Diffusivity Of H2O Research Articles

Modeling the influence of water on the performance of non-aqueous Li-O2 batteries

Unresolved controversy regarding the influence of water (H2O) on the action mechanisms and performance of lithium-oxygen (Li-O2) batteries hinders the ultimate goal of operating the Li-O2 batteries in ambient air instead of pure O2. Herein, we explored the influence of H2O on the discharge/charge behavior and cycling performance of Li-O2 batteries by establishing a mathematical model. In the presence of H2O, the generation reaction of lithium hydroxide (LiOH) lowers the volume fraction of lithium peroxide (Li2O2) in the cathode near the O2 side. Although the presence of H2O with a higher concentration enhances the initial discharge capacity of Li-O2 batteries, the discharge capacity gradually decreases with cycles due to the continuous deposition of undecomposed LiOH in the cathode. The cycle termination of the Li-O2 batteries at lower H2O concentrations is primarily attributed to the porous cathode blocking caused by LiOH accumulation, but the effect of H2O crossover on Li anode corrosion becomes more crucial as H2O concentration increases. The initial discharge capacity increment of Li-O2 batteries is limited by increasing H2O diffusivity, while the battery cycle life decreases obviously with the increase of H2O diffusivity. For high H2O diffusivities, the cycle termination of the Li-O2 batteries is no longer controlled by LiOH accumulation in the cathode but by Li anode corrosion due to H2O crossover. The proposed model can correctly predict the performance of the Li-O2 batteries incorporating H2O and is conducive to designing protection strategies to enhance the cycle life of Li-O2 batteries working in ambient air.

The role of oxidation level in mass-transport properties and dehumidification performance of graphene oxide membranes

Here we report on gas and vapor transport properties of ultra-thin graphene oxide (GO) membranes, with various C:O ratios. Graphene oxide nanosheets with an average lateral size of 800 nm and C:O ratio ranging from 2.11 to 1.81 have been obtained using improved Hummers’ method by variation of graphite:KMnO4 ratio. Thin-film selective layers based on the obtained graphene oxide have been spin-coated onto porous substrates. To extend the C:O range to 2.60, thermal reduction of GO membranes was applied. A decrease in C:O ratio leads to significant water vapor permeance growth to over 60 m3(STP)·m−2·bar−1·h−1 while the permeance towards permanent gases reduces slightly. According to the permeation and sorption measurements, a decisive role of H2O diffusivity has been established, while the water sorption capacity of the graphene oxide stays nearly independent of C:O ratio in GO. The result is supported by semi-empirical modeling which reveals diminution of H2O jump activation barriers with both increasing GO interlayer spacing and its oxidation degree. The height of the activation barriers was found to vary up to an order of magnitude within the entire range of relative humidity (0–100% RH), lowering significantly for strongly oxidized GO. Our results evidence the necessity of attaining maximum GO oxidation degree for improving water transport in GO, especially at low partial pressures.

Effect of alkalis on water diffusion in silicate melts based on new experimental data for a shoshonitic melt

Water diffusion is an important mechanism affecting magmatic water content, vapor bubble growth, and the dynamics of volcanic eruptions. Water diffusivity in silicate melts is dependent on temperature, pressure, H2O content and speciation, and melt composition. While a general model has recently been developed for water diffusivity in calc-alkaline (subalkaline) melts as a function of silica content, the effect of alkalis on water diffusion is less clear due to the lack of experimental data for alkali-rich melts and a systematic evaluation. Here, water diffusion in shoshonitic melt with up to 1.6 wt% H2O was investigated at 1654–1926 K and 1 GPa in a piston cylinder apparatus using both hydration and diffusion couple methods. Diffusion profiles measured by FTIR microspectroscopy are in accordance with a modified speciation-based diffusion model which takes into account different mobility of hydroxyl (OH) and molecular H2O (H2Om). The diffusivity ratio DOH/DH2Om in shoshonitic melt increases with increasing temperature and falls into the range of 0.24–0.55, greater than the values for all of the melts previously investigated. The remarkable contribution of OH to water diffusion in shoshonitic melt is attributed to a substantial presence of free OH species (OH bonding with network-modifying cations but here especially with K or Na). The total H2O diffusivity in shoshonitic melt can be expressed as:DH2Ot=e-12.11-16765T1+Ce-11.14+17381-262.6CT,where D is in m2/s, T is temperature in K, and C is H2Ot content in wt% (DH2Ot reduced to DOH at C = 0). The above equation replicates the experimental data within 0.1 natural log unit. The total H2O diffusivities at 1 wt% H2Ot in three alkali-rich melts, including shoshonite, trachyte and phonolite, appear to fall onto a single Arrhenian trend line over a broad temperature range of 1100–1900 K. Compared to the general water diffusivity model for calc-alkaline melts, water diffusivities in alkali-rich silicate melts are generally higher and roughly twofold at the same P-T-H2Ot-XSi conditions, with XSi being the mole fraction of Si among all cations. Excess alkalis decrease degree of melt polymerization and promote formation of mobile K-OH and Na-OH species, thereby having a positive influence on water diffusion and vapor bubble growth in magma.

An experimentally validated numerical model for bubble growth in magma

Volcanic eruptions are driven by the growth of gas bubbles in magma. The timing and rate of bubble growth are important because they determine whether enough gas pressure can develop to fragment the melt. Bubbles grow in response to decompression and diffusive transport of dissolved volatiles (predominantly H2O) that exsolve into the bubbles. Growth is resisted by the viscosity of the melt. Both melt viscosity and H2O diffusivity have non-linear dependence on the concentration of H2O dissolved in the melt, which necessitates a numerical approach to modelling bubble growth. Several bubble growth models have previously been published and applied, but none of them has been validated against continuous, in situ experimental data or provided as a user-friendly tool. Here we present a numerical bubble growth model, implemented in MATLAB, which allows for arbitrary temperature and pressure pathways, and accounts for the impact of spatial variations in dissolved H2O concentration on viscosity and diffusivity. We validate the model against two sets of experimental data: (1) New continuous data for gas-volume fraction as a function of time, collected using optical dilatometry of vesiculating hydrous obsidian samples which were heated from 930 °C to 1000 °C at atmospheric pressure. This dataset captures isobaric, isothermal bubble growth under strongly disequilibrium conditions. (2) Discrete data from published decompression experiments at 825 °C and pressures from 200 MPa to ~5 MPa with decompression rates from 0.1 MPa s−1 to 10 MPa s−1. These experiments represent isothermal, decompression-driven bubble growth spanning equilibrium to strongly disequilibrium conditions. The numerical model closely reproduces the experimental data across all conditions, providing validation against contrasting bubble growth scenarios. The validated model has a wide range of potential volcanological applications, including forward modelling of bubble growth phenomena, and inverse modelling to reconstruct pressure–temperature–time pathways from textures and volatile contents of eruptive products.

Effect of an Electric Field on Surface Properties of Hydrophobic Particles during a Flotation Process in Salt Water.

Under air flotation conditions, the effect of an electric field (EF) to the interfacial properties of hydrophobic particles in salt solution was investigated by using molecular dynamics simulation. The varieties of thickness of the hydrophobic layer on the surface of the hydrophobic particles, radial distribution function of Na+, diffusivity of CO2 and H2O, residence time of H2O in the solvent shell of Na+, and the distribution of CO2 near the graphene caused by the applied EF in different NaCl systems were calculated, and all those results were compared with those where there is no EF applied. The results reveal that the thickness of the hydrophobic layer near the hydrophobic particles is decreased under an EF. The coordination number and residence time of H2O in the solvent shell of Na+ are also decreased, which indicates that the restraint of Na+ on surrounding H2O is weakened, and the H2O molecules are more mobile. Under an external EF, the diffusion coefficient of CO2 and H2O is increased, and the quantity of CO2 near the surface of graphene is decreased according to the numerical results of the radial distribution of graphene-CO2. All the results indicate that the EF will decrease the thickness of the hydrophobic layer near hydrophobic particles, which can therefore weaken the adsorption between the hydrophobic particles and the gas molecules. The simulation results in the present study can provide theoretical support for brine mineral flotation under an external EF.

Solubility, diffusivity, and O isotope systematics of H2O in rhyolitic glass in hydrothermal temperature experiments

In many volcanic settings, eruptive deposits experience prolonged cooling in the presence of water, such as in subglacial or submarine eruptions. Under these conditions, volcanic glass will rehydrate and record the isotopic composition of the water. This isotope exchange is moderated by H2O solubility and diffusivity in the glass. In this study, we report results from glass hydration experiments conducted at 175–375 °C to constrain H2O solubility and diffusivity under these hydrothermal conditions over timescales lasting hours to months. We use anhydrous high and low silica rhyolites as well as hydrous high silica rhyolite (perlites) with isotopically labeled water as starting materials. Measurements of bulk H2O by TC/EA of experimental glasses provide minimum H2O solubility estimates. High-Si rhyolitic glass has an H2O solubility between 2.75 wt.% (175 °C, 0.89 MPa) and 4.1 wt.% (375 °C, 21 MPa) while low-Si rhyolite H2O solubility is uniformly ∼0.5 wt.% higher at each temperature. We find a roughly linear relationship of solubility vs 1/T that is ∼1–2 wt.% greater than extrapolations from magmatic temperature solubility relationships. Furthermore, three independent methods of diffusion modeling – one in situ and two mass balance approaches – all produce H2O diffusivity (DH2O) values that up to 5.5 times greater than predicted by extrapolation of the 1/T – DH2O relationships above 400 °C to the experimental P-T-XH2O conditions. In situ H2O profiles in rhyolite particles measured by NanoSIMS have the characteristic “snowplow ” functional form that arises from the H2O concentration dependence of DH2O. We cannot detect diffusively driven kinetic fractionation of D relative to H with the NanoSIMS data. Diffusion and mass balance calculations that fit TC/EA time series of bulk H2O in particles of a single size distribution, and calculations that reconcile two sets of different sized particles at a single experimental duration, return similar DH2O constraints. We also present time series δ18O of bulk glass (δ18Obulk) and the δ18O of water-in-glass (δ18Owig) measurements, which indicate that molecular water (H2Om) dissolved in the glass is the primary driver of subsequent oxygen isotope exchange between glass and an external fluid. Local equilibrium between the δ18Owig and the δ18Obulk is rapidly established and ranges from approximately −14‰ at 175 °C to −10‰ at 375 °C. Both the δ18Obulk and δ18Owig then increase with time moving slowly towards estimated bulk glass δ18O equilibrium with the external experimental water. Oxygen isotope exchange between glass and a fluid is therefore strongly linked to – and is limited by – H2O diffusivity, which is slower at lower P-T conditions and lower H2O solubilities as H2Om diffusion is the main exchange mechanism.

Second-law thermodynamic analysis on premixed syngas flames

Exergy destructions in laminar premixed syngas flames, with varied fuel compositions and changed pressures, were numerically studied. Chemical reactions are found to be the dominant source for exergy destruction, followed by heat conduction and mass diffusion. As a result of the trade-off between the increased exergy destruction rates and narrowed flame thickness, the total exergy destruction increases by about 1% with hydrogen enrichment, but is nearly unchanged with pressure elevation. Specifically, with hydrogen enrichment, the exergy destruction from heat conduction decreases due to the narrowed flame thickness, whereas those from mass diffusion and chemical reactions increase because of higher diffusivity of H2, H and H2O and higher reactivity of H2 than CO, respectively. In contrast, with pressure elevation, the higher temperature gradient and mole fraction gradients result in increased exergy destruction from heat conduction, but the exergy destruction from chemical reactions decrease due to the narrowed flame thickness.

Second-law thermodynamic analysis on premixed syngas flames

Exergy destructions in laminar premixed syngas flames, with varied fuel compositions and changed pressures, were numerically studied. Chemical reactions are found to be the dominant source for exergy destruction, followed by heat conduction and mass diffusion. As a result of the trade-off between the increased exergy destruction rates and narrowed flame thickness, the total exergy destruction increases by about 1% with hydrogen enrichment, but is nearly unchanged with pressure elevation. Specifically, with hydrogen enrichment, the exergy destruction from heat conduction decreases due to the narrowed flame thickness, whereas those from mass diffusion and chemical reactions increase because of higher diffusivity of H2, H and H2O and higher reactivity of H2 than CO, respectively. In contrast, with pressure elevation, the higher temperature gradient and mole fraction gradients result in increased exergy destruction from heat conduction, but the exergy destruction from chemical reactions decrease due to the narrowed flame thickness.

Chitosan/graphene oxide mixed matrix membrane with enhanced water permeability for high-salinity water desalination by pervaporation

Pervaporation is a promising technology for water desalination. In this work, graphene oxide (GO) was incorporated into chitosan (CS) to fabricate mixed matrix membranes (MMMs) to explore the enhancement of water permeability for high-salinity water desalination and to understand the transport mechanism in the MMMs. The effect of GO content on the morphology, wettability and desalination performance of the membranes as well as the thermodynamic and kinetic behavior of water and salt permeation in the membranes were investigated. The MMMs have rougher and more hydrophilic surface, dense structure and improved mechanical stability owing to the good chemical compatibility of GO with CS matrix. The apparent activation energy results indicate the advantage of the MMMs in desalinating high-salinity water. The enhanced permeate flux of 30.0 kg/m2·h was obtained in desalinating 5 wt% aqueous NaCl solution at 81 °C with 1 wt% GO content and 99.99% of salt rejection was achieved. The solubility and diffusivity of H2O and NaCl in the MMMs demonstrate a trade-off effect on the water permeability and water/salt selectivity with increasing of GO content. The NaCl diffusion coefficient in the membranes was lower than that of H2O by two orders of magnitude, contributing to the high selectivity of the membrane.

The impact of dissolved fluorine on bubble nucleation in hydrous rhyolite melts

Surface tension of hydrous rhyolitic melt is high enough that large degrees of supersaturation are needed to homogeneously nucleate H2O bubbles during eruptive magma ascent. This study examines whether dissolved fluorine lowers surface tension of hydrous rhyolite, and thus lowers the supersaturation required for bubble nucleation. Fluorine was targeted because it, like H2O, changes melt properties and is highly soluble, unlike all other common magmatic volatiles. Rhyolite melts were saturated at Ps = 245 MPa with H2O fluid that contained F, generating rhyolite with 6.7 ± 0.4 wt.% H2O and 1.1–1.3 wt.% F. When these melts were decompressed rapidly to Pf = 149–202 MPa and quenched after 60 s, bubbles nucleated at supersaturations of ΔP = Ps − Pf ≥52 MPa, and reached bubble number densities of NB = 1012–13 m−3 at ΔP = 78–101 MPa. In comparison, rhyolite saturated with 6.34 ± 0.09 wt.% H2O, but only 0.25 wt.% F, did not nucleate bubbles until ΔP ≥ 100–116 MPa, and even then, at significantly lower NB (<1010 m−3). Numerical modeling of bubble nucleation and growth was used to estimate the values of surface tension required to generate the observed values of NB. Slight differences in melt compositions (i.e., alkalinity and H2O content), H2O diffusivity, or melt viscosity cannot explain the observed differences in NB. Instead, surface tension of F–rich rhyolite must be lower by approximately 4% than that of F–poor rhyolite. This difference in surface tension is significant and, for example, exceeds that found between hydrous basaltic andesite and hydrous rhyolite. These results suggest that is likely that surface tension for F–rich magmas, such as topaz rhyolite, is significantly lower than for F–poor magmas.

A general model of water diffusivity in calc-alkaline silicate melts and glasses

The calc-alkaline magma series (basalt, andesite, dacite, and rhyolite) commonly occurring at subduction zones are typically hydrous. Water diffusivity in silicate melt is a crucial parameter controlling the transport and evolution of water throughout magma differentiation. Based on experimental data accumulated over the past three decades, we develop a unified, mechanistic and quantitative model of water diffusion and diffusivity in calc-alkaline silicate melts and glasses. This model involves the following features: (1) the coupling of water diffusion and water speciation; (2) an exponential increase of molecular H2O (H2Om) diffusivity with the mole fraction of total water (X), DH2Om=D0exp(aX), and OH diffusivity (DOH) independent of total water content; (3) the influence of temperature and melt/glass composition on water speciation, the parameter a, and the ratio DOH/D0; and (4) the influence of temperature, composition and pressure on D0. Using the mole fraction of Si among all cations (YSi) to index melt/glass composition, we find that total water (H2Ot) diffusivity at 1wt% H2Ot decreases with increasing YSi (i.e., from basaltic to rhyolitic composition) at high temperature (T>1300K) but this trend is reversed in the low temperature range (T<900K). A vast majority of diffusivity data, including those used for model development and those excluded, can be reproduced by our model within a factor of 2, only slightly greater than experimental uncertainty. This new model allows to trace the evolution of water diffusivity along the liquid line of descent, and is particularly useful when the composition of interest is intermediate between the specific compositions previously investigated.

Details of the gabbro‐to‐eclogite transition determined from microtextures and calculated chemical potential relationships

AbstractPermian‐aged metagabbros from the eclogite type‐locality in the eastern European Alps were partially to completely transformed to eclogite during Eoalpine intracontinental subduction. Microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro‐to‐eclogite transition, allowing the details of the eclogitization process to be investigated. Original, anorthite‐rich igneous plagioclase is pervasively replaced by fine‐grained intergrowths of clinozoisite, kyanite and Na‐rich plagioclase. Where plagioclase was in contact with igneous orthopyroxene, 100–200 μm thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains. The rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet. Igneous clinopyroxene does not show any breakdown textures; however, jadeite content gradually increases towards the rims. In addition, exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds. Given that the igneous plagioclase is pervasively replaced by clinozoisite, kyanite and Na‐rich plagioclase, whereas kyanite–diopside symplectites are confined to narrow rim zones, we suggest that the development of these textures was controlled by the (im)mobility of different elements on different length scales. The presence of hydrous minerals in the core of anhydrous plagioclase indicates that H2O diffusivity occurred on a mm‐scale. By contrast, the size of the anhydrous diopside–kyanite and diopside–garnet symplectites indicate that Fe–Mg–Ca–Na diffusivity was limited to a 10s of μm scale. Chemical potential relations calculated in the idealized NCASH chemical system show that the clinozoisite–kyanite–albite intergrowths formed due to an increase of μH2O to plagioclase, whereas all other elements remained effectively immobile on the scale of this texture. Fluid conditions indicated by this texture span from virtually dry conditions (0.15) to H2O‐saturation, and therefore does not imply that the rocks were ever fluid‐saturated. Calculations in the CMAS and NCFMAS systems show that the gabbro‐to‐eclogite transition is characterized by the growth of garnet, diopsidic clinopyroxene and kyanite due to diffusion of Ca (+ Na) and Mg (+ Fe) along a μCaO (+ Na2O)–μMgO (+ FeO) chemical potential gradient developed between orthopyroxene and plagioclase compositional domains. The anhydrous nature of the textures indicate that the gabbro‐to‐eclogite transition is not driven by hydration; however, increased μH2O acts as a catalyst that increases diffusivity of all elements and rates of dissolution–precipitation, allowing the overstepped metamorphic reactions to occur. Our results show that crustal eclogite formation requires low H2O content, confirming that true eclogites are dry rocks.

Generalized Langevin Methods for Calculating Transmembrane Diffusivity

The membrane permeability coefficient of a solute can be estimated using the solubility-diffusion model. This model requires the diffusivity profile (D(z)) of the solute as it moves along the transmembrane axis, z. The generalized Langevin equation provides one strategy for calculating position-dependent diffusivity from straightforward molecular dynamics simulations where the solute is restrained to a series of positions on the z-coordinate by a harmonic potential. The diffusivity of the solute is calculated from its correlation functions, which are related to the friction experienced by the solute. Roux and Hummer have derived expressions for the diffusion coefficient from the velocity autocorrelation function (VACF) and position autocorrelation function (PACF), respectively. In this work, these methods are validated by calculating the diffusivity of H2O and O2 in homogeneous liquids. These methods are then used to calculate transmembrane diffusivity profiles. The VACF method is less sensitive to thermostat forces and has incrementally lower errors but is more sensitive to the spring constant of the harmonic restraint. For the permeation of a solute through a lipid bilayer, the diffusion coefficients calculated using these methods provided significantly different results. Long-lived correlations of the restrained solute due to inhomogeneities in the bilayer can result in spuriously low diffusivity when using the PACF method. The method based on the VACF does not have this issue and predicts higher rates of diffusion inside the bilayer.

Viscosity controls humidity dependence of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; uptake to citric acid aerosol

Abstract. The heterogeneous loss of dinitrogen pentoxide (N2O5) to aerosol particles has a significant impact on the night-time nitrogen oxide cycle and therefore the oxidative capacity in the troposphere. Using a 13N short-lived radioactive tracer method, we studied the uptake kinetics of N2O5 on citric acid aerosol particles as a function of relative humidity (RH). The results show that citric acid exhibits lower reactivity than similar dicarboxylic and polycarboxylic acids, with uptake coefficients between ∼ 3 × 10-4–∼ 3 × 10-3 depending on humidity (17–70 % RH). At RH above 50 %, the magnitude and the humidity dependence can be best explained by the viscosity of citric acid as compared to aqueous solutions of simpler organic and inorganic solutes and the variation of viscosity with RH and, hence, diffusivity in the organic matrix. Since the diffusion rates of N2O5 in highly concentrated citric acid solutions are not well established, we present four different parameterizations of N2O5 diffusivity based on the available literature data or estimates for viscosity and diffusivity of H2O. Above 50 % RH, uptake is consistent with the reacto-diffusive kinetic regime whereas below 50 % RH, the uptake coefficient is higher than expected from hydrolysis of N2O5 within the bulk of the particles, and the uptake kinetics is most likely limited by loss on the surface only. This study demonstrates the impact of viscosity in highly oxidized and highly functionalized secondary organic aerosol material on the heterogeneous chemistry of N2O5 and may explain some of the unexpectedly low loss rates to aerosol derived from field studies.

CO2/H2 separation using a highly permeable polyurethane membrane: Molecular dynamics simulation

In this study, Molecular Dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations were conducted to investigate the diffusivity, solubility, and permeability of CO2, CO, H2, and H2O in a polyurethane membrane at three different temperatures. The characterization of the simulated structures was carried out using XRD, FFV, Tg and density calculation, and cavity size distribution. The obtained results were within the expectations reported data in the literature based on the experimental approach, indicating the authenticity of approached in this work. The results showed that the highest diffusivity and permeability coefficients were observed for H2; while the highest values of solubility coefficient were found for H2O and CO2 gases. The increase of operating temperature from 298 K to 318 K has a positive effect on the permeation of all gases and a corresponding negative effect on the selectivity of the gas pair CO2/H2. Also, the results vividly showed that CO2 and H2O gases have a profound affinity with hard phase of polyurethane, while H2 and CO were conversely adsorbed by soft one. Moreover, the enhancement of permeability and permselectivity of CO2/H2 pair confirmed using Robeson Upper-Bond graph showed its good capacity for CO2/H2 separation application.

Diffusive fractionation of H2O and CO2 during magma degassing

H2O–CO2 systematics of melt inclusions and obsidian pyroclasts has been used widely to explore the behaviour of fluids in magmatic systems under the assumption that a fluid–melt equilibrium is attained. However, fluid exsolution is a process involving the diffusive transfer of volatiles, and kinetic effects may control the composition of volatiles. In this study, single-step decompression experiments were carried out on H2O–CO2-bearing basaltic and rhyolitic melts to investigate the evolution of H2O–CO2 compositions during vesiculation. Microanalysis of the volatiles using Raman and infrared spectroscopy showed that CO2 content decreased towards bubbles, while H2O content was almost constant throughout the quenched glass samples. This resulted in higher CO2/H2O ratios than estimated from the equilibrium degassing, which is interpreted as a kinetic effect: the diffusivity of H2O is higher than that of CO2. A simple model for the diffusive degassing of ascending magma was developed to investigate the disequilibrium evolution of its H2O–CO2 content. CO2/H2O ratios were strongly dependent on the magma ascent velocity. The model was applied to melt inclusions with high CO2/H2O ratios from Etna and Stromboli volcanoes, and it was shown that the high ratios could not be explained by diffusive fractionation under a typical magma ascent velocity. Rather, these ratios are affected by other processes, such as CO2 fluxing.

Atomistic molecular dynamics simulations of H2O diffusivity in liquid and supercritical CO2

Molecular dynamics simulations were employed for the calculation of diffusion coefficients of pure CO2 and of H2O in CO2 over a wide range of temperatures (298.15 K < T < 523.15 K) and pressures (5.0 MPa < P < 100.0 MPa), that are of interest to CO2 capture-and-sequestration processes. Various combinations of existing fixed-point-charge force-fields for H2O (TIP4P/2005 and Exponential-6) and CO2 (elementary physical model 2 [EPM2], transferable potentials for phase equilibria [TraPPE], and Exponential-6) were tested. All force-field combinations qualitatively reproduce the trends of the experimental data for infinitely diluted H2O in CO2; however, TIP4P/2005–EPM2, TIP4P/2005–TraPPE and Exponential-6–Exponential-6 were found to be the most consistent. Additionally, for H2O compositions ranging from infinite dilution to , the Maxwell–Stefan diffusion coefficient is shown to have a weak non-linear composition dependence.

Hydroxyl and molecular H2O diffusivity in a haploandesitic melt

H2O diffusion in a haploandesitic melt (a high-silica and Fe-free andesitic melt, NBO/T=0.173) has been investigated at 1GPa in a piston-cylinder apparatus. We adopted a double diffusion couple technique, in which one couple was composed of a nominally anhydrous glass with 0.01wt.% H2O and a hydrous glass with 5.7wt.% H2O, and the other contained the same nominally anhydrous glass and a hydrous glass with 3.3wt.% H2O. Both couples were annealed in a single experimental run and hence experienced exactly the same P–T history, which is crucial for constraining the dependence of H2O diffusivity on water content. H2O concentration profiles were measured by both Fourier transform infrared (FTIR) microspectroscopy and confocal Raman microspectroscopy. Nearly identical profiles were obtained from Raman and FTIR methods for profile length >1mm (produced at 1619–1842K). By contrast, for profile lengths <100μm (produced at 668–768K), FTIR profiles show marked convolution effects compared to Raman profiles. A comparison between the short FTIR and Raman profiles indicates that the real spatial resolution (FWHM) of FTIR analyses is about 28μm for a 7μm wide aperture on ∼200μm thick glasses.While the short profiles are not reliable for quantitative modeling, the long diffusion profiles at superliquidus temperatures can be fit reasonably well by a diffusivity model previously developed for felsic melts, in which molecular H2O (H2Om) is the only diffusive species and its diffusivity (DH2Om) increases exponentially with the content of total water (H2Ot). However, there is noticeable misfit of the data at low H2Ot concentrations, suggesting that OH diffusivity (DOH) cannot be neglected in this andesitic melt at high temperatures and low water contents. We hence develop a new fitting procedure that simultaneously fits both diffusion profiles from a single experimental run and accounts for the roles of both OH and H2Om diffusion. With this procedure, DOH/DH2Om is constrained to be 0.1–0.2 at 1619–1842K as H2Ot concentration approaches zero. The obtained OH diffusivity is similar to fluorine diffusivity but is much higher than Eyring diffusivity.

Erratum to: Thermal Diffusivity of H2O Near the Critical Point

The following is a correction to the paper titled “Thermal Diffusivity of H2O near the Critical Point” by J. V. Sengers, R. A. Perkins, M. L. Huber, and B. Le Neindre that appeared in Int. J. Thermophys. 30: 1453 (2009). The information for RD in Table 1 yielding RD = 1.05 ± 0.05 was obtained by using the IAPWS Formulation 1985 for the Thermal Conductivity [49] for estimating the background thermal conductivity of H2O and not the estimate proposed by Le Neindre et al. [58] as was stated in the paper. If we do use the estimate proposed by Le Neindre et al. [58] for the background thermal conductivity, we find RD = 1.01± 0.05.

H2-H2O Effective Binary Diffusivity Measurement in Solid Oxide Fuel Cell Anode Using an Electrochemical Cell

The effective binary diffusivity of H2 and H2O in a Ni and yittria-stabilized zirconia (YSZ) anode of the solid oxide fuel cells (SOFCs) was measured between 650 and 800oC using an electrochemical cell consisting of an oxygen pump, an oxygen sensor, and a porous SOFC anode pellet. The effective binary diffusivity was obtained from the relationship between the current density across the oxygen pump, and the H2 partial pressure gradient across the anode sample measured using the oxygen sensor. The anode limiting current density and concentration polarization were estimated using the experimental results.