Study On The Effect Of Electrolyte Additives On The Supercapacitor Performances Of Citrate-based Carbon Materials

In this dissertation, a series of nanoporous carbon materials with different internal structures and the electrochemical performances was prepared by the template method, and the influence of the carbonization temperature and template type for the properties of carbon materials was studied deeply. On this basis, some redox additives were introduced into electrolytes to further improve the electrochemical performances of supercapacitors, in which the reaction mechanisms were investigated in detail. The specific content is as follows:1. A novel redox additive of calcon carboxylic acid sodium salt (abbr. CS) has been for the first time implemented in KOH electrolyte for largely improving the electrochemical performance of supercapacitor. By directly carbonizing potassium citrate monohydrate at 800 ℃, giving rise to the Carbon-800 sample, and the resulting specific capacitance can reach up to 128 F g-1 at 3 A g-1 when measured in a three-electrode system with the conventional 6 mol L-1 KOH electrolyte. To further elevate these capacitances, certain amount of CS as redox additive is introduced into KOH electrolyte.Remarkably, the Carbon-800-15 sample by adding CS (15 mmol L-1) into 6 mol L-1 KOH electrolyte displays a highly improved specific capacitance of 237 F g-1 at 3 A g-1 in a three-electrode system, which is almost the 2.1 times than that of the pristine one. In addition, the Carbon-800-15 sample also possesses an elevated energy density of 11.4 Wh kg-1 at the power density of 1.5 kW kg-1 when measured in a two-electrode system,much larger than the 7.3 Wh kg-1 of the Carbon-800 sample. The improved performance incurred by CS additive is attributed to the fast redox reactions at the electrolyte/electrode interface. Apparently, the present efficient and low-cost redox additive of CS is expected to be implemented for largely improving the performances of supercapacitor.2. In this work, by simply heating trisodium citrate dihydrate as sole precursor at 800 ℃, novel 2D amorphous carbon material (the SC-blank sample) occurs, whose high surface area and large pore volume can reach up to 1015.8 m2 g-1 and 2.0 cm3 g- 1,respectively. Furthermore, we present a synchronous carbonization and graphitization method to produce partially graphitic carbon materials. When heating the mixture of trisodium citrate dihydrate and nickel nitrate hexahydrate as template and graphitization catalyst (the mass ratio of 7:1) at 800 ℃, the resulting SC-7:1 sample’s surface area has decreased to be 682.8 m2 g-1 due to its improved graphitization degree, together with a lower capacity, but a better rate capability. More importantly, in order to further enhance capacity, certain amount of sulfanilic acid azochromotrop (abbr. SAA) serving as redox additive has been introduced into KOH electrolyte. Notably, the SC-blank sample incorporated with SAA redox additive (12 mmol L-1) indicates a largely enhanced capacity of 200 F g-1 when measured at 3 A g-1, which is much larger than that of the pristine one (66 F g-1). To sum up, the SAA is an efficient redox additive for elevating the capacity.3. We demonstrate a simple but effective redox additive of pyrocatechol violet(abbr. PCV) to largely promote the capacitive performances especially when carried out in three different kinds of electrolytes (H2SO4, Na2SO4 and KOH),mostly due to its fast electron and proton transfer occurring in the electrode/electrolyte interface. It reveals that the PCV dosage incorporated into electrolyte plays a crucial role in the determination of capacitive performance. When conducted in a two-electrode system,incorporating 0.06 mol L-1 PCV into 1 mol L-1 H2SO4 can achieve large capacitance up to 200 F g-1, which is almost 2.06 times than the one without the addition of PCV.Besides, the corresponding energy density is of 28 Wh kg-1 (also increasing 2 times).What’s more, PCV has been extended as the redox additive in other electrolytes such as Na2SO4 and KOH, and remarkable promotion in capacitance and energy density also occur, well evincing the high efficiency and universal applicability of PCV for the large promotion of supercapacitors’ performances. PCV is particularly excellent mediating agent for redox-active electrolyte due to its excellent electrochemical reversibility,universal applicability and high solubility.