Co-reporter:Jay B. Benziger;May Jean Cheah;Vaclav Klika;Michal Pavelka
Journal of Polymer Science Part B: Polymer Physics 2015 Volume 53( Issue 22) pp:1580-1589
Publication Date(Web):
DOI:10.1002/polb.23794
ABSTRACT
Water and proton transport across a Nafion membrane are measured as functions of water activity and applied electric potential with a polymer electrolyte hydrogen pump. Water and proton transport across the membrane must match water and proton transport entering and leaving the electrode/membrane/vapor three phase interfaces at the anode and cathode. At low applied electric potential proton and water fluxes are correlated. At moderate to high applied electric potential the proton current is constant, independent of applied electric potential, while the water transport increases with increasing electric potential. At high applied electric potential water and proton transport become uncoupled at the membrane interfaces; water is transported across the membrane/vapor interface and protons are transported across the membrane/electrode interface. The applied electric potential drives electro-osmosis to redistribute the water in the membrane. Water redistribution is limited by the interfacial transport of water across the membrane/vapor interface. © 2015 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2015, 53, 1580–1589
Co-reporter:Donghao Ye, Eric Gauthier, Jay B. Benziger, Mu Pan
Journal of Power Sources 2014 Volume 256() pp:449-456
Publication Date(Web):15 June 2014
DOI:10.1016/j.jpowsour.2014.01.082
•Direct measurement of gas diffusion layer bulk and contact resistances.•Teflon treatment increases GDL contact resistance with no change of bulk resistance.•Microporous layer decreases contact resistance.•Uneven compression under channels and ribs deforms GDL, breaking electrical contact.A multi-electrode probe is employed to distinguish the bulk and contact resistances of the catalyst layer (CL) and the gas diffusion layer (GDL) with the bipolar plate (BPP). Resistances are compared for Vulcan carbon catalyst layers (CL), carbon paper and carbon cloth GDL materials, and GDLs with microporous layers (MPL). The Vulcan carbon catalyst layer bulk resistance is 100 times greater than the bulk resistance of carbon paper GDL (Toray TG-H-120). Carbon cloth (CCWP) has bulk and contact resistances twice those of carbon paper. Compression of the GDL decreases the GDL contact resistance, but has little effect on the bulk resistance. Treatment of the GDL with polytetrafluoroethylene (PTFE) increases the contact resistance, but has little effect on the bulk resistance. A microporous layer (MPL) added to the GDL decreases the contact resistance, but has little effect on the bulk resistance. An equivalent circuit model shows that for channels less than 1 mm wide the contact resistance is the major source of electronic resistance and is about 10% of the total ohmic resistance associated with the membrane electrode assembly.
Co-reporter:Kevin B. Daly, Athanassios Z. Panagiotopoulos, Pablo G. Debenedetti, and Jay B. Benziger
The Journal of Physical Chemistry B 2014 Volume 118(Issue 48) pp:13981-13991
Publication Date(Web):November 12, 2014
DOI:10.1021/jp509061z
The design of fuel cells and lithium ion batteries is constrained, in part, by mechanical creep and perforation of the polymer electrolyte, a process that is poorly understood at the molecular level. The mechanical stiffness (quantified as shear viscosity) and structure of a widely used polymer electrolyte, Nafion, are studied in the limit of a low solvent volume fraction (≤26% v/v H2O) using molecular dynamics simulations. The viscosity is shown to increase by up to 4 orders of magnitude in response to changes in composition representing as little as 2 wt % of system. Two types of compositional changes are considered, changes in solvent volume fraction and counterion type. A system with a counterion Xv+ for every v Nafion monomers and y water molecules is denoted as (RSO3)vX·(H2O)y. The following trend is observed in viscosity: (RSO3)2Ca > RSO3Na > RSO3H·(H2O)3 > RSO3H ≈ RSO3H·(H2O)10. This trend correlates with changes in the strength of the SO3–/Xv+/SO3– cross-links and the size of the cross-link networks. Counterion type is shown to strongly influence the morphology. The simulations are able to reproduce some important experimental trends without crystalline domains or high-MW effects like entanglements, providing a simplified understanding of the mechanical properties of Nafion.
Co-reporter:Thomas Hellstern, Eric Gauthier, May Jean Cheah, Jay B. Benziger
International Journal of Hydrogen Energy 2013 Volume 38(Issue 35) pp:15414-15427
Publication Date(Web):22 November 2013
DOI:10.1016/j.ijhydene.2013.09.073
•Slugs form in gas flow channels but don't penetrate the GDL.•Gas flow can bypass slugs flowing through the GDL.•Slugs detach when pressure drop through GDL overcome surface tension of slug in channel.•Gas flow under ribs can cause slugs to slow stall and block channel.Water drops emerge from large pores of the hydrophobic Gas Diffusion Layers (GDL) into the cathode gas flow channel of Polymer Electrolyte Membrane (PEM) Fuel Cells. The drops grow into slugs that span the cross-section of the flow channels. The slugs detach and are forced out the gas flow channel by the air flow. An acrylic micro-fluidic flow cell with a 1.6 mm gas flow channel and a 100 μm liquid pore through a carbon paper GDL has been used to quantitatively determine slug volumes, velocity of slug motion, and the force required to move slugs as functions of the gas and liquid flow rates. In a channel with 4 acrylic walls, slugs detach immediately upon formation. A porous GDL wall allows gas flow to bypass the slugs, thus allowing slugs to continue to grow after spanning the open area of the channel. The differential pressure to detach and move slugs is equal to the dynamic interfacial force on a slug normalized by the cross-sectional area of the channel. The dynamic interfacial force is equal to the difference between the downstream (advancing) and upstream (receding) contact lines of the water with the channel walls. Slugs will stop moving if the differential pressure drop for gas flow to bypass the slug and flow through the GDL under the rib separating the channels is less than the differential pressure required to move the slug. The results improve our physical insight into the state of water hold up in PEM fuel cells.
Co-reporter:Xiaoming Yan, Gaohong He, Xuemei Wu, Jay Benziger
Journal of Membrane Science 2013 429() pp: 13-22
Publication Date(Web):
DOI:10.1016/j.memsci.2012.11.026
Co-reporter:Qiao Zhao ;Jay Benziger
Journal of Polymer Science Part B: Polymer Physics 2013 Volume 51( Issue 11) pp:915-925
Publication Date(Web):
DOI:10.1002/polb.23284
ABSTRACT
Water sorption and mechanical properties of perfluoro sulfonated acids (PFSAs), Aquivion and Nafion, are compared under environmentally controlled conditions from 25 to 120 °C and water activities of 0–0.95. Under dry conditions Nafion and Aquivion have thermal transitions at 60 °C and 95 °C, respectively, where the elastic modulus decreases rapidly with increasing temperature. Below their thermal transition temperatures water sorption plasticizes both PFSAs; the elastic moduli decrease with increasing water activity. Above the thermal transition water sorption stiffens both polymers; increasing the water activity from 0 to 0.01 increases the elastic moduli by a factor >10. Plasticization and stiffening are reversible with changing water activity at constant temperature. The thermal transition in PFSAs is suggested to result from reversible clustering of ionic groups. The higher thermal transition temperature for Aquivion is suggested to reduce the risk of membrane thinning and failure due to creep. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2013, 51, 915–925
Co-reporter:May J. Cheah, Ioannis G. Kevrekidis, and Jay B. Benziger
Langmuir 2013 Volume 29(Issue 31) pp:9918-9934
Publication Date(Web):June 26, 2013
DOI:10.1021/la4011967
Water emerging from ∼100 μm pores into millimeter-size gas flow channels forms drops that grow and become slugs which span the flow channel. Flowing gas causes the slugs to detach and move down the channel. The effect of channel geometry, surface wettability, and gravity on the formation and motion of water slugs has been analyzed using high-speed video images of the drops and differential pressure–time traces. Drops grow and appear, assuming a sequence of shapes that minimize the total interfacial energy of the gas–liquid and liquid–solid interfaces. The drops are initially spherical caps centered on the pore (the liquid contacts one wall). Above a certain size, the drops move to the corner, forming “corner drops” (the liquid contacts two walls). Corner drops grow across the channel, evolving into partial liquid bridges (drops confined by three walls), and finally the drops span the channel cross-section forming slugs (contacting all four walls). Smaller slugs are formed in channels with hydrophobic walls than in channels with hydrophilic walls. Smaller slugs are formed in channels with curved walls than in square or rectangular channels. Slugs move when the differential gas pressure overcomes the force to move the advancing and receding gas–liquid–solid contact lines of the slugs. Residual water left behind in corners by moving slugs reduces the barriers for drops to form slugs, causing the steady-state slug volumes to be smaller than those seen at start-up in dry channels.
Co-reporter:May Jean Cheah, Ioannis G. Kevrekidis, and Jay B. Benziger
Langmuir 2013 Volume 29(Issue 48) pp:15122-15136
Publication Date(Web):2017-2-22
DOI:10.1021/la403057k
Water emerging from micrometer-sized pores into millimeter-sized gas-flow channels forms drops. The drops grow until the force from the flowing gas is sufficient to detach the drops as either (1) slugs that completely occlude the cross section of the channel and move at the superficial gas velocity, (2) drops that partially occlude the channel and move at a velocity that is less than the gas velocity, or (3) films that flow continuously, occluding part of the channel. At steady state, small residual water droplets, ∼100 μm in diameter, left in corners and on surface defects from previous drops, are key in determining the shape of water drops at detachment. Slugs are formed at low-gas-phase Reynolds numbers (ReG) in both hydrophilic and hydrophobic channels. Drops are shed in Teflon-coated hydrophobic channels for ReG > 30. Films are formed in acrylic hydrophilic channels for ReG > 30. Slugs form when growing drops encounter residual water droplets that nucleate the drop to slug transition. Drops are shed when the force exerted by the flowing gas on growing drops exceeds the force needed to advance the gas/liquid/solid contact line before they grow to the critical size for the drop to slug transition. Drops grow by “stick–slip” of the solid–liquid–gas contact lines and with pinned contact lines until the force on the drops results in either the downstream contact angle becoming greater than the dynamic advancing contact angle or the upstream contact angle becoming less than the dynamic receding contact angle. The upstream contact line never detaches for hydrophilic channels, which is why films form. The shape of water drops and the detachment energies are shown to be well approximated by the force balance between the force needed to advance the drop’s contact lines and the force that the flowing gas exerts on a stationary drop.
Co-reporter:Eric Gauthier, Thomas Hellstern, Ioannis G. Kevrekidis, and Jay Benziger
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 2) pp:761
Publication Date(Web):December 27, 2011
DOI:10.1021/am201408t
Liquid water is pushed through flow channels of fuel cells, where one surface is a porous carbon electrode made up of carbon fibers. Water drops grow on the fibrous carbon surface in the gas flow channel. The drops adhere to the superficial fiber surfaces but exhibit little penetration into the voids between the fibers. The fibrous surfaces are hydrophobic, but there is a substantial threshold force necessary to initiate water drop motion. Once the water drops begin to move, however, the adhesive force decreases and drops move with minimal friction, similar to motion on superhydrophobic materials. We report here studies of water wetting and water drop motion on typical porous carbon materials (carbon paper and carbon cloth) employed in fuel cells. The static coefficient of friction on these textured surfaces is comparable to that for smooth Teflon. But the dynamic coefficient of friction is several orders of magnitude smaller on the textured surfaces than on smooth Teflon. Carbon cloth displays a much smaller static contact angle hysteresis than carbon paper due to its two-scale roughness. The dynamic contact angle hysteresis for carbon paper is greatly reduced compared to the static contact angle hysteresis. Enhanced dynamic hydrophobicity is suggested to result from the extent to which a dynamic contact line can track topological heterogeneities of the liquid/solid interface.Keywords: carbon; carbon fibers; contact angle; contact angle hysteresis; hydrophobic; Teflon; wetting;
Co-reporter:Xuemei Wu, Jay Benziger, Gaohong He
Journal of Power Sources 2012 Volume 218() pp:424-434
Publication Date(Web):15 November 2012
DOI:10.1016/j.jpowsour.2012.07.002
Hydrogen recovery from CO2/H2 reformate mixtures by selective electrochemical pumping was compared from carbon supported Pt and Pd catalysts. Catalyst coated membranes were prepared by air-brushing a suspension of commercially available 20 wt% Pt/C or 20 wt% Pd/C catalysts and solubilized Nafion in methanol onto Nafion 115 membranes. Electrochemical activity and separation efficiency for the different catalyst layer formulations were evaluated by cyclic voltammetry, polarization and potentiostatic hydrogen pumping. The effective membrane-electrode-assembly (MEA) resistance increased due to dilution of H2 by CO2; the effective MEA resistance was greater for Pd/C catalysts than for Pt/C catalysts. CO2 adsorbed more strongly to Pd catalysts than Pt catalysts reducing the electrochemical active surface area available for hydrogen oxidation/reduction. Pd/C catalysts had an energy efficiency for hydrogen recovery from reformate mixtures approximately 80% that of Pt catalysts. Because Pd is ten times less costly than Pt the results presented here suggest that Pd/C catalysts would be a promising candidate for hydrogen pumps to recover H2 from reformate mixtures.Highlights► H2 was purified with a PEM hydrogen pump with Pd/C catalysts. ► Pd/C catalysts were less efficient than Pt/C catalysts for H2 purification. ► CO2 adsorbs on Pd catalysts reducing the electroactive surface area. ► The effective MEA resistance increases with decreases in H2 partial pressure. ► Gas phase diffusion contributes to the effective MEA resistance.
Co-reporter:Qiongjuan Duan, Huaping Wang, Jay Benziger
Journal of Membrane Science 2012 Volumes 392–393() pp:88-94
Publication Date(Web):1 March 2012
DOI:10.1016/j.memsci.2011.12.004
The flux of liquid water through Nafion membranes of different thickness and equivalent weight was measured as a function of hydrostatic pressure and temperature. Hydraulic water transport across Nafion membranes increases with temperature and equivalent weight of the Nafion. Hydraulic permeability increases with temperature due to both decreased water viscosity and increased hydrophilic volume fraction. Convective flow from the applied hydrostatic water pressure is an order of magnitude greater than the estimated diffusive water flux associated with the water activity gradient. Water sorption and hydraulic permeability data predict a hydrophilic pore network with hydrophilic domains 2.5 nm in diameter spaced 5.5 nm apart. The pore network structure from water sorption and hydraulic permeability are consistent with the spacing between hydrophilic domains observed with small angle X-ray scattering experiments.Graphical abstractHighlights► Liquid water is forced through hydrophilic domains in Nafion by hydraulic permeation. ► Hydraulic permeation increases with hydrophilic volume fraction. ► Hydrophilic domains consist of hydrophilic channels ∼2.5 nm in diameter spaced 5.5 nm apart.
Co-reporter:Qiao Zhao, Nicole Carro, Ho Youn Ryu, Jay Benziger
Polymer 2012 Volume 53(Issue 6) pp:1267-1276
Publication Date(Web):9 March 2012
DOI:10.1016/j.polymer.2012.01.050
Methanol and ethanol sorption and transport in 1100 equivalent weight H+Nafion were compared to water sorption and transport. Sorption isotherms for methanol and ethanol were fit to a solvation shell model, with four molecules in the first solvation shell. The larger molar volume of alcohols resulted in greater swelling from sorption. Proton conductivity is five times greater for water saturated Nafion than alcohol saturated Nafion. Alcohol pervaporation and alcohol vapor permeation is slower than water pervaporation and water vapor permeation. The Nafion/vapor interfacial transport coefficients for alcohols and water scale with vapor pressure. The diffusivity of water is 3–4 times greater than the diffusivity of methanol and ethanol. The results indicate that alcohols sorb by solvating the sulfonic acid groups, similar to the interaction of water with Nafion. Larger alcohol molecules diffuse slower in the hydrophilic channels of Nafion than the smaller water molecules.
Co-reporter:Carlos E. Colosqui, May J. Cheah, Ioannis G. Kevrekidis, Jay B. Benziger
Journal of Power Sources 2011 Volume 196(Issue 23) pp:10057-10068
Publication Date(Web):1 December 2011
DOI:10.1016/j.jpowsour.2011.08.084
A microfluidic device is employed to emulate water droplet emergence from a porous electrode and slug formation in the gas flow channel of a PEM fuel cell. Liquid water emerges from a 50 μm pore forming a droplet; the droplet grows to span the entire cross-section of a microchannel and transitions into a slug which detaches and is swept downstream. Droplet growth, slug formation, detachment, and motion are analyzed using high-speed video images and pressure–time traces. Slug volume is controlled primarily by channel geometry, interfacial forces, and gravity. As water slugs move downstream, they leave residual micro-droplets that act as nucleation sites for the next droplet-to-slug transition. Residual liquid in the form of micro-droplets results in a significant decrease in slug volume between the very first slug formed in an initially dry channel and the ultimate “steady-state” slug. A physics-based model is presented to predict slug volumes and pressure drops for slug detachment and motion.Highlights► We study (ex situ) droplet/slug formation in gas channels of PEM fuel cells. ► Physical modeling successfully estimates slug volume/pressure drops. ► Gravity orientation affects slug formation; pendant droplets form smaller slugs. ► Residual micro-droplets/films alter surface wettability and slug formation/volume. ► Insights from experiments/modeling can enhance fuel cell flow design/operation.
Co-reporter:Qiao Zhao, Paul Majsztrik, and Jay Benziger
The Journal of Physical Chemistry B 2011 Volume 115(Issue 12) pp:2717-2727
Publication Date(Web):March 3, 2011
DOI:10.1021/jp1112125
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 cm3/mol at λ = 0.5 H2O/SO3 to 1.5 cm3/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.
Co-reporter:Xuemei Wu;Xiaowen Wang;Gaohong He;Jay Benziger
Journal of Polymer Science Part B: Polymer Physics 2011 Volume 49( Issue 20) pp:1437-1445
Publication Date(Web):
DOI:10.1002/polb.22326
Abstract
Water sorption, volumetric expansion, and proton conductivity of 1100 EW Nafion and 555 EW sulfonated polyetheretherketone (SPEEK) were compared as functions of water activity at 60 and 80 °C. Water sorption in Nafion occurs with a small positive volume of mixing, ∼0.005 cm3/cm3. In contrast, water sorption in SPEEK has a large negative volume of mixing ∼−0.05 cm3/cm3. The percolation thresholds for proton conduction occur at hydrophilic volume fractions of 0.10 in Nafion and 0.30 in SPEEK. Proton conductivity increases quadratically with hydrophilic volume fraction above the percolation threshold. The different percolation thresholds suggest the hydrophilic domains in Nafion grow from lamella, whereas the hydrophilic domains in SPEEK grow from spheres. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1437–1445, 2011
Co-reporter:May Jean Cheah, Ioannis G. Kevrekidis, and Jay Benziger
The Journal of Physical Chemistry B 2011 Volume 115(Issue 34) pp:10239-10250
Publication Date(Web):July 22, 2011
DOI:10.1021/jp204785t
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.
Co-reporter:Jay Benziger and James Nehlsen
Industrial & Engineering Chemistry Research 2010 Volume 49(Issue 21) pp:11052
Publication Date(Web):June 11, 2010
DOI:10.1021/ie100631a
Decene and acetone were hydrogenated over Ni, Pd, Pt, Cu, Ag, and Au catalysts in a polymer electrolyte hydrogen pump (PEHP) reactor. Water was oxidized over a Pt mesh anode and protons were pumped to the catalysts supported on porous carbon cathodes in contact with an organic liquid phase. The protons are reduced at the cathode to adsorbed hydrogen atoms which hydrogenate adsorbed olefins and carbonyl groups by heterogeneous catalysis. At low current density decene hydrogenation over Pd and Pt increased with the current density to the 1/2 power, indicating the surface reaction of adsorbed hydrogen with adsorbed decene was the rate limiting step. At high current density the reaction rate decreased linearly with current density, indicating adsorbed hydrogen inhibited decene adsorption and decene adsorption was the rate limiting step. Decene hydrogenation at 50 °C was 100 times slower over Ni, Cu, Ag, and Au catalysts compared to Pd and Pt. Acetone hydrogenation over Pt increased linearly with proton current density and was 10 times slower than decene hydrogenation. Acetone and decene hydrogenation rates at 50 °C were almost the same over Cu catalysts. Data were fit with modified Langmuir−Hinshelwood kinetics; the rate limiting steps were identified as the first hydrogen addition to adsorbed decene and the second hydrogen addition to adsorbed acetone.
Co-reporter:M. Barclay Satterfield
Journal of Polymer Science Part B: Polymer Physics 2009 Volume 47( Issue 1) pp:11-24
Publication Date(Web):
DOI:10.1002/polb.21608
Abstract
Tensile stress–strain and stress relaxation properties of 1100 equivalent weight Nafion have been measured from 23 to 120 °C at 0–100% relative humidity. At room temperature, the elastic modulus of Nafion decreases with water activity. At 90 °C, the elastic modulus goes through a maximum at a water activity of ∼ 0.3. At temperatures ≥90 °C, hydrated membranes are stiffer than dry membranes. Stress-relaxation was found to have two very different rates depending on strain, temperature, and water content. At high temperature, low water activity, and small strain, the stress relaxation displays a maximum relaxation time with stress approaching zero after 103–104 s. Water absorption slows down stress-relaxation rates. At high water activity, the maximum stress relaxation time was >105 s at all temperatures. No maximum relaxation time was seen at T ≤ 50 °C. Increasing the applied strain also resulted in no observed upper limit to the stress relaxation time. The results suggest that temperature, absorbed water, and imposed strain alter the microstructure of Nafion inducing ordering transitions; ordered microstructure increases the elastic modulus and results in a stress relaxation time of >105 s. Loss of microphase order reduces the elastic modulus and results in a maximum stress relaxation time of 103–104 s. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 11–24, 2009.
Co-reporter:Paul Majsztrik, Andrew Bocarsly and Jay Benziger
The Journal of Physical Chemistry B 2008 Volume 112(Issue 51) pp:16280-16289
Publication Date(Web):November 21, 2008
DOI:10.1021/jp804197x
The permeation of water through 1100 equivalent weight Nafion membranes has been measured for film thicknesses of 51−254 μm, temperatures of 30−80 °C, and water activities (aw) from 0.3 to 1 (liquid water). Water permeation coefficients increased with water content in Nafion. For feed side water activity in the range 0 < aw < 0.8, permeation coefficients increased linearly with water activity and scaled inversely with membrane thickness. The permeation coefficients were independent of membrane thickness when the feed side of the membrane was in contact with liquid water (aw = 1). The permeation coefficient for a 127 μm thick membrane increased by a factor of 10 between contacting the feed side of the membrane to water vapor (aw = 0.9) compared to liquid water (aw = 1). Water permeation couples interfacial transport across the fluid membrane interface with water transport through the hydrophilic phase of Nafion. At low water activity the hydrophilic volume fraction is small and permeation is limited by water diffusion. The volume fraction of the hydrophilic phase increases with water activity, increasing water transport. As aw → 1, the effective transport rate increased by almost an order of magnitude, resulting in a change of the limiting transport resistance from water permeation across the membrane to interfacial mass transport at the gas/membrane interface.
Co-reporter:Sharonmoyee Goswami, Shannon Klaus and Jay Benziger
Langmuir 2008 Volume 24(Issue 16) pp:8627-8633
Publication Date(Web):July 9, 2008
DOI:10.1021/la800799a
Water drops on Nafion films caused the surface to switch from being hydrophobic to being hydrophilic. Contact angle hysteresis of >70° between advancing and receding values were obtained by the Wilhelmy plate technique. Sessile drop measurements were consistent with the advancing contact angle; the sessile drop contact angle was 108°. Water drop adhesion, as measured by the detachment angle on an inclined plane, showed much stronger water adhesion on Nafion than Teflon. Sessile water and methanol drops caused dry Nafion films to deflect. The flexure went through a maximum with time. Flexure increased with contact area of the drop, but was insensitive to the film thickness. Methanol drops spread more on Nafion and caused larger film flexure than water. The results suggest that the Nafion surface was initially hydrophobic but water and methanol drops caused hydrophilic sulfonic acid domains to be drawn to the Nafion surface. Local swelling of the film beneath the water drop caused the film to buckle. The maximum flexure is suggested to result from motion of a water swelling front through the Nafion film.
Co-reporter:Paul W. Majsztrik, M. Barclay Satterfield, Andrew B. Bocarsly, Jay B. Benziger
Journal of Membrane Science 2007 Volume 301(1–2) pp:93-106
Publication Date(Web):1 September 2007
DOI:10.1016/j.memsci.2007.06.022
Water sorption, desorption, and permeation in and through Nafion 112, 115, 1110 and 1123 membranes were measured as functions of temperature between 30 and 90 °C. Water permeation increased with temperature. Water permeation from liquid water increased with the water activity difference across the membrane. Water permeation from humidified gas into dry nitrogen went through a maximum with the water activity difference across the membrane. These results suggested that the membrane was less swollen in the presence of water vapor and that a thin skin formed on the dry side of the membrane that reduced permeability to water. Permeation was only weakly dependent on membrane thickness; results indicated that interfacial mass transport at the membrane/gas interface was the limiting resistance. The diffusivity of water in Nafion deduced from water sorption into a dry Nafion film was almost two orders of magnitude slower than the diffusivity determined from permeation experiments. The rate of water sorption did not scale with the membrane thickness as predicted by a Fickian diffusion analysis. The results indicated that water sorption was limited by the rate of swelling of the Nafion. Water desorption from a water saturated film was an order of magnitude faster than water sorption. Water desorption appeared to be limited by the rate of interfacial transport across the membrane/gas interface. The analysis of water permeation and sorption data identifies different regimes of water transport and sorption in Nafion membranes corresponding to diffusion through the membrane, interfacial transport across the membrane–gas interface and swelling of the polymer to accommodate water.
Co-reporter:Paul W. Majsztrik;Hitoshi Ota;M. Barclay Satterfield;Andrew B. Bocarsly
Journal of Polymer Science Part B: Polymer Physics 2006 Volume 44(Issue 16) pp:2327-2345
Publication Date(Web):13 JUL 2006
DOI:10.1002/polb.20857
Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion-based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. Thecomposite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long-time creep of ∼40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of ∼0.55 MPa in 125-μm membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 °C and 100% relative humidity, the resistivity of Nafion increases by ∼15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006
Co-reporter:Chris Yang, S. Srinivasan, A.B. Bocarsly, S. Tulyani, J.B. Benziger
Journal of Membrane Science 2004 Volume 237(1–2) pp:145-161
Publication Date(Web):1 July 2004
DOI:10.1016/j.memsci.2004.03.009
The physiochemical properties of Nafion 115 and a composite Nafion 115/zirconium phosphate (∼25 wt.%) membranes are compared. The composite membrane takes up more water than Nafion at the same water activity. However, the proton conductivity of the composite membrane is slightly less than that for Nafion 115. Small angle X-ray scattering shows that the hydrophilic phase domains in the composite membrane are spaced further apart than in Nafion 115, and the composite membrane shows less restructuring with water uptake. Despite the lower proton conductivity of the composite membranes they display better fuel cell performance than Nafion 115 when the fuel cell is operated at reduced humidity conditions. It is suggested that the composite membrane has a greater rigidity that accounts for its improved fuel cell performance.
Co-reporter:J. Benziger, L. Cadonati, F. Calaprice, M. Chen, A. Corsi, F. Dalnoki-Veress, R. Fernholz, R. Ford, C. Galbiati, A. Goretti, E. Harding, Aldo Ianni, Andrea Ianni, S. Kidner, M. Leung, F. Loeser, K. McCarty, D. McKinsey, A. Nelson, A. Pocar, C. Salvo, et al.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (21 September 2009) Volume 608(Issue 3) pp:464-474
Publication Date(Web):21 September 2009
DOI:10.1016/j.nima.2009.07.035
The system for controlled filling of the nested flexible scintillator containment vessels in the Borexino solar neutrino detector is described. The design and operation principles of pressure and shape monitoring systems are presented for gas filling, gas displacement by water, and water displacement by scintillator. System specifications for safety against overstressing the flexible nylon vessels are defined as well as leak-tightness and cleanliness requirements. The fluid-filling system was a major engineering challenge for the Borexino detector.
Co-reporter:Xiaoming Yan, Shuang Gu, Gaohong He, Xuemei Wu, Jay Benziger
Journal of Power Sources (15 March 2014) Volume 250() pp:
Publication Date(Web):15 March 2014
DOI:10.1016/j.jpowsour.2013.10.140
•Imidazolium-functionalized poly(ether ether ketone) was successfully synthesized.•PEEK-ImOH HEMs exhibit improved dimensional stability.•PEEK-ImOH HEMs exhibit high hydroxide conductivity (e.g., 52 mS cm−1 at 20 °C).•PEEK-ImOH HEMs exhibit good mechanical property (e.g., 78 MPa of tensile strength).•The methanol/O2 fuel cell employing PEEK-ImOH shows high performance.A series of imidazolium-functionalized poly(ether ether ketone)s (PEEK-ImOHs) were successfully synthesized by a two-step chloromethylation–Menshutkin reaction followed by hydroxide exchange. PEEK-ImOH membranes with ion exchange capacity (IEC) ranging from 1.56 to 2.24 mmol g−1 were prepared by solution casting. PEEK-ImOHs show selective solubility in aqueous solutions of acetone and tetrahydrofuran, but are insoluble in lower alcohols. PEEK-ImOH membranes with IEC of 2.03 mmol g−1 have high hydroxide conductivity (52 mS cm−1 at 20 °C), acceptable water swelling ratio (51% at 60 °C), and great tensile strength (78 MPa), and surprising flexibility (elongation-to-break of 168%), and high thermal stability (Decomposition temperature: 193 °C). In addition, PEEK-ImOH membranes show low methanol permeability (1.3–6.9 × 10−7 cm2 s−1). PEEK-ImOH membrane was tested in methanol/O2 fuel cell as both the HEM and the ionomer impregnated into the catalyst layer; the open circuit voltage is 0.84 V and the peak power density is 31 mW cm−2.
Co-reporter:G. Alimonti, C. Arpesella, M.B. Avanzini, H. Back, M. Balata, D. Bartolomei, A. de Bellefon, G. Bellini, J. Benziger, A. Bevilacqua, D. Bondi, S. Bonetti, A. Brigatti, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, G. Cecchet, R. Cereseto, A. Chavarria, M. Chen, et al.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (1 October 2009) Volume 609(Issue 1) pp:58-78
Publication Date(Web):1 October 2009
DOI:10.1016/j.nima.2009.07.028
The successful deployment of the Borexino solar neutrino detector required assorted physical and chemical operations to produce exceptional pure fluids and fill multiple detector zones. The composition and flow rates of high purity gases and liquids had to be precisely controlled to maintain liquid levels and pressures. The system was required to meet exceptional requirements for cleanliness and leak-tightness. A large scale modular system connecting fluid receiving, purification and fluid delivery processes was developed for Borexino. At the core is a flow control system that delivers scintillator components to plants for purification, and then fills the Borexino detector volumes with ultrahigh purity buffer or ultrahigh purity scintillator. The liquid handling system maintains precise control over the liquid levels and differential pressures between the different volumes of the detectors that are separated by flexible nylon vessels. The preparation, commissioning and operation of the system for filling the Borexino detector with scintillator is described.
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