| Supercapacitor is a kind of energy storage device that has many advantages,such as high power density,long cycle life,easy maintainence,broad working temperature and high safety compared with batteries.Therefore,it can be used in the fields where batteries are not suitable.The advantages of high conductivity,good stability,low price and easy access make carbon a main electrode material for supercapacitor.Although carbon based supercapacitor is commercially available,the electrochemical performances are not satisfied and need to be further improved.In this thesis,carbon materials with hierarchical porous structures were prepared.The effects of introduction of heteroatom N and graphene on the structure and capacitor performance were investigated:1.Carbon xerogels with 3D network skeleton are prepared from resorcinol and formaldehyde.Graphene quantum dots and N are used to improve the capacitor performance.(1)The skeleton of carbon xerogel is strengthened by decorating with graphene quantum dots,which results in maintaining more pores and surface area and enhancing capacitance.Meanwhile,the addition of graphene quantum dots improves the conductivity of carbon xerogel,which improves the performance in large charge/discharge current.(2)Melamine is sued as N source to prepare N-rich carbon xerogel.With the increase of melamine,the inside channels become larger which leads to faster ion transport and smaller Rca.Meanwhile,the increasing content of pyridinic N and pyrrolic N in carbon xerogel results in more pseudocapacity induced in RM-Xs.However,more N in carbon lattice introduces more defects and larger degree of disorder,which leads to lower graphitization degree and the degradation of rate performance of RM-Xs.The sample RM-6-4 demonstrates the best electrochemical performance with the largest capacitance of 139 F g-1 at a current density of 0.5 A g-1.2.F-127 is used as soft template to prepare spherical resin starting with resorcinol and formaldehyde.Graphite oxide is wrapped on the spherical resin by electrostatic adsorption and thus constructes 3D framework.Heattreating at high temperature,spherical resin is carbonized and graphite oxide is reduced to be graphene.The wrinkled graphene sheets introduce a large amount of mesopores and macropores,which makes the specific surface area of the samples increased from 585.8 m2 g-1 to 914.7 m2 g-1 and thusimproves the specific capacitance from 78 F g-1to 122 F g-1.The addition of graphene also increases the conductivity and improvs the rate performance.Graphene/porous carbon spheres shows 87.5%of the capacitance at current density of 10 A g-1 compared to that at 0.5 A g-1(65.8%for bare carbon spheres),and the retention at 100 A g-1 is 65%.3.Camellia pollen with a protein content of 29%is used as carbon source to prepare N-rich hierarchical carbon materials by facile hydrothermal process with an"etching regent" of NH4BF4.The specific surface area(SSA)of the sample increases from 401 m2g-1(CH-0)to 526 m2g-1(CH-0.015)at first and then decreases to 431 m2 g-1(CH-0.02)as the amount of NH4BF4 used during the hydrothermal process increases.Meanwhile,the conductivity of the CH-Xs increases with the increase of the amount of NH4BF4.It was found that the SSA plays a very important role on the electrical double-layer capacitance while the pseudocapacitance is dominated by the content of N(especially the pyrollic N).The sample CH-0.015 shows the highest specific surface area of 526 m2 g-1 and the largest specific capacitance of 205 F g-1 at a current density of 0.5 A g-1.Camellia pollen is completely reacted at a hydrothermal temperature higher than 160 ?.The sample hydrothermal treated at 180 ? shows the largest capacitance.The structure of the sample is damaged when the treat temperature is higher than 180 ?,which deteriorates the capacitive performance. |
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