Indium-based Catalysts For Carbon Dioxide Electrochemical Reduction In Flow Cell

The emergence of electrocatalytic CO₂ reduction technology has successfully combined the development of new energy with CO₂ conversion.It can convert CO₂ into high-value-added fuels and chemicals driven by renewable energy power,while reducing CO₂emissions.It can also store intermittent clean energy in the form of chemical bonds to promote the even distribution of energy.There are many products of electrocatalytic CO₂reduction,of which formic acid is one of the most promising options for economic applications.Among many catalysts,In-based catalysts show extremely high selectivity to formate products and have attracted the attention of many researchers due to their low toxicity and environmental friendliness.However,in the CO₂ catalyzed reaction,In-based catalysts have disadvantages such as low current density and poor stability.Therefore,it is necessary to optimize In-based materials to obtain stable and efficient electrocatalysts.In addition,the reaction device is also closely related to the overall performance of CO₂electroreduction.At present,most studies still use H-type electrolytic cell as the reaction device,and its limited CO₂ solubility and mass transfer efficiency greatly limit the further improvement of current density.Therefore,it is especially important to explore and upgrade the performance and structural characteristics of different types of electrolytic cell in the CO₂electroreduction reaction.This thesis focuses on the development of high-efficiency In-based catalysts and the optimization of advanced reaction devices.Based on the use of modified strategies to obtain high catalytic activity and selectivity catalysts,combined with the structural characteristics of different flow cell reactors,to achieve stable catalytic reduction of CO₂ to formate under high current density and high selectivity.The main research contents and results are as follows:(1)Ultrathin Zn In₂S₄ nanosheets modified with Zn vacancies are prepared by the ultrasonic-assisted exfoliation method.The test results show that,compared with the bulk Zn In₂S₄ material,Zn In₂S₄ nanosheets exhibit significantly enhanced catalytic performance in the reaction of reducing CO₂ to form formate.Electrochemical analysis and theoretical calculations show that larger active specific surface area,faster charge transfer rate and optimized binding energy of reaction intermediates are the main reasons for the excellent catalytic activity of Zn In₂S₄ nanosheets.The application of the new liquid-phase flow cell further enhances the overall performance of the CO₂ reduction reaction.The initial potential of the Zn In₂S₄ nanosheets is reduced by 700 m V compared with the H-type electrolytic cell.The selectivity of formate reaches 94%,and its partial current density is-245 m A cm^-2,which is 83 times that of the H-type electrolytic cell.(2)Based on the research results in the previous chapter,adjusting the active site density and electronic structure of the material can effectively enhance its catalytic performance.Therefore,MOFs with abundant metal active sites are selected as the research object,and the metal sites can be modified by-NH₂ through ligand functionalization and the electronic environment can be changed.Benefiting from the surface-NH₂ functional groups,MIL-68(In)-NH₂ can effectively adsorb and activate CO₂ and stabilize the reactive intermediates.In the liquid-phase flow cell,the selectivity of MIL-68(In)-NH₂ to formate is as high as 94.4%at-1.1 V vs.RHE,and the partial current density is-108 m A cm^-2.The application of the new gas-phase flow cell further increases the formate partial current density to 332 m A cm^-2.Its unique zero-gap configuration can effectively reduce the internal resistance of the device and avoid the overflow phenomenon in the liquid-phase flow cell.(3)In view of the instability of the In-based MOFs material in the previous chapter,the carbon nanorod confined In₂O₃ nanoparticles(In₂O₃@C)are obtained by high-temperature calcination of MIL-68(In).The tight coating of the carbon layer significantly improves the conductivity of the material,which can effectively prevent In₂O₃ from being reduced to metal In at the reduction potential and reduce the catalytic performance.The electronic interaction between the carbon layer and In₂O₃ also further enhances the intrinsic activity of catalyst.In the liquid-phase flow cell,within the potential window of-0.8 V vs.RHE to-1.3 V vs.RHE,In₂O₃@C can convert CO₂ to formate with a Faraday efficiency of more than 90%,and its partial current density can also reach-137.3 m A cm^-2.In addition,in view of the problem that the gas-phase flow cell is not suitable for producing liquid products,solid electrolyte components are introduced to upgrade,and finally In₂O₃@C directly reduces CO₂ with a current density of 30 m A cm^-2 to obtain a liquid formic acid solution.(4)Based on the application research of MOFs-derived composite materials in CO₂electroreduction in the previous chapter,the heteroatom doping strategy is used to further enhance its catalytic ability.N-doped In₂O₃@C(N-In₂O₃@C)is prepared by high-temperature pyrolysis of In-rho-ZMOFs.The selectivity to formate in the liquid-phase flow cell is up to 95.5%,and partial current density also reaches-147.6 m A cm^-2.The excellent catalytic activity of N-In₂O₃@C is mainly due to the fact that doping with N atoms can change the local electronic structure of the material and optimize its binding energy with the intermediate,so that CO₂ can be quickly activated to form formate.At the same time,optimization is made for the anode half reaction that is often overlooked in the reaction device.After the oxygen evolution reaction is replaced by methanol oxidation reaction,the overpotential of driving the electrolytic cell to 50 m A cm^-2 can be reduced from 4 V to 2.8V under the condition that the cathode and anode can react at the same time to produce formate.