Coal-Based 2D Ultrathin N‑Doped Hierarchical Porous Carbon Nanosheets Induced by Dual-Salt System of Na2CO3–K2CO3 for High-Performance Supercapacitors

2D carbon materials like graphene and carbon nanosheets excel in energy storage and conversion due to their unique microstructure, pores, and electrochemical properties. Our study successfully synthesized coal-based 2D ultrathin N-doped hierarchical porous carbon nanosheets (NCPNs) using a dual-salt system of Na2CO3–K2CO3 for high-performance supercapacitors (SCs). To achieve this, we used coal and urea as carbon and nitrogen sources, respectively, with K2CO3 as an activator and Na2CO3 as a template in an annealing process. Atomic force microscopy (AFM) showed the thickness of NCPNs to be 10.2 nm. We also successfully prepared nanosheets from the other three carbon sources (coal tar, coal pitch, and pyrene), thereby indicating the method’s universality. Na2CO3 was crucial for nanosheet formation. NCPNs exhibited a 2D structure, high surface area (1923 m2 g–1), and capacitance (305 F g–1 at 0.2 A g–1). For symmetrical SCs, it can work properly in three different electrolytes: 6 M KOH (0–1.2 V), 1 M Na2SO4 (0–1.6 V), and 1 M TEABF4/AN (0–3.2 V), with energy densities of 10.4, 17.9, and 45 Wh kg–1, respectively. Its stability was exceptional, retaining 99.73% capacity over 10,000 cycles at 10 A g–1. These results suggest that NCPNs possess a significant potential for energy storage.