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Dynamic Article LinksC<Journal ofMaterials Chemistry
Cite this: DOI: 10.1039/c1jm13314d
www.rsc.org/materials PAPER
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Preparation and hydrogen storage capacity of templated and activatedcarbons nanocast from commercially available zeolitic imidazolateframework†
A. Almasoudi and R. Mokaya*
Received 14th July 2011, Accepted 29th September 2011
DOI: 10.1039/c1jm13314d
A commercially available zeolitic imidazolate framework (ZIF), namely Basolite Z1200 (BASF), has
been used as template for nanocasting of highly microporous ZIF-templated carbon. The ‘‘hard
template carbonization technique’’ consists of liquid impregnation of furfuryl alcohol into the pores of
the ZIF followed by polymerization and then carbonization during which the ZIF template is removed
to generate the microporous carbon (90–95% microporosity) with a surface area of 900–1100 m2 g�1
and a pore volume of ca. 0.7 cm3 g�1. Chemical activation (with KOH at a relatively low temperature of
700 �C for 1 h and a carbon/KOH weight ratio of 1 : 4) of the ZIF-templated carbons increases their
porosity by between 30 and 240% depending on their carbonization temperature, to achieve a surface
area of up to 3200 m2 g�1 and a pore volume of 1.94 cm3 g�1. Despite the drastic increase in porosity, the
activated ZIF-templated carbons retain significant microporosity with micropores contributing 80–
90% of surface area and 60–70% of pore volume. This occurs because the activation process mainly
enhances existing porosity rather than creating new larger pores. The activation enhances the hydrogen
uptake capacity of the ZIF-templated carbons by between 25 and 140% from 2.6–3.1 wt% to the range
3.9–6.2 wt% (at�196 �C and 20 bar). The increase in hydrogen uptake after activation is closely related
to rises in the micropore surface area and micropore volume rather than overall increase in porosity.
Due to their microporous nature, the carbons exhibit high hydrogen storage density in the range 13.0–
15.5 mmol H2 m�2, which is much higher than that of most high surface area activated carbons.
1 Introduction
Porous carbon materials are widely used in industry due to their
hydrophobic nature, high surface area, good thermal and
mechanical stability, chemical inertness and high physisorption
capacity. This last property is useful in addressing one of the
main current challenges in energy research, i.e., hydrogen
storage.1 This is due to the fact that hydrogen physisorbed on
porous carbon can be released reversibly. The physisorption of
hydrogen on porous solid state materials, including metal–
organic frameworks,2,3 zeolites,4 templated carbons5–7 or
School of Chemistry, University of Nottingham, University Park,Nottingham, NG7 2RD, UK. E-mail: [email protected]; Fax:+44 (0)115 9513562
† Electronic supplementary information (ESI) available: Sevenadditional figures, powder XRD pattern, TGA curve, nitrogen sorptionisotherm and PSD curve of commercially available ZIF (BasoliteZ1200), powder XRD pattern and TGA curve of ZIF/FA composite(after heating at 80 �C for 24 h and then at 150 �C for 6 h under Ar),PSD curves of ZIF templated carbons, nitrogen sorption isotherms ofZIF templated carbons before and after activation, and plot ofhydrogen storage capacity of ZIF templated carbons before and afteractivation as a function of micropore surface area and microporevolume. See DOI: 10.1039/c1jm13314d
This journal is ª The Royal Society of Chemistry 2011
activated carbons8,9 and other forms of carbon nanostructures,10
is currently under intense study. Traditionally, highly porous
carbon materials that are useful for sorption applications such as
hydrogen storage have been prepared via physical (gas) or
chemical activation of suitable carbon precursors.11,12 Recently
a new ‘‘hard template carbonization’’ technique has been devel-
oped that allows a more precise control of the porous structure of
carbons.13,14 The technique consists of the carbonization of an
organic precursor in the nanospace of a template inorganic
material and the liberation of the resultant carbon network via
dissolution of the template. This methodology allows control of
porosity in the resulting templated carbons due to the spatial
regulation imposed by the template nanospace, leading to
materials with narrow pore size distribution, which usually also
exhibit high surface area and pore volume.14 The type of inor-
ganic template used determines the porous structure of the
templated carbon material.14 Structurally well ordered ‘hard’
templates that have so far been used include zeolites, mesoporous
silicates and metal–organic frameworks (MOFs).14–16
Recently, there have been reports of carbonmaterials prepared
via templating (hard or soft) methods followed by activation.17–20
It has been reported that the porosity of templated microporous,
mesoporous or macroporous carbons can be varied beneficially
J. Mater. Chem.
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by physical or chemical activation.17–20 The activation of porous
carbons to enhance properties is not restricted to templated
carbons; other types of moderate to high surface area carbons
such as carbide-derived-carbons can also be activated to greatly
enhance their porosity and energy or hydrogen storage
capacity.21 The aim of the work reported here was therefore first
to synthesise porous materials via a hitherto unexplored tem-
plating route that utilises commercially available MOFs of the
zeolitic imidazolate framework (ZIF) type namely Basolite
Z1200 as a hard template. Secondly, we aimed to further modify
the textural properties of the ZIF-templated carbons via mild
chemical activation with the hope of enhancing textural prop-
erties and hydrogen storage capacity. We have used the
commercially available zeolitic imidazolate framework (ZIF)
that is readily available, namely Basolite Z1200 (BASF) as
a template for nanocasting carbon. The generated carbons were
then subjected to chemical activation with KOH at a relatively
low temperature of 700 �C. The relatively low temperature was
used in an attempt to optimize the pore space of the ZIF-tem-
plated carbons as opposed to creation of totally ‘new’ porosity.
We explore and discuss the hydrogen storage properties of the
ZIF-templated and activated carbons.
2 Experimental
2.1 Material synthesis
The ZIF template, ZIF-8 (Basolite Z1200� Sigma-Aldrich), was
degassed at 200 �C for 3 h to remove any water. The degassed
ZIF template was soaked in furfuryl alcohol (FA), stirred for 1 h
and the resulting mixture was allowed to stand overnight (so that
the FA could fully infiltrate the ZIF-template pores), followed by
filtration and washing with dimethyl formamide to remove any
externally adsorbed FA. The FA/ZIF composite was then
transferred into a quartz boat and placed in a furnace, under
flowing Ar, for 6 h to exclude air and then heated at 80 �C for
24 h and then at 150 �C for 6 h under Ar. Subsequently,
carbonization of the composite was performed under Ar at 900,
1000, 1050, and 1100 �C for 8 h with a heating ramp of 3 �Cmin�1. The resulting samples were labelled BF-T, where T is the
carbonization temperature in �C.For the chemical activation, the BF-T carbon samples were
thoroughly mixed with KOH at a carbon/KOH weight ratio of
1/4. The mixture was then heat treated in a horizontal furnace
under a nitrogen flow at 700 �C for 1 h with a heating ramp rate
of 3 �Cmin�1. The resulting mixture was washed three times with
2 M HCl at room temperature to remove any inorganic salts and
then with distilled water until neutral pH was achieved. Finally,
the resultant activated ZIF-templated carbon was dried in an
oven at 120 �C for 3 h. The activated ZIF-templated carbons
were denoted as ACX, where X is the templated carbon.
2.2 Material characterisation
Powder XRD analysis was performed on a Bruker D8 Advance
powder diffractometer using CuKa radiation (l ¼ 1.5406 �A) and
operating at 40 kV and 40 mA, with 0.02� step size and 2 s step
time. Thermogravimetric analysis was performed using a TA
Instruments SDT Q600 analyzer under flowing gas (air or
nitrogen) conditions. For porosity analysis, each sample was
J. Mater. Chem.
pre-dried in an oven and then degassed overnight at 200 �C under
high vacuum. The textural properties were determined by
nitrogen sorption at �196 �C using a Micromeritics ASAP 2020
volumetric sorptometer. The surface area was calculated by using
the BET method applied to adsorption data in the relative
pressure (P/Po) range of 0.06–0.22. The total pore volume was
determined from the amount of nitrogen adsorbed at P/Po ¼0.99. The pore size distribution was determined by a non-local
density functional theory (NLDFT) method using nitrogen
adsorption isotherms. Scanning electron microscopy (SEM)
images were recorded using a FEI XL30 microscope. The
samples for SEM analysis were prepared by ultrasonic dispersion
of the powder products in ethanol, which were then deposited
and dried on a holey carbon film on a copper supported grid.
2.3 Hydrogen uptake measurements
An intelligent gravimetric analyser (IGA) was used to measure
the hydrogen storage capacity using high purity hydrogen
(99.9999%). The carbon samples were dried in an oven for 24 h at
80 �C overnight and then placed in the analysis chamber and
degassed at 200 �C and 10�10 bar for 4–6 h. The hydrogen uptake
measurements were performed in the 0–20 bar pressure range at
�196 �C (liquid nitrogen bath).
3 Results and discussion
3.1 Nature of ZIF template and ZIF/carbon composites
Zeolitic imidazolate frameworks (ZIFs) are nanoporous mate-
rials which consist of tetrahedral clusters of MN4 (M ¼ Zn,
linked by imidazolate ligands) with a SOD (sodalite) zeolite-type
structure.22 The ZIF used as template, i.e., commercially avail-
able Basolite Z1200, has a well defined XRD pattern with sharp
peaks characteristic of a crystalline solid (Fig. S1†). The ZIF was
assessed, by thermogravimetric analysis (TGA), to be stable in
nitrogen up to 500 �C (Fig. S2a†).22 Thermal treatment of the
ZIF under nitrogen causes continuous mass loss between 500 and
1000 �C with distinct mass loss events at 605, 640 and 950 �C, asshown by the differential thermogravimetric (DTG) profile
(Fig. S2b†). The residual mass at 1000 �C is ca. 35 wt%. This
residual mass is presumed to be largely due to the Zn contained
in the ZIF structure, and is consistent with the empirical struc-
tural formula of C8H12N4Zn. Under our thermal analysis
conditions Zn metal is not vaporised until ca. 1100 �C (inset,
Fig. S2a†). The porous structure of the ZIF was ascertained by
nitrogen sorption analysis (Fig. S3†). The ZIF adsorbs nitrogen
mainly at relative pressure (P/Po) < 0.1, which corresponds to
micropore filling. The isotherm is type I, typical of a microporous
material. The microporosity of the ZIF is confirmed by the pore
size distribution (PSD) data (Fig. S3†), which shows unimodal
PSD with pore maxima at ca. 11 �A. The ZIF template had
a surface area and pore volume of 1417 m2 g�1 and 0.77 cm3 g�1
respectively with a micropore surface area of 1397 m2 g�1 and
a micropore volume of 0.67 cm3 g�1.
As described in the Experimental section, the templated
carbon materials were prepared using furfuryl alcohol (FA) as
carbon precursor wherein the first step was ingress of the FA into
the pores of the ZIF via liquid impregnation and heating of the
resulting composite at between 80 and 150 �C under Ar, followed
This journal is ª The Royal Society of Chemistry 2011
Fig. 2 TGA curves of ZIF-templated carbon materials carbonised at
various temperatures.
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by a second thermal treatment step involving carbonisation of
the ZIF/FA composite at 900–1100 �C. The XRD pattern of the
ZIF/FA composite following impregnation with furfuryl alcohol
and heating at 80 �C for 24 h and then at 150 �C for 6 h under Ar
but prior to the carbonization process (Fig. S4a†) shows peaks at
positions similar to those of the ZIF pattern but with lower
intensity. This indicates that the FA impregnation process does
not alter the crystalline structure of the ZIF. Thermogravimetric
analysis (in air) of the ZIF/FA composite (Fig. S4b†) indicates
mass loss in the temperature range 350–550 �C, with a residual
mass of ca. 33%. Considering that the ZIF is stable up to ca.
500 �C (Fig. S2†), the early mass loss from the composite
(350–500 �C) is due to decomposition of the FA. Beyond 500 �C,two mass loss processes are superimposed: (i) mass loss from
decomposition/combustion of the carbon precursor, FA, and (ii)
mass loss due to the decomposition of the ZIF. Indeed, an
inflection point observed at ca. 450 �C may indicate the point at
which the ZIF starts burning off.
3.2 ZIF-templated carbons
The XRD patterns of the carbons prepared at various carbon-
isation temperatures are shown in Fig. 1a. The patterns show two
broad features centered at 2q ¼ 24� and 44�. These very broad
and low intensity diffraction bands are at positions where (002)
and (10) diffraction peaks of graphitic carbon would occur. The
broad nature of the diffraction bands indicates that the ZIF-
templated carbon materials are essentially amorphous. The
sample to sample variation of the intensity of the diffraction
band at 2q¼ 44� suggests that higher carbonisation temperatures
generate slightly more turbostratic/graphitic carbons. Overall,
however, the carbonisation temperature has only a very slight
effect on the extent of graphitisation. This implies that most of
the FA (carbon precursor) was deposited inside the porosity of
the template wherein it cannot graphitise at the carbonisation
temperatures used.23 It is also the case that FA does not easily
undergo graphitization, especially at the temperatures used. On
the other hand, no diffraction peaks of the Basolite ZIF frame-
work are observed. This is due to decomposition of the ZIF
during the high temperature carbonisation step.
The TGA curves of the ZIF-templated carbon materials are
shown in Fig. 2. All the samples exhibit a small initial mass loss
below 200 �C, which can be attributed to removal of physisorbed
water. The main mass loss event, which is due to combustion of
the carbon, occurs in the temperature range 500–600 �C. Thetemperature at which maximum mass loss occurs varies
Fig. 1 Powder XRD patterns of ZIF-templated carbon materials
carbonised at various temperatures before (a) and (b) after activation
with KOH (at a KOH/carbon ratio of 4) at 700 �C for 1 h.
This journal is ª The Royal Society of Chemistry 2011
depending on the carbonisation temperature. The sample
prepared at 900 �C burns off at a lower temperature compared to
the other samples. The residual mass at 1000 �C varies with
carbonisation temperature, being ca. 11.5 wt% for sample
BF-900 and ca. 2 wt% for samples carbonised at 1000, 1050 and
1100 �C. The much higher residual mass for sample BF-900 is
due to incomplete removal of Zn during the carbonisation step.
The residual mass of the other samples indicates that they are
virtually ZIF free due to their higher carbonisation temperature.
The TGA data therefore indicate that carbonisation of the
ZIF/FA composite at temperature above 1000 �C removes the
ZIF template and generates Zn-free carbons, with a carbon yield
(excluding water and residual mass) of ca. 97 wt%.
Fig. 3a shows the nitrogen sorption isotherms for the ZIF-
templated carbons. The isotherms are all type I and exhibit
virtually no hysteresis between adsorption and desorption
branches. The type I nature of the isotherms, with significant
nitrogen uptake at relative pressure (P/Po) below 0.1, indicates
that all the BF-T carbon samples have a microporous structure.
All the carbons also have some adsorption at P/Po above 0.95,
which we attribute to interparticle voids. The isotherms indicate
relatively comparable porosity for the BF-T carbons with surface
area in the range 900–1100 m2 g�1 and a pore volume of between
0.6 and 0.7 cm3 g�1 as shown in Table 1. The highest surface area
is achieved for carbonisation temperatures of 1000 and 1100 �C(1067 and 1131 m2 g�1 respectively), as well as the largest pore
volume (ca. 0.7 cm3 g�1). Sample BF-900 has lower surface area
and pore volume presumably due to the presence of a significant
Fig. 3 Nitrogen sorption isotherms of ZIF-templated carbon materials
carbonised at various temperatures before (a) and (b) after activation
with KOH (at a KOH/carbon ratio of 4) at 700 �C for 1 h.
J. Mater. Chem.
Table 1 Textural properties and hydrogen uptake capacity of ZIF-templated carbonmaterials before (BF-T) and after (ACBF-T) activation with KOH(at a KOH/carbon ratio of 4) at 700 �C for 1 h
Sample Surface areaa/m2 g�1 Pore volumeb/cm3 g�1 Pore sizec/�AH2 uptake(wt%)
H2 uptakedensity/mmol H2 m
�2
BF-900 933 (872) 0.57 (0.41) 8/15/20 2.6 13.9BF-1000 1131 (1055) 0.69 (0.49) 8/15/20 3.0 13.3BF-1050 1069 (979) 0.67 (0.46) 5/15/20 3.1 14.5BF-1100 1067 (954) 0.69 (0.45) 8/15/20 3.0 14.1ACBF-900 3188 (2529) 1.94 (1.16) 6/12/25 6.2 9.7ACBF-1000 1893 (1678) 1.13 (0.78) 8/13/21 4.9 12.9ACBF-1050 1425 (1204) 0.91 (0.57) 8/13/21 3.9 13.7ACBF-1100 1523 (1356) 0.95 (0.63) 8/13/21 4.7 15.4
a Values in parenthesis are micropore surface area. b Values in parenthesis are micropore volume. c Pore size maxima from NLDFT pore analysis.
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amount of residual ZIF template (i.e., Zn) as indicated by
thermal analysis data (Fig. 2). For all the BF-T carbons, the
proportion of micropore surface area is high at ca. 90% while the
micropore volume is ca. 70%. Both are consistent with the highly
microporous nature of the ZIF-templated carbons.
The pore size distribution (PSD) of all the ZIF-templated
carbons, determined via a DFT model using adsorption data
(Fig. S5†), exhibits three maxima suggesting a tri-modal pore size
distribution within the pore size range 5–30�A. The smallest pores
are centred at 6–8�A, with two further pore size maxima at 15 and
20 �A. The majority of the pores are however in the 10–25 �A size
range, with no pores above 30 �A. This is consistent with the
highly microporous nature of the ZIF-templated carbons. The
carbonisation temperature appears to have some effect on the
pore size distribution; the higher the temperature, the broader
the PSD, with an increase in the proportion of pore channels of
size ca. 20 �A. Overall, however, the fairly narrow distribution of
the pores suggests that a pore-templating process occurs within
the ZIF template particles.24 Representative scanning electron
microscopy (SEM) images of sample BF-900 (shown in Fig. 4)
indicate the presence of particles with size between 50 and 100 nm
that are similar to those of the ZIF template.25 The morphology
of the ZIF template was therefore transferred to the carbon
material. Such a transfer of particle morphology is expected to
occur in a templating mechanism whereby the carbon is
predominantly nanocast within the pore channels of the ZIF-
template. Furthermore, the smooth surface and sharp particle
edges of sample BF-900 are consistent with the absence of
externally deposited carbon that would otherwise generate
a separate phase of irregular shaped particles.
3.3 Activated ZIF-templated carbons
TheXRD patterns of the activated ZIF-templated carbons, shown
in Fig. 1b, are very similar to those of the carbons before activation
Fig. 4 Representative SEM images of ZIF-templated carbon BF-900.
J. Mater. Chem.
(Fig. 1a). The XRD patterns exhibit broad and very low intensity
peaks at 2q¼ 25� and 43�. The low intensity and broadness of these
peaks suggest that the amorphous nature of the carbon remains
unchanged after activation. Themain aim of the activation process
was to enhance the porosity of the ZIF-templated carbons while
retaining the dimensions of the pore channels. Fig. 3b shows the
nitrogen sorption isotherms of the activated ZIF-templated
carbons. The isotherms show that the effect of the activation
process depends on the temperature at which the ZIF-templated
carbon was carbonised. For samples carbonised at 1000, 1050 and
1100 �C, the activation process leads to a modest increase in the
amount of nitrogen adsorbed (Fig. S6†). The modest increase in
adsorption is accompanied by a slight widening of the isotherm
‘knee’, which indicates the formation of slightly larger micropores.
On the other hand, for sample BF-900, the activation generates an
activated carbon that exhibits amuch larger increase in the amount
of nitrogen adsorbed and also a more extensive widening of the
isotherm ‘knee’. The isotherm ‘knee’widens to cover theP/Po range
between 0.1 and 0.3, which indicates that the activation increases
the proportion (and amount) of large micropores and generates
small mesopores. Nevertheless, despite the tendency to larger
micropores and slight mesoporosity, the activated ZIF-templated
carbons still remain predominantly microporous as indicated by
their type I isotherms with significant adsorption at
P/Po below 0.2.
In all cases, the activation has little effect on the amount of pores
smaller than 10 �A, and the pores originally present in the ZIF-tem-
plated carbons are retained after activation (Fig. 5). The effect of
activationon the pores centred at 15�Avaries from sample to sample.
While samplesACBF-1050andACBF-1100show little change to the
proportion of these pores, there is a significant increase for BF-1000
andamuch larger increase in their number forBF-900.Nevertheless,
in all cases the actual size of the pores remains at ca. 15 �A after
activation regardless of the extent of increase in their proportion.On
the other hand, for all samples, the size and proportion of pores
centred at ca. 20�A increase significantly after activation (Fig. 5). The
extent of increase in size and proportion is higher for carbons
generated at lower carbonisation temperature. In particular, activa-
tion of sample BF-900 causes a drastic increase in the proportion of
pores larger than 15�A, and the poremaxima shift from20�A to 25�A.
The overall picture that emerges from the pore size distribution
curves is that for the activated ZIF-templated carbons, the propor-
tionof largermicroporesandsmallmesopores (15–25�A)exceeds that
of smaller (<15 �A) pores. Furthermore, activated ZIF-templated
carbons possess somepores that are larger than 30�A,whichwere not
This journal is ª The Royal Society of Chemistry 2011
Fig. 5 Pore size distribution curves of ZIF-templated carbon materials
carbonised at various temperatures before (filled symbols) and after
(empty symbols) activation with KOH.
Fig. 6 Hydrogen uptake isotherms of ZIF-templated carbon materials
carbonised at various temperatures before (a) and (b) after activation
with KOH (at a KOH/carbon ratio of 4) at 700 �C for 1 h.
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present before activation. The main effect of KOH activation on
ZIF-templated carbons is, therefore, to generate pores of size
between 15 and 25�A,while largely retainingpores smaller than 15�A.
A similar trend has previously been observed for KOH activation of
zeolite-templated carbons andactivatedcarbide-derivedcarbons.20,21
(We, however, note that the apparent size of the pores (1.2 and 2.2–
3.4 nm)may be overestimated, which is a general feature ofNLDFT
pore size obtained from nitrogen sorption data.)26 Despite the
uncertainty about the actual pore size, the presented data are suffi-
ciently robust for the comparative analysis in this work and do not
affect the observed trends in pore size changes.
The textural properties of the activated ZIF-templated
carbons are summarized in Table 1. A clear increase in the total
surface area and pore volume is registered after the activation
process. The total surface area increases from 933–1131 m2 g�1 to
1608–3188 m2 g�1 whereas the pore volume rises from ca.
0.7 cm3 g�1 to between 0.9 and 1.94 cm3 g�1. Especially remark-
able is the case of ACBF-900, where there is an increase of 240%
to the total surface area and pore volume. The large increase in
textural properties for sample ACBF-900 may in part be
explained by the removal of ZIF residues during the activation
process. It is also likely that carbonisation at 900 �C generates
a ZIF-templated carbon framework that is more active than
those carbonised at higher temperature. The samples prepared at
1000 �C and above undergo surface area and pore volume
increases of between 30 and 70% depending on their carbon-
isation temperature. Despite the large increases in textural
properties, the proportion of micropore surface area (80–90%)
and micropore volume (60–70%) remains high in the activated
ZIF-templated carbons. This trend is rather different from that
observed for KOH activated zeolite-templated carbons where,
although activation caused an increase in the total surface area
and pore volume, in some cases this was accompanied by
a drastic decrease in microporosity (i.e., increase in overall
surface area and pore volume was accompanied by decrease in
micropore surface area and volume).20 Indeed it is noteworthy
that for the ZIF-templated carbons, especially those carbonised
This journal is ª The Royal Society of Chemistry 2011
at 1000 �C and above, activation causes a fairly uniform
percentage increase in the total surface area, pore volume,
micropore surface area and micropore volume. For example, in
the case of sample ACBF-1000, the percentage increase in the
total surface area, pore volume, micropore surface area and
micropore volume are 67%, 64%, 60% and 60% respectively.
Overall, therefore the proportion of microporosity for the
ACBF-T samples remains high after activation, which may be
explained by the generally non-changing pore size distribution
after activation (Fig. 5) due to the relatively mild activation
temperature of 700 �C. This means that activation of BF-T
carbons (where T ¼ 1000–1100 �C) allows the formation of
carbons with higher textural properties but with no substantial
change in the pore size distribution. For sample ACBF-900,
where significant pore enlargement occurs after activation, the
percentage increase in the total surface area and pore volume (ca.
240%) is higher than the increase in the micropore surface area
(ca. 190%) and micropore volume (ca. 180%) due to formation of
pores of size larger than the micropore range.
3.4 Hydrogen storage
The hydrogen sorption isotherms of the ZIF-templated carbon
materials, measured by gravimetric analysis at �196 �C and
20 bar, are shown in Fig. 6a. The hydrogen uptake in wt% was
calculated on the basis of a density of 1.5 g cm�3 for the carbon
samples and a density of 0.04 g cm�3 for the hydrogen. All the
hydrogen uptake isotherms are completely reversible, with the
absence of hysteresis between the adsorption and desorption
processes, and no saturation is achieved in the 20 bar pressure
range, which suggests that higher hydrogen adsorption capacity
can be obtained at pressures above 20 bar. The hydrogen uptake
capacity of the carbons at 20 bar is summarised in Table 1. The
BF-T samples carbonised at 1000, 1050 or 1100 �C have higher
hydrogen uptake capacity of between 2.9 and 3.1 wt% compared
to 2.6 wt% for sample BF-900 which was carbonised at 900 �C.The hydrogen uptake capacity therefore to some extent corre-
lates with total surface area. Thus, in general, the lower the total
surface area of the ZIF-templated carbon, the lower the
hydrogen uptake capacity at �196 �C and 20 bar.
The hydrogen sorption isotherms for the activated ZIF-tem-
plated carbons are displayed in Fig. 6b, and the corresponding
uptake at 20 bar is summarised in Table 1. The hydrogen storage
capacity of the activated ZIF-templated carbons (at �196 �C and
20 bar) is between 3.9 and 6.2wt%,which is comparable or superior
J. Mater. Chem.
Fig. 7 Plot of hydrogen storage capacity as a function of (A) surface
area or (B) pore volume of ZIF-templated carbons before (B) and after
(C) chemical activation with KOH (at a KOH/carbon ratio of 4) at 700�C for 1 h. The line in (A) is a Chahine plot.34
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to other activated carbons.8,27–29 Activation enhances the hydrogen
uptake of all the ZIF-templated carbons by between 25 and 140%.
The desired effect of enhancing the hydrogen storage capacity of
ZIF-templated carbons via KOH activation is therefore achieved.
This enhancement is clearly a consequence of increases in the
textural properties after the chemical activation process. Especially
remarkable is again the case of ACBF-900, where a hydrogen
uptake as high as 6.2 wt% (140% enhancement) is achieved. A
hydrogen uptake of 6.2 wt% at�196 �C and 20 bar is at the highest
end of values so far reported for carbon materials.5–10,16,27–29
It is interesting to consider how changes in the textural prop-
erties after activation match with the increase in hydrogen
storage capacity. We first note that there is no close match
between increases in total surface area and pore volume with
hydrogen uptake. For example, sample ACBF-900 has a surface
area and pore volume increase of ca. 240% while the hydrogen
uptake increases by only 140%. Therefore, some of the new
porosity in sample ACBF-900 is not as efficient in hydrogen
storage. This is unsurprising given that there is some pore
enlargement when BF-900 is activated to ACBF-900. On the
other hand, there is a closer match between increases in textural
properties for the other three samples after activation and the
enhancement in hydrogen uptake. For example, sample
ACBF-1000 has a surface area and pore volume increase of ca.
65% and the hydrogen uptake also increases by ca. 65%. In this
case the new porosity after activation is of similar dimensions to
that present before activation (Fig. 5) and therefore equally
efficient in hydrogen storage. Indeed for all the samples there is
a much closer match between increase in micropore surface area
andmicropore volume and the enhancement in hydrogen uptake.
This observation is further evidence that micropores are the more
important spaces for hydrogen storage.5–10,30–32
The effect of thehighmicroporosity of the presentZIF templated
carbons is illustrated in Table 1 by the high hydrogen storage
density values of (excepting sample ACBF-900) between 13.3 and
15.4mmolH2m�2. The hydrogen storage density values (a snapshot
of uptake per surface area) are superior to those reported in the
literature for a variety of carbons such as (i) KOH activated CDCs
(10 � 0.7 mmol H2 m�2) and CO2 activated CDCs (9 � 0.1 mmol
H2 m�2) (measured at �196 �C and 60 bar),7c (ii) composites of
activated carbon and CNTs (9.55 mmol H2 m�2) (�196 �C and 60
bar)8a and activated carbons, SWNTs, SWNHs, and GCFs
(11.75 mmol H2 m�2) (�196 �C and 20 bar)28 and (iii) chemically
activated carbons obtained from anthracite (9� 0.1 mmol H2 m�2)
(�196 �C and 20 bar).8b The hydrogen storage density values are
also slightly higher than those we have recently reported for acti-
vated zeolite templated carbons20 and activated CDCs,21 and are
comparable to theuptakedensityof activated carbons derived from
hydrochar.33 Sample ACBF-900 has a lower hydrogen uptake
density (9.7 mmol H2 m�2) due to the presence of mesopores.
Fig. 7a shows a plot of the hydrogen uptake (at 20 bar) as
a function of surface area of the ZIF-templated carbons before
and after activation wherein an approximately linear relationship
is observed. A similar relationship is obtained between hydrogen
uptake and pore volume as shown in Fig. 7b. It is noteworthy
that all the carbons (except for sample ACBF-900) store more
hydrogen than would be expected according to the Chahine rule
(i.e., 1 wt% hydrogen stored per 500 m2 g�1 of carbon).34
Therefore the hydrogen uptake of these carbons does not
J. Mater. Chem.
generally fit into the Chahine rule due to higher uptakes per given
surface area. We have previously observed similar behaviour for
KOH activated zeolite-templated and carbide-derived carbons
(CDCs).20,21 On the other hand, the hydrogen uptake of sample
ACBF-900 fits into the Chahine rule. We attribute the behaviour
of sample ACBF-900 to the presence of small mesopores, while
all the other samples possess roughly similar pore size distribu-
tion mainly within the micropore range (Fig. 5). This clearly
implies that although the mesopores in ACBF-900 contribute to
the enhancement in hydrogen uptake, they store less per unit
surface area compared to micropores. Indeed a plot of hydrogen
uptake as a function of micropore surface area or pore volume
(Fig. S7†) shows a linear relationship wherein sample ACBF-900
fits in with the other samples. This observation is consistent with
our recent study where we have shown that a linear relationship
can exist between the micropore surface area and hydrogen
uptake for carbon samples that possess very similar pore size
distribution.35 These observations clarify the fact that although
a high pore volume is desirable for hydrogen storage, it is more
advantageous if a significant proportion or all of the volume is
from micropores.
4 Conclusions
We have shown that a commercially available zeolitic imidazo-
late framework (ZIF), namely Basolite Z1200, may be used to
nanocast highly microporous carbon with a surface area of ca.
1000 m2 g�1 and a pore volume of ca. 0.7 cm3 g�1. The ZIF-
templated carbons are prepared via liquid impregnation of fur-
furyl alcohol (FA) into the pores of the ZIF followed by poly-
merization of the FA and finally carbonization at 900–1100 �C.The ZIF framework is effectively removed during the carbon-
ization step. The ZIF-templated carbons have 90–95% of their
surface area arising from micropores. On chemical activation
(with KOH at 700 �C for 1 h and a carbon/KOH weight ratio of
1 : 4), ZIF-templated carbons undergo enhancement of their
porosity of between 30 and 240% depending on their carbon-
ization temperature. A sample carbonized at 900 �C has the
highest increase of 240% with surface area increasing to ca.
3200 m2 g�1 and pore volume to 1.94 cm3 g�1. Despite the drastic
increase in porosity, the activated ZIF-templated carbons retain
a predominantly microporous nature with micropores contrib-
uting 80–90% of surface area and 60–70% of pore volume. In
general, the micropores present in the ZIF-templated carbons are
This journal is ª The Royal Society of Chemistry 2011
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retained after activation with most of the ‘new’ pores generated
having dimensions similar to that of the ‘original’ pores. The
activation results in an increase in hydrogen uptake capacity (at
�196 �C and 20 bar) of between 25 and 140% from 2.6–3.1 wt%
to the range 3.9–6.2 wt%. The increase in hydrogen uptake is
strongly linked to rises in the micropore surface area and
micropore volume. Especially remarkable is the case of the
activated carbon obtained via carbonisation at 900 �C, whichexhibits a surface area of �3200 m2 g�1 after activation and
a hydrogen storage capacity of 6.2 wt% (at 20 bar and �196 �C).
Acknowledgements
This research was funded by the University of Nottingham. A. A.
thanks King Abdulaziz University, Saudi Arabia, for
a scholarship.
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