Development of high performance alpha-Co(OH)2/reduced graphene oxide microfilm for flexible in-sandwich and planar micro-supercapacitors

Herein, a series of alpha-Co(OH) 2 /reduced graphene oxide (rGO) microfilms and film-based nanodevices were developed via a new scalable technique. Due to the unique hexagonal nanoplates of ultrathin alpha-Co(OH) 2 and the intrinsically conductive nature of rGO sheets, such thin films not only can improve the conductivity of alpha-Co(OH) 2 and prevent the re-stacking of alpha-Co(OH) 2 and rGO sheets but also short the transport routes of electrons and ions between the electrode and the electrolyte. The optimized alpha-Co(OH) 2 /rGO flexible electrode presents high specific capacitance (273.86 mF/cm 2 at 0.1 mA/cm 2 ), advanced rate capability, and excellent coulombic efficiency. Simultaneously, in-sandwich symmetric and asymmetric supercapacitors assembled with polyvinyl alcohol-KOH gel as the solid-state electrolyte achieved high areal and volumetric specific capacitances. Furthermore, a self-assembled planar alpha-Co(OH) 2 /rGO micro-supercapacitor (MSC) delivers high specific area capacitance (130F/cm 2 at 0.5 mA/cm 2 ) and excellent energy density (20 mWh/cm 3 @56 mW/cm 3 ), which are superior to most of the recently reported carbon-based and metal hydroxides/oxides/sulfides-based planar MSCs. Also, our planar MSC shows excellent cycling performance, good flexibility, and mechanical stability. This work promotes the syntheses of other two-dimensional metal hydroxides/rGO composite film for high-performance flexible micro-electronics.