Since the industrial revolution began in the 18th cen-tury, fossil fuels in the form of coal, oil, and natural gas

have powered the technology and transportation networksthat drive society. But continuing to power the world fromfossil fuels threatens our energy supply and puts enor-mous strains on the environment. The world’s demand forenergy is projected to double by 2050 in response to popu-lation growth and the industrialization of developing coun-tries.1 The supply of fossil fuels is limited, with restrictiveshortages of oil and gas projected to occur within our life-times (see the article by Paul Weisz in PHYSICS TODAY,July 2004, page 47). Global oil and gas reserves are con-centrated in a few regions of the world, while demand isgrowing everywhere; as a result, a secure supply is in-creasingly difficult to assure. Moreover, the use of fossilfuels puts our own health at risk through the chemical andparticulate pollution it creates. Carbon dioxide and othergreenhouse gas emissions that are associated with globalwarming threaten the stability of Earth’s climate.

A replacement for fossil fuels will not appearovernight. Extensive R&D is required before alternativesources can supply energy in quantities and at costs com-petitive with fossil fuels, and making those alternativesources available commercially will itself require develop-ing the proper economic infrastructure. Each of those stepstakes time, but greater global investment in R&D willmost likely hasten the pace of economic change. Althoughit is impossible to predict when the fossil fuel supply willfall short of demand or when global warming will becomeacute, the present trend of yearly increases in fossil fueluse shortens our window of opportunity for a managedtransition to alternative energy sources.

Hydrogen as energy carrierOne promising alternative to fossil fuels is hydrogen2,3 (seethe article by Joan Ogden, PHYSICS TODAY, April 2002,page 69). Through its reaction with oxygen, hydrogen re-

leases energy explosively in heat en-gines or quietly in fuel cells to producewater as its only byproduct. Hydrogenis abundant and generously distrib-uted throughout the world without re-gard for national boundaries; using itto create a hydrogen economy—a fu-ture energy system based on hydrogenand electricity—only requires technol-ogy, not political access.

Although in many ways hydrogenis an attractive replacement for fossil

fuels, it does not occur in nature as the fuel H2. Rather, itoccurs in chemical compounds like water or hydrocarbonsthat must be chemically transformed to yield H2. Hydro-gen, like electricity, is a carrier of energy, and like elec-tricity, it must be produced from a natural resource. Atpresent, most of the world’s hydrogen is produced fromnatural gas by a process called steam reforming. However,producing hydrogen from fossil fuels would rob the hydro-gen economy of much of its raison d’être: Steam reformingdoes not reduce the use of fossil fuels but rather shiftsthem from end use to an earlier production step; and it stillreleases carbon to the environment in the form of CO2.Thus, to achieve the benefits of the hydrogen economy, wemust ultimately produce hydrogen from non-fossil re-sources, such as water, using a renewable energy source.

Figure 1 depicts the hydrogen economy as a networkcomposed of three functional steps: production, storage,and use. There are basic technical means to achieve eachof these steps, but none of them can yet compete with fos-sil fuels in cost, performance, or reliability. Even whenusing the cheapest production method—steam reformingof methane—hydrogen is still four times the cost of gaso-line for the equivalent amount of energy. And productionfrom methane does not reduce fossil fuel use or CO2 emis-sion. Hydrogen can be stored in pressurized gas contain-ers or as a liquid in cryogenic containers, but not in den-sities that would allow for practical applications—drivinga car up to 500 kilometers on a single tank, for example.Hydrogen can be converted to electricity in fuel cells, butthe production cost of prototype fuel cells remains high:$3000 per kilowatt of power produced for prototype fuelcells (mass production could reduce this cost by a factor of10 or more), compared with $30 per kilowatt for gasolineengines.

The gap between the present state of the art in hy-drogen production, storage, and use and that needed for acompetitive hydrogen economy is too wide to bridge in in-cremental advances. It will take fundamental break-throughs of the kind that come only from basic research.

Beyond reformingThe US Department of Energy estimates that by 2040 carsand light trucks powered by fuel cells will require about150 megatons per year of hydrogen.3 The US currently pro-duces about 9 megatons per year, almost all of it by re-forming natural gas. The challenge is to find inexpensive

© 2004 American Institute of Physics, S-0031-9228-0412-010-3 December 2004 Physics Today 39

George Crabtree is a physicist in the materials science divisionat Argonne National Laboratory in Illinois. Mildred Dresselhausis a professor in the department of physics and the departmentof electrical engineering and computer science at the Massa-chusetts Institute of Technology in Cambridge. MichelleBuchanan is a chemist in the chemical sciences division at OakRidge National Laboratory in Tennessee.

If the fuel cell is to become the modern steam engine, basicresearch must provide breakthroughs in understanding,materials, and design to make a hydrogen-based energysystem a vibrant and competitive force.

George W. Crabtree, Mildred S. Dresselhaus, and Michelle V. Buchanan

The Hydrogen Economy

and efficient routes to create hydrogen in sufficient quanti-ties from non-fossil natural resources. The most promisingroute is splitting water, which is a natural carrier of hydro-gen. It takes energy to split the water molecule and releasehydrogen, but that energy is later recovered during oxida-tion to produce water. To eliminate fossil fuels from thiscycle, the energy to split water must come from non-carbonsources, such as the electron–hole pairs excited in a semi-conductor by solar radiation, the heat from a nuclear reac-tor or solar collector, or an electric voltage generated by re-newable sources such as hydropower or wind.

The direct solar conversion of sunlight to H2 is one ofthe most fascinating developments in water splitting.4 Es-tablished technology splits water in two steps: conversionof solar radiation to electricity in photovoltaic cells fol-lowed by electrolysis of water in a separate cell. It is wellknown that the photovoltaic conversion occurs with an ef-ficiency up to 32% when expensive single-crystal semicon-ductors are used in multi-junction stacks, or about 3% withmuch cheaper organic semiconductors; remarkably, thecost of delivered electricity is about the same in both cases.Advanced electrolyzers split water with 80% efficiency.

The two processes, however, can be combined in a sin-gle nanoscale process: Photon absorption creates a localelectron–hole pair that electrochemically splits a neighbor-ing water molecule. The efficiency of this integrated photo-chemical process can be much higher, in principle, than thetwo sequential processes; it has now reached 8–12% in thelaboratory4 and has prospects for much greater gains as re-searchers learn to better control the nanoscale excitationand photochemistry. The technical challenge is finding ro-bust semiconductor materials that satisfy the competing re-quirements of nature. The Sun’s photons are primarily inthe visible, a wavelength that requires semiconductors withsmall bandgaps—below 1.7 eV—for efficient absorption.Oxide semiconductors like titanium dioxide that are robustin aqueous environments have wide bandgaps, as high as3.0 eV, and thus require higher-energy photons for excita-tion. The use of dye-sensitized photocells that accumulateenergy from multiple low-energy photons to inject higher-energy electrons into the semiconductor is a promising di-rection for matching the solar spectrum. Alternatively, oxidesemiconductors can be doped with impurities that reducetheir bandgap energies to overlap better with the solar spec-trum. In both cases, new strategies for nanostructured hy-brid materials are needed to more efficiently use solar en-ergy to split water.

Water can be split in thermochemical cycles operatingat elevated temperatures to facilitate the reaction kinetics.5Heat sources include solar collectors operating up to 3000°Cor nuclear reactors designed to operate between 500°C and

900°C (see the article by GailMarcus and Alan Levin,PHYSICS TODAY, April 2002,

page 54). More than 100 types of chemical cycles have beenproposed, including systems based on zinc–oxygen operat-ing at 1500°C, sulfur–iodine at 850°C, calcium–bromine at750°C, and copper–chlorine at 550°C. At high temperatures,thermochemical cycles must deal with the tradeoff betweenfavorable reaction kinetics and aggressive chemical corro-sion of containment vessels. Separating the reaction prod-ucts at high temperature is a second challenge: Unsepa-rated mixtures of gases recombine if allowed to cool. Butidentifying effective membrane materials that selectivelypass hydrogen, oxygen, water, hydrogen sulfate, or hydro-gen iodide, for example, at high temperature remains aproblem. Dramatic improvements in catalysis could lowerthe operating temperature of thermochemical cycles, andthus reduce the need for high-temperature materials, with-out losing efficiency. Molecular-level challenges, with whichresearchers are fast making progress using nanoscale de-sign, include accelerating the kinetics of reactions throughcatalysis, separating the products at high temperature, anddirecting products to the next reaction step.

Bio-inspired processes offer stunning opportunities toapproach the hydrogen production problem anew.6 Thenatural world began forming its own hydrogen economy 3 billion years ago, when it developed photosynthesis to con-vert CO2, water, and sunlight into hydrogen and oxygen.Plants use hydrogen to manufacture the carbohydrates intheir leaves and stalks, and emit oxygen to the atmospherefor animals to breathe. Single-cell organisms such as algaeand many microbes produce hydrogen efficiently at ambi-ent temperatures by molecular-level processes. These nat-ural mechanisms for producing hydrogen involve elaborateprotein structures that have only recently been partiallysolved. For billions of years, for instance, plants have useda catalyst based on manganese–oxygen clusters to splitwater efficiently at room temperature, a process that freesprotons and electrons. Likewise, bacteria use iron andnickel clusters as the active elements both for combiningprotons and electrons into H2 and splitting H2 into protonsand electrons (see figure 2). The hope is that researcherscan capitalize on nature’s efficient manufacturingprocesses by fully understanding molecular structures andfunctions and then imitating them using artificial materi-als in such applications as fuel-cell anodes and cathodes.

Storing hydrogen Storing hydrogen in a high-energy-density form that flex-ibly links its production and eventual use is a key elementof the hydrogen economy. Unlike electricity, which must beproduced and used at the same rate, stored hydrogen canbe stockpiled for much later use, or used as ballast tobridge the differing temporal cycles of energy productionand consumption.

Gas orhydridestorage

H2 H2

Production Storage Use

Biological andbio-inspired Automotive

fuel cells

Consumerelectronics

Stationaryelectricity/heat

generation

Fossil fuelreforming

+Carbon capture

Nuclear/solarthermochemical

cycles

SolarWindHydro

H O2

Figure 1. The hydrogen econ-omy as a network of primaryenergy sources linked to multi-ple end uses through hydrogenas an energy carrier. Hydrogenadds flexibility to energy pro-duction and use by linking nat-urally with fossil, nuclear, re-newable, and electrical energyforms: Any of those energysources can be used to makehydrogen.

40 December 2004 Physics Today http://www.physicstoday.org

The traditional storage options are conceptually sim-ple—cylinders of liquid and high-pressure gas. Industrialfacilities and laboratories are already accustomed to han-dling hydrogen both ways. These options are viable for thestationary consumption of hydrogen in large plants thatcan accommodate large weights and volumes. Storage asliquid H2 imposes severe energy costs because up to 40%of its energy content can be lost to liquefaction.

For transportation use, the on-board storage of hy-drogen is a far more difficult challenge. Both weight andvolume are at a premium, and sufficient fuel must bestored to make it practical to drive distances comparableto gas-powered cars.3 Figure 3 illustrates the challenge byshowing the gravimetric and volumetric energy densitiesof fuels, including the container and apparatus needed forfuel handling. For hydrogen, that added weight is a majorfraction of the total. For on-vehicle use, hydrogen needstore only about half of the energy that gasoline providesbecause the efficiency of fuel cells can be greater by a fac-tor of two or more than that of internal combustion en-gines. Even so, the energy densities of the most advancedbatteries and of liquid and gaseous hydrogen pale in com-parison to gasoline.

Meeting the volume restrictions in cars or trucks, forinstance, requires using hydrogen stored at densitieshigher than its liquid density. Figure 4 shows the volumedensity of hydrogen stored in several compounds and insome liquid hydrocarbons.7 All of those compounds storehydrogen at higher density than the liquid or the com-pressed gas at 10 000 psi (�700 bar), shown as points onthe right-hand vertical axis for comparison. The most ef-fective storage media are located in the upper-right quad-rant of the figure, where hydrogen is combined with lightelements like lithium, nitrogen, and carbon. The materi-als in that part of the plot have the highest mass fractionand volume density of hydrogen. Hydrocarbons likemethanol and octane are notable as high-volume-densityhydrogen storage compounds as well as high-energy-density fuels, and cycles that allow the fossil fuels to re-lease and recapture their hydrogen are already in use instationary chemical processing plants.7

The two challenges for on-vehicle hydrogen storageand use are capacity and cycling performance under the ac-cessible on-board conditions of 0–100°C and 1–10 bars. Toachieve high storage capacity at low weight requires strongchemical bonds between hydrogen and light-atom host ma-terials in stable compounds, such as lithium borohydride(LiBH4). But to achieve fast cycling at accessible conditionsrequires weak chemical bonds, fast kinetics, and short dif-fusion lengths, as might be found in surface adsorption.Thus, the high-capacity and fast-recycling requirementsare somewhat in conflict. Many bulk hydrogen-storage

compounds, such as metallic magnesium nitrogen hydride(Mg2NH4) and ionic sodium borohydride (Na+(BH4)–), con-tain high volumetric hydrogen densities but require tem-peratures of 300°C or more at 1 bar to release their H2.Compounds with low-temperature capture and release be-havior, such as lanthanum nickel hydride (LaNi5H6), havelow hydrogen-mass fractions and are thus heavy to carry.

Hydrogen absorption on surfaces is a potential routeto fast cycling, but has been explored relatively little ex-cept for carbon substrates. Hydrogen can be adsorbed inmolecular or atomic form on suitable surfaces, using pres-sure, temperature, or electrochemical potential to controlits surface structure and bonding strength. A major chal-lenge is controlling the bonding and kinetics of multiplelayers of hydrogen. The first layer is bonded by van derWaals or chemical forces specific to the substrate; the sec-ond layer sees primarily the first layer and therefore bondswith very different strength. The single-layer properties ofadsorbed hydrogen on carbon can be predicted rather ac-curately and are indicated by the solid curve in figure 4;the behavior of multiple layers is much less well under-stood. But experience with carbon suggests that multiplelayers are needed for effective storage capacity. One routefor overcoming the single-layer limitation is to adsorb hy-drogen on both sides of a substrate layer, arranged withothers in nanoscale stacks that allow access to both sides.

Nanostructured materials offer a host of promisingroutes for storing hydrogen at high capacity in compoundsthat have fast recycling. Large surface areas can be coatedwith catalysts to assist in the dissociation of gaseous H2,and the small volume of individual nanoparticles producesshort diffusion paths to the material’s interior. Thestrength of the chemical bonds with hydrogen can be weak-ened with additives7 such as titanium dioxide in sodiumaluminum hydride (NaAlH4). The capture and releasecycle is a complex process that involves molecular dissoci-ation, diffusion, chemical bonding, and van der Waals at-traction. Each of the steps can be optimized in a specificnanoscale environment that includes appropriate cata-lysts, defects, and impurity atoms. By integrating thesteps into an interactive nanoscale architecture where hy-drogen molecules or atoms are treated in one environment

http://www.physicstoday.org December 2004 Physics Today 41

2H O2 4H + 4e+ –

Mn

Mn

MnMn

Mn

MnMn Mn

OOO

OO

O

OO

O

O

cubane

2H + 2e H+ –2E

H+

H2e–

Fe Fe

d+H

S– [4Fe4S]

CNHd+COC

O

SS

OCNC

Figure 2. Nature has developed remarkably simple and efficient methods to split water and transform H2 into its com-

ponent protons and electrons. The basic constituent of the cat-alyst that splits water during photosynthesis is cubane (top)—

clusters of manganese and oxygen. Researchers are onlybeginning to understand cubane’s oxidation states using crys-tallography and spectroscopy (see J.-Z. Wu et al., ref. 6). Bac-

teria use the iron-based cluster (bottom, circled) to catalyzethe transformation of two protons and two electrons into H2.

The roles of this enzyme’s complicated structural and elec-tronic forms in the catalytic process can be imitated in the

laboratory. The hope is to create synthetic versions of thesenatural catalysts (see F. Gloaguen et al. and J. Alper, ref. 6).

for dissociation, for example, and handed off to the nextenvironment for diffusion, nanoscience engineers could simultaneously optimize all the desired properties. Another approach is to use three-dimensional solids withopen structures, such as metal–organic frameworks8 inwhich hydrogen molecules or atoms can be adsorbed oninternal surfaces. The metal atoms that form the verticesof such structures can be catalysts or dopants that facili-tate the capture and release cycle. Designed nanoscale architectures offer unexplored options for effectively con-trolling reactivity and bonding to meet the desired stor-age requirements.

Realizing the promiseA major attraction of hydrogen as a fuel is its natural com-patibility with fuel cells. The higher efficiency of fuelcells—currently 60% compared to 22% for gasoline or 45%for diesel internal combustion engines—would dramati-cally improve the efficiency of future energy use. Couplingfuel cells to electric motors, which are more than 90% ef-ficient, converts the chemical energy of hydrogen to me-chanical work without heat as an intermediary. This at-tractive new approach for energy conversion could replacemany traditional heat engines. The broad reach of that ef-ficiency advantage is a strong driver for deploying hydro-gen fuel cells widely.

Although fuel cells are more efficient, there are alsogood reasons for burning hydrogen in heat engines for trans-portation. Jet engines and internal combustion engines canbe rather easily modified to run on hydrogen instead of hy-drocarbons. Internal combustion engines run as much as25% more efficiently on hydrogen compared to gasoline andproduce no carbon emissions. The US and Russia have test-flown commercial airliners with jet engines modified to burnhydrogen.9 Similarly, BMW, Ford, and Mazda are road-testing cars powered by hydrogen internal combustion en-gines that achieve a range of 300 kilometers, and networksof hydrogen filling stations are being implemented in someareas of the US, Europe, and Japan. Such cars and fillingstations could provide an early start and a transitionalbridge to hydrogen fuel-cell transportation.

The versatility of fuel cells makes them workable innearly any application where electricity is useful. Station-ary plants providing 200 kilowatts of neighborhood elec-trical power are practical and operating efficiently. Suchplants can connect to the electrical grid to share power butare independent of the grid in case of failure. Fuel-cellpower for consumer electronics like laptop computers, cellphones, digital cameras, and audio players provide more

hours of operation than batteries at the samevolume and weight. Although the cost per kilo-watt is high for these small units, the unit costcan soon be within an acceptable consumerrange. Electronics applications may be thefirst to widely reach the consumer market, es-tablish public visibility, and advance the

learning curve for hydrogen technology.The large homogeneous transportation market offers

enormous potential for hydrogen fuel cells to dramaticallyreduce fossil fuel use, lower harmful emissions, and im-prove energy efficiency. Fuel cells can be used not only incars, trucks, and buses, but also can replace the diesel elec-tric generators in locomotives and power all-electric ships.8

Europe already has a demonstration fleet of 30 fuel-cellbuses running regular routes in 10 cities, and Japan ispoised to offer fuel-cell cars for sale.

A host of fundamental performance problems remain tobe solved before hydrogen in fuel cells can compete withgasoline.10 The heart of the fuel cell is the ionic conductingmembrane that transmits protons or oxygen ions betweenelectrodes while electrons go through an external load to dotheir electrical work, as shown in figure 5. Each of the halfreactions at work in that circuit requires catalysts inter-acting with electrons, ions, and gases traveling in differentmedia. Designing nanoscale architectures for these triplepercolation networks that effectively coordinate the inter-action of reactants with nanostructured catalysts is a majoropportunity for improving fuel-cell performance. The trickis to get intimate contact of the three phases that coexist inthe cell—the incoming hydrogen or incoming oxygen gasphase, an electrolytic proton-conducting phase, and a metal-lic phase in which electrons flow into or from the externalcircuit (see PHYSICS TODAY, July 2001, page 22).

A primary factor limiting proton-exchange-membrane(PEM) fuel-cell performance is the slow kinetics of the oxy-gen reduction reaction at the cathode. Even with the bestplatinum-based catalysts, the sluggish reaction reducesthe voltage output of the fuel cell from the ideal 1.23 V to0.8 V or less when practical currents are drawn. This volt-age reduction is known as the oxygen overpotential. Thecauses of the slow kinetics, and solutions for speeding upthe reaction, are hidden in the complex reaction pathwaysand intermediate steps of the oxygen reduction reaction.It is now becoming possible to understand this reaction atthe atomic level using sophisticated surface-structure andspectroscopy tools such as vibrational spectroscopies, scan-ning probe microscopy, x-ray diffraction and spectroscopy,and transmission electron microscopy.11,12 In situ electro-chemical probes, operating under reaction or near reactionconditions, reveal the energetics, kinetics, and intermedi-ates of the reaction pathway and their relation to the sur-face structure and composition of the reactants and cata-lysts. These powerful new experimental probes, combinedwith equally powerful and impressive computationalquantum chemistry using density functional theory,13 are

42 December 2004 Physics Today http://www.physicstoday.org

Gasoline

Proposed DOE goal

Liquid H2

Chemical hydrides

Compressed gas H2

Batteries

30

30 40

20

20

10

10

0

0

VO

LU

ME

TR

IC E

NE

RG

Y D

EN

SIT

Y (

MJ/

L)

GRAVIMETRIC ENERGY DENSITY (MJ/kg)

Figure 3. The energy densities of hydrogenfuels stored in various phases and materials areplotted, with the mass of the container and ap-paratus needed for filling and dispensing thefuel factored in. Gasoline significantly outper-forms lithium-ion batteries and hydrogen ingaseous, liquid, or compound forms. The pro-posed DOE goal refers to the energy densitythat the US Department of Energy envisions asneeded for viable hydrogen-powered trans-portation in 2015.

opening a new chapter in atomic-level understanding ofthe catalytic process. The role of such key features as theatomic configuration of catalysts and their supports, andthe electronic structure of surface-reconstructed atomsand adsorbed intermediate species, is within reach of fun-damental understanding. These emerging and incisive ex-perimental and theoretical tools make the field ofnanoscale electrocatalysis ripe for rapid and comprehen-sive growth. The research is highly interdisciplinary, in-corporating forefront elements of chemistry, physics, andmaterials science.

Beyond the oxygen reduction reaction, fuel cells providemany other challenges. The dominant membrane for PEMfuel cells is perfluorosulfonic acid (PFSA), a polymer builtaround a C–F backbone with side chains containing sulfonicacid groups (SO3

⊗) (for example, Nafion). Beside its high cost,this membrane must incorporate mobile water moleculesinto its structure to enable proton conduction. That restrictsits operating temperature to below the boiling point ofwater. At this low temperature—typically around 80°C—expensive catalysts like platinum are required to make theelectrochemical reactions sufficiently active, but even traceamounts of carbon monoxide in the hydrogen fuel streamcan poison the catalysts. A higher operating temperaturewould expand the range of suitable catalysts and reducetheir susceptibility to poisoning. Promising research direc-tions for alternative proton-conducting membranes that op-erate at 100–200°C include sulfonating C–H polymersrather than C–F polymers, and using inorganic polymercomposites and acid–base polymer blends.14

Solid oxide fuel cells (SOFCs) require O2⊗ transport

membranes, which usually consist of perovskite materialscontaining specially designed defect structures that be-come sufficiently conductive only above 800°C. The hightemperature restricts the constructionmaterials that can be used in SOFCsand limits their use to special environ-ments like stationary power stations orperhaps large refrigerated trucks whereadequate thermal insulation and safetycan be ensured. Finding new materialsthat conduct O2

⊗ at lower temperatureswould significantly expand the range ofapplications and reduce the cost ofSOFCs.

OutlookThe hydrogen economy has enormoussocietal and technical appeal as a po-tential solution to the fundamental en-ergy concerns of abundant supply andminimal environmental impact. The ul-timate success of a hydrogen economydepends on how the market reacts: Doesemerging hydrogen technology providemore value than today’s fossil fuels? Al-though the market will ultimately drivethe hydrogen economy, governmentplays a key role in the move from fossil-fuel to hydrogen technology. The invest-ments in R&D are large, the outcome forspecific, promising approaches is uncer-tain, and the payoff is often beyond themarket’s time horizon. Thus, early gov-ernment investments in establishinggoals, providing research support, andsharing risk are necessary to prime theemergence of a vibrant, market-drivenhydrogen economy.

The public acceptance of hydrogen depends not onlyon its practical and commercial appeal, but also on itsrecord of safety in widespread use. The special flamma-bility, buoyancy, and permeability of hydrogen presentchallenges to its safe use that are different from, but notnecessarily more difficult than, those of other energy car-riers. Researchers are exploring a variety of issues: hy-drodynamics of hydrogen–air mixtures, the combustion ofhydrogen in the presence of other gases, and the embrit-tlement of materials by exposure to hydrogen, for exam-ple. Key to public acceptance of hydrogen is the develop-ment of safety standards and practices that are widelyknown and routinely used—like those for self-service gaso-line stations or plug-in electrical appliances. The techni-cal and educational components of this aspect of the hy-drogen economy need careful attention.

Technical progress will come in two forms. Incremen-tal advances of present technology provide low-risk com-mercial entry into the hydrogen economy. Those advancesinclude improving the yield of natural-gas reforming tolower cost and raise efficiency; improving the strength ofcontainer materials for high-pressure storage of hydrogengas; and tuning the design of internal combustion enginesto burn hydrogen. To significantly increase the energy sup-ply and security, and to decrease carbon emission and airpollutants, however, the hydrogen economy must go wellbeyond incremental advances. Hydrogen must replace fos-sil fuels through efficient production using solar radiation,thermochemical cycles, or bio-inspired catalysts to splitwater. Hydrogen must be stored and released in portablesolid-state media, and fuel cells that convert hydrogen toelectrical power and heat must be put into widespread use.

Achieving these technological milestones while satisfy-ing the market discipline of competitive cost, performance,

http://www.physicstoday.org December 2004 Physics Today 43

~ ~~ ~

H absorbed on graphite monolayer

Liquidhydrogen

Gaseoushydrogen(700 bar)

0.5 g cm–31 g cm–35 g cm–3

LiBH4

KBH4

NaBH4

LiAlH4

NaA H4l

Mg NH2 4

MgH2

0 5 10 15 20 25 1000

50

150

100

LaNi H5 6

FeTiH1.7

HYDROGEN MASS DENSITY (mass %)

HY

DR

OG

EN

VO

LU

ME

DE

NS

ITY

(kg

Hm

)

2–3

CH (liq)4C H OH2 5C H8 18

C H3 8

C H2 6NH3

CH OH3

Figure 4. The storage density of hydrogen in compressed gas, liquid, ad-sorbed monolayer, and selected chemical compounds is plotted as a func-tion of the hydrogen mass fraction. All compounds here except thegraphite monolayer store hydrogen at greater than its liquid density at atmospheric pressure; green data points on the right-hand side indicateliquid and gaseous H2 densities. The straight lines indicate the total densityof the storage medium, including hydrogen and host atoms. Of the inor-ganic materials (plotted as triangles), the compounds shown in boxes all form the alanate structure, composed of a tetrahedral, methanelikeAlH4

– or BH4– anion and a metal cation. Those compounds are among the

most promising hydrogen-storage fuel-cell materials. (Adapted from L. Schlapbach and A. Zuttel, ref. 7.)

and reliability requires tech-nical breakthroughs thatcome only from basic re-search. The interaction of hy-drogen with materials encom-passes many fundamentalquestions that can now be ex-plored much more thoroughlythan ever before using so-phisticated atomic-level scan-ning probes, in situ structuraland spectroscopic tools atx-ray, neutron, and electronscattering facilities, and pow-erful theory and modelingusing teraflop computers.The hope is to solve mysteriesthat Nature has long kept hidden, such as the molecularbasis of catalysis and the mechanism that allows plants to split water at room temperature using sunlight.Nanoscience provides not only new approaches to basicquestions about the interaction of hydrogen with materi-als, but also the power to synthesize materials with custom-designed architectures. This combination ofnanoscale analysis and synthesis promises to create newmaterials technology, such as orderly control of the elec-tronic, ionic, and catalytic processes that regulate thethree-phase percolation networks in fuel cells. Such ex-quisite control over materials behavior has never been sonear at hand.

The international character of the hydrogen economyis sure to influence how it develops and evolves globally.Each country or region of the world has technological andpolitical interests at stake. Cooperation among nations toleverage resources and create innovative technical and or-ganizational approaches to the hydrogen economy is likelyto significantly enhance the effectiveness of any nationthat would otherwise act alone. The emphasis of the hy-drogen research agenda varies with country; communica-tion and cooperation to share research plans and resultsare essential.

Will the hydrogen economy succeed? Historical prece-dents suggest that it might. New energy sources and car-riers have flourished when coupled with new energy con-verters. Coal became king as fuel for the steam engine topower the industrial revolution—it transformed the faceof land transportation from horse and buggy to rail, andon the sea from sail to steamship.15 Oil fueled the internalcombustion engine to provide automobiles and trucks thatcrisscross continents, and later the jet engine to conquerthe skies. Electricity coupled with light bulbs and with ro-tary motors to power our homes and industries. Hydrogenhas its own natural energy-conversion partner, the fuelcell. Together they interface intimately with the broadbase of electrical technology already in place, and they canexpand to propel cars, locomotives, and ships, power con-sumer electronics, and generate neighborhood heat andlight. Bringing hydrogen and fuel cells to that level of im-

44 December 2004 Physics Today http://www.physicstoday.org

pact is a fascinating challenge and opportunity for basic sci-ence, spanning chemistry, physics, biology, and materials.

References1. M. I. Hoffert et al., Nature 395, 891 (1998); Energy Informa-

tion Administration, International Energy Outlook 2004, rep.no. DOE/EIA-0484 (2004), available at http://www.eia.doe.gov/oiaf/ieo.

2. J. A. Turner, Science 285, 687 (1999); Special Report: Towarda Hydrogen Economy, Science 305, 957 (2004).

3. US Department of Energy, Office of Basic Energy Sciences,Basic Research Needs for the Hydrogen Economy, US DOE,Washington, DC (2004), available at http://www.sc.doe.gov/bes/hydrogen.pdf; Basic Energy Sciences Advisory Com-mittee, Basic Research Needs to Assure a Secure Energy Fu-ture, US DOE, Washington, DC (2003), available athttp://www.sc.doe.gov/bes/reports/files/SEF_rpt.pdf; Commit-tee on Alternatives and Strategies for Future Hydrogen Pro-duction and Use, The Hydrogen Economy: Opportunities,Costs, Barriers, and R&D Needs, National Research Council,National Academies Press, Washington, DC (2004), availableat http://www.nap.edu/catalog/10922.html.

4. O. Khasalev, J. A. Turner, Science 280, 425 (1998); S. U. M.Khan, M. Al-Shahry, W. B. Ingler, Science 297, 2189 (2002);N. S. Lewis, Nature 414, 589 (2001).

5. C. Perkins, A. W. Weimar, Int. J. Hydrogen Energy (in press).6. J. Alper, Science 299, 1686 (2003); F. Gloaguen et al., Inorg.

Chem. 41, 6573 (2002); J.-Z. Wu et al., Inorg. Chem. 43, 5795(2004); K. N. Ferreira et al., Science 303, 1831 (2004).

7. L. Schlapbach, A. Zuttel, Nature 414, 353 (2001); W.Grochala, P. P. Edwards, Chem. Rev. 104, 1283 (2004).

8. N. L. Rosi et al., Science 300, 1127 (2003).9. P. Hoffmann, Tomorrow’s Energy: Hydrogen, Fuel Cells, and

the Prospects for a Cleaner Planet, MIT Press, Cambridge,MA (2001).

10. J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd ed.,Wiley, Hoboken, NJ (2003).

11. N. M. Markovic, P. J. Ross, Surf. Sci. Rep. 45, 117 (2002).12. P. L. Gai, Top. Catal. 21, 161 (2002).13. A. Mattsson, Science 298, 759 (2002).14. Q. Li, R. He, J. O. Jensen, N. J. Bjerrum, Chem. Mater. 15,

4896 (2003).15. B. Freese, Coal: A Human History, Perseus, Cambridge, MA

(2003). �

+ ++

+

+

+ ++

e–

Anode Cathode

H2 O2

H O2

Protonexchange

membrane

Three-phase percolation networks

H 2H 2e2O ⊕⊕ ⊗ 2H 2e O H O⊕ ⊗⊕ ⊕ O2 212

Load

Figure 5. In a proton-exchange-membrane fuel cell, hy-drogen and oxygen react electrochemically. At the anode,hydrogen molecules dissociate, the atoms are ionized, andelectrons are directed to an external circuit; protons arehanded off to the ion-exchange membrane and passthrough to the cathode. There, oxygen combines with pro-tons from the ion-exchange membrane and electrons fromthe external circuit to form water or steam. The energyconversion efficiency of the process can be 60% or higher.

http://www.physicstoday.org December 2004 Physics Today 45