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Effects of composite cathode on electrochemical and redox properties for intermediate-temperature solid oxide fuel cells
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Effects of composite cathode on electrochemical and redox properties for intermediate-temperature solid oxide fuel cells Changmin Kim a, Junyoung Kim a, Jeeyoung Shin b,**, Guntae Kim a,* a
Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea b Department of Mechanical Engineering, Dong-Eui University, Busan 614-714, Republic of Korea
article info
abstract
Article history:
A composite cathode composed of NdBa0.5Sr0.5Co1.5Fe0.5O5þd (NВSCF) and Ce0.9Gd0.1O1.95
Available online 23 July 2014
(GDC) has been investigated to evaluate its electrochemical properties for intermediatetemperature solid oxide fuel cells (IT-SOFCs) based on structural characteristics and oxy-
Keywords:
gen redox stability. The composite cathode has a lower polarization loss than a pure NBSCF
Solid oxide fuel cells (SOFC)
cathode because the specific addition of GDC provides extended electrochemically active
Layered perovskite
sites where the oxygen reduction reaction (ORR) occurs. Accordingly, the optimized NBSCF-
Composite cathode
40GDC cathode material had the lowest ASR, 0.074 U cm2 at 873 K, resulting in excellent cell
Electrochemical performance
performance of 1.83 W cm2 at 873 K. In particular, investigation into the oxygen redox stability reveals that the composite cathode has superior redox stability under the operating conditions than a bulk NBSCF cathode material, which affects the long-term stability of the cathode performance. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are devices that convert chemical energy to electrical energy directly through electrochemical oxidation, providing advantages of fuel flexibility, low pollutant emission, and high efficiency. The requirement of high operating temperature of 1073e1273 K, however, gives rise to considerable issues such as high costs and material compatibility originating from the difference in the thermal expansion coefficient (TEC). Significant efforts therefore have been devoted to lowering the operation temperature of SOFCs toward an intermediate range (873e1073 K) to enhance long-
term stability and economic feasibility. Lowering of the operating temperature, however, results in serious problems including degradation of the electrocatalytic activity over the cathode, which is associated with the oxygen reduction reaction (ORR) [1e4]. In this regard, mixed ionic and electronic conductors (MIECs) with perovskite oxides based on transition metal (e.g. Mn, Fe, Co, and Ni) have been extensively researched as promising cathode materials of IT-SOFCs, due to their capability to conduct electrons and oxygen ions [5]. Among the various MIECs oxides, cobalt based perovskite oxides, such as BaCoO3 [6], Sm0.5Sr0.5CoO3 [7], La0.6Sr0.4Co0.2Fe0.8O3d [8], and
* Corresponding author. Fax: þ82 52 217 2909. ** Corresponding author. Fax: þ82 51 890 2232. E-mail addresses: [email protected] (J. Shin), [email protected] (G. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.07.007 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Pr1xSrxCoO3d [9], have displayed excellent electrocatalytic activity for the oxygen reduction reaction. Recently, layered perovskite oxides have been studied as novel cathode materials due to their remarkable oxygen kinetics rate relative to those of ABO3-type simple perovskite oxides [10,11]. The general formula of the layered perovskite oxides can be described as AA0 B2O5þd, where A ¼ trivalent lanthanide ion (Ln ¼ Pr, Nd, Sm, and Gd), A0 ¼ Ba or Sr, and B ¼ a first row transition metal ion or a mixture thereof. The layered perovskite consists of two layers with alternating stacking of … jAOjBO2jA0 OdjBO2j … along the c-axis. The lanthanide layers provide mobile oxygen species channels by reducing the oxygen bonding strength, which enhances oxygen ion diffusivity [11]. On the basis of these favorable properties of layered perovskite oxides, such as LnBaCo2O5þd (Ln ¼ Pr, Nd, Sm, and Gd) layered perovskite oxides, several groups have studied their capability as IT-SOFCs cathode materials. Kim et al. [12] reported that PrBaCo2O5þd (PBCO) is a promising material for IT-SOFC cathodes, as it showed a high surface exchange coefficient and rapid oxygen ion diffusivity, resulting in low cathodic polarization. The partial cosubstitution of Co for Fe and Ba for Sr, in particular, improves the ORR activity, electrical conductivity, oxygen ion diffusivity, and stability of the cathode [13e18]. Upon this background, our group reported the layered perovskite cathode materials, LnBa0.5Sr0.5Co1.5Fe0.5O5þd (Ln ¼ Pr and Nd), which have high electrochemical performance with excellent stabilities under operating conditions [19]. Based on a DFT analysis presented in our previous study [19], the layered structure provides pore channels for ion motion in the [LneO] and [CoeO] planes, which could provide fast paths for oxygen transport and these oxygen ion diffusion paths follow a zigezag type trajectory through the CoeO plane perpendicular to the Ln-O plane. It has been reported that the composite cathode prepared by specific addition of ionic conducting materials on a MIEC cathode contributes to improvement of electrochemical performance. Among these ionic conducting materials, Ce0.9Gd0.1O1.95 (GDC) can improve the electrocatalytic activity of the cathode by providing additional triple phase boundary (TPB) sites where the electrochemical reaction occurs [20,21]. Kim et al. [22] reported on the mechanism of the ORR for a MIEC-GDC composite cathode consisting of NdBa0.5Sr0.5Co2O5þd (NBSCO) and GDC. The authors reported that the composite cathode has a lower polarization loss than a pure NBSCO cathode due to the addition of GDC followed by extension of the TPB sites. Together with the substantial enhancement of single cell performance of Fe doped NBSCO, NdBa0.5Sr0.5Co1.5Fe0.5O5þd (NBSCF), long-term stability of the cell is also an important requirement for IT-SOFCs [19]. Low p(O2) operation may cause critical redox degradation of the cathode at the interface between the electrolyte and the cathode, which affects the longterm stability of the cathode performance [23,24]. Kim et al. [25] reported that a ceria based YSZ electrolyte composite anode exhibits enhanced reducibility relative to that of bulk ceria. In this regard, improvement of oxygen redox properties can be expected through the addition of an electrolyte material on the cathode. Meanwhile, there have been no reports on the effects of NBSCF-GDC composite cathodes, particularly
regarding the oxygen redox stability of NВSCF. In this study, therefore, we conducted a systematic investigation to optimize the electrochemical properties of the NBSCF cathode by optimizing the ratio of GDC to NBSCF based on the structural characteristics and oxygen redox stability of these materials.
Experimental NdBa0.5Sr0.5Co1.5Fe0.5O5þd oxides were synthesized using the Pechini process. Stoichiometric amounts of Nd(NO3)3$6H2O (Aldrich, 99.9%, metal basis), Ba(NO3)2 (Aldrich, 99 þ %), Sr(NO3)2 (Aldrich, 99 þ %), Co(NO3)2$6H2O (Aldrich, 98 þ %), and Fe(NO3)3$9H2O (Aldrich, 98 þ %) were dissolved in distilled water under continuous heating and stirring. A proper amount of citric acid and ethylene glycol were added into the beaker after the mixture was dissolved. After a viscous resin was formed, the mixture was heated around 473 K. The resultant products were pre-calcined at 873 K for 4 h, and ballmilled in acetone for 24 h. The pre-calcined NBSCF powder and Ce0.9Gd0.1O1.95 (GDC) were mixed at weight ratio of 10:0, 8:2, 6:4, and 5:5 and ball-milled in acetone for 24 h. The abbreviations used to identify various samples are given in Table 1. The mixtures of the NBSCF and GDC were blended with a binder (Heraeus V006) to form slurries for both symmetrical cell and single cell fabrication. The structure of NBSCF-xGDC cathode (x ¼ 0 and 40) were characterized by using X-ray powder diffraction (XRD) (Rigaku diffractometer, Cu Ka radiation) with a scan rate of 0.5 min1. The powder pattern and lattice parameters were analyzed by Rietveld refinement using GSAS program. In situ XRD of the NBSCF-xGDC cathode (x ¼ 0 and 40) was obtained from room temperature to operation temperature (Bruker, D8 Advance). The microstructures and morphologies of NBSCF-xGDC cathode sample (x ¼ 0, 20, 40, and 50) were observed using a field emission scanning electron microscope (SEM) (Nova SEM). To prepare anode supported single cell, NiO powder, GDC powder, and starch were mixed at weight ratio of 6:4:2 and ball-milled in ethanol for 24 h. NiO powder and GDC powder was prepared by glycine nitrate process. The detailed procedure has been described elsewhere [26]. After dried, the NiOGDC mixture was pressed into a pellet which has 0.6 mm thickness and 15 mm diameter. Thin GDC electrolyte membranes were prepared by a refined particle suspension coating technique. A GDC suspension was used for the electrolyte prepared by dispersing GDC powders (Aldrich) in ethanol with a proper amount of binder (Polyvinyl butyral, B-98) and dispersant (Triethanolamine, Alfa Aesar) at a ratio of 1:10. The GDC suspension was applied to a NiO-GDC anode support by
Table 1 e Abbreviation of NdBa0.5Sr0.5Co1.5Fe0.5O5þdexCe0.9Gd0.1O1.95. Abbreviation
NBSCF-0GDC NBSCF-20GDC NBSCF-40GDC NBSCF-50GDC
Composition (wt%) NdBa0.5Sr0.5Co1.5Fe0.5O5þd
Ce0.9Gd0.1O1.95
100 80 60 50
0 20 40 50
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drop-coating, followed by drying in air and subsequent cosintering at 1673 K for 5 h. Electrochemical impedance spectroscopy of NBSCF-xGDC (x ¼ 0, 20, 40, and 50) was carried out using a symmetrical cell. The GDC electrolyte powders were pressed into pellets, and then sintered at 1623 K for 4 h in air to obtain a dense electrolyte substrate. The slurries were screen-printed onto both sides of the GDC electrolytes to form symmetrical cells, followed by sintering at 1223 K. The silver wire and silver paste was used as a current collector for the electrodes. Impedance spectra were recorded under OCV in a frequency range of 1 mHze500 kHz with AC perturbation of 14 mV from 773 K to 923 K. Electrochemical performances were evaluated with NiGDC anode supported single cells. To determine the optimized cell performance, NBSCF-xGDC (x ¼ 0 and 40) slurries were screen-printed on the GDC electrolyte as a cathode. The cells were finally sintered at 1223 K for 4 h in air with an active electrode area of 0.36 cm2. The silver wires were used as a current collector onto both side of the cathode and anode of single cell using a silver paste. Each cell was mounted on alumina tube using a ceramic adhesive (Aremco, Ceramabond 553) to fix the single cell. Humidified hydrogen (3% H2O) was applied as fuel through a water bubbler with a flow rate of 20 mL min1. IeV curves were examined using a BioLogic Potentiostat at operating temperature from 773 K to 923 K. The redox properties and oxygen nonstoichiometry of NBSCF cathode material and NBSCF-40GDC composite were measured using coulometric titration (CT) as a function of the oxygen partial pressure, p(O2). A yttria-stabilized zirconia (YSZ) tube (McDanel Advanced Ceramic Technologies, Z15410630) was used both to electrochemically pump oxygen out of the system and to sense the equilibrium in coulometric titration. The oxygen sensor was part of the container wall and could also be used to add or remove oxygen from the system through application of a potential across the ionconducting YSZ tube. The oxide-sample was placed in a sealed container at the temperature of interest and equilibrated sufficiently by purging 5% O2eAr gas over it in the tube for 24 h. The detailed procedure has been described elsewhere [27]. The initial stoichiometric oxygen content of the sample is determined by iodometric titration and thermogravimetric analysis (TGA) in air at 973 K [19]. Oxygen partial pressure of the internal tube was determined from the OCV through the Nernst equation by the following Equation (1): 4EF
ex RT pin O2 ¼ pO2 $e
(1)
pðOex 2 Þ,
where the E, F, R, and T stand for the oxygen partial pressure of the external tube (ca. 0.21 atm), OCV, Faraday constant, gas constant, and temperature, respectively. The sample was allowed to equilibrate with the surrounding atmosphere in the tube. Oxygen nonstoichiometry during coulometric titration is calculated from the Equation (2): Dd ¼
2M It V $Dpin O2 m 4F RT
(2)
where the M, m, I, t, F, V, R, T, and DpðOin 2 Þ represent for molar mass of sample, sample mass, applied current, duration time, Faraday constant, free volume of the tube, gas constant,
Fig. 1 e (a) XRD patterns of pure NBSCF sintered at 1423 K. (b) Observed, and calculated XRD profiles and the difference between them for pure NBSCF. (c) XRD patterns of NBSCFexGDC (x ¼ 0 and 40) mixture after sintering at 1223 K in air.
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temperature, and change of oxygen partial pressure of the internal tube, respectively. The criterion of thermodynamic equilibrium of oxygen concentration is within 1 mV h1.
Results and discussion Fig. 1(a) presents XRD patterns of NBSCF sintered at 1423 K for 12 h. The patterns show that NBSCF has a single-phase layered perovskite structure without any detectable secondary phase. The powder patterns and lattice parameters were analyzed by Rietveld refinement as described in Fig. 1(b). The lattice parameters are a ¼ b ¼ 3.858 Å and c ¼ 7.710 Å, respectively, indicating that NBSCF can be indexed to a tetragonal structure (space group: P4/mmm) [28]. Under the actual fuel cell operating conditions, the reaction between the electrolyte and cathode materials can form an insulating layer that obstructs the electron and oxygen ion diffusion [29]. In order to evaluate the chemical compatibility of the
Fig. 2 e (a) In-situ XRD patterns of NBSCF-40GDC sintered at 1223 K, measured at various temperatures, and (b) partially enlarged in-situ XRD data.
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components, XRD measurements were carried out using a mixture of NBSCF and GDC powders sintered at 1223 K for 4 h. As shown in Fig. 1(c), there is no solid state reaction between the NBSCF cathode and the GDC electrolyte, indicating that the chemical compatibility between NBSCF and GDC is suitable under the present processing condition. Fig. 2(a) presents in-situ XRD patterns of NBSCF-40GDC measured from 373 K to 973 K. The XRD patterns indicate that the NBSCF remains in its layered perovskite structure over the entire temperature range without any chemical reaction with GDC. Fig. 2(b) shows partially enlarged diffraction peak of Fig. 2(a). As the temperature increases, the main diffraction peaks shift to the lower 2 theta, indicating that the volume of unit cells increases due to the larger size of reduced B-site cations [30].
Fig. 3 e SEM images show the microstructure of a single cell (a) the cross-section of NBSCF-40GDC, and the microstructure of (b) NBSCF-0GDC, (c) NBSCF-20GDC, (d) NBSCF-40GDC, and (e) NBSCF-50GDC composite cathode.
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Fig. 4 e (a) Experimental and fitted impedance spectra of NBSCF-xGDC symmetrical cells (x ¼ 0, 20, 40, and 50) by the equivalent circuit shown as an inset, at 873 K. (b) Comparison of NBSCF-xGDC composite cathode (x ¼ 0, 20, 40, and 50) ASR plotted versus inverse temperatures. (c) The ASR of symmetrical cells (Rp) measured at 873 K and fitted R2 and R3 for NBSCF-xGDC (x ¼ 0, 20, 40, and 50) shown as an inset.
The microstructures of the NBSCF-xGDC (x ¼ 0, 20, 40, and 50) cathode are identified by SEM and are presented in Fig. 3 The microstructure of the electrode is related to the characteristics of the surface, the TPB area, volume fraction of chemical phases, and electron transport [31,32]. The cathode layer has a highly porous morphology that ensures good oxygen diffusion and its thickness is about 15e20 mm as seen in Fig. 3(a). The porous NBSCF cathode is adhered well onto the dense GDC electrolyte (ca. 15 mm) without any cracks or delamination, which should enhance the mechanical compatibility. Fig. 3(b) to (e) show the microstructure of the NBSCF-xGDC (x ¼ 0, 20, 40, and 50) cathode. The small GDC particles are distributed well over the NBSCF-xGDC composite cathode (x ¼ 20, 40, and 50) compared to the single-phase NBSCF cathode, and it is anticipated that this will increase the electrochemical active sites [22]. The impedance spectra for the symmetrical cells (NBSCFxGDC/GDC/NBSCF-xGDC) were obtained by AC impedance spectroscopy under various temperatures in air. Fig. 4(a) shows the representative impedance spectra of NBSCF-xGDC (x ¼ 0, 20, 40, and 50) composites cathodes at 873 K under an OCV condition. Electrochemical impedance spectroscopy is typically used to describe all resistances related with the electrode and electrolyte of the cell, including the gasecathode interface, and the cathode-electrolyte interface. From the spectra, the difference between the intercepts at the real axis of the Nyquist plots indicates the area specific resistance (ASR), which is the non-ohmic resistance of the composite cathode. Arrhenius plots of the cathode non-ohmic resistances are provided in Fig. 4(b). The ASR of NBSCF-0GDC, NBSCF-20GDC, NBSCF-40GDC, and NBSCF50GDC composites are 0.126, 0.094, 0.074, and 0.082 U cm2, respectively. The ASR value decreases with increasing GDC ratio up to 40 wt% and then increases beyond 40 wt% of GDC. In order to identify the factors of the non-ohmic resistance, the impedance spectra are fitted by the equivalent circuit. Fig. 3(c) shows the non-ohmic resistances (Rp) and fitting parameters (R2 and R3) of NBSCF-xGDC (x ¼ 0, 20, 40, and 50) composites shown as an inset, measured at 873 K under an OCV condition. The impedance at high and intermediate frequency, R2, is associated with electron, and ion transfer at the electrode, electrolyte, and interface of the collector and electrode. Meanwhile, the impedance at low frequency, R3, is related with non-charge transfer, such as oxygen surface exchange and gas-phase diffusion on the electrode layer. With an increase of the amount of GDC up to 40 wt%, the noncharge transfer resistance (R3) dramatically decreases while the charge transfer resistance (R2) shows similar values, indicating that the addition of GDC can improve the rate of oxygen surface exchange at the interface of MIEC and GDC, where the ORR occurs. Increasing the amount of GDC beyond 40 wt%, however, leads to a decrease of the electrochemical performance, because the excessive amount of GDC hinders the conduction of electrons due to its poor electronic conductivity. Consequently, excessive addition of GDC beyond 40 wt% can impede the ORR, thus indicating that NBSCF40GDC is the most optimized composition for the NBSCF cathode system. Fig. 5(a) and (b) show the IeV curve and the corresponding power density of NBSCF-xGDC/GDC/Ni-GDC (x ¼ 0 and 40)
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anode-supported single cells at various temperatures using humidified H2 (3% H2O) as a fuel and air as an oxidant. As expected from the lower ASR values of NBSCF-40GDC compared to those of NBSCF-0GDC, the NBSCF-40GDC single cell shows excellent cell performance. The maximum power density of the NBSCF-0GDC and NBSCF-40GDC composite cathodes was 1.55 and 1.83 W cm2 at 873 K, respectively. Fig. 6 shows the equilibrium oxygen non-stoichiometries for bulk NBSCF and the NBSCF-40GDC composite cathode determined by coulometric titration (CT) as a function of p(O2) at 973 K. The initial oxygen content of the NBSCF samples are taken from our previous study [19]. Under actual fuel cell conditions, the interface of SOFCs between the electrolyte and the cathode experiences a low p(O2), which may cause redox degradation of the cathode and affect the long term stability [23,24]. As can be seen from the data, the NBSCF-0GDC sample starts to decay at a p(O2) of around ~106 atm, which is possibly the starting point of decomposition. Meanwhile, the NBSCF-40GDC composite is stable down to a lower p(O2) of ~107 atm, suggesting that it has superior redox stability under the operating conditions over the NBSCF bulk cathode material. Interfacial interactions between NBSCF and GDC seem to cause the NBSCF to be more reducible. Enhanced reducibility of the cathode in contact with the electrolyte has been reported in the previous studies [25,33,34]. The fact that
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Fig. 6 e Oxygen non-stoichiometry of NBSCF-xGDC composite cathode (x ¼ 0 and 40) as a function of p(O2) at 973 K.
the electrode in contact with the electrolyte material, i.e., composite cathode, can endure more reduced state may indicate that addition of GDC on NBSCF cathode improve the oxygen redox stability, although a definitive explanation is obviously not yet available. Therefore, specific addition of GDC on a bulk NBSCF cathode can be a relevant process for enhancement of oxygen redox stability.
Conclusions The electrochemical properties of NBSCF-xGDC cathode materials were systematically investigated to optimize the GDC ratio based on the structural characteristics, the electrochemical performance, and the redox stability of the cathode for IT-SOFC application. The favorable electrochemical properties of NBSCF-xGDC composites originate from an electrocatalytic effect stemming from the provision of additional electrochemically active sites where the electrochemical reaction occurs. Accordingly, the optimized NBSCF-40GDC cathode material had the lowest ASR, 0.074 U cm2 at 873 K, resulting in remarkable cell performance of 1.83 W cm2 at 873 K. In addition, the oxygen redox stability is an important factor that influences the stability of cathode materials. The isotherms of NBSCF-40GDC obtained from a coulometric titration experiment reveal higher redox stability at lower p(O2). This study demonstrates that optimized addition of GDC on the NBSCF cathode not only improves the electrochemical performance but also enhances the oxygen redox stability.
Acknowledgments Fig. 5 e IeV curves and corresponding power density curves of single cells (NBSCF-xGDC/GDC/Ni-GDC) under various temperatures; (a) x ¼ 0 and (b) x ¼ 40.
This research was supported by the Mid-career Researcher Program (2013R1A2A2A04015706) and funded by the Ministry of Science, ICT and Future Planning, and the BK21 Plus
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Program (META-material-based Energy Harvest and Storage Technologies, 10Z20130011057) and the Basic Science Research Program (2010-0021214) funded by the Ministry of Education (MOE, Korea) and National Research Foundation of Korea (NRF).
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[17] Kim JH, Cassidy M, Irvine JTS, Bae J. Advanced electrochemical properties of LnBa0.5Sr0.5Co2O5þd (Ln ¼ Pr, Sm, and Gd) as cathode materials for IT-SOFC. J Electrochem Soc 2009;156:B682e9. [18] Jun A, Kim J, Shin J, Kim G. Optimization of Sr content in layered SmBa1xSrxCo2O5þd perovskite cathodes for intermediate-temperature solid oxide fuel cells. Int J Hydrogen Energy 2012;37:18381e8. [19] Choi S, Yoo S, Kim J, Park S, Jun A, Sengodan S, et al. Highly efficient and robust cathode materials for low-temperature solid oxide fuel cells: PrBa0.5Sr0.5Co2xFexO5þd. Sci Rep 2013;3:2426e31. [20] Dyck CR, Yu ZB, Krstic VD. Thermal expansion matching of Gd1-xSrxCoO3-d composite cathodes to Ce0.8Gd0.2O1.95 IT-SOFC electrolytes. Solid State Ionics 2004;171:17e23. [21] Leng Y, Chan SW, Liu Q. Development of LSCFeGDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte. Int J Hydrogen Energy 2008;33:3808e17. [22] Kim J, Seo W-Y, Shin J, Liu M, Kim G. Composite cathodes composed of NdBa0.5Sr0.5Co2O5þd and Ce0.9Gd0.1O1.95 for intermediate-temperature solid oxide fuel cells. J Mater Chem A 2013;1:515e9. [23] Adler SB. Factors governing oxygen reduction in solid oxide fuel cell cathodes. Chem Rev 2004;104:4791e843. [24] Bastidas DM, Tao S, Irvine JTS. A symmetrical solid oxide fuel cell demonstrating redox stable perovskite electrodes. J Mater Chem 2006;16:1603e5. [25] Kim G, Vohs JM, Gorte RJ. Enhanced reducibility of ceriaeYSZ composites in solid oxide electrodes. J Mater Chem 2008;18:2386e90. [26] Jun A, Shin J, Kim G. High redox and performance stability of layered SmBa0.5Sr0.5Co1.5Cu0.5O5þd perovskite cathodes for intermediate-temperature solid oxide fuel cells. Phys Chem Chem Phys 2013;15:19906e12. [27] Yoo S, Shin JY, Kim G. Thermodynamic and electrical characteristics of NdBaCo2O5þd at various oxidation and reduction states. J Mater Chem 2011;21:439e43. [28] Maignan A, Martin C, Pelloquin D, Nguyen N, Raveau B. Structural and magnetic studies of ordered oxygen-deficient perovskites LnBaCo2O5þd, closely related to the “112” structure. J Solid State Chem 1999;260:247e60. [29] Rossignol C, Ralph JM, Bae J-M, Vaughey JT. Ln1xSrxCoO3 (Ln ¼ Gd, Pr) as a cathode for intermediate-temperature solid oxide fuel cells. Solid State Ionics 2004;175:59e61. [30] Nakayama M, Ikuta H, Uchimoto Y, Wakihara M. Ionic conduction of lithium in B-site substituted perovskite compounds, (Li0.1La0.3)yMxNb1xO3 (M ¼ Zr, Ti, Ta). J Mater Chem 2002;12:1500e4. n B. Review on modeling [31] Andersson M, Yuan J, Sunde development for multiscale chemical reactions coupled transport phenomena in solid oxide fuel cells. Appl Energy 2010;87:1461e76. [32] Nam JH, Jeon DH. A comprehensive micro-scale model for transport and reaction in intermediate temperature solid oxide fuel cells. Electrochim Acta 2006;51:3446e60. [33] Costa-Nunes O, Ferrizz R, Gorte RJ, Vohs JM. Structure and thermal stability of ceria films supported on YSZ(1 0 0) and aAl2O3(0 0 0 1). Surf Sci 2005;592:8e17. [34] Costa-Nunes O, Gorte RJ, Vohs JM. High mobility of ceria films on zirconia at moderate temperatures. J Mater Chem 2005;15:1520e2.
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Effects of composite cathode on electrochemical and redox properties for intermediate-temperature solid oxide fuel cells Changmin Kim a, Junyoung Kim a, Jeeyoung Shin b,**, Guntae Kim a,* a
Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea b Department of Mechanical Engineering, Dong-Eui University, Busan 614-714, Republic of Korea
article info
abstract
Article history:
A composite cathode composed of NdBa0.5Sr0.5Co1.5Fe0.5O5þd (NВSCF) and Ce0.9Gd0.1O1.95
Available online 23 July 2014
(GDC) has been investigated to evaluate its electrochemical properties for intermediatetemperature solid oxide fuel cells (IT-SOFCs) based on structural characteristics and oxy-
Keywords:
gen redox stability. The composite cathode has a lower polarization loss than a pure NBSCF
Solid oxide fuel cells (SOFC)
cathode because the specific addition of GDC provides extended electrochemically active
Layered perovskite
sites where the oxygen reduction reaction (ORR) occurs. Accordingly, the optimized NBSCF-
Composite cathode
40GDC cathode material had the lowest ASR, 0.074 U cm2 at 873 K, resulting in excellent cell
Electrochemical performance
performance of 1.83 W cm2 at 873 K. In particular, investigation into the oxygen redox stability reveals that the composite cathode has superior redox stability under the operating conditions than a bulk NBSCF cathode material, which affects the long-term stability of the cathode performance. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are devices that convert chemical energy to electrical energy directly through electrochemical oxidation, providing advantages of fuel flexibility, low pollutant emission, and high efficiency. The requirement of high operating temperature of 1073e1273 K, however, gives rise to considerable issues such as high costs and material compatibility originating from the difference in the thermal expansion coefficient (TEC). Significant efforts therefore have been devoted to lowering the operation temperature of SOFCs toward an intermediate range (873e1073 K) to enhance long-
term stability and economic feasibility. Lowering of the operating temperature, however, results in serious problems including degradation of the electrocatalytic activity over the cathode, which is associated with the oxygen reduction reaction (ORR) [1e4]. In this regard, mixed ionic and electronic conductors (MIECs) with perovskite oxides based on transition metal (e.g. Mn, Fe, Co, and Ni) have been extensively researched as promising cathode materials of IT-SOFCs, due to their capability to conduct electrons and oxygen ions [5]. Among the various MIECs oxides, cobalt based perovskite oxides, such as BaCoO3 [6], Sm0.5Sr0.5CoO3 [7], La0.6Sr0.4Co0.2Fe0.8O3d [8], and
* Corresponding author. Fax: þ82 52 217 2909. ** Corresponding author. Fax: þ82 51 890 2232. E-mail addresses: [email protected] (J. Shin), [email protected] (G. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.07.007 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Pr1xSrxCoO3d [9], have displayed excellent electrocatalytic activity for the oxygen reduction reaction. Recently, layered perovskite oxides have been studied as novel cathode materials due to their remarkable oxygen kinetics rate relative to those of ABO3-type simple perovskite oxides [10,11]. The general formula of the layered perovskite oxides can be described as AA0 B2O5þd, where A ¼ trivalent lanthanide ion (Ln ¼ Pr, Nd, Sm, and Gd), A0 ¼ Ba or Sr, and B ¼ a first row transition metal ion or a mixture thereof. The layered perovskite consists of two layers with alternating stacking of … jAOjBO2jA0 OdjBO2j … along the c-axis. The lanthanide layers provide mobile oxygen species channels by reducing the oxygen bonding strength, which enhances oxygen ion diffusivity [11]. On the basis of these favorable properties of layered perovskite oxides, such as LnBaCo2O5þd (Ln ¼ Pr, Nd, Sm, and Gd) layered perovskite oxides, several groups have studied their capability as IT-SOFCs cathode materials. Kim et al. [12] reported that PrBaCo2O5þd (PBCO) is a promising material for IT-SOFC cathodes, as it showed a high surface exchange coefficient and rapid oxygen ion diffusivity, resulting in low cathodic polarization. The partial cosubstitution of Co for Fe and Ba for Sr, in particular, improves the ORR activity, electrical conductivity, oxygen ion diffusivity, and stability of the cathode [13e18]. Upon this background, our group reported the layered perovskite cathode materials, LnBa0.5Sr0.5Co1.5Fe0.5O5þd (Ln ¼ Pr and Nd), which have high electrochemical performance with excellent stabilities under operating conditions [19]. Based on a DFT analysis presented in our previous study [19], the layered structure provides pore channels for ion motion in the [LneO] and [CoeO] planes, which could provide fast paths for oxygen transport and these oxygen ion diffusion paths follow a zigezag type trajectory through the CoeO plane perpendicular to the Ln-O plane. It has been reported that the composite cathode prepared by specific addition of ionic conducting materials on a MIEC cathode contributes to improvement of electrochemical performance. Among these ionic conducting materials, Ce0.9Gd0.1O1.95 (GDC) can improve the electrocatalytic activity of the cathode by providing additional triple phase boundary (TPB) sites where the electrochemical reaction occurs [20,21]. Kim et al. [22] reported on the mechanism of the ORR for a MIEC-GDC composite cathode consisting of NdBa0.5Sr0.5Co2O5þd (NBSCO) and GDC. The authors reported that the composite cathode has a lower polarization loss than a pure NBSCO cathode due to the addition of GDC followed by extension of the TPB sites. Together with the substantial enhancement of single cell performance of Fe doped NBSCO, NdBa0.5Sr0.5Co1.5Fe0.5O5þd (NBSCF), long-term stability of the cell is also an important requirement for IT-SOFCs [19]. Low p(O2) operation may cause critical redox degradation of the cathode at the interface between the electrolyte and the cathode, which affects the longterm stability of the cathode performance [23,24]. Kim et al. [25] reported that a ceria based YSZ electrolyte composite anode exhibits enhanced reducibility relative to that of bulk ceria. In this regard, improvement of oxygen redox properties can be expected through the addition of an electrolyte material on the cathode. Meanwhile, there have been no reports on the effects of NBSCF-GDC composite cathodes, particularly
regarding the oxygen redox stability of NВSCF. In this study, therefore, we conducted a systematic investigation to optimize the electrochemical properties of the NBSCF cathode by optimizing the ratio of GDC to NBSCF based on the structural characteristics and oxygen redox stability of these materials.
Experimental NdBa0.5Sr0.5Co1.5Fe0.5O5þd oxides were synthesized using the Pechini process. Stoichiometric amounts of Nd(NO3)3$6H2O (Aldrich, 99.9%, metal basis), Ba(NO3)2 (Aldrich, 99 þ %), Sr(NO3)2 (Aldrich, 99 þ %), Co(NO3)2$6H2O (Aldrich, 98 þ %), and Fe(NO3)3$9H2O (Aldrich, 98 þ %) were dissolved in distilled water under continuous heating and stirring. A proper amount of citric acid and ethylene glycol were added into the beaker after the mixture was dissolved. After a viscous resin was formed, the mixture was heated around 473 K. The resultant products were pre-calcined at 873 K for 4 h, and ballmilled in acetone for 24 h. The pre-calcined NBSCF powder and Ce0.9Gd0.1O1.95 (GDC) were mixed at weight ratio of 10:0, 8:2, 6:4, and 5:5 and ball-milled in acetone for 24 h. The abbreviations used to identify various samples are given in Table 1. The mixtures of the NBSCF and GDC were blended with a binder (Heraeus V006) to form slurries for both symmetrical cell and single cell fabrication. The structure of NBSCF-xGDC cathode (x ¼ 0 and 40) were characterized by using X-ray powder diffraction (XRD) (Rigaku diffractometer, Cu Ka radiation) with a scan rate of 0.5 min1. The powder pattern and lattice parameters were analyzed by Rietveld refinement using GSAS program. In situ XRD of the NBSCF-xGDC cathode (x ¼ 0 and 40) was obtained from room temperature to operation temperature (Bruker, D8 Advance). The microstructures and morphologies of NBSCF-xGDC cathode sample (x ¼ 0, 20, 40, and 50) were observed using a field emission scanning electron microscope (SEM) (Nova SEM). To prepare anode supported single cell, NiO powder, GDC powder, and starch were mixed at weight ratio of 6:4:2 and ball-milled in ethanol for 24 h. NiO powder and GDC powder was prepared by glycine nitrate process. The detailed procedure has been described elsewhere [26]. After dried, the NiOGDC mixture was pressed into a pellet which has 0.6 mm thickness and 15 mm diameter. Thin GDC electrolyte membranes were prepared by a refined particle suspension coating technique. A GDC suspension was used for the electrolyte prepared by dispersing GDC powders (Aldrich) in ethanol with a proper amount of binder (Polyvinyl butyral, B-98) and dispersant (Triethanolamine, Alfa Aesar) at a ratio of 1:10. The GDC suspension was applied to a NiO-GDC anode support by
Table 1 e Abbreviation of NdBa0.5Sr0.5Co1.5Fe0.5O5þdexCe0.9Gd0.1O1.95. Abbreviation
NBSCF-0GDC NBSCF-20GDC NBSCF-40GDC NBSCF-50GDC
Composition (wt%) NdBa0.5Sr0.5Co1.5Fe0.5O5þd
Ce0.9Gd0.1O1.95
100 80 60 50
0 20 40 50
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drop-coating, followed by drying in air and subsequent cosintering at 1673 K for 5 h. Electrochemical impedance spectroscopy of NBSCF-xGDC (x ¼ 0, 20, 40, and 50) was carried out using a symmetrical cell. The GDC electrolyte powders were pressed into pellets, and then sintered at 1623 K for 4 h in air to obtain a dense electrolyte substrate. The slurries were screen-printed onto both sides of the GDC electrolytes to form symmetrical cells, followed by sintering at 1223 K. The silver wire and silver paste was used as a current collector for the electrodes. Impedance spectra were recorded under OCV in a frequency range of 1 mHze500 kHz with AC perturbation of 14 mV from 773 K to 923 K. Electrochemical performances were evaluated with NiGDC anode supported single cells. To determine the optimized cell performance, NBSCF-xGDC (x ¼ 0 and 40) slurries were screen-printed on the GDC electrolyte as a cathode. The cells were finally sintered at 1223 K for 4 h in air with an active electrode area of 0.36 cm2. The silver wires were used as a current collector onto both side of the cathode and anode of single cell using a silver paste. Each cell was mounted on alumina tube using a ceramic adhesive (Aremco, Ceramabond 553) to fix the single cell. Humidified hydrogen (3% H2O) was applied as fuel through a water bubbler with a flow rate of 20 mL min1. IeV curves were examined using a BioLogic Potentiostat at operating temperature from 773 K to 923 K. The redox properties and oxygen nonstoichiometry of NBSCF cathode material and NBSCF-40GDC composite were measured using coulometric titration (CT) as a function of the oxygen partial pressure, p(O2). A yttria-stabilized zirconia (YSZ) tube (McDanel Advanced Ceramic Technologies, Z15410630) was used both to electrochemically pump oxygen out of the system and to sense the equilibrium in coulometric titration. The oxygen sensor was part of the container wall and could also be used to add or remove oxygen from the system through application of a potential across the ionconducting YSZ tube. The oxide-sample was placed in a sealed container at the temperature of interest and equilibrated sufficiently by purging 5% O2eAr gas over it in the tube for 24 h. The detailed procedure has been described elsewhere [27]. The initial stoichiometric oxygen content of the sample is determined by iodometric titration and thermogravimetric analysis (TGA) in air at 973 K [19]. Oxygen partial pressure of the internal tube was determined from the OCV through the Nernst equation by the following Equation (1): 4EF
ex RT pin O2 ¼ pO2 $e
(1)
pðOex 2 Þ,
where the E, F, R, and T stand for the oxygen partial pressure of the external tube (ca. 0.21 atm), OCV, Faraday constant, gas constant, and temperature, respectively. The sample was allowed to equilibrate with the surrounding atmosphere in the tube. Oxygen nonstoichiometry during coulometric titration is calculated from the Equation (2): Dd ¼
2M It V $Dpin O2 m 4F RT
(2)
where the M, m, I, t, F, V, R, T, and DpðOin 2 Þ represent for molar mass of sample, sample mass, applied current, duration time, Faraday constant, free volume of the tube, gas constant,
Fig. 1 e (a) XRD patterns of pure NBSCF sintered at 1423 K. (b) Observed, and calculated XRD profiles and the difference between them for pure NBSCF. (c) XRD patterns of NBSCFexGDC (x ¼ 0 and 40) mixture after sintering at 1223 K in air.
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temperature, and change of oxygen partial pressure of the internal tube, respectively. The criterion of thermodynamic equilibrium of oxygen concentration is within 1 mV h1.
Results and discussion Fig. 1(a) presents XRD patterns of NBSCF sintered at 1423 K for 12 h. The patterns show that NBSCF has a single-phase layered perovskite structure without any detectable secondary phase. The powder patterns and lattice parameters were analyzed by Rietveld refinement as described in Fig. 1(b). The lattice parameters are a ¼ b ¼ 3.858 Å and c ¼ 7.710 Å, respectively, indicating that NBSCF can be indexed to a tetragonal structure (space group: P4/mmm) [28]. Under the actual fuel cell operating conditions, the reaction between the electrolyte and cathode materials can form an insulating layer that obstructs the electron and oxygen ion diffusion [29]. In order to evaluate the chemical compatibility of the
Fig. 2 e (a) In-situ XRD patterns of NBSCF-40GDC sintered at 1223 K, measured at various temperatures, and (b) partially enlarged in-situ XRD data.
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components, XRD measurements were carried out using a mixture of NBSCF and GDC powders sintered at 1223 K for 4 h. As shown in Fig. 1(c), there is no solid state reaction between the NBSCF cathode and the GDC electrolyte, indicating that the chemical compatibility between NBSCF and GDC is suitable under the present processing condition. Fig. 2(a) presents in-situ XRD patterns of NBSCF-40GDC measured from 373 K to 973 K. The XRD patterns indicate that the NBSCF remains in its layered perovskite structure over the entire temperature range without any chemical reaction with GDC. Fig. 2(b) shows partially enlarged diffraction peak of Fig. 2(a). As the temperature increases, the main diffraction peaks shift to the lower 2 theta, indicating that the volume of unit cells increases due to the larger size of reduced B-site cations [30].
Fig. 3 e SEM images show the microstructure of a single cell (a) the cross-section of NBSCF-40GDC, and the microstructure of (b) NBSCF-0GDC, (c) NBSCF-20GDC, (d) NBSCF-40GDC, and (e) NBSCF-50GDC composite cathode.
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Fig. 4 e (a) Experimental and fitted impedance spectra of NBSCF-xGDC symmetrical cells (x ¼ 0, 20, 40, and 50) by the equivalent circuit shown as an inset, at 873 K. (b) Comparison of NBSCF-xGDC composite cathode (x ¼ 0, 20, 40, and 50) ASR plotted versus inverse temperatures. (c) The ASR of symmetrical cells (Rp) measured at 873 K and fitted R2 and R3 for NBSCF-xGDC (x ¼ 0, 20, 40, and 50) shown as an inset.
The microstructures of the NBSCF-xGDC (x ¼ 0, 20, 40, and 50) cathode are identified by SEM and are presented in Fig. 3 The microstructure of the electrode is related to the characteristics of the surface, the TPB area, volume fraction of chemical phases, and electron transport [31,32]. The cathode layer has a highly porous morphology that ensures good oxygen diffusion and its thickness is about 15e20 mm as seen in Fig. 3(a). The porous NBSCF cathode is adhered well onto the dense GDC electrolyte (ca. 15 mm) without any cracks or delamination, which should enhance the mechanical compatibility. Fig. 3(b) to (e) show the microstructure of the NBSCF-xGDC (x ¼ 0, 20, 40, and 50) cathode. The small GDC particles are distributed well over the NBSCF-xGDC composite cathode (x ¼ 20, 40, and 50) compared to the single-phase NBSCF cathode, and it is anticipated that this will increase the electrochemical active sites [22]. The impedance spectra for the symmetrical cells (NBSCFxGDC/GDC/NBSCF-xGDC) were obtained by AC impedance spectroscopy under various temperatures in air. Fig. 4(a) shows the representative impedance spectra of NBSCF-xGDC (x ¼ 0, 20, 40, and 50) composites cathodes at 873 K under an OCV condition. Electrochemical impedance spectroscopy is typically used to describe all resistances related with the electrode and electrolyte of the cell, including the gasecathode interface, and the cathode-electrolyte interface. From the spectra, the difference between the intercepts at the real axis of the Nyquist plots indicates the area specific resistance (ASR), which is the non-ohmic resistance of the composite cathode. Arrhenius plots of the cathode non-ohmic resistances are provided in Fig. 4(b). The ASR of NBSCF-0GDC, NBSCF-20GDC, NBSCF-40GDC, and NBSCF50GDC composites are 0.126, 0.094, 0.074, and 0.082 U cm2, respectively. The ASR value decreases with increasing GDC ratio up to 40 wt% and then increases beyond 40 wt% of GDC. In order to identify the factors of the non-ohmic resistance, the impedance spectra are fitted by the equivalent circuit. Fig. 3(c) shows the non-ohmic resistances (Rp) and fitting parameters (R2 and R3) of NBSCF-xGDC (x ¼ 0, 20, 40, and 50) composites shown as an inset, measured at 873 K under an OCV condition. The impedance at high and intermediate frequency, R2, is associated with electron, and ion transfer at the electrode, electrolyte, and interface of the collector and electrode. Meanwhile, the impedance at low frequency, R3, is related with non-charge transfer, such as oxygen surface exchange and gas-phase diffusion on the electrode layer. With an increase of the amount of GDC up to 40 wt%, the noncharge transfer resistance (R3) dramatically decreases while the charge transfer resistance (R2) shows similar values, indicating that the addition of GDC can improve the rate of oxygen surface exchange at the interface of MIEC and GDC, where the ORR occurs. Increasing the amount of GDC beyond 40 wt%, however, leads to a decrease of the electrochemical performance, because the excessive amount of GDC hinders the conduction of electrons due to its poor electronic conductivity. Consequently, excessive addition of GDC beyond 40 wt% can impede the ORR, thus indicating that NBSCF40GDC is the most optimized composition for the NBSCF cathode system. Fig. 5(a) and (b) show the IeV curve and the corresponding power density of NBSCF-xGDC/GDC/Ni-GDC (x ¼ 0 and 40)
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anode-supported single cells at various temperatures using humidified H2 (3% H2O) as a fuel and air as an oxidant. As expected from the lower ASR values of NBSCF-40GDC compared to those of NBSCF-0GDC, the NBSCF-40GDC single cell shows excellent cell performance. The maximum power density of the NBSCF-0GDC and NBSCF-40GDC composite cathodes was 1.55 and 1.83 W cm2 at 873 K, respectively. Fig. 6 shows the equilibrium oxygen non-stoichiometries for bulk NBSCF and the NBSCF-40GDC composite cathode determined by coulometric titration (CT) as a function of p(O2) at 973 K. The initial oxygen content of the NBSCF samples are taken from our previous study [19]. Under actual fuel cell conditions, the interface of SOFCs between the electrolyte and the cathode experiences a low p(O2), which may cause redox degradation of the cathode and affect the long term stability [23,24]. As can be seen from the data, the NBSCF-0GDC sample starts to decay at a p(O2) of around ~106 atm, which is possibly the starting point of decomposition. Meanwhile, the NBSCF-40GDC composite is stable down to a lower p(O2) of ~107 atm, suggesting that it has superior redox stability under the operating conditions over the NBSCF bulk cathode material. Interfacial interactions between NBSCF and GDC seem to cause the NBSCF to be more reducible. Enhanced reducibility of the cathode in contact with the electrolyte has been reported in the previous studies [25,33,34]. The fact that
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Fig. 6 e Oxygen non-stoichiometry of NBSCF-xGDC composite cathode (x ¼ 0 and 40) as a function of p(O2) at 973 K.
the electrode in contact with the electrolyte material, i.e., composite cathode, can endure more reduced state may indicate that addition of GDC on NBSCF cathode improve the oxygen redox stability, although a definitive explanation is obviously not yet available. Therefore, specific addition of GDC on a bulk NBSCF cathode can be a relevant process for enhancement of oxygen redox stability.
Conclusions The electrochemical properties of NBSCF-xGDC cathode materials were systematically investigated to optimize the GDC ratio based on the structural characteristics, the electrochemical performance, and the redox stability of the cathode for IT-SOFC application. The favorable electrochemical properties of NBSCF-xGDC composites originate from an electrocatalytic effect stemming from the provision of additional electrochemically active sites where the electrochemical reaction occurs. Accordingly, the optimized NBSCF-40GDC cathode material had the lowest ASR, 0.074 U cm2 at 873 K, resulting in remarkable cell performance of 1.83 W cm2 at 873 K. In addition, the oxygen redox stability is an important factor that influences the stability of cathode materials. The isotherms of NBSCF-40GDC obtained from a coulometric titration experiment reveal higher redox stability at lower p(O2). This study demonstrates that optimized addition of GDC on the NBSCF cathode not only improves the electrochemical performance but also enhances the oxygen redox stability.
Acknowledgments Fig. 5 e IeV curves and corresponding power density curves of single cells (NBSCF-xGDC/GDC/Ni-GDC) under various temperatures; (a) x ¼ 0 and (b) x ¼ 40.
This research was supported by the Mid-career Researcher Program (2013R1A2A2A04015706) and funded by the Ministry of Science, ICT and Future Planning, and the BK21 Plus
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Program (META-material-based Energy Harvest and Storage Technologies, 10Z20130011057) and the Basic Science Research Program (2010-0021214) funded by the Ministry of Education (MOE, Korea) and National Research Foundation of Korea (NRF).
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