Synthesis And Capacitance Performance Of Manganese Oxide/Reduced Graphene Double-Shelled Hollow Microspheres

Supercapacitors are regarded as a new type of energy storage devices with high power density and long lifespan. The study of supercapacitor electrode material is very important for the development of supercapacitors. Manganese dioxide (MnO2) is considered as a good candidate of supercapacitor electrode material because of its electrochemical behavior, low cost, and environmental compatibility, therefore, it has very important applied research value. The electrochemical energy storage performance of electrode materials is closely related to the theoretical capacitance of material, the microstructure of electrode and the conductivity of material. As MnO2 has good theory Faraday capacitance, Graphene (RGO) owns excellent conductivity, we made MnO2/RGO double-shelled hollow microspheres, the larger surface area and good ionic transport can lead to a higher current density and facilitate the fast transport of electrolyte ions. This paper mainly studied the electrode material of Double-shelled Manganese dioxide/Graphene (Double-shelled MnO2/RGO), Double-shelled Na+intercalate into Manganese dioxide/Graphene (Double-shelled NaxMnO2/RGO), Li+intercalated into Manganese dioxide/Graphene (Double-shelled LixMnO2/RGO). The cyclic voltammetry (CV) measurement, the galvanostatic charge-discharge (GCD) measurement, the electrochemical impedance spectroscopy (EIS) measurement were used to study the electrochemical capacitance performance of these materials. The main work are listed as follows:(1) The preparation and electrochemical properties of Mn2/RGO double-shelled hollow microspheres.PS microsphere was used as a self-sacrificing template, Firstly, GO/PS core-shell microsphere was formed by coating GO layer on the surface of PS microsphere, Then, MnO2/RGO/PS core-shell-shell microsphere was synthesized through in-situ chemical reduction of KMnO4 using benzyl alcohol which was initially anchored to RGO/PS. Finally, MnO2/RGO double-shelled hollow microsphere was obtained by chemically etching PS microsphere from MnO2/RGO/PS using tetrahy drofuran (THF) solvent. The morphology, mocrostructure and electrochemical energy storage of MnO2/RGO double-shelled hollow microspheres have been investigated. The microstructure characterization shows that MnO2/RGO/PS core-shell-shell microsphere included a double-shelled layer of MnO2/RGO with a layer thickness of 210-270 nm and the core of PS microsphere with a diameter of 540-580 nm. As-prepared MnO2/RGO double-shelled hollow microsphere had a pore diameter of 500-580 nm. Dipped MnO2/RGO double-shelled hollow microsphere thick solution onto nickel foam (2.5 mgcm-2) to make electrode. The electrochemical measurement shows that the MnO2/RGO double-shelled hollow microsphere yielded a high specific capacitance of 450.1 F·g-1 at a current density of 0.2 A·g-1,153.2 F·g-1 at a current density of 10 A·g-1 at the potential window of 0-0.8 V in 0.5 M Na2SO4 solution, showing a poor rate capability. The power density was ascended from 0.08 to 4 W Kg-1 and the energy density accordingly declined from 40.1 to 11.4 Wh·Kg-1 when the current density was increased from 0.2 to 10 A·g-1. It kept high capacitance retention of 79.7% after 1000 cycles and good rate capability at a current density of 5 A·g-1. EIS measurement shows that MnO2/RGO double-shelled hollow microsphere material owns smaller Ro, Ret and Zw,, The Ztotal was 2.37 ?. It indicates that the well-design structure of double-shelled hollow microsphere could effectively improve the electrical conductivity and ion diffusion of MnO2/RGO electrode material and improve its capacitive performance, therefore, the rate capability of MnO2/RGO double-shelled material needs to be improved.(2) The preparation and electrochemical properties of NaxMnO2/RGO double-shelled hollow microspheres.RGO shell microspheres were painted onto pre-processed carbon paper (CP), in Mn(CH3COO)2, Na2SO4 solution, using shell RGO/CP as working electrode anodic deposited NaxMnO2 in a three electrode system, then NaxMnO2/RGO double-shelled hollow microsphere was obtained. The morphology, mocrostructure and electrochemical energy storage of NaxMnO2/RGO double-shelled hollow microspheres have been investigated. Raman spectra shows the feature of Na+can be recognized at 484 cm-1, Na+exists in NaxMnO2/RGO double-shelled composite materials. The electrochemical measurement shows that the NaxMnO2/RGO/CP has a high specific capacitance of 317.4 mF cm-2 at a current density of 1.0 mA cm"2. The NaxMnO2/RGO double-shelled composite material shows mass specific capacitance of 568.2 F g-1 at a current density of 0.2 A g-1.at the potential window of 0-0.8 V in 0.5 M Na2SO4 solution, and the mass capacitance was 568.2 F·g-1 to 252.5 F·g-1 when the current density was increased from 0.2 to 10 A·g-1, rate capability was 44.4%. It kept high capacitance retention of 83.4% after 1000 cycles and good rate capability at a high current density of 6.7 A·g-1. The rate capability and capacitive of NaxMnO2/RGO double-shelled material has been improved.An aqueous symmetric supercapacitor was constructed using NaxMnO2/RGO/CP electrode and Polyvinyl Alcohol-sodium sulfate (PVA-Na2SO4) solid-state gel electrolytes. The power density of NaxMn02/RGO/CP was improved from 0.5 to 10 kW kg"1, the energy density was reduced from 183.0 to 27.7 Wh kg-1 as current density was improved from 0.5 to 10 A g-1. A small supercapacitor with a size 10 mm X 10 mmX 1.1mm could light a LED.(3) The preparation and electrochemical properties of LixMnO2/RGO double-shelled hollow microspheres.RGO shell microspheres were painted onto pre-processed titanium nitride (TiN), in Mn(CH3COO)2, LiCCIO4 solution, using RGO/TiN as working electrode anodic deposited LixMnO2 in a three electrode system, then LixMnO2/RGO double-shelled hollow microsphere was obtained. The morphology, mocrostructure and electrochemical energy storage of LixMnO2/RGO shell hollow microspheres have been investigated. The SEM characterization indicates that the LixMnO2/RGO double-shelled microspheres were deposited on the mouth of ordered TiN nanotube arrays. The GCD measurement shows that the specific capacitance of shell LixMnO2/RGO/TiN was 430.2 mF cm-2 at a current density of 0.2 mA cm-2, at the potential window of 0-0.8 V in 0.5 M Li2SO4 solution, higher than shell LixMnO2/RGO/TiN (342.5 mF cm-2). The LixMnO2/RGO double-shelled composite material shows mass specific capacitance of 568.2 F g-1 at a current density of 0.2 A g-1. at the potential window of 0-0.8 V in 0.5 M Li2SO4 solution, and the mass capacitance was 671.3 F g-1 to 459.4 F·g-1 when the current density was increased from 0.2 to 10 A·g-1, rate capability was 31.2%. It kept high capacitance retention of 91.6% after 1000 cycles at a current density of 7.5 A g-1.Supercapacitor was constructed using LixMnO2/RGO/TiN electrode and Polyvinyl Alcohol-Lithium sulfate (PVA-Li2SO4) solid-state gel electrolytes. The power density of LixMnO2/RGO/TiN was improved from 0.2 to 10 kW kg-1, the energy density was reduced from 186 Wh·kg-1 to 29.6 Wh·kg-1 as current density was improved from 0.2 to 10 A g-1. The supercapacitor could light a LED very well.