Lignin-derived carbon nanofibers-laminated redox-active-mixed metal sulfides for high-energy rechargeable hybrid supercapacitors

The design of inexpensive, binder-free electrodes with high energy densities from naturally occurring compounds poses an interesting challenge for the construction of high-performance supercapacitors. In particular, the synthesis of highly conductive carbon electrodes starting from miscanthus-derived (silver grass) lignin with inorganic active sites is of interest for hybrid supercapacitors. In this work, Ni-Mn sulfide nanograin networks are embedded on lignin-derived carbon nanofibers (LCNFs) by electrospinning, followed by either direct vapor-phase sulfidation or peroxidation-induced sulfidation in an inert atmosphere. After either of the two different sulfidation processes, the direct transformation of Ni-Mn sulfide on carbon nanofibers results in highly mechanically flexible freestanding electrodes, with a good specific capacity of 652.3 C g⁻¹ at 1 A g⁻¹, excellent current rate properties (73.6% capacity retention at 20 times the initial current density), and a cycling stability of 91.3% after 5000 cycles, even at the high current density of 10 A g⁻¹. These outstanding properties are attributed to the rational design of surface-embedded Ni-Mn sulfide nanograins on the 3D LCNFs and the fascinating synergetic and graphitic nature of the fibrous web, which increases conductivity, influences the interfacial contact, and promotes highly electrochemically active sites on the electrode surface. Furthermore, an aqueous hybrid supercapacitor assembly of LCNFs-NiMnS//activated carbon (AC) is demonstrated, with a maximum energy density of 52.4 W h kg⁻¹ at a power density of 800 W kg⁻¹, as well as 92.3% capacitance retention even after 10 000 cycles. This work not only establishes a high-capacity anode but also provides an ideal strategy for designing flexible and conductive heterostructure network for various energy storage applications.