## PERSPECTIVE

## Don't Forget Long-Term Fundamental
Research in Energy

George M. Whitesides" and George W. Crabtree/

Achieving a fundamental understanding of the phenomena that will underpin both global stewardship and future technologies in energy calls for a thoughtful balance between large-scale immediate solutions using existing technology and the fundamental research needed to provide better solutions in the 50-year period.

E

nimy engineer-ing pre-occupations and and society. limate shared There change by is science. a range are now of views advo-cacy on energy for particular and almost technologies. religious levels There of is also agree-ment surprisingly that there broad is (although no single not solution universal) to the dual problems of meeting future demands for energy and managing the environmental consequences of energy production. Whatever strategy emerges will be a quilt made up of patches representing almost every imaginable technology.

The energy problem is often phrasal in terms of developing a strategy that roughly doubles the global production ofenergy by 2050 (from 13 to about 30 terawatts) (Fig. I) (1-9). The problem of climate change includes two especially important components: (i) understanding the chem-ist*, relationship of the between atmosphere the climate and oceans and the and (ii) predicting the impact on climate of different at-mospheric strategies for C'0, energy is the production. dominant Because greenhouse gas. and because coal is the carbon-rich fossil fuel whose use can most readily be expanded (especially in the rapidly growing economy of China). understanding the linkage between coal and climate is particularly important 16).

There is a pervasive sense that "We must do something soon." This urgency mny be justified. but we must also remember that the problems of environ-ment providing arc not energy about and to maintaining go away, no the matter how hard we try using current technologies. In the rush to do something to find technological solutions to global-scale problems—we should not forget that we must ultimately understand sci-ence a:s-tainable than and if solutions. we engineering are to Fundamental find is important. the most research Understanding effective, in <u>environ-ment</u> phenomena <u>leads to</u> relevant <u>new technologies</u> to energy and <u>and to</u> the <u>the ability</u>

to control the economic and environmental out-
comes of their applications (7).

The cost of large technology demonstration projects is enormous and the time to develop them decades, and it is easy to overlook the fundamental research that nourishes them. Today, we have a growing thicket of energy and environmental problems and great enthusiasm for solving them quickly. In fact, 50 years from now, most of these problems (and more) will still remain unsolved. Developing the best patches we can for the immediate problems is one approach. Understanding the underlying problems is another, and one that is at least as important, much less expensive, and perhaps ultimately time-saving. Energy and climate are problems that will extend over decades or centuries, and the unimaginable technologies of 100 years from now will rest on fundamental research that must start now.

What follows is a sketch of nine representative long-term problems in research that are vital to the development of future technology for energy. We emphasize that this list is personal and idiosyncratic; it tends to emphasize materials. Others might select differently, although most lists would probably have areas of broad overlap.

## The Oxygen Electrode Problem

A hydrogen fuel cell operates by extracting electrons from \( \mathrm{H}_{2} \) and transferring them through an external circuit to \( \mathrm{O}_{2} \) (to generate \( \mathrm{H}_{2} \mathrm{O} \) ). If the \( \mathrm{H}_{2} \) is generated electrochemically, the reverse reactions take place. In either event, the transfer of electrons from \( \mathrm{H}_{2} \mathrm{O} \) to one electrode and to \( \mathrm{O}_{2} \) from another are slow reactions and lower the efficiency of practical fuel cells (considering the free energy of the reactions involved). The slow rates of interconversion of \( 4 \mathrm{e}^{-}+\mathrm{O}_{2}+4 \mathrm{H}^{+} \)and \( 2 \mathrm{H}_{2} \mathrm{O} \) exemplify a broader class of reactions in which a single process requires the transfer of multiple electrons. Understanding these reactions and finding strategies for circumventing their limitations are important in developing new, more practical procedures for reactions ranging from the electrochemical production of \( \mathrm{H}_{2} \) and the use

of O2 in fuel cells to the reduction of nitrogen to ammonia.

## Catalysis by Design

Many of the reactions that occur in the production of energy involve catalysis: the full set used in the processing of crude oil to fuels; all of the biological reactions involved in photosynthesis, in fixing \( \mathrm{CO}_{2} \), and in biodegradation; the hydration of \( \mathrm{CO}_{2} \) to carbonate ion; the movement of electrons in batteries; the operation of fuel cells; the cleanup of exhaust gas from internal combustion engines; and many others. Given the enormous importance of catalysis in the production and storage of energy, in the production of petrochemicals and the materials derived from them, and in all biological and most geochemical processes, it is astonishing (and a little disheartening) how little is known of the fundamentals of catalysis: how catalysts operate, how to control them, and especially how to generate new ones. Catalysis by design has periodically been embraced as a grand challenge, and periodically abandoned as too difficult, but nanoscience and surface science offer new approaches to this problem. The fundamental study of catalysis must be reanimated across the full spectrum of processes involved in energy and the environment.

## Transport of Charge and Excitation

Pholoexcitation of the semiconductor or dye component of a solar cell creates an exciton: a separated but associated hole and electron (4, 5). To generate current the electron must move to one electrode, and the hole to the otha, before they combine. These processes arc inefficient in photo-cells: materials defective. that might polyclystalline, make inexpensive disordered, or quantum-dot semiconductors (whether inorganic circum-venting cost-effective or organic). their solar Understanding deficiencies cells. is them one key and to

## Chemistry of CO2

CO2 is a key molecule in global warming (6). in chemical and biological fuel production, and in fuel use. We must know everything possible about its physical and chemical interactions. Important topics include new uses of CO, in large-scale chemistry (where it has the attractive feature that it has negative cast), new chemical reactions of CO2. the movement and reactions of CO2 in the earth, the role of CO2 in determining the behavior of the atmosphere and oceans. and the chemistry and properties of CO2 at high pressures. For decades, there has been little msearch, whether fundamental or exploratory, in this area: it was considered a solved problem.

## Improving on Photosynthesis

The process of uptake and fixation of CO2 in biological photosynthesis is not an optimized

## Improving on Photosynthesis (cont.)

## SPECIALSECTION

| Primary energy source | Value (as shown) |
|---|---:|
| Petroleum | 40.13 |
| Natural Gas | 22.66 |
| Coal | 22.87 |
| Nuclear | 8.13 |
| Biomass | 2.78 |
| Hydro | 2.71 |
| Geothermal | 0.33 |
| Wind | 0.15 |
| Solar | 0.006 |

| Petroleum / Natural Gas breakdown | Value (as shown) |
|---|---:|
| Petroleum — Domestic | 16.13 |
| Petroleum — Net imports | 24.0 |
| Natural Gas — Domestic | 19.06 |
| Natural Gas — Net imports | 3.6 |

| Electricity / transmission | Value (as shown) |
|---|---:|
| Electricity generation | 38.0 |
| Distributed electricity | 12.46 |
| Transmission & distribution losses | 2.54 |

| End‑use sectors (final flows) | Value (as shown) |
|---|---:|
| Residential | 11.60 |
| Commercial | 8.47 |
| Industrial | 24.33 |
| Light‑duty vehicles | 17.01 |
| Freight / Other | 7.60 |
| Aircraft | 3.37 |

Values transcribed directly from the figure; units not shown in the image.

ency requires understanding the fundamental phenomena of existing and alternative energy conversions. Solid-state lighting, for example, can achieve efficiencies of \( 50 \% \) or more, provided that we understand the mechanisms controlling the conversion of electronic energy to photons. New understanding of mechanisms of friction, wear, and corrosion also provides new strategies for reducing losses.

## The Chemistry of Small Molecules

The chemistry of small molecules dominates many aspects of energy and climate: \( \mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2}, \mathrm{O}_{2}, \mathrm{CO}_{2}, \mathrm{CO} \) (for Fischer-Tropsch chemistry), \( \mathrm{NO}_{\mathrm{x}}, \mathrm{O}_{3}, \mathrm{NH}_{3}, \mathrm{SO}_{2}, \mathrm{CH}_{4} \), \( \mathrm{CH}_{3} \mathrm{OH}, \mathrm{HCl} \), and others are all vitally important components of these discussions. There remains a wide range of information about these molecules and their combinations that is needed to un-

Fig. 1. The complex system of energy flows in the United States in 2005 (1). More than half of the energy produced is wasted. Units are in quads; 1 quad = 1015 British thermal units = 1.055 exajoules. [Figure prepared by Lawrence Livermore National Laboratory, University of California, and the U.S. Department of Energy]

marvel: It is fairly inefficient thermodynamical-
ly, and several key reactions [for example, the
key step involving the reaction of CO2 with
ribulose diphosphate (with its competing reac-
tion with O2)] have a surprisingly poor yield.
These inefficiencies are an opportunity. Photo-
synthesis has immense appeal for the closed-
cycle capture of energy from the Sun in forms
that are useful as fuels (4). The prospects of re-
engineering biological photosynthesis for greater
efficiency, maximizing metabolic flux through
specific biosynthetic pathways, growing plants
in regimes of temperature or salinity where they
normally do not flourish, and directly produc-
ing fuels such as H2, CH4, or alcohols are all
alluring ones, but ones that will require decades
of imaginative research to realize. The prospect
of non-biological photosynthesis (where "pho-
tosynthesis" might include bio-inspired physical
and chemical reactions or more straightforward
photochemical or photothermal processes that
generate fuels or store energy) also warrants
new research.

derstand the complex systems of which they are a part.

## New Ideas: Separating Wheat from Chaff

The spectrum of ideas for dealing with problems of energy and global stewardship is not complete, based just on what we now know. We need new ideas, and we need to know which of the current smorgasbord of unexplored and unproved ideas will work (9). Developing affordable technologies for removing carbon from the atmosphere (for example, by growing biomass and converting it to a stable form of carbon) must be explored now, if they are to be options in the future. Changing the albedo of Earth, stimulating photosynthesis in the oceans by the addition of essential trace elements such as iron, developing new nuclear power cycles, a hydrogen economy, new methods for separating gases (such as \( \mathrm{CO}_{2} \) from air) and liquids, room-temperature superconductivity to carry electric power without loss, biological \( \mathrm{H}_{2} \) production, new concepts in batteries, and nuclear fusion-all must be explored fundamentally and realistically.

linearly. These systems are the natural home of big surprises—often referred to as emergent behavior. Our difficulty in understanding and modeling these systems leads to uncertainties that cloud most discussions of energy and the environment and of the costs and impacts of almost any technology (5). What really is the cost of a kilowatt produced by silicon solar cells? Now important will the burning of coal be to global warming? What, in &tail, are the global sources and sinks for carbon and how do they interact? What can one say about the impact of technologies for generating nuclear nu-clear power weapons? on the potential Development for proliferation of the theory of of re-liable complex results systems (or at to least the point results where whose it reliability gives ca-pability, min-imizing can unintended be quantified) and the consequences. potential is probably remains miseries the a best key of way enabling the of law of

## Complex Systems

The Efficiency of Energy Use

Understanding energy and the environment analytically poses a series of problems that we presently have neither the mathematical tools nor the data to solve. Most global systems are "complex" in physicists' definition of the word: They comprise many components, with many degrees of freedom, usually interacting non-

Increasing the efficiency of energy conversion
and storage is a major opportunity. Many of
our standard energy conversion routes are far
Electric-ity from production their Camot with efficiency the present limits: mix of fuels

is only 37% efficient on average, the typical
automobile engine is perhaps 25% efficient,
and an incandescent bulb is only 5% efficient
for producing visible light. Increasing eflici-

These problems all require long-term, patient investment in fundamental research to yield new and validated ideas. These problems are also, in some cases, sufficiently technical that their importance is most obvious to specialists. The oxygen electrode (as one example) might seem an exotic problem in science, but it is hard to

## Complex Systems (cont.)

## Sustainability and Energy

believe that a hydrogen economy that used electrolysis to generate H2 and O2 from water. and a fuel cell to convert H2 and Q back to water and electrom, could make a substantial much-improved contribution oxygen to global electrode. energy The without identification a of this problem is not in any sense new: The redox chemistry of oxygen has been a subject of active interest (but limited success) for decades. We simply need new ideas.

Another reason to work on these big problems is that they will attract the most talented young people. Over the past 30 years, the National Institutes of Health has used stable and generous support to recruit and build a very effective community of biomedical scientists.

Solving the problems of energy and global stewardship will require the same patient, flexible, and broadly based investment, if society believes that the problems in these area arc sufficiently important to provide a life's work for its most talenteel young people.

## References and Notes

1. President's Council of Advisors on Science and Technology (PCAST), The Energy Imperative. Technology and the Role of Emerging Companies (2006), www.ostp.gov/PCAST/pcast.html.

2. World Energy Outlook 2004 (International Energy Agency, Paris, 2004), www.worldenergyoutlook.org/.

3. Basic Research Needs to Assure a Secure Energy Future. J. Stringer, L. Horton, Chairs [workshop report, U.S. Department of Energy (DOE) Office of Basic Energy Sciences, 2003], www.sc.doe.gov/bes/reports/abstracts.html#SEC.

	4. Basic Research Needs for Solar Energy Utilization, N. S. Lewis, G. W. Crabtree, Chairs (workshop report, DOE Office of Basic Energy Sciences, 2005), www.sc.doe.gov/bes/reports/abstracts.html#SEU.

	5. Systems and life-cycle energy Technology Analyses (National Renewable Energy Laboratory), www.nrel.gov/ analysis/tech_analysis.html.

	6. See discussions of global climate science from the National Center for Atmospheric Research, vnew.uut.edur researchAtimate.

	7. J. M. Deutch, R. K. Lester, Making Technology Work: Applications in Energy and the Environment (Cambridge Univ. Press, New York, 2004).

	B. N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. U.S.A. 103, 15729 (2006).

	9. M. S. Dresselhaus, I. L. Thomas, Nature 414, 332 (2001).

10.1126/science.1140362

PERSPECTIVE

## Toward Cost-Effective
Solar Energy Use

Nathan 5. Lewis

At present, solar energy conversion technologies face cost and scalability hurdles in the technologies required for a complete energy system. To provide a truly widespread primary energy source, solar energy must be captured, converted, and stored in a cost-effective fashion. New developments in nanotechnology, biotechnology, and the materials and physical sciences may enable step-change approaches to cost-effective, globally scalable systems for solar energy use.

More energy from sunlight strikes Earth in 1 hour than all of the energy consumed by humans in an entire year. In fact, the solar energy resource dwarfs all other renewable and fossil-based energy resources combined (1). With increasing attention toward carbon-neutral energy production, solar electricity-or photovoltaic (PV) technology-is receiving heightened attention as a potentially widespread approach to sustainable energy production. The global solar electricity market is currently more than $10 billion/year, and the industry is growing at more than \( 30 \% \) per annum (2). However, low-cost, base-loadable, fossilbased electricity has always served as a formidable cost competitor for electrical power generation. To provide a truly widespread primary energy source, solar energy must be captured, converted, and stored in a cost-effective fashion. Even a solar electricity device that operated at near the theoretical limit of \( 70 \% \) efficiency would not provide the needed technology if it were expensive and if there were no cost-effective mechanism to store and dispatch the converted

solar energy upon demand (3). Hence, a complete solar-based energy system will not only require cost reduction in existing PV manufacturing methods, but will also require science and technology breakthroughs to enable, in a convenient, scalably manufacturable form, the ultralowcost capture, conversion, and storage of sunlight.

One key step is the capture and conversion of the energy contained in solar photons. Figure elec-tricity as I a shows function the fully of the amortized efficiency cost and of cost of an installed IN module (2, 4). Because the total energy provided by the Sun is fixed over the 30- year lifetime of a PV module, once the energy estab-lished, conversion the total efficiency amount of of a PV "product" module electricity is mid-latitude produced system. location The by theoretical the module is known al for a representative the lifetime limit for of even the efficiency an optimal single band gap solar conversion device is 31%, because photons having energies lower than the absorption threshold ofthe active PV material are not absorbed, whereas photons having energies much higher than the band gap rapidly release heat to the lattice ofthe solid and in-ternal therefore energy ultimately equal to contain that of only the band a useful gap (2). Small test cells have demonstrated efficiencies of >20%. with the remaining losses almost en-

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. Shipped PV modules now have efficiencies of 15 to \( 20 \% \) in many cases. At such an efficiency, if the cost of a module is \( \sim \$ 300 / \mathrm{m}^{2}(2) \), and if we take into account the accompanying fixed costs in the so-called "balance of systems" (such as the inverter, grid connection, etc., which add a factor of \( \sim 2 \) to the total installed system cost), then the sale price of grid-connected PV electricity must be \( \$ 0.25 \) to \( \$ 0.30 \) per kilowatt-hour (kWh) to recover the initial capital investment and cost of money over the lifetime of the PV installation (2, 4). Currently, however, utilityscale electrical power generation costs are much less, with current and new installations costing \( \sim \$ 0.03 \) to \( \$ 0.05 \) per kWh (1). Hence, for solar electricity to be cost-competitive with fossilbased electricity at utility scale, improvements in efficiency are helpful, but manufacturing costs must be substantially reduced.

In current manufacturing schemes for Si-based solar cells, the cost of the processed and purified Si is only about \( 10 \% \) of the final cost of the PV module. Some of the Si is lost in cutting up boules into wafers, and other costs are incurred in polishing the wafers, making the diffused junction in the Si into a photovoltaic device, fabricating the conducting transparent glass, masking and making the electrical contacts, sealing the cells, connecting the cells together reliably into a module, and sealing the module for shipment. Hence, in such systems, the energy conversion efficiency is at a premium so as to better amortize these other fixed costs involved with making the final PV module.

Improvements in efficiency above the 31% theoretical limit are possible ifthe constraints that Shockley-Qucisser are (2). incorporated For theoretical example. into if efficiency photons the so-called limit having are energies relaxed greater than the band gap of the absorbing material did not dissipate their excess energy as heat. but instead produced more voltage or