Green Water-Soluble Salt-Templated Synthesis of 3D Hierarchical Porous Hard Carbon for High-Performance Supercapacitors

A green and scalable water-soluble salt coprecipitation strategy is developed for the synthesis of three-dimensional hierarchical porous hard carbons (3DPHCs) with tailored pore architectures for high-performance supercapacitors. Using low-cost and environmentally benign salts (KCl, NaCl, Na2SO4) as templates and ammonium citrate as the carbon source, a composite precursor is formed via rapid coprecipitation in an ethanol/water system, followed by carbonization and facile water washing. The optimized material, 3DPHCKCl-700, exhibits an ultrahigh specific surface area of 1931 m2 g–1, a large pore volume of 0.93 cm3 g–1, and a predominantly microporous structure (88.2%). When evaluated as a supercapacitor electrode, it delivers a remarkable specific capacitance of 422 F g–1 at 0.5 A g–1 and maintains 225 F g–1 at 20 A g–1 in 1 M H2SO4, along with outstanding cycling stability (94.45% retention after 10,000 cycles). Symmetric supercapacitors assembled with 3DPHCKCl-700 achieve high energy densities of 11.42 Wh kg–1 (300 W kg–1) in acidic electrolyte and 22.21 Wh kg–1 (500 W kg–1) in neutral 1 M Na2SO4, demonstrating practical potential by powering an LED. The work presents a sustainable and efficient route to advanced carbon materials, bridging synthetic simplicity with high electrochemical performance for next-generation energy storage devices.