Research On Synthesis And Volumetric Capacitance Properties Of Holey Graphene And MXene-based Composite Films

Portable electronic devices have become an indispensable consumer product in our daily life.Importantly,miniaturized energy storage units in these devices need to store more energy in as little space as possible,which means that volumetric performance instead of common gravimetric performance is the crucial evaluation index.Note that the excellent volumetric performance of supercapacitor electrode requires the gravimetric performance and packing density to simultaneously reach the optimum values.Unfortunately,there is a contradiction between the gravimetric performance and packing density for most supercapacitor electrodes,usually resulting in an inferior volumetric energy density.In this paper,holey graphene with abundant nanoporous structure and graphene-like material?MXene?with high volumetric capacitance are used as the matrix to regulate and optimize the structure between holey graphene and conductive polymer as well as MXene,and the designed electrode materials possess a dense interconnected pore structure.Furthermore,the paper is devoted to seriously explore the influencing factors and laws of the capacitance performance parameters of the electrodes'volumetric capacitance at the nanoscale.Simultaneously,the mechanism of ion transport across the electrode materials is also revealed.The free-standing holey graphene/polypyrrole composite film is constructed by a simple one-step hydrothermal and mechanical compression of graphene oxide,hydrogen peroxide and pyrrole.By controlling the amount of hydrogen peroxide and pyrrole to adjust the nanopore size of the graphene surface and the pore structure between the composite layers,and then the preparation of electrode with a dense interconnected pore structure is realized.Experimental results show that the electrode material has excellent volumetric capacitance and high rate capability.Based on the above,the holey graphene hydrogel is treated with nitrogen doping to induce polyaniline nanoparticles on the surface of holey graphene.Then through mechanical compression and capillary shrinkage,the controllable build of nitrogen-doped holey graphene/polyaniline film with dense structure and highly interconnected pore channel is achieved.The results indicate that the gravimetric capacitance and volumetric capacitance of the electrode material can reach up to730 F g^-1 and 1058 F cm^-3,respectively,and thus overcoming the drawback that the excellent gravimetric capacitance and volumetric capacitance can not be realized for the same electrode.The 3D MXene aerogel is constructed by using water molecules as sacrificial templates.Meanwhile,the influence mechanism of compression strength on the pore structures,density,conductivity and electrochemical properties of MXene aerogel is studied.In addition,the expendable Fe?OH?₃ nanoparticle template is introduced into the MXene flakes by electrostatic self-assembly,and the compact MXene film with highly interconnected nanopore channels is constructed.Furthermore,the effect of the introduced pore structure and heat treatment temperature on the volumetric capacitance of electrode materials is studied.Consequently,the results show that the volumetric capacitance of the assembled electrode material can reach 1142 F cm^-3,and maintain up to 749 F cm^-3 even at high electrode mass loading(11.2 mg cm^-2).Considering the negative surface charge of holey graphene oxide and MXene,those two materials are evenly mixed and modified with sodium hydroxide,and then the modified MXene/holey graphene composite film can be obtained by filtering and low-temperature heat treatment.The effect of holey graphene embedding and dosage on the interlayer pore structure and density of MXene is investigated to realize controllable build of electrode materials with ultra-high volumetric capacitance(1445 F cm^-3),excellent rate capability and large active material mass loading.The design strategy and constructed electrode materials used in this paper are not only limited to supercapacitors,but also widely applicable to other energy storage devices requiring high volumetric capacitance performance.