One-step solid-state pyrolysis of bio-wastes to synthesize multi-hierarchical porous carbon for ultra-long life supercapacitors

Porous carbon is highly desired in supercapacitor electrodes due to its high specific surface area, ample pore size and superior electrochemical stability. Yet, the development of a general and simple synthetic method to prepare porous carbon remains challenging. Meanwhile, recycling waste to obtain high value-added materials is an effective way to solve environmental pollution and resource shortage problems. Herein, a general one-step solid-state pyrolysis method is developed to synthesize multi-hierarchical porous carbon using bio-wastes as precursors and potassium ferrate as the pore-forming agent. This method is superior to the traditional two-step or multi-step method due to its simple procedure, low cost, little pollution and time-saving features. The multiple pore-forming effect derived from potassium ferrate is responsible for this multi-hierarchical porous structure. The resulting porous carbon is used to fabricate symmetrical supercapacitors, exhibiting specific capacitances of 291.2 F g-1 at 1 A g-1 and 240.1 F g-1 at 10 A g-1, and exceptional cyclic stability with 93.2% capacitance retention over 100?000 cycles. Furthermore, this method has been applied to five other types of bio-wastes, verifying its universality. In addition, the multiple pore-forming mechanism of potassium ferrate is investigated. This work provides a simple and general method to convert abandoned bio-wastes into ideal supercapacitor electrode materials, which hold great potential in energy storage applications.