Mesoporous NiO nanosheets were directly grown on three-dimensional (3D) carbon cloth substrate, which can be used as binder-free anode for lithium-ion batteries (LIBs). These mesoporous nanosheets were interconnected with each other and forming a network with interval voids, which give rise to large surface area and efficient buffering of the volume change. The integrated hierarchical electrode maintains all the advantageous features of directly building two-dimensional (2D) nanostructues on 3D conductive substrate, such as short diffusion length, strain relaxation and fast electron transport. As the LIB anode, it presents a high reversible capacity of 892.6 mAh g⁻¹ after 120 cycles at a current density of 100 mA g⁻¹ and 758.1 mAh g⁻¹ at a high charging rate of 700 mA g⁻¹ after 150 cycles. As demonstrated in this work, the hierarchical NiO nanosheets/carbon cloth also shows high flexibility, which can be directly used as the anode to build flexible LIBs. The introduced facile and low-cost method to prepare NiO nanosheets on flexible and conductive carbon cloth substrate is promising for the fabrication of high performance energy storage devices, especially for next-generation wearable electronic devices.

In this paper, ZnCo₂O₄ nanowires have been grown and self-assembled as hierarchical structures on a 3D conductive Ni foam substrate. Both leaf-like ZnCo₂O₄ and dandelion-like ZnCo₂O₄ assemblies were synthesized through a hydrothermal process followed by a post-annealing treatment. It is shown that leaf-like assemblies are directly grown on the substrate while dandelion-like assemblies are adsorbed on the surface of the structures. A possible formation mechanism of ZnCo₂O₄ hierarchical structures was proposed. It is shown that these nanowires are porous structures which provide much increased specific surface area. Further work was conducted by taking these Ni foam supported ZnCo₂O₄ structures as binder-free electrodes for Li-ion batteries. Remarkably, the leaf-like ZnCo₂O₄/Ni foam electrode exhibits greatly improved electrochemical performance with high capacity and excellent cycling stability. A high reversible capacity of 1050 mA h g⁻¹ at the rate of 100 mA g⁻¹ was obtained after 60 cycles. Meanwhile, the electrode showed a high rate of 416 mA g⁻¹ with a high capacity of 850 mA h g⁻¹ even after 50 cycles. Our work demonstrates that this unique nanowire self-assembled ZnCo₂O₄ hierarchical structure is promising for high-performance electrochemical energy applications.

Free-standing NiO/MnO2 core-shell nanoflake structure was deposited on flexible carbon cloth (CC) used as electrode for high-performance supercapacitor (SC). The NiO core was grown directly on CC by hydrothermal process and the following annealing treatment. MnO2 thin film was then covered on NiO structures via a self-limiting process in aqueous solution of 0.5 M KMnO4 and 0.5 M Na2SO4 with a carbon layer serving as the sacrificial layer. Both the core and shell materials are good pseudocapacitive materials, the compounds of binary metal oxides can provide the synergistic effect of all individual constituents, and thus enhance the performance of SC electrode. The obtained CC/NiO/MnO2 heterostructure was directly used as SC electrodes, showing an enhanced electrochemical performance including areal capacitance of 316.37 mF/cm2 and special gravimetric capacitance of 204.3 F/g at the scan rate of 50 mV/s. The electrode also shows excellent cycling stability, which retains 89% of its initial discharge capacitance after 2200 cycles with >97% Coulombic efficiency. The synthesized binder-free hierarchical composite electrode with superior electrochemical properties demonstrates enormous potential in the application of flexible SCs.

The carbon-based micro/nano electromechanical system (MEMS/NEMS) technique provides a powerful approach to large-scale manufacture of high-aspect-ratio carbon structures for wafer-level processing. The fabricated three-dimensional (3D) carbon structures have the advantages of excellent electrical and electrochemical properties, and superior biocompatibility. In order to improve their performance for applications in micro energy storage devices and microsensors, an increase in the footprint surface area is of great importance. Various approaches have been proposed for fabricating large surface area carbon-based structures, including the integration of nanostructures such as carbon nanotubes (CNTs), graphene, nanowires, nanofilms and nanowrinkles onto 3D structures, which has been proved to be effective and productive. Moreover, by etching the 3D photoresist microstructures through oxygen plasma or modifying the photoresist with specific materials which can be etched in the following pyrolysis process, micro/nano hierarchical carbon structures have been fabricated. These improved structures show excellent performance in various applications, especially in the fields of biological sensors, surface-enhanced Raman scattering, and energy storage devices such as micro-supercapacitors and fuel cells. With the rapid development of microelectronic devices, the carbon-based MEMS/NEMS technique could make more aggressive moves into microelectronics, sensors, miniaturized power systems, etc. In this review, the recent advances in the fabrication of micro/nano hierarchical carbon-based structures are introduced and the technical challenges and future outlook of the carbon-based MEMS/NEMS techniques are also analyzed.

MnO₂ nanorod/carbon cloth (MnO₂/CC) composites were prepared through in situ redox deposition as freestanding electrodes for flexible supercapacitors. The CC substrates possessing porous and interconnecting structures enable the uniform decoration of MnO₂ nanorods on each fiber, thus forming conformal coaxial micro/nanocomposites. Three-dimensional CC can provide considerable specific surface area for high mass loading of MnO₂, and the direct deposition process without using polymeric binders enables reliable electrical connection of MnO₂ with CC. The effect of MnO₂ decoration on the electrochemical performances was further investigated, indicating that the electrode prepared with 40 min deposition time shows high specific capacitance (220 F/g at a scan rate of 5 mV/s) and good cycling property (90% of the initial specific capacitance was maintained after 2500 cycles) in 1 M Na₂SO₄ aqueous solution. This enhanced electrochemical performance is ascribed to the synergistic effect of good conductivity of carbon substrates as well as outstanding pseudocapacitance of MnO₂ nanorods. The obtained MnO₂/CC compositing electrode with the advantages of low cost and easy fabrication is promising in applications of flexible supercapacitors.