As this MRS Bulletin issue on “Advanced Materials for Energy Storage” was being finalized, the North American continent suffered one of its warmest summers ever recorded. The past two to three years have also been the warmest on record, August 1999 being the 21st consecutive month with above-average temperatures. Such deviations could be quite natural. Indeed, these temperature increases can be explained in part by increased solar activity. Yet, burning of fossil fuels and other human factors are considered likely contributors. Whether from human activities or cosmic effects, for human survival, technological solutions may be needed to keep the global temperature within tolerable margins.

In a parallel development, the price of oil on the New York market has nearly doubled since the beginning of 1999. This current increase is directly related to self-controls imposed by the oil-producing countries. The price will probably not climb to excessive values, mainly because the oil-producing countries do not want other nations to develop competitive alternative technologies. However, these increases in oil prices could have devastating effects on the general economy, and again, the future is unpredictable.

This situation poses an interesting dilemma for the scientific community. Scientists are trained to base conclusions on facts and respond with appropriate engineering solutions. Yet the extrapolation of global climate change is far from conclusive; the economies of nations can be volatile and often are determined by politics as opposed to science; and the needs of individual nations are varied and continue to evolve based on their local resources, economies, and environmental regulations.

Membrane separation science has enjoyed tremendous progress since the first synthesis of membranes almost 40 years ago, which was driven by strong technological needs and commercial expectations. As a result, the range of successful applications of membranes and membrane processes is continuously broadening. An additional change lies in the nature of membranes, which is now extended to include liquid and gaseous materials, biological or synthetic. Membranes are understood to be thin barriers between two phases through which transport can take place under the action of a driving force, typically a pressure difference and generally a chemical or electrical potential difference.

An attempt at functional classification of membranes would have to include diverse categories such as gas separation, pervaporation, reverse osmosis, micro- and ultrafiltration, and biomedical separations. The dominant application of membranes is certainly the separation of mixed phases or fluids, homogeneous or heterogeneous. Separation of a mixture can be achieved if the difference in the transport coefficients of the components of interest is sufficiently large. Membranes can also be used in applications other than separation targeting: They can be employed in catalytic reactors, energy storage and conversion systems, as key components of artificial organs, as supports for electrodes, or even to control the rate of release of both useful and dangerous species.

In order to meet the requirements posed by the aforementioned applications, membranes must combine several structural and functional properties.

Corrosion is a branch of materials research that is, by its nature, “schmutzy” (dirty). It has a reputation as an unglamorous subject and certainly suffers from an image problem in the materials community: the guest editors would like to have a dollar for every time we sat down and discussed “How can we make corrosion sound positive?” Solving corrosion problems requires an understanding of corrosion mechanisms, and corrosion mechanisms are infinitely subtle and interesting. Corrosion processes can often encompass concepts from a variety of disciplines, including thermodynamics, surface science, electrochemistry, physical metallurgy, and solid-state and statistical physics. While corrosion of metals is the subject of this issue of MRS Bulletin, ceramics, polymers, and geological materials are all greatly affected by corrosion.

Corrosion of metals is an electrochemical process, composed of anodic (metal oxidation) and cathodic (reduction) reactions occurring on a surface. The rates of these reactions are measured as current densities (in units of A m−2). On a single piece of metal, the anodic and cathodic current densities can vary arbitrarily from point to point, as long as the total anodic and cathodic currents are equal. This variation, which is associated with local differences in pH, surface composition, or defects in a surface film, leads to various localized phenomena, such as pitting and stress-corrosion cracking, and also to intriguing oscillatory dynamics.

Microporous inorganic membranes are potentially useful in gas separation in emerging areas such as catalytic reactors, gasification of coal, molten-carbonate and solid-electrolyte fuel cells, and water decomposition by thermochemical reactions. If the feed or product gases can be separated at elevated temperatures specific to each process, the energy required for purification could be greatly reduced. Advances in the development of inorganic membranes have been quite rapid in recent years. For example, in 1991 the reported CO2/N2 selectivity at ambient temperature was less than 10, but by 1997 it had improved to approximately 100.

The permeation rate and permselectivity of porous inorganic membranes are dependent on the microstructures of membrane/support composites such as pore size and distribution, porosity, tortuosity, and the affinity between permeating species and pore walls. Figure 1 shows the relationship between molecular weight and kinetic diameter (calculated from minimum equilibrium cross-sectional diameter) for a selected series of molecules. Hydrogen and helium are smaller and lighter than the others. Structural isomers such as n-C4H10 and i-C4H10 have the same mass but quite different sizes. Therefore, the control of micropores is of critical importance in these cases. However, the molecular masses and sizes of CO2 and N2 are not greatly different; thus the difference in affinity is important for separation of these molecules.

In order to achieve effective separation of small-molecule gases, the membrane pores should be smaller than 2 nm. In the case of mesopores or macropores, gases permeate with low selectivities through these pores. In this article, preparation processes and permeation properties of porous inorganic membranes are reviewed, and permeation mechanisms are discussed.

Although conventional lead-acid batteries are considered a rather mature technology, significant research and development efforts are currently under way to enhance their performance and operating life. These efforts are being driven by the demands of both the automotive and stationary (or standby) market sectors. Both major markets have need of lead-acid batteries with higher energy density or reduced size and weight; however, the automotive sector is also driven to mitigate the cycle-life reduction of its “starter, lighting, and ignition” (SLI) batteries that results from rising “under the hood” temperatures in modern automobiles.

The operating and cycle lives of leadacid batteries are limited by the resistance of the positive Pb-alloy electrodes to intergranular-degradation processes (i.e., corrosion, cracking, and creep). Figure 1 shows an example of near-through-wall cracking and some inter-granular corrosion (grain-dropping at surface) observed in a Pb-lwt%Sb positive battery grid following approximately four years of service. In addition to the breaching of grid electrical continuity by corrosion and cracking processes (as indicated in Figure 1), the relatively high homologous temperature of operation for lead-acid batteries (i.e., >0.6 Tm, where Tm is the melting temperature) promotes intergranular-creep processes that result in dimensional changes in the electrodes over time (i.e., grid “growth”); this causes adjacent plates to short, leading to reduced battery capacity.