No abstract

In the past decade, there have been exciting developments in the fi eld of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not suffi cient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal-air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li-air and Zn-air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li-air and Zn-air batteries, with the aim of providing a better understanding of the new electrochemical systems.

Nitrogen‐doped graphene (NG) is a promising metal‐free catalyst for the oxygen‐reduction reaction (ORR). A facile and low‐cost synthesis of NG via the pyrolysis of graphene oxide and urea is reported. The N content in NG can be up to 7.86%, with a high percentage of graphitic N (≈24%), which gives rise to an excellent catalytic activity toward the ORR.

A series of flexible nanocomposite electrodes were fabricated by facile electro-deposition of cobalt and nickel double hydroxide (DH) nanosheets on porous NiCo2O4 nanowires grown radially on carbon fiber paper (CFP) for high capacity, high energy, and power density supercapacitors. Among different stoichiometries of CoxNi1-xDH nanosheets studied, Co0.67Ni0.33 DHs/NiCo2O4/CFP hybrid nanoarchitecture showed the best cycling stability while maintaining high capacitance of ∼1.64 F/cm(2) at 2 mA/cm(2). This hybrid composite electrode also exhibited excellent rate capability; the areal capacitance decreased less than 33% as the current density was increased from 2 to 90 mA/cm(2), offering excellent specific energy density (∼33 Wh/kg) and power density (∼41.25 kW/kg) at high cycling rates (up to150 mA/cm(2)).

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (> or =2,000 degrees C). Solid silicon has recently been generated directly from silica at much lower temperatures (< or =850 degrees C) via electrochemical reduction in molten salts. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants. Here we demonstrate a low-temperature (650 degrees C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m(2) g(-1)), and contained a significant population of micropores (< or =20 A). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications.

The anode materials that have been developed for solid oxide fuel cells (SOFCs) are vulnerable to deactivation by carbon buildup (coking) from hydrocarbon fuels or by sulfur contamination (poisoning). We report on a mixed ion conductor, BaZr(0.1)Ce(0.7)Y(0.2-)(x)Yb(x)O(3-delta), that allows rapid transport of both protons and oxide ion vacancies. It exhibits high ionic conductivity at relatively low temperatures (500 degrees to 700 degrees C). Its ability to resist deactivation by sulfur and coking appears linked to the mixed conductor's enhanced catalytic activity for sulfur oxidation and hydrocarbon cracking and reforming, as well as enhanced water adsorption capability.

Recently, there has been great interest in flexible and wearable energy devices for applications in flexible and stretchable electronics. Even though future developments are moving toward thinner, lighter, and cheaper solutions, [1,2] many existing energy-harvesting and storage devices are still too bulky and heavy for intended applications. For example, high-efficiency dye-sensitized solar cells (DSSCs) employ fluorine-doped tin oxide (FTO) glass as the substrate of working electrode. However, the use of rigid FTO glass has restricted adaptability of DSSCs during transportation, installation, and application, [2] requiring further development of flexible cells to improve DSSC adaptability. To develop flexible and wearable electronics, not only new materials for the substrates used in energy storage devices such as batteries and supercapacitors need to be explored, but future development of higher performance energy systems still depends on the employment of new and lighter electrode materials.In recent years, electrochemical supercapacitors have attracted much attention as novel energy-storage devices because of their high power density, long life cycles, and high efficiency. [3][4][5][6][7][8][9][10][11][12] Supercapacitors can deliver higher power than batteries and store more energy than conventional capacitors. [13,14] Current research on supercapacitors has focused on their applications in electric vehicles, hybrid electric vehicles, and backup energy sources. Thus, conventional supercapacitors are heavy and bulky, and it is still a challenge to achieve high efficiency miniaturized energy-storage devices that are compatible with the flexible/wearable electronics. [15] Herein, we present a prototype of a high-efficiency fiberbased electrochemical microsupercapacitor using ZnO nanowires (NWs) as electrodes. These fiber supercapacitors, which have great potential for scale-up, comprise two electrodes that employ a flexible plastic wire and a Kevlar fiber as a substrate. Both wire and fiber are covered with arrays of highquality ZnO NWs grown by the hydrothermal method, and ZnO NWs on a Kevlar fiber was coated with a thin gold film to improve the charge-collection capacity. Furthermore, employment of ZnO NWs could provide exciting solutions to the future development of wearable energy devices. Our fiber-based microsupercapacitor would be large enough to be used in self-powering nanosystems, such as a power shirt using piezoelectric ZnO NWs grown radially around textile fibers.Even though conventional research efforts on bulky supercapacitors have focused on the use of carbon-based materials, such as activated carbons, and some transition metal oxides, such as RuO 2 and ZnO, could have several advantages over the conventional electrode materials of supercapacitors for the wearable electronics. First, it can be grown at low temperatures (less than 100 8C) by a chemical approach on any substrate and any shape substrate. Second, it is biocompatible and environmentally friendly material. Furthermore, ZnO NWs [16][17...

Electrocatalysts for oxygen reduction are a critical component that may dramatically enhance the performance of fuel cells and metal-air batteries, which may provide the power for future electric vehicles. Here we report a novel bio-inspired composite electrocatalyst, iron phthalocyanine with an axial ligand anchored on single-walled carbon nanotubes, demonstrating higher electrocatalytic activity for oxygen reduction than the state-of-the-art Pt/C catalyst as well as exceptional durability during cycling in alkaline media. Theoretical calculations suggest that the rehybridization of Fe 3d orbitals with the ligand orbitals coordinated from the axial direction results in a significant change in electronic and geometric structure, which greatly increases the rate of oxygen reduction reaction. Our results demonstrate a new strategy to rationally design inexpensive and durable electrochemical oxygen reduction catalysts for metal-air batteries and fuel cells.