A perspective on two-dimensional materials for planar micro-supercapacitors

The increasing demand for portable, wearable, and miniaturized electronics has substantially promoted the immense development of planar microsupercapacitors (MSCs) built on a single substrate. Atomically thin two-dimensional (2D) nanosheets, by virtue of their intrinsically unique structure and fascinating electrochemical properties, provide a new material platform for the creation of high performance planar MSCs in which the electrolyte ions can completely utilize flat architecture and ultrathin thickness of 2D nanosheets, parallel to the direction of ionic diffusion along the plane of 2D nanosheets. Herein, we present an overview and perspective on diverse 2D materials for planar MSCs. First, an introduction is presented to highlight the advances of MSCs, the uniqueness of 2D materials in the assembly of planar MSCs with three different configurations, i.e., stacked, interdigital, and 3D planar geometries, and the progress of microfabrication techniques for microelectrodes of MSCs. Second, the state-of-the-art 2D materials to manufacture planar MSCs, including graphene, transition metal oxides/hydroxides, transition metal dichalcogenides, metal carbides, metal nitrides, phosphorene, boron nitride, metal-organic frameworks, and covalent-organic frameworks, are systemically discussed in detail. Special emphasis is given to the multiple roles of 2D materials for functional components as active materials, current collectors, additives/binders, and separators for planar MSCs. Finally, the existing challenges and prospective solutions of 2D materials for planar MSCs with high performance and various innovative form factors are proposed.