Preparation And Electrochemical Performances Of Mxene / Metal Oxide Composites

To address the problems of energy demand and environment pollution,a variety of new energy technologies such as fuel cells,lithium-ion batteries and supercapacitors have emerged.Supercapacitors have high power density and long cycling stability,but their low energy density still cannot meet the requirements of commercialization,which greatly restricts their wide application.Therefore,it is ergent to develop high performance supercapacitors.Most of the electrode materials used in supercapacitors are carbon materials with low specific capacity.In this thesis,the anode pseudocapacitive material with high specific capacity is selected as the research object,which is combined with the highly conductive MXene to assemble the asymmetric supercapacitor(ASC).In addition,the electrochemical properties of electrodes were studied in detail.Fe₂O₃ nanoparticles were firstly synthesized by a solvothermal method,and then Fe₂O₃/MXene composite was prepared by simple physical mixing.As a result,the specific capacitance of the Fe₂O₃/MXene composite is 850.8 F·g^-1 at 0.5 A·g^-1.When the current density is increased to 50 A·g^-1,the specific capacitance can still retain 345.5 F·g^-1.In addition,the asymmetric supercapacitor(ASC)was fabricated with the previously synthesized nickel-cobalt layered double hydroxide/graphene(NiCo-LDH/RGO)as positive electrode and the Fe₂O₃/MXene composite as negative electrode.The ASC device achieves an energy density up to 89.2 Wh·kg^-1 at 678.0 W·kg^-1 and exhibits good cyclic stability(90.5%capacity retention after 10000 cycles).Co-doped Fe₃O₄(Co-Fe₃O₄)nanoparticles were prepared by a simple coprecipitation method,and then three-dimensional Co-Fe₃O₄/MXene composite was prepared by physical mixing method.The doped Co and MXene conductive substrate enable the Co-Fe₃O₄/MXene composite to achieve a specific capacitance up to 888.2 F·g^-1 at 0.5 A·g^-1.In addition,an ASC device was assembled with NiCo-LDH/RGO and Co-Fe₃O₄/MXene composite as the positive and negative electrode,respectively.The ASC exhbits excellent electrochemical performance,which can achieve an energy density of 74.7 Wh·kg^-1(volume energy density of 145.3 Wh·L^-1)and good cyclic stability(92.4%capacity retention after 10000 cycles).Co-Fe₃O₄nanoparticles also shows excellent electrochemical performance for lithium ion batteries,which can achieve a specific capacity of 1112 mAh·g^-1 and could retain 450 mAh·g^-1 at 10A·g^-1.The MoO₃ nanoribbons were prepared through a hydrothermal method.The internal and external conductivity of the MoO₃ nanoribbons was improved through the fabrication of vacancy by hydrazine hydrate and combination with MXene,leading to enhanced electrochemical performances.When the current density is 0.5 A·g^-1,the specific capacitance of MoO_3-x/MXene composite can reach 423.0 F·g^-1,and could maintain 242.9 F·g^-1 when the current density is increased to 50 A·g^-1.An ASC has been fabricated using the synthesized manganese dioxide/graphene nanosheets as the positive electrode and the MoO_3-x/MXene composite as the negative electrode.It shows high energy density(88.3 Wh·kg^-1)and good cycling stability(91.3%capacity retention after 10000 cycles).MoO_3-x/MXene composite also exhibits excellent lithium-ion storage performances,which achieves a high specific capacity of 1258 mAh·g^-1 at 0.1 A·g^-1 and 474 mAh·g^-1 can be remained at 10 A·g^-1.