Hollow core–shell ZnO@ZIF-8 on carbon cloth for flexible supercapacitors with ultrahigh areal capacitance

Rational design and synthesis of hollow core-shell hetero-structures with high complexity still remains challenging for high-performance supercapacitors. Here, a simple and effective strategy that involves a 'root-etch-wrap' process was developed to synthesize hollow core-shell hetero-structured electrodes. Specifically, ZnO hollow spheres take root on carbon cloth via an in situ growth routine, then are etched to aid the generation of a ZIF-8 shell. As-synthesized hollow core-shell ZnO@ZIF-8 is wrapped by PANI nanocoating, resulting in a flexible conductive porous electrode (denoted as PANI/ZnO@ZIF-8-CC). The optimized electrode exhibits an ultrahigh areal capacitance (4839-3987 mF cm-2 at 5-30 mA cm-2), which is at least 3 times higher than that of PANI-CC and ZnO@ZIF-8-CC owing to the synergistic effect. In addition, a symmetric flexible supercapacitor fabricated by PANI/ZnO@ZIF-8-CC exhibits a high energy density of 0.137-0.0891 mW h cm-3 (at a power density of 1.421-23.629 W cm-3) and a good long-term cycling ability (87% for 10 000 cycles at 5 mA cm-2). All of these results make unique core-shell structured PANI/ZnO@ZIF-8-CC a promising electrode material for advanced energy storage and conversion applications.