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.

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.

Traditional batteries stored energy in thick plates made from heavy metals like lead, nickel, and zinc. They delivered from 15 to 20 Wh/kg. Lighter-weight lithium anodes were used in some military batteries. Then came the need for lightweight batteries for powering cellular telephones and laptop computers. Lithiumion batteries were developed, and the worldwide demand, just for use in laptop computers, has grown to 150 million units per year.

The need to reduce air pollution in downtown areas has created a market for battery-powered electric vehicles. Clara Ford chose to drive an electric car, even though her husband, Henry Ford, was making gasoline-powered cars. However, the cost of replacing worn-out leadacid batteries soon ended the electric-car age of the early 1900s. The need for lightweight, long-life batteries for zero-emission cars has produced unprecedented investments in battery technology. The lithium-battery technology used in laptop-computer batteries did not support the requirements of high power and long life for the charge/discharge cycling needed in electric cars. An executive of a lithium-battery manufacturer was asked what he was doing about the cycle life of his batteries. His answer: “The life of a laptop computer is nine months. Then a newer model makes it obsolete. We meet this requirement!”

Magnesium can reversibly store about 7.7 wt% hydrogen, equivalent to more than twice the density of liquid hydrogen. This high storage capacity, coupled with a low price, suggests that magnesium and magnesium alloys could be advantageous for use in battery electrodes and gaseous-hydrogen storage systems. The use of a hydrogen-storage medium based on magnesium, combined with a fuel cell to convert the hydrogen into electrical energy, is an attractive proposition for a clean transportation system. However, the advent of such a system will require further research into magnesium-based alloys that form less stable hydrides and proton-conducting membranes that can raise the operating temperature of the current fuel cells.

Following the U.S. oil crisis of 1974, research into alternative energy-storage and distribution systems was vigorously pursued. The controlled oxidation of hydrogen to form water was proposed as a clean energy system, creating a need for light and safe hydrogen-storage media. Extensive research was done on inter-metallic alloys, which can store hydrogen at densities of about 1500 cm3-H2 gas/ cm3-hydride, higher than the storage density achieved in liquid hydrogen (784 cm3/cm3 at –273°C) or in pressure tanks (˜200 cm3/cm3 at 200 atm). The interest in metal hydrides accelerated following the development of portable electronic devices (video cameras, cellular phones, laptop computers, tools, etc.), which created a consumer market for compact, rechargeable batteries. Initially, nickel-cadmium batteries fulfilled this need, but their relatively low energy density and the toxicity of cadmium helped to drive the development of higher-energy-density, less toxic, rechargeable batteries.

Recent reports of very high, reversible adsorption of molecular hydrogen in pure nanotubes, alkali-doped graphite, and pure and alkali-doped graphite nanofibers (GNFs) have aroused tremendous interest in the research community, stimulating much experimental work and many theoretical calculations worldwide. The U.S. Department of Energy (DOE) Hydrogen Plan has seta standard for this discussion by providing a commercially significant benchmark for the amount of reversible hydrogen adsorption. This benchmark requires a system-weight efficiency (the ratio of stored H2 weight to system weight) of 6.5-wt% hydrogen and a volumetric density of 63 kg H2/m. If the encouraging experimental reports (summarized in Table I) are reproducible, it may be possible to reach the goals of the DOE Hydrogen Plan. On the other hand, the community still awaits confirmation of these experimental results by workers in other laboratories. Of additional concern is the fact that theoretical calculations have been unable to identify adsorption mechanisms compatible with the requirements of the DOE Hydrogen Plan.

An economical, safe, hydrogen-storage medium is a critically needed component of a hydrogen-fueled transportation system. Hydrogen storage in a carbon-based material offers further advantages associated with its low mass density. Furthermore, fuel cell technology involving the conversion of hydrogen into protons, or hydrogen and oxygen into electric current, is being vigorously researched for both transportation and small power-plant applications.

Flywheels are mechanical devices that store kinetic energy in a rotating mass. A simple example is the potter's wheel. For energy storage and conversion, an efficient method to exchange energy with a flywheel device is by converting the energy between mechanical and electrical forms. Typically a flywheel designed to perform this type of energy exchange is a combination of a motor and a generator. Energy is transferred into the device for storage by using it as a motor to consume electrical energy and spin the mass. This energy can be recovered with an efficiency exceeding 80% by using the flywheel as an electrical generator. Although the concept of storing energy in a rotating mass is an ancient idea, the relatively recent advent of advanced fiber-composite materials offers the potential for improved energy storage and conversion using rotating electromechanical devices.

The achievable energy density (energy/weight) of a simple flywheel design, such as that shown schematically in Figure 1, is proportional to the specific strength (strength/density) of the material. The particular type of composite flywheel shown in this figure is composed entirely of circumferentially wrapped fiber. Although other designs have been suggested and constructed, this version is the most common for energy-storage applications. As discussed in the earliest papers on the subject, such as the article by Post and Post, the proportionality between energy density and specific strength favors the use of fiber composites. The remarkable combination of mechanical properties and low density achieved with fiber composites has made them attractive and nearly essential for aerospace applications.