Study On The Effect Of Sulfide In Precursor On The Capacitance Performance Of Supercapacitor Activated Carbon Electrode

With the rapid development of science and technology, more and more attention was paid to the growing demand of high performance power supply. Supercapacitor, a new type of energy storage element, has become one of the research focuses in the field of new energy resource around the world. At present, the researches mainly focus on the development of high-performance electrode materials. Activated carbon (AC) electrode material is the firstly investigated and commercially produced due to its advantage such as good electrical conductivity, high surface area, large capacitance, long cycling life, good electrochemical stability in various acid and alkali solutions, relatively low cost and abundant raw materials. It is well-known that mineral substances and their derivatives (such as petroleum coke, coal, etc), a type of high carbon content, abundant and low-priced raw materials, are regarded as good and important precursors for preparing AC.In this work, dibenzothiophene (DBT) and pyrite (FeS2), the representative of organic and inorganic sulfide compound, were added into pitch and petroleum coke to prepare analogous sulfur-containing precursors (ASCPs) with different sulfur contents, correspondingly. AC as supercapacitor electrode material was prepared by ASCPs using a high-temperature KOH activation treatment. We systematically investigated the effect of sulfide on the structure and capacitive performance of AC by the single factor analysis. In addition, the sulfur content and morphological, surface functional groups of AC are tested by a variety of means, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), ion chromatography, element analysis and so on. The pore structure and specific surface area of AC are measured by scanning electron microscope (SEM), transmission electron microscope (TEM) and low temperature N2physical sorption. The electrochemical properties of AC are tested by electrochemical methods, such as cycile voltammetry (CV), galvanostatic charge and discharge (GCD), alternating-current impedance (EIS) and so on. According to the capacitance performance of AC electrode, we explored the influence law and mechanism of sulfide on the electrochemical behavior of AC. The main contents and results are as follows:(1) Effect of organic sulfide DBT in the precursor on the pore structure and capacitance performance of prepared AC. According to the sulfur analyses, the DBT in the precursors can react with KOH and produce K2S and K2SO4during activation process, leading to the decrease of actual KOH/char ratio. The pore texture analysis of ACs shows that the DBT in precursor can cause the decrease of the BET specific surface area, total pore volume, micropore specific surface area, mesopore specific surface area and mean pore size of resultant AC samples. Electrochemical measurements demonstrated that the specific capacitance, power performance and rate capability of ACs also deteriorate with the increase of DBT content in the precursors. As to quantitative analysis, the negative influence of DBT on the pore structure and capacitive performance of ACs becomes larger with the rise of DBT content in the ASCPs. This could be explained by the fact that more KOH are consumed by the DBT during chemical activation. However, there is no residual sulfur found in the resultant ACs regardless of various DBT content in the ASCPs. It can be concluded that the KOH consumption resulted from DBT reaction in precursor is the most important negative reason, leading to insufficient activation, less developed porosity and decreasing capacitance performance of AC. Experimental data clarified that if the KOH dosage is increased enough to compensate the KOH amount consumed by DBT, the cheap DBT-containing minerals with much low price can be potentially employed to prepare high quality AC for supercapacitor.(2) Effect of inorganic sulfide FeS2in the precursor on the pore structure and capacitance performance of prepared AC. As evidenced by the sulfur analyses, the FeS2in ASCPs can react with KOH to produce Fe3O4, K2SO4, K2SO3, K2S2O3, K2S and organic sulfur thioether (C-S-C) during activation process, which leads to actual KOH/PC gradually decreased. And, eventually, the residual sulfur in the resultant AC exists in the form of sulfate and sulfur-containing function group. Pore texture analysis reveals that the FeS2in ASCPs make the porosity undeveloped, such as BET specific surface area, total pore volume, mesopore specific surface area and volume, micropore specific surface area and volume, mean pore size. Electrochemical measurements demonstrated that the increase of FeS2content in ASCPs lead to a decrease in the capacitance performance, rate capability, cycle stability and a increase of impedance and leakage current. These results can be interpreted by the fact that FeS2in ASCPs reacts with and consume the activation agent KOH, leading to the insufficient activation, less developed pore structure, and deteriorating capacitance performance of the resultant AC. In addition, the by-product Fe3O4exists in AC sample as impurities, reducing the mass specific capacitance and increasing the leakage current of AC sample. However, of special interest is the fact that the FeS2in ASCPs can be partially converted into organic sulfur-containing group thioether C-S-C as a basic part of resultant AC, enhancing specific capacitance to some extent. In general, the FeS2in ASCPs impose obviously negative influence on capacitance performance of AC electrode. Furthermore, it is difficult to alleviate or eliminate the negative influence by increasing KOH amounts, compared with organic sulfur DBT. As a consequence, for practical application, pyrite FeS2content in mineral precursors for preparing supercapacitor AC electrode materials should be strictly controlled as low as possible in order to reduce its negative influence.