Preparation and Electrochemical Performance of Polycrystalline Mesoporous Co3O4 Nanosheets/N-Rgo as Superior Materials for Electrochemical Energy Storage Systems

Introduction

     In the last decade, two-dimensional (2D) inorganic nanosheets have constituted an important domain of the nanostructure, its unique structure, intriguing property and potential application which differ from those of the other bulk-state materials ^1-2. Graphene is 2D crystalline form of carbon possesses unique properties. The presence of heteroatoms at the carbon surface can enhance the reactivity and electric conductivity. Porous nanostructure and incorporation of heteroatoms are both desirable for Li⁺ ion storage ³. The mesoporous nature in Co₃O₄ nanosheets provides extra space for Li⁺ storage and significantly reduces paths for both Li⁺ ion and electron diffusion^1-2. Notably, the graphene shells not only act as buffers to accommodate the volume expansion of Co₃O₄but also serve as the reliable conductive channels of the electrode.

    In this work, self-supported 2D Mesoporous Co₃O₄ nanosheets (NS) and N-Graphene (N-rGO) have been successfully synthesized through a facile hydrothermal method to display single crystalline. The mesoporous Co₃O₄ NS/N-rGO composite were prepared by an infiltration method, the morphology, crystal structure, and electrochemical properties of Co₃O₄ nanosheets with graphene composites were systematically investigated in detail.

Experimental

     Mesoporous Co₃O₄ nanosheets was synthesized by using 1g CoCl₂. 6 H₂O, and 4g Urea were dissolved in water then 4g of Polyvinylpyrrolidone, The resulting suspension was transferred into a Teflon- lined stainless steel autoclave, tightly sealed and heated at a temperature of 120 ^oC in an electric oven for 24h and the precipitates were filtered and washed thoroughly with water and ethanol, Finally, the above precursors were calcined in a tube furnace at 400 ^oC for 3h in air to obtain the final product. GO was synthesized from graphite flake using well reported   modified Hummer method ⁴. Chemical reduction of GO solution was achieved using ammonia and hydrazine hydrate (N₂H₄-64-65 %) as reducing agents under a hydrothermal environment ⁵. The mesoporous Co₃O₄NS/N-rGO composite were prepared by an infiltration method

     The electrochemical characterizations were performed using CR2032 coin-type cell. The test cell was made of a cathode and a lithium metal anode separated by a porous polypropylene film. The electrolyte used was a mixture of 1M LiPF₆-EC/DMC (1:1 by vol.). The charge/discharge current density was 0.2 mA/cm²with a voltage of 0 to 3 V at room temperature.

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Results and Discussion

     Fig. 1a is the high-magnification SEM image to demonstrate the uniformity and regularity in the ultimate Co₃O₄NS /N-rGO . The selected area electron diffraction (SAED) pattern (inset in figure 1b) and high resolution TEM (HRTEM) image (figure 1b) clearly demonstrate the well-textured and single crystalline nature, Co₃O₄ NPs are attached to each other by using carbon, The electrochemical performance of the as-prepared  Co₃O4 NS/N-rGO composite and Co₃O₄ NS was first evaluated by Galvanostatic charge/discharge cycling at a current density of 80 mA g^-1(fig-2). The Co₃O₄ NS/NrGO (1523 mAh g^-1) and bare Co₃O₄ NS (1327 mAh g^-1) recover its original capacity or even little bit higher for the 50^th cycle. The Co₃O₄ NS/N-rGO composite exhibits much better rate capability compared to the Co₃O₄ NS electrode operated at various rates between 80 and 2000 mA g^-1 (Figure 3). 

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