2009 Fuels Hydrogen Storage Carbon Ma

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Carbon Materials

M Hirscher, Max-Planck-Institut fur Metallforschung, Stuttgart, Germany

& 2009 Elsevier B.V. All rights reserved.

Introduction

The major bottleneck for commercializing fuel cell ve-

hicles is onboard hydrogen storage. The presently avail-

able systems are high-pressure tanks or liquefied

hydrogen in cryogenic vessels, which both possess severe

disadvantages. Storage in lightweight solids could be the

solution to this problem, because an onboard storage

system should possess a high gravimetric storage capacity

(US Department of Energy (DoE) system target of 6 and

9 wt% for 2010 and 2015, respectively). The gravimetric

storage capacity of the system is defined as mass of

hydrogen divided by the sum of the masses of storage

system and hydrogen given in percent, whereas forstorage materials often the wt% are based on the material

weight instead of the whole system weight. There are two

principal mechanisms: (1) adsorption of hydrogen mol-

ecules on surfaces, i.e., physisorption and (2) hydrogen

atoms dissolved or forming chemical bonds, i.e., chemi-

sorption. This article focuses on the interaction of

hydrogen with carbon materials. Carbon is one of the

very few light materials that are solid at room tempera-

ture. Furthermore, different carbon structures can be

produced with large inner surfaces, e.g., activated carbon,

fullerenes, carbon nanotubes, and carbon nanofibers.

Typically, at room temperature carbon only interacts

very weakly with hydrogen through van der Waals forces.

Only at elevated temperatures or strong mechanical

treatment, e.g., high-energy ball milling, are CH bonds

formed. These bonds are strong and hydrogen is only

released at temperatures far above 600 K, which is too

high for automotive applications. Activated carbon

structures with high surface areas of up to 3000 m2 g1

are commercially available and show, at liquid nitrogen

temperature, a sufficient hydrogen storage capacity by

physisorption of hydrogen molecules. The newly avail-

able nanostructured carbon materials (Figure 1), espe-

cially single-walled carbon nanotubes (SWNTs), have

given rise to speculation that capillary forces could en-

hance gas or hydrogen adsorption as well as drawing up

liquids by capillarity. The adsorption would be a con-

sequence of the attractive potential of the pore walls due

to the strong curvature. Unfortunately, however, the ef-

fect of a so-called nanocapillarity could be neither ex-

perimentally nor theoretically substantiated.This article addresses, first, the false disclosure of

spectacular results for novel carbon nanostructures and,

second, the capability of carbons possessing a high specific

surface area for hydrogen storage by cryo-adsorption.

Spurious Measurements of HydrogenStorage in Novel Carbon Nanostructures

The first publication concerning the hydrogen storage

capability of carbon nanotubes caused enormous interest.

This work claimed that hydrogen can condense to a high

density inside narrow SWNTs and estimated a gravi-

metric storage capacity of 510 wt%, which would be

highly attractive for automotive applications. The total

amount of hydrogen desorbed from the total sample with

only a small concentration of SWNTs and determined

with thermal desorption spectroscopy (TDS) was about

0.01 wt% considering the whole weight of the sample.

The reported large storage values were estimated after

attributing all desorbed hydrogen to the small weight

fraction of SWNTs present in the sample. Three years

later, the same authors affirmed that they had directly

measured a hydrogen storage capacity of about 7 wt% by

purifying the samples. This time, hydrogen was releasedat a significantly higher temperature than that required in

the first study. Furthermore, the SWNTs, which are

typically closed at the ends by fullerene-like caps, had

been opened. For opening the SWNTs the carbon

nanotubes were dispersed in HNO3and treated by high-

power ultrasonication using an ultrasonic horn, typically

made from a titanium alloy. After heating in high vac-

uum, the opened SWNTs could adsorb this high amount

of hydrogen within a few minutes at ambient pressure

and room temperature. In 2001, another laboratory found

5200 nm 3050 nm ~1 nm

Figure 1 Carbon nanostructures assembled from different

arrangements of graphene sheets. Left to right: graphitic

nanofibers, multiwalled carbon nanotubes, bundle of single-

walled carbon nanotubes.

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these results to be erroneous. They carefully investigated

the hydrogen uptake in various carbon materials after

high-power ultrasonication. The characteristic shape of

the TDS spectrum was more or less similar to the prior

data, but the storage capacity of the sonicated material

was reduced by a factor of three. Further investigations

revealed that owing to cavitation, the high-power ultra-

sonication transferred metal particles from the horn intothe samples. By using the titanium-alloy horn for soni-

cation, hydrogen desorption could be observed in all the

investigated carbon materials, e.g., purified SWNTs,

graphite, and even diamond powder. Thus, all hydrogen

storage of the sonicated samples could be attributed to

hydrogen storage in the titanium-alloy particles. Indeed,

the formation of titanium hydride was confirmed by X-

ray analysis. By contrast, no hydrogen storage was found

(i.e., below 0.005 wt%) when the SWNTs were sonicated

with a stainless-steel horn.

Very spectacular hydrogen storage capacities of up to

67 wt% were claimed for a certain type of carbon nanofiberin 1998. High hydrogen adsorption was measured for dif-

ferent carbon materials by applying a hydrogen pressure of

about 11 MPa at room temperature and monitoring for

24 h. The highest value of nearly 67 wt% was obtained for

herringbone-type carbon nanofibers, but even for graphite

itself a very high storage capacity of 4.5 wt% was reported,

measured at room temperature. To date, however, the ex-

tremely high values have not been confirmed in any other

laboratory. It should be noted that also in 1998, another

group found a capacity less than 0.2 wt% for comparable

carbon nanofibers examined by the same method under

similar experimental conditions.

In 1999, yet another group published promising data

obtained through a volumetric method, which measures

pressure changes at a constant volume that are due to

hydrogen adsorption or desorption of the sample. It was

claimed that multiwalled carbon nanotubes (MWNTs)

gave a hydrogen uptake of 1013 wt% at room tem-

perature and at a pressure of 11 MPa. The nanotubes had

to be boiled in hydrochloric acid. In a further publication,

however, the same group reduced the storage capacity of

the MWNTs by a factor of two. In the same year, other

workers used thermo-gravimetric analysis (TGA) to

measure the hydrogen adsorption of carbon nanofibers

and activated carbon at high pressure. They observed amaximum weight increase at 12.5 MPa and room tem-

perature that corresponded to a hydrogen uptake of

1.6 wt% in activated carbon and of 1.2wt% in fibrous

material. Furthermore, they found a linear correlation

between the specific surface area of the materials and the

maximum hydrogen uptake. Again in 1999, a group

measured SWNTs with large diameters treated with

hydrochloric acid and obtained a storage capacity of

4.2 wt% at room temperature. The purity of the samples

was about 5060%. High hydrogen storage capacities of

10 wt%, and later 15 wt% were claimed from another

laboratory for herringbone-type carbon nanofibers in

2001. But none of these high values has survived cross-

checking by independent research groups repeating the

measurements on the same sample in different labora-

tories. One US laboratory carefully examined different

carbon materials at high pressures but found no hydrogen

sorption appreciably above background at room tem-perature. Very low storage capacities, i.e., below 1 wt%,

have been independently observed at room temperature

and 8 MPa by several European laboratories. Latest re-

sults show values of storage capacity below 0.5 wt% at

room temperature for a large variety of carbon nanos-

tructures with specific surface areas up to 2500 m2 g1. In

essence, the measured isotherms are in agreement with

theoretical calculations based on the physisorption of

hydrogen molecules.

In 1999, a group from Asia announced storage capacities

of up to 20 wt% for lithium-treated MWNTs and even up

to 14 wt% for lithium-treated graphite. They tried tointercalate different carbon materials by mixing with alkali

salts and performing a so-called solid-state reaction by a

heat treatment under hydrogen flow. Subsequent gravi-

metric measurements performed under hydrogen flow

yielded dramatic and reversible weight changes of the al-

kali-treated carbon samples that were attributed to

hydrogen adsorption and release. The measurements were

revisited by two US laboratories and it was discovered that

the majority of the weight changes were due to the ad-

sorption and release of water. The gravimetric experiment

on lithium-treated graphite was reproduced under hydro-

gen flow in a European laboratory and yielded weight

changes very similar to those obtained in the first publi-

cation. The characterization of the intercalation product by

X-ray diffraction did not, however, provide any evidence of

intercalated graphite. The so-called solid-state reaction

yields a mixture of graphite, lithium oxide, and lithium

hydroxide. The lithium oxide is highly hygroscopic, and

the release and reabsorption of water of crystallization

leads to the observed weight changes.

Specific Surface Area andCryo-Adsorption Storage

The correlation between the specific surface of the carbon

materials and their ability to adsorb hydrogen was care-

fully examined in 2001. Applying the volumetric method,

different adsorbents were investigated for hydrogen at

77 K and atmospheric pressure. An approximately linear

correlation was found between the specific surface area

determined by BrunauerEmmetTeller (BET) model

and the hydrogen adsorption. The highest storage capacity

of about 2 wt% was obtained for activated carbon with a

surface area of about 2000m2 g1, whereas carbon

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nanofibers possessing lower surface areas showed the

storage capacity less than 1 wt%. Another group deter-

mined the hydrogen storage capacity for different carbon

nanostructures by applying an electrochemical method

that measures the electric charge of the carbon electrode

against a counterelectrode in a galvanostatic setup at room

temperature. They included these values in one plot

together with the low-temperature adsorption data. Allexperimental data may be described by a storage capacity

of 1.5 wt% per 1000 m2 g1 specific surface area. Slightly

higher values were obtained in later investigations of dif-

ferent nanostructured carbon materials at higher pressures

of 5 MPa and at 77 K. A Canadian group has reported the

hydrogen storage capacities of different types of activated

carbon at cryogenic temperature. The authors compare

regular-grade activated carbon with AX-21 activated car-

bon, which is produced by reaction with potassium hy-

droxide and has cage-like porosity and a very high surface

area of the order of 3000 m2 g1. AX-21 has a hydrogen

storage capacity of about 5 wt% at 77 K and 2 MPa, i.e.,twice as much as regular-grade activated carbon with

surface area ranging between 700 and 1800 m2 g1. These

data are in very good agreement with the results obtained

by volumetric measurements on a similar type of activated

carbon with a specific surface area of 2500 m2 g1. In this

recent study, the saturation values have been systematic-

ally investigated for hydrogen adsorption at 77 K on vari-

ous carbon nanostructures from different suppliers

worldwide. The results confirm the linear correlation be-

tween the specific surface area and the hydrogen storage

capacity independent of the type of carbon material

(Figure 2) with a slope of 1.9wt% per 1000m2 g1 of

specific surface area. This clearly indicates that the

hydrogen adsorption process is based on the local inter-

action between the hydrogen molecule and the surface; it

is independent of the long-range order, curvature, or

ordered arrays of graphene sheets. Therefore, the pre-

requisite to reach high gravimetric storage capacity for a

cryo-adsorption device is a porous material possessing a

high specific surface area.

The remaining problem is the volumetric storage

capacity that is limited physically by the density of liquid

hydrogen. Typically, the activated carbon materials with a

very high specific surface area are fine fluffy powders

showing a very low packing density. Recently, thisvolumetric problem was overcome by preparing activated

carbon monoliths (Figure 3), which show a volumetric

storage density of 29.7 kg H2 m3 at 77K and 4MPa.

Furthermore, if the compressed gas within the cavities of

the activated carbon monolith is included, a total volu-

metric storage density of 39.3 kg H2 m3 will be reached.

These results together with the fast kinetics and the total

reversibility, i.e., high cyclic stability, are very promising

for future applications of activated carbon as cryo-ad-

sorption storage devices. If an operating pressure of less

than 2 MPa can be achieved, it will be possible to use

conformable tank systems in contrast to high-pressure

cylinders.

BET specific surface area (m2g1)

Hydrogenstorage(wt%)

77 K

RT

5000

0

1

2

3

4

5

1000 1500 2000 2500 3000

Figure 2 Relationship between maximum hydrogen storage

capacity of different carbon nanostructures and their BET specific

surface area (determined by the BrunauerEmmetTeller (BET)

model) at 77 K and room temperature (RT). Reproduced from

Hirscher M and Panella B (2005) Nanostructures with high

surface area for hydrogen storage. Journal of Alloys and

Compound404406: 399401.

Figure 3 Examples of activated carbon monoliths with a high

micropore volume and a diameter of 4.5 and 1.6 cm, respectively.

Reproduced from Jorda-Beneyto M, Lozano-CastelloD, Suarez-

Garca F, Cazorla-Amoros D, and Linares-Solano A (2008)

Advanced activated carbon monoliths and activated carbons for

hydrogen storage. Microporous and Mesoporous Materials112:

235242.

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Concluding Remarks

The chemical bond between carbon and hydrogen is too

strong to be used for reversible hydrogen storage. On the

contrary, the interaction between hydrogen molecules

and the carbon surface through van der Waals forces is

typically weak. Unfortunately, however, the hopes placed

in novel carbon nanostructures proved falsely optimisticas none of the spectacular results that claimed high

storage capacities could be independently validated.

At low temperatures, carbon materials with high

specific surface areas show gravimetric hydrogen storage

capacities of around 5 wt%. Recently, the volumetric

storage capacity of these activated carbons has been im-

proved by preparing monoliths. Owing to the very fast

kinetics and total reversibility of the cryo-adsorption

process, very short refueling times and long cycle life-

times can be realized. Even carbon materials cannot meet

the gravimetric storage capacity and the operating tem-

perature for onboard hydrogen storage set by the DoE for2010; these materials are very promising for transporta-

tion applications because they meet the targets for ma-

terial cost, refueling rate, cycle lifetime, and can almost

reach the volumetric storage capacity.

Nomenclature

Abbreviations

BET Brunauer-Emmet-Teller

DoE US Department of Energy

MWNT multiwalled carbon nanotube

SWNT single-walled carbon nanotubeTDS thermal desorption spectroscopy

TGA thermo-gravimetric analysis

See also: Fuels Hydrogen Storage: MetalOrganic

Frameworks; Zeolites.

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