Effect Of Sulfate And Elemental Sulfur In Supercooler Of Activated Carbon In Mineral Precursors

Supercapacitors, also known as electrochemical capacitors, as a promising energy storage device due to the advantages of fast charge-discharge, high power density and long cycle life, have been widely applied from portable electronic devices to hybrid electric vehicles. It is important to develop high performance electrode materials, among which activated carbon (AC) is considered as one of the most attractive electrode materials for its good electrical conductivity, large specific surface area, physicochemical stability, well-controlled pore structure and low price.Minerals, such as coal, petroleum coke (PC) and pitch, have been acted as good precursors to prepare AC for their rich carbon content, abundant resources and low price. However, sulfur-containing substances commonly exist in minerals, and they have an obvious influence on the pore structure and supercapacitive behaviors of AC. Therefore, the effect of sulfur species and content in minerals on relative performance of AC is worth studying in detail, In general, the sulfur-containing species in minerals are diversified and complicated, mainly including inorganic sulfur, such as sulfate and pyrite, organic sulfur and elemental sulfur. In this paper, the inorganic sulfate and elemental sulfur were selected as the research object, and the main purpose is to study the effect of sulfate and elemental sulfur in minerals on the porosity and capacitance performance of AC, thus providing the data and theoretical basis for sulfur-containing minerals as precursors of energy storage AC.In this work, elemental sulfur and the representative sulfate, FeSO4, CaSO4 and K2SO4, were added into PC to obtain analog sulfur-containing precursors (ASCPs) respectively, and then ASCPs were activated by KOH to prepare AC. The role of ASCPs is to ensure single factor study being of accuracy and scientificity, namely, just focusing on the relation of sulfur species in minerals with resultant AC quality. The relative results are as follows:(1) The influence of representative sulfate, K2SO4, FeSO4 and CaSO4, on pore structure of AC and its capacitance performance was investigated, respectively. The results show:1) K2SO4 in ASCPs can participate actively in KOH activation reaction, and plays an effective assistant activation in AC preparation, namely K2SO4 can increase the porosity and static capacitance performance of AC.2) CaSO4 in ASCPs has low reactivity in activation process-only a small part of CaSO4 was involved in activation reaction, while most of which was transformed into CaCO3 residued in AC after washing. Due to the coexistence of byproduct CaCO3 with AC, the porosity and capacitance performance of AC was decreased obviously. Therefore, CaSO4 in ASCPs has the negative influence on AC performance.3) FeSO4 in ASCPs has dual influence. On the one hand, FeSO4 can take active part in activation reaction and reduce the actual KOH amount, thus decreasing AC porosity and its capacitance performance. On another hand, the sulfur (+6) in FeSO4 can be transformed into organic thioether, which can cause the redox reaction to generate psedocapacitance for AC. In this case, FeSO4 plays a positive role for AC capacitance behavior. Therefore, as for the minerals with sulfate used for supercapcacitor AC precursors in practice, K2SO4 should be retained for its beneficial effect during AC activation, while CaSO4 should be removed as much as possible for its negative impact. FeSO4 should play its positive role as far as possible, namely promoting inorganic sulfur into organic thioether to contribute to AC psedocapacitance.(2) The relation of elemental sulfur in ASCPs with AC porosity and its capacitance behavior was studied intensively. The results show:elemental sulfur in ASCPs can react with activator KOH and consume part of KOH, thus leading to the deficiency of effective KOH amount for PC activation, eventually reducing the porosity and capacitance performance of AC. However, it is noteworthy that elemental sulfur in ASCPs can be converted into thioether, K2S2O3 and K2SO4. As thioether can improve AC psedocapacitance, elemental sulfur also shows beneficial effect on AC capacitance performance. Therefore, as for the use of minerals with abundant elemental sulfur for supercapcacitor AC precursors, the AC psedocapacitance improvement from sulfur transformation should be utilized fully. Meanwhile, the dosage of activator KOH should be increased correspondingly to compensate for the consumption amount of KOH reacting with elemental sulfur, thus ensuring that the resultant AC was activated fully to be of advanced porosity and good capacitance performance.