Nanotechnology and the Environment - American Chemical Society

Chapter 44

Energy and the Environment: Perpetual Dilemma or Nanotechnology-Enabled Opportunity?

Downloaded by UNIV OF NORTH CAROLINA on October 23, 2014 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch044

Debra R. Rolison Surface Chemistry Branch, Code 6170, Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, DC 20375 ([email protected]; fax: 1-202-767-3321)

The global demand for the energy that sustains human-based activity is unceasing—and increasing. This global need drives production processes (including extraction, chemical manipulation, and distribution) and in-use processes that potentially can compromise environmental quality. Yet the global and local environment can only be sustained, cleaned, and preserved through the expenditure of energy. A perpetual irony it may be, but thermodynamics demands that truism. Are there environmentally green opportunities foreseeable by re-thinking and re-designing energy production and power generation from a nanoscopic perspective? The heterogeneous catalysis necessary to process petroleum into liquid transportation fuels is (and has always been) innately nanoscopic (1). Even with a history that long precedes today's focus on nanoscale science and engineering, improved catalytic chemistries are still sought, particularly to desulfurize fuels, in order to lower the environmental impact of the use of fossil fuels (2). Breakthroughs in energy storage and conversion are arising from the confluence of nanoscale S&T, environmentally green considerations, and the importance of alternate and distributed energy. Multifunctional materials are prerequisite to electrochemical and thermal-to-electric power sources. In order to deliver high performance in such devices, multifunctional materials must exhibit some combination of the following properties: electronic conductivity, ionic conductivity, thermal conductivity, separation of electron-hole pairs, 324

© 2005 American Chemical Society

In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.


325 catalytic selectivity and activity, as well as facile mass transport of molecules in the electrochemical devices. Independent control of the elementary processes that give rise to energyrelevant functionalities is difficult-to-impossible with bulk materials. Scientists and engineers who design, fabricate, and study matter on the nanoscale are recognizing that an obvious research opportunity lies in determining how to assemble nanoscopic building blocks into multifunctional architectures that store or convert energy and produce power (3-6). Another critical design parameter arises from the need to move beyond the high level of understanding that has been developed for electron-transfer reactions between molecules or between electrodes and molecules (7) and to understand charge transfer on the nanoscale (8). This knowledge base is also critical to advances in molecular electronics (P). Progress in the areas of nanomaterials and nanoarchitectures, and new conceptual insights into charge transfer in molecular wires will ultimately contribute to the design of improved devices for energy storage and conversion. The electrified interfaces that power electrochemical storage and conversion devices, such as batteries, fuel cells (including biofuel cells), electrochemical capacitors, and dye-sensitized photovoltaic cells, require the motion of mobile ions in the electrolyte to counterbalance surface charges that arise as the energy of the electrons (i.e., the potential) varies. When the electrode becomes nanoscopic, and especially when it contacts a volume of electrolyte or fuel/oxidant in a nanoscale pore, size matters. One well-studied porous electrode, carbon, alerts those interested in devising nanostructured electrochemical power sources that the critical factor is not high surface area, but rather molecule-, ion-, and solvent-accessible surface area on the time scale of the electrochemical processes that store or generate ionic and electronic charge (10,11). In particular, the surface area that resides behind pore openings sized at < 1.5 nm, such as is typical in single-walled carbon nanotubes does not contribute to the electrochemically active surface area. Mesoporous nanostructured materials comprising bicontinuous networks of porosity and electrochemically active nanoscopic solid have already demonstrated the importance of arranging nanoscale building blocks in space to create multifunctional power-source architectures (3). Sol-gel-derived materials, such as aerogels, in which the porous network retains throughconnectivity (by minimizing compressive forces during processing), provide a means to investigate how electrochemical processes are influenced by (1) minimal solid-state diffusion lengths; (2) effective mass transport of ions and solvent to the nanoscale, networked (indeed, self-wired) electrode material; and (3) amplification of the surface (and surface-defect) character of the electrochemically active material. Recent work with nanoscopic, highly mesoporous, networked chargeinsertion oxides has indicated that such architectures store electrical charge by


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three, not two, means: (1) double-layer capacitance (as measured using bulky, non-insertion cations in which normalizing die capacitance by the physisorptionderived surface area yields standard values); (2) pseudocapacitance readily accessible at high rates of discharge (ideally expressed as a constant 9C/3E , with charge stored over a range of potentials atypical of a localized faradaic redox process); and (3) energetic-specific insertion of cations, as would be seen in a standard insertion battery material. These "colors" of capacitance at cationinsertion materials are depicted schematically in Figure 1.

Figure 1. The three types of charge stored at an electrified cation-insertion material; (a) formation of double-layer capacitance at an electrified interface (in this scheme at a potential negative of the point of zero charge); (b) electrochemical intercalation of Li* into a battery cathode; (c) proposed cation vacancies and accommodation of vacancy-balancing protons in Μη0 · (Reproducedfrom reference 5. Copyright 200J Royal Society of Chemistry.) 2


327 Materials that blend all three mechanisms of charge/energy storage are innately hybrid and should be able to provide future functions now served by the physical/electrical coupling of battery and capacitor in applications that require both peak power and high energy density expressed as low levels of power sustained over long duration use (5). The ability to store both battery-like charge and ultracapacitor-like charge is attributed to the disorder present in the nanoscale materials, which derive from sol-gel processing (5). Inducing protonstabilized cation vacancies in polycrystalline, Ι-μιη-sized V 0 powder (by heating at 460 °C under an oxygen/water atmosphere) was recently shown to increase the Li-ion capacity of the oxide relative to the as-received material, while inducing anion vacancies (by heating at 460 °C under a low partial pressure of oxygen) lowered the lithium-ion capacity (12). Coupling an ability to make disordered nanoscopic insertion materials with a computational understanding of how to increase the number of electrons transferred per transition metal cation, while minimizing deleterious structural or phase changes in the insertion material (13), will spur improvements in batteries impossible to achieve with mesoscale materials and structures. The energy conversion realized by fuel cells is innately nanoscopic, because the electroreactions in H2/O2 or direct methanol fuel cells are catalyzed, usually by carbon-supported nanoparticulate Pt-group-metals. But rather than just resolve the demands of standard heterogeneous catalysis (activity, selectivity, facile transport of reactants to and products from the catalytic site), the electrocatalysis that is essential in a fuel cell adds the need for high mobility of electrons and ions to and from the catalytic site. These additional demands create what is known as the three-phase boundary (14) between (i) the solid electrocatalyst (perched on or nestled in an electron-conductive porous carbon, which serves as the current collector that transports electrons—the true reactants and products of the fuel cell), (ii) the molecular fuel or oxidant (frequently a gas), and (Hi) the electrolytic medium that is liquid, gel-like, or polymeric and contains solvated mobile ions. The only truly reactive zone is the point (or line) of contact at the junction of the three phases (see Figure 2 (15)). Even with decades of work on this issue, fuel cells still require the design of improved structures to maximize the effective area of the three-phase boundary as well as the transport of all species to and from it. These challenges are opportunities awaiting creative nanoarchitectural design (4). Advances in nanoscale science and technology will improve the nature of the electrocatalysis and architectures critical to high-performance fuel cells, but nanoS&T already plays a key role in electrochemical devices that convert solar energy into electrons (e.g., in the dye-sensitized, wide-bandgap nanocrystalline seminconductor photovoltaic cells (3)), or solar energy into fuel (16), or in the control of enzymatic or biomimetic catalysis in order to convert non-petroleumderived fuels into electron energy (17). In the realm of thermal/electric, rather than chemical/electric, controlled fabrication on the nanoscale of thin-film


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superlattices of known thermoelectric materials has broken decades of stalemate to lead to improved thermoelectric performance relative to the performance of devices using the bulk material (18,19).

Figure 2. The three-phase boundary phase at acarbon-supported Pt electrocatalyst. To electrogenerate power (electrons per second at a specific potential) the reactive interphase requires transport of a gas-phase reactant (H in this example) to and adsorption at a Pt nanoparticle while maintaining intimate contact to a protonconducting phase (such as an acid electrolyte or a proton-exchange polymer). (Micrograph is reproducedfromreference 15. Copyright 2002 American Chemical Society.) 2

Future contributions to the materials of importance in energy storage and conversion will also require better theoretical, synthetic, and characterization approaches to disordered materials. A key opportunity/challenge will be to "pin" the most active, highest performance physicochemical state of the material even when exposed to thermodynamic forces (temperature, pressure, electrical potential, photochemical energy) that would otherwise drive restructuring, crystallization, denaturation, or densification of the nanoscale energy-storage or -conversion material. The session on "Nanotech-enabled green energy sources," held as part of the symposium on "Nanotechnology and the Environment" at the 225 Meeting of the American Chemical Society, 23-27 March 2003 in New Orleans, th



329 explored electrical energy as derived from electrochemical, thermal, solar, and biological conversion processes from the perspective of re-design and optimization on the nanoscale. Six experts described their latest work on the importance of nanoscience on fundamental processes that produce energy. The papers derived from this session follow and explore (i) the nature of charge transfer on the nanoscale; (ii) mesoporous battery nanoarchitectures; (Hi) nanostructured thin-film superlattices in thermoelectric devices; (iv) membraneless, microfluidic-based biofuel cells; and (v) hybrid energy converters that couple dye-sensitized, wide-bandgap semiconductor photovoltaics and biofuel cells. The fundamental science in all these areas is well underway. The future research will be richly influenced by nanoS&T, inevitably yielding new design strategies for nanotech-enabled green energy.

Acknowledgements The author is grateful for the sustained support of her team's research on multifunctional nanoarchitectures by the Office of Naval Research and the Defense Applied Research Projects Agency (DARPA).

References 1. Kaufmann, T. G.; Kaldor, Α.; Stuntz, G. F.; Kerby, M . C.; Ansell, L. L. Catalysis science and technology for cleaner transportation fuels. Catal. Today 2000, 62, 77-90. 2. Rossini, S. The impact of catalytic materials on fuel reformulation. Catal. Today 2003, 77, 467-484. 3. Grätzel, M . Photoelectrochemical cells. Nature 2001, 414, 338-344. 4. Rolison, D. R. Catalytic nanoarchitectures—the importance of nothing and the unimportance of periodicity. Science 2003, 299, 1698-1701. 5. Rolison, D. R.; Dunn, B. Electrically conductive oxide aerogels: new materials in electrochemistry. J. Mater. Chem. 2001, 11, 963-980. 6. Cava, R. J.; DiSalvo, F. J.; Brus, L. E.; Dunbar, K. R.; Gorman, C. B.; Haile, S. M.; Interrante, L. V.; Musfeldt, J. L.; Navrotsky, Α.; Nuzzo, R. G.; Pickett, W. E.; Wilkinson, A. P.; Ahn, C.; Allen, J. W.; Burns, P. C.; Ceder, G.; Chidsey, C. E. D.; Clegg, W.; Coronado, E.; Dai, H. J.; Deem, M . W.; Dunn, B. S.; Galli, G.; Jacobson, A. J.; Kanatzidis, M . ; Lin, W. B.; Manthiram, Α.; Mrksich, M.; Norris, D. J.; Nozik, A. J.; Peng, X. G.; Rawn, C.; Rolison, D.; Singh, D. J.; Toby, B. H.; Tolbert, S.; Wiesner, U. B.; Woodward, P. M.; Yang, P. D. Future directions in solid state chemistry:



330 report of the NSF-sponsored workshop. Progress Solid State Chem. 2002, 30, 1-101. 7. Jortner, J.; Bixon, M . Electron transfer-from isolated molecules to biomolecules. Adv. Chem. Phys. 1999, 106, 35-202, and references therein. 8. Adams, D. M . ; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P.; Lieberman, M.; Lindsay, S.; Marcus, R. Α.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M . D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X . Charge transfer on the nanoscale: current status. J. Phys. Chem. Β 2003, 107, 6668-6697. 9. Nitzan, Α.; Ratner, M . A. Electron transport in molecular wire junctions. Science, 2003, 300, 1384-1389. 10. Koresh, J.; Soffer, A. Double-layer capacitance and charging rate of ultramicroporous carbon electrodes. J. Electrochem. Soc. 1977, 124, 13791385. 11. Yang, K. L.; Yiacoumi, S.; Tsouris, C. Electrosorption capacitance of nanostructured carbon aerogel obtained by cyclic voltammetry. J. Electroanal. Chem. 2003, 540, 159-167. 12. Swider-Lyons, K. E.; Love, C. T.; Rolison, D. R. Improved lithium capacity of defective V O materials. SolidState Ionics 2002, 152-153, 99-104. 13. Hwang, B. J.; Tsai, Y. W.; Carlier, D.; Ceder, G. A combined computational/experimental study on LiNi Co Mn O . Chem. Mater. 2003, 15, 3676-3682. 14. Bockris, J. O'M.; Reddy, Α. Κ. N. Modern Electrochemistry 2; Plenum Press: New York, 1970; pp 1382-1385. 15. Anderson, M . L.; Stroud, R. M.; Rolison, D. R. Enhancing the activity of fuel-cell reactions by designing three-dimensional nanostructured architectures: catalyst-modified carbon-silica composite aerogels. Nano Lett. 2002, 2, 235-240 [correction: Nano Lett. 2003, 3, 1321]. 16. Millsaps, J. F.; Bruce, B. D.; Lee, J. W.; Greenbaum, E. Nanoscale photosynthesis: Photocatalytic production of hydrogen by platinized photosystem I reaction centers. Photochem.Photobiol.2001, 630-635. 17. Palmore, G. T. R.; Whitesides, G. M. Microbial and enzymatic biofuel cells. 2

5

1/3

1/3

1/3

2

In Enzymatic Conversion of Biomass for Fuels Production; Himmel, M. E.,

Baker, J. O., Overend, P., Eds.; Am. Chem. Soc. Symp. Ser. 566: Washington, DC, 1994; pp 271-290. 18. Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O'Quinn, B. Thin-film thermoelectric devices with high room-temperaturefiguresof merit. Nature 2001, 413, 597-602. 19. Harman, T.; Taylor, P. J.; Walsh, M . P.; LaForge, B . E . Quantum dot superlattice thermoelectric materials and devices. Science 2002, 297, 22292232.


Nanotechnology and the Environment - American Chemical Society

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