Preparation And Electrochemical Performance Of Low-Dimensional Porous Carbon Electrode Materials With High Specific Capacitance In Supercapacitors

Micro-supercapacitor（MSC）has become one of the most potential micro-energy storage devices due to its advantages of high-power density,fast charge-discharge rate,easy integration,and maintenance-free.However,the low capacitance of MSC is still an important factor limiting its development,which is closely related to the electrode materials.Among electrode materials,graphene has attracted much attention as an electrode material for MSC in recent years due to its unique two-dimensional structure and high theoretical specific surface area.However,limited by the severe stacking caused by its interlayer van der Waals forces and the lack of in-plane pores,the capacitive performance exhibited by graphene is still far lower than expected.To this end,this dissertation research explores new principles and methods to construct low-dimensional porous carbon structures using graphene as a carbon source to meet the urgent needs of MSC for high specific capacitance electrode materials,and the main research works and results are as follows:In view of the shortcomings of the current chemical exfoliation method for preparing graphene,such as complicated steps and long cycle,the feasibility of using perchloric acid as a single chemical exfoliation system to prepare graphene with high-quality was studied.The results show that perchloric acid can act as an oxidant,intercalation agent and exfoliator at the same time to achieve the fast and effective exfoliation of graphite,and the entire process takes only～15 min at the fastest.In addition,the obtained graphene has the characteristics of few layers,large size and few structural defects.The graphene samples obtained by the one-step heating method and the two-step microwave route both possess a layer number of≤5 layers,with the average size reach up to 262.2 and 256.8μm²,respectively.Moreover,the I_D/I_G values in the Raman spectra of graphene obtained by the one-step heating method and the two-step microwave route are only 0.052 and 0.056,respectively,with the electrical conductivity of the resultant graphene can be as high as 2.1×10⁵S m^-1and 2.01×10⁵S m^-1,respectively.Using graphene oxide as the carbon source and ammonium molybdate as the Mo source,a graphene（RGO）-Mo₂C integrated structure was constructed through an in-situ carbothermal reaction.Then the RGO-carbide derived carbon（CDC）integrated structure was obtained after a chlorine etching process at 800℃.Next,two MSC devices-RGO-CDC-MSC and RGO-MSC were fabricated by using the RGO-CDC and RGO as electrode materials,respectively,with the use of the highly conductive graphene obtained from the above perchloric acid system as the current collector,and packaged with PVA/H₃PO₄ as the hydrogel electrolyte.Furthermore,the electrochemical performances of the obtained two MSC devices were also tested.The results show that the introduction of CDC can not only prevent the stacking of RGO,but also increase its specific surface area and provide ion channels perpendicular to the RGO sheets.Therefore,RGO-CDC-MSC exhibits better capacitance performance than RGO-MSC,with a specific capacitance as high as 40.26 mF cm^-2,which is much higher than that of RGO-MSC of 3.81 mF cm^-2.In addition,RGO-CDC-MSC also exhibits excellent series-parallel integration capability,mechanical flexibility,and long-cycle stability（capacitance retention rate of 97.83%after 10,000 cycles）.Taking the graphene exfoliated from the above perchloric acid system as the carbon source,and the Ti powder as the Ti source,a 0D/2D TiC integrated structure was constructed by a molten salt method.Then,a unique 0D/2D porous carbon integrated structure（PC-IA）was obtained after a chlorine etching process at 800℃.The electrochemical properties of PC-IA-MSC and GNS-MSC devices constructed with PC-IA and graphene as electrodes,respectively,were tested.The results show that PC-IA-MSC has significantly better capacitance performance,and its specific capacitance increases from 3.89 mF cm^-2of GNS-MSC to 70.12 mF cm^-2.This can be attributed to the structural superiority of PC-IA compared to graphene,including higher specific surface area,better electrolyte wettability,and the ion channels exist in the interlayer and vertical directions,respectively.Taking the graphene obtained by exfoliating natural graphite after KOH activation asthe carbon source,and the Ti powder as the Ti source,a two-dimensional polycrystalline TiC structure was synthesized by a molten salt method.Then,porous carbon nanosheets with a unique polycrystalline-like micro-morphological feature（PL-PCN）were obtained through a chlorine etching process at 800℃,and the electrochemical performance of the PL-PCN-MSC devices constructed by using PL-PCN samples as electrodes were investigated.The results show that the introduction of KOH activation can significantly change the morphology,pore structure and capacitance properties of the obtained PL-PCN samples.Moreover,the specific capacitance of the obtained PL-PCN-MSC can reach up to 108.88 mF cm^-2,which is significantly higher than that of the previously reported carbon-based MSCs.The promising electrochemical performance of the fabricated MSC should result from the synergism of the high specific surface area(up to 1649.97 m²g^-1),the excellent wettability to electrolyte and the unique hierarchical porous structure of PL-PCN,in which the mesopores existing in the form of“grain boundary”and the micropores inside the“grain”can effectively provide abundant ion transport channels and accessible electrochemical active sites,respectively.