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.     

A Porous Silicon‐Based Ionomer‐Free Membrane Electrode Assembly for Miniature Fuel Cells

Previous work showed the pertinence of using grafted porous silicon as the proton exchange membrane for miniature fuel cells. One of the limitations was the membrane‐electrodes assembly, which required an ionomer, in the current study a 5% Nafion®‐117 solution, to ensure a proton‐conducting link between the commercial carbon cloth electrodes and the membrane. Here, new developments for this fuel cell, with a totally Nafion®‐free process, are reported. The Pt catalyst is sputtered and electrodeposited onto the surface of the proton conducting porous silicon membrane. The initial performance of this fuel cell is shown and demonstrates the validity of the technique.     

Relationship Between Methanol Permeability and Structure of Different Radiation‐Grafted Membranes

Styrene grafted and sulfonated poly(vinylidene fluoride) and poly(vinylidene fluoride‐co‐hexafluoropropylene) films are candidates as electrolytes in direct methanol fuel cells. Their behaviour in water, 1 and 3 mol dm^–3 aqueous methanol, and pure methanol were studied. According to SAXS results, water and methanol‐water solutions have similar effects on the membranes, i.e., the lamellar period increases and the ionic domains enlarge. Furthermore, differences in the ionic domain structures in pure methanol and water were observed. These structural changes together with dissimilar liquid uptakes in water and in methanol are reflected as changes in the conductivities. An increase in the SAXS intensity and changes in the Bragg distance of the ionic peak were observed in methanol compared to aqueous solutions. This may be related to the hydrophobicity of the CH₃ group on methanol. Dissimilarities in methanol permeability through the radiation‐grafted membrane can be related to structural differences in membranes observed with SAXS. Permeabilities were observed to be lower for the radiation‐grafted membranes compared to Nafion® 115, which compensates for the higher area resistance of the experimental membranes and thus improves their performance in a fuel cell.     

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     

Effect of Compression Cycling on Polybenzimidazole‐based High‐Temperature Polymer Electrolyte Membrane Fuel Cells

Contact pressure cycling experiments have been conducted with various commercial high temperature PEM membrane‐electrode‐assemblies based on phosphoric acid doped PBI. Two different membrane‐electrode‐assembly types have been electrochemically investigated employing linear sweep voltammetry, electrochemical impedance spectroscopy and polarization curves, but also micro‐computed tomography imaging has been used as post‐mortem investigation technique. Thickness displacement changes on the membrane‐electrode‐assemblies (MEA) during the experiences have also been recorded. Reversible and irreversible effects have been observed in MEA behavior during the three contact pressure cycles. Furthermore, the micro‐computed tomography tool allows a detailed visual insight into the structural effects of compression forces on the MEA. The electrochemical characterization has revealed that damages under contact pressure cycling have been induced in both kinds of MEAs. Moreover, once MEA damages have appeared, they are facilitated from cycle to cycle. These damages are related to hydrogen crossover and short circuit formation that develop fuel cell performance deterioration. Thus, micro‐computed tomography imaging investigations reveal defects, pin holes or cracks within the catalyst layer and membrane e.g., which may cause degradation aspects like hydrogen crossover or loss of electrical isolation already observed by the electrochemical characterization.     

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.     

Ionomer Membrane and MEA Development for DMFC

Abstract

Membrane-electrode assemblies (MEAs) have been prepared from different acid-base-blends consisting of different sulfonated arylene main-chain polymers and polybenzimidazole PBI and a microphase-separated arylene main-chain block copolymer consisting of a sulfonated and proton-conducting and a hydrophobic microphase. The MEAs have been prepared using 4 different methods: Method A: The membrane has been prepared first as a free film, and the electrodes have subsequently been coated onto the membrane; Method B: The membrane has been prepared first as a polyester-supported film, and the electrodes have thereafter been coated onto the membrane; Method C: The MEA has been built up from the anode; Method D: The MEA has been built up from the cathode. The MEAs have been tested under different temperatures and different meOH concentrations. Three different polyacid-PBI blend membranes could be identified which showed comparable or even better DMFC performance than Nafion®105: a sulfonated polyethersulfone-PBI blend membrane, a sulfonated polyetherketone-PBI blend membrane, and a partially fluorinated sulfonated polyether-PBI blend membrane. The proton-conducting block co-ionomer membrane initially showed an excellent DMFC performance due to reduced meOH permeability, compared to the polyacid-PBI blend membranes, which however degraded with time of the DMFC operation probably being due to irreversible morphology changes. Among all tested MEAs the MEAs prepared by Method B showed the best DMFC performance. The DMFC performance of the MEAs prepared by Method C and Method D was slightly worse than that of the MEAs made via Method B. The DMFC performance of a MEA from the sulfonated polyetherketone-PBI blend membrane which was built up using Method D improved steadily during 4 weeks of DMFC operation.     

Characterization of Membrane Electrode Assemblies for High‐Temperature PEM Fuel Cells

This paper will present the characterization of two types of membrane‐electrode‐assemblies (MEAs) for high‐temperature polymer electrolyte membrane fuel cells (HT‐PEMFC) working under reformate stream. The important aspects to be considered in the characterization of these MEAs are: (i) presence of contaminants, and (ii) composition of the anode. Start/stop cycling test were performed for two different Dapozol® MEAs using different GDL materials, using first hydrogen and then synthetic reformate as a fuel gas, both with a dew point of 80 °C. With these results the influence of contaminants present in the reformate was compared for the two types of MEAs, showing the superior performance of the Dapozol® 101 MEA under these conditions. The possibility to further enhance the MEAs' resilience against the operation of reformates by changing the anode catalyst composition was evaluated in a half MEA configuration, considering that the impact of the H₂S present in the fuel presents a major issue. For this reason the hydrogen oxidation reaction (HOR) was evaluated for two types of Pt‐based electrocatalysts in an anodic half MEA configuration using different hydrogen‐rich fuel mixtures. These results provide valuable information for the optimization of the MEA and the anode catalyst for HT‐PEMFC.     

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.     

The Ionic Conductivity of a Nafion® 1100 Series of Proton‐exchange Membranes Re‐cast from Butan‐1‐ol and Propan‐2‐ol

The ionic conductivity of Nafion® 1100 extruded membranes re‐cast from solutions of butan‐1‐ol and propan‐2‐ol is measured in 0.5 mol dm^–3 H₂SO₄ at 295 K, using an immersed, four‐electrode d.c. technique. The general trend is an increasing conductivity for the thicker membranes. Materials which were solution‐cast from butan‐1‐ol yielded the highest conductivity while a series of membranes with lower conductivities (similar to those of an extruded Nafion® 1100 series of membranes) was found using propan‐2‐ol. The conductivity results indicate that membranes manufactured by extrusion and casting from various solvents might have different structures. Differences in the water content and conductivity of the membranes are considered to arise from the impact of processing conditions on the surface and bulk structure of the membranes.     

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.     

Nanostructured membrane electrode assemblies from layer‐by‐layer composite/catalyst containing membranes and their fuel cell performances

In this study, two approaches are compared to develop nanostructured membrane electrode assemblies (MEA) using layer‐by‐layer (lbl) technique. The first is based on the direct deposition of polyallylamine hydrochloride (PAH) and sulfonated polyaniline (sPAni) on Nafion support to prepare lbl composite membrane. In the second approach, sPAni is coated on the support in the presence of platinum (Pt) salt, Nafion solution and Vulcan for obtaining catalyst containing membranes (CCMs). SEM and UV–vis analysis show that the multilayers are deposited on both sides of Nafion successfully. Although H₂/O₂ single cell performances of acid doped lbl composite membrane based MEA are found to be at the range of 126 and 160 mW cm^?2 depending on the number of deposited layers, the cell performance of MEA obtained from catalyst containing lbl self‐assembled thin membrane (PAH/sPAni‐H⁺)_10‐Pt is found to be 360 mW cm^?2 with a Pt utilization of 720 mW mgPt^?1. This performance is 82% higher as compared to original Nafion^®117 based MEA (198 mW cm^?2). From the cell performance evaluations for different structured MEAs, it is mainly found out that the use of lbl CCMs instead of composite membranes and fabrication of thinner electrolytes result in a higher H₂/O₂ cell activity due to significant reduction in ohmic resistivity. Also, it is observed that the use of sPAni slightly improves the cell performance due to an increased probability of the triple phase contact and it can lead to superior physicochemical properties such as conductivity and thermal stability. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40314.     

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.     

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.     

Gas Diffusion Layers for Proton Exchange Membrane Fuel Cells Using In situ Modified Carbon Papers with Multi‐walled Carbon Nanotubes Nanoforest

Gas diffusion layers (GDLs) in the proton exchange membrane fuel cells (PEMFCs) enable the distribution of reactant gases to the reaction zone in the catalyst layers by controlling the water in the pore channels apart from providing electrical and mechanical support to the membrane electrode assembly (MEA). In the present work, we report the in situ growth of carbon nanotubes nanoforest (CNN) directly onto macro‐porous carbon paper substrates. The surface property as analysed by a Goniometer showed that the CNN/carbon paper surface is highly hydrophobic. CNN/carbon paper was employed as a GDL in an MEA using Nafion‐212 membrane as an electrolyte and evaluated in single cell PEMFCs. While the GDLs prepared by wire‐rod coating process have major performance losses at lower humidities, the in situ CNN/carbon paper, developed in this work, shows very stable performance at all humidity conditions demonstrating a significant improvement for fuel cell performance. The CNN/carbon‐based MEAs showed very stable performance with power density values of ∼1,100 and 550 mW cm^–2, respectively, both using O₂ and air as oxidants at ambient pressure.     

Performance and durability of sulfonated poly(arylene ether sulfone) membrane-based membrane electrode assemblies fabricated by decal method for polymer electrolyte fuel cells

The combination of Nafion-based electrode and hydrocarbon-based membrane is an ideal choice for researcher in making membrane electrode assemblies (MEAs) containing alternative membranes replacing Nafion for polymer electrolyte fuel cells (PEFCs) due to their intrinsic properties. This advantage, however, is limited by the incompatibility between the membrane and the electrode, which results in MEA performance decay and low durability. In this study, we propose fabrication of MEA made of sulfonated poly(aryl ether sulfone) (SPES) membrane and Nafion-based electrode using the decal process. The decal process was found to be very effective in forming good interface between SPES and the electrode, although hot pressing temperature was relatively low (140 °C). The SPES-MEA revealed comparable performance to conventional Nafion-MEA at high humidity, indicating negligible contact resistance in the SPES–electrode interface. Open circuit voltage (OCV) drop of SPES-MEA during OCV holding at 40% RH for 200 h was from 0.975 V to 0.8 V, implying slight chemical degradation of SPES leading to increased hydrogen crossover in the membrane. However, it seems that the interfacial damage between the SPES and Nafion electrode in the SPES-MEA is negligible during the OCV test. Nonetheless, further investigation is necessary to confirm the long-term stability of the SPES-MEA fabricated by the decal process under harsher conditions such as dry/wet and freeze/thaw cycling.     

A Novel Design of Wave‐Like PEMFC Stack with Undulate MEAs and Perforated Bipolar Plates

Commercialisation of proton exchange membrane fuel cell (PEMFC) technology depends on high volumetric power density and specific power for a given cost. In the present study, a novel wave‐like architecture for PEMFC stack based on undulate membrane electrode assembles (MEAs) and perforated bipolar plates (PBPs) was presented. Different from conventional plate‐and‐frame architecture, this design increased active area and achieved higher volumetric power density due to undulate MEAs. Moreover, perforated sheet metal was used as bipolar plates so that it could improve specific power. A single cell was designed and fabricated in house to evaluate the performance of the novel architecture stack. Stamped PBPs with open rate of 28.26% and hot pressed 5‐layer undulate MEAs with Nafion® 112 were adopted. The results indicated that the peak volumetric power density and specific power are 2,715.94 W L^–1 and 2,157.86 W kg^–1, respectively, while they are 2,151.28 W L^–1 and 1,709.22 W kg^–1 at the output voltage of 0.6 V. This study may propose a possible means to meet the DOE's 2010 technical target that volumetric power density is 2,000 W L^–1 and specific power is 2,000 W kg^–1 for stack.     

SAXS Analysis of Catalyst Degradation in High Temperature PEM Fuel Cells Subjected to Accelerated Ageing Tests

The loss in performance during fuel cell operation is one of the critical factors that hamper fuel cells commercialization. This paper presents a research activity related to high temperature polymer electrode membrane fuel cell (HT‐PEMFC) degradation. The aim of the study is to investigate catalyst degradation of membrane electrode assemblies (MEAs) subjected to load cycles. Two HT‐PEM MEAs have been subjected to accelerated ageing tests based on load cycling. The cycles profile has been chosen in order to enhance catalyst degradation. Both the tests show a fuel cell performance loss lower than 30 mV after 100,000 cycles at 600 mA cm⁻². In order to analyze the catalyst evolution, synchrotron small angle X‐ray scattering (SAXS) has been employed. The catalyst degradation of the two conditioned samples has been compared with the data obtained from a new MEA that has been used as reference sample. The SAXS results showed a mean size increase of the platinum nanoparticles up to the 100%.     

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.     

On a new degradation mode for high-temperature polymer electrolyte fuel cells: How bipolar plate degradation affects cell performance

The contribution of the bipolar plate material to the overall degradation of a high temperature membrane electrode assembly (HT MEA) for polymer electrolyte fuel cells (PEFCs) is studied in terms of performance decrease, phosphoric acid uptake in the bipolar plates and change of surface morphology of the bipolar plates. Two different high temperature graphite composites, a surface treated graphite and a gold coated stainless steel flowfield and the respective MEAs are compared after operation at 180 °C. Both graphite surface treatment and gold coating lead to negligible uptake of the electrolyte and ensure low degradation rates, whereas the composite plates exhibit high uptake of acid from the MEA into the surface near bulk. Apparent MEA degradation caused by acid redistribution from the MEA to the increasingly porous plates is observed in terms of increased ohmic cell resistances and reduction of catalyst utilization as consequence of acid loss from the catalyst layers.