Surface electronics regulation and enhanced electrochemical performance of nitrogen-doped carbon layer encapsulated NiCo₂S₄ nanosheets for supercapacitors

The rational construction of nano-architecture and the regulation of surface electronics in active materials are crucial for high-energy supercapacitors. Herein, a nitrogen-doped carbon layer (NC) was encapsulated on the electrodeposited NiCo2S4 nanosheets on nickel foam (NF) via the thermal vapor deposition polymerization technique, which effectively mitigated the volume fluctuation of NiCo2S4 nanosheets and significantly enhanced the electrical conductivity. Additionally, the electronic structure of the carbon skeleton and the adsorption energy of OH- can be tuned by the incorporation of pyridinic-N and pyrrolic-N in the NC, as revealed by density functional theory (DFT) calculations. Consequently, the binder-free NiCo2S4@NC/NF electrode exhibits superior storage capacity (1800 and 1420.5 F/g at 1 and 20 A/g, respectively). Moreover, the assembled hybrid supercapacitor with NiCo2S4@NC/NF as cathode and N-doped activated carbon (N-AC) as anode exhibits high energy density (46.0 Wh/kg) and long-cycling stability (91.4 % retention after 4000 cycles). Our work unveils the mechanism of NC encapsulation in modulating the surface electronic structure, the adsorption energy towards the electrolyte ions and the electrochemical performance of the materials at the theoretical calculation level. This finding provides a promising avenue for engineering the surface electronics in energy storage devices.