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Sustainability and Energy of 0 2 in fuel cells to the reduction of nitrogen PERSPECTIVE to ammonia. Don't Forget Long-Term Fundamental Catalysis by Design Many of the reactions that occur in the pro- duction of energy involve catalysis: the full set Research in Energy used in the processing of crude oil to fucks; all of the biological reactions involved in photo- George M. Whitesides" and George W. Crabtree/ synthesis, in fixing CO,, and in biodegrada- tion; the hydration of CO, to carbonate ion: the Achieving a fundamental understanding of the phenomena that will underpin both global movement of electrons in batteries: the opera- stewardship and future technologies in energy calls for a thoughtful balance between large-scale tion of fuel cells: the cleanup of exhaust gas immediate solutions using existing technology and the fundamental research needed to provide from internal combustion engines: and many better solutions in the 50-year period. others. Given the enormous importance of ca- talysis in the production and storage of energy. nimy and limate change are now pre- to control the avnomic and environmental out- in the production of petrochemicals and the ma- E occupations shared by science. engineer- ing and society. There is a range of views on energy and almost religious levels of advo- comes of their applications (7). The cost of large technology demonstra- tion projects is enormous and the time to de- tenals derived from them, and in all biological and most geochemical processes it is astonish- ing (and a little disheartening) how little is cacy for particular technologies. There is also velop them decades, and it is easy to overlook known of the fundamentals of catalysis: how surprisingly broad (although not universal) agree- the fundamental research that nourishes them. catalysts operate, how to control them, and es- ment that there is no single solution to the dual Today, we have a growing thicket of energy pecially how to generate new ones. Catalysis by problems of meeting future demands for energy and environmental problems and great enthu- design has periodically been embraced as a and managing the environmental consequences siasm for solving them quickly. In fact. 50 grand challenge, and periodically abandoned as of energy production. Whatever strategy emerges years from now, most of these problems (and too difficult, but nanoseicncc and surface sci- will be a quilt made up of patches representing more) will still remain unsolved. Developing ence offer new approaches to this problem. The almost every imaginable technology. the best patches we can for the immediate fundamental study of catalysis roust be re- The energy problem is often phrasal in terms problems is one approach. Understanding the animates! across the full spectrum of processes of developing a strategy that roughly doubles the underlying problems is another, and one that involved in energy and the environment. global production of energy by 2050 (from 13 to is at least as important, much less expensive, about 30 terawatts) (Fig. I) (1-9). The problem and perhaps ultimately time-saving. Energy Transport of Charge and Excitation of climate change includes two especially and climate are problems that will extend over Pholoexcitation of the semiconductor or dye important components: (i) understanding the decades or centuries, and the unimaginable component of a solar cell creates an exciton: a relationship between the climate and the chem- technologies of 100 years from now will reit separated but associated hole and electron (4, 5). ist*, of the atmosphere and oceans and (ii) on fundamental research that must start now. To generate current the electron must move to predicting the impact on climate of different What follows is a sketch of nine represent- one electrode, and the hole to the otha, before strategies for energy production. Because at- ative long-tem problems in research that are they combine. These processes arc inefficient in mospheric C'0, is the dominant greenhouse vital to the development of future technology materials that might make inexpensive photo- gas. and because coal is the carbon-rich fossil for energy. We emphasize that this list is per- cells: defective. polyclystalline, disordered, or fuel whose use can most readily be expanded sonal and idiosyncratic; it tends to emphasize quantum-dot semiconductors (whether inorganic (especially in the rapidly growing economy of materials. Others might select differently, al- or organic). Understanding them and circum- China). understanding the linkage between coal though most lists would probably have areas venting their deficiencies is one key to cost- and climate is particularly important 16). of broad overlap. effective solar cells. There is a pervasive sense that "We must do something soon." This urgency mny be justified. The Oxygen Electrode Problem Chemistry of CO2 but we must also remember that the problems of A hydrogen fuel cell operates by extracting eke- CO2 is a key molecule in global warming (6). providing energy and maintaining the environ- trons from i12 and transferring them through in chemical and biological fuel production, and ment arc not about to go away, no matter how an external circuit to 0 2 (to generate I-120). If in fuel use. We must know everything possible hard we try using current technologies. In the the H2 is generated electrochemically, the re- about its physical and chemical interactions. rush to do something to find technological verse reactions take place. In either event, the Important topics include new uses of CO, in solutions to global-scale problems—we should transfer of electrons from H2O to one elec- large-scale chemistry (where it has the attractive not forget that we must ultimately understand trode and to 0 2 from another arc slow reac- feature that it has negative cast), new chemical than if we are to find the most effective, a:s- tions and lower the efficiency of practical fuel reactions of CO2. the movement and reactions of tainable solutions. Fundamental research in sci- cells (considering the free energy of the reac- CO2 in the earth, the role of CO2 in determining ence and engineering is important. Understanding tions involved). The slow rates of interconversion the behavior of the atmosphere and oceans. phenomena relevant to energy and the environ- of 4 e + 02 + 4 W and 2 H,0 exemplify a and the chemistry and properties of CO2 at high ment leads to new technologies and to the ability broader clam of reactions in which a single pressures. For decades, there has been little process requires the transfer of multiple elec- msearch, whether fundamental or exploratory, in 'Department of Chemistry and Chemical Biology, Harvard trons. Understanding these reactions and find- this area: it was considered a solved problem. University. Cambridge, IAA 02138. USA, IMaterlals Science Division. Argonne National Laboratory. Argonne, IL 60439, ing strategies for circumventing their limitations USA. arc important in developing new, more prac- Improving on Photosynthesis 'Co %horn correspondence should be addressed. E-mail: tical procedures for reactions ranging from the The process of uptake and fixation of CO2 in [email protected] electrochemical production of H2 and the use biological photosynthesis is not an optimized 796 9 FEBRUARY 2007 VOL 315 SCIENCE www.sciencemag.org EFTA_R1_01521504 EFTA02444589  SPECIALSECTION ency requires understanding l•Pct , PC at we Omar obt.off•• wnes the fundamental phenome- IFS iry na of existing and alternative (144.tPGA energy conversions. Solid- Oninthowe state lighting, for example. Orsects Ite.3.n.a.1 can achieve efficiencies of *.ittl Cr ) 50% or inure, provided that .a Commeetsal we understand the mecha- nisms controlling the conva- sion of electronic energy to photons. New understand- ing of mechanisms of fric- tion. wear, and corrosion also provides new strategies for re- ducing lasses. The Chemistry of Small Molecules Thc chemistry of small mol- ecules dominates many as- pects of energy and climate: 112O, 112, O2. CO2. CO (for Fischer-Tropsch chemistry). NO,. O3. NH,.. SO2. O14. ClItOll, NCI, and others arc all vitally important corn- Fitments of these dens:ions. Ng. 1. The complex system of energy Rows in the United States in 2005 (V. More than half of the energy produced is T ere remains a wide range wasted. Units are in quads; 1 quad = 1015 British thermal units = 1.055 exajoules. [Figure prepared by Lawrence of inlimnation about these Livermore National Laboratory, University of California, and the U.S. Department of Energy] molecules and their combi- nations that is needed to un- marvel: It is fairly inefficient thermodynamical- linearly. These systems are the natural home derstand the complex systems of which they ly. and several key reactions [for example, the of big surprises—often referred to as emergent are a part. key step involving the reaction of CO2 with behavior. Our difficulty in understanding and ribulose diphosphate (with its competing reac- modeling these systems leads to uncertainties New Ideas: Separating Wheat from Chaff tion with th)) have a surprisingly poor yield. that cloud most discussions of energy and the The spectum of ideas for dealing with prob- These inefficiencies are an opportunity. Photo- environment and of the costs and impacts of lems of energy and global stewardship is not synthesis has immense appeal for the closed. almost any technology (5). What really is the complete. based just on what we now know. cycle capture of energy from the Sun in forms cost of a kilowatt produced by silicon solar We need new ideas, and we need to know that arc useful as fuels (4). Thc prospects of re- cells? Now important will the burning of coal which of the current smorgasbord of unex- engineering biological photosynthesis for greater be to global warming? What, in &tail, are the plored and unproved ideas will work (9). efficiency, maximizing metabolic flux through global sources and sinks for carbon and how Developing affordable technologies for remov- specific biosynthetic pathways. growing plants do they interact? What can one say about the ing carbon from the atmosphere (for example. in regimes of temperature or salinity where they impact of technologies for generating nuclear by growing biomass and converting it to a stable normally do not flourish, and directly produc- power on the potential for proliferation of nu- form of carbon) must be explored now, if they ing fuels such as 112, 0114, or alcohols arc all clear weapons? Development of the theory of are to be options in the future. Changing the alluring ones, but ones that will require decades complex systems to the point where it gives re- albedo of Earth, stimulating photosynthesis in of imaginative research to realize. The prospect liable results (or at least results whose reliability the oceans by the addition of essential trace ek- of non-biological photosynthesis (where 'pho- can be quantified) remains a key enabling ca- malts such as iron, developing new nuclear tosynthesis" might include No-inspired physical pability, and is probably the best way of min- power cycles, a hydrogen economy, new meth- and chemical reactions or more straightfonvard imizing the potential miseries of the law of orb for separating gases (such as CO2 from air) photochemical or photothennal processes that unintended consequences. and liquids, mom-temperature superconductivity generate fuels or store energy) also warrants to carry electric power without loss biological new research. The Efficiency of Energy Use II, production, new concepts in batteries, and Increasing the efficiency of energy conversion nuclear fusion all must be explored fundamen- Complex Systems and storage is a major opportunity. Many of tally and realistically. Understanding energy and the environment our standard energy conversion routes are far These problems all require long-tens, patient analytically poses a series of problems that we from their Camot efficiency limits: Electric- investment in fundamental research to yield new presently have neither the mathematical tools ity production with the present mix of fuels and validated ideas. These problems arc also. nor the data to solve. Most global systems are is only 37% efficient on average, the typical in some cases, sufficiently technical that their "complex" in physicists' definition of the word: automobile engine is perhaps 25% efficient, importance is most obvious to specialists. The They comprise many components, with many and an incandescent bulb is only 5% efficient oxygen electrode (as one example) might seem degrees of freedom, usually interacting non- for producing visible light. Increasing eflici- an exotic problem in science, but it is hard to www.sciencemag.org SCIENCE VOL 315 9 FEBRUARY 2007 797 EFTA_R1_01521505 EFTA02444590  Sustainability and Energy believe that a hydrogen economy that used Solving the problems of energy and global 4. Bask Research Needs for Solar Energy Utilkorion, electrolysis to generate H2 and O2 from water. stewardship will require the same patient, N. S. Lerns. G. W. Crabtree, Chairs bookshop report, DOE Office of Bask Energy Selerkes, 2005). moise doe. and a fuel cell to convert H2 and Q back to flexible, and broadly based investment, if goctecreponstabstracts.hlmlOSEU. water and electrom, could make a substantial society believes that the problems in these area 5. Systems and tile-cycle energy Technology Analyses contribution to global energy without a much- arc sufficiently important to provide a life's (Nelanal Renewable Energy laboratory!, onwrirel god improved oxygen electrode. The identification work for its most talenteel young people. anahuvton_analysts.hunl. 6. See discussions of global climate science from the of this problem is not in any sense new: The National Center for Atmospheric Research, vnew.uut.edur redox chemistry of oxygen has been a subject of References and Notes researchAtimate. active interest (but limited success) for decades. I. President's Council of Adviseas on Science and 7. ). M. Omit R. K. Lester. Meting Technology Wart: Technology MASI). the Energy Imperative reehnolo9Y Applkohons In Energy and the Environment (Cambridge We simply need new ideas. and the Role of Emerging Companies 12006% mwrostp. unw. Press. New Yak, 2004/ Another mason to work on these big prob- gowPCASI/pcast.lumt. 8. N. S. Lewis. D. G. Noses. Pre. Holt Arad. Sci. USA lems is that they will attract the most talented 2. World Energy Outfooi 1004 (International Energy 103. 15729 (2006). young people. Over the past 30 years, the Na- Agency. Paris. 2004). vnrwriondenergroutIookorgt. 9. M. S. orenethair. I. L. Thomas. Nature 414. 332 3. Bask Research Needs to Assure o Secure Energy Future. (2001). tional Institutes of Health has used stable and 1 Stringer, L. Nato. Gins loorkihap report. IJS generous support to recruit and build a very Department of Energy (001) Office of Bask Energy Sciences. effective community of biomedical scientists. 20031. woctsc.doe.godbegrepenstabstracts.htnal6EC. 10.112Edscience.1140362 PERSPECTIVE tirely due to small reflection losses, grid shading losses• and other losses at the 5 to 10% level that any practical system will have to some extent. Toward Cost-Effective Shipped PV modules now have efficiencies of IS to 20% in many cases. At such an efficiency, Solar Energy Use if the cost ofa module is —53001m2 (2), and if we take into account the accompanying fixed costs in the so-called "balance of system" (such as Nathan 5. Lewis the inverter, grid connection, etc., which add a factor of -2 to the total installed system cost), At present, solar energy conversion technologies face cost and scalability hurdles in the then the sale price of grid-connected PV elec- technologies required for a complete energy system. To provide a truly widespread primary energy tricity must be 50.25 to 50.30 per kilowatt-hour source, solar energy must be captured, converted, and stored in a cost-effective fashion. New (kWh) to recover the initial capital investment developments in nanotechnology, biotechnology, and the materials and physical sciences may and cost of money over the lifetime of the PV enable step-change approaches to cost-effective, globally scalable systems for solar energy use. installation (2. 4). Qurently, however, utility- scale electrical power generation costs BM much ore energy from sunlight strikes Earth solar energy upon demand (3). Hence, a com- less, with current and new installations costing m in I hour than all of the energy con- sumed by humans in an entire year. In f ct, the solar energy resource dwarfs all other plete solar-basal energy system will not only require cost reduction in existing IN manufac- turing methods, but will also requite science and -S0.03 to 50.05 per kWh (1). Hence. for solar electricity to be cost-competitive with fossil- based electricity at utility scale, improvements renewable and fossil-based energy resources technology breakthroughs to enable, in a conve- in efficiency are helpful, but manufacturing costs combined (I). With increasing attention to- nient, scalably manufacturablc tom . the ultralow- must be substantially reduced. ward carbon-neutral energy production, solar cost capture, onwasion, and storage of sunlight. In current manufasiuring schemes for Si- electricity—or photovoltaic (PV) technology—is One key step is the capture and conversion based solar cells, the cost of the processed and receiving heightened attention as a potentially of the energy contained in solar photons. purified Si is only about 10% of the final cost of widespread approach to sustainable energy pro- Figure I shows the fully amortized cost of elec- the PV module. Some of the Si is lost in cutting duction. The global solar electricity market is tricity as a function of the efficiency and cost of up boules into wafers, and other costs arc currently more than S I 0 billion/year. and the in- an installed IN module (2, 4). Because the total incurred in polishing the wafers, making the dustry is growing at more than 30% per annum energy provided by the Sun is fixed over the 30- diffused junction in the Si into a photovoltaic (2). However, low-cost, base-loadable. fossil- year lifetime of a PV module, once the energy device, fabricating the conducting transparent based electricity has always served as a for- conversion efficiency of a PV module is estab- glass, masking and making the electrical con- midable cost competitor for electrical power lished, the total amount of "product" electricity tacts. sealing the cells, connecting the cells generation. To provide a truly widespread primary produced by the module al a representative mid- together reliably into a module, and sealing the energy source, solar energy must be captured, latitude location is known for the lifetime of the module for shipment. I knee, in such systems, convened, and stored in a cast-effective fashion. system. The theoretical efficiency limit for even the energy conversion efficiency is at a premium Even a solar electricity device that operated at an optimal single band gap solar conversion so as to bate amortize these other fixed costs near the theoretical limit of70% efficiency would device is 31%, because photons having energies involved with snaking the final PV module. not provide the needed technology if it were lower than the absorption threshold of the active Improvements in efficiency above the 31% expensive and if there were no cost-effective PV material are not absorbed, whereas photons theoretical limit are possible if the constraints that mechanism to store and dispatch the converted having energies much higher than the band gap are incorporated into the so-called Shockley- rapidly release heat to the lattice of the solid and Qucisser theoretical efficiency limit are relaxed therefore ultimately contain only a useful in- (2). For example. if photons having energies Beckman Institute and Ko4i Nanosoence Institute, 210 Noyes Laboratory, 127-72, California Institute of Tedinal. ternal energy equal to that of the band gap (2). greater than the band gap of the absorbing ogy, Pasadena, CA 91125. USA E-mail: nslovisapits. Small test cells have demonstrated efficiencies material did not dissipate their excess energy as caltech.edu of >20%. with the remaining losses almost en- heat. but instead produced more voltage or 798 9 FEBRUARY 2O07 VOL 315 SCIENCE www.sciencemag.org EFTA_R1_01521506 EFTA02444591