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Characterisation of the electrode–electrolyte BIMEVOX system for oxygen separation
Solid State Ionics 159 (2003) 181 – 191 www.elsevier.com/locate/ssi
Characterisation of the electrode–electrolyte BIMEVOX system for oxygen separation: Part II. Thermal studies under controlled atmosphere C. Pirovano *, R.N. Vannier, G. Nowogrocki, J.C. Boivin, G. Mairesse Laboratoire de Cristallochimie et Physicochimie du Solide, Universite´ des Sciences et Technologies de Lille, UMR 8012, ENS Chimie Lille, B.P. 108, Villeneuve d’Ascq Cedex 59652, France Received 3 November 2002; received in revised form 21 January 2003; accepted 24 January 2003
Abstract The reduction of Bi4V2O11 and BIMEVOX powders was investigated using two methods: thermogravimetric analyses and high-temperature X-ray diffraction under hydrogen flow. The chemical reduction, due to a VV – VIV reduction, was compared to the evolution of cathodes of dense BIMEVOX membranes under current bias which had previously been characterised using in situ X-ray synchrotron diffraction. This correlation enables to quantify the level of reduction reached on Bi4V2O11 membranes under operating conditions. This explains the oxygen transfer in the materials, at least at the cathode under bias. D 2003 Elsevier Science B.V. All rights reserved. Keywords: BIMEVOX; Oxygen separation; Thermogravimetry; Thermodiffraction; Oxygen
1. Introduction Oxygen ion-conducting ceramics represent an attractive class of materials, world-wide investigated for use in practical devices such as solid oxide fuel cells (SOFC), catalytic membrane reactors and ceramic oxygen generators (COG). Yttria-stabilised zirconia (YSZ) is the most commonly used electrolyte in these applications. However, its domain of use is typically between 800 and 1000 jC. These rather high temperatures give rise to technological problems. Two trends are considered in order to design new devices working * Corresponding author. Tel.: +33-3-20-43-65-83; fax: +33-320-43-48-95. E-mail address: [email protected] (C. Pirovano).
at lower temperature. The first one is for keeping the well-known zirconia as electrolyte but decreasing the thickness of the membrane in order to decrease its resistance at lower temperature. The second way is for developing a new material with higher oxygen ion conductivity. Mainly three classes of materials are known to have better conducting properties than YSZ in air: doped lanthanum gallate, doped ceria and bismuth-based phases. In this field, the so-called BIMEVOX family displays the best properties at moderate temperatures (300 –600 jC). It derives from Bi4V2O11 by partial substitution for vanadium with a variety of metals (ME). Depending on the temperature, Bi4V2O11 exhibits three polymorphs denoted by a, h and g which have been widely described (see, for example, Ref. [1]). The high-temperature g-prototype
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is the most disordered form and thereby the most interesting for its conducting properties (r>10 1 Scm 1 at T >500 jC). These properties are mainly due to the bidimensional character of its structure which consists of highly covalent [Bi2O2]2+ layers alternating with highly disordered oxygen-deficient perovskite-like sheets [VO3.550.5]2 . This g-type structure can be stabilised at room temperature by appropriate doping on the vanadium site with other aliovalent or isovalent cations, which prevent ordering phenomena in the perovskite slabs and, as a result, the formation of the h and/or a polymorphs. The acronym BIMEVOX is used and BIMEVOX.x corresponds to a Bi2V1 xMExO5.5 d composition. Several BIMEVOX materials have been tested as membrane for ceramic oxygen generators. High oxygen flows were obtained using membrane simply made of a dense BIMEVOX electrolyte sandwiched in between two gold grids acting as current collectors [2]. It was shown that, in contrast to classical electrolytes (YSZ, stabilised ceria, etc.), in the case of BIMEVOX materials, the oxygen transfer occurred at the electrolyte surface itself. Indeed in classical devices, the kinetics of transfer at the electrolyte surface are usually slow and electrode materials which are mixed (electronic and ionic) conductors must be added to allow the oxygen transfer into the membrane. To understand this unusual behaviour of BIMEVOX materials, the same membranes were characterised under operating conditions by X-ray synchrotron radiation [3]. Three compositions were checked, a slightly Bienriched non-doped phase (BIBIVOX.02), a BICOVOX and a BICUVOX. At 600 jC, the structure of these materials can be described in a tetragonal unit ˚ and ci15.4 A ˚ . Under cathodic cell with ai3.9 A polarisation, an evolution of the unit cell parameters corresponding to the BIMEVOX phases at the surface membrane was clearly observed. A decrease of 0.45% for a and an increase of 0.35% for c were noticed for BIBIVOX.02. The evolution was smoother for the doped compositions. The same evolution of the unit cell parameters is observed when studying these materials under reducing atmosphere. Owing to the susceptibility of bismuth-based materials to reduction under certain atmospheres, these materials cannot be used directly in a fuel cell but they
exhibit attractive properties, which could be fully exploited under nonreducing conditions as in oxygen separation. Bi4V2O11 is able to lose some oxygen when the experimental environment is modified by thermal treatment and/or by variations in oxygen partial pressure, leading to a Bi4V2O11 d compound. This oxygen non-stoichiometry is related to a variation in the VIV content within the compound. Under hydrogen atmosphere, this VV – VIV reduction is limited to one third of the V cations leading to the formation of Bi6V3O16 (Bi4V2O10.66) phase before ultimate destruction into Bi and V2O3. The crystal structure of this phase was solved by Joubert et al. [4]. It can be considered as an ordered form of Bi2(V1V x VIV x )O5.5 x/2 with x = 0.33. The bidimensional structure of BIMEVOX is maintained and from a structural point of view, the main difference concerns the vanadium sheets, which transforms into independent ribbons of one octahedron in between two tetrahedra. When studying these materials under operating conditions by X-ray diffraction, because of grain orientations and the low oxygen X-ray diffusion factor compared to bismuth, it was not possible to extract any further information other than the unit cell parameters [3]. Their evolution under cathodic polarisation is likely due to a reduction of VV into VIV. In this paper, the transformations of BIMEVOX compounds under controlled atmosphere were investigated by thermogravimetric analyses and X-ray thermodiffraction. The evolutions observed under chemical reduction were then compared to those noticed on the cathode of a dense membrane using in situ X-ray diffraction at the ESRF [3].
2. Experimental Powders of Bi4V2O11 or BIMEVOX (ME = Co, Cu mainly) were prepared by conventional solid state synthesis from commercial products as previously described [2]. High-temperature X-ray diffraction was carried out on a Siemens D5000 diffractometer, Cu Ka radiation, equipped with a HTK 1200 Anton Paar device and a PSD detector. The calibration in temperature of the heating device was checked prior to the experiments
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using gold powder. For characterisation under reducing atmosphere, the samples were heated under air atmosphere from room temperature up to 350 jC with a 0.2 jC/s rate. This temperature was chosen in order to ensure a limited modification speed. The atmosphere was then purged from oxygen with nitrogen over about 45 min. After a first reference X-ray diffractogram under N2, the sample was then studied under a N2/H2 flow (2.5/2.5 l/h, 1 atm). A diffractogram was recorded every 15 min in the 10– 70j 2h domain, with a 5-min delay before each measurement, a step of 0.0146j and a counting time of 0.15 s (i.e. a diffractogram was recorded in about 10 min). BIBIVOX.02 was also studied at 600 jC. In this case, the measurement was recorded with the same conditions under a N2/H2 3% flow (5 l/h) using a commercial gas mixture. Thermogravimetric analyses were carried out on a Setaram 92-16.18 thermobalance under a gas flow of 1 l/h. All the analyses were performed on samples which were previously annealed for 12 h at 800 jC and slowly cooled down to room temperature under oxygen. This was to make sure that the starting powders were fully oxidised (d = 0 using the Bi2VO5.5 d formula, for example). The measurements under air were carried out in a platinum crucible containing about 250 mg powder. In order to avoid any mass loss, which could result from moisture absorption, the samples were, prior to the analysis, in situ treated for 1 h at 300 jC in the thermobalance; the temperature being chosen after preliminary tests showed that no oxygen loss occurred. They were then heated from room temperature up to 800 jC with a 5 jC/min rate and cooled down with the same rate after 2 h at 800 jC. The measurements under reducing atmosphere were carried out in a silica crucible using about 150 mg powder. They were performed under isothermal conditions at 350 jC in a N2/H2 3% flow. Only 3% of hydrogen was used as safety precaution, in order to prevent chemical reaction between eventually formed metallic Bi and the platinum of the thermobalance. The increase in temperature to 350 jC was performed under the same flow at a 10 jC/min rate in order to avoid any artefact due to the change of flow. All the measurements were systematically corrected from the Archimede pressure by subtraction of a blank measured under the same conditions.
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Both for the HTDX and TG analyses, experiments were performed with dry hydrogen in order to be in the must reducing conditions.
3. Results and discussion 3.1. Thermogravimetric measurements 3.1.1. Oxygen non-stoichiometry under air The first experiments were performed under air from 20 up to 800 jC. Fig. 1 exhibits the results obtained for Bi4V2O11 and three doped materials (BICOVOX, BICUVOX, BIZNVOX). In the case of Bi4V2O11, as can be observed in Fig. 1a, the
Fig. 1. TGA analyses under air (a): Bi4V2O11, (b): BIMEVOX materials (ME: Co, Cu, Zn).
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material underwent a very small oxygen evolution. The oxygen loss started above 500 jC. The decrease of the oxygen stoichiometry continued until 800 jC (the maximum measurement temperature) but did not increase after 2 h at 800 jC. The transformation was reversible on cooling and the initial oxygen stoichiometry was recovered on returning to room temperature. The oxygen non-stoichiometry remained very low with d = 10 10 3 for Bi2VO5.5 d at 800 jC. This result is in agreement with the one obtained by Joubert [5] for a similar measurement. For the doped materials (Fig. 1b), the same evolution was observed but with a lower variation of oxygen stoichiometry. Table 1 displays the oxygen non-stoichiometry reached for several BIMEVOX materials heated in air up to 800 jC. The oxygen loss occurs between 500 and 800 jC. In general, BIMEVOX phases appear to be less sensitive to reduction than the undoped parent compound, except for BICUVOX and BIMNVOX materials, which exhibit stoichiometry variation comparable to that of Bi4V2O11. A similar study was performed by Audinot [6] on La1 xSrxFe1 yCoyO3 d (LSFC), La1 xSrxFe1 y NiyO3 d (LSFN) perovskite phases and LaGa1 x NixO3 (LGN) gallate lanthanum phase. In the case of LSFC and LSFN, the oxygen losses by heating started at 500 jC and were about 1% to 1.4% between 500 and 800 jC. For LGN, the oxygen losses began at higher temperature around 800 jC and were about 0.7% between 800 and 1050 jC. Whereas in the case of BIMEVOX materials, the losses are at maximum 0.2% for Bi4V2O11 and BIMNVOX.10
Table 1 Oxygen atom losses for different BIMEVOX under air from 500 to 800 jC Compound
Formulation (stoichiometric compound)
Oxygen losses (per mole of compound) between 500 and 800 jC d( F 2 10 3)
Bi2VO5.5 BICOVOX.10 BICUVOX.10 BINIVOX.10 BIZNVOX.10 BIMNVOX.10 BINBVOX.25
Bi2VO5.5 Bi2V0.9Co0.1O5.35 Bi2V0.9Cu0.1O5.35 Bi2V0.9Ni0.1O5.35 Bi2V0.9Zn0.1O5.35 Bi2V0.9Mn0.1O5.40a Bi2V0.9Nb0.1O5.5
10 10 4 10 7 10 2 10 2 10 10 10 4 10
a
3 3 3 3 3 3 3
For the manganese with an oxidation state of III.
and are about 0.1% for the other BIMEVOX studied. LSCF, LSFN and LGN are mixed conductors. The huge difference in oxygen losses is an indication of the low electronic contribution to the conductivity in BIMEVOX phases. 3.1.2. Thermogravimetric measurements under reducing atmosphere The thermogravimetric analysis performed on a non-doped phase, Bi2V0.98Bi0.02O5.48 (BIBIVOX.02), under N2/H2 3% atmosphere at 350 jC is reported in Fig. 2. An enlargement of the starting evolution is given. Zone corresponds to the increase in temperature to 350 jC. Next, a slope change is observed between zones and , this breaking-down point coincides with the oxygen stoichiometry corresponding to a Bi4V2O10.66 type phase with 2% Bi enrichIV III ment, that is to say Bi 2VV 0.653V 0.327 Bi 0.02O 5.317 . Beyond this stage (zone ), the reduction process increases considerably. It likely corresponds to the beginning of the material decomposition as proved by X-ray analysis of the powder after experiment, which revealed bismuth metal and V2O3 to be present. Similar experiments were carried on BIMEVOX phases (BICOVOX, BICUVOX, BIZNVOX). In those cases, plateaux were observed after a decrease of the oxygen stoichiometry (Fig. 3), indicating a better stability of the doped material toward reduction. The corresponding evolutions of the oxygen stoichiometry are reported in Table 2. The experiments were performed at 350 jC under N2/H2 3% atmosphere (1 l/h) for 30 h or more, but since the starting powder grain size was not standardised, no conclusion can be drawn about the relative reduction rate of the three studied materials. The difference in behaviour between the different samples could be due to surface kinetics effects and possibly will also depend on the flow rate of the gas. However, the same tendency as under air atmosphere was observed: a quite important change for BICUVOX, compared to BICOVOX and especially BIZNVOX. In contrast to the non-doped phase, X-ray diffraction performed after these TGA analyses revealed no extra phase. The structure of the materials was maintained except for BICUVOX for which a change of symmetry from tetragonal to orthorhombic was observed.
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Fig. 2. TGA analyses under N2/H2 3% at 350 jC for the BIBIVOX.02 sample.
3.2. High-temperature X-ray diffraction under reducing atmosphere 3.2.1. Bi4V2O11 and the solid solution Bi2V1 y BiyO5.5 d The diffractograms corresponding to the reduction of a BIBIVOX.02 sample performed at 350 jC under H2/N2 atmosphere are reported in Fig. 4. Its behaviour is similar to that observed in the case of the reduction of Bi4V2O11. This process has already been described in details [1,7]. Therefore, we will
just recall it briefly. At the beginning, the characteristic (hkl)/(khl) doublets of the orthorhombic form of Bi4V2O11 progressively merge into a unique reflection characterising the transformation to a tetragonal polymorph. Using the general formalism adopted to describe the BIMEVOX compounds, this phase corresponds to a BIVIVVOX with formula Bi2V1V x VIV x O5.5 x/2. Then, new Bragg peaks corresponding to a-Bi4V2O10.66 begin to evolve. This new phase can be considered as an ordered form of Bi2V1V x VIV x O5.5 x/2 with x = 0.33. The two phases coexist for about 3 h but the amount of Bi4V2O10.66 progressively increases while that of the tetragonal phase concomitantly decreases. If the reduction is further pursued, the Bi4V2O10.66 phase would pro-
Table 2 Oxygen atom losses for different BIMEVOX at 350 jC under N2/ H2 3%
Fig. 3. TGA analyses under N2/H2 3% at 350 jC for the BIMEVOX materials (ME: Co, Cu, Zn).
Compound
Formulation (stoichiometric compound)
Oxygen loss (per mole of compound)
BICOVOX BICUVOX BIZNVOX
Bi2V0.85Co0.13Bi0.02O5.285 Bi2V0.9Cu0.1O5.35 Bi2V0.9Zn0.1O5.35
0.09 0.19 0.05
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Fig. 4. Reduction of a Bi4V2O11 sample under N2/H2 at 350 jC.
gressively decompose into Bi and V2O3. Otherwise, if this stage is not reached, the reoxidation remains reversible and occurs at about 300 jC in air. The solid solution Bi2V1 yBiyO5.5 d was studied for y = 0, 0.02, 0.04 [7], and similar behaviour was observed with the exception of the highly enriched composition ( y = 0.04) where the reduction process at 350 jC occurred via an orthorhombic phase, which corresponds to the intermediate h-Bi4V2O10.66 polymorph. 3.2.2. BIMEVOX materials Diffractograms obtained at 350 jC under H2/N2 for a BICOVOX sample are reported in Fig. 5. In this
case, compared to the parent compound Bi4V2O11, only a smooth evolution of the diffractograms is observed. However, as soon as the reducing flow is applied, a shift of the 100 reflection towards higher 2h values is observed. It corresponds to a decrease of the a parameter. Concomitantly, an opposite shift of the 006 reflection towards smaller 2h values occurs, corresponding to an increase of the c parameter which characterises the stacking direction of the bismuth oxide and vanadium oxide sheets. The evolutions of the unit cell parameters are reported in Fig. 6. In the case of the BICUVOX materials, a different behaviour was shown. After 90 min under N2/H2
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Fig. 5. Reduction of a BICOVOX.10 sample under N2/H2 at 350 jC.
Fig. 6. Evolution of the unit cell parameters for BICOVOX.10 under N2/H2 at 350 jC.
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atmosphere, a change in symmetry was observed: it decreased from tetragonal to orthorhombic. Diagrams obtained at 350 jC under H2/N2 are reported in Fig. 7 and the corresponding evolutions of the unit cell parameters in Fig. 8. 3.2.3. Comparison with the evolution of BIMEVOX cathode under operating conditions The same evolution as that observed under reducing atmosphere was evident for BIBIVOX.02 and BICOVOX cathodes under operating conditions [3]. This confirms the superficial and partial reduction of the material on the cathodic side. However, under electrical bias, smaller unit cell parameter evolutions were noticed. In the case of the BICOVOX composition, we observed a variation from 0.1% to 0.2% for a and c for
the BICOVOX cathode against 0.7% for a and 0.5% for c under reducing atmosphere. Even if the temperature conditions are different between these two experiments, the in situ characterisation of the cathode was performed at 620 jC as against 300 jC for the chemical reduction, this shows that, in all cases, the oxygen losses reached under chemical reduction were likely larger than those reached under electrical bias. This, consequently, proves that the partial reduction of the porous part of the membranes (cathode) was not carried out too far, being very likely buffered by the oxygenated atmosphere around the porous surface of the membrane. To deduce an estimation of the oxygen content of the BIBIVOX.02 cathode under operating conditions, the unit cell parameters reached under bias
Fig. 7. Reduction of a BICUVOX.10 sample under N2/H2 at 350 jC.
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Fig. 8. Evolution of the unit cell parameters for BICUVOX.10 under N2/H2 at 350 jC.
were compared with those corresponding to the nonreduced phase and the reduced one at 620 jC. Indeed since the parent compound transformed into Bi4V2O10.66 under reducing atmosphere, Vegard’s law can be used in order to determine the oxygen stoichio-
metry variation reached under polarisation. It was assumed that, for a given temperature, the lattice parameters varied linearly with the oxygen stoichiometry. The BIBIVOX.02 (Bi2V0.98Bi0.2O5.317) diffractogram was recorded at 620 jC under air atmosphere
Fig. 9. Comparison of the diffractograms of Bi2V0.98Bi0.02O5.48 and Bi2V0.98Bi0.02O5.317 at 600 jC.
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∆
Fig. 10. Estimated reduction domain for the BIBIVOX.02 sample.
and its unit cell parameters deduced by pattern matching using Fullprof software program [8]. The same composition was rapidly heated up to 620 jC under a N2/H2 3% atmosphere and the data recorded. Depending on the temperature, Bi6V3O16 exhibits three polymorphs, a, h and g. The two hightemperature g-Bi4V2O11 and g-Bi6V3O16 are not easy to distinguish. However, a characteristic peak around 59j in 2h allows for a distinction to be made between these phases (Fig. 9), and enables us to check if we were effectively dealing with the reduced phase. This was confirmed when recording the X-ray diffractogram on returning to room temperature. An aBi6V3O16 type form was easily identified. The oxygen stoichiometry under air was 5.48, neglecting the small evolution observed when heating the material under air atmosphere. It was 5.317 for the reduced form. When comparing the unit cell parameters observed for the BIBIVOX.02 cathode with those deduced for the oxidised and reduced powder, an estimation of the cathode oxygen stoichiometry can be deduced. It is reported in Fig. 10 and lies in between 5.40 and 5.36. This result shows that even in the case of very severe conditions (1 A/cm2, 620 jC), the material reduction at the cathode surface is limited to the reversible domain of BIBIVOX.02.
4. Conclusion The reduction of Bi4V2O11 and BIMEVOX powders was investigated using two methods: thermogravimetric analyses and high-temperature X-ray diffraction under hydrogen flow. The chemical reduction, due to a VV – VIV reduction, was compared to the evolution of cathodes of dense BIMEVOX membranes under current bias, which had previously been characterised using in situ X-ray synchrotron diffraction. This correlation enables quantification of the level of reduction reached on Bi4V2O11 membranes under operating conditions. This explains the oxygen transfer in these materials, at least at the cathode under bias.
Acknowledgements We thank the Centre National de la Recherche Scientifique and the Re´gion Nord-Pas de Calais for the financial support of one of the authors (C.P.). Michel Drache and Nora Bouremma are acknowledged for their help for the TGA measurements and Laurence Burylo for DXHT measurements. The development of BIMEVOX-based COG was the
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subject of a ‘‘Contrat de Programme de Recherche’’ (CPR) between l’Air Liquide and the CNRS within the CNRS ‘‘Programme Mate´riaux’’ framework.
References [1] M. Huve´, R.N. Vannier, G. Nowogrocki, G. Mairesse, G. Van Tendeloo, J. Mater. Chem. 6 (8) (1996) 1339. [2] J.C. Boivin, C. Pirovano, G. Nowogrocki, G. Mairesse, Ph. Labrune, G. Lagrange, Solid State Ionics 113 – 115 (1998) 639.
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[3] C. Pirovano, R.N. Vannier, E. Capoen, G. Nowogrocki, M. Anne, J.C. Boivin, G. Mairesse, M. Anne, E. Dooryhee, P. Strobel, Solid State Ionics 159 (in press). [4] O. Joubert, A. Jouanneaux, M. Ganne, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 97 (1 – 4) (1995) 119. [5] O. Joubert, The`se, Universite´ de Nantes, Nantes, 1993. [6] J.N. Audinot, The`se, Universite´ de Bordeaux I, Bordeaux, 1999. [7] S. Patoux, R.N. Vannier, G. Nowogrocki, J.M. Tarascon, Chem. Mater. 13 (2001) 500. [8] R. Carjaval, FULLPROF Program, Version 0.2, Mars 1998.
Characterisation of the electrode–electrolyte BIMEVOX system for oxygen separation: Part II. Thermal studies under controlled atmosphere C. Pirovano *, R.N. Vannier, G. Nowogrocki, J.C. Boivin, G. Mairesse Laboratoire de Cristallochimie et Physicochimie du Solide, Universite´ des Sciences et Technologies de Lille, UMR 8012, ENS Chimie Lille, B.P. 108, Villeneuve d’Ascq Cedex 59652, France Received 3 November 2002; received in revised form 21 January 2003; accepted 24 January 2003
Abstract The reduction of Bi4V2O11 and BIMEVOX powders was investigated using two methods: thermogravimetric analyses and high-temperature X-ray diffraction under hydrogen flow. The chemical reduction, due to a VV – VIV reduction, was compared to the evolution of cathodes of dense BIMEVOX membranes under current bias which had previously been characterised using in situ X-ray synchrotron diffraction. This correlation enables to quantify the level of reduction reached on Bi4V2O11 membranes under operating conditions. This explains the oxygen transfer in the materials, at least at the cathode under bias. D 2003 Elsevier Science B.V. All rights reserved. Keywords: BIMEVOX; Oxygen separation; Thermogravimetry; Thermodiffraction; Oxygen
1. Introduction Oxygen ion-conducting ceramics represent an attractive class of materials, world-wide investigated for use in practical devices such as solid oxide fuel cells (SOFC), catalytic membrane reactors and ceramic oxygen generators (COG). Yttria-stabilised zirconia (YSZ) is the most commonly used electrolyte in these applications. However, its domain of use is typically between 800 and 1000 jC. These rather high temperatures give rise to technological problems. Two trends are considered in order to design new devices working * Corresponding author. Tel.: +33-3-20-43-65-83; fax: +33-320-43-48-95. E-mail address: [email protected] (C. Pirovano).
at lower temperature. The first one is for keeping the well-known zirconia as electrolyte but decreasing the thickness of the membrane in order to decrease its resistance at lower temperature. The second way is for developing a new material with higher oxygen ion conductivity. Mainly three classes of materials are known to have better conducting properties than YSZ in air: doped lanthanum gallate, doped ceria and bismuth-based phases. In this field, the so-called BIMEVOX family displays the best properties at moderate temperatures (300 –600 jC). It derives from Bi4V2O11 by partial substitution for vanadium with a variety of metals (ME). Depending on the temperature, Bi4V2O11 exhibits three polymorphs denoted by a, h and g which have been widely described (see, for example, Ref. [1]). The high-temperature g-prototype
0167-2738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-2738(03)00078-X
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is the most disordered form and thereby the most interesting for its conducting properties (r>10 1 Scm 1 at T >500 jC). These properties are mainly due to the bidimensional character of its structure which consists of highly covalent [Bi2O2]2+ layers alternating with highly disordered oxygen-deficient perovskite-like sheets [VO3.550.5]2 . This g-type structure can be stabilised at room temperature by appropriate doping on the vanadium site with other aliovalent or isovalent cations, which prevent ordering phenomena in the perovskite slabs and, as a result, the formation of the h and/or a polymorphs. The acronym BIMEVOX is used and BIMEVOX.x corresponds to a Bi2V1 xMExO5.5 d composition. Several BIMEVOX materials have been tested as membrane for ceramic oxygen generators. High oxygen flows were obtained using membrane simply made of a dense BIMEVOX electrolyte sandwiched in between two gold grids acting as current collectors [2]. It was shown that, in contrast to classical electrolytes (YSZ, stabilised ceria, etc.), in the case of BIMEVOX materials, the oxygen transfer occurred at the electrolyte surface itself. Indeed in classical devices, the kinetics of transfer at the electrolyte surface are usually slow and electrode materials which are mixed (electronic and ionic) conductors must be added to allow the oxygen transfer into the membrane. To understand this unusual behaviour of BIMEVOX materials, the same membranes were characterised under operating conditions by X-ray synchrotron radiation [3]. Three compositions were checked, a slightly Bienriched non-doped phase (BIBIVOX.02), a BICOVOX and a BICUVOX. At 600 jC, the structure of these materials can be described in a tetragonal unit ˚ and ci15.4 A ˚ . Under cathodic cell with ai3.9 A polarisation, an evolution of the unit cell parameters corresponding to the BIMEVOX phases at the surface membrane was clearly observed. A decrease of 0.45% for a and an increase of 0.35% for c were noticed for BIBIVOX.02. The evolution was smoother for the doped compositions. The same evolution of the unit cell parameters is observed when studying these materials under reducing atmosphere. Owing to the susceptibility of bismuth-based materials to reduction under certain atmospheres, these materials cannot be used directly in a fuel cell but they
exhibit attractive properties, which could be fully exploited under nonreducing conditions as in oxygen separation. Bi4V2O11 is able to lose some oxygen when the experimental environment is modified by thermal treatment and/or by variations in oxygen partial pressure, leading to a Bi4V2O11 d compound. This oxygen non-stoichiometry is related to a variation in the VIV content within the compound. Under hydrogen atmosphere, this VV – VIV reduction is limited to one third of the V cations leading to the formation of Bi6V3O16 (Bi4V2O10.66) phase before ultimate destruction into Bi and V2O3. The crystal structure of this phase was solved by Joubert et al. [4]. It can be considered as an ordered form of Bi2(V1V x VIV x )O5.5 x/2 with x = 0.33. The bidimensional structure of BIMEVOX is maintained and from a structural point of view, the main difference concerns the vanadium sheets, which transforms into independent ribbons of one octahedron in between two tetrahedra. When studying these materials under operating conditions by X-ray diffraction, because of grain orientations and the low oxygen X-ray diffusion factor compared to bismuth, it was not possible to extract any further information other than the unit cell parameters [3]. Their evolution under cathodic polarisation is likely due to a reduction of VV into VIV. In this paper, the transformations of BIMEVOX compounds under controlled atmosphere were investigated by thermogravimetric analyses and X-ray thermodiffraction. The evolutions observed under chemical reduction were then compared to those noticed on the cathode of a dense membrane using in situ X-ray diffraction at the ESRF [3].
2. Experimental Powders of Bi4V2O11 or BIMEVOX (ME = Co, Cu mainly) were prepared by conventional solid state synthesis from commercial products as previously described [2]. High-temperature X-ray diffraction was carried out on a Siemens D5000 diffractometer, Cu Ka radiation, equipped with a HTK 1200 Anton Paar device and a PSD detector. The calibration in temperature of the heating device was checked prior to the experiments
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using gold powder. For characterisation under reducing atmosphere, the samples were heated under air atmosphere from room temperature up to 350 jC with a 0.2 jC/s rate. This temperature was chosen in order to ensure a limited modification speed. The atmosphere was then purged from oxygen with nitrogen over about 45 min. After a first reference X-ray diffractogram under N2, the sample was then studied under a N2/H2 flow (2.5/2.5 l/h, 1 atm). A diffractogram was recorded every 15 min in the 10– 70j 2h domain, with a 5-min delay before each measurement, a step of 0.0146j and a counting time of 0.15 s (i.e. a diffractogram was recorded in about 10 min). BIBIVOX.02 was also studied at 600 jC. In this case, the measurement was recorded with the same conditions under a N2/H2 3% flow (5 l/h) using a commercial gas mixture. Thermogravimetric analyses were carried out on a Setaram 92-16.18 thermobalance under a gas flow of 1 l/h. All the analyses were performed on samples which were previously annealed for 12 h at 800 jC and slowly cooled down to room temperature under oxygen. This was to make sure that the starting powders were fully oxidised (d = 0 using the Bi2VO5.5 d formula, for example). The measurements under air were carried out in a platinum crucible containing about 250 mg powder. In order to avoid any mass loss, which could result from moisture absorption, the samples were, prior to the analysis, in situ treated for 1 h at 300 jC in the thermobalance; the temperature being chosen after preliminary tests showed that no oxygen loss occurred. They were then heated from room temperature up to 800 jC with a 5 jC/min rate and cooled down with the same rate after 2 h at 800 jC. The measurements under reducing atmosphere were carried out in a silica crucible using about 150 mg powder. They were performed under isothermal conditions at 350 jC in a N2/H2 3% flow. Only 3% of hydrogen was used as safety precaution, in order to prevent chemical reaction between eventually formed metallic Bi and the platinum of the thermobalance. The increase in temperature to 350 jC was performed under the same flow at a 10 jC/min rate in order to avoid any artefact due to the change of flow. All the measurements were systematically corrected from the Archimede pressure by subtraction of a blank measured under the same conditions.
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Both for the HTDX and TG analyses, experiments were performed with dry hydrogen in order to be in the must reducing conditions.
3. Results and discussion 3.1. Thermogravimetric measurements 3.1.1. Oxygen non-stoichiometry under air The first experiments were performed under air from 20 up to 800 jC. Fig. 1 exhibits the results obtained for Bi4V2O11 and three doped materials (BICOVOX, BICUVOX, BIZNVOX). In the case of Bi4V2O11, as can be observed in Fig. 1a, the
Fig. 1. TGA analyses under air (a): Bi4V2O11, (b): BIMEVOX materials (ME: Co, Cu, Zn).
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material underwent a very small oxygen evolution. The oxygen loss started above 500 jC. The decrease of the oxygen stoichiometry continued until 800 jC (the maximum measurement temperature) but did not increase after 2 h at 800 jC. The transformation was reversible on cooling and the initial oxygen stoichiometry was recovered on returning to room temperature. The oxygen non-stoichiometry remained very low with d = 10 10 3 for Bi2VO5.5 d at 800 jC. This result is in agreement with the one obtained by Joubert [5] for a similar measurement. For the doped materials (Fig. 1b), the same evolution was observed but with a lower variation of oxygen stoichiometry. Table 1 displays the oxygen non-stoichiometry reached for several BIMEVOX materials heated in air up to 800 jC. The oxygen loss occurs between 500 and 800 jC. In general, BIMEVOX phases appear to be less sensitive to reduction than the undoped parent compound, except for BICUVOX and BIMNVOX materials, which exhibit stoichiometry variation comparable to that of Bi4V2O11. A similar study was performed by Audinot [6] on La1 xSrxFe1 yCoyO3 d (LSFC), La1 xSrxFe1 y NiyO3 d (LSFN) perovskite phases and LaGa1 x NixO3 (LGN) gallate lanthanum phase. In the case of LSFC and LSFN, the oxygen losses by heating started at 500 jC and were about 1% to 1.4% between 500 and 800 jC. For LGN, the oxygen losses began at higher temperature around 800 jC and were about 0.7% between 800 and 1050 jC. Whereas in the case of BIMEVOX materials, the losses are at maximum 0.2% for Bi4V2O11 and BIMNVOX.10
Table 1 Oxygen atom losses for different BIMEVOX under air from 500 to 800 jC Compound
Formulation (stoichiometric compound)
Oxygen losses (per mole of compound) between 500 and 800 jC d( F 2 10 3)
Bi2VO5.5 BICOVOX.10 BICUVOX.10 BINIVOX.10 BIZNVOX.10 BIMNVOX.10 BINBVOX.25
Bi2VO5.5 Bi2V0.9Co0.1O5.35 Bi2V0.9Cu0.1O5.35 Bi2V0.9Ni0.1O5.35 Bi2V0.9Zn0.1O5.35 Bi2V0.9Mn0.1O5.40a Bi2V0.9Nb0.1O5.5
10 10 4 10 7 10 2 10 2 10 10 10 4 10
a
3 3 3 3 3 3 3
For the manganese with an oxidation state of III.
and are about 0.1% for the other BIMEVOX studied. LSCF, LSFN and LGN are mixed conductors. The huge difference in oxygen losses is an indication of the low electronic contribution to the conductivity in BIMEVOX phases. 3.1.2. Thermogravimetric measurements under reducing atmosphere The thermogravimetric analysis performed on a non-doped phase, Bi2V0.98Bi0.02O5.48 (BIBIVOX.02), under N2/H2 3% atmosphere at 350 jC is reported in Fig. 2. An enlargement of the starting evolution is given. Zone corresponds to the increase in temperature to 350 jC. Next, a slope change is observed between zones and , this breaking-down point coincides with the oxygen stoichiometry corresponding to a Bi4V2O10.66 type phase with 2% Bi enrichIV III ment, that is to say Bi 2VV 0.653V 0.327 Bi 0.02O 5.317 . Beyond this stage (zone ), the reduction process increases considerably. It likely corresponds to the beginning of the material decomposition as proved by X-ray analysis of the powder after experiment, which revealed bismuth metal and V2O3 to be present. Similar experiments were carried on BIMEVOX phases (BICOVOX, BICUVOX, BIZNVOX). In those cases, plateaux were observed after a decrease of the oxygen stoichiometry (Fig. 3), indicating a better stability of the doped material toward reduction. The corresponding evolutions of the oxygen stoichiometry are reported in Table 2. The experiments were performed at 350 jC under N2/H2 3% atmosphere (1 l/h) for 30 h or more, but since the starting powder grain size was not standardised, no conclusion can be drawn about the relative reduction rate of the three studied materials. The difference in behaviour between the different samples could be due to surface kinetics effects and possibly will also depend on the flow rate of the gas. However, the same tendency as under air atmosphere was observed: a quite important change for BICUVOX, compared to BICOVOX and especially BIZNVOX. In contrast to the non-doped phase, X-ray diffraction performed after these TGA analyses revealed no extra phase. The structure of the materials was maintained except for BICUVOX for which a change of symmetry from tetragonal to orthorhombic was observed.
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Fig. 2. TGA analyses under N2/H2 3% at 350 jC for the BIBIVOX.02 sample.
3.2. High-temperature X-ray diffraction under reducing atmosphere 3.2.1. Bi4V2O11 and the solid solution Bi2V1 y BiyO5.5 d The diffractograms corresponding to the reduction of a BIBIVOX.02 sample performed at 350 jC under H2/N2 atmosphere are reported in Fig. 4. Its behaviour is similar to that observed in the case of the reduction of Bi4V2O11. This process has already been described in details [1,7]. Therefore, we will
just recall it briefly. At the beginning, the characteristic (hkl)/(khl) doublets of the orthorhombic form of Bi4V2O11 progressively merge into a unique reflection characterising the transformation to a tetragonal polymorph. Using the general formalism adopted to describe the BIMEVOX compounds, this phase corresponds to a BIVIVVOX with formula Bi2V1V x VIV x O5.5 x/2. Then, new Bragg peaks corresponding to a-Bi4V2O10.66 begin to evolve. This new phase can be considered as an ordered form of Bi2V1V x VIV x O5.5 x/2 with x = 0.33. The two phases coexist for about 3 h but the amount of Bi4V2O10.66 progressively increases while that of the tetragonal phase concomitantly decreases. If the reduction is further pursued, the Bi4V2O10.66 phase would pro-
Table 2 Oxygen atom losses for different BIMEVOX at 350 jC under N2/ H2 3%
Fig. 3. TGA analyses under N2/H2 3% at 350 jC for the BIMEVOX materials (ME: Co, Cu, Zn).
Compound
Formulation (stoichiometric compound)
Oxygen loss (per mole of compound)
BICOVOX BICUVOX BIZNVOX
Bi2V0.85Co0.13Bi0.02O5.285 Bi2V0.9Cu0.1O5.35 Bi2V0.9Zn0.1O5.35
0.09 0.19 0.05
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Fig. 4. Reduction of a Bi4V2O11 sample under N2/H2 at 350 jC.
gressively decompose into Bi and V2O3. Otherwise, if this stage is not reached, the reoxidation remains reversible and occurs at about 300 jC in air. The solid solution Bi2V1 yBiyO5.5 d was studied for y = 0, 0.02, 0.04 [7], and similar behaviour was observed with the exception of the highly enriched composition ( y = 0.04) where the reduction process at 350 jC occurred via an orthorhombic phase, which corresponds to the intermediate h-Bi4V2O10.66 polymorph. 3.2.2. BIMEVOX materials Diffractograms obtained at 350 jC under H2/N2 for a BICOVOX sample are reported in Fig. 5. In this
case, compared to the parent compound Bi4V2O11, only a smooth evolution of the diffractograms is observed. However, as soon as the reducing flow is applied, a shift of the 100 reflection towards higher 2h values is observed. It corresponds to a decrease of the a parameter. Concomitantly, an opposite shift of the 006 reflection towards smaller 2h values occurs, corresponding to an increase of the c parameter which characterises the stacking direction of the bismuth oxide and vanadium oxide sheets. The evolutions of the unit cell parameters are reported in Fig. 6. In the case of the BICUVOX materials, a different behaviour was shown. After 90 min under N2/H2
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Fig. 5. Reduction of a BICOVOX.10 sample under N2/H2 at 350 jC.
Fig. 6. Evolution of the unit cell parameters for BICOVOX.10 under N2/H2 at 350 jC.
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atmosphere, a change in symmetry was observed: it decreased from tetragonal to orthorhombic. Diagrams obtained at 350 jC under H2/N2 are reported in Fig. 7 and the corresponding evolutions of the unit cell parameters in Fig. 8. 3.2.3. Comparison with the evolution of BIMEVOX cathode under operating conditions The same evolution as that observed under reducing atmosphere was evident for BIBIVOX.02 and BICOVOX cathodes under operating conditions [3]. This confirms the superficial and partial reduction of the material on the cathodic side. However, under electrical bias, smaller unit cell parameter evolutions were noticed. In the case of the BICOVOX composition, we observed a variation from 0.1% to 0.2% for a and c for
the BICOVOX cathode against 0.7% for a and 0.5% for c under reducing atmosphere. Even if the temperature conditions are different between these two experiments, the in situ characterisation of the cathode was performed at 620 jC as against 300 jC for the chemical reduction, this shows that, in all cases, the oxygen losses reached under chemical reduction were likely larger than those reached under electrical bias. This, consequently, proves that the partial reduction of the porous part of the membranes (cathode) was not carried out too far, being very likely buffered by the oxygenated atmosphere around the porous surface of the membrane. To deduce an estimation of the oxygen content of the BIBIVOX.02 cathode under operating conditions, the unit cell parameters reached under bias
Fig. 7. Reduction of a BICUVOX.10 sample under N2/H2 at 350 jC.
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Fig. 8. Evolution of the unit cell parameters for BICUVOX.10 under N2/H2 at 350 jC.
were compared with those corresponding to the nonreduced phase and the reduced one at 620 jC. Indeed since the parent compound transformed into Bi4V2O10.66 under reducing atmosphere, Vegard’s law can be used in order to determine the oxygen stoichio-
metry variation reached under polarisation. It was assumed that, for a given temperature, the lattice parameters varied linearly with the oxygen stoichiometry. The BIBIVOX.02 (Bi2V0.98Bi0.2O5.317) diffractogram was recorded at 620 jC under air atmosphere
Fig. 9. Comparison of the diffractograms of Bi2V0.98Bi0.02O5.48 and Bi2V0.98Bi0.02O5.317 at 600 jC.
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∆
Fig. 10. Estimated reduction domain for the BIBIVOX.02 sample.
and its unit cell parameters deduced by pattern matching using Fullprof software program [8]. The same composition was rapidly heated up to 620 jC under a N2/H2 3% atmosphere and the data recorded. Depending on the temperature, Bi6V3O16 exhibits three polymorphs, a, h and g. The two hightemperature g-Bi4V2O11 and g-Bi6V3O16 are not easy to distinguish. However, a characteristic peak around 59j in 2h allows for a distinction to be made between these phases (Fig. 9), and enables us to check if we were effectively dealing with the reduced phase. This was confirmed when recording the X-ray diffractogram on returning to room temperature. An aBi6V3O16 type form was easily identified. The oxygen stoichiometry under air was 5.48, neglecting the small evolution observed when heating the material under air atmosphere. It was 5.317 for the reduced form. When comparing the unit cell parameters observed for the BIBIVOX.02 cathode with those deduced for the oxidised and reduced powder, an estimation of the cathode oxygen stoichiometry can be deduced. It is reported in Fig. 10 and lies in between 5.40 and 5.36. This result shows that even in the case of very severe conditions (1 A/cm2, 620 jC), the material reduction at the cathode surface is limited to the reversible domain of BIBIVOX.02.
4. Conclusion The reduction of Bi4V2O11 and BIMEVOX powders was investigated using two methods: thermogravimetric analyses and high-temperature X-ray diffraction under hydrogen flow. The chemical reduction, due to a VV – VIV reduction, was compared to the evolution of cathodes of dense BIMEVOX membranes under current bias, which had previously been characterised using in situ X-ray synchrotron diffraction. This correlation enables quantification of the level of reduction reached on Bi4V2O11 membranes under operating conditions. This explains the oxygen transfer in these materials, at least at the cathode under bias.
Acknowledgements We thank the Centre National de la Recherche Scientifique and the Re´gion Nord-Pas de Calais for the financial support of one of the authors (C.P.). Michel Drache and Nora Bouremma are acknowledged for their help for the TGA measurements and Laurence Burylo for DXHT measurements. The development of BIMEVOX-based COG was the
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subject of a ‘‘Contrat de Programme de Recherche’’ (CPR) between l’Air Liquide and the CNRS within the CNRS ‘‘Programme Mate´riaux’’ framework.
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