Design And Preparation Of MnO₂ Nanomaterials And Their Composites With High Cycle Stability

Due to the increasing demand for clean,sustainable energy,portable electrical equipment and hybrid vehicles in society,in order to meet the hard requirements of high power and long life of equipment,supercapacitors are expected to become a breakthrough,and the types and structures of supercapacitor electrode materials.It has a decisive influence on its electrochemical performance.At present,the bottleneck of supercapacitor development is its low energy density and poor cycle life.Therefore,it is an urgent problem to improve the cycle stability under the premise of ensuring higher capacitance of electrode materials.Transition metal oxide is a kind of electrode material with great research value.MnO₂ as a pseudocapacitance electrode has abundant reserves,low cost and large theoretical capacitance.As a battery type electrode material,Ni Co₂O₄ has high conductivity and has more electrochemical reaction sites.However,the transition metal oxide has insufficient cycle stability as an electrode material,which is caused by poor conductivity of the electrode material and shrinkage of the reactant volume during the redox reaction,and expansion causes the active material to fall off.Therefore,it is a very challenging task to prepare a novel and excellent electrode material by a simple and easy-to-operate synthesis method.In this paper,a simple hydrothermal synthesis method was used to design and prepare electrode materials with special morphology and excellentelectrochemical properties,which provided a new idea for the further development of supercapacitor electrode materials.The main research contents are as follows:1.Firstly,MnOOH/Mn₃O₄ nanocomposites with different morphologies were prepared by adjusting the hydrothermal reaction time.On the basis of this,MnO₂ nanomaterials were obtained by further oxidation.The belt-like MnO₂ nanomaterial,transformed by hydrothermal reaction for 1 hour,has a specific surface area of 135 m2 g-1.As an electrode material of supercapacitor,the specific capacitance of the belt-like MnO₂ is 193.5 F g-1?the current density is 1 A g-1?,after 20,000 cycles,the capacitance retention is 109%.Finally,a ribbon-type asymmetric supercapacitor with a voltage window of 2.0 V was assembled with belt-like MnO₂ as the positive electrode material and activated carbon as the negative electrode material.When the power density is 100 W kg^-1,the corresponding energy density is 33.8 Wh kg^-1.2.A sandwich structure of Ni Co₂O₄@MnO₂/nickel foam?NF?/MnO₂ electrode material is synthesized on a NF substrate by combination of hydrothermal conversion and oxidation process.This structure is highly stable during charge and discharge.Among them,the Ni Co₂O₄ nanowire has a good bonding force with the NF substrate,and the MnO₂,covering the surface of the Ni Co₂O₄ nanowire and the NF,has a good protection effect on the electrode material.The active material is prevented from falling off due to volume change during the redox reaction,thereby ensuring the stability of the electrode material.Therefore,this novel sandwich structure gives the composite a satisfactory electrochemical performance.Significantly,the Ni Co₂O₄@MnO₂/NF/MnO₂ electrode was used as a positive electrode,and An AC electrode was used as a negative electrode to assemble a hybrid supercapacitor.The highest energy density is 53.5 Wh kg^-1 and the maximum power density is 8 k W kg^-1.After50,000 cycles,the capacity retention rate of the hybrid supercapacitor is still above 80%.Proving that this is a promising energy storage device,and the created synthetic strategy could stimulate further capability on engineering novel hierarchical architecture for various device applications.