Efficient and stable symmetrical electrode La0.6Sr0.4Co0.2Fe0.7Mo0.1O3–δ for direct hydrocarbon solid oxide fuel cells

Electrochimica Acta 323 (2019) 134857

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Efficient and stable symmetrical electrode La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed for direct hydrocarbon solid oxide fuel cells Chunling Lu a, b, Bingbing Niu b, Shenglong Yu a, Wendi Yi b, Shijing Luo b, Baomin Xu b, *, Yuan Ji a, ** a b

Key Laboratory of Physics and Technology for Advanced Batteries, Ministry of Education, College of Physics, Jilin University, Changchun, 130012, China Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, 518055, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 April 2019 Received in revised form 29 August 2019 Accepted 8 September 2019 Available online 12 September 2019

The single-phase La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed symmetrical electrode is obtained by doping Mo at the B site of the La0.6Sr0.4Co0.2Fe0.8O3ed cathode. La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed shows good structural stability in reducing and oxidizing conditions. At 800
C, the polarization resistances are 0.041 and 0.226 U cm2 in air and H2, respectively. The electrochemical impedance stability measurement results show that La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed demonstrates satisfying electrochemical stability either as cathode or anode. The output performances of the La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed symmetrical electrode are 929 and 481 mW cm2 under H2 and ethanol fuel at 850
C, respectively. The output performance stability test of a single cell is carried out in liquid petroleum gas and the La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed electrode shows good ability of tolerating carbon deposition and resisting sulfur poisoning. The good structure stability, high performance, and satisfying electrochemical stability make La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed a promising symmetrical electrode for SOFC application. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Symmetrical solid oxide fuel cell Stability Electrochemical performance Hydrocarbon

1. Introduction Solid oxide fuel cells (SOFCs) are a promising technology that can convert chemical energy stored in gaseous, liquid, and solid state fuels to electric energy with high efficiency and low pollution [1]. Nickel-based materials are traditional anode for SOFCs, however, they encountered many problems, such as carbon deposition when using the hydrocarbon fuels, sulfur poisoning when the fuels contain sulfide, and the huge volume change during cycling test [2]. A single cell of SOFCs is usually made up of different materials, anode and cathode, and requires at least two thermal steps to fabricate. Symmetrical solid oxide fuel cells (SSOFCs) can improve the thermomechanical compatibility between the electrode and the electrolyte, simplify the fabrication process, and enhance the carbon- and sulfur-tolerant ability by operating the anode as cathode in turn, which have attracted extensive attentions [3]. Usually, the symmetrical electrodes can be divided into three

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Xu), [email protected] (Y. Ji). https://doi.org/10.1016/j.electacta.2019.134857 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

categories: single-phase oxides with stable structure, composite oxides, and quasi-symmetrical materials with a reducible electrode. The single-phase oxides are generally very attractive as symmetrical electrodes for SOFCs because of their simplicity, easy fabrication, good coking resistance, and low cost. The typical single-phase symmetrical electrode oxides include the LnMO3-based oxides (Ln ¼ La, Pr; M ¼ Cr, Mn), SrTiO3-based oxides, LaFeO3ed-based oxides, and SrCoO3ed-based oxides. For example, the La0.75Sr0.25Cr0.5Mn0.5O3ed (LSCM) as a symmetrical electrode was first reported by Tao and Irvine [4]. The maximum power densities of 300 and 230 mW cm2 were obtained in H2 and CH4 with the configuration of LSCM/YSZ(200mm)/LSCM at 900
C [4]. La0.5Sr0.5Co0.5Ti0.5O3ed (LSCT) perovskite oxide has a stable structure in both reducing and oxidizing atmospheres at high temperature when it was used as the cathode and the anode of SOFCs, and a maximum power density of 110 mW cm2 was achieved at 800
C in H2 [5]. Wang et al. [6] reported a directemethane symmetrical electrode La0.6Ce0.1Sr0.3Fe0.9Ni0.1O3ed (CLSFN), which showed good performance (522 mW cm2 at 850
C) and stability in wet CH4. La0.6Sr0.4Co0.2Fe0.8O3ed (LSCF) [7] and Nb-doped LSCF [8] have been considered as symmetrical electrode materials and some good

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achievements have been made. In addition, many double perovskite materials have been applied as symmetrical electrodes for SOFCs. For example, Sr2Fe1.5Mo0.5O6ed (SFM) has been regarded as the most promising symmetrical electrode for SOFCs [9]. SFM material has good redox stability and high conductivity of 310 S cm1 in H2 and 550 S cm1 in air at 780
C. However, with the further development of research, it has been found that SFM material reacted with the vapor at low temperatures, which is a potential shortcoming for applications of SOFCs [10]. The other double perovskite materials including Sr2Co1.15Mo0.85O6ed [11], Sr2TiFeO6ed (STF) [12], and Sr2TiFe0.9Mo0.1O6ed (STFM01) [13] have been reported as symmetrical electrodes for SOFCs and some good results are achieved. However, some problems still exist in these single-phase oxides, such as the sulfur poisoning [14], low catalytic activity and electrochemical performance [15], inferior conductivity in reducing atmosphere [6,8], and insufficient structure stability [16]. The composite electrodes may benefit to solve these problems as it is difficult for single-phase materials to have all of the properties required for a symmetrical electrode. In order to improve the oxygen ion conductivity of electrode materials the electrolyte materials of oxygen ion conductor are usually incorporated to form the composite electrodes, such as, the physically mixed composite electrodes including LSCMþYSZþGd0.2Ce0.8O2ed (GDC0.2) [17] and LaSr2Fe2CrO9ed(LSFCr)þGDC0.1 [18], La0.7Ca0.3CrO3 (LCC)þGDC0.2 [19], and La0.7Ca0.3Cr0.97O3 (LCC97)þYSZ [20]. The impregnated composite electrodes including LSCM-infiltrated YSZ backbone [21], LSFSc impregnated LSGM scaffold [22], and SrFe0.75Mo0.25O3ed (SFM) impregnated LSGM scaffold [23] also show much performance improvement. For these composite electrodes, the electrochemical performance has been improved, but the composite method leads to the complicated preparation process of electrode, thus increases the fabrication cost and weakens the advantages of SSOFCs. In recent years, the exsolution technique with in-situ growth of metal nanoparticles on the perovskite electrodes has attracted much attention because the nanoparticles have excellent catalytic activity to O2 or fuel gas [1]. This technique is also widely applied in the SSOFCs. Yang et al. [24] have reported Pr0.4Sr0.6Co0.2Fe0.7 Nb0.1O3ed (P-PSCFN) would be partially reduced in reducing condition and transformed into a K2NiF4-type structure Pr0.8Sr1.2(CoFe)0.8Nb0.2O4þd (K-PSCFN) main phase with Co-Fe alloy. An SSOFC with the configuration of P-PSCFN/LSGM/K-PSCFN showed the maximum power density of 960 mW cm2 and 600 mW cm2 at 800
C in H2 and CH4, respectively. Jardiel and Delahaye et al. [25,26] used Ni to substitute Mn or Cr in LSCM to form La0.75Sr0.25Cr0.5Mn0.5-xNixO3-d or La0.75Sr0.25Cr0.5-xNixMn0.5O3d as symmetrical electrodes for SOFCs. After reducing in H2, the Ni nanoparticles were observed on the sample surface and the main perovskite structure was still stable. The electrochemical performance was improved due to the Ni nanoparticles exsolution which has good catalytic activity. However, it needs to pay attention to the condition of controlling the nanoparticles exsolution and whether exsoluted nanoparticles can efficiently act as catalysts in natural gas or coal gas (containing H2S or S and P compounds) during the longterm operation. La0.6Sr0.4Co0.2Fe0.8O3ed (LSCF) was an outstanding cathode for SOFCs and has been commercialized. In recent years, LSCF and its ramifications serving as SOFCs anode have been studied extensively, such as the LSCF|YSZ|LSCF micro-SOFC [27], La0.6Sr0.4Co0.2Fe0.7Nb0.1O3ed symmetrical electrode [28], and A-site deficient (La0.6Sr0.4)1exCo0.2Fe0.6Nb0.2O3ed (x ¼ 0, 0.05 and 0.1) symmetrical electrodes [8]. Nevertheless these materials either have good structural stability but low catalytic activity or have excellent electrocatalytic properties but unsatisfying structural stability. The Mo element in perovskite oxides has multiple valences of Mo6þ, Mo5þ, and Mo4þ, which can provide conductive

pathways for electrons and Mo-based oxides have been reported showing high catalytic activity as SOFCs anode [29,30]. Moreover, Mo-doped Pr0.4Sr0.6Co0.2Fe0.8O3ed oxides [31] and Mo-doped Sr2TiFeO6ed oxides [12] as symmetrical electrodes have been reported and good structural stability and electrochemical performance have been obtained. Therefore, we used the Mo to substitute the Fe element in LSCF to design an efficient and stable symmetrical electrode. It is very difficult for a single oxide material showing not only good phase stability under oxidizing and reducing atmosphere, but also high electrocatalytic activity for the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). In this paper, the single-phase La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed symmetrical electrode demonstrated good structural stability, excellent electrochemical activity for fuel oxidation, and satisfying electrochemical stability. 2. Experimental 2.1. Synthesis and sample preparation La0.6Sr0.4Co0.2Fe0.7Mo0.1O3ed (LSCFM) was prepared by solidstate reaction. Stoichiometric amounts of La2O3 (99.9%), SrCO3 (99.0%), Co2O3 (99.0%), Fe2O3 (99.5%), and MoO3 (99.0%) were mixed and ground in a mortar. The mixture was sintered at 1000 and 1200
C in air for 10 h, respectively, then it was reduced in 5% H2/Ar at 1000
C for 10 h to obtain the pure phase. La0.9Sr0.1Ga0.8Mg0.2O3 was used as the electrolyte material and was synthesized by a conventional solid-state reaction. The La2O3 (99.9%), SrCO3 (99.0%), Ga2O3 (99.99%), MgO (98.0%) were mixed as the stoichiometric ratio and were balling in ethyl alcohol for 5 h. After drying under a heating lamp, the powders were pelleted and calcined at 1200
C for 10 h in air. Subsequently, the pellets were ground in a mortar for 2 h, then uniaxially pressed into a disk. Afterwards, it was followed by a sintering at 1450
C for 10 h to obtain the final LSGM electrolyte. Ce0.8Sm0.2O2ed (SDC) powders were prepared by the glycine-nitrate combustion as described in literature [13]. Electrochemical performances were measured on the electrolyteesupported single cell with a 200-mm-thick LSGM as the electrolyte. The thickness of SDC buffer layer is about 10 mm [32,33]. The area and thickness of the electrode are 0.12 cm2 and 15e20 mm, respectively. The Ag grids were printed on the anode and cathode surfaces with Ag paste as current collectors. The single cells were sealed onto alumina tubes using Ag paste as sealant. 2.2. Material characterization The crystal structure of LSCFM was identified by X-ray diffractometer (XRD, Rigaku D/Maxe2550) with Cu-Ka radiation. The thermal expansion coefficient (TEC) was measured by a dilatometer (Netzsch DIL 402C) in air and 5% H2/Ar. Electrical conductivity was tested by the four-terminal DC method in air and H2, respectively. The composition and the chemical state of the as-synthesized LSCFM sample were characterized by the X-ray photoelectron spectrometer (XPS) (VG Scientific ESCALAB Mk II) with a monochromatized microfocused Al Ka (1486.6 eV) radiation source. A scanning electron microscope (SEM, JEOL JSM-6701F) was used to inspect the surface morphologies of the samples. The AC impedance spectra were obtained by a potentiostat (Solarton 1260 and 1287) with the configuration of LSCFM/SDC/LSGM/SDC/LSGM in air and H2. The maximum power densities of the single cell with LSCFM symmetrical electrode were measured at different temperatures using H2, C2H5OH, liquid petroleum gas (LPG) as fuels and ambient air as oxidant. The flowing rates of H2 and LPG (3% H2O) were controlled to 100 ml/min. The ethanol gas was provided by bubbling N2 through anhydrous ethanol at 60
C with a flowing rate

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100 ml/min. After the stability test, the electrode surface was evaluated by a Raman test to detect the possible carbon deposition. Raman spectroscopy measurement was conducted using a Raman spectrometer (RenishawinVia) equipped with an argon-ion laser operation at an excitation wavelength of 532 nm. 3. Results and discussion 3.1. Structural analysis Fig. 1 shows the XRD patterns of LSCFM after calcining at 1200
C in air and reducing in 5%H2/Ar at 1000
C. After calcining at 1200
C in air, the main phase of LSCFM is formed and a secondary phase of SrMoO4 was appeared. SrMoO4 impurity often appears in the Mo-containing materials during the synthetic process [32,34]. This impurity disappears after the sample reduced at 1000
C in 5% H2/Ar for 10 h, indicating the high solubility of Mo in LSCF under reducing conditions. LSCFM exhibits an orthorhombic structure with lattice parameters of a ¼ 5.5328 Å, b ¼ 5.5458 Å, c ¼ 7.8447 Å and V ¼ 240.7 Å3. The LSCF was synthesized at 1200
C in air and then calcined at 1000
C in 5% H2/Ar for 10 h to investigate the structural stability. Obviously, LSCF has been decomposed to complex oxides (SrFeO3, Co7Fe3, SrLaFeO4) after reducing in 5% H2/Ar and the perovskite structure is completely destroyed. Therefore, LSCFM has better structural stability than LSCF in reducing condition. To investigate the structural stability of LSCFM in oxidizing condition, the LSCFM is calcined at 850
C in air for 10 h and the XRD result was shown in Fig. 1. The peak position of the oxidized sample shifts to the higher angles, indicating that the unit-cell volume of LSCFM sample decreased. The lattice parameters of the oxidized LSCFM are a ¼ 5.4795 Å, b ¼ 5.5162 Å, c ¼ 7.9066 Å and V ¼ 238.9 Å3, decreased by 0.74% compared to the original LSCFM sample. This is due to the valences of Co, Fe, and Mo elements increased to their corresponding higher valences after oxidizing in air and the ionic radius of Co (Coþ2/þ3/þ4 ¼ 0.065/0.055/0.053 nm), Fe (Feþ2/þ3/þ4 ¼ 0.078/0.065/0.059 nm), and Mo (Moþ4/þ5/ þ6 ¼ 0.065/0.061/0.059 nm) become smaller. Similar variations of unit-cell volume in reduced and oxidized samples are shown in the LSCT symmetrical electrode [35] and Zr-doped SrFeO3 [36] symmetrical electrode. No obvious secondary phases are detected in the oxidized LSCFM sample and it remains to keep the original perovskite structure. The SEM images of the as-prepared and

Fig. 1. XRD patterns of LSCFM and LSCF in air and H2.

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oxidized LSCFM samples are depicted in Fig. 2 (a) and (b), respectively. Fig. 2 (a) reveals that the as-prepared LSCFM sample exhibit a porous structure with a tight network. After calcined in air at 850
C, the porous network still existed with a smooth surface morphology. Based on the these results, we can conclude that LSCFM can keep good structural stability under oxidizing and reducing conditions and can be applied as a symmetrical electrode for SOFCs. The chemical compatibility of the LSCFM sample with the LSGM and SDC electrolytes in air and 5% H2/Ar was assessed by calcining their respective mixtures (in a weight ratio of 1:1) at 850
C for 10 h, and the XRD results are shown in Fig. 3 (a) and (b). In comparison with the XRD patterns of LSCFM, LSGM, and SDC, neither additional diffraction peaks nor obvious XRD peak shifts are observed in the XRD patterns of the LSCFMeLSGM and LSCFMeSDC mixtures. These findings indicate that the LSCFM sample is chemically compatible with both the LSGM and SDC electrolytes in air and 5% H2/Ar. 3.2. XPS analysis XPS is carried out to determine the valences of Co, Fe and Mo elements in the as-synthesized LSCF and LSCFM samples. Fig. 4(a) shows the Co 2p core-level XPS spectra for LSCF and LSCFM samples. The spin-orbital splitting between the Co 2p3/2 and Co 2p1/2 peaks are 15.8 eV, indicating the coexistence of the Co3þ and Co2þ species [37e39]. The binding energies at 779.2 and 794.7 eV are assigned to Co3þspecies [40] and the peaks at 780.9 eV and 796.1 eV are attributed to Co2þ species [30]. The presence of an additional peaks located at 785.0 eV for LSCFM sample is a further evidence for Co2þ species similar to the results that Zhang and Oku reported [39,40]. The ratios of Co3þ/Co2þ are 1:1.2 for LSCF and 1:2.9 for LSCFM, indicating that the Mo doped and the synthesized condition of reducing atmosphere decreased the valence of Co element. The Fe 2p3/2 and Fe 2p1/2 core level spectra are deconvoluted in peaks at the binding energies of 709.9 and 722.9 eV for Fe3þ and 711.9 and 724.5 eV for Fe4þ, respectively, which are accordant coexistence with those reported in literatures [41,42]. The ratios of Fe4þ/Fe3þ are 1.2:1 for LSCF and 1:3.3 for LSCFM. This is because the higher valence of Mo element substituted the Fe, and thus the valence of Fe element decreased in order to keep electroneutrality. Fig. 4 (c) shows the XPS results of Mo 3d core level spectra. The curvefitting analysis of Mo excitations brings about four peaks that are assigned to the Mo6þ (232.1 and 235.3 eV) and Mo5þ (231.1 and 234.9 eV) species. These results are similar to those that literatures [41,43] reported. Based on the XPS analysis, the Co2þ/Co3þ, Fe3þ/Fe4þ and Mo5þ/ Mo6þ coexist in the LSCFM sample. No Co0þ or Fe0þ elements are detected in the XPS test, indicating that no mental nanoparticles exsoluted from the LSCFM sample. In the anodic operation condition of SOFCs, the mixed valences of Co, Fe, and Mo species can sufficiently accept the electrons from H2 or hydrocarbons and

Fig. 2. The SEM images of the as-prepared and oxidized LSCFM samples.

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Fig. 3. The chemical compatibility of LSCFM with LSGM and SDC (a) in air (b) in H2.

Fig. 4. Coreelevel spectra of the LSCFM and LSCF samples at room temperature: (a) Co 2p, (b) Fe 2p, and (c) Mo 3d.

exhibit good catalytic activity. In the cathode operating condition, the coexist mixed valences of Co2þ/Co3þ and Fe3þ/Fe4þ can transform between each other at high temperature. Moreover, the Co is chemically active for oxygen reduction reaction in perovskite structure [44]. Therefore, the LSCFM sample as symmetrical electrode for SOFCs should show a good electrochemical performance.

3.3. Thermal expansion and electrical conductivity analysis Thermal expansion coefficient (TEC) is a critical parameter during the cell assemble and operation for SOFCs. Fig. 5 (a) gives the TEC curves of LSCFM and LSCF at 30e850
C in air and 5%H2/Ar. The TEC curves of LSGM and SDC electrolytes are included in Fig. 3 for comparison. The average TEC values of LSCF and LSCFM are 16.6 and 12.9  106 K1, respectively, which demonstrates that the Mo

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5

Fig. 5. (a) TEC curves of LSCFM in air and H2, and LSCF, LSGM, and SDC in air. (b) Electrical conductivity results of LSCFM in air and H2 at 300e850
C.

substitution of Fe element for LSCF improves the thermal compatibility with LSGM electrolyte. The decreased TEC comes from two aspects: the first reason, the Mo substituted for Fe element, the content of Fe element was decreased, therefore, the chemical expansion of Fe4þ(0.059 nm) to Fe3þ(0.065 nm) [45] was improved. The second reason is that the Mo5þ (0.061 nm) species are oxidized to Mo6þ (0.059 nm) during the heating and the chemical expansion is relieved due to the ionic radius becomes smaller. In 5% H2/Ar, the average TEC value is 13.3  106 K1 for LSCFM sample. The average TEC values of LSCFM are slightly higher than that of LSGM and SDC electrolyte. The SDC buffer layer was introduced between the electrode and electrolyte material. The SDC buffer layer is porous and rough, which can improve the thermal expansion compatibility between the electrode and electrolyte. In addition, the Ce4þ of SDC can be reduced to Ce3þ in reducing condition, which can improve the charge transfer at the interface between the electrolyte and electrode. Moreover, SDC is an excellent oxygen ion conductor. Therefore, the SDC buffer layer can improve the thermal compatibility between the electrode and electrolyte and extend the TPB lengths and enhance the cell performance [13,32]. Shown in Fig. 5 (b) are the electrical conductivity results of LSCFM in air and H2 as a function of temperature. LSCFM shows semiconducting behavior both in oxidizing and in reducing conditions, implying the temperature activated polaron hopping mechanism. The electrical conductivity values are 2.5e8.2 S cm1 at 300e850
C in air, which are higher than those of La0.6Sr0.4Co0.2Fe0.6Nb0.2O3ed (0.5e5.5 S cm1 at 400e850
C) [8], STFM (1.2 S cm1 at 500
C) [13] and Sr2CoMoO6ed (0.88 S cm1 at 800
C) [11] symmetrical electrodes. However, the conductivity of LSCFM is much lower than that of LSCF sample (usually higher than 100 S cm1 [46,47]) in air. The reason and conduction mechanism can be explained as followings. The relationship between the charge carriers Fe4þ/Co3þ and oxygen vacancies can be described as:

1  € $ 2Fe Fe þ V O þ O2 42FeFe þ OO 2

(1)

1  € $ 2Co Co þ V O þ O2 42CoCo þ OO 2

(2)

3þ $ where Fe and Fe4þ, respectively. Co Fe and 2FeFe represent Fe Co and Co$Co denote the Co2þ and Co3þ, respectively. According to the XPS results, the number of charge carriers Fe4þ and Co3þ have been decreased obviously for Mo-doped sample, thus resulting in the decreased of electrical conductivity. Similar results have been

reported in the Mo doped SrFeO3 based material [13,48]. The other reason is that the LSCFM is synthesized in reducing condition at 1000
C with larger unit-cell volume, the density of LSCFM is inferior to that of LSCF synthesized in air at high temperature. In H2, the electrical conduction behavior can be explained by the double exchange mechanism with the charges transfer through the Co2pO2p, Fe3d-O2p, and Mo3d-O2p orbitals. The conductivity values are measured as 1.3e3.5 S cm1 at 300e850
C, which are higher than those of La0.6Ce0.1Sr0.3Fe0.9Ni0.1O3ed (CLSFNi, 0.61e0.45 S cm1 at 500e850
C in 5% H2/Ar) [6] and nanoparticles modified La0.8Sr1.2Fe0.9Co0.1O4ed (about 0.3 S cm1 at 800
C in 5% H2/Ar) symmetrical electrodes [49]. In terms of the electrical conductivity, LSCFM applied as a symmetrical electrode for SOFCs should have good electrochemical performance compared with the existed symmetrical electrodes. 3.4. Interfacial polarization resistance In order to assess the electrochemical performance and electrochemical stability of LSCFM symmetrical electrode, the electrochemical impedance tests were carried out for the symmetrical cells on LSGM electrolyte with a SDC buffer layer. Fig. 6 (a) shows the polarization resistance (Rp) values of LSCFM sample in air. The Rp values are 0.148, 0.075, 0.041, and 0.025 U cm2 at 700, 750, 800 and 850
C, respectively, which are lower than those of Nb doped LSCF symmetrical electrodes, such as the La0.6Sr0.4Co0.2Fe0.6Nb0.2O3ed (0.355 U cm2 at 800
C) [8] and La0.4Sr0.6Co0.2Fe0.7Nb0.1O3ed (0.147 U cm2 at 800
C) [50]. Compared to the Rp value of LSCF cathode (0.03 U cm2 at 800
C in air) [51], LSCFM sample shows a slightly higher Rp values. It is that the lower conductivity of LSCFM sample may lead to the problem of charge transfer, thus increasing the Rp. Besides, the oxygen vacancy concentration would be decreased due to the higher valence Mo substituted Fe element, which may influence the part of gas absorption and diffusion. It is worth noting that the Rp is lower than the expected value of 0.15 U cm2 for cathode at 700
C [52], and LSCFM is suited to a cathode for SOFCs. Fig. 6 (b) shows the impedance spectra of LSCFM in H2. The Rp values are 0.553, 0.364, 0.266, and 0.211 U cm2 at 700e850
C, respectively. The significantly higher Rp values in reducing condition may be at least partly due to the lower electronic conductivity of LSCFM in H2. Similar results are reported in La0.5Sr0.5Fe0.9Nb0.1O3-d symmetrical electrode [53]. For comparison, we summarize Rp values of the existing typical symmetrical electrodes as shown in Table 1. It is worth noting that the Rp values in air of the LSCFM symmetrical electrode obtained here are much lower than those of other previous common symmetrical electrodes (Table 1), and the Rp values in H2 can

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Fig. 6. Electrochemical impedance results of LSCFM (a) in air (b) in H2 at 700e850
C. (c) Electrochemical impedance stability test results for 50 h at 700
C in air and H2.

reach the better level compared to the typical symmetrical electrodes. Therefore, the LSCFM as symmetrical electrode for SOFC should have satisfying electrochemical performance. In order to further investigate the electrochemical stability of LSCFM as cathode and anode, 50-h continuity tests of polarization resistance are conducted as shown in Fig. 6 (c). The cathode Rp remains

unchanged of about 0.147 U cm2 at 700
C, which implies that LSCFM applied as cathode has satisfied electrochemical performance stability. However, for the LSCF cathode, it has been reported that the Rp values increase from 0.105 to 0.212 U cm2 at 800
C for 50 h [50] and 2.27 U cm2 to 4.18 U cm2 at 600
C for 100 h [61]. In addition, inadequate long-term stability of LSCF is a primary factor

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Table 1 The electrochemical performance of various symmetrical electrodes. Pmax (mW cm2)

Rp(U cm2) (800
C)

SmBaMn2O5þd Sr2Fe1.5Mo0.5O6 Sr2Fe1.4Nb0.1Mo0.5O6 Sr2Co1.15Mo0.85O6 Sr2TiFe0.9Mo0.1O6 La0.5Sr0.5Co0.5Ti0.5O3-d Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-d La0.8Sr1.2 Fe0.9Co0.1O4 ±d La0.6Ca0.4Fe0.8Ni0.2O3-d La0.8Sr0.2Fe0.7Ni0.3O3-d La0.6Ce0.1Sr0.3Fe0.9Ni0.1O3-d La0.6Sr0.4Co0.2Fe0.6Nb0.2 O3-d La0.5Sr0.5Fe0.9Nb0.1 O3-d Sm0.8Sr0.2Fe0.8Ti0.15Ru0.05 O3-d (La0.8Sr0.2)0.8Sc0.2Mn0.775Ru0.025 O3-d La0.8Sr0.2MnO3 La0.6Sr0.4Co0.2Fe0.7Mo0.1O3-d

Air

H2

H2

0.27 0.24 0.10 0.15 0.06 e e 0.29 0.22 0.10 e 0.35 0.06 0.16 0.23 1.44 0.04

1.23 0.27 0.22 0.46 0.27 e 0.44 1.14 e 0.06 e 0.59 0.24 0.19 0.52 5.19 0.27

326 (800
C, 300 mm LSGM) [54] 500 (800
C, 265 mm LSGM) [9] 531 (800
C, 243 mm LSGM) [55] 460 (800
C, 300 mm LSGM) [11] 444 (800
C, 200 mm LSGM) [13] 110 (800
C, 300 mm LSGM) [5] 780 (850
C, 265 mm LSGM) [34] 237 (800
C, 1000 mm LSGM) [27] 140 (800
C, 280 mm SDC) [56] 540 (800
C, 350 mm LSGM) [57] 889 (850
C, 300 mm LSGM) [6] 400 (800
C, 200 mm LSGM) [8] 1000 (850
C, 300 mm LSGM) [53] 417 (800
C, 600 mm SDC) [58] 318 (800
C, 200 mm SSZ) [59] 151 (800
C, 400 mm YSZ) [60] 929 (850
C, 270 mm LSGM) 737 (800
C, 270 mm LSGM)

impeding the widely practical commercial applications, which have been much reported by literatures [62,63]. Therefore, the LSCFM sample has better electrochemical stability as a cathode than that of LSCF. In H2, the Rp increases from 0.552 to 0.581 U cm2, which shows acceptable electrochemical performance stability. The LSCFM sample is synthesized in reducing condition, so it should keep the structure stable in H2. We speculate that the increased Rp is the reason of the exfoliation of current collector Ag because of the unmatched thermal expansion coefficients (TECs). The detailed degradation mechanism will be further researched in the future. 3.5. Fuel cell performance and stability Fig. 7 (a) provides the I-V and I-P curves of the electrolytesupported single cell with LSCFM electrode at 550e850
C under H2 fuel. The Pmax values are 265, 425, 570, 737, and 929 mWcm2 at 650e850
C. The performances in H2 are relatively satisfied, which are higher than those of Sr2Fe1.5Mo0.5O6ed [9] and the nanoparticles decorated La0.6Ce0.1Sr0.3Fe0.9Ni0.1O3ed (CLSFNi) [6] symmetrical electrodes as shown in Table 1. Even if at the low temperatures of 550 and 600
C, the Pmax of 82 and 168 mW cm2 can be obtained, which indicates that LSCFM may be applied as low-temperature symmetrical electrode for SOFCs. Recently, ethanol as a fuel can be easily obtained from biomass fermentation and easy to store and transport, which has attracted extensive attentions for SOFCs. The conventional nickel-cermet anode is liable to inactive due to the quick carbon build-up when directly fed with ethanol fuel. Fig. 7 (b) shows the electrochemical performance of a single cell in ethanol. The Pmax reaches 99, 303, 405, and 481 mW cm2 at 700e850
C. The performances are superior to that of (La0.6Sr0.4)0.90Co0.2Fe0.6Nb0.2O3 (62 mW cm2 at 800
C in ethanol) [8] and slightly lower than that of the anode supported cell with Cu-CeO2 impregnated Ni-YSZ anode (400 mW cm2) [64]. Under ethanol fuel, the followed reactions may occur:

C2 H5 OH þ 6O2 / 2CO2 þ3H2 O þ 12e

(1)

C2 H5 OH þ 3H2 O/2CO2 þ6H2

(2)

C2 H5 OH / C2 H4 O þ H2 /CH4 þCO þ H2

(3)

CH4 / C þ 2H2

(4)

(i) The C2H5OH was converted by a direct electrochemical reaction by oxide ions from the electrolyte as Eq. (1); (ii) The ethanol steam reforming (ESR) as Eq. (2) at the anode side during the cell operation; (iii) The ethanol decomposition reaction (3) and methane cracking (4) with the carbon deposited electrode surface. Overmuch carbon can be eliminated by operating the anode as a cathode. In order to assess the electrochemical performance of the single cell in the hydrocarbon containing sulfide, the Liquid Petroleum Gas (LPG including C3H8 (90%), C4H10, olefins, and H2S (<100 ppm)) was used as the fuel and the result is shown in Fig. 6 (c). At 700e800
C, the Pmax values are 146, 352, and 394 mW cm2, respectively, exhibiting satisfying power density. The stability of electrochemical performance is significant for the single cell. The stability tests of the single cell with LSCFM symmetrical electrode were carried out in H2 and LPG at 700
C and 850
C, respectively. Fig. 8 (a) shows the stability test results with the constant voltage discharge at 0.6 V and 0.4 V at 700
C and a constant current discharge at 0.05 A at 850
C in H2. It can be seen that the output performance almost keeps unchanged during the test process except for a slight fluctuation, indicating the single cell with LSCFM symmetrical electrode can operate well in H2 at 700
C and 850
C, and the LSCFM material can keep structural stable for a long time in reducing condition. For SOFCs, one characteristic is that it can directly feed with the hydrocarbon fuel. The stability test of a single cell with LSCFM symmetrical electrode was carried out in LPG with the constant current discharge at 700
C and 850
C as shown in Fig. 8 (b). It can be seen that in the figure, there was no obvious decline of the voltage during the measurement at both 700
C and 850
C. It indicates that the single cell with LSCFM symmetrical electrode can operate stability in hydrocarbon fuels containing sulfur compound. Fig. 9 shows the Raman result on the electrode surface after stability test. The deposited carbons by two forms can be detected: disordered carbons (D-band) and graphitic carbons (D0 -band) [13]. A small number of deposited carbons can improve the electrical conductivity of electrode, thus are benefit for improving electrode performance. Due to the complex gas composition in LPG, the reaction mechanism in LPG is difficult to identify. Fig. 10 (a) shows the SEM result of electrode surface after stability test of the single cell in LPG at 850
C. It can be seen that some disorder carbon distributed on the electrode surface and not block the gas channels, which is in agreement with the Raman result. Fig. 10 (b) shows the SEM result of the interface between the

8

C. Lu et al. / Electrochimica Acta 323 (2019) 134857

Fig. 7. I-V and I-P curves of single cell with LSCFM (a) at 550e850
C in H2, (b) at 700e850
C in ethanol, and (c) at 700e800
C in Liquid Petroleum Gas.

current collector Ag and electrode surface. No phenomenon of Ag diffused to electrode is observed, however, large amount of carbon deposited on the surface of Ag due to no oxygen ions are proved to oxidize the carbon. The EDS and XPS results of LSCFM electrode surface after stability test are shown in Fig. 10 (c) and (d), respectively. The elements of La, Sr, Co, Fe, Mo, and C are detected on the electrode surface (Fig. 10 (c)) from the EDS result. No elements of Ag

or S are detected by the EDS test. For XPS spectra, the binding energies of sulfide or S are usually at around 160e170 eV [65e67], no peaks (the blue circle area) were observed in the XPS survey result of LSCFM electrode surface in Fig. 10(d). The EDS and XPS results indicate that no diffusion of Ag and no any sulfur compounds are deposited on the electrode surface. Based on the above results, we can conclude that LSCFM electrode can tolerate carbon deposition

C. Lu et al. / Electrochimica Acta 323 (2019) 134857

9

Fig. 8. Electrochemical performance stability test of single cell (a) in H2, (b) in Liquid Petroleum Gas at 700
C and 850
C.

SOFCs. The incorporation of Mo element on the B-site in La0.6Sr0.4Co0.2Fe0.8O3 significantly improves the chemical and phase stability under the reducing condition. The LSCFM possesses satisfied structural and electrochemical stability both in oxidizing and in reducing conditions. At 850
C, the polarization resistance values of the symmetrical cells with LSCFM electrode are 0.025 U cm2 and 0.211 U cm2 in air and H2. Even at 700
C, the Rp is 0.147 U cm2, satisfying the requirement of the traditional cathode for SOFCs. LSGM-supported SSOFCs with LSCFM electrode shows good electrochemical performance in H2, C2H5OH, and LPG. The single cell tests of LSCFM symmetrical electrode demonstrate the characteristic of fuel flexibility for SOFCs. Stable electrochemical performances of the single cell with LSCFM electrode have been obtained in H2 and ethanol fuels. In whole, LSCFM is a promising candidate material for SSOFCs. Acknowledgment Fig. 9. Raman spectra of the LSCFM electrode surface after stability test.

This work was supported by the National Natural Science Foundation of China (No.9174523) and the Fundamental Research (Discipline Arrangement) project funding from Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20170412154554048). References

Fig. 10. The SEM images of (a) the LSCFM electrode surface after stability test at 850
C in LPG, (b) the interface between current collector Ag and electrode. (c) The EDS and (d) XPS results of the LSCFM electrode surface after stability test at 850
C in LPG.

and resist sulfur poisoning as long as the abundant O2 is provided.

4. Conclusions A single-phase oxide of La0.6Sr0.4Co0.2Fe0.7Mo0.1O3d has been synthesized and carefully evaluated as a symmetrical electrode for

[1] Y.F. Li, W.Q. Zhang, Y. Zheng, J. Chen, B. Yu, Y. Chen, M, L. Liu, Controlling cation segregation in perovskite based electrodes for high electro-catalytic activity and durability, Chem. Soc. Rev. 46 (2017) 6345e6378. [2] N. Mahato, A. Banerjee, A. Gupta, S. Omar, K. Balani, Progress in material selection for solid oxide fuel cell technology: a review, Prog. Mater. Sci. 72 (2015) 141e337. , Z.P. Shao, Progress and prospects in [3] C. Su, W. Wang, M.L. Liu, M.O. Tade symmetrical solid oxide fuel cells with two identical electrodes, Adv. Energy Mater. 5 (2015) 150e188. [4] D.M. Bastidas, S.W. Tao, J.T.S. Irvine, A symmetrical solid oxide fuel cell demonstrating redox stable perovskite electrodes, J. Mater. Chem. 16 (2006) 1603e1605. [5] R. Martı'nez-Coronado, A. Aguadero, D. Pe'rez-Coll, L. Troncoso, J.A. Alonso, M.T. Ferna'ndez-Dı'az, Characterization of La0.5Sr0.5Co0.5Ti0.5O3 as symmetrical electrode material for intermediate-temperature solid-oxide fuel cells, Int. J. Hydrogen Energy 37 (2012) 18310e18318. [6] L.Z. Bian, C.C. Duan, L.J. Wang, R. O’Hayre, J. Cheng, K.C. Chou, Ce-doped La0.7Sr0.3Fe0.9Ni0.1O3 as symmetrical electrodes for high performance direct hydrocarbon solid oxide fuel cells, J. Mater. Chem. A 5 (2017) 15253e15259. [7] A. Hartley, M. Sahibzada, M. Weston, I.S. Metcalfe, D. Mantzavinos, La0.6Sr0.4Co0.2Fe0.8O3 as the anode and cathode for intermediate temperature solid oxide fuel cells, Catal. Today 55 (2000) 197e204. [8] B.B. Niu, F.J. Jin, T. Feng, L.L. Zhang, Y. Zhang, T.M. He, A-site deficient (La0.6Sr0.4)1exCo0.2Fe0.6Nb0.2O3 symmetrical electrode materials for solid oxide fuel cells, Electrochim, Acta 270 (2018) 174e182. [9] Q. Liu, X. Dong, G. Xiao, F. Zhao, F. Chen, A novel electrode material for symmetrical SOFCs, Adv. Mater. 22 (2010) 5478e5482. [10] J.H. Wright, A.V. Virkar, Q. Liu, F. Chen, Electrical characterization and water sensitivity of Sr2Fe1.5Mo0.5O6 as a possible solid oxide fuel cell electrode, J. Power Sources 237 (2013) 13e18. [11] T. Wei, Q. Zhang, Y.H. Huang, J.B. Goodenough, Cobalt-based double

10

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

C. Lu et al. / Electrochimica Acta 323 (2019) 134857 perovskite symmetrical electrodes with low thermal expansion for solid oxide fuel cells, J. Mater. Chem. 22 (2012) 225. W. Li, Y. Cheng, Q.J. Zhou, T. Wei, Z. Li, H. Yan, Z. Wang, X. Han, Evaluation of double perovskite Sr2FeTiO6 as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceram. Int. 41 (2015) 12393e12400. B.B. Niu, F.J. Jin, L.L. Zhang, P.F. Shen, T.M. He, Performance of double perovskite symmetrical electrode materials Sr2TiFe1-xMoxO6 (x ¼ 0.1, 0.2) for solid oxide fuel cells, Electrochim. Acta 263 (2018) 217e227. Y.Q. Li, Z.H. Wang a, J.W. Li, X.B. Zhu, Y.H. Zhang, X.Q. Huang, Y.J. Zhou, L. Zhu, Z. Lü, Sulfur poisoning and attempt of oxidative regeneration of La0.75Sr0.25Cr0.5Mn0.5O3ed anode for solid oxide fuel cell, J. Alloy. Comp. 698 (2017) 794e799. S.P. Jiang, Y.M. Ye, T.M. He, S.B. Ho, Nanostructured palladiumeLa0.75Sr0.25Cr0.5Mn0.5O3/Y2O3eZrO2 composite anodes for direct methane and ethanol solid oxide fuel cells, J. Power Sources 185 (2008) 179e182. L. Zhu, B. Wei, Y.H. Zhang, Z. Lü, Z.H. Wang, X.Q. Huang, Z.Q. Cao, W. Jiang, Y.Q. Li, Investigation on a novel composite solid oxide fuel cell anode with La0.6Sr0.4Co0.2Fe0.8O3 derived phases, Electrochim. Acta 160 (2015) 89e93. zquez, B. Ballesteros-Pe rez, J. Pen ~ a-Martínez, J.C. Ruiz-Morales, J. Canales-Va  pez, J.T.S. Irvine, P. Nún ~ ez, J. Eur. Ceram. Soc. 27 (2007) D. Marrero-Lo 4223e4227. Q. Zhou, C. Yuan, D. Han, T. Luo, J.L. Li, Z.L. Zhan, Evaluation of LaSr2Fe2CrO9 as a Potential Electrode for symmetrical solid oxide fuel, Electrochim. Acta 133 (2014) 453e458. Y.C. Zhang, Q.J. Zhou, T.M. He, La0.7Ca0.3CrO3eCe0.8Gd0.2O1.9 composites as symmetrical electrodes for solid-oxide fuel cells, J. Power Sources 196 (2011) 76e83. B. Lin, S.L. Wang, X.Q. Liu, G.Y. Meng, Simple solid oxide fuel cells, J. Alloy. Comp. 490 (2010) 214e222. X.B. Zhu, Z. Lü, B. Wei, X.Q. Huang, Y.H. Zhang, W.H. Su, A symmetrical solid oxide fuel cell prepared by dry-pressing and impregnating methods, J. Power Sources 196 (2011) 729e733. X.J. Liu, D. Han, Y.C. Zhou, X. Meng, H. Wu, J.L. Li, F.R. Zeng, Z.L. Zhan, Scsubstituted La0.6Sr0.4FeO3 mixed conducting oxides as promising electrodes for symmetrical solid oxide fuel cells, J. Power Sources 246 (2014) 457e463. X. Meng, X.J. Liu, D. Han, H. Wu, J.L. Li, Z.L. Zhan, Symmetrical solid oxide fuel cells with impregnated SrFe0.75Mo0.25O3 electrodes, J. Power Sources 252 (2014) 58e63. C.H. Yang, Z.B. Yang, C. Jin, G.L. Xiao, F.L. Chen, M.F. Han, Sulfur-tolerant redoxreversible anode material for direct hydrocarbon solid oxide fuel cells, Adv. Mater. 24 (2012) 1439e1443. T. Jardiel, M.T. Caldes, F. Moser, J. Hamon, G. Gauthier, O. Joubert, New SOFC electrode materials: the Ni-substituted LSCM-based compounds (La0.75Sr0.25)(Cr0.5Mn0.5xNix)O3d and (La0.75Sr0.25)(Cr0.5xNixMn0.5)O3d, Solid State Ion. 181 (2010) 894. T. Delahaye, T. Jardiel, O. Joubert, R. Laucournet, G. Gauthier, M.T. Caldes, Electrochemical properties of novel SOFC dual electrode La0.75Sr0.25Cr0.5Mn0.3Ni0.2O3d, Solid State Ion. 184 (2011) 39. B.-K. Lai, K. Kerman, S. Ramanathan, Nanostructured La0.6Sr0.4Co0.8Fe0.2O3/ Y0.08Zr0.92O1.96/La0.6Sr0.4Co0.8Fe0.2O3 (LSCF/YSZ/LSCF) symmetric thin film solid oxide fuel cells, J. Power Sources 196 (2011) 1826e1832. Z.B. Yang, N. Xu, M.F. Han, F.L. Chen, Performance evaluation of La0.4Sr0.6Co0.2Fe0.7Nb0.1O3 as both anode and cathode material in solid oxide fuel cells, Int. J. Hydrogen Energy 39 (2014) 7402e7406. N.B. Childs, A. Wesinstein, R. Smith, S. Sofie, C. Key, Electrical conductivity of Sr2-xVMoO6 (X¼0.0, 0.1, 0.2) double perovskite, J. Appl. Phys. 113 (2013) 243506. P. Zhang, Y.H. Huang, J.G. Cheng, Z.Q. Mao, J.B. Goodenough, Sr2CoMoO6 anode for solid oxide fuel cell running on H2 and CH4 fuels, J. Power Sources 196 (2011) 1738e1743. P. Zhang, G.Q. Guan, D.S. Khaerudini, X.G. Hao, C.F. Xue, M.F. Han, Y. Kasai, A. Abudula, B-site Mo-doped perovskite Pr0.4Sr0.6 (Co0.2Fe0.8)1-xMoxO3 (x ¼0, 0.05, 0.1 and 0.2) as electrode for symmetrical solid oxide fuel cell, J. Power Sources 276 (2015) 347e356. B.B. Niu, F.J. Jin, R.J. Fu, T. Feng, Y. Shen, J.C. Liu, T.M. He, Pd-impregnated Sr1.9VMoO6 double perovskite as an efficient and stable anode for solid-oxide fuel cells operating on sulfur-containing syngas, Electrochim. Acta 274 (2018) 91e102. B.B. Niu, F.J. Jin, J.C. Liu, Y. Zhang, P.P. Jiang, T. Feng, B.M. Xu, T.M. He, Highly carbon- and sulfur-tolerant Sr2TiMoO6 double perovskite anode for solid oxide fuel cells, Int. J. Hydrogen Energy 44 (2019), 20404e20415. Z.X. Xie, H.L. Zhao, T. Chen, X. Zhou, Z.H. Du, Synthesis and electrical properties of Al-doped Sr2MgMoO6 as an anode material for solid oxide fuel cells, Int. J. Hydrogen Energy 36 (2011), 7527e7264. R. Martı'nez-Coronado, A. Aguadero, D. Perez-Coll, L. Troncoso, J.A. Alonso, M.T. Fernandez-Diaz, Characterization of La0.5Sr0.5Co0.5Ti0.5O3 as symmetrical electrode material for intermediate-temperature solid-oxide fuel cells, Int. J. Hydrogen Energy 37 (2012) 18310e18318. L. Santos-Gomez, J.M. Compana, S. Bruque, E.R. Losilla, D. Marrero-Lopez, Symmetric electrodes for solid oxide fuel cells based on Zr-doped SrFeO3, J. Power Sources 279 (2015) 419e427. L.Q. Chen, F.L. Yuan, Z.B. Lia, X.Y. Niu, Y.J. Zhu, Synergistic effect between the redox property and acidity on enhancing the low temperature NH3-SCR

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52] [53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

activity for NOx removal over the Co0.2CexMn0.8-xTi10 (x¼0e0.40) oxides catalysts, Chem. Eng. J. 354 (2018) 393e406. F.X. Zhang, S.J. Zhang, N.J. Guan, E. Schreier, M. Richter, R. Eckelt, R. Fricke, NO SCR with propane and propene on Co-based alumina catalysts prepared by coprecipitation, Appl. Catal., B 73 (2007) 209e219. P. Zhang, G.Q. Guan, D.S. Khaerudini, X.G. Hao, C.F. Xue, M.F. Han, Y. Kasai, A. Abudul, Evaluation of performances of solid oxide fuel cells with symmetrical electrode material, J. Power Sources 266 (2014) 241e249. M. Oku, Y. Sato, In-situ X-ray photoelectron spectroscopic study of the reversible phase transition between CoO and Co3O4 in oxygen of 10-3 Pa, Appl, Surf. Sci. 55 (1992) 37e41. F.J. Jin, L. Li, T.M. He, NdBaCo2/3Fe2/3Cu2/3O5 double perovskite as a novel cathode material for CeO2- and LaGaO3-based solid oxide fuel cells, J. Power Sources 273 (2015) 591e599. T. Chen, S.L. Pang, X.Q. Shen, X.N. Jiang, W.Z. Wang, Evaluation of Ba-deficient PrBa1-xFe2O5 oxides as cathode materials for intermediate-temperature solid oxide fuel cells, RSC Adv. 6 (2016) 13829e13836. K.N. Sun, J. Liu, J. Feng, H. Yuan, M.J. He, C.M. Xu, Z.H. Wang, W. Sun, J.S. Qiao, Investigation of B-site doped perovskites Sr2Fe1.4X0.1Mo0.5O6 (X¼Bi, Al, Mg) as high-performance anodes for hybrid direct carbon fuel cell, J. Power Sources 365 (2017) 109e116. R. Jacobs, T. Mayeshiba, J. Booske, D. Morgan, Material discovery and design principles for stable, high activity perovskite cathodes for solid oxide fuel cells, Adv. Energy Mater. 8 (2018) 1702708. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751e767. A. Mineshige, J. Izutsu, M. Nakamura, K. Nigaki, J. Abe, M. Kobune, S. Fujii, T. Yazawa, Introduction of A-site deficiency into La0.6Sr0.4Co0.2Fe0.8O3 and its effect on structure and conductivity, Solid State Ion. 176 (2005) 1145e1149. F.Y. Shen, K. Lu, Perovskite-type La0.6Sr0.4Co0.2Fe0.8O3, Ba0.5Sr0.5Co0.2Fe0.8O3, and Sm0.5Sr0.5Co0.2Fe0.8O3 cathode materials and their chromium poisoning for solid oxide fuel cells, Electrochim. Acta 211 (2016) 445e452. G.L. Xiao, Q. Liu, S.W. Wang, V.G. Komvokis, M.D. Amiridis, A. Heyden, S.G. Ma, F.L. Chen, Synthesis and characterization of Mo-doped SrFeO3 as cathode materials for solid oxide fuel cells, J. Power Sources 202 (2012) 63e69. J. Zhou, T.-H. Shin, C.S. Ni, G. Chen, K. Wu, Y.H. Cheng, J.T.S. Irvine, In situ growth of nanoparticles in layered perovskite La0.8Sr1.2Fe0.9Co0.1O4d as an active and stable electrode for symmetrical solid oxide fuel cells, Chem. Mater. 28 (2016) 2981e2993. Z.B. Yang, Y. Chen, N. Xu, Y.S. Niu, M.F. Han, F.L. Chen, Stability investigation for symmetric solid oxide fuel cell with La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-d electrode, J. Electrochem. Soc. 162 (7) (2015) F718eF721. A. Esquirol, N.P. Brandon, J.A. Kilner, M. Mogensen, Electrochemical characterization of La0.6Sr0.4Co0.2Fe0.8O3 cathodes for intermediate-temperature SOFCs, J. Electrochem. Soc. 151 (11) (2004). A1847A1855. B.C.H. Steele, Survey of materials selection for ceramic fuel cells II. Cathodes and anodes, Solid State Ion. 8688 (1996) 1223e1234. L.Z. Bian, C.C. Duan, L.J. Wang, L.Z. Zhu, R. O’Hayre, K.-C. Chou, Electrochemical performance and stability of La0.5Sr0.5Fe0.9Nb0.1O3-d symmetric electrode for solid oxide fuel cells, J. Power Sources 399 (2018) 398e405. Y. Zhang, H.L. Zhao, Z.H. Du, K. Swierczek, Y.Y. Li, High performance SmBaMn2O5þd electrode for symmetrical solid oxide fuel cell, Chem. Mater. (2019), https://doi.org/10.1021/acs.chemmater.9b01012. M.L. Gou, R.Z. Ren, W. Sun, C.M. Xu, X.G. Meng, Z.H. Wang, J.S. Qiao, K.N. Sun, Nb-doped Sr2Fe1.5Mo0.5O6-d electrode with enhanced stability and electrochemical performance for symmetrical solid oxide fuel cells, Ceram. Int. (2019). https://doi.org/10.1016/j.ceramint.2019.03.130. X.F. Ding, H. Liu, Z.P. Gao, G.X. Hua, L.X. Wang, L.M. Ding, G.L. Yuan, La0.6Ca0.4Fe0.8Ni0.2O3-Sm0.2Ce0.8O1.9 composites as symmetrical bi-electrodes for solid oxide fuel cells through infiltration and in-situ exsolution, Int. J. Hydrogen Energy 42 (39) (2017) 1e10. O.B. Mya, L.D. Santos-Gomez, J.M. Porras-Vazquez, M. Omari, J.R. RamosBarrado, D. Marrero-Lopez, La1-xSrxFe0.7Ni0.3O3 as both cathode and anode materials for solid oxide fuel cells, Int. J. Hydrogen Energy 42 (2017) 23160e23169. W.W. Fan, Z. Sun, Y. Bai, K. Wu, Y.H. Cheng, Highly stable and efficient perovskite ferrite electrode for symmetrical solid oxide fuel cells, ACS Appl. Mater. Interfaces 11 (2019) 23168e23179. J. Zhou, N. Wang, J.J. Cui, J.K. Wang, J.M. Yang, Q.C. Chen, X.C. Zheng, K. Wu, Structural and electrochemical properties of B-site Ru-doped (La0.8Sr0.2)0.9Sc0.2Mn0.8O3-d as symmetrical electrodes for reversible solid oxide cells, J. Alloy. Comp. 792 (2019) 1132e1140. X.Y. Luo, Y. Yang, Y. Yang, D. Tian, X.Y. Lu, Y.H. Chen, Q. Huang, B. Lin, Reduced-temperature redox-stable LSM as a novel symmetrical electrode material for SOFCs, Electrochim. Acta 260 (2018) 121e128. T. Hong, K. Brinkman, C.R. Xia, Copper oxide as a synergistic catalyst for the oxygen reduction reaction on La0.6Sr0.4Co0.2Fe0.8O3-d perovskite structured electrocatalyst, J. Power Sources 329 (2016) 281e289. M.E. Lynch, L. Yang, W.T. Qin, J.-J. Choi, M.F. Liu, K. Blinn, M.L. Liu, Enhancement of La0.6Sr0.4Co0.2Fe0.8O3-d durability and surface electrocatalytic activity by La0.85Sr0.15MnO3-d investigated using a new test electrode platform, Energy Environ. Sci. 4 (2011) 2249e2258. S.P. Simner, M.D. Anderson, M.H. Engelhard, J.W. Stevenson, Degradation mechanisms of LaeSreCoeFeeO3 SOFC cathodes, Electrochem. Solid State

C. Lu et al. / Electrochimica Acta 323 (2019) 134857 Lett. 9 (10) (2006) A478eA481. [64] E.N. Armstrong, J.W. Park, N.Q. Minh, High-performance direct ethanol solid oxide fuel cells, Electrochem. Solid State Lett. 15 (5) (2012) B75B77. [65] T. Ohnoa, M. Akiyoshi, T. Umebayashi, K. Asai, Takahiro Mitsui, M. Matsumura, Preparation of S-doped TiO2 photocatalysts and their photocatalytic activities under visible light, Appl. Catal., A 265 (2004) 115e121.

11

[66] T. Boningari, S.N.R. Inturi, M. Suidan, P.G. Smirniotisa, Novel one-step synthesis of sulfur doped-TiO2 by flame spray pyrolysis for visible light photocatalytic degradation of acetaldehyde, Chem. Eng. J. 339 (2018) 249e258. [67] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Physical electronics, Inc., 1995, p. 61.