Covalent Graphene-MOF Hybrids for High-Performance Asymmetric Supercapacitors

In this work, the covalent attachment of an amine functionalized metal-organic framework (UiO-66-NH₂  = Zr₆ O₄ (OH)₄ (bdc-NH₂ )₆ ; bdc-NH₂  = 2-amino-1,4-benzenedicarboxylate) (UiO-Universitetet i Oslo) to the basal-plane of carboxylate functionalized graphene (graphene acid = GA) via amide bonds is reported. The resultant GA@UiO-66-NH₂ hybrid displayed a large specific surface area, hierarchical pores and an interconnected conductive network. The electrochemical characterizations demonstrated that the hybrid GA@UiO-66-NH₂ acts as an effective charge storing material with a capacitance of up to 651 F g^-1 , significantly higher than traditional graphene-based materials. The results suggest that the amide linkage plays a key role in the formation of a π-conjugated structure, which facilitates charge transfer and consequently offers good capacitance and cycling stability. Furthermore, to realize the practical feasibility, an asymmetric supercapacitor using a GA@UiO-66-NH₂ positive electrode with Ti₃ C₂ T_X MXene as the opposing electrode has been constructed. The cell is able to deliver a power density of up to 16 kW kg^-1 and an energy density of up to 73 Wh kg^-1 , which are comparable to several commercial devices such as Pb-acid and Ni/MH batteries. Under an intermediate level of loading, the device retained 88% of its initial capacitance after 10 000 cycles.