Antonio Redondo

Recent advances in solid polymer electrolyte fuel cell technology with low platinum loading electrodes

The Gemini Space program demonstrated the first major application of fuel cell systems. Solid polymer electrolyte fuel cells were used as auxiliary power sources in the spacecraft. There has been considerable progress in this technology since then, particularly with the substitution of Nafion for the polystyrene sulfonate membrane as the electrolyte. Unt_l recently the performance was good only with high platinum loading (4 mg/cm 2) electrodes. This paper presents methods to advance the technology by (i) use of low platinum loading (0.35 mg/cm 2) electrodes; (ii) optimization of anode/membrane/cathode interfaces by hot-pressing; (i{i) pressurization of reactant gases, which is most important when air is used as cathodic reactant; and (iv) adequate humidification of reactant gases to overcome the water management problem. The high performance of the fuel cell with the low loading of platinum appears to be due to the extension of the three dimensional reaction zone by introduction of a proton conductor, Nafion. This was confirmed by cyclic voltammetry.

Investigation of Early Events in FcεRI-Mediated Signaling Using a Detailed Mathematical Model

Aggregation of FcεRI on mast cells and basophils leads to autophosphorylation and increased activity of the cytosolic protein tyrosine kinase Syk. We investigated the roles of the Src kinase Lyn, the immunoreceptor tyrosine-based activation motifs (ITAMs) on the β and γ subunits of FcεRI, and Syk itself in the activation of Syk. Our approach was to build a detailed mathematical model of reactions involving FcεRI, Lyn, Syk, and a bivalent ligand that aggregates FcεRI. We applied the model to experiments in which covalently cross-linked IgE dimers stimulate rat basophilic leukemia cells. The model makes it possible to test the consistency of mechanistic assumptions with data that alone provide limited mechanistic insight. For example, the model helps sort out mechanisms that jointly control dephosphorylation of receptor subunits. In addition, interpreted in the context of the model, experimentally observed differences between the β- and γ-chains with respect to levels of phosphorylation and rates of dephosphorylation indicate that most cellular Syk, but only a small fraction of Lyn, is available to interact with receptors. We also show that although the β ITAM acts to amplify signaling in experimental systems where its role has been investigated, there are conditions under which the β ITAM will act as an inhibitor.

The Hydration Number of Li+ in Liquid Water

The hydration of ions in water is not only fundamental to physical chemistry, but is also relevant to the current issue of selectivity of biological ion channels. In the context of potassium channels for example, the free energies for replacement of inner shell water ligands with peptide carbonyls donated by proteins of the channel, specifically for the preference of K{sup +} over Na{sup +}. Studies to elucidate the thermodynamic features of such inner shell exchange reactions require prior knowledge of the ion hydration structures and energetics. Simulations have produced a range of results including both four and six inner shell water neighbors with considerable statistical dispersion. Simulations are typically not designed to provide sole determinations of such properties, although they do shed light on the issues determining the hydration number of ions in water. The theoretical scheme used here to address these problems for the Li{sup +}(aq) ion is based upon the quasi-chemical organization of solution theory, which is naturally suited to these problems.

Resistance-induced peaks in cyclic voltammograms of systems that can be switched electrochemically between an insulating and conductive state

In the cyclic voltammetry of certain films that exhibit in general a reversible or quasi-reversible behavior, prepeaks associated with the main oxidation peak may be found under certain conditions. In the present paper we demonstrate that this phenomenon can be explained by consideration of resistive effects in the film. A simple model presumes that the electrochemistry is reversible and that the film resistivity (Ro) is inversely related to the amount of (oxidizing) charge (qt) by a simple power relationship: (Ro) = a(qmax/qt)P. The shape of the theoretical cyclic voltammogram is a function of a, P, and the scan rate, and it mimics qualitatively many of the experimentally observed features, including prepeaks and shoulders.

Simultaneous ellipsometric and microgravimetric measurements during the electrochemical growth of polyaniline

Abstract We describe an experimental system for simultaneous quartz crystal microbalance (QCM) and ellipsometric measurements performed in situ during the growth of a thin film on an electrode surface. The film studied is grown on a Pt electrode sputtered onto a quartz crystal, which serves as the microbalance. The quartz crystal is positioned in a semicylindrical cell equipped with glass windows, allowing continuous optical probing of the electrode surface. The automatic elllipsometer readings are taken simultaneously with the QCM measurements. The information obtained from the two techniques is complementary: the QCM measures the overall mass increase during film growth, whereas the ellipsometer measures the optical properties of the film and its thickness. From such combined simultaneous measurements, the apparent density of the film can be derived. We describe here results of such combined measurements performed during the anodic growth of polyaniline in aqueous acid solutions showing substantial variations in film density as a function of electrochemical growth conditions.

Dielectric relaxation and underlying dynamics of acetonitrile and 1-ethyl-3-methylimidazolium triflate mixtures using THz transmission spectroscopy

We use terahertz (THz) transmission spectroscopy to obtain the frequency dependent complex dielectric functions for pure acetonitrile, pure 1-ethyl-3-methylimidazolium triflate (emim triflate, a room temperature molten salt), and mixtures of the two liquids. The behavior of the pure liquids is modeled with either two (acetonitrile) or three (emim triflate) Debye relaxations. We then discuss the interactions of the molten salt and solvent based on the modified Debye relaxations evident in the mixtures. We determine that at low molten salt concentrations, the mixtures behave like electrolyte solutions of a crystalline salt dissolved in a solvent. At higher molten salt concentrations, the behavior is that of a mixture of two liquids.

Kinetic proofreading models for cell signaling predict ways to escape kinetic proofreading

In the context of cell signaling, kinetic proofreading was introduced to explain how cells can discriminate among ligands based on a kinetic parameter, the ligand-receptor dissociation rate constant. In the kinetic proofreading model of cell signaling, responses occur only when a bound receptor undergoes a complete series of modifications. If the ligand dissociates prematurely, the receptor returns to its basal state and signaling is frustrated. We extend the model to deal with systems where aggregation of receptors is essential to signal transduction, and present a version of the model for systems where signaling depends on an extrinsic kinase. We also investigate the kinetics of signaling molecules, “messengers,” that are generated by aggregated receptors but do not remain associated with the receptor complex. We show that the extended model predicts modes of signaling that exhibit kinetic discrimination for some range of parameters but for other parameter values show little or no discrimination and thus escape kinetic proofreading. We compare model predictions with experimental data.

Modeling the early signaling events mediated by FcεRI

We present a detailed mathematical model of the phosphorylation and dephosphorylation events that occur upon ligand-induced receptor aggregation, for a transfectant expressing FcepsilonRI, Lyn, Syk and endogenous phosphatases that dephosphorylate exposed phosphotyrosines on FcepsilonRI and Syk. Through model simulations we show how changing the ligand concentration, and consequently the concentration of receptor aggregates, can change the nature of a cellular response as well as its amplitude. We illustrate the value of the model in analyzing experimental data by using it to show that the intrinsic rate of dephosphorylation of the FcepsilonRI gamma immunoreceptor tyrosine-based activation motif (ITAM) in rat basophilic leukemia (RBL) cells is much faster than the observed rate, provided that all of the cytosolic Syk is available to receptors.

Dielectric relaxation of electrolyte solutions using terahertz transmission spectroscopy

We use terahertz (THz) transmission spectroscopy to obtain the frequency dependent complex dielectric constants of water, methanol, and propylene carbonate, and solutions of lithium salts in these solvents. The behavior of the pure solvents is modeled with either two (water) or three (methanol and propylene carbonate) Debye relaxations. We discuss the effects of ionic solvation on the relaxation behavior of the solvents in terms of modifications to the values of the Debye parameters of the pure solvents. In this way we obtain estimates for numbers of irrotationally bound solvent molecules, the numbers of bonds broken or formed, and the effects of ions on the higher-frequency relaxations.

Thermal conductivity of optical coatings

A model capable of explaining recent experimental results showing a considerable decrease in the thermal diffusivity of optical coatings is presented. The model assumes that the coatings contain microcracks. It predicts that such imperfections can cause reduction factors of 2 or more orders of magnitude in the thermal conductivity of the films, when compared to the corresponding bulk values. If the cracks enclose a region where light is absorbed the thermal isolation can lead to damage of the coating. A similar model consisting of a random distribution of voids only predicts a modest reduction of the net thermal conductivity.