International Journal of Hydrogen Energy 33 (2008) 455–462www.elsevier.com/locate/ijhydene

Synthesis and characterization of bismuth vanadate electrolyte materialwith aluminium doping for SOFC application

Ravi Kant∗, K. Singh, O.P. PandeySchool of Physics and Materials Science, Thapar Institute of Engineering and Technology, Deemed University, Patiala 147004, Punjab, India

Received 19 June 2007; accepted 9 July 2007Available online 10 September 2007

Abstract

Solid electrolytes are the most important and indispensable part of a solid oxide fuel cell (SOFC) where hydrogen is used as one of the fuelsto obtain electricity. Bismuth vanadate used as an electrolyte in SOFC when doped with different divalent and trivalent metal ions provides goodionic conductivity. Higher temperature �-phase (600.700 ◦C) of bismuth vanadate which exhibits a higher ionic conductivity can be stabilizedat room temperature by various substitutions such as copper, aluminium and titanium and is denoted as BIMEVOX. In the present study a newseries of Bi4V2−xAlxO11−� (0�x �0.4) samples were prepared by taking an appropriate amount of constituent oxides. The sintered pelletswere characterized by a scanning electron microscope (SEM) and an X-ray diffraction (XRD) technique. An AC conductivity measurement ofall the samples in the temperature range of 200.700 ◦C was done. The conductivity measurement data exhibited a higher ionic conductivity forthe sample doped with aluminium for the composition x = 0.2 as compared with other doped samples of the same series. Since the grain sizeand its phase distribution influence the conductivity to a great extent, the sintering parameter for the sample x = 0.2 was varied between 750and 825 ◦C in an interval of 25 ◦C. The details of the conductivity behaviour of these samples along with their microstructural characteristicsare presented in this paper. The Arrhenius plots clearly indicate the various slope changes, which are in agreement with the phase transitionsthat occur in these samples. The results are discussed in the light of vacancy formation and disorder enhancement in the doped samples.� 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Solid electrolyte; Ionic conductivity; Solid oxide fuel cell; Phase transition

1. Introduction

Hydrogen, which is a future source of energy, finds its ap-plication as a fuel in fuel cells to generate electricity. In mostof the designs, the primary reaction involved is the oxidationof hydrogen by oxygen. In the system under development, foruse in the existing fossil fuel-based energy systems, the hydro-gen as reactant is produced by reforming gaseous hydrocarbon.Therefore, a growing use of high-efficiency fuel cell will con-tribute to a net reduction in the rate of CO2 emissions to theatmosphere by lowering the amount of hydrocarbons that mustbe oxidized per unit of energy produced. The fuel cell consistsof an electrolyte layer in contact with an anode and cathodeelectrode on either side of the electrolyte as shown in Fig. 1.

∗ Corresponding author.E-mail address: ravikant_mail@rediffmail.com (R. Kant).

0360-3199/$ - see front matter � 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2007.07.025

The electrolyte provides a physical barrier to prevent the directmixing of fuel and oxidant from the anode and cathode sidesrespectively, but allows the conduction of ionic charge betweenthe electrodes.

There has been a lot of research to develop materials for solidoxide fuel cell (SOFC) applications [1–6], which are given inTable 1. It is apparent from the table that bismuth vanadate-based materials are promising candidates for their applicationsin SOFC due to a high ionic conductivity at lower temperatureand stability with time.

The purpose of this paper is to develop a bismuth vanadate-based new oxide ion conducting material for SOFC with the aimof enhancing their performance by selecting proper dopants.

Solid electrolytes obtained by doping Bi4V2O11 are afamily of oxide ion conductors known as BIMEVOX [7–9].Since their discovery in 1988, the BIMEVOXes have beenthe subject of significant research due to their high oxide ion

456 R. Kant et al. / International Journal of Hydrogen Energy 33 (2008) 455–462

conductivity at relatively low temperatures. The aliovalentmetal cation doped Bi4V2O11 exhibits three crystallographicpolymorphs �, � and � in the range of room temperature to800 ◦C [9,10]. A number of studies have been reported duringrecent years on pure and doped Bi4V2O11. These studies havebeen very useful in understanding the electrical and structuralproperties of these materials [11–13]. Depending upon thevalency of the doping cation, the anionic vacancy rate can bemodified and, as a result, it influences the symmetry and theconnectivity of the polyhedra inside the perovskite like layer.Consequently, the ionic conductivity performances are alsoexpected to depend drastically on structural defects and theirtrapping effects on O2−-diffusion pathways. The anisotropy ofthe ionic conductivity study by Pho and Son [14] in a singlecrystal observed that conductivity values along the paralleldirection are about two orders of magnitude larger than theperpendicular direction. Almost all BIMEVOX phases that areessentially two dimensional O2− anionic conductors exhibit asimilar level of conductivity in the high-temperature � range,but striking differences can be observed at low temperaturesaccording to (i) the structural type stabilized at room tem-perature (�, �, �); and (ii) within a given structural type, thechemical nature of the dopant [15–17].

Electron

Flow

Load

Hydrogen

Water

Anode Electrolyte

Oxygen

lons

Cathode

Oxygen

Fig. 1. Solid oxide fuel cell.

Table 1Comparison of electrolyte materials for SOFC

Electrolyte Properties

Temperature ofoperation (◦C)

Ionic conductivity� (S/cm)

Other features Ref.

ZnO-doped YSZ 800 0.0289 Degradation of the cell componentsand interfacial reaction

[1,2]

GdAlO3, Ca-doped GdAlO3 1000 0.0132, 0.057 New material with little data on itsoxygen ion conductivity

[3]

ZrO2 co-doped with Sc2O3 and CeO2 800 0.120 High sintering temperature [4,5]BIMEVOX 400 0.02 Good stability over time, high density [6]

2. Structural aspects

The parent material (Bi4V2O11−�) crystallizes as �-phasewith orthorhombic symmetry at room temperature [9]. It ex-hibits an orthorhombic structure with cell dimensions a=5.53,b = 5.61, c = 15.25 A. On heating, the �-phase undergoes twostructural transitions to � (447 ◦C) and � (567 ◦C) polymorphs.The existence of two other phases, one �′ just before meltingduring heating and a second �′ during cooling, has also beenreported [9]. However, these phases have never been fully char-acterized.

The phase transitions are associated with vacancy orderingin the oxide sublattice and the crystallographic relationships ofthe various polymorphs have been characterized with respectto a mean orthorhombic unit cell of dimensions am ≈ 5.53,bm ≈ 5.61, cm ≈ 15.28 A, viz, �-orthorhombic, a ≈ 3am,b ≈ bm, c ≈ cm; �(orthorhombic), a ≈ 2am, b ≈ bm, c ≈cm; �(tetragonal), a ≈ am/

√2, c ≈ cm [18–20]. Among these

phases, the interest of most researchers is in the fully disordered�-phase, which shows an exceptionally high conductivity atrelatively low temperatures.

The structure of Bi4V2O11−� is close to the mean struc-ture of Bi2MoO6 [21] and is a member of the aurivlliusfamily. It consists of alternating layers of [Bi2O2]n2+

n and

[VO3.5−��0.5+�]n2−n (where � represents an oxide ion va-

cancy). The [Bi2O2]n2+n layers exhibit Bi in a square pyramidal

coordination with four Bi–O bonds of an approximate lengthof 2.3 A. The Bi 6s2 lone pairs are stereochemically active andpoint to vacant sites between the four corner sharing vanadiumpolyhedra in the vanadate layers.

Vanadium is in a regular octahedral environment with oxy-gen in the structure of Bi4V2O11 (Fig. 2). Vanadium octahedralshares corners to give the two-dimensional network. How-ever, the incorporation of significant numbers of vacanciesin the vanadate layer requires lower coordination numbersfor some of the vanadium polyhedra. In the crystal structureof �-Bi4V2O11−�, significant deviations from the idealizedstructure are observed [18]. The environment of vanadium in�-Bi4V2O11 was investigated by solid-state NMR studies [22].This effect is more pronounced for the parent compound thanfor the doped one and this is likely due to a slight reductionof VV into VIV [23]. In fact, recent studies suggest the like-lihood of both four and five coordinate vanadium polyhedra

R. Kant et al. / International Journal of Hydrogen Energy 33 (2008) 455–462 457

Fig. 2. Structure of Bi4V2O11−�.

[18]. Distortions within the vanadium polyhedra lead to a fer-roelectric behaviour with maximum permittivity at the � ↔ �phase transition [10,24]. Substituted Bi4V2O11−� representedas BIMEVOX quenched from high temperatures show the regu-lar �-phase structure with an absence of superlattice reflectionsand have enabled detailed studies of the defect structure of the�-phase to be carried out at ambient temperatures [25–27]. Sub-sequent studies on BIMEVOX using neutron diffraction con-firmed significant disorder in both apical and equatorial oxy-gen in the vanadate layers [28], and electrochemical reductionof the compound [29] was also investigated using a structuralmodel given by Mairesse [30] in order to explain order–disordertransition.

Over the number of years, detailed investigations of the de-fect structures of a range of divalent substituted �-BIMEVOXeshave been carried out using a combination of high-resolutionpowder neutron and X-ray diffraction (XRD) [25]. On the basisof electrical studies [31] and structure, a model for ionic con-duction based on the defect structure is proposed [32]. Studieson the quenched �-BIMEVOXes have confirmed that there isa significant disorder in the oxide sites in the vanadate layerand that the high-temperature tetragonal �-phase is preservedby rapid quenching. The disorder is limited to the vanadate

layer oxygens, while vacancies appear to be concentrated in theequatorial planes of these layers.

3. Experimental

Bi4V2−xAlxO11−� (0�x�0.4) powders were prepared bysolid-state reaction from stoichiometric amounts of the follow-ing oxides: Bi2O3 (99%), V2O5 (99%) and Al2O3 (99.9%). Thestarting powders were mixed in an acetone media by a mortarpestle for an hour to break any large agglomerates and then ballmilled for 2 h to achieve a fully homogeneous powder mixture.The resulting mixtures were dried and thoroughly ground andthen calcined at 700 ◦C in silica crucibles for 12 h in air. Cal-cined powders were ground, mixed and refired at 800 ◦C for12 h. The sintered powders were further ground and compactedat a pressure of 10 ton after mixing with a binder to make pel-lets of approximately same dimensions. The so prepared pel-lets were sintered at optimum temperature of 800 ◦C for 10 h.The X-ray diffractograms of the samples of the pellets of dif-ferent compositions were obtained by Rigaku (Model Geigerflex) at the scan speed of 5◦/ min. Differential scanning calori-metric (DSC) measurement was performed by Mettler Toledoat the heating rate of 10 ◦C/ min in air. Ionic conductivity wasmeasured by an AC impedance analyser technique with Model4274A multi-frequency Hewlett-Packard LCR meter in the fre-quency range of 100 Hz–100 kHz. The gold sputtered pelletswere used to carry out AC conductivity measurements in thetemperature range of 200–700 ◦C during the cooling cycle.

4. Results and discussion

4.1. X-ray diffraction

The structure of Al-doped Bi4V2−xAlxO11−� compoundssynthesized in the present study in the composition range0�x�0.4 was analysed by XRD study. The compound isorthorhombic with the �-form of the Bi4V2O11 for x�0.1while the room temperature unit cell is tetragonal of the �-typeBi4V2O11 in the range of 0.2�x�0.4. The XRD patterns ob-served for Bi4V2−xAlxO11−� (0�x�0.4) are shown in Fig. 3.

X-ray powder diffraction data confirmed that samples ofBi4V2−xAlxO11−� (0�x�0.4) are single phase. In XRD pat-terns of samples of the above series, initially peaks were ob-served to shift at lower angles from x = 0 (pure) to x = 0.1 andcould be indexed to an orthorhombic unit cell with lattice pa-rameters given in Table 2. The XRD patterns of x=0 (pure) andx = 0.1 of Bi4V2−xAlxO11−� at room temperature show veryweak reflections at 24.18◦ and 24.14◦, respectively, which isattributed to superstructure of �-form as reported in Ref. [33].

Data for compositions for x�0.2 were fitted to tetragonalmean cell. No significant changes are observed in XRD patternof samples of x = 0.2, 0.3 and 0.4. Interestingly, XRD patternsof 0.2�x�0.4 are indexed to the �-phase with decreasing lat-tice parameters of the cell, whereas other authors [16,34] re-ported �-phase for a value of x = 0.2. The gradual decrease incell dimensions ‘a’ and ‘c’ from x =0.2 to 0.4, except for somedeviation from curvature (Fig. 4) that is seen at x = 0.3 for

458 R. Kant et al. / International Journal of Hydrogen Energy 33 (2008) 455–462

2θ (degree)

In

ten

sity (

arb

. u

nits)

5.00 20.00 40.00 60.00 80.00 100.00

a

b

c

d

e

Fig. 3. X-ray diffraction pattern of Bi4V2−xAlxO11−�. (a) x =0, (b) x =0.1,(c) x = 0.2, (d) x = 0.3 and (e) x = 0.4.

Table 2Composition and lattice parameters of Bi4V2−xAlxO11−� (0�x �0.4)

Composition a (A) b (A) c (A)

x = 0 5.52 5.60 15.24x = 0.1 5.52 5.60 15.30x = 0.2 3.96 15.36x = 0.3 3.94 15.51x = 0.4 3.91 15.26

c-axis, reflects the substitution of VV by the smaller AlIII(0.59and 0.50 A, respectively) [35]. The maxima in the a-axis di-mension at x = 0.1 can be correlated to the stabilization of the�-phase above this composition range. The difference in varia-tion between basal a-parameter and the axial c-parameter (Ta-ble 2) reflects the fact that the ordering phenomenon observedin the BIMEVOXes is restricted to the basal plane through or-dering of equatorial oxide vacancies in the vanadate layer [26].

4.2. AC impedance measurement

Conductivity values were extracted from impedance com-plex plane plots. The impedance spectra obtained for BIALVOXare similar to those observed for other BIMEVOXes. The gen-

0

2

4

6

8

10

12

14

16

18

0 0.1 0.2 0.3 0.4 0.5

composition (x) in Bi4V2-xAlxO11-δ

lattic

e p

ara

mete

rs a

,b a

nd c

(A

°)

a

bc

Fig. 4. Composition vs lattice parameters (0�x �0.1 = �-phase) and(0.2�x �0.4 = �-phase).

-7.2

-6.7

-6.2

-5.7

-5.2

-4.7

-4.2

-3.7

-3.2

-2.7

-2.2

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2

1000/T(K-1)

log

σ (S

/cm

)

x= 0

x= 0.1

Fig. 5. Arrhenius plot of Bi4V2−xAlxO11−� (x = 0 and 0.1).

Heat flow

(E

nd

o)

100 200 300 400 500 600 700

Temperature (°C)

Fig. 6. DSC plot of pure sample.

eral features of the impedance spectra and their evolution withtemperature are similar for all of the studied compositions.For temperatures below 400 ◦C, the impedance spectra con-sisted of one semicircle. Contribution from the bulk sample andgrain boundaries could not be distinguished, except a broad-ened semicircle with a low-frequency spike. As the temperature

R. Kant et al. / International Journal of Hydrogen Energy 33 (2008) 455–462 459

-7

-6.5

-6

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

-2

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2

1000/T (K-1)

log

σ (S

/cm

)

x = 0.2

x = 0.3

x = 0.4

Fig. 7. Arrhenius plot of �-Bi4V2−xAlxO11−� (0.2�x �0.4).

Fig. 8. SEM photograph of fractured surface of (a) Al = 0.2 and (b) Al = 0.4sintered at 800 ◦C.

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0 0.1 0.2 0.3 0.4 0.5

composition (x) in Bi4V2-xAlxO11-δ

condu c

tivity σ

(S/c

m)

500°C600°C700°C

Fig. 9. Conductivity isotherm for Bi4V2−xAlxO11−� (0�x �0.4) forAl-doped materials with varying x.

Table 3The ionic conductivity at 500 ◦C and activation energies of Bi4V2−xAlxO11−�

Composition �500 ◦C (S/cm) Ea (eV) Density (g/cm3)

x = 0 7.7961 × 10−5 0.3427 6.2821x = 0.1 1.9869 × 10−4 0.2456 6.1158x = 0.2 6.6384 × 10−4 0.2256 6.4489x = 0.3 3.5493 × 10−4 0.2316 6.0596x = 0.4 2.5283 × 10−4 0.2400 6.2063

-7

-6.5

-6

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

-2

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2

1000/T(K-1)

log

σ (S

/cm

)

825°C775°C750°C

Fig. 10. Arrhenius plot of Bi4V2−xAlxO11−� (x = 0.2) at different sinteringtemperatures.

increases, the resistance decreases and higher frequencies arerequired to measure complete semicircles due to total conduc-tivity (bulk + grain boundary contributions). Since the avail-able maximum frequency of the impedance analyser is fixed,the semicircle corresponding to the bulk resistance may go onvanishing on the high-frequency side. At the highest tempera-tures, a spike only at very low frequencies associated with the

460 R. Kant et al. / International Journal of Hydrogen Energy 33 (2008) 455–462

Table 4Conductivity and activation energy at different sintering temperature of Bi4V1.8Al0.2O11−�

Sintering temperature (◦C) �400 ◦C (S/cm) �500 ◦C (S/cm) Ea (eV) < 400 ◦C Ea (eV) > 500 ◦C

750 7.1680 × 10−5 5.1200 × 10−4 0.3272 0.1972775 9.1522 × 10−5 6.2350 × 10−4 0.2928 0.2443825 4.5329 × 10−5 4.0362 × 10−4 0.2917 0.1949

0.000003

0.000006

0.000009

725 750 775 800 825 850

Sintering temperature (°C)

Co

nd

uctivity σ

(S

/cm

)

at 300°C

Fig. 11. Conductivity variation with sintering temperature ofBi4V1.8Al0.2O11−�.

polarization of the electrode is observed [12] and suggests thatthe compound is primarily an ionic conductor [36]. The con-ductivity behaviour of all the BIALVOX compositions gener-ally shows two linear regions with different activation energiesin their Arrhenius plots: a low-temperature region and a high-temperature region. The link between these two linear regionsis either in the form of a simple but distinct change in slope atabout 450–500 ◦C or a discontinuity around this temperaturerange. The particular behaviour adopted is composition depen-dent.

The transitions corresponding to � → � and � → � areevident in the sample of pure at 450 and 565 ◦C and at 440 ◦Cfor x = 0.1, respectively, as shown in Arrhenius plot (Fig. 5).This is in good agreement with the representative DSC plotat 452 and 565 ◦C for pure (Fig. 6). The slight discrepanciesobserved in the two different measurements indicate that thestructural changes involved in these transitions are sluggish innature.

The Arrhenius plots for the composition range (0.2�x�0.4)

corresponding to the �-phase are shown in Fig. 7, with small orsharp discontinuity but with different activation energies of thetwo regions, indicating the phase transitions at 450, 490 and460 ◦C for x = 0.2, 0.3 and 0.4, respectively. The highest andlowest values of the ionic conductivity are obtained with x=0.2and 0.4, respectively (Table 2), which are further supported byfractured surface scanning electron microscope (SEM) micro-graphs for x = 0.2 and 0.4. The grains are observed to be moreuniform and of a smaller size with lesser porosity for x = 0.2(Fig. 8a). The big hole existing on the right side of the micro-graph is because of material removal when the pellet for SEMstudy was fractured (Fig. 8a). The material is detached from

Fig. 12. SEM photographs of Al = 0.2 sintered at (a) 750 ◦C, (b) 775 ◦C and(c) 825 ◦C.

R. Kant et al. / International Journal of Hydrogen Energy 33 (2008) 455–462 461

this area. When compared to a sample of x =0.4 where cluster-type non-uniform pattern is observed for (Fig. 8b), this structure(Fig. 8a) exhibits a lesser amount of porosity and the patternis uniformly distributed throughout the structure. The conduc-tivity at different temperatures with respect to composition isshown in Fig. 9 which shows that the increase in conductivityis the highest for x = 0.2.

The decrease of ionic conductivity as shown in Table 3 andFig. 9 from x = 0.2 to 0.4 with increasing oxygen vacancyconcentrations can be attributed to the formation of defect pairs,i.e. the effect of higher concentrations of vacancies is offset bythe locally ordered states [37]. For the x = 0.2 phase, both thevacancy concentration and the pathways for O2− motion areoptimized.

Detailed conductivity along with SEM studies was carriedout further, for the high-conducting composition of x = 0.2 ofBi4V2−xAlxO11−� compound. In these studies, samples wereprepared by the same procedure but at different sintering tem-peratures of 750, 775 and 825 ◦C in addition to 800 ◦C. Theexperimental data are presented in the form of Arrhenius plotsin Fig. 10. Quantitative values of the activation energies wereextracted from these data plots.

The data do not show simple Arrhenius-type behaviour withsingle activation energy. The curves consist of two linear re-gions with different activation energies Ea, one at temperaturegreater than 500 ◦C and the other below 400 ◦C. In the inter-mediate temperature regions, a pronounced curvature is alsopresent in the data plots at 430, 420 and 440 ◦C of the pelletssintered at 750, 775 and 825 ◦C, respectively. For the linear re-gions below 400 ◦C and above 500 ◦C, Ea values along withconductivities at 400 and 500 ◦C are summarized in Table 4.

The highest conductivity is obtained for a sample sinteredat 800 ◦C for x = 0.2 as compared to all the other selectedtemperatures followed by 775 ◦C in the following sequence ofconductivity at 300 ◦C: �825 < �750 < �775 < �800 as shown inFig. 11. High conductivity obtained at 800 ◦C may be accountedfor a higher density of the sample at 800 ◦C, which is in goodagreement with the work of Krok et al. [38]. The above re-sults of conductivity variations are further supported by frac-tured surface SEM photographs of the samples at the studiedtemperature range as shown in Fig. 12a–c. A closer look at themicrographs shows higher porosity and non-uniform pattern ofgrains for the samples exhibiting low conductivity.

5. Conclusion

A new BIMEVOX series in which VV is substituted withtrivalent cation AlIII was studied for ionic conductivity in thecomposition range 0�x�0.4 at different sintering tempera-ture. �-phase is stabilized when the substitution is done in therange of 0.2�x�0.4 in Bi4V2−xAlxO11−� compound. Thehighest ionic conductivity was found for the Bi4V1.8Al0.2O11−�compounds with �700 ◦C = 3.1177 × 10−3 S/cm at sinteringtemperature of 800 ◦C as compared to samples sintered at750, 775 and 825 ◦C. This accounts for the higher densityof the sample sintered at 800 ◦C for x = 0.2. The highestionic conductivity obtained at a low temperature appears to

be dependent on structural type (�-phase) and dopant rate.In general, the conductivity behaviour is in the order of�0.1 < �0.4 < �0.3 < �0.2 with respect to composition range0�x�0.4 of Al in Bi4V2−xAlxO11−�. The increased con-ductivity trend is inconsistent with the trend of decreasedactivation energy (Ea : x = 0.1 > 0.4 > 0.3 > 0.2).

Acknowledgement

The financial support provided by All India Council of Tech-nical Education (AICTE), New Delhi (India), to carry out thework is highly acknowledged.

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