Electrospun NiCo2S4 nanofibers decorated by rGO wrapping with high OH- adsorption ability for alkaline supercapacitors

The bimetallic sulfide of NiCo2S4 generally show better electrochemical performance than its bimetallic oxide counterpart in supercapacitor electrode applications. In this work, we theoretically explain their essential differences in reaction mechanisms through density functional theory, including their OH- adsorption energy, energy band structure, total and projected density of states, as well as d-band center of Ni and Co, which confirms the intrinsic advantage of NiCo2S4 over NiCo2O4 as electrode material for alkaline supercapacitors. Then, through electrospinning, we fabricate porous nanofibers of NiCo2S4-NF and NiCo2O4-NF with similar porous, fibrous morphology stacked by nanoparticles, of which NiCo2S4-NF gives obviously superior performance in alkaline electrolyte as supercapacitor electrodes, verifying the theoretical prediction. Moreover, an rGO coverage layer was in-situ incorporated to fabricate porous nanofibers of rGO@NiCo2S4-NF, which includes an additional surface rGO wrapping layer. The 1D and porous architecture renders rGO@NiCo2S4-NF more exposed OH- adsorption sites to induce active redox reactions, while the rGO coverage further stabilized the nanofibers and ensures inter-fiber charge transfer. Due to the combination of morphology optimization and carbon combination, rGO@NiCo2S4-NF delivers enhanced capacity, cycle stability and rate ability than NiCo2S4-NF and NiCo2O4-NF. Using rGO@NiCo2S4-NF, asymmetric supercapacitors are assembled, which also exhibits promising energy densities and power densities.