- Email: [email protected]
Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition
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 x x x ( 2 0 1 4 ) 1 e6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition Hunhyeong Lee a, Jinyi Choi b, Inyu Park b, Dongwook Shin a,b,* a b
Division of Materials Science & Engineering, Hanyang University, Republic of Korea Department of Fuel Cells and Hydrogen Technology, Hanyang University, Republic of Korea
article info
abstract
Article history:
The cobalt-based cathode, LSCF, BSCF and SSC, have high thermal expansion coefficients,
Received 25 November 2013
which cause poor thermal compatibility with electrolytes and limited cell performance. In
Received in revised form
this work, a continuously gradient Sm0.5Sr0.5CoO3d (SSC) e BaCe0.7Zr0.2Y0.1O3d (BCZY)
24 March 2014
composite cathode was developed for BCZY electrolyte based fuel cells by electrostatic
Accepted 14 April 2014
spray slurry deposition (ESSD). In the deposited cathode the content of BCZY gradually
Available online xxx
decreased and that of SSC gradually increased in the direction away from the electrolyte ecathode interface. The single cell, consisting of a BCZYeNiO anode substrate, a BCZY
Keywords:
electrolyte and a compositionally gradient SSC-BCZY cathode layer, was assembled and
Proton conducting fuel cell
tested from 550 to 700 C under humidified hydrogen atmosphere (w3% H2O) and static air
Continuously gradient composite
as an oxidant. The cross-sectional morphology of the cathode was examined by energy
cathode
dispersive X-ray (EDX) analysis. The comparison of electrochemical performances between
Electrostatic slurry spray deposition
various cathode configurations was conducted to demonstrate the effect of gradient
Perovskite electrolyte
cathode. The power density and impedance spectra indicate that the optimized cathode structure greatly improves the polarization resistance and ohmic resistance. The results suggest that the migration of protons to triple phase boundaries (TPBs) and the surface diffusion of O might be enhanced in the gradient SSCeBCZY composite cathodes. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells are very promising technology because of the potential as a replacement of conventional stationary power-plants [1,2]. Nevertheless, high operating temperature seriously inhibits long term stability and durability of material systems, which limits wide commercialization [3]. Since the
proton conductors were found by Iwahara et al. [4], proton conducting fuel cell (PCFC) attracted lots of researches because of its advantages such as low activation energy [5] and high energy efficiency [6] compared with conventional fuel cells based on oxygen-ion conducting electrolytes. Many perovskite-type oxides, particularly the rare earth doped BaCeO3 oxides which exhibit the highest conductivities have been extensively employed as the electrolytes [7].
* Corresponding author. Department of Fuel Cells and Hydrogen Technology, Hanyang University, Republic of Korea. Tel.: þ82 2 2220 0503; fax: þ82 2 2220 4011. E-mail address: [email protected] (D. Shin). http://dx.doi.org/10.1016/j.ijhydene.2014.04.106 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
2
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 x x x ( 2 0 1 4 ) 1 e6
Cobalt-containing perovskite materials as mixed ionic and electronic conductors (MIEC) have been extensively studied as cathode materials [8], since these materials are believed to potentially extend three phase boundary (TPB). These materials exhibit excellent oxygen catalytic activity by the disorder-free oxygen ions migration mechanism. Unfortunately, these active cobalt-based cathodes in practical application often suffer from some disadvantages, in which the high thermal expansion coefficients (TECs) always cause electrolyteecathode interface delamination resulting in dramatic degradation of cell performance. The strategy adopted to solve the thermal mismatch is mixing these cathodes with electrolyte material to form composite cathodes. Addition of a sufficient amount of electrolyte to the cathode materials results in a significant improvement in the thermal mismatch problem between the electrolyte and the cathode [9]. However, composite cathodes with single composition cannot solve this inherent problem thoroughly because the amount of electrolyte material added into cathode has a limit. Therefore, the composite cathode is not the best functional architecture to develop the high potential power output. To reduce the undesirable ohmic drop and avoid discontinuities in thermal expansion coefficients, many researches have recently examined the effects of gradient layered cathodes [10,11]. However, gradient layered cathodes still remain the fundamental problems of composite cathode unsolved. Furthermore, each cathode layer needs slurries with different composition and the complexity of the fabrication process is tremendously increased. Hence, the precisely graded structures require more tedious and costly process. Another disadvantage of gradient layered cathode is that the thickness of cathode layer becomes greater than that of composite cathode due to the fabrication limitation. In this work, the cathode with continuously gradient structure is introduced to solve aforementioned problems and maximize the effect of heterogeneous structure of composite cathode. To realize continuously gradient composite cathode, electrostatic slurry spray deposition (ESSD) technique with substrate rotation was applied. ESSD utilizes electrostatic spraying of slurries containing powders and organic additives for slurry stabilization and rheological control [12]. This deposition method has several advantages; high deposition rate, the stability of the crystal phase, much less formation of crack and better adhesion to substrate caused by deposition condition at room temperature. The ESSD with substrate rotation method can deposit two materials simultaneously with independent control of flow rates. The composite cathodes fabricated in this work were designed to BCZY content gradually decreased and SSC content gradually increase in the direction away from the electrolyte/cathode interface. The fabricated cathodes showed excellent adhesion with the electrolyte and significantly improved the electrochemical performance of single cell.
Experimental The electrolyte BaCe0.7Zr0.15Y0.15O3d (BCZY) powders were prepared by a citric acid-nitrate solegel method. Appropriate stoichiometric ratio of nitrate powders (Ba(NO3)2, Ce(NO3)3,
ZrO(NO3)2 and Y(NO3)3) were dissolved in the citric acid diluted 50 ml de-ionized water with the molar ratio of citric acid to total metal ions at 5:1. The pH value of mixed solution was adjusted to 7 by adding a small amount of liquid ammonia and a small quantity of ethylene glycol was added as polymerization agent. A metal-citrate complex was dried to evaporate excess water and to form a plump dried gel at 150 C in electric oven for 24 h after stirring at room temperature for 30 min. During hydration process, poly-condensation reaction happened between citric acid and nitrates and bloated gel formed. The dried powders were calcined at 1000 C in air for 2 h. And commercial SSC powders from Winner Technology Co. were used. Two cathode types were fabricated, homogeneous SSCBCZY composite (cell type A) and continuously gradient composite (cell type B). Both cases, NiOeBCZY anode supports with 1 wt.% pore former, carbon black, were pressed into a disc with 1 mm in thickness and 25 mm in diameter. The prepared BCZY slurry was deposited on the NiOeBCZY anode without pre-sintering by ESSD method. The electrolyte deposited half-cell was co-sintered with the anode at 1400 C for 2 h. For the deposition of the composite cathode in cell type A, the slurries were prepared by mixing BCZY and SSC in weight ratio 4:6. For cell type B, two separate slurries of BCZY and SSC were prepared and contained into separate slurry reservoirs connected to spray nozzles. Prepared slurries were composed by isopropyl alcohol and toluene at a ratio of 70:30. A polyvinyl butyral of 0.2 wt.% as a binder was dissolved in solvents. The cathodes area of 1 cm2 were deposited by ESSD technique from the aforementioned slurries: (i) the composite cathode of cell A was deposited onto the previously prepared anode-electrolyte pellet by normal ESSD, (ii) the gradient composite cathode of cell B was deposited by the ESSD with a rotating stage as shown in Fig. 1(a). Electrostatic slurry spray deposition (ESSD) system use high positive voltage. When the simply two nozzles are used for deposition at one place, sprayed aerosol distribution will be distorted each other because of strong electric fields. Therefore, two nozzles need appropriate distant. For this reason, we use two nozzles with a rotating disk. The flow rate of BCZY slurry was changed from 8 ml h1 to 2 ml h1 and that of SSC slurry was changed from 2 ml h1 to 8 ml h1 for 2 h. The deposition rate of SSC is slightly higher than BCZY. Under this condition, gradient cathode is deposited by mixture SSC and BCZY with the weight ratio of 6:4. The rotating speed of the disk was 2 RPM. Higher RPM reduces deposition rate, while lower RPM produces layer-by-layer deposition. The expected structure is schematically shown in Fig. 1(b). Subsequently, cells were sintered at 1050 C for 2 h in air. An electrolyte-supported symmetric cell was fabricated for characterization of the cathodes. The BCZY powders with 1 wt.% NiO as a sintering aid were pressed into a pellet with 1 mm in thickness and 25 mm in diameter. The cathode layers were deposited on both sides of the electrolyte pellet by aforementioned method. Energy dispersive X-ray spectroscopy (EDX, Hitachi S-4800) was used to verify compositional gradient. A scanning electron microscope was used to observe the microstructure of the cells after full cell test. Electrochemical measurements of
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
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 x x x ( 2 0 1 4 ) 1 e6
3
Fig. 1 e (a) Illustration of ESSD system with rotating substrate and (b) schematic diagram of the continuously gradient composite cathode.
single cells (A and B) were performed from 550 to 700 C with humidified hydrogen as fuel and the static air as oxidant. Hydrogen gas is bubbled through water at room temperature to produce about 3% relative humidity. The impedance spectra were recorded under open circuit condition (OCV) using a 20 mV amplitude AC signal over a frequency range of 1 MHz to 0.01 Hz at temperature ranging from 550 C to 700 C. The IeV curve and impedance tests were performed at intervals of 50 C and all data were taken 1 h after the desired temperature was reached. The cell performance was measured with a Potentiostat/Galvanostat (Solartron 1287) coupled with a 1260 frequency response analyzer using the software; Corr-Ware and Z-Plot. The voltageecurrent curves were recorded by DC Electronic Load at a scanning rate of 20 mV s1.
Results and discussion Fig. 2 shows the cross-sectional image of the cathode gradient cathode with the thickness of w20 mm. The fabricated single cell was used for the cross-sectional EDX analysis before impedance and power test. EDX signal was taken for 20 min to
Fig. 2 e EDX mapping of continuously gradient composite cathode.
ensure averaged value without local variation or error. The EDX mapping reveals the compositional gradient; the cathode composition consists of a gradually increasing SSC and decreasing BCZY contents from the underlying dense electrolyte to the current collector on the top. At the interface to the electrolyte, BCZY is dominant. The compositional dominance of BCZY is expected to not only reduce thermal mismatch, but also lower ohmic resistance due to good adhesion. At the top of the cathode, on the contrary, the SSC dominant part enables rapid oxygen adsorption and ion diffusion into the triple phase boundary. The nature of the TPB is determined by the combination of the reaction processes that occur at the cathode/electrolyte interface and by the diffusion processes which supply reactants to this interface [13]. Therefore, the better adhesion and increasing diffusion by continuously gradient composite cathode would cause performance improvement of cathode. After cell fabrication, all the cells were checked by EDX analysis, and found that the compositional variation between two cells is less than w1%. Therefore it is certain that the difference in performance is due to gradient structure, not due to unintended. Fig. 3(a and b) show the cross-sectional SEM images of the tested cell. Fig. 3(a) shows the microstructure of cell A with cathode of the weight ratio of SSC:BCZY ¼ 60:40. Cell B with the gradient cathode with varying SSC:BCZY ratio is shown in Fig. 3(b). In overall, both cells have same thickness of electrolyte and the cathodes seem to maintain uniform and crackfree interface implying good adhesion to the electrolyte substrate. Electrochemical performances of the single cell A and B were tested to demonstrate the effects of continuously gradient composite cathode. As seen from Fig. 4(a), the open-circuit voltage (OCV) of the cell A and B varied from 1.05 V to 0.98 V in the temperature range from 550 C to 700 C did not show noticeable difference between cells. The maximum power density of cell A was reached 248 mW cm2 at 700 C. For cell B, the maximum power density was improved to 412 mW cm2 at 700 C as shown in Fig. 4(b). Although the maximum power density of cell B was increased 60% than cell A, at low temperature 550 C, the power density of cell B was greatly improved by 140% compared with cell A.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
4
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 x x x ( 2 0 1 4 ) 1 e6
Fig. 3 e Cross section SEM images of sing cell with (a) composite cathode and (b) continuously gradient composite cathode.
There are isolated BCZY which do not connected to electrolyte in composite cathode. These isolated BCZY particles disturb ion and electron migration. The farther site from the interface of electrolyte/cathode, the worse this phenomenon gets. Thus the common composite cathode exhibited relatively high polarization resistance, resulting in large ohmic drop due to the difficulty in current collection [14]. In case of gradient cathode, the continuous connections between electrolyte and cathode are widespread because of dominant BCZY ratio nearby the interface like Fig. 1(b). The continuously gradient cathode reduces delamination and improves the connectivity between particles in electrolyte and cathode. Therefore, this improved connectivity will result in the increased path for proton conduction from electrolyte to cathode [15]. Also the isolated BCZY particles are hardly observed in the part far from electrolyte because of dominant SSC ratio. For these reasons, the power measurement results explained that the continuously gradient composite cathode elevated cell performance compared to composite cathode. The Impedance spectra shown in Fig. 5 were analyzed to evaluate the single cell resistances. The ohmic resistance and polarization resistance of cell B were lower than those of cell A. At 700 C, the ohmic resistance of the cell A and B was 0.45 and 0.26 U cm2, respectively. The cell ohmic resistances are composed of those of anode, electrolyte, cathode, and interfacial resistances of anode-electrolyte and cathodeelectrolyte. In this work, both cells had the same compositions of 1 mm anode substrate and 20 mm electrolyte layers like as Fig. 3. The increase in ohmic resistance of a cell is caused mainly by electrode delamination, local shortage of
fuel, anode re-oxidation and electrolyte thickness. Since the thickness of electrolyte, anode composition and microstructure, and cell measurement conditions are all same in both cells, the difference of ohmic resistance of two cells is originated from electrolyteecathode interface. Thereby, the reduction of ohmic resistance might be resulted from the interfacial environment of cathode-electrolyte. In cell B, the mismatch of thermal expansion coefficients was minimized due to gradient cathode, which greatly decreased delamination in cathode-electrolyte interface. Hence, the continuously gradient cathode showed lower ohmic resistance than that of composite cathode at the same operating temperatures. Also, the polarization resistance was reduced from 0.75 to 0.23 U cm2 at 700 C, 3.8 to 2.9 at 550 C. With ohmic resistance, the polarization resistances were reduced evidently at cell B. However, it is too difficult to define the polarization resistances in detail because of the over-lapped polarization resistances of anode, cathode, and electrolyte [16]. To analyze precisely the cathode performances with impedance spectra, the symmetric cells with BCZY electrolyte of 1 mm thickness were fabricated. The cathode electrodes layers of both cells were deposited by ESSD about 20 mm thickness. Fig. 6(a) shows the impedance spectra of symmetric cells at 550 C and 700 C. Impedance spectra of symmetric cells represent the ohmic resistance and the cathode polarization. The ohmic resistance of symmetric cell includes the resistance of electrolyte, the resistance of cathode, and the interfacial resistance between electrolyte and cathode [17]. Since the conductivity and thickness of electrolyte are same in two cells, the difference in ohmic resistances will be
Fig. 4 e Performances of the single cells with different cathodes (a) composite cathode and (b) continuously gradient composite cathode under wet hydrogen atmosphere at 550e700 C. Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
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 x x x ( 2 0 1 4 ) 1 e6
5
w2 U cm2 to w0.8 U cm2. As mentioned earlier, the ohmic resistance of symmetric cell B was decreased by the same reason about the impedance data of Fig. 5. The polarization resistances of cell B were lower than cell A at the every frequency. Besides, the noteworthy changes at low frequency in polarization resistance were observed in Fig. 6(b). The high, middle, and low frequency arcs correspond to the diffusion of protons from electrolytes to TPBs, the reduction of O to O, and the diffusion of O to TPBs, respectively [18]. In composite cathode, the aforementioned isolated BCZY particles exist far away from electrolyte and these particles hinder adsorption and diffusion of oxygen ion. On the other hand, the SSC dominant part in cathode of cell B enables more oxygen gas adsorption to SSC and rapidly moving into the cathode interior without isolated BCZY. As a result, the continuously gradient composite cathode makes a remarkable improvement of resistance at low frequency.
Conclusion Fig. 5 e Impedance spectra of the two cells with (a) composite cathode and (b) continuously gradient composite cathode measured under open-circuit conditions at different temperatures.
originated from the cathode resistance and interfacial resistance between cathode and electrolyte. The ohmic resistance of the cell B was reduced from 5.41 to 5.07 U cm2 at 700 C. The polarization resistance of the cell B was also reduced from
In order to improve the performance of gradient cathode, a continuously gradient cathode was fabricated based on BCZY electrolyte. The compositions of the cathodes were gradually changed from the BCZY electrolyte to the upper cathode. The continuously gradient cathode has lower ohmic resistance and good adhesion at cathode-electrolyte interface. The impedance results of symmetric cathode cell indicate that the gradient composite cathode structure significantly improved the ohmic resistance and polarization resistance. The results demonstrate that this configuration not only buffers the TEC mismatch but also optimizes the cathode structure for oxygen reduction reaction. Particularly, the lowfrequency related polarization resistance was decreased dramatically with the continuously gradient cathode. Under these experiments, diffusion processes appear to dominate cathode performance. The ESSD is simple and cost-effective method for preparing the components of fuel cells. The continuously gradient composite cathode by ESSD became significant performance improvement and this cathode is good candidate for fuel cell.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2011-0019319 and also No. NRF2013R1A2A2A01015189).
references
Fig. 6 e (a) Electrochemical impedance spectra and (b) bode plot of symmetric cells with different cathode, measured at 550 and 700 C in ambient air and open circuit conditions.
[1] Hirabayashi Daisuke, Tomita Atsuko, Teranishi Shinya, Hibino Takashi. Improvement of a reduction-resistant Ce0.8Sm0.2O1.9 electrolyte by optimizing a thin BaCe1xSmxO3d layer for intermediate-temperature SOFCs. Solid State Ionics 2005;176:881e7. [2] Singhal S. Ceramic fuel cells for stationary and mobile applications. Am Ceram Soc Bull 2003;82:9601e10.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
6
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 x x x ( 2 0 1 4 ) 1 e6
[3] Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935e9. [4] Iwahara H, Esaka T, Uchida H, Maeda N. Proton conduction in sintered oxides and its application to steam electrolysis for hydrogen production. Solid State Ionics 1981;3-4:359e63. [5] Sammells AF, Cook RL, White JH, Osborne JJ, MacDuff RC. Rational selection of advanced solid electrolytes for intermediate temperature fuel cells. Solid State Ionics 1992;52:111e23. [6] Demin AK, Tsiakaras PE, Sobyanin VA, Yu Hramova S. Thermodynamic analysis of a methane fed SOFC system based on a protonic conductor. Solid State Ionics 2002;198:555e60. [7] Ricote S, Bonanos N, Marco de Lucas MC, Caboche G. Structural and conductivity study of the proton conductor BaCe(0.9x)ZrxY0.1O(3d) at intermediate temperatures. J Power Source 2009;193:1189e93. [8] Sun Wenping, Yan Litao, Lin Bin, Zhang Shangquan, Liu Wei. High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3deCe0.8Sm0.2O2d composite cathode. J Power Source 2010;195:3155e8. [9] Fabbri E, Licoccia S, Traversa E, Wachsman ED. Composite cathodes for proton conducting electrolytes. Fuel Cells 2009;9:128e38. [10] Song Hwa Seob, Lee Seungho, Lee Daehee, Moon Jooho. Functionally-graded composite cathodes for durable and high performance solid oxide fuel cells. J Power Source 2010;195:2628e32. [11] Hart NT, Brandon NP, Day MJ, Lapen˜a-Rey N. Functionally graded composite cathodes for solid oxide fuel cells. J Power Source 2002;106:42e50.
[12] Choi Jinyi, park Inyu, Lee Hunhyeong, Shin Dongwook. Effect of enhanced reaction area in double layered Ba0.5Sr0.5Co0.8Fe0.2O3ed cathode for intermediate temperature solid oxide fuel cells. Solid State Ionics 2012;216:54e7. [13] O’Hayre Ryan, Barnett David M, Prinz Fritz B. The triple phase boundary: a mathematical model and experimental investigations for fuel cells. J Electrochem Soc 2005;152:A439e44. [14] Li Chien-Hung, Hu Shao-Hua, Tay Kok-Wan, Fu Yen-Pei. Electrochemical characterization of gradient Sm0.5Sr0.5CoO3d cathodes on Ce0.8Sm0.2O1.9 electrolytes for solid oxide fuel cells. Ceram Int 2012;38:1557e62. [15] Magnie`re C, Di Iorio S, Morel B. Experimental study of the ohmic resistance between the interconnect and the Ni-CGO Cermet. ECS Trans 2011;35:1841e9. [16] Sumi Hirofumi, Yamaguchia Toshiaki, Matsuib Toshiaki. AC impedance characteristics for anode-supported microtubular solid oxide fuel cells. Electrochim Acta 2012;67:159e65. [17] Jin Hyun Woo, Kim Ju Hee, Park Young Min, Kim Haekyoung. Lanthanum based iron and cobalt-containing perovskite using an inorganic nano dispersants aqueous solution. J Ceram Process Res 2012;13:286e90. [18] He Fei, Wu Tianzhi, Peng Ranran, Xia Changrong. Cathode reaction models and performance analysis of Sm0.5Sr0.5CoO3deBaCe0.8Sm0.2O3d composite cathode for solid oxide fuel cells with proton conducting electrolyte. J Power Source 2009;194:263e8.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition Hunhyeong Lee a, Jinyi Choi b, Inyu Park b, Dongwook Shin a,b,* a b
Division of Materials Science & Engineering, Hanyang University, Republic of Korea Department of Fuel Cells and Hydrogen Technology, Hanyang University, Republic of Korea
article info
abstract
Article history:
The cobalt-based cathode, LSCF, BSCF and SSC, have high thermal expansion coefficients,
Received 25 November 2013
which cause poor thermal compatibility with electrolytes and limited cell performance. In
Received in revised form
this work, a continuously gradient Sm0.5Sr0.5CoO3d (SSC) e BaCe0.7Zr0.2Y0.1O3d (BCZY)
24 March 2014
composite cathode was developed for BCZY electrolyte based fuel cells by electrostatic
Accepted 14 April 2014
spray slurry deposition (ESSD). In the deposited cathode the content of BCZY gradually
Available online xxx
decreased and that of SSC gradually increased in the direction away from the electrolyte ecathode interface. The single cell, consisting of a BCZYeNiO anode substrate, a BCZY
Keywords:
electrolyte and a compositionally gradient SSC-BCZY cathode layer, was assembled and
Proton conducting fuel cell
tested from 550 to 700 C under humidified hydrogen atmosphere (w3% H2O) and static air
Continuously gradient composite
as an oxidant. The cross-sectional morphology of the cathode was examined by energy
cathode
dispersive X-ray (EDX) analysis. The comparison of electrochemical performances between
Electrostatic slurry spray deposition
various cathode configurations was conducted to demonstrate the effect of gradient
Perovskite electrolyte
cathode. The power density and impedance spectra indicate that the optimized cathode structure greatly improves the polarization resistance and ohmic resistance. The results suggest that the migration of protons to triple phase boundaries (TPBs) and the surface diffusion of O might be enhanced in the gradient SSCeBCZY composite cathodes. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells are very promising technology because of the potential as a replacement of conventional stationary power-plants [1,2]. Nevertheless, high operating temperature seriously inhibits long term stability and durability of material systems, which limits wide commercialization [3]. Since the
proton conductors were found by Iwahara et al. [4], proton conducting fuel cell (PCFC) attracted lots of researches because of its advantages such as low activation energy [5] and high energy efficiency [6] compared with conventional fuel cells based on oxygen-ion conducting electrolytes. Many perovskite-type oxides, particularly the rare earth doped BaCeO3 oxides which exhibit the highest conductivities have been extensively employed as the electrolytes [7].
* Corresponding author. Department of Fuel Cells and Hydrogen Technology, Hanyang University, Republic of Korea. Tel.: þ82 2 2220 0503; fax: þ82 2 2220 4011. E-mail address: [email protected] (D. Shin). http://dx.doi.org/10.1016/j.ijhydene.2014.04.106 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
2
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 x x x ( 2 0 1 4 ) 1 e6
Cobalt-containing perovskite materials as mixed ionic and electronic conductors (MIEC) have been extensively studied as cathode materials [8], since these materials are believed to potentially extend three phase boundary (TPB). These materials exhibit excellent oxygen catalytic activity by the disorder-free oxygen ions migration mechanism. Unfortunately, these active cobalt-based cathodes in practical application often suffer from some disadvantages, in which the high thermal expansion coefficients (TECs) always cause electrolyteecathode interface delamination resulting in dramatic degradation of cell performance. The strategy adopted to solve the thermal mismatch is mixing these cathodes with electrolyte material to form composite cathodes. Addition of a sufficient amount of electrolyte to the cathode materials results in a significant improvement in the thermal mismatch problem between the electrolyte and the cathode [9]. However, composite cathodes with single composition cannot solve this inherent problem thoroughly because the amount of electrolyte material added into cathode has a limit. Therefore, the composite cathode is not the best functional architecture to develop the high potential power output. To reduce the undesirable ohmic drop and avoid discontinuities in thermal expansion coefficients, many researches have recently examined the effects of gradient layered cathodes [10,11]. However, gradient layered cathodes still remain the fundamental problems of composite cathode unsolved. Furthermore, each cathode layer needs slurries with different composition and the complexity of the fabrication process is tremendously increased. Hence, the precisely graded structures require more tedious and costly process. Another disadvantage of gradient layered cathode is that the thickness of cathode layer becomes greater than that of composite cathode due to the fabrication limitation. In this work, the cathode with continuously gradient structure is introduced to solve aforementioned problems and maximize the effect of heterogeneous structure of composite cathode. To realize continuously gradient composite cathode, electrostatic slurry spray deposition (ESSD) technique with substrate rotation was applied. ESSD utilizes electrostatic spraying of slurries containing powders and organic additives for slurry stabilization and rheological control [12]. This deposition method has several advantages; high deposition rate, the stability of the crystal phase, much less formation of crack and better adhesion to substrate caused by deposition condition at room temperature. The ESSD with substrate rotation method can deposit two materials simultaneously with independent control of flow rates. The composite cathodes fabricated in this work were designed to BCZY content gradually decreased and SSC content gradually increase in the direction away from the electrolyte/cathode interface. The fabricated cathodes showed excellent adhesion with the electrolyte and significantly improved the electrochemical performance of single cell.
Experimental The electrolyte BaCe0.7Zr0.15Y0.15O3d (BCZY) powders were prepared by a citric acid-nitrate solegel method. Appropriate stoichiometric ratio of nitrate powders (Ba(NO3)2, Ce(NO3)3,
ZrO(NO3)2 and Y(NO3)3) were dissolved in the citric acid diluted 50 ml de-ionized water with the molar ratio of citric acid to total metal ions at 5:1. The pH value of mixed solution was adjusted to 7 by adding a small amount of liquid ammonia and a small quantity of ethylene glycol was added as polymerization agent. A metal-citrate complex was dried to evaporate excess water and to form a plump dried gel at 150 C in electric oven for 24 h after stirring at room temperature for 30 min. During hydration process, poly-condensation reaction happened between citric acid and nitrates and bloated gel formed. The dried powders were calcined at 1000 C in air for 2 h. And commercial SSC powders from Winner Technology Co. were used. Two cathode types were fabricated, homogeneous SSCBCZY composite (cell type A) and continuously gradient composite (cell type B). Both cases, NiOeBCZY anode supports with 1 wt.% pore former, carbon black, were pressed into a disc with 1 mm in thickness and 25 mm in diameter. The prepared BCZY slurry was deposited on the NiOeBCZY anode without pre-sintering by ESSD method. The electrolyte deposited half-cell was co-sintered with the anode at 1400 C for 2 h. For the deposition of the composite cathode in cell type A, the slurries were prepared by mixing BCZY and SSC in weight ratio 4:6. For cell type B, two separate slurries of BCZY and SSC were prepared and contained into separate slurry reservoirs connected to spray nozzles. Prepared slurries were composed by isopropyl alcohol and toluene at a ratio of 70:30. A polyvinyl butyral of 0.2 wt.% as a binder was dissolved in solvents. The cathodes area of 1 cm2 were deposited by ESSD technique from the aforementioned slurries: (i) the composite cathode of cell A was deposited onto the previously prepared anode-electrolyte pellet by normal ESSD, (ii) the gradient composite cathode of cell B was deposited by the ESSD with a rotating stage as shown in Fig. 1(a). Electrostatic slurry spray deposition (ESSD) system use high positive voltage. When the simply two nozzles are used for deposition at one place, sprayed aerosol distribution will be distorted each other because of strong electric fields. Therefore, two nozzles need appropriate distant. For this reason, we use two nozzles with a rotating disk. The flow rate of BCZY slurry was changed from 8 ml h1 to 2 ml h1 and that of SSC slurry was changed from 2 ml h1 to 8 ml h1 for 2 h. The deposition rate of SSC is slightly higher than BCZY. Under this condition, gradient cathode is deposited by mixture SSC and BCZY with the weight ratio of 6:4. The rotating speed of the disk was 2 RPM. Higher RPM reduces deposition rate, while lower RPM produces layer-by-layer deposition. The expected structure is schematically shown in Fig. 1(b). Subsequently, cells were sintered at 1050 C for 2 h in air. An electrolyte-supported symmetric cell was fabricated for characterization of the cathodes. The BCZY powders with 1 wt.% NiO as a sintering aid were pressed into a pellet with 1 mm in thickness and 25 mm in diameter. The cathode layers were deposited on both sides of the electrolyte pellet by aforementioned method. Energy dispersive X-ray spectroscopy (EDX, Hitachi S-4800) was used to verify compositional gradient. A scanning electron microscope was used to observe the microstructure of the cells after full cell test. Electrochemical measurements of
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
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 x x x ( 2 0 1 4 ) 1 e6
3
Fig. 1 e (a) Illustration of ESSD system with rotating substrate and (b) schematic diagram of the continuously gradient composite cathode.
single cells (A and B) were performed from 550 to 700 C with humidified hydrogen as fuel and the static air as oxidant. Hydrogen gas is bubbled through water at room temperature to produce about 3% relative humidity. The impedance spectra were recorded under open circuit condition (OCV) using a 20 mV amplitude AC signal over a frequency range of 1 MHz to 0.01 Hz at temperature ranging from 550 C to 700 C. The IeV curve and impedance tests were performed at intervals of 50 C and all data were taken 1 h after the desired temperature was reached. The cell performance was measured with a Potentiostat/Galvanostat (Solartron 1287) coupled with a 1260 frequency response analyzer using the software; Corr-Ware and Z-Plot. The voltageecurrent curves were recorded by DC Electronic Load at a scanning rate of 20 mV s1.
Results and discussion Fig. 2 shows the cross-sectional image of the cathode gradient cathode with the thickness of w20 mm. The fabricated single cell was used for the cross-sectional EDX analysis before impedance and power test. EDX signal was taken for 20 min to
Fig. 2 e EDX mapping of continuously gradient composite cathode.
ensure averaged value without local variation or error. The EDX mapping reveals the compositional gradient; the cathode composition consists of a gradually increasing SSC and decreasing BCZY contents from the underlying dense electrolyte to the current collector on the top. At the interface to the electrolyte, BCZY is dominant. The compositional dominance of BCZY is expected to not only reduce thermal mismatch, but also lower ohmic resistance due to good adhesion. At the top of the cathode, on the contrary, the SSC dominant part enables rapid oxygen adsorption and ion diffusion into the triple phase boundary. The nature of the TPB is determined by the combination of the reaction processes that occur at the cathode/electrolyte interface and by the diffusion processes which supply reactants to this interface [13]. Therefore, the better adhesion and increasing diffusion by continuously gradient composite cathode would cause performance improvement of cathode. After cell fabrication, all the cells were checked by EDX analysis, and found that the compositional variation between two cells is less than w1%. Therefore it is certain that the difference in performance is due to gradient structure, not due to unintended. Fig. 3(a and b) show the cross-sectional SEM images of the tested cell. Fig. 3(a) shows the microstructure of cell A with cathode of the weight ratio of SSC:BCZY ¼ 60:40. Cell B with the gradient cathode with varying SSC:BCZY ratio is shown in Fig. 3(b). In overall, both cells have same thickness of electrolyte and the cathodes seem to maintain uniform and crackfree interface implying good adhesion to the electrolyte substrate. Electrochemical performances of the single cell A and B were tested to demonstrate the effects of continuously gradient composite cathode. As seen from Fig. 4(a), the open-circuit voltage (OCV) of the cell A and B varied from 1.05 V to 0.98 V in the temperature range from 550 C to 700 C did not show noticeable difference between cells. The maximum power density of cell A was reached 248 mW cm2 at 700 C. For cell B, the maximum power density was improved to 412 mW cm2 at 700 C as shown in Fig. 4(b). Although the maximum power density of cell B was increased 60% than cell A, at low temperature 550 C, the power density of cell B was greatly improved by 140% compared with cell A.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
4
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 x x x ( 2 0 1 4 ) 1 e6
Fig. 3 e Cross section SEM images of sing cell with (a) composite cathode and (b) continuously gradient composite cathode.
There are isolated BCZY which do not connected to electrolyte in composite cathode. These isolated BCZY particles disturb ion and electron migration. The farther site from the interface of electrolyte/cathode, the worse this phenomenon gets. Thus the common composite cathode exhibited relatively high polarization resistance, resulting in large ohmic drop due to the difficulty in current collection [14]. In case of gradient cathode, the continuous connections between electrolyte and cathode are widespread because of dominant BCZY ratio nearby the interface like Fig. 1(b). The continuously gradient cathode reduces delamination and improves the connectivity between particles in electrolyte and cathode. Therefore, this improved connectivity will result in the increased path for proton conduction from electrolyte to cathode [15]. Also the isolated BCZY particles are hardly observed in the part far from electrolyte because of dominant SSC ratio. For these reasons, the power measurement results explained that the continuously gradient composite cathode elevated cell performance compared to composite cathode. The Impedance spectra shown in Fig. 5 were analyzed to evaluate the single cell resistances. The ohmic resistance and polarization resistance of cell B were lower than those of cell A. At 700 C, the ohmic resistance of the cell A and B was 0.45 and 0.26 U cm2, respectively. The cell ohmic resistances are composed of those of anode, electrolyte, cathode, and interfacial resistances of anode-electrolyte and cathodeelectrolyte. In this work, both cells had the same compositions of 1 mm anode substrate and 20 mm electrolyte layers like as Fig. 3. The increase in ohmic resistance of a cell is caused mainly by electrode delamination, local shortage of
fuel, anode re-oxidation and electrolyte thickness. Since the thickness of electrolyte, anode composition and microstructure, and cell measurement conditions are all same in both cells, the difference of ohmic resistance of two cells is originated from electrolyteecathode interface. Thereby, the reduction of ohmic resistance might be resulted from the interfacial environment of cathode-electrolyte. In cell B, the mismatch of thermal expansion coefficients was minimized due to gradient cathode, which greatly decreased delamination in cathode-electrolyte interface. Hence, the continuously gradient cathode showed lower ohmic resistance than that of composite cathode at the same operating temperatures. Also, the polarization resistance was reduced from 0.75 to 0.23 U cm2 at 700 C, 3.8 to 2.9 at 550 C. With ohmic resistance, the polarization resistances were reduced evidently at cell B. However, it is too difficult to define the polarization resistances in detail because of the over-lapped polarization resistances of anode, cathode, and electrolyte [16]. To analyze precisely the cathode performances with impedance spectra, the symmetric cells with BCZY electrolyte of 1 mm thickness were fabricated. The cathode electrodes layers of both cells were deposited by ESSD about 20 mm thickness. Fig. 6(a) shows the impedance spectra of symmetric cells at 550 C and 700 C. Impedance spectra of symmetric cells represent the ohmic resistance and the cathode polarization. The ohmic resistance of symmetric cell includes the resistance of electrolyte, the resistance of cathode, and the interfacial resistance between electrolyte and cathode [17]. Since the conductivity and thickness of electrolyte are same in two cells, the difference in ohmic resistances will be
Fig. 4 e Performances of the single cells with different cathodes (a) composite cathode and (b) continuously gradient composite cathode under wet hydrogen atmosphere at 550e700 C. Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
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 x x x ( 2 0 1 4 ) 1 e6
5
w2 U cm2 to w0.8 U cm2. As mentioned earlier, the ohmic resistance of symmetric cell B was decreased by the same reason about the impedance data of Fig. 5. The polarization resistances of cell B were lower than cell A at the every frequency. Besides, the noteworthy changes at low frequency in polarization resistance were observed in Fig. 6(b). The high, middle, and low frequency arcs correspond to the diffusion of protons from electrolytes to TPBs, the reduction of O to O, and the diffusion of O to TPBs, respectively [18]. In composite cathode, the aforementioned isolated BCZY particles exist far away from electrolyte and these particles hinder adsorption and diffusion of oxygen ion. On the other hand, the SSC dominant part in cathode of cell B enables more oxygen gas adsorption to SSC and rapidly moving into the cathode interior without isolated BCZY. As a result, the continuously gradient composite cathode makes a remarkable improvement of resistance at low frequency.
Conclusion Fig. 5 e Impedance spectra of the two cells with (a) composite cathode and (b) continuously gradient composite cathode measured under open-circuit conditions at different temperatures.
originated from the cathode resistance and interfacial resistance between cathode and electrolyte. The ohmic resistance of the cell B was reduced from 5.41 to 5.07 U cm2 at 700 C. The polarization resistance of the cell B was also reduced from
In order to improve the performance of gradient cathode, a continuously gradient cathode was fabricated based on BCZY electrolyte. The compositions of the cathodes were gradually changed from the BCZY electrolyte to the upper cathode. The continuously gradient cathode has lower ohmic resistance and good adhesion at cathode-electrolyte interface. The impedance results of symmetric cathode cell indicate that the gradient composite cathode structure significantly improved the ohmic resistance and polarization resistance. The results demonstrate that this configuration not only buffers the TEC mismatch but also optimizes the cathode structure for oxygen reduction reaction. Particularly, the lowfrequency related polarization resistance was decreased dramatically with the continuously gradient cathode. Under these experiments, diffusion processes appear to dominate cathode performance. The ESSD is simple and cost-effective method for preparing the components of fuel cells. The continuously gradient composite cathode by ESSD became significant performance improvement and this cathode is good candidate for fuel cell.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2011-0019319 and also No. NRF2013R1A2A2A01015189).
references
Fig. 6 e (a) Electrochemical impedance spectra and (b) bode plot of symmetric cells with different cathode, measured at 550 and 700 C in ambient air and open circuit conditions.
[1] Hirabayashi Daisuke, Tomita Atsuko, Teranishi Shinya, Hibino Takashi. Improvement of a reduction-resistant Ce0.8Sm0.2O1.9 electrolyte by optimizing a thin BaCe1xSmxO3d layer for intermediate-temperature SOFCs. Solid State Ionics 2005;176:881e7. [2] Singhal S. Ceramic fuel cells for stationary and mobile applications. Am Ceram Soc Bull 2003;82:9601e10.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106
6
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 x x x ( 2 0 1 4 ) 1 e6
[3] Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935e9. [4] Iwahara H, Esaka T, Uchida H, Maeda N. Proton conduction in sintered oxides and its application to steam electrolysis for hydrogen production. Solid State Ionics 1981;3-4:359e63. [5] Sammells AF, Cook RL, White JH, Osborne JJ, MacDuff RC. Rational selection of advanced solid electrolytes for intermediate temperature fuel cells. Solid State Ionics 1992;52:111e23. [6] Demin AK, Tsiakaras PE, Sobyanin VA, Yu Hramova S. Thermodynamic analysis of a methane fed SOFC system based on a protonic conductor. Solid State Ionics 2002;198:555e60. [7] Ricote S, Bonanos N, Marco de Lucas MC, Caboche G. Structural and conductivity study of the proton conductor BaCe(0.9x)ZrxY0.1O(3d) at intermediate temperatures. J Power Source 2009;193:1189e93. [8] Sun Wenping, Yan Litao, Lin Bin, Zhang Shangquan, Liu Wei. High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3deCe0.8Sm0.2O2d composite cathode. J Power Source 2010;195:3155e8. [9] Fabbri E, Licoccia S, Traversa E, Wachsman ED. Composite cathodes for proton conducting electrolytes. Fuel Cells 2009;9:128e38. [10] Song Hwa Seob, Lee Seungho, Lee Daehee, Moon Jooho. Functionally-graded composite cathodes for durable and high performance solid oxide fuel cells. J Power Source 2010;195:2628e32. [11] Hart NT, Brandon NP, Day MJ, Lapen˜a-Rey N. Functionally graded composite cathodes for solid oxide fuel cells. J Power Source 2002;106:42e50.
[12] Choi Jinyi, park Inyu, Lee Hunhyeong, Shin Dongwook. Effect of enhanced reaction area in double layered Ba0.5Sr0.5Co0.8Fe0.2O3ed cathode for intermediate temperature solid oxide fuel cells. Solid State Ionics 2012;216:54e7. [13] O’Hayre Ryan, Barnett David M, Prinz Fritz B. The triple phase boundary: a mathematical model and experimental investigations for fuel cells. J Electrochem Soc 2005;152:A439e44. [14] Li Chien-Hung, Hu Shao-Hua, Tay Kok-Wan, Fu Yen-Pei. Electrochemical characterization of gradient Sm0.5Sr0.5CoO3d cathodes on Ce0.8Sm0.2O1.9 electrolytes for solid oxide fuel cells. Ceram Int 2012;38:1557e62. [15] Magnie`re C, Di Iorio S, Morel B. Experimental study of the ohmic resistance between the interconnect and the Ni-CGO Cermet. ECS Trans 2011;35:1841e9. [16] Sumi Hirofumi, Yamaguchia Toshiaki, Matsuib Toshiaki. AC impedance characteristics for anode-supported microtubular solid oxide fuel cells. Electrochim Acta 2012;67:159e65. [17] Jin Hyun Woo, Kim Ju Hee, Park Young Min, Kim Haekyoung. Lanthanum based iron and cobalt-containing perovskite using an inorganic nano dispersants aqueous solution. J Ceram Process Res 2012;13:286e90. [18] He Fei, Wu Tianzhi, Peng Ranran, Xia Changrong. Cathode reaction models and performance analysis of Sm0.5Sr0.5CoO3deBaCe0.8Sm0.2O3d composite cathode for solid oxide fuel cells with proton conducting electrolyte. J Power Source 2009;194:263e8.
Please cite this article in press as: Lee H, et al., Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.04.106