Performance and Durability of Membrane Electrode Assemblies Based on Radiation‐Grafted FEP‐g‐Polystyrene Membranes

Membrane electrode assemblies (MEAs) based on radiation‐grafted proton exchange membranes developed at PSI have shown encouraging performance in the past in hydrogen and methanol fuelled polymer electrolyte fuel cells. In this study, the effect of the pre‐treatment of crosslinked radiation‐grafted FEP membranes prior to lamination with the electrodes on the performance of the MEAs was investigated. Two approaches were assessed separately and in combination: (1) the impregnation of the radiation‐grafted membranes with solubilised Nafion®, and (2) the use of a swollen vs. dry membrane. It is found that the combination of coating the membrane with Nafion® ionomer and hot‐pressing the MEA with the membrane in the wet state produce the best single cell performance. In the second part of the study, the durability of an MEA, based on a radiation‐grafted FEP membrane, was investigated. The performance was stable for 4,000 h at a cell temperature of 80 °C. Then, a notable degradation of the membrane, as well as the electrode material, started to occur as a consequence of either controlled or uncontrolled start‐stop cycles of the cell. It is assumed that particular conditions, to which the cell is subjected during such an event, strongly accelerate materials degradation, which leads to the premature failure of the MEA.     

Evaluating the Effectiveness of Using Flexography Printing for Manufacturing Catalyst‐Coated Membranes for Fuel Cells

This study is an evaluation of the effectiveness of the flexography printing process for manufacturing catalyst‐coated membranes (CCMs) for use in proton exchange membrane fuel cells (PEMFCs). Flexography is a maskless and continuous process that is used in large‐scale production with water‐based inks to reduce the cost of production of PEMFC components. Unfortunately, water has undesirable effects on the Nafion® membrane: water wets the membrane surface poorly and causes the membrane to bulge outwards significantly. Membrane printability was improved by pre‐treating membrane samples by water immersion for short periods (<2 min). This pre‐treatment was used to control the membrane deformation before printing to limit the impact of the ink transfer. Water and ink drop deposition experiments were performed to estimate the liquid‐air‐Nafion® apparent contact angle and the locally induced membrane deformation. Despite the short immersion times used in the tests, the immersion pre‐treatment appeared to induce structural modifications that enhanced both the membrane wettability and the dimensional stability. Flexography printability tests were performed on these treated membranes and showed that the dimensional instability of the Nafion® membrane was the critical parameter for limiting the ink transfer. The immersion pre‐treatment improved the printability of the Nafion® membranes, which were used to fabricate cathodes that were tested in an operational fuel cell.     

A Sulfophenylated Polysulfone as the DMFC Electrolyte Membrane – an Evaluation of Methanol Permeability and Cell Performance

A sulfophenylated polysulfone (PSU‐sph), carrying 0.8 sulfonic acid units per repeating unit of the polymer, is evaluated as a membrane electrolyte for DMFC applications. The liquid uptake, methanol transport characteristics, electrolyte conductivity, and fuel cell performance are investigated. The methanol transport and DMFC performance results are compared to those of Nafion® 117. The PSU‐sph membrane investigated shows superior qualities with regard to methanol crossover, with a methanol permeability of approximately 25% compared to that of Nafion®. The conductivity was measured to be 15% compared to that of Nafion®. However, this could not fully account for the internal resistance of the cell, implying that the contact resistance between the electrodes and electrolyte is higher when PSU‐sph is used, probably because the electrodes are developed for use with Nafion® membranes. The stability of the PSU‐sph membrane seems promising, with very low degradation observed over a period of 72 hours. It was concluded that although the mass transport properties of the PSU‐sph membrane sample investigated were superior, it could not match the performance of Nafion® 117 in a DMFC application. However, a higher degree of sulfonation may have a significant positive effect on cell performance. The results also showed that a fully intergrated MEA is needed to fully assess new menbrane materials.     

Durability enhancement of Nafion® fuel cell membranes via in situ sol‐gel‐derived titanium dioxide reinforcement

To improve durability of Nafion® membranes, samples were modified via an in situ sol‐gel polymerization of titanium isopropoxide to generate titania quasi‐networks in the polar domains. The incorporated titania reduced water uptake but equivalent weight was essentially unchanged. Fuel cell performance of the modified membrane was inferior to that of the unfilled membrane although these were considered as model studies with focus on mechanical durability. Mechanical analysis of contractile stress buildup during drying from a swollen state in samples clamped at constant length demonstrated considerable reinforcement of Nafion® by the titania structures. Tensile studies showed that at 80°C and 100% relative humidity the dimensional change of the composite membrane is one half and the initial modulus is three times higher than that of the unmodified membrane. During an open circuit voltage decay test the voltage decay rate for the modified membrane is 3.5 times lower than that of control Nafion®. Fluoride emission for the composite is at least an order of magnitude lower than that of the control Nafion® membrane indicating reduced chemical degradation. These model studies indicate that this in situ inorganic modification offers a way to enhance fuel cell membrane durability by reducing both physical and chemical degradation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Performance Study of Direct Borohydride Fuel Cells Employing Polyvinyl Alcohol Hydrogel Membrane and Nickel‐Based Anode

A direct borohydride fuel cell (DBFC) employing a polyvinyl alcohol (PVA) hydrogel membrane and a nickel‐based composite anode is reported. Carbon‐supported platinum and sputtered gold have been employed as cathode catalysts. Oxygen, air and acidified hydrogen peroxide have been used as oxidants in the DBFC. Performance of the PVA hydrogel membrane‐based DBFC was tested at different temperatures and compared with similar DBFCs employing Nafion® membrane electrolytes under identical conditions. The borohydride–oxygen fuel cell employing PVA hydrogel membrane yielded a maximum peak power density of 242 mW cm^–2 at 60 °C. The peak power densities of the PVA hydrogel membrane‐based DBFCs were comparable or a little higher than those using Nafion® 212 membranes at 60 °C. The fuel efficiency of borohydride–oxygen fuel cell based on PVA hydrogel membrane and Ni‐based composite anode was found to be between 32 and 41%. The cell was operated for more than 100 h and its performance stability was recorded.     

Temperature Effects on the Molecular Dynamics of Modified Nafion® Membranes Incorporating Ionic Liquids' Cations: A 1H NMRD Study

Nafion® is the polymer electrolyte membrane most used in fuel cells (PEMFC), however Nafion® presents some limitations due to the water loss with increasing temperature. In this work is presented the study of the molecular dynamics in modified Nafion®/ionic liquids (IL) cations membranes with increasing temperature, by proton NMR relaxometry (¹H NMRD). Three Nafion® membranes modified with Phenyltrimethylammonium (TMPA⁺), 1‐n‐butyl‐3‐methylimidazolium (BMIM⁺) and n‐dodecyltrimethylammonium (DTA⁺) IL cations were considered. This study allowed the evaluation of the effect of the IL cations incorporation in Nafion® membranes and to assess the degree of confinement of the IL cations in the membranes matrix in relation with the water content. Thermogravimetry analysis was also performed to study the water loss with increasing temperature of the Nafion®/IL cations membranes. It was possible to establish a correlation between the water content and the IL cations self‐diffusion coefficients. The sequential order of the hydration level obtained for the studied systems was Nafion®/DTA⁺>Nafion®/BMIM⁺ ≳ Nafion®/H⁺>Nafion®/TMPA⁺, while the water loss follows the sequence Nafion®/H⁺>Nafion®/BMIM⁺∼Nafion®/DTA⁺>Nafion®/TMPA⁺. The Nafion®/BMIM⁺ presented the largest temperature variation of both the self‐diffusion coefficient and the hydration conditions. In terms of PEMFCs efficiency, the Nafion®/DTA⁺ modified membrane seems to offer the largest and stable hydration conditions with temperature.     

Waterproofing of carbon paper by plasma polymerization

Waterproof treatment by plasma polymerization was applied to the porous material of proton exchange membrane fuel cells (PEMFCs). Carbon paper used for PEMFCs was waterproofed by the gas plasma of hexafluoropropylene (HFP) prior to binding to the polymer electrolyte membrane, Nafion®. A plasma‐polymerized HFP thin film on the surface of the carbon fiber increased the contact angle of the carbon paper. The inner surface of the carbon paper was also waterproofed as well as its outer surface. It was found from the fuel cell performance test that this waterproofing method of the carbon paper was effective in the high‐current density region where the condensed water product tends to accumulate. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1748–1753, 2006     

Grafting of 4‐Hydroxybenzenesulfonic Acid onto Commercially Available Poly(VDF‐co‐HFP) Copolymers for the Preparation of Membranes

The grafting of a phenate bearing sulfonate group in solution onto commercially available poly(VDF‐co‐HFP) copolymers, where VDF and HFP stand for vinylidene fluoride and hexafluoropropene, respectively, is presented. This reaction leads to novel fluoropolymers, bearing aryl sulfonic acid side functions, which are fuel cell membrane precursors. A mechanism similar to the grafting of bisphenol onto VDF‐containing copolymers is discussed. First, the sulfonate phenate is modified to give the didecyldimethylammonium bromide sulfonate phenate salt, in order to promote the substitution onto a fluorine atom in VDF unit adjacent to one HFP unit onto a fluorine atom in the copolymer. The substitution of this salt onto the fluorinated copolymer yields low molar percentages of grafted phenate, ranging from 1.8 to 5.1 mol‐%, whereas it reaches values up to 13 mol‐% grafting when the NH₂‐CH₂‐CH₂‐S‐CH₂‐CH₂‐C₆H₄‐SO₃Na amine is used as the grafting agent. NMR characterization is used to monitor the grafting process. The electrochemical properties of the resulting phenate grafted‐poly(VDF‐co‐HFP) copolymer are studied. The theoretical ion exchange capacities are half that of Nafion®. The proton conductivities are also lower than that of Nafion®, although one conductivity measurement reached a value of 5.1 mS cm^–1, showing a non‐negligible conductivity. The water uptake is lower than these noted for a sulfonated amine‐grafted copolymer, and is of the same order as that for Nafion®. Finally, it is shown that these novel materials start to decompose above 200 °C, showing a similar thermostability as that of an amino‐containing aryl sulfonate‐grafted poly(VDF‐co‐HFP) copolymer.     

Stereoselective Behavior of Nafion® Membranes towards (+)‐α‐Pinene and (−)‐α‐Pinene

α‐Pinene enantiomers were sorbed in Nafion® membranes. The membranes included a commercial extruded Nafion® 115 membrane as well as membranes prepared by casting a Nafion® solution, evaporating the solvent, and a thermal treatment at different temperatures. The microstructure of membranes was studied by small‐angle and wide‐angle X‐ray scattering, and magic‐angle spinning nuclear magnetic resonance spectroscopy. The change of membrane weight during the sorption process was determined with a sorption microbalance. Noticeable differences concerning the sorption behavior of the various membranes could be stated. The sorption of (+)‐α‐pinene and (?)‐α‐pinene in an extruded Nafion® membrane turned out to be rather low.     

Influence of Radiation‐Induced Grafting Process on Mechanical Properties of ETFE‐Based Membranes for Fuel Cells

The mechanical stability is, in addition to thermal and chemical stability, a primary requirement of polymer electrolyte membranes in fuel cells. In this study, the impact of grafting parameters and preparation steps on stress–strain properties of ETFE‐based proton conducting membranes, prepared by radiation‐induced grafting and subsequent sulphonation, was studied. No significant change in the mechanical properties of the ETFE base film was observed below an irradiation dose of 50 kGy. It was shown that the elongation at break decreases with increasing both the crosslinker concentration and graft level (GL). However, the tensile strength was positively affected by the crosslinker concentration. Yield strength and modulus of elasticity are almost unaffected by the introduction of crosslinker. Interestingly, yield strength and modulus of elasticity increase gradually with GL without noticeable change of the inherent crystallinity of grafted films. The most brittle membranes are obtained via the combination of high GL and crosslinker concentration. The optimised ETFE‐based membrane (GL of ∼25%, 5% DVB v/v), shows mechanical properties superior to those of Nafion® 112 membrane. The obtained results were correlated qualitatively to the other ex situ properties, including crystallinity, thermal properties and water uptake of the grafted membranes.     

Sulfonated polyimide/chitosan composite membrane for vanadium redox flow battery: Membrane preparation,characterization, and single cell performance

A novel sulfonated polyimide/chitosan (SPI/CS) composite membrane was prepared from self‐made SPI (50% of sulfonation degree) through an immersion and self‐assembly method, which was successfully applied in vanadium redox flow battery (VRB). The proton conductivity of SPI/CS composite membrane is effectively improved compared to the plain SPI membrane. The VO²⁺ permeability coefficient across SPI/CS composite membrane is 1.12 × 10^?7 cm² min^?1, which is only one tenth of that of Nafion® 117 membrane. Meanwhile, the proton selectivity of SPI/CS composite membrane is about eight times higher than that of Nafion® 117 membrane. In addition, the oxidative stability SPI/CS composite membrane is superior to that of pristine SPI membrane. The VRB single cell using SPI/CS composite membrane showed higher energy efficiency (88.6%) than that using Nafion® 117 membrane, indicating that SPI/CS composite membrane is a promising proton conductive membrane for VRB application. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Transport properties of sulfonated poly (styrene‐isobutylene‐styrene) membranes with counter‐ion substitution

In this study, the transport properties of poly(styrene‐isobutylene‐styrene) (SIBS) were determined as a function of sulfonation level (0–94.9%) and counter‐ion substitution (Ba⁺², Ca⁺², Mg⁺², Mn⁺², Cu⁺², K⁺¹) for fuel cell applications. Increasing the sulfonation level improved the ion exchange capacity (IEC) of the membranes up a maximum (1.71 mequiv/g), suggesting a complex three‐dimensional network at high sulfonation levels. Results show that proton conductivity increases with IEC and is very sensitive to hydration levels. Methanol permeability, although also sensitive to IEC, shows a different behavior than proton conductivity, suggesting fundamental differences in their transport mechanism. The incorporation of counter‐ion substitution decreases both methanol and proton transport. Methanol permeability seems to be related to the size of the counter‐ion studied, while proton conductivity is more sensitive to water content, which is also reduced upon the incorporation of counter‐ions. To complement the studies, selectivity (i.e., proton conductivity/methanol permeability) of the studied membranes was determined and compared to Nafion® 117. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nafion® reinforced with polyacrylonitrile/ZrO2 nanofibers for direct methanol fuel cell application

Nafion® membrane blended with polyacrylonitrile nanofibers decorated with ZrO₂ was successfully fabricated. The composite membrane showed improved proton conductivity, swelling ratio, thermal and mechanical stability, reduced methanol crossover, and enhanced fuel cell efficiency. The nanocomposite membranes achieved a reduced methanol crossover of 5.465 × 10⁻⁸ cm² S⁻¹ compared to 9.118 × 10⁻⁷ cm² S⁻¹ of recast Nafion® membrane using a 5 M methanol solution at 80°C. The composite membrane also showed an ion conductivity of 1.84 compared to 0.25 S cm⁻¹ recast Nafion® at 25°C. The composite membranes showed a peak power density of 68.7 mW·cm⁻² at 25°C, these results show a promising composite membrane for fuel cell application.     

Sorption and transport properties of the semi‐interpenetrating network based on crosslinked poly(vinyl alcohol) and poly(styrene sulfonic acid‐co‐maleic anhydride) as proton‐conducting membranes

This study investigates the sorption and transport properties of hydrocarbon membranes based on poly(vinyl alcohol) network and poly(styrene sulfonic acid‐co‐maleic acid) (PSSA‐MA). The water and methanol self‐diffusion coefficients through an 80 wt % PSSA‐MA interpenetrating SIPN‐80 membrane measured 3.75 × 10^?6 and 5.47 × 10^?7 cm²/s, respectively. These results are lower than the corresponding values of Nafion® 115 (8.89 × 10^?6 cm²/s for water and 8.63 × 10^?6 cm²/s for methanol). The methanol permeability of SIPN‐80 membrane is 4.1 × 10^?7 cm²/s, or about one‐fourth that of Nafion® 115. The difference in self‐diffusion behaviors of Nafion® 115 and SIPN‐80 membranes is well correlated with their sorption characteristics. The solvent uptake of Nafion® 115 increased as the methanol concentration increased up to a methanol mole fraction of 0.63, and then decreased. However, the solvent uptake of the SIPN‐80 membranes decreased sluggishly as the methanol concentration increased. The λ values of water and methanol (i.e., λ and λ) in Nafion® 115 are quite close, indicating no sorption preference between water and methanol. In contrast, the λ value is only one‐third λ for a SIPN‐80 membrane. Accordingly, the SIPN membranes are regarded as candidates for direct methanol fuel cell applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

High Performance Fuel Cell Operation with a Non‐fluorinated Polyphosphazene Electrode Binder

Non‐fluorinated sulphonated polyphosphazene (SPOP) was synthesised and characterised for utilisation as an electrode binder. Polarisation curves were obtained at 80 °C and 95% RH in a H₂/air fuel cell. SPOP has the proper properties for a cathode binder in a fuel cell, as its polarisation curve traced that of Nafion® binder for the same operating conditions.     

Sulfonated polyimide and poly (ethylene glycol) diacrylate based semi‐interpenetrating polymer network membranes for fuel cells

Semi‐interpenetrating polymer network (semi‐IPN) membranes based on novel sulfonated polyimide (SPI) and poly (ethylene glycol) diacrylate (PEGDA) have been prepared for the fuel cell applications. SPI was synthesized from 1,4,5,8‐naphthalenetetracarboxylic dianhydride, 4,4′‐diaminobiphenyl 2,2′‐disulfonic acid, and 2‐bis [4‐(4‐aminophenoxy) phenyl] hexafluoropropane. PEGDA was polymerized in the presence of SPI to synthesize semi‐IPN membranes of different ionic contents. These membranes were characterized by determining, ion exchange capacity, water uptake, water stability, proton conductivity, and thermal stability. The proton conductivity of the membranes increased with increasing PEGDA content in the order of 10^?1 S cm^?1 at 90°C. These interpenetrating network membranes showed higher water stability than the pure acid polyimide membrane. This study shows that semi‐IPN SPI membranes based on PEGDA which gives hydrophilic group and structural stability can be available candidates comparable to Nafion® 117 over 70°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Fuel Cell Membranes Based on Grafted and Post‐Sulfonated Glycidyl Methacrylate (GMA)

Glycidyl methacrylate (GMA) was pre‐irradiation grafted into ETFE base film of 25 μm thickness up to graft levels of 300%. The grafted films were sulfonated using a mixture of sulfite and bisulfite. FTIR and SEM–EDX analysis of the synthesized films and membranes was performed to confirm the grafting and the sulfonation. A pronounced front mechanism for grafting of GMA into ETFE was found. Regarding ex situ fuel cell relevant properties, conductivities of up to 0.25 S cm^–1 were attained. For the first time, fuel cell testing of this type of membrane is reported. These grafted membranes performed comparable to a commercial benchmark membrane (Nafion® 212) and better than a styrene‐based grafted membrane with similar conductivity. Post‐test FTIR analysis showed that a fraction of the grafted chains was lost during the test under constant current conditions, yet the membrane still exhibited superior durability compared to a styrene‐based grafted membrane. Hydrolysis of the methacrylate groups was shown not to be the principle cause of the loss of sulfonic acid groups.     

Sulfonated poly(arylene ether ketone)s prepared by direct copolymerization as proton exchange membranes: Synthesis and comparative investigation on transport properties

Sulfonated poly(aryl ether ketone)s (SPAEK) copolymers were synthesized by aromatic nucleophilic polycondensation from 3,3′, 5,5′‐tetramethyl‐4, 4′–biphenol, 1,4‐bis(4‐fluorobenzoyl) benzene, and disulfonated difluorobenzophenone. The SPAEK membranes did not exhibit excessive swelling in hot water and at the same time show the proton conductivities in the range of 0.030 S/cm to 0.099 S/cm at 80°C. The methanol diffusion coefficients of the SPAEK membranes were in the range of 4.7 × 10^?7 to 8.1 × 10^?7cm²/s measured at 25°C. The transport properties of this series of SPAEK copolymers were compared to poly(aryl ether ether ketone)s (SPEEK), poly(aryl ether ether ketone ketone)s (SPEEKK), and Nafion® membranes. It was found that the transport properties (including proton conductivity and methanol permeability) follows the trend of SPEEKK‐60 < SPAEK‐60 < SPEEK‐60 < Nafion® 117, the order of which is also attributed to the differences in the chemical structure of the polymers and the membrane morphology. In general, this novel series of SPAEK membranes possess various advantages, such as low cost of the initial monomers, high thermal and mechanical stability, and low methanol permeability while simultaneously possessing sufficient proton conductivity, which makes them notably promising as proton exchange membrane (PEM) materials in direct methanol fuel cell (DMFC) applications. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Fabrication of Function‐Graded Proton Exchange Membranes for Direct Methanol Fuel Cells Using Electron Beam‐Grafting

Function‐graded proton exchange membranes (G‐PEMs) based on poly(tetrafluoroethylene‐co‐hexafluoropropylene) were fabricated for direct methanol fuel cells (DMFCs) via electron beam‐grafting using the heterogeneous energy deposition technique. The G‐PEMs had a water uptake gradient in the proton transfer direction, originating from the sulfonic acid group gradient. The distribution of sulfonic acid groups in the various G‐PEMs was evaluated using X‐ray photoelectron spectroscopy. Four types of PEMs (flat‐type, strong‐gradient, meso‐gradient, and weak‐gradient types) were fabricated. By varying the direction of the G‐PEMs, the methanol permeation test and DMFC operation were performed with two orientations of the sulfonic acid group gradient, decreasing from the methanol injection (anode) side (decrease‐type) or the other (cathode) side (increase‐type). The methanol permeability of the strong‐gradient, meso‐gradient, and weak‐gradient G‐PEMs was lower than that of Nafion®117 and the flat‐type PEM. The “increase‐type” orientation of the strong‐gradient G‐PEM resulted in the lowest methanol permeability. The DMFC performance of the G‐PEMs was influenced by the thickness direction, such as “decrease‐type” and “increase‐type.” The performance of the “decrease‐type” assembly was higher than that of the “increase‐type.” The “decrease‐type” assembly with P‐200 k (weak‐gradient G‐PEM) exhibited the highest performance of the fabricated PEMs, comparable to that of Nafion®117.     

Tubular Cathode Prepared by a Dip‐Coating Method for Low Temperature DMFC

A novel tubular cathode for the direct methanol fuel cell (DMFC) is proposed, based on a tubular titanium mesh. A dip‐coating method has been developed for its fabrication. The tubular cathode is composed of titanium mesh, a cathode diffusion layer, a catalyst layer, and a recast Nafion® film. The titanium mesh is present at the inner circumference of the diffusion layer, while the recast Nafion® film is at the outer circumference of the catalyst layer. A DMFC single cell with a 3.5 mgPt cm^–2 tubular cathode was able to perform as well, in terms of power density, as a conventional planar DMFC. A peak power density of 9 mW cm^–2 was reached under atmospheric air at 25 °C.