Largely enhanced pseudocapacitance by a facile in-situ decoration of MoS2 nanosheets with CoFe2O4 nanoparticles

Using a facile in-situ hydrothermal method, few-layer MoS2 nanosheets were decorated with CoFe2O4 nanoparticles (similar to 18 nm in size) in the presence of Ni foam. Then the CoFe2O4/MoS2 composite coated on Ni foam was studied as a supercapacitor electrode. Atomic force microscopy (AFM) shows the successful exfoliation of bulk MoS2 powder into few-layer MoS2 nanosheets. X-ray diffraction (XRD) patterns confirm the crystal structure formation of CoFe2O4/MoS2 nanostructures and transmission electron microscopy (TEM) demonstrate their morphologies. The electrochemical energy storage tests were conducted using a three-electrode setup. We found that MoS2 nanosheets considerably enhance the pseudocapacitance properties of CoFe2O4 nanoparticles, especially its specific capacitance from 500 to 1013 F/g, its power density from 125.0 to 166.6 W/kg, and its energy density from 11.1 to 22.5 Wh/kg. More importantly, we found that the incorporation of MoS2 nanosheets enhances the charge-discharge cycling stability of CoFe2O4. We also constructed a two-electrode (asymmetric) setup using the CoFe2O4/MoS2 composite coated on Ni foam as one electrode and activated carbon as the other electrode, and showed it can light up a red LED for several minutes. We also conducted some calculations in the framework of density functional theory to study how the interface of MoS2 and CoFe2O4 would help them toward better charge storage. We found that the built-in electric field developed across the interface can pull electrons from CoFe2O4 toward MoS2 for better charge redistribution, and the incorporation of MoS2 would add some energy states near the Fermi level that can increase the capacitance of CoFe2O4.