VO quantum dot-enhanced nitrogen-sulfur dual-doped hierarchical porous carbon electrodes from waste eggshell membranes for advanced flexible supercapacitors.

The weathering of rocks naturally creates abundant pore structures on their surfaces. Drawing inspiration from this, we present a simple yet effective approach-combining hydrothermal carbonization and pyrolysis carbonization-to synthesize multivalent vanadium oxide (VO) quantum dot-enhanced nitrogen- and sulfur-doped hierarchical porous carbon materials derived from waste biomass of eggshell membranes. These carbon materials, termed VO-S@CESM, are used as electrodes for supercapacitors. The results demonstrate that the multivalent VO quantum dot structure effectively increases the active sites and enhances the pseudocapacitance, particularly the pseudocapacitance associated with V in the composites. The optimal VO-S@CESM sample achieves a capacitance of 355 F/g at 0.5 A/g. The flexible VO-S@CESM symmetrical supercapacitor retains more than 80 % of its capacity across various bending angles (0°-180°). It also exhibits a high energy density of 27.9 Wh kg and a power density of 906 W kg. Density functional theory (DFT) calculations confirmed that the introduction of VO quantum dots significantly increases the adsorption energy of Na ions and induces polarization in the carbon materials. This quantum dot-enhanced carbon material design opens new avenues for the development of advanced energy storage materials.