DFT Study Of Oxygen Reduction Reaction On Transition Metal Supported By Graphene-like Materials

Proton exchange membrane fuel cells?PEMFCs?provide many outstanding properties,such as high energy efficiency and environmentally friendly,thereby exhibiting promising prospective in conversion device.However,its commercialization process still encounters many problems.On one hand,the kinetics of oxygen reduction reaction?ORR?occurring at cathode is sluggish,which requires a highly Pt loading mainly accounted for PEMFCs high cost;On the other hand,the insufficient stability of Pt/C,especially caused by electrochemical oxidation of carbon support,seriously shorten the life time and reduce the stability of the catalyst.Meanwhile,Pt is prone to dissolution and agglomeration,which degrades the performance of PEMFCs.Therefore,reducing the load of precious metal and improving the stability of substrate is important to satisfy fuel cells demand.Recently,single-atom catalysts and sub-nanocluster metal catalysts have attracted special attention.These catalysts with highly metal utilization exhibit extraordinary activity towards many reactions,including ORR.However,single-atom and sub-nanocluster have a high surface energy.Therefore,suitable substrates are important to stabilize the single-atom and sub-nanocluster catalyst.Defective two-dimensional boron nitride and graphitic carbon nitride materials provided highly stability.The cavities in these structure formed by N atoms are ideal anchored sites for single-atom and sub-nanocluster.However,there are few reports on the ORR systematic study of single-atom and sub-nanocluster supported by defective two-dimensional boron nitride materials as well as graphitic carbon nitride material and many foundamental issues still need to be further studied,such as the origin of catalytic activity and mechanism of precise cobalt-nitrogen?Co-N₃?active centre,the origin of catalytic activity for single transition metal atom supported by boron defective two-dimensional boron nitride material and the corresponding tendency,the influence of transition metal single atom supported by the boron defective two-dimensional boron nitride material on ORR activity and foundamental origin,the reason for weakening of adsorption scaling relations in Pd_xCu_4-x subnanometer supported by defective two-dimensional boron nitride materials,the relationship between the ORR activity and the single 3d transition metal atom supported on graphitic carbon nitride substrate as well as the effect of loading density.In this work,the density functional theory?DFT?was used to investigate the stability and properties of catalysts,adsorption behavior and reaction mechanism to clarify the underlying origin of the above problems.These results provide theoretical basis for the design of novel electrocatalysts with higher activity.?1?The experimentally available two-dimensional boron nitride material with boron vacancies was adopted as the support to anchore single Co atom?Co/BN?,which has similar active center structure with CoN₃ embedded graphene,leading to the formation of precise Co-N₃ active centre.The DFT calculation was used to explore the ORR mechanism and activity origin of Co/BN.The results both in thermodynamics and dynamics indicated that Co atom can be adsorbed stably on the boron vacany,owning to the strongle interaction between the 3d states of Co atom and 2p states of N atoms in the boron vacany.This interaction promotes electron transfer from Co atom to the support and enhances the stability of Co/BN.Because of the electron transfer,the Co atom becomes the positive charge center,thus it can adsorb and activate ORR intermediate effectively.Especially,H₂O₂ can not maintain stably configuration,decomposing into two OH species,indicating that only 4e^-pathway occur on Co/BN.Through mechanism analysis,ORR prefers associative mechanism on Co/BN and the rate-limiting step with an activation barrier of 0.30 eV.Compared with boron defective 2D–BN and pristine 2D–BN,Co/BN exhibits higher ORR activity.The reason for this is that introducing single Co atoms enhances the interaction between OH species and active centers and facilitates ORR process.?2?The results of work?1?indicated that Co/BN is an efficient ORR catalyst.Therefore,we designed a series of single transition metal atom supported on the defective 2D–BN substrate with a boron vacancy?M/BN?to explore the effect of single transition metal atom in M/BN on ORR activity and foundamental origin.Calculation results showed that a strong interaction between the studied single transition metal atom and supporting defective 2D–BN is formed,indicating M/BN is stable in thermodynamics.The adsorption free energy(?G_adsshowed that the adsorption free energy of OOH,O and OH species display scaling relations with each other.This means only one independent variable can be used to describe the adsorption strength of the other two species.We adopted?G_*OH as the single descriptor to construct activity volcano curve.This curve indicated that only with moderate binding strength between OH species and catalyst surface,the catalyst exhibits outstanding ORR activity.Combined with the results of the energy difference between HOMO and LUMO,it is found that a volcano relationship can be still obtained between HOMO-LUMO and ORR limiting potential.It can be inferred that a high activity occurs on those M/BN catalyst,which has a moderate HOMO-LUMO gap.Finally,through analyzing the stability of H₂O₂ on M/BN surface,it is found that only 4e^-take place on the M/BN with only d valence electrons of the single metal atom.?3?According to the work?2?,the adsorption free energy of oxygenated intermediates displays scaling relations with each other.It is convenient to construct volcano curve.However,the volcano curve also limits the maximum catalytic activity.Compared with the results of M/BN in work?2?,the linear relationship among?G_adsds of different oxygenated adsorbates decrease significantly on catalyst of Pd_xCu_4-x subnanometer supported by the B defective 2D–BN.There are two main aspects contribute to this phenomenon:?1?As compared to single-atom catalysts,the active sites of Pd_xCu_4-x subnanometer are more diversity,thus different adsorbed species prefers different active site;?2?There is a difference among Pd_xCu_4-x subnanometer in fluxional behavior.Therefore,Pd_xCu_4-x subnanometer will facilitate the adsorption of oxygen species by adjusting its configuration.Compared with Cu/BN,the studied Pd_x Cu_4-x/BN catalysts have higher selectivity because of only 4e^-pathway taking place.Compared with Pd/BN,Pd₃Cu₁/BN exhibits higher ORR activity,owning to the overpotential of Pd₃Cu₁/BN?0.77 V?is lower than that of Pd/BN?0.92 V?.Therefore,ORR selectivity and activity can be improved by reasonable design of subnanometer catalyst.?4?Graphitic carbon nitride has a unique structure with regular distribution of cavity,which is an ideal anchored site for single transition metal atom.Meanwhile,the interaction between the single metal atom and the N atoms around the cavity leads to formation of M-N_x structure.We performed a systematic DFT investigations on the ORR activity of single 3d transition metal atoms supported on graphitic carbon nitride substrate?M/C₃N₄?.The results show that the binding energy of all studied M/C₃N₄ is negative with formation of different M-N_x structure.By analysis of the adsorption free energy,it is found that linear relationships are still existing among the adsorption free energy of OOH,O and OH species on various M/C3N4 surface.Moreover,Ni/C3N4locates the top region of volcano curve with limiting potential of 0.49 V,comparable to that of Pt?111??0.40 V?.The volcano curve manifested that only with moderate binding strength between OH species M/C3N4 surface,the catalyst exhibits excellent ORR activity.Finally,we took Ni atom as an example to explore the effects of different loading density on ORR activity.The results showed that the polarization charge of the active center decreases with the increase of Ni loading density,which can not active oxygenated intermediates sufficiently.Therefore,the ORR activity can be regulated by the loading density of metal atom on graphitic carbon nitride.