Properties Of Proton Exchange Membranes Research Articles

Mechanical durability of proton exchange membranes with catalyst platinum dispersion

The durability of proton exchange membrane (PEM) fuel cells remains a challenging issue for their long term operational use. Degradation of the PEM related to dissolution of the adjacent catalyst and re-deposition into the PEM significantly reduces cell efficiency. We investigate the effects of platinum (Pt) dispersions intended to simulate the re-deposited catalyst on the mechanical durability of the PEM. The bulge technique was applied to characterize the mechanical properties of PEMs simulating pressure loading on fully hydrated membranes in fuel cells. The results showed that with increasing Pt dispersion concentration the stiffness of the PEMs increased, and the membranes became less ductile and inclined to fracture at lower stresses under pressure loading. We also used the out-of-plane tearing test to characterize membrane fracture behavior which revealed the harmful effects of Pt dispersion on the fracture resistance under different environmental conditions. Deterioration in fracture resistance was explained in terms of the Pt distribution and aggregation as defects inside the membranes as characterized by electron microscopy. Fracture was shown to initiate preferentially at the interface of Pt particles and the polymer matrix, and propagate through the defect regions in polymer with lower energy, thus reducing the overall fracture resistance of the PEM.

Experimental Measurement of Stress and Strain in Nafion Membrane during Hydration Cycles

Proton exchange membranes (PEMs) in fuel cell stacks develop biaxial hygrothermal fatigue stresses in response to changing operation conditions. Determination of these stresses is complicated as both constitutive properties of PEM and the hygrothermal strain in PEM are highly dependent on time, temperature and water absorption. In this study, by tracking the spherical bending curvature of a bimaterial strip composed of a 25-μm thick PEM (Nafion) and a 250-μm thick PEEK (polyether ether ketone) substrate using a Digital Image Correlation (DIC) system, the biaxial tensile and compressive stresses in the plane of the PEM during humidity cycles are recorded. Due to the relatively high bending stiffness ratio between PEEK and PEM, the hygral stresses in the PEM are determined from the bimaterial curvature without requiring knowledge of the constitutive properties of the PEM. As the strain in the PEM released due to bending of the bimaterial strip is limited, the stresses measured using the bimaterial strip are very close to that in a fully constrained membrane, mimicking those in a fuel cell. The hygral strain in a freestanding membrane placed next to the bimaterial structure is recorded at the same time using the DIC system. The full stress and strain histories during incremental relative humidity (RH) test between 10%RH and liquid wet as well as during larger RH cycles are presented. The hygral expansion of Nafion during the increasing and decreasing RH step changes exhibits an asymmetry that resembles the behavior of glassy polymers during the classic Kovacs temperature jump test. The stress history also reveals the RH-dependent viscoelasticity of Nafion.

Effect of cation contamination and hydrated pressure loading on the mechanical properties of proton exchange membranes

Perfluorosulfonic acid (PFSA) polymer membranes are widely used as electrolyte thin films to transport protons in proton exchange membrane (PEM) fuel cells. The mechanical degradation of the membrane represents a common failure mode that limits the operational life of the fuel cells. In the present work, effect of contamination related to cation exchange on the mechanical reliability of PEMs was investigated. We applied the bulge test technique to assess the mechanical properties of Nafion ® PFSA membranes simulating pressure loading on hydrated PEMs in fuel cells. The corresponding elastic moduli of Nafion ® before and after cation exchange were analyzed and compared with the results measured by uniaxial tension experiments at selected humidity conditions, showing increasing stiffness with the increase of cation radius. We also used the out-of-plane tearing test method to characterize the fracture behaviors of PEMs. The effects of cation exchange and water absorption on mechanical and fracture properties of PEMs at different temperatures are discussed in terms of cation and water interactions with the molecular structure of PFSA polymers.

Mesoscale modeling of the influence of morphology on the mechanical properties of proton exchange membranes

The nano-scale morphology of a proton exchange membrane (PEM) strongly influences its proton conductivity and mechanical performance. In this paper, a multi-scale modeling approach has been developed to obtain the morphologies of hydrated perfluorosulfonic acid (PFSA) membranes and then to predict their mechanical properties based on the simulated morphology. Two representative ionic domain morphologies were compared, spherical and cylindrical, to represent cast and extruded membranes. The calculated overall elastic Young’s moduli are very close for both morphologies and agree well with experiments. The nano-scale phase segregation in hydrated PFSA induces a non-uniform distribution of local stress. The peak stress is localized at the smeared water/PFSA interface. The cylindrical morphology develops much lower peak stress than the isotropic spherical morphology under the same level of strain. These results explain why the extruded membranes show more than 10 times longer life in durability tests than recast membranes, despite having similar bulk moduli.

Review of the proton exchange membranes for fuel cell applications

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for clean and efficient power generation in the twenty-first century. Proton exchange membranes (PEMs) are the key components in fuel cell system. The researchers have focused to reach the proton exchange membrane with high proton conductivity, low electronic conductivity, low permeability to fuel, low electroosmotic drag coefficient, good chemical/thermal stability, good mechanical properties and low cost. These are classified into the “iron triangle” of performance, durability, and cost. Current PEMFC technology is based on expensive perflourinated proton-exchange membranes (PEMs) that operate effectively only under fully hydrated conditions. There is considerable application-driven interest in lowering the membrane cost and extending the operating window of PEMs. PEMFC system complexity could be reduced by the development of ‘water-free’ electrolytes that do not require hydration. It also enables the PEMFC to be operated under ‘warm’ conditions (i.e. above 100 °C) thus further improving its efficiency. Capital cost could also be further reduced because at warmer conditions less Pt could be used. This paper presents an overview of the key requirements for the proton exchange membranes (PEM) used in fuel cell applications, along with a description of the membrane materials currently being used and their ability to meet these requirements. A number of possible alternative candidates are reviewed and presented in this paper. Also discussed are some of the new materials, technologies, and research directions being pursued to try to meet the demanding performance and durability needs of the PEM fuel cell industry. The alternative PEMs are classified into three categories: (1) modified Nafion ® composite membranes; (2) functionalized non-fluorinated membranes and composite membranes therein; and (3) acid–base composite membranes. Several commonly used inorganic additives are reviewed in the context of composite membranes. Finally, the general methods of the measuring and evaluating of proton exchange membrane properties have been investigated such as proton conductivity, ion exchange capacity, water uptake, gas permeability, methanol permeability, durability, thermal stability and fuel cell performance test.

Syntheses of sulfonated star-hyperbranched polyimides and their proton exchange membrane properties

We synthesized novel sulfonated star-hyperbranched polyimides composed of a hydrophobic hyperbranched polymer for polymer stability and a hydrophilic sulfonated polyimide as the proton-transport site in the core–shell structure. The proton conductivities of the star-hyperbranched polyimide membranes were measured as functions of the relative humidity and temperature using electrochemical impedance spectroscopy. Although the water uptake and IEC value for the sulfonated star-hyperbranched polyimide membranes were almost constant, the proton conductivity of the membrane strongly depended on the molecular weight of the hydrophilic sulfonated polyimide as the shell. Especially, the conductivity of the high molecular weight star-hyperbranched polyimide membranes was significantly superior to that determined in Nafion ® at all temperatures and was 0.51 S cm −1 at 80 °C and 98% RH, which may suggest that a good proton-transport pathway in the core–shell structure is formed. Consequently, this material proved to be promising as a proton exchange membrane and may have potential applications for use in fuel cells.

Locating sulfonic acid groups on various side chains to poly(arylene ether sulfone)s: Effects on the ionic clustering and properties of proton-exchange membranes

Poly(arylene ether sulfone)s carrying various aromatic mono-, di-, and trisulfonated side chains have been investigated with respect to their nanoscale structure and key membrane properties. Sulfobenzoyl, sulfonaphthoxybenzoyl, disulfonaphthoxybenzoyl, and trisulfopyrenoxybenzoyl side chains were attached to the poly(arylene ether sulfone) main chain by employing different combinations of metallation and nucleophilic aromatic substitution reactions. The nature of the sulfonated side chains was found to either promote or suppress the formation of ionic clusters, in relation to the ionic clustering occurring in corresponding polymers carrying sulfonic acid groups directly on the main chain. Analysis by small angle X-ray scattering (SAXS) of solvent cast membranes showed that the ionic clustering was promoted by placing the sulfonic acid groups on the relatively long naphthoxybenzoyl or pyrenoxybenzoyl side chains. This resulted in SAXS profiles that indicated larger characteristic separation lengths and narrower ionomer peaks, as compared with corresponding main chain sulfonated polymers. On the other hand, the ionic clustering was almost completely suppressed in membranes based on polymers functionalized with short 2-sulfobenzoyl side chains. Proton conductivity measurements at low or moderate water contents showed a trend of increasing conductivities with the length and the sulfonic acid functionality of the side chain. The structure of the side chain also influenced the thermal stability and glass transition temperature of the membranes.

Surface Properties of PEMFC Gas Diffusion Layers

The wetting properties of proton exchange membrane fuel cell (PEMFC) gas diffusion layers (GDLs) were quantified by surface characterization measurements and modeling of material properties. Single-fiber contact-angle and surface-energy (both Zisman and Owens–Wendt) data of a wide spectrum of GDL types are presented to delineate the effects of hydrophobic postprocessing treatments. Modeling of the basic sessile-drop contact angle demonstrates that this value only gives a fraction of the total picture of interfacial wetting physics. Polar forces contributed less than dispersive forces to the composite wetting of GDLs. Internal water contact angles obtained from the Owens–Wendt analysis were measured at 13–19° higher than their single-fiber counterparts. An inverse relationship was found between internal contact angle and both Owens–Wendt surface energy and percent polarity of the GDL. The most sophisticated PEMFC mathematical models use either experimentally measured capillary pressures or the standard Young–Laplace capillary-pressure equation. Based on the results of the Owens–Wendt analysis, an advancement to the Young–Laplace equation was proposed for use in these mathematical models, which utilizes only solid surface energies and a fractional surface coverage of a fluoropolymer. Capillary constants for the spectrum of analyzed GDLs are presented for the same purpose.

The effects of wetproofing on the capillary properties of proton exchange membrane fuel cell gas diffusion layers

In the analysis of proton exchange membrane (PEM) fuel cell components, the capillary pressure vs. saturation ( P C ( S L )) curve is an increasingly popular tool for understanding the interaction of liquid water with the porous gas diffusion layer (GDL) material. In this study, hysteretic water/air P C ( S L ) measurements were combined with mercury intrusion porosimetry (MIP) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging to quantify the effects of fluoropolymer loading on GDL samples. Commercially wetproofed carbon fiber papers with 0–40 wt.% Teflon loading were investigated. MIP showed a slight reduction in characteristic pore radii and a significant loss of pore volume at the highest Teflon loading. Water/air P C ( S L ) measurements showed a significant reduction in water wetting between samples with 0 and 5 wt.% Teflon loading, but negligible additional wetproofing at loadings from 10 to 40 wt.%. ToF-SIMS imaging, a technique that is sensitive to monolayer surfaces coverages, found that GDL materials with 5 wt.% Teflon loading displayed nearly complete fluoropolymer coverage on the carbon substrate, confirming P C ( S L ) measurements showing that all of the wetproofing occurs in a narrow range of Teflon loadings. Results for P C ( S L ) measurements were fitted using a bundle-of-capillaries model. The apparent water intrusion contact angles fell between 130° and 133° in the rough Teflonated pore space (regardless of loading), whereas the apparent gas intrusion contact angles fell between 66° and 70° for the same materials.

Novel hydrophilic–hydrophobic multiblock copolyimides as proton exchange membranes: Enhancing the proton conductivity

A series of novel multiblock copolymers based on sulfonated copolyimides were developed and evaluated for use as proton exchange membranes (PEMs). In these multiblock copolyimides, the hydrophilic blocks were composed of the sulfonated dianhydride and the sulfonated diamine, with sulfonic acid groups on every aromatic ring (i.e., fully sulfonated). This molecular design was implemented to effectively enhance the proton conductivity. The properties of the multiblock copolyimides with varying IEC values or block lengths were investigated to obtain a better understanding of the relationship between molecular structure and properties of proton exchange membranes. The water uptake and proton conductivity were found to be highly dependent upon their structure. The block copolymers displayed significantly higher proton conductivities, especially at low relative humidity than the random copolymers with a similar IEC. The results indicated that the distribution of sulfonic acid and the length of the blocks play a key role on properties of proton exchange membranes.

Phase transition and morphology of polydispersed ABA′ triblock copolymers determined by continuous and discrete simulations

The purpose of this study is to consider the effect of polydispersity in triblock copolymer on its order-to-disorder phase transition and morphology, by supplementing continuous description of polymer indices carried out using random phase approximation by a discrete sampling of the polydispersity performed at the mesoscale level. Both methods confirm that the increase in polydispersity involves a greater stability of the ordered state, in agreement with published data on the effect of polydispersity on diblock copolymers. The morphology simulations reveal that longer blocks are phase segregated and shorter blocks are more or less uniformly distributed throughout the polymer. It is proposed that avoiding very short polymer chains will increase the degree of phase segregation, thus improve the mechanical and conductive properties of proton exchange membrane more efficiently.

Thermal stability of proton exchange fuel-cell membranes based on crosslinked-polytetrafluoroethylene for membrane-electrode assembly preparation

We investigated thermal properties of proton exchange membranes (PEMs) prepared by the radiation-induced grafting of styrene into crosslinked-polytetrafluoroethylene films and the subsequent sulfonation for fuel-cell applications. A conventional thermogravimetric analysis was found to be unreliable because the resulting curve varied greatly with the heating rate. Thus, in order to obtain accurate information, we performed an ex-situ heat-treatment analysis, which involved heating of the PEMs at fixed temperatures of 200–350 °C and measurement of their remaining weight, ion exchange capacity (IEC) and proton conductivity ( σ) after washing in pure water. The IEC and σ did not change at any temperature up to 200 °C, indicating high thermal stability. At 250 °C, however, the PEM properties deteriorated probably via radical cleavage of the C–S bond between a sulfonic acid group and an aromatic ring, and condensation of two sulfonic acid groups. Finally, the PEM was hot-pressed with two electrodes at 200 °C to produce a good membrane-electrode assembly for a fuel cell.

Influence of microstructure and chemical composition on proton exchange membrane properties of sulfonated–fluorinated, hydrophilic–hydrophobic multiblock copolymers

Nanophase separated ionic-hydrophobic block copolymers have shown promise as proton exchange membranes than random copolymers. A series of multiblock copolymers composed of alternating segments of fully disulfonated poly(arylene ether sulfone)s and highly fluorinated poly(arylene ether sulfone)s were evaluated for their potential as proton exchange membranes. Block length and chemical composition were the two major structural variables influencing the properties. Significantly, higher water and proton transport properties were observed for the higher block length materials. Water uptake was controlled by controlling the ratio between the hydrophobic and hydrophilic block lengths. These ion containing multiblock copolymers showed promise as some of the more selective PEMs in terms of achieving both high proton conductivity and low water uptake. Detailed structure–property relationships are provided in the paper.

Toward Improved Conductivity of Sulfonated Aromatic Proton Exchange Membranes at Low Relative Humidity

Three sulfonated aromatic polymers with different sequence lengths were studied in order to better understand the relationship between molecular structure, morphology, and properties of proton exchange membranes as a function of relative humidity. A random copolymer with a statistical distribution of sulfonic acid groups had very small domain sizes, whereas an alternating polymer with sulfonic acid groups spaced evenly along the polymer chain was found to have larger, but quite isolated, domains. The multiblock copolymer studied herein showed highly phase-separated hydrophilic and hydrophobic domains, with good long-range connectivity. Scanning force microscopy as a function of relative humidity was used to observe water absorption and swelling of the hydrophilic domains in each of the three membranes. The conductivity, water sorption kinetics, and fuel cell performance, especially at low relative humidity, were found to be highly dependent upon the morphology. The multiblock copolymer outperformed both t...

Structured polymer electrolyte blends based on sulfonated polyetherketoneketone (SPEKK) and a poly(ether imide) (PEI)

The performance and economics of proton-exchange membrane (PEM) fuel cells are highly dependent on the membranes used to separate the fuel and oxidant. While maintaining reasonable cost, the membrane must feature a number of desirable properties including high proton conductivity. Blends of polymers are one approach to tailoring PEM properties; however, blending to achieve mechanically and chemically robust membranes has generally resulted in reduced conductivity. The objective of this work was to demonstrate the use of field alignment of the proton-conducting domains to increase the conductivity in a polymer blend PEM. A blend of sulfonated poly(etherketoneketone) (SPEKK) and a polyether imide (PEI) was used to illustrate this method. Blends of SPEKK/PEI with a 3:7 mass ratio were aligned using electric field strengths varying from 0 to 30 V/mm and frequencies varying from 0 to 10 kHz. In general, the degree of alignment agreed with theoretical predictions for the alignment of drops or particles suspended in a fluid with a different dielectric constant, e.g., when the frequency of the applied ac field was increased, the threshold field for phase alignment increased and the diameter of the oriented columnar structures decreased. Alignment resulted in up to three orders of magnitude increase in conductivity at low humidity. By careful selection of temperature and residual solvent content, alignment was shown to be possible in the melt state, which is essential for an economic process for producing alignment-enhanced membranes.

Comparison of PEM Properties of Copoly(aryl ether ether nitrile)s Containing Sulfonic Acid Bonded to Naphthalene in Structurally Different Ways

The study represents a comparative investigation of three series of high molecular weight aromatic poly(aryl ether ether nitrile) copolymers (SPAEEN-n), containing different sulfonated naphthalene groups, where n (in the range 0−80 mol %) refers to molar percentage of repeat units containing the sulfonic acid function. The three series of SPAEEN-n copolymers were synthesized via potassium carbonate-mediated nucleophilic polycondensation reactions of commercially available monomers: 2,6-difluorobenzonitrile (2,6-BFBN), one of three dihydroxynaphthalenesulfonate (DHNS) monomers, and unsulfonated 4,4‘-biphenol (BP), incorporated for the purpose of adjusting the sulfonic acid content. The incorporation of the sulfonated repeat unit into copolymers and their structures were confirmed by 1H NMR and FT-IR spectroscopy. Products were solvent-cast into membranes of varying ionic content as proton exchange membrane (PEM) materials for fuel cells applications. Ion exchange capacity (IEC), thermal stabilities, swelling, and proton conducting properties of the membranes were investigated in relation to their structures and compared with those of perfluorinated ionomer (Nafion 117). Films of SPAEENH copolymers in acid form all showed high thermal stabilities with glass transition temperatures (Tg) > 224 °C and decomposition temperatures (Td) > 264 °C. In general, the SPAEENH copolymers exhibited low moisture absorption and in some instances high proton conductivities (from 10-2 to 10-1 S/cm) comparable to that of Nafion 117 at similar water uptake, swelling, and H2O/SO3H ratio.

Transport Properties of Proton Exchange Membranes

Disulfonated poly(arylene ether sulfone) random copolymers have shown promise as potential candidates for proton exchange membranes. The chemical structure of these copolymers has served as a basis for the molecular design of new random and multiblock copolymers. In random copolymers the sulfonic acid is distributed statistically, while in the multiblock copolymers the chemical structure is designed to have co continuous hydrophilic and hydrophobic domains. The effect of these variables on the transport properties of the membranes was investigated. For the random copolymers, high proton conductivity was observed at higher IEC along with high methanol permeability. But for the multiblock copolymers, it was possible to combine high proton conductivity with low methanol permeability. Improved proton conductivity was observed under partially hydrated conditions for samples with higher block lengths. Self diffusion coefficient of water was also measured by NMR which provided better understanding of the transport process.

Highly Fluorinated Comb‐Shaped Copolymers as Proton Exchange Membranes (PEMs): Improving PEM Properties Through Rational Design

AbstractA new class of comb‐shaped polymers for use as a proton conducting membrane is presented. The polymer is designed to combine the beneficial physical, chemical, and structural attributes of fluorinated Nafion‐like materials with higher‐temperature, polyaromatic‐based polymer backbones. The comb‐shaped polymer unites a rigid, polyaromatic, hydrophobic backbone with lengthy hydrophilic polymer side chains; this combination affords direct control over the polymer nanostructure within the membrane and results in distinct microphase separation between the opposing domains. The microphase separation serves to compartmentalize water into the hydrophilic polymer side chain domains, resulting in effective membrane water management and excellent proton conductivities.

Influence of chemical composition and sequence length on the transport properties of proton exchange membranes

AbstractOne of the integral parts of the fuel cell is the proton exchange membrane. Our research group has been engaged in the past few years in the synthesis of several sulfonated poly(arylene ether) random copolymers. The copolymers were varied in both the bisphenol structure as well as in the functional groups in the backbone such as sulfone and ketones. To compare the effect of sequence length, multiblock copolymers based on poly(arylene ether sulfone)s were synthesized. This paper aims to describe our investigation of the effect of chemical composition, morphology, and ion exchange capacity (IEC) on the transport properties of proton conducting membranes. The key properties examined were proton conductivity, methanol permeability, and water self diffusion coefficient in the membranes. It was observed that under fully hydrated conditions, proton conductivity for both random and block copolymers was a function of IEC and water uptake. However, under partially hydrated conditions, the block copolymers showed improved proton conductivity over the random copolymers. The proton conductivity for the block copolymer series was found to increase with increasing block lengths under partially hydrated conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2226–2239, 2006

Measuring moisture sorption and diffusion kinetics on proton exchange membranes using a gravimetric vapor sorption apparatus

The understanding of water sorption and diffusion properties of proton exchange membranes is crucial to the fuel cell's ultimate performance. In this study, a dynamic gravimetric vapor sorption (DVS) instrument was used to measure the water vapor sorption properties of three Nafion ® based fuel cell membranes: N-117 (extruded film, 183 μm thick); N-112 (extruded film, 51 μm thick); and NR-112 (dispersion cast film, 51 μm thick). Water sorption characteristics were studied between 0 and 95% relative humidity (RH) at 30, 40, 50, 70, and 80 °C. The thicker dispersion cast, N-117, film had a lower water vapor sorption capacity (based on percentage weight gain) than the thinner, N-112 sample. The dispersion cast, NR-112, film had a lower percentage water uptake than the extruded, N-112, film. Below 80% RH, the water sorption capacity increases with temperature for all three samples. Above 80% RH, the moisture sorption capacity increases from 30 to 50 °C, but decreases at 70 and 80 °C compared to the lower temperature data. Moisture diffusion coefficients were also calculated over the humidity and temperature range studied. In general, maximum diffusion coefficients were measured at intermediate humidities. Water heat of sorption calculations at low coverages yielded higher values for the extruded (N-112) film compared to the dispersion cast (NR-112) film indicating a higher affinity for water.