Interpenetrating network structures assembled by “string of candied haws”-like PPY nanotube-interweaved NiCo-MOF-74 polyhedrons for high-performance supercapacitors

Due to the insufficient performance, it is still a challenge to combine conventionally synthesized metal-organic framework (MOF-74) with other materials to fabricate composite electrodes for high-performance supercapacitors. Herein, we synthesized a series of NiCo-MOF-74 composite materials with different amounts of polypyrrole nanotubes (T-PPy) denoted as NiCo-MOF-74/T-PPy-m (m = 2.5, 5, 10, 15 and 20) through in situ growth of the bimetallic MOF around the PPy nanotubes by a simple solvothermal method. By comparison, the NiCo-MOF-74(1)/T-PPy-10 composite, with a Ni/Co molar ratio of 1:1 obtained in the presence of 10 mg of T-PPy, possessed the highest specific capacitance of 849 C·g−1 at 1 A·g−1 and excellent cyclic stability with a capacitance retention rate of 90.0% after 10,000 cycles as well as good rate performance. Moreover, the assembled asymmetric supercapacitor respectively using NiCo-MOF-74(1)/T-PPy-10 and active carbon as positive electrode and negative electrode achieved a superior energy density of 58.4 Wh kg−1 at the power density of 747.6 W kg−1 and high cyclic lifespan (91.4% capacitance retention after 10,000 cycles). The mechanism of capacitance increase was studied in detail, influences of metal molar ratios, morphology and addition amounts of PPy on electrochemical performances of the formed composition materials were systematically discussed, and it was found that the interpenetrating network structure assembled by “string of candied haws”-like T-PPy-interweaved NiCo-MOF-74(1) polyhedrons possessed the best capacitive performance for enhanced conductivity of the resulted composite, improved dispersity of the bimetallic MOF and more exposed active sites. And the interwoven structure alleviated the stress change-induced structure or phase variation caused by repeated volumetric swelling and shrinking during charge/discharge cycle and enhanced the rate capability and cycling stability.