Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Air oxidation of carbon supports boosts the low-humidity fuel cell performance

We introduce a straightforward, yet effective strategy to combat the performance decline of proton-exchange membrane fuel cells in low-humidity environments. Our method centers on air-oxidizing carbon supports, significantly improving proton and oxygen transport within the cathode catalyst layer.

Modification of the Physical Properties of a Nafion Film Due to Inclusion of n-Dodecyltriethylammonium Cation: Time Effect

This study investigates the effects of modifying commercial Nafion-212 thin films with dodecyltriethylammonium cation (DTA+) on their electrical resistance, elastic modulus, light transmission/reflection and photoluminescence properties. The films were modified through a proton/cation exchange process for immersion periods ranging from 1 to 40 h. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to analyze the crystal structure and surface composition of the modified films. The electrical resistance and the different resistive contributions were determined via impedance spectroscopy. Changes in the elastic modulus were evaluated using stress-strain curves. Additionally, optical characterization tests, including light/reflection (250-2000 nm) and photoluminescence spectra, were also performed on both unmodified and DTA⁺-modified Nafion films. The results reveal significant changes in the electrical, mechanical and optical properties of the films, depending on the exchange process time. In particular, the inclusion of the DTA⁺ into the Nafion structure improved the elastic behavior of the films by significantly decreasing the Young modulus. Furthermore, the photoluminescence of the Nafion films was also enhanced. These findings can be used to optimize the exchange process time to achieve specific desired properties.

Impact of Initial Cyclic Loading on Mechanical Properties and Performance of Nafion

Nafion possesses many interesting properties such as a high ion-conductivity, hydrophilicity, and thermal and chemical stability that make this material highly suitable for many applications including fuel cells and various (bio-)chemical and physical sensors. However, the mechanical properties of a Nafion membrane that are known to be affected by the viscoplastic characteristics of the material itself have a strong impact on the performance of Nafion-based sensors. In this study, the mechanical properties of Nafion under the cyclic loading have been investigated in detail. After cyclic tensile loading (i.e., maximum elongation about 25% at a room temperature and relative humidity about 40%) a time-dependent recovery comes into play. This recovery process is also shown being strain-rate dependent. Our results reveal that the recovery behavior weakens after performing several stress-strain cycles. Present findings can be of a great importance in future design of various chemical and biological microsensors and nanosensors such as hydrogen or glucose ones.

Influence of sulfonated SBA - 15 on fuel cell performance of sulfonated polysulfone electrolyte membranes

The prepared mesoporous SBA-15 (Santa Barbara Amorphous-15) was sulfonated and used as filler for the preparation of sulfonated polysulfone based composite electrolyte membranes. The SBA-15 and polysulfone were sulfonated using 3-mercaptopropyl trimethoxysilane and trimethylsilyl chlorosulfonate, respectively. The different weight percentages (1, 3, and 5 wt%) of sulfonated SBA-15 (SSBA-15) were used to prepare composite electrolyte membranes. Water uptake, ion exchange capacity, swelling ratio and proton conductivity of the composite membranes were studied for assessing the suitability of the electrolyte membranes for use in fuel cells. Characterization techniques such as FT-IR, XRD, SEM, TEM and Brunauer–Emmett– Teller were used to study the physico-chemical properties of the electrolyte membranes. TEM and BET analysis showed that SBA -15 retained its mesoporous structure even after sulfonation process. The prepared membranes were then tested in an in-house built single-cell fuel cell using hydrogen as fuel and oxygen as the oxidant. The fuel cell study showed that the presence of Sulfonated SBA-15 in the polymer matrix provided additional ion exchange sites and retained water for proton transfer which resulted in higher power density of 815 mW/cm2 with SPSU + 3% SSBA-15 membrane as compared with Nafion 117®.

Anion Exchange Membranes for Alkaline Polymer Electrolyte Fuel Cells-A Concise Review

Solid anion exchange membrane (AEM) electrolytes are an essential commodity considering their importance as separators in alkaline polymer electrolyte fuel cells (APEFC). Mechanical and thermal stability are distinguished by polymer matrix characteristics, whereas anion exchange capacity, transport number, and conductivities are governed by the anionic group. The physico-chemical stability is regulated mostly by the polymer matrix and, to a lesser extent, the cationic head framework. The quaternary ammonium (QA), phosphonium, guanidinium, benzimidazolium, pyrrolidinium, and spirocyclic cation-based AEMs are widely studied in the literature. In addition, ion solvating blends, hybrids, and interpenetrating networks still hold prominence in terms of membrane stability. To realize and enhance the performance of an alkaline polymer electrolyte fuel cell (APEFC), it is also necessary to understand the transport processes for the hydroxyl (OH^-) ion in anion exchange membranes. In the present review, the radiation grafting of the monomer and chemical modification to introduce cationic charges/moiety are emphasized. In follow-up, the recent advances in the synthesis of anion exchange membranes from poly(phenylene oxide) via chloromethylation and quaternization, and from aliphatic polymers such as poly(vinyl alcohol) and chitosan via direct quaternization are highlighted. Overall, this review concisely provides an in-depth analysis of recent advances in anion exchange membrane (AEM) and its viability in APEFC.

High Proton Conductivity of 3D Graphene Oxide Intercalated with Aromatic Sulfonic Acids

The development of efficient proton conductors that are capable of high power density, sufficient mechanical strength, and reduced gas permeability is challenging. Herein, we report the development of a series of aromatic sulfonic acid/graphene oxide hybrid membranes incorporating benzene sulfonic acid (BS), naphthalene sulfonic acid (NS), naphthalene disulfonic acid (DS) or pyrene sulfonic acid (PS) using a facile freeze dried method. For out-of-plane proton conductivity, the 3DGO-BS and 3DGO-NS yielded proton conductivities of 4.4×10-2  S cm-1 and 3.1×10-2  S cm-1 , respectively; this represents a two-times higher value than that which occurs for three dimensional graphene oxide (3DGO). Additionally, the respective prepared films as membranes in a proton exchange membrane fuel cell (PEMFC) show maximum power density of 98.76 mW cm-2 for 3DGO-NS while it is 92.75 mW cm-2 for 3DGO-BS which are close to double that obtained for 3DGO (50 mW cm-2 ).

Novel Filler-Filled-Type Polymer Electrolyte Membrane for PEFC Employing Poly(vinylphosphonic acid)-b-polystyrene-Coated Cellulose Nanocrystals as a Filler

Low-acidity polymer electrolyte membranes are essential to polymer electrolyte fuel cells (PEFCs) and water electrolysis systems, both of which are expected to be next-generation energy and hydrogen sources. We developed a new type of high-performance polymer electrolyte membrane (PEM) in which the core particles are precisely electrolyte polymer coated and filled into binder resin. Cellulose nanocrystals (CNCs), which have attracted attention as light, rigid, and sustainable materials, were selected as the core material for the filler. The CNC surface was coated with a new block copolymer containing a proton conductive polymer of poly(vinylphosphonic acid) (PVPA) and a hydrophobic polymer of polystyrene (PS) using RAFT polymerization with particles (PwP) we developed. The pelletized fillers and the filler-filled polycarbonate membranes achieved proton conductivities of over 10^-2 S/cm with lower activation energies and much weaker acidity than the Nafion membrane.

Influence of Mixed Imide Composition and Thermal Annealing on Ionic Liquid Uptake and Conductivity of Polyimide-Poly(ethylene glycol) Segmented Block Copolymer Membranes

Understanding the impact of different bridging groups in the two-step polymerization of poly(ethylene glycol) (PEG)-incorporated polyimide (PI) materials is significant. It is known that the proton exchange membranes (PEMs) used in industry today can experience performance degradation under rising temperature conditions. Many efforts have been devoted to overcoming this problem by improving the physical and mechanical properties that extend the hygrothermal life of a PEM. This work examines the effect of oxygenated and fluorinated bridging anhydrides in the production of PI-PEG PEMs. It is shown that the dianhydride identity and the amount incorporated in the synthesis influences the properties of the segmented block copolymer (SBC) membranes, such as increased ionic liquid uptake (ILU), enhanced conductivity and higher Young's modulus favoring stiffness comparable to Nafion 115, an industrial standard. Investigations on the ionic conductivity of PI-PEG membranes were carried out to determine how thermal annealing would affect the material's performance as an ion-exchange membrane. By applying a thermal annealing process at 60 °C for one hour, the conductivities of synthesized segmented block copolymer membranes values were increased. The effect of thermal annealing on the mechanical properties was also shown for the undoped SBC via measuring the change in the Young's modulus. These higher ILU abilities and mechanical behavior changes are thought to arise from the interaction between PEG molecules and ethylammonium nitrate (EAN) ionic liquid (IL). In addition, higher interconnected routes provide a better ion-transfer environment within the membrane. It was found that the conductivity was increased by a factor of ten for undoped and a factor of two to seven for IL-doped membranes after thermal annealing.

Hybrid Proton-Exchange Membrane Based on Perfluorosulfonated Polymers and Resorcinol-Formaldehyde Hydrogel

Organic resorcinol-formaldehyde (RF) hydrogels were introduced into a hybrid cation-exchange membrane in order to enhance its following properties: water uptake, thermal stability, and ionic conductivity. This study was aimed to investigate the modifications induced by the RF organic clusters that form a uniform distributed network within the perflourosulfonated acid (PFSA) matrix. RF concentration was controlled by resorcinol and formaldehyde impregnation time using water or ethanol solvents. The specific morphological and structural properties were characterized by atomic force microscopy, UV-Vis, and Fourier transform infrared spectroscopy. Thermo-gravimetric analysis was employed to study the thermal stability and degradation processes of the composite membranes. Proton conductivity, as a function of relative humidity (RH) at 80 °C, was measured using in-plane four-point characterization technique. Compared to the pristine membrane, the PFSA-RF hybrid membranes showed improved thermal stability at up to 46 °C and higher ionic conductivity for low RF content, especially at low relative humidity, when using ethanol-based solvents. Single fuel cell testing on RF-based membrane-electrode assembly revealed impeccable fuel crossover and power performance at 80 °C and 40% relative humidity, delivering a 76% increase in power density compared to a reference assembled with a pristine membrane and the same catalyst loadings.

Effect of Increased Ionic Liquid Uptake via Thermal Annealing on Mechanical Properties of Polyimide-Poly(ethylene glycol) Segmented Block Copolymer Membranes

Proton exchange membranes (PEMs) suffer performance degradation under certain conditions-temperatures greater than 80 °C, relative humidity less than 50%, and water retention less than 22%. Novel materials are needed that have improved water retention, stability at higher temperatures, flexibility, conductivity, and the ability to function at low humidity. This work focuses on polyimide-poly(ethylene glycol) (PI-PEG) segmented block copolymer (SBC) membranes with high conductivity and mechanical strength. Membranes were prepared with one of two ionic liquids (ILs), either ethylammonium nitrate (EAN) or propylammonium nitrate (PAN), incorporated within the membrane structure to enhance the proton exchange capability. Ionic liquid uptake capacities were compared for two different temperatures, 25 and 60 °C. Then, conductivities were measured for a series of combinations of undoped or doped unannealed and undoped or doped annealed membranes. Stress and strain tests were performed for unannealed and thermally annealed undoped membranes. Later, these experiments were repeated for doped unannealed and thermally annealed. Mechanical and conductivity data were interpreted in the context of prior small angle X-ray scattering (SAXS) studies on similar materials. We have shown that varying the compositions of polyimide-poly(ethylene glycol) (PI-PEG) SBCs allowed the morphology in the system to be tuned. Since polyimides (PI) are made from the condensation of dianhydrides and diamines, this was accomplished using components having different functional groups. Dianhydrides having either fluorinated or oxygenated functional groups and diamines having either fluorinated or oxygenated diamines were used as well as mixtures of these species. Changing the morphology by creating macrophase separation elevated the IL uptake capacities, and in turn, increased their conductivities by a factor of three or more compared to Nafion 115. The stiffness of the membranes synthesized in this work was comparable to Nafion 115 and, thus, sufficient for practical applications.

Mechanical Properties and Chemical Durability of Nafion/Sulfonated Graphene Oxide/Cerium Oxide Composite Membranes for Fuel-Cell Applications

To improve both the mechanical and chemical durability of Nafion membranes for polymer electrolyte membrane fuel-cells (PEMFCs), Nafion composite membranes containing sulfonated graphene oxide (SGO) and cerium oxide (CeO₂; ceria) were prepared by solution casting. The structure and chemical composition of SGO were investigated by FT-IR and XPS. The effect of the sulfonation, addition of SGO and ceria on the mechanical properties, proton conductivity, and chemical stability were evaluated. The addition of SGO gave rise to an increase in the number of sulfonic acid groups in Nafion, resulting in a higher tensile strength and proton conductivity compared to that of graphene oxide (GO). Although the addition of ceria was found to decrease the tensile strength and proton conductivity, Nafion/SGO/ceria composite membranes exhibited a higher tensile strength and proton conductivity than recast Nafion. Measurement of the weight loss and SEM observations of the composite membranes after immersing in Fenton's reagent indicate an excellent radical scavenging ability of ceria under radical degradation conditions.

Electrochemical characterisation and comparison of transport in Nafion films and particles

The diffusional transport of the dication and monocation of methyl viologen, MV2+ and MV+, in hydrated Nafion films and particles is studied electrochemically. Methodology is established for characterising the relative rates of transport and for evaluating the diffusion coefficient of the two ions. The transport in the particles is shown to be significantly faster than in films of similar thickness whilst the diffusion of MV2+ is confirmed to be faster than that of MV+.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.

Nafion-stabilised bimetallic Pt-Cr nanoparticles as electrocatalysts for proton exchange membrane fuel cells (PEMFCs)

The current study investigated the unique combination of alloying (Pt with Cr) and Nafion stabilisation to reap the benefits of catalyst systems with enhanced catalytic activity and improved durability in PEMFCs.

The current study investigated the unique combination of alloying (Pt with Cr) and Nafion stabilisation to reap the benefits of catalyst systems with enhanced catalytic activity and improved durability in PEMFCs. Pt–Cr alloy nanoparticles stabilised with Nafion were chosen in the current study owing to their higher stability in acidic and oxidising media at high temperatures compared to other Pt-transition metal alloys (e.g. Pt–Ni, Pt–Co). Two different precursor : reducing agent (1 : 10 and 1 : 20) ratios were used in order to prepare two different alloys, denoted as Pt–Cr 10 and Pt–Cr 20. The Pt–Cr 20 alloy system (with composition Pt₈₀Cr₂₀) demonstrated higher electrocatalytic activity for the oxygen reduction reaction compared to commercial Pt/C (TKK) catalysts. Accelerated stress tests and single cell tests revealed that Nafion stabilised alloy catalyst systems displayed significantly enhanced durability (only ∼20% loss of ECSA) compared with Pt/C (50% loss of ECSA) due to improved catalyst–ionomer interaction. Furthermore, the Pt–Cr 20 alloy system demonstrated a current density comparable to that of Pt/C making them promising potential electrocatalysts for proton exchange membrane fuel cells.

Mechanical Behavior of Free-Standing Fuel Cell Electrodes on Water Surface

Fundamental understanding of the mechanical behavior of polymer electrolyte fuel cell electrodes as free-standing materials is essential to develop mechanically robust fuel cells. However, this has been a significant challenge due to critical difficulties, such as separating the pristine electrode from the substrate without damage and precisely measuring the mechanical properties of the very fragile and thin electrodes. We report the mechanical behavior of free-standing fuel cell electrodes on the water surface through adopting an innovative ice-assisted separation method to separate the electrode from decal transfer film. It is found that doubling the ionomer content in electrodes increases not only the tensile stress at the break and the Young's modulus (E) of the electrodes by approximately 2.1-3.5 and 1.7-2.4 times, respectively, but also the elongation at the break by approximately 1.5-1.7 times, which indicates that stronger, stiffer, and tougher electrodes are attained with increasing ionomer content, which have been of significant interest in materials research fields. The scaling law relationship between Young's modulus and density (ρ) has been unveiled as E ∼ ρ(1.6), and it is compared with other materials. These findings can be used to develop mechanically robust electrodes for fuel cell applications.

A simple UV-ozone surface treatment to enhance photocatalytic performance of TiO2 loaded polymer nanofiber membranes

UV-ozone treated electrospun nanofiber membranes for increased photocatalytic activity.

In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles

Perfluorinated ionomers, in particular Nafion, are an essential component in hydrogen fuel cells, as both the proton exchange membrane and the binder within the catalyst layer. During normal operation of a hydrogen fuel cell, the ionomer will progressively swell and deswell in response to the changes in hydration, resulting in mechanical fatigue and ultimately failure over time. In this study, we have developed and implemented a cantilever bending technique in order to investigate the swelling-induced stresses in biaxially constrained Nafion thin films. When the deflection of a cantilever beam coated with a polymer film is monitored as it is exposed to varying humidity environments, the swelling induced stress-thickness product of the polymer film is measured. By combining the stress-thickness results with a measurement of the swelling strain as a function of humidity, as measured by quartz crystal microbalance (QCM) and X-ray reflectivity (XR), the swelling stress can be determined. An estimate of the Young's modulus of thin Nafion films as a function of relative humidity is obtained. The Young's modulus values indicate orientation of the ionic domains within the polymer films, which were confirmed by grazing incidence small-angle X-ray scattering (GISAXS). This study represents a measurement platform that can be expanded to incorporate novel ionomer systems and fuel cell components to mimic the stress state of a working hydrogen fuel cell.

Highly Enhanced Force Generation of Ionic Polymer-Metal Composite Actuators via Thickness Manipulation

On purpose to enhance the generating force of ionic polymer-metal composite (IPMC) actuators, the thickness of the ion-exchange membrane is manipulated in two different ways. One is grafting poly(styrenesulfonic acid) onto poly(vinylidene fluoride-co-hexafluoropropylene) films with varying thickness, and the other is stacking pre-extruded Nafion films to thicker films by pressing at high temperatures. For both groups of the membranes, ionic properties including ion-exchange capacity and ionic conductivity are maintained similarly inside the groups regardless of the thickness. The actuation tests clearly show the increase in generating force with increasing thickness of the IPMCs prepared. It is due to a larger bending stiffness of thicker IPMCs, which is consistent with the predicted result from the cantilever beam model. The increase in force is more remarkable in Nafion-stacked IPMCs, and a thick IPMC lifts a weight of 100 g, which far exceeds the reported values for IPMCs.

Preparation, characterization, and enhanced thermal and mechanical properties of epoxy-titania composites

This paper presents the synthesis and thermal and mechanical properties of epoxy-titania composites. First, submicron titania particles are prepared via surfactant-free sol-gel method using TiCl₄ as precursor. These particles are subsequently used as inorganic fillers (or reinforcement) for thermally cured epoxy polymers. Epoxy-titania composites are prepared via mechanical mixing of titania particles with liquid epoxy resin and subsequently curing the mixture with an aliphatic diamine. The amount of titania particles integrated into epoxy matrix is varied between 2.5 and 10.0 wt.% to investigate the effect of sub-micron titania particles on thermal and mechanical properties of epoxy-titania composites. These composites are characterized by X-ray photoelectron (XPS) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric (TG), and mechanical analyses. It is found that sub-micron titania particles significantly enhance the glass transition temperature (>6.7%), thermal oxidative stability (>12.0%), tensile strength (>21.8%), and Young's modulus (>16.8%) of epoxy polymers. Epoxy-titania composites with 5.0 wt.% sub-micron titania particles perform best at elevated temperatures as well as under high stress.