Transition metal hydroxides@conducting MOFs on carbon nanotube yarns for ultra-stable quasi-solid-state supercapacitors with a ship-in-a-bottle architecture

Abstract

Yarn-shaped supercapacitors (SCs) functionalized with pseudocapacitive materials show promise in wearable electronics. However, their development was hindered by poor electrochemical properties, especially long-term cycling stability, owing to the volumetric change during charging/discharging. Herein, we report a ship-in-a-bottle architecture on carbon nanotube yarn (CNTY) based SCs, in which transition metal hydroxide (TMH) nanoparticles (Ni(OH)₂ or Co(OH)₂) are confined in conducting nanoporous metal–organic frameworks (MOFs, Ni₃(HITP)₂) which anchor onto CNTY, involves the synergy of nanoconfinement and hydrogen bonding (H-bonding) network to mutually support each phase toward improved electrochemical performance. The Ni(OH)₂@Ni₃(HITP)₂@CNTY electrode possesses an areal specific capacitance of 496 mF cm⁻² at 0.4 mA cm⁻² due to the hierarchical structure which led to facilitated charge transport and enhanced ion storage. Moreover, the ternary CNTY-based SCs demonstrate exceptional cycle performance (90.9–92.3% capacitance retention after 10 000 cycles at 5 mA cm⁻²). Importantly, the nanoconfinement is confirmed by field emission scanning electron microscopy, transmission electron microscopy-energy dispersive spectroscopy, and energy dispersive spectroscopy, and Brunauer–Emmett–Teller and cryogenic-TEM characterization studies. The H-bonding (O⋯H–N) network between Ni(OH)₂ and Ni₃(HITP)₂ is confirmed by Fourier transform infrared spectroscopy and density functional theory calculations. Both nanoconfinement and the H-bonding network contribute to an ultra-stable Ni(OH)₂@Ni₃(HITP)₂ structure due to its high durability to volumetric change caused by phase separation and structural collapse during charging/discharging.