Fuel cells have the potential to become an important energy conversion technology. Research efforts directed toward the widespread commercialization of fuel cells have accelerated in light of ongoing efforts to develop a hydrogen-based energy economy to reduce dependence on foreign oil and decrease pollution. Proton exchange membrane (also termed “polymer electrolyte membrane”) (PEM) fuel cells employing a solid polymer electrolyte to separate the fuel from the oxidant were first deployed in the Gemini space program in the early 1960s using cells that were extremely expensive and had short lifetimes due to the oxidative degradation of their sulfonated polystyrene-divinylbenzene copolymer membranes. These cells were considered too costly and short-lived for real-world applications. The commercialization of Nafion by DuPont in the late 1960s helped to demonstrate the potential interest in terrestrial applications for fuel cells, although its major focus was in chloroalkali processes. PEM fuel cells are being developed for three main applications: automotive, stationary, and portable power. Each of these applications has its unique operating conditions and material requirements. Common themes critical to all high performance proton exchange membranes include (1) high protonic conductivity, (2) low electronic conductivity, (3) low permeability to fuel and oxidant, (4) low water transport through diffusion and electro-osmosis, (5) oxidative and hydrolytic stability, (6) good mechanical properties in both the dry and hydrated states, (7) cost, and (8) capability for fabrication into membrane electrode assemblies (MEAs). Nearly all existing membrane materials for PEM fuel cells rely on absorbed water and its interaction with acid groups to produce protonic conductivity. Due to the large fraction of absorbed water in the membrane, both mechanical properties and water transport become key issues. Devising systems that can conduct protons with little or no water is perhaps the greatest challenge for new membrane materials. Specifically, for automotive applications the U.S. Department of Energy has currently established a guideline of 120 °C and 50% relative humidity as target operating conditions, and a goal of 0.1 S/cm for the protonic conductivity of the membrane. New membranes that have significantly reduced methanol permeability and water transport (through diffusion and electro-osmotic drag) are required for portable power oriented direct methanol fuel cells (DMFCs), where a liquid methanol fuel highly diluted in water is used at generally <90 °C as the source of protons. Unreacted methanol at the anode can diffuse through the membrane and react at the cathode, lowering the voltage efficiency of the cell and reducing the system’s fuel efficiency. The methanol is usually delivered to the anode as a dilute, for example, 1 M (or less), solution (3.2 wt %), and relatively thick Nafion 117 (1100 EW, 7 mil ∼ 178 μm thick) is used to reduce methanol crossover. The dilute methanol feed increases the system’s complexity and reduces the energy density of the fuel, while the thick Nafion membrane increases the resistive losses of the cell, especially when compared to the thinner membranes that are used in hydrogen/air systems. The presence of excessive amounts of water at the cathode through diffusion and electro-osmosis * To whom correspondence should be addressed. E-mail: jmcgrath@vt.edu. † Sandia National Laboratory. ‡ Case Western Reserve University. § Los Alamos National Laboratory. | Virginia Polytechnic Institute and State University. 4587 Chem. Rev. 2004, 104, 4587−4612

Alternative Polymer Systems for Proton Exchange Membranes (PEMs)

Poly(ethylene oxide) (PEO) based materials are widely considered as promising candidates of polymer hosts in solid-state electrolytes for high energy density secondary lithium batteries. They have several specific advantages such as high safety, easy fabrication, low cost, high energy density, good electrochemical stability, and excellent compatibility with lithium salts. However, the typical linear PEO does not meet the production requirement because of its insufficient ionic conductivity due to the high crystallinity of the ethylene oxide (EO) chains, which can restrain the ionic transition due to the stiff structure especially at low temperature. Scientists have explored different approaches to reduce the crystallinity and hence to improve the ionic conductivity of PEO-based electrolytes, including blending, modifying and making PEO derivatives. This review is focused on surveying the recent developments and issues concerning PEO-based electrolytes for lithium-ion batteries.

Poly(ethylene oxide)-based electrolytes for lithium-ion batteries

Electrochemical measurement of transference numbers in polymer electrolytes

The motivation for lithium battery development and a discussion of ion conducting polymers as separators begin this review, which includes a short history of polymer electrolyte research, a summary of the major parameters that determine lithium ion transport in polymer matrices, and consequences for solid polymer electrolyte development. Two major strategies for the application of ion conducting polymers as separators in lithium batteries are identified: One is the development of highly conductive materials via the crosslinking of mobile chains to form networks, which are then swollen by lithium salt solutions ("gel electrolytes"). The other is the construction of solid polymer electrolytes (SPEs) with supramolecular architectures, which intrinsically give rise to much enhanced mechanical strength. These materials as yet exhibit relatively common conductivity levels but may be applied as very thin films. Molecular composites based on poly(p-phenylene)- (PPP)-reinforced SPEs are a striking example of this direction. Neither strategy has as yet led to a "breakthrough" with respect to technical application, at least not for electrically powered vehicles. Before being used as separators, the gel electrolytes must be strengthened, while the molecularly reinforced solid polymer electrolytes must demonstrate improved conductivity.

Polymer Electrolytes for Lithium-Ion Batteries

The COMPASS force field: parameterization and validation for phosphazenes

: The phosphazene polymer (NP(OC2H40C2H4OCH3)n, MEEP, was synthesized and amorphous solvent-free salt complexes were performed with LiSo3CF3, NaSO3CF3, Sr(SO3CF3)2, and AgSO3CF3. A material with the composition (LiSO3CF3)0.25. MEEP has a conductivity of .00008 ohm/cm at 30 C, which is much higher than corresponding poly (ethylene oxide) complexes. The phosphazene electrolytes are promising materials for ambient-temperature high energy density batteries.

Polyphosphazene solid electrolytes

: Poly(organophosphazenes) have been synthesized with alkyl-ether-alkoxy groups attached to the phosphorus atoms of the skeleton. These species are water-stable and either water-soluble or hydrophilic polymers. Specific members of this series form complexes with metal salts, which are excellent solid electrolyte materials. Mixed substituent polymers with hydrophobic trifluoroethoxy and alkyl-ether alkoxy side groups have also been prepared and these are of interest as membranes and biomedical materials. (Author)

Polyphosphazenes with etheric side groups: prospective biomedical and solid electrolyte polymers

Complex formation and ionic conductivity of polyphosphazene solid electrolytes

A solid state laminar electrochemical cell comprising: an alkali metal anode layer; a solid ionically conducting electrolyte layer; and a cathode/current collector layer; wherein said electrolyte layer is interposed between said alkali metal anode layer and said cathode/current collector layer and wherein said cathode/current collector layer comprises an electrically conductive substrate having a plurality of surface voids and a composite cathode composition comprising an intercalation compound, an electrically conductive filler and an ionically conductive electrolyte, said cathode composition being coated on the surface of said substrate facing said electrolyte layer and being maintained in the voids of said surface is disclosed.

Solid state electrochemical cell having porous cathode current collector

A button-type battery has an anode, a cathode, a separator, and an electrolyte encased within a housing and surrounding by a fluid-tight peripheral seal. The battery is capable of being deflected such that a first portion of the battery is angled relative to a second portion through an internal angle between the first and second portions in a range from 175° to at least 90° without destroying operation of the button-type battery or rupturing the fluid-tight peripheral seal.

Peter M. Blonsky

Papers

Polyphosphazene solid electrolytes

Polyphosphazenes with etheric side groups: prospective biomedical and solid electrolyte polymers

Complex formation and ionic conductivity of polyphosphazene solid electrolytes

Solid state electrochemical cell having porous cathode current collector

Button-type battery having bendable construction, and angled button-type battery