Amorphous bimetal phosphide hierarchical nanostructures enrooted on Ni–Fe foam have been developed as efficient nonprecious bifunctional electrocatalysts for overall water splitting with high activity and superb stability at high current density.
Flute-like porous alpha-Fe2O3 nanorods and branched nanostructures such as pentapods and hexapods were prepared through dehydration and recrystallisation of hydrothermally synthesised beta-FeOOH precursor. Transmission electron microscopy (TEM), high-resolution TEM and selected area electron diffraction analyses reveal that the nanorods, which grow along the [110] direction, have nearly hollow cavities and porous walls with a pore size of 20-50 nm. The hexapods have six symmetric arms with a diameter of 60-80 nm and length of 400-900 nm. The growth direction of the arms in the hexapod-like nanostructure is also along the [110] direction, and there is a dihedral angle of 69.5 degrees between adjacent arms. These unique iron oxide nanostructures offer the first opportunity to investigate their magnetic and gas sensing properties. The nanostructures exhibited unusual magnetic behaviour, with two different Morin temperatures under field-cooled and zero-field-cooled conditions, owing to their shape anisotropy and magnetocrystalline anisotropy. Furthermore, the alpha-Fe2O3 nanostructures show much better sensing performance towards ethanol than that of the previously reported polycrystalline nanotubes. In addition, the alpha-Fe2O3 nanostructure based sensor can selectively detect formaldehyde and acetic acid among other toxic, corrosive and irritant vapours at a low working temperature with rapid response, high sensitivity and good stability.
Higher capacity rechargeable batteries can be constructed by using mesoscale nickel hydroxide tubes as the positive-electrode material. The tubes were prepared by deposition of nickel ions and aqueous ammonia in an anodic alumina membrane as the template. The picture shows a high-resolution TEM image of the tubes. Rechargeable batteries are becoming more and more important in everyday life, especially in consumer electronic devices such as cellular telephones, notebook computers, compact camcorders, and electric vehicles.1–3 The nickel hydroxide/nickel oxyhydroxide couple Ni(OH)2/NiOOH is the primary redox system used as the positive electrode of alkaline rechargeable batteries including nickel/cadmium (Ni/Cd), nickel/iron (Ni/Fe), nickel/metal hydride (Ni/MH), and nickel/zinc (Ni/Zn).4, 5 These batteries are usually positive-electrode-limited for reasons of proper recombination reactions and battery safety. It follows that increasing the energy density of the nickel hydroxide electrode is essential for raising the energy density of such batteries. That is, nickel hydroxide Ni(OH)2 is oxidized on charging to nickel oxyhydroxide (NiOOH). Thus, one mole of Ni(OH)2 (92.7 g) can theoretically yield a capacity of 289(26 800/92.7) mA h g−1. However, the charge/discharge reactions which take place in the nickel hydroxide electrode are much more complex. Four phases are reported to be produced over the lifetime of a nickel hydroxide electrode, namely, β-Ni(OH)2, β-NiOOH, γ-NiOOH, and α-Ni(OH)2.6, 7 α-Ni(OH)2 can also be oxidized to γ-NiOOH at a lower potential than the corresponding oxidation state compared with β-Ni(OH)2, with a higher discharge capacity than β-Ni(OH)2/β-NiOOH, since the nickel oxidation state in γ-NiOOH is known to exceed 3 because of Ni4+ defects.8 The formation of these phases is associated with volume expansion or swelling of the nickel hydroxide electrode, which interferes with effective contact between particles of the active material, and this increases the resistance of the electrode reaction, especially at high-rate or high-temperature charge/discharge.9 To improve the characteristics of the nickel hydroxide electrode, much work has focused on the development of spherical Ni(OH)2 powder and related composite materials.10–14 Nickel hydroxide particles with nanostructural multiphase exhibit superior electrochemical behavior and higher proton diffusion coefficients.15, 16 On the one hand, spherical nickel hydroxide powder, which suppresses the development of inner-pore volume, makes it possible to increase the density of the active material itself. On the other hand, the core of spherical nickel hydroxide powder is still inactive at high-rate and high-temperature charge/discharge due to the diffusion barrier. Recently, single-crystal Ni(OH)2 nanorods17 and β-Ni(OH)2 nanosheets18 have been prepared by the hydrothermal method. However, no attempt has been made to fabricate Ni(OH)2 micro- and/or nanotubes and test their electrochemical activity. Here we report that Ni(OH)2 tubes with mesoscale dimensions, produced by chemical deposition of nickel ions and ammonia within anodic alumina membranes, have superior capacity and good cycling reversibility, and are promising positive-electrode materials for alkaline rechargeable batteries. Ni(OH)2 tubes were synthesized by a template method,19–22 as shown schematically in Figure 1. The preparation process mainly involves 1) impregnating nickel ions in the pore walls of the alumina template; 2) dripping ammonia solution through the alumina membrane to precipitate nickel hydroxide; 3) dissolving the alumina membrane in NaOH solution to obtain tube bundles. Schematic diagram showing the template preparation of Ni(OH)2 tubes. The homogeneity and crystallinity of the product was examined by powder XRD (Figure 2). All the diffraction peaks can be indexed as a hexagonal β-Ni(OH)2 structure with the lattice constants a=3.127 Å and c=4.606 Å, in good agreement with that of the standard values (ICDD-JCPDS card No. 14-0117). The broadening of peaks in the XRD pattern indicates that the component crystallites are of nanoscale character. No peaks from other phases were found, that is, the as-synthesized β-Ni(OH)2 is of high purity. The contents of Ni and Al in the product, analyzed by inductively coupled plasma (ICP) emission spectroscopy (Model P-5200 from Hitachi), were 62 and 0.1 %, respectively, that is, traces of alumina remain. XRD pattern of the as-prepared β-Ni(OH)2. Figure 3 shows SEM images of the as-prepared sample at different magnifications. Figure 3 a is an overall view of the product, from which it can be seen that a large quantity of filament bundles were obtained. The length of the bundles is approximately 60 μm, which corresponds to the thickness of the template membrane employed in the synthesis process. Figure 3 b shows an SEM image of one bundle at a higher magnification. This shows that the Ni(OH)2 tubes are arranged roughly parallel to one another with a smooth-surface alignment. This might be due to the remaining trace of the alumina matrix rather than a kind of self-organization. Figure 3 c is a typical SEM image with end-on view, showing the open ends of a tube bundle. The tubes are not all 200 nm in diameter, because of the pore-diameter distribution of the template around 200 nm. The tubes are cylinders with a wall thickness of 20–30 nm. Typical SEM images of as-synthesized β-Ni(OH)2. a) Overall view at low magnification. b) Walls of the tube bundles. c) Tube-bundle tips at high magnification. To further understand the detailed structure of the tubes, TEM and HRTEM were performed (Figure 4). Figure 4 a is the TEM image of a group of Ni(OH)2 tubes scattered on the copper mesh. The central parts of the mesostructures are bright in contrast to their edges, confirming their hollow-tube nature. However, the tubes are only several micrometers long, much shorter than those observed in SEM images (ca. 60 μm). This phenomenon arises from the strong supersonic vibrations prior to the sample preparation for TEM analysis, that is, the long tubes were broken into shorter ones. The open-ended walls surrounding the central hollow core with an average outer diameter of 200 nm and a smooth surface are clearly visible in the TEM image of Figure 4 b. The wall is complete and is composed of an array of needlelike and multilayered nanoparticles (Figure 4 c). This was confirmed by the electron diffraction pattern of the tube (inset of Figure 4 c), which shows the coexistence of nanocrystallites and amorphous structures. This result indicates that the crystallites have no preferred orientation. The mechanism of formation presumably resembles that of the noble-metal nanoparticle nanotubes reported by Rubinstein et al.22 Note that under our synthesis conditions, the homogeneity of the Ni(OH)2 tubes is high (>98 % based on the SEM and TEM observations). Therefore, the lengths and diameters of the tubes are definitely of mesoscale dimensions. Representative TEM (a, b) and HRTEM (c) images of the as-prepared Ni(OH)2 tubes. The inset in c) is the corresponding electron diffraction pattern. Figure 5 shows the cyclic voltammograms of the two spherical-powder and tube Ni(OH)2 electrodes in the first cycle. Similarly shaped voltammograms were also obtained for five-potential cycling. Reversible peaks are observed for the spherical-powder and tube electrodes, but their characteristics are different. The features of the voltammograms are summarized in Table 1: 1) When the electrodes were scanned cathodically, two peaks, for the oxidation potential EO and oxygen-evolution potential EOE, appeared; during the following anodic polarization, only one peak was observed, which was assigned to the reduction potential ER. 2) The difference EO−ER between the oxidation potential and the reduction potential is taken as a measure of the reversibility of the electrode reaction. The smaller this value, the more reversible the electrode reaction. 3) The intensities of EO and ER for the tube electrode were much larger than those of the spherical-powder electrode, that is, the energy density in the tube electrode is higher. 4) The difference between the oxygen-evolution potential and the oxidation potential EOE−EO, is also an important parameter for judging the performance of the nickel electrode. The value of EOE−EO increased from 25 mV for the spherical-powder electrode to 77 mV for the tube electrode. This allows the tube electrode to be charged fully (i.e., complete oxidation of Ni2+ to Ni3+) before oxygen evolution. Consequently, it demonstrates that the tube electrode exhibits much better electrochemical-cyclic properties than the spherical-powder electrode. Cyclic voltammograms of Ni(OH)2 tube (solid line) and spherical-powder (dotted line) electrodes at 20 °C and a scan rate of 0.5 mV s−1. Electrode Potentials [mV] ER EO EOE EO−ER EOE−EO Tube 365 485 562 120 77 Spherical powder 335 515 540 180 25 Figure 6 shows the charge–discharge curves of the tube and spherical-powder electrodes in the tenth cycle. The discharge curve of the tube electrode displays a higher discharge voltage and a longer plateau than that of the spherical-powder electrode, while its charge curve has a lower plateau. The highest discharge capacity of 315 mA h g−1 for the tube electrode, achieved at 50 mA g−1 and 20 °C, illustrates that in addition to the phase transformation of β-Ni(OH)2 to β-NiOOH, partial formation of γ-NiOOH occurred. Charge/discharge curves as a function of capacity for nickel hydroxide tube (solid line) and spherical-powder (dotted line) electrodes at 20 °C. Table 2 summarizes the effect of the working temperature and the discharge current density on the electrode capacity. At 20 °C and 150 mA g−1, the tube electrode still retained 265 mA h g−1, corresponding to about 84.1 % of the electrode capacity at 50 mA g−1, while the spherical-powder electrode only showed 203 mA h g−1 (about 76.6 % of its capacity at 50 mA g−1). Furthermore, it can also be seen that as the working temperature increases, the tube electrode has much higher capacities than the spherical-powder electrode. The tube electrode shows improved high-rate and high-temperature discharge ability due to the faster diffusion processes in the hollow tubes than in the dense spherical powder. This feature is critical for alkaline rechargeable batteries for high-power output at high temperatures. Electrode T [°C] Current density [mA g−1] Discharge capacity [mA h g−1] Tube 20 50 315 100 289 150 265 40 50 281 100 257 150 232 60 50 251 100 229 150 205 Spherical powder 20 50 265 100 235 150 203 40 50 233 100 182 150 154 60 50 198 100 155 150 123 After a preliminary test of 50 charging/discharging cycles at 150 mA g−1 with 100 % depth of charge and discharge, the capacity of the tube electrode decreased by only about 4 %, that is, an average capacity fading of 0.2 mA h g−1 per cycle. Therefore, the hollow structure of the nickel hydroxide tube, which allows diffusion and oxidation/reduction to occur easily, contributes to superior capacity, reversibility, high-rate discharge, high-temperature performance, and cycling stability. In summary, Ni(OH)2 tubes prepared by a template method under ambient conditions can be used as the positive-electrode materials of alkaline rechargeable batteries showing important advantages in terms of capacity, high-rate discharge, and high-temperature performance. Anodic aluminum oxide membranes (Whatman, Φ47 mm with 0.2 μm pores and 60 μm thickness) were used as the templates. NiCl2⋅6 H2O, ammonia, and other reagents were all of analytical grade and used without further purification. The as-prepared samples were characterized by powder XRD (Rigaku INT-2000 X-ray generator, CuKα radiation), SEM (Philips XL-30, 20 kV), TEM, and high-resolution TEM (HRTEM; Philips Tecnai F20, 200 kV). Nickel hydroxide electrodes were prepared by inserting an active paste into a nickel foam substrate. A paste containing 85 wt % nickel hydroxide tubes or spherical powder (Tanaka Chemical, Japan), 10 wt % carbon black, and 5 wt % polytetrafluoroethylene (PTFE) was used. The electrode was dried at 80 °C for 1 h and cut into a disk (1.2×1.2 cm), which was pressed at a pressure of 100 kg cm−2 to a thickness of 0.4 mm. Then the electrode was spot-welded to a nickel sheet for electrical connection. Electrochemical performance was measured with a Solartron SI 1260 Potentionstat Analyzer with 1287 Interface and an Arbin charge/discharge unit at controlled temperatures in an electrochemical cell, which contained the nickel hydroxide working electrode, a metal hydride electrode, a Hg/HgO reference electrode, and 6 M KOH solution as the electrolyte. The discharge capacity of the nickel hydroxide in the positive electrode was based on the amount of active material (Ni(OH)2) excluding the weight of carbon black and PTFE in the electrode. The discharge capacity of each electrode was expressed in mA h per gram of active material.
In the Communication by D. Li et al., the wrong journal was cited in reference 3c, the correct reference is included below. We apologize for this oversight.
Metal-free phosphorus-doped graphene nanosheets (P-TRG) with large surface area (496.67 m2 g−1) and relatively high P-doping level (1.16 at.%) were successfully prepared by thermal annealing a homogenous mixture of graphene oxide and 1-butyl-3-methlyimidazolium hexafluorophosphate under argon atmosphere. It was found that the P atoms were substitutionally incorporated into the carbon framework and were partially oxidized, which created new active sites for the oxygen reduction reaction (ORR). Accordingly, the ORR catalytic performance of the P-doped graphene was demonstrated to be better than or at least comparable to that of the benchmark Pt/C catalyst.
Monodisperse α-Fe2O3 porous nanospheres with uniform shape and size have been synthesized via a facile template-free route. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and Raman spectroscopy were employed to characterize the product, showing the high quality of the as-prepared α-Fe2O3 porous nanospheres. Furthermore, the α-Fe2O3 porous nanospheres can selectively detect ethanol, formaldehyde and acetic acid, with a rapid response and high sensitivity, from a series of flammable and toxic/corrosive gases, indicating their potential applications for high sensitivity gas sensors.
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