High potential of microalgal sludge biochar for a flexible all-solid-state microsupercapacitor

Biorefinery is drawing attention as an environmentally friendly technology due to increasing interest in sustainable development. Microalgal biomass is a promising feedstock for biorefinery because of its fast growth rate and low requirement for land. It is essential to treat the remained microalgal sludge (90 wt% of total microalgal biomass) after biorefinery to construct a true zero-waste microalgal biorefinery. However, few studies have reported methods of utilizing microalgal sludge, a by-product of biorefinery. In this study, the microalgal sludge was converted to biochar through pyrolysis. Physicochemical and electrochemical properties of microalgal sludge biochar (MASB) were then analyzed. A flexible all-solid-state microsupercapacitor was fabricated by manufacturing a composite with a conductive polymer. Element components of MASB presented the inherent presence of doping nitrogen compared to those of lignocellulosic biochar such as Miscanthus biochar (MB) and rice straw biochar (RB). The pyrolysis temperature of MASB was optimized to obtain a high degree of graphitization and 900 °C (range, 600 °C–1000 °C) was found to be an appropriate pyrolysis temperature. Nitrogen doping and graphitic structure of MASB-900 (MASB produced at 900 °C) affected the capacitive performance of the fabricated microsupercapacitor. The capacitance was 262.8 F g⁻¹ at a current density of 0.1 A g⁻¹ and the energy density was 36.5 Wh kg⁻¹ at a power density of 224.4 W kg⁻¹. These results were superior to those of other previously studied biochar-based supercapacitors. The proposed microsupercapacitor demonstrates the possibility of zero-waste biorefinery and a high value-added application of microalgal sludge.