Synthesis of Metal @ Carbon Sphere/Graphene Film Electrodes for Flexible Asymmetric Supercapacitors

Abstract

Free-standing, binder-free and high-performance metal @ carbon sphere/graphene (Fe@C/G and Co@C/G) film electrodes were synthesized by using two-dimensional graphene sheets as adhesives and the toroidal shaped β-cyclodextrins (β-CDs) as carbon sources. The mulberry-like Fe@C spheres and urchin-like Co@C spheres were obtained by a simple hydrothermal method through adjusting the proportions of carbon and metal. And the flexible Fe@C/G and Co@C/G film electrodes were prepared by water bath and vacuum filtration methods. Most interestingly, the Fe@C/G and Co@C/G film electrodes exhibited the maximum areal specific capacitances of 2.19 F cm-2 and 0.64 F cm-2 at a current density of 0.5 mA cm-2, respectively (438 F g-1 and 128 F g-1 at a current density of 0.1 A g-1, respectively). Furthermore, a flexible solid-state asymmetric Fe@C/G//Co@C/G supercapacitor with high flexibility, good cycle stability and high energy density was assembled together in parallel with PVA/H2SO4 gel electrolyte by using Fe@C/G film as positive electrode and Co@C/G film as negative electrode. As expected, the device displayed an operating voltage of 0-1.8 V and delivered a high volumetric specific capacitance of about 9.05 F cm-3 at 0.5 mA cm-2. Moreover, it showed an excellent volumetric energy density of up to 4.07 mW cm-3 at a volumetric power density 0.009 W cm-3 and a good cycle stability of 87.3% capacitance retention after 10000 charge/discharge cycles. Even after 500 bending/unbending cycles, still about 80.5% of its initial capacitance was retained. Carbon coating on metal surface could introduce pseudocapacitive metal oxides into graphene composites to prepare graphene flexible films. Such results well demonstrated its strong potential for the practical applications of flexible and wearable energy storage devices. Figure 1