The Preparation And Electrochemical Performances Of Supercapacitors Based On Two-dimensional Conductive Nanomaterials

Two-dimensional（2D）conductive nanomaterials have unique structural and electrochemical characteristics,exhibiting wide application prospects in the field of electrode materials for supercapacitors.However,2D conductive nanomaterials are prone to stacking during the assemble process,resulting in limited electrochemical active area on the surface of the electrodes and the obstruction of ion transport in the electrolyte,limiting their electrochemical performance.Therefore,in this dissertation,by using 2D conductive nanomaterials including reduced graphene oxide（RGO）and transition metal carbide（MXene）as structural units,three assembly strategies are proposed to construct porous and channel structures in fiber and film electrodes to improve the electrochemical reaction kinetics and charge storage ability.Porous heteroatom dual-doped graphene fiber electrodes,MXene hybrid fiber electrodes and MXene composite film electrodes with fast ion transport pathways are designed and fabricated.At the same time,the electrochemical performance of the supercapacitor decreases significantly due to the water electrolyte solidification at low temperatures,to solve this problem,three kinds of low-temperature resistant electrolytes are prepared by introducing the component with solar-thermal conversion effect,the ionic liquid with low freezing point and the design of polymer double cross-linked network structure,so as to improve the low-temperature electrochemical performance of the supercapacitor.The main research contents and results of this dissertation are as follows:（1）Preparation and electrochemical performance of low-temperature-resistant supercapacitors based on porous nitrogen/sulfur dual-doped graphene fibers:In order to restrain the stacking of nanosheets in the assembly process and the low electrochemical activity of graphene fiber electrodes,porous nitrogen/sulfur dual-doped graphene fibers（NS-GF₂）with high specific surface area,high conductivity,and high capacitance are prepared by hydrothermal synthesis combined with thermal annealing process.The porous structure improves the specific surface area of the electrode materials,and the synergistic effect of nitrogen/sulfur dual-doping increases the surface electrochemical active sites and improves the charge storage ability of the graphene fibers.Therefore,the NS-GF₂ exhibit good electrochemical performance with a specific capacitance of 401 F cm^-3.In addition,in order to increase the capacitance of the supercapacitors at zero temperature,the PVA/H₂SO₄/GO low-temperature-resistant electrolyte is prepared by introducing graphene oxide（GO）nanosheets with solar-thermal energy conversion effect into the polyvinyl alcohol（PVA）electrolyte.The all-solid-state flexible supercapacitor is assembled by NS-GF₂ electrode and PVA/H₂SO₄/GO electrolyte.At room temperature,the specific volume capacitance and energy density of the supercapacitor can reach 221 F cm^-3 and7.7 m W h cm^-3,respectively.After 10000 charge/discharge cycles,the capacitance retention of the supercapacitor is 86%,exhibiting good cycle stability.When the ambient temperature decreases to 0℃under 1-sun solar illumination(100 m W cm^-2)the photothermal convention effect of the GO nanosheets in the electrolyte can increase the temperature of the device by 15℃,thus increasing its capacitance to 182 F cm^-3,reaching 82%of its capacitance at room temperature.（2）Preparation and electrochemical performance of low-temperature-resistant supercapacitors based on MXene/RGO/PEDOT:PSS hybrid fibers:In view of the difficulties in forming fiber electrodes due to the weak gelation ability of MXene nanosheets and the long ion transport path due to limited contact area between fiber electrodes and electrolyte,MXene/RGO/PEDOT:PSS hybrid fiber（D-MGP₃）with radially oriented channel structure is prepared by radial freezing after hydrothermal assembly assisted with RGO and poly（3,4-ethylenedioxythiophene）:poly（styrene sulfonate）（PEDOT:PSS）.The radially oriented channels in the fiber not only inhibit the stacking of MXene nanosheets and increase the contact between electrodes and electrolyte,but also provide the shortest transmission pathways for ions in the electrolyte,speeding up the ion transmission rate and improving the electrochemical reaction kinetics of the electrodes.Therefore,the D-MGP₃hybrid fiber electrode shows excellent electrochemical performance,with a specific capacitance of 475 F g^-1 at 5 m V s^-1,in addition,it can provide a specific capacitance of 366 F g^-1 at a high scan rate of 1000 m V s^-1,with a good rate performance.In order to improve the capacitance retention of the supercapacitors at low temperatures,the EMIES/PVA/H₂SO₄ ionogel electrolyte is prepared by introducing 1-ethyl-3-methylimidazole-ethyl sulfate（EMIES）with a low freezing point.The all-solid-state fiber supercapacitor assembled with D-MCP₃ fiber electrodes and EMIES/PVA/H₂SO₄ electrolyte exhibits a maximum power density and an energy density reached 1600 W kg^-1and 10.11 W h kg^-1,respectively,at room temperature.Benefiting from the low-temperature-resistant ionogel electrolyte,the supercapacitor can still maintain good flexibility and electrochemical performance at-40℃,and the capacitance retention can reach 71%.（3）Preparation and electrochemical performance of low-temperature-resistant supercapacitors based on MXene/carbon nanotubes（CNT）composite film electrodes and ionogel electrolyte:In view of the poor ion accessibility and long ion transport pathways caused by the dense internal structure of 2D MXene thin film electrode materials,MXene/CNT composite film electrode（v-MCF₄）with vertical channel structure is prepared by scraping molding and directional freezing.The vertically arranged channel structure increases the specific surface area of the MXene-based thin film electrode,which is conducive to sufficient charge transfer at the electrode/electrolyte interface and provides fast pathways for ion transport.v-MCF₄ composite film electrode has good electrochemical performance with a specific capacitance of421 F g^-1 at 2 m V s^-1 and the capacitance retention of 86%(363 F g^-1)at 1000m V s^-1.In order to further reduce the operating temperature of the supercapacitors and improve the capacitance retention at low temperatures,a double cross-linked polymer network of PVA and the copolymer（P（AA-co-AMPS）of acrylamide-2-methylpropanesulfonic acid）in the binary solvent EMIES/H₂O system is constructed to inhibit the growth of ice crystals.The low-temperature-resistant ionogel electrolyte with double-crosslinked network named PE5/P（AA-co-AMPS）have a freezing point as low as-54℃.The integrated all-solid-state supercapacitor assembled by v-MCF₄ composite film electrode and the PE5/P（AA-co-AMPS）ionogel electrolyte exhibits excellent electrochemical performance and low temperature resistance,with a high specific capacitance of 231 F g^-1 and a maximum energy density of 10.17 W h kg^-1 at room temperature.Even when the temperature decreases to-50℃,the specific capacitance can still reach 213 F g^-1,and the capacitance retention is as high as 92%.