MXene-Based Flexible Transparent Supercapacitors

Transparent flexible energy storage device is an important part of future wearable electronic system.Compared with other flexible energy storage devices,the flexible and transparent supercapacitor not only has fast charging/discharging characteristic,high power density and long cycle life,but also can greatly enhance the comfort and aesthetics of the product.It can be used to fill the shortage of battery power density and energy density of traditional capacitors,which has great potential for application and is the current research hot spot in flexible and transparent energy storage.To achieve flexible and transparent design and fabrication of supercapacitor electrodes,ensure high energy storage capacity,long cycle life and high charging/discharging efficiency,while taking into account the high transparency and mechanical properties of the electrodes,is the top priority of flexible and transparent supercapacitor research.MXenes are attractive for energy storage applications due to their unique layered structure,large specific surface area,high conductive properties and tunable surface groups,exhibiting high electrochemical properties.However,the electrode design is influenced by the intrinsic light transmitance of MXenes,which greatly limit the further improvement of electrochemical performance.How to break the limitations of material selection brought by flexible transparent design and propose a universal design strategy for flexible transparent supercapacitor electrodes from the structure is the key to build and realize flexible transparent supercapacitor electrodes at present.This dissertation intends to design and prepare MXene-based flexible transparent supercapacitors with high electrochemical performance in neutral aqueous electrolytes.Firstly,two-dimensional Ti3C2Tx MXene electrode with larger layer spacing and more favorable-O surface functional groups were synthesized by improved preparation method to achieve high specific capacitance in neutral aqueous electrolytes.Secondly,the Ti3C2Tx MXene/PEDOT:PSS composite film electrode was prepared by spin-coating aimed at the low transparency and low multiplicity characteristics of the prepared MXene film and can be used for flexible transparent supercapacitors.Then,in view of the problems that Ti3C2Tx MXene/PEDOT:PSS composite film prepared by spin-coating in pursuing transparency and resulting in lower capacitance performance,a one-step in situ codeposition method was initially explored to prepare MXene/PPy composite film electrode on a flexible micromesh to achieve high capacitance supercapacitor electrode.Finally,the MXene/PPy composite with high capacitance and self-supporting flexible transparent micromesh structure was combined to construct a flexible transparent supercapacitor electrode with "nickel mesh "core"-MXene/PPy composite "shell" structure by in situ electrodeposition,and the MXene based supercapacitor electrode with "high light transmission-high capacitance-high mechanical properties" was prepared.This dissertation includes the following contents:1.High specific capacitance MXene in neutral aqueous electrolytes were synthesized.Ti3C2Tx-TMAOH MXene electrode for neutral aqueous electrolytes were prepared by a two-step synthesis process.It showed that the prepared 2D nanosheets have expanded layer spacing and more abundant-O surface functional groups.Combined with electrochemical tests,it demonstrated that this special structure and surface properties not only have faster ion transport efficiency,but also provide more active sites for charge transfer.The electrode showed a high specific capacitance(149.8 F/g)and high rate performance in 1 M Li2SO4 electrolyte.The freestanding film electrode obtained by filtration of the MXene exhibited a high bulk capacitance of 325.7 F/cm3,which was attributed to the good layer structure and high conductivity of the film.Flexible solidstate supercapacitor assembled by the film electrode also exhibits a high surface capacitance of 161.3 mF/cm2 and high safety.2.Preparation of Ti3C2Tx MXene/PEDOT:PSS composite film electrode for flexible transparent supercapacitors.The MXene/PEDOT:PSS composite film electrode was prepared by mixing MXene with PEDOT:PSS by a one-step spin-coating method for the study of flexible transparent supercapacitors by selecting a flexible PET substrate with high conductivity and low multiplicity characteristics.It showd the optimal mixing mass ratio of MXene to PEDOT:PSS is 2:1,and the prepared devices have better optoelectronic properties(62.0%)as well as mechanical flexibility(performance degradation of only 4%at a radius of curvature of 1 mm).Combining the special structure of Ti3C2Tx and high conductivity of PEDOT:PSS as well as their interaction,it reduced the ion diffusion path in the electrochemical reaction and accelerated the electrochemical reaction process.The assembled flexible transparent supercapacitor exhibits a surface capacitance of 3.1 mF/cm2 and a high energy density of 0.18 μW·h/cm2,showing great potential in the application of flexible transparent devices.Although the composite film achieves flexible transparency,the low capacitance caused by limiting the film thickness while improving transparency limits its application in flexible transparent supercapacitors.3.Exploring the preparation of high specific capacitance MXene/PPy composite membrane electrodes.To address the low capacitance of MXene/PEDOT:PSS electrode prepared by spin-coating,flexible Ti3C2Tx MXene/PPy-ECC composite film electrodes were prepared by a one-step in situ co-deposition strategy under the selected optimal electrodeposition conditions.The prepared MXene/PPy-ECC composite membrane electrode exhibited a high surface capacitance of 1210mF/cm2(0.1 mA/cm2)in a neutral aqueous three-electrode system.The excellent electrochemical performance stems from the three-dimensional structure and high conductivity of the composite film,which reduces the transport path of electrolyte ions.In addition,interaction of ECC substrate with MXene and PPy and the synergistic effect generated during the electrochemical reaction slowed down the volume change of PPy during charging/discharging.The flexible solid-state supercapacitor as well as the fiber supercapacitor exhibited a surface capacitance of 253.3 mF/cm2 and a length specific capacitance of 41.6 mF/cm.providing a reference for the preparation of flexible solid-state supercapacitor devices.A feasible method is provided for the preparation of MXene based flexible transparent supercapacitor electrodes with high capacitance.4.Construction of high capacitance and highly flexible transparent MXene/PPy-Ni mesh grid structure supercapacitor electrodes.Based on the high capacitance of MXene/PPy electrode material in neutral aqueous electrolyte,the MXene/PPy-Ni grid electrode with "high transmittance-high capacitance-high mechanical properties" was prepared by electrochemical co-deposition of "nickel grid "core"-MXene/PPy composite"shell" structure.By combining laser direct writing technology and selective electrodeposition process to prepare flexible transparent Ni grid electrode,which showed high permeability of 2600 mm/s(at 10 Pa atm),optical transparency of 85%(visible range)and excellent mechanical flexibility.Owing to the high capacitive properties of the MXene/PPy active material and the highly flexible and transparent properties of the Ni grid electrode,the MXene/PPy-Ni grid electrode has an optical transparency of 65%in the visible range and exhibits a high surface capacitance of 347.4 mF/cm2 in the threeelectrode system.This high capacitance can be attributed to the special structure of freestanding Ni mesh electrode and the synergistic effect with the electrochemically active materials of MXene and PPy,which provide conductive paths for the electrode and promote the ion and electron transport and good electrochemical behavior,providing a reference for the construction and preparation of flexible transparent high-capacity MXene/polymer mesh supercapacitor electrodes.In summary,this dissertation focuses on the design and preparation of MXene based electrode with high electrochemical performance and composite thin film electrodes,and explores the possibility of their application in flexible transparent solid-state supercapacitors.This study provides a reference for expanding the application of MXenebased electrode in flexible wearable supercapacitors.