Preparation And Capacitive Performance Of Porous Carbons

As new energy storge devices, supercapacitors have received extensive attention because of their unique including long cycle life, superior reversibility, and high energy and power density. In general, the electrode material, as an important part of the supercapacitor, is crucial to ensure a good performance of the supereapacitor. Among the exploration of supercapacitor electrode materials, porous carbon materials are considered as the main candidate for supercapacitors in terms of large surface area, high conductivity, suitable pore size distribution and regular structure, long-term cycle stablity, as well as electrochemical reproducibility.Based on the consideration, a series of carbon materials with different surface areas and pore structures were prepared by steam activation from the industrial pyrolytic char, tenplate-solvothermal method and tenplate-solvent evaporation method. And the electrochemical performances of the carbons as supercapacitor electrode materials were investigated in6M KOH electrolyte. The main results are shown as follows:(1) Activated carbons (ACs) derived from the industrial pyrolytic tire char were prepared by steam activation and first evaluated as potential electrode materials for supercapacitor. With the activation temperature and time increasing, the surface areas and pore volumes of ACs increase gradually, while the micropore volume decreases. This suggests a continuous widening of pores featured in the development of mesopores and macropore at the expense of micropores and surface area. Base on the yield and textural properties, the ACs obtained at800℃for4h (AC-800-4) and850℃for2h (AC-850-2) are selected to study the electrochemical performances as supercapacitor electrode material. The results demonstrate that the obtained ACs display good eletrochemical properties. Compared to the AC-850-2electrode, the AC-800-4electrode has a higher capacitance (110F g-1), lower equivalent series resistance (0.34Ω), but processes a larger charge transfer resistance and time constant, which could be attributed to less pore size and higher content of oxygen-containing functional groups.The AC-800-4was further modified by concentrated nitric acid (labeled as m-AC) and then used as electrode material for supercapacitors. It is found that the morphology, porous texture and localized graphitic structure for the AC-800-4and m-AC have little difference, but the oxygen content increases and the functional groups change after acid treatment. The electrochemical results demonstrate that the m-AC electrode displays higher specific capacitance (140F g-1), better stability and cycling performance, which can be attributed to the increase of surface oxygen-containing functional groups introduced by acid treatment. These groups can improve the wettability of the electrode to enhance the utilization efficiency of the surface area and bring stable pseudocapacitance, leading to a notable improvement of the specific capacitance and the cycle performance.(2) Carbon hollow-spheres with different structure and surface chemical properties were prepared by hydrothermal treatment directly using colloidal silica as template, glucose and phenolic resol as carbon source (marked as g-CHS and p-CHS), respectively.The carbon hollow-spheres using glucose as carbon source (g-CHS) have regular shape, uniform size, smooth surface and homogeneous dispersion. In the meantime, the as-prepared sample possesses a hierarchical porous structure with micropore shell and meso/macropore core and moderate oxygen functional groups. While, the carbon hollow-spheres using phenolic resol as carbon source (p-CHS) have rough surface, nonuniform wall thickness and serious partical agglomeration. Compared with the g-CHS, the p-CHS have more micropore structure, lower degree of graphitization and higher oxygen content.On the comparison study for the electrochemical performances as supercapacitor electrode materials, it shows that the g-CHS electrodes possess higher specific capacitance (about266F g-1at1A g-1in KOH aqueous electrolyte), longer cyclic life and more excellent rate capability, which can be attributed to the higher ratio of mesopores/macropores, facilitating fast ion diffusion and offering a good charge accommodation. Furthermore, the p-CHS show the characteristics of the hybrid capacitor due to the high oxygen content, which enhance the specific capacitance but negatively influence the cycle stability.(3) Honeycomb hierarchical porous carbon was prepared by an easy solvent evaporation method directly using colloidal silica as template, glucose as carbon source (marked as g-HHPC). The as-prepared sample possesses three-dimensionally honeycomb-like sturcture with interconnected meso/macropore and abundant micropore, low degree of graphitization and moderate oxygen functional groups.Compared with the g-CHS with similar specific surface area, pore volume and degree of graphitization, the obtained HHPC as an electrode material exhibits better capacitive performances with higher specific capacitances (about292F g-1and44.5μF cm-2at1A g-1in KOH aqueous electrolyte), better electrochemical stability and rate capability, and less time constant, which can be mainly attributed to the unique three-dimensional hierarchical porous structure, not only promoting the mass transfer/diffusion of ions into the pores effectively but also offering a good environment for charge accumulation. While, the higher capacitance loss of22%could be due to the slow reduction of functional groups during cycling in a non-reversible way.In summary, the electrochemical properties of the electrode materials depend upon several parameters, for instance, the porous structure and the surface chemical properties. And the effects of these parameters are interlinked and revealed in combination. For the porous structure, mesopores and macropores can promote the mass transfer/diffusion of ions into the pores, while micropores can yield a higher surface area for charge accumulation. For the surface oxygen-containing functional groups, on the one hand, these groups can improve the wettability of the electrode to enhance the utilization efficiency of the surface area and bring pseudocapacitance, leading to a notable improvement of the capacitive performance. On the other hand, the introduced functional groups may result in pore obstruction and increase the charge transfer resistance due to the redox reaction of the functional groups.