MOF-reinforced Co9S8 self-supported nanowire arrays for highly durable and flexible supercapacitor

Abstract

The structural stability of the electrode material is very important to achieve a long cycle life of a supercapacitor. In this work, we demonstrate a multi-step preparation of metal organic framework (MOF)-reinforced Co9S8 self-supported nanowire arrays for high-performance supercapacitor with very high durability. Co-based nanowires with a composition of Co2(OH)2(CO3)2 are first synthesized on the nickel foam substrate by a facile hydrothermal method. After converting them into Co3O4 nanowires by calcination, leaf-like Co-based MOF (Co-MOF) has been deposited on the surface of the oxide nanoarrays (NAs), forming Co3O4 NAs@Co-MOF core-shell structure. Co9S8 nanoarrays are finally obtained by an in-situ hydrothermal sulfurization, and this process not only realizes the chemical transformation, but also changes the morphology of the Co-MOF from leaf-like microplatelets into nanosheets wrapping around the one-dimensional (1D) nanowires. When this sample has been applied for supercapacitor, it exhibits a 4.48 F cm−2 at a current density of 2 mA cm−2. The long-term cycling test is conducted at 25 mA cm−2, and the sample can maintain a high and stable capacitance of 1.6 F cm−2 after 100k cycles. This gives rise to a capacitance loss of only 5.1 × 10−4% per cycle, demonstrating an exceptional cycling stability. Furthermore, an asymmetric supercapacitor is assembled by pairing the Co9S8 nanoarrays with activated carbon, and the device delivers excellent cycle stability with a reversible capacitance of 416 mF cm−2 after 100k cycles, leading to a capacitance loss of 2.8 × 10−4% per cycle. Furthermore, a quasi-solid-state asymmetric supercapacitor is able to light up 16 red LEDs, demonstrating great potential for practical application with excellent cycle stability and mechanical flexibility.