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Enhanced durability of a proton conducting oxide fuel cell with a purified yttrium-doped barium zirconate-cerate electrolyte
Journal of Power Sources 278 (2015) 320e324
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Enhanced durability of a proton conducting oxide fuel cell with a purified yttrium-doped barium zirconate-cerate electrolyte Muhammad Hakim a, Chung-Yul Yoo b, Jong Hoon Joo b, Ji Haeng Yu a, b, * a b
Advanced Energy Technology, Korea University of Science and Technology, Daejeon 305-350, Republic of Korea Advanced Materials & Devices Laboratory, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The effect of BZCY refinement on the cell stability were investigated. The fuel cell prepared using the refined BZCY shows a stable performance for 480 h. The fuel cell with as-calcined BZCY displays a rapid degradation over 110 h. Ba(OH)2 causes cathode delamination from the as-calcined BZCY electrolyte.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2014 Received in revised form 26 November 2014 Accepted 13 December 2014 Available online 15 December 2014
The aim of this study is to investigate the effect of yttrium-doped barium zirconate-cerate (BaZr0.3Ce0.5Y0.2O3d, BZCY) refinement on cell stability under operational fuel cell conditions. For this purpose, anode-supported cells, comprised of a nickel oxide (NiO)-BZCY anode, a BZCY electrolyte, and a BZCY-La0.6Sr0.4Co0.2Fe0.8O3d composite cathode are successfully prepared with refined or as-calcined BZCY powder. The long-term fuel cell performance is evaluated under a potentiostatic measurement at 600 C. The cell with the refined BZCY electrolyte shows a modest power density of 47 mW cm2 at a 600 C operating temperature over 480 h without any significant performance loss, whereas the cell with the as-calcined BZCY electrolyte displays a rapid degradation of cell performance over 110 h. A posttesting analysis of the cell with the refined BZCY does not reveal any evidence of delamination resulting from electrolyte surface decomposition. These results demonstrate that the refinement process significantly enhances the chemical stability of BZCY-based proton conducting fuel cells, which produce a high content of water vapor on the cathode side. © 2014 Elsevier B.V. All rights reserved.
Keywords: High temperature proton conductor Fuel cell Barium zirconate-cerate solid solution Refinement Durability
1. Introduction High temperature proton conductors (HTPCs) have received increasing attention as alternative electrolyte materials for solid oxide fuel cells over the past few decades. HTPC electrolytes, when
* Corresponding author. Advanced Materials & Devices Laboratory, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea. E-mail address: [email protected] (J.H. Yu). http://dx.doi.org/10.1016/j.jpowsour.2014.12.046 0378-7753/© 2014 Elsevier B.V. All rights reserved.
exposed to a humid atmosphere, show an ionic (nearly protonic) conductivity comparable with doped cerium(IV) oxide (ceria) in the intermediate temperature range (approximately 102 S cm1 at 600 C), and even higher conductivity below 500 C [1,2]. This high protonic conductivity originates from its large quantity of water uptake and fast diffusion of protons. Additionally, there is another advantage in that the fuel is not diluted by water in HTPCs since the water is produced at the cathode side rather than the anode side € ger-Vink notation, the cathodic reaction is [3]. Using the Kro
M. Hakim et al. / Journal of Power Sources 278 (2015) 320e324
described by Equation (1): 0 1 2OH$O þ O2 ðgÞ þ 2e /2O O þ H2 OðgÞ 2
(1)
Perovskite-type oxides based on barium cerate (BaCeO3) and barium zirconate (BaZrO3) have been extensively studied as a proton conducting electrolytes with trivalent cations based on yttrium (Y3þ), gadolinium (Gd3þ), and ytterbium (Yb3þ) substituted into the cerium or zirconium sites [3e9]. Despite the excellent proton conductivity of a Y-doped BaCeO3, its poor chemical stability in a carbon dioxide (CO2)- and/or water (H2O)-containing atmosphere limits its practical applications [1,10]. In comparison, the Ydoped BaZrO3 exhibits an excellent chemical stability towards CO2, H2O, and hydrogen sulfide (H2S), but its proton conductivity is an order of magnitude lower than that of the BaCeO3-based materials [2,11]. Furthermore, the refractory nature of the BaZrO3-based materials requires both a high temperature (1700 C) and prolonged time (20 h) for densification [12], and the consequent barium evaporation lowers the bulk- and grain-boundary conductivities [13,14]. The BaCeO3eBaZrO3 solid solution has been systematically investigated in order to enhance the chemical stability of doped BaZrO3 and maintain its proton conductivity [15e18]. A yttriumdoped barium zirconate-cerate (BaZrxCeyY1xyO3d) with a low Zr content (i.e., BaZr0.1Ce0.7Y0.2O3d) still decomposed when exposed to an atmosphere containing 3% CO2 [19] and 2.76% H2O [20], while a BaZr0.3Ce0.5Y0.2O3d was reported as a promising proton conducting electrolyte with acceptable chemical stability and conductivity [16,20]. Yoo and Lim reported a relatively stable NiOeBa0.98Zr0.2Ce0.6Y0.2O3d/Ba0.98Zr0.2Ce0.6Y0.2O3d/ Ba0.5Sr0.5Co0.8Fe0.2O3d cell tested at 600 C for 600 h [21]; however, more research is necessary to evaluate the long-term durability and performance of thin film cells with a BZCY electrolyte under fuel cell operation conditions. Slodczyk et al. [22,23] have noted the formation of bariumcontaining secondary phases (carbonates, hydroxides, hydrates, etc.) on the surface of a dense Y-doped BaZrO3 through in-situ and ex-situ Raman spectroscopy. These minor impurities cannot be detected by X-ray diffraction and have a detrimental effect on the stability and transport properties of Ba-containing proton conductors. Recently, we proposed a new procedure of refining ascalcined BaZr0.3Ce0.5Y0.2O3d (hereafter, BZCY) using deionized water to remove the water soluble barium hydroxide (Ba(OH)2) from the BZCY powder prepared through a solid state reaction [24]. Thermogravimetry and conductivity measurements confirmed that the refinement process enhanced the chemical stability of BZCY in a CO2-containing atmosphere. In this study, we have investigated the effect of BZCY refinement on the durability of a BZCY-based proton conducting oxide fuel cell. For this purpose, anode-supported cells are prepared with a refined BZCY powder. The long-term performance of the cell is evaluated under a potentiostatic measurement at 600 C, and its results are compared with a cell fabricated using the as-calcined BZCY. To the best of our knowledge, this is the first report describing the long-term performance of an anodesupported cell using a BZCY electrolyte.
321
then calcined in air at 1200 and 1300 C for 3 h with intermediate grinding. The formation of a BZCY single phase was confirmed by Xray diffraction (XRD, Rigaku, D-Max-2200). To alleviate the effect of water-soluble impurities on BZCY's stability and performance, the powder was refined using multiple washings of an as-calcined BZCY powder with deionized water. The as-calcined BZCY powder was stirred in deionized water (pH ¼ 6.7) for 2 h. The solution was kept in steady state for 1 h and decanted carefully to remove insoluble BaCO3 on the surface of the solution which was formed due to possible reaction of Ba(OH)2 or Ba2þ ion with atmospheric CO2. Then, the solution was passed through a Büchner filter funnel using an Advantec filter paper (Grade 5C, Pore size: <5 mm) to separate the refined BZCY powder. The pH of the filtrate was used as an indicator of the refinement progress. At the initial refinement step, the pH was about 12, and subsequent refinements were performed until the filtrate's pH reached approximately 7.5. The refined BZCY powder was dried in an oven for 24 h prior to the electrolyte and cathode slurry preparation. In order to investigate the effect of BZCY powder refinement on the fuel cell performance, two anode-supported cells were fabricated with the as-calcined or refined BZCY powder used in the
2. Experimental The BZCY powder was prepared by a conventional solid-state reaction. Stoichiometric amounts of barium carbonate (BaCO3, Aldrich, >99%), cerium(IV) oxide (CeO2, High Purity Chemicals, 99.9%), yttrium(III) oxide (Y2O3, High Purity Chemicals, 99.9%), and zirconium dioxide (ZrO2, Tereo Corporation, 99.9%) were ballmilled in ethanol for 48 h. The powder mixture was dried and
Fig. 1. (a) Currentevoltage characteristics of cells with the as-calcined (open symbols) or the refined BZCY (solid symbols) electrolyte at 600 C (squares) and 750 C (circles), and (b) the open circuit voltage and maximum power density as a function of temperature for the same cells.
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electrolyte and composite cathode. The anode was prepared by ball-milling NiO (High Purity Chemicals, 99.97%) with the ascalcined BZCY at a volume ratio of Ni:BZCY ¼ 45:55 in ethanol for 48 h. The anode powder mixture was uniaxially pressed (∅ ¼ 25.4 mm) at 25 MPa for 30 s, followed by pre-sintering at 1150 C for 3 h in air. The BZCY electrolyte was deposited onto the NiO-BZCY substrate by dip coating, and then the electrode was cosintered at 1400 C for 3 h in O2 at ~50 mL min1 to obtain a dense electrolyte film. The cathode slurries were prepared by mixing the BZCY (refined or as-calcined) with La0.6Sr0.4Co0.2Fe0.8O3d (LSCF, Kceracell) at a volume ratio of 35:65 in isopropyl alcohol with a plastic binder. The slurry was deposited onto the sintered BZCY electrolyte by spray coating, giving a 0.5 cm2 effective area, then cured at 1000 C in O2 at ~50 mL min1 for 1 h. The sintering of the BZCY electrolyte and firing of the composite cathode were both conducted in O2 to avoid reaction between the CO2 or H2O in air with the Ba in the BZCY. A Pt paste (Engelhard, Platinum Ink 6926) was applied to the cathode and anode as a current collecting layer. The as-calcined and refined BZCY anode-supported cells were sealed with gold rings in an alumina tube by holding them at a temperature of 1000 C. A humidified H2:Ar mixture (70:30) with 3% H2O was fed to the anode side at a flow rate of 100 mL min1, while dry air was fed to the cathode at an identical flow rate. Electrochemical measurements (currentevoltage (IeV) characteristics and impedance measurements) were conducted from 750 C down to 600 C using an AC impedance spectrometer (Autolab, PGSTAT302N). The impedance of the cell was recorded in the range of 1 MHz down to 0.1 Hz under open circuit voltage (OCV) with an oscillation voltage of 50 mV. To evaluate the long-term performance, the cell was tested at 600 C under potentiostatic conditions at a 0.7 V cell voltage. The microstructure of the cell after the longterm test was examined using a Field Emission Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy (FESEMEDX, Hitachi S-4700). Additionally, a glancing incidence angle XRD with a 1.5 incidence angle was used to investigate the phase structure of the thick BZCY electrolyte film after the long-term test. 3. Results and discussion Fig. 1 shows the IeV characteristics of the as-calcined and refined BZCY cells under the wet H2:Ar and dry air at temperatures ranging from 750 to 600 C with 50 C decrements. The OCV is
Fig. 2. The long-term fuel cell performances of cells with the as-calcined (blue line) and refined (red line) BZCY electrolyte at a cell voltage of 0.7 V and 600 C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
approximately 1.0 V across the entire temperature range for both the as-calcined and refined BZCY cells, in good agreement with previously reported results [20]. Despite the slightly lower OCV (0.92 V) at 750 C, the maximum power output of the as-calcined BZCY cell (127 mW cm2) is comparable to that of the refined one (125 mW cm2) as presented in Fig. 1b. This result implies that the polarization resistance of as-calcined cell is less than that of refined one, which is consistent with the as-calcined cell showing a lower slope in the IeV curve than the refined cell at 750 C (Fig. 1a). However, as the electrochemical measurement proceeds with a cooler cell temperature, the decreasing power output of the ascalcined BZCY cell is greater than the refined one, as shown in Fig. 1b. As a result, the refined BZCY cell presents a considerably improved performance over the as-calcined one below 750 C. Since the OCV values of the as-calcined and refined cells are not significantly different compared to the difference between their maximum power densities, as shown in Fig. 1b, the effect of gas leakage (either through the electrolyte or the sealing material) on the power generation can be neglected. Fig. 2 compares the long-term performances of the as-calcined and refined BZCY cells at an operating temperature of 600 C and a cell voltage of 0.7 V. The IeV curves and impedance spectra at the OCV are repeatedly measured at 4-h time intervals during the longterm testing. The fluctuation in the current density curves with regular intervals results from the dynamic load. The as-calcined BZCY cell shows a rapid current density degradation, by 33%, in 110 h. However, the refined BZCY cell shows only a 10% loss of current density within 480 h of fuel cell operation. This observation suggests that the stability of the anode-supported cell is significantly enhanced by the refined BZCY in both the electrolyte and composite cathode. The maximum power density value at 600 C of the refined BZCY cell is 45 mW cm2, comparable to the 60 mW cm2 of the NieBaCe0.9Y0.1O3d/BaCe0.9Y0.1 O3d/Nd2NiO4d cell [25], which was tested for 1000 h. Fig. 3 shows the IeV characteristics of the as-calcined and refined BZCY cells before and after the long-term test. The refined and as-calcined BZCY cells have nearly identical OCV values of ~1.0 V, which remains constant regardless of operation time. The refined BZCY cell at 600 C exhibits ~7% degradation in its maximum power output, from 45 mW cm2 to 42 mW cm2 after 480 h. On the other hand, the as-calcined BZCY cell shows a 42% drop in its maximum power density from 27 mW cm2 to
Fig. 3. Cell voltage vs. current and power density curves of cells with the as-calcined (triangles) and refined (circles) BZCY electrolyte at 600 C for both initial (open symbols) and end (solid symbols) times.
M. Hakim et al. / Journal of Power Sources 278 (2015) 320e324
16 mW cm2 after only 110 h. In-situ electrochemical impedance measurements of the ascalcined and refined BZCY cells under OCV during the long-term testing at 600 C are shown in Fig. 4. The intercept with the real axis at a high frequency is taken as the ohmic resistance (Rohm) of the cell, which primarily originates from the electrolyte resistance. The difference between the high frequency intercept and the lowest frequency intercept (with the real axis) is the polarization resistance (Rp) of the electrodes. The total resistance (Rtot) of the cell is the sum of the Rohm and Rp, and their sum (¼ R1 þ R2) is deconvoluted by fitting the equivalent circuit model: L$Rohm(R1Q1) (R2Q2) [26,27]. The values from the equivalent circuit fitting are summarized in Table 1. The conductivity of the refined BZCY electrolyte is estimated from the cell's Rohm at 600 C. The calculated conductivity of the refined BZCY electrolyte with a 15 mm thickness is 1.5 103 S cm1, which is in line with the electrical conductivity of the bulk BZCY reported in the literature [18,20]. The refined BZCY cell displays a negligible change in its Rohm, but it has a 33% increase in its Rp after 480 h of operation; whereas the as-calcined cell BZCY exhibits substantial increases in both its Rohm and Rp (32% and 40%, respectively) by 110 h of operation. It appears that this increase in the Rohm and Rp values of the as-calcined BZCY cell is mainly due to cathode delamination, as reported in the literature [28,29]. Fig. 5 presents cross section images of the ascalcined and refined BZCY cells after the long-term test. The BZCY electrolyte looks gas-tight and well attached to the anode for both cells. However, the cathode layer of the as-calcined BZCY cell is partially delaminated, while the cathode of the refined BZCY cell is stable, remaining in contact with the electrolyte even after 480 h of operation. From a finite element model for anode supported cell, ohmic and polarization resistance changes (Roohm =Rohm and Rop =Rp ) are calculated to be 0.712 and 0.713, respectively, after 30% cathode area delamination [30]. Assuming that the increase in Rohm and Rp of the as-calcined BZCY cell originates from the cathode delamination, the intact area would be between 60 and 70% of initial cathode area. On the other hand, the Rp increase of the refined BZCY cell implies that refined BZCY used for cathode is still not stable enough in humid atmosphere. It is also reported that water vapor accelerates the decomposition of LSCF and (La0.6Sr0.4)0.98MnO3 (LSM) cathodes in solid oxide fuel cell (SOFC), which becomes more serious at lower temperature [31]. Since the protonic ceramic fuel cell (PCFC) is operated at low temperature and, furthermore, water vapor is produced at cathode side differently from SOFC, the
323
Table 1 Area-specific resistances of the BZCY cells calculated from the electrochemical impedance spectroscopy measurement. Sample
Time (h)
Rohm (U cm2)
Rp (U cm2)
Rtot (U cm2)
As-calcined
0 110 0 480
1.69 2.49 0.92 1.11
5.79 9.73 3.24 4.82
7.48 12.22 4.35 5.74
Refined
Fig. 5. Cross section SEM images of cells with (a) the as-calcined and (b) refined BZCY electrolyte after the long-term tests. The insets show a higher magnification image of the electrolyte/cathode interface.
degradation of cell performance from the cathode materials would become severe. However, a detail study is required to understand the degradation of the composite cathode. It should be noted that the refinement of BZCY is effective for enhancing the stability of anode-supported cells under operational fuel cell conditions despite the fast degradation rate (14% per 1000 h) compared to SOFC. In previous experiments [24] we demonstrated that the BZCY prepared through the solid state reaction contains a barium oxide (BaO) residue, making the sintered ceramic unstable in the presence of CO2 or H2O. Since the BaO is only stable at high temperatures and in low CO2 and H2O content environments, it transforms into BaCO3 and Ba(OH)2 by reacting with trace amounts of CO2 and H2O vapor in the atmosphere as follows.
C
Fig. 4. Impedance spectra of the cells under OCV at 600 with the (a) as-calcined and (b) refined BZCY electrolyte for initial (squares) and end (circles) times. The numbers in the spectra denote the logarithmic value of the frequency (Hz).
BaO þ CO2 /BaCO3
(2)
BaO þ H2 O/BaðOHÞ2
(3)
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M. Hakim et al. / Journal of Power Sources 278 (2015) 320e324
If the electrolyte material in the as-calcined BZCY cell contains the BaO residue, Ba(OH)2 will form at the BZCY/LSCF interface due to an increasing water vapor pressure resulting from the cathodic reaction (Equation (1)). The formation of hydroxide at the BZCY/ LSCF interface may cause the cathode layer delamination during long-term operation, consequently degrading the as-calcined BZCY cell. As presented in Fig. 1, the cell performance of as-calcined BZCY cell is comparable with that of refined at 750 C but becomes poorer with decreasing operation temperature. This is because the formation of hydroxide (Equation (3)) is an endothermic reaction and thus favored in the unrefined BZCY especially at low temperatures. Fig. 6 shows the phase analysis results of the BZCY electrolyte after long-term operation. The presence of a small Ni peak in the XRD is due to the anode substrate (Ni-BZCY). As the Pt paste is used for the current collecting layer, it is clearly seen in the XRD pattern of refined BZCY cell. However, the Pt peaks are hardly seen in the XRD patterns of as-calcined BZCY cell because the Pt layer was considerably detached from the cell by the delamination of cathode. The peaks resulting from Ba(OH)2$8H2O are evident in the ascalcined BZCY electrolyte, while they are not apparent for the refined BZCY electrolyte after the long-term test. Therefore, the formation of the Ba(OH)2 phase is the most probable origin of cathode delamination from the electrolyte and the rapid degradation in the fuel cell performance seen for the as-calcined BZCY cell. This result confirms that the refinement process of the BZCY powder to decrease the water-soluble Ba(OH)2 residue from the electrolyte and composite cathode material (BZCY and LSCF) is effective for enhancing the stability and performance of proton conducting oxide fuel cells. This is vital in real world applications, where a high concentration of water is produced on the cathode side. 4. Conclusions The fuel cell prepared using the refined BZCY powder showed a stable performance for 480 h, while the cell prepared with the ascalcined BZCY displayed a severe deterioration, mainly resulting from delamination of the cathode layer. In the case of using the ascalcined BZCY powder for the electrolyte and cathode, Ba(OH)2$8H2O was observed on the electrolyte after 110 h of
operation. The formation of Ba(OH)2 from the water vapor produced at the protonic ceramic fuel cell's (PCFC's) cathode is the most likely reason for cathode delamination from the electrolyte and consequent rapid increase in the cell's ohmic and polarization resistances. This investigation shows that the BZCY refinement plays a significant role in increasing stability and performance of the cell. These results could be useful for optimizing Ba-based electrolytes used in proton conducting ceramic fuel cells. Further work will focus on optimizing electrolyte thickness, anode porosity, use of an anode functional layer (AFL), and other parameters to improve cell performance while retaining the long-term stability properties.
Acknowledgments This research was supported by the Fusion Research Program for Green Technologies though the National Research Foundation (NRF) of Korea, funded by the Ministry of Science, ICT & Future Planning (NRF-2011-0019302). Asif Mahmood is gratefully thanked for his fruitful discussions.
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Fig. 6. XRD patterns of the electrolyte surfaces after long-term cell testing for the ascalcined (green line) and refined (blue line) BZCY electrolyte, with the marked peaks showing the different components, including the BZCY and the Ba(OH)2$8H2O. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Enhanced durability of a proton conducting oxide fuel cell with a purified yttrium-doped barium zirconate-cerate electrolyte Muhammad Hakim a, Chung-Yul Yoo b, Jong Hoon Joo b, Ji Haeng Yu a, b, * a b
Advanced Energy Technology, Korea University of Science and Technology, Daejeon 305-350, Republic of Korea Advanced Materials & Devices Laboratory, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The effect of BZCY refinement on the cell stability were investigated. The fuel cell prepared using the refined BZCY shows a stable performance for 480 h. The fuel cell with as-calcined BZCY displays a rapid degradation over 110 h. Ba(OH)2 causes cathode delamination from the as-calcined BZCY electrolyte.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2014 Received in revised form 26 November 2014 Accepted 13 December 2014 Available online 15 December 2014
The aim of this study is to investigate the effect of yttrium-doped barium zirconate-cerate (BaZr0.3Ce0.5Y0.2O3d, BZCY) refinement on cell stability under operational fuel cell conditions. For this purpose, anode-supported cells, comprised of a nickel oxide (NiO)-BZCY anode, a BZCY electrolyte, and a BZCY-La0.6Sr0.4Co0.2Fe0.8O3d composite cathode are successfully prepared with refined or as-calcined BZCY powder. The long-term fuel cell performance is evaluated under a potentiostatic measurement at 600 C. The cell with the refined BZCY electrolyte shows a modest power density of 47 mW cm2 at a 600 C operating temperature over 480 h without any significant performance loss, whereas the cell with the as-calcined BZCY electrolyte displays a rapid degradation of cell performance over 110 h. A posttesting analysis of the cell with the refined BZCY does not reveal any evidence of delamination resulting from electrolyte surface decomposition. These results demonstrate that the refinement process significantly enhances the chemical stability of BZCY-based proton conducting fuel cells, which produce a high content of water vapor on the cathode side. © 2014 Elsevier B.V. All rights reserved.
Keywords: High temperature proton conductor Fuel cell Barium zirconate-cerate solid solution Refinement Durability
1. Introduction High temperature proton conductors (HTPCs) have received increasing attention as alternative electrolyte materials for solid oxide fuel cells over the past few decades. HTPC electrolytes, when
* Corresponding author. Advanced Materials & Devices Laboratory, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea. E-mail address: [email protected] (J.H. Yu). http://dx.doi.org/10.1016/j.jpowsour.2014.12.046 0378-7753/© 2014 Elsevier B.V. All rights reserved.
exposed to a humid atmosphere, show an ionic (nearly protonic) conductivity comparable with doped cerium(IV) oxide (ceria) in the intermediate temperature range (approximately 102 S cm1 at 600 C), and even higher conductivity below 500 C [1,2]. This high protonic conductivity originates from its large quantity of water uptake and fast diffusion of protons. Additionally, there is another advantage in that the fuel is not diluted by water in HTPCs since the water is produced at the cathode side rather than the anode side € ger-Vink notation, the cathodic reaction is [3]. Using the Kro
M. Hakim et al. / Journal of Power Sources 278 (2015) 320e324
described by Equation (1): 0 1 2OH$O þ O2 ðgÞ þ 2e /2O O þ H2 OðgÞ 2
(1)
Perovskite-type oxides based on barium cerate (BaCeO3) and barium zirconate (BaZrO3) have been extensively studied as a proton conducting electrolytes with trivalent cations based on yttrium (Y3þ), gadolinium (Gd3þ), and ytterbium (Yb3þ) substituted into the cerium or zirconium sites [3e9]. Despite the excellent proton conductivity of a Y-doped BaCeO3, its poor chemical stability in a carbon dioxide (CO2)- and/or water (H2O)-containing atmosphere limits its practical applications [1,10]. In comparison, the Ydoped BaZrO3 exhibits an excellent chemical stability towards CO2, H2O, and hydrogen sulfide (H2S), but its proton conductivity is an order of magnitude lower than that of the BaCeO3-based materials [2,11]. Furthermore, the refractory nature of the BaZrO3-based materials requires both a high temperature (1700 C) and prolonged time (20 h) for densification [12], and the consequent barium evaporation lowers the bulk- and grain-boundary conductivities [13,14]. The BaCeO3eBaZrO3 solid solution has been systematically investigated in order to enhance the chemical stability of doped BaZrO3 and maintain its proton conductivity [15e18]. A yttriumdoped barium zirconate-cerate (BaZrxCeyY1xyO3d) with a low Zr content (i.e., BaZr0.1Ce0.7Y0.2O3d) still decomposed when exposed to an atmosphere containing 3% CO2 [19] and 2.76% H2O [20], while a BaZr0.3Ce0.5Y0.2O3d was reported as a promising proton conducting electrolyte with acceptable chemical stability and conductivity [16,20]. Yoo and Lim reported a relatively stable NiOeBa0.98Zr0.2Ce0.6Y0.2O3d/Ba0.98Zr0.2Ce0.6Y0.2O3d/ Ba0.5Sr0.5Co0.8Fe0.2O3d cell tested at 600 C for 600 h [21]; however, more research is necessary to evaluate the long-term durability and performance of thin film cells with a BZCY electrolyte under fuel cell operation conditions. Slodczyk et al. [22,23] have noted the formation of bariumcontaining secondary phases (carbonates, hydroxides, hydrates, etc.) on the surface of a dense Y-doped BaZrO3 through in-situ and ex-situ Raman spectroscopy. These minor impurities cannot be detected by X-ray diffraction and have a detrimental effect on the stability and transport properties of Ba-containing proton conductors. Recently, we proposed a new procedure of refining ascalcined BaZr0.3Ce0.5Y0.2O3d (hereafter, BZCY) using deionized water to remove the water soluble barium hydroxide (Ba(OH)2) from the BZCY powder prepared through a solid state reaction [24]. Thermogravimetry and conductivity measurements confirmed that the refinement process enhanced the chemical stability of BZCY in a CO2-containing atmosphere. In this study, we have investigated the effect of BZCY refinement on the durability of a BZCY-based proton conducting oxide fuel cell. For this purpose, anode-supported cells are prepared with a refined BZCY powder. The long-term performance of the cell is evaluated under a potentiostatic measurement at 600 C, and its results are compared with a cell fabricated using the as-calcined BZCY. To the best of our knowledge, this is the first report describing the long-term performance of an anodesupported cell using a BZCY electrolyte.
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then calcined in air at 1200 and 1300 C for 3 h with intermediate grinding. The formation of a BZCY single phase was confirmed by Xray diffraction (XRD, Rigaku, D-Max-2200). To alleviate the effect of water-soluble impurities on BZCY's stability and performance, the powder was refined using multiple washings of an as-calcined BZCY powder with deionized water. The as-calcined BZCY powder was stirred in deionized water (pH ¼ 6.7) for 2 h. The solution was kept in steady state for 1 h and decanted carefully to remove insoluble BaCO3 on the surface of the solution which was formed due to possible reaction of Ba(OH)2 or Ba2þ ion with atmospheric CO2. Then, the solution was passed through a Büchner filter funnel using an Advantec filter paper (Grade 5C, Pore size: <5 mm) to separate the refined BZCY powder. The pH of the filtrate was used as an indicator of the refinement progress. At the initial refinement step, the pH was about 12, and subsequent refinements were performed until the filtrate's pH reached approximately 7.5. The refined BZCY powder was dried in an oven for 24 h prior to the electrolyte and cathode slurry preparation. In order to investigate the effect of BZCY powder refinement on the fuel cell performance, two anode-supported cells were fabricated with the as-calcined or refined BZCY powder used in the
2. Experimental The BZCY powder was prepared by a conventional solid-state reaction. Stoichiometric amounts of barium carbonate (BaCO3, Aldrich, >99%), cerium(IV) oxide (CeO2, High Purity Chemicals, 99.9%), yttrium(III) oxide (Y2O3, High Purity Chemicals, 99.9%), and zirconium dioxide (ZrO2, Tereo Corporation, 99.9%) were ballmilled in ethanol for 48 h. The powder mixture was dried and
Fig. 1. (a) Currentevoltage characteristics of cells with the as-calcined (open symbols) or the refined BZCY (solid symbols) electrolyte at 600 C (squares) and 750 C (circles), and (b) the open circuit voltage and maximum power density as a function of temperature for the same cells.
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electrolyte and composite cathode. The anode was prepared by ball-milling NiO (High Purity Chemicals, 99.97%) with the ascalcined BZCY at a volume ratio of Ni:BZCY ¼ 45:55 in ethanol for 48 h. The anode powder mixture was uniaxially pressed (∅ ¼ 25.4 mm) at 25 MPa for 30 s, followed by pre-sintering at 1150 C for 3 h in air. The BZCY electrolyte was deposited onto the NiO-BZCY substrate by dip coating, and then the electrode was cosintered at 1400 C for 3 h in O2 at ~50 mL min1 to obtain a dense electrolyte film. The cathode slurries were prepared by mixing the BZCY (refined or as-calcined) with La0.6Sr0.4Co0.2Fe0.8O3d (LSCF, Kceracell) at a volume ratio of 35:65 in isopropyl alcohol with a plastic binder. The slurry was deposited onto the sintered BZCY electrolyte by spray coating, giving a 0.5 cm2 effective area, then cured at 1000 C in O2 at ~50 mL min1 for 1 h. The sintering of the BZCY electrolyte and firing of the composite cathode were both conducted in O2 to avoid reaction between the CO2 or H2O in air with the Ba in the BZCY. A Pt paste (Engelhard, Platinum Ink 6926) was applied to the cathode and anode as a current collecting layer. The as-calcined and refined BZCY anode-supported cells were sealed with gold rings in an alumina tube by holding them at a temperature of 1000 C. A humidified H2:Ar mixture (70:30) with 3% H2O was fed to the anode side at a flow rate of 100 mL min1, while dry air was fed to the cathode at an identical flow rate. Electrochemical measurements (currentevoltage (IeV) characteristics and impedance measurements) were conducted from 750 C down to 600 C using an AC impedance spectrometer (Autolab, PGSTAT302N). The impedance of the cell was recorded in the range of 1 MHz down to 0.1 Hz under open circuit voltage (OCV) with an oscillation voltage of 50 mV. To evaluate the long-term performance, the cell was tested at 600 C under potentiostatic conditions at a 0.7 V cell voltage. The microstructure of the cell after the longterm test was examined using a Field Emission Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy (FESEMEDX, Hitachi S-4700). Additionally, a glancing incidence angle XRD with a 1.5 incidence angle was used to investigate the phase structure of the thick BZCY electrolyte film after the long-term test. 3. Results and discussion Fig. 1 shows the IeV characteristics of the as-calcined and refined BZCY cells under the wet H2:Ar and dry air at temperatures ranging from 750 to 600 C with 50 C decrements. The OCV is
Fig. 2. The long-term fuel cell performances of cells with the as-calcined (blue line) and refined (red line) BZCY electrolyte at a cell voltage of 0.7 V and 600 C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
approximately 1.0 V across the entire temperature range for both the as-calcined and refined BZCY cells, in good agreement with previously reported results [20]. Despite the slightly lower OCV (0.92 V) at 750 C, the maximum power output of the as-calcined BZCY cell (127 mW cm2) is comparable to that of the refined one (125 mW cm2) as presented in Fig. 1b. This result implies that the polarization resistance of as-calcined cell is less than that of refined one, which is consistent with the as-calcined cell showing a lower slope in the IeV curve than the refined cell at 750 C (Fig. 1a). However, as the electrochemical measurement proceeds with a cooler cell temperature, the decreasing power output of the ascalcined BZCY cell is greater than the refined one, as shown in Fig. 1b. As a result, the refined BZCY cell presents a considerably improved performance over the as-calcined one below 750 C. Since the OCV values of the as-calcined and refined cells are not significantly different compared to the difference between their maximum power densities, as shown in Fig. 1b, the effect of gas leakage (either through the electrolyte or the sealing material) on the power generation can be neglected. Fig. 2 compares the long-term performances of the as-calcined and refined BZCY cells at an operating temperature of 600 C and a cell voltage of 0.7 V. The IeV curves and impedance spectra at the OCV are repeatedly measured at 4-h time intervals during the longterm testing. The fluctuation in the current density curves with regular intervals results from the dynamic load. The as-calcined BZCY cell shows a rapid current density degradation, by 33%, in 110 h. However, the refined BZCY cell shows only a 10% loss of current density within 480 h of fuel cell operation. This observation suggests that the stability of the anode-supported cell is significantly enhanced by the refined BZCY in both the electrolyte and composite cathode. The maximum power density value at 600 C of the refined BZCY cell is 45 mW cm2, comparable to the 60 mW cm2 of the NieBaCe0.9Y0.1O3d/BaCe0.9Y0.1 O3d/Nd2NiO4d cell [25], which was tested for 1000 h. Fig. 3 shows the IeV characteristics of the as-calcined and refined BZCY cells before and after the long-term test. The refined and as-calcined BZCY cells have nearly identical OCV values of ~1.0 V, which remains constant regardless of operation time. The refined BZCY cell at 600 C exhibits ~7% degradation in its maximum power output, from 45 mW cm2 to 42 mW cm2 after 480 h. On the other hand, the as-calcined BZCY cell shows a 42% drop in its maximum power density from 27 mW cm2 to
Fig. 3. Cell voltage vs. current and power density curves of cells with the as-calcined (triangles) and refined (circles) BZCY electrolyte at 600 C for both initial (open symbols) and end (solid symbols) times.
M. Hakim et al. / Journal of Power Sources 278 (2015) 320e324
16 mW cm2 after only 110 h. In-situ electrochemical impedance measurements of the ascalcined and refined BZCY cells under OCV during the long-term testing at 600 C are shown in Fig. 4. The intercept with the real axis at a high frequency is taken as the ohmic resistance (Rohm) of the cell, which primarily originates from the electrolyte resistance. The difference between the high frequency intercept and the lowest frequency intercept (with the real axis) is the polarization resistance (Rp) of the electrodes. The total resistance (Rtot) of the cell is the sum of the Rohm and Rp, and their sum (¼ R1 þ R2) is deconvoluted by fitting the equivalent circuit model: L$Rohm(R1Q1) (R2Q2) [26,27]. The values from the equivalent circuit fitting are summarized in Table 1. The conductivity of the refined BZCY electrolyte is estimated from the cell's Rohm at 600 C. The calculated conductivity of the refined BZCY electrolyte with a 15 mm thickness is 1.5 103 S cm1, which is in line with the electrical conductivity of the bulk BZCY reported in the literature [18,20]. The refined BZCY cell displays a negligible change in its Rohm, but it has a 33% increase in its Rp after 480 h of operation; whereas the as-calcined cell BZCY exhibits substantial increases in both its Rohm and Rp (32% and 40%, respectively) by 110 h of operation. It appears that this increase in the Rohm and Rp values of the as-calcined BZCY cell is mainly due to cathode delamination, as reported in the literature [28,29]. Fig. 5 presents cross section images of the ascalcined and refined BZCY cells after the long-term test. The BZCY electrolyte looks gas-tight and well attached to the anode for both cells. However, the cathode layer of the as-calcined BZCY cell is partially delaminated, while the cathode of the refined BZCY cell is stable, remaining in contact with the electrolyte even after 480 h of operation. From a finite element model for anode supported cell, ohmic and polarization resistance changes (Roohm =Rohm and Rop =Rp ) are calculated to be 0.712 and 0.713, respectively, after 30% cathode area delamination [30]. Assuming that the increase in Rohm and Rp of the as-calcined BZCY cell originates from the cathode delamination, the intact area would be between 60 and 70% of initial cathode area. On the other hand, the Rp increase of the refined BZCY cell implies that refined BZCY used for cathode is still not stable enough in humid atmosphere. It is also reported that water vapor accelerates the decomposition of LSCF and (La0.6Sr0.4)0.98MnO3 (LSM) cathodes in solid oxide fuel cell (SOFC), which becomes more serious at lower temperature [31]. Since the protonic ceramic fuel cell (PCFC) is operated at low temperature and, furthermore, water vapor is produced at cathode side differently from SOFC, the
323
Table 1 Area-specific resistances of the BZCY cells calculated from the electrochemical impedance spectroscopy measurement. Sample
Time (h)
Rohm (U cm2)
Rp (U cm2)
Rtot (U cm2)
As-calcined
0 110 0 480
1.69 2.49 0.92 1.11
5.79 9.73 3.24 4.82
7.48 12.22 4.35 5.74
Refined
Fig. 5. Cross section SEM images of cells with (a) the as-calcined and (b) refined BZCY electrolyte after the long-term tests. The insets show a higher magnification image of the electrolyte/cathode interface.
degradation of cell performance from the cathode materials would become severe. However, a detail study is required to understand the degradation of the composite cathode. It should be noted that the refinement of BZCY is effective for enhancing the stability of anode-supported cells under operational fuel cell conditions despite the fast degradation rate (14% per 1000 h) compared to SOFC. In previous experiments [24] we demonstrated that the BZCY prepared through the solid state reaction contains a barium oxide (BaO) residue, making the sintered ceramic unstable in the presence of CO2 or H2O. Since the BaO is only stable at high temperatures and in low CO2 and H2O content environments, it transforms into BaCO3 and Ba(OH)2 by reacting with trace amounts of CO2 and H2O vapor in the atmosphere as follows.
C
Fig. 4. Impedance spectra of the cells under OCV at 600 with the (a) as-calcined and (b) refined BZCY electrolyte for initial (squares) and end (circles) times. The numbers in the spectra denote the logarithmic value of the frequency (Hz).
BaO þ CO2 /BaCO3
(2)
BaO þ H2 O/BaðOHÞ2
(3)
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If the electrolyte material in the as-calcined BZCY cell contains the BaO residue, Ba(OH)2 will form at the BZCY/LSCF interface due to an increasing water vapor pressure resulting from the cathodic reaction (Equation (1)). The formation of hydroxide at the BZCY/ LSCF interface may cause the cathode layer delamination during long-term operation, consequently degrading the as-calcined BZCY cell. As presented in Fig. 1, the cell performance of as-calcined BZCY cell is comparable with that of refined at 750 C but becomes poorer with decreasing operation temperature. This is because the formation of hydroxide (Equation (3)) is an endothermic reaction and thus favored in the unrefined BZCY especially at low temperatures. Fig. 6 shows the phase analysis results of the BZCY electrolyte after long-term operation. The presence of a small Ni peak in the XRD is due to the anode substrate (Ni-BZCY). As the Pt paste is used for the current collecting layer, it is clearly seen in the XRD pattern of refined BZCY cell. However, the Pt peaks are hardly seen in the XRD patterns of as-calcined BZCY cell because the Pt layer was considerably detached from the cell by the delamination of cathode. The peaks resulting from Ba(OH)2$8H2O are evident in the ascalcined BZCY electrolyte, while they are not apparent for the refined BZCY electrolyte after the long-term test. Therefore, the formation of the Ba(OH)2 phase is the most probable origin of cathode delamination from the electrolyte and the rapid degradation in the fuel cell performance seen for the as-calcined BZCY cell. This result confirms that the refinement process of the BZCY powder to decrease the water-soluble Ba(OH)2 residue from the electrolyte and composite cathode material (BZCY and LSCF) is effective for enhancing the stability and performance of proton conducting oxide fuel cells. This is vital in real world applications, where a high concentration of water is produced on the cathode side. 4. Conclusions The fuel cell prepared using the refined BZCY powder showed a stable performance for 480 h, while the cell prepared with the ascalcined BZCY displayed a severe deterioration, mainly resulting from delamination of the cathode layer. In the case of using the ascalcined BZCY powder for the electrolyte and cathode, Ba(OH)2$8H2O was observed on the electrolyte after 110 h of
operation. The formation of Ba(OH)2 from the water vapor produced at the protonic ceramic fuel cell's (PCFC's) cathode is the most likely reason for cathode delamination from the electrolyte and consequent rapid increase in the cell's ohmic and polarization resistances. This investigation shows that the BZCY refinement plays a significant role in increasing stability and performance of the cell. These results could be useful for optimizing Ba-based electrolytes used in proton conducting ceramic fuel cells. Further work will focus on optimizing electrolyte thickness, anode porosity, use of an anode functional layer (AFL), and other parameters to improve cell performance while retaining the long-term stability properties.
Acknowledgments This research was supported by the Fusion Research Program for Green Technologies though the National Research Foundation (NRF) of Korea, funded by the Ministry of Science, ICT & Future Planning (NRF-2011-0019302). Asif Mahmood is gratefully thanked for his fruitful discussions.
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Fig. 6. XRD patterns of the electrolyte surfaces after long-term cell testing for the ascalcined (green line) and refined (blue line) BZCY electrolyte, with the marked peaks showing the different components, including the BZCY and the Ba(OH)2$8H2O. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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