Biomass Residue Streams as Potential Feedstocks for the Production of Activated-Carbon-Based Electrodes for Supercapacitors

Abstract

Currently, activated carbon production relies heavily on unsustainable feedstocks, e.g., coal and fresh wood. To phase out these practices, biomass residue streams offer a valid alternative. Therefore, this research screens different biomass streams to be converted into top-tier activated carbon. These should preferably have a well-developed porosity and high nitrogen content to maximize their potential as electrode materials in supercapacitors. Seven different biomass streams were selected in this study: common ivy trimmings (CI), brewer’s spent grain (BSG), macadamia nut shells (MNS), chicken feathers (CF), coffee husks (CH), and the microalgae species Spirulina sp. (SP) and Chlorella vulgaris (CV). Activated carbons were produced in a two-step process comprising carbonization at 700 °C in an inert atmosphere, followed by physical activation using CO2 at 800 °C. The most promising activated carbons were tested in symmetric coin cell supercapacitors. The biomass’s mineral composition had a large impact on creating porous activated carbon structures. The activated carbon yields decreased for the samples with large mineral fractions due to a relative increase in carbon burnoff. In terms of creating nitrogen-rich activated carbons, CF proved best with a nitrogen content of 8.2%. In terms of porosity, MNS outperformed the other investigated biomass streams with a BET specific surface area of 693.7 m2/g. A correlation between the activated carbon’s microporosity and their specific capacitance was established. The MNS-derived activated carbon performed as the best of the screened streams. Future research should focus on optimizing the activated carbon production process (physical activation and demineralization) to produce highly porous, nitrogen-rich activated carbon.