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A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres Sapna Gupta a,b,n, Prabhakar Singh b,c a
Intel Corporation, Hillsboro, OR, 97124 USA Center for Clean Energy Engineering, Department of Materials Science and Engineering, University of Connecticut, Storrs, CT, 06269 USA c Fraunhofer Center for Energy Innovation, University of Connecticut, Storrs, CT, 06268 USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 June 2016 Received in revised form 27 August 2016 Accepted 27 August 2016
This paper reports a comparative assessment of the chemical and structural stability of LSCM perovskite composites fabricated with 8YSZ and 10ScSZ. Role of oxidizing and reducing atmospheres are examined on the processing and electrochemical performance of the composites. Higher density is obtained during the sintering of LSCM mixed with 10ScSZ when compared to composite mixtures containing 8YSZ in oxidizing as well as reducing atmosphere. Above composites also densified more in Ar-3%H2-3%H2O atmosphere than air along with the formation of SrZrO3 in reducing atmosphere. MnCr2O4 formation is found only in LSCM-8YSZ composite in Ar-3%H2-3%H2O. Electrochemical tests conducted using symmetric cell configurations indicates low polarization resistance and higher performance for Gas (air/fuel)/ LSCM-10ScSZ//8YSZ/LSCM-10ScSZ/Gas (air/fuel). Unlike cell containing LSCM-8YSZ composite, no significant changes are identified in the polarization resistance of LSCM-10ScSZ cell for 80 h. Sr-segregation on the surface of LSCM in electrically tested LSCM-8YSZ cell is attributed to performance degradation in the reducing atmosphere. & 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: LSCM-8YSZ and LSCM-10ScSZ comparison Processing Interfacial reaction – SrZrO3 Reducing atmosphere Electrochemical performance Oxygen transport membrane
1. Introduction Perovskite-fluorite based oxygen transport membrane (OTM) system is currently being developed for a wide range of industrial applications for hydrocarbon conversion to syngas which can be further processed to hydrogen/liquid fuel for transportation [1– 16]. OTM's also find application in advanced coal based power plants for clean and efficient energy production via oxy-combustion with minimal gas emissions [1–16]. The membrane system can also easily be incorporated in the integrated gasification combined cycle (IGCC) and coal gasification fuel cell (CGFC) systems for efficient power generation along with CO2 capture [1–6]. During the operation of oxygen transport membrane system, one side of the dense membrane is exposed to air and the other side to fuel (e.g. H2, CO, CH4) [1,9]. Due to the oxygen partial pressure difference across the membrane, oxygen selectively permeates through the membrane To improve on the surface exchange kinetics and the oxygen flux, membranes utilize porous air and fuel electrodes in the OTM system [1,9]. Porous electrodes offer enhanced triple phase boundary sites for electron exchange n
Corresponding author at: Intel Corporation, Hillsboro, OR 97124, USA. E-mail address: [email protected] (S. Gupta).
and electrochemical reactions as well as reduce gas phase diffusional resistance. Similar to SOFC electrodes, obtaining long term stable performance with no or negligible degradation of OTM air/ fuel electrode is found to be a challenge for the prolonged stable operation of OTM system [1–3,9,16]. Existing fuel and air electrodes, fabricated using perovskites, remain unsuitable for the long-term stable operation of OTM system due to interfacial reactions and related compound formation as well as surface morphological changes due to exolution of dopants. This is due to insufficient structural-chemical-thermal stability of materials, processing abnormalities, chemical decomposition, phase separation, and interfacial stability [1,3,8,9,14]. For example, LaCoO3 d decomposes and form La2CoO4 and CoO at PO2 o 10 7 atm and 1000 °C [1]. Likewise, LaFeO3 d decomposes and form La2O3 and Fe metal at PO2 o10 17 atm and 1000 °C [1]. LSM-YSZ can be considered to be used for air electrode of the OTM system as it is already being used as the state of the art for SOFC. However, it degrades with time and tend to delaminate after long term electrical testing. Interfacial stability and cation migration are a few known challenges with LSM-YSZ [17–19]. Interfacial compound formation (e.g. La2Zr2O7 and SrZrO3) at the interface of LSM and YSZ results in increase in ohmic resistance contributing to lower performance [17–19]. Similar to SOFC anode, Ni-YSZ can be used as OTM fuel electrode due to its high conductivity and
http://dx.doi.org/10.1016/j.ceramint.2016.08.172 0272-8842/& 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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electro-catalytic activity. However, its structural stability, volume expansion, limited resistance to sulfur poisoning and carbon deposition are major concerns in reducing atmosphere under OTM operating conditions [3,20]. To minimize and overcome the above mentioned challenges with the electrode materials, A and B-site doped perovskites are being investigated for OTM electrodes. (Ln, A)(B)O3 δ (Ln¼Lanthanide group: La), A ¼(Alkaline earth: Sr, Ca) and B ¼(Transition metal: Mn, Cr) based single-phase perovskites have been investigated for the air and fuel electrodes [1,5,9–12]. Long-term thermal/chemical stability and performance of these perovskites are still a challenge for stable operation of OTM [1,9,13]. Perovskite-fluorite based mixed ionic and electronic conductor (MIEC) and dual phase composites are currently being investigating for dual benefit i.e. high performance and stability under OTM system operating conditions [1,10,14–16]. It is found that the performance and stability of the composites are higher when compared to single-phase perovskite oxides [1,13]. Due to higher stability at high temperature (Z1000 °C) in a wide range of oxygen partial pressure (1–10 24 atm), lanthanum chromite based materials are considered to be a promising material for electrodes [1,2,5,6,11,14]. When mixed with fluorite phase, high performance can be obtained as required for OTM [1,13]. A review article on lanthanum chromite based materials for oxygen transport membrane system is published recently by the authors [1]. In this study, the authors have chosen strontium and manganese co-doped lanthanum chromite based perovskite of composition (La0.75Sr0.25)0.95Cr0.7Mn0.3O3 (LSCM) for the OTM electrode investigation as it can provide superior performance and long term structural stability when mixed with fluorite phase based on the critical literature review [1,21–23]. 8YSZ and 10ScSZ are two commonly reported stable fluorite materials [1,24–29]. Most of the literature reports result on LSCM and 8YSZ [27,28]. 10ScSZ is stable and provides higher ionic conductivity than 8YSZ which appears to be promising for higher performance along with its stability. It is important to find the right combination of perovskite and fluorite phases to achieve higher performance and stability under OTM system operating conditions. Therefore, in this study, the authors investigate a comparative study on LSCM processing (under OTM system fabricating conditions) with 8YSZ and 10ScSZ. Electrochemical performance (under OTM system operating conditions) and stability of LSCM are also investigated when combined with 8YSZ and 10ScSZ under cathodic (oxidizing) and anodic (reducing) OTM operating conditions. This study can also be utilized for other high temperature electrochemical systems (e.g. solid-oxide fuel and electrolysis cells – SOFC and SOEC) as similar MIEC materials are currently being developed for these applications [2,5,21–23,30–39].
10ScSZ samples was measured. Scanning electron (FEI – ESEM Quanta 250) and transmission electron (FEI Tecnai T12 S/TEM) microscopes were utilized for microstructural analysis of the sintered and tested samples. Elemental profile analysis of the samples was conducted using Energy dispersive spectroscopy (EDS) attached to the SEM as well as TEM. Focused ion beam (FEI Strata 400S DUALBEAM FIB) technique was used to prepare samples for TEM microstructural analysis. X-ray diffraction (BRUKER-D8 ADVANCE, Bruker AXS Inc.) was utilized to determine the crystal structure of LSCM composite with 8YSZ and 10ScSZ and secondary compounds, if any when exposed to oxidizing and reducing gas atmosphere. The scan step used for X-ray diffraction was 0.02° using CuKα radiation (λ ¼1.5406 Å). 2.3. Symmetric cell fabrication LSCM-8YSZ and LSCM-10ScSZ pastes were prepared using ink vehicle (Fuel Cell Materials). As working and counter electrodes (thickness: 20 mm, diameter: 10 mm), LSCM-8YSZ and LSCM10ScSZ composites were subsequently screen-printed on both sides of (ZrO2)0.92(Y2O3)0.08(YSZ) electrolyte (200 mm thick, Fuel Cell Materials). The electrodes were then dried at room temperature and subsequently sintered at 1200 °C (heating rate of 3 °C/ min) for 2 h in air. Lower processing temperature (1200 °C) was chosen for the symmetrical cells to achieve porous electrode layers structure and also to avoid secondary phase formations that were identified at 1400 °C. The electrochemical active area of the cell electrode was 0.8 cm2. Platinum screen current collector (Alfa Aesar – 50 mesh) and platinum wires (Alfa Aesar – 0.25 mm) were attached to both the electrodes using platinum paste from ElectroScience Laboratories Inc. Curing of platinum paste was obtained in air at 900 °C with 1 h (3 °C/min) dwell time. As-assembled LSCM8YSZ//8YSZ//LSCM-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetric cells were then installed in a tubular alumina chamber and placed in the constant temperature furnace zone. Multichannel potentiostat (VMP2, Bio-Logic) leads were attached to the assembled symmetric cell for electrochemical impedance measurement. 2.4. Electrochemical testing LSCM-8YSZ and LSCM-10ScSZ based symmetrical cells were heated up to 950 °C with the heating rate of 3 °C/min in air and Ar3%H2-3%H2O with flow rate of 300sccm. The cells were tested for 80 h under the constant bias of 0.5 V. The impedance measurement was performed (at two hour intervals) in the frequency range starting from 100 mHz to 200 kHz using a 10 mV alternating current. Experiments were repeated a couple of times under same condition to ensure reproducibility. Post-test characterizations of the samples were performed using SEM-EDS (FEI – ESEM Quanta 250).
2. Experimental procedure 2.1. (La0.75Sr0.25)0.95Cr0.7Mn0.3O3 δ and 8YSZ/10ScSZ synthesis
3. Results and discussion
LSCM powder was received from Praxair Inc. The powder was mixed with 8YSZ and10ScSZ in the weight ratio of 50:50. Mixed powders were then uniaxially pressed into cylindrical pellets and subsequently sintered in oxidizing (air) and reducing (Ar-3%H2-3% H2O) atmosphere at 1400 °C with 10 h dwell time. To ensure the maintenance of the oxygen pressure during cooling, samples were cooled in the flowing gas environment.
3.1. Crystal structure
2.2. Characterization Using Archimedes principle, density of LSCM-8YSZ and LSCM-
Figs. 1 and 2 shows the XRD pattern of LSCM composite with 8YSZ and 10ScSZ sintered in oxidizing (air 0.21 atm) and reducing atmosphere (Ar-3%H2-3%H2O 10 10 atm) at 1400 °C. LSCM crystal structure is rhombohedral (JCPDS 75-9872) and 8YSZ XRD peaks corresponds to cubic phase (JCPDS 70-4436) in air. However, in both the cases (LSCM-8YSZ and LSCM-10ScSZ), it is identified that the peaks splitting associated with the LSCM rhombohedral phase disappears for the samples sintered in Ar-3%H2-3%H2O as shown in Fig. 1(e.g. inserted plot peak position). This corresponds
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Table 1 Relative density of LSCM-8YSZ and LSCM-10ScSZ sintered at 1400 °C for 10 h.
Material
Relative density (%) Air
Ar-3%H2-3%H2O
LSCM–8YSZ LSCM–10ScSZ
597 2.2 78.17 1.5
74.8 7 1.3 88.2 70.9
compared to 10ScSZ. 3.2. Sintering behavior and microstructural analysis
Fig. 1. XRD pattern of LSCM and 8YSZ/10ScSZ composites when exposed to air and Ar-3%H2-3%H2O at 1400 °C for 10 h: (a) LSCM – 8YSZ and (b) LSCM – 10ScSZ.
Fig. 2. XRD comparison plot of LSCM when mixed with 8YSZ and 10ScSZ in Ar-3% H2-3%H2O at 1400 °C for 10 h.
to phase transformation to a higher symmetry cubic phase (JCPDS 074-1961) [2,5]. The authors have identified and reported similar phase transformation in reducing atmosphere for single phase LSCM perovskite [11]. Furthermore, it indicates that the cubic phase becomes dominating in Ar-3%H2-3%H2O atmosphere as the peak splitting corresponding to rhombohedral phase almost disappears. Extra peaks corresponding to the strontium zirconate (SrZrO3) (JCPDS-01-074-1297) are identified in Ar-3%H2-3%H2O sintered samples unlike air. Fig. 2 shows XRD pattern comparison of LSCM composite with 8YSZ and 10ScSZ when processed in Ar3%H2-3%H2O atmosphere at 1400 °C. Peak intensity of SrZrO3 is higher in case of LSCM-8YSZ composite when compared to LSCM10ScSZ under the same conditions. This corresponds to higher amount of SrZrO3 formation when LSCM is mixed with 8YSZ when
Table 1 shows relative density of LSCM-8YSZ and LSCM-10ScSZ sintered in oxidizing (air) and reducing atmosphere (Ar-3%H2-3% H2O) at 1400 °C for 10 h. Higher density of LSCM-8YSZ composite is obtained in Ar-3%H2-3%H2O (74.8 71.3%) atmosphere when compared to air (59 72.2%). On the other hand, LSCM-10ScSZ composite density increases from 78.1 71.5% to 88.2 70.9% when gas atmosphere is switched from oxidizing (air) to reducing (Ar-3% H2-3%H2O) respectively. In both the cases, it is noted that lower density is obtained when the composites are processed in air atmosphere. This is due to the fact that Cr-evaporates (Cr2O3 þ3/2 O2-2CrO3↑) from the bulk and condensates (2CrO3↑-Cr2O3 þ3/ 2O2) at the grain boundaries in oxidizing atmosphere and that inhibits further mass transport and grain growth of lanthanum chromite based materials resulting in lower densification in air [1,2]. On comparison with LSCM-8YSZ composite, LSCM-10ScSZ achieves higher density in oxidizing as well as reducing atmosphere. This is attributed to the higher density of 10ScSZ (88.5 71.5%) itself when compared to 8YSZ (83.0 71.8%) in reducing gas atmosphere. Moreover, the ionic radii of Zr4 þ ¼0.84 Å matches with Sc3 þ ¼0.87 Å compared to Y3 þ ¼ 1.02 Å. This may attribute to the closed packed and dense structure of ScSZ fluorite and the composite with LSCM when compared to LSCM-YSZ. Fig. 3 shows the SEM microstructures of LSCM composite with 8YSZ and 10ScSZ, sintered in air and Ar-3%H2-3%H2O gas atmosphere. Unlike fluorite phase, it is found that the morphology of perovskite (LSCM) phase changes in reducing atmosphere. The surface rearrangement may attribute to the decrease in the intensity of the corresponding XRD pattern (Fig. 1) in reducing atmosphere [2,40]. No secondary phases have been identified in oxidizing as well as reducing atmosphere using SEM. However, SrZrO3 secondary phase formation is detected using XRD technique for the samples sintered in Ar-3%H2-3%H2O. To confirm on the secondary phase formation, TEM microstructural analysis is conducted on LSCM-8YSZ and LSCM-10ScSZ samples sintered in air as well as Ar-3%H2-3%H2O atmosphere as shown in Fig. 4. The sample preparation for TEM analysis is performed using FIB as shown in the insert of Fig. 4a. It is clear that the denser microstructure is obtained for LSCM when mixed with 10ScSZ than 8YSZ. No secondary phase is identified in the samples sintered in air atmosphere. On the other hand, as expected from the XRD pattern and analysis, secondary phases are identified in both the composites in reducing gas atmosphere (Fig. 4c and d). TEM elemental analysis reveals that the secondary phase predominantly consists of Sr and Zr corresponding to SrZrO3 formation as shown in Table 2 (LSCM-8YSZ) and Table 3 (LSCM-10ScSZ). However, a second new phase is identified only in Ar-3%H2-3%H2O sintered LSCM-8YSZ composite as shown in the TEM micrograph (Fig. 4c). The secondary phase is enriched in Mn and Cr as shown in the Table 2. This corresponds to MnCr2O4 spinel formation attributing to Mn:Cr::1:2. The phase is not identified in the XRD. This is probably due to small amount of phase is present which is outside the detection limit of XRD. Fig. 5 shows the TEM elemental mapping of FIB-cross-section of
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 3. SEM micrographs of LSCM containing 8YSZ and10ScSZ composite sintered at 1400 °C for 10 h in air (a) LSCM-8YSZ, (b) LSCM-10ScSZ; in Ar-3%H2-3%H2O (c) LSCM8YSZ and (d) LSCM-10ScSZ.
sintered LSCM-8YSZ in Ar-3%H2-3%H2O. It is clearly identified that one of the secondary phase is enriched in Sr and Zr and other in Mn and Cr. However, TEM elemental mapping of LSCM-10ScSZ indicates only one secondary phase formation enriched in Sr and Zr as shown in Fig. 6. Fig. 7 shows a schematic of mechanism steps for the formation of SrZrO3 and MnCr2O4 in LSCM-8YSZ and LSCM-10ScSZ in reducing atmosphere. Step 1 represents an initial stage of LSCM-8YSZ and LSCM-10ScSZ system heating to the temperature of the reaction. In step 2, reduction of Mn takes place from Mn4 þ -Mn3 þ Mn2 þ resulting in diffusion of Mn into the fluorite phase. It is found that the solubility of lower valence state Mn is higher in fluorite phase (e.g. 8YSZ) when exposed to reducing gas atmosphere due to better matching ionic radius with Zr4 þ [16,41]. Chemical diffusion coefficient of Mn in 8YSZ is 10 13 cm2/s at 1000 °C [42]. Moreover, as Mn diffuses into the fluorite phase, A-site gets enriched in LSCM (step 2) and therefore, Sr reacts with Zr resulting in the formation of SrZrO3 (step 3) at the interface of LSCM and 8YSZ [3,16]. Concurrently, the Cr and Mn components at the B-site, now in excess within the perovskite lattice, react to form MnCr2O4 to keep the mass balance as shown in step 3. Unlike LSCM-10ScSZ, MnCr2O4 formation is identified in the case of LSCM-8YSZ (step 3). Small amount of SrZrO3 forms in case of LSCM-10ScSZ composite when compared to LSCM-8YSZ. This is in agreement with the observed lower SrZrO3 peaks intensity (Fig. 2.) in the XRD pattern of LSCM-10ScSZ when compared to LSCM-
8YSZ. This is attributed to the lower amount of Mn diffusion in ScSZ due to the higher stability and better ionic radius match of Zr with Sc than Y. Transmission electron microscopy elemental analysis also reveals that Mn diffusion is lower in 10ScSZ ( 1.1 at%) than 8YSZ ( 3.5 at%) in reducing atmosphere as shown in Tables 2 and 3. Therefore, LSCM perovskite lattice is not significantly enriched in B-site after small amount of SrZrO3 formation. Concurrently, the Cr and Mn components at the B-site, now not significantly enriched within the perovskite lattice to react and form MnCr2O4. Overall, this attributes to the higher stability of LSCM-10ScSZ composite when compared to LSCM-8YSZ in reducing gas atmosphere. 3.3. Electrochemical measurements and post-test characterization Fig. 8a and b shows comparison of the Nyquist plots of impedance spectra acquired from LSCM-8YSZ//8YSZ//LSCM-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetrical cells electrochemical testing at 950 °C (0–80 h) and 0.5 V. Only two spectra (0 and 80 h) are shown for clarity. The intercept associated with the high frequency represents ohmic (electrolyte) resistance and on the other hand, semi-circle diameter represents polarization resistance which includes oxygen surface exchange and migration and electrochemical kinetics associated with electrode layer [3,43]. A significant difference is identified in the impedance spectra of both the cells as shown in Fig. 8a and b. Higher performance and
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 4. TEM micrographs of LSCM containing 8YSZ and10ScSZ composite sintered at 1400 °C for 10 h in air (a) LSCM-8YSZ, (b) LSCM-10ScSZ; in Ar-3%H2-3%H2O, (c) LSCM8YSZ and (d) LSCM-10ScSZ.
Table 2 STEM-EDS elemental analysis of LSCM-8YSZ sintered in Ar-3%H2-3%H2O. Element
La
Sr
Cr
Mn
Y
Zr
LSCM (at%) YSZ (at%) Sr-Zr rich phase (at%) Mn-Zr rich phase (at%)
42.1 0 3.5 0
9.8 1.6 38 0
35.6 0.3 4.8 65.4
10.6 3.5 0 32.6
0 14.1 0.4 0.2
1.1 80.5 53.3 1.8
Table 3 STEM-EDS elemental analysis of LSCM-10ScSZ sintered in Ar-3%H2-3%H2O. Element
La
Sr
Cr
Mn
Sc
Zr
LSCM (at%) ScSZ (at%) Sr-Zr rich phase (at%)
41.5 0.1 12.7
9.7 0 40.8
36.7 0.1 6.9
10.7 1.1 0.4
0 12.9 1.4
1.3 85.7 37.7
lower polarization resistance is reflected from the impedance spectra of LSCM-10ScSZ//8YSZ//LSCM-10ScSZ when compared to LSCM73/8YSZ//8YSZ//LSCM73/8YSZ in oxidizing (Fig. 8a) and reducing atmosphere (Fig. 8b) during the time interval of 0–80 h. Fig. 9a and b shows comparison plot of polarization resistances of symmetrical cells of configuration Gas (air/ Fuel)/LSCM-8YSZ// 8YSZ//LSCM-8YSZ/Gas (air/fuel) and Gas (air/fuel)/LSCM-10ScSZ//
8YSZ//LSCM-10ScSZ/Gas (air/fuel) as a function of time (80 h) at 950 °C in air and Ar-3%H2-3%H2O respectively. Constant bias of 0.5 V is applied. In oxidizing atmosphere (air), the polarization resistance of LSCM-8YSZ//8YSZ//LSCM-8YSZ increases and then stabilizes after 30 h. There is no significant difference observed after the stabilization up to 80 h. The polarization resistance increases from 12.0 to 12.1 Ω cm2 as time increases from 30 h to 80 h. In case of LSCM-10ScSZ//8YSZ//LSCM-10ScSZ exposed to air, there is also no large variation in the performance and polarization resistance. It increases from 1.3 (30 h) to 1.5 Ω cm2 (80 h). On comparison, LSCM-10ScSZ based symmetrical cell takes less time for stabilization than LSCM-8YSZ (Fig. 9a). Moreover, LSCM-10ScSZ provides higher performance and lower polarization resistance ( 9 times) than LSCM-8YSZ as shown in Fig. 9a. This may correspond to the higher conductivity of 10ScSZ fluorite phase when compared to 8YSZ [1,29]. On the other hand, in reducing atmosphere, the polarization resistance of LSCM-8YSZ based symmetrical cell increases continuously with time (Fig. 9b). The initial resistance of the cell is 12.0 Ω cm2 (0 h) which increases to 24.9 Ω cm2 (80 h) in Ar3%H2-3%H2O at 950 °C under the constant bias of 0.5 V. However, unlike LSCM-8YSZ system, LSCM-10ScSZ based symmetrical cells polarization resistance does not vary significantly with increase in time as shown in Fig. 9b. On comparison, even in reducing atmosphere, lower resistance ( 3.5 times) and higher stability is
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 5. Elemental mapping of FIB cross-section of sintered LSCM-8YSZ in Ar-3%H2-3%H2O.
Fig. 6. Elemental mapping of FIB cross-section of sintered LSCM-10ScSZ in Ar-3%H2-3%H2O atmosphere.
obtained for LSCM-10ScSZ based composite cell when compared to LSCM-8YSZ. This corresponds to lower stability and degradation of the LSCM-8YSZ cell when compared to LSCM-10ScSZ in Ar-3% H2-3%H2O atmosphere. Overall, for both the cases (oxidizing and reducing), LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetrical cells achieved higher performance and lower resistance when compared to LSCM-8YSZ//8YSZ//LSCM-8YSZ. This corresponds to the higher conductivity and stability of LSCM-10ScSZ on comparison with LSCM-8YSZ. Microstructural analysis of the tested symmetrical cells were performed after completion of the electrochemical tests to understand the degradation of the tested symmetrical cells. Fig. 10a– c (a: as-sintered, b: tested in air, c: tested in Ar-3%H2-3%H2O) shows LSCM-8YSZ microstructures after the electrochemical testing. There is no significant difference identified in the air tested sample on comparison with the as-sintered sample. This attributes to the stable performance of the cell with time up to 80 h.
However, in reducing atmosphere, it is found that the surface morphology of the LSCM phase is modified with nano particles on surface after the testing. Using SEM-EDS analysis, the modified surface of LSCM is found to be enriched in Sr when compared to the as-sintered sample. An average ( 20 points) of Sr:La ratio of the LSCM phase surface is 0.43 70.04 for the as-sintered cell, 0.44 70.05 for the air tested cell and 0.65 70.05 for the reducing gas atmosphere tested cell. Unlike Ar-3%H2-3%H2O, the Sr: La ratio matches for the as-sintered and air tested cell. This confirms on the Sr-enrichment of the LSCM surface in reducing gas atmosphere. This corresponds to the lower polarization resistance and stability of the cell tested in reducing atmosphere. Similar performance degradation due to Sr-enrichment on LSM surface is identified for (La0.8Sr0.2)0.98MnO3 (LSM) based cell tested for 100 h at 850 °C [44]. Fig. 10d–f (d: as-sintered, e: tested in air, f: tested in Ar-3%H23%H2O) shows microstructure of LSCM-10ScSZ based cells. No
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 7. Schematic of mechanism steps for the formation of SrZrO3/MnCr2O4 in LSCM-8YSZ and LSCM-10ScSZ composites in reducing atmosphere.
Fig. 8. Comparison of the Nyquist plots of impedance spectra acquired from LSCM-/8YSZ//8YSZ//LSCM7-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetrical cells testing at 950 °C (0–80 h) and 0.5 V: (a) air, and (b) Ar-3%H2-3%H2O. Only two spectra (0 and 80 h) are shown for clarity.
changes are identified in the cells after testing in oxidizing as well reducing atmosphere. This attributes to higher performance and stability of LSCM-10ScSZ based cells. Unlike LSCM-8YSZ cell, no LSCM surface modification is observed in the reducing atmosphere tested cells. On comparison, LSCM-10ScSZ based cells are more stable and provides higher performance than LSCM-8YSZ cells in both the oxidizing and reducing atmosphere.
4. Conclusion
Fig. 9. Comparison plots of resistance (non-ohmic) changes with time for LSCM8YSZ//8YSZ//LSCM-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ cell tested at 0.5 V: (a) in air, and (b) Ar-3%H2-3%H2O.
Composites consisting of LSCM-8YSZ and LSCM-10ScSZ have been processed in oxidizing and reducing atmosphere (Ar-3%H2Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 10. SEM micrographs of the tested symmetrical cell of LSCM-8YSZ//8YSZ//LSCM-8YSZ (anode surface): (a) as-sintered, (b) in air, (c) in Ar-3%H2-3%H2O; LSCM-10ScSZ// 8YSZ//LSCM-10ScSZ: (d) as-sintered, (e) in air, and (f) Ar-3%H2-3%H2O.
Table 4 Summary and comparison of LSCM-8YSZ and LSCM-10ScSZ composites. LSCM-8YSZ
LSCM-10ScSZ
Lower relative density (%) Air 597 2.2 Ar-3%H2-3%H2O 74.8 7 1.3
Higher relative density (%) Air 78.17 1.5 Ar-3%H2-3%H2O 88.2 7 0.9
Secondary Phases (More) Air No Ar-3%H2-3%H2O SrZrO3 and MnCr2O4
Secondary Phases (Less) Air No Ar-3%H2-3%H2O SrZrO3
Mn diffusion into YSZ (Higher) Air No Ar-3%H2-3%H2O 3.5 at%
Mn diffusion into ScSZ (Lower) Air No Ar-3%H2-3%H2O 1 at%
Polarization resistance (Higher) Air 7.1 Ω cm2 Ar-3%H2-3%H2O 12.0 Ω cm2
Polarization resistance (Lower) Air 0.8 Ω cm2 Ar-3%H2-3%H2O 3.4 Ω cm2
3%H2O) at 1400 °C. Both the composites are characterized using XRD, SEM, TEM (with EDS) and EIS. Symmetrical cells based on LSCM-8YSZ and LSCM-10ScSZ electrodes are also tested using electrochemical impedance measurement technique. Results of this study and comparison of LSCM-8YSZ and LSCM-10ScSZ composites are listed in Table 4. Our key findings include: 1. Higher density is achieved for LSCM when mixed with 10ScSZ compared to 8YSZ. 2. Interfacial reaction is identified in reducing atmosphere with higher amount of SrZrO3 formation in LSCM-8YSZ when exposed to reducing atmosphere. 3. Mn diffusion into the fluorite phase drives interfacial reaction resulting in SrZrO3 formation. 4. Mechanistic steps for secondary phase formation's (SrZrO3 and MnCr2O4) are explained and reported in this study.
5. Lower polarization resistance is obtained from LSCM-10ScSZ based symmetrical cell when compared to LSCM-8YSZ in oxidizing as well as reducing atmosphere. 6. Higher stability is achieved for LSCM-10ScSZ cell compared to LSCM-8YSZ in reducing atmosphere. 7. LSCM-10ScSZ composite is more beneficial to be used for higher stability and performance of high temperature electrochemical devices such as OTM.
Acknowledgments This work was performed as part of doctoral research (SG). Authors sincerely thank Center for Clean Energy Engineering for providing laboratory facilities and advanced characterization equipment's for conducting experimental work. Helpful discussions with Manoj K. Mahapatra is gratefully acknowledged.
References [1] S. Gupta, M.K. Mahapatra, P. Singh, Lanthanum chromite based perovskites for oxygen transport membrane, MSE: R: Rep. 90 (2015) 1–36. [2] S. Gupta, J.J. Adams, J.R. Wilson, E.G. Eddings, M.K. Mahapatra, P. Singh, Performance and post-test characterization of an OTM system in an experimental coal gasifier, Appl. Energy 165 (2016) 72–80. [3] S. Gupta, Development of Perovskite/Fluorite Based Materials for Oxygen Transport Membrane System (thesis), 2015. 〈http://digitalcommons.uconn. edu/dissertations/799〉. [4] S. Gupta, M.K. Mahapatra, P. Singh, Phase transformation, thermal expansion and electrical conductivity of lanthanum chromite, Mater. Res. Bull. 48 (2013) 3262–3267. [5] S. Gupta, P. Singh, Manganese doped lanthanum strontium chromite fuel electrode for solid oxide fuel cell and oxygen transport membrane systems, ECS Trans. 66 (2015) 117–123. [6] K.T. Jacob, S. Gupta, P. Singh, Thermodynamic properties of LaCrO4, La2CrO6 and La2Cr3O12, and sub-solidus phase relations in the system lanthanumchromium-oxygen, J. Am. Ceram. Soc. 96 (2013) 3933–3938. [7] K.T. Jacob, S. Gupta, P. Singh, Electrochemical determination of Gibbs energy of formation of LaCrO3 using a composition- graded bi-electrolyte, J. Am. Ceram.
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
S. Gupta, P. Singh / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Soc. 96 (2013) 3272–3278. [8] S. Gupta, P. Singh, Structural and electrochemical performance stability of perovskite-fluorite composite for high temperature electrochemical devices, Adv. Solid Oxide Fuel Cells Electron. Ceram. (2015) 67–74. [9] H.J.M. Bouwmeester, A.J. Burggraaf, Fundamentals of inorganic membrane science and technology, in: A.J. Burggraaf, L. Cot (Eds.), Membrane Science and Technology Series 4, Elsevier, Amsterdam, 1996, pp. 435–510. [10] N. Nagabhushana, J.A. Lane, G.M. Christie, B.A.V. Hassel, Composite Oxygen Ion Transport Membrane, US Patent 7556676B2, 2009. [11] S. Gupta, Y. Zhong, M.K. Mahapatra, P. Singh, Processing and electrochemical performance of manganese-doped lanthanum-strontium chromite in oxidizing and reducing atmospheres, Int. J. Hydrog. Energy 40 (2015) 13479–13489. [12] J.J. Liu, T. Liu, W.D. Wang, J.F. Gao, C.S. Chen, Zr0.84Y0.16O1.92– La0.8Sr0.2Cr0.5Fe0.5O3 δ dual-phase composite hollow fiber membrane targeting chemical reactor applications, J. Membr. Sci. 389 (2012) 435–440. [13] J. Sunarso, S. Baumann, J.M. Serra, W.A. Meulenberg, S. Liu, Y.S. Lin, J.C. Diniz da Costa, Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation, J. Membr. Sci. 320 (2008) 13–41. [14] S. Gupta, J.A. Lane, J.R. Wilson, P.J. Plonczak, P. Singh, Interfacial stability of iron doped lanthanum strontium chromite and Scandia stabilized zirconia in oxidizing and reducing atmosphere, Solid State Ion. 288 (2016) 10–13. [15] W. Fang, Y. Zhang, J. Gao, C. Chen, Oxygen permeability of asymmetric membrane of functional La0.8Sr0.2Cr0.5Fe0.5O3 δ (LSCrF)–Zr0.8Y0.2O2-δ (YSZ) supported on porous YSZ, Ceram. Int. 40 (2014) 799–803. [16] S. Gupta, P. Singh, Nickel and titanium doubly doped lanthanum strontium chromite for high temperature electrochemical devices, J. Power Sources 306 (2016) 801–811. [17] M. Keane, M.K. Mahapatra, A. Verma, P. Singh, LSM-YSZ interactions and anode delamination in solid oxide electrolysis cell, Int. J. Hydrog. Energy 37 (2012) 16776–16785. [18] N. Li, M. Keane, M.K. Mahapatra, P. Singh, Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ, Int. J. Hydrog. Energy 38 (2013) 6298–6303. [19] B. Hu, M. Keane, M.K. Mahapatra, P. Singh, Stability of strontium-doped lanthanum manganite cathode in humidified air, J. Power Sources 248 (2014) 196–204. [20] V.B. Vert, F.V. Melo, L. Navarrete, J.M. Serra, Redox stability and electrochemical study of nickel doped chromites as anodes for H2/CH4-fueled solid oxide fuel cells, Appl. Catal. B Environ. 115 (2012) 115. [21] S. Tao, J.T.S. Irvine, Phase transition in perovskite oxide La0.75Sr0.25Cr0.5Mn0.5O3 δ observed by in situ high temperature neutron powder diffraction, Chem. Mater. 18 (2006) 5453–5460. [22] S. Tao, J.T.S. Irvine, Synthesis and characterization of (La0.75Sr0.25)Cr0.5Mn0.5O3– δ – redox-stable, efficient perovskite anode for SOFCs, J. Electrochem. Soc. 151 (2004) A252–A259. [23] S. Tao, J.T.S. Irvine, A redox-stable efficient anode for solid-oxide fuel cells, Nat. Mater. 2 (2003) 320–323. [24] M. Mori, Y. Hiei, T. Yamamoto, Control of the thermal expansion of strontiumdoped lanthanum chromite perovskites by B-site doping for high-temperature solid oxide fuel cell separators, J. Am. Ceram. Soc. 84 (2001) 781–786. [25] M. Mori, T. Yamamoto, H. Itoh, H. Inaba, H. Tagawa, Thermal expansion of nickel-zirconia anodes in solid oxide fuel cells during fabrication and operation, J. Electrochem. Soc. 145 (1998) 1374–1381. [26] A. Bieberle, L.J. Gauckler, in: H. Tuller, J. Schoonman, I. Riess (Eds.), Oxygen Ion
[27]
[28]
[29]
[30] [31]
[32] [33]
[34]
[35]
[36]
[37]
[38]
[39]
[40] [41]
[42] [43]
[44]
9
and Mixed Conductors and Their Technological Applications, Kluwer Academic Publishers, Dordrecht, 2000, pp. 347–358. S.P. Jiang, X.J. Chen, S.H. Chan, J.T. Kwok, K.A. Khor, (La0.75Sr0.25)(Cr0.5Mn0.5)O3/ YSZ composite anodes for methane oxidation reaction in solid oxide fuel cells, Solid State Ion. 177 (2006) 149–157. I. Jung, D. Lee, S.O. Lee, D. Kim, J. Kim, S.H. Hyun, J. Moona, LSCM–YSZ nanocomposites for a high performance SOFC anode, Ceram. Int. 39 (2013) 9753–9758. S. Kazlauskas, A. Kežionis, T. Šalkus, A.F. Orliukas, Sintering of oxygen ion conductive ceramics and their electrical properties, Lith. J. Phys. 52 (2012) 231–237. R.E. Williford, P. Singh, Engineered cathodes for high performance SOFCs, J. Power Sources 128 (2004) 45–53. V.V. Kharton, E.V. Tsipis, I.P. Marozau, A.P. Viskup, J.R. Frade, J.T.S. Irvine, Mixed conductivity and electrochemical behavior of (La0.75Sr0.25)0.95Cr0.5Mn0.5O3 δ, Solid State Ion. 178 (2007) 101–113. J. Liu, B. Madsen, Z. Ji, S. Barnett, A fuel-flexible ceramic-based anode for solid oxide fuel cells, Solid State Lett. 5 (2002) A122–A124. S. Zha, P. Tsang, Z. Cheng, M. Liu, Electrical properties and sulfur tolerance of La0.75Sr0.25Cr1 xMnxO3 under anodic conditions, J. Solid State Chem. 175 (2005) 1844–1850. D.M. Bastidas, S. Tao, J.T.S. Irvine, A symmetrical solid oxide fuel cell demonstrating redox stable perovskite electrodes, J. Mater. Chem. 16 (2006) 1603–1605. M.F. Lu, E.V. Tsipis, J.C. Waerenborgh, A.A. Yaremchenko, V.A. Kolotygin, S. Bredikhin, V.V. Kharton, Thermomechanical, transport and anodic properties of perovskite-type (La0.75Sr0.25)0.95Cr1 xFexO3 δ, J. Power Sources 206 (2012) 59–69. F. Chen, M.L. Liu, Preparation of yttria stabilized zirconia (YSZ) films on La0.85Sr0.15MnO3 (LSM) and LSM-YSZ substrates using electrophoretic deposition (EPD) process, J. Eur. Ceram. Soc. 21 (2001) 127–134. Z. Jiang, Z. Lei, B. Ding, C. Xia, F. Zhao, F. Chen, Electrochemical characteristics of solid oxide fuel cell cathodes prepared by infiltrating (La,Sr)MnO3 nanoparticles into yttria-stabilized bismuth oxide backbones, Int. J. Hydrog. Energy 35 (2010) 8322–8330. C. Yang, C. Jin, F. Chen, Characterization of infiltrated (La0.75Sr0.25)0.95MnO3 as oxygen electrode for solid oxide electrolysis cells, Int. J. Hydrog. Energy 35 (2010) 5187–5193. W. Tan, Q. Zhong, D. Xu, H. Yan, X. Zhu, Catalytic activity and sulfur tolerance for Mn-substituted La0.75Sr0.25CrO3 7 δ in gas containing H2S, Int. J. Hydrog. Energy 36 (2013) 16656–16664. H. Chen, Surface/interface X-ray diffraction review, Mater. Chem. Phys. 43 (1996) 116–125. J.W. Stevenson, P.F. Hallman, T.R. Armstrong, L.A. Chick, Sintering behavior of doped lanthanum and yttrium manganite, J. Am. Ceram. Soc. 78 (1995) 507–512. M.B. Ricoult, M. Badding, Y. Thibault, Advances in electronic and electrochemical ceramics, Ceram. Trans. 179 (2005) 173. M. Keane, M.K. Mahapatra, A. Verma, P. Singh, LSM-YSZ interactions and anode delamination in solid oxide electrolysis cell, Int. J. Hydrog. Energy 37 (2012) 16776–16785. B. Hu, M. Keane, M.K. Mahapatra, P. Singh, Stability of strontium-doped lanthanum manganite cathode in humidified air, J. Power Sources 248 (2014) 196–204.
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres Sapna Gupta a,b,n, Prabhakar Singh b,c a
Intel Corporation, Hillsboro, OR, 97124 USA Center for Clean Energy Engineering, Department of Materials Science and Engineering, University of Connecticut, Storrs, CT, 06269 USA c Fraunhofer Center for Energy Innovation, University of Connecticut, Storrs, CT, 06268 USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 June 2016 Received in revised form 27 August 2016 Accepted 27 August 2016
This paper reports a comparative assessment of the chemical and structural stability of LSCM perovskite composites fabricated with 8YSZ and 10ScSZ. Role of oxidizing and reducing atmospheres are examined on the processing and electrochemical performance of the composites. Higher density is obtained during the sintering of LSCM mixed with 10ScSZ when compared to composite mixtures containing 8YSZ in oxidizing as well as reducing atmosphere. Above composites also densified more in Ar-3%H2-3%H2O atmosphere than air along with the formation of SrZrO3 in reducing atmosphere. MnCr2O4 formation is found only in LSCM-8YSZ composite in Ar-3%H2-3%H2O. Electrochemical tests conducted using symmetric cell configurations indicates low polarization resistance and higher performance for Gas (air/fuel)/ LSCM-10ScSZ//8YSZ/LSCM-10ScSZ/Gas (air/fuel). Unlike cell containing LSCM-8YSZ composite, no significant changes are identified in the polarization resistance of LSCM-10ScSZ cell for 80 h. Sr-segregation on the surface of LSCM in electrically tested LSCM-8YSZ cell is attributed to performance degradation in the reducing atmosphere. & 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: LSCM-8YSZ and LSCM-10ScSZ comparison Processing Interfacial reaction – SrZrO3 Reducing atmosphere Electrochemical performance Oxygen transport membrane
1. Introduction Perovskite-fluorite based oxygen transport membrane (OTM) system is currently being developed for a wide range of industrial applications for hydrocarbon conversion to syngas which can be further processed to hydrogen/liquid fuel for transportation [1– 16]. OTM's also find application in advanced coal based power plants for clean and efficient energy production via oxy-combustion with minimal gas emissions [1–16]. The membrane system can also easily be incorporated in the integrated gasification combined cycle (IGCC) and coal gasification fuel cell (CGFC) systems for efficient power generation along with CO2 capture [1–6]. During the operation of oxygen transport membrane system, one side of the dense membrane is exposed to air and the other side to fuel (e.g. H2, CO, CH4) [1,9]. Due to the oxygen partial pressure difference across the membrane, oxygen selectively permeates through the membrane To improve on the surface exchange kinetics and the oxygen flux, membranes utilize porous air and fuel electrodes in the OTM system [1,9]. Porous electrodes offer enhanced triple phase boundary sites for electron exchange n
Corresponding author at: Intel Corporation, Hillsboro, OR 97124, USA. E-mail address: [email protected] (S. Gupta).
and electrochemical reactions as well as reduce gas phase diffusional resistance. Similar to SOFC electrodes, obtaining long term stable performance with no or negligible degradation of OTM air/ fuel electrode is found to be a challenge for the prolonged stable operation of OTM system [1–3,9,16]. Existing fuel and air electrodes, fabricated using perovskites, remain unsuitable for the long-term stable operation of OTM system due to interfacial reactions and related compound formation as well as surface morphological changes due to exolution of dopants. This is due to insufficient structural-chemical-thermal stability of materials, processing abnormalities, chemical decomposition, phase separation, and interfacial stability [1,3,8,9,14]. For example, LaCoO3 d decomposes and form La2CoO4 and CoO at PO2 o 10 7 atm and 1000 °C [1]. Likewise, LaFeO3 d decomposes and form La2O3 and Fe metal at PO2 o10 17 atm and 1000 °C [1]. LSM-YSZ can be considered to be used for air electrode of the OTM system as it is already being used as the state of the art for SOFC. However, it degrades with time and tend to delaminate after long term electrical testing. Interfacial stability and cation migration are a few known challenges with LSM-YSZ [17–19]. Interfacial compound formation (e.g. La2Zr2O7 and SrZrO3) at the interface of LSM and YSZ results in increase in ohmic resistance contributing to lower performance [17–19]. Similar to SOFC anode, Ni-YSZ can be used as OTM fuel electrode due to its high conductivity and
http://dx.doi.org/10.1016/j.ceramint.2016.08.172 0272-8842/& 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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electro-catalytic activity. However, its structural stability, volume expansion, limited resistance to sulfur poisoning and carbon deposition are major concerns in reducing atmosphere under OTM operating conditions [3,20]. To minimize and overcome the above mentioned challenges with the electrode materials, A and B-site doped perovskites are being investigated for OTM electrodes. (Ln, A)(B)O3 δ (Ln¼Lanthanide group: La), A ¼(Alkaline earth: Sr, Ca) and B ¼(Transition metal: Mn, Cr) based single-phase perovskites have been investigated for the air and fuel electrodes [1,5,9–12]. Long-term thermal/chemical stability and performance of these perovskites are still a challenge for stable operation of OTM [1,9,13]. Perovskite-fluorite based mixed ionic and electronic conductor (MIEC) and dual phase composites are currently being investigating for dual benefit i.e. high performance and stability under OTM system operating conditions [1,10,14–16]. It is found that the performance and stability of the composites are higher when compared to single-phase perovskite oxides [1,13]. Due to higher stability at high temperature (Z1000 °C) in a wide range of oxygen partial pressure (1–10 24 atm), lanthanum chromite based materials are considered to be a promising material for electrodes [1,2,5,6,11,14]. When mixed with fluorite phase, high performance can be obtained as required for OTM [1,13]. A review article on lanthanum chromite based materials for oxygen transport membrane system is published recently by the authors [1]. In this study, the authors have chosen strontium and manganese co-doped lanthanum chromite based perovskite of composition (La0.75Sr0.25)0.95Cr0.7Mn0.3O3 (LSCM) for the OTM electrode investigation as it can provide superior performance and long term structural stability when mixed with fluorite phase based on the critical literature review [1,21–23]. 8YSZ and 10ScSZ are two commonly reported stable fluorite materials [1,24–29]. Most of the literature reports result on LSCM and 8YSZ [27,28]. 10ScSZ is stable and provides higher ionic conductivity than 8YSZ which appears to be promising for higher performance along with its stability. It is important to find the right combination of perovskite and fluorite phases to achieve higher performance and stability under OTM system operating conditions. Therefore, in this study, the authors investigate a comparative study on LSCM processing (under OTM system fabricating conditions) with 8YSZ and 10ScSZ. Electrochemical performance (under OTM system operating conditions) and stability of LSCM are also investigated when combined with 8YSZ and 10ScSZ under cathodic (oxidizing) and anodic (reducing) OTM operating conditions. This study can also be utilized for other high temperature electrochemical systems (e.g. solid-oxide fuel and electrolysis cells – SOFC and SOEC) as similar MIEC materials are currently being developed for these applications [2,5,21–23,30–39].
10ScSZ samples was measured. Scanning electron (FEI – ESEM Quanta 250) and transmission electron (FEI Tecnai T12 S/TEM) microscopes were utilized for microstructural analysis of the sintered and tested samples. Elemental profile analysis of the samples was conducted using Energy dispersive spectroscopy (EDS) attached to the SEM as well as TEM. Focused ion beam (FEI Strata 400S DUALBEAM FIB) technique was used to prepare samples for TEM microstructural analysis. X-ray diffraction (BRUKER-D8 ADVANCE, Bruker AXS Inc.) was utilized to determine the crystal structure of LSCM composite with 8YSZ and 10ScSZ and secondary compounds, if any when exposed to oxidizing and reducing gas atmosphere. The scan step used for X-ray diffraction was 0.02° using CuKα radiation (λ ¼1.5406 Å). 2.3. Symmetric cell fabrication LSCM-8YSZ and LSCM-10ScSZ pastes were prepared using ink vehicle (Fuel Cell Materials). As working and counter electrodes (thickness: 20 mm, diameter: 10 mm), LSCM-8YSZ and LSCM10ScSZ composites were subsequently screen-printed on both sides of (ZrO2)0.92(Y2O3)0.08(YSZ) electrolyte (200 mm thick, Fuel Cell Materials). The electrodes were then dried at room temperature and subsequently sintered at 1200 °C (heating rate of 3 °C/ min) for 2 h in air. Lower processing temperature (1200 °C) was chosen for the symmetrical cells to achieve porous electrode layers structure and also to avoid secondary phase formations that were identified at 1400 °C. The electrochemical active area of the cell electrode was 0.8 cm2. Platinum screen current collector (Alfa Aesar – 50 mesh) and platinum wires (Alfa Aesar – 0.25 mm) were attached to both the electrodes using platinum paste from ElectroScience Laboratories Inc. Curing of platinum paste was obtained in air at 900 °C with 1 h (3 °C/min) dwell time. As-assembled LSCM8YSZ//8YSZ//LSCM-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetric cells were then installed in a tubular alumina chamber and placed in the constant temperature furnace zone. Multichannel potentiostat (VMP2, Bio-Logic) leads were attached to the assembled symmetric cell for electrochemical impedance measurement. 2.4. Electrochemical testing LSCM-8YSZ and LSCM-10ScSZ based symmetrical cells were heated up to 950 °C with the heating rate of 3 °C/min in air and Ar3%H2-3%H2O with flow rate of 300sccm. The cells were tested for 80 h under the constant bias of 0.5 V. The impedance measurement was performed (at two hour intervals) in the frequency range starting from 100 mHz to 200 kHz using a 10 mV alternating current. Experiments were repeated a couple of times under same condition to ensure reproducibility. Post-test characterizations of the samples were performed using SEM-EDS (FEI – ESEM Quanta 250).
2. Experimental procedure 2.1. (La0.75Sr0.25)0.95Cr0.7Mn0.3O3 δ and 8YSZ/10ScSZ synthesis
3. Results and discussion
LSCM powder was received from Praxair Inc. The powder was mixed with 8YSZ and10ScSZ in the weight ratio of 50:50. Mixed powders were then uniaxially pressed into cylindrical pellets and subsequently sintered in oxidizing (air) and reducing (Ar-3%H2-3% H2O) atmosphere at 1400 °C with 10 h dwell time. To ensure the maintenance of the oxygen pressure during cooling, samples were cooled in the flowing gas environment.
3.1. Crystal structure
2.2. Characterization Using Archimedes principle, density of LSCM-8YSZ and LSCM-
Figs. 1 and 2 shows the XRD pattern of LSCM composite with 8YSZ and 10ScSZ sintered in oxidizing (air 0.21 atm) and reducing atmosphere (Ar-3%H2-3%H2O 10 10 atm) at 1400 °C. LSCM crystal structure is rhombohedral (JCPDS 75-9872) and 8YSZ XRD peaks corresponds to cubic phase (JCPDS 70-4436) in air. However, in both the cases (LSCM-8YSZ and LSCM-10ScSZ), it is identified that the peaks splitting associated with the LSCM rhombohedral phase disappears for the samples sintered in Ar-3%H2-3%H2O as shown in Fig. 1(e.g. inserted plot peak position). This corresponds
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Table 1 Relative density of LSCM-8YSZ and LSCM-10ScSZ sintered at 1400 °C for 10 h.
Material
Relative density (%) Air
Ar-3%H2-3%H2O
LSCM–8YSZ LSCM–10ScSZ
597 2.2 78.17 1.5
74.8 7 1.3 88.2 70.9
compared to 10ScSZ. 3.2. Sintering behavior and microstructural analysis
Fig. 1. XRD pattern of LSCM and 8YSZ/10ScSZ composites when exposed to air and Ar-3%H2-3%H2O at 1400 °C for 10 h: (a) LSCM – 8YSZ and (b) LSCM – 10ScSZ.
Fig. 2. XRD comparison plot of LSCM when mixed with 8YSZ and 10ScSZ in Ar-3% H2-3%H2O at 1400 °C for 10 h.
to phase transformation to a higher symmetry cubic phase (JCPDS 074-1961) [2,5]. The authors have identified and reported similar phase transformation in reducing atmosphere for single phase LSCM perovskite [11]. Furthermore, it indicates that the cubic phase becomes dominating in Ar-3%H2-3%H2O atmosphere as the peak splitting corresponding to rhombohedral phase almost disappears. Extra peaks corresponding to the strontium zirconate (SrZrO3) (JCPDS-01-074-1297) are identified in Ar-3%H2-3%H2O sintered samples unlike air. Fig. 2 shows XRD pattern comparison of LSCM composite with 8YSZ and 10ScSZ when processed in Ar3%H2-3%H2O atmosphere at 1400 °C. Peak intensity of SrZrO3 is higher in case of LSCM-8YSZ composite when compared to LSCM10ScSZ under the same conditions. This corresponds to higher amount of SrZrO3 formation when LSCM is mixed with 8YSZ when
Table 1 shows relative density of LSCM-8YSZ and LSCM-10ScSZ sintered in oxidizing (air) and reducing atmosphere (Ar-3%H2-3% H2O) at 1400 °C for 10 h. Higher density of LSCM-8YSZ composite is obtained in Ar-3%H2-3%H2O (74.8 71.3%) atmosphere when compared to air (59 72.2%). On the other hand, LSCM-10ScSZ composite density increases from 78.1 71.5% to 88.2 70.9% when gas atmosphere is switched from oxidizing (air) to reducing (Ar-3% H2-3%H2O) respectively. In both the cases, it is noted that lower density is obtained when the composites are processed in air atmosphere. This is due to the fact that Cr-evaporates (Cr2O3 þ3/2 O2-2CrO3↑) from the bulk and condensates (2CrO3↑-Cr2O3 þ3/ 2O2) at the grain boundaries in oxidizing atmosphere and that inhibits further mass transport and grain growth of lanthanum chromite based materials resulting in lower densification in air [1,2]. On comparison with LSCM-8YSZ composite, LSCM-10ScSZ achieves higher density in oxidizing as well as reducing atmosphere. This is attributed to the higher density of 10ScSZ (88.5 71.5%) itself when compared to 8YSZ (83.0 71.8%) in reducing gas atmosphere. Moreover, the ionic radii of Zr4 þ ¼0.84 Å matches with Sc3 þ ¼0.87 Å compared to Y3 þ ¼ 1.02 Å. This may attribute to the closed packed and dense structure of ScSZ fluorite and the composite with LSCM when compared to LSCM-YSZ. Fig. 3 shows the SEM microstructures of LSCM composite with 8YSZ and 10ScSZ, sintered in air and Ar-3%H2-3%H2O gas atmosphere. Unlike fluorite phase, it is found that the morphology of perovskite (LSCM) phase changes in reducing atmosphere. The surface rearrangement may attribute to the decrease in the intensity of the corresponding XRD pattern (Fig. 1) in reducing atmosphere [2,40]. No secondary phases have been identified in oxidizing as well as reducing atmosphere using SEM. However, SrZrO3 secondary phase formation is detected using XRD technique for the samples sintered in Ar-3%H2-3%H2O. To confirm on the secondary phase formation, TEM microstructural analysis is conducted on LSCM-8YSZ and LSCM-10ScSZ samples sintered in air as well as Ar-3%H2-3%H2O atmosphere as shown in Fig. 4. The sample preparation for TEM analysis is performed using FIB as shown in the insert of Fig. 4a. It is clear that the denser microstructure is obtained for LSCM when mixed with 10ScSZ than 8YSZ. No secondary phase is identified in the samples sintered in air atmosphere. On the other hand, as expected from the XRD pattern and analysis, secondary phases are identified in both the composites in reducing gas atmosphere (Fig. 4c and d). TEM elemental analysis reveals that the secondary phase predominantly consists of Sr and Zr corresponding to SrZrO3 formation as shown in Table 2 (LSCM-8YSZ) and Table 3 (LSCM-10ScSZ). However, a second new phase is identified only in Ar-3%H2-3%H2O sintered LSCM-8YSZ composite as shown in the TEM micrograph (Fig. 4c). The secondary phase is enriched in Mn and Cr as shown in the Table 2. This corresponds to MnCr2O4 spinel formation attributing to Mn:Cr::1:2. The phase is not identified in the XRD. This is probably due to small amount of phase is present which is outside the detection limit of XRD. Fig. 5 shows the TEM elemental mapping of FIB-cross-section of
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 3. SEM micrographs of LSCM containing 8YSZ and10ScSZ composite sintered at 1400 °C for 10 h in air (a) LSCM-8YSZ, (b) LSCM-10ScSZ; in Ar-3%H2-3%H2O (c) LSCM8YSZ and (d) LSCM-10ScSZ.
sintered LSCM-8YSZ in Ar-3%H2-3%H2O. It is clearly identified that one of the secondary phase is enriched in Sr and Zr and other in Mn and Cr. However, TEM elemental mapping of LSCM-10ScSZ indicates only one secondary phase formation enriched in Sr and Zr as shown in Fig. 6. Fig. 7 shows a schematic of mechanism steps for the formation of SrZrO3 and MnCr2O4 in LSCM-8YSZ and LSCM-10ScSZ in reducing atmosphere. Step 1 represents an initial stage of LSCM-8YSZ and LSCM-10ScSZ system heating to the temperature of the reaction. In step 2, reduction of Mn takes place from Mn4 þ -Mn3 þ Mn2 þ resulting in diffusion of Mn into the fluorite phase. It is found that the solubility of lower valence state Mn is higher in fluorite phase (e.g. 8YSZ) when exposed to reducing gas atmosphere due to better matching ionic radius with Zr4 þ [16,41]. Chemical diffusion coefficient of Mn in 8YSZ is 10 13 cm2/s at 1000 °C [42]. Moreover, as Mn diffuses into the fluorite phase, A-site gets enriched in LSCM (step 2) and therefore, Sr reacts with Zr resulting in the formation of SrZrO3 (step 3) at the interface of LSCM and 8YSZ [3,16]. Concurrently, the Cr and Mn components at the B-site, now in excess within the perovskite lattice, react to form MnCr2O4 to keep the mass balance as shown in step 3. Unlike LSCM-10ScSZ, MnCr2O4 formation is identified in the case of LSCM-8YSZ (step 3). Small amount of SrZrO3 forms in case of LSCM-10ScSZ composite when compared to LSCM-8YSZ. This is in agreement with the observed lower SrZrO3 peaks intensity (Fig. 2.) in the XRD pattern of LSCM-10ScSZ when compared to LSCM-
8YSZ. This is attributed to the lower amount of Mn diffusion in ScSZ due to the higher stability and better ionic radius match of Zr with Sc than Y. Transmission electron microscopy elemental analysis also reveals that Mn diffusion is lower in 10ScSZ ( 1.1 at%) than 8YSZ ( 3.5 at%) in reducing atmosphere as shown in Tables 2 and 3. Therefore, LSCM perovskite lattice is not significantly enriched in B-site after small amount of SrZrO3 formation. Concurrently, the Cr and Mn components at the B-site, now not significantly enriched within the perovskite lattice to react and form MnCr2O4. Overall, this attributes to the higher stability of LSCM-10ScSZ composite when compared to LSCM-8YSZ in reducing gas atmosphere. 3.3. Electrochemical measurements and post-test characterization Fig. 8a and b shows comparison of the Nyquist plots of impedance spectra acquired from LSCM-8YSZ//8YSZ//LSCM-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetrical cells electrochemical testing at 950 °C (0–80 h) and 0.5 V. Only two spectra (0 and 80 h) are shown for clarity. The intercept associated with the high frequency represents ohmic (electrolyte) resistance and on the other hand, semi-circle diameter represents polarization resistance which includes oxygen surface exchange and migration and electrochemical kinetics associated with electrode layer [3,43]. A significant difference is identified in the impedance spectra of both the cells as shown in Fig. 8a and b. Higher performance and
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 4. TEM micrographs of LSCM containing 8YSZ and10ScSZ composite sintered at 1400 °C for 10 h in air (a) LSCM-8YSZ, (b) LSCM-10ScSZ; in Ar-3%H2-3%H2O, (c) LSCM8YSZ and (d) LSCM-10ScSZ.
Table 2 STEM-EDS elemental analysis of LSCM-8YSZ sintered in Ar-3%H2-3%H2O. Element
La
Sr
Cr
Mn
Y
Zr
LSCM (at%) YSZ (at%) Sr-Zr rich phase (at%) Mn-Zr rich phase (at%)
42.1 0 3.5 0
9.8 1.6 38 0
35.6 0.3 4.8 65.4
10.6 3.5 0 32.6
0 14.1 0.4 0.2
1.1 80.5 53.3 1.8
Table 3 STEM-EDS elemental analysis of LSCM-10ScSZ sintered in Ar-3%H2-3%H2O. Element
La
Sr
Cr
Mn
Sc
Zr
LSCM (at%) ScSZ (at%) Sr-Zr rich phase (at%)
41.5 0.1 12.7
9.7 0 40.8
36.7 0.1 6.9
10.7 1.1 0.4
0 12.9 1.4
1.3 85.7 37.7
lower polarization resistance is reflected from the impedance spectra of LSCM-10ScSZ//8YSZ//LSCM-10ScSZ when compared to LSCM73/8YSZ//8YSZ//LSCM73/8YSZ in oxidizing (Fig. 8a) and reducing atmosphere (Fig. 8b) during the time interval of 0–80 h. Fig. 9a and b shows comparison plot of polarization resistances of symmetrical cells of configuration Gas (air/ Fuel)/LSCM-8YSZ// 8YSZ//LSCM-8YSZ/Gas (air/fuel) and Gas (air/fuel)/LSCM-10ScSZ//
8YSZ//LSCM-10ScSZ/Gas (air/fuel) as a function of time (80 h) at 950 °C in air and Ar-3%H2-3%H2O respectively. Constant bias of 0.5 V is applied. In oxidizing atmosphere (air), the polarization resistance of LSCM-8YSZ//8YSZ//LSCM-8YSZ increases and then stabilizes after 30 h. There is no significant difference observed after the stabilization up to 80 h. The polarization resistance increases from 12.0 to 12.1 Ω cm2 as time increases from 30 h to 80 h. In case of LSCM-10ScSZ//8YSZ//LSCM-10ScSZ exposed to air, there is also no large variation in the performance and polarization resistance. It increases from 1.3 (30 h) to 1.5 Ω cm2 (80 h). On comparison, LSCM-10ScSZ based symmetrical cell takes less time for stabilization than LSCM-8YSZ (Fig. 9a). Moreover, LSCM-10ScSZ provides higher performance and lower polarization resistance ( 9 times) than LSCM-8YSZ as shown in Fig. 9a. This may correspond to the higher conductivity of 10ScSZ fluorite phase when compared to 8YSZ [1,29]. On the other hand, in reducing atmosphere, the polarization resistance of LSCM-8YSZ based symmetrical cell increases continuously with time (Fig. 9b). The initial resistance of the cell is 12.0 Ω cm2 (0 h) which increases to 24.9 Ω cm2 (80 h) in Ar3%H2-3%H2O at 950 °C under the constant bias of 0.5 V. However, unlike LSCM-8YSZ system, LSCM-10ScSZ based symmetrical cells polarization resistance does not vary significantly with increase in time as shown in Fig. 9b. On comparison, even in reducing atmosphere, lower resistance ( 3.5 times) and higher stability is
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 5. Elemental mapping of FIB cross-section of sintered LSCM-8YSZ in Ar-3%H2-3%H2O.
Fig. 6. Elemental mapping of FIB cross-section of sintered LSCM-10ScSZ in Ar-3%H2-3%H2O atmosphere.
obtained for LSCM-10ScSZ based composite cell when compared to LSCM-8YSZ. This corresponds to lower stability and degradation of the LSCM-8YSZ cell when compared to LSCM-10ScSZ in Ar-3% H2-3%H2O atmosphere. Overall, for both the cases (oxidizing and reducing), LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetrical cells achieved higher performance and lower resistance when compared to LSCM-8YSZ//8YSZ//LSCM-8YSZ. This corresponds to the higher conductivity and stability of LSCM-10ScSZ on comparison with LSCM-8YSZ. Microstructural analysis of the tested symmetrical cells were performed after completion of the electrochemical tests to understand the degradation of the tested symmetrical cells. Fig. 10a– c (a: as-sintered, b: tested in air, c: tested in Ar-3%H2-3%H2O) shows LSCM-8YSZ microstructures after the electrochemical testing. There is no significant difference identified in the air tested sample on comparison with the as-sintered sample. This attributes to the stable performance of the cell with time up to 80 h.
However, in reducing atmosphere, it is found that the surface morphology of the LSCM phase is modified with nano particles on surface after the testing. Using SEM-EDS analysis, the modified surface of LSCM is found to be enriched in Sr when compared to the as-sintered sample. An average ( 20 points) of Sr:La ratio of the LSCM phase surface is 0.43 70.04 for the as-sintered cell, 0.44 70.05 for the air tested cell and 0.65 70.05 for the reducing gas atmosphere tested cell. Unlike Ar-3%H2-3%H2O, the Sr: La ratio matches for the as-sintered and air tested cell. This confirms on the Sr-enrichment of the LSCM surface in reducing gas atmosphere. This corresponds to the lower polarization resistance and stability of the cell tested in reducing atmosphere. Similar performance degradation due to Sr-enrichment on LSM surface is identified for (La0.8Sr0.2)0.98MnO3 (LSM) based cell tested for 100 h at 850 °C [44]. Fig. 10d–f (d: as-sintered, e: tested in air, f: tested in Ar-3%H23%H2O) shows microstructure of LSCM-10ScSZ based cells. No
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 7. Schematic of mechanism steps for the formation of SrZrO3/MnCr2O4 in LSCM-8YSZ and LSCM-10ScSZ composites in reducing atmosphere.
Fig. 8. Comparison of the Nyquist plots of impedance spectra acquired from LSCM-/8YSZ//8YSZ//LSCM7-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ symmetrical cells testing at 950 °C (0–80 h) and 0.5 V: (a) air, and (b) Ar-3%H2-3%H2O. Only two spectra (0 and 80 h) are shown for clarity.
changes are identified in the cells after testing in oxidizing as well reducing atmosphere. This attributes to higher performance and stability of LSCM-10ScSZ based cells. Unlike LSCM-8YSZ cell, no LSCM surface modification is observed in the reducing atmosphere tested cells. On comparison, LSCM-10ScSZ based cells are more stable and provides higher performance than LSCM-8YSZ cells in both the oxidizing and reducing atmosphere.
4. Conclusion
Fig. 9. Comparison plots of resistance (non-ohmic) changes with time for LSCM8YSZ//8YSZ//LSCM-8YSZ and LSCM-10ScSZ//8YSZ//LSCM-10ScSZ cell tested at 0.5 V: (a) in air, and (b) Ar-3%H2-3%H2O.
Composites consisting of LSCM-8YSZ and LSCM-10ScSZ have been processed in oxidizing and reducing atmosphere (Ar-3%H2Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
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Fig. 10. SEM micrographs of the tested symmetrical cell of LSCM-8YSZ//8YSZ//LSCM-8YSZ (anode surface): (a) as-sintered, (b) in air, (c) in Ar-3%H2-3%H2O; LSCM-10ScSZ// 8YSZ//LSCM-10ScSZ: (d) as-sintered, (e) in air, and (f) Ar-3%H2-3%H2O.
Table 4 Summary and comparison of LSCM-8YSZ and LSCM-10ScSZ composites. LSCM-8YSZ
LSCM-10ScSZ
Lower relative density (%) Air 597 2.2 Ar-3%H2-3%H2O 74.8 7 1.3
Higher relative density (%) Air 78.17 1.5 Ar-3%H2-3%H2O 88.2 7 0.9
Secondary Phases (More) Air No Ar-3%H2-3%H2O SrZrO3 and MnCr2O4
Secondary Phases (Less) Air No Ar-3%H2-3%H2O SrZrO3
Mn diffusion into YSZ (Higher) Air No Ar-3%H2-3%H2O 3.5 at%
Mn diffusion into ScSZ (Lower) Air No Ar-3%H2-3%H2O 1 at%
Polarization resistance (Higher) Air 7.1 Ω cm2 Ar-3%H2-3%H2O 12.0 Ω cm2
Polarization resistance (Lower) Air 0.8 Ω cm2 Ar-3%H2-3%H2O 3.4 Ω cm2
3%H2O) at 1400 °C. Both the composites are characterized using XRD, SEM, TEM (with EDS) and EIS. Symmetrical cells based on LSCM-8YSZ and LSCM-10ScSZ electrodes are also tested using electrochemical impedance measurement technique. Results of this study and comparison of LSCM-8YSZ and LSCM-10ScSZ composites are listed in Table 4. Our key findings include: 1. Higher density is achieved for LSCM when mixed with 10ScSZ compared to 8YSZ. 2. Interfacial reaction is identified in reducing atmosphere with higher amount of SrZrO3 formation in LSCM-8YSZ when exposed to reducing atmosphere. 3. Mn diffusion into the fluorite phase drives interfacial reaction resulting in SrZrO3 formation. 4. Mechanistic steps for secondary phase formation's (SrZrO3 and MnCr2O4) are explained and reported in this study.
5. Lower polarization resistance is obtained from LSCM-10ScSZ based symmetrical cell when compared to LSCM-8YSZ in oxidizing as well as reducing atmosphere. 6. Higher stability is achieved for LSCM-10ScSZ cell compared to LSCM-8YSZ in reducing atmosphere. 7. LSCM-10ScSZ composite is more beneficial to be used for higher stability and performance of high temperature electrochemical devices such as OTM.
Acknowledgments This work was performed as part of doctoral research (SG). Authors sincerely thank Center for Clean Energy Engineering for providing laboratory facilities and advanced characterization equipment's for conducting experimental work. Helpful discussions with Manoj K. Mahapatra is gratefully acknowledged.
References [1] S. Gupta, M.K. Mahapatra, P. Singh, Lanthanum chromite based perovskites for oxygen transport membrane, MSE: R: Rep. 90 (2015) 1–36. [2] S. Gupta, J.J. Adams, J.R. Wilson, E.G. Eddings, M.K. Mahapatra, P. Singh, Performance and post-test characterization of an OTM system in an experimental coal gasifier, Appl. Energy 165 (2016) 72–80. [3] S. Gupta, Development of Perovskite/Fluorite Based Materials for Oxygen Transport Membrane System (thesis), 2015. 〈http://digitalcommons.uconn. edu/dissertations/799〉. [4] S. Gupta, M.K. Mahapatra, P. Singh, Phase transformation, thermal expansion and electrical conductivity of lanthanum chromite, Mater. Res. Bull. 48 (2013) 3262–3267. [5] S. Gupta, P. Singh, Manganese doped lanthanum strontium chromite fuel electrode for solid oxide fuel cell and oxygen transport membrane systems, ECS Trans. 66 (2015) 117–123. [6] K.T. Jacob, S. Gupta, P. Singh, Thermodynamic properties of LaCrO4, La2CrO6 and La2Cr3O12, and sub-solidus phase relations in the system lanthanumchromium-oxygen, J. Am. Ceram. Soc. 96 (2013) 3933–3938. [7] K.T. Jacob, S. Gupta, P. Singh, Electrochemical determination of Gibbs energy of formation of LaCrO3 using a composition- graded bi-electrolyte, J. Am. Ceram.
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i
S. Gupta, P. Singh / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Soc. 96 (2013) 3272–3278. [8] S. Gupta, P. Singh, Structural and electrochemical performance stability of perovskite-fluorite composite for high temperature electrochemical devices, Adv. Solid Oxide Fuel Cells Electron. Ceram. (2015) 67–74. [9] H.J.M. Bouwmeester, A.J. Burggraaf, Fundamentals of inorganic membrane science and technology, in: A.J. Burggraaf, L. Cot (Eds.), Membrane Science and Technology Series 4, Elsevier, Amsterdam, 1996, pp. 435–510. [10] N. Nagabhushana, J.A. Lane, G.M. Christie, B.A.V. Hassel, Composite Oxygen Ion Transport Membrane, US Patent 7556676B2, 2009. [11] S. Gupta, Y. Zhong, M.K. Mahapatra, P. Singh, Processing and electrochemical performance of manganese-doped lanthanum-strontium chromite in oxidizing and reducing atmospheres, Int. J. Hydrog. Energy 40 (2015) 13479–13489. [12] J.J. Liu, T. Liu, W.D. Wang, J.F. Gao, C.S. Chen, Zr0.84Y0.16O1.92– La0.8Sr0.2Cr0.5Fe0.5O3 δ dual-phase composite hollow fiber membrane targeting chemical reactor applications, J. Membr. Sci. 389 (2012) 435–440. [13] J. Sunarso, S. Baumann, J.M. Serra, W.A. Meulenberg, S. Liu, Y.S. Lin, J.C. Diniz da Costa, Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation, J. Membr. Sci. 320 (2008) 13–41. [14] S. Gupta, J.A. Lane, J.R. Wilson, P.J. Plonczak, P. Singh, Interfacial stability of iron doped lanthanum strontium chromite and Scandia stabilized zirconia in oxidizing and reducing atmosphere, Solid State Ion. 288 (2016) 10–13. [15] W. Fang, Y. Zhang, J. Gao, C. Chen, Oxygen permeability of asymmetric membrane of functional La0.8Sr0.2Cr0.5Fe0.5O3 δ (LSCrF)–Zr0.8Y0.2O2-δ (YSZ) supported on porous YSZ, Ceram. Int. 40 (2014) 799–803. [16] S. Gupta, P. Singh, Nickel and titanium doubly doped lanthanum strontium chromite for high temperature electrochemical devices, J. Power Sources 306 (2016) 801–811. [17] M. Keane, M.K. Mahapatra, A. Verma, P. Singh, LSM-YSZ interactions and anode delamination in solid oxide electrolysis cell, Int. J. Hydrog. Energy 37 (2012) 16776–16785. [18] N. Li, M. Keane, M.K. Mahapatra, P. Singh, Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ, Int. J. Hydrog. Energy 38 (2013) 6298–6303. [19] B. Hu, M. Keane, M.K. Mahapatra, P. Singh, Stability of strontium-doped lanthanum manganite cathode in humidified air, J. Power Sources 248 (2014) 196–204. [20] V.B. Vert, F.V. Melo, L. Navarrete, J.M. Serra, Redox stability and electrochemical study of nickel doped chromites as anodes for H2/CH4-fueled solid oxide fuel cells, Appl. Catal. B Environ. 115 (2012) 115. [21] S. Tao, J.T.S. Irvine, Phase transition in perovskite oxide La0.75Sr0.25Cr0.5Mn0.5O3 δ observed by in situ high temperature neutron powder diffraction, Chem. Mater. 18 (2006) 5453–5460. [22] S. Tao, J.T.S. Irvine, Synthesis and characterization of (La0.75Sr0.25)Cr0.5Mn0.5O3– δ – redox-stable, efficient perovskite anode for SOFCs, J. Electrochem. Soc. 151 (2004) A252–A259. [23] S. Tao, J.T.S. Irvine, A redox-stable efficient anode for solid-oxide fuel cells, Nat. Mater. 2 (2003) 320–323. [24] M. Mori, Y. Hiei, T. Yamamoto, Control of the thermal expansion of strontiumdoped lanthanum chromite perovskites by B-site doping for high-temperature solid oxide fuel cell separators, J. Am. Ceram. Soc. 84 (2001) 781–786. [25] M. Mori, T. Yamamoto, H. Itoh, H. Inaba, H. Tagawa, Thermal expansion of nickel-zirconia anodes in solid oxide fuel cells during fabrication and operation, J. Electrochem. Soc. 145 (1998) 1374–1381. [26] A. Bieberle, L.J. Gauckler, in: H. Tuller, J. Schoonman, I. Riess (Eds.), Oxygen Ion
[27]
[28]
[29]
[30] [31]
[32] [33]
[34]
[35]
[36]
[37]
[38]
[39]
[40] [41]
[42] [43]
[44]
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and Mixed Conductors and Their Technological Applications, Kluwer Academic Publishers, Dordrecht, 2000, pp. 347–358. S.P. Jiang, X.J. Chen, S.H. Chan, J.T. Kwok, K.A. Khor, (La0.75Sr0.25)(Cr0.5Mn0.5)O3/ YSZ composite anodes for methane oxidation reaction in solid oxide fuel cells, Solid State Ion. 177 (2006) 149–157. I. Jung, D. Lee, S.O. Lee, D. Kim, J. Kim, S.H. Hyun, J. Moona, LSCM–YSZ nanocomposites for a high performance SOFC anode, Ceram. Int. 39 (2013) 9753–9758. S. Kazlauskas, A. Kežionis, T. Šalkus, A.F. Orliukas, Sintering of oxygen ion conductive ceramics and their electrical properties, Lith. J. Phys. 52 (2012) 231–237. R.E. Williford, P. Singh, Engineered cathodes for high performance SOFCs, J. Power Sources 128 (2004) 45–53. V.V. Kharton, E.V. Tsipis, I.P. Marozau, A.P. Viskup, J.R. Frade, J.T.S. Irvine, Mixed conductivity and electrochemical behavior of (La0.75Sr0.25)0.95Cr0.5Mn0.5O3 δ, Solid State Ion. 178 (2007) 101–113. J. Liu, B. Madsen, Z. Ji, S. Barnett, A fuel-flexible ceramic-based anode for solid oxide fuel cells, Solid State Lett. 5 (2002) A122–A124. S. Zha, P. Tsang, Z. Cheng, M. Liu, Electrical properties and sulfur tolerance of La0.75Sr0.25Cr1 xMnxO3 under anodic conditions, J. Solid State Chem. 175 (2005) 1844–1850. D.M. Bastidas, S. Tao, J.T.S. Irvine, A symmetrical solid oxide fuel cell demonstrating redox stable perovskite electrodes, J. Mater. Chem. 16 (2006) 1603–1605. M.F. Lu, E.V. Tsipis, J.C. Waerenborgh, A.A. Yaremchenko, V.A. Kolotygin, S. Bredikhin, V.V. Kharton, Thermomechanical, transport and anodic properties of perovskite-type (La0.75Sr0.25)0.95Cr1 xFexO3 δ, J. Power Sources 206 (2012) 59–69. F. Chen, M.L. Liu, Preparation of yttria stabilized zirconia (YSZ) films on La0.85Sr0.15MnO3 (LSM) and LSM-YSZ substrates using electrophoretic deposition (EPD) process, J. Eur. Ceram. Soc. 21 (2001) 127–134. Z. Jiang, Z. Lei, B. Ding, C. Xia, F. Zhao, F. Chen, Electrochemical characteristics of solid oxide fuel cell cathodes prepared by infiltrating (La,Sr)MnO3 nanoparticles into yttria-stabilized bismuth oxide backbones, Int. J. Hydrog. Energy 35 (2010) 8322–8330. C. Yang, C. Jin, F. Chen, Characterization of infiltrated (La0.75Sr0.25)0.95MnO3 as oxygen electrode for solid oxide electrolysis cells, Int. J. Hydrog. Energy 35 (2010) 5187–5193. W. Tan, Q. Zhong, D. Xu, H. Yan, X. Zhu, Catalytic activity and sulfur tolerance for Mn-substituted La0.75Sr0.25CrO3 7 δ in gas containing H2S, Int. J. Hydrog. Energy 36 (2013) 16656–16664. H. Chen, Surface/interface X-ray diffraction review, Mater. Chem. Phys. 43 (1996) 116–125. J.W. Stevenson, P.F. Hallman, T.R. Armstrong, L.A. Chick, Sintering behavior of doped lanthanum and yttrium manganite, J. Am. Ceram. Soc. 78 (1995) 507–512. M.B. Ricoult, M. Badding, Y. Thibault, Advances in electronic and electrochemical ceramics, Ceram. Trans. 179 (2005) 173. M. Keane, M.K. Mahapatra, A. Verma, P. Singh, LSM-YSZ interactions and anode delamination in solid oxide electrolysis cell, Int. J. Hydrog. Energy 37 (2012) 16776–16785. B. Hu, M. Keane, M.K. Mahapatra, P. Singh, Stability of strontium-doped lanthanum manganite cathode in humidified air, J. Power Sources 248 (2014) 196–204.
Please cite this article as: S. Gupta, P. Singh, A comparative study on manganese-doped lanthanum-strontium chromite mixed with 8YSZ and 10ScSZ in oxidizing and reducing atmospheres, Ceramics International (2016), http://dx.doi.org/10.1016/j. ceramint.2016.08.172i