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Electrochemical characterization of YBaCo3ZnO7 + Gd0.2Ce0.8O1.9 composite cathodes for intermediate temperature solid oxide fuel cells
Electrochimica Acta 55 (2010) 5312–5317
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical characterization of YBaCo3 ZnO7 + Gd0.2 Ce0.8 O1.9 composite cathodes for intermediate temperature solid oxide fuel cells J.-H. Kim a , Y.N. Kim a , S.M. Cho b , H. Wang b , A. Manthiram a,∗ a b
Electrochemical Energy Laboratory & Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712, USA Department of Electrical and Computer Engineering, Texas A & M University, College Station, TX 77843, USA
a r t i c l e
i n f o
Article history: Received 18 January 2010 Received in revised form 15 April 2010 Accepted 16 April 2010 Available online 22 April 2010 Keywords: Solid oxide fuel cell Composite cathode Polarization resistance
a b s t r a c t YBaCo3 ZnO7 + Gd0.2 Ce0.8 O1.9 (GDC) composites with various GDC contents (0–70 wt.%) have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (SOFC). The effect of GDC incorporation on the microstructure, electrochemical properties, and thermal expansion behavior of the YBaCo3 ZnO7 + GDC composites has been studied. The composite cathodes consist of smaller particles with larger surface area compared to the pure YBaCo3 ZnO7 cathode, which is beneficial for providing extended triple-phase boundary (TPB) where the oxygen reduction reaction (ORR) occurs. Among the various compositions investigated, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite is found to be optimum with the lowest polarization resistance (0.28 cm2 at 600 ◦ C) compared to that of pure YBaCo3 ZnO7 (0.62 cm2 at 600 ◦ C). Anode-supported single cell SOFC fabricated with the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode also exhibits excellent performance with a maximum power density of 743 mW/cm2 at 750 ◦ C. Additionally, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite shows a low thermal expansion coefficient (TEC) of 10.7 × 10−6 ◦ C−1 , which provides good compatibility with those of standard SOFC electrolytes. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The RBaCo4 O7+ı (Dy, Ho, Er, Tm, Yb, Lu, Y, Ca, In) oxides have drawn attention recently as they exhibit interesting magnetic behavior [1–4] and oxygen absorption/desorption properties [5–9]. Especially, the latter property has stimulated interest in their application as oxygen storage materials. For example, RBaCo4 O7 absorbs extra oxygen into the lattice (up to 1.5 oxygen atoms per formula unit) in a narrow temperature range (200 ◦ C ≤ T ≤400 ◦ C) and lose the extra oxygen rapidly at T > 400 ◦ C [5–7]. The amount of oxygen absorption into the lattice strongly depends on the Rn+ ion and oxygen partial pressure, which is accompanied by structural transitions. RBaCo4 O7 with a stoichiometric amount of oxygen (ı ≈ 0) consists of corner-shared CoO4 tetrahedra with the Ba2+ and Rn+ ions adopting, respectively, 12- and 6-fold oxygen coordinations with a hexagonal structure. In contrast, YBaCo3 ZnO7+ı with extra oxygen (ı > 1.0) in the lattice has an orthorhombic structure with the formation of a supper-lattice and the cobalt ions occupying both edge-shared octahedra and corner-shared tetrahedra [7–9]. However, details of the mechanisms of oxygen adsorption/desorption into/from the RBaCo4 O7+ı lattice remain to be established with future studies.
∗ Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681. E-mail address: [email protected] (A. Manthiram). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.04.058
Unfortunately, the RBaCo4 O7+ı samples suffer from phase decomposition at elevated temperatures of 700–800 ◦ C, which prevents their use in high-temperature applications such as cathode materials in SOFC [5–7]. For example, the YBaCo4−x Fex O7 (0 ≤ x ≤0.8) samples were found to decompose into cobalt-based perovskite oxides and Co3 O4 with severe volume shrinkage at 700–800 ◦ C although it has an attractive low thermal expansion coefficient (TEC) of ∼10 × 10−6 ◦ C−1 below 700 ◦ C [10]. However, Hao et al. [11] reported recently a low oxygen adsorption by the zinc-substituted YBaCo4−x Znx O7 (1 ≤ x ≤3) at high temperatures, suggesting their improved phase stability [11]. Encouraged by this earlier report, we presented recently a direct investigation of the phase stability of the RBaCo4−x Mx O7 (R = Y, Ca, In, or mixtures thereof and M = Zn, Fe, Al, or mixtures thereof) series and their potential for SOFC applications [12]. Among them, only YBaCo4−x Znx O7 (1 ≤ x ≤2) showed long-term stability at high temperatures (600–800 ◦ C). In particular, the RBaCo4−x Mx O7 samples have the advantage of low TECs of 9.5 × 10−6 –13 × 10−6 ◦ C−1 in the range of 80–900 ◦ C, which provides good thermal expansion compatibility with the standard SOFC electrolytes. The low TEC can be understood to be due to the absence of spin-state transitions with the tetrahedral-site Co2+/3+ ions and relatively small amount of oxygen loss with increasing temperature. In addition, the YBaCo3 ZnO7 sample was found with electrolyte-supported single cell SOFC to show fuel cell performance comparable to that of the well-studied La0.6 Sr0.4 Co0.2 Fe0.8 O3 (LSCF) cathode. Further
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Fig. 1. Scanning electron microscope (SEM) images showing the cross sections of the cathode–GDC electrolyte portion in the symmetrical cells: (a and b) simple YBaCo3 ZnO7 , (c and d) YBaCo3 ZnO7 + GDC (50:50 wt.%) composite, (e and f) YBaCo3 ZnO7 + GDC (30:70 wt.%) composite cathodes after the AC impedance measurements.
enhancement in the cathode performance was also achieved on employing a YBaCo3 ZnO7 + GDC composite cathode due to the extended triple-phase boundary (TPB) and enhanced oxide-ion conductivity. Based on the above findings, we present here a systematic investigation of the YBaCo3 ZnO7 + GDC composite cathode with varying ratios between YBaCo3 ZnO7 and GDC and an exploration of their use as cathodes in anode-supported SOFC. The influence of GDC incorporation into the YBaCo3 ZnO7 cathode on the microstructure, TEC, and catalytic activity for the oxygen reduction reaction (ORR) in SOFC are discussed. 2. Experimental The YBaCo3 ZnO7 (YBC3Z) sample was synthesized by conventional solid-state reaction methods. Required amounts of Y2 O3 , BaCO3 , Co3 O4 , and ZnO were thoroughly mixed by ball-milling in ethanol for 24 h and calcined at 1000 ◦ C for 12 h in air. The calcined powders were then ground, pressed into pellets, and sintered at 1200 ◦ C for 12 h in air. The YBaCo3 ZnO7 + GDC (Nextech, Micro
grade) composite cathodes with different ratios (0–70 wt.% GDC) were prepared by ball-milling appropriate amounts of YBaCo3 ZnO7 and GDC in ethanol for 3 days. The polarization resistance (Rp ) of the YBaCo4−x Znx O7 + GDC composite cathode in contact with GDC pellets was measured using symmetrical cells in the temperature range of 375–750 ◦ C by AC impedance spectroscopy (Solartron 1260 FRA). The GDC electrolyte disks were prepared by pelletizing and sintering required amounts of Gd2 O3 and CeO2 at 1600 ◦ C for 10 h. All the cathode materials were mixed with an organic binder (Heraeus V006) to form a slurry and then applied onto both the sides of a dense GDC pellet (0.75 mm thickness) by screen printing. The YBaCo4−x Znx O7 + GDC cathodes were heated at 900 ◦ C for 3 h. The microstructures of the cathodes were observed with a Hitachi S-5500 scanning electron microscope (SEM). Thermal expansion data were collected with a dilatometer (Linseis L75H) during three consecutive heating/cooling cycles at a rate of 3 ◦ C min−1 between 20 and 900 ◦ C with an intermediate dwelling at 900 ◦ C for 1 h in air. Fuel cell performances of the YBaCo4−x Znx O7 + GDC (50:50 wt.%) cathode were evaluated with the anode-supported
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single cells consisting of YBaCo3 ZnO7 |YBaCo3 ZnO7 + GDC composite|GDC|YSZ|Ni + GDC. The NiO + GDC (65:35 wt.% with 7 wt.% carbon) cermet anode disk was prepared by pelletizing and sintering at 1350 ◦ C for 3 h. Subsequent Y0.15 Zr0.85 O1.93 (YSZ, Tosoh) and GDC (Nextech) thin electrolyte films were deposited using the pulsed laser deposition (PLD) technique performed in a multitarget chamber with a KrF excimer laser (Lambda Physik 210, = 248 nm, 5–10 Hz). The experimental details are available elsewhere [13]. For the cathode, the YBaCo3 ZnO7 or the YBaCo4−x Znx O7 + GDC layers were applied to the thin electrolyte layer by heating at 900 ◦ C for 3 h. Pt meshes and wires were attached to each electrode using Pt paste as current collector. During the single cell SOFC operation, humidified H2 (∼3% H2 O at 25 ◦ C) and air were supplied, respectively, as fuel and oxidant at a rate of 100 cm3 min−1 . The area of each electrode was 0.5 cm2 . After the single cell SOFC tests, the microstructures and elemental profile of the cathodes were observed with a JEOL JSM-5610 scanning electron microscope (SEM) combined with energy dispersive X-ray spectroscopy (EDS). 3. Results and Discussion 3.1. Microstructure of the composite cathodes Among the various RBa(Co,M)4 O7 (R = Y, Ca, In, and M = Zn, Fe, Al) samples investigated with the hexagonal structure, the YBaCo4−x Znx O7 (1 ≤ x ≤2) samples were found to be promising candidates by us before with exceptionally low TEC and good high-temperature stability [12]. The YBaCo3 ZnO7 cathode showed enhanced electrochemical performance on employing it as a YBaCo3 ZnO7 + GDC composite cathode. Fig. 1 shows the cross-sectional SEM images of the pure YBaCo3 ZnO7 and the YBaCo3 ZnO7 + GDC composite cathodes attached onto the top of the GDC electrolyte. While the bottom of the micrograph indicates the dense GDC electrolyte, the upper portion shows the porous cathodes with approximate thicknesses of ∼20 m. From the SEM images, all the samples are found to have good bonding with continuous contact at the cathode|GDC interfaces. In addition, it is apparent that the composite cathodes have smaller particle sizes compared to the pure YBaCo3 ZnO7 cathode. For example, while the pure YBaCo3 ZnO7 cathode consists of larger particles (>300 nm in diameter) in Fig. 1(b), the composite cathodes consist of much smaller particles (∼200 nm in diameter) in Fig. 1(d) and (f). In the composite cathodes, the growth of the YBaCo3 ZnO7 particles is suppressed by the neighboring GDC particles on heating at 900 ◦ C. Since the GDC electrolyte requires much higher sintering temperature compared to YBaCo3 ZnO7 , the smaller particles in the composite cathodes in Fig. 1(d) and (f) are presumed to be the GDC particles. The smaller particles and the consequent larger surface area of the composite cathodes are beneficial for providing extended TPB where the ORR reaction occurs. Similarly, the La1−x Srx MnO3 (LSM) + YSZ and LSCF + GDC composite cathodes have been found to exhibit better electrochemical performances with extended TPB compared to the pure LSM and LSCF cathodes [14–18].
Fig. 2. Area-normalized AC impedance spectra obtained with the YBaCo3 ZnO7 |GDC|YBaCo3 ZnO7 symmetrical cell at various temperatures in air: (a) 650 ◦ C; (b) 600 ◦ C; (c) 550 ◦ C; (d) 500 ◦ C.
GDC electrolyte have been evaluated in the literature from their well-separated AC impedance spectra [18–24]. However, these electrolyte resistances are strongly influenced by several factors such as temperature [18,20,21], grain size [22,23], and impurities at grain boundaries [24]. Therefore, we first characterized our homemade GDC electrolyte pellet using the Pt|GDC|Pt cell. The bulk and grain boundary semicircles can be observed up to 300 and 650 ◦ C, respectively, while they become hard to discern above those temperature due to the decreasing time constant beyond the limit of the instrument [18,20,21]. Fig. 3 shows the grain boundary conductance data obtained from the home-made GDC electrolyte, and the results agree well with the literature data [24] with an activation energy (Ea ) of 1.09 eV. It is interesting to note that the high frequency semicircles in both the pure and composite cathodes also match well with the grain boundary conductance obtained from the GDC electrolyte in Fig. 3. Furthermore, the high frequency semicircles have capacitance values of 0.3–0.4 F at T ∼ 500 ◦ C, which are close to the grain boundary capacitance in GDC [21]. These results indicate that the high frequency semicircles observed in our symmetrical cells are associated with the grain boundary resistance of the GDC electrolyte. 3.3. Effect of composite cathode on the polarization resistance Fig. 4(a) shows the electrical circuit model employed to fit the impedance spectra of the various cathodes investigated in this study. The constant phase element (CPE) has been adopted to provide a better fitting result for the depressed semicircle. The CPE has been widely used to describe the inhomogeneous porous electrode|electrolyte systems. The RS is from the Pt wire and the bulk resistance of the GDC electrolyte, followed by the grain boundary resistance (RGB /CPEGB ) at the high frequency region.
3.2. Characterization of the electrolyte portion in the total impedance spectra Fig. 2 shows the AC impedance spectra obtained with the YBaCo3 ZnO7 |GDC|YBaCo3 ZnO7 symmetrical cell at various temperatures in air. The impedance spectra consist of a high frequency semicircle and a suppressed arc from medium to low frequency region. Under the symmetrical cell configuration, it is necessary to assess the portion of the GDC electrolyte in the total impedance spectra [19]. The inter-grain and grain boundary resistance of the
Fig. 3. Grain boundary conductance of the GDC electrolyte measured with symmetrical cells in comparison to the literature data [24].
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Fig. 4. (a) Electrical circuit model and area-normalized AC impedance spectra of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode at (b) 500 ◦ C and (c) 600 ◦ C.
The lower frequency arc is associated with the cathode polarization resistance, which has been fitted using two consecutive (R1 /CPE1 ) and (R2 /CPE2 ) circuits. The (R1 /CPE1 ) observed at higher frequencies is related to the charge transfer of oxide ions at the electrode|electrolyte interface, while the (R2 /CPE2 ) at lower frequencies is attributed to the diffusion of oxygen. In Fig. 4(b) and (c), the representative area-normalized impedance spectra of the YBaCo3 ZnO7 + GDC composite cathode show good fitting based on the circuit model in Fig. 4(a). Due to the small difference in the time constants, the cathode polarization resistance does not show a distinct separation between (R1 /CPE1 ) and (R2 /CPE2 ). The polarization resistance arc of the composite cathode is similar in shape to that in the pure YBaCo3 ZnO7 cathode (Fig. 2), but with different resistance values. This indicates that GDC incorporation into the YBaCo3 ZnO7 cathode affects the total polarization of the cathode (R1 + R2 ) with extended TPB, and the ORR is expected to be governed by similar rate-determining mechanism. Fig. 5 shows the variations of the total polarization resistance (Rp ) of the YBaCo3 ZnO7 + GDC composite cathodes with various GDC contents in air. Considering the symmetrical cell configuration, the Rp of the cathode was determined by (R1 + R2 )(Area)/2. In Fig. 5(a), both the Rp value and its slope increase with increasing GDC content from 0 to 30 wt.%. However, the composite cathodes show similar Rp values of ∼0.16 cm2 at 750 ◦ C because of the increase in Ea with GDC content. Further increase in GDC content from 30 to 50 wt.% in the YBaCo3 ZnO7 + GDC leads to a decrease in Rp , which becomes more prominent with increasing temperature in Fig. 5(b). The incorporation of GDC into YBaCo3 ZnO7 offers an extended TPB and thereby enhances the catalytic activity for ORR. Furthermore, oxide-ion bulk diffusion will be improved through the GDC portion within the composite cathode. Additional incorporation of GDC (60 and 70 wt.% GDC) into the composite cathode again increases the Rp values in Fig. 5(c). Fig. 6 shows the variation of Ea with the GDC content in the YBaCo3 ZnO7 + GDC composite cathodes. The data indicate a linear increase in Ea from 0.66 eV (0 wt.% GDC) to 1.08 eV (70 wt.% GDC) with increasing GDC content. This can be understood to be due
Fig. 5. Temperature dependence of Rp in the YBaCo3 ZnO7 + GDC composite cathodes with various GDC content in air: (a) 0–30 wt.%; (b) 30–50 wt.%; (c) 50–70 wt.% GDC.
to the higher Ea (∼1.0 eV) for the ionic conductivity of the GDC electrolyte compared to that of YBaCo3 ZnO7 with an Ea = 0.66 eV [25]. Fig. 7 demonstrates the variation of Rp in the YBaCo3 ZnO7 + GDC composite cathode with the GDC content and temperature. These curves were derived using the linear fitting data from the Rp curves in Fig. 5. The lowest Rp value is found with the composite cathode
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Fig. 6. Variation of the activation energy with the GDC content in the YBaCo3 ZnO7 + GDC composite cathode in air.
with 50 wt.% GDC at T > 475 ◦ C and the pure YBaCo3 ZnO7 cathode at T < 475 ◦ C. This discrepancy is attributed to the higher Ea of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode (0.95 eV) compared to that of the pure YBaCo3 ZnO7 cathode (0.66 eV). Considering the typical operating temperature of intermediate temperature SOFC (600 ◦ C ≤ T ≤800 ◦ C), the composite with 50 wt.% GDC is the optimum cathode with the lowest Rp value. Similarly, the LSCF + GDC composite cathode system also exhibits the lowest Rp value with the 50:50 wt.% ratio [17]. 3.4. Thermal expansion behavior of the composite cathodes Fig. 8 shows the thermal expansion behavior of the YBaCo3 ZnO7 , GDC, and YBaCo3 ZnO7 + GDC (50:50 wt.%) composite specimens. The pure YBaCo3 ZnO7 exhibits lower TEC than pure GDC. For example, the TECs of GDC and YBaCo3 ZnO7 are, respectively, 12.8 × 10−6 and 9.5 × 10−6 ◦ C−1 in the temperature range of 20–800 ◦ C in air. The low TEC of YBaCo3 ZnO7 is attributed to the absence of spinstate transitions with the tetrahedral Co2+/3+ ions and a smaller amount of oxygen loss on heating compared to those normally encountered with octahedral Co3+ ions in perovskite oxides [12].
Fig. 7. Variation of Rp with temperature and GDC content in the YBaCo3 ZnO7 + GDC composite cathode in air.
Fig. 8. Thermal expansion (dL/Lo ) curves of (a) GDC, (b) YBaCo3 ZnO7 + GDC (50:50 wt.%) composite, and (c) YBaCo3 ZnO7 specimens in air.
Nevertheless, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite specimen provides better thermal expansion compatibility with the GDC electrolyte with a TEC value of 10.7 × 10−6 ◦ C−1 in Fig. 8(b). 3.5. Fuel cell performance of the composite cathodes The fuel cell performances of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathodes have been evaluated with anode-supported single cells. The single cell consists of YBaCo3 ZnO7 |YBaCo3 ZnO7 + GDC (50:50 wt.%) composite|GDC|YSZ|Ni+GDC. Here, the YBaCo3 ZnO7 cathode layer placed on top of the composite cathode layer was employed as a current collector. Fig. 9 shows that the Rp of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode is not influenced by the additional YBaCo3 ZnO7 cathode layer on top of it. This result suggests that the YBaCo3 ZnO7 cathode layer acts only as a current collector without contributing to the ORR, which is most likely to occur near the
Fig. 9. Comparison of the Rp of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode before and after applying additional YBaCo3 ZnO7 layer on top of it. YBC3Z refers to YBaCo3 ZnO7 .
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the EDS analysis correspond well with the chemical compositions of the different layers and ensures absence of interfacial reactions between the various layers. 4. Conclusions
Fig. 10. Fuel cell performances of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode in the temperature range of 750–600 ◦ C.
With an aim to improve the electrochemical performance of the YBaCo3 ZnO7 cathode, the YBaCo3 ZnO7 + GDC composite cathodes with different GDC contents (0–70 wt.%) have been investigated. The composite cathodes consist of smaller particles with larger surface area compared to the pure YBaCo3 ZnO7 cathode as evidenced by the SEM data. The polarization resistances of the composite cathodes have been measured with AC impedance spectroscopy. The high frequency semicircle in the impedance spectra has been identified as the grain boundary resistance in the GDC electrolyte pellet. Among the various composite cathodes investigated, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite was found to be the optimum cathode with the lowest Rp value. Fuel cell performances of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode collected with anode-supported single cell SOFC reveal a maximum power density value of 743 mW/cm2 at 750 ◦ C. Additionally, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode shows a TEC of 10.7 × 10−6 ◦ C−1 , which provides a better thermal expansion compatibility with the GDC electrolyte compared to the pure YBaCo3 ZnO7 cathode. Acknowledgements Financial support by the Welch Foundation grant F-1254 is gratefully acknowledged. H.W. and S.M. Cho thank the support from National Science Foundation (NSF 0709831).
Fig. 11. SEM image showing the cross section of the single cell SOFC after the fuel cell performance tests. The YSZ and GDC portions in the electrolyte are indicated in the image. The elemental profiles were recorded at the yellow line.
electrolyte within the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite layer. In addition, the long distance from the electrolyte to the YBaCo3 ZnO7 cathode (>20 m) will lead to a large resistance for the oxide-ion transport. Fig. 10 shows the I–V curves and the corresponding power density curves recorded at 750, 700, 650, and 600 ◦ C. The thin layer of YSZ was employed in between the anode and the GDC electrolyte, which prevents the reduction of GDC electrolyte in hydrogen atmosphere and thereby maintains high open-circuit voltages (OCV) of ∼1.1 V at all temperatures [26]. Reduction of Ce4+ into Ce3+ ions in GDC in hydrogen generally leads to an increase in electronic conductivity and a consequent decrease in OCV, which becomes more severe at high temperatures (T > 650 ◦ C) [27]. The maximum power density values of the single cells were found to be 743, 526, 300, and 143 mW/cm2 , respectively, at 750, 700, 650, and 600 ◦ C. However, concentration polarization was observed under high current drain (>1.4 A/cm2 ) due to the effect of anode porosity and limited fuel supply, which prevents us from evaluating the true maximum power density of the cell at 750 ◦ C. Fig. 11 shows the cross-sectional SEM image of the single cell after the fuel cell performance test. While the middle of the micrograph indicates the dense YSZ|GDC electrolyte with a total thickness of ∼8 m, each side of the electrolyte film shows the porous anode and cathode. The elemental profiles obtained from
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical characterization of YBaCo3 ZnO7 + Gd0.2 Ce0.8 O1.9 composite cathodes for intermediate temperature solid oxide fuel cells J.-H. Kim a , Y.N. Kim a , S.M. Cho b , H. Wang b , A. Manthiram a,∗ a b
Electrochemical Energy Laboratory & Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712, USA Department of Electrical and Computer Engineering, Texas A & M University, College Station, TX 77843, USA
a r t i c l e
i n f o
Article history: Received 18 January 2010 Received in revised form 15 April 2010 Accepted 16 April 2010 Available online 22 April 2010 Keywords: Solid oxide fuel cell Composite cathode Polarization resistance
a b s t r a c t YBaCo3 ZnO7 + Gd0.2 Ce0.8 O1.9 (GDC) composites with various GDC contents (0–70 wt.%) have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (SOFC). The effect of GDC incorporation on the microstructure, electrochemical properties, and thermal expansion behavior of the YBaCo3 ZnO7 + GDC composites has been studied. The composite cathodes consist of smaller particles with larger surface area compared to the pure YBaCo3 ZnO7 cathode, which is beneficial for providing extended triple-phase boundary (TPB) where the oxygen reduction reaction (ORR) occurs. Among the various compositions investigated, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite is found to be optimum with the lowest polarization resistance (0.28 cm2 at 600 ◦ C) compared to that of pure YBaCo3 ZnO7 (0.62 cm2 at 600 ◦ C). Anode-supported single cell SOFC fabricated with the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode also exhibits excellent performance with a maximum power density of 743 mW/cm2 at 750 ◦ C. Additionally, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite shows a low thermal expansion coefficient (TEC) of 10.7 × 10−6 ◦ C−1 , which provides good compatibility with those of standard SOFC electrolytes. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The RBaCo4 O7+ı (Dy, Ho, Er, Tm, Yb, Lu, Y, Ca, In) oxides have drawn attention recently as they exhibit interesting magnetic behavior [1–4] and oxygen absorption/desorption properties [5–9]. Especially, the latter property has stimulated interest in their application as oxygen storage materials. For example, RBaCo4 O7 absorbs extra oxygen into the lattice (up to 1.5 oxygen atoms per formula unit) in a narrow temperature range (200 ◦ C ≤ T ≤400 ◦ C) and lose the extra oxygen rapidly at T > 400 ◦ C [5–7]. The amount of oxygen absorption into the lattice strongly depends on the Rn+ ion and oxygen partial pressure, which is accompanied by structural transitions. RBaCo4 O7 with a stoichiometric amount of oxygen (ı ≈ 0) consists of corner-shared CoO4 tetrahedra with the Ba2+ and Rn+ ions adopting, respectively, 12- and 6-fold oxygen coordinations with a hexagonal structure. In contrast, YBaCo3 ZnO7+ı with extra oxygen (ı > 1.0) in the lattice has an orthorhombic structure with the formation of a supper-lattice and the cobalt ions occupying both edge-shared octahedra and corner-shared tetrahedra [7–9]. However, details of the mechanisms of oxygen adsorption/desorption into/from the RBaCo4 O7+ı lattice remain to be established with future studies.
∗ Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681. E-mail address: [email protected] (A. Manthiram). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.04.058
Unfortunately, the RBaCo4 O7+ı samples suffer from phase decomposition at elevated temperatures of 700–800 ◦ C, which prevents their use in high-temperature applications such as cathode materials in SOFC [5–7]. For example, the YBaCo4−x Fex O7 (0 ≤ x ≤0.8) samples were found to decompose into cobalt-based perovskite oxides and Co3 O4 with severe volume shrinkage at 700–800 ◦ C although it has an attractive low thermal expansion coefficient (TEC) of ∼10 × 10−6 ◦ C−1 below 700 ◦ C [10]. However, Hao et al. [11] reported recently a low oxygen adsorption by the zinc-substituted YBaCo4−x Znx O7 (1 ≤ x ≤3) at high temperatures, suggesting their improved phase stability [11]. Encouraged by this earlier report, we presented recently a direct investigation of the phase stability of the RBaCo4−x Mx O7 (R = Y, Ca, In, or mixtures thereof and M = Zn, Fe, Al, or mixtures thereof) series and their potential for SOFC applications [12]. Among them, only YBaCo4−x Znx O7 (1 ≤ x ≤2) showed long-term stability at high temperatures (600–800 ◦ C). In particular, the RBaCo4−x Mx O7 samples have the advantage of low TECs of 9.5 × 10−6 –13 × 10−6 ◦ C−1 in the range of 80–900 ◦ C, which provides good thermal expansion compatibility with the standard SOFC electrolytes. The low TEC can be understood to be due to the absence of spin-state transitions with the tetrahedral-site Co2+/3+ ions and relatively small amount of oxygen loss with increasing temperature. In addition, the YBaCo3 ZnO7 sample was found with electrolyte-supported single cell SOFC to show fuel cell performance comparable to that of the well-studied La0.6 Sr0.4 Co0.2 Fe0.8 O3 (LSCF) cathode. Further
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Fig. 1. Scanning electron microscope (SEM) images showing the cross sections of the cathode–GDC electrolyte portion in the symmetrical cells: (a and b) simple YBaCo3 ZnO7 , (c and d) YBaCo3 ZnO7 + GDC (50:50 wt.%) composite, (e and f) YBaCo3 ZnO7 + GDC (30:70 wt.%) composite cathodes after the AC impedance measurements.
enhancement in the cathode performance was also achieved on employing a YBaCo3 ZnO7 + GDC composite cathode due to the extended triple-phase boundary (TPB) and enhanced oxide-ion conductivity. Based on the above findings, we present here a systematic investigation of the YBaCo3 ZnO7 + GDC composite cathode with varying ratios between YBaCo3 ZnO7 and GDC and an exploration of their use as cathodes in anode-supported SOFC. The influence of GDC incorporation into the YBaCo3 ZnO7 cathode on the microstructure, TEC, and catalytic activity for the oxygen reduction reaction (ORR) in SOFC are discussed. 2. Experimental The YBaCo3 ZnO7 (YBC3Z) sample was synthesized by conventional solid-state reaction methods. Required amounts of Y2 O3 , BaCO3 , Co3 O4 , and ZnO were thoroughly mixed by ball-milling in ethanol for 24 h and calcined at 1000 ◦ C for 12 h in air. The calcined powders were then ground, pressed into pellets, and sintered at 1200 ◦ C for 12 h in air. The YBaCo3 ZnO7 + GDC (Nextech, Micro
grade) composite cathodes with different ratios (0–70 wt.% GDC) were prepared by ball-milling appropriate amounts of YBaCo3 ZnO7 and GDC in ethanol for 3 days. The polarization resistance (Rp ) of the YBaCo4−x Znx O7 + GDC composite cathode in contact with GDC pellets was measured using symmetrical cells in the temperature range of 375–750 ◦ C by AC impedance spectroscopy (Solartron 1260 FRA). The GDC electrolyte disks were prepared by pelletizing and sintering required amounts of Gd2 O3 and CeO2 at 1600 ◦ C for 10 h. All the cathode materials were mixed with an organic binder (Heraeus V006) to form a slurry and then applied onto both the sides of a dense GDC pellet (0.75 mm thickness) by screen printing. The YBaCo4−x Znx O7 + GDC cathodes were heated at 900 ◦ C for 3 h. The microstructures of the cathodes were observed with a Hitachi S-5500 scanning electron microscope (SEM). Thermal expansion data were collected with a dilatometer (Linseis L75H) during three consecutive heating/cooling cycles at a rate of 3 ◦ C min−1 between 20 and 900 ◦ C with an intermediate dwelling at 900 ◦ C for 1 h in air. Fuel cell performances of the YBaCo4−x Znx O7 + GDC (50:50 wt.%) cathode were evaluated with the anode-supported
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single cells consisting of YBaCo3 ZnO7 |YBaCo3 ZnO7 + GDC composite|GDC|YSZ|Ni + GDC. The NiO + GDC (65:35 wt.% with 7 wt.% carbon) cermet anode disk was prepared by pelletizing and sintering at 1350 ◦ C for 3 h. Subsequent Y0.15 Zr0.85 O1.93 (YSZ, Tosoh) and GDC (Nextech) thin electrolyte films were deposited using the pulsed laser deposition (PLD) technique performed in a multitarget chamber with a KrF excimer laser (Lambda Physik 210, = 248 nm, 5–10 Hz). The experimental details are available elsewhere [13]. For the cathode, the YBaCo3 ZnO7 or the YBaCo4−x Znx O7 + GDC layers were applied to the thin electrolyte layer by heating at 900 ◦ C for 3 h. Pt meshes and wires were attached to each electrode using Pt paste as current collector. During the single cell SOFC operation, humidified H2 (∼3% H2 O at 25 ◦ C) and air were supplied, respectively, as fuel and oxidant at a rate of 100 cm3 min−1 . The area of each electrode was 0.5 cm2 . After the single cell SOFC tests, the microstructures and elemental profile of the cathodes were observed with a JEOL JSM-5610 scanning electron microscope (SEM) combined with energy dispersive X-ray spectroscopy (EDS). 3. Results and Discussion 3.1. Microstructure of the composite cathodes Among the various RBa(Co,M)4 O7 (R = Y, Ca, In, and M = Zn, Fe, Al) samples investigated with the hexagonal structure, the YBaCo4−x Znx O7 (1 ≤ x ≤2) samples were found to be promising candidates by us before with exceptionally low TEC and good high-temperature stability [12]. The YBaCo3 ZnO7 cathode showed enhanced electrochemical performance on employing it as a YBaCo3 ZnO7 + GDC composite cathode. Fig. 1 shows the cross-sectional SEM images of the pure YBaCo3 ZnO7 and the YBaCo3 ZnO7 + GDC composite cathodes attached onto the top of the GDC electrolyte. While the bottom of the micrograph indicates the dense GDC electrolyte, the upper portion shows the porous cathodes with approximate thicknesses of ∼20 m. From the SEM images, all the samples are found to have good bonding with continuous contact at the cathode|GDC interfaces. In addition, it is apparent that the composite cathodes have smaller particle sizes compared to the pure YBaCo3 ZnO7 cathode. For example, while the pure YBaCo3 ZnO7 cathode consists of larger particles (>300 nm in diameter) in Fig. 1(b), the composite cathodes consist of much smaller particles (∼200 nm in diameter) in Fig. 1(d) and (f). In the composite cathodes, the growth of the YBaCo3 ZnO7 particles is suppressed by the neighboring GDC particles on heating at 900 ◦ C. Since the GDC electrolyte requires much higher sintering temperature compared to YBaCo3 ZnO7 , the smaller particles in the composite cathodes in Fig. 1(d) and (f) are presumed to be the GDC particles. The smaller particles and the consequent larger surface area of the composite cathodes are beneficial for providing extended TPB where the ORR reaction occurs. Similarly, the La1−x Srx MnO3 (LSM) + YSZ and LSCF + GDC composite cathodes have been found to exhibit better electrochemical performances with extended TPB compared to the pure LSM and LSCF cathodes [14–18].
Fig. 2. Area-normalized AC impedance spectra obtained with the YBaCo3 ZnO7 |GDC|YBaCo3 ZnO7 symmetrical cell at various temperatures in air: (a) 650 ◦ C; (b) 600 ◦ C; (c) 550 ◦ C; (d) 500 ◦ C.
GDC electrolyte have been evaluated in the literature from their well-separated AC impedance spectra [18–24]. However, these electrolyte resistances are strongly influenced by several factors such as temperature [18,20,21], grain size [22,23], and impurities at grain boundaries [24]. Therefore, we first characterized our homemade GDC electrolyte pellet using the Pt|GDC|Pt cell. The bulk and grain boundary semicircles can be observed up to 300 and 650 ◦ C, respectively, while they become hard to discern above those temperature due to the decreasing time constant beyond the limit of the instrument [18,20,21]. Fig. 3 shows the grain boundary conductance data obtained from the home-made GDC electrolyte, and the results agree well with the literature data [24] with an activation energy (Ea ) of 1.09 eV. It is interesting to note that the high frequency semicircles in both the pure and composite cathodes also match well with the grain boundary conductance obtained from the GDC electrolyte in Fig. 3. Furthermore, the high frequency semicircles have capacitance values of 0.3–0.4 F at T ∼ 500 ◦ C, which are close to the grain boundary capacitance in GDC [21]. These results indicate that the high frequency semicircles observed in our symmetrical cells are associated with the grain boundary resistance of the GDC electrolyte. 3.3. Effect of composite cathode on the polarization resistance Fig. 4(a) shows the electrical circuit model employed to fit the impedance spectra of the various cathodes investigated in this study. The constant phase element (CPE) has been adopted to provide a better fitting result for the depressed semicircle. The CPE has been widely used to describe the inhomogeneous porous electrode|electrolyte systems. The RS is from the Pt wire and the bulk resistance of the GDC electrolyte, followed by the grain boundary resistance (RGB /CPEGB ) at the high frequency region.
3.2. Characterization of the electrolyte portion in the total impedance spectra Fig. 2 shows the AC impedance spectra obtained with the YBaCo3 ZnO7 |GDC|YBaCo3 ZnO7 symmetrical cell at various temperatures in air. The impedance spectra consist of a high frequency semicircle and a suppressed arc from medium to low frequency region. Under the symmetrical cell configuration, it is necessary to assess the portion of the GDC electrolyte in the total impedance spectra [19]. The inter-grain and grain boundary resistance of the
Fig. 3. Grain boundary conductance of the GDC electrolyte measured with symmetrical cells in comparison to the literature data [24].
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Fig. 4. (a) Electrical circuit model and area-normalized AC impedance spectra of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode at (b) 500 ◦ C and (c) 600 ◦ C.
The lower frequency arc is associated with the cathode polarization resistance, which has been fitted using two consecutive (R1 /CPE1 ) and (R2 /CPE2 ) circuits. The (R1 /CPE1 ) observed at higher frequencies is related to the charge transfer of oxide ions at the electrode|electrolyte interface, while the (R2 /CPE2 ) at lower frequencies is attributed to the diffusion of oxygen. In Fig. 4(b) and (c), the representative area-normalized impedance spectra of the YBaCo3 ZnO7 + GDC composite cathode show good fitting based on the circuit model in Fig. 4(a). Due to the small difference in the time constants, the cathode polarization resistance does not show a distinct separation between (R1 /CPE1 ) and (R2 /CPE2 ). The polarization resistance arc of the composite cathode is similar in shape to that in the pure YBaCo3 ZnO7 cathode (Fig. 2), but with different resistance values. This indicates that GDC incorporation into the YBaCo3 ZnO7 cathode affects the total polarization of the cathode (R1 + R2 ) with extended TPB, and the ORR is expected to be governed by similar rate-determining mechanism. Fig. 5 shows the variations of the total polarization resistance (Rp ) of the YBaCo3 ZnO7 + GDC composite cathodes with various GDC contents in air. Considering the symmetrical cell configuration, the Rp of the cathode was determined by (R1 + R2 )(Area)/2. In Fig. 5(a), both the Rp value and its slope increase with increasing GDC content from 0 to 30 wt.%. However, the composite cathodes show similar Rp values of ∼0.16 cm2 at 750 ◦ C because of the increase in Ea with GDC content. Further increase in GDC content from 30 to 50 wt.% in the YBaCo3 ZnO7 + GDC leads to a decrease in Rp , which becomes more prominent with increasing temperature in Fig. 5(b). The incorporation of GDC into YBaCo3 ZnO7 offers an extended TPB and thereby enhances the catalytic activity for ORR. Furthermore, oxide-ion bulk diffusion will be improved through the GDC portion within the composite cathode. Additional incorporation of GDC (60 and 70 wt.% GDC) into the composite cathode again increases the Rp values in Fig. 5(c). Fig. 6 shows the variation of Ea with the GDC content in the YBaCo3 ZnO7 + GDC composite cathodes. The data indicate a linear increase in Ea from 0.66 eV (0 wt.% GDC) to 1.08 eV (70 wt.% GDC) with increasing GDC content. This can be understood to be due
Fig. 5. Temperature dependence of Rp in the YBaCo3 ZnO7 + GDC composite cathodes with various GDC content in air: (a) 0–30 wt.%; (b) 30–50 wt.%; (c) 50–70 wt.% GDC.
to the higher Ea (∼1.0 eV) for the ionic conductivity of the GDC electrolyte compared to that of YBaCo3 ZnO7 with an Ea = 0.66 eV [25]. Fig. 7 demonstrates the variation of Rp in the YBaCo3 ZnO7 + GDC composite cathode with the GDC content and temperature. These curves were derived using the linear fitting data from the Rp curves in Fig. 5. The lowest Rp value is found with the composite cathode
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Fig. 6. Variation of the activation energy with the GDC content in the YBaCo3 ZnO7 + GDC composite cathode in air.
with 50 wt.% GDC at T > 475 ◦ C and the pure YBaCo3 ZnO7 cathode at T < 475 ◦ C. This discrepancy is attributed to the higher Ea of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode (0.95 eV) compared to that of the pure YBaCo3 ZnO7 cathode (0.66 eV). Considering the typical operating temperature of intermediate temperature SOFC (600 ◦ C ≤ T ≤800 ◦ C), the composite with 50 wt.% GDC is the optimum cathode with the lowest Rp value. Similarly, the LSCF + GDC composite cathode system also exhibits the lowest Rp value with the 50:50 wt.% ratio [17]. 3.4. Thermal expansion behavior of the composite cathodes Fig. 8 shows the thermal expansion behavior of the YBaCo3 ZnO7 , GDC, and YBaCo3 ZnO7 + GDC (50:50 wt.%) composite specimens. The pure YBaCo3 ZnO7 exhibits lower TEC than pure GDC. For example, the TECs of GDC and YBaCo3 ZnO7 are, respectively, 12.8 × 10−6 and 9.5 × 10−6 ◦ C−1 in the temperature range of 20–800 ◦ C in air. The low TEC of YBaCo3 ZnO7 is attributed to the absence of spinstate transitions with the tetrahedral Co2+/3+ ions and a smaller amount of oxygen loss on heating compared to those normally encountered with octahedral Co3+ ions in perovskite oxides [12].
Fig. 7. Variation of Rp with temperature and GDC content in the YBaCo3 ZnO7 + GDC composite cathode in air.
Fig. 8. Thermal expansion (dL/Lo ) curves of (a) GDC, (b) YBaCo3 ZnO7 + GDC (50:50 wt.%) composite, and (c) YBaCo3 ZnO7 specimens in air.
Nevertheless, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite specimen provides better thermal expansion compatibility with the GDC electrolyte with a TEC value of 10.7 × 10−6 ◦ C−1 in Fig. 8(b). 3.5. Fuel cell performance of the composite cathodes The fuel cell performances of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathodes have been evaluated with anode-supported single cells. The single cell consists of YBaCo3 ZnO7 |YBaCo3 ZnO7 + GDC (50:50 wt.%) composite|GDC|YSZ|Ni+GDC. Here, the YBaCo3 ZnO7 cathode layer placed on top of the composite cathode layer was employed as a current collector. Fig. 9 shows that the Rp of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode is not influenced by the additional YBaCo3 ZnO7 cathode layer on top of it. This result suggests that the YBaCo3 ZnO7 cathode layer acts only as a current collector without contributing to the ORR, which is most likely to occur near the
Fig. 9. Comparison of the Rp of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode before and after applying additional YBaCo3 ZnO7 layer on top of it. YBC3Z refers to YBaCo3 ZnO7 .
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the EDS analysis correspond well with the chemical compositions of the different layers and ensures absence of interfacial reactions between the various layers. 4. Conclusions
Fig. 10. Fuel cell performances of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode in the temperature range of 750–600 ◦ C.
With an aim to improve the electrochemical performance of the YBaCo3 ZnO7 cathode, the YBaCo3 ZnO7 + GDC composite cathodes with different GDC contents (0–70 wt.%) have been investigated. The composite cathodes consist of smaller particles with larger surface area compared to the pure YBaCo3 ZnO7 cathode as evidenced by the SEM data. The polarization resistances of the composite cathodes have been measured with AC impedance spectroscopy. The high frequency semicircle in the impedance spectra has been identified as the grain boundary resistance in the GDC electrolyte pellet. Among the various composite cathodes investigated, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite was found to be the optimum cathode with the lowest Rp value. Fuel cell performances of the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode collected with anode-supported single cell SOFC reveal a maximum power density value of 743 mW/cm2 at 750 ◦ C. Additionally, the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite cathode shows a TEC of 10.7 × 10−6 ◦ C−1 , which provides a better thermal expansion compatibility with the GDC electrolyte compared to the pure YBaCo3 ZnO7 cathode. Acknowledgements Financial support by the Welch Foundation grant F-1254 is gratefully acknowledged. H.W. and S.M. Cho thank the support from National Science Foundation (NSF 0709831).
Fig. 11. SEM image showing the cross section of the single cell SOFC after the fuel cell performance tests. The YSZ and GDC portions in the electrolyte are indicated in the image. The elemental profiles were recorded at the yellow line.
electrolyte within the YBaCo3 ZnO7 + GDC (50:50 wt.%) composite layer. In addition, the long distance from the electrolyte to the YBaCo3 ZnO7 cathode (>20 m) will lead to a large resistance for the oxide-ion transport. Fig. 10 shows the I–V curves and the corresponding power density curves recorded at 750, 700, 650, and 600 ◦ C. The thin layer of YSZ was employed in between the anode and the GDC electrolyte, which prevents the reduction of GDC electrolyte in hydrogen atmosphere and thereby maintains high open-circuit voltages (OCV) of ∼1.1 V at all temperatures [26]. Reduction of Ce4+ into Ce3+ ions in GDC in hydrogen generally leads to an increase in electronic conductivity and a consequent decrease in OCV, which becomes more severe at high temperatures (T > 650 ◦ C) [27]. The maximum power density values of the single cells were found to be 743, 526, 300, and 143 mW/cm2 , respectively, at 750, 700, 650, and 600 ◦ C. However, concentration polarization was observed under high current drain (>1.4 A/cm2 ) due to the effect of anode porosity and limited fuel supply, which prevents us from evaluating the true maximum power density of the cell at 750 ◦ C. Fig. 11 shows the cross-sectional SEM image of the single cell after the fuel cell performance test. While the middle of the micrograph indicates the dense YSZ|GDC electrolyte with a total thickness of ∼8 m, each side of the electrolyte film shows the porous anode and cathode. The elemental profiles obtained from
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