Water Transport Resistance Research Articles

Influence of elevated temperatures on the mechanical properties and microstructure of self consolidating lightweight aggregate concrete

Self consolidating lightweight concrete (SCLWC) mixtures were prepared by using two different lightweight coarse aggregates and by replacing normal weight crushed coarse limestone aggregate at a constant water/powder ratio. One of the SCLWC mixtures was also prepared at different water/powder ratios. All the mixtures were exposed to 300, 600 and 900°C, respectively. Lightweight aggregate type and water/powder ratio affect water transport characteristics and resistance of the mixtures to elevated temperatures. The microstructural investigation findings are consistent with thermal strain and residual mechanical properties of the mixtures after exposure to high temperatures.

A Steady Permeation Method for Measuring Water Transport Properties of Fuel Cell Membranes

This paper describes a new steady permeation method for measuring water transport properties of fuel cell membranes. It is distinguished from most previous experiments of this type by employing a separate procedure for quantifying the extraneous sources of water transport resistance in the measurement cell that do not originate in the membrane (the so-called device resistance). This result is then used to determine the membrane water permeability in cells built with Nafion membranes in three thicknesses (25, 50, and 125 microns) operating at three different temperatures (70, 90 and 110°C) over a range of relative humidities from 30–90%. The permeability data lie along three parallel straight lines (one for each temperature) on semi-log plots vs. average membrane relative humidity. There is no discernible dependence of permeability on membrane thickness, suggesting that any localized transport resistance that may exist at the membrane surface was too small to be detected by this method. The permeability data can be fit to a simple 3-constant empirical expression for the membrane permeability as a function of temperature and relative humidity. The largest difference between the present and previous measurements occurs at higher relative humidities, where the device resistance is large compared with the membrane resistance.

Effect of Interfacial Water Transport Resistance on Coupled Proton and Water Transport Across Nafion

Dynamic and steady-state water flux, current density, and resistance across a Nafion 115 membrane-electrode-assembly (MEA) were measured as functions of temperature, water activity, and applied potential. After step changes in applied potential, the current, MEA resistance, and water flux evolved to new values over 3000-5000 s, indicating a slow redistribution of water in the membrane. Steady-state current density initially increased linearly with increasing potential and then saturated at higher applied potentials. Water flux increases in the direction of current flow resulting from electro-osmotic drag. The coupled transport of water and protons was modeled with an explicit accounting for electro-osmotic drag, water diffusion, and interfacial water transport resistance across the vapor/membrane interface. The model shows that water is dragged inside the membrane by the proton current, but the net water flux into and out of the membrane is controlled by interfacial water transport at the membrane/vapor interface. The coupling of electro-osmotic drag and interfacial water transport redistributes the water in the membrane. Because water entering the membrane is limited by interfacial transport, an increase in current depletes water from the anode side of the membrane, increasing the membrane resistance there, which in turn limits the current. This feedback loop between current density and membrane resistance determines the stable steady-state operation at a fixed applied potential that results in current saturation. We show that interfacial water transport resistance substantially reduces the impact of electro-osmotic drag on polymer electrolyte membrane fuel cell operation.

The function of hydrophobic cathodic backing layers for high energy passive direct methanol fuel cell

Homemade wet-proofing carbon papers with back-flow effect were applied as backing layers in the cathode of passive air-breathing direct methanol fuel cell (DMFC) fed by pure methanol. With the increase of polytetrafluoroethylene (PTFE) content, the carbon papers exhibited different water transport resistance and generated different back-flow effects. Moreover, PTFE-treated carbon papers were observed by scanning electronic microscope (SEM) to investigate the function of the cross-linked microstructure. Maximum energy density (438 Wh L −1) of the improved pure methanol DMFC was obtained by using carbon paper with 40 wt.% PTFE content as the cathodic backing layer. This value was 6 times larger than that of the conventional DMFC fed by 2 M methanol solution.

Effects of cathode gas diffusion layer design on polymer electrolyte membrane fuel cell water management and performance

The effects of a microporous layer (MPL) on performance and water management of polymer electrolyte fuel cells are investigated. The presence of an MPL on the cathode side is found to slightly improve performance, although the voltage gain is less significant than that obtained by wetter reactants. The effect of the MPL on water management depends on the cathode inlet-gas humidity. Differences in water crossover rate are insignificant for wet cathode feed (RH = 75%), while they are significant for dry feed (RH = 25%). A model based on transport resistance of the MPL is proposed to explain the experimental trends observed. Modeling results suggest that the presence of the MPL on the cathode side causes a reduction of the water flux from the cathode catalyst layer to the flow channels, effectively promoting water back diffusion through the membrane. Higher cathode humidity reduces the driving force for water transport from the electrode to the gas channels, also reducing the importance of the water transport resistance due to the presence of the MPL.

Diffusion and Interfacial Transport of Water in Nafion

Water absorption, membrane swelling, and self-diffusivity of water in 1100 equivalent weight Nafion were measured as functions of temperature and water activity. Free volume per water at 80 °C, determined from water uptake and volume expansion data, decreases with water content in the membrane from 12 cm(3)/mol at λ = 0.5 H(2)O/SO(3) to 1.5 cm(3)/mol at λ = 4. The change in free volume with water content displays a transition at λ = 4. Limiting water self-diffusivity in Nafion was determined by pulsed gradient spin echo NMR at long delay times. The limiting self-diffusivity increases exponentially with water activity; the rate of increase of diffusivity with water content shows a transition at λ = 4. The tortuosity of the hydrophilic domains in Nafion decreased from 20 at low membrane water activity to 3 at λ = 4. It suggested a change in the connectivity of the hydrophilic domains absorbed water occurs at λ ∼ 4. The diffusivity results were employed to separate the contributions of diffusional and interfacial resistance for water transport across Nafion membranes, which enabled the determination of the interfacial mass transport coefficients. A diffusion model was developed which incorporated activity-dependent diffusivity, volume expansion, and the interfacial resistance, and was used to resolve the water activity profiles in the membrane.

Interfacial Water Transport Effects in Proton-Exchange Membranes

It is well known that the proton-exchange membrane is perhaps the most critical component of a polymer-electrolyte fuel cell. Typical membranes, such as Nafion®, require hydration to conduct efficiently and are instrumental in cell water management. Recently, evidence has been shown that these membranes might have different interfacial morphology and transport properties than in bulk. In this paper, experimental data combined with theoretical simulations that explore the existence and impact of interfacial resistance on water transport for Nafion®21x membranes will be presented. A mass-transfer coefficient for the interfacial resistance is calculated from experimental data using different permeation cells. This coefficient is shown to depend exponentially on relative humidity or water activity. The interfacial resistance does not seem to exist for liquid/membrane or membrane/membrane interfaces. The effect of the interfacial resistance is to flatten the water content profiles within the membrane during operation. Under typical operating conditions, the resistance is on par with the water transport resistance of the bulk membrane. Thus, the interfacial resistance can be dominant especially in thin, dry membranes and can affect overall fuel cell performance.

Analysis of in situ water transport in Nafion ® by confocal micro-Raman spectroscopy

A proton exchange membrane fuel cell (PEMFC) is a promising alternative source of clean power for automotive applications, but its acceptance in such applications depends on reducing its costs and increasing its power density to achieve greater compactness. To meet these requirements, further improvements in cell performance are required. In particular, when the fuel cell is operating at high current density, the transport of water through the membrane has considerable impacts on the performance because of the large concentration gradient of water between the cathode and anode. Through-plane water permeation across the membrane is therefore a fundamental process in operational PEMFCs. Recently, resistance to water transport at the membrane–gas interface has been reported, and this is affected by the temperature and relative humidity. We investigated the distribution of water inside a proton exchange membrane during a water permeation test by using confocal micro-Raman spectroscopy with a fine spatial resolution (2–3 μm). In the presence of a water flux, the local water content is not necessarily in equilibrium with the water activity in the gas phase. Interfacial water-transport resistance due to the presence of a non-equilibrium membrane structure at the interface cannot be neglected.

Polymer Electrolyte Fuel Cell Transport Mechanisms: A Universal Approach to Multilayer Two-Phase Modeling Through the General Transport Equation

A mathematical multilayer, multispecies two-phase model for polymer electrolyte fuel cells (PEFCs) is presented based on fundamental molecular theory using the general transport equation (GTE). The GTE was previously developed and applied to bridge the gap that exists between the benchmark modeling philosophies in the literature for transport across the PEFC. In the current work, the GTE is applied with Darcy’s law to describe water transport and water uptake through the porous and quasiporous layers of a PEFC under single- and two-phase operating conditions. The characteristic transport equations and available material properties from the literature are translated into a single-cell fuel cell model, which is implemented using the object modeling technique. The PEFC model is applied to predict and validate the net water transport ratio and water content under a range of operating conditions. The numerical model exhibits good agreement with experimental data under both vapor- and liquid-equilibrated conditions. The model is then applied in a water transport study to determine the effects of cell compression on local water content, liquid water intrusion, water transport, and Ohmic resistance across nonreinforced polymer electrolyte membranes (PEMs) under two-phase operating conditions. The modeling results suggest that the presence of liquid water at the cathodic boundary of the PEM and a well-established liquid water network can affect water uptake and water transport and can reduce the Ohmic resistance of the PEM.

Water management of the DMFC passively fed with a high-concentration methanol solution

The methanol barrier layer adopted for high-concentration direct methanol fuel cells (HC-DMFCs) increases water transport resistance, and makes water management in HC-DMFCs more challenging and critical than that in the conventional direct methanol fuel cell (DMFC) without a methanol barrier layer. In the semi-passive HC-DMFC used in this work, oxygen was actively supplied to the cathode side while various concentrated methanol solutions, 4 M, 8 M, 16 M, and neat methanol, were passively supplied from the anode fuel reservoir. The effects of the cathode relative humidity, cathode pressure, and oxygen flow rate on the water crossover coefficient, fuel efficiency, and overall performance of the fuel cell were studied. Results showed that electrolyte membrane resistance, which was determined by its water content, was the predominant factor that determined the performance of a HC-DMFC, especially at a high current density. A negative water crossover coefficient, which indicated that water flowed back from the cathode through the electrolyte membrane to the anode, was measured when the methanol concentration was 8 M or higher. The back flow of water from the cathode is a very important water supply source to hydrate the electrolyte membrane. The water crossover coefficient was decreased by increasing the cathode relative humidity and back pressure. Water flooding at the cathode was not severe in the HC-DMFC, and a low oxygen flow rate was preferred to decrease water loss and yield a better performance. The peak power density generated from the HC-DMFC fed with 16 M methanol solution was 75.9 mW cm −2 at 70 °C.

Double-Skinned Forward Osmosis Membranes for Reducing Internal Concentration Polarization within the Porous Sublayer

A scheme to fabricate forward osmosis membranes comprising a highly porous sublayer sandwiched between two selective skin layers via phase inversion was proposed. One severe deficiency of existing composite and asymmetric membranes used in forward osmosis is the presence of unfavorable internal concentration polarization within the porous support layer that hinders both (i) separation (salt flux) and (ii) the performance (water flux). The double skin layers of the tailored membrane may mitigate the internal concentration polarization by preventing the salt and other solutes in the draw solution from penetrating into the membrane porous support. The prototype double-skinned cellulose acetate membrane displayed a water flux of 48.2 L·m−2·h−1 and lower reverse salt transport of 6.5 g·m−2·h−1 using 5.0 M MgCl2 as the draw solution in a forward osmosis process performed at 22 °C. This can be attributed to the effective salt rejection by the double skin layers and the low water transport resistance within the p...

Analysis of Water Transport in Catalyst Layers

Water transport phenomena in catalyst layers (CLs) of a polymer electrolyte fuel cell were investigated experimentally and analytically. An experimental technique was developed for evaluating water transport resistance in the CLs based on the water permeation rate through the membrane electrode assembly (MEA). Water transport resistance in the CLs was regarded as being driven by the water activity gradient and was examined for MEAs with different CL thicknesses using an equivalent circuit model. This model consists of three transport mechanisms--vapor transport in pores, liquid transport in the ionomer and the phase change at the interface between the ionomer and pores. Experimental results showed that MEA resistance increased almost linearly with increasing CL thickness. Compared with the results calculated with the model, the phase change resistance was deemed to be negligibly small under our test conditions.

Analysis of Water Transport in Catalyst Layers: The Effect of Carbon Supports

Water transport phenomena in catalyst layers (CLs) were investigated experimentally and analytically. Water transport resistance in CLs was regarded as being driven by the water activity gradient and examined using an equivalent circuit model consisting of two transport mechanisms in parallel. In this study, the effect of different carbon supports on water transport resistance was investigated. The experimental results showed that CLs with a carbon support of graphitized ketjen black displayed lower water transport resistance than CLs with a carbon support of ketjen black. One possible reason for that difference is that the tortuosity of the ionomer in the CLs differed depending on the carbon support shape. The water resistance values in the CLs calculated with the model were higher in both cases than the experimental results. This suggests the existence of another water transport path that is faster than the path in the ionomer.

Surface Structure Analysis of Polymer Electrolyte Membrane under Water Permeation Process by Tapping-Mode Atomic Force Microscopy

Through-plane water permeation across the membrane is one of the fundamental processes involved in operational PEMFCs. In recent years, water transport resistance at the membrane-gas interface has been reported and is related to relative humidity applied to the membrane. In this study, we investigated variation of surface structure of the membrane under water permeation process by using tapping-mode atomic force microscopy (AFM). AFM results revealed that surface structure of the membrane depends on relative humidity at the both side of the membrane and also suggested that local equilibrium was not necessarily established at the membrane-gas interface under water permeation associated in the membrane.

An Analysis of The Water Transport Properties of Polymer Electrolyte Membrane

Water transport resistance through the polymer electrolyte membrane was evaluated by measuring the water permeability of it. From the results of a thickness sensitivity evaluation, two types of water transport resistance were found. One was "internal diffusion resistance," which was proportional to the membrane thickness, and the other was "interfacial transport resistance," which was independent of thickness. This interfacial resistance was found not to be negligible. Furthermore, both resistances showed sensitivity to temperature and relative humidity. The internal diffusion coefficient estimated from the results of water permeation experiments correlated well with the results obtained by the pulse field gradient nuclear magnetic resonance (PFG-NMR) method. And, the electro-osmotic drag coefficient was also measured by the proton pump balance method based on the water permeation test system. It was concluded that water permeation experiments are helpful for obtaining a better fundamental understanding of water transport mechanism through the membrane.

Internal Resistance to Water Mobility in Seafood during Warm Air Drying and Microwave-Vacuum Drying

We present a novel microwave-vacuum drying technique as an effective method for seafood. Microwave is irradiated just for the latent heat supply to keep the material at room temperature. We made several experiments using the scallop and examined the internal resistance for water transport based on the porous media model. The experimental and theoretical study indicates that the microwave-vacuum drying keeps the channel for water transportation and results in the high permeability at the wide range of moisture content. The drying time was significantly shortened as compared with the warm-air drying and the room-temperature drying results in good quality of dried seafood.

A theoretical and experimental study of simultaneous heat and mass transport resistances in a shallow fluidized bed dryer of mate leaves

A theoretical and a semi-theoretical modeling approach were applied in order to predict drying kinetics of mate leaves in a shallow fluidized bed dryer. The first procedure involves an internal diffusive mechanism of mass transport (Fick's second law), while the second one assumes that the resistance for water transport is represented by an apparent convective term analogous to the Newton's law of cooling (Lewis model). Since heat and mass transfer occurs at the same time, an energy equation assuming negligible internal conduction was written to the solid phase and it was coupled to the mass balance representing the mechanism of mass transfer. Model parameters were simultaneously tuned on experimental transient moisture content and on temperature profiles of mate leaves, which were obtained by varying the equivalent particle diameter approximately from 5.2 × 10 −3 to 1.1 × 10 −2 m at the drying temperatures of 52 and 101 °C. A regression analysis based on the uncertainties in the calculated parameters as well as on the identification of possible tendencies in residuals corroborates the assumption of negligible internal heat transfer conduction and evidences that the semi-theoretical model of Lewis describes better than the purely diffusive model the transport of water over the whole period of drying. The estimated Biot number (0 < Bi < 100) reveals that both internal and external mass transport resistances play an important role for mate leaves drying and demonstrates that the single parameter of the Lewis model represents an effective coefficient that takes into accounts both diffusion and convection. A significant effect of the equivalent particle diameter and temperature on the drying constant and on the external heat transfer coefficient is also evidenced.

Water transport in the proton exchange-membrane fuel cell: Comparison of model computation and measurements of effective drag

Water transport through the membrane of a PEMFC was investigated by measurement of the net drag under various feed gas humidity. Measured data were compared with computed results obtained using a two-dimensional cell model. Considering the change in the gas content related to the flow configuration, the humidity of the supply gas, reaction rates, and the mass balance of each gas species were derived at five sections along the flow channels. By solving these mass balance equations, the water transport rates and current density distribution were obtained along the flow channels for various feed gas humidity. The results for net drag computed from the model show a similar tendency, but are slightly higher than the measured values. This suggests that there is a certain resistance related to water transport at the cathode membrane interface in association with water production. The cause of this water transport resistance is discussed.

Influence of porous substrate on mesopore structure and water permeability of surfactant templated mesoporous silica membranes

The formation of silica films via sol–gel route on disordered mesoporous and macroporous supports is reported. These films are structurally characterised by TEM, XRD, XPS and permporometry. It has been found that the ordered mesoporous silica layer does not grow directly on a mesoporous support. Instead it grows on a structurally disordered interface of 10–20 nm thickness. This observation differs considerably from studies on film formation on dense supports as reported in the literature. Water transport experiments were carried out on the silica films deposited on porous supports. The results suggest that the disordered interface layer of the film deposited on the mesoporous support does not contribute significantly to the total resistance for water transport.

Evaluation of Internal Resistance of Sea Food in Drying Process (Comparison between Microwave-vacuum Drying and Warm-air Drying)

We present a novel microwave drying as an effective drying method for seafood. The drying time was successfully shortened an compared with the warm-air drying and the drying at room-temperature results in good quality for dried seafood. In this study, we examined the internal resistance for water transport based on the porous media model. The permeability of scallop was estimated experimentally by measuring the perfusion rate of water injected from a syring into the scallop. And we examined the morphological change of the muscular fiver-cell. It is found that the microwave-vacuum drying can prevent the surface shrinkage and the expansion of inner-cell due to the increase of osmotic pressure observed in the warm-air drying. The internal channel for the water transportation can be kept during the microwave-vacuum drying and this results in the high permeability at the wide range of moisture content. The porous media model agrees very well with both experiments of the microwave-vacuum drying and warm-air drying.