Ionomer Membranes Research Articles

Sulfonated Cellulose Membranes: Physicochemical Properties and Ionic Transport versus Degree of Sulfonation

AbstractThe next generation of green ion selective membranes is foreseen to be based on cellulosic nanomaterials with controllable properties. The introduction of ionic groups into the cellulose structure via chemical modification is one strategy to obtain desired functionalities. In this work, bleached softwood fibers are oxidatively sulfonated and thereafter homogenized to liberate the cellulose nanofibrils (CNFs) from the fiber walls. The liberated CNFs are subsequently used to prepare and characterize novel cellulose membranes. It is found that the degree of sulfonation collectively affects several important properties of the membranes via the density of fixed charged groups on the surfaces of the CNFs, in particular the membrane morphology, water uptake and swelling, and correspondingly the ionic transport. Both ionic conductivity and cation transport increase with the increased level of sulfonation of the starting material. Thus, it is shown that the chemical modification of the CNFs can be used as a tool for precise and rational design of green ion selective membranes that can replace expensive conventional fluorinated ionomer membranes.

Exponential Water Uptake in Ionomer Membranes Results from Polymer Plasticization

Water content is the most influential factor in the performance of ion-exchange membranes (IEMs), controlling their mechanical rigidity, ionic conductivity, and degradation rates. Membranes absorb water exponentially at high water activity (aw), making that region of the sorption isotherm the most influential on membrane properties. Plasticization of the polymer by water has been proposed to cause this exponential uptake at high aw. However, an integrated microscopic picture of the structure, thermodynamic, and mechanical properties of IEMs as a function of aw has remained elusive. Here, we use large-scale molecular simulations validated with experimental measurements to compute the sorption isotherms, Young’s modulus, polymer dynamics, and structure of IEMs. The simulations unveil that the exponential increase in water uptake (WU) coincides in all cases with the glass to rubber transition of the membrane, as measured through its Young’s modulus and segmental polymer dynamics. Functionalization of the polymer with alkyl groups further contributes to its plasticization, increasing the WU at a given aw and ion-exchange capacity (IEC). The alkyl chains act synergistically with water to plasticize the polymer matrix and allow water penetration in the membrane. The simulations reveal that the width of the water channels depends on the ratio λ of water to ions in the polymer but is independent of its IEC. We conclude that differences in the polymer matrix─not the water channels─are responsible for the distinct uptake response of IEMs to the thermodynamic driving force of water activity.

Sulfonated Cellulose Membranes Improve the Stability of Aqueous Organic Redox Flow Batteries

The drawbacks of current state‐of‐the‐art selective membranes, such as poor barrier properties, high cost, and poor recyclability, limit the large‐scale deployment of electrochemical energy devices such as redox flow batteries (RFBs) and fuel cells. In recent years, cellulosic nanomaterials have been proposed as a low‐cost and green raw material for such membranes, but their performance in RFBs and fuel cells is typically poorer than that of the sulfonated fluoropolymer ionomer membranes such as Nafion. Herein, sulfonated cellulose nanofibrils densely cross‐linked to form a compact sulfonated cellulose membrane with limited swelling and good stability in water are used. The membranes possess low porosity and excellent ionic transport properties. A model aqueous organic redox flow battery (AORFB) with alizarin red S as negolyte and tiron as posolyte is assembled with the sulfonated cellulose membrane. The performance of the nanocellulose‐based battery is superior in terms of cyclability in comparison to that displayed by the battery assembled with commercially available Nafion 115 due to the mitigation of crossover of the redox‐active components. This finding paves the way to new green organic materials for fully sustainable AORFB solutions.

Novel Sulfonated and Phosphonated Ionomers and Ionomer (blend) Membranes for Electrochemical Applications

The heart of a HT-PEM fuel cell is represented by the proton conductive membrane. Nowadays PFSA-based materials e.g. Nafion are state of the art in large scale energy applications. However, the PFSAs are relatively expensive, and give rise to concerns regarding toxic intermediates in the production process, and the questionability of their recyclability. This leads to a strong need for development of cost effective alternatives. Such alternatives have to exihibit customized properties, like reduced gas crossover, high ionic conductivity, mechanical strength, thermal stability and durability.In this work, we present novel cation-exchange ionomers, which are suitable as proton exchange membranes in electrochemical applications such as fuel cells, electrolysers, and redox-flow batteries. For our approach we developed novel fully and partially aromatic polymers with customized mechanical behavior, using two different synthetic pathways, the first being modification of the aromatic building blocks to more flexible ones and/or introduction of flexible side chains into the monomers or polymers.With these two approaches it was possible to synthesize ionomer materials which are less brittle than our earlier developed sulfonated[1] or phosphonated[2] poly(pentafluorostyrene) ionomers. Our development lead to ionomers which are suitable in pure form or as blend in proton exchange membranes with promising proton conductivity, mechanical and thermal stability.The novel ionomers and ionomer membranes were characterized in terms of ex-situ characterization including impedance spectroscopy, IR spectroscopy, NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Moreover, preliminary in-situ tests of the ionomer membranes in low and high-temperature fuel cells have been performed, yielding promising results. Furthermore the synthesized membranes were characterized in-situ in redox-flow batteries, yielding promising results. [1] Highly phosphonated polypentafluorostyrene. V. Atanasov, J. Kerres, Macromolecules 2011, 44, 6416–6423; DOI: 10.1021/ma2011574 [2] Sulfonated poly(pentafluorostyrene): synthesis & characterization. V. Atanasov, M. Bürger, S. Lyonnard, G. Gebel, J. Kerres, Solid State Ionics, 2013, 252, 75-83; DOI: 10.1016/j.ssi.2013.06.010

Effects of Reinforcement Type on the Structure and Properties of Perfluorosulphonic Acid Membranes for Polymer Electrolyte Membrane Fuel Cells

Fuel cell membrane durability remains one of the key challenges limiting the wide scale adoption of fuel cell technology. Membrane degradation in polymer electrolyte membrane (PEM) fuel cells limits the operational lifetime of the fuel cell and prevents the industrial targets from being met. A common approach to increasing the lifetime of membranes includes the addition of a reinforcement layer to increase the mechanical durability of the membrane while not adversely affecting the performance [1,2]. Although reinforced membranes are widely used in industry, there is a literature gap considering membrane structure and properties in relation to durability. This work contributes to characterizing the effects of reinforcement type on the membrane properties. In this study a selection of perfluorosulphonic acid (PFSA) ionomer membranes with expanded polytetrafluoroethylene (ePTFE) reinforcements were tested, these include two novel DMR100 membranes with different reinforcements and Nafion XL compared to a conventional, non-reinforced Nafion NRE-211 for reference. All of these membranes contain common PFSA ionomer and differ primarily in the type of reinforcement layer and membrane thickness. The study addresses comparison between the membrane chemical composition, water uptake, crystallinity, and mechanical strength. Methods of ex situ characterization include solid state nuclear magnetic resonance (SS_NMR), small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA). Membrane chemical structure and properties are measured by SS-NMR and FTIR, whereas the water uptake, domain spacing, and crystallinity are assessed by SAXS/WAXS study of hydrated and dry membranes [3-5]. Tensile mechanical properties are measured under room temperature (23°C, 50% RH) and fuel cell conditions (80°C, 90% RH) by DMA [6]. Overall, this paper contributes both qualitative and quantitative understanding of the key structural properties of membranes with different reinforcement resulting in novel knowledge about membranes that can be leveraged for improved fuel cell durability.

Ionomer Membranes Produced from Hexaarylbenzene-Based Partially Fluorinated Poly(arylene ether) Blends for Proton Exchange Membrane Fuel Cells.

In this study, a series of high molecular weight ionomers of hexaarylbenzene- and fluorene-based poly(arylene ether)s were synthesized conveniently through condensation and post-sulfonation modification. The use a of blending method might increase the stacking density of chains and affect the formation both of interchain and intrachain proton transfer clusters. Multiscale phase separation caused by the dissolution and compatibility differences of blend ionomer in high-boiling-point solvents was examined through analysis and simulations. The blend membranes produced in this study exhibited a high proton conductivity of 206.4 mS cm−1 at 80 °C (increased from 182.6 mS cm−1 for precursor membranes), excellent thermal resistance (decomposition temperature > 200 °C), and suitable mechanical properties with a tensile strength of 73.8–77.4 MPa. As a proton exchange membrane for fuel cell applications, it exhibits an excellent power efficiency of approximately 1.3 W cm−2. Thus, the ionomer membranes have strong potential for use in proton exchange membrane fuel cells and other electrochemical applications.

Three-Electrode Study of Electrochemical Ionomer Degradation Relevant to Anion-Exchange-Membrane Water Electrolyzers.

Among existing water electrolysis (WE) technologies, anion-exchange-membrane water electrolyzers (AEMWEs) show promise for low-cost operation enabled by the basic solid-polymer electrolyte used to conduct hydroxide ions. The basic environment within the electrolyzer, in principle, allows the use of non-platinum-group metal catalysts and less-expensive cell components compared to acidic-membrane systems. Nevertheless, AEMWEs are still underdeveloped, and the degradation and failure modes are not well understood. To improve performance and durability, supporting electrolytes such as KOH and K2CO3 are often added to the water feed. The effect of the anion interactions with the ionomer membrane (particularly other than OH-), however, remains poorly understood. We studied three commercial anion-exchange ionomers (Aemion, Sustainion, and PiperION) during oxygen evolution (OER) at oxidizing potentials in several supporting electrolytes and characterized their chemical stability with surface-sensitive techniques. We analyzed factors including the ionomer conductivity, redox potential, and pH tolerance to determine what governs ionomer stability during OER. Specifically, we discovered that the oxidation of Aemion at the electrode surface is favored in the presence of CO32-/HCO3- anions perhaps due to the poor conductivity of that ionomer in the carbonate/bicarbonate form. Sustainion tends to lose its charge-carrying groups as a result of electrochemical degradation favored in basic electrolytes. PiperION seems to be similarly negatively affected by a pH drop and low carbonate/bicarbonate conductivity under the applied oxidizing potential. The insight into the interactions of the supporting electrolyte anions with the ionomer/membrane helps shed light on some of the degradation pathways possible inside of the AEMWE and enables the informed design of materials for water electrolysis.

Formation and characterization of Pt-Cu-IPMC with high specific surface area and dendritic electrode

ABSTRACT Ionic polymer–metal composite (IPMC) is a kind of electroactive intelligent material. The dendritically interfacial electrodes (DIEs) is very important to the performance of IPMC. Herein, the copper–platinum composite electrode (Pt-Cu-IPMC) with a high specific area DIEs was prepared by combining impregnation-reduction plating platinum (IRP-Pt), autocatalytic plating platinum (ACP-Pt) and chemical reduction plating copper (CRP-Cu). The effects of DIEs and the cycles of ACP-Pt have been systematically investigated on the electrode’s thickness, micro-morphology and electromechanical response characteristics of all samples. Importantly, when the cycles of ACP-Pt increases, the platinum and copper particles in the ionomer membrane are highly dispersed. For example, when the cycles of ACP-Pt is 4, the penetration depth of the DIEs reaches 31.98 µm. Thus, copper and platinum dendrites were clearly observed in the samples. Combined with the analysis of the strain energy density, it showed that the presence of DIEs has some impact on the driving performance of the IPMC, the tip displacement, and blocking force of 67.27 mm and 27.21 mN have been achieved in the Pt-Cu-IPMC after three cycles of ACP-Pt.

Modeling the actuation of curved ionic polymer metal composites

An ionic polymer metal composites (IPMC) is a soft actuator that consists of an ionomer membrane, neutralized by mobile counterions and plated by metal electrodes. Despite their early promise in robotics, medical devices, and microsystem technologies, widespread application of IPMC actuators is far from being reached. Recent advancements in additive manufacturing technologies have the potential to expand the reach of IPMCs by affording the realization of complex, design-specific geometries that were impossible to attain with standard manufacturing techniques. For this potential to be attained, it is critical to establish physically-based models that could inform 3D printing, beyond the flat, thin, non-tapered geometries that have been the object of investigation for almost three decades. Here, we bridge this gap by presenting an analytical framework to study actuation of a double-clamped IPMC arch under an applied voltage. We adopt a thermodynamically the consistent continuum model to describe the coupled electrochemo-mechanical phenomena taking place within the IPMC. We establish an analytical solution for the electrochemistry using the method of matched asymptotic expansions, which is, in turn, utilized to compute osmotic pressure and Maxwell stress. The mechanical response of the IPMC arch is modeled as a plane strain problem with an induced state of eigenstress, which is solved with the use of a smooth Airy function. The accuracy of our analytical solution is validated through finite element simulations. Through a parametric analysis, we investigate the effect of curvature on the deformation and the reaction forces exerted by the clamps. The proposed analytical framework offers new insight into the response of curved IPMCs, in which progress on 3D printing should be grounded.

Gas Mass-Transport Coefficients in Ionomer Membranes Using a Microelectrode.

Gas permeability, the product of gas diffusivity and Henry's gas-absorption constant, of ionomer membranes is an important transport parameter in fuel cell and electrolyzer research as it governs gas crossover between electrodes and perhaps in the catalyst layers as well. During transient operation, it is important to divide the gas permeability into its constituent properties as they are individually important. Although transient microelectrode measurements have been used previously to separate the gas permeability into these two parameters, inconsistencies remain in the interpretation of the experimental techniques. In this work, a new interpretation methodology is introduced for determining independently diffusivity and Henry's constant of hydrogen and oxygen gases in ionomer membranes (Nafion 211 and Nafion XL) as a function of relative humidity using microelectrodes. Two time regimes are accounted for. At long times, gas permeability is determined from a two-dimensional numerical model that calculates the solubilized-gas concentration profiles at a steady state. At short times, permeability is deconvoluted into diffusivity and Henry's constant by analyzing transient data with an extended Cottrell equation that corrects for actual electrode surface area. Gas permeability and diffusivity increase as relative humidity increases for both gases in both membranes, whereas Henry's constants for both gases decrease with increasing relative humidity. In addition, results for Nafion 211 membranes are compared to a simple phase-separated parallel-diffusion transport theory with good agreement. The two-time-regime analysis and the experimental methodology can be applied to other electrochemical systems to enable greater precision in the calculation of transport parameters and to further understanding of gas transport in fuel cells and electrolyzers.

DFT Study on Side Chain Detachment of Perfluorosulfonic Acid Ionomers by Radical-Assisted Nucleophilic Attack of Water

Chemical degradation of perfluorosulfonic acid (PFSA) ionomer membranes is still an unresolved critical issue in the development of polymer electrolyte fuel cells (PEFCs). Its reaction mechanism has not been settled yet even after experimental and computational studies for decades. In this study, the role of hydroxyl radical to side chain degradation is revisited and a new reaction mechanism is proposed. Its nucleophilic attack on the carbon atom in the ether group induces detachment of the side chain. The reactions are supposed to be feasible as their reaction barrier heights are only slightly higher than those of tertiary fluorine atom abstraction reactions by hydrogen radical in absence of the solvation effect. When a single water molecule is present with hydroxyl or hydroperoxyl radical it is the oxygen in the water that undergoes the nucleophilic attack on the carbon in the ether groups and the calculated barrier heights are lowered to 70∼100 kJ mol−1 as the radical assists the reaction as a hydrogen acceptor. Possible reaction pathways of the products are also calculated to explain the occurrence of the experimentally observed degradation products of Nafion after chemical durability tests.

Failure kinetics of ionomer membranes

The equivalence of the role of thermal fluctuations in the failure of fuel cell ionomer membranes is investigated in light of the kinetic theory of the strength of solids. Specifically, the activation energy of the membrane material is determined using two physically distinct methods, each exemplifying the role of thermal fluctuations. The failure kinetics of ionomer membranes is a function of the applied load as well as the temperature under which the membranes operate in a fuel cell. Time-to-failure data is thus generated for varied environmental conditions, and kinetic parameters are calculated by treatment using established methods. Separately, thermal decomposition data of the membrane material is generated for various heating rates, and the kinetic parameters so obtained are compared with those from the mechanically induced failure approach. The results from the two methods are found to confirm each other and may be used in the investigation of fracture modeling and lifetime prediction of fuel cell membranes.

Determination of the through-plane profile of vanadium species in hydrated Nafion studied with micro X-ray absorption near-edge structure spectroscopy - proof of concept.

Vanadium-ion transport through the polymer membrane results in a significant decrease in the capacity of vanadium redox flow batteries. It is assumed that five vanadium species are involved in this process. Micro X-ray absorption near-edge structure spectroscopy (micro-XANES) is a potent method to study chemical reactions during vanadium transport inside the membrane. In this work, protocols for micro-XANES measurements were developed to enable through-plane characterization of the vanadium species in Nafion 117 on beamline P06 of the PETRA III synchrotron radiation facility (DESY, Hamburg, Germany). A Kapton tube diffusion cell with a diameter of 3 mm was constructed. The tube diameter was chosen in order to accommodate laminar flow for cryogenic cooling while allowing easy handling of the cell components by hand. A vertical step size of 2.5 µm and a horizontal step size of 5 µm provided sufficient resolution to resolve the profile and good statistics after summing up horizontal rows of scan points. The beam was confined in the horizontal plane to account for the waviness of the membrane. The diffusion of vanadium ions during measurement was inhibited by the cryogenic cooling. Vanadium oxidation, e.g. by water radiolysis (water percentage in the hydrated membrane ∼23 wt%), was mitigated by the cryogenic cooling and by minimizing the dwell time per pixel to 5 ms. Thus, the photo-induced oxidation of V3+ in the focused beam could be limited to 10%. In diffusion experiments, Nafion inside the diffusion cell was exposed on one side to V3+ electrolyte and on the other side to VO2+. The ions were allowed to diffuse across the through-plane orientation of the membrane during one of two short defrost times (200 s and 600 s). Subsequent micro-XANES measurements showed the formation of VO2+ from V3+ and VO2+ inside the water body of Nafion. This result proves the suitability of the experimental setup as a powerful tool for the determination of the profile of vanadium species in Nafion and other ionomeric membranes.

Mitigation of mechanical membrane degradation in fuel cells – Part 1: Gas diffusion layers with low surface roughness

Hygrothermal variations that arise during dynamic fuel cell operation are known to generate mechanical stresses in the ionomer membrane. Previous research has indicated that membrane electrode assembly (MEA) interaction effects may influence membrane degradation under such loads. The present objective is therefore to evaluate novel MEA design strategies for mitigating mechanical membrane degradation in fuel cells. In this case (Part 1), a gas diffusion layer (GDL) with low surface roughness is applied to suppress buckling-driven membrane failures. Laboratory-based X-ray computed tomography is used in a customized, time-resolved workflow for non-invasive four-dimensional characterization of membrane damage evolution during accelerated stress testing. Membrane crack development is the key failure mode preceded by fracture of the cathode catalyst layer. In comparison to high surface roughness GDL, the severity of membrane buckling is substantially reduced by adoption of the smoother GDL, contributing 2x greater lifetime. Accompanying finite element simulations of the unit fuel cell assembly show plastic strain accumulation in the buckled membrane and identified a critical range of GDL void sizes that influence membrane buckling. Overall, the improvement in GDL surface demonstrates substantial mitigation effect against fatigue-driven mechanical membrane degradation and failure, which is also corroborated by the numerical simulation results. • Novel MEA design strategies are shown to mitigate mechanical membrane degradation • CCM buckling into GDL voids is identified as the key mechanism for membrane failure • 2× higher RH cycling lifetime is achieved with low surface roughness GDLs • Complementary finite element simulations corroborate the experimental findings • A critical GDL pore radius is demonstrated for CCM buckling to occur

Bipolar Membranes for Ion Management in (Photo)Electrochemical Energy Conversion

Conspectus(Photo)electrochemical energy conversion is important in the development of a carbon-neutral energy economy because it can provide a pathway for mitigating the intermittency of renewable energy sources such as wind and solar. In order to operate efficiently, these technologies, which include photoelectrochemical cells, water and CO2 electrolyzers, fuel cells, and redox flow batteries, require fast charge transfer kinetics at the electrode/electrolyte interface as well as robust ion management in the electrolyte.In conventional electrolyzers and fuel cells, the electrolyte is strongly acidic or basic and ionic current is carried by H+ or OH– ions. In contrast, photoelectrodes and electrocatalysts for water splitting are often studied in buffered solutions. The question of ion balance in these systems led us to analyze the polarization losses due to ion concentration gradients in cells that employed various buffer–membrane combinations. Continuously driving the buffer ions across an ionomer membrane not only lowers the buffer capacity of an aqueous electrolyte but also introduces pH gradients that result in significant energy losses.To address the problem, we and other groups have studied the use of reverse-biased bipolar membranes (BPMs) in (photo)electrolytic cells. BPMs consist of an anion exchange layer (AEL) laminated with a cation exchange layer (CEL) and are usually equipped with a catalytic layer in between to accelerate the water dissociation reaction. At the AEL/CEL interface, water dissociates into protons and hydroxide ions, which replenish those consumed at the cathode and anode. Compared to conventional water electrolyzers with proton/anion exchange membranes (PEM/AEM), BPM electrolyzers provide the unique advantage of continuously operating the cathode and anode under different pH conditions, which is desirable when the two electrode reactions have different pH requirements.BPMs also enable the use of buffered electrolytes at pH values that are optimized for electrode stability and product selectivity in applications such as CO2 electrolysis. Product crossover losses and CO2 pumping can be dramatically reduced in BPM-based CO2 electrolyzers, relative to conventional alkaline membranes, by electrostatic repulsion (of anionic products) and electroosmotic drag (for neutral products). BPM-based gas fed CO2 electrolyzers can achieve high current density, but they suffer from low Faradaic efficiency (FE) due to the acidic local environment of the CEL. This problem can be mitigated by adding an aqueous buffering layer or by creating a weak acid cation exchange film on the CEL face of the membrane.The use of BPMs in fuel cells and redox flow batteries offers some interesting advantages. Configurations with both reverse and forward bias have been studied, but forward bias has been favored due to material compatibility, reaction kinetics, and thermodynamic considerations. The net reaction at the AEL/CEL interface is the acid–base neutralization reaction, which has a high inherent reaction rate constant, but in the BPM is limited to a nanometric space-charge layer and requires efficient catalysis to achieve high current density. Understanding the mechanism of the acid–base neutralization and the opposite process, the water dissociation reaction, will be essential for improving the performance of forward-biased BPMs. This Account reviews our current understanding of the working mechanisms of BPMs and discusses how we can use them to effectively manage ions for various (photo)electrochemical applications.

Ionomer Transport Properties Using Interdigitated and Microelectrodes

Transport through the ionomer as both a membrane and in the catalyst layer is critical for efficient fuel-cell operation. To gain this understanding, we explore the behavior of gas, water, and ion transport in ionomer membranes and thin films using novel microelectrode and interdigitated electrodes. In this work, several common ionomer membranes (including both PFSA and AEM chemistries) are evaluated using a custom microelectrode cell to determine the mass transport parameters of hydrogen and oxygen gas. These parameters are determined as a function of both water content and temperature. Numerical fitting of the current transient is used to separate diffusivity and solubility from a single chronoamperometry experiment. In addition, the electro-osmotic coefficients of these membranes are determined as a function of temperature. Finally, the proton conductivities of both thick and thin ionomer films are reported as a function of temperature and humidity evaluated using interdigitated electrodes. Together, gas transport parameters, electro-osmotic coefficients, and proton conductivities present a more complete picture of ionomer transport properties, allowing for an enhanced understanding of the limiting factors for fuel-cell performance.This material is based on work performed by the Million Mile Fuel Cell Truck (M2FCT), which is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, under contract number DE-AC02-05CH11231.

(Invited) Ionomer Adhesives, Coatings, and Membranes for Electrochemical Separations

A growing population striving for higher standards of living are stressing water resources around the globe. Because of the anticipated water scarcity and security challenges, water futures began trading as a commodity, like gold, oil and other precious commodities, on the stock market in late 2020. Ensuring water security necessitates water treatment and remediation technologies that are low cost, scalable, and energy efficient. Furthermore, advances in electrochemical separations technologies have been useful for targeted nutrient recovery, carbon valorization, and pollutant removal from liquids. In this talk, I will present our group’s work on addressing ohmic related resistances in membrane capacitive deionization and electrodeionization with new membrane and porous ion conducting materials. Borrowing from advances in fuel cell electrode binder technology, we show that solution processable ionomers are versatile for arriving at a new class of porous ionic conductors that address spacer channel resistances in the said deionization platforms under low TDS (total dissolved solids) concentrations. The new materials improve the separation efficiency and energy efficiency of deionization processes.

Visualization Guided Mitigation of Mechanical Membrane Degradation in Fuel Cells

Dynamic operational power demands from a fuel cell, especially in transportation applications, are known to causefluctuations in its relative humidity (RH) and temperature [1]. This leads to repetitive in-plane compressive and tensile stress build-up in the confined perfluorosulfonic acid (PFSA) ionomer membranes due to constraining of their cyclic swelling and shrinkage under hydrated and dehydrated states, respectively. The ionomer’s dynamic water sorption characteristics during these hygrothermal variations lead to catalyst coated membrane (CCM) deformation and cyclic mechanical stresses resulting in structural fatigue and micro-crack development at high stress accumulation sites [2]. Lately, laboratory-based X-ray computed tomography (XCT) was used for non-destructive four-dimensional (4D) visualization and microstructural analysis of fuel cell parts showing novel insights on their failure processes [3,4]. The objective of the present work is to leverage 4D XCT visualization to design and demonstrate novel mitigation strategies for mechanical membrane degradation in fuel cells. A custom designed X-ray transparent fixture housing a small-scale single cell fuel cell was subjected to 2 min. each of wet and dry cycles with N2 gas at anode and cathode compartments, thereby causing fatigue-based membrane damage [5]. 3D tomographic data sets of same membrane electrode assembly (MEA) locations were acquired as a function of degradation time, which facilitated to study the damage development and growth. As a baseline for this work, a non-bonded MEA with high surface roughness gas diffusion layers (GDLs) demonstrated that the primary membrane failure mode was CCM buckling into GDL voids/pores [6]. To overcome this failure mode, two separate mitigation approaches are explored in the present work: (i) a commercial GDL with low surface roughness; and (ii) bonding of MEA to arrest buckling. Furthermore, a multi-physics finite element method (FEM) model is developed to simulate the contact interaction at the catalyst layer | GDL interface and non-uniformity in the GDL microstructure. The XCT findings are correlated with the numerical model to perceive the specific impact of GDL surface features on the development and distribution of membrane failures.Similar to the baseline MEA, membrane crack development was observed to be the key failure mode for both mitigation approaches. When the two mitigation designs are compared at similar cycling stages with the baseline (Fig. 1), however, the size and density of membrane cracks is drastically reduced. Since both mitigation strategies are tested using the same accelerated stress test protocol as the baseline, it is hereby established that each approach provides substantial mitigation against fatigue driven mechanical membrane degradation, predominantly due to reduced severity of CCM buckling deformation, resulting in a doubling of the membrane lifetime in each case. Complementary finite element simulations corroborate the experimental findings and further estimate the critical GDL void sizes to prevent CCM buckling and the required interfacial MEA adhesion quality to stabilize the MEA for overall membrane durability. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program.

Wet/dry cycle durability of polyphenylene ionomer membranes in PEFC

The mechanical durability of our hydrocarbon polymer electrolyte membrane, poly(sulfophenylene quinquephenylene) (SPP-QP) or polyphenylene ionomer, was evaluated in wet/dry cycle tests in fuel cells according to the US-DOE protocol, where the effect of gas diffusion layers (hard or soft GDL) was investigated. The membrane exhibited mechanical failure with the hard GDL and H2 crossover (permeation through the membrane) jumping from 0.01% to ca. 2% after 4,000 cycles. Post-test analyses indicated that the edge of the membrane under the gasket was the most damaged, where the dimensional change upon humidification/dehumidification was restricted. Use of the soft GDL significantly improved the wet/dry cycle durability of the membrane with no practical changes in the H2 crossover, even after 30,000 cycles, due to the strong adhesion of the GDL to the catalyst layers. The mechanical durability of the SPP-QP membrane was better than that of our previous aromatic-based ionomer membrane containing ether and ketone groups in the main chain. The loss of molecular weight and the sulfonic acid groups was rather minor for the SPP-QP membrane, indicating chemical robustness of the membrane under the severe wet/dry cycle conditions.

Electrospun Ion-Conducting Composite Membrane with Buckling-Induced Anisotropic Through-Plane Conductivity.

Fuel cell (FC) is an attractive green alternative for today's fuel combustion systems. In common FCs, a polymer electrolyte membrane selectively conducts protons but blocks the passage of electrons and fuel. Nafion, the current benchmark membrane material, has a superior conductivity owing to unique morphology comprising randomly oriented elongated ionic nanochannels within its Teflon-like matrix. Channel orientation enhances Nafion conductivity, yet there has been no facile method to induce a stable alignment in the desired through-plane (TP) direction. Here, we report an approach based on dual electrospun Nafion-PVDF nanofiber composites that yields a stable TP alignment. It utilizes extreme thinness and strong inherent orientation within electrospun nanofibers, which is readily converted to TP alignment by plunging an electrospun nanofiber mat into a thin slit, resulting in nanofiber buckling and subsequent consolidation. Using TEM and SAXS, we demonstrate a pronounced and sustained TP ion channel orientation in prepared membranes, yielding a highly anisotropic swelling and conductivity exceeding that of bulk Nafion when normalized to Nafion content. The analysis also highlights the importance of PVDF as a stabilizing component, preserving orientation upon annealing, while a similarly prepared pure Nafion membrane loses anisotropy. The approach holds potential to advance the FC technology by overcoming current limitations of ionomeric membranes.