Hierarchical bimetallic hydroxide/chalcogenide core–sheath microarrays for freestanding ultrahigh rate supercapacitors

Transition metal compounds (TMCs) either crystalline or amorphous exhibit specific advantages in electrochemical energy storage. To integrate their merits into one electrode, we have herein developed hierarchical bimetallic hydroxide/chalcogenide core-sheath microarrays on nickel foam (NF) for freestanding high-efficiency supercapacitors, wherein interior crystalline metal chalcogenides serve as highly conductive pivots and exterior amorphous bimetallic hydroxides provide rich ion diffusion channels. With the synergic effect of the unique structure and bimetallic composition, the as-prepared Ni(OH)₂-Co(OH)₂/NiSe-Ni₃S₂/NF electrode displays an ultrahigh areal specific capacitance of 19.01 F cm^-2 at 15 mA cm^-2, which can be retained as 6.01 F cm^-2 even at 125 mA cm^-2. To the best of our knowledge, such excellent tolerance of ultrafast ion insertion/extraction at high current density is rare among NF-based free-standing electrodes. The asymmetric supercapacitor by assembling with activated carbon as the negative electrode delivers a volumetric capacitance of 3.93 F cm^-3 at 30 mA cm^-2, corresponding to an energy density of 13.9 mW h cm^-3 at a power density of 200 mW cm^-3. A capacitance retention of 82.5% was observed after 4000 cycles, together with an average 97% coulombic efficiency. This work may provide a facile strategy to construct hierarchical microarrays for efficient energy storage devices.