In Situ Synthesized La0.6Sr0.4Co0.2Fe0.8O3âÎ´ ... - ACS Publications

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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In Situ Synthesized La0.6Sr0.4Co0.2Fe0.8O3−δ−Gd0.1Ce0.9O1.95 Nanocomposite Cathodes via a Modified Sol−Gel Process for Intermediate Temperature Solid Oxide Fuel Cells Dong Woo Joh,† Areum Cha,‡ Jeong Hwa Park,† Kyeong Joon Kim,† Kyung Taek Bae,† Doyeub Kim,† Young Ki Choi,‡ Hyegsoon An,§,¶ Ji Su Shin,∥ Kyung Joong Yoon,*,⊥ and Kang Taek Lee*,† †

Department of Energy Science and Engineering, DGIST, Daegu 42988, Republic of Korea R&D Center, Samchun Pure Chemical Co., Pyeongtaek 1774522, Republic of Korea § High-Temperature Energy Materials Research Center, KIST, Seoul 02792, Republic of Korea ¶ Department of Fuel Cells and Hydrogen Technology and ∥Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea ⊥ High-Temperature Energy Materials Research Center, KIST, Seoul 02792, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Composite cathodes comprising nanoscale powders are expected to impart with high specific surface area and triple phase boundary (TPB) density, which will lead to better performance. However, uniformly mixing nanosized heterophase powders remains a challenge due to their high surface energy and thus ease with which they agglomerate into their individual phases during the mixing and sintering processes. In this study, we successfully synthesized La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)−Gd0.1Ce0.9O1.95 (GDC) composite cathode nanoscale powders via an in situ sol−gel process. High-angle annular dark field scanning transmission electron microscopy analysis of in situ prepared LSCF−GDC composite powders revealed that both the LSCF and GDC phases were uniformly distributed with a particle size of ∼90 nm without cation intermixing. The in situ LSCF−GDC cathode sintered on a GDC electrolyte showed a low polarization resistance of 0.044 Ω cm2 at 750 °C. The active TPB density and the specific two phase (LSCF/pore) boundary area of the in situ LSCF− GDC cathode were quantified via a 3D reconstruction technique, resulting in 12.7 μm−2 and 2.9 μm−1, respectively. These values are significantly higher as compared to reported values of other LSCF−GDC cathodes, demonstrating highly well-distributed LSCF and GDC at the nanoscale. A solid oxide fuel cell employing the in situ LSCF−GDC cathode yielded excellent power output of ∼1.2 W cm−2 at 750 °C and high stability up to 500 h. KEYWORDS: solid oxide fuel cells, nanocomposite, LSCF−GDC, in situ sol−gel process, 3D reconstruction, oxygen reduction reactions

1. INTRODUCTION Solid oxide fuel cells (SOFCs) have attracted significant attention due to their high energy conversion efficiency and fuel flexibility.1−4 However, commercialization of SOFC technology has been bottlenecked by its high operating temperature of over 800 °C, which limits the choice of materials and causes the rapid degradation rate of the system components. Over the past several decades, research has focused on lowering the SOFC operating temperature to below 750 °C, thereby lowering the system cost, improving the longterm stability, and shortening start-up time.5,6 However, lowering operating temperatures consequently leads to increase in cathode polarization resistance associated with the thermally activated nature of the oxygen reduction reaction (ORR), which deteriorates the SOFC performance.7 Recently, cobaltite-based perovskite materials such as La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) have been studied as promising © XXXX American Chemical Society

cathode materials at intermediate temperatures (IT, 600−750 °C) due to their rapid surface activation and mixed ionic and electronic ionic conductivity (MIEC).8−10 However, owing to the high activation energy for oxygen ion diffusion of LSCFs (186 ± 5 kJ mol−1), their ionic conductivity drastically decreases as the operation temperature is lowered.11 To compensate for this effect, dual phase composite cathodes with the addition of ionic conducting electrolyte such as LSCF−gadolinium doped ceria (GDC) have been developed, which demonstrated lower cathodic polarization resistance as compared to that of a single phase cathode (e.g., LSCF).12,13 Additionally, decreasing the particulate size of the cathode at the nanoscale can effectively enhance the ORR activity of the Received: April 4, 2018 Accepted: May 3, 2018 Published: May 3, 2018 A

DOI: 10.1021/acsanm.8b00566 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

(99.0% pure, Samchun Pure Chemical), which are chelating agents, are added with stirring (for 30 min) so that the total metal ions:CA:glycine molar ratio becomes 2:1:4. Then, the GDC nanopowders prepared by the coprecipitation method17 were dispersed in the LSCF precursor solution. The solution was mixed and stirred at 90 °C for 1 h, and the water was evaporated at 130 °C to obtain a viscous LSCF gel containing GDC nanopowders, which is dark brown in color. The resulting gel was preheated at 300 °C for 5 h to form dark-gray ash and calcined at 900 °C for 6 h in ambient air to form nanocomposite powders, which are referred to as in situ LSCF− GDC. For comparison, LSCF nanopowders were synthesized by exactly the same sol−gel process described above, except for the dispersion process of GDC nanopowders. To fabricate the reference LSCF−GDC cathode, the LSCF nanopowders was mixed with GDC nanopowders using a conventional ball mill method with zirconia ball media in ethanol for 24 h, which are denoted as BM LSCF−GDC. 2.2. Fuel Cell Fabrication. To prepare a LSCF−GDC|GDC| LSCF−GDC symmetric cells, a GDC electrolyte pellet was prepared by uniaxial pressing and heat treatment at 1400 °C for 10 h. For the cathode ink, the prepared LSCF−GDC powders were mixed with an ESL 441 (Electro Science, United States) binder system using a planetary centrifugal mixer (Thinky Corp., Japan). The prepared cathode inks were coated on both sides of the sintered GDC electrolyte and then heat treated at 1050 °C for 3 h. An SOFC button cell was fabricated with the structure of NiO− yttria stabilized zirconia (YSZ) anode|NiO−YSZ anode functional layer (AFL)|YSZ electrolyte|GDC buffer layer|LSCF−GDC cathodes. The anode support was fabricated by tape casting with NiO (Alfa Aesar, United States)−YSZ (Tosoh, Japan) by 65:35 wt %. Submicron-sized AFL (NiO (JT Baker, United States)−YSZ) and YSZ electrolyte were sequentially coated on the anode support by slurry coating and then cosintered at 1400 °C for 4 h. The GDC buffer layer was screen-printed on YSZ electrolyte and sintered at 1250 °C for 5 h. The LSCF−GDC cathodes with an area of 1 cm2 were screenprinted on GDC and then heat-treated at 1050 °C for 3 h. 2.3. Characterization. The crystallinity of the synthesized in situ LSCF−GDC powders were analyzed by X-ray diffraction (XRD, Rigaku, MiniFlex 600). XRD patterns were collected using Cu Ka radiation (λ = 1.5418 Å) in the range of 20° ≤ 2θ ≤ 80° with scan rate of 2.4 o/min, a step size of 0.02°, a voltage of 40 kV, and current of 15 mA. The specific surface area of the powders was analyzed by the Brunauer−Emmett−Teller (BET) surface area technique at 77.37 K using an accelerated surface area and porosimetry system (Micromeritics, ASAP 2020) by nitrogen adsorption. The particle-size distribution of the in situ LSCF−GDC powders was measured using a dynamic light-scattering (DLS) spectrophotometer (Malvern, Zetasizer Nano ZS90). The morphology of the synthesized powders was observed by a high-resolution transmission electron microscope (TEM) and HAADF−STEM using a Cs-corrected FEI Titan 80-300 microscope. For elemental analysis, energy dispersive X-ray spectroscopy (EDS) mapping analysis was performed using a TEM-attached EDS spectrometer (Super-X EDS detector). The microstructure of the SOFC button cell was observed using scanning electron microscopy (SEM, Hitachi, S-4800). The elemental line scan was performed using the EDS in the SEM. The ORR performance of the in situ LSCF−GDC cathode on GDC electrolyte was measured by EIS using a potentiostat (Bio-Logic, VMP-300). EIS tests were performed in the frequency range of 7 MHz to 0.1 Hz with an AC amplitude signal of 50 mV at open circuit voltage (OCV). The current−voltage−power density (I−V−P) characteristics of the SOFC button cell were determined using the potentiostat under conditions of dry air (200 sccm) and 3% humidified hydrogen (200 sccm) at the cathode and anode side, respectively. 2.4. 3D Reconstruction. 3D reconstruction of LSCF−GDC cathodes was performed using a focused ion beam (FIB)-SEM dual beam system (FEI, Helios Nanolab G3 UC). To obtain cross-sectional images of LSCF−GDC by FIB tomography, open pores of the cathode sample were infiltrated with epoxy (EpoVac System, Struers), and the sample was cured for a day. A backscattered electron detector (BSD) was used to obtain the best contrast and resolution. Repeated SEM

cathode by extending the electrochemically active sites from its high surface area. For example, Zhou et al. reported that the polarization resistance for a nanosized (∼42 nm) Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) cathode was as low as 0.36 Ω cm2 at 600 °C.14 Baqué et al. synthesized nanoscale LSCF powders by a modified sol−gel process using hexamethylenetetramine and demonstrated a low area specific resistivity (ASR) (0.4 Ω cm2 at 450 °C), which was 2 orders of magnitude lower than that with a microsized cathode of the same composition.15 It is evident that a dual phase composite cathode at the nanoscale is expected to further improve cathode performance. However, upon mixing, heterophase nanoparticles are easily agglomerated to form clusters of the same phase due to their high surface energy,16 resulting in significant coarsening between adjacent particles during high-temperature sintering. To address this issue, we developed a modified sol−gel method to synthesize a uniform mixture of LSCF−GDC nanocomposite powders, as schematically shown in Figure 1.

Figure 1. Schematic diagram of the in situ sol−gel process of synthesizing nanocomposite LSCF−GDC powders.

Unlike the conventional mechanical-mixing process of LSCF and GDC nanopowders, when nanoscale LSCFs are synthesized by a sol−gel process, the presynthesized GDC nanoparticles are added to the LSCF precursor solution to produce a uniformly distributed nanoparticle mixture. Furthermore, the in situ formed nanocomposite powders (referred to as in situ LSCF−GDC) are expected to act as spacers to suppress the coarsening of each phase during the high-temperature sintering process. In this study, we prepared LSCF−GDC nanocomposite powders by the in situ sol−gel process. The phase stability and morphology of the in situ LSCF−GDC powders were investigated. The electrochemical performance of the in situ LSCF−GDC cathode on the GDC electrolyte was measured by electrochemical impedance spectroscopy (EIS). In addition, the microstructural features of the in situ LSCF−GDC cathode were quantified via a 3D reconstruction technique and correlated to the electrochemical performance. Finally, the performance of an SOFC comprising the nanocomposite cathode was evaluated.

2. EXPERIMENTAL DETAILS 2.1. Material Synthesis. For in situ synthesis of LSCF−GDC (50:50 wt %), the LSCF precursor solution was prepared by dissolving the stoichiometric metal nitrates, i.e., La(NO3)3·6H2O (>99.99% pure, Kanto), Sr(NO3)2 (>99.0% pure, Samchun Pure Chemical), Co(NO3)2·6H2O (98.0% pure, Samchun Pure Chemical), and Fe(NO3)2· 9H2O (98.5% pure, Samchun Pure Chemical) in DI water. Subsequently, 0.43 M citric acid (CA, C6H8O7·H2O) (99.5% pure, Samchun Pure Chemical) and 2.22 M glycine (NH2CH2COOH) B


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Figure 2. (a) XRD patterns of as-synthesized in situ LSCF−GDC, LSCF, and GDC nanopowders and in situ LSCF−GDC annealed at 1050 °C for 10 h. (b) TEM image and (c) HADDF-STEM images of in situ LSCF−GDC powders. imaging and FIB slice processes were automatically controlled using Auto Slice and View software from FEI. The slicing distance in the Zaxis direction was 60 nm. The 3D reconstruction of the in situ LSCF− GDC cathode was performed by alignment, segment, cropping, and labeling of the sliced images, and various microstructural parameters were calculated using Avizo 9.0 software package (FEI VSG).

S1). The crystallite size of the LSCF in in situ LSCF−GDC was smaller than that of LSCF nanopowders synthesized by simple sol−gel method (31.50 nm). This is because the second oxide phase (e.g., GDC) that has dispersed between the primary nanoparticles (e.g., LSCF) limits the growth of the primary nanoparticles at elevated temperatures. On the other hand, due to the double heat treatment, the crystallite size of the GDC in in situ LSCF−GDC was slightly larger than that of GDC nanopowders synthesized by coprecipitation method (34.71 nm). The calculated crystallite sizes are shown in Table S1. To determine the overall particle size of in situ LSCF−GDC powders, BET surface area analysis by N2 absorption technique was carried out. The surface area associated particle size (DSAA) from the BET results can be calculated by

3. RESULTS AND DISCUSSION 3.1. Powder Analysis. Figure 2a shows the XRD patterns of as-prepared in situ LSCF−GDC, GDC, and LSCF powders. The XRD peaks of the in situ LSCF−GDC are clearly indexed to the cubic perovskite phase of LSCF (JCPDS no. 75-0161) and the cubic fluorite phase of GDC (JCPDS no. 48-0124) without any impurity phases. Moreover, indexing the patterns of in situ LSCF−GDC powder after annealing at 1050 °C for 10 h shows that there was no reactivity between LSCF and GDC up to 1050 °C, the highest processing temperature for fabricating LSCF−GDC cathodes. From the XRD patterns, the crystallite sizes (L) of the powders were calculated by Scherrer’s eq 1. L (nm) =

kλ β cos θB

DSAA (nm) =

6 SBETρth

(2)

where ρth is the theoretical density of the in situ LSCF−GDC (50:50 wt %) composite and SBET is the BET specific surface area of the powders. The theoretical density (ρth) can be calculated by the following equation:

(1)

ρth = VLSCFρLSCF + (1 − VLSCF)ρGDC

where, k is the shape factor (0.9), λ is the wavelength of Cu Kα radiation (λ = 1.5418 Å), β is the peak width at half the maximum intensity (fwhm), and θB is a Bragg’s angle. The crystallite size of the LSCF and GDC in in situ LSCF−GDC were calculated as 21.00 and 38.14 nm, respectively (Figure

(3)

where VLSCF is the LSCF vol %, ρLSCF the density of the LSCF (6.21 g cm−3), and ρGDC the density of GDC (7.24 g cm−3). The SBET of the powders was measured as 22.77 m2 g−1, and the calculated DSAA was ∼39 nm. In addition, DLS measurement of C


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Figure 3. Nyquist plots of impedance spectra of the in situ LSCF−GDC cathode and the BM LSCF−GDC cathode under open circuit condition in ambient air at (a) 750, (b) 700, (c) 650, and (d) 600 °C.

particle size of in situ LSCF−GDC powders shows a monomodal distribution with the average diameters of 233.6 ± 68.3 nm and the median diameter (D50) of 221.7 nm (Figure S2). Mean particle size measured by DLS was larger than the calculated value from BET. This deviation could be a result of aggregation of the individual nanoparticles that might have been induced during dispersion in ethanol. Figure 2b shows a TEM image of the in situ LSCF−GDC powders after calcination at 900 °C. The powder size was evaluated as ∼56 nm. This result is comparable to the calculated DSAA (∼39 nm) from the BET result. Figure 2c displays the STEM-HAADF image of the soft agglomerated in situ LSCF−GDC powders and shows the average particle size of ∼90 nm. The elemental analysis by EDS mapping in the same region reveals that synthesized nanoparticles were clearly divided into two phases, GDC (Ce and Gd) and LSCF (La, Sr,

Figure 4. Arrhenius plots of the ASRs of in situ LSCF−GDC, BM LSCF−GDC, and other reported LSCF−GDCs in literature. The symmetric cell information and ASR values are summarized in Table 1.

Table 1. Comparison of the ASR of Various LSCF-Doped Ceria-Based Cathodes in This Study and Reported in Literature electrolyte

cathode

materials

materials

GDCa GDC GDC SDCb YSZc SDC/YSZ/SDC

LSCFd−GDC LSCF−GDC LSCF−GDC L58SCFe−SDC L58SCF−GDC LSCF−SDC

GDC

LSCF#f−GDC

powder synthesis method and mixing technique in situ sol−gel process LSCF (sol−gel), GDC (coprecipitation)/ball milling LSCF (Kceracell, Korea), GDC (coprecipitation)/ball milling LSCF58 (citrate complexation), SDC (coprecipitation) L58SCF (citrate method), GDC (coprecipitation)/ball milling LSCF (Fuelcell Material Co., United States), SDC (glycine nitrate solution)/ infiltration LSCF (dip pyrolysis)

ASR(750 °C) (Ω cm2)

reference

0.044 0.265 0.105 0.277 0.109 0.084

this work this work 17 18 19 20

0.059

21

GDC: Gd0.1Ce0.9O1.95. bSDC: Sm0.2Ce0.8O2−δ. cYSZ: Y0.16Zr0.84O1.92. dLSCF: La0.6Sr0.4Co0.2Fe0.8O3−δ. eL58SCF: La0.58Sr0.4Co0.2Fe0.8O3−δ. fLSCF#: (La0.6Sr0.4)1−xCo0.2Fe0.8O3−δ (0 ≤ x ≤ 0.1).

a

D


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3.2. Area Specific Resistance of LSCF−GDC Cathodes. Figures 3a−d show the impedance spectra of the in situ LSCF− GDC|GDC|in situ LSCF−GDC and BM LSCF−GDC|GDC| BM LSCF−GDC symmetric cells measured over the temperature range of 750−600 °C under OCV. To directly compare the cathodic polarization resistance, the ohmic resistance associated with the GDC electrolyte and current collectors (corresponding to the high frequency intercept at the real-axis of the Nyquist plot) was set to zero. The total electrode polarization resistance was obtained as the difference between high and low frequency intercepts at the real-axis. Considering the active cathode area, the calculated cathodic polarization ASR (Rp) of the LSCF−GDC cathodes was plotted as a function of inverse temperatures (Figure 4). Compared to other LSCF-doped ceria-based cathodes reported in the literature and BM LSCF−GDC, the results indicate superior ORR activity of the in situ LSCF−GDC nanocomposite cathode (Figure 4 and Table 1).17−22 For example, the ASR of the in situ LSCF−GDC cathode was as low as 0.044 Ω cm2 at 750 °C, which is ∼83 and ∼63% lower compared to those of BM LSCF−GDC and the LSCF−GDC cathode fabricated by ball-milling with nanoparticles of LSCF (∼95.10 nm) and GDC (∼73.91 nm), respectively. This result demonstrates that in situ sol−gel process produces homogeneously and well-mixed nanocomposites with highly extended electrochemically active sites, thus effectively enhancing ORR activity. 3.3. Microstructural Analysis. Figure 5 shows crosssectional FIB-SEM images of the BM LSCF−GDC and in situ LSCF−GDC cathodes after EIS measurement. Although nanosized powders of both LSCF and GDC phases were used as starting materials, the microstructure of the BM LSCF− GDC cathode was highly agglomerated in each phase and densified (Figure 5a). This coarsening could be triggered during the mixing process and accelerated upon the sintering process. In comparison, the in situ LSCF−GDC cathode shows

Figure 5. Cross-sectional microstructures of the (a) BM LSCF−GDC and (b) in situ LSCF−GDC cathodes after ASR test.

Co, and Fe), indicating homogeneously mixed nanocomposites without intermixing of cations between two phases.

Figure 6. (a) 3D reconstruction of the total volume. Magnified images with different microstructural features; (b) LSCF (blue) and GDC (yellow) phases, (c) pore (translucent gray) and centroid pore skeleton (red), (d) interfacial area of the LSCF/pore phase (red), and (e) active (red line) and inactive (green line) TPBs. E


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Table 2. Summary of Microstructural Features of the in Situ LSCF−GDC Cathode Quantified via the 3D Reconstruction phases

mean particle diameter (nm)

standard deviation

volume fraction (vol.%)

connectivity (%)

specific two phase boundary area (LSCF−pore) (μm2/μm3)

TPB density (μm/μm3)

active TPB density (μm/μm3)

LSCF GDC pore

266.1 288.0 289.6

11.7 14.0 18.3

32.1 34.7 33.2

92.3 92.2 92.1

2.9

13.5

12.7

Figure 7. Gradient plots of (a) particle diameters and (b) volume fractions of pore (red line), LSCF (blue line), and GDC (yellow line) phases as a function of distance.

Figure 8. (a) I−V−P plots of the NiO−YSZ anode supported SOFC with the in situ LSCF−GDC and BM LSCF−GDC cathode at 750 °C, and (b) long-term stability test of the in situ LSCF−GDC cathode SOFC for 500 h.

highly homogeneous mixture of heterophase in nanoscale (∼200 nm) with high porosity, which is beneficial for high TPB density. This microstructural analysis is coincident with the high ORR activity (i.e., low ASR) of in situ LSCF−GDC, as shown in Figure 4, and supports our theory (Figure 1) that the in situ sol−gel process effectively disperses the nanoscale powders and prevents the coarsening of each phase during the high-temperature sintering. For in-depth analysis of the microstructures of the in situ LSCF−GDC cathode, we conducted 3D reconstruction of the cathode using a FIB/SEM dual beam system. Figure 6a shows the 3D reconstruction of the in situ LSCF−GDC cathode with the total volume of 8.04 × 7.56 × 7.52 μm (457.08 μm3) in x, y, and z directions, respectively. Based on this total volume, various microstructural features, including particle diameter, volume fraction, phase connectivity, specific two-phase boundary area, and TPB density of the nanocomposite cathode, were quantified using an Avizo software package. The calculated values are presented in Table 2. For better understanding, Figures 6b−e show the magnified views of the same localized region of the 3D reconstruction with different microstructural features. Figure 6b shows the magnified images of the microstructure of the LSCF (blue), GDC (yellow), and pore (translucent/gray) phases. The calculated mean particle

sizes of LSCF, GDC, and pore were 266.1, 288.0, and 289.5 nm, respectively. The volume fractions of LSCF, GDC, and pore phases were estimated to be 32.1, 34.7, and 33.2%, respectively. Considering the initial mixing ratio of LSCF to GDC (1:1), this result confirms and corroborates our quantitative analysis. Figure 6c shows the centroid 3D skeleton of pores, which provides the connectivity of the phase. The calculated connectivity of LSCF, GDC, and pore phases was almost identical (∼92%), indicating that each phase formed a highly uniform 3D network. Due to the MIEC nature of LSCF, ORR at the LSCF−GDC composite cathode occurred both on the two phase (LSCF/ pore) boundary area as well as the TPB sites. Figure 6d displays the interface area between LSCF and pore. The estimated specific surface area of LSCF was 2.9 μm−1. Figure 6e shows the TPB network. The calculated active TPB (red lines in Figure 6e) density of in situ LSCF−GDC was 12.7 μm−2. Recently, Kim et al. reported that the calculated specific surface area and active TPB densities of the LSCF−GDC (50:50 vol %) cathode fabricated with commercial powders were calculated to be 1.9 μm−1 and 4.1 μm−2, respectively.23 Compared to these values, the LSCF specific surface area and the active TPB density of the in situ LSCF−GDC cathode were significantly increased by 53.0 and 209.8%, respectively. F


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ACS Applied Nano Materials Table 3. Comparison of the Performance of Various SOFCs in This Study and Reported in the Literature anode

a

electrolyte thickness (μm)

buffer layer

cathode

materials

materials

materials

Ni−YSZa

YSZ

14

GDCb

∼7

LSCFc−GDC

in situ sol−gel process

20−25

1.00

1.180

Ni−YSZ

YSZ

14

GDC

∼7

LSCF−GDC

20−25

0.99

0.696

Ni−YSZ

YSZ

15

GDC

3−5

LSCF−GDC/ LSCF

LSCF (sol−gel) and GDC (coprecipitation) LSCF (Fuel Cell Materials Co., United States) and GDC (Anan Kasei Co., Japan)

8/8

3.84

0.715

this work this work 24

Ni−YSZ

YSZ

∼10

GDC

∼5

15/40

2.50

0.860

25

Ni−YSZ

YSZ

∼10

GDC

1−2

15/40

81.00

0.540

26

Ni−YSZ

YSZ

324.00

0.450

27

Ni−YSZ

YSZ

LSCF−GDC/ LSCF LSCF−GDC/ LSCF LSCF−GDC/ LSCF LSCF8228d− GDC

0.50

0.570

28

GDC 3

GDC

7

materials

powder synthesis method

LSCF (Seimi Chemicals, Japan) and GDC (Annan Kasei, Japan)

thickness (μm)

LSCF8228 (Thin Film Components, United States) and GDC (Nextech, United States)/ball milling

area (cm2)

MPD (750 °C) (W cm−2)

thickness (μm)

reference

YSZ: Y0.16Zr0.84O1.92. bGDC: Gd0.1Ce0.9O1.95. cLSCF: La0.6Sr0.4Co0.2Fe0.8O3−δ. dLSCF8228: La0.8Sr0.2Co0.2Fe0.8O3−δ.

proving that the three phases were uniformly mixed at the nanoscale. Consequently, the quantitative microstructural results of in situ LSCF−GDC cathode are strongly correlated to the excellent electrochemical performance of the cathode. 3.4. SOFC Performance. Figure 8a presents the I−V−P characteristics of a NiO−YSZ anode supported SOFC with the in situ LSCF−GDC and BM LSCF−GDC cathode at 750 °C. The detailed structural parameters of the SOFCs are summarized in Table 3. The OCV of the both SOFCs was ∼1.1 V, close to the theoretical value, indicating the highly dense YSZ electrolytes. The maximum power density (MPD) of the fuel cell using the in situ LSCF−GDC cathode was 1.18 W cm−2, which is superior (∼2 times higher) compared to that of the BM LSCF−GDC cathodes (0.69 W cm−2) and performance compared to that of the similarly structured SOFCs with LSCF−GDC composite cathodes previously reported, as shown in Table 3.24−28 For example, Lim et al. reported that the SOFC using the LSCF−GDC cathode with a 10 μm thick YSZ electrolyte yielded a MPD of only 0.54 W cm−2 at 750 °C.26 Figure 8b shows the long-term stability result of the SOFC employing in situ LSCF−GDC cathode at 750 °C in a galvanostatic mode with an applied current density of 200 mA. The output voltage was maintained for over 500 h with no measurable degradation, indicating high stability. Cross-sectional SEM images of the in situ LSCF−GDC cathode cell reveals that the cathode structure retained its porous, finegrained nanostructures even after this extensive operation (Figure 9a). In addition, the elemental composition profile of the SOFC by the EDS line scan analysis (Figure 9b) indicates no reactivity or interfacial diffusion between adjacent layers after long-term test.

Figure 9. (a) Cross-sectional SEM image and (b) EDS line scan after a 500 h long-term test of an SOFC with the in situ LSCF−GDC cathode.

4. CONCLUSION In summary, LSCF−GDC nanocomposite powders were prepared via an in situ sol−gel process. HADDF-STEM analysis confirmed that ∼90 nm sized LSCF and GDC nanoparticles were homogeneously mixed. Due to the uniformly mixed nanostructure, the ASR of the in situ LSCF−GDC cathode on GDC electrolyte was as low as 0.044 Ω cm2 at 750 °C. Quantitative analysis of microstructures

Figures 7a and b show the distributions of the average particle size and the volume fraction of LSCF, GDC, and pore phases as a function of distance in the total analytic volume, respectively. Each phase had a narrow size distribution in the nanoscale and a similar volume fraction in the overall volume, G


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by the 3D reconstruction revealed that the specific two phase (LSCF/pore) boundary area and the active TPB density of the in situ LSCF−GDC cathode were exceptionally high, 2.9 μm−1 and 12.7 μm−2, respectively. When this nanocomposite cathode was employed in an SOFC, the MPD was ∼1.2 W cm−2 at 750 °C, which yields an approximate ∼37% higher performance, as compared to other reported values with similarly structured SOFCs. Moreover, the tested SOFC showed excellent longterm stability up to 500 h. These results demonstrate that the in situ LSCF−GDC nanocomposite as an IT-SOFC cathode is highly feasible.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00566. Comparison of GDC and LSCF peaks, table of measured parameters and calculated crystallite sizes, and analysis of size distribution of in situ LSCF-GDC nanoparticles after calcination (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.T.L). *E-mail: [email protected] (K.J.Y). ORCID

Kyung Joong Yoon: 0000-0002-4161-5111 Kang Taek Lee: 0000-0002-3067-4589 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Fundamental Research and Development Program for Core Technology of Materials funded by the Ministry of Trade, Industry, & Energy, Korea (Grant 10050985). This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry, & Energy (MOTIE) of the Republic of Korea (Grant 20173010032120)

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REFERENCES

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ACS Applied Nano Materials (28) Liu, Q. L.; Khor, K. A.; Chan, S. H.; Chen, X. J. AnodeSupported Solid Oxide Fuel Cell with Yttria-Stabilized Zirconia/ Gadolinia-Doped Ceria Bilalyer Electrolyte Prepared by Wet Ceramic Co-Sintering Process. J. Power Sources 2006, 162, 1036−1042.

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