High-Energy-Density Asymmetric Supercapacitor Based on a Nickel Cobalt Double Hydroxide/Reduced-Graphene-Oxide Fiber Electrode

With high electrical conductivity, good mechanical strength, and excellent multidimensional flexibility, graphene fibers are more suitable for wearable devices than other flexible materials. However, challenges still exist in increasing their energy density. Here, we overcome this disadvantage by developing a fibrous supercapacitor loaded with battery-type active material. First, reduced-graphene-oxide fibers (rGOFs) with high conductivity and high toughness are fabricated from flake graphite by a series of redox reactions and wet-spinning processes. Second, Co-based zeolite imidazole framework (Co-ZIF) nanosheets are grown in situ on the surfaces of the rGOFs at room temperature and then converted into nickel cobalt layered double hydroxide (NCLDH) nanosheets by an etching codeposition method. The as-prepared NCLDH@rGOFs with a unique one-dimensional cladding structure exhibit an outstanding area specific capacitance of 2020 mF cm-2 at 5 mA cm-2 in a three-electrode system. When the NCLDH@ rGOFs and FeOOH@rGOFs are assembled into a flexible asymmetric supercapacitor (NF-FASC), the NF-FASC shows high area specific capacitance (351.6 mF cm-2), high energy density (max. 117.3 mu Wh cm-2), excellent power density (max. 34 000 mu W cm-2), and acceptable cycle stability (75.5% capacitance retention after 5000 cycles).