High-Performance Anode Material Sr2FeMo0.65Ni0.35O6−δ with In

Aug 16, 2016 - In past decades, significant efforts have been devoted to developing cost-effective and high-performance electrode materials for solid ...
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High-Performance Anode Material Sr2FeMo0.65Ni0.35O6−δ with In Situ Exsolved Nanoparticle Catalyst Zhihong Du,† Hailei Zhao,*,†,‡ Sha Yi,† Qing Xia,† Yue Gong,⊥ Yang Zhang,† Xing Cheng,† Yan Li,§ ∥ ́ Lin Gu,⊥ and Konrad Swierczek †

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, Beijing 100083, China § Department of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China ∥ Faculty of Energy and Fuels, Department of Hydrogen Energy, AGH University of Science and Technology, 30-059 Krakow, Poland ⊥ Beijing National Laboratory for Condensed Matter Physics Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: A metallic nanoparticle-decorated ceramic anode was prepared by in situ reduction of the perovskite Sr2FeMo0.65Ni0.35O6−δ (SFMNi) in H2 at 850 °C. The reduction converts the pure perovksite phase into mixed phases containing the Ruddlesden− Popper structure Sr3FeMoO7−δ, perovskite Sr(FeMo)O3−δ, and the FeNi3 bimetallic alloy nanoparticle catalyst. The electrochemical performance of the SFMNi ceramic anode is greatly enhanced by the in situ exsolved Fe−Ni alloy nanoparticle catalysts that are homogeneously distributed on the ceramic backbone surface. The maximum power densities of the La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolyte supported a single cell with SFMNi as the anode reached 590, 793, and 960 mW cm−2 in wet H2 at 750, 800, and 850 °C, respectively. The Sr2FeMo0.65Ni0.35O6−δ anode also shows excellent structural stability and good coking resistance in wet CH4. The prepared SFMNi material is a promising high-performance anode for solid oxide fuel cells. KEYWORDS: nanoparticle catalysts, in situ exsolution, electrochemical performance, anode, solid oxide fuel cells via appropriate chemical substitutions at A and/or B sites.14,15 Nevertheless, the catalytic activity of these perovskite anodes is still far from that of Ni-based anodes.1 One of the common approaches to improve the catalytic activity of ceramic anodes is to incorporate them with nanosized catalysts, such as Pt, Pd, and Ni, as well as ceria.11,16−19 Although the cell performance is considerably improved, it depends greatly on the uniformity and the morphology of catalysts within these dual-phase anodes.18−20 Besides, reactions between the incorporated catalyst and the electrode backbone would result in a negative impact on the long-term stability. In this regard, the development of an easy preparation method that can uniformly distribute a stable nanoparticle catalyst on the surface of porous anodes has been the focus of extensive investigations in recent years.18,19,21

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n past decades, significant efforts have been devoted to developing cost-effective and high-performance electrode materials for solid oxide fuel cells (SOFCs).1,2 To overcome the problems concerning the conventional anode material Ni/YSZ, such as poor redox ability, Ni agglomeration, deactivation of Ni by coking, and sulfur poisoning,3 various anodes have been proposed and investigated, for example, metal fluorites cermets (Co0.5Fe0.5Sm0.2Ce0.8O1.9, Sn−Ni−Gd0.1Ce0.9O1.95),4,5 perovskites (La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM),6 La- and Y-doped SrTiO3,7,8 Sr2MgMoO6−δ,9 Sr2Fe1.5Mo0.5O6−δ,10 and PrBaMn2O5+δ);11 tungsten bronze,12 and pyrochlore-structured materials.13 Among these anodes, perovskite ceramics have been identified as promising anode materials due to their inherent redox stability in a wide range of oxygen partial pressure, variety of physical and electrochemical properties, and ease of production. Most importantly, the flexibility of the perovskite-type structure allows further optimization of the mixed ionic− electronic conductivity and the catalytic activity of these ceramics © 2016 American Chemical Society

Received: June 16, 2016 Accepted: August 16, 2016 Published: August 16, 2016 8660

DOI: 10.1021/acsnano.6b03979 ACS Nano 2016, 10, 8660−8669

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ACS Nano Traditional infiltration has been widely used to fabricate catalyst-modified porous electrodes for SOFC. It usually requires multiple deposition steps and expensive precursors, as well as complicated morphology control.23 In addition, it remains a challenge to infiltrate a cell with a large electrode area or thickness.23 Alternatively, in situ growth of metal nanoparticles from ceramic oxides has been recently demonstrated as a more effective approach to obtain the supported catalysts in certain material systems.21,22,24−27 In this method, the catalytic active element, such as Pd, Ru, Co, and Ni, is first incorporated into the host ceramic lattice during the material synthesis in air, and then it segregates from the backbone and precipitates on the particle surface in the form of highly dispersed nanoparticles under reducing operation conditions at the anode side. In comparison with the infiltration technique, the latter approach can produce finer and more uniformly distributed catalyst nanoparticles on the porous electrode surface under the operation conditions of SOFCs. Moreover, this method is more time- and cost-effective and less affected by other factors such as the morphology of the ceramic backbones, viscosity of the infiltration precursor, and chemical compatibility between electrode materials and the infiltrated material.21,23 So far, a few material systems have been demonstrated to be capable of generating catalyst nanoparticles in situ under reducing atmosphere (anode side of SOFC or cathode side of SOEC), including La0.52Sr0.28Ti0.94(Ni/Fe)0.06O3,15,21 (NiO)0.05(SrTi0.8Nb0.2O3)0.95,24 Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ,25 NbTi0.5Ni0.5O4,26 La0.8Sr0.2Cr1−xMxO3−δ (M = Pd, Ru, Ni),27 and La0.8Sr1.2Fe0.9Co0.1O4±δ.28 For these nanoparticle-decorated anodes, the extracted nanometallic catalyst together with the reduced ceramic backbone that sustains the required ionic−electronic conductivity and the chemical and structural stability delivers good fuel cell performance and excellent redox stability. In this work, we evaluated the potential of double perovskite Sr2FeMoO6−δ (SFMO) as a host ceramic to exsolve catalytic active metallic nanoparticles with the aim of improving the electrochemical activity of electrode materials. We found that Sr2FeMoO6−δ doped with the catalytically active and easily reducible element Ni can generate uniformly distributed Ni−Fe alloy nanoparticle catalysts on the surface of a ceramic anode via in situ reduction under the cell operating conditions, while the host ceramic partially transforms into the Ruddlesden−Popper (RP) phase Sr3FeMoO7−δ and perovskite Sr(FeMo)O3−δ, maintaining mixed ionic and electronic conduction ability. This endows the material with a low polarization resistance and an excellent anode performance. We demonstrated that Sr2FeMo1−xNixO6−δ is a promising anode material with excellent electrochemical performance and durability.

Figure 1. Room-temperature XRD patterns of Sr2FeMo1−xNixO6−δ (x = 0.25, 0.30, 0.35) calcined at 1100 °C for 10 h in air: (a) 10−90°; (b) 17.5−45°.

confirms the structure type (see Figure S2, Supporting Information), with SFMNi possessing tetragonal structure (I4/m). The detailed refinement parameters are listed in Table S1 (Supporting Information). Although the SFMNi sample shows B-site-ordered structure, the low intensity of typical peaks of double perovskite at 19 and 38° suggests the low ordering degree of B site atom arrangement. Figure 2 shows the room-temperature XRD pattern of SFMNi after reduction at 850 °C in pure H2. As expected, the

Figure 2. Room-temperature XRD pattern of Sr2FeMo0.65Ni0.35O6−δ after reduction at 850 °C in pure H2 for 10 h.

RESULTS AND DISCUSSION Although pure Sr2FeMoO6−δ double perovskite cannot be synthesized in air due to the preferable high oxidation state of Fe3+ and Mo6+ under oxidizing atmosphere (Figure S1, Supporting Information), substitution of low-valence element Ni for Mo in Sr2FeMo1−xNixO6−δ (SFMNi, x = 0.25−0.35) can stabilize the single-phase perovskite structure in air. As shown in Figure 1a, when the doping content x is 0.25 and 0.3, a little amount of SrMoO4 still remains, while when the doping level reaches 0.35, the characteristic peaks of SrMoO4 diminish and the Sr2FeMo0.65Ni0.35O6−δ exhibits a pure perovskite structure. The two weak peaks at 19 and 38° (Figure 1b) indicate the double perovskite structure characteristics of the SFMNi. Rietveld refinement conducted on the X-ray diffraction (XRD) data further

subsequent reduction in H2 converts the pure perovksite phase into mixed phases containing a metallic phase, RP phase Sr3FeMoO7−δ (JCPDS 52-1715), and perovskite phase. Because Fe and Ni are adjacent to each other in the periodic table, they are apt to form a solid solution phase. In this work, the metallic phase in the reduced SFMNi can be well indexed with FeNi3 (JCPDS 65-3244, Pm3̅m, a = 3.55 Å). Actually, FeNi3 alloy can form a solid solution with a floating content of Fe and Ni.29 Based on this information, the phase transformations of the compound under reducing atmosphere can be speculated as eq 1. 8661

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fraction is reduced to metal from the substrate. Figure 3b depicts the Fe 2p core-level spectra of the reduced SFMNi. Most of the iron exists in a mixed oxidation states of Fe2+ (709.8 eV) and Fe3+ (∼710.8, 711.6, 713.4, and ∼719.2 eV), while a small amount of Fe is reduced to metallic iron Fe0 (706.7 eV).36,37 These results are consistent with XRD investigation that some peaks are well indexed with FeNi3. The Mo 3d XPS spectra (Figure 3c) show two broad peaks, which can be fitted into two valence states of Mo6+ (232.3 and 235.4 eV) and Mo5+ (231.6 and 234.7 eV).37,38 It can be seen from Figure 3d that the O 1s consists of two main peaks around 530 and 531.6 eV. The former comes from lattice oxygen (Olattice), and the latter is typical of adsorbed oxygen species (Oad) including OH−, O−, O22−, and carbonate.39,40 The high peak ratio of Oad/Olattice demonstrates a high concentration of adsorbed oxygen species. Such weakly bonded Oad can be easily released at evaluated temperature, leaving plenty of active sites,40 which benefit the surface reaction of the electrode. The XPS analysis confirms the mixed valence state couples of Fe2+/Fe3+ and Mo6+/Mo5+, metallic Ni0 and Fe0, and the high ratio adsorbed oxygen on the surface of the reduced SFMNi. Figure 4 displays the microstructure of the as-prepared and reduced perovskite SFMNi. The as-prepared SFMNi displays porous structure with homogeneous particle size distribution and a glossy particle surface (Figure 4a,b). After reduction, many nanosized particles (FeNi3 alloy) exsolve out and distribute on the SFMNi substrate surface (Figure 4c,d). The average diameter of Fe−Ni alloy nanoparticles is about 50−60 nm. In order to confirm the exsolution of FeNi3 alloy nanoparticles, scanning transmission electron microscopy (STEM) along with energydispersive X-ray spectroscopy (EDX) analysis is conducted on the reduced SFMNi powders. The STEM and elemental EDX mapping images, as shown in Figure 5, demonstrate the exsolution of Fe−Ni alloy nanoparticles on the surface of the Sr2FeMo0.65Ni0.35O6−δ particle. The existence of Fe−Ni alloy nanoparticles is clearly confirmed by the elemental mapping results, which present agglomerated Ni and Fe distribution (Figure 5d,e), while uniform Sr, Mo, and O distribution (Figure 5b,c,f) occurs in the reduced sample. The high-angle annular dark-field (HAADF) images (in HAADF, image contrast is proportional to atomic number Z, and heavier atoms appear brighter) further confirm that the exsolved nanoparticles are FeNi3 alloy (Figure 6). The high-magnification HAADF observation on the FeNi3 alloy nanoparticle along the [110] zone axis reveals a well-developed crystal lattice structure with obvious Z contrast change along the AB direction (Figure 6b,c), which is indexed as a cubic structure (Pm3̅m, a = 3.57 Å). The structural model of FeNi3 along the [110] zone axis is given in Figure 6d, from which evident distinction of atom distribution can be easily observed along the AB direction, corresponding well to the element intensity change in Figure 6c. The crystal structure transformation and surface morphology evolution of SFMNi under reducing atmosphere is schematically illustrated in Figure 7. Under reducing conditions, the perovskite phase SFMNi is reduced to metallic alloy particles, RP phase as well, as a new kind of perovskite phase. The RP phase Sr3FeMoO7−δ is a layered perovskite, consisting of n (here n = 2) layers of ABO3 (here SrFe0.5Mo0.5O3−δ) perovskite blocks intercalated by rock salt AO (here SrO) layers, which can be represented by a chemcial formula An+1BnO3n+1 (n = 1, 2,...∞). The Mo(V)/Mo(VI) and Fe(II)/Fe(III) redox couples in (Fe)O6 and (Mo)O6 octahedral units can accept electrons while losing lattice oxygen, which is beneficial for electrical conduction and catalytic activity toward the oxidation of fuel.

Sr2FeMo0.65Ni 0.35O6 − δ 1 1 1 = SrFe0.7Mo0.3O3 − δ + Sr3FeMoO7 − δ + Fe1.2Ni 2.8 2 2 8 1 + O2 (g) (1) 2

The perovskite phase SrFe1−yMoyO3−δ (y = 0.3) is a mixed oxygen ionic and electronic conductor,10,30−33 whereas the RP phase Sr3FeMoO7−δ is at least an electronic conductor because of its mixed valence state of Fe and Mo ions according to the reported X-ray absorption near-edge spectra (XANES) and neutron diffraction results.34,35 Therefore, the reduced products, Fe−Ni alloy nanoparticles decorated on Sr3FeMoO7−δ and SrFe1−yMoyO3−δ ceramics, are expected to contribute a good anode performance on the electrode. The phase transformation most likely occurs only on the SFMNi particle surface, while the inside of the particle maintains the pristine perovskite structure because the decomposed phases form a dense outer layer on the particles, which may impede further reduction of the inside pristine perovskite. This ensures the structural stability of the SFMNi anode in long-term operation conditions. X-ray photoelectron spectroscopy (XPS) was used to examine oxidation states of elements in the reduced SFMNi sample. The Ni 2p, Fe 2p, Mo 3d, and O 1s spectra were recorded at room temperature. As shown in Figure 3a, the peaks of binding

Figure 3. XPS spectra of reduced Sr2FeMo0.65Ni0.35O6−δ: (a) Ni 2p; (b) Fe 2p; (c) Mo 3d; and (d) O 1s.

energy at ∼852.6/858.6, ∼854.7/856.5, and ∼861.4 eV can be assigned to metallic nickel Ni0, divalent valence state Ni2+, and the satellite peak, respectively.24,36 Clearly, most of the nickel is still in the lattice, taking the form of Ni2+, and only a small 8662

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Figure 4. Low and high-magnification field emission SEM images of (a,b) as-prepared and (c,d) reduced perovskite Sr2FeMo0.65Ni0.35O6−δ. The reduction treatment was performed at 850 °C for 2 h in H2. Nanosized FeNi3 particles exsolved from the backbone disperse uniformly on the reduced electrode surface.

Figure 5. (a) HAADF (Z-contrast) STEM image of reduced Sr2FeMo0.65Ni0.35O6−δ anode and (b−f) corresponding elemental mapping images of Sr (magenta), Mo (cyan), Fe (green), Ni (yellow), and O (red).

Figure 8 shows the electrical conductivity of the reduced SFMNi sample recorded in 5% H2/95% Ar from 250 to 900 °C. It exhibits high conductivity in reducing atmosphere, which decreases with increasing temperature but still remains 55.4 S cm−1 at 800 °C. This value is much higher than that of most of the

The newly formed SrFe1−yMoyO3−δ (y = 0.3) phase is also a mixed conductor and shows good structural stability in reducing atmosphere.10,30−33 Meanwhile, the in situ precipitated FeNi3 alloy nanoparticles are expected to greatly enhance the catalytic performance of anodes. 8663

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Figure 6. HAADF (Z-contrast) STEM image of reduced Sr2FeMo0.65Ni0.35O6−δ anode (a); high-magnification HAADF image of FeNi3 alloy nanoparticle along the [110] zone axis (b); obvious Z-contrast change along the AB direction (c); and structural model of FeNi3 alloy along [110] zone axis (d).

Figure 7. Illustration models for (a) structure transformation and (b) surface morphology evolution of Sr2FeMo0.65Ni0.35O6−δ anodes under reducing atmosphere.

typical ceramic anodes, such as LSCM (1−3 S cm−1),41 La-doped SrTiO3−δ (30−40 S cm−1),42−44 Sr2MgMoO6−δ (1−8 S cm−1),9,45 and the RP phase (Pr0.4Sr0.6)3(Fe0.85Nb0.15)2O7−δ (19 S cm−1),22 and five times higher than the reported requirement of conducitivity (10 S cm−1) for intrinsic material in dense form.46 After reduction in H2-containing atmosphere, nanosized FeNi3 bialloy particles were exsolved and coated on the backbone surface, which is beneficial to improve the electronic conductivity. Nevertheless, at the lower-temperature range, the linear relationship in

Arrhenius plots (Figure 8 inset) indicates that the reduced SFMNi anode obeys small polaron conduction behavior with activation energy being 0.05 eV for electron hopping. It should be the Fe2+/Fe3+ and Mo5+/Mo6+ redox couples in the reduced substrate that provide the high electronic conductivity and dominate the electron conduction behavior of alloy (metal) nanoparticledecorated SFMNi. At elevated temperatures, the Arrhenius plots deviate from linear behavior, and the electrical conductivity decreases relatively fast with increasing temperature. This can be 8664

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The electrochemical impedance spectra (EIS) of the SFMNi anode were measured with a LSGM (La0.8Sr0.2Ga0.8Mg0.2O3−δ)supportd symmetric cell configuation SFMNi|LDC|LSGM|LDC| SFMNi in pure H2 (50 mL min−1) under zero dc conditions. The symmetric cells were reduced at 850 °C for 2 h before the impedance spectra were recorded at different temperatures from 850 to 700 °C. Figure 9a,b displays the Nyquist and Bode plots of the EIS data and the corresponding fitting results. The employed equivalent circuits are illustrated in Figure 9b. In the equivalent circuits, L is the inductance, and (RHQ H), (RMQ M), and (RLQ L) represent the constant phase element and resistance of the processes at high, medium, and low frequency (HF, MF, and LF), respectively. The ohmic resistances corresponding to the high-frequency intercepts are normalized to zero to highlight the size differences between impedance arcs at different temperatures. For the EIS data collected in the high-temperature range of 750−850 °C, three obvious EIS responses can be observed from the Bode plot (Figure 9b), so a high goodness of fit for the Nyquist plots can be obtained by fitting the data with equivalent circuit L(RHQ H)(RMQ M)(RLQ L). For the EIS data tested at 700 °C, the large low frequency arc in the Bode plot is almost perfectly symmetrical compared to other EIS data, suggesting that its Nyquist plot could be fitted with two responses, high- and low-frequency responses, by using the equivalent circuit L(RHQ H)(RLQ L). The area specific resistances (ASR) of the SFMNi anode, characteristic capacitances, and frequencies (C = (QR)1/n/R, f = 1/2π(QR)1/n) of each fitted arc at different temperatures are calculated and summarized in Table S2. With increasing temperature, the polarization resistances decrease, demonstrating the thermal activation process of electrode reactions. The ASR of the SFMNi anode are 0.565, 0.290,

Figure 8. Temperature dependence of electrical conductivity of reduced Sr2FeMo0.65Ni0.35O6−δ in 5% H2/95% Ar. Inset is the corresponding Arrhenius plot.

explained by the fact that the increased oxygen vacancies caused by lattice oxygen loss at high temperature block the electron hopping along Fe2+(Mo5+)−O−Fe3+(Mo6+) bonds and hinder the electron conduction. In spite of this, the increased oxygen vacancies can improve the ionic conductivity of the reduced ceramic substrate, which is beneficial to the electrode reactions. On the other hand, the decreasing behavior of conductivity with temperature may come partially from the influence of nanometal particles at high temperatures.

Figure 9. (a) Nyquist and (b) Bode plots of electrochemical impedance data of symmetrical cell SFMNi|LDC|LSGM|LDC|SFMNi measured in pure H2 (50 mL min−1) under open-circuit voltage at different temperature, (c) area specific resistance of SFMNi as a function of testing time in pure H2 (50 mL min−1) at 800 °C. Insets in (b) are equivalent circuits. High-frequency intercepts (Ohmic resistances) are normalized to zero to highlight the size differences between impedance arcs at different temperatures. 8665

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Table 1. ASR of the Typical Reported Anode Materials and the Prepared Sr2FeMo0.65Ni0.35O6−δ in This Work under Similar Conditions composition

temperature (°C)

ASR (Ω cm2)

electrolyte

ref

La4Sr8Ti11Mn0.5Ga0.5O38−δ/YSZ La0.2Sr0.8TiO3−GDC (30:70) LST2D/YSZ LSCM Sr2MgMoO6−δ Sr2Fe1.5Mo0.5O6−δ Sr2Co1+xMo1−xO6−δ Co−Fe alloy modified Pr0.8Sr1.2(Co,Fe)0.8Nb0.2O4+δ Ni-YSZ Sr2FeMo0.65Ni0.35O6−δ

850 800 900 900 850 850 800 850 800 750 800 850

∼0.25 2.1 1.17 0.26 0.48 0.21 0.35−0.7 0.31 0.16 0.290 0.163 0.106

YSZ LSGM YSZ YSZ LSGM LSGM LSGM LSGM YSZ LSGM

47 48 49 50 51 10 52 25 53 this work

0.163, and 0.106 Ω·cm2 at 700, 750, 800, and 850 °C, respectively, demonstrating excellent electrochemical performance. The derived capacitance and frequency of the semicircles at high, intermediate, and low frequency (see Table S2) indicate that the HF responses are likely related to the interface process between the electrolyte and electrode, while the LF response at 700 °C and MF response at 750−850 °C should be associated with the surface reaction processes.54 The large capacitance of the SFMNi anode in the low-frequency range of the Nyquist plots at 750−850 °C indicates that these LF responses are from a gasphase diffusion process.54 At similar conditions, the obtained polarization resistance is smaller than that of most promising ceramic anodes, as shown in Table 1. Moreover, due to the in situ generation feature and the high dispersity of catalyst particles, the metal nanoparticledecorated SFMNi anode exhibits good structural stability at 800 °C in H2. The impedance spectra of the symmetric cell as a function of time were measured for about 24 h with 1 h intervals. As shown in Figure 9c, the SFMNi anode displays very stable polarization resistance during the testing, suggesting its durable and robust structural characteristics. The SFMNi anode performance was studied in LSGM electrolyte-supported SFMNi/LDC/LSGM/LSCF single cells with LSCF (La0.58Sr0.4Co0.2Fe0.8O3−δ) as the cathode. The voltage and power density versus current density plots are shown in Figure 10a. The open-circuit voltage (OCV) at 850 °C is about 1.1 V, which is very close to the Nernst theoretical value, indicating the good sealing state of the tested cell. The cell maximum power density (Pmax) in H2 reaches 390, 590, 792, and 960 mW cm−2 at 700, 750, 800, and 850 °C, respectively. The observed Pmax of SFMNi is much higher than that of the recently reported SrTi0.3Fe0.7O3−δ/Ce0.9Gd0.1O2−δ composite anode (337 mW cm−2 at 800 °C),55 LSCM (300 mW cm−2 at 900 °C),6 and Sr2Fe1.5Mo0.5O6−δ (650 mW at 850 °C).10 The SFMNi anode also delivers a stable power output at 750 °C under a constant current density of 0.55 A cm−2 (Figure 10b), indicating the excellent electrochemical performance and good short-term structual stability. The excellent electrochemical performance of SFMNi could be mainly attributed to the high catalytic activity of FeNi3 bimetal alloy nanoparticles toward H2 oxidation. The FeNi3 nanoparticles exsolved in situ from the backbone and homogeneously coated on the substrate surface can effectively accelerate the electrode reaction processes. These results demonstrate that such a strategy, in situ exsolution of metallic catalyst nanoparticles from a ceramic backbone in

Figure 10. (a) Voltage and power density versus current density curves of LSGM electrolyte-supported single-cell SFMNi/LDC/ LSGM/LSCF operated in humidified H2 (40 mL min−1). (b) Cell voltage as a function of testing time for single cell operated under a constant current density of 0.55 A cm−2 at 750 °C.

operating conditions, is very efficient in improving the electrochemical performance of ceramic anode materials. Finally, we evaluated the electrocatalytic activity of the SFMNi electrode toward hydrocarbon fuel by feeding humidified CH4 (3% H2O) to the anode of a single cell. The current− voltage and power density curves of the fuel cells are presented in Figure 11a. The Pmax value of SFMNi/LDC/LSGM/LSCF reaches 500 mW cm−2 in humidified CH4 (3% H2O) at 850 °C. The OCV in Figure 11a is lower than the theoretical OCV value (∼1.33 V at 1000 °C, 3% H2O/97% CH4) calculated from the 8666

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alloy nanoparticles homogeneously decorate the ceramic backbone surface and, hence, greatly enhance the anode’s electrochemical performance. The Sr2FeMo0.65Ni0.35O6−δ anode exhibits high electronic conductivity, impressive catalytic activity toward the oxidation of H2 and hydrocarbons (CH4), and good structural stability in reducing atmosphere, leading to excellent electrochemical performances as an anode in single cells. The maximum power density of the LSGM electrolyte-supported single cell with SFMNi as an anode reaches 590, 793, and 960 mW cm−2 in wet H2 at 750, 800, and 850 °C, respectively. The excellent electrochemical performance and durability of the electrolyte-supported single cell with the Sr2FeMo0.65Ni0.35O6−δ as an anode strongly suggest that Sr2FeMo0.65Ni0.35O6−δ is a promising anode material for high-performance SOFCs.

MATERIALS AND METHODS Material Preparation. Sr2FeMo1−xNixO6−δ (SFMNi, x = 0.25−0.35) powders were synthesized by a citric nitrate combustion method. First, a stoichiometric amount of high-purity Sr(NO3)2, (NH4)6Mo7O24·4H2O, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O (all from Sinopharm), and the chelating agent citric acid monohydrate (C6H8O7·H2O, Guangdong Xilong) was dissolved in deionized water to form a metal ion solution. The pH value of the solution was adjusted to 4 with ammonia (AR, Sinopharm). The amount of C6H8O7·H2O was fixed at a 2:1 molar ratio to the total amount of metal ions. The obtained solution was heated in a water bath at 80 °C until a gel was formed. The resultant gel was slowly heated to 250 °C in an oven for self-combustion. The as-synthesized fluffy products were grinded and subsequently calcined at 800 °C for 6 h to remove the organic residues. The precursors were calcined at 1100 °C for 10 h to obtain the pure powders. The La0.4Ce0.6O2−δ (LDC) buffer layer and La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode powders were also synthesized in the same way. Finally, SFMNi powder was uniaxially pressed into a rectangular bar and then sintered at 1400 °C for 4 h to get dense bars (the relative density of these samples is greater than 90%) for electrical conductivity measurements. The SFMNi samples were reduced at 1000 °C for 12 h under a flow of 5% H2/95% Ar before conductivity testing. Fabrication of Electrolyte-Supported Symmetric and Single Cells. The LSGM (fuelcellmaterials) pellets were sintered at 1450 °C for 8 h and then polished to a thickness of 300 μm. LDC ink was pasted on the electrolyte and sintered at 1400 °C for 2 h, which was employed as a buffer layer between the anode and the electrolyte to prevent the interface reaction and interdiffusion of ionic species.63 The electrode slurries were prepared by homogeneously mixing the electrode powders (SFMNi or LSCF) with poly(methyl methacrylate) (pore former, 5 wt % to the electrode powders) and α-terpineol solution of 6 wt % ethylene cellulose (50 wt % to the electrode powders). For symmetrical cells, a SFMNi slurry was screen-printed on both sides of the LDCcoated LSGM electrolyte symmetrically with an active area of 0.5 cm2, followed by calcining at 1150 °C for 2 h. The single-cell performance of SFMNi was tested with the cell configuration of SFMNi|LDC|LSGM| LSCF. The SFMNi slurry was screen-printed on the LDC-coated LSGM electrolyte and then sintered in the same way with symmetrical cells. Then LSCF ink was applied as a cathode by screen-printing and firing at 1000 °C for 2 h. The active area of both the anode and the cathode is 0.5 cm2, and the electrode thickness is about 30−40 μm. Ag paste was used as the current collector, which was painted in a grid structure and fired at 700 °C for 0.5 h. Single cells were sealed on an alumina tube using a ceramic-based material (Cerama-bond 552-VFG, Aremco). The humidified H2 and CH4 were fed as fuel to the anode side with a flow rate of 40 mL min−1, and air was conducted to the cathode side at a flow rate of 100 mL min−1 as oxidant. Material Characterizations. The XRD patterns were recorded on PANalytical Empyrean diffractometer (with Cu Kα radiation, Panalytical, The Netherlands). The Rietveld refinements of the XRD patterns were performed using GSAS/EXPGUI software.64,65 The micromorphology of the powders and electrodes were characterized using a field emission SEM (CARL ZEISS, SUPRA55, 10 kV). XPS was collected on

Figure 11. (a) Voltage and power density versus current density curves of LSGM electrolyte-supported single-cell SFMNi/LDC/ LSGM/LSCF operated in humidified CH4 (40 mL min−1) at 850 °C. (b) Cell voltage as a function of testing time for single-cell SFMNi/ LDC/LSGM/LSCF operated under a constant current density of 0.2 A cm−2 in humidified CH4 (40 mL min−1) at 850 °C.

Nernst equation56 but is similar to many reported works running on CH4.6,9,10,17,25 The possible reason is related to the incomplete oxidation of CH4 to H2O and CO2 at the anode side. The short-term stability of the SFMNi/LDC/LSGM/LSCF cell in CH4 atmosphere is shown in Figure 11b. When a constant current density of 0.2 A cm−2 was applied to the cell, the cell voltage increased initially and reached a stable value of 0.8 V in the following 24 h. After the durability test in CH4, the cell was disassembled, and the microstructures of the porous SFMNi anode and LSCF cathode were examined. No carbon or carbon fibers were observed at the anode side (Figure S3, Supporting Information), indicating the relatively good coking resistance of the SFMNi electrode. It is interesting to note that the FeNi3 alloy catalyzes CH4 oxidation without promoting coking, although both Fe and Ni are known to be good catalysts for carbon deposition.57,58 This phenomenon is also encountered in other alloys5,22,25,59 and can be explained by density functional theory computation that a bimetallic alloy can oxidize the hydrocarbon fuel rather than forming carbon−carbon bonds.60,61 It is also thought that the exsolved nanoparticles, which anchor well on the surface of backbone, would decrease the propensity of hydrocarbon coking.62 Nevertheless, further investigation is definitely needed to elucidate the detailed mechanism of a bimetallic alloy toward coking resistance.

CONCLUSIONS In summary, in situ growth of a metallic nanoparticle catalyst on ceramic anodes has been achieved by annealing the porous perovskite Sr2FeMo0.65Ni0.35O6−δ in H2 at 850 °C. Under reducing atmosphere, Sr2FeMo0.65Ni0.35O6−δ partially transforms into RP-type layered perovskite Sr3FeMoO7−δ, perovskite Sr(FeMo)O3−δ, as well as Fe−Ni bimetal alloy. The precipitated Ni-based 8667

DOI: 10.1021/acsnano.6b03979 ACS Nano 2016, 10, 8660−8669

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ACS Nano

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ESCALAB 250Xi, (Al Kα radiation, Thermofisher, USA) to identify the oxidation state of elements in the reduced SFMNi. To investigate the morphology and composition of the exsolved nanoparticles, a highresolution aberration-corrected scanning transmission electron microscope (JEOL-ARM200CF) equipped with a Schottky cold emission gun, operated at 200 kV, was used to perform HAADF and EDX mapping. Electrical conductivity of the sintered SFMNi bar was measured using a dc four-terminal measurement. The electrical conductivity of reduced SFMNi samples was tested in 5% H2/95% Ar in a temperature range of 250−900 °C. The electrochemical characterizations of symmetrical and single cells were measured with a Solartron 1260 impedance gain/phase analyzer (Solartron Analytical, England) in combination with 1287 electrochemical interface (Solartron Analytical, England) as described in detail elsewhere.47 Before testing, the cells were first reduced in H2 at 850 °C for 2 h, and then the impedance spectra and cell performance were recorded. To investigate the cell performance in hydrocarbon fuel, CH4 was fed to the reduced anode and the cell performance and impedance spectra of the cells were recorded.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03979. XRD patterns, Rietveld refinement results and parameters, polarization resistances, and SEM images (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by National Basic Research Program of China (2013CB934003, 2012CB215405), Guangdong Industry-Academy-Research Alliance (2012B091100129), National Nature Science Foundation of China (51302275, 51402019) and Program of Introducing Talents of Discipline to Universities (B14003), Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB07030200), National Program on Key Basic Research Project (2014CB921002), and National Natural Science Foundation of China (51522212, 51421002). REFERENCES (1) Cowin, P. I.; Petit, C. T. G.; Lan, R.; Irvine, J. T. S.; Tao, S. Recent Progress in the Development of Anode Materials for Solid Oxide Fuel Cells. Adv. Energy Mater. 2011, 1, 314−332. (2) Ge, X. M.; Chan, S. H.; Liu, Q. L.; Sun, Q. Solid Oxide Fuel Cell Anode Materials for Direct Hydrocarbon Utilization. Adv. Energy Mater. 2012, 2, 1156−1181. (3) Jiang, S. P.; Chan, S. H. A Review of Anode Materials Development in Solid Oxide Fuel Cells. J. Mater. Sci. 2004, 39, 4405−4439. (4) Lu, Z. G.; Zhu, J. H.; Bi, Z. H.; Lu, X. C. A Co−Fe Alloy as Alternative Anode for Solid Oxide Fuel Cell. J. Power Sources 2008, 180, 172−175. (5) Myung, J. H.; Kim, S. D.; Shin, T. H.; Lee, D.; Irvine, J. T. S.; Moon, J.; Hyun, S. H. Nano-Composite Structural Ni−Sn Alloy Anodes for High Performance and Durability of Direct Methane-Fueled SOFCs. J. Mater. Chem. A 2015, 3, 13801−13806. (6) Bastidas, D. M.; Tao, S.; Irvine, J. T. S. A Symmetrical Solid Oxide Fuel Cell Demonstrating Redox Stable Perovskite Electrodes. J. Mater. Chem. 2006, 16, 1603−1605. (7) Slater, P. R.; Fagg, D. P.; Irvine, J. T. S. Synthesis and Electrical Characterisation of Doped Perovskite Titanates as Potential Anode 8668

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