Application Of MXene-based Composites In Flexible Supercapacitors

The development of wearable electronic devices greatly stimulates the demand for flexible energy storage devices.As the general name of two-dimension(2D)carbides and nitrides of transition metals,MXenes have great application potential in flexible energy storage field,among which Ti₃C₂T_x is the most extensively studied MXene material.Ti₃C₂T_x have unique combinations of graphene and graphene oxide`s characters such as distinctive 2D layered structure,remarkable surface chemistry with highly hydrophilic surface combined with superexcellent metallic conductivity which have emerged potential applications in the field of flexible supercapacitors.However,due to van der Waals forces,Ti₃C₂T_x flakes are easy to self-stack which limits the accessibility of ions and impede the full usage of their functional surface.In this thesis,Ti₃C₂T_x MXene was compounded with high theoretical specific capacitance materials to improve the restacking and inactive capacity,which provided guidelines for the development of flexible energy storage devices.The specific research contents were as follows:(1)The chemical etching method was used to exfoliate Ti₃C₂T_x MXene from Ti₃AlC₂ raw material.Ti₃C₂T_x MXene was compounded Fe OOH with high theoretical capacitance to be free-standing supercapacitor electrodes by electrostatic self-assembly and vacuum filtration.Through series of characterizations,it was found that the Fe OOH/MXene composites effectively inhibited the self-stacking of Ti₃C₂T_x MXene flakes and improved the conductivity of Fe OOH.The electrochemical performance of Fe OOH/MXene with different contents were investigated in detail.The specific capacitance of Fe OOH/MXene electordes were increased first and then declined with increasing Fe OOH material contents,reaching an optimum value at 20 wt.%Fe OOH content.The specific capacitance of Fe OOH/MXene was 237 F g^-1 at a scan rate of 1m V s^-1 and no capacitance decayed after 10000 cycles.(2)Based on Fe OOH/MXene,the Fe₂O₃@MXene composite electrodes were fabricated through annealing treatment.Fe OOH converted into Fe₂O₃ nanoparticles at high temperatures and anchored on the Ti₃C₂T_x MXene layer to form a stable Fe₂O₃@MXene structure.The results showed Fe₂O₃ nanoparticles effectively improved the self-stacking of Ti₃C₂T_x MXene,increasing the interconnected mesopore channel and providing more electrochemically active sites,while the Ti₃C₂T_x MXene highly conductive film enhanced the electrochemical activity of Fe₂O₃ nanoparticles anchored on it.Besides,the layer structure of Ti₃C₂T_x MXene effectively suppressed the volume expansion of Fe₂O₃ during the electrochemical reaction.The synergy effect between two materials obtained an electrode with enhanced electrochemical performance.The optimal specific capacitance was obtained when Fe₂O₃ content was 9.1 wt.%in the composite.Fe₂O₃@MXene electrodes had a wide potential window of 1 V,an ultrahigh volumetric capacitance of 2607 F cm^-3at a scan rate of 1 m V s^-1,and an exceptional cycle lifespan(121%retained after 13000 cycles).Finally,the flexible solid-state supercapacitors were fabricated by sandwiching the PVA/H₂SO₄ gel electrolyte between two Fe₂O₃@MXene electrodes to explore application in advanced flexible energy storage devices.The results showed the supercapacitor devices had a potential window of 1.2 V,a high volumetric energy density of 29.3 Wh L^-1,a maximum volumetric power density of 213.8 W L^-1.The capacitance remained 95%after 10000cycles.(3)Based on the above research results,we have combined different metal oxides with Ti₃C₂T_x MXene to explore the universality of synergies.Mn₂O₃,Mg O,Co₃O₄ and Ni O were compounded with Ti₃C₂T_x MXene respectively to obtain corresponding flexible electrodes by self-assembly and vacuum filtration.The synergistic effect of the two phases provided new design concept of MXene based supercapacitor electrodes in flexible energy storage systems.