Hierarchical optimization of metal–organic frameworks <i>via</i> bimetallic incorporation and ligand-induced defects towards enhanced capacitive performance

Rationally tailoring the crystal structures of metal-organic frameworks (MOFs) to improve their capacitive performance is a challenge. This study demonstrates a sequential two-step modification strategy to enhance the electrochemical activity of MOFs, including secondary metal ion incorporation and ligand defect engineering. The synergistic effect between two metals in bimetallic MOFs could improve the charge transfer efficiency, stability and overall electrochemical performance. On the other hand, ligand-deficient defects in MOFs can result in coordinative unsaturation at metal centers, enhancing the exposure and availability of active sites. Herein, the crystal structure was modulated by doping Co²⁺ ions into a Ni-MOF and simultaneously introducing the isophthalic acid (IPA) ligand structurally analogous to 1,3,5-benzenetricarboxylic acid (H₃BTC), yielding a defect-rich bimetallic MOF. The resulting defective bimetallic Co/Ni-MOF-10 exhibits the highest specific capacitance of 1179.8 F g^-1 (589.9 C g^-1), surpassing that of the pristine Ni-MOF by 77.9%. Significant enhancements in both cycling stability and rate capability have also been achieved. Moreover, the assembled Co/Ni-MOF-10//AC ASC device shows suitable energy density and stable cycling performance over 3000 cycles, retaining 85.1% of its capacitance and maintaining 96.0% efficiency. This work suggests that rational manipulation of the coordination environment at metal centers constitutes an effective strategy to enhance capacitive performance.