| With the rapid development of the global economy, the depletion of fossil fuels, and increasing environmental pollution, there is an urgent need for efficient, clean, and sustainable sources of energy, as well as new technologies associated with energy conversion and storage. Among various power sources, supercapacitor is a mature technology and has the potential to enable major advances in energy storage. Compared with conventional capacitors, the specific energy of supercapacitors is several orders of magnitude higher. Supercapacitors also have a greater power density and longer cycle life than most batteries, but their specific energy is somewhat lower. Doping of nitrogen atoms into graphitic networks is considered one of the best approaches to eliminate this disadvantage. The nitrogen-doping could produce a pseudocapacitance besides the electric double-layer capacitance, enhance the wettability in electrode electrolyte interface. Moreover, introduction of nitrogen can induce a large number of defects in the carbon structure and improve the capacitance of electrode.As we known, the electrolyte is an important component which plays a fundamental role during charge-discharge process in supercapacitors. Therefore, an electrolyte with good electrochemical properties is very significant for an efficient super capacitor. Introducing redox additives into the electrolyte is an efficient and simple way to improve the specific capacitance. The quick reversible Faradic reactions of redox additives at the electrode-electrolyte interface bring additional pseudocapacitive contribution for the system. Consequently, it is expected that the electrochemical performances of carbon-based supercapacitors can be improved prominently. The major research contents of the work are as follows:1. A simple template carbonization method has been developed to produce nitrogen-containing nanoporous carbon from diphenylcarbazide, using Mg(OH)2 as hard template. The carbonization temperature has a crucial role in determining the carbon structure. The carbon-3:1-800 sample obtained with the mass ratio of diphenylcarbazide and Mg(OH)2 as 3:1 at 800 ℃ exhibits the optimum pore structure as well as the resultant best electrochemical performance. It has a large BET surface area of 1538.0 m2 g-1, high pore volume of 3.48 cm3 g-1, and hierarchical pore size distribution. As a result, it delivers superior electrochemical behaviors in a three-electrode system using 6 mol L-1 KOH as electrolyte, whose specific capacitance calculated from galvanostatic charge-discharge curve can reach up to 517.4 F g-1 at a current density of 1 A g-1, which is much larger than most of the nanocarbons ever reported in the literature. The carbon-3:1-800 sample also exhibits good cycling stability within 10000 cycles.2. Nitrogen-doped nanoporous carbon materials have been prepared by a template carbonization method, in which diphenylcarbazide serves as carbon/nitrogen source and Mg(NO3)2-6H2O powder as hard template, respectively. The mass ratio of diphenylcarbazide and Mg(NO3)2·6H2O powder plays a crucial role in determining the pore structures and electrochemical performances. The resulting carbon-2:1 sample displays large BET surface area of 1366 m2 g-1, and high pore volume of 2.18 cm3 g-1. It also exhibits good cycling stability and superior electrochemical behaviors, including high specific capacitance of 323.5 F g-1 at 1 A g-1 in a three-electrode system using 6 mol L-1 KOH as electrolyte. More importantly, to further improve the electrochemical performance, different amounts (8,16 and 24 mg) diphenylcarbazide herein serving as novel redox additive is introduced into the carbon-2:1 system to form the carbon-2:1-8/16/24 electrodes. As a result, the specific capacitances of the carbon-2:1-8/16/24 electrodes at 2 A g-1 have been improved up to be 345.4,540.9 and 808.6 F g-1, respectively, which are 1.54,2.21, and 3.53 times than that of the pristine system (-228.8 F g-1). Furthermore, the carbon-2:1-24 also retains high cycling stability as 77.5% after 5000 cycles. The present method of incorporating diphenylcarbazide (redox additive) into carbon system is simple but efficient for large improvement of supercapacitor performances.3. In this work, we demonstrate two kinds of novel redox additives, diphenylcarbazide and phenylazofonnic acid 2-phenylhydrazide, with quick and reversible redox-reaction incorporated into carbon-based supercapacitors. It is revealed that the 4-electrons and 4-protons transfer occurring in the redox additive-carbon materials system within the electrode can result in additional capacitance. When introducing 24 mg diphenylcarbazide or phenylazoformic acid 2-phenylhydrazide substance into 24 mg carbon system, largely improved specific capacitances of 544.3 or 427.6 F g-1 are achieved, which are almost 4.31 and 3.39 times, respectively, than that of the pristine carbon (126.1 F g-1) at the current density of 2 A g-1. Besides, their corresponding capacitance retentions can reach up to 78.9% and 75.6%, respectively, after 5000 charge/discharge cycles, and both of them are somewhat smaller than that of the pristine carbon (96.8%) due to the incremental electronic resistances. The present commercially available, low-cost and highly effective redox additives of diphenylcarbazide and phenylazoformic acid 2-phenylhydrazide are quite promising and expected to be implemented for the high performance supercapacitors. |
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