Fast microwave self-activation from chitosan hydrogel bead to hierarchical and O, N co-doped porous carbon at an air-free atmosphere for high-rate electrodes material

As ZnCl 2 is a good microwave absorber, it was usually used as an porogen to produce porous carbon from biomass by microwave heating . In this study, a facile microwave self-activation with the absence of ZnCl 2 was proposed to produce chitosan-hydrogel-bead-based porous carbons (CPC-Air) for a short duration of 10 min. KOH was used as a substitute for the typically used NaOH to form chitosan-based hydrogen beads. During microwave irradiation , KOH acted as a chemical porogen to generate abundant micropores with a pore volume of 0.39 cm 3  g −1 in CPC-Air, while water wrapped in hydrogel beads was transformed into steam, serving as a physical porogen to form a considerable content of mesopores (41.8%) and creating a air-free atmosphere. Microwave irradiation led to fast pyrolysis of chitosan, generating meso-/macropores. Combined with self-activation process, it resulted in a high defects degree in CPC-Air. Nitrogen with a high content of 3.4% remained in CPC-Air after self-activation. In addition, a significantly high oxygen content of 17.5% was introduced by the post-oxidation in air atmosphere. Consenquently, the proposed approach can produce a hierarchical and O, N co-doped CPC material with a high defects degree at air-free atmosphere, and with comparable electrochemical properties but with much simpler process and less production duration than the conventional method using ZnCl 2 . Particularly, owing to the combined effects of those characteristics in CPCs, an excellent rate capability was achieved in the prepared electrode, demonstrating an ultra-high capacitance retention of 87.5% when the current load was increased from 0.5–10 A g −1 . The supercapacitor prepared by this electrode can deliver the energy densities of 17.3–20.6 W h kg −1 and the power densities of 0.5–9.0 kW kg −1 .