Electro-osmotic Drag Coefficient Research Articles

Operating Temperature Effects on Water Transport Behavior in a Single Cell PEMFC

Flooding and membrane dehydration are phenomena that must be avoided in a proton exchange membrane fuel cell (PEMFC) operation. It needs a sufficient knowledge about water transport behaviors. Electro-osmotic drag and back diffusion are the dominant water transport mechanisms through the membrane in PEMFC. In this study, the relative humidity (RH) profile along the channel at both anode and cathode sides have been recorded. The experiment was conducted in a single cell PEMFC with single serpentine flow field design. The water content profile was strongly influenced by RH profile thus in turn influenced the electro-osmotic drag coefficient, water diffusion coefficient and back diffusion flux. The operating temperatures of cell also influence those water transport behavior. The temperature was varied at 25, 40, 50 and 60 oC, while the pressure at the anode and the cathode was fixed at 1 bar. The higher the temperature, the smaller the water contents but with higher electro-osmotic drag coefficient, water diffusion coefficient and back diffusion flux. After all, the strategy of using saturated hydrogen and dry air in this study successfully prevents flooding and membrane dehydrating in the system – that are the major problems in PEMFC operation.

Dual-Feed Balanced High-Pressure Electrolysis of Water in a Lightweight Polymer Electrolyte Membrane Stack

Efficient water electrolyzers that generate high-pressure hydrogen and oxygen are useful for aerospace and civilian applications. A lightweight polymer-electrolyte-membrane electrolyzer capable of producing oxygen and hydrogen at a balanced pressure of 13.8 MPa has been designed and fabricated by Hamilton Sundstrand Space Systems International (HSSSI). This electrolyzer uses a unique metal-laminate cell design that reduces the weight of traditional high-pressure water electrolysis stacks by eliminating the need to place the stack within a pressure vessel. The electrolyzer operates on water fed to the anode and cathode (dual-feed mode). This design simplifies the balance of plant for a regenerative fuel cell system and enhances system safety. The electrolyzer was tested at balanced pressure in the range of 1.72–13.8 MPa using a test system designed and constructed at the Jet Propulsion Laboratory. Current efficiency of 96 to 97% was achieved at a balanced product pressure of 13.8 MPa. The polarization characteristics were governed by the kinetics of oxygen evolution and the ionic resistance of the membrane electrolyte. The electro-osmotic water drag coefficient was determined to be in the range of 2.3–2.4 and this value did not vary with the operating pressure up to 13.8 MPa. The dual-feed electrolyzer is a promising design for achieving high energy efficiencies at high product pressure.

Development and experimental validation of a dynamic thermal and water distribution model of an open cathode proton exchange membrane fuel cell

Abstract Water concentration in proton exchange membrane (PEM) fuel cells strongly influences performance and durability which demands for fundamental understanding of water transport mechanisms. The system efficiency can be significantly improved with greater understanding of water flux dynamics through the membrane and its dependence on the internal conditions of the fuel cell. Therefore, a two-dimensional, non-isothermal, dynamic model of a 100 W open cathode, self-humidified PEM fuel cell system has been developed, that is capable of representing system specific control mechanisms for water and thermal management. The model consists of three coupled submodels based on energy, momentum and water mass balance of the system. The work is based on experimental observations of the investigated fuel cell stack, for which the crucial coefficients for water transport, namely the diffusion and the electroosmotic drag (EOD) coefficient have been determined. The diffusivity of water vapor through the MEA at 30 °C was determined to be 3.3 × 10 −8 m 2 s −1 and increases by 3 × 10 −10 m 2 s −1 °C −1 up to 50 °C stack temperature. The EOD coefficient was measured as 0.47–0.48 water molecules per proton at stack currents from 1 to 3 A. Validation of the steady state and the dynamic model by using experimental data, directly obtained from laboratory tests, has shown that the model predictions match the experimental data well.

Review of the proton exchange membranes for fuel cell applications

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for clean and efficient power generation in the twenty-first century. Proton exchange membranes (PEMs) are the key components in fuel cell system. The researchers have focused to reach the proton exchange membrane with high proton conductivity, low electronic conductivity, low permeability to fuel, low electroosmotic drag coefficient, good chemical/thermal stability, good mechanical properties and low cost. These are classified into the “iron triangle” of performance, durability, and cost. Current PEMFC technology is based on expensive perflourinated proton-exchange membranes (PEMs) that operate effectively only under fully hydrated conditions. There is considerable application-driven interest in lowering the membrane cost and extending the operating window of PEMs. PEMFC system complexity could be reduced by the development of ‘water-free’ electrolytes that do not require hydration. It also enables the PEMFC to be operated under ‘warm’ conditions (i.e. above 100 °C) thus further improving its efficiency. Capital cost could also be further reduced because at warmer conditions less Pt could be used. This paper presents an overview of the key requirements for the proton exchange membranes (PEM) used in fuel cell applications, along with a description of the membrane materials currently being used and their ability to meet these requirements. A number of possible alternative candidates are reviewed and presented in this paper. Also discussed are some of the new materials, technologies, and research directions being pursued to try to meet the demanding performance and durability needs of the PEM fuel cell industry. The alternative PEMs are classified into three categories: (1) modified Nafion ® composite membranes; (2) functionalized non-fluorinated membranes and composite membranes therein; and (3) acid–base composite membranes. Several commonly used inorganic additives are reviewed in the context of composite membranes. Finally, the general methods of the measuring and evaluating of proton exchange membrane properties have been investigated such as proton conductivity, ion exchange capacity, water uptake, gas permeability, methanol permeability, durability, thermal stability and fuel cell performance test.

Oscillations of PEM fuel cells at low cathode humidification

Oscillatory fluctuations of a single polymer electrolyte fuel cell appear upon operation with a dry cathode air supply and a fully humidified anode stream. Periodic transitions between a low and a high current operation point of the oscillating state are observed. The transition time of 20–25 s for the change from the low to the high operation is fast and does not depend on the operating parameters. Contrasting with this behavior, the downward transition depends strongly on the operating conditions. Impedance measurements indicate a high ionic resistance with low water content for the low current operation and a low ionic resistance of the membrane with high water content for the high current operation. An insight into the transitions is obtained by current density distributions at distinct times indicating a propagating active area with defined boundaries. The observations are in agreement with assuming a liquid water reservoir at the anode with a downward transition period depending on the operation conditions. The high current operation possesses a high electro-osmotic drag and a high permeation rate (corresponding to liquid–vapor permeation) leading to a large water flux to the cathode. Subsequently, the liquid reservoir at the anode is consumed leading to an anode drying. The system establishes a new quasi-stable operation point associated with a low current, low electro-osmotic drag coefficient, and a low water permeation (corresponding to vapor–vapor permeation). When liquid water is formed at the anode interface after some time the fast transition to the high current operation occurs. This interpretation is supported by conductive atomic force microscopy current images of the membrane showing a strong dependence of the ionic conductivity on the activation procedures with or without liquid water and also showing oscillatory behavior after the membrane is activated. Specifically, activation with liquid water yields a high conductivity with currents larger by three orders of magnitude.

Analysis of water transport in a high pressure PEM electrolyzer

This paper analyzes, through experimental data and a transport model, the water transported through the membrane under different operating conditions in a on a Proton Exchange Membrane (PEM) electrolyzer operating with a high-pressure gradient across the membrane from the cathode (high-pressure) side to the anode (nearly ambient-pressure) side. The phenomena involved in this movement are described and analyzed, with a focus on the electro-osmotic drag coefficient, n eo. We have observed that the behavior of the hydraulic percolation determines the results obtained for the electro-osmotic drag, while the contribution of the water diffusion is negligible. In general, the cathode pressure significantly reduces the water transport (a positive effect). Also, operation at lower current density reduces the net electro-osmotic drag coefficient, n g; therefore, the best operation strategy for obtaining dried hydrogen at the cathode is to impose high cathode pressure and low current density.

Measurements of water uptake and transport properties in anion-exchange membranes

This paper reports on the measured water uptake and water transport properties through an anion-exchange membrane, including the water diffusivity, the electro-osmotic drag (EOD) coefficient, and the mass-transfer coefficient of water at the cathode catalyst layer/membrane interface. The water uptake of the membrane in equilibrium with liquid water and water vapor at different values of relative humidity is measured at 30, 40 and 60 °C. The measured water uptake at each measured temperature is correlated in terms of the relative humidity. Like in Nafion ® membranes, the Schroeder’s paradox phenomenon is also found to exist in the anion-exchange membrane. The water diffusivity of the anion-exchange membrane shows the same order of 10 −10 m 2 s −1 as that of the Nafion ® membrane. The measured mass-transfer coefficient of water at the cathode catalyst layer/membrane interface falls in the range of 1.0 × 10 −6 to 1.0 × 10 −5 m s −1. The EOD coefficients measured at 30 °C and 40 °C are, respectively, 2.3 and 3.6.

Electroosmotic Drag of Water in Hydrated Potassium Perfluorosulfonated Polymer Membrane in External Electric Fields

The electroosmotic drag and the corresponding mechanism of water molecules in hydrated potassium perfluorosulfonate electrolyte polymer membrane were studied using molecular dynamics simulations,and the relationship between the membrane structure and electroosmotic drag characteristics was analyzed.It is concluded that velocities of both H2O and K+ obey the Maxwell velocity distribution function without external electric field applied.If an appropriate electric field is applied,the velocities of H2O and K+ still obey the Maxwell velocity distribution in the direction perpendicular to the electric field,and obey the peak shifted Maxwell velocity distribution in the direction parallel to the electric field.The peak shifting velocities coincide with the average transport velocities of H2O and K+ induced by the applied electric field,and could be applied to evaluate the electroosmotic drag coefficient of water.The results also show that the average number of water molecules in the first coordination shell of K + is 4.04,and the average transport velocity of these water molecules is about 57% of that of K+.The electroosmotic drag coefficient contributed by these water molecules is about 77% of total the electroosmotic drag coefficient(2.97).

Electro-osmotic drag coefficient and proton conductivity in Nafion ® membrane for PEMFC

A new method based on hydrogen pump is presented to measure the electro-osmotic drag coefficient and proton conductivity in Nafion ® 117 membrane under similar condition to operating PEMFC. For membranes in contact with liquid water on both sides, electro-osmotic drag coefficient and proton conductivity increase with increasing temperature over the range of 20–90 °C while keeping water content inside the membrane constant ( λ = 22.5) and decrease with decreasing water content in the membrane while keeping temperature of system constant ( T = 25 °C). On the basis of cluster-network model and transport mechanisms of proton and water in ionomeric membrane, the results are discussed.

Membrane water-flow rate in electrolyzer cells with a solid polymer electrolyte (SPE)

Water-flow rate across Nafion membrane in SPE electrolyzer cells was measured and modelled. From the analysis of water transport mechanisms in SPE water electrolysis, the water-flow rate through membrane can be described by the electro-osmotic drag. The calculated electro-osmotic drag coefficients, n d, for the membrane in SPE electrolysis cells at different temperatures were compared with literature and in good agreement with those of Ge et al. and Ise et al. To describe the water-flow rate through membrane more accurately, a linear fit of n d as a function of temperature for the membrane in SPE water electrolysis was proposed in this paper. This paper studied the membrane water-flow rate experimentally and mathematically, which is of importance in the designing and optimization of the process of SPE water electrolysis. This paper also provided a novel method for measuring the electro-osmotic drag coefficient of Nafion membrane in contact with liquid water, acid and methanol solutions, etc.

Electro-osmosis and Water Uptake in Polymer Electrolytes in Equilibrium with Water Vapor at Low Temperatures

Water uptake and electro-osmosis are investigated to improve the understanding and aid the modeling of water transport in proton-exchange membrane fuel cells (PEMFCs) below . Measurements of water sorption isotherms show a significant reduction in the water capacity of polymer electrolytes below . This reduced water content is attributed to the lower vapor pressure of ice compared to supercooled liquid water. At , 1100 equivalent weight Nafion in equilibrium with vapor over ice has of water per sulfonic acid group. Measurements of the electro-osmotic drag coefficient for Nafion and both random and multiblock copolymer sulfonated poly(arylene ether sulfone) (BPSH) chemistries are reported for vapor equilibrated samples below . The electro-osmotic drag coefficient of BPSH chemistries is found to be , and that of Nafion is . No significant temperature effect on the drag coefficient is found. The implication of an electro-osmotic drag coefficient less than unity is discussed in terms of proton conduction mechanisms. Simulations of the ohmically limited current below show that a reduced water uptake below results in a significant decrease in PEMFC performance.

An experimental investigation of electro-osmotic drag coefficients in a polymer electrolyte membrane fuel cell

Through the use of a water balance experiment, the electro-osmotic drag coefficients of Nafion 115 were obtained under several conditions (as a function of water content and thermodynamics conditions). For the cases when the anode was fully hydrated (corresponding to water content λ ≈ 14 in the adjacent membrane) and the cathode suffered from drying when dry air was supplied ( λ ≈ 2), the electro-osmotic drag coefficients varied from 0.82 (±0.06) to 0.50 (±0.03) H 2O/H + when the current density varied from 0.4 to 1.0 A cm −2 (95% confidence level). When the current density increased, the electro-osmotic drag coefficient decreased. When the water content at the anode increased from λ ≈ 5 to λ ≈ 14, the cathode was supplied with dry air (λ ≈ 2), and the fuel cell discharged constant current density at 0.6 A cm − 2, the electro-osmotic drag coefficient increased from 0.44 (±0.06) to 0.68 (±0.06) H 2O/H + (95% confidence level). Higher relative humidity gas leads to a higher electro-osmotic drag coefficient at constant current density.

Methanol Distribution and Electroosmotic Drag in Hydrated Poly(perfluorosulfonic) Acid Membrane

The methanol distribution and electroosmotic drag in hydrated poly(perfluorosulfonic) acid electrolyte membrane are studied using molecular dynamics simulations under various electric fields applied. The results indicate that the methanol molecules are preferentially distributed near the hydrophobic PFSA backbones with their methyl groups in contact with the fluorine atoms and their hydroxyl groups pointing to the hydrophilic subphase. As the hydroxyl groups of methanol forming hydrogen bonds, hydroxyl groups are more likely to accept hydrogen atoms than to donate hydrogen atoms. The calculated methanol diffusion coefficient is in good correspondence with experimental values, and the electroosmotic drag coefficient for methanol is much smaller than that of water molecules.

Evaluation of electroosmotic drag coefficient of water in hydrated sodium perfluorosulfonate electrolyte polymer

The electroosmotic drag coefficient of water molecules in hydrated sodium perfluorosulfonate electrolyte polymer is evaluated on the basis of the velocity distribution functions of the sodium cations and water molecules with an electric field applied using molecular dynamics simulations. The simulation results indicate that both velocity distribution functions of water molecules and of sodium cations agree well with the classic Maxwellian velocity distribution functions when there is no electric field applied. If an electric field is applied, the distribution functions of velocity component in directions perpendicular to the applied electric field still agree with the Maxwellian velocity distribution functions but with different temperature parameters. In the direction of the applied electric field, the electric drag causes the velocity distribution function to deviate from the Maxwellian velocity distribution function; however, to obey the peak shifted Maxwellian distribution function. The peak shifting velocities coincide with the average transport velocities induced by the electric field, and could be applied to the evaluation of the electroosmotic drag coefficient of water. By evaluation of the transport velocities of water molecules in the first coordination shells of sodium cations, sulfonate anion groups, and in the bulk, it is clearly shown that the water molecules in the first coordination shell of sodium cations are the major contribution to the electroosmotic drag and momentum transfer from water molecules within the first coordination shell to the other water molecules also contributes to the electroosmotic drag.

Molecular Analysis on Methanol Diffusion in a Model Nafion Membrane

Methanol crossover, mainly due to diffusion mechanism, deteriorates the performance of direct methanol fuel cells. Using a molecular simulation technique, we try to reproduce the methanol diffusion event in a model Nafion membrane with an atomistic resolution. The simulation system comprises molecules of the Nafion-water-methanol-hydronium ions at four different concentrations of methanol. Molecular simulation results show that methanol molecules migrate with water clusters and hydronium ions via the sulfonic acid groups of the side chains of the electrolyte. Electro-osmotic drag coefficients and diffusion coefficients of methanol were evaluated at varied methanol concentration and compared to reported experimental results. The thermal effect on the methanol diffusion was also investigated using this molecular analysis technique.

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.

Electroosmotic Drag Coefficient of Proton Exchange Membrane as a Function of Relative Humidity

A novel technique for measuring the electroosmotic drag coefficient (EODC) of proton exchange membranes using a non flow through, or "dead-ended" hydrogen pump is presented here over a full range of non-condensing humidification. This experiment is conducted in situ under "fuel cell like" conditions, so the transport phenomena observed are directly applicable to an operating fuel cell without any extrapolation. Experiments were conducted on 50 μm and 500 μm Nafion® 1100 EW membranes. The measurements consistently demonstrated that for low to medium humidification, the EODC observed is below unity. This observation confirms that the vehicular proton transport mechanism cannot be the sole source of proton transport at low relative humidity, leading one to conclude that the Grotthus "hopping" mechanism is active even at low levels of humidification.

Electro-osmosis and Water Uptake in Polymer Electrolytes in Equilibrium with Water Vapor at Low Temperatures

Measurements of the electro-osmotic drag coefficient for Nafion® and both random and multi-block co-polymer sulfonated poly(arylene ether sulfone) (BPSH) chemistries are reported for vapor equilibrated samples below 0 {degree sign}C. No significant change in the drag coefficient behavior for Nafion from that reported above 0 {degree sign}C is found. However BPSH is found to have a drag coefficient of 0.4. The implication of a drag coefficient less than unity in the interpretation of conduction mechanisms is discussed. Measurements of water sorption isotherms below 0 {degree sign}C are also presented. A significant reduction in the capacity of polymer electrolytes to store water below 0 {degree sign}C is found. This reduced water content is a result of the lower vapor pressure of ice compared to supercooled liquid.

Electro-osmotic Drag Coefficient and Water Uptake in Polymer Electrolytes in Equilibrium with Water Vapor at Low Temperatures

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Electroosmotic Drag Coefficient of Proton Exchange Membrane as a Function of Relative Humidity

not Available.