Perfluorosulphonic Acid Research Articles

Prototype Demonstration of Innovative and Sustainable Recycling of Materials in Polymer Electrolyte Membrane Fuel Cells and Electrolyzers

Low-temperature (LT) polymer electrolyte membrane (PEM) fuel cells (FCs) and electrolysis cells (ECs) contain valuable materials such as platinum-group metals (PGMs) and perfluorosulphonic acid (PFSA) polymer. The Danish companies IRD Fuel Cells A/S and CriMaRec ApS together with the University of Southern Denmark are developing and scaling up efficient and environmentally friendly processes to recycle PGMs, mainly platinum (Pt) and iridium (Ir), with the aim of establishing production alternatives to the costly, energy-intensive, and pollutive conventional processes. The approach is based on patented electrochemical methods involving potential cycling [1,2]. The efforts also comprise recycling or reuse of PFSA polymer as an alternative to incineration. The established outcomes are excellently performing LT-PEMFC&EC membrane–electrode assemblies (MEAs) containing PFSA material recovered from spent MEAs and PGMs recycled from MEAs [3,4] as well as from existing scrap resources such as spent catalysts from exhaust systems of automobiles [5].In this presentation we shall provide the main results proving our PGM-recycling concepts for Pt and Ir catalysts. Pt and Ir compounds (such as (NH4)2[PtCl6], Pt(NO3)2, and IrCl3) obtained from the processing of spent MEAs and spent autocatalysts were found to have the same qualities as those of commercial equivalents, and new electrocatalyst products (Pt/C and IrO x nanoparticles) made from these recycling compounds show properties and performances that are similar to or better than those produced from fresh precursors. Our development of the PGM-recovery processes into pilot-scale production is also presented. Two reactors with volumes of 18 dm3 and 50 dm3, respectively, have been designed, constructed, and optimized for extraction of PGMs from autocatalyst material and MEAs, respectively. In preliminary runs validating the recovery processes in these prototype reactors, 7.3 g of PGM (mainly Pt and palladium (Pd)) were recovered from a 4.5-kg batch of autocatalyst material with an estimated efficiency of 91 %, while 15.4 g of PGM (Pt and ruthenium (Ru)) were recovered from a 0.7-kg batch of LT-PEMFC MEAs with an efficiency of around 93 %. The reactor processes involve low energy consumption, low emissions, and a cycle time less than 2 h – in very great contrast to the conventional recovery processes – and the associated costs are in the order of 10 % of the PGM market price. Furthermore, we present our optimization of the recovery and regeneration of PFSA from the membranes of spent MEAs and the successful reapplication of this material as ionomer in new LT-PEMFC&EC MEAs. The feasibility of the PFSA recycling has been demonstrated as the performances of these new MEAs are similar to or better than those of MEAs prepared using fresh commercial ionomer. On top of these results, we present strong performances of LT-PEMFC&EC MEAs with recycled Pt/C cathode and IrO x anode electrocatalysts, respectively.The perspectives of this work are that a highly competitive large-scale production of LT-PEMFC&EC MEAs containing recycled PGM and PFSA will allow the MEA manufacturer to offer its customers the cost-reducing possibility of returning the spent MEAs for recycling. It can also ensure adequate reuse and/or recycling of production process waste, thus reducing the amount of scrap material. Moreover, it will facilitate the flow of secondary (recycled) PGMs from declining industrial sectors into the hydrogen-energy sector, securing for the latter the supply of these critical raw materials and decreasing the dependence on primary (mined) PGM. The green transition from fossil-fuel-based energy to renewable energy is thus accelerated, with the cost of the PEMFC&EC technology being considerably reduced and its deployment accordingly promoted. Acknowledgement The work is supported by the Energy Technology Development and Demonstration Programme (EUDP) of the Danish Energy Agency through the project 64019-0551 3R – Recycle, Reuse, Reduce.

Rapid detection of per- and polyfluoroalkyl substances (PFAS) using paper spray-based mass spectrometry

Traditional PFAS analysis by mass spectrometry (MS) is time-consuming, as laborious sample preparation (e.g., extraction and desalting) is necessary. Herein, we report fast detection of PFAS by paper spray (PS)-based MS techniques, which employs a triangular-shaped filter paper for sample loading and ionization (≤ 3 min per sample). In this study, PS-MS was first used for direct PFAS analysis of drinking water, tap water, and wastewater. Interestingly, food package paper materials can be directly cut and examined with PS-MS for possible PFAS contamination. For samples containing salt matrices which would suppress PFAS ion signal, desalting paper spray mass spectrometry (DPS-MS), was shown to be capable of rapidly desalting, ionizing and detecting PFAS species such as per-fluorooctanoic acid (PFOA) and per-fluorosulphonic acid (PFOS). The retention of PFAS on paper substrate while salts being washed away by water is likely due to hydrophilic interaction between the PFAS polar head (e.g., carboxylic acid, sulfonic acid) with the polar filter paper cellulose surface. The DPS-MS method is highly sensitive (limits of detection:1.2–4.5 ppt) and can be applicable for directly analyzing soil extract and soil samples. These results suggest the high potential of PS-MS and the related DPS-MS technique in real-world environmental analysis of PFAS.

Effects of Reinforcement Type on the Structure and Properties of Perfluorosulphonic Acid Membranes for Polymer Electrolyte Membrane Fuel Cells

Fuel cell membrane durability remains one of the key challenges limiting the wide scale adoption of fuel cell technology. Membrane degradation in polymer electrolyte membrane (PEM) fuel cells limits the operational lifetime of the fuel cell and prevents the industrial targets from being met. A common approach to increasing the lifetime of membranes includes the addition of a reinforcement layer to increase the mechanical durability of the membrane while not adversely affecting the performance [1,2]. Although reinforced membranes are widely used in industry, there is a literature gap considering membrane structure and properties in relation to durability. This work contributes to characterizing the effects of reinforcement type on the membrane properties. In this study a selection of perfluorosulphonic acid (PFSA) ionomer membranes with expanded polytetrafluoroethylene (ePTFE) reinforcements were tested, these include two novel DMR100 membranes with different reinforcements and Nafion XL compared to a conventional, non-reinforced Nafion NRE-211 for reference. All of these membranes contain common PFSA ionomer and differ primarily in the type of reinforcement layer and membrane thickness. The study addresses comparison between the membrane chemical composition, water uptake, crystallinity, and mechanical strength. Methods of ex situ characterization include solid state nuclear magnetic resonance (SS_NMR), small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA). Membrane chemical structure and properties are measured by SS-NMR and FTIR, whereas the water uptake, domain spacing, and crystallinity are assessed by SAXS/WAXS study of hydrated and dry membranes [3-5]. Tensile mechanical properties are measured under room temperature (23°C, 50% RH) and fuel cell conditions (80°C, 90% RH) by DMA [6]. Overall, this paper contributes both qualitative and quantitative understanding of the key structural properties of membranes with different reinforcement resulting in novel knowledge about membranes that can be leveraged for improved fuel cell durability.

Solvent absorption rate of perfluorosulphonic acid membranes towards understanding direct coating processes

Here we present a method for measurement of the rate of solvent absorption by perfluorosulfonic acid (PFSA) membranes using a force tensiometer. The method presented here can be used as a tool to understand solvent absorption and should provide a rationale for designing catalyst inks for direct coating processes since it is necessary to understand how the absorption rate – on a time scale of seconds to minutes – compares to the time scales of the coating process in order to minimize membrane swelling. This method allows for rapid screening of the absorption and swelling behavior of different solvents and mixtures. Using this method, the absorption of water/1-propanol mixtures were measured for three thicknesses of Nafion PFSA membrane – Nafion 1135, Nafion 115, and Nafion 117. We find that the absorption rate is dependent on the ratio of the solvents as well as the thickness of the membrane. The analysis indicates that the highest absorption rate occurs when the mass percentage of 1-propanol in the mixture is 50%, whereas the lowest rates are for pure water and 1-propanol. In addition, membrane distortion also occurs most quickly for the 50% 1-propanol mixture. Our results suggest that catalyst inks that are highly rich (≥90%) in water or 1-propanol are likely best for direct coating as such formulations will minimize absorption and swelling.

An investigation into the long-term binding and uptake of PFOS, PFOA and PFHxS in soil – plant systems

This study investigated the potential aging and plant bioaccumulation of three perfluoroalkyl acids (PFAAs), perfluorosulphonic acid (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexanesulphonic acid (PFHxS) in 20 soils over a six-month period. Sorption coefficients (Log Kd) ranged from 0.13–1.28 for PFHxS, 0.17–1.06 for PFOA and 0.98–2.03 for PFOS, respectively, and bioaccumulation factors (Log BAFs) ranged from 0.29–1.24, 0.22–1.46 and 0.05–0.65 for PFHxS, PFOA and PFOS, respectively. Over the six-month period, Kd values significantly increased for PFHxS and PFOA but the magnitude of the increase was very small and did not translate into differences in plant PFAA-concentrations between aged and freshly spiked treatments. The Kd and BAF values were modelled by multiple linear regression (MLR) to soil physico-chemical properties and by partial least squares regression to soil spectra acquired by mid-infrared spectroscopy (DRIFT−PLSR). Modelling of each PFAA was influenced by different soil properties, including organic carbon, pH, CEC, exchangeable cations (Ca2+, Mg2+, Na+ and K+) and oxalate extractable Al. BAF values were not strongly correlated to any soil property but were inversely correlated to Kd values. Our results indicate that limited aging occurred in these soils over the six-month period.

Synthesized Yttria Stabilised Zirconia as filler in Proton Exchange Membranes (PEMs) with enhanced stability

Composite membranes based on Perfluorosulphonic Acid (PSFA) represent a possible solution to operate at temperatures over 100 °C and low relative humidity (RH) levels in Polymer Electrolyte Fuel Cells (PEFCs). Yttria Stabilised Zirconia (YSZ) containing an 8 mol.% of yttrium oxide was synthesised and used to obtain Nafion® membranes with a variable content of YSZ (3, 5, 10 wt%). The membranes were characterised in terms of XRD, Ion Exchange Capacity (IEC), water uptake and swelling at different temperatures (room T, 80 and 95 °C). Proton conductivity, single cell tests in H2/air (I-V curves) and Accelerated Degradation Tests (ADTs) were conducted at 120 °C 75% RH. The composite membrane containing a 5 wt% of synthesized YSZ (NsynYSZ5) had the best performance and the highest electrochemical stability, reaching 48 cycles against to 28 cycles of a pristine Nafion membrane.

Structural and spectroscopic features of proton hydrates in the crystalline state. Solid-state DFT study on HCl and triflic acid hydrates

ABSTRACTCrystalline HCl and CF3SO3H hydrates serve as excellent model systems for protonated water and perfluorosulphonic acid membranes, respectively. They contain characteristic H3O+, H5О+2, H7О+3 and H3O+(H2O)3 (the Eigen cation) structures. The properties of these cations in the crystalline hydrates of strong monobasic acids are studied by solid-state density function theory (DFT). Simultaneous consideration of the HCl and CF3SO3H hydrates reveals the impact of the size of a counter ion and the crystalline environment on the structure and infrared active bands of the simplest proton hydrates. The H7O+3 structure is very sensitive to the size of the counter ion and symmetry of the local environment. This makes it virtually impossible to identify the specific features of H7O+3 in molecular crystals. The H3O+ ion can be treated as the Eigen-like cation in the crystalline state. Structural, infrared and electron-density features of H5О+2 and the Eigen cation are virtually insensitive to the size of the counter ion and the symmetry of the local crystalline environment. These cations can be considered as the simplest stable proton hydrates in the condensed phase. Finally, the influence of the Grimme correction on the structure and harmonic frequencies of the molecular crystals with short (strong) intermolecular O–H···O bonds is discussed.

Coupled Membrane Transport Parameters for Ionic Species in All-Vanadium Redox Flow Batteries

One of the major sources of capacity loss in all-vanadium redox flow batteries (VRFBs) is the undesired transport of active vanadium species across the ion-exchange membrane, generically termed crossover. In this work, a novel system has been designed and built to investigate the concentration- and electrostatic potential gradient-driven crossover for all vanadium species through the membrane in real-time. For this study, a perfluorosulphonic acid membrane separator (Nafion® 117) was used. The test system utilizes ultraviolet/visible (UV/Vis) spectroscopy to differentiate vanadium ion species and separates contributions to crossover stemming from concentration and electrostatic potential gradients. It is shown that the rate of species transport through the ion-exchange membrane is state of charge dependent and, as a result, interaction coefficients have been deduced which can be used to better estimate expected crossover over a range of operating conditions. The electric field was shown to increase the negative-to-positive transport of V(II)/V(III) and suppress the positive-to-negative transport of V(IV)/V(V) during discharge, with an inverse trend during charging conditions. Electric-field-induced transport coefficients were deduced directly from experimental data.

Study of Alcohol Sensing Devices Using DMFC Technology

Sensors for volatile organic compounds like alcohols (methanol and ethanol) can be fabricated using Direct Methanol Fuel Cell (DMFC) technology. The main problem is such a device is alcohol crossover. The alcohol diffuses from the anode through the electrolyte (perfluoro sulphonic acid membrane) to the cathode, where it reacts directly with the oxygen and produces no electrical current from the cell. This also results in poisoning of the cathode catalysts. The designed and fabrication of the sensor is by means of micro electro mechanical systems (MEMS) fabrication technology with the electrochemical inputs using a standard DMFC single cell arrangement. An ampherometric detection technique will be employed because of the redox reaction involved. The accuracy will be determined by the accurate detection of electric current or changes in electric current We have used a passive mode design protocol using COMSOL Multiphysics in the simulation. The design and simulation would involve optimization of various parameters, in the construction of the cell. We can optimize the overall power density and hence the sensitivity of the sensor by the modification of various parameters like the area of the working electrodes, separation distance and the electrode-electrolyte interface. A passive mode design protocol, for a cm cell area, using various parametric functions, and interfacing Darcy’s law of fluidic flow through a porous medium, under specific pressure and temperature, was applied. The designing involves the construction of gas diffusion layers using carbon matrix for electrodes with various parametric variations. Platinum and various alloy catalysts like Pt-Ru, Pt-Fe, Pt-Sn and Pt-Mo was chosen as the Oxygen Reduction Reaction (ORR) catalysts. The output of the cell was optimized for ampherometric detection. A MEMS based sensor with microfludic interconnects and its response characteristics will be presented.

Synthesis, characterization and fuel cell performance tests of boric acid and boron phosphate doped, sulphonated and phosphonated poly(vinyl alcohol) based composite membranes

The aim of this study is to synthesize a composite membrane having high proton conductivity, ion exchange capacity and chemical stability. In order to achieve this aim, poly(vinyl alcohol) (PVA) based composite membranes are synthesized by using classic sol–gel method. Boric acid (H3BO3) and boron phosphate (BPO4) are added to the membrane matrix in different ratios in order to enhance the membrane properties. Characterization tests, i.e; FT-IR analysis, mechanical strength tests, water hold-up capacities, swelling properties, ion exchange capacities, proton conductivities and fuel cell performance tests of synthesized membranes are carried out.As a result of performance experiments highest performance values are obtained for the membrane containing 15% boron phosphate at 0.6 V and 750 mA/cm2. Water hold-up capacity, swelling ratio, ion exchange capacity and proton conductivity of this membrane are found as 56%, 8%, 1.36 meq/g and 0.37 S/cm, respectively. These values are close to the values obtained ones for perfluorosulphonic acid membranes. Therefore this membrane can be regarded as a promising candidate for usage in fuel cells.

Effect of Water Transport on Swelling and Stresses in PFSA Membranes

AbstractThe mechanical durability of membranes used in proton exchange membrane fuel cells (PEMFC) is directly linked to the stresses that evolve in the membrane during fuel cell operation. The stresses are primarily induced due to the swelling of the membrane as it absorbs water, within the mechanical constraints in the fuel cell assembly. Thus, in order to predict the membrane stresses, the water content in the perfluorosulphonic acid (PFSA) membranes is determined numerically via three different absorption models based on experimentally determined water uptake data from the literature. Two models are based on a single, humidity‐dependent Fickian transport coefficient for the bulk PFSA membrane. In the third, two transport properties are modeled to account for a possible difference in transport resistance between the bulk membrane and the outer surface. The membrane sorption behaviors characterized from these three models are then independently incorporated in a representative fuel cell finite element model and subjected to a standard relative humidity (RH) protocol designed for measuring mechanical durability of PEMFCs. The results show that the sorption behavior has a significant effect on the membrane stresses and therefore, may impact the lifetime of the membrane.

Water permeation and sorption properties of Nafion 115 at elevated temperatures

There is the potential to use membrane technology to recover high purity water from power station flue gas streams. In this work, the permeation of water, CO2 and N2 through Nafion 115 was investigated as a function of water activity at 70–150°C. It was found that all permeances increased with increasing water activity but reduced with increasing temperature. This data was supplemented by sorption analysis at lower temperature which conversely showed decreasing solubility as temperature increased. The changes in solubility and permeance with water activity were attributed to membrane swelling as water activity increased. A comparison with a thinner perfluoro-sulphonic acid polymer, from an alternate supplier, suggested that permeance did not scale linearly with membrane thickness, possibly reflecting inhomogeneity in membrane swelling. The results were used to simulate the capture of water from a brown coal power station flue gas and it was found that a permeate pH of 3.3–4.0 is achievable with Nafion 115 at 150°C. This suggests that for use as boiler feed this stream will need to undergo further treatment.

Investigation of the temperature-related performance of proton exchange membrane fuel cell stacks in the presence of CO

SUMMARY H2 is generally used as the fuel in proton exchange membrane fuel cells (PEMFCs). However, H2 produced from reformate gas usually contains a trace of CO, which may severely affect the fuel cell performance. With the adoption of domestic short side chain, low equivalent weight perfluorosulphonic acid (PFSA) membrane, a 100 W stack is built and evaluated at elevated temperature of 95 °C for the purpose of improving its CO tolerance. The stack is operated with 5 ppm, 10 ppm and 20 ppm CO/H2, respectively; better performance is obtained as expected. Furthermore, a 5 kW PEMFC stack is prepared with home-made Ir–V/C and Pt/C as anode catalysts for the membrane electrode assemblies to compare their CO tolerance. Physical and electrochemical characterizations, such as transmission electron microscope and linear scan voltammogram are employed for catalyst investigation. The results demonstrate that the employment of domestic PFSA membrane enables the stack to be operated at 95 °C, which can improve the CO tolerance of all the anode catalysts. In addition, the effect of CO on cell polarization is insignificant at lower current densities. Under the same operating conditions, cells with Ir–V/C catalyst show better CO tolerance. Copyright © 2013 John Wiley & Sons, Ltd.

STXM Characterization of PEM Fuel Cell Catalyst Layers

The synchrotron based scanning transmission soft X-ray microscopy (STXM) utilizing near edge X-ray absorption fine structure (NEXAFS) is a novel technique for characterization of membrane electrode assembly (MEA) components in proton exchange membrane fuel cells (PEMFC). Component maps for carbon support, Pt and ionomer in the cathode catalyst layer and perfluorosulphonic acid (PFSA) membrane including the reinforced layer have been obtained from full C 1s multi-energy image sequences and maps from images at four energies in the C 1s and F 1s regions. The ability of this technique to distinguish both structural and chemical differences in cathode catalyst layers from two significantly different designs has been demonstrated. The ionomer distribution in the cathode catalyst layer of an advanced catalyst coated membrane (CCM) structure is compared to that of the cathode in a gas diffusion electrode (GDE).

Study of Carbon Based Solid State EDLCs at High Sweep Rates

Study of carbon based solid state EDLCs at high sweep rates. C.K. Subramaniam, K. K. Cheralathan, G.Velayutham and Sri Bollepalli a Materials Division, School of Advanced Sciences, VIT University, Vellore, 632014, TN, INDIA. b Anabond Sainergy Fuel Cell India Pvt., Ltd., 288 Burma Colony, Perungudi Chennai 600096, INDIA. c Sainergy Tech, Inc., 869 Pickens Industrial Drive, Suite 11, Marietta, GA 30062,USA, Solid state carbon based Electrochemical Double Layer capacitors (EDLC) was assembled and cyclic voltammetry (CV) was studied at high sweep rates. The carbons used were Vulcan XC carbon with typical surface area of 260 m2/gm, templated mesoporous carbon, (CMK-3), typical surface area of 1260 m2/gm, and a blend of Vulcan XC carbon and mesoporous carbon (90:10 % by weight). Solid ionic polymers of perfluorosulphonic acid were used as electrolyte. CV was performed for scan rates as high as 800V/s. The polymer electrolyte membrane can support high scan rates.

Effect of time-dependent material properties on the mechanical behavior of PFSA membranes subjected to humidity cycling

A viscoelastic-plastic constitutive model is developed to characterize the time-dependent mechanical response of perfluorosulphonic acid (PFSA) membranes. This model is then used in finite element simulations of a representative fuel cell unit, (consisting of electrodes, gas diffusion layer and bipolar plates) subjected to standardized relative humidity (RH) cycling test conditions. The effects of hold times at constant RH, the feed rate of humidified air and sorption rate of water into the membrane on the stress response are investigated. While the longer hold times at high and low humidity lead to considerable redistribution of the stresses, the lower feed and sorption rates were found to reduce the overall stress levels in the membrane. The redistribution and reduction in stress magnitudes along with inelastic deformation during hydration eventually lead to development of residual tensile stresses after dehydration. Simulations indicate that these tensile stresses can be on the order of 9–10 MPa which may lead to mechanical degradation of the membrane. The simulation results show that time-dependent properties can have a significant effect on the in-plane stress response of the membrane.

PFSA-TiO2(or Al2O3)-PVA/PVA/PAN difunctional hollow fiber composite membranes prepared by dip-coating method

PFSA-TiO2(or Al2O3)-PVA/PVA/PAN difunctional hollow fiber composite membranes with separation performance and catalytic activity have been prepared by dip-coating method. The good separation performance was brought about by the glutaraldehyde (GA) surface cross-linked PVA/PAN composite membrane, and the good catalytic activity of the membrane was achieved by the perfluorosulphonic acid (PFSA) used. The difunctional hollow fiber membranes were characterized by XRD, TGA, EDX, SEM, and FTIR. The separation performance was measured by dehydration of azeotropic top product of ethanol-acetic acid esterification, and the catalytic activity was obtained by investigating the esterification of ethanol and acetic acid. The FTIR spectra and the morphologies of difunctional hollow fiber composite membranes were similar for samples prior to esterification and post-esterification with ethanol and acetic acid for 24 h. Difunctional hollow fiber composite membranes with 2% PFSA, 8% TiO2 (named as DM-T1), and 2% PFSA, 8% Al2O3 (named as DM-A1) (all by weights) showed the best catalytic activity. They displayed fluxes of 165 and 173 g/m2 h, separation factors of water to ethanol of 279 and 161, PFSA contents in difunctional hollow fiber composite membrane of 3.2 and 2.4%, the ratios of PFSA to feed solution (acetic acid–ethanol) of 0.031 and 0.023%, and the equilibrium conversion of ethanol at 53.5 and 57.6%, in the given order for TiO2 and Al2O3 containing samples.

Toxicology of perfluorinated compounds

Perfluorinated compounds [PFCs] have found a wide use in industrial products and processes and in a vast array of consumer products. PFCs are molecules made up of carbon chains to which fluorine atoms are bound. Due to the strength of the carbon/fluorine bond, the molecules are chemically very stable and are highly resistant to biological degradation; therefore, they belong to a class of compounds that tend to persist in the environment. These compounds can bioaccumulate and also undergo biomagnification. Within the class of PFC chemicals, perfluorooctanoic acid and perfluorosulphonic acid are generally considered reference substances. Meanwhile, PFCs can be detected almost ubiquitously, e.g., in water, plants, different kinds of foodstuffs, in animals such as fish, birds, in mammals, as well as in human breast milk and blood. PFCs are proposed as a new class of 'persistent organic pollutants'. Numerous publications allude to the negative effects of PFCs on human health. The following review describes both external and internal exposures to PFCs, the toxicokinetics (uptake, distribution, metabolism, excretion), and the toxicodynamics (acute toxicity, subacute and subchronic toxicities, chronic toxicity including carcinogenesis, genotoxicity and epigenetic effects, reproductive and developmental toxicities, neurotoxicity, effects on the endocrine system, immunotoxicity and potential modes of action, combinational effects, and epidemiological studies on perfluorinated compounds).

The effect of relative humidity on the gas permeability and swelling in PFSI membranes

The permeation of helium, oxygen and nitrogen in perfluorosulphonic acid ionomeric (PFSI) membranes with short and long side chains, namely Aquivion® and Nafion® 117, was studied in the relative humidity range between 0 and 90%, and at temperatures between 35 °C and 65 °C. The presence of humidity enhances, up to a factor of 100, the gas permeability in the membranes, due to the permeation of gas molecules in the hydrophilic domains: the enhancement is rather pronounced for O2 and N2, and less marked for He permeability. The relative permeability increase, Pgas/Pgas,0, shows a complex dependence on the relative humidity, as the water content in the membrane is itself a non linear function of this parameter. The water volume fraction in the membrane at each activity was accurately estimated from measurements of vapor-induced swelling, which indicate that the partial molar volume of water is smaller than its pure liquid value, in both membranes at 35 °C. When plotted against water volume fraction, the gas permeability increases exponentially in the range between 2% and 20%; the slope of the curve is higher for Nafion® than for Aquivion®, as it is reasonable due to their different microstructures. The ideal selectivity of the two membranes for He over N2, O2 over N2 and He over O2, decreases markedly with increasing water content in the membrane.

Phosphotungstic Acid Modified Expanded PTFE based Nafion Composites

Composite membranes have been prepared by impregnation of PFSI(perfluorosulphonic acid ionomer) solution (Nafion®) into the expanded polytetrafluoroethylene (EPTFE) matrix. The matrix was modified by Na/Naphthalene treatment and with phosphotungstic acid (PTA). The matrix modifications helped in reducing the contact angle thereby improving the impregnation process by reducing the hydrophobicity of the matrix. The fuel cell made with these composite membranes showed that the performance of PTA modified matrix is comparable to that of Na/ naphthalene modified matrix. Further the performances of matrix modified membranes were higher than that of the as received matrix-composites. PTA treatment offers a simple method of matrix modification to achieve high performance.