The effects of amine/nitro/hydroxyl groups on the benzene rings of redox additives on the electrochemical performance of carbon-based supercapacitors

Abstract

In this work, a series of porous carbon materials with hierarchical porosities have been synthesized via a template carbonization method, in which cheap CaCO₃ serves as a template and glucose as a carbon precursor. During the carbonization process, CO₂ produced by the decomposition of the CaCO₃ template can act as an internal activating agent, significantly improving microporosity and mesoporosity. All the carbon materials obtained by regulating the ratio of glucose to CaCO₃ exhibit the amorphous features with a low graphitization degree. Among them, the carbon-1 : 2 sample shows 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 a specific 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 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 show a 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 excellent cyclic stability. The remarkable improvement of supercapacitors is attributed to the quick reversible Faradaic reactions of amine and hydroxyl groups adhering to the phenyl rings, which largely accelerates electron migration and brings additional pseudocapacitive contribution for carbon-based supercapacitors.