbonding from the oxygen to the neighboring cations [18]; (iii) the

presence of oxygen vacancies [19,20] and (iv) the importance of theelectrical conductivity [17].

From these points of view, the half-metallic double perovskitesseem to be good candidates to show interesting electrocatalyticproperties for oxygen reduction and oxygen evolution reactions.

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volume was also found to increase upon La doping. The ionic radiusof La3+ (1.36 Å) is smaller than that of Sr2+ (1.44 Å), and thereforethe cell expansion is not motivated by the steric effects associ-ated with the ionic sizes but it reflects electronic effects [10,23].

Table 1The unit cell parameters a, c and the unit cell volume (V) as a function of La dop-ing (x) in Sr2−xLaxFeMoO6 (obtained from X-ray diffraction refinements). Unit cellparameter values from Ref. [10] are also included.

56 F. Azizi et al. / Journal of Alloys

eeping this purpose in mind, we present the results concern-ng La doping effects on the structural, grain morphology, andlectric properties of Sr2−xLaxFeMoO6 oxides and investigate thelectrochemical behavior of such compounds prepared as oxygenlectrode films. To the best of our knowledge, no report dealing withhe electrochemical activity of these double perovskites for oxygeneactions exists.

. Experimental details

Polycrystalline Sr2−xLaxFeMoO6 (0 ≤ x ≤ 1) samples have beenrepared by standard solid state reaction. Stoichiometric amountsf SrCO3, Fe2O3, MoO3 (Aldrich) and La2(CO3)3 (Aldrich) wererounded together and the resulting mixture was pressed into pel-ets for 5 min at 7 tons/cm2. The pellets were sintered at 1100 ◦Cn a platinum crucible for 3 hours in a stream of 5% H2/N2.oom-temperature X-ray diffraction (XRD) patterns were collectedsing a Siemens D-500 diffractometer equipped with a primaryeam quartz monochromator (Co K�1 = 0.178897 nm). Using the-FIT program [21], the cell parameters a and c of the tetrago-al structure were refined. Quantitative elemental analysis of Fe,o, La and Sr and oxide morphology were examined by energy-

ispersive spectroscopy (EDS) and scanning electron microscopySEM), respectively, using a JEOL (JSM840) scanning electron micro-cope.

The sample specific surface area (SBET) was measured onowders by the BET method using a Perkin-Elmer Shell Sorp-ometer. Electrical resistivity � of the samples was measured atoom temperature on pressed pellets using a Z-computer Tacusselmpedancemeter and a PJT 120-1 Tacussel potentiostat–galvanostat

ith a PE 8116 Sefram recorder. In order to make the electricalontacts, conducting silver paint was applied on both sides of theellets.

The electrochemical experiments for O2 reduction and evolutionere performed using a Volta Lab 40 potentiostat/galvanostat con-

rolled by a PC. The electrochemical measurements were carried outn a three-compartment cell. Potassium hydroxide electrolyte solu-ion (1 M) was prepared by dissolving the required amount of KOHMerck) into bidistilled water. The working electrodes (1 cm2) werebtained by painting, with an oxide suspension in isopropanol, aolyisobutylene foil (Nikolaus Branz, Berlin) charged with graphitef the type used in the battery industry [22]. To evaporate theolvent, the samples were then dried in an oven at 80 ◦C for 1 h.he conducting and inactive carbon material towards oxygen reac-ions was used to ensure that no electrochemical activity is dueo the substrate. The latter, before being coated, was mechanicallyolished with sand paper (grade 1200) and then degreased withcetone. The loading of catalyst films was 12 ± 2 mg cm−2 in eachase. The counter electrode used was a Pt plate with large surfacerea. A mercury/mercurous oxide electrode (Hg/HgO, 1 mol/dm3

OH) was used as the reference electrode.

. Results and discussion

X-ray diffraction patterns recorded on Sr2−xLaxFeMoO6 pow-ers with different La content x are presented in Fig. 1. All intenseiffraction peaks can be identified according to a double perovskitetructure with tetragonal I4/mmm symmetry. Results [10] haveeported that a structural phase transition from a tetragonal to anrthorhombic lattice can occur for x > 0.4. The difference between

he tetragonal and the orthorhombic structure may be difficult toetect in the present study since the temperature and the annealingime are lower than in Ref. [10]. Observed additional Bragg peaksorresponding to a minor amount of the non-conducting SrMoO4mpurity phase were also observed. The pattern of the SFMO parent

Fig. 1. X-ray diffraction patterns for Sr2−xLaxFeMoO6 (x = 0, 0.25, 0.50 and 1). Thestar indicates the reflexion of SrMoO4 impurity phase.

compound revealed a weak super cell reflection (1 0 1), observedat 22.4◦. This latter disappears for all La doped samples indicatingan increased disorder of the Fe and Mo ions. Table 1 summarizesthe values of the lattice parameters a and c and the volume V ofthe tetragonal cell as a function of the doping level x in the SLFMOcompounds. It is clearly seen that La doping leads to an increase ofthe cell parameter a while c remains nearly unchanged. Values ofparameter a from the paper of Navarro et al. [10], using X-ray diffrac-tion analysis, have been also included for comparison. The cell

La content, x a (Å) a (Å) (Ref. [10]) c (Å) c (Å) (Ref. [10]) V (Å3)

0 5.5682 5.5705 7.8668 7.8970 243.12700.25 5.5749 5.5750 7.8737 7.9150 244.71890.5 5.6107 5.6050 7.8668 7.8800 245.97281 5.6290 5.6150 7.8440 7.9105 246.4144

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Fig. 2. SEM micrographs of Sr2FeMoO6 (a), Sr1.75La0

ndeed, La doping reduces the average valence of the Fe cations,hus increasing their ionic radii.

In Fig. 2, showing SEM pictures, we compare the morphologyf the oxides. The samples with higher La content (x = 0.5 and 1)Fig. 2c and d) exhibit sphere-shaped small grains which appearo be better separated while in samples with lower x (x = 0 and.25) (Fig. 2a and b) the grains are much larger and with no well-efined shape. Lanthanum seems to act as a nucleation center forhe growth of the perovskite crystalline phase. As the La content isncreased, the number of such nucleation centers increases as well,

hich effectively raises the number of grains with small sizes. Toxamine grain-to-grain chemical homogeneity and determine ele-ental composition of the compounds, EDS microanalyses were

erformed on different powder surface regions for each sample.he EDS spectrum corresponding to Sr1.5La0.5FeMoO6 sample indi-ated only Sr, La, Fe, Mo and O peaks (not shown here). The actualhemical composition of all the samples was determined and thetomic concentration for each chemical element was found in goodgreement with the nominal values.

Fig. 3 shows the variation of BET surface area of ther2−xLaxFeMoO6 series. In accordance with the evolution of theirrain size (c.f., Fig. 2), the surface values increase monotonicallyith increasing x. These values are relatively low and the largest

ne is obtained for the higher La doped sample. As can be seen

oO6 (b), Sr1.5La0.5FeMoO6 (c) and SrLaFeMoO6 (d).

below, evolution in electrode activity cannot be explained in termsof specific BET areas.

For an application of the compounds as electrocatalysts it is ofcourse necessary that these materials should be conductive. Theresistivity of the samples was therefore tested as function of Ladoping. Results from resistivity measurements, at room temper-ature, obtained by impedancemetric and voltammetric methodsare presented in Fig. 4. The resistivity of the undoped sample ishigher than those of the La doped samples and is very close tothat reported by Niebieskikwiat et al. [24] for Sr2FeMoO6.04 calci-nated under an Ar–H2 mixture. In doped compounds, � decreasesnotably with increasing La content x up to x = 0.5, then increasesfor x = 1. The decrease with the La content could be understood byconsidering the additional electron of La3+ with respect to Sr2+. Kimet al. [25], using GdBa1−xSrxO6+� double perovskite as cathode forsolid oxide fuel cells, have reported that the electrical conductivityof the compounds increases and then decreases with Sr content.This increase followed by a decrease is accompanied by a struc-tural change from orthorhombic (x = 0) to tetragonal (0.2 ≤ x ≤ 0.6)

and then to orthorhombic (x = 1). No structural evolution has beenobserved in our compounds. The drop in resistivity for x = 1 in ourcase may be understood based on the increase of the anti-site con-centration, i.e. increased disorder in Fe/Mo sublattice as mentionedin our previous work [26] and as reported in literature [27,28].

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558 F. Azizi et al. / Journal of Alloys and Compounds 484 (2009) 555–560

Fig. 3. Specific surface area SBET dependence on lanthanum content (x) forSr2−xLaxFeMoO6 series.

Fig. 4. Electrical resistivity � dependence on lanthanum content (x) forSr2−xLaxFeMoO6 series.

Fig. 5. The i–E polarization curves of oxygen reduction (a) and evolution (b) forSr2−xLaxFeMoO6 electrodes in 1 M KOH (currents are based on geometric areas).

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F. Azizi et al. / Journal of Alloys a

It seems also important to point out that the SrMoO4 spurioushase does not form in the grain boundaries, which would lead ton enhancement of the resistivity for the SrMoO4 rich samples [24].

The oxygen reduction and evolution reactions according to:

2 + 2H2O + 4e− ⇔ 4OH−(E◦ = 0.401 V vs. SHE)

ere chosen as the test reactions, operating as main processes inuel cells, metal–air batteries, or alkaline water electrolysers. Thepen-circuit potentials of the tested electrodes series, in air, shiftedrom −100 mV vs. Hg/HgO for Sr2FeMoO6 to about −30 mV for thether compositions. However, under pure oxygen flow, these val-es increased slowly to reach an average value of 100 mV for allompositions. The electrochemical activity for both oxygen reduc-ion and evolution reactions was investigated on Sr2−xLaxFeMoO6oated graphited isobutylene substrate C. Polarization studiesnder potentiostatic conditions were carried out. According to O2volution or reduction, the electrode material was polarized at aower potential and then the potential of the test was increaseddecreased) in constant steps in the anodic (cathodic) directionnd the corresponding steady value of the current to each potentialas measured. The time taken for the current to reach steady stateas observed to vary with applied potential: it was 10–15 min at

ow overpotentials and 5-8 min at high overpotentials. Fig. 5a andshows, respectively, the cathodic and anodic current-potential

urves plotted for the C/Sr2−xLaxFeMoO6 and for the bare support.he coated electrode films showed good adherence during polariza-ion. The very small current, observed for the bare support, is related

o the electrode capacitive behavior. Electrode reactions over theurface of the oxides exhibit high currents. Compounds with large xalues (i.e. x = 0.5 and 1) show simultaneously higher cathodic andnodic currents than those with smaller x. Compared to the x = 1,

ig. 6. Electrode performance as a function of x at E = −500 and 850 mV for reduction�) and evolution (©), respectively, for Sr2−xLaxFeMoO6 series.

mpounds 484 (2009) 555–560 559

the Sr1.5La0.5FeMoO6 one appears to be more active. It is importantto point out that the Pt electrode remains the best material for O2reduction. Therefore, a further activity enhancement (equal at leastto that of Pt electrode) of the sample with x = 0.5 should be requiredby improving the electrical behavior. By way of illustration, Fig. 6shows the cathodic and anodic current densities obtained at poten-tials of E = −500 and 850 mV. The highest electrode performance isachieved, for both cases, with x = 0.5. It is noteworthy to point outthe evolution of the resistivity (cf. Fig. 4) and the parallel trend of theoxygen evolution and reduction rates, displaying simultaneouslythe highest values of the electrochemical activity of both reactionsand the lowest resistivity at x = 0.5. This parallel variation, mostlyestablished for the electrochemical activity and the resistivity ofperovskite-type oxides [25,29,30] supports our results obtained onthe present compounds. Therefore, the surface area of the com-pounds as the determining factor in the reduction and evolutionrates should be excluded, as the electrochemical activity of the x = 1electrode does not increase with its surface area. Such a correlationbetween the activity and the resistivity evinces the key role of theelectrical parameter as a determining factor in the reaction kinet-ics. Indeed, this fact is explained as a function of the better overlapbetween Fe and Mo orbitals [31,32] and implies a better electrontransfer between neighboring Fe and Mo cations, between the twoextreme electronic configurations (Fe2+–Mo6+ versus Fe3+–Mo5+).

4. Conclusions

This work was undertaken for the purpose of elucidating theinfluence of the crystalline structure, grain morphology, electricalproperty on the electrochemical activity of Sr2−xLaxFeMoO6 (x = 0,0.25, 0.5 and 1) samples. The latter have been synthesized via asolid-state reaction process. The structural study for all compounds,presenting a minor amount of SrMoO4 impurity, indicates that allintense diffraction peaks can be indexed according to a doubleperovskite structure with tetragonal I4/mmm symmetry. La dop-ing causes an increase of the cell volume and the cell parameter awhile c remains unchanged. As we increase La content the numberof nucleation centers increases as well. These nucleation centersraise the number of grains with small sizes. The resistivity of thedoped compounds is lower than those of the parent compound.This decrease with La content could be attributed to the effectarising from the electron-doping. Compared to all studied composi-tions, Sr1.5La0.5FeMoO6 electrode exhibits a greater electrochemicalactivity, indicating that this material is the best electrocatalyst forboth oxygen reduction and evolution. The increase in activity doesnot originate from the surface area effect, but is merely due to theresistivity being a much more effective parameter which controlsthese electrochemical processes. In this context, Sr1.5La0.5FeMoO6electrode film showing the best activity appears to be a promis-ing bifunctional material for the cathode in fuel cells and theanode in electrolysis cells. The work is being completed by furtherexperiments to improve the electrical behavior for developing newmaterials to optimize renewable energy sources.

Acknowledgment

This study has been supported by the Algerian Ministry of HigherEducation and Scientific Research.

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