Optimizing the Structure and Performances of Cu-MOF@Ti₃C₂T_X Hybrid Electrodes by Introducing Modulated Ligand

To date, two-dimensional metal-organic frameworks (2D MOFs) have attracted much attention in many fields. Owing to their ultra-high porosity and specific surface area, great structural diversity and functional tunability, as well as feasible precision design at the molecular level, 2D MOFs have won rapid development in the field of energy storage. However, as a coordination compound, MOFs possess poor structural stability and are prone to structural collapse in electrochemical reactions, which seriously limits their electrochemical performance. Therefore, there is an urgent need to improve the structural stability of MOF electrode materials. In this study, a 2D MOF@Ti₃C₂T_X hybrid was constructed, in which urea pyrimidinone isocyanate (UPy-NCO) units were introduced via a condensation reaction with the active functional groups on MOFs, thus forming multiple hydrogen bonds among MOF frameworks to strengthen their structural stability. Importantly, 2,6-diaminopyridine was utilized to modulate the structure and properties. Initially, the mono-coordination model of the N atom on a pyridine ring with metal ions could create defects and form further pores. Two -NH₂ groups helped to improve the grafting reaction degree of UPy-NCO, leading to an increased ratio of forming quadruple hydrogen bonds (H-bonds), further strengthening the structure of the hybrid. As expected, the Cu-MOF@Ti₃C₂T_X-20%DAP-UPy hybrid exhibited a specific capacitance of 148 F g^-1 at 1 A g^-1, which is 45% higher than that of Cu-MOF@Ti₃C₂T_X-UPy (102 F g^-1). A good capacitance retention of 88% was obtained as the current density increased from 0.2 to 5 A g^-1. Moreover, excellent cycling stability (91.1%) was obtained at 1 A g^-1 after 5000 cycles.