Study Of Electrochemical Energy Storage Properties Of Carbon Fiber Composite Cobalt-Based Oxide/Phosphide

Recently years,with the perpetual evolution of portable electronic devices and the rapid progress of new energy vehicle industry,the performance requirements of energy storage materials are becoming more and more demanding.As a new type of energy storage device,supercapacitor,with its advantages of fast charging and discharging capability,high power density and long cycle life,has become a hot spot for research in the field of energy storage.Among many materials,cobalt-based oxides/phosphides have the characteristics of low cost,good cycle stability and no pollution to the environment,but they still have many problems in practical applications.In this paper,with the preparation of supercapacitor electrode materials for excellent electrochemical properties as the starting point,one-dimensional nanofibers cobalt-based oxide（phosphide）/carbon composites were prepared by electrospinning method.The carbon fibers provided the stable microstructure for the electrode materials,increased the specific surface area,reduced the internal resistance,shortened the distance between electrons conduction and ions diffusion,significantly improved the specific capacitance and the rate capability,and played an important role in enhancing the cycle stability.The structure,morphology,and electrochemical energy storage properties were studied in depth by X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy,transmission electron microscopy,laser Raman spectroscopy,cyclic voltammetry,galvanostatic charge/discharge,electrochemical impedance spectroscopy and cycle life tests.The specific studies are as follows.1.Preparation and electrochemical energy storage characteristics of manganese cobaltate/carbon nanofibersMn Co₂O₄/C precursor yarns were prepared by electrospinning,Mn（CH₃COO）₂·4H₂O and Co（CH₃COO）₂·4H₂O as the manganese and cobalt sources,and PAN and PVP as the carbon source and pore-making agent,with three concentrations precursor solutions of 5 wt%,10 wt%and 15 wt%,respectively.By comparing the spinning process and the morphology of the samples,it was determined that the spinning process of the precursor solution of 10 wt%concentration was smooth and the morphology of the obtained samples were as expected.The Mn Co₂O₄/C composites were successfully prepared by the three-step temperature-controlled annealing method,characterized in terms of morphology and structure,and the working electrodes were prepared by the conventional coating method.After analysis,it could be judged that the annealing temperature as well as the microscopic morphology affected the electrochemical properties of the materials.The Mn Co₂O₄/C electrode material was prepared at 600°C,and it could reach a specific capacitance of202 F/g at a current density of 1 A/g,which was higher than that of the composites obtained at 500°C and 700°C（167 F/g and 129 F/g at a current density of 1 A/g,respectively）.Compared with Mn Co₂O₄ nanoparticles,Mn Co₂O₄/C nanofibers had a large number of active sites and prevented particles agglomeration after multiple redox reactions.And they had a crucial effect on improving the electrochemical properties,reflected in the cycle life test.After 5000 charge/discharge cycles at a current density of 10 A/g,99.97%of the initial capacity could still be maintained.2.Preparation and electrochemical energy storage characteristics of cobalt nickel phosphide/carbon nanofibersNi Cl₂·6H₂O,Co Cl₂·6H₂O and phenylphosphonic acid were used as nickel,cobalt and phosphorus sources,respectively.The spinning precursor solution with a concentration of 10 wt%was configured,and Ni Co P/C composites were successfully prepared by electrospinning.Since the phosphorus source was directly added to the precursor solution,there was no PH₃ gas was emitted in the calcination process and no pollution to the environment.After the morphological characterization and electrochemical testing of the Ni Co P/C composites,it was found that increased the inorganic salt concentration not only did not enhance the energy storage capacity,but also damaged the nanofibers structure.After XRD test,it could be learned that the crystallization temperature of Ni Co P should be higher than 700°C,otherwise,the pure Ni Co P materials would not be obtained.The Raman test proved that there were a large number of defective structures on the surface of carbon fibers,which increased the specific surface area of the materials,reduced the internal resistance,and facilitated the rapid conduction of electrons and ions.The specific capacitance of Ni Co P/C obtained at 700°C could reach 290 F/g at a current density of 1 A/g,and could be maintained at67.6%of 1 A/g at a current density of 10 A/g.The Ni Co P particles were highly dispersed in the carbon nanofibers,and the carbon nanofibers effectively inhibited the destruction of the crystal structure of Ni Co P particles after multiple charge/discharge,and the synergistic effect of the two results in the loss of only 0.17%of the original specific capacitance after 5000 charge/discharge cycles at a current density of 10 A/g,showing excellent cycle stability.In summary,cobalt-based oxide/phosphide as supercapacitor electrode materials is a good choice.The preparation of composite nanomaterials by electrospinning is a simple process,low cost,adjustable structure,and the electrode materials have excellent electrochemical properties,which have broad application prospects in the field of supercapacitors.