Preparation And Electrocatalytic Oxygen Reduction Research Of Transition Metal Single Atom Supported Carbon Materials

Oxygen reduction reaction（ORR）is an important reaction in clean energy conversion and storage devices,but its slow electron transfer kinetics greatly limits the energy conversion efficiency of these devices.Consequently,efficient ORR catalysts are the key to improve the efficiency of such energy devices.Commercial platinum/carbon（Pt/C）catalysts have been proven to be the most effective ORR catalysts.However,due to its high cost and low reserves,the new energy field in recent years mainly tends to develop economical,efficient,durable and reversible non-precious metal catalysts.At present,researchers have found that single atom catalysts（SACs）have ultra-high atomic utilization and good stability,and are expected to become an alternative to commercial Pt/C catalysts.Therefore,a variety of transition metal single atom supported carbon materials（M-N/C）were synthesized in this study,and the ORR catalytic activity and mechanism were investigated,which will provide experimental reference and theoretical support for the design and development of low-cost,high-performance ORR catalysts in the future.The specific research content of this paper is as follows:（1）we propose a simple pyrolysis method to synthesize a series of M-N/C with inorganic metal salts,1,10-phenanthroline,hydroxylamine hydrochloride and cherry kernel powder as raw materials,and the characterization and electrocatalytic ORR performance tests were carried out.Aberration corrected scanning transmission electron microscopy（AC-STEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and other characterization methods show that such catalysts have similar physicochemical properties and clear M-N_xcoordination configurations.The electrochemical performance tests suggest that the ORR catalytic activity of different catalysts follows the trend Fe_0.05-N/C>Co_0.05-N/C≈Pt/C>Ni_0.05-N/C>N/C.Particularly,the prepared Fe_0.05-N/C and Co_0.05-N/C with the half-wave potential(E_1/2)of 0.88 V and 0.85 V,respectively.The stability and methanol resistance tests indicate that Fe_0.05-N/C and Co_0.05-N/C have good cycle stability and resistance to methanol toxicity.This study could deepen our understanding of designing efficient ORR electrocatalysts at the atomic level.（2）By a similar synthesis process,a series of M-N/C were prepared with polyamide-amine dendrimer（PAMAM）as nitrogen source,and used for electrocatalysis ORR.These catalysts possess large surface area and abundant pore structure,and there are atomically dispersed M-N_x in the carbon matrix.The electrochemical test results show that the ORR activity sequence of each catalyst is Fe_0.05-N/C>Pt/C>Co_0.05-N/C>Ni_0.05-N/C>N/C.Especially,the Fe_0.05-N/C catalyst with an onset potential(E_onset)of 0.98 V,a E_1/2 of 0.86 V and a limited current density（J_L）of 5.17 m A·cm^-2.In addition,single-atom models of Fe,Co and Ni based on pyridine nitrogen and pyrrole nitrogen were constructed with graphene configuration as support by density functional theory（DFT）,and the catalytic activities and mechanisms in ORR were investigated.The results manifest that Fe-N₄/C has the strongest catalytic activity,follows by Co-N₄/C,and Ni-N₄/C is the worst,which satisfactorily agrees with the experimental results.This study provides a new idea for the rational design of novel ORR catalysts for electrocatalysis.