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

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.