Facile Preparation of High-Performance Free-Standing Micro-Supercapacitors by Optimizing Oxygen Groups on Graphene

Free-standing micro-supercapacitor (MSC) devices without substrate or current collectors are promising for practical applications. However, it is still difficult to prepare high-performance free-standing MSC devices because of the requirement of optimized active sites, conductivity, ion diffusion, controlled patterns, moisture susceptibility, etc. Here, it is proposed that the optimization of oxygen level on graphene is promising to solve these requirements because of the balance of sp² and sp³ hybridization. Using the medium-oxidized graphene, the flexible, conductive, hydro-stable, easy-processing film can be facilely obtained, which facilitates the preparation of free-standing MSC electrodes. After constructing with gel electrolyte, the free-standing MSC device shows a high capacitance of 898.4 mF cm^-2 using aqueous-gel electrolyte and 383.6 mF cm^-2 using ion-gel electrolyte with mass loading of ca. 10 mg cm^-2. Correspondingly, the MSC device can achieve a landmark energy density of 42.6 µWh cm^-2 at 0.85 mW cm^-2 (7.1 mWh cm^-3 at 141.7 mW cm^-3). The advantages of high performance, facile preparation, and low inactive components make the free-standing MSC device promising for practical applications.