Study On Carbonaceous Fuel Oxidation And CO₂ Reduction Based On Solid Oxide Fuel Cell

The energy problem affects the development social economy and is valued by the whole world.Traditional fossil fuels,such as coal,natural gas and petroleum,are still the main energy resources at present,but the traditional energy conversion technology is inefficient and emits a lot of CO₂,NO_x and other environmental pollutants.China has set a strategic goal of achieving peak carbon emissions by 2030 and carbon neutrality by2060,so it is imperative to develop low-carbon and energy-efficient technologies.Solid oxide cell（SOC）is an all-Solid energy conversion device,which can be divided into Solid oxide fuel cell（SOFC）and Solid oxide electrolysis cell（SOEC）.In SOFC mode,chemical energy in fuel can be directly converted into electricity with the characteristic of high conversion efficiency and environment friendly.General SOFC operating temperature is about 600-1000 ^oC,so SOFC can also use natural gas,synthetic gas and other biomass gas and hydrocarbons as fuel.At present,the main challenge to SOFC is that the traditional nickel-based fuel electrode has a weak coke resistance,which leads to the degradation of cell performance.Perovskite oxides have good coke resistance and excellent redox stability.At present,perovskite oxide as fuel electrode is a hot research topic,but its catalytic activity is limited,so it needs further modification to improve its catalytic activity.At present,the commonly used methods including impregnation,in-situ exsolution and composite electrode,etc.Based on this,we designed a new type of perovskite material achieving the exsolution of nano-particles under the oxidized conditions,and the metal catalyst was impregnated on its surface to further optimize its catalytic performance,therefore,the excellent fuel electrode material is obtained.In SOEC mode,the excess electrical energy can be directly converted into chemical energy,with high electrolysis performance and clean.In addition,SOEC can be combined with renewable energy technology,electrolysis of CO₂ can not only reduce carbon emissions,and electrolysis of CO can be combined with H₂ and further through Fischer-tropsch synthesis of high-value chemicals.However,since the thermodynamic stability and kinetic inertia of CO₂,its high-temperature electrolysis performance is poor,it is very important to develop new cathode materials with high catalytic performance,high carbon deposition resistance,high current density and good stability.Therefore,a new type of SOEC symmetric electrode material has been developed to realize the co-exsolution of oxide and metal alloy nanoparticles under reducing condition,combined with Density functional theory（DFT）,the mechanism of electrocatalytic activation of CO₂ by the fuel electrode is revealed.Firstly,the research on SOFC.La_0.8Ce_0.2FeO₃（LCF）,La_0.7Pr_0.1Ce_0.2FeO₃（LPCF）and La_0.7Gd₀.₁Ce_0.2FeO₃（LGCF）were prepared by solution method by doping Ce³⁺,Ce³⁺/Pr³⁺and Ce³⁺/Gd³⁺on the A site of LaFeO₃ perovskite oxide.In addition,in order to prevent the oxidation of Ce³⁺and Pr³⁺in air,20%Ar-CO₂ was used to prevent the oxidation.Because Ce and Pr are easy to be oxidized,we construct in-situ exsolution of Ce O₂,Pr-doped Ce O₂（PDC）and Gd-doped Ce O₂（GDC）for LCF,LPCF and LGCF in air,respectively.At the same time,we also found that the exsolution of Ce O₂-based accompanied by the exsolution of Fe₂O₃ from B-site,which can further improve the catalytic activity of the material.Compared with the traditional exsolution of nanoparticles under the reduction condition,the exsolution of nanoparticles under the oxidation condition saves the preparation process and technology.When H₂ is used as fuel,LCF,LPCF and LGCF anodes are painted on the one side of electrolyte La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ（LSGM）respectively,and the same La_0.8Sr_0.2Co_0.2Fe_0.8O₃（LSCF）and Gd_0.1Ce_0.9O_1.95（GDC）composite cathodes（mass ratio 6:4）were painted on the other side of LSGM electrolyte for the preparation of full cells and the peak power density（PPD）with power of 285,738 and 379 m W cm^-2,respectively,the lowest polarization impedance（R_p）was 0.12Ωcm² among three cells.In order to improve the cell performance,Ni-Fe alloy nanoparticles were prepared by impregnating 1wt%Ni on the anode surface.When H₂ is used as fuel,the PPD of the cell with 1 wt%Nii+LCF,1 wt%Ni+LPCF and 1 wt%Ni+LGCF anodes reached 714,808 and 606 m W cm^-2,respectively,while when C₃H₈ is used as fuel,the PPD the cell with 1 wt%Ni+LPCF and 1 wt%Ni+LGCF anodes reached 714,808 and 606 m W cm^-2,respectively,its power reaches 434,280 and 381 m W cm^-2,respectively,and the three cells show excellent stability under C₃H₈ condition.In addition,when H₂ and CH₄ mixture gas（1:4）were used as fuel,the PPD of the cells with 1wt%Ni+LCF,1wt%Ni+LPCF and 1wt%Ni+LGCF reached 663,501 and 745 m W cm^-2,respectively.The exsolution of Ce O₂-based oxides was successfully achieved under the oxidized condition by doping Ce³⁺,Ce³⁺/Pr³⁺or Ce³⁺/Gd³⁺on the A site of LaFeO₃,and metal nanoparticles were impregnated on the surface of the oxides,the co-exsolution of oxide and metal alloy nanoparticles can be induced,and the coke resistance the cells under hydrocarbon fuel can be greatly improved.Then,the research on SOEC.A series of Sr_1-xCe_xFe_1-y Co_yO₃（x=0.0.05,0.01;y=0.0.05,0.1）were prepared by solution method by doping Ce and Co at A and B sites of SrFeO₃,respectively.It is found that a small amount of Ce doping（less than 10%）cannot stabilize the crystal structure of perovskite,and Ruddlesden-Popper（R-P）phase is easily formed.The exsolution of Co-Fe alloy and Ce O₂nanoparticles can be achieved by reduction treatment for Sr_0.95Ce_0.05Fe_0.9Co_0.1O₃（5Ce10Co F）and Sr_0.9Ce_0.1Fe_0.9Co_0.1O₃（10Ce10Co F）symmetric electrodes for SOEC and the catalytic activity for CO₂ in the cathode chamber can be promoted.The cells with 5Ce10Co F and 10Ce10Co F symmetric electrode show no obvious degradation after 48 h durability test for CO₂ electrolysis at a bias voltage of 1.3 V,and the current densities were 0.98 and 1.30 A cm^-2,respectively,the Faraday efficiencies of two cells were up to 94%and 98%,respectively.In addition,DFT calculation indicated that the formation of oxygen vacancy is contributed to the CO₂electrolysis,and Ce-doped Se FeO₃ is more advantageous to the electrolysis of CO₂ than that of Co doping.The results show that Ce/Co co-doped SrFeO₃ as a symmetrical electrode of SOEC exhibits excellent cell performance for CO₂ electrolysis at high temperature.