Design Hierarchical Electrodes with Highly Conductive NiCo₂S₄ Nanotube Arrays Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors

We report on the development of highly conductive NiCo₂S₄ single crystalline nanotube arrays grown on a flexible carbon fiber paper (CFP), which can serve not only as a good pseudocapacitive material but also as a three-dimensional (3D) conductive scaffold for loading additional electroactive materials. The resulting pseudocapacitive electrode is found to be superior to that based on the sibling NiCo₂O₄ nanorod arrays, which are currently used in supercapacitor research due to the much higher electrical conductivity of NiCo₂S₄. A series of electroactive metal oxide materials, including Co_<i>x</i>Ni_1–<i>x</i>(OH)₂, MnO₂, and FeOOH, were deposited on the NiCo₂S₄ nanotube arrays by facile electrodeposition and their pseudocapacitive properties were explored. Remarkably, the as-formed Co_<i>x</i>Ni_1–<i>x</i>(OH)₂/NiCo₂S₄ nanotube array electrodes showed the highest discharge areal capacitance (2.86 F cm^–2 at 4 mA cm^–2), good rate capability (still 2.41 F cm^–2 at 20 mA cm^–2), and excellent cycling stability (∼4% loss after the repetitive 2000 cycles at a charge–discharge current density of 10 mA cm^–2)