Theoretical Study On Graphene-supported Single-Atom Catalysts Towards Oxygen Reduction Reaction

At present,the development of sustainable and clean forms of energy is particularly urgent due to the ever-increasing demand for energy induced by the rapid growth of the world population,which results in the growingly serious energy crisis and environmental problems.In recent years,the emergence of various fuel cells has been extensively regarded as an alternative form of fossil energy,whereas the oxygen reduction reaction（ORR）occurred on the cathode of fuel cells tends to be very sluggish,which greatly jeopardize the comprehensive performance of the fuel cell and thus hinders their practical application to a great extent.So far,the precious metal platinum（Pt）-based catalyst is still considered as the benchmark of ORR catalyst and the solo catalyst for commercial application.Howerver,its high cost and scarcity preclude its large-scale application.In this regard,single-atom catalysts（SACs）are seen as an ideal and reliable potential replacement for Pt-based ORR catalysts due to their high atom utilization,high catalytic activity selectivity and stability.Moreover,a large number of experimental and theoretical studies have shown that graphene can act as a very superior support material of SACs,so the theoretical study and screening of graphene-supported SACs towards ORR is of great significanceOn the basis of that,in this paper,through the variation of the types of vacancy in graphene supports accompanied by the introduction of N doping as well as via changing the type of metal atoms to reasonably control the coordination environment of single metal atoms and structure of active centers in SACs,27 kinds of graphene-supported non-noble metal and precious metal SACs are theoretically designed,including 11 kinds of Fe catalysts,4 kinds of Co,Ni,Pt,Pd catalysts for each,followed by an in-depth study of their ORR performance aiming to clarify the catalytic reaction mechanism of ORR over these SACs and compare their activity at the same time.The main research results of this paper are as follows:1.Besides Pd/SVG-N₃ in which SVG denotes the single-vacancy graphene and N₃denotes the three N atoms doping,the stability of other catalysts is fairly good.As for single-vacancy graphenen-supported SACs,its corresponding stability is reduced with the introduction of N-atom doping.Considerable charge is transferred from the metal atoms to the graphene supports,which renders the metal atoms with different positively charged properties,and thus enhances the activity of the metal center and also indicates that the strong metal-support interaction to ensures the excellent stability of the catalyst.In addition,it is also found that the formation energy of all catalysts is very small,indicating that all catalysts are easy to synthesize and do not require a large amount of energy input.The doping of N atoms significantly reduces the formation of catalysts,which is conducive to the preparation of catalysts.2.Except for Ni/DVG-N₄,Pt/DVG-N₄ and Pd/DVG-N₄,all other catalysts can stably adsorb O₂,and their adsorption energies are relatively large.Moreover,theΔG in the O₂adsorption process are all negative,illuminating that O₂ is stable spontaneous adsorption.A large number of electrons are injected into O₂ from the catalysts,and meanwhile the bond length of O=O in the adsorption of O₂ was prolonged to different degrees,indicating the activation of O₂ by the catalyst.3.By calculating the adsorption of ORR key intermediates O and OH groups over various catalysts,it was found that all catalysts could adsorb the two intermediates stably.By comparing the differences in adsorption energies,the ORR activities of Fe/DVG-N₁,Co/DVG-N₄,Ni/DVG,Pt/DVG and Pd/DVG were predicted to be better.4.Through the adsorption of HOOH and H₂O on different catalysts,it was found that HOOH could not exist on the catalyst surface with a spontaneious decomposition process,while H₂O could exist stably and the adsorption energy is small,indicating that H₂O was easy to desorb from catalysts and form the final product,which proved that the catalyst had a good selectivity for ORR and all followed the four-electron path.The specific reaction mechanism of ORR was studied through theΔG of each step of elementary reaction of possible path.It was discovered the rate-determining step of ORR over various catalysts was the process by which adsorbed OH group combined a proton and an electron to form H₂O,and the overpotential was obtained to compare and reflect the activity of catalyst.It was found that the catalyst with the best performance among Fe catalysts was Fe/DVG-N₁ via HOOH dissociation mechanism with an overpotential of 0.62V.Among the Co and Ni catalysts,the Co/DVG-N₄ and Ni/DVG were projected to be better than others with the overpotentials of 0.56 V,0.50 V,respectively.In the midst of precious metal Pt and Pd catalysts,the better ORR performance was found over Pt/DVG and Pd/DVG,both of which is the OOH dissociation path,and the reaction overpotential is0.37 V,0.34 V,respectively.In general,the vacancy type of graphene substrater,the doping of N atoms and the types of metal atoms have a significant impact on the performance of ORR.By comparison,it is found that the performance of the double-vacancy graphene-supported catalyst is better than that with single vacancy,indicating that the catalyst with coordination of four atoms is beneficial to ORR.The ORR performance of noble metal-based SACs was found to be slightly better than non-noble metal one,but the value of overpotential was still lower than most Pt bulk,nanoparticles,nanoclusters and even Pt alloys.As a result,these nonprecious metal SACs still can be used as the potential alternative to Pt-based catalysts.Through a series of theoretical screening,this paper will not only promote the understanding of the ORR of graphene-supported SACs,but also serve as an example of the rational design of ORR catalysts,which will also offer reference for the development of other catalysts towards other meaningful reactions.