Progress and prospects of graphene for in-plane micro-supercapacitors

Micro-supercapacitors hold great promise for powering the Internet of Things devices owing to their high power density and long cycling life. However, the limited energy density hinders their practical use. Electrode materials play an important role in the performance of micro-supercapacitors. With the advantages of a large specific surface area and a high electrical conductivity, graphene has been considered a good candidate for the electrode material of micro-supercapacitors. The two-dimensional surface of graphene is parallel to the direction of transport of the electrolyte ions for micro-supercapacitors with an in-plane structure, which helps improve the ion accessibility of the electrodes. Therefore, the construction of graphene-based in-plane micro-supercapacitors has aroused great interest among researchers. Here, we summarize the recent advances in graphene and graphene-based materials for in-plane micro-supercapacitors from the perspective of electrode material design. The electrode materials include graphenes produced by chemical vapor deposition, liquid-phase exfoliation, reduction of graphene oxide, laser induction and heteroatom doping, as well as graphene-based composites, such as carbon nanotube/graphene, transition metal oxide/graphene, conducting polymer/graphene and two-dimensional material/graphene composites. Challenges and opportunities in graphene-based in-plane micro-supercapacitors are discussed, and future research directions and development trends are proposed.