MnO 2 Micro-nanostructures Can Be Controlled And Their Electrochemical Properties Are Studied

MnO₂ is a promising electrode material for supercapacitors.Controlling synthesis of MnO₂ micro/nano structures has been received great attention in recent years.In this paper,template method with MnCO₃ templates was used to synthesize MnO₂ micro/nano structures,including micro hollow sphere,micro hollow cube,nanowire and nanolayer.The products were characterized by using X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,Field emission scanning electron microscopy?FESEM?,transmission electron microscopy?TEM?and Brunauer-Emmett-Teller?BET?.Electrochemical workstation and land cycler were carried to measure the electrochemical performance of the materials.Besides,the factors which effect morphologies,structures and electrochemical performance were discussed,and the formation mechanisms of the different MnO₂ micro/nano structures were studied.The specific research results are as follows:1.MnSO₄ and NaHCO₃ were used as raw materials.The evolution was based on the morphological evolution of MnCO₃ by controlling the concentration of MnSO₄ solution,which can be oxidized by KMnO₄ solution to form ?-MnO₂ shells and followed by the removal of residual inner MnCO₃ with HCl.Finally,the evolution of ?-MnO₂ from hollow cubes to hollow spheres was achieved.The evolution of ?-MnO₂ from hollow cubes to hollow spheres was based on the morphological evolution of MnCO₃ templates,which was controlled by the concentration of MnSO₄ solution.It has been found that,the initial specific capacitance at a current density of 1 A g-1 of the as-prepared ?-MnO₂ hollow spheres is 203 F g-1,which is higher than that of ?-MnO₂ hollow cubes?152 F g-1?.In addition,the ?-Mn O2 hollow cubes retain 93% and the ?-MnO₂ hollow spheres retain 80% of the initial specific capacitance after 2000 charge/discharge cycles at 2 A g-1.2.MnSO₄ and NaHCO₃ were used as raw materials to synthesize MnCO₃ spheres,and MnCO₃ cubes were synthesized when?NH4?2SO₄ was added the initial mixture.Two types of the as obtained MnCO₃ were oxidized by K2S2O8 solution to obtain ?-MnOOH nanowires in a certain temperature.The as-obtained ?-MnOOH nanowires were translated into MnO₂ nanowires in two different methods: one method was that the as-obtained ?-MnOOH nanowires was calcined in the air to synthesize ?-MnO₂ nanowires;the other was that the as-obtained ?-MnOOH nanowires were oxidized into ?-MnO₂ by KMnO₄ solution in a hydrothermal method with autoclaves.MnOOH nanowires were synthesized from MnCO₃ sphere and MnCO₃ cube templates,which was an in-situ transformation.The specific capacitance of ?-MnO₂ nanowires obtained from MnCO₃ sphere templates at the current density of 1 A g-1 is 71.4 F g-1,which is higher than that of ?-Mn O2 nanowires obtanined from the same templates?37.5 F g-1?.?-MnO₂ nanowires obtained from MnCO₃ cube templates had a specific capacitance of 87.5 F g-1,which is also higher than that of ?-MnO₂ nanowires obtanined from the same templates?38.7 F g-1?.By comparing the ?-and ?-MnO₂ obtained from the two templates,it proved that ?-MnO₂ has a better capacitance performance than ?-MnO₂.3.KMnO₄,C₆H₁₂O₆·H₂O and C₄H₆O₆ were used as raw materials,and MnCO₃ ellipsoid templates were synthesized in an autoclave.The as-synthesized MnCO₃ ellipsoid templates were oxidized by K2S2O8 solution to obtain MnO₂ nanoflakes in a certain temperature.It has been found that,MnO₂ nanoflakes were obtained by peeling off the as-synthesized MnCO₃ ellipsoid templates,which is an efficient method to obtain MnO₂ nanoflakes.The as-synthesized MnO₂ nanoflakes have a specific capacitance of 92.5 F g-1 at the current density of 1 A g-1 and 50 F g-1 at the current density of 5 A g-1,which showed good rate capacitance?54%?.Nyquist plot of the as-obtanied MnO₂ nanoflakes consists of a semicircleand a line corresponding the high frequency and low frequency region,which shows good capacitance performance and a promising electrode material for supercapacitors.