2D coordination unsaturated Ni-MOFs hierarchical nanosheets with internal electric fields for high-performance hybrid supercapacitors

The poor intrinsic electronic conductivity and low-exposure electroactive sites of MOFs limit their practical application in energy storage. Herein, the coordination unsaturated nickel-based metal organic frameworks (Ni-MOFs) were prepared by one-step solvothermal method via adjusting the molar ratio of metal salts to organic ligands (RM/L). The coordination unsaturated degree and morphology of the materials can be controlled by adjusting RM/L. When the RM/L is 2, the obtained Ni-MOF 2:1 is a two-dimensional (2D) hierarchical nanosheets made of interwoven ultra-thin nanoribbons. The 2D hierarchical nanosheets structure of Ni-MOF 2:1 can not only expose more electroactive sites, but also promote ions diffusion. The density functional theory (DFT) simulations reveal that the coordination unsaturated centers can narrow the band gap, thus resetting the internal electric field and improving the conductivity of Ni-MOF 2:1. Benefit from abundant electroactive sites, rapid ions diffusion and improved electrical conductivity, the Ni-MOF 2:1 shows a high specific capacity of 746 C g−1 at 1 A g−1 and excellent cycling stability of 89.7 % capacity retention after 10,000 cycles at 10 A g−1. The hybrid supercapacitor (HSC) assembled with Ni-MOF 2:1 as cathode and activated carbon (AC) as anode has a maximum energy density of 53.1 Wh Kg−1 at 799.9 W Kg−1. This work provides a promising strategy for the rational design of MOFs-based electrode materials for high performance HSC. Moreover, compared with the studies using MOFs derivatives as electrode materials, the direct use of Ni-MOFs as electrode materials in this study is of great significance to avoid energy waste and maintain the inherent advantages of MOFs.