Side Chain Perfluorosulfonic Acid Research Articles

All-Perfluorosulfonated-Ionomer Composite Membranes Containing Blow-Spun Fibers: Effect of a Thin Fiber Framework on Proton Conductivity and Mechanical Properties.

In this study, thin fiber composite polymer electrolyte membranes (PEMs) were prepared using short side-chain perfluorosulfonic acid (PFSA) ionomers, Aquivion, to create composite PEMs with improved proton conductivity and improved mechanical properties. PFSA thin fiber webs prepared by blow spinning and successive hot pressing were used as the porous substrate. Herein, PFSA ionomers were used for both the substrate and the matrix of the composite PEMs, and their structures, properties, and fuel cell performance were characterized. By adding the PFSA thin fiber webs to the matrix, the proton conductivity was enhanced and the mechanical properties were slightly improved. The prepared PFSA thin fiber composite PEM showed better FC performance than that of the pristine PFSA one for the high-temperature low-humidity condition in addition to the low-temperature high-humidity one. To the best of our knowledge, this is the first report on the all PFSA composite membranes containing a PFSA thin fiber framework.

Short side chain perfluorosulfonic acid composite membrane with covalently grafted cup stacked carbon nanofibers for polymer electrolyte fuel cells

Efficient proton conduction through polymer electrolyte membrane is critical in the performance of low temperature polymer electrolyte fuel cells (LT-PEFC). Herein, polystyrene sulfonic acid grafted cup stacked carbon nanofiber (PSSA-g-CSNF) incorporated short side chain (SSC)-perfluorosulfonic acid (PFSA) ionomer composite polymer electrolyte is discussed. An optimum level of PSSA-g-CSNF introduced as an additive to SSC-PFSA matrix exhibit higher proton conductivity, improved mechanical, chemical stability, and limited hydrogen crossover towards PEFC application in comparison to recast pristine SSC-PFSA membranes (Aquivion®-720). Maximum current density of 2.5 A cm−2 at 0.5 V and peak power density of 1.4 W cm−2 under H2 and O2 condition is seen for the optimized nanocomposite membrane. Besides, the peak power density of the nanocomposite membrane shows an enhancement higher than 22% and 42% in relation to recast pristine Aquivion and Nafion212 respectively. In addition to this, nanocomposite membrane shows higher chemical stability and retention of 80% initial OCV even after 100 h of accelerated stress test (AST) as compared to 73% for pristine Aquivion. Uniform dispersion of PSSA-g-CSNF in Aquivion facilitates facile proton transport by limiting fuel cross-over via hydrogen bonding interaction with closely spaced hydrophilic groups in the membrane matrix in LT-PEFC.

Improved Thermo-Mechanical Properties and Reduced Hydrogen Permeation of Short Side-Chain Perfluorosulfonic Acid Membranes Doped with Ti3C2Tx.

Benefiting from its large specific surface with functional -OH/-F groups, Ti3C2Tx, a typical two-dimensional (2D) material in the recently developed MXene family, was synthesized and used as a filler to improve the properties of the short side-chain (SSC) perfluorosulfonic acid (PFSA) proton exchange membrane. It is found that the proton conductivity is enhanced by 15% while the hydrogen permeation is reduced by 45% after the addition of 1.5 wt% Ti3C2Tx filler into the SSC PFSA membrane. The improved proton conductivity of the composite membrane could be associated with the improved proton transport environment in the presence of the hydrophilic functional groups (such as -OH) of the Ti3C2Tx filler. The significantly reduced hydrogen permeation could be attributed to the incorporation of the impermeable Ti3C2Tx 2D fillers and the decreased hydrophilic ionic domain spacing examined by the small angle X-ray scattering (SAXS) for the composite membrane. Furthermore, improved thermo-mechanical properties of the SSC/Ti3C2Tx composite membrane were measured by dynamic mechanical analyzer (DMA) and tensile strength testing. The demonstrated higher proton conductivity, lower hydrogen permeation, and improved thermo-mechanical stability indicate that the SSC/Ti3C2Tx composite membranes could be a potential membrane material for PEM fuel cells operating above the water boiling temperature.

Wire based electrospun composite short side chain perfluorosulfonic acid/polyvinylidene fluoride membranes for hydrogen-bromine flow batteries

A main component of a hydrogen-bromine flow battery (HBFB) is the ion exchange membrane. Available membranes have a trade-off between the major requirements: high proton conductivity, low bromine species crossover, and high mechanical and chemical stability. To overcome this, electrospinning of a highly proton conductive polymer (short side chain perfluorosulfonic acid (SSC PFSA)) and a hydrophobic inert polymer (polyvinylidene fluoride (PVDF)) was used to electrospin composite polymer fiber mats. Piles of multiple mats were hot pressed resulting in dense ion exchange membranes. Membranes with three different SSC PFSA/PVDF ratios were prepared, characterized, and subjected to short and long term (1500 h) HBFB testing. The electrospun membranes have performances very comparable to those of commercial membranes. For the SSC PFSA/PVDF electrospun membrane, a higher SSC PFSA loading gives a higher membrane proton conductivity compared to a lower loading, but at the expense of a higher bromine species crossover.The SSC PFSA/PVDF (50/50 wt%) membrane shows a coulombic efficiency of 98%, a voltaic efficiency of 80% and an initial available capacity of 105 Ah L−1 at a current density of 150 mA cm−2, which equals that of the current benchmark long side chain PFSA membrane. This performance is constant over 200 cycles during 2 months of continuous HBFB operation.

In situ long-term membrane performance evaluation of hydrogen-bromine flow batteries

Because of the impracticality of long life time testing of hydrogen bromine flow batteries (HBFBs) under real, cyclic operating conditions, we utilized a high-frequency cycling load accelerated lifetime test (ALT) to evaluate the membrane electrode assembly performance in HBFBs. Four different membrane chemistries were tested to assess the relative long-term performance and durability of the corresponding HBFBs and to understand the main long-term failure mechanism in HBFB technology. The high-frequency cycling load ALT is a highly valuable method to assess long-term HBFB (components) stability and to understand the degradation/failure mechanism. The results showed that the utilization of long side chain perfluorosulfonic acid (LSC PFSA, i.e. Nafion®) membranes result in stable HBFB operation until a sudden cell failure at the end of the cycle life. The use of a more selective (lower bromine species crossover) reinforced LSC PFSA membrane results in approx. five times longer life times. In contrast, the use of a grafted polyvinylidene fluoride (SPVDF) membrane results in a slow incremental performance decrease (or area resistance increase) during the ALT and, again, a sudden cell failure at the end of the cycle life. After the ALT, the LSC PFSA membrane still shows high chemical stability. On the other hand, the grafted SPVDF and SPE membranes show clear membrane degradation visible as a strong decrease in IEC and an increase in ohmic AR. Gradual Pt catalyst degradation-dissolution, that results in insufficient Pt catalyst loading and subsequently low hydrogen reaction kinetics, is the main failure mechanism of the HBFBs. The membrane bromine species crossover rate is directly related to the rate of Pt catalyst degradation-dissolution and scales almost linearly with the cell total ampere-hours. As long as the catalyst loading is sufficient and does not reach a minimum value, this observed Pt degradation-dissolution rate does not significantly impact the cell performance.

Study of Annealed Aquivion® Ionomers with the INCA Method †.

We investigated the possibility to increase the working temperature and endurance of proton exchange membranes for fuel cells and water electrolyzers by thermal annealing of short side chain perfluorosulfonic acid (SSC-PFSA) Aquivion® membranes. The Ionomer nc Analysis (INCA method), based on nc/T plots where nc is a counter elastic force index, was applied to SSC-PFSA in order to evaluate ionomer thermo-mechanical properties and to probe the increase of crystallinity during the annealing procedure. The enhanced thermal and mechanical stability of extruded Aquivion® 870 (equivalent weight, EW = 870 g·mol−1) was related to an increase of long-range order. Complementary differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements confirmed the increase of polymer stiffness by the annealing treatment with an enhancement of the storage modulus over the whole range of temperature. The main thermomechanical relaxation temperature is also enhanced. DSC measurements showed slight base line changes after annealing, attributable to the glass transition and melting of a small amount of crystalline phase. The difference between the glass transition and melting temperatures derived from INCA plots and the ionic-cluster transition temperature derived from DMA measurements is consistent with the different experimental conditions, especially the dry atmosphere in DMA. Finally, the annealing procedure was also successfully applied for the first time to an un-crystallized cast membrane (EW = 830 g·mol−1) resulting in a remarkable mechanical and thermal stabilization.

Ionomer and protein size analysis by analytical ultracentrifugation and electrospray scanning mobility particle sizer.

By combining analytical ultracentrifugation (AUC) in liquid phase and scanning mobility particle sizer (SMPS) in the gas phase, additional information on the particle size and morphology has been obtained for rigid particles. In this paper, we transfer this concept to soft particles, allowing us to analyze the size and molar mass of the short side chain perfluorosulfonic acid ionomer Aquivion® in a dilute aqueous suspension. The determination of the primary size and exact molar mass of this class of polymers is challenging since they are optically transparent and due to the formation of different aggregate structures depending on the concentration and solvent properties. First, validation of AUC and SMPS measurements was carried out using the well-defined biopolymers bovine serum albumin (BSA) and lysozyme (LYZ) to confirm the reliability of the results of the two unique and independent classifying methods. Then, the ionomer Aquivion® was studied using both techniques. From the mean molar mass of 185 ± 14kDa obtained by AUC, a mean hydrodynamic diameter of 7.6 ± 0.5nm was calculated. The particle size obtained from SMPS (7.1nm) agrees very well with the results from AUC showing that the molecule was transferred into the gas phase without significantly changing its structure. In conclusion, the Aquivion® is molecularly dispersed in the used aqueous buffer solution without any aggregate formation in the investigated concentration range (< 2gl-1).

Water transport through short side chain perfluorosulfonic acid ionomer membranes

Liquid and water vapour transport through short side chain (SSC) perfluorosulfonic acid (PFSA) ionomer membranes are measured as a function of membrane thickness (24–96µm) and temperature (25–85°C). Three types of water permeation techniques are used in this work and are based on applied chemical potential gradients across a membrane. These are: liquid-liquid permeation (LLP) in which both sides of membrane are in contact with liquid water; liquid-vapour permeation (LVP) where one side of the membrane is exposed to liquid and the other is exposed to vapour; and vapour-vapour permeation (VVP) where both sides of the membrane are exposed to water vapour. Within the SSC series, the increase in permeance for VVP upon decreasing membrane thickness is not as significant as compared to LLP due to the larger interfacial water transport resistance (Rinterfacial) observed for membranes exposed to vapour. The study of LVP and VVP transport resistances indicates that Rinterfacial plays dominate role in determining the overall membrane resistance, even for a thick SSC membrane. It is speculated that Rinterfacial is the result of water depletion layer caused by membrane dehydration at the membrane interface. The LVP interfacial resistance for SSC membranes is similar to that found for long side chain PFSA ionomer membranes such as, Nafion®, even though SSC membranes possess significant lower internal LVP permeation resistance. Water permeance determined by LLP, LVP and VVP measurements are all found to increase with temperature, due to the increased water volume fraction (Xv) and an increase in the effective hydrophilic pore size. The activation energy of LLP, LVP and VVP water permeation are calculated to be 16.3, 40.2 and 43.1kJmol−1 respectively, which is lower than that of Nafion under similar conditions.

Morphological Differences in Short Side Chain and Long Side Chain Perfluorosulfonic Acid Proton Exchange Membranes at Low and High Water Contents

Ionomer membrane materials such as Dupont’s Nafion® (a sulfonated tetrafluoroethylene based fluoropolymer) have long been used as the proton conducting medium in proton exchange membrane (PEM) fuel cells. It has recently been shown that short side chain perfluorosulfonic acid membranes such as Hyflon Ion® developed by Solexis can outperform the benchmark long side chain material, DuPont’s Nafion®. Despite these promising results, a detailed understanding of the relationship between membrane morphology, hydration, and fuel cell performance has yet to be achieved. Using atomic force microscopy (AFM) phase imaging over a wide range of relative humidity and in solution, we compare a Nafion 212 membrane to Hyflon Ion® E87-05 and find evidence of the improved performance in the latter. Hyflon exhibits a higher hydrophilic surface area across all water contents, explaining its higher proton conductivity and fuel cell performance. At high water contents, the surface of Nafion® undergoes significant structural rearrangement and forms large rod-like features parallel to the surface, whereas the Hyflon Ion® membrane undergoes little if any such transition. [see the figure] We show using conductive AFM that these features are insulating, which indicates they most likely consist of a fluorocarbon layer surrounding a hydrophilic core and do not contribute to fuel cell performance. The ability of Hyflon to undergo changes in water content with minimal rearrangement of hydrophilic domains implies an ability to operate more effectively under variable relative humidity.

Lifetime Prediction Approach Applied to the Aquivion™ Short Side Chain Perfluorosulfonic Acid Ionomer Membrane for Intermediate Temperature Proton Exchange Membrane Fuel Cell Application

AbstractDegradation and durability studies of the short side chain perfluorosulfonic acid ionomer membrane, Aquivion™, of Solvay Specialty Polymers, are described in a fuel cell operation beyond 100 °C. Specific electrochemical accelerated stress tests under a dynamic electric load cycling profile were developed to submit the membrane to hydration–dehydration cycles, while the effect of operation temperature was also investigated to reproduce typical power demands and/or thermal gradient periods likely to be encountered during seasonal variation of a micro‐combined heat and power system. Aquivion™ is a high performance membrane which is shown in this study to sustain operation until 110 °C, higher than was possible with Nafion® under similar conditions. Post‐test observations studies by electron microscopy and ex situ tensile measurements bring to light modifications of membrane thickness and mechanical properties that rationalize an upper operating temperature of 120 °C. Membrane thermal annealing is shown to increase lifetime with the ageing procedures used. A predictive lifetime model is proposed by correlating the estimated lifetime values and the experimental data via a lifetime coefficient.

Morphological differences in short side chain and long side chain perfluorosulfonic acid proton exchange membranes at low and high water contents

Short side chain (SSC) perfluorosulfonic acid membranes such as Hyflon Ion® have shown improved proton conductivity and the ability to outperform the benchmark long side chain (LSC) material, Nafion®. Using atomic force microscopy phase imaging over a wide range of relative humidity, we compare the morphology of Nafion to Hyflon and find evidence for the improved performance in the latter. While the surface of Nafion undergoes significant structural rearrangement and forms large worm-like features at high water contents, Hyflon undergoes little, if any, transition. We also provide evidence of improved water retention in Hyflon at low water contents, explaining the improved conductivity under these conditions. Liquid phase imaging in water and methanol solutions was used to probe the extremes of swelling in both membranes and conductive AFM was used to characterize electrochemical activity at the membrane surface.

Polymer electrolyte membrane water electrolyser with Aquivion® short side chain perfluorosulfonic acid ionomer binder in catalyst layers

The Aquivion® short-side-chain (SSC) perfluorosulfonic acid (PFSA) ionomer was adopted in catalyst layers (CL) of polymer electrolyte membrane water electrolysers (PEMWE) instead of long-side-chain (LSC) Nafion® ionomer. The effects of SSC ionomer content in CL for oxygen evolution reaction were studied in half cell with cyclic voltammetry and steady state linear sweep. In a single cell test the MEA with SSC-PFSA Aquivion® ionomer exhibited better thermal stability than the one with LSC-PFSA Nafion® ionomer at 90 °C. The cell voltage at a current density of 1 A cm−2 was 1.63 V at 90 °C using the SSC-PFSA Aquivion® ionomer binder, Nafion® 117 membrane, and without back pressurizing. In a continuous operation the cell voltage degradation rate of the MEA using Aquivion® ionomer binder was only about 0.82 mV h−1.

Short side chain perfluorosulfonic acid membranes and their composites with nanosized zirconium phosphate: hydration, mechanical properties and proton conductivity

Aqueous dispersions of a short side chain perfluorosulfonic acid ionomer (equivalent weight 830 g eq.−1) and gels of nanosized zirconium phosphate (ZrP) in propanol are used for the preparation, by solution casting, of composite membranes with ZrP loading up to 13 wt%. These membranes, together with reference neat ionomer membranes, are characterized by mechanical stress–strain tests and in-plane conductivity determinations under different conditions of temperature and relative humidity (RH). The membrane hydration is also determined under the environmental conditions of mechanical and conductivity measurements. The conductivity of the neat ionomer membrane, at 90 and 120 °C, is weakly dependent on temperature but strongly influenced by changes in RH going from values around 0.02 S cm−1 at 25% RH to values around 0.25 S cm−1 at 90% RH. The conductivity of the composite membranes decreases with increasing filler content being however in the range of 0.01–0.02 S cm−1 at 25% RH and in the range of 0.16–0.23 S cm−1 at 90% RH at both temperatures. On the other hand the presence of the filler results in a significant increase in the Young's modulus (up to 80%) and in the yield stress (up to 124%) not only under ambient conditions but also at 80 °C and 80% RH.

Enhanced Activity of Ir-V as Anode Catalyst for PEMFC at Elevated Temperature

Using shorter side chain perfluorosulfonic acid (PFSA) membrane, PEMFC can be operated at elevated temperature up to 95åC. Five synthesized catalysts, with different Ir:V ratios, made via an ethylene glycol (EG) method were used as the anode catalysts respectively, with a loading of 0.4 mg (Ir) cm-2. With 40%Ir-10%V anode catalyst, the highest power density, 728 mW cm-2, can be reached under 95åC for a 50cm2 H2/air single cell. This is much higher than that with Pt/C as the anode catalyst and this trend is further proved by linear sweep voltammetry (LSV). XRD patterns show that mean 2.7nm particle size has been obtained, and the addition of V to Ir/C may decrease the lattice parameter of Ir (fcc) crystal.

Dielectric relaxation studies of water-containing short side chain perfluorosulfonic acid membranes

The storage and loss components of the complex dielectric permittivities of short side chain perfluorosulfonic acid membranes were determined as a function of water content, temperature, and membrane equivalent weight. As in our previous studies of electrolyte-containing long and short side chain perfluorosulfonate ionomers, a low-frequency graphical feature on isothermal loss spectra was identified that is suggested to be reflective of, and that crudely quantifies the tortuousity of, long-range (intercluster) protonconductive pathways