RUAN Wenji, NI Jiupai, NI Chengsheng

(College of Research and Environment, Southwest University, Chongqing 400715, China)

Extended Abstract:[Background and purpose] Through solid oxide electrolysis cell (SOEC) technology, CO₂ can be converted into CO or other fuels. Since CO₂ is reduced on the cathode surface of the SOEC, the cathode materials should have high structural stability, catalytic activity anti-carbon deposition, etc. Traditional Ni(O)-based ceramics possess excellent catalytic performance, but carbon deposition is prone to occur during the electrolysis of CO₂, which leads to a rapid decline in performance. It has been shown that high-entropy oxides (e.g., Fe_0.6Mn_0.6Co_0.6Ni_0.6Cr_0.6O₄,Ni@Sr₂Fe_1.0Ti_0.2Cr_0.2Mn_0.2Mo_0.2Ni_0.2O_6−δ, La_0.2Pr_0.2Sm_0.2- Sr_0.2Ca_0.2Fe_0.9Ni_0.1O_3−δ, etc.) have good catalytic activity and structural stability and exhibit high stability and catalytic activity towards CO₂.Rutile-type oxides have high electronic conductivity, high stability and catalytic activity. In this work, rutile-type high-entropy oxides (Co_0.2Zr_0.2Fe_0.2Ni_0.2Ti_0.2)Nb_0.5Ta_0.5O₄ (ZNT) and (Co_0.2Mn_0.2Fe_0.2Ni_0.2Ti_0.2)Nb_0.5Ta_0.5O₄ (MNT) were synthesized by using the solid state reaction method. Their structural stability, morphological characteristics, electrical conductivity and other aspects were explored. The catalytic performance and stability of ZNT and MNT as cathode materials for SOEC in the electrolysis of pure CO₂ were tested, while the reaction mechanism of their direct electrocatalytic reduction of CO₂ was studied.[Methods] The ZNT and MNT cathode materials were synthesized by using the solid-state reaction method, while the samples were sintered at 1300 ℃ for 5 h. A planetary ball mill was used to mill the samples for 2 h to obtain finer powder. La_0.8Sr_0.2MnO₃ (LSM) and Gd_0.15Ce_0.85O_0.1 (GDC) were prepared by using combustion method. The LSM powder was obtained by calcining at 900 ℃ for 5 h, while the GDC powder was calcined at 850 ℃ for 10 h. LSM-GDC anode was obtained by uniformly mixing the LSM and GDC powders at a mass ratio of 7:3. The electrolyte Sc_0.18Ce_0.01Zr_0.81O₂−d (SCZ) was prepared by using tape casting method, followed by sintering at 1410 ℃ for 5 h. After the electrode materials were uniformly mixed with an equal amount of organic vehicle, the cathode and anode pastes were respectively coated on both sides of the electrolyte and then dried. After sintering at 950 ℃ for 2 h, button cells were obtained. The cells with silver wires were fixed on the electrolytic cell testing device for performance test. X-ray diffraction (XRD), thermogravimetric analysis (TGA), hydrogen temperature-programmed reduction (H₂-TPR), thermal expansion coefficient (TEC), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) were used to characterize and test the samples. The four-probe method was used to measure electrical conductivity of the samples, while an electrochemical workstation was used to characterize the electrochemical impedance spectroscopy (EIS), current-voltage (I-V) and stability of the cells.[Results] MNT consisted of insulating oxide (Mn, Fe, Co)Nb₂O₆ under reducing conditions. ZNT was more stable than MNT, with exsolution of Ni⁰ and Co_0.72Fe_0.28 from ZNT and MNT at 800 ℃ in 5% H₂. MNT containted more metal and alloy nanoparticles than ZNT, while MNT (1.20%) exhibited larger expansion volume than ZNT (0.37%). EDS and TEM results indicated that the elements in ZNT were uniformly distributed. However, Co and Ni showed aggregation phenomenon, suggesting the exsolution of metal and alloy nanoparticles in ZNT after reduction at 800 ℃. XPS, TGA and TPR results of ZNT and MNT also demonstrated that the valence states of cations and the content of oxygen vacancies increased in the samples reduced at 800 ℃, indicating that the change in the valence states of metal cations induced oxygen vacancies, while the change in ion valence states and the exsolution of metal nanoparticles were responsible for the loss of oxygen in the samples. In addition, both ZNT (0.42 S·cm⁻¹) and MNT (0.17 S·cm⁻¹) had high electrical conductivity after reduction at 800 ℃. Meanwhile, ZNT had higher electrical conductivity than MNT, due to the formation of the insulating oxide (Mn, Fe, Co)Nb₂O₆ in MNT. During the electrolysis of CO₂, the ZNT cell exhibited higher catalytic performance and stability than the MNT cell. The formation of (Mn, Fe, Co)Nb₂O₆ and coke on the surface of the MNT electrode hindered electron conduction, making the electrochemical performance of the MNT cell poorer than that of the ZNT cell and also leading to the gradual decline of the stability of the MNT cell.[Conclusions] ZNT and MNT were synthesized by using the solid-state method. ZNT showed higher structural stability than MNT, while more Ni nanoparticles and Co-Fe alloy nanoparticles were exsolved from MNT than from ZNT after reduction in 5% H₂ at 800 ℃. However, the generation of (Mn, Fe, Co)Nb₂O₆ on the surface of MNT hindered the electron conduction. Therefore, ZNT (0.42 S·cm⁻¹) achieved higher electrical conductivity than MNT (0.17 S·cm⁻¹) after reduction. During the electrolysis of CO₂ at 800 ℃, the ZNT cell (0.96 A·cm⁻², 1.3 V) exhibited higher catalytic activity than the MNT cell (0.56 A·cm⁻², 1.3 V), because the ZNT had entropy-stabilized crystal structure than MNT, while the generation of (Mn, Fe, Co)Nb₂O₆ in MNT would hinder the catalytic reaction of CO₂ on the electrode. In addition, the stability of the MNT cell gradually declined, due to the carbon deposition on the surface of the MNT electrode during the electrolysis of CO₂ at 800 ℃ and 1.3 V. In summary, the ZNT cell has higher stability and catalytic activity than the MNT cell.

Key words: rutile structure; high-entropy oxides; CO₂ catalysis; nanoparticles