Atmosphere-free pyrolysis of harakeke fiber: A new chamber-induced activation methodology for porous carbon electrodes in supercapacitors

Currently available synthetic methods of producing activated carbons have triggered a huge demand for uneconomical activation agents. Herein, a simple single-step ‘atmosphere-free’ pyrolysis process was developed and applied to harakeke fiber to produce activated carbon with a highly porous structure. With the unique honeycomb-like framework serving as both a container and a quasi-enclosed reactor, the ‘chamber-induced’ self-activation is enabled simultaneously during the carbonization of harakeke fiber. Through raising the pyrolysis temperature and the (sample) mass-to-(furnace) volume ratio, the specific surface area and the mesoporosity of the product increases from 420 to 2204 m² g⁻¹, and 12.5 to 74.4% respectively. We have proved that H₂O released during the whole pyrolysis process plays a critical role in the ‘chamber-induced’ self-activation. The harakeke-derived hierarchical porous carbon (CAHF) exhibits a high specific capacitance of 295 F g⁻¹ with superior rate performance (180 F g⁻¹ at 40 A g ⁻¹). A symmetric supercapacitor employing CAHF as electrodes delivers a maximum energy density of 25.8 W h kg⁻¹ at a power density of 661 W kg⁻¹. This work offers a novel strategy for the preparation of porous carbon and provides a new vision for utilizing the pyrolysis gas. It also emphasizes the importance of the mass-to-volume ratio to the pyrolysis process.