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

Abstract

As ZnCl2 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 ZnCl2 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 cm3 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 ZnCl2. 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.