A porous Co₉S₈ nanosheet array on Ni foam can be realized via a facile 2D ZIF templated method.

Metal–organic frameworks (MOFs) have emerged as a new platform for the construction of various functional materials for energy related applications. Here, a facile MOF templating method is developed to fabricate a hierarchical nickel–cobalt sulfide nanosheet array on conductive Ni foam (Ni–Co–S/NF) as a binder-free electrode for supercapacitors. A uniform 2D Co-MOF nanowall array is first grown in situ on Ni foam in aqueous solution at room temperature, and then the Co-MOF nanowalls are converted into hierarchical Ni–Co–S nanoarchitectures via an etching and ion-exchange reaction with Ni(NO₃)₂, and a subsequent solvothermal sulfurization. Taking advantage of the compositional and structural merits of the hierarchical Ni–Co–S nanosheet array and conductive Ni foam, such as fast electron transportation, short ion diffusion path, abundant active sites and rich redox reactions, the obtained Ni–Co–S/NF electrode exhibits excellent electrochemical capacitive performance (1406.9 F g⁻¹ at 0.5 A g⁻¹, 53.9% retention at 10 A g⁻¹ and 88.6% retention over 1000 cycles), which is superior to control CoS/NF. An asymmetric supercapacitor (ASC) assembled by using the as-fabricated Ni–Co–S/NF as the positive electrode and activated carbon (AC) as the negative electrode delivers a high energy density of 24.8 W h kg⁻¹ at a high power density of 849.5 W kg⁻¹. Even when the power density is as high as 8.5 kW kg⁻¹, the ASC still exhibits a high energy density of 12.5 W h kg⁻¹. This facile synthetic strategy can also be extended to fabricate other hierarchical integrated electrodes for high-efficiency electrochemical energy conversion and storage devices.

Metal–organic frameworks (MOFs) show great advantages as new kinds of active materials for energy storage. In this study, bimetallic metal–organic frameworks (Ni/Co-MOFs) with nanosheet-assembled flower-like structures were synthesized by etching Ni-MOF microspheres in a cobalt nitrate solution. It can be clearly observed that the amount of Co(NO₃)₂ and etching time play crucial roles in the formation of Ni/Co-MOF nanosheets. The Ni/Co-MOFs were used as electrode materials for supercapacitors and the optimized Ni/Co-MOF-5 exhibited the highest capacitances of 1220.2 F g⁻¹ and 986.7 F g⁻¹ at current densities of 1 A g⁻¹ and 10 A g⁻¹, respectively. Ni/Co-MOF-5 was further sulfurized, and the derived Ni–Co–S electrode showed a higher specific capacitance of 1377.5 F g⁻¹ at a current density of 1 A g⁻¹ and a retention of 89.4% when the current density was increased to 10 A g⁻¹, indicating superior rate capability. Furthermore, Ni/Co-MOF-5 and Ni–Co–S showed excellent cycling stability, i.e. about 87.8% and 93.7% of initial capacitance can be still maintained after 3000 cycles of charge–discharge. More interestingly, the Ni/Co-MOF-5//AC ASC shows an energy density of 30.9 W h kg⁻¹ at a power density of 1132.8 W kg⁻¹, and the Ni–Co–S//AC ASC displays a high energy density of 36.9 W h kg⁻¹ at a power density of 1066.42 W kg⁻¹. These results demonstrate that the as-synthesized bimetallic Ni/Co-MOF nanosheets and their derived nickel–cobalt sulfides have promising applications in electrochemical supercapacitors.

Metal–organic frameworks (MOFs) have emerged as promising electrode materials for supercapacitors (SCs), due to their diverse functionalities and high porosity. However, the applications of MOFs in practical SC devices are restricted by their intrinsic low conductivity and poor stability. Herein, a thin layer of conductive polyaniline (PANI) was decorated on Ni foam (NF) before the growth of Ni-MOF to tackle these issues. PANI not only improves the conductivity but also promotes the formation of Ni-MOF nanosheet arrays and ensures good mechanical adhesion. The as-prepared Ni-MOF/PANI/NF exhibits a high areal capacitance (3626.4 mF cm⁻² at 2 mA cm⁻²) and good rate capacity (71.3% at 50 mA cm⁻²). Moreover, an asymmetric supercapacitor (ASC) device using Ni-MOF/PANI/NF and activated carbon (AC) can deliver a maximum energy density of 45.6 W h kg⁻¹ (850.0 W kg⁻¹) with excellent cycling stability (capacitance retention of 81.6% after 10 000 cycles), outperforming most of the reported pristine MOF-based ASC devices.