Oxygen-rich hierarchical porous carbon nanosheets derived from the KOH/KNO3 co-activation treatment of soybean straw for high-performance supercapacitors

Abstract

It is attractive to transform waste biomass into active carbon materials for energy storage via green chemical methods. In this study, we successfully prepared porous carbon nanosheets (PCNs) for carbon-based supercapacitors with a hierarchical porous structure and abundant oxygen-containing functional groups via a novel KOH/KNO₃ co-activation method as an alternative to the common single KOH activation method, adopting soybean straw (SS) as a precursor. PCNs were obtained via a mild two-step conversion method with a high specific surface area of 1658.8 m² g⁻¹, a large mesopore and macropore ratio of 34.61%, and a considerable oxygen content of 16.77 at%, mainly consisting of hydrophilic carbonyl, carboxyl, and N–O functional groups compared to that in the single KOH activated sample, and are also outstanding for SS-based carbon nanomaterials. In a three-electrode system, the PCN electrode exhibited a high specific capacitance of 438 F g⁻¹ at a current density of 1 A g⁻¹, as well as an excellent capacitance retention of 91.32% when the current density increased to 20 A g⁻¹. Moreover, the symmetrical supercapacitor based on the PCN electrode provided a large energy density of 16.09 W h kg⁻¹ at a power density of 249.55 W kg⁻¹, and a significant energy density of 9.39 W h kg⁻¹ was achieved at a relatively high power density of 9141 W kg⁻¹. In addition, the PCN-based supercapacitor exhibited excellent stability with 96.6% initial capacitance retention after 14 000 cycles at a large current density of 10 A g⁻¹. Therefore, the novel KOH/KNO₃ co-activation method can not only improve the specific surface area and the porosity of SS-derived active carbon materials, but also implant abundant oxygen-containing functional groups for superior charge storage. Our work provides a compact co-activation method for the high-valued utilization of waste biomass resources.