Highly Interconnected Nanorods and Nanosheets Based on a Hierarchically Layered Metal-Organic Framework for a Flexible, High-Performance Energy Storage Device

The pseudocapacitors based on metal-organic frameworks (MOFs) are one of the state-of-the-art candidates to improve the future energy storage systems because of their high porosity, diverse functional groups, and potential as the template. In this study, the direct growth of a urea-assisted hierarchically layered cobalt-based MOF (u-hl-MOF) on Ni foam and direct growth of nanorods and nanosheets on the surface of the u-hl-MOF via the MOF-mediated approach without the addition of a cobalt precursor are reported. After the annealing process with sulfur powder under an Ar flow is complete, only the surface of the u-hl-MOF is converted into cobalt sulfide@carbon (Co_1-xS@C), whereas the bulk internal u-hl-MOF remains without any degradation. Therefore, a core-shell structure of u-hl-MOF@Co_1-xS@C (u-hl-MSC) is formed, which exhibits a high areal specific capacitance of 13.1 F cm^-2 with a three-electrode system. A solid-state flexible asymmetric supercapacitor (ASC) is also assembled using poly(vinyl alcohol)/KOH, u-hl-MSC, and activated carbon/Super P as the electrolyte, cathode, and anode materials, respectively. This solid capacitor exhibits one of the highest areal capacitance of 1.35 F cm^-2 and an energy density of 270 mW h cm^-2 with a high power density of 0.6 W cm^-2 among all the MOF- or sulfide-based ASCs.