Microwave-assisted high-entropy pyrolysis of walnut-shell cellulose to catalyst-free carbon nanotubes for high-performance supercapacitors

Electrode materials fundamentally govern the electrochemical performance of supercapacitors. Carbon nanotubes (CNTs) have been widely adopted for electric double-layer capacitors due to their favorable conductivity and structural integrity. However, their practical application is often limited by high cost, intricate synthesis, and restricted specific surface area, which collectively constrain the achievable specific capacitance. In this work, CNTs were prepared from walnut shell cellulose under microwave irradiation without a catalyst. The resulting material was impregnated with KOH, and element doping was promoted by the entropy increase caused by microwave-induced internal heating in the N 2 /CO 2 /SO 2 /NH 3 atmosphere. The high defect density in the SBC-3-800 and NBC-3-800 samples severely undermined the structural integrity of the carbon framework. In contrast, the CBC-3-800 sample retained a moderate defect density while offering a substantially larger specific surface area, which facilitated more efficient charge transport. The assembled symmetric supercapacitors delivered a specific capacitance of 51.8 F·g −1 at 1 A·g −1 , with an energy density of 7.19 Wh·kg −1 and a power density of 999.38 W·kg −1 . Furthermore, the CBC-3-800 electrode demonstrates exceptional cycling stability, retaining 97.1% of their initial capacity after 30,000 galvanostatic charge-discharge cycles at 1 A·g −1 .