Supercapacitor Based On Three-dimensional Porous Nitrogen-doped Graphene

With the rapid development of science,technology and society,fossil energy is gradually exhausted.It is urgent to develop renewable energy and corresponding energy storage technologies.With the advantages of excellent energy storage performances,high safety and environmental suitability,supercapacitor plays an important role in energy storage.On the other hand,supercapacitor is required to possess functions of portability and flexibility due to people’s improved material and spiritual life.Graphene is one of the ideal electrode materials for supercapacitors because of its large specific surface area and excellent conductivity.At present,the key point is how to prepare highly qualified flexible graphene and simultaneously improve the energy storage performances.In this thesis,highly qualified free-standing and flexible graphene with three-dimensional foam structure was synthesized by chemical vapor deposition(CVD).And then it was combined with polyaniline to improve its energy storage performance.Following aspects were focused:CVD growth of three-dimensional porous graphene: In CVD process,carbon source,nitrogen doping and CVD growth time are decisive factors for determining the quality of graphene and its composition effect with polyaniline.Firstly,the carbon source,methane and ethylene,were compared.It was found that using ethylene as carbon source can obtain highly qualified graphene at lower growth temperature than methane.Secondly,nitrogen doping can effectively affect the charge distribution in graphene.It was indicated that,adding ammonia gas as nitrogen source,the produced graphene was nitrogen-doped can be composited with polyaniline better.Thirdly,CVD growth time is a key factor in determining the quality of the obtained graphene.Adjusting the growth time between 5 min to 30 min.It was found that the graphene/polyaniline composite electrode with growth time of 10 min has the best energy storage performance and can be free standing.The composition of three-dimensional porous graphene with polyaniline:Polyaniline is a conductive polymer with good pseudocapacitor energy storage performance.Compositing with graphene can effectively improve the energy storage performance of the produced composite electrode.However,too high polyaniline content will cause polyaniline to form agglomerates,and thus the energy storage performance will be seriously deducted.For graphene obtained by ethylene as carbon source,ammonia as nitrogen source and CVD growth for 10 min,polyaniline with different content(5%-80% mass of the nitrogen-doped graphene)was deposited in situ by electrochemical deposition.It was founded that,when the mass of polyaniline was50% of nitrogen-doped graphene,polyaniline was evenly composited on graphene,and the resulted composite electrode exhibited the best energy storage performance,and the specific capacitance reached 283 F/g at current density of 1 A/g.Assembly of flexible supercapacitors: Using gel electrolyte,the above polyaniline/graphene composite electrode was assembled into symmetric supercapacitor.The energy storage performance and flexibility of the supercapacitor were systematically studied.The specific capacitance of the supercapacitor electrode reached 276.6 F/g at a current density of 1 A/g.And the specific capacitance can maintain 82.6% after 10 000 cycles.In addition,the resulting supercapacitor exhibited good flexibility.Being bent for 180°,the specific capacitance can still maintain89.66%.After bending 1000 times,71.3% of the specific capacitance can be well maintained.