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1 TOPIC 6: REVIEW OF THE NATIONAL SCIENCE FOUNDATION REPORT APPROACHES TO COMBAT TERRORISM: OPPORTUNITIES FOR BASIC RESEARCH IN ENERGY/POWER SOURCES Debra Rolison, a member of this committee and of the committee that wrote the NSF report, Approaches to Combat Terrorism: Opportunities for Basic Research in Energy/Power Sources summarized the November 2002 meeting at which the report had been presented. That report set forth the current options for portable/mobile/leave-behind power sources for integrated circuits. To get improved performance, we need improved materials. A key opportunity is to create multifunctional architectures using nanoscopic components that produce power. Particularly promising are porous, disordered materials that can be synthesized by soft methods such as self-organization. In batteries, we want to achieve higher capacities by transferring more electrons per metal center. We also want to maximize charge transfer by reducing cathode/electrolyte/anode separation dimensions to the nanoscale. By synthesizing an open, intercalated V2Os aerogel for Li-ion batteries, one can increase the Li ion uptake by a factor of 4 compared with the best dense V2Os, creating a capacity of 1,600 Wh/kg. In batteries made from nanoscale materials, one sees a blurring of the properties usually ascribed to discrete components; for example, on discharge, batteries show voltage changes more commonly associated with capacitors. This introduces a degree of multifunctionality to these architecturese.g., the battery power supply does not require a separate capacitor. As an example of a three-dimensional nanostructured battery architecture, Rolison showed an all- silid-state battery with a sol-gel-derived MnO2 ambigel cathode coated with an electrodeposited polymer electrolyte separator, and the remaining mesoporous volume filled with Li metal anode. This integrated, interpenetrating architecture maximizes the interface between anode and cathode (and minimizes the distance between them), more effectively utilizes the available volume, and results in a battery with both high energy and high power density. Using a similar approach, high-quality nanowire and superior ultracapacitors featuring polymer-modified carbon nanofoams can be produced. In these m~croporous structures, it is important to realize that the walls of the micropores cannot be wetted with liquid electrolyte on any practical time scale one must rely on solid-state charge transfer. This has huge implications for nanotubes and other ultra-high-surface-area materials. 21
1 22 Summary of the Power Systems Workshop In fuel cells, we need better electrocatalysts that are not poisoned by less-than-pristine fuels and would like to get rid of proton-conducting membranes altogether. Disordered Pt/Ru blacks can be used as catalysts in direct methanol fuel cells, which can be made carbon-free and membrane-free through a nanowired architecture design. The key is to design multifunctional disordered electrode architectures rather than using a "masonry" (layer-on-layer) approach. Similarly, in thermoelectric, photovoltaic, and thermionic power sources, nanostructured materials/processing approaches may break the historical limitations of low efficiency and high- temperature requirements. In energy-harvesting applications, one typically needs to tap into low-power, low-temperature distributed sources. Pulsed power is possible if one could use the low-power continuous source to trickle charge a capacitor or fill the "fuel tank" of a leave-behind, direct methanol fuel cell. Ultraporous nanoarchitectures may find uses as catalysts or capacitor materials in these systems. For instance, it is possible to generate an aerogel of nanosized gold particles and cytochrome C that is stable at room temperature for 6 weeks. This suggests that it may be possible to self-assemble an artificial energy transport chain that mimics biological energy transport chains. In another example, state-of-the-art CO oxidation catalysts can be created by using sol-gel chemistry to generate golcI-titanium oxide composite aerogels in which 6-nary gold particles intermingle with 10-nm titania particles. This like-sized neighbor architecture enables catalytic activity that is not available in the older architectures in which a 3-nary gold particle rides on a 40-nm titania particle. Rolison concluded by noting that if disorder is good in nanoarchitectures, almost all of our analytical/characterization tools that depend on order (e.g., x-ray diffraction, EXAFS) are inadequate. Thus, new characterization methods will have to be developed. Also, a key goal for the future is to understand how these disordered structures can be chemically and physically stabilized. TOPIC 6 DISCUSSION It was remarked earlier (see Topic 1 discussion) that no rechargeable battery uses more than 25 percent of its volume actively. How, it was asked, should we think about the percentage volume that is unused? The response was that if one charges and recharges slowly, one can "talk" to most of the battery volume. The issue is how much of the active material one can utilize in fast charging and discharging. If one is smart in the architectural design and uses electrode materials that can withstand the mechanical strains associated with ion movement, one can get more battery capacity. However, as we move to smaller and smaller length scales, we begin to smear the definitions of battery, capacitor, and the like. The best present batteries have power densities of 25-30 Wig. How much better can we expect batteries to get, and when? According to Rolison, with MnO2 porous architectures, one can double the C rate, since one can "talk" to nearly all of the surface area. If the electrode/electrolyte structures are thin, one can get around many of the problems faced by standard batteries. An important area for future study is hydrogen storage, either in hydrides or butadiene. Rolison concluded by saying that in the future, power for integrated circuits will be "all-nano all the time." 1