Electrocatalytic ORR Activity Of Transition Metal Modified Graphene Edges

Fuel cells have the characteristics of high performance and low cost for energy conversion and storage.Oxygen reduction reaction(ORR)is an important chemical reaction in energy conversion.The oxygen reduction reaction at the cathode is slower than that at the anode,which limits the efficiency of the battery.Therefore,it is very important to find efficient cathode catalysts.Carbon-based ORR catalysts with high cost performance have been recognized as one of the promising materials to replace precious metals.The porous structure of the graphene layer can provide an ideal support for the metal activity center,and its large surface area can ensure contact and interaction between metal atoms and reactant molecules.The structure of metal embedded graphene catalyst has attracted people’s attention.In this paper,a series of single-atom and dual-atom catalyst models of transition metal atom modified graphene edges were established,and their catalytic activity to ORR was investigated by density functional theory(DFT)calculation method.The single-atom catalysts in this system were composed of six metal monatoms(Pt,Pd,Ag,Au,Cu and Ru)at the two edges(zigzag and armchair)of graphene nanoribbons(M@GNRs).The adsorption structures of various oxygen-containing intermediates were calculated by density functional theory,the ORR paths and their overpotentials were predicted,and the catalytic activity of ORR on Pt(111)was compared.Our calculations showed that the adsorption energy of OH group can be used as a main index to reflect the performance of catalytic materials.When the OH group adsorption energy is greater than 3.5 e V,it is difficult for OH group to desorb to form water molecules,and the ORR performance is general.When the OH group adsorption energy is less than 3.0 e V,the ORR activity is better.Among them,2Pd@GNR has the best ORR activity,and the overpotential is only 0.388 V,far less than0.597 V of Pt(111).For OH pre-adsorption of M(OH)@GNRs,the adsorption energy of most materials is too small to be used as ORR catalyst.Only 2Cu(OH)@GNR has a good electrocatalytic performance with an overpotential of 0.528 V.The dual-atom catalysts in this system were three kinds of metal dimers(Pt,Pd and Ag)embedded at the edge of graphene(MM@GNRs).We found that the adsorption of OH group on MM@GNRs is generally large by calculating,which is not conducive to the formation of water in the next step.Only the adsorption energy of 2Ag Ag@GNR is less than 3.5 e V,and its over-potential is 0.856 V.For the systems with OH group pre-adsorption(MM(OH)@GNRs),their adsorption energy for oxygen-containing intermediates decreased significantly compared with MM@GNRs,and the overpotential changed significantly.The overpotential of 2Pd Pd(OH)@GNR and 2Ag Ag(OH)@GNR are 0.369 V and 0.381 V respectively,which are far less than 0.597 V of Pt(111).They have good catalytic performance.OH group adsorption energy can be used as a good indicator to preliminarily judge the ORR activity of MM@GNRs and MM(OH)@GNRs.