Ce0.6Mn0.3Fe0.1O2-δ membrane fuel electrode fabricated by infiltration method for solid oxide electrochemical cells

Electrochimica Acta 235 (2017) 365–373

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Cu/Ce0.6Mn0.3Fe0.1O2-d membrane fuel electrode fabricated by infiltration method for solid oxide electrochemical cells Limin Liua,b , Yong Wanga , Xiaoliang Zhoud,* , Yixuan Yanga , Chao Maa , Yuan Lia , Chen Wangc , Xiaohong Zhaoa,* , Bin Lia,b,** a Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials, College of Science, Northeast Forestry University, Harbin, 150040, China b Post-doctoral Mobile Research Station of Forestry Engineering, Northeast Forestry University, Harbin, 150040, China c Material Science and Engineering College, Northeast Forestry University, Harbin, 150040, China d Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, Heilongjiang, 150080, PR China

A R T I C L E I N F O

Article history: Received 6 December 2016 Received in revised form 8 March 2017 Accepted 10 March 2017 Available online 12 March 2017 Keywords: Solid oxide electrochemical cell Doped ceria Cermet electrode CO oxidation CO2 reduction

A B S T R A C T

A cermet fuel electrode Cu/Ce0.6Mn0.3Fe0.1O2-d (Cu/CMF) is proposed for a reversible solid oxide electrochemical cell (SOC) with CO/CO2 as the shuttle. Cu is inert for the carbon deposition and CMF is electrochemically active for CO oxidation and CO2 reduction. The electrode polarization resistances in CO at 700, 750 and 800
C are 0.328, 0.207 and 0.155 V cm2, respectively, presenting high electrochemical activity towards CO oxidation. The maximal power densities can reach up to 303.3, 482.2 and 691.9 mW/ cm2 at 700, 750 and 800
C, respectively. When the reversible SOCs are operated in solid oxide electrolysis cell mode, the polarization resistance of the electrolysis cell for pure CO2 reduction at 800
C has the minimal value of 0.126 V cm2 at 1.8 V. At 2.0 V, the current densities can reach up to 0.584 1.219 and 2.204 A/cm2 at 700, 750 and 800
C, respectively. The durability of the cell in CO for as long as 200 h indicates the electrode has remarkable stability. And the short-term stability characteristics for CO2 electrolysis at different voltages illustrate that the cell performs well below 2.0 V. The harsh reducing environment at 2.0 V may be detrimental to the Cu/CMF electrode. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Renewable energy from wind and solar power provides the options of using the surplus electricity from these intermittent sources to mitigate greenhouse emission and supply sustainable energy to replace limited fossil fuels. Electrolyzing CO2 to CO, H2O to H2, and also co-electrolyzing CO2 and H2O to form synthesis gas (H2 + CO) when electricity demand is low have been an active research area recently. Under the background of mitigating climate change, the concept of carbon capture and utilization is gaining worldwide attention, in which waste CO2 is used as a feedstock for both chemical and fuel production [1,2]. Due to the intermittent nature of wind and solar power, converting method to another

* Corresponding authors. ** Corresponding author at: Post-doctoral Mobile Research Station of Forestry Engineering, Northeast Forestry University, Harbin, 150040, China. E-mail addresses: [email protected] (X. Zhou), [email protected] (X. Zhao), [email protected] (B. Li). http://dx.doi.org/10.1016/j.electacta.2017.03.081 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

form energy is required with respect to energy storage and regeneration of electricity. For these reasons, the development of reversible sold oxide electrochemical cells (SOCs) has been taken as a feasible alternative [3–5]. SOCs are able to realize the storage and conversion of renewable energy sources by operating reversibly in solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes [6–8]. In the SOFC mode, various fuels, such as H2, natural gas, hydrocarbons or syngas, are converted spontaneously to electricity and heat. When excess electricity is available, the device can run in the SOEC mode to convert the electrical energy back to chemical energy by the electrolysis of various feedstock, such as H2O, CO2 or CO2 + H2O to fuel. As shown in Fig. 1 is a reversible SOC that can be considered as a special type of rechargeable flow battery. This reversible SOC system can produce/consume a wide variety of fuels including hydrocarbons not only H2. The fuel used in this case is CO which can be electrochemically converted to CO2 in the fuel electrode in SOFC mode and the generated CO2 can later be converted back to CO in SOEC mode. Compared to solution-based electrolysis cells, SOECs, which are essentially the inverse running of SOFCs, are

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Fig. 1. Schematic illustration of a reversible solid oxide electrochemical cell using CO2 and CO as the shuttle.

capable of higher electrolysis efficiencies due to the higher operating temperatures typically higher than 650
C [9–11]. The higher operating temperature can result in a lower Nernst potential, the thermodynamic potential required for water or CO2 splitting, and in lower electrode overpotential during the electrochemical processes [11,12]. Coelectrolysis of both CO2 and H2O can be performed to obtain synthesis gas, which can be catalytically converted using well-known processes to produce hydrocarbons such as gasoline, diesel and methanol [12]. Power consumption and performance for the electrolysis and fuel cells are related to the overall cell resistances. Therefore, highperformance electrode materials are required in order to reduce the power consumption. Conventionally, the fuel electrode is a composite of Ni and YSZ (Y2O3 stabilized ZrO2) [13–15]. However, the Ni-YSZ fuel electrodes suffer from several important limitations: Firstly, the electrode overpotential of Ni-YSZ for CO oxidation is higher than that for H2 oxidation [16]. Secondly, the stability of Ni-YSZ composite in the environment of CO-CO2 is under concern because Ni carbonyls are highly volatile, making it indispensable to select appropriate operating conditions to avoid the carbonyl formation [17]. Thirdly, because Ni is also a superb catalyst for the Boudouard reaction, 2CO ! C + CO2, the operation should be limited to higher temperatures and CO2:CO ratios to avoid equilibrium conditions favorable for this reaction [18,19]. Finally, Ni-YSZ is susceptible to sulfur poisoning, which is another major issue during the long term operation of the reversible SOCs, leading to degradation of the cell as a result of loss of triple phase boundary (TPB) length, the region where the redox reaction occurs [20]. In contrast to Ni-YSZ composite electrode, perovskite oxides have attracted a lot of attentions due to their high redox-stability and reasonable activity [21,22]. However, the p-type conductor of the well-known LSCM (La0.75Sr0.25Cr0.5Mn0.5O3-d) is not well suitable for the strong reducing potential which inevitably produces large electrode polarization resistance and the strong reducing potential can also cause chemical and structural changes of the LSCM electrodes [23]. Although the n-type conductor of LST (La0.2Sr0.8TiO3-d) fits well the strong reducing conditions in the fuel electrode compartment and presents better electrode performances than LSCM [24,25], the electrode performances and Faradic efficiencies are still challenging due to the insufficient catalytic activity when compared to the conventional Ni metal based electrode [26–29]. Therefore, searching for alternative fuel electrode materials for SOCs with CO-CO2 as the oxidation and reduction shuttle is still a task of top priority.

In order to focus on the CO2/CO electrochemical conversion reaction, this study will be directly relevant to the development of novel fuel electrode for the reversible SOCs running primarily on CO to produce CO2 in SOFC mode, and thus the natural reverse of the cell in SOEC mode involving reducing CO2 to CO. Another incentive of this work is to develop a fuel electrode that can be utilized for CO2 reduction to produce CO which could be used in chemical production or react with H2 to produce liquid fuel via the Fischer–Tropsch reaction, thus lowering our carbon footprint. For these reasons, a cermet fuel electrode Cu/Ce0.6Mn0.3Fe0.1O2-d (Cu/ CMF) is proposed in this paper for a reversible SOC with CO/CO2 as the shuttle, in which Mn and Fe co-doped CeO2 was reported as an active electrode system [30]. Cu was selected as a substitute for the commonly used Ni because Cu does not catalyze the formation of carbon fibers in the way that Ni does [31]. In the cell with this electrode, the conversion between electricity energy and chemical energy can be accomplished. Our study is primarily focused on this fuel electrode and employed galvanostatic and electrochemical impedance spectroscopy (EIS) techniques to establish the performance and long term stability of the cell with this fuel electrode during CO2 reduction and CO oxidation at different temperatures. The Cu/CMF electrode presented good performance for CO oxidation and CO2 reduction and is a promising electrode material for reversible SOCs. 2. Experimental 2.1. Preparation and characterization of Ce0.6Mn0.3Fe0.1O2-d powder In this study, the CMF (Ce0.6Mn0.3Fe0.1O2-d) powder was prepared by the citric acid-nitrate method. The stoichiometric amounts of the nitrates Ce(NH4)2(NO3)6 (Aladdin, 99.99%), Mn (NO3)2 (Aladdin, AR, 50 wt.% in H2O) and Fe(NO3)39H2O (Aladdin, 99.99%) were dissolved into the deionized water under strong stirring. Then citric acid as the complexing agent was added at the ratio of 1.8:1 of citric acid to all metal cations. After continuous stirring for 1 h, a transparent brown solution was obtained. And then about 5 mL solution was dried in a ceramic crucible at 120
C and calcined at 850
C for 4 h in air at the heating rate of 5
C/min. After the heat treatment, the grey CMF powder was obtained. In order to investigate the phase stability of CMF powder in reducing atmosphere at high temperatures, the CMF powder was treated by firing at 850
C for 10 h in pure H2. The phase structures of the asprepared powders fired in air and in H2 were analyzed by an X-ray diffractometer (XRD, PANalytical, Netherlands, CuKa radiation at

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40 kV, 40 mA at the scanning rate of 2
/min). The identification of phases was performed using commercially available software Jade (version 6.5) with the 2004 PDF cards. 2.2. Single cell fabrication The electrolyte-supported asymmetrical cells with a configuration of Cu/CMF|ScSZ|LSM/ScSZ (ScSZ((Sc2O3)0.1(CeO2)0.01(ZrO2)0.89), Tianyao materials, Qingdao China), LSM (La0.8Sr0.2MnO3-d, Fuel Cell Materials, USA) were prepared as follows. The commercial ScSZ powder was ball-milled thoroughly with ethanol solvent and polyvinyl butyral binder for 48 h to prepare the electrolyte slurry. A certain proportion of graphite as the pore former agent was then added to a part of the electrolyte slurry in order to prepare the fuel electrode slurry after stirring for another 24 h. The fuel electrode and the electrolyte slurries were tape cast, dried in air and cut into round discs. The bi-layer film was prepared by the lamination method. The bi-layer film was subsequently sintered at 1550
C for 4 h in air and the porosity of the porous fuel electrode layer before impregnation was estimated to be around 65%. The LSM/ScSZ powder with weight ratio of 60:40 was dispersed in terpineol to form the oxygen electrode slurry for screen printing. After screen printing LSM/ScSZ on the other side of the electrolyte, the obtained tri-layer cell was sintered at 1100
C for 2 h in order to obtain good binding between oxygen electrode and the electrolyte. The porous fuel electrode skeleton of the cells was then successively infiltrated with the asprepared CMF precursor solution and calcined at 850
C for 4 h. The concentration of the CMF solution used in the infiltration process was 0.5 mol/L. The infiltration-calcination procedure was repeated until the desired weight increment was achieved and the weight increment of CMF was about 30%. After the preparation of CMF, the Cu was introduced into the porous layer by the same method with Cu(NO3)2 solution with the concentration of 1 M. The weight increment of Cu was about 15% with regard to the porous ScSZ layer. The symmetrical cell with Cu/CMF fuel electrode on both side of ScSZ electrolyte layer for the polarization behavior test in H2 was prepared with the same method as mentioned above. The effective electrode area of the as-prepared cells was 0.16 cm2, which was determined by the area of the porous ScSZ layer after sintered at high temperature. The thicknesses of the components of ScSZ electrolyte, Cu/CMF fuel electrode and LSM/ScSZ oxygen electrode were about 70, 45 and 15 mm, respectively. 2.3. Electrochemical performance measurement and microstructure characterization of the cell In order to measure the electrochemical performance, the single cells were sealed to alumina tubes using Ceramabond 552 adhesive (Aremco products, USA) as the sealant. On both sides of the cell, the silver paste (DAD-87, Shanghai Research Institute of Synthetic Resin, China) and silver wires were employed as the current collector and the leads, respectively. The testing temperature in this study was ranged from 700 to 800
C at the temperature interval of 50
C. H2 and CO were taken as the fuels when the single cell was operated in the mode of solid oxide fuel cell, and when the cell was operated reversibly, the fuel was switched to pure CO2. The flow rate of all the gases in the fuel compartment was fixed at 50 sccm and the oxygen electrode was exposed to the ambient air. Before testing, the single cell was heated up to 800
C at the rate of 5
C/min and held at this temperature in H2 at the flow rate of 50 sccm until the OCV (open circuit voltage) of the cell was stable, ensuring the CuO in the Cu/CMF electrode was reduced to metal Cu which serves as the electron conductor in the electrode. The current-voltage curves (I–V) and galvanostatic experiments were measured by using Arbin fuel cell testing system (Arbin

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instruments, USA). The electrochemical impedance spectra (EIS) measurement was carried out by a PARSTAT 2273 advanced electrochemical system in the frequency range from 0.1 Hz to 1 MHz with the applied amplitude of 10 mV under OCV and various terminal voltages during the operation. The cell microstructures were observed using a scanning electron microscope (SEM, FEI, Quanata 200). 3. Results and discussion 3.1. Phase structure characterization of the CMF powder The XRD patterns of the CMF powder prepared by the solution combustion method using the corresponding nitrate and citric acid as the complexing agent were collected. As shown in Fig. 2 is the XRD patterns of CMF powders after fired in air for 4 h and in H2 for 10 h at 850
C, respectively. The XRD results indicate that the CMF powder fired in air display reflections only from cubic CeO2 (fluorite structure, Fm3m) [32]. The homogeneous cubic ceria-like solid solution in air can be obtained through doping ceria with Fe and Mn. The structure of the solid solution in air is corresponded to that of cubic ceria in which some of the Ce4+ cations were substituted by Fe3+ and Mn3+ cations. In order to achieve the electrical neutrality, the solid solution is formulated as Ce0.6Mn0.3Fe0.1O2-d with the oxygen vacancy formation in it. The substitution of Ce4+ by Fe3+ and Mn3+ resulted in a contraction of the unit cell due to the smaller sizes of the Fe3+ cation (0.55 Å) and the Mn3+ cation (0.58 Å) as compared to that of the Ce4+ cation (0.92 Å) [33,34]. This contraction is evident from the unit cell parameters: 5.4170 Å for the CeO2 and 5.3704 Å for the CMF sample in air [33]. With regard to the Fe3+ and Mn3+ doped CeO2 materials, T. Ishihara et al. found that the impurity phase of Mn3O4 was detected in the sample Ce0.7Mn0.3O2-d without Fe and this phase can be removed by increasing the amount of Fe. The limit of Mn solid solution in the CeO2 lattice can be expanded by doping Fe3+ and no impurity phase was observed in the samples with Fe contents of up to 0.2 [30]. However, the refection of the CMF sample after calcined at 850
C for 10 h in H2 can be assigned to cubic CeO2 and a small amount of secondary phase. The diffraction patterns of the secondary phase indicate the presence of MnO in the sample. Therefore, the heat treatment of the CMF sample in reducing atmosphere at high temperature resulted in the exsolution of MnO from the lattice. Although the addition of Fe can expand the limit of Mn content in the solid solution in air, the secondary in the form of MnO was still detected in the harsh reducing conditions at high temperature. Also, it can be found that the intensity of the XRD pattern for the reduced CMF powder was drastically reduced compared to the same sample treated in air, indicating the lower crystallinity of the CMF sample after reduction, which probably resulted from partial amorphization after reduction at 850
C. As shown in Fig. 2, the reactivity between ScSZ and Cu(CuO) was focused because RuisMorales et al. found that CuO reacted with cubic YSZ generating monoclinic ZrO2 at high temperatures and the reaction started at 900
C in their experiment [35]. As shown in Fig. 2, the structure of ScSZ did not change and no secondary phases or the XRD patterns of monoclinic ZrO2 were found after firing the composite at 850
C. Consequently, these results indicated that no reactivity between ScSZ and Cu (CuO) was observed under our experimental conditions. 3.2. Cu/CMF electrode microstructure fabricated by infiltration method With regard to the Cu/CMF electrode we used in this study, Cu in the electrode can avoid the carbon deposition during the operation because Cu is an excellent electronic conductor but a poor catalyst

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Fig. 2. XRD patterns for (a) CMF powder after fired in H2 at 850
C for 10 h, (b) CMF powder after fired in air at 850
C for 4 h, (c) ScSZ powder, (d) a composite CuO/CMF/ScSZ (15: 30: 100 weight) after fired in air at 850
C for 4 h and (e) a composite CuO/CMF/ScSZ (15: 30: 100 weight) after fired in H2 at 850
C for 4 h.

for carbon formation [36]. However, because Cu2O and CuO melt at 1235 and 1326
C, respectively, the conventional methods used for fabricating Ni-YSZ cermets cannot be applied to Cu/CMF electrode [31]. Our solution is to synthesize a highly porous ScSZ layer, which is calcined together with the ScSZ electrolyte at high temperature, and then to introduce the CMF and Cu by infiltration of the porous ScSZ layer with soluble nitrates of the constituent metal ions. SEM images of the as-prepared cell with the configuration of Cu/CMF| ScSZ|LSM/ScSZ are shown in Fig. 3. The cell configuration of the Cu/ CMF|ScSZ|LSM/ScSZ cell adopted in this study was ScSZ electrolyte supported type with the dense ScSZ electrolyte (70 mm) sandwiched between the porous Cu/CMF fuel electrode (45 mm) and LSM/ScSZ oxygen electrode (15 mm). As shown in Fig. 3(a), the porous ScSZ with Cu/CMF in it was well connected with the ScSZ electrolyte and the Ag current collector was also adherent to the porous Cu/CMF electrode very well. Although there were some pin holes in the electrolyte, the pin holes were disconnected with each other which can prevent the gases in both compartments from transferring through the electrolyte. The scaffold of the porous ScSZ layer before infiltration is shown in Fig. 3(b), the average particle size in the porous layer was 1  5 mm and the particles were well connected which presumably can facilitate the transmission of oxygen ions during the cell operation. The porosity of the porous ScSZ layer before infiltration was about 65%. After infiltration, the porous ScSZ scaffold was uniformly covered by Cu/CMF layer as shown in Fig. 3(c). It can be seen that there were some pores on the surface of Cu/CMF layer which will be beneficial for the gas transport from the surface to the electrochemical reaction sites. However, the porosity of the Cu/ CMF layer estimated from the surface area is not very big. In order to clearly show the Cu/CMF microstructure, the image in Fig. 3(c) was zoomed in as shown in Fig. 3(d) and (e). On the magnified surface, the particles in the Cu/CMF layer can be seen clearly and the particle boundary is also obvious. Besides, some small pores

can be seen on the surface of the Cu/CMF layer. Fig. 3(f) shows fracture cross-sectional details of the Cu/CMF layer on the ScSZ scaffold. The thin layer and a part of the ScSZ are shown. The thin Cu/CMF layer appears to be quite dense, which might be explained by the less refractory nature of Cu. It is clear that the Cu/CMF electrode microstructure features with the intimate contact between the electrode and ScSZ scaffold. Although the porosity of the Cu/CMF layer is small, the porosity of the Cu/CMF electrode adherent to the ScSZ scaffold after infiltration was determined to be about 42%, reduced by 23% compared with that before infiltration. However, from the perspective of porosity of the Cu/ CMF layer, the electrochemical performance of the cell with Cu/ CMF electrode is probably compromised. 3.3. Electrochemical performance of the cell with Cu/CMF fuel electrode in SOFC mode For the reversible SOC shown in Fig. 1, the electrochemical activity of the fuel electrode of the SOC is crucial for the energy efficiency of the system. In comparison to H2, CO has not received much attention for SOFCs, which may be partly due to its relatively high overvoltage and accordingly slower oxidation kinetics for CO than for H2 [37]. Fig. 4 shows the current-voltage (I–V) and I-P curves for the cell using Cu/CMF as the electrode at various temperatures, i.e. 700, 750 and 800
C. The open-circuit potentials were close to the theoretical values at all temperatures examined in both H2 and CO as shown in Fig. 4(a). At high current density, no obvious concentration polarization phenomenon was observed. As discussed above, the Cu/CMF fuel electrode featured with the relatively dense layer on the ScSZ scaffold after sintered at 850
C for 4 h. The microstructure of this layer appears to be negative for the gas diffusion during the electrode processes, consequently resulting in significant concentration polarization resistance. However, the actual electrode polarization behavior performed

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Fig. 3. SEM images of the cell with the configuration of Cu/CMF|ScSZ|LSM/ScSZ. (a) Cross-sectional image of the cell with Cu/CMF electrode, (b) SEM image of the porous SzSZ scaffold before infiltration, (c)-(e) SEM images of Cu/CMF electrode surface with different magnifications, (e) Magnified cross-sectional image of the Cu/CMF electrode.

differently. This might be attributed to high surface area of this layer which was largely determined by the porous ScSZ scaffold and the electrochemical reactions were mainly taking place on the surface of this layer instead of inside of it. As shown in Fig. 4(b), the maximal power densities in H2 were 315.9, 518.6 and 749.3 mW/ cm2 at 700, 750 and 800
C, respectively. When the fuel was switched to CO, the maximal power densities were 303.3, 482.2 and 691.9 mW/cm2 at 700, 750 and 800
C, respectively. The maximal power densities in CO at 700, 750 and 800
C were lower than those in H2 by 4%, 7% and 7.7%, respectively, accordingly indicating the kinetics of the Cu/CMF electrode was slightly slower for CO oxidation than for H2 oxidation. However, as far as the absolute values in maximal power densities at different temperatures in either CO or H2 are concerned, the Cu/CMF electrode can work well as the anode at the intermediate temperatures as the SOCs operate in SOFC mode. The observed high power density of the cell using Cu/CMF fuel electrode could be attributed to the high surface activity of the composite towards CO oxidation. In order to further determine the internal resistance of the cell, the impedance analysis was conducted and the results are shown in Fig. 5. Fig. 5(a) and 5(b) show the impedance spectra of the cell with Cu/CMF electrode under open circuit conditions when H2 and CO were used as fuel. Evidently, the impedance spectrum mainly consists of the ohmic and polarization resistances. The latter was

attributed to the activation and diffusion overpotential [38]. The polarization resistances of the cell in H2 at 700, 750 and 800
C were 0.666, 0.343 and 0.223 Vcm2, respectively. And in CO, the polarization resistances were 0.804, 0.419 and 0.285 Vcm2, respectively. Apparently, the size of the electrode impedance increased slightly as the fuel was switched from H2 to CO, which is consistent with the cell performance as discussed above. It is worth noting in both H2 to CO, the diffusion resistance was not observed, suggesting the gas surface diffusion to three phase boundary (TPB) regions or the reaction sites may be fast. In order to separate the polarization resistance of the fuel electrode from the whole cell, the temperature dependence of the reaction in the fuel electrode was further studied by using the symmetrical cell of Cu/CMF|ScSZ|Cu/CMF. Fig. 5(c) shows the area resistances of the fuel electrode and the oxygen electrode and the apparent activation energy of the Cu/CMF electrode in H2 and CO and the LSM/ScSZ electrode in air. In this study, the area resistances of the oxygen electrode (Rp,c) were obtained by subtracting the Rp, a of the symmetrical cell of Cu/CMF|ScSZ|Cu/CMF from the Rp of the Cu/CMF|ScSZ|LSM/ScSZ cell at various temperatures. The Rp,a of the Cu/CMF electrode in H2 at 700, 750 and 800
C were 0.190, 0.131 and 0.093 V cm2, respectively. And in CO, the Rp,a of the Cu/ CMF electrode increased to 0.328, 0.207 and 0.155 V cm2, respectively. This suggests that the electrochemical oxidation of

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in H2 at 800 C

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Current density (mA/cm ) Fig. 4. Power-generation curves of the cell using Cu/CMF for fuel electrode and LSM/ScSZ for oxygen electrode in H2 and CO at different temperatures. (a) I–V curves and (b) IP curves.

CO occurred on the Cu/CMF was slightly harder than that of H2. The activation energy of the Cu/CMF electrode in H2 was 0.70 eV, which is similar to 077 eV for the electrode in CO under the same conditions. The large apparent activation energy (1.22 eV in Fig. 5(c)) for the oxygen electrode when air was used as the oxidant also suggests limited O2 reduction of LSM/ScSZ electrode. Accordingly, the higher electrochemical performance could be expected when using the air electrode materials with higher catalytic ability towards O2 reduction than LSM/ScSZ electrode. According to the above results, it is reasonable to assume that the Cu/CMF electrode is a suitable fuel electrode for CO electrochemical oxidation. 3.4. Electrochemical performance of the cell with Cu/CMF fuel electrode in SOEC mode Fig. 6 shows the recorded voltage as a function of the current density of the cell with the configuration of Cu/CMF|ScSZ|LSM/ ScSZ for the electrolysis of pure CO2 with the applied voltage ranging from 0 to 2.0 V at different temperatures. The slope change of I–V curves indicate that there exist different cell processes in the voltage region. At lower voltage, the Cu/CMF fuel electrode was electrochemically reduced. This processes appeared related with the temperatures and the current density increased with the increase of temperature. And as the temperature increased, two platforms in the curve below 1.1 V were observed, which probably resulted from the valence change of the doping ions in the electrode. Above 1.1 V, the current densities of the electrolysis cell increased significantly quickly. The exact onset potential for CO2 reduction at 800
C determined by software HSC 6.0 was 1.004 V. At the applied voltage of 2.0 V, the current densities reached up to 0.584 1.219 and 2.204 A/cm2 at 700, 750 and 800
C, respectively. Fig. 7(a) shows the electrochemical impedance spectra of the cell for pure CO2 electrolysis at 1.6 V at different temperatures. Increasing temperature is expected to favor the electrode

polarization, following the improved kinetics process of the electrode. The Rp values of the electrolysis cell were 0.620, 0.269 and 0.151 V cm2 at 1.6 V at 700, 750 and 800
C, respectively. Evidently, the internal resistance of the electrolysis cell was significantly reduced when the temperature was elevated from 700 to 800
C. The reduced internal resistance is theoretically expected with regard to increasing the system efficiency. Fig. 7(b) shows the electrochemical impedance spectra under a series of applied voltages ranging from 1.2 to 2.0 V at 800
C. The internal resistance of the electrolysis cell i.e. the total cell resistance decreased with the increase of the applied voltage. The ohmic resistance monotonously decreased at the voltages over 1.2 V, which was attributed to Joule heat effect of the high current density. In addition, the polarization resistance (Rp) was obviously reduced with the voltage increase with the exception at the applied voltage of 2.0 V, demonstrating faster electrode kinetics at higher applied voltage below 2.0 V [28,39]. The reason for this phenomenon is unknown yet. Therefore, in the whole applied voltage, the Rp reached the minimal value at the voltage of 1.8 V which is 0.126 V cm2. 3.5. Durability test of the cell of Cu/CMF|ScSZ|LSM/ScSZ in both SOFC and SOEC mode In addition to the high electrode catalytic capability in SOCs, good durability is also essential for the practical applications. Therefore, in the study, the stability performance of the cell with Cu/CMF as the fuel electrode and LSM/ScSZ as the oxygen electrode was investigated. Fig. 8(a) and 8(b) shows the long-term stability of the cell in SOFC mode at 750
C at the constant current density of 750 mA/cm2 in H2 and CO. At first, the cell was operated in H2 at 750
C for 300 h. During the initial 44 h, the terminal voltage of the cell increased from 0.744 to 0.842 V, which probably resulted from the activation of water generated in the anode compartment when the electrochemical reaction proceeds [40]. Afterwards, the cell performance gradually decreased. However, the voltage of 0.796 V

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Fig. 5. Electrochemical impedance spectra of the cell with the configuration of Cu/ CMF|ScSZ|LSM/ScSZ under open-circuit at different temperatures in H2 (a) and CO (b), (c) Area specific resistance (ASR) changing as a function of temperature for Cu/ CMF fuel electrode and LSM/ScSZ oxygen electrode under open circuit. The ASR values calculated from the high and low frequency intercepts in the impedance diagram for the symmetrical cell of Cu/CMF|ScSZ|Cu/CMF.

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Fig. 7. Electrochemical impedance spectra of the cell with the configuration of Cu/ CMF|ScSZ|LSM/ScSZ under different voltages for pure CO2 electrolysis with the oxygen electrode exposed to static air. (a) under 1.6 V at different temperatures and (b) under voltages ranging from 1.2 to 2.0 V at 800
C.

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Current density (A/cm ) Fig. 6. The current density dependence of the applied voltage profile of the cell with the configuration of Cu/CMF|ScSZ|LSM/ScSZ for pure CO2 direct electrolysis at different temperatures.

at 300 h was still higher than that at the beginning of the durability test. When the fuel was switched to CO, the cell was held at the same current density for another 200 h, and after running for 200 h, the terminal voltage was 0.756 V. The cell performance decreased by 2.3% during the whole holding time. The low melting point of Cu in the Cu/CMF may be the reason for the performance gradual degradation during the long-term test. Apart from the long-term test, the durability of the cell in SOEC mode was measured as well in order to study the electrolysis performance at different applied voltage. As shown in Fig. 8(c), the durability of the electrolysis cell with Cu/CMF fuel electrode was tested in short

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(a)

1.0 2

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o

at 750 mA/cm in H2 at 750 C

0.9 0.8 0.7 0.6 0

(b) 0.90

50

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60

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0.0

o

800 C

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(c)

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at 1.2 V at 1.4 V at 1.6 V at 1.8 V at 2.0 V

-1.5 -2.0 -2.5 0

2

4

6

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o

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at 0.75 V in SOFC mode 0.8 0.4 0.0 -0.4 -0.8 at 1.4 V in SOEC mode -1.2 0

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30

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Time (h) Fig. 8. Durability test of the cell with the configuration of Cu/CMF|ScSZ|LSM/ScSZ. Long-term performance in H2 (a) and in CO (b) at the constant current density of 750 mA/ cm2 at 750
C, (c) Short-term durability test for pure CO2 electrolysis with different applied voltage at 800
C and (d) Cycled performance between SOEC mode and SOFC mode for 100 h at 800
C.

term from 1.2 to 2.0 V with the voltage interval of 0.2 V. At each applied voltage, the electrolysis cell was held for 2 h. The electrolysis cell shows excellent stability at all applied voltages except 2.0 V at which the performance of the electrolysis cell decreased significantly during the test. Xie at al. [28] found the current density at 2.0 V rapidly degraded with time when they studied the La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM) electrode for reducing pure CO2 and they attributed it to the chemical changes of LSCM at strong reducing potential of 2.0 V. For instance, it was reported that LSCM was reduced into a cubic structure in 5%H2/Ar at 900
C [41,42]. Therefore, the structure stability of the fuel electrode in reducing atmosphere or under cathodic polarization is critical for the electrochemical stability especially at higher

electrolysis voltage between the electrolysis cell. As discussed in the section 3.1, it was found that the secondary phase of MnO exsoluted from the bulk in H2 at high temperature. Therefore, the structure stability of CMF in reducing atmosphere at high temperature is probably the reason for the performance degradation at high applied voltage. Accordingly, operating the electrolysis at 2.0 V is supposed to be avoided. In Fig. 8(d) is the durability performance at 800
C when the cell was cycled between in SOFC mode and in SOEC mode. The voltage was 0.75 V in SOFC mode where the fuel was pure CO and the applied voltage was 1.4 V when the cell was switched to SOEC mode for CO2 reduction. From this figure, it can be seen that the cell presented impressive durability during the test indicating that this cell is highly robust for CO

L. Liu et al. / Electrochimica Acta 235 (2017) 365–373

oxidation and CO2 reduction. In summary, the cell of Cu/CMF|ScSZ| LSM/ScSZ in both SOFC and SOEC modes exhibited remarkable durability, which is supposed to be very important for the practical applications. 4. Conclusion In this paper, a solid oxide electrochemical cell (SOC) was studied, in which CO used as the fuel can be oxidized to CO2 when the cell is operated as a solid oxide fuel cell and the produced CO2 can be subsequently electrochemically reduced to CO when the cell is operated in reverse. In each cycle, the conversion between electricity energy and chemical energy can be accomplished. In this study, a cermet fuel electrode Cu/Ce0.6Mn0.3Fe0.1O2-d (Cu/CMF) is proposed for this SOC. In the Cu/CMF electrode, Cu is inert for the carbon deposition and CMF is electrochemically active for CO oxidation and CO2 reduction. By using the infiltration method, the Cu/CMF electrode uniformly adherent to the ScSZ porous scaffold was obtained. When the cell with the Cu/CMF electrode was operated in SOFC mode, the cell presented high catalytic ability towards CO oxidation. The polarization resistances of the Cu/CMF electrode in CO at 700, 750 and 800
C were 0.328, 0.207 and 0.155 V cm2, respectively. When the cell was operated in SOEC mode, the Cu/CMF was electrochemically active towards the CO2 reduction as well. The polarization resistance of the electrolysis cell at 800
C had the minimal value of 0.126 V cm2 at the applied voltage of 1.8 V. From the results of durability test, it was found that the cell with the Cu/CMF electrode presented remarkable stability during the long-term test both in H2 and CO. As far the stability of the cell in SOEC mode, the short-term performance at different applied voltages indicated that the electrolysis cell can perform well the voltages below 2.0 V. The high voltage will be detrimental to the cell due to the harsh reducing environment in the fuel compartment which can possibly result in the decomposition of the electrode materials. In summary, the Cu/CMF fuel electrode exhibited promising electrochemical performance for the SOC operated in CO in SOFC mode and in CO2 in SOEC mode and could be a practical electrode material for reversible SOCs. Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities (2572015CB20), China Postdoctoral Science Foundation Funded Project (2015M571379) and the National Natural Science Foundation of China (21406033, 31370709).

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References [1] P.K. Addo, B. Molero-Sanchez, M. Chen, S. Paulson, V. Birss, Fuel cells 15 (2015) 689. [2] R.M. Cullar-Franca, A. Azapagic, J. CO2 Util. 9 (2015) 82. [3] D. Honnery, P. Moriarty, Int. J. Hydrogen Energy 36 (2011) 3283. [4] C. Graves, S.D. Ebbesen, M. Mogensen, K.S. Lackner, Renewable Sustainable Energy Rev. 15 (2011) 1. [5] S.D. Ebbesen, S.H. Jensen, A. Hauch, M.B. Mogensen, Chem. Rev. 114 (2014) 10697. [6] S.P. Jiang, Asia-Pac. J. Chem. Eng. 11 (2016) 386. [7] J.C. Ruiz-Morales, D. Marrero-Lopez, J. Canales-Vazquez, J.T.S. Irvine, RSC Advances 1 (2011) 1403. [8] S. Gamble, Mater. Sci. Technol. 27 (2011) 1485. [9] F. Bidrawn, G. Kim, G. Corre, J.T.S. Irvine, J.M. Vohs, R.J. Gorte, Electrochem. Solid State Lett. 11 (2008) B167. [10] T. Ishihara, N. Jirathiwathanakul, H. Zhong, Energy Environ. Sci. 3 (2010) 665. [11] G. Tsekouras, J.T.S. Irvine, J. Mater. Chem. 21 (2011) 9367. [12] C. Graves, S.D. Ebbesen, S.H. Jensen, S.B. Simonsen, M.B. Mogensen, Nature Mater. 14 (2015) 239. [13] O.A. Marina, L.R. Pederson, M.C. Williams, G.W. Coffey, K.D. Meinhardt, D.D. Nguyen, E.C. Thomsen, J. Electrochem. Soc. 154 (2007) B452. [14] A. Hauch, S.H. Jensen, J.B. Bilde-Sorensen, M. Mogensen, J. Electrochem. Soc. 154 (2007) A619. [15] S. Elangovan, J.J. Hartvigsen, L.J. Frost, Int. J. Appl. Ceram. Technol. 4 (2007) 109. [16] O. Costa-Nunes, R.J. Gorte, J.M. Vohs, J. Power Sources 141 (2005) 241. [17] K. Morikawa, T. Shirasaki, M. Okada, Adv. Catal. 20 (1969) 98. [18] M.L. Toebes, J.H. Bitter, A.J. van Dillen, K.P. de Jong, Catal. Today 76 (2002) 33. [19] E. Perry Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. [20] S. Gamble, Mater. Sci. Technol. 27 (2011) 1485. [21] S.W. Tao, J.T.S. Irvine, Nature Mater. 2 (2003) 320. [22] C. Jin, C.H. Yang, F.L. Chen. J. Electrochem Soc. 158 (2011) B1217. [23] S.S. Xu, S.G. Chen, M. Li, K. Xie, Y. Wang, Y.C. Wu, J. Power Sources 239 (2013) 332. [24] K. Xie, Y.Q. Zhang, G.Y. Meng, J.T.S. Irvine, Energy Environ. Sci. 4 (2011) 2218. [25] Y. Gan, Q.Q. Qin, S.G. Chen, Y. Wang, D.H. Dong, K. Xie, J. Power Sources 245 (2014) 245. [26] J. Yan, H. Chen, E. Dogdibegovic, J. Stevenson, M. Cheng, X. Zhou, J. Power Sources 252 (2014) 79. [27] Z. Zhan, L. Zhao, J. Power Sources 195 (2010) 7250. [28] S. Xu, S. Li, W. Yao, D. Dong, K. Xie, J. Power Sources 230 (2013) 115. [29] X. Yue, J. Irvine, J. Electrochem. Soc. 159 (2012) F442. [30] T. Ishihara, T.H. Shin, P. Vanalabhpatana, K. Yonemoto, M. Matsuka, Electrochem. Solid State Lett. 13 (8) (2010) B95. [31] H.P. He, J.M. Vohs, R.J. Gorte, J. Electrochem Soc. 150 (11) (2003) A1470. [32] B.A. Boukamp, Solid State Ionics 20 (1986) 31. [33] H. Lv, H.Y. Tu, B.Y. Zhao, Y.J. Wu, K.A. Hu, Solid State Ionics 177 (2007) 3467. [34] A. Gupta, U.V. Waghmare, M.S. Hegde, Chem. Mater. 22 (2010) 5184. [35] J.C. Ruiz-Morales, J. Canales-Vázquez, D. Marrero-López, J. Peña-Martínez, A. Tarancón, J.T.S. Irvine, P. Núñez, J. Mater. Chem. 18 (2008) 5072. [36] S. Park, J.M. Vohs, R.J. Gorte, Nature 265 (404) (2000). [37] M. Homel, T.M. Gür, J.H. Koh, A.V. Virkar, J. Power Sources 195 (2010) 6367. [38] M.J. Jørgensen, M. Mogensen, J. Electrochem Soc. 148 (5) (2001) A433. [39] X. Yue, J. Irvine, Electrochem. Solid-State Lett. 15 (2012) B31. [40] J.T.S. Irvine, D. Neagu, M.C. Verbraeken, C. Chatzichristodoulou, C. Graves, M.B. Mogensen, Nature Energy 1 (2016) 1. [41] Y. Zhang, J. Li, Y. Sun, B. Hua, J. Luo, ACS Appl. Mater. Interfaces 8 (2016) 6457. [42] S.W. Tao, J.T.S. Irvine, Chem. Mater. 18 (2006) 5453.