NEW Ni MIEC CERMET ANODE FOR SOF

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NEW Ni MIEC CERMET ANODE FOR SOF Powered By Docstoc
					 NEW Ni/ MIEC CERMET ANODE FOR SOFC APPLICATIONS BASED ON
        THE IRON SUBSTITUTED Ba2In2(1-x)Ti2xO5+x 1-x PHASES


                F. MOSER1, M.T. CALDES1, O. JOUBERT1, V. GARCIA-ROJAS1, M.ZAHID2
 1
     CNRS, Université de Nantes, Nantes Atlantique Universités - Institut des Matériaux Jean ROUXEL, UMR
      6502 - 2, rue de la Houssinière, BP 32229 - 44322 Nantes, FRANCE; 2European Institute for Energy
                   Research (Eifer) Emmy Noether Strasse 11 D-76131 Karlsruhe, GERMANY

ABSTRACT
Ni/BIT07 cermet was used as anode in single-cells based on BIT07 (BaIn0.3Ti0.7O2.85) electrolyte and using
La0.8Sr0.2MnO3 as cathode The value of Pmax at 690°C under dry hydrogen was found to be 14 mWcm−2
whereas that under dry natural gas was 8 mWcm−2. To improve the stability of the Ni/BIT07 cermets under
methane, electronic conductivity was induced into BIT07 compounds by coupled substitution Fe→Ti and
La→Ba. The electronic conductivity of Ni/Ba0.7La0.3In0.3Ti0.1Fe0.6O2.8 (Ni/BLITFe) cermets is higher than
that of 18.7%-vol Ni/BIT07 (σ700 °C ≈ 102 S cm−1), even for a lower Ni content (16.1%-vol Ni/BLITFe σ700°C ≈
200 S cm−1). Ni/BLITFe cermets exhibit an activity towards dry reforming of methane and a good resistance
to coking.

1 INTRODUCTION
The development of new electrolyte materials with improved properties is essential to the future of Solid
Oxide Fuel Cells (SOFC) technology. Recently, Jayaraman et al. [1] reported a new family of anionic
conductors Ba2In2(1-x)Ti2xO5+x 1-x with 0 ≤ x ≤ 0.7 (called BITx) which can be considered as potential
electrolytes for SOFC, mainly the phase BaIn0.3Ti0.7O2.85 (BIT07).
In order to design electrodes chemically and mechanically compatible with BIT07 electrolytes, Ni/BIT07
cermets were prepared and characterized [2]. Authors obtained a cermet containing only 18.7 vol.% of Ni
with an open porosity of 40%. Its electronic conductivity (σ700 °C ≈ 102 S cm−1) is higher than that of Ni/YSZ
cermets with a larger Ni content and the thermal expansion coefficient measured, 11.4×10−6 K−1, is close to
that of the electrolyte compound BIT07 (9.9×10−6 K−1). This Ni/BIT07 cermet was used as anode in single-
cells based on BIT07 electrolyte and using La0.8Sr0.2MnO3 as cathode [3]. The value of Pmax at 690°C under
dry hydrogen was found to be 14 mWcm−2 whereas that under dry natural gas was 8 mWcm−2.
However Ni-based anodes are usually unstable in the presence of hydrocarbons because Ni is a catalyst for
the formation of coke unless large amounts of steam are also present. High currents and low concentration
(4–9%) of dry CH4 can also prevent coke build-up for operating temperatures less than 700°C. Recently,
Hamakawa et al. [4] has shown a Ni-based catalyst composed of a mixed conducting support oxide which
have the function of self anti-coking phenomenon in the methane conversion. In fact, the self-migration of
lattice oxygen inside the support regulating the balance between the oxide ionic and electronic conductivities
plays an important role to prevent accumulation of deposited carbon over the catalysts.
Therefore, one way to improve the stability of the Ni/BIT07 cermets under dry natural gas would be to
induce electronic conductivity into BIT07 compounds by substitution of Ti by Fe (Fe→Ti).
In this work a new family of MIEC compounds derivate from BIT07 has been prepared and characterized.
The Ni/MIEC cermet anodes based on these iron substituted BIT07 phases have been also studied.

2. EXPERIMENTAL
BaIn0.3Ti0.7-yFeyO3-δ (BITFe0.7) compounds have been prepared by solid state reaction of BaCO3, In2O3,TiO2
and Fe2O3 These reactants were ground thoroughly in acetone and calcined at 1200 °C for 24 h, then ground
and compacted into a pellet. This compact was then heated at 1350 °C for 24 h, ground and passed through
mesh 100.
X-ray powder diffraction (XRD) patterns of all materials were recorded using a Brüker "D8 Advance"
powder diffractometer operated in Bragg-Brentano reflection geometry with a Cu anode X-ray source, a
focusing Ge(111) primary monochromator (selecting the Cu Kalpha1 radiation) and a 1-D position-sensitive
detector ("Vantec" detector). Refinements of cell parameters were carried out using the program
FULLPROF.
Dilatometry measurements were made with a Netzsch DIL 102C system under air. The standard DC four-
probe method was used to measure the electrical conductivity in air and reducing atmosphere (95%Ar/5%
H2) over a range of temperatures between 200 °C and 800 °C. Powders were uniaxially pressed into a pellet
and then sintering in air at 1200°C < T < 1400°C. Sintered pellets densified to over 90% were cut to
rectangular-shaped samples with dimensions 2x2x10 mm and subjected directly to electrical conductivity
measurements.
The temperature-programmed reduction (TPR) studies were performed in a chemisorption unit Micromeritics
AutoChem 2910 with powder samples of 50 mg. Before reaction, the samples were treated with helium at
500 °C for 10 min and cooled to room temperature. The TPR experiments were carried out under a 5.1%
H2/Ar mixture flowed at 50 mL.min-1 through the sample, raising the temperature at 10 °C.min-1 up to 850
°C. The consumption of hydrogen was monitored on-line with a thermal conductivity detector.
The reforming reaction was carried out in a continuous-flow quartz fixed-bed under atmospheric pressure, by
passing a flow of methane (50%), carbon dioxide (50%) without dilution, over 50 mg of catalyst at 800 °C.
The temperature was increased at 10 °C min_1 from room temperature to 800 °C and maintained for several
hours. The catalyst was reduced under pure hydrogen for 1 h at 700 °C prior to each catalytic reaction run.
The composition of the reactants / products mixture was analysed with an on-line mass spectrometer. All the
catalytic experiments were performed twice.
Transmission electron microscopy (TEM) was carried out on a field emission gun microscope Hitachi
HF2000 operating at 200 kV, equipped with an energy dispersive X-ray (EDX) analyzer. The compounds
were gently ground in ethanol and microcrystals were deposited on a holed carbon film supported by a
copper grid.

3. THE IRON SUBSTITUTED BIT07 PHASES (BITFe0.7)

3.1 Structural characterization
The XRD patterns of BaIn0.3Ti0.7-yFeyO3- δ compounds prepared as described in the experimental section are
shown in Figure 1a. Single phase compounds were obtained in the composition range 0 ≤ y ≤ 0.4. They
exhibit a cubic perovskite structure which can be described in the space group Pm-3m. The cell parameter a
increases weakly with increasing Fe content as illustrated in Figure 1b.




 Figure 1 : a) XRD patterns of BaIn0.3Ti0.7-yFeyO3- δ with 0 ≤ y ≤ 0.7 under air at RT, b) lattice parameter and
                                FWHM of the (110) reflection vs. Fe content

For y ≥ 0.5, even if no extra peaks corresponding to secondary phases were observed in XRD patterns, the
full width at half maximum (FWHM) of the diffraction peaks (e.g. see Figure1b for the (110) reflection (2θ ≈
31°)) is much larger than for other y values, a feature indicating a structural distortion or a biphased
compound.
Mössbauer spectroscopy was used to determine the average valence of iron in these materials which was
above 3.5 at room temperature [5]. Therefore, the aliovalent substitution of Ti (IV) by Fe(III/IV) implies the
formation of charge-compensating oxygen vacancies which number increases with the iron content. For y ≥
0.5, the oxygen vacancies content becomes high enough to destabilize the cubic perovskite structure. This
feature is translated in the XRD by a broadening of the diffraction peaks. In order to go further in the
characterization of this specific aspect, a transmission electron microscopy study is in progress [5].
To stabilize the cubic structure for high iron rates the substitution of Ti by Fe was coupled to that of Ba by
La (La→Ba). This coupled substitution is expected to limit the number of charge-compensating oxygen
vacancies formed during reaction. Thus, the La-derivates compounds Ba1-zLazIn0.3Ti0.7-yFeyO3- δ (y=0.5 and
0.6; 0 ≤ z ≤ 0.7) were synthesised and characterised.

3.1.1 The Ba1-zLazIn0.3Ti0.7-yFeyO3- δ compounds
Ba1-zLazIn0.3Ti0.7-yFeyO3- δ compounds were prepared according to the same protocol as BITFe. La2O3 was
used as precursor of lanthanum. The coupled substitution Fe→Ti and La→Ba allowed us to stabilise the
cubic perovskite structure for high iron contents (y = 0.6). The XRD patterns of Ba1-zLazIn0.3Ti0.1Fe0.6O3- δ
samples (0 ≤ z ≤ 0.7) are shown in figure 2a.




Figure 2 : a) XRD patterns of Ba1-zLazIn0.3Ti0.7-yFeyO3- δ with 0 ≤ z ≤ 0.7 under air at RT, b) lattice parameter
                             and FWHM of the (110) reflection vs. La content


As illustrated in Figure 2b, for the composition range 0 ≤ z ≤ 0.2, the FWHM of the diffraction peaks
decreases with increasing La content. For 0.2 ≤ z ≤ 0.5 the reflection width is narrow enough to refine lattice
parameters in the cubic SG Pm-3m. As expected, the replacement of Ba2+(effective ionic radii: re= 1.42 Å)
by smaller cation La3+ (re= 1.16 Å) induces a cell parameter decrease. For 0.5 ≤ z ≤ 0.7 a broadening of the
diffraction peaks is observed once more.
The Fe valence in these compounds was estimated (Mössbauer spectroscopy) to range from a value of 3.3 to
3.2. According to electrical neutrality principle, this result means that for Ba1-zLazIn0.3Ti0.1Fe0.6O3- δ
compounds oxygen content increase slightly with La content.
3.2 Electrical Conductivity
The Arrhenius plots of electrical conductivity of BaIn0.3Ti0.7-yFeyO3-δ (y = 0, 0.2, 0.4) and Ba1-zLazIn0.3Ti0.1
Fe0.6O3-δ ( z= 0.1,0.2,0.3) materials are shown in Figure 3.




 Figure 3 : DC conductivity vs. temperature of BaIn0.3Ti0.7O2.85 (hexagon), BaIn0.3Ti0.3Fe0.2O3-δ (up triangle),
    BaIn0.3Ti0.3Fe0.4O3-δ (lozenge), Ba0.9La0.1In0.3Ti0.1Fe0.6O3-δ (square), Ba0.8La0.2In0.3Ti0.1Fe0.6O3-δ (star),
 Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ under air (black circle) and Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ under Ar/H2 (5%) (empty
                                                        circle)

For low iron rates, the temperature dependence of the electrical conductivity indicates a semiconducting
behaviour which changes to pseudo metallic above y = 0.4. Furthermore, the total conductivity increases
significantly with the Fe and La content, at least 2 orders of magnitude. Thus, BaIn0.3Ti0.7O3-δ (BIT07) yields
a conductivity of 1x10-2 S.cm-1 at 700 °C whereas Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ (BLITFe) shows a conductivity
of 3 S.cm-1. In reducing atmosphere the electrical conductivity decreases drastically due to lowered mean Fe
valence. Iron reduction was confirmed by temperature-programmed reduction (TPR).




               Figure 4 : displays the normalized TPR profile of the Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ.
The peak at ≈ 400°C can be assigned to iron reduction. The consumption of hydrogen measured indicates a
decrease of Fe valence from 3.3 to 3. Moreover, no extra reduction peaks were observed up to 850°C,
attesting to the redox stability of this material under reducing atmosphere.
Due to the rather low electrical conductivity exhibited by these materials at low pO2, only
Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ was retained for cermet elaboration. Indeed, this compound shows the best
conductivity at 700 °C in both air (3 S.cm-1) and under Ar/ 5% H2 (2.10-2 S.cm-1).

3.4 Thermal expansion coefficient
The thermal expansion coefficient (TEC) of Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ was established by dilatometric
measurement in air. The value obtained, 30.10-6 K-1, is exceptionally high compared with those of BIT07
(9.9.10-6 K-1) and others ferrites-based perovskites [6]. As illustrated in Figure 5, the slope of TEC curve
increases with temperature, which can be explained by a loss of oxygen due to iron reduction. As has already
been outlined [6] higher oxygen vacancy concentrations lead to higher TECs values. This feature could
partially explain that BLITFe exhibits a TEC that is well above the TEC of BIT07. Thus, BLITFe shows, at
room temperature, an oxygen vacancy content higher than BIT07 (8% vs. 5%). Furthermore, while for
BLITFe the oxygen vacancy number increase with temperature, for BIT07 it remains stable. Finally, it must
be noted that cationic substitutions Fe→Ti and La→Ba could noticeable modify the strength of the metal-
oxygen bonds which influences strongly the TEC value.




            Figure 5 : Relative dilatation of pellets vs. temperature for Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ



4. THE Ni/BLITFe CERMETS

4.1 Elaboration and characterization
Powders of Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ (as prepared 2.45 g) and NiO (1.05 g, grain size 0.5 to 1 µm) with
70:30 weight ratio, were introduced with twenty glass balls (Ø 5 mm) into a 30 mL glass vessel, then placed
on a roller bank for 2 h at 30 rpm. Samples of the resulting Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ/NiO mixture were
pressed to obtain pellets of Ø 10 mm (thickness 2 mm) that were heated at 1200°C for 6h in order to get a
porous matrix, while minimizing shrinkage. This thermal treatment does not induce any chemical reaction
between constituents, as illustrated in Figure 6.
                                                                                    (c)



                                                                                    (b)



                                                                                    (a)



      Figure 6 : XRD patterns of a) Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ , b) Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ + NiO, c)
              Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ + Ni cermet, Ni reflections and ▲ NiO reflections

This was confirmed by a refinement of cubic cell parameter of Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ by full pattern
matching, indicating no change with respect of the initial value. At this stage, the density reaches 75±2%
theoretical density. Finally, rectangular shaped samples were subjected to a reducing process for 5h at 750°C
under flowing Ar/H2 (5%).The weight loss observed during TG analysis of Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ /NiO
mixture under these reducing conditions correspond to a complete reduction of NiO to Ni. An extra weight
loss of 0.6% related to reduction of FeIV → FeIII was also observed (see Figure 7).




                               Figure 7 : TG curves of NiO/BLITFe and BLITFe

This result is confirmed by XRD pattern of the final material which shows a mixture of Ni and
Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ. The reduction step does not induce any measurable further shrinkage of the
samples. Therefore, due to oxygen loss, the density decrease down to 60% that is overall porosity reaches
40%.

4.2. Influence of the Ni content on the DC conductivity.
DC conductivity measurements between RT and 700 °C under Ar/5%H2 mixture were made on
Ni/Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ cermets with various Ni contents (27.7, 19.8 and 16.1 vol.%). These cermets
were prepared as described in Section 4.1. A comparison of the total DC resistivity has been made with data
reported for pure nickel [7], a 18.7% vol Ni/BIT07 cermet [2] and a Ni/YSZ cermet with a larger Ni content
(30 vol.%) [8]. In Figure 7 a sharp change in slope can be observed for all the curves in correspondence to
the Curie temperature of Ni (at 350 °C). The increase in resistivity with increasing temperature indicates a
metallic behaviour. Interestingly, the electronic conductivity of each Ni/BLITFe-type cermets is higher than
that of 18.7%-vol Ni/BIT07 (σ700°C ≈ 102 S cm−1), even for a lower Ni content (16.1%-vol Ni/BLITFe σ700 °C
≈ 200 S cm−1). That a Ni/YSZ cermet with a larger Ni content exhibits a lower conductivity is likely not an
intrinsic property of Ni/YSZ cermets but more probably related with the morphology of this specific example
(not described in Ref. [8]), which depends on the preparation process. As expected, the conductivity of
Ni/BLITFe cermets increases with Ni rate. The best results were obtained for 27.7%-vol Ni/BLITFe cermet
with σ700 °C≈1000 S cm−1




Figure 8 : log(ρ) vs. temperature for (a) pure Ni ; (b) 27.7%-vol Ni/BLITFe ; (c) 19.8%-vol Ni/BLITFe ; (d)
                  16.1%-vol Ni/BLITFe ; (e) 18.7%-vol Ni/BIT07 and (f) 30%-vol Ni/YSZ

4.3 Catalytic dry reforming of methane
Among the methane reforming reactions, dry reforming appears as the one for which coking phenomena is
most important. Thus to evaluate carbon deposition characteristics of the Ni/BLITFe-type cermets and
compare to those of Ni/BIT07, carbon dioxide reforming of methane was studied over these cermets.
Ni/BaIn0.3Ti0.7O2.85 (Ni/BIT07) and Ni/Ba0.7La0.3In0.3Ti0.1Fe0.6O2.8 (Ni/BLITFe), cermets containing 3.7% vol
of Ni were prepared by impregnation method. Ni/BaTiO3, which has been extensively studied as methane
reforming catalyst was also prepared and used as a reference. A low Ni content (3.7%vol) was deliberated
chosen to enhance the influence of the BIT-type phases on the coking characteristics of the cermets.
To prepare the cermets, powder samples sieved through 100 mesh, were impregnated with a nickel nitrate
solution, dried at room temperature and calcinated at 750 °C for 3 hours.
The observed conversions of methane and carbon dioxide are listed in Table 1.

        Table 1. CH4, CO2 conversion, after 1 h and 5 h of reaction at 800 °C over 3.7%vol cermets

         Cermet            Initial Conversion (1h)    Final Conversion (5h)        % coke (5h)
                                 CH4    CO2                CH4    CO2
        Ni/BIT07                   12    18                 23    36                no carbon
                                                                                    deposition
        Ni/BLITFe                  12     20                 15      20             no carbon
                                                                                    deposition
        Ni/BaTiO3                  64     75                 56      69                0.6

All cermets exhibit an activity towards dry reforming of methane. The lower conversion of methane
compared to carbon dioxide suggests that the reverse water gas shift reaction (RWGS) takes place.
Ni/BIT07 and Ni/BLITFe present low conversions compared to those of Ni/BaTiO3, but their catalytic
activity increases or remains nearly constant in time. However a noticeable deactivation is observed for
Ni/BaTiO3. Since coke deposition is the major cause of deactivation, the amount of coke formed during
reaction was determined by TGA under air.
The TG curve of Ni/BaTiO3 is shown in Figure 9a. A weight gain is first observed corresponding to the
oxidation of Ni metal. Between 400 °C and 900 °C two weight losses are observed attributed to coke
oxidation (corroborated by mass spectrometry signal-44). The total mass loss is 0.53%.
In the case of Ni/BIT07 and Ni/BLITFe no loss weight was observed in TGA indicating that no carbon
deposition occurred during the reaction (5h). This feature was corroborated by transmission electron
microscopy (TEM). In Figure 9b a TEM micrograph of Ni/BLITFe after 5 hours in stream is shown. Nearly
spherical Ni particles of 40–60 nm in size without carbonaceous deposits were systematic observed at the
edge of BLITFe crystals.




 Figure 9 : (a) TG curve of Ni/BaTiO3 heated in air after 5h on stream, (b) TEM micrograph of Ni/BLITFe


5. CONCLUSION
The coupled substitution Fe→Ti and La→Ba allowed us to induce electronic conductivity into BIT07
compounds. In fact, BaIn0.3Ti0.7O3-δ (BIT07) yields a total conductivity in air of 1x10-2 S.cm-1 at 700°C
whereas Ba0.7La0.3In0.3Ti0.1Fe0.6O3-δ (BLITFe), shows a conductivity of 3 S.cm-1. The electronic conductivity
of Ni/BLITFe cermets is higher than that of Ni/BIT07 even for Ni a lower content. Furthermore, Ni/BLITFe
cermets exhibit an activity towards dry reforming of methane and a good resistance to coking.
In order to determine the catalytic activity and selectivity of Ni/BLITFe cermets toward methane total
oxidation, in the absence of gas-phase oxygen, experiments using a pulse reactor system are in progress.
.
REFERENCES
1. V. Jayaraman, A. Magrez, M. Caldes, O. Joubert, M. Ganne, Y. Piffard, L. Brohan, 2004, Solid State
Ionics, vol. 170, pp. 17
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3. D. Prakash 1, T. Delahaye, O. Joubert, M.-T. Caldes, Y. Piffard, 2007, Journal of Power Sources, vol. 167,
pp. 111–117
4. Satoshi Hamakawa , Koichi Sato, Tomoya Inoue, Masateru Nishioka, Kiyoshi Kobayashi, Fujio
Mizukami, 2006, Catalysis Today, vol. 117, pp. 297
5. F. Moser et al., in progress
6. A. Mai, Vincent A.C. Haanappel, S. Uhlenbruck, F. Tietz, D. Stöver, 2005, Solid State Ionics, vol. 176,
pp. 1341
7. H. Ullmann, N. Trofimenko, F. Tietz, D. Stöver, A. Ahmad-Khanlou, 2000, Solid State Ionics, vol. 138,
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8. U. Anselmi-Tamburini, G. Chiodelli, M. Arimondi, F. Maglia, G. Spinolo, Z.A. Munir, 1998, Solid State
Ionics, vol. 110, pp. 35

				
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