In the past seventy years, the concentration of CO₂ in atmosphere has increased by about 0.012%, which is the main reason of greenhouse effect inducing a series of problems like global warming and climate change^[1]. Capture and utilization of CO₂ is of great significance to sustainable development of human society. Nevertheless, high thermodynamic stability and kinetic inertness of CO₂ limit its conversion and applications^[2]. Solid oxide electrolysis cells (SOECs) are one of the most promising technologies to convert CO₂ into high energy density fuels and value- added chemicals using the surplus renewable electricity, promoting CO₂ utilization and penetration of renewable electricity in the current energy regime as well^[3,4].

Ni-based cermets are the commonly used cathodes in SOECs, but suffer from some limitations such as nickel oxidation in CO₂-rich atmospheres and carbon deposition in CO-rich atmosphere at high CO₂ conversions, which may prevent their practical applications^[5,6]. Due to their distinctive features such as brilliant redox stability and excellent resistance against coking formation and sulfur poisoning, mixed ionic-electronic conducting oxides have been widely investigated as alternative cathodes for CO₂ electrolysis. Furthermore, some mixed conducting oxides can be simultaneously used as both anodes and cathodes to obtain symmetrical SOECs, which greatly simplifies the fabrication process and reduces complications associated with the different electrode|electrolyte interfaces. Extensive efforts have been made to explore perovskite and double-perovskite electrodes in symmetrical SOECs, such as Sr₂Fe_1.5Mo_0.5O_6-δ^[7,8], La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ^[9], La_xSr_1-xTiO_3-δ^[10,11] and La_0.6Sr_0.4Fe_0.8Ni_0.2O_3-δ^[12]. The symmetrical electrodes should be stable with sufficient electrical conductivities (e.g., >1 S∙cm^-1) in both air and fuel environments, and have good chemical compatibility with the other cell components. La_0.6Ca_0.4Fe_0.8Ni_0.2O_3-δ (LCaFN) was employed as the electrodes for symmetrical SOECs, showing a self-recovery from the performance degradation due to formation of carbonates by air treatment^[13]. Sr₂Fe_1.5Mo_0.5O_6-δ (SFM), the symmetrical SOFC electrodes developed by Liu, et al. exhibited high electrocatalytic activities for both CO₂ reduction and oxygen evolution reactions in symmetrical SOECs^[14]. Partial substitution in the A or B site is an effective strategy to optimize the catalytic activity of perovskite oxides. Ce⁴⁺ was introduced into the A site of La_0.7Sr_0.3Cr_0.5Fe_0.5O_3-δ (LSCrF) to increase oxygen vacancies in the lattice by in situ reduction under the operational conditions and thereby enhance the catalytic activities^[15]. In this work, La³⁺ doped Sr₂Fe_1.5Ni_0.1Mo_0.4O_6-δ (L_xSFNM) was investigated as the symmetrical SOEC electrodes for direct CO₂ electrolysis. The influence of the La³⁺ content on the oxygen vacancy concentration, electrical conductivity, and catalytic activity were analyzed. Electrochemical performance was evaluated on symmetrical "L_xSFNM@LSGM|LSGM| L_xSFNM@LSGM" cells for pure CO₂ electrolysis.

L_xSFNM powders were prepared using the Sol-Gel method. The precursor solution was prepared as follows: stoichiometric amounts of the nitrate salts La(NO₃)₃∙6H₂O, Sr(NO₃)₂, Fe(NO₃)₃∙9H₂O, Ni(NO₃)₃∙6H₂O and (NH₄)₆Mo₇O₂₄∙4H₂O were firstly dissolved in distilled water, and then citric acid was added with the molar ratio of the citric acid to the total metal ions at 1.2 : 1. The precursor solution was subsequently heated at 80 ℃ until a gel was formed, followed by drying at 200 ℃ for 5 h and calcination at 1000 ℃ in air for 5 h to obtain pure L_xSFNM powders.

The tri-layer structure of “porous|dense|porous” LSGM (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ) was prepared by laminating three tape-cast green tapes at 75 ℃ and 20 MPa, with 40wt% rice starch and graphite used as the pore-forming material in the porous layers. The laminated layers were co-fired at 1380 ℃ to produce the final ceramic structures. L_xSFNM catalysts were added into the porous LSGM scaffolds by impregnating the precursor solution with the total concentration of metal ions at 1 mol∙L^-1, followed by calcination at 1000 ℃ for 5 h. The impregnation and calcination cycle was repeated for 16 times so as to achieve an L_xSFNM loading of ~25wt% relative to the LSGM scaffolds.

The X-ray diffraction (XRD) pattern of as-synthesized powders was examined at room temperature on an Rigaku D/Max 2100 Powder X-ray Diffractometer with a monochromatic Cu Kα and the diffraction data were recorded in the range 2θ=20^o-80^o with scan rate of 5 (^o)/min. The cell structure was examined using the scanning electron microscope (SEM) in a FEI Magellan 400 microscope. The conductivities of L_xSFNM were tested in air and 50% CO-50% CO₂ using the DC four-probe method. For fuel cell measurements, silver ink (DAD87, Shanghai Institute of Synthetic Resin) and silver wires was applied on the anode and cathode electrode surface as the current collectors. Impedance measurements were performed on the symmetrical fuel cells in a homogeneous environment of air or 50% CO-50% CO₂. The electrolysis performance of the symmetrical fuel cells was evaluated using IM6 Electrochemical Workstation (ZAHNER, Germany), with pure CO₂ fed to the cathodes at 50 sccm and ambient air in the anodes.

The XRD patterns of La_xSr_2-xFe_1.5Ni_0.1Mo_0.4O_6-δ oxides at room temperature are shown in Fig. 1(a). All samples showed a pure cubic perovskite structure without any extra peaks of impurities observed. Fig. 1(b) shows that the (110) peak shifts to a higher scattering angle at x=0.1, which can be explained by smaller ionic radius for La³⁺ than that for Sr²⁺ (0.136 nm vs. 0.144 nm). With the La³⁺ content further increasing, the diffraction peak shifted to lower scattering angles. Prior reports showed that the cell expansion was inhibited due to the steric effects associated with anti-site defects^[16,17]. This indicates the existence of a disordered crystal structure in L_xSFNM double perovskites.

Fig. 2 compares the thermogravimetric curves of L_xSFNM powders in air, showing a loss of 1%-1.5% in the powder weight due to release of the lattice oxygen with the temperature increasing from 30 to 900 ℃. Note that the largest weight loss was observed for L_0.3SFNM, indicating the most abundant presence of oxygen vacancies at high temperatures. Fig. 3(a, b) show the conductivities of L_xSFNM measured in air or 50% CO-50% CO₂ at 650-800 ℃. Due to their p-type conducting nature, the conductivities were much higher in air than those in 50% CO-50% CO₂ (17.3-38.1 vs. 2.7-5.5 S∙cm^-1). Fig. 3 also shows that L_0.3SFNM had the highest conductivities among four samples at all temperatures. For x≤0.3, partial substitution of La³⁺ for Sr²⁺ increased the concentration of anti-site defects and the electron density of Fermi level. It has been reported that the half metallic properties of SFM was quite sensitive to the anti-site defect concentration as it altered the Fe-O-Mo bonding network and the electronic hopping pathway^[18]. Due to a decrease in the crystal symmetry and the cation order, the double exchange interaction between Fe-O-Mo may be inhibited at x≥0.4, leading to decreased conductivities^[19,20,21].

In order to study the effect of different La³⁺ contents on the activities of L_xSFNM oxides toward oxygen evolution and CO₂ reduction reactions, electrochemical impedance measurements were performed on electrolyte- supported symmetrical cells with impregnated L_xSFNM electrodes, i.e., L_xSFNM@LSGM|LSGM|L_xSFNM@LSGM (Fig. 4(a)). The dense LSGM electrolytes were 210 μm in thickness, whereas the porous electrodes were around 20 μm in thickness. Fig. 4(b) shows excellent interfacial bonding between dense electrolytes and the porous scaffolds that is conducive to minimizing the contact resistance. As shown in Fig. 4(c), nano-scale L_xSFNM catalysts were deposited onto the internal surfaces of porous LSGM scaffolds to form a well-interconnected network to facilitate electronic conduction and thus enhance their catalytic activities.

Electrochemical impedance measurements were performed for symmetrical cells in the homogeneous environment of dry air or 50% CO-50% CO₂, with the typical Nyquist plots of impedance data as compared in Fig. 5. Note that the electrolyte resistances were subtracted from the cell impedance, and the polarization resistances were divided by two due to the symmetrical configurations. Fig. 5(a) shows that the anode polarization resistances (R_P,A) at 800 ℃ are 0.12, 0.09, 0.07 and 0.10 Ω∙cm² for L_0.1SFNM, L_0.2SFNM, L_0.3SFNM and L_0.4SFNM in dry air, respectively. L_0.3SFNM exhibited much smaller R_P,A compared with some previously reported air electrodes, such as Sr₂FeMoO₆ (0.1 Ω∙cm² at 850 ℃)^[22], Sr₂Fe_1.4Ni_0.1Mo_0.5O_6-δ (0.22 Ω∙cm² at 750 ℃)^[23], Pr_0.6Sr_0.4Fe_0.7Ni_0.1Mo_0.1O_3-δ (0.4 Ω∙cm² at 800 ℃)^[24] and La_0.6Sr_0.4Fe_0.9Mn_0.1O_3-δ-GDC (0.24 Ω∙cm² at 800 ℃)^[25]. Distribution of relaxation time (DRT) analyses were conducted on impedance data so as to identify the number of polarization processes involved in the oxygen evolution reaction. With spectra transferred from the frequency into time domain, the overlapped polarization processes can be clearly distinguished. Fig. 5(b) shows that all DRT curves have three distinctive peaks, corresponding to three elementary reaction processes in the oxygen evolution reaction. The low-frequency peaks (1-10 Hz) were usually assigned to gas diffusion in the porous electrodes, while the high frequency peaks (300-2000 Hz) were related to the charge transfer process across the electrode- electrolyte interfaces. R_LF and R_HF were almost independent of the La³⁺ content. Note that the intermediate frequency peaks (50-200 Hz) dropped pronouncedly in the order of L_0.1SFNM > L_0.4SFNM > L_0.2SFNM > L_0.3SFNM. Given that the intermediate frequency peaks were usually associated with the bulk transport of oxygen surface exchange and O^2- bulk diffusion^[26], it can be concluded that L_0.3SFNM had the highest surface oxygen exchange rate. The highest activity of L_0.3SFNM toward oxygen evolution was also consistent with the smallest activation energy of R_P,A among the investigated L_xSFNM (Fig. 6(a)). It should also be pointed out that the intermediate frequency peaks were much larger than the low- or high-frequency peaks for all L_xSFNM samples, indicating that surface oxygen exchange is the rate-limiting step in oxygen evolution reaction.

Fig. 5(c) shows an order of the cathode polarization resistances (R_P,C) - L_0.3SFNM (0.62 Ω∙cm²)0.2SFNM (0.86 Ω∙cm²) < L_0.4SFNM (0.98 Ω∙cm²) < L_0.1SFNM (1.24 Ω∙cm²) measured in 50% CO-50% CO₂, demonstrating that L_0.3SFNM had the highest activities toward CO₂ reduction reactions. Fig. 5(d) shows the DRT analysis results of impedance data in Fig. 5(c), also showing three well distinguished peaks. The low-, intermediate- and high-frequency peaks were associated with dissociative adsorption of CO₂ molecules on the surface, surface diffusion and the charge transfer process, respectively. R_HF and R_IF remained independent of the La³⁺ content, while R_LF showed the lowest at x=0.3. Much higher R_LF than R_HF and R_IF indicated that the overall CO₂ electro- reduction was limited by dissociative adsorption of CO₂ on the surface of LSFNM. Moreover, the cathode polarization resistances (R_P,C) was much larger than R_P,A, demonstrating that the electrochemical performance of the CO₂ electrolysis cells mainly depended on the catalytic activities of cathodes toward CO₂ reduction reactions. Fig. 6(b) shows that the activation energies of R_P,C are 0.83-0.89 eV.

The electrochemical performance of L_0.3SFNM electrode for pure CO₂ electrolysis was further evaluated using the LSGM-electrolyte supported SOECs at 650-800 ℃, with pure CO₂ in the cathodes at 50 sccm and ambient air in the anodes. Fig. 7(a) shows the curves of measured voltages as a function of current densities. At the applied voltage of 1.5 V, the electrolysis current densities increased from 0.4 A∙cm^-2 at 650 ℃ to 1.17 A∙cm^-2 at 800 ℃. These values were competitive when compared with previously reported results listed in Table 1. Fig. 7(b) compares the corresponding EIS spectra measured at 1.5 V. The Ohmic resistance and polarization resistance both increased obviously with reduced cell operating temperature. The interfacial polarization resistances were 0.13 Ω∙cm² at 800 ℃ and 0.52 Ω∙cm² at 650 ℃. Fig. 7(c) summarizes the correlation between the doping amount of La³⁺ ions and the electrolysis current density at 1.5 V, showing that L_0.3SFNM yielded the highest performance at all measurement temperatures. Fig. 7(d) shows the preliminary short-term stability of symmetrical L_0.3SFNM electrode cells measured at 800 ℃ and 1.3 V in pure CO₂. The current density slightly dropped from 0.68 to 0.64 A∙cm^-2 during the first 2 h, and then remained stable, indicating good stability of the nano-scale L_0.3SFNM electrodes.

In summary, a series of perovskite oxides La_xSr_2-xFe_1.5Ni_0.1Mo_0.4O_6-δ (L_xSFNM, x=0.1, 0.2, 0.3 and 0.4) have been synthesized and evaluated as symmetrical electrodes for solid oxide electrolysis cells. Impedance measurements show that the electrode performance strongly depended upon the La³⁺ doping content in L_xSFNM. Both of the highest activities for oxygen evolution and CO₂ reduction observed both at x=0.3. LSGM electrolyte- supported SOECs produce an electrolysis current density of 1.17 A∙cm^-2 at 800 ℃ and 1.5 V in pure CO₂, and good stability was observed during a preliminary 50 h measurement. These results demonstrate that L_0.3SFNM could be a promising alternative as symmetrical electrodes in SOECs for pure CO₂ electrolysis.