Properties Of Ionomers Research Articles

(Keynote) Electrochemical Conversion of Atmospheric CO2 to Value-Added Products

Climate change is an existential threat to humanity that calls for immediate reduction of CO2 concentration in the atmosphere. While drastic reduction in green-house gas emissions needs to remain the primary objective, possible integration of the direct air capture (DAC) and electrocatalytic conversion of CO2 represents a promising strategy to aid in averting a climate catastrophe. In this presentation, we will summarize the recent progress in electrochemical conversion of CO2 to value-added products at Los Alamos, in which we combine experimental and modeling approaches to tackle challenges in the development of materials for the integration of DAC with electrochemical CO2 reduction reaction (CO2RR), mostly concentrating on the conversion part. We use a “5M” (make, measure, model, machine-learn, and modify) approach to accelerate the understanding of how CO2 sorbents and CO2RR catalysts synthesis impacts their structure and performance.Enhancing electrocatalytic activity and selectivity for the desired products are the main priorities in the development of electrocatalysts for CO2RR.1 Understanding local environment, e.g., local pH and triple phase boundary at the reaction area is also essential for long-term operation of CO2RR devices.2 We will discuss the development of atomically dispersed Ni-N-C catalysts with high selectivity for CO2 conversion to CO (faradaic efficiency in excess of 90%) and the effect that the ionomer used in the electrode preparation has on the Ni-N-C catalyst performance. In particular, we will shed light on how the ionomer properties such as hydrophobicity, local pH, and backbone structure affect the CO2RR activity and selectivity by modifying the ionomer-catalyst interface. In addition, we will outline our approach to facilitating the highly challenging integration of the DAC and CO2RR components via the use of electronically conductive carbon nanofibers with high CO2 sorption capacity and thermo-oxidative resistance. Acknowledgement Research presented in this work has been supported by the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory under project number 20230065DR. References (1) Li, Y.; Wang, H.; Yang, X.; O'Carroll, T.; Wu, G. Designing and Engineering Atomically Dispersed Metal Catalysts for CO2 to CO Conversion: From Single to Dual Metal Sites. Angewandte Chemie International Edition 2024, 63 (12), e202317884.(2) Chen, J.; Wang, L. Effects of the Catalyst Dynamic Changes and Influence of the Reaction Environment on the Performance of Electrochemical CO2 Reduction. Advanced Materials 2022, 34 (25), 2103900.

Effects of Ionomer Film Thickness and Water Content on Oxygen Permeation Properties of PEFC Cathode Catalyst

This study explores the effects of film thickness and water content on the oxygen permeation properties of ionomers in the cathode catalyst layer of Polymer Electrolyte Fuel Cells (PEFCs) using Molecular Dynamics (MD) simulations. The aim is to understand in detail how these parameters influence the efficiency of oxygen transport within the ionomer of the catalyst layer, which is important for optimizing the performance of PEFCs.In this study, we adopt the MD simulation, which computes the time evolution of the positions and velocities of particles based on Newton's equations of motion.In this study, Nafion, commonly used in PEFC, served as the ionomer material, employing a simplified model with 10 sulfonic acid groups per chain to lower computational costs, yet reflecting its actual structural and transport properties[1]. For the oxygen molecule, a model in which the intramolecular force is represented by the Morse potential is used. For the water molecule, the F3C model in which the oxygen and hydrogen atoms move independently and are combined with BOND and ANGLE potentials to form a molecule was used.The number of Nafion chain in the ionomer was changed to 14, 25, and 35 to control the film thickness. From here, the difference of thickness is expressed by Thin, Medium, and Thick. The thicknesses correspond to 3.5 nm, 5.0 nm, and 7.0 nm, respectively. The water content was varied with λ=3, 7, and 11 to investigate the effect of water content in the ionomer on oxygen permeability. Water content is the ratio of the sum of water molecules and hydronium ions to the number of hydrophilic groups in the polymer.In this study, the oxygen permeability of the ionomer membrane was determined using equation (1). The number of atoms in a water cluster that contains the most atoms was determined as the maximum cluster size to analyze the extent to which water clusters expand. Furthermore, a group of water molecules and hydronium ions, in which the distance between the oxygen atoms within the water molecules and hydronium ions is within 3.5 Å, has been defined as a water cluster. 3.5 Å is the end value of the first peak in the Radial Distribution Function (RDF) between the oxygen atoms of the water molecule in the Nafion bulk film. RDF is a function that shows the average density of particles as a function of distance from a specific point. The density distribution of Nafion and water vertical to the platinum interface was calculated to analyze the layer structure of the film.Fig. 1 shows the film thickness dependence of maximum cluster size for each water content condition. The vertical axis of the graph is the common logarithm of the maximum cluster size. For water content λ=3, the maximum cluster size is the highest when the film thickness is Medium. However, for water contents λ=7 and 11, the maximum cluster size increases as the film thickness increases.Fig. 2 shows the film thickness dependence of oxygen permeability for each water content condition. For a water content of λ=3, the oxygen permeability reaches a minimum value when the film thickness is medium, and for water contents of λ=7 and 11, the oxygen permeability decreases as the film thickness increases. Moreover, oxygen permeability decreases with increasing water content across different film thicknesses.Fig. 3 shows the vertical density distribution of water molecules and Nafion at the Pt interface for each film thickness condition at a water content of λ=3. In this graph, the water density has a second and third peak in addition to the first peak above the Pt interface only when the film thickness is Medium, suggesting that a thick layer of water formed near the interface hinders the permeability of oxygen. In addition, when the film thickness is Thick, there are two peaks in water density at the top of the film, suggesting that a layer of water has formed at the top. However, the water layer formed in a thicker film is considered to be more mobile than that on the hydrophilic platinum surface, the effect of the water layer on oxygen permeability is considered to be smaller than that of the water layer formed in the intermediate film thickness.From the above results, designing the catalyst layer to avoid forming water layers near the platinum interface that hinder oxygen permeability is important.Refarence:[1]Venkatnathan A, Devanathan R, Dupuis MJournal of Physical Chemistry B (2007) 111(25) 7234-7244 Figure 1

Deconvoluting the through-Plane Degradation Mechanisms of the Pt-Co/C Cathode Catalyst Layer in Polymer Electrolyte Membrane Fuel Cells Under Load Cycling Conditions

Polymer electrolyte membrane fuel cells (PEMFCs) are an important technology to reduce greenhouse gas emissions in heavy duty vehicles (HDVs). State-of-the-art cathode catalyst layers (CCLs) that drive the kinetically slow oxygen reduction reaction (ORR) are Pt-M (M = first row transition metal) nanoparticles (NPs) supported on a carbon support (Pt-M/C). Pt-M NPs improve initial ORR kinetics and lower nominal Pt content, however, their degradation under load cycling conditions is still poorly understood due to the multidimensional physiochemical problem driven by both thermodynamics and kinetics. Alloy leaching leads to lower kinetic performance of the catalyst and affects ionomer properties, particle growth lowers efficiency of the catalyst, and changes in the CCL structure effects long-term transport efficiency. These phenomena occur at different rates and length scales. Furthermore, the initial material can be quite heterogenous in both NP size and composition, which adds further complexity in understanding the aging phenomena in Pt-M/C.This study focuses on understanding degradation mechanisms on commercially available membrane electrode assemblies (MEAs) using Pt-M/C CCLs to enable better understanding of potential pathways to more novel materials, including more complex ‘dealloyed’ bimetallic catalysts, and electrode design. Samples were extracted from aged MEAs tested using a voltage cycling protocol after 0 (BOL), 1000, 3000, 30000, and 108000 cycles. Scanning transmission electron microscopy and energy dispersive spectroscopy (STEM/EDS) studies have been instrumental to understanding localized heterogeneities in the catalyst aging phenomena in tested MEAs, aiding in identification of contributing degradation mechanisms. STEM/EDS provides both visualization of the initial and aged materials and allows to quantify composition and size changes throughout the entire CCL. We approach the imaging and mapping from a through-plane spatial perspective (ex-situ) including near the gas diffusion layer – CCL interface (GDL-CCL), middle of the catalyst layer (MID-CCL), and CCL-membrane (CCL-PEM) interface. Relative changes in the particle size and elemental ratios were used to quantify the losses of catalyst and compare to overall losses for each respective number of cycles. In the past, we have used the F/Pt ratio (average, in-plane, and through-plane) to quantify Pt (catalyst) loss relative to F (ionomer) for Pt/C CCLs and particle size analysis in the three locations in the CCL to track spatial particle growth. Here, we present additional parameters, including the F/Co and Pt/Co ratios, as metrics to track Co loss and Pt loss or growth. X-ray photoelectron spectroscopy (XPS) was also used to deconvolute both elemental and chemical changes as a function of number of cycles. However, the signal acquisition depth of XPS is limited up to 10 nm and can only be used to complement trends in the STEM/EDS quantification near the GDL-CCL interface. Nevertheless, a mechanistic approach involving both techniques highlights STEM/EDS and XPS as complementary characterization techniques for assessment of degradation of Pt-Co/C occurring at different length scales as a function of number of cycles.

Investigating Cathode Ionomer Content and Assembly Techniques for Anion Exchange Water Electrolysers

An ionomer is often used in catalyst layers to a) adhere catalysts to the membrane or transport layer, b) facilitate ionic conductivity for OH- species through catalyst layers, and c) achieve ideal catalyst dispersions in catalyst inks used in spraying methods[1]. The choice of ionomer plays a crucial role in determining ion conductivity, water management, and mechanical stability within the membrane electrode assembly of anion exchange membrane water electrolysers (AEMWEs). Therefore, optimizing ionomer properties is crucial to achieving high-performance AEMWEs with efficient hydroxide transport.Both low and high ionomer loadings can lead to challenges in charge transport resistance. At low loadings, poor electronic conductivity and reduced contact between the catalyst layer and the membrane surface can occur due to a rough electrode interface[2]. This can result in inefficient charge transfer kinetics and hinder the overall efficiency (Figure 1c). Conversely, high ionomer loadings can lead to increased charge transport resistance, attributed to decreased gas permeability to the catalyst surface. The excessive presence of ionomer within the catalyst layer can block gas-liquid transport pores inside catalyst aggregates, limiting mass transport of reactants to the active sites[2]. This phenomenon can impede gas diffusion and oxygen reduction kinetics, leading to decreased performance and lower efficiency in AEMWEs (Figure 1d). Achieving an optimal ionomer loading balance is therefore essential for promoting efficient charge transfer and gas transport processes (Figure 1e), ultimately enhancing the overall performance and durability of AEMWE devices. The use of a proton conducting ionomer such as Nafion seems counterintuitive for use within AEMWEs, its hydroxide conductivity is comparatively lower, potentially limiting electrolyser performance. However, compared to Nafion, OH- conductive binders (Sustainion and PiperION) typically yield poorer electrodes, with larger catalyst aggregates[3].Two anion exchange ionomers and one ion exchange ionomer are evaluated across a range of loadings, and two different cathode assembly techniques, Catalyst coated substrates (CCS) and catalyst coated membranes (CCM) (Figure 1a). Since different catalyst surfaces are observed to influence ionomer degradation differently[4], commercially available Pt/C was utilised as a catalyst to focus on ionomer content and fabrication techniques. To investigate the effect of using hydrophobic or hydrophilic ionomers on catalyst layer homogeneity, the content of Nafion, Sustainion, and PiperION ionomers in the catalyst layers was varied (Figure 1b). Electrochemical impedance spectroscopy was used to determine the trade-off between high and low ionomer contents, polarisation curves found the in-situ electrocatalytic performance and highlighted the best performing ionomer and fabrication technique. Scanning electron microscopy was used to highlight the differences in catalyst layer structures and homogeneity.[1] Volk et al. Nov. 2023, EES. Catal. 2(1). 109-137. DOI: 10.1039/D3EY00193H.[2] Zhao et al. Feb. 2023, Int. J. Hydrogen Energy. 48(13). 5266-5275. DOI: 10.1016/j.ijhydene.2022.11.057.[3] Jervis et al. Nov. 2017, J. Electrochem. Soc. 164(14). 1551-1555. DOI: 10.1149/2.0441714jes.[4] Li et al. Mar. 2019, ACS Appl. Mater. Interfaces. 11(10). 9696–9701. DOI: 10.1021/acsami.9b00711.Figure 1: Schematic illustrations of: a) Membrane electrode assembly fabrication techniques, catalyst coated membrane and catalyst coated substrate. b) The chemical structures of PiperION A5 by Versogen, Sustainion XA-9 by Dioxide Materials, and Nafion by Liquion. c) Catalyst layer with low ionomer loading. d) Catalyst layer with moderate ionomer loading. e) Catalyst layer with high ionomer loading. Figure 1

Effects of Ionomer Film Thickness and Water Content on Oxygen Permeation Properties of PEFC Cathode Catalyst

In order to promote the widespread use of Polymer Electrolyte Fuel Cell (PEFC), enhancing cell performance is necessary. One significant factor affecting cell performance is the oxygen permeability in the ionomer, which depends on its nano-scale structure. In this study, to investigate the relationship between structures of catalyst layers and cell performances, we analyzed the effects of film thickness and water content on the oxygen permeation properties of ionomer in the cathode catalyst layer of PEFC using molecular dynamics simulations. We constructed a system in which oxygen molecules permeated ionomer on a platinum (Pt) surface. We found that sulfonic acid groups have a significant effect on oxygen permeation in low water content conditions.

Confocal Raman Microscopy – Shedding Light on the Ionomer Properties of PFSA Membranes

Perfluorinated sulfonic acid (PFSA) polymers are the state-of-the-art material for the proton exchange membrane (PEM) in various electrochemical energy conversion devices, e.g., PEM fuel cells, PEM water electrolyzers, or redox flow batteries, combining chemical and mechanical durability with high proton conductivity.The most prominent example of a PFSA material is NafionTM, which is a so-called long side chain (LSC) ionomer due to its comparably long side chain between the polymer backbone and the charged sulfonyl end group. On the other hand, short side chain (SSC) ionomers like Aquivion® and 3M Ionomer or composite membranes are promising approaches to improve performance in harsh conditions, such as low humidity or high operating temperatures.1 SSC PEMs feature increased water uptake, proton conductivity,2 and crystallinity3 compared to LSC PEMs. Composite PEMs can be specifically tailored to the required operating conditions by carefully designing the interlayers or additives. As the performance and longevity of electrochemical cells are a delicate convolution of various (composite) membrane properties and parameters, a method for evaluating ionomer membrane properties is crucial for rationally engineering optimized PEMs.Here, we use confocal Raman microscopy (CRM) to investigate application-relevant properties of PFSA PEMs and for high-resolution imaging of composite membranes.4 Thus, the high spatial resolution of confocal optical microscopy and the chemical sensitivity of Raman scattering are combined in a single measurement approach. NafionTM, 3M Ionomer, and Aquivion® at varying equivalent weights (EW) feature distinctive Raman spectra (Fig. 1) that show characteristic spectral changes depending on the ionomer’s side chain structure and EW. We show that the EW of these PFSA types can be reliably measured using Raman spectroscopy. Further, parameters such as swelling, ionizable group content, and water uptake can be quantified by CRM non-destructively and contact-free. The diffraction-limited resolution of CRM of less than 2 µm enables a view of the local distribution of these properties and, therefore, allows to image and analyze composite membranes. In summary, we comprehensively study ionomer properties of (composite) membranes and establish CRM as a powerful platform for characterizing advanced PEMs for electrochemical energy deviceReferences A. S. Aricò, A. Di Blasi, G. Brunaccini, F. Sergi, G. Dispenza, L. Andaloro, M. Ferraro, V. Antonucci, P. Asher, S. Buche, D. Fongalland, G. A. Hards, J. D. B. Sharman, A. Bayer, G. Heinz, N. Zandonà, R. Zuber, M. Gebert, M. Corasaniti, A. Ghielmi and D. J. Jones, Fuel Cells, 10(6), 1013–1023 (2010).Y.-C. Park, K. Kakinuma, H. Uchida, M. Watanabe and M. Uchida, Journal of Power Sources, 275, 384–391 (2015).K. D. Kreuer, M. Schuster, B. Obliers, O. Diat, U. Traub, A. Fuchs, U. Klock, S. J. Paddison and J. Maier, Journal of Power Sources, 178(2), 499–509 (2008).M. Maier, D. Abbas, M. Komma, M. S. Mu'min, S. Thiele and T. Böhm, Journal of Membrane Science, 669, 121244 (2023). Figure 1

Decoupling and quantifying the mass transfer resistance of the gas diffusion electrode for CO2 electrochemical reduction reaction

Mass transfer capacity ultimately dominates and limits the performance of CO2 electrochemical reduction reaction (CO2RR), which is closely related to industrial applications. Hence, the quantification of CO2 transport resistance is urgent and necessary. In addition, the transport of CO2 involves the entire process from the gas phase source to the catalytic site, resulting in the superposition of transport resistance from each component of the electrolyzer, which makes the transport resistance decoupling challenging. Based on the limiting current method, an experimental method is developed here to quantify the transport resistance of the complex porous electrode of the carbon fiber substrate (CFS), microporous layer (MPL) and catalyst layer (CL). Additionally, a dummy catalyst layer (DCL) is designed, which has a similar pore structure to CL but no catalytic sites for CO2RR, to further quantify the resistance in CL formed from pore structures and ionomer properties. The experimental results can provide the optimization direction and experimental basis for the subsequent electrode design.

Does Confinement Mediate Cerium Exchange and Its Impact on Ionomer Properties?

Does Confinement Mediate Cerium Exchange and Its Impact on Ionomer Properties?

In-Situ Analysis of Electrodes Using Fine-Tuned Poly(Terphenyl Piperidinium) Ionomers in Anion Exchange Membrane Fuel Cells

In the field of low temperature polymer electrolyte fuel cells, anion exchange membrane fuel cells (AEMFCs) have been considered a promising alternative by circumventing the obstacles to commercialization faced by the established PEMFC technology. The application of platinum group metal-free catalysts for the oxygen reduction reaction, the potential to utilize cheaper and more sustainable polymers than the industry benchmark Nafion®, and the generally less corrosive alkaline operation conditions, have all been identified as inherent advantages of the AEMFC technology, potentially enabling more cost-effective and environmental-friendly mass production of AEMFC system.The first major challenge in developing AEMFC systems was the lack of stable and conductive anion exchange polymers to be used as membrane and ionomer materials. This challenge was at least partly overcome by the successful development of a variety of next-generation polymers. Among those, poly(arylene piperidinium) show great potential through the combination an ether-free aromatic backbone for increased mechanical stability with a piperidinium-based cationic groups displaying excellent resistance to hydroxide attack in highly alkaline environment [1]. Furthermore, the crucial parameters ion exchange capacity (IEC), water uptake and dimensional swelling can be finely controlled via copolymerization and partial substitution of the cationic functional group in the repeating unit [2].In a previous study using a series of poly(terphenyl piperidinium) membranes with different IECs, we established the importance of these three parameters, in particular for the ionomer used in the catalyst layer of the electrode [3]. Furthermore, it was established that water balance of AEMFC systems proved to be one of the greatest challenges in reaching and maintaining high cell performance. The asymmetrical nature of the simultaneous electrochemical production and consumption of water at the anode and cathode, respectively, necessitates fine-tuning the ionomer properties, as well as selecting operational parameters for each electrode individually in order to strike an optimal water balance [4].In this study, we expand on our previous work by analyzing a matrix of electrode combination based on three poly(terphenyl piperidinium)ionomers with different IEC values. Furthermore, we complemented our analysis by evaluating the effects of substituting a fraction of regular dissolvable ionomer with insoluble non-conformal particles of cross-linked poly(terphenyl piperidinium). The beneficial effects of insoluble non-conformal ionomer particles in the catalyst layer has been previously shown by Varcoe and coworkers utilizing radion-grafted anion exchange membranes [5], and by Holdcroft and coworkers utilizing phenylated poly(phenylene) ionomers for PEMFCs [6].Different electrode combinations were evaluated while varying operational parameters such as gas flow rates, gas relative humidities and back pressure. The recording of observed cell performances were complemented by in-situ analysis of in-let/out-let gas relative humidities, H2/O2 electrochemical impedance spectroscopy for kinetic analysis and information on ohmic losses, H2/Ar electrochemical impedance spectroscopy to measure ionic conductivity, cyclic voltammetry to measure electrochemically active surface area as well as ex-situ characterization of the electrodes before/after the test run.The study was carried out with the aim to establish a correlation between the properties of individually tuned ionomers and operational parameters, and to investigate reasons for performance losses due to mass transport limitations, drop in ionomer conductivity, loss of active area, and decreased kinetics caused by phenyl adsorption.Finally, we demonstrate how the application of individually optimized electrodes using fine-tuned poly(terphenyl piperidinium) based ionomers in conjunction with suitable operating conditions will lead to a drastic increase in fuel cell performance compared to symmetrical non-optimized electrodes.A selection of experimental data is shown in Figure 1.[1] J. S. Olsson, T. H. Pham, P. Jannasch, Adv. Funct. Mater. 2017, 28, 1702758.[2] J. S. Olsson, T. H. Pham, P. Jannasch, Tuning poly(arylene piperidinium) anion-exchange membranes by copolymerization, partial quaternization and crosslinking, J. Membrane Sci., 2019, 578, 183-195.[3] T. Novalin, D. Pan, G. Lindbergh, C. Lagergren, P. Jannasch, R.W. Lindström, Electrochemical performance of poly(arylene piperidinium) membranes and ionomers in anion exchange membrane fuel cells, J. Power Sources, 2021, 507, 230287. [4] D. P. Leonard, S. Maurya , E. J. Park , S. Noh , C. Bae , E. D. Baca , C. Fujimoto and Y. S. Kim , J. Mater. Chem. A, 2020, 8 , 14135-14144.[5] L. Q. Wang , J. J. Brink , Y. Liu , A. M. Herring , J. Ponce-Gonzalez , D. K. Whelligan and J. R. Varcoe, Energy Environ. Sci., 2017, 10 , 2154-2167.[6] E. Balogun, S. Cassegrain, P. Mardle, M. Adamski, T. Saatkamp, and S. Holdcroft, ACS Energy Lett. 2022, 7, 2070-2078. Figure 1

Catalyst Layers with Blended Ionomer Using Short- and Long-Side Chain Dispersions

Hydrogen-based energy conversion technology is based on the membrane electrode assembly (MEA) comprising of an electrolyte and a pair of catalyst layers. It should have high efficiency and allow dynamic operation at low temperature. Electrochemical reactions for hydrogen oxidation reaction and oxygen reduction reaction occur in the catalyst layers in the MEA. Catalyst layers for MEA is prepared by catalyst inks comprising of electrocatalyst, ionomer dispersion and additional solvents. Ionomers plays an important roles for the formation of porous structure of catalyst layer for transportation of gas and reactant and for the ionic conduction. The ionomers are divided that long side chain (LSC) and short side chain (SSC) depending on the length of side chain. Long side chain (LSC) ionomers shows high performance at low temperatures and high humidity, while short side chain (SSC) ionomers shows high performance at high temperatures and low humidity. It has been reported that the properties of ionomer dispersions substantially influence the performance of catalyst layers due to the different interaction between electrocatalysts and ionomer. In this study, to investigate the effect of the properties of ionomer, various contents of blended ionomer dispersions were prepared. Properties of single and blended ionomers were characterized using catalysts layers in terms of I-V polarization, cyclic voltammetry, electrochemical impedance spectroscopy and microscopic evaluation such as porosimetry. Acknowledgment This research was supported in part by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning(NRF-2019M3E6A1063677) and by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040520).

A comprehensive study on the ionomer properties of PFSA membranes with confocal Raman microscopy

The proton exchange membrane is the heart of many electrochemical energy systems, and their performance and longevity depend on a careful selection and processing of membrane materials. In this work, we use confocal Raman microscopy to investigate application-relevant properties of perfluorinated sulfonic acid (PFSA) membranes. Raman spectra of Nafion, Aquivion, and 3M Ionomer at different equivalent weights show differences in the spectral area and position of various Raman bands but reveal a clear trend of band intensities versus equivalent weight. Characteristics of these ionomers such as swelling and water uptake were determined with through-plane imaging. The technique offers advantages over conventional measurements, such as contact-free and non-destructive data acquisition. Calibration curves for the equivalent weight were derived for the different PFSAs with excellent linear fits that allow a quantification of the ionizable group content. The high spatial resolution of confocal Raman microscopy of less than 2 μm enables the local evaluation of all these ionomer properties. Furthermore, Raman imaging was employed to characterize composite membranes, which shows that it can resolve individual phases and features in multi-layered membranes. This study provides a comprehensive summary of confocal Raman microscopy as a powerful tool for the analysis of ionomer and membrane properties.

Ion-Transport Properties of Polydimethylsiloxane-Based Ionomers with Amidinium or Imidazolinium Alkyldithiocarbamate Pendant Groups in Low Dielectric Solvents or as Neat Liquids.

The ion transport properties of ionomers comprised of polydimethylsiloxanes with amidinium or imidazolinium attached side chains and alkyldithiocarbamate anions (where alkyl is hexyl or octadecyl) have been investigated in chloroform solutions principally and as neat liquids. The influence of modifying the molecular weights of the polydimethylsiloxanes, the frequency of their amidininium or imidazolinium side groups, and temperature on the conductivity have been explored. When a solvent more polar than chloroform, syn-tetrachloroethane, was employed, a large increase in the ionic conductivity was found despite there being an increase in the viscosity of the solution. At least in these two solvents, polarity is more important in determining the conductivity than the viscosity. When normalized for ion content, Walden plots of the ionomer solutions at different ionomer concentrations approached values found for 1 M aqueous KCl. As neat liquids, the amidinium and imidazolinium hexyldithiocarbamate ionomers exceeded the values associated with the "superionic" region of the Walden plot (i.e., above the conductivity values for 1 M aqueous KCl). As ion content and polymer molecular weight increased, larger decoupling between bulk viscosity and ionic conductivity was noted, probably as a result of changes in the dynamic fragility of the ionomers.

Anion exchange membranes and ionomer properties of a polyfluorene-based polymer with alkyl spacers for water electrolysis

Aryl ether-free polymers have attracted considerable attention in recent years for their application in anion exchange membrane (AEM) fuel cells (AEMFCs) and water electrolysis (AEMWE). Here, an aryl ether-free polyfluorene-based polymer bearing pendent ammonium groups in side chains and propyl spacers in its polymer backbone is synthesized through the Suzuki cross-coupling reaction. The propyl spacer introduced into the backbone improves the flexibility of the polymer and allows effective ionic cluster formation by the polymer. Compared to polyphenylene oxide (PPO), which has a similar molecular weight, quaternized poly[9,9-bis(6-bromohexyl)fluorene]–co-[4,4-bis((4-phenyl)propyl)biphenyl)] (PFPB-QA) polymers show significantly superior membrane-forming properties owing to the propyl spacers. The PFPB-QA membrane also exhibits high ionic conductivity (122 mS cm−1 at 80 °C) and excellent alkaline stability (1 M KOH at 80 °C for 720 h). Moreover, the AEMWE utilizing PFPB-QA exhibits a current density of 1.53 A cm−2 at 2.0 V in 1 M KOH at 70 °C. Therefore, the present study shows that an aryl ether-free polymer with an alkyl spacer has excellent mechanical properties, ionic conductivity, alkaline stability, and cell performance.

Humidity-Dependent Hydration and Proton Conductivity of PFSA Ionomer Thin Films at Fuel-Cell-Relevant Temperatures: Effect of Ionomer Equivalent Weight and Side-Chain Characteristics.

Studies on the hydration properties, proton conductivity, and water content of perfluorinated ionomer thin films at temperatures relevant to fuel cell operation temperatures (around 80 °C) and the effect of ionomer chemistry are scarce. In this work, we report the water content and proton conductivity properties of thin-film ionomers (30 nm) at 80 °C over a wide range of relative humidity (0-90%) for seven different ionomers differing in the side-chain structure, including the number of protogenic groups, with the equivalent weight ranging from 620 to 1100 g/mol of sulfonic acid. The results show that the acid content or equivalent weight of the ionomer is the strongest determinant of both the swelling and the proton conductivity of ionomer films at a given relative humidity. The molar water content (λ) of ionomer films normalized to the molar protogenic group is observed to be equivalent-weight-dependent, implying that the affinity for water is acid-content-dependent. At high relative humidity conditions (>70%) pertinent to fuel cell operations, the proton conductivity of low-equivalent-weight ionomers was higher than that of higher-equivalent-weight ionomers. However, upon correlating the proton conductivity with molar water content (λ), the differences reduce dramatically, highlighting that water content is the controlling factor for proton conduction. Significantly higher values of both water content and proton conductivity are observed at 80 °C compared to those at 30 °C, implying that room temperature data are not reliable for estimating ionomer properties in the fuel cell catalyst layer.

Evolution During Accelerated Stress Test of Performance and of the Catalyst Layer's Ionomer Physical and Chemical Structures in Proton Exchange Membrane Fuel Cell

Catalyst layer’s ionomer plays a crucial role in the operation of Proton Exchange Membrane Fuel Cell (PEMFC) not simply because it binds the catalyst nanopowder but mostly because it allows protons transport and affects both gas diffusivity and water management [1]. In the catalyst layer, the ionomer is a perfluorosulfonic acid polymer similar to that use in the membrane as electrolyte. It is in the form of a few nanometer thick film dispersed around the carbon support or as aggregates larger than 150 nm within the pores of the carbon network [2,3]. In contrast with bulk ionomer membrane or model thin films that have been extensively studied [ref], little is known on the structural and functional properties of this proton conducting ionomer inside the electrode, and even less on their evolution upon aging. To date, it has proven difficult to selectively probe the fluorinated polymer dispersed at the nanoscale in a few micrometer thick electrode using conventional laboratory characterization techniques (electrochemistry, spectroscopy, and microscopy). Especially, its evolution upon aging is still an open question whereas the degradation of the ionomer due to radical attack was demonstrated and extensively investigated. Only Morawietz and co-workers evidence a thinning of the ionomer after operation [4]In this study, we studied the evolution of performance of a Membrane Electrode Assembly (MEA) after Accelerated Stress Test (AST) known to induce severe membrane degradation due to radicals, as well as the physical and chemical structures of the ionomer in the catalyst layer. Small Angle Neutron Scattering (SANS) spectra have been extensively analysed to characterise its swelling behaviour as a function of relative humidity. Its chemical structure was investigated by elemental analyses and XPS measurements, in addition to measurements of ion exchange capacity and of vapour sorption isotherms. Electrochemical characterisations including polarisation curve, impedance measurements and limiting current analyses in dry and wet conditions were performed to assess limiting phenomena and to try to unravel the contribution of the ionomer properties in the loss of performance. The SANS measurements clearly evidence an evolution of the structure and swelling behaviour of the ionomer after AST but, despite multiple characterisations, they can be hardly related to the evolution of electrochemical characteristics. Jinnouchi, R. et al. The role of oxygen-permeable ionomer for polymer electrolyte fuel cells. Commun. 12, (2021).Ueda, S., Koizumi, S., Ohira, A., Kuroda, S. & Frielinghaus, H. Grazing-incident neutron scattering to access catalyst for polymer electrolyte fuel cell. Phys B Condens Matter 551, 309–314 (2018).Morawietz, T., Handl, M., Oldani, C., Friedrich, K. A. & Hiesgen, R. Influence of Water and Temperature on Ionomer in Catalytic Layers and Membranes of Fuel Cells and Electrolyzers Evaluated by AFM. Fuel Cells 18, 239–250 (2018).Morawietz, T. et al. High-resolution analysis of ionomer loss in catalytic layers after operation. J Electrochem Soc 165, F3139–F3147 (2018). Figure 1

Understanding Performance and Durability with KOH and DI-Water Fed Anion Exchange Membrane Electrolyzers

Anion Exchange Membrane electrolyzers (AEMELs) have increased in popularity in recent years due to their potential to combine the benefits from proton exchange membrane electrolyzers and traditional alkaline electrolyzers – namely, high current density operation, pressurized H2 discharge and lowering cost by utilizing low-cost earth abundant electrocatalysts and inexpensive component materials. In the AEMEL, the oxygen evolution reaction (OER) electrode plays a critical role dictating the overall efficiency of the cell due to sluggish OER activity and species transport. In our previous study, we optimized the OER electrode structure by investigating the effects of catalyst loading, catalyst type, the porous transport substrate and additive carbon [1] – resulting in an excellent performance of 1.55 V operation at 1.0 A/cm2.Though that study led to high performance, all of those gains were made using KOH electrolyte being fed to the cell, not deionized (DI) water. The use of DI water complicates AEMEL operation as the liquid phase can no longer be relied on to carry any of the ionic charge – this possibly can reduce the electrochemically active surface area (ECSA) [2]. It has also been stated that pure water operation may have negative consequences on cell durability. Therefore, to allow for AEMELs to operate efficiently on DI water, it is important to understand how the ion transport and charge transfer resistance change as cells are transitioned from KOH to DI water operation. It is also important to understand what effects even trace amounts of KOH can have on behavior. Lastly, the effect of the ionomer properties on the OER anode performance should be well-understood.In this study, we investigate the role of alkaline feed pH on the performance and durability of AEMELs. When shifting from alkaline feed to DI water, it will be shown that there is an increase in ohmic and charge transfer resistance, resulting in significant voltage loss. Because the alkaline feed enhances electrode conductivity by connecting catalyst active sites to the conductive ionomer and AEM [2-3], new electrode structures are needed that allow for enhanced ECSA and lower resistances with DI water feeds. In this work, that was accomplished by creating a four-layer electrode structure as well as manipulating the cell operating conditions. Lastly, our team enabled the use of a high IEC ionomer in the anode catalyst layer by introducing a cross-linker and introducing smaller, cryo-milled ionomer particles. This was important to overcome adhesion issues that can come from high water uptake and swelling in high-IEC ionomers [4-5]. This paper will focus not only on performance, but also longevity, with several cells being stably operated for more than 500 hours continuously.

Evaluation of ionomer distribution on porous carbon aggregates in catalyst layers of polymer electrolyte fuel cells

Understanding ionomer distribution properties that facilitate proton conduction and oxygen transfer to Pt particles in the cathode catalyst layer (CCL) of the polymer electrolyte fuel cell (PEFC) is essential for optimized design of CCL with high cell performance. In this study, the model structure of Ketjen black (KB) as porous carbon was numerically simulated. After validating the model, the relationship between the weight ratio of ionomer/carbon (I/C) and ionomer coverage was investigated. Moreover, relative proton conductivity of simulated KB was compared with the reference data of Vulcan XC-72 (VB) as non-porous carbon. Under the same I/C ratio conditions, ionomer coverage significantly differed depending on the carbon support. Moreover, under the same carbon volume ratio conditions, simulated KB exhibited lower relative proton conductivity than VB because simulated KB had the lower ionomer volume ratio than that of simulated VB. The relationship between ionomer content and ionomer properties differ depending on the carbon support. The results of our study can contribute to designing an optimal catalyst layer. • Actual ketjen black structure is numerically simulated and validated. • Ionomer thickness, coverage, conductivity on ketjen black are estimated. • At the same ionomer content, ionomer coverage differs depending on carbon support.

Aliphatic Anion Exchange Ionomers with Long Spacers and No Ether Links by Ziegler-Natta Polymerization: Properties and Alkaline Stability.

In this work we report the synthesis of poly(vinylbenzylchloride-co-hexene) copolymer grafted with N,N-dimethylhexylammonium groups to study the effect of an aliphatic backbone without ether linkage on the ionomer properties. The copolymerization was achieved by the Ziegler–Natta method, employing the complex ZrCl4 (THF)2 as a catalyst. A certain degree of crosslinking with N,N,N′,N′-tetramethylethylenediamine (TEMED) was introduced with the aim of avoiding excessive swelling in water. The resulting anion exchange polymers were characterized by 1H-NMR, FTIR, TGA, and ion exchange capacity (IEC) measurements. The ionomers showed good alkaline stability; after 72 h of treatment in 2 M KOH at 80 °C the remaining IEC of 76% confirms that ionomers without ether bonds are less sensitive to a SN2 attack and suggests the possibility of their use as a binder in a fuel cell electrode formulation. The ionomers were also blended with polyvinyl alcohol (PVA) and crosslinked with glutaraldehyde. The water uptake of the blend membranes was around 110% at 25 °C. The ionic conductivity at 25 °C in the OH− form was 29.5 mS/cm.

Tailored catalyst microenvironments for CO2 electroreduction to multicarbon products on copper using bilayer ionomer coatings

Electrochemical carbon dioxide reduction (CO2R) provides a promising pathway for sustainable generation of fuels and chemicals. Copper (Cu) electrocatalysts catalyse CO2R to valuable multicarbon (C2+) products, but their selectivity depends on the local microenvironment near the catalyst surface. Here we systematically explore and optimize this microenvironment using bilayer cation- and anion-conducting ionomer coatings to control the local pH (via Donnan exclusion) and CO2/H2O ratio (via ionomer properties), respectively. When this tailored microenvironment is coupled with pulsed electrolysis, further enhancements in the local ratio of CO2/H2O and pH are achieved, leading to selective C2+ production, which increases by 250% (with 90% Faradaic efficiency and only 4% H2) compared with static electrolysis over bare Cu. These results underscore the importance of tailoring the catalyst microenvironment as a means of improving overall performance in electrochemical syntheses.

High O2 Permeability Ionomers for Improved Fuel Cell Performance

In a proton exchange membrane (PEM) fuel cell, the local oxygen transport across the ionomer film in the catalyst layer has a significant impact on electrode performance especially at high current density.1 Therefore, using ionomers with higher oxygen permeability than Nafion can significantly improve high current density performance. In this work, high O2 permeability ionomers (HOPIs) have been synthesized by copolymerization of perfluoro-2,2-dimethyl-1,3-dioxole (PDD) with perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride (PFSVE) and a ter-monomer. PDD’s ring structure is the main source of higher permeability, while PFSVE provides ionic conductivity. Some of the recently developed HOPIs have two to ten times higher permeability than Nafion, which should result in a significant improvement in fuel cell performance. HOPIs with different PDD content and equivalent weight were studied to establish the correlation between ionomer properties and MEA performance, and identify the best HOPI composition. Local oxygen resistance, ionomer sheet resistance, ionomer coverage, and SO3 - group coverage was evaluated and correlated to electrode performance. Improved performance of the electrodes using HOPI was correlated to the interaction of the ionomer with catalyst particles. The best performing HOPI was also significantly more durable than Nafion in the 30,000 square wave accelerated stress test with voltage cycling between 0.6-0.95 V. Ex situ ionomer characterization methods such as Fenton tests, H2O2 vapor cell tests and NMR have been used to elucidate the reason for improved durability of the PDD ionomer.This work will provide a comprehensive understanding of interactions among Pt, carbon, ionomer and theirimpact on the electrode structure and fuel cell performance and durability. The attained information will beused to improve fuel cell electrode design. Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell TechnologyOffice under the Grant DE-SC0018597.