Production of pure molecular hydrogen is essential to the realization of the proposed “hydrogen economy” that could ultimately provide hydrogen as a clean, renewable source of energy; eliminate the industrialized world's dependence on petroleum; and reduce the generation of greenhouse gases linked to global warming. A crucial step in obtaining pure hydrogen is separating it from other gaseous compounds—mainly CO2—that often accompany hydrogen in industrial chemical reactions. Advanced membrane technology may prove to be the key to the successful, economical production of molecular hydrogen.

Size-sieving glassy polymer membranes can separate H2 on the basis of its small size. Alternatively, reverse-selective rubbery polymers can expedite the passage and, hence, removal of CO2 due to its relatively high solubility in such membranes alone or in conjunction with dissociative chemical reactions. Transition-metal membranes and their alloys can adsorb H2 molecules, dissociate the molecules into H atoms for transport through interstitial sites, and subsequently recombine the H atoms to form molecular H2 again on the opposite membrane side. Microporous amorphous silica and zeolite membranes comprising thin films on a multilayer porous support exhibit good sorption selectivity and high diffusion mobilities for H2, leading to high H2 fluxes. Finally, carbon-based membranes, including carbon nanotubes, may be viable for H2 separation on the basis of selective surface flow and molecular sieving. A wide variety of materials challenges exist in hydrogen purification, and the objective of this issue of MRS Bulletin is to address those challenges and their potential solutions from basic principles.

The development of a hydrogen-based economy would generate a substantial necessity for efficient means of collecting hydrogen with a relatively high purity. Membrane separations play a major role in the separation of hydrogen gas from various gas mixtures, and this article discusses the use of polymeric materials to produce these membranes. After a review of the historical use of polymeric membranes and some background information regarding mechanisms of gas transport in membranes, this article will review the work that has been done in the two major classes of hydrogen separation membranes: hydrogen-selective membranes and hydrogen-rejective membranes. In hydrogen-selective membranes, the very small size of the hydrogen molecule is exploited to allow rapid diffusion of hydrogen through the membrane while excluding other gases. Hydrogen-rejective membranes use the significantly higher sorption of other gases to overcome the advantages of the small size of the hydrogen molecule. The discussion of these two types of membranes will be followed by a presentation of the current state of the art with regard to polymeric membranes for hydrogen separation and a discussion of the predictions for future applications and advancements in this area.

The most widely used method of hydrogen production, steam methane reforming, yields a product stream consisting mainly of hydrogen (H2) and carbon dioxide (CO2). Purification of this product is currently accomplished using amine-based acid gas scrubbers or pressure swing adsorption technology. Membranes are well suited to bulk CO2 removal and offer a viable alternative to these established technologies. This review considers one type of such membranes, polymeric facilitated transport membranes. These membranes selectively permeate CO2 by means of a reversible reaction between the gas and the membrane material. In addition, the membrane provides a barrier to H2 permeation. The result is removal of the CO2 contaminant and recovery of the H2 product at high pressure, eliminating the need for recompression prior to use or storage. A wide range of polymeric materials have been investigated, including ion-exchange resins, hydrophilic polymers blended with CO2-reactive salts, polyelectrolytes, fixed-site carrier polymers, and biomimetic materials. This review provides a description of the reaction chemistry of facilitated transport, a summary of membrane permselective properties, and suggestions for future research efforts.

Microporous amorphous silica and zeolite membranes are made as thin films on a multilayer porous support. The membranes have a network of connected micropores with ∼0.5–nm diameters. Net transport of small molecules on this network occurs under the driving force of a gradient in chemical potential. Favorable combinations of sorption selectivity and diffusion mobility in the membrane materials lead to high H2 fluxes and good selectivity with respect to other gases. The membranes show potential for application in H2 separation under harsh conditions. Amorphous silica membranes show very high H2 fluxes because they can be made very thin; silicalite-type zeolite membranes are expected to have a better operational stability. To make the membranes a viable option, improvements are needed in reducing membrane defects and manufacturing costs and enhancing reproducibility and operational stability. This article summarizes the state of the art, provides relevant definitions, and outlines the base design and long-term specifications of viable supported membrane structures. This is followed by an overview of transport properties, synthesis, and operational stability of the membrane and the supporting structures. Directions for future research programs are provided by demonstrating how the selection of the actual membrane composition and supporting structure can be derived from an application-based design. The success of such a design depends critically on fundamental studies of membrane transport, strength, and operational stability.

Inorganic carbon-based membranes for gas separation comprise materials that are fabricated through pyrolysis of a precursor material (often a synthetic polymer), and the more recently discovered carbon nanotubes. Fabrication, assembly into different architectures, and mechanism of operation are summarized for precursor-based carbon membranes, with a focus on selective surface flow and molecular sieving. Only preliminary work on carbon nanotube-based membranes for gas separation has been published. Their unusual transport properties, however, promise their use in gas separation in the future. In light of this application, structural properties and results relating to flow through these tubular structures are summarized.

Dense metal membranes are a well-developed technology for the production of high-purity hydrogen. The physical mechanism of hydrogen transport across metal films—dissociation of molecular hydrogen, diffusion of interstitial atomic hydrogen, and subsequent recombinative desorption of molecular hydrogen—means that metal membranes can have extremely high selectivities for hydrogen transport relative to other gases. We describe current experimental and theoretical trends in the development of metal alloy membranes for hydrogen purification in practical, chemically robust processes.