High-quality Graphite Nanosheets Prepared By Electrochemical Intercalation And Their Application To Supercapacitor Electrodes

Graphite nanosheets are the basic units of expanded graphite (EG) and composed of a singlelayer or multilayer graphene with a thickness of0.34-100nm and particle size in micrometer scale,which are a class of two-dimensional carbon nanomaterials and usually produced by thermalexpansion of graphite intercalation compounds (GIC). Compared with graphene, graphitenanosheets are easier to be prepared and inherit many excellent physical and chemical propertiesof gaphene, owing a very wide range of applications. To prepare high quality graphite nanosheetswith thin layer and high specific surface area (SSA), we need synthesize uniformly andsufficiently intercalated GICs at first. As we know, chemical oxidation intercalation andtraditional electrochemical intercalation are the two main methods to prepare GICs. Graphitesnanosheets prepared by weak chemical oxidation have less defect but thick layer and low SSA.By deep chemical oxidation, however they become thinner and have higher SSA. But deepchemical oxidation is complicated and results in more defects on the nanosheets. As for traditionalelectrochemical intercalation, it is a convenient and efficient route but the produced nanosheetshave thick layer and low SSA. In this thesis, we have developed a fast electrochemical exfoliationcombined with low thermal expansion (250℃) route to prepare high SSA graphite nanosheetsbased on the deeply understanding of graphite intercalation and exfoliation mechanism, in orderto meet their huge application demands.We also measured the performance of the as-preparedgraphite nanosheet as electrode material in supercapacitors. The main contents are summarized asfollows:1. A fast electrochemical exfoliation combined with low-thermal expansion route to preparegraphite nanosheets has been developed. The exfoliation process was carried out on a two electrodeelectrochemical exfoliation system. Where pure graphite rod, metal titanium plate act as anode andcathode, respectively. The electrolyte is sulphuric acid and sodium nitrate mixed aqueous solution.At a constant current4.5A, GIC-H2SO4with production rate of10g/h can be achieved. Then10gof the so-obtained GIC was thermally expanded at250℃to prepare exapnded graphite(GIC-H2SO4,7.8g). The GIC-H2SO4is composed of a large number of porous graphite nanosheetswith the thickness of4-10nm and its SSA is145m2/g. The specific capacitance of GNs-H2SO4based two electrode supercapacitor can reach61.86F/g at a charging/discharging current density of0.6A/g. After one thousand cycling test, the capacitance retention rate is98.9%, suggesting its goodperformance.In order to understand the effect of oxidation degree on the electrochemical intercalation, we use acetic acid instead of sulphuric acid to carry out intercalation experiments to prepareGIC-CH3COOH. The production rate is1g/h. Using low temperature expansion,0.65g ofexpanded graphite (GNs-CH3COOH) can be prepared from1g of GIC-CH3COOH The SSA ofGNs-CH3COOH is75m2/g, lower than that of GIC-H2SO4, and the thickness of the graphitenanosheets is about10nm. This means high oxidation degree is beneficial for graphite intercalation.The specific capacitance of GNs-CH3COOH based supercapacitor is41.75F/g at acharging/discharging current density of0.6A/g. After one thousand cycling test, the capacitanceretention rate is99.1%.2. We used GNs-H2SO4as substrate to grow carbon nanotubes (CNTs) by chemical vapordeposition to form three-dimensional CNT/GNs composite material as well as support MnO2tofabricate MnO2/GNs by one step chemical reaction. The specific capacitance of CNT/GNs basedtwo electrode devices reached114.8F/g at a charging/discharging current density of0.4A/g. Afterone thousand cycling test, the capacitance retention rate is99.1%. The specific capacitance ofMnO2/GNs is442.1F/g at a charging/discharging current density of0.4A/g. After one thousandcycling test, the capacitance retention rate is87%. These results suggest that our graphitenanosheets has potential application in supercapacitors.