Cost effective cation exchange membranes: A review

This paper will look at developments of new polymer electrolyte membranes to replace high cost ion exchange membranes such as Nafion^®, Flemion^® and Aciplex^®. These perfluorinated polymer electrolytes are currently the most commercially utilized electrolyte membranes for polymer electrolyte fuel cells, with high chemical stability, proton conductivity and strong mechanical properties. While perfluorinated polymer electrolytes have satisfactory properties for fuel cell applications, they limit commercial use due to significant high costs as well as reduced performance at high temperatures and low humidity. A promising alternative to obtain high performance proton-conducting polymer electrolyte membranes is through the use of hydrocarbon polymers. The need for inexpensive and efficient materials with high thermal and chemical stability, high ionic conductivity, miscibility with other polymers, and good mechanical strength is reviewed in this paper. Though it is difficult to evaluate the true cost of a product based on preliminary research, this paper will examine several of the more promising materials available as low cost alternatives to ion exchange membranes. These alternative membranes represent a new generation of cost effective electrolytes that can be used in various ion exchange systems. This review will cover recent and significant patents regarding low cost polymer electrolytes suitable for ion exchange membrane applications. Promising candidates for commercial applications will be discussed and the future prospects of cost effective membranes will be presented.     

Trends for fuel cell membrane development

Fuel cells are considered a promising energy conversion technology of the future owing to inherent advantages of electrochemical conversion over thermal combustion processes. In the polymer electrolyte fuel cell (PEFC) a proton-conducting polymer membrane is utilized as solid electrolyte, having to allow the transport of protons from anode to cathode yet block the passage of reactants (e.g. H₂, O₂) and electrons. Although PEFC technology has matured substantially over the past two decades, technological barriers, such as insufficient durability and high cost, still delay commercialization in many applications. In this contribution, we review current fuel cell membrane technology and outline approaches that are taken to improve the functionality as well as the chemical and mechanical stability of proton conducting polymers in fuel cells.     

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.     

Proton-conducting alkaline earth hafnates: A review of manufacturing technologies,physicochemical properties and electrochemical performance

Perovskite-type oxides exhibiting oxide-ion and proton conductivity at high temperatures are promising functional materials for electrochemical energy conversion and storage technologies. Alkaline earth hafnates are representatives of the proton-conducting electrolytes and characterized by excellent chemical stability. However, to date, alkaline-earth hafnates have been less studied than other families of perovskites, such as the cerate-, zirconate- and zirconate-cerate-based electrolytes. This article provides a review on the currently known data on alkaline-earth hafnates, including their structure, phase transitions, synthesis technologies, sintering ability, thermal properties, chemical stability, electrical conductivity, and transference numbers. Furthermore, this review highlights the applications of alkaline-earth hafnates as an electrolyte in different types of electrochemical devices.     

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.     

Li+电池固态聚合物电解质研究进展

固态聚合物电解质具有高安全性、高成膜性和黏弹性等优点，并与电极具有良好的接触性和相容性，是实现高安全性和高能量密度固态Li⁺电池的重要电解质体系。然而聚合物电解质室温离子电导率较低（10^-8~10^-6 S·cm^-1），不能满足固态聚合物电池在常温运行的需求。因此，在提高离子电导率、机械强度和电化学稳定性等本征属性的基础上，同时探究改善电解质/电极的界面处及电极内部的离子输运是研发固态聚合物Li⁺电池面临的关键问题。主要从改性聚合物电解质用以提高Li⁺电池电化学性能的角度出发，综述了凝胶聚合物电解质、全固态聚合物电解质和复合固态电解质中的离子输运机制及其关键参数，总结了近年来聚合物电解质的最新研究进展和未来的发展方向。     

Recent progress in barium zirconate proton conductors for electrochemical hydrogen device applications: A review

Electrochemical hydrogen devices like fuel cells are widely investigated as promising technologies to mitigate the rising environmental challenges and enhance the renewable energy economy. In these devices, proton-conducting oxides (PCOs) are applied as electrolyte materials to transport protons. Excellent physical stability and higher proton transport number are two essential properties of electrolyte materials. Doped BaZrO₃ (BZO) is a solid ion-conducting perovskite material with high chemical stability and good proton-conducting properties at an intermediate temperature range of 400–650 °C. Therefore, BZO is an attractive material among the exciting proton-conducting oxides as electrolyte material. To enhance the proton transport properties and improve the material fabrication process of BZO, techniques such as the use of dopants, sintering aid, synthesis methods are crucial. The present review work highlights the applications of BZO as electrolyte material in electrochemical hydrogen devices such as hydrogen isotopes separation systems, hydrogen sensors, hydrogen pumps, and protonic ceramic fuel cells (PCFCs) or solid oxide fuel cells (SOFCs). The central section of this review summarizes the recent research investigations of these applications and provides a comprehensive insight into the various synthesis process, doping, sintering aid, operating environments, and operating condition's impact on the composition, morphology, and performance of BZO electrolyte materials. Based on the reviewed literature, remarks on current challenges and prospects are provided. The presented information on in-depth analysis of the physical properties of barium zirconate electrolyte's along with output performance will guide aspirants in conducting research further on this field.     

A composite electrolyte membrane containing high-content sulfonated carbon spheres for proton exchange membrane fuel cells

Sulfonated carbon spheres (SCS) were employed with perfluorinated ionomers as a binder to make proton-conducting electrolyte membranes for polymer electrolyte membrane fuel cells (PEMFC). Hot-pressing produced a symmetric, thin membrane with SCS particles concentrated in the center of the membrane. Relative to Nafion, the SCS materials showed higher density of sulfonic acid groups and increased water retention capacity of the membrane. This is the favorable condition for effective back diffusion of water from cathode preventing dehydration of membrane. As a result, the SCS membrane showed much better performance in single cell PEMFC operation than Nafion membrane. The membrane also showed much improved tolerance to chemical degradation by oxygen radical species.     

Characteristics of PVdF-HFP/TiO2 composite membrane electrolytes prepared by phase inversion and conventional casting methods

Porous poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP)-based polymer membranes filled with various contents of titania (TiO₂) nanocrystalline particles are prepared by phase inversion technique and, along with conventional casting method for comparison. N-methyl-2-pyrrolidone (NMP) as a solvent is used to dissolve the polymer and to make the slurry with TiO₂. Cast film is obtained by spreading the slurry and evaporating NMP in a dry oven, while phase inversion membrane by promptly immersing the spread slurry into flowing water as a non-solvent. Physical and electrochemical characterizations, such as morphology, thermal and crystalline behavior, and other transport properties of lithium ionic species, are carried out for the polymer films/membranes and the polymer electrolytes with absorbing an electrolyte solution. Phase inversion polymer electrolytes are proved to show superior behaviors in electrochemical properties, such as ionic conductivity, electrochemical and interfacial stability, than cast film electrolytes. This is greatly owed to highly porous structure of phase inversion membranes. Even including the feature of interfacial resistance with lithium electrode, phase inversion polymer electrolytes of PVdF-HFP/(5-20 wt.% TiO₂) can be optimized as the adequate ones in applying to the electrolyte medium of lithium rechargeable batteries.     

A panoramic view of Li7P3S11 solid electrolytes synthesis,structural aspects and practical challenges for all-solid-state lithium batteries

The development of an inorganic electrochemical stable solid-state electrolyte is essentially responsible for future state-of-the-art all-solid-state lithium batteries (ASSLBs). Because of their advantages in safety, working temperature, high energy density, and packaging, ASSLBs can develop an ideal energy storage system for modern electric vehicles (EVs). A solid electrolyte (SE) model must have an economical synthesis approach, exhibit electrochemical and chemical stability, high ionic conductivity, and low interfacial resistance. Owing to its highest conductivity of 17 mS·cm^-1, and deformability, the sulfide-based Li₇P₃S₁₁ solid electrolyte is a promising contender for the high-performance bulk type of ASSLBs. Herein, we present a current glimpse of the progress of synthetic procedures, structural aspects, and ionic conductivity improvement strategies. Structural elucidation and mechanistic approaches have been extensively discussed by using various characterization techniques. The chemical stability of Li₇P₃S₁₁ could be enhanced via oxide doping, and hard and soft acid/base (HSAB) concepts are also discussed. The issues to be undertaken for designing the ideal solid electrolytes, interfacial challenges, and high energy density have been discoursed. This review aims to provide a bird's eye view of the recent development of Li₇P₃S₁₁-based solid-state electrolyte applications and explore the strategies for designing new solid electrolytes with a target-oriented approach to enhance the efficiency of high energy density all-solid-state lithium batteries.     

Electrosynthesis and electrochemical characterisation of phenazine polymers for application in biosensors

A comparative investigation has been undertaken of the electrosynthesis and electrochemical properties of three different electroactive polymers on carbon film electrode substrates: poly(neutral red) from the phenazine dye neutral red, and poly(methylene green) and poly(methylene blue), from the corresponding phenothiazine dyes. The formation of the radical cation at different potentials and the chemical structures of the monomers both influence the electropolymerisation process of the three polyaromatic dyes. Of the three, poly(neutral red) is shown to have the best adhesion at carbon film electrodes. The influence of the electrolyte and pH on film growth and on electrochemical properties was investigated. The formal potential decreased linearly with increase in pH, in the pH range from 1 to 7 for all three polymers. The modified electrodes were also characterised by electrochemical impedance spectroscopy. The bulk and interfacial characteristics of the two phenothiazine polymers were similar and oxygen-dependent, but different to those of the phenazine polymer, poly(neutral red), which were not significantly influenced by the presence of oxygen in solution. Perspectives for use in electrochemical biosensors are indicated.     

A Sol-Gel Derived Organic/Inorganic Hybrid Membrane for Intermediate Temperature PEFC

Intermediate temperature operation of polymer electrolyte fuel cells has been pointed out to be a promising option to overcome most of the technological problems of the current PEM system and new classes of electrolyte membrane have been investigated elsewhere. Proton conducting organic/inorganic nano-hybrid polymer electrolyte membranes have been synthesized in the present work. The membranes were synthesized by bridging temperature tolerant polyether polymers such as PEO or PTMO to inorganic silicate moieties to form organic/inorganic hybrid macromolecules. The hybrid membranes become proton conducting polymer electrolytes by doping with heteropolyacids such as 12-phosphotungstic acid (PWA). The conducting properties of the membrane were studied by modifying the polyether structure, molecular weight, PWA concentration, water content, and also various processing conditions. The membranes are flexible and thermally stable due to the temperature tolerant inorganic frameworks of the macromolecules. The proton conductivity of the membranes is in a range from 10^–3–10^–2 S/cm up to 140 °C under controlled humidity.     

Polymer electrolyte membranes for high‐temperature fuel cells based on aromatic polyethers bearing pyridine units

This review is focused on the design and synthesis of new high‐temperature polymer electrolytes based on aromatic polyethers bearing polar pyridine moieties in the main chain. Such materials are designed to be used in polymer electrolyte fuel cells operating at temperatures higher than 100 °C. New monomers and polymers have been synthesized and characterized within this field in respect of their suitability for this specific application. Copolymers with optimized structures in order to combine excellent film‐forming properties with high mechanical, thermal and oxidative stability and controlled acid uptake have been synthesized which, after doping with phosphoric acid, result in ionically conducting membranes. Such materials have been studied in respect of their conductivity under various conditions and used for the construction of membrane‐electrode assemblies (MEAs) which are used for fuel cells operating at temperatures up to 180 °C. New and improved, in terms of oxidative stability and mechanical properties in the doped state, polymeric membranes have been synthesized and used effectively for MEA construction and single‐cell testing. Copyright © 2009 Society of Chemical Industry     

Determination of the physicochemical characteristics and electrical performance of postsulfonated and grafted sulfonated derivatives of poly(para‐phenylene) as new proton‐conducting membranes for direct methanol fuel cell

Poly(para‐phenylene)s (PPPs) are an interesting class of rigid‐rod polymers that have excellent thermal and mechanical properties. Because of their high degree of crystallinity and lower permeability to methanol, PPPs are insoluble and infusible. A number of methods have been developed to synthesize substituted sulfonated PPPs bearing lateral chains to improve their solubility. In this work, a comparison of the physicochemical properties of three PPP‐based polymers is made with respect to Nafion membranes. One of these polymers was prepared with the postsulfonation method, and the other two were made with a new method of grafting developed in the Commissariat à l'Energie Atomique laboratory (a grafted sulfonated PPP polymer and a grafted perfluorinated sulfonated polymer). The sulfonated PPP polymers were examined for their mechanical properties, small‐angle X‐ray scattering, water absorption, proton conductivity, and methanol permeability. Relations between structures and properties were also investigated. Performances in fuel‐cell tests were also investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 944–952, 2006     

Alternatives toward proton conductive anhydrous membranes for fuel cells: Heterocyclic protogenic solvents comprising polymer electrolytes

Fuel cells are gaining increasing attention as a clean and promising technology for energy conversion. One of the key benefits of fuel cells compared to other methods is the direct energy conversion that enables the achievement of high efficiency. The electrolyte membrane is the most essential parts of a fuel cell unit, and consequently has been the subject of considerable research and development. Among the various types of proton conducting electrolytes examined for fuel cell applications, polymer electrolyte membranes (PEMs) are regarded as viable candidates since they enable operation of the cells at desirably low temperatures. This review describes recent progress in the design and development of high performance proton conducting PEMs, including the analysis of the design requirements and strategies for development of advanced PEMs for operation in anhydrous conditions. Some of the most widely used types of azole heterocycles are introduced and compared, particularly in terms of their performance characteristics in polyacids containing different functional groups. In addition, the latest research studies and progress in the field of azole-containing and azole-functionalized electrolyte systems are discussed and reviewed.     

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.     

A microporous gel electrolyte based on poly(vinylidene fluoride-co-hexafluoropropylene)/fully cyanoethylated cellulose derivative blend for lithium-ion battery

A gel polymer electrolyte based on the blend of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and fully cyanoethylated cellulose derivative (DH-4-CN) was prepared and characterized. Thermal, mechanical, swelling, liquid electrolyte retention and electrochemical properties, as well as microstructures of the prepared polymer electrolytes, were investigated using thermogravimetric analysis, electrochemical impedance spectroscopy, linear sweep voltammetry, and scanning electron microscopy. The results showed that the addition of DH-4-CN could obviously improve the conductivity of PVDF-HFP based electrolyte. The maximum ionic conductivity of 4.36 mS cm⁻¹ at 20 °C can be obtained for PVDF-HFP/DH-4-CN 14:1 in the presence of 1 M LiPF₆ in EC and DMC (1:1, w/w). The dry blend membranes exhibit excellent thermal behavior. All the blend electrolytes are electrochemically stable up to about 4.8 V vs. Li/Li⁺ for all compositions. The results reveal that the composite polymer electrolyte qualifies as a potential application in lithium-ion battery.     

Fabrication and characterization of electrolyte membranes based on organoclay/tripropyleneglycol diacrylate/poly(vinylidene fluoride) electrospun nanofiber composites

Utilizing polymer electrospinning technology, novel electrolyte membranes based on poly(vinylidene fluoride) (PVDF)/organomodified clay (OC)/tripropyleneglycol diacrylate (TPGDA) composite nanofibers with a diameter of 100–400 nm were fabricated for application in lithium batteries. Ultraviolet photo‐polymerization of electrospun PVDF/OC/TPGDA nanofibers generated chemically crosslinked TPGDA‐grafted PVDF/OC nanofibers exhibiting robust mechanical and electrochemical properties. The prepared fibrous PVDF/OC/TPGDA electrolytes were characterized in terms of morphology, crystallinity, electrochemical stability, ionic conductivity and cell cycleability. Based on differential scanning calorimetry analysis, the crystallinity of PVDF decreased by ca 10% on employing the OC and TPGDA. Compared with pure PVDF film‐based electrolyte membranes, the TPGDA‐ and OC‐modified PVDF electrolyte membranes exhibited improved mechanical properties and various electrochemical properties. The OC‐ and TPGDA‐modified microporous membranes are promising candidates for overcoming the drawbacks of the lower mechanical stability of fibrous‐type electrolytes with further improvement of electrochemical performance. Copyright © 2009 Society of Chemical Industry     

Aprotic ionic liquids as electrolyte components in protonic membranes

In this paper we describe the preparation and the properties of a series of aprotic ionic liquid-based, proton-conducting membranes. The ionic liquids (ILs) 1,2-dimethyl-3-n-propylimidazolium bis(trifluoromethanesulfonyl)imide and the 3-methyl-1-n-propylpyridinium bis(trifluoromethanesulfonyl)imide are used as the casting solvents of PVdF gel-type membranes; the proton conductivity is achieved by the addition of a superacid component, namely, trifluoromethanesulfonic acid (HTf) or N,N-bis(trifluoromethanesulfonyl)imide (HTFSI). The polymer electrolytes showed good thermal and electrochemical properties in the temperature range of interest for PEMFC applications. The strong coordination between the ILs and the HTFSI, which have the same anion, improves the thermal stability of this kind of membrane, but lowers the chemical properties and the conductivity, due to an increase in viscosity. HTf-added samples have an ionic conductivity of 2 × 10⁻² S cm⁻¹ at 100 °C, showing the best overall properties and making these membranes of interest applications in fuel cells.