Preparation Of Asphaltene-based Heteroatom Co-doped Porous Carbons And Their Electrochemical Performance For Supercapacitors

Porous carbons which are characterized by high surface area and excellent physico-chemical stability and they are considered as an excellent candidate for electrode, but the energy density is rekatively low, which is preventing its application in large scare. To incorporate heteroatom into carbon materials can introduce pseudocapacitive effects and then improve the energy density. Lots of researchs are focusing on the electrode material of supercapacitors, carbon materials are synthesized from different carbon precursor though different methods and using heavy organic matters as carbon precursor are considered as a more significant method. In this paper, we synthesized of amphiphilic carbon material (ACM) from asphaltene using HNO3/H2SO4as oxidizing agent and then synthesized B/N co-doped porous carbons and P/N co-doped porous carbons. The morphology was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystal structure was studied by X-ray diffraction (XRD) and Raman spectroscopy. The pore-structure was studied by nitrogen adsorption, the composition was studied by element analysis, inductively coupled plasma (ICP) and X-ray photoelectron spectroscopy (XPS) and the electrochemical performance was examined by an electrochemical workstation and cell test system.The major studies and results are showing below,(1) ACM was synthesized from asphaltene which was extracted from coal liquefaction residue using HNO3/H2SO4as oxidizing agent. Results indicate that there are much of hydrophilic functional groups such as-OH,-COOH and nitrogen functional groups such as-NO2in ACM, the former will improve the dispersibility of ACM in aqueous solution and the latter makes it a superior nitrogen precursor to prepare N-doped carbon materials.(2) B/N co-doped porous carbons have been synthesized using as preprared ACM as carbon and nitrogen precursor, H3BO3as a boron source and a pore-forming agent. The influence of the ratio of H3BO3/ACM and the heat treatment temperature on the pore-structure and surface chemical properties was investigated, the electrochemical performance in relation to the pore-structure and surface chemical properties was discussed, and the preparation process mechanism of ACM based B/N co-doped porous carbons was discussed. The results indicated that the crystal structure, pore-structure and surface properties were influenced significantly by the ratio of H3BO3/ACM and the heat treatment process. With the increase of the ratio of H3BO3/ACM and heat treatment temperature, the specific surface area and total pore volume increases gradually and then decline, but the content of mesopore and average pore size keep increasing, the content of nitrogen always decline and the variation tendency of boron content is on the contrary. The sample synthesis at the ratio of H3BO3/ACM is5and the heat treatment temperature is900℃, which with specific surface area of1103m2·g-1, pore volume of0.921cm3·g-1, nitrogen content of5.256wt.%, boron content of1.703wt.%get the the maximal specific capacitance of349F·g-1at100mA·g-1in6M aqueous solution of KOH. However, the sample subjected to a heat treatment at1000℃had the best rate capability, with a capacity retention of75%when the current density increased from100mA·g-1to10A·g-1. The abundant pore-structure, suitable surface chemical properties and superior electrochemical performance of B/N co-doped porous carbons are attributed to the excellent characteristics of ACM.(3) P/N co-doped porous carbons have been synthesized using as preprared ACM as carbon and nitrogen precursor, H3PO4as a phosphorus source and an activated agent. The influence of the ratio of H3PO4/ACM on the pore-structure and surface chemical properties was investigated and the electrochemical performance in relation to the pore-structure and surface chemical properties was discussed. The results indicated that the pore-structure of porous carbon was abundant significantly after the H3PO4activated process, with the increase of the ratio of H3PO4/ACM, the specific surface area and micropore volume increases gradually, but the total pore volume and the content of mesopore increases gradually and then decline, the content of nitrogen and phosphorus always decline and the variation tendency of oxygen content is on the contrary. The sample synthesis at the ratio of H3PO4/ACM is4, which with specific surface area of1914m2·g-1, pore volume of1.166cm3·g-1, nitrogen content of1.14at.%and phosphorus content of0.27at.%shows superior electrochemical performance, the maximal specific capacitance is296F·g-1at100mA·g-1in1M aqueous solution of H2SO4. It is also shows best rate capability, with a capacity retention of76%when the current density increased from100mA·g-1to30A·g-1.