Tingting Liu

In this paper, δ-MnO₂ with layered structure was prepared by a facile liquid phase method, and exfoliated MnO₂ nanosheet (e-MnO₂) was obtained by ultrasonic exfoliation, whose surface was negatively charged. Then, positive charges were grafted on the surface of MnO₂ nanosheets with a polycation electrolyte of polydiallyl dimethylammonium chloride (PDDA) in different concentrations. A series of e-MnO₂@reduced graphene oxide (rGO) composites were obtained by electrostatic self-assembly combined with hydrothermal chemical reduction. When PDDA was adjusted to 0.75 g/L, the thickness of e-MnO₂ was ~1.2 nm, and the nanosheets were uniformly adsorbed on the surface of graphene, which shows layer-by-layer morphology with a specific surface area of ~154 m²/g. On account of the unique heterostructure, the composite exhibits good electrochemical performance as supercapacitor electrodes. The specific capacitance of e-MnO₂-0.75@rGO can reach 456 F/g at a current density of 1 A/g in KOH electrolyte, which still remains 201 F/g at 10 A/g. In addition, the capacitance retention is 98.7% after 10000 charge-discharge cycles at 20 A/g. Furthermore, an asymmetric supercapacitor (ASC) device of e-MnO₂-0.75@rGO//graphene hydrogel (GH) was assembled, of which the specific capacitance achieves 94 F/g (1 A/g) and the cycle stability is excellent, with a retention rate of 99.3% over 10000 cycles (20 A/g).

Manganese oxides, as a type of two-dimensional (2D) material with high specific area and low cost, are considered promising energy storage materials. Here, we report novel AgMn₂O₄/Na_0.55Mn₂O₄ nanosheets created by a popular liquid precipitation method with different AgNO₃ contents, and their corresponding physical and electrochemical characterizations are performed. The results show that the ultra-thin Na_0.55Mn₂O₄ nanosheets were combined with the AgMn₂O₄ nanoparticles and an enhancement in their specific capacity was observed compared to the pristine sheets. This electrode material displays a peak specific capacitance of 335.94 F g⁻¹ at 1 A g⁻¹. Using an asymmetric supercapacitor (ASC) assembled using a positive electrode made of AgMn₂O₄/Na_0.55Mn₂O₄ nanosheets and a reduced graphene oxide (rGO) negative electrode, a high energy density of 65.5 Wh kg⁻¹ was achieved for a power density of 775 W kg⁻¹. The ASC showed good cycling stability with a capacitance value maintained at 90.2% after 10,000 charge/discharge cycles. The excellent electrochemical performance of the device was ascribed to the heterostructures and the open space formed by the interconnected manganese oxide nanosheets, which resulted in a rapid and reversible faraday reaction in the interface and further enhanced its electrochemical kinetics.

Anisotropic NiCo₂O₄ nanoneedle arrays grew directly on Ni foam in the presence of rGO via the hydrothermal method followed by annealing assisted by hexadecyl trimethyl ammonium bromide (CTAB).