Theoretical Study Of Heteroatom Doped Graphene With Defects Electrocatalysts For Oxygen Reduction Reaction

Polymer electrolyte membrane fuel cells（PEMFC）,as clean energy devices,have attracted extensive interest over the past few decades because of their high conversion efficiency,high power density,and zero/low emissions.Currently,Pt-based materials are the most widely used catalysts for the Oxygen Reduction Reaction（ORR）but their high cost,poor CO tolerance,and low durability and stability hinder their commercial applications in PEMFC.Therefore,the exploration of inexpensive and sustainable nonprecious metal or metal-free electrocatalysts with high performance has been the key issue in the development of high-efficiency PEMFC.Recently,sp² nanocarbon materials such as doped graphene and nanotubes have become important candidates and hotspots of nonprecious metal or metal-free ORR electrocatalysts due to their excellent electrical conductivity and high surface area.This thesis systematically studied different nonprecious metal and metal-free doped defective graphene（M-Gra）formation and electrocatalytic performance for the ORR within a quantum chemistry calculation and unified electrochemical thermodynamic framework,in order to understand the cause of the catalytic activity of M-Gra.The main contents and results are summarized as follows:1.Phosphorus（P）doped graphene with three kinds of defects as electrocatalysts for the ORR.The calculated formation energy showed that P-doped divacancy graphene（P-GDV）is the most stable than P-doped monovacancy graphene and P-doped graphene with Stone-Wales defect.Therefore,P-GDV is selected as electrocatalyst for the ORR.The calculated results indicated that O₂ molecule could be chemisorbed on P-GDV surface,which is the first necessary step to initialize the ORR.Following the adsorption of O₂ molecule on P-GDV,there are three competitive reaction pathways through 4e-process to form H2 O molecules.The kinetically most favorable is the hydrogenation of O₂ molecule which forms OOH and hydrogenation of OOH which forms H₂O+O.The rate-determining step is the final step in the pathway,i.e.,the hydrogenation of OH into H2 O.The free energy diagram of the ORR predicted that for the most favorable pathway,the working potential is 0.27 V.2.Silicon（Si）doped graphene with defects as electrocatalysts for the ORR.The calculated results showed that O₂ dissociation and OOH hydrogenation into O+H₂O are the most favorable pathways kinetically for Si-doped divacancy graphene.The rate-determining step is the O atom hydrogenation into OH.However,the rate-determining step is the OH hydrogenation into the second H2 O as the same as reaction pathway on Si-doped monovacancy graphene.This result showed that the same metal-free doped graphene with different defects can lead to the different catalytic activity.The possible ORR elementary steps and charge transfer could take place within a small active region around the Si atom and its adjacent four carbon atoms by charge transfer and the differential charge density analysis.3.Manganese and Phosphorus MnP_x（x=1-4）codoped graphene with defects as electrocatalysts for the ORR.The calculated formation energy indicated that MnP₂ codoped divacancy graphene（MnP₂-Gra）is energetically the most stable in the MnP_x（x=1-4）codoped graphene.The MnP₂-Gra as electrocatalyst,the MnP₂ moiety and its adjacent six C atoms are catalytically active sites for the ORR.In acidic medium,the kinetically most favorable pathway is the hydrogenation of OOH to form O+H₂O,which is a four-electron process.