Highly Hydrophilic Carbon Dots' Decoration on NiCo2O4 Nanowires for Greatly Increased Electric Conductivity, Supercapacitance, and Energy Density

The design and exploration of the transition metal oxides (TMOs)-carbon composite are greatly desired for enhanced supercapacitor application due to the low hydrophilicity and electrical conductivity of the prime TMOs. In this work, the highly hydrophilic carbon dots (CDs) with an average size of 2.56 nm are synthesized by a microwave thermolysis method, and then used as a decoration agent on the surface of NiCo2O4 nanowires. The CDs-decorated NiCo2O4 (CDs/NiCo2O4) with a high surface area of 120.0 m(2) g(-1) is prepared via a one-pot hydrothermal method and subsequent annealing. Only a small amount of CDs' decoration not only makes the surface more hydrophilic and easier for aqueous electrolyte infiltration into electrode, but can also promote the electrical conductivity of the as-synthesized CDs/NiCo2O4 to 30-fold higher than the prime NiCo2O4, and greatly decrease the internal resistance of the electrode material. Therefore, the CDs/NiCo2O4-based three-electrode system shows an extremely high specific capacitance (2202 F g(-1)) while a symmetrical CDs/NiCo2O4 supercapacitor offers a desirable energy density up to 73.5 Wh kg(-1). This work provides a new strategy to improve supercapacitance as well as energy density of TMO-based electrodes via a convenient surface decoration method by using carbon nanomaterials.