Solvothermal synthesis of flower-string-like NiCo-MOF/MWCNT composites as a high-performance supercapacitor electrode material

Abstract

In view of the poor electronic conductivity and inferior structural stability, the direct application of Metal-Organic Frameworks (MOFs) as an electrode material is limited. Herein, the carboxylated multi-wall carbon nanotubes (MWCNT) are utilized as a substrate for the in-situ growth of bimetallic NiCo-based MOFs (NiCo-MOF). The flower-string-like NiCo-MOF/MWCNT composites are successfully synthesized using 4, 4′-biphenyldicarboxylic acid (BPDC) as a ligand via a simple solvothermal approach. In the composites, MWCNT is wrapped with 2D NiCo-MOF nanosheets through some bonds or affinity between MWCNT and NiCo-MOF. Meanwhile, the MWCNT presence does not affect the formation of the NiCo-MOF crystals. Compared with the hydrangea-like NiCo-MOF microspheres, the flower-string-like composite structures are more loosely assembled by NiCo-MOF nanosheets. Owing to the guiding effect of MWCNT on the NiCo-MOF growth, many well-dispersed flower-string-like structures can be achieved by adjusting MWCNT dosages. Due to the unique hierarchical structure and good synergistic effect, the NiCo-MOF/MWCNT composites exhibit excellent capacitance performances. The optimal composites demonstrate a large specific capacitance of 1010 F g−1 at 0.5 A g−1, a good rate capability (760 F g−1 even at 10 A g−1), a good cycling stability and reversibility with the capacitance retention of 828 F g−1 (100% of the initial capacitance) and coulombic efficiency of approximate 98% at 5 A g−1 after 3000 cycles, a low internal resistance and charge transfer resistance. The asymmetric supercapacitor assembled by NiCo-MOF/MWCNT composites as electrode materials has high energy density even at high power density, good rate capability, excellent cycling stability and reversibility. Hence, the as-synthesized NiCo-MOF/MWCNT composites are a promising high-performance electrode material for supercapacitors.