Self-generation of low ash carbon microspheres from the hydrothermal supernatant of anaerobic digestate: Formation insights and supercapacitor performance

Abstract

This work provides the first observations of and insights into the self-generation of carbon microspheres from the supernatant after hydrothermal carbonization of anaerobic digestate has been completed and the hydrochar removed. Solid State NMR and XPS revealed that the carbon microspheres were comprised of decomposed fragments of proteins, carbohydrates and lignin. The carbon microspheres were significantly lower in ash content (3.1%), compared to the hydrothermal solid (41.2%) and precursor (25.2%) and their formation reduced the total organic carbon load of the supernatant. The low ash content allowed them to be easily activated, achieving a surface area of 1711.0 m2 g−1, compared to 51.4 m2 g−1 for the activated hydrothermal solid and 12.8 m2 g−1 for the activated precursor. The microcarbon spheres achieved a specific capacitance from cyclic voltammetry of 86 F g−1 at 100 mV s−1 to 176 F g−1 at 1 mV s−1, while the gravimetric capacitance was 42 F g−1 at 25 A g−1 and 140 F g−1 at 0.5 A g−1 in 0.5 M Li2SO4 and a 1.8V potential window. Overall, this study highlights the importance of exploring this new product and its valorisation potential for the hydrothermal carbonization of ash-rich precursors.