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Electrochemical reduction of CO2 in a symmetrical solid oxide electrolysis cell with La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ electrode
Journal of CO₂ Utilization 33 (2019) 445–451
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Electrochemical reduction of CO2 in a symmetrical solid oxide electrolysis cell with La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ electrode ⁎
T
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Zhibin Yanga, , Chaoyang Maa, Ning Wanga, Xinfang Jinb, Chao Jinc, , Suping Penga a
Research Center of Fuel Cell, School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, China Department of Mechanical Engineering, University of Massachusetts, Lowell, 01854, USA c College of Energy, Soochow Institute for Energy and Materials Innovations, Soochow University, Suzhou 215006, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 electrolysis Solid oxide electrolysis cell Perovskite oxides Electrode materials La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ
La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN) perovskite oxide is investigated as electrode materials of a symmetrical solid oxide electrolysis cell (SOEC) for CO2 reduction reaction. A high current density of 0.442 A cm−2 and a low polarization resistance are obtained @1.5 V with pure CO2 at 800 °C because of excellent CO2 adsorbability of LSCFN. When reducing carrier gas, such as CO and/or H2 is flowed, the polarization resistance decreases further. Interesting, when H2/CO contents increase, the current density firstly increases and then reaches a plateau, the behavior is different from the conventional Ni-cermet based SOEC. Due to the reverse water gas shift reaction when H2 exists, the cell performance of CO2 electrolysis with H2 as carrier gas is better than that with CO as carrier gas. The SOEC can be continuously operated for 150 h without any attenuation when H2/CO2 = 3/7 mixture gas is filled @ 0.24 A cm−2 at 800 °C. The study indicates that LSCFN is a promising electrode material for CO2 electrolysis.
1. Introduction Nowadays, the continuous growth of the global population and unprecedented prosperity of human society lead to a dramatic increase in energy demand by the whole world. The three major fossil fuels—petroleum, natural gas, and coal—combined accounted for more than 80% of the worldwide energy production in 21 st century. The burning of fossil fuels in the conversion to electric energy has brought the CO2 concentration in the atmosphere to 29% above the pre-industrial level [1,2]. Excessive CO2 emission is one of the important causes of climate change and ecosystem deterioration, and has a great impact on the development of human societies. Thus, it is imperative to promote efficient reduction and conversion of CO2 into usable energy. As one of the most promising CO2 conversion technologies, solid oxide electrolysis cell (SOEC) has attracted extensive attention in recent years. It can convert CO2 into fuels with high efficiency. As early as the 1970s, SOEC was proposed to electrolyze CO2 to provide oxygen and fuel from the Martian atmosphere [3–5]. Then, the high-temperature steam electrolysis using SOEC had been widely studied. Most recently, significant efforts have been made to develop steam and CO2 co-electrolysis to generate syngas [6–8]. Meanwhile, the intermittent electrical energy from new energy resources is used as the power to electrolyze CO2. For instance, an activated carbon recovery energy system, which ⁎
combines SOEC with nuclear power, has been proposed in 2013. It utilizes thermal and electrical energy from nuclear power for CO2 reduction reaction [9]. SOEC operates under the reverse mode of solid oxide fuel cell (SOFC). Therefore they share a lot of similar materials in different components [10]. Cathode materials used in SOEC include Ni-YSZ, La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM), La0.2Sr0.8TiO3+δ (LST), etc. Among them, Ni-YSZ cermet, a popular SOEC cathode material, is widely used as the anode in SOFC [11]. The Ni-YSZ electrode exhibits excellent catalytic properties but also has several limitations in application. Nickel could be easily oxidized to nickel oxide in pure CO2, resulting in the loss of electronic conductivity and then accelerating cell degradation. So, it is necessary to flow some reducing carrier gas to prevent the oxidation of Ni metal during electrolysis. Even so, carbon deposition is inevitable under high CO2/H2 ratio, which is harmful to the long-term stability of the cell. To overcome these drawbacks, perovskite-type oxides with good redox stability and high carbon deposition resistance have received extensive attention. LSCM [12] and LST [13] are two well-established electrode materials, but the reported cell performance was poor because of their deficient catalytic and adsorption activity. To solve the problem, many methods were investigated to improve electrode performance. Firstly, LSCM-GDC composite has been proved to be a good
Corresponding authors. E-mail addresses: [email protected] (Z. Yang), [email protected] (C. Jin).
https://doi.org/10.1016/j.jcou.2019.07.021 Received 8 April 2019; Received in revised form 13 July 2019; Accepted 18 July 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 33 (2019) 445–451
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2.2. Single cell fabrication and test
electrode because of the enhanced catalytic property introduced by GDC [14]. Secondly, impregnation of metal nanoparticles on the electrode also greatly improves the electrode activity. By impregnating Ni, Cu and NiCu metals on LSCM significantly improved the catalytic activity for CO2 reduction reaction [15]. Also, there are many alternative perovskite materials, which have been extensively studied. La0.3Sr0.7Fe0.7Ti0.3O3 (LSFT)was explored as both anode and cathode materials for direct electrolysis with pure CO2. It showed a low polarization resistance of 0.08 Ω cm2 at 2.0 V and 800 ℃ [16]. Ni-doped LaSrFeO3-δ cathode material was also studied as a cathode for hightemperature CO2 electrolysis and showed superior electro-catalytic activity for CO2 reduction. A current density of 0.85 A cm−2 at 1.55 V and 800 ℃ in a CO2/CO (70:30) mixture was reported [17]. As all know, in situ nano-metal particles precipitation from perovskite has become a research hotspot in recent years, which enhanced the electrode catalytic activity, such as nano Co-Fe from Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ [18], Ni from Sr2FeMo1-xNixO6-δ [19]. Active Co metallic nanoparticles were also successfully exsolved and evenly distributed on the porous cubic SrMo0.8Co0.1Fe0.1O3-δ framework to improve electrochemical performance in SOFC [20]. Further, highly stable and efficient catalyst with in situ exsolved Fe − Ni Alloy nanospheres for direct CO2 electrolysis also be studied in SOEC [21]. In our previous work, La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN) has been proved to be a good symmetric electrode under SOFC mode [22–24]. Although, the measured electrical conductivity of LSCFN is 271 S cm−1 in air and 35.8 S cm−1 in 5% H2-95% Ar [25], the conductivity is still lower than that of Ni-YSZ electrode. More interestingly, some Co-Fe alloy nanoparticles were exsolved from perovskite skeleton in the reduced atmosphere, which demonstrates good size stability, and could increase the lengths of triple phase boundary and then enhance the cell catalytic activity and stability for H2 oxidation. In this work, LSCFN was examined as electrode material under SOEC mode for pure CO2 electrolysis. The CO2 reduction and adsorption properties of the LSCFN perovskite material was evaluated. The electrode polarization resistance in various gas mixtures was studied by AC impedance spectra. Furthermore, the electrochemical performance and stability of the cell were tested.
The single cell consists of three components: YSZ dense electrolyte, GDC barrier layer, and LSCFN electrode. The YSZ electrolyte was prepared by tape-casting with a thickness of 200 μm after sintering. Then a thin GDC barrier layer was applied on both sides of the YSZ electrolyte by the screen-printing method to prevent electrode from reacting with the electrolyte. The LSCFN electrode ink was applied on both sides of the barrier layers by the same method. The effective area of both electrodes is 0.5 cm2 with a thickness of approximately 30 μm. Finally, Au paste was painted on both surfaces of the LSCFN electrode and connected to the silver wire for current collection. The electrode polarization resistance in different atmospheres at evaluated temperature was studied by the AC impedance. The single cell electrochemical impedance spectroscopy (EIS) and current-voltage (I–V) curve were tested using Autolab electrochemical workstation (Metrohm, Switzerland). The long-term stability was evaluated in a 30% H2/CO2 mixture atmosphere at 800 ℃. The electrode morphology before and after the test was obtained by SEM. The Raman Microscope was used to detect the coke deposition and generation of secondary phases after the test. 3. Results and discussion 3.1. Crystal structure Fig. 1 shows the XRD patterns of as-prepared and reduced LSCFN powders. It is shown that the as-prepared LSCFN powders were in a pure perovskite phase. It can be also observed that the peak is shifted to a lower angle after reduction for 10 h at 850 ℃, suggesting the lattice expansion. The change of valence states may be the main reason for the lattice expansion because of the lattice oxygen loss in reducing atmosphere, such as Co4+/Co3+ to Co2+ and/or Fe4+/Fe3+ to Fe2+. Research has shown that the formation of oxygen vacancies could effectively enhance the chemical adsorption because of the CO2 could be incorporated into the oxygen vacancy sites and form strong chemical bonding [26]. Moreover, a small amount of metallic phase precipitated from the LSCFN perovskite skeleton, which was beneficial to the cell performance and prevent carbon deposition.
2. Experimental section 2.1. Material synthesis and test
3.2. Redox and adsorption properties
LSCFN powders were prepared by conventional solid-state reaction. The details have been described in our previous work [22]. The obtained powders were then processed and characterized by X-ray diffraction (XRD), Temperature-programed desorption (TPD), and Temperature-programmed reduction (TPR), respectively. Specifically, the prepared LSCFN powders were firstly reduced in a 5% H2/N2 atmosphere under 850 ℃ for 10 h. Then, the phase of the prepared and reduced powders was analyzed by XRD. Meanwhile, the prepared powders were characterized by TPR. The procedure of the TPR experiment was as follows: the powders were first loaded in a U-shaped quartz tube and treated in a He atmosphere (50 ml min−1) at 300 ℃ for 2 h, cooled down to 50 ℃ and then exposed in 50 ml min−1 of 10% H2/He mixture atmosphere for 0.5 h; after a steady state was reached, the prepared LSCFN powders were heated up to 1000 ℃ in 10% H2/He atmosphere; and the TPR profile was recorded by a thermal conductivity detector (TCD). After that, the reduced powders were characterized by CO2-TPD. The procedure of the TPD experiment was similar as TPR: the powders were treated in a He atmosphere at 300 ℃ for 2 h; then cooled down to 100 ℃ and treated in 10% CO2/H2 atmospheres for 1 h to adsorb CO2; finally, the powders were heated to 1000 ℃ in the He atmosphere to get TPD profile.
Fig. 2(a) shows the H2-TPR profiles of LSCFN powders. The TPR profiles mainly reflect the changes in the valence of B-site metal cation, from which the redox properties can be obtained [26]. It can be seen that the TPR profile of LSCFN contains four reduction peaks: the first three peaks between 150 ℃ and 500 ℃ corresponds to the reduction
Fig. 1. XRD of (A1) as-prepared LSCFN powders; (A2) LSCFN annealed in 5% H2/N2 atmosphere at 850 ℃ for 10 h. 446
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Fig. 2. (a) H2-TPR profiles of LSCFN and (b) CO2-TPD profiles of LSCFN and reduced LSCFN.
Fig. 3. the impedance spectroscopy of LSCFN: (a) different temperature in CO2 atmosphere; (b) different H2/CO2 ratios at 800 ℃; (c) different CO/CO2 ratios at 800 ℃; (d) Comparison of polarization impedances in different atmospheres.
which was obtained in our previous work [22]. Excellent redox properties of B-site cation in LSCFN perovskite also was obtained; the reduction of Co4+/Co3+ and Fe4+/Fe3+ increased oxygen vacancy concentration and was beneficial for CO2 electrolysis. Fig. 2(b) shows the CO2-TPD profiles of LSCFN and reduced LSCFN. Generally, the adsorption of CO2 on the surface of catalysts can be classified into the physical adsorption and chemical adsorption [27,30,31]. Specifically, the desorption peak located between 100˜300 °C can be ascribed to the physical desorption of CO2. Fig. 2(b) shows, reduced LSCFN powders exhibited much higher desorption peak which may suggested it’s have much higher surface area due to nano metallic phase precipitated from the LSCFN perovskite. However, unlike pure LSCFN, which had no obvious higher temperature peak, the reduced LSCFN demonstrated a strong chemical desorption peak between 600˜800 °C, which suggested that the reduced LSCFN powders are able to more strongly adsorb the CO2 molecules on the electrode surface, in which the more CO2 molecules could remain bonded to the electrode even at high temperature about 800 °C (corresponds to the operating temperature). Hence, more CO2 molecules are possibly involved in the reaction, resulting in the better electrolysis performance. This is mainly due to the existence of large amount of oxygen vacancies in reduced LSCFN. Since CO2 molecules could be incorporated into oxygen vacancies in the lattice, sufficient oxygen vacancies in the LSCFN could provide accommodation sites for CO2 molecules at
Fig. 4. CO2 electrolysis current density vs. cell voltage curve with the cell configuration of LSCFN-GDC|YSZ|GDC-LSCFN.
from Co4+/Co3+ to Co2+ (298.5 ℃), from Co2+ to Co0 (344.1 ℃) and from Fe4+/Fe3+ to Fe2+ (401.3 ℃) [28]; the last one corresponds to the reduction from Fe3+ to Fe2+ and from Fe2+ to Fe0 (886.08 ℃) [29]. The peak temperature (optimal reduction temperature) of the reduction from Fe2+ to Fe0 is 886.08 ℃; therefore, only part of the iron precipitated at 850 ℃ reduction and the alloy phase in the XRD pattern is not obvious. The alloy phase was visible after 2 h of reduction at 900 ℃, 447
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Fig. 5. (a) I–V curves and (b) AC impendence of LSCFN-GDC|YSZ|GDC-LSCFN for pure CO2 electrolysis at evaluated temperature.
Fig. 6. (a) I–V curves and (b) AC impendence of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in different ratios of H2/CO2 at 800℃; (c) I–V curves and (d) AC impendence of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in different ratios of CO/CO2 at 800℃.
elevated temperatures. For another, it can be concluded that the reduced LSCFN had a capacity for CO2 adsorption before it reached 800 ℃. 3.3. Electrode impedance Fig. 3a shows the impedance spectrum of the symmetric cells with the configuration of LSCFN-GDC|YSZ|GDC-LSCFN tested in a CO2 atmosphere under a series of temperatures. The polarization resistance of the LSCFN electrode decreased verse temperature, from 3.53 Ω cm2 at 750 ℃, 3.02 Ω cm2 at 800 ℃, to 2.82 Ω cm2 at 850 ℃. The AC impedance in various mixtures of reducing gas (H2 and CO) and carbon dioxide at 800 ℃ are shown in Fig. 3(b) and (c). The polarization impedance decreased with the increasing of hydrogen (or carbon monoxide) contents, resulting from the increasing of oxygen vacancy concentration and the exsolution of CoFe alloy nanoparticles in reducing atmosphere, as shown in the XRD and TPR profiles. Fig. 3(d) compares
Fig. 7. Stability test of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in H2/ CO2 = 3/7 atmosphere with constant current density 0.24 A cm−2 at 800℃.
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Fig. 8. SEM images of the LSCFN-GDC|YSZ|GDC-LSCFN cell: (a) Cross-sectional microstructures of the cell after the stability test. (b) Cathode after the stability test. (c) Anode after the stability test. (d) The as-prepared electrode (e) Raman spectra collected from cathode surface before and after the stability test.
Based on the current density characteristics, three regions are identified: the negative current density region I, the small current density region II and the large current density region III, similar to the results reported in references [3,16]. An initial open circuit voltage of 0.11 V is observed when CO2 was fed into the cathode. When the cell voltage is less than the OCV voltage, a negative current density is obtained in region I because oxygen ions flow in the reversed direction from the anode to the cathode. In region II, as the voltage increases, the current density becomes positive and then increases marginally. In region III, the current density increases sharply with the increase of voltage,
the polarization resistance both in a H2/CO2 atmosphere and in a CO/ CO2 atmosphere with the similar fuel molar fraction ratio at 800 ℃. It can be seen that a smaller polarization of the LSCFN electrode was obtained in H2/CO2 mixture, indicating that strong reducing atmospheres contribute to the enhancement of the electrode activity of cells. 3.4. Electrolysis performance Fig. 4 shows the typical current density versus cell voltage curve of LSCFN-GDC|YSZ|GDC-LSCFN in a pure CO2 atmosphere at 850 ℃. 449
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4. Conclusions
because the CO2 electrolysis occurred in this region. It has been proved that impurities in O2 were electrolyzed in the first two regions, in which CO2 electrolysis has not yet began. Fig. 5(a) shows the I–V curves of LSCFN-GDC|YSZ|GDC-LSCFN cell for the electrolysis of pure CO2 with the cell voltages of 0.4–1.5 V from 750 ℃ to 850 ℃. The current density is 442 mA cm−2 at an overpotential of 1.407 V (corresponding to the applied voltage of 1.5 V) at 800 ℃, which shows a much higher current density at a relative small overpotential compared with some reported literatures, such as LSCM (180 mA cm−2 @ over potential of 1.62 V at 800 ℃) [12], or LST (125 mA cm−2 @ applied voltage of 2 V and 750 ℃) [13]. Fig. 5(b) exhibits the AC impedance spectra of the cell measured at OCV for CO2 electrolysis. As the temperature increases from 750 ℃ to 850 ℃, the ohmic impedance decreases from 0.98 Ω cm2 to 0.555 Ω cm2 due to the increase of the electrolyte conductivity at high temperature, while the polarization resistance at low frequency (RLF, with magnitude below 10 HZ) increases from 0.293 Ω cm2 to 0.829 Ω cm2. Generally, RLF is related to the surface exchange reaction and O2 diffusion. TPD results show CO2 desorption will occur when the temperature increases to 850 ℃. So, the RLF increases with increasing of the operated temperature. Fig. 6(a) shows the I–V curves of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis with different ratios of H2/CO2 at 800 ℃. As hydrogen content increased, the performance in the SOFC mode was gradually improved. Different from Ni/YSZ cathode based SOEC with worse performance of CO2 electrolysis [14], the SOEC performance based on LSCFN cathode was relatively stable even in the lower content of CO2. This result show that LSCFN cathode has better catalytic activity and stability than normal Ni/YSZ cathode. The I–V curves of the latter for CO2 electrolysis in different atmospheres were parallel lines, indicating the performance in the SOEC mode decreased with the reducing gas molar fraction ratio increased. Therefore, LSCFN cathode works well in a reducing atmosphere, and its performance remains stable without deterioration. The corresponding impedance spectra are shown in Fig. 6(b). The ohmic and polarization resistance decreased with increasing hydrogen content. Fig.6 (c) and (d) shows the I–V curves of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in different ratios of CO/CO2 at 800 ℃. The trending is the same as that with hydrogen but slightly decreases because the cell resistance was higher in a CO atmosphere. Also, reverse water-gas shift (RWGS) reaction also helps to promote CO2-H2O co-electrolysis [32].
In summary, the LSCFN perovskite has been fabricated and investigated for CO2 electrolysis as electrode materials in a wide range of operating conditions. The current density reached 0.442 A cm−2 @ 1.5 V and 800 ℃ in a pure CO2 atmosphere, which was much higher than that reported for LST and LSCM. When a reducing gas was added, the electrode resistance, especially the polarization resistance, decreased as the amount of reducing gas increased. It was led by the increase in oxygen vacancy concentration and the precipitation of a small amount of CoFe alloy in the reducing atmosphere, as observed in the XRD and TPR profiles. With the increase of H2/CO content, the current density approached to a plateau, which was different from those of Nicermet and any other cathodes. The cell performance in CO2/H2 atmosphere was better than that in CO2/CO, resulted from two facts: the reverse water gas shift reaction occurred when H2 existed; and the polarization resistance of the cell with H2 atmosphere was lower than that under the CO atmosphere. The cell performance had a small degradation after 150 hs stability test with 70% CO2-30% H2 under 800 °C, which maybe resulted from the pore structure deterioration. However, no significant agglomerations, carbon deposit or other secondary phase generation were observed. Therefore, it can be concluded that LSCFN is a promising electrode material for CO2 electrolysis in SOEC with enhanced catalytic activity and extended stability, even in a reducing atmosphere. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Financial supports from the National Key Research and Development Program of China (2017YFB0601904), Beijing Municipal Science and Technology Project (Z181100005118008), National Natural Science Foundation of China (21773167), and the Yueqi Young Scholar Project (CUMTB) are greatly appreciated. References
3.5. Long-term stability test
[1] Y. Zheng, W. Zhang, Y. Li, J. Chen, B. Yu, J. Wang, L. Zhang, J. Zhang, Nano Energy 40 (2017) 512. [2] V.G. Azevedo, S. Sartori, L.M.S. Campos, Renewable Sustainable Energy Rev. 81 (2018) 107. [3] K.R. Sridhar, Solid State Ion. 93 (1997) 321. [4] R.A. De Souza, J.A. Kilner, Solid State Ion. 106 (1998) 175. [5] G. Tao, K.R. Sridhar, C.L. Chan, Solid State Ion. 175 (2004) 621. [6] Z. Yang, Y. Liu, T. Zhu, Y. Chen, M. Han, C. Jin, Int. J. Hydrogen Energy 41 (1997) (2016). [7] Y. Zheng, J. Wang, B. Yu, W. Zhang, J. Chen, J. Qiao, J. Zhang, Chem. Soc. Rev. 46 (1427) (2017). [8] M. Ni, M. Leung, D. Leung, Int. J. Hydrogen Energy 33 (2337) (2008). [9] A.L. Dipu, Y. Ujisawa, J. Ryu, Y. Kato, Nucl. Eng. Des. 271 (2014) 30. [10] L. Zhang, S. Hu, X. Zhu, W. Yang, J. Energy Chem. 26 (2017) 593. [11] R. Xing, Y. Wang, S. Liu, J. Chao, J. Power Sources 208 (2012) 276. [12] S. Xu, S. Li, W. Yao, D. Dong, K. Xie, J. Power Sources 230 (2013) 115. [13] Y. Li, J. Zhou, D. Dong, Y. Wang, J.Z. Jiang, H. Xiang, K. Xie, Physical chemistry chemical physics: PCCP 14 (2012) 15547. [14] X. Yue, J.T.S. Irvine, J. Electrochem. Soc. 159 (2012) F442. [15] C. Zhu, L. Hou, S. Li, L. Gan, K. Xie, J. Power Sources 363 (2017) 177. [16] Z. Cao, B. Wei, J. Miao, Z. Wang, Z. Lü, W. Li, Y. Zhang, X. Huang, X. Zhu, Q. Feng, Y. Sui, Electrochem. commun. 69 (2016) 80. [17] S. Liu, Q. Liu, J.L. Luo, J. Mater. Chem. A 5 (2673) (2017). [18] C.H. Yang, Z.B. Yang, C. Jin, G.L. Xiao, F.L. Chen, M.F. Han, Adv. Mater. 24 (1439) (2012). [19] Z. Du, H. Zhao, S. Yi, Q. Xia, Y. Gong, Y. Zhang, X. Cheng, Y. Li, L. Gu, K. Swierczek, ACS Nano 10 (2016) 8660. [20] S. Liu, Q. Liu, X.-Z. Fu, J.-L. Luo, Appl. Catal. B 220 (2018) 283. [21] S. Liu, Q. Liu, J.L. Luo, ACS Catal. 6 (2016) 6219. [22] Z. Yang, N. Xu, M. Han, F. Chen, Int. J. Hydrogen Energy 39 (2014) 7402. [23] N. Xu, T. Zhu, Z. Yang, M. Han, J. Mater. Sci. Technol. 33 (1329) (2017).
Fig. 7 shows the long-term test of the LSCFN-GDC|YSZ|GDC-LSCFN cell for CO2 in H2/CO2 = 3/7 atmosphere with constant current density 0.24 A cm−2 at 800 ℃. The cell voltage increases from 1.305 to 1.379 V, exhibiting only 5.6% degradation after 150 h stability test, which is better than that of La0.3Sr0.7Fe0.7Ti0.3O3 electrode material, approximately 5% decrease during 24 h operation likely due to the delamination of LSFT oxygen electrode under anodic polarization [16]. The SEM images of the LSCFN-GDC|YSZ|GDC-LSCFN cell before and after stability test are presented in Fig. 8. The anode and cathode thickness are about 20 and 28 μm respectively. The electrode isolation layer and electrolyte interfaces show good contact without obvious delamination. By comparing Fig. 8(b) and (d), it implies that the electrode pore structure deteriorated after the stability test, which should be the signs for the slight performance degradation. No significant agglomerations can be observed after the test. Fig. 8(e) shows the Raman spectra collected from cathode surface before and after the stability test. No observable carbon peaks are detected from the cathode surface because two typical carbon characteristic peaks at 1338 (D-band) and 1568 cm−1 (G-band) were not observed in the Raman spectrum [21]. Similarly, no other secondary phase is observed. Therefore, the LSCFN is an electronically active and stable electrode materials for CO2 electrolysis. 450
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Electrochemical reduction of CO2 in a symmetrical solid oxide electrolysis cell with La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ electrode ⁎
T
⁎
Zhibin Yanga, , Chaoyang Maa, Ning Wanga, Xinfang Jinb, Chao Jinc, , Suping Penga a
Research Center of Fuel Cell, School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, China Department of Mechanical Engineering, University of Massachusetts, Lowell, 01854, USA c College of Energy, Soochow Institute for Energy and Materials Innovations, Soochow University, Suzhou 215006, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 electrolysis Solid oxide electrolysis cell Perovskite oxides Electrode materials La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ
La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN) perovskite oxide is investigated as electrode materials of a symmetrical solid oxide electrolysis cell (SOEC) for CO2 reduction reaction. A high current density of 0.442 A cm−2 and a low polarization resistance are obtained @1.5 V with pure CO2 at 800 °C because of excellent CO2 adsorbability of LSCFN. When reducing carrier gas, such as CO and/or H2 is flowed, the polarization resistance decreases further. Interesting, when H2/CO contents increase, the current density firstly increases and then reaches a plateau, the behavior is different from the conventional Ni-cermet based SOEC. Due to the reverse water gas shift reaction when H2 exists, the cell performance of CO2 electrolysis with H2 as carrier gas is better than that with CO as carrier gas. The SOEC can be continuously operated for 150 h without any attenuation when H2/CO2 = 3/7 mixture gas is filled @ 0.24 A cm−2 at 800 °C. The study indicates that LSCFN is a promising electrode material for CO2 electrolysis.
1. Introduction Nowadays, the continuous growth of the global population and unprecedented prosperity of human society lead to a dramatic increase in energy demand by the whole world. The three major fossil fuels—petroleum, natural gas, and coal—combined accounted for more than 80% of the worldwide energy production in 21 st century. The burning of fossil fuels in the conversion to electric energy has brought the CO2 concentration in the atmosphere to 29% above the pre-industrial level [1,2]. Excessive CO2 emission is one of the important causes of climate change and ecosystem deterioration, and has a great impact on the development of human societies. Thus, it is imperative to promote efficient reduction and conversion of CO2 into usable energy. As one of the most promising CO2 conversion technologies, solid oxide electrolysis cell (SOEC) has attracted extensive attention in recent years. It can convert CO2 into fuels with high efficiency. As early as the 1970s, SOEC was proposed to electrolyze CO2 to provide oxygen and fuel from the Martian atmosphere [3–5]. Then, the high-temperature steam electrolysis using SOEC had been widely studied. Most recently, significant efforts have been made to develop steam and CO2 co-electrolysis to generate syngas [6–8]. Meanwhile, the intermittent electrical energy from new energy resources is used as the power to electrolyze CO2. For instance, an activated carbon recovery energy system, which ⁎
combines SOEC with nuclear power, has been proposed in 2013. It utilizes thermal and electrical energy from nuclear power for CO2 reduction reaction [9]. SOEC operates under the reverse mode of solid oxide fuel cell (SOFC). Therefore they share a lot of similar materials in different components [10]. Cathode materials used in SOEC include Ni-YSZ, La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM), La0.2Sr0.8TiO3+δ (LST), etc. Among them, Ni-YSZ cermet, a popular SOEC cathode material, is widely used as the anode in SOFC [11]. The Ni-YSZ electrode exhibits excellent catalytic properties but also has several limitations in application. Nickel could be easily oxidized to nickel oxide in pure CO2, resulting in the loss of electronic conductivity and then accelerating cell degradation. So, it is necessary to flow some reducing carrier gas to prevent the oxidation of Ni metal during electrolysis. Even so, carbon deposition is inevitable under high CO2/H2 ratio, which is harmful to the long-term stability of the cell. To overcome these drawbacks, perovskite-type oxides with good redox stability and high carbon deposition resistance have received extensive attention. LSCM [12] and LST [13] are two well-established electrode materials, but the reported cell performance was poor because of their deficient catalytic and adsorption activity. To solve the problem, many methods were investigated to improve electrode performance. Firstly, LSCM-GDC composite has been proved to be a good
Corresponding authors. E-mail addresses: [email protected] (Z. Yang), [email protected] (C. Jin).
https://doi.org/10.1016/j.jcou.2019.07.021 Received 8 April 2019; Received in revised form 13 July 2019; Accepted 18 July 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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2.2. Single cell fabrication and test
electrode because of the enhanced catalytic property introduced by GDC [14]. Secondly, impregnation of metal nanoparticles on the electrode also greatly improves the electrode activity. By impregnating Ni, Cu and NiCu metals on LSCM significantly improved the catalytic activity for CO2 reduction reaction [15]. Also, there are many alternative perovskite materials, which have been extensively studied. La0.3Sr0.7Fe0.7Ti0.3O3 (LSFT)was explored as both anode and cathode materials for direct electrolysis with pure CO2. It showed a low polarization resistance of 0.08 Ω cm2 at 2.0 V and 800 ℃ [16]. Ni-doped LaSrFeO3-δ cathode material was also studied as a cathode for hightemperature CO2 electrolysis and showed superior electro-catalytic activity for CO2 reduction. A current density of 0.85 A cm−2 at 1.55 V and 800 ℃ in a CO2/CO (70:30) mixture was reported [17]. As all know, in situ nano-metal particles precipitation from perovskite has become a research hotspot in recent years, which enhanced the electrode catalytic activity, such as nano Co-Fe from Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ [18], Ni from Sr2FeMo1-xNixO6-δ [19]. Active Co metallic nanoparticles were also successfully exsolved and evenly distributed on the porous cubic SrMo0.8Co0.1Fe0.1O3-δ framework to improve electrochemical performance in SOFC [20]. Further, highly stable and efficient catalyst with in situ exsolved Fe − Ni Alloy nanospheres for direct CO2 electrolysis also be studied in SOEC [21]. In our previous work, La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN) has been proved to be a good symmetric electrode under SOFC mode [22–24]. Although, the measured electrical conductivity of LSCFN is 271 S cm−1 in air and 35.8 S cm−1 in 5% H2-95% Ar [25], the conductivity is still lower than that of Ni-YSZ electrode. More interestingly, some Co-Fe alloy nanoparticles were exsolved from perovskite skeleton in the reduced atmosphere, which demonstrates good size stability, and could increase the lengths of triple phase boundary and then enhance the cell catalytic activity and stability for H2 oxidation. In this work, LSCFN was examined as electrode material under SOEC mode for pure CO2 electrolysis. The CO2 reduction and adsorption properties of the LSCFN perovskite material was evaluated. The electrode polarization resistance in various gas mixtures was studied by AC impedance spectra. Furthermore, the electrochemical performance and stability of the cell were tested.
The single cell consists of three components: YSZ dense electrolyte, GDC barrier layer, and LSCFN electrode. The YSZ electrolyte was prepared by tape-casting with a thickness of 200 μm after sintering. Then a thin GDC barrier layer was applied on both sides of the YSZ electrolyte by the screen-printing method to prevent electrode from reacting with the electrolyte. The LSCFN electrode ink was applied on both sides of the barrier layers by the same method. The effective area of both electrodes is 0.5 cm2 with a thickness of approximately 30 μm. Finally, Au paste was painted on both surfaces of the LSCFN electrode and connected to the silver wire for current collection. The electrode polarization resistance in different atmospheres at evaluated temperature was studied by the AC impedance. The single cell electrochemical impedance spectroscopy (EIS) and current-voltage (I–V) curve were tested using Autolab electrochemical workstation (Metrohm, Switzerland). The long-term stability was evaluated in a 30% H2/CO2 mixture atmosphere at 800 ℃. The electrode morphology before and after the test was obtained by SEM. The Raman Microscope was used to detect the coke deposition and generation of secondary phases after the test. 3. Results and discussion 3.1. Crystal structure Fig. 1 shows the XRD patterns of as-prepared and reduced LSCFN powders. It is shown that the as-prepared LSCFN powders were in a pure perovskite phase. It can be also observed that the peak is shifted to a lower angle after reduction for 10 h at 850 ℃, suggesting the lattice expansion. The change of valence states may be the main reason for the lattice expansion because of the lattice oxygen loss in reducing atmosphere, such as Co4+/Co3+ to Co2+ and/or Fe4+/Fe3+ to Fe2+. Research has shown that the formation of oxygen vacancies could effectively enhance the chemical adsorption because of the CO2 could be incorporated into the oxygen vacancy sites and form strong chemical bonding [26]. Moreover, a small amount of metallic phase precipitated from the LSCFN perovskite skeleton, which was beneficial to the cell performance and prevent carbon deposition.
2. Experimental section 2.1. Material synthesis and test
3.2. Redox and adsorption properties
LSCFN powders were prepared by conventional solid-state reaction. The details have been described in our previous work [22]. The obtained powders were then processed and characterized by X-ray diffraction (XRD), Temperature-programed desorption (TPD), and Temperature-programmed reduction (TPR), respectively. Specifically, the prepared LSCFN powders were firstly reduced in a 5% H2/N2 atmosphere under 850 ℃ for 10 h. Then, the phase of the prepared and reduced powders was analyzed by XRD. Meanwhile, the prepared powders were characterized by TPR. The procedure of the TPR experiment was as follows: the powders were first loaded in a U-shaped quartz tube and treated in a He atmosphere (50 ml min−1) at 300 ℃ for 2 h, cooled down to 50 ℃ and then exposed in 50 ml min−1 of 10% H2/He mixture atmosphere for 0.5 h; after a steady state was reached, the prepared LSCFN powders were heated up to 1000 ℃ in 10% H2/He atmosphere; and the TPR profile was recorded by a thermal conductivity detector (TCD). After that, the reduced powders were characterized by CO2-TPD. The procedure of the TPD experiment was similar as TPR: the powders were treated in a He atmosphere at 300 ℃ for 2 h; then cooled down to 100 ℃ and treated in 10% CO2/H2 atmospheres for 1 h to adsorb CO2; finally, the powders were heated to 1000 ℃ in the He atmosphere to get TPD profile.
Fig. 2(a) shows the H2-TPR profiles of LSCFN powders. The TPR profiles mainly reflect the changes in the valence of B-site metal cation, from which the redox properties can be obtained [26]. It can be seen that the TPR profile of LSCFN contains four reduction peaks: the first three peaks between 150 ℃ and 500 ℃ corresponds to the reduction
Fig. 1. XRD of (A1) as-prepared LSCFN powders; (A2) LSCFN annealed in 5% H2/N2 atmosphere at 850 ℃ for 10 h. 446
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Fig. 2. (a) H2-TPR profiles of LSCFN and (b) CO2-TPD profiles of LSCFN and reduced LSCFN.
Fig. 3. the impedance spectroscopy of LSCFN: (a) different temperature in CO2 atmosphere; (b) different H2/CO2 ratios at 800 ℃; (c) different CO/CO2 ratios at 800 ℃; (d) Comparison of polarization impedances in different atmospheres.
which was obtained in our previous work [22]. Excellent redox properties of B-site cation in LSCFN perovskite also was obtained; the reduction of Co4+/Co3+ and Fe4+/Fe3+ increased oxygen vacancy concentration and was beneficial for CO2 electrolysis. Fig. 2(b) shows the CO2-TPD profiles of LSCFN and reduced LSCFN. Generally, the adsorption of CO2 on the surface of catalysts can be classified into the physical adsorption and chemical adsorption [27,30,31]. Specifically, the desorption peak located between 100˜300 °C can be ascribed to the physical desorption of CO2. Fig. 2(b) shows, reduced LSCFN powders exhibited much higher desorption peak which may suggested it’s have much higher surface area due to nano metallic phase precipitated from the LSCFN perovskite. However, unlike pure LSCFN, which had no obvious higher temperature peak, the reduced LSCFN demonstrated a strong chemical desorption peak between 600˜800 °C, which suggested that the reduced LSCFN powders are able to more strongly adsorb the CO2 molecules on the electrode surface, in which the more CO2 molecules could remain bonded to the electrode even at high temperature about 800 °C (corresponds to the operating temperature). Hence, more CO2 molecules are possibly involved in the reaction, resulting in the better electrolysis performance. This is mainly due to the existence of large amount of oxygen vacancies in reduced LSCFN. Since CO2 molecules could be incorporated into oxygen vacancies in the lattice, sufficient oxygen vacancies in the LSCFN could provide accommodation sites for CO2 molecules at
Fig. 4. CO2 electrolysis current density vs. cell voltage curve with the cell configuration of LSCFN-GDC|YSZ|GDC-LSCFN.
from Co4+/Co3+ to Co2+ (298.5 ℃), from Co2+ to Co0 (344.1 ℃) and from Fe4+/Fe3+ to Fe2+ (401.3 ℃) [28]; the last one corresponds to the reduction from Fe3+ to Fe2+ and from Fe2+ to Fe0 (886.08 ℃) [29]. The peak temperature (optimal reduction temperature) of the reduction from Fe2+ to Fe0 is 886.08 ℃; therefore, only part of the iron precipitated at 850 ℃ reduction and the alloy phase in the XRD pattern is not obvious. The alloy phase was visible after 2 h of reduction at 900 ℃, 447
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Fig. 5. (a) I–V curves and (b) AC impendence of LSCFN-GDC|YSZ|GDC-LSCFN for pure CO2 electrolysis at evaluated temperature.
Fig. 6. (a) I–V curves and (b) AC impendence of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in different ratios of H2/CO2 at 800℃; (c) I–V curves and (d) AC impendence of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in different ratios of CO/CO2 at 800℃.
elevated temperatures. For another, it can be concluded that the reduced LSCFN had a capacity for CO2 adsorption before it reached 800 ℃. 3.3. Electrode impedance Fig. 3a shows the impedance spectrum of the symmetric cells with the configuration of LSCFN-GDC|YSZ|GDC-LSCFN tested in a CO2 atmosphere under a series of temperatures. The polarization resistance of the LSCFN electrode decreased verse temperature, from 3.53 Ω cm2 at 750 ℃, 3.02 Ω cm2 at 800 ℃, to 2.82 Ω cm2 at 850 ℃. The AC impedance in various mixtures of reducing gas (H2 and CO) and carbon dioxide at 800 ℃ are shown in Fig. 3(b) and (c). The polarization impedance decreased with the increasing of hydrogen (or carbon monoxide) contents, resulting from the increasing of oxygen vacancy concentration and the exsolution of CoFe alloy nanoparticles in reducing atmosphere, as shown in the XRD and TPR profiles. Fig. 3(d) compares
Fig. 7. Stability test of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in H2/ CO2 = 3/7 atmosphere with constant current density 0.24 A cm−2 at 800℃.
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Fig. 8. SEM images of the LSCFN-GDC|YSZ|GDC-LSCFN cell: (a) Cross-sectional microstructures of the cell after the stability test. (b) Cathode after the stability test. (c) Anode after the stability test. (d) The as-prepared electrode (e) Raman spectra collected from cathode surface before and after the stability test.
Based on the current density characteristics, three regions are identified: the negative current density region I, the small current density region II and the large current density region III, similar to the results reported in references [3,16]. An initial open circuit voltage of 0.11 V is observed when CO2 was fed into the cathode. When the cell voltage is less than the OCV voltage, a negative current density is obtained in region I because oxygen ions flow in the reversed direction from the anode to the cathode. In region II, as the voltage increases, the current density becomes positive and then increases marginally. In region III, the current density increases sharply with the increase of voltage,
the polarization resistance both in a H2/CO2 atmosphere and in a CO/ CO2 atmosphere with the similar fuel molar fraction ratio at 800 ℃. It can be seen that a smaller polarization of the LSCFN electrode was obtained in H2/CO2 mixture, indicating that strong reducing atmospheres contribute to the enhancement of the electrode activity of cells. 3.4. Electrolysis performance Fig. 4 shows the typical current density versus cell voltage curve of LSCFN-GDC|YSZ|GDC-LSCFN in a pure CO2 atmosphere at 850 ℃. 449
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4. Conclusions
because the CO2 electrolysis occurred in this region. It has been proved that impurities in O2 were electrolyzed in the first two regions, in which CO2 electrolysis has not yet began. Fig. 5(a) shows the I–V curves of LSCFN-GDC|YSZ|GDC-LSCFN cell for the electrolysis of pure CO2 with the cell voltages of 0.4–1.5 V from 750 ℃ to 850 ℃. The current density is 442 mA cm−2 at an overpotential of 1.407 V (corresponding to the applied voltage of 1.5 V) at 800 ℃, which shows a much higher current density at a relative small overpotential compared with some reported literatures, such as LSCM (180 mA cm−2 @ over potential of 1.62 V at 800 ℃) [12], or LST (125 mA cm−2 @ applied voltage of 2 V and 750 ℃) [13]. Fig. 5(b) exhibits the AC impedance spectra of the cell measured at OCV for CO2 electrolysis. As the temperature increases from 750 ℃ to 850 ℃, the ohmic impedance decreases from 0.98 Ω cm2 to 0.555 Ω cm2 due to the increase of the electrolyte conductivity at high temperature, while the polarization resistance at low frequency (RLF, with magnitude below 10 HZ) increases from 0.293 Ω cm2 to 0.829 Ω cm2. Generally, RLF is related to the surface exchange reaction and O2 diffusion. TPD results show CO2 desorption will occur when the temperature increases to 850 ℃. So, the RLF increases with increasing of the operated temperature. Fig. 6(a) shows the I–V curves of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis with different ratios of H2/CO2 at 800 ℃. As hydrogen content increased, the performance in the SOFC mode was gradually improved. Different from Ni/YSZ cathode based SOEC with worse performance of CO2 electrolysis [14], the SOEC performance based on LSCFN cathode was relatively stable even in the lower content of CO2. This result show that LSCFN cathode has better catalytic activity and stability than normal Ni/YSZ cathode. The I–V curves of the latter for CO2 electrolysis in different atmospheres were parallel lines, indicating the performance in the SOEC mode decreased with the reducing gas molar fraction ratio increased. Therefore, LSCFN cathode works well in a reducing atmosphere, and its performance remains stable without deterioration. The corresponding impedance spectra are shown in Fig. 6(b). The ohmic and polarization resistance decreased with increasing hydrogen content. Fig.6 (c) and (d) shows the I–V curves of LSCFN-GDC|YSZ|GDC-LSCFN for CO2 electrolysis in different ratios of CO/CO2 at 800 ℃. The trending is the same as that with hydrogen but slightly decreases because the cell resistance was higher in a CO atmosphere. Also, reverse water-gas shift (RWGS) reaction also helps to promote CO2-H2O co-electrolysis [32].
In summary, the LSCFN perovskite has been fabricated and investigated for CO2 electrolysis as electrode materials in a wide range of operating conditions. The current density reached 0.442 A cm−2 @ 1.5 V and 800 ℃ in a pure CO2 atmosphere, which was much higher than that reported for LST and LSCM. When a reducing gas was added, the electrode resistance, especially the polarization resistance, decreased as the amount of reducing gas increased. It was led by the increase in oxygen vacancy concentration and the precipitation of a small amount of CoFe alloy in the reducing atmosphere, as observed in the XRD and TPR profiles. With the increase of H2/CO content, the current density approached to a plateau, which was different from those of Nicermet and any other cathodes. The cell performance in CO2/H2 atmosphere was better than that in CO2/CO, resulted from two facts: the reverse water gas shift reaction occurred when H2 existed; and the polarization resistance of the cell with H2 atmosphere was lower than that under the CO atmosphere. The cell performance had a small degradation after 150 hs stability test with 70% CO2-30% H2 under 800 °C, which maybe resulted from the pore structure deterioration. However, no significant agglomerations, carbon deposit or other secondary phase generation were observed. Therefore, it can be concluded that LSCFN is a promising electrode material for CO2 electrolysis in SOEC with enhanced catalytic activity and extended stability, even in a reducing atmosphere. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Financial supports from the National Key Research and Development Program of China (2017YFB0601904), Beijing Municipal Science and Technology Project (Z181100005118008), National Natural Science Foundation of China (21773167), and the Yueqi Young Scholar Project (CUMTB) are greatly appreciated. References
3.5. Long-term stability test
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Fig. 7 shows the long-term test of the LSCFN-GDC|YSZ|GDC-LSCFN cell for CO2 in H2/CO2 = 3/7 atmosphere with constant current density 0.24 A cm−2 at 800 ℃. The cell voltage increases from 1.305 to 1.379 V, exhibiting only 5.6% degradation after 150 h stability test, which is better than that of La0.3Sr0.7Fe0.7Ti0.3O3 electrode material, approximately 5% decrease during 24 h operation likely due to the delamination of LSFT oxygen electrode under anodic polarization [16]. The SEM images of the LSCFN-GDC|YSZ|GDC-LSCFN cell before and after stability test are presented in Fig. 8. The anode and cathode thickness are about 20 and 28 μm respectively. The electrode isolation layer and electrolyte interfaces show good contact without obvious delamination. By comparing Fig. 8(b) and (d), it implies that the electrode pore structure deteriorated after the stability test, which should be the signs for the slight performance degradation. No significant agglomerations can be observed after the test. Fig. 8(e) shows the Raman spectra collected from cathode surface before and after the stability test. No observable carbon peaks are detected from the cathode surface because two typical carbon characteristic peaks at 1338 (D-band) and 1568 cm−1 (G-band) were not observed in the Raman spectrum [21]. Similarly, no other secondary phase is observed. Therefore, the LSCFN is an electronically active and stable electrode materials for CO2 electrolysis. 450
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