Compact energy storage: Opportunities and challenges of graphene for supercapacitors

Gravimetric energy densities of nanocarbons continuously improved over the years. However, due to their low densities, these nanomaterials normally suffer from relatively low volumetric performance, and their high gravimetric energy densities cannot be translated to the practical devices. Developing electrode materials with high volumetric performance and achieving compact energy storage on a device level is highly important to promote the materials and devices for energy storage into real applications. As a basic unit for all types of sp2 carbons and flexible 2D material, graphene has many intrinsic characteristics beneficial to compact energy storage. Based on the applications of graphene in supercapacitors, the paper presents design principles for practical energy storage devices from the perspective of material, electrode and devices respectively. The recent efforts to prepare electrode materials with a high volumetric performance, particularly the design concept on the high density electrode materials for high volumetric performance are also introduced. This review further highlights the importance to design energy storage material from a device perspective and discusses the opportunities and challenges toward compact energy storage.