Iron-assisted molten salt synthesis of highly graphitized hierarchical porous N-doped carbon for enhanced aqueous energy storage

The development of advanced carbon electrodes with high conductivity and a large specific surface area is crucial for electrochemical energy storage applications. In this work, we propose an iron-assisted molten salt (NaCl) strategy to fabricate highly graphitized hierarchical porous N-doped carbon (GHPNC) as an efficient electrode for high-performance supercapacitors. Ferric chloride (FeCl₃) is employed as an activator to promote the formation of a micro-mesoporous structure, while NaCl and the resulting iron oxide facilitate the development of macropores. The optimized GHPNC-5.0-1.0 sample (where 5.0 represents the FeCl₃-to-carbon precursor mass ratio and 1.0 denotes the NaCl-to-carbon precursor mass ratio) exhibits a well-developed hierarchical porous structure and high graphitization, leading to excellent electrochemical performance. In a 6 M KOH electrolyte, the GHPNC-5.0-1.0 electrode achieves a capacitance of 201.5 F g⁻¹ at 0.5 A g⁻¹. A symmetric supercapacitor (SSC) assembled with this electrode delivers an energy density of 9.84 Wh kg⁻¹ at a power density of 250.17 W kg⁻¹. In a 2 M ZnSO₄ electrolyte, the GHPNC-5.0-1.0//Zn hybrid supercapacitor (ZHSC) demonstrates an energy density of 79.87 Wh kg⁻¹ at 625 W kg⁻¹. Notably, the ZHSC exhibits outstanding cycling stability, retaining nearly 100 % of its capacitance after 10,000 cycles.