Engineering the structures of ZnCo-MOFs via a ligand effect for enhanced supercapacitor performance

Tuning the structures, compositions and morphologies of metal-organic frameworks (MOFs) is critical to boosting their supercapacitor performances. In this study, a ligand-engineering strategy was adopted to fabricate ZnCo-bimetallic MOFs with unique properties using three different ligands (2-methylimidazole, terephthalic acid and 2-amino terephthalic acid) under the same synthesis protocol. The variation in the electron-donating ability of the three ligands gave rise to changes in their structural, morphological and electrochemical properties. Compared to other MOFs, the imidazole-based ZnCo-MOF (ZnCo-MOF-HMIM) with a dodecahedron morphology, good specific surface area and moderate pore characteristics provided considerable electron transport paths for ion migration on the electrode surface site, which guarantees a greater charge storage. Specifically, ZnCo-MOF-HMIM delivered the best specific capacity of 176.8 m h A g^-1 at 1 A g^-1 specific current and retains about 87.5% of its capacity at 10 A g^-1 after 5000 cycles. Furthermore, the asymmetric device achieved a specific energy of 28.2 W h kg^-1 at a specific power of 1025.4 W kg^-1 and demonstrates remarkable coulombic efficiency and capacity retention of 98.4% and 80.0% at 10 A g^-1 over 10 000 cycles respectively. The presence of an N donor atom in the imidazole ligand which imparts high hydrophobicity, and the synergistic effects of Zn and Co ions could predispose ZnCo-MOF-HMIM to have more active sites and greater stability for enhanced performance. This work provides insight into the key role of ligands in the formation mechanism of bimetallic MOFs for enhanced electrochemical energy storage.