Preparation Of Carbon-based Catalysts For Application In Electrocatalytic Reduction Reaction

Carbon-based materials own several advantages such as plentiful supply,adjustable structure,excellent electrical conductivity and better environmental compatibility.Researchers all over the world have made unremitting efforts to design them as highly efficient and durable catalysts.This dissertation aims to turn carbonbased materials into highly active catalysts through tuning their electronic structure for application in Oxygen Reduction Reaction(ORR),Nitrogen Reduction Reaction(NRR)and Carbon Dioxide Reduction Reaction(CO2RR)and explore its "structure-activity"relationship by structural characterization and DFT calculation.The specific contents are as follows.1.Graphene with π-conjugated electron structure is inactive to ORR because of the weak adsorption of oxygenated species at the electroneutral carbon site.Due to their difference in electronegativity,N doping into carbon-based materials is an efficient strategy to make local charge imbalance,however,most N-doped carbon catalysts are still inferior to transition metal based catalysts,because it is difficult for N atoms to change π-conjugated system to a level high enough to break the oxygen-oxygen bond thus activate oxygen adsorption process.In order to improve the ORR performance of carbon based catalysts,an Ag@NC core-shell structure was prepared.Ag@NC exhibits an onset potential of 1.013 V and a half-wave potential of 0.906 V,which is superior to 20 wt%commercial Pt/C and most carbon-based electrocatalysts.Further studies revealed that N doping can alter the average electron structure,and Ag core can inject extra electron to outer graphene layer,which enhances the adsorption of oxygen species.Density functional theory calculation shows that every carbon site in Ag@4C model is activated by extra transferred π electrons of 0.0572 e-and showed the best performance.The Ediff of the carbon site in Ag@4C is lower than other models,and the activity of Ag@4C is highest with the lowest energy barrier value of 0.75 eV in rate-determining step.2.As for NRR,the critical bottleneck lies in adsorbing and activating nitrogen for the high bond energy of nonpolar N=N triple bond.Numerous metal-based catalysts were reported especially transition-metal-based catalysts,due to their available d-orbital and donable d-electrons which can act as electrophile or nucleophile and easier to form intermediates,but their favorable affinity towards H+over N2 impedes their wider application.Carbon-based materials are naturally inert to HER and rich in oxygen species like-OH and-COOH,which have an affinity for electrons.Defects engineering and oxygen groups modifying in carbon networks can improve NRR performance.The highly electronegative halogen atoms can modify oxygen-containing groups through substitution reaction,which is a suitable choice for tuning their electron distribution.This work introduced Cl atoms into carbon matrix thus forming-Cl and-COCl species in defect-engineering area by pyrolysis.This catalyst achieves a high ammonia yield of 103.96 μg mgcat-1 and a faradaic efficiency of 21.71%,which is 7.0 times as high as that without Cl modulation and exceeds most metal-based catalysts.The newly produced-Cl and-COCl functional groups could attract more electrons from adjacent carbon atoms due to their stronger electronegativity,thus improving their affinity towards N2 and weakening the bond strength of N≡N.Moreover,Cl doping can effectively inhibit HER process and improve the kinetic process of NRR.3.Pure carbon materials are chemically inert so that they cannot activate intermediate products like CO2*,that is the reason why they are not appropriate for CO2RR.Herein,this work designed a N and Cl co-doped carbon matrix to improve CO2RR activity.The faradaic efficiency of this catalyst exceeds 95%at multiple voltages and shows excellent stability in 60 h I-t test.The heteroatom Cl not only making more defects and larger apertures to provide efficient paths for electron and proton transport,but also increasing pyridine nitrogen in the material.The lone-pair electrons in pyridine nitrogen can bind with CO2 molecules and induce positive polarization of adjacent carbon atoms,which promote the formation of*COOH intermediates.It shows that the synergistic effect of N and Cl can reduce the redox potential and further promote CO2RR process far more than single atom doped carbon-based catalyst.In addition,a novel catalyst with EDTMPA molecules depositing on carbon nanotubes was prepared,which can improve CO Faraday efficiency to 79%.In this catalytic system,EDTMPA molecules as catalysts were connected with carbon nanotubes through PO32-,and carbon nanotubes act as conductive carriers,which can transfer electrons to the active center.