Chemoselective Synthesis Of Nitrogen-doped Graphene For Electrochemical Energy Storage And Electrocatalysis

Graphene,as a kind of special two-dimensional(2D)material,has attracted great interests for applications in energy conversion and storage,due to its excellent properties such as large specific surface area(2600 m2 g-1),high conductivity,superior mechanical behavior and excellent chemical stability.However,random aggregation and overlaying of graphene sheets often cause the loss of accessible surface and result in diminished performance in practical applications.Among most of the strategies to rmodify graphene,nitrogen doping is an effective way to tailor the properties of graphene.But the current preparation condition of nitrogen-doped graphene is so harsh that the preparation cost is high and can hardly control the type and content of nitrogen bonding.At the same time,constructing a three-dimensional(3D)structure is also a way to avoid restacking.Currently,the process of fabricating 3D structure is cumbersome due to it always needs template.And the 3D graphene tends to have a lower conductivity.Herein,we want to develop a chemoselective synthesis method to prepare the nitrogen-doped graphene at a mild condition.Then we could regulate the type and content of nitrogen atom,obtain the high-performance nitrogen-doped graphene and characterize its electrochemical performance.The main research contents are as follows:1.Inspired by the specific convenient ring formation reaction between the aminoguanidine and ?,?-unsaturated ketone,we used aminoguanidine as reducing agent to obtain pyrazolic-structure-rich nitrogen-doped graphene(PNG)at a mild condition(60?).The nitrogen content was as high as 9.6 wt%.Moreover,PNG exhibits excellent capacitive and electrocatalytic activities.The PNG supercapacitor with acid electrolytes can provide a specific capacitance of 226 F g-1,an energy density of 30.3 Wh kg-1 and superb cycle stability.Under neutral electrolytes,PNG supercapacitors can be operated at higher voltages up to 1.4 V,providing an ultra-high energy density of 35.8 Wh kg-1.As an electrocatalyst for oxygen evolution reaction(OER)in alkaline solution,PNG needs only 394 mV overpotential to drive a current density of 20 mA cm-2 and remains stable within 16 h.The OER catalytic activity and stability of PNG are superior to that of noble metal-based OER catalyst IrO2.These outstanding properties make PNG a bifunction active material for supercapacitors and electrocatalysis.2.Combining of nitrogen-doping and constructing 3D structure,we report a three-step strategy to prepare flexible 3D nitrogen-doped graphene monoliths.1)a facile solution synthesis of pyrazolic-structure-rich nitrogen-doped graphene(NG)with high specific capacitance;2)self-assembly of NG to form nitrogen graphen monoliths(NGM)with cellular structure by filtration and freeze-drying;and 3)microwave treatment of NGM to give microwaved NGM(M-NGM)with unique nanosac-in-sheet structure.With the hierarchical porous structure,high-content nitrogen(6.6 wt%)and outstanding conductivity(15 S cm-1),M-NGM can be readily assembled to form an all-in-one configured supercapacitor without using current collectors.This flexible supercapacitor provides superior specific capacitance of 364 F g-1 and areal capacitance of 894 mF cm-2,an energy density of 12.2 Wh kg-1,and great cycle stability(?100%capacitance retention after 10000 cycles)and mechanical robustness(-97%capacitance retention after 500 bend cycles),whose performances are among the best of carbon materials-based flexible supercapacitors.