Importance of Turning to Renewable Energy Resources with Hydrogen
as a Promising Candidate and on-board Storage a Critical Barrier
A.C. Dillon*, B. P. Nelson, Y. Zhao, Y-H. Kim, C. E. Tracy and S. B. Zhang
National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401
ABSTRACT
The majority of the world energy consumption is derived from fossil fuels. Furthermore,
the United States (US) consumption of petroleum vastly exceeds its production, with the
majority of petroleum being consumed in the transportation sector. The increasing
dependency on foreign fuel resources in conjunction with the severe environmental
impacts of a petroleum-based society dictates that alternative renewable energy resources
be developed. The US Department of Energy’s (DOE’s) Office of Energy Efficiency and
Renewable Energy and the Office of Basic Energy Sciences are currently promoting a
vehicular hydrogen-based energy economy. However, none of the current on-board
storage technologies are suitable for practical and safe deployment. Significant
scientific advancement is therefore still required if a viable on-board storage technology
is to be developed. A detailed discussion of the benefits of transitioning to a hydrogen-
powered automotive fleet as well as the tremendous technical hurdles faced for the
development of an on-board hydrogen storage system are provided here. A novel class
of theoretically predicted nanostructured materials that could revolutionize hydrogen
storage materials is also presented.
INTRODUCTION
Currently over 80% of the world’s energy consumption relies on fossil fuel
resources. Furthermore, the United States now relies of foreign sources for the majority
of the petroleum it consumes. Also, approximately one-half of the total world oil
supply has already been consumed. It is therefore imperative for several reasons that
renewable energy resources be developed. First, the environmental impacts of
employing petroleum as a primary energy resource cannot be sustained. Also the
increased dependence on foreign resources greatly threatens national security. Finally,
if the current petroleum consumption rate is maintained all of the world oil supplies will
be completely depleted. Thus the growing number of densely populated metropolitan
cities with poor local air quality, and the increasing uncertainty associated with access
to foreign fuel sources have spurred a Presidential initiative to implement hydrogen as
an energy carrier for transportation needs. Hydrogen can be generated by a variety of
means including the electrolysis of water using electricity derived from wind power,
photovoltaics (PV) or by thermo-chemical processing of biomass. Hydrogen can then
be reacted with oxygen in fuel cells to generate electricity, combusted in an engine to
generate mechanical energy, or simply burned to generate heat. In each of these cases,
0895-G05-03.1Mater. Res. Soc. Symp. Proc. Vol. 895 © 2006 Materials Research Society
water is produced in a virtually pollution-free process. Thus, a hydrogen-based energy
economy could supply a closed pollution-free cycle that relies entirely on renewable
resources.
Unfortunately, significant scientific advancement is still required for the long-
term use of hydrogen as an energy carrier, particularly in the transportation sector
Furthermore, the Department of Energy (DOE) has concluded that hydrogen on-board
storage is the most challenging aspect for a successful transition to a hydrogen
economy. Specifically, DOE has determined that a hydrogen storage density of ~9 wt.%
and ~ 45 kg/m
3
will be required for fuel cell powered vehicles to supplant petroleum-
fueled vehicles. None of the potential technologies satisfy DOE’s technical targets that
would allow for the safe and convenient deployment of hydrogen in America’s
automotive fleet. Hydrogen (H
2
) is a non-polarizable molecule, and is therefore a gas
at room temperature and ambient pressure. In order to achieve even a moderate driving
range of ~ 150 miles / fill-up, the H
2
would have to be pressurized to extremely high
pressures (~10,000 p.s.i. or 680 atms.), and the container must be reasonably shaped
such that too much space is not compromised. Additionally, an adsorption process for
on-board vehicular storage will require a hydrogen binding energy between ~10-40
kJ/mol to allow for near-room temperature operation at reasonable pressures. A
moderate binding energy is also crucial for managing the heat load during refueling.
Recently, carbon-based materials have been studied as potential adsorbents for
hydrogen that could be employed for on-board vehicular storage
1
. Typically, non-
dissociative physisorption on a carbon surface such as graphite involves a binding
energy of only ~ 4 kJ/mol due purely to van der Waals forces. A chemical bond is of
course much stronger e.g. ~ 400 kJ/mol in methane. The desired binding energy range
for vehicular hydrogen storage therefore dictates that molecular H
2
be stabilized in an
unusual manner. Thus it seems likely that it is necessary to develop a new class of
compounds that will store H
2
in sufficient quantities to allow for a reasonable range
between refueling. Recent theoretical studies predict that fullerene-based organometallic
complexes, where dihydrogen ligands are stabilized with moderate binding energies,
could be the solution to vehicular hydrogen storage
2
. The tremendous need for a
transition to renewable energy resources, such as hydrogen, and the prediction of this
novel class of materials for on-board H
2
storage is presented here.
DISCUSSION
Necessity of Turning to Renewable Energy
In 2005 the entire world energy consumption was approximately 13 terra-watts
(TW), the equivalent of 87 billion barrels of oil. As stated previously, over 80% of the
energy utilized was derived from fossil fuel resources. Figure 1 displays a pie chart
breakdown of the 2005 world energy consumption. Note that 7% of the world energy
use was derived from nuclear power. Nuclear power is desirable as long as the isotope
uranium, 235 (U
235
) is employed
3
. However, a less than twenty-year supply of U
235
that
may be recovered at a reasonable cost is available. It will then be necessary to employ
breeder reactors. Unfortunately the byproduct of this reaction is the very hazardous
element, plutonium, 239 (U
238
-> P
239
)
3
. This generation of large quantities of weapons
0895-G05-03.2
grade waste thus presupposes moral leadership for hundreds of years. Figure 2 displays
that since the industrial age, war and genocide have been the predominant cause of
events with deaths numbering ~100,000. The total number of deaths represented by this
figure is 332 million. Although 10% of the world energy consumption is derived form
biomass, this is not nearly representative of what could be accomplished
4
. Finally, 2%
of the energy use relies on hydroelectric sources and a mere 1% of the energy consumed
is obtained from alternate renewable resources. A blow up of this tiny 1% fraction
shows that solar energy in the form of photovoltaics as well as wind power are
drastically underemployed. These, of course, represent two renewable resources of
which there is an essentially infinite supply.
It is thus apparent that the world energy consumption relies heavily on
petroleum. Also this dependency on fossil fuels has multiple catastrophic implications.
The United States of America (USA) represents only 4.6% of the world's population yet
annually consumes ~25% of the world's energy resources and is therefore the largest
single energy consumer
5
. In addition, USA production of petroleum peaked in
approximately 1970. A roughly fifteen-year plateau and a significant decline in
production then followed. However, USA petroleum consumption has been increasing
dramatically since the onset of the industrial age. In order to maintain this incongruity,
foreign fuel sources, primarily from the Middle East, have been employed. These trends
are very explicitly demonstrated in Fig. 3. Unfortunately the societal impacts of
increasing petroleum consumption are often disregarded. The production of green house
gases and the tremendous cost of environmental pollution including the clean up of oil
spills is seldom considered. Furthermore, military protection of foreign oil sources is
not only expensive but may lead to life-loss in combat
6
.
It seems tremendously obvious that both for environmental and energy security
reasons the necessity of turning to renewable energy resources is imperative.
Unfortunately, the long-term implications of remaining primarily dependent on fossil
fuel sources for energy needs may still not be enough to spur significant research thrusts
focused on renewable energy paths. Figure 4 displays the total number of giga-barrels
Figure 1: Breakdown of the 2005 world energy consumption with the total energy consumed
being ~ 13 TW and roughly the equivalent of 87 billion barrels of oil.
0895-G05-03.3
(GB) of oil that has been discovered since 1930, as well as, the barrels of oil that are
projected to be discovered up to the year 2050
7
. Note that although there are now very
sophisticated technologies, it is still only possible to discover very small pockets of
remaining oil reserves. Overlaid on this plot is the amount of oil that has already been
recovered
7
. This comparison of the current rate of recovery versus the available
resources indicates that if efforts are not taken to become exceedingly less dependent on
petroleum as an energy carrier, the petroleum reserves will be depleted completely by
the turn of the century.
Figure 2: Pie-chart breakdown of events with deaths numbering ~100,000. The total number of
deaths represented is 332 million.
Figure 3: Barrels of oil produced in the USA and the world form 1900-2005 along with USA
consumption. Extrapolations of USA production and consumption are also provided out to 2040.
Note the ever-increasing dependency on foreign sources.
0895-G05-03.4
Hydrogen, a Promising Alternative: On-board Vehicular Storage is Major Hurdle
A USA Presidential initiative to implement hydrogen as the primary energy
carrier by 2015 has been decreed. An intensive planning and evaluation process carried
out by the DOE’s Office of Basic Sciences and the Office of Energy Efficiency and
Renewable Energy has resulted in two
major documents describing the challenges
and needs associated with implementing a
hydrogen energy economy
8
. Both studies
conclude that vehicular hydrogen storage
is the cornerstone technology for the
deployment of hydrogen-powered
vehicles. However, the scientific
underpinning related to hydrogen storage
requires significant advancement if a
viable on-board storage technology is to be
developed. None of the current vehicular
storage methods meet both the DOE
volumetric and gravimetric targets for
vehicular storage. The DOE 2015 targets
are 0.09 kg H
2
/ kg (gravimetric) and 45 kg
H
2
/m
3
(volumetric) with both of these goals including the weight and volume of the
Binding Type
Figure 5. Schematic of the continuum of
hydrogen binding energies for different
Figure 4: Giga-barrels of oil discovered from 1930 to present as well as a projection of the oil
barrels to be discovered until 1950. The total world consum
p
tion of GB from 1930 to the
p
resent is
also displayed.
0895-G05-03.5
container. A refueling weight of 2 kg H
2
/ min and a cost equivalent to $ 1.50 / gallon of
gasoline are also required
8
. Finally, an ideal binding energy that would allow for
reversible hydrogen storage at near ambient conditions is ~10-40 kJ/mol. This binding
energy is stronger than that expected for physisorption but significantly weaker than
chemisorption. Figure 5 is a cartoon of a continuum of binding energies with the
window where on-board hydrogen storage would be reversible at ambient conditions.
Note metal hydrides, that have been studied extensively for H
2
storage, do not fall
within this reversible window
9
. The unique binding energy range likely dictates that
molecular H
2
be stabilized via enhanced physisorption or by complexing dihydrogen
with a transition metal atom. The later employs molecular H
2
as a ligand in
organometallic complexes and is often labeled a “Kubas” interaction
10
.
Recent theoretical calculations have shown that by complexing fullerenes with a
transition metal, H
2
molecules may be bound with binding energies between ~30-45
kJ/mol. The spin-polarized calculations were carried out using density-functional
formalism within the Generalized Gradient Approximation. In an optimal structure,
scandium has been predicted to complex with a fullerene. Here the scandium shares
charge with all of the carbon atoms in the pentagon through a Dewar coordination. The
C
60
[ScH
2
(H
2
)
4
]
12
complex, shown in Fig. 6, has a theoretical total hydrogen capacity of
8.77 wt.%. However, the theoretical investigations showed that the first hydrogen to
complex with a given scandium atom dissociates forming two monhydride species with
a binding energy of ~ 100 kJ/mol. The next four H
2
molecules are stabilized as
dihydrogen ligands with the more
moderate binding energy of ~30 kJ/mol.
A compound with a theoretical reversible
hydrogen capacity of 7.0 wt% and a stable
“18-electron complex” then results.
A boron-doped fullerene, C
48
B
12
,
has also been examined. Here the weight
of the fullerene is reduced, and it has been
predicted that more dihydrogen ligands
bind the Sc atoms again complexed to
five-membered rings. The B dopants create holes in the electronic structure and pull
more charge from the Sc transition metal atom. This charge transfer leaves each Sc in
the hydrogen-bare C
48
B
12
Sc
12
with only one valence electron. Then, each Sc in
C
48
B
12
Sc
12
binds only a single monohydride to form C
48
B
12
[ScH]
12
. More importantly,
it is now possible to bind five dihydrogen ligands to form C
48
B
12
[ScH(H
2
)
5
]
12
having a
reversible hydrogen capacity of 8.77 wt%. The stable monohydride, C
48
B
12
[ScH]
12
,
species (discharged) and the (charged) complex, C
48
B
12
[ScH(H
2
)
5
]
12
, are shown in
Figures 7 a) and b), respectively. Note that the Sc-fullerene complexes in their
discharged states have outer metallic surfaces that have a partial positive charge causing
a repulsive van der Walls interaction and reducing the likelihood that the molecules will
Figure 6: Three-dimensional Scandium (pink) C
60
(green) complex with 8.7 wt.% total H
2
(white)
capacity and 7.0 wt% reversible hydrogen
storage.
0895-G05-03.6
dimerize. Also, Assuming a 3Å van der Walls distance between the metal-coated
fullerenes, the volumetric storage capacity was calculated to be 34.2 kg/m
3
. The is
approximately 25% less than the DOE volumetric targets implying that lightweight
materials with more efficient packing densities must still be discovered. Alternatively,
passenger/storage space will be sacrificed, or it will be necessary to refuel more
frequently.
The chemistry of C
60
is generally olefinic (i.e. the metal is coordinated to the
fullerene through 2 carbon atoms contributing 2 electrons to the bonding). Thus, the
above synthesis of fullerene-metal-H
2
complexes, where the metal is coordinating with
five carbon atoms, is not expected to be easy. Rational synthetic methods to
experimentally demonstrate five-fold coordination between a fullerene and a transition
metal are currently being explored. Also, investigations probing the substitution of more
strongly bound ligands by weakly bound dihydrogen ligands are underway.
CONCLUSIONS
The imperativeness of turning to renewable energy resources for the replacement
of fossil fuels has been clearly demonstrated. Environmental impacts together with
energy security issues due to increased reliance on foreign petroleum have spurred a
Presidential initiative to implement hydrogen as an energy carrier by 2015. Hydrogen
can be generated by a variety of renewable energy sources. It can then be reacted with
oxygen in fuel cells to generate electricity or simply burned to generate heat, where
water is produced in a virtually pollution-free process. However, none of the current on-
board storage technologies are suitable for practical and safe deployment. Significant
scientific advancement is therefore still required if a viable on-board storage technology
is to be developed. The development of a novel class of fullerene-based organometallic
molecules could possibly provide the solution to vehicular hydrogen storage. Rational
Figure 7: Stable boron- (blue) doped fullerene (green), scandium (pink) complexes showing
a) the discharged molecule with a monohydride (white) species on each Sc atom and b) the
fully charged molecule with five dihydrogen ligands (white) on each scandium atom having a
reversible hydrogen capacity of 8.77 wt%.
a) b)
0895-G05-03.7
synthetic routes towards the bulk synthesis of these new compounds is currently the
focus of on going research.
ACKNOWLEDGMENTS
This work was funded by the U.S. Department of Energy Office of Energy
Efficiency and Renewable Energy under subcontract DE-AC36-99GO10337 to NREL.
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http://www.eia.doe.gov/fueloverview.html.
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C. Sagan, Billions & Billions. (Ballantine Publishing Group, New York,
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G. J. Kubas, Acc. Chem. Res. 21, 120 (1988).
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