Non-noble Metal Catalysts Synthesis And Ionic Liquid Sboosting Electrocatalytic Reduction Of CO₂

Since the First Industrial Revolution,a large amount of fossil fuels has been used to promote the development of human society and economy.As a result,the concentration of CO2 in the atmosphere continues to increase,which has caused a series of ecological and environmental problems.At the same time,CO2 is also a cheap,abundant and renewable C1-feedstock,which can be converted into value-added fuels or chemicals,providing an efficient way to solve environmental and energy problems.Among various CO2 conversion methods,electrocatalytic reduction of CO2 is considered to be one of the most promising CO2 utilization methods due to its mild and controllable reaction conditions,the utilization of electric energy generated by clean energy,and the easy assembly to scale up.However,there are still many problems in the electrocatalytic reduction of CO2 field.For example,it is difficult to trade off the high faraday efficiency and current density of reduction products,the structure and catalytic mechanism of active sites are unclear,and the catalysts with low cost and high activity need to be synthesized urgently.Developing new non-boble catalysts and screening electrolytes are the key to solve the above problems.In this work,three catalyst design strategies were used to improve the stability of non-noble metal catalysts for the reaction intermediates,and further combining ionic liquid electrolyte to construct reaction system of CO2 electrocatalytic reduction.The synthesis process,activity evalution,and reaction mechanism of the catalysts were explored,and highly efficient CO2 conversion was successfully achieved.The main reaserch contents and results are as follows:(1)Based on the strategy of modulating the morphology of materials.The flowerlike indium sulfide(In2S3)assembled by two-dimensional nanoflakes was synthesized via ionothermal method and applied into electrocatalytic reduction of CO2 for the first time.Due to the structure formed through ionothermal method,the flowerlike In2S3 has larger specific surface area and faster interfacial electron transfer rate than bulk In2S3 synthesized by hydrothermal method.In ionic liquid electrolyte,flowerlike In2S3 exhibited higher electrocatalytic reduction of CO2 activity than bulk In2S3,achieving the highest HCOOH faraday efficiency of 86%with 25.6 mA cm-2 partial current density and 478 ?mol h-1 cm-2 formate formation rate.The theoretical calculation showed that the unique crystal plane composition of flowerlike In2S3 has a significant effect on CO2 reduction activity.The(400)crystal plane has the strongest stability to the reaction intermediate and is more conducive to the formation of formate.The proportion of(400)crystal plane in flowerlike In2S3 was significantly enhanced,further combining with the co-catalysis of ionic liquid eletrolyte,the efficiency of CO2 reduction to formate was significantly improved.(2)Considering the high cost of indium-based catalysts and the low utilization rate of metal atoms in metal compounds,manganese(Mn)single-atom catalyst with low cost and high utilization rate of metal atoms was synthesized by loading and roasting methods.XRD,XAFS,HAAD-STEM and other characterization methods confirmed that Mn existed as a single atom on the substrate through N coordination,and the coordination number of Mn-N is 3,which is the first discovered.The Mn single atom catalyst showed 98.8%CO faraday efficiency with 14.0 mA cm-2 partial current sensity at 0.44 V overpotential in KHCO3 aqueous electrolyte.Considering the advantages of ionic liquids,the activity of Mn single atom catalyst was further tested in the ionic liquid electrolyte.The results indicated that the potential range of CO faraday efficiency over 90%was obviously widened,and the CO partial current sensity can reach 18.6 mA cm-2 and 29.7 mA cm-2 at the overpotential of 0.42 V and 0.62 V,respectively.In-situ characterization and theoretical calculation showed that Mn atom is the active site,on which CO2 was adsorbed,activated and converted.The Gibbs free energy of the key intermediate in the reaction process has less change in the three-coordination structure,which is in favor of the electrocatalytic reduction of CO2.(3)In order to solve the problem that the catalysts showed high CO faraday efficiency only within a very narrow potential range in aqueous electrolyte.Copper-nitrogen-doped carbon nanotube catalyst was fabricated by loading and roasting methods,using metal ionic liquid as precursor.XRD,HAAD-STEM,XAFS and other characterization methods confirmed that Cu mianly existed in the catalyst as Cu-N3 structure.The prepared catalyst exhibited over 90%CO faraday efficiency and high CO partial current density in a wide potential range from-0.42 to-0.92 V,and the maximum CO faraday efficiency value of 98.7%was obtained at-0.82 V with CO partial current sensity of 234.3 mA cm-2.The CO formation rate can achieve 5130 ?mol h-1 cm-2 at-1.02 V.Theoretical calculation demonstrated that Cu-N3 site facilitated the formation of COOH*and the binding of reaction intermediates,thereby,accelerating electrochemical reduction of CO2.(4)Considering the problems of high cost and environmental pollution after waste of metal-based catalyst,a low-cost and environment-friendly non-metallic N-doped carbon material was prepared by pyrolyzing polyaniline with iodine treatment.When the pyrolysis temperature was 900?,the obtained carbon material exhibited a nearly 100%CO faraday efficiency with 38.2 mA cm-2 CO partial current density in ionic liquid electrolyte.However,the highest CO faraday efficiency of the carbon material without iodine treatment is 86.2%,and the CO partial current density is only 13.5 mA cm-2.By analyzing the distribution of N species content and the change of activity of the carbon material pyrolyzing at different temperatures,pyrrole N was identified as the active site.Through a series of characterization,it was proved that iodine treatment not only increased the pyrrole N content,but also increased the electrochemical surface area,CO2 adsorption capacity and decreased the interfacial charge-transfer resistance of the N-doped carbon material,which is in favor of the electrocatalytic reduction of CO2.