Theoretical Design And Mechanism Study Of Oxygen Reduction And Carbon Dioxide Electroreduction Catalysts Based On Two-dimensional Carbon Materials

With the increase of global population and competition for resources,energy crisis and environmental pollution have become increasingly serious.To solve these problems,it is an urgent to develop efficient and clean energy technologies.Proton exchange membrane fuel cells(PEMFCs)and carbon dioxide electrolytic cells,as two kinds of clean energy conversion devices,have received extensive attention because they are environmentally friendly,high energy conversion efficiency,low temperature performance and pollution-free.In these two energy conversion devices,oxygen reduction reaction(ORR)is very important for PEMFCs,while CO2 reduction reaction(CO2RR)is an effective strategy for reducing CO2 to valuable chemicals or fuels.Precious metal Pt-based catalysts and Cu-based catalysts are efficient ORR and CO2RR electrocatalysts,but they have high cost and poor stability.In addition,the reaction overpotential is high in these two electrochemical reaction processes.These problems limit the large-scale commercial application of PEMFCs and electrocatalytic CO2 reduction technology.Therefore,the development of inexpensive,efficient and stable ORR and CO2RR catalysts that reduce the overpotential of the reaction is critical to promoting the commercialization of PEMFCs and electrocatalytic CO2 reduction technologies.In recent years,two-dimensional materials such as graphene and metal-organic frames(MOFs)have been widely used in the field of electrocatalysis due to their low cost,high surface area and adjustable structure.Based on density functional theory and electrochemical thermodynamic model analysis,a series of ORR electrocatalysts modified with different axial ligands composed of non-noble metal and nitrogen co-doped graphene were systematically studied.The catalytic performance of transition metal-doped two-dimensional MOF materials as CO2RR electrocatalysts was investigated.The main research contents and results are as follows:(1)Diverse axial ligands have been used to modify the ORR activity of transition metal and nitrogen codoped graphene(MN4-Gra).The weakened adsorption of the ORR intermediates induced by the axial ligand would activate these intermediates efficiently and improve the catalytic activity.Our study indicated that among investigated the axial ligands,nitrobenzene and benzene exhibited the good performance in improving the ORR activity.Several promising ORR catalysts were predicted.Among them,FeN4-Gra modified by nitrobenzene,i.e.,FeN4-Gra/ben/NO2 possessed the best catalytic activity with the small overpotential of 0.21 V.In addition,these catalysts were demonstrated to be stable by the calculated formation energies and dissolution potentials.Scaling relationships,i.e.,ΔG*OH vs ΔG*O,ΔG*OH vs ΔG*OOH,were obtained.Volcano plots,i.e.,ΔG*OH vs equilibrium potential,ΔG*OH overpotential and d-band center vs overpotential,were obtained.The scaling relationships suggested that the adsorption energy of*OH and d-band center would be good descriptors for predicting the highly efficient ORR catalysts.Therefore,the axial ligand engineering would be a useful strategy to enhance the efficiency of ORR catalysts.(2)A series of transition metal 1,2,3,4,5,6,7,8,9,10,11,12-perthiolated coronene frameworks(TM-PTC,TM=Sc-Zn)were investigated as CO2RR electrocatalysts based on the density functional theory(DFT)calculations.The calculated results showed that the main products were CH4 for TM-PTC(TM=Sc-Mn).Axial oxygen modified TM-PTC,i.e.,TM-O-PTC has different catalytic mechanisms compared to TM-PTC.Cr-O-PTC is predicted to be the best CO2RR catalyst due to the small overpotential of 0.62 V in the gas phase and 0.55 V in solution.It is expected that this work could provide a new guidance for developing high-performance CO2RR electrocatalysts.