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Short Communication
Ceria-based electrolyte reinforced by solegel technique for intermediate-temperature solid oxide fuel cells Yun-Gyeom Choi a,b, Jun-Young Park b, Ji-Won Son a, Jong-Ho Lee a, Hae-June Je a, Byung-Kook Kim a, Hae-Weon Lee a, Kyung Joong Yoon a,* a
High-Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea b Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, Republic of Korea
article info
abstract
Article history:
High performance solid oxide fuel cells (SOFCs) based on gadolinia-doped ceria (GDC)
Received 2 April 2013
electrolyte are demonstrated for intermediate temperature operation. The inherent tech-
Received in revised form
nical limitations of the GDC electrolyte in sinterability and mechanical properties are
16 May 2013
overcome by applying solegel coating technique to the screen-printed film. When the
Accepted 28 May 2013
quality of the electrolyte film is enhanced by the additional solegel coating, the OCV and
Available online 24 June 2013
maximum power density increase from 0.73 to 0.90 V and from 0.55 to 0.95 W cm2,
Keywords:
impedance analysis reveals that the reinforcement of the thin electrolyte with solegel
Gadolinia-doped ceria
coating significantly reduces the polarization resistance. Elementary reaction steps for the
Electrolyte
anode and cathode are analyzed based on the systematic impedance study, and the rela-
Solegel
tion between the structural integrity of the electrolyte and the electrode polarization is
Impedance spectroscopy
discussed in detail.
Solid oxide fuel cells
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
respectively, at 650 C with humidified hydrogen (3% H2O) as fuel and air as oxidant. The
reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) represent one of the most efficient ways to generate electricity from a variety of fuels with low levels of pollutant emissions. Currently, the key issues for the successful development and deployment of SOFC technology on a commercial scale are costs and reliability, which are closely related to its high operating temperature. Therefore, over the past decade, considerable efforts have been made to reduce the operating temperature to the intermediate range (w650 C), which would lower materials and
manufacturing costs, improve reliability, simplify the balance-of-plant (BOP) components, and enable thermal cycling. Gadolinia-doped ceria (GDC) is considered to be one of the most promising electrolyte materials for intermediate temperature SOFCs due to its outstanding ionic conductivity and chemical compatibility with highly active cobaltcontaining cathode materials [1]. In addition, NieGDC cermet is considered to be the ideal anode material to match with GDC electrolyte due to its excellent catalytic activity, carbon tolerance, and sulfur resistance [2]. Therefore, anodesupported SOFCs with NieGDC anode, GDC electrolyte, and
* Corresponding author. Tel.: þ82 2 958 5515; fax: þ82 2 958 5529. E-mail addresses:
[email protected],
[email protected] (K.J. Yoon). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.161
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Co-containing cathode would be one of the most promising propositions for high performance intermediate-temperature SOFCs. For fabrication of GDC-based anode-supported SOFCs, thin electrolyte is generally co-sintered with the anode support, and various coating techniques have been explored for deposition of gas-tight electrolyte film on the anode support, such as screen printing [3], tape casting [4], spin coating [2,5], spray coating [6,7] and dry co-pressing [8]. In these processes, it is known to be extremely difficult to obtain fully dense GDC electrolyte at the practical co-sintering temperatures because of its inferior sintering behavior [9,10]. In addition, since GDC exhibits lower mechanical strength than YSZ, GDC electrolyte is prone to processing defects such as micro-cracks after cosintering due to the internal stress caused by the sintering shrinkage mismatch [11]. Consequently, open circuit voltage (OCV) values for ceria-based SOFCs are inconsistent in the literature [12,13], which could possibly be explained by the variation in the quality of the electrolyte film. Since the crossleakage of the gases through the electrolyte reduces the overall efficiency of the SOFC system, dense and gas-tight GDC electrolyte film is highly desirable for successful development of intermediate-temperature SOFC technology. In this study, high performance SOFCs based on GDC electrolyte were fabricated for intermediate temperature operation. Thin film GDC electrolyte was deposited by screen printing, and solegel coating was additionally applied to remove open pores and processing flaws such as microcracks. The fabricated cells were electrochemically characterized, and the effect of the solegel coating process on the performance of the GDC-based SOFCs was discussed in detail.
2.
functional layer (La0.8Sr0.2CoO3 (LSC) þ GDC) and cathode current collecting layer (LSC) were screen printed and sintered at 950 C in air. The effective electrode area was 1 cm 1 cm. The fabricated cells were tested with humidified hydrogen (3% H2O) as fuel and air as oxidant at 650 C. Electrochemical measurements were performed using Solartron 1260/1287 frequency response analyzer and potentiostat. After testing, the cells were sectioned and impregnated with epoxy in vacuum. After epoxy was hardened, they were polished down to 0.25 mm, and the cross-sections were examined using scanning electron microscopy (SEM) analysis (Philips FEI XL-30 FEG). The microstructural features such as thickness and porosity were measured using the image analysis software program ImageJ.
3.
Results and discussion
Sol-gel coating process was developed to enhance the quality of the GDC electrolyte fabricated by screen printing. Fig. 1(a) shows the SEM image of the surface of the GDC film prepared by conventional solegel process using the chemical solution composed of nitrate precursors and solvent. The film was
Experimental
Chemical solution for GDC solegel coating was prepared by mixing stoichiometric amount of Gd(NO3)3$6H2O and Ce(NO3)3$6H2O in a solvent composed of dimethylformamide (DMF), ethanol (EtOH), water, acetylacetone (Acac), and acetic acid (Ac). For sol infiltration, glycerin was added to the chemical solution as a drying control chemical agent (DCCA), while glycerin along with polyvinylpyrrolidone (PVP) binder was added for thin film deposition. For cell fabrication, GDC, NiO, and poly(methyl methacrylate) (PMMA) were ball-milled for 24 h in ethanol with dispersant (0.2 wt%), binder (1.5 wt%), and plasticizer (1.5 wt %), and the granules were obtained by spray drying. Volume ratio of GDC, NiO, and PMMA was 0.37:0.33:0.3. Anode substrates (2 cm 2 cm) were fabricated by uni-axially pressing the granules at 60 MPa. The slurries for anode functional layer, electrolyte, cathode functional layer, and cathode current collecting layer were prepared by mixing ceramic powders with dispersant (0.5 wt%), binder (2.5 wt%), and plasticizer (2.5 wt%) in a-terpineol using planetary mill for screen printing. The anode functional layer (NiO/GDC) and electrolyte (GDC) were screen printed sequentially, followed by cosintering at 1430 C. On top of the co-sintered GDC electrolyte, the chemical solutions for GDC infiltration and thin film deposition were spin-coated subsequently, followed by thermal treatment at 600 C for 5 min. Then, the cathode
Fig. 1 e SEM images of the surface of GDC film prepared by spin coating of chemical solution (a) without and (b) with DCCA. Images were taken after drying at 300 C in air.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 9 8 6 7 e9 8 7 2
deposited on a sapphire substrate by spin coating and dried at 300 C in air. The surface cracks were clearly observed after drying. In general, drying-related defects result from the differential drying stresses [14,15]. During the drying process, evaporation of the solvents from the micro-pores in the gel network generates the capillary forces, leading to the overall drying stresses. In particular, non-uniform distribution of pore sizes results in local differential drying stresses, eventually leading to drying-related cracks. Crack formation during drying process could be avoided by employing DCCA, which lowers the drying stresses by controlling the hydrolysis and condensation reaction rates, pore size distribution, hardness of the gel, and liquid vapor pressure [16,17]. In Fig. 1(b), drying-related macro-defects were completely removed by addition of glycerol as DCCA. Glycerol is known to be an effective DCCA which lowers the surface tension of the chemical solution and improves the uniformity of the pore size distribution in the gel network [18]. In this solegel technique, glycerol was found to be an excellent DCCA to eliminate drying-related defects from the GDC film. The microstructure of the fabricated cell with the GDC electrolyte reinforced by additional solegel coating is shown in Fig. 2. The NieGDC anode support was w850 mm thick and w35% porous. The NieGDC anode functional layer was w10 mm thick and w22% porous. The GDC electrolyte was w10 mm thick and dense. The LSCeGDC cathode functional layer was w15 mm thick and w30% porous. The LSC cathode current collecting layer was w20 mm thick and w35% porous. The GDC electrolyte was fabricated by screen printing and sintering at 1430 C, followed by multiple solegel coatings and thermal treatments at 600 C. Solegel coating process was performed in two steps; sol infiltration and thin film deposition. The first step was intended to infiltrate the GDC sol into the pores and micro-defects by capillary action, and the chemical solution was tuned to lower the viscosity and enhance the wetting properties. This step was repeated three times. In the second step, the highly viscous chemical solution containing organic binder was used to form the blocking thin film on top of the screen-printed GDC film. This process was
Fig. 2 e SEM images of the cross-section of the cell with GDC electrolyte formed by screen printing and additional solegel coating.
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repeated twice. Fig. 3 compares the cross-sectional images of the unmodified electrolyte (Fig. 3(a)) and reinforced electrolyte (Fig. 3(b)) in high magnification. In Fig. 3(a), no distinct layer formed by solegel process was observed, and the alteration of the microstructural features such as thickness and morphology due to the additional solegel coating was not detected. The reinforced electrolyte in Fig. 3(b) appears slightly denser than the unmodified one in Fig. 3(a). However, the micro-defects in the GDC electrolyte, which cause substantial gas cross-leakage and reduction of OCV, could hardly be recognized in the SEM investigation as we reported earlier [19], and, in practice, it is extremely difficult to measure the amount of the solution infiltrated into the GDC electrolyte. GDC particles derived from the solegel coating were not observed, which indicates that the small particles were completely sintered after high temperature processing. Therefore, the effect of the solegel coating on the quality of the electrolyte film should be examined by electrochemical characterization. In addition, the structural change of the anode functional layer due to infiltration was not observed in Fig. 3, which indicates that the amount of solution reaching the anode functional layer through the electrolyte is negligible. Fig. 4 shows the effect of the additional solegel coating on the current density-voltage (IeV) characteristics and corresponding power densities. The electrochemical characterization was performed at 650 C with humidified hydrogen (3% H2O) as fuel and air as oxidant. The OCV increased from 0.73 V to 0.90 V with the additional spin coating process, which indicates that solegel coating effectively blocks the open pores and micro-cracks, and prevents the gas cross-leakage through the electrolyte. In general, the OCV of the GDC-based SOFCs is lower than the theoretical Nernst potential because of the two reasons; the leakage current induced by the mixed ionic- and electronic-conduction, and the gas cross-leakage through the processing defects generated during the co-sintering process. GDC develops the mixed ionic- and electronic-conductivity in typical SOFC operating conditions, and the magnitude of the leakage current depends on the thickness of the electrolyte; OCV increases with increasing the electrolyte thickness [20]. Since the effect of the additional solegel coating on the total thickness of the electrolyte is negligible as confirmed by SEM investigation, the remarkable increase of OCV with solegel coating observed in Fig. 4 could be ascribed to the prevention of gas cross-leakage through the processing defects. Namely, the results in Fig. 4 indicate that substantial amount of micropores and defects exist in the GDC electrolyte after cosintering, and additional solegel coating process effectively blocks the processing defects and prevents the gas crossleakage through the electrolyte. The cell performance was substantially improved by enhancing the quality of GDC electrolyte. Maximum power density increased from 0.55 W cm2 to 0.95 W cm2 at 650 C by addition of solegel coating process. In order to understand the effect of the electrolyte film quality on the electrochemical characteristics of the GDCbased SOFCs, impedance spectra were collected and analyzed in Fig. 5. In the impedance spectra, the high frequency intercept corresponds to the area specific ohmic resistance, and the area specific polarization resistance is obtained by subtracting the high frequency intercept from the low frequency intercept [21]. In Fig. 5, ohmic resistances of the
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Fig. 3 e SEM images of (a) unmodified and (b) reinforced electrolytes.
1.0
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0.6
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0.4
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0.2 0.0 0.0
Screen Printing + Sol-Gel Coating Screen Printing Only 0.5
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Power Density (W-cm-2)
Voltage (V)
two cells were similar at w0.09 U cm2 and not significantly influenced by additional solegel coating, which reconfirms that the effect of the additional solegel coating on the total electrolyte thickness is negligible. Polarization resistance of the cell decreased from 0.25 U cm2 to 0.17 U cm2 with the addition of solegel coating, indicating that the improvement of cell performance with the enhancement of the film quality of GDC electrolyte results not only from increase of OCV but also from reduction of the electrode polarization resistance. In general, the impedance spectra of the SOFCs are composed of a number of overlapping depressed semicircles reflecting physical and/or chemical processes associated with the electrode reaction [22]. Fig. 5 shows that enhancing the quality of the GDC electrolyte substantially decreases low-frequency part (103 Hz) of the impedance and slightly increases highfrequency part (103 Hz), which suggests that the individual electrode processes are differently influenced by the crossleakage through the GDC electrolyte. In order to clarify the effect of the electrolyte film quality on the electrode polarization, it is necessary to identify the elementary electrode reactions associated with the each impedance arc. In Fig. 6, impedance spectra were collected while separately varying the anodic and cathodic gas compositions to
0.0 2.0
Current Density (A-cm-2) Fig. 4 e IeV curves and power densities of the GDC-based cells with the electrolytes fabricated by screen printing only and reinforced with additional solegel coating. Electrochemical measurements were performed at 650 C with humidified hydrogen (3% H2O) as fuel and air as oxidant.
distinguish the contribution of the each electrode. Fig. 6(a) shows the Bode plot of the imaginary part of the impedance spectra measured with the various oxidant compositions (20% O2-80% N2 and 10% O2-90% N2) on the cathode and fixed fuel composition (97% H2-3% H2O) on the anode. High-frequency impedance (>103 Hz) increases and low-frequency impedance (103 Hz) of the mixed ionic- and electronic-conducting perovskite cathode, which shows the positive dependence on pO2, is related to solid-state oxygen ionic diffusion [23,24]. The oxygen vacancy concentration in the mixed conducting electrode decreases with increasing pO2, and the bulk diffusion resistance varies inversely with the concentration of oxygen vacancies since oxygen ions move via a vacancy mechanism [25,26]. Consequently, the cell impedance associated with the solid state oxygen diffusion in the cathode increases with increasing pO2 on the cathode side. The impedance arc of the cathode at 10e103 Hz could be associated with adsorption and surface exchange involving atomic oxygen [27,28], and decreases with increasing pO2 because of the increased amount of oxygen available for the surface chemical exchange [25]. In Fig. 6(b), the fuel composition was varied (97% H2-3% H2O and 48.5% H248.5% N2-3% H2O) while the oxidant composition was fixed (20% O2-80% N2). With increasing pH2, decrease of the impedance arc at the characteristic frequency of 10e103 Hz is pronounced. The electrode process on the Ni-based cermet anode within this frequency range could be associated with the gasesolid interaction (adsorption, dissociation, desorption, etc.) or surface diffusion of the adsorbed species [29,30]. Such impedance arc decreases with increasing pH2 due to the increased amount of the reactant species. High-frequency impedance (>103 Hz) also slightly decreases with increasing pH2 on the anode in Fig. 6(b). It could be attributed to the contribution of the mixed conducting cathode because increasing pH2 on the anode side increases pO2 gradient across the GDC electrolyte, leading to higher leakage current and lower pO2 on the cathode side, which increases the oxygen vacancy concentration of the cathode material. The details of the electrode reaction mechanisms and impedance characteristics of the individual electrodes were discussed thoroughly in our earlier works [31,32]. Based on the information obtained from Fig. 6, the effect of the electrolyte film quality on the impedance spectra in Fig. 5 can be interpreted. Improving the quality of the electrolyte by
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 9 8 6 7 e9 8 7 2
- ZIM (Ohm-cm2)
0.3 Temperature: 650oC Open Circuit Fuel: Humidified H2 (3% H2O)
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Fig. 5 e Nyquist plot of impedance spectra of the GDCbased cells with the electrolytes fabricated by screen printing only and reinforced with additional spin coating. Impedance measurements were performed at 650 C with humidified hydrogen (3% H2O) as fuel and air as oxidant.
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additional solegel coating considerably reduces the gas crossleakage through the micro-defects, resulting in higher pO2 and pH2 on the cathode-electrolyte and anode-electrolyte interfaces, respectively. Consequently, the high-frequency impedance, which is associated with the oxygen vacancy concentration of the mixed conducting cathode, slightly increases, but the low-frequency impedance arcs, which are determined by the surface concentrations of the reactant species on the electrodes, significantly decreases, resulting in remarkable reduction of the overall polarization resistance. In addition, the low-frequency impedance at 1e10 Hz could include the substantial contribution of the anodic concentration polarization in GDC-based anode-supported SOFCs based on our earlier paper [19]. Concentration polarization due to the limited gas transport across the porous electrode becomes severe when the reactant species is depleted and/or the product species is accumulated at the electrode-electrolyte interface [33]. Reduced gas cross-leakage through the electrolyte due to enhanced electrolyte film quality increases the concentration of the reactant species ( pH2) and decreases the concentration of the product species ( pH2O) at the anodeelectrolyte interface, resulting in reduced concentration polarization at the low frequency range as shown in Fig. 5. Thus, the analysis on impedance spectra in Figs. 5 and 6 indicates that enhancement of the film quality of the GDC electrolyte is clearly reflected in the impedance spectra, and remarkably reduces the polarization resistance, resulting in substantial improvement of the cell performance.
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Frequency (Hz) Fig. 6 e Bode plots of the imaginary part of the impedance spectra of the cells with GDC electrolyte fabricated by screen printing, and screen printing and additional spin coating, measured at 650 C with (a) various oxidant compositions (20% O2-80% N2 and 10% O2-90% N2) and fixed fuel composition (humidified hydrogen (3% H2O)), and (b) various fuel compositions (97% H2-3% H2O and 48.5% H2-48.5% N2-3% H2O) and fixed oxidant composition (20% O2-80% N2).
Conclusion
In this work, dense and gas-tight GDC electrolyte was fabricated by reinforcing the screen-printed film with solegel coating process. The OCV increased from 0.73 to 0.90 V by applying the solegel coating process, and the maximum power density of 0.95 W cm2 was achieved at 650 C with humidified hydrogen (3% H2O) as fuel and air as oxidant. Enhancement of the electrolyte film quality contributed to the performance improvement through increasing the OCV and reducing the polarization resistance. Impedance analysis indicated that reduced gas cross-leakage results in increased pO2 and pH2 at the cathode-electrolyte and anode-electrolyte interfaces, respectively, leading to decreased activation and concentration polarizations. The materials system, fabrication process, and cell configuration developed in this work are suitable for intermediate-temperature operation, and the knowledge presented in this paper could be utilized for successful commercialization of SOFC technology.
Acknowledgment This work was financially supported by the Institutional Research Program of Korea Institute of Science and Technology (2E24042), and the Fundamental Research and Development Program for Core Technology of Materials, funded by the Ministry of Knowledge Economy, Republic of Korea.
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