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Enhancing electrode performance by exsolved nanoparticles: a superior cobalt-free perovskite electrocatalyst for solid oxide fuel cells Guangming Yang, Wei Zhou, Meilin Liu, and Zongping Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12157 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 4, 2016

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ACS Applied Materials & Interfaces

Enhancing electrode performance by exsolved nanoparticles: a superior cobalt-free perovskite electrocatalyst for solid oxide fuel cells Guangming Yang1, Wei Zhou1, Meilin Liu2*, Zongping Shao1,3,4* 1

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key

Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, P.R. China E-mail: [email protected] 2

Center for Innovative Fuel Cell and Battery Technologies, School of Materials Science and

Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA E-mail: [email protected] 3

College of Energy, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, P.R.

China 4

Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845,

Australia

ACS Paragon Plus Environment

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ABSTRACT: The successful development of low-cost, durable electrocatalysts for oxygen reduction reaction (ORR) at intermediate temperatures is critical for broad commercialization of solid oxide fuel cells (SOFCs). Here we report our findings in design, fabrication, and characterization of a cobalt-free SrFe0.85Ti0.1Ni0.05O3-δ cathode decorated with NiO nanoparticles. Exsolved from and well bonded to the parent electrode under well-controlled conditions, the NiO nanoparticles uniformly distributed on the surface of the parent electrode greatly enhance cathode performance, demonstrating ORR activity better than that of the benchmark cobalt-based Ba0.5Sr0.5Co0.8Fe0.2O3-δ. Further, a process for regeneration of the NiO nanoparticles was also developed to mitigate potential performance degradation due to coarsening of NiO particles under practical operating conditions. As a general approach, this exsolution-dissolution of electro-catalytically active nanoparticles on an electrode surface may be applicable to the development of other high-performance cobalt-free cathodes for fuel cells and other electrochemical systems.

KEYWORDS: solid oxide fuel cell, cathode, oxygen reduction reaction, Nickel nanoparticle, surface modification


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INTRODUCTION Solid-oxide fuel cells (SOFCs) continue to attract tremendous attentions as potential alternative to conventional fire-power plant for electric power generation because of their high efficiency, low emission, and excellent fuel flexibility.1-5 Decreasing the operating temperature to the intermediate range (500-750 °C) is an important step towards the wide-spread commercialization of SOFCs technology, while the lack of suitable cathode material with sufficiently high activity for oxygen reduction reaction (ORR) and stability has become the main obstacle. During the past decade, tremendous efforts have been directed towards the development of alternative cathode materials with enhanced ORR activity at intermediate temperatures, and some cobalt-based perovskite oxides have showed favorable activity for ORR.6-11 For practical use, however, a compromise must be made between cost, performance, and durability; for this reason, most cobalt-based perovskite oxides have not been used in practical devices because of their poor stability, easy poisoning by CO2, and large thermal expansion coefficient.12, 13 Currently, an important direction in SOFC research is the development of high-performance cobalt-free perovskite oxides as alternative electrode materials for operation at intermediate temperatures, and iron-based perovskites have received particular attention because of the high abundance, low cost, and favorable stability of iron oxide.14-18 Unfortunately, the electro-catalytic activity of most iron-based perovskite electrode materials for ORR is still not high enough at intermediate temperatures. For example, as a promising cathode material for SOFCs with superior phase stability and tolerance against CO2 in ambient air and as one of the most active cobalt-free electrocatalysts for ORR, perovskite oxide SrFe0.9Ti0.1O3-δ demonstrated an area specific resistance (ASR) of around 0.16 Ω cm2 at 600 °C,18 which is still much worse than that


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of the benchmark cobalt-contained perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) electrode (0.078 Ω cm2).6 Recently, the formation of composite electrodes,19,

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and the design of unique electrode

architectures,21-25 have also emerged as alternative strategies for improving the electrode performance and stability from different aspects. In addition, the modification of selected electrodes with some precious metals (Ag, Pd, and Pt),26-29 or perovskite oxides-based nanoparticles,21-23 is reported to significantly enhance the catalytic activity of the electrodes for ORR, while infiltration has been the most applied technique for introducing such nanoparticles onto electrode surfaces.30-33 Some potential drawbacks of the infiltration method may include non-uniform distribution of the as-prepared nanoparticles, reduced porosity of the electrode, and complicated processing procedures.34 Coarsening of nanoparticles at elevated temperatures is also a serious concern for most nanoparticles-modified electrodes.35 In this study, we reported that a cobalt-free, SrFe0.85Ti0.1Ni0.05O3-δ electrode can be converted into a superior cathode for ORR with electro-activity at intermediate temperatures even higher than that of the benchmark cobalt-based BSCF through surface modification with uniformly distributed nickel oxide nanoparticles. Surprisingly, NiO is seldom applied as a component for cathode. More specifically, the active nickel oxide nanoparticles were introduced by an in-situ growth technique based on the exsolution of nickel ion from the bulk of perovskite phase. The nickel nanoparticles are uniformly distributed on and well bonded to the surface of parent electrodes. A process for electrode regeneration was further developed to mitigate the potential problem of performance degradation due to coarsening of NiO particles under practical operation conditions. As a general approach, it may be applied to the development of other highperformance cobalt-free electrocatalysts for ORR at intermediate temperatures. Further, it may


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also be applicable to the development of novel catalysts for other applications, such as oxygen electrode for metal-air batteries and catalysts for chemical oxidative reactions. EXPERIMENTAL SECTION Powders and symmetrical cells preparation SrFe0.9Ti0.1O3-δ (SFT) and SrFe0.85Ti0.1Ni0.05O3-δ (SFTNi) powders were synthesized via a conventional solid-state reaction process. SrCO3, Fe2O3, TiO2 and NiO of analytical grade, purchased form Sinopharm Chemical Reagent Co., Ltd, were used as the starting materials and mixed in stoichiometric amount according to the nominal composition of the target products by ball milling for 2 h. The precursor powders were firstly fired at 1100 °C for 10 h in air, then ground with a mortar and pestle, and further sintered at 1200 °C for 10 h to obtain single-phase perovskite-type SFT or SFTNi oxides. The symmetrical cells with the configuration SFT (SFTNi) | SDC | SFT (SFTNi) were fabricated with the Sm0.2Ce0.8O1.9 (SDC, Fuel Cell Materials, Inc.) electrolyte as substrate. The SDC discs were firstly dry pressed uniaxially in a stainless steel die to form dense substrates and subsequently sintered in air at 1400 °C for 5 h. The electrodes were deposited by the wet powder spraying method. The electrode slurries were milled (Fritsch, Pulverisette 6) at a rotating speed of 400 rpm for 0.5 h, with the powder and a mixed solution of isopropyl alcohol, ethylene glycol, and glycerol. The resulting slurries were sprayed onto both surfaces of the SDC disks using an air gun and subsequently sintered in air at 1050 °C for 2 h. Single cells were fabricated with two different typical half-cells. One is a dry pressed electrolyte-supported half-cell with 0.4 mm thick SDC, and 60 wt.% NiO +40 wt.% SDC composite as the anode, which was sintered at 1300 °C for 5 h. The other is an anode-supported half-cell prepared through tape-casting method with 60 wt.% NiO+ (Y2O3)0.08(ZrO2)0.92 (YSZ) as the anode, YSZ as the electrolyte and SDC as the


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buffer layer. The cathode layer was prepared by depositing the electrode slurry onto the SDC electrolyte and calcined at 1050 °C for 2 h. The effective area of cathode was 0.48 cm2. The Niexsolved SFTNi powders and electrodes were prepared in a tube furnace and testing mould with the 10% H2-Ar atmosphere, respectively. Characterizations The crystal structures of the fresh and treated with the diluted hydrogen samples were characterized by X-ray diffraction (XRD) using a Rigaku Smartlab X-ray diffractometer. The XRD spectra were recorded with filtered Cu-Kα radiation in a 2-theta range of 20-90 ° with scanning steps of 0.02 °. The detailed structural information was conducted by a Rietveld refinement of the XRD patterns. Microstructures of the powders and the electrodes were determined with a field emission scanning electron microscope (SEM, HITACHI-S4800) and a high resolution-transmission electron microscope (HR-TEM, JEOL JEM-2100). The bright-field scanning transmission electron microscopy (STEM) images and the corresponding energydispersive X-ray (EDX) mappings were acquired using an FEI Tecnai G2 T20 electron microscope and an FEI Tecnai G2 F30 S-TWIN field emission transmission electron microscope (TEM) equipped with an EDX analyzer, respectively. The electrical conductivity of the SFTNi sample under different conditions (or atmosphere) was measured via a four-probe DC method with a Keithley 2420 source-meter. The bar-shaped dense pellet of the sample with dimensions of ~1.7 mm × 4.5 mm × 10.5 mm for conductivity measurement was prepared by sintering at 1250 °C for 10 h. Electrochemical measurements Symmetrical cells SFT (SFTNi) | SDC | SFT (SFTNi) were fabricated for the electrochemical impedance spectra (EIS) measurements, and the test was conducted in air atmosphere using a


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Solartron 1287 potentiostat/galvanostat and a Solartron 1260A frequency response analyzer. The cells were tested between 750-550 °C under air at a fixed flow rate of 100 mL min-1 [STP], and the frequency range was recorded from 100 kHz to 0.1 or 0.01 Hz with a signal amplitude of 10 mV. I-V polarization was conducted using a Keithley 2420 source meter to assess the performance of the single SOFC fuel cells with two different configurations in the temperature range of 750-550 °C. The anode chamber was supplied with hydrogen fuel at a flow rate of 80 mL min-1 [STP], and cathode side was directly exposed to the air. A thin layer of silver was used as current collectors, and silver wires were attached to both electrodes as the leads. RESULTS AND DISCUSSION To disperse nickel oxide nanoparticles on the surface of the perovskite, we first designed and synthesized a material with the nominal composition of SrFe0.85Ti0.1Ni0.05O3-δ (SFTNi). For comparison, nickel-free SrFe0.9Ti0.1O3-δ (SFT) perovskite was also synthesized. Both materials were then treated in a reducing atmosphere (10% H2-Ar) at 600 °C for 24 h, and the obtained samples were named SFTNi-H2 and SFT-H2, respectively. The H2-treated samples were then exposed to air at 600 °C for 10 min (to allow re-oxidation of the samples, including oxidation of nickel metal to nickel oxide) and the resulted samples were named SFTNi-H2-Air and SFT-H2Air correspondingly. According to the XRD patterns (Figure S1), the SFT sample has a cubic perovskite structure as expected (Figure S2a).18 The diffraction patterns of the SFTNi sample matched well with those of the SFT without additional diffraction peaks assignable to free nickel oxide, suggesting the successful doping of the nickel into the perovskite lattice. The Rietveld refinement of the XRD pattern for the SFTNi sample confirmed the cubic perovskite structure of the oxide (Figure S2b). Although a phase transition was observed for the SFTNi sample after the hydrogen treatment, the cubic perovskite structure was restored after exposure to air at 600 oC


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for only 10 min (Figure S2c), suggesting that the phase structures of SFTNi are reversible. In the SFTNi-H2-Air sample, no peaks of NiO were observed, due presumably to the small size and low content of NiO nanoparticles. To reveal the microscopic details of the samples after exsolution of nickel from the SFTNi oxide lattice after the hydrogen treatment, we used SEM and HR-TEM to examine the fresh SFTNi and SFTNi-H2-Air samples. The sample was exposed to the diluted hydrogen at 600 °C for 40 h to allow complete exsolution of nickel from the oxide bulk phase, and then was reoxidized at 600 °C for 10 min to allow the conversion of phase structure of the main grains back to cubic perovskite. As depicted in Figure 1a&b, the surface of fresh SFTNi grains was clean and smooth, while many nanoparticles appeared to uniformly decorate the parent SFTNi grains after the hydrogen treatment. As shown in Figure 1c, the surface of the large grains in the reoxidized sample was still decorated uniformly with many spherical nanoparticles of ~15 nm in diameter. A HR-TEM image of such a nanoparticle (Figure 1d) showed diffraction fringes with a distance of 2.40 Å, that matches well with the spacing of (111) planes for NiO, suggesting the nickel oxide nature of such nanoparticles, resulting from oxidation of the exsolved metallic nickel. For the main large grains, diffraction fringes with a distance of 2.65 Å were observed, which agreed well with the spacing of the (110) planes for the SFT perovskite phase. Shown in Figure 1e are the STEM images of a selected part of the sample, confirming the nickel nature of the nanoparticles. It is noted that no iron was exsolved from the perovskite phase according to the HR-TEM analysis because no nanoparticles were observed on the SFT oxide surface after the same treatment of the sample in the same hydrogen atmosphere, even at 800 °C, as shown in Figure S3.


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Figure 1. SEM images of SFTNi before (a) and after (b) the prolonged treatment in the diluted hydrogen, and the TEM (c), HR-TEM (d) and EDS (e) images of the hydrogen treated sample (re-oxidized in air). The electrical conductivities under different atmospheres give further support of the nickel incorporation and exsolution. As shown in Figure S4, a maximum conductivity of 35 S cm-1 was demonstrated for SFT, which is consistent with our previous report.18 As to fresh SFTNi, the conductivity reached a maximum value of 130 S cm-1 in air, indirectly supported the successful


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doping of nickel into the perovskite oxide lattice.36,

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We then further tested the electrical

conductivity of SFTNi sample at the fixed temperature of 600 °C but under different atmospheres. The electrical conductivity of the fresh SFTNi was ~68 S cm-1 at 600 °C, which decreased to ~30 S cm-1 after operating in argon at 600 °C for 40 min. The decrease in conductivity is typical for p-type conductors due to partial reduction in argon.38, 39 A further decrease in electrical conductivity to ~10 S cm-1 was observed after exposure of the sample to hydrogen atmosphere at 600 °C for 30 min; this observation is expected because of the exsolution of nickel from the perovskite oxide lattice under the hydrogen atmosphere. After switching back to air atmosphere, the electrical conductivity remained stable at ~27 S cm-1 for about 10 h, similar to the conductivity of the nickel-free SFT, but much lower than that of the fresh SFTNi (68 S cm-1). These observations suggest that the exsolved nickel oxide phase is stable under the testing conditions; the nickel oxides are unable to incorporate into the perovskite oxide lattice after the re-exposure of the reduced SFTNi sample to an oxidizing atmosphere (air) at 600 °C. To evaluate the effect of the exsolved nickel oxide nanoparticles on the electrochemical performance of SFT for ORR, we fabricated symmetrical cells with SFTNi electrodes and SDC electrolyte. The area specific resistances (ASR) of the electrodes are determined from electrochemical impedance spectra (EIS) acquired at 550 to 750 °C. The symmetrical cells were first tested in air, providing ASR of the homogenous SFTNi electrode (without exsolution of Ni). Subsequently, the cells were exposed to a 10% H2-Ar atmosphere at 600 °C for 10 min, allowing exsolution of metallic nickel nanoparticles to the surface. These cells were then exposed to air for 10 min before performing impedance measurements to allow the restoration of the main perovskite phase and the oxidation of metallic nickel to nickel oxide. Shown in Figure 2a are the


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typical EIS at 600 oC before and after the hydrogen treatment. The intercept of the arc at the high frequency with the real axis could be reasonably assigned to the ohmic resistance from the electrolyte, while the depressed arcs at the intermediate-to-low frequency range are related to the electrode process.40, 41 The size of the arcs associated with the electrode process was significantly reduced after the formation of nickel nanoparticles. The electrode processes may be separated into the charge transfer process and the mass transfer (e.g., surface diffusion) process, which are often associated with the impedance arcs in the high frequency and the intermediate-to-low frequency range, respectively. Indeed, the EIS for both fresh and hydrogen treated SFTNi can be well fitted by an equivalent circuit shown in the inset of Figure 2a. The total resistance of the cell consists of an electrolyte ohmic resistance (R), a resistance to charge transfer (the size of the high frequency arc, R1), and a resistance to mass transfer (the size of the intermediate frequency arc, R2). Listed in Table S1 are the fitted values for R1 and R2 for the SFTNi electrode before and after the hydrogen treatment at 600 °C. A substantial decrease in the polarization resistance at the intermediate-to-low frequency range was induced by the creation of NiO nanoparticles, suggesting that the rate of surface process is obviously enhanced by the introduction of the NiO nanoparticles. Figure 2b gives a comparison of the overall ASRs for the SFTNi electrode before and after the nickel exsolution at different temperatures. After the exsolution of nickel with the formation of homogeneous nickel oxide nanoparticles, the ASR at 600 °C was reduced to only 0.07 Ω cm2, which is even slightly lower than that of BSCF (0.078 Ω cm2).6


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Figure 2. (a) The typical EIS at 600 °C before and after the hydrogen treatment for the SFTNi electrode. The inset is the equivalent circuit adopted for fitting the EIS data. (b) The comparison of the overall ASRs for the SFTNi electrode before and after the exsolution of nickel at 600 °C. (c) ASRs of the SFTNi electrode in air at 600 °C with respect to time after the exsolution of nickel.


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As demonstrated previously that the hydrogen treatment of SFT did not lead to the formation of any nanoparticles. Interestingly, a slight increase in ASRs was observed for the SFT electrode after the hydrogen treatment as compared to that of the fresh one (Figure S5 and Table S2). It thus strongly suggests that the substantially increased electrochemical performance of the SFTNi electrode after the hydrogen treatment was related to the formation of NiO nanoparticles that decorated the perovskite surface. It should be mentioned that the fresh SFTNi electrode with the nickel presented in the oxide lattice showed only slightly better electrochemical performance than the nickel-free SFT electrode. The morphologies of SFT and SFTNi electrodes after treatment with the diluted hydrogen and subsequent electrochemical performance tests were depicted in Figure S6. As expected, the surface of the SFT electrode was still clean, while the SFTNi electrode was covered with uniformly distributed nanoparticles of 10-20 nm in diameter. Since nickel oxide nanoparticles play a vital role in enhancing the ORR activity of the electrode, any change in size and morphology of the nickel oxide particles due to coarsening or dissolution of nickel into SFT oxide lattice during operation could lead to performance degradation of the electrode. To examine the stability of the SFTNi electrodes with exsolved nickel oxide particles, they were annealed in air at 600 °C for 80 h or at 800 °C for 20 h. Note that the temperature of 800 °C was chosen primarily to examine whether NiO would be reincorporated into the oxide lattice at high temperatures. This does not imply that it would be impossible for NiO nanoparticles to be re-incorporated into the oxide lattice below 800 °C. As shown in Figure S7a, the nanoparticles appear largely the same after the treatment at 600 °C for 80 h, suggesting that the as-formed nickel oxide particles are largely stable at 600 °C. In addition to the spherical nickel oxide nanoparticles, some plate-shaped nanoparticles of SrO with the size


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of ~50 nm also appeared. As we know, the exsolution of nickel from the oxide lattice of SFTNi would have a tendency to create B-site cation deficiency in the perovskite lattice: SrFe0.85Ti0.1Ni0.05O3-δ+H2→SrFe0.85Ti0.1O3-δ+H2O+Ni However, it is generally believed that the formation of B-site cation deficiency in perovskite oxide lattice is energetically unfavorable.42, 43 Thus, exsolution of some Sr may be possible to allow the A and B site cations in the perovskite oxide lattice to reach new cation stoichiometry. Due to the much larger cation size of Sr2+(1.18 Å) than Ni2+(0.69 Å), the diffusion rate of Sr2+ in perovskite is much lower than Ni2+, consequently longer time was required for the egress of Sr2+ from the perovskite oxide lattice. The NiO nanoparticles disappeared after exposure of the reduced SFTNi to air at 800 °C for 20 h (Figure S7b), suggesting the re-incorporation of nickel into the oxide lattice. Thus the in-situ created nickel oxide modified electrode can only be operated at temperatures lower than 800 °C. Shown in Figure 2c is the time dependence of the ASRs of the reduced SFTNi electrode on the time exposed to air at 600 °C. The ASRs remained relatively stable at ~0.075 Ω cm2 for about 270 h, suggesting that the newly-created SrO phase had little effect on the electrode performance for ORR. The effect of the hydrogen treatment temperature on the size of the nickel oxide nanoparticles was investigated in order to get proper temperature of exsolution for optimal electrode performance. Four different temperatures were investigated: 500, 600, 700, and 800 °C. Shown in Figure 3a are the SEM images of the SFTNi samples after different treatments. The treatment at 500 °C did not obviously lead to the formation of Ni nanoparticles, while they appeared at 600 °C or higher. It suggests a treatment temperature of 600 °C or higher is necessary to make the insitu creation of nickel nanoparticles through the exsolution method kinetically feasible. Significant increase in particle size of the nickel nanoparticles was observed with the increase of


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hydrogen treatment temperature for the SFTNi sample, especially for temperatures higher than 700 °C. For example, the sizes of the exsolved nickel particles reached 40-50 nm at 800 °C, suggesting that a lower temperature is preferred for smaller nickel particles. Figure 3b shows the corresponding ASRs of the SFTNi electrode reduced at different temperatures. An increase in the particle size of the nickel oxide resulted in deterioration in the electrode performance. The electrode reduced at 600 °C for 10 min displayed the lowest ASR.

Figure 3. (a) SEM images of the SFTNi samples after the treatment in diluted hydrogen at 500, 600, 700, and 800 °C for 10 h, respectively. (b) Comparison of ASRs for the SFTNi electrodes after treatment in diluted hydrogen at different temperatures for 10 min. On the other hand, infiltration/impregnation method is widely used for the preparation of nanoparticles modified electrodes.30-33 In a typical preparation process, a porous scaffold or


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electrode backbone is first prepared, and then nanoparticles of a catalyst are introduced by an infiltration or impregnation process.44, 45 We also fabricated the NiO-modified SFT and SFTNi electrodes using a solution infiltration process. The same amount of nickel oxide (calculated based on the amount of nickel in SFTNi) was applied. Listed in Table S3 is a comparison of the ASRs of the SFT, SFTNi, NiO-SFT prepared from the exsolution method, and NiO-SFT and NiO-SFTNi prepared from the infiltration method. Interestingly, the NiO-modified electrodes derived from the impregnation method did not show any improvement in electrochemical activity for ORR as compared to the pristine SFT and SFTNi electrodes. Shown in Figure S8 are the SEM images of the as-prepared, NiO-modified SFT electrode through impregnation. The NiO particles were not uniformly distributed but seriously aggregated, suggesting that small size and uniform distribution of NiO nanoparticles is critical to enhancing electrochemical activity for ORR. Although the SFTNi electrodes with exsolved nickel oxide nanoparticles has displayed stable performance for a period of 270 h at 600 °C, the long-term stability of the nanoparticles on electrode surface is yet to be demonstrated under realistic operating conditions, for thousands of hours, or even longer. The concerns are originated from the possibility of NiO nanoparticles coarsening or dissolution into the parent phase under operating conditions. Thus, the development of a proper way to regenerate the desired nanostructure is vital to maintaining longterm stability and reliability. It was previously reported that Pd nanoparticle catalyst can be insitu regenerated through the dissolution-exsolution of Pd in a perovskite lattice by changing the surrounding atmosphere.46-48 Our results (Figure S7b) also showed that nickel can dissolve into the oxide lattice at temperature of 800 °C and higher. Note that it does not mean that NiO nanoparticles would not be re-incorporated into the oxide lattice below 800 °C. It suggests that


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we could regenerate the desired nanoparticles of nickel oxide electrocatalyst by annealing the electrode at 800 °C or higher in air to allow dissolution of nickel into the oxide lattice and then re-treating the electrode in a hydrogen atmosphere at 600 °C for 10 min to allow re-exsolution of nickel nanoparticles. Shown in Figure 4a is a comparison of the ASRs for the SFTNi electrodes prepared under different conditions. As expected, the ASRs of the treated SFTNi electrode increased after annealing at 800 °C in air for 20 h because most NiO nanoparticles were reincorporated into the oxide lattice (Figure S9). After the second around in-situ exsolution at 600 °C, the regenerated electrode showed even slightly better electrode performance than the one after the first around exsolution of NiO nanoparticles, which may suggest that more nickel nanoparticles were exsolved to decorate the parent grains after the second process. Figure 4b schematically illustrate the change in surface morphologies of the SFTNi electrode under different conditions.


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Figure 4. (a) ASRs for the SFTNi electrodes after the various treatments (fresh, in-situ reduced at 600 °C, further calcined at 800 °C for 20 h, and in-situ re-reduced at 600 °C), (b) Schematic of the morphologies of SFTNi electrode at different conditions. To evaluate the performance of the new cathode under practical fuel cell operation conditions, we fabricated single cells with SFTNi cathodes. A thick (~ 0.4 mm) SDC electrolyte supported cell was used to demonstrate the effect of exsolved Ni nanoparticles on cell performance. As shown in Figure S10, peak power densities (PPDs) of the cell were increased from 160 to 340 mW cm-2 at 750 °C and from 55 to 158 mW cm-2 at 600 °C after the SFTNi cathode was subjected to the conditions for nickel exsolution. The cell voltage remained fairly stable for a period of 140 h at a constant polarization current density of 100 mA cm-2 at 600 °C (Figure S11), which further confirmed the high stability of NiO nanoparticles modified electrode for ORR. Finally, we also fabricated and tested an anode-supported cell with a thin-film YSZ electrolyte, a SDC buffer layer, and a Ni-exsolved SFTNi cathode, considering the serious leaking current of SDC thin-film electrolyte at temperatures higher than 600 °C. As shown in Figure S12, very attractive peak power densities of 1650, 1420, 1100, 684, and 340 mW cm-2 were achieved at 750, 700, 650, 600, and 550 °C, respectively, which are among the best results ever reported for YSZ-based fuel cells (Table S4).

CONCLUSIONS In summary, we have successfully developed a superior SOFC cathode composed of a porous cobalt-free strontium titanium ferrite perovskite backbone and uniformly distributed NiO nanoparticles as surface modifier, derived from a chemical reduction-induced exsolutionoxidation process. The nickel was first incorporated into the perovskite oxide lattice, and then


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selectively exsolved from the lattice under a reducing condition to modify the parent oxide surface in the form of uniformly-distributed nanoparticles of around 15 nm in diameter. After reexposure of the electrode to air, the metallic nickel nanoparticles were quickly oxidized to nickel oxide nanoparticles, which were stable at 600 °C for hundreds of hours. Such NiO nanoparticles significantly enhanced the ORR kinetics on the surface of strontium titanium ferrite perovskite oxide to improve the electrode performance. At 800 °C, the nickel oxide was re-incorporated into the main perovskite lattice, allowing for re-exsolution of nickel nanoparticles under proper conditions. It offers a possible approach for regeneration of the nanoparticles decorated on electrode surfaces and a solution to solve the potential problem of performance degradation due to coarsening of NiO nanoparticles under practical operation conditions. This exsolutiondissolution of electro-catalytically active nanoparticles on an electrode surface may be applicable to other electrode systems.

ASSOCIATED CONTENT Supporting Information More detailed figures and tables about the structure, morphology, electrical conductivity, symmetrical cell performance, single cell performance are available. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email address: [email protected]


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*Email address: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the Major Project of Educational Commission of Jiangsu Province of China under contract No. 13KJA430004, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Changjiang Scholars Program (T2011170), and CAS Interdisciplinary Innovation Team. G.M. Yang also thanks the Innovation Project of Graduate Research by Jiangsu Province (No. KYLX15-0788). REFERENCES (1) Ormerod, R. M. Solid Oxide Fuel Cells. Chem. Soc. Rev. 2003, 32 (1), 17-28. (2) Hibino, T.; Hashimoto, A.; Inoue, T.; Tokuno, J.; Yoshida, S.; Sano, M. A Low-OperatingTemperature Solid Oxide Fuel Cell in Hydrocarbon-Air Mixtures. Science 2000, 288 (5473), 2031-2033. (3) Stambouli, A. B.; Traversa, E. Solid Oxide Fuel Cells (SOFCs): A Review of an Environmentally Clean and Efficient Source of Energy. Renew. Sust. Energy Rev. 2002, 6 (5), 433-455. (4) Sengodan, S.; Choi, S.; Jun, A.; Shin, T. H.; Ju, Y. W.; Jeong, H. Y.; Shin, J.; Irvine, J. T.; Kim, G. Layered Oxygen-Deficient Double Perovskite as an Efficient and Stable Anode for Direct Hydrocarbon Solid Oxide Fuel Cells. Nat. Mater. 2015, 14 (2), 205-209.


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Table of Contents Graphic and Synopsis

Synopsis of this work: SrFe0.85Ti0.1Ni0.05O3-δ perovskite oxide is demonstrated for the first time as superior electrode for oxygen reduction reaction (ORR) through a chemical reduction-induced exsolution-oxidation process. The exsolved nickel nanoparticles uniformly dispersed on the electrode surface significantly enhanced the ORR kinetics, resulting in electrode performance superior to that of the benchmark cobalt-based perovskite cathodes.


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Figure 1. SEM images of SFTNi before (a) and after (b) the prolonged treatment in the diluted hydrogen, and the TEM (c), HR-TEM (d) and EDS (e) images of the hydrogen treated sample (re-oxidized in air). 268x288mm (96 x 96 DPI)



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Figure 2. (a) The typical EIS at 600 °C before and after the hydrogen treatment for the SFTNi electrode. The inset is the equivalent circuit adopted for fitting the EIS data. (b) The comparison of the overall ASRs for the SFTNi electrode before and after the exsolution of nickel at 600 °C. (c) ASRs of the SFTNi electrode in air at 600 °C with respect to time after the exsolution of nickel. 382x814mm (96 x 96 DPI)


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Figure 3. (a) SEM images of the SFTNi samples after the treatment in diluted hydrogen at 500, 600, 700, and 800 °C for 10 h, respectively. (b) Comparison of ASRs for the SFTNi electrodes after treatment in diluted hydrogen at different temperatures for 10 min. 482x707mm (96 x 96 DPI)



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Figure 4. (a) ASRs for the SFTNi electrodes after the various treatments (fresh, in-situ reduced at 600 °C, further calcined at 800 °C for 20 h, and in-situ re-reduced at 600 °C), (b) Schematic of the morphologies of SFTNi electrode at different conditions. 314x389mm (96 x 96 DPI)


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A Superior Cobalt-Free Perovskite Electrocatalyst ... - ACS Publications

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