Scripta Materialia 56 (2007) 803–808

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Viewpoint Paper

Storage of hydrogen by physisorption on carbon andnanostructured materials

Pierre Benard and Richard Chahine*

Institut de recherche sur l’hydrogene, Universite du Quebec a Trois-Rivieres, CP 500, Trois-Rivieres, Que., Canada G9A 5H7

Received 14 September 2006; revised 22 November 2006; accepted 2 January 2007Available online 14 February 2007

Abstract—Nanoporous materials such as high surface area activated carbons (AC), single-walled carbon nanotubes (SWNTs) andmetal–organic frameworks (MOFs) have been proposed as storage media for hydrogen. In this paper we discuss the use andlimitations of physisorption as a storage technology for hydrogen. Currently physisorption of hydrogen on undoped carbon nano-structures and MOFs falls short of the DOE targets for vehicular applications. Meeting these targets will require qualitative changesto the sorption process beyond structural optimization.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Carbon and graphite; Hydrogen storage; Nanostructure; Hydrogen adsorption

1. Introduction

Significant research and development activities arebeing carried out in order to increase the efficiency ofhydrogen storage systems to make them competitivewith current fossil fuels for transportation and station-ary applications, and with batteries for portable applica-tions. In order to develop and demonstrate viablehydrogen storage technologies, a set of objectives havebeen proposed by the US Department of Energy(DOE), based on achieving a driving range of 500 kmfor a hydrogen powered vehicle. These objectives [1]fix a target of 2 kW h kg�1 (6 wt.%), 1.5 kW h l�1 and$4 kW h�1 for 2010 and of 3 kW h kg�1 (9 wt.%),2.7 kW h l�1 and $2 kW h�1 for 2015. It is importantto note that these are system targets, which implies thatthe weight of the storage system as a whole must betaken into account.

Hydrogen is currently stored in vehicles as a gas inhigh pressure cylinders (at up to 700 bar) or as a liquidat 20 K in cryogenic reservoirs. According to the DOE,the maximum storage densities that have been achievedso far using these storage technologies are 1.2 kW h l�1

and 1.7 kW h kg�1 for liquid and 0.8 kW h l�1 and1.6 kW h kg�1 for high pressure [2]. The use of light-weight materials such as aluminium could push the

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* Corresponding author. Tel.: +1 819 376 5139; fax: +1 819 3765164; e-mail: richard.chahine@uqtr.ca

gravimetric storage density of cryogenic liquid hydrogenstorage up to 15 wt.% and even 18% [3]. However, thevolumetric density cannot be pushed beyond2.3 kW h l�1, the energy density of liquid hydrogen atthe normal boiling point (without the volume of thestorage system). Although liquid hydrogen could inprinciple meet the 2010 targets, it cannot meet the volu-metric requirements of the 2015 targets, althoughtheoretically it could come close. The compressed gasdensities still fall short of the 2010 targets and it is diffi-cult to imagine how they could be improved to meet the2015 targets [2]. Designing a storage system that couldoperate at room temperature and that would meet the2015 targets requires going beyond the classic storagestrategy.

A promising storage alternative for hydrogen systemsrelies on the chemical or physical binding of hydrogenwith other elements. This can be achieved through sorp-tion of hydrogen on or in a solid substrate, relying eitheron physisorption or chemisorption processes (or a com-bination of both), or through the chemical storage ofhydrogen in organic liquids or others.

Storage by sorption processes on the surface ofmaterials has been widely studied for hydrogen storageapplications in the last few years. Carbon-based nano-porous materials such as activated carbons (AC),single-walled carbon nanotubes (SWNTs) and metal–organic frameworks (MOFs) have been proposed aspromising adsorbents for hydrogen. The interest in thesematerials lies in the fact that they can be optimized for

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Figure 1. Adsorption isotherms of hydrogen on AX-21 powder overthe temperature and pressure ranges of interest to storage applications,obtained using a volumetric system. The points represent the exper-imental data. The lines show a five parameter fit of the experimentaldata to a modified Dubinin isotherm.

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hydrogen storage through various physical and chemicaltreatments.

Finding the best adsorbents for hydrogen storagebasically involves optimizing three parameters: the char-acteristic binding energy of the hydrogen molecule withthe material, the available surface for adsorption pro-cesses and the bulk density of the adsorbate. Both ofthe latter can be combined into an average surface avail-able per unit volume of the adsorbent which should bemaximized. The binding energy determines the operat-ing temperature of a hydrogen storage system basedon the solid.

The objective of this paper is to discuss the use ofphysisorption for hydrogen storage in relation to thestructural and energetic parameters, particularly as itpertains to the specific surface of the nanostructures,in order to find optimal nanostructure geometries. Thispaper is divided as follows. We first present the basicproperties of hydrogen sorption on activated carbonover the supercritical range of temperature. Nanostruc-tured carbon materials such as single-walled nanotubesare then discussed and compared. This is followed bya discussion of the potential of metal organic frame-works as adsorbents for hydrogen storage and the useof dopants to improve the storage densities.

2. Physisorption of hydrogen on carbon structures

Pure carbon materials have hydrogen bindingenergies in the range of 4–15 kJ mol�1. The lower boundis typical of activated carbon and graphite, and theupper bound is typical of internal and interstitial sitesof SWNT and SWNT bundles. The higher energy bind-ing sites are associated with confined geometry effectsand as such are associated with smaller specific surfaces.

2.1. Activated carbons

Activated carbon has been proposed as storagemedium for natural gas at ambient temperature andlow pressures. For hydrogen, a density of about 40% thatof LH2 at 20 K can be achieved at 5 MPa using densifiedactivated carbon powder with a high surface area and ahigh bulk density [4–6]. However, a temperature of77 K is required to obtain this level of performance.

The excess adsorbed density as a function of temper-ature over the supercritical range (35–300 K) of interestto storage applications is shown in Figure 1 for apowdered AX-21 activated carbon with a 2800 m2 g�1

specific surface and a density of 0.3 g ml�1. The excessadsorbed density is defined as the difference betweenthe amount of hydrogen stored on the surface of anadsorbent at a certain temperature and pressure andthe amount that would be present in the porous materialunder the same conditions in the absence of surface–gasinteractions. The storage capacity is thus the sum of theexcess amount adsorbed and of the amount of adsorbatein the bulk gas phase. The isotherms are strongly tem-perature dependent, and storage densities of practicalinterest occur only at low temperatures. The isothermsare fully reversible and exhibit no hysteresis. The pres-ence in the isotherms of a maximum at high pressure

and low temperature is a typical feature of excessadsorption isotherms. It occurs when the density ofthe coexistent bulk gas phase increases faster than theadsorbed phase.

As shown in Figure 1, the experimental data can befitted over the entire range using a five parameter fit ofa modified supercritical Dubinin isotherm [7]. Thisapproach, which is useful for engineering applications,does not have a proper low pressure limit. The Ono–Kondo self-consistent equations adapted by Aranovitchet al. to supercritical adsorption on carbon slit-pores [8]can also be used to model [9] the isotherms over theexperimental range of interest with similar accuracy.This model has a well-defined low pressure limit(Henry’s law) but does not take into account the de-tailed porous structure of the carbon. These effects areincorporated into a temperature-dependent prefactor[6].

The adsorption isotherms of hydrogen on activatedcarbons with pore structure similar to AX-21 can bemodeled using these approaches provided that the iso-therms are multiplied by the ratio of the specific surfaceof the adsorbent to the specific surface of AX-21(2800 m2 g�1).

The design of physisorption-based systems raisesthermal management issues due to the thermalexchanges during adsorption and desorption cycles.However, such effects are much less severe than chemi-sorption. These thermal effects can be characterized bythe isosteric heat of adsorption, which is about6 kJ mol�1 [10] for sorption of hydrogen on activatedcarbons.

Computer simulations on carbon slit pores andexperimental results show that little improvement inthe storage capacity can be expected using pure acti-vated carbons. Larger values could be obtained at theprice of lowering the operating temperature of the stor-age system (see Fig. 1). Although activated carbonexhibits important storage densities of hydrogen at77 K, the thermal management issues associated withthe operation of a cryogenic storage system hasprompted work on other carbon structures that couldadsorb hydrogen in significant quantity at highertemperatures.

Figure 2. Contour plot of excess density adsorbed as a function ofSWNT diameter and lattice spacing at 77 K and 1 atm (bundle ofseven units), obtained using grand canonical Monte Carlo simulationsof adsorption of hydrogen on a SWNT. The hydrogen–carboninteractions were modeled using standard Lennard–Jones potential.

P. Benard, R. Chahine / Scripta Materialia 56 (2007) 803–808 805

2.2. Carbon nanostructures

Various carbon nanostructures have been investi-gated in order to obtain higher storage density. Carbonnanofibers are layered graphitic nanostructures whichinitially generated significant interest and great contro-versy. The reported values of the hydrogen storagecapacity of such structures ranged from less than1 wt.% to several tens of weight percentages at moderatepressures and temperatures. This wide spread of resultshas now been attributed to sample preparation andexperimental issues [11,12]. A reproducible, detailedstudy of various types of carbon nanofibers shows that,from 0.1 to 105 bars and from 77 to 295 K, the maxi-mum adsorbed density obtained is 0.7 wt.% at 105 bars[13], implying that these structures present little or nointerest for hydrogen storage applications.

The adsorption of hydrogen in single-walled carbonnanotubes has also been the object of much controversy.The narrow pore size distribution of SWNTs makesthem attractive candidates as adsorbents for hydrogen.Their cylindrical geometry can lead to a deeper potentialwell inside, which is a function of radius. Calculationsbased on the minimization of the energy of a hydrogenmolecule located at the center of a SWNT using acontinuum cylindrical Lennard–Jones potential [14]show that the minimum sorption binding energy insidethe SWNT would be 12.3 kJ mol�1, corresponding to aSWNT radius of about 3.2 A. This binding energy istwo to three times larger than the one usually measuredfor activated carbon. Very small radii, however, can leadto repulsive interactions inside the nanotube. Earlyexperiments on SWNTs reported adsorbed amountsbetween 0 and 10 wt.%. The spread of values was mainlydue to the small amounts of samples available for sorp-tion measurements, which requires highly sensitivesystems, and to sample preparation. These issues havenow been generally addressed. For instance, excessadsorption measurements of hydrogen at 77 K from 0to 60 bars on a round-robin sample of IRH-3 carbonat four different laboratories agreed within 10%, compa-rable to the accuracy of the measurement systems.

Most measurements show that under ambient condi-tions, the amount of hydrogen adsorbed in SWNTs issmall (<1 wt.%) [15]. Under cryogenic conditions,hydrogen uptakes ranging from about 1 to 2.4 wt.%have been reported [16–18]. Pradhan et al. [19] foundthat SWNTs can reversibly store 6 wt.% hydrogen at77 K and 2 atm. Ye et al. [20] obtained 8 wt.% adsorp-tion at 40 atm and 80 K on SWNT bundles. Poirieret al. [21] observed a maximum hydrogen uptake at77 K and 1 atm ranging from 1.5 to 2.5 wt.%, dependingon the sample preparation. An adsorption enthalpy(averaged over coverage) of 4.3–4.5 kJ mol�1 wasreported, which is typical of carbon adsorbents. Thesemeasurements were confirmed by measuring the adsorp-tion density using both a gravimetric and a volumetricsystem.

Thermal treatments under high vacuum and hightemperature can enhance the adsorption properties ofSWNTs by removing guest species limiting the accessto adsorption sites and increasing the specific surfaceavailable to the hydrogen molecules. A detailed study

of the effect of acid treatment and consecutive exposuresto heat of SWNTs on the excess adsorbed density ofhydrogen at 77 K and 1 bar and on the specific surfaceof the SWNT has been performed by Lafi et al. Themaximum excess adsorbed density (4.6 wt.%) wasobtained after treatment with HF and heat at 600 �C,corresponding to the highest specific surface obtained(1555 m2 g�1) [22]. The adsorption properties wereshown to be very sensitive to sample preparation, andthat some measure of optimization of the adsorbed den-sity was possible.

2.3. Physisorption of hydrogen on carbons andstructural properties

The high pressure, low temperature adsorbed densityof hydrogen on carbon nanostructures generally corre-lates linearly with the pore volume and the specific sur-face of the adsorbent. Monte Carlo grand canonicalsimulations of hydrogen molecules (using standardLennard–Jones interactions) show that the excess ad-sorbed density is a function of the geometry of SWNTbundles (radius and tube spacing) [23,24]. This excessdensity is defined here as the difference between the den-sity in the presence of the adsorbent in a given simula-tion volume and the density of the gas in the samevolume. Figure 2 shows that, for a hexagonal arrange-ment of SWNT bundles, a maximum of about 3 wt.%(equivalent to 4–5 wt.% when the volume of the adsor-bent is taken into account) can be expected at 77 Kand 1 bar for tube diameters ranging between 12 and14 A and a distance of about 6 A between walls of adja-cent tubes [25]. A clear correlation between the adsorbeddensity, the specific surface and the distance between thenanotubes in a bundle is observed. The specific surfacesaturates when the distance between the nanotubes issufficiently large and the interactions between hydrogenand adjacent carbon structures no longer overlap. Amaximum of the adsorbed density is observed becausefurther separating the nanotubes decreases the overlapof the attractive part of the interaction potentials with-out increasing the surface available for sorption. Themaximum value that can be expected is 6 wt.% at40 bars. Quantum effects can only lower this value [25].

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Figure 3. Adsorption enthalpy DH0 as a function of fractional filling ofhydrogen on two different single-walled nanotube samples, theactivated carbon IRH-33 and MOF-5.

806 P. Benard, R. Chahine / Scripta Materialia 56 (2007) 803–808

A investigation of physisorption of hydrogen onlayers of four different graphite fragments using grandcanonical Monte Carlo simulations as a function ofpressure, temperature and the accessible surface of theadsorbent shows the effect of structural parameters onthe sorption process. These carbon fragments are con-trived structures designed to study the effect of densityand the specific surface on physisorption of hydrogenon carbon nanostructures [26]. They have a specific sur-face ranging from 2700 to 9220 m2 g�1 (with decreasingbulk density as the specific surface is increased). At lowpressure, the gravimetric adsorbed density of hydrogenwas observed to increase with the density of the carbonnanostructure. This is because the greater nanostructuredensity reflects narrower pores and increased surface–hydrogen interactions due to surface potential overlaps.This trend is reversed at high pressure, where the specificsurface available for adsorption plays a more importantrole. This study also concluded that the high pressuretrend is reversed when considering the volumetricadsorbed density: per unit volume, the high densitymaterials always have the largest adsorbed density.The linear correlation between adsorbed density andspecific surface was also observed for these structuresat 35 bars and 77 K.

For physisorbed hydrogen, these results confirm thatmaximizing the specific surface of the adsorbent seemsto remain the optimal strategy for the design of materi-als for hydrogen storage if volumetric considerations arenot critical.

2.4. Metal–organic frameworks

Metal–organic frameworks are networks of transitionmetal atoms bridged by ligands that have been pro-posed as structured nanoporous materials for hydrogenstorage. They exhibit very large specific surfaces (1000–6000 m2 g�1) [27]. At room temperature (295 K), revers-ible hydrogen uptakes of less than 1 wt.% have beenobtained [21].

Wong-Foy et al. [28] have recently obtained maxi-mum excess adsorption densities ranging from 2.0 to7.3 wt.% of hydrogen at 77 K for seven MOF com-pounds. The largest uptakes were 7.3 wt.% for IRMOF177 and 6.7 wt.% for IRMOF-20 at 70–80 bars. Theyobtained a linear correlation between the specific sur-face of the MOF structures investigated and themaximum excess adsorbed densities, IRMOF 177 hav-ing a specific surface in excess of 5500 m2 g�1. They alsoobtained large volumetric storage densities (about32 g l�1 for IRMOF 177), although no clear correlationbetween specific surface and volumetric storage densitycould be established. Interestingly, they also observedthat at low pressures the three lowest surface areamaterials had the largest excess adsorbed density, inagreement with the trend observed in the grand canon-ical simulations discussed earlier. This suggests thatgravimetric adsorption is determined by the bindingenergy at low pressure and by the specific surface athigh pressure.

Other related compounds include Al-BDC and Cr-BDC (MILS-53), which can adsorb 2.8–3.9 wt.% at77 K at pressures of up to 50 bars [29].

Figure 3 compares the adsorption enthalpy as a func-tion of coverage for activated carbon, two differentSWNTs and MOF-5, obtained using a Dubinin analysis.The enthalpies are clearly distinctive in the low coverageregion, and converge at high coverage. At low filling, theenthalpy of adsorption is largest for SWNTs and smallestfor MOF-5, reflecting the stronger surface interactionsfor SWNTs. The enthalpies remain below 5 kJ mol�1,which is consistent with physisorption [21].

2.5. Doping nanostructures with metals: beyondcryogenic storage

The excess density of hydrogen stored on carbon andMOF nanostructures has been limited so far to about 6–7.5 wt.% at 77 K and 30–40 bars. A study of microporefilling of activated carbons shows that at these tempera-tures the density of hydrogen in the pores approachesthat of liquid hydrogen. Any further improvement ofthe storage density would entail storing hydrogen inthe porous structure at densities beyond liquid hydro-gen. Increasing the operating temperature of a physi-sorption storage system will require going beyondcurrent carbon and MOF nanostructures.

A possible strategy is to enhance the sorption capac-ity of carbon nanostructures by doping them withcertain metals, which could lead to non-dissociativebinding of hydrogen molecules and enable them to beused to reversibly store hydrogen [30]. Kim et al. havepredicted a room temperature reversible storage of1.7 wt.% of hydrogen on C54Be. Ab initio quantumchemistry calculations by Yildirim et al. initially sug-gested that a titanium atom embedded on a single-walled nanotube or a C60 could possibly lead to astorage density of 7–8 wt.% at high titanium coverageunder ambient conditions [31,32]. Metals such as Scand Cr were also predicted to enhance sorption effectson C60 and SWNTs. However, recent work by Sunet al. has shown that clustering of the titanium atomswould lower this number to a maximum value of2.85% for titanium-doped C60 [33]. In addition, these ef-fects have not yet been demonstrated experimentally.

The effect of functionalizing SWNT with platinumand palladium was also investigated [34], showingthat Pt can bind up to two hydrogen molecules in the

P. Benard, R. Chahine / Scripta Materialia 56 (2007) 803–808 807

chemisorption range, and that it could dissociate molec-ular hydrogen on SWNT with large diameter (Pd exhib-iting similar but weaker effects).

Hydrogen storage by spillover has been proposed as amechanism to enhance the storage density of carbon-based and MOF nanostructures. This approach relieson the use of a supported metallic catalyst to dissociatemolecular hydrogen, relying on surface diffusionthrough a bridge to store atomic hydrogen in a receptor.An increase by a factor of 2.9 for the activated carbonAX-21 and 1.6 for single-walled nanotubes at 298 Kand 1 bar was obtained using a Pd/carbon catalyst[35]. Li et al. also used a spillover mechanism to enhancethe adsorption density in the MOF-5 and IRMOF-8metal organic framework by a factor of 3.3 and 3.1,respectively [36]. This was achieved using a catalyst con-taining 5% Pt supported by activated carbon (primaryreceptor). The spillover effect was obtained by grindingthe catalyst and the MOF in a 1:9 ratio (wt.). Themaximum density observed was 1.8% at 298 K and10 MPa for IRMOF-8.

3. Conclusion

The hydrogen uptake in nanoporous materials such ascarbon nanostructures and MOFs is due mostly to phys-isorption. The maximum gravimetric excess adsorptiondensity correlates linearly with the specific surface andthe pore volume of the adsorbent. This suggests thatthe best adsorbents for practical storage applications willgenerally be the one with the largest specific surface perunit volume. The low pressure region of the adsorbentisotherms depends on the adsorbate–adsorbent interac-tion, leading to a reverse trend with respect to the higherpressure region. This is observed experimentally forMOF adsorbents and in hydrogen sorption simulationson carbon nanostructures. The excess hydrogen densityadsorbed on single wall nanotubes is strongly dependenton sample preparation. Notwithstanding costs issues,single wall carbon nanotubes are more advantageousthan activated carbons, as they exhibit higher hydrogencoverage per unit area. They also have a larger bulkdensity, leading to better volumetric storage.

Currently physisorption of hydrogen on undopedcarbon nanostructures and MOFs falls short of theDOE targets for the use of hydrogen as a transportationfuel. Promising applications remain limited to certainniche applications, such as bulk transportation ofhydrogen, with large storage densities compared withcompressed gas cylinders and lower boil-off expectedcompared with liquid hydrogen cryogenic reservoirs[37], and where thermal management issues would belimited due to few fill/refill operations. Meeting theambitious DOE target will require going beyond struc-tural optimization, and qualitatively change the interac-tions involved in the sorption process, by investigatingthe effect of dopants, catalysts and substitution.

We gratefully acknowledge the support of NaturalResources Canada and of the Natural Science and Engi-neering Research Council of Canada.

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