Hydrothermal synthesis of NiCo-based bimetal-organic frameworks as electrode materials for supercapacitors

NiCo-based bimetal-organic frameworks (NiCo-MOF) are synthesized via a hydrothermal approach, using 4,4'-biphenyldicarboxylic acid (BPDC) as a ligand. Structural characterizations find that NiCo-MOF is composed of one-dimensional micro-nanorods and regular three-dimensional cuboids assembled by one-dimensional micronanorods , possessing a good crystalline structure, in which nickel and cobalt ions have been coordinated to the BPDC ligands. Moreover, NiCo-MOF can be stable below 300 degrees C in air. The electrochemical measurements show that GCD activation at a high current density can significantly increase the capacity of NiCo-MOF due to decreasing the charge-transfer resistance and ion diffusion impendence. The specific capacitance of NiCo-MOF reaches 990.7 F/g from 156.5 F/g at 1 A/g, and the coulombic efficiency increases to 98.0% from 77.6% after the CV-GCD activation. Activated NiCo-MOF exhibits prominent pseudocapacitive behaviors, high specific capacitances at different current densities and a good cycling stability after 1000 cycles. The capacity of NiCo-MOF is up to 405.0 F/g when the current density increases to 20 A/g, and remains 553.2 F/g at 1 A/g after 3000 cycles. Compared with monometallic Ni-MOF and Co-MOF, bimetallic NiCo-MOF possesses a much higher capacity at different current densities due to the lower intrinsic resistance, charge-transfer resistance and ion diffusion impendence.