Template-free synthesis of nitrogen doped carbon materials from an organic ionic dye (murexide) for supercapacitor application

Abstract

The present study describes a template-free single step carbonization route to prepare hierarchically structured nitrogen-doped carbon materials (NCMs) by using an organic ionic dye (OID), ammonium purpurate (murexide). These NCMs exhibited moderate specific surface area (307 m² g⁻¹ for NCM_(MDE)-900) and hierarchical macro/meso/microporous structures with abundant nitrogenous functionalities that contributed towards the high specific capacitance displayed by pseudocapacitive contribution. In particular, the NCM_(MDE)-800 displayed superior specific capacitance (222 F g⁻¹ at 3 A g⁻¹) and excellent cyclic stability of ∼10 000 cycles at 10 A g⁻¹ in 1 M 1.0 M H₂SO_4(aq). The material prepared at higher temperature, viz. NCM_(MDE)-900, exhibited high rate capability at 50 A g⁻¹, which is 92% of their specific capacitances at 10 A g⁻¹. Further systematic investigations of NCM_(MDE)-800 in three different electrolytes, viz. 1.0 M H₂SO_4(aq), 6 M KOH_(aq) and 0.5 M Na₂SO_4(aq), revealed that 1.0 M H₂SO_4(aq) is a promising electrolyte for achieving good specific capacitance, high capacitive retention and long term cyclic stability, which might have its origins in the formation of ionic interactions between the active protons in 1.0 M H₂SO_4(aq) and the alpha carbon atoms with Lewis basic character next to the nitrogen in these NCMs.