Study On Efficient Electrocatalytic Reduction Of CO₂ By Carbon-based Materials Rich In Ni-N Structure

The continuous increase of carbon dioxide concentration in the environment is bringing a lot ofdisaster consequences to all mankind.Electrocatalytic reduction of greenhouse gas carbon dioxide to valuable fuels and chemicals,this technology can be well combined with sustainable energy,easy to industrialization,easy to control and other advantages,which is attracting more and more attention.However,there are still some problems in ECR,such as expensive catalyst raw materials,unstable performance and low selectivity.In order to solve these problems,it is a research direction to select general low-cost raw materials to prepare high selectivity,high activity and stability catalytic materials by a simple method.Carbon based material is a good matrix material because of its wide sources,large specific surface area,good stability and high electron transfer.When the transition metal nickel atom and nitrogen atom are anchored to carbon-based material together,the utilization of metal can be improved to the greatest extent.When combined with nitrogen atom,the ability of receiving electron for nickel atom can be increased,showing good catalytic performance in electrocatalytic reduction of carbon dioxide,such as high activity,high selectivity and high stability.However,there are many problems in the preparation of carbon-based materials with Ni-N structure,such as many preparation steps,high equipment requirements,expensive raw materials and so on.At the same time,the mechanism of electrocatalytic reduction of carbon dioxide for this kind of materials is not very clear.In view of the above considerations,in this paper,the carbon-based materials rich in Ni-Nstructure are prepared by using conventional equipment to prepare carbon based materials rich in Ni-N structure by using common public chemicals as raw materials,and conducts a large number of electrochemical tests and calculation simulation to comprehensively study the activity,selectivity and catalytic mechanism for electrocatalytic reduction of CO₂.This paper mainly elaborates on the following three aspects:(1)Ni-ZIF-8 bimetallic nano materials were prepared by rapid and simple ultrasonic assisted method,and Ni-N-doped carbon materials(Ni NPIC)were successfully prepared by pyrolysis.The key role of ultrasonic cavitation in the rapid preparation of interconnected Zn-Ni bimetallic MOFs precursors was discussed,and the formation of cross-linked layered porous Ni NPIC materials by carbonization of precursors was analyzed;In this experimental scheme,by changing the amount of Ni in the bimetal MOF precursor,the Ni loading of Ni NPIC can be accurately controlled between 0.62 wt%and 5.26 wt%.In addition,the high surface area and interconnected porous structure of the catalyst provide a highly accessible Ni-N site and a convenient mass diffusion channel,which improves the electron transfer,reduces the interface resistance and improves the electrolyte/gas transfer rate in ECR.The optimized catalyst can effectively reduce CO₂to CO at a medium overpotential of 0.54V(vs.REH),with FE_COof 95.1%,current density of 10.2 m A cm^-2(113 m A mg^-1)and conversion rate of 542?mol m^-2s^-1(36.1?mol mg^-1min^-1).The possible path of ECR and the relationship between the structure and properties of the catalyst are further explored by impedance and Tafel slope measurements.(2)Common mixtures of melamine,nickel nitrate and citric acid were prepared by a simple one-step in-situ pyrolysis method to produce Ni-N-CNSs with thickness of about 1.5 nm.The ultra-thin porous structure of Ni-N-CNSs has large surface area,abundant mesopores and a large number of uniformly dispersed Ni atoms.A series of catalytic materials,Ni NPs-N-CNSs,N-CNSs,Co-N-CNSs and Fe-N-CNSs,were prepared by similar methods.The electrochemical properties of Ni-N-CNSs showed excellent activity and selectivity for ECR.The CO conversion current is 121.4 m A mg^-1and Faraday efficiency is close to 100%CO,and the CO generation rate is as high as 37.7?mol mg^-1min^-1and super durability.This is because Ni-N-CNSs will expose as many Ni atoms as possible.These Ni atoms anchored by N atoms will provide a special electronic structure that is conducive to both CO₂adsorption and CO desorption,and inhibit the competition of HER in ECR.Density functional theory(DFT)was used to simulate the ECR reaction process,revealing the high activity and selectivity of Ni-N-CNSs for ECR in water.(3)Nickel phthalocyanine(NiPc)rich in Ni-N structure was ultrasonically dispersed in a defect-rich Ketjen Black(KB)solution,and then a NiPc/KB composite catalyst can be simply obtained through a drying step.In this complex,the large specific surface area and good electronic conductivity of ketjenblack are fully utilized,and nickel phthalocyanine provides a stable Ni-N structure.In 0.5 M KHCO₃electrolyte solution,it was found that the catalytic performance of NiPc was greatly improved by changing the electrochemical potential of NiPc and KB,showing a FE_COof up to 98.2%,and the FE_COof no less than90%at the potential range of-0.65 V?-1.05 V.After the composition of NiPc and ketjenblack,not only the selectivity of the product was improved,but also the catalytic activity was greatly improved.Density functional theory(DFT)calculation basically revealed that part of the electrons of nickel phthalocyanine molecules transferred to the ketjenblack substrate,enhanced the adsorption of COOH by the complex,and inhibited the competitive hydrogen evolution reaction(HER),so that the NiPc/KB composite catalyst showed a very high catalytic activity and selective in ECR.