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Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-δ as air electrode for solid oxide steam electrolysis cells
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Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d as air electrode for solid oxide steam electrolysis cells Ze-Tian Tao a,b, Yan-Mei Jiang b,**, Libin Lei b,c, Fanglin Chen b,* a
Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng, Jiangsu, China b Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA c Smart Energy Research Center, School of Materials and Energy, Guangdong University of Technology, Guangzhou, Guangdong 510006, China
highlights Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d (PBCFN) was evaluated as air electrode for SOECs. SOECs with PBCFN air electrode achieved a relatively high performance. Electrolysis current of 0.53 A/cm2 was achieved in SOECs at 800oC and 1.3V. SOECs with PBCFN air electrode showed good short-term stability.
article info
abstract
Article history:
Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d (PBCFN) is synthesized and evaluated as air electrode for solid
Received 22 March 2019
oxide steam electrolysis cells (SOECs). X-ray diffraction and TEM analysis show that PBCFN
Received in revised form
has a pure tetragonal perovskite structure with a lattice fringe spacing of 0.39 nm for the
24 June 2019
(110) planes. When the applied voltage is set at 1.3 V, a relatively high electrolysis current
Accepted 8 July 2019
density of 470 mA cm2 at 800 C is achieved for electrolyte-supported electrolysis cells with
Available online 31 July 2019
the cell configuration of Sr2Fe1.5Mo0.5O6 (SFM)eSm0.2Ce0.8O2 (SDC)/La0.8Sr0.2Ga0.87Mg0.13O3 (LSGM)/SDCePBCFN. In addition, there is no obvious performance degradation during the
Keywords:
short-term stability test of the above cells at a constant electrolysis voltage of 1.3 V,
Air electrode materials
indicating that PBCFN is a promising air electrode for SOECs.
Chemical stability
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Electrochemical performance Perovskite structure Solid oxide electrolysis cells
Introduction Clean and renewable energy resources will play a pivotal role in achieving a sustainable global energy supply and
combating the environmental issues associated with the increasing consumption of fossil fuels [1,2]. Hydrogen has been regarded as an environmentally benign and highly efficient energy carrier because the emission product is water and the exhaust heat can also be utilized. H2 can be produced
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.-M. Jiang), [email protected] (F. Chen). https://doi.org/10.1016/j.ijhydene.2019.07.050 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Fig. 1 e XRD pattern of the Pr0.5Ba0.5 (Co0.7Fe0.25Nb0.05) O3 d sample after calcination at 1100 C for 10 h in air.
from steam electrolysis using solid oxide electrolysis cells (SOECs) under the external driving force of renewable electricity. To ensure successful and large-scale deployment of the SOEC system, breakthroughs in the new electrode materials development are vital for the SOEC technology. Ni-based cermets are the most studied and the state-ofthe-art hydrogen electrode materials for SOECs due to their excellent electrical and catalytic properties [3]. However, they also face disadvantages such as poor redox stability, susceptible to vaporization and coarsening at a high temperature and humid atmosphere [4e6]. Consequently, alternative hydrogen electrode material such as Sr2Fe1.5Mo0.5O6 (SFM) has recently been explored due to its enhanced redox cycling ability and relatively high melting point [7e9].
Considering the air electrode materials, the performance of electrolysis cells is highly related to the sluggish rate of oxygen evolution reaction (OER). In solid oxide fuel cells (SOFCs), the materials with mixed ionic and electronic conducting (MIEC) ability are fabricated to enhance the triple-phase boundary (TPB) sites, thus reducing the polarization resistance. For SOECs, the cell catalytic ability and performance can also be significantly improved by using MIECs as the air electrode. Among the air electrode materials developed for SOECs, LaxSr1-xCoyFe1-yO3 (LSCF), which has been widely studied in oxygen-ion conducting SOFC (OeSOFC), has also been applied in SOECs for its high electronic and ionic conductivities [10,11]. However, the relatively slow kinetics of oxygen dissociation-absorption process has been reported to limit the performance of LSCF as air electrode material for SOFCs [12]. Recently, PrBaCo2O6-based material has been studied as air electrode materials for SOFCs and shown excellent cell performance because of its high catalytic ability [13]. In addition, it has been demonstrated that the substitution of a small amount of Fe for Co can further improve the catalytic performance [14]. As it has been extensively reported, it is a great challenge to solve the problem of the longterm electrochemical stability of the cells and the aging of the electrode materials under different water content [15e17]. However, the volatility of Co element limits the cell operation life. On the other hand, Nb doping has been proved to be an effective way to improve the electrode stability for cobaltite perovskite by several groups [18e23]. In this study, Nb and Fe doped Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3d (PBCFN) is synthesized and characterized as air electrode material for SOECs based on La0.8Sr0.2Ga0.87Mg0.13O3 (LSGM) electrolyte. XRD and TEM are used to characterize the crystal structure of the synthesized PBCFN powders. Cell voltagecurrent relationship as well as the impedance diagram of the cells is evaluated in both SOFC and SOEC modes. In addition, the short-term stability of the SOECs is also investigated.
Fig. 2 e (a) TEM image of the Pr0.5Ba0.5 (Co0.7Fe0.25Nb0.05) O3-d. (b) Partial enlarged details of the part of red box in Fig. 2a. The inset in b shows the corresponding crystal structure of cubic perovskite in projection along [100], expanded view of the framework built on Co/Fe/NbO6 octahedra, with Pr/Ba ions in green. (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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Experimental Powder syntheses The PBCFN powders were synthesized via a wet Pechini method. All of the starting chemicals, Pr(NO3)3$6H2O(99.9%), BaCO3(99%), Co(NO3)4∙4H2O(99%), C4H4NNbO9 (99.9%), and Fe(NO3)3$6H2O(99.9%) are analytical reagents purchased from Sigma-Aldrich. Appropriate amount of the starting materials was dissolved in EDTA-NH3 aqueous solution under the conditions of heating and stirring. Appropriate amount of citric acid was subsequently added into the solution to form sol complexes. The molar ratio was set at 1: 1.5: 1 for EDTA: citric acid: metal cations. The mixture was heated under stirring and became gel. It finally ignited to flame and resulted in an
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ash-like material. The as-prepared material was calcined in air at 1100 C for 10 h to obtain final powders. In addition, the SFM powders were obtained through a wet Pechini method as described in our previous paper [24]. La0.8Sr0.2Ga0.87Mg0.13O3 (LSGM) electrolyte powders and SDC powders were both purchased from FuelCell Materials Inc.
Fabrication of single cells The electrolyte supported steam electrolysis cells with the configuration of SFMeSDC/LSGM/SDCePBCFN were prepared for electrochemical performance evaluations. LSGM powders were pressed under uniaxially pressure (300 MPa) to form LSGM pellets with a diameter of 13 mm and then sintered at 1500 C for 5 h to obtain dense LSGM electrolyte disks with a diameter of about 10 mm. The composite
Fig. 3 e The cross-section morphology of the assembled single cells (a) and enlarged morphology of LSGM electrolyte (b).
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electrode slurries of PBCFN/SDC and SFM/SDC were prepared by mixing SDC powders with the corresponding electrode powders at a weight ratio of 30:70. Ink vehicle purchased from FuelCell Materials Inc was used as binder. The electrode slurries were then screen-printed on the LSGM electrolyte surface in symmetric positions with an area of 0.3 cm2 and calcined at 1000 C for 3 h to form porous electrode. Ag paste was painted on the electrode surface as a current collector.
Characterization The phase structures of the PBCFN powders were studied by X-ray diffractometer (XRD) using Cu-Ka radiation (1.5418 A) at 40 kV and 40 mA. The crystal structure was further observed through TEM to establish the lattice fringe spacing. The crosssectional microstructure of the single cells was observed by scanning electron microscope (SEM, Zeiss Ultra Plus FESEM). The prepared single cells were sealed in a home-made testing system using an electrical conductive paste (DAD-87, Shanghai Research Institute of Synthetic Resins, China). Humidified hydrogen with various steam concentrations (3 vol%42 vol%) which was controlled by heating water to a certain temperature and then feeding into the hydrogen electrode chamber at a flow rate of 30 mL min1, while the air electrode side was exposed to ambient air. The mass flow rate of H2 was controlled by using a digital mass flow controller (APEX, AlicatScientific) and the actual water partial pressure was determined using an on-line humidity sensor (HMP 337, Vaisala). Electrochemical performance of electrolysis cell including current density-voltage (IeV) and impedance diagram was measured on an electrochemical test system (Versa STAT 3e400, Princeton Applied Research, USA).
Result and discussion Fig. 1 shows the XRD pattern of the Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d sample obtained after calcination at 1100 C for 10 h in air. The sharp peaks located at 22.9 , 32.6 , 40.2 , 46.8 , 52.7 , 58.2 , 68.3 , 73.1 , and 77.7 can be readily indexed to the (100), (110), (112), (200), (202), (114), (204), (214), and (302) diffractions, respectively, with a tetragonal structure (space group P4/ mmm). In addition, there are still some small peaks which can be indexed to (212), (220) and other diffractions. This indicates that moderate substitution of Fe by Nb in PrBaCoFeO5þd materials does not change their phase structure, while the lattice parameter (0.39 nm) increases slightly caused by the larger ionic radii of Nb4þ/5þ (0.068 (HS)/0.064(LS) nm) compared to those of Fe3þ/4þ (0.0645 (HS)/0.0585 (LS) nm) [25]. To identify more crystal structure information, Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d sample was characterized by TEM observation (Fig. 2a and b). Fig. 2a shows a typical bright-field TEM image of single solid nanoparticle. The corresponding high resolution TEM image (Fig. 2b) demonstrates the presence of crystalline fringes, corresponding to the (110) plane of the cubic single perovskite structure, with a lattice fringe spacing of 0.39 nm, which is in agreement with that of the XRD analysis (d110 ¼ 0.39 nm). Single perovskite phase has the advantage of intrinsic 3D diffusion channels. The channels facilitate
Fig. 4 e IeV Curves and impedance spectrum of the single cell tested at different temperatures in SOFC mode.
the fast penetration of oxygen ions into the Pr0.5Ba0.5 (Co0.7Fe0.25Nb0.05) O3-d lattice (Fig. 2b inset). Fig. 3 presents the microstructures of the typical crosssection of the single electrolysis cell with configuration of PBCFN-SDC/LSGM/SFM-SDC. From Fig. 3(a), it can be seen that the electrolyte is about 450um. The thick electrolyte may lead to high ohmic resistance of the total cell, thus lowering the cell performance, which can be improved by further reducing the electrolyte thickness in future work. In addition, the electrode
Fig. 5 e IeV curves of the single electrolysis cell under different steam concentrations at 800 C.
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Table 1 e Cell performance of solid oxide electrolysis cells reported in literatures. Cell configuration
SFM/LSGM/SFM K-PSCFN-CFA/LSGM/BCFN NieYSZ/YSZ/YSZeLSM LST/YSZ/YSZeLSM SFMeSDC/LCO/LSGM/SDCeLSCF LSCNNi-YSZ/YSZ/LSM-YSZ NieYSZ/YSZ/YSZeLSCF Ni-YSZ/YSZ/GDC/PBSCF PSTFjYSZjLSM-YSZ Ni-SDC/LSGM/BLC Ni/GDCj6Sc1CeSZjLSCF SFMeSDC/LSGM/SDCePBCFN
Test conditions 40% H2Oe60% H2, 800 C 40% H2Oe60% H2, 800 C 50% H2Oe50% H2, 800 C 47% H2Oe50% N2e3% H2, 900 C 42% H2Oe58% H2, 850 C 3% H2OeAre5% H2, 800 C 40% H2Oe60% H2, 800 C 50% H2Oe50% H2, 800 C 60% H2Oe40%Ar, 800 C 20%steam/1%H2/79%Ar, 800 C 75% Absolute feed humidity 3% H2Oe97% H2, 800 C 42% H2Oe58% H2, 800 C 3% H2Oe97% H2, 850 C
Electrochemical performance i @ 1.3 V (A cm2)
R @ OCV (U cm2)
0.48 0.75 0.3 0.065 0.64 0.031 0.61 1.30 0.22 0.52 >0.90 0.41 0.53 0.65
0.68 0.67 1.0 1.7 0.44 4.2 0.42 0.31 0.61 0.51 0.48 0.30
Ref.
[26] [27] [28] [29] [10] [30] [31] [32] [33] [34] [35] This work
K-PSCFN-CFA: K-Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3deCo-Fe Alloy; BCFN: Ba0.9Co0.7Fe0.2Nb0.1O3d; YSZ: Yttria stabilized zirconia; LSM: Strontium doped lanthanum manganite; LST: La0.3Sr0.7TiO3þd; LCO: La0.5Ce0.5O1.5; LSCNNi: (La0.75Sr0.25)0.95(Cr0.8Ni0.2)0.95Ni0.05O3d; GDC: Gd0.1Ce0.9O1.95; PBSCF: PrBa0.5Sr0.5Co1.5Fe0.5O5þd; PSTF: Pr0.3Sr0.7Ti0.3Fe0.7O3d; BLC: Ba0.6La0.4CoO3-d; 6Sc1CeSZ: Scandia/ceria doped zirconia.
is attached very well to the electrolyte without any obvious cracks from the enlarged morphology in Fig. 3(b). Fig. 4a shows the current density-cell voltage (IeV) curves and power density-cell voltage (PeV) curves of the electrolysis cells in SOFC mode by exposing the anode to a 3% H2Oe97% H2 atmosphere and the cathode to ambient air at different temperatures. The OCVs at 800 and 850 C are 1.008 and 1.000 V, respectively. Moreover, the maximum power densities (MPDs) of single cells at 850 and 800 C are 0.406 W cm2 and 0.246 W cm2, respectively. From Fig. 4a, it can be noted that the cell performance increases with the operating temperature. In addition, the cell resistances are also investigated by the impedance diagram in Fig. 4b. The Rp calculated from the real axis between low frequency and high frequency significantly decreases with the increase of the temperature, from 0.52 U cm2 at 800 C to 0.31 U cm2 at 850 C. Meanwhile, the ohmic resistance occupies a large part of the total resistance by comparing the impedance diagram, particularly at the low operating temperature, showing that the cell performance can be further improved by reducing electrolyte thickness.
The single cells in electrolysis mode at different steam concentrations are also observed at 800 C and shown in Fig. 5. It can be seen that the OCV is influenced by the steam concentrations, and increases when the steam concentrations decrease, as predicted from the Nernst equation [25], and thus the current densities are also affected by the steam concentrations. For example, when the applied voltage is at 1.3 V, the current density is 0.41 A cm2 under steam concentration of 3% while it increases to 0.53 A cm2 when the steam concentration is 42%. The cell performance is also compared with the electrolysis cells with different electrodes reported in the literature and shown in Table 1 [10,26e35]. It can be seen that the electrochemical performance of PBCFN is comparable to that of Ba0.9Co0.7Fe0.2Nb0.1O3d (BCFN) and SFM electrode and better than most other air electrode materials such as (La0.75Sr0.25)0.95MnO3 (LSM) and LaxSr1-xCoyFe1-yO3 (LSCF). However, it also can be found that Schefold group has obtained a high electrolysis current at almost the same operating conditions because of the thin electrolyte thickness, providing an efficient approach to improve cell performance in our future work.
Fig. 6 e The impedance diagram of the single electrolysis cell operated under the steam concentration of 30% at 800 C with different applied voltages.
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In order to further study the electrolysis cell, the impedance diagram at various applied voltages under the steam concentration of 30% at 800 C is shown in Fig. 6. An equivalent circuit (inserted in the figure) which consists of R0, L, CPE-RH, and CPERL is applied to fit the impedance diagram using Zview program. In general, the impedance diagram observed in this study is consisted of the ohmic resistance and polarization resistance. The ohmic resistances are almost the same at different applied voltages while the polarization resistances show significant differences, i.e., 0.99 U cm2 in the SOFC mode (0.7 V), 0.19 U cm2 under 1.3 V and 0.49 U cm2 under OCV in the SOEC mode. The EIS profiles feature a depressed arc shape, which consists of two arcs associated with two different electrode processes. The high frequency arc is corresponding to the charge transfer while the low frequency arc is dominated by the mass transfer of the solid oxide electrolyser [36]. When the EIS is tested in the SOEC mode, the electrode reactions such as surface diffusion of O ad will be highly improved thus enhancing the electrode process and decreasing the electrode polarization resistance at the high cell voltage [37]. In addition, the impedance diagrams under different steam concentration from 3% H2O to 20% H2O are characterized and shown in Fig. 7(a). The impedance diagrams are also fitted with the same equivalent circuit described above by using Zview program and presented in Fig. 7(b). It can be found that the polarization resistance slightly decreases with the increase of steam concentrations, which is consistent with the IeV curves in Fig. 6, while the ohmic resistance remains almost the same, indicating that increase in the concentration of the reactant can promote electrolysis reaction [15,36]. By comparison, it can be discovered that the ohmic resistance mainly comes from the electrolyte which is related to the
Fig. 8 e Time dependence of the cell voltage for the electrolysis cell measured at a constant voltage of 1.3 V at 850 C under the conditions of 3% H2Oe97% H2. electrolyte thickness and operation temperature, while the polarization resistance is associated with the electrode process influenced by many factors such as steam concentration and applied voltage. To investigate the short-term stability of SFMeSDC/LSGM/ SDCePBCFN electrolysis cells at 850 C, the cells are operated at 1.3 V with the cathode exposed to 3% H2Oe97% H2 for 10 h, as shown in Fig. 8. During the operation life, the current density is slightly decreased and then stable at 495 mA cm2, indicating that PBCFN is relatively a stable catalyst and the cell using this catalyst as the air electrode can obtain a short-term stable operation performance, because SFM had been demonstrated to be a stable fuel electrode material under the electrolysis process in the previous study [10].
Conclusion Stable perovskite structure PBFCN air electrode is synthesized and applied on the solid oxide steam electrolysis cells with LSGM as electrolyte and SFM as the hydrogen electrode material. The doping of Nb in the air electrode has improved the phase stability of PBCF and thus enhanced the electrolysis cell stability under the operation conditions. The electrochemical test showed that the cell performance can obtain a high current density of 0.53 A cm2 when the steam concentrations is 42% with the applied voltage of 1.3 V. Moreover, the relationship of current densities and operation temperatures are also observed and tested. Consequently, the SOEC with PBCFN air electrode shows stable and good performance at electrolysis mode (applied voltage is set at 1.3 V) for 10 h, showing great potential for further research.
Acknowledgements Fig. 7 e The impedance diagram of the single electrolysis cell operated under the different steam concentration at 800 C.
This work is supported by the National Natural Science Foundation of China (Grant Number: 21406190), Natural
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 4 ( 2 0 1 9 ) 2 3 5 3 9 e2 3 5 4 6
Science Foundation of the Higher Education Institutions of Jiangsu Province (No. 18KJA430017) and the U.S. National Science Foundation (DMRe1832809).
Appendix A. Supplementary data
[17]
[18]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.050. [19]
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Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d as air electrode for solid oxide steam electrolysis cells Ze-Tian Tao a,b, Yan-Mei Jiang b,**, Libin Lei b,c, Fanglin Chen b,* a
Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng, Jiangsu, China b Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA c Smart Energy Research Center, School of Materials and Energy, Guangdong University of Technology, Guangzhou, Guangdong 510006, China
highlights Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d (PBCFN) was evaluated as air electrode for SOECs. SOECs with PBCFN air electrode achieved a relatively high performance. Electrolysis current of 0.53 A/cm2 was achieved in SOECs at 800oC and 1.3V. SOECs with PBCFN air electrode showed good short-term stability.
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abstract
Article history:
Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d (PBCFN) is synthesized and evaluated as air electrode for solid
Received 22 March 2019
oxide steam electrolysis cells (SOECs). X-ray diffraction and TEM analysis show that PBCFN
Received in revised form
has a pure tetragonal perovskite structure with a lattice fringe spacing of 0.39 nm for the
24 June 2019
(110) planes. When the applied voltage is set at 1.3 V, a relatively high electrolysis current
Accepted 8 July 2019
density of 470 mA cm2 at 800 C is achieved for electrolyte-supported electrolysis cells with
Available online 31 July 2019
the cell configuration of Sr2Fe1.5Mo0.5O6 (SFM)eSm0.2Ce0.8O2 (SDC)/La0.8Sr0.2Ga0.87Mg0.13O3 (LSGM)/SDCePBCFN. In addition, there is no obvious performance degradation during the
Keywords:
short-term stability test of the above cells at a constant electrolysis voltage of 1.3 V,
Air electrode materials
indicating that PBCFN is a promising air electrode for SOECs.
Chemical stability
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Electrochemical performance Perovskite structure Solid oxide electrolysis cells
Introduction Clean and renewable energy resources will play a pivotal role in achieving a sustainable global energy supply and
combating the environmental issues associated with the increasing consumption of fossil fuels [1,2]. Hydrogen has been regarded as an environmentally benign and highly efficient energy carrier because the emission product is water and the exhaust heat can also be utilized. H2 can be produced
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.-M. Jiang), [email protected] (F. Chen). https://doi.org/10.1016/j.ijhydene.2019.07.050 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Fig. 1 e XRD pattern of the Pr0.5Ba0.5 (Co0.7Fe0.25Nb0.05) O3 d sample after calcination at 1100 C for 10 h in air.
from steam electrolysis using solid oxide electrolysis cells (SOECs) under the external driving force of renewable electricity. To ensure successful and large-scale deployment of the SOEC system, breakthroughs in the new electrode materials development are vital for the SOEC technology. Ni-based cermets are the most studied and the state-ofthe-art hydrogen electrode materials for SOECs due to their excellent electrical and catalytic properties [3]. However, they also face disadvantages such as poor redox stability, susceptible to vaporization and coarsening at a high temperature and humid atmosphere [4e6]. Consequently, alternative hydrogen electrode material such as Sr2Fe1.5Mo0.5O6 (SFM) has recently been explored due to its enhanced redox cycling ability and relatively high melting point [7e9].
Considering the air electrode materials, the performance of electrolysis cells is highly related to the sluggish rate of oxygen evolution reaction (OER). In solid oxide fuel cells (SOFCs), the materials with mixed ionic and electronic conducting (MIEC) ability are fabricated to enhance the triple-phase boundary (TPB) sites, thus reducing the polarization resistance. For SOECs, the cell catalytic ability and performance can also be significantly improved by using MIECs as the air electrode. Among the air electrode materials developed for SOECs, LaxSr1-xCoyFe1-yO3 (LSCF), which has been widely studied in oxygen-ion conducting SOFC (OeSOFC), has also been applied in SOECs for its high electronic and ionic conductivities [10,11]. However, the relatively slow kinetics of oxygen dissociation-absorption process has been reported to limit the performance of LSCF as air electrode material for SOFCs [12]. Recently, PrBaCo2O6-based material has been studied as air electrode materials for SOFCs and shown excellent cell performance because of its high catalytic ability [13]. In addition, it has been demonstrated that the substitution of a small amount of Fe for Co can further improve the catalytic performance [14]. As it has been extensively reported, it is a great challenge to solve the problem of the longterm electrochemical stability of the cells and the aging of the electrode materials under different water content [15e17]. However, the volatility of Co element limits the cell operation life. On the other hand, Nb doping has been proved to be an effective way to improve the electrode stability for cobaltite perovskite by several groups [18e23]. In this study, Nb and Fe doped Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3d (PBCFN) is synthesized and characterized as air electrode material for SOECs based on La0.8Sr0.2Ga0.87Mg0.13O3 (LSGM) electrolyte. XRD and TEM are used to characterize the crystal structure of the synthesized PBCFN powders. Cell voltagecurrent relationship as well as the impedance diagram of the cells is evaluated in both SOFC and SOEC modes. In addition, the short-term stability of the SOECs is also investigated.
Fig. 2 e (a) TEM image of the Pr0.5Ba0.5 (Co0.7Fe0.25Nb0.05) O3-d. (b) Partial enlarged details of the part of red box in Fig. 2a. The inset in b shows the corresponding crystal structure of cubic perovskite in projection along [100], expanded view of the framework built on Co/Fe/NbO6 octahedra, with Pr/Ba ions in green. (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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Experimental Powder syntheses The PBCFN powders were synthesized via a wet Pechini method. All of the starting chemicals, Pr(NO3)3$6H2O(99.9%), BaCO3(99%), Co(NO3)4∙4H2O(99%), C4H4NNbO9 (99.9%), and Fe(NO3)3$6H2O(99.9%) are analytical reagents purchased from Sigma-Aldrich. Appropriate amount of the starting materials was dissolved in EDTA-NH3 aqueous solution under the conditions of heating and stirring. Appropriate amount of citric acid was subsequently added into the solution to form sol complexes. The molar ratio was set at 1: 1.5: 1 for EDTA: citric acid: metal cations. The mixture was heated under stirring and became gel. It finally ignited to flame and resulted in an
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ash-like material. The as-prepared material was calcined in air at 1100 C for 10 h to obtain final powders. In addition, the SFM powders were obtained through a wet Pechini method as described in our previous paper [24]. La0.8Sr0.2Ga0.87Mg0.13O3 (LSGM) electrolyte powders and SDC powders were both purchased from FuelCell Materials Inc.
Fabrication of single cells The electrolyte supported steam electrolysis cells with the configuration of SFMeSDC/LSGM/SDCePBCFN were prepared for electrochemical performance evaluations. LSGM powders were pressed under uniaxially pressure (300 MPa) to form LSGM pellets with a diameter of 13 mm and then sintered at 1500 C for 5 h to obtain dense LSGM electrolyte disks with a diameter of about 10 mm. The composite
Fig. 3 e The cross-section morphology of the assembled single cells (a) and enlarged morphology of LSGM electrolyte (b).
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electrode slurries of PBCFN/SDC and SFM/SDC were prepared by mixing SDC powders with the corresponding electrode powders at a weight ratio of 30:70. Ink vehicle purchased from FuelCell Materials Inc was used as binder. The electrode slurries were then screen-printed on the LSGM electrolyte surface in symmetric positions with an area of 0.3 cm2 and calcined at 1000 C for 3 h to form porous electrode. Ag paste was painted on the electrode surface as a current collector.
Characterization The phase structures of the PBCFN powders were studied by X-ray diffractometer (XRD) using Cu-Ka radiation (1.5418 A) at 40 kV and 40 mA. The crystal structure was further observed through TEM to establish the lattice fringe spacing. The crosssectional microstructure of the single cells was observed by scanning electron microscope (SEM, Zeiss Ultra Plus FESEM). The prepared single cells were sealed in a home-made testing system using an electrical conductive paste (DAD-87, Shanghai Research Institute of Synthetic Resins, China). Humidified hydrogen with various steam concentrations (3 vol%42 vol%) which was controlled by heating water to a certain temperature and then feeding into the hydrogen electrode chamber at a flow rate of 30 mL min1, while the air electrode side was exposed to ambient air. The mass flow rate of H2 was controlled by using a digital mass flow controller (APEX, AlicatScientific) and the actual water partial pressure was determined using an on-line humidity sensor (HMP 337, Vaisala). Electrochemical performance of electrolysis cell including current density-voltage (IeV) and impedance diagram was measured on an electrochemical test system (Versa STAT 3e400, Princeton Applied Research, USA).
Result and discussion Fig. 1 shows the XRD pattern of the Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d sample obtained after calcination at 1100 C for 10 h in air. The sharp peaks located at 22.9 , 32.6 , 40.2 , 46.8 , 52.7 , 58.2 , 68.3 , 73.1 , and 77.7 can be readily indexed to the (100), (110), (112), (200), (202), (114), (204), (214), and (302) diffractions, respectively, with a tetragonal structure (space group P4/ mmm). In addition, there are still some small peaks which can be indexed to (212), (220) and other diffractions. This indicates that moderate substitution of Fe by Nb in PrBaCoFeO5þd materials does not change their phase structure, while the lattice parameter (0.39 nm) increases slightly caused by the larger ionic radii of Nb4þ/5þ (0.068 (HS)/0.064(LS) nm) compared to those of Fe3þ/4þ (0.0645 (HS)/0.0585 (LS) nm) [25]. To identify more crystal structure information, Pr0.5Ba0.5Co0.7Fe0.25Nb0.05O3-d sample was characterized by TEM observation (Fig. 2a and b). Fig. 2a shows a typical bright-field TEM image of single solid nanoparticle. The corresponding high resolution TEM image (Fig. 2b) demonstrates the presence of crystalline fringes, corresponding to the (110) plane of the cubic single perovskite structure, with a lattice fringe spacing of 0.39 nm, which is in agreement with that of the XRD analysis (d110 ¼ 0.39 nm). Single perovskite phase has the advantage of intrinsic 3D diffusion channels. The channels facilitate
Fig. 4 e IeV Curves and impedance spectrum of the single cell tested at different temperatures in SOFC mode.
the fast penetration of oxygen ions into the Pr0.5Ba0.5 (Co0.7Fe0.25Nb0.05) O3-d lattice (Fig. 2b inset). Fig. 3 presents the microstructures of the typical crosssection of the single electrolysis cell with configuration of PBCFN-SDC/LSGM/SFM-SDC. From Fig. 3(a), it can be seen that the electrolyte is about 450um. The thick electrolyte may lead to high ohmic resistance of the total cell, thus lowering the cell performance, which can be improved by further reducing the electrolyte thickness in future work. In addition, the electrode
Fig. 5 e IeV curves of the single electrolysis cell under different steam concentrations at 800 C.
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Table 1 e Cell performance of solid oxide electrolysis cells reported in literatures. Cell configuration
SFM/LSGM/SFM K-PSCFN-CFA/LSGM/BCFN NieYSZ/YSZ/YSZeLSM LST/YSZ/YSZeLSM SFMeSDC/LCO/LSGM/SDCeLSCF LSCNNi-YSZ/YSZ/LSM-YSZ NieYSZ/YSZ/YSZeLSCF Ni-YSZ/YSZ/GDC/PBSCF PSTFjYSZjLSM-YSZ Ni-SDC/LSGM/BLC Ni/GDCj6Sc1CeSZjLSCF SFMeSDC/LSGM/SDCePBCFN
Test conditions 40% H2Oe60% H2, 800 C 40% H2Oe60% H2, 800 C 50% H2Oe50% H2, 800 C 47% H2Oe50% N2e3% H2, 900 C 42% H2Oe58% H2, 850 C 3% H2OeAre5% H2, 800 C 40% H2Oe60% H2, 800 C 50% H2Oe50% H2, 800 C 60% H2Oe40%Ar, 800 C 20%steam/1%H2/79%Ar, 800 C 75% Absolute feed humidity 3% H2Oe97% H2, 800 C 42% H2Oe58% H2, 800 C 3% H2Oe97% H2, 850 C
Electrochemical performance i @ 1.3 V (A cm2)
R @ OCV (U cm2)
0.48 0.75 0.3 0.065 0.64 0.031 0.61 1.30 0.22 0.52 >0.90 0.41 0.53 0.65
0.68 0.67 1.0 1.7 0.44 4.2 0.42 0.31 0.61 0.51 0.48 0.30
Ref.
[26] [27] [28] [29] [10] [30] [31] [32] [33] [34] [35] This work
K-PSCFN-CFA: K-Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3deCo-Fe Alloy; BCFN: Ba0.9Co0.7Fe0.2Nb0.1O3d; YSZ: Yttria stabilized zirconia; LSM: Strontium doped lanthanum manganite; LST: La0.3Sr0.7TiO3þd; LCO: La0.5Ce0.5O1.5; LSCNNi: (La0.75Sr0.25)0.95(Cr0.8Ni0.2)0.95Ni0.05O3d; GDC: Gd0.1Ce0.9O1.95; PBSCF: PrBa0.5Sr0.5Co1.5Fe0.5O5þd; PSTF: Pr0.3Sr0.7Ti0.3Fe0.7O3d; BLC: Ba0.6La0.4CoO3-d; 6Sc1CeSZ: Scandia/ceria doped zirconia.
is attached very well to the electrolyte without any obvious cracks from the enlarged morphology in Fig. 3(b). Fig. 4a shows the current density-cell voltage (IeV) curves and power density-cell voltage (PeV) curves of the electrolysis cells in SOFC mode by exposing the anode to a 3% H2Oe97% H2 atmosphere and the cathode to ambient air at different temperatures. The OCVs at 800 and 850 C are 1.008 and 1.000 V, respectively. Moreover, the maximum power densities (MPDs) of single cells at 850 and 800 C are 0.406 W cm2 and 0.246 W cm2, respectively. From Fig. 4a, it can be noted that the cell performance increases with the operating temperature. In addition, the cell resistances are also investigated by the impedance diagram in Fig. 4b. The Rp calculated from the real axis between low frequency and high frequency significantly decreases with the increase of the temperature, from 0.52 U cm2 at 800 C to 0.31 U cm2 at 850 C. Meanwhile, the ohmic resistance occupies a large part of the total resistance by comparing the impedance diagram, particularly at the low operating temperature, showing that the cell performance can be further improved by reducing electrolyte thickness.
The single cells in electrolysis mode at different steam concentrations are also observed at 800 C and shown in Fig. 5. It can be seen that the OCV is influenced by the steam concentrations, and increases when the steam concentrations decrease, as predicted from the Nernst equation [25], and thus the current densities are also affected by the steam concentrations. For example, when the applied voltage is at 1.3 V, the current density is 0.41 A cm2 under steam concentration of 3% while it increases to 0.53 A cm2 when the steam concentration is 42%. The cell performance is also compared with the electrolysis cells with different electrodes reported in the literature and shown in Table 1 [10,26e35]. It can be seen that the electrochemical performance of PBCFN is comparable to that of Ba0.9Co0.7Fe0.2Nb0.1O3d (BCFN) and SFM electrode and better than most other air electrode materials such as (La0.75Sr0.25)0.95MnO3 (LSM) and LaxSr1-xCoyFe1-yO3 (LSCF). However, it also can be found that Schefold group has obtained a high electrolysis current at almost the same operating conditions because of the thin electrolyte thickness, providing an efficient approach to improve cell performance in our future work.
Fig. 6 e The impedance diagram of the single electrolysis cell operated under the steam concentration of 30% at 800 C with different applied voltages.
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In order to further study the electrolysis cell, the impedance diagram at various applied voltages under the steam concentration of 30% at 800 C is shown in Fig. 6. An equivalent circuit (inserted in the figure) which consists of R0, L, CPE-RH, and CPERL is applied to fit the impedance diagram using Zview program. In general, the impedance diagram observed in this study is consisted of the ohmic resistance and polarization resistance. The ohmic resistances are almost the same at different applied voltages while the polarization resistances show significant differences, i.e., 0.99 U cm2 in the SOFC mode (0.7 V), 0.19 U cm2 under 1.3 V and 0.49 U cm2 under OCV in the SOEC mode. The EIS profiles feature a depressed arc shape, which consists of two arcs associated with two different electrode processes. The high frequency arc is corresponding to the charge transfer while the low frequency arc is dominated by the mass transfer of the solid oxide electrolyser [36]. When the EIS is tested in the SOEC mode, the electrode reactions such as surface diffusion of O ad will be highly improved thus enhancing the electrode process and decreasing the electrode polarization resistance at the high cell voltage [37]. In addition, the impedance diagrams under different steam concentration from 3% H2O to 20% H2O are characterized and shown in Fig. 7(a). The impedance diagrams are also fitted with the same equivalent circuit described above by using Zview program and presented in Fig. 7(b). It can be found that the polarization resistance slightly decreases with the increase of steam concentrations, which is consistent with the IeV curves in Fig. 6, while the ohmic resistance remains almost the same, indicating that increase in the concentration of the reactant can promote electrolysis reaction [15,36]. By comparison, it can be discovered that the ohmic resistance mainly comes from the electrolyte which is related to the
Fig. 8 e Time dependence of the cell voltage for the electrolysis cell measured at a constant voltage of 1.3 V at 850 C under the conditions of 3% H2Oe97% H2. electrolyte thickness and operation temperature, while the polarization resistance is associated with the electrode process influenced by many factors such as steam concentration and applied voltage. To investigate the short-term stability of SFMeSDC/LSGM/ SDCePBCFN electrolysis cells at 850 C, the cells are operated at 1.3 V with the cathode exposed to 3% H2Oe97% H2 for 10 h, as shown in Fig. 8. During the operation life, the current density is slightly decreased and then stable at 495 mA cm2, indicating that PBCFN is relatively a stable catalyst and the cell using this catalyst as the air electrode can obtain a short-term stable operation performance, because SFM had been demonstrated to be a stable fuel electrode material under the electrolysis process in the previous study [10].
Conclusion Stable perovskite structure PBFCN air electrode is synthesized and applied on the solid oxide steam electrolysis cells with LSGM as electrolyte and SFM as the hydrogen electrode material. The doping of Nb in the air electrode has improved the phase stability of PBCF and thus enhanced the electrolysis cell stability under the operation conditions. The electrochemical test showed that the cell performance can obtain a high current density of 0.53 A cm2 when the steam concentrations is 42% with the applied voltage of 1.3 V. Moreover, the relationship of current densities and operation temperatures are also observed and tested. Consequently, the SOEC with PBCFN air electrode shows stable and good performance at electrolysis mode (applied voltage is set at 1.3 V) for 10 h, showing great potential for further research.
Acknowledgements Fig. 7 e The impedance diagram of the single electrolysis cell operated under the different steam concentration at 800 C.
This work is supported by the National Natural Science Foundation of China (Grant Number: 21406190), Natural
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Science Foundation of the Higher Education Institutions of Jiangsu Province (No. 18KJA430017) and the U.S. National Science Foundation (DMRe1832809).
Appendix A. Supplementary data
[17]
[18]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.050. [19]
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