Construction Of MXene-based Film Electrode And Its Supercapacitor Performance Investigation

Supercapacitors have an irreplaceable position in the field of energy storage due to their high power density,long cycle life and good safety.However,traditional supercapacitors have a low energy density and inferior rate performance,and meanwhile,the cycle stability need to be improved because of the demand for high-performance electric vehicles/electronic equipment.Developing new electrode materials for supercapacitors is one of the keys to solving the above problems and improving the performance of supercapacitors.In addition,the use of conductive agents and binders in the electrode materials further reduces the energy density of supercapacitors.Therefore,it is the heart of current research to construct and optimize film electrode materials without conductive agents and binders to improve the energy density,rate performance and cycle stability of supercapacitors.Owing to the large specific surface area,multiple active sites,short ion transport path and excellent mechanical properties,two-dimensional materials have become a research hotspot in the field of supercapacitors.MXenes are a new type of two-dimensional transition metal carbides/nitrides with combination of hydrophilicity and conductivity.Their layer spacing,composition and surface functional groups are tunable.Furthermore,they can be easily assembled into free-standing films and directly used as electrodes without any conductive agents and binders.All these characteristics make MXenes great potential in the field of energy storage especially supercapacitors since their discovery.However,the composition tunability of MXenes needs to be further explored;the unavoidable functional groups on the surface of MXenes reduce their conductivity;the two-dimensional structure of MXenes is prone to collapse and re-stacking,slowing ion/electron migration rate and affecting the electrochemical performance.Therefore,overcoming the above problems and accelerating the development of MXenes in supercapacitors are an urgent research topic.Making full use of MXenes'unique characteristics combination,the composition,structure and composite of MXene based film electrodes?taking d-Ti₃C₂ as a typical representative of MXene family?were optimized from the atomic/micro-nano/macroscopic scale to improve the electrochemical properties,such as specific capacitance,rate performance and cycle stability.At the atomic scale,nitrogen-doped d-Ti₃C₂ nanosheets were successfully prepared by hydrothermal method,and the effects of different nitrogen contents on the electrochemical performance of d-Ti₃C₂ were investigated.The introduction of N increases the spacing of d-Ti₃C₂ layer,the electrical conductivity and the psudocapacitance.Combining first principles calculation,the position of doped N and its influence mechanism on electrochemical performance of N-doped d-Ti₃C₂ nanosheets were revealed.At the micro-nano scale,taking advantage of the susceptibility of titanium atoms on the surface of d-Ti₃C₂ nanosheets,the in-plane porous structure of d-Ti₃C₂ nanosheets was constructed by in situ oxidation-etching process,realizing the optimization of microstructure.The in-plane porous structure shortens the ion transport path,mitigates d-Ti₃C₂ nanosheets re-stacking,and increases the effective specific surface area and the active sites,so as to improve its specific capacitance,rate performance and cycle stability.At the macroscopic scale,CNTs?1D?and rGO?2D?heterogeneous phases were introduced to construct d-Ti₃C₂/CNTs and nitrogen-doped d-Ti₃C₂/rGO films by electrophoretic deposition and vacuum filtration,respectively,optimizing the d-Ti₃C₂-based film electrodes.The introduction of the heterogeneous phases effectively alleviates the re-stacking of d-Ti₃C₂ nanosheets,exerts synergetic effects between the components,thereby obtaining excellent electrochemical performance.Subsequently,the pore forming agents?nano-TiO₂?were in situ introduced during the nitrogen doping process,and then etched off to realize the regulation of chemical composition and physical structure of d-Ti₃C₂ nanosheets from the atomic and micro-nano scales.The modification of composition and structure of d-Ti₃C₂ enhances the conductivity and psedocapacitance,provides more effective specific surface area and shortens diffusion path of ions,thereby improving the electrochemical performance.Finally,combining the introduction of doping,porous structure and heterogeneous phases,d-Ti₃C₂ based electrode was conducted by the multi-scale regulation to improve its electrochemical performance.Via the polydopamine?PDA?coating and carbonization strategy,we prepared a porous d-Ti₃C₂/nitrogen-doped carbon free-standing film with a specific capacitance of 488.1 F g^-1 at1 A g^-1;even at ultra-high current density(100 A g^-1),its specific capacitance still can reach as high as 411.2 F g^-1,with the capacitance retention rate of 84.2%;After 10,000 cycles at 10 A g^-1,the capacitance retention rate is 95.4%,showing good cycling performance.The excellent electrochemical performance is ascribed to the synergetic effect from composition,structure and material designing,including the enhanced conductivity and psedocapacitance by the nitrogen-doping,the more effective specific surface area and shorter the ion diffusion path from the porous structure and the alleviation of the collapse and re-stacking of d-Ti₃C₂nanosheets from the heterogeneous phase introduction.This multi-scale design provides a potential for the film preparation of other two-dimensional materials.In summary,MXenes were tuned from the chemical composition,microstructure and material design,to construct MXene based film electrodes,achieving the improvements of specific capacitance,rate performance and cycle stability.The results are beneficial to understand the effects of MXenes'chemical composition and microstructure on their electrochemical performance,give full play to the advantages of MXenes in supercapacitors,accelerate the practical application of MXenes,and promote the development of new energy storage technologies.