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Sc and Ta-doped SrCoO3-δ perovskite as a high-performance cathode for solid oxide fuel cells
Journal Pre-proof Sc and Ta-doped SrCoO3-δ perovskite as a high-performance cathode for solid oxide fuel cells Xiaoyong Xu, Jie Zhao, Mengran Li, Linzhou Zhuang, Jinxuan Zhang, Sathia Aruliah, Fengli Liang, Hao Wang, Zhonghua Zhu PII:
S1359-8368(19)34260-X
DOI:
https://doi.org/10.1016/j.compositesb.2019.107491
Reference:
JCOMB 107491
To appear in:
Composites Part B
Received Date: 21 August 2019 Revised Date:
26 September 2019
Accepted Date: 27 September 2019
Please cite this article as: Xu X, Zhao J, Li M, Zhuang L, Zhang J, Aruliah S, Liang F, Wang H, Zhu Z, Sc and Ta-doped SrCoO3-δ perovskite as a high-performance cathode for solid oxide fuel cells, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107491. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Sc and Ta-doped SrCoO3-δ Perovskite as a High-Performance Cathode for Solid Oxide Fuel Cells Xiaoyong Xu
a,£
b£
a
a
a
a
, Jie Zhao , Mengran Li , Linzhou Zhuang , Jinxuan Zhang , Sathia Aruliah , Zhonghua Zhu
a,*
a. School of Chemical Engineering, The University of Queensland, St Lucia, Queensland, 4072, Australia b. School of Chemistry and Chemical Engineering, Anhui University, Hefei, China.
6 SSTC
5
SSNC[10] SSC[8]
-LN(ASR)
4 3 2 1 0 1
1.1
1.2 1000/T
1.3
1.4
Average area-specific resistance (ASR) of SSTC within a cathode|SDC|cathode symmetrical cell in air (SrSc0.175Ta0.025Co0.8O3-δ, SSTC; SrSc0.175Nb0.025Co0.8O3-δ, SSNC; SrSc0.2Co0.8O3-δ, SSC)
Sc and Ta-doped SrCoO3-δ Perovskite as a High-Performance Cathode for Solid Oxide Fuel Cells Xiaoyong Xu1£, Jie Zhao2£, Mengran Li1, Linzhou Zhuang1, Jinxuan Zhang1, Sathia Aruliah1, Fengli Liang1, Hao Wang3, Zhonghua Zhu1* 1
School of Chemical Engineering, The University of Queensland, Brisbane 4072 Australia
2
College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, PR China
3
Centre for Future Materials, University of Southern Queensland, Springfield Central, QLD 4300, Australia
£
Xu and Zhao made equal contributions to this paper
*Corresponding author, email: [email protected]
1
ABSTRACT Sealing and stability is the challenge for solid oxide fuel cells (SOFCs) at high temperature. It is crucial to operate SOFCs at a low temperature to avoid these issues. Thus, it is necessary to improve the sluggish oxygen reduction reaction (ORR) activity of cathode. The SrCoO3-δ perovskite with a cubic phase is identified as a potential cathode material for SOFCs due to its high oxygen permeability and reasonable electrical conductivity. However, it readily decomposes into the orthorhombic structured brownmillerite Sr2Co2O5 or hexagonal Sr6Co5O15 during high-temperature sintering resulting in the loss of a significant amount of lattice oxygen. Herein, we partially replace Co with Sc and Ta at SrCoO3-δ to stabilize its cubic perovskite structure and restrain the lattice oxygen loss, thus improving the oxygen-ionic conductivity, and the structural and chemical stability. The synthesized SrSc0.175Ta0.025Co0.8O3-δ (SSTC) cathode achieves a remarkably high ORR performance and an area-specific resistance obtained in this study reaches as low as 0.233, 0.033, and 0.004 Ω cm2 at 500, 600, and 700 °C, respectively. The new Sc and Ta-doped SrCoO3-δ perovskite cathode even present a better ORR activity than the previously reported Sc and Nb-doped SrCoO3-δ cathode. The reason for the improved performance by Ta doping is possible that Ta–O bond is stronger than Nb–O bond and the electronegativity of Ta5+ is lower than that of Nb5+, resulting in a lower valence state of cobalt and a higher oxygen vacancies concentration. KEYWORDS: Perovskite, cathode material, Solid oxide fuel cell, SrCoO3-δ, Electrochemistry, Oxygen reduction reaction
2
1
Introduction Solid oxide fuel cells (SOFCs) can produce electricity by directly converting
the chemical energy in fuel, and they are promising electrochemical energy conversion technology with excellent fuel flexibility and high energy efficiency [1-5]. The researchers[4] have devoted intensive effort in recent years on intermediate temperature SOFCs (IT-SOFCs) to make SOFC technology more stable and affordable; the operating temperature is decreased to reduce fabrication cost and sealing issues. Other benefits, such as quick start and cooling cycles, can also be enhanced as a result of reducing operating temperatures to 600 °C or below. However, the electrochemical activity of the cathode drops sharply with decreasing temperature[3, 6-13]. The development of advanced cathode materials with high-performance of ORR is crucial for IT-SOFCs. Researchers reached a consensus that mixed ionicelectronic conducting oxides (MIECs) should be a potentially promising candidate
for
IT-SOFCs
cathode.
MIECs
as
the
cathode
extend
the
electrochemical reaction zone from the limited three-phase boundary area (TPB) between cathode and electrolyte to the overall surface area of MIEC cathode leading to a higher electrochemical performance [6, 14, 15]. Among the cobaltcontaining materials, SrCoO3-δ (SCO) is identified as a promising perovskite parent compound to develop a series of cubic perovskite materials with high electronic conductivity and oxygen ionic conductivity [16]. However, it readily decomposes into the orthorhombic structured brownmillerite Sr2Co2O5 or hexagonal Sr6Co5O15 at high temperature by losing a significant amount of lattice oxygen[17].
3
Substituted A- or B- sites have been widely used to stabilize the cubic perovskite of SrCoO3-δ. It is a common strategy to dope a metal oxide with higher oxidation states to increase static electrical repulsion to prevent tilting of octahedral and compensate for the loss of lattice oxygen chemically, thus stabilize SCO's cubic perovskite structure. Several perovskites, such as Sm0.5Sr0.5CoO3-δ[18], δ[17],
SrSc0.2Co0.8O3-δ[16,
19],
SrCo1-xTaxO3-
SrNb0.2Co0.8O3-δ[20] and SrNb0.1Co0.9O3-δ[21], have been developed as the
cathode for IT-SOFCs operating at the temperature to 600 °C or below. Zeng et al. stabilized the cubic structure of SrCoO3-δ oxides by replacing Co by Sc[22]. Furthermore, the doping of a bit amount of Sc3+ could stabilize the cobalt ion at the lower valence state and the high spin state[16, 19]. The synergistic substitution is a common strategy to improve the oxygen-ionic conductivity.
The
perovskites,
SrSc0.175Nb0.025Co0.8O3-δ
(SSNC)[12]
and
SrTa0.1Nb0.1Co0.8O3-δ (STNC)[13] were reported by Zhou et al and our group with a high-performance of ORR at 550 °C. SSNC improved the oxygen reduction reactivity by more than 100% at 500 °C compared with the prominent cathode, Ba0.5Sr0.5Co0.2Fe0.8O3-δ (BSCF). Tantalum and niobium are under the same group on the periodic table. Furthermore, Ta5+ and Nb5+ have an identical ionic radius [17]. Previous researchers have consistently confirmed that the Nb–SCO system presents worse thermal / chemical/electrochemical/electrical stability than Ta–SCO[17, 20] although there are different explanations on it. Herein, we partially replace Co with Sc and Ta in SrCoO3-δ to stabilize its beneficial cubic perovskite structure and restrain the
4
lattice oxygen loss, thus improving the ionic conductivity, and the structural and chemical stability. The perovskite oxide SrSc0.175Ta0.025Co0.8O3-δ (SSTC) was synthesized and evaluated as a potential cathode material by a solid-state reaction. The phase structure and chemical compatibility with electrolyte material were evaluated by X-ray diffraction measurements (XRD), as well as electrical conductivity by four-probe measurement, and polarization resistance of the SSTC using symmetric cell test was assessed. The performance and electrochemical stability of the single-cell with SSTC as a cathode were also presented. Sc and Ta-doped SCO presents a very high electrochemical performance for SOFCs in the future.
2
Experimental Section
2.1 Material synthesis and characterization The perovskite oxide SrSc0.175Ta0.025Co0.8O3-δ (SSTC) was synthesized by solid-state reaction. A stoichiometric amount of oxide powders Sc2O3, SrCO3, Ta2O5 and Co3O4 (Sigma-Aldrich) in the ethanol media were mixed in a ball mill (Fritsch, Pulverisette 5) at a rotation rate of 300 rpm for 24 hours. Then, dry it in the oven at 80ºC for 24 hours. The dried solid powders were pressed into granules and calcined in static air at 1200°C for 20 hours to form the perovskite oxide. SSTC powder was compressed into discs and then sintering at 1200 °C for 20 hours and then polishing the discs into a bar shape for electrical conductivity and thermal expansion coefficient (TEC) testing. The dimensions are 7.5 mm × 4.0 mm × 12.5 mm. Netsch DIL 402C dilatometer was used to measure TECs from
5
25 °C to 850 °C with a heating rate of 5 °C/min and 50 ml min-1 of airflow rate. The relative density of the specimen is over 95%. A four-probe DC technique was used to measure the electrical conductivities by an Autolab PGSTAT30 electrochemical workstation. Oxygen temperature-programmed desorption (O2-TPD, Belcat, Japan) was used to monitor the oxygen desorption properties of the cathode materials from room temperature to 850 °C with a heating rate of 10 °C min-1. For an O2-TPD measurement, about 50 mg powders were kept in a U-type quartz tube. 30 ml min-1 of the carrier gas (pure argon) was passed through the tube. A mass spectrometer (Belmass, Japan) was used to monitor the effluent gases. Netsch DIL 402C dilatometer was used to measure TECs of SSTC and a fourprobe DC technique was used to measure the electrical conductivities by an Autolab
PGSTAT30
electrochemical
workstation.
The
initial
oxygen
stoichiometry (δ) is determined by the iodometric titration method[23]. The details are described in the supporting information. The crystalline structure and chemical compatibility of the SSTC sample were determined by XRD on a Bruker-AXS D8 Advance diffractometer using Cu-Kα radiation at 40 kV and 40 mA equipped with an Anton-Paar high-temperature HTK-1200N cell. In situ X-ray diffraction measurements, a high-temperature Rigaku Model Multiflex X-ray diffractometer using Cu-Kα radiation at 45 kV and 200mA was utilized to analyze the crystal structure. The data sets were recorded in a step-scan mode in the range of 20–80° with intervals of 0.02°.
6
X-ray photoelectron spectra (XPS) were obtained on a Kratos Axis ULTRA xray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer and 150 W (15 kV, 10 mA) monochromatic Al Kα (1486.6 eV) radiation. The binding energy was determined using a 284.8 eV C1s line from adventitious carbon as a reference. 2.2 Cell fabrication and testing For symmetrical cells, SDC powders were dry-pressed into discs, followed by sintering 1400°C for 5 hours to form dense SDC disks=. The SSTC slurry was prepared by mixing binder, a solvent with SSTC powder by ball milling Fritsch, Pulverisette 5) at a rotation rate of 400 rpm for 2 hours. The SSTC slurry was sprayed onto both sides of the SDC pellet symmetrically and subsequently calcined at 1000°C for 2 hours in static air. The silver slurry was brushed on the surface of the pellet as the current collector. The thickness of the cathode was controlled by spraying time (10 seconds, 6 times) and quantified by a scanning electron microscope (SEM). Electrochemical impedance spectra (EIS) of the symmetrical cells were measured in air using an Autolab PGSTAT30 electrochemical workstation. The frequency range was 100 kHz to 0.01 Hz, and the signal amplitude was 10 mV under open cell voltage conditions. Anode-supported cells were prepared via dual dry-pressing followed by hightemperature sintering. 40 wt% SDC and 60 wt% NiO were well mixed by ball milling as anode powders. The electrolyte was densified through firing at 1350 °C for 3 h in air. Afterward, the cathode slurry was sprayed onto the central surface of the dense electrolyte and fired at 1000°C for 2 h in air. The cathode
7
was approximately10 µm thick, and an effective area is about 0.28 cm2. The I–V polarization was measured using an Autolab PGSTAT30 with a four-terminal configuration. 100 ml min−1 (STP) of humidified (3% H2O) H2 fuel was passed into the anode chamber and 100 ml min−1 (STP) of instrument air wad fed into the cathode side.
3
Results and Discussion
3.1 Structural characterization Pure SSTC was synthesized by calcining the mixture of oxides at 1200 °C for 20 h in air. These peaks indicate that the crystal synthesis of SSTC is good, and the diffraction peaks can be well identified as a pure cubic perovskite structure[12]. The Rietveld refinement fitting results are depicted in Fig.1a with reliability factors of Rwp = 0.0124 and Rp = 0.0124 with the goodness of fit = 2.13 and the d spacing value of 0.277 nm for 110 planes. The d spacing value is consistent with high-resolution TEM result (Fig. 1b) which presents a similar d spacing value for 110 planes, denoting the oxygen vacancy ordering in the cubic perovskite. To confirm the stability of SSTC in the working condition, high-temperature XRD measurement was performed in air from 25 °C to 800 °C. No decomposition was observed for SSTC in this temperature range (Fig. 1c), indicating that SSTC is thermally stable. Additionally, the XRD diffraction peaks shift to a lower diffraction angle at a higher temperature, revealing an obvious lattice expansion at high temperature. Such expansion is attributed to the
8
reduction of tri- and tetra-valent Co species to a lower valence with larger ionic sizes at elevated temperature. In Fig 1 (a) the first peak is observed at approximately 23º but no peaks in Fig 1 (c) at the same position. This may be caused by a different accuracy of two XRD instruments. XPS results in Fig. 1(d) presents that the binding energy (B.E.) of Ta 4f7/2 (25.64 eV) assigned to Ta5+, indicating the 5+ charge on Ta cations in SSTC[20]. For Sc3+, interestingly, there is no obvious peak in the binding energy of 410 eV to 400 eV. Based on the previous research on SSC[19] and SSNC[12], we consider the same 3+ charge on Sc cations in SSTC. 3.2 Electrical conductivity The electrical conductivity of SSTC in the air between 400 and 700 °C was carried out by a 4-probe DC conductivity method. Fig.2 presents a drop in electrical conductivity with temperature. This behaviour of electrical conductivity against temperature is similar to a semiconductor, caused by a decrease in the concentration of electronic defects with increasing temperature due to more oxygen vacancy in SSTC as elevated the temperature. SrSc0.175Ta0.05Co0.8O3-δ has an electrical conductivity of 12-23.7 S cm-1 in the air between 400 and 700 °C. Compared to SrCoO3-δ (4.6–20 S cm-1) and SrSc0.2Co0.8O3-δ (4.6-25 S cm-1) in air between 300 and 900 °C [16], SrSc0.175Ta0.05Co0.8O3-δ perovskite didn’t show the improvement in the electrical conductivity. Both Sc3+ and Ta5+ have the fixed valence states, which may act as a block for the electrons between oxygens and cobalt ions in the Zerner double exchange[22].
9
3.3 Oxygen stoichiometry TGA studies were performed to investigate the oxygen vacancy content (δ) for SSTC at different temperatures. The initial oxygen stoichiometry (δ0) in the SSTC perovskite was obtained to be 0.40 using the iodometric titration. The results are shown in Fig. 3. SSTC oxygen vacancy concentration steadily increases with temperature due to the reduction of the cobalt ions as evidenced by the lattice expansion shown in Fig. 1(c). The oxygen nonstoichiometry of SrSc0.2Co0.8O3-δ reached a value as high as 0.548 at room temperature[10]. Substituting Ta for Sc in SrSc0.2Co0.8O3-δ decreases the oxygen non-stoichiometry because Ta has a high 5+ fixed oxidation state, which increases the total charge of the cations, and therefore requires more oxygen ions to compensate for the charge imbalance[23]. 3.4 Thermal expansion coefficient Fig. 4 presents the comparison of thermal expansion curves of SSTC and SSC in the range of room temperature to 850 °C in air. Both curves show an inflexion at 450 ~500 °C corresponding to a reduction of teltra-cobalt ion to tri-cobalt ion [24]. This inflexion is also indicated by oxygen temperature-programmed desorption (O2-TPD) results (Fig. 5). Then, there is a reason to believe that the increased slope above 500 °C is related to the loss of lattice oxygen. With Sc and Ta-doped SC, SSTC shows a slightly lower thermal expansion coefficient than SSC. It can be explained that SSC converts more Co4+ to Co3+ or Co3+ to Co2+ than SSTC, and consequently expanding the lattice.
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3.5 Electrode performance Fig. 6 shows that the porous cathode is still attached intimately to the SDC surface after testing. This demonstrates that SSTC cathode material adheres well to the electrolyte and the thickness of SSTC cathode is approximately 15 µm. We also checked the compatibility of SSTC with the electrolyte SDC by mixing SDC with SSTC (1:1 wt%) followed by sintering at 1000 °C in static air for 2 h. Fig. 7 shows that no extra peaks were detected for the mixture of SSTC and SDC, which indicates that the chemical interactions between the SSTC and SDC at 1000 °C are negligible. ORR activity of SSTC cathodes was characterized by electrochemical impedance spectroscopy (EIS) using symmetrical cells with SDC as an electrolyte. ASR is normalized by the electrode area of the cathode polarization resistance and it. may also be calculated by impedance spectra obtained under open-circuit conditions. ASR is the difference between the real axis intercepts of the impedance spectrum, that is the electrode polarization resistance (Rp). ASR can describe the performance of SOFC cathodes: higher ASR value reflects less active ORR activity. Fig. 8 shows the ASR values of SSTC cathode between 500 and 700 °C. The ASR values are extremely low. They are only 0.233, 0.090, 0.033, 0.011, and 0.004 Ω cm2 at 500, 550, 600, 650 and 700 °C, respectively. As a reference, Wei et al[19] reported the ASRs of the SrSc0.2Co0.8O3-δ cathode are 0.415, 0.206, 0.095, 0.044, and 0.023 Ω cm2 at 500, 550, 600, 650 and 700 °C, respectively. Later, they replaced a small amount of Sc by Nb in SrSc0.2Co0.8O3-δ (SSCO) and demonstrate that there are two oxygen ion transport paths with very
11
low migration barrier energies, which are produced by partially replacing Co atoms with Nb and Sc atoms into SrCoO3-δ. The ASRs of the SSNC cathode are 0.32, 0.11, 0.04, 0.02, and 0.009 Ω cm2 at 500, 550, 600, 650 and 700 °C, respectively, approximately 120-230% higher than those of the SSTC cathode. This indicates that the SSNC cathode has a worse performance of ORR than SSTC at intermediate temperatures. Previous research show consistently that Ta–SCO system presents better thermal/electrical/electrochemical/chemical stability than Nb–SCO because Ta–O bonding is stronger than Nb–O [17, 20]. In this study, the Ta-SSCO cathode also presents the advantage over the Nb-SSCO system. To understand the reason why SSTC cathode presents the excellent performance, we measured the electrical conductivity of SSTC, which is even lower than its counterpart, SSNC. This indicates that the high electronic conductivity is not the main reason for the enhanced ORR activity.. This phenomenon is also observed by a previous study[24]. We also measured the oxygen vacancy concentration of SSTC, which has a slightly higher oxygen vacancy of 0.43-0.50 in the air between 400 and 700 °C than SSNC (0.41-0.465 in the air between 400 and 700 °C). The formation of oxygen vacancies will diminish the charge carriers and thereby degrade the electronic conductivity. Therefore, a lower electrical conductivity of SSTC than SSNC can be explained by their higher oxygen vacancy formed in the lattice. Oxygen vacancies is a crucial parameter in perovskite oxides owing to its impact on material properties including the ionic charge transport[25]. Itis generally expected that increasing the oxygen vacancies will increase oxygen ionic conductivity[15]. This may contribute to the high ORR of SSTC. If compared to
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SSNC, the SSTC presents a greater ORR activity. As explained by the previous researchers[17, 20], Nb5+ and Ta5+ have different electronegativity although similar ionic radii.. The main reason for the enhanced performance is that Ta–O bond is stronger than Nb–O bond and the lower electronegativity of Ta5+ leads to a bit lower valence of cobalt and more oxygen vacancies. XPS results in Fig. S2 confirms that SSTC presents a lower valence of cobalt than SSNC. To further investigate the effect of Ta in SSTC cathode, Nyquist plots of the EIS of SSTC cathode at 500, 550 and 600˚C are presented in Fig.9a. Based on an equivalent circuit, Rohm(RE1 - CPE1)(RE2 -CPE2), the impedance spectra can be divided into two half-cycles as shown in Fig. 9b, the details of this theory is described in the supporting information[10]. The calculated values of RE1 and RE2 of the SSTC cathode are presented in Fig. 10, and the values for SSC from other researchers[10] are also included. For SSTC, RE2 values are much higher than RE1 values at the low temperature (500°C, 550°C and 600°C) while they are almost same at the high temperature (650°C and 700°C). RE2 values are much higher than RE1 values at the low temperatures range of 500°C to 600°C, which means that the diffusion process is the rate-controlling step of ORR for SSTC. Therefore, with a small amount of Tadoped SSC, the rate-controlling step of ORR for SSC changes from the chargetransfer process to the diffusion process in the SSTC, indicating that Ta5+ in the SSTC cathode benefits the mobility of oxygen ion.
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The performance of an anode-supported single fuel cell with SSTC cathode was then investigated. Scanning electron microscopy is used to characterize the cross-section y of this single fuel cell; as shown in Fig. 11, the single cell is composed of a porous anode supporting layer, a 42 µm-thick dense SDC electrolyte layer, and a 15 µm-thick porous SSTC cathode layer, and a porous NiSDC anode. Fig. 12 presents the single-cell performance with SSTC cathode in the range of 550°C and 650°C with 100ml of humidified hydrogen as a fuel and 100ml of air as an oxidant. The maximum power density of this fuel cell increased with increasing temperature while the polarization resistance decreased with increasing temperature (Fig. 13). The maximum power densities reached at 300, 570 and 964 mW cm-2 at 550, 600 and 650 °C, respectively. The polarization resistance of this single-cell decreased from 0.232 Ω cm-2 at 550 °C to0.038 Ω cm-2 at 650 °C. Due to a 42 µm-thick dense SDC electrolyte layer in this study, the single-cell does not present the outstanding results compared to other research with only a 5~10 µm-thick dense SDC or GDC electrolyte[12, 13, 26]. We compare SSTC with BSCF using the same anode-supported single cell. In Fig. S1, SSTC obtained the maximum power density of 964 mW cm-2 while BSCF only obtained 880 mW cm-2 at 650 °C. This performance illustrates that SSTC could be a highperformance cathode for low-temperature SOFC.
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4. Conclusions In summary, a new SrSc0.175Ta0.025Co0.8O3-δ perovskite has been evaluated as the cathode for SOFCs, and it shows excellent electrochemical performance in the temperature range of 500°C and 700°C. This excellent performance is attributed to its parent compound, SrSc0.2Co0.8O3-δ. Partial replacement of Sc with Ta enhanced the ORR activity and reduced the TECs slightly. Nb5+ and Ta5+ have different electronegativity although they have similar ionic radii. The Ta-SSCO system presents a greater ORR activity than Nb-SSCO system. Ta–O bond is stronger than Nb–O bond and the lower electronegativity of Ta5+ leads to a lower valence of cobalt and more oxygen vacancies, resulting in the better performance of Ta-SSCO system.
Conflicts of interest There are no conflicts to declare. Associated content Supporting Information The supporting information to this article can be found.
Acknowledgements The research work was supported by the Australian Research Council Discovery Projects (DP170104660 and DP190101782).
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[18] Xia C, Rauch W, Chen F, Liu M. Sm0.5Sr0.5CoO3 cathodes for low-temperature SOFCs. Solid State Ionics. 2002;149(1):11-9. [19] Zhou W, Shao Z, Ran R, Cai R. Novel SrSc0.2Co0.8O3−δ as a cathode material for low temperature solid-oxide fuel cell. Electrochem Commun. 2008;10(10):1647-51. [20] Li M, Zhou W, Peterson VK, Zhao M, Zhu Z. A comparative study of SrCo0.8Nb0.2O3-d and SrCo0.8Ta0.2O3-d as low-temperature solid oxide fuel cell cathodes: effect of non-geometry factors on the oxygen reduction reaction. J Mater Chem A. 2015;3(47):24064-70. [21] Cascos V, Martínez-Coronado R, Alonso JA. New Nb-doped SrCo1−xNbxO3−δ perovskites performing as cathodes in solid-oxide fuel cells. Int J Hydrogen Energ. 2014;39(26):14349-54. [22] Zeng P, Ran R, Shao Z, Yu H, Liu S. Effects of scandium doping concentration on the properties of strontium cobalt oxide membranes. Brazilian Journal of Chemical Engineering. 2009;26:563-74. [23] Li M, Zhou W, Zhu Z. Comparative Studies of SrCo1−xTaxO3−δ (x=0.05–0.4) Oxides as Cathodes for Low-Temperature Solid-Oxide Fuel Cells. ChemElectroChem. 2015;2(9):1331-8. [24] Zhu Y, Sunarso J, Zhou W, Jiang S, Shao Z. High-performance SrNb0.1Co0.9xFexO3-d perovskite cathodes for low-temperature solid oxide fuel cells. J Mater Chem A. 2014;2(37):15454-62. [25] Eichel RA. Structural and dynamic properties of oxygen vacancies in perovskite oxides-analysis of defect chemistry by modern multi-frequency and pulsed EPR techniques. Phys Chem Chem Phys. 2011;13(2):368-84. [26] Lee JG, Park JH, Shul YG. Tailoring gadolinium-doped ceria-based solid oxide fuel cells to achieve 2 W cm−2 at 550 °C. Nat Commun. 2014;5:4045.
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Fig. 1 (a) Rietveld refinement plot of SSTC (blue—observed; orange—calculated; grey— the difference between observed and calculated) sintered at 1200 °C for 20 h in air.
(b) High-resolution TEM (HRTEM) and corresponding inverse fast
Fourier transform (IFFT) images showing the d-spacing for [110] plane. (c) In-situ XRD patterns for SSTC tested in air from 25 to 800 °C. (d) X-ray photoelectron spectroscopy profile of Ta cation in SSTC at room temperature
18
Fig. 2 Electrical conductivity of SSTC in air
100.5
0.65 SSTC-weight% SSTC-weight% SSTC-δ SSTC-δ 0.6
SSC-δ[17] SSC-δ [19]
99.5
0.55
δ
Weight (%)
100
99
0.5
98.5
0.45
98
0.4 300
400
500
600
700
800
Temperature (°C) Fig. 3 Oxygen vacancy content ( δ) and weight change of SSTC against temperature 19
Fig. 4 Thermal expansions of SSTC and SSC in the range of room temperature to 850 ºC
20
Fig.5 Oxygen temperature programmed desorption profiles of SSTC and SSC
Fig. 6 Cross-sections of SSTC cathode in a configuration of the symmetrical cell
21
Fig.7 XRD results of SSTC-SDC powders sintered at 1000 °C in air for 2 h
22
700
650
600
T / (°C) 550
500
10 SSC[10]
Rp (Ω cm2)
1
SSNC[12] SSTC
0.1
0.01
0.001 0.98
1.08
1.18
1.28
1.38
1000/T (K -1) Fig. 8 Area-specific resistance of SSTC in an SSTC|SDC|SSTC symmetrical cell in air
23
Fig. 9 Nyquist plots of the SSTC cathode at 500, 550, and 600ºC; (b) An equivalent circuit, Rohm(RE1 - CPE1)(RE2 -CPE2).
24
1
RE (Ω cm2)
0.1
R E1 - SSTC
0.01
R E2 - SSTC R E1 - SSC[10] R E2 - SSC[10] 0.001 450
500
550
600
650
700
Temperature (ºC) Fig.10 Temperature dependence of RE1 and RE2 for SSTC and SSC cathode
25
750
Figure 11. Cross-section morphology of an SSTC-based single fuel cell
26
1.0
1000 V-I 650°C
0.9
V-I 600°C
900
V-I 550°C P-I 650°C
800
P-I 600°C
Voltage ( V )
0.7
P-I 550°C
700
0.6
600
0.5
500
0.4
400
0.3
300
0.2
200
0.1
100
0.0 0
500
1000
1500
2000
2500
3000
3500
Power density (mW cm-2)
0.8
0 4000
Current density (mA cm-2) Fig. 12 I–V and I–P curves of the anode-supported single cell with SSTC at 550, 600 and 650 °C in wet hydrogen
27
Fig. 13 Impedance spectra of the anode-supported single cell with SSTC at open cell voltage at 550, 600 and 650 °C in wet hydrogen
28
Highlights A new SrSc0.175Ta0.025Co0.8O3-δ (SSTC) perovskite cathode synthesized for SOFCs Excellent electrochemical performance achieved between 500°C and 700°C The stronger Ta–O bond compared to Nb–O bond may lead to the improved performance of SSTC compared to SrSc0.175Nb0.025Co0.8O3-δ (SSNC)
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1359-8368(19)34260-X
DOI:
https://doi.org/10.1016/j.compositesb.2019.107491
Reference:
JCOMB 107491
To appear in:
Composites Part B
Received Date: 21 August 2019 Revised Date:
26 September 2019
Accepted Date: 27 September 2019
Please cite this article as: Xu X, Zhao J, Li M, Zhuang L, Zhang J, Aruliah S, Liang F, Wang H, Zhu Z, Sc and Ta-doped SrCoO3-δ perovskite as a high-performance cathode for solid oxide fuel cells, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107491. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Sc and Ta-doped SrCoO3-δ Perovskite as a High-Performance Cathode for Solid Oxide Fuel Cells Xiaoyong Xu
a,£
b£
a
a
a
a
, Jie Zhao , Mengran Li , Linzhou Zhuang , Jinxuan Zhang , Sathia Aruliah , Zhonghua Zhu
a,*
a. School of Chemical Engineering, The University of Queensland, St Lucia, Queensland, 4072, Australia b. School of Chemistry and Chemical Engineering, Anhui University, Hefei, China.
6 SSTC
5
SSNC[10] SSC[8]
-LN(ASR)
4 3 2 1 0 1
1.1
1.2 1000/T
1.3
1.4
Average area-specific resistance (ASR) of SSTC within a cathode|SDC|cathode symmetrical cell in air (SrSc0.175Ta0.025Co0.8O3-δ, SSTC; SrSc0.175Nb0.025Co0.8O3-δ, SSNC; SrSc0.2Co0.8O3-δ, SSC)
Sc and Ta-doped SrCoO3-δ Perovskite as a High-Performance Cathode for Solid Oxide Fuel Cells Xiaoyong Xu1£, Jie Zhao2£, Mengran Li1, Linzhou Zhuang1, Jinxuan Zhang1, Sathia Aruliah1, Fengli Liang1, Hao Wang3, Zhonghua Zhu1* 1
School of Chemical Engineering, The University of Queensland, Brisbane 4072 Australia
2
College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, PR China
3
Centre for Future Materials, University of Southern Queensland, Springfield Central, QLD 4300, Australia
£
Xu and Zhao made equal contributions to this paper
*Corresponding author, email: [email protected]
1
ABSTRACT Sealing and stability is the challenge for solid oxide fuel cells (SOFCs) at high temperature. It is crucial to operate SOFCs at a low temperature to avoid these issues. Thus, it is necessary to improve the sluggish oxygen reduction reaction (ORR) activity of cathode. The SrCoO3-δ perovskite with a cubic phase is identified as a potential cathode material for SOFCs due to its high oxygen permeability and reasonable electrical conductivity. However, it readily decomposes into the orthorhombic structured brownmillerite Sr2Co2O5 or hexagonal Sr6Co5O15 during high-temperature sintering resulting in the loss of a significant amount of lattice oxygen. Herein, we partially replace Co with Sc and Ta at SrCoO3-δ to stabilize its cubic perovskite structure and restrain the lattice oxygen loss, thus improving the oxygen-ionic conductivity, and the structural and chemical stability. The synthesized SrSc0.175Ta0.025Co0.8O3-δ (SSTC) cathode achieves a remarkably high ORR performance and an area-specific resistance obtained in this study reaches as low as 0.233, 0.033, and 0.004 Ω cm2 at 500, 600, and 700 °C, respectively. The new Sc and Ta-doped SrCoO3-δ perovskite cathode even present a better ORR activity than the previously reported Sc and Nb-doped SrCoO3-δ cathode. The reason for the improved performance by Ta doping is possible that Ta–O bond is stronger than Nb–O bond and the electronegativity of Ta5+ is lower than that of Nb5+, resulting in a lower valence state of cobalt and a higher oxygen vacancies concentration. KEYWORDS: Perovskite, cathode material, Solid oxide fuel cell, SrCoO3-δ, Electrochemistry, Oxygen reduction reaction
2
1
Introduction Solid oxide fuel cells (SOFCs) can produce electricity by directly converting
the chemical energy in fuel, and they are promising electrochemical energy conversion technology with excellent fuel flexibility and high energy efficiency [1-5]. The researchers[4] have devoted intensive effort in recent years on intermediate temperature SOFCs (IT-SOFCs) to make SOFC technology more stable and affordable; the operating temperature is decreased to reduce fabrication cost and sealing issues. Other benefits, such as quick start and cooling cycles, can also be enhanced as a result of reducing operating temperatures to 600 °C or below. However, the electrochemical activity of the cathode drops sharply with decreasing temperature[3, 6-13]. The development of advanced cathode materials with high-performance of ORR is crucial for IT-SOFCs. Researchers reached a consensus that mixed ionicelectronic conducting oxides (MIECs) should be a potentially promising candidate
for
IT-SOFCs
cathode.
MIECs
as
the
cathode
extend
the
electrochemical reaction zone from the limited three-phase boundary area (TPB) between cathode and electrolyte to the overall surface area of MIEC cathode leading to a higher electrochemical performance [6, 14, 15]. Among the cobaltcontaining materials, SrCoO3-δ (SCO) is identified as a promising perovskite parent compound to develop a series of cubic perovskite materials with high electronic conductivity and oxygen ionic conductivity [16]. However, it readily decomposes into the orthorhombic structured brownmillerite Sr2Co2O5 or hexagonal Sr6Co5O15 at high temperature by losing a significant amount of lattice oxygen[17].
3
Substituted A- or B- sites have been widely used to stabilize the cubic perovskite of SrCoO3-δ. It is a common strategy to dope a metal oxide with higher oxidation states to increase static electrical repulsion to prevent tilting of octahedral and compensate for the loss of lattice oxygen chemically, thus stabilize SCO's cubic perovskite structure. Several perovskites, such as Sm0.5Sr0.5CoO3-δ[18], δ[17],
SrSc0.2Co0.8O3-δ[16,
19],
SrCo1-xTaxO3-
SrNb0.2Co0.8O3-δ[20] and SrNb0.1Co0.9O3-δ[21], have been developed as the
cathode for IT-SOFCs operating at the temperature to 600 °C or below. Zeng et al. stabilized the cubic structure of SrCoO3-δ oxides by replacing Co by Sc[22]. Furthermore, the doping of a bit amount of Sc3+ could stabilize the cobalt ion at the lower valence state and the high spin state[16, 19]. The synergistic substitution is a common strategy to improve the oxygen-ionic conductivity.
The
perovskites,
SrSc0.175Nb0.025Co0.8O3-δ
(SSNC)[12]
and
SrTa0.1Nb0.1Co0.8O3-δ (STNC)[13] were reported by Zhou et al and our group with a high-performance of ORR at 550 °C. SSNC improved the oxygen reduction reactivity by more than 100% at 500 °C compared with the prominent cathode, Ba0.5Sr0.5Co0.2Fe0.8O3-δ (BSCF). Tantalum and niobium are under the same group on the periodic table. Furthermore, Ta5+ and Nb5+ have an identical ionic radius [17]. Previous researchers have consistently confirmed that the Nb–SCO system presents worse thermal / chemical/electrochemical/electrical stability than Ta–SCO[17, 20] although there are different explanations on it. Herein, we partially replace Co with Sc and Ta in SrCoO3-δ to stabilize its beneficial cubic perovskite structure and restrain the
4
lattice oxygen loss, thus improving the ionic conductivity, and the structural and chemical stability. The perovskite oxide SrSc0.175Ta0.025Co0.8O3-δ (SSTC) was synthesized and evaluated as a potential cathode material by a solid-state reaction. The phase structure and chemical compatibility with electrolyte material were evaluated by X-ray diffraction measurements (XRD), as well as electrical conductivity by four-probe measurement, and polarization resistance of the SSTC using symmetric cell test was assessed. The performance and electrochemical stability of the single-cell with SSTC as a cathode were also presented. Sc and Ta-doped SCO presents a very high electrochemical performance for SOFCs in the future.
2
Experimental Section
2.1 Material synthesis and characterization The perovskite oxide SrSc0.175Ta0.025Co0.8O3-δ (SSTC) was synthesized by solid-state reaction. A stoichiometric amount of oxide powders Sc2O3, SrCO3, Ta2O5 and Co3O4 (Sigma-Aldrich) in the ethanol media were mixed in a ball mill (Fritsch, Pulverisette 5) at a rotation rate of 300 rpm for 24 hours. Then, dry it in the oven at 80ºC for 24 hours. The dried solid powders were pressed into granules and calcined in static air at 1200°C for 20 hours to form the perovskite oxide. SSTC powder was compressed into discs and then sintering at 1200 °C for 20 hours and then polishing the discs into a bar shape for electrical conductivity and thermal expansion coefficient (TEC) testing. The dimensions are 7.5 mm × 4.0 mm × 12.5 mm. Netsch DIL 402C dilatometer was used to measure TECs from
5
25 °C to 850 °C with a heating rate of 5 °C/min and 50 ml min-1 of airflow rate. The relative density of the specimen is over 95%. A four-probe DC technique was used to measure the electrical conductivities by an Autolab PGSTAT30 electrochemical workstation. Oxygen temperature-programmed desorption (O2-TPD, Belcat, Japan) was used to monitor the oxygen desorption properties of the cathode materials from room temperature to 850 °C with a heating rate of 10 °C min-1. For an O2-TPD measurement, about 50 mg powders were kept in a U-type quartz tube. 30 ml min-1 of the carrier gas (pure argon) was passed through the tube. A mass spectrometer (Belmass, Japan) was used to monitor the effluent gases. Netsch DIL 402C dilatometer was used to measure TECs of SSTC and a fourprobe DC technique was used to measure the electrical conductivities by an Autolab
PGSTAT30
electrochemical
workstation.
The
initial
oxygen
stoichiometry (δ) is determined by the iodometric titration method[23]. The details are described in the supporting information. The crystalline structure and chemical compatibility of the SSTC sample were determined by XRD on a Bruker-AXS D8 Advance diffractometer using Cu-Kα radiation at 40 kV and 40 mA equipped with an Anton-Paar high-temperature HTK-1200N cell. In situ X-ray diffraction measurements, a high-temperature Rigaku Model Multiflex X-ray diffractometer using Cu-Kα radiation at 45 kV and 200mA was utilized to analyze the crystal structure. The data sets were recorded in a step-scan mode in the range of 20–80° with intervals of 0.02°.
6
X-ray photoelectron spectra (XPS) were obtained on a Kratos Axis ULTRA xray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer and 150 W (15 kV, 10 mA) monochromatic Al Kα (1486.6 eV) radiation. The binding energy was determined using a 284.8 eV C1s line from adventitious carbon as a reference. 2.2 Cell fabrication and testing For symmetrical cells, SDC powders were dry-pressed into discs, followed by sintering 1400°C for 5 hours to form dense SDC disks=. The SSTC slurry was prepared by mixing binder, a solvent with SSTC powder by ball milling Fritsch, Pulverisette 5) at a rotation rate of 400 rpm for 2 hours. The SSTC slurry was sprayed onto both sides of the SDC pellet symmetrically and subsequently calcined at 1000°C for 2 hours in static air. The silver slurry was brushed on the surface of the pellet as the current collector. The thickness of the cathode was controlled by spraying time (10 seconds, 6 times) and quantified by a scanning electron microscope (SEM). Electrochemical impedance spectra (EIS) of the symmetrical cells were measured in air using an Autolab PGSTAT30 electrochemical workstation. The frequency range was 100 kHz to 0.01 Hz, and the signal amplitude was 10 mV under open cell voltage conditions. Anode-supported cells were prepared via dual dry-pressing followed by hightemperature sintering. 40 wt% SDC and 60 wt% NiO were well mixed by ball milling as anode powders. The electrolyte was densified through firing at 1350 °C for 3 h in air. Afterward, the cathode slurry was sprayed onto the central surface of the dense electrolyte and fired at 1000°C for 2 h in air. The cathode
7
was approximately10 µm thick, and an effective area is about 0.28 cm2. The I–V polarization was measured using an Autolab PGSTAT30 with a four-terminal configuration. 100 ml min−1 (STP) of humidified (3% H2O) H2 fuel was passed into the anode chamber and 100 ml min−1 (STP) of instrument air wad fed into the cathode side.
3
Results and Discussion
3.1 Structural characterization Pure SSTC was synthesized by calcining the mixture of oxides at 1200 °C for 20 h in air. These peaks indicate that the crystal synthesis of SSTC is good, and the diffraction peaks can be well identified as a pure cubic perovskite structure[12]. The Rietveld refinement fitting results are depicted in Fig.1a with reliability factors of Rwp = 0.0124 and Rp = 0.0124 with the goodness of fit = 2.13 and the d spacing value of 0.277 nm for 110 planes. The d spacing value is consistent with high-resolution TEM result (Fig. 1b) which presents a similar d spacing value for 110 planes, denoting the oxygen vacancy ordering in the cubic perovskite. To confirm the stability of SSTC in the working condition, high-temperature XRD measurement was performed in air from 25 °C to 800 °C. No decomposition was observed for SSTC in this temperature range (Fig. 1c), indicating that SSTC is thermally stable. Additionally, the XRD diffraction peaks shift to a lower diffraction angle at a higher temperature, revealing an obvious lattice expansion at high temperature. Such expansion is attributed to the
8
reduction of tri- and tetra-valent Co species to a lower valence with larger ionic sizes at elevated temperature. In Fig 1 (a) the first peak is observed at approximately 23º but no peaks in Fig 1 (c) at the same position. This may be caused by a different accuracy of two XRD instruments. XPS results in Fig. 1(d) presents that the binding energy (B.E.) of Ta 4f7/2 (25.64 eV) assigned to Ta5+, indicating the 5+ charge on Ta cations in SSTC[20]. For Sc3+, interestingly, there is no obvious peak in the binding energy of 410 eV to 400 eV. Based on the previous research on SSC[19] and SSNC[12], we consider the same 3+ charge on Sc cations in SSTC. 3.2 Electrical conductivity The electrical conductivity of SSTC in the air between 400 and 700 °C was carried out by a 4-probe DC conductivity method. Fig.2 presents a drop in electrical conductivity with temperature. This behaviour of electrical conductivity against temperature is similar to a semiconductor, caused by a decrease in the concentration of electronic defects with increasing temperature due to more oxygen vacancy in SSTC as elevated the temperature. SrSc0.175Ta0.05Co0.8O3-δ has an electrical conductivity of 12-23.7 S cm-1 in the air between 400 and 700 °C. Compared to SrCoO3-δ (4.6–20 S cm-1) and SrSc0.2Co0.8O3-δ (4.6-25 S cm-1) in air between 300 and 900 °C [16], SrSc0.175Ta0.05Co0.8O3-δ perovskite didn’t show the improvement in the electrical conductivity. Both Sc3+ and Ta5+ have the fixed valence states, which may act as a block for the electrons between oxygens and cobalt ions in the Zerner double exchange[22].
9
3.3 Oxygen stoichiometry TGA studies were performed to investigate the oxygen vacancy content (δ) for SSTC at different temperatures. The initial oxygen stoichiometry (δ0) in the SSTC perovskite was obtained to be 0.40 using the iodometric titration. The results are shown in Fig. 3. SSTC oxygen vacancy concentration steadily increases with temperature due to the reduction of the cobalt ions as evidenced by the lattice expansion shown in Fig. 1(c). The oxygen nonstoichiometry of SrSc0.2Co0.8O3-δ reached a value as high as 0.548 at room temperature[10]. Substituting Ta for Sc in SrSc0.2Co0.8O3-δ decreases the oxygen non-stoichiometry because Ta has a high 5+ fixed oxidation state, which increases the total charge of the cations, and therefore requires more oxygen ions to compensate for the charge imbalance[23]. 3.4 Thermal expansion coefficient Fig. 4 presents the comparison of thermal expansion curves of SSTC and SSC in the range of room temperature to 850 °C in air. Both curves show an inflexion at 450 ~500 °C corresponding to a reduction of teltra-cobalt ion to tri-cobalt ion [24]. This inflexion is also indicated by oxygen temperature-programmed desorption (O2-TPD) results (Fig. 5). Then, there is a reason to believe that the increased slope above 500 °C is related to the loss of lattice oxygen. With Sc and Ta-doped SC, SSTC shows a slightly lower thermal expansion coefficient than SSC. It can be explained that SSC converts more Co4+ to Co3+ or Co3+ to Co2+ than SSTC, and consequently expanding the lattice.
10
3.5 Electrode performance Fig. 6 shows that the porous cathode is still attached intimately to the SDC surface after testing. This demonstrates that SSTC cathode material adheres well to the electrolyte and the thickness of SSTC cathode is approximately 15 µm. We also checked the compatibility of SSTC with the electrolyte SDC by mixing SDC with SSTC (1:1 wt%) followed by sintering at 1000 °C in static air for 2 h. Fig. 7 shows that no extra peaks were detected for the mixture of SSTC and SDC, which indicates that the chemical interactions between the SSTC and SDC at 1000 °C are negligible. ORR activity of SSTC cathodes was characterized by electrochemical impedance spectroscopy (EIS) using symmetrical cells with SDC as an electrolyte. ASR is normalized by the electrode area of the cathode polarization resistance and it. may also be calculated by impedance spectra obtained under open-circuit conditions. ASR is the difference between the real axis intercepts of the impedance spectrum, that is the electrode polarization resistance (Rp). ASR can describe the performance of SOFC cathodes: higher ASR value reflects less active ORR activity. Fig. 8 shows the ASR values of SSTC cathode between 500 and 700 °C. The ASR values are extremely low. They are only 0.233, 0.090, 0.033, 0.011, and 0.004 Ω cm2 at 500, 550, 600, 650 and 700 °C, respectively. As a reference, Wei et al[19] reported the ASRs of the SrSc0.2Co0.8O3-δ cathode are 0.415, 0.206, 0.095, 0.044, and 0.023 Ω cm2 at 500, 550, 600, 650 and 700 °C, respectively. Later, they replaced a small amount of Sc by Nb in SrSc0.2Co0.8O3-δ (SSCO) and demonstrate that there are two oxygen ion transport paths with very
11
low migration barrier energies, which are produced by partially replacing Co atoms with Nb and Sc atoms into SrCoO3-δ. The ASRs of the SSNC cathode are 0.32, 0.11, 0.04, 0.02, and 0.009 Ω cm2 at 500, 550, 600, 650 and 700 °C, respectively, approximately 120-230% higher than those of the SSTC cathode. This indicates that the SSNC cathode has a worse performance of ORR than SSTC at intermediate temperatures. Previous research show consistently that Ta–SCO system presents better thermal/electrical/electrochemical/chemical stability than Nb–SCO because Ta–O bonding is stronger than Nb–O [17, 20]. In this study, the Ta-SSCO cathode also presents the advantage over the Nb-SSCO system. To understand the reason why SSTC cathode presents the excellent performance, we measured the electrical conductivity of SSTC, which is even lower than its counterpart, SSNC. This indicates that the high electronic conductivity is not the main reason for the enhanced ORR activity.. This phenomenon is also observed by a previous study[24]. We also measured the oxygen vacancy concentration of SSTC, which has a slightly higher oxygen vacancy of 0.43-0.50 in the air between 400 and 700 °C than SSNC (0.41-0.465 in the air between 400 and 700 °C). The formation of oxygen vacancies will diminish the charge carriers and thereby degrade the electronic conductivity. Therefore, a lower electrical conductivity of SSTC than SSNC can be explained by their higher oxygen vacancy formed in the lattice. Oxygen vacancies is a crucial parameter in perovskite oxides owing to its impact on material properties including the ionic charge transport[25]. Itis generally expected that increasing the oxygen vacancies will increase oxygen ionic conductivity[15]. This may contribute to the high ORR of SSTC. If compared to
12
SSNC, the SSTC presents a greater ORR activity. As explained by the previous researchers[17, 20], Nb5+ and Ta5+ have different electronegativity although similar ionic radii.. The main reason for the enhanced performance is that Ta–O bond is stronger than Nb–O bond and the lower electronegativity of Ta5+ leads to a bit lower valence of cobalt and more oxygen vacancies. XPS results in Fig. S2 confirms that SSTC presents a lower valence of cobalt than SSNC. To further investigate the effect of Ta in SSTC cathode, Nyquist plots of the EIS of SSTC cathode at 500, 550 and 600˚C are presented in Fig.9a. Based on an equivalent circuit, Rohm(RE1 - CPE1)(RE2 -CPE2), the impedance spectra can be divided into two half-cycles as shown in Fig. 9b, the details of this theory is described in the supporting information[10]. The calculated values of RE1 and RE2 of the SSTC cathode are presented in Fig. 10, and the values for SSC from other researchers[10] are also included. For SSTC, RE2 values are much higher than RE1 values at the low temperature (500°C, 550°C and 600°C) while they are almost same at the high temperature (650°C and 700°C). RE2 values are much higher than RE1 values at the low temperatures range of 500°C to 600°C, which means that the diffusion process is the rate-controlling step of ORR for SSTC. Therefore, with a small amount of Tadoped SSC, the rate-controlling step of ORR for SSC changes from the chargetransfer process to the diffusion process in the SSTC, indicating that Ta5+ in the SSTC cathode benefits the mobility of oxygen ion.
13
The performance of an anode-supported single fuel cell with SSTC cathode was then investigated. Scanning electron microscopy is used to characterize the cross-section y of this single fuel cell; as shown in Fig. 11, the single cell is composed of a porous anode supporting layer, a 42 µm-thick dense SDC electrolyte layer, and a 15 µm-thick porous SSTC cathode layer, and a porous NiSDC anode. Fig. 12 presents the single-cell performance with SSTC cathode in the range of 550°C and 650°C with 100ml of humidified hydrogen as a fuel and 100ml of air as an oxidant. The maximum power density of this fuel cell increased with increasing temperature while the polarization resistance decreased with increasing temperature (Fig. 13). The maximum power densities reached at 300, 570 and 964 mW cm-2 at 550, 600 and 650 °C, respectively. The polarization resistance of this single-cell decreased from 0.232 Ω cm-2 at 550 °C to0.038 Ω cm-2 at 650 °C. Due to a 42 µm-thick dense SDC electrolyte layer in this study, the single-cell does not present the outstanding results compared to other research with only a 5~10 µm-thick dense SDC or GDC electrolyte[12, 13, 26]. We compare SSTC with BSCF using the same anode-supported single cell. In Fig. S1, SSTC obtained the maximum power density of 964 mW cm-2 while BSCF only obtained 880 mW cm-2 at 650 °C. This performance illustrates that SSTC could be a highperformance cathode for low-temperature SOFC.
14
4. Conclusions In summary, a new SrSc0.175Ta0.025Co0.8O3-δ perovskite has been evaluated as the cathode for SOFCs, and it shows excellent electrochemical performance in the temperature range of 500°C and 700°C. This excellent performance is attributed to its parent compound, SrSc0.2Co0.8O3-δ. Partial replacement of Sc with Ta enhanced the ORR activity and reduced the TECs slightly. Nb5+ and Ta5+ have different electronegativity although they have similar ionic radii. The Ta-SSCO system presents a greater ORR activity than Nb-SSCO system. Ta–O bond is stronger than Nb–O bond and the lower electronegativity of Ta5+ leads to a lower valence of cobalt and more oxygen vacancies, resulting in the better performance of Ta-SSCO system.
Conflicts of interest There are no conflicts to declare. Associated content Supporting Information The supporting information to this article can be found.
Acknowledgements The research work was supported by the Australian Research Council Discovery Projects (DP170104660 and DP190101782).
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Fig. 1 (a) Rietveld refinement plot of SSTC (blue—observed; orange—calculated; grey— the difference between observed and calculated) sintered at 1200 °C for 20 h in air.
(b) High-resolution TEM (HRTEM) and corresponding inverse fast
Fourier transform (IFFT) images showing the d-spacing for [110] plane. (c) In-situ XRD patterns for SSTC tested in air from 25 to 800 °C. (d) X-ray photoelectron spectroscopy profile of Ta cation in SSTC at room temperature
18
Fig. 2 Electrical conductivity of SSTC in air
100.5
0.65 SSTC-weight% SSTC-weight% SSTC-δ SSTC-δ 0.6
SSC-δ[17] SSC-δ [19]
99.5
0.55
δ
Weight (%)
100
99
0.5
98.5
0.45
98
0.4 300
400
500
600
700
800
Temperature (°C) Fig. 3 Oxygen vacancy content ( δ) and weight change of SSTC against temperature 19
Fig. 4 Thermal expansions of SSTC and SSC in the range of room temperature to 850 ºC
20
Fig.5 Oxygen temperature programmed desorption profiles of SSTC and SSC
Fig. 6 Cross-sections of SSTC cathode in a configuration of the symmetrical cell
21
Fig.7 XRD results of SSTC-SDC powders sintered at 1000 °C in air for 2 h
22
700
650
600
T / (°C) 550
500
10 SSC[10]
Rp (Ω cm2)
1
SSNC[12] SSTC
0.1
0.01
0.001 0.98
1.08
1.18
1.28
1.38
1000/T (K -1) Fig. 8 Area-specific resistance of SSTC in an SSTC|SDC|SSTC symmetrical cell in air
23
Fig. 9 Nyquist plots of the SSTC cathode at 500, 550, and 600ºC; (b) An equivalent circuit, Rohm(RE1 - CPE1)(RE2 -CPE2).
24
1
RE (Ω cm2)
0.1
R E1 - SSTC
0.01
R E2 - SSTC R E1 - SSC[10] R E2 - SSC[10] 0.001 450
500
550
600
650
700
Temperature (ºC) Fig.10 Temperature dependence of RE1 and RE2 for SSTC and SSC cathode
25
750
Figure 11. Cross-section morphology of an SSTC-based single fuel cell
26
1.0
1000 V-I 650°C
0.9
V-I 600°C
900
V-I 550°C P-I 650°C
800
P-I 600°C
Voltage ( V )
0.7
P-I 550°C
700
0.6
600
0.5
500
0.4
400
0.3
300
0.2
200
0.1
100
0.0 0
500
1000
1500
2000
2500
3000
3500
Power density (mW cm-2)
0.8
0 4000
Current density (mA cm-2) Fig. 12 I–V and I–P curves of the anode-supported single cell with SSTC at 550, 600 and 650 °C in wet hydrogen
27
Fig. 13 Impedance spectra of the anode-supported single cell with SSTC at open cell voltage at 550, 600 and 650 °C in wet hydrogen
28
Highlights A new SrSc0.175Ta0.025Co0.8O3-δ (SSTC) perovskite cathode synthesized for SOFCs Excellent electrochemical performance achieved between 500°C and 700°C The stronger Ta–O bond compared to Nb–O bond may lead to the improved performance of SSTC compared to SrSc0.175Nb0.025Co0.8O3-δ (SSNC)
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: