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Effect of GDC interlayer on the degradation of solid oxide fuel cell cathode during accelerated current load cycling
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 9 ( 2 0 1 4 ) 2 0 7 9 9 e2 0 8 0 5
Available online at www.sciencedirect.com
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Effect of GDC interlayer on the degradation of solid oxide fuel cell cathode during accelerated current load cycling Muhammad Zubair Khan a,b, Rak-Hyun Song a,b,*, Seung-Bok Lee a, Jong-Won Lee a, Tak-Hyoung Lim a, Seok-Joo Park a a
Fuel Cell Research Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea b Department of Advanced Energy Technology, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea
article info
abstract
Article history:
The effect of gadolinium-doped ceria (GDC) interlayer on the cathode degradation of solid
Available online 30 July 2014
oxide fuel cell (SOFC) during accelerated current load cycling was investigated. The SOFC half-cells with and without GDC interlayer were prepared and tested under 400 rapid
Keywords:
current load cycles. The half-cells consisted of lanthanum strontium cobalt ferrite (LSCF)-
SOFC
GDC composite cathode, GDC interlayer, scandia ceria stabilized zirconia (ScCeSZ) elec-
Current load cycling
trolyte, and platinum anode as a counter electrode. The area specific resistance (ASR) of the
GDC interlayer
half-cell was measured every 10 current load cycles. The ASR of the half-cell without GDC
Area specific resistance
interlayer greatly increased with current load cycling, which is attributed to the delami-
Sr diffusion
nation of the cathode/electrolyte interface due to SrZrO3 formation during sintering. On the other hand, the half-cell with GDC interlayer showed a minute increase in ASR during current load cycling due to very small elemental diffusion across the GDC interlayer/ electrolyte interface. These results mean that the GDC interlayer produced high resistance to cathode degradation under the current load cycling due to effective suppression of Sr diffusion across the interface. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices that convert chemical energy into electrical power in an electrochemical manner at high temperatures (600e1000 C). Considerable industrial interest is given to
them because of fuel flexibility, high efficiency, and low emissions [1]. At present, the degradation of SOFC components due to prolonged exposure at high temperatures is a key challenge for the development of SOFCs. Lifetime evaluation of SOFC demands long-term tests with massive experimental efforts. During long-term tests, gradual degradation of SOFC may also not clarify the leading degradation mechanisms. In
* Corresponding author. Department of Advanced Energy Technology, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea. Tel.: þ82 42 860 3578; fax: þ82 42 860 3180. E-mail address: [email protected] (R.-H. Song). http://dx.doi.org/10.1016/j.ijhydene.2014.07.022 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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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 9 ( 2 0 1 4 ) 2 0 7 9 9 e2 0 8 0 5
reliability engineering, accelerated life testing is a well-known method for lifetime evaluation during a short time [2], but the accelerated test tool was not established in the SOFC system up to now. In an SOFC system, a cell must have the ability to withstand current load cycles. However, these cycles cause a mismatch in the thermal expansion coefficient (TEC) between the electrolyte and cathode at the cathode/electrolyte interface because cathode materials show chemical expansion due to an increased oxygen non-stoichiometry in polarized state [3]. As a result, the mechanical stresses build up at the interface and lead to the detachment of the cathode. The cobaltite perovskite type oxides are good candidates for SOFC cathodes because of their mixed conductive properties, but during high temperature operation, they can react with zirconia-based electrolytes to form insulating phases like La2Zr2O7 (LZO) or SrZrO3 (SZO), which reduce the ionic conductivity of an electrolyte and, therefore, decrease the cell performance [4]. To avoid such a phenomenon, the diffusion barrier interlayer of Gd-doped ceria (GDC) is coated at the interface and has effectively reduced the formation of such insulating e [6] studied the influence of phases [5]. Heneka and Ivers-Tiffe high current load cycling on the performance of SOFC single cells without GDC interlayer, in which the formation of insulating phases like La2Zr2O7 with different TEC led to the delamination of the cathode layer and resulted in performance degradation. However, although GDC interlayer has been used practically in making the SOFC cells, there has been no report on the effect of the GDC interlayer during the current load cycle on the cathode performance. In this work, the effect of the GDC interlayer on the long-term performance of the SOFC cells was investigated in detail to understand and prevent their degradation behaviors under accelerated current load conditions.
Experimental Commercially available (Sc2O3)0.10(CeO2)0.01(ZrO2)0.89 (ScCeSZ) (Fuel Cell Materials, USA) was used to make circular disc type pellets by uniaxial pressing. The ScCeSZ pellets were presintered at 1050 C for 3 h. The pre-sintered electrolyte pellets were ultrasonically cleaned in acetone. The thin and dense Gd0.10Ce0.90O1.95 (GDC) interlayer on the pre-sintered electrolyte was coated by a vacuum slurry coating process that is described in more detail in a previous work [7]. The ScCeSZ electrolyte pellet without GDC interlayer was directly
sintered at 1400 C for 5 h. A sample coated with GDC was also sintered at 1400 C for 5 h. The cathode paste was a composite of 50 wt.% (La0.60Sr0.40)0.95(Co0.20Fe0.80)O3x (LSCF) and 50 wt.% GDC (Fuel Cell Materials, USA). The cathode paste was coated on surface of the sintered pellets by a screen printing process, followed by sintering at 1150 C for 5 h. A Pt anode as counter electrode was developed by coating Pt paste on the opposite side of the cathode. The final dimensions of the half-cells were 24 mm in diameter, 600 mm in electrolyte thickness with 0.2 cm2 of effective electrode area. Pt wires of 0.2 mm diameter and Pt mesh were used as current collectors. La0.6Sr0.4CoO3 (LSCO) paste was painted on the cathode surface and Pt mesh to reduce the contact resistance. Fig. 1(a and b) are schematics of a half-cell without GDC interlayer and with a 3.4 mm GDC interlayer, respectively. Fig. 2 illustrates experimental conditions for current load cycling. Half-cells were operated in air at 900 C, and the cathode was polarized by an external direct current using a Solatron 1287 potentiostat. Accelerated current load cycling was performed between open circuit voltage (OCV) and 1.0 A/cm2. A holding time of 10 min was given after each current cycle to be stable voltage across the cell. The cell voltages between the cathode and reference electrode were measured every 10 current load cycles at a constant current density of 0.1 A/cm2 to evaluate the ASR. This current density was applied for 50 min to obtain stable values of ASR. During current cycle test, we can measure the ASR value at 1 A/cm2. However, at this high current density the ASR decreased rapidly due to cathode activation process. Therefore, the reference value of small current density was selected to evaluate the effects of current cycling on the ASR of half cells. The reference electrode was placed at the edge of ScCeSZ electrolyte pellet. After current load cycling, microstructures of half-cells were examined by scanning electron microscopy (SEM; S4700, Hitachi Ltd., Japan), and the results were compared with those of the sintered cells. Elemental diffusions for the sintered cells were examined by wavelength dispersive spectroscopy (WDS; EPMA 1600, Shimadzu Japan). Figs. 7(a) and 8(a) show the microstructures of sintered half-cells without and with GDC interlayer, respectively.
Results and discussion Degradation under rapid current load cycling was determined by means of ASR measurement. The cells were connected in series and operated under the same current density. ASR was
Fig. 1 e Schematic of the half-cells (a) without GDC interlayer and (b) with 3.4 mm GDC interlayer.
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 9 ( 2 0 1 4 ) 2 0 7 9 9 e2 0 8 0 5
Fig. 2 e Experimental conditions for current load cycling.
defined by dividing the voltage measured between the cathode and reference electrode by applying a current density of 0.1 A/cm2. The measured ASR is the sum of cathode polarization and ohmic resistance. Fig. 3 is an illustration of the actual ASR across half-cells with the number of current cycles. Initially, a half-cell without GDC interlayer showed a higher ASR of 1.10U,cm2 as compared to a cell with GDC interlayer. The higher ASR is attributed to the formation of insulating phases like SZO during the sintering stage in the half-cell without GDC interlayer, which causes ohmic loss and cathodic polarization. In Fig. 7(a), the high concentration of Sr at the cathode/electrolyte interface in the half-cell without GDC interlayer indicates the formation of SZO phases. The cell without GDC interlayer showed a rapid increase in ASR of up to 250 cycles, and then it remained almost constant. This increase in ASR is considered to be irreversible damage in the cell. After 400 current cycles, the ASR with final value was 1.46 U cm2. A half-cell without GDC interlayer showed a
Fig. 3 e Actual ASRs developed across the cathode and reference electrode of the half-cells with and without GDC interlayer as a function of the current load cycle.
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degradation rate of 0.08% per cycle. A half-cell with a 3.4 mm GDC interlayer showed an initial ASR of 0.60 U cm2, which is almost half of the initial ASR of a cell without GDC interlayer. After first 10 current load cycles, there was no change in ASR, and as current load cycling proceeded, the increase in ASR was negligibly small. Over 400 current load cycles, the final ASR of a cell with GDC interlayer was 0.66U,cm2. The degradation rate for cell with GDC interlayer was 0.025% per cycle. Fig. 4 shows the increment in ASR from the initial values as the current load cycling progressed. The ASR increment is sharp for the cell without GDC interlayer, showing an increment of 0.36 U cm2 from the initial value, while that with GDC interlayer is 0.06 U cm2. To understand the reason for the ASR increment, microstructural analysis was performed after current load cycling. Fig. 5(a) and (b) show microstructures of a half-cell without GDC interlayer before and after current load cycling, respectively. In the case of no GDC interlayer, delamination at the interface of the cathode and ScCeSZ electrolyte is observed after the current load cycling, but delamination is not observed with the just sintered cell. The delaminated cell shows many small particles and cracks near the interface of cathode/electrolyte in the half-cell without GDC interlayer, which is attributed to the enhanced difference in the thermal expansion between cathode and ScCeSZ electrolyte due to the combined effect of local Joule heating and the presence of a second phase under high current density. This will be discussed in next section in more detail. ASR increment data shows that the delamination occurred gradually. Cathode delamination leads to a reduction of active sites at the interface and, therefore, to an increase in the interfacial resistance of the cell [8]. Fig. 6(a) and (b) represent the microstructure of the cathode/GDC interlayer in the halfcells with GDC interlayer, before and after current load cycling, respectively. No delamination near the interface was observed. A small increase of the ASR in the half-cells with GDC interlayer may be attributed to presence of a small
Fig. 4 e ASR increment (DASR) of the half-cells with and without GDC interlayer as a function of the current load cycle. DASR is defined by the difference between ASRs at each current load cycle and the least ASR at the initial current load cycle.
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Fig. 6 e Microstructures of half-cells with GDC interlayer (a) before and (b) after current load cycling. Fig. 5 e Microstructures of the half-cells without GDC interlayer (a) before and (b) after current load cycling.
amount of secondary phase at the interface of GDC interlayer/ ScCeSZ electrolyte and cathode microstructural changes. The microstructural comparison of the half-cells before and after current load cycling is presented in Figs. 5 and 6. Cathode microstructural change is visible after the current cycle test in both half-cells, with and without GDC interlayer. The cathode particles became fine after the cathodic current cycle test, which is consistent with the results of the references [9,10] showing that microstructural changes occurs
after cathodic polarization. These changes are closely associated with diffusion and migration of oxygen vacancies generated on the electrode surface under the current passage [10]. In Fig. 7(a), elemental mapping of Sr and La for the sintered half-cell without GDC interlayer showed strontium (Sr) and lanthanum (La) enrichment near the cathode/electrolyte interface. After current load cycling, enhanced concentration of Sr was observed at the delaminated region of the half-cell, which is an indication that application of cyclic current caused further migration of Sr towards the interface of cathode/electrolyte. The presence of Sr and La near the interface
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Fig. 7 e Backscattered electron image and WDS elemental mapping of La and Sr for the half-cell without GDC interlayer (a) before and (b) after current load cycling.
formed insulating phases like SZO and LZO while reacting with zirconia at the sintering stage. High concentration of Sr and the presence of La in Fig. 7(b) reveals that the formation of such insulating phases was further enhanced during current load cycling. Insulating phases like SZO have a high thermal expansion coefficient (TEC) of 29.8 106/K below 700 C and 26.1 106 between 700 and 1500 C [11], and LZO has low TEC of 7 106/K in the range of 20e1000 C [12]. On the other hand, TEC values of cathode and electrolyte materials are about 13 106/K in the range of 20e1000 C [13]. Therefore, due to the formation of insulating phases near the interface, thermal expansion mismatch was enhanced, and it caused thermal stress build up, which is considered to produce delamination between the cathode and electrolyte during the current load cycling. During operation at high current density, Joule heating occurs. According to Joules Law, heat ¼ I2Rt where I is direct current, R is the resistance of a cell, and t is time in s. Application of 1 A/cm2 for 10 min per current cycle produces 132 J of additional heat. This heat will lead to local temperature change (DT) at an active area. As the cell is electrolytesupported, the major contribution of temperature change comes from the zirconia electrolyte. The specific heat of zirconia (Cp) at 900 C is 0.6804 J/g C [14], and with 1.5 g mass (m) of an electrolyte pellet, a temperature change of 129 C occurs,
which has been calculated by Q ¼ m Cp DT. Due to rapid current load cycling, such temperature changes lead to an accumulation of thermal stresses at the cathode/electrolyte interface. Gradually, the thermal stress at the interface with the insulating phase reaches the critical value to start delamination of the cathode from an electrolyte in the cell without GDC interlayer [6]. However, Sr diffusion was suppressed at the interface of the cathode/GDC interlayer in the sintered half-cell with GDC interlayer. Fig. 8(a and b) show the elemental distribution of La and Sr in the half-cell with GDC interlayer before and after current load cycling. Before the current load cycle, there is no diffusion of Sr and La across the interface. However, only a small amount of Sr diffuses across the GDC interlayer/electrolyte interface during the current load cycling. The accumulation of Sr at the GDC interlayer/ ScCeSZ electrolyte interface indicates that this migration was caused by a gas phase diffusion mechanism through the GDC interlayer, causing the formation of an SZO phase, which somehow led to a reduction in electrolyte ionic conductivity. Moreover, the formation of a small amount of SZO phase could not affect the mechanical stability between ScCeSZ electrolyte and GDC interlayer because of the close matching of their thermal expansion coefficients. Results reported in Refs. [15e17] also show Sr enrichment at the GDC interlayer/ electrolyte interface during the sintering stage, and after long-
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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 9 ( 2 0 1 4 ) 2 0 7 9 9 e2 0 8 0 5
Fig. 8 e Backscattered electron image and WDS elemental mapping of La and Sr for the half-cell with GDC interlayer (a) before and (b) after current load cycling.
term testing at normal operating conditions, further migration of Sr caused a decrease in cell performance. Thus, the cell with GDC interlayer had no delamination but showed only a small increase in ASR due to a small amount of Sr diffusion at the interface and cathode microstructure change caused by the cathodic polarization effect.
and electrolyte effectively suppressed Sr migration towards the electrolyte. During the current load cycling, there was no delamination in the cell with GDC interlayer but only its small degradation of 0.025% per cycle, which could be due to a small amount of Sr migration and cathode microstructural changes.
Conclusions
Acknowledgment
In this paper, the effect of the GDC interlayer on the cathode degradation of SOFC half-cells under accelerated current load cycling has been studied. The degradation of half-cells was evaluated by means of increments in ASR. Microstructural analysis was performed before and after the current cycle test. A half-cell without GDC interlayer showed a higher degradation rate of 0.08% per cycle due to the formation of an insulating phase by more Sr diffusion at the cathode and electrolyte interface. Insulating phases like SZO and LZO with different TECs enhance the TEC mismatch near the interface of the cathode and electrolyte, and additionally, a temperature gradient by Joule heating occurs during the current load cycling. Thus, thermal stress builds up, which leads to delamination of the cathode in the cell without GDC interlayer. The introduction of GDC interlayer between the cathode
This work was supported by the New & Renewable Energy Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) in the form of a grant funded by the Korean government's Ministry of Trade, Industry & Energy. (No. 20113020030010 & 20113020030050), and was also supported by Korea CCS R&D Center (KCRC) grant funded by the Korean government (Ministry of Science, ICT & Future Planning).
references
[1] Tietz T. Materials development for advanced planar solid oxide fuel cells. Int J Appl Ceram Technol 2013;4:436e45.
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[2] Meeker WQ, Escobar LA, Lu CJ. accelerated degradation tests: modeling and analysis. Technometrics 2013;40:37e41. [3] Weber A. Fuel cells-solid oxide fuel cells j life-limiting considerations. Encycl Electrochem Power Sources 2009:120e34. [4] Ralph JM, Rossignol C, Kumar R. Cathode materials for reduced-temperature SOFCs. J Electrochem Soc 2003;150(11):A1518e22. K, Vulliet J, Blein F, Robert CL, et al. [5] Hierso J, Boy P, Valle Nanostructured ceria based thin films (1 mm) as cathode/electrolyte interfaces. J Solid State Chem 2013;197:113e9. e EI. Influence of high current cycling on the [6] Heneka MJ, Tiffe performance of SOFC single cells. J Fuel Cell Sci Technol 2012;9(1):011001e6. [7] Son HJ, Song RH, Lim TH, Lee SB, Kim SH, Shin DR. Effect of fabrication parameters on coating properties of tubular solid oxide fuel cell electrolyte prepared by vacuum slurry coating. J Power Sources 2010;195:1779e85. e EI. Thermocyclic load [8] Herbstritt D, Kru¨gel A, Weber A, Tiffe delamination defects and electrical performance of single cells. In: Proceeding of the 7th international symposium on solid oxide fuel cells, The Electrochemical Society, Inc.; 2001 June 3e8; Tsukuba, Ibarki, Japan; 2001. [9] Mizusaki J, Takatoshi S, Tagawa H. A Chemical diffusioncontrolled electrode reaction at the compact L1xSrxMnO3/
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stabilized zirconia interface in oxygen atmospheres. J Electrochem Soc 1996;143:3065e73. Jiang SP, Love JG. Observation of structural change induced by cathodic polarization on (La, Sr) MnO3 electrodes of solid oxide fuel cells. Solid State Ionics 2003;158:45e53. Ligny DD, Richet P. High-temperature heat capacity and thermal expansion of SrTiO3 and SrZrO3 perovskites. Am Phys Soc 1996;53:3013e22. Marques FMB, Labrincha JA, Frade JR. La2Zr2O7 formed at ceramic electrode/YSZ contacts. J Mater Sci 1993;28:3809e15. Singhal SC, Kendall K, editors. High temperature solid oxide fuel cells: fundamentals, design and applications. Oxford: Elsevier; 2003. Victor AC, Douglas TB. Physical properties of high temperature materials. Part VI. Enthalpy and heat capacity of magnesium oxide, zirconium oxide, and zirconium silicate from 0 to 900 C. WADC Technical Report; 1960. 57e374. Kim WH, Song HS, Moon J, Lee HW. Intermediate temperature solid oxide fuel cell using (La,Sr)(Co,Fe)O3-based cathodes. Solid State Ionics 2006;177(35e36):3211e6. Simner SP, Anderson MD, Engelhard MH, Stevenson JW. Degradation mechanisms of LaeSreCoeFeeO3 SOFC cathodes. Electrochem Solid-State Lett 2006;9(10):A478e81. € ller P, Kivi I, Lust E. Application of FIB-TOFKanarbik R, Mo SIMS and FIB-SEM-EDX methods for the analysis of element mobility in solid oxide fuel cells. ECS Trans 2013;57(1):581e7.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of GDC interlayer on the degradation of solid oxide fuel cell cathode during accelerated current load cycling Muhammad Zubair Khan a,b, Rak-Hyun Song a,b,*, Seung-Bok Lee a, Jong-Won Lee a, Tak-Hyoung Lim a, Seok-Joo Park a a
Fuel Cell Research Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea b Department of Advanced Energy Technology, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea
article info
abstract
Article history:
The effect of gadolinium-doped ceria (GDC) interlayer on the cathode degradation of solid
Available online 30 July 2014
oxide fuel cell (SOFC) during accelerated current load cycling was investigated. The SOFC half-cells with and without GDC interlayer were prepared and tested under 400 rapid
Keywords:
current load cycles. The half-cells consisted of lanthanum strontium cobalt ferrite (LSCF)-
SOFC
GDC composite cathode, GDC interlayer, scandia ceria stabilized zirconia (ScCeSZ) elec-
Current load cycling
trolyte, and platinum anode as a counter electrode. The area specific resistance (ASR) of the
GDC interlayer
half-cell was measured every 10 current load cycles. The ASR of the half-cell without GDC
Area specific resistance
interlayer greatly increased with current load cycling, which is attributed to the delami-
Sr diffusion
nation of the cathode/electrolyte interface due to SrZrO3 formation during sintering. On the other hand, the half-cell with GDC interlayer showed a minute increase in ASR during current load cycling due to very small elemental diffusion across the GDC interlayer/ electrolyte interface. These results mean that the GDC interlayer produced high resistance to cathode degradation under the current load cycling due to effective suppression of Sr diffusion across the interface. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices that convert chemical energy into electrical power in an electrochemical manner at high temperatures (600e1000 C). Considerable industrial interest is given to
them because of fuel flexibility, high efficiency, and low emissions [1]. At present, the degradation of SOFC components due to prolonged exposure at high temperatures is a key challenge for the development of SOFCs. Lifetime evaluation of SOFC demands long-term tests with massive experimental efforts. During long-term tests, gradual degradation of SOFC may also not clarify the leading degradation mechanisms. In
* Corresponding author. Department of Advanced Energy Technology, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea. Tel.: þ82 42 860 3578; fax: þ82 42 860 3180. E-mail address: [email protected] (R.-H. Song). http://dx.doi.org/10.1016/j.ijhydene.2014.07.022 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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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 9 ( 2 0 1 4 ) 2 0 7 9 9 e2 0 8 0 5
reliability engineering, accelerated life testing is a well-known method for lifetime evaluation during a short time [2], but the accelerated test tool was not established in the SOFC system up to now. In an SOFC system, a cell must have the ability to withstand current load cycles. However, these cycles cause a mismatch in the thermal expansion coefficient (TEC) between the electrolyte and cathode at the cathode/electrolyte interface because cathode materials show chemical expansion due to an increased oxygen non-stoichiometry in polarized state [3]. As a result, the mechanical stresses build up at the interface and lead to the detachment of the cathode. The cobaltite perovskite type oxides are good candidates for SOFC cathodes because of their mixed conductive properties, but during high temperature operation, they can react with zirconia-based electrolytes to form insulating phases like La2Zr2O7 (LZO) or SrZrO3 (SZO), which reduce the ionic conductivity of an electrolyte and, therefore, decrease the cell performance [4]. To avoid such a phenomenon, the diffusion barrier interlayer of Gd-doped ceria (GDC) is coated at the interface and has effectively reduced the formation of such insulating e [6] studied the influence of phases [5]. Heneka and Ivers-Tiffe high current load cycling on the performance of SOFC single cells without GDC interlayer, in which the formation of insulating phases like La2Zr2O7 with different TEC led to the delamination of the cathode layer and resulted in performance degradation. However, although GDC interlayer has been used practically in making the SOFC cells, there has been no report on the effect of the GDC interlayer during the current load cycle on the cathode performance. In this work, the effect of the GDC interlayer on the long-term performance of the SOFC cells was investigated in detail to understand and prevent their degradation behaviors under accelerated current load conditions.
Experimental Commercially available (Sc2O3)0.10(CeO2)0.01(ZrO2)0.89 (ScCeSZ) (Fuel Cell Materials, USA) was used to make circular disc type pellets by uniaxial pressing. The ScCeSZ pellets were presintered at 1050 C for 3 h. The pre-sintered electrolyte pellets were ultrasonically cleaned in acetone. The thin and dense Gd0.10Ce0.90O1.95 (GDC) interlayer on the pre-sintered electrolyte was coated by a vacuum slurry coating process that is described in more detail in a previous work [7]. The ScCeSZ electrolyte pellet without GDC interlayer was directly
sintered at 1400 C for 5 h. A sample coated with GDC was also sintered at 1400 C for 5 h. The cathode paste was a composite of 50 wt.% (La0.60Sr0.40)0.95(Co0.20Fe0.80)O3x (LSCF) and 50 wt.% GDC (Fuel Cell Materials, USA). The cathode paste was coated on surface of the sintered pellets by a screen printing process, followed by sintering at 1150 C for 5 h. A Pt anode as counter electrode was developed by coating Pt paste on the opposite side of the cathode. The final dimensions of the half-cells were 24 mm in diameter, 600 mm in electrolyte thickness with 0.2 cm2 of effective electrode area. Pt wires of 0.2 mm diameter and Pt mesh were used as current collectors. La0.6Sr0.4CoO3 (LSCO) paste was painted on the cathode surface and Pt mesh to reduce the contact resistance. Fig. 1(a and b) are schematics of a half-cell without GDC interlayer and with a 3.4 mm GDC interlayer, respectively. Fig. 2 illustrates experimental conditions for current load cycling. Half-cells were operated in air at 900 C, and the cathode was polarized by an external direct current using a Solatron 1287 potentiostat. Accelerated current load cycling was performed between open circuit voltage (OCV) and 1.0 A/cm2. A holding time of 10 min was given after each current cycle to be stable voltage across the cell. The cell voltages between the cathode and reference electrode were measured every 10 current load cycles at a constant current density of 0.1 A/cm2 to evaluate the ASR. This current density was applied for 50 min to obtain stable values of ASR. During current cycle test, we can measure the ASR value at 1 A/cm2. However, at this high current density the ASR decreased rapidly due to cathode activation process. Therefore, the reference value of small current density was selected to evaluate the effects of current cycling on the ASR of half cells. The reference electrode was placed at the edge of ScCeSZ electrolyte pellet. After current load cycling, microstructures of half-cells were examined by scanning electron microscopy (SEM; S4700, Hitachi Ltd., Japan), and the results were compared with those of the sintered cells. Elemental diffusions for the sintered cells were examined by wavelength dispersive spectroscopy (WDS; EPMA 1600, Shimadzu Japan). Figs. 7(a) and 8(a) show the microstructures of sintered half-cells without and with GDC interlayer, respectively.
Results and discussion Degradation under rapid current load cycling was determined by means of ASR measurement. The cells were connected in series and operated under the same current density. ASR was
Fig. 1 e Schematic of the half-cells (a) without GDC interlayer and (b) with 3.4 mm GDC interlayer.
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Fig. 2 e Experimental conditions for current load cycling.
defined by dividing the voltage measured between the cathode and reference electrode by applying a current density of 0.1 A/cm2. The measured ASR is the sum of cathode polarization and ohmic resistance. Fig. 3 is an illustration of the actual ASR across half-cells with the number of current cycles. Initially, a half-cell without GDC interlayer showed a higher ASR of 1.10U,cm2 as compared to a cell with GDC interlayer. The higher ASR is attributed to the formation of insulating phases like SZO during the sintering stage in the half-cell without GDC interlayer, which causes ohmic loss and cathodic polarization. In Fig. 7(a), the high concentration of Sr at the cathode/electrolyte interface in the half-cell without GDC interlayer indicates the formation of SZO phases. The cell without GDC interlayer showed a rapid increase in ASR of up to 250 cycles, and then it remained almost constant. This increase in ASR is considered to be irreversible damage in the cell. After 400 current cycles, the ASR with final value was 1.46 U cm2. A half-cell without GDC interlayer showed a
Fig. 3 e Actual ASRs developed across the cathode and reference electrode of the half-cells with and without GDC interlayer as a function of the current load cycle.
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degradation rate of 0.08% per cycle. A half-cell with a 3.4 mm GDC interlayer showed an initial ASR of 0.60 U cm2, which is almost half of the initial ASR of a cell without GDC interlayer. After first 10 current load cycles, there was no change in ASR, and as current load cycling proceeded, the increase in ASR was negligibly small. Over 400 current load cycles, the final ASR of a cell with GDC interlayer was 0.66U,cm2. The degradation rate for cell with GDC interlayer was 0.025% per cycle. Fig. 4 shows the increment in ASR from the initial values as the current load cycling progressed. The ASR increment is sharp for the cell without GDC interlayer, showing an increment of 0.36 U cm2 from the initial value, while that with GDC interlayer is 0.06 U cm2. To understand the reason for the ASR increment, microstructural analysis was performed after current load cycling. Fig. 5(a) and (b) show microstructures of a half-cell without GDC interlayer before and after current load cycling, respectively. In the case of no GDC interlayer, delamination at the interface of the cathode and ScCeSZ electrolyte is observed after the current load cycling, but delamination is not observed with the just sintered cell. The delaminated cell shows many small particles and cracks near the interface of cathode/electrolyte in the half-cell without GDC interlayer, which is attributed to the enhanced difference in the thermal expansion between cathode and ScCeSZ electrolyte due to the combined effect of local Joule heating and the presence of a second phase under high current density. This will be discussed in next section in more detail. ASR increment data shows that the delamination occurred gradually. Cathode delamination leads to a reduction of active sites at the interface and, therefore, to an increase in the interfacial resistance of the cell [8]. Fig. 6(a) and (b) represent the microstructure of the cathode/GDC interlayer in the halfcells with GDC interlayer, before and after current load cycling, respectively. No delamination near the interface was observed. A small increase of the ASR in the half-cells with GDC interlayer may be attributed to presence of a small
Fig. 4 e ASR increment (DASR) of the half-cells with and without GDC interlayer as a function of the current load cycle. DASR is defined by the difference between ASRs at each current load cycle and the least ASR at the initial current load cycle.
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Fig. 6 e Microstructures of half-cells with GDC interlayer (a) before and (b) after current load cycling. Fig. 5 e Microstructures of the half-cells without GDC interlayer (a) before and (b) after current load cycling.
amount of secondary phase at the interface of GDC interlayer/ ScCeSZ electrolyte and cathode microstructural changes. The microstructural comparison of the half-cells before and after current load cycling is presented in Figs. 5 and 6. Cathode microstructural change is visible after the current cycle test in both half-cells, with and without GDC interlayer. The cathode particles became fine after the cathodic current cycle test, which is consistent with the results of the references [9,10] showing that microstructural changes occurs
after cathodic polarization. These changes are closely associated with diffusion and migration of oxygen vacancies generated on the electrode surface under the current passage [10]. In Fig. 7(a), elemental mapping of Sr and La for the sintered half-cell without GDC interlayer showed strontium (Sr) and lanthanum (La) enrichment near the cathode/electrolyte interface. After current load cycling, enhanced concentration of Sr was observed at the delaminated region of the half-cell, which is an indication that application of cyclic current caused further migration of Sr towards the interface of cathode/electrolyte. The presence of Sr and La near the interface
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Fig. 7 e Backscattered electron image and WDS elemental mapping of La and Sr for the half-cell without GDC interlayer (a) before and (b) after current load cycling.
formed insulating phases like SZO and LZO while reacting with zirconia at the sintering stage. High concentration of Sr and the presence of La in Fig. 7(b) reveals that the formation of such insulating phases was further enhanced during current load cycling. Insulating phases like SZO have a high thermal expansion coefficient (TEC) of 29.8 106/K below 700 C and 26.1 106 between 700 and 1500 C [11], and LZO has low TEC of 7 106/K in the range of 20e1000 C [12]. On the other hand, TEC values of cathode and electrolyte materials are about 13 106/K in the range of 20e1000 C [13]. Therefore, due to the formation of insulating phases near the interface, thermal expansion mismatch was enhanced, and it caused thermal stress build up, which is considered to produce delamination between the cathode and electrolyte during the current load cycling. During operation at high current density, Joule heating occurs. According to Joules Law, heat ¼ I2Rt where I is direct current, R is the resistance of a cell, and t is time in s. Application of 1 A/cm2 for 10 min per current cycle produces 132 J of additional heat. This heat will lead to local temperature change (DT) at an active area. As the cell is electrolytesupported, the major contribution of temperature change comes from the zirconia electrolyte. The specific heat of zirconia (Cp) at 900 C is 0.6804 J/g C [14], and with 1.5 g mass (m) of an electrolyte pellet, a temperature change of 129 C occurs,
which has been calculated by Q ¼ m Cp DT. Due to rapid current load cycling, such temperature changes lead to an accumulation of thermal stresses at the cathode/electrolyte interface. Gradually, the thermal stress at the interface with the insulating phase reaches the critical value to start delamination of the cathode from an electrolyte in the cell without GDC interlayer [6]. However, Sr diffusion was suppressed at the interface of the cathode/GDC interlayer in the sintered half-cell with GDC interlayer. Fig. 8(a and b) show the elemental distribution of La and Sr in the half-cell with GDC interlayer before and after current load cycling. Before the current load cycle, there is no diffusion of Sr and La across the interface. However, only a small amount of Sr diffuses across the GDC interlayer/electrolyte interface during the current load cycling. The accumulation of Sr at the GDC interlayer/ ScCeSZ electrolyte interface indicates that this migration was caused by a gas phase diffusion mechanism through the GDC interlayer, causing the formation of an SZO phase, which somehow led to a reduction in electrolyte ionic conductivity. Moreover, the formation of a small amount of SZO phase could not affect the mechanical stability between ScCeSZ electrolyte and GDC interlayer because of the close matching of their thermal expansion coefficients. Results reported in Refs. [15e17] also show Sr enrichment at the GDC interlayer/ electrolyte interface during the sintering stage, and after long-
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Fig. 8 e Backscattered electron image and WDS elemental mapping of La and Sr for the half-cell with GDC interlayer (a) before and (b) after current load cycling.
term testing at normal operating conditions, further migration of Sr caused a decrease in cell performance. Thus, the cell with GDC interlayer had no delamination but showed only a small increase in ASR due to a small amount of Sr diffusion at the interface and cathode microstructure change caused by the cathodic polarization effect.
and electrolyte effectively suppressed Sr migration towards the electrolyte. During the current load cycling, there was no delamination in the cell with GDC interlayer but only its small degradation of 0.025% per cycle, which could be due to a small amount of Sr migration and cathode microstructural changes.
Conclusions
Acknowledgment
In this paper, the effect of the GDC interlayer on the cathode degradation of SOFC half-cells under accelerated current load cycling has been studied. The degradation of half-cells was evaluated by means of increments in ASR. Microstructural analysis was performed before and after the current cycle test. A half-cell without GDC interlayer showed a higher degradation rate of 0.08% per cycle due to the formation of an insulating phase by more Sr diffusion at the cathode and electrolyte interface. Insulating phases like SZO and LZO with different TECs enhance the TEC mismatch near the interface of the cathode and electrolyte, and additionally, a temperature gradient by Joule heating occurs during the current load cycling. Thus, thermal stress builds up, which leads to delamination of the cathode in the cell without GDC interlayer. The introduction of GDC interlayer between the cathode
This work was supported by the New & Renewable Energy Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) in the form of a grant funded by the Korean government's Ministry of Trade, Industry & Energy. (No. 20113020030010 & 20113020030050), and was also supported by Korea CCS R&D Center (KCRC) grant funded by the Korean government (Ministry of Science, ICT & Future Planning).
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