Microporous and Mesoporous Materials 129 (2010) 335–339
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso
Metal-organic frameworks for hydrogen storage
Michael Hirscher a,*, Barbara Panella b, Barbara Schmitz a
a
b
Max-Planck-Institut für Metallforschung, Heisenbergstr 3, 70569 Stuttgart, Germany
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, Hönggerberg, Wolfgang-Pauli-Str. 10, 8093 Zürich, Switzerland
a r t i c l e
i n f o
Article history:
Received 4 November 2008
Accepted 3 June 2009
Available online 11 June 2009
Keywords:
Hydrogen storage
Metal-organic frameworks
Porous materials
Physisorption
Coordination polymers
a b s t r a c t
Physisorption of hydrogen in porous materials at cryogenic conditions is a viable mechanism for hydrogen storage in mobile applications. This storage mechanism has the advantage of possessing fast kinetics,
low heat of adsorption and being completely reversible. Among all porous materials, metal-organic
frameworks (MOFs) are the best candidates for H2-adsorption, since they consist of light atoms, are
highly porous and their pore dimensions can be tailored by chemical engineering. Additionally, MOFs
show the highest storage capacity of any other porous material. Different properties of the material, like
specific surface area, composition and pore size can influence the storage capacity. Therefore, an understanding about the correlation between adsorption properties and structure of MOFs is necessary to specifically improve these materials for hydrogen storage. Our main achievements in the investigation of H2
storage in MOFs are discussed and compared to results reported in literature.
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
Global warming and limited oil reserves will require a change
from fossil fuels to cleaner and renewable energy sources in the
near future. Presently, for mobile applications two main technologies are considered as energy storage: batteries for electrical vehicles and hydrogen for internal combustion engines or fuel cells.
Batteries are considered as very promising since they do not require the additional production of a synthetic fuel. However, besides the long recharging time and the low cycling stability both
the heavy weight and the large volume occupied by these batteries
are large drawbacks [1]. Another possibility could be the use of
hydrogen as energy carrier produced from clean energy sources.
But the lack of an efficient storage system for hydrogen is the major bottleneck for the development of fuel cell driven vehicles. The
commercially available hydrogen storage technologies for mobile
applications are presently compressed gas in tanks at pressures
up to 70 MPa and liquid hydrogen in cryogenic vessels at temperatures of approximately 20 K. These technologies have severe disadvantages and limitations. On the one hand, compressed
hydrogen occupies large volumes, possesses low gravimetric densities and the high pressures find low consumer acceptance. On
the other hand, the liquefaction process of hydrogen which occurs
at very low temperature, has the drawback of consuming 30–40%
of the energy content of H2 and the storage at 20 K additionally implies a heavy insulation system to limit the boil-off of the liquid.
Therefore, alternative possibilities for hydrogen storage have to
* Corresponding author. Fax: +49 711 689 1952.
E-mail address: [email protected] (M. Hirscher).
1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2009.06.005
be considered. One strategy to increase the stored hydrogen density is to exploit the interaction between H2 and a solid material
as e.g. by physisorption (i.e. molecular adsorption) of hydrogen
on porous materials. The mechanism of physisorption possesses
several advantages like being completely reversible and possessing
very fast kinetics of adsorption and desorption. Additionally, due to
the small enthalpy of adsorption for molecular H2 storage, only a
small amount of heat is produced during onboard refuelling which
is a great advantage compared to chemical hydrides. Among all
porous materials metal-organic frameworks (MOFs) show very
large storage capacities due to their extremely high internal porosity [2–5]. This paper summarises the major achievements in the
fundamental understanding of hydrogen adsorption in metal-organic frameworks.
2. Storage capacity and specific surface area
Fig. 1 shows as a typical example for metal-organic frameworks
the hydrogen adsorption isotherm of MOF-5 at different temperatures. At 77 K the isotherm shows a typical type I isotherm according to the IUPAC classification with an initial steep increase in the
hydrogen uptake, followed by the saturation at higher pressures.
At 2 MPa saturation is not reached and a slightly lower hydrogen
uptake (4.5 wt%) than the maximum storage capacity is obtained
[6,7]. These results were independently confirmed by three other
groups [3,8,9] all showing a similar maximum hydrogen uptake
of 5 wt% for MOF-5 at 77 K. Owing to the small heat of adsorption
of H2 on porous materials, at higher temperatures both the initial
slope of the isotherm and the maximum achievable hydrogen uptake decrease, reaching a storage capacity of only 0.1 wt% at room
336
M. Hirscher et al. / Microporous and Mesoporous Materials 129 (2010) 335–339
The scatter in the data is due to the fact that the commonly used
N2-BET specific surface area applied in the classical pressure range
(0.05 < p/p0 < 0.3) is often not an appropriate model for micorporous materials [13]. Nevertheless, a clear trend for all these microporous materials can be recognized. Among these materials
MOFs possess the highest specific surface area and are therefore
presently the best candidates for hydrogen storage by
physisorption.
3. Adsorption sites
Fig. 1. Hydrogen adsorption (full symbols) and desorption (open symbols)
77 K (liquid N2),
87 K (liquid Ar),
87 K,
97 K,
isotherms of MOF-5 at
107 K, 117 K, 296 K.
temperature and 2 MPa. Physisorption of hydrogen in metal-organic frameworks like for other porous materials is therefore presently limited to cryogenic conditions, i.e. at the temperature of
liquid nitrogen (77 K).
The hydrogen adsorption in a variety of metal-organic frameworks with different building blocks and possessing a high porosity like e.g. Cu-BTC, IRMOF-8, MIL-53 was investigated. For
different metal-organic frameworks it was shown that the maximum hydrogen storage capacity at 77 K depends linearly on the
specific surface area (SSA) and is independent of the composition
and the structure [7]. This indicates that for MOFs a high specific
surface area is a necessary prerequisite for reaching a high storage
capacity. A similar correlation between the SSA and the hydrogen
uptake was previously obtained for other microporous materials
like carbon nanostructures [10], zeolites [11] and Prussian blue
analogues [12] as shown in Fig. 2 [4].
Fig. 2. Maximum hydrogen storage capacity versus BET SSA of different porous
materials: MOFs (open squares) [7], filled squares [3], carbon materials (triangles)
[10], zeolites (crosses) [11], Prussian blue analogues (circles) [12]. Reprinted from
Hirscher et al. [4], Copyright (2007), with permission from Elsevier.
At low hydrogen pressures the SSA is not the main property
which influences the hydrogen uptake in MOFs. Indeed, if the
adsorption isotherms of MOF-5 and Cu-BTC are compared, two different pressure regions can be distinguished (Fig. 3). At pressures
above 0.6 MPa MOF-5 possesses a higher storage capacity due to
the larger SSA, while at lower pressures a crossover in the adsorption isotherms is observed and Cu-BTC stores more hydrogen.
Responsible for this crossover in the hydrogen uptake of the two
MOFs are the higher heat of adsorption of Cu-BTC, which determines the storage capacity at low pressures and the larger SSA of
MOF-5, which is responsible for the maximum storage capacity
at high H2 pressures. Therefore, a MOF structure possessing a large
SSA together with a high heat of adsorption would allow to reach
high storage capacities already at low pressures. Different opinions
exist about the influence of the MOF structure and composition on
the heat of adsorption for hydrogen storage. Metal-centres, especially with unsaturated coordination positions, were shown to be
preferential adsorption sites for hydrogen at very low coverage.
As an example for the influence of metal sites Rowsell et al. showed
by inelastic neutron scattering that hydrogen molecules in MOF-5
first adsorb at the metal-oxide units since they interact more
strongly with H2 than the organic parts of the framework [14].
Additional experiments indicate that also the pore size in the MOFs
has an influence on the heat of adsorption of hydrogen. By thermal
desorption spectroscopy of H2 adsorbed in several metal-organic
frameworks it was possible to identify different adsorption sites
in these materials [15]. With this technique hydrogen is first adsorbed on the metal-organic framework at room temperature
and then cooled down to 20 K. At this low temperature it is possible to remove the free hydrogen molecules in the gas phase by
applying a high vacuum (10 5 Pa), while the adsorbed H2 molecules stick on the surface of the framework. The amount of hydrogen adsorbed under these conditions corresponds to a storage
Fig. 3. Hydrogen uptake of MOF-5 (squares) and Cu-BTC (triangles) at 77 K.
M. Hirscher et al. / Microporous and Mesoporous Materials 129 (2010) 335–339
capacity close to the maximum value at 77 K. When the temperature is increased with a constant heating rate the desorbed hydrogen molecules are detected with a mass spectrometer. Since the
hydrogen, which is adsorbed in sites possessing different energy,
is released at different temperatures, the resulting desorption
spectrum reflects the number of adsorption sites in the metal-organic frameworks. For Cu-BTC, MOF-5, MIL-53 and IRMOF-8 the
hydrogen desorption spectra can be well correlated to the pore
structure of their framework. As an example the thermal desorption spectrum of hydrogen on Cu-BTC is shown in Fig. 4 which presents three maxima at distinct temperatures. The maximum at
very low temperature, which is not material specific, can be assigned to some liquid hydrogen or hydrogen adsorbed in multilayers. The two additional maxima are characteristic for the material
and can be assigned to hydrogen adsorbed in the smaller cavities of
Cu-BTC which is released at higher temperatures (approx. 52 K)
and to H2 adsorbed in the large central pore which is released already at 35 K. Hence, for the investigated MOFs the desorption
temperature, which at a constant heating rate depends on the
interaction energy with H2, can be directly correlated with the size
of the pores present in the framework, i.e. hydrogen adsorbed in
larger pores is released at lower temperatures and H2 molecules
more strongly adsorbed in smaller pores are desorbed at higher
temperatures (Fig. 5). These results indicate that at high hydrogen
concentrations, i.e. when the metal-centres are already saturated
with H2 and further molecules are introduced in the framework,
the pore size mainly determines the interaction between hydrogen
and the metal-organic frameworks. This is due to the fact that in
smaller pores the van der Waals potential from the wall surface
can more strongly overlap than in larger pore enhancing the interaction between the framework and the H2 molecules.
4. Heat of adsorption
The temperature and pressure dependence of the adsorption
isotherms (fig. 1) are governed by the heat of adsorption. Measurements over a wide temperature and pressure range allow to determine very accurately the isosteric heat of adsorption (DH) applying
the Clausius–Clapeyron equation at different temperatures and in a
Fig. 4. Thermal desorption spectrum of H2 adsorbed on Cu-BTC. The maxima are
assigned to the cavities with different size. The inset shows the larger central cavity
along the [1 0 0] direction (left) and the smaller cavities with triangular window
along the [1 1 1] direction of Cu-BTC. Panella et al. [15]. Copyright Wiley-VCH
Verlag GmbH & Co. KGaA. Reproduced with permission.
337
Fig. 5. Hydrogen desorption temperature versus the pore size of MOFs. Panella
et al. [15]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with
permission.
large range of surface coverage up to 90% [6]. In Fig. 6 the isosteric
heat of adsorption versus the surface coverage is shown for various
MOFs. The values for the different materials range between 3.5 kJ/
mol and 7 kJ/mol. The decrease of the isosteric heat of adsorption
with increasing coverage is on the one hand due to the existence
of different adsorption sites in these porous materials where the
strongest binding sites are occupied at lower coverage, while those
possessing a lower adsorption enthalpy are occupied only at higher
hydrogen concentrations. On the other hand repulsive interaction
between the adsorbed hydrogen molecules might appear at higher
loadings which then decrease the heat of adsorption. The mean
values of the isosteric heat of adsorption for each porous material
can be correlated to the pore size in an analogous way as for the
desorption temperature in thermal desorption spectroscopy which
depends on DH. Similarly, the higher heat of adsorption values at
lower coverage are correlated to smaller pores since these are
occupied first while the smaller values of DH obtained at high coverage are assigned to adsorption in the larger pores. Fig. 7 indicates
an increase of the heat of adsorption with decreasing pore size.
Fig. 6. Isosteric heat of adsorption in dependence of the surface coverage of
5, Cu-BTC, MIL-101 and MIL53.
MOF-
338
M. Hirscher et al. / Microporous and Mesoporous Materials 129 (2010) 335–339
Fig. 7. The average isosteric heat of adsorption for hydrogen decreases with
increasing pore size.
Fig. 8. Isosteric heat of adsorption for MOF-5 versus the excess hydrogen uptake
(red dots: present work, green solid line: Dinca et al. [16], black dashed line: Zhou
et al. [17]). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
5. Discussion
Results from literature and ours are compared to achieve a comprehensive picture on the nature of hydrogen adsorption in MOFs.
In Fig. 8 the isosteric heat of adsorption for MOF-5 is compared to
the heat of adsorption found by Dinca et al. [16] and Zhou et al.
[17]. Dinca et al. used low pressure adsorption isotherms up to
0.1 MPa at 77 K and 87 K to determine the isosteric heat of adsorption. At low hydrogen concentrations they found a strong initial
decrease in the isosteric heat of adsorption with increasing uptake,
i.e. from an initial value of 6.5 kJ/mol to 4.5 kJ/mol at a storage
capacity of 1 wt%. These results are in good agreement with our
measurements. Measurements on other MOFs possessing open
metal sites, i.e. unsaturated coordination positions, show even
higher heat of adsorption at very low coverage and attribute these
to adsorption sites at the metal centres [18]. Zhou et al. calculated
the isosteric heat of adsorption in MOF-5 from isotherms at temperatures between 30 K and 300 K and pressures up to 6.5 MPa
[17]. Therefore these measurements provide very precise isosteric
heat of adsorption values at higher uptake. They found only a small
decrease in the isosteric heat of adsorption of about 1 kJ/mol when
the uptake increases from 1 wt% to 4.5 wt% compared to the low
Fig. 9. Isosteric heat of adsorption for Cu-BTC versus the excess hydrogen uptake
(blue solid line: present work, red dashed line: Lee et al. [19]). (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
coverage region. This shows that few very strong adsorption sites
exist which are occupied at low hydrogen uptake (<1 wt%) and
the main hydrogen adsorption takes place at almost equally strong
binding sites.
While MOF-5 does not possess metal sites with unsaturated
coordination positions, in Cu-BTC, after removal of a water solvent,
open metal sites are accessible to H2 molecules. In Fig. 9 the isosteric heat of adsorption for Cu-BTC is compared to measurements
from Lee et al. [19]. The isosteric heat of adsorption decreases from
around 6 kJ/mol to 5 kJ/mol at higher coverage. The isosteric heat
of adsorption reported by Lee et al., which is calculated from
hydrogen adsorption data in the low pressure region up to
0.1 MPa at 77 K and 87 K, is with an initial value of 6.5 kJ/mol
slightly higher. However, at higher coverage the DH decreases to
less than 6 kJ/mol at 1 wt% which is in good agreement with our
measurements. Again, a few stronger adsorption sites are occupied
at low coverage and with increasing coverage the isosteric heat is
decreasing slowly. These results can be compared to neutron powder diffraction data reported by Peterson et al. [20] who found that
at a storage capacity of 0.5 wt% the hydrogen is adsorbed close to
the metal centres of Cu-BTC. This is followed by adsorption at sites
located in the small pore of Cu-BTC and finally at an uptake above
2 wt%, hydrogen is adsorbed in the larger pore. These results are
consistent with the idea that hydrogen adsorption is governed by
metal centres only at very low uptake but when increasing the
H2 concentration to technologically relevant amounts the pore
dimension determine the interaction with hydrogen. Since the
storage capacity at low pressures, i.e. the initial steep increase in
the isotherm, depends on the heat of adsorption, small pores are
responsible for large uptake values at pressures below saturation
conditions.
6. Conclusion
The hydrogen storage properties have been studied for a variety
of metal-organic frameworks possessing different structures and
compositions. At high pressure the maximum hydrogen uptake
in MOFs is determined by the specific surface area. Therefore, a
high SSA is the first prerequisite for the application of MOFs in
cryoadsorption systems for hydrogen storage. Furthermore, both
thermal desorption studies and temperature-dependent measurements of the adsorption isotherms indicate clearly that at techno-
M. Hirscher et al. / Microporous and Mesoporous Materials 129 (2010) 335–339
logically relevant hydrogen concentrations the pore size of the cavities in the framework determines the heat of adsorption and
therefore the hydrogen uptake at low pressure. Only at very low
hydrogen concentrations, H2 is preferentially adsorbed on metal
centres. Therefore, the use of MOFs for hydrogen storage requires
a combination of small pores, which enable a lower pressure for
storage, and a larger specific surface area, which is needed for a
high storage capacity. However, the presently existing MOFs with
the highest surface area possess relatively large pores (e.g. 11–
34 Å [2,3]) and the frameworks with very small cavities show a
low SSA [21]. For the future the great challenge will be the synthesis of new frameworks consisting of a network of small accessible
pores and possessing still a large specific surface area.
Acknowledgment
Partial funding by the European Commission DG Research (contract SES6-2006-518271/NESSHY) is gratefully acknowledged.
References
[1] R. von Helmolt, U. Eberle, J. Power Sources 165 (2007) 833–843.
[2] M. Latroche, S. Surble, C. Serre, C. Mellot-Draznieks, P.L. Llewellyn, J.H. Lee, J.S.
Chang, S.H. Jhung, G. Ferey, Angew. Chem. Int. Ed. 45 (2006) 8227–8231.
339
[3] A.G. Wong-Foy, A.J. Matzger, O.M. Yaghi, J. Am. Chem. Soc. 128 (2006) 3494–
3495.
[4] M. Hirscher, B. Panella, Scripta Mater. 56 (2007) 809–812.
[5] R.E. Morris, P.S. Wheatley, Angew. Chem. Int. Ed. 47 (2008) 4966–4981.
[6] B. Schmitz, U. Müller, N. Trukhan, M. Schubert, G. Férey, M. Hirscher, Chem.
Phys. Chem. 9 (2008) 2181–2184.
[7] B. Panella, M. Hirscher, H. Pütter, U. Müller, Adv. Funct. Mater. 16 (2006) 520–
524.
[8] A. Dailly, J.J. Vajo, C.C. Ahn, J. Phys. Chem. B 110 (2006) 1099–1101.
[9] E. Poirier, R. Chahine, P. Benard, L. Lafi, G. Dorval-Douville, P.A. Chandonia,
Langmuir 22 (2006) 8784–8789.
[10] B. Panella, M. Hirscher, S. Roth, Carbon 43 (2005) 2209–2214.
[11] H.W. Langmi, D. Book, A. Walton, S.R. Johnson, M.M. Al-Mamouri, J.D. Speight,
P.P. Edwards, I.R. Harris, P.A. Anderson, J. Alloys Compd. 404–406 (2005) 637–
642.
[12] S.S. Kaye, J.R. Long, J. Am. Chem. Soc. 127 (2005) 6506–6507.
[13] K.S. Walton, R.Q. Snurr, J. Am. Chem. Soc. 129 (2007) 8552–8556.
[14] J.L.C. Rowsell, J. Eckert, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005)
14904–14910.
[15] B. Panella, K. Hönes, U. Müller, N. Trukhan, M. Schubert, H. Pütter, M. Hirscher,
Angew. Chem. Int. Ed. 47 (2008) 2138–2142.
[16] M. Dinca, W.S. Han, Y. Liu, A. Dailly, C.M. Brown, J.R. Long, Angew. Chem. Int.
Ed. 46 (2007) 1419–1422.
[17] W. Zhou, H. Wu, M.R. Hartman, T. Yildirim, J. Phys. Chem. C 111 (2007) 16131–
16137.
[18] M. Dinca, J.R. Long, Angew. Chem. Int. Ed. 47 (2008) 6766–6779.
[19] J. Lee, J. Li, J. Jagiello, J. Solid State Chem. 178 (2005) 2527–2532.
[20] V.K. Peterson, Y. Liu, C.M. Brown, C.J. Kepert, J. Am. Chem. Soc. 128 (2006)
15578–15579.
[21] M. Dinca, J.R. Long, J. Am. Chem. Soc. 127 (2005) 9376–9377.