DFT Study Of The Catalytic Performance Of Heteroatoms-doped Carbon-based Catalysts For Oxygen Reduction Reaction

The key factor of large-scale commercialization of Polymer Electrolyte Fuel Cells(PEFC)is reducing their production cost.At present,the main stack cost contributor(?40%)is the expensive Pt-based noble metal electrocatalysts.And the oxygen reduction reaction(ORR)at the cathode is inherently slower by several orders of magnitude than the hydrogen oxidation reaction(HOR)at the anode and contributes more to this cost.Owing to the scarcity of precious Pt,searching for earth-abundant alternatives is a key chanllenge to fuel-cell technology.In other words,the development of high-performance and durable Pt-free ORR catalysts plays an imoprotant role in large-scale commercialization of PEFC-powered vehicles.Herein,we focused on carbon-based metal-free ORR electrocatalysts to explore their ORR activity and stability by using density functional theory(DFT)calculation.To understand the nature of active sites and essential reasons of poor durability of doped carbon-based catalysts,it is necessary to explore the key factors of determining the catalytic activity for ORR,and the corrosion mechanism during ORR.And these results of theory calculation would provide some useful guidance to design doped carbon-based catalysts with high activity and stability.(1)To identify the most optimum P-doped graphene configuration for ORR,by using density functional theory(DFT)calculation,we explored the relationships among the catalytic activity,stability,and the local chemical bonding states of P-doped graphene for ORR.The doped structures show that P atom can substitute one or two C atoms to form P-doped graphene structures with three or four P-C bonds(PC3G or PC4G),respectively,and these structures are easily oxidized into the OPC3 G and OPC4 G models with P-O bond.The calculations reveal that the stability,band structure,surface charge distribution,potential active sites,and free energy of the rate-determining step of P-doped graphene can be modulated effectively by the chemical bonding states of P atom and the formation of C-P-O bond.The OPC3 G model is the most effective and stable P-doped graphene for ORR due to its stability,activity,and the amount of the potential active sites.(2)To enhance the intrinsic activity of and increase the number of active sites in heteroatom-doped graphene(doped-G),it is necessary to recognize the origin of its catalytic activity,and to search for a universal definition and description of the different active sites.We report an evaluation of a series of heteroatom-doped graphene materials as ORR catalysts with the aid of density functional theory(DFT)calculations.The results indicate that the intrinsic catalytic activity and the ORR mechanism depend on the triple effect,that is,the charge,the spin density and the coordinate state(ligand effect)of the carbon sites.The contribution of the above effects towards increasing the binding energies of *OOH or *OH has the following order: negative charge effect < positive charge effect< low spin effect < ligand effect < high spin effect.For nonmetallic single-heteroatomdoped-G,the triple effect separately activates the carbon sites around the doped atom.On each single carbon active site,only the end-on adsorption of *OOH is preferred,leading to the ORR following the associative mechanism,which has an intrinsic limitation in its overpotential of 0.44 V.However,for metal-doped-G and dual-heteroatom-doped-G,the triple effect can activate double carbon sites,and lead to the ORR following the dissociative mechanism,which breaks through the activity limitation of the associative mechanism,and further enhances the catalytic activity for the ORR.(3)To understand the essential reasons of poor durability and rapid initial performance loss of heteroatoms doped graphene catalyst during electrochemical ORR process,it is necessary to explore the detailed mechanism of carbon active sites oxidation(COR)at different electrode potentials,as it may greatly influence the ORR activity.Density functional theory(DFT)calculation is used to investigate all possible COR mechanism,including Direct-COR and Indirect-COR,on four typical doped-graphene,and understand the competition relation between COR and ORR from thermodynamic point of view.The systematic calculations found that,during ORR process,forming surface oxidation intermediates(partial oxidation)on doped-graphene is much easier than losing the carbon site(complete oxidation),and the Indirect-COR occurred on doped-graphene is relatively higher than the Direct-COR.The competition relation between COR and ORR is mainly influenced by the interaction between the doped-graphene and reaction species,stability of doped-structure,ORR mechanism,and electrode potential.For COR,partial oxidation of doped-graphene is the dominant oxidation reaction compared to complete oxidation in ORR potential range.More importantly,both partial and complete oxidation of doped-graphene can remarkably depress the ORR activity.Then COR should be one of major contributors to the rapid initial performance loss of carbon based catalysts in the stability testing.