Abstract:

Owing to their high power density, excellent rate performance, and good cycle performance, supercapacitors are widely used for energy storage applications. Two-dimensional (2D) layered materials are structural compounds having a layered host structure with a layer thickness of nanometers and a lateral dimension of several micrometers. Their layer spacing can be controlled by changing the interaction between the layers. 2D layered materials can be delaminated into nanosheets. Exfoliated nanosheet materials provide a new strategy for improving the performance of supercapacitors. Unlike bulk layered materials, the exfoliated nanosheets not only provide a unique nano-scale reaction space for electrochemical reactions, but also offer the possibility for improving the specific capacitance and storage rate of the supercapacitor. However, in 2D layered materials, the ion or electron transport in the vertical direction is obstructed, despite their fast ion and electron transport in the horizontal direction. As a result, the occurrence of electrode reactions, and hence the realization of rapid storage become difficult. It is detrimental to the power and energy densities and rapid energy storage of supercapacitors. Therefore, it is imperative to develop novel electrode materials in order to fabricate supercapacitors showinghigh energy densities at high power densities.

Porous 2D layered electrode materials offer two major advantages. First, the porous structure can alleviate the problems caused by the stacking of nanosheets during the assembly process. Second, the porous structure can effectively promote the electrolyte penetration of the electrode, which alleviates the volume changes of the electrode material during the charging-discharging process and releases the structural strain. Hence, such materials facilitate ion or electron transport, thus increasing the specific capacitance of the supercapacitor. Over the past few years, 2D nanosheet holeization has evolved as a promising approach to improve the energy density of supercapacitors at high power densities. Various porous 2D layered supercapacitor electrode materials have been developed. This paper reviews the exfoliation of 2D layered materials, nanosheet holeization strategy, and the application of assembled porous layered electrode materials in supercapacitors. In this paper, we have reviewed the exfoliation of 2D layered materials with different electric properties and the performance of 2D nanosheets. Different methods used for the preparation of holey 2D nanosheets have also been discussed. We prepared holey MnO₂ nanosheets and reduced graphite oxide via redox holeization mechanism, and 2D porous nanomaterials were also prepared by using suitable templates such as hard or self-sustaining templates. These holey 2D nanosheets were used to prepare porous 2D layered electrode materials such as holey graphene/manganese dioxide composite fibers and holey graphene/polypyrrole hybrid aerogels. The capacitance of these electrode materials was investigated systematically. Finally, the prospects for the development of porous 2D layered electrode materials such as the optimization of theirrate performance, flexibility, and energy density were discussed. Novel holeization methods should be developed in order to prepare metal oxide nanosheets with controllable hole sizes. In addition, other 2D materials such as MXene should be explored.

Key words: Two-dimensional layered material, Delamination, Nanosheet holeization, Holey electrode material, Supercapacitor