Speciation and Proton Conductivity of Phosphoric Acid Confined in Mesoporous Silica.

Phosphoric acid (PA) confined in a commercial mesoporous silica (CARIACT G) with porous size in the range of 3 to 10 nm was studied in relation to its coordination with the silanol groups on the silica surface as a function of temperature, up to 180 °C, using 31P and 29Si MAS NMR spectroscopy. As the temperature increases, the coordination of Si and P in the mesopores depends on the pore size, that is, on the area/volume ratio of the silica matrix. In the mesoporous silica with the higher pore size (10 nm), a considerable fraction of PA is nonbonded to the silanol groups on the surface, and it seems to be responsible for its higher conductivity at temperatures above 120 °C as compared to the samples with a smaller pore size. The electrical conductivity of the functionalized mesoporous silica was higher than that reported for other silico-phosphoric composites synthesized by sol-gel methods using soft templates, which require high-temperature calcination and high-cost reagents and are close to that of the best PA-doped polybenzimidazole membranes used in high-temperature proton exchange membrane fuel cells (HT-PEMFCs). The rate of PA release from the mesoporous silica matrix when the system is exposed to water has been measured, and it was found to be strongly dependent on the pore size. The low cost and simplicity of the PA-functionalized mesoporous silica preparation method makes this material a promising candidate to be used as an electrolyte in HT-PEMFCs.

Assessing dead time effects when attempting isotope ratio quantification by time-of-flight secondary ion mass spectrometry.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a quasi-non-destructive technique capable of analyzing the outer monolayers of a solid sample and detecting all elements of the periodic table and their isotopes. Its ability to analyze the outer monolayers resides in sputtering the sample surface with a low-dose primary ion gun, which, in turn, imposes the use of a detector capable of counting a single ion at a time. Consequently, the detector saturates when more than one ion arrives at the same time hindering the use of TOF-SIMS for quantification purposes such as isotope ratio estimation. Even though a simple Poisson-based correction is usually implemented in TOF-SIMS acquisition software to compensate the detector saturation effects, this correction is only valid up to a certain extent and can be unnoticed by the inexperienced user. This tutorial describes a methodology based on different practices reported in the literature for dealing with the detector saturation effects and assessing the validity limits of Poisson-based correction when attempting to use TOF-SIMS data for quantification purposes. As a practical example, a dried lithium hydroxide solution was analyzed by TOF-SIMS with the aim of estimating the 6Li/7Li isotope ratio. The approach presented here can be used by new TOF-SIMS users on their own data for understanding the effects of detector saturation, determine the validity limits of Poisson-based correction, and take into account important considerations when treating the data for quantification purposes.

Structure and dynamics of nanoconfined water and aqueous solutions.

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Advances in the study of supercooled water.

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed.

Revisiting the glass transition temperature of water-glycerol mixtures in the bulk and confined in mesoporous silica.

In this work, we revisited the glass transition temperature (Tg) behavior of bulk and confined water-glycerol solutions as a function of the mixture composition and size of the confinement media, with the aim to shed some light on some controversies found in the literature. In the case of bulk mixtures, some discrepancies are observed due to the differences in the way of calculating Tg from the DSC experiments and differences in the protocols of cooling/reheating. However, unphysical behavior observed below the eutectic composition can be due to the crystallization of water during the cooling of the mixture. We also analyzed the effect of confinement on the glass transition of glycerol aqueous solutions, with glycerol mass fraction, wG, between 0.5 and 1.0, in silica mesoporous samples with pore diameters between 2 and 58 nm. Our results show that the the Tg dependence on pore size changes with the mixture composition. For glycerol-rich samples, Tg decreases with a decreasing pore size. This tendency changes with increasing water concentration below wG ∼ 0.6 for samples with dp between 2 and 8 nm, where two glass transition temperatures appear. We hypothesize that this effect is related to the existence of two liquid phases with different densities. The Tg composition dependence in confined glycerol-water mixtures was analyzed with the Gordon-Taylor equation modified for confined mixtures, which allowed us to calculate the Tg of the pure components as a function of the pore size. This analysis shows that for pores with dp > 20 nm, and for pure water and pure glycerol, Tg decreases with the pore size, attaining an almost constant value for samples with pore sizes between 2 and 8 nm. This Tg pore size dependence is explained considering the competition of two opposite effects: a reduction in Tg with a decreasing pore size given when the length scale of dynamics is comparable to the pore size, and an increment in Tg with a decreasing pore size as a result of increasing interactions of the confined liquid with the pore walls.

In-situ characterization of discharge products of lithium-oxygen battery using Flow Electrochemical Atomic Force Microscopy.

The increasing interest in lithium-oxygen batteries (LOB), having the highest theoretical energy densities among the advanced lithium batteries, has triggered the search for in-situ characterization techniques, including Electrochemical Atomic Force Microscopy (EC-AFM). In this work we addressed the characterization of the formation and decomposition of lithium peroxide (Li2O2) on a carbon cathode using a modified AFM technique, called Flow Electrochemical Atomic Force Microscopy (FE-AFM), where an oxygen-saturated solution of the non-aqueous lithium electrolyte is circulated through a liquid AFM cell. This novel technique does not require keeping the AFM equipment inside a glove-box, and it allows performing a number of experiments using the same substrate with different electrolytes without disassembling the cell. We study the morphology of Li2O2 on graphite carbon using lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in dimethyl sulphoxide (DMSO) as electrolyte under different operational conditions, in order to compare our results with those reported using other electrolytes and in-situ and ex-situ EC-AFM.

The Nanostructure of Water-in-Salt Electrolytes Revisited: Effect of the Anion Size.

The increasing interest in developing safe and sustainable energy storage systems has led to the rapid rise in attention to superconcentrated electrolytes, commonly called water-in-salt (WiS). Several works indicate that the transport properties of these liquid electrolytes are related to the presence of nanodomains, but a detailed characterization of such structure is missing. Here, the structural nano-heterogeneity of lithium WiS electrolytes, comprising lithium trifluoromethanesulfonate (LiTf) and bis(trifluoromethanesulfonyl)imide (LiTFSI) solutions as a function of concentration and temperature, was assessed by resorting to the analysis of small-angle neutron scattering (SANS) patterns. Variations with the concentration of a correlation peak, rather temperature-independent, in a Q range around 3.5-5 nm-1 indicate that these electrolytes are composed of nanometric water-rich channels percolating a 3D dispersing anion-rich network, with differences between Tf and TFSI anions related to their distinct volumes and interactions. Furthermore, a common trend was found for both systems' morphology above a salt volume fraction of ∼0.5. These results imply that the determining factor in the formation of the nanostructure is the salt volume fraction (related to the anion size), rather than its molality. These findings may represent a paradigm shift for designing WiS electrolytes.

Environmental chamber with controlled temperature and relative humidity for ice crystallization kinetic measurements by atomic force microscopy.

The present work describes the development of an environmental chamber (EC), with temperature and humidity control, for measuring ice growth kinetics over a substrate with an atomic force microscope (AFM). The main component of the EC is an AFM fluid glass cell. The relative humidity (RH) inside the EC is set by the flow of a controlled ratio of dry and humid nitrogen gases. The sample temperature is fixed with an AFM commercial accessory, while the temperature of the nitrogen gas inside the EC is controlled by circulating cold nitrogen vapor through a copper cooler, specially designed for this purpose. With this setup, we could study the growth rate of ice crystallization over a mica substrate by measuring the force exerted between the tip and the sample when they approach each other as a function of time. This experimental development represents a significant improvement with respect to previous experimental determinations of ice growth rates, where RH and temperature of the air above the sample were determined far away from the ice crystallization regions, in opposition to the present work.

Effect of bimodal mesoporous carbon as PtRu catalyst support for direct methanol fuel cells.

Mesoporous carbons (MCs) with different pore sizes were synthesized and evaluated as a catalyst support for fuel cells. The MCs were obtained from resorcinol-formaldehyde precursors, polymerized in the presence of polydiallyldimethylammonium chloride (cationic polyelectrolyte) as a structuring agent and commercial silica (Sipernat® or Aerosil®) as the hard template. The MC obtained with Aerosil® shows a broad pore size distribution with a maximum at 21 nm. On the other hand, the MCs with Sipernat® show a bimodal pore size distribution, with a narrow peak centered at 5 nm and a broad peak with a maximum ca. 30 nm. All MCs present a high specific surface area (800-1000 m2 g-1) and total pore volume ranging from 1.36 to 1.69 cm3 g-1. PtRu nanoparticles were deposited onto the MC support by an impregnation-reduction method with NaBH4 at 80 °C in basic media. The electrochemical characterization reveals improved electrocatalysis towards the methanol oxidation for the catalyst deposited over the carbon with the highest total pore volume. This catalyst also presented the highest CO2 conversion efficiency, ca. 80%, for the methanol oxidation as determined by differential electrochemical mass spectroscopy analysis. Moreover, the catalyst as a fuel cell anode showed the best performance, reaching a power density of 125 mW cm-2 at 90 °C with methanol as fuel and dry O2.

Effect of halogen dopants on the properties of Li2O2: is chloride special?

There is consensus on the fact that one of the main limitations of Li air batteries (LABs) is the insulating character of Li2O2 and that it becomes crucial to explore new conduction paths. Recent studies indicate that doping with chloride increases the ion conductivity of Li2O2, although to a much lesser extent than expected if chloride is assumed to be a donor dopant [Gerbig et al., Adv. Mater., 2013, 25, 3129]. Subsequently, it has been shown that the addition of lithium chloride, LiCl, to the battery electrolyte increases its discharge capacity, while this effect is not observed with other halogens [Matsuda et al., J. Phys. Chem. C, 2016, 120, 13360]. This fact was attributed to an increase in the conductivity of Cl-doped Li2O2, but still the responsible mechanism is not clear. In this work, we have performed first principle calculations to study the effect of the different halogens (F, Cl, Br, I) as substitutional defects on the electronic and transport properties of Li2O2. We have calculated the formation energies of the different defects and impurities and we analysed how they affect the activation barriers and diffusion coefficients. We have demonstrated that the chloride does not behave like a donor dopant, thus explaining the meager increase of the ionic conductivity experimentally observed, and neither does it promote polaron formation and mobility. We have also found that chloride does not present any special behaviour among the halogen series. Our results reveal that all the studied configurations associated with the halogen defects do not derive metallic states nor extra polarons that would increase considerably the electronic conductivity. This is mainly due to the ionic characteristics of the Li2O2 crystal and the capability of the oxygen dimers to adapt its valence rather than to the nature of the dopant itself.

Calorimetric study of water's two glass transitions in the presence of LiCl.

A DSC study of dilute glassy LiCl aqueous solutions in the water-dominated regime provides direct evidence of a glass-to-liquid transition in expanded high density amorphous (eHDA)-type solutions. Similarly, low density amorphous ice (LDA) exhibits a glass transition prior to crystallization to ice Ic. Both glass transition temperatures are independent of the salt concentration, whereas the magnitude of the heat capacity increase differs. By contrast to pure water, the glass transition endpoint for LDA can be accessed in LiCl aqueous solutions above 0.01 mole fraction. Furthermore, we also reveal the endpoint for HDA's glass transition, solving the question on the width of both glass transitions. This suggests that both equilibrated HDL and LDL can be accessed in dilute LiCl solutions, supporting the liquid-liquid transition scenario to understand water's anomalies.

Ionic Transport and Speciation of Lithium Salts in Glymes: Experimental and Theoretical Results for Electrolytes of Interest for Lithium-Air Batteries.

Glycol ethers, or glymes, have been recognized as good candidates as solvents for lithium-air batteries because they exhibit relatively good stability in the presence of superoxide radicals. Diglyme (bis(2-methoxy-ethyl)ether), in spite of its low donor number, has been found to promote the solution mechanism for the formation of Li2O2 during the discharge reaction, leading to large deposits, that is, high capacities. It has been suggested that lithium salt association in these types of solvents could be responsible for this behavior. Thus, the knowledge of the speciation and transport behavior of lithium salts in these types of solvents is relevant for the optimization of the lithium-air battery performance. In this work, a comprehensive study of lithium trifluoromethanesulfonate (LiTf) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-di-methoxyethane (DME) and diglyme, over a wide range of concentrations, have been performed. Consistent ion pairs and triplet ions formation constants have been obtained by resorting to well-known equations that describe the concentration dependence of the molar conductivities in highly associated electrolytes, and we found that the system LiTf/DME would be the best to promote bulky Li2O2 deposits. Unexpected differences are observed for the association constants of LiTf and, to a lesser extent, for LiTFSI, in DME and diglyme, whose dielectric constants are similar. Molecular dynamics (MD) simulations allowed us to rationalize these differences in terms of the competing interactions of the O-sites of the ethers and the SO x groups of the corresponding anions with Li+ ion. The limiting Li+ diffusivity derived from the fractional Walden rule agrees quite well with those obtained from MD simulations, when solvent viscosity is conveniently rescaled.

Glass Transition Temperature of Saccharide Aqueous Solutions Estimated with the Free Volume/Percolation Model.

The glass transition temperature of trehalose, sucrose, glucose, and fructose aqueous solutions has been predicted as a function of the water content by using the free volume/percolation model (FVPM). This model only requires the molar volume of water in the liquid and supercooled regimes, the molar volumes of the hypothetical pure liquid sugars at temperatures below their pure glass transition temperatures, and the molar volumes of the mixtures at the glass transition temperature. The model is simplified by assuming that the excess thermal expansion coefficient is negligible for saccharide-water mixtures, and this ideal FVPM becomes identical to the Gordon-Taylor model. It was found that the behavior of the water molar volume in trehalose-water mixtures at low temperatures can be obtained by assuming that the FVPM holds for this mixture. The temperature dependence of the water molar volume in the supercooled region of interest seems to be compatible with the recent hypothesis on the existence of two structure of liquid water, being the high density liquid water the state of water in the sugar solutions. The idealized FVPM describes the measured glass transition temperature of sucrose, glucose, and fructose aqueous solutions, with much better accuracy than both the Gordon-Taylor model based on an empirical kGT constant dependent on the saccharide glass transition temperature and the Couchman-Karasz model using experimental heat capacity changes of the components at the glass transition temperature. Thus, FVPM seems to be an excellent tool to predict the glass transition temperature of other aqueous saccharides and polyols solutions by resorting to volumetric information easily available.

Impact of supramolecular interactions of dextran-ß-cyclodextrin polymers on invertase activity in freeze-dried systems.

ß-Cyclodextrin (ß-CD)-grafted dextrans with spacer arms of different length were employed to evaluate the impact of supramolecular interactions on invertase activity. The modified dextrans were used as single additives or combined with trehalose in freeze-dried formulations containing invertase. Enzyme activity conservation was analyzed after freeze-drying and thermal treatment. The change of glass transition temperature (Tg ) was also evaluated and related to effective interactions. Outstanding differences on enzyme stability were mainly related to the effect of the spacer arm length on polymer-enzyme interactions, since both the degree of substitution and the molecular weight were similar for the two polymers. This change of effective interactions was also manifested in the pronounced reduction of Tg values, and were related to the chemical modification of the backbone during oxidation, and to the attachment of the ß-CD units with spacer arms of different length on dextran.

Diffusion-viscosity decoupling in supercooled glycerol aqueous solutions.

The diffusion of ferrocene methanol in supercooled glycerol-water mixtures has been measured over a wide viscosity range, which allowed analyzing the composition dependence of the Stokes-Einstein breakdown (diffusion-viscosity decoupling). The observed decoupling exhibits a common behavior for all studied compositions (glycerol mass fractions between 0.7 and 0.9), determined by the reduced temperature (T/Tg) of the mixtures. This result differs from that reported previously for the diffusion of glycerol in its aqueous solutions, where the reduced temperature for the decoupling decreases with increasing water content. We conclude that the contradictory results are only apparent, and they can be explained by the use of inconsistent extrapolated values of the viscosity of the glycerol-water mixtures in the supercooled region.

Concentration and temperature dependence of the viscosity of polyol aqueous solutions.

The concentration and temperature dependence of the viscosity of supercooled polyol (sucrose, trehalose, glucose and glycerol) aqueous solutions was analyzed with the aim of finding simple and accurate correlation equations for the description of this transport property. Three different equations were examined and compared, two empirical equations and an equation derived from the Avramov-Milchev (AM) model. If a description of the viscosity temperature dependence is intended, the AM model gives the best representation of the experimental data with only two adjustable parameters, which have a clear physical meaning. However, if we focus on both, temperature and concentration dependence, the empirical equations are found to be superior to the AM model, except for the glycerol aqueous system. The AM model includes a parameter related to the system fragility, which was obtained for all the aqueous polyol mixtures previously mentioned as a function of concentration, and also for water-trehalose-sodium tetraborate mixtures as a function of the electrolyte content. The results show that the fragility parameter increases with polyol concentration in the series glycerol

High-activity mesoporous Pt/Ru catalysts for methanol oxidation.

High activity mesoporous Pt/Ru catalysts with 2D-hexagonal structure were synthesized using a triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymer (Pluronic F127) template. The normalized mass activities for the methanol oxidation reaction (MOR) of the Pt/Ru catalysts with a regular array of pores is higher than those reported for nanoparticulated Pt/Ru catalysts. Different kinetic parameters, as Tafel slope and activation energy, were obtained for the MOR on the mesoporous catalysts. Results indicated that catalysts performance depends on pore size. Mass activities and the CO2 conversion efficiency for large pore size mesoporous catalysts (10 nm) are greater than those reported for smaller pore size mesoporous catalysts with similar composition. The effect of pore size on catalysts performance is related to the greater accessibility of methanol to the active areas inside large pores. Consequently, the overall residence time of methanol increases as compared with mesoporous catalyst with small pores.

Polyethylene glycol-based low generation dendrimers functionalized with ß-cyclodextrin as cryo- and dehydro-protectant of catalase formulations.

Polyethylene glycol (PEG)-based low generation dendrimers are analyzed as single excipient or combined with trehalose in relation to their structure and efficiency as enzyme stabilizers during freeze-thawing, freeze-drying, and thermal treatment. A novel functional dendrimer (DGo -CD) based on the known PEG's ability as cryo-protector and ß-CD as supramolecular stabilizing agent is presented. During freeze-thawing, PEG and ß-CD failed to prevent catalase denaturation, while dendrimers, and especially DGo -CD, offered the better protection to the enzyme. During freeze-drying, trehalose was the best protective additive but DGo -CD provided also an adequate catalase stability showing a synergistic behavior in comparison to the activities recovered employing PEG or ß-CD as unique additives. Although all the studied dendrimers improved the enzyme remaining activity during thermal treatment of freeze-dried formulations, the presence of amorphous trehalose was critical to enhance enzyme stability. The crystallinity of the protective matrix, either of PEG derivatives or of trehalose, negatively affected catalase stability in the freeze-dried systems. When humidified at 52% of relative humidity, the dendrimers delayed trehalose crystallization in the combined matrices, allowing extending the protection at those conditions in which normally trehalose fails. The results show how a relatively simple covalent combination of a polymer such as PEG with ß-CD could significantly affect the properties of the individual components. Also, the results provide further insights about the role played by polymer-enzyme supramolecular interactions (host-guest crosslink, hydrogen bonding, and hydrophobic interactions) on enzyme stability in dehydrated models, being the effect on the stabilization also influenced by the physical state of the matrix.

Viscosity of supercooled aqueous glycerol solutions, validity of the Stokes-Einstein relationship, and implications for cryopreservation.

The viscosity of supercooled glycerol aqueous solutions, with glycerol mass fractions between 0.70 and 0.90, have been determined to confirm that the Avramov-Milchev equation describes very well the temperature dependence of the viscosity of the binary mixtures including the supercooled regime. On the contrary, it is shown that the free volume model of viscosity, with the parameters proposed in a recent work (He, Fowler, Toner, J. Appl. Phys. 100 (2006) 074702), overestimates the viscosity of the glycerol-rich mixtures at low temperatures by several orders of magnitude. Moreover, the free volume model for the water diffusion leads to predictions of the Stokes-Einstein product, which are incompatible with the experimental findings. We conclude that the use of these free volume models, with parameters obtained by fitting experimental data far from the supercooled and glassy regions, lead to incorrect predictions of the deterioration rates of biomolecules, overestimating their life times in these cryopreservation media.

Heat capacity and glass transition in P2O5-H2O solutions: support for Mishima's conjecture on solvent water at low temperature.

The P(2)O(5)-water system has the widest range of continuously glass-forming compositions known for any glassformer + water binary system. Despite the great range of structures explored by the glasses and liquids in this system, the glass transition temperature (T(g)) itself varies in a simple monotonic fashion. However the values of T(g) reported in the literature show wide disagreement, linked to the different methods of measurement employed. In this work we use differential scanning calorimetry (DSC) to obtain both T(g) itself and the jump in heat capacity that occurs as the metastable equilibrium of the supercooled liquid relieves the non-ergodic glassy state. Our study covers the molar ratio range of H(2)O/P(2)O(5) from 1.5 to 14 (corresponding to the mass fraction of P(2)O(5) between 0.36 and 0.84), which includes the compositions corresponding to pyrophosphoric acid (H(4)P(2)O(7)) and orthophosphoric acid (H(3)PO(4)). The theoretical model of Couchman and Karasz predicts very well the glass transition temperatures of the P(2)O(5)-H(2)O system over the whole composition range if the relatively large heat capacity change associated with water in aqueous solutions at the glass transition temperature is adopted, instead of the vanishingly small value observed for vapor deposited or hyperquenched pure water. Therefore, solvent water in this ambient pressure P(2)O(5)-H(2)O system behaves like a different liquid, more closely resembling a high-density liquid (HDL) polyamorph, as suggested by Mishima for electrolytes at high pressures.