Mechanism Study Of Oxygen Reduction Reaction Based On Nitrogen-doped Graphene Quantum Dots With Surface Modification And Vacancy Defects

The fuel cells are promising way to solve climate and energy crisis due to their clean,environmentally friendly,and efficient,etc.The electrochemical reactions on electrodes of fuel cells are the key to their efficient and stable operation,which require efficient catalysts,especially for the oxygen reduction reactions（ORR）with slow reaction kinetics on cathode.Currently,the noble metal platinum（Pt）is the cathode catalyst mainly used.However,its limited reserves and high price greatly increase the manufacturing cost of fuel cells.Hence,in recent years,lots of studies were conducted on materials that can replace Pt.Among these materials,carbon-based nanomaterials（graphene,carbon nanotubes,etc.）can be employed as an ideal electrocatalyst due to their wide sources,high conductivity and stability,and large specific surface area,etc.Among them,graphene quantum dots（GQDs）are graphene nanosheets with extremely small size,which have larger specific surface area and better electrical properties than graphene,and will have more advantages in electrocatalysis.Therefore,this paper will employ GQDs as study object and introduce three modification methods:N-doping,vacancy defects and surface modification functional groups.The process of GQDs catalyzing ORR were studied through theoretical calculations,and the relationship between electronic properties and ORR catalytic activity of GQDs were evaluated to reveal the reasons that affect activity,as well as find the best methods to improve activity.The contents of this paper consists of following three parts:（1）Effects of different ions/groups on ORR catalytic activity and electronic properties of pyridinic-N and pyrrolic-N doped GQDs.The differences of ORR catalytic activity between pyridinic-N（p N）and pyrrolic-N（pr N）doped GQDs were studied,as well as the effects of two ions OH^-,H₃O⁺,and two groups OH,H₃O on activity and electronic properties of GQDs.The calculations of ORR free energy change show that p N-doped GQDs possess higher catalytic activity than pr N-doped GQDs,and their activity can be enhanced by adsorption of OH and H₃O groups.The reason why OH and H₃O groups increase more activity is that they greatly alter the electronic structure of GQDs,effectively control the adsorption strength of reaction species,and thus change the free energy change of reaction process.（2）Effects of different oxygen-containing groups on ORR catalytic activity and electronic properties of GQDs.Based on previous work,the effects of more other types of oxygen-containing groups on GQDs catalytic activity were studied.In this chapter,three oxygen-containing groups,carbonyl（O）,hydroxyl（OH）and ether（COC）were selected to investigate their effects on GQDs activity.Among them,only COC increases GQDs activity,while the remaining groups decrease GQDs activity.The analysis of molecular orbital show that GQDs adsorbed with COC have smaller LUMO-HOMO energy gap,which is conducive to electron transfer during the reaction,thus improve the activity.These results show that it is a feasible method to improve ORR catalytic activity of GQDs and other carbon nanomaterials by adsorbing functional groups.（3）Effects of different N-doping and vacancy defects on ORR catalytic activity and electronic properties of GQDs.This chapter introduced different vacancy defect states into graphitic-N（g N）,p N and pr N-doped GQDs.It was found that the order of activity improvement is:g N>pr N>p N>defects.Among them,g N has the greatest degree of improvement,while pure defects only slightly increase the activity,and GQDs with the combination of g N and divacancy defects has the highest activity,indicating that N-doping have greater impact on GQDs ORR activity than defects.The analysis of electronic structure and molecular orbital show that the contribution of N-doping to density of states at partial energy levels are greater than that of defect rings,and g N significantly reduces the LUMO-HOMO energy gap,indicating that N-doping have greater impact on electronic and orbital properties of GQDs.Finally,based on the results of activity and stability calculations,the combination of g N and single or double vacancies,or single g N,are ideal methods to improve ORR catalytic activity of carbon nanomaterials.To sum up,it is expected that this work will provide useful theoretical guidance for design and synthesis of modified carbon nanomaterials,and contribute to promote application of carbon nanomaterials as electrocatalysts in fuel cells and other energy storage and conversion devices.