Optimization Of Performance Of The Porous Carbon-based Supercapacitors:Study On Mechanism Of The Nitrogen Doping And Electrolyte Additives

Supercapacitors are devices that are used for the rapid storage and release of electric energy, which can bridge the gap between conventional capacitors and rechargeable batteries. It has the superior properities of long cycling life, large specific capacitances,high power densities and energy densities. Studies have shown that the electrode material plays a decisive impact on its performance. Therefore,high-performance materials research in this area has been the focus of attention. Compared to other supercapacitor electrode materials, carbon-based electrode materials have attracted widespread attention due to their unique physical and chemical properties. This paper uses the template carbonization to prepare the porous carbon materials, by changing the experimental parameters to get a series of controlled structure, excellent electrochemical properties of nanoporous carbon materials. In addition, the capacitor performances are further improved by introducing redox active additive into electrolyte. The main content of the paper are as follows:1. A series of nitrogen-doped nanoporous carbons are prepared via a synchronous template carbonization and nitridation method, in which glucose serves as carbon precursor,azodicarbonamide ?ADC?,urea and melamine as nitridation agents and templates. Results indicate that all the obtained carbon materials deliver the amorphous features with low graphitization degree. The Carbon-G-A sample prepared by heating glucose and ADC at 800 ? exhibits high surface area of 624.8 m²g^-1,large total pore volume of 0.53 cm3 g^-1 and fairly high nitrogen content of 10.35%. The Carbon-G-A sample delivers superior electrochemical performance, whose specific capacitance can reach up to 202.7 F g^-1 at a current density of 1 A g^-1. What's more, it exhibits long-term cycling ability of 92.3% retention even after 5000 cycles. Besides,in the cases of urea and melamine as nitrogen sources,the resultant nitrogen-containing carbons deliver smaller porosities and lower nitrogen contents,but also acceptable specific capacitances of 148.7 and 126.1 F g^-1 at 1 A g^-1,respectively. It opens a straightforward and easy protocol to convert inexpensive glucose into highly capacitive nitrogen-doped nanoporous carbon materials, using ADC, urea and melamine as nitrogen sources.2. In this work, using folic acid as a carbon/nitrogen precursor and Zn?NO₃?2·6H2O as the template, largely nitrogenated ?12.4%? and highly porous carbon material has been produced by a synchronous template carbonization and nitridation approach. More importantly, we demonstrate two novel redox additives of magneson I ?MI? and alizarin yellow R ?AYR?, which have been implemented in KOH electrolyte for highly improving the performance of carbon-based supercapacitors. Both of these redox additives have exhibited the quick electron transfer and reversible redox reactions in the mixed electrolytes. In the case of MI as redox additive, the M-10 sample delivers a largely improved capacitance of 451 F g^-1 at 3 A g^-1, compared with the pristine one without any additives (180 F g^-1). As for the case of AYR, the resultant A-10 sample's capacitance also has highly elevated up to 405 F g^-1. It also reveals that the pseudocapacitance contribution of MI and AYR substances comes from both the amine group ?due to the reduction of nitro? and phenolic hydroxyl, which can undergo a rapid redox reaction along with the electronic gain and loss. The present work has provided a simple but highly effective method for elevating the electrochemical performance of supercapacitors.3. In this work, a series of porous carbon materials with hierarchical porosities have been synthesized via a template carbonization method, in which the cheap CaCO₃ serves as template and glucose as carbon precursor. During the carbonization process, the CO₂ produced by the decomposition of CaCO₃ template can act as an internal activating agent,quite improving the microporosity and mesoporosity. All the carbon materials obtained by regulating the ratio of glucose to CaCO₃ exhibit the amorphous features with low graphitization degree. Among them, the carbon-1:2 sample delivers a high BET surface area of up to 818.5 m² g^-1 and a large total pore volume of 1.78 cm3 g^-1 as well as aspecific capacitance of 107.0 F g^-1 at 1 A g^-1. In addition, a series of hydroquinone ?HQ?,p-aminophenol ?PAP? and p-nitrophenol ?PNP? as the novel redox additives that can produce pseudo-capacitances have been added into the KOH electrolyte for promoting the total capacitive performances via redox reactions at the electrode-electrolyte interface. As expected, a 2.5-fold increase in the galvanostatic capacitance of 240.0 F g^-1 in the HQ-0.5 electrolyte occurs, compared with the conventional KOH electrolyte.Similarly, the PAP-0.5 electrolyte and the PNP-0.5 electrolyte also deliver high specific capacitance of 184.0 F g^-1 at 2 A g^-1 (156.6 F g^-1 at 3 A g^-1) and 153.0 F g^-1 at 3 A g^-1,respectively. Additionally, the three kinds of electrolytes exhibit an excellent cyclical stability. The remarkable improvement of the supercapacitors are attributed to the quick reversible faradaic reactions of the amine and hydroxyl groups adhering to the phenyl rings,which largely accelerates the electronic migration and brings additional pseudocapacitive contribution for the carbon-based supercapacitors.