N-doped sawdust-based activated biocarbons prepared by microwave-assisted heat treatment as potential electrode materials for supercapacitors

Abstract

Microwave and conventional heating have been combined to obtain highly microporous nitrogen-doped activated biocarbons. In order to generate nitrogen groups bonded in different ways into the carbonaceous matrix, the modification with urea was performed at two different stages of processing, i.e., precursor or char. Additionally, the effects of chemical and thermal (physical) activation on the physicochemical parameters and capacitance behavior of the biocarbons prepared were tested. All the materials under investigation were characterized by elementary analysis, surface area measurements as well as estimation of the number of surface functional groups. Depending on the variant of preparation, the final products were microporous nitrogen-doped activated biocarbons of well-developed surface area ranging from 314 to 1483 m²/g, showing acidic or intermediate acidic–basic character of the surface and different contents of nitrogen functional groups, varying from 0.8 to 8.6 wt.%. Finally, the electrode materials were manufactured using prepared biocarbons and tested as components of symmetric supercapacitors, containing organic electrolyte. The electrochemical performance of these materials was investigated in Swagelok® type cells by using CV, GCD and EIS techniques. The results obtained in this study showed that incorporation of nitrogen and chemical activation of sawdust can led to activated biocarbons with very good electrochemical parameters. Most promising materials have exhibited high specific capacitance values (ca. 50 − 100 F g⁻¹), good rate capability, and excellent cycling stability (ca. 86–88% capacitance retention) after thousands of GCD cycles.