In situ growth of two-dimensional thienyl based bimetallic nickel‑cobalt metal-organic framework nanosheet arrays for enhanced electrochemical energy storage

The design and fabrication of novel two-dimensional (2D) metal-organic framework (MOF) nanosheets with excellent electrochemical performance are significant in the field of energy storage. Herein, a series of 2D bimetallic nickel‑cobalt MOFs (NiCo-MOFs) nanosheets based on thiophene ligand with adjustable components are synthesized using an in situ approach and a facile one-step hydrothermal reaction in DMF/H2O mixed solvent. These thienyl based NiCo-MOF nanosheets, grown directly on nickel foam (NF), function as electrodes for supercapacitors. The optimal NiCo-MOF/NF-3-2 (Ni/Co = 3:2) obtained by modulating the Ni/Co molar ratio demonstrates an ultrahigh specific capacity of 615.7 C g−1 (1243 F g−1) at the current density of 1 A g−1 and an excellent rate capacity (63.9% retention at 10 A g−1). DFT calculations indicate that adjustable bimetallic composition can modulate the electronic structure of metal centers and improve conductivity of MOF materials. The exceptional electrochemical performance of the NiCo-MOF/NF-3-2 electrode can be attributed to regular 2D nanosheet arrays on a conductive substrate, which provides open spaces and short transport distance between abundance active sites and electrolyte ions. Moreover, an asymmetric supercapacitor device composed of NiCo-MOF/NF-3-2 and active carbon exhibits a high energy density of 54.6 Wh kg−1 at a power density of 800 W kg−1 and an outstanding cycle life with a capacitance retention of 84.7% after 5000 cycles. This study demonstrates the remarkable potential of novel 2D bimetallic NiCo-MOF nanosheets for energy storage and conversion applications.