Ionomer Membranes Research Articles

Impact of Dispersion Solvent on Ionomer Thin Films and Membranes

Perfluorosulfonic acid (PFSA) ionomers are an important class of materials that many electrochemical devices rely on as their ion-conducting electrolyte. Often, PFSA films are prepared through solu...

Thermal desorption and pyrolysis direct analysis in real time mass spectrometry of Nafion membrane

AbstractPerfluorosulfonic acid ionomer membranes have been widely used as proton conducting membranes in various electrochemical processes such as polymer electrolyte fuel cells and water electrolysis. While their thermal stability has been studied by thermogravimetry and analysis of low molecular weight products, their decomposition mechanism is little understood. In this study a newly developed methodology of thermal desorption and pyrolysis in combination with direct analysis in real time mass spectrometry is applied for Nafion membrane. An ambient ionization source and a high‐resolution time‐of‐flight mass spectrometer enabled unambiguous assignment of gaseous products. Thermal decomposition is initiated by side chain detachment above 350°C, which leaves carbonyls on the main chain at the locations of the side chains. Perfluoroalkanes are released above 400°C by main chain scission and their further decomposition products dominate above 500 °C. DFT calculation of reaction energies and barrier heights of model compounds support proposed decomposition reactions.

(Invited) Liquid Chromatography/Mass Spectrometry Analysis of Effluent Water from Perfluoroalkylsulfonic Acid (PFSA) Membrane Degradation Studies at Accelerated Durability Test Conditions

Perfluoroalkylsulfonic acid (PFSA) ionomer membranes degrade under accelerated testing conditions such as open circuit voltage (OCV). Analyzing effluent water for fluoride ion release rate is a primary method for evaluating the membrane degradation rate. However, many proposed degradation reaction pathways should result in the release of small molecule polymer fragments. Liquid chromatography/mass spectrometry (LC/MS) methods are well suited to analyze for these fragments and provide insight into the degradation reactions. Accelerated OCV durability tests were conducted on membrane electrode assemblies made with 3M Ionomer or Nafion™ membranes. Effluent water was analyzed for fluoride, sulfate, and trifluoroacetic acid by ion chromatography (IC) and other polymer fragments by LC/MS. The results of this study imply measurable degradation occurs at the tertiary fluoride on the polymer backbone or the ether oxygen connecting the side chain to the backbone. In Nafion™, these structural features also exist in the ionomer sidechain.

Effects of melt flow index and equivalent weight on the dimensional stability and mechanical behavior of perfluorosulfonic acid ionomer membranes

The effect of melt flow index (MFI) and equivalent weight (EW) on three aspects relating to perfluorosulfonic acid membrane mechanical durability were studied: dimensional stability, hygral stress during humidity cycles and resistance to rupture under biaxial stresses. Membranes used in this study have nominal EWs of 800 and 700 g/mol –SO3H and MFI ranging from ≫ 10 to 0.005 g/10 min. The total water-uptake, water-uptake per sulfonic acid group, and coefficient of hygral expansion all increase with decreasing EW. For the same EW, the ionomer with lower MFI absorbs less water, swells less and experiences less hygral stress during humidity cycles. Although a 700 EW membrane with an MFI of 0.005 g/10 min swells more than the 800EW ionomers, it is projected to be the most durable because it experiences the least hygral stress during humidity cycles and is the most resistant to rupture under biaxial stress. These findings are supported by in-situ RH-cycling tests, which show a strong inverse correlation between log[MFI] and cycles to failure, and a weaker inverse dependence on EW. The results suggest that high-performing, low-EW ionomers can be mechanically robust enough for fuel cell applications provided they have a sufficiently low MFI.

Fabrication of Cu/Nafion-Based Ionic Polymer Metal Composites by Electroless Plating Method

The Cu/Nafion-based ionic polymer metal composites were analyzed in terms morphology, proton conductivity, water loss, displacement and blocking force performance were examined. The SEM images revealed that Cu particles distribution is uniform and compact on the surface of Nafion membrane due to continuous heterogeneous nucleation and grain growth. The IPMC is composed of the ionomer membrane layer, the middle layer and the surface electrode layer. A less water restricts the migration of the hydrated cation transfer, and causes the migration barrier, resulting in reducing conductivity of the IPMCs. The maximum displacement and maximum force are 30 mm and 9.1 mN.

Morphology and Transport of Multivalent Cation-Exchanged Ionomer Membranes Using Perfluorosulfonic Acid–CeZ+ as a Model System

Perfluorosulfonic acids (PFSAs) are commonly used as solid polymer electrolyte membranes (PEMs) in electrochemical energy devices, where they are vulnerable to attack by radical species during oper...

Perfluorinated membrane electrode assembly containing metal-free-catalyst cathode for anion exchange membrane fuel cells

An important advantage of anion-exchange membrane fuel cells (AEMFCs) is non-noble metal catalyst (NMCs) can be used in cathode thus drastically reducing the cost compared to proton exchange membrane fuel cells (PEMFCs). However, development of ionomer binder and membrane in perfluorinated membrane electrode assembly (MEA) for NMCs-based cathode are still insufficient. Here, we created a perfluorinated MEA containing metal-free-catalyst cathode derived from soluble perfluorinated ionomer and its membrane via catalyst-ink-based methods. Meanwhile, metal-free nitrogen-carbon (N, S-P-CNFs) electrocatalyst structure with soluble ionomer as the catalyst blinder was prepared and exhibits obviously 4e ORR catalysts activities. The resulting perfluorinated MEA exhibits a peak power density of 105.1 mW cm-2 at 80°C (Pt/C cathode and N, S-P-CNFs anode) in H2/O2 (CO2-free) fuel cells.

Effect of N-cyclic cationic groups in poly(phenylene oxide)-based catalyst ionomer membranes for anion exchange membrane fuel cells

Herein, as a binder (or catalyst ionomer) for AEMFCs, we investigated the effect of two different cationic copolymers based on poly(phenylene oxide) (PPO) with N-cyclic quaternary ammonium (QA) groups, including six-membered dimethyl piperidinium (DMP) and bis-six-membered azonia-spiro undecane (ASU). An earlier report on the same polymers for membranes in AEMFCs indicated the better electrochemical performance of PPO-ASU compared with PPO-DMP. Therefore, we would like to investigate these two polymers for catalyst ionomers. The outcome in this study using these two copolymers as catalyst ionomers indicates the opposite result; the electrochemical performance of the PPO-DMP ionomer is much better than the PPO-ASU ionomer. The commercial Fumion ionomer was used for the qualitative comparison. The density functional theory (DFT) calculation of the adsorption energy according to different orientations of the cationic groups on the catalyst surface shows that there is no difference between the adsorption energy of DMP and ASU cations, in compliance with the orientations of the cations. Although the PPO-ASU ionomer membrane has the highest hydroxide conductivity at 60 °C in liquid water, the hydrogen oxidation/reduction (HOR) activity of PPO-DMP and PPO-ASU showed similar values with the Fumion ionomer. While the PPO-DMP ionomer membrane shows relatively large fuel gas (hydrogen) permeability in dry and wet conditions, due to the chain flexibility and the presence of two methyl groups compared to the single methyl groups and lower flexibility of the PPO-ASU and Fumion ionomers. The electrochemical performance of a membrane electrode assembly (MEA) using the PPO-DMP ionomer exhibited an exceptional peak power density of 335 mW cm−2 compared to lower peak power densities the of PPO-ASU and Fumion ionomers under 60 °C and a fully humidified condition (H2/O2). The SEM images of MEAs after testing supports the conclusion that the PPO-DMP ionomer forms a uniform catalyst interface that is very well bound between the electrode and membrane, unlike the PPO-ASU and Fumion ionomers. The PPO-DMP ionomer membrane also showed better tensile strength and elongation at break than the PPO-ASU ionomer membrane. Therefore, we conclude that the well-prepared three-phase boundary structure played a critical role for the catalyst ionomer in each electrode, overcoming one of the critical performance-limiting factors.

Characterization of Ionomer Membranes with Confocal Raman Microscopy

Ionomer membranes are crucial components of electrochemical energy systems such as fuel cells, electrolyzers, or redox-flow batteries. They need to meet demanding requirements for high-performance and long-term stable devices, and a targeted optimization of the membranes requires exact knowledge of parameters such as membrane thickness, ion exchange capacity, or water uptake. We employ confocal Raman microscopy as a tool for in-depth characterization of membranes that are used in proton exchange membrane fuel cells and water electrolyzers. Raman spectroscopy as a vibrational spectroscopy technique is well suited to characterize ionomer membranes, and the combination of Raman spectroscopy with a confocal microscope additionally adds spatial resolution to this chemical analysis. Confocal Raman microscopy is a valuable tool to complement classical characterization of ionomer membranes that provides a plethora of chemical information about the investigated specimen on a single-digit micron scale.We present our latest results on application-relevant ionomer degradation and water management in membranes determined by confocal Raman microscopy. We found that a Nafion NR-211 membrane that was aged in an accelerated stress test in a fuel cell suffered from severe thinning and an anisotropic increase in its equivalent weight between the electrodes.1 Both are signs of chemical-mechanical degradation and were quantified with high spatial resolution using confocal Raman microscopy (Fig. 1A). Further, we used confocal Raman microscopy to characterize ionomer composite membranes. Composite membranes are a promising approach to optimize membrane properties by e.g. integrating mechanically stabilizing porous networks. We quantified the equivalent weight along the depth profile of a Nafion XL membrane, and also analyzed the influence of the reinforcement layer on water distribution within the hydrated membrane (Fig. 1B).2 Figure 1: Results of ionomer membrane characterization by Raman microscopy. A) Quantification of degradation of Nafion NR-211 in a fuel cell (open circuit voltage (OCV) hold as stress test). B) Through-plane Raman image of water distribution in a Nafion XL membrane. 1. T. Böhm, R. Moroni, M. Breitwieser, S. Thiele and S. Vierrath, J. Electrochem. Soc., 166(7), F3044-F3051 (2019).2. M. S. Mu’min, T. Böhm, R. Moroni, R. Zengerle, S. Thiele, S. Vierrath and M. Breitwieser, J. Memb. Sci., 585, 126–135 (2019). Figure 1

Sulfonated and Partially Fluorinated Poly(aryl) Multiblock-Co-Ionomer- and Blend Membranes As Proton Conductors for PEM Electrolysis

The core of a PEM water electrolyzer is represented by the proton conductive membrane. While nowadays PFSA-based materials such as Nafion are state of the art in large scale energy applications, there is a strong need for development of cost efficient alternatives, which exhibit reduced gas crossover, high ionic conductivity and durability.Here, we present proton exchange membranes based on two different concepts: Blends of partially fluorinated polybenzimidazole and pyridine side-chain-modified polysulfones (3K-BM) as well as arylene main-chain multiblock co-ionomers (MB-O).The 3K-BM ionomer membranes represent a blend of hydrophilic partially fluorinated and sulfonated aromatic polyethers with two hydrophobic components: F6-PBI and pyridine-modified PSU [1]. Proton conductivity is achieved by the hydrophilic parts of the material whereas the hydrophobic components ensure stability and enable adjustment of mechanical properties.MB-O consists of hydrophobic and hydrophilic building blocks, which are synthesized via step-growth polycondensation. The former blocks are partially fluorinated aromatics, whereas the latter ones are based on sulfonated nonfluorinated aromatics [2].Multiblock-co-ionomers are capable of forming nanophase-separated structures [2,3], which are tunable by varying the ratio of hydrophilic and hydrophobic proportions. In addition to the polymer synthesis and membrane preparation, ex-situ characterization including NMR spectroscopy, gel permeation chromatography, thermogravimetric analysis, H2-crossover analysis via linear-sweep-voltammetry and AC impedance spectroscopy have been performed with our novel materials. First preliminary in-situ tests in a PEM electrolysis cell have already yielded high performances, which can potentially be increased further by optimization of electrodes with respect to binders for the respective membrane material. Acknowledgements This work has been funded by the German Federal Ministry of Education and Research (BMBF) as part of the project: “Increasing Lifespan and Power of Polymer Electrolyte Membrane Electrolysers through High Power Membrane Electrode Assemblies (POWER-MEE)“ References [1] J. Kerres, A. Ullrich, M. Hein, J. Polym. Sci.: Part A: Polym. Chem. 39 (2001) 2874–2888[2] F. Schönberger, J. Kerres, J.Polym. Sci. Part A: Polym. Chem. 45 (2007) 5237–5255.[3] H. Ghassemi, J. E. McGrath, T. A. Zawodzinski, Polymer 47 (2006) 4132-4139

Laboratory XANES to study vanadium species in vanadium redox flow batteries

The speciation of vanadium in the electrolyte of vanadium redox flow batteries (VRFBs) is important to determine the state of charge of the battery. To obtain a better understanding of the transport of the different vanadium species through the separator polymer electrolyte membranes, it is necessary to be able to determine concentration and species of the vanadium ions inside the nanoscopic water body of the membranes. The speciation of V in the electrolyte of VRFBs has been performed by others at the synchrotron by X-ray absorption near-edge structure analysis (XANES). However, the concentrations are quite high and not necessarily justify the use of a large-scale facility. Here, we show that vanadium species in the electrolyte and inside the ionomeric membranes can be determined by laboratory XANES. We were able to determine V species in the 1.6 M electrolyte with a measurement time of 2.3 h and V species having a concentration of 9.8 g kg−1 inside the membranes (178 µm thick) with a measurement time of 5 h. Our results show that laboratory XANES is an appropriate tool to study these kind of samples.

Optimizing polymer electrolyte membrane thickness to maximize fuel cell vehicle range

Automotive hydrogen polymer electrolyte membrane (PEM) fuel cell systems require periodic purges to remove nitrogen and water from the anode. Purging increases system performance by limiting anode hydrogen dilution, but reduces hydrogen utilization. State of the art fuel cell membrane electrode assemblies utilize thin ionomer membranes in an effort to increase performance and reduce cost. Thinner membranes also increase the required anode purge rates due to the increased transport of inert gases. A model was developed to examine the relationship between membrane thickness and vehicle range which takes into account anode purge rate. The model includes changes in efficiency and hydrogen utilization as a function of PEM thickness for a variety of operating conditions. The model predicts that an optimal membrane thickness which maximizes vehicle range exists, but this thickness is highly dependent on other system conditions. The results of this study can be extended to help optimize stack development and balance of plant design.

Nafion Ionomer-Based Single Component Electrolytes for Aqueous Zn/MnO2 Batteries with Long Cycle Life

Recently, aqueous rechargeable Zn/MnO2 batteries are emerging as promising energy storage aids owing to their improved safety, low cost of fabrication, and high energy density. However, the rapid decay of capacity during extended charge-discharge cycles hinders the prospect of this technology beyond lab-scale. In the conventional Zn/MnO2 cell, additives such as Mn2+ have been used to tackle the stability issue. Here, we demonstrate that cycling performance of the Zn/MnO2 cell can be improved substantially by using Nafion ionomer as the separator in combination with zinc-ion conducting electrolytes. The Nafion ionomer-based Zn/MnO2 cells do not require any Mn2+ additive in the electrolyte and hence termed as single component electrolytes. The postmortem study of the post-cycled electrodes reveals that the structural evolution of both the anode and cathode in various electrolytes (1 M Zn(CF3SO3)2, 1 M ZnSO4·7H2O, and 3 M ZnSO4·7H2O) during prolonged cycling significantly influences the cycle life of the respective cells. Optimizing the Nafion ionomer membrane with a suitable electrolyte could render the desired combination of high capacity and high cycle life for a Zn/MnO2 cell.

4D in situ visualization of mechanical degradation evolution in reinforced fuel cell membranes

Composite ionomer membranes with ePTFE reinforcement have been developed to improve operational durability of polymer electrolyte fuel cells by creating a more mechanically robust membrane electrode assembly. The present objective is to determine the morphological damage evolution of a reinforced membrane subjected to pure mechanical degradation by wet/dry cycling in a fuel cell. Identical-location four-dimensional in situ visualization by X-ray computed tomography is used to reveal the progressive degradation stages from initiation via propagation to failure. The observed degradation process is dominated by fatigue driven membrane fracture which is primarily confined under the channel area. When compared to degradation of non-reinforced membranes, the results for the reinforced membrane demonstrate similar non-linear progression of membrane fracture, but at 2-3x lower rate due to the improved fracture resistance of ePTFE reinforcement. Membrane-catalyst layer delamination and catalyst layer cracks are identified as preceding drivers of local membrane fracture, while wet and dry phase in situ imaging demonstrates hydration induced through-plane swelling of 30% and widespread crack closure at advanced degradation states. Overall, these results provide new understanding of mechanical degradation in composite membranes and prospective directions for further durability enhancements.

FeNx/FeSx-Anchored Carbon Sheet–Carbon Nanotube Composite Electrocatalysts for Oxygen Reduction

Even though various Pt-free electrocatalysts for oxygen reduction reaction (ORR) have been introduced, many of them are found to be active only in alkaline conditions. Considering Nafion, phosphoric acid-doped polybenzimidazole (PBI), and so on as the prominent ionomer membranes, used in the commercially available polymer electrolyte membrane fuel cells (PEMFCs), it becomes important that any development on the Pt-free catalysts should ensure the better ORR performance under acidic conditions. The present work effectively tackles this issue, where an ORR-based catalyst could be prepared with simultaneous incorporation of both Fe–N and Fe–S active sites on in situ generated carbon sheets which are spatially separated by the carbon nanotube (CNT) network. This catalyst shows ability to perform under both acidic and basic conditions. This has been achieved by growing a polyethylenedioxythiophene polymer network in the presence of CNT and melamine followed by its pyrolysis under an inert atmosphere. The catalyst formed at 900 °C (PMCNT-900) displays 0.94 V onset potential for ORR under acidic electrolyte conditions, which corresponds to 60 mV overpotential compared to its 40 wt % Pt/C counterpart. Interestingly, in single cell demonstration of Nafion-based PEMFC with PMCNT-900 as the cathode catalyst, the system delivered a maximum power density (PD) of 500 and 275 mW/cm2 at 60 °C under H2–O2 and H2–air feed conditions, respectively. On the other hand, in a single cell test in the anion exchange membrane fuel cell (AEMFC) mode, a maximum power density of 65 mW/cm2 at 50 °C could be achieved with the same cathode catalyst, which is a comparable value obtained while employing Pt/C as the cathode. These results, thus, infer to the efficiency of the catalyst to facilitate ORR under the extreme pH conditions, and particularly its performance under acidic condition reveals its prospect as a potential Pt-free electrocatalyst to serve in the Nafion-based systems.

Group Vibrational Mode Assignments as a Broadly Applicable Tool for Characterizing Ionomer Membrane Structure as a Function of Degree of Hydration

Infrared spectra of Nafion, Aquivion, and the 3 M membrane were acquired during total dehydration of fully hydrated samples. Fully hydrated exchange sites are in a sulfonate form with a C3V local s...

An AC-driven desalination/salination system based on a Nafion cationic rectifier

Cationic diodes based on Nafion ionomer coated microhole array substrates are combined with an anion conducting membrane to give an AC-electricity driven desalination or salination system. The combination of cationic diode with anionic resistor allows a 3D-printed four-chamber system to be configured with two internal chambers to extract and/or accumulate salt. Three configurations are demonstrated combining desalination and salination for an aqueous 250 mM NaCl solution. For a proof-of-concept device, 50% desalination/salination is demonstrated. The charge efficiency is estimated to be only 12%, limited mainly due to (i) resistivity lowering rectification, (ii) rectification ratio limitations for the array of microholes, (iii) insufficient stability of ionomer membranes under AC-pulse-driven operation. Perspectives for improvements and for future desalination/salination applications are given.

Enhancement of service life of polymer electrolyte fuel cells through application of nanodispersed ionomer.

In polymer electrolyte fuel cells (PEFCs), protons from the anode are transferred to the cathode through the ionomer membrane. By impregnating the ionomer into the electrodes, proton pathways are extended and high proton transfer efficiency can be achieved. Because the impregnated ionomer mechanically binds the catalysts within the electrode, the ionomer is also called a binder. To yield good electrochemical performance, the binder should be homogeneously dispersed in the electrode and maintain stable interfaces with other catalyst components and the membrane. However, conventional binder materials do not have good dispersion properties. In this study, a facile approach based on using a supercritical fluid is introduced to prepare a homogeneous nanoscale dispersion of the binder material in aqueous alcohol. The prepared binder exhibited high dispersion characteristics, crystallinity, and proton conductivity. High performance and durability were confirmed when the binder material was applied to a PEFC cathode electrode.

Improving the water management in anion-exchange membrane fuel cells via ultra-thin, directly deposited solid polymer electrolyte.

Thin ionomer membranes are considered key to achieve high performances in anion exchange membrane fuel cells. However, the handling of unsupported anion exchange membranes with thicknesses below 15 μm is challenging. Typical pre-treatments of KOH-soaking, DI-water rinsing and/or wet assembly with sub-15 μm thin films are particularly problematic. In this work, we report configurations of membrane electrode assemblies with solid polymer electrolyte thicknesses equivalent to 3, 5 and 10 μm, made possible by direct coating of the ionomer onto gas diffusion electrodes (direct membrane deposition). The anion-conducting solid polymer electrolyte employed is hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), which is known for its high mechanical stability and low rate of gas crossover. By fabricating membrane-electrode-assemblies with PtRu/C anodes and Pt/C cathodes with a low precious metal loading of <0.3 mg cm−2, reproducible performances beyond 1 W cm−2 in H2/O2 atmosphere are achieved. The thin membranes enable excellent performance robustness towards changes in relative humidity, as well as low ionic resistances (<40 mOhm cm2).

An ultrathin ionomer interphase for high efficiency lithium anode in carbonate based electrolyte

High coulombic efficiency and dendrite suppression in carbonate electrolytes remain challenges to the development of high-energy lithium ion batteries containing lithium metal anodes. Here we demonstrate an ultrathin (≤100 nm) lithium-ion ionomer membrane consisting of lithium-exchanged sulfonated polyether ether ketone embedded with polyhedral oligosilsesquioxane as a coating layer on copper or lithium for achieving efficient and stable lithium plating-stripping cycles in a carbonate-based electrolyte. Operando analyses and theoretical simulation reveal the remarkable ability of the ionomer coating to enable electric field homogenization over a considerably large lithium-plating surface. The membrane coating, serving as an artificial solid-electrolyte interphase filter in minimizing parasitic reactions at the electrolyte-electrode interface, enables dendrite-free lithium plating on copper with outstanding coulombic efficiencies at room and elevated (50 °C) temperatures. The membrane coated copper demonstrates itself as a promising current collector for manufacturing high-quality pre-plated lithium thin-film anode.