Screening of biomass residue streams for their applicability as feedstocks for activated carbon production and their compliance as electrode material

Abstract

Currently, activated carbon production still relies heavily on unsustainable feedstocks, e.g., coal or fresh wood. To be able to phase out these practices, biomass residue streams offer a valid alternative, both from an economic and ecological perspective. Therefore, this research screens different promising biomass streams for their potential to be converted into top-tier activated carbon. These should preferably have a well-developed porosity and high nitrogen content to maximize their energy storage capacity and potential applicability as electrode material in supercapacitors. Seven different types of biomass were selected in this study based on their physicochemical characteristics (e.g., lignocellulosic composition and nitrogen content): 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). The biomass streams were transformed into biochars and activated carbons using a home-built stainless steel screw reactor [1,2]. Activated carbon was produced in a two step-process comprising a carbonization step at 700 °C in an inert atmosphere, followed by a physical activation step using CO2 at 800 °C. Biomass, biochars, and activated carbon were characterized for their ultimate and proximate analysis, biochemical composition, and elemental composition via inductively coupled plasma–atomic emission spectroscopy. Their surface functional groups were determined via FT-IR. Lastly, the porosity of the resulting activated carbons was measured via nitrogen physisorption experiments. The most promising activated carbons were incorporated in coin cell supercapacitors. The results demonstrate the significant impact of the biomass’s mineral composition on creating highly porous activated carbon structures. Furthermore, the overall activated carbon yields decreased for the samples with large ash fractions due to a relative increase in carbon burn-off. In terms of creating nitrogen-rich activated carbons, CF proved best with a resulting nitrogen content of 8.2%, in contrast with MNS, which exhibited the lowest percentage (0.54 %). However, in terms of porosity, this sample (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 porosity and their specific capacitance could be made when verifying the electrode material performance. Thus, the MNS-derived activated carbon performed best of the screened biomass streams with a specific capacitance of 53 F/g. In conclusion, an investigation on the screening of different biomass residue streams was performed. It became clear that the low-ash content, lignocellulosic biomass MNS performed best compared to the other tested biomass streams. Future research should focus on combining different biomass streams to produce a highly porous nitrogen-rich biomass stream that would be perfectly suitable as electrode material.