Preparation And Electrocatalytic Performance Of Non-metallic Doped Graphene Nanoribbons For Oxygen Reduction

Energy is an important factor for the development of national economy and the improvement of people's living standard,and it is also an important factor that directly affects economic development.Since the beginning of the 21st century,resource shortage,environmental pollution and greenhouse effect caused by traditional energy utilization have become increasingly prominent,and it is increasingly difficult for traditional energy to meet the needs of human survival and development.In order to solve the increasingly serious energy crisis and environmental pollution problems,people have developed a variety of new green energy,including metal air cell and fuel cell.In fuel cells,oxygen reduction reaction(ORR)is a key step to determine the reaction rate,and a catalyst is needed to accelerate the reaction process.The precious metal Pt is widely used as ORR catalyst,but its high cost limits the wide application of fuel cells.Therefore,the development of low-cost and high-performance substitutes,such as non-precious metal and metal-free electrocatalysts,has become a research hotspot of fuel cell catalysts.Graphene-based catalyst has become a new type of catalyst with great development prospect because of its advantages such as low density,high specific surface area,good electrical conductivity and chemical stability.Graphene nanoribbons(GNRs)not only retain the characteristics of graphene,such as light weight,large surface area and good electrical conductivity,but also have high aspect ratio and irregular edges.GNRs can be doped with heteroatom to further modulate the electron gap,which can improve its reactivity and electrocatalytic ability as ORR catalyst.In this paper,the design and regulation of N doping,structure and morphology of GNRs were carried out to prepare metal-free carbon nano-catalysts,and the influence of morphology,structure and elemental composition of the material on its ORR electrocatalytic performance was explored,thus realizing the controllable synthesis of efficient non-metallic ORR catalysts.The main research contents are as follows:(1)Graphene oxide nanoribbon(GONRs)was prepared by longitudinal decompression of multi-walled carbon nanotubes(MWCNTs).Then,N-doped graphene nanoribbon(N-GNRs-900)catalysts with porous structure were prepared by using g-C₃N₄ as N source combined with hydrothermal and high temperature annealing.SEM,TEM,XRD and XPS test results show that MWCNTs are successfully shear to GONRs rich in edge defects and N successful doping of GNRs.Electrochemical test results show that the ORR electrocatalytic activity of N_1.5-GNRs-900 is the highest.In the 0.1 M KOH electrolyte,the half-wave potential is 0.82 V and the initial potential is 0.93 V.The H₂O₂ yield test and K-L calculation prove that the oxygen reduction reaction of N_1.5-GNRs-900 is a four-electron transfer process.N_1.5-GNRs-900 also shows excellent performance in terms of stability and methanol tolerance.(2)To solve the problem that GNRs is easy to stick,GNRs was composite with three-dimensional porous graphene,and then the composite carbon material was N-doped to prepare an N-doped graphene nanoribbons/graphene composite(N-GNRs/G-900),which was a porous aerogel with three-dimensional network structure.In aerogel,3D graphene acts as a porous skeleton,while GNRs with irregular edges are built on the 3D skeleton,revealing abundant edge defects.The synergistic effect of 3D porous graphene and GNRs increases the amount of N doping and the content of Pyridinic N and Graphitic N in carbon materials.The initial potential and half-wave potential of N-GNRs₃/G-900 electrocatalyst in 0.1M KOH electrolyte reach 0.98 V and 0.856 V,which is similar to that of commercial Pt/C.The oxygen reduction reaction of N-GNRs₃/G-900 electrocatalyst is a four-electron transfer process.N-GNRs₃/G-900 also shows good performance in stability and methanol tolerance.The DFT calculation shows the effect of GNR_S rich in edge defects in the material,and the simulation results agree with the experimental results.This work provides an effective way to regulate surface charge redistribution through morphology,structure control and heteroatom doping.