Template Synthesis of Nitrogen-Doped Carbon Nanosheets for High-Performance Supercapacitors Improved by Redox Additives

In this work, nitrogen-doped sheetlike carbon materials have been synthesized by a template carbonization method using 1,5-diphenylcarbazide and MgCl₂·6H₂O as carbon source and template, respectively. The method presents a carbon sample with amorphous characteristics as well as high surface area and large pore volume. More importantly, introducing the redox additives 4-hydroxybenzoic acid (HBA), 3,4-dihydroxybenzoic acid (DHBA), and 3,4,5-trihydroxybenzoic acid (THBA) with functional hydroxyl groups into 1 mol L^–1 H₂SO₄ has largely improved the capacitances as well as the energy density. As expected, the supercapacitor with the redox additive HBA exhibits higher capacitances, with an increase of 1.57 times compared with the conventional H₂SO₄ electrolyte. Besides, compared with the supercapacitor without any redox additive, the redox additive DHBA produces a large improvement of capacitances, increasing by 3.18 times. In addition, the redox reactions of HBA and DHBA are reversible, while that of THBA is irreversible. Moreover, HBA with a hydroxyl group can release/gain a proton/electron, and DHBA, which owns a pair of hydroxyl groups, can release two protons/electrons. Moreover, both of the redox processes of HBA and DHBA are controlled by a diffusion mechanism.