Modelling of morphology and proton transport in PFSA membranes

Measurement of Water Uptake and States in Nafion Membranes Using Humidity-Controlled Terahertz Time-Domain Spectroscopy

Perfluorinated sulfonic acid ionomers are well known for their unique water uptake properties and chemical/mechanical stability. Understanding their performance-stability trade-offs is key to realizing membranes with optimal properties. Terahertz time-domain spectroscopy has been demonstrated to resolve water states inside industrially relevant membranes, producing qualitatively agreeable results to conventional gravimetric analysis and prior demonstrations. Using the proposed humidity-controlled terahertz time-domain spectroscopy, here we quantify this detailed water information inside commercially available Nafion membranes at various humidities for direct comparison against literature values from dynamic vapor sorption, differential scanning calorimetry, and Fourier transform infrared spectroscopy on selected samples. Using this technique therefore opens up opportunities for rapid future parameter space investigation for membrane optimization.

Ion-Specific Effects on Ion and Polyelectrolyte Solvation

Ion-specific effects on aqueous solvation of monovalent counter ions, Na+ ${^+ }$ , K+ ${^+ }$ , Cl- ${^- }$ , and Br- ${^- }$ , and two model polyelectrolytes (PEs), poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) were here studied with ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations based on the OPLS-aa force-field which is an empirical fixed point-charge force-field. Ion-specific binding to the PE charge groups was also characterized. Both computational methods predict similar response for the solvation of the PEs but differ notably in description of ion solvation. Notably, AIMD captures the experimentally observed differences in Cl- ${^- }$ and Br- ${^- }$ anion solvation and binding with the PEs, while the classical MD simulations fail to differentiate the ion species response. Furthermore, the findings show that combining AIMD with the computationally less costly classical MD simulations allows benefiting from both the increased accuracy and statistics reach.

Effect of ionic groups on the morphology and transport properties in a novel perfluorinated ionomer containing sulfonic and phosphonic acid groups: a molecular dynamics study

Combining the phosphonic acid group with the sulfonic acid group in PEMs has been shown to be an effective strategy for improving the fuel cell performance. However, the interplay of two different ionic groups and the resulting effect on the membrane properties have not been fully elucidated. Here, we used classical molecular dynamics simulation to investigate the morphologies, transport properties and effects of ionic groups in a novel perfluorinated PEM containing two ionic groups (PFSA-PFPA) in comparison to the corresponding homopolymers. Phase separations between hydrophilic and hydrophobic domains are confirmed in these PEMs and result from the evolution of water clusters formed around the ionic groups. The combination of both ionic groups brings a complicated morphological feature in PFSA-PFPA, with near-cylindrical aqueous domains of large length scales interconnected by tortuous domains of small sizes. And we found that the self-diffusion coefficients of water molecules are strongly related to morphologies, with the water transport in PFSA-PFPA lying between two analogous homopolymers. At the molecular level, we found that the sulfonic and phosphonic acid groups have distinct effects on the coordination behaviors and the dynamics of water molecules and hydronium ions. Strong electrostatic interactions lead to compact coordination structures and sluggish dynamics of hydronium ions around phosphonic acid groups, which determine the morphological evolution and transport properties in PFSA-PFPA. Our study affords insights into the relationship between molecular characteristics and transport properties bridged by phase-separated morphologies in a novel PEM containing both sulfonic acid and phosphonic acid groups, which deepens the understanding of the interplay between two ionic groups and may inspire the rational design of high-performance PEMs.

Transcendent Aspects of Proton Channels

A handful of biological proton-selective ion channels exist. Some open at positive or negative membrane potentials, others open at low or high pH, and some are light activated. This review focuses on common features that result from the unique properties of protons. Proton conduction through water or proteins differs qualitatively from that of all other ions. Extraordinary proton selectivity is needed to ensure that protons permeate and other ions do not. Proton selectivity arises from a proton pathway comprising a hydrogen-bonded chain that typically includes at least one titratable amino acid side chain. The enormously diverse functions of proton channels in disparate regions of the phylogenetic tree can be summarized by considering the chemical and electrical consequences of proton flux across membranes. This review discusses examples of cells in which proton efflux serves to increase pHi, decrease pHo, control the membrane potential, generate action potentials, or compensate transmembrane movement of electrical charge.

Organic Supercapacitors as the Next Generation Energy Storage Device: Emergence, Opportunity, and Challenges

Harnessing new materials for developing high-energy storage devices set off research in the field of organic supercapacitors. Various attractive properties like high energy density, lower device weight, excellent cycling stability, and impressive pseudocapacitive nature make organic supercapacitors suitable candidates for high-end storage device applications. This review highlights the overall progress and future of organic supercapacitors. Sustainable energy production and storage depend on low cost, large supercapacitor packs with high energy density. Organic supercapacitors with high pseudocapacitance, lightweight form factor, and higher device potential are alternatives to other energy storage devices. There are many recent ongoing research works that focus on organic electrolytes along with the material aspect of organic supercapacitors. This review summarizes the current research status and the chemistry behind the storage mechanism in organic supercapacitors to overcome the challenges and achieve superior performance for future opportunities.

Modified Cellulose Proton-Exchange Membranes for Direct Methanol Fuel Cells

A direct methanol fuel cell (DMFC) is an excellent energy device in which direct conversion of methanol to energy occurs, resulting in a high energy conversion rate. For DMFCs, fluoropolymer copolymers are considered excellent proton-exchange membranes (PEMs). However, the high cost and high methanol permeability of commercial membranes are major obstacles to overcome in achieving higher performance in DMFCs. Novel developments have focused on various reliable materials to decrease costs and enhance DMFC performance. From this perspective, cellulose-based materials have been effectively considered as polymers and additives with multiple concepts to develop PEMs for DMFCs. In this review, we have extensively discussed the advances and utilization of cost-effective cellulose materials (microcrystalline cellulose, nanocrystalline cellulose, cellulose whiskers, cellulose nanofibers, and cellulose acetate) as PEMs for DMFCs. By adding cellulose or cellulose derivatives alone or into the PEM matrix, the performance of DMFCs is attained progressively. To understand the impact of different structures and compositions of cellulose-containing PEMs, they have been classified as functionalized cellulose, grafted cellulose, acid-doped cellulose, cellulose blended with different polymers, and composites with inorganic additives.

Morphological Effect of Side Chain Length in Sulfonated Poly(arylene ether sulfone)s Polymer Electrolyte Membranes via Molecular Dynamics Simulation

With the recognition of the multiple advantages of sulfonated hydrocarbon-based polymers that possess high chemical and mechanical stability with significant low cost, we employed molecular dynamics simulation to explore the morphological effects of side chain length in sulfonated polystyrene grafted poly(arylene ether sulfone)s (SPAES) proton exchange membranes. The calculated diffusion coefficients of hydronium ions (H₃O⁺) are in range of 0.61-1.15 × 10^-7 cm²/s, smaller than that of water molecules, due to the electrical attraction between the oppositely charged sulfonate group and H₃O⁺. The investigation into the radial distribution functions suggests that phase segregation in the SPAES membrane is more probable with longer side chains. As the hydration level of the membranes in this study is relatively low (λ = 3), longer side chains correspond to more water molecules in the amorphous cell, which provides better solvent effects for the distribution of sulfonated side chains. The coordination number of water molecules and hydronium ions around the sulfonate group increases from 1.67 to 2.40 and from 2.45 to 5.66, respectively, with the increase in the side chain length. A significant proportion of the hydronium ions appear to be in bridging configurations coordinated by multiple sulfonate groups. The microscopic conformation of the SPAES membrane is basically unaffected by temperature during the evaluated temperature range. Thus, it can be revealed that the side chain length plays a key role in the configuration of the polymer chain and would contribute to the formation of the microphase separation morphology, which profits proton transport in the hydrophilic domains.

Coarse-Grained Modeling of Ion-Containing Polymers

Ion-containing polymers have continued to be an important research focus for several decades due to their use as an electrolyte in energy storage and conversion devices. Elucidation of connections between the mesoscopic structure and multiscale dynamics of the ions and solvent remains incompletely understood. Coarse-grained modeling provides an efficient approach for exploring the structural and dynamical properties of these soft materials. The unique physicochemical properties of such polymers are of broad interest. In this review, we summarize the current development and understanding of the structure-property relationship of ion-containing polymers and provide insights into the design of such materials determined from coarse-grained modeling and simulations accompanying significant advances in experimental strategies. We specifically concentrate on three types of ion-containing polymers: proton exchange membranes (PEMs), anion exchange membranes (AEMs), and polymerized ionic liquids (polyILs). We posit that insight into the similarities and differences in these materials will lead to guidance in the rational design of high-performance novel materials with improved properties for various power source technologies.

A phosphonic acid anion and acid dimer dianion stabilized by proton transfer in OHN hydrogen bonds - models of structural motifs in blend polymer membranes

The structure of intermolecular hydrogen-bonded complexes formed between tert-butylphosphonic acid and trimethylpyridine molecules has been experimentally studied as the simplest model system of the structural motifs in blend proton-conducting polymer membranes based on phosphonic acid residues. The stoichiometry of the formed complexes and proton positions in OHO and OHN hydrogen bonds were established by the H/D isotope effects and temperature dependences of the signals in ¹H and ³¹P NMR spectra. Two structural motifs, namely, 1 : 2 and 2 : 2 acid-base complexes, were identified at the low temperature in a polar aprotic environment. In the 1 : 2 complex, one proton has passed through the hydrogen bond center creating a chain of two cooperatively coupled OHN bonds, while in the 2 : 2 complex both OHN bonds are zwitterionic and anti-cooperatively coupled to each other via a dianionic cyclic dimer of phosphonic acid in the middle. The dianionic cyclic dimer is metastable by itself, but under the used experimental conditions it is stabilized by complexation with two trimethylpyridinium cations. Additionally, quantum chemical calculations using the DFT method were carried out to support the experimental data.

Intermediate Temperature PEFC's with Nafion® 211 Membrane Electrolytes: An Experimental and Numerical Study

This paper evaluates the performance of Nafion 211 at elevated temperatures up to 120 °C using an experimentally validated model. Increasing the fuel cell operating temperature could have many key benefits at the cell and system levels. However, current research excludes this due to issues with membrane durability. Modelling is used to investigate complex systems to gain further information that is challenging to obtain experimentally. Nafion 211 is shown to have some interesting characteristics at elevated temperatures previously unreported, the first of which is that the highest performance reported is at 100 °C and 100% relative humidity. The model was trained on the experimental data and then used to predict the behaviour in the membrane region to understand how the fuel cell performs at varying temperatures and pressures. The model showed that the best membrane performance comes from a 100 °C operating temperature, with much better performance yielded from a higher pressure of 3 bar.

Controlling Hydronium Diffusivity in Model Proton Exchange Membranes

Fuel-cell-based proton exchange membranes (PEMs) show great potential as cost-effective and clean energy conversion devices. In our recent work, we found that for the low-hydrated model PEMs with a inhomogeneous water distribution and a sulfonate anionic functional end group (SO₃^-), the H₃O⁺ reacts with SO₃^- according to SO₃^- + H₃O⁺ ↔ SO₃H + H₂O, indicating that the anions in PEMs become active participants in the hydronium diffusion. In this work, we use fully atomistic ab initio molecular dynamics simulations to elucidate the optimal conditions that would promote the participation of SO₃^- in the hydronium diffusion mechanism by increasing the H₃O⁺/SO₃^- reactivity, thus increasing the hydronium diffusivity along the cell. The results presented in this work allow us to suggest a set of design rules for creating novel, highly conductive PEMs operating at high temperatures under a nonuniform water distribution using a linker/anion with a relatively high pK_a such as (CH₂)₂SO₃. We expect that the discovery of these key design principles will play an important role in the synthesis of high-performing materials for emerging PEM-based fuel cell technologies.

Patterned Membranes for Proton Exchange Membrane Fuel Cells Working at Low Humidity

High performing proton exchange membrane fuel cells (PEMFCs) that can operate at low relative humidity is a continuing technical challenge for PEMFC developers. In this work, micro-patterned membranes are demonstrated at the cathode side by solution casting techniques using stainless steel moulds with laser-imposed periodic surface structures (LIPSS). Three types of patterns, lotus, lines, and sharklet, are investigated for their influence on the PEMFC power performance at varying humidity conditions. The experimental results show that the cathode electrolyte pattern, in all cases, enhances the fuel cell power performance at 100% relative humidity (RH). However, only the sharklet pattern exhibits a significant improvement at 25% RH, where a peak power density of 450 mW cm⁻² is recorded compared with 150 mW cm⁻² of the conventional flat membrane. The improvements are explored based on high-frequency resistance, electrochemically active surface area (ECSA), and hydrogen crossover by in situ membrane electrode assembly (MEA) testing.

Pseudo Jahn-Teller Origin of the Proton-transfer Energy Barrier in the Hydrogen-bonded [FHF]-System

The results of ab initio calculations of the adiabatic potential energy surfaces for the proton-bound [FHF]- system at different F-F distances have been rationalized in the framework of the vibronic theory. It is shown that the instability of the symmetric D∞h structure at increased F∙∙∙F distances and the proton displacement to one of the fluorine atoms is due to the pseudo Jahn–Teller mixing of the ground 1Σg electronic state with the lowest excited state of 1Σu symmetry through the asymmetric σu vibrational mode.

A broad-range variable-temperature solid state NMR spectral and relaxation investigation of the water state in Nafion 117

Understanding the water state in Nafion is not only crucial for operating a proton-exchange membrane (PEM)-based fuel cell, but also intimately related to the elucidation of the proton transport mechanism in a PEM. Although many studies have been published on this subject, some controversies and ambiguities remain unresolved. In this work, we design three different types of Nafion samples by substituting protons with lithium or sodium cations. We also pay special attention to the preparation of samples for carrying out broad-range variable temperature solid state NMR experiments so that no membrane dehydration occurs during the long experimental time at low temperatures. With these precautions and improvements, clear and largely straightforward information could be obtained to ensure minimal ambiguity and complexity in the interpretation of the experimental data. Our results show that about 40-60% of water remains unfrozen at -70 °C, depending on the type of the substituting cation. Both the ¹H and ²H spectral and relaxation results indicate that water freezing starts from the center of the nanopores inside Nafion and increases gradually as the temperature decreases. The protons remain dissociated with sulfonate groups even at the lowest temperature we reached (-70 °C), whereas both lithium and sodium are associated with sulfonate groups at most temperatures below 0 °C. The experimental data also suggest that besides frozen and unfrozen water, there is broad distribution of water state and dynamics in Nafion as the temperature is lowered from above zero down to -70 °C. The effect of the size of the substituting cation significantly affects the properties of supercooled water by modifying the cation-water interaction and impeding the rotation of sulfonate groups. These novel results not only help us in establishing a better understanding of the water state in Nafion and its performance as a proton exchange mebrane, but also provide insights into water freezing, antifreeze and supercooling in other nanoscopic environments.

Tuning proton conductivity and energy barriers for proton transfer

Proton transport is critical for many technologies and for a variety of biochemical and biophysical processes. Proton transfer between molecules (via structural diffusion) is considered to be an efficient mechanism in highly proton conducting materials. Yet, the mechanism and what controls energy barriers for this process remain poorly understood. It was shown that mixing phosphoric acid (PA) with lidocaine leads to an increase in proton conductivity at the same liquid viscosity. However, recent simulations of mixtures of PA with various bases, including lidocaine, suggested no decrease in the proton transfer energy barrier. To elucidate this surprising result, we have performed broadband dielectric spectroscopy to verify the predictions of the simulations for mixtures of PA with several bases. Our results reveal that adding bases to PA increases the energy barriers for proton transfer, and the observed increase in proton conductivity at a similar viscosity appears to be related to the increase in the glass transition temperature (T_g) of the mixture. Moreover, the energy barrier seems to increase with T_g of the mixtures, emphasizing the importance of molecular mobility or interactions in the proton transfer mechanism.

Morphological effect of side chain on H3O+ transfer inside polymer electrolyte membranes across polymeric chain via molecular dynamics simulation

Performance and durability of polymer electrolyte membrane are critical to fuel cell quality. As fuel cell vehicles become increasingly popular, membrane fundamentals must be understood in detail. Here, this study used molecular dynamic simulations to explore the morphological effects of perfluorosulfonic acid (PFSA)-based membranes on ionic conductivity. In particular, I developed an intuitive quantitative approach focusing principally on hydronium adsorbing to, and desorbing from, negatively charged sulfonate groups, while conventional ionic conductivity calculations featured the use of mean square displacements that included natural atomic vibrations. The results revealed that shorter side-chains caused more hydroniums to enter the conductive state, associated with higher ion conductivity. In addition, the hydronium path tracking showed that shorter side-chains allowed hydroniums to move among host groups, facilitating chain adsorption, in agreement with a mechanism suggested in earlier studies.

Removal of Etalon Features in the Far-Infrared-Terahertz Transmittance Spectra of Thin Polymer Films

Etalon features in infrared spectra of stratified samples, their influence on the interpretation, and methods to circumvent their presence in infrared spectra have been in discussion for decades. This paper focuses on the application of a method originally developed to remove interference fringes in the mid-infrared spectra for far-infrared Fourier transform spectroscopy on thin polymer films. We show that the total transmittance reflectance technique, commonly used for mid-infrared, also works successfully in the far-infrared spectral range where other approaches fail. Experimental spectra obtained by such technique are supported by model calculations and reveal the possibility and limits to obtain almost undisturbed far-infrared spectra which are suitable to determine low-energy vibrations of ionomer salts under certain sample conditions.

Lamellar Lyotropic Liquid Crystal Superior to Micellar Solution for Proton Conduction in an Aqueous Solution of 1-Tetradecyl-3-methylimidazolium Hydrogen Sulfate

Humidified perfluorosulfonic acid polymers with a nanoscopic phase-separated morphology are highly proton-conductive materials for fuel cells, yet morphology tuning of the acidic materials for enhanced conduction remains a challenge. Aqueous acidic lyotropic liquid crystals (LLCs) provide a powerful platform to construct well-defined nanostructures for proton conduction. We report an aqueous LLC formed by 1-tetradecyl-3-methylimidazolium hydrogen sulfate, exhibiting a proton conductivity of 210 mS cm^-1 at 25 °C, which surpasses that formed by alkylsulfonic acid, thus demonstrating that a mobile acidic anion is more efficient than constrained sulfonic acid functionality to transport protons in LLCs. For an aqueous solution of 1-alkyl-3-methylimidazolium hydrogen sulfate, a lamellar LLC results in higher conductivity than a micellar solution under the same hydration conditions. The peak power density of the fuel cell fabricated from porous membranes filled with the lamellar LLC is four times as high as that filled with the micellar solution. The work offers an efficient way to construct highly proton-conductive LLC materials for fuel cell application.

Proton Transfer in Phosphoric Acid-Based Protic Ionic Liquids: Effects of the Base

Electronic structure calculations were performed to understand highly decoupled conductivities recently reported in protic ionic liquids (PILs). To develop a molecular-level understanding of the mechanisms of proton conductivity in PILs, minimum-energy structures of trimethylamine, imidazole, lidocaine, and creatinine (CRT) with the addition of one to three phosphoric acid (PA) molecules were determined at the B3LYP/6-311G** level of theory with the inclusion of an implicit solvation model (SMD with ε = 61). The proton affinity of the bases and zero-point energy corrected binding energies were computed at a similar level of theory. Proton dissociation from PA occurs in all systems, resulting in the formation of ion pairs due to the relatively strong basicity of the bases (proton acceptors) and the effect of the high dielectric constant solvent in stabilizing the charge separation. The second and third PA molecules preferentially form "ring-like" hydrogen bonds with one another instead of forming hydrogen bonds at the donor and acceptor sites of the bases. Potential energy scans reveal that the bases with stronger proton affinity exert greater influence on the energetics of proton transfer between the individual PA molecules. However, the effects are minimal when shifted into a single-well from a double-well potential. Barrierless proton transfer was observed to occur in the CRT system with several PA molecules present, implying that the CRT may be a promising PA-based PIL.

Simulation study of the effects of phase separation on hydroxide solvation and transport in anion exchange membranes

Anion exchange membranes (AEMs) can be cheaper alternatives than proton exchange membranes, but a key challenge for AEMs is to archive good ionic conductivity while maintaining mechanical strength. Diblock copolymers containing a mechanically strong hydrophobic block and an ion-conducting hydrophilic block have been shown to be viable solutions to this challenge. Using our recently developed reactive hydroxide model, we investigate the effects of block size on the hydroxide solvation and transport in a diblock copolymer (PPO-b-PVBTMA) in its highly hydrated state. Typically, both hydroxide and water diffusion constants decrease as the hydrophobic PPO block size increases. However, phase separation takes place above a certain mole ratio of hydrophobic PPO to hydrophilic PVBTMA blocks and we found it to effectively recover the diffusion constants. Extensive analyses reveal that morphological changes modulate the local environment for hydroxide and water transport and contribute to that recovery. The activation energy barriers for hydroxide and water diffusion show abrupt jumps at the same block ratios when such recovery effects begin to appear, suggesting transformation of the structure of water channels. Taking the advantages of partial phase separation can help optimize both ionic conductivity and mechanical strength of fuel cell membranes.

Effect of the Side-Chain Length in Perfluorinated Sulfonic and Phosphoric Acid-Based Membranes on Nanophase Segregation and Transport: A Molecular Dynamics Simulation Approach

The effect of side-chain length on the nanophase-segregated structure and transport in perfluorinated sulfonic acid (PFSA)-based and perfluorinated phosphoric acid (PFPA)-based membranes is investigated at 20 and 5 wt % water content conditions using a molecular dynamics simulation method. It is found using the pair correlation analysis that the longer side chain leads to more developed local water structures in the water phase at 20 wt % water content, observable in both membrane chemistries albeit more distinct in PFPA-based membranes. It is also confirmed from the structure factor analysis that large-scale nanophase segregation is enhanced with increasing side-chain length for PFPA membranes, whereas no significant change is observed at these scales for PFSA membranes. Next, it is revealed that the proton transport is increased by 0.004 S/cm in PFSA-based membranes with increasing side-chain length due to the enhanced vehicular and hopping mechanisms, whereas the proton transport in PFPA-based membranes is decreased by 0.002 S/cm despite improved nanophase segregation. As confirmed by the pair correlation function analysis, the diminished proton transport in PFPA-based membranes is attributed to the molecular association of phosphate groups with hydronium ions via hydrogen bond in the longer side-chain case, which is namely a hydronium-mediated bridge configuration. Such bridge configurations and correspondingly similar trends in proton transport are also observed at 5 wt % water content condition to a lesser extent. Our simulation study demonstrates that the proton transport is affected by the hydrogen-bonding network as well as by the nanophase segregation.

Molecular Modeling of Structure and Dynamics of Nafion Protonation States

We present the results of the atomistic molecular dynamics modeling of different protonation states of Nafion at varying hydration levels. Previous experiments have shown that the degree of deprotonation (DDP) of the sulfonic acid groups in a Nafion membrane varies significantly upon hydration. Our goal is to provide insights into the effects of variable protonation states and water content on the internal structure and vehicular transport inside the Nafion membrane. The Nafion side chain lengths showed a weak increasing trend with increasing DDP at all hydration levels, exposing more of the sulfonic acid groups to the hydrophilic/water phase. The water-phase characteristic size/diameter decreased with increasing DDP, but, interestingly, the average number of water molecules per cluster increased. The probability of water-hydronium hydrogen bond formation decreased with increasing DDP, despite an increase in the total number of such hydrogen bonds. The water diffusion was largely unaffected by the state of deprotonation. In contrast to that, the hydronium ion diffusion slowed down with increasing DDP in the overall membrane. The hydronium ion residence times around the sulfonic acid group increased with increasing DDP. Our simulations show a strong connection between the morphology of the water domains and protonation states of Nafion. Such a connection can also be expected in polyelectrolyte membranes similar to Nafion.

Understanding of Nanophase Separation and Hydrophilic Morphology in Nafion and SPEEK Membranes: A Combined Experimental and Theoretical Studies

The understanding of the relationship between the chemical structure and the hydrophilic structure is crucial for the designing of high-performance PEMs. Comparative studies in typical Nafion and sulfonated poly (ether ether ketone) (SPEEK) were performed using a combined experimental and theoretical method. SPEEK showed suppressed fuel crossover and good mechanical property but low water uptake, weak phase separation, and inadequate proton conductivity. Molecular dynamics (MD) simulation approaches were employed to get a molecular-level understanding of the structure-property relationship of SPEEK and Nafion membranes. In SPEEK membranes, the local aggregation of hydrophilic clusters is worse, and much stronger electrostatic interaction between Os-Hh was verified, resulting in less delocalized free H3O+ and much lower DH3O+. In addition, the probability of H2O-H3O+ association varied with water content. Particularly, SPEEK exhibited much lower H9O4+ probability at various relative water contents, leading to lower structural diffusivity than Nafion. Eventually, SPEEK possessed low vehicular and structural diffusivities, which resulted in a low proton conductivity. The results indicated that the structure of hydrated hydronium complexes would deform to adapt the confining hydrophilic channels. The confinement effect on diffusion of H2O and H3O+ is influenced by the water content and the hydrophilic morphologies. This study provided a new insight into the exploration of high-performance membranes in fuel cell.

Coarse-grained model of nanoscale segregation, water diffusion, and proton transport in Nafion membranes

We present a coarse-grained model of the acid form of Nafion membrane that explicitly includes proton transport. This model is based on a soft-core bead representation of the polymer implemented into the dissipative particle dynamics (DPD) simulation framework. The proton is introduced as a separate charged bead that forms dissociable Morse bonds with water beads. Morse bond formation and breakup artificially mimics the Grotthuss hopping mechanism of proton transport. The proposed DPD model is parameterized to account for the specifics of the conformations and flexibility of the Nafion backbone and sidechains; it treats electrostatic interactions in the smeared charge approximation. The simulation results qualitatively, and in many respects quantitatively, predict the specifics of nanoscale segregation in the hydrated Nafion membrane into hydrophobic and hydrophilic subphases, water diffusion, and proton mobility. As the hydration level increases, the hydrophilic subphase exhibits a percolation transition from a collection of isolated water clusters to a 3D network of pores filled with water embedded in the hydrophobic matrix. The segregated morphology is characterized in terms of the pore size distribution with the average size growing with hydration from ∼1 to ∼4 nm. Comparison of the predicted water diffusivity with the experimental data taken from different sources shows good agreement at high and moderate hydration and substantial deviation at low hydration, around and below the percolation threshold. This discrepancy is attributed to the dynamic percolation effects of formation and rupture of merging bridges between the water clusters, which become progressively important at low hydration, when the coarse-grained model is unable to mimic the fine structure of water network that includes singe molecule bridges. Selected simulations of water diffusion are performed for the alkali metal substituted membrane which demonstrate the effects of the counter-ions on membrane self-assembly and transport. The hydration dependence of the proton diffusivity reproduces semi-qualitatively the trend of the diverse experimental data, showing a sharp decrease around the percolation threshold. Overall, the proposed model opens up an opportunity to study self-assembly and water and proton transport in polyelectrolytes using computationally efficient DPD simulations, and, with further refinement, it may become a practical tool for theory informed design and optimization of perm-selective and ion-conducting membranes with improved properties.

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.

Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion

For the first time, we report the effects of elevated temperatures, from 80 to 100 °C, on the changes in the states of water and ion-water channels and their correlation with the proton conductivity of Nafion NR212, which was investigated using a Fourier transform infrared spectroscopy study. Experimentally, three types of water aggregates, protonated water (H+(H2O) n ), nonprotonated hydrogen (H)-bonded water (H2O···H2O), and non-H-bonded water, were found in Nafion, and the existence of those three types of water was confirmed through ab initio molecular dynamics simulation. We found that the proton conductivity of Nafion increased for up to 80 °C, but from 80 to 100 °C, the conductivity did not increase; rather, all of those elevated temperatures showed identical conductivity values. The proton conductivities at lower relative humidities (RHs) (up to 50%) remained nearly identical for all elevated temperatures (80, 90, and 100 °C); however, from 60% RH (over λ = 4), the conductivity remarkably jumped for all elevated temperatures. The results indicated that the amount of randomly arranged water gradually increased and created more H-bonded water networks in Nafion at above 60% RH. From the deconvolution of the O-H bending band, it was found that the volume fraction f i (i=each deconvoluted band) of H-bonded water for elevated temperatures (>80-100 °C) increased remarkably higher than for 60 °C.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.

Ab initio and density functional theory (DFT) studies on triflic acid with water and protonated water clusters

The structure, stability and infrared spectral signatures of triflic acid (TA) with water clusters (W_n) and protonated water clusters (TAH⁺W_n, n = 1 - 6) were computed using DFT and MP2 methods. Our calculations show that a minimum of three water molecules are necessary to stabilize the dissociated zwitterionic form of TA. It can be seen from the results that there is no significant movement of protons in smaller (n = 1 and 2) and linear (n = 1 - 6) types of water clusters. Further, the geometries of TAW_n clusters first form a neutral pair (NP) to contact ion pair (CIP), then form a solvent separated ion pair (SSIP) in a water hexamer. These findings reveal that proton transfer may take place through NP to CIP and then CIP to SSIP. The calculated binding energies (BEs) of ion pair clusters is always higher than that of NP clusters (i.e., more stable than the NP). Existing excess proton linear chain clusters transfer a proton to adjacent water molecules via a Grotthuss mechanism, whereas the same isomers in the branched motifs do not conduct protons. Examination of geometrical parameters and infrared frequencies reveals hydronium ion (H₃O⁺ also called Eigen cation) formation in both TAW_n and protonated TAW_n clusters. The stability of Eigen water clusters is three times higher than that of other non-Eigen water clusters. Our study shows clearly that formation of ion pairs in TAW_n and TAH⁺W_n clusters greatly favors proton transfer to neighboring water molecules and also enhances the stability of these complexes.

Water Diffusion Dependence on Amphiphilic Block Design in (Amphiphilic-Hydrophobic) Diblock Copolymer Membranes

Polyelectrolyte membranes (PEMs) are applied in polyelectrolyte fuel cells (PEFC). The proton conductive pathways within PEMs are provided by nanometer-sized water containing pores. Large-scale application of PEFC requires the production of low-cost membranes with high proton conductivity and therefore good connected pore networks. Pore network formation within four alternative model diblock (hydrophobic_amphiphilic) copolymers in the presence of water is studied by dissipative particle dynamics. Each hydrophobic block contains 50 consecutively connected hydrophobic (A) fragments, and amphiphilic blocks contain 40 hydrophobic A beads and 10 hydrophilic C beads. For one amphiphilic block the C beads are distributed uniformly along the backbone. For the other architectures C beads are located at the end of the side chains attached at regular intervals along the backbone. Water diffusion through the pores is modeled by Monte Carlo tracer diffusion through mapped morphologies. Diffusion is highest for the grafted architectures and increases with increase of length of the side chains. A consistent picture emerges in which diffusion strongly increases with the value of ⟨Nbond⟩ within the amphiphilic block, where ⟨Nbond⟩ is the average number of bonds between hydrophobic A beads and the nearest C bead.

Modelling linear and branched amphiphilic star polymer electrolyte membranes and verification of the bond counting method

Water diffusion through hydrated amphiphilic star polymer membranes depends strongly on hydrophilic position within the linear and Y-shaped arms.