Electrochemical characterization of Fe–rich BaFe0.7Co0.2Nb0.1O3–δ as cathode material for IT–SOFC

Journal Pre-proof Electrochemical characterization of Fe–rich BaFe0.7Co0.2Nb0.1O3–δ as cathode material for IT–SOFC

Yue–e Chen, Hongdong Cai, Jingsheng Xu, Lin Qu, Leilei Zhang PII:

S1293-2558(19)30476-5

DOI:

https://doi.org/10.1016/j.solidstatesciences.2019.106005

Article Number:

106005

Reference:

SSSCIE 106005

To appear in:

Solid State Sciences

Received Date:

21 April 2019

Accepted Date:

11 September 2019

Please cite this article as: Yue–e Chen, Hongdong Cai, Jingsheng Xu, Lin Qu, Leilei Zhang, Electrochemical characterization of Fe–rich BaFe0.7Co0.2Nb0.1O3–δ as cathode material for IT– SOFC, Solid State Sciences (2019), https://doi.org/10.1016/j.solidstatesciences.2019.106005

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Journal Pre-proof

Electrochemical characterization of Fe–rich BaFe0.7Co0.2Nb0.1O3–δ as cathode material for IT–SOFC Yue–e Chena, Hongdong Caib, Jingsheng Xub, Lin Qub, Leilei Zhangb* a Yanshan

b

University, Qinhuangdao, 066004, P.R. China

College of Sciences, Liaoning Shihua University, Fushun, 113001, PR China

Abstract Fe–rich BaFe0.7Co0.2Nb0.1O3–δ (BFCNb) oxide has been evaluated as the highly active cathode catalyst for IT–SOFC. Cubic structure with space group of Pm3m is achieved through Co and Nb co–doping at B–sites of BaFeO3–δ. Though BFCNb exhibits poor electronic conductivity, the oxygen non–stoichiometry (δ) was demonstrated to be as high as 0.564–0.602 at 600–800 oC. Such high oxygen vacancy concentration obviously facilitates the catalytic activation for oxygen reduction. As expected, the BFCNb cathode exhibits excellent electrochemical performance: the polarization resistance of 0.017–0.056 Ω cm2 is achieved at 800–700 oC for BFCNb cathode on GDC electrolyte; the power output for a 300–μm–thick GDC supported single cell with BFCNb cathode attains 583–320 mW cm–2 at 800–700 oC and 0.5 V. However, the SOFC exhibits a performance loss of ~1.7%–0.86% after a 40 h short–term stability test. This may be due to the agglomeration of the BFCNb cathode particles and/or the strong chemical expansion of the BFCNb cathode. Keywords: IT–SOFC; BaFe0.7Co0.2Nb0.1O3–δ cathode; polarization resistance; oxygen

* Corresponding author: Leilei Zhang (Liaoning Shihua University), Tel.: +86 024 57605555. E–mail address: [email protected] (L.L. Zhang) 1

Journal Pre-proof reduction mechanism; electrochemcial stability 1. Introduction Converting the chemical energy of fuels directly into electrical energy using the SOFC system has been paid more and more attention due to the advantages of high conversion efficiency, low emission, all solid–state configuration, and so on. At present, much effort has been devoted to intermediate temperature (IT, 600–800 oC) SOFCs, which can prolong the lifetime of cell components, reduce the running and manufacturing cost of SOFCs, and enlarge the choice of SOFC materials. Nowadays, mixed

ionic

and

electronic

conducting

(MIEC)

perovskite

oxide

La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) was widely investigated as one of the most potential candidate materials for the intermediate-low temperature SOFC cathode [1–4]. Compared with traditional LSM/YSZ composite cathode, these MIEC cathodes enlarge the reaction active sites from triple phase boundaries (TPB, including electrode, electrolyte and reactant) to the whole cathode surface, and reduce the operating temperature of ~100 oC while maintaining the former performance of the SOFC with LSM/YSZ cathode [5]. However, to our knowledge, the biggest issue for the LSCF cathode is the Sr segregation from the cathode surface to form second phase, such as SrO or SrCO3, which has been confirmed to degrade the performance of SOFCs [6]. Additionally, the reaction between LSCF cathode and YSZ electrolyte to form SrZrO3 phase also contributes to the performance degradation of the SOFCs with YSZ electrolyte [7].

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Journal Pre-proof Co–rich perovskite oxide BaCo0.7Fe0.2Nb0.1O3–δ was reported to possess especially high oxygen permeability and good structure stability [8]. Here, the Nb5+ is a stable cation which plays an important role in the stability of perovskite structure [9]. Recently, the BaCo0.7Fe0.2Nb0.1O3–δ was evaluated as a promising candidate cathode material for IT–SOFC and the maximum power density (Pmax) of the SOFC using BaCo0.7Fe0.2Nb0.1O3–δ as cathode and 300 μm La0.8Sr0.2Ga0.83Mg0.17O3–δ (LSGM) as electrolyte attains 550 mW cm–2 at 700 oC [10]. The main drawbacks for this cathode is its poor chemical stability [11] and high thermal expansion coefficient (TEC, 18.2 × 10–6 K–1 at 25–800 oC) [10], which may result in negative effect on the performance of SOFC due to the chemical and/or thermal expansion incompatibility problems between the cathode and electrolyte components. Chen et al. [12] reported that with increasing Fe content in the BaCo0.9–yFeyNb0.1O3–δ system, the TEC was found to reduce monotonously from y=0.0 to 0.6 and the composition with y=0.6 reaches a TEC as low as 16.1 × 10–6 K–1 at 25–800 oC. Thus it is reasonable to expect that increasing Fe content to y=0.7 is very promising for a further decrease in TEC. However, up to date, no study has been reported on the performance of Fe–rich BaFe0.7Co0.2Nb0.1O3–δ (BFCNb) cathode for IT–SOFC. In addition, no studies on the mechanism of oxygen reduction for the BaFeO3–based perovskite cathodes have been published. In the present study, Fe–rich BaFe0.7Co0.2Nb0.1O3–δ (BFCNb) perovskite oxide was evaluated as cathode of IT–SOFC based on GDC electrolyte. The oxygen reduction mechanism of the BFCNb cathode was elucidated and discussed and the main

3

Journal Pre-proof rate–limiting steps for the cathode oxygen reduction were also determined. Furthermore, the oxygen non–stoichiometry (δ), electrical conductivity, thermal expansion and electrochemical performance of the BFCNb cathode were systematically studied. In the end, a 40–hour performance stability for the BFCNb|GDC|NiO–SDC was detected to monitor the stability of the BFCNb cathode. 2. Experimental 2.1 Sample synthesis and cell fabrication The BFCNb sample were prepared by the solid–state reaction method. Stoichiometric amounts of BaCO3, Fe2O3, Co3O4 and Nb2O5 were selected as starting materials to yield BFCNb. The mixture of these raw materials was thoroughly ground and calcined at 900 oC for 10 hours. The obtained precursor was ground again and then pressed into pellets. The pellets were subsequently divided into two parts for final sintering at different temperatures: one part pellets were sintered at 1000 oC for 10 hours, while the other pellets were sintered at 1100 oC for 10 hours. The Gd0.1Ce0.9O1.95 (GDC), Sm0.2Ce0.8O1.9 (SDC) and NiO powders used in this study were prepared by a glycine–nitrate process (GNP) [13]. The dense GDC electrolyte disks were obtained by sintering at 1400 oC for 10 hours. The NiO–SDC composite anode with a weight ratio of 60:40 was prepared by physical mixing. The BFCNb|GDC|BFCNb symmetrical cell fabricated by a screen–printing technology was used for the measurement of impedance spectra. The BFCNb slurry was printed onto both side of the GDC electrolyte substrate (~300 μm thick) and then calcined at 950

oC

for 2 hours to obtain the final symmetrical cell. The

4

Journal Pre-proof BFCNb|GDC|NiO–SDC single cell was used to characterize the comprehensive performance of the BFCNb cathode on GDC electrolyte. The same fabrication technology to the symmetrical cell was adopted. The NiO–SDC slurry was firstly painted onto one surface of GDC substrate (~300 μm thick) and calcined at 1250 oC for 4 hours. And then the BFCNb slurry was printed onto the other surface of GDC substrate

and

finally

calcined

at

950

oC

for

2

hours

to

obtain

the

BFCNb|GDC|NiO–SDC single cell. The slurries used for cell fabrication were prepared by mixing the electrode powders with a homemade organic binder consisting of 10 wt.% ethyl cellulose and 90wt.% terpineol. The effective area of the electrode for both the symmetrical cell and the single cell was controlled to be ~0.2 cm2. Ag paste was painted onto the cathode and anode surfaces to be used as current collector. To reduce the effects of Ag current collector on the effective contact area and the electrochemical performance of the cathode, the current collectors were prepared in a mesh form. 2.2 Characterization The BFCNb samples sintered at 1000 and 1100 oC were ground into powders and the phase composition and crystal structure for the respective BFCNb powders were analyzed by an X–ray diffractometer (XRD, Rigaku−D−Max γA) with Cu Kα radiation (λ=0.15418 nm) at a scanning step of 0.02o. The BFCNb pellet sintered at 1000 oC was used for electrical conductivity measurement using a four–probe DC technology. The oxygen nonstoichiometry (δ) of the BFCNb was determined by the combination of thermogravimetric analysis (TGA) and iodometric titration. The TGA

5

Journal Pre-proof was performed on a TGA Q55 analyzer (TA instruments) with heating rate of 10 K min–1. The weight mass of BFCNb powder used for TGA test is 23.68 mg. Temperature program desorption of oxygen (O2–TPD) was conducted by a chemisorption analyzer (PCA−1200, Builder, China) using He (99.999%) as carrier gas at a flow rate of 30 mL min–1. The

measurement of thermal expansion behavior

for the BFCNb was performed using a DIL 801 dilatometer (TA instruments) with heating rate of 5 K min–1 under ambient air. AC impedance spectra based on BFCNb|GDC|BFCNb symmetrical cell were conducted under open circuit voltage (OCV) with a perturbation voltage of 10 mV. Linear voltammetry based on BFCNb|GDC|NiO–SDC single cell were used to given power output of the single cell using dry H2 as fuel with flow rate of 60 mL min–1. The static ambient air was used as oxidant. A short–term stability with constant voltages of 0.2 V and 0.5 V for 40 h was carried out to monitor the stability of the single cell with BFCNb cathode. All electrochemical characterizations were conducted using an Autolab–PGSTAT302N electrochemical workstation. 3. Results and discussion 3.1 Phase composition and phase structure Figs. 1 (a) and (b) show the XRD patterns of the BFCNb powders prepared at 1100 oC

for 10 h and 1000 oC for 10 h, respectively. The diffraction peaks for the sample

sintered at 1000 oC can be indexed well to a cubic structure with space group Pm3m, indicating that the pure BFCNb phase with respect to XRD result has been synthesized and the Co and Nb co–doping successfully stabilizes the cubic phase of

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Journal Pre-proof the BF oxide. The miller indices based on cubic perovskite with a Pm3m space group have been marked in Fig. 1. It is widely accepted that the cubic phase is more beneficial for fast oxygen ion transport [14,15], which significantly facilitates the catalytic activity for ORR. For the sample sintered at 1100 oC, a weak diffraction peak (see the pattern (b)) is observable at around 27.9o, which can be assigned to the BaFe2O4 impurity phase, while no peak at the same position can be observed for the 1000 oC sintered sample. It is then inferred that the presence of BaFe2O4 impurity is closely related to the sintering temperature. The BaFe2O4 impurity has also been detected in the preparation of BF–based perovskites in other studies [12,16]. As we know, the BaFe2O4 oxide is a electronic and oxygen–ionic insulating phase [17], which may block the electrons and oxygen–ions transport and then play a negative effect on the electrochemical performance of the cathode. This implies that the optimal temperature for sample preparation is 1000 oC. Using a CELREF V3 software, the calculated lattice parameters for the BFCNb sample sintered at 1000 oC are a=b=c=0.4058(2) nm. Considering that the sintering temperature of the BFCNb cathode on the GDC electrolyte was 950 oC for 2 h, the 1000 oC prepared BFCNb powder was re–calcined at the same condition and then was checked by XRD. The obtained pattern was shown in Fig. 1(c). It can be seen that, in comparison to the 1000 oC

prepared sample, neither additional peak nor peak shift can be observed for the 950

oC

re–calcined sample. This result indicates that the BFCNb is stable below 1000 oC

in air. 3.2 TGA and O2–TPD

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Journal Pre-proof Fig. 2(a) shows the TG curve of BFCNb sample in air. The first weight lose takes place at 100–200 oC, which can be attributed to lattice water evaporation; however, the second weight lose takes place beyond 300 oC, which is caused by the release of lattice oxygen accompanied by thermal reduction of Fe4+/Co4+ cations. Similar thermal reduction phenomena have also been reported on other Fe/Co–containing perovskites [12,18,19]. The oxygen vacancies are then formed with the oxygen release, as described by the following equations:

1   2CoCo （Co 4） OO  2CoCo (Co3 )  VO  O2 ( g ) 2 1   2 FeFe ( Fe 4 )  OO  2 FeFe ( Fe3 )  VO  O2 ( g ) 2

(1) (2)

The transport of oxygen ions and oxygen surface exchange are closely associated with the concentration of oxygen vacancy [20], which is thus accepted to be an important index for the electrode materials of SOFCs. Then the oxygen nonstoichiometry δ is determined by a combination of TG and iodometric titration. The δ of BFCNb at room temperature is ~0.480. Considering that the weight lose at 200–300 oC is mainly due to lattice water evaporation, the δ value would keep a constant value of 0.480 unchanged until 300 oC. Thus the temperature dependence of the oxygen nonstoichiometry (see the red curve in Fig. 2(a)) is given at an initial temperature of 300 oC. As can be seen, the δ attains 0.564–0.602 at 600–800 oC. So high concentration of oxygen vacancy at the cell operating temperatures is clearly beneficial for the fast oxygen ion transport and then the electrocatalytic activity for oxygen reduction at cathode region. Fig. 2(b) shows the O2–TPD curve of the BFCNb using He as carrier gas. Clearly,

8

Journal Pre-proof there are two peaks existing in the curve, denoted as Peak–α and Peak–β, respectively. As supported by above TGA result, the Peak–α in a lower temperature zone can be ascribed to the release of oxygen from the intrinsic oxygen vacancies, which is accompanied by the reduction of Co4+/Fe4+ to Co3+/Fe3+ [11,16]. The Peak–β at around 760 oC also indicates a further oxygen release, which may be caused by the thermal reduction of Co3+/Fe3+ to Co2+/Fe2+ [11,16,21]. As shown in Fig. 2(a), the δ is ~0.480 at 300 oC while ~0.625 at 800 oC. In view of the fixed valence of +5 for the Nb cation, the average valence states of Co/Fe cations were calculated to be +2.82 at 300 oC and +2.50 at 850 oC based on Pauling's principle of charge neutrality, which means no or small amounts of Co4+/Fe4+ ions in BFCNb at 300–800 oC. However, the notable Peak–α at 300–600 oC confirmed the presence of amounts of Co4+/Fe4+ ions in BFCNb. The Co4+/Fe4+ ions may originate from a thermally excited charge disproportion reaction of Co3+/Fe3+, as described in following Eqs. (3) and (4):

2Co3 (CoCo )  Co2 (CoCo )  Co4 (CoCo )

(3)

2Fe3 (FeFe )  Fe 2 (FeFe )  Fe 4 (FeFe )

(4)

3.3 Electrical conductivity The temperature dependence of electrical conductivity of BFCNb was presented in Fig. 3. In the temperature range of 300–450 oC, the electrical conductivity increases with temperature, indicating a semiconductor–like behavior; however, with further increasing temperature, the electrical conductivity tends to decrease, indicating a metal–like behavior. This result is consistent with the previous studies on the electrical conductivity of doped BF perovskites [11,18]. Thus, like other BF–based

9

Journal Pre-proof oxides, the BFCNb should also follow a p–type small polaron hopping along B–O–B bond with conduction mechanism similar to the Zerner double exchange [11,12]. With respect to thermally activated hopping of the small polarions, the electrical conductivity of BFCNb oxide is expected to increase with increasing temperature. However, with increasing temperature to some extent, the thermal reduction of Fe/Co cations takes place and the accompanied formation of oxygen vacancies would break the transport paths of electrons along B–O–B bonds and then contribute to negative effect on the conductivity [11,12]. Thus, the decreased conductivity with increasing temperature above 450 oC can be ascribed to the oxygen release. Based on the results of the TGA and O2–TPD, the oxygen release from BFCNb lattice takes place at an initial temperature of ~300 oC. Considering the increased conductivity of BFCNb with temperature at 300–450 oC, the effect of thermal activation (see Eqs. (2) and (3)) clearly plays a more predominant role than that of the formation of oxygen vacancy. However, beyond 450 oC, the oxygen vacancy formation obviously plays a more predominant role and then leads to the decrease in conductivity with further increasing temperature. 3.4 Thermal expansion behavior Fig. 4 presents the temperature dependence of the relative expansion (ΔL/L0) and TEC for the BFCNb in air. The average TEC for BFCNb is 16.0 × 10–6 K–1 at 30–930 oC.

Though this average TEC value is close to that of GDC electrolyte (~13 × 10–6

K–1), the TECs at 400–700 oC are much higher than that of GDC. For example, the TEC of BFCNb at 450 oC is as high as ~40×10–6 K–1. This abrupt increase in TEC at

10

Journal Pre-proof around 450 oC is generally assumed to be associated with the chemical expansion induced by the transition of oxidation state and spin state of B–site cations with increasing temperature [22]. The oxygen release with temperature induced by the thermal reduction of B–site cations has been demonstrated by the TGA, O2–TPD and electrical conductivity measurements. According to the literature reports, the strong chemical expansion of the cathode materials was apt to cause large tensile stress at the cathode/electrolyte interface [23,24], where a mechanical failure and cathode spallation from electrolyte substrate may take place. Such a strong chemical expansion of the BFCNb cathode is clearly not a desirable characteristic of the SOFC cathode material. This can be mitigated by mixing this cathode material with other materials with a lower TEC such as GDC or LaNi0.6Fe0.4O3 [25,26]. Further work aimed at reducing the chemical expansion of the BFCNb cathode material will be done in our future work. 3.5 Oxygen reduction mechanism As mentioned above, the BFCNb samples were prepared at two different temperatures, i.e., 1000 and 1100 oC. To determine the optimal preparation condition, the two BFCNb samples were ground and annealed on the GDC electrolytes under the same condition for the electrochemical characterization. Fig. 5 gives a comparison of the electrochemical impedance spectra (EIS) for the two BFCNb cathodes on GDC electrolytes based on the symmetrical cells BFCNb|GDC|BFCNb. Here the impedance spectra have been divided by two to give a half symmetrical cell data. The low–frequency intercept on real axis (the right intercept) denotes the total resistance

11

Journal Pre-proof including the ohmic resistance (R0) mainly contributed from electrolyte and polarization resistance (Rp) resulting from the charge transfer and gas diffusion and adsorption/desorption, while the high–frequency intercept on the real axis (the left intercept) denotes R0. It is known that the cathode polarization in SOFCs has a negative effect on the cell performance especially at lower temperatures. The study of impedance spectra is then mainly focused on the Rp. From Fig. 5, the BFCNb prepared at 1000 oC obviously shows a lower Rp than that of the 1100 oC prepared sample, indicating a higher electrochemical performance of the 1000 oC prepared sample. In view of this, the following electrochemical characterizations for the BFCNb cathode are focused on the sample prepared at 1000 oC. In this study, the Rp values for the BFCNb cathode on GDC electrolyte are 0.017, 0.056 and 0.866 Ω cm2 at 800, 700 and 600 oC, respectively. Clearly, these Rp values are much smaller than or comparable to those of some famous Fe–rich cathode materials, e.g., BaCo0.7Fe0.2Nb0.1O3−δ (Rp=0.12 Ω cm2 at 700

oC)

[10],

Ba0.9Co0.3Fe0.6Nb0.1O3−δ (Rp=0.13 Ω cm2 at 700 oC) [12], La0.6Sr0.4Co0.2Fe0.8O3−δ (Rp=~0.09 Ω cm2 at 700 oC) [27] and SrNb0.1Fe0.9O3−δ (Rp=0.058 Ω cm2 at 700 oC) [28]. The excellent electrochemical performance for BFCNb may be due to the high oxygen vacancy concentration that contributes to the fast oxygen surface exchange and bulk diffusion. To understand the mechanism of oxygen reduction in the cathode, the impedance spectra is fitted using a model of R0–L0–R1//CPE1–R2//CPE2–R3//CPE3–R4//CPE4, as shown in Figs. 6(a)–(f). In this model, R0 denotes ohmic resistance, L0 denotes

12

Journal Pre-proof inductance, CPEn (n=1–4) is a constant phase element denoting non–ideal capacitance and Rn (n=1–4) denotes polarization resistance corresponding to different sub–steps in the process of oxygen reduction. It can be seen that the spectra could be well fitted using this equivalent circuit. The fitted parameters are listed in Table 1. The derived capacitance (C) and angular relaxation frequency (f), calculated by using Eqs. (5) and (6), are used to identify the different sub–steps. At 550–600 oC, the derived C1 is 1.06 × 10–6–1.63 × 10–6 F cm–2 and f1 is 88561.39–203094.35 Hz, which are comparable to the reported values for the grain boundaries of GDC electrolyte [29,30]. Thus the Arc1 can be attributed to the oxygen ionic diffusion through the grain boundaries of GDC electrolyte. It can be seen that the Arc1 totally disappears above 650 oC. ( RiTi )1/ pi Ri

(5)

( RiTi ) 1/ pi 2

(6)

Ci 

fi 

The C2 and f2 for Arc2 are ~10–4 F cm–2 and ~10000 Hz, respectively, which are similar to the reported values for oxygen–ionic charge transfer through cathode/electrolyte boundary [31,32]. With increasing the operating temperature, the size of Arc2 tends to decrease and above 750 oC, this arc is nearly non–observable. The Arc3 is characterized by the f3 of 116.85–2351.49 Hz and C3 of 7.17 × 10–3–3.37 × 10–3 F cm–2, which are typical values for electronic charge transfer during the oxygen reduction process. As for the Arc4, especially higher C4 (i.e., 12–20 F cm–2) and lower f4 (i.e., 6.34–7.51 Hz) are obtained. This high capacitance at low–frequency region cannot be assigned to any charge transfer process but to the process of gas

13

Journal Pre-proof diffusion and adsorption/desorption. According to the references [32–34], so large a capacitance is a so–called chemical capacitance, which is related to the oxygen stoichiometry variation in the electrode bulk and/or the electrode/electrolyte interface. This point of view is well supported by our TPD result in section 3.2, in which the BFCNb cathode material has been demonstrated to release a large amount of lattice oxygen at high temperatures (700–800 oC). Based on above discussion, it is concluded that the polarization processes associated with BFCNb cathode correspond to the Arc2, Arc3 and Arc4, while the Arc1 is related to the process of oxygen–ionic diffusion through grain boundaries of GDC electrolyte. From Table 1, one can see that the Arc3 always exists in the whole electrode reaction at different temperatures; furthermore, the polarization resistance (R3) associated with Arc3 occupies above 2/3 that of total polarization resistance of BFCNb cathode in the whole operating temperature range. These results indicate that the process of electronic charge transfer plays the rate–limiting role in the cathode oxygen reduction. This is well supported by the poor electrical conductivity of BFCNb cathode, as demonstrated in the Section 3.3. Fig. 7 shows the Arrhenius curve of the cathode polarization resistance. The derived activation energy is 108.2 kJ mol–1, which is also much smaller than or comparable

to

some

reported

Fe–based

cathode

materials,

such

as

La0.6Sr0.4Co0.2Fe0.8O3−δ (Ea=~159 kJ mol–1) [35], BaNb0.05Fe0.95O3−δ (Ea=~117 kJ mol–1) [22] and BaFe0.75Ni0.25O3−δ (Ea=~115 kJ mol–1) [36]. The lower activation energy means a faster oxygen permeation in the cathode. Thus, the BFCNb is very

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Journal Pre-proof promising to possess higher catalytic activity than La0.6Sr0.4Co0.2Fe0.8O3−δ and BaNb0.05Fe0.95O3−δ cathodes. 3.6 Cell performance and stability A 300–μm–thick GDC supported single cell was used to evaluate the performance of BFCNb cathode. Fig. 8(a) presents the curves of current density vs voltage/power density at 600–800 oC. Ag paste painted in a meshed form was used for current collection. Dry H2 with flow rate of 60 mL min–1 was used as the fuel gas in anode while the static ambient air was used as the oxidant in cathode. The single cell exhibits a power output of 320 mW cm–2 at 700 oC/0.5 V. The high performance of the BFCNb cathode is much higher than (or comparable to) those of state–of–the–art intermediate cathode materials, such as Ba0.9Co0.3Fe0.6Nb0.1O3–δ (based on 350 μm LSGM electrolyte: 180 mW cm–2 at 700 oC/0.5 V) [12], BaFe0.8Cu0.2O3–δ (based on 300 μm SDC electrolyte: 325 mW cm–2 at 700

oC/0.5

V) [37] and

SrCo0.7Fe0.2Nb0.1O3–δ (based on 300 μm SDC electrolyte: 210 mW cm–2 at 700 oC/0.5 V) [38]. As demonstrated above, such a high performance for BFCNb cathode can be attributed to the low polarization resistance of the BFCNb cathode. In addition, it is known that Ag possesses good electrical conductivity and excellent catalytic activity for oxygen reduction [39,40]. In view of this fact, though the Ag current collector layer is prepared in a mesh form, the limited amount of Ag may also contribute to the electrochemical performance of the cathode to some extent. Fig. 8(b) shows a comparison of the cell performances for two same SOFCs with Ag current collectors painted in a meshed form and in a full–covered form, respectively. The difference in

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Journal Pre-proof maximum power density caused by the difference of Ag amount is ~40 mW cm–2, which corresponds to approximately 6% of the total cell performance. This result indicates that using Ag as current collector has a positive effect on the electrochemical performance of the cathode, but such effect is very limited. To evaluate the performance stability of the BFCNb cathode, the single cell was run at 700 oC for 40 hours using H2 as fuel with constant voltages of 0.2 and 0.5 V. The short–term electrochemical behavior of the SOFC with BFCNb cathode was shown in Fig. 9(a). It can be seen that there is a performance loss of ~0.02–0.01 A cm–2 during the 40 h test, i.e., ~1.7%–0.86% per 40 h. If the cell performance follows the same time dependence later on, it can be predicted that the performance loss would attain ~42.5–21.5% after 1000 h. Clearly, this value is rather high degradation rate for the application of single cell. To further understand the possible reasons for the performance degradation of the SOFC, the cross–sectional SEM image of the BFCNb cathode on GDC electrolyte after the short–term stability test is shown in Fig. 9(b). It can be seen that the GDC electrolyte component is very dense with no pores observed in the SEM. The BFCNb cathode layer exhibits porous and well–connected three–dimensional microstructure, which is beneficial for oxygen diffusion, surface oxygen exchange and charge conduction. However, slight agglomeration of particles (see the part of the red dotted frame) due to sintering effect is visible after cell testing. So the degradation in cell performance during the stability test may be associated with the particle agglomeration. In addition, as discussed in the thermal expansion section, the tensile stress at the cathode/electrolyte interface caused by the mismatch between

16

Journal Pre-proof the BFCNb cathode and GDC electrolyte may be another reason for the performance degradation of the SOFC. It is concluded that, in order to maintain good performance stability of the SOFC, the BFCNb cathode need to be further optimized to minimize the cathode/electrolyte mismatch and suppress the particle agglomeration at high temperatures. 4. Conclusions In this study, a solid–state reaction method was used for the synthesis of Fe–rich BFCNb perovskite cathode. Co and Nb co–doping has succeeded in stabilizing the cubic phase of BF perovskite. The O2–TPD measurement determined that the BFCNb experiences a first thermal reduction from Co4+/Fe4+ to Co3+/Fe3+ at ~460 oC under He ambient condition while it experimences a second reduction from Co3+ to Co2+ or Fe3+ to Fe2+ at 760 oC. A combination of TGA and iodometric titration measurements determines that the oxygen non–stoichiometry δ of BFCNb in air is 0.564–0.602. Though the average TEC for BFCNb is around 16.1 × 10–6 K–1 at 30–930 oC, the TEC at 450 oC is as high as ~40 × 10–6 K–1, which leads to a TEC mismatching with common electrolytes such as GDC. At 700–800 oC, the polarization resistance (Rp) attains 0.056–0.017 Ω cm2. The fitting analysis for the impedance spectra demonstrates that the mechanism of oxygen reduction is mainly rate–limited by the process of electronic charge transfer. Using BFCNb as cathode and Ni–SDC as anode, the 300–μm–thick GDC electrolyte supported single cell shows a power output of 583–320 mW cm–2 at 800–700 oC and 0.5 V. In addition, short–term stability test indicates that the single cell with BFCNb cathode exhibits a performance degradation

17

Journal Pre-proof of ~1.7%–0.86% after a 40–hour test. The performance loss for the cell stability may be due to the high chemical expansion of BFCNb cathode and/or the agglomeration of BFCNb cathode particles. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 11504320 and 21403101), by the Foundation of Education Department of Liaoning Province (No. L2012135), by the Foundation of the Science and Technology Department of Liaoning Province (No. 201602475) and by the Liaoning Revitalization Talents Rrogram (No. XLYC1807179). References [1] L.M.P. Garcia, D. A. Macedo, G.L. Souza, F.V. Motta, C.A.Paskocimas, R.M. Nascimento, Citrate–hydrothermal synthesis, structure and electrochemical performance of La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes for IT–SOFCs, Ceramics International 39, 2013, 8385–8392. [2]

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Figure captions Fig. 1. XRD patterns for BFCNb powders: (a) prepared at 1100 oC, (b) prepared at 1000 oC and (c) re–calcined at 950oC for the 1000 oC prepared sample. Fig. 2. (a) TGA curve (blue line) and temperature dependence of oxygen

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Journal Pre-proof nonstoichiometry δ (red line) for BFCNb; (b) O2–TPD curve for BFCNb. Fig. 3. Temperature dependence of electrical conductivity for BFCNb in air at 300–850 oC. Fig. 4. Thermal expansion curve and corresponding TEC curve of BFCNb. Fig. 5. Comparison of the EIS for the BFCNb cathode materials prepared at 1000oC and 1100 oC, respectively. Fig. 6. Experimental and fitting results of the EIS of BFCNb cathode on GDC electrolyte based on a half symmetrical cell: (a) 800 oC, (b) 750 oC, (c) 700 oC, (d) 650 oC, (e) 600 oC and (f) 550oC. The number in the spectra denotes the logarithm of frequency. Fig. 7.Arrhenius plot of polarization resistances at 650–800 oC. Fig. 8. (a) Dependence of current density on voltage and power density at 600–800 oC for the SOFC with Ag current collector in a meshed form; (b) comparison of cell performance at 800 oC for the SOFCs with Ag current collectors painted in a meshed form and in a full–covered form. Fig. 9. (a) The 40 h short–term stability of the single cell BFCNb|GDC|NiO–SDC; (b) the cross–sectional SEM image of the BFCNb cathode on GDC electrolyte after the short–term stability test. Table caption Table 1. Fitting parameters for electrochemical impedance spectra based on a half symmetrical cell BFCNb|GDC.

26

Journal Pre-proof

27

Journal Pre-proof 

Fe-rich BaFe0.7Co0.2Nb0.1O3-δ (BFCNb) was evaluated as cathode of IT-SOFC.



BFCNb cathode exhibits high oxygen vacancy concentration at 600–800 oC.



The Rp of BFCNb cathode on GDC electrolyte is as low as 0.056 Ω cm2 at 700 oC.



Power output of a single cell with BFCNb cathode attains 320 mW cm-2 at 700 oC/5

V.

Journal Pre-proof Table 1. Fitting parameters for electrochemical impedance spectra based on a half symmetrical cell BFCNb|GDC. T (oC)

550

600

650

700

750

800

R0 (Ω cm2) L0 (H)

1.20 1.33×10-7

0.83 1.08×10-7

0.78 4.07×10-8

0.50 1.43×10-7

0.35 1.03×10-7

0.26 1.21×10-7

R1 (Ω cm2) CPE1-T (F cm-2) CPE1-P C1 (F cm-2)

1.70 2.67×10-6 0.93 1.06×10-6

0.48 1.88×10-6 0.99 1.63×10-6

-

-

-

-

F1 (Hz)

88561.39

203094.35

-

-

-

-

R2 (Ω cm2) CPE2-T (F cm-2) CPE2-P C2 (F cm-2)

0.26 9.87×10-3 0.74 1.21×10-3

9.88×10-2 6.71×10-3 0.69 2.50×10-4

4.18×10-2 7.61×10-3 0.71 2.84×10-4

1.60×10-2 1.73×10-3 0.93 7.85×10-4

-

-

F2 (Hz)

504.66

6435.64

13427.99

12674.68

-

-

Arc3

R3 (Ω cm2) CPE3-T (F cm-2) CPE3-P C3 (F cm-2) F3 (Hz)

0.57 6.51×10-2 0.65 1.11×10-2 25.29

0.19 4.55×10-2 0.72 7.17×10-3 116.85

8.98×10-2 4.21×10-2 0.73 5.35×10-3 331.48

4.99×10-2 4.14×10-2 0.73 4.21×10-3 758.55

3.22×10-2 4.30×10-2 0.71 2.92×10-3 1691.78

2.01×10-2 4.50×10-2 0.73 3.37×10-3 2351.49

Arc4

R4 (Ω cm2) CPE4-T (F cm-2) CPE4-P C4 (F cm-2) F4 (Hz)

-

-

-

-

1.21×10-3 12.90 1.129 20.75 6.34

1.71×10-3 10.63 1.04 12.4 7.51

Arc1

Arc2