Configuration Regulation And Capacitance Properties Of Graphene Doped Nitrogen

Graphene is one of the most popular electrode materials for supercapacitors.Nitrogen doping can improve the electrochemical properties of graphene.Different doped nitrogen configurations have different effects on the performance of supercapacitors.At present,there is still no unified scientific conclusion on the mechanism of doping nitrogen to improve the capacitance performance of graphene.It`s still a huge challenge to effectively control the structure configurations and doping amount of doped nitrogen atoms in graphene,and even accurately control the position of nitrogen atoms in the lattice plane of graphene.In this paper,graphene was synthesized by self-propagating combustion reaction between Mg and CO.Solid nitrogenous precursors were pyrolyzed simultaneously by large amount of heat from combustion reaction.The carbon atoms produced by the reaction and nitrogen-containing small molecules generated after pyrolysis of nitrogen precursors were deposited on Mg O template to prepare nitrogen doped graphene with specific configuration.And the effects of different nitrogen precursors and process conditions on the physical structure and properties of nitrogen doped graphene were studied.Finally,single-configuration doped nitrogen was used in supercapacitor and discussed the relationship between nitrogen configuration and electrochemical performance.The effects of different nitrogen precursors on graphene doped nitrogen were investigated firstly.Nitrogen doped graphene was prepared by using nitrogen-containing five membered and six membered heterocyclic organics as nitrogen precursors,among which single-configuration pyrrolic and single-configuration pyridinic nitrogen doped graphene can be synthesized by using melamine and poly(4-vinylpyridine)as nitrogen precursors respectively.Moreover,the content of doped nitrogen in graphene can be easily adjusted by adjusting the amount of nitrogen precursor.The content of doping nitrogen of pyrrolic configuration can be adjusted from 3.5 at%to 9.2 at%and pyridinic configuration can be adjusted from 2.5 at%to 6.5 at%.Effective regulation of pyrrolic and pyridinic doped nitrogen in graphene can be realized by using mixed nitrogen precursors.And pyridinic nitrogen has priority over pyrrolic configuration when co-doped in graphene.Different process conditions perform important effects on the preparation of graphene.The increase of Mg O/Mg ratio will absorb the reaction heat and reduce the system temperature.Thus,the nitrogen source cannot be fully pyrolyzed and amorphous carbon was produced,which reduces the specific surface area of graphene and the amount of doped nitrogen.Nitrogen doped graphene was prepared with the same proportion of raw materials under atmospheric pressure and 1MPa high pressure.The samples prepared under high pressure possess multilayer carbon cage structure,low specific surface area,and contains both pyrrole and pyridine doped nitrogen.While under atmospheric pressure,thin layer of graphene with single-configuration doped nitrogen was synthesized,which means atmospheric pressure is more suitable for the preparation of single configuration nitrogen doped graphene.In order to clarify the formation mechanism of single-configuration doped nitrogen,the pyrolysis process of different nitrogen sources at different temperatures was analyzed by pyrolysis mass spectrometry.The mechanism of nitrogen doped graphene with single configuration formed by melamine and poly(4-vinylpyridine)was revealed:Melamine and poly(4-vinylpyridine)were rapidly pyrolyzed under the high amount of heat generated by self-propagating combustion reaction.The pyrolysis products only contain nitrogen-containing fragments with molecular weights of 43 and 78,respectively,which were co-deposited with free C atoms produced by the reaction to form graphene.It is the homogeneous fragments that ensure the singleness of doped nitrogen configuration.The single-configuration nitrogen doped graphene were used as supercapacitors electrode in EMI-TFSI for capacitance performance test.Both pyrrolic and pyridinic nitrogen doped graphene show excellent rate performance.At the scanning speed of 500m V/s,pyrrolic nitrogen doped graphene maintains a good rectangular shape.When the scanning speed increases to 1000 m V/s,the curve of pyrrolic nitrogen doped graphene began to deform,while pyridinic nitrogen doped graphene still maintained a good shape,indicating that pyridinic nitrogen is more conducive to the improvement of graphene rate performance.At a current density of 1 A/g,pyrrolic nitrogen doped graphene with 4.6 at%nitrogen content and pyridinic nitrogen doped graphene with 5.2 at%nitrogen content showed area specific capacity of 33.54μF/cm~2and 43.07μF/cm~2respectively,which is increased by 67.7%and 115.35%compared with monolayer graphene.In order to eliminate the interference caused by different specific surface areas,the nitrogen content per unit area was used as a parameter to compare the effects of two nitrogen doped configurations on improving the specific capacity of graphene.Pyrrolic doped nitrogen has a higher area specific capacitance at a lower nitrogen content per unit area,indicating that pyrrolic doped nitrogen is more conducive to the improvement of the specific capacity of graphene than pyridinic doped nitrogen.