Perfluorosulphonic Acid Membranes Research Articles

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.

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.

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.

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.

High Temperature Operation of a Solid Polymer Electrolyte Fuel Cell Stack Based on a New Ionomer Membrane

AbstractPolymer electrolyte fuel cell stacks assembled with Johnson Matthey Fuel Cells and SolviCore MEAs based on the Aquivion™ E79‐03S short‐side chain (SSC), chemically stabilised perfluorosulphonic acid membrane developed by Solvay Solexis were investigated at CNR‐ITAE in the EU Sixth Framework ‘Autobrane' project. Electrochemical experiments in fuel cell short stacks were performed under practical automotive operating conditions at pressures of 1–1.5 bar abs. over a wide temperature range, up to 130 °C, with varying levels of humidity (down to 18% R. H.). The stacks using large area (360 cm2) MEAs showed elevated performance in the temperature range from ambient to 100 °C (cell power density in the range of 600–700 mWcm–2) with a moderate decrease above 100 °C. The performances and electrical efficiencies achieved at 110 °C (cell power density of about 400 mWcm–2 at an average cell voltage of about 0.5–0.6 V) are promising for automotive applications. Duty‐cycle and steady‐state galvanostatic experiments showed excellent stack stability for operation at high temperature. A performance comparison of AquivionTM and NafionTM‐based MEAs under practical operating conditions showed a significantly better capability for the Solvay Solexis membrane to sustain high temperature operation.

Sub‐Freezing Conductivity of PFSA Membranes

AbstractSome properties of perfluorosulphonic acid (PFSA) membranes at sub‐freezing temperatures are presented in this work. Both commercial PFSAs and PFSA‐based membranes, developed within the Autobrane project, have been investigated. The proton conductivity at temperatures between –25 and 0 °C, has been determined by AC Impedance Spectroscopy. At each given condition, the different PFSA membranes exhibit similar values of proton conductivity. At –25 °C, the proton conductivities under ice‐saturated conditions were three to ten times higher than under reduced p conditions. As expected, proton conductivities decreased with decreasing temperature. The phase behaviour of water in the PFSA membranes has been investigated by differential scanning calorimetry (DSC) at temperatures between 20 and –60 °C. Water contents of membrane samples were estimated by thermo‐gravimetrical analysis (TGA) before the measurement (in fully hydrated state) and after measurement at –25 °C. For all the membranes, there was a remarkable decrease in the hydration number after the experiment at –25 °C. This fact may also contribute to the decrease in conductivity observed. No evidence of damage as a consequence of the measuring conditions could be detected for any of the membranes.

The U.S. DOEs High Temperature Membrane Effort

AbstractThe U.S. Department of Energy (DOE) is sponsoring research and development (R&D) efforts emphasising fuel cell membrane materials that can operate at temperatures up to 120 °C with no inlet humidification and at total pressures of <2.5 atm. Several different strategies are being investigated and will be discussed. Recent results have demonstrated improved conductivity at lower relative humidity (RH) compared to standard perfluorosulphonic acid membranes. At 120 °C, conductivity approaching 0.1 S cm–1 has been achieved at 50% RH, an improvement over Nafion 112 by more than a factor of three. Detailed results of DOE sponsored research will be discussed, as well as incorporation of membranes into membrane electrode assemblies. Finally, the impact of the different strategies on durability will be discussed.

Characteristics of Polypyrrole/Nafion Composite Membranes in a Direct Methanol Fuel Cell

Commercial perfluorosulphonic acid membranes (Nafion) have been impregnated with polypyrrole by in situ polymerization to decrease the crossover of methanol in direct methanol fuel cells (DMFCs). Modified membranes produced by polymerization of the pyrrole with hydrogen peroxide and iron(III) have been evaluated in a DMFC. Both methods produce membranes that can provide enhanced cell performance, although membranes produced with iron(III) as the oxidizing agent for the polymerization require additional treatments to restore their conductivity and promote bonding to the electrodes. Performance gains result from substantial reductions of the cathode overpotential, while anode overpotentials increase due to the lower conductivities of the modified membranes. Part of the beneficial effect at the cathode appears to be due to lower water crossover from the anode to the cathode. © 2003 The Electrochemical Society. All rights reserved.

The contact angle between water and the surface of perfluorosulphonic acid membranes

Recently studies aimed at clarifying the effect of perfluorinated sulphonic acid membrane surface properties on their behaviour in fuel cells were initiated. These polymers generally consist of a poly(tetrafluoroethylene) (PTFE) backbone with sulphonateterminated sidechains. A primary influence of membrane surface properties in fuel cells could be through surface effects on membrane hydration. The difference in the timescale for membrane equilibration with water in liquid and vapour phases may be explained by the effect of surface properties on membrane hydration. A membrane of thickness 178/~m equilibrates with liquid water within, at most, a few tens of seconds [1], whereas it takes much longer for this membrane to completely equilibrate with vapour phase water [2, 3]. (This is not simply because of the low concentration in the gas phase; a simple calculation shows that, at 1 atm vapour pressure, the membrane would be saturated with water in less than a second if the process were limited by the number of collisions of gas-phase water molecules with the membrane surface.) This difference in hydration dynamics may cause the apparent difference in equilibrium water uptake by ionomers from liquid and saturated vapour phases [3]. Surface effects may also account for the differences in water diffusion coefficients measured by (i) classical vapour-phase sorption methods, which involve transport of isotopically labelled water into, across, and out of a membrane sample, and (ii) N M R methods, which probe transport within the membrane. The vapourphase sorption measurement yields apparent water diffusion coefficients significantly lower [4] than those determined by N M R [5]. The self-diffusion coefficients measured by the sorption method are weakly dependent on membrane water content while those measured by N M R depend strongly on water content. A hydrophobic membrane surface could control the flux observed in the sorption experiment and likely acts as a barrier to water transport into, and possibly out of, the membrane. Our work described here aims to determine whether or not such a hydrophobic 'skin' exists. We report here on water contact angles measured on the perfluorosulfonate ionomer membranes to obtain some information on the hydrophilicity of the ionomer surface.

Aspects of charge propagation through Nafion/Os(bipy) 32+/3+ films on glassy-carbon electrodes

According to a kinetic model, the maximum currents, i L, max that can be obtained from the mediated oxidation of ferrocyanide at glassy-carbon electrodes covered with Nation ® 1 1 Registered trademark, perfluorosulphonic acid membrane, DuPont. /Os(bipy) 3 3+ films are determined jointly by film conductivity and by a saturation in the rate of the surface reaction. Values of i L, max, predicted on the basis of chronocoulometrically measured charge-propagation currents, i E, agreed with those actually recorded by rotating-disc techniques. These observations confirmed the validity of the model and further indicated that i E for these systems was the same for both transient and steady-state conditions, in contradiction with earlier published results. Significant electron hopping between neighbouring redox sites within these coatings was revealed by the dependence of the apparent charge-transport diffusion coefficient, D E(app), on mediator concentration, c T. Attempts were made to explain these results in terms of parallel conduction pathways, variable electron-hopping distances, and migrational effects.

A comparison of the electro-osmotic properties of charged polymer membranes and those predicted from binary electrolyte solution models Part I. Univalent forms of sulphonate membranes and chloride electrolyte models

By changing from the usual solvent-fixed frame of reference for flows to one based upon a fixed anion, electro-osmotic transference numbers are defined for any electrolyte, for which transport numbers are known. For sulphonate membranes, chloride electrolyte analogues were chosen. Agreement between observed transference numbers and those of the model electrolytes are shown to be excellent for both polystyrene based and perfluoro-sulphonic acid membranes. From irreversible thermodynamics it is shown that the transference number for any membrane will have a maximum value equal to the molar ratio of water to fixed charge in the membrane and independent of ionic form. The observed value is in addition, proportional to the fraction of the total water friction, which is due to water interaction with counterion. It is the latter which is estimated successfully from model electrolytes. The ionic forms used were Li +, Na +, K +, Rb +, Cs + and H + at 25°C in membranes in which electrolyte exclusion was almost complete.