Gradient synthesis of carbon quantum dots and activated carbon from pulp black liquor for photocatalytic hydrogen evolution and supercapacitor

Black liquor (BL) is a by-product of the chemical pulping industry and is mainly used as a low-value fuel; however, its potential to produce high-value products has not been fully exploited. In this study, a green and simple strategy is reported for the gradient production of Na+-functionalized carbon quantum dots (Na+-CQDs) for the first time, N and S co-doped CQDs (N/S-CQDs), and N and S co-doped KOH-activated carbon (N/S-KAC) from BL by dialysis, hydrothermal carbonization and activation-carbonization, respectively. Due to the good electron trapping ability, photoluminescence and promising up-conversion luminescence of CQDs, the hydrogen evolution efficiency of Na+-CQDs/TiO2 and N/S-CQDs/TiO2 photocatalysts was improved by 2.45 and 1.46 times, respectively, compared with pure TiO2. N/S-KAC with a high specific surface area of 2294 m2 g− 1 provides an excellent specific capacitance of 253 F g− 1 at 0.5 A g− 1 and a promising energy density of 26.92 Wh kg− 1 under a power density of 566 W kg− 1 for the fabricated symmetrical supercapacitor. Moreover, the electrode material has good cycling stability with a capacitance retention of ~ 93.91% after 5000 cycles. This pathway provides a versatile and scalable approach for the construction and co-production of nanostructured materials, photocatalysts and energy storage devices.