Preparation Of MXene-based Composite Materials And Research On Capacitance Performance

Energy and environmental issues have become the major challenge for human beings.With the rapid development of portable electronic products,clean energy,microelectronic equipment and electric vehicles,People's requirements for energy storage devices are getting higher and higher.Supercapacitors?SC?have received increasing attention due to their high power and energy density,excellent charge and discharge characteristics and excellent recyclability.In recent years,two-dimensional layered materials have great application potential in the field of supercapacitors due to their large specific surface area,good electrical conductivity,long cycle life and a large number of surface-active sites.MXene is a new type of two-dimensional materials.It is a new family of two-dimensional transition metal carbides and/or nitrides.The general formula is M_n+1X_nT_x,where"M"stands for early transition metals and"X"stands for C and/or N,“T_x”represents surface functional groups including O,OH and F,n=1,2 or 3.MXene has good conductivity,high specific surface area and excellent electrochemical performance,making it widely used in the direction of electrochemical energy storage.Nevertheless,some intrinsic properties of MXene such as being easy to be oxidized and layer-stacking aggregation severely limit the accessibility of electrolyte ions,leading to compromised energy density and specific capacitance.To solve these problems,great efforts have been devoted to the design and prepare hybrid nanocomposites with different hierarchical structures to enhance the electrical conductivity and consequently improve the electrochemical properties.In this work,we design and prepare a series of Co₃O₄ doped MXene/RGO hybrid porous aerogels through facile in-situ reduction and thermal annealing process.Ti₃C₂T_x MXene is prepared through the etching of Ti₃AlC₂?MAX?in the presence of HF acid and the graphene oxide?GO?is prepared through a modified Hummers'method.The prepared Ti₃C₂T_x MXene is then applied as the structural substrate to load Co₃O₄ nanoparticles,followed by mixing with GO to obtain Co₃O₄-MXene/RGO hybrid porous aerogels after the freeze-drying process and thermal calcination at 600 ^oC under nitrogen atmosphere.We find that when the Co₃O₄-MXene/RGO hybrid porous aerogel prepared with the mass ratio of Co₃O₄-MXene:RGO at 3:1?CMR31?is utilized as electrode for supercapacitor,the superior specific capacitance of 345 F g^-1 at the current density of 1 A g^-1 is achieved,which is significantly higher than those of Ti₃C₂T_x MXene,RGO and MXene/RGO electrodes.In addition,a high capacitance retention(85%of the initial capacitance after 10,000 cycles at a high current density of 3 A g^-1)and a low internal resistance R_s?0.44??can be achieved.Additionally,an all-solid-state asymmetric supercapacitor is assembled using the CMR31and has the ability to light up a blue LED indicator for 5 min when four ASCs are connected in series.Therefore,these novel Co₃O₄-MXene/RGO hybrid porous aerogels should envision potential practical applications in high energy storage devices.In two-dimensional nanomaterials,the electrochemical performance of MXene nanosheets is limited because being easy to aggregate or re-stack,which hinders the penetration and diffusion of ions in the electrolyte.The large and low-defect single-layer MXene nanosheet network structure can more improve the conductivity of the material more effectively,thereby further improving the electrochemical performance.In order to make full use of MXene nanosheets for energy storage,a series of methods and strategies to prevent re-stacking were proposed.Here,we have seriously designed a simple method for preparing Ti₃C₂T_x MXene foam without any template.The porous Ti₃C₂T_x MXene foam can effectively prevent the re-stacking of MXene nanosheets and shorten the diffusion distance of ions.Therefore,the MXene foam electrode shows improved electrochemical performance,with high surface capacitance,good rate performance and stable cycle life.We used a simple thermal reduction process to prepare a light,flexible and hydrophobic MXene foam,which has high strength,high conductivity and excellent electrochemical performance.The surface wettability of MXene film has successfully changed from typical hydrophilicity to hydrophobicity,resulting in excellent water resistance and good durability.In addition,the advantageous porous structure shortens the diffusion distance of electrolyte ions and accelerates the electron transfer rate during charging and discharging,so it has better electrochemical performance.We found that when the MXene foam added with 100?L hydrazine is used as the electrode of supercapacitor,an ultra-high surface capacitance of 256 mF cm^-2 is achieved at a current density of 1 mA cm^-2,which is much higher than that of MXene film and the MXene foam added with 50?L hydrazine electrode.In addition,a high capacitance retention rate(the capacitance can maintain 91.7%of the initial capacitance after 6,000 cycles at a high current density of 10 mA cm^-2)and a low internal resistance Rs?0.64??.This new method provides new ideas and methods for the subsequent application of MXene in the direction of supercapacitors,and expands the horizon for the application of MXene in other hydrophobic directions and improves the availability of MXene.