Study On Supercapacitor Performance Of Phosphorus-Nitrogen Co-doped Graphene Composites

Supercapacitor has the advantages of high power density,fast charge and discharge rate,friendly environment and long service life.It is a hot topic in the field of energy conversion and storage.Electrode material is one of the key factors to determine the performance of supercapacitor.The exploration and development of electrode materials with high specific capacitance,high energy density and long cycle performance have become highly demanded to promote the development and application of supercapacitor technology.At present,the carbon-based supercapacitor electrodes are widely used.However,its further development is limited by the low specific capacity and low energy density,which cannot meet the demand of electronicl devices,especially the fast development of power equipment for the high capacity and high efficiency of power supply.Functionalized graphene composites are promising supercapacitor electrode materials,due to their unique advantages such as rich active sites,large specific surface area,high specific capacitance and high energy density.In this paper,phosphor and nitrogen co-doped graphene composite was prepared using suitable phosphorus and nitrogen sources with graphene oxide or holey graphite oxide.The electrochemical performance of the composite was studied.The main contents are as follows:(1)Three-dimensional structure composite hydrogel was prepared using o-phenylenediamine(OPD)as a nitrogen source and phosphoric acid(H3PO4)as a phosphorus source to perform hydrothermal reaction with graphene oxide(GO),with subsequent polymerization to ferric chloride hexahydrate(FeCl3·6H2O).High-temperature carbonization and activation were then proceeded to obtain hierarchical porous phosphorus and nitrogen co-doped graphene/carbon composites(P/N-GC).The as-prepared P/N-GC sample exhibited typical hierarchical porous structure with high specific surface area and a relatively high doping level of phosphorus and nitrogen.The investigation of its electrochemical properties was then carried out to evaluate its potential application in supercapacitors.In three-electrode systems,the P/N-GC electrode showed the maximum gravimetric specific capacitance and volumetric capacitance of up to 440 F g'1 and 387.2 F cm-3 in alkaline electrolyte at a current density of 0.5 A g-1,respectively.It showed a high capacitance retention rate of 62.5%with the current density increasing from 0.5 to 20 A g-1,and can also maintain 96.3%of the initial specific capacitance after 3000 cycles.Furthermore,the P/N-GC assembled symmetric supercapacitor presented ahighest energy density and power density of 25.5 Wh kg-1 and 14960 W kg-1 in 6 M KOH aqueous electrolyte,respectively.The supercapacitor also showed high energy density and power density of 62.5 Wh kg-1 and 29480 W kg-1 in 1 M LiPF6 organic electrolyte.(2)The nitrogen-phosphorus doped holey graphene/carbon composites(N-P-HGC)were prepared by mixing o-phenylenediamine and hydroxy ethylidene two phosphonic acid(HEDP)with holey graphite oxide(HGO)solution with subsequent hydrothermal process and high-temperature carbonization.N2 isothermal adsorption characterization demonstrated that the as-prepared materials possessed obvious mesopores and macropores.This hierarchical structure is beneficial to facilitate the effective contact between electrolyte and electrode and provide fast ion channels for electrolytes and shortens the diffusion path.The N-P-HGC-700 electrode exhibited specific capacitance of 334 F g-1 in 1 M H2SO4 electrolyte at a current density of 0.5 A g-1.It can maintained 53.3%(178.8 F g-1)of the initial specific capacitance at a high current density of 50 A g-1,indicating excellent capacitance performance.It also showed a good cycle stability with only 3.8%decay in specific capacitance after 5000 cycles.Remarkly,an all solid-state two-electrode symmetric supercapacitor has been fabricated from N-P-HGC-700 electrode using PVA/H2SO4 gel electrolyte.The two-electrode symmetric supercapacitor exhibited a specific capacitance of 290 F g-at a current density of 0.5 A g-1 and a power density of 349 W kg-1 at an energy density of 19.7 Wh kg-1.In addition,it showed high energy density of 29.8 Wh kg-1 at a power density of 349 W kg-1 in 1 M Na2SO4 neutral electrolyte as well as the superior cycle stability with 93%capacitance retention after 4000 cycles.