Fabrication Of Phosphorus Doped Polyacrylonitrile-Based Carbons And Their Applications In Supercapacitors

The introduction of heteroatoms into carbon framework or onto the surface of carbon is a simple and effective method to increase the capacitance of carbons by introducing pseudocapacitance through reversible redox reactions of heteroatom groups. Nitrogen doped carbons have been attracted much attention due to their outstanding electrochemical performance. Recently, phosphorus doped carbons present high specific capacitance and widened working voltage, thus attracting more and more interests. It seems to be very promising that nitrogen/phosphorus co-doped carbon will be a high-performance electrode material, but only a few papers reported nitrogen/phosphorus co-doped carbons for supercapacitors. Although the mechanism of nitrogen groups in enhancing the capacitance of carbons is known, but it is unclear for phosphorus groups. Thus, it makes great sense to identify the true role of phosphorus groups in enhancing the capacitance of carbons. On the other hand, the porosity of porous carbons makes it complex to study the role of phosphorus groups, so nonporous carbons decorated with heteroatom groups are highly expected. Also, nonporous carbons will produce high volumetric capacitance, which is very important for their practical use. In his paper, the nitrogen/phosphorus co-doped carbons and nitrogen-doped carbons were prepared; the electrochemical performance of the nitrogen/phosphorus co-doped carbons and nitrogen-doped carbons were:omprehensively investigated; the mechanism of phosphorus groups in enhancing the capacitance of carbons is discussed.(1) Nitrogen/phosphorus co-doped porous carbons were fabricated by solvent evaporation and subsequent pre-oxidation and carbonization at different carbonization temperature. Nitrogen adsorption and X-ray photoelectron spectroscopy analyses showed that the specific surface area and heteroatom content are highly dependent on the carbonization temperature. Electrochemical measurements showed that nitrogen/phosphorus co-doped porous carbons prepared at800℃possessed the best electrochemical properties with the specific capacitance and volumetric capacitance reaching as high as205.7F/g and261F/cm3at0.5A/g, respectively. The outstanding capacitance can be ascribed to the synergetic effect of nitrogen and phosphorus groups.(2) Nitrogen/phosphorus co-doped nonporous carbon nanofibers with different contents of phosphorus were prepared by electrospinning and subsequent pre-oxidation and carbonization at800℃. Nitrogen adsorption analysis showed that nitrogen/phosphorus co-doped nonporous carbon nanofibers exhibited nonporous characteristics. Electrochemical measurements showed that the sample with the phosphorus content of～9 wt.%presented the highest specific and volumetric capacitances, corresponding to224.9F/g and265F/cm3at0.5A/g, respectively.(3) The capacitive behaviors of the nitrogen/phosphorus co-doped nonporous carbon nanofibers and the pure nitrogen-doped nonporous carbon nanofibers were comprehensively and comparatively investigated in different electrolytes (1M H2SO4,0.5M Li2SO4,1M Na2SO4and0.5M K2SO4) in order to study the true role of phosphorus groups in enhancing the capacitance of carbons. The analyses showed that the phosphorus groups are very attractive to the electrolyte ions, especially the protons, thus greatly enhancing the electric double layer capacitance.