Mechanism Study On Electrocatalytic Oxygen Reduction Reactions Of Doped Graphene Quantum Dots

Fuel cells can directly convert chemical energy into electrical energy,and conversion processes are simple,efficient,and environmentally friendly,known for their efficiency and green.The key problem is to promote large-scale commercial applications of fuel cells and develop low cost and stable cathodic oxygen reduction reaction(ORR)catalysts.However,the main reason for preventing their practical applications is that platinum and other precious metals are required to be used as ORR catalysts,and platinum has limited reserves,high price and poor stability at high current density.In recent years,sp² hybrid carbon nanomaterials have become one of the most promising candidates due to their excellent activity and stability,excellent resistance to fuel penetration and toxicity.Studies have shown that carbon-based nanomaterials have ORR activity as free-metal catalysts,but the mechanism of ORR catalysis is still unclear.In addition to applications,ORR is also one of important reactions in fundamental electrochemical research.Due to quantum effects and edge effects,graphene quantum dots(GQDs)have demonstrated excellent electrocatalytic activity compared with other carbon-based nanomaterials.In this study,by using density functional theory(DFT)calculations,the ORR reaction mechanisms of GQDs were investigated,and the structure-activity relationship between the microstructures of GQDs and their ORR catalytic activity was revealed.This thesis mainly includes the following contents:(1)The stability,electronic structure and other properties of graphene quantum dots were studied to investigate the effects of N and S doping.The heteroatom doping is an effective method to regulate electrochemical properties of GQDs.The influences of different atom doping types on electronic structures of GQDs were revealed through calculating the formation energy,electronic structure and electrostatic potential of pristine and co-doped GQDs.The highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)energy gaps of GQDs can be reduced due to the synergistic effect of N and N/S co-doping.The introduction of S can effectively change the charge distribution around N atom,and the charge density of C atom adjacent to N is significantly enhanced,indicating that heteroatom doping GQDs can improve charge distributions,and their electronic structures can be effectively regulated by changing the type of atomic doping.This work will provide a reasonable mechanism to explain internal relationships between atom doping and ORR active sites of GQDs,and offer theoretical guidance for the applications of nanomaterials in ORRs.(2)The mechanism of ORR of N and N/S co-doped GQDs is revealed.The energy diagrams of four-electron and two-electron reduction reactions were studied by DFT,and the effects of atom doping on the ORR electrocatalytic activity and reaction processes of GQDs were investigated.The results show that the doped GQDs with pyridine N atoms have higher electrocatalytic activity than the doped GQDs with graphitic N.Compared with N doping,N/S co-doping can enhance the ORRs activity of GQDs.The introduction of S atom can change the state of N doping,resulting in asymmetry of electron spin and charge density.In addition,the introduction of S atom can significantly reduce the free energy of the reaction paths and improve electrocatalytic performances of ORRs.This work will provide reasonable guidance for the rational design of N/S co-doped carbon materials for ORR electrocatalysts and the investigation of their catalytic mechanisms.(3)The effects of different forms of H₃O⁺,OH^–,H₃O and OH on the ORR electrocatalytic properties of GQDs were investigated.The results show that charged H₃O⁺and OH^–ions are not favorable to ORR reactions.However,when H₃O and OH are adsorbed on g N-GQD to generate functionalizations,ORR reaction is promoted.The electrocatalytic activity of different ions/groups on GQDs is ranked as follows:g N-H₃O-GQDs>g N-OH-GQDs>g N-GQDs>g N-H₃O⁺-GQDs>g N-OH^–-GQDs.The adsorption of H₃O on the surface of g N-GQDs has the best ORR catalytic performance and can effectively promote oxygen reduction reactions.By analyzing differential electron density,it can be seen that when H₃O is adsorbed to g N-GQDs,the adsorption electron transfer of O₂ is more obvious,which can effectively promote the adsorption of O₂.The results can reveal the influences of different ions/groups on ORR catalytic mechanisms of GQDs and provide guidances for the design of ORR electrocatalysts.