In the past decades, the energy consumption of nonrenewable fossil fuels has been increasing, which severely threatens human life. Thus, it is very urgent to develop renewable and reliable energy storage devices with features of environmental harmlessness and low cost. High power density, excellent cycle stability, and a fast charge/discharge process make supercapacitors a promising energy device. However, the energy density of supercapacitors is still less than that of ordinary batteries. As is known to all, the electrochemical performance of supercapacitors is largely dependent on electrode materials. In this review, we firstly introduced six typical transition metal oxides (TMOs) for supercapacitor electrodes, including RuO2, Co3O4, MnO2, ZnO, XCo2O4 (X = Mn, Cu, Ni), and AMoO4 (A = Co, Mn, Ni, Zn). Secondly, the problems of these TMOs in practical application are presented and the corresponding feasible solutions are clarified. Then, we summarize the latest developments of the six TMOs for supercapacitor electrodes. Finally, we discuss the developing trend of supercapacitors and give some recommendations for the future of supercapacitors.

With the development of wearable and flexible electronic devices, there is an increasing demand for new types of flexible energy storage power supplies. The flexible supercapacitor has the advantages of fast charging and discharging, high power density, long cycle life, good flexibility, and bendability. Therefore, it exhibits great potential for use in flexible electronics. In flexible supercapacitors, graphene materials are often used as electrode materials due to the advantages of their high specific surface area, high conductivity, good mechanical properties, etc. In this review, the classification of flexible electrodes and some common flexible substrates are firstly summarized. Secondly, we introduced the advantages and disadvantages of five graphene-based materials used in flexible supercapacitors, including graphene quantum dots (GQDs), graphene fibers (GFbs), graphene films (GFs), graphene hydrogels (GHs), and graphene aerogels (GAs). Then, we summarized the latest developments in the application of five graphene-based materials for flexible electrodes. Finally, the defects and outlooks of GQDs, GFbs, GFs, GHs, and GAs used in flexible electrodes are given.

Ni-metal–organic framework (MOF)/nano carbon (NC) electrode materials were synthesized by a one-step hydrothermal method. The prepared Ni-MOF/NC presented in the form of NC distributed uniformly on the Ni-MOF sheets, which effectively increased the specific surface area of the material and the rate of redox reaction. The Ni-MOF/NC electrode delivered 828 F/g at 1 A/g. In addition, the asymmetric supercapacitor (ASC) assembled by Ni-MOF/NC and active carbon retained 100% of specific capacitance after 5000 cycles. Furthermore, the ASC delivered energy density of 23.84 W h/kg at 849.74 W/g.

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The flower-like NiCo MOF prepared by a hydrothermal has a specific capacitance of 927.1 F g−1 at 1 A g−1 and a capacitance retention of 69.7% from 1 A g−1 to 10 A g−1, showing excellent electrochemical performance.