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Chemical stability study of Barium Cerate - Based Ionic Conducting Materials

K. Przybylski1*, J. Prazuch1, T. Brylewski1, R. Amendola2, S. Presto2, M. Viviani2 1 AGH, Department of Solid State Chemistry, Al. Mickiewicza 30, 30-059, Krakow POLAND 2 CNR-IENI, Via De Marini 6, 16149 Genova ITALY kaz@agh.edu.pl

Abstract The chemical stability of BaCe0.85Y0.15O2.925 (BCY15), BaCe0.7In0.3O2.85 (BCI30) and a series of BCY15-YDC15 (Ce0.85Y0.15O1.925) mixtures with different compositions was studied in atmospheres containing CO2/H2O/N2 in different ratios and at different temperatures (25 and 600 °C). Thermal analysis coupled with mass spectrometry (DTA-TG-MS) was employed to quantitatively compare the amount of water uptake and of barium carbonate formation during exposition to reacting atmospheres. The investigation was carried out on both loose powders and sintered ceramics. At room temperature only pure BCY15 showed a remarkable tendency to reaction with CO2, while at 600 °C all materials contained huge amounts of BaCO3. On the contrary, when saturation was done in H2O/N2, a good chemical stability was obtained for all materials over the whole temperature range.

Keywords: barium cerate; stability in CO2; proton conductors;

1. Introduction

BaCeO3-based compounds have been investigated as promising proton conducting materials for fuel cells and electrolysis applications since the early work of Iwahara [1]. Basic features of the conduction mechanism and chemical stability issue have been explored by several authors [2-4]

In particular, stability problems in atmospheres containing water and carbon dioxide were reported, especially for Y-doped cerate (BCY), sometimes with contradictory conclusions, likely due to differences in the overall stoichiometry. At temperatures required for sintering, i.e. 1300-1400 °C, Barium is easily lost from BCY. This leads to the formation of Ba-deficient phases and BaO (Eq. (1)), which is readily transformed into BaCO3 (Eq.(3)) on cooling, i.e. below 1000 °C [5].

xBaOOYCeBaOYCeBa xx +→ −− 313 ),(),( (1)

Ba(Ce,Y )O3 → BaO+CeO2 +Y2O3 (2)

BaO+CO2 → BaCO3 (3)

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A kinetic stabilization of BCY has been registered below 500 °C, which is no longer occurring in partially carbonated materials, giving rise to a complete decomposition into parent oxides (Eq.(2)) and taking place in the range 850-1200 °C [4]. In fact, as sintering is carried out in air, it is likely that this situation is the standard one.

It has been also demonstrated that structural deformation given by foreign atoms as well as Ba excess in the perovskite structure increase reactivity with CO2/H2O. The use of BCY in proton conducting fuel cells or electrolyzers in the range 600-700 °C is therefore potentially prevented by chemical reactivity, unless a special care is placed in the control of feeding gases at both electrodes.

Strategies to improve the stability of barium cerate without affecting transport properties rely on the possibility to: i) obtain solid solutions with another stable perovskite (e.g. zirconates or titanates), ii) find a doping element with high solubility and ionic radius similar to the one of Ce3+, iii) use composites with CeO2, in order to prevent reaction (2) by adding products of the reaction.

The aim of this study is to test the stability of barium cerate – based materials, obtained by doping with In instead of Y (case ii) or mixing with Y-doped CeO2 (case iii), at room temperature and at 600 °C and make a comparison with BCY.

2. Experimental

Fine powders of BaCe0.85Y0.15O3-δ (BCY15) and Ce0.85Y0.15O2-δ (YDC) were prepared by a combustion route (Marion Technologies SA, Toulouse, France), while BaCe0.70In0.30O3-δ (BCI30) powders were prepared by solid state reaction. Mixtures of BCY15 and YDC15 were prepared by overnight ball milling in plastic jars with zirconia balls and in n-propanol. Table 1 reports all the samples analyzed.

Table 1. Name, composition and volume fraction of each sample. Name Composition

BCY15 BaCe0.85Y0.15O3-δ BCI30 BaCe0.70In0.30O3-δ B90Y10 90% BCY15 + 10% YDC15 B70Y30 70% BCY15 + 30% YDC15 Y90B10 10% BCY15 + 90% YDC15 Y80B20 20% BCY15 + 80% YDC15 Y70B30 30% BCY15 + 70% YDC15

Afore-mentioned samples were studied before and after CO2/H2O or N2/H2O saturation both at room temperature and 600 °C, by using Differential Thermal Analysis (DTA), Thermogravimetry (TG) and Mass Spectrometry (MS). Moreover, each powder has been pressed into 3 pellets with 5mm diameters and about 2mm height. All pellets have been sintered at 1200oC in flowing synthetic air for 2 hrs. The experimental analysis provided for the bulk samples is the same as for the powders.

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Before inserting the powders into CO2/H2O rich atmosphere at both room temperature and 600oC, the powders were heated up to 1100oC with a rate of 10oC/min and then annealed at 1100oC for 2.5 h in flowing synthetic air in order to eliminate the CO2 originating from organic material, such as PVA and paraffin oil. After such treatment, each powder was divided into 3 parts. Two parts of the powders were introduced to a furnace where they were exposed to 600oC for 600 h in a flowing gas mixture containing 100% CO2/H2O or N2/H2O. The other part was placed into a desiccator filled with selected atmosphere, in which the powder was kept at room temperature for 600 h.

The diagram of the experimental setup for saturating the samples at 600oC is given in Fig.1. Each powder was put into a different quartz crucible, which in turn was located in the isothermal zone of the electrically heated furnace inside quartz glass, as shown in Fig.1. Besides the furnace, quartz glass and quartz crucibles, this system consists of a mass flow controller that measures the gas flow, a thermocouple inserted into the furnace to ensure the correct operation temperature, and a saturator, where the flowing gas is moisturized with water vapor. For N2, the water saturator was heated at 82 °C in order to increase steam partial pressure. The afore-mentioned mixture enters into the bottom of the quartz glass, through a spiral quartz tube attached to the quartz glass, and leaves through the top of the quartz glass.

Fig.1. Scheme of the experimental setup used for exposing the samples at 600oC in the N2/H2O gas mixture.

In the DTA-TG-MS analysis of both the powder and the bulk samples a microthermogravimetric apparatus (SDT-2960 by TAInstruments, USA) was used, coupled with a quadruple gas analyzer (Thermostar GSD 300 by Balzers, Liechtenstein). The procedure of the analysis was as follows. The samples were

CO2+H2O,

CO2,

CO2,

CO2+H2O,

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heated up to 1200oC continuously with a rate of 20oC/min in a stream of helium with a rate of 100 cm3/min, and the mass changes of the samples were registered continuously with simultaneous gas analysis in the direct vicinity of the sample. The sensitivity of the microgravimetric apparatus was 1 μg. The sensitivity of the gas analyzer, on the other hand, was about several ppm, depending on the analyzed gas.

3. Results and discussion

3.1. Powders

Particle size of powders employed was of the order of 200 nm (d50) for BCY15 and YDC15, while BCI30 powder was in the 500 nm range. It is worth noting that the BCY15 had slight Ba excess, i.e. BA/(Ce+Y) = 1.02 (ICP). Fig. 2 shows SEM images of the three powders: in BCY15 (a) particles are round shaped and non agglomerated, in YDC15 (b) some large aggregates are also visible (d90 = 2.7 μm), in BCI30 (c)

Fig.2. SEM images of powders analyzed. BCY15 (a), YDC15 (b), BCI30 (c).

3.2. Stability in CO2/H2O

Figures 3-4 show the thermal analysis results obtained on powders exposed to wet carbonated atmosphere at room temperature and at 600 °C.

a b

c

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Fig.3. DTA, TG and MS curves of the BCY15, BCI30, B90Y10, B70Y30 powders after extended treatment in CO2/H2O rich atmosphere at: a) room temperature and b) 600oC.

At room temperature the major effect is a surface hydration, although BCY15 shows clearly the formation of BaCO3 which decomposes partly at 800 °C and partly at 1100 °C.

On the contrary, at 600 °C all materials reacts and some BaCO3 formation takes place. Also in this case, the carbonate is decomposed during the test in two steps, in the range 900-1100 °C. A correlation between the amount of carbonate and the content of cerate in the sample can be also drawn (see also Fig. 7).

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Fig.4. DTA, TG and MS curves of the Y90B10, Y80B20, and Y70B30 powders after extended treatment in CO2/H2O rich atmosphere at: a) room temperature and b) 600oC.

Results on sintered samples are presented in Figs. 5-6. At room temperature there is not appreciable difference with powders: some hydration at the surface, readily eliminated at 100 °C and a clear carbonation of BCY15 and a fair one in BCI30.

At 600 °C again, barium carbonate is detected in all samples. Major losses are found in composites, while BCY15 and BCI30 revealed a less severe phenomenon. Such a behavior is better seen in Fig. 7, where the molar fraction of cerate transformed into barium carbonate (eqs. 1, 3), and the water uptake as calculated from TG measures, are reported for all samples.

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Fig.5. DTA, TG and MS curves of the a) BCY15, BCI30, B90Y10, B70Y30 and b) Y90B10, Y80B20 and Y70B30 bulk samples after extended treatment in CO2/H2O rich atmosphere at room

temperature.

Fig.6. DTA, TG and MS curves of the a) BCY15, BCI30, B90Y10, B70Y30 and b) Y90B10, Y80B20 and Y70B30 bulk samples after extended treatment in CO2/H2O rich atmosphere at 600oC.

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While at room temperature both water and CO2 contents can be considered the same in powders and ceramics, a large discrepancy appears at 600 °C. In general, sintering drastically reduces the exposed surface and consequently the rate of any reaction involving a gas phase. This is also valid for surface adsorption of water molecules.

For this reason a huge limitation of carbonation in ceramics is expected. Such a limitation is higher for BCY15 and reduces progressively as the BCY15 fraction in the sample is reduced. Given the wide particle size distribution of YDC powder, it is highly probable that sintering at 1200 °C was not sufficient to get dense ceramics when YDC was included, i.e. in all samples except BCY15. Porous ceramics will therefore react with gases at similar rate as loose powders, as found for samples with higher YDC fraction.

Therefore, given that carbonation is due to BCY only, the amount of BaCO3 that can be expected in dense BCY-YDC composites is significantly lower. For BCI30, the values obtained are: 6.4% at room temperature and 39.5% at 600 °C, i.e. much better than BCY at room temperature but almost the same at 600 °C

0.2 0.4 0.6 0.8 1.0

20

40

60

80

100 Bulk samples (room temperature) Powders (room temperature) Bulk samples (600oC) Powders (600oC)

% o

f con

vers

ion

into

BaC

O3

Molar procent of BCY150.2 0.4 0.6 0.8 1.0

2

4

6

8

10b)a)

Bulk samples (room temperature) Powders (room temperature) Bulk samples (600oC) Powders (600oC)

% o

f H2O

in th

e sa

mpl

es

Molar procent of BCY15

Fig.7. The correlation between the molar fraction of BCY15 in the initially received sample and a) the molar fraction of barium converted into BaCO3 and b) the correlation between the amount of

H2O present in both the bulk samples and powders at room temperature and 600oC

3.2. Stability in N2/H2O

The results of the stability test under steam at 600 °C are reported in Table 2 and Fig. 8.

Under saturation in water vapor (about 47 mol.% of H2O), all the materials show excellent stability and the coincidence of carbonate content in all samples can be considered a baseline due to manipulation of samples in air.

This result is encouraging for the application of barium cerates in the central membrane of the IDEAL-Cell, where comparable conditions are envisaged, expect for the total pressure, which is expected to increase in real conditions.

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Table 2. Measured molar fractions of CO2 and H2O for the bulk samples after the effect of N2/H2O rich atmosphere at 600°C for 600 h

Formula % CO2 % H2O 1 BaCe0,85Y0,15O2.925 0,2 0,1 2 BaCe0,7In0,3O2.85 0,15 0,15 3 0,9BaCe0,85Y0,15O3 + 0,1Ce0,85Y0,15O2 0,25 0,05 4 0,7BaCe0,85Y0,15O3 + 0,3Ce0,85Y0,15O2 1 0,05 5 0,1BaCe0,85Y0,15O3 + 0,9Ce0,85Y0,15O2 0,1 0,05 6 0,2BaCe0,85Y0,15O3 + 0,8Ce0,85Y0,15O2 0,18 0,05 7 0,3BaCe0,85Y0,15O3 + 0,7Ce0,85Y0,15O2 0,16 0,05

Fig.8. TG and MS curves of the a) BCY15, BCI30, B90Y10, B70Y30 and b) Y90B10, Y80B20 and Y70B30 powders after extended treatment in N2/H2O rich atmosphere at 600ºC for 600 hrs.

4. Conclusions

The reactivity of BCY15 with CO2 has been confirmed and a total decomposition of the cerate powder was obtained already after 600 h at 600 °C. This would of course prevent any application in hydrocarbon fuelled cell but also makes it questionable for hydrogen-air cells.

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Alternative materials tested in this work showed very good stability at room temperature, although a significant reactivity at 600 °C was detected for both BCI and BCY-YDC composites.

However, the possibility to easily manipulate and store materials at room temperature has to be considered a great advance.

From the TG-MS measurements, it can be concluded that all the samples are stable at 600ºC for 600 h during exposure to atmosphere containing N2 and water vapor of about 46,8 mol. %, which make them suitable for central membranes in IDEAL-Cells.

Acknowledgements

The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No 213389.

References

[1] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359. [2] K.D. Kreuer, E. Schönherr, J. Maier, Solid State Ionics 70/71 (1994) 278. [3] N. Bonanos, Solid State Ionics 145 (2001) 265. [4] N. Zakowsky, S. Williamson, J.T.S. Irvine, Solid State Ionics 176 (2005) 3019. [5] S. Gopalan, A.V. Virkar, Journal of the Electrochemical Society 140 (1993) 1060.