Self-Decorated MnO Nanoparticles on Double Perovskite Solid Oxide

Research Article pubs.acs.org/journal/ascecg

Self-Decorated MnO Nanoparticles on Double Perovskite Solid Oxide Fuel Cell Anode by in Situ Exsolution Sivaprakash Sengodan,† Young-Wan Ju,‡ Ohhun Kwon,† Areum Jun,† Hu Young Jeong,§ Tatsumi Ishihara,∥ Jeeyoung Shin,*,⊥ and Guntae Kim*,† †

School of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea Department of Chemical Engineering, Wonkwang University, Iksan, Jeonbuk, Republic of Korea § UNIST Central Research Facilities and School of Mechanical and Advanced Materials Engineering, UNIST, Ulsan 689-798, Republic of Korea ∥ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ⊥ Division of Mechanical Systems Engineering, Sookmyung Women’s University, Seoul 04310, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Surface decorated electrocatalytic nanoparticles coupled with oxide materials can effectively improve the electrochemical catalytic properties in energy storage and conversion application, such as chemical processes, electrolysis, batteries, and fuel cells. Particularly, Mn rich simple perovskite-type R0.5Ba0.5MnO3‑δ (R = Pr and Nd) undergoes a phase transition to layered perovskite RBaMn2O5+δ at high temperature reduced condition. During this phase transition, the exsolution of MnO nanoparticles (MnO-NP) from the bulk layered perovskite NdBaMn2O5+δ is observed. For in-depth investigation on the exsolution of MnO, a layered NdBaMn2O5+δ thin film is fabricated with pulsed laser deposition and characterized by transmission electron microscopy. For the first time, this paper reports clear evidence of selfdecorated MnO nanoparticles on a layered NdBaMn2O5+δ matrix via exsolution process and their electro catalytic effect in solid oxide fuel cells. KEYWORDS: Solid oxide fuel cells, Ceramic anode, Layered perovskite, Exsolution, Oxygen nonstoichiometry

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INTRODUCTION Surface decorated electrocatalytic nanoparticles coupled with oxide materials can effectively improve the electrochemical catalytic properties in energy storage and conversion applications, such as chemical processes, electrolysis, batteries, and fuel cells. In particular, solid oxide fuel cells (SOFCs) offer opportunities to achieve efficient energy conversion by an electrochemical process to transform chemical energy to electrical energy, which thereby emits less CO2 in comparison to most of the other conventional power generation systems.1−7 In the current state-of-the-art SOFC technology, the conventional Ni-based composite anode has been widely used as a SOFC anode material, because Ni exhibits excellent catalytic activity and electrical conductivity and high temperature stability. However, this Ni-based anode material suffers from serious drawbacks, such as carbon build-up (coking), sulfur poisoning, and low tolerance to redox cycling.8,9 Therefore, several approaches and materials have been considered to design and/or discover new anode materials that simultaneously possess good coking tolerance, sulfur tolerance, and excellent catalytic activity. In general, a new anode material should possess some primary requirements, such as high mixed electronic/oxygen ion conductivity over a © 2017 American Chemical Society

wide range of oxygen partial pressure (pO2), good chemical stability and compatibility with other fuel cell components, high surface exchange kinetics, and good catalytic properties for fuel oxidation.8,9 Because of these requirements, it is difficult to find an anode material that can meet all of the requirements, and consequently anode materials are often compromised in physical and chemical properties, particularly in terms of redox stability, catalytic activity, and electrical conductivity. In this context, several perovskite type oxide (ABO3) materials have been considered alternative anode materials for SOFCs, because they are very flexible in choosing components, and therefore, catalytic properties can be tailored significantly by A- and B-site doping.10−12 Perovskite structures containing a transition metal are particularly interesting because they can incorporate multiple oxidation state cations, which essentially determines the electrocatalytic process, electrical conducting paths, oxygen nonstoichiometry, and surface exchange kinetics. Early studies on perovskite anode materials focused on La0.75Sr0.25Cr0.5Mn0.5O3‑δ (LSCM),2 La0.8Sr0.2ScxMn1−xO3‑δ Received: June 30, 2017 Revised: August 15, 2017 Published: August 23, 2017 9207

DOI: 10.1021/acssuschemeng.7b02156 ACS Sustainable Chem. Eng. 2017, 5, 9207−9213

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. X-ray diffraction (XRD) patterns of (a) Nd0.5Ba0.5MnO3 and layered NdBaMn2O5+δ (NBMO). Nd0.5Ba0.5MnO3 is synthesized at 950 °C in air, which indicates a mixture of cubic (C) and hexagonal (H) phase. The phase change from the Nd0.5Ba0.5MnO3 to the layered NBMO occurs via a reducing process in H2 at 800 °C. (b) Oxygen nonstoichiometry of layered NBMO as a function of pO2 at 650, 700, and 750 °C. (c) The Arrhenius plot of total electrical conductivity of layered NBMO measured in air and humidified 5% H2-95% N2. (d) Partial molar enthalpy (ΔH) and partial molar entropy (ΔS) of layered NBMO.

(LSSM),13 donor−acceptor doped SrTiO3,14 Sr2MgMoO6‑δ (SMMO),15 Sr2Fe1.5Mo0.5O6‑δ (SFM),16 and Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3‑δ (PSCFN).17 These perovskite type ceramic anode materials demonstrate different degrees of improved coking and sulfur tolerance under fuel cell operating condition. However, the power density, catalytic activity, and electrical conductivity of these perovskite anodes are much lower than those of the conventional Ni-based anode materials. Among the numerous other perovskite systems that have been studied, layered perovskite containing manganese has continued to attract attention, because manganese remains mixed valence Mn4+/Mn3+/Mn2+ under SOFC anode operating conditions.6 This active mixed Mn4+/Mn3+/Mn2+ valence redox couple can accept electrons to promote its dissociative chemisorption on the anode surface. The catalytic activity of the redox couple often requires an easy release of the oxidized product as well as rapid exchange of oxide ions at the surface of the anode. Infiltration of Pd/CeO2,10,12 Ni, Co−Fe alloy,6 etc. is generally useful to improve the perovskite anode catalytic activity. Although infiltration technique is widely used to introduce the catalyst in SOFC electrodes, they offer limited control over particle distribution and often lead to the degradation by agglomeration. Compared to the conventional catalyst deposition methods, perovskite electrode materials capable of self-regeneration or partial exsolution of the nanoparticle catalyst during operation are cost-effective and attractive in terms of simplifying the deposit process and enhancing the lifetime of the catalyst.18−20 The exsolution of nanoparticle catalysts on perovskite oxide has been demonstrated only for a few easily reducible cations with Pd4+, Ni2+, Ru2+, Pt2+, and Rh4+ with a stoichiometric composition and some A-site deficient nonstoichiometric composition (for example, Ni 2 + exsolution from La0.52Sr0.28Ni0.06Ti0.94O3).20−24 A-site nonstoichiometry could serve as a driving force to exsolve cations (Ni, Cu, Fe, Co, etc.) to form nanoparticles dispersed on the perovskite surface.24,25 This exsolution in stoichiometric compounds is not clearly

understood yet and believed to take place with cations which is reduced to metals.24 The Gibbs free energy of reduction (ΔGred) of MnO to metallic Mn by hydrogen or CO is thermodynamically limited at temperatures between 127 and 727 °C because the ΔGred value for the MnO reduction to metallic Mn is greater than 100 kJ mol−1.26,27 For the first time, this paper shows the in situ exsolution of MnO nanoparticles (MnO-NP) demonstrated by PLD thin film and the electrochemical and thermodynamic properties of a stoichiometric layered oxygen deficient perovskite, NdBaMn2O5+δ (NBMO).

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EXPERIMENTAL SECTION

A-site layered NBMO anode was prepared by annealing Nd0.5Ba0.5MnO3‑δ oxide in a reducing atmosphere (in H2) at 800 °C. The Nd0.5Ba0.5MnO3‑δ powders were prepared via Pechini synthesis. The required amounts of nitrate salts of Nd, Ba, and Mn were dissolved in distilled water along with citric acid as a complexing agent. The solution was heated to form an organic resin containing metals in the solid solution followed by a combustion reaction to form fine powders. The powders were slowly decomposed at 600 °C and calcined in air at 950 °C for 4 h. A-site layered NBMO was obtained by annealing Nd0.5Ba0.5MnO3‑δ in H2 at 800 °C in a sealed alumina tube furnace for 4 h. A thin NBMO film was deposited on the dense Al2O3 substrate by PLD by using a commercial system (PLD-7; PASCAL, Japan). The oxygen pressure was adjusted to 0.67 Pa before the deposition process by introducing commercially available oxygen (without further purification), and the substrate was heated to 800 °C by using an infrared heater. An excimer laser was used with a power of 180 mJ pulse−1 and a frequency of 10 Hz to deposit the film. The crystal structures of the sample were analyzed using X-ray diffractometry by a Rigaku diffractometer (Cu Kα radiation, 40 kV, 30 mA). The morphological features of the samples were examined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Electrical conductivity was measured as a function of temperature using the standard four-probe technique with a BioLogic Potentiostat. The electrical conductivity samples for NBMO were prepared by pressing the powder of Nd0.5Ba0.5MnO3 into pellets followed by 9208


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ACS Sustainable Chemistry & Engineering sintering at 1500 °C for 12 h. The electrical conductivity was measured in 5% H2 to convert Nd0.5Ba0.5MnO3 to layered NBMO and then that of layered NBMO was measured in an air atmosphere. Coulometric titration (CT) was used to measure the oxygen nonstoichiometry of NBMO as a function of temperature and oxygen partial pressure pO2, and drive thermodynamic properties, such as partial molar enthalpy (ΔH) and entropy (ΔS), as reported elsewhere.13 About 0.2 g of the sample is placed inside an oxygen ion (O2− ) conducting yttria stabilized zirconia (YSZ) tube (Z15410630, McDanel Advanced Ceramic Technologies) that is sealed from the outside atmosphere. Ag paste (05063-AB, SPI Supplies) is painted on the inner and outer walls of the YSZ tube as electrodes. Pt wire is used as a lead wire to make electrical connections to the instruments. The Ag electrodes on either side of the tube are alternatively used for pumping in/out oxygen and to measure the potential across the membrane. The potential across the membrane is given by the ratio of the pO2 outside and inside the cell according to the Nernst equation. The electrolyte support cell used in this study was prepared by a tape casting process, with the outer two layers having pore formers, as reported elsewhere.10 A 120 μm dense yttria stabilized zirconia (YSZ) electrolyte disk was laminated between two 50 μm porous YSZ layers. The diameter of the porous YSZ region was 0.67 cm. To prepare composites of the NBMO-YSZ electrolyte supported cell, first prepared precursor solutions were prepared by dissolving neodymium(III) nitrate hexahydrate (Nd(NO3)3·6H2O), barium nitrate (Ba(NO3)2), and manganese(II) nitrate hydrate (Mn(NO3)3·6H2O) in distilled water (molar ratio of 0.5:0.5:1) with the addition of quantitative amounts of citric acid. The citric acid-to-metal ions ratio in solution was 1:1. A NBMO-YSZ anode was prepared by infiltrating the precursor solution into the anode side of the three-layered YSZ backbone. NBMO was infiltrated into the porous YSZ backbone by a multistep process followed by heating at 450 °C to decompose nitrates and citric acid. The infiltration process was repeated until 50 wt % loading of oxide was achieved. Finally, NBMO-YSZ anode wafers were calcined in air at 950 °C. The LSF (La0.8Sr0.2FeO3)-YSZ cathode was fabricated by infiltration with an aqueous solution of nitrate salts of La, Sr, and Fe with citric acid on a porous YSZ backbone opposite the anode layer and then calcined in air at 850 °C. The infiltrated anode and cathode had an active area of 0.36 cm2. For fuel cell performance tests, the cells were mounted on alumina tubes with ceramic adhesives (Ceramabond 552, Aremco). Ag paste and Ag wire were used for electrical connections to both the anode and the cathode. The entire cell was placed inside a furnace and heated to the desired temperature. The anode was exposed to humidified (3% H2O) H2 at a flow rate of 20 mL min−1 and the cathode was left open in air. V−I polarization curves were measured using a BioLogic Potentiostat in a temperature range of 700−800 °C.

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Figure 2. (a) High-angle annular dark-field scanning TEM image of Nd0.5Ba0.5MnO3 film and (b) high-angle annular dark-field scanning TEM image of NdBaMn2O5+δ (NBMO) film. An EDS point spectrum of a MnO particle is shown in the right inset. (Cu peaks in EDS spectrum originate from TEM copper grid.)

partially or entirely removed, creating many oxygen vacant sites in the crystal sites. At this stage, it is difficult to determine the exsolution phenomenon from the XRD data, due to the small number of the nanoparticles. Figure 1b shows the equilibrium oxygen nonstoichiometry for NBMO measured by coulometric titration as a function of pO2 at a temperature of 650−750 °C. The oxygen isotherm of NBMO was measured from 10−25 to 10−2 atm of pO2, which corresponds to the actual operational pO2 range of the fuel cell electrode. The plot shows a plateau region above 10−10 atm, which corresponds to the oxygen stoichiometric composition for NBMO, and the low pO2 region corresponds to the oxygen deficient region. At the high pO2 region, with partial substitution of Ba2+ in Nd3+ site, the electrical charge is balanced by the formation of Mn4+ in the Mn3+ site. The formula of NBMO can be expressed as

RESULTS

Figure 1a shows the typical XRD patterns of as prepared Nd0.5Ba0.5MnO3‑δ calcined in air and NBMO after annealing in H2 at 800 °C. As indicated in Figure 1a, the XRD pattern of Nd0.5Ba0.5MnO3‑δ is a mixture of cubic and hexagonal phases. However, after reduction in H2 at 800 °C, Nd0.5Ba0.5MnO3‑δ undergoes a phase transition to form A-site layered perovskite, NdBaMn2O5+δ.6,28 The crystal structures were refined by a Rietveld analysis of the XRD results (Figure S1). The Ba ordered oxygen nonstoichiometric NBMO phase is described by a tetragonal unit cell (P4/mmm) with cell parameters a = b = 3.893 Å and c = 7.799 Å and a volume V = 118.08 Å3. Due to the difference between the size of Nd and Ba ions, NBMO adopts an ordered crystal structure, in which a MnO2 square sublattice is sandwiched between two types of rock salt layers, NdO and BaO layers, along the c-axis ([BaO]-[MnO2]-[NdO][MnO2]-[BaO].29 A-site-ordered NBMO has a remarkable structural feature: oxygen atoms in the NdOx plane can be

2− + + + + [(Nd3Nd )(Ba 2Nd )][(Mn 3Mn )(Mn 4Mn )][OO ]6

Under the low pO2 condition, oxygen vacancies V•• 0 become the main defect species, and the electroneutrality condition is maintained by successive reduction of Mn4+ to Mn3+ and Mn3+ to Mn2+. The interaction between the surrounding oxygen gas and the other defect species in NBMO can be expressed as 1 × 2Mn ×Mn + O2 + V •O• ⇄ OO + 2Mn•Mn (1) 2 4+ At elevated temperature in NBMO oxides, the Mn /Mn3+/ Mn2+ redox species are expected to maintain a mixed valence 9209


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Figure 3. (a) Bright-field (BF) TEM image and (b) high-angle annular dark-field scanning TEM image of Nd0.5Ba0.5MnO3 film and its atomic arrangement viewed in the [100] direction. (c) A bright-field (BF) TEM image and (d) a bright-field (BF) scanning TEM image of a layered NdBaMn2O5+δ (NBMO) film and its atomic arrangement viewed in the [110] direction.

state in a reducing atmosphere. Moreover, the ability to form a Mn4+/Mn3+ redox couple allows a 6-fold coordinated Mn4+ to accept an electron while losing an oxide ligand and Mn3+/Mn2+ allows a 5-fold coordinated Mn3+ to accept an electron while losing an oxide ligand; this ability of NBMO is the basic catalytic activity toward oxidation of hydrocarbon and hydrogen fuels. The electrical conductivity of NBMO has been measured in 5% H2−N2, and an Arrhenius plot for electrical conductivity as a function of temperature is shown in Figure 1c. In NBMO layered perovskite oxides, electrical conduction takes place by an electron hopping mechanism between oxygen ions and Mnn+/Mnn+1 as delineated below:

ΔG = −RT ln K =

(3)

where K denotes the equilibrium constant for eq 1. At a constant δ, the partial molar enthalpy of oxygen vacancy formation as a function of temperature in NBMO oxide is given by Gibbs−Helmholtz equation

( ΔTG ) = R ∂ ln[p(O2)] 1 1 2 ∂( T ) ∂( T ) δ

∂ ΔH =

(4)

Also, the partial molar entropy of oxygen vacancy formation as a function of temperature in NBMO oxide is given by the Maxwell relation

Mn(n + 1) + − O2 − − Mn n + ↔ Mn n + − O− − Mn n + ↔ Mn n + − O2 − − Mn(n + 1) +

1 RT ln p(O2 ) 2

ΔS =

(2)

Partial substitution of Ba2+ in the Nd3+ site leads to the formation of Mn4+ ions, which produces more hole (h•) carriers and results in good electronic conductivity. Under a low pO2 condition, the reduction of NBMO leads to the formation of oxygen vacancies V•• O and consequently decreases the hole concentration, h• = Mn4+. This oxygen vacancy formation apparently blocks the electron conduction hopping between Mn(n+1)+ and Mnn+ in the Mn−O−Mn network, leading the lowering of electrical conductivity. The Gibbs energy of oxidation (ΔG) of NBMO is expressed as

∂ΔG R ∂ ln[p(O2 )] = ∂T 2 ∂T

δ

(5)

The partial molar enthalpy of oxygen vacancy formation calculated using eq 4 is shown in Figure 1d. The partial molar enthalpy of oxygen vacancy formation for NBMO oxides ranges from −310 to −175 kJ mol−1 between δ = 5.83 and 5.35. The partial molar enthalpy of oxidation (ΔH) are a strong function of oxygen nonstoichiometry in the NBMO oxides and ΔH increases with increasing δ. This suggest that under low p(O2) condition the interaction between the randomly distributed defects plays a major role in oxygen vacancy formation. In NBMO oxide, the ΔH values increase with increasing δ, suggesting that more energy is needed for lift-off of the oxygen 9210


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substrate has a dense surface morphology with well-defined large polygonal grains ∼20 μm in diameter. However, when the sample was annealed in H2 for 10 h, the exsolution phenomenon occurred throughout the surface. More importantly, the exsolved nanoparticles were well distributed and more abundant over the surface of the NBMO thin film, compared to Nd0.5Ba0.5MnO3, which indicates that annealing in H2 has a crucial effect on the exsolution process. To investigate the exact phase of the exsolved nanoparticles, we employed cross-sectional transmission electron microscopy (TEM). High-angle annular dark-field (HAADF) images of the sample before and after being reduced in H2 at 800 °C are shown in Figure 2a,b, respectively. The surface of the simple Nd0.5Ba0.5MnO3 perovskite is flat, and there are no remarkable nanoparticles on the surface except Pt nanoparticles coated for SEM observation. When the sample is reduced in H2 at 800 °C for 10 h, new nanoparticles with a diameter of 100 nm formed and homogeneously covered the surface of the NBMO film (Figure 2b).1 From the contrast difference between a particle and film in the HAADF STEM image (Figure 2b), we can estimate that these particles have a different phase compared with NBMO. To confirm the composition of the exsolved nanoparticles, we performed an energy-dispersive X-ray spectroscopy (EDX) point analysis (Figure S4). The composition was thereupon identified as the cubic MnO phase.24,25 Furthermore, the phase transition of the Nd0.5Ba0.5MnO3 film after reduction also can be observed by HAADF STEM atomic images (Figure 3). Compared to the general cubic perovskite structure of Nd0.5Ba0.5MnO3, as seen in Figure 3b, the layered NBMO film shows a periodic contrast change along the [110] direction due to A-site ordering of NdO and BaO layers (Figure 3d).6 The TEM study clearly shows a phase transition from a cubic perovskite to A-site ordered layer structure and exsolution of MnO nanoparticles, which decorated the surface of the layered perovskite. The electrochemical performance of NBMO as a SOFC anode has been evaluated by using a 120 μm thick YSZ electrolyte supported cell. Figure 4a shows the V−I and corresponding power densities of the NBMO anode with a NBMO-YSZ/YSZ/LSF-YSZ configuration at 700−800 °C using H2 (3 vol % H2O) as the fuel and ambient air as the oxidant. The NBMO-YSZ cell delivers a peak power density of 580 mW cm−2 at 800 °C in H2. The observed electrochemical performance of the prepared single cells with the NBMO-YSZ anode is higher than those of other ceramic anodes prepared under a similar configuration, fabrication process, and testing conditions (Table S1).10,30−32 Figure 4b shows the corresponding impedance plots of the NBMO-YSZ anode measured in H2 at 700−800 °C under an open circuit voltage (OCV) condition. The ohmic losses for the fuel cell are about 0.272, 0.420, and 0.686 Ω cm2 at 800, 750, and 700 °C, respectively, which are in good agreement with the estimated ohmic loss related with a 120 μm thick YSZ electrolyte. Moreover, it should be noted that, in the previously reported ceramic anodes, an oxidation catalyst (1 wt % Pd supported on 10 wt % CeO2) is added to the anodes to enhance the electrochemical performance of the fuel cells. However, even without any additional oxidation catalysts such as Pd, Pt, or Ni, superior fuel cell performance was achieved due to MnO exsolution and its excellent catalytic activity toward oxidation of fuels.

Figure 4. (a) Current density−voltage and the corresponding power density curves of YSZ electrolyte supported (120 μm) SOFCs with a layered NdBaMn2O5+δ (NBMO) anode and La0.8Sr0.2FeO3 (LSF) as a cathode in wet H2 (∼3 vol % H2O) at 700−800 °C. (open symbols designate V and closed symbols designate power density). (b) Corresponding impedance plots of the YSZ electrolyte supported (120 μm) SOFCs with a layered NdBaMn2O5+δ (NBMO) anode measured in H2 at 700−800 °C under an open circuit voltage (OCV) condition.

inside the lattice as the reduction proceeds. In other words, due to the variation of interaction between the randomly distributed defects, it becomes more difficult for oxygen vacancy to exist in NBMO as the oxygen nonstoichiometry changes from δ = 5.83 to 5.35. The partial molar entropy (ΔS) of oxygen vacancy formation is calculated using eq 5 and are between −110 and −57 J mol−1 K−1 as shown in Figure 1d. The partial molar entropy (ΔS) of oxygen vacancy formation is almost independent of oxygen nonstoichiometry between δ = 5.83 to 5.77. This suggests that the probability of oxygen vacancy formation in NBMO is almost constant as the reduction proceeds. From Figure 1d, it is observed that the energy required to create oxygen vacancy (enthalpy, ΔH) increases with oxygen nonstoichiometry (δ) and the probability of oxygen vacancy formation (entropy, ΔS) is almost constant with oxygen nonstoichiometry. Since it is difficult to clearly identify the exsolved nanosized particles at the surface of NBMO, we used a different approach. A polycrystalline film of pure Nd0.5Ba0.5MnO3 was grown on an alumina substrate by pulse laser deposition (PLD). Figure S2 shows the XRD patterns of a NBMO thin film annealed in a H2 atmosphere at 800 °C for 4 h. After postannealing, it is seen that the layered perovskite structure of NBMO was successfully formed in the deposited film. However, there are still no significant secondary peaks on the deposited NBNO film. To understand the surface characteristics of pristine NBMO, the thin film sample was characterized by SEM. Figure S3 shows the surface morphology of the NBMO substrate before and after annealing in H2. The as-deposited Nd0.5Ba0.5MnO3−Al2O3 9211


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(5) Chen, D.; Xu, y.; Tade, M. O.; Shao, Z. General Regulation of Air Flow Distribution Characteristics within Planar Solid Oxide Fuel Cell Stacks. ACS Energy Lett. 2017, 2, 319−326. (6) Sengodan, S.; Choi, S.; Jun, A.; Shin, T. H.; Ju, Y. W.; Jeong, H. Y.; Shin, J.; Irvine, J. T. S.; Kim, G. Layered oxygen-deficient double perovskite as an efficient and stable anode for direct hydrocarbon solid oxide fuel cells. Nat. Mater. 2014, 14, 205−209. (7) Choi, S.; Yoo, S.; Kim, J.; Park, S.; Jun, A.; Sengodan, S.; Kim, J.; Shin, J.; Jeong, H. Y.; Choi, Y.; Kim, G.; Liu, M. Highly efficient and robust cathode materials for low-temperature solid oxide fuel cells: PrBa0.5Sr0.5Co2−xFexO5+δ. Sci. Rep. 2013, 3, 2426. (8) McIntosh, S.; Gorte, R. J. Direct hydrocarbon solid oxide fuel cells. Chem. Rev. 2004, 104, 4845−4865. (9) Wang, W.; Su, C.; Wu, Y.; Ran, R.; Shao, Z. Progress in solid oxide fuel cells with nickel-based anodes operating on methane and related fuels. Chem. Rev. 2013, 113, 8104−8151. (10) Sengodan, S.; Yeo, H. J.; Shin, J. Y.; Kim, G. Assessment of perovskite-type La0.8Sr0.2ScxMn1−xO3−δ oxides as anodes for intermediate-temperature solid oxide fuel cells using hydrocarbon fuels. J. Power Sources 2011, 196, 3083−3088. (11) Sengodan, S.; Yoon, J. S.; Yoon, M. Y.; Hwang, H. J.; Shin, J.; Kim, G. Electrochemical performance of YST infiltrated and Fe doped YST infiltrated YSZ anodes for IT-SOFC. ECS Electrochem. Lett. 2013, 2, F45−F49. (12) Kim, G.; Corre, G.; Irvine, J. T. S.; Vohs, J. M.; Gorte, R. J. Engineering composite oxide SOFC anodes for efficient oxidation of methane. Electrochem. Solid-State Lett. 2008, 11, B16−B19. (13) Sengodan, S.; Ahn, S.; Shin, J.; Kim, G. Oxidation−reduction behavior of La0.8Sr0.2ScyMn1−yO3±δ (y= 0.2, 0.3, 0.4): Defect structure, thermodynamic and electrical properties. Solid State Ionics 2012, 228, 25−31. (14) Ruiz-Morales, J. C.; Canales-Vázquez, J.; Savaniu, C.; MarreroLópez, D.; Zhou, W.; Irvine, J. T. S. Disruption of extended defects in solid oxide fuel cell anodes for methane oxidation. Nature 2006, 439, 568−571. (15) Huang, Y.-H.; Dass, R. I.; Xing, Z.-L.; Goodenough, J. B. Double perovskites as anode materials for solid-oxide fuel cells. Science 2006, 312, 254−257. (16) Liu, Q.; Dong, X.; Xiao, G.; Zhao, F.; Chen, F. A novel electrode material for symmetrical SOFCs. Adv. Mater. 2010, 22, 5478−5482. (17) Yang, C.; Li, J.; lin, Y.; Liu, J.; Chen, F.; Liu, M. In situ fabrication of CoFe alloy nanoparticles structured (Pr0.4Sr0.6)3(Fe0.85Nb0.15)2O7 ceramic anode for direct hydrocarbon solid oxide fuel cells. Nano Energy 2015, 11, 704−710. (18) Kobsiriphat, W.; Madsen, B. D.; Wang, Y.; Shah, M.; Marks, L. D.; Barnett, S. A. Nickel-and ruthenium-doped lanthanum chromite anodes: effects of nanoscale metal precipitation on solid oxide fuel cell performance. J. Electrochem. Soc. 2010, 157, B279−B284. (19) Xiao, G.; Wang, S.; Lin, Y.; Zhang, Y.; An, K.; Chen, F. Releasing metal catalysts via phase transition:(NiO) 0 .05 (SrTi0.8Nb0.2O3)0.95 as a redox stable anode material for solid oxide fuel cells. ACS Appl. Mater. Interfaces 2014, 6, 19990−19996. (20) Katz, M. B.; Graham, G. W.; Duan, Y.; Liu, H.; Adamo, C.; Schlom, D. G.; Pan, X. Self-regeneration of Pd−LaFeO3 catalysts: new insight from atomic-resolution electron microscopy. J. Am. Chem. Soc. 2011, 133, 18090−18093. (21) Tanaka, H.; Uenishi, M.; Taniguchi, M.; Tan, I.; Narita, K.; Kimura, M.; Kaneko, K.; Nishihata, Y.; Mizuki, J. I. The intelligent catalyst having the self-regenerative function of Pd, Rh and Pt for automotive emissions control. Catal. Today 2006, 117, 321−328. (22) Madsen, B. D.; Kobsiriphat, W.; Wang, Y.; Marks, L. D.; Barnett, S. A. Nucleation of nanometer-scale electrocatalyst particles in solid oxide fuel cell anodes. J. Power Sources 2007, 166, 64−67. (23) Kobsiriphat, W.; Madsen, B. D.; Wang, Y.; Marks, L. D.; Barnett, S. A. La0.8Sr0.2Cr1−xRuxO3−δ−Gd0.1Ce0.9O1.95 solid oxide fuel cell anodes: Ru precipitation and electrochemical performance. Solid State Ionics 2009, 180, 257−264.

CONCLUSION An A-site layered oxygen deficient perovskite, NdBaMn2O5+δ (NBMO), has been investigated to find out its potential application as a solid oxide fuel cell anode. Nd0.5Ba0.5MnO3 has been shown to form a cubic/hexagonal perovskite type structure when sintered in air, and during annealing in a reducing condition, it undergoes a phase transition to form an A-site layered perovskite structure. The NBMO sample has semiconductor type behavior, reaching a maximum conductivity of 3 S cm−1 in 5% H2 at 800 °C, and shows excellent stability in a reducing condition. The TEM analysis showed structural evidence of MnO exsolution over the surface of the NBMO during reduction. The MnO nanoparticle decorated NBMO as an anode exhibited excellent power generation performance even without any additional oxidation catalysts. The unique structural phase transition in the perovskite ceramic anode potentially offers a new approach to produce a nanoparticle decorated perovskite for next generation electrodes for SOFCs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02156. Additional information including XRD patterns, SEM images of NBMO thin film, EDS, and mapping. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +82 2 2077 7863. *E-mail: [email protected]. Fax: +82 52 217 2909. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2016H1D3A1909709), Basic Science Research Program through the NRF funded by the Ministry of Education (2016-0790), Mid-Career Researcher Program (2016R1A2B4008514). This research was also supported by Sookmyung Women’s University Research Grant (1-17032016).

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REFERENCES

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Self-Decorated MnO Nanoparticles on Double Perovskite Solid Oxide

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