Capacitive Energy Storage Mechanism And Applications Of Ti₃C₂T_x MXene

MXene,a new class of two-dimensional transition metal carbon/nitride,has been proved to be an intercalation pseudocapacitive supercapacitor electrode material with excellent electrochemical performance,which delivers up to 1500 F/Crcm3 in H2SO4 solution.It has been demonstrated that there is a continuous change in the transition metal oxidation state during charge and discharge.However,an in-depth understanding of energy storage mechanism of this new material remains elusive:What is the specific reaction process?What are key issues that determine electrochemical performance.At present,a large number of research works focused on the modification of MXenes have not made significant progress,which is also due to the lack of understanding of the energy storage mechanism in a large extent.This dissertation takes Ti3C2Tx MXene as the research object,proceeds from the comprehending of energy storage mechanism of MXenes,and aims to shed light on the direction for the preparation of high-performance MXenes and promote their practical application.The main conclusions are as follow:(1)A Ti3C2Tx MXene electrode is prepared by solution-based nanosheet-self-assembly method,which exhibited different electrochemical performance in different electrolytes.The in situ electrochemical Raman spectroscopy demonstrates that the charge storage mechanism is dependent on electrolytes:in H2SO4 solution,Ti3C2Tx MXene stores pseudocapacitive charge induced by-O functional groups involved redox reaction,while in(NH4)2SO4 and MgSO4 solutions,NH4+ or Mg2+ cation is adsorbed on the surface of Ti3C2Tx and only double electrical layer capacitance is recognized.In addition,the manner of ion intercalation is dependent on electrolytes:ion exchange dominates in H2SO4,while counterion adsorption in the rest.The two factors,i.e.,surface functional group-involved redox action-induced pseudocapacitance,and ion exchange-featured ion intercalation,simultaneously contribute to the superior capacitance of Ti3C2Tx MXene in H2SO4 solution.(2)Based on the understanding of the energy storage mechanism,we proceed from the preparation of MXenes and comprehensively study the correlation between structural feature,chemical composition/state and electrochemical performance(H2SO4 solution)of Ti3C2Tx MXene.It is demonstrated that the larger interlayer slits resulting from the more high-mobility water intercalated between the MXene interlayers and the higher ratio of the-O functional groups benefit the electrochemical performance.The above two key factors simultaneously account for the superior capacitance of the Ti3C2Tx MXene electrode prepared in the low concentration HF solution.This result not only reasonably explains the difference in the capacitance of Ti3C2Tx prepared by different methods in different research groups,but also provides a synthesis strategy for the preparation of high-electrochemical-performance MXenes.(3)Based on the understanding of the energy storage mechanism,we proceed from the modification of MXenes and comprehensively further study the correlation between structural feature,chemical composition/state and electrochemical performance(H2SO4 solution)of Ti3C2Tx MXene.Simultaneously creating a broadened yet uniform interlayer spacing and incorporating heteroatom with lower electronegativity between the interlayers,enables high capacitance.Following this concept of interlayer engineering,a higher capacitance is achieved for Ti3C2Tx MXene by readily annealing in ammonia atmosphere.Moreover,the engineered MXene exhibits excellent rate performance and cyclability.This result breaks the ice that the regulation and modification of MXenes has been in the trial and error stage for a long time,which helps MXene to achieve higher capacity.(4)In order to advance the practical production applications of MXenes,the application of MXene in supercapacitor devices,including flexible symmetric supercapacitor devices and asymmetric supercapacitor devices,is explored.The commercially available silver-plated nylon yarns with good electrical and mechanical properties or carbonized waste silk clothes used as current collectors are coated by Ti3C2Tx nanosheets to form the flexible linear or planar electrode.The assembled all-solid-state supercapacitor exhibits a high length or area specific capacitance with good cycle stability and flexibility,which satisfies the operating requirements of flexible and woven electronic devices.In addition,the hydrogen ion rocking chair asymmetric supercapacitor is assembled by exploiting the redox-active hydroquinone in carbon nanotubes positive electrode and Ti3C2Tx MXene negative electrode.The supercapacitor operates in a 1.6 V voltage window and delivers a high energy density of 62 Wh/kg,which substantially exceeds those of the state-of-the-art aqueous asymmetric supercapacitors reported so far.These encouraging results will open the way to a new area of developing high-energy-density asymmetric supercapacitors for practical applications.