Cationic starch as a precursor to prepare porous activated carbon for application in supercapacitor electrodes

Three activated carbons (ACs) for the electrodes of supercapacitor were prepared from cationic starch using KOH, ZnCl₂ and ZnCl₂/CO₂ activation. The BET surface area, pore volume and pore size distribution of the ACs were evaluated using density functional theory method, based on N₂ adsorption isotherms at 77 K. The surface morphology was characterized with SEM. Their electrochemical performance in prototype capacitors was determined by galvanostatic charge/discharge characteristics and cyclic voltammetry, and compared with that of a commercial AC, which was especially prepared for use in supercapacitors. The KOH-activated starch AC presented higher BET surface area (3332 m² g⁻¹) and larger pore volume (1.585 cm³ g⁻¹) than those of the others, and had a different surface morphology. When used for the electrodes of supercapacitors, it exhibited excellent capacitance characteristics in 30 wt% KOH aqueous electrolytes and showed a high specific capacitance of 238 F g⁻¹ at 370 mA g⁻¹, which was nearly twice that of the commercial AC.     

Effects of carbonization temperature on microstructure and electrochemical performances of phenolic resin-based carbon spheres

Activated phenol resin-based carbon spheres (APCS) electrodes with high double layer capacitance and good rate capability were prepared from phenol resin-based spheres (PS) at different carbonization temperatures prior to KOH activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the APCS in 6 M KOH electrolyte. APCS carbonized at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the APCS in 6 M KOH aqueous solution can be as high as 282 F g⁻¹. It remains 252 F g⁻¹ as the current density increases to 1000 mA g⁻¹.     

Potato starch-based activated carbon spheres as electrode material for electrochemical capacitor

Potato starch-based activated carbon spheres (PACS) were prepared from potato starch by stabilization, carbonization followed by activation with KOH. The obtained PACS are hollow and retain the original morphology of potato starch with decrease in size, as shown by scanning electron microscopy. Modification of textural properties of the PACS was achieved by varying the carbonization temperature and the ratio of KOH/PCS. The results of N₂ adsorption isotherms indicate that the samples prepared are mainly microporous. The electrochemical behaviors of the hollow PACS were studied by galvanostatic charge-discharge, cyclic voltammetry, and impedance spectroscopy. The results indicate that high specific capacitance of 335 F/g is obtained at current density 50 mA/g for PACS with specific surface area 2342 m²/g. Only a slight decrease in capacitance, to 314 F/g, was observed when the current density increases to 1000 mA/g, indicating a stable electrochemical property.     

Magnetism of nanographene network influenced by the interaction with acid species

The magnetism of nanographite (stacked nanographene sheets)-based nanoporous carbon is investigated in relation to the interaction with acid guest species. The concentration of the localized spins of non-bonding π-electron state (edge state) localized in the nanographene edges decreases upon the sulfonation of nanographene edges through charge-transfer interaction with sulfonic groups. The sulfonation of nanographene edges enhances the hydrophilic nature of the edges, resulting in the easiness in the water adsorption into the nanopores. This enhances the mechanical compression effect of water molecules condensed in the nanopores on the nanographite domains, resulting in the decrease in the spin concentration. The change in the magnetism upon water uptake reveals ferrimagnetic nature of individual nanographene sheets. The adsorption of HCl having no oxidation ability shows a mechanical effect on the edge-state spins similar to water adsorption. The spin concentration is reduced in two-step manner by the charge-transfer interaction with guest concentrated HNO₃ that is strong oxidant. In the presence of H₂O molecules in diluted HNO₃ the cooperation of mechanical and charge-transfer interactions creates also a two-step change in the magnetism.     

Hierarchically porous nanoflowers from TiO2–B nanosheets with ultrahigh surface area for advanced lithium-ion batteries

In this work, hierarchically porous TiO₂–B nanoflowers have been successfully synthesized via a facile solvothermal method followed by calcination treatment. The TiO₂–B nanoflowers are constructed by thin nanosheets, presenting ultrahigh specific surface area, up to 214.6 m² g⁻¹. As anode materials for Li-ion batteries, the TiO₂–B sample shows high reversible capacity, excellent cycling performance and superior rate capability. The specific capacity of TiO₂–B could remain over 285 mA h g⁻¹ at 1 C and 181 mA h g⁻¹ at 10 C rate after 100 cycles. We believe that the pseudocapacitive mechanism, ultrahigh surface area and scrupulous nanoarchitecture of the TiO₂–B are responsible for the enhancement of electrochemical properties.     

Strong dependence of ferrimagnetic properties on Na concentration in Na-K alloy clusters incorporated in low-silica X zeolite

Na concentration (x) dependence of ferrimagnetic properties is investigated for Na-K alloy clusters incorporated in low-silica X (LSX) zeolite. In the LSX zeolite, β-cages of inner diameter ≈7 Å are arranged in a diamond structure, and supercages of inner diameter ≈13 Å are formed among them. The used LSX zeolite contains xNa⁺ and (12−x)K⁺ cations per β-cage or supercage. Guest nK atoms are loaded into the zeolite, namely the loading density is given by n per β-cage. The samples at x=4 have been reported to show Néel’s N-type ferrimagnetism in the specific region of n. This ferrimagnetism is explained by the model of antiferromagnetic coupling between two non-equivalent magnetic sublattices of clusters, the ones in β-cages and the others in supercages. In the present study, the value of x is changed from 4 to 0. Ferrimagnetic properties are found to show strong x-dependence. A systematic increase in loading densities of ferrimagnetic region is clearly observed with decreasing x. A remarkable change in temperature dependence of spontaneous magnetization is observed depending on x. Na⁺ cations are known to be mainly distributed in β-cages. Hence, the decrease in Na concentration is proposed to change the electronic potential depth for clusters in β-cage, which leads to important differences in the interaction between electrons localized in β-cages and those in supercages.     

Self-discharge performance of Ni-MH battery by using electrodes with hydrophilic/hydrophobic surface

The polytetrafluoroethylene (PTFE) and carboxymethyl cellulose (CMC) film is separately coated on the surface of the metal hydride (MH) and Ni(OH)₂ electrodes to obtain the electrodes with hydrophobic or hydrophilic surface. The effects of the surface treatment on the oxygen and hydrogen evolution from the electrodes are studied by using cyclic voltammetry tests. Although the positive and negative active materials of the Ni-MH batteries show a lower self-decomposition rate after the CMC treatment, the self-discharge rate of the batteries show little change. On the contrary, the self-discharge rate of the batteries decreases from 35.9% to 27.1% by using the PTFE-treated Ni(OH)₂ electrodes, which might be related to the suppression of the reaction between NiOOH and H₂ by the hydrophobic film.     

Antiferromagnetic resonance study of sodalite loaded with sodium by multi-frequency ESR

We have carried out electron spin resonance (ESR) measurements on powder samples of sodalite loaded with Na at several frequencies between 9.7 and 35 GHz and at temperatures between 1.5 and 60 K. The ESR absorption spectrum below a Néel temperature T_N turns into an asymmetric spectrum with a long tail at low fields from a symmetric one above T_N. The line shape of the spectra below T_N is analyzed by a powder pattern simulation of the antiferromagnetic resonance spectra with easy-plane anisotropy. The calculated line shape reproduces the experimental one considerably well by assuming a Gaussian distribution of the zero-field energy gap. We have evaluated a small anisotropy field of about 2×10⁻⁴ T by using the exchange coupling constant calculated from the Weiss and the Néel temperatures. This result indicates that the sodalite loaded with Na is quite an ideal Heisenberg antiferromagnet as expected from the s-electron character of Na clusters and the cubic arrangement of nano-spaces in the sodalite.     

Effect of carbon nanotubes on the anode performance of natural graphite for lithium ion batteries

A comparative investigation was carried out on carbon black and multiwalled carbon nanotubes as conductive additives in spherical natural graphite for lithium ion batteries. Scanning electron microscopy images showed that carbon nanotubes interlaced graphite particles in series to form a three-dimensional network. The constant current charge-discharge experiments showed that carbon nanotubes were more effective in improving reversible capacity and cycle stability. The reversible capacity was improved to 366 mAh/g and the cycle stability was improved effectively when carbon nanotubes were used. The research is of potential interest to the application of carbon nanotubes as conductive additives in anode materials for high-power lithium ion batteries.     

NMR property of sodalite loaded with potassium

²⁷Al NMR property of potassium-loaded sodalite, which shows antiferromagnetic transition around 70 K, is reported. Monotonic narrow spectrum above 70 K is broadened below 70 K. The broadened spectrum is analyzed with an assumption that the broadening is given by dipolar field from magnetic moment in the cage of sodalite. Two Gaussian functions are used for fitting. Widths of each component proportionally scale each other. The ratio between the Gaussian widths of each component of the narrower and the broader components is 15±3.     

Fabrication and field emission properties of multi-walled carbon nanotube/silicon nanowire array

Silicon nanowire (SiNW) arrays were fabricated on silicon wafers by the metal-assisted chemical etching method. Varied average diameters of SiNW arrays were realized through further treatment in a mixed agent of HF and HNO₃ of certain concentrations. After the treatment, there were more than 93% SiNWs with diameters smaller than 100 nm. The tip of each SiNW was subsequently wrapped with multi-walled carbon nanotubes (MWCNTs) with chemical vapor deposition method. The as-fabricated MWCNT/SiNW arrays were fabricated into electric field emitters, with turn-on field of 2.0 V/μm (current density: 10 μA/cm²), much lower than that of SiNW array (5.0 V/μm). The turn-on electric field of MWCNT/SiNW array decreased with the decreasing of the average diameter of SiNWs, indicating the performance of the field emission is relative to the morphology of SiNWs. As the SiNW array is uniform in height and easy to fabricate, the MWCNT/SiNW array shows potential applications in flat electric display.     

Vibration and instability of a single-walled carbon nanotube in a three-dimensional magnetic field

The vibration and instability of a single-walled carbon nanotube (SWCNT) under a general magnetic field are of particular interest to researchers. Using nonlocal Rayleigh beam theory and Maxwell’s equations, the dimensionless governing equations pertinent to the free vibration of a SWCNT due to a general magnetic field were derived. The effects of the longitudinal and transverse magnetic fields on the longitudinal and flexural frequencies as well as their corresponding phase velocities were addressed and are discussed below. The critical transverse magnetic field (CTMF) associated with the lateral buckling of the SWCNT was obtained. The obtained results reveal that the CTMF increases with the longitudinally induced magnetic field. Further, its value decreases as the effect of the small-scale parameter increases.     

Characterization and organic electric-double-layer-capacitor application of KOH activated coal-tar-pitch-based carbons: Effect of carbonization temperature

The present study reports the influence of pre-carbonization on the properties of KOH-activated coal tar pitch (CTP). The change of crystallinity and pore structure of pre-carbonized CTPs as well as their activated carbons (ACs) as function of pre-carbonization temperature are investigated. The crystallinity of pre-carbonized CTPs increases with increasing the carbonization temperature up to 600 °C, but a disorder occurs during the carbonization around 700 °C and an order happens gradually with increasing the carbonization temperatures in range of 800–1000 °C. The CTPs pre-carbonized at high temperatures are more difficult to be activated with KOH than those pre-carbonized at low temperatures due to the increase of micro-crystalline size and the decrease of surface functional groups. The micro-pores and meso-pores are well developed at around 1.0 nm and 2.4 nm, respectively, as the ACs are pre-carbonized at temperatures of 500–600 °C, exhibiting high specific capacitances as electrode materials for electric double layer capacitor (EDLC). Although the specific surface area (SSA) and pore volume of ACs pre-carbonized at temperatures of 900–1000 °C are extraordinary low (non-porous) as compared to those of AC pre-carbonized at 600 °C, their specific capacitances are comparable to each other. The large specific capacitances with low SSA ACs can be attributed to the structural change resulting from the electrochemical activation during the 1st charge above 2.0 V.     

Variation of spark-processing parameters on the photoluminescence properties of spark-processed silicon

Electrical and physical parameters, which influence the photoluminescence (PL) properties of spark-processed silicon (sp-Si), were systematically varied in order to obtain optimal PL emission. Among these parameters are the average spark current, the pulse width of the spark events, the frequency of the pulses, the processing time, the electrode diameter, the distance between the electrodes, the spark-processing environment, and the gas ambient pressure. It was found that for optimal PL emission the processing current needs to be between 20 and 40 mA, and the pulse frequency of the sparks between 10 and 15 kHz. Further, the N₂/O₂ ratio of the processing environment needs to be about 7:3 and the ambient gas pressure and the processing time as large as feasible. The conditions that are favorable for green PL are a small pulse width, a small counter electrode diameter, a small gap between electrodes, a relatively large nitrogen concentration in the processing chamber, and a comparatively large spark frequency. In the opposite cases, a UV/blue PL is predominantly observed. The results are discussed in terms of various thermal effects on the resulting molecules or defects, which are believed to be important for the PL emission.     

Mesoporous carbon-titania nanocomposites for high-power Li-ion battery anode material

We have investigated the Li-ion battery anode properties of several kinds of mesoporous composites of carbon and titanium dioxides (titania, TiO₂) prepared by tri-constituent co-assembly method. The maximum reversible capacity (197 mAh/g) at current density of 50 mA/g was obtained for the composite of TiO₂:carbon=7:3 calcined at 600 °C. It was also found that the composite maintained the high reversible capacity as large as 109 mAh/g even at the high current density of 1000 mA/g.     

Fabrication of Ru(bpy)3-titanate nanotube nanocomposite and its application as sensitive solid-state electrochemiluminescence sensor material

A novel and facile method for effective immobilization of Ru(bpy)₃²⁺ within titanate nanotubes (TiNTs) and its application as a sensitive solid-state electrochemiluminescence (ECL) sensor material was studied. The process involved the formation of Ru(bpy)₃²⁺-titanate nanotube nanocomposite (Ru-TiNTs) via electrostatic interactions by mixing TiNTs and Ru(bpy)₃(ClO₄)₂ in aqueous medium. Then Ru-TiNTs were attached to the surface of a Pt electrode to form an ECL sensor. Characterization of Ru(bpy)₃²⁺-titanate nanotube nanocomposite was accomplished by transmission electron microscopy, X-ray photoelectron spectrum, and field emission scanning electron microscope. The electrochemistry and ECL behavior of Ru(bpy)₃²⁺ immobilized on TiNTs were studied with tripropylamine as a coreactant. As-prepared Ru-TiNTs exhibited very good stability and Ru(bpy)₃²⁺ species contained showed excellent ECL behavior. Therefore, the as-prepared Ru(bpy)₃²⁺-titanate nanotube nanocomposite exhibited great promise as new luminescent materials for solid-state ECL detection.     

Simple chemical aqueous synthesis of dahlia nanoflower consisting of finger-like ZnO nanorods and observation of stable ultraviolet photoluminescence emission

In this work, we have reported the synthesis of dahlia flower-like ZnO nanostructures consisting of human finger-like nanorods by the hydrothermal method at 120 °C and without using any capping agent. Optical properties of the samples, including UV–vis absorption and photoluminescence (PL) emission characteristics are determined by dispersing the samples in water as well as in ethanol media. The quenching of PL emission intensity along-with the red shifting of the PL emission peak are observed when the samples are dispersed in water in comparison to those obtained after dispersing the samples in ethanol. It has been found that PL emission characteristic, particularly the spectral nature of PL emission, of the samples remains almost unaltered (except some improvement in UV PL emission) even after thermally annealing it for 2 h at the temperature of 300 °C. Also the synthesized powder samples, kept in a plastic container, showed a very stable PL emission even after 15 months of synthesis. Therefore, the synthesized samples might be useful for their applications in future optoelectronics devices.     

Ion exchange and adsorption equilibrium studies on clinoptilolite, bentonite and vermiculite

In the present paper three natural minerals used in many industrial and environmental applications namely zeolite clinoptilolite and the clays bentonite and vermiculite are studied by utilising ion exchange and adsorption. In particular, the Dubinin-Astakhov adsorption isotherm is used modified by introducing a solubility-normalized adsorption potential for studying the ion exchange process. The equation, is applied in experimental isotherms in order to determine adsorption energy and heterogeneity parameter for the ion exchange of Pb²⁺ in the natural minerals. The results indicate that the modified Dubinin-Astakhov adsorption isotherm represents the experimental data well and at the same time provides the heterogeneity parameter of the materials, which is an important adsorbent physical parameter as well as the adsorption energy. In order to deepen the study and link the results to the pore structure BET analysis is presented as well.     

One-step synthesis of pure Cu nanowire/carbon nanotube coaxial nanocables with different structures by arc discharge

Pure Cu nanowire/carbon nanotube (Cu@C) coaxial nanocables are one-step fabricated by arc discharge. The microstructure and morphology of the Cu@C nanocables are investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). XRD results reveal that the copper carbide nano-crystals were formed in the nanocables and it plays an important role in the growth of the Cu@C nanocables. As-prepared Cu@C nanocables exhibit three different structures, including well-filled Cu@C nanocables, symmetrically trifurcate Cu@C nanocables, and twice capsulated Cu@C nanocables. The fabrication of Cu@C nanocables with different structures offers more opportunities for the development of nanoelectronic devices. The formation mechanisms of Cu@C nanocables with different structures are discussed as well.     

Facile synthesis and electrochemical properties of hierarchical MnO2 submicrospheres and LiMn2O4 microspheres

Hierarchical MnO₂ submicrospheres have been successfully synthesized by a wet chemical method. The as-prepared products were characterized by means of XRD, SEM, FTIR, TG, and TEM. With the as-prepared MnO₂ submicrospheres as precursors, LiMn₂O₄ microspheres were conveniently prepared by a simple solid-state reaction between MnO₂ and LiOH at a temperature as low as 600 °C. Electrochemical properties of the as-prepared MnO₂ submicrospheres and LiMn₂O₄ microspheres as cathode materials in lithium ion cells were investigated by galvanostatic charge/discharge tests.