Perfluorosulfonic Acid Polymer Research Articles

An Optimization Study of Nafion-1,2,3-Triazole - H3PO4 Composite Membrane for High Temperature- Non Humidity Condition of PEMFC

The proton exchange membrane fuel cells (PEMFCs) are receiving increasing attention as one of the clean energy and suitable primary power sources for transportation and stationary applications. The proton exchange membrane of recent PEMFCs is commonly used perfluorosulfonic acid polymer membranes (e.g., Nafion). This polymeric membrane is depend on the presence of water to solvate the protons from the sulfonic acid groups. Consequently, the operational cell temperature is limited to below 100oC. However, the PEMFC when operating at high temperature (above 120oC) is having numerous advantages because include increased reaction kinetics, easier water management and a better tolerance to CO.In this study, we examined Nafion-based composite membrane that can be used at high temperature and non-humidity condition. A novel Nafion-based composite membrane with 1,2,3-triazole and phosphoric acid was prepared by autoclave solution processing [1-3]. Nafion-1,2,3-triazole-H3PO4 (Na-Tri-PA) composite membranes were used as an electrolyte membrane for PEMFC at non-humidity condition of 150oC. Here, we obtained homogeneous membrane as adding 1,2,3-triazole in the Na-PA membrane, and from an optimization process for MEA, Na-Tri-PA composite membrane showed the power density of about 70mW/cm2.AcknowledgementsThis work was partially supported by MEXT program for development of environmental technology using nanotechnology.[1] J. –D. Kim, M. –S. Jun, C. Nishimura, Y. –W. Choi, ECS transactions, 35, 241 (2011).[2] J.-D. Kim, A. Suzuki, M.-S. Jun, ECS transactions, 58, 1185 (2013).[3] M.-S. Jun, J.-D. Kim, ECS transactions, 58, 1195 (2013).

High-Resolution Mapping of the PFSA Polymer Distribution in PEFC Electrode Layers

Perfluorosulfonic acid (PFSA) ionomer is a key component within the electrode layers of polymer electrolyte fuel cells (PEFCs) since this solid electrolyte provides an efficient proton transport path to the active catalyst sites. Both the average loading on a 100-nm scale as well as the actual film thickness distribution on a 1-nm scale of the PFSA ionomer in the porous layer structure are critical to electrode performance so quantitative measurements of both properties are highly desired.Scanning transmission electron microscopy (STEM) is one useful tool to map this PFSA distribution in the electrode layer [1]. However, it has long been recognized that fluorinated compounds are sensitive to electron beam radiation damage [2,3]. By maintaining an electron dose below 1x104 e-/nm2 at 100-nm lateral resolution and cooling the specimen below -100ºC, the total fluorine mass loss for Nafion® ionomer is held below 10% a/a whereby the electron beam damage threshold for a typical Pt/C electrode layer is improved by 2-3 orders-of-magnitude at this sample temperature. Atomic force microscopy (AFM) [4] is a complementary tool suitable to map the Nafion® coverage on a 1-nm length scale on the electrode surface. The adhesion of the AFM tip to Nafion® in air was measured to be very low as the fluorocarbon backbone was preferentially exposed at polymer-air interface. In this paper, we present ionomer mapping measurements of two common PEFC electrode constructions. Figure 1 shows Pt/C/Nafion® layers at 8 μm dry thickness that are coated from the same electrode ink onto either a decal substrate (CCD) or directly onto the gas diffusion media. The other electrode layer comprises a PtCoMn nanostructured thin film (NSTF) electrode [5] that is overcoated with Nafion® ionomer using a preferred adsorption scheme to a targeted 2.0 nm dry thickness. Figure 2 shows that a significantly different through-layer ionomer/carbon (I/C w/w) profile is measured by energy-dispersive X-ray spectroscopy (EDS) for the CCD and CCDM electrode layers that can be ascribed to partial permeation of the electrode ink solution into the porous gas diffusion media substrate. In addition, the AFM adhesion measurement in Figure 3 shows a linear increase in ionomer coverage on the NSTF electrode surface to the adsorption limit (at 2 nm dry thickness). ORNL Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) the Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy.

Study of storage capacity in various carbon/graphene-based solid-state supercapacitors

Solid-state electrochemical double-layer capacitor (SEDLC) forms excellent energy storage device for high-power applications. They are highly reliable, with no electrolyte leaks, and can be packaged to suit various applications. The electrode material can be activated carbon to graphene. These can have a range of particle size, surface area, pore size and pore distribution for charge storage. The emphasis will be to optimize the graphene to carbon blend in the electrodes which would provide appreciable storage density of the SEDLC. We can use perfluorosulfonic acid polymer as the solid electrolyte in the SEDLC assembly. They have high ionic conductivity, good thermal stability, and mechanical strength. They also have excellent long-term chemical stability. Carbon is widely used for many practical applications, especially for the adsorption of ions and molecules, as it is possible to synthesize one-, two- or three-dimensional (1-, 2-, or 3-D) carbons. Some of the problems in activated carbon like varying micro or mesopores, poor ion mobility due to varying pore distribution, low electrical conductivity, can be overcome using graphene and blends of graphene with carbon of the right pore dimension and distribution. Graphene in various structural nomenclatures have been used by various groups for charge storage. Graphene nanoplates (GNP), with narrow mesopore distributions have been effectively used for SEDLCs. SEDLCs assembled with GNP and blends of GNP with Vulcan XC and solid polymer electrolyte like Nafion show exceptional performance. The cyclic voltammetric studies show that they support high scan rates with substantial smaller capacitance drop as we increase scan rates. Optimization of the electrode structure in terms of blend percentage, binder content and interface character in the frequency and time domain provides excellent insight into the double-layer interface.

Proton conducting membranes based on semi-interpenetrating polymer network of fluorine-containing polyimide and perfluorosulfonic acid polymer via click chemistry

Fluorine-containing polyimide (FPI) with hydroxyl groups is synthesized from 4,4′-(hexafluoro isopropylidene) diphthalic anhydride (6FDA), 2,2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane (6FAP), and 4,4′-diaminodiphenyl ether (ODA) via high temperature polycondensation. Thereafter, alkynyl groups are introduced into FPI. During the preparation of the composite membrane, alkynyl groups on FPI react with azido methyl of 4,4′-bis(azido methyl) biphenyl via click chemistry and form semi-interpenetrating polymer network (semi-IPN) structure within the composite membranes. The mechanical properties, thermal behavior, water uptake, swelling ratio, proton conductivity, oxidative stability, as well as the performance in single cell operation are investigated. Compared to pure perfluorosulfonic acid (PFSA) polymer membrane, the composite membranes based on semi-IPN of FPI and PFSA exhibit improved mechanical properties, excellent thermal and dimensional stabilities, and suitable proton conductivity. The tensile strength of the composite membranes ranges from 28.0 to 67.0MPa. With increasing FPI content in the membranes, the dimensional stability of the composite membranes increases. The composite membranes have the proton conductivity from 4.3×10−2 S·cm−1 to 1.0×10−1 S·cm−1 at 100°C and also have good performances as proton exchange membrane (PEM) in single cell at 80°C.

Porous Metal Oxide Nanotubes Modified Proton Exchange Membranes for Fuel Cells Operating at Elevated Temperature and Low Humidity

Polymer electrolyte membrane fuel cells (PEMFC) have been widely accepted as one of the most promising power sources for automobile application due to high energy conversion efficiency and environmental compatibility. However, the current PEMFC technology, utilizes perfluorosulfonic acid (PFSA) polymer membranes, e.g., Nafion, as electrolyte, is limited owing to expensive components and poor performance at elevated temperature (100 – 200 °C). PEMFC operated at elevated temperature results in enhancing the performance due to improving electrode kinetic and reduction of adsorption poisoned species such as CO [1]. The deterioration of PEMFC performance at elevated temperature is mainly attributed to the loss of proton conductivity of the Nafion membrane which results in drastic increase in the ohmic over potential [2]. Proton conductivity of Nafion membrane is strongly influenced by the water content and the conductivity is maximum when the membrane is fully hydrated. Operating PEFC at elevated temperature leads to membrane dehydration which dramatically reduces the proton conductivity in order of magnitude. Thus, it is highly desirable to enhance proton conductivity Nafion membrane at elevated temperature in order to achieve high PEFC performance. Porous metal oxides nanotubes (TiO2, ZrO2, and CeO2) with diameter in a range of 30 – 180 nm were synthesized by calcining electrospun metal ions embedded polyacrylonitrile nanofiber, which was prepared using a conventional single spinneret electrospinning technique, at 600 oC under an air atmosphere. Compared to a commercial membrane (Nafion®212) operated at 80, 100, and 110 oC, the Nafion-TiO2 membrane delivered improved power density at 0.6 V. The performance of composite membranes was evaluated by making MEA and subsequently the composite membranes were investigated the H2/O2 fuel cell at 100 oC under 100 %RH and at 80 oC under 18 %RH. Fig. 1B illustrates the polarization and power density plots obtained at 100 oC under 100 %RH. It was found that the power density at 0.6 V obtained from Nafion-TiO2 composite membrane operated at 100 oC was 730 mW/cm2, which is 1.6 times higher than commercial membrane (NRE-212) and recast Nafion membrane. On the other hand, the Nafion-TiO2 composite membrane delivered maximum power density of 640 mW cm-2, which is 3 times higher than that of the performance obtained from NRE-212 and recast Nafion membranes (Fig. 1C). The higher PEMFC performance at 100 oC and low RH was due to the hygroscopic property of TiO2 which can retain water at 100 oC and due to the tubular morphology which facilitates the proton transport channel through membrane. We also fabricated inorganic proton cluster modified graphene oxide Nafion composite membrane. The deposition of phosphotungstic acid (PW) on the modified graphene oxide sheet with positively charged amine was carried out through an electrostatic interaction between PW and the modified graphene sheets to make heterogeneous system. The modified graphene oxide (mGO) was synthesized through a condensation reaction. A mixture of mGO and PW aqueous dispersion was stirred to fabricate PW-mGO. The interaction between inorganic cluster and the modified graphene oxide was confirmed by UV-visible, IR and cyclic voltammetry studies and these results revealed that that an alteration of the electronic structure of deposited cluster as a result of strong hybridization between the graphene oxide and PW [3]. The NRE-212/PW-mGO composite membrane exhibited much higher proton conductivity at relatively low humidity than the Nafion-212 (NRE-212), and the composite membrane showed 4-fold higher maximum power density PEMFC operated under 18 % RH than that of pristine Nafion-212 membrane. The synthesis of composite perfluorosulfonic acid membrane, conductivity and fuel cell performance at elevated temperature under low relative humidity will be discussed in this presentation. References E. Chalkova, M. B. Pague, M. V. Fedkin, D. J. Wesolowski, S. N. Lvov, J. Electrochem. Soc. 2005, 152, A1035.K. A. Mauritz and R. B. Moore, Chem. Rev. 104 (2004) 4535.Y. Kim, and S. Shanmugam, ACS Appl. Mat. Int., (2013) DOI:10.1021/am4043245.

Chemistry and Degradation Mitigation Effect of Cerium Oxide in Polymer Electrolyte Membranes

One of the main obstacles to wide-spread commercialization of polymer electrolyte membrane fuel cells (PEMFC) is the short operational lifetime of perfluorosulfonic acid (PFSA) membranes used in these devices. Various radical oxygen species are considered to be the main degrading species for PFSA membranes.[1]Radical scavenging properties of cerium oxide Ce(III)/Ce(IV) redox couple in the biological environment are well-known and hence its application in PEMFC, as a degradation mitigator, has received considerable attention.[2, 3] In this work, two cerium oxide nanoparticle formulations with an order of magnitude difference in average size (2-5nm and 20-150nm) were added to 1100 EW PFSA membranes in 0.5, 1.0 and 2.0 wt% loading. We will discuss that ceria, in the acidic environment of heated/humidified membranes, is reduced to Ce(III) ions which negatively impacts proton conductivity. Both liquid and gas Fenton tests were conducted. These tests showed that the fluoride emission is reduced by an order of magnitude with the polymer end groups as the main point of attack for liquid Fenton tests and additional side-chain attack for gas Fenton tests. The extent of durability improvement is found to be independent of cerium oxide particle sizes.[4] The membranes were subjected to 94 and 500 h open-circuit voltage hold accelerated durability tests. The open-circuit voltage decay rate was shown to be decreased by half and seven-fold for the 94 h and 500 h tests respectively. In 500 h tests, ceria-containing MEAs demonstrated at least two orders of magnitude reduction in fluoride emission rates with minimal change in the integrity of the MEA while the baseline MEA underwent catastrophic failure. In the presence of cerium oxide nanoparticles, the platinum deposition in the membrane was found at previously reported distance from cathode; however, the platinum particles showed an increase in size and caused a broadening with particles reaching further into the membrane.[5][1] J.M. Fenton, M.P. Rodgers, D.K. Slattery, X. Huang, V.O. Mittal, L.J. Bonville, H.R. Kunz, Membrane Degradation Mechanisms and Accelerated Durability Testing of Proton Exchange Membrane Fuel Cells, ECS Trans. 25 (2009) 233-247.[2] P. Trogadas, J. Parrondo, V. Ramani, Degradation mitigation in polymer electrolyte membranes using cerium oxide as a regenerative free-radical scavenger, Electrochemical and Solid State Letters 11 (2008) B113-B116.[3] F.D. Coms, H. Liu, J.E. Owejan, Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions, ECS Trans. 16 (2008) 1735-1747.[4] B.P. Pearman, N. Mohajeri, D.K. Slattery, M.D. Hampton, S. Seal, D.A. Cullen, The chemical behavior and degradation mitigation effect of cerium oxide nanoparticles in perfluorosulfonic acid polymer electrolyte membranes, Polymer Degradation and Stability 98 (2013) 1766-1772.[5] B.P. Pearman, N. Mohajeri, R.P. Brooker, M.P. Rodgers, D.K. Slattery, M.D. Hampton, D.A. Cullen, S. Seal, The degradation mitigation effect of cerium oxide in polymer electrolyte membranes in extended fuel cell durability tests, Journal of Power Sources 225 (2013) 75-83.

Morphology and properties of perfluorosulfonic acid polymer/perfluorocarboxylic acid polymer blend membranes

A melt‐extruded method has been used to prepare blend membranes of Perfluorosulfonic Acid Polymer (PFSA) and Perfluorocarboxylic Acid Polymer (PFCA) with different combination ratios. Fourier Transform Infrared (FT‐IR) and X‐ray Diffraction (XRD) analyses have showed that the chemical structure and crystallization behaviour of both polymers remained unchanged in the blend membrane, and the crystallinity has been increased for the blend membrane with a higher weight fraction of PFCA. According to the results obtained from Scanning Electron Microscope (SEM), PFSA and PFCA polymers are well compatible, forming mixed aggregates containing both sulphonyl and carboxyl ions. Surprisingly, a membrane containing a 50:50 ratio of PFSA/PFCA has showed microphase separation, and it possesses the smallest ionic cluster size among all the blend membranes prepared, as illustrated by the Small Angle X‐ Ray Scattering (SAXS) analysis. In addition, when the PFCA content was increased, the ion exchange capacity (IEC) for blend membranes remained the same, while their water uptake and proton conductivity have decreased rapidly. Compared with the PFSA/PFCA two‐layered membrane, this PFSA/blend/PFCA membrane not only maintained respectable electrolysis property, also exhibited significantly higher peel strength, especially after the hydrolization process; plus, it has showed excellent resistance to the peeling damage under the operational conditions of chlor‐alkali electrolytic cells. POLYM. ENG. SCI., 55:180–189, 2015. © 2014 Society of Plastics Engineers

Temperature‐ and Humidity‐Controlled SAXS Analysis of Proton‐Conductive Ionomer Membranes for Fuel Cells

We report herein temperature- and humidity-controlled small-angle X-ray scattering (SAXS) analyses of proton-conductive ionomer membranes. The morphological changes of perfluorosulfonic acid polymers (Nafion and Aquivion) and sulfonated aromatic block copolymers (SPE-bl-1 and SPK-bl-1) were investigated and compared under conditions relevant to fuel cell operation. For the perfluorinated ionomer membranes, water molecules were preferentially incorporated into ionic clusters, resulting in phase separation and formation of ion channels. In contrast, for the aromatic ionomer membranes, wetting led to randomization of the ionic clusters. The results describe the differences in the proton-conducting behavior between the fluorinated and nonfluorinated ionomer membranes, and their dependence on the humidity.

A versatile strategy towards semi-interpenetrating polymer network for proton exchange membranes

A universal method to prepare proton exchange membrane (PEM) with semi-interpenetrating polymer network (semi-IPN) via a versatile crosslinker 1,3-benzenedisulfonyl azide (1,3-BDSA) is proposed. Sulfonyl azide groups can react with any polymer with carbon-hydrogen bonds via hydrogen abstraction. Polyvinylidene fluoride (PVDF) and perfluorosulfonic acid (PFSA) polymer are selected as two precursors for semi-IPN. When 1,3-BDSA is heated to a sufficiently high temperature it can form the nitrene and connect with PVDF via hydrogen abstraction reaction, recombination, or CH-bond insertion. The crosslinking structure of PVDF is formed, and semi-IPN is constructed in the presence of PFSA at the same time. The mechanical properties, degree of crosslinking, water uptake, and proton conductivity of semi-IPN membranes are investigated. Compared with pure PFSA membrane, the mechanical properties and the dimensional stability of the semi-IPN membranes are improved significantly. The tensile strength of the composite membrane (27.2 MPa) is much higher than that of PFSA membrane (10.1 MPa). The maximum power density of the composite membrane can reach 382 mW cm−2 at 940 mA cm−2. Sulfonyl azide based crosslinkers can be used to prepare semi-IPN or IPN PEMs from any paired polymers with carbon–hydrogen bonds.

The chemical behavior and degradation mitigation effect of cerium oxide nanoparticles in perfluorosulfonic acid polymer electrolyte membranes

Perfluorosulfonic acid membranes are susceptible to degradation during hydrogen fuel cell operation due to radical attack on the polymer chains. Mitigation of this attack by cerium-based radical scavengers is an approach that has shown promise. In this work, two formulations of crystalline cerium oxide nanoparticles, with an order of magnitude difference in particle size, are incorporated into said membranes and subjected to proton conductivity measurements and ex-situ durability tests. We found that ceria is reduced to Ce(III) ions in the acidic environment of a heated, humidified membrane which negatively impacts proton conductivity. In liquid and gas Fenton testing, fluoride emission is reduced by an order of magnitude, drastically increasing membrane longevity. Sideproduct analysis demonstrated that in the liquid Fenton test, the main point of attack is weak polymer end groups, while in the gas Fenton test, there is additional side-chain attack. Both mechanisms are mitigated by the addition of the ceria nanoparticles, whereby the extent of the concentration-dependent durability improvement is found to be independent of particle size.

Effect of Surface Hydrophilicity on the Formation of Nafion Thin Films Inside PEMFC Catalyst Layers: A Computational Study

Structure of Nafion inside Polymer Electrolyte Membrane Fuel Cells (PEMFC) catalyst layers is suspected to have a significant impact on the electrochemical activity and transport phenomena that determine the overall cell performances. In those regions, Nafion can be found as an ultra-thin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic character of these surfaces on the structural formation of the films has not been explored in details yet. Here, we report about our Molecular Dynamic computer simulations studies of the formation of Nafion thin films in contact with unstructured continuous surfaces, characterized by their global wetting properties only. We investigate changes of morphology and nanostructure of the perfluorosulfonic acid polymer as a result of water and polymer-surface affinities. Charge and water distributions are also analyzed. Possible impact of Nafion thin film self-assembly on the actual activity properties of realistic catalysts layers is shortly discussed.

Portable method of measuring gaseous acetone concentrations

Measurement of acetone in human breath samples has been previously shown to provide significant non-invasive diagnostic insight into the control of a patient's diabetic condition. In patients with diabetes mellitus, the body produces excess amounts of ketones such as acetone, which are then exhaled during respiration. Using various breath analysis methods has allowed for the accurate determination of acetone concentrations in exhaled breath. However, many of these methods require instrumentation and pre-concentration steps not suitable for point-of-care use. We have found that by immobilizing resorcinol reagent into a perfluorosulfonic acid polymer membrane, a controlled organic synthesis reaction occurs with acetone in a dry carrier gas. The immobilized, highly selective product of this reaction (a flavan) is found to produce a visible spectrum color change which could measure acetone concentrations to less than ppm. We here demonstrate how this approach can be used to produce a portable optical sensing device for real-time, non-invasive acetone analysis.

Degradation Study of Perfluorosulfonic Acid Polymer Electrolytes: Approach from Decomposition Product Analysis

Operation tests were applied to fuel cell with perfluorosulfonic acid (PFSA) polymer for membranes and electrodes. Degradation of PFSA polymer in catalyst layers after the tests was investigated using 19F NMR. As the decomposition products, six kinds of the side-chain fragments were acquired from the water and the methanol extracts, and the each weight of the fragment was acquired. The side-chain fragments indicate that side-chain scission occurs at two positions in the side-chain. Moreover, reduction of relative equivalent weight for all PFSA polymer (EW*all) and amounts of PFSA polymer in catalyst layer after the tests suggests that main-chain scission occurs. Hydrogen peroxide vapor (HPV) exposure tests were applied to PFSA polymer membranes. PFSA polymer degradation after the tests was investigated using ion chromatography and total organic carbon analysis for trap water, and using 19FNMR for water/methanol extracts of membranes. Decomposition products other than fluoride ion accounted for a significant portion of degradation weight loss, and their generation rates were not proportional to fluoride ion release rate (FRR). The differences between the results of FRR and other decomposition product analysis strongly suggest the presence of two or more decomposition mechanisms in PFSA degradation.

Molecular dynamics simulations of side chain pendants of perfluorosulfonic acidpolymer electrolyte membranes

Perfluorosulfonic acid (PFSA) polymer electrolyte membranes like Dow, Aciplex and Nafion have similar backbones but different side chain pendants. The effect of hydration and temperature on the side chain pendant nanostructure, and water and hydronium ion dynamics, are investigated by employing classical molecular dynamics simulations at 300 K and 350 K. The 60% longer side chain pendant length in Aciplex compared to Dow results in phase segregation. The presence of an extra ether oxygen atom in the Nafion side chain pendant provides more flexibility (∼20% chain length contraction caused by flexibility and the hydrophobic force of the pendant CF3 group) where the sulfonate group tends to drift from the hydrophilic–hydrophobic domain, which gives rise to a hydrosphere region at higher hydration. The calculated structure factors and scattering intensities reproduce features of SANS and SAXS profiles for Dow and Nafion, and confirm the existence of spherical water aggregates in the rod shaped pendant nanostructure of Nafion. The effect of hydration on the mobility of hydronium ions at 300 K in Nafion is insignificant at higher hydration (λ ≥ 9), and trends are in agreement with experimental data. The activation energy of the diffusion of hydronium ions and water molecules in Nafion side chain pendant–water mixtures (14–25 kJ mol−1) validate experimental observations (16–22 kJ mol−1).

Degradation behaviors of perfluorosulfonic acid polymer electrolyte membranes for polymer electrolyte membrane fuel cells under varied acceleration conditions

AbstractThe degradation of perfluorosulfonic acid (PFSA) membranes (e.g., Nafion membranes) in polymer electrolyte membrane fuel cells has caused wide widespread concern. However, their degradation behaviors, which lead to the damage of fuel cells, need to be investigated under alternative accelerating environments by the simulation of fuel‐cell operating conditions. Nafion membranes showed a homogeneous degradation behavior during hydrogen peroxide (H2O2) aging, whereas a nonhomogeneous (or crack‐type) degradation behavior occurs for Nafion membranes aged in an H2O2/Fe2+ system (Fenton's reagent), where plenty of the typical microcracks appeared. Interestingly, in the case of nonhomogeneous degradation, the membrane presented a lower fluoride emission rate than that with the homogeneous degradation; this indicates a possible selective attack model of free radicals to both CF2 and the defect end groups in PFSA membranes. In addition, the effects of the different degradation behaviors on the thermal stability and water uptake of membranes were examined by thermogravimetric analyses. H2 crossover and single‐fuel‐cell tests were carried out to evaluate the influence of the degradation behaviors on the fuel‐cell performance. These showed that the membrane with a nonhomogeneous degradation behavior had a higher hydrogen crossover and was more destructive than that with a homogeneous behavior. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Depth‐resolved micro‐Raman spectroscopy of tri‐layer PFSA membrane for PEM fuel cells: how to obtain reliable inner water contents

Raman depth‐profiling microspectroscopy is currently emerging as a fast and non‐invasive method for the local content measurement of water diffusing across the perfluorosulfonic acid polymer used as electrolyte in low‐temperature fuel cells. However, water depth profiles obtained thereby are affected by the gradual degradation of the Raman signal as the exciting radiation is focused deeper into the sample and, thus, usually exhibit artificial gradients. The appropriate way to rectify raw data, in order to measure reliable inner water contents, is discussed in the case of perfluorosulfonic acid membranes soaked in water. The method is tested on a tri‐layer material composed by stacked ionomers with different chemical composition, ionic exchange capacity and swelling. Comparison of Raman spectra of the different ionomers, obtained under strictly the same hydration and optical conditions, allows critical discussion of previously reported band assignments. It is shown that Raman bands arising from the polymer backbone and from the side chain can be readily discriminated. Reliable water concentration profiles can then be obtained from the ratio between the Raman signal of sorbed water and of the polymer phase. Comparison with depth profiles obtained by using the pure water signal as internal reference shows that quantitative measurement requires the careful choice of the adequate Raman bands for representing the ionomer occupation of the volume probed by the exciting radiation. Different to what usually performed, the ionomer Raman signal to be integrated for compensating optical losses should include both bands arising from the polymer backbone and side chains. Last, obtaining accurate water concentration profiles also needs the knowledge of the so‐called optical factor β, i.e. the ratio between the scattering cross sections of ionomer chemical groups and sorbed water, which has to be measured by independent Raman experiments. Copyright © 2012 John Wiley & Sons, Ltd.

Synthesis and Properties of Sulfonated Poly(arylene ether)s with Flexible Oligomeric Phenylene Ether Segments

Novel sulfonated poly(arylene ether)s derived from ω,ω′-dihydroxypoly(1,4-phenylene ether)s (DHPE) and mixed dihalide monomers of 3,3′-disulfo-4,4′-dichlorodiphenylsulfone disodium salt (S-DCDPS) and 2,6-dichlorobenzonitrile were synthesized. As DHPE has molecular weight distribution with phenylene rings lower than ca. 10 units, experimental determination of its average molecular weight made it possible to be used as a monomer to prepare sulfonated poly(arylene ether)s. Membranes derived from resultant polymers showed a similar level of proton conductivity to those of other common sulfonated poly(arylene ether)s; however, its softening temperature was much lower. Therefore, good MEA was able to be obtained by hot-pressing at relatively lower temperature without shrinking around electrode. The membrane with an ion exchange capacity of 0.9 showed lower methanol permeability and dimensional change in methanol solution assuming a condition of DMFC than standard perfluorosulfonic acid polymer membrane (PFSA).

Proton exchange membranes based on semi-interpenetrating polymer networks of polybenzimidazole and perfluorosulfonic acid polymer with hollow silica spheres as micro-reservoir

Abstract A new method to prepare reinforced proton exchange membranes (PEMs) for polymer electrolyte membrane fuel cells (PEMFCs), which can work either in humidified environment or at anhydrous state, is proposed via constructing semi-interpenetrating polymer network (semi-IPN) of polybenzimidazole (PBI) and perfluorosulfonic acid polymer, with poly(vinylbenzyl phosphonic acid) functionalized hollow silica spheres (HPSS) as the additive. Vinyl functionalized PBI, which is synthesized from poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and p-vinylbenzyl chloride, is thermally crosslinked in the presence of Nafion ® as well as HPSS during the membrane preparation. Thus semi-IPN structure is formed in the composite membranes. Compared with Nafion ® , the composite membranes show higher thermal stability and water uptake, wider range of working temperature, improved mechanical properties, as well as better dimensional stability. Under humidified condition, the proton conductivity of the composite membranes is mainly provided by hydrated Nafion ® , whereas PBI and the acidic moieties in the membranes contribute to the proton conductivity at higher temperature and anhydrous state. HPSS in the composite membranes behave like the micro-reservoir and help provide proton conductivity at the state between hydrated and anhydrous states.

A new structured composite membrane for fuel cell applications

A new proton conducting membrane was prepared by confining sub-nanoliter volumes of perfluorosulfonic acid (PFSA) polymer within rigid pores cladded with zeolite layers. Tests showed membrane-electrode assembly (MEA) made of confined Nafion PFSA-HZSM-5/Sil-1 structured composite membrane delivered an order of magnitude higher MPD (maximum power density) compared to a standard Nafion 117 MEA in a hydrogen PEMFC at high temperature operation under dry feed conditions. Similarly, 900% higher MPD was obtained from direct methanol fuel cell (DMFC) made of the new membrane. The zeolites are believed to play an active role in the water and methanol managements through adsorption and transport, and the confinement of the PFSA within zeolite environment changes its microstructure resulting in the improved thermal stability.

Irradiation Effect of Gamma Ray on Electrical Characteristics of Perfluorosulfonic Acid Polymer Membrane

Perfluorosulfonic acid polymer membranes, such as Nafion®, Aciplex®, and Flemion®, are attracting great attention as electrolyte membranes for fuel cells because they have high proton conductivity. In this paper, the effect of γ rays on the electrical properties of perfluorosulfonic acid membranes by applying a variable voltage and measuring the current and dimension of the sample have been investigated. In order to understand the irradiation effects on conductivity changes, we have used optical absorption measurement in the wavelength of 190 ∼ 500 nm. The results show that the electrical conductivity has been increased by increasing the absorption dose, and intensity of the absorption bands have increased by increasing the absorption dose. The electrical conductivities of the irradiated membranes at room temperature are enhanced about one order of magnitude higher than the conductivities of an unirradiated membrane. Moreover, an optical absorption measurement shows that the structures of the membranes are modified by γ radiation. Intensity absorption bands associated to fluorocarbon and peroxy radicals and C=O groups increased with increasing the dose. The change in electrical conductivity seems to be related to the high production of peroxy radicals in the irradiated membrane.