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Cite this: DOI: 10.1039/c1jm13314d

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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: r.mokaya@nottingham.ac.uk; 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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