Influence of ionomer content on the structure and performance of PEFC membrane electrode assemblies

Nafion^® ionomer content of the cathode catalyst-layer of a polymer electrolyte fuel cell (PEFC), made by the “decal” hot pressing method, has been investigated for its effect on performance and structure of the membrane electrode assembly (MEA). Varying Nafion^® content was shown to have an effect on performance within the entire range of polarization curves (i.e. kinetic, ohmic, and mass-transport regions) as well as on the structure. AFM analysis shows the effect of Nafion on the dispersion of carbon aggregates. Further analysis using TEM demonstrates the effect of Nafion on both the dispersion of carbon aggregates and the distribution and thickness of the Nafion ionomer films surrounding the catalyst/carbon aggregates. The MEA structure change correlates well with the MEA performance on both kinetics and mass-transport region. The determining factors on the performance of MEA are the interfacial zone (between the ionomer and catalyst particle), the dispersion of catalyst/carbon aggregates and the distribution/thickness of Nafion films. An optimized Nafion^® content in the range of 27 ± 6 wt.% for the cathode was determined for an E-TEK 20% Pt₃Cr/C catalyst at a loading of 0.20 mg Pt/cm².     

Radiation-grafted membrane/electrode assemblies with improved interface

Recent progress is reported in preparing membrane/electrode assemblies for polymer electrolyte fuel cells based on radiation-grafted FEP-g-poly(styrenesulfonic acid) membranes. MEAs with an improved interface between the membrane and commercially available gas diffusion electrodes were obtained by Nafion®-coating of the membrane and hot-pressing. These improved MEAs showed both, performance data comparable to those of MEAs based on Nafion® 112 and an operation lifetime in H₂/O₂ fuel cells of more than 2000 h at 60 °C and 500 mA cm⁻² current density.     

Microscopic characterizations of membrane electrode assemblies prepared under different hot-pressing conditions

The durability of the membrane electrode assembly (MEA) for direct methanol fuel cells (DMFCs) is one of the most critical issues to be addressed before widespread commercialization of the DMFC technology. In this work, we investigated the effect of the hot-pressing duration on the performance and durability of the MEA prepared by hot-pressing technique. It was found that the 60-min hot pressing at 135 °C under the pressure of 4.0 MPa yielded a significantly improved MEA durability than did the 3-min hot pressing (a typical duration in practice) under the same condition, but no substantial difference was found in the cell performance of the MEAs prepared with the two different hot-pressing durations. The reason why the hot-pressing duration had no significant effect on cell performance is explained based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) characterizations of the changes in the physiochemical properties of MEAs and their constituent components, including the anode, cathode and Nafion membrane, before and after hot pressing with different durations.     

Characteristics and performance of membrane electrode assemblies with operating conditions in polymer electrolyte membrane fuel cell

The degradation behavior of a membrane-electrode assembly (MEA) was investigated in accelerated degradation tests under constant voltage (0.8 V and 0.7 V) and load cycling (from open circuit voltage to 0.35 V) conditions. Changes in the structural and electrochemical characteristics of MEA after the durability tests give information as to the degradation mechanism of MEAs. The results of cyclic voltammogram and postmortem analysis by X-ray diffraction and high resolution-transmission electron microscopy indicate that the cathode catalyst layers of the MEAs showed no extreme degradation under constant voltage mode, whereas MEAs under repetition of load cycling mode showed very severe degradation after 280 h. However, the single cell performance of the MEA under repetition of load cycling mode was higher than under constant voltage mode. In addition, although the Pt band in the membrane of the MEA under repetition of load cycling mode was observed by field emission scanning electron microscopy, it did not affect the ohmic resistance.     

质子交换膜燃料电池膜电极制备方法的研究进展

燃料电池技术作为一种绿色能源技术,在减少能源消耗、环境污染等方面具有巨大潜力。膜电极（MEA）是质子交换膜燃料电池（PEMFC）的核心部件,MEA中电化学反应的顺利进行需要各功能层的协调配合,MEA各功能层涉及的传质、导电、导质子、催化等方面均影响燃料电池的性能。根据制备方法,可以将MEA分为催化剂涂敷基底（CCS）型MEA、催化剂涂敷膜（CCM）型MEA、有序化MEA和一体化MEA。MEA的性能不仅由催化剂本身载量决定,也受其结构设计和制备工艺的影响。本文介绍了MEA制备过程中常见的改进方法,分别从催化剂喷涂、刮涂、模槽挤出涂覆方式,催化剂浆料组成中Nafion含量和溶剂极性选择,催化层梯度化、图案化及界面结构改进,PEM结构增强、图案化、成膜方式等方面的研究进展进行讨论。但是目前对于MEA各功能层界面间的研究较少,应该注意的是催化层/质子交换膜（PEM）界面以及催化层/气体扩散层（GDL）界面设计也将直接影响MEA的性能。     

A glue method for fabricating membrane electrode assemblies for direct methanol fuel cells

We present a simple glue method for fabricating membrane electrode assemblies (MEA) for direct methanol fuel cells (DMFC). Rather than the conventional “dry” hot-pressing method that relies solely on hot-pressing at a high pressure and temperature to form a MEA, the “wet” method developed in this work introduces a binding agent, consisting of Nafion^® solution, between a polymer electrolyte membrane (PEM) and an anode/cathode. The introduced binding agent can provide a better adhesion and stronger binding force between a membrane and an electrode, thereby facilitating a better interfacial contact between the electrode and the Nafion^® membrane, which has been proved by scanning electron microscopy (SEM) analyses to the cross-sectional morphology of the MEA after long-term operation. The cell performance characterization showed the MEA fabricated by the glue method was more stable in cell performance than that fabricated by the conventional hot-pressing method. Cyclic voltammetry (CV) results also demonstrated the MEA fabricated by the glue method exhibited a higher electrochemical surface area (ESA) as a result of the improved interfacial contact between the Nafion^® membrane and the electrodes. Finally, the DMFC with the MEA fabricated by the glue method was characterized by the electrochemical impedance spectroscopy (EIS).     

Electrochemically promoted olefin isomerization reactions at polymer electrolyte fuel␣cell membrane electrode assemblies

The isomerization of 2-3-dimethyl-1-butene was enhanced over a thousand fold (vs the open circuit value) by spillover protons generated by low currents (electrochemical promotion) on carbon supported Pd catalysts in a polymer electrolyte fuel cell. There was substantial proton spillover catalyzed shift of the double bond of 2-3-dimethyl-1-butene. With 3-3-dimethyl-1-butene, the proton spillover catalyzed methyl shift occurred at low levels and 2-2-dimethyl-butane was the primary product from the simple reduction reaction. Although the substantial non-Faradaic electrochemical modification of catalysis (NEMCA) of the double bond isomerization of an olefin was further demonstrated, the more challenging electrochemical promotion of an olefin methyl shift at the polymer electrolyte Pd/C cathode was less pronounced.     

Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy

The most common methods used to characterize the electrochemical performance of fuel cells are to record current–voltage U(i) curves. However, separation of electrochemical and ohmic contributions to the U(i) characteristics requires additional experimental techniques. The application of electrochemical impedance spectra (EIS) is an approach to determine parameters which have proved to be indispensable for the development of fuel cell electrodes and membrane electrode assemblies (MEAs). This paper proves that it is possible to split the cell impedance into electrode impedances and electrolyte resistance by varying the operating conditions of the fuel cell (current load) and by simulation of the measured EIS with an equivalent circuit. Furthermore, integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell and the determination of the voltage loss fractions.     

Ameliorating effect of silica addition in the anode-catalyst layer of the membrane electrode assemblies for polymer electrolyte fuel cells

Incorporation of silica particles through a sol-gel process into the anode-catalyst layer with a sol-gel modified Nafion-silica composite membrane renders easy retention of back-diffused water from the cathode to anode through the composite membrane electrolyte, increases the catalyst-layer wettability and improves the performance of the Polymer Electrolyte Fuel Cell (PEFC) while operating under relative humidity (RH) values ranging between 18% and 100% with gaseous hydrogen and oxygen reactants at atmospheric pressure. A peak power density of 300 mW cm⁻² is achieved at a load current-density value of 1200 mA cm⁻² for the PEFC employing a sol-gel modified Nafion-silica composite membrane and operating at 18% RH. Under similar operating conditions, the PEFC with a Membrane Electrode Assembly (MEA) comprising Nafion-silica composite membrane with silica in the anode-catalyst layer delivers a peak power density of 375 mW cm⁻². By comparison, the PEFC employing commercial Nafion membrane fails to deliver satisfactory performance at 18% RH due to the limited availability of water at its anode, acerbated electro-osmotic drag of water from anode to cathode and insufficient water back diffusion from cathode to anode causing the MEA to dehydrate.     

Oxygen reduction on Ru-oxide pyrochlores bonded to a proton-exchange membrane

Oxygen reduction at a gas-fed, porous, ruthenium-pyrochlore electrode attached to a Dow Developmental Fuel Cell Membrane was measured in solutions of various pH. Electrode assemblies containing high surface area Pb₂Ru_2–x Pb _x O_7–y or Bi₂Ru_2–x Bi _x O_7–y with different amounts of Teflon content with/without the incorporation of Dow gel in the active part of the electrode with/without a CO₂-treated Vulcan XC-72 carbon substrate were tested. The oxide pyrochlores were found to be chemically stable and to show their lowest overpotential if separated from a 2.5 M H₂SO₄ proton reservoir by the membrane. Interesting oxygen reduction activity at room temperature was obtained with the Pb₂Ru_1.74Pb_0.26O_7–y electrode bonded with 22% by weight Teflon and incorporating 5% by weight Dow gel. The performance of the oxides against B-site Pb concentration and a measurement of the surface charge on the particles indicate that, in this configuration, the active sites for the oxygen reduction reaction are OH^– species at the O-site positions of the A₂B₂O₆O_1–y pyrochlores, especially the bridging oxygen with one Ru and one Pb near neighbour, i.e. Pb–O_b–Ru. Evidence that oxide particles precipitated on CO₂-treated carbon transfer electrons to the substrate is also presented.     

The influence of hydrogen- and cation-underpotential deposition on oxide-mediated Pt dissolution in proton-exchange membrane fuel cells

In order to fully understand the influence of a lower potential limit on platinum dissolution and the likely mechanism for mass and surface-area loss under potential cycling conditions, the dissolution of a Pt catalyst in a N₂-saturated 0.5 M H₂SO₄ solution was examined using an electrochemical quartz nanobalance (EQCN) flow cell, a rotating ring-disk electrode (RRDE) and inductively coupled plasma mass spectroscopy (ICP-MS). Due to the observation that cycling to a lower potential limit, which coincides with the hydrogen under-potential (H_UPD) region, results in a decrease in the dissolution rate, cations capable of interfering with the hydrogen UPD process (Zn²⁺, Li⁺, Na⁺, K⁺, and Cd²⁺) were introduced to the solution. Larger rates of mass loss were observed in the presence of these cations during the cycling process in the UPD region, despite apparently negligible effects on the behavior with more positive lower potential limits or on oxide formation and stripping. It was found that the quantity of soluble Pt species produced during the electrochemical reduction of PtO₂ was proportional to the charge associated with oxide stripping at the disk electrode during the RRDE experiment.     

Development of functional nanofibrous membrane assemblies towards biological sensing

A novel research approach was investigated that has the potential to improve sample preparation for complex matrices. Generation of high surface area nanofibrous membranes with covalently attached molecular recognition elements for selective capture of target biological agents were developed using the electrospinning fabrication technique. Two types of electrospun capture membranes were fabricated containing either carboxyl (COOH) or amine (NH₂) functional groups for covalent attachment of antibodies. The carboxyl functional membrane was produced by electrospinning polyvinyl chloride (PVC) formulated to be 1.8% carboxylated. The amine functional membrane was made by co-electrospinning two polymers, water-soluble polyamine and water insoluble polyurethane. Linking of molecular recognition groups, antibodies, to the carboxylated PVC was performed using established crosslinking chemistries. Antigen/antibody experiments were tested on the electrospun membranes. Results showed that electrospun membranes, treated with a secondary antibody used as the analyte, reacted only with its complement as indicated with a chemiluminescent signal. Toxin studies with Staphylococcal enterotoxin B (SEB) were conducted using avidin/biotin chemistries on the electrospun membranes. Experiments were performed using a modified ELISA sandwich assay on the fibrous membranes in the following configuration: avidin–biotinylated SEB antibody–SEB toxin–SEB antibody–HRP. Results have shown that 1–100 ng/ml concentrations of toxin were detected using a chemiluminescent signal detection scheme.     

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.     

Chemical and mechanical degradation of silicone rubber under two compression loads in simulated proton-exchange membrane fuel-cell environments

Long-term chemical and mechanical stability of gaskets in proton-exchange membrane (PEM) fuel cells is critical to the sealing and normal operation of these fuel cells. In this study, the chemical and mechanical degradation of a silicone rubber was investigated. Two compression loads and two simulated environments were used. The weight change of sample was monitored. Optical microscopy was applied to observe the morphological changes on the specimen surface. Attenuated total reflection (ATR)–Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the chemical changes on the surface of specimens after exposure to the simulated solutions and subjection to compression loads over time. A compression stress-relaxation test was used to elucidate the stress-relaxation property changes of the specimens after exposure to the simulated fuel-cell environments and the compression loads. Optical microscopy showed that the surface morphology of the specimens changed from initially smooth to slightly rough followed by crack initiation and finally propagation. The ATR–FTIR and XPS results indicate that the surface chemistry of the specimens significantly changed via decrosslinking and chain scission after exposure to the simulated environments and compression loads over time. The compression stress-relaxation test results indicate that the mechanical properties of the silicone rubber specimens changed significantly. We found that both the acid concentration of the test solution and the compression load significantly affected the degradation of the silicone rubber material. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47855.     

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.