Theoretical Study Of Doped Graphene Electrocatalysts For Oxygen Reduction Reaction

The polymer electrolyte membrane fuel cells?PEMFCs?as a device that converts chemical energy into electrical energy by the reaction of hydrogen and oxygen to form water,has attracted extensive interest due to the zero pollution and the high energy conversion efficiency.However,the slow reaction rate of oxygen reduction reaction?ORR?at cathode severely hinders the development of PEMFCs.Until now,the Pt and its alloys are still recognized as the best cathode catalysts of PEMFCs.But their shortcomings such as low durability,high cost,low reserves and poor CO tolerance hinder their commercial applications.Thus,finding a promising alternative to Pt and its alloys of ORR is an important part of the development of PEMFCs.In the past few years,the heteroatom doped carbon materials in especial of the metal and N atoms co-doped carbon materials were found have excellent catalytic performance of ORR and also have advantages of low cost,high stability,etc.,attracting the attention of many scholars.In this paper,based on the density functional theory,we systematical studied the ORR electrocatalytic activity of different metals and N atoms co-doped divacancy graphene by calculation the formation energy,the charge distribution,the transition state and the free energy.And their catalytic reaction mechanism was also discussed in-depth.The main results as follows:1.Iridium and nitrogen atoms co-doped divacancy graphene.In this part,the reaction mechanism and free energy change of ORR on IrN₄ doped graphene has been investigated.The calculated bader charge and charge density difference indicated the IrN₄and its ten adjacent C atoms are active center.The O₂ on the catalyst surface is a chemical adsorption.Subsequently,the ORR mechanism study indicated the activated O₂can complete 4e^-ORR process through three possible pathways and finally reduction into two H₂O molecules.The kinetically most favorable is the O₂ hydrogenation into OOH and then OOH hydrogenation into 2OH.The rate determining step of this pathway is OOH hydrogenation into 2OH.From the free energy diagrams of this pathway,the free energies of all steps are natative values that indicated the ORR is a spontaneous exothermic.After consideration of the positive electrode potential,the predicted working potential of this catalyst is 0.42 V.2.Iron porphyrin carbon matrix.The catalytic active center of this electrocatalyst is the FeN₄ and its adjacent eight C atoms.The study of the reaction mechanism indicated the most favorable pathway is the process of O₂ hydrogenation into OOH and then OOH hydrogenation into O+H₂O.The rate determining step is O₂ hydrogenation into OOH with the energy barrier of 0.09 eV.The thermodynamic studies showed that the working potential of Fe porphyrin carbon matrix is 0.53 V.3.Ruthenium and nitrogen atoms co-doped divacancy graphene oxide.The results shown the O atom modified RuN₄ doped divacancy graphene has better ORR catalytic performance than the RuN₄ doped divacancy graphene both in kinetically and thermodynamically.For the O atom modified RuN₄ doped divacancy graphene,the RuN₄and its adjacent eight C atoms form the catalytic active center.HOOH can't keep its stable chemical structure on the surface of RuN₄ doped divacancy graphene oxide.Thus,the ORR is a 4e^-transfer process.After calculation of transition state,the results indicated the kinetically most favorable pathway is the process of O₂ to form OOH and then the OOH to form O+H₂O.The rate determining step of this pathway is O hydrogenation into OH.The predicted working potential is 0.54 V by calculated the free energy change of ORR under different potential.