Study On Synthesizing Functional Carbon Material Base On Biomass And Its Application For Supercapacitors

Supercapacitor is an important energy storage device and its electrochemical properties of the electrode material basically determine its capacity. Besides, biomass is one of the most important renewable energies and rationally using it to replace the fossil energy could alleviate environmental degradation and global warming to some extent. However, in our country, the utilization rate of biomass is not high at present. Many biomass, such as straws, corn stalks and corn cobs, were burned or abandoned, causing resources waste and environment pollution. Although the compositions of various biomass are different, their basic elements are similar. For example, plant biomass mainly contains cellulose, hemicellulose, lignin, plant protein, plant lipid and so on, and substantially consists of carbon, hydrogen, oxygen, nitrogen, sulfur, especially carbon occupying the main element. Therefore, utilizing biomass to prepare cost-effective carbon materials for the electrode of supercapacitor not only improves the utilization rate of renewable energy but also provides a promising way to solve the key problems of energy storage technology. In this essay, corn cobs were selected to synthesize supercapacitor electrode material with high capacitance and stable performance, which is a promising material for high-power supercapacitor energy storage device. The characterization results show that this nitrogen doping carbon nanocomposite owns a three-dimensional pore-network structure, loaded with transition metal nanoparticles. The pore-network structure of the carbon nanomaterial provides high specific surface and good electrical conductivity, and the doped nitrogen atom greatly improves the charge distribution and electron transfer. Meanwhile, transition metal oxide possesses a certain pseudocapacitance to enhance the capacitive property of the material. The results obtained are as follows:(1) The corncobs activated carbon nanomaterial was prepared by one-step calcining method. The optimal heating rate, annealing temperature and holding time were determined by regulating the temperature program. The optimized sample was measured with nitrogen adsorption-desorption test to characterize its specific surface area and pore size distribution, and the results show that its specific surface area reaches as high as 1055 m2 g-1 while the pore size distribution uniformly concentrates at 4-5 nm. Moreover, scanning electron microscopy (SEM) and X-ray diffractometer (XRD) were used to characterize its morphology and composition, finding that the material possesses a three-dimensional pore-network structure and is uniformly loaded with Fe2O3 nanoparticles. Electrochemical capacitance performance was tested in 6 M KOH solution using electrochemical workstation with cyclic voltammetry and chronopotentiometry, and the result demonstrates that the capacitance is 345 F g-1 at a current density of 1A g-1.(2) Melamine served as a nitrogen precursor, was mixed with corncobs powder and then calcined together to prepare nitrogen-doped functional carbon nanomaterial. The calcining temperature, calcining time and melamine ratio were adjusted to determine the optim?m condition. The sample prepared under the optimized condition was characterized by a BET test, and the results show that the specific surface area is up to 1100 m2 g-1 while the pore size distributes between 40-50 nm. Further, XRD, X-ray photoelectron spectroscopy (XPS), SEM, high resolution transmission electron microscopy (HRTEM) and mapping were used to characterize its morphology and composition. The results display that the nitrogen-doped carbon nanomaterial shows a pore-network structure with plenty of thin-walled carbon nanotubes growing on the surface of mesoporous carbon and some spherical particles attaching on the port of nanotubes. This carbon nanomaterial is made up of four main elements (C, N, O and Fe) and few S element. The nanotubes consist of two elements (C and N) and the spherical particles on the port are Fe2O3 nanoparticles. The electrochemical performance was also tested in 6 M KOH solution, and the result shows that the capacitance value is up to 579 F g-1.