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High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer
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High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer Nanqi Duan, Jiyang Ma, Jin Li, Dong Yan, Bo Chi*, Jian Pu, Jian Li Center for Fuel Cell Innovation, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
article info
abstract
Article history:
Dense bismuth oxides stabilized with lanthanide dopants (d-Bi2O3) are always used as
Received 10 January 2018
interlayer for thin layer GDC (gadolinia-doped ceria) or YSZ (yttria-stabilized zirconia) elec-
Received in revised form
trolyte solid oxide fuel cell to improve the cell performance. Dense ESB (Bi1.6Er0.4O3) layer
2 March 2018
preparation needs special equipment or well synthesized nano-sized powders, which is not
Accepted 22 March 2018
friendly for large scale and commercial application. In this paper, anode support unsintered
Available online xxx
cathode cell with porous ESB interlayer is prepared through simple screen print and its electrochemical performance is tested. No matter sintered or not, porous ESB interlayer
Keywords:
lowers both the polarization resistance and ohmic resistance, and enhances cell perfor-
Bi2O3-Er2O3
mance, especially for the ohmic resistance of unsintered cells. The cell with ESB porous
Unsintered cathode
interlayer and SSC (Sm0.5Sr0.5CoO3-d)-ESB unsintered cathode achieves a peak power density
Solid oxide fuel cell
of 1.329 W cm2 at 700 C, which is 1.61 times higher than that of the cell without ESB
Interlayer
interlayer and 0.93 times higher than that of the sintered cell. However, the durability of
Durability
porous ESB interlayer enhanced cells is not ideal and it should be improved in further works. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cell (SOFC), consisting of porous anode and cathode separated by a dense electrolyte, is an electrochemical and environmental friendly device, which efficiently converts the chemical energy of fossil and hydrocarbon fuels into electricity and heat without combustion and mechanical motion involved [1]. The operating temperature of primary thick electrolyte-supported SOFCs is near 1000 C, which has been lowered to the intermediate-temperature range of 600e800 C by accepting anode-supported configuration, which naturally come with the benefits in more extensive
materials selection, higher materials stability, better performance durability, and lower manufacturing cost [2,3]. Yttria-stabilized zirconia (YSZ) is the most widely used electrolyte material for both commercial products and laboratory scale researches, which benefits from its high stability at a wide range of oxygen partial pressure [4]. Other electrolyte materials, such as Gadolinia-doped Ceria (GDC) and Bismuth oxides stabilized with lanthanide dopants (d-Bi2O3, e.g., Bi1.6Er0.4O3 (ESB)), have higher oxygen ion conductivity but worse stability at low oxygen partial atmosphere [5,6]. At a typical anode atmosphere, pure H2, GDC is partially reduced and then gets certain electronic conductivity leading to a lower OCV (open circuit voltage) [7], while ESB is easily
* Corresponding author. E-mail address: [email protected] (B. Chi). https://doi.org/10.1016/j.ijhydene.2018.03.168 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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reduced leading to the birth of metal Bi [5,8]. Compared with YSZ, GDC has a better chemical compatibility with many perovskite cathode materials, such as La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF). GDC was usually used as the barrier layer between YSZ electrolyte and cathode to prevent the unexpected interactions [9,10]. At most conditions, the GDC barrier layer, usually prepared by screen print, was porous, because it's expensive to prepare dense GDC thin layer on YSZ surface. What's more, it has been reported that the inter-diffusion between doped-ceria and stabilized zirconia during high temperature sintering (over 1200 C) results in insulating phases, thus significantly increasing ohmic resistance by at least 1 order of magnitude compared to that of pure YSZ [11]. Bismuth oxide stabilized with lanthanide dopants (d-Bi2O3) has been known as a promising electrolyte material with higher oxygen ions conductivity than both GDC and YSZ, but its application is limited by its weak tolerance of reduction atmosphere. So, different with GDC, dense ESB is always prepared between YSZ or GDC electrolyte and cathode constituting a bilayer electrolytes SOFC [12]. Wachsman et al. pioneered bismuth oxide/ceria bilayer electrolytes SOFCs: ceria electrolyte at the fuel side can shield the bismuth oxide from the reduction atmosphere and maintain the chemical stability; at the same time ESB will block the electrons transportation through GDC layer; moreover, total area specific resistance is decreased [13e15]. Joh et al. [16] replaced GDC with YSZ and used LSM (La0.8Sr0.2MnO3d)-ESB as cathode, and a higher OCV than that of GDC electrolyte condition and a peak power density of 2.08 W cm2 at 700 C was achieved. But the establishment of such dense ESB layer requires special equipment, such as pulsed laser deposition (PLD) [12], or nano-sized ESB powers prepared by special method, such as wet chemical co-precipitation [16]. For cell with ESB interlayer, cathode was usually prepared through the strategy of compositing with Bi2O3 based oxygen ions conductor, such as Er0.4Bi1.6O3 decorated La0.76Sr0.19MnO3þd [17], (La0.8Sr0.2)0.95MnO3-d-(Er0.2Bi0.8)2O3 [18], La0.76Sr0.19MnO3þd decorated Er0.4Bi1.6O3 [19], Sr doped LaMnO3 (LSM)-Y & Ce codoped Bi2O3 (BYC7) [20], and (Bi2O3)0.7(Er2O3)0.3-Ag [21]. These composite cathode cells achieved reasonable performance at temperature below 750 C and partial materials also showed good durability performance. However, bare work about the function of a porous ESB layer has been reported. It is still not clear whether a porous ESB layer has the same ability to improve electrochemical performance. Therefore, in this paper, a solid oxide fuel cell with porous ESB (Bi1.6Er0.4O3) interlayer between the dense YSZ electrolyte and unsintered cathode was fabricated by a low cost and easy prepared screen print method. Moreover, novel SSC (Sm0.5Sr0.5CoO3-d)-ESB composite was used as the cathode for solid oxide fuel cell.
Experimental ESB and SSC powers preparation ESB powers were synthesized through a citrate process (CP). Stoichiometric amounts of Er(NO3)3$5H2O (99.9%, Sinopharm)
and Bi(NO3)3$5H2O (99.9%, Sinopharm) were added into distilled water contained in a Pyrex container, to which ethylene glycol and citric acid were added. This solution was slowly heated to 80 C for 2 h and white precipitates appeared, and then it was dried at 200 C for more than 10 h to get the finally white-yellow ash, which at last was calcined in air at 700 C for 5 h to form the fluorite structure. SSC powders were synthesized through a similar process using corresponding metal nitrates. The difference was that no precipitate appeared in the solution at 80 C and the final calcine temperature was 900 C.
Cells preparation The anode substrate of the cell was prepared by tape casting technique and the details can be found in previous reports [22,23]. A mixture of nickel oxide (NiO) powders (Type A, Inco.) and YSZ powders (TZ-8YS, Tosoh) with a mass ratio of 57:43, adding fish oil as dispersant, polyvinyl butyral as organic binders, and polyaleneglycol as plasticizer, was ball milled for 36 h to from a uniform slurry with toluene and ethanol as solvent. Then the slurry was collected to form band shape by tape casting process. After dried at room temperature for more than 24 h, the green substrate was cut into circular samples with target size. At last, the anode functional layer and electrolyte layer was screen printed on the substrate in sequence and co-sintered at 1400 C for 4 h. ESB slurry with a weight ratio of 60% was screen printed on the surface of YSZ electrolyte, after dried at 80 C for 6 h, SSCESB composite cathode slurry with the same weight percentage was screen printed on the surface of ESB interlayer, which repeated twice. For the cell without porous ESB interlayer, SSC-ESB composite was directly screen printed on the surface of YSZ. At last, partial cells were sintered at 750 C for 2 h to decompose the organics.
Characterizations A Solartron 1260 frequency response analyzer and a Solartron 1287 electrochemical interface (Solartron Analytical) were employed to measure the impedance of the cell in a frequency range between 1000 kHz and 0.1 Hz with a signal amplitude of 20 mV at open circuit, as well as the current-voltage (I-V), current-power (I-P) and time-voltage curves. The X-ray curves of ESB and SSC were examined by X-ray diffraction (XRD) (X'Pert PRO, PANalytical B.V.) The cell microstructure was examined by a scanning electron microscope (SEM, Sirion 200 and Quanta 200, FEI).
Results and discussion Citrate process has been known as a mature and widely used method to synthesize nano-sized and micron-sized powders. As shown by the X-ray spectra of ESB and SSC in Fig. 1, both powders were well fitted with previous reported results [20,24,25]. ESB and SSC mix powders with a weight ratio of 1:1 were well manual grinded for more than 2 h to form a uniform cathode slurry. Two type cells were fabricated: type A without porous ESB interlayer and type B with porous ESB interlayer. A
Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 1 e The X-ray spectra of synthesized ESB and SSC powders.
part of both type cells were sintered at 750 C for 2 h before testing, which called cathode-sintered cells; the left cells, called cathode-unsintered cells, were directly tested without any sintering procedure before.
Sintered cells performance A Ceramabond glass sealant (Aremco Product, Inc.) was used to seal the anode compartment to an alumina tube. Pt paste was used as current collector for both anode and cathode and silver wires were used as the measuring lead. From room temperature to 500 C, a heating rate of 5 C min1 was accepted, during that process, 5 mol % H2-N2 with a flux of 50 ml min1 was inlet into anode side to slowly and partially reduce anode. Then, wet H2 (3 mol % H2O) with a flux of 50 ml min1 was used to reduce nickel oxide to nickel at 500 C
for 0.5 h as well as the fuel gas for the later performance testing. The cathode was naked in static air for all the heating and testing procedures. I-V and I-P curves of sintered cells at the temperature range of 500e700 C were shown in Fig. 2. The OCV of both type cells were above 1.1 V at 500e700 C, specifically, it was about 1.127 V, 1.119 V, and 1.112 V at 600, 650, and 700 C, respectively. The peak power density of type B cell was only a bit lower than that of type A cell below 600 C. The peak power density of type B cell was 0.309 and 0.688 W cm2 at 650 and 700 C, respectively, which was higher than 0.292 and 0.583 W cm2 of type A cell. The EIS (electrochemical impedance spectra) of two type cells at the temperature range of 500e700 C were shown in Fig. 3. Both cells had very closed ohmic resistance; the specific value of that for type B cell was 0.494, 0.275, and 0.163 U cm2 at 600, 650, and 700 C, respectively. Without porous ESB
Fig. 2 e IeV and I-P curves of sintered type A (a) and type B (b) cells at 500e700 C. Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 3 e Electrochemical impedance spectra of sintered type A (a) and type B (b) cells at 500e700 C.
interlayer, the ohmic resistance of type A cell was 0.523, 320, and 0.187 U cm2 at these three temperatures, respectively, which was only a little higher than that of type B cell. The total resistance of type A cell was 5.140, 2.106, and 0.835 U cm2 at 600, 650, and 700 C, respectively, which was higher than that of type B cell. Enhanced by porous ESB interlayer, the total resistance of type B cell was 2.973, 1.392, and 0.720 U cm2 at 600, 650, and 700 C, respectively. After sintered at 750 C, the porous ESB interlayer seemed have very limited influence on the electrochemical performance. The microstructure pictures of sintered type B cell after tested were shown in Fig. 4. The thickness of anode functional layer and YSZ electrolyte was about 25 and 20 mm, respectively. The ESB porous interlayer was about 6 mm thick, and the SSC-ESB cathode was about 30 mm thick. Visually observation form the SEM picture, SSC-ESB cathode had a higher porosity than ESB interlayer.
Unsintered cells performance As shown in Fig. 5, the peak power density of type A cell was 0.246, 0.385, and 0.509 W cm2 at 600, 650, and 700 C, respectively, which was lower than that of sintered type A cell. The ohimc resistance of type A cell was 0.707, 0.439, and 0.354 U cm2 at 600, 650, and 700 C, respectively, and the total resistance was 1.780, 1.130, and 0.819 U cm2 at those three temperatures, respectively, as EIS shown in Fig. 6. It has been widely reported that the oxygen conductivity of ESB would decrease during later annealing [8,26,27]. So, the performance of solid oxide fuel cell accepting stabilized Bi2O3 material will benefit from less heating procedure. The ohmic resistance
was higher but the total resistance was smaller than that of sintered cell, indicating that the polarization of type A cell increased after sintering. Although sintered SSC-ESB cathode had a better connection between particles which more sturdy paths for both electrons and oxygen ions conduction, the degradation caused by sintering was dominated. Different with type A cells, unsintered type B cell had a much higher performance than sintered type B cell. The peak power density was 0.505, 0.861, and 1.329 W cm2 at 600, 650, and 700 C, respectively. The ohmic resistance was 0.328, 0.173, and 0.104 U cm2 at 600, 650, and 700 C, respectively, and the total resistance was 1.184, 0.780, and 0.548 U cm2 at these three temperatures, respectively. For a more intuitive look, the ohmic (Ro), polarization (Rp), and total (R) resistance of these four different cells was shown in Fig. 7. The unsintered type A cell had the biggest Ro at 600e700 C, and the Ro of other three cells was in a sequence of sintered type A > sintered type B > unsintered type B. It could be concluded that the interface resistance between unsintered SSC-ESB cathode and YSZ electrolyte was rather high. Porous ESB interlayer might improve the oxygen ions transportation from YSZ electrolyte to SSC-ESB cathode by acting a bridge, thus the interface resistance between YSZ electrolyte and SSC-ESB cathode was eliminated and the ohmic resistance was decreased. While, Rp of these four cells was in a sequence of sintered type A > sintered type B > unsintered type A > unsintered type B at the temperature range of 600e700 C. With the increase of temperature, the difference of Rp between sintered and unsintered SSC-ESB cathode became smaller, especially at 700 C, which might due to the slowly sintering of SSC-ESB cathode during heating and testing. The
Fig. 4 e The microstructure pictures of sintered type B cell after tested: full cell (a), ESB porous interlayer (b), and SSC-ESB cathode (c). Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 5 e IeV and I-P curves of unsintered type A (a) and type B (b) cells at 500e700 C.
Fig. 6 e EIS of unsintered type A (a) and type B (b) cells at 500e700 C. unsintered SSC-ESB cathode had smaller polarization resistance than the sintered SSC-ESB cathode, but the unsintered SSC-ESB cathode had high interface resistance when contacting directly with YSZ electrolyte without porous ESB interlayer. No matter sintered or not, type B cell always has smaller ohmic and polarization resistance and higher I-P performance than that of type A cell.
Durability testing
Fig. 7 e The ohmic (Ro), polarization (Rp), and total (R) resistance of four kinds of cells.
At a current density of 200 mA cm2, the working voltage of sintered type A cell increased a little at first and then showed remarkable stability, as shown in Fig. 8a. The working voltage increase was contributed by the decreased polarization resistance, as shown in Fig. 8b. At a current density of 400 mA cm2, the working voltage of sintered type B cell appeared a very slow decreasing and the polarization resistance increased a little (Fig. 8c and d). With porous ESB interlayer, the sintered cell experienced a slow degradation during working test. Previous reports have demonstrated that the d-Bi2O3 would transform into a hexagonal phase during annealing [8]. Kruidhof et al. showed that the d phase (Bi2O3)1-x (Er2O3)x was metastable below 740 C, when x < 0.275; their
Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 8 e Working voltage curves (a, c) and EIS (b, d) of sintered cells (type A: a & b, type B: c & d) at 600 C.
sample was found to gradually transform to the hexagonal phase during long term annealing at 650 C [28]. Heating experience would accelerate the degradation speed of ESB. In order to less the heating experience of ESB material, unsintered type B cell was fabricated and it showed reasonable high performance, but the durability of unsintered type B cell was still unclear. An unsintered type B cell was heated to 500 C for 2 h to reduce anode and then directly heated to 600 C for current static testing, the results were shown in Fig. 9.
The degradation speed was about 4.875 mV h1 and 1.135 mV h1 in the first 24 h and the later 96 h, respectively. The cell's ohmic resistance deceased a little in the first 12 h and then increased slowly but still smaller than the original value. After the early 48 h, the polarization resistance increased very slowly as well as the peak power density. The degradation was mainly due to the annealing of ESB in both interlayer and cathode, which was accelerated by released heat under static current testing.
Fig. 9 e Durability testing of unsintered type B cell at 600 C: working voltage at a current density of 300 mA cm¡2 (a), eis (b), and I-V and I-P curves (c). Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Conclusion [11]
ESB and SSC powders were well synthesized through a citrate process. Four kinds of cells with SSC-ESB cathode were fabricated and distinguished based on whether they were sintered and whether they had porous ESB interlayer. The unsintered cell had a smaller polarization resistance than the sintered cell, and the cell with porous ESB interlayer had a smaller ohmic resistance than the cell without porous ESB interlayer. Especially, the electrochemical performance of the unsintered cell was dramatically enhanced by porous ESB interlayer. A peak power density of 0.505, 0.861, and 1.329 W cm2 was achieved at 600, 650, and 700 C, respectively. The electrochemical performance of unsintered cell with porous ESB interlayer degraded slowly during durability testing, which was mainly due to the annealing of ESB.
Acknowledgments This research was financially supported by National Natural Science Foundation of China (51702109, U1601207) and National Key Research & Development Project-International Cooperation Program (2016YFE0126900). The SEM and XRD characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
references
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High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer Nanqi Duan, Jiyang Ma, Jin Li, Dong Yan, Bo Chi*, Jian Pu, Jian Li Center for Fuel Cell Innovation, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
article info
abstract
Article history:
Dense bismuth oxides stabilized with lanthanide dopants (d-Bi2O3) are always used as
Received 10 January 2018
interlayer for thin layer GDC (gadolinia-doped ceria) or YSZ (yttria-stabilized zirconia) elec-
Received in revised form
trolyte solid oxide fuel cell to improve the cell performance. Dense ESB (Bi1.6Er0.4O3) layer
2 March 2018
preparation needs special equipment or well synthesized nano-sized powders, which is not
Accepted 22 March 2018
friendly for large scale and commercial application. In this paper, anode support unsintered
Available online xxx
cathode cell with porous ESB interlayer is prepared through simple screen print and its electrochemical performance is tested. No matter sintered or not, porous ESB interlayer
Keywords:
lowers both the polarization resistance and ohmic resistance, and enhances cell perfor-
Bi2O3-Er2O3
mance, especially for the ohmic resistance of unsintered cells. The cell with ESB porous
Unsintered cathode
interlayer and SSC (Sm0.5Sr0.5CoO3-d)-ESB unsintered cathode achieves a peak power density
Solid oxide fuel cell
of 1.329 W cm2 at 700 C, which is 1.61 times higher than that of the cell without ESB
Interlayer
interlayer and 0.93 times higher than that of the sintered cell. However, the durability of
Durability
porous ESB interlayer enhanced cells is not ideal and it should be improved in further works. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cell (SOFC), consisting of porous anode and cathode separated by a dense electrolyte, is an electrochemical and environmental friendly device, which efficiently converts the chemical energy of fossil and hydrocarbon fuels into electricity and heat without combustion and mechanical motion involved [1]. The operating temperature of primary thick electrolyte-supported SOFCs is near 1000 C, which has been lowered to the intermediate-temperature range of 600e800 C by accepting anode-supported configuration, which naturally come with the benefits in more extensive
materials selection, higher materials stability, better performance durability, and lower manufacturing cost [2,3]. Yttria-stabilized zirconia (YSZ) is the most widely used electrolyte material for both commercial products and laboratory scale researches, which benefits from its high stability at a wide range of oxygen partial pressure [4]. Other electrolyte materials, such as Gadolinia-doped Ceria (GDC) and Bismuth oxides stabilized with lanthanide dopants (d-Bi2O3, e.g., Bi1.6Er0.4O3 (ESB)), have higher oxygen ion conductivity but worse stability at low oxygen partial atmosphere [5,6]. At a typical anode atmosphere, pure H2, GDC is partially reduced and then gets certain electronic conductivity leading to a lower OCV (open circuit voltage) [7], while ESB is easily
* Corresponding author. E-mail address: [email protected] (B. Chi). https://doi.org/10.1016/j.ijhydene.2018.03.168 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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reduced leading to the birth of metal Bi [5,8]. Compared with YSZ, GDC has a better chemical compatibility with many perovskite cathode materials, such as La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF). GDC was usually used as the barrier layer between YSZ electrolyte and cathode to prevent the unexpected interactions [9,10]. At most conditions, the GDC barrier layer, usually prepared by screen print, was porous, because it's expensive to prepare dense GDC thin layer on YSZ surface. What's more, it has been reported that the inter-diffusion between doped-ceria and stabilized zirconia during high temperature sintering (over 1200 C) results in insulating phases, thus significantly increasing ohmic resistance by at least 1 order of magnitude compared to that of pure YSZ [11]. Bismuth oxide stabilized with lanthanide dopants (d-Bi2O3) has been known as a promising electrolyte material with higher oxygen ions conductivity than both GDC and YSZ, but its application is limited by its weak tolerance of reduction atmosphere. So, different with GDC, dense ESB is always prepared between YSZ or GDC electrolyte and cathode constituting a bilayer electrolytes SOFC [12]. Wachsman et al. pioneered bismuth oxide/ceria bilayer electrolytes SOFCs: ceria electrolyte at the fuel side can shield the bismuth oxide from the reduction atmosphere and maintain the chemical stability; at the same time ESB will block the electrons transportation through GDC layer; moreover, total area specific resistance is decreased [13e15]. Joh et al. [16] replaced GDC with YSZ and used LSM (La0.8Sr0.2MnO3d)-ESB as cathode, and a higher OCV than that of GDC electrolyte condition and a peak power density of 2.08 W cm2 at 700 C was achieved. But the establishment of such dense ESB layer requires special equipment, such as pulsed laser deposition (PLD) [12], or nano-sized ESB powers prepared by special method, such as wet chemical co-precipitation [16]. For cell with ESB interlayer, cathode was usually prepared through the strategy of compositing with Bi2O3 based oxygen ions conductor, such as Er0.4Bi1.6O3 decorated La0.76Sr0.19MnO3þd [17], (La0.8Sr0.2)0.95MnO3-d-(Er0.2Bi0.8)2O3 [18], La0.76Sr0.19MnO3þd decorated Er0.4Bi1.6O3 [19], Sr doped LaMnO3 (LSM)-Y & Ce codoped Bi2O3 (BYC7) [20], and (Bi2O3)0.7(Er2O3)0.3-Ag [21]. These composite cathode cells achieved reasonable performance at temperature below 750 C and partial materials also showed good durability performance. However, bare work about the function of a porous ESB layer has been reported. It is still not clear whether a porous ESB layer has the same ability to improve electrochemical performance. Therefore, in this paper, a solid oxide fuel cell with porous ESB (Bi1.6Er0.4O3) interlayer between the dense YSZ electrolyte and unsintered cathode was fabricated by a low cost and easy prepared screen print method. Moreover, novel SSC (Sm0.5Sr0.5CoO3-d)-ESB composite was used as the cathode for solid oxide fuel cell.
Experimental ESB and SSC powers preparation ESB powers were synthesized through a citrate process (CP). Stoichiometric amounts of Er(NO3)3$5H2O (99.9%, Sinopharm)
and Bi(NO3)3$5H2O (99.9%, Sinopharm) were added into distilled water contained in a Pyrex container, to which ethylene glycol and citric acid were added. This solution was slowly heated to 80 C for 2 h and white precipitates appeared, and then it was dried at 200 C for more than 10 h to get the finally white-yellow ash, which at last was calcined in air at 700 C for 5 h to form the fluorite structure. SSC powders were synthesized through a similar process using corresponding metal nitrates. The difference was that no precipitate appeared in the solution at 80 C and the final calcine temperature was 900 C.
Cells preparation The anode substrate of the cell was prepared by tape casting technique and the details can be found in previous reports [22,23]. A mixture of nickel oxide (NiO) powders (Type A, Inco.) and YSZ powders (TZ-8YS, Tosoh) with a mass ratio of 57:43, adding fish oil as dispersant, polyvinyl butyral as organic binders, and polyaleneglycol as plasticizer, was ball milled for 36 h to from a uniform slurry with toluene and ethanol as solvent. Then the slurry was collected to form band shape by tape casting process. After dried at room temperature for more than 24 h, the green substrate was cut into circular samples with target size. At last, the anode functional layer and electrolyte layer was screen printed on the substrate in sequence and co-sintered at 1400 C for 4 h. ESB slurry with a weight ratio of 60% was screen printed on the surface of YSZ electrolyte, after dried at 80 C for 6 h, SSCESB composite cathode slurry with the same weight percentage was screen printed on the surface of ESB interlayer, which repeated twice. For the cell without porous ESB interlayer, SSC-ESB composite was directly screen printed on the surface of YSZ. At last, partial cells were sintered at 750 C for 2 h to decompose the organics.
Characterizations A Solartron 1260 frequency response analyzer and a Solartron 1287 electrochemical interface (Solartron Analytical) were employed to measure the impedance of the cell in a frequency range between 1000 kHz and 0.1 Hz with a signal amplitude of 20 mV at open circuit, as well as the current-voltage (I-V), current-power (I-P) and time-voltage curves. The X-ray curves of ESB and SSC were examined by X-ray diffraction (XRD) (X'Pert PRO, PANalytical B.V.) The cell microstructure was examined by a scanning electron microscope (SEM, Sirion 200 and Quanta 200, FEI).
Results and discussion Citrate process has been known as a mature and widely used method to synthesize nano-sized and micron-sized powders. As shown by the X-ray spectra of ESB and SSC in Fig. 1, both powders were well fitted with previous reported results [20,24,25]. ESB and SSC mix powders with a weight ratio of 1:1 were well manual grinded for more than 2 h to form a uniform cathode slurry. Two type cells were fabricated: type A without porous ESB interlayer and type B with porous ESB interlayer. A
Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 1 e The X-ray spectra of synthesized ESB and SSC powders.
part of both type cells were sintered at 750 C for 2 h before testing, which called cathode-sintered cells; the left cells, called cathode-unsintered cells, were directly tested without any sintering procedure before.
Sintered cells performance A Ceramabond glass sealant (Aremco Product, Inc.) was used to seal the anode compartment to an alumina tube. Pt paste was used as current collector for both anode and cathode and silver wires were used as the measuring lead. From room temperature to 500 C, a heating rate of 5 C min1 was accepted, during that process, 5 mol % H2-N2 with a flux of 50 ml min1 was inlet into anode side to slowly and partially reduce anode. Then, wet H2 (3 mol % H2O) with a flux of 50 ml min1 was used to reduce nickel oxide to nickel at 500 C
for 0.5 h as well as the fuel gas for the later performance testing. The cathode was naked in static air for all the heating and testing procedures. I-V and I-P curves of sintered cells at the temperature range of 500e700 C were shown in Fig. 2. The OCV of both type cells were above 1.1 V at 500e700 C, specifically, it was about 1.127 V, 1.119 V, and 1.112 V at 600, 650, and 700 C, respectively. The peak power density of type B cell was only a bit lower than that of type A cell below 600 C. The peak power density of type B cell was 0.309 and 0.688 W cm2 at 650 and 700 C, respectively, which was higher than 0.292 and 0.583 W cm2 of type A cell. The EIS (electrochemical impedance spectra) of two type cells at the temperature range of 500e700 C were shown in Fig. 3. Both cells had very closed ohmic resistance; the specific value of that for type B cell was 0.494, 0.275, and 0.163 U cm2 at 600, 650, and 700 C, respectively. Without porous ESB
Fig. 2 e IeV and I-P curves of sintered type A (a) and type B (b) cells at 500e700 C. Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 3 e Electrochemical impedance spectra of sintered type A (a) and type B (b) cells at 500e700 C.
interlayer, the ohmic resistance of type A cell was 0.523, 320, and 0.187 U cm2 at these three temperatures, respectively, which was only a little higher than that of type B cell. The total resistance of type A cell was 5.140, 2.106, and 0.835 U cm2 at 600, 650, and 700 C, respectively, which was higher than that of type B cell. Enhanced by porous ESB interlayer, the total resistance of type B cell was 2.973, 1.392, and 0.720 U cm2 at 600, 650, and 700 C, respectively. After sintered at 750 C, the porous ESB interlayer seemed have very limited influence on the electrochemical performance. The microstructure pictures of sintered type B cell after tested were shown in Fig. 4. The thickness of anode functional layer and YSZ electrolyte was about 25 and 20 mm, respectively. The ESB porous interlayer was about 6 mm thick, and the SSC-ESB cathode was about 30 mm thick. Visually observation form the SEM picture, SSC-ESB cathode had a higher porosity than ESB interlayer.
Unsintered cells performance As shown in Fig. 5, the peak power density of type A cell was 0.246, 0.385, and 0.509 W cm2 at 600, 650, and 700 C, respectively, which was lower than that of sintered type A cell. The ohimc resistance of type A cell was 0.707, 0.439, and 0.354 U cm2 at 600, 650, and 700 C, respectively, and the total resistance was 1.780, 1.130, and 0.819 U cm2 at those three temperatures, respectively, as EIS shown in Fig. 6. It has been widely reported that the oxygen conductivity of ESB would decrease during later annealing [8,26,27]. So, the performance of solid oxide fuel cell accepting stabilized Bi2O3 material will benefit from less heating procedure. The ohmic resistance
was higher but the total resistance was smaller than that of sintered cell, indicating that the polarization of type A cell increased after sintering. Although sintered SSC-ESB cathode had a better connection between particles which more sturdy paths for both electrons and oxygen ions conduction, the degradation caused by sintering was dominated. Different with type A cells, unsintered type B cell had a much higher performance than sintered type B cell. The peak power density was 0.505, 0.861, and 1.329 W cm2 at 600, 650, and 700 C, respectively. The ohmic resistance was 0.328, 0.173, and 0.104 U cm2 at 600, 650, and 700 C, respectively, and the total resistance was 1.184, 0.780, and 0.548 U cm2 at these three temperatures, respectively. For a more intuitive look, the ohmic (Ro), polarization (Rp), and total (R) resistance of these four different cells was shown in Fig. 7. The unsintered type A cell had the biggest Ro at 600e700 C, and the Ro of other three cells was in a sequence of sintered type A > sintered type B > unsintered type B. It could be concluded that the interface resistance between unsintered SSC-ESB cathode and YSZ electrolyte was rather high. Porous ESB interlayer might improve the oxygen ions transportation from YSZ electrolyte to SSC-ESB cathode by acting a bridge, thus the interface resistance between YSZ electrolyte and SSC-ESB cathode was eliminated and the ohmic resistance was decreased. While, Rp of these four cells was in a sequence of sintered type A > sintered type B > unsintered type A > unsintered type B at the temperature range of 600e700 C. With the increase of temperature, the difference of Rp between sintered and unsintered SSC-ESB cathode became smaller, especially at 700 C, which might due to the slowly sintering of SSC-ESB cathode during heating and testing. The
Fig. 4 e The microstructure pictures of sintered type B cell after tested: full cell (a), ESB porous interlayer (b), and SSC-ESB cathode (c). Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 5 e IeV and I-P curves of unsintered type A (a) and type B (b) cells at 500e700 C.
Fig. 6 e EIS of unsintered type A (a) and type B (b) cells at 500e700 C. unsintered SSC-ESB cathode had smaller polarization resistance than the sintered SSC-ESB cathode, but the unsintered SSC-ESB cathode had high interface resistance when contacting directly with YSZ electrolyte without porous ESB interlayer. No matter sintered or not, type B cell always has smaller ohmic and polarization resistance and higher I-P performance than that of type A cell.
Durability testing
Fig. 7 e The ohmic (Ro), polarization (Rp), and total (R) resistance of four kinds of cells.
At a current density of 200 mA cm2, the working voltage of sintered type A cell increased a little at first and then showed remarkable stability, as shown in Fig. 8a. The working voltage increase was contributed by the decreased polarization resistance, as shown in Fig. 8b. At a current density of 400 mA cm2, the working voltage of sintered type B cell appeared a very slow decreasing and the polarization resistance increased a little (Fig. 8c and d). With porous ESB interlayer, the sintered cell experienced a slow degradation during working test. Previous reports have demonstrated that the d-Bi2O3 would transform into a hexagonal phase during annealing [8]. Kruidhof et al. showed that the d phase (Bi2O3)1-x (Er2O3)x was metastable below 740 C, when x < 0.275; their
Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Fig. 8 e Working voltage curves (a, c) and EIS (b, d) of sintered cells (type A: a & b, type B: c & d) at 600 C.
sample was found to gradually transform to the hexagonal phase during long term annealing at 650 C [28]. Heating experience would accelerate the degradation speed of ESB. In order to less the heating experience of ESB material, unsintered type B cell was fabricated and it showed reasonable high performance, but the durability of unsintered type B cell was still unclear. An unsintered type B cell was heated to 500 C for 2 h to reduce anode and then directly heated to 600 C for current static testing, the results were shown in Fig. 9.
The degradation speed was about 4.875 mV h1 and 1.135 mV h1 in the first 24 h and the later 96 h, respectively. The cell's ohmic resistance deceased a little in the first 12 h and then increased slowly but still smaller than the original value. After the early 48 h, the polarization resistance increased very slowly as well as the peak power density. The degradation was mainly due to the annealing of ESB in both interlayer and cathode, which was accelerated by released heat under static current testing.
Fig. 9 e Durability testing of unsintered type B cell at 600 C: working voltage at a current density of 300 mA cm¡2 (a), eis (b), and I-V and I-P curves (c). Please cite this article in press as: Duan N, et al., High performance cathode-unsintered solid oxide fuel cell enhanced by porous Bi1.6Er0.4O3 (ESB) interlayer, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.168
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Conclusion [11]
ESB and SSC powders were well synthesized through a citrate process. Four kinds of cells with SSC-ESB cathode were fabricated and distinguished based on whether they were sintered and whether they had porous ESB interlayer. The unsintered cell had a smaller polarization resistance than the sintered cell, and the cell with porous ESB interlayer had a smaller ohmic resistance than the cell without porous ESB interlayer. Especially, the electrochemical performance of the unsintered cell was dramatically enhanced by porous ESB interlayer. A peak power density of 0.505, 0.861, and 1.329 W cm2 was achieved at 600, 650, and 700 C, respectively. The electrochemical performance of unsintered cell with porous ESB interlayer degraded slowly during durability testing, which was mainly due to the annealing of ESB.
Acknowledgments This research was financially supported by National Natural Science Foundation of China (51702109, U1601207) and National Key Research & Development Project-International Cooperation Program (2016YFE0126900). The SEM and XRD characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
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