Shaohui Xu

Electrochemically-deposited nanostructured Co(OH)2 flakes on three-dimensional ordered nickel/silicon microchannel plates for miniature supercapacitors

Silicon microchannel plates (Si-MCPs) coated with a nickel layer are an excellent substrate in miniature supercapacitors. Nanometer-sized Co(OH)2 flakes serving as the active materials are electrodeposited on ordered three-dimensional (3D) Ni/Si-MCPs and the Co(OH)2 flakes have different structures depending on the solvent used. The cobalt hydroxide synthesized from a de-ionized water solvent is composed of compact nano-flakes, whereas that synthesized from an alcohol containing solvent is composed of loosely packed nano-flakes, and that from acetone are nano-flakes with nano-particles. The three types of electrode materials are investigated from the perspective of electrochemical capacitors by means of cyclic voltammogram, galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy. The highest specific capacitance of 6.90 F cm−2 is achieved from the samples prepared in acetone at a discharge current density of 10 mA cm−2 and it is much better than the 1.46 F cm−2 observed in previous studies, thus demonstrating excellent capacity retention.

Obtaining a high area ratio free-standing silicon microchannel plate via a modified electrochemical procedure

A silicon microchannel plate (Si MCP) is of large interest for electron multiplication. Conventional fabrication processes utilize an etching process on the front side and wafer thinning on the backside. In this note, a process based on electrochemical etching, developed to generate a gap between the device layer and the substrate, is presented. A sample containing a microchannel through-hole structure can be directly obtained with a laser cut by scanning the laser beam on the surface without cutting through the wafer. An oxidation step is used to increase the MCPs electrical resistance. After dry oxidation at 900 °C, structure distortion is observed in the free-standing sample. Thus, oxidizing before cutting the microchannel plate from the substrate is recommended.

Large-size P-type silicon microchannel plates prepared by photoelectrochemical etching

The influence of backside illumination and temperature on the fabrication of large and high aspect ratio silicon microchannel plates (MCPs) by photoelectrochemical (PEC) process is described. Backside illumination is provided by three 150-W tungsten halogen lamps with a feedback loop, keeping a constant current density. The etching temperature is maintained by a circulation system. Proper backside illumination and the lower temperature can provide better integrated etching conditions compared to that without illumination and temperature control. Etching under the improved conditions results in smoother undercutting and better surface topography for large (effective diameter of about 80 mm for 4-inch silicon substrates) silicon microchannel plates. Enhancing the backside illumination within the etching temperature range ensures that the aspect ratio is more than 40, boding well for applications of silicon microchannel plates.

Miniature supercapacitors composed of nickel/cobalt hydroxide on nickel-coated silicon microchannel plates

Silicon microchannel plates with a large surface area coated with a nickel layer are excellent templates for miniature supercapacitors. Three-dimensional Si-MCP/Ni/Ni(OH)2 and Si-MCP/Ni/Co(OH)2 sandwiched structures are produced with the Ni(OH)2 and Co(OH)2 thin films serving as the active materials in the supercapacitors which consist of microcrystalline and nano-sized flakes or rods. The specific capacitances derived from the CV and typical charge/discharge curves exhibit good consistency. The highest specific capacitance of 3.75 F cm−2 is obtained at a discharge current density of 10 mA cm−2 from Si-MCP/Ni/Ni(OH)2 and 1.46 F cm−2 from Si-MCP/Ni/Co(OH)2 at a scanning rate of 10 mV s−1. At high rates, the specific capacitances measured from both samples are relatively low, but the excellent capacitive properties are restored at low rates after being subjected to several high rate charge/discharge cycles. Owing to the large specific capacitance per unit area, good cycling performance, small volume, and unique three-dimensional structure, these electrodes are excellent supercapacitors suitable for secondary power sources. In addition, the potential of nickel coated Si-MCPs as templates for miniaturization and integration of devices in practical applications is discussed and demonstrated.

Hierarchical 3-dimensional CoMoO4 nanoflakes on a macroporous electrically conductive network with superior electrochemical performance

Nanoscale cobalt molybdate (CoMoO4) particles are fabricated hydrothermally on the surface and sidewall of three-dimensional nickel-coated silicon microchannel plates (also called macroporous electrically conductive network, MECN) as the active electrode in a miniature energy storage device. The relationships between the reaction time, morphology, formation mechanism of the CoMoO4 nanostructure, and electrochemical performance are studied. After an optimal hydrothermal synthesis time of 2.5 h, the CoMoO4 electrode has a capacity of 32.40 mA h g−1 (492.48 μA h cm−2) at constant current density of 1 A g−1 and retention ratio of 85.98% after 5000 cycles. The large specific capacity and excellent rate capability can be attributed to the unique 3D ordered porous architecture which facilitates electron and ion transport, enlarges the liquid–solid interfacial area, and enhances the utilization efficiency of the active materials. Furthermore, the weight and size of the device are reduced. By using the CoMoO4/MECN electrode as the positive electrode and carbon/nickel foam (carbon/NF) as the negative electrode, the faradaic electrode was packaged by a CR2025 battery cell as the miniature hybrid device exhibits stable power characteristics (5000 cycle times with 71.82% retention). After charging each hybrid device for 10 s, three devices in series can power two 5 mm diameter light-emitting diodes (LED) efficiently.

Hybrid MnO2/C nano-composites on a macroporous electrically conductive network for supercapacitor electrodes

A two-step hydrothermal process is designed to synthesize hybrid MnO2/C nano-composites on a macroporous electrically conductive network (MECN) via a redox reaction in a 30 mM KMnO4 solution with carbon microspheres. The microstructure, surface morphology, and electrochemical properties of the MnO2/C coated on MECN are determined systematically. The MnO2 nanoflakes, which are about 40–200 nm, are deposited regularly on the carbon microspheres coated on the MECN electrode. The MnO2 nano-lamellas offer fast ion transport and adsorption–desorption to/from the MnO2 surface, and the microporous carbon microspheres enhance the electrical storage and ion transfer. The in situ growth of MnO2 on the carbon microspheres on the MECN substrate leads to a small contact resistance and short current transfer length. The materials are demonstrated to be excellent electrodes in supercapacitors boasting a high capacitance of 497 F g−1 at 1 A g−1 with a 1 cm2 electrode and excellent long-term cycling stability over 5000 cycles in 1 M Na2SO4. The MnO2/C/MECN||active carbon/Ni-foam asymmetrical supercapacitors (ASCs) deliver an energy density of 0.50 mW h cm−3 (55.5 W h kg−1 at a power density of 4000 W kg−1) with 87.6% retention of the specific capacitance after 5000 cycles.

Preparation of multi-layer graphene on nickel-coated silicon microchannel plates by a hydrothermal carbonization procedure and its improved field emission properties

An emission cell comprising multi-layer graphene (MLG) on nickel-coated silicon microchannel plates (Ni/Si-MCPs) was prepared. The Ni3C film was formed on the Si-MCPs by hydrothermal carburization in a polyol solution containing a small amount of NaAc as the carbon source and thermal annealing was performed to produce the vertically and horizontally aligned multi-layer graphene field-emission cathode on the surface of the Ni/Si-MCPs (MLG-MCPs). The microstructure and surface morphology were investigated and field emission (FE) studies indicated that the MLG-MCPs delivered better FE performance than Ni/Si-MCPs due to characteristics such as sharp edges, large aspect ratio, and the vertically and horizontally aligned and patterned MLG with good electrical conductivity. The turn-on field of the sample annealed at 800 °C was 2.0 V μm−1 at a current density of 10 μA cm−2 and the field emission threshold was 3.2 V μm−1 at 1 mA cm−2. The structure was very stable showing 97.5% retention after continuous operation for over 6 h at 2 × 10−5 Pa, suggesting a promising candidate for FE devices. This would open up possibilities for the next generation FE electron sources from well-aligned macroporous graphene with skeleton and extend their practical applications.

Three-dimensional homo-nanostructured MnO2/nanographene membranes on a macroporous electrically conductive network for high performance supercapacitors

A three-step hydrothermal route was designed to fabricate three-dimensional (3D) homo-nanostructured MnO2 (MnO2–MnO2)/nanographene membranes on a macroporous and electrically conductive network (MECN). The preparation technology, structure and morphology, and electrochemical properties of samples are determined systematically. The nanographene/MECN electrode with more defects as the active surface had been synthesized by hydrothermal carbonization. The in situ growth of δ-MnO2 with a carbon-assisted reaction on the nanographene/MECN was strongly adhered to the substrate. The additional α-MnO2 with a redox reaction enhanced the mass loading of MnO2, developing the specific capacitance of the MnO2–MnO2/nanographene/MECN electrode. The materials are demonstrated as an electrode with a maximum capacitance of 4.5 F cm−2 or 179 F cm−3 (894 F g−1) at 1 mA cm−2 for 1 cm2 samples and retaining over 83% after 20 000 cycles in 1 M Na2SO4. The MnO2–MnO2/nanographene/MECN||AC/Ni-foam supercapacitors with high volumetric energy densities exhibit the ideal performance of a supercapacitor (1 mW h cm−3, 40.3 W h kg−1, at 1000 W kg−1), indicating a promising future for supercapacitors.

Three-dimensional tetsubo-like Co(OH)2 nanorods on a macroporous electrically conductive network as an efficient electroactive framework for the hydrogen evolution reaction

Conducting the hydrogen evolution reaction (HER) in an alkaline environment using a non-precious transition metal catalyst with high efficiency is challenging. Here, we report excellent HER activity achieved using three-dimensional (3D) tetsubo-like Co(OH)2 nanorods on a macroporous electrically conductive network (MECN) synthesized by a hydrothermal method. This unique framework comprises three levels of porous structures, including a bottom-ordered MECN substrate, an intermediate layer of vertically porous Co(OH)2 nanowires with a mean diameter of 100 nm and length of about 2 μm, and outmost Co(OH)2 nanosheets (≈20 nm). The 3D array structure with a large aspect ratio provides a large specific surface area and exposes more active sites to catalyze electrochemical reactions at the electrode–electrolyte interface. Compared with Co(OH)2 nanosheets on an MECN and foamy Co(OH)2 on an MECN structure, the synthesized architecture has excellent HER catalytic reactivity, including a low potential of −69.2 mV vs. RHE, a cathodic current density of 10 mA cm−2, a small Tafel slope of 61.9 mV dec−1, a high current density, and robust catalytic stability in 1 M KOH, and is promising in HER applications.

Nitrogen-doped multilayered nanographene derived from Ni3C with efficient electron field emission

Stability and durability are crucial to graphene-based field emitting materials. Although well-aligned N-doped graphene has a large aspect ratio and good electrical conductivity, it suffers from weak adhesion to the substrate, electric field shielding, and Joule heating effect and possible damage and collapse may result in dissatisfied field emission properties. Herein, field emitters based on N-doped multilayered nanographene derived from Ni3C films are demonstrated to have strong adhesion to the substrate and a uniform large-aspect-ratio morphology. Field-emission (FE) measurements, from the channel edges of 250 microns in depth and 1 micron in width covered with N-doped multilayer nanographene, were performed on N-doped multilayered nanographene on Ni/Si-MCPs (N-doped MLG-MCPs), revealing a small turn-on field of 0.5 V μm−1, a low threshold field of 1.1 V μm−1, and a large enhancement factor β of 9012 at a distance of 100 μm. In addition, the current density is 2.85 mA cm−2 and 96.2% retention is observed after operation for 6 h. The performance and stability of N-doped MLG-MCPs are better than those reported previously from doped graphene nanostructures and comparable to those of carbon nanotubes and carbon-based nanocomposites. The materials with a well-aligned nanographene skeleton have great potential as next-generation FE electron sources.