Phil. Trans. R. Soc. A (2010) 368, 3365–3377doi:10.1098/rsta.2010.0115

REVIEW

Materials towards carbon-free, emission-freeand oil-free mobility: hydrogen fuel-cell

vehicles—now and in the futureBY KATSUHIKO HIROSE*

Toyota Motor Corporation, Fuel Cell Development Group, 1 Toyota,Toyota, Aichi, Japan

In the past, material innovation has changed society through new material-inducedtechnologies, adding a new value to society. In the present world, engineers and scientistsare expected to invent new materials to solve the global problem of climate change. Forthe transport sector, the challenge for material engineers is to change the oil-based worldinto a sustainable world. After witnessing the recent high oil price and its adverse impacton the global economy, it is time to accelerate our efforts towards this change.

Industries are tackling global energy issues such as oil and CO2, as well as localenvironmental problems, such as NOx and particulate matter. Hydrogen is the mostpromising candidate to provide carbon-free, emission-free and oil-free mobility. Assuch, engineers are working very hard to bring this technology into the real society.This paper describes recent progress of vehicle technologies, as well as hydrogen-storage technologies to extend the cruise range and ensure the easiness of refuellingand requesting material scientists to collaborate with industry to fight againstglobal warming.

Keywords: fuel cell; fuel-cell vehicle; hydrogen; hydrogen storage

1. Introduction

Mobility is one of the most basic desires of a human being. Even in the prehistoricage, there are many evidences of people travelling all over the world. Once thesociety was exposed to an affordable and easy means of mobility, the ‘automobile’,the idea of personal and quick mobility was enthusiastically accepted all over theworld. The growth of automobile population is still very high. Typically, oncea person attains a certain economic threshold, the desire to acquire a personalmeans of mobility becomes very strong. It is estimated that the vehicle populationwill be doubled within the next one or two decades (figure 1). However, since

*hirose@katsuhiko.tec.toyota.co.jp

One contribution of 13 to a Discussion Meeting Issue ‘Energy materials to combat climate change’.

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3366 K. Hirose

era society material mobility energy limitation

stone age village stone, bone tool

bronze age city state bronze, pottery foot, horse wood food

foot wood food

iron age nation

industrial urbanization steel railway, steamship coal energyrevolution

globalization steel, copper,aluminum energy

21st century environmental ? informationtechnology

renewable Earth

• stone tool

• bronze

wood foodiron, porcelain horse, carriage, ship

20th century automobile,aeroplane

oil,nuclear

societalchange

change ofvalue

technologyinnovation

materialinnovation

• bronze age• start of city state • rich and poor

• division of labour

• metallurgy

Figure 1. Material innovation and society.

the late twentieth century, we have faced negative aspects of this veryattractive mobility

— local pollution,— accidents,— global warming (CO2), and— energy shortage.

Local pollution and accidents are now well tackled owing to technologicaldevelopments, such as catalysts and airbags, although improvement is stillnecessary in some areas. But global warming (CO2) and energy shortage arenow becoming more serious and confronted globally.

The energy demand of mobility favours only one side: ‘oil’. Although energydiversification is inevitable, escaping from the oil-based world, for the automobile,is not easy owing to a high energy density of petrol or diesel oil. Nonetheless, weare forced to re-realize the uncertainty of oil price and its influence on the worldeconomy, as observed during 2007–2008.

The automobile industry has been working hard to introduce ‘non’-oil-basedautomobiles, but has not yet been widely successful because of the strongadvantage of oil-based fuel and the cost differences that exist between thoseso-called alternative fuel technologies and conventional oil-burning technologies.However, recent progress of electric propulsion systems, led by the hybridelectric car highlights two kinds of pure electric cars as a near-future alternativeautomobile—the battery electric vehicle (BEV) and the fuel-cell vehicle (FCV).

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Review. Hydrogen fuel-cell vehicles 3367

2. What is the future of mobility?

What should future mobility be like? This needs to be discussed because futuretransport requires increased/new infrastructures and further material research,resulting in a huge cost to the society in terms of money and resources.

‘Efficiency’ (energy/man-km) is a commonly used term in discussionspertaining to future transport, but in addition, the ‘quality of mobility’ (QOM)should be discussed more for the future quality of life. The reason behind ‘whypeople spend relatively large amounts of money to purchase an automobile’ needsto be considered and is thought to be as follows. QOM is not easily quantifiable,but to make the discussion simple, QOM is defined as ‘the time to reach adestination from home’. Figure 2 shows an image where a comparison is drawnbetween public transport and a private passenger car based on typical dailyactivities (commute, shopping, meeting someone). In many cities, the commutingtime is shorter by car, except during certain peak hours when use of the car isless advantageous owing to traffic congestion and difficulty in parking. On theother hand, the efficiency of the public transport is heavily dependent on trafficdensity (i.e. number of people on board). As a result, the efficiency of the publictransport drops in the countryside owing to under-capacity operation, and sodoes the QOM, owing to the low frequency of the service.

There is a need to discuss ways to improve mobility, but the superiority ofprivate cars cannot change in many cities and countries. This aspect needs to beconsidered when cities redesign future traffic. However, as previously described,the negative impact of this mobility must be overcome, otherwise we would beforced to sacrifice the QOM. At least, the energy efficiency must be improvedby improving the fuel economy (alternative fuel or new power trains such as fuelcells) and by the improvement of traffic management in cities to avoid traffic jamsand congestion, in turn preventing the deterioration of efficiency and QOM. Anideal scenario of future mobility would be one where, on the one hand, publictransport and private cars help each other in the city by sharing the road withoptimum efficiency and QOM, while on the other hand, the usage of private carsbecomes the main mobility tool in the countryside.

3. Comparison of two pure electric vehicles

BEVs and FCVs share almost the same components, such as electric motorsand power controllers or inverters, to control the speed of the vehicle. However,big differences remain in the main energy source. While BEVs uses the energystored in the battery, FCVs use the electricity generated by the fuel cell, with theprimary energy stored in the hydrogen tank.

Although similar in components, characteristics of the BEV and the FCV arequite different owing to the difference in energy density of the battery and thehydrogen tank (figure 3). The BEV’s cruising range is restricted because of thesmall quantity of energy stored in the battery.

On the other hand, the FCV has a longer cruising range because of its highenergy density and the easiness of refuelling. Battery-technology development hasfocused on increasing the energy density, but there still remains a very big gapbetween the battery pack and the hydrogen tank.

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why do people use cars?

is a modal shift to public transport a solution for the future?

high efficiencyenergy/man-km

advancedpassenger

car

passengercar

advancedpower train

shorter time to reach the destination

quality of mobility (QOM)congestionparking

trainin big city

busin big city

busin small

townquality of life (QOL)

trafficmanagement

train insmalltown

passenger

car

in big city

frequen

cy

ditstan

ce fr

om home to

statio

n

Figure 2. Quality of mobility and car.

diesel(Toyota calculation)

100

50

0electricity gaseous fuel liquid fuel

gasoline

ethanol

high-pressurehydrogen(35 MPa)

high-pressurehydrogen(70 MPa)

hydrogen-absorbing

alloy (2 wt%)compressednatural gas(20 MPa)

lithium-ion battery

volu

met

ric

ener

gy d

ensi

ty (

gaso

line

= 1

00)

Figure 3. Energy density.

The automobile industry is of the view that there is a clear difference of usebetween the two technologies and hence both can coexist (figure 4). While theBEV is suitable for short and small mobilities, the FCV is for larger or long-rangevehicles (US Department of Energy 2008).

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city

long-distance trucksexpressway buses

large

middle

EVFCHV: inter-city travel

Commsmall

long-rangemiddle-rangeshort-range

EV

FCHV

Middle and largepassenger cars

city buscourier vehicles

EV: inner-city

PHV(biofuel)

commutertown use

Figure 4. Positioning of the BEV and FCV. EV, electric vehicle; FCHV, fuel-cell hybrid vehicle;PHV, plug-in hybrid vehicle.

4. Fuel-cell vehicles; carbon-free, emission-free and oil-free vehicles and itsprogress

(a) Recent progress of fuel-cell vehicles

In order to accomplish FCV commercialization, five major issues (figure 5) shouldbe solved, as stated previously on many occasions. At present, the industry claimsto have solved most of the technical issues and the main focus is on reducing thecost for commercialization.

The Fuel Cell Commercialization Conference of Japan (an initiative bringingtogether the automobile industry and the energy industry) has announced thatthey are now targeting the start of FCV commercialization by 2015.

(b) Potential of hydrogen

Hydrogen FCVs do not emit CO2 or hazardous emissions during operation.However, hydrogen is the secondary energy carrier, and is at present mostlyproduced from carbon-based primary energy sources. So, the CO2 emission issolely owing to the hydrogen production process. Currently, industries are mass-producing hydrogen several ways. The oil industry is one of the biggest usersof hydrogen where, on the one hand, hydrogen is produced by steam-reformingnatural gas or oil residuals. On the other hand, hydrogen is also producedby electrolysis while producing chemical products, wherein hydrogen is a by-product. As such, it is difficult to calculate the efficiency in the latter case.A common view of hydrogen vision is shown in figure 6. It is economical tostart producing hydrogen with the existing facilities that oil refineries currentlyhave, and gradually increase production through either carbon capture andsequestration or renewable-energy production. This could be accelerated eitherbased on the needs of society to reduce carbon or by new ways of hydrogenproduction from renewable sources.

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compactness/high power density stack durability high and low temperature

oper

atin

g te

mpe

ratu

re

goal

cost

next targets approaching target

× 1.5 ~ 2 × 1.5 ~ 2

105°C

30°C

1/10by design/materials

× 1/10by mass

production

2008model

goal2008model

goal2008model

goal2008modelgoal2008

model

outp

ut

dens

ity

dura

bilit

y

cost

crui

sing

ran

ge

cruising range

Figure 5. Recent progress and remaining issues.

electricity/hydrogen sustainable societyglobal warming: carbon-freeenergy security: oil-freekeywords

socialneeds

vehicles

fossil based hydrogen production

.

′30′20′10 ′50′15

commercializationmassspreaddemonstration

start spread

engineering low-cost station pipeline (large scale)pipeline (small scale)

oil-free

efficient compressor

hydrogenproducts

compact stationinfra.

technologiesCO

2 sequestrationCO2 separation

further reduction

sites increase

fossil + carbon capture and sequestration

mass sequestration

renewable hydrogensustainable

carbon-free productsdirect productssolar wind

— economical benefits— energy security

penalty for CO2 emissionincentive for CO2 reduction

socialactions

incentives for infra.

incentives for renewable

incentives for vehicles and station

consensus for the low carbon society

— environmental benefits

deregulationcode & standard preparation

CO2 –20% =50%⇒HV PHV FCV·EV increase

oil price >$100 barrel–1 further fluctuation supply shortage CO restrictions

Figure 6. Hydrogen vision. HV, hybrid vehicle; PHV, plug-in hybrid vehicle; EV, electric vehicle.

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Review. Hydrogen fuel-cell vehicles 3371

well-to-wheel *1*1

natural gas

20% 40%

FCHV-adv 67% 40%59%*2

natural gashydrogen (70 MPa)

EV 39% 33%85%39% 33%85%

electricity

refine 84% 40% 34%84% 40% 34%

gasoline

gasoline ICE

(Toyoto calculation)

84% 19%23%refine 84% 19%

gasoline

*11 tank-to-wheel efficiency: measured in the Japanese 10–15 test cycle*2 efficiency difference between 35 MPa and 70 MPa: approx. 2%

energypathway

well-to-tank50%

tank-to-wheel50%

crude oil

crude oil

gasoline HV(Prius)

membrane separation reform

gas-fired powergeneration

Figure 7. Efficiency comparison of power train. FCHV, fuel-cell hybrid vehicle; EV, electric vehicle;HV, hybrid vehicle; ICE, internal combustion engine.

well-to-well to wheel CO2 emission

CO2 emission (gasoline vehicle = 1)

diesel vehicle

gasoline vehicle

0 0.2 0.4 0.6 0.8 1.0

gasoline HV

diesel HV

EV

future

future

biomass, nuclear power coal-firedpower

FCHV (natural gas; current)FCHV (natural gas; achieved)

well-to-tank CO2

tank-to-wheel CO2

(coal)

(biomass decomposition)

(water electrolysis/renewables)

():source of hydrogen Source : Mizuho Information and Research Institute report/Toyota calculation ( excluding FCHV). FCHV : Toyota calculation hydrogen fuelled, Toyota in-house testing in the Japanese 10–15 test cycles.

Figure 8. Comparison of CO2 reduction potential. FCHV, fuel-cell hybrid vehicle; HV, hybridvehicle; EV electric vehicle.

Figures 7 and 8 show the latest comparison of well-to-wheel efficiency fromnatural gas to vehicle propulsion energy.

Owing to the latest improvement in vehicle efficiency, FCVs consume the leastamount of primary energy. This shows the advantage of FCVs, even if they usehydrogen from fossil fuels.

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5. Materials to be prepared for mass introduction

(a) Materials innovations for fuel-cell stacks

The use of expensive platinum as a catalyst is commonly stated as one of theweakest points of fuel cells. However, recent progress in the development ofplatinum alloy and structured catalysts shows the potential to reduce the amountof platinum usage towards a level similar to the early years of the current three-way catalyst. But the understanding of the mechanism of catalyst deteriorationand migration into the membrane surface has to be developed further beforecommercialization and mass usage.

In addition, catalysts other than platinum are the most required innovationwe expect from scientists. Currently, carbon alloys or metal oxide catalysts aredeveloped through laboratory research, but are not as active as platinum.

(b) Materials innovations for hydrogen storage

This has long been thought of as one of the biggest hurdles for hydrogenFCVs. It is still a difficult challenge, but the cruising range of certain FCVshas now exceeded conventional petrol vehicles with the development of 70 MPahigh-pressure carbon fibre composite tanks, in addition to the improvements infuel-cell efficiency. However, this is not a sustainable solution since the currenthigh-pressure tank uses expensive carbon fibres in large quantities, thus makingit expensive and not easy to produce in large numbers. We still need a cheap, safeand easily producible hydrogen-storage material and, yet again, we are expectingan innovative and intelligent breakthrough from the scientific community.

6. Hydrogen-storage technologies now and in the future

(a) State of the art

Figure 9 shows state-of-the-art hydrogen-storage technologies on the scale ofgravimetric density and volumetric density, together with the target. There is abig gap between the target and the current status. In addition, many technologiesare still at the laboratory level, because of which, only a few technologies, such ashigh-pressure tanks and liquid-hydrogen tank systems, have been tested in realvehicle conditions. Requirements for vehicle conditions are provided in the listin figure 10.

‘Why is hydrogen storage difficult?’ This question is related to the energyrequired to conceal hydrogen in a small area. There are several ways to concealhydrogen in small areas, but all of them fall either under physical or chemicalways. Current tank systems, both high pressure and liquid temperature, are basedon physical ways of storing hydrogen. Physical ways have a big advantage as theenergy required to extract hydrogen for use is very low. However, the tank itselfis very expensive and complicated since the system has to maintain extremepressure and temperature conditions, i.e. either 70 MPa or 20 K.

Other ways of storing hydrogen is by chemical bonding, wherein the hydrogenis stored in the lattices of host atoms. There are many challenges in thisdirection, but for chemical storage, the energy required to store hydrogen in asmall area is small, but the energy needed to extract hydrogen for use (DH )

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hydrogen: 5 kg

35 MPa

compressed cryo-adsorption

70 MPa chemical hydride

sciencecarbon5wt%

area8wt% unstabilizationMOF

syst

em v

olum

e

LH2

metal-dopedMOF metal hydride

BCC

Laves

Mg(AlH4)2

BN series

12wt% 2.0 mass%AlH32.5 mass%

ICE

4 mass%

system weight

finaltargetconventional

Figure 9. Current hydrogen storage. MOF, metal organic framework; ICE, internal combustionengine; BCC, body-centred cubic.

— safety1. non-hazardous, non-toxicity

1. durability— reliability

— performance

— cost

1. high volumetric gravimetric density, charge discharge2. high efficiency

1. low system cost, low maintenance cost— infrastructure

— scalability1. installable for small and large vehicles

1. well-to-tank, hydrogen production, distribution/delivery

Figure 10. List of requirements.

is relatively large because it involves breaking the chemical bond in orderto release hydrogen. Efforts to find a material with a low DH are ongoing(figure 11).

Liquid hydrogen is another good candidate as it has characteristically goodgravimetric and volumetric density. In fact, in the industry, the gas deliverysystem commonly uses liquid hydrogen. However, energy required for liquefyinghydrogen is much larger than in theory and is normally not recoverable.Furthermore, flash gas during the refuelling and boil off gas while the systemis idling and not consuming gas does not make this technology very attractive.

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3374 K. Hirose

basic sciencehydrogen storage

physical: small heat exchange, simple but container difficult

chemical: large heat exchange, complex regeneration system

physical containment limitation since hydrogen no longer behaves as ideal gas

free space (100 kPa, 300K) molecular energy 3.6 kJ mol–1pressure to close the molecular distance

room temperature35 MPa 70 MPa

energy necessary to keepthe distance closer

high-pressuretank

surface potentialadsorption surface potential to eliminate the force between H2assumption higher adsorption term, higher physisorption

physisorptioncryo-technology

adsorption

molecular to atomsurface potential to stick H2

chemisorption

3–5 kJ mol–1latent heat

latent heat

density 0.07 lowtemperature

LH2

20°C (–253°C)chemical containment

highpressure

30–100kJ mol–1

Figure 11. Schematic picture of hydrogen storage.

–200 –100 0 100 200 3000.01

0.1

1

10

100

ambient

roomtemperature

pres

sure

(M

Pa)

temperature (°C)

70 MPa compressed H2

H2 absorbing alloy and compressedhydrogen gas (hybrid tank)

chemical hydridepossible usable area

ideal areaLH2

cryogenic adsorption

Figure 12. Directions of developments.

(b) New development trends

Current development trends are shown in figure 12. The emphasis is on loweringthe pressure for metal hydride tank systems and lowering the temperature forchemical hydride tank systems.

A new approach is extending the temperature range above the liquid hydrogentemperature and the room temperature.

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RT35 MPa5%100 kg280 l

RT3.5 MPa9.2%46 kg2800 l

RT70 MPa5%126 kg + a140 l + a

LT35 MPa10% adsorber50 + 71 kg166 l

adsorber50 kg10% 5%

bulk density 0.3l

adsorber50 kg10%

density 1.0

LT35 MPa10% adsorber50 + 32 kg50 l

LT3.5 MPa10% adsorber50 + 3 kg50 l

estimated adsorber tank system

lowerpressure

bulk densityup

lower pressure

higher pressure

adsorber weight reduction

LT35 MPa5% adsorb100 + 71 kg332 l

adsorber100 kg

bulk density 0.3l

effective

adsorber performance

bulk density up

lower pressure

Figure 13. Influence of physical property. LT, low temperature (or cryo temperature range around77 K); RT, room temperature.

Hydrogen absorbing alloy (metal hydride) and its improved hybrid tanksystem are also very attractive, but material cost and high-performance materialdevelopment stall implementation (Darkrim et al. 1999; Lamari et al. 2000;Bénard & Chahine 2001; Gardiner et al. 2004; Kojima et al. 2005, 2006; Moriet al. 2005a,b,c; Bhatia & Myers 2006; Kabbour et al. 2006; Poirier et al. 2006;Furukawa et al. 2007; Liu et al. 2007; Vasiliev et al. 2007; Richard et al. 2009a,b).

7. Important message for material scientists

Tank weight and volume is a function of pressure, temperature, materialperformance and bulk density. Most of the hydrogen-storage material discussionis on the gravimetric and volumetric index. However, influences of the materialcharacteristics other than these performances are ignored easily.

Figure 13 shows the influence of the physical property of the material into thetank system. This shows the effect of the different parameters on the hydrogenadsorption tank system size and weight.

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Pressure and bulk density, rather than the adsorption performance, are moreinfluential factors for tank size and weight. An example is shown for the case ofthe adsorption system. There are other characteristics, and these are differentfrom one system to another. It is important for both scientists and engineersto work together from the early stage of the material development to meet therequired material target (Mori & Hirose 2009; Richard et al. 2009c).

8. Conclusion

In order to tackle the current problems, it is very urgent to bring new technologiessuch as the fuel cell into the real world as quickly and as much as possible.Engineers and scientists need to communicate with each other more often anddeeply than ever to accelerate developing innovative materials in order to bringthose technologies into real life.

— Future mobility must meet to improve energy efficiency, QOM and reduceemissions and global warming gas, thus giving a good QOM.

— FCVs have very high energy efficiency and retain good usability, such asrange and short charging time, so that they reduce global warming gasemissions and oil usage.

— FCVs are now in the process of commercialization. Cost reduction is thekey for this technology so that the material innovations (fuel stack andhydrogen storage) are essential for the mass introduction of this newtechnology.

— For the hydrogen-storage-material development, physical properties, suchas bulk density, are very important for the size and weight of the tanksystem.

— Scientists and engineers must work closely to accelerate materialdevelopment and bring new materials into commercialization, thus beingeffective for climate change.

References

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