- Email: [email protected]
Study of CaZr0.9In0.1O3−δ based reversible solid oxide cells with tubular electrode supported structure
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 4 2 ( 2 0 1 7 ) 2 3 1 8 9 e2 3 1 9 7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Study of CaZr0.9In0.1O3¡d based reversible solid oxide cells with tubular electrode supported structure Xiao-Feng Ye a, Y.B. Wen a, S.J. Yang a, Y. Lu a, W.H. Luo b, Z.Y. Wen a,*, J.B. Meng b,** a
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Dingxi Road, Shanghai 200050, PR China b China Academy of Engineering Physics, 64 Mianshan Road, Mianyang, Sichuan 621900, PR China
article info
abstract
Article history:
In recent years, the Reversible Solid Oxide Cells (RSOCs) are regarded as direct energy
Received 19 May 2017
converters between hydrogen and electricity. The proton conducting oxides proposed as
Received in revised form
electrolyte materials for RSOCs have several advantages. Perovskite type oxides CaZ-
21 July 2017
r0.9In0.1O3-d (CZI) are known as high temperature proton conductors with high chemical
Accepted 24 July 2017
stability and mechanical properties. In this paper, Ni-CZI electrode supported tubular
Available online 18 August 2017
single cells with CZI thin film electrolyte are fabricated for the first time. Composite air
Keywords:
applied to improve the electrochemical performance. Three single cells with different LSC
Reversible solid oxide cells
impregnation loadings are tested and compared. The modified single cell achieves
Proton conducting
hydrogen production rate of 2.1 mL$min1cm2 at 1.3 V and 850 C. After the initial 18%
Electrode-supported
performance degradation, the cell performance almost kept constant in 3 SOEC-SOFC cy-
Tubular
cles. All these results show the potential application of CZI materials and electrode sup-
Impregnation
ported tubular structure for RSOCs.
electrodes with La0.6Sr0.4CoO3 (LSC) nano-particles impregnated into porous CZI matrix are
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered to be one of the leading candidates as alternative fuels. Potential large-scale hydrogen applications include the production of synthetic hydrocarbon fuels via the Fischer-Tropsch process, and direct use as transportation fuel in emerging hydrogen fuel cell vehicles. Currently, 96% of the commercial hydrogen is produced by conventional steam reforming and partial oxidation of abundant natural gas and
liquid hydrocarbons [1,2]. Therefore, large-scale hydrogen production without fossil fuel consumption and greenhouse gas emissions has become the key to achieving the “hydrogen economy” [3,4]. The high-temperature steam electrolysis using solid oxide cells offers a promising method for highly efficient hydrogen production because of the higher electrode activity and following lower polarization losses at high temperatures. Solid Oxide Electrolysis Cells (SOEC) could be an effective pathway from power to gas. Ideally, if the SOEC (electrical to chemical) and SOFC (chemical to electrical) can
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.Y. Wen), [email protected] (J.B. Meng). http://dx.doi.org/10.1016/j.ijhydene.2017.07.195 0360-3199/© 2017 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 4 2 ( 2 0 1 7 ) 2 3 1 8 9 e2 3 1 9 7
Fig. 1 e The flow chart of the fabrication process of tubular proton conducting RSOCs.
occur in the same device with high reversible efficiencies, i.e., reversible solid oxide cells (RSOCs), there can be significant overall cost benefits [5]. RSOCs can take the advantage of excess electrical capacity in the grid during off-peak hours to produce hydrogen and then utilize it later during period of high electrical power demand. Consequently, RSOC can be regarded as a reciprocal direct energy converter between hydrogen and electricity. Proton conducting oxides have been proposed as electrolyte materials for RSOCs due to several advantages such as easier water management and possible lower operation
temperatures [6]. The most intensely studied proton conducting materials during the past years are ABO3 (A ¼ Sr, Ba, Ca; B¼Ce, Zr) perovskite type oxides [7e9]. Among them, CaZr0.9In0.1O3-d (CZI) are known as high temperature proton conductors with high chemical stability and mechanical properties. A Japanese company TYK had developed a series of hydrogen analyzers for molten aluminum and commercialized them [10,11]. However, its proton conductivity is only 103 Scm1 at 850 C [12], which limits its application in fuel cells. Therefore, thin film electrolyte should be used to reduce the ohmic resistance for the CZI-based RSOCs. Dip coating,
Fig. 2 e The schematic drawing of the testing setup for the characterization of the single cells.
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Fig. 4 e Arrhenius plots of the conductivity of the CZI electrolyte in both wet hydrogen and wet air.
used to prepare the electrolyte slurry. The Pechini method, a sol-gel process, has been widely used to synthesis ceramic oxide composites at low temperatures. As for the cell structure, current research are mainly focused on the planar button cells [13,14] and their electrode materials, and only a few works had been done about the proton conducting tubular cells, which are supposed to have advantages such as easier sealing, high mechanical strength and higher thermal cycling ability [15e17]. The thermal expansion coefficient (TEC) of CZI materials is about 3 106 K1, which is much lower than that of common air electrode materials. Therefore, we made a porous CZI matrix and then impregnated electrode catalysts into it to form a composite air electrode, which can obtain good electrochemical performance and match the TEC between the electrolyte and the air electrode layers. In this paper, we reported for the first time the study of tubular electrode supported proton conducting RSOCs by
Fig. 3 e The XRD patterns of the CZI powders fabricated by sol-gel method (a), solid reaction method (b) and treated in wet air for different time (c).
also known as slurry coating, is a conventional simple, fast and cost-effective wet chemical method used for the fabrication of ceramic membrane. In order to obtain dense electrolyte film, nano-CZI powders fabricated by Pechini methods were
Fig. 5 e The linear shrinkage curves of L-CZI, S-CZI and NiO/S-CZI pellets.
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fabricating Ni-CZI electrode supported CZI electrolyte composite tube using isostatic pressing, dip-coating and cosintering technologies. Then CZI-LSC composite electrode was fabricated by dip-coating and wet-impregnation methods. Single cells with electrode support and tubular structure were fabricated and reversibly operated as fuel cell and as electrolysis cell for 6 cycles. The modified single cell achieved good electrochemical performance and cycle stability. The possible issues to improve the cell performance were also considered. These results demonstrate the potential application of CZI materials in RSOCs for energy storage.
Experimental Synthesis and characterization of materials The CaZr0.9In0.1O3 material used in the anode support was synthesized from the raw materials CaCO3 (99.9%), ZrO2(99.9%) and In2O3 (99.9%) by solid-state reaction method. All of these oxides and carbonates used in the synthesis were supplied by Shanghai Chem.Ltd., China. The powders were mixed in stoichiometric proportions and ball-milled with ethanol for 3 h. The dried mixture was sintered at 1250 C for 10 h in air to decompose the carbonate and form the perovskite phase. After smashing, ball-milling, and sieving, CZI
powder was finally obtained, which was denoted as S-CZI in this paper. The CaZr0.9In0.1O3 material used in the electrolyte was fabricated by the citric method as follows. An aqueous solution containing all required ions as metal nitrates (Ca(NO3)2$3H2O, Zr(NO3)4$5H2O and In(NO3)3$3H2O) was first stirred at room temperature, to which an equivalent of citric acid (C6H8O7$H2O) of per total metals was then added. The solution was stirred for 30 min and held at boiling temperature for 60 min to evaporate water, resulting in fine ash of black color. The resultant ash was then fired at 800e1100 C for 5 h to form the CZI powders, which were denoted as LCZI in this paper. The phases of S-CZI and L-CZI powders were identified by means of a Rigaku XRD diffractometer at room temperature, using monochromatic Cu-Ka radiation. S-CZI powders preheated at 1350 C for 10 h were put in the wet air atmosphere at 850 C and then characterized by XRD analysis to ensure their chemical stability. CZI electrolyte pellet with diameter of 2 cm and thickness of 1 mm was dry-pressed and sintered at 1550 C for 10 h, and Pt paste was applied onto both sides of the pellet and sintered at 800 C to fabricate the porous electrodes. Electrochemical impedance spectra (EIS) of the CZI electrolyte pellet in wet hydrogen and wet air were obtained using an Electrochemical Workstation (Autolab, Switzerland) in the temperature range of 600e850 C.
Fig. 6 e The photos of the tubular single cells during different steps of the fabrication process.
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 4 2 ( 2 0 1 7 ) 2 3 1 8 9 e2 3 1 9 7
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Fig. 7 e The cross-sectional SEM image of tubular proton conducting SOCs (a), the electrolyte film (b), the hydrogen electrode (c), the air electrode (d) and the EDX results (e) of the particles in air electrode.
These obtained S-CZI and L-CZI powders were respectively pressed into cylindrical samples with a diameter of 5 mm and a length of 10 mm. The sintering behavior and shrinkage of the samples was measured by means of a NETZSCH DIL 402 PC dilatometer. The measurements were carried out over the temperature range from 25 C to 1500 C at heating rate of 5 K min1.
Fabrication of the single cell The electrode-supported electrolyte composite tube is composed of a NiO-CZI supported layer and a CZI electrolyte layer. The tubes used in this work are fabricated by isostatic pressing, dip-coating and co-sintering procedures. The powders of NiO and S-CZI powders as well as pore formers were mixed and then isostatically pressed at 200 MPa into a green support tube. After pre-heating at 1000 C for 1 h, the L-CZI slurry was dip-coated onto the outer surface of the pre-heated tube. The composite tube was then sintered at 1450 C for 5 h to make the electrolyte dense. To prepare the composite air electrode of the cell, a slurry containing CZI powders with graphite pore formers (30 wt%) was then dip-coated onto the electrolyte surface and then sintered at 1300 C to form the porous CZI matrix. CZI-LSC composite electrode was obtained by impregnating a 4 M
aqueous metal nitrate solution into the porous CZI matrix, followed by calcinations at 900 C for 30 min. La(NO3)2, Sr(NO3)3$9H2O and Co(NO3)3$9H2O were used as metal precursors. The wet impregnation and calcinations treatment was repeated until the final LSC loading in the composite electrode is achieved. The whole fabrication process of the single cells is illustrated in Fig. 1. Three tubular single cells were fabricated with different LSC loadings of 30 wt%, 40 wt% and 50 wt%. The obtained single cells have diameters of 8 mm and lengths of 4 cm with the cathode effective area of 7.54 cm2.
Characterization of the RSOCs A schematic of the experimental setup used for single cells testing is presented in Fig. 2. It is composed of gas supply cylinders, mass-flow controllers, a furnace, a water bath, high temperature furnace, a single cell, sealant and an electrochemical workstation. Flow rates of nitrogen, hydrogen, and air input are established by means of precise mass-flow controllers. The water steam content in the air was controlled by the temperature of the water bath. Glass sealants were placed between the single cell and the alumina tube to prevent gas leakage. The current collector layers used in hydrogen and air sides were nickel foam and Ag mesh, which were respectively
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Fig. 8 e The I-V curves of the single cells between the fuel cell and steam electrolysis modes (a) with LSC loadings and operation temperatures (c) and their electrochemical impedance spectra under open circuit voltage (b, d).
attached to the electrodes surface using Pt paste. Pt lead wires and a four-probe configuration were adopted in the electrochemistry test. Hydrogen and air was adopted as the fuel and oxidant, respectively. The setup was put in a furnace and heated to 800 C for sealing. The hydrogen electrode was fully reduced in H2 atmosphere for several hours prior to cell testing, and the electrochemical tests were carried out at 800 C, 850 C and 900 C. The hydrogen and air flow rates were both set at 100 mL min1. In our test, the water steam content in the air was always controlled at 20% by controlling the water bath temperature. The current-voltage curves and EIS were obtained using an Electrochemical Workstation. The impedance spectra of the cells were recorded at open circuit voltage (OCV) over the frequency range from 500 kHz to 50 mHz with an excitation potential of 10 mV. The morphology of the single cells and their layers structure were examined by scanning electron microscopy (SEM, S-3400 N, JEOL Co. Ltd., Japan).
Results and discussion Materials characterization Fig. 3 shows the X-ray diffraction (XRD) patterns of L-CZI powders (a), S-CZI powders (b) preheated at different temperatures and S-CZI powders treated at high temperature in
wet air for 200 h (c). It indicated the formation of the perovskite phase of the CaZr0.9In0.1O3 powder even after preheating at 800 C except for a little ZrO2 phase while using sol-gel technologies. The ZrO2 phase intensity decreased 50% when pre-heating temperature increased to 900 C. The intensity continued to decrease with increasing temperature and totally disappeared in samples calcinated at 1100 C. For S-CZI powders, there is still a little CaIn2O4 hase after sintering at 1250 C for 10 h. Although the CaIn2O4 phase disappears after sintering above 1350 C, the obtained powders will have low sintering activity. To make sure that the electrolyte film is dense enough, we should use the L-CZI powders to prepare the electrolyte slurry. Fig. 3 (c) indicates the good chemical stability of CZI materials in wet air atmosphere, in which no apparent new phase was observed after 200 h. The conductivity of CZI in wet hydrogen and wet air were measured and calculated, and the results were shown in Fig. 4. The measured conductivity of CZI in hydrogen and air at 850 C is 1.20 103 S cm1 and 0.85 103 S cm1, respectively. According to the Arrhenius curves in Fig. 4, the activation energy of CZI in hydrogen and air is 0.79 eV and 1.02 eV, respectively. These results are very close to the values in previous report [18], which also indicates the proton conducting characteristics of CZI materials. It is observed form Fig. 5 that both the NiO/S-CZI pellet and L-CZI pellet start to shrink from about 900 C. The shrinking rate of NiO/S-CZI pellet is larger than that of the L-CZI pellet, which is probably due to the high sintering activity of fine
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 4 2 ( 2 0 1 7 ) 2 3 1 8 9 e2 3 1 9 7
nickel powders. However, it is noticed that the final shrinkage percentage of both pellets is about 13%. TEC and linear shrinkage percentage matching of the supported electrode and the electrolyte layer has been achieved, which makes it possible to fabricate a NiO-CZI/CZI composite structure.
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800 Ce900 C for the single cell with 50 wt% LSC loading. The open circuit voltage (OCV) of the cell is 0.972 V, 0.951 V and 0.918 V, which are close to the theoretical values calculated from Nernst formula and decreased with the operating temperature. In the electrolysis mode, the current densities of the cells are 182, 300 and 480 mA cm2 at 1.3 V, respectively.
Morphology of the single cells The photographs of the tubular cell during the fabrication process are shown in Fig. 6. A tubular single cell with electrode-supported structure was successfully fabricated with no obvious deformation or cracks. The linear shrinkage percentage of the single cell is about 15% to make sure the electrolyte layer is dense, which is also consistent with the thermal behavior results mentioned above. Fig. 7 shows the cross-sectional SEM images of the Ni-CZI/ CZI/CZI-LSC single cell (a), the electrolyte layer (b), the hydrogen electrode (c) and the air electrode (d). As it can be seen in Fig. 7 (a), both a porous anode and a dense electrolyte are obtained, and the two layers have good contact without obvious segregation at the interface. A good contact can also be seen at the interface between the electrolyte and the cathode layer. The three layers (anode, electrolyte and cathode) have average thicknesses of about 1200, 15, and 40 mm, respectively. In the impregnated air electrode (d), we can see many LSC particles formed in the pore of the CZI matrix, and the particle size of the impregnated LSC is about 500 nm. The EDX results in Fig. 7 (e) can confirm that the small particles formed in the CZI matrix are LSC catalysts.
Electrochemical performance and stability Fig. 8 (a) shows the I-V curves in reversible cell operation between the fuel cell and steam electrolysis modes at 800 C for single cells with different LSC loadings. The open circuit voltages (OCV) of the cells are all around 0.97 V while the LSC loadings increased from 30% to 40% and 50%, which is close to the theoretical values calculated from Nernst formula. In the electrolysis mode, the current densities of the cells are 92, 170 and 182 mA cm2 at 1.3 V, respectively. The electrolysis performance apparently increases with the increasing LSC loading. The maximum power density in the SOFC mode is 101 mW cm2, which is lower than that of most oxygen ionic conducting fuel cells [19]. In an attempt to examine the electrochemical behaviors among the cells with different LSC loadings, we measured their impedance spectra under open circuit voltage. The polarization resistance (Rp) results in Fig. 8 (b) clearly show a decrease in the Rp with increasing LSC loading in the air electrode, which is consistent with the results of impregnated electrodes as reported in Refs. [20,21]. More impregnated LSC catalyst increases three phases' boundaries in the composite electrodes and improves the electrochemical performance as a result. It is also observed from the EIS in Fig. 8 (b) that the Rp (0.72 U cm2) of the single cell with 50 wt% LSC loading is much smaller than the Ro (2.43 U cm2) of the cell, indicating that the ohmic resistance of the cells is the main reason for the low cell performance. Fig. 8 (c) shows the I-V curves in reversible cell operation between the fuel cell and steam electrolysis modes at
Fig. 9 e The cell current density (a), the electrochemical impedance spectra curves (b) and the I-V curves (c) as a function of operation during SOFC/SOEC cycling for the single cell with 50 wt% LSC loading at 850 C.
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The EIS results in Fig. 8 (d) show that the ohmic resistance of the cells decrease from 2.44 U cm2 to 1.85 and 1.17 U cm2 with the increase of the operating temperature. Compared to the ohmic resistance, the polarization resistance of the single cell is much smaller. The conductivity of the CZI materials is not high enough to improve the cell performance, and thinner electrolyte layer or new materials with higher proton conductivity should be applied to reduce the ohmic resistance. A stability test of the single cell with 50 wt% LSC loading in reversible modes run under a constant voltage of 1.2 V and 0.8 V was carried out at 850 C, and the results are shown in Fig. 9. Three SOEC/SOFC cycles were operated in a period of 8 h. The cell current density decreased from 195 to 160 mA cm2 in the first 3 h in the electrolysis mode, which is probably due to the electrode sintering phenomena proved by the EIS results discussed later. After the initial 3 h, the cell current density almost remained constant, demonstrating the stability in both the electrolysis and the fuel cell modes. In an attempt to examine the reason for different cell performances in thermal cycles and SOFC/SOEC cycles, we measured the impedance spectra of the cell under open circuit voltage as a function of operating time. After the initial 3 h, the ohmic resistance increased by 9%, from 1.85 U cm2 to 2.01 U cm2, and the ohmic resistance increased slightly to 2.03 U cm2 after 3 SOFC-SOEC cycles in next 5 h. The I-V curves in Fig. 9 (c) also showed the similar degradation tendency. The degradation is possibly due to the change in anode microstructure resulted from nickel coarsening at high operation temperatures reported before [22], which is also illustrated by the largely grown nickel particles shown in Fig. 7 (c). However, the polarization resistance almost kept constant at 0.38 U cm2 in the total period. These results demonstrate the stability of CZI materials for the R-SOC applications.
Conclusions In this paper, Ni-CZI electrode supported CZI electrolyte tube was fabricated by isostatic pressing, dip-coating and cosintering technologies. Nano-CZI powders obtained by Pechini methods were used for preparing the coating slurry to make the electrolyte layer dense enough. Then CZI-LSC composite electrode was fabricated by dip-coating and wetimpregnation methods. The single cells with electrode support and tubular structure are reversibly operated as fuel cell and as electrolysis cell, which illustrate that air electrode with 50 wt% LSC loading showed the best electrochemical performance, especially for hydrogen production mode. The hydrogen production rate of the single cell achieved 2.1 mL$min1cm2 at 1.3 V and 850 C. The electrochemical performance remained almost constant after initial degradation probably due to the particle coarsening phenomena in the electrode. These results demonstrate the potential application of CZI materials and the fabrication technology of protonconducting tubular cell structure in RSOCs for energy storage. The thickness of the CZI electrolyte layer and the microstructure of the electrodes could be modified to improve the cell performance in our future work, and the fabrication methods can also be applied to other proton conducting materials with higher proton conductivity.
Acknowledgements The authors are grateful for the financial support from National Natural Science Foundation of China (51672294, 51302296) and the Science and Technology Commission of Shanghai Municipality (No. 15DZ2281200).
references
[1] Ramachandran R, Menon RK. An overview of industrial uses of hydrogen. Int J Hydrogen Energy 1999;23:593e8. [2] Ni M. Computational fluid dynamics modeling of a solid oxide electrolyzer cell for hydrogen production. Int J Hydrogen Energy 2009;34:7795e806. [3] Mingyi Liu, Bo Yu, Jingming Xu, Jing Chen. Thermodynamic analysis of the efficiency of high-temperature steam electrolysis system for hydrogen production. J Power Sources 2008;177:493e9. [4] Brisse A, Schefold J, Zahid M. High temperature water electrolysis in solid oxide cells. Int J Hydrogen Energy 2008;33:5375e82. [5] Marina OA, Pederson LR, Williams MC, Coffey GW, Meinhardt KD, Nguyen CD. Electrode performance in reversible solid oxide fuel cells. J Electrchem Soc 2007;154:B452e9. [6] Bi L, Boulfrad S, Traversa E. Steam electrolysis by solid oxide electrolysis cells (SOECs) with proton-conducting oxides. Chem Soc Rev 2014;43(24):8255e70. [7] Iwahara H, Esaka T, Uchida H, Maeda N. Proton conduction in sintered oxides and its application to steam electrolysis for hydrogen production. Solid State Ionics 1981;3:359e63. [8] Iwahara H, Uchida H, Ono K, Ogaki K. Proton conduction in sintered oxides based on BaCeO3. J Electrochem Soc 1988;135(2):529e33. [9] Mitsui A, Miyayama M, Yanagida H. Evaluation of the activation energy of proton conduction in perovskite-type oxides. Solid State Ionics 1987;22(2):213e7. [10] Yajima T, Iwahara H, Uchida H, Koide K. Relation between proton conduction and concentration of oxide ion vacancy in SrCeO3 based sintered oxides. Solid State Ionics 1990;40e41(4):914e7. [11] Yajima T, Koide K, Takai H, Fukatsu N, Iwahara H. Application of hydrogen sensor using proton conductive ceramics as a solid electrolyte to aluminum casting industries. Solid State Ionics 1995;79(C):333e7. [12] Zhang Jingchao, Wen Zhaoyin, Chi Xiaowei, Han Jinduo, Wu Xiangwei, Wen Tinglian. Proton conducting CaZr0.9In0.1O3d ceramic membrane prepared by tape casting. Solid State Ionics 2012;225:291e6. [13] Stuart PA, Unno Toshiyuki, Kilner JA, Skinner SJ. Solid oxide proton conducting steam electrolyzers. Solid State Ionics 2008;179(21e26):1120e4. [14] Gan Lizhen, Ye Lingting, Liu Mingzhou, Tao Shanwen, Xie Kui. A scandium-doped manganate anode for a protonconduction solid oxide steam electrolyzer. RSC Adv 2015;6(1):641e7. [15] Suzuki T, Yamaguchi T, Fujishiro Y, Awano M. Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature. J Power Sources 2006;160(1):73e7. [16] Kendall K. Progress in micro tubular solid oxide fuel cells. Int J Appl Ceram Technol 2010;7(1):1e9. [17] Lawlor V, Griesser S, Buchinger G, Olabi AG, Cordiner S, Meissner D. Review of the micro-tubular solid oxide fuel cell:
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Part I. Stack design issues and research activities. J Power Sources 2009;193(195):387e99. [18] Shao L, Wang S, Qian J, Xue Y, Liu R. Fabrication of cathodesupported tubular solid oxide electrolysis cell for high temperature steam electrolysis. J New Mater Electrochem Syst 2011;14(3):179e82. [19] Laguna-Bercero MA, Skinner SJ, Kilner JA. Performance of solid oxide electrolysis cells based on Scandia stabilized zirconia. J Power Sources 2009;192(1):126e31.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Study of CaZr0.9In0.1O3¡d based reversible solid oxide cells with tubular electrode supported structure Xiao-Feng Ye a, Y.B. Wen a, S.J. Yang a, Y. Lu a, W.H. Luo b, Z.Y. Wen a,*, J.B. Meng b,** a
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Dingxi Road, Shanghai 200050, PR China b China Academy of Engineering Physics, 64 Mianshan Road, Mianyang, Sichuan 621900, PR China
article info
abstract
Article history:
In recent years, the Reversible Solid Oxide Cells (RSOCs) are regarded as direct energy
Received 19 May 2017
converters between hydrogen and electricity. The proton conducting oxides proposed as
Received in revised form
electrolyte materials for RSOCs have several advantages. Perovskite type oxides CaZ-
21 July 2017
r0.9In0.1O3-d (CZI) are known as high temperature proton conductors with high chemical
Accepted 24 July 2017
stability and mechanical properties. In this paper, Ni-CZI electrode supported tubular
Available online 18 August 2017
single cells with CZI thin film electrolyte are fabricated for the first time. Composite air
Keywords:
applied to improve the electrochemical performance. Three single cells with different LSC
Reversible solid oxide cells
impregnation loadings are tested and compared. The modified single cell achieves
Proton conducting
hydrogen production rate of 2.1 mL$min1cm2 at 1.3 V and 850 C. After the initial 18%
Electrode-supported
performance degradation, the cell performance almost kept constant in 3 SOEC-SOFC cy-
Tubular
cles. All these results show the potential application of CZI materials and electrode sup-
Impregnation
ported tubular structure for RSOCs.
electrodes with La0.6Sr0.4CoO3 (LSC) nano-particles impregnated into porous CZI matrix are
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered to be one of the leading candidates as alternative fuels. Potential large-scale hydrogen applications include the production of synthetic hydrocarbon fuels via the Fischer-Tropsch process, and direct use as transportation fuel in emerging hydrogen fuel cell vehicles. Currently, 96% of the commercial hydrogen is produced by conventional steam reforming and partial oxidation of abundant natural gas and
liquid hydrocarbons [1,2]. Therefore, large-scale hydrogen production without fossil fuel consumption and greenhouse gas emissions has become the key to achieving the “hydrogen economy” [3,4]. The high-temperature steam electrolysis using solid oxide cells offers a promising method for highly efficient hydrogen production because of the higher electrode activity and following lower polarization losses at high temperatures. Solid Oxide Electrolysis Cells (SOEC) could be an effective pathway from power to gas. Ideally, if the SOEC (electrical to chemical) and SOFC (chemical to electrical) can
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.Y. Wen), [email protected] (J.B. Meng). http://dx.doi.org/10.1016/j.ijhydene.2017.07.195 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Fig. 1 e The flow chart of the fabrication process of tubular proton conducting RSOCs.
occur in the same device with high reversible efficiencies, i.e., reversible solid oxide cells (RSOCs), there can be significant overall cost benefits [5]. RSOCs can take the advantage of excess electrical capacity in the grid during off-peak hours to produce hydrogen and then utilize it later during period of high electrical power demand. Consequently, RSOC can be regarded as a reciprocal direct energy converter between hydrogen and electricity. Proton conducting oxides have been proposed as electrolyte materials for RSOCs due to several advantages such as easier water management and possible lower operation
temperatures [6]. The most intensely studied proton conducting materials during the past years are ABO3 (A ¼ Sr, Ba, Ca; B¼Ce, Zr) perovskite type oxides [7e9]. Among them, CaZr0.9In0.1O3-d (CZI) are known as high temperature proton conductors with high chemical stability and mechanical properties. A Japanese company TYK had developed a series of hydrogen analyzers for molten aluminum and commercialized them [10,11]. However, its proton conductivity is only 103 Scm1 at 850 C [12], which limits its application in fuel cells. Therefore, thin film electrolyte should be used to reduce the ohmic resistance for the CZI-based RSOCs. Dip coating,
Fig. 2 e The schematic drawing of the testing setup for the characterization of the single cells.
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Fig. 4 e Arrhenius plots of the conductivity of the CZI electrolyte in both wet hydrogen and wet air.
used to prepare the electrolyte slurry. The Pechini method, a sol-gel process, has been widely used to synthesis ceramic oxide composites at low temperatures. As for the cell structure, current research are mainly focused on the planar button cells [13,14] and their electrode materials, and only a few works had been done about the proton conducting tubular cells, which are supposed to have advantages such as easier sealing, high mechanical strength and higher thermal cycling ability [15e17]. The thermal expansion coefficient (TEC) of CZI materials is about 3 106 K1, which is much lower than that of common air electrode materials. Therefore, we made a porous CZI matrix and then impregnated electrode catalysts into it to form a composite air electrode, which can obtain good electrochemical performance and match the TEC between the electrolyte and the air electrode layers. In this paper, we reported for the first time the study of tubular electrode supported proton conducting RSOCs by
Fig. 3 e The XRD patterns of the CZI powders fabricated by sol-gel method (a), solid reaction method (b) and treated in wet air for different time (c).
also known as slurry coating, is a conventional simple, fast and cost-effective wet chemical method used for the fabrication of ceramic membrane. In order to obtain dense electrolyte film, nano-CZI powders fabricated by Pechini methods were
Fig. 5 e The linear shrinkage curves of L-CZI, S-CZI and NiO/S-CZI pellets.
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fabricating Ni-CZI electrode supported CZI electrolyte composite tube using isostatic pressing, dip-coating and cosintering technologies. Then CZI-LSC composite electrode was fabricated by dip-coating and wet-impregnation methods. Single cells with electrode support and tubular structure were fabricated and reversibly operated as fuel cell and as electrolysis cell for 6 cycles. The modified single cell achieved good electrochemical performance and cycle stability. The possible issues to improve the cell performance were also considered. These results demonstrate the potential application of CZI materials in RSOCs for energy storage.
Experimental Synthesis and characterization of materials The CaZr0.9In0.1O3 material used in the anode support was synthesized from the raw materials CaCO3 (99.9%), ZrO2(99.9%) and In2O3 (99.9%) by solid-state reaction method. All of these oxides and carbonates used in the synthesis were supplied by Shanghai Chem.Ltd., China. The powders were mixed in stoichiometric proportions and ball-milled with ethanol for 3 h. The dried mixture was sintered at 1250 C for 10 h in air to decompose the carbonate and form the perovskite phase. After smashing, ball-milling, and sieving, CZI
powder was finally obtained, which was denoted as S-CZI in this paper. The CaZr0.9In0.1O3 material used in the electrolyte was fabricated by the citric method as follows. An aqueous solution containing all required ions as metal nitrates (Ca(NO3)2$3H2O, Zr(NO3)4$5H2O and In(NO3)3$3H2O) was first stirred at room temperature, to which an equivalent of citric acid (C6H8O7$H2O) of per total metals was then added. The solution was stirred for 30 min and held at boiling temperature for 60 min to evaporate water, resulting in fine ash of black color. The resultant ash was then fired at 800e1100 C for 5 h to form the CZI powders, which were denoted as LCZI in this paper. The phases of S-CZI and L-CZI powders were identified by means of a Rigaku XRD diffractometer at room temperature, using monochromatic Cu-Ka radiation. S-CZI powders preheated at 1350 C for 10 h were put in the wet air atmosphere at 850 C and then characterized by XRD analysis to ensure their chemical stability. CZI electrolyte pellet with diameter of 2 cm and thickness of 1 mm was dry-pressed and sintered at 1550 C for 10 h, and Pt paste was applied onto both sides of the pellet and sintered at 800 C to fabricate the porous electrodes. Electrochemical impedance spectra (EIS) of the CZI electrolyte pellet in wet hydrogen and wet air were obtained using an Electrochemical Workstation (Autolab, Switzerland) in the temperature range of 600e850 C.
Fig. 6 e The photos of the tubular single cells during different steps of the fabrication process.
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Fig. 7 e The cross-sectional SEM image of tubular proton conducting SOCs (a), the electrolyte film (b), the hydrogen electrode (c), the air electrode (d) and the EDX results (e) of the particles in air electrode.
These obtained S-CZI and L-CZI powders were respectively pressed into cylindrical samples with a diameter of 5 mm and a length of 10 mm. The sintering behavior and shrinkage of the samples was measured by means of a NETZSCH DIL 402 PC dilatometer. The measurements were carried out over the temperature range from 25 C to 1500 C at heating rate of 5 K min1.
Fabrication of the single cell The electrode-supported electrolyte composite tube is composed of a NiO-CZI supported layer and a CZI electrolyte layer. The tubes used in this work are fabricated by isostatic pressing, dip-coating and co-sintering procedures. The powders of NiO and S-CZI powders as well as pore formers were mixed and then isostatically pressed at 200 MPa into a green support tube. After pre-heating at 1000 C for 1 h, the L-CZI slurry was dip-coated onto the outer surface of the pre-heated tube. The composite tube was then sintered at 1450 C for 5 h to make the electrolyte dense. To prepare the composite air electrode of the cell, a slurry containing CZI powders with graphite pore formers (30 wt%) was then dip-coated onto the electrolyte surface and then sintered at 1300 C to form the porous CZI matrix. CZI-LSC composite electrode was obtained by impregnating a 4 M
aqueous metal nitrate solution into the porous CZI matrix, followed by calcinations at 900 C for 30 min. La(NO3)2, Sr(NO3)3$9H2O and Co(NO3)3$9H2O were used as metal precursors. The wet impregnation and calcinations treatment was repeated until the final LSC loading in the composite electrode is achieved. The whole fabrication process of the single cells is illustrated in Fig. 1. Three tubular single cells were fabricated with different LSC loadings of 30 wt%, 40 wt% and 50 wt%. The obtained single cells have diameters of 8 mm and lengths of 4 cm with the cathode effective area of 7.54 cm2.
Characterization of the RSOCs A schematic of the experimental setup used for single cells testing is presented in Fig. 2. It is composed of gas supply cylinders, mass-flow controllers, a furnace, a water bath, high temperature furnace, a single cell, sealant and an electrochemical workstation. Flow rates of nitrogen, hydrogen, and air input are established by means of precise mass-flow controllers. The water steam content in the air was controlled by the temperature of the water bath. Glass sealants were placed between the single cell and the alumina tube to prevent gas leakage. The current collector layers used in hydrogen and air sides were nickel foam and Ag mesh, which were respectively
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Fig. 8 e The I-V curves of the single cells between the fuel cell and steam electrolysis modes (a) with LSC loadings and operation temperatures (c) and their electrochemical impedance spectra under open circuit voltage (b, d).
attached to the electrodes surface using Pt paste. Pt lead wires and a four-probe configuration were adopted in the electrochemistry test. Hydrogen and air was adopted as the fuel and oxidant, respectively. The setup was put in a furnace and heated to 800 C for sealing. The hydrogen electrode was fully reduced in H2 atmosphere for several hours prior to cell testing, and the electrochemical tests were carried out at 800 C, 850 C and 900 C. The hydrogen and air flow rates were both set at 100 mL min1. In our test, the water steam content in the air was always controlled at 20% by controlling the water bath temperature. The current-voltage curves and EIS were obtained using an Electrochemical Workstation. The impedance spectra of the cells were recorded at open circuit voltage (OCV) over the frequency range from 500 kHz to 50 mHz with an excitation potential of 10 mV. The morphology of the single cells and their layers structure were examined by scanning electron microscopy (SEM, S-3400 N, JEOL Co. Ltd., Japan).
Results and discussion Materials characterization Fig. 3 shows the X-ray diffraction (XRD) patterns of L-CZI powders (a), S-CZI powders (b) preheated at different temperatures and S-CZI powders treated at high temperature in
wet air for 200 h (c). It indicated the formation of the perovskite phase of the CaZr0.9In0.1O3 powder even after preheating at 800 C except for a little ZrO2 phase while using sol-gel technologies. The ZrO2 phase intensity decreased 50% when pre-heating temperature increased to 900 C. The intensity continued to decrease with increasing temperature and totally disappeared in samples calcinated at 1100 C. For S-CZI powders, there is still a little CaIn2O4 hase after sintering at 1250 C for 10 h. Although the CaIn2O4 phase disappears after sintering above 1350 C, the obtained powders will have low sintering activity. To make sure that the electrolyte film is dense enough, we should use the L-CZI powders to prepare the electrolyte slurry. Fig. 3 (c) indicates the good chemical stability of CZI materials in wet air atmosphere, in which no apparent new phase was observed after 200 h. The conductivity of CZI in wet hydrogen and wet air were measured and calculated, and the results were shown in Fig. 4. The measured conductivity of CZI in hydrogen and air at 850 C is 1.20 103 S cm1 and 0.85 103 S cm1, respectively. According to the Arrhenius curves in Fig. 4, the activation energy of CZI in hydrogen and air is 0.79 eV and 1.02 eV, respectively. These results are very close to the values in previous report [18], which also indicates the proton conducting characteristics of CZI materials. It is observed form Fig. 5 that both the NiO/S-CZI pellet and L-CZI pellet start to shrink from about 900 C. The shrinking rate of NiO/S-CZI pellet is larger than that of the L-CZI pellet, which is probably due to the high sintering activity of fine
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nickel powders. However, it is noticed that the final shrinkage percentage of both pellets is about 13%. TEC and linear shrinkage percentage matching of the supported electrode and the electrolyte layer has been achieved, which makes it possible to fabricate a NiO-CZI/CZI composite structure.
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800 Ce900 C for the single cell with 50 wt% LSC loading. The open circuit voltage (OCV) of the cell is 0.972 V, 0.951 V and 0.918 V, which are close to the theoretical values calculated from Nernst formula and decreased with the operating temperature. In the electrolysis mode, the current densities of the cells are 182, 300 and 480 mA cm2 at 1.3 V, respectively.
Morphology of the single cells The photographs of the tubular cell during the fabrication process are shown in Fig. 6. A tubular single cell with electrode-supported structure was successfully fabricated with no obvious deformation or cracks. The linear shrinkage percentage of the single cell is about 15% to make sure the electrolyte layer is dense, which is also consistent with the thermal behavior results mentioned above. Fig. 7 shows the cross-sectional SEM images of the Ni-CZI/ CZI/CZI-LSC single cell (a), the electrolyte layer (b), the hydrogen electrode (c) and the air electrode (d). As it can be seen in Fig. 7 (a), both a porous anode and a dense electrolyte are obtained, and the two layers have good contact without obvious segregation at the interface. A good contact can also be seen at the interface between the electrolyte and the cathode layer. The three layers (anode, electrolyte and cathode) have average thicknesses of about 1200, 15, and 40 mm, respectively. In the impregnated air electrode (d), we can see many LSC particles formed in the pore of the CZI matrix, and the particle size of the impregnated LSC is about 500 nm. The EDX results in Fig. 7 (e) can confirm that the small particles formed in the CZI matrix are LSC catalysts.
Electrochemical performance and stability Fig. 8 (a) shows the I-V curves in reversible cell operation between the fuel cell and steam electrolysis modes at 800 C for single cells with different LSC loadings. The open circuit voltages (OCV) of the cells are all around 0.97 V while the LSC loadings increased from 30% to 40% and 50%, which is close to the theoretical values calculated from Nernst formula. In the electrolysis mode, the current densities of the cells are 92, 170 and 182 mA cm2 at 1.3 V, respectively. The electrolysis performance apparently increases with the increasing LSC loading. The maximum power density in the SOFC mode is 101 mW cm2, which is lower than that of most oxygen ionic conducting fuel cells [19]. In an attempt to examine the electrochemical behaviors among the cells with different LSC loadings, we measured their impedance spectra under open circuit voltage. The polarization resistance (Rp) results in Fig. 8 (b) clearly show a decrease in the Rp with increasing LSC loading in the air electrode, which is consistent with the results of impregnated electrodes as reported in Refs. [20,21]. More impregnated LSC catalyst increases three phases' boundaries in the composite electrodes and improves the electrochemical performance as a result. It is also observed from the EIS in Fig. 8 (b) that the Rp (0.72 U cm2) of the single cell with 50 wt% LSC loading is much smaller than the Ro (2.43 U cm2) of the cell, indicating that the ohmic resistance of the cells is the main reason for the low cell performance. Fig. 8 (c) shows the I-V curves in reversible cell operation between the fuel cell and steam electrolysis modes at
Fig. 9 e The cell current density (a), the electrochemical impedance spectra curves (b) and the I-V curves (c) as a function of operation during SOFC/SOEC cycling for the single cell with 50 wt% LSC loading at 850 C.
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The EIS results in Fig. 8 (d) show that the ohmic resistance of the cells decrease from 2.44 U cm2 to 1.85 and 1.17 U cm2 with the increase of the operating temperature. Compared to the ohmic resistance, the polarization resistance of the single cell is much smaller. The conductivity of the CZI materials is not high enough to improve the cell performance, and thinner electrolyte layer or new materials with higher proton conductivity should be applied to reduce the ohmic resistance. A stability test of the single cell with 50 wt% LSC loading in reversible modes run under a constant voltage of 1.2 V and 0.8 V was carried out at 850 C, and the results are shown in Fig. 9. Three SOEC/SOFC cycles were operated in a period of 8 h. The cell current density decreased from 195 to 160 mA cm2 in the first 3 h in the electrolysis mode, which is probably due to the electrode sintering phenomena proved by the EIS results discussed later. After the initial 3 h, the cell current density almost remained constant, demonstrating the stability in both the electrolysis and the fuel cell modes. In an attempt to examine the reason for different cell performances in thermal cycles and SOFC/SOEC cycles, we measured the impedance spectra of the cell under open circuit voltage as a function of operating time. After the initial 3 h, the ohmic resistance increased by 9%, from 1.85 U cm2 to 2.01 U cm2, and the ohmic resistance increased slightly to 2.03 U cm2 after 3 SOFC-SOEC cycles in next 5 h. The I-V curves in Fig. 9 (c) also showed the similar degradation tendency. The degradation is possibly due to the change in anode microstructure resulted from nickel coarsening at high operation temperatures reported before [22], which is also illustrated by the largely grown nickel particles shown in Fig. 7 (c). However, the polarization resistance almost kept constant at 0.38 U cm2 in the total period. These results demonstrate the stability of CZI materials for the R-SOC applications.
Conclusions In this paper, Ni-CZI electrode supported CZI electrolyte tube was fabricated by isostatic pressing, dip-coating and cosintering technologies. Nano-CZI powders obtained by Pechini methods were used for preparing the coating slurry to make the electrolyte layer dense enough. Then CZI-LSC composite electrode was fabricated by dip-coating and wetimpregnation methods. The single cells with electrode support and tubular structure are reversibly operated as fuel cell and as electrolysis cell, which illustrate that air electrode with 50 wt% LSC loading showed the best electrochemical performance, especially for hydrogen production mode. The hydrogen production rate of the single cell achieved 2.1 mL$min1cm2 at 1.3 V and 850 C. The electrochemical performance remained almost constant after initial degradation probably due to the particle coarsening phenomena in the electrode. These results demonstrate the potential application of CZI materials and the fabrication technology of protonconducting tubular cell structure in RSOCs for energy storage. The thickness of the CZI electrolyte layer and the microstructure of the electrodes could be modified to improve the cell performance in our future work, and the fabrication methods can also be applied to other proton conducting materials with higher proton conductivity.
Acknowledgements The authors are grateful for the financial support from National Natural Science Foundation of China (51672294, 51302296) and the Science and Technology Commission of Shanghai Municipality (No. 15DZ2281200).
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