Theoretical Investigation On The Oxygen Reduction Reaction Mechanism Of Non-Noble Metal Doped Graphene

In recent years,the energy shortage and environmental pollution become more and more serious.Searching for environmental friendly and high efficient energy sources has become an urgent task.As a kind of environmental friendly energy storage equipment,proton exchange membrane fuel cell(PEMFC)has high energy density and energy conversion efficiency,which has attracted extensive attention in the scientific community.However,compared with the fast reaction rate in the anode,the slow ORR reaction rate in the cathode hinders seriously the high efficiency of fuel cells.By far,platinum and platinum based alloys are the best catalysts for fuel cells.However,the high price and poor durability of platinum catalyst seriously hinder the large-scale commercial applications.Therefore,developing new catalysts with low cost and high energy storage efficiency has become a huge challenge.As a cheap carbon material,graphene has attracted extensive attention in the field of electrocatalysis due to its advantages such as large surface area,excellent mechanical stability,high energy density and long life.However,graphene itself has no catalytic activity.To create the active sites,we need to dope heteroatoms into graphene,thus the catalytic active sites can be created by adjusting the electronic structure and breaking the charge neutrality.In this study,my work is mainly focused on the graphene doped by the non-noble metals.The doped graphene maintains similar configuration to perfect graphene and has large surface area,high carrier mobility,excellent mechanical properties and excellent chemical stability.At the same time,the catalytic properties are very different due to the change of composition.In this work,the main results are as follows:(1)Experimental studies have shown that for Co and N co-doped graphene,the reaction is generally a 2×2e-process.Namely,the intermediate HOOH is produced at one active site,but its decomposition is at another active site,that is,the dual-site 2x2e"process.However,the specific reactive sites and reaction mechanism are not available from the experimental studies.Therefore,we studied Co and N co-doped graphene theoretically.The structural study shows that the formation energy of CoN4 doped graphene(CoN4-Gra)is lower than that of CoN2-Gra,indicating that the former is more stable.For CoN4-Gra,the optimal reaction pathway is the formation of HOOH,and HOOH is the final product.For CoN2-Gra,the optimal reaction pathway is the formation and decomposition of HOOH.Since both CoN4-Gra and CoN2-Gra structures were observed experimentally and were found in the same material,this means that CoN2-Gra could be used as the active site for HOOH decomposition,which explained the experimental observation.Our study provides the first direct theoretical evidence for a two-site 2 x2e-ORR process.In addition,we also discussed the free energy of the intermediates at different electrode potentials.The experimental study also shows that besides CoN2-Gra and CoN4-Gra,CoN3-Gra may also exist.Therefore,we have also studied the ORR mechanism for CoN3-Gra.The results indicated that the process of OOH hydrogenated to O+H2O is the most favorable pathway.The energy barrier in the rate-determining step is 0.38 eV,much lower than?0.80 eV for pure Pt.In addition to the low reaction barrier,the low tafel slope also indicates that CoN3-Gra is a very promising ORR catalyst.(2)It has been reported in the previous study that single atom Sn-doped graphene is a good candidate for ORR.In order to demonstrate the fact,in this work,Sn-doped divacancy graphene(Sn-Gra)was designed.First,the optimal adsorption site was confirmed by calculating the adsorption energy of different adsorption intermediates.Second,the reaction barrier and free energy change of each step are discussed in order to determine the most favorable reaction pathway.The results showed that Sn-Gra is a promising catalyst both thermodynamically and dynamically.The most favorable reaction pathway is a four-electron reaction process.The energy barrier of the optimal reaction pathway(O2-OOH-H2O)is 0.75 eV,slightly lower than 0.88 eV on P-doped single vacancy graphene,0.85 eV on P-doped divacancy graphene,0.80 eV on Pt.(3)The most stable La and N doped graphene(LaNx-Gra,x=1-4)was determined by comparing the formation energy for different numbers of N atoms.Our results indicated that LaN4-Gra is the most stable.For this structure,we explored all possible elementary reactions,and analyzed the mechanism of ORR by comparing different reaction pathways.The most stable adsorption sites of the intermediates were determined and their ORR mechanism on was studied.The results showed that the oxygen molecule hydrogenation is the the most favorable ORR pathway.The formation of the second H2O molecule in this pathway is the rate-determining step,with an energy barrier of 0.89 eV.ORR mechanism on LaNa-Gra is a four-electron process.Therefore,LaN4-Gra is a better ORR catalyst compared with pure Pt with an energy barrier of?0.8 eV.