Metal oxides and carbon-based materials are the most promising electrode materials for a wide range of low-cost and highly efficient energy storage and conversion devices. Creating unique nanostructures of metal oxides and carbon materials is imperative to the development of a new generation of electrodes with high energy and power density. Here we report our findings in the development of a novel graphene aerogel assisted method for preparation of metal oxide nanoparticles (NPs) derived from bulk MOFs (Co-based MOF, Co(mIM) (mIM = 2-methylimidazole). The presence of cobalt oxide (CoO) hollow NPs with a uniform size of 35 nm monodispersed in N-doped graphene aerogels (NG-A) was confirmed by microscopic analyses. The evolved structure (denoted as CoO/NG-A) served as a robust Pt-free electrocatalyst with excellent activity for the oxygen reduction reaction (ORR) in an alkaline electrolyte solution. In addition, when Co was removed, the resulting nitrogen-rich porous carbon-graphene composite electrode (denoted as C/NG-A) displayed exceptional capacitance and rate capability in a supercapacitor. Further, this method is readily applicable to creation of functional metal oxide hollow nanoparticles on the surface of other carbon materials such as graphene and carbon nanotubes, providing a good opportunity to tune their physical or chemical activities.

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α-NiS/rGO nanohybrids synthesized from water-refluxed Ni-MOF-74/rGO were applied as a high-performance hybrid supercapacitor positive electrode for the first time.

A hybrid supercapacitor consisting of a battery-type electrode and a capacitive electrode could exhibit dramatically enhanced energy density compared with a conventional electrical double-layer capacitor (EDLCs). However, advantages for EDLCs such as stable cycling performance will also be impaired with the introduction of transition metal-based species. Here, we introduce a facile hydrothermal procedure to prepare highly porous MOF-74-derived double hydroxide (denoted as MDH). The obtained 65%Ni-35%Co MDH (denoted as 65Ni-MDH) exhibited a high specific surface area of up to 299 m2 g–1. When tested in a three-electrode configuration, the 65Ni-MDH (875 C g–1 at 1 A g–1) exhibited excellent cycling stability (90.1% capacity retention after 5000 cycles at 20 A g–1). After being fabricated as a hybrid supercapacitor with N-doped carbon as the negative electrode, the device could exhibit not only 81 W h kg–1 at a power density of 1.9 kW kg–1 and 42 W h kg–1 even at elevated working power of 11.5 kW kg–1, but also encouraging cycling stability with 95.5% capacitance retention after 5000 cycles and 91.3% after 10 000 cycles at 13.5 A g–1. This enhanced cycling stability for MDH should be associated with the synergistic effect of hierarchical porous nature as well as the existence of interlayer functional groups in MDH (proved by Fourier transform infrared spectroscopy (FTIR) and in situ Raman spectroscopy). This work also provides a new MOF-as-sacrificial template strategy to synthesize transition metal-based hydroxides for practical energy storage applications.

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Composition design and morphology control of electrode materials are effective strategies to enhancing the specific capacity, rate capability, and cycling life of electrochemical energy storage devices. Here we report our findings in the design and synthesis of a three-phase nickel sulfide (NiS-Ni 3 S 2-Ni 3 S 4 , denoted as TP-Ni x S y) with 3D flower-like architecture assembled from interconnected nanoflakes, which delivers a specific capacity of 724 C g-1 at a current density of 1 A g-1. When integrated with reduced graphene oxide (rGO), a TP-Ni x S y /rGO composite nanoflake electrode, derived from a hydrothermal process, demonstrates not only higher specific capacity (807 C g-1 at 1 A g-1) but also better rate capability (~72% capacity retention as the current density was increased from 1 to 20 A g-1). Moreover, a hybrid energy storage device, constructed from a TP-Ni x S y /rGO positive electrode and a graphene-based negative electrode, shows a high energy density of 46 Wh kg-1 at a power density of 1.8 kW kg-1. It retained an energy density of 32 Wh kg-1 at power density of 17.2 kW kg-1 , demonstrating its viability and potential for practical applications.