Record High Hydrogen Storage Capacity in the Metal-Organic Framework Ni2(m-dobdc) at Near-Ambient Temperatures

Article abstract of DOI:10.1021/acs.chemmater.8b03276

Hydrogen holds promise as a clean alternative automobile fuel, but its on-board storage presents significant challenges due to the low temperatures and/or high pressures required to achieve a sufficient energy density. The opportunity to significantly reduce the required pressure for high density H₂ storage persists for metal-organic frameworks due to their modular structures and large internal surface areas. The measurement of H₂ adsorption in such materials under conditions most relevant to on-board storage is crucial to understanding how these materials would perform in actual applications, although such data have to date been lacking. In the present work, the metal-organic frameworks M₂(m-dobdc) (M = Co, Ni; m-dobdc^4- = 4,6-dioxido-1,3-benzenedicarboxylate) and the isomeric frameworks M₂(dobdc) (M = Co, Ni; dobdc^4- = 1,4-dioxido-1,3-benzenedicarboxylate), which are known to have open metal cation sites that strongly interact with H₂, were evaluated for their usable volumetric H₂ storage capacities over a range of near-ambient temperatures relevant to on-board storage. Based upon adsorption isotherm data, Ni₂(m-dobdc) was found to be the top-performing physisorptive storage material with a usable volumetric capacity between 100 and 5 bar of 11.0 g/L at 25 °C and 23.0 g/L with a temperature swing between -75 and 25 °C. Additional neutron diffraction and infrared spectroscopy experiments performed with in situ dosing of D₂ or H₂ were used to probe the hydrogen storage properties of these materials under the relevant conditions. The results provide benchmark characteristics for comparison with future attempts to achieve improved adsorbents for mobile hydrogen storage applications.

Full text of DOI:10.1021/acs.chemmater.8b03276

Article
pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Record High Hydrogen Storage Capacity in the Metal−Organic
Framework Ni2(m‑dobdc) at Near-Ambient Temperatures
†,‡
Matthew T. Kapelewski,†,‡ Tomce Runcevski, Jacob D. Tarver,⊥,§ Henry Z. H. Jiang,†,‡
Katherine E. Hurst,§ Philip A. Parilla,∥ Anthony Ayala,⊥,¶ Thomas Gennett,§,# Stephen A. FitzGerald,□
Craig M. Brown,⊥,○ and Jeffrey R. Long*,†,‡,△
̌
̌
†Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States
‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
⊥Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
§Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States
∥Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States
¶Department of Chemistry, University of Maryland, College Park, Maryland 20742, United States
#Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States
□Department of Physics, Oberlin College, Oberlin, Ohio 44074, United States
○Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States
△Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, United
States
S
* Supporting Information
ABSTRACT: Hydrogen holds promise as a clean alternative
automobile fuel, but its on-board storage presents significant
challenges due to the low temperatures and/or high pressures
required to achieve a sufficient energy density. The
opportunity to significantly reduce the required pressure for
high density H2 storage persists for metal−organic frameworks
due to their modular structures and large internal surface
areas. The measurement of H2 adsorption in such materials
under conditions most relevant to on-board storage is crucial
to understanding how these materials would perform in actual
applications, although such data have to date been lacking. In
the present work, the metal−organic frameworks M2(m-
dobdc) (M = Co, Ni; m-dobdc4− = 4,6-dioxido-1,3-benzenedicarboxylate) and the isomeric frameworks M2(dobdc) (M = Co,
Ni; dobdc4− = 1,4-dioxido-1,3-benzenedicarboxylate), which are known to have open metal cation sites that strongly interact
with H2, were evaluated for their usable volumetric H2 storage capacities over a range of near-ambient temperatures relevant to
on-board storage. Based upon adsorption isotherm data, Ni2(m-dobdc) was found to be the top-performing physisorptive
storage material with a usable volumetric capacity between 100 and 5 bar of 11.0 g/L at 25 °C and 23.0 g/L with a temperature
swing between −75 and 25 °C. Additional neutron diffraction and infrared spectroscopy experiments performed with in situ
dosing of D2 or H2 were used to probe the hydrogen storage properties of these materials under the relevant conditions. The
results provide benchmark characteristics for comparison with future attempts to achieve improved adsorbents for mobile
hydrogen storage applications.
cells to produce water and electricity that power the vehicle
and close the cycle. Achieving such an economy, however,
requires the successful development of each aspect of this
process to both efficiently produce H2 for use in fuel cells and
consume H2 in the production of electricity.
INTRODUCTION
■
Molecular hydrogen (H2) holds significant promise as a
transportation fuel and is already used in some motor vehicles
and for certain specialty applications such as forklifts. Because
water is the only byproduct of the fuel cell cycle, hydrogen fuel
cell vehicles could, in principle, provide zero-emission
transportation.1 An economy can be envisioned in which
solar energy is used to inexpensively produce hydrogen and
oxygen from water; these products are then consumed in fuel
Received: August 2, 2018
Revised: October 25, 2018
© XXXX American Chemical Society
A
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Significant investment in infrastructure supporting hydrogen
fuel cell vehicles is underway around the world. As of 2017, the
United States has 34 publicly accessible hydrogen fueling
stations, with 31 of these in California.2 The “California
Hydrogen Highway” is a planned expansion of the current
distribution network to 100 hydrogen fueling stations in
California, primarily linking San Diego, Los Angeles, and the
San Francisco Bay Area.3 Other countries including Japan,
France, Germany, and the United Kingdom have made
significant investments in hydrogen infrastructure both in
anticipation of and to help bring about the wider use of
hydrogen fuel cell vehicles.4 Public−private partnerships
further these efforts and provide a basis for the future of
hydrogen fuel cell vehicles to provide a clean alternative to
traditional fossil-fuel-based transportation.5 In addition to
infrastructure developments, further scientific advances are
imperative to realize the widespread adoption of hydrogen as a
commercial fuel. Notable among such desired advances is the
development of efficient hydrogen storage systems.6 While
containing 2.6−3 times more energy per unit mass than
gasoline,7,8 hydrogen poses challenges in the pursuit of storage
at high volumetric densities. Hydrogen is a weakly interacting
gas at ambient temperature and pressure and thus requires
cooling and/or compression for storage at densities sufficient
for acceptable driving ranges in automobiles. However,
cryogenic storage requires the use of large, expensive, and
well-insulated systems to maintain a low temperature.6,9
Similarly, compression of H2 at high pressures, typically up
to 700 bar, is costly and requires heavy, expensive, and bulky
storage tanks.10,11 Both of these solutions therefore add to the
price of the vehicle in addition to providing significant
engineering challenges given the wide operating temperature
range for passenger vehicles (−40 to 60 °C). Furthermore,
compression to 700 bar results in a hydrogen volumetric
energy density of only 5.6 MJ/L at 298 K, significantly lower
than the 32.4 MJ/L for gasoline.8 While the use of a metal or
chemical hydride as a storage medium could mitigate the need
for low temperature or high pressure storage vessels, these
materials tend to suffer from either capacity limitations or
problems arising from large activation energies and reversibility
issues.12−15
An alternative to either cryogenic or compressive storage
involves the use of an adsorbent material such as a zeolite16 or
activated carbon17 to boost the hydrogen density in a tank
under more ambient conditions. With just two electrons and a
low polarizability, H2 is capable of engaging in only weak van
der Waals interactions, leading to an adsorption enthalpy that
is typically on the order of −5 kJ/mol. Accordingly, adsorption
sites capable of strongly polarizing H2 must be introduced to
achieve sufficient densification and a reasonable driving range.
Cryo-adsorption, which entails a combination of adsorption
and cryogenic storage, is one possible strategy to yield high
capacities.18,19 However, the ideal situation would involve
adsorption under ambient temperature conditions with a
relatively low fill pressure of 100 bar or lower. Such a system
would be expected to lower costs significantly because a
conformable, lightweight storage vessel could potentially be
used, and no on-board cooling system would be required.
Metal−organic frameworks (MOFs) are a class of materials
with great potential for hydrogen storage, among other
applications related to gas storage and separations.20 The
inherent synthetic tunability of these structures has led to a
wide range of interesting properties such as high surface
areas,21 negative gas adsorption,22 and precisely engineered
pore environments.23 Such tunability can be used to improve
their properties for a desired application, including hydrogen
storage,24−27 and has made MOFs one of the most intensely
studied fields in modern inorganic chemistry. For example, it is
possible to create MOFs featuring pore surfaces with a high
concentration of strong H2 adsorption sites, a feature less
readily achieved in zeolites and activated carbon adsorbents.
Computationally predicted hydrogen adsorption isotherms in
MOFs have shown high hydrogen capacities at near-ambient
temperatures, but these materials have yet to be evaluated
experimentally.28−31 MOFs can thus, in principle, be designed
to exhibit H2 binding enthalpies in the optimal range of −15 to
−20 kJ/mol,32 leading to a high storage capacity under
conditions relevant to light-duty fuel cell vehicles.33 The appeal
of this approach is apparent in the many studies of MOFs for
H2 storage that have often focused on materials containing
coordinatively unsaturated (open) metal sites.34 These
exposed positive charges are able to polarize H2 more strongly
than the typical surfaces available for physisorption in most
storage materials.35−38 Thus far, however, no MOFs have been
shown to achieve the necessary binding enthalpies or the
capacity metrics set forth by the United States Department of
Energy (US DOE).33
The most promising metal−organic framework identified to
date for H2 storage is Ni2(m-dobdc) (m-dobdc4− = 4,6-
dioxido-1,3-benzenedicarboxylate), which was shown previ-
ously to display an H2 binding enthalpy of −13.7 kJ/mol, as
measured by variable-temperature infrared spectroscopy and
representing the largest value yet observed in a MOF by this
method.39 Ni2(m-dobdc) is a structural isomer of Ni2(dobdc)
(dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; Ni-MOF-
74), and its record binding enthalpy is largely a result of a
higher charge density at its coordinatively unsaturated Ni2+
centers. These sites strongly polarize H2, providing the primary
binding sites for H2 within the pores of the material and
leading to a high gravimetric storage capacity of greater than
11 mmol/g (2.2 wt %) at 77 K and 1 bar. Recent reports have
shown that the material Cu(I)-MFU-4l exhibits an H2 isosteric
heat of adsorption of −32 kJ/mol;40 however, the volumetric
density of these open metal coordination sites in this material
is about 10% of that in Ni2(m-dobdc), rendering it perhaps
more suitable for H2/D2 separations than H2 storage.41
In this work, we investigated the hydrogen storage properties
of Ni2(m-dobdc) and other related top-performing MOFs,
specifically Co2(m-dobdc), Co2(dobdc), and Ni2(dobdc),
under more practical conditions. Adsorption isotherms at
multiple temperatures in the range of 198 to 373 K were
measured to determine capacities at pressure up to 100 bar,
while in situ powder neutron diffraction and infrared
spectroscopy experiments were employed to probe the nature
of the interactions of hydrogen within the pores of the
materials.
EXPERIMENTAL SECTION
General Synthesis. The compounds M2(m-dobdc) (M = Co, Ni)
were synthesized and activated according to modified versions of the
large-scale literature procedure.39
■
Synthesis of H4(m-dobdc). Resorcinol (1,3-dihydroxybenzene;
37.6 g, 0.341 mol) was pulverized and dried under vacuum. KHCO3
(100 g, 0.99 mmol) was separately pulverized and dried under
reduced pressure. The two powders were mixed together thoroughly
and placed in a glass jar, which was sealed in a Parr reaction bomb
equipped with an internal thermocouple and a pressure gauge. The
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reaction bomb was evacuated under vacuum, and then CO2 was dosed
to a pressure of 40 bar. The bomb was heated to 250 °C (as measured
by the internal thermocouple) in a sand bath for 24 h and then slowly
cooled to room temperature. The pressure was vented; 1 L of water
was added to the solid, which was broken up mechanically, and the
mixture was sonicated. The resulting suspension was filtered, and the
filtrate was acidified with 12 M HCl until a pH < 2 was achieved and a
white solid had precipitated. This solid was collected by filtration and
and fit using a third-order polynomial, which was then used to
perform a background correction on all subsequently collected data at
each temperature. The uptake in these background adsorption
isotherms was typically on the order of 10 v/v (volume of H2 per
volume of MOF), and the measured values were subtracted from the
total adsorption isotherms of the metal−organic frameworks. Such
error primarily stems from minor temperature fluctuations in the
three-zone experimental setup as well as small valve volumes. Pore
volumes were determined experimentally using N2 adsorption
isotherm data. Pore volumes were determined to be 0.53, 0.56,
0.52, and 0.54 cm3/g for Co2(m-dobdc), Ni2(m-dobdc), Co2(dobdc),
and Ni2(dobdc), respectively. Crystallographic densities were used in
all calculations to obtain volumetric capacities.
1
dried in air to yield 53.2 g (79%) of product. H NMR (400 MHz,
DMSO-d6) δ 9.22 (br, 4H), 8.28 (s, 1H), 6.22 (s, 1H); 13C NMR
(400 MHz, DMSO-d6) δ 172.0, 167.7, 134.3, 107.3, 103.0.
Synthesis of Co2(m-dobdc). Aliquots of 310 mL of methanol
and 310 mL of N,N-dimethylformamide (DMF) were added to a 1-L
three-neck round-bottom flask equipped with a reflux condenser and
sparged with N2 with stirring for 1 h. The solids H4(m-dobdc) (2.00
g, 10.1 mmol) and CoCl2 (3.27 g, 25.2 mmol) were added under N2
pressure, and the reaction mixture was vigorously stirred and heated at
120 °C for 18 h. The mixture was then cooled to ambient temperature
and filtered, affording a pink microcrystalline powder. The powder
was soaked in 500 mL of DMF for 24 h, then soaked in three
successive aliquots of 500 mL of methanol for 24 h each. The
resulting pink powder was collected by filtration and heated at 180 °C
under dynamic vacuum until the outgas rate was <1 μbar/min,
yielding 1.71 g (54.3%) of activated product.
Synthesis of Ni2(m-dobdc). An identical procedure was used as
for Co2(m-dobdc) above, except that the solvent consisted of 220 mL
of methanol and 405 mL DMF, and NiCl2 (3.27 g, 25.2 mmol) was
used in place of CoCl2. The reaction yielded 1.69 g (54.4%) of
activated product.
Synthesis of M2(dobdc) (M = Co, Ni). These materials were
synthesized using identical procedures to their M2(m-dobdc)
congeners above, with the substitution of like amounts of the
isomeric H4(dobdc) ligand for the H4(m-dobdc) ligand. These
reactions yielded 2.06 g (65.4%) of activated Co2(dobdc) and 2.25 g
(80.1%) of activated Ni2(dobdc).
Adsorption isotherms at 77 and 100 K (Figures S2 and S3) were
collected on a custom-built volumetric adsorption apparatus at the
National Renewable Energy Laboratory (NREL), details of which can
be found in the Supporting Information.
Temperature-Programmed Desorption. The temperature-
programmed desorption (TPD) data were collected on a custom-
built NREL TPD apparatus that allows for identification and
quantification of effluent gases, as described elsewhere.44 In summary,
calibrated adsorption capacities and desorption activation energies
and kinetics can be investigated using the system, in which it is
possible to heat or cool samples in vacuum to temperatures between
77 and 1200 K. Samples may be exposed to hydrogen (99.9999%) at
pressures up to ∼1000 Torr, and the system can achieve pressures as
low as 10−9 Torr. The TPD system is equipped with a mass
spectrometer with detection range of 0−100 atomic mass units to
detect impurities present in materials both during degas and after
hydrogen exposures.
Powder Neutron Diffraction Measurements. Powder neutron
diffraction data were collected on the high resolution neutron powder
diffractometer, BT-1, at the National Institutes of Standards and
Technology (NIST) Center for Neutron Research (NCNR), with a
Ge-(311) monochromator using an in-pile 60’ collimator correspond-
ing to a wavelength of 2.077 Å. Measurements were performed on
1.11 g of activated Co2(m-dobdc). The activated sample was
transferred into a He-purged glovebox equipped with oxygen and
water monitors. The sample was loaded into an aluminum can
equipped with a valve for gas loading up to pressures of 100 bar and
loaded into a top-loading closed-cycle refrigerator. Data collection was
performed at 77 and 198 K for the activated sample. At 77 K, one
loading of 78 bar of D2 was measured. At 198 K, the sample was
initially exposed to 79 bar of D2 and allowed to reach equilibrium.
Additional measurements were performed at reduced pressures of 54
and 36 bar of D2. Aluminum Bragg peaks were removed from the data
during analysis.
In Situ Infrared Spectroscopy. Infrared spectra were acquired
using a Bomem DA3Michelson interferometer equipped with a
quartz-halogen source, a CaF2 beamsplitter, and a liquid nitrogen-
cooled mercury−cadmium−telluride detector. A cutoff filter above
9000 cm−1 was used to prevent unwanted sample heating from the IR
source. A custom-built diffuse reflectance system with a sample
chamber that allows both the temperature and atmosphere of the
material to be controlled was utilized for all experiments.45 Activated
powder samples (∼10 mg) were transferred to a Cu sample holder
within an Ar-purged glovebox. The samples were sealed within a
dome containing sapphire windows and a valve for gas loading. Seals
were achieved using either indium or Teflon gaskets depending on the
pressure and temperature of the specific experiment. The dome was
bolted to a copper slab providing thermal contact to a coldfinger
cryostat (Janis ST-300T). The sample temperature was monitored by
a Si-diode thermometer bolted directly to the copper slab. A reference
infrared spectrum was obtained at each temperature. Hydrogen gas
was introduced from a dosing manifold to a desired pressure while
maintaining the sample at constant temperature. Multiple infrared
spectra were obtained at each pressure step up to a maximum pressure
of 100 bar. These spectra were then referenced to the initial spectrum
without H2.
Synthesis of MOF-5. The synthesis of MOF-5 was carried out
according to a previously published procedure.42,43
Measurement of Gas Adsorption Isotherms. All gas
adsorption isotherms in the range 198 to 373 K were measured on
a Particulate Systems HPVA II instrument. The sample holder was
custom-built using a Swagelok valve connected to a sample holder.
Typically, 1.0−2.0 g of sample was used for each measurement to
ensure that measurement and mass errors were minimized. These
samples were activated in standard glass sample tubes as loose
powders on a Micromeritics ASAP 2420 instrument and transferred to
the custom HPVA sample holder in a drybox. Once the sample holder
was connected to the HPVA instrument, the sample was immersed in
a recirculating fluid bath connected to a Julabo FP89-HL/TK filled
with Dow Syltherm fluid. During the data collection, a portion of the
sample holder was exposed above the fluid in the temperature bath
but below the temperature-controlled dosing manifold of the HPVA-
II instrument. The resulting existence of three temperature zones
leads to challenges in performing the required volume calibrations,
which are essential to properly determining the gas uptake of a
sample. The volume of each temperature zone was therefore
experimentally determined based on He measurements at multiple
temperatures, and the results were applied in obtaining corrected
adsorption data. A more complete discussion of this calibration
method can be found in a similar paper discussing the measurement
of methane adsorption in MOFs.42
Importantly, the background adsorption of H2 within the sample in
an empty sample holder should be close to zero at all pressures,
assuming the proper calibrations are in place. While this is true for
isotherms being measured at close to ambient temperature (at which
the temperatures of the two parts of the sample holder are very
similar, resulting in a minimal temperature gradient), isotherms
measured at temperatures further from ambient will see a larger
temperature gradient and a commensurate deviation from null
adsorption. To account for this deviation, background adsorption
measurements for H2 were repeated three times at each temperature
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studies without needing to account for sample preparation or
measurement of other densities. The actual storage capacity in
a system, however, will depend on the bulk density, shaping,
and packing of the storage material, which is outside the scope
of this report.48
Furthermore, the volumetric usable capacity is the most
important consideration when evaluating adsorbents for
hydrogen storage. For the purposes of this work, usable
capacity is defined as the total amount of H2 adsorbed between
5 and 100 bar in the total adsorption isotherm (Figure 1). The
RESULTS AND DISCUSSION
■
General Considerations for H2 Storage in Adsorb-
ents. As introduced earlier, adsorbent materials have the
potential to store H2 at reduced pressures and temperatures
relative to cryogenic or high-pressure technologies and
therefore offer a more energetically and financially promising
solution. The US DOE has released guidelines for hydrogen
storage in light-duty and specialty vehicles (e.g., passenger
vehicles, forklifts, golf carts, and specialized airport vehicles,
among others). A subset of the system-based targets associated
with these guidelines and relevant to adsorbent-based storage
is reproduced in Table 1.
Table 1. Selected US DOE Targets for the Onboard Storage
of Hydrogen in Light-Duty Fuel Cell Vehicles33
storage parameter
units
2020
ultimate
system gravimetric H2
capacity
kg H2/kg system,
kWh/kg
0.045,
1.5
0.065,
2.2
system volumetric H2
capacity
g H2/L system,
kWh/L
30, 1.0
50, 1.7
storage system cost
$/kg H2 stored, $/gge 333, 4
at pump
266, 4
operating ambient
temperature
°C
−40 to
−40 to
60
60
min/max delivery
temperature
°C
−40 to
−40 to
85
85
operational cycle life (1/4 tank cycles
to full)
1500
5
1500
5
min delivery pressure from
storage system
bar (abs)
max delivery pressure from
storage system
system fill time (5 kg)
bar (abs)
min
12
12
Figure 1. An illustration of how usable capacity is calculated,
considering adsorption at 100 bar and desorption at 5 bar. For MOFs,
usable volumetric capacity is determined from the total uptake and
crystallographic density of the material for easy comparison across
multiple studies.
3−5
3−5
To date, no adsorbents have been produced that satisfy the
2020 target capacity requirements of 4.5 wt % and 30 g/L H2.
The trade-off between volumetric and gravimetric H2 density
in MOFs has been previously studied, however, showing
maximization of both to be difficult.46,47 While pressure ranges
are not explicitly given, operating pressures below 100 bar have
the potential to reduce storage vessel and compression costs
while maintaining reasonable capacities. Importantly, these
target capacity requirements are full system capacities.
Therefore, potential adsorbent materials must actually exceed
target capacities, as the full system will involve more mass and
volume than that of the adsorbent alone.
The volumetric capacity is the primary consideration when
evaluating MOF materials for H2 storage, because in light-duty
vehicles, the available volume for a tank for adsorbent-based
storage of H2 is the limiting factor in determining the driving
range of a vehicle. This concept has been discussed in detail
elsewhere for natural gas storage,42 and the same principles will
apply to H2 storage. For example, a given percent increase in
volumetric storage capacity will yield a commensurate percent
increase in driving range assuming a fixed-volume tank. In
contrast, the same percent increase in gravimetric capacity will
yield only a small percent increase in driving range due to the
savings in weight of the adsorbent in the fuel tank; therefore,
targeting materials based on their total volumetric capacity is a
more useful means of identifying candidate materials for H2
storage. Crystallographic densities are used herein to calculate
volumetric capacities as an upper bound of storage capacity, as
these represent an intrinsic property of each material and allow
for the comparative evaluation of materials across multiple
total adsorption isotherm is calculated by accounting for the
excess capacity plus the amount of bulk H2 present under the
conditions at which the isotherm was measured. The total
adsorption thus gives the total amount of gas contained within
the volume of a crystal of the adsorbent. A minimum pressure
of 5 bar is assumed to be necessary for the fuel injector in the
vehicle, such that any H2 stored below 5 bar is inaccessible as
fuel. Thus, all H2 uptake would ideally occur after 5 bar, and
the total capacity would be equal to the usable capacity.49 In
practice, however, materials that strongly bind H2 typically
adsorb large quantities of H2 at lower pressures, which are then
inaccessible to use in the fuel cell.
There are many considerations when measuring adsorption
isotherms at high pressures that are crucial for properly
evaluating materials for their H2 adsorption properties. For
example, it is important to use a large mass of material to
minimize mass errors that may significantly affect the gas
uptake. Furthermore, all volumes must be carefully calibrated
to ensure reproducibility and accuracy of measurements.
Maintaining isothermal control is also essential; regardless of
the number of temperature zones in the measurement,
consistent volumes at consistent temperatures must be
maintained to ensure accuracy across multiple isotherm
collections.
The adsorbent cost, which impacts the entire system cost, is
another important metric, as the H2 storage system must be
economically competitive with gasoline storage tanks. This
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Figure 2. Hydrogen adsorption isotherms for (a) Co2(m-dobdc), (b) Ni2(m-dobdc), (c) Co2(dobdc), and (d) Ni2(dobdc) at −75 (black circles),
−50 (navy squares), −40 (blue triangles), −25 (green upside-down triangles), 0 (gold diamonds), 25 (yellow hexagons), 50 (orange stars), 75
(dark red pentagons), and 100 °C (bright red crosses) measured between 0 and 100 bar and plotted in terms of total volumetric and gravimetric
capacity. The black line in each plot represents the volumetric density of pure compressed H2 at 25 °C.
necessity is quite challenging, owing to the relative difficulty of
containing a compressed gas versus a liquid fuel. Further, the
complexity of synthesis and high precursor expenses for many
metal−organic frameworks can render them costly to prepare,
limiting their industrial application in gas storage, gas
separations, and catalysis. Zeolites currently used in such
applications are generally less expensive based on their
aluminosilicate composition, although a recent report shows
that alternative synthetic routes for MOFs can significantly
reduce their cost, making some competitive with zeolites.50
Furthermore, among MOFs, the M2(m-dobdc) series of
materials is particularly poised as a low-cost adsorbent with
useful gas adsorption properties. The cost of the H4(m-dobdc)
linker is low, as it can be formed in a one-step reaction from
cheaply available resorcinol, potassium bicarbonate, and CO2,
with no solvent needed other than water during isolation of the
product. The overall cost of M2(m-dobdc) itself is thus largely
dependent on the metal salt but can be as low as ∼$3/kg for
raw materials for the Mg2(m-dobdc) analogue. Such economic
considerations are paramount to the successful deployment of
MOFs in gas storage applications.
MOF-5 have been reported previously.20,37,39,43 In this study,
the high-pressure H2 adsorption isotherms of these 5 materials
were measured between 0 and 100 bar at temperatures of −75,
−50, −40, −25, 0, 25, 50, 75, and 100 °C. Increments of 25 °C
were chosen to provide a wide range of conditions for
considering temperature swings when determining the
volumetric usable capacity of these materials; −40 °C was
also measured because it is the temperature at which hydrogen
is stored at and dispensed from fueling stations.51 These
isotherms for Co2(m-dobdc), Ni2(m-dobdc), Co2(dobdc), and
Ni2(dobdc) materials are shown in Figure 2 and for MOF-5 in
Figure S1.
Among the five measured materials, Ni2(m-dobdc) exhibits
the highest adsorption capacities at all temperatures and
pressures, and all isotherms in this material at 75 °C and below
exhibit a H2 capacity higher than that of pure compressed H2
at 25 °C. At 25 °C and 100 bar, Ni2(m-dobdc) takes up 11.9 g
of H2 per L of crystal, which is the highest among the MOFs
measured in this study and, to our knowledge, the highest for
any known adsorbent. The usable capacity under these
conditions is slightly reduced to 11.0 g/L, however, due to
the uptake of 0.9 g/L at 5 bar. This still outperforms
compressed hydrogen, which would require compression to
over 150 bar to obtain the same total volumetric usable
High-Pressure H2 Adsorption Isotherms. Structural
characterization and low-pressure H2 adsorption isotherms of
Co2(m-dobdc), Ni2(m-dobdc), Co2(dobdc), Ni2(dobdc), and
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capacity at 25 °C. At 100 bar and the lowest measured
temperature of −75 °C, Ni2(m-dobdc) takes up a total of 23.8
g/L H2, corresponding to a total usable capacity of 19.0 g/L.
Notably, H2 adsorption data collected at 75.6 K exhibit a total
capacity of 57.3 g/L at 105 bar (Figure S2), a value that
exceeds the DOE system capacity target, albeit at cryogenic
temperatures. Furthermore, data collected at 100 K show
capacities at the DOE system capacity targets at 100 bar, which
is notable given that measured sample densities were used in
calculating the capacity (Figure S3). It is important to note for
all of these capacities for Ni2(m-dobdc) and the other materials
discussed later that the targets are whole system targets using a
material’s actual density, while the data presented here is for
crystallographic density (except the 100 K isotherm in Figure
S3) and simply the material capacity and not the whole system
capacity, which is estimated to require 1.2−2 times the target
capacities, depending on the material and system design.52
However, a recent report outlined the synthesis of a high-
density HKUST-1 monolith with improved CH4 storage
capacity relative to that of the bulk material; such a strategy
could potentially be applied to Ni2(m-dobdc) as well to retain
H2 storage capacity in a real system.53
density at its open metal coordination sites compared to the
other materials. Volumetrically, MOF-5 is inferior to the
M2(m-dobdc) and M2(dobdc) adsorbents due to a lack of
strong adsorption sites within its pores. While cycling
experiments were not completed, we would expect the
hydrogen storage capacity to be retained in all of these
materials over many cycles, as seen previously in MOF-5.54
It is important to understand the benefits that an adsorbent
can offer over compression of pure H2. To that end, a
comparison of volumetric H2 storage capacities at all of the
measured temperatures shows that Ni2(m-dobdc) imparts a
clear enhancement in capacity relative to the compressed gas
(Figure 3). Furthermore, this advantage increases substantially
If it is possible to use a temperature swing in a storage
system through application of active cooling at high filling
levels, the usable capacities attained with Ni2(m-dobdc) are
even higher. For example, adsorption at −40 °C with
desorption at 25 °C affords a usable capacity of 18.2 g/L.
An even more extreme temperature swing from adsorption at
−75 °C to desorption at 25 °C gives a usable capacity of 23.0
g/L. This enhanced usable capacity represents 77% of the
DOE system target of 30 g/L, which is the highest H2
volumetric usable capacity achieved to date for an adsorbent
operating in this temperature range. It is relevant to note that
increasing the desorption temperature to 100 °C offers only an
additional 0.4 g/L of usable capacity over desorption at 25 °C,
which is not likely to be worthwhile given the additional
system complexity required to heat the MOF above ambient
temperature.
Figure 3. Comparison of the total volumetric capacities of Ni2(m-
dobdc) (green circles) and pure compressed H2 (blue squares), both
at 100 bar. Decreasing temperature leads to an increase in the
advantage Ni2(m-dobdc) has over pure H2 in terms of total
volumetric capacity.
The related MOFs Co2(m-dobdc), Co2(dobdc), and
Ni2(dobdc) were also evaluated for their H2 storage perform-
ance under various temperature swings, and the results are
summarized in Table 2. As the best known adsorbent for
with decreasing temperature. Even at 100 °C, the volumetric
H2 capacity of a crystal of Ni2(m-dobdc) is 121% of the
capacity of pure H2. This advantage increases to 155% at 25 °C
and 209% at −75 °C, highlighting the utility of Ni2(m-dobdc)
for increasing the density of hydrogen in a storage cylinder
filled at 100 bar.
Table 2. Comparison of the Volumetric Usable Capacities in
g/L for Selected Temperature Swings
Temperature-Programmed Desorption of H2. Phys-
isorptive storage of H2 (such as in MOFs) has the advantage
over chemisorptive storage (such as in metal hydrides) in that
the gas is accessible without large energy inputs. As an
illustration of this accessibility and the stronger binding in the
M2(m-dobdc) series, we carried out temperature-programmed
desorption (TPD) experiments on samples of Ni2(m-dobdc)
and Ni2(dobdc) loaded with H2.
The results of the TPD measurements indicate that Ni2(m-
dobdc) binds H2 more strongly, given the shift in the
desorption profile of H2 as compared with Ni2(dobdc) (Figure
4). These desorption peaks, centered at −165 and −175 °C for
Ni2(m-dobdc) and Ni2(dobdc), respectively, appear to indicate
that both materials polarize H2 strongly enough that it desorbs
above liquid nitrogen temperature (−198 °C at the NREL
altitude). Empirical differences in desorption temperature
between materials typically arise due to differences in pore
shape or size, which impact the diffusion of hydrogen through
Co2(m-
Ni2(m-
dobdc)
dobdc) Co2(dobdc) Ni2(dobdc) MOF-5
25 °C, no
10.5
11.0
8.8
9.9
8.8
swing
−75 °C, no
18.2
19.0
16.5
18.4
15.8
swing
−40 to 25 °C
−75 to 25 °C
17.3
21.9
22.3
18.2
23.0
23.4
14.0
18.3
18.6
16.6
21.4
21.8
12.8
16.5
16.7
−75 to
100 °C
cryogenic hydrogen storage, MOF-5 was also measured for
comparison (Figure S1), and the data agree well with a
previous measurement performed at 25 °C.43 From the results
in Table 2, Ni2(m-dobdc) is clearly the top-performing
material for all of the considered temperature swings. This
superiority arises from it having the highest capacity under all
conditions, which is a consequence of the greater charge
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Figure 4. Temperature-programmed desorption of H2 in Ni2(m-
dobdc) and Ni2(dobdc). Note the difference in desorption temper-
ature between Ni2(m-dobdc) and Ni2(dobdc).
Figure 5. A single pore of Co2(m-dobdc) showing the seven distinct
D2 binding sites as determined from neutron diffraction data. Purple,
red, gray, white, and yellow spheres represent Co, O, C, and H atoms
and D2 molecules, respectively.
the pores. However, due to the similar pore shapes and sizes
exhibited by these two MOFs, the higher desorption
temperature for Ni2(m-dobdc) is indicative of a stronger H2
binding at the open Ni2+ sites.
occupancy. Importantly, a comparison of the adsorption
isotherm data collected at 198 K and the D2 loadings observed
by powder neutron diffraction at the same temperature reveal a
quantitative agreement between the two methods for
measuring storage capacity (Figure S4).
Notably, the D2···D2 distances (Table 3) measured for
certain sites within the pores of Co2(m-dobdc) are very short.
In Situ Powder Neutron Diffraction. Powder neutron
diffraction experiments were undertaken at high pressures to
further understand hydrogen adsorption in the M2(m-dobdc)
frameworks. The measurements were performed on Co2(m-
dobdc), as its greater degree of crystallinity allowed for
structure solutions of the D2-dosed samples and the refinement
of the D2 adsorption positions within the pores. While not a
direct measure of the performance of Ni2(m-dobdc), the
similar structure and adsorption behavior of Co2(m-dobdc)
should provide a representative example of the Ni2(m-dobdc)
material. Additionally, D2 and H2 have previously been shown
to behave nearly identically in powder neutron diffraction
experiments.39 Samples were measured at 198 K at pressures of
36, 54, and 79 bar, as well as at 77 K at a pressure of 78 bar to
most closely simulate the adsorption isotherm conditions while
retaining the ability to crystallographically locate each D2
binding site within the pores.
Table 3. Selected D2···D2 Distances within Co2(m-dobdc) as
Determined from Powder Neutron Diffraction Collected at
a
77 K and 78 bar
D2···D2 interaction
distance (Å)
1···2
2···2
3···4
4···5
2.86(3)
3.08(3)
3.12(5)
3.41(3)
3.21
55
solid H2
a
Numbers in parentheses indicate one standard deviation in the value.
At 77 K, the sample of Co2(m-dobdc) loaded with D2 at 78
bar revealed 7 distinct adsorption sites (Figure 5). At site 1, the
strongest adsorption site, the D2 is bound to the open Co2+
coordination site with a Co···D2(centroid) separation of
2.25(7) Å. The D2 at site 2 is directly adjacent, interacting
with both the D2 bound at site 1 as well as ligand O atoms
from a hydroxide and a carboxylate. Site 3 occupies a position
above the center of the aromatic ring of the m-dobdc4− linker,
while site 4 lies adjacent to this. These first four adsorption
sites were previously observed in neutron diffraction experi-
ments carried out on Co2(m-dobdc) at 4 K and pressures
below 1 bar.39 Adsorption sites 5−7, which become occupied
only at the higher D2 pressures measured here, could likely
have been located in the previous study if higher dosings were
used. Sites 5 and 6 lie at the center of the hexagonal channels
of the framework, while site 7 resides 3.10(3) Å from the D2
located at site 5 and primarily relies on D2···D2 interactions for
stabilization. At 77 K and 78 bar, sites 1−6 show full
occupancy of D2, and site 7 shows approximately half
For example, the distance between the D2 molecules at sites 1
and 2 is only 2.86(3) Å. This is significantly shorter than the
H2···H2 separation of 3.21 Å in solid hydrogen,55 and is
approaching the H2···H2 distance of 2.656 Å in solid H2
pressurized to 54 kbar at 300 K.56 These comparisons to
solid hydrogen powerfully illustrate the ability of materials in
the M2(m-dobdc) series to densify hydrogen within their
pores. Other notably short D2···D2 distances within Co2(m-
dobdc) can be seen in Table 3, further illustrating this
principle.57 Significantly, the high charge density on the metals
not only strongly polarizes D2 bound at the coordinatively
unsaturated Co2+ center, but additionally impacts D2 bound in
more weakly physisorbing secondary sites as well, leading to a
high hydrogen packing density within the adsorbent.
In Situ Infrared Spectroscopy. High-pressure H2-dosed
in situ infrared spectroscopy was used to further understand H2
loading in Ni2(m-dobdc). Spectra were collected in the
pressure range 10−90 bar at multiple temperatures ranging
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from 198 K (Figure 6) to 298 K (Figures S11−S19). Adsorbed
H2 in MOFs has been shown to exhibit a vibrational frequency
Figure 7. Comparison of infrared spectra with approximately constant
adsorption of H2 in Ni2(m-dobdc) based on total peak area for each
spectrum. Note the change in relative peak areas from approximately
equal loading of open Ni2+ sites (∼4035 cm−1) and other sites
(∼4125 cm−1) at 273 K and 70 bar to a much higher concentration of
H2 bound to the open Ni2+ sites at 198 K and 10 bar.
Figure 6. In situ H2-dosed infrared spectroscopy of Ni2(m-dobdc) at
198 K with H2 pressure between 10 and 90 bar. Note that the spectra
have been offset for clarity. The peak on the right corresponds to H2
bound to the open Ni2+ site, and the peak on the left corresponds to
H2 bound at secondary sites within the pores.
that is lower than that of free gaseous H2 (4161 cm−1) and
generally correlates with the H2 binding energy at a given
site.58 In Figure 6, the peak at ∼4035 cm−1 corresponds to H2
bound to the open Ni2+ sites in the framework, while the peak
at ∼4125 cm−1 corresponds to H2 adsorbed at more weakly
interacting secondary sites within the pores. At lower pressures,
the peak area of the Ni2+-bound H2 is significantly larger than
that of the peak area at the secondary sites, indicating a
substantially higher H2 binding enthalpy.
As the gas pressure is increased, the area of the secondary
site peak grows with the corresponding increase in adsorbed
H2 within the pores. A commensurate increase is not seen for
the Ni2+-bound H2, as saturation of these sites prior to the
occupation of secondary sites is likely. A comparison of the
peak areas calculated from these spectra, which should be
proportional to the H2 loading, shows good agreement with
the isotherm data when a single linear scaling factor (used to
compare absolute adsorption from isotherms to the relative
adsorption determined by infrared spectroscopy) is applied to
the peaks areas at each temperature (Figures S11−S19),
especially at pressures below 60 bar. The small standard
deviations for the observed scaling factors (<0.8 for all
temperatures and <0.3 for 198 and 233 K) support the validity
of this method (Table S6).
Figure 7 displays infrared spectra collected for Ni2(m-
dobdc) at approximately equivalent H2 loadings at various
temperatures and pressures. The results illustrate how the
loading of each of the two types of adsorption sites (Ni2+
centers at 4035 cm−1 and more weakly physisorbing sites at
4125 cm−1) changes as a function of temperature. At 273 K
and 70 bar, the area under the peaks for each binding site are
approximately equal, indicating an even distribution of bound
H2 between the open Ni2+ sites and other sites within the
pores. As the temperature is decreased, the pressure drops as
more H2 adsorbs in the material, and the peak at 4035 cm−1
begins to grow while the peak at 4125 cm−1 shrinks, indicating
a shift toward more adsorption at the open Ni2+ sites. At 198 K
and 10 bar, most of the adsorbed H2 is bound to the open
metal sites. This confirmation of the temperature dependence
of the binding site population, while expected, is quite
interesting and illustrates the importance of operating
conditions when considering the use of an adsorbent in a
hydrogen storage system.
CONCLUSIONS
■
Selected high-performance metal−organic frameworks were
evaluated for their H2 adsorption properties under conditions
relevant to on-board storage in motor vehicles. Adsorption
isotherms in the pressure range of 0−100 bar were measured
for the materials Co2(m-dobdc), Ni2(m-dobdc), Co2(dobdc),
and Ni2(dobdc), which contain a high density of coordina-
tively unsaturated metal sites, as well as for MOF-5, which
does not. Ni2(m-dobdc) is the top-performing material with
respect to the critical metric of usable volumetric H2 capacity
at pressures between 5 and 100 bar and near-ambient
temperatures. To our knowledge, this compound displays the
highest physisorptive hydrogen storage capacity of any known
adsorbent under these conditions. Its high capacity is
attributable to the presence of highly polarizing Ni2+
adsorption sites, which lead to large binding enthalpies and a
dense packing of H2 within the material. This conclusion is
supported by the results of temperature-programmed desorp-
tion, in situ powder neutron diffraction, and in situ infrared
spectroscopy experiments performed under relevant condi-
tions. The results provide benchmark data for comparison with
future generations of adsorbents designed for hydrogen
storage. In particular, efforts are underway to create new
metal−organic frameworks with low-coordinate metal cations
capable of binding multiple H2 molecules at enthalpies in the
optimal range of −15 to −20 kJ/mol. Lastly, this study
highlights the importance of adsorption conditions in the
evaluation of materials and the superior performance of
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metal−organic frameworks containing open metal coordina-
warranty, expressed or implied, or assumes any legal liability
or responsibility for the accuracy, completeness, or usefulness
of any information, apparatus, product, or process disclosed, or
represents that its use would not infringe privately owned
rights.
tion sites for physisorptive H2 storage.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.8b03276.
REFERENCES
■
(1) Hoffmann, P. Tomorrow’s Energy: Hydrogen, Fuel Cells, and the
Prospects for a Cleaner Planet; The MIT Press: Cambridge, MA, 2012.
(2) Alternative Fuels Data Center. http://www.afdc.energy.gov/
fuels/hydrogen_locations.html (accessed April 23, 2017).
Additional experimental data, H2 adsorption isotherms,
powder neutron diffraction structures, H2-dosed infrared
spectroscopy data, and other data (PDF)
(3) AB-8 Alternative Fuel and Vehicle Technologies: Funding
Programs. http://leginfo.legislature.ca.gov/faces/billNavClient.
xhtml?bill_id=201320140AB8 (accessed April 25, 2017).
(4) Fuel Cell Technologies Market Report 2015. https://energy.
gov/sites/prod/files/2016/10/f33/fcto_2015_market_report.pdf
(accessed April 25, 2017).
Crystallographic information files (ZIP)
AUTHOR INFORMATION
Corresponding Author
*E-mail: jrlong@berkeley.edu.
■
(5) H2USA. http://www.h2usa.org/ (accessed April 25, 2017).
International Partnership for Hydrogen and Fuel Cells in the
Economy. http://www.iphe.net/ (accessed April 25, 2017). California
Fuel Cell Partnership. http://cafcp.org/ (accessed April 25, 2017).
Low Carbon Vehicle Partnership. http://www.lowcvp.org.uk/ (ac-
cessed April 25, 2017). U.S. Drive Partnership. https://www.
hydrogen.energy.gov/usdrive_partnership.html (accessed April 25,
2017). Scandinavian Hydrogen Highway Partnership. http://www.
scandinavianhydrogen.org/shhp/vehicles/ (accessed April 25, 2017).
ORCID
Stephen A. FitzGerald: 0000-0001-9492-9256
Jeffrey R. Long: 0000-0002-5324-1321
Author Contributions
M.T.K. and J.R.L. formulated the project. M.T.K. synthesized
the compounds. M.T.K., K.E.H., P.A.P., and T.G. collected and
analyzed isotherm data. K.E.H. and T.G. collected temper-
ature-programmed desorption data. T.R., J.D.T., A.A., and
C.M.B. collected powder neutron diffraction data and solved
crystal structures. T.R., H.Z.H.J., and S.A.F. collected and
analyzed infrared spectra. M.T.K. and J.R.L. wrote the paper,
and all authors contributed to revising it. All authors have
given approval to the final version of the manuscript.
(6) Schlapbach, L.; Zuttel, A. Hydrogen-storage materials for mobile
̈
applications. Nature 2001, 414, 353−358.
(7) Midilli, A.; Ay, M.; Dincer, I.; Rosen, M. A. On hydrogen and
hydrogen energy strategies: I: current status and needs. Renewable
Sustainable Energy Rev. 2005, 9, 255−271.
(8) Mazloomi, K.; Gomes, C. Hydrogen as an energy carrier:
prospects and challenges. Renewable Sustainable Energy Rev. 2012, 16,
3024−3033.
Notes
The authors declare the following competing financial
interest(s): J.R.L. has a financial interest in Mosaic Materials,
Inc., a startup company working to commercialize metal-
organic frameworks, including the M2(m-dobdc) materials.
The University of California, Berkeley has applied for a patent
on some of the materials discussed herein, on which J.R.L. and
M.T.K. are listed as inventors.
(9) Arnold, G.; Eberle, U.; Hasenauer, D. Apparatus for optimal
adsorption and desorption of gases utilizing highly porous gas storage
materials. US Patent US7517396 B2, 2009.
(10) Zu
24−33.
̈
ttel, A. Materials for hydrogen storage. Mater. Today 2003, 6,
(11) H35 (35 MPa/350 bar) vs. H70 (70 MPa/700 bar) Fueling
Mirai Refueling Tips. https://ssl.toyota.com/mirai/Mirai_Fueling.pdf
(accessed January 15, 2017).
ACKNOWLEDGMENTS
(12) Jepsen, L. H.; Ley, M. B.; Lee, Y.-S.; Cho, Y. W.; Dornheim,
M.; Jensen, J. O.; Filinchuk, Y.; Jørgensen, J. E.; Besenbacher, F.;
Jensen, T. R. Boron-nitrogen based hydrides and reactive composites
for hydrogen storage. Mater. Today 2014, 17, 129−135.
(13) Zhu, Q.-L.; Xu, Q. Liquid organic and inorganic chemical
hydrides for high-capacity hydrogen storage. Energy Environ. Sci. 2015,
8, 478−512.
■
This work was fully supported by the United States
Department of Energy (USDOE), Office of Energy Efficiency
and Renewable Energy (EERE), Fuel Cell Technologies Office
(FCTO), under Contract DE-AC02-05CH11231. We thank
the NSF for providing graduate fellowship support for M.T.K.,
the US Department of Energy, Office of Energy Efficiency and
Renewable Energy, Fuel Cell Technologies Office, for support
of J.D.T. under Contract DE-AC36-08GO28308, and the NSF
for support of S.A.F. through Grant CHE-1111896. We also
thank Rodolfo Torres-Gavosto for the improved H2(m-dobdc)
synthesis; Q. Barker-Plummer, D. Mukasa, J. Parker, and E.
Rappeport for experimental assistance; and Dr. Katie R.
Meihaus for editorial assistance. Certain commercial equip-
ment, instruments, or materials are identified in this document.
Such identification does not imply recommendation or
endorsement by the National Institute of Standards and
Technology nor does it imply that the products identified are
necessarily the best available for the purpose. The views and
opinions of the authors expressed herein do not necessarily
state or reflect those of the United States Government or any
agency thereof. Neither the United States Government nor any
agency thereof, nor any of their employees, makes any
(14) He, T.; Pachfule, P.; Wu, H.; Xu, Q.; Chen, P. Hydrogen
carriers. Nat. Rev. Mater. 2016, 1, 16059.
(15) Rusman, N. A. A.; Dahari, M. A review on the current progress
of metal hydrides material for solid-state hydrogen storage
applications. Int. J. Hydrogen Energy 2016, 41, 12108−12126.
(16) Dundar-Tekkaya, E.; Yurum, Y. Mesoporous MCM-41 material
̈ ̈ ̈
for hydrogen storage: A short review. Int. J. Hydrogen Energy 2016, 41,
9789−9795.
(17) Blankenship, T. S.; Mokaya, R. Cigarette butt-derived carbons
have ultra-high surface area and unprecedented hydrogen capacity.
Energy Environ. Sci. 2017, 10, 2552−2562.
(18) Ahluwalia, R. K.; Peng, J. K. Automotive hydrogen storage
system using cryo-adsorption on activated carbon. Int. J. Hydrogen
Energy 2009, 34, 5476−5487.
́
(19) Petitpas, G.; Benard, P.; Klebanoff, L. E.; Xiao, J.; Aceves, S. A
comparative analysis of the cryo-compression and cryo-adsorption
hydrogen storage methods. Int. J. Hydrogen Energy 2014, 39, 10564−
10584.
I
DOI: 10.1021/acs.chemmater.8b03276
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
(20) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.;
O’Keeffe, M.; Yaghi, O. M. Hydrogen Storage in Microporous Metal-
Organic Frameworks. Science 2003, 300, 1127.
(39) Kapelewski, M. T.; Geier, S. J.; Hudson, M. R.; Stuck, D.;
̈
Mason, J. A.; Nelson, J. N.; Xiao, D. J.; Hulvey, Z.; Gilmour, E.;
FitzGerald, S. A.; Head-Gordon, M.; Brown, C. M.; Long, J. R. M2(m-
dobdc) (M = Mg, Mn, Fe, Co, Ni) Metal-Organic Frameworks
Exhibiting Increased Charge Density and Enhanced H2 Binding at the
Open Metal Sites. J. Am. Chem. Soc. 2014, 136, 12119−12129.
(40) Denysenko, D.; Grzywa, M.; Jelic, J.; Reuter, K.; Volkmer, D.
Scorpionate-Type Coordination in MFU-4l Metal-Organic Frame-
works: Small-Molecule Binding and Activation upon the Thermally
Activated Formation of Open Metal Sites. Angew. Chem., Int. Ed.
2014, 53, 5832−5836.
(21) Farha, O. K.; Eryazici, I.; Jeong, N. C.; Hauser, B. G.; Wilmer,
C. E.; Sarjeant, A. A.; Snurr, R. Q.; Nguyen, S. T.; Yazaydın, A. O.;
Hupp, J. T. Metal-Organic Framework Materials with Ultrahigh
Surface Areas: Is the Sky the Limit? J. Am. Chem. Soc. 2012, 134,
15016−15021.
(22) Krause, S.; Bon, V.; Senkovska, I.; Stoeck, U.; Wallacher, D.;
̈
Tobbens, D. M.; Zander, S.; Pillai, R. S.; Maurin, G.; Coudert, F.-X.;
Kaskel, S. A pressure-amplifying framework material with negative gas
adsorption transitions. Nature 2016, 532, 348−352.
(41) Weinrauch, I.; Savchenko, I.; Denysenko, D.; Souliou, S. M.;
Kim, H.-H.; Le Tacon, M.; Daemen, L. L.; Cheng, Y.; Mavrandonakis,
̈
A.; Ramirez-Cuesta, A. J.; Volkmer, D.; Schutz, G.; Hirscher, M.;
(23) Yuan, S.; Chen, Y.-P.; Qin, J.-S.; Lu, W.; Zou, L.; Zhang, Q.;
Wang, X.; Sun, X.; Zhou, H.-C. Linker Installation: Engineering Pore
Environment with Precisely Placed Functionalities in Zirconium
MOFs. J. Am. Chem. Soc. 2016, 138, 8912−8919.
Heine, T. Capture of heavy hydrogen isotopes in a metal-organic
framework with active Cu(I) sites. Nat. Commun. 2017, 8, 14496.
(42) Mason, J. A.; Veenstra, M.; Long, J. R. Evaluating metal-organic
frameworks for natural gas storage. Chem. Sci. 2014, 5, 32−51.
(43) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Long, J. R. Impact of
Preparation and Handling on the Hydrogen Storage Properties of
Zn4O(1,4-benzenedicarboxylate)3 (MOF-5). J. Am. Chem. Soc. 2007,
129, 14176−14177.
(44) Hurst, K. E.; Heben, M. J.; Blackburn, J. L.; Gennett, T.; Dillon,
A. C.; Parilla, P. A. A dynamic calibration technique for temperature
programmed desorption spectroscopy. Rev. Sci. Instrum. 2013, 84,
025103.
(45) FitzGerald, S. A.; Churchill, H. O. H.; Korngut, P. M.;
Simmons, C. B.; Strangas, Y. E. Cryogenic apparatus for diffuse
reflection infrared spectroscopy with high-pressure capabilities. Rev.
Sci. Instrum. 2006, 77, 093110.
(46) Goldsmith, J.; Wong-Foy, A. G.; Cafarella, M. J.; Siegel, D. J.
Theoretical Limits of Hydrogen Storage in Metal-Organic Frame-
works: Opportunities and Trade-Offs. Chem. Mater. 2013, 25, 3373−
3382.
̆
(24) Murray, L. J.; Dinca, M.; Long, J. R. Hydrogen storage in metal-
organic frameworks. Chem. Soc. Rev. 2009, 38, 1294−1314.
(25) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Hydrogen
Storage in Metal-Organic Frameworks. Chem. Rev. 2012, 112, 782−
835.
(26) Basdogan, Y.; Keskin, S. Simulation and modelling of MOFs for
hydrogen storage. CrystEngComm 2015, 17, 261−275.
́
́
́
(27) Gomez-Gualdron, D. A.; Colon, Y. J.; Zhang, X.; Wang, T. C.;
Chen, Y.-S.; Hupp, J. T.; Yildirim, T.; Farha, O. K.; Zhang, J.; Snurr,
R. Q. Evaluating topologically diverse metal-organic frameworks for
cry-adsorbed hydrogen storage. Energy Environ. Sci. 2016, 9, 3279−
3289.
(28) Rahali, S.; Belhocine, Y.; Seydou, M.; Maurel, F.; Tangour, B.
Multiscale study of the structure and hydrogen storage capacity of an
aluminium metal-organic framework. Int. J. Hydrogen Energy 2017, 42,
15271−15282.
(29) Han, S. S.; Goddard, W. A., III Lithium-Doped Metal-Organic
Frameworks for Reversible H2 Storage at Ambient Temperature. J.
Am. Chem. Soc. 2007, 129, 8422−8423.
(30) Rao, D.; Lu, R.; Xiao, C.; Kan, E.; Deng, K. Lithium-doped
MOF impregnated with lithium-coated fullerenes: A hydrogen storage
route for high gravimetric and volumetric uptakes at ambient
temperatures. Chem. Commun. 2011, 47, 7698−7700.
(31) Meng, Z.; Lu, R.; Rao, D.; Kan, E.; Xiao, C.; Deng, K.
Catenated metal-organic frameworks: Promising hydrogen purifica-
tion materials and high hydrogen storage medium with further lithium
doping. Int. J. Hydrogen Energy 2013, 38, 9811−9818.
(32) Bhatia, S. K.; Myers, A. L. Optimum Conditions for Adsorptive
Storage. Langmuir 2006, 22, 1688−1700.
́
́
(47) Gomez-Gualdron, D. A.; Wang, T. C.; García-Holley, P.;
Sawelewa, R. M.; Argueta, E.; Snurr, R. Q.; Hupp, J. T.; Yildirim, T.;
Farha, O. K. Understanding Volumetric and Gravimetric Hydrogen
Adsorption Trade-off in Metal-Organic Frameworks. ACS Appl.
Mater. Interfaces 2017, 9, 33419−33428.
(48) Parilla, P. A.; O'Neil, K. J.; Hurst, K.; Gennett, T. An
international multi-laboratory investigation of carbon-based hydrogen
sorbent materials. Appl. Phys. A: Mater. Sci. Process. 2016, 122, 42.
(49) Mason, J. A.; Oktawiec, J.; Taylor, M. K.; Hudson, M. R.;
Rodriguez, J.; Bachman, J. E.; Gonzalez, M. I.; Cervellino, A.;
Guagliardi, A.; Brown, C. M.; Llewellyn, P. L.; Masciocchi, N.; Long,
J. R. Methane storage in flexible metal-organic frameworks with
intrinsic thermal management. Nature 2015, 527, 357−361.
(50) DeSantis, D.; Mason, J. A.; James, B. D.; Houchins, C.; Long, J.
R.; Veenstra, M. Techno-Economic Analysis of Metal-Organic
Frameworks for Hydrogen and Natural Gas Storage. Energy Fuels
2017, 31, 2024−2032.
(51) Parks, G.; Boyd, R.; Cornish, J.; Remick, R.; Popovich, N.
Hydrogen Station Compression, Storage, and Dispensing Technical Status
and Costs; NREL: Golden, CO, 2014 http://www.nrel.gov/docs/
fy14osti/58564.pdf.
(52) Satyapal, S.; Petrovic, J.; Read, C.; Thomas, G.; Ordaz, G. The
U.S. Department of Energy’s National Hydrogen Storage Project:
Progress towards meeting hydrogen-powered vehicle requirements.
Catal. Today 2007, 120, 246−256.
(33) DOE. Technical Targets for Onboard Hydrogen Storage for
Light-Duty Vehicles, https://energy.gov/eere/fuelcells/doe-technical-
targets-onboard-hydrogen-storage-light-duty-vehicles (accessed April
25, 2017).
̆
(34) Dinca, M.; Long, J. R. Hydrogen Storage in Microporous
Metal-Organic Frameworks with Exposed Metal Sites. Angew. Chem.,
Int. Ed. 2008, 47, 6766−6779.
̆
(35) Dinca, M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.;
Long, J. R. Hydrogen Storage in a Microporous Metal-Organic
Framework with Exposed Mn2+ Coordination Sites. J. Am. Chem. Soc.
2006, 128, 16876−16883.
(36) Vitillo, J. G.; Regli, L.; Chavan, S.; Ricchiardi, G.; Spoto, G.;
Dietzel, P. D. C.; Bordiga, S.; Zecchina, A. Role of Exposed Metal
Sites in Hydrogen Storage in MOFs. J. Am. Chem. Soc. 2008, 130,
8386−8396.
(53) Tian, T.; Zeng, Z.; Vulpe, D.; Casco, M. E.; Divitini, G.;
Midgley, P. A.; Silvestre-Albero, J.; Tan, J.-C.; Moghadam, P. Z.;
Fairen-Jimenez, D. A sol-gel monolithic metal-organic frameworks
with enhanced methane uptake. Nat. Mater. 2017, 17, 174−179.
(54) Ming, Y.; Purewal, J.; Yang, J.; Xu, C.; Veenstra, M.; Gaab, M.;
(37) Zhou, W.; Wu, H.; Yildirim, T. Enhanced H2 Adsorption in
Isostructural Metal-Organic Frameworks with Open Metal Sites:
Strong Dependence of the Binding Strength on Metal Ions. J. Am.
Chem. Soc. 2008, 130, 15268−15269.
(38) Oh, H.; Savchenko, I.; Mavrandonakis, A.; Heine, T.; Hirscher,
M. Highly Effective Hydrogen Isotope Separations in Nanoporous
Metal-Organic Frameworks with Open Metal Sites: Direct Measure-
ment and Theoretical Analysis. ACS Nano 2014, 8, 761−770.
Muller, U.; Siegel, D. J. Stability of MOF-5 in a hydrogen gas
̈
environment containing fueling station impurities. Int. J. Hydrogen
Energy 2016, 41, 9374−9382.
(55) van Kranendonk, J.; Gush, H. P. The crystal structure of solid
hydrogen. Phys. Lett. 1962, 1, 22−23.
J
DOI: 10.1021/acs.chemmater.8b03276
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
(56) Mao, H. K.; Jephcoat, A. P.; Hemley, R. J.; Finger, L. W.; Zha,
C. S.; Hazen, R. M.; Cox, D. E. Synchotron X-ray Diffraction
Measurements of Single-Crystal Hydrogen to 26.5 Gigapascals.
Science 1988, 239, 1131−1134.
(57) Liu, Y.; Kabbour, H.; Brown, C. M.; Neumann, D. A.; Ahn, C.
C. Increasing the Density of Adsorbed Hydrogen with Coordinatively
Unsaturated Metal Centers in Metal-Organic Frameworks. Langmuir
2008, 24, 4772−4777.
(58) FitzGerald, S. A.; Burkholder, B.; Friedman, M.; Hopkins, J. B.;
Pierce, C. J.; Schloss, J. M.; Thompson, B.; Rowsell, J. L. C. Metal-
Specific Interactions of H2 Adsorbed within Isostructural Metal-
Organic Frameworks. J. Am. Chem. Soc. 2011, 133, 20310−20318.
K
DOI: 10.1021/acs.chemmater.8b03276
Chem. Mater. XXXX, XXX, XXX−XXX