Equivalent weight (EW) is the number of grams of dry Nafion per mole of sulfonic acid groups when the material is in the acid form. This is an average EW in the sense that the comonomer sequence distribution (that is usually unknown to the investigator and largely unreported) gives a distribution in m in this formula. EW can be ascertained by acid-base titration, by analysis of atomic sulfur, and by FT-IR spectroscopy. The relationship between EW and m is EW ) 100m + 446 so that, for example, the side chains are separated by around 14 CF2 units in a membrane of 1100 EW. Common at the time of this writing are Nafion 117 films. The designation “117” refers to a film having 1100 EW and a nominal thickness of 0.007 in., although 115 and 112 films have also been available. Early-reported studies involved 1200 EW samples as well as special experimental varieties, some being rather thin. The equivalent weight is related to the property more often seen in the field of conventional ion exchange resins, namely the ion exchange capacity (IEC), by the equation IEC ) 1000/EW. The mention of the molecular weight of high equivalent weight (EW > 1000 g‚mol-1) Nafion is almost absent in the literature, although the range 105-106 Da has been mentioned. As this polymer does not form true solutions, the common methods of light scattering and gel permeation chromatography cannot be used to determine molecular weight as well as the size and shape of isolated, truly dissolved molecules. Studies of the structure of this polymer in solvent (albeit not a true solution) will be mentioned in the scattering section of this review. It should be noted that Curtin et al. performed size exclusion chromatography determinations of the molecular weight distribution in Nafion aqueous dispersions after they were heated to high temperatures (230, 250, and 270 °C).1 Before heating, there was a high molecular weight shoulder on a bimodal distribution, due to molecular aggregates, but this shoulder disappeared upon heating, which indicated that the aggregates were disrupted. The peaks for the monomodal distribution for the heated samples were all located at molecular weights slightly higher than 105 g‚mol-1. Also, light scattering experiments revealed that the radius of gyration had a linear dependence on the molar mass of the aggregates, which suggests that the particles are in the form of rods or ribbons, or at least some elongated structure. Nafion ionomers are usually derived from the thermoplastic -SO2F precursor form that can be extruded into sheets of required thickness. Strong interactions between the ionic groups are an obstacle to melt processing. This precursor does not possess the clustered morphology that will be of great concern in this article but does possess Teflon-like crystallinity which persists when the sulfonyl fluoride form is converted to, for example, the K+ form by reacting it with KOH in water and DMSO. Thereafter, the -SO3H form is achieved by soaking the film in a sufficiently concentrated aqueous acid solution. Extrusion of the sulfonyl fluoride precursor can cause microstructural orientation in the machine direction, * Address correspondence to either author. Phone: 601-266-5595/ 4480. Fax: 601-266-5635. E-mail: Kenneth.Mauritz@usm.edu; RBMoore@usm.edu. 4535 Chem. Rev. 2004, 104, 4535−4585

State of Understanding of Nafion

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

New Insights into Perfluorinated Sulfonic-Acid Ionomers

Flow Batteries: Current Status and Trends

Energy storage is increasingly seen as a valuable asset for electricity grids composed of high fractions of intermittent sources, such as wind power or, in developing economies, unreliable generation and transmission services However, the potential of batteries to meet the stringent cost and durability requirements for grid applications is largely unquantified We investigate electrochemical systems capable of economically storing energy for hours and present an analysis of the relationships among technological performance characteristics, component cost factors, and system price for established and conceptual aqueous and nonaqueous batteries We identified potential advantages of nonaqueous flow batteries over those based on aqueous electrolytes; however, new challenging constraints burden the nonaqueous approach, including the solubility of the active material in the electrolyte Requirements in harmony with economically effective energy storage are derived for aqueous and nonaqueous systems The attributes of flow batteries are compared to those of aqueous and nonaqueous enclosed and hybrid (semi-flow) batteries Flow batteries are a promising technology for reaching these challenging energy storage targets owing to their independent power and energy scaling, reliance on facile and reversible reactants, and potentially simpler manufacture as compared to established enclosed batteries such as lead–acid or lithium-ion

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Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries

The continued search for routes to improve the power and energy density of lithium ion batteries for electric vehicles and consumer electronics has resulted in significant innovation in all cell components, particularly in electrode materials design. In this Review, we highlight an often less noted route to improving energy density: increasing the Li+ transference number of the electrolyte. Turning to Newman’s original lithium ion battery models, we demonstrate that electrolytes with modestly higher Li+ transference numbers compared to traditional carbonate-based liquid electrolytes would allow higher power densities and enable faster charging (e.g., >2C), even if their conductivity was substantially lower than that of conventional electrolytes. Most current research in high transference number electrolytes (HTNEs) focuses on ceramic electrolytes, polymer electrolytes, and ionomer membranes filled with nonaqueous solvents. We highlight a number of the challenges limiting current HTNE systems and suggest a...

Promising Routes to a High Li+ Transference Number Electrolyte for Lithium Ion Batteries

Relationship between ionic conductivity of perfluorinated ionomeric membranes and nonaqueous solvent properties

High ionic conductivities of 10-4 to 10-2 S/cm are achieved with Li+-form perfluorosulfonate ionomers over a wide temperature range by swelling in nonaqueous organic solvents. The dependence of ionic conductivity on temperature, solvent absorption, and membrane equivalent weight is examined for Nafion perfluorinated ionomer membranes. These results are compared with other ionomer membranes, including those having hydrocarbon backbones and weaker acid groups, to correlate ionic conductivity with ionomer structure. The most important factors determining ionic conductivity in membranes swollen with polar nonaqueous solvents, beyond the solvent properties such as viscosity and molecular weight, are the basicity of the fixed anion group and the solvent uptake by the membrane. Ionic conductivity is generally limited by dissociation of the cation from the fixed anion site. Several means for increasing conductivity are demonstrated including the use of cation complexing agents to increase ionic dissociation.

Ionic Conductivity of Nonaqueous Solvent-Swollen Ionomer Membranes Based on Fluorosulfonate, Fluorocarboxylate, and Sulfonate Fixed Ion Groups

Ultrasonically Induced Cavitation Studies of Electrochemical Passivity and Transport Mechanisms

A sensor which allows dielectric measurements to be made on a high temperature product such as a molten plastic flowing in a conduit. It comprises a ceramic cylinder with an interdigitated electrode capacitor patterned into the inside wall. Uses of the sensor include continuous determination of copolymer composition and polymer viscosity (molecular weight) in a completely non-obstructive manner.

Noninvasive dielectric sensor and technique for measuring polymer properties

Disclosed is a highly conductive polymer electrolyte membrane and a process for producing it. This invention also describes batteries which employ the polymer electrolyte membrane of this invention.

Stephen A. Perusich

Papers

Relationship between ionic conductivity of perfluorinated ionomeric membranes and nonaqueous solvent properties

Ionic Conductivity of Nonaqueous Solvent-Swollen Ionomer Membranes Based on Fluorosulfonate, Fluorocarboxylate, and Sulfonate Fixed Ion Groups

Ultrasonically Induced Cavitation Studies of Electrochemical Passivity and Transport Mechanisms

Noninvasive dielectric sensor and technique for measuring polymer properties

Highly conductive ion exchange polymer and process