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Highly active and CO2-tolerant Sr2Fe1.3Ga0.2Mo0.5O6-δ cathode for intermediate-temperature solid oxide fuel cells
Journal of Power Sources 450 (2020) 227722
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Highly active and CO2-tolerant Sr2Fe1.3Ga0.2Mo0.5O6-δ cathode for intermediate-temperature solid oxide fuel cells Chunming Xu a, Kening Sun a, Xiaoxia Yang a, Minjian Ma a, Rongzheng Ren a, Jinshuo Qiao a, Zhenhua Wang a, Shuying Zhen b, Wang Sun a, * a b
Beijing Key Laboratory for Chemical Power Source and Green Catalysis, Beijing Institute of Technology, Beijing, 100081, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� SFGM are firstly evaluated as the cath ode for flat tubular solid oxide fuel cell. � Ga-doping enhanced the oxygen-ion transport capability of SFM-based materials. � A low polarization resistance of 0.099 Ω cm2 at 750 � C for SFGM cathode. � SFGM exhibits outstanding CO2 tolerance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid oxide fuel cells Cathode Perovskite oxides CO2 tolerance Oxygen reduction reaction
The development of cathode materials, with high catalytic activity toward oxygen reduction reaction (ORR), structural stability and CO2 tolerance, is an important research direction for the successful realization of intermediate-temperature solid oxide fuel cells (IT-SOFCs). Herein, a novel double perovskite mixed ionic conductor, i.e., Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM), is developed by Ga-doping at Fe-sites of Sr2Fe1.5Mo0.5O6-δ (SFM) and evaluated as a cathode material in IT-SOFCs. At 750 � C, the area specific resistance (ASR) of SFGM cathode is found to be 0.099 Ω cm2, which is ~50% lower than SFM cathode (0.224 Ω cm2) in 20% O2/N2 atmosphere. Moreover, SFGM exhibits outstanding CO2 tolerance due to its excellent CO2 adsorption resistance compared with SFM. The ASR of SFGM remains stable at ~0.13 Ω cm2 during 100 h of continuous operation in 5% CO2containing air. In addition, a large-sized SFGM cathode (30 cm2) is utilized in anode-supported flat-tube SOFCs to demonstrate the potential of SFGM in practical applications. At 750 � C, the as-prepared SFGM-based single-cell provides a stable power of 12 W for 291 h in 5% CO2-containing air. The superior electrochemical performance and outstanding CO2 tolerance of SFGM are promising features for the rapid development of IT-SOFCs.
1. Introduction Solid oxide fuel cells (SOFCs) are considered as promising energy
conversion devices due to their high efficiency and environmentfriendly nature. Moreover, SOFCs superior fuel adaptability than other fuel cell systems, which makes them an attractive choice for fossil-fuel to
* Corresponding author. E-mail address: [email protected] (W. Sun). https://doi.org/10.1016/j.jpowsour.2020.227722 Received 21 October 2019; Received in revised form 16 December 2019; Accepted 6 January 2020 Available online 29 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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hydrogen-fuel transition [1–3]. In order to accelerate the commerciali zation of SOFCs, the operating temperature should be reduced, from the traditional high temperature (HT, 900–1000 � C) to an intermediate-temperature (IT, 600–800 � C), to decrease the stack cost and increase the lifetime [4]. However, despite the enhanced durability, the lower operating temperature leads to the inferior electrochemical activity of electrode materials [5–7]. Therefore, a lot of research effort is directed to enhance the durability, reduce the cost and prolong the service life of SOFC cathodes, while maintaining the high oxygen reduction activity at a lower operating temperature. During the past 20 years, simple perovskites, double perovskites, Ruddlesden–Popper phases and other kinds of mixed ionic electronic conducting (MIEC) oxides have been widely investigated as cathode materials and rendered substantial achievements in the field of ITSOFCs. In order to develop a cathode material with excellent oxygen reduction reaction (ORR) properties, a great deal of research effort has been focused on perovskites (ABO3) and double perovskites (AA’B2O6), such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), doped SrCoO3-δ (SCO) and PrBaCo2O5þδ (PBCO), demonstrating outstanding performance in ITand LT-SOFCs [3,8–11]. However, a large number of studies have shown that, when the cathode gas contains CO2 impurities, the alkaline earth elements (Ba, Sr) at the A-site of perovskite segregate and react with CO2 to form a high-impedance carbonate phase, leading to reduced ORR activity [12,13]. Hence, the practical utilization of perovskites is hin dered by the inherent chemical instability in CO2 environment. For instance, a promising intermediate temperature cathode, i.e., BSCF, renders performance degradation due to CO2 poisoning, where CO2 in duces Ba/Sr enrichment at A-site and leads to the formation of Ba-rich hexagonal secondary phase, resulting in poor durability of the elec trode material [12,14]. For example, the Ba0.5Sr0.5Co0.8Fe0.2O3-δ cath ode was exposed to an air containing 10% CO2 for 5 min at 600 � C, and the impedance value decreased by about 20 times [15]. In practical applications, the cathode material must possess excellent chemical sta bility to resist the corrosion induced by CO2 and other impurity gases. It has been reported that the partial substitution of B-site transition metal cation by Ti4þ, Zr4þ, Ta5þ, Sb5þ or Nb5þ can lead to enhanced CO2 resistance of various MIEC perovskite oxides, such as SrCoO3 δ [16,17], SrFe0.8Mo0.2O3 δ [18], Bi0.5Sr0.5FeO3 δ [19] and Pr0.6Sr0.4FeO3-δ [20]. However, the doping of high valence cation reduces the concentration of oxygen vacancies, leading to inferior oxygen transport performance [21]. For instance, when the temperature is 800 � C, the impedance value of the SrFe1 xSbxO3 δ (SFS x ¼ 0.05, 0.10, 0.15, 0.20) cathode is higher than that of SrFeO3 (~0.044 Ω cm2) and increases from 0.11 to 0.13 Ω cm2 as the Sb content increases [21]. In recent years, the rapidly developing Fe-based double perovskite Sr2Fe1.5Mo0.5O6-δ (SFM) is considered as a promising IT-SOFC cathode due to its structural stability, matching thermal expansion coefficient (TEC) with electrolyte, excellent ion exchange capacity and excellent ORR performance [22,23]. In addition to SOFCs, SFM-based perovskite materials are also being studied as cathode materials in solid oxide electrolytic cells for direct electrolysis of CO2, rendering splendid chemical stability under CO2 atmosphere [24–26]. One should note that, despite the lower electro-catalytic performance of SFM-based perov skites than SrCoO3-based materials, such as the superior structural sta bility and optimal TEC make them promising cathode materials for SOFCs. Previously reports, we have demonstrated that the replacement of Bsite Fe element with a transition metal, such as nickel or cobalt, induces the formation of oxygen vacancies, affects the electronic structure of Bsite cation and, thereby, improves the electronic and ionic conductivity of SFM [27]. However, despite the improved ORR performance, Ni- and Co-doping degrades the structural stability of SFM [28]. In another report, we have utilized a low-valence Sc element to replace Fe, which improved the ORR performance without destroying the structure of cathode. These results indicate the potential of another low-valence element, i.e., Ga3þ [29].
Recently, Manthiram et al. have reported that the stability and per formance of YBaCo4O7 materials can be significantly improved by Gadoping [30,31]. And, the addition of Ga into Pr2(Ni0.75Cu0.25) O4-δ-based MIEC increased the oxygen permeability [32,33]. In addi tion, Ga-doping also contributed to the improved structural stability of Fe-based perovskite, i.e., La0.6Sr0.4Ga0.3Fe0.7O3 (LSGF) [34–36]. Herein, we aimed to replace the B-site Fe element in SFM with Ga element to simultaneously optimize the structural stability and ORR performance as cathode material in SOFCs. The influence of Ga-doping and CO2 poisoning on structural stability, ORR performance and re covery capability has been systematically investigated. Moreover, in order to comprehensively investigate the practical application potential of SFGM, it was applied to large-size flat-tube SOFCs. The current research provides a novel strategy to fabricate novel high-performance cathode materials for IT-SOFCs. 2. Experimental procedure 2.1. Synthesis Sr2Fe1.5Mo0.5O6-δ (SFM) and Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM) pow ders were synthesized by using the combustion method. Briefly, stoi chiometric quantities of Sr(NO3)2, Fe(NO3)3�9H2O, Ga(NO3)2�xH2O and (NH4)6Mo7O24�4H2O salts were dissolved in an appropriate amount of deionized water and a certain proportion of citric acid, followed by the addition of glycine as a combustion promoter. The resulting clear solution was stirred in a water bath at 80 � C until it attained a sol-gel form and, then, the combustion was carried out at 250 � C for 1 h. Then, the as-received powder was ground and calcined in a muffle furnace at 1050 � C for 4 h in air [37–39]. 2.2. Characterization The crystal structure and phase analysis were performed by using Xray diffractometer (XRD, Panalytical X’Pert Pro). The XRD data were analyzed by using the Rietveld analysis with GSAS program and EXPGUI interface. The microstructure of the as-received powders was observed by scanning electron microscopy (SEM, FEI QUANTA-250) and highresolution transmission electron microscopy (HRTEM, JEM-2100F) The chemical composition and oxidation state of different atoms were determined by X-ray photoelectron spectroscopy (XPS, MULT1 LAB2000, VG). The electrical conductivity was measured by using a Keithley 2400 source meter and four-probe method in air. The electrical conductivity relaxation (ECR) measurements were performed by using the four-probe method. O2-temperature-programmed desorption (TPD) experiments were carried out by using a micromeritics apparatus (Chembet Pulsar TPR/TPD). The Electrochemical impedance spectroscopy (EIS) was measured by using LSGM (La0.9Sr0.1Ga0.8Mg0.2O3, Fuel Cell, USA), as the electrolyte, and GDC (Gd0.2Ce0.8O1.95, GDC, Fuel cell materials Co., USA), as the barrier layer. The electrochemical properties were assessed by assem bling SFM|GDC|LSGM|GDC|SFM and SFGM|GDC|LSGM|GDC|SFGM symmetric cells. Herein, the electrode materials were prepared by screen printing and sintered at 1100 � C for 2 h. EIS was carried out by using the symmetrical cell in N2/O2 mixture. The electrochemical workstation (Autolab 302 N), with an AC amplitude of 10 mV, was used to collect EIS spectra in the frequency range of 0.01–100 kHz. 2.3. Single-cell fabrication The electrochemical performance of the single-cell was assessed by using an anode-supported flat-tube SOFC with an effective cathode area of 30 cm2. A flat-tube anode carrier was prepared by extrusion molding. First, NiO (55 wt %, JT Baker Co., USA) and yttria-stabilized zirconia (45 wt %, 8 mol% YSZ, Tosoh Co., Japan) graphite, as pore former, were ball-milled in ethanol for 24 h, followed by drying. Then, an organic 2
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binder and distilled water were added to prepare an anode slurry. The extruded anode support was dried at 80 � C for 48 h and pre-sintered at 1100 � C [40]. The YSZ electrolyte layer was prepared by dip-coating on the flat-tube anode and ceramic interconnector was screen-printed on one side of the anode. After sintering at 1400 � C for 5 h, the barrier slurry (Gd0.2Ce0.8O1.95, GDC, Fuel cell materials Co., USA) was pasted on the electrolyte and sintered at 1300 � C for 5 h, which was employed as a buffer layer between the cathode and the electrolyte to prevent the interface reaction and inter-diffusion of ionic species [41]. Finally, as-prepared SFGM and SFM cathodes were screen-printed on the barrier layer and sintered at 1150 � C for 2 h [42].
stoichiometric ratio of Sr, Fe, Ga and Mo elements was found to be 2.00, 1.24, 0.19, 0.46, respectively, which is consistent with the designed composition of SFGM. 3.2. Physicochemical characterization The catalytic oxygen activation and oxygen-ion transfer capability are defined as performance indices for cathode materials in IT-SOFCs. Fig. 2a presents the oxygen-ion transport mechanism of perovskite materials, which indicates that the surface exchange of oxygen-ions and chemical bulk diffusion dictate the oxygen-ion transport process [45]. The electrical conductivity relaxation (ECR) measurements directly reflect the oxygen-ion conductivity of SFM and SFGM electrodes. As presented in Fig. 2b, the normalized test results show that, at 750 � C, the relaxation time is obviously decreased from 7520 to 2513 s, the surface oxygen exchange coefficient (Kchem) is increased from 8.71 � 10 5 cm s 1 to 23.2 � 10 5 cm s 1, and the chemical bulk diffusion coefficient (Dchem) increased from 9.03 � 10 6 cm2 s 1 to 32.1 � 10 6 cm2 s 1 after Ga-doping. The increased Kchem and Dchem indicate that the Ga-doping enhanced the oxygen-ion transport capability of SFM-based materials, which may lead to superior ORR performance. In order to further reveal the influ ence of Ga-doping on ORR performance, the valence state and electronic structure of surface elements, which are closely related to the ORR ki netics, have been analyzed by XPS. Fig. 2c presents the high-resolution O 1s XPS spectrum from SFGM powder, which can be deconvoluted into three peaks. The peak, located at 528.5 eV, can be assigned to the lattice oxygen (lat-O), whereas the peak, located at 529.7 eV, can be ascribed to chemically adsorbed oxygen (chem-O, O-/O2 ). The 3rd peak, located at ~531.3 eV), can be assigned to the physically adsorbed oxygen due to hydroxyls (OH ) or carbonate (CO23 ) species [25,43,46]. The fitting results showed that the proportion of chem-O in SFM and SFGM mate rials was 19.9% and 24.9%, respectively. One should note that the proportion of chem-O is directly associated with the amount of surface oxygen vacancies [47], which indicates that Ga-doping increased the concentration of oxygen vacancies. Moreover, the binding energy of lattice oxygen in SFM and SFGM cathodes was found to be 528.5 and 528.7eV, respectively, which may be caused by the difference in elec tronegativity between Fe2þ and Fe4þ [37,48]. The decrease of coulombic force between B-site ions and O2 indicates the increase of oxygen activity in the lattice and enhanced oxygen-ion transfer
3. Results and discussion 3.1. Element composition and crystal structure In order to determine the influence of Ga-doping on crystal structure, the synthesized Sr2Fe1.5Mo0.5O6-δ (SFM) and Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM) were analyzed by X-ray diffraction (XRD) and the general structure analysis system (GSAS) was utilized for Rietveld refinement. As shown in Fig. 1a and b, the refined XRD patterns confirm the cubic double perovskite structure, with a space group of F m-3m, of both SFM and SFGM cathodes. However, Ga-doping increased the lattice param eter from a ¼ 7.838 Å and V ¼ 481.522 Å3 to a ¼ 7.906 Å and V ¼ 494.163 Å3, respectively. Furthermore, selected area electron diffrac tion (SAED) and high-resolution transmission electron microscopy (HRTEM) were employed to further analyze the crystal structure of SFGM. SAED patterns and HRTEM images, along [001] (Fig. 1c, e) and [111] (Fig. 1d, f) axes, confirm the cubic double perovskite structure, with a space group of F m-3m, of SFGM. Moreover, the observed lattice parameters are consistent with XRD results. X-ray photoelectron spectroscopy (XPS) of SFM and SFGM powders was carried out to confirm the successful doping of Ga into the perov skite crystal lattice. As shown in Fig. S1, Sr 3d (133 eV), Fe 2p (710 eV), O 1s (532 eV) and Mo 3d (232 eV) have been detected in XPS spectra of both SFM and SFGM cathodes [37,43]. Fig. S2 shows that a weak Ga peak (�1120 eV) is present in XPS spectrum of SFGM, which has not been observed in high-resolution XPS spectrum of SFM powder [44]. In addition, the energy dispersive X-ray (EDX) results (Fig. S3) confirmed that Ga is uniformly distributed in SFGM without any observable sepa ration. The distribution of Ga is as uniform as Sr, Fe, Mo and O, and the
Fig. 1. Crystal structure of SFM and SFGM cathodes: (a) and (b) Rietveld-refined XRD profiles; SAED patterns along (c) [001] and (d) [111] zone axis for SFGM; and HRTEM images along (e) [001] and (f) [111] direction for SFGM. 3
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Fig. 2. Physicochemical properties of SFM and SFGM cathodes: (a) the oxygen-ion transport mechanism of perovskite materials; (b) ECR, measured at 750 � C; (c) high-resolution O 1s XPS spectrum; (d) high-resolution Fe 2p XPS spectrum; (e) O2-TPD curves; and (f) TG curves in the temperature range of 400–800 � C in air.
capability [43]. Fig. 2d, Fig. S4 and Fig. S5 provide high-resolution Fe 2p, Mo 3d and Sr 3d XPS spectra of SFM and SFGM cathodes, respec tively. And, the area ratios of each peak for Fe 2p and Mo 3d are sum marized in Table S1 and Table S2, respectively. Owing to Ga-doping, the peak position of Fe4þ, Fe3þ, Fe2þ and Sr2þ moved towards lower binding energy, which indicates lower bonding energy between metallic ions and oxygen in perovskite oxides, leading to superior oxygen-ion transfer capability [49]. The influence of Ga-doping on the formation of oxygen vacancies at elevated temperatures has been investigated by oxygen temperature program desorption (O2-TPD) test and thermos gravimetric analysis (TGA) in air. Fig. 2e presents the O2-TPD curves of SFM and SFGM cathodes, where the area of the desorption peak is approximately equal to the amount of chemically absorbed oxygen. Moreover, the content of
lost lat-O due to the change of valence state of metallic ions decreased with increasing temperature. The desorbed peak area of chemically absorbed oxygen, around 400 � C, for SFGM cathode is found to be higher than SFM, which is consistent with XPS analysis. Furthermore, TGA results are completely consistent with O2-TPD results (Fig. 2f). In TGA curves, the weight loss corresponds to the desorption peak area in O2TPD and, at 800 � C, the weight loss of SFGM (1.82%) is found to be higher than SFM (0.92%) due to Ga-doping. 3.3. Electrochemical activity Furthermore, electrochemical impedance spectroscopy (EIS) was employed to assess the electro-catalytic activity of SFM and SFGM electrodes in symmetric cell configuration. Fig. 3a shows the area
Fig. 3. Electrochemical characterization of SFM and SFGM cathodes: (a) ASR values in the temperature range of 650–800 � C; (b) EIS of SFGM cathode as a function of oxygen partial pressure at 750 � C; (c) DRT of SFGM cathode as a function of oxygen partial pressure at 750 � C; (d) DRT results of SFM and SFGM cathodes at 750 � C and the oxygen partial pressure of 0.2 atm; and impedance spectra of (e) SFM- and (f) SFGM-based symmetric cells at 750 � C and the oxygen partial pressure of 0.2 atm. 4
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specific resistance (ASR) of SFM and SFGM cathodes in the temperature range of 650–800 � C, measured in ambient air. The Nyquist plots of both cathodes at different temperatures are presented in Fig. S6. It is worth noting that the value of ASR is an important criterion to evaluate the activity of SOFCs electrode, where the lower ASR value corresponds to better the electro-catalytic performance. As shown in Fig. 3a, both SFM and SFGM electrodes rendered typical electrochemical impedance spectra (EIS) in symmetric cells, i.e., SFM|GDC|LSGM|GDC|SFM and SFGM|GDC|LSGM|GDC|SFGM. The area specific resistance (ASR) of SFGM cathode is significantly lower than the SFM cathode. At 750 � C, the ASR value of SFGM and SFM cathodes was found to be 0.245 Ω cm 2 and 0.628 Ω cm2, respectively, which indicates that SFGM cathode renders superior electro-catalytic performance. The ORR kinetics of the cathode are also influenced by external factors, such as reaction temperature and cathode gas composition. In order to clarify the ORR mechanism of SFGM electrode and influence of Ga-doping on ORR performance, the electrochemical impedance spec troscopy of SFM- and SFGM-based symmetric cells has been carried out under different partial pressures of oxygen at 750 � C. The EIS data have been further analyzed by DRT. Fig. 3b and Fig. S7 show the Nyquist plots of SFGM- and SFM-based symmetric cells, respectively. The EIS data were first analyzed by using the relaxation time distribution (DRT), as shown in Fig. 3c and Fig. S7. The characteristic time constants can be obtained from the high-frequency (HF), intermediate-frequency (IF) and low-frequency (LF) regions of the Nyquist plot. Based on DRT analysis, the Nyquist plot of SFGM- and SFM-based symmetric cells can be fitted by equivalent circuits, which are shown in Fig. 3e and 3f, respectively. The EIS fitting results are summarized in Table S3. The relationship between interfacial polarization resistance (Rp) and oxygen partial pressure (pO2) can be given as [50–52].
in-situ temperature-dependent XRD. The high-temperature (HT)-XRD patterns of SFGM cathode were measured under a constant flow of CO2-containing air in the temperature range of 100 � C–800 � C, as shown in Fig. 4c. One should note that the high-temperature and CO2-con taining air did not induce any impurity phase. However, the diffraction peaks, at 2θ ¼ 31� to 33� , shifted towards a lower angle with increasing temperature, which is directly related to the expansion of the perovskite crystal lattice. HT-XRD analysis confirmed the excellent thermal sta bility of SFGM perovskite in CO2-containing air. Apart from the chemical and structural stability, the competitive adsorption of carbon dioxide and oxygen on the cathode surface also influences the electro-catalytic properties and dictates the catalyst per formance. In the presence of CO2, CO2 occupies the oxygen active sites and reduces the overall electrochemical performance. The influence of CO2 on ORR performance of SFM and SFGM cathodes is schematically illustrated in Fig. 4a and 4b. CO2-assisted temperature-programmed desorption (CO2-TPD) is an effective tool to evaluate the adsorption/ desorption behavior of CO2. The CO2-TPD curves of SFM and SFGM cathodes are shown in Fig. 4. It can be readily observed that the chemical adsorption capacity of SFM and SFGM cathode is significantly different at 750 � C. Briefly, the desorption peak area of SFM was significantly larger than that of SFGM, which confirms the weak chemical adsorption of CO2 on SFGM cathode. Moreover, we have utilized EIS to analyze the variation in impedance with respect to CO2 concentration in air. The Nyquist plots of SFGM cathode, measured in CO2-containing air at 750 � C, are shown in Fig. 4f. Obviously, the ASR value of SFGM electrode exhibited better stability at different concentrations of CO2 in the air than SFM electrode. For instance, the ASR value of SFGM and SFM electrodes remained constant at a CO2 concentration of ~1%. However, when CO2 concentration was increased to 10%, the ASR value of SFM increased from 0.202 Ω cm2 to 0.354 Ω cm2, whereas the ASR value of SFGM electrode increased from 0.117 Ω cm2 to 0.138 Ω cm2, indicating the superior stability of SFGM electrode. In addition, when CO2 concentration was reduced to 0, the ASR values of both electrodes have been restored to the initial state after a certain period of time, which indicates that CO2 did not render an irreversible impact on the electrode structure. Moreover, various steps of the electrode reaction process exhibited different levels of sensitivity to CO2 concentration. DRT analysis was carried out to attain further insights into the degradation mechanism of the reaction process [55]. As shown in Fig. 4e, DRT analysis was performed on impedance data of SFGM electrode, measured in a constant flow of air, 1% CO2, 5% CO2, 10% CO2 and 20% CO2. The results reveal that the low-frequency impedance is significantly increased with increasing CO2 concentra tion, which indicates that the process of oxygen dissociation into oxygen ions is inhibited due to the occupation of ORR active site with increasing CO2 concentration. Therefore, as shown in Fig. 4e and 4f, SFM is more sensitive to the variation in CO2 concentration because of its stronger affinity towards chemical adsorption of CO2. Therefore, the Rp value of SFM cathode has exhibited a larger change with respect to CO2 con centration than SFGM, as shown in Fig. 4f. In addition, EIS is employed to evaluate the short-term stability of symmetric cell, i.e., SFGM|SDC| LSGM|SFGM, in CO2-containing air for 100 h. As shown in Fig. 4g, the ASR value of SFGM cathode is stabilized at ~0.13 Ω cm2 within 100 h, which indicates that SFGM cathode has excellent ORR activity and good CO2 tolerance in 5% CO2-containing air at 750 � C.
Rp ¼ k(pO2)n Hence, the impedance values at different frequencies can be related to the oxygen partial pressure, as shown in Fig. 3b. One should note that the slope of each line corresponds to the value of n for each process, which is found to be 0.44 in the low-frequency region and represents the dissociation of O2 into 2Oads. Similarly, the n value of 0.27 in the intermediate-frequency region corresponds to the migration of oxygenions to the three-phase boundary region. Furthermore, the n value of 0.08 in the high-frequency region indicates the transfer of O2 at the electrode/electrolyte interface [53]. Fig. 3e and 3f presents Nyquist plots of SFGM- and SFM-based symmetric cell, measured at 750 � C and under the constant flow of 100 sccm mixture gas (80% N2 and 20% O2). The experimentally measured EIS data were fitted by using an equivalent circuit [LR (Q1R1) (Q2R2) (Q3R3)] and analyzed by using DRT. Fig. 3d compares DRT curves of both SFM and SFGM cathodes. The ASR value of SFM and SFGM electrodes was found to be 0.224 Ω cm2 and 0.099 Ω cm2, respectively. In addition, owing to Ga-doping, the impedance values (R1, R2 and R3) of the symmetric cells at HF, IF and LF are significantly reduced from 0.018, 0.068 and 0.138 Ω cm2 to 0.006, 0.024 and 0.069 Ω cm2, respectively. It can be seen from Fig. 3d, 3e and 3f that Ga-doping significantly reduced the ASR values in intermediate-frequency and high-frequency regions, which further confirms the great improvement in surface exchange and ion diffusion processes. Besides, compared with some other cobalt-free cathodes, SFGM cathode also demonstrated an excellent ORR electro-catalytic activity (Table S4).
3.5. Single-cell performance and stability test Lastly, SFGM- and SFM-based anode-supported flat-tube SOFCs, with YSZ electrolyte, have been characterized to comprehensively assess the cathode performance and demonstrate the potential of Ga-doped SFM in practical applications. The digital photograph and cross-sectional SEM image of anode-supported flat-tube SOFC, with SFGM cathode and YSZ electrolyte, are presented in Fig. 5a and 5b, respectively. The anodesupported flat-tube SOFCs has a total length of 15 cm, where the
3.4. Resistance to CO2 poisoning In addition to the superior ORR activity, the durability of SOFC cathodes in environmentally benign gases, e.g., CO2, plays a critical role in practical applications [54]. The chemical and structural stability of cathode materials at high temperature are also important parameters. Herein, the structural stability of SFGM cathode has been verified by 5
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Fig. 4. CO2 durability of SFM and SFGM cathodes: schematic illustration of CO2 influence on ORR per formance of (a) SFM and (b) SFGM cathodes; (c) HTXRD patterns of SFGM powder in the temperature range of 100 � C–800 � C, measured in CO2-containing; (d) CO2-TPD profiles; (e) DRT curve of SFGM as a function of different CO2; (f) Time-dependent ASR values of SFM and SFGM cathodes treated in different CO2 concentrations in air (0%,1%, 2%, 5%,10% and 20%) at 750 � C. (g) Durability test of a Symmetric cell with the SFGM cathode at CO2 concentrations in air at 750 � C for 100 h.
cathode has a length of 10 cm and an area of 30 cm2. SEM image con firms the dense structure of YSZ electrolyte. Moreover, SFGM cathode is in close contact with the GDC barrier layer, which ensures the reliability of the single-cell. Fig. 5c presents the maximum power density of SFGM- and SFMbased anode-supported flat-tube SOFCs in the temperature range of 650–800 � C, where the anode and cathode are supplied with humidified H2 (97% H2/3% H2O, 200 sccm) and air (200 sccm), respectively. At 750 � C, the maximum power density of SFGM and SFM cathodes was 482.4 mW cm 2 and 245.4 mW cm 2, respectively. Moreover, the opencircuit voltage (OCV) values were close to the Nernst potential,
suggesting the excellent sealing of single-cell and gas-tight characteris tics of the electrolyte. These results confirm a superior electro-catalytic activity for ORR of the SFGM cathode under realistic operating condi tions. Fig. 5d and Fig. S9 presents I–V and I–P curves of anode-supported flat-tube SOFC for SFGM and SFM cathode, measured at different tem peratures (650–800 � C). The maximum power at 750 � C was found to be 15.12 W, as shown in Fig. 5d. To demonstrate the excellent CO2-tolerance of SFGM and SFM cathode, Fig. 5e and Fig. S10 presents the long-term stability of anodesupported flat-tube SOFCs, measured at an operating temperature of 750 � C. The cells were operated in pure air for 97 h and, then, switched 6
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Fig. 5. (a) The digital photograph of flat-tube SOFCs; (b) cross-sectional SEM images of anode-supported single-cell with SFGM cathode; (c) peak power density of SFM- and SFGM-based single-cells at different temperatures (650–800 � C); (d) typical I–V and I–P curves of anode-supported single-cell with SFGM cathode; and (e) durability of anode-supported single-cell with SFGM cathode, measured at a cell current of 16 A and 750 � C.
to 5% CO2-containing air, followed by continuous operating for 291 h. A slight decrease in Fig. 5e is observed when the cathode gas is switched from pure air to 5% CO2-containing air, which is similar to the discharge test results (Fig. S8). One should note that the discharge test is per formed in air with different concentrations of CO2. As mentioned earlier, CO2 occupies the ORR active site and leads to a decreased discharge performance. Despite the slightly lower performance of SFGM-based single-cell due to the introduction of CO2-containing air, the singlecell exhibited a stable performance for 291 h because of CO2-tolerant chemical structure of SFGM electrode. It is worth emphasizing that a large-sized SFGM cathode is utilized in flat-tube SOFCs, rendering su perior performance, desirable CO2 tolerance and excellent durability, which demonstrates the potential of Ga-doped SFM cathodes for prac tical applications.
Acknowledgements This work is supported by National Natural Science Foundation of China (21506012 and 51802018), China Postdoctoral Science Founda tion (No. 2017M620642) and Analysis & Testing Center (Beijing Insti tute of Technology). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227722. References [1] Y. Chen, B. deGlee, Y. Tang, Z. Wang, B. Zhao, Y. Wei, L. Zhang, S. Yoo, K. Pei, J. H. Kim, Y. Ding, P. Hu, F.F. Tao, M. Liu, Nat. Energy 3 (2018) 1042–1050. [2] Y. Lei, W. Shizhong, B. Kevin, L. Mingfei, L. Ze, C. Zhe, L. Meilin, Science 326 (2009) 126–129. [3] Y. Zhang, R. Knibbe, J. Sunarso, Y. Zhong, W. Zhou, Z. Shao, Z. Zhu, Adv. Mater. 29 (2017) 1700132. [4] E.D. Wachsman, K.T. Lee, Science 334 (2011) 935–939. [5] C. Yu, C. Yan, D. Dong, D. Yong, Y. Choi, Z. Lei, S. Yoo, D. Chen, B. Deglee, X. Han, Energy Environ. Sci. 10 (2017) 964–971. [6] Y. Chen, Y. Choi, S. Yoo, Y. Ding, R. Yan, K. Pei, C. Qu, L. Zhang, I. Chang, B. Zhao, Joule, 2 (2018) 938-949. [7] R. Pelosato, G. Cordaro, D. Stucchi, C. Cristiani, G. Dotelli, J. Power Sources 298 (2015) 46–67. [8] M.E. Lynch, Y. Lei, W. Qin, J.J. Choi, M. Liu, K. Blinn, M. Liu, Energy Environ. Sci. 4 (2011) 2249–2258. [9] J. Kim, S. Choi, A. Jun, H.Y. Jeong, J. Shin, G. Kim, Chemsuschem 7 (2014) 1669–1675. [10] L. Zhu, B. Wei, Z. Wang, K. Chen, H. Zhang, Y. Zhang, X. Huang, Z. Lü, ChemSusChem 9 (2016) 2443–2450. [11] M. Li, W. Zhou, Z. Zhu, ChemElectroChem 2 (2015) 1331–1338. [12] Y. Zhu, J. Sunarso, W. Zhou, Z. Shao, Appl. Catal. B Environ. 172–173 (2015) 52–57. [13] S.Y. Lai, D. Ding, M. Liu, M. Liu, F.M. Alamgir, Chemsuschem 7 (2015) 3078–3087. [14] K. Efimov, X. Qiang, A. Feldhoff, Chem. Mater. 22 (2010). [15] W. Zhou, F. Liang, Z. Shao, Z. Zhu, Sci. Rep. 2 (2012) 327. [16] W. Sun, P. Li, C. Xu, L. Dong, J. Qiao, Z. Wang, D. Rooney, K. Sun, J. Power Sources 343 (2017) 237–245. [17] J. Wang, T. Yang, L. Lei, K. Huang, J. Mater. Chem. 5 (2017) 8989–9002.
4. Conclusions In summary, Ga-doped SFM cathode, i.e., Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM), has been successfully fabricated and demonstrated as a prom ising cathode material for IT-SOFCs. The results revealed that Ga-doping increased the ORR by promoting the oxygen-ion transfer capability and lowering the ASR value of SFGM cathode. At 750 � C, SFGM and SFM cathodes exhibited an ASR value of 0.099 Ω cm2 and 0.224 Ω cm2, respectively. Furthermore, SFGM cathode demonstrated excellent elec trochemical stability and CO2 tolerance. For instance, at a cell current of 16 A and an operating temperature of 750 � C, SFGM-based single-cell has been continuously operating for 291 h, without any significant performance attenuation, in CO2-containing air. These results demon strate the potential of SFGM perovskite as a cathode material in ITSOFCs. Notes The authors declare no competing financial interest.
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Highly active and CO2-tolerant Sr2Fe1.3Ga0.2Mo0.5O6-δ cathode for intermediate-temperature solid oxide fuel cells Chunming Xu a, Kening Sun a, Xiaoxia Yang a, Minjian Ma a, Rongzheng Ren a, Jinshuo Qiao a, Zhenhua Wang a, Shuying Zhen b, Wang Sun a, * a b
Beijing Key Laboratory for Chemical Power Source and Green Catalysis, Beijing Institute of Technology, Beijing, 100081, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� SFGM are firstly evaluated as the cath ode for flat tubular solid oxide fuel cell. � Ga-doping enhanced the oxygen-ion transport capability of SFM-based materials. � A low polarization resistance of 0.099 Ω cm2 at 750 � C for SFGM cathode. � SFGM exhibits outstanding CO2 tolerance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid oxide fuel cells Cathode Perovskite oxides CO2 tolerance Oxygen reduction reaction
The development of cathode materials, with high catalytic activity toward oxygen reduction reaction (ORR), structural stability and CO2 tolerance, is an important research direction for the successful realization of intermediate-temperature solid oxide fuel cells (IT-SOFCs). Herein, a novel double perovskite mixed ionic conductor, i.e., Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM), is developed by Ga-doping at Fe-sites of Sr2Fe1.5Mo0.5O6-δ (SFM) and evaluated as a cathode material in IT-SOFCs. At 750 � C, the area specific resistance (ASR) of SFGM cathode is found to be 0.099 Ω cm2, which is ~50% lower than SFM cathode (0.224 Ω cm2) in 20% O2/N2 atmosphere. Moreover, SFGM exhibits outstanding CO2 tolerance due to its excellent CO2 adsorption resistance compared with SFM. The ASR of SFGM remains stable at ~0.13 Ω cm2 during 100 h of continuous operation in 5% CO2containing air. In addition, a large-sized SFGM cathode (30 cm2) is utilized in anode-supported flat-tube SOFCs to demonstrate the potential of SFGM in practical applications. At 750 � C, the as-prepared SFGM-based single-cell provides a stable power of 12 W for 291 h in 5% CO2-containing air. The superior electrochemical performance and outstanding CO2 tolerance of SFGM are promising features for the rapid development of IT-SOFCs.
1. Introduction Solid oxide fuel cells (SOFCs) are considered as promising energy
conversion devices due to their high efficiency and environmentfriendly nature. Moreover, SOFCs superior fuel adaptability than other fuel cell systems, which makes them an attractive choice for fossil-fuel to
* Corresponding author. E-mail address: [email protected] (W. Sun). https://doi.org/10.1016/j.jpowsour.2020.227722 Received 21 October 2019; Received in revised form 16 December 2019; Accepted 6 January 2020 Available online 29 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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hydrogen-fuel transition [1–3]. In order to accelerate the commerciali zation of SOFCs, the operating temperature should be reduced, from the traditional high temperature (HT, 900–1000 � C) to an intermediate-temperature (IT, 600–800 � C), to decrease the stack cost and increase the lifetime [4]. However, despite the enhanced durability, the lower operating temperature leads to the inferior electrochemical activity of electrode materials [5–7]. Therefore, a lot of research effort is directed to enhance the durability, reduce the cost and prolong the service life of SOFC cathodes, while maintaining the high oxygen reduction activity at a lower operating temperature. During the past 20 years, simple perovskites, double perovskites, Ruddlesden–Popper phases and other kinds of mixed ionic electronic conducting (MIEC) oxides have been widely investigated as cathode materials and rendered substantial achievements in the field of ITSOFCs. In order to develop a cathode material with excellent oxygen reduction reaction (ORR) properties, a great deal of research effort has been focused on perovskites (ABO3) and double perovskites (AA’B2O6), such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), doped SrCoO3-δ (SCO) and PrBaCo2O5þδ (PBCO), demonstrating outstanding performance in ITand LT-SOFCs [3,8–11]. However, a large number of studies have shown that, when the cathode gas contains CO2 impurities, the alkaline earth elements (Ba, Sr) at the A-site of perovskite segregate and react with CO2 to form a high-impedance carbonate phase, leading to reduced ORR activity [12,13]. Hence, the practical utilization of perovskites is hin dered by the inherent chemical instability in CO2 environment. For instance, a promising intermediate temperature cathode, i.e., BSCF, renders performance degradation due to CO2 poisoning, where CO2 in duces Ba/Sr enrichment at A-site and leads to the formation of Ba-rich hexagonal secondary phase, resulting in poor durability of the elec trode material [12,14]. For example, the Ba0.5Sr0.5Co0.8Fe0.2O3-δ cath ode was exposed to an air containing 10% CO2 for 5 min at 600 � C, and the impedance value decreased by about 20 times [15]. In practical applications, the cathode material must possess excellent chemical sta bility to resist the corrosion induced by CO2 and other impurity gases. It has been reported that the partial substitution of B-site transition metal cation by Ti4þ, Zr4þ, Ta5þ, Sb5þ or Nb5þ can lead to enhanced CO2 resistance of various MIEC perovskite oxides, such as SrCoO3 δ [16,17], SrFe0.8Mo0.2O3 δ [18], Bi0.5Sr0.5FeO3 δ [19] and Pr0.6Sr0.4FeO3-δ [20]. However, the doping of high valence cation reduces the concentration of oxygen vacancies, leading to inferior oxygen transport performance [21]. For instance, when the temperature is 800 � C, the impedance value of the SrFe1 xSbxO3 δ (SFS x ¼ 0.05, 0.10, 0.15, 0.20) cathode is higher than that of SrFeO3 (~0.044 Ω cm2) and increases from 0.11 to 0.13 Ω cm2 as the Sb content increases [21]. In recent years, the rapidly developing Fe-based double perovskite Sr2Fe1.5Mo0.5O6-δ (SFM) is considered as a promising IT-SOFC cathode due to its structural stability, matching thermal expansion coefficient (TEC) with electrolyte, excellent ion exchange capacity and excellent ORR performance [22,23]. In addition to SOFCs, SFM-based perovskite materials are also being studied as cathode materials in solid oxide electrolytic cells for direct electrolysis of CO2, rendering splendid chemical stability under CO2 atmosphere [24–26]. One should note that, despite the lower electro-catalytic performance of SFM-based perov skites than SrCoO3-based materials, such as the superior structural sta bility and optimal TEC make them promising cathode materials for SOFCs. Previously reports, we have demonstrated that the replacement of Bsite Fe element with a transition metal, such as nickel or cobalt, induces the formation of oxygen vacancies, affects the electronic structure of Bsite cation and, thereby, improves the electronic and ionic conductivity of SFM [27]. However, despite the improved ORR performance, Ni- and Co-doping degrades the structural stability of SFM [28]. In another report, we have utilized a low-valence Sc element to replace Fe, which improved the ORR performance without destroying the structure of cathode. These results indicate the potential of another low-valence element, i.e., Ga3þ [29].
Recently, Manthiram et al. have reported that the stability and per formance of YBaCo4O7 materials can be significantly improved by Gadoping [30,31]. And, the addition of Ga into Pr2(Ni0.75Cu0.25) O4-δ-based MIEC increased the oxygen permeability [32,33]. In addi tion, Ga-doping also contributed to the improved structural stability of Fe-based perovskite, i.e., La0.6Sr0.4Ga0.3Fe0.7O3 (LSGF) [34–36]. Herein, we aimed to replace the B-site Fe element in SFM with Ga element to simultaneously optimize the structural stability and ORR performance as cathode material in SOFCs. The influence of Ga-doping and CO2 poisoning on structural stability, ORR performance and re covery capability has been systematically investigated. Moreover, in order to comprehensively investigate the practical application potential of SFGM, it was applied to large-size flat-tube SOFCs. The current research provides a novel strategy to fabricate novel high-performance cathode materials for IT-SOFCs. 2. Experimental procedure 2.1. Synthesis Sr2Fe1.5Mo0.5O6-δ (SFM) and Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM) pow ders were synthesized by using the combustion method. Briefly, stoi chiometric quantities of Sr(NO3)2, Fe(NO3)3�9H2O, Ga(NO3)2�xH2O and (NH4)6Mo7O24�4H2O salts were dissolved in an appropriate amount of deionized water and a certain proportion of citric acid, followed by the addition of glycine as a combustion promoter. The resulting clear solution was stirred in a water bath at 80 � C until it attained a sol-gel form and, then, the combustion was carried out at 250 � C for 1 h. Then, the as-received powder was ground and calcined in a muffle furnace at 1050 � C for 4 h in air [37–39]. 2.2. Characterization The crystal structure and phase analysis were performed by using Xray diffractometer (XRD, Panalytical X’Pert Pro). The XRD data were analyzed by using the Rietveld analysis with GSAS program and EXPGUI interface. The microstructure of the as-received powders was observed by scanning electron microscopy (SEM, FEI QUANTA-250) and highresolution transmission electron microscopy (HRTEM, JEM-2100F) The chemical composition and oxidation state of different atoms were determined by X-ray photoelectron spectroscopy (XPS, MULT1 LAB2000, VG). The electrical conductivity was measured by using a Keithley 2400 source meter and four-probe method in air. The electrical conductivity relaxation (ECR) measurements were performed by using the four-probe method. O2-temperature-programmed desorption (TPD) experiments were carried out by using a micromeritics apparatus (Chembet Pulsar TPR/TPD). The Electrochemical impedance spectroscopy (EIS) was measured by using LSGM (La0.9Sr0.1Ga0.8Mg0.2O3, Fuel Cell, USA), as the electrolyte, and GDC (Gd0.2Ce0.8O1.95, GDC, Fuel cell materials Co., USA), as the barrier layer. The electrochemical properties were assessed by assem bling SFM|GDC|LSGM|GDC|SFM and SFGM|GDC|LSGM|GDC|SFGM symmetric cells. Herein, the electrode materials were prepared by screen printing and sintered at 1100 � C for 2 h. EIS was carried out by using the symmetrical cell in N2/O2 mixture. The electrochemical workstation (Autolab 302 N), with an AC amplitude of 10 mV, was used to collect EIS spectra in the frequency range of 0.01–100 kHz. 2.3. Single-cell fabrication The electrochemical performance of the single-cell was assessed by using an anode-supported flat-tube SOFC with an effective cathode area of 30 cm2. A flat-tube anode carrier was prepared by extrusion molding. First, NiO (55 wt %, JT Baker Co., USA) and yttria-stabilized zirconia (45 wt %, 8 mol% YSZ, Tosoh Co., Japan) graphite, as pore former, were ball-milled in ethanol for 24 h, followed by drying. Then, an organic 2
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binder and distilled water were added to prepare an anode slurry. The extruded anode support was dried at 80 � C for 48 h and pre-sintered at 1100 � C [40]. The YSZ electrolyte layer was prepared by dip-coating on the flat-tube anode and ceramic interconnector was screen-printed on one side of the anode. After sintering at 1400 � C for 5 h, the barrier slurry (Gd0.2Ce0.8O1.95, GDC, Fuel cell materials Co., USA) was pasted on the electrolyte and sintered at 1300 � C for 5 h, which was employed as a buffer layer between the cathode and the electrolyte to prevent the interface reaction and inter-diffusion of ionic species [41]. Finally, as-prepared SFGM and SFM cathodes were screen-printed on the barrier layer and sintered at 1150 � C for 2 h [42].
stoichiometric ratio of Sr, Fe, Ga and Mo elements was found to be 2.00, 1.24, 0.19, 0.46, respectively, which is consistent with the designed composition of SFGM. 3.2. Physicochemical characterization The catalytic oxygen activation and oxygen-ion transfer capability are defined as performance indices for cathode materials in IT-SOFCs. Fig. 2a presents the oxygen-ion transport mechanism of perovskite materials, which indicates that the surface exchange of oxygen-ions and chemical bulk diffusion dictate the oxygen-ion transport process [45]. The electrical conductivity relaxation (ECR) measurements directly reflect the oxygen-ion conductivity of SFM and SFGM electrodes. As presented in Fig. 2b, the normalized test results show that, at 750 � C, the relaxation time is obviously decreased from 7520 to 2513 s, the surface oxygen exchange coefficient (Kchem) is increased from 8.71 � 10 5 cm s 1 to 23.2 � 10 5 cm s 1, and the chemical bulk diffusion coefficient (Dchem) increased from 9.03 � 10 6 cm2 s 1 to 32.1 � 10 6 cm2 s 1 after Ga-doping. The increased Kchem and Dchem indicate that the Ga-doping enhanced the oxygen-ion transport capability of SFM-based materials, which may lead to superior ORR performance. In order to further reveal the influ ence of Ga-doping on ORR performance, the valence state and electronic structure of surface elements, which are closely related to the ORR ki netics, have been analyzed by XPS. Fig. 2c presents the high-resolution O 1s XPS spectrum from SFGM powder, which can be deconvoluted into three peaks. The peak, located at 528.5 eV, can be assigned to the lattice oxygen (lat-O), whereas the peak, located at 529.7 eV, can be ascribed to chemically adsorbed oxygen (chem-O, O-/O2 ). The 3rd peak, located at ~531.3 eV), can be assigned to the physically adsorbed oxygen due to hydroxyls (OH ) or carbonate (CO23 ) species [25,43,46]. The fitting results showed that the proportion of chem-O in SFM and SFGM mate rials was 19.9% and 24.9%, respectively. One should note that the proportion of chem-O is directly associated with the amount of surface oxygen vacancies [47], which indicates that Ga-doping increased the concentration of oxygen vacancies. Moreover, the binding energy of lattice oxygen in SFM and SFGM cathodes was found to be 528.5 and 528.7eV, respectively, which may be caused by the difference in elec tronegativity between Fe2þ and Fe4þ [37,48]. The decrease of coulombic force between B-site ions and O2 indicates the increase of oxygen activity in the lattice and enhanced oxygen-ion transfer
3. Results and discussion 3.1. Element composition and crystal structure In order to determine the influence of Ga-doping on crystal structure, the synthesized Sr2Fe1.5Mo0.5O6-δ (SFM) and Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM) were analyzed by X-ray diffraction (XRD) and the general structure analysis system (GSAS) was utilized for Rietveld refinement. As shown in Fig. 1a and b, the refined XRD patterns confirm the cubic double perovskite structure, with a space group of F m-3m, of both SFM and SFGM cathodes. However, Ga-doping increased the lattice param eter from a ¼ 7.838 Å and V ¼ 481.522 Å3 to a ¼ 7.906 Å and V ¼ 494.163 Å3, respectively. Furthermore, selected area electron diffrac tion (SAED) and high-resolution transmission electron microscopy (HRTEM) were employed to further analyze the crystal structure of SFGM. SAED patterns and HRTEM images, along [001] (Fig. 1c, e) and [111] (Fig. 1d, f) axes, confirm the cubic double perovskite structure, with a space group of F m-3m, of SFGM. Moreover, the observed lattice parameters are consistent with XRD results. X-ray photoelectron spectroscopy (XPS) of SFM and SFGM powders was carried out to confirm the successful doping of Ga into the perov skite crystal lattice. As shown in Fig. S1, Sr 3d (133 eV), Fe 2p (710 eV), O 1s (532 eV) and Mo 3d (232 eV) have been detected in XPS spectra of both SFM and SFGM cathodes [37,43]. Fig. S2 shows that a weak Ga peak (�1120 eV) is present in XPS spectrum of SFGM, which has not been observed in high-resolution XPS spectrum of SFM powder [44]. In addition, the energy dispersive X-ray (EDX) results (Fig. S3) confirmed that Ga is uniformly distributed in SFGM without any observable sepa ration. The distribution of Ga is as uniform as Sr, Fe, Mo and O, and the
Fig. 1. Crystal structure of SFM and SFGM cathodes: (a) and (b) Rietveld-refined XRD profiles; SAED patterns along (c) [001] and (d) [111] zone axis for SFGM; and HRTEM images along (e) [001] and (f) [111] direction for SFGM. 3
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Fig. 2. Physicochemical properties of SFM and SFGM cathodes: (a) the oxygen-ion transport mechanism of perovskite materials; (b) ECR, measured at 750 � C; (c) high-resolution O 1s XPS spectrum; (d) high-resolution Fe 2p XPS spectrum; (e) O2-TPD curves; and (f) TG curves in the temperature range of 400–800 � C in air.
capability [43]. Fig. 2d, Fig. S4 and Fig. S5 provide high-resolution Fe 2p, Mo 3d and Sr 3d XPS spectra of SFM and SFGM cathodes, respec tively. And, the area ratios of each peak for Fe 2p and Mo 3d are sum marized in Table S1 and Table S2, respectively. Owing to Ga-doping, the peak position of Fe4þ, Fe3þ, Fe2þ and Sr2þ moved towards lower binding energy, which indicates lower bonding energy between metallic ions and oxygen in perovskite oxides, leading to superior oxygen-ion transfer capability [49]. The influence of Ga-doping on the formation of oxygen vacancies at elevated temperatures has been investigated by oxygen temperature program desorption (O2-TPD) test and thermos gravimetric analysis (TGA) in air. Fig. 2e presents the O2-TPD curves of SFM and SFGM cathodes, where the area of the desorption peak is approximately equal to the amount of chemically absorbed oxygen. Moreover, the content of
lost lat-O due to the change of valence state of metallic ions decreased with increasing temperature. The desorbed peak area of chemically absorbed oxygen, around 400 � C, for SFGM cathode is found to be higher than SFM, which is consistent with XPS analysis. Furthermore, TGA results are completely consistent with O2-TPD results (Fig. 2f). In TGA curves, the weight loss corresponds to the desorption peak area in O2TPD and, at 800 � C, the weight loss of SFGM (1.82%) is found to be higher than SFM (0.92%) due to Ga-doping. 3.3. Electrochemical activity Furthermore, electrochemical impedance spectroscopy (EIS) was employed to assess the electro-catalytic activity of SFM and SFGM electrodes in symmetric cell configuration. Fig. 3a shows the area
Fig. 3. Electrochemical characterization of SFM and SFGM cathodes: (a) ASR values in the temperature range of 650–800 � C; (b) EIS of SFGM cathode as a function of oxygen partial pressure at 750 � C; (c) DRT of SFGM cathode as a function of oxygen partial pressure at 750 � C; (d) DRT results of SFM and SFGM cathodes at 750 � C and the oxygen partial pressure of 0.2 atm; and impedance spectra of (e) SFM- and (f) SFGM-based symmetric cells at 750 � C and the oxygen partial pressure of 0.2 atm. 4
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specific resistance (ASR) of SFM and SFGM cathodes in the temperature range of 650–800 � C, measured in ambient air. The Nyquist plots of both cathodes at different temperatures are presented in Fig. S6. It is worth noting that the value of ASR is an important criterion to evaluate the activity of SOFCs electrode, where the lower ASR value corresponds to better the electro-catalytic performance. As shown in Fig. 3a, both SFM and SFGM electrodes rendered typical electrochemical impedance spectra (EIS) in symmetric cells, i.e., SFM|GDC|LSGM|GDC|SFM and SFGM|GDC|LSGM|GDC|SFGM. The area specific resistance (ASR) of SFGM cathode is significantly lower than the SFM cathode. At 750 � C, the ASR value of SFGM and SFM cathodes was found to be 0.245 Ω cm 2 and 0.628 Ω cm2, respectively, which indicates that SFGM cathode renders superior electro-catalytic performance. The ORR kinetics of the cathode are also influenced by external factors, such as reaction temperature and cathode gas composition. In order to clarify the ORR mechanism of SFGM electrode and influence of Ga-doping on ORR performance, the electrochemical impedance spec troscopy of SFM- and SFGM-based symmetric cells has been carried out under different partial pressures of oxygen at 750 � C. The EIS data have been further analyzed by DRT. Fig. 3b and Fig. S7 show the Nyquist plots of SFGM- and SFM-based symmetric cells, respectively. The EIS data were first analyzed by using the relaxation time distribution (DRT), as shown in Fig. 3c and Fig. S7. The characteristic time constants can be obtained from the high-frequency (HF), intermediate-frequency (IF) and low-frequency (LF) regions of the Nyquist plot. Based on DRT analysis, the Nyquist plot of SFGM- and SFM-based symmetric cells can be fitted by equivalent circuits, which are shown in Fig. 3e and 3f, respectively. The EIS fitting results are summarized in Table S3. The relationship between interfacial polarization resistance (Rp) and oxygen partial pressure (pO2) can be given as [50–52].
in-situ temperature-dependent XRD. The high-temperature (HT)-XRD patterns of SFGM cathode were measured under a constant flow of CO2-containing air in the temperature range of 100 � C–800 � C, as shown in Fig. 4c. One should note that the high-temperature and CO2-con taining air did not induce any impurity phase. However, the diffraction peaks, at 2θ ¼ 31� to 33� , shifted towards a lower angle with increasing temperature, which is directly related to the expansion of the perovskite crystal lattice. HT-XRD analysis confirmed the excellent thermal sta bility of SFGM perovskite in CO2-containing air. Apart from the chemical and structural stability, the competitive adsorption of carbon dioxide and oxygen on the cathode surface also influences the electro-catalytic properties and dictates the catalyst per formance. In the presence of CO2, CO2 occupies the oxygen active sites and reduces the overall electrochemical performance. The influence of CO2 on ORR performance of SFM and SFGM cathodes is schematically illustrated in Fig. 4a and 4b. CO2-assisted temperature-programmed desorption (CO2-TPD) is an effective tool to evaluate the adsorption/ desorption behavior of CO2. The CO2-TPD curves of SFM and SFGM cathodes are shown in Fig. 4. It can be readily observed that the chemical adsorption capacity of SFM and SFGM cathode is significantly different at 750 � C. Briefly, the desorption peak area of SFM was significantly larger than that of SFGM, which confirms the weak chemical adsorption of CO2 on SFGM cathode. Moreover, we have utilized EIS to analyze the variation in impedance with respect to CO2 concentration in air. The Nyquist plots of SFGM cathode, measured in CO2-containing air at 750 � C, are shown in Fig. 4f. Obviously, the ASR value of SFGM electrode exhibited better stability at different concentrations of CO2 in the air than SFM electrode. For instance, the ASR value of SFGM and SFM electrodes remained constant at a CO2 concentration of ~1%. However, when CO2 concentration was increased to 10%, the ASR value of SFM increased from 0.202 Ω cm2 to 0.354 Ω cm2, whereas the ASR value of SFGM electrode increased from 0.117 Ω cm2 to 0.138 Ω cm2, indicating the superior stability of SFGM electrode. In addition, when CO2 concentration was reduced to 0, the ASR values of both electrodes have been restored to the initial state after a certain period of time, which indicates that CO2 did not render an irreversible impact on the electrode structure. Moreover, various steps of the electrode reaction process exhibited different levels of sensitivity to CO2 concentration. DRT analysis was carried out to attain further insights into the degradation mechanism of the reaction process [55]. As shown in Fig. 4e, DRT analysis was performed on impedance data of SFGM electrode, measured in a constant flow of air, 1% CO2, 5% CO2, 10% CO2 and 20% CO2. The results reveal that the low-frequency impedance is significantly increased with increasing CO2 concentra tion, which indicates that the process of oxygen dissociation into oxygen ions is inhibited due to the occupation of ORR active site with increasing CO2 concentration. Therefore, as shown in Fig. 4e and 4f, SFM is more sensitive to the variation in CO2 concentration because of its stronger affinity towards chemical adsorption of CO2. Therefore, the Rp value of SFM cathode has exhibited a larger change with respect to CO2 con centration than SFGM, as shown in Fig. 4f. In addition, EIS is employed to evaluate the short-term stability of symmetric cell, i.e., SFGM|SDC| LSGM|SFGM, in CO2-containing air for 100 h. As shown in Fig. 4g, the ASR value of SFGM cathode is stabilized at ~0.13 Ω cm2 within 100 h, which indicates that SFGM cathode has excellent ORR activity and good CO2 tolerance in 5% CO2-containing air at 750 � C.
Rp ¼ k(pO2)n Hence, the impedance values at different frequencies can be related to the oxygen partial pressure, as shown in Fig. 3b. One should note that the slope of each line corresponds to the value of n for each process, which is found to be 0.44 in the low-frequency region and represents the dissociation of O2 into 2Oads. Similarly, the n value of 0.27 in the intermediate-frequency region corresponds to the migration of oxygenions to the three-phase boundary region. Furthermore, the n value of 0.08 in the high-frequency region indicates the transfer of O2 at the electrode/electrolyte interface [53]. Fig. 3e and 3f presents Nyquist plots of SFGM- and SFM-based symmetric cell, measured at 750 � C and under the constant flow of 100 sccm mixture gas (80% N2 and 20% O2). The experimentally measured EIS data were fitted by using an equivalent circuit [LR (Q1R1) (Q2R2) (Q3R3)] and analyzed by using DRT. Fig. 3d compares DRT curves of both SFM and SFGM cathodes. The ASR value of SFM and SFGM electrodes was found to be 0.224 Ω cm2 and 0.099 Ω cm2, respectively. In addition, owing to Ga-doping, the impedance values (R1, R2 and R3) of the symmetric cells at HF, IF and LF are significantly reduced from 0.018, 0.068 and 0.138 Ω cm2 to 0.006, 0.024 and 0.069 Ω cm2, respectively. It can be seen from Fig. 3d, 3e and 3f that Ga-doping significantly reduced the ASR values in intermediate-frequency and high-frequency regions, which further confirms the great improvement in surface exchange and ion diffusion processes. Besides, compared with some other cobalt-free cathodes, SFGM cathode also demonstrated an excellent ORR electro-catalytic activity (Table S4).
3.5. Single-cell performance and stability test Lastly, SFGM- and SFM-based anode-supported flat-tube SOFCs, with YSZ electrolyte, have been characterized to comprehensively assess the cathode performance and demonstrate the potential of Ga-doped SFM in practical applications. The digital photograph and cross-sectional SEM image of anode-supported flat-tube SOFC, with SFGM cathode and YSZ electrolyte, are presented in Fig. 5a and 5b, respectively. The anodesupported flat-tube SOFCs has a total length of 15 cm, where the
3.4. Resistance to CO2 poisoning In addition to the superior ORR activity, the durability of SOFC cathodes in environmentally benign gases, e.g., CO2, plays a critical role in practical applications [54]. The chemical and structural stability of cathode materials at high temperature are also important parameters. Herein, the structural stability of SFGM cathode has been verified by 5
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Fig. 4. CO2 durability of SFM and SFGM cathodes: schematic illustration of CO2 influence on ORR per formance of (a) SFM and (b) SFGM cathodes; (c) HTXRD patterns of SFGM powder in the temperature range of 100 � C–800 � C, measured in CO2-containing; (d) CO2-TPD profiles; (e) DRT curve of SFGM as a function of different CO2; (f) Time-dependent ASR values of SFM and SFGM cathodes treated in different CO2 concentrations in air (0%,1%, 2%, 5%,10% and 20%) at 750 � C. (g) Durability test of a Symmetric cell with the SFGM cathode at CO2 concentrations in air at 750 � C for 100 h.
cathode has a length of 10 cm and an area of 30 cm2. SEM image con firms the dense structure of YSZ electrolyte. Moreover, SFGM cathode is in close contact with the GDC barrier layer, which ensures the reliability of the single-cell. Fig. 5c presents the maximum power density of SFGM- and SFMbased anode-supported flat-tube SOFCs in the temperature range of 650–800 � C, where the anode and cathode are supplied with humidified H2 (97% H2/3% H2O, 200 sccm) and air (200 sccm), respectively. At 750 � C, the maximum power density of SFGM and SFM cathodes was 482.4 mW cm 2 and 245.4 mW cm 2, respectively. Moreover, the opencircuit voltage (OCV) values were close to the Nernst potential,
suggesting the excellent sealing of single-cell and gas-tight characteris tics of the electrolyte. These results confirm a superior electro-catalytic activity for ORR of the SFGM cathode under realistic operating condi tions. Fig. 5d and Fig. S9 presents I–V and I–P curves of anode-supported flat-tube SOFC for SFGM and SFM cathode, measured at different tem peratures (650–800 � C). The maximum power at 750 � C was found to be 15.12 W, as shown in Fig. 5d. To demonstrate the excellent CO2-tolerance of SFGM and SFM cathode, Fig. 5e and Fig. S10 presents the long-term stability of anodesupported flat-tube SOFCs, measured at an operating temperature of 750 � C. The cells were operated in pure air for 97 h and, then, switched 6
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Fig. 5. (a) The digital photograph of flat-tube SOFCs; (b) cross-sectional SEM images of anode-supported single-cell with SFGM cathode; (c) peak power density of SFM- and SFGM-based single-cells at different temperatures (650–800 � C); (d) typical I–V and I–P curves of anode-supported single-cell with SFGM cathode; and (e) durability of anode-supported single-cell with SFGM cathode, measured at a cell current of 16 A and 750 � C.
to 5% CO2-containing air, followed by continuous operating for 291 h. A slight decrease in Fig. 5e is observed when the cathode gas is switched from pure air to 5% CO2-containing air, which is similar to the discharge test results (Fig. S8). One should note that the discharge test is per formed in air with different concentrations of CO2. As mentioned earlier, CO2 occupies the ORR active site and leads to a decreased discharge performance. Despite the slightly lower performance of SFGM-based single-cell due to the introduction of CO2-containing air, the singlecell exhibited a stable performance for 291 h because of CO2-tolerant chemical structure of SFGM electrode. It is worth emphasizing that a large-sized SFGM cathode is utilized in flat-tube SOFCs, rendering su perior performance, desirable CO2 tolerance and excellent durability, which demonstrates the potential of Ga-doped SFM cathodes for prac tical applications.
Acknowledgements This work is supported by National Natural Science Foundation of China (21506012 and 51802018), China Postdoctoral Science Founda tion (No. 2017M620642) and Analysis & Testing Center (Beijing Insti tute of Technology). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227722. References [1] Y. Chen, B. deGlee, Y. Tang, Z. Wang, B. Zhao, Y. Wei, L. Zhang, S. Yoo, K. Pei, J. H. Kim, Y. Ding, P. Hu, F.F. Tao, M. Liu, Nat. Energy 3 (2018) 1042–1050. [2] Y. Lei, W. Shizhong, B. Kevin, L. Mingfei, L. Ze, C. Zhe, L. Meilin, Science 326 (2009) 126–129. [3] Y. Zhang, R. Knibbe, J. Sunarso, Y. Zhong, W. Zhou, Z. Shao, Z. Zhu, Adv. Mater. 29 (2017) 1700132. [4] E.D. Wachsman, K.T. Lee, Science 334 (2011) 935–939. [5] C. Yu, C. Yan, D. Dong, D. Yong, Y. Choi, Z. Lei, S. Yoo, D. Chen, B. Deglee, X. Han, Energy Environ. Sci. 10 (2017) 964–971. [6] Y. Chen, Y. Choi, S. Yoo, Y. Ding, R. Yan, K. Pei, C. Qu, L. Zhang, I. Chang, B. Zhao, Joule, 2 (2018) 938-949. [7] R. Pelosato, G. Cordaro, D. Stucchi, C. Cristiani, G. Dotelli, J. Power Sources 298 (2015) 46–67. [8] M.E. Lynch, Y. Lei, W. Qin, J.J. Choi, M. Liu, K. Blinn, M. Liu, Energy Environ. Sci. 4 (2011) 2249–2258. [9] J. Kim, S. Choi, A. Jun, H.Y. Jeong, J. Shin, G. Kim, Chemsuschem 7 (2014) 1669–1675. [10] L. Zhu, B. Wei, Z. Wang, K. Chen, H. Zhang, Y. Zhang, X. Huang, Z. Lü, ChemSusChem 9 (2016) 2443–2450. [11] M. Li, W. Zhou, Z. Zhu, ChemElectroChem 2 (2015) 1331–1338. [12] Y. Zhu, J. Sunarso, W. Zhou, Z. Shao, Appl. Catal. B Environ. 172–173 (2015) 52–57. [13] S.Y. Lai, D. Ding, M. Liu, M. Liu, F.M. Alamgir, Chemsuschem 7 (2015) 3078–3087. [14] K. Efimov, X. Qiang, A. Feldhoff, Chem. Mater. 22 (2010). [15] W. Zhou, F. Liang, Z. Shao, Z. Zhu, Sci. Rep. 2 (2012) 327. [16] W. Sun, P. Li, C. Xu, L. Dong, J. Qiao, Z. Wang, D. Rooney, K. Sun, J. Power Sources 343 (2017) 237–245. [17] J. Wang, T. Yang, L. Lei, K. Huang, J. Mater. Chem. 5 (2017) 8989–9002.
4. Conclusions In summary, Ga-doped SFM cathode, i.e., Sr2Fe1.3Ga0.2Mo0.5O6-δ (SFGM), has been successfully fabricated and demonstrated as a prom ising cathode material for IT-SOFCs. The results revealed that Ga-doping increased the ORR by promoting the oxygen-ion transfer capability and lowering the ASR value of SFGM cathode. At 750 � C, SFGM and SFM cathodes exhibited an ASR value of 0.099 Ω cm2 and 0.224 Ω cm2, respectively. Furthermore, SFGM cathode demonstrated excellent elec trochemical stability and CO2 tolerance. For instance, at a cell current of 16 A and an operating temperature of 750 � C, SFGM-based single-cell has been continuously operating for 291 h, without any significant performance attenuation, in CO2-containing air. These results demon strate the potential of SFGM perovskite as a cathode material in ITSOFCs. Notes The authors declare no competing financial interest.
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