Ionomer Structure Research Articles

Controlled Ionomer Deposition into the Cathode Catalyst Layer By Inkjet Printer for PEM Fuel Cells

Proton exchange membrane (PEM) fuel cells are being intensively investigated as alternative energy conversion systems for both residential and automotive applications. In order for this technology to become fully commercial, the reduction of cost and improvements in performance and durability of PEM fuel cells membrane electrode assemblies (MEAs) are still required. The ionomer plays several important roles in the catalyst layer (CL), as the binder, proton conductor as well as oxygen transport media. It has been shown that amount of ionomer and its morphology and interaction with catalyst particles play an important role in Pt utilization and cell performance. However, controlling the ionomer structure and morphology inside the CL with conventional method of MEA preparation has been a challenge. There are several reports describing the effect of ionomer loading in catalyst layers on the fuel cell performance, ionic conductivity, and microstructure of MEA but less attention has been paid to the role of the distribution of ionomer within the catalyst layer [1–4]. It is conjectured that incorporating a graded distribution of ionomer such that its content is higher at the CL/membrane interface and lower at the CL/carbon paper interface might be beneficial to proton migration and mass transport, respectively. That is, the region of the catalyst layer possessing the highest ionic current density would be designed to possess the higher proton conductivity, and the region associated with the highest flux of gaseous oxygen would be bestowed with the higher porosity. Some studies that examined novel methodologies for MEA preparation have specifically tried to achieve a controlled ionomer distribution inside the CL [1–4], with the aim of improving three-phase boundary and catalyst utilization. For instance, Xie et al. [1] prepared a gas diffusion electrode (GDE) that contains a graded distribution of ionomer. The performance was improved when the Nafion® content in the GDE was higher toward the CL/membrane interface and lower toward the CL/carbon paper interface. It was hypothesized that this maximizes proton transport in the GDE in the region of greatest ion flux and maximizes porosity in the region of greatest gaseous flux, respectively. Shin et al. [2] impregnated lower ionomer content into a catalyst layer and coated additional higher ionomer content on its surface in order to increase proton conductivity at the catalyst layer/membrane interface. Cheng et al. [3] also suggested that impregnating the catalyst layer with additional Nafion® ionomer improves proton conduction. A theoretical study of PEMFC cathodes [4] suggested that the catalyst layer possessing a graded distribution of ionomer in which the Nafion® content was larger toward the catalyst layer/membrane interface should exhibit a slightly higher performance in fuel cells. Recently, great interest has arisen with respect to inkjet printing technology for manufacturing CL’s [5–7]. Our recent publications on inkjet printing techniques show great potential for increasing Pt utilization by reducing amount of ionomer [6]. The high-precision of inkjet printing allows for controlled catalyst deposition, especially for low Pt loadings, as well as ionomer patterning and gradient structures inside the CL. Therefore, in this work, inkjet printing technology was used to study ionomer distribution inside the CL. An experimental strategy is devised and implemented to examine the influence of a gradient of ionomer printed by inkjet nozzles into the CL and to verify the theoretical prediction of Wang et al. [4]. The effect of a graded distribution of ionomer content on ionic conductivity, active catalyst area, and porosity as well as performance results under different simulated automotive conditions are examined.

(Invited) Ionomer Membrane and Thin-Film Diagnostics

Ionomers are used as the solid-electrolyte in polymer-electrolyte fuel-cells (PEFCs) where they provide multiple functionalities such as ion conductivity, gas separation, and water transport in a mechanically robust polymer matrix. In addition, these ionomers are found as nanometer-thick electrolyte thin films within fuel-cell catalyst layers (CLs) wherein electrochemical reactions occur.1 The performance and durability of the ionomers in PEFCs are related to their properties governed by the physiochemical interactions between chemical structure, water uptake, and morphology, all of which change with the environment. Moreover, the ionomer behaves differently from the bulk (polymer-electrolyte) when it is confined to nanometer thicknesses (as in CL),2,3 where its behavior seems to be strongly influenced by its interfaces with the air and the catalytic sites, such as carbon and platinum.1,2 Thus, it is of great interest to measure in order to understand and optimize transport functionalities and mechanical stability across the lengthscales, i.e. from bulk to thin film. In this talk, the structure/function relationship of perfluorosulfonic acid (PFSA) ionomers will be examined from micrometer to nanometer lengthscales, with a focus on the related advanced diagnostics and the information they can provide to enable correlations between an ionomer’s phase-separated nanostructure and its transport and mechanical properties. The morphology of PFSA ionomers of different equivalent weights and side-chain chemistries will be discussed based on small- and wide-angle X-ray scattering (SAXS/WAXS) experiments. We will then present an overview of the membrane's hydration, mechanical and transport properties in response to humidity, time, temperature, and thermal history. Such measurements utilize various traditional diagnostics like impedance spectroscopy and diffusion cells, as well as advanced diagnostic methods, including time-resolved SAXS, computed x-ray tomography, and conductive AFM. In addition, applicability of the diagnostics to the thin-film regime (4 to 400 nm) will be discussed, including the use of state-of-the-art in-situ measurement techniques, such as Grazing-Incidence SAXS (GISAXS). We will report results showing how an ionomer's structure and swelling properties deviate from the bulk when it is thinner than 50 nm due to confinement2,3 and how these deviations are controlled by the substrate/film interactions.2 The collected data set will be analyzed to illustrate the bulk-to-film transition of ionomers of various equivalent weights used in fuel-cell membranes and CLs.

How mobile are protons in the structure of dental glass ionomer cements?

The development of dental materials with improved properties and increased longevity can save costs and minimize discomfort for patients. Due to their good biocompatibility, glass ionomer cements are an interesting restorative option. However, these cements have limited mechanical strength to survive in the challenging oral environment. Therefore, a better understanding of the structure and hydration process of these cements can bring the necessary understanding to further developments. Neutrons and X-rays have been used to investigate the highly complex pore structure, as well as to assess the hydrogen mobility within these cements. Our findings suggest that the lower mechanical strength in glass ionomer cements results not only from the presence of pores, but also from the increased hydrogen mobility within the material. The relationship between microstructure, hydrogen mobility and strength brings insights into the material's durability, also demonstrating the need and opening the possibility for further research in these dental cements.

Structure and Dynamics of Coarse-Grained Ionomer Melts in an External Electric Field

We perform molecular dynamics simulations on a set of ionomer melts in the presence of a static, external electric field. We employ the same coarse-grained bead–spring model from our previous simulations, which characterized the zero-field morphologies and dynamics of the isolated or percolated ionic aggregates observed in these systems. Here we investigate the electric field effects on these aggregates. In the linear response regime, the morphology of both isolated and percolated aggregates is unaltered because the force between the two ions at contact is much stronger than the force on an ion due to the external field. However, the same fields are strong enough to bias the local ion dynamics so that ions in the percolated systems, which contain a continuous ionic network, transition to the steady state drift regime. Furthermore, the field biases the motion of oppositely charged ions in opposite directions and decorrelates the ionic aggregates along the field direction. In the systems with isolated ionic...

Effects of Ionomer Morphology on Oxygen Reduction on Pt

The oxygen reduction reaction (ORR) at the interface between platinum and Nafion 1100 equivalent weight was studied as a function of temperature (20–80 °C), humidity (10–100%), scan rate, the manner in which Nafion film was deposited, and the state of the Pt surface using ultramicroelectrodes employing cyclic voltammetry and chronoamperometry. ORR on smooth electrodes was strongly inhibited under specific conditions dependent on temperature, humidity, and scan rate. From the data presented, we postulate that dynamic changes in the molecular structure of the ionomer at the platinum interface result in differences in ORR voltammetry for films prepared and equilibrated under different conditions. The lack of similar changes for rough, platinized electrodes has been attributed to differences in initial ionomer structure and a higher energy barrier for ionomer restructuring. These model system studies yield insight into the ionomer-catalyst interface of particular interest for polymer electrolyte fuel cells.

Analysis of Oxygen Transport in Cathode Catalyst Layer of Low‐Pt‐Loaded Fuel Cells

AbstractOxygen transport resistance, one of the causes of large polarization in the cathode catalyst layer (CL), is intensified in low‐Pt‐loaded polymer electrolyte membrane fuel cells (PEMFCs). In order to explore operation strategies and cathode design to mitigate the large oxygen transport resistance of low‐Pt‐loaded fuel cells, the influence of operating conditions and ionomer structure on oxygen transport in the CL is investigated. Remarkably, the oxygen transport resistance data for different operation conditions and ionomer structures lie on a single curve when they are plotted as a function of the water partial pressure of the feed. At a high water partial pressure of 80 kPa, the oxygen transport resistance of the low‐Pt‐loaded CL (0.14±0.03 mg−Pt cm−2) becomes comparable to that of the high‐Pt‐loaded CL (0.40±0.04 mg−Pt cm−2) as a result of the opposing influences of Pt loading on Knudsen and ionomer film diffusion. This emphasizes the importance of the water uptake in the ionomer film for reducing oxygen transport in the CL. From a fuel cell design perspective, the operation strategy and CL design to maintain high water partial pressure in the cathode CL are extremely important for realizing low‐Pt‐loaded fuel cells.

Direct Imaging of Nanoscale Morphology of Perfluorosulfonic Acid Ionomers

Understanding the nanostructure and morphology of perfluorosulfonic acid (PFSA) ionomer and its correlation to functional properties are extremely important for designing new ionomer materials. A number of structural models of ionomer (mainly Nafion®) have been proposed based on x-ray scattering data to explain the impact of morphology on its water swelling and transport properties. One of the earliest models is cluster network model where spherical ionic clusters, connected by a narrow channel are embedded by amorphous fluorocarbon matrix.1 Alternatively, Gebel and coworkers have proposed fibrillar model, which consists of fluorocarbon crystallites surrounded by ionic groups.2 Most recently, parallel water channel model is proposed by Schmidt-Rohr et al. where large fluorocarbon crystallites consists the parallel channel that are decorated with the sulfonate group inside.3The common element to these models is a phase separated morphology – fluorocarbon phase and ionic cluster phase. However, disagreement regarding the shape, size and behaviour of the phases remain. To resolve the controversy, direct imaging technique such as electron microscope have been employed but never been successful to resolve the real nanostructure and morphology. In our approach, we have employed transmittance electron microscope (TEM) to directly image the well-defined crystalline phase (fluorocarbon matrix) and amorphous phase (ionic cluster).The ionomer film was prepared on the spongy carbon film supported by Cu grid adopting self-assembly method4,5. Both free-standing and carbon-supported films ranging in thickness of around 4 to 50 nm were generated and investigated by the instrument Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA) at 200kV acceleration voltage. The image was captured by Gatan UltraScan 4000 CCD (Gatan, Pleasanton, California, USA) at 2048x2048 pixels.Although a number of research papers have discussed the crystalline phase in ionomer structure but found it challenging to image because of the high degree of disorderdness of the ionomer materials. Therefore, researchers mostly relied on SAXS/SANS data to depict the structural morphology. Hence, it is very exciting to present images where the crystalline phase is clearly evident by direct TEM imaging of Nafion® ionomer Nanofilm without any chemical modification and treatment (Figure 1). The highly ordered crystalline phase consists of a number of lamellae that are surrounded by the amorphous phase. The crystalline phase varies from 4 to 6 nm in size. The d spacing of those lamellae was calculated to be 0.37 nm. This particular structure of ionomer is consistent with the Fibrillar model proposed by Gebel et al.2where the majority of hydrophobic, Teflon-like phase stay in the high density crystalline region and amorphous phase consist of ionic domains (Figure 2). These results may help resolve the debate around ionomer structure model.It is also interesting to see that the crystalline phase is evident regardless of equivalent weight of ionomers whereas the size, shape and extent of the crystalline phase depends on the equivalent weight. We plan to discuss the thermal annealing and dispersion effect on the morphological change of the ionomer in the nanofilms. The morphology of supported film comparing with freestanding nanofilm also will be discussed. Acknowledgements Financial Early Researcher Awards (Ontario Ministry of Research and Innovation) and Natural Sciences and Engineering Research Council of Canada (NSERC), MITACS Elevate PDF Fellowship (Devproshad K. Paul), Automotive Fuel Cell Cooperation (AFCC).

Invited: Study of Fundamental Aspects of Ionomer Structure in Different States

In PEFC system, the ionomer is used as a binder and an electrolyte membrane. The characteristics of low gas permeability and high proton conductivity are required for the electrolyte membrane, while, if the ionomer is used as a binder, conductivity should be high as well as electrolyte membrane, but in this case, high gas permeability should be better which is opposed to the case of membrane characteristic. Nafion® is used for binder and electrolyte membrane, but there still has been unclear factor about structural difference in both binder and membrane states, especially we do not know about the effect of structure and physical properties of ionomer in a catalyst layer on cell performance. In this study, we prepared model ionomer materials and electrodes, and conducted the structural analysis in order to understand the effect on the cell performance systematically. Perfluorinated sulfonic acid ionomer Nafion® and di-block copoly(p-phenylene) ionomer having sequentially controlled hydrophilic and hydrophobic parts were targeted as the model ionomer1 (Scheme 1). As the model electrode, we also used Pt-dispersed carbon electrodes in which several nanometers Pt particles dispersed2. We performed microscopic observation and scattering analysis in order to understand the structures in solution and thin film solid states. Here Nafion® thin film was prepared on platinum nano-particle dispersed carbone electrode as a model for the ionomer film within a catalyst layer. We found that aggregated structure of ionomer changed with carbon surface area, the size of aggregated structure became large, and proton conductive domain also enlarged with the increase in carbon surface area. These results imply that the morphology and the property of ionomer covering catalyst support could be dominated by balance of interaction between carbon and platinum. In the case of hydrocarbon ionomer, it has been found that di-block copolymer forms higher order structure over a range of several hundred nm by means of AFM observation. Furthermore, the gas permeability of di-block copolymer film was comparable to that of Nafion®. Details will be discussed at presentation. Acknowledgement Ministry of Economy, Trade and Industry (METI), Japan and New Energy and Industrial technology Development Organization (NEDO) are acknowledged for granting the work.

Kinetic Modeling of Chemical Degradation Phenomena in Fuel Cell Ionomers

A kinetic model has been developed to describe the chemical degradation of Nafion-type polymer electrolyte membranes. The model accounts for pathways of radical formation and consumption along with mechanisms of ionomer degradation through radical attack. Systematic simplifications on the set of model equations lead to analytical expressions for concentration of aggressive hydroxyl radical as a function of initial iron content and hydrogen peroxide concentration. The ionomer degradation model employs a coarse-grained structure of Nafion ionomer. . It distinguishes units corresponding to head groups and trunk segments of sidechains. The backbone segment between two sidechains is the third unit considered in the degradation model. A set of differential equations is formulated to describe the consumption of these units and determine the associated rate of fluorine loss and the change in the ion exchange capacity (IEC). A parametric study was performed to study the impact of different degradation mechanisms, degradation conditions, and chemistries of the PFSA on fluorine loss and IEC change. The rate constants of degradation processes are found through comparison with the experimental degradation data. Results will be discussed in view a structural stabilization of PEMs and the mitigation of chemical degradation.

The Effect of Carbon Support Structure on Ionomer Distributions in PEM Fuel Cell Catalyst Layers

Ionomer thin films form the proton-conducting path within the catalyst layers (CL) of polymer electrolyte membrane fuel cells (PEMFCs) and the web-like ionomer network provides support for the porous CL structure. The recast ionomer is assumed to be relatively uniform in thickness, structure, and chemistry across the entire CL thickness, but there are numerous factors that can significantly influence the homogeneity and dispersion of the ionomer as well as its stability during electrochemical aging. These factors include (1) the type of carbon support material used, including the extent of graphitization (or lack thereof) and the presence of surface oxide functional groups, (2) the loading, dispersion, and size/shape of Pt-based electrocatalysts, and (3) the CL pore structure. Recently, extensive studies have been conducted to identify the optimum conditions for examining ionomer thin films using electron microscopy and spectroscopy techniques1 and molecular dynamics (MD) simulations have been performed to identify the structure and adhesion of ionomer thin films on carbon and Pt surfaces.2 The current work will expand on this microscopy research to apply specific microscopy methods, including high-resolution imaging and spectroscopy coupled with electron tomography, to identify the nature of ionomer film dispersions within CLs prepared with different carbon supports having varying degrees of graphitization (high surface area carbon, Vulcan, and fully graphitized low surface area carbon) and Pt loadings. Additionally, changes to these ionomer films within the different CLs during accelerated stress testing (AST), specifically the AST for carbon corrosion (1.2V hold for 100-400 hours), will be analyzed and compared in terms of changes to the ionomer distributions on the changing carbon surfaces. Microscopy observations will be coupled with MD simulations to better understand the correlations between ionomer dispersion and structure and the materials comprising the CLs.

Mechanism of Hydroxyl Radical Attack to Chemical Structure of PFSA Ionomer: A Solid State Q-NMR Study

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Experimental Study of the Activity Change Due to Operation History in PEMFC

We have developed a new method of precisely determining the activity of cathode catalyst layer in MEA by maintaining the effect of platinum oxides at a constant level and have subsequently studied activity changes resulting from various operational histories. Cyclic voltammetry was used to quantitatively correlate activity changes in the cathode catalyst layer with various operational conditions and demonstrated that activity decreased after low humidity operation and recovered after high humidity operation in a N2 atmosphere at potentials below 0.2 V. The activity changes were found to be very slow and a span of more than 8 hours was required for the activity to reach the steady state. This phenomenon is presumably caused by the adsorption/desorption of ionomer sulfonate groups on the Pt surface, based on observations that the activity changes are consistent with sulfonate group adsorption and dependent on the ionomer structure.

An Experimental Study of the Effects of Operational History on Activity Changes in a PEMFC

The accurate evaluation of the activation overpotential of a membrane electrode assembly (MEA) is essential for fuel cell design. We have developed a new method of precisely determining the activity of cathode catalyst layer in MEA by maintaining the effect of platinum oxides at a constant level and have subsequently studied activity changes resulting from various operational histories. Cyclic voltammetry was used to quantitatively correlate activity changes in the cathode catalyst layer with various operational conditions and demonstrated that activity decreased after low humidity operation and recovered after high humidity operation in a N2 atmosphere at potentials below 0.2 V. The activity changes were found to be very slow and a span of more than 8 hours was required for the activity to reach the steady state. This phenomenon is presumably caused by the adsorption/desorption of ionomer sulfonate groups on the Pt surface, based on observations that the activity changes are consistent with sulfonate group adsorption and dependent on the ionomer structure. Oxygen transport resistance in the catalyst layer also varied in conjunction with the activity changes. Two possible Pt/ionomer interface models are proposed in an attempt to explain the above observations.

Differences between polymer/salt and single ion conductor solid polymer electrolytes

PEO/salts and PEO-based ionomers have been previously investigated for battery applications. Although both contain a PEO backbone, reports so far show different polymer dynamics, cation solvation, and ion conduction mechanisms. The different properties may be attributed to their lattice energy, ion content, molecular weight, and molecular structure. As the ion identities from which this information has been obtained differ, we have used molecular dynamics simulation to study a PEO/salt system that has identical cations and anions as a PEO-based ionomer previously studied in our group. This enables us to isolate the effect of covalent bonding of the anion to the backbone. Our study shows that the ionomer structure reduces PEO flexibility, PEO mobility, and cation solvation. In the PEO/salt, PEO segments crosslinking via ion aggregation does not occur because the anions are not incorporated in the polymer. The absence of ionic crosslinking results in PEO mobility evenly distributed along the polymer backbone in contrast to the gradient mobility exhibited in the PEO-based ionomer. Due to the more mobile PEO chains in the PEO/salt, ion transport is controlled by the dynamics of the polymer matrix, whereas ion hopping is more important within more rigid ionomer systems. To improve cation mobility in the PEO/salt, the focus should be on polymer dynamics. For the PEO-based ionomer, additional attention should be focused on attaining cation states that promote cation hopping.

Relating Ionomer and Electrode Structure to H2-Air Fuel Cell Performance and Durability

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Hydration structure of trifluoromethanesulfonate studied by quantum chemical calculations

Our recent study based on infrared spectroscopy reported hydration structure of Nafion side chain terminal (S. Morita, K. Kitagawa, Temperature-dependent structure changes in Nafion ionomer studied by PCMW2D IR correlation spectroscopy, J. Mol. Struct. 974 (2010) 56–59). In the present study, detailed hydration structure of Nafion ionomer was explored by means of quantum chemical calculations (QCCs) using a model side chain terminal of trifluoromethanesulfonate (TFMS). The result indicates that the hydration of TFMS induces deprotonation of sulfonic acid group in TMFS and formation of hydronium anion with hydrating water molecules. Hydration energies of TFMS deprotonated anion with H3O+, H5O2+ and H7O3+ are calculated to be 599.6, 474.1 and 464.6kJmol−1, respectively.

Investigation of ionomer structure through its dependence on ion exchange capacity (IEC)

The ion exchange capacity (IEC) of conventional ion-exchange perfluorinated membranes based on the so-called Long-Side-Chain (LSC) polymers and their Short-Side-Chain (SSC) Aquivion ® derivatives was determined using a series of experimental techniques. Newly developed and already used experimental analyses were compared in terms of sensitivity to the determination of the ion exchange capacity. The use of complementary techniques allows better determination of IEC with an uncertainty of about 3%. This IEC determination method was then applied to membranes with large chemical differences such as copolymers and reinforced membranes. In addition, based on these analyses, relationship between DMA, WAXS, TGA and conductivity parameters and either IEC or the molar number of tetrafluoroethylene (TFE) per comonomer unit are underlined, leading to a more comprehensive approach of architecture in perfluorosulfonic acid (PFSA) polymer membranes.

Understanding the Ionomer Structure and the Proton Conduction Mechanism in PEFC Catalyst Layer: Adsorbed Nafion on Model Substrate

To understand the structure/property of the ionomer in the CL, adsorbed Nafion thin films were prepared on model substrate, SiO2 terminated silicon wafer, by spontaneous adsorption of Nafion from its 0.1 to 5 wt% solution. Fitting of Variable Angle Spectroscopic Ellipsometry (VASE) data yielded thickness of adsorbed films ranging from 4 nm at low concentrations (0.1 wt% film) to 306 nm at high concentrations (5wt% film). Contact angle measurements indicated that the low (0.1 wt%) and high (3 wt%) films to posses hydrophobic surface but the intermediate concentration (0.5 wt% and 1.0 wt%) film surface to be hydrophilic. Conductivity measurements of adsorbed films over a range of RH showed that all films prepared from 1 wt% and lower concentrations solutions to possess similar conductivities at high RH. However, the 5wt% film showed much higher conductivity indicating a different proton conduction mechanism for low and high concentration films.

Effect that the chemical structure of ionomer has on the morphology of water channels of ion-exchange membranes: A mesoscopic simulation

The effect that the features of the chemical structure of oligomer chains of an ionomer has on the morphology of water channels formed in ion exchange membranes based on sulfonated polyether ether ketones (SPEEK) has been studied by the dynamic density functional theory. The characteristics of mem� branes based on oligomeric SPEEK chains with the regular, partially distorted, and completely random dis� tribution of hydrophobic and hydrophilic monomers have been compared. The results indicate that ionomer membranes based on regular multiblock AB copolymers can perform better than membranes based on multi� block copolymers with a random chain structures.

Understanding the Ionomer Structure and the Proton Conduction Mechanism in PEFC Catalyst Layer: Adsorbed Nafion on Model Substrate

not Available.