Performance evaluation of an anode-supported solid oxide fuel cell with Ce0.8Sm0.2O1.9 impregnated GdBaCo2O5+δ cathode

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Performance evaluation of an anode-supported solid oxide fuel cell with Ce0.8Sm0.2O1.9 impregnated GdBaCo2O5Dd cathode Bo Wei a,*, Zhe Lu¨ a, Weiping Pan a, Xiqiang Huang a, Yaohui Zhang a, Wenhui Su a,b a b

Department of Physics, Harbin Institute of Technology, Harbin 150080, China International Center for Materials Physics, Academia Sinica, Shenyang 110015, China

article info

abstract

Article history:

The performance of an anode-supported solid oxide fuel cell with nanosized Ce0.8Sm0.2O1.9

Received 28 April 2012

(SDC) impregnated GdBaCo2O5þd (GBCO) cathode is evaluated under various operation

Received in revised form

conditions. An SDC interlayer is applied between 8 mol% yttria-stabilized zirconia (8YSZ)

12 June 2012

electrolyte and cathode to prevent undesired chemical reaction. At 700  C, the fuel cell

Accepted 27 June 2012

yields a maximum power density of 790 mW cm2 when using simulated air as oxidant,

Available online 24 July 2012

which is promoted to 1100 mW cm2 in oxygen flow. The activation energies of ohmic resistance are smaller than that of bulk YSZ due to non-YSZ contribution. Electrode

Keywords:

polarization resistance changes with fuel cell voltage while ohmic resistance nearly

Solid oxide fuel cells

remains constant. The fuel cell output is mainly determined by the electrode polarization.

Double perovskite cathode

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Impregnation

reserved.

Electrochemical performance Impedance spectra

1.

Introduction

With the advantages of high efficiency and low emissions, solid oxide fuel cell (SOFC) is widely considered as a promising technique for future power generation [1]. Intermediate temperature SOFC (IT-SOFC) operating at 500  Ce750  C will greatly facilitate the commercialization, because of the utilization of low cost metallic interconnect, the broadening of material selection and the prolonged the lifetime [2]. La1xSrxMnO3 (LSM) cathodes are extensively used for conventional SOFCs operating at 800  C, as they exhibit good catalytic activity, chemical and thermal compatibility with 8 mol% yttria-stabilized zirconia (8YSZ) electrolyte [3]. For IT-SOFCs, however, LSM cathodes confront with large polarization resistance that is caused by slow oxygen reduction

kinetics, limiting the SOFC performance. Therefore, the development of high performance alternative cathode materials is very important [4,5]. Perovskite-type mixed ionic and electronic conductors (MIECs) generally involve reasonable or high concentration of oxygen vacancy, which enables higher oxygen-ion conductivity than basically electronic conducting LSM cathodes. Accordingly, the application of MIEC cathodes can obviously reduce the polarization resistance, because the oxygen reduction can occur on the whole cathode surface instead of the electrolyteeelectrodeeair triple phase boundaries. Great efforts have been devoted to study the MIEC-based cathodes, and excellent catalytic property for oxygen reduction reactions has been demonstrated in representative alternatives like La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) [6,7], Sm0.5Sr0.5CoO3d (SSC)

* Corresponding author. Tel.: þ86 451 86418430; fax: þ86 451 86418420. E-mail addresses: [email protected], [email protected] (B. Wei). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.102

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[8,9] and Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) [10,11] oxides. Recent years, layered perovskite MIECs, LnBaCo2O5þd (Ln ¼ rare earth elements, like Pr, Nd, Sm, Gd) [12e21], have attracted extensive interests. Fast oxygen-ion transport properties and high electrical conductivities have been confirmed in these compounds, which make them to be potential cathodes for IT-SOFCs. Based on doped ceria electrolytes, anode-supported fuel cells with GdBaCo2O5þd and PrBaCo2O5þd cathodes generated considerable peak power densities of 560 mW cm2 and 620 mW cm2 at 600  C [18], respectively. These cathodes were also applied on proton conducting electrolytes [22e24]. Using a BaZr0.1Ce0.7Y0.2O3d (BZCY) thin film electrolyte, peak power densities of 520 mW cm2 and 382 mW cm2 were obtained at 700  C for fuel cells with PrBaCo2O5þd [22]and SmBaCo2O5þd cathodes [24], respectively. For practical application, the chemical compatibility between LnBaCo2O5þd and electrolytes is a very important concern. Good compatibility with ceria based electrolytes is determined, and several studies have showed that no detectable secondary phase is examined in GdBaCo2O5þdeCe0.9Gd0.1O1.95 (GBCOeGDC) and PrBaCo2O5þdeCe0.8 Sm0.2O1.9 (PBCOeSDC) [18] mixtures after heat-treatment at 1100e1200  C. But high reactivity is observed between GBCO and YSZ at a low temperature of 700  C [25], which limits the application in YSZ electrolyte. Up to now, YSZ is still widely used as the state-of-the-art electrolyte material due to its high stability and mechanical strength. It is known that many cobalt-based perovskite oxides can readily react with YSZ electrolyte. To block the detrimental reactions, an SDC or GDC interlayer (buffer layer) is often deposited between YSZ and cathodes with effective improvements [26e29]. In a previous study, GBCO cathode was directly used on YSZ film fuel cell without a protective layer [14]. As the GBCO cathode was fired at 975  C for 2 h, undesirable elements interdiffusion is expected. In this study, an SDC interlayer is deposited on the surface of YSZ electrode before the fabrication of GBCO-based cathode. Here, nanosized SDC particles were introduced onto the GBCO surface using an impregnation method, which can significantly enhance the electrocatalytic activity toward the oxygen reduction reactions. The performance of a Ni þ YSZ anode-supported fuel cell with a GBCOeSDC composite cathode was evaluated under various conditions and considerable output was achieved.

resulted in a purple viscous gel, which was heat-treated at 200  C for 10 h and followed by calcination at 1000  C for 4 h to obtain the final product. Room temperature X-ray diffractometer (XRD, Rigaku D/max-rB XRD, Japan) was used to check the phase purity, which can be indexed as a desired orthorhombic perovskite structure. Ce0.8Sm0.2O1.9 powder was synthesized via a modified solegel process using citric acid as complexing agent, and the cation to citric acid ratio of 1:1.2. As-obtained precursor was calcinated at 800  C for 2 h and yellowish SDC powder was obtained. GBCO and SDC powders were separately mixed with 6 wt.% ethyl cellulose and terpineol binder to prepare the GBCO and SDC inks.

2.2.

0.25 g 8YSZ (TZ8Y, Tosoh, Japan) powder was pressed under w200 MPa with a 13 mm stainless steel die and followed by a sintering at 1400  C for 4 h. SDC barrier layers were further fabricated between YSZ and GBCO to prevent the chemical reactions. SDC slurry was symmetrically coated on both surfaces of YSZ substrate and sintered at 1300  C for 1 h. Then, GBCO ink was applied onto SDC interlayers and sintered at 950  C for 4 h (0.28 cm2). 3 mol L1 SDC impregnation solution containing Sm3þ and Ce4þ nitrate salts with a cation ratio of 1:4 was dropped onto GBCO cathode surface. The solution was quickly soaked into the porous GBCO backbone. After drying at 150  C, the cathodes were further calcinated at 850  C for 1 h in air. The mass change was weighted before and after impregnation process to estimate the value of SDC loading (w2.0 mg cm2).

2.3.

Experimental

2.1.

Materials synthesis

The GdBaCo2O5þd oxide powder was synthesized by a combined ethylenediamine tetraacetic acid (EDTA)ecitrate complexing technique [18,19]. Analytical grade Gd2O3, Ba(NO3)2, Co(NO3)2$H2O, EDTA, NH3$H2O and citric acid were used as raw materials. Stoichiometric amount of Gd2O3 oxide was first dissolved in hot nitric acid, which was then mixed with pre-prepared EDTAeNH3$H2O solution. Rest metal nitrates and citric acid were added in sequence under heating (80  C) and stirring. A mole ratio of 1:1:2 was controlled for total metal ions:EDTA:citric acid and the pH value of solution was adjusted to w7 using NH3$H2O. The evaporating of water

Single cell fabrication

Powders of NiO (Inco, Canada), YSZ and starch were mixed together at a weight ratio of 5:5:2, forming the anode mixture, which was pressed into circular pellets (13 mm) and precalcinated at 1000  C for 2 h. A YSZ film was deposited on one side of anode substrate via a slurry coating method. The resultant anodeeelectrolyte bi-layer was co-sintered at 1400  C for 4 h to densify the film. The preparation of SDC interlayer, GBCO cathode and impregnation treatment were similar to the aforementioned process in Section 2.2.

2.4.

2.

Symmetrical half-cell fabrication

Characterizations

The impedance spectra of symmetrical half-cell were collected using a combined electrochemical system (Solartron SI 1287 þ Solartron SI 1260, England). The frequency was ranged from 91 kHz to 0.1 Hz with a singal amplitude of 10 mV. A two-electrode four-wire configuration was used for fuel cell performance evaluation, which was performed between 550  C and 800  C. Hydrogen with a fixed flow rate of 150 mL min1 was fed to the anode chamber while oxygen and nitrogen mixture was fed to cathode side at a total rate of 100 mL min1. The gas flow rate and composition were controlled by mass flow controllers (Sevenstar, China). Impedance spectra were collected under open-circuit condition or specific polarization conditions. As-obtained impedance spectra were fitted using a Zview 2.3f software (Scribner Associates). After testing, the microstructure and

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morphologies of the single cell were observed by a scanning electron microscope (SEM, Hitachi-S4800, Japan). The elements analysis of the cathode surface was studied by an energy dispersive X-ray spectroscopy (EDS, Oxford).

3.

Results and discussion

3.1.

Symmetrical half-cells

In our previous study, it was found that, the SDC impregnated GBCO cathode on SDC electrolyte exhibited obviously improved activity toward oxygen reduction reaction [30]. Here, the electrochemical performance was also examined on YSZ electrolyte with SDC buffer layers. Fig. 1 shows the AC impedance spectra of pure-GBCO and SDC impregnated GBCO cathodes under open circuit conditions in air. At 700  C, the polarization resistance of pure-GBCO cathode is 0.50 U cm2, which is significantly reduced to 0.19 U cm2 after impregnation treatment. The enhancement is attributed to the extension of electrochemically active sites with the introduction of many SDC nanoparticles. In the following sections, the performance of GBCOeSDC cathode is evaluated in a completed anode-supported fuel cell.

3.2.

Single cell microstructure

Fig. 2(a)e(d) shows the microstructures of the tested single cell. From the surface view of SDC interlayer in Fig. 2(a), it can be found that the SDC is partially sintered after heattreatment at 1300  C, forming an island-like structure. The grain size is below 0.5 mm. For SDC interlayers prepared by ceramic methods like screen printing, dip/slurry coating, it is difficult to achieve full dense at a sintering temperature 1300  C [31,32]. There are two reasons, one is that the loss of oxygen and cerium reduction may lead to stoichiometry deviation, which hence lowers the sinterability [33]. And the other is due to the as-sintered anode þ YSZ substrate, which hardly showed shrinkage during the sintering of the interlayer. Fig. 2(b) gives the cross-sectional view of the fuel cell and the Ni þ YSZ anode support, YSZ film, SDC interlayer and cathode exhibit good contact. The YSZ film is relatively dense with a thickness of w25 mm. Although some closed pores can be found, it is expected that the fuel cell voltage will not be

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Measured at 700 C Pure GBCO GBCO-SDC

0.2

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Fig. 1 e AC impedance spectra of pure-GBCO and GBCOeSDC cathodes at 700  C. Ohmic resistances from electrolyte were subtracted for comparison.

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affected. The cathodeeelectrolyte interface in Fig. 2(c) reveals that GBCOeSDC composite exhibits good adhesion to SDC porous layer. The SDC interlayer is not clear after impregnation, and from the image of non-impregnated fuel cell, the thickness is 2e3 mm (not shown here). An enlarged view of the impregnated cathode in Fig. 2(d) shows that many SDC particles (30e50 nm) cover on the surface of GBCO backbone. The corresponding EDS profile in Fig. 2(e) clearly confirms the existence of Ce and Sm elements with a molar ratio of w4:1. The introduction of SDC nanoparticles can greatly extend the length of triple phase boundaries, and thus promote oxygen reduction reactions.

3.3.

Single cell performance

Fig. 3(a) and (b) shows the currentevoltageepower density curves of the single cell with GBCOeSDC composite cathode operating in simulated air and oxygen flow, respectively. With air flow in cathode chamber (O2 ¼ 21 mL min1, N2 ¼ 79 mL min1), the open circuit voltage (OCV) changes from 1.07 V to 1.04 V between 600 and 750  C. These results are very close to theoretical value, implying that the YSZ film is quite dense and negligible gas is permeated through the electrolyte. Fuel cell performance increases with the increasing of temperature. The peak power densities (PPDs) of 290, 515, 790 and 1056 mW cm2 are achieved at 600, 650, 700 and 750  C, respectively. These results are comparable with that obtained from similar fuel cell with a BSCF cathode, which generated PPDs of 1160 mW cm2 at 750  C and 520 mW cm2 at 650  C for an optimized single cell [33]. Tarancon et al. have evaluated the performance of GBCO cathode on anode-supported YSZ (15 mm) film cells [14]. Peak power densities of 300 mW cm2 at 700  C and 500 mW cm2 at 800  C were yielded. GBCO cathode on YSZ surface was fired at 975  C for 2 h, but it is reported that obvious reaction occurred at a lower temperature of 700  C. Strong reactivity results in the decomposition of GBCO and the formation of new insulating phases like BaZrO3 and Y2O3 [34]. And therefore, the cell output was restricted by high interfacial resistance. In our study, the introduction of SDC protective layer can effectively avoid the direct contact between YSZ and GBCO and thus decreases the detrimental element interdiffusion. In addition, the SDC layer with higher ionic conductivity is beneficial for oxygen reduction reactions [35]. Both factors result in the performance enhancement. When cathode gas is switched to oxygen (100 mL min1), the OCV slightly increase to 1.1e1.05 V, due to the increase of oxygen partial pressure at the cathode side. More importantly, the fuel cell performance is substantially promoted using the oxygen as an oxidant, the PPD values of 400, 726, 1100 and 1606 mW cm2 are reached at 600, 650, 700 and 750  C, respectively. The results are 38e52% higher than that obtained with simulated air, because an increased oxygen concentration can facilitate the oxygen dissociative adsorption process at the cathode surface. From the voltageecurrent traces, a concave-up curvature pffiffiffiffiffiffiffi ð 1Þ can be found at low current densities, followed by an approximately linear behavior. Usually, the electrode activation overpotential (hact)ecurrent density (i) relationship can be described by the well-known ButlereVolmer equation,

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Fig. 2 e SEM images of the single cell (a) surface view of the SDC barrier layer, (b) cross-sectional view of the single cell, (c) cathodeeelectrolyte interface and (d) surface view of SDC impregnated GBCO cathode, and (e) the EDS profile of (d).

     azFhact ð1  aÞzFhact  exp i ¼ i0 exp RT RT

(1)

where i0 is the exchange current density, a is the transfer coefficient, z is the number of electrons transferred in the electrode reaction, F is the Faraday constant, R is the gas constant, and T is the absolute temperature. The values of z and a are set as 2 and 0.5, respectively. In the low current density regime, the fuel cell performance is mainly dominated by activation or charge-transfer processes of electrode, and Eq. (1) becomes, RT i ¼ Rct i hact z zFi0

(2)

where Rct is the charge transfer resistance. Using the low voltage data (below 0.08 V) of IeV curves, the exchange current

densities at various temperatures are determined. At 700  C, the obtained results are 94.4 mA cm2 and 113.6 mA cm2 for the fuel cell operated in air and oxygen atmospheres, respectively. These data are higher than that reported of 65 mA cm2 for La0.6Sr0.4Co0.8Fe0.2O3 cathode at 700  C in air [36], but smaller than that of 182 mA cm2 for Ba0.5Sr0.5Co0.8Fe0.2O3 cathode [11].

3.4.

Impedance spectra

Fig. 4(a) and (b) shows the corresponding impedance spectra with cathode in simulated air and oxygen flow, respectively. These data obtained under OCV condition which are basically characterized by a larger arc at high frequency range and a smaller one at low frequency range. These spectra are fitted

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Fig. 3 e Cell voltages and power densities as a function of current density with cathode in simulated air (a) and in oxygen flow (b).

using an equivalent circuit in Fig. 4(c) with two semicircles, LRohm(R1CPE1)( R2CPE2). In this circuit, L is the inductance from silver wires and testing system, Rohm is the ohmic resistance, R1, R2 are the electrode polarization resistances, and CPE1, CPE 2 are constant phase elements. The expression of CPE is CPE ¼ Y0( jw)n, where Y0 is the admittance, w is the angular pffiffiffiffiffiffiffi frequency, j is the imaginary unit ðj ¼ 1Þ and n is an exponent (0 < n < 1). For n ¼ 1, the CPE is a pure capacitance and for n ¼ 0.5, the CPE is a Warburg-type diffusion impedance. The sum of R1, R2 gives the total electrode polarization resistance (Rp), which contains the contributions from both anode and cathode. The high frequency arc (R1, CPE1) reflects the charge transfer of oxide ions, while the low frequency one (R2, CPE2) is mostly influenced by the oxygen dissociative adsorption and/ or surface diffusion of oxygen species in cathode [37]. In the present study, the low frequency semicircle becomes much smaller when the cathode gas was changed from simulated air to pure oxygen, suggesting that the cathode process mainly contributed to the low frequency one. It can also be observed that, not like high frequency arc, the low frequency one exhibits a weak temperature dependence, which is consistent with the fact that the concentration polarization is weakly dependent on temperature [38].

2

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1

c

Fig. 4 e AC impedance curves of the single cell with cathode operating in air (a) oxygen flow (b) and the equivalent circuit for data fitting (c).

As it is difficult to separate the specific contribution with reasonable accuracy (especially for spectra obtained in oxygen), here a simple analysis is made by dividing them into Rohm and Rp. Fig. 5(a) and (b) gives the changes of ohmic resistance, polarization resistance and total cell resistance as a function of temperature. As clearly shown, under both atmospheres, Rohm is smaller than Rp in the tested temperature region. The Rp values are reduced when cathode atmosphere is changed from simulated air to oxygen flow. The lnRp and lnRohm vs. reciprocal of absolute temperature (1/T ) are plotted in Fig. 6. Good linear relationships can be observed for all four curves and correspondingly, the activation energies (Ea) can be calculated using the slopes of linear fitting. The Ea values obtained for Rohm in air and oxygen are basically identical (70.88  3.24 kJ mol1, 74.79  1.91 kJ mol1), which are smaller than that of bulk 8YSZ (95 kJ mol1) [39,40].

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Fig. 5 e Changes of ohmic, electrode polarization and total resistance as a function of temperature, (a) in simulated air and (b) in oxygen flow.

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Rohm-oxygen 74.79+/-1.91 kJ mol-1 -1 Rp -oxygen 80.02+/-4.32 kJ mol -1 70.88+/-3.24 kJ mol Rohm -air -1 78.45+/-4.40 kJ mol R -air p

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1.25

From the literature, we can found that the activation energies of ohmic resistance from anode-supported, 8YSZ film fuel cells are very different. For example, Kim et al. obtained a result of 65 kJ mol1 for a fuel cell (10 mm film) with LSM þ YSZ cathode [41], while Yoon et al. got two values of 89.4 kJ mol1 for cell 1 and 77.6 kJ mol1 for cell 2 (graded anode) with Ca-doped LaMnO3 cathode (w10 mm film for both cells) [42]. By varying the thickness of YSZ film, Lu et al. have carried out detailed study on the ohmic loss in anodesupported fuel cells [43]. They concluded that the ohmic resistances can be divided into two parts, YSZ contribution and non-YSZ contribution (intercept resistance). For the fuel cell with an SDC interlayer, the non-YSZ part includes current collector, cathode, SDC layer, Ni þ YSZ anode, and the contacts between neighboring components. When using LSM or LSCF as cathodes, the activation energy values of intercept resistance were 29.9 kJ mol1 and 65.6 kJ mol1, respectively. Clearly, the difference of Ea between bulk YSZ and ohmic resistance of YSZ film fuel cell is caused by the contribution of intercept resistance. It seems that the density of the ceria interlayer plays an important role in determining the fuel cell performance. For interlayers prepared by ceramic techniques, the relative poor sinterability of doped ceria electrolytes makes it difficult to achieve full dense. Adding sintering aids like cobalt oxide [27] or doping with transition metals (2 mol% Cu, Co, Fe, Ni) [28] can result in a denser microstructure, but cracks or delamination caused by shrinkage mismatch still existed. Mai et al. have prepared a dense GDC layer using reactive sputtering method [27]. At 700  C and 0.7 V, the fuel cell has exhibited a higher power density of 0.9 W cm2 than that with screenprinted interlayer (0.7 W cm2). Similarly, Lu et al. have fabricated a dense SDC interlayer using pulsed laser deposition (PLD) [29]. At 750  C and 0.7 V, the fuel cell gave a power density of over 1.4 W cm2, significantly higher than the fuel cell with porous SDC (0.8 W cm2). Dense ceria layers deposited at low deposition temperature can not only exhibit an increased conductivity but effectively minimize the formation of SrZrO3 solid solution between YSZ and interlayer. Both factors contributed to the improved fuel cell output. It is reasonably expected that the performance of our fuel cell with SDC impregnated GBCO cathode can be further improved, if the preparation of SDC interlayer is optimized. During operation, the interdiffusion between YSZ and ceria interlayer can be not obvious, as the operation temperature (<800  C) is much lower than the interlayer sintering temperature. Mai et al. [44] have carried out the long-term stability testing (2000e5000 h) of single cells with Ce0.8Gd0.2O1.95 interlayers at 750  C and 0.5 A cm2. The degradation rate was below 1.5% per 1000 h and the performance loss was probably caused by the SrZrO3 formation and coarsening of the microstructure. Impedance spectra given in Fig. 4 are obtained under OCV condition (namely, equilibrium state), but they cannot reflect the realistic SOFC operation with current flow. It is known that, in most cases, the total cell resistance is a dynamic property, which is dependent on the current density. Herein, impedance spectra were also collected under various polarization voltages in simulated air, and the typical results at 700  C are shown in Fig. 7. The charge-transfer resistance decreases obviously with the decreasing of the cell voltage

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(increasing current density). However, Rp exhibits a different behavior, which shows a minimum value at the cell voltage of w0.65 V (1.12 A cm2). Below this current density, the electrode polarization decreases with the increasing of current density, which can be attributed to the increasing of electrode activity. Under OCV, the Rohm value accounts for w20% of total resistance, which becomes larger under polarization. At 0.65 V, the percent of Rohm value increases to w40%. This suggests that the fuel cell performance is primarily limited by electrode polarization resistance.

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Z' ( Ωcm ) Fig. 7 e AC impedance spectra of fuel cell obtained under various polarization conditions at 700  C.

(0.55 V), as the activation polarization is reduced with the increasing of current density. Furthermore, when the cell voltage is 0.85 V, an additional low frequency arc is observed. These impedance spectra are correspondingly fitted using the equivalent circuit with three arcs in Fig. 4(c). The n values of the additional arc are close to 1 (0.92e1), and therefore, a pure capacitance behavior is expected for this process, which is most likely attributed to the oxygen gas diffusion within the pores of impregnated GBCO cathode. This new arc increases with current density, suggesting the gas diffusion problem becomes evident at the high oxygen consumption rate. Jiang and Wang have observed similar phenomenon for GDC impregnated LSM cathodes at low oxygen partial pressure [45]. For fuel cell operation, higher current density can also results in the decreasing of the local oxygen partial pressure at the cathode surface, as the oxygen is continuously consumed. Fitted data of ohmic and total electrode polarization resistances are given in Fig. 8. As clearly shown, Rohm remains almost constant with the decreasing of operating voltage

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Fig. 8 e Change of cell resistance with polarization voltage.

Conclusion

In this paper, an SDC interlayer is deposited between YSZ electrolyte and SDC impregnated GBCO cathode to block the detrimental reactions and the performance of anodesupported fuel cell is characterized under various conditions. At 700  C, considerable peak power densities of 790 mW cm2 and 1100 mW cm2 are achieved when using simulated air and oxygen as oxidants, respectively. Analysis of impedance spectra shows that electrode resistance varied with current density, which mainly determines the fuel cell performance. With a dense SDC interlayer and an optimized cathode, a further enhanced performance is expected.

Acknowledgments This work was supported by the Natural Science Foundation of China (20901020), and the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT.NSRIF.2012066).

references

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