Tunable Hierarchical Pore Structure of Nickel–Cobalt Bimetallic Organic Framework Materials for High-Performance Supercapacitor

Abstract

In this work, a series of Ni/Co-MOFs with high specific capacitances were synthesized as anode materials using a one-step hydrothermal reaction method. NaOH in different amounts (3, 4, 5, and 6 mmol) was added during the synthesis to tune the pore structure of Ni/Co-MOFs. It was found that the Ni/Co-MOF-3 with a NaOH amount of 5 mmol exhibits the largest specific surface area and pore volume, which provides more active sites for the electrochemical reaction and facilitates ion diffusion at the interface of the electrolyte solution/active material, thus increasing the capacitance of the electrode material. The electrochemical test results show that the specific capacitance of Ni/Co-MOF-3 reaches 1361 F g^-1 at 1 A g^-1. Impressively, the specific capacitance is still as high as 1214 F g^-1 when the current density increases from 1 to 20 A g^-1, with a high capacitance retention rate of about 89.2%. In addition, Ni/Co-MOF-3 and activated carbon were used as positive and negative materials, respectively, to assemble an asymmetric capacitor, which has a specific capacitance of 134.4 F g^-1 at 1 A g^-1 and an energy density of 47.69 Wh kg^-1 at a power density of 800 W kg^-1. The specific capacitance is 60% of the initial specific capacitance at 5 A g^-1 after 10,000 cycles. At the same time, two such asymmetric capacitors can light up the red LED indicator for more than 30 min after being connected in series with a full charge, indicating that they have outstanding application potential.