Graphene Encapsulated Fe₃N Electrocatalyst For Water Oxidation And Catalytic Mechanism Study

Oxygen evolution reaction（OER）is an important electrode reaction involving various new technologies for energy conversion and storage.It has become a research hotspot in the field of energy research.OER is a kinetically slow process with multi-electron transfer,so it is often to be the rate-determining step in reaction,which hinders the further development.Therefore,developing high-efficiency,low-cost,and commercially favourable OER electrocatalytic materials has been the focus and difficulty.In decades,transition metal and graphene composite materials,as low-cost materials,showed great OER catalytic activity and stability,and became the candidate for future large-scale applications.Thus,this article designed a facile method to synthesize nitrogen-doped graphene encapsulated iron nanoparticles（Fe₃N@NG）as a model material,and then studied its（photo）electrocatalytic performance and mechanism by comparison experiments and characterization.This article provides a comprehensive explanation of the active sites,reaction kinetics,and changes in electron transfer during the OER.The specific research contents are showed as below:（1）The materials of Fe₃N@NG with good encapsulated structure and uniform dispersion were prepared by a facile method.OER electrochemical measurement showed that it had a lower onset potential of 290 m V and an overpotential of 380 m V at 10 m A·cm^-2.And the current density of 25 m A·cm^-2 was obtained at 1.7 V vs.RHE.By comparing the performance of Fe₃N@NG and Fe₃N/NG materials with similar specific surface area and the same phase,it is found that Fe₃N@NG with encapsulated structure had higher OER catalytic activity and stability.In the measurement of photo-electrochemical water splitting,Fe₃N@NG,serving as a co-catalyst of Fe₂O₃photoanode,also had a good performance.It enhanced the electron-hole separation of the photoanode,making it shows the highest IPCE value of 23.3%at 350 nm.The load of Fe₃N@NG increased the photocurrent density at 1.23 V vs.RHE by 1.8 times,reaching 2.40 m A·cm^-2 and decreased the onset potential from 0.89 V to 0.84 V.（2）Analyzing the XPS and in situ M（?）ssbauer spectrum results,it was found that part of Fe in Fe₃N@NG was oxidized to Fe⁴⁺during the OER process.Combining theoretical calculation results,we verified the kinetically superiority of Fe⁴⁺@NG and pointed out the active sites of OER catalysis—pyridinic nitrogen interacting with iron.Further analyzing the experimental results and theoretical calculation results,we elaborated the OER catalytic mechanism of graphene-encapsulated Fe₃N@NG.Under the applied voltage,internal Fe³⁺of Fe₃N@NG was partially oxidized to Fe⁴⁺by electron transfer through the graphene nanoshells.The highly electronegative Fe⁴⁺which is interacted with pyridinic nitrogen doped in graphene,will promote the electron transfer from oxygen intermediates to graphene,thereby making the desorption of oxygen easier,and finally effectively promoting the OER reaction.Meanwhile,the graphene nanoshell also plays a role of protection for maintaining Fe₃N and Fe⁴⁺,which contributes to better stability.（3）Nitrogen-doped graphene encapsulated transition metals（Co,Ni,Mn）were prepared with the same method.Their noticeable OER performance proved that the facile method is convenient for various metal materials.