Operation of an Oxygen Gas Diffusion Electrode in a Hybride Fuel Cell

The paper is devoted to the operation of a hybride fuel cell (i.e. in the course of simultaneous electrosynthesis of a chemical product), in particular, the polarization characteristic of an oxygen electrode and the ratio of the produced electric energy to the amount of the produced target product. An equation for the dependence of the polarization of an oxygen gas diffusion hydrophobized electrode of a hydrogen-oxygen fuel cell on the concentration of the produced electroactive soluble product H₂O₂ in an inner-kinetic regime is suggested. It is established that the polarization variation does not depend on its initial magnitude but does depend on the concentration of the target product and the ratio between currents of the side and target reactions. The ratio of the produced electric energy to the amount of the produced target product is found to depend on the concentration of the produced target product, the electrode polarization, and the ratio between exchange currents of the side and target reactions.     

The ion adsorption effect on selectivity of liquid state, O,O'-didecylodithiophosphate chelate based ion-selective electrodes

Ion-selective electrodes with liquid membranes including O,O′-didecylo-dithiophosphate complexes of Tl(I), Pb(II), Cd(II) and Ni(II) are characterised and results of the study on their selectivity are reported. A short review of problems related to determination and interpretation of selectivity coefficients of ion-selective electrodes is presented with particular emphasis on the drawbacks of the hitherto used methods. A new method is proposed, which in the experimental part is close to that of mixed solutions recommended by IUPAC but can be applied also when the latter is of no use. The method proposed for determination of selectivity coefficients simultaneously allows concluding about the mechanism of potential generation. A few examples of relations between selectivity coefficients of the electrodes and concentrations of disturbing ions in solutions, are given. An interpretation of the above relations as results of the processes of ion adsorption at the interface of the electrode membrane and water solution is proposed. The results obtained have confirmed the hypothesis given by Pungor, according to which the main role in the mechanism of generation of ion-selective electrodes potential is played by the processes of ion chemisorption at the interface of the membrane and water solution.     

The effect of sonication on glass electrodes

This communication describes observations made on the disturbing effect of ultrasound on the functioning of glass electrodes. It was observed that the signal of glass electrodes drops significantly, with as much as several pH units, if sonication is applied to the sample solution. The effect was studied for a variety of electrolyte solutions and for several glass electrode makes, and was always found to be reversible and reproducible. The quality and the concentration of the electrolyte, as well as the ultrasonic power applied were seen to strongly influence the magnitude of the effect. These findings suggests that (i) if a glass electrode is being used to follow chemical reactions in sonicated solutions, irradiation should be intermitted for the time of measurements, and (ii) the phenomenon may provide an alternative method for sensing ultrasound or mapping ultrasound energy distribution.     

Hydrogen-oxygen (air) fuel cell with nafion and platinum: Calculating overall characteristics,comparing the performance of a cathode with polymeric electrolyte with a hydrophobized cathode with liquid electrolyte

Results of calculating the major overall characteristics of both an individual cathode and the whole hydrogen-oxygen (air) fuel cell with Nafion and platinum are shown. The effect of varying the parameters of both the active layer and the polymeric-electrolyte membrane on the overall characteristics of such a fuel cell is analyzed. The mechanisms of operation of active layers of hydrophobized cathodes and cathodes containing Nafion are compared. These two electrode types demonstrate a qualitative difference in the current generation mechanisms. As a result, the current in cathodes with Nafion increases more actively with the increase in over-potential (in proportion with exp [η₀/2], where η₀ is the cathodic overpotential) as compared with the case of hydrophobized cathodes (here the current ～ exp[η₀/4]). This explains the fact that a fuel cell with Nafion demonstrates so high power characteristics as compared with a fuel cell with hydrophobized electrodes and liquid electrolyte.     

Influence of column characteristics on retention and selectivity in RP-HPLC

Summary Eight commercially available reversed-phase (RP) columns of widely different characteristics were evaluated and compared using the linear solvation energy relationships (LSER). Retention factors of 32 solutes of different types were determined under isocratic conditions using an acetonitrile-water (30∶70) mobile phase. Stationary phase properties were compared by the fitting coefficients of the LSER-based regression equations which are characteristic of the individual stationary phases and represent the extent of various molecular interactions contributing to the retention process. The good agreement between the calculated and measured logk values for different type of compounds support the adequacy and applicability of the LSER model to describe chromatographic retention. Characterization of column performance for the separation of various type of compounds was established by the determination of the different selectivity factors representing hydrophobic selectivity, polar selectivity and specific selectivity.     

The nitrogen position effect on the selectivity of diazacrown ethers to metal ion

In this paper, the nitrogen position isomers of diaza-18-crown-6, diaza-15-crown-5, diaza-12-crown-4, and their complexes with Na⁺ ion are studied by the density functional theory. Their stable structures are identified by geometry optimization without strict for their geometry. The binding capability that reflects the selectivity of these diazacrown ethers to metallic ions is estimated by the binding energy. The position effects of nitrogen on the stability of these complexes are also investigated by the explicit natural bond orbital and atoms-in-molecule analysis. It is found that different nitrogen position in crown ring will result in different relative energy. To those diazacrown ethers under consideration, the calculation results show that 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane, 1,4-diaza-7,10,13-trioxacyclopentadecane, and 1,7-diaza-4,10-dioxacyclododecane exhibit higher metal binding selectivity than their corresponding nitrogen position isomers, respectively. Clearly, the position of nitrogen plays an important role in the selectivity of diazacrown ethers to metal ions in the system involved. This work will be help for the material design of ionic recognition and other related fields.     

Effect of alkali metal additives on the activity and selectivity of structured silver catalysts in epoxidation of ethylene by nitrogen(I) oxide

We have shown that introducing Cs, K, or Na additives into the composition of silver catalysts (supported on ceramic honeycomb monoliths) for epoxidation of ethylene by nitrogen(I) oxide leads to an increase in the yield of the target product (ethylene oxide) by a factor of 1.5–2, while the presence of lithium in the composition of the composites inhibits the process. The optimal content of alkali promoters in the composition of the catalysts decreases in the series Na, K, Cs antibatically relative to the ionic radii of the elements. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 41, No. 6, pp. 360–363, November–December, 2005.     

Simulation of synergistic effect at coadsorption of two organic substances on electrode

Based of numerical calculations, it is shown that for two organic substances with similar dependences of adsorption on the electrode potential, the overall surface coverage is well described by the ordinary Frumkin isotherm with a certain effective attraction constant a _eff. A correlation between the a _eff value and the constants of intermolecular interaction in the adsorption layer was revealed for co-adsorbing molecules 1 and 2, namely, a ₁₁, a ₂₂, and a ₁₂.     

The effect of electrolysis conditions on the oxidation of styrene in methanol

Anodic oxidation of styrene (1) in methanol and the effect of the anion of the supporting electrolyte and of the anode material on this process have been studied. The most efficient conversion of1 into (1,2-dimethoxyethyl)benzene (2) occurs when the electrolysis is carried out with a platinum anode and with potassium fluoride or tetrabutylammonium tetrafluoroborate as the supporting electrolyte. Cleavage of the C-C bond in2 to give benzaldehyde dimethylacetal (3) is the most efficient at a graphite anode in the presence of sodium tosylate.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1624–1628, September, 1994.     

Mechanism of coadsorption of two organic substances on electrodes at strong attractive interaction between their molecules in the adsorption layer

Based on the model of three parallel capacitors, the differential capacitance and the surface concentration of two organic compounds coadsorbed on an electrode are calculated as a function of the potential. It is shown that for the strong attractive interaction of two different adsorbed molecules that occupy equal areas on the surface and for different combinations of the other adsorption parameters is well described by the model of two parallel capacitors with a simple Frumkin isotherm. However, in this case, the effective attraction constant in this isotherm should depend on the electrode potential. The obtained results show that the good agreement of experimental data with the calculations based on the model of two parallel capacitors is insufficient for assuming that the orientation of adsorbed molecules of a given organic compound is potential-independent. On the other hand, if the adsorbate molecule in two different orientations occupies different areas on the electrode surface, then the model of two parallel capacitors does not allow one to describe the dependence of the total surface concentration on the electrode potential even under the conditions where this model adequately describes the differential capacitance curves.     

The effect of carbon black mixture composition on the structural and electrochemical characteristics of gas diffusion electrodes for electrosynthesis of hydrogen peroxide

Gas-diffusion electrodes containing polytetrafluoroethylene, acetylene black A437E and its mixtures with two types of furnace black (P702 and P268E) with different size of particles are studied. The electrode capacitance, the average diameter and volume of pores and their surface area are investigated as a function of the electrode composition. The kinetic characteristics of electrodes in the oxygen electroreduction are assessed. The slope of polarization curves in Tafel coordinates and the transfer coefficient value are shown to be virtually independent of the electrode composition. The preparative synthesis of Н₂О₂ is carried out.     

The effect of the electrolyte concentration in the solution on the voltammetric response of insertion electrodes

A solid-state redox reaction involving an insertion of ions is analyzed with respect to the influence of the concentration of inserting ions in the solution phase. The voltammetric response is independent of the mass transfer in the solution provided that z = (D _ss/D _aq)^1/2 ρ/[C⁺]* is smaller than 0.1 (D _ss: diffusion coefficient of the cation C⁺ in the crystal; D _aq: diffusion coefficient of the cation C⁺ in the solution; ρ: density of the solid compound; [C⁺]*: concentration of cations in the bulk of the solution). In real cases this condition will be satisfied at solution concentrations above 1 mol/l. Received: 15 December 1997 / Accepted: 5 March 1998     

Mercury(II) binding to thiol-functionalized mesoporous silicas: critical effect of pH and sorbent properties on capacity and selectivity

The binding properties of mesoporous thiol-functionalized silica sorbents towards mercury(II) species were studied as a function of pH in a wide range (0-8), in the absence or in the presence of competing metal ions, from batch equilibration experiments. To this end, a series of thiol-functionalized adsorbents characterized by different structures (from completely disordered amorphous solids to highly ordered mesostructures), variable density of organic ligands (from 1 to 4 mmol g⁻¹), and various degrees of porosity, have been prepared either by post-synthesis grafting or by the co-condensation route. Hg(II) binding to these thiol-functionalized silica samples is strongly dependent on pH, especially in acidic medium (pH < 4) where non-hydrolyzed Hg²⁺ species become dominant. This behavior was found to be significantly affected by the degree of structural organization of the materials (amorphous or ordered mesoporous solids, short-range versus long-range structural order) and the adsorbent composition (density of functional groups). A beneficial effect of high structural order was observed in both the capacity (access to a high number of binding sites) and selectivity (towards other metal ions) for the ordered mesoporous sorbents in comparison to the amorphous gels, but this was only true for pH values down to 4, where Hg(II) species are mainly in the form of Hg(OH)₂. In more acidic medium, however, the sorption of the non-hydrolyzed Hg²⁺ species underwent dramatic loss of effectiveness, which resulted in both lower capacities and worse selectivity. These restrictions were more marked when increasing the density of functional groups in the materials and, to lesser extent, when decreasing their level of structural ordering. They were interpreted on the basis of electrostatic considerations as the binding of Hg²⁺ to thiol groups leads to the generation of positively charged complexes in the host material while that of Hg(OH)₂ involves the formation of neutral moieties. Possible regeneration of sorbents and re-use were also discussed.     

Effect of rate and direction of solution flow on metal deposition in porous electrodes: The final weight and distribution of the deposit

Using an earlier-developed dynamic model for a porous flow-through electrode (PFE) with a high initial conductivity, the effect of the solution’s flow rate (0.05–10 cm/s) and direction on the final metal weight m _f and uniformity of the metal distribution in the porous matrix is studied. It is found that m _f increases with increasing flow rate. However, the dependence is nonmonotonic: it peaks at intermediate flow rates. The peak is most pronounced in the case of rear supply. At high and very low flow rates, m _f is independent of the flow direction. In the first case, the metal distribution profiles almost coincide, while in the second case they are mirror-opposite. The deposit weight correlates well with the index of uniformity of its distribution: all other factors being equal, the more uniform the deposit distribution in PFE, the larger the m _f. These effects are explained by taking into account the joint effect of profiles of cathodic polarization and concentration of metal ions in PFE.     

The alkali metal cation effect on the surface-enhanced Raman spectra of phosphate anions adsorbed at silver electrodes

The interaction of adsorbed phosphate anions with alkali metal cations at the Ag|aqueous solution interface has been investigated by surface-enhanced Raman spectroscopy (SERS). Formation of ion pairs at the interface was evident from the cation-induced perturbations in the SER spectra of anions. The frequency of the external vibration, silver–oxygen (Ag---O′), was not sensitive to the nature of cation, while the relative intensity of this mode was cation-dependent and was explored as a sensitive probe for the monitoring of coadsorption of ions at the interface. From the internal phosphate vibrations, both asymmetric modes, δ_as(PO) and ν_as(PO), were found to be the most sensitive to the nature of the cation. At a relatively positive potential (0.00 V vs. Ag | AgCl) the spectral parameters for the Cs⁺ and K⁺ cations were very similar indicating the same bonding type with anions. A more inhomogeneous chemical environment for the phosphate oxygen atoms was detected in the case of Na⁺ and Li⁺ cations. An increase in ν_as(PO) frequency by ca. 10 cm⁻¹ was the characteristic spectral signature for the interaction of phosphates with Li⁺. The formation of water-shared ion pairs at the interface was suggested based on the absence of splitting in the ν_as(PO) mode and the previously observed frequency sensitivity of this band to solvent H₂O substitution by D₂O. At negative potential (−0.80 V), a stabilization effect of Cs⁺ on the phosphate adlayer was detected based on the twofold increase in intensity of the ν(Ag---O′) mode compared with Li⁺. Splitting of the ν_as(PO) mode suggested the contact interaction of anions with specifically adsorbed Cs⁺ cations.     

The effect of the binder morphology on the cycling stability of Li–alloy composite electrodes

It is demonstrated that the design of the composite electrode, or more precisely the morphology and distribution of the binder poly(vinylidine fluoride) (PVdF) within the composite electrode, has a significant impact on the cycling performance of Li storage alloy (Sn/SnSb) electrodes. Different binder morphologies and distributions have been obtained by using different solvents for the slurry preparation, such as 1-methyl-2-pyrrolidinone (NMP), in which PVdF is dissolved, yielding electrodes with a homogeneously and finely distributed binder, or decane, in which PVdF is only dispersed, yielding electrodes in which the original particle morphology of the binder powder is preserved. In constant current cycling tests carried out in an excess of electrolyte, the electrodes with the ‘dispersed’ binder show far better cycling capacities and stabilities than those with the ‘dissolved’ binder. This is explained by the different binding strengths, swelling behaviour in the electrolyte, electrode porosities, and possible ‘buffer’ effects of the compact and the finely distributed binders.     

The effect of ‘crystallinity’ and structural disorder on the electrochemical performance of substituted nickel hydroxide electrodes

β_bc-Nickel hydroxide exhibit non-uniform broadening reflections in their PXRD pattern due to the presence of structural disorder. β_bc-Nickel hydroxide electrodes with smaller crystallite size and structural disorder reversibly exchanges 0.9e/Ni. Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide samples also display non-uniform broadening of reflections in their powder X-ray diffraction patterns with smaller crystallite size and exchanges 0.7–0.8e/Ni. Hydrothermal treatment of β_bc-nickel hydroxide slurry at 170 °C results in an ordering of reflections in their powder X-ray diffraction pattern with an increased crystallite size. Crystalline β-nickel hydroxide electrode reversibly exchanges 0.3–0.4e/Ni. Hydrothermal-treated Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide slurries at 170 °C display sharp reflections with similar crystallite size and electrochemical activities as that of crystalline β-nickel hydroxide. This clearly demonstrates that partial substitution of Co/Zn/Ca/Cd in the nickel hydroxide matrix does not show any dramatic improvement in their electrochemical activity at 25–30 °C. Structural disordered material with smaller crystallite size delivers electrochemical activity close to theoretical capacity.     

The effect of the curing agent content on the curing and liquid‐crystalline phase of liquid‐crystalline epoxy resin

The effect of the curing agent content on the curing behavior and liquid‐crystalline (LC) phase of the liquid‐crystalline epoxy (LCE) resin 4,4′‐di(2,3‐epoxypropyloxy)phenyl benzoate was studied. Diaminodiphenylester (DDE) was used as a curing agent. The curing behavior was observed via differential scanning calorimetry, and the LC phase was investigated with a polarized optical microscopy. The LC phase in the LCE/DDE mixture with a high DDE content was developed during curing. The onset time was inversely proportional to the DDE content. The mesophase stability of LCE/DDE was enhanced by the addition of large amounts of DDE. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 374–379, 2001     

The effect of grass fiber filler on curing characteristics and mechanical properties of natural rubber

Natural rubber is reinforced with a novel type of grass fiber (Cyperus Tegetum Rox b). The effects of fiber loading of different mesh sizes on curing characteristics and mechanical properties of grass fiber filled natural rubber composite are studied. Since 400 mesh grass fiber loaded natural rubber composite shows superior mechanical properties, therefore the effect of silane coupling agent was studied for this particular composite. Here composites were prepared by using water leached grass fiber. Optimum cure time increases with the increase in fiber loading but the change in scorch time is less. The same trend of increase in optimum cure time is observed in the presence of Si69. But the value is higher compared to that of rubber composite without Si69. With increase in the fiber loading, modulus and hardness of the composite increases but tensile strength decreases. The mechanical properties of the composite, namely moduli at 200 and 300% elongation and hardness increase in the presence of Si69 but tensile strength is less compared to that of the composite without Si69. Elongation at break is not much affected due to the presence of Si69. Copyright © 2004 John Wiley & Sons, Ltd.     

Polyoxometalate immobilization in Cu/Ag-pz porous coordination polymers: The influences of them on the structural properties of frameworks

Three new high dimensional Cu^I/Ag-pz porous coordination polymers (PCPs) with different Keggin polyoxometalate templates have been hydrothermally synthesized: [Cu^I₅(pz)₆Cl][HPMo^VI₁₀Mo^V₂O₄₀] (1) [Ag₅(pz)₇(BW₁₂O₄₀)] (2) and [Cu^I₅(pz)₆H(H₂W₁₂O₄₀)]·4H₂O (3) (pz=pyrazine). The choice of the particular Keggin POM templates is shown to influence the structural properties of the Cu/Ag PCPs. Compound 1 shows a Cl-bridged Cu-pz-Cl double layer, between which two kinds of [HPMo^VI₁₀Mo^V₂O₄₀]⁴⁻(PMo₁₂) polyanions are located. Compound 2 presents a 3D Ag-pz framework with parallelogram-like voids, into which BW₁₂O₄₀⁵⁻ (BW₁₂) Keggin ions are incorporated. Compound 3 contains a Cu-pz cationic layer framework, between which are located [H(H₂W₁₂O₄₀)]⁵⁻ (W₁₂) Keggin ions. Primary photocatalytic experiment indicates that compound 1 presents excellent photocatalytic activity. The photoluminescence properties and electrochemistry properties of the compounds are also discussed.