Proton-Conductivity Enhancement in Polymer Thin Films

Design of Functional Gyroid Minimal Surfaces Transporting Proton Based Solely on Surface Hopping Conduction Mechanism

Surface proton hopping conduction (SPHC) mechanisms is an important proton conduction mechanism in conventional polymer electrolytes, along with the Grotthuss and vehicle mechanisms. Due to the small diffusion coefficient of protons in the SPHC mechanism, few studies have focused on the SPHC mechanism. Recently, it has been found that a dense alignment of SO3 - groups significantly lowers the activation energy in the SPHC mechanism, enabling fast proton conduction. In this study, a series of polymerizable amphiphilic-zwitterions is prepared, forming bicontinuous cubic liquid-crystalline assemblies with gyroid symmetry in the presence of suitable amounts of bis(trifluoromethanesulfonyl) imide (HTf2N) and water. In situ polymerization of these compounds yields gyroid-nanostructured polymer films, as confirmed by synchrotron small-angle X-ray scattering experiments. The high proton conductivity of the films on the order of 10-2 S cm-1 at 40 °C and relative humidity of 90% is based solely on the SPHC mechanism.

Unravel-engineer-design: a three-pronged approach to advance ionomer performance at interfaces in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs), which use hydrogen as fuel, present an eco-friendly alternative to internal combustion engines (ICEs) for powering low-to-heavy-duty vehicles and various devices. Despite their promise, PEMFCs must meet strict cost, performance, and durability standards to reach their full potential. A key challenge lies in optimizing the electrode, where a thin ionomer layer is responsible for proton conduction and binding catalyst particles to the electrode. Enhancing ion transport within these sub-μm thick films is critical to improving the oxygen reduction reaction (ORR) at the cathodes of PEMFCs. For the past 15 years, our research has targeted this limitation through a comprehensive "Unravel - Engineer - Design" approach. We first unraveled the behavior of ionomers, gaining deeper insights into both the average and distributed proton conduction properties within sub-μm thick films and at interfaces that mimic catalyst binder layers. Next, we engineered ionomer-substrate interfaces to gain control over interfacial makeup and boost proton conductivity, essential for PEMFC efficiency. Finally, we designed novel nature-derived or nature-inspired, fluorine-free ionomers to tackle the ion transport limitations seen in state-of-the-art ionomers under thin-film confinement. Some of these ionomers even pave the way to address cost and sustainability challenges in PEMFC materials. This feature article highlights our contributions and their importance in advancing PEMFCs and other sustainable energy conversion and storage technologies.

Order-Order Transition in Statistical Copolymer Thin Film Induced by LCST-Type Behavior

In this paper, we describe the formation of an ordered structure in a copolymer thin film through hydration, which subsequently transitions to a different ordered structure upon dehydration. A statistical copolymer of poly(N-octadecyl acrylamide-stat-hydroxymethyl acrylamide) with a comonomer content ratio of 1:1, denoted as p(ODA50/HEAm50), was synthesized via free radical copolymerization. We prepared a thin film of this copolymer on a solid substrate and annealed it at 60 °C under humid conditions. This treatment formed a side-chain segregated lamellar (SCSegL) structure, in which the ODA and HEAm units are oriented perpendicularly to the polymer backbone and opposite each other. Increasing the annealing temperature to 90 °C led to a transition to a side-chain mixed lamellar (SCMixL) structure, where the ODA and HEAm units are also oriented perpendicularly to the polymer backbone but in both directions. The quartz crystal microbalance (QCM) data indicate that p(ODA50/HEAm50) exhibits LCST-like behavior with a transition temperature of approximately 50 °C. We conclude that the formation of the SCSegL structure at 60 °C is due to pronounced segregation between the water-adsorbed HEAm groups and the hydrophobic ODA. Conversely, dehydration at 90 °C reduces the segregation forces, forming the SCMixL structure, which exhibits lower strain. These results demonstrate that the p(ODA50/HEAm50) film undergoes an order-to-order transition driven by the hydration-dehydration process. Additionally, we found that changes in the lamellar structure significantly alter the swelling properties of the film.

Surface proton hopping conduction mechanism dominant polymer electrolytes created by self-assembly of bicontinuous cubic liquid crystals

For the development of the next generation of fuel cells, it is essential to create an innovative design principle of polymer electrolytes that is beyond extension of the existing strategy. In the present study, we focused on the surface hopping proton conduction mechanism where an activation energy for proton conduction is greatly reduced by decreasing the distance between SO3- groups. Our gyroid nanostructured polymer film (Film-G), with a hydrophilic surface where the SO3- groups are aligned densely and precisely, shows high proton conductivity of the order of 10-2 S cm-1 when the water content is about 15 wt%. We reveal that the high proton conductivity of Film-G is attributed to the exhibition of an extremely-fast surface hopping conduction mechanism due to the reduced activation energy barrier along the gyroid minimal surface. This finding should introduce a game-changing novel opportunity in polymer electrolyte design.

Electrochemical Detection of Selective Anion Transport through Subnanopores in Liquid-Crystalline Water Treatment Membranes

The anion-selective transport through subnanoporous liquid-crystalline (LC) water treatment membranes was quantitatively detected by the deposition and electrochemical analysis of the LC membrane on the GaN electrode. The time course of the capacitance and Warburg resistance of the LC membrane suggest that the interaction of the LC membrane with monovalent Cl- ions is distinctly different from that with SO42- ions. A continuous decay in capacitance suggests the condensation of Cl- ions in subnanopores, whereas the interaction between SO42- ions and the inner wall of subnanopores is much weaker. The chronoamperometry data further suggest that SO42- ions are transported through subnanoporous channels 10 times faster than Cl- ions. These results, together with the previous X-ray emission spectroscopy, suggest that SO42- ions, which possess similar hydrogen-bonded structures to the hydrogen-bonded networks inside the subnanopores, can exchange the associated water molecules and hop along the network of water molecules, but Cl- ions bind and accumulate inside subnanopores. The well-controlled supramolecular self-assembly of LC building blocks opens a large potential toward the fine adjustment of hydrogen-bonding networks in nanospace providing materials new functions, which cannot be realized by bulk water.

Structure and conductivity of ionomer in PEM fuel cell catalyst layers: a model-based analysis

Efforts in design and optimization of catalyst layers for polymer electrolyte fuel cells hinge on mathematical models that link electrode composition and microstructure with effective physico-chemical properties. A pivotal property of these layers and the focus of this work is the proton conductivity, which is largely determined by the morphology of the ionomer. However, available relations between catalyst layer composition and proton conductivity are often adopted from general theories for random heterogeneous media and ignore specific features of the microstructure, e.g., agglomerates, film-like structures, or the hierarchical porous network. To establish a comprehensive understanding of the peculiar structure-property relations, we generated synthetic volumetric images of the catalyst layer microstructure. In a mesoscopic volume element, we modeled the electrolyte phase and calculated the proton conductivity using numerical tools. Varying the ionomer morphology in terms of ionomer film coverage and thickness revealed two limiting cases: the ionomer can either form a thin film with high coverage on the catalyst agglomerates; or the ionomer exists as voluminous chunks that connect across the inter-agglomerate space. Both cases were modeled analytically, adapting relations from percolation theory. Based on the simulated data, a novel relation is proposed, which links the catalyst layer microstructure to the proton conductivity over a wide range of morphologies. The presented analytical approach is a versatile tool for the interpretation of experimental trends and it provides valuable guidance for catalyst layer design. The proposed model was used to analyze the formation of the catalyst layer microstructure during the ink stage. A parameter study of the initial ionomer film thickness and the ionomer dispersion parameter revealed that the ionomer morphology should be tweaked towards well-defined films with high coverage of catalyst agglomerates. These implications match current efforts in the experimental literature and they may thus provide direction in electrode materials research for polymer electrolyte fuel cells.

Lyotropic Liquid Crystalline Property and Organized Structure in High Proton-Conductive Sulfonated Semialicyclic Oligoimide Thin Films

Fully aromatic sulfonated polyimides with a rigid backbone can form lamellar structures under humidified conditions, thereby facilitating the transmission of protons in ionomers. Herein, we synthesized a new sulfonated semialicyclic oligoimide composed of 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA) and 3,3'-bis-(sulfopropoxy)-4,4'-diaminobiphenyl to investigate the influence of molecular organized structure and proton conductivity with lower molecular weight. The weight-average molecular weight (M _w) determined by gel permeation chromatography was 9300. Humidity-controlled grazing incidence X-ray scattering revealed that one scattering was observed in the out-of-plane direction and showed that the scattering position shifted to a lower angle as the humidity increased. A loosely packed lamellar structure was formed by lyotropic liquid crystalline properties. Although the ch-pack aggregation of the present oligomer was reduced by substitution to the semialicyclic CPDA from the aromatic backbone, the formation of a distinct organized structure in the oligomeric form was observed because of the linear conformational backbone. This report is the first-time observation of the lamellar structure in such a low-molecular-weight oligoimide thin film. The thin film exhibited a high conductivity of 0.2 (±0.01) S cm^-1 under 298 K and 95% relative humidity, which is the highest value compared to the other reported sulfonated polyimide thin films with comparable molecular weight.

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.

Molecular-Level Control over Ionic Conduction and Ionic Current Direction by Designing Macrocycle-Based Ionomers

Poor ionic conductivity of the catalyst-binding, sub-micrometer-thick ionomer layers in energy conversion and storage devices is a huge challenge. However, ionomers are rarely designed keeping in mind the specific issues associated with nanoconfinement. Here, we designed nature-inspired ionomers (calix-2) having hollow, macrocyclic, calix[4]arene-based repeat units with precise, sub-nanometer diameter. In ≤100 nm-thick films, the in-plane proton conductivity of calix-2 was up to 8 times higher than the current benchmark ionomer Nafion at 85% relative humidity (RH), while it was 1-2 orders of magnitude higher than Nafion at 20-25% RH. Confocal laser scanning microscopy and other synthetic techniques allowed us to demonstrate the role of macrocyclic cavities in boosting the proton conductivity. The systematic self-assembly of calix-2 chains into ellipsoids in thin films was evidenced from atomic force microscopy and grazing incidence small-angle X-ray scattering measurements. Moreover, the likelihood of alignment and stacking of macrocyclic units, the presence of one-dimensional water wires across this macrocycle stacks, and thus the formation of long-range proton conduction pathways were suggested by atomistic simulations. We not only did see an unprecedented improvement in thin-film proton conductivity but also saw an improvement in proton conductivity of bulk membranes when calix-2 was added to the Nafion matrices. Nafion-calix-2 composite membranes also took advantage of the asymmetric charge distribution across calix[4]arene repeat units collectively and exhibited voltage-gating behavior. The inclusion of molecular macrocyclic cavities into the ionomer chemical structure can thus emerge as a promising design concept for highly efficient ion-conducting and ion-permselective materials for sustainable energy applications.

Accumulation of Sulfonic Acid Groups Anchored in Covalent Organic Frameworks as an Intrinsic Proton-Conducting Electrolyte

Covalent organic frameworks (COFs) are a novel class of crystalline porous polymers, which possess high porosity, excellent stability, and regular nanochannels. 2D COFs provide a 1D nanochannel to form the proton transport channels. The abovementioned features afford a powerful potential platform for designing materials as proton transportation carriers. Herein, the authors incorporate sulfonic acid groups on the pore walls as proton sources for enhancing proton transport conductivity in the 1D channel. Interestingly, the sulfonic acid COFs (S-COFs) electrolytes being binder free exhibit excellent proton conductivity of ≈1.5 × 10^-2 S cm^-1 at 25 ℃ and 95% relative humidity (RH), which rank the excellent performance in standard proton-conducting electrolytes. The S-COFs electrolytes keep the high proton conduction over the 24 h. The activation energy is estimated to be as low as 0.17 eV, which is much lower than most reported COFs. This research opens a new window to evolve great potential of structural design for COFs as the high proton-conducting electrolytes.

OH- Conductive Properties and Water Uptake of Anion Exchange Thin Films

Several investigations have indicated that proton conduction and hydration properties of acidic ionomers differ from those of membranes. However, relations between the OH^- conductivity and water uptake in thin film forms of anion exchange membranes have not been reported yet. For this study, new in situ measurements were established to elucidate the OH^- conductivity and water uptake without allowing any influence of CO₂ from the air. Poly[(9,9-bis(6'-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt-(1,4-benzene)], denoted as PFB⁺ , was synthesized as a model ionomer. The highest OH^- conductivity of 273 nm-thick PFB⁺ film was 5.3×10^-2  S cm^-1 at 25 °C under 95 % relative humidity (RH), which is comparable to the reported OH^- conductivity of PFB⁺ membrane. Reduced OH^- conductivity was found in the thinner film at 95 % RH. The decreased OH^- conductivity is explainable by the reduced number of water molecules contained in the thinner film. The OH^- conductivity was reduced only slightly under the same water uptake.

Gemini Thermotropic Smectic Liquid Crystals for Two-Dimensional Nanostructured Water-Treatment Membranes

We have developed a two-dimensional (2D) liquid-crystalline (LC) nanostructured water-treatment membrane showing high virus rejection ability (over 99.99997% for bacteriophage Qβ) and improved water permeation. Polymerizable gemini amphiphiles have been designed and synthesized. They have H-shaped gemini-type structures of thermotropic smectic liquid crystals composed of cationic imidazolium moieties. One of the gemini amphiphiles shows a smectic A phase with an interdigitated bilayer structure. A cross-linked self-standing 2D nanostructured polymer film has been obtained by in situ photopolymerization of the gemini amphiphile in the smectic phase. The length of linkers in gemini amphiphiles affects the formation of LC phases. The 2D nanostructured membrane also showed selective salt rejection.

Intrinsic proton conduction in 2D sulfonated covalent organic frameworks through a post-synthetic strategy

A 2D sulfonated COF showed intrinsic proton conductivity up to 10−3 at 25 °C and 100% relative humidity and high conductivity up to 10−2 S cm−1 at 70 °C and 100% RH.

Functional Ionic Liquid Crystals

Ionic liquid crystals have emerged as a new class of functional soft materials in the last two decades, and they exhibit synergistic characteristics of ionic liquids and liquid crystals such as macroscopic orientability, miscibility with various species, phase stability, nanostructural tunability, and polar nanochannel formation. Owing to these characteristics, the structures, properties, and functions of ionic liquid crystals have been a hot topic in materials chemistry, finding various applications including host frameworks for guest binding, separation membranes, ion-/proton-conducting membranes, reaction media, and optoelectronic materials. Although several excellent review articles of ionic liquid crystals have been published recently, they mainly focused on the fundamental aspects, structures, and specific properties of ionic liquid crystals, while these applications of ionic liquid crystals have not yet been discussed at one time. The aim of this feature article is to provide an overview of the applications of ionic liquid crystals in a comprehensive manner.

Simultaneous Analysis of Molecular Orientation and Quantity Change of Constituents in a Thin Film Using pMAIRS

Spectral analysis using chemometrics is extensively used for quantitative chemical analysis in a mixture, but it works powerfully only when the peak intensity is solely proportional to the quantity of chemical components. In this sense, thin films on a solid substrate are not suitable for chemometric analysis, because the molecular orientation also influences the peak intensity via the surface selection rules. In the present study, this long-term analytical issue has readily been overcome by using p-polarized multiple-angle incidence resolution spectrometry (pMAIRS), which has a characteristic that the in-plane (IP) and out-of-plane (OP) vibrational spectra of a thin-film sample are obtained simultaneously in a common ordinate scale. Thanks to this unique power of pMAIRS, the average of the IP and OP spectra annihilates optical anisotropy, yielding an orientation-free spectrum, which enables us to perform the simultaneous quantitative analysis of both quantity change and molecular orientation of the constituents in a thin film. Now, we are ready to examine chemical reactions quantitatively in a thin film.

Progress on highly proton-conductive polymer thin films with organized structure and molecularly oriented structure

Several current topics are introduced in this review, with particular attention to highly proton-conductive polymer thin films with organized structure and molecularly oriented structure. Organized structure and molecularly oriented structure are anticipated as more promising approaches than conventional less-molecular-ordered structure to elucidate mechanisms of high proton conduction and control proton conduction. This review introduces related polymer materials and molecular design using lyotropic liquid crystals and hydrogen bond networks for high proton conduction. It also outlines the use of substrate surfaces and external fields, such as pressure and centrifugal force, for organizing structures and molecularly oriented structures.

Synthesis of self-assembled nucleobases and their anhydrous proton conductivity

We synthesized self-assembled nucleobases (SANs), such as 1-dodecylthymine (DOT) or 9-dodecyladenine (DOA), in which the nucleobase is immobilized on a long alkyl chain. The thermal stability of the SAN was increased by mixing with the acidic surfactant mono-dodecyl phosphate (MDP). Additionally, the SAN-MDP composite material showed proton conductivity of 4.62 × 10-4 S cm-1 at 160 °C under anhydrous conditions. Additionally, the activation energy of the proton conduction was approximately 0.2 eV and this value was one order of magnitude higher than that of a typical humidified perfluorinated membrane, in which the proton can be moved by vehicle molecules, such as water molecules. In contrast, when the nucleobase without the immobilization of a long alkyl chain was mixed with MDP, the proton conductivity of these composite materials was two orders of magnitude less than that of the SAN-MDP composite. Therefore, we measured the XRD spectra of the SAN-MDP composite material. As a result, the SAN-MDP composite material showed a self-assembled structure with a two-dimensional proton conducting pathway, such as a lamellar structure, and that the anhydrous proton conduction was related to the interaction between the nucleobase of the SAN and the phosphate group of MDP. Consequently, the self-assembled nucleobase derivatives have the potential for use as novel anhydrous proton conductors with a two-dimensional proton conducting pathway.

Interesting Facets of Surface, Interfacial, and Bulk Characteristics of Perfluorinated Ionomer Films

Ion-containing perfluorinated polymers possess unique viscoelastic properties, excellent proton conductivity, and nanophase-segregated structure all arising from the clustering of hydrophilic sulfonic acid groups within a matrix of hydrophobic fluorocarbons. When these ionomers are confined to nanothin films, a broad swathe of structural organization imparting a rich variety of surface, interfacial, and bulk characteristics can be expected. However, our understanding of perfluorinated ionomer thin film behavior is still in a rudimentary stage, and much of the research focus to date has been on its hydration-related structure and properties pertinent to electrochemical applications. Thus, many hidden gems-their interesting surface and interfacial properties-have been overlooked. In this Invited Feature Article, which is a summary of the key contributions by the author's group, including several collaborative publications on ionomer thin films, we unravel many of these facets. In addition, the article attempts to integrate knowledge acquired from a variety of investigations of different aspects of the ionomer thin films to refine and develop a consistent picture of their structure and behavior. First, we focus on the self-assembly of ionomers and show that dispersion media and hydrophobicity/hydrophilicity of the substrate can result in partial or even no coverage of substrates, shedding light on the complexity of polymer-substrate, polymer-solvent, and polymer-polymer interactions, an insight completely obscured when the spin-coating method is adopted for film creation. We demonstrate that the same ionomer can be used to create a variety of surfaces ranging from superhydrophilic to highly hydrophobic by controlling the film thickness or through the choice of substrate material. The ultrathin, hydrophilic surfaces of self-assembled Nafion ionomer films exhibit wettability switching behavior which opens the door to creating stimuli-responsive smart surfaces. The thermoresponsive behavior of the films is discussed in the context of surface (wettability) and bulk (thermal expansion) characteristics as well as a newly discovered vibrational mode. The substrate- and film thickness-dependent thermal expansion coefficients reinforce the importance of interfacial interactions and confinement on the structure/properties of these films. They also open up the potential of tuning ionomer bulk properties via substrate chemistry. The discovery of a vibrational mode that becomes thermally activated at high temperature has provided new insights into the origins of the molecular motions responsible for the α-relaxation of the Nafion ionomer as well as the underlying reason for wettability switching. Our recent neutron reflectometry study of different ionomers varying in side-chain composition/length on a platinum substrate shows that the interfacial hydration level is correlated to the side-chain length, which opens up the possibility of the controlling the interfacial electrochemistry. Finally, a systematic analysis of factors affecting proton conduction is presented to elucidate the yet-unresolved origins of the suppressed conduction of nanothin ionomer films compared to that of the bulk membrane. By revealing these interesting yet poorly understood facets of ionomer thin films, the article aims to stimulate further scientific pursuit on this topic.

Gyroid structured aqua-sheets with sub-nanometer thickness enabling 3D fast proton relay conduction

A polymerizable amphiphile having two zwitterionic head-groups has been designed. This compound co-organizes with an acid, bis(trifluoromethanesulfonyl)imide (HTf2N), into a gyroid bicontinuous cubic liquid-crystalline phase. In situ polymerization of this phase has been successfully achieved by UV irradiation in the presence of a photoinitiator, yielding a self-standing gyroid-nanostructured polymer film. When the polymer film is placed under different relative humidity conditions or in water, it absorbs water owing to the strong hydration ability of the zwitterionic parts. It has been found that the polymer film preserves the gyroid nanostructure after the water absorption. Based on reconstructed electron density maps, it is assumed that the absorbed water molecules form a 3D continuous network along the gyroid minimal surface, which satisfies several key conditions for inducing fast proton conduction. As expected, such hydrated films show high ionic conductivities in the order of 10-1 S cm-1 when the water content of the film reaches 15.6 wt% at RH = 90%. The high conductivity is attributed to the induction of the Grotthuss mechanism, that is, proton conduction via the hydrogen-bonding network of the incorporated water molecules.

Biomimetic Polyelectrolytes Based on Polymer Nanosheet Films and Their Proton Conduction Mechanism

We report a biomimetic polyelectrolyte based on amphiphilic polymer nanosheet multilayer films. Copolymers of poly( N-dodecylacrylamide- co-vinylphosphonic acid) [p(DDA/VPA)] form a uniform monolayer at the air-water interface. By depositing such monolayers onto solid substrates using the Langmuir-Blodgett (LB) method, multilayer lamellae films with a structure similar to a bilayer membrane were fabricated. The proton conductivity at the hydrophilic interlayer of the lamellar multilayer films was studied by impedance spectroscopy under temperature- and humidity-controlled conditions. At 60 °C and 98% relative humidity (RH), the conductivity increased with increasing mole fraction of VPA ( n) up to 3.2 × 10^-2 S cm^-1 for n = 0.41. For a film with n = 0.45, the conductivity decreased to 2.2 × 10^-2 S cm^-1 despite the increase of proton sources. The reason for this decrease was evaluated by studying the effect of the distance between the VPAs ( l_VPA) on the proton conductivity as well as their activation energy. We propose that for n = 0.41, l_VPA is the optimal distance not only to form an efficient two-dimensional (2D) hydrogen bonding network but also to reorient water and VPA. For n = 0.45, on the other hand, the l_VPA was too close for a reorientation. Therefore, we concluded that there should be an optimal distance to obtain high proton conductivity at the hydrophilic interlayer of such multilayer films.

Thermal Transitions in Perfluorosulfonated Ionomer Thin-Films

Thin perfluorosulfonated ion-conducting polymers (PFSI ionomers) in energy-conversion devices have limitations in functionality attributed to confinement-driven and surface-dependent interactions. This study highlights the effects of confinement and interface-dependent interactions of PFSI thin-films by exploring thin-film thermal transition temperature (T_T). Change in T_T in polymers is an indicator for chain relaxation and mobility with implications on properties like gas transport. This work demonstrates an increase in T_T with decreasing PFSI film thickness in acid (H⁺) form (from 70 to 130 °C for 400 to 10 nm, respectively). In metal cation (M⁺) exchanged PFSI, T_T remained constant with thickness. Results point to an interplay between increased chain mobility at the free surface and hindered motion near the rigid substrate interface, which is amplified upon further confinement. This balance is additionally impacted by ionomer intermolecular forces, as strong electrostatic networks within the PFSI-M⁺ matrix raises T_T above the mainly hydrogen-bonded PFSI-H⁺ ionomer.

Molecular Dynamics Simulations of Substrate Hydrophilicity and Confinement Effects in Capped Nafion Films

Nafion nanocomposites for energy-related applications are being used extensively because of the attractive properties such as enhanced water retention, low unwanted crossover of electrolytes, and high proton conductivity. We present the results of the molecular dynamics modeling of Nafion films confined between two walls (substrates) of different polymer-wall interaction strengths and of different separation distances to model Nafion nanocomposites. Our goal is to provide insights into the effects of varying hydrophilicity and volume fraction of fillers/nanoparticles on the internal structure and water transport inside the Nafion membrane. The sulfur-sulfur radial distribution function first peak distance and the sulfur-oxygen (water) coordination number in the first hydration shell were negligibly affected by the wall (substrate) hydrophilicity or the film thickness. The Nafion side chains were found to bend toward the substrates with high hydrophilicity which is in qualitative agreement with existing experiments. The amount of bending was observed to reduce with increasing film thickness. However, the side-chain length did not show any noticeable variation with wall (substrate) hydrophilicity or film thickness. The water clusters became smaller and more isolated clusters emerged for highly hydrophilic substrates. In addition, the water cluster sizes showed a decreasing trend with decreasing film thickness in the case of hydrophilic substrates, which has also been observed in experiments of supported Nafion films. The in-plane water diffusion was enhanced considerably for hydrophilic substrates, and this mechanism has also been proposed previously in experiments. The in-plane water diffusion was also found to be a strong function of the substrate selectivity toward the hydrophilic phase. Our simulations can help provide more insights to experimentalists for choosing or modifying nanoparticles for Nafion nanocomposites.