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Research Article pubs.acs.org/journal/ascecg

Mixed-Conductor Sr2Fe1.5Mo0.5O6−δ as Robust Fuel Electrode for Pure CO2 Reduction in Solid Oxide Electrolysis Cell Yihang Li, Xinran Chen, Yi Yang, Yunan Jiang, and Changrong Xia* CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province 230026, P. R. China ABSTRACT: Electrolysis of carbon dioxide to carbon monoxide, through which the greenhouse gas could be effectively utilized, using solid oxide electrolysis cells is now attracting much interest. Here, we show for the first time that the redox-stable Sr2Fe1.5Mo0.5O6−δ (SFM) ceramic electronic-ionic conductor can be used as the electrocatalyst to electrolyze and convert 100% CO2 to CO without using any safe gases like H2 and CO. SFM maintained its cubic structure and had an electrical conductivity of 21.39 S cm−1 at 800 °C in 1:1 CO−CO2 atmosphere. Its surface reaction coefficient for CO2 reduction is 7.15 × 10−5 cm s−1 at 800 °C. Compared with those reported for the typical oxide ceramic electrodes, high electrochemical performance has been demonstrated for single phase SFM cathode using 100% CO2 as the feeding gas. For example, a current density of 0.71 A·cm−2 was obtained using a fuel cell supported on LSGM (La0.9Sr0.1Ga0.8Mg0.2O3−δ) electrolyte operated at 800 °C and an applied voltage of 1.5 V. The electrolysis performance was further improved by using SFM−Sm0.2Ce0.8O2−δ composite cathode, and the current density increased to 1.09 A·cm−2 under the same operation conditions. Durability test at 800 °C for 100 h demonstrated a relatively stable performance for CO2 electrolysis under harsh conditions of 100% CO2 without safe gas and above 1 A cm−2 current density, which is seldom achieved in the literature but highly desirable for the commercial application, indicating that SFM is a highly promising ceramic fuel electrode for CO2 electrolysis. KEYWORDS: Solid oxide electrolysis cells, Mixed ionic and electronic conductor, Fuel electrode, CO2 reduction, Sr2Fe1.5Mo0.5O6−δ

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where e− is an excess free electron in n-type conductor, V•• O is the oxygen vacancy provided by the electrode and/or electrolyte materials, and O×O is the lattice oxygen atom. Based on the oxygen reduction reaction mechanism proposed by Muñoz-Garciá and Pavone,6 the CO2RR processes might consist of elementary steps as follows,

INTRODUCTION CO2 electrolysis was initially investigated by NASA in 1960s to supply O2 for Mars explorations because of 95% CO2 in the atmosphere on the Mars.1 In recent years, the electrolysis process has become more and more focused since it is the key reaction for utilizing CO2 and thus reducing CO2 emission, which is important for the sustainable development of human society. The inherently stable carbon−oxygen bond in CO2 makes the electrolysis reaction particularly challenging to catalyze at room temperature.2 It is thus impressive to develop CO2 electrolysis techniques operated at elevated temperatures. Solid oxide electrolysis cells (SOECs), which are operated at temperatures up to 1000 °C, are widely considered as a promising technique that could electrolyze CO2 to CO and meanwhile convert renewable energy such as solar and wind energies to chemical energy.3,4 Under external voltage, the pure or high concentrated CO2 captured from the atmosphere can be electrolyzed into CO on the fuel electrode (cathode) of SOEC while releasing oxygen on the oxygen electrode (anode) with negligible side reactions. The cathodic CO2 reduction reaction (CO2RR) is usually expressed in the Kröger-Vink notation as5 CO2 + 2e′ +

V •• O

= CO +

× OO

© 2017 American Chemical Society

CO2 adsorption,

CO2 (g) ⇄ CO2 (ad)

(1a)

CO2 activation,

•• CO2 (ad) + V •• O ⇄ (CO2 )O

(1b)

Charge transfer,

• × (CO2 )•• O + e′ ⇄ (CO2 )O + OO

(1c)

Charge transfer,

× (CO2 )•O + e′ ⇄ OO + CO(ad)

(1d)

CO desorption,

CO(ad) ⇄ CO(g)

(1e)

So, the cathode must have high catalytic activity and electronic and ionic conductivities. The state-of-the-art fuel electrodes are Ni-YSZ (yttria stabilized zirconia) cermets, where Ni serves as not only the electrocatalyst but also the electronic conductor. However, NiReceived: July 24, 2017 Revised: October 11, 2017 Published: October 30, 2017

(1) 11403

DOI: 10.1021/acssuschemeng.7b02511 ACS Sustainable Chem. Eng. 2017, 5, 11403−11412

Research Article

ACS Sustainable Chemistry & Engineering

LSGM and SFM in the cell fabrication and operation processes. The LSGM and LSCF powders from the Fuel Cell Materials were used without further treatment. SFM powders were synthesized by citric acid−glycine combustion method, while SDC and LDC powders were prepared by the glycine−nitrate combustion process starting with nitrate precursors. Details of the synthetic procedures have been reported in our previous work.28,29 The LSGM electrolyte substrates were fabricated by dry-pressing LSGM powder uniaxially under 300 MPa and then sintering at 1450 °C for 5 h in air. The electrolyte pellets were ∼230-μm thick and 15 mm in diameter. LDC paste, prepared with the LDC powder and terpilenol, was printed on one side of the LSGM pellet, followed by sintering at 1250 °C for 5 h, forming an ∼5 μm thick LDC, the interlayer between SFM electrode and LSGM electrolyte. For a half cell, the SFM work electrode (WE) was screen-printed on the LDC side, followed by drying and sintering at 1000 °C for 2 h, while 5-μmthick Pt counter electrode (CE) was on the other side. Au paste (Sinoplatinum metals Co., Ltd.) brushed onto the surface of SFM electrode and heated at 500 °C for 30 min served as the current collector. For a full cell, SFM or SFM-SDC fuel electrode was screen-printed on the surface of the LDC layer while the LSCF-SDC oxygen electrode was on the other side of the LSGM electrolyte (an effective area of 0.2376 cm2), followed by sintering at 1000 °C for 2 h. Characteristics. X-ray diffraction (XRD) analysis was performed at room temperature using a Philips X’Pert Pro MPD diffractometer with Cu Kα radiation, tube voltage 45 kV, and tube current 40 mA. Intensities were collected with a step size of 0.05° and a measuring time of 5 s at each step. More detailed structural information was analyzed by the Rietveld method with the GSAS program and the EXPGUI interface to determine the space group and to approximate the lattice parameters. Subsequently, XPS (X-ray photoelectron spectroscopy, Kratos Analytical AXIS 165 with monochromatic Al Kα source) was performed to determine the oxidation states of Fe and Mo. The microstructures of the post-test cells were investigated by means of field emission scanning electron microscopy (FE-SEM, JEOL JSM7600F). The high-resolution transmission electron microscopy (HRTEM) was performed on a JEOL JEM-ARM200F TEM/STEM with a spherical aberration corrector. In all of this work, the electrochemical measurements to evaluate the cell performance were performed with a total flow rate of 20 mL min−1 for feeding gas and in a temperature range of 650−850 °C. For the half cell as shown in Figure 1, it was exposed to a 1:1 mixture of CO and CO2. At open

YSZ could not be used for electrolysis of pure or concentrated CO2, because the oxidation of Ni and carbon deposition are two big issues when Ni particles are constantly exposed to concentrated CO2 atmosphere, in which the oxygen partial pressure is high enough to convert Ni to NiO that has neither catalytic activity nor electrical conductivity.4,7 Thus, safe gas like H2 and CO must be supplied to the Ni-YSZ electrode to maintain nickel in the metal state, which requires a more complicated and higher cost feeding system.8−12 So, alternative redox stable electrode materials have been developed for the electrolysis of concentrated CO2. Pt has been investigated by Tao et al. for the electrolysis of pure CO2 using symmetrical cells with YSZ electrolytes. The activation overpotentials were very large.13 Subsequently, Pt-YSZ composites were studied as the cathodes, which exhibited significantly improved performance.14 The nobel metal-based fuel electrodes are too expensive for commercial application. To reduce the cost, mixed ionic-electronic conducting (MIEC) perovskites have been developed as the potential cathode materials. Typical redox stable perovskites include strontium titanate-based oxides (e.g., La0.2Sr0.8TiO3+δ, LST for short) and lanthanum chromate-based oxides such as La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM).15−18 These cathode materials have demonstrated great stability at 600−850 °C for electrolyzing pure CO2 without the addition of any safe gases. For example, Li et al. have fabricated YSZ electrolyte-supported SOECs with LST based fuel electrodes. A current density of 0.05 A cm−2 was obtained at 800 °C and 1.5 V, while it was increased to 0.15 A cm−2 through Mn doping due to the enhanced chemical adsorption of CO2.19,20 Besides, a current density of 0.09 Acm−2 was reported for a SOEC with LSCMbased fuel electrode under the same conditions.18 However, their performance is still insufficient for future practical application. Therefore, it is highly desired to develop fuel electrode materials to enable direct CO2 electrolysis. A well-established Sr2Fe1.5Mo0.5O6−δ (SFM) has shown good redox stability.21 It thus has been investigated as both the cathode and the anode for solid oxide fuel cells (SOFCs).22,23 Our recent studies demonstrated that SFM displayed fast surface exchange rates in both oxidizing and reducing conditions, especially when doped ceria cooperated.24,25 We have also found that SFM is very impressive in application for CO2 electrolysis. When SFM nanoparticles were deposited in porous YSZ backbones using the infiltration technique, the cathode exhibited excellent performance for CO2−H2O coelectrolysis.26 Improved performance in CO2 electrolysis was observed when Ni−Fe alloy nanoparticles formed on SFM using the exsolvation process with nonstoichiometric SFM as the precursors.27 However, studies are rarely conducted to reveal the inherently physical and chemical properties of SFM as the cathode for CO2 electrolysis. In this regard, this work presents a comprehensive study toward SFM as the cathode material, including structural stability, conductivity, and durability under the conditions for CO2 electrolysis.

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Figure 1. Schematic illustration of a half cell comprising a Pt counter electrode and SFM working electrode (a) and electrode reactions under positive (b) and negative (c) bias for SOEC and SOFC model, respectively.

EXPERIMENTAL METHODS

circuit conditions, there was not any net current flowing through the cell. The SFM electrode materials reached equilibrium with CO and CO2 in the gas phase. An applied bias could drive the electrode reactions and thus caused net current. A positive potential drove electrons from the Au current collector to SFM electrode, reacted with CO2 to form oxygen ion as shown in eq 1, and moved the oxide ions from the SFM electrode to the LSGM electrolyte, and finally the reaction oxidized CO at the Pt electrode. Thus, the positive biases

Materials and Cell Preparation. Full single cells and half single cells were prepared for electrochemistry analysis. The full cells were supported on LSGM (La0.9Sr0.1Ga0.8Mg0.2O3−δ) electrolytes with SFM (Sr2Fe1.5Mo0.5O6−δ) or SFM-SDC (Sm0.2Ce0.8O1.9) fuel electrodes and LSCF (La0.6Sr 0.4 Co0.2 Fe 0.8 O3−δ)-SDC oxygen electrodes. LDC (La0.4Ce0.6O2−δ) was used as the interlayer between LSGM electrolyte and fuel electrode to prevent the mutual element diffusion between 11404


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Figure 2. Room-temperature powder X-ray diffraction patterns for (a) oxidized SFM heated at 1000 °C in air for 5 h and (b) reduced SFM heated at 800 °C in 1:1 CO−CO2 stream for 5 h; Corresponding Rietveld refinement for (c) the oxidized and (d) the reduced powders (Obs, Observed; Cal, Calculated; Bac, Background); XPS spectra of the oxidized and the reduced SFM powders for (e) Fe 2p and (f) Mo 3d signals. Electrical Conductivity Relaxation Measurements. The electrical conductivity relaxation (ECR) measurements were performed by the standard 4-probe technique. To prepare the bar samples, SFM powders were mixed with 3 wt % PVA (poly(vinyl alcohol)) with ball milling for 2 h. The powder mixtures were uniaxially pressed at 300 MPa and sintered in air at 1350 °C for 5 h to obtain rectangular bars with dimensions of about 20 × 4.8 × 0.6 mm3. Their relative density measured by the Archimedes method was well above 98%. Electrical connections to the sample were established using silver wires. Silver conducting resin (Shanghai Institute of synthetic resin, DAD87) was used to improve the contact between the bar specimen and the wires. The sample was fixed in a quartz tube and heated to the measured temperatures. Then, the conductivity response to a stepwise change in atmosphere was measured as a function of time. The atmosphere change was performed to conduct the CO2 reduction reaction by increasing the CO2 content from 33.3% to 50.0% in CO−CO2 mixtures, i.e., from 2:1 CO/CO2 to 1:1 CO/CO2, corresponding to increasing the oxygen partial pressure, PO2, from 9.49 × 10−20 to 3.79 × 10−19 atm at 800 °C. Meanwhile, CO oxidation

promoted electrochemical CO2 reduction reactions in the SFM electrode and CO oxidation reactions at the Pt electrode. The reverse reactions occurred when the potential bias was switched to negative, i.e., electrochemical oxidation of CO at the SFM electrode.30 I−V measurements (Line Scanning Voltage, sweep rate of 10 mV·s−1) between −1.5 and 1.5 V were performed using a Solartron 1287 interface, with control and data collection handled by Corrware software. For the full cell measurement, CO2 (99.999%, Nanjing special gas Factory Co., Ltd.) was fed to the SFM and SFM-SDC cathodes while the LSCF-SDC anodes were exposed to ambient air. I− V was performed from 0 to 1.6 V for CO2 electrolysis. Electrochemical impedance spectra (EIS) were collected under open circuit and polarization conditions through a Solartron 1260 analyzer using an amplitude of 10 mV in the frequency range of 0.01 to 1 MHz. The durability was evaluated by monitoring the current density under a constant applied voltage. The CO2 and CO contents of outlet gas were determined by online gas chromatography (FULI, GC9790II) using a thermal conductivity detector. 11405


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ACS Sustainable Chemistry & Engineering Table 1. Summary of the Rietveld Refinement Results for the Oxidized and Reduced SFM Powders sample

space group

a (Å)

b (Å)

c (Å)

ωRp (%)

Rp (%)

χ2

oxidized SFM reduced SFM

Pm3̅m Pm3̅m

3.916 3.925

3.916 3.925

3.916 3.925

4.43 4.92

5.98 6.30

1.267 1.416

reaction was performed by decreasing the CO2 content from 50.0% to 33.3% in CO−CO2 mixtures. The gas flow rate was 200 mL min−1. Detailed descriptions of the ECR technique and the model used for data fitting could be found in our previous work.24,25,31,32

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RESULTS AND DISCUSSION Figure 2a shows the room-temperature XRD pattern of the asprepared SFM powder, which was heated in an oxidized atmosphere of air at 1000 °C for 5 h. The oxidized SFM powder showed a single phase of cubic perovskite structure, as identified by the XRD analysis and previously reported results.21 When the powder was treated with a reducing atmosphere (PO2 = 3.79 × 10−19 atm) of 1:1 CO−CO2 gas mixture for 5 h at 800 °C, not any impurity phases are observed as shown in Figure 2b, suggesting the structural and chemical stability under the typical conditions of electrolyzing CO2 at temperatures up to 800 °C. It is noted that the diffraction peaks were shifted slightly to lower angles by the treatment in the CO−CO2 mixture. For example, the diffraction angle was decreased from 32.30° to 32.06° for peak (110). Normally, lower 2θ angles indicate lattice expansion. It is probably due to the lattice oxygen loss, reducing the average valence of Fe and Mo ions as confirmed by the XPS fitting results of Fe 2p3/2 and Mo 3d5/2 spectra. In Figure 2e, Fe 2p3/2 signal is fitted by three peaks, which correspond to Fe4+ (r = 0.585 Å), Fe3+ (r = 0.645 Å), and Fe2+ (r = 0.78 Å).33 The concentrations are 40.22% for Fe4+, 36.18% for Fe3+, and 23.60% for Fe2+. When SFM is treated in CO−CO2 atmosphere, they are changed to 35.65%, 34.46%, and 29.89%, respectively. In this regard, the average oxide state of Fe can be determined, from +3.16 to +3.06. In the case of Mo 3d spectra as shown in Figure 2f, the former contains only Mo6+, while the latter indicates two contributions, 87.19% for Mo6+ (r = 0.59 Å) and 12.81% for Mo5+ (r = 0.61 Å).33 Therefore, the average oxide state of Mo is reduced from +6 to +5.87. The lattice expansion phenomenon was further analyzed by refining the room temperature XRD patterns based on the cubic structure (space group: Pm3̅m, No. 221), Figure 2c,d. The reliability of the Rietveld refinement is determined by ωRp, Rp, and χ2. The refinement gave ωRp, Rp, and χ2 values of 4.43%, 5.98%, and 1.267 for the oxidized SFM as well as 4.92%, 6.30%, and 1.416 for the reduced SFM, respectively, indicating the goodness of fit. The results showed that the reduced SFM has a slightly larger lattice constant of a = b = c = 3.925 Å than the oxidized SFM (a = b = c = 3.916 Å), Table 1. Such fully disordered distribution over the Fe and Mo sites with space group Pm3̅m has already reported for SFM structure by several groups,33−35 such as Liu and co-workers using powder X-ray diffraction method.33 However, Muñoz-Garciá suggested an orthorhombic structure with Pnma for SFM based on powder neutron diffraction analysis.22 In addition, a tetragonal I4/mcm and a Fe−Mo ordered structure with Fm3̅m have been also reported for SFM.34,36 The observed discrepancy between the different research groups could be attributed to the differences in the material preparation conditions.34 The particle size of the SFM powder was in the range of 200−300 nm, as indicated by the TEM image in Figure 3a. The

Figure 3. (a) TEM image of the reduced SFM powder and (b) HRTEM image of a typical SFM particle. Inset at the lower-left is the corresponding fast Fourier transformation pattern.

high-resolution TEM image is shown in Figure 3b. The two mutually perpendicular facets are indexed to (100) and (010) facets, and the facet at the angle of 45° can be indexed to the (110) facet, while the Fourier transformation (FFT) pattern is depicted as in the lower-left inset of Figure 3b. They could be indexed to the simple cubic perovskite structure where the reflection condition corresponds to space group Pm3̅m, consistent with the Rietveld analysis. The electrical conductivity is critical for electrode materials. Figure 4 shows the temperature-dependent conductivity of

Figure 4. Temperature dependence of total conductivity of SFM in air and 1:1 CO−CO2 gas mixture.

SFM in both oxidizing and reducing atmospheres. At 800 °C, the conductivity in air is 115.92 S cm−1 while it is 21.39 S cm−1 in 1:1 CO−CO2. In the oxidizing atmosphere with high oxygen partial pressure (PO2), the electrical charge is mainly balanced by Fe4+ and Mo6+, and the charge carriers are dominated by 1 × • electronic holes, 2 O2 + V •• O = OO + 2h . In the reducing conditions with low-PO2, electroneutrality is maintained by the 1 × formation of oxygen vacancies (OO = 2 O2 + V •• O + 2e′) associated with the reduction of Fe4+ to Fe3+/2+ and Mo6+ to Mo5+.21,37 The reduction reaction creates charge carriers of oxygen vacancies as shown by eqs 2 and 3, in which × and ′ 11406


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measurements, which were conducted to study the surface exchange properties.24 CO2 reduction was performed by changing the atmosphere from 2:1 CO−CO2 (PO2 = 9.49 × 10−20 atm at 800 °C) to 1:1 CO−CO2 (PO2 = 3.79 × 10−19 atm). The increase in CO2 concentration increases PO2 via the gas phase equilibrium reaction of

refer to neutrality and the unit negative charge of Fe or Mo with respect to the lattice. 1 × OO + 2Fe×Fe = 2Fe′Fe + O2 + V •• O (2) 2 1 O2 + V •• O (3) 2 Consequently, the oxygen ionic conductivity could be increased due to the increase in the concentration of oxygen vacancies. Meanwhile, the reduction also increases the concentration of electrons and, thus, increases the total conductivity.37−39 The I−V responses at the SFM electrodes were investigated using a half cell for the CO2 electro-reduction as well as the reverse reaction, the CO electro-oxidation, Figure 5a. Positive × OO + 2Mo×Mo = 2Mo′Mo +

CO2 = CO +

1 O2 2

(4)

Combining eqs 2,3, and 4 gives eqs 5 and 6, which are both equivalent to eq 1a in SFM structure. × × 2Fe′Fe + CO2 + V •• O = OO + 2Fe Fe + CO

(5)

× × 2Mo′Mo + CO2 + V •• O = OO + 2Mo Mo + CO

(6)

Thus, the conductivity relaxation process associated with the increase of CO2 concentration corresponds to the CO2 reduction reaction.24 On the contrary, CO oxidation occurs when the atmosphere is switched to low CO2 content. As shown in Figure 5b, the re-equilibrium time is about 1930 s for CO2 reduction, much lower than 2560 s for CO oxidation. The smaller re-equilibrium time demonstrates faster surface reaction rate since the relaxation process is dominated by the surface reaction.42 The surface reaction rate constant, Kchem, obtained by fitting the measured ECR curves, is 7.15 × 10−5 cm s−1 at 800 °C for CO2 reduction, much higher than 4.91 × 10−5 cm s−1 for CO oxidation. The difference in Kchem further suggests that SFM is even better as a catalyst for CO2 reduction in SOEC than for CO oxidation in SOFC. To characterize the electrochemical performance of the SFM cathode for CO2 reduction in SOEC, bilayer electrolytesupported cells were used with an ∼230-μm-thick LSGM electrolyte and a 5-μm LDC barrier layer. Figure 6a shows the temperature-dependent performance for a single cell consisting of a SFM cathode, a bilayer electrolyte, and an LSCF-SDC composite anode using 100% CO2 as the feeding gas to the cathode. A higher operating temperature results in faster reaction rate, i.e., higher current density, which can be attributed to the increased conductivity and enhanced catalytic activity at elevated temperature. The current densities were 0.26, 0.33, 0.49, and 0.71 A·cm−2 at operation temperatures of 650, 700, 750, and 800 °C, respectively, when 1.5 V was applied. The current density, 0.71 A·cm−2, is higher than almost all of the reports, from 0.05 to 0.47 A·cm−2, for electrolyzing pure CO2 at 800 °C using redox stable oxide electrode materials, Table 2. It is also comparable to 0.75 A·cm−2 reported by Liu et al. using a YSZ electrolyte-supported cell with a LSFN (La0.6Sr0.4Fe0.8Ni0.2O3−δ) cathode and a LSCFGDC (gadolinia doped ceria) anode.43 It is noted that Ni could be exsolved from perovskite LSFN in the reducing atmosphere (3:7 CO:CO2). The EIS studies were further carried out at open circuit voltages (Voc), Figure 6b. The ohmic resistance (Ro) is the spectra intercept with the real-axis at high frequency, indicative of the electrolyte resistance, while the total polarization resistance (Rp) is calculated from the distance between the two intercepts of the depressed arcs with the real axis.16 As summarized in Table 3, the Rp values are 6.703, 3.152, 1.589, and 0.928 Ω cm2 at 650, 700, 750, and 800 °C, respectively. The Rp at 800 °C, 0.928 Ω cm2, is much lower than those previously reported at Voc using redox stable oxide cathodes (Table 2), such as 12 Ω cm2 for LST-SDC,19 3.2 Ω cm2 for LSCM-SDC,18 and 1.33 Ω cm2 for LSCrFe.44 Besides, in Figure 6c, the Rp was as low as 0.239 Ω cm2 at 800 °C when

Figure 5. (a) Polarization curves for SFM in a half-cell configuration at various 700−850 °C; (b) electrical conductivity relaxation curves (Normalized conductivity as a function of time) of CO2 reduction and CO oxidation for a SFM bar at 800 °C.

current density refers to CO2 reduction, the cathode reaction in SOEC, while negative current density refers to CO oxidation, the anode reaction in SOFC. It is reported that SFM is a good anode material for SOFC using H2, hydrocarbon, and CO fuels.40,41 Figure 5a shows that SFM appears to be a significantly better catalyst for CO2 reduction in SOEC than CO oxidation in SOFC. For example, at 800 °C, the current density is 0.902 A·cm−2 under an applied bias of +1.5 V, much higher than 0.665 A·cm−2 under −1.5 V. The CO2 reduction and CO oxidation reactions were further compared with electrical conductivity relaxation (ECR) 11407


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Table 2. Current Density at 1.5 V and Polarization Resistance for Pure CO2 Electrolysis Using Different Fuel Electrodes That Are Operated at 800 °Ca fuel electrode

electrolyte

LSTSDC LSTMn

YSZ

LSTFe LSCMSDC LSCrFe CeLSCrFe LSFN

YSZ YSZ

YSZ

YSZ YSZ YSZ

LSFM SFM

LSGM LSGM

SFMSDC

LSGM

oxygen electrode

polarization resistance (Ω cm2)

current density (A cm−2)

refs

LSMSDC LSMSDC LSTFe LSCMSDC LSCrFe LSCF

12 at Voc

0.05

19

1.5 at 1.2 V

0.15

20

0.35 at 1.2 V 3.2 at Voc

0.28 0.09

45 18

1.33 at Voc 0.21 at 1.2 V

0.32 0.47

44 46

LSCFGDC BLC LSCFSDC LSCFSDC

0.16 at Voc

0.75

47

-0.239 at 1.5 V

0.21 0.71

0.190 at 1.5 V

1.09

48 this work this work

a

LST = La0.2Sr0.8TiO3−δ; LSM = (La0.8Sr0.2)0.95MnO3−δ; LSTFe La0.3Sr0.7Ti0.3Fe0.7O3−δ; LSTMn = La0.2Sr0.8Ti0.9Mn0.1O3−δ; LSFN La 0.6 Sr 0.4 Fe 0.8 Ni 0.2 O 3−δ ; GDC = Gd 0.2 Ce 0.8 O 2−δ ; LSFM La 0.4 Sr 0.4 Fe 0.9 Mn 0.1 O 3−δ ; BLC = La 0.4 Ba 0.6 CoO 3 ; LSCrFe La0.3Sr0.7Cr0.3Fe0.7O3−δ.

= = = =

Table 3. Fitting Results of EIS Data for SOECs with SFM and SFM-SDC Cathodes under Open-Circuit Conditions for CO2 Electrolysis samples SFM

Figure 6. Electrochemical performance of the cell with SFM electrode at 650−800 °C: (a) I−V curves; (b) electrochemical impedance spectra at Voc; and (c) electrochemical impedance spectra at 800 °C under 1.5 V.

SFM-SDC

resistance (Ω cm2)

800 °C

750 °C

700 °C

650 °C

Ro Rp Ro Rp

0.509 0.928 0.486 0.512

0.823 1.589 0.731 1.126

1.305 3.153 1.059 2.642

2.030 6.703 1.630 6.392

increment of about 53.5% at 800 °C and 1.5 V applied voltage. The improvement must be attributed to the SFM-SDC cathode since the electrolyte and LSCF-SDC anode were generally the same for the two cells. To the best of our knowledge, this is the best performance among all the reported oxide electrodes to date for the direct CO2 electrolysis, verifying the promise of using SFM as a SOEC cathode material for CO2 electrolysis. The impedance spectra were measured at 650−800 °C under open circuit conditions, Figure 7b. The corresponding simulated values are summarized in Table 3. In comparison with the cell with SFM cathode, the interfacial polarization resistance of the cell with SFM-SDC cathode was significantly reduced. For example, at 800 °C, the Rp value at Voc was reduced from 0.928 to 0.512 Ω cm2, while Rp at 1.5 V was reduced from to 0.239 to 0.190 Ω cm2, indicative of the remarkable enhancement of the electrochemical CO2 reduction reaction kinetics.49 It should be also noted that Ro was slightly reduced, possibly due to the increased ionic conductivity of the electrode as a result of SDC addition. Figure 8a presents the short-term variations of current density recorded versus time and the corresponding applied voltages. The current densities increased with the applied voltages, consistent with the performance response shown in Figure 7a. The CO production rate increased from 3.62 to 9.01 mL min−1 cm−2 with the Faradaic efficiency above 95% when the applied voltage was increased from 1.0 to 1.6 V. The

the applied voltage was 1.5 V, as expected from the I−V characteristics. The reduction in Rp could be explained by the applied potential, which creates strong reducing conditions,17 leading to faster charge transfer and enhanced CO2 adsorption. The relatively high current density and low polarization resistance for CO2 electrolysis in full cells further demonstrates that SFM is an excellent candidate for CO2 reduction reactions. When SFM is used as the anode material for SOFC, it is demonstrated that its electrochemical performance could be improved by adding a second phase electrolyte such as doped ceria.28 Adding oxygen ion-conducting materials into electrodes often results in enhanced surface reaction rate in addition to a spatial expansion of the triple phase boundaries (electrochemical reaction zone), thus reducing the electrode polarization losses.28 So, SDC was cooperated to improve the catalytic activity for CO2 reduction since SDC is not only a good catalyst for electrochemical redox reactions but also an excellent oxygen ion conductor.49 Figure 7a shows the temperature-dependent I−V curves of the SOEC using SFMSDC cathode for 100% CO2 electrolysis. The current densities were 0.40, 0.54, 0.75, and 1.09 A cm−2 at 650, 700, 750, and 800 °C, respectively, when 1.5 V was applied. It is clear that the cell performance is substantially improved by replacing SFM cathode with SFM-SDC composite. For example, the current density was improved from 0.71 to 1.09 A cm−2 with an 11408


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Figure 7. Electrochemical performance of the full cell with SFM-SDC composite electrode: (a) I−V curves at various temperatures; (b) electrochemical impedance spectra at Voc and different temperatures; (c) electrochemical impedance spectra at 800 °C and 1.5 V.

Figure 8. Durability test for pure CO2 electrolysis at SFM-SDC cathode. (a) Variations of current density under different applied voltages; (b) current density recorded at 800 °C under constant applied voltage of 1.5 V; (c) EIS plots measured at Voc for the SOEC before and after 100-h durability test.

durability for direct CO2 electrolysis was examined at a current density above 1 A cm−2, a very harsh condition that is seldom achieved in the literature but highly desirable for commercial applications.4 The potentiostatic test was conducted for more than 100 h under a constant applied potential of 1.5 V. The current density variation as a function of time is presented in Figure 8b. There is only a slight drop in current density during the initial 8 h operation. Subsequently, a steady state is observed at current density above 1.0 A cm−2, with very impressive stable cell performance under such harsh conditions including over 1.0 A cm−2 current density and pure CO2 electrolysis. Figure 8c compares the impedance spectra, which were measured at 800 °C under open circuit conditions before and after the 100-h durability test. The ohmic resistance was negligibly changed, from 0.484 to 0.488 Ω cm2. The total interfacial polarization resistance was slightly increased from 0.512 to 0.557 Ω cm2, in agreement with the slight decrease in current density during the initial 8 h operation, Figure 8b. Consequently, it is concluded that SFM is an excellent fuel electrode for direct CO2 electrolysis since it has demonstrated impressive electrochemical performance and good stability under harsh operation conditions. After the durability testing, the cell was broken into small pieces. Figure 9a shows the XRD pattern for a small piece of the SFM-SDC electrode that had been used for durability measurement. It can be seen that the main phases are still SFM

and SDC, and no obvious impurity peaks are observed, indicating that both SFM and SDC kept their phase structure in the harsh test conditions. Figure 9d,e shows the microstructures of the SOEC after the durability test. No delamination is observed at the electrode/electrolyte interfaces, suggesting high thermal compatibility between the SFM-SDC composite electrode and the electrolyte, Figure 9b. Similarly, LSCF-SDC shows very good bonding with the dense LSGM electrolyte, Figure 9d. In addition, high-resolution images of SFM-SDC (Figure 9c) cathode and LSCF-SDC anode (Figure 9e) demonstrate that the electrodes maintained the porous structure in the durability test.

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CONCLUSIONS SFM, which has been reported as the anode material for SOFC, was examined as the cathode toward pure CO2 electrolysis in SOEC. SFM demonstrated chemical and structural stability at 800 °C in CO−CO2 gas mixture. Its conductivity was 21.39 S cm−1 at 800 °C in 1:1 CO−CO2 atmosphere. Its catalytic activity for CO2 electrolysis in SOEC was much higher than that for CO oxidation in SOFC. Under an applied voltage of 1.5 V, the current density of 0.71 A cm−2 was obtained at 800 °C with fuel cells consisting of SFM cathodes, bilayer LDC/LSGM electrolytes, and LSCF-SDC anodes. The current density increased to 1.09 A cm−2 when SDC was added to SFM to 11409


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Figure 9. Microstructures of the fuel cell after the 100 h durability test. (a) XRD pattern of the SFM-SDC electrode; (b) cross-sectional SEM image for SFM-SDC cathode and the bilayer electrolyte; (c) high-resolution image of the SFM-SDC cathode; (d) cross-sectional image of LSCF-SDC anode and LSGM electrolyte, and (e) high-resolution image of the LSCF-SDC anode.

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from a SFM-SDC composite electrode. Meanwhile, the Rp value was reduced from 0.239 Ω cm2 to 0.190 Ω cm2. The SFM-SDC cathode demonstrated good durability when the cell was operated at 1.5 V and 800 °C for more than 100 h under harsh conditions of 1 A·cm−2 current density and using 100% CO2 as the feed gas. Therefore, this study reveals that SFM is a promising fuel electrode for direct CO2 electrolysis in SOECs.

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AUTHOR INFORMATION

Corresponding Author

*(C.X.) Tel.: +86-551-63607475. Fax: +86-551-63601696. Email: [email protected]. ORCID

Changrong Xia: 0000-0002-4254-1425 Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

The authors are grateful to the financial support from National Natural Science Foundation of China (91645101 and 51372239) and Anhui Estone Materials Technology Co., Ltd. (2016340022003195). 11410


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