Electrochemical Energy Storage Of Activated Graphene-based Supercapacitors

Graphene has become one of the most popular electrode materials of supercapacitor by considering its large specific surface area,excellent electrical conductivity and electrochemical stability.Nevertheless,they suffer from the weakness of particular low capacity.In principle,topological defects such as stress asymmetry and structural distortion of graphene have the improved density of state（DOS）at near the Fermi level,which could contribute the capacitance to some extent,whilst the details are not clear.Unlike traditional pseudocapacitive materials which cannot compactly stack with graphene（the structure reduces power density and sacrifies the electron/charge transfer）,two-dimensional transition metal dichalcogenides（2D TMDs）have also 2D surface properties（e.g.Faradic and non-Faradic behavior）.Considering the 2D feature of both graphene and 2D TMDs,the close connection between both materials is possible,which has the potential to improve the capacity of the composite electrode,with the rapid charge/discharge process remained.On this basis,the thesis proposed to modify graphene materials with aboundant surface corrugations and to investigate the underlying mechanism on the capacitance improvement.Building on this target,the 2D Mo-S nanocrystals（NCs）were loaded onto the activated graphene wall,aiming to fabricate a composite electrode with elevalated energy storage performance.The work here could potentially guide the design and fabrication of novel all 2D based composite electrode with more excellent energy storage performance.The content below lists main findings of this thesis work:（1）A simple thermal expansion method was developed to create surface corrugations of graphene material from graphene oxide（GO）films.Optimization of experiments confirmed the best reaction temperature of 375℃（as-prepared sample was denoted as a-G-375）.a-G-375 based symmetric supercapacitor delivered the specific capacitance of 342 F g^-1 at 5 m V s^-1,power density of 214 W kg^-1 and corresponding energy density of 11.9 Wh kg^-1.Density functional theory（DFT）calculations show that the corrugation structure improved the local DOS around the Fermi level,which enhanced the adsorption energy to electrolyte ions,compressed the electric double layer and increased the number of electron transfers,thus improved electrochemical energy storage capacitance of graphene materials.（2）2D MoS₂ NCs were fabricated using“K-intercalated unzipping”technique.Ion exchange was used to desulfurize the 2D surface,yielding MoS_1.5 NCs.The ultra-small size and surface S vacancy of 2D MoS_1.5 NCs provided more electrochemical active sites.Besides,S vacancy boosted DOS around the Fermi level,thereby enhancing conductivity and adsorption energy with electrolyte ions,and considerably improved specific capacitance.As a result,MoS_1.5 NCs exhibited the specific capacitance of 178,206 and 215 F g^-1 at 5 m V s^-1 in 1 M Li Cl,Na Cl and KCl electrolytes respectively,which is much greater than that of MoS₂ NCs.（3）The MoS_1.5/a-G-375 composite was prepared by loading 2D MoS_1.5 NCs onto a-G-375 surface through“stirring-adsorption-curing”method.It delivered specific capacitance of 620 F g^-1 at 1 m V s^-1in 1 M Li Cl.The specific capacitance was still remained as high as 332 F g^-1 at 5 m V s^-1,rationally higher than those of pure a-G-375(225 F g^-1)and 2D MoS_1.5 NCs(178 F g^-1).