A cathode for solid polymer electrolyte fuel cells: Designing the optimal structure of the active layer

The complete computer simulation of the cathodic active layer with solid polymer electrolyte (Nafion) is carried out. The active layer structure can be described by 8 parameters. In designing the optimal structure, it is shown that to provide the high overall characteristics of the cathode and save the catalyst, 0.5 of the active layer volume should be set aside for the support grains (agglomerates of carbon particles covered with platinum and containing Nafion incorporations and microvoids). Protons and oxygen molecules must be supplied to the active layer by means of peculiar combined percolation clusters. The latter consist of a combination of support grains with either Nafion grains (to produce “protonic” clusters) or grains-voids (to afford “gas” clusters). The volume fractions of Nafion grains and grain-voids are assumed to be 0.25 and 0.25. The computer simulation of the support grain structure is also carried out. Their composition, i.e., the volume fractions of the carbon component (g _e), Nafion (g _ii), and microvoids (g _gg), is varied. The support grains play the key role in the active layer functioning. It is impossible to organize three full-value percolation clusters (electronic, protonic, and gas); hence, one has to have one or two combined clusters in the active layer. Thus the double load fells on the support grains. Their optimal structure should not only sustain the transport of protons and electrons in the active layer but also create the best conditions for the electrochemical process in each grain. The maximum current I _max (realized upon reaching the optimal active layer thicknesses Δ*) is calculated. The dependences of I _max and Δ* on the main parameters characterizing the support grains (g _e and g _ii) are analyzed. Here, two goals are sought: (1) to obtain the high currents, (2) to provide the low consumption of platinum per power unit. To solve the first problem, one has to work with high values of g _e. The second problem requires the opposite: the values of g _e must be minimal possible.     

Core edge energy loss studies of Pd clusters on graphite

In order to verify that the d-band of Pd clusters moves towards higher binding energies when the cluster size decreases, we have carried out energy loss measurements of the M_4,5 core edge of Pd clusters with different diameters evaporated on polycristalline graphite. Generally the shifts have been attributed in the past both to initial and or final state effects. In the last case one could not observe variations with our technique which probes the partial unfilled density of states. Our experimental results show an enhancement of the absorption and a narrowing of the lineshape with increasing cluster size. We attribute the observed effects to a genuine change in the electronic structure which occurs in the initial state of the cluster. Comparative studies of this effect have been performed following the evolution of the M_4,5 VV Auger levels and the collective excitations in the range 0–30 eV far from the elastic peak.     

Identification of diluted small defect clusters in silicon by electron paramagnetic resonance

The possibilities to identify small diluted paramagnetic cluster defects in solids by Electron Paramagnetic Resonance are demonstrated in two examples: the Mn ₄ ⁰ cluster and a Cu-Au cluster in silicon crystals. A comprehensive picture of the clusters can be given containing the chemical nature of the constituents, the spatial arrangement, the charge states and the electronic structure.     

Laser photoionization and spectroscopy of gas phase silver clusters

Silver clusters containing up to 40–50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd:YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N=3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag₃, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via theE state at 249 nm establishes the dissociation energy of Ag ₂ ⁺ to be <2.1 eV.     

Trends in Geometric,Energetic, Electronic,and Magnetic Properties of Vanadium–Copper Clusters Cu<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>V with <Emphasis Type="Italic">n</Emphasis> = 1–12: Density Functional Calculations

A systematic quantum chemical investigation on the geometric, energetic, electronic and magnetic properties of vanadium-copper nanoalloy clusters (n = 1–12) is performed by using BPW91/LanL2DZ calculations. The calculated results show that the structural evolution of Cu _n V clusters favors a compact and icosahedral growth pattern and V atom favors occupying the most highly coordinated position. Energetic properties show that doping of one V atom contributes to strengthening the stability of the copper clusters with the growth of the clusters. The stacking mode of clusters apparently has a more important effect on the clusters stability than the electronic structure. However, electronic structures have some contribution to the stability of Cu _n V clusters as well. The electronic properties of Cu _n V are analyzed through vertical ionization potential (VIP), vertical electron affinity (VEA) and chemical hardness (η). The magnetism calculations show that when doping V atom in copper clusters, the cluster system generate a very large magnetic moment and its contribution mainly comes from the 3d orbital of doping-V atom.     

Photodissociation of sulfur dioxide cluster anions

Photodissociation of negatively charged sulfur dioxide clusters (SO₂) _n ^? , 2≦n≦11, was investigated in the wavelength range between 458 nm and 600 nm using a tandem mass spectrometer. The spectral position of the absorption band remains unchanged, however it exhibits narrowing with increasing cluster size. The smooth evolution of the spectra shows that the clusters are composed of a dimer anion core surrounded by neutral molecules. The analysis of the fragmentation products reveals that the absorption of a photon is followed by statistical evaporation of neutrals with a mean energy loss of 0.28±0.05 eV per evaporated monomer in the large cluster limit.     

TiO2 and Al2O3 nanoporous oxide layers decorated with silver nanoparticles—active substrates for SERS measurements

Self-organized nanoporous oxide layers (TiO₂, Al₂O₃) exhibiting specific properties, obtained by anodic oxidation at a constant voltage in neutral electrolyte, may serve as attractive SERS substrates for investigating the interactions between an adsorbate and adsorbent, or as a stable platform for detecting various organic compounds. This paper presents the influence of the size of the nanotubes/nanopores and the structure of the porous oxide layers on the SERS enhancement factor, E _F. We used pyridine and mercaptobenzoic acid as probe molecules, since they have a large cross-section for Raman scattering. To characterize the morphology and structure of the oxide layer substrates, before and after vacuum vapor deposition of silver nanoparticles, we applied scanning electron microscopy, X-ray diffraction and surface analytical techniques: AES, XPS and SERS. The results obtained show that for the same amount of Ag (0.02 mg/cm²) the size of the nanopores significantly affects the E _F, which reaches, at a properly chosen nanopore size, distinctly higher values than that characteristic of a standard silver surface roughened by electrochemical cycling, i.e. E _F?>?10⁶. The new Ag/MeO _x –NT composites layer, ensure a good reproducibility of the SERS measurements and exhibit stability over a limited period of time.     

Adsorption and inhibition effect of curcumin on mild steel corrosion in hydrochloric acid

The inhibition effect of curcumin on the corrosion of mild steel in 1.0 M HCl solution was studied by weight loss, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. The experimental results suggest that this compound is an efficient corrosion inhibitor and the inhibition efficiency increases with the increase in inhibitor concentration. Adsorption of this compound on mild steel surface obeys Langmuir isotherm. Also the objective of this work is to attempt to find relationships between electronic structure and inhibition efficiency. The structural parameters, such as the frontier molecular orbital energies (E _HOMO and E _LUMO), gap of energy ΔE, from the molecule to iron as well as electronic parameters such as Mulliken atomic populations were calculated and discussed using the Density Functional Theory method (DFT).     

Computational Studies on the Mo-Doped Gold Nanoclusters Au<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>Mo(<Emphasis Type="Italic">n</Emphasis> = 1–10): Structures,Stabilities and Magnetic Properties

The geometric structures, relative stabilities, magnetic properties of Mo-doped gold clusters Au _n Mo(n = 1–10) have been investigated at the PBE1PBE/def2TZVP level of theory. The results show that molybdenum doping has a significant effect on the geometric structures and electronic properties of Au _n Mo(n = 1–10) clusters. For the lowest energy structures of Au _n Mo(n = 1–10), the two dimensional to three dimensional transition occurs at cluster size n ≥ 8, and their relative stabilities exhibit odd–even oscillation with the change of Au atom number. It is found that charge in corresponding Au _n Mo clusters transfers from Mo atom to Au _n host in the size range n = 1–7, whereas the charge in opposition direction in the size range n = 8–10. In addition, the magnetic properties of Au _n Mo clusters are enhanced after doping single Mo atom into the corresponding gold clusters. Our results are valuable for the design of magnetic material.     

Structure of Ag,Fe and Ge microclusters

The structures of Ag, Fe and Ge microclusters were determined using EXAFS. The measurements were performed over a wide range of clusters sizes. The clusters were prepared using the gas aggregation technique and isolated in solid argon at 4.2 K. The measurements were performed at the National Synchrotron Light Source (NSLS) at beam line X-18B. A strong contraction of the interatomic distances was observed for Ag dimers and multimers. Silver clusters larger than 12 A mean diameter show a small contraction of thenn distance and a structure consistent with an fcc lattice. By contrast clusters smaller than 12 A show the presence of a small expansion and a strong reduction or absence ofnnn in the EXAFS signal. This points towards a different crystallographic structure for Ag microclusters with diameter less than 12 A. In iron clusters we observe a gradual reduction of thenn distance as the cluster size decreases. The interatomic distance for iron dimers was determined to be 1.94 A, in good agreement with earlier measurements. The iron microclusters show a bcc structure down to a mean diameter of 9 A. Iron clusters with 9 A mean diameter show a structure inconsistent with a bcc lattice. The new structure is consistent with an fcc or hcp lattice. The measurements on Ge clusters show the presence of only nearest neighbors. There was clear evidence of temporal annealing as determined by variations in the near edge structure of the K-absorption edge. Absorption edge measurements were also performed for free Ge clusters travelling perpendicular to the direction of the synchrotron radiation beam. The measurements performed on the free clusters were consistent with those obtained for matrix isolated clusters.     

Electron attachment to HCl clusters

Negatively charged cluster ions of hydrogen chloride are formed by electron attachment to HCl clusters, which are produced in a seeded supersonic beam traversing a sustained gas discharge. Cluster ions of (HCl) _n ^? , withn=2, and tentatively withn=3 and 4 are observed. Cluster ions like Cl _n ^? , Cl _n ^? (HCl) _m , and withAr attached to them are also seen. The relevance to radiation chemistry of HCl if briefly discussed. Atoms evaporating from the hot, thoriated tungsten filament of the glow discharge lead to clusters such as Th _n ^? and its oxides.     

Enrichment and segregation in alkali heteroclusters

A spherical average pseudopotential method (SAPS) is used to investigate some properties of compound alkali clusters. The effect that the substitution of a Sodium atom by a Lithium atom in a Na _n cluster has on the stabilities and geometries is studied forn≤21. We have found that substitution is always energetically possible. On the other hand equiatomic Na _n Cs _n clusters are considered in the size rangen≤16. We find a strong segregation effect of the Cesium atoms towards the cluster surface. This agrees with what happens in liquid Na _x Cs_1?x alloys.     

Structural and dynamical properties of magnesium microclusters

We present systematic Density Functional Theory-Local Density Approximation computations for neutral Magnesium clusters Mg _n withn≤13. For the smaller sizes the ground state structure is optimized starting from selected symmetries and allowing for relaxation, Jahn-Teller distorsion and spin polarization. For the larger sizes we perform a simulated annealing based on the ab-initio Molecular Dynamics. By the same method, we study the thermal and dynamical properties of Mg₁₀ and Mg₁₆. The general picture emerging from these computations shows that already atn ≈10 these clusters have acquired many characteristic features of metallic Magnesium.     

First principle study of magnetic and electronic properties of single X (X = Al,Si) atom added to small carbon clusters (C<Subscript> <Emphasis Type="Italic">n</Emphasis> </Subscript>X, <Emphasis Type="Italic">n</Emphasis> = 2–10)

In this paper, the magnetic and electronic properties of single aluminum and silicon atom added to small carbon clusters (C_nX; X = Al, Si; n = 2–10) are studied in the framework of generalized-gradient approximation using density functional theory. The calculations were performed for linear, two dimensional and three dimensional clusters based on full-potential local-orbital (FPLO) method. The total energies, HOMO–LUMO energy gap and total magnetic moments of the most stable structures are presented in this work. The calculations show that C_nSi clusters have more stability compared to C_nAl clusters. In addition, our magnetic calculations were shown that the C_nAl isomers are magnetic objects whereas C_nSi clusters are nonmagnetic objects.     

A core-shell surface magnetic molecularly imprinted polymers with fluorescence for λ-cyhalothrin selective recognition

In this study, we report here a general protocol for making core-shell magnetic Fe₃O₄/SiO₂-MPS/MIPs (MPS = 3-(methacryloxyl) propyl trimethoxysilane, MIPs = molecularly imprinted polymers, Fe₃O₄/SiO₂-MPS as core, MIPs as shell) via a surface molecular imprinting technique for optical detection of trace λ-cyhalothrin. The fluorescent molecularly imprinted polymer shell was first prepared by copolymerization of acrylamide with a small quantity of allyl fluorescein in the presence of λ-cyhalothrin to form recognition sites without doping. The magnetic Fe₃O₄/SiO₂-MPS/MIPs exhibited paramagnetism, high fluorescence intensity, and highly selective recognition. Using fluorescence quenching as a detecting tool, Fe₃O₄/SiO₂-MPS/MIPs were successfully applied to selectively and sensitively detect λ-cyhalothrin, and a linear relationship could be obtained covering a wide concentration range of 0–50 nM with a correlation coefficient of 0.9962 described by the Stern-Volmer equation. The experimental results of practical detection revealed that magnetic Fe₃O₄/SiO₂-MPS/MIPs as an attractive recognition element was satisfactory for determination of trace λ-cyhalothrin in honey samples. This study, therefore, demonstrated the potential of MIPs for detection of λ-cyhalothrin in food.     

Preparation and characterization of cationic pH-sensitive SiO2/polymer core-shell nanoparticles with amino groups in the shell

Silica/poly(styrene-N,N′-dimethylaminoethyl methacrylate) (SiO₂/P(St-DMAEMA)) cationic pH-responsive core-shell particles with a narrow size distribution and diameter of less than 200 nm were synthesized by emulsion polymerization. The effects of the St/DMAEMA molar ratio, SiO₂ core size, monomer amount, and cross-linking degree on the morphology and pH-responsiveness of the core-shell particles were investigated by transmission electron microscopy, dynamic light scattering, and conductometric titration. The results showed that core-shell particles with only one SiO₂ core could be obtained when the cross-linker divinyl benzene (DVB) was used, and the diameter of the core-shell particles increased with the size of the SiO₂ core and the total amount of monomer. It was observed that the amount of surface amino groups, zeta potential, and volume swelling ratio of the core-shell particles were significantly affected by the St/DMAEMA molar ratio, and a high volume swelling ratio was achieved at pH 4 and a DVB content of 3 mol%. The zeta potential was observed to be a function of pH, and the particles were positively charged when the pH value was below approximately 7.2.     

Spectroscopy of free sodium-ammonia clusters

Neutral sodium ammonia clusters are formed in apickup source by injecting a beam of sodium atoms into the expansion zone of a pulsed nozzle beam of neat ammonia gas. The clusters are studied by one-photon ionisation in the range of 266 nm to 500 nm with pulsed lasers and Time-of-Flight mass spectroscopy. Na(NH₃) _n cluster ion signals up ton=35 are observed. The ionisation potentials of complexes up ton=9 are reported.     

X-ray spectroscopy study of lithiated graphite obtained by thermal deposition of lithium

X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy are used for in situ studies of the electronic structure of lithiated natural graphite produced by thermal deposition of lithium upon graphite in a vacuum. By XPS and NEXAFS spectroscopy it is found that lithium vapor thermal deposition results in the formation of a lithiated graphite surface layer and a change in its electronic structure. Based on the quantum chemical simulation of the experimental СK_α XES spectrum of lithiated graphite, it is found that lithium atoms are located mostly on the edges of graphite crystallites. Atomic force microscopy reveals that the size of natural graphite flakes varies from 50 nm to 200 nm.     

Compared electronic structures of negative ionsM p C n − : II. Transition metals in Hückel theory

SIMS experiments on transition metal carbides produce negative cluster ionsM _p C _n ^? (n < 10) where the transition metalM is Ti, Zr, V, Cr, W, Fe and Ni withp=1 and V withp=2. TheM _p C _n ^? ions show very pronounced alternations in their emission intensities I(M _p C _n ^? ) versusn with strong maxima for evenn whateverM. Since such phenomena are due to the stability properties of the clusters themselves (correspondence rule), it means that the negative ions are the most stable ones for evenn. It is thus possible to get the general outlines of their electronic structures from the Pitzer and Clementi model (sp hybridization in Hückel approximation): the clusters are assumed to be linear chains of “cumulene”-type :=C=..C=C=M and the alternations in the relative stabilities of these chains are due to the fact that the HOMO (highest occupied molecular orbital) of the clusters lies in a double degenerate π level band. Now HOMO may be either full (or almost full) or half-filled (or nearly half-filled), and an aggregate with a complete (or almost complete) HOMO is more stable than an aggregate with a half-filled HOMO. Consequently, the number of π electrons is governing the parity effect in the stability alternations. However, this number is depending on the number of σ electrons of the chain and on the existence of onedδ level (due to theM atom) which is either empty for deficientd electron elements (Ti, Zr, V, Cr, W) or filled for richd electron elements (half-filled for Fe or full up for Ni). As theMC _n ^? chain must have a full (or nearly full) HOMO ifn is even, it is then possible to infer a likely electronic configurations of these clusters, and hence to determine their relative stabilities, and to verify that “even” clusters are more stable than “odd” ones. Thus such Hückel model results can be used as a guide for more sophisticated calculations (ab initio, etc...).