Influence Of The Construction Of 3D Conductive Networks On The Energy Storage Of MnMoO₄

In order to promote clean energy,electric and hybrid vehicles,portable electronic devices,supercapacitors with high power density and good cycling stability have gradually become a hot research topic.At present,supercapacitors still have the disadvantage of relatively low energy density and cannot fully meet the demand for high performance energy storage devices.How to improve the energy density under the premise of ensuring the power density of devices is a huge challenge for supercapacitors.The energy density of supercapacitor is mainly affected by the reaction depth and the electronic conductivity of the electrode.Aiming at this problem,this thesis selects MnMoO₄?Manganese Molybdate?with good electronic conductivity as the electrode material and builds three-dimensional conductive networks to improve the low energy density of the supercapacitor.The main research contents are as follows:?1?CNF-MnMoO₄?three-dimensional structure of carbon nanofiber-MnMoO₄ composite?was prepared by electrospinning method to improve the electronic and ionic conductivity of the electrode.Compared with MnMoO₄ NP?MnMoO₄ nanoparticles?,the composite structure of CNF-MnMoO₄has smaller grain size and larger contact area with the electrolyte.Meanwhile,the CNF extra electron transport channel outside the MnMoO₄ particles allows faster electrons transfer between the active material and the current collector.Electrochemical tests show that the CNF-MnMoO₄electrode has a specific capacitance of 328.1 F g^-1 at the scan rate of 5 mV s^-1,which is 10%higher than that of MnMoO₄ NP.However,the CNF coated on the outside of the MnMoO₄ particles will prevent the contact of the active material with the electrolyte to some extent.Under the offset of the two aspects,this three-dimensional network has limited effect on improving electrochemical performance.?2?The MnMoO₄ nanosheet is grown on the surface of Nickel foam by hydrothermal method as a positive material to increase the contact area with the electrolyte and the bonding stability with the substrate.This method can fabricate a binder-free electrode,and the nanosheet array grown vertically on the substrate not only prevents stacking between the nanosheets but also ensures contact of the nanosheets with the electrolyte.The MnMoO₄ nanosheets has a specific capacitance of 1241 F g^-1 at the current density of 3 A g^-1,which is significantly improved compared with CNF-MnMoO₄.Commercial AC?activated carbon?was used as the negative electrode material to construct an aqueous asymmetric device with an energy density of 31.6 Wh kg^-1 at a power density of 935 W kg^-1.When this work was completed,it was the first reported MnMoO₄ based asymmetric device.The high specific capacitance of MnMoO₄ electrode,and high energy and power density of asymmetric device lay the foundation for the application of MnMoO₄ material in the supercapacitor,and promote its process in the practical application.?3?The CNT/MnMoO₄ composite electrode is constructed,and the active material mass loading of the electrode is increased under the premise of ensuring the specific.In order to further improve the electrochemical performance of MnMoO₄ electrode,a layer of CNTs?carbon nanotubes?is grown on the surface of Nickel foam by CVD method before hydrothermal.The specific surface area of substrate is increased by the CNT,and then a layer of MnMoO₄ nanosheet is grown on the surface by hydrothermal method.Compared with the MnMoO₄ nanosheets grown directly on the Nickel foam,the composite structure has a double mass loading and the specific capacitance reaches1150.37 F g^-1(the areal capacitance is 0.78 F cm^-2)at the scan rate of 2 mV s^-1,which is equivalent to the specific capacitance and nearly doubled areal capacitance(0.42 F cm^-2)of the MnMoO₄nanosheet electrode.This improved electrochemical performance is not only attribute to the expansion of the specific surface area of current collector by the CNT,but also because that CNT is used as an external electron high-speed transmission channel to increase the electron conductivity of the electrode.The CNT/MnMoO₄ composite electrode provides the possibility to further improve the electrochemical performance of MnMoO₄ electrode.?4?The three-dimensional structure of CNT/rGO/MnMoO₄ composite electrode and the CNT/rGO/MnMoO₄//AC asymmetric device were constructed to improve the energy density and power density of the device.This composite structure can improve the cycling stability of the electrode while ensuring that the specific capacitance and mass loading of the electrode are not reduced.The rGO?reduced graphene oxide?was introduced between CNT and MnMoO₄ nanosheets as an intermediary material.A more stable 3D conductive network could be established due to the advantages of good contact with CNTs and more defects?suitable for MnMoO₄ nanosheet growth?.Electrochemical tests show that the CNT/rGO/MnMoO₄ composite structure electrode has a specific capacitance of 2374.9 F g^-1 and areal capacitance of 1.59 F cm^-2 at the scan rate of 2 mV s^-1,which is more than doubled areal capacitance compared with the CNT/MnMoO₄ electrode and the cycling stability has been significantly improved.In order to match the high mass loading and specific capacitance with the positive electrode material,AC was sprayed onto the Nickel foam with CNT to obtain a CNT-AC negative electrode with improved electrochemical performance.The specific capacitance of CNT-AC reaches 234 F g^-1 at the current density of 5 mA cm^-2,which is double of the AC?AC coated on Nickel foam?.The aqueous asymmetric device CNT/rGO/MnMoO₄//CNT-AC was fabricated with the CNT/rGO/MnMoO₄ composite structure as the positive electrode and the CNT-AC as the negative electrode.When the power density is 1367.9 W kg^-1,the energy density is as high as 59.4 Wh kg^-1.Its energy density and power density are superior to most of the reported MnMoO₄ based supercapacitors.Therefore,the high performance of CNT/rGO/MnMoO₄//CNT-AC devices indicate that a reasonable three-dimensional conductive network can greatly increase the utilization rate of active materials,improve the performance of supercapacitors in an all-round way,and promote the application of supercapacitors.