Constructing Nanoporous Ir/Ta2 O5 Interfaces on Metallic Glass for Durable Acidic Water Oxidation.

Although proton exchange membrane water electrolyzers (PEMWE) are considered as a promising technique for green hydrogen production, it remains crucial to develop intrinsically effective oxygen evolution reaction (OER) electrocatalysts with high activity and durability. Here, a flexible self-supporting electrode with nanoporous Ir/Ta2O5 electroactive surface is reported for acidic OER via dealloying IrTaCoB metallic glass ribbons. The catalyst exhibits excellent electrocatalytic OER performance with an overpotential of 218 mV for a current density of 10 mA cm-2 and a small Tafel slope of 46.1 mV dec-1 in acidic media, superior to most electrocatalysts. More impressively, the assembled PEMWE with nanoporous Ir/Ta2 O5 as an anode shows exceptional performance of electrocatalytic hydrogen production and can operate steadily for 260 h at 100 mA cm-2 . In situ spectroscopy characterizations and density functional theory calculations reveal that the modest adsorption of OOH* intermediates to active Ir sites lower the OER energy barrier, while the electron donation behavior of Ta2 O5 to stabilize the high-valence states of Ir during the OER process extended catalyst's durability.

Sulfonamide-Sulfonimide Copolymers as Novel, Fluorine-Lean Type of Proton Exchange Membranes for Fuel Cell Application.

In this work, a novel type of fluorine-lean proton exchange membranes is presented, using sulfonamide-sulfonimide functional groups for ion conduction. These groups are constructed on a polystyrene backbone for simple and cost-efficient usage as well as rapid scalability. The polymer is further tailored by adjusting the sulfonamide functionality with various end-groups, namely pentafluorophenyl, 4-fluorophenyl, butyl and octyl groups. These groups affect the pKa, leading to pKa values of 5.7 for the pentafluorophenyl substitution and pKa 10.5 for the alkyl chain. The glass transition temperature of the sulfonamide homopolymers can be reduced from Tg=151 °C (Pentafluorophenyl) to 49 °C (Octyl), making the ionomer more flexible at room temperature. The combination of the non-swelling sulfonamide further mitigates the high water uptake of the sulfonimide while maintaining the nominal ion exchange capacity. This combination leads to extremely high proton conductivities with up to σ=283 mS cm-1 at room temperature, which is clearly outperforming Nafion and approaches values for acid doped systems. This approach can pave the way to a novel type of ion conducting class in proton exchange membrane fuel cells.

The Hybridization of Polymers with Metal Oxide Clusters for the Design of Non-Fluorinated Proton Exchange Membranes.

As the key component of various energy storage and conversion devices, proton exchange membranes (PEMs) have been attracting significant interest. However, their further development is limited by the high cost of perfluorosulfonic acid polymers and the poor stability of acid-dopped non-fluorinated polymers. Recently, a new group of PEMs has been developed by hybridizing polyoxometalates (POMs), a group of super acidic sub-nanoscale metal oxide clusters, with polymers. POMs can serve simultaneously as both proton sponges and stabilizing agents, and their complexation with polymers can further improve polymers' mechanical performance and processability. Enormous efforts have been focused on studying supramolecular complexation or covalent grafting of POMs with various polymers to optimize PEMs in terms of cost, mechanical properties and stabilities. This concept summarizes recent advances in this emerging field and outlines the design strategies and application perspectives employed for using POM-polymer hybrid materials as PEMs.

Structural Characterization and Physicochemical Properties of Functionally Porous Proton-Exchange Membrane Based on PVDF-SPA Graft Copolymers.

Fluorinated proton-exchange membranes (PEMs) based on graft copolymers of dehydrofluorinated polyvinylidene fluoride (D-PVDF), 3-sulfopropyl acrylate (SPA), and 1H, 1H, 2H-perfluoro-1-hexene (PFH) were prepared via free radical copolymerization and characterized for fuel cell application. The membrane morphology and physical properties were studied via small-(SAXS) and wide-angle X-ray scattering (WAXS), SEM, and DSC. It was found that the crystallinity degree is 17% for PEM-RCF (co-polymer with SPA) and 16% for PEM-RCF-2 (copolymer with SPA and PFH). The designed membranes possess crystallite grains of 5-6 nm in diameter. SEM images reveal a structure with open pores on the surface of diameters from 20 to 140 nm. Their transport and electrochemical characterization shows that the lowest membrane area resistance (0.9 Ωcm2) is comparable to perfluorosulfonic acid PEMs (such as Nafion®) and polyvinylidene fluoride (PVDF) based CJMC cation-exchange membranes (ChemJoy Polymer Materials, China). Key transport and physicochemical properties of new and commercial membranes were compared. The PEM-RCF permeability to NaCl diffusion is rather high, which is due to a relatively low concentration of fixed sulfonate groups. Voltammetry confers that the electrochemical behavior of new PEM correlates to that of commercial cation-exchange membranes, while the ionic conductivity reveals an impact of the extended pores, as in track-etched membranes.

Supramolecular Modifying Nafion with Fluoroalkyl­Functionalized Polyoxometalate Nanoclusters for High­Selective Proton Conduction.

Fluoroalkyl-grafted polyoxometalate nanoclusters are used as supramolecular additives to precisely modify the ionic domains of Nafion, which can increase the proton conductivity and selectivity simultaneously. The resulting hybrid membranes show significantly enhanced power density in fuel cells and improved energy efficiency in vanadium flow batteries.

New triazinephosphonate dopants for Nafion proton exchange membranes (PEM).

A new paradigm for energy is underway demanding decarbonized energy systems. Some of them rely on emerging electrochemical devices, crucial in hydrogen technologies, including fuel cells, CO2 and water electrolysers, whose applications and performances depend on key components such as their separators/ion-exchange membranes. The most studied and already commercialized Nafion membrane shows great chemical stability, but its water content limits its high proton conduction to a limited range of operating temperatures. Here, we report the synthesis of a new series of triazinephosphonate derivatives and their use as dopants in the preparation of new modified Nafion membranes. The triazinephosphonate derivatives were prepared by substitution of chlorine atoms in cyanuric chloride. Diverse conditions were used to obtain the trisubstituted (4-hydroxyphenyl)triazinephosphonate derivatives and the (4-aminophenyl)triazinephosphonate derivatives, but with these amino counterparts, only the disubstituted compounds were obtained. The new modified Nafion membranes were prepared by casting incorporation of the synthesized 1,3,5-triazinephosphonate (TPs) derivatives. The evaluation of the proton conduction properties of the new membranes and relative humidity (RH) conditions and at 60 °C, showed that they present higher proton conductivities than the prepared Nafion membrane and similar or better proton conductivities than commercial Nafion N115, in the same experimental conditions. The Nafion-doped membrane with compound TP2 with a 1.0 wt % loading showed the highest proton conductivity with 84 mS·cm-1.

Highly Elastic, Healable, and Durable Anhydrous High-Temperature Proton Exchange Membranes Cross-Linked with Highly Dense Hydrogen Bonds.

Proton exchange membranes (PEMs) with excellent durability and working stability are important for PEM fuel cells with extended service life and enhanced reliability. In this study, highly elastic, healable, and durable electrolyte membranes are fabricated by the complexation of poly(urea-urethane), ionic liquids (ILs), and MXene nanosheets (denoted as PU-IL-MX). The resulting PU-IL-MX electrolyte membranes have a tensile strength of ≈3.86 MPa and a strain at break of ≈281.89%. The PU-IL-MX electrolyte membranes can act as high temperature PEMs to conduct protons under an anhydrous condition of the temperatures above 100 °C. Importantly, the ultrahigh density of hydrogen-bond-cross-linked network renders PU-IL-MX membranes excellent IL retention properties. The membranes can maintain more than ≈98% of their original weight and show no decline of proton conductivity after being placed under highly humid conditions of ≈80 °C and relative humidity of ≈85% for 10 days. Moreover, due to the reversibility of hydrogen bonds, the membranes can heal damage under the working conditions of fuel cells to restore their original mechanical properties, proton conductivities, and cell performances.

Iridium Doped Pyrochlore Ruthenates for Efficient and Durable Electrocatalytic Oxygen Evolution in Acidic Media.

Developing highly active, durable, and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is of prime importance in proton exchange membrane (PEM) water electrolysis techniques. Ru-based catalysts have high activities but always suffer from severe fading and dissolution issues, which cannot satisfy the stability demand of PEM. Herein, a series of iridium-doped yttrium ruthenates pyrochlore catalysts is developed, which exhibit better activity and much higher durability than commercial RuO2 , IrO2 , and most of the reported Ru or Ir-based OER electrocatalysts. Typically, the representative Y2 Ru1.2 Ir0.8 O7 OER catalyst demands a low overpotential of 220 mV to achieve 10 mA cm-2 , which is much lower than that of RuO2 (300 mV) and IrO2 (350 mV). In addition, the catalyst does not show obvious performance decay or structural degradation over a 2000 h stability test. EXAFS and XPS co-prove the reduced valence state of ruthenium and iridium in pyrochlore contributes to the improved activity and stability. Density functional theory reveals that the potential-determining steps barrier of OOH* formation is greatly depressed through the synergy effect of Ir and Ru sites by balancing the d band center and oxygen intermediates binding ability.

Correlated interfacial water transport and proton conductivity in perfluorosulfonic acid membranes.

Water must be effectively transported and is also essential for maximizing proton conductivity within fuel-cell proton-exchange membranes (PEMs). Therefore, identifying relationships between PEM properties, water transport, and proton conductivity is essential for designing optimal PEMs. Here, we use coherent Raman spectroscopy to quantify real-time, in situ diffusivities of water subspecies, bulk-like and nonbulk-like (interfacial) water, in five different perfluorosulfonic acid (PFSA) PEMs. Although the PEMs were chemically diverse, water transport within them followed the same rule: Total water diffusivity could be represented by a linear combination of the bulk-like and interfacial water diffusivities. Moreover, the diffusivity of interfacial water was consistently larger than that of bulk-like water. These measurements of microscopic transport were combined with through-plane proton conductivity measurements to reveal the correlation between interfacial water transport and proton conductivity. Our results demonstrate the importance of maximizing the diffusivity and fractional contribution of interfacial water to maximize the proton conductivity in PFSA PEMs.

Biomimetic N-Doped Graphene Membrane for Proton Exchange Membranes.

Proton exchange membranes (PEMs) with both high selectivity and high permeance are of great demand in hydrogen-based applications, especially in fuel cells. Although graphene membranes have shown high selectivity of protons over other ions and molecules, the relatively low permeance of protons through perfect pristine graphene restricts its practical applications. Inspired by the nitrogen-assisted proton transport in biological systems, we introduced N-doping to increase the proton permeance and proposed a type of N-doped graphene membranes (NGMs) for proton exchange, which have both high proton permeance and high selectivity. Compared to the state-of-the-art commercial PEMs, the NGMs show significant increases in both areal proton conductivity (2-3 orders of magnitude) and selectivity of proton to methanol (1-2 orders of magnitude). The work realized the controllable tuning of proton permeance of the graphene membrane with N-doping and developed a new type of graphene-based PEMs with high performance for practical applications.

Applications of Nanomaterials in Microbial Fuel Cells: A Review.

Microbial fuel cells (MFCs) are an environmentally friendly technology and a source of renewable energy. It is used to generate electrical energy from organic waste using bacteria, which is an effective technology in wastewater treatment. The anode and the cathode electrodes and proton exchange membranes (PEM) are important components affecting the performance and operation of MFC. Conventional materials used in the manufacture of electrodes and membranes are insufficient to improve the efficiency of MFC. The use of nanomaterials in the manufacture of the anode had a prominent effect in improving the performance in terms of increasing the surface area, increasing the transfer of electrons from the anode to the cathode, biocompatibility, and biofilm formation and improving the oxidation reactions of organic waste using bacteria. The use of nanomaterials in the manufacture of the cathode also showed the improvement of cathode reactions or oxygen reduction reactions (ORR). The PEM has a prominent role in separating the anode and the cathode in the MFC, transferring protons from the anode chamber to the cathode chamber while preventing the transfer of oxygen. Nanomaterials have been used in the manufacture of membrane components, which led to improving the chemical and physical properties of the membranes and increasing the transfer rates of protons, thus improving the performance and efficiency of MFC in generating electrical energy and improving wastewater treatment.

Light-Modulated Cationic and Anionic Transport across Protein Biopolymers*.

Light is a convenient source of energy and the heart of light-harvesting natural systems and devices. Here, we show light-modulation of both the chemical nature and ionic charge carrier concentration within a protein-based biopolymer that was covalently functionalized with photoacids or photobases. We explore the capability of the biopolymer-tethered photoacids and photobases to undergo excited-state proton transfer and capture, respectively. Electrical measurements show that both the photoacid- and photobase-functionalized biopolymers exhibit an impressive light-modulated increase in ionic conductivity. Whereas cationic protons are the charge carriers for the photoacid-functionalized biopolymer, water-derived anionic hydroxides are the suggested charge carriers for the photobase-functionalized biopolymer. Our work introduces a versatile toolbox to photomodulate both protons and hydroxides as charge carriers in polymers, which can be of interest for a variety of applications.

Precise Molecular-Level Modification of Nafion with Bismuth Oxide Clusters for High-performance Proton-Exchange Membranes.

Fabricating proton exchange membranes (PEMs) with high ionic conductivity and ideal mechanical robustness through regulation of the membrane microstructures achieved by molecular-level hybridization remains essential but challenging for the further development of high-performance PEM fuel cells. In this work, by precisely hybridizing nano-scaled bismuth oxide clusters into Nafion, we have fabricated the high-performance hybrid membrane, Nafion-Bi12 -3 %, which showed a proton conductivity of 386 mS cm-1 at 80 °C in aqueous solution with low methanol permeability, and conserved the ideal mechanical and chemical stabilities as PEMs. Moreover, molecular dynamics (MD) simulation was employed to clarify the structural properties and the assembly mechanisms of the hybrid membrane on the molecular level. The maximum current density and power density of Nafion-Bi12 -3 % for direct methanol fuel cells reached to 432.7 mA cm-2 and 110.2 mW cm-2 , respectively. This work provides new insights into the design of versatile functional polymer electrolyte membranes through polyoxometalate hybridization.

Non-Fluorinated Polymer Composite Proton Exchange Membranes for Fuel Cell Applications - A Review.

The critical component of a proton exchange membrane fuel cell (PEMFC) system is the proton exchange membrane (PEM). Perfluorosulfonic acid membranes such as Nafion are currently used for PEMFCs in industry, despite suffering from reduced proton conductivity due to dehydration at higher temperatures. However, operating at temperatures below 100 °C leads to cathode flooding, catalyst poisoning by CO, and complex system design with higher cost. Research has concentrated on the membrane material and on preparation methods to achieve high proton conductivity, thermal, mechanical and chemical stability, low fuel crossover and lower cost at high temperatures. Non-fluorinated polymers are a promising alternative. However, improving the efficiency at higher temperatures has necessitated modifications and the inclusion of inorganic materials in a polymer matrix to form a composite membrane can be an approach to reach the target performance, while still reducing costs. This review focuses on recent research in composite PEMs based on non-fluorinated polymers. Various inorganic fillers incorporated in the PEM structure are reviewed in terms of their properties and the effect on PEM fuel cell performance. The most reliable polymers and fillers with potential for high temperature proton exchange membranes (HTPEMs) are also discussed.

Polyoxometalate-Polymer Hybrid Materials as Proton Exchange Membranes for Fuel Cell Applications.

As one of the most efficient pathways to provide clean energy, fuel cells have attracted great attention in both academic and industrial communities. Proton exchange membranes (PEMs) or proton-conducting electrolytes are the key components in fuel cell devices, which require the characteristics of high proton conductivity as well as high mechanical, chemical and thermal stabilities. Organic-inorganic hybrid PEMs can provide a fantastic platform to combine both advantages of two components to meet these demands. Due to their extremely high proton conductivity, good thermal stability and chemical adjustability, polyoxometalates (POMs) are regarded as promising building blocks for hybrid PEMs. In this review, we summarize a number of research works on the progress of POM-polymer hybrid materials and related applications in PEMs. Firstly, a brief background of POMs and their proton-conducting properties are introduced; then, the hybridization strategies of POMs with polymer moieties are discussed from the aspects of both noncovalent and covalent concepts; and finally, we focus on the performance of these hybrid materials in PEMs, especially the advances in the last five years. This review will provide a better understanding of the challenges and perspectives of POM-polymer hybrid PEMs for future fuel cell applications.

Highly Stable, Low Gas Crossover, Proton-Conducting Phenylated Polyphenylenes.

Two classes of novel sulfonated phenylated polyphenylene ionomers are investigated as polyaromatic-based proton exchange membranes. Both types of ionomer possess high ion exchange capacities yet are insoluble in water at elevated temperatures. They exhibit high proton conductivity under both fully hydrated conditions and reduced relative humidity, and are markedly resilient to free radical attack. Fuel cells constructed with membrane-electrode assemblies containing each ionomer membrane yield high in situ proton conductivity and peak power densities that are greater than obtained using Nafion reference membranes. In situ chemical stability accelerated stress tests reveal that this class of the polyaromatic membranes allow significantly lower gas crossover and lower rates of degradation than Nafion benchmark systems. These results point to a promising future for molecularly designed sulfonated phenylated polyphenylenes as proton-conducting media in electrochemical technologies.

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes.

This study describes a novel approach for the in situ synthesis of metal oxide-polyelectrolyte nanocomposites formed via impregnation of hydrated polyelectrolyte films with binary water/alcohol solutions of metal salts and consecutive reactions that convert metal cations into oxide nanoparticles embedded within the polymer matrix. The method is demonstrated drawing on the example of Nafion membranes and a variety of metal oxides with an emphasis placed on zinc oxide. The in situ formation of nanoparticles is controlled by changing the solvent composition and conditions of synthesis that for the first time allows one to tailor not only the size, but also the nanoparticle shape, giving a preference to growth of a particular crystal facet. The high-resolution TEM, SEM/EDX, UV-vis and XRD studies confirmed the homogeneous distribution of crystalline nanoparticles of circa 4 nm and their aggregates of 10-20 nm. The produced nanocomposite films are flexible, mechanically robust and have a potential to be employed in sensing, optoelectronics and catalysis.

Nanoscale Distribution of Sulfonic Acid Groups Determines Structure and Binding of Water in Nafion Membranes.

The connection between the nanoscale structure of two chemically equivalent, yet morphologically distinct Nafion fuel-cell membranes and their macroscopic chemical properties is demonstrated. Quantification of the chemical interactions between water and Nafion reveals that extruded membranes have smaller water channels with a reduced sulfonic acid head group density compared to dispersion-cast membranes. As a result, a disproportionally large amount of non-bulk water molecules exists in extruded membranes, which also exhibit larger proton conductivity and larger water mobility compared to cast membranes. The differences in the physicochemical properties of the membranes, that is, the chemical constitution of the water channels and the local water structure, and the accompanying differences in macroscopic water and proton transport suggest that the chemistry of nanoscale channels is an important, yet largely overlooked parameter that influences the functionality of fuel-cell membranes.

Nanosized IrO(x)-Ir Catalyst with Relevant Activity for Anodes of Proton Exchange Membrane Electrolysis Produced by a Cost-Effective Procedure.

We have developed a highly active nanostructured iridium catalyst for anodes of proton exchange membrane (PEM) electrolysis. Clusters of nanosized crystallites are obtained by reducing surfactant-stabilized IrCl3 in water-free conditions. The catalyst shows a five-fold higher activity towards oxygen evolution reaction (OER) than commercial Ir-black. The improved kinetics of the catalyst are reflected in the high performance of the PEM electrolyzer (1 mg(Ir) cm(-2)), showing an unparalleled low overpotential and negligible degradation. Our results demonstrate that this enhancement cannot be only attributed to increased surface area, but rather to the ligand effect and low coordinate sites resulting in a high turnover frequency (TOF). The catalyst developed herein sets a benchmark and a strategy for the development of ultra-low loading catalyst layers for PEM electrolysis.

Advances in ionogels for proton-exchange membranes.

To ensure the long-term performance of proton-exchange membrane fuel cells (PEMFCs), proton-exchange membranes (PEMs) have stringent requirements at high temperatures and humidities, as they may lose proton carriers. This issue poses a serious challenge to maintaining their proton conductivity and mechanical performance throughout their service life. Ionogels are ionic liquids (ILs) hybridized with another component (such as organic, inorganic, or organic-inorganic hybrid skeleton). This design is used to maintain the desirable properties of ILs (negligible vapor pressure, thermal stability, and non-flammability), as well as a high ionic conductivity and wide electrochemical stability window with low outflow. Ionogels have opened new routes for designing solid-electrolyte membranes, especially PEMs. This paper reviews recent research progress of ionogels in proton-exchange membranes, focusing on their electrochemical properties and proton transport mechanisms.