Fabrication and performance investigation of black liquor lignin-modified carbon/MnO2 electrode materials in sodium-ion capacitors

To overcome the long-lasting low conductivity and sluggish ion transfer kinetics of conventional MnO2/carbon electrode materials, here black liquor lignin modified graphene oxide (GO) is adopted as the carbon skeleton to grow active MnO2 by hydrothermal deposition. The numerous nitrogen- and oxygen-containing functional groups on lignin promote the conductivity and wettability of the carbon substrate. The introduction of lignin and parameters adjustment of hydrothermal treatment effectively inhibits MnO2 aggregation during depositing, and enables uniform and stable δ-MnO2 growth with platelet structure, which benefits to facilitate the formation of oxygen vacancies to improve reaction kinetics. The synergistic effect of the carbon substrate and δ-MnO2 endows GO/BL/MnO2 composite with high crystallinity, developed hierarchical porous structure, and good ionic/electronic conductivity. The specific capacitance of GO/BL0.5/MnO2-20-150 in 1 M Na2SO4 electrolyte reaches 1043.0 mF/cm2 at 1 mA/cm2 (179 F/g at 0.5 A/g), ascribed from pseudocapacitance from the Mn4+/Mn3+ redox reaction and double-layer capacitance from carbon substrate. The all-biomass-based solid-state capacitor ASCGO/BL0.5 shows a specific capacitance of 272.3 mF/cm2 at 3 mA/cm2 (98 F/g at 0.5 A/g) and delivers an energy density of 0.151 mWh/cm2 at 3000 mW/cm2 (23.1 Wh/kg at 3000 W/kg). Moreover, the fabricated devices excitingly possess excellent flexibility and sensitivity with good applicability, demonstrating their broader application prospects in sensors.