Improved hydrogen release from ammonia borane confined in microporous carbon with narrow pore size distribution

Article abstract of DOI:10.1039/c7ta02461d

Ammonia borane is a promising hydrogen storage candidate due to its high hydrogen capacity and good stability at room temperature, but there are still some barriers to be overcome before it can be used for practical applications. We present the hydrogen release from ammonia borane confined in templated microporous carbon with extremely narrow pore size distribution. Compared with neat ammonia borane, the hydrogen release temperature of ammonia borane confined in microporous carbon with a pore size of 1.05 nm is significantly reduced, starting at 50 °C and with the peak dehydrogenation temperature centred at 86 °C. The dehydrogenation kinetics of ammonia borane confined in templated microporous carbon is significantly improved and by-products including ammonia and diborane are also completely prohibited without any catalysts involved. The remarkably fast hydrogen release rate and high hydrogen storage capacity from ammonia borane confined in microporous carbon are due to the dramatic decrease in the activation energy of ammonia borane. This has been so far the best performance among porous carbon materials used as the confinement scaffolds for ammonia borane in hydrogen storage, making AB confined in microporous carbon a very promising candidate for hydrogen storage.

Full text of DOI:10.1039/c7ta02461d

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Kun Chang, Zhaorong Chang et al.
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DOI: 10.1039/C7TA02461D
Improved Hydrogen Release from Ammonia Borane Confined in
Microporous Carbon with Narrow Pore Size Distribution
Zhuxian Yang,a Dan Zhou,b Binling Chen,a Zongjian Liu,c Qinghua Xia,b Yanqiu Zhu,a and
Yongde Xiaa,*
a College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter
EX4 4QF, United Kingdom. E-mail: y.xia@exeter.ac.uk
b
Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials, &
Ministry of Education Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, Hubei University, Wuhan 430062, China
c
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014,
China
1

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Abstract
DOI: 10.1039/C7TA02461D
Ammonia borane is a promising hydrogen storage candidate due to its high hydrogen
capacity and good stability at room temperature, but there are still some barriers to be
overcome before it can be used for practical applications. We present the hydrogen release
from ammonia borane confined in templated microporous carbon with extremely narrow pore
size distribution. Compared with neat ammonia borane, hydrogen release temperature of
ammonia borane confined in microporous carbon with pore size of 1.05 nm is significantly
reduced, starting at 50 C and with peak dehydrogenation temperature centred at 86 C. The
dehydrogenation kinetics of ammonia borane confined in templated microporous carbon is
significantly improved and by-products including ammonia and diborane are also completely
prohibited without any catalysts involved. The remarkable fast hydrogen release rate and high
hydrogen storage capacity from ammonia borane confined in microporous carbon is due to
the dramatic decrease in the activation energy of ammonia borane. This is so far the best
performance among porous carbon materials used as the confinement scaffolds for ammonia
borane in hydrogen storage, making AB confined in microporous carbon a very promising
candidate for hydrogen storage.
2

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Introduction
DOI: 10.1039/C7TA02461D
The current widely used fossil fuels are non-renewable energy resources which will be
depleted in the next century. Meanwhile, the use of fossil fuels inevitably emits greenhouse
gas carbon dioxide into the air that has caused issues such as atmosphere pollution and global
warming. Therefore, intensive efforts have been devoted to developing renewable energy
resources that can replace fossil fuels in the near future. Hydrogen has been proposed as an
ideal alternative energy resource since the burning of hydrogen only generates water, which
is green and has no environmental concerns. One of the major issues for hydrogen as an
energy vector is how to store this highly flammable gas effectively. Generally, four main
methods to store hydrogen have been investigated so far including high pressure tanks,
sorbents (such as porous carbons, metal-organic-frameworks), metal hydrides (such as LiBH4
and AlBH4) and chemical hydrides (such as ammonia borane). Each method has some
2
advantages and drawbacks.1,
Amongst these different hydrogen storage approaches,
chemical hydrides are very promising and show higher gravimetric hydrogen capacities
because they consist of lighter elements.
Ammonia borane (AB, NH3·BH3), a typical chemical hydride, has been intensively
studied as a promising hydrogen storage media because of its high hydrogen capacities of
19.6 wt% and good stability at room temperature.2, 3 AB dehydrogenates in three thermolysis
steps at around 110, 150, and > 500 °C for the first, second, and third equivalents of H2
respectively, with each step generating about 6.5 wt% hydrogen, which exceeds the revised
target of hydrogen storage materials for light-duty vehicles as set by the US Department of
Energy. Furthermore, recent reports have demonstrated that the spent fuel type derived from
the removal of greater than two equivalents of H2 per molecule of AB (i.e., polyborazylene)
can be converted back to AB nearly quantitatively by 24-hour treatment with hydrazine in
liquid ammonia at 40°C.4, 5 However, despite these merits, there are technical barriers
3

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preventing AB to be practically used as an on-board energy source, such as its slow thermal
DOI: 10.1039/C7TA02461D
kinetics below 100 C and unwanted by-products including ammonia (NH3), diborane (B2H6),
and borazine (B3H6N3), which will poison the catalyst in a proton membrane fuel cell.2, 3, 6-8
To date, a number of ways have been investigated to tackle the above mentioned issues
including the use of metal catalysts3, 9-17 or metal-free catalysts,18-20 the formation of metal
hydrides,21-23 the realisation of metal or methane substitution,24-26 the utilisation of ionic
28
30
31-40
liquid,27,
additives,29,
or nanoconfinement8,
etc. Significant improvements on
lowering the hydrogen release temperatures, improving the kinetics and avoiding the
emissions of those harmful by-products have been made.
Confinement within porous materials has been reported to affect chemical reactions by
modification of the thermodynamic properties due to adsorption, geometrical constraints in
pores comparable to the molecular sizes, selective adsorption of reacting molecules, and
changes of the potential energy surface, etc.41 In particular, nano-confinement of hydrides
into porous scaffold hosts has been reported to be able to alter the thermodynamics and
kinetics of the hydrides significantly by reducing the dimension of hydride particles to
nanoscale and therefore shorten the mass diffusion lengths; It is generally accepted that the
confinement effect is inversely correlated to the average pore size of scaffold, i.e. the smaller
the pore, the faster the kinetics.42-44 The first report on nanoconfinement of AB in
mesoporous silica demonstrated a lower dehydrogenation temperature, suppression of
borazine release, and lower enthalpy of the decomposition,31 which has triggered a number of
studies in this area. Nanoscaffolds including mesoporous silica,31, 45-47 mesoporous carbon,32,
38, 48, 49
activated carbon,50 carbon nanotubes,39, 40 metal-organic frameworks (MOFs),8, 33, 36,
37, 51-53
35
porous MnO2 and low-density porous aromatic framework34 have been studied.
Particularly, MOFs have attracted much attention because of the combination of nanoporosity
4

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and active metal sites in MOFs, which can offer a synergistic effect of nanoconfinement and
DOI: 10.1039/C7TA02461D
metal site based catalysis.8
However, compared with porous carbon materials, MOFs and porous silicas are relatively
heavy, which will inevitably compromise the hydrogen capacity of the nanocomposites. Yet
only a couple of studies on porous carbon materials as the confinement hosts have been
carried out so far.32, 48-50 In particular, mesoporous carbon materials including coherent
carbon cryogel32, 48 and CMK-349 have been investigated. The coherent carbon cryogel/AB
nanocomposite has exhibited lower dehydrogenation temperature and suppression of
borazine;32 and the mesoporous carbon CMK-3/AB nanocomposite has shown
dehydrogenation in one-step at a significantly lower temperature but with the emission of
ammonia, which can be avoided when a lithium catalyst is applied.49 In addition, Sepehri et al
have studied the effect of pore size on dehydrogenation temperature and kinetics of
mesoporous coherent carbon/AB nanocomposites, showing that the smaller the pore, the
better the dehydrogenation performance.48 However, so far only mesoporous carbons with
pore size in the range of 4.5 to 16 nm32, 48, 49 have been investigated as scaffolds, and it is
highly desirable and interesting to study the confinement effect when the pore of the carbon
materials is further reduced down to micropore level (below 2 nm). Although recently
activated carbons have been investigated as the scaffold, the composite dehydrogenates at
room temperature causing safety issues (the composite is unstable).50 Given that generally
activated carbon exhibits a broad pore size distribution in the range of 0.3-2.0 nm,54, 55
templated microporous carbon with narrow pore size distribution will be expected to perform
better, which has not been explored in previous research.
In this regard, for the first time we report the hydrogen release from AB confined in
microporous carbon with extremely narrow pore size distribution of 1.0 -1.5 nm, which was
nanocasted using zeolite EMC-2 as the hard template. Without any catalysts involved,
5

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hydrogen release temperature of AB from the nanocomposite is significantly reduced down to
DOI: 10.1039/C7TA02461D
50 C with dehydrogenation peak centred at 86 C. The dehydrogenation kinetics is improved
and by-products including ammonia and diborane are prohibited in the nanocomposite system.
The reason for the remarkable fast hydrogen release rate and high hydrogen storage capacity
from ammonia borane confined in microporous carbon was addressed by comparing the
activation energy of neat AB and AB confined microporous carbon.
Experimental Section
Sample preparation
Zeolite EMC-2 was synthesized following reported method.56 The microporous carbon
material was prepared via chemical vapor deposition (CVD) method using zeolite EMC-2 as
the template and ethylene as the carbon precursor at 800 C. In brief, 1 g of zeolite ECM-2
was put in a tube furnace under Ar flow, and when the temperature reached 800 C, Ar flow
was replaced by ethylene flow of 100 mL/m for 3 h, followed by cooling down under Ar
flow. The resulting carbon/zeolite composite was washed with 10% hydrofluoric acid several
times, followed by refluxing with concentrated hydrochloric acid at 60 C to completely
remove the zeolite framework. Finally, the resulting carbon material was dried in an oven at
120 C overnight and named as CEMC. Thermogravimetric analysis (TGA) of the
synthesized carbon was performed using a TA SDT Q600 instrument with a heating rate of
10 C/min under an air flow of 100 mL/min, confirming the complete removal of the zeolite
template. SEM-EDX also confirmed that only C and O were detected in the obtained carbon
material.
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AB was loaded into the microporous carbon CEMC by infusion of equivalenDtOvI:o1l0u.1m039e/Co7TfA02461D
AB-methanol solution to ECMC. Briefly, AB was dissolved in anhydrous methanol to form
1.0 M solution at room temperature, and the carbon was added into the calculated amount of
methanol solution (based on the pore volume of ECMC) and stirred for several hours at room
temperature. AB was incorporated into the pores of the carbon by the capillary effect. The
solvent was removed via vacuum at room temperature overnight.
Sample characterisation
The X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance
diffractometer working with CuKα (Ni-filtered) radiation λ = 0.15418 nm and a scanning step
size of 2θ = 0.02o. The textural properties were determined via N2 sorption at -196 °C on a
Quantachrome Autosorb iQ sorptometer. The surface area was calculated using the Brunauer-
Emmett-Teller (BET) method based on adsorption data in the partial pressure (P/P0) range of
0.02-0.22 and the total pore volume was determined from the amount of nitrogen adsorbed at
P/P0 of ca. 0.99. The partial pressure range (P/P0) 0.02 – 0.22 was selected for the calculation
of surface area by taking into account previous report which indicates that low partial
pressure range of P/P0 0.01 – 0.05 will overestimate the surface area while the partial
pressure range of P/P0 0.1 – 0.3 can underestimate the surface area.57 The pore size
distribution (PSD) was obtained using the non-local density functional theory (NLDFT)
method for slit/cylinder pores using the software provided by Quantachrome. The thermal
gravimetric analysis (TGA) and decomposition process was carried out on a TA SDT Q600
instrument coupled with a mass spectrometer. The TGA was run with a heating ramp rate of
2 C/min under argon flow of 100 mL/min, and the gaseous compositions from AB
decomposition were determined by the coupled mass spectrometer. In the isotherm process,
the sample was heated up to the target temperatures (70, 80 and 90 C respectively) at a ramp
rate of 20 C/min under argon, then held at the isotherm temperature for 2 hours. Differential
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scanning calorimetry (DSC) was determined with a Netszch DSC (204 HP Phoenix) at
DOI: 10.1039/C7TA02461D
various ramp rates under argon flow. The amount of H2 released from the sample was
calculated based on the mass of AB in the material.
Results and discussion
The structure of neat AB, porous carbon CEMC, AB/CEMC before and after thermolysis at
200 C was examined by XRD and shown in Fig. 1A. The XRD pattern of the carbon
material CEMC shows two well resolved peaks at 2θ of 6.2o and 6.8o, which were at the same
position as the (100) and (101) diffraction of the zeolite ECM-2. This observation clearly
indicated that a high level of replication of zeolite-type structural ordering in the carbon
material was realised, which is also in highly agreement with previous report results.58-60
A
further hump peak at 2θ of 26o is due to the (002) diffraction line of partially graphitised
carbon. The XRD pattern of AB/CEMC suggests that the loading of AB does not change the
structure of the carbon CEMC. No XRD peaks of AB can be observed in AB/CEMC
composite sample, implying that AB is indeed fully incorporated inside the pores rather than
deposited on the outer surface of CECM matrix. The XRD pattern of AB/CEMC after
thermolysis indicates that the structure of carbon CEMC does not change after the
thermolysis of AB. Fig. 1B shows the nitrogen adsorption and desorption of CEMC and
AB/CEMC, and the inset is the pore size distribution of CEMC. CEMC shows isotherm
typical for microporous materials with surface area of 1652 m2 g-1 and pore volume of 0.87
cm3 g-1, and very narrow pore size distribution in the range of 1-1.5 nm, centred at 1.05 nm. It
is worth noting that the pore size distributions of those mesoporous carbon materials reported
as scaffolds are in a much broader range of 3-6 nm (for CMK-349, 61), 3-20 nm (carbon
cryogel32) and 5-20 nm (carbon cryogel48). However, the isotherm of the AB/CEMC
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composite is almost a straight line with surface area of 56 m g and pore volumeDoOfI:010..010439c/Cm7TA02461D
g-1, implying that the pores of CEMC are almost completely occupied by the infiltrated AB,
which is consistent with the XRD results.
600
B
A
CEMC
500
400
300
200
100
0
Neat AB
CEMC
AB/CEMC
0
5 10 15 20 25 30 35
Pore size (Å)
AB/CEMC
AB/CEME after thermolysis
0.0 0.2 0.4 0.6 0.8 1.0
10
20
2
30
40
50
Relative pressure (P/P0)
(degree)
Fig. 1 Powder XRD patterns (A) of neat AB, porous carbon CEMC, and AB/CEMC before
and after thermolysis at 200 C, and N2 adsorption (solid) and desorption (empty) isotherms
(B) of neat CEMC (black) and AB/CEMC (red).
Fig. 2 presents the dehydrogenation profiles of AB and AB/CEMC obtained from
thermogravimetric analysis (TGA) combined with mass spectroscopy (MS). Neat AB starts to
dehydrogenate at above 100 C via a two-step process and centres at 114 C and 140C
respectively, with the emission of hydrogen, ammonia and diborane simultaneously, as
evidenced by the MS signals, which is in good agreement with literature report.8, 31 In the
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case of AB/CEMC, dehydrogenation starts at much lower temperature of around 50 C and
DOI: 10.1039/C7TA02461D
peak dehydrogenation temperature is centred at 86 C without any by-products detectable by
MS. The absence of MS signals for by-products indirectly confirms that AB was totally
deposited inside the pores of carbon scaffold rather than on the outside surface of the carbon.
It has been demonstrated that when AB is fully located inside the pores of the scaffold, the
formation of by-products is completely suppressed, whilst overloading of AB leads to partial
AB deposited on the outside surface of carbon scaffold, which performs similar to bulk AB
and results in the generation of by-products, due to the size effect.62 Such a remarkable
decrease of decomposition temperature for AB in CEMC is due to the effect of nano-
confinement. It is worth pointing out that this is significant improvement compared to the
mesoporous carbons as the scaffolds. For example, the peak dehydrogenation temperature of
AB in mesoporous CMK-3 scaffold is 95 C yet accompanied by the emission of ammonia
and borazine; even when Li was introduced as the catalyst, the AB/Li-CMK-3 composite
shows peak dehydrogenation temperature of 90 C.49 In addition, those coherent carbon
cryogels as scaffolds results in peak dehydrogenation temperatures at 90, 98, 102 and 110 C
48
respectively.32,
When activated carbon is used as the scaffold, the composite
dehydrogenates at room temperature, causing safety issues.50 Therefore this is the best result
so far for porous carbon materials as the scaffold. It outperforms the poly(methyl acrylate) as
scaffold too, which gives a peak temperature of 95 C.63 It is also better than33 or comparable
to those AB/MOF composites (84 C).8 We believe it is due to the small pore size and very
narrow pore size distribution of the templated carbon CEMC, which enhances the
confinement effect dramatically, in very good agreement with published results that smaller
pores lead to better dehydrogenation performance.48
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DOI: 10.1039/C7TA02461D
114 oC
H
m/e = 2
Net AB
AB/CEMC
2
86 oC
80
40
60
100 120 140 160 180 200
107 oC
Net AB
NH
3
AB/CEMC
m/e = 17
40
60
80
100
120
140
160
180
200
B2H6
m/e = 27
Net AB
AB/CEMC
100 oC
149 oC
40
60
80
100
120
140
o
160
180
200
Temperature ( C)
Fig. 2 The MS signals of H2, NH3 and B2H6 from neat AB and AB/CEMC composite
between 40 and 200 oC at a heating rate of 2 oC/min in Ar.
The dehydrogenation rate of AB and AB/CEMC at various temperatures are shown in Fig.
3. AB/CEMC releases 7.6, 5.0 and 3.6 wt % hydrogen within 5 min at 90, 80 and 70 °C
respectively, while neat AB does not release any hydrogen in the first 10 min regardless the
temperatures under study, due to the long induction period for dehydrogenation of neat AB.
Moreover, Around 9.9 and 7.0 wt% hydrogen released from AB confined in CEMC within 10
min at 90 and 80 C respectively, which is much higher than the DOE bench mark hydrogen
storage target of 5.5 wt% by 2015.7 There have been no reported results on the
dehydrogenation rate of AB confined in carbon materials but the dehydrogenation rate of AB
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in CEMC is comparable to the results of AB confined in MOF JUC-32-Y.DOIT: 1h0.e103h9/iCg7hTA02461D
hydrogen storage capacities combined with the fast hydrogen release rate of AB in CECM
enables AB/CEMC promising for on-board applications. As shown in Fig 3B, after 40 min
the hydrogen release of AB/CEMC at different isotherm temperature only shows slightly
increase and at 70 °C the hydrogen release from AB/CEMC even remains unchanged, but at
90 °C the hydrogen release from AB/CEMC is gradually up to ~13.0 wt % within 2 h, which
is the theoretical hydrogen for the first two-step dehydrogenation of AB.
14
12
10
8
12
10
8
90 oC
90 oC
B
A
80 oC
70 oC
80 oC
70 oC
6
12
10
8
Neat AB
6
4
6
4
o
90
C
4
80 o
C
2
2
2
0
0
5
10 15 20 25 30
Time (min)
0
0
0
5
10 15 20 25 30
Time (min)
0
20 40 60 80 100 120
Time (min)
Fig. 3 Time dependence of hydrogen release of AB/CEMC at various temperatures for 30
min (A) or 2 h (B) respectively. The inset in left figure is the time dependence of hydrogen
release of neat AB.
Due to the remarkable improvement in lowering the dehydrogenation temperature and fast
dehydrogenation rate of AB confined in CEMC, we therefore evaluated the activation energy
of the dehydrogenation for both neat AB and AB/CEMC to gain insight into the driving force
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of the dramatic improvement. The activation energy was calculated by non-isothermal DSC
DOI: 10.1039/C7TA02461D
runs under various heating rates, based on the Kissinger equation as follows:64
푛−1
(
)
훽
푇푚2푎푥
퐸푎
1
푛퐴푅 1 − 푎푚
푙푛
= −
(
) + 푙푛
(1)
푅 푇푚푎푥
퐸푎
Where 훽 is the heating rate; Tmax is the maximum dehydrogenation temperature at different
heating rates; Ea is the apparent dehydrogenation activation energy value; R is the universal
gas constant and the A is a pre-exponential factor. The DSC profiles of neat AB at ramp rate
of 2, 5, 10 15 °C/min and AB/CEMC at 2, 10 and 20 °C/min respectively are shown in Fig.
S1 of ESI, and the Kissinger plots of AB and AB/CEMC derived from those DSC profiles are
presented in Fig. 4. The activation energy of neat AB is about 131 kJ mol-1, a little bit lower
than reported values (180 kJ mol-1,31 160 kJ mol-1 48). The activation energy of AB/CEMC is
about 75 kJ mol-1, which is 56 kJ mol-1 lower than that of neat AB, indicating 43% decrease
in activation energy. This is significant improvement compared to the AB/coherent carbon
composites, which showed only 10 and 40 kJ mol-1 reduction respectively in the activation
energy.48 Due to the geometrical constraints of porous scaffold, the nano-confinement effect,
as demonstrated in this work and the literature reports, can significantly lower the activation
energy of the dehydrogenation of AB, which favours the breaking of B-H bond and N-H
bond to produce H2, but prevents the breaking of B-N bond, consequently supresses the
formation of NH3 and B2H6. In this work, the very narrow pore size distribution of the studied
microporous carbon contributes to the remarkable decrease in activation energy of AB,
therefore it accelerates the hydrogen release rate and depresses the emission of by-product
including ammonia and diborane, consequently it results in high capacity of pure hydrogen
release from the AB confined in CEMC.
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DOI: 10.1039/C7TA02461D
-8.5
-9.0
AB/CEMC
-9.5
y = - 9.0022x + 13.4900
Ea = 75 kJ mol-1
-10.0
-10.5
-11.0
-11.5
Neat AB
y = - 15.7125x + 29.7633
Ea = 131 kJ mol-1
2.45 2.50 2.55 2.60 2.65 2.70 2.75
1000/T (K-1)
Fig. 4 Kissinger plots for net AB and AB/CEMC obtained from DSC with different heating
rates.
Conclusions
This report has demonstrated that confining AB in microporous carbon with very narrow pore
size distribution of 1-1.5 nm without any catalysts involved can significantly lower the
o
hydrogen release temperature of AB from 114 C down to 50C with peak dehydrogenation
temperature centred at 86 C, remarkably improve the dehydrogenation kinetics and
completely prohibit the emission of by-products including ammonia and diborane. This
improvement is due to the dramatic decrease in the activation energy of AB confined in
microporous carbon. This is the best performance to date for AB confined in porous carbon
materials, making AB/CEMC a very promising candidate for hydrogen storage.
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Acknowledgements
DOI: 10.1039/C7TA02461D
The financial support by the Royal Society and University of Exeter is greatly
acknowledged.
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DOI: 10.1039/C7TA02461D
Table of Contents entry:
Ammonia borane confined in microporous carbon with narrow pore size significantly reduces
dehydrogenation temperature, prohibits by-products and improves dehydrogenation rate.
12
H2 release
o
AB/CEMC at 90
C
114 oC
10
8
6
86 oC
4
AB/CEMC
Neat AB
2
o
Neat AB at 90
C
0
40 60 80 100 120 140 160 180 200
Temperature (oC)
0
5
10
15
20
25
30
Time (min)