Perfluorosulfonated Ionomer Membranes Research Articles

On-body hypoxia monitor based on lactate biosensors with a tunable concentration range

We report on a noninvasive hypoxia monitor based on a biosensor linearly responding to lactate through its whole concentration range in human sweat. The biosensor is designed with an additional perfluorosulfonated ionomer (Nafion analogue) membrane acting as a diffusion barrier for lactate due to its intrinsic negative charge. The resulting biosensor linearly responds to lactate in the range from 1 to 100 mM and displays significantly improved operational stability, maintaining its initial response during 3 h in human sweat flow. Thus, the elaborated monitor has been validated through on-body continuous sweat analysis during exhaustive physical exercise. In contrast to periodic sweat sampling, the elaborated monitor allows recording of the true extremes of sweat lactate, which are of particular importance for diagnostics.

Nanozymes “Artificial Peroxidase”: Enzyme Oxidase Mixtures for Single‐Step Fabrication of Advanced Electrochemical Biosensors

AbstractWe report the single‐step fabrication of advanced electrochemical biosensors upon drop‐casting of “artificial peroxidase”, that is, oxidase‐containing mixtures. Compared to bienzyme biosensors, we substitute peroxidase with more active and stable nanozymes based on catalytically synthesized Prussian Blue. The electroactivity of nanozymes in the sensing membrane was facilitated with carbon black nanoparticles, which resulted in record sensitivity of the H2O2 transducer exceeding 1.1 A M−1 ⋅ cm−2. Both glucose and lactate oxidases were co‐immobilized with nanozymes in alkoxysilane or perfluorosulfonated ionomer membranes. The proposed drop‐casting approach results in biosensors that are advantageous over those conventionally produced upon layer‐by‐layer immobilization in terms of i) almost an order of magnitude increased sensitivity, ii) unsurpassed biosensor‐to‐transducer sensitivity ratio reaching the value of 0.3, and iii) three times extended operational lifetime.

Effects of melt flow index and equivalent weight on the dimensional stability and mechanical behavior of perfluorosulfonic acid ionomer membranes

The effect of melt flow index (MFI) and equivalent weight (EW) on three aspects relating to perfluorosulfonic acid membrane mechanical durability were studied: dimensional stability, hygral stress during humidity cycles and resistance to rupture under biaxial stresses. Membranes used in this study have nominal EWs of 800 and 700 g/mol –SO3H and MFI ranging from ≫ 10 to 0.005 g/10 min. The total water-uptake, water-uptake per sulfonic acid group, and coefficient of hygral expansion all increase with decreasing EW. For the same EW, the ionomer with lower MFI absorbs less water, swells less and experiences less hygral stress during humidity cycles. Although a 700 EW membrane with an MFI of 0.005 g/10 min swells more than the 800EW ionomers, it is projected to be the most durable because it experiences the least hygral stress during humidity cycles and is the most resistant to rupture under biaxial stress. These findings are supported by in-situ RH-cycling tests, which show a strong inverse correlation between log[MFI] and cycles to failure, and a weaker inverse dependence on EW. The results suggest that high-performing, low-EW ionomers can be mechanically robust enough for fuel cell applications provided they have a sufficiently low MFI.

Cyclic Hygral Swelling and Shrinkage Behavior of Fuel Cell Membranes

Mechanical membrane durability is a key factor for the overall lifetime of PEM fuel cells. It is therefore important to understand the impact of the dynamic membrane hygral swelling and shrinkage characteristics on the in-situ mechanical stresses and the associated mechanical durability. In this work, an in-depth experimental investigation was carried out to determine the cyclic hygral swelling and shrinkage properties of four commonly used types of fuel cell membranes: Nafion® NRE211, two different expanded polytetrafluoroethylene (ePTFE) reinforced perfluorosulfonic acid ionomer membranes, and one hydrocarbon membrane. In addition to the bare membranes, the effect of coating the membranes with catalyst layers on the hygral swelling/shrinkage behavior was also investigated. Tensile specimens were subjected to ex-situ hydration and dehydration cycles using a dynamic mechanical analyzer (TA Instruments Q800 DMA) equipped with an environmental chamber, wherein the dynamic swelling and shrinkage characteristics as well as the residual stresses were measured for each membrane by displacement sensor and load cell, respectively. Hydrocarbon and reinforced PFSA ionomer membranes revealed higher shrinkage than swelling at each RH cycle, resulting in an overall shrinkage. Inside a fuel cell stack where membrane swelling and shrinkage is constrained, the observed shrinkage would result in internal tensile stress in the membrane and adjacent components, and hence reduce the overall membrane mechanical durability. The accumulation of residual stress during successive RH cycles was therefore measured explicitly using a custom-designed constrained shrinkage test and compared to the tensile properties of each individual membrane. For example, the hydrocarbon membranes analyzed in this work showed large residual stresses (severe contraction) during confined shrinkage at 80oC that exceeded their yield stress, and are therefore deemed prone to failure after repeated RH cycles. In addition, the hygral swelling and shrinkage behavior of each membrane was compared to the corresponding catalyst-coated membranes and the reinforcement offered by the catalyst layers was determined. The identified swelling and shrinkage properties, generalized into three different categories of overall cyclic behavior, have vital implications for the development of durable fuel cell membranes and membrane electrode assemblies. In addition to providing good mechanical strength, the membranes are also expected to accommodate or preferably eliminate progressive shrinkage stresses during dynamic fuel cell operation.

Theory of Multi-Ion Transport and Thermodynamic Phenomena in Solvent-Filled Membranes

Solvent-filled membranes, such as Nafion, are an important class of polymer electrolytes because the imbibed solvent imbues the material with high ionic conductivity, while still allowing the membrane to maintain its mechanical stability. Both of these attributes are essential for membrane electrolytes in numerous energy-storage and -conversion devices including redox-flow batteries. The transport phenomena is complicated due to the multiple ions in the electrolytes employed in these applications and the higher concentrations desired for energy density. Furthermore, while there is understanding on the single-ion transport in these materials, there is a need to develop a self-consistent theory for describing the ion transport and ion partitioning and thermodynamics when interacting with the charged polymer membrane. Moreover, there are few quantitative material design guides for polymers that balance the movement of certain species that are desirable, such as current-carrying ions, against transport of contaminants, additives, or redox-active species, which decrease device performance. To address the above gaps and provide material design insights, this work describes the theory of multi-ion transport in polymer-electrolyte membranes. To specify the thermodynamics of the system, we formulate the membrane and electrolyte solution free energies. The model accounts for long-range electrostatic and short-range solvation and physical interactions between ions. Externally validated solution and membrane properties parameterize the model. The model calculations agree with measured water and ion uptake in dilute and concentrated binary and ternary salt electrolytes, with the uptake being a balance between osmotic and elastic pressures that are impacted by ion specificity and membrane pretreatment.1-2 To model the transport phenomena, we use Stefan-Maxwell-Onsager theory. To connect performance to the molecular-level structure of the polymer, hydrodynamic theory provides constitutive relations for the involved ion/solvent and ion/ and solvent/polymer friction coefficients, with classical porous-media theories scaling tortuosity. The model compares favorably to a number of published measured membrane transport properties1-2 (i.e. conductivity, transference numbers, electroosmosis, and water-transport coefficient) in dilute and concentrated binary and ternary electrolytes. Importantly, the model isolates the contribution of each type of molecular-scale to the measured properties. Acknowledgements: This study was funded with support of the Fuel Cell Performance and Durability Consortium (FC-PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02- 05CH11231 and, in part, by the Advanced Research Projects Agency - Energy (ARPA-E), U.S. Department of Energy (DOE) under Award Number DEAR0000149 . References Okada, T.; Satou, H.; Okuno, M.; Yuasa, M., Ion and Water Transport Characteristics of Perfluorosulfonated Ionomer Membranes with H+ and Alkali Metal Cations. J. Phys. Chem. B 2002, 106 (6), 1267-1273.Tang, Z.; Svoboda, R.; Lawton, J. S.; Aaron, D. S.; Papandrew, A. B.; Zawodzinski, T. A., Composition and Conductivity of Membranes Equilibrated with Solutions of Sulfuric Acid and Vanadyl Sulfate. J. Electrochem. Soc. 2013, 160 (9), F1040-F1047.

Characterization of Membrane Degradation Growth in Fuel Cells Using X-ray Computed Tomography

Perfluorosulfonic acid ionomer membranes are subjected to simultaneous chemical and mechanical degradation under fuel cell operation. Despite the importance of membrane durability, the understanding of its structural degradation and failure modes has been considerably restricted by conventional 2D imaging. In this work, non-invasive micro X-ray computed tomography (XCT) is adopted to visualize the 3D membrane decay at different life stages during combined chemical and mechanical degradation. A detailed survey exhibits damage density of 6 and 10 cracks per mm2 observed at the near-final and final end of life stages respectively. Through-thickness membrane cracks with unbranched I-shaped cracks and Y- and X- shaped cracks with one and two branches respectively are observed. The observed damage development at each life stage is correlated to supplementary diagnostic data including hydrogen leak rate, open circuit voltage, and tensile strength. In particular, large X-shaped cracks formed due to embrittlement from underlying chemical degradation are deemed to have a critical impact on the eventual failure development by facilitating large hydrogen leaks. Overall, the comprehensive 3D perspective enabled by XCT imparts new knowledge pertaining to the degradation process, and could also be extended to other fuel cell failure modes and degradation mechanisms.

Observation of Photovoltaic Action from Photoacid-Modified Nafion Due to Light-Driven Ion Transport.

Replacing passive ion-exchange membranes, like Nafion, with membranes that use light to drive ion transport would allow membranes in photoelectrochemical technologies to serve in an active role. Toward this, we modified perfluorosulfonic acid ionomer membranes with organic pyrenol-based photoacid dyes to sensitize the membranes to visible light and initiate proton transport. Covalent modification of the membranes was achieved by reacting Nafion sulfonyl fluoride poly(perfluorosulfonyl fluoride) membranes with the photoacid 8-hydroxypyrene-1,3,6-tris(2-aminoethylsulfonamide). The modified membranes were strongly colored and maintained a high selectivity for cations over anions. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and ion-exchange measurements together provided strong evidence of covalent bond formation between the photoacids and the polymer membranes. Visible-light illumination of the photoacid-modified membranes resulted in a maximum power-producing ionic photoresponse of ∼100 μA/cm2 and ∼1 mV under 40 Suns equivalent excitation with 405 nm light. In comparison, membranes that did not contain photoacids and instead contained ionically associated RuII-polypyridyl coordination compound dyes, which are not photoacids, exhibited little-to-no photoeffects (∼1 μA/cm2). These disparate photocurrents, yet similar yields for nonradiative excited-state decay from the photoacids and the RuII dyes, suggest temperature gradients were not likely the cause of the observed photovoltaic action from photoacid-modified membranes. Moreover, spectral response measurements supported that light absorption by the covalently bound photoacids was required in order to observe photoeffects. These results represent the first demonstration of photovoltaic action from an ion-exchange membrane and offer promise for supplementing the power demands of electrochemical processes with renewable sunlight-driven ion transport.

Cerium Ion Mobility and Diffusivity Rates in Perfluorosulfonic Acid Membranes Measured via Hydrogen Pump Operation

Ion mobility and diffusivity coefficients were determined for cerium ions in Nafion XL perfluorosulfonic acid ionomer membranes at 100% and 50% relative humidity in a conductivity cell using a hydrogen pump. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). In order to decouple simultaneous effects of Ce ion mobility and back-diffusion which occur due to potential and concentration gradients, respectively, a one-dimensional model was developed and fit to these intermittent XRF profiles. The resulting mobility and diffusivity coefficients demonstrate the dramatic effects of potential and concentration gradients on Ce ion migration during PEM fuel cell operation.

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.

Characterization of polymer electrolyte membranes for application in glucose fuel cells

We report studies into the effects of electrolyte membrane characteristics on power generation in glucose fuel cells. Commercially available perfluo-rosulfonate ionomer membranes of various thicknesses were characterized by quantitatively measuring and analyzing electrical power generation. Characterization suggested that thinner Nafion® membranes could lead to improved performance (higher peak power, lower ohmic loss, and lower activation loss) over thicker membranes, however extremely thin membranes gave low performance due to their lack of stability, stemming from fabrication failure. The dependency on fuel concentration was also identified and a glucose concentration of 1 M gave the best performance. Finally, the prepared ion-exchange polymer composite membrane (formed by sorbing a Nafion® membrane into a polycarbonate track-etched (PCTE) membrane) was tested, and it was observed that cell performance was remarkably improved when membrane porosity was considered quantitatively. Our study is expected to be useful in the design of biofuel cells that can be utilized in low-concentration environments, such as living or-ganisms.

Performance Study on Magnetic Field Inducing Perfluorosulfonate Ionomer Membranes for PEMFC

A through-plane oriented perfluorosulfonate ionomer (PFSI) membrane was fabricated using a magnetic field inducing method. The magnetic field inducing PFSI membrane showed improved fuel cell performance compared to the pristine PFSI and Nafion 212 membranes under the investigated conditions. Especially under low humidity, the M-PM had a superior cell performance over the pristine membrane with a maximum power density more than twice of the latter. The hydrogen permeability of the magnetic field inducing membrane was higher than the pristine membrane under high humidity but much lower than the latter under low humidity. The cell resistance of proton exchange membrane fuel cell (PEMFC) with the magnetic field inducing membrane was lower than that observed with the pristine membrane in the testing range of relative humidity (RH), especially under low humidity. Therefore, the advantages of cell resistance under low humidity brought about the superior fuel cell performance, which may be attributed to the through-plane oriented structure in the magnetic field inducing membrane. In addition, the tensile strength of the magnetic field inducing membrane remained unchanged compared to the pristine membrane. The high performance of the magnetic field inducing PFSI membrane makes it a competitive candidate for PEMFC applications.

Interplay between α-relaxation and morphology transition of perfluorosulfonate ionomer membranes

Nafion α-relaxation is the subject of intense investigations as it regulates the performance of electric actuators and polymer electrolyte fuel cells. Dielectric spectroscopy and atomic force microscopy measurements of Nafion membranes allow identifying the conformation transition of the polymeric aggregates as the process underlying the α-transition. The dielectric permittivity curves of Nafion membranes show that for relative humidity higher than ∼60% and for temperatures higher than ∼120 °C, the α-relaxation displaces to lower frequencies. Such unusual behavior is attributed to an elongation of Nafion polymeric aggregates occurring at high temperatures (T > 120 °C) and is in perfect agreement with morphological changes inferred from atomic force microscopy analyses.

Radically coarse-grained approach to the modeling of chemical degradation in fuel cell ionomers.

We present a kinetic model of chemical degradation in perfluorosulfonic acid ionomer membranes. It accounts for pathways of radical formation along with mechanisms of ionomer degradation through radical attack. Simplifications in the set of model equations leads to analytical expressions for the concentration of hydroxyl radicals as a function of initial concentrations of iron ions and hydrogen peroxide. The coarse-grained ionomer degradation model distinguishes units that correspond to ionomer head groups, trunk segments of ionomer side chains, and backbone segments between two side chains. A set of differential equations is formulated to describe changes in concentrations of these units. The model is used to study the impact of different degradation mechanisms and ionomer chemistries on fluorine loss and change in ion exchange capacity. Comparison of the model with experimental degradation data for Nafion and Aquivion membranes allows rate constants of degradation processes to be determined. Results of these analyses are discussed in view of strategies to mitigate chemical degradation of ionomer membranes.

Origin of α and β relaxations of Nafion.

Our experiments disentangle the low and high frequency dispersions in perfluorosulfonate ionomer solutions and membranes, providing a reasonable model for understanding these relaxations. Dielectric spectroscopy (DS) and small angle x-ray scattering (SAXS) measurements revealed that the dielectric relaxations observed at low (α relaxation) and high (β relaxation) frequencies show typical features of the longitudinal and radial polarization, respectively, of rodlike polymeric aggregates. Such relaxations were attributed to counterion fluctuations in the vicinity of sulfonic acid groups, in resemblance with polyelectrolytes. Characteristic correlation lengths calculated from both DS and SAXS data are in good agreement adding further evidence to the proposed model. Such description provides insights for the understanding of the crossover from polyelectrolytes, dominated by charge repulsion, to ionomers, dominated by dipolar attraction.

On the constitutive relations for catalyst coated membrane applied to in-situ fuel cell modeling

The elastic–viscoplastic behavior of catalyst coated membranes (CCMs) used in polymer electrolyte membrane fuel cells is investigated in this work. Experimental results reveal significant differences between the mechanical properties of a pure perfluorosulfonic acid ionomer membrane and the corresponding CCM under uniaxial tension and cyclic loading. An elastic–viscoplastic constitutive model that is capable of capturing the time dependent response of the CCM at different humidity and temperature conditions is developed and validated against ex-situ experimental results. The validated model is then utilized to simulate the in-situ mechanical response of the CCM when treated as a composite object bonded through the ionomer phase. When compared to a conventional membrane model, the CCM model predicts considerably lower maximum stress and higher plastic strain under typical fuel cell operating conditions and improved plastic strain recovery during hygrothermal unloading. These results reflect the weaker nature of the CCM material which yields at a lower stress than the membrane and may lead to elevated plastic deformation when exposed to hygrothermal cycles in a constrained fuel cell environment. Hence, coupled CCM implementation is generally recommended for finite element modeling of fuel cells.

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.

Evidence for a crystallite-rich skin on perfluorosulfonate ionomer membranes

Perfluorosulfonate ionomer (PFSI) membranes are widely used in electrochemical applications such as fuel cells, chlor-alkali cells, and actuators/sensors. In this work, the surface crystallization of a solution-cast PFSI membrane is quantitatively characterized for the first time by synchrotron grazing incidence X-ray diffraction (GIXRD) and small angle X-ray scattering (GISAXS). A crystallite-rich skin is observed on the PFSI membrane surface, which is 4 nm thick, possesses a crystallinity which is 40% higher than the bulk, with the crystallites aligned parallel to the membrane surface. An intermediate layer exists between the skin and the bulk. The discovery of a crystallite-rich skin may help to explain the surface properties of PFSI membranes, and facilitate an understanding of the transport properties across the membrane interface.

Properties of precursor solution cast PFSI membranes with various ion exchange capacities and annealing temperatures

Perfluorosulfonated ionomer (PFSI) membranes were successfully prepared through a novel perfluorinated sulfonyl fluoride (PFSF) solution casting method. Compared with those manufactured by traditional ways, these membranes have a more ordered inner structure, which give them a higher ionic conductivity and chemical stability, as well as a better methanol crossover resistance. It is crucial to choose suitable ion exchange capacity (IEC) values and preparation conditions for the PFSI membrane manufacture, however, related data are unavailable for this newly discovered technique. Herein, to optimize the preparation conditions, membrane properties, including dimensional stability, water uptake, proton conductivity, solubility and methanol permeability, that vary with IEC values and annealing temperatures were systematically investigated. It is found that an IEC value between 0.94 and 1.02 meq g−1 and an annealing temperature higher than 115 °C are preferred. The result provides first-hand data for the high quality PFSI membrane preparation approach and will be instructive for future membrane studies and applications.

Perfluorosulfonate ionomer membranes with improved through-plane proton conductivity fabricated under magnetic field

Magnetic field treated perfluorosulfonate ionomer (PFSI) membranes (M-PMs) with improved through-plane proton conductivity are facilely fabricated though solvent casting of PFSI/Fe3O4 nanocomposite dispersion without any third additive under magnetic field followed by discarding the Fe3O4 nanoparticles. In the PFSI/Fe3O4 nanocomposite membrane, Fe3O4 nanoparticles are clearly uniaxially aligned by magnetic field. Subsequently, M-PMs are obtained by removing Fe3O4 from the nanocomposite membranes to eliminate the negative effect of Fe3O4 on proton conducting. The effect of magnetic field treatment on M-PMs' performance is investigated by both in- and through-plane proton conductivity. The results demonstrate that the through-plane proton conductivity of the M-PMs prepared from PFSI/Fe3O4 nanocomposite membrane with optimized Fe3O4 content is enhanced, while the in-plane proton conductivity is reduced compared to the pristine PFSI membrane, which indicate through-plane anisotropic phenomenon of proton conducting in the membranes. Such magnetic field treated PFSI membranes exhibit good appropriateness for using in electrochemical applications, for example fuel cells.

Biocompatibility and anti-cracking performance of perfluorocarboxylic acid ionomer membranes for implantable biosensors

Perfluorosulfonate ionomer (PFSI) membranes can be used to protect biosensors in biological environment. Perfluorocarboxylate ionomer (PFCI) has similar structure to PFSI, and is thus expected to be applicable in vivo. In this study, biocompatibility and anti-cracking performance of PFCI membranes were first investigated. Among various kinds of PFCI membranes, the Na+-type exhibited best blood compatibility, as evidenced by the human platelet adhesion and hemolysis rate experiments. When cultured on Na+-type PFCI membranes, endothelial cells spread and proliferated to colonize the entire surface, indicating good cell adhesion activity of the membranes. The Na+-type membranes were also subcutaneously implanted into guinea pigs, whose incisions were healed after operation without significant rejection phenomenon. After incubation in cell culture medium for 12 weeks, PFCI membranes kept intact only with trace amount of calcium phosphate deposition. XRD and SAXS results demonstrated that PFCI membranes possessed higher crystallinity and smaller ion cluster size than PFSI membranes, thereby endowing the membranes with much better anti-cracking performance. The excellent biocompatibility and anti-cracking performance of Na+-type PFCI membranes make them promising implantable biomaterials and protective layers in biosensors.