Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers

This paper presents an overview of the synthesis, chemical and electrochemical properties, and polymer electrolyte fuel cell applications of new proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Due to their chemical stability, high degree of proton conductivity, and remarkable mechanical properties, perfluorinated polymer electrolytes such as Nafion^®, Aciplex^®, Flemion^®, and Dow membranes are some of the most promising electrolyte membranes for polymer electrolyte fuel cells. A number of reviews on the synthesis, electrochemical properties, and fuel cell applications of perfluorinated polymer electrolytes have also appeared during this period. While perfluorinated polymer electrolytes have satisfactory properties for a successful fuel cell electrolyte membrane, the major drawbacks to large-scale commercial use involve cost and low proton-conductivities at high temperatures and low humidities. Presently, one of the most promising ways to obtain high performance proton-conducting polymer electrolyte membranes is the use of hydrocarbon polymers for the polymer backbone. The present review attempts for the first time to summarize the synthesis, chemical and electrochemical properties, and fuel cell applications of new proton-conducting polymer electrolytes based on hydrocarbon polymers that have been made during the past decade.     

Air-breathing microbial fuel cell with enhanced performance using nanocomposite proton exchange membranes

This work aims to improve the performance of air-breathing microbial fuel cells (MFCs) through using hydrocarbon polymer based nanocomposite proton exchange membranes. Accordingly, nanocomposite membranes based on sulfonated poly(ether ether ketone) (SPEEK) and montmorillonite (MMT) were investigated for such an application. Although the incorporation of MMT into SPEEK membranes resulted in reduced oxygen permeability as well as proton conductivity, but the overall selectivity was found to be improved. MFC tests revealed that using the optimized nanocomposite membrane (SPEEK-70/MMT-3 wt%) results in a considerably higher open circuit voltage (OCV) compared to the corresponding neat membrane. Moreover, it was found that the SPEEK-70/MMT-3 wt% membrane is able to provide about 40% more power output than Nafion^®117. On the account of high proton conductivity, low oxygen permeability, high electrochemical performance, ease of preparation and low cost, hydrocarbon based nanocomposite PEMs could be considered as promising electrolytes to enhance the performance of MFCs.     

A synthesis study of phosphonated PSEBS for high temperature proton exchange membrane fuel cells

A series of novel phosphonated proton exchange membranes has been prepared using poly(styrene‐ethylene/butylene‐styrene) block copolymer (PSEBS) as base material. Phosphonic acid functionalization of the polymer was performed by a simple two‐step process, via chloromethylation of PSEBS followed by phosphonation utilizing the Michaels–Arbuzov reaction. The successful phosphonation of the polymers were characterized by NMR and Fourier transform infrared. The phosphonated ester form of the membranes were obtained by solvent evaporation method and hydrolyzed to get a proton conducting membrane. The membrane properties such as ion exchange capacity, water uptake and proton conductivity at various temperatures were examined for their suitability to be utilized as a high temperature polymer electrolyte. Additionally, the morphology, thermal, and mechanical properties of the synthesized membranes were investigated, using scanning electron microscope, thermogravimetric analysis, and tensile test, respectively. The effective (anhydrous) proton conductivity was studied with respect to various degrees of functionalization. From the studies, the membranes were found to have a comparatively good conductivity and one of the membranes reached the maximum value of 5.81 mS/cm² at 140 °C as measured by impedance analyzer. It was found that the synthesized membranes were mechanically durable, chemically, and thermally stable. Hence, the synthesized phosphonated membranes could be a promising candidate for high temperature polymer electrolyte fuel cell applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45954.     

Recycling and regeneration of used perfluorosulfonic membranes for polymer electrolyte fuel cells

This paper describes a method for the recycling and regeneration of used perfluorosulfonic Nafion^® (Dupont) membranes by dissolution and recasting. The dissolution of the used Nafion^® membranes from polymer electrolyte fuel cells is realized using dimethyl sulfoxide as a solvent under atmospheric pressure and 190 °C. A mechanically robust membrane can be reproduced by a recast process of the dissolved Nafion^® solution at 170 °C. The recycled membrane has shown a good crystalline structure and high mechanical strength. Membrane properties, including water uptake, exchange capacity and resistance are similar to that of the as-received Nafion^® 115 membrane. Fuel cells prepared by the recycled membrane demonstrate a comparable performance to that of the fresh fuel cell.     

Electrochemical characterization of radiation-grafted ion-exchange membranes based on different matrix polymers

Seven proton conducting membranes based on different commercial fluoropolymer films were prepared by radiation grafting with styrene followed by sulfonation. These membranes were studied as candidates for fuel cell electrolyte membranes and compared to Nafion^® 105 and 117 with respect to conductivity, oxygen and hydrogen permeability, kinetics of the oxygen reduction reaction (ORR) and performance in a fuel cell. The dependence of the conductivity of the membranes on the relative humidity (RH) and temperature was also determined. The conductivity was observed to depend on the membrane thickness and the water uptake. The dependence of the conductivity on the temperature and the RH was the same for all of the experimental membranes. Reactant gas permeabilities appeared to depend only slightly on the matrix material and no major differences in the Tafel slopes and exchange current densities of the ORR were observed. Membranes with high water uptakes appeared to be less durable in the fuel cell than membranes with low water uptakes. Thus to prepare a membrane that is durable under the fuel cell conditions, the water uptake must remain low even at the expense of the conductivity.     

Multiblock sulfonated-fluorinated poly(arylene ether)s for a proton exchange membrane fuel cell

New proton exchange membranes were prepared and evaluated as polymer electrolytes for a proton exchange membrane fuel cell (PEMFC). Sulfonated-fluorinated poly(arylene ether) multiblocks (MBs) were synthesized by nucleophilic aromatic substitution of highly activated fluorine terminated telechelics made from decafluorobiphenyl with 4,4′-(hexafluoroisopropylidene)diphenol and hydroxyl-terminated telechelics made from 4,4′-biphenol and 3,3′-disulfonated-4,4′-dichlorodiphenylsulfone. Membranes with various sulfonation levels were successfully cast from N-methyl-2-pyrrolidinone. An increase sulfonated block size in the copolymer resulted in enhanced membrane ion exchange capacity and proton conductivity. The morphological structure of MB copolymers was investigated by tapping mode atomic force microscopy (TM-AFM) and compared with those of Nafion^® and sulfonated poly(arylene ether) random copolymers. AFM images of MBs revealed a very well defined phase separation, which may explain their higher proton conductivities compared to the random copolymers. The results are of particular interest for hydrogen/air fuel cells where conductivity at high temperature and low relative humidity is a critical issue.     

Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs)

This review summarizes efforts in developing sulfonated hydrocarbon proton exchange membranes (PEMs) with excellent long-term electrochemical fuel cell performance in medium-temperature and/or low-humidity proton exchange membrane fuel cell (PEMFC) applications. Sulfonated hydrocarbon PEMs are alternatives to commercially available perfluorosulfonic acid ionomers (PFSA, e.g., Nafion^®) that inevitably lose proton conductivity when exposed to harsh operating conditions. Over the past few decades, a variety of approaches have been suggested to optimize polymer architectures and define post-synthesis treatments in order to further improve the properties of a specific material. Strategies for copolymer syntheses are summarized and future challenges are identified. Research pertaining to the sulfonation process, which is carried out in the initial hydrocarbon PEM fabrication stages, is first introduced. Recent synthetic approaches are then presented, focusing on the polymer design to enhance PEM performance, such as high proton conductivity even with a low ion exchange capacity (IEC) and high dimensional stability. Polymer chemistry methods for the physico-chemical tuning of sulfonated PEMs are also discussed within the framework of maximizing the electrochemical performance of copolymers in membrane-electrode assemblies (MEAs). The discussion will cover crosslinking, surface fluorination, thermal annealing, and organic–inorganic nanocomposite approaches.     

Sulphonated Poly(ether ether ketone) Proton Exchange Membranes for Fuel Cell Applications

Polymer electrolyte membrane fuel cells (PEMFCs) are promising new power sources for automotive and portable devices. Nafion® is the currently used membrane in PEMFCs. Although these membranes show high proton conductivity and excellent chemical stability, their high cost makes them unpractical for commercial purposes. Sulphonated poly(ether ether ketone) (SPEEK) ionomers were synthesized using chlorosulphonic acid as the sulphonating agent in dichloromethane medium. Homogeneous proton-conducting membranes were developed from the obtained SPEEK by solvent casting method. Membranes were assessed for their suitability in fuel cell applications. The extent of sulphonation was controlled by varying the reaction time, concentration of polymer, and concentration of sulphonating agent. The SPEEK membranes exhibit degree of sulphonation from 10 to 66%, ion exchange capacity from 0.29 to 1.92 meq/g and maximum water and methanol uptake up to 54 and 22%, respectively, at 25°C. The membranes were characterized by FTIR to confirm sulphonation, and DSC and TGA to investigate the thermal stability. The proton conductivities of such membranes were found to be excellent in the order of 10^?2 S/cm in the fully hydrated condition at room temperature as measured by impedance spectroscopy. The durability of the membranes was also tested. The study revealed the possibility of a cheaper alternative membrane for use in PEMFC.     

Sulfonated aromatic hydrocarbon polymers as proton exchange membranes for fuel cells

This article reviews recent studies on proton exchange membrane (PEM) materials for polymer electrolyte fuel cells. In particular, it focuses on the development of novel sulfonated aromatic hydrocarbon polymers for PEMs as alternatives to conventional perfluorinated polymers. It is necessary to improve proton conductivity especially under low-humidity conditions at high operating temperatures to breakthrough the current aromatic PEM system. Capable strategies involve the formation of well-connected proton channels by microphase separation between hydrophilic and hydrophobic domains and the increase of the ion exchange capacity of PEMs while keeping water resistance. Herein, we introduce novel molecular designs of sulfonated aromatic hydrocarbon polymers and their performance as PEMs.     

Proton conducting membranes based on semi-interpenetrating polymer network of Nafion and polybenzimidazole

A new strategy to prepare the reinforced composite membranes for polymer electrolyte membrane fuel cells (PEMFCs), which can work both in humidified and anhydrous state, was proposed via constructing semi-interpenetrating polymer network (semi-IPN) structure from polybenzimidazole (PBI) and Nafion^®212, with N-vinylimidazole as the crosslinker. The crosslinkable PBI was synthesized from poly(2,2′-(m-phenylene)-5,5′-bibenzimidazole) and p-vinylbenzyl chloride. The semi-IPN structure was formed during the membrane preparation. The composite membranes exhibit excellent thermal stability, high-dimensional stability, and significantly improved mechanical properties compared with Nafion^®212. The proton transport in the hydrated composite membranes is mainly contributed by the vehicle mechanism, with proton conductivity from ∼10⁻² S/cm to ∼10⁻¹ S/cm. When the temperature exceeds 100 °C, the proton conductivity of the semi-IPN membranes decreases quickly due to the dehydration of the membranes. Under anhydrous condition, the proton conductivity of the membranes will drop to ∼10⁻⁴ S/cm, which is also useful for intermediate temperature (100-200 °C) PEMFCs. The benzimidazole structure of PBI and the acidic component of Nafion^® provide the possibility for the proton mobility via structure diffusion involving proton transfer between the heterocycles with a corresponding reorganization of the hydrogen bonded network.     

Characterization of proton exchange membranes based on SPSEBS/SPSU blends

Proton-conducting polymer membranes are used as an electrolytes in proton exchange membrane fuel cells (PEMFCs). The most widely used commercially available membrane electrolytes are perfluorosulfonic acid polymers, an expensive class of ionomers. In this study, the potential of polymer blends derived from sulfonated polystyrene ethylene butylene polystyrene (SPSEBS) and sulfonated polysulfone (SPSU) for use in electrolyte applications was examined. Although SPSEBS by itself exhibits good conductivity, flexibility, and chemical stability, it has poor mechanical stability. So, in an effort to improve the mechanical properties of SPSEBS while maintaining its good conductivity, it was blended with SPSU. SPSEBS/SPSU blends were therefore prepared by a solvent evaporation method, and the resulting blend membranes were characterized in terms of conductivity, ionic exchange capacity, and water uptake. Sulfonation was confirmed and the crystallinity of the blend membranes was studied by FTIR spectroscopy and X-ray diffraction. The morphologies of the membranes were studied by scanning electron microscope (SEM), and their thermal stabilities by TGA and DSC. Finally, the mechanical strength of SPSEBS was studied using a UTM (universal testing machine). This paper presents the results of recent investigations aimed at developing an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Easy steps were chosen for this preparation technique in order to simplify the method, thus minimizing costs. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of the hydrogen fuel cell was studied by performing single cell measurements during H₂/O₂ operation. Carbon cloth was used as a gas diffusion layer (GDL), and the composition of the electrode ink was optimized to maximize fuel cell performance. A commercial E-TEK catalyst was used for the anode and cathode, with Pt loadings of 0.125 and 0.37 mg/cm², respectively. The MEA with the best performance delivered approximately 0.50 W/cm² at room temperature. The methanol permeability and the selectivity ratio strongly influenced DMFC performance. Both direct methanol fuel cells (DMFCs) and PEMFCs are discussed in this paper.     

Examination of the fundamental relation between ionic transport and segmental relaxation in polymer electrolytes

Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. Despite significant advantages for applications in electrochemical devices, the use of solid polymer electrolytes is strongly limited by their poor ionic conductivity. The classical theory predicts that the ionic transport is dictated by the segmental motion of the polymer matrix. As a result, the low mobility of polymer segments is often regarded as the limiting factor for development of polymers with sufficiently high ionic conductivity. Here, we show that the ionic conductivity in many polymers can be strongly decoupled from their segmental dynamics, in terms of both temperature dependence and relative transport rate. Based on this principle, we developed several polymers with “superionic” conductivity. The observed fast ion transport suggests a fundamental difference between the ionic transport mechanisms in polymers and small molecules and provides a new paradigm for design of highly conductive polymer electrolytes.     

Preparation of proton exchange membranes based on crosslinked polytetrafluoroethylene for fuel cell applications

We prepared proton exchange membranes by the γ-ray-induced post grafting of styrene into crosslinked polytetrafluoroethylene (PTFE) films and subsequent sulfonation. The degree of grafting was controlled in the range of 7-75% by the crosslinking density of the PTFE matrix as well as the grafting conditions. Under our preparation conditions, the films at the grafting yield of ≥30% were found to produce ion exchange membranes with a homogeneous distribution of sulfonic acid groups. The resulting membranes showed a large ion exchange capacity up to 2.9 meq g⁻¹, which exceeded the performance of commercially available perfluorosulfonic acid films such as Nafion; nevertheless, they appeared to be dimensionally stable in water. These should undoubtedly result from the use of the crosslinked PTFE films as graft substrates and make our ion exchange membranes promising for applications to polymer electrolyte fuel cells.     

High-throughput screening of ionic conductivity in polymer membranes

Combinatorial and high-throughput techniques have been successfully used for efficient and rapid property screening in multiple fields. The use of these techniques can be an advantageous new approach to assay ionic conductivity and accelerate the development of novel materials in research areas such as fuel cells. A high-throughput ionic conductivity (HTC) apparatus is described and applied to screening candidate polymer electrolyte membranes for fuel cell applications. The device uses a miniature four-point probe for rapid, automated point-to-point AC electrochemical impedance measurements in both liquid and humid air environments. The conductivity of Nafion^® 112 HTC validation standards was within 1.8% of the manufacturer's specification. HTC screening of 40 novel Kynar^® poly(vinylidene fluoride) (PVDF)/acrylic polyelectrolyte (PE) membranes focused on varying the Kynar^® type (5×) and PE composition (8×) using reduced sample sizes. Two factors were found to be significant in determining the proton conducting capacity: (1) Kynar^® PVDF series: membranes containing a particular Kynar^® PVDF type exhibited statistically identical mean conductivity as other membranes containing different Kynar^® PVDF types that belong to the same series or family. (2) Maximum effective amount of polyelectrolyte: increments in polyelectrolyte content from 55 wt% to 60 wt% showed no statistically significant effect in increasing conductivity. In fact, some membranes experienced a reduction in conductivity.     

Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for efficient power generation in the 21st century. Currently, high temperature proton exchange membrane fuel cells (HT-PEMFC) offer several advantages, such as high proton conductivity, low permeability to fuel, low electro-osmotic drag coefficient, good chemical/thermal stability, good mechanical properties and low cost. Owing to the aforementioned features, high temperature proton exchange membrane fuel cells have been utilized more widely compared to low temperature proton exchange membrane fuel cells, which contain certain limitations, such as carbon monoxide poisoning, heat management, water leaching, etc. This review examines the inspiration for HT-PEMFC development, the technological constraints, and recent advances. Various classes of polymers, such as sulfonated hydrocarbon polymers, acid-base polymers and blend polymers, have been analyzed to fulfill the key requirements of high temperature operation of proton exchange membrane fuel cells (PEMFC). The effect of inorganic additives on the performance of HT-PEMFC has been scrutinized. A detailed discussion of the synthesis of polymer, membrane fabrication and physicochemical characterizations is provided. The proton conductivity and cell performance of the polymeric membranes can be improved by high temperature treatment. The mechanical and water retention properties have shown significant improvement., However, there is scope for further research from the perspective of achieving improvements in certain areas, such as optimizing the thermal and chemical stability of the polymer, acid management, and the integral interface between the electrode and membrane.     

Lithium ion transport through nonaqueous perfluoroionomeric membranes

The transport properties of lithiated perfluorinated ionomers imbibed with nonaqueous solvents and solvent mixtures were studied. Polymeric ion‐exchange membranes have potential use in the next generation single‐ion secondary lithium polymer batteries, where the lithiated form of the membrane is used as a polymer electrolyte. The novelty of the approach for lithium battery applications lies in the advantage offered by a transference number of unity, no additional salt (e.g., LiPF₆) is needed, and the excellent physical and chemical stability of the fluoropolymers. Ion‐exchange membranes were converted to the Li⁺ salt form and analyzed for total conversion using FT‐IR. Nonaqueous solvents and solvent mixtures were imbibed into the membranes in a glove box, and the uptake was measured over time. A four‐point probe was used to determine the ionic conductivity based on impedance measurements performed over a frequency range of 10 to 35,000 Hz. Conductivities exceeding 10^?4 S/cm with transference numbers of unity were achieved making these ionomeric membranes potentially useful in rechargeable lithium polymer batteries.     

Structure-property-performance relationships of sulfonated poly(arylene ether sulfone)s as a polymer electrolyte for fuel cell applications

This article focuses on structure-property-performance relationships of directly copolymerized sulfonated polysulfone polymer electrolyte membranes. The chemical structure of the bisphenol-based disulfonated polysulfones was systematically alternated by introducing fluorine moieties or other polar functional groups such as benzonitrile or phenyl phosphine oxide in the copolymer backbone. Ac impedance measurements of the polymer electrolyte membranes indicated that fluorine incorporation increased proton conductivity, while polar functional group incorporation decreased conductivity. Likewise, other properties such as water uptake and ion exchange capacity are impacted by the incorporation of fluorine moiety or polar groups. These properties are critically tied with H₂/air and direct methanol fuel cell performance. We have rationalized fuel cell performance of these selected copolymers in light of structure-property relationships, which gives useful insight for the development and application of next generation polymer electrolytes.     

聚合物基镁离子固态电解质的研究进展Q

镁离子电池因其比容量高、资源丰富、环境友好、安全性高(无枝晶)等优势,在储能电池领域脱颖而出.然而,镁金属负极在液态电解质中易钝化,导致其电化学性能不佳.因此,开发高效适用的固态电解质对实现高性能、实用化镁离子电池至关重要.聚合物电解质具有优异的机械稳定性、电化学稳定性、热稳定性且离子电导率高、成本低.但镁离子较高的电荷密度和较强的溶剂化作用限制了其在固态电解质中的解离与扩散.从纯固态聚合物电解质、凝胶聚合物电解质、复合聚合物电解质3个方面综述了国内外聚合物基镁离子固态电解质的离子电导率对解决镁金属负极钝化效应的贡献及其应用研究进展,指出聚合物基镁离子固态电解质当前面临的挑战并对其研究方向进行了建议和展望.     

A study on sulfonated poly(arylene ether sulfone) membranes containing two different types of SiO2 for a high temperature and low-humidified polymer electrolyte fuel cell

Two different types of silica oxide were prepared as filler in sulfonated polymers for fuel cell applications operated under water deficient environment. SiO₂ nanoparticle and thiol-embedded SiO₂ nanoparticles were mechanically mixed with sulfonated (arylene ether sulfone) solutions, and then the mixtures were cast to prepare composite membranes. The composite membranes with different amount of SiO₂ were prepared to investigate the effect of two types of SiO₂ nanoparticles on ionic conductivity with relative humidity at 120 °C. In addition, ion exchange capacity, water uptake, thermogravitational analysis, differential scanning calorimetry were studied. As results, the composite membranes containing thiol-embedded SiO₂ showed better water-channel forming ability at low relative humidity less than 50% in this study. Under full hydration of the composite membranes, the composite membranes containing pure SiO₂ nano-particles have higher ionic conductivity since the thiol-embedded SiO₂ might cause steric hindrance to make water channel well connected. Thus, below 50% relative humidity, the composite membranes containing 10 wt% of thiol-embedded SiO₂ showed the best ionic conductivity. It is very promising for polymer electrolyte fuel cells operated normally under 50% relative humidity at cathode.     

Novel polysulfone/sulfonated polyaniline/niobium pentoxide polymer blend nanocomposite membranes for fuel cell applications

New series of polymer nanocomposite membranes were prepared from polysulfone (PSU), sulfonated polyaniline (SPANI) and niobium pentoxide (Nb₂O₅) by solution casting technique. In order to assess the suitability of the polymer electrolytes in fuel cell applications, the membranes were characterized with respect to their physicochemical properties. Scanning electron microscope, X-ray diffraction, and X-ray photoelectron spectroscopy data confirmed the successful incorporation of nanofillers into the polymer matrix. The membrane loaded with 10 wt% of niobium pentoxide into PSU/SPANI exhibited a proton conductivity of 0.0674 S cm⁻¹, whereas the control membrane showed 0.0110 S cm⁻¹. The incorporation of niobium pentoxide into pristine polymer not only improved the ionic conductivity but also enhanced the thermal and oxidative stabilities. The substantial results achieved with the organic–inorganic polymer composites derived from PSU-SPANI and Nb₂O₅ have been established and can be viable materials for electrolyte in fuel cell applications.