Synthesis Of Carbon Dots And Its Applications In Electrochemistry Reduction CO₂

The large-scale production of value-added chemicals and chemical raw materials using renewable energy and carbon dioxide is conducive to the full utilization of resources and the alleviation of the dual pressure of energy crisis and environmental problems caused by fossil fuels.Although electrocatalytic carbon dioxide reduction reaction（CO₂RR）can obtain various more valuable C₂ products,some C₁ products（such as CO and HCOOH）can be obtained with high selectivity and low energy consumption,which is very important for the chemical industry.At present,catalysts used for CO₂RR mainly include metal catalysts,non-metallic catalysts,and molecular catalysts,but these catalysts have problems such as high cost,poor selectivity,and stability.Therefore,developing economical and efficient catalysts has become an emergent for modern industry.In this study,cobalt phthalocyanine was used as a precursor and carbon dots（CDs）were used as a coating layer,resulting in the synthesis of a unique CDs coating Cobalt phthalocyanine（CoPc）structure.A series of CDs were prepared by adjusting the type and content of precursors,and composite materials were prepared by combining them with CoPc（CDs@CoPc）.The structure,morphology,elemental composition,elemental content,valence state,and conducted electrochemical performance were tested.The main research content and results are as follows:（1）The precursor 1,4-butanediol reacted with p-phenylenediamine to obtain CDs,then the synthesis of CDs@CoPc is achieved through the hydrothermal reaction of CDs with CoPc,where CDs are anchored on the surface of CoPc（CDs@CoPc）.This material achieves the regulation and efficient conversion of CO₂.Among the CDs,CoPc and CDs@CoPc,CDs@CoPc shows excellent catalytic performance.At-0.85 V（vs.RHE）,the current density of CDs@CoPc is 13.2 mA cm^-2 and the FE_co reaches75.36%,15%higher than pure CoPc.Electrochemical tests show that the material has a large C_dl(5.71 mF cm^-2)and a small charge transfer resistance.Subsequent characterization shows that the excellent electrochemical performance of the material is mainly due to the coating effect of CDs:CDs on the material surface prevent the active site from contacting H⁺in the electrolyte,simultaneously suppress HER.（2）By adjusting the type and quality of precursors,replacing 1,4-butanediol with dithiothreitol,S element was introduced into CDs,and a series of catalysts with different sulfur contents were successfully prepared（SCDs@CoPc-x（1,2,3））.The FE_coof SCDs@CoPc-2 reaches 93.31%at-0.75 V（vs.RHE）,which is 1.5 times that of pure CoPc,and the CO₂RR catalytic efficiency was maintained at FE_co>85%between-0.65 V and-0.95 V（vs.RHE）.In addition,as the S content increases,SCDs@CoPc-3 efficiency has also been further improved(FE_co=99.56%,j_co=11.06 mA cm^-2).The electrochemical tests show that SCDs@CoPc-2 has the largest electrochemical active surface area and the smallest charge transfer resistance.The characterization analysis showed that the combination of H⁺and active site in the electrolyte was blocked,which inhibited HER,but also slowed down the proton coupled electron transfer process of CO₂RR to a certain extent.The introduction of S promotes the decomposition of H₂O,which improves the local proton concentration,meets the H⁺required for the decisive step（CO₂+H⁺+*→*COOH）reaction,and promotes the normal progress of CO₂RR.