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

Abstract

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 m2 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.

Carbon-Based Quantum Dots for Supercapacitors: Recent Advances and Future Challenges

“…Initially, the use of carbon dots as tandem materials in SC aimed to exploit their surface functionalization to increase SC performances. As they have a lot of hydrophilic functional groups on the surface, CDs can increase the electrodes’ wettability as reported in their utilization together with NiCo 2 O 4 nanowires, as shown in Figure 7 a [ 132 ]. Furthermore, CDs also may increase the active surface area, provide unique morphology, and modify the conductivity.…”

“…( c ). The specific capacitance of NiCo 2 O 4 and CDs/NiCo 2 O 4 in varying current densities: reproduced from Reference [ 132 ]. Copyright John Wiley and Sons, 2019.…”

Rational Design of Carbon Layer-Decorated Metal Oxide/Nickel Cobalt Sulfide-Based Composite with Faster Energy Storage and Long Cyclic Life

“…At present, Co and Ni TMOs have shown great advantages toward SCs because of their remarkable specific discharge capacity and cost-effectiveness. − Nevertheless, the unsatisfactory electrochemical performance of a TMO in SC applications is due to the material’s low electronic conductivity and poor rate performance. Thus, many strategies have been proposed to solve these problems.…”

Porous NiCo2O4–FeCo2O4 Nanowire Arrays as Advanced Electrodes for High-Performance Flexible Asymmetric Supercapacitors

“…Moreover, vertically aligned nanowire arrays grown on a conductive substrate could increase the surface area and reduce the ion transport distance. Furthermore, CC can be tightly bound to the active material to realize high-quality loading . Thus, these advantages encourage the formation of nanocomposites with a 3D hierarchical structure, instead of transition metals; such structures can boost the efficiency of active substances and enhance the stability. ,, …”

“…Furthermore, CC can be tightly bound to the active material to realize high-quality loading. 36 Thus, these advantages encourage the formation of nanocomposites with a 3D hierarchical structure, instead of transition metals; such structures can boost the efficiency of active substances and enhance the stability. 3,37,38 To address these issues while maintaining the advantages, we developed a novel NiCo 2 O 4 −FeCo 2 O 4 nanowire array electrode via surface activation of CC, hydrothermal growth of the precursor nanowire array, and transformation by fast calcination.…”