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NiCo2O4is a promising electrode material for supercapacitors due to its low cost, high theoretical specific capacitance and good cycling stability. However, NiCo2O4has the disadvantages of poor electronic and ionic conductivities and easy agglomeration. Moreover, the irreversible structural damage caused by the NiCo2O4volume expansions during charging and discharging limits its use in supercapacitors. Herein, we fabricated sandwich Ti3C2TxMXene/NiCo2O4composites via an electrostatic self-assembly method. The problem of easy agglomeration of the NiCo2O4nanospheres was solved by intercalating Ti3C2TxMXene layers with the NiCo2O4nanospheres. Furthermore, the Ti3C2TxMXene layer was also utilized as a protective layer to limit serious volume expansion and protect the structure of NiCo2O4nanospheres from destruction. The results showed that the prepared Ti3C2TxMXene/NiCo2O4composite exhibited a high specific capacitance of 1025 F g−1at a current density of 1 A g−1and had an initial capacity of 81 % when the current density was increased to 10 A g−1. The Ti3C2TxMXene/NiCo2O4composite cathode and activated carbon anode were assembled in an asymmetric supercapacitor, which showed an energy density of 36.67 Wh kg−1at 800 W kg−1and a capacity retention of 88.2 % after 5000 cycles.

In this paper, we successfully anchor NiCo2S4(NCS) hollow spheres (HS) on carbon cloth (CC) via an electrostatic adsorption-assisted template strategy. Synthesized NCS HS/CC composites show high conductivity and excellent cycle performance due to the synergistic effect of their structural properties. Compared to NCS HS, NCS HS/CC exhibits a high specific capacity of 724.9C/g, good rate performance (88.5 %, 20 A/g) and excellent cycle stability (91.06 %, 5000 cycles). In addition, the fabricated hybrid supercapacitor (HSC) devices have excellent energy/power density (35.44 Wh/kg, 800 W/kg) and superior cycling stability, with 97.5 % capacity retention after 5000 cycles at 10 A/g. This electrostatic adsorption-assisted hard template strategy provides a method for preparing electrode materials with unique structures and excellent performance.

Despite the significant progress in bimetallic MOFs electrodes, strategies to seamlessly interconnect of hollow spherical structure MOFs and electrically conductive substrate materials without adhesives are still deficient. In this study, we designed an original method for in situ synthesis of hollow spherical bimetallic MOF-derived sulfides (HN2CMS) on nickel foam by using self-grown ZIF-67 on nickel foam as precursor and template followed by two simple hydrothermal reactions. The prepared electrodes exhibit a high specific capacitance of 938.1C g−1at the current density of 1 A g−1, low electrochemical resistance and excellent cycling stability (85.0 % after 5000 cycles) in a 3 M KOH aqueous electrolyte. More importantly, an assembled asymmetric supercapacitor based on HN2CMS provides an ultra-high energy density of 51.2 Wh kg−1with a power density of 800 W kg−1. And it exhibits an outstanding lifetime of 81.1 % capacity remained after 5000 cycles, demonstrating that hollow spherical bimetallic MOF-derived sulfides are a prospective candidate for SC electrode materials. This synthesis method breaks a new path to developing binder-free electrodes with hollow structures based on bimetallic MOFs, thus improving the electrochemical performance of asymmetric SC.