[ECS 216th ECS Meeting - Vienna, Austria (October 4 - October 9, 2009)] ECS Transactions -...

Investigation of Sr-doped and Sr-free LaMn1-yCryO3±δ Perovskites As Sulfur Tolerant SOFC Anodes

L. Deleebeeck and V. Birss

Department of Chemistry, University of Calgary, Calgary, Alberta T2N 1N4, Canada

Sr-doped and Sr-free La1-xSrxMn1-yCryO3±δ (LSMC) perovskite-type oxides were synthesized and tested for use as intermediate temperature anodes for SOFC applications. Thermo-chemical and chemical stabilities were tested in hydrogen and low ppm H2S: H2 at 800˚C. Relative reducibility was determined by temperature program reduction using thermo-gravimetric analysis. This was correlated with oxygen non-stoichiometry determined by permanganate titration. The final phase after reduction at 1100˚C was determined by powder X-ray diffraction and the ability of these materials to recover after reduction was tested by re-sintering in air. Electrochemical testing revealed catalytic activity towards hydrogen as a function of changing sintering temperature and operating temperature, as well as Sr and Cr content LSMC materials were found to be chemical stable in upto 10 ppm H2S in humidified hydrogen at 800˚C, and Sr-free LMC materials were shown to have reasonable catalytic activity towards the hydrogen oxidation reaction at 700 and 800˚C.

Introduction

La1-xSrxMn1-yCryO3±δ (LSMC) perovskite-type oxides have been investigated as possible replacement anodes for Ni/yttria-stabilized zirconia (YSZ) due to their reasonable catalytic activity towards the oxidation of hydrogen (1) and methane (2) along with good sulfur and coke tolerance. Under high temperature (600-1000˚C) reducing conditions (pO2 ~10-20 atm), typical of SOFC anode environments, LSMC materials are mixed electronic and ionic conductors (MEIC), show good chemical stability when in contact with standard electrolyte and interconnect materials, and have a matching thermal expansion coefficient with YSZ. LSMCs undergo only a slight volume change of approximately 0.20% upon oxidation/reduction cycling (3). LSMC materials have been shown to coke under open circuit conditions (4), while being coke-resistant and capable of catalyzing the complete oxidation of CH4 under polarized conditions (5).

LSMC materials have been reported to be unstable in H2 environments containing

0.5% H2S (6) and 10% H2S (7). In both cases, the secondary phases formed were MnS, α-MnOS and La2O2S. However, it has also been reported that La0.8Sr0.2Mn0.2Cr0.8O3:YSZ anodes experience a 150 mV loss at 500 mA/cm2 when exposed to 1000 ppm H2S, a 50 mV loss in 100 ppm H2S, but no more than a 10 mV loss in 10 ppm H2S (8). This suggests that LSMC materials may be sulfur tolerant at low ppm H2S concentrations. Here, we show that both Sr-free and Sr-doped LSMC materials possess these desirable properties.

ECS Transactions, 25 (2) 2231-2239 (2009)10.1149/1.3205773 © The Electrochemical Society

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Experimental Procedures

La1-xSrxMn1-yCryO3±δ (xSr = 0, 0.1, 0.2 and yCr = 0.4, 0.5, 0.6) oxides were synthesized by co-precipitation, followed by calcination at 1260˚C in air, and their composition was verified by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and phase purity by powder X-ray Diffraction (pXRD). pXRD data were acquired between 10 and 80˚ 2-theta (2θ), with a scan width of 0.01˚ and a scan rate of 0.5 2θ/min. Their thermo-chemical stability was determined by pXRD after exposure to 800˚C in H2:3% H2O, and in 10 ppm H2S and 30 ppm H2S (in N2:H2:3% H2O) for 5 days. The reducibility of these oxides was determined by temperature programmed reduction (TPR) between 200 and 1100˚C in 20% H2:80% He using Thermo-Gravimetric Analysis (TGA) at a ramp rate of 1˚C/min. The oxygen non-stoichiometry and amount of Mn4+ in each material was determined by permanganate titration (9). The phase produced following TGA was determined by pXRD after 24 hrs at 1100˚C in 100% H2. Finally, these LSMC materials reduced for 24hrs at 1100˚C were re-sintered at 1260˚C in air.

LSMC materials were sintered in air in a half-cell configuration on YSZ for 2 hrs at

1100 or 1200˚C. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) studies were performed at 700 and 800˚C in H2: 3% H2O. EIS was carried out at the open circuit potential (OCP) between 105 to 0.005 Hz, with AC amplitude of 10 mV. CV was performed between –0.8V to +0.8V vs. the Pt reference electrode (RE) using a scan rate between 5 and 10 mV/s.

Results and Discussion Material Synthesis and Characterization

The La1-xSrxMn1-yCryO3±δ (xSr = 0, 0.1, 0.2 and yCr = 0.4, 0.5, 0.6) oxides were

synthesized by co-precipitation and the molar stoichiometries of La, Sr, Mn and Cr were determined by ICP-AES (Table I). The phase of the as-synthesized oxides was determined by pXRD, showing that all of the materials are perovskite-type oxides, as shown in Figs. 1a and 2a for LaMn0.6Cr0.4O3±δ and La0.8Sr0.2Mn0.6Cr0.4O3±δ, respectively.

TABLE I. LSMC perovskite stoichiometry and oxygen non-stoichiometry. Stoichiometry Redox Titration Name

Expected Obtained by ICP 3±δ LMC64 LaMn0.6Cr0.4O3±δ La0.92Mn0.66Cr0.43O3±δ 3.173 LSMC9164` La0.9Sr0.1Mn0.6Cr0.4O3±δ La0.86Sr0.10Mn0.65Cr0.41O3±δ 3.026 LSMC8264 La0.8Sr0.2Mn0.6Cr0.4O3±δ La0.72Sr0.20Mn0.67Cr0.43O3±δ 3.047 LMC55 LaMn0.5Cr0.5O3±δ La0.96Mn0.52Cr0.52O3±δ 3.182 LSMC9155 La0.9Sr0.1Mn0.5Cr0.5O3±δ La0.86Sr0.097Mn0.51Cr0.54O3±δ 3.120 LSMC8255 La0.8Sr0.2Mn0.5Cr0.5O3±δ La0.81Sr0.18Mn0.51Cr0.51O3±δ LMC46 LaMn0.4Cr0.6O3±δ La0.99Mn0.42Cr0.60O3±δ LSMC9146 La0.9Sr0.1Mn0.4Cr0.6O3±δ La0.90Sr0.11Mn0.41Cr0.59O3±δ LSMC8246 La0.8Sr0.2Mn0.4Cr0.6O3±δ La0.79Sr0.21Mn0.43Cr0.59O3±δ

ECS Transactions, 25 (2) 2231-2239 (2009)



Figure 1. Powder X-ray diffraction patterns of LaMn0.6Cr0.4O3±δ after (a) sintering at 1260˚C in air, reduction at 800˚C for 120 hrs in (b) H2: 3% H2O, (c) 10 ppm H2S: N2 : H2 : 3% H2O, (d) 30 ppm H2S: N2 : H2 : 3% H2O, (e) reduction at 1100˚C for 24 hrs in 100% H2, and (f) re-sintered at 1260˚C in air.

Figure 2. Powder X-ray diffraction patterns of La0.8Sr0.2Mn0.6Cr0.4O3±δ after (a) sintering at 1260˚C in air, reduction at 800˚C for 120hrs in (b) H2: 3% H2O, (c) 10 ppm H2S: N2 : H2 : 3% H2O, (d) 30 ppm H2S: N2 : H2 : 3% H2O, (e) reduction at 1100˚C for 24 hrs in 100% H2, and (f) re-sintered at 1260˚C in air. (La,Sr)2Mn0.6Cr0.4O3±δ, indicated by (·) is a Ruddlesden-Popper (R-P) phase.




The thermo-chemical stability of the LSMC materials was tested at 800˚C in humidified H2 for 5 days. All materials were found to be stable under reducing conditions, as shown for LaMn0.6Cr0.4O3±δ in Fig. 1b, except for La0.8Sr0.2Mn0.6Cr0.4O3±δ, shown in Fig. 2b, which formed an additional Ruddlesden-Popper (R-P) phase ((La,Sr)2Mn0.6Cr0.4O3±δ). This is consistent with the R-P phase, (La,Sr)2MnO3, formed when La0.72Sr0.25Mn0.5Cr0.5O3±δ was quenched at 900˚C in a reducing atmosphere (pO2 = 10-20 bar) (10).

. The relative reducibility of the LSMC materials was determined by temperature

programmed reduction using thermo-gravimetric analysis (TGA), these results show clear trends in terms of the Mn:Cr and La:Sr ratios. LSMC materials with the most Mn (least Cr) were found to be more reducible than those with more Cr, as shown for xSr = 0.2 in Fig. 3, regardless of the Sr content. Sr-free LMC materials (xSr = 0) were found to be more reducible, followed by LSMC materials with xSr = 0.2, and then LSMC with xSr = 0.1, as shown for the yCr = 0.4 materials in Fig. 4.

Figure 3. Temperature programmed reduction for xSr=0.2 obtained by thermogravimetric analysis in 20% H2: 80% He: (i) La0.8Sr0.2Mn0.6Cr0.4O3±δ, (ii) La0.8Sr0.2Mn0.5Cr0.5O3±δ and (iii) La0.8Sr0.2Mn0.4Cr0.6O3±δ.

These trends are dependent on the initial Mn4+ content (oxygen non-stoichiometry) of

each material, as determined by redox titration (9). The oxygen non-stoichiometry (3±δ) of the LSMC materials (yCr = 0.4, 0.5) is shown in Table 1, with the trends with respect to Sr content (xSr) following the TGA trend in Fig. 4. However, the variation in 3±δ with respect to Cr content (yCr = 0.4 and 0.5) is opposite to the TGA trend shown in Fig. 3. LSMC materials with higher Cr content (yCr = 0.5 and 0.6) were found to have very limited solubility in a Fe(II) solution (Fe(NH4)2(SO4)2·6H2O in 10% H2SO4), which rendered the determination of the average Mn oxidation state (Mny+) difficult

After TPR up to 1100˚C, as also achieved by exposure to 24 hrs at 1100˚C in H2, all

of the LSMC materials show the formation of new phases. LMC (xSr = 0) is seen to form La2O3 (Fig. 1e), while LSMC (xSr > 0) forms a R-P phase (Fig. 2e). The reversibility of this phase formation (La2O3 and the R-P phase) was determined by re-sintering these reduced materials (1100˚C/24 hrs) at 1260˚C in air. Both LMC (Fig. 1f) and LSMC (Fig. 2f) show the disappearance of these reduced-state phases, and restoration of the as-synthesized (Figs. 1a and 2a) perovskite-type pXRD patterns.




Figure 4. Temperature programmed reduction for yCr=0.4 obtained by thermogravimetric analysis in 20% H2: 80% He: (i) LaMn0.6Cr0.4O3±δ, (ii) La0.9Sr0.1Mn0.6Cr0.4O3±δ and (iii) La0.8Sr0.2Mn0.6Cr0.4O3±δ.

Electrochemistry in H2:3% H2O

Figure 5. Area corrected Nyquist plots of (A) LaMn0.5Cr0.5O3±δ, (B) La0.9Sr0.1Mn0.5Cr0.5O3±δ, and (C) La0.8Sr0.2Mn0.5Cr0.5O3±δ in half-cell configuration, sintered at 1200˚C, and operating at 800˚C in humidified hydrogen. Showing variation of open circuit potential electrochemical impedance spectroscopy with increasing time of polarization at +0.6 V vs. reference between measurements: (1) before polarization, after (2) 2 min, (3) 4 min, (4) 8 min, (5) 16 min, (6) 32 min, (7) 64 min, (8) 128 min, and (9) 256 min.




EIS measurements were carried out to determine the average area specific series (R1, Rs) and polarization (Rp) resistances, as well as the constant phase element (CPE) characteristics, of the LSMC anodes in humidified H2. The results were compiled from three replicate samples electrochemically tested at the open circuit potential (OCP), first at 700˚C and then at 800˚C. Typical OCP Nyquist plots are shown in Fig. 5 for the La1-

xSrxMn0.5Cr0.5O3±δ (xSr = 0, 0.1 and 0.2) series, sintered at 1200˚C and operating at 800˚C. The EIS data were collected first from high to low frequencies and then back, with good overlap seen in all cases, indicative of the absence of changing properties during the collection of a set of data.

The Nyquist plots in Fig. 5a (xSr = 0) show the presence of two time constants, while

Figs. 5b and 5c (xSr = 0.1 and 0.2, respectively) are better fitted by three time constants. This is consistent with the literature, where two (11 or three (12) arcs have been reported for LSMC anodes. While the Rs values remain constant, the Rp values increase as a function of time following polarization at +0.6 V for successively longer time periods (2 to 256 min). This may reflect the effect of either a changing anode composition (oxidation of Mn sites, which only slowly reduce back again at the OCP), or an altered microstructure, as a result of anodic polarization. Because Rs does not change, this argues against the development of delamination or other interfacial problems.

The Rs values in these experiments are a summation of the ohmic resistances of the Pt

leads, the solid electrolyte, and the WE interfaces, while Rp, the sum of the resistances of all of the arcs, reflects the quality of contact to the current collector, as well as inter-particle contact within the anode microstructure.

R2 (high frequency arc) and R4 (low frequency arc) are typically smaller than R3

(intermediate frequency arc) in both Figs. 5b and 5c. Notably, both the Rs and Rp values are quite high in the data shown in Fig. 5. Therefore, further improvement of the properties of these anodes will be required, including the testing of composite anode materials (using YSZ). However, for the purpose of verifying the anode sulfur tolerance (predicted from the XRD results), the anode performance may not need to be fully optimized.

Based on the EIS data, it was found that the average Rp values are a function of

sintering (1100 vs. 1200˚C) and operating temperatures (700 vs. 800˚C), as well as Sr and Cr content. Fig. 5 shows that both Rs and Rp decrease with increasing Sr content at 800oC. While Fig. 6 shows the variation of Rp with Sr content is a function of both the operating temperature and Cr content. Fig. 6b confirms that at 800˚C, when yCr = 0.5, Rp tends to decrease with increasing Sr content (Fig 5), while when yCr = 0.6, Rp increases with increasing Sr content (Fig. 6a). This results in La0.8Sr0.2Mn0.5Cr0.5O3±δ showing the best catalytic activity at 800˚C (Fig. 6b).

At 700˚C, Rp tends to be lowest for xSr = 0.1 (Fig. 6a-b). At first glance, these trends

appear to follow those shown by TGA with respect to Sr content (Fig. 4). However, TGA shows that the xSr = 0.1 LSMC materials are the least reducible (least % mass loss between 150 to 1100˚C) and so should have the largest Rp (least catalytically active). These TGA results are inconsistent with the electrochemical data, which indicates that, instead, xSr = 0.1 materials have the highest catalytic activity at 700˚C.




Figure 6. Average area specific resistance (R3 + R4) for La1-xSrxMn1-yCryO3±δ (xSr = 0, 0.1, 0.2 and yCr = 0.5, 0.6) at (i) 700˚C and (ii) 800˚C in humidified hydrogen: yCr = 0.6, half-cells sintered at (A) 1200˚C for 2hrs, (B) yCr = 0.5, sintered at 1200˚C for 2hrs. Average resistance (R3 + R4) for each material is calculated from three replicate samples, with a sequence of open circuit potential, cyclic voltammetry and potentiostatic polarization measurements between successive electrochemical impedance spectroscopy measurements.

Rs and Rp tend to increase as Cr content increases (Fig 6a vs. 6b). A lower Rs value

may be due to better contact to the current collector, as well as better inter-particle contact within the anode microstructure, both expected to lead to a higher density of triple phase boundary (TPB) sites and hence a higher reactivity (lower Rp). The trend with Cr content is expected, since a decrease in the Mn content (increase in Cr content) of LSMC materials has previously been shown to cause a decrease in catalytic activity towards the hydrogen oxidation reaction (7). This is also consistent with the TGA data (Fig. 3), which show that LSMC materials with higher Cr content are more thermo-chemically stable, but should be less catalytically active (5) (higher Rp) than those with lower Cr (higher Mn) content. The agreement between the Rp and TGA trends with Cr content is in stark contrast to the opposing trends with Sr content at 700˚C. This may indicate that the TGA mass loss data correlate well with Mn content (yCr) but poorly with the Mn4+ content of these LSMC materials. The latter is thought to give rise to catalytic activity (5). In summary, the electrochemical data suggest that La0.9Sr0.1Mn0.5Cr0.5O3±δ exhibits the highest H2 oxidation catalytic activity at 700˚C (Fig. 6b).

Finally, average Rp tends to decrease with sintering temperature for xSr = 0 and 0.1

LSMC materials. This is probably due to improved adhesion between the anode layer and




YSZ electrolyte, leading to better, sustained activity as a function of operating time, since the anode layer is not as readily disconnecting from the O2- source (YSZ electrolyte). However, for xSr = 0.2, Rp is seen to increase as a function of sintering temperature. This indicates that a lower sintering temperature is likely the optimum electrode preparation condition, at least for La0.8Sr0.2Mn0.4Cr0.6O3±δ. Chemical Stability in H2S:N2:H2:3% H2O

LSMC materials have been reported to be unstable in 0.5% H2S:H2 (6) and 10% H2S:H2

(7). In both cases, the secondary phases formed were MnS, α-MnOS and La2O2S. After exposure to 10 ppm H2S, all LSMC materials were stable in the present work (Fig. 1c) except for La0.8Sr0.2Mn0.6Cr0.4O3±δ, which formed an additional R-P phase in H2: 3% H2O (Fig. 2c), unrelated to the H2S content. LSMC materials with higher Mn content have been found to be more likely to form additional sulfur-containing phases upon exposure to 10% H2S: H2 (7). The chemical stability at 30 ppm H2S was tested with the three yCr = 0.4 LSMC materials. After exposure to 30 ppm H2S (in N2: H2: 3% H2O), LaMn0.6Cr0.4O3±δ (Fig. 1d), La0.9Sr0.1Mn0.6Cr0.4O3±δ and La0.8Sr0.2Mn0.6Cr0.4O3±δ (Fig. 2d), formed an additional phase, La2O2S. La0.8Sr0.2Mn0.6Cr0.4O3±δ (Fig. 2d) additionally shows formation of an R-P phase, as expected from other reduction studies (Fig. 2b and 2c). These results show that all of the LSMC materials studied here are sulfur tolerant upon exposure to 10 ppm H2S in humidified H2, but materials with lower Cr content (yCr) are shown to be unstable in 30 ppm H2S: H2 at 800˚C.

Conclusions

La1-xSrxMn1-yCryO3±δ (xSr = 0, 0.1, 0.2 and yCr = 0.4, 0.5, 0.6) perovskite-type oxides were successfully synthesized by co-precipitation. All materials, except La0.8Sr0.2Mn0.6Cr0.4O3±δ, were found to be stable in humidified hydrogen and 10 ppm H2S : H2 at 800˚C. Sr-free LMC materials were found to form an additional La2O3 phase at 1100˚C in hydrogen, while Sr-doped LSMC materials formed an additional Ruddlesden-Popper (R-P) phase. This additional phase formation was found to be reversible for all materials upon re-sintering at 1260˚C in air. Lower Cr content materials (yCr = 0.4) were found to form an additional La2O2S phase upon exposure to 30 ppm H2S in humidified hydrogen at 800˚C.

The oxygen non-stoichiometry of low Cr content (yCr = 0.4) LSMC materials was determined by permanganate titration, Sr-doped LSMC (xSr = 0.1) was found to have the lowest oxygen non-stoichiometry. This trend was reproduced by temperature programmed reduction between 150 to 1100˚C, where xSr = 0.1 materials were found to be the least reducible (lowest initial Mn4+ content).

Electrochemical experimentation with the anode materials in a half-cell configuration gave average area specific series resistances (Rs) and polarization resistances (Rp). Rp was found to deteriorate as a function of increasing duration of anodic polarization (+ 0.6V vs. reference) probably due to changing Mn oxidation state or anode microstructure as a function of potential.




Acknowledgements

We gratefully acknowledge the support of this work and scholarship funding of LD from National Science and Engineering Research Council. We also extend our thanks to Drs. S. Paulson, J. Fournier, V. Thangadurai, S. Xia, H. Shahbaazi and K. Karan for technical assistance and useful discussions.

References

1. S. Tao and J. T. S. Irvine, The Chemical Record, 4, 83 (2004).

2. S. P. Jiang, X. J. Chen, S. H. Chan, and J. T. Kwok, J. Electrochem. Soc., 153, A580 (2006).

3. S. Tao and J. T .S. Irvine, J. Electrochem. Soc., 151, A252 (2004).

4. M. K. Bruce, M. van den Bossche, and S. McIntosh, J. Electrochem. Soc., 155, B1202 (2008).

5. M. van den Bossche, and S. McIntosh, J. Catal., 255, 313 (2008).

6. J. Chen, Q. L. Liu, S. H. Chan, N. P. Brandon, and K. A. Khor, J. Electrochem. Soc., 154, B1206 (2007).

7. S. Zha, P. Tsang, Z. Cheng, and M. Liu, J. Solid State Chem., 178, 1844 (2005).

8. R. Mukundan, E. L. Brosha, and F. H. Garzon, Electrochem. Solid St., 7, A5 (2004).

9. E. Bloom Jr., T. Y. Kometani, and J. W. Mitchell, J. Inorg. Nucl. Chem., 40, 403 (1978).

10. M. Oishi, K. Yashiro, K. Sato, J. Mizusaki and T. Kawada, J. Solid St. Chem., 181, 3177 (2008).

11. X. C. Lu and J. H. Zhu, Solid State Ionics, 178, 1467 (2007)

12. J. C. Ruiz-Morales, J. Canales-Vazquez, D. Marrero-Lopez, J. T. S. Irvine, and P. Nunez, Electrochem. Acta, 52, 7217 (2007).




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