subscriber access provided by universitaetsbibliothek ...kg) (see chapter 3 of ref 1). different...

Subscriber access provided by Universitaetsbibliothek | Erlangen-Nuernberg

Inorganic Chemistry is published by the American Chemical Society. 1155 SixteenthStreet N.W., Washington, DC 20036

Article

Preface: Overview of the Forum on Solar and Renewable EnergyRichard Eisenberg, and Daniel G. Nocera

Inorg. Chem., 2005, 44 (20), 6799-6801• DOI: 10.1021/ic058006i • Publication Date (Web): 26 September 2005

Downloaded from http://pubs.acs.org on May 5, 2009

More About This Article

Additional resources and features associated with this article are available within the HTML version:

• Supporting Information• Links to the 13 articles that cite this article, as of the time of this article download• Access to high resolution figures• Links to articles and content related to this article• Copyright permission to reproduce figures and/or text from this article

http://pubs.acs.org/doi/full/10.1021/ic058006i

ForumPreface: Overview of the Forum on Solar and Renewable Energy

Richard Eisenberg* ,† and Daniel G. Nocera ‡

Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and Departmentof Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Received August 2, 2005

Inorganic Chemistryis pleased to present a Forum on Solarand Renewable Energy. The contributions that followrepresent a sampler of cutting edge research that deals withdifferent aspects of the science related to our Forum themefrom the considered perspective of chemistry and, mostespecially, inorganic chemistry. The Articles are authoredby leaders in the field and represent accounts of research intheir respective laboratories ranging from dye-sensitized solarcells (DSSCs) and the photophysics of irradiated semicon-ductors to hydrogen generation and design considerationsfor artificial photosynthetic systems. They represent someof the science on which tomorrow’s technology for energygeneration will be built. There is no more serious challengefacing society today than energy for sustainable development.

In 1957, Charles David Keeling started a life-long seriesof measurements of global CO2 levels. The activity wasinitiated as part of the International Geophysical year andwas performed at Mauna Loa, HI, to remove local biases inthe data and provide a reliable and reproducible baselinemeasurement of CO2 in the atmosphere. The initial measure-ments revealed a surprising and unexpected annual oscillationthat arose from different levels of photosynthetic activitybetween winter and summer in a world where most of theland mass is in the northern hemisphere. It was said thatKeeling was observing the planet breathing. However, whatwas even more striking than the annual CO2 oscillation of aliving planet was the relentless upward bias in the baselinevalue of atmospheric CO2 from year to year. The averagebaseline level of CO2, which in 1957 was 316 ppm, hasincreased to 370 ppm in 2003. It has been estimated that theaverage global CO2 level at the dawn of the industrialrevolution was 280 ppm based on trapped air analyzed inice cores, meaning that, since the latter part of the 19thcentury, this value has increased by 32%. These numbersare facts. Data through 1999 along with primary referencesup to that point may be found in theWorld EnergyAssessment,1 which is available online (http://www.undp.org/

seed/eap/activities/wea/). The growth in the atmospheric CO2inventory tracks the increase in global burning of fossil fuelover the past 150 years. Detailed analysis of relative carbonisotope abundances confirms that the observed CO2 increasecomes from fossil fuels. The rate of CO2 increase, whichitself is accelerating, is by far the largest for any comparableperiod of time and reflects a world of both growingpopulation and industrialization as people conduct all mannerof activities associated with the functioning of modernsocieties: transportation, agriculture, manufacturing, con-struction, and heating, lighting, and cooling of homes andbusinesses. All of these activities consume energy.

In 1998, global annual energy consumption was 402exajoules (12.7 TW), with the United States portion of thatamount corresponding to approximately 25%. Of that globalamount, 80% was generated from fossil fuels with aconsequent annual emission of nearly 7 Gt C (1 Gt) 1012kg) (see Chapter 3 of ref 1). Different scenarios have beenproposed for future global annual energy needs with valuesof 837-1041 exajoules (26.4-32.9 TW) estimated formiddle to high growth by 2050 and 1464-1859 exajoules(46.3-58.7 TW) by 2100. In an analysis of these numbersand their implications for the future, many projections ofannual emissions and atmospheric inventory of CO2 havebeen made.2,3 The resultant numbers vary widely dependingon the assumed composition of energy sources, the efficiencyof energy production and consumption, the global economy,and different intervention scenarios to control CO2 levels.Modestly stringent interventions are based on stabilizingatmospheric CO2 in the 550-650 ppm range with substan-tially higher values projected (>750 ppm) if the present mixof global energy sources is maintained.

Clearly, CO2 levels will continue to increase well into thefuture. It is important to recognize, at least briefly, what thismeans from an environmental standpoint. Records already

* To whom correspondence should be addressed. E-mail: [email protected].

† University of Rochester.‡ Massachusetts Institute of Technology.

(1) World Energy Assessment: Energy and the Challenge of Sustainability;United Nations: New York, 2000.

(2) Wigley, T. M. L.; Richels, R.; Edmonds, J. A.Nature1996, 379, 240-243.

(3) Hoffert, M. I.; Caldeira, K.; Jain, A. K.; Haites, E. F.; Harvey, L. D.;Potter, S. D.; Schlesinger, M. E.; Wigley, T. M. L.; Wuebbles, D. J.Nature1998, 395, 881-884.

Inorg. Chem. 2005, 44, 6799−6801

10.1021/ic058006i CCC: $30.25 © 2005 American Chemical Society Inorganic Chemistry, Vol. 44, No. 20, 2005 6799Published on Web 09/26/2005

indicate that we are in a period of global warming, with afurther rise in temperature to be expected. Climate sensitivityis a parameter that can be used to estimate that increase andis defined as the change in the mean global temperatureresulting from a doubling of atmospheric CO2.4 While therange of values estimated for climate sensitivity may notappear large (1.5-4.5 °C), the consequences will be sub-stantial, and if actual climate sensitivity lies in the upperhalf of the estimated range, the results may be devastating,at least for some regions, habitats, and species. Temperatureincreases alone tell only a fraction of the story. Highertemperatures lead to increased cycling of water and watervapor, higher levels of atmospheric water vapor (also agreenhouse gas), changing and more severe weather patterns,and rising sea levels from thinning polar icecaps. While theseconsequences are predicted and not guaranteed, what iscertain is that we are perturbing the planet on a scale neverdone before by any of its inhabitants with consequentialeffects that are unquestionably serious and possibly cata-strophic. It is thus imperative that the global communitymoves, and moves quickly, to a more CO2-neutral energyproduction profile.

If we consider where the world stands today in terms ofenergy use and where it will be in 2050 assuming continuedeconomic development, we are faced with a dauntingchallenge of where that energy will come from if our energyprofile is to be more CO2-neutral. It is likely, in fact, thatreserves of readily accessible oil will be declining over thiscentury, and while coal is abundant, it offers a host ofenvironmental problems in addition to large-scale CO2generation that must be addressed (coal is the least hydrogen-rich of fossil fuels, meaning that more carbon is oxidized toCO2 per gram of fuel). We need to look beyond somethingincremental becausethe additional energy needed is greaterthan the total of all of the energy currently produced. Todemonstrate the challenges confronting the world in findingan additional 14-20 TW by 2050, consider thetotal amountsthat may be possible for each as presented in the WEA andsummarized on the webpage of one of the Forum contributors(Nathan S. Lewis, http://nsl.caltech.edu/energy.html):bio-mass, 7-10 TW available from the entire agricultural landmass of the planet (excluding the area required to house apopulation of 9 billion people);wind on land, 2.1 TW fromsaturating the entire class 3 (the wind speed required forsustainable energy generation, 5.1 m/s at 10 m above theground) global land mass with wind mills;nuclear, 8 TWof nuclear energy requiring the construction of 8000 newnuclear power plants (to generate this energy, one newnuclear power plant needs to be built every 2 days until2050); andhydroelectric, 1.5 TW left to tap by dammingall available rivers. These unrealistic energy scenarioshighlight that the additional energy needed per year by 2050over the current 12.8 TW fossil fuel energy base is simplynot attainable from these much discussed sources: the globalappetite for energy is simply too great. While, in the shortterm, an “energy mix” will most likely be employed as astop-gap measure to satisfy the world’s growing need for

energy, the answer to this dilemma over the long term mustbe solar energy, which is the theme of this Forum.

Solar energy is an inexhaustible (at least for several billionyears) and freely available energy source. More energy comesfrom the sun in 1 h each day than is used by all humankindactivities in 1 year. The challenge today is to capture andutilize solar energy for sustainable development on a grandscale. There are different manifestations of solar energyconversion, of which one relies fundamentally on chemistryfor its scientific underpinnings, namely, the conversion oflight into stored chemical energy in the form of fuel. We allknow that photosynthesis powers life on the planet, and wecan be inspired by the elaborate constructions of light-harvesting assemblies and photosynthetic reaction centerscoupled with the multistep sequences that lead to theconversion of CO2 and H2O to carbohydrates and O2. Thereis still much that remains to be understood about naturalphotosynthesis, particularly with regard to the oxidation ofwater to O2, which may be a Forum subject in the future.

The execution of natural photosynthesis for energy (asopposed to food, clothing, and shelter) corresponds toobtaining energy from biomass. While large scale programshave been adopted in Brazil and are advocated in pilotprojects here for ethanol as a biomass fuel, questions existabout the amount of net energy obtained in such an approach,and for the conversion of corn to ethanol in the United States,whether the energy balance overall is positive enough tomerit use of this strategy for the long term. An analysis ofcorn-to-ethanol conversion needs to take into account thework done and energy consumed in planting, growing,fertilizing, harvesting, processing, and fermenting the corn.

Photosynthesis need not be limited to what is done innature. The light-driven splitting of water into its constituentelements is one such example. Hydrogen and oxygen canthen be run through a fuel cell to reform the water and giveelectrical energy with greater efficiency than was possiblewith conventional electrical generators. The sequence ofwater splitting and constituent element recombination is theessential linchpin of the much-discussed hydrogen economy.The development of a photosynthetic system that wouldaccomplish the front end of this task using a large portionof the solar spectrum in a cost-effective way would be amajor advance in energy production and the critical break-through in moving the hydrogen economy toward reality.The design of such a system can be guided by the key stepsand requirements of natural photosynthesis such as theefficient capture of visible light photons, electron/holeseparation through electron transfer to give energetic oxidiz-ing and reducing equivalents, charge accumulation for theenergy storing steps that follow, and catalysis of thosereactions. Some of the efforts in these directions arechronicled in the Articles appearing in this Forum.

In the approaches being followed with respect to artificialphotosynthesis and water splitting, one can find molecular,supramolecular, semiconductor, and hybrid systems beingformulated, fabricated, and studied. The activity is at thecutting edge of architecture and engineering at the nanometerand sub-nanometer (i.e., molecular) scales. Scientists, andchemists foremost among them, are creating new moleculesand materials with the specific focus of satisfying each

(4) Caldeira, K.; Jain, A. K.; Hoffert, M. I.Science2003, 299, 2052-2054.

Eisenberg and Nocera

6800 Inorganic Chemistry, Vol. 44, No. 20, 2005

criterion for photosynthesis in ways that vary from what hasbeen done before. When successful, the chemistry andscience will form the basis on which new technologies forenergy production can be built. This is truly a “grandchallenge”, using sunlight and water to produce clean,environmentally friendly energy for sustainable developmenton a massive scale.

Another important line of attack in solar energy utilizationsand one that is closer to widespread commercial implementa-tionsis the direct conversion of light to electrical energy.Photovoltaic devices have been around for decades and arewidely known for their use in space programs. However,their cost of production and their efficiency of operation havelimited their utilization to special cases such as remotelocations in which less expensive electrical energy is notavailable. On the basis of costs for turnkey (or complete)installations, electrical energy from photovoltaic devices was6-10 times more expensive in 1999 than electrical energygenerated using coal or oil. While the efficiency of photo-voltaic panels has been improving, further development isnecessary to make such systems economically viable on alarger scale with a more favorable energy payback time (thatis, the time to recoup the complete costs of the installation).

An alternative approach to the photovoltaic device for thedirect conversion of sunlight to electrical energy is the DSSC,in which an irradiated sensitizer transfers an electron to awide band-gap semiconductor electrode, leading to photo-current. The oxidized sensitizer or dye is then regeneratedusing a solution redox couple that also serves to acceptelectrons from the external circuit. The device is really alight-driven electrochemical cell with a dye-sensitized pho-toanode. The most successful and extensively studied DSSCis the Grätzel cell, which Michael Gra¨tzel describes in hisForum contribution. The Gra¨tzel cell is based on mesoscopicmetal oxide, most notably TiO2, thin films that are sensitizedby surface-bound metal complexes and function through theagency of a redox couple in a fluid eletrolyte. The cell iselegantly conceived, as well as durable and robust. GeraldMeyer writes in his Forum Article about specific consider-ations regarding the photoanode in DSSCs and, in particular,about the thermodynamics, kinetics, and dynamics of inter-facial charge separation in such systems.

A recent report from the U.S. Department of Energy basedon a workshop held in Washington, DC, in April 2005 onSolar Energy (Chem. Eng. News2005, 82 (No. 22), 30) hasidentified a number of fundamental research needs that haveto be addressed for the development of new solar energybased technology. While the entire report is available on theweb at http://www.sc.doe.gov/bes/reports/abstracts.html#SEU,we highlight just a few of those needs that should be ofinterest to readers ofInorganic Chemistry:

(i) The development of structured assemblies that alloworganization of the active units (light harvesting, chargeconduction, and catalytic) for optimum coupling for efficientfuel production. This includes the further development ofsynthetic methods to accomplish the needed couplingbetween components contained in the assemblies.

(ii) The synthesis of new semiconductor nanocrystalscontaining p-n homo- and heterojunctions and their studyfor light-driven charge separation.

(iii) The development of novel methods for compartmen-talizing oxidizing and reducing sites by nanostructure designand the development of nanoscale pore architectures that steerreaction intermediates to desired fuel products.

(iv) The design, synthesis, and study of efficient, high-turnover catalysts for the splitting of water and the reductionof carbon dioxide. To effect these reactions, robust ligands,multimetallic active sites and secondary environments willneed to be designed and synthesized. The functioning of thenew catalysts will rely on complex mechanisms that incor-porate multielectron, atom, and proton-coupled electron-transfer reactions. The new catalysts will serve as compo-nents to be integrated into the higher order assemblies ofpart i.

(v) To design better interfacial catalysts for fuel formationand water oxidation, the detailed understanding developedfor these reactions on the molecular level will need to betranslated to surface-bound molecular and colloidal catalysts.

The Forum Articles cover ongoing research on differentaspects of these highlighted needs. In addition to the Gra¨tzeland Meyer contributions on DSSCs, Nathan Lewis discussescharge transfer and recombination at semiconductor-solutioninterfaces as well as chemical modification of semiconductorsurfaces to modify and control interfacial reactions, whileArthur Nozik covers photoconversion efficiencies in semi-conductor quantum dots with special focus on impactionization that yields more excitons than photons absorbed.New approaches such as Nozik’s offer the promise of greatlyincreasing the efficiency of solar cells. From the laboratoryof Thomas Mallouk, efforts directed to the construction andutilization of integrated assemblies for catalytic hydrogenand oxygen evolution are described, including a novel layer-by-layer approach to achieve efficient light-driven chargeseparation with the ultimate goal of splitting water withvisible sunlight. A contribution by Thomas J. Meyer and co-workers describes in depth approaches to artificial photo-synthesis based on functional elements and modular com-ponents using ruthenium(II) tris diimine complexes assensitizers in various constructions. Finally, the authors ofthis overview have written in separate contributions about(1) light-driven H2 production and multielectron processes,notably water oxidation to O2, that rely on proton-coupledelectron transfer (Nocera) and (2) the design and synthesisof molecular systems having platinum(II) chromophores withcovalently linked components for photoinduced chargeseparation (Eisenberg).

Clearly, the challenge for the 21st century is energy, andthe answers to that challenge lie mainly in chemistry, withthe discipline of inorganic chemistry playing a central role.What chemists do to address this challenge will have impactreaching far beyond our laboratories and institutions.

IC058006I

Forum on Solar and Renewable Energy

Inorganic Chemistry, Vol. 44, No. 20, 2005 6801

Powering the planet: Chemical challenges in solarenergy utilizationNathan S. Lewis*† and Daniel G. Nocera†‡

*Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125; and‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139-4307

Edited by Edward I. Solomon, Stanford University, Stanford, CA, and approved August 11, 2006 (received for review May 25, 2006)

Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold bymidcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossilenergy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atmosphere demands that holding atmo-spheric CO2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deploymentof schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supplyfrom all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providingmore energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency ofinsolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. Anespecially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at ayear-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific chal-lenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form ofchemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbonspecies.

The supply of secure, clean, sus-tainable energy is arguably themost important scientific andtechnical challenge facing hu-

manity in the 21st century. Energy secu-rity, national security, environmentalsecurity, and economic security canlikely be met only through addressingthe energy problem within the next10–20 yr. Meeting global energy de-mand in a sustainable fashion willrequire not only increased energyefficiency and new methods of usingexisting carbon-based fuels but also adaunting amount of new carbon-neutralenergy. The various factors that con-spire to support the above far-reachingconclusions and the basic scienceneeded for the development of a large-scale cost-effective carbon-neutral en-ergy system are the focus of this paper.

The Global Energy PerspectiveIn 2001, worldwide primary energy con-sumption was 425 � 1018 J, which is anaverage energy consumption rate of 13.5terawatt (TW) (1). Eight-six percent ofthis energy was obtained from fossilfuels, with roughly equal parts from oil,coal, and natural gas. Nuclear poweraccounted for �0.8 TW of primary(thermal) energy, and the remainder ofthe energy supply came mostly from un-sustainable biomass, with a relativelysmall contribution from renewablesources (1).

Future energy demand is projected toincrease considerably relative to that in2001. The most widely used scenarios forfuture world energy consumption havebeen those developed by the Intergovern-mental Panel on Climate Change, an or-

ganization jointly established by the WorldMeteorological Organization and theUnited Nations Environment Program(after Scenario B2 in ref. 2; Ė � (869 EJ�yr)�(106 TJ�EJ)�(60�60�24�365 s�yr) �27.54 TW (TJ, terajoule; and EJ, exa-joule). The scenario outlined in the lasttwo columns of Table 1 is based on ‘‘mod-erate’’ assumptions and hence is reason-ably viewed as neither overly conservativenor overly aggressive.

To better understand this scenario,the top half of Table 1 breaks down therate of energy consumption, Ė, intothree fundamental factors (3):

Ė � N�(GDP�N)�(Ė�GDP), [1]

where N is the global population, GDP�Nis the globally averaged gross domesticproduct (GDP) per capita, and Ė�GDP isthe globally averaged energy intensity (i.e.,the energy consumed per unit of GDP).The world population was �6.1 billion in2001, and in the scenario represented inTable 1, the global population is projectedto increase by 0.9% yr�1 to �9.4 billionby 2050. World per capita GDP was�$7,500 per capita in 2001. In the Table 1scenario, GDP�N is projected to increaseat the historical average rate of 1.4% yr�1to �$15,000 per capita by 2050. No coun-try has a policy against economic growth,so this increase in GDP�N seems quitereasonable and in fact may well be modestgiven the rapid economic growth beingexperienced by China and India atpresent. With no changes in the globallyaveraged energy intensity, the world en-ergy consumption rate would grow, due topopulation growth and economic growth,by 2.3% yr�1, from 13.5 TW in 2001 to

�40.8 TW in 2050. However, the globalaverage energy intensity has declined con-tinuously over the past 100 yr, due to im-provements in technology throughout theenergy production, distribution, and end-use chain. In anticipation of continuedimprovements in technology, the globalaverage energy intensity in the Table 1scenario is projected to decrease at ap-proximately the historical average rate of0.8% yr�1, from 0.29 W�($ yr�1) in 2001to 0.20 W�($ yr�1) by 2050. This decreaseoffsets somewhat the projected increasesin population and per capita GDP, so thatthe world energy consumption rate is in-stead projected to grow by 2.3% yr�1 �0.8% yr�1 � 1.5% yr�1, from 13.5 TW in2001 to �27 TW by 2050. Hence, evenfactoring in a decrease in energy intensity,the world energy consumption rate is pro-jected to double from 13.5 TW in 2001 to27 TW by 2050 and to triple to 43 TW by2100 (4).

The Global Energy Challenge Presentedby Consumption of Fossil FuelsMany sources indicate there are amplefossil energy reserves, in one form oranother, to supply this energy at somereasonable cost. The World Energy As-sessment Report estimates of the totalreserves (i.e., 90% confidence that the

Author contributions: N.S.L. and D.G.N wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS direct submission.

Abbreviations: TW, terawatt; GDP, gross domestic product;PV, photovoltaics; GtC, metric tons of carbon.

†To whom correspondence may be addressed. E-mail:[email protected] or [email protected].

© 2006 by The National Academy of Sciences of the USA

www.pnas.org�cgi�doi�10.1073�pnas.0603395103 PNAS � October 24, 2006 � vol. 103 � no. 43 � 15729–15735

PE

RS

PE

CT

IVE

reserves exist) and of the global re-source base (5), including both con-ventional and unconventional sources,provide a benchmark for evaluating thetotal available global fossil energy base.Based on 1998 consumption rates,40–80 yr of proven conventional andunconventional oil reserves exist glo-bally, and 50–150 yr of oil are availableif the estimated resource base is in-cluded. Sixty to 160 yr of reserves ofnatural gas are present, and between207 and 590 yr of gas resources, notincluding the natural gas potentiallyavailable as methane clathrates in thecontinental shelves, are in the estimatedresource base. Similarly, a 1,000- to2000-yr supply of coal, shales, and tarsands is in the estimated resource base.Hence the estimated fossil energy re-sources could support a 25- to 30-TWenergy consumption rate globally for atleast several centuries.

Consumption of fossil energy at thatrate, however, will produce a potentiallysignificant global issue. Historically, themean carbon intensity (kg of C emitted tothe atmosphere as CO2 per year per W ofpower produced from the fuel) of theglobal energy mix has been declining. Inthe past two centuries, the energy mix hasshifted from being dominated by wood tocoal to oil and now more to natural gas.This shift has produced a decrease in theaverage carbon intensity of the energymix, because oil and gas have higher H�Cratios and hence upon combustion pro-duce more water and less CO2 per unit ofheat released than does coal. If the car-bon intensity were to remain at the year2001 value (approximately equal partscoal, oil, and natural gas), the world car-bon emission rate would grow due to theprojected growth in the energy consump-tion from 6.6 billion metric tons of carbon(GtC) yr�1 in 2001 to 13.5 GtC yr�1 by2050. The Intergovernmental Panel on

Climate Change ‘‘business as usual’’ sce-nario of Table 1 projects, arguably opti-mistically, that the historical trend ofmean carbon intensity decline with timewill continue through 2050, producing anenergy mix continually favoring cleaner-burning fuels from a carbon emissionsviewpoint, until the average in 2050 is be-low that of the least carbon-intensive fossilenergy source, natural gas. This decreasein carbon intensity would offset somewhatthe increase in the rate of energy con-sumption. But even with this projecteddecrease in carbon intensity, the worldcarbon emissions rate in this scenario isprojected to nearly double from 6.6 GtCyr�1 in 2001 to 11.0 GtC yr�1 by 2050 (2).

On the timescale of many centuries,CO2 emissions are essentially cumulativein the atmosphere. The CO2 equilibrateson an �10- to 30-yr timescale betweenthe atmosphere and the near-surfacelayer of the oceans (6), which accountsfor why only �50% of the anthropo-genic CO2 emissions remain in the at-mosphere (the remainder partitioninginto the biosphere and the oceans). Be-cause there are no natural destructionmechanisms of CO2 in the atmosphere,the long-term removal of atmosphericCO2 must occur by convection. The rel-evant mixing time between the near-surface ocean layer and the deep oceansis between 400 and several thousandyears (6, 7). Hence, in the absence ofgeoengineering or active intervention,whatever environmental effects might becaused by this atmospheric CO2 accu-mulation over the next 40–50 yr willpersist globally for the next 500–2,000 yror more.

Although the precise future effects ofsuch anthropogenic CO2 emissions arestill somewhat uncertain, the emissionlevels can certainly be viewed rigorouslywithin a historical perspective. The datafrom the Vostok ice core indicate that

the atmospheric CO2 concentration hasbeen between 210 and 300 ppm for thepast 420,000 yr (8), and more recentstudies of Dome Concordia ice coreshave extended this time period to650,000 yr (9). Over this same time pe-riod, the atmospheric CO2 concentrationhas been highly correlated with, but isnot necessarily the cause of, tempera-ture swings that have repeatedly causedice ages on the planet. The CO2 concen-trations in the past 50 yr have beenrising because of anthropogenic CO2emissions from fossil fuel consumption,and they are now in excess of 380 ppm.Without intervention, even the Table 1scenario produces, within the 21st cen-tury, atmospheric CO2 concentrationsthat are more than double the prean-thropogenic values (4, 6). The exactlevels vary depending on the assumedcomposition of energy sources, the effi-ciency of energy production and con-sumption, the global economy, anddifferent intervention scenarios to con-trol CO2 levels. Modestly stringentinterventions are based on stabilizingatmospheric CO2 in the 550- to 650-ppmrange, with substantially higher valuesprojected (�750 ppm) if the Table 1scenario is followed. Climate modelspredict a variety of different global re-sponses to levels of CO2 at or in excessof 550 ppm in the atmosphere. In somemodels, moderate changes are predicted,whereas in others, relatively serious sealevel rises, changes in the hydrologicalcycle, and other effects are predicted(10). Tipping points involving positivefeedback, such as the accelerated loss ofpermafrost, which could release furtherCO2 which then could accelerate stillfurther permafrost loss, are of substan-tive concern. What can be said withcertainty is that the atmospheric CO2concentrations are being increased andwithout severe intervention will con-

Table 1. World energy statistics and projections

Quantity Definition Units 2001* 2050† 2100‡

N Population B persons 6.145 9.4 10.4GDP GDP§ T $/yr 46 140¶ 284�

GDP�N Per capita GDP $�(person-yr) 7,470 14,850 27,320Ė/GDP Energy intensity W�($�yr) 0.294 0.20 0.15Ė Energy consumption rate TW 13.5 27.6 43.0C�E Carbon intensity KgC�(W�yr) 0.49 0.40 0.31Ċ Carbon emission rate GtC�yr 6.57 11.0 13.3Ċ Equivalent CO2 emission rate GtCO2�yr 24.07 40.3 48.8

*Ė � (403.9 Quads�yr)�(33.4 GWyr�Quad)�(10–3 TW�GW) � 13.5 TW; and Ċ � (24.072 GtCO2�yr)�(12�44 GtC�GtCO2) � 6.565 GtC (adapted from ref. 1).

†Ė � (869 EJ�yr)�(106 TJ�EJ)�(60�60�24�365 s�yr) � 27.5 TW [adapted from ref. 2 (Scenario B2), pp. 48–55].‡Ė � (1,357 EJ�yr)�(106 TJ�EJ)�(60�60�24�365 s�yr) � 43.0 TW [adapted from ref. 2 (Scenario B2), pp. 48–55].§All in year 2000 U.S. dollars, using the inflation-adjusted conversions: $2000 � 1�0.81590 $1990 (adapted from ref.1), and �purchasing power parity� exchange rates.

¶In year 2000 U.S. dollars: (113.9 T$1990)�(1�0.81590 $2000�$1990) � 139.6 T$2000.�In year 2000 U.S. dollars: (231.8 T$1990)�(1�0.81590 $2000/$1990) � 284.1 T$2000.

15730 � www.pnas.org�cgi�doi�10.1073�pnas.0603395103 Lewis and Nocera

tinue to increase, because of anthropo-genic sources, to levels that have notbeen present on the planet in at leastthe past 650,000 yr and probably in thepast 20 million yr.

Carbon-Neutral Fuel Sources and theSolar OpportunityTo meet the (arguably optimistic) Inter-governmental Panel on Climate Changeprojection in the Table 1 scenario forthe average carbon intensity in 2050, theprojected carbon intensity in 2050 is�0.45 kg of C yr�1 W�1, which is lowerthan that of any of the fossil fuels. Theonly way one can reach this value of themean carbon intensity is through a sig-nificant contribution of carbon-freepower to the total energy mix. This con-clusion holds for an economy entirelybased on natural gas; to the extent thatthe mix of consumed fossil fuels is not100% natural gas but is roughly alsoequal parts oil and coal, even morecarbon-free energy is required to main-tain the average of the energy mix atthe 0.45 kg of C yr�1 W�1 value. In fact,the amount of carbon-free power re-quired in 2050 to meet these carbon in-tensity targets is �10 TW and is muchgreater than 10 TW if emissions are tobe lowered such that CO2 can be stabi-lized at 550 ppm. Even more carbon-free power will be required later in the21st century if CO2 levels are to be keptbelow 550 ppm or if a lower atmo-spheric CO2 target level is desired. Byalmost any reasonable estimate, stabili-zation of atmospheric CO2 levels at 550ppm or lower will require as muchcarbon-neutral power by approximatelythe year 2050 as the amount of powerproduced at present from all energysources combined (4). Furthermore, be-cause CO2 emissions are cumulative ona century-level timescale, even higherlevels of carbon-neutral power are re-quired by 2050 if their introduction doesnot start immediately with a constantrampup but instead are delayed by 20 yrfor their commissioning while awaitingtechnology development and�or policyand socioeconomic interventions.

Three general routes are available toproduce such large amounts of carbon-neutral power.

Nuclear fission is one method, but itwould require widespread implementa-tion of breeder reactors (11). Estimatedterrestrial U resources are sufficient toproduce �100 TW-yr of electricity usingconventional once-through U reactortechnology. Hence, if 10 TW of powerwere obtained from conventional nu-clear fission, the terrestrial U resourcebase would be exhausted at that level inless than a decade (in fact, it would beexhausted after the first 30 yr of reactor

construction because of the fuel con-sumed during the rampup phase). More-over, construction of nuclear powerplants would need to proceed at a veryrapid rate by historical standards (one1-GWe (gigawatt-electric) power plantevery 1.6 days for the next 45 yr). Theinternational tokamak (magnetic con-finement fusion) experiment (ITER) isnow scheduled to demonstrate an en-ergy breakeven point in 35 yr for a fewminutes of operational time. Althoughfusion might possibly provide significantcommercial energy late in the 21st cen-tury, the ITER time line is much too farin the future to provide a credible op-tion to make a significant contributionto the amount of cost-effective carbon-neutral energy production needed tomeet any reasonable atmospheric CO2concentration target in the next 40–50 yr.

Carbon capture and storage comprisea second general approach (12). In thisapproach, the carbon dioxide is dis-solved in the underground aquifers. Tobe a viable option technically, the CO2must not leak at a globally averaged rateof 1% for a timescale of centuries. Oth-erwise, the emitted flux will be greaterthan or equal to that intended to bemitigated initially. Experiments at scaleare needed, along with extensive model-ing, simulation, monitoring, and valida-tion, to ascertain with �99% confidencethat the leak rate will be acceptably lowfor a 500- to 1,000-yr period. Further-more, each reservoir is different geolog-ically, so proof that sequestration workstechnically at one reservoir is not gen-eral proof that the process will work atthe required level globally. The globalreservoir capacity has been estimated tobe equivalent to �100–150 yr of carbonemissions. Hence, sequestration couldbuy time if it works technically and is sovalidated within the next 10–20 yr. Anadditional condition is that the energydistribution and end-use chain must betransformed to handle massive quanti-ties of carbon-free fuels (hydrogen) orelectricity on the needed timescale tomitigate carbon emissions.

The third general approach is to userenewable energy. Of the various renew-able energy sources, by far the largestresource is provided by the sun. Moreenergy from sunlight strikes the earth in1 hr (4.3 � 1020 J) than all of the en-ergy currently consumed on the planetin 1 yr (4.1 � 1020 J in 2001) (5). Yet,in 2001, only �0.1% of electricity and�1.5% of fuels (mostly from biomass)were provided by a solar source (1).Against the backdrop of the dauntingcarbon-neutral energy needs of ourglobal future, the large gap between ourpresent use of solar energy and its enor-mous undeveloped potential defines a

compelling imperative for science andtechnology in the 21st century.

Solar Energy UtilizationSolar energy utilization requires solar (i)capture and conversion and (ii) storage.Solar capture and conversion may beaccomplished by photovoltaics (PVs).The challenge here is to dramaticallyreduce the cost per W of delivered solarelectricity. Compared with fossil energy,solar energy is diffuse, and hence mate-rials costs must be very inexpensive tomake a solar-based process economical.Knowing the insolation striking an areaof the earth for a 30-yr period, it is rela-tively simple to calculate the sale priceof the converted energy that is neededto pay back at least the initial cost thatis required to cover that area with thesolar energy conversion system. At 10%efficiency, and a cost of $300 m�2, bothtypical of current Si-based solar electric-ity modules, along with a balance ofsystems cost of $3 Wp�1 (peak W), anelectricity price of $0.35 [kW-hr]�1 isrequired to cover the initial system costs(13). By comparison, fossil-derived elec-tricity (high-value energy) now costs ap-proximately $0.02–0.05 [kW-hr]�1, andthat cost includes storage and distribu-tion costs. To reach a cost point nearthat of fossil-derived energy will thusrequire improvements in efficiency butadditionally will require large decreasesin cost, into a range below $100 m�2.For comparison, the cost of paint isabout $1 m�2, so the solar energy con-version system can cost �10 times morethan the cost of paint, but not muchmore if it is to provide cost-effectiveprimary energy.

In the absence of cost-effective stor-age, solar electricity can never be a pri-mary energy source for society, becauseof the diurnal variation in local insola-tion. In principle, storage of electricitycould be obtained using batteries, butat present no battery is inexpensiveenough, when amortized over the 30-yrlifetime of a solar device, to satisfy theneeded cost per W targets for the wholesystem. A second method is to store theelectrical energy mechanically. For in-stance, electricity could be used to driveturbines to pump water uphill. This ap-proach is relatively inexpensive forstoring large amounts of energy at mod-est charge and discharge rates, but is notwell matched to being charged and dis-charged every 24 h to compensate forthe diurnal cycle. For example, bufferingthe day�night cycle in the U.S. energydemand by this approach would requirea pumping capacity equivalent of�5,000 Hoover Dams, filling and empty-ing reservoirs every day and every night.Currently, the cheapest method of solar

Lewis and Nocera PNAS � October 24, 2006 � vol. 103 � no. 43 � 15731

energy capture, conversion, and storageis solar thermal technology, which cancost as little as $0.10–0.15 per kW-hrfor electricity production. Advances inthis potentially very important approachto solar energy utilization will requirenew materials for the focusing and ther-mal capture of the energy in sunlight, aswell as new thermochemical cycles forproducing useful fuel from the capturedsolar energy. The possibility of inte-grated capture, conversion, and storagefunctions makes solar thermal technol-ogy an option that should be vigorouslypursued to exploit the large untappedsolar energy resource for carbon-neutralenergy production

A third method of storage is to bor-row the design of nature, in whichchemical bonds are broken and formedto produce solar fuels in an artificialphotosynthesis process. Photosynthesisitself is relatively inefficient, when mea-sured on a yearly average basis per unitarea of insolation. For example, switch-grass, one of the fastest-growing crops,yields energy stored in biomass at ayearly averaged rate of �1 W�m2 (5).Because the averaged insolation at atypical midlatitude is 200–300 W�m2(5), the yearly averaged energy conver-sion and storage efficiency of the mostrapidly growing large area crops is cur-rently �0.5%. Even if this efficiencycould be reached with no energy inputsinto the farm or any energy losses dueto outputs from the utilization of thebiomass, growth of energy crops on allof the naturally irrigated cultivatableland on earth that is not currently usedfor food crops would yield perhaps 5–10TW of total power. Whereas biofuelsderived from existing plants could pro-vide a potentially significant contribu-tion to liquid fuels for transportationuses (if cellulosic conversion technologycan be successfully developed and de-ployed economically) increased energyconversion and storage efficiency arehighly desirable to remove land area asa serious constraint on the amount ofenergy that can be obtained from thesun and stored in chemical bonds. Oneapproach is to develop an artificial pho-tosynthetic process with an average effi-ciency significantly higher than plants oralgae.

The primary steps of photosynthesisinvolve the conversion of sunlight into a‘‘wireless current.’’ In all cases, to forma useful fuel, O2 must be evolved, so itcan be released into our oxygen-contain-ing atmosphere and used elsewhere asan oxidation reagent for fuel consump-tion. The reduced fuel could be eitherhydrogen from water reduction, or itcould be an organic species, such asmethanol or methane, that is derived

from the fixation of atmospheric CO2.Recombination of the reduced fuel withreleased O2 would then regenerate theoriginal species, closing the cycle in acarbon-neutral fashion.

In natural photosynthesis, the anodiccharge of the wireless current is used atthe oxygen-evolving complex to oxidizewater to oxygen, with the concomitantrelease of four protons. The cathodiccharge of the wireless current is cap-tured by Photosystem I to reduce theprotons to ‘‘hydrogen,’’ with the reducedhydrogen equivalents stored through theconversion of NADP to NADPH. Thus,the overall primary events of photosyn-thesis store sunlight by the rearrange-ment of the chemical bonds of water,to form oxygen and Nature’s form ofhydrogen.

An artificial photosynthetic systemcould be realized by spatially separatingsolid-state or molecular water reductionand oxidation catalysts and connectingthem to a light collection and chargeseparation system. In one such con-struct, the spatially separated electron–hole pairs provided by a photovoltaicassembly based on a solid-state junction,on either the macroscale or thenanoscale, are captured by the catalysts,and the energy is stored in the bondrearrangement of water to H2 and O2.Other concepts involve more intimateintegration of the charge separation andchemical bond-forming functions, toavoid costs and system constraints asso-ciated with electrical contacts, wires,inverters, etc., involved with converting1-eV photons into 1-eV chemical bondsthrough electricity as a discrete interme-diary. One approach to this type of sys-tem is depicted in Fig. 1, in which thetightly integrated system is modeled af-ter natural photosynthesis and serves as

a model for the artificial photosyntheticsystems that are discussed below.

The Basic Science Needs for PVsOne of the key issues in solar captureand conversion is how to separatecharge efficiently over macroscopic dis-tances without using expensive, highlypure, semiconductor materials. This ef-fort requires the development of newchemical and materials methods tomake polycrystalline and nanocrystallinesemiconductors perform as if they wereexpensive single crystals. Numerous re-search approaches are being pursued(13). Materials consisting of a networkof interpenetrating regions can facilitateeffective charge separation and collec-tion, thus relaxing the usual constraintin which the photogenerated carriersmust exist long enough to traverse theentire distance of the cell. Presentphoton conversion devices based on asingle-bandgap absorber, including semi-conductor PV, have a theoretical ther-modynamic conversion efficiency of32% in unconcentrated sunlight. How-ever, the conversion efficiency can beincreased, in principle, to 45–65% if car-rier thermalization can be prevented (byovercoming the so-called Shockley–Queisser limit). Multiple-bandgapabsorbers in a cascaded junction config-uration can result in high photoconver-sion efficiencies, particularly when cellsare designed to sustain the operatingconditions (e.g., elevated temperatures)associated with highly concentrated sun-light. It is expected that mature high-concentration PV systems can provide10–20% more energy than standard PVsystems with the same installed powerrating.

In addition to making evolutionarychanges to existing PV technologies,

Fig. 1. H2 and O2 are combined in a fuel cell to generate a flow of electrons and protons across amembrane, producing electrical energy. The solar fuel cell uses light to run the electron and proton flowin reverse. Coupling the electrons and protons to catalysts breaks the bonds of water and makes the bondsH2 and O2 to effect solar fuel production.


new materials for next-generation PVsare needed. Building upon the recentsuccess in developing efficient molecularorganic PVs and the recent advances inthe controlled assembly of hybrid organ-ic�inorganic nanostructures, organic andhybrid PV cells could possibly exceed10% energy conversion efficiency, whileoffering a potentially inexpensive manu-facturing paradigm (e.g., casting fromemulsions, printing, and use of flexiblesubstrates for production of large-areathin-film cells; ref. 14). To guide the PVnanostructure assembly, biologically de-rived and�or genetically engineeredsystems might be used to control thecrystal structure, phase, orientation, andnanostructural regularity of inorganicmaterials. Genetically modified photo-synthetic complexes from plants andbacteria can also convert incident lightinto photocurrent. Although the presentenergy conversion efficiencies of suchsystems are low, the projected maximumcould be possibly as high as 10%. Fi-nally, the Shockley–Queisser limit maybe overcome by using multilayer junc-tions of semiconductor quantum dots,quantum wells and related nanostruc-tures, and new inorganic materials andphotoassemblies. Such materials couldchannel the excess energy of electron�hole pairs into photovoltages and pho-tocurrents, with the design guided by arefined detailed understanding of pho-ton absorption, charge creation, andcharge separation processes.

The Basic Science Needs for Solar FuelsAs described above, an important storageapproach involves conversion of the en-ergy captured in the charge-separatedstates of a solar capture and conversionsystem into chemical bonds. Water split-ting is an example of a more general con-version to a solar fuel cycle that involvesevolution of oxygen as one componentand formation of a reduced fuel as theother. Unexplored basic science issues areimmediately confronted when the prob-lem is posed in the simplest chemistryframework (see Scheme 1).

The overall transformation is a mul-tielectron process promoted by photo-catalyst and light. Elucidation of thefundamental principles of single elec-tron-transfer reactions represented

such an important milestone in chemis-try that two Nobel Prizes wereawarded for such work (15, 16). Al-though dramatic advances have oc-curred in our understanding of singleelectron-transfer reactions, especiallythose in biology (17), a similar level ofunderstanding of multielectron redoxreactions has yet to be realized. More-over, to ensure charge neutrality in thesystem, proton transfer must accom-pany electron transfer (i.e., proton-coupled electron transfer; ref. 18);hence, electron and proton inventoriesboth need to be managed (19). Watersplitting additionally presents sizablethermodynamic and kinetics barriersto making and breaking the bonds re-quired to facilitate the desired chemi-cal reactions. This is especiallypertinent to the water-splitting prob-lem, because the byproduct of wateractivation at the catalyst, whether mo-lecular or solid, will invariably yieldspecies that have strong metal–oxygenbonds. To close a catalytic cycle, thesestable bonds need to be activated bythe captured solar energy either di-rectly or indirectly. More generally, theactivation of all small molecules ofconsequence to carbon-neutral solarenergy storage, including CO2, O2, and

H2O, share the reaction commonalitiesof bond-making and -breaking pro-cesses that require multielectron trans-fers coupled to proton transfer.

The Reaction Chemistry of Solar EnergyStorage in Chemical BondsPerhaps the most straightforward water-splitting scheme is to have catalysts actdirectly on water, as exemplified by thetwo half-reactions denoted as WS1 (WS1,water-splitting strategy 1) in Scheme 2.The spatial separation of the catalysts re-quires that the charge-separation functionbe imbedded in some type of membrane,so that the protons generated on the an-odic side of the cell are transported to thecathodic side of the cell for reduction. Ineffect, the system must be run in theopposite direction of a fuel cell, withsunlight providing the thermodynamicimpetus to drive the cell in the desiredfuel-forming direction.

The preparation of hydrogen-produc-ing catalysts constitutes an intense areaof study. Fe-only hydrogenases, com-prised of small dithiolate-bridged bime-tallic iron cofactors coordinated by COand cyanide ligands, provide a bench-mark for the efficient evolution of H2 inmolecular systems (20, 21). Structural,and in some cases, functional, analoguesScheme 1.

Scheme 2.


of such enzymatic active sites have beenprepared (22–27); however, none ofthese Fe-only hydrogenase biomimics yetproduce H2 efficiently at low overpoten-tial. Synthetic catalysts compare favor-ably to, and in some cases exceed, theefficiency of the biomimetic models. Inthe presence of sacrificial chemical re-ductants, mononuclear and binuclearmetal complexes of Co, Ni, and Rh areknown to effect catalytic hydrogen evo-lution electrochemically or photochemi-cally (28–36). Intimate mechanisticdetails, however, are known in only afew cases (37), and the different possi-bilities, such as protonation of a hydridevs. uni- or bimolecular reductive elimi-nation (right side, WS1, Scheme 2), ingeneral have not yet been unraveled.

Other water-splitting cycles can alsobe developed. The water-splittingschemes WS2 and WS3 presented inScheme 2 use basic reaction types thatare common to organometallic catalysis.However, for the water-splitting prob-lem, O, as opposed to C or N, needs tobe managed. Every reaction, however,does have a precedent for carbon or ni-trogen. In WS2 in Scheme 2, oxidativeaddition across XOH (X � C, N) bondsis a basic reaction of organometallicchemistry but is not yet well establishedfor water (38–43). If this reaction canbe achieved cleanly, hydrogen may begenerated by �-H abstraction, which is acommon reaction in organometallicchemistry and is used to generate met-al–ligand multiple bonds. For instance,the �-H abstraction of metal alkylidenesproduces alkylidynes (44). But �-H ab-straction to produce metal-oxo species,and H2 is uncommon for well definedhydroxo–hydrido complexes. In the caseof WS3, the water–gas shift reactionproduces H2 from H2O using CO as thereductant. An intense research effort,beginning in the 1970s and ending in the1980s, provided the basic science for thedevelopment of catalysts to effect thewater–gas shift reaction (45). However,the reaction must be closed by the con-version of CO2 to CO. On this front,little is known. Some inroads to CO2reduction have been made on photo–(46, 47) and electro– (48–50) catalyticfronts, but generally the precise path toCO2 reduction is ill-defined, making itdifficult to improve these systems bydesign. A recent report of CO2 reduc-tion by a well defined homogeneousmetal complex operating at high turn-over number and frequency (51) is aharbinger of the promise that basic sci-ence holds for the design of efficientCO2 reduction catalysts.

As in WS1, WS2 and WS3 cycles areclosed by oxygen production, providinga further imperative for the develop-

ment of reactions of the type describedby Schemes 3–5. However, very few cat-alysts are known to oxidize water nearthe thermodynamic potential. Again, themost notable system is in biology, spe-cifically involving the oxygen-evolvingcomplex (OEC) of Photosystem II. TheOEC comprises a cluster of four Mncenters and a Ca center (52–54), but nofunctional or structural models of thecatalytically active site are yet available(55). At present, the ruthenium dimer[(bpy)2(OH2)RuIIIORuIII(OH2)(bpy)2]4�(bpy � 2,2�-bipyridine) (56) and itsrelatives (57–59) represent the only un-equivocally established molecular elec-trocatalysts for generating O2 fromH2O. However, at present, this reactionproceeds at a high overpotential andwith modest turnover numbers.

The success of WS1, WS2, and WS3and other yet-undefined water-splittingschemes is predicated on systems thatpromote the conversion of oxygen frommetal oxos. Many mechanistic possibili-ties for this conversion await discovery.They include the following.

(i) Nucleophilic Attack of Hydroxide onHigh-Valent Metal Oxos (Scheme 3). Thisbasic reaction type is the foundation onwhich oxidation catalysts have been devel-oped in the disciplines of organometallicand organic chemistry (60). Here the ole-fin bond attacks a metal oxo species toform two carbon–oxygen bonds. The re-placement of the two-electron bond of theolefin by the lone pair of hydroxide wouldlead to the oxygen–oxygen bond-formingreaction that is critical for water oxidation.The substitution, however, is not trivial.OH� is thermodynamically more difficultto oxidize than are olefins. Also, the over-all reaction to produce oxygen involves afour-electron change at the metal, sothere may be benefits to examining reduc-tive elimination from more than onemetal center, in which the multielectron

equivalency can be shared by metal cen-ters working in concert.

(ii) Radical Coupling of Two Oxos (Scheme4). As shown, the oxygen radical may bedelivered directly from a high-valentmetal. Alternatively, the oxo speciescould be delivered from a multiplybonded metal–ligand species (61). Thislatter approach represents a paradigmshift in oxygen chemistry, because thestrong metal–oxo double and triplebonds may be avoided, potentially low-ering the activation barrier for oxygenatom delivery from a reactive multiplybonded metal–ligand center.

(iii) Reductive Elimination of bis Hydroxosor bis �-Oxos (Scheme 5). These unknownreaction types encompass a four-electronchange to make oxygen. A shared electronequivalency among a multimetallic centermay expedite the reaction, such as thatshown for Scheme 6.

In Scheme 2, the WS cycles are com-pleted by the same parent metal complex.This does not have to be the case. As hasrecently been demonstrated, metal com-plexes working in tandem can promotereactions of energy consequence (62). Ac-cordingly, the water-splitting schemes maybe accomplished by two different metalcomplexes working in concert. Regardlessof the precise details of the reaction de-sign, oxygen production invariably will bean energetically demanding process thatmust be coupled to a charge-separatedstate to capture, convert, and store solarenergy in the form of chemical bonds. Byuse of a photovoltaic assembly to accom-plish solar-driven charge separation, theconstraints on the catalyst design are re-laxed solely to provide storage. However,in bringing catalysts to a charge-separatingassembly, the reaction chemistry will beperformed in a heterogeneous and�orinterfacial environment. Accordingly, theneed to acquire a molecular-level under-standing of reactions at the surfaces ofsolids represents another scientific chal-lenge confronting the effective utilizationof solar energy. Finally, inasmuch as theaforementioned reactions and schemes areall enacted at a metal-based platform, therole of inorganic chemistry, whether at a

Scheme 3.

–

Scheme 4.

Scheme 5.

Scheme 6.


molecule or a surface, will be pivotal tothe development of the aforementionedwater-splitting cycles. Ingenious ap-proaches to water splitting may be possi-ble using organic catalysts and biocatalystsas well, although the ability to operatethese reactions at low overpotential willrepresent a significant challenge.

ConclusionsThe sun has a unique role in sustainableenergy production, in that it is the un-disputed champion of energy; theresource base presented by terrestrialinsolation far exceeds that of all otherrenewable energy sources combined.The solar energy resource additionallyfar exceeds what can possibly be envi-sioned as a level of human consumption

necessary to support even the mosttechnologically advanced society. How-ever, to be a material contribution toprimary energy supply, solar energymust be captured, converted, and storedto overcome the diurnal cycle and theintermittency of the terrestrial solar re-source. Arguably the most attractivemethod for this energy conversion andstorage is in the form of chemicalbonds, by production of cheap solarfuels. Significant advances in basic sci-ence, however, are needed for thistechnology to attain its full potential.Chemistry will assume a special role inthis endeavor, because new materialsmust be created for solar capture andconversion, and because new catalystsare needed for the desired chemical

bond conversions. Here we present ablueprint for a reaction chemistry, wheninterfaced to a charge-separation struc-ture, that permits artificial photosynthe-sis to be envisioned. The progress ofscientists in chemistry, biology, engi-neering, materials science, and physicsin addressing the basic science chal-lenges involved with realizing this artifi-cial photosynthesis will be critical toenable humans to use the sun sustain-ably as their primary energy source.

We acknowledge sustained support from theU.S. Department of Energy (Office of BasicEnergy Sciences) and the National ScienceFoundation (and in particular, ChemicalBonding Center CP-CP0533150) for basicresearch in renewable energy and for facili-tating our ongoing perspective on global en-ergy options.

1. Energy Information Administration (2005) An-nual Energy Outlook (US Dept of Energy, Wash-ington, DC).

2. Nakicenovic N, Swart, R, eds (2000) in SpecialReport on Emissions Scenarios (IntergovernmentalPanel on Climate Change, Washington, DC), pp48–55.

3. Kates R (2000) Environment 42:10–19.4. Hoffert MI, Caldeira K, Jain AK, Haites EF,

Harvey LD, Potter SD, Schlesinger ME, WigleyTML, Wuebbles DJ (1998) Nature 395:881–884.

5. United Nations Development Program (2003)World Energy Assessment Report: Energy and theChallenge of Sustainability (United Nations, NewYork).

6. Wigley TML, Richels R, Edmonds JA (1996)Nature 379:240–243.

7. Maier-Reimer E, Hasselmann K (1987) ClimateDyn 2:63–90.

8. Petit JR, Jouzel J, Raynaud D, Barkov NI, BarnolaJ-M, Basile I, Bender M, Chappellaz J, Davis M,Delaygue G, et al. (1999) Nature 399:429–436.

9. Siegenthaler U, Stocker TF, Monnin E, Lüthi D,Schwander J, Stauffer B, Raynaud D, BarnolaJ-M, Fischer H, Masson-Delmotte V, et al. (2005)Science 310:1313–1317.

10. Intergovernmental Panel on Climate Change(2001) Climate Change 2001, Synthesis ReportSummary for Policymakers (IntergovernmentalPanel on Climate Change, Washington, DC),Third Assessment Report.

11. Moniz E, Deutch, J, eds (2003) The Future ofNuclear Power (Massachusetts Institute of Tech-nology, Cambridge, MA).

12. Metz B, Davidson O, de Coninck, Loos HM,Meyer L, eds (2005) Carbon Dioxide Capture andStorage (Intergovernmental Panel on ClimateChange, Washington, DC).

13. Solar Energy Utilization Workshop (2005) BasicScience Needs for Solar Energy Utilization (USDept of Energy, Washington, DC).

14. Shaheen SE, Ginley DS, Jabbour GE (2005) MRSBull 30:10–19.

15. Marcus RA (1993) Angew Chem Int Ed Engl32:1111–1121.

16. Taube H (1984) Angew Chem Int Ed Engl 23:329–339.

17. Gray HB, Winkler JR (2005) Proc Natl Acad SciUSA 102:3534–3539.

18. Cukier RI, Nocera DG (1998) Annu Rev PhysChem 49:337–369.

19. Chang CJ, Chang MCY, Damrauer NH, NoceraDG (2004) Biophys Biochim Acta 1655:13–28.

20. Darensbourg MY, Lyon EJ, Zhao X, GeorgakakiIP (2003) Proc Natl Acad Sci USA 100:3683–3688.

21. Peters JW, Lanzilotta WN, Lemon BJ, SeefeldtLC (1998) Science 282:1853–1858.

22. Mejia-Rodriguez R, Chong DS, Reibenspies JH,Soriaga MP, Darensbourg MY (2004) J Am ChemSoc 126:12004–12014.

23. Ott S, Kritikos M, Akermark B, Sun LC, LomothR (2004) Angew Chem Int Ed 43:1006–1009.

24. Borg SJ, Behrsing T, Best SP, Razavet M, Liu XM,Pickett CJ (2004) J Am Chem Soc 126:16988–16999.

25. Borg SJ, Bondin MI, Best SP, Razavet M, Liu X,Pickett CJ (2005) Biochem Soc Trans 33:3–6.

26. Gloaguen F, Lawrence JD, Schmidt M, WilsonSR, Rauchfuss TB (2001) J Am Chem Soc123:12518–12527.

27. Gloaguen F, Lawrence JD, Rauchfuss TB (2001)J Am Chem Soc 123:9476–9477.

28. Gray HB, Maverick AW (1981) Science 214:1201–1205.

29. Koelle U (1992) New J Chem 16:157–169.30. Koelle U, Ohst S (1986) Inorg Chem 25:2689–

2694.31. Koelle U, Infelta PP, Gratzel M (1998) Inorg

Chem 27:879–883.32. Connolly P, Espenson JH (1986) Inorg Chem

25:2684–2688.33. James TL, Cai LS, Muetterties MC, Holm RH

(1996) Inorg Chem 35:4148–4161.34. Collin JP, Jouaiti A, Sauvage JP (1988) Inorg

Chem 27:1986–1990.35. Hu X, Cossairt BM, Brunschwig BS, Lewis NS,

Peters JC (2005) Chem Comm 4723–4725.36. Heyduk AF, Nocera DG (2001) Science 293:1639–

1641.37. Esswein AJ, Veige AS, Nocera DG (2005) J Am

Chem Soc 127:16641–16651.38. Milstein D, Calabrese JC, Williams ID (1986)

J Am Chem Soc 108:6387–6389.39. Dorta R, Rozenberg H, Shimon LJW, Milstein D

(2002) J Am Chem Soc 124:188–189.

40. Burn MJ, Fickes MG, Hartwig JF, Hollander FJ,Bergman RG (1993) J Am Chem Soc 115:5875–5876.

41. Tani K, Iseki A, Yamagata T (1998) Angew ChemInt Ed 37:3381–3383.

42. Dorta R, Togni A (1998) Organometallics17:3423–3428.

43. Morales-Morales D, Lee DW, Wang Z, JensenCM (2001) Organometallics 20:1144–1147.

44. Schrock RR, Seidel SW, Mosch-Zanetti NC,Shih K-Y, O’Donoghue MB, Davis WM, ReiffWM (1997) J Am Chem Soc 119:11876–11893.

45. Ford PC (1981) Acc Chem Res 14:31–37.46. Lin W, Frei H (2005) J Am Chem Soc 127:1610–1611.47. Fujita E, Brunschwig BS (2001) in Catalysis of

Electron Transfer, Heterogeneous and Gas-PhaseSystems, ed Balzani V (Wiley-VCH, Weinheim,Germany),Vol 4, pp 88–126.

48. Simón-Manso E, Kubiak CP (2005) Organometal-lics 24:96–102.

49. Hammouche M, Lexa D, Momenteau M, SavéantJ-M (1991) J Am Chem Soc 113:8455–8466.

50. Beley M, Collin J-P, Ruppert R, Sauvage J-P(1986) J Am Chem Soc 108:7461–7467.

51. Laitar DS, Müller P, Sadighi JP (2005) J Am ChemSoc 127:17196–17197.

52. Ferreira KN, Iverson TM, Maghlaoui K, Barber J,Iwata S (2004) Science 303:1831–1838.

53. Sauer K, Yano J, Yachandra VK (2005) Photo-synth Res 85:73–86.

54. Loll B, Kern J, Saenger W, Zouni A, Biesiadka J(2005) Nature 438:1040–1044.

55. Ruttinger W, Dismukes GC (1997) Chem Rev 97:1–24.56. Gersten SW, Samuels GJ, Meyer TJ (1982) J Am

Chem Soc 104:4029–4030.57. Sens C, Romero I, Rodriguez M, Llobet A, Parella

T, Benet-Buchholz J (2004) J Am Chem Soc126:7798–7799.

58. Wada T, Tsuge K, Tanaka K (2000) Angew ChemInt Ed 39:1479–1482.

59. Zong R, Thummel RP (2005) J Am Chem Soc127:12802–12803.

60. Jacobsen EN (2000) Acc Chem Res 33:421–431.

61. Odom AL, Cummins CC, Protasiewicz JD (1995)J Am Chem Soc 117:6613–6614.

62. Goldman AS, Roy AH, Huang Z, Ahuja R,Schinski W, Brookhart M (2006) Science312:257–261.


Corrections

BIOPHYSICS. For the article ‘‘Drift and breakup of spiral waves inreaction–diffusion–mechanics systems,’’ by A. V. Panfilov, R. H.Keldermann, and M. P. Nash, which appeared in issue 19, May8, 2007, of Proc Natl Acad Sci USA (104:7922–7926; first pub-lished April 27, 2007; 10.1073�pnas.0701895104), the authorsnote that on page 7922, right column, the first sentence inMathematical Model, ‘‘Our RDM model is based on a three-variable Fenton–Karma RD model for cardiac excitation (15),coupled with the soft-tissue mechanics equations described inrefs. 12 and 16 . . . , where �(x) is the standard Heaviside stepfunction: �(x) � 1 for x � 0 and �(x) � 0 for x � 0,’’ shouldinstead read: ‘‘Our RDM model consists of RD equationsdeveloped by F. H. Fenton (personal communication) and isbased on a three-variable Fenton–Karma RD model for cardiacexcitation (15), coupled with the soft-tissue mechanics equationsdescribed in refs. 12 and 16 . . . , where �(x) is the standardHeaviside step function: �(x) � 1 for x � 0 and �(x) � 0 for x� 0.’’ Additionally, on page 7923, left column, beginning on line10 of the text, the formula for Isi is incorrect in part. The portionof the formula appearing as ‘‘(0.46 � 0.085 � tanh[k(u � 0.5)])’’should instead appear as: ‘‘(0.23 � 0.085tanh[10(u � 0.65)]).’’Thus, the corrected formula should read Isi(u, w) � �(u �0.2)uw(0.23 � 0.085tanh[10(u � 0.65)]). Finally, on page 7926,in the first sentence of the Acknowledgments, the authors wouldlike to more specifically acknowledge the assistance of Dr.Fenton. Therefore, ‘‘We thank Dr. F. Fenton, Prof. P. J. Hunter,and Dr. P. Kohl for valuable discussions’’ should instead read:‘‘We are grateful to Dr. F. H. Fenton, who kindly providedequations used in the construction of our RDM model, and toProf. P. J. Hunter and Dr. P. Kohl for valuable discussions.’’These errors do not affect the conclusions of the article.

www.pnas.org�cgi�doi�10.1073�pnas.0710559104

IN THIS ISSUE, MEDICAL SCIENCES. For the ‘‘In This Issue’’ summaryentitled ‘‘Carvedilol sidesteps G proteins,’’ appearing in issue 42,October 16, 2007, of Proc Natl Acad Sci USA (104:16392), thefigure caption appeared incorrectly. The online version has beencorrected. The figure and its corrected caption appear below.

Carvedilol recruits �-arrestin to the �2-adrenergic receptor. The �-arrestin2-GFP is shown in green.


PERSPECTIVE. For the article ‘‘Powering the planet: Chemicalchallenges in solar energy utilization,’’ by Nathan S. Lewis andDaniel G. Nocera, which appeared in issue 43, October 24, 2006,of Proc Natl Acad Sci USA (103:15729–15735; first publishedOctober 16, 2006; 10.1073�pnas.0603395103), the authors notethat in Fig. 1, the charges shown in the solar fuel cell are on thewrong sides of the cell. The holes should be at the anode, andthe electrons should be at the cathode. This error does not affectthe conclusions of the article. The corrected figure and its legendappear below.

Fig. 1. H2 and O2 are combined in a fuel cell to generate a flow of electronsand protons across a membrane, producing electrical energy. The solar fuelcelluses light to run the electron and proton flow in reverse. Coupling theelectrons and protons to catalysts breaks the bonds of water and makes thebonds H2 and O2 to effect solar fuel production.


20142 � PNAS � December 11, 2007 � vol. 104 � no. 50 www.pnas.org

DOI: 10.1126/science.1162018 , 1072 (2008); 321Science

et al.Matthew W. Kanan,2+Neutral Water Containing Phosphate and Co

In Situ Formation of an Oxygen-Evolving Catalyst in

www.sciencemag.org (this information is current as of May 5, 2009 ):The following resources related to this article are available online at

http://www.sciencemag.org/cgi/content/full/321/5892/1072version of this article at:

including high-resolution figures, can be found in the onlineUpdated information and services,

http://www.sciencemag.org/cgi/content/full/1162018/DC1 can be found at: Supporting Online Material

found at: can berelated to this articleA list of selected additional articles on the Science Web sites

http://www.sciencemag.org/cgi/content/full/321/5892/1072#related-content

http://www.sciencemag.org/cgi/content/full/321/5892/1072#otherarticles, 3 of which can be accessed for free: cites 19 articlesThis article

4 article(s) on the ISI Web of Science. cited byThis article has been

http://www.sciencemag.org/cgi/collection/chemistryChemistry

: subject collectionsThis article appears in the following

http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this article

permission to reproduce of this article or about obtaining reprintsInformation about obtaining

registered trademark of AAAS. is aScience2008 by the American Association for the Advancement of Science; all rights reserved. The title

CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience

on

May

5, 2

009

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

http://oascentral.sciencemag.org/RealMedia/ads/click_lx.ads/sciencemag/cgi/reprint/L22/1299801664/Top1/AAAS/PDF-USB-4.1.09-6.30.09/usb_2009.raw/673778354e456f414879774141565232?xhttp://www.sciencemag.org/cgi/content/full/321/5892/1072http://www.sciencemag.org/cgi/content/full/1162018/DC1http://www.sciencemag.org/cgi/content/full/321/5892/1072#related-contenthttp://www.sciencemag.org/cgi/content/full/321/5892/1072#otherarticleshttp://www.sciencemag.org/cgi/collection/chemistryhttp://www.sciencemag.org/about/permissions.dtlhttp://www.sciencemag.org

In Situ Formation of anOxygen-Evolving Catalyst in NeutralWater Containing Phosphate and Co2+Matthew W. Kanan and Daniel G. Nocera*

The utilization of solar energy on a large scale requires its storage. In natural photosynthesis,energy from sunlight is used to rearrange the bonds of water to oxygen and hydrogenequivalents. The realization of artificial systems that perform “water splitting” requires catalyststhat produce oxygen from water without the need for excessive driving potentials.Here we report such a catalyst that forms upon the oxidative polarization of an inert indiumtin oxide electrode in phosphate-buffered water containing cobalt (II) ions. A variety of analyticaltechniques indicates the presence of phosphate in an approximate 1:2 ratio with cobalt in thismaterial. The pH dependence of the catalytic activity also implicates the hydrogen phosphateion as the proton acceptor in the oxygen-producing reaction. This catalyst not only forms in situfrom earth-abundant materials but also operates in neutral water under ambient conditions.

Sunlight is the only renewable and carbon-neutral energy source of sufficient scale toreplace fossil fuels and meet rising globalenergy demand (1). The diurnal variation in localinsolation, however, demands a cost-effectivestorage of solar energy for its large-scale utili-zation. Of the possible storage methods, natureprovides the blueprint for storing sunlight in theform of chemical fuels (1, 2). The primary stepsof natural photosynthesis involve the absorptionof sunlight and its conversion into spatially sepa-rated electron/hole pairs. The holes of this wirelesscurrent are then captured by the oxygen-evolvingcomplex (OEC) to oxidize water to oxygen andthe electrons are captured by photosystem I toreduce NADP+ (nicotinamide adenine dinucleo-tide phosphate) to NADPH (the reduced form ofNADP+), nature’s form of hydrogen (3). Thus,the overall primary events of photosynthesis storesolar energy in a fuel by rearranging the chemi-cal bonds of water to form H2 (i.e., NADPH)and O2.

An approach to duplicating photosynthesisoutside of a photosynthetic membrane is to con-vert sunlight into spatially separated electron/hole pairs within a photovoltaic cell and thencapture the charges with catalysts that mediate“water splitting” (1, 4). The four holes arecaptured by a catalyst at the anode to produceoxygen, and the four electrons are captured by aseparate catalyst at the cathode to producehydrogen. The net result is the storage of solarenergy in the chemical bonds of H2 and O2.

A key determinant of energy storage in ar-tificial photosynthesis is the efficiency of thewater-splitting catalysts. Electrocatalysts that areefficient for solar-to-fuels conversion must oper-ate close to the Nernstian potentials (E) for theH2O/O2 and H2O/H2 half-cell reactions shown in

Scheme 1 (half-cell potentials given in the con-vention of reduction potentials).

The voltage in addition to E that is requiredto attain a given catalytic activity—referred to asoverpotential—limits the efficiency of convertinglight into catalytic current. Of the two reactions,the H2O/O2 reaction is considerably more com-plex (5). This reaction requires a four-electronoxidation of two water molecules coupled to theremoval of four protons to form a relatively weakoxygen-oxygen bond. In addition to controllingthis proton-coupled electron transfer (PCET)(6, 7), a catalyst must tolerate prolonged exposureto oxidizing conditions. Even at the thermody-namic limit, water oxidation requires an oxidiz-

ing power that causes most chemical functionalgroups to degrade. Accordingly, the generation ofoxygen from water presents a substantial chal-lenge toward realizing artificial photosynthesis (8).

The fine-tuned molecular machinery of theOEC oxidizes water at a low overpotential usinga Mn4O4Ca cluster (9–12). Outside the OEC, ex-amples of water oxidation catalysts include first-row spinel and perovskite metal oxides, whichrequire concentrated basic solutions (pH > 13)and moderate overpotentials (

tion may be exploited to prepare electrocatalystsin situ if the precipitated material remains cat-alytically active and can be oxidized at an electrodesurface. To explore this possibility forCo-catalyzedwater oxidation, we examined electrochemical oxi-dations of aqueous solutions containing phosphateand Co2+. Cyclic voltammetry of a 0.5 mM solu-tion of Co(NO3)2 in 0.1 M potassium phosphatepH7.0 (KPi electrolyte) exhibits an oxidationwaveatEp = 1.13V (whereEp is the peakpotential) versusthe normal hydrogen electrode (NHE), followed bythe onset of a strong catalytic wave at 1.23 V (Fig.1A). A broad, relatively weak reduction wave is ob-served on the cathodic scan. The presence of a cat-alytic wave prompted us to examine the electrodeactivity during controlled-potential electrolysis.

Indium tin oxide (ITO) was used as the elec-trode for bulk electrolysis to ensure a minimalbackground activity for O2 production. An elec-trolysis at 1.29 V without stirring in neutral KPielectrolyte containing 0.5 mM Co2+ exhibits arising current density that reaches a peak value>1 mA/cm2 after 7 to 8 hours (Fig. 1B). Duringthis time, a dark coating forms on the ITOsurface, and effervescence from this coating be-

comes increasingly vigorous (19).The same resultsare observed with either CoSO4, Co(NO3)2, orCo(OTf )2 (where OTf = triflate) as the Co

2+

source, which indicates that the original Co2+

counterion is unimportant and that this activitydoes not depend on an impurity found in a specificsource. The amount of charge passed during thecourse of an 8-hour electrolysis far exceeds whatcould be accounted for by stoichiometric oxidationof the Co2+ in solution (20). These observations areindicative of the in situ formation of an oxygen-evolving catalyst. Catalyst formation also pro-ceeds on a fluorine tin oxide electrode and if KPiis replaced by NaPi electrolyte. In a control ex-periment, the current density during bulk elec-trolysis under identical conditions in the absence ofCo2+ rapidly drops to a baseline level of ~25 nA/cm2

(inset in Fig. 1B).The morphology of the electrode coating

formed during electrolysis in the presence of Co2+

was examined by scanning electron microscopy(SEM). The electrodeposited material consists ofparticles that have coalesced into a thin film andindividual micrometer-sized particles on top ofthe film (Fig. 2A). The ITO substrate can be seen

through cracks in the film that form upon drying,as evidenced by particles that are split into com-plementary pieces. The film thickness graduallyincreases over the course of the electrodeposition(see fig. S4 for additional images). At maximumactivity under these electrolysis conditions, thefilm is >2 mm thick. The x-ray powder diffractionpattern of an electrodeposited catalyst showsbroad amorphous features and no peaks indica-tive of crystalline phases other than the peaks as-sociated with the ITO layer (fig. S1).

In the absence of detectable crystallites, thecomposition of the electrodeposited material wasanalyzed by three complementary techniques.Energy-dispersive x-ray analysis (EDX) spectrawere obtained from multiple 100-to-300–m2

regions of several independently prepared sam-ples. These spectra identify Co, P, K, andO as theprincipal elemental components of the material(Fig. 2B). Although the material’s morphology isnot ideally suited for quantitative EDX, the analy-ses consistently indicate a Co:P:K ratio between~2:1:1 and 3:1:1. To obtain an independent deter-mination of elemental composition, electrolysiswas performedwith several larger ITO electrodes;the deposited material was scraped off and com-bined for a total yield of ~3 mg. Microanalyticalelemental analysis of the combined material in-dicates 31.1% Co, 7.70% P, and 7.71% K, cor-responding to a 2.1:1.0:0.8 Co:P:K ratio. Finally,the surface of an electrodeposited catalyst on theITO substrate was analyzed by x-ray photoelectronspectroscopy (XPS). All peaks in theXPS spectraare accounted for by the elements detected above,in addition to In and Sn from the ITO substrate.The high-resolution P 2p peak at 133.1 eV isconsistent with phosphate. The Co 2p peaks at780.7 and 795.7 eVare in a range typical of Co2+

or Co3+ bound to oxygen (fig. S2) (21). Together,the x-ray diffraction and analytical results indi-cate that electrolysis of a Co2+ solution in neutralKPi electrolyte results in the electrodeposition ofan amorphous Co oxide or hydroxide incorporat-ing a substantial amount of phosphate anion at astoichiometric ratio of roughly 2:1:1 for Co:P:K.

A B

5µm 5µm

CoPKO KCo

1 2 3 4 5 6 7 8 9 10E / keV

0.2

0.4

0.6

0.8

1.0

1.2

cps

/ eV

Fig. 2. (A) SEM image (30° tilt) of the electrodeposited catalyst after 30 C/cm2 were passed in 0.1 M KPielectrolyte at pH 7.0, containing 0.5 mM Co2+. The ITO substrate can be seen through cracks in the driedfilm. (B) Typical EDX histogram acquired at 12 kV. cps, counts per second.

0.5 1.0 1.5 2.0 2.5 3.0 3.50

1000

2000

3000

4000

cou

nts

1.0 1.5 2.0 2.50

20

40

60

80

% A

bund

ance

time / h

2.1 ± 0.2%

24.5 ± 0.6%

73.4 ± 0.6%

1.0 2.0 3.00

40

80

120

time (h)

A B

µmol

O2

C

0 2 4 6 8 10 12 14 160

20

40

60

80

100

time / htime / h

Fig. 3. (A) Mass spectrometric detection of isotopically labeled 16,16O2 (blackline), 16,18O2 (blue line), and

18,18O2 (red line) during electrolysis of a catalystfilm on ITO in a KPi electrolyte containing 14.6% 18OH2. The green arrowindicates the initiation of electrolysis at 1.29 V (NHE), and the red arrowindicates the termination of electrolysis. (Inset) Expansion of the 18,18O2signal. (B) Percent abundance of each isotope over the course of the

experiment. Average observed abundance of T2s is indicated above each line.Statistical abundances are 72.9, 24.9, and 2.1%. (C) O2 production measuredby fluorescent sensor (red line) and theoretical amount of O2 produced (blueline), assuming a Faradic efficiency of 100%. The green arrow indicates theinitiation of electrolysis at 1.29 V, and the red arrow indicates the terminationof electrolysis.

www.sciencemag.org SCIENCE VOL 321 22 AUGUST 2008 1073

REPORTS

on

May

5, 2

009

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

http://www.sciencemag.org

Three experiments were performed to estab-lish that the catalytic activity observed with thismaterial corresponds to authentic water oxida-tion. Each of these experiments was performed inneutral KPi electrolyte in the absence of Co2+.Catalyst coatings (~1.3 cm2) were prepared in apreliminary step as described above and storedunder ambient laboratory conditions until theywere used.

To confirm that water is the source of the O2produced, electrolysis was performed in helium-saturated electrolyte containing 14.6% 18OH2 ina gas-tight electrochemical cell in line with a massspectrometer. The helium carrier gas was contin-uously flowed through the headspace of theanodic compartment into the mass spectrometer,and the relative abundances of 32O2,

34O2, and36O2 were monitored at 2-s intervals. Withinminutes of initiating electrolysis at 1.29 V, thesignals for the three isotopes began to rise abovetheir background levels as the O2 produced bythe catalyst escaped into the headspace. Uponterminating the electrolysis 1 hour later, these sig-nals slowly returned to their background levels(Fig. 3A). The 32O2,

34O2, and36O2 isotopes were

detected in the statistical ratio (72.9, 24.9, and2.1% relative abundances, respectively) (Fig. 3B).

The Faradaic efficiency of the catalyst wasmeasured with a fluorescence-based O2 sensor.Electrolysis was performed in KPi electrolyte in agas-tight electrochemical cell under an N2 atmo-sphere with the sensor placed in the headspace.After initiating electrolysis at 1.29 V, the percent-age of O2 detected in the headspace rose in ac-cord with what was predicted by assuming thatall of the current was caused by 4e– oxidation ofwater to produce O2 (Fig. 3C). The amount of O2produced (95 mmol, 3.0 mg) greatly exceeds theamount of catalyst (~0.2 mg), which shows noperceptible decomposition during the course ofthe experiment.

The stability of phosphate under catalytic con-ditions was assayed by 31P nuclear magnetic res-onance (NMR). Electrolysis in a two-compartmentcell with 10 mL of KPi electrolyte (1 mmol phos-phate) on each side was allowed to proce

subscriber access provided by universitaetsbibliothek ...kg) (see chapter 3 of ref 1). different...

Documents