Reduced Thermal Expansion and Enhanced Redox

Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4244−4254

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Reduced Thermal Expansion and Enhanced Redox Reversibility of La0.5Sr1.5Fe1.5Mo0.5O6−δ Anode Material for Solid Oxide Fuel Cells He Qi,† Tony Thomas,† Wenyuan Li,† Wei Li,† Fang Xia,‡ Nan Zhang,† Edward M. Sabolsky,† John Zondlo,† Richard Hart,§ and Xingbo Liu*,† †

Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, United States Discipline of Chemistry and Physics, College of Science, Health, Engineering and Education, Murdoch University, Perth, WA 6150, Australia § GE, Global Research Centre, Niskayuna, New York 12309, United States Downloaded via NOTTINGHAM TRENT UNIV on August 13, 2019 at 10:15:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: High performance anode materials with suitable thermal and chemical expansions are highly desirable for solid oxide fuel cells. In this work, we report a promising anode material La0.5Sr1.5Fe1.5Mo0.5O6‑δ (LSFM) synthesized in nitrogen at 1050 °C. Its phase stability, mechanical behavior, redox stability, and electrochemical performance were studied. The electrical conductivity of LSFM reaches 23 S cm−1 in 5% H2−95% N2 at 800 °C with excellent reversibility over three redox cycles. After lanthanum doping, the coefficient of thermal expansion (CTE) is reduced from 17.12 × 10−6 K−1 (SF1.5M) to 15.01 × 10−6 K−1 (LSFM), and this value can be lowered further with a higher lanthanum content. Dilatometry testing at 800 °C shows that the chemical expansion behavior of LSFM is highly reversible during the oxidation−reduction cycling. These results indicate that the thermal and chemical expansion of the crystal lattice can be reduced by a stronger metal−oxygen (M−O) bond strength, leading to an improvement in redox reversibility. The polarization resistance of the LSFM symmetrical cell at 800 °C in humidified hydrogen is 0.16 Ω cm2, and the active region is ∼4.5 μm. The half-tear-drop-shaped impedance spectroscopy indicates an oxygen bulk diffusion and surface reaction colimited process. The maximum power density of the LSFM single cell reaches 1156 mW cm−2 at 800 °C within humidified H2. The new ceramic material LSFM is a promising anode for high performance solid oxide fuel cells. KEYWORDS: lanthanum-doped perovskite as anode material, thermal expansion coefficient modification, chemical and mechanical stability, conductivity reversibility, colimitation mechanism, active region estimation

1. INTRODUCTION The solid oxide fuel cell (SOFC) is an efficient hightemperature device that can directly convert chemical energy to electrical energy.1−4 Because of its high efficiency5 and environmental friendliness, extensive investigations have been made worldwide by researchers in the past decades. Recently, nickel−yttria stabilized zirconia (Ni-YSZ) is widely used as the anode in SOFCs.6 As a common anode material, Ni-YSZ possesses (1) attractive catalytic activity for hydrogen oxidation reaction,7 (2) good electronic and ionic conductivity (around 102−103 S cm−1 at 600−1000 °C in H2),8,9 and (3) lower cost than platinum (Pt), praseodymium (Pr), and other precious metals. However, the nature of Ni displays some drawbacks which are very difficult to overcome. First, the volume expansion of the oxidation from Ni (6.589 cm3 mol−1) to NiO (11.198 cm3 mol−1) is ∼70%;10 this gives rise to extra mechanical stress leading to cracks and fuel leakage and finally causing damage to the entire stack. Second, severe carbon deposition and sulfur poisoning on Ni are responsible for the © 2019 American Chemical Society

rapid performance decay when hydrocarbon fuels are applied as fuel gas.11−13 To avoid these shortcomings of Ni-YSZ and improve the lifetime of the SOFC, Ni-free anode materials with a suitable coefficient of thermal expansion (CTE), high redox reversibility, and great electrochemical performance are strongly desired. CTE is an important physical property for materials used in SOFCs. The CTE of common electrolytes such as LSGM and YSZ is in the range of (10−12) × 10−6 K−1.14−18 However, most of SOFC anodes have higher CTE values. A big CTE mismatch between anode and electrolyte may cause loss of anode contact with electrolyte and the failure of the cell during temperature ramping or thermal cycles. Besides a suitable CTE, a high redox reversibility is also critical for an anode material as it can prevent the degradation and failure of the cell when encountering an air leak from cathode Received: March 6, 2019 Accepted: May 31, 2019 Published: May 31, 2019 4244

DOI: 10.1021/acsaem.9b00494 ACS Appl. Energy Mater. 2019, 2, 4244−4254

Article

ACS Applied Energy Materials

ions:citric acid:EDTA was 1:1.25:1. The solution was adjusted to pH = 6 with ammonia−water and held on a hot plate at about 80 °C for several hours with stirring until gelation occurred. The gel was first heated at 200 °C overnight to evaporate the liquid and then heated at 600 °C for over 12 h until nitrates and organic residues were totally decomposed. The resultant powder was separated into three parts and calcined at 1050 °C in air, 5% H2−95% N2, and nitrogen, respectively. In the following chemical compatibility test, dilatometry test, conductivity test, and cells test, only LSFM synthesized in nitrogen was used. Sr2Fe1.5Mo0.5O6−δ (SF1.5M) anode material was also synthesized by the same method without the addition of lanthanum nitrate and calcined at 1050 °C in air. 2.2. Chemical Compatibility Test. The chemical compatibility between LSFM and electrolytes at the sintering condition (at 1150 °C in N2 for 2 h) and the operation condition (at 800 °C in H2 for 10 h) was tested. For the compatibility test at sintering conditions, LSFM pure-phase powder was mixed with YSZ (TOSOH), GDC (fuelcellmaterials), and LSGM (fuelcellmaterials) with 1:1 mass ratio and pressed into pellets to ensure a large contact area and a close contact of two phases. Under operation conditions, the mixture pellets of LSFM+GDC and LSFM+LSGM were tested. After heat treatment, pellets were crushed and thoroughly grinded into powders and characterized by XRD. 2.3. Bar Samples Fabrication for Dilatometry and Conductivity Test. Cuboid bar samples were fabricated for dilatometry and four-point dc electrical conductivity measurements. After mixing LSFM with 3 wt % polyvinyl butyral (PVB, Alfa Aesar) uniformly, the mixtures were pressed into pellets (diameter was 31 mm) and sintered at 1250 °C for 4 h in nitrogen. Bar samples were cut from the sintered pellets with a diamond saw blade and polished with sandpaper to achieve a cuboid shape and smooth surfaces. Before cutting, the porosity of the sintered pellet was measured with Archimedes’ principle. First, the weight of the dry pellet in air was measured with an analytical balance (A&D GH-202), and the average value was noted as M1. Second, the pellet was immersed in water in a beaker, and the beaker was put in a vacuum chamber for several minutes to eject air in the pellet. The weight of the saturated pellet was measured again in water, and the average weight was noted M −M as M2. The real volume of the pellet can be calculated as V = ρ1 2 .

side, fuel exhausting, or power outage in practical applications. Redox reversibility involves chemical expansion reversibility and electrical conductivity reversibility, both of which affect the microstructure and the performance of the cell among redox cycling. Ceramic anode materials normally have higher resistance to carbon deposition. Therefore, various ceramic anode candidates have been extensively explored with a focus on several structure types, such as perovskite,19−21 fluorite,22,23 pyrochlore,22,24,25 and tungsten bronze.26,27 Among them, Sr2FexMo2−xO6−δ based perovskite materials have emerged as promising anodes and been studied with different Fe/Mo ratios.28−34 Unfortunately, Sr2FexMo2−xO6−δ is unstable in air when x is lower than 1.5.31,33,34 The CTE property and redox reversibility of Sr2FexMo2−xO6−δ based anodes remain unclear. This work finds that the CTE of Sr2Fe1.5Mo0.5O6−δ (SF1.5M) is higher than that of common electrolytes, leading to CTE mismatch. We hypothesize that the thermal and chemical expansion behaviors may be influenced by metal−oxygen (M− O) bond strength in the perovskite crystal structure. Increasing the bond strength of the M−O bond can probably stabilize the crystal lattice and reduce the expansion. Though no work has been done to demonstrate this hypothesis, a similar idea and phenomenon have been mentioned in the literature. For example, Forbess et al. discussed the effect of M−O bond strength on the transition temperature and the activation energy of La-doped material Sr1−xLaxBi2Nb2O9.35 Because of the larger bond strength of the La−O bond (799 ± 4 kJ mol−1 at RT) than the Sr−O bond (425.5 ± 16.7 kJ mol−1 at RT), the substitution of La3+ into Sr2+ sites enhanced the stability of the crystal structure. In addition, with the increment of La content in La1−xSrxCr0.2Fe0.8O3, a reduction in CTE was observed by Chou et al.36 Therefore, we envisage that the modification of A-site metal elements by substituting partial Sr with La in the perovskite structure may improve the resistance of the material to environment-induced thermal and chemical expansions. Herein, we report that the doping of La in the A-site of Sr2Fe1.5Mo0.5O6‑δ can lower the CTE and enhance the redox reversibility. Increasing the La content reduces the CTE of LaxSr2−xFe1.5Mo0.5O6−δ. In particular, La0.5Sr1.5Fe1.5Mo0.5O6−δ (LSFM) synthesized in nitrogen at 1050 °C demonstrates a favorable CTE of 15.01 × 10−6 K−1 and a remarkable electrical conductivity reversibility over three redox cycles. These results indicate that the thermal and chemical expansion of the crystal lattice can be reduced by a larger metal−oxygen (e.g., La−O) bond strength. The polarization resistance of LSFM symmetrical cell at 800 °C in humidified hydrogen is as low as 0.16 Ω cm2, and the active region is ∼4.5 μm. The LSFM full cell shows high power density within humidified H2. Such new ceramic material LSFM with stable structure is a promising anode for high performance solid oxide fuel cells.

water

Here, ρwater = 1 g cm−3 is the density of water. Also, the theoretical volume of the pellet can be obtained as V0 = πr2h. Here, r is the radius and h is the thickness of the pellet. Hence, the porosity (ϕ) of the pellet can be written as ij V yz ϕ = jjj1 − zzz × 100% j z V 0{ k

(1)

After sintering, the porosity of all samples was ≤5%. 2.4. Symmetrical Cell Fabrication. In this work, symmetrical cells with LSGM electrolyte (LSFM/LSGM/LSFM) were fabricated by the screen-printing method. LSGM pellets (diameter 16 mm) were sintered at 1400 °C in air for 4 h. The diameter of the sintered electrolyte discs was ∼13 mm. The synthesized LSFM electrode slurry was screen-printed on both sides of the electrolyte symmetrically with an active area of 0.7 cm2, followed by sintering at 1150 °C in nitrogen for 2 h to achieve a good interface connection and a porous electrode (Figure S1). Silver current collectors with gold leads were applied on both sides of the symmetrical cells. 2.5. Single Cell Fabrication. The electrolyte supported single cell LSCF/LSGM/LSFM was utilized in this work. LSCF ((La0.6Sr0.4)0.95Co0.2Fe0.8O3−δ, fuelcellmaterials) cathode paste and LSFM anode paste were printed on dense LSGM (∼130 μm thick, Figure S2) by the screen-printing method. The active areas of the cathode and anode were 0.3 and 0.7 cm2, respectively. After electrode printing, the cell was sintered at 1150 °C in N2 for 2 h. The thickness of porous electrodes was ∼20 μm (Figure S2). Silver current collectors and gold leads were applied on both sides of the cell. 2.6. Characterization. Compositions of the synthesized powders and mixtures were characterized by X-ray diffraction (XRD,

2. METHODS 2.1. Powder Preparation. La0.5Sr1.5Fe1.5Mo0.5O6−δ (LSFM) was synthesized via the EDTA−citric acid sol−gel method. Stoichiometric amounts of raw materials (Alfa Aesar), La(NO3)3·6H2O, Sr(NO3)2, and Fe(NO3)3·9H2O, were dissolved in distilled water in one beaker (solution 1). In another beaker, the mixture of ethylenediaminetetraacetic acid (EDTA) (Alfa Aesar) and a suitable amount of ammonia−water (Alfa Aesar) was heated and stirred until a clear solution was obtained (solution 2). Solution 2 was mixed into solution 1, and then (NH4)6Mo7O24·4H2O (Alfa Aesar) and citric acid (Alfa Aesar) were added one by one. The molar ratio of metal 4245


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ACS Applied Energy Materials PANalytical X’pert PRO) with Cu Kα radiation. Scanning electron microscopy (SEM, Hitachi S-4700) was used to observe the microstructure of cells. The Rietveld refinement of the LSFM crystal structure was performed with TOPAS Academic v6 software. The instrument profile and detector zero were obtained from the refinement of an external silicon standard. The background was modeled using a fifthorder polynomial function; the peak shape was modeled by using the modified Thompson−Cox−Hastings pseudo-Voigt function.37 The peak broadening was modeled by crystallite size function. The initial structural model was from the Crystallography Open Database (COD), #96-154-2033.38 The site occupancies of the elements (La, Sr, Fe, and Mn) were from the nominal chemical composition. The expansion behavior of materials at high temperature was characterized by a simple push rod type dilatometer (Netzsch DIL 402C). Details of the test can be found elsewhere.39,40 Briefly, the sample was heated in air from room temperature to 800 °C at a rate of 3 °C min−1, followed by two 5% H2−air redox cycles with each cycle lasting for 10 h. A cooling rate of 3 °C min−1 was applied as the sample cooled in air. Because the operation temperatures of most solid oxide fuel cells (SOFCs) including ours are ≤800 °C, the temperature range of our coefficient of thermal expansion (CTE) test was set to be from 25 to 800 °C. The electrical conductivity was measured by the four-point dc method at 800 °C for two to three 5% H2−air redox cycles. The soaking time for each atmosphere was 6−8 h until the conductivity value was stable. Gold leads were fixed on the sample by platinum paste. Electrochemical characterization was performed on the LSFM/ LSGM/LSFM symmetrical cells with two-electrode configuration. Electrochemical impedance spectroscopies (EIS) were recorded between 600 and 800 °C in humidified H2 at open circuit condition over the frequency range from 0.01 Hz to 100 kHz with 10 mV perturbation. All electrochemical experiments were performed using an AUTOLAB PGSTAT302N potentiostat/galvanostat (Metrohm, Netherlands) controlled by NOVA 1.11 software. Prereduction in humidified H2 at 800 °C for 3 h was performed before electrochemical tests. Zview software was used for impedance spectra analysis and fitting. For the single cell test, 97% H2−3% H2O as fuel was flowed on the anode side and air as the oxidant was flowed on the cathode side. The flow rates on both sides were 100 sccm. Current−voltage (I−V) curves were recorded with a NOVA 1.11 at 700, 750, and 800 °C.

Figure 1. (a) XRD patterns of LSFM powders calcined in 5% H2− 95% N2, air, and nitrogen. (b) XRD pattern with Rietveld refinement result of LSFM calcined in nitrogen. The crystal structure of LSFM is shown in the inset.

3. RESULTS AND DISCUSSION 3.1. Phase Characterization. 3.1.1. Phase Formation. Initially, the calcination conditions for the LSFM were investigated, and the phases were characterized by XRD. Figure 1a display the XRD patterns of LSFM calcined in air, 5% H2−95% N2 and nitrogen, respectively. Pure LSFM with perovskite structure is obtained in nitrogen (Figure 1a and Figure S3). According to Rietveld refinement (Figure 1b and Table S1), the as-prepared pure LSFM shows a cubic structure (Pm-3m space group) with a = b = c = 3.93584(2) Å and α = β = γ = 90° (Rwp = 4.61%, χ2 = 1.56, Rp = 3.38%). In 5% H2− 95% N2, the synthesized material is far from the perovskite structure. According to the XRD data analysis, the calcined powder is composed of pure metal iron (Fe, JCPDS No. 060696) and several kinds of metal oxides such as LaSrFeO4 (JCPDS No. 71-1745), Sr3Fe2O6 (JCPDS No. 82-0426), and SrMoO3 (JCPDS No. 81-0640), as marked in Figure 1a. The crystal structure of the powder calcined in air showed more similarity to a cubic perovskite structure, except for several impurity peaks corresponding to SrMoO4 (JCPDS No. 080482) and LaFeO3 (JCPDS No. 75-0541). According to the literature,41,42 SF1.5M pure phase is usually obtained in air. The difference of synthesis atmospheres

Figure 2. XRD patterns of the LSFM pure phase subjected to heat treatment at 850 °C for 20 h in air and 5% H2−95% N2.

between LSFM and SF1.5M probably comes from the different valence states of La3+ and Sr2+. The reaction between oxygen gas and defects in LSFM can be expressed as 1 × × O2 (g) + V •• O + 2M′M = OO + 2MM (2) 2 Here, M presents B-site cations Fe and Mo, M′M corresponds to Fe2+ and Mo5+, and M×M corresponds to Fe3+ and Mo6+. When partially replacing the divalent cation Sr2+ by the trivalent cation La3+, the oxygen vacancy concentration decreases to maintain the electrical neutrality. If B-site cations are assumed to be Fe3+/Fe4+ and Mo6+ in air, the theoretical 4246


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Figure 5. Coefficient of thermal expansion (CTE) from 25 to 800 °C in air.

on the XRD results, LSFM synthesized in nitrogen would be used in the following tests. 3.1.2. Chemical Stability. Figure 2 shows the XRD patterns of LSFM pure-phase powder before and after heat treatment at 850 °C for 20 h in air and 5% H2−95% N2, respectively. Compared with the original XRD pattern, no new peaks are observed from both samples. This result demonstrates the high phase stability of LSFM in both reducing and oxidizing atmospheres and indicates the great potential for acting as redox stable electrode. In addition, the chemical stability of LSFM in pure H2 for short- and long-term reduction is further investigated. After 5 h reduction in pure H2, minor new peaks could be detected by the XRD (Figure 3b). The peak at 45° corresponds to metal Fe (marked as a red star, JCPDS No. 06-0696), the one at 31.5°

Figure 3. (a−c) Phase stability of LSFM in H2 at 800 °C for (a) 0, (b) 5, and (c) 105 h. (d, e) SEM images of LSFM powders after reduction for (d) 5 h and (e) 105 h.

value of δ become 0 or negative, which is unfavorable to perovskite phase formation. Thus, a lower PO2 environment (such as nitrogen or argon) would be preferred to push reaction 2 backward and produce some oxygen vacancies. Nevertheless, a reducing atmosphere (such as 5% H2) would go too far to reduce cations into metal (such as Fe) and metal oxides which are more stable in a reducing environment. Based

Figure 4. XRD patterns of (a) LSFM+YSZ, (b) LSFM+GDC, and (c) LSFM+LSGM mixtures sintered at 1150 °C in nitrogen for 2 h and (d) LSFM+GDC and (e) LSFM+LSGM mixtures sintered at 800 °C in hydrogen for 10 h. 4247


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Figure 6. Dilatometry behavior of (a) LSFM and (b) SF1.5M from room temperature to 800 °C in air followed by two redox cycles at 800 °C.

Figure 7. Electrical conductivity performance with redox cycling for (a) LSFM and (b) SF1.5M measured at 800 °C. O and R on the X-axis denote air and 5% H2, respectively. The following number stands for the cycle number.

utilized with LSGM as electrolyte. In this work, the chemical compatibility of LSFM with electrolytes under sintering conditions (at 1150 °C in N2 for 2 h) and operation conditions (at 800 °C in H2 for 10 h) is verified. First, mixture pellets were sintered at 1150 °C in nitrogen for 2 h, and XRD patterns after treatment are shown in Figure 3a−c. The XRD pattern reveals that LSFM is very unstable with YSZ. During the sintering process LSFM reacts severely with YSZ and decomposes into several oxides in a short time. The main peak of LSFM, which should be at around 32.1°, disappeared after sintering. Besides YSZ, SrMoO4, SrZrO3, and LaFeO3 are also detected. LSFM shows perfect compatibility with both GDC and LSGM, as shown in Figure 4b,c. Based on this result, the chemical compatibility of LSFM with GDC and LSGM at operating conditions (800 °C, H2) is further evaluated. Mixture pellets of LSFM+GDC and LSFM+LSGM were sintered at 800 °C in hydrogen for 10 h. Both GDC and LSGM exhibit good chemical compatibility with LSFM in hydrogen (Figure 4d,e). These results guarantee a clear electrode/electrolyte interface (Figures S1 and S4) as well as a clean LSFM−GDC composite electrode during the sintering and operation process. Based on the favorable compatibility with LSFM, LSGM will be applied as the electrolyte without GDC barrier layer. 3.2. Dilatometry Performance. According to the literature, the CTE of LSGM is (10−12) × 10−6 K−1,14−16 and that for YSZ is (10−11) × 10−6 K−1.17,18 Experimental results reveal that lanthanum doping on the A-site of SF1.5M is

corresponds to K2NiF4-type structure (purple diamond, according to Yang’s report43), and the one at 35° could be Fe2MoO4 (blue solid circle, JCPDS No. 25-1403). Exsolved Fe nanoparticles can be observed from the SEM image (Figure 3d). In fact, the Fe exsolution and phase transition seem to mainly occur in the first few hours, and then further decomposition is dramatically restrained. The LSFM powder that had been subjected to 5 h reduction in pure H2 at 800 °C was further calcined under the same conditions for another 100 h. As shown in Figure 3a−c, the XRD peaks for minor decomposition product phases in LSFM subject to 100 h reduction are not obviously intensified in comparison with those of the counterpart subject to 5 h reduction. Both samples maintain the major phase of LSFM similar to the pristine LSFM synthesized in N2. The SEM images also demonstrate a similar and small quantity of exsolution particles in LSFM upon 5 and 100 h reduction without obvious change (Figure 3d,e). The Fe exsolution and phase transition of LSFM powder are much weakened compared to the significant phase decomposition of other perovskite materials with similar structure such as Sr2FeMo0.65Ni0.35O6−δ,44 La0.6Sr0.4Fe0.8Ni0.2O3−δ,45 and Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ,43 which usually have severe metal exsolution and phase transformation in H2 at similar temperatures. The pure LSFM powder demonstrates high stability against H 2 reduction. Previous literature has reported that Sr2FeMoO6,32,33 Sr2Fe1.5Mo0.5O6,29,30 and other similar compositions are all 4248


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Figure 8. Symmetrical cell LSFM/LSGM/LSFM tested in wet H2 from 600 to 800 °C. (a) Nyquist plot of impedance. The ohmic resistance R1 is subtracted from the impedance plot. (b) Arrhenius plot and activation energy. (c) Raw data and fitting line at 800 °C. The equivalent circuit is shown in the inset. (d) Variation of Rchem and Cchem as a function of temperature.

Table 1. Comparison of Polarization Resistance of Ceramic Anodes in Wet H2 material

Rp (Ω cm2)

temp (°C)

ref

SF1.5M SF1.5M Sr1.9MgMoO6−δ La0.75Sr0.25Cr0.5Mn0.5O3 Gd2Ti1.4Mo0.6O7 LSFM YSZ+SF1.5M (infiltration) SF1.5M + 20−50% SDC SF1.5M + SDC

0.356 0.27 0.32 0.26 0.2 0.16 0.235 0.16 0.15

800 800 800 900 950 800 800 800 800

42 29 57 58 59 this work 34 42 60

helpful in reducing the CTE from room temperature to 800 °C. After replacing 1/4 of the strontium with lanthanum, the CTE is reduced from 17.12 × 10−6 K−1 (SF1.5M) to 15.01 × 10−6 K−1 (LSFM). When 1/2 of the strontium is replaced, this value drops to 13.3 × 10−6 K−1 (LaSrFe1.5Mo0.5O6−δ, XRD pattern is shown in Figure S5) (Figure 5), which is very close to that reported for LSGM. The underlying reason probably relates to the stronger La−O bond in the lattice. It is known that the La−O bond (799 ± 4 kJ mol−1) has a higher bond strength than the Sr−O bond (425.5 ± 16.7 kJ mol−1);35 it requires more energy to stretch or break a La−O bond. In other words, with the same temperature increment, La−O can help to reduce the crystal size expansion and lower the CTE. On the other hand, with lanthanum doping the lattice stability in a redox environment is also improved. Figure 6a,b shows the dilatometry behavior of LSFM and SF1.5M during

Figure 9. Voltage (open symbols) and power density (filled symbols) as a function of current density for an electrolyte-supported single cell LSCF/LSGM/LSFM under humidified H2 from 700 to 800 °C.

redox cycling. Compared with SF1.5M, LSFM presents a smaller volume change under redox environment and better reversibility during two redox cycles. For SF1.5M, the volume in air before and after redox cycles is a clear ladder shape. After each expansion−contraction cycle, some volume expansion still remains which can be easily observed in Figure 6b. In contrast, the lanthanum doped sample shows no expansion remnant (Figure 6a). This excellent chemical expansion reversibility benefits from the high structure stability and good phase stability of LSFM. Moreover, different chemical expansion behaviors of LSFM and SF1.5M in 5% H2 are noticed. LSFM exhibits milder 4249


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ACS Applied Energy Materials Table 2. Maximum Power Density of Anode Materials with Wet H2 material SF1.5M

SF1.5M+SDC YSZ+SF1.5M (infiltration) YSZ+SF1.5M (infiltration) Sr2FeMoO6−δ Sr2CoMoO6−δ Sr2NiMoO6−δ Sr2NiMoO6−δ Sr2Fe1.4Ni0.1Mo0.5O6‑δ (PrBa)0.95(Fe0.9Nb0.1)2O5+δ Pr0.5Ba0.5MnO3 LSFM

T (°C)

Pmax (mW cm−2)

La0.6Sr0.4Co0.2Fe0.8O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ SF1.5M SF1.5M SF1.5M Sr2Fe1.4Ni0.1Mo0.5O6−δ SF1.5M+SDC YSZ+La0.6Sr0.4Fe0.9Sc0.1O3−δ (infiltration) YSZ+SF1.5M (infiltration) Ba0.5Sr0.5Co0.8Fe0.2O3−δ SrCo0.8Fe0.2O3

800 800 800 800 800 800 800 800 800 800 800

Ba0.5Sr0.5Co0.8Fe0.2O3−δ Sr2Fe1.4Ni0.1Mo0.5O6−δ PrBaCo2O5+δ NdBa0.5Sr0.5Co1.5Fe0.5O5+δ + Ce0.9Gd0.1O2−δ (La0.6Sr0.4)0.95Co0.2Fe0.8O3−δ

800 800 800 800 800

291 520 500 500 530 260 500 462 1030 600 735 500 595 530 1050 1070 1156

cathode

ref 71 72 29 73 41 74 75 34 76 33 20 77 74 78 79 this work

3.3. Electrical Conductivity during Redox Cycling. Electrical conductivity performance of LSFM under redox cycling was performed with the dense bar sample by the dc four-point method. In air−5% H2 redox cycling, air in each cycle is written as O-1, O-2, and O-3. 5% H2 in each cycle is written as R-1, R-2, and R-3, and the following number is the cycle number. As shown in Figure 7a, LSFM exhibits excellent conductivity reversibility. There is no conductivity degradation in both air and 5% H2−95% N2 among three redox cycles. Meanwhile, the conductivity of LSFM in 5% H2 reaches 23 S cm−1. This electrical conductivity performance is very attractive compared with other anode materials like Sr2MgMoO6 (0.8 S cm−1 in 5% H2−Ar at 800 °C),48 Sr1.4La0.6MgMoO6 (5.0 S cm−1 in 5% H2−Ar at 800 °C),49 La0.8Sr0.2Sc0.2Mn0.8O3 (6.1 S cm−1 in wet 5% H2−Ar at 850 °C),50 and La0.8Sr0.2Fe0.9Nb0.1O3 (1.5 S cm−1 in 5% H2−N2 at 800 °C).51 Compared with SF1.5M (Figure 7b), lanthanum doping can increase the conductivity in 5% H−95% N2 and improve the reversibility of electrical conductivity during redox cycling. For LSFM, both the high phase stability in oxidation− reduction environment and the ideal chemical expansion behavior in redox cycling could be beneficial to the high reversibility of electrical conductivity. 3.4. Impedance Spectroscopy on Symmetrical Cells. After sintering in nitrogen at 1150 °C for 2 h, a porous electrode with a thickness of ∼30 μm (Figure S7a) was obtained. The SEM images of the porous electrode and the good connection at electrode/electrolyte interface can be found in Figure S1. As reported by Watanabe et al.52 and Xiong et al.,53 platinum can dramatically promote a cell’s electrochemical performance. To avoid the extra performance promotion from platinum, a silver current collector was applied on all cells in this work. The silver current collector is porous and uniform (Figure S7b) which leaves enough space for fuel (wet H2) to saturate in the electrode while also allowing the product (H2O) to leave the electrode. The polarization resistance (Rp) of an anode material in SOFCs is an effective parameter to assess the catalytic activity on the hydrogen oxidation reaction (HOR). Figure 8a presents the Nyquist plot of LSFM/LSGM/LSFM symmetrical cell from 700 to 800 °C in wet H2. An attractive polarization resistance is obtained at 800 °C as 0.16 Ω cm2. This

chemical expansion and slight contraction when soaked in 5% H2, while SF1.5M presents larger chemical expansion and extra expansion in 5% H2. Chemical expansion is the result of two competing processes: lattice contraction around oxygen vacancies and lattice expansion from the increase of cation radius. In general, the quantity of cation radius increment is much larger, leading to a net expansion.46 For perovskite structure, subtle symmetry changes during expansion47 and the degree of electronic charge localization will also contribute to the chemical expansion behavior. Compared with SF1.5M, more B-site cations in LSFM would be reduced to a lower valence state after switching the gas from air to 5% H2 (from A to B in Figure 6). As the cation radius increment dominates the expansion behavior, a larger chemical expansion in LSFM is expected. However, a milder chemical expansion is observed from our dilatometry test. Specifically, the volume expansion in SF1.5M from A to B is 0.2216% (as shown in Figure 6b). This value increased to 0.2409% (from A to C) after reduction for 5 h. For LSFM (as shown in Figure 6a), the volume change from A to B is 0.1706%, which is smaller than SF1.5M, and after 5 h reduction this value dropped to 0.1062% (from A to C). Based on this experimental phenomenon, a stronger M−O bond probably also contribute to a smaller chemical expansion (from A to B) and a stronger maintenance with respect to the original structure (point A). Considering the different radius of La3+ and Sr2+, the smaller La3+ (117.2 pm) can make more space for octahedral tilting and structure distortion than Sr2+ (132 pm). Thus, it is more likely for La-doped compositions to perform a structure contraction during relaxation (from B to C). This conclusion is consistent with our experimental observations on La-doped compositions (La0.5Sr1.5Fe1.5Mo0.5O6−δ and LaSrFe1.5Mo0.5O6−δ). Both samples exhibited similar volume contraction behavior in reducing atmosphere (Figure S6). In the macroscopic view, the appropriate thermal expansion and mild chemical expansion of LSFM can help to avoid the poor internal connections and microcracks in stacks and maintain the electrical and electrochemical performance during redox cycling. In that case the good dilatometry performance may also contribute to the excellent reversibility of the electrical conductivity. 4250


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applied the colimited theory on the anode. Impedances of the LSFM symmetrical cell are fitted well (Figure 8c) with the R1(R2CPE1)Lo1 equivalent circuit (inset in Figure 8c). R1 represents ohmic resistance from electrolyte resistance and electrode ohmic resistance34 (subtracted from the impedance plot). The green semicircle (Figure 8c and Figure S8) in the high frequency region corresponds to the oxygen ion transfer process at the electrode/electrolyte interface (R2CPE1).63,64 The blue half-tear-drop curve (Figure 8c and Figure S8) in the low frequency region presents the oxygen bulk diffusion and surface chemical exchange (gas adsorption, dissociation, and oxygen exchange) colimited process (Lo1).61,65,66 Rchem and Cchem as a function of temperature are plotted in Figure 8d. As expected, Rchem decreases as the temperature increases, which implies a faster surface kinetics at higher temperature. The chemical capacitance Cchem calculated from fitting result is very large (1−1.3 F cm−2), which accords with Kawada’s result.62 From the proportional relationship between Cchem and lδ (Cchem ∝ lδ) from eq 6, the increase of Cchem with temperature illustrates an expansion of the active region in the porous electrode. Furthermore, the size of the active region is estimated. Calculated from Rietveld refinement, the molar volume Vm of this material is 73.41 cm3 mol−1. According to the literature,67−69 the thermodynamic factor f is estimated as 10, and the porosity of the electrode ε is 0.5. From eq 6 the size of the active region lδ is ∼4.5 μm at 800 °C and consistent with the typical values (3−5 μm).61 This value demonstrates that a large portion of the electrode surface is active for HOR, which indicates some level of ionic conductivity, a good catalytic activity, and a sufficient electrical conductivity of LSFM. 3.5. Single-Cell Performance. Full cell performance of the LSFM single-phase anode was evaluated on an LSGM electrolyte-supported cell operated in humidified hydrogen from 700 to 800 °C. The LSCF single phase was applied as the cathode. Current−voltage (I−V) and current−power density (I−P) curves are shown in Figure 9. The open circuit voltages (OCV) are 1.09−1.10 V at operating temperatures. Such high values, which are very close to the theoretical OCV (1.10 V at 800 °C and 1.12 V at 700 °C in humidified H2)70 calculated from the Nernst equation, confirm a well-formed dense LSGM electrolyte and good sealing of the cell. The maximum power density (Pmax) achieved at 700, 750, and 800 °C is 329, 693, and 1156 mW cm−2, respectively. To the best of our knowledge, this value is higher than cells employing undoped Sr2Fe1.5Mo0.5O6−δ (SF1.5M) single phase or SF1.5M+GDC/ YSZ composite as anodes reported to date and can be comparable to the performance of many other promising ceramic anodes under similar conditions (Table 2). Meanwhile, the 100 h long-term stability test at 800 °C under a current density of 600 mA cm−2 was performed and indicates a good stability (Figure S9). The high electrical conductivity and good electrochemical performance are responsible for the high power density.

performance is prominent compared with that of Sr2Fe1.5Mo0.5O6−δ, Sr1.9MgMoO6−δ, and other ceramic anodes published in recent years and also competitive with that of ceramic+SDC composite anodes, as shown in Table 1. The activation energy (Ea) of LSFM is 1.2 eV (Figure 8b). This value is comparable to those of similar ceramic materials, such as Sr2Fe1.5Mo0.5O6−δ (1.07 eV),42 Ba2PrMoO6−δ (1.09 eV),54 Sr2Mg(Mo0.8Nb0.2)O6−δ (1.16 eV),55 Ba2NdMoO6‑δ (1.19 eV),54 and Sr2FeTi0.75Mo0.25O6−δ (1.27 eV).56 As shown in Figure 8a, the impedance spectroscopy of LSFM symmetrical cell is a half-tear-drop-shaped impedance which is composed of a diagonal of ∼45° at the high-frequency region and followed by a semicircle at the low-frequency region. The half-tear-drop-shaped impedance arises from a colimitation of surface oxygen exchange and oxygen bulk diffusion in the oxygen reduction reaction (ORR) or the hydrogen oxidation reaction (HOR). This phenomenon is first reported based on ORR and then reviewed by Adler.61 When electrode presents a facile solid-state diffusion and electrochemical process is only limited by the oxygen exchange at surface, the impedance is composed of several semicircles and can be fitted by equivalent circuits such as Rs(R1C1)(R2C2). However, if solid-state diffusion is slowed down or surface oxygen exchange rate is promoted, at some point the electrochemical process will be colimited by the surface reaction and the bulk diffusion. In that case impedance will transform from a semicircle shape to a half-tear-drop shape. This colimitation process can be described as Zchem =

Cchem =

f=

R chem 1 + (jωtchem)α

(3)

tchem R chem

(4)

−∂μO /∂δ 2

RT

∂μO yz 4F 2(1 − ε)lδ ijj 4F 2 (1 − ε)lδ 2 z jj − zz = j z ∂ δ RT fV m Vm k {

(5)

−1

Cchem =

(6)

Here Zchem is the chemical impedance colimited by surface oxygen exchange and solid-state diffusion. Rchem is the characteristic resistance from chemical reaction. tchem is the characteristic time. f is a thermodynamic factor reflecting the ease with which to change the stoichiometry (δ) in a given change of PO2. ε is the porosity of the electrode. lδ is the characteristic “utilization” length which indicates the thickness of the active region in a porous electrode, normally at a range of several micrometers. Vm is the molar volume of the electrode. Cchem is the chemical capacitance related to the bulk oxidation/reaction of the material and usually very large. As shown by Kawada et al.,62 for a very thin (1.5 μm) La0.6Sr0.4CoO3−δ mixed-conducting film the chemical capacitance is about 0.1−1 F cm−2. This value is ∼3 orders larger than the oxygen adsorption and transport dominated pseudocapacitance of Pt (∼10−3 F cm−2) and 5 orders larger than the interfacial polarization capacitance of the Pt/YSZ interface (10−6−10−5 F cm−2). From the view of oxygen diffusion in the bulk and oxygen exchange at surface, HOR is similar to ORR. In that case we

4. CONCLUSIONS Based on the A-site doped anode material LSFM, phase purity and stability, thermal expansion, redox reversibility, and electrochemical performance have been studied. LSFM was successfully synthesized with the EDTA−citric acid sol−gel method, and a pure phase was easily obtained upon calcination in nitrogen. The LSFM pure phase is stable in both air and 5% H2. When reducing LSFM in H2 at 800 °C, minor Fe 4251


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product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

nanoparticles exsolved from the perovskite structure during the first few hours, and the material reaches a stable state after that. Compared with SF1.5M, lower CTE and better chemical expansion reversibility of LSFM during redox cycling are proved. As the bond strength of La−O bond is much larger than that of Sr−O bond, resistance of the crystal lattice to thermal and chemical expansion is probably improved by a stronger La−O bond in the structure. The electrical conductivity performance reaches 23 S cm−1 in 5% H2−95% N2 at 800 °C and reveals an excellent reversibility during redox cycling. With porous and uniform microstructure, the LSFM/ LSGM/LSFM symmetrical cell presents a polarization resistance of 0.16 Ω cm2, which is lower than that of many reported anode materials. The half-tear-drop-shaped symmetrical cell impedance implies a colimitation mechanism in the HOR process. Higher surface kinetics and broader active region are expected at a higher temperature. The maximum power density of the LSGM electrolyte-supported single cell is 1156 mW cm−2 at 800 °C. This attractive performance suggests that LSFM is a promising anode candidate material for the next generation of Ni-free SOFCs.

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ABBREVIATIONS SOFC, solid oxide fuel cell; LSFM, La0.5Sr1.5Fe1.5Mo0.5O6−δ; SF1.5M, Sr2Fe1.5Mo0.5O6−δ; CTE, coefficient of thermal expansion; XRD, X-ray diffraction; SEM, scanning electron microscopy; EIS, electrochemical impedance spectroscopy; Rp, polarization resistance; HOR, hydrogen oxidation reaction; ORR, oxygen reduction reaction; OCV, open circuit voltages; I−V, current−voltage; I−P, current−power density; Pmax, maximum power density.

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ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00494. Figure S1: cross section of LSGM/LSFM interface after sintering; Figure S2: cross section of LSCF/LSGM/ LSFM single cell; Figure S3: XRD patterns of LSFM synthesized in N2; Figure S4: elements distribution nearby the LSGM/LSFM interface; Figure S5: XRD pattern of LaSrFe1.5Mo0.5O6−δ; Figure S6: dilatometry behavior of LSFM and LaSrFe1.5Mo0.5O6−δ; Figure S7: morphologies of LSFM electrode and current collector; Figure S8: Nyquist plots and fitting results of LSFM symmetrical cell; Figure S9: long-term stability test of LSCF/LSGM/LSFM single cell; Table S1: final refined structure parameters (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.L.). ORCID

Xingbo Liu: 0000-0001-8720-7175 Funding

This work was supported by the Department of Energy National Energy Technology Laboratory under Award DEFE0026169. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We extend our sincere appreciation to the DOE-NETL project manager Mr. Steven Markovich. We acknowledge the use of the WVU Shared Research Facilities to complete this work. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, 4252


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Reduced Thermal Expansion and Enhanced Redox

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