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)

In the last years membrane processes for gas separation are gaining a larger acceptance in industry and in the market are competing with consolidated operations such as pressure swing absorption and cryogenic distillation. The key for new applications of membranes in challenging and harsh environments (e.g., petrochemistry) is the development of new tough, high performance materials. The modular nature of membrane operations is intrinsically fit for process intensification, and this versatility might be a decisive factor to impose membrane processes in most gas separation fields, in a similar way as today membranes represent the main technology for water treatment. This review highlights the most promising areas of research in gas separation, by considering the materials for membranes, the industrial applications of membrane gas separations, and finally the opportunities for the integration of membrane gas separation units in hybrid systems for the intensification of processes.

Membrane Gas Separation: A Review/State of the Art

Progress in the production and modification of PVDF membranes

Metal-organic framework (MOF) materials, which are constructed from metal ions or metal ion clusters and bridging organic linkers, exhibit regular crystalline lattices with relatively well-defined pore structures and interesting properties. As a new class of porous solid materials, MOFs are attractive for a variety of industrial applications including separation membranes - a rapidly developing research area. Many reports have discussed the synthesis and applications of MOFs and MOF thin films, but relatively few have addressed MOF membranes. This critical review provides an overview of the diverse MOF membranes that have been prepared, beginning with a brief introduction to the current techniques for the fabrication of MOF membranes. Gas and liquid separation applications with different MOF membranes are also included (175 references).

Metal–organic framework membranes: from synthesis to separation application

Application and modification of poly(vinylidene fluoride) (PVDF) membranes – A review

Pervaporation is one of the most active areas in membrane research, and the pervaporation process has been shown to be an indispensable component for chemical separations. In this paper, the recent development in pervaporation membranes and pervaporation processes is reviewed, and some outstanding questions involved in membrane pervaporation are discussed with emphasis on the following issues:  mass transport in the membrane, membrane material selection, concentration polarization in the boundary layer, pressure buildup in hollow fiber membranes, asymmetric and composite membranes, and the activation energy for permeation. We attempt to provide insight into this dynamic field and to highlight some of the outstanding problems yet to be solved or clarified.

Liquid Separation by Membrane Pervaporation: A Review

Hydrophobic poly(ether ether ketone) (PEEK) were modified by sulfonation at different temperatures (22, 36, 45, and 55°C) and varying period of time with concentrated sulfuric acid used as solvent A kinetic study was carried out based on the assumption that sulfonation reaction is a second-order reaction, which takes place preferentially in the aromatic ring between the two ether (O) links (the first type substitution), and there is only one substituent attached to each repeat unit of the PEEK before the complete substitution of this preferred aromatic ring More than 100% substitution was observed in experiment All the data with substitution degree less than 95% agree fairly well with the kinetic behavior of the second-order reaction The reaction rate coefficient and activation energy for first type substitution were obtained The sulfonated PEEK samples were characterized in terms of ion-exchange capacity (IEC), 1H-NMR, contact angle, and solubility © 2001 John Wiley & Sons, Inc J Appl Polym Sci 82: 2651–2660, 2001

Xianshe Feng

Papers

Liquid Separation by Membrane Pervaporation: A Review

Sulfonation of poly(ether ether ketone)(PEEK): Kinetic study and characterization

Estimation of activation energy for permeation in pervaporation processes

Separation of acetone–butanol–ethanol (ABE) from dilute aqueous solutions by pervaporation

Thin film composite membranes embedded with graphene oxide for water desalination