Development of softwood kraft lignin-based conductive carbon for sustainable supercapacitor

Electric double-layer capacitors (EDLCs) are promising devices for sustainable energy storage. However, EDLC components, such as separators and electrodes composed of activated carbon and conductive additives, are derived from fossil resources. To reduce this dependency, an EDLC was assembled using a separator and electrodes derived from hardwood kraft lignin, while still relying on fossil-based carbon black (CB) as the conductive additive. To achieve more sustainable EDLCs, this study developed all the conductive carbon, separator, and electrodes from softwood kraft lignin (SKL). When SKL was carbonized at 900 °C, it showed poor electrical conductivity and was unsuitable as a conductive additive. The carbon structures became more ordered with higher temperatures, and SKL-carbons prepared at 1,300–2,000 °C showed comparable conductivity to CB. The EDLCs with 1 wt % of these SKL-carbons exhibited higher capacitance and energy density than reference EDLCs with 1 and 5 wt % CB. Furthermore, a turbostratic (T) structure formed at 2,500 °C, enhancing conductivity and EDLC performance. SKL-carbon prepared at 2,800 °C exhibited a graphite structure in addition to the T structure, achieving the highest conductivity (0.54 S cm⁻1), but the resulting EDLC showed low power density. Thus, SKL-carbon prepared at 2,500 °C was the best conductive additive for EDLCs.