| Supercapacitors and fuel cells are key components in energy storage and conversion.Graphene is considered to be a perfect energy storage material due to its excellent physicochemical properties and unique structural characteristics.Because of its high specific surface area and abundant pore structure,porous carbon is used as a support material or an electrode material in the field of energy storage.For energy storage materials,the specific surface area,pore structure,surface functional groups and electrical conductivity are four major factors affecting their performance.From the above points,in this thesis,functional graphene and cysteine-based carbon are prepared by using graphene and cysteine as precursors taking advantages of the latter’s reducibility and containing nitrogen and sulfur,and their super capacitance and electrocatalytic activity toward oxygen reduction reaction have been tested.The effects of preparation conditions on the morphology and structure,and ultimately the capacitive and catalytic properties of the materials in detail.The specific contents are as follows:1.Cysteine that has the reducibility and the oxygen groups that have oxidability on the GO layer take an oxidation-reduction reaction at 95℃,thus making GO self-assemble into a three-dimensional graphene hydrogel.A large number of macropores will be produced directly after the freeze drying of hydrogels,which will cause the density of materials decrease sharply,further result in low volume specific capacity of materials.Therefore,we have obtained the high density reduced graphite oxide aerogel with a thickness of about 20μm with a micro-mesoporous structure by using the method of mechanical compression and then freeze drying to eliminate the macroporous structure.The bulk density of the sample obtained by reaction for 8h is 1.44g cm-3,about 65%of the theoretical density of graphite material,and the volume specific capacity is 293.6 F cm-3 under the 0.5 A g-1 current density in the alkaline solution.Moreover,capacitance of the material in the three electrode system did not decay after 300000 cycles of charging and discharging,showing a very long cycle life.2.By using cysteine as a carbon,nitrogen and sulfur source directly and NaCl and KOH as a hard template and activator,a high specific surface area of nitrogen and sulfur co-doped porous carbon was prepared in situ.When the ratio of m KOH:m NaCl:m L-Cys is 20:1:20 and the carbonization temperature is 700℃,the highest capacitance and catalytic activity of oxygen reduction are obtained。The material shows a high specific capacitance(0.5 A g-1 current density,363.1 F g-1),a good multiplier performance(the current density is 100 A g-1,the capacitance still reaches 201.2 F g-1)and an excellent cyclic stability（10000 cycles charge and discharge,and the capacitance retention rate is 97.8%）in alkaline solution.In addition,the material has a corrected starting potential（0.94V vs RHE）and a large limit diffusion current density(5.05 m A cm-2 at 0V vs RHE)in the ORR test.The excellent electrochemical properties of the materials benefited from the high specific surface area(1551.66 m2g-1),unique pore structure and suitable amount of nitrogen（3.13%）,sulfur（1.84%）,and oxygen（12.37%）.These factors facilitate the rapid transmission of ions,the conduction of electrons and the increase of the active site of the material.3.In order to improve the oxygen reduction catalytic activity of porous carbons,we have introduced GO to improve the conductivity of the materials.By adjusting the amount of GO,it is found that the sample NS-G@CNS-20 has the best oxygen reduction catalytic activity when m L-Cys:m GO is 20.The conductivity increased by an order of magnitude（from 0.29 to5.01 S/m）compared with that without GO.In the ORR test,the material shows a correct starting potential（0.998V vs RHE）.Compared without GO,the starting potential moves right to 0.058V,which is closer to commercial Pt/C（1.05 V vs RHE）.In addition,NS-G@CNS-20also has the most corrected half wave potential（0.77 V）and the largest limiting current density(6.01 mA cm-2 in 0V vs RHE)compared with other samples,which confirms that increasing the conductivity of the catalyst has a positive effect on its catalytic performance. |
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