BaCo0.7Fe0.22Y0.08O3−δ as an Active Oxygen ... - ACS Publications

BaCo0.7Fe0.22Y0.08O3−δ as an Active Oxygen Reduction Electrocatalyst for LowTemperature Solid Oxide Fuel Cells below 600 °C Wei He,† Xuelian Wu,‡ Guangming Yang,§ Huangang Shi,∥ Feifei Dong,*,† and Meng Ni*,† †

Building Energy Research Group, Department of Building and Real Estate, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 999077, China ‡ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China § State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, China ∥ School of Environmental Engineering, Nanjing Institute of Technology, Nanjing 211167, China S Supporting Information *

ABSTRACT: Solid oxide fuel cells (SOFCs) offer great promise as sustainable energy conversion devices due to their high chemical-toelectrical conversion efficiency, flexible fuel sources, and low pollutions. In recent years, much effort has been devoted to developing intermediate temperature SOFCs. Central to the devices is the availability of a highly effective electrocatalyst for oxygen reduction reaction with reduced temperature operation, especially below 600 °C. Here we present a novel B-site Y-doped perovskitetype oxide BaCo0.7Fe0.22Y0.08O3−δ (BCFY) with extremely low polarization resistances (e.g., 0.10 Ω cm2 at 550 °C), which is ascribed to high cubic symmetry structure and fast oxygen kinetics. The superior electrocatalytic activity and stability enable BCFY to be a promising cathode candidate toward the application of reduced temperature SOFCs.

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(LSCF), Sm0.5 Sr 0.5 CoO 3−δ (SSC), and SrNb 0.1 Co0.9O 3−δ (SNC),5,12−16 have been widely investigated as effective cathode materials for low-temperature SOFCs (LT-SOFCs). As a significant parent material, many perovskite-type composite oxides with a variety of properties can be exploited based on BaCo1−xFexO3 (BCFx), which exhibits favorable oxygen-ion migration because of enlarged lattice free volume and good catalytic activity for ORR due to variable valence state of Co/Fe ions.17 However, because of the large mismatch in ionic size between Ba and Co/Fe ions, the BCFx usually prefers the low-symmetry hexagonal phase. The oxygen vacancy formation energy in the hexagonal phase is much higher than that in the cubic phase.18 To stabilize the cubic lattice structure, a doping strategy is often applied. Smaller ions such as Sr and

olid oxide fuel cells (SOFCs) are sustainable energy conversion devices that can directly convert chemical energy in fuel to electrical power, with high chemical-toelectrical conversion efficiency, flexible fuel sources, and low emissions.1−3 Recently, extensive efforts for SOFCs have been focused on lowering the operating temperature, especially below 600 °C,4−6 which allows more inexpensive interconnecting materials (stainless steel), more easy-to-accomplish sealing, and better thermal and chemical stability of the cell components.7 Central to the devices is the availability of a highly active cathode material for electrochemical reduction reaction of oxygen in the reduced temperature regime.8−11 The mixed ionic−electronic conductors (MIECs) can extend active sites for oxygen reduction reaction (ORR) from a traditional electrolyte−cathode−gas triple phase boundary (TPB) region to the entire electrode surface, thus significantly enhancing the catalytic activity of the electrode. Among them, cobalt-containing perovskite-type oxides, such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF), La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ © XXXX American Chemical Society

Received: November 20, 2016 Accepted: January 6, 2017 Published: January 6, 2017 301

DOI: 10.1021/acsenergylett.6b00617 ACS Energy Lett. 2017, 2, 301−305

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Figure 1. (a) XRD pattern of BCFY powder, (b) TEM image of BCFY, (c) high-resolution TEM image, (d) corresponding SEAD patterns.

fraction profiles. A scanning transmission electron microscope (STEM) is employed to further check the morphology and structure of BCFY. Figure 1b is a typical TEM image of BCFY grain synthesized by a sol−gel method. The high-resolution TEM image (Figure 1c) exhibits the presence of well-defined crystalline fringes with a lattice fringe spacing of 2.87 Å, corresponding to the (110) crystal plane of the cubic lattice structure. The crystal structure is also confirmed to be perovskite from the corresponding selected area electron diffraction (SAED) patterns (Figure 1d) along the [1̅11] zone axis. The result above is in accordance with the XRD data (d110 = 2.90 Å). The existence of Ba, Co, Fe, and Y can be validated by the energy dispersive X-ray (EDX) diffraction spectrum (Figure S2). As reported before, BCF has a hexagonal perovskite structure, and its Goldschmidt tolerance factor t = (rA + rO)/ 2 (rB + rO) is estimated to be bigger than 1. The partial substitution of Y ions with a larger ionic radius (rY3+ = 0.90 Å) for B-site Fe ions (rFe3+ = 0.645 Å, rFe4+ = 0.585 Å) enables it to approach 1, which contributes to the construction of cubic perovskite structure.27 Results for the variation in electrical conductivity of BCFY with respect to temperature (400−700 °C) and oxygen partial pressure (PO2 = 0.21 and 0.1 atm) are shown in Figure S3. The electrical conductivity increases monotonously with an increase in temperature, suggesting semiconducting behavior. Identification of the oxygen bulk diffusion coefficient (Dchem) and surface exchange coefficient (kchem), determined using the electrical conductivity relaxation (ECR) method, can be helpful in evaluating the electrochemical activity of cathode materials. Figure 2a depicts the typical transient conductivity response of BCFY over the temperature range of 450−750 °C upon abruptly shifting the partial pressure of oxygen from 0.21 to 0.1 atm. The Arrhenius plots of Dchem and kchem for BCFY perovskite are shown in Figure 2b, and both Dchem and kchem values rise with increasing temperature, indicating high activity

La were utilized to partly replace the large Ba ions in the Asite,19,20 or cations with a large ionic radius and high oxidation state were doped into the Co/Fe ions in the B-site to stabilize the cubic lattice structure of BCFx; as a result, the resulting composite oxides exhibited very promising electrocatalytic activity for ORR.21−25 Recently, cubic perovskite-type oxides BaCo0.7Fe0.3−xYxO3−δ (BCFYx) have attracted much attention as ceramic membranes for oxygen separation from air.26 It was claimed that the Y-substitution stabilized the cubic structure because of its large size and increased the oxygen vacancy concentration for its low valence state, and the ceramic membranes showed a high oxygen permeation flux. Until now, however, detailed knowledge about the electrochemical properties of BCFYx as an oxygen reduction electrocatalyst for LTSOFCs is still unclear. Herein, we present a B-site Y3+-doped perovskite oxide with a nominal composition of BaCo0.7Fe0.22Y0.08O3−δ (BCFY) toward the application as an oxygen reduction electrode. The cubic phase structure and fast oxygen kinetics of BCFY perovskite render it a promising cathode material with the desirable electrochemical activity for LT-SOFCs. Figure S1 exhibits the X-ray diffraction (XRD) patterns of assynthesized BaCo0.7Fe0.3O3−δ (BCF) and BCFY oxides. The diffraction peaks of BCF oxide cannot be well indexed into a single perovskite phase, and the BCF sample consists of a major hexagonal phase and a trace amount of impurity phase. After substituting Y3+ cations for the Fe-site, the single perovskite phase is successfully formed, suggesting that Y doping can effectively stabilize the phase structure of BCF. Figure 1a shows the refined diffraction patterns of the as-prepared BCFY powder. It is clear that the diffraction patterns can be assigned to a cubic symmetrical perovskite structure in space group Pm3m with unit cell data of a = 4.12 Å. The low converged reliability factors (Rp = 2.58%, Rwp = 3.26%, χ2 = 1.32) indicate good agreement between experimental and calculated dif302

DOI: 10.1021/acsenergylett.6b00617 ACS Energy Lett. 2017, 2, 301−305

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ACS Energy Letters

− QPELF) (Figure S4c), it is deducible that the ORR is dominated by impedance in the low-frequency arc associated with noncharge transfer processes (e.g., adsorption−desorption and diffusion of oxygen) in view of larger RLF over RHF.32 As shown in Figure 3, the BCFY cathode exhibits lower ASRs than

Figure 3. Arrhenius-type plots of ASR values for the BCFY cathode and other predominant cobalt-containing perovskite cathodes at various temperatures.

those of the predominant cobalt-containing cathode materials, such as SSC and LSCF.13,15 Additionally, the ASR values of the BCFY cathode are much lower than those of other BCFderived cathode materials like BaCo0.6Fe0.3Nb0.1O3−δ and BaCo0.6Fe0.3Sn0.1O3−δ.24 It thus suggests the superior electrocatalytic activity of BCFY for the ORR, which relates well with a high cubic symmetry structure and fast oxygen bulk diffusion and oxygen surface exchange kinetics. Fuel cell tests offer further verification of the electrochemical performance of the BCFY cathode for ORR in a single-cell configuration under real operating conditions. Figure S5 exhibits a typical scanning electron microscope (SEM) image of the anode-supported triple-layered fuel cell. The dense electrolyte film is ∼10 μm thick and is well-adhered to the electrodes, without any noticeable cracking or delamination. Figure 4 shows the I−V polarization curves and corresponding

Figure 2. (a) ECR responses of BCFY at various temperatures after a sudden change of oxygen partial pressure from 0.21 to 0.1 atm, (b) variation of Dchem and kchem for BCFY with regard to the temperature dependence derived from the ECR measurement.

at elevated temperatures. Lower activation energies for Dchem and kchem indicate that the oxygen kinetics has low-temperature dependence, in favor of reduced temperature operation. The high Dchem and kchem values of 2.24 × 10−4 cm2 s−1 and 2.23 × 10−3 cm s−1 at 600 °C are acquired, which are much larger than those of other prevalent cathode materials, such as LSCF and BSCF.28,29 Additionally, the electrical conductivity of BCFY is smaller than that of LSCF and BSCF,30,31 implying that the improved activity is not attributed to the electronic conductivity. The area-specific resistances (ASRs) of BCFY are measured by electrochemical impedance spectroscopy (EIS) for the BCFY| samaria-doped ceria (SDC)|BCFY symmetrical cell configuration. The ASR values are accessed by the impedance intercept between high and low frequency on the real axis of Nyquist plots, and representative impedance spectra are exhibited in Figure S4a. The ASR values are 0.03, 0.10, and 0.43 Ω cm2 at 600, 550, and 500 °C, respectively. The extremely low electrode polarization resistance reflects high activity for ORR. For comparison, the ASRs of a benchmark cathode BSCF are measured to be 0.09, 0.23, and 1.1 Ω cm2 at 600, 550, and 500 °C, respectively (Figure S4b), approximately 1−2 times higher than those of the BCFY cathode. By fitting the EIS data of BCFY and BSCF at 600 °C to an equivalent circuit model with a configuration of Rohm(RHF − QPEHF)(RLF

Figure 4. I−V curves and corresponding power densities of a single cell at different temperatures. 303

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National Natural Science Foundation of China (Contract No. 51302135).

power density curves obtained for the single cell with humidified hydrogen (3% H2O) as a fuel and static ambient air as an oxidant over the temperature range of 450−600 °C. The cell exhibits superior open-circuit voltages (OCVs) from 0.86 (600 °C) to 0.94 V (500 °C), which are lower than the theoretical values because of some internal short circuits in SDC electrolyte induced by partial reduction of Ce4+ ions to Ce3+ ions.33 The cell delivers an attractive power output with peak power densities of 967, 687, and 423 mW cm−2 at 600, 550, and 500 °C, respectively, which are comparable to those of BSCF under similar operation conditions.5 The fuel cell performance can be affected by many resistances, such as contact resistance between the electrode and current collector, electrode polarization resistance, concentration polarization resistance, electrolyte resistance, and so on. The fuel cells with the BCFY cathode do not show a higher performance though the cathode exhibits a lower ASR value than that of BSCF, which might be related to other resistances. More competitive electrochemical performance can be expected to be achieved by optimization of the cathode and other component processing. As shown in Figure S6, under a constant current density of 400 mA cm−2, a stable output voltage at around 0.75 V is achieved throughout the 50 h test period at 550 °C. The favorable cell performance and stability test suggest that BCFY holds great promise as a potential ORR electrocatalyst for application in the LT-SOFC field. In summary, a cubic symmetry perovskite oxide BCFY was achieved by substituting 8% mol Y for Fe at the B-site, and its properties were characterized. The obtained extremely low polarization resistance values can be attributed to the cubic symmetrical structure and fast oxygen bulk diffusion rate and oxygen surface exchange kinetics. The favorable electrochemical activity and stability justify the potential of BCFY as an active oxygen reduction electrocatalyst for reduced temperature SOFCs.

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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/acsenergylett.6b00617. Experimental details, XRD pattern, EDX diffraction spectrum, electrical conductivity, EIS data, SEM image, and short-term stability measurement (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: ff[email protected] (F.D.). *E-mail: [email protected] (M.N.). ORCID

Guangming Yang: 0000-0003-3792-5018 Feifei Dong: 0000-0002-3512-3145 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a grant (Project Number: PolyU 152127/14E) from the Research Grant Council, University Grants Committee, Hong Kong SAR, a grant from The Hong Kong Polytechnic University (Account: 1-ZVFQ), and the 304

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DOI: 10.1021/acsenergylett.6b00617 ACS Energy Lett. 2017, 2, 301−305