Preparation Of Nitrogen-doped Graphene Hydrogels And The Study On Their Electrochemical Performances

In this thesis,two kinds of nitrogen-doped graphene hydrogels were prepared using graphene oxide as the main precursors,and the electrochemical properties of these hydrogels were studied as unsupported electrode materials for supercapacitors.One is nitrogen-doped graphene hydrogel(H-NGH)by hydrothermal reduction with melamine as the nitrogen source;the other is nitrogen-doped graphene hydrogel(C-NGH)by chemical reduction with urea as the nitrogen source and ascorbic acid as the reducing agent.Based on electrochemical performance evaluation,we determine the addition amount of melamine is 50 mg and the temperature of hydrothermal reduction reaction is 180?when H-NGH is prepared.Similar to hydrothermal graphene hydrogel(H-GH)without nitrogen doping,the oxygen-containing functional groups of H-NGH were largely removed with a layer spacing of 0.346 nm and it has a three-dimensional cross-linked network structure with pore sizes mostly at 0.5 to 1?m.H-NGH has an I_D/I_G of 1.135 and a moisture content of up to 96.5%.The 100 g of the weight was pressed against two H-NGH cylinders having a diameter of 1.4 cm and a height of 1.5 cm without collapse and deformation.The nitrogen content of H-NGH is 8.61 at%,and the nitrogen elements are evenly distributed,mainly composed of pyridine and pyrrole nitrogen.At a current density of 0.5 A/g,the specific capacitance of H-NGH is 335.5 F/g,which is 81.3 F/g higher than that of H-GH.With the current density ranging from 0.5 A/g to 10 A/g,the capacitance retention rate of H-NGH is 76.8%,while that of H-GH is 73.8%.After a long cycle of20000 cycles at 10 A/g,the capacitance of H-NGH is not attenuated,and the coulomb efficiency is always around 100%.Nitrogen doping increases the pseudocapacitance of H-GH.According to the electrochemical properties,the addition amount of urea is determined to be 100mg and the time of chemical reduction reaction is 4h when C-NGH is prepared.Similar to the chemical reduction graphene hydrogels(C-GH)without nitrogen doping,C-NGH has a three-dimensional cross-linked porous structure with pore sizes about 2 to 4?m.After the reduction process,the internal oxygen-containing functional groups were largely removed with a layer spacing of 0.367 nm.The water content of C-NGH was 98.9%and the value of I_D/I_G was 1.17.C-NGH cylinder with a diameter of1.8 cm and a height of 2.0 cm does not deform under a 100 g weight.The nitrogen content of C-NGH is 10.50at%,and the nitrogen elements are evenly distributed,which can be divided into pyridine nitrogen,pyrrole nitrogen and graphite nitrogen.The specific surface area of C-NGH is 132 m~2/g,which is similar to the 122 m~2/g of C-GH,and the pore distribution of the two is basically the same.They are both graded mesoporous structures,with the majority of pores at 2-5 nm.At a current density of 0.5 A/g,the specific capacitance of C-NGH is 231.9 F/g,which is 58.4F/g higher than that of C-GH.With the current density ranging from 2 A/g to 50 A/g,the capacitance of C-NGH can still maintain62.6%.After the long cycle test of 20000 cycles at 10 A/g,the C-NGH capacitance retention rate is 97%and the coulomb efficiency is always 100%.Nitrogen doping increases the electric double layer capacitance of C-GH.