Development of MEMS-based direct methanol fuel cell with high power density using nanoimprint technology

Performance of MEMS-based DMFC is low, because graphite-based porous electrodes show poor compatibility with MEMS technology. Nanoimprint technology was adopted in this paper to prepare fine pattern on proton exchange membrane (PEM) in MEMS-based DMFC as a promising alternative to the graphite-based porous electrodes. Micro-convex with the diameter of about 600 nm and the height of 50–70 nm was prepared on Nafion^® 117 membrane by the nanoimprint at 130 °C using silicon mold. Thick Pt film (20 nm) was deposited as catalyst directly on the nanoimprinted Nafion^® 117 membrane. Then the Pt-coated PEM was sandwiched with micro-channeled silicon plates to form a micro-DMFC. With passively feeding of 1 M methanol solution and air at room temperature, the as-prepared cell had the open circuit voltage (OCV) of 0.74 V and the maximum power density of 0.20 mW/cm². The measured OCV was higher than those (0.1–0.3 V) of the state-of-the-art MEMS-based DMFC with planar electrode and pure Pt catalyst.     

Surface Polaron Effect on the Ion Transfer across the Interface of Two Immiscible Electrolytes

An ion disposed near the interface of two immiscible liquids creates a protrusion of one solvent into the other, subject to hydrophobic or hydrophilic interaction with these solvents. Due to a finite relaxation time the protrusion may not be able to spontaneously follow the ion when it moves across the interface. This, as well as any improper thermal fluctuation of the protrusion, will cause slowdown of the ion transfer. Here we present the results of theoretical analysis in which the stochastic motions of the ion and protrusion are coupled and considered on the same footing. We show that if the equilibrium electrochemical potential for the ion has no barrier, the ion transport is purely diffusional with D _eff = k _B T/{6[r + (4/3)(h _max/)² L]} being the effective diffusion coefficient. Here, is the average viscosity of the liquids, r is the Stokes radius of an ion, L and h _max are the average lateral size and the maximal height of the protrusion, and is the half-width of the function h _eq(z) which characterizes the equilibrium ion-interface coupling [h _eq(z) is the height of the protrusion for a given distance of the ion to the unperturbed interface, if the ion had been infinitely slowly moving]. This function could be obtained from model calculations or molecular dynamic simulations. The second term in the denominator is the one that causes the slowdown of ion transfer across the interface. This expression can easily rationalize an order of magnitude slowdown, but hardly much more than that. This is not inconsistent with recent more careful claims on the true scale of the observed effect. Otherwise, one should invoke models that rest on the existence of the barrier in the electrochemical potential.     

Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study

In order to probe the ligand properties we have examined a series of Cr(CO)₅L and Ni(CO)₃L complexes using density functional theory (DFT). The ligands included in our study are N-heterocyclic carbenes (NHCs) and Bertrand-type carbenes. Our study shows that the carbene–metal bonds of imidazol-2-ylidenes (1), imidazolin-2-ylidenes (2), thiazo-2-ylidenes (3), and triazo-5-ylidenes (4) are significantly stronger than those of Bertrand-type carbenes (5–7). The force constants of C–O in complexes are related to the property of isolated carbenes such as proton affinity (PA), electronegativity (χ), and charge transfer (ΔN). NHCs and Bertrand-type carbenes are identified as nucleophilic, soft ligands. Carbene stabilization energy (CSE) computations indicate that carbenes 1 and 4 are the most stable species, while 2 and 3 are less stable. In contrast to NHCs, CSE of carbenes 5–7 are much smaller, and their relative stabilities are in the order (amino)(aryl) carbenes 7e–7g > (amino)(alkyl) carbenes 7a–7d > (phosphino)(aryl) 6d–6e, and (phosphino)(silyl) carbenes 5a–5c > (phosphino)(alkyl) carbenes 6a–6c.     

Transient absorption spectra of 2-hydroxybenzophenone photostabilizers

Transient absorption spectra (400–600 nm) of 2-hydroxybenzophenone and the methyl methacrylate copolymer of 2-hydroxy, 3-allyl, 4,4'-dimethoxybenzophenone following 355 nm excitation (7–480 ps delay) are reported. A short-lived, 435 nm transient (τ ≈ 10 ps in CH₂Cl₂) for both molecules is assigned to the lowest excited singlet before internal proton transfer. Weaker, broad T-T absorption is observed after 480 ps.     

Two-dimensional TiO2 (001) nanosheets as an effective photo-assisted recyclable sensor for the electrochemical detection of bisphenol A

Electrochemical detection is an efficient method for the detection of Bisphenol A (BPA). Herein, a sensitive photo-electrochemical sensor based on two-dimensional (2D) TiO₂ (001) nanosheets was fabricated and then used for BPA electrochemical detection. Upon light irradiation, the 2D TiO₂ (001) nanosheets electrode provided a lower detection limit of BPA detection compared with an ambient electrochemical determination. The low detection limit is ∼5.37 nmol/L (S/N = 3). Furthermore, profiting from the photoelectric characteristics, the 2D TiO₂ (001) nanosheets electrode exhibits a nice regeneration property. After 45 min of light irradiation, the electrochemical signal was regenerated from 14.7% to 82.9% of the original signal at the 6^th cycle. This is attributed to the non-selective OH mediation produced by the 2D TiO₂ (001) nanosheets mineralizing anodic polymeric products and resuming surface reactive sites. This investigation indicates that photo-assistance is an efficient method to improve the electrochemical sensor for detecting BPA in water environments.     

Determination of butyltins in environmental samples using sodium tetraethylborate derivatisation: characterisation and minimisation of interferences

Interferences affecting the determination of butyltin species by sodium tetraethylborate (STEB) derivatisation followed by purge-trap preconcentration were systematically studied using synthetic solutions, natural water samples and sediment extracts. Substances that did not cause interferences included most common cations (apart from those metal ions listed below), anions, metalloids and polar organic compounds. Natural organic matter (NOM) specifically interfered with tributyltin (TBT) due to a mechanism involving partitioning of the butyltin to the hydrophobic portions of the NOM. The metal ions Ag(I) (≥2 μM), Cd(II) (≥2 μM), Cu(II) (≥0.5 μM) interfered predominantly with the determination of monobutyltin (MBT) due to catalytic degradation of the STEB reagent. Pb(II) (≥14 μM) interfered with butyltin determination by an unknown mechanism. Other interferences to the purge-trap method were shown to occur in the presence of chelating agents (e.g. EDTA) or hydrophobic liquids such as diesel fuel. A mixture comprising methanol (MeOH), EDTA and Mn(II) was used to partially mask the effect of interfering NOM and metals. Spike recoveries (mean±S.D. of n=7 different samples) of MBT, dibutyltin (DBT) and TBT in contaminated natural water samples were improved from 70±36,90±11 and 91±24 to 102±10,98±3 and 98±4%, respectively. Spike recoveries (mean±S.D. of n=5 different samples) of MBT, DBT and TBT in aliquots of sediment extracts were improved from 86±17,79±18 and 59±32 to 97±6.2,103±3.6 and 103±5.0%, respectively. The ability to analyse larger aliquots of sediment extracts in the presence of the masking mixture improved the detection limit four-fold if MBT and DBT determination was required and 10-fold if only TBT determination was required.     

Review of absolute doubly differential cross-section experiments and cross-section ratios for electron bremsstrahlung from rare gas atom and thin-film targets

We discuss doubly differential cross-section experiments for electron bremsstrahlung from free gas atom and thin-film targets for electron energies of 100 keV or less. We compare cross-section ratios for different target atoms with two theoretical models: ordinary bremsstrahlung and total bremsstrahlung calculated in the stripping approximation. Ratios of cross sections have been used to improve the comparison between experiment and theory when only relative cross sections are available or when the error in the absolute cross section is large. We also discuss additional background processes that may be more important in gas target experiments.     

On-line preconcentration and determination of chromium in parenteral solutions by inductively coupled plasma optical emission spectrometry

A method for the preconcentration and speciation of chromium was developed. On-line preconcentration and determination were obtained using inductively coupled plasma optical emission spectrometry (ICP-OES) coupled with flow injection. To determinate the chromium (III) present in parenteral solutions, chromium was retained on activated carbon at pH 5.0. On the other hand, a step of reduction was necessary in order to determine total chromium content. The Cr(VI) concentration was then determined by difference between the total chromium concentration and that of Cr(III). A sensitivity enrichment factor of 70-fold was obtained with respect to the chromium determination by ICP-OES without preconcentration. The detection limit for the preconcentration of 25 ml of sample was 29 ng l⁻¹. The precision for the 10 replicate determinations at the 5 μg l⁻¹ Cr level was 2.3% relative standard deviation, calculated with the peak heights. The calibration graph using the preconcentration method for chromium species was linear with a correlation coefficient of 0.9995 at levels near the detection limits up to at least 60 μg l⁻¹. The method can be applied to the determination and speciation of chromium in parenteral solutions.     

Characterization of a new magnesium hydrogen orthophosphate salt,Mg3.5H2(PO4)3, synthesized in supercritical water

Beige crystals of a new magnesium hydrogen orthophosphate salt, Mg_3.5H₂(PO₄)₃, as well as nanoparticles of an amorphous, non-Mg-containing phosphate material, Fe_1−yK_yPO₄ (0 < y < 1), have been produced by hydrothermal reactions in supercritical water (SCW) of equivalent quantities of aqueous MgCl₂·6H₂O (2 M), and K₄P₂O₇ (1 M) in concentrated HCl in a stainless-steel batch reactor at 400–450 °C and 25–32 MPa. The new salt has been characterized by single-crystal X-ray diffraction and IR and Raman spectroscopies. It crystallizes in the triclinic space group Pī, Z = 2 with the following unit-cell parameters: a = 6.438(1), b = 7.856(1), c = 9.438(1) Å; α = 104.57(1), β = 108.61(1), γ = 101.28(1)°, V = 739.99 Å³. The effects of the SCW conditions on the nature of the products and their yields and morphologies have been studied by IR and Raman spectroscopies, X-ray powder diffraction, X-ray energy dispersive analysis, scanning electron microscopy, transmission electron microscopy and inductively coupled plasma analysis.     

New lithium ion conducting polymer electrolytes based on polysiloxane grafted with Si-tripodand centers

New kind of polymer host for lithium cations has been synthesized by catalyzed hydrosilylation reaction involving hydrogen atoms of a polysiloxane and double bonds of vinyl tris-2-methoxyethoxy silane. The obtained macromolecule can be regarded as siloxane backbone grafted with silicon tripodand elements with very short polyether chains. A family of Li ion conducting polymer electrolyte membranes have been prepared by dissolving LiPF₆ in thus obtained polymer matrix. Exceptionally high room temperature specific conductivities, exceeding 10⁻³ S/cm at 25 °C, have been measured for the studied polymer electrolytes. It is proposed that polyether chains tend to self-assembly in the presence of Li cations and this highly organized arrangement of Li coordination sites creates pathways of high lithium conductivity along the polysiloxane backbones. In addition to that, strong shielding of Li-cations suppresses the formation of ion pairs, thus increasing the charge carrier concentration. The electrolytes can be easily formed into dimensionally stable, flexible membranes.