Structure Design And Mechanism Study Of High-Efficient Oxygen Reduction Catalysts Based On Two-Dimensional Materials

As a clean energy device,proton exchange membrane fuel cells(PEMFCs)has received significant attention due to the high energy conversion efficiency,low emission,and non-pollution properties.In PEMFCs,compared with the fast reaction in the anode,the slow oxygen reduction reaction(ORR)kinetics at the cathode prohibits its reaction efficiency.The key to break this bottleneck is to search highly efficient cathode electrocatalyst.Up to now,the most widely used electrocatalysts for ORR are Pt and Pt-based alloys.Nonetheless,the unaffordable high cost,limited resources,and poor stability hinder their large-scale commercial applications for PEMFCs.Thus,it is vital to develop the catalysts with high efficiency,low cost,and high stability for the large-scale commercial applications of PEMFCs.Recently,two-dimensional materials(such as graphene,phthalocyanine,etc.)have been widely studied in electrocatalysis,due to their large specific surface area,high energy density,and low cost.Studies have found that transition metal doped two-dimensional materials exhibited better ORR catalytic activity,and when multiple metals act as active sites,the catalytic activity will be significantly improved due to the synergistic effect.Thus,in this work,the reaction mechanisms and catalytic activity of oxygen reduction reaction on the transition metal(dimer)doped two-dimensional materials have been investigated systemically by means of the density functional theory methods combined with the microkinetic model and the computational hydrogen electrode model.In this work,the main results are as follows:(1)The reaction mechanism for ORR on FeN₃ doped divacancy graphene(FeN₃-Gra)has been investigated.The results show that FeN₃-Gra is thermodynamically stable.Bader charge analysis indicates that FeN₃ moiety and its adjacent eleven carbon atoms are the catalytic active sites.The geomerical optimization and Quantum Chemical Molecular dynamics reveals that HOOH cannot stably exist on the catalyst surface and is easily decomposed into*OH+*OH,implying that ORR is a four-electron process.Compared with O2 dissociation,O2 hydrogenation is much easier occur and the most favorable pathway is the*OOH hydrogenation to form*O+H2O.The formation of the second H2O molecule is the rate-determining step with an energy barrier of 0.48 eV.This barrier is much lower than 0.80 eV for Pt,also lower than 0.56 eV for FeN4 doped divacancy graphene.In addition,the calculated Tafel slope is also smaller than Pt in both low and high overpotential regions.The above results indicate that FeN₃-Gra is a potential ORR catalyst.(2)The reaction mechanism for ORR on RhNx doped divacancy graphene(RhNx-Gra)has been investigated.The geometries and stability of RhNx-Gra(x=1-4)have been examined.The calculation results show that RhNx-Gra(x=2-4)is thermodynamically stable,and different nitrogen numbers affect ORR catalytic activity and reaction mechanism.For RhNx-Gra(x=2-4),the ORR proceed a four-electron transfer process,and the rate-determining steps are the formation of*OH,the generation of*OH+*OH,and formation of the second H2O mocule,respectively,with the energy barrier of 1.08,0.54,and 0.24 eV,respectively.Thus,RhNx-Gra(x=3,4)have lower energy barrier compared with 0.80 eV for Pt,revealing that they exhibit high ORR activity.In addition,the free energy calculation shows that the working potentials for RhNx-Gra(x=3,4)are 0.33 and 0.34 V,respectively.Based on the above calculation results,it can be seen that RhNx-Gra(x=3,4)has good ORR catalytic activity,and the catalytic activity increases with the increase of the number of nitrogen atoms.Thus,ORR activity could be adjusted by changing the number of nitrogen atoms.(3)The processes for ORR in TM2Pc with a series of transition metal dimers(M=Mn-Cu and Ru-Pd)has been studied and the influences of different transition metal dimers on ORR catalytic activity are also discussed.The results indicate that there is volcano relationship between ORR activity and ?G*OH.Volcano plot suggests that Fe2Pc has the best ORR activity thermodynamically,with the overpotential of 0.25 V,which is much lower than 0.45 V for pure Pt.To further confirm good ORR catalytic activity of Fe2Pc,the kinetic properties of Fe2Pc is also explored.The energy barrier calculations show that for Fe2Pc,*O2->*OOH->*OH+*OH?*OH+H2O?H2O is the most favorable pathway,in which the rate-determining step is the*OOH hydrogenation to form*OH+*OH,with an energy barrier of 0.25 eV,much smaller than 0.80 eV for Pt.Therefore,Fe2Pc has good ORR activity compared with Pt and will be an efficient and cheap ORR electrocatalyst.(4)Experimental studies have found that FeCo and nitrogen codoped graphene exhibit excellent ORR catalytic activity.Herein,a series of homonuclear and heteronuclear binuclear transition metal and nitrogen codoped graphene(M2N6/FeMN6-Gra,M=Mn-Cu)as ORR catalysts have been investigated by means of density functional method.The calculated formation energies and molecular dynamics simulations indicate that these catalysts are stable thermodynamically.Scaling relationships,i.e.,?G*O vs ?G*OH,?G*O vs ?G*OOH and ?G*OH vs ?G*OOH,are obtained.Interestingly,there is a strong linear relationship for overpotential and the electronegativity difference(between Fe and another metal).Notably,volcano plots,i.e.,?G*O vs equilibrium potential,?G*OH vs overpotential and d-band center vs overpotential,are established.This means that ?G*O,?G*OH and d-band center are good descriptors to describe the catalytic activity.Volcano plots show that among theses studied compounds,FeMN6-Gra(M=Co,Fe and Ni)has high catalytic activity.Particularly,FeCoN6-Gra exhibits the best ORR catalytic activity,with the working potential is 0.97 V,better than 0.78 V for Pt.Furthermore,kinetic calculation reveals that energy barrier is 0.34 eV in the rate-determining step,much lower than 0.80 eV for Pt.Besides 3d transition metal dimers,reaction mechanisms for ORR on a series of 4d homonuclear and heteronuclear transition metals and nitrogen codoped graphene(M2N6/FeMN6-Gra,M=Ru-Ag)have been also explored.Among these studied catalysts,FeRhN6-Gra exhibits the best catalytic activity,with low overpotential of 0.26 V.The kinetic calculation indicates that the formation of*OH is the rate-determining step with a low energy barrier of 0.40 eV.More importantly,the ratio between electronegativity and atomic radius of transition metals is found to be a good descriptor for evaluating the catalytic activity.In a word,the above results suggest that binuclear transition metal and nitrogen codoped graphene are good ORR catalysts and further study toward this direction would open a new avenue for the design of highly efficient catalysts.