Design of ultra-thin nanosheet bimetallic NiCo MOF with binary ligand via solvent-assisted ligand exchange (SALE) reaction for high performance supercapacitors

Metal-organic frameworks (MOFs) are attractive active materials for pseudocapacitor electrodes owing to their high porosities, large specific surface areas, and unique morphologies. MOFs comprising various metal ions have been studied to improve the electrochemical performance of pseudocapacitor electrodes. Bimetallic MOFs form more abundant redox sites owing to the synergetic effect of metal ions. In this work, we fabricated a NiCo-BDC ligand based bimetallic MOF (NiCo-BDC) directly on a Ni-foam substrate via a facile hydrothermal process. After the in-situ growth of NiCo-BDC, some of the 1,4-benzenedicarboxylic acid (BDC) ligands combined with metal ions on the surface and were exchanged to form a new ligand, 2-methylimidazole (MIM), through a solvent-assisted ligand exchange (SALE) reaction. Although the chemical composition of the MOF changed during the SALE reaction, the original nanosheet morphology and high specific surface area were maintained. The chemical state of the metal ions as well as the elemental composition were changed during the SALE reaction, resulting in an increase in the number of electrochemically active sites. Furthermore, this unique binary ligand system decreased the charge transfer resistance and facilitated redox reactions. The NiCo-BDC@MIM binary ligand based bimetallic MOF with a Ni/Co molar ratio of 2:3 (NiCo-23-BDC@MIM) exhibited a specific capacitance of 2252 F g−1 at 1 A g−1, which was considerably improved compared to that before the SALE reaction (1047 F g−1). Moreover, it not only showed a high energy density of 40.2 Wh kg−1 at a power density of 972 W kg−1, but maintained 73.2% of its initial capacitance and 100% Coulombic efficiency after 2000 cycles. These results demonstrate the potential of MOFs with binary ligand systems as well as bimetallic structures for application as energy storage materials.