State of understanding of nafion

Metaplastic and energy-efficient biocompatible graphene artificial synaptic transistors for enhanced accuracy neuromorphic computing

CMOS-based computing systems that employ the von Neumann architecture are relatively limited when it comes to parallel data storage and processing. In contrast, the human brain is a living computational signal processing unit that operates with extreme parallelism and energy efficiency. Although numerous neuromorphic electronic devices have emerged in the last decade, most of them are rigid or contain materials that are toxic to biological systems. In this work, we report on biocompatible bilayer graphene-based artificial synaptic transistors (BLAST) capable of mimicking synaptic behavior. The BLAST devices leverage a dry ion-selective membrane, enabling long-term potentiation, with ~50 aJ/µm2 switching energy efficiency, at least an order of magnitude lower than previous reports on two-dimensional material-based artificial synapses. The devices show unique metaplasticity, a useful feature for generalizable deep neural networks, and we demonstrate that metaplastic BLASTs outperform ideal linear synapses in classic image classification tasks. With switching energy well below the 1 fJ energy estimated per biological synapse, the proposed devices are powerful candidates for bio-interfaced online learning, bridging the gap between artificial and biological neural networks.

Three layered cucurbit[6]uril-based metal-organic rotaxane networks functionalized by sulfonic groups for proton conduction

Three new cucurbit[6]uril (CB[6])-based metal-organic rotaxane networks (MORNs) (named CUST-711, CUST-712, and CUST-713) functionalized by a sulfonic group (-SO₃H) have been designed and synthesized via a hydrothermal method. All three compounds exhibited similar two-dimensional (2D) wave layer structures. Their stability under different temperature and relative humidity conditions has been investigated and all the compounds showed excellent stability. Furthermore, their proton conduction properties were also discussed in detail. Due to different structures and sulfonic group sites, the three compounds exhibited different proton conduction abilities of which CUST-712 exhibited an intrinsic relatively high proton conductivity (1.75 × 10^-4 S cm^-1 at 85 °C and 97% relative humidity). These results provide ideas for the design and synthesis of functional CB[6]-based metal-organic rotaxane frameworks (MORFs) as proton conducting materials.

Facile construction of a highly proton-conductive matrix-mixed membrane based on a -SO3H functionalized polyamide

Developing a facile strategy to construct low-cost and efficient proton-conductive electrolytes is pivotal in the practical application of proton exchange membrane (PEM) fuel cells. Herein, a polyamide with in-built -SO₃H moieties, PA(PhSO3H)2, was synthesized via a simple one-pot polymeric acylation process. Investigations via electrochemical impedance spectroscopy (EIS) measurements revealed that the fabricated PA(PhSO3H)2 displays a proton conductivity of up to 5.54 × 10^-2 S cm^-1 at 353 K under 98% relative humidity (RH), which is more than 2 orders of magnitude higher than that of its -SO₃H-free analogue PA(Ph)2 (2.38 × 10^-4 S cm^-1) under the same conditions. Therefore, after mixing with polyacrylonitrile (PAN) at different ratios, PA(PhSO3H)2-based matrix-mixed membranes were subsequently made and the analysis results revealed that the proton conductivity can reach up to 5.82 × 10^-2 S cm^-1 at 353 K and 98% RH when the weight ratio of PA(PhSO3H)2 : PAN is in 3 : 1 (labeled as PA(PhSO3H)2-PAN(3 : 1)), the value of which is comparable even to those of commercially available electrolytes that are used in PEM fuel cells. In addition, continuous testing shows that PA(PhSO3H)2-PAN(3 : 1) possesses long-life reusability. This work demonstrates that, utilizing the simple reaction of polymeric acylation with a sulfonated module as a precursor, highly effective proton-conductive membranes for PEM fuel cells can be achieved in a facile manner.

High Hydrogen Isotope Separation Efficiency: Graphene or Catalyst?

Single-layer graphene has been demonstrated to be a high-efficiency hydrogen isotope sieving membrane in the electrochemical hydrogen pumping system. In this work, we transferred this membrane to proton exchange membrane water electrolysis (PEMWE), which has wide industrial applications. Two membrane electrode assemblies with decorated Pt and ink-coated Pt were investigated. The graphene with the decorated Pt scheme acquired the reported highest proton-to-tritium separation factor of 19.50 in PEMWE. However, rather than graphene, the decorated catalyst was demonstrated to be responsible for this remarkable separation efficiency. Previous studies from Geim's group underestimated the enhanced separation efficiency of decorated Pt over ink-coated Pt, resulting in an exaggerated separation efficiency for graphene. The behavior of proton transfer with hydrogen isotope separation through graphene was interpreted by a serial-parallel circuit model, which suggested that hydrogen isotope separation occurs at defect sites. The limited separation efficiency for graphene was also well understood by a density functional theory (DFT) calculation using an SW 55-77 model and the transition state theory for the kinetic isotope effect. This research provides a thorough understanding of proton transfer with hydrogen isotope separation through graphene.

Microkinetic Analysis of the Oxygen Evolution Performance at Different Stages of Iridium Oxide Degradation

The microkinetics of the electrocatalytic oxygen evolution reaction substantially determines the performance in proton-exchange membrane water electrolysis. State-of-the-art nanoparticulated rutile IrO₂ electrocatalysts present an excellent trade-off between activity and stability due to the efficient formation of intermediate surface species. To reveal and analyze the interaction of individual surface processes, a detailed dynamic microkinetic model approach is established and validated using cyclic voltammetry. We show that the interaction of three different processes, which are the adsorption of water, one potential-driven deprotonation step, and the detachment of oxygen, limits the overall reaction turnover. During the reaction, the active IrO₂ surface is covered mainly by *O, *OOH, and *OO adsorbed species with a share dependent on the applied potential and of 44, 28, and 20% at an overpotential of 350 mV, respectively. In contrast to state-of-the-art calculations of ideal catalyst surfaces, this novel model-based methodology allows for experimental identification of the microkinetics as well as thermodynamic energy values of real pristine and degraded nanoparticles. We show that the loss in electrocatalytic activity during degradation is correlated to an increase in the activation energy of deprotonation processes, whereas reaction energies were marginally affected. As the effect of electrolyte-related parameters does not cause such a decrease, the model-based analysis demonstrates that material changes trigger the performance loss. These insights into the degradation of IrO₂ and its effect on the surface processes provide the basis for a deeper understanding of degrading active sites for the optimization of the oxygen evolution performance.

A Review on the Long-Term Performance of Proton Exchange Membrane Fuel Cells: From Degradation Modeling to the Effects of Bipolar Plates, Sealings, and Contaminants

Proton-exchange membrane fuel cells (PEMFCs) are regarded as promising alternatives to internal combustion engines (ICEs) to reduce pollution. Recent research on PEMFCs focuses on achieving higher power densities, reducing the refueling time, mitigating the final price, and decreasing the degradations, to facilitate the commercialization of hydrogen mobility. The design of bipolar plates and compression kits, in addition to their coating, can effectively improve performance, increase durability, and support water/thermal management. Past reviews usually focused on the specific aspect, which can hardly provide readers with a complete picture of the key challenges facing and advances in the long-term performance of PEMFCs. This paper aims to deliver a comprehensive source to review, from both experimental, analytical and numerical viewpoints, design challenges, degradation modeling, protective coatings for bipolar plates, and key operational challenges facing and solutions to the stack to prevent contamination. The significant research gaps in the long-term performance of PEMFCs are identified as (1) improved bipolar-plate design and coating, (2) the optimization of the design of sealing and compression kits to reduce mechanical stresses, and (3) stack degradation regarding fuel contamination and dynamic operation.

Global diffusion of hydrogen molecules in the heterogeneous structure of polymer electrolytes for fuel cells: Dynamic Monte Carlo combined with molecular dynamics calculations

Using our recently developed dynamic Monte Carlo (MC) method [Nagai et al., J. Chem Phys. 156, 154506 (2022)], we investigated global diffusion of hydrogen molecules over structural heterogeneities of polymer electrolyte membranes in fuel cells. The three-dimensional position-dependent free energies and the diffusion constants of the hydrogen molecules, required by the present dynamic MC calculations, were taken from our previous study [Nagai et al., J. Chem. Phys. 156, 044507 (2022)] and newly evaluated in this work, respectively. The calculations enabled evaluating the hydrogen dynamics over long-time scales, including global diffusion constants. Based on the calculated global diffusion constants and free energies, the permeability of hydrogen molecules was estimated via the solubility-diffusion model. The estimated values were in good agreement with reported experimental data, thus validating the present methodology. The analysis of the Monte Carlo trajectories indicated that the main permeation paths are located in the polymer and interfacial phases, although the water phase may make a non-negligible contribution to mass transport.

Photocatalytic CO2 Conversion Using Metal-Containing Coordination Polymers and Networks: Recent Developments in Material Design and Mechanistic Details

International guidelines have progressively addressed global warming which is caused by the greenhouse effect. The greenhouse effect originates from the atmosphere’s gases which trap sunlight which, as a consequence, causes an increase in global surface temperature. Carbon dioxide is one of these greenhouse gases and is mainly produced by anthropogenic emissions. The urgency of removing atmospheric carbon dioxide from the atmosphere to reduce the greenhouse effect has initiated the development of methods to covert carbon dioxide into valuable products. One approach that was developed is the photocatalytic transformation of CO2. Photocatalysis addresses environmental issues by transferring CO2 into value added chemicals by mimicking the natural photosynthesis process. During this process, the photocatalytic system is excited by light energy. CO2 is adsorbed at the catalytic metal centers where it is subsequently reduced. To overcome several obstacles for achieving an efficient photocatalytic reduction process, the use of metal-containing polymers as photocatalysts for carbon dioxide reduction is highlighted in this review. The attention of this manuscript is directed towards recent advances in material design and mechanistic details of the process using different polymeric materials and photocatalysts.

Mechanochemical Synthesis of Phosphonate-Based Proton Conducting Metal-Organic Frameworks

Water-stable metal-organic frameworks (MOFs) with proton-conducting behavior have attracted great attention as promising materials for proton-exchange membrane fuel cells. Herein, we report the mechanochemical gram-scale synthesis of three new mixed-ligand phosphonate-based MOFs, {Co(H₂PhDPA)(4,4'-bipy)(H₂O)·2H₂O}_n (BAM-1), {Fe(H₂PhDPA)(4,4'-bipy) (H₂O)·2H₂O}_n (BAM-2), and {Cu(H₂PhDPA)(dpe)₂(H₂O)₂·2H₂O}_n (BAM-3) [where H₂PhDPA = phenylene diphosphonate, 4,4'-bipy = 4,4'-bipyridine, and dpe = 1,2-di(4-pyridyl)ethylene]. Single-crystal X-ray diffraction measurements revealed that BAM-1 and BAM-2 are isostructural and possess a three-dimensional (3D) network structure comprising one-dimensional (1D) channels filled with guest water molecules. Instead, BAM-3 displays a 1D network structure extended into a 3D supramolecular structure through hydrogen-bonding and π-π interactions. In all three structures, guest water molecules are interconnected with the uncoordinated acidic hydroxyl groups of the phosphonate moieties and coordinated water molecules by means of extended hydrogen-bonding interactions. BAM-1 and BAM-2 showed a gradual increase in proton conductivity with increasing temperature and reached 4.9 × 10^-5 and 4.4 × 10^-5 S cm^-1 at 90 °C and 98% relative humidity (RH). The highest proton conductivity recorded for BAM-3 was 1.4 × 10^-5 S cm^-1 at 50 °C and 98% RH. Upon further heating, BAM-3 undergoes dehydration followed by a phase transition to another crystalline form which largely affects its performance. All compounds exhibited a proton hopping (Grotthuss model) mechanism, as suggested by their low activation energy.

Highly Effective Proton-Conduction Matrix-Mixed Membrane Derived from an -SO3H Functionalized Polyamide

Developing a low-cost and effective proton-conductive electrolyte to meet the requirements of the large-scale manufacturing of proton exchange membrane (PEM) fuel cells is of great significance in progressing towards the upcoming “hydrogen economy” society. Herein, utilizing the one-pot acylation polymeric combination of acyl chloride and amine precursors, a polyamide with in-built -SO₃H moieties (PA-PhSO₃H) was facilely synthesized. Characterization shows that it possesses a porous feature and a high stability at the practical operating conditions of PEM fuel cells. Investigations of electrochemical impedance spectroscopy (EIS) measurements revealed that the fabricated PA-PhSO₃H displays a proton conductivity of up to 8.85 × 10⁻² S·cm⁻¹ at 353 K under 98% relative humidity (RH), which is more than two orders of magnitude higher than that of its -SO₃H-free analogue, PA-Ph (6.30 × 10⁻⁴ S·cm⁻¹), under the same conditions. Therefore, matrix-mixed membranes were fabricated by mixing with polyacrylonitrile (PAN) in different ratios, and the EIS analyses revealed that its proton conductivity can reach up to 4.90 × 10⁻² S·cm⁻¹ at 353 K and a 98% relative humidity (RH) when the weight ratio of PA-PhSO₃H:PAN is 3:1 (labeled as PA-PhSO₃H-PAN (3:1)), the value of which is even comparable with those of commercial-available electrolytes being used in PEM fuel cells. Additionally, continuous tests showed that PA-PhSO₃H-PAN (3:1) possesses a long-life reusability. This work demonstrates, using the simple acylation reaction with the sulfonated module as precursor, that low-cost and highly effective proton-conductive electrolytes for PEM fuel cells can be facilely achieved.

The importance of water content on the conductivity of biomaterials and bioelectronic devices

Conductive biocompatible-, bioinspired- and biomaterials are increasing in importance, especially in bioelectronic applications where these materials are used in a variety of devices. Given the intended purpose of many of these devices is to interface with the human body, a pertinent issue is the effect of water from the environment on the electrical properties of the materials and devices. A researcher on biomaterials may currently not be aware, but the conductivity of these materials and device performances can be significantly altered with the presence of hydration in the environment. Examples will be given to highlight the problem that the conductivity of biomaterials can change by orders of magnitude depending on water content. Furthermore, case studies will be discussed in which control of the water content was key to understanding the underlying charge transport mechanism of conductive biomaterials. Examples of various devices and their response to hydration content will also be covered. Finally, this perspective will also mention the various methods of hydration control (including contrast studies) that can be used to perform careful work on conductive biomaterials and devices. Overall, water content should be considered an environmental variable as important as temperature to control for sound scientific investigation and to yield understanding of conductive biomaterials and bioelectronic devices.

Possibility to Alter Dynamics of Luminescence from Surface of Polymer Membrane with Ultrasonic Waves

The temporal dynamics of luminescence from the surface of Nafion polymer membranes have been studied. In fact, the polymer membrane was soaked in liquids with different contents of deuterium. The test liquids were ordinary (natural) water (deuterium content equal to 157 ppm) and deuterium-depleted water (deuterium content is equal to 3 ppm). Simultaneously with the excitation of luminescence, the Nafion plate was irradiated with ultrasonic pulses, having a duration of 1 μs. The ultrasonic waves were generated with different repetition rates and amplitudes, and irradiated the surface of Nafion in the geometry of grazing or normal incidence. Luminescence regimes were studied when the membrane was irradiated with one ultrasonic wave (one piezoelectric transducer) or two counter-propagating waves (two piezoelectric transducers). It turned out that ultrasonic waves, which fall normal to the membrane interface, do not affect the dynamics of luminescence. At the same time, in the case of ultrasonic irradiation in the grazing incidence geometry, sharp jumps in the luminescence intensity occur, and the behavior of these jumps substantially depends on the mode of irradiation: one or two piezoelectric transducers. This allows for control of the dynamics of luminescence from the polymer surface. In accordance with this model, the possibility of altering the luminescence dynamics is due to the effect of unwinding the polymer fibers from the surface toward the liquid bulk upon soaking. It is important that such unwinding does not occur in deuterium-depleted water, which was confirmed in a direct experiment with dynamic light scattering from polydisperse aqueous suspensions of Nafion nanometer-sized particles; these suspensions were prepared in ordinary water and deuterium-depleted water. Thus, ultrasonic irradiation affects the dynamics of luminescence only when Nafion is swollen in ordinary water; in the case of deuterium-depleted water this effect is missed.

Development of WO3-Nafion Based Membranes for Enabling Higher Water Retention at Low Humidity and Enhancing PEMFC Performance at Intermediate Temperature Operation

The proton exchange membrane (PEM) represents a pivotal material and a key challenge in developing fuel cell science and hydrogen technology. Nafion is the most promising polymer which will lead to its commercialisation. Hybrid membranes of nanosized tungsten oxide (WO₃) and Nafion were fabricated, characterised, and tested in a single cell. The incorporation of 10 wt% WO₃ resulted in 21% higher water uptake, 11.7% lower swelling ratio, almost doubling the hydration degree, and 13% higher mechanical stability of the hybrid membrane compared to the Nafion XL. Compared to commercial Nafion XL, the rNF-WO-10 hybrid membrane showed an 8.8% and 20% increase in current density of the cell at 0.4 V operating at 80 and 95 °C with 1.89 and 2.29 A/cm², respectively. The maximum power density has increased by 9% (0.76 W/cm²) and 19.9% (0.922 W/cm²) when operating at the same temperatures compared to the commercial Nafion XL membrane. Generally, considering the particular structure of Nafion XL, our Nafion-based membrane with 10 wt% WO₃ (rNF-WO-10) is a suitable PEM with a comparable performance at different operating conditions.

Gas Mass-Transport Coefficients in Ionomer Membranes Using a Microelectrode

Gas permeability, the product of gas diffusivity and Henry's gas-absorption constant, of ionomer membranes is an important transport parameter in fuel cell and electrolyzer research as it governs gas crossover between electrodes and perhaps in the catalyst layers as well. During transient operation, it is important to divide the gas permeability into its constituent properties as they are individually important. Although transient microelectrode measurements have been used previously to separate the gas permeability into these two parameters, inconsistencies remain in the interpretation of the experimental techniques. In this work, a new interpretation methodology is introduced for determining independently diffusivity and Henry's constant of hydrogen and oxygen gases in ionomer membranes (Nafion 211 and Nafion XL) as a function of relative humidity using microelectrodes. Two time regimes are accounted for. At long times, gas permeability is determined from a two-dimensional numerical model that calculates the solubilized-gas concentration profiles at a steady state. At short times, permeability is deconvoluted into diffusivity and Henry's constant by analyzing transient data with an extended Cottrell equation that corrects for actual electrode surface area. Gas permeability and diffusivity increase as relative humidity increases for both gases in both membranes, whereas Henry's constants for both gases decrease with increasing relative humidity. In addition, results for Nafion 211 membranes are compared to a simple phase-separated parallel-diffusion transport theory with good agreement. The two-time-regime analysis and the experimental methodology can be applied to other electrochemical systems to enable greater precision in the calculation of transport parameters and to further understanding of gas transport in fuel cells and electrolyzers.

Industrial-Current-Density CO2-to-C2+ Electroreduction by Anti-swelling Anion-Exchange Ionomer-Modified Oxide-Derived Cu Nanosheets

CO₂ electroreduction to high-energy-density C₂₊ products is highly attractive, whereas the C₂₊ selectivity under industrial current densities is still unsatisfying. Here, an anti-swelling anion exchange ionomer (AEI) was first proposed to optimize the local environment for promoting industrial-current-density CO₂-to-C₂₊ electroreduction. Taking the anti-swelling AEI-modified oxide-derived Cu nanosheets as an example, in situ Raman spectroscopy and contact angle measurements revealed that the OH^--accumulated -N(CH₃)₃⁺ groups and anti-swelling backbone of AEI could synergistically regulate the local pH level and water content. In situ Fourier-transform infrared spectroscopy and theoretical calculations demonstrated that the higher local pH value could lower the energy barrier for the rate-limiting COCO* hydrogenated to COCOH* from 0.08 to 0.04 eV, thereby boosting the generation of C₂₊ products. Owing to the anti-swelling backbone, the optimized water content of 3.5% could suppress the competing H₂ evolution and hence facilitate the proton-electron transfer step for C₂₊ production. As a result, the anti-swelling AEI-modified oxide-derived Cu nanosheets achieved a C₂₊ Faradaic efficiency of 85.1% at a current density up to 800 mA cm^-2 with a half-cell power conversion efficiency exceeding 50%, outperforming most reported powder catalysts.

Spectroscopic Investigation of Catalyst Inks and Thin Films Toward the Development of Ionomer Quality Control

As the production of polymer electrolyte fuel cells expands, novel quality control methods must be invented or adapted in order to support expected rates of production. Ensuring the quality of deposited catalyst layers is an essential step in the fuel cell manufacturing process, as the efficiency of a fuel cell is reliant on the catalyst layer being uniform at both the target platinum loading and the target ionomer content. Implementing a quality control method that is sensitive to these aspects is imperative, as wasting precious metals and other catalyst materials is expensive, and represents a potential barrier to entry into the field for manufacturers experimenting with novel deposition processes. In this work, we analyzed catalyst inks to determine if their ionomer content could be quantized spectroscopically. Attenuated total reflection (ATR) Fourier transform infrared spectroscopic technique was investigated producing a signal proportional to the ionomer content. ATR spectroscopy was able to quantitatively differentiate samples in which the ionomer to carbon mass ratio (I/C) varied between 0.9 and 3.0. The I/C ratio was correlated to the measured ATR signal near the CF₂ vibrational bands located between 1100 cm^-1 and 1400 cm^-1. The experimental results obtained constitute a step toward the development of novel quality control methodologies for catalyst inks utilized by the fuel cell industry.

Switched Proton Conduction in Metal-Organic Frameworks

Stimuli-responsive materials can respond to external effects, and proton transport is widespread and plays a key role in living systems, making stimuli-responsive proton transport in artificial materials of particular interest to researchers due to its desirable application prospects. On the basis of the rapid growth of proton-conducting porous metal-organic frameworks (MOFs), switched proton-conducting MOFs have also begun to attract attention. MOFs have advantages in crystallinity, porosity, functionalization, and structural designability, and they can facilitate the fabrication of novel switchable proton conductors and promote an understanding of the comprehensive mechanisms. In this Perspective, we highlight the current progress in the rational design and fabrication of stimuli-responsive proton-conducting MOFs and their applications. The dynamic structural change of proton transfer pathways and the role of trigger molecules are discussed to elucidate the stimuli-responsive mechanisms. Subsequently, we also discuss the challenges and propose new research opportunities for further development.

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.

From radial to unidirectional water pumping in zeta-potential modulated Nafion nanostructures

Chemically propelled micropumps are promising wireless systems to autonomously drive fluid flows for many applications. However, many of these systems are activated by nocuous chemical fuels, cannot operate at high salt concentrations, or have difficulty for controlling flow directionality. In this work we report on a self-driven polymer micropump fueled by salt which can trigger both radial and unidirectional fluid flows. The micropump is based on the cation-exchanger Nafion, which produces chemical gradients and local electric fields capable to trigger interfacial electroosmotic flows. Unidirectional pumping is predicted by simulations and achieved experimentally by nanostructuring Nafion into microarrays with a fine tune modulation of surrounding surface zeta potentials. Nafion micropumps work in a wide range of salt concentrations, are reusable, and can be fueled by different salt cations. We demonstrate that they work with the common water-contaminant cadmium, using the own capture of this ion as fuel to drive fluid pumping. Thus, this system has potential for efficient and fast water purification strategies for environmental remediation. Unidirectional Nafion pumps also hold promise for effective analyte delivery or preconcentration for (bio)sensing assays.

Chemically propelled micropumps are wireless fluid flow driving systems with many potential applications. Here, the authors report a self-driven reusable Nafion micropump fueled by different salt cations in a wide range of concentrations that triggers both radial and unidirectional flows, showing efficient water remediation capabilities.

Electro-osmotic Drag and Thermodynamic Properties of Water in Hydrated Nafion Membranes from Molecular Dynamics

One of the important parameters in water management of proton exchange membranes is the electro-osmotic drag (EOD) coefficient of water. The value of the EOD coefficient is difficult to justify, and available literature data on this for Nafion membranes show scattering from in experiments and simulations. Here, we use a classical all-atom model to compute the EOD coefficient and thermodynamic properties of water from molecular dynamics simulations for temperatures between 330 and 420 K, and for different water contents between λ = 5 and λ = 20. λ is the ratio between the moles of water molecules to the moles of sulfonic acid sites. This classical model does not capture the Grotthuss mechanism; however, it is shown that it can predict the EOD coefficient within the range of experimental values for λ = 5 where the vehicular mechanism dominates proton transfer. For λ > 5, the Grotthuss mechanism becomes dominant. To obtain the EOD coefficient, average velocities of water and ions are computed by imposing different electric fields to the system. Our results show that the velocities of water and hydronium scale linearly with the electric field, resulting in a constant ratio of ca. 0.4 within the error bars. We find that the EOD coefficient of water linearly increases from 2 at λ = 5 to 8 at λ = 20 and the results are not sensitive to temperature. The EOD coefficient at λ = 5 is within the range of experimental values, confirming that the model can capture the vehicular transport of protons well. At λ = 20, due to the absence of proton hopping in the model, the EOD coefficient is overestimated by a factor of 3 compared to experimental values. To analyze the interactions between water and Nafion, the partial molar enthalpies and partial molar volumes of water are computed from molecular dynamics simulations. At different water uptakes, multiple linear regression is used on raw simulation data within a narrow composition range of water inside the Nafion membrane. The partial molar volumes and partial molar excess enthalpies of water asymptotically approach the molar volumes and molar excess enthalpies of pure water for water uptakes above 5. This confirms the model can capture the bulklike behavior of water in the Nafion at high water uptakes.

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.

Phosphinate MOFs Formed from Tetratopic Ligands as Proton-Conductive Materials

Metal-organic frameworks (MOFs) are attracting attention as potential proton conductors. There are two main advantages of MOFs in this application: the possibility of rational design and tuning of the properties and clear conduction pathways given by their crystalline structure. We hereby present two new MOF structures, ICR-10 and ICR-11, based on tetratopic phosphinate ligands. The structures of both MOFs were determined by 3D electron diffraction. They both crystallize in the P3̅ space group and contain arrays of parallel linear pores lined with hydrophilic noncoordinated phosphinate groups. This, together with the adsorbed water molecules, facilitates proton transfer via the Grotthuss mechanism, leading to a proton conductivity of up to 4.26 × 10^-4 S cm^-1 for ICR-11. The presented study demonstrates the high potential of phosphinate MOFs for the fabrication of proton conductors.

Role of nanoparticle size and surface chemistry on ion transport and nanostructure of perfluorosulfonic acid ionomer nanocomposites

Herein, we present a systematic investigation of the impact of silica nanoparticle (SiNP) size and surface chemistry on the nanoparticle dispersion state and the resulting morphology and vanadium ion permeability of the composite ionomer membranes. Specifically, Nafion containing a mass fraction of 5% silica particles, ranging in nominal diameters from 10 nm to >1 μm and with both sulfonic acid- and amine-functionalized surfaces, was fabricated. Most notably, an 80% reduction in vanadium ion permeability was observed for ionomer membranes containing amine-functionalized SiNPs at a nominal diameter of 200 nm. Further, these membranes exhibited an almost 400% increase in proton selectivity when compared to pristine Nafion. Trends in vanadium ion permeability within a particular nominal diameter were seen to be a function of the surface chemistry, where, for example, vanadyl ion permeability was observed to increase with increasing particle size for membranes containing unfunctionalized SiNPs, while it was seen to remain relatively constant for membranes containing amine-functionalized SiNPs. In general, the silica particles tended to exhibit a higher extent of aggregation as the size of the particles was increased. From small-angle neutron scattering experiments, an increase in the spacing of the hydrophobic domains was observed for all composite membranes, though particle size and surface chemistry were seen to have varying impacts on the spacing of the ionic domains of the ionomer.

Adhesive lift method for patterning arbitrary-shaped thin ion-selective films in micro/nanofluidic device

Micro/nanofluidic platforms with nanoporous films have been utilized as research tools for studying electrokinetic phenomena occurring not only in macro-scale systems such as electro-desalination but also in micro-scale systems such as bio-molecular preconcentrators. However, due to the limitations of fabrication techniques, studies with nanoporous films are mainly limited to vary the physicochemical properties of the films such as surface charge and pore size, despite the enormous effect of the membrane morphology on the phenomena that is to be expected. Therefore, we propose an economic and feasible nanofabrication method called the "adhesive lift method" for patterning thin arbitrarily-shaped nanoporous film to integrate it into micro/nanofluidic platforms. The conformal patterning of the nanoporous films (Nafion or poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) in this work) was accomplished with spin coating, oxygen plasma treatment and the "adhesive lift technique". Using the fabricated platforms, the initiation of ion concentration polarization along the film with various shapes was demonstrated. In particular, various electrokinetic characteristics of overlimiting conductance depending on the length scale of the microchannels were successfully demonstrated. Therefore, the presented adhesive lift method would provide platforms which can nearly mimic practical macro-scale fluidic systems so that the method would be very useful for studying various electrokinetic phenomena inside it.

Ion and Molecular Transport in Solid Electrolytes Studied by NMR

NMR is the method of choice for molecular and ionic structures and dynamics investigations. The present review is devoted to solvation and mobilities in solid electrolytes, such as ion-exchange membranes and composite materials, based on cesium acid sulfates and phosphates. The applications of high-resolution NMR, solid-state NMR, NMR relaxation, and pulsed field gradient ¹H, ⁷Li, ¹³C, ¹⁹F, ²³Na, ³¹P, and ¹³³Cs NMR techniques are discussed. The main attention is paid to the transport channel morphology, ionic hydration, charge group and mobile ion interaction, and translation ions and solvent mobilities in different spatial scales. Self-diffusion coefficients of protons and Li⁺, Na⁺, and Cs⁺ cations are compared with the ionic conductivity data. The microscopic ionic transfer mechanism is discussed.

High-Resolution Ion-Flux Imaging of Proton Transport through Graphene|Nafion Membranes

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This intrinsic proton conductivity of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions, and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) graphene|Nafion membranes using an "electrochemical ion (proton) pump cell" mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes, etc.) and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20-60 localized sites across a ≈0.003 mm² area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites "opening" and a small number of sites "closing" under an applied electric field on the seconds time scale. Applying a simple equivalent circuit model of ion transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer scale, implying that rare atomic defects are responsible for proton conductance. Overall, this work reinforces SECCM as a premier tool for the structure-property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, for example in diagnosing the failure mechanisms of protective surface coatings.

Supramolecular Proton Conductors Self-Assembled by Organic Cages

Proton conduction is vital for living systems to execute various physiological activities. The understanding of its mechanism is also essential for the development of state-of-the-art applications, including fuel-cell technology. We herein present a bottom-up strategy, that is, the self-assembly of Cage-1 and -2 with an identical chemical composition but distinct structural features to provide two different supramolecular conductors that are ideal for the mechanistic study. Cage-1 with a larger cavity size and more H-bonding anchors self-assembled into a crystalline phase with more proton hopping pathways formed by H-bonding networks, where the proton conduction proceeded via the Grotthuss mechanism. Small cavity-sized Cage-2 with less H-bonding anchors formed the crystalline phase with loose channels filled with discrete H-bonding clusters, therefore allowing for the translational diffusion of protons, that is, vehicle mechanism. As a result, the former exhibited a proton conductivity of 1.59 × 10^-4 S/cm at 303 K under a relative humidity of 48%, approximately 200-fold higher compared to that of the latter. Ab initio molecular dynamics simulations revealed distinct H-bonding dynamics in Cage-1 and -2, which provided further insights into potential proton diffusion mechanisms. This work therefore provides valuable guidelines for the rational design and search of novel proton-conducting materials.

Preparation and Properties of Sulfonated Poly(phthalazinone ether ketone) Membranes for Electrodialysis

Sulfonated poly(phthalazinone ether ketones) (SPPEK) with ion exchange capacities from 0.77 to 1.82 mmol·g⁻¹ are synthesized via an electrophilic substitution reaction. Nuclear magnetic resonance and infrared absorption spectroscopy are used to characterize the chemical structure of the obtained polymers for confirming the successful introduction of sulfonic groups. SPPEKs show excellent thermal stability; their temperature required to achieve 5% weight loss is about 360 °C. Accordingly, the obtained membranes possess high ion perm-selectivity, proton conductivity, and low area resistance. Regarding the electrodialysis-related performance of the membranes, the SPPEK-4 membrane has the highest limiting current density (39.8 mA·cm²), resulting from its high content of sulfonic groups. In a desalination test of standard solution, SPPEK-3 and SPPEK-4 membranes exhibit both better salt removal rate and acceptable energy consumption than commercial membrane. Additionally, SPPEK-3 membrane shows outstanding performance in terms of high concentration rate and low energy consumption during saline water treatment, which indicates the feasibility of novel membranes in electrodialysis application.

Dynamic Monte Carlo calculation generating particle trajectories that satisfy the diffusion equation for heterogeneous systems with a position-dependent diffusion coefficient and free energy

A series of new Monte Carlo (MC) transition probabilities was investigated that could produce molecular trajectories statistically satisfying the diffusion equation with a position-dependent diffusion coefficient and potential energy. The MC trajectories were compared with the numerical solution of the diffusion equation by calculating the time evolution of the probability distribution and the mean first passage time, which exhibited excellent agreement. The method is powerful when investigating, for example, the long-distance and long-time global transportation of a molecule in heterogeneous systems by coarse-graining them into one-particle diffusive molecular motion with a position-dependent diffusion coefficient and free energy. The method can also be applied to many-particle dynamics.

High performance poly(isatin alkyl-terphenyl)s proton exchange membranes with flexible alkylsulfonated side groups

Hydrocarbon-based polymer proton exchange membranes (PEMs) free of heteroatom linkages are supposed to be an attractive alternative for the most advanced perfluorosulfonic acid PEMs, but it is challenging to synthesize them. Here we disclosed a series of aliphatic chain-containing poly(isatin diphenyl-co-terphenyl)(PIDT) copolymers, which were conveniently prepared by superacid-catalyzed Friedel-Crafts polycondensation. Subsequently, the sulfonated copolymer (SPIDT) membranes were prepared by the grafting of side-chain sulfonic acid groups. Due to the formed continuous and efficient nanoscale proton transport channel, these PEMs exhibited excellent proton conductivity showing 186 mS/cm at 80°C, higher than Nafion115 (150 mS/cm). Meanwhile, the prepared membranes exhibited good oxidative stability. The residual weight of the membranes is still greater than 98 wt % after 1 h immersion in Fenton’s reagent at 80°C. Notably, the direct borohydride-hydrogen fuel cell (DBHFC) equipped with SPIDT-50 membrane as the diaphragm showed the peak power density of 71 mW•cm−2 at 25°C, which was greater than that of Nafion115 (63 mW•cm−2). Therefore, the hydrocarbon-based PEMs prepared in this study show promise for application in fuel cells.

A Short Overview of Biological Fuel Cells

This short review summarizes the improvements on biological fuel cells (BioFCs) with or without ionomer separation membrane. After a general introduction about the main challenges of modern energy management, BioFCs are presented including microbial fuel cells (MFCs) and enzymatic fuel cells (EFCs). The benefits of BioFCs include the capability to derive energy from waste-water and organic matter, the possibility to use bacteria or enzymes to replace expensive catalysts such as platinum, the high selectivity of the electrode reactions that allow working with less complicated systems, without the need for high purification, and the lower environmental impact. In comparison with classical FCs and given their lower electrochemical performances, BioFCs have, up to now, only found niche applications with low power needs, but they could become a green solution in the perspective of sustainable development and the circular economy. Ion exchange membranes for utilization in BioFCs are discussed in the final section of the review: they include perfluorinated proton exchange membranes but also aromatic polymers grafted with proton or anion exchange groups.

Electrochemical CO2 reduction - The macroscopic world of electrode design, reactor concepts & economic aspects

For the efficient electrochemical conversion of CO2 into valuable chemical feedstocks, a well-coordinated interaction of all electrolyzer compartments is required. In addition to the catalyst, whose role is described in detail in the part "Electrochemical CO2 Reduction toward Multicarbon Alcohols - The Microscopic World of Catalysts & Process Conditions" of this divided review, the general cell setups, design and manufacture of the electrodes, membranes used, and process parameters must be optimally matched. The authors' goal is to provide a comprehensive review of the current literature on how these aspects affect the overall performance of CO2 electrolysis. To be economically competitive as an overall process, the framework conditions, i.e., CO2 supply and reaction product treatment must also be considered. If the key indicators for current density, selectivity, cell voltage, and lifetime of a CO2 electrolyzer mentioned in the techno-economic consideration of this review are met, electrochemical CO2 reduction can make a valuable contribution to the creation of closed carbon cycles and to a sustainable energy economy.