Ligand-Controlled\nGrowth of Different Morphological\nBimetallic Metal–Organic Frameworks for Enhanced Charge-Storage\nPerformance and Quasi-Solid-State Hybrid Supercapacitors

Abstract

The present research work facilitates a ligand-mediated\neffective\nstrategy to achieve different morphological surface structures of\nbimetallic (Ni and Co) metal–organic frameworks (MOFs) by utilizing\ndifferent types of organic ligands like terephthalic acid (BDC), 2-methylimidazole\n(2-Melm), and trimesic acid (BTC). Different morphological structures,\nrectangular-like nanosheets, petal-like nanosheets, and nanosheet-assembled\nflower-like spheres (NSFS) of NiCo MOFs, are confirmed from the structural\ncharacterization for ligands BDC, 2-Melm, and BTC, respectively. The\nbasic characterization studies like scanning electron microscopy,\nX-ray diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller\nrevealed that the NiCo MOF prepared by using trimesic acid as the\nligand (NiCo MOF_BTC) with a long organic linker exhibits a three-dimensional\narchitecture of NSFS that possesses higher surface area and pore dimensions,\nwhich enables better ion kinetics. Also, the NiCo MOF_BTC delivered\nthe highest capacity of 1471.4 C g^–1 (and 408 mA\nh g^–1) at 1 A g^–1 current density,\ncompared to the other prepared NiCo MOFs and already reported different\nNiCo MOF structures. High interaction of trimesic acid with the metal\nions confirmed from ultraviolet–visible spectroscopy and X-ray\nphotoelectron spectroscopy leads to a NSFS structure of NiCo MOF_BTC.\nFor practical application, an asymmetric supercapacitor device (NiCo\nMOF_BTC//AC) is fabricated by taking NiCo MOF_BTC and activated carbon\nas the positive and negative electrode, respectively, where the PVA\n+ KOH gel electrolyte serves as a separator as well as an electrolyte.\nThe device delivered an outstanding energy density of 78.1 Wh kg^–1 at a power density of 750 W kg^–1 in an operating potential window of 1.5 V. In addition, it displays\na long cycle life of 5000 cycles with only 12% decay of the initial\nspecific capacitance. Therefore, these findings manifest the morphology\ncontrol of MOFs by using different ligands and the mechanism behind\nthe different morphologies that will provide an effective way to synthesize\ndifferently structured MOF materials for future energy-storage applications.