Solvent-Controlled Morphology of Amino-Functionalized Bimetal Metal-Organic Frameworks for Asymmetric Supercapacitors

The composition-tuned, structure-modified, and morphology-controlled nanoscale metal-organic frameworks (MOFs) are quite important to improve the electrochemical performances for supercapacitors. In this work, a solvent-controlled method to prepare amino-functionalized bimetal MOFs with various morphologies is proposed. Three different morphologies of NiCo-MOFs, such as nanospheres, nanosheet-assembled hollow spheres (NSHSs), and rhombus sheets, have been successfully synthesized by using different solvents. The as-prepared three nanoscale NiCo-MOFs are comparatively characterized and are endowed a possible mechanism on nucleation and crystal growth controlling morphology. When used as electrode materials for supercapacitors, all NiCo-MOFs have excellent electrochemical properties. Specifically, the NiCo-MOF NSHS owns the best specific capacitance, which can achieve 1126.7 F g^-1 at the current density of 0.5 A g^-1 and maintain 93% of its original capacitance at the current density of 10 A g^-1 after 3000 charge-discharge cycles. Moreover, an asymmetric supercapacitor device (NiCo-MOF NSHS//AC) assembled with NiCo-MOF NSHS as the positive electrode and activated carbon (AC) as the negative electrode achieves an energy density of 20.94 Wh kg^-1 at a power density of 750.84 W kg^-1. This work is facile and highly reproducible and can be extended to prepare other nano-MOFs in energy storage and conversion fields. In addition, it opens up an effective approach to synthesizing amino-functionalized MOFs by a solvent-controlled method without any other changes in the experimental conditions.