Reversible hydrogen desorption from LiBH4 catalyzed by graphene supported Pt nanoparticles

Article abstract of DOI:10.1039/c3dt50933h

The thermally induced de-/rehydrogenation performance of the graphene supported Pt nanoparticles (Pt/G) doped LiBH₄ was greatly improved even at very low catalyst content due to a synergetic effect of Pt addition and nanoconfinement in graphene. For the 5 wt% Pt/G doped LiBH₄ sample, the onset hydrogen desorption temperature is about 140°C lower than that of the pure LiBH₄. With increasing loading of the Pt/G catalysts in LiBH₄ samples, the onset dehydrogenation temperature and the two main desorption peaks from LiBH₄ were found to decrease while the hydrogen release amount increased. About 17.8 wt% can be released from the 50 wt% Pt/G doped LiBH₄ sample below 500°C. Moreover, variation of the equilibrium pressure (350-450°C) indicates that the dehydrogenation enthalpy is reduced from 74 kJ mol^-1 H₂ for the pure LiBH₄ to ca. 48 kJ mol^-1 H₂ for the 10 wt% Pt/G doped LiBH₄, showing improved thermodynamic properties. More importantly, a reversible capacity of ca. 8.1 wt% in the 30th de-/rehydrogenation cycle was achieved under 3 MPa H₂ at 400°C for 10 h, indicating that the Pt/G catalysts play a crucial role in the improvement of the hydrogen uptake reversibility of LiBH₄ at lower temperature and pressure conditions. Especially, LiBH₄ was reformed and a new product, Li₂B₁₀H₁₀, was detected after the rehydrogenation process.

Full text of DOI:10.1039/c3dt50933h

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Reversible hydrogen desorption from LiBH catalyzed
4
by graphene supported Pt nanoparticles
Cite this: Dalton Trans., 2013, 42, 12926
a,b
a,b
a
a
a
Juan Xu, Zhongqing Qi, Jianyu Cao, Rongrong Meng, Xiaofang Gu,
a
a,b
Wenchang Wang and Zhidong Chen*
The thermally induced de-/rehydrogenation performance of the graphene supported Pt nanoparticles
Pt/G) doped LiBH was greatly improved even at very low catalyst content due to a synergetic effect of
Pt addition and nanoconfinement in graphene. For the 5 wt% Pt/G doped LiBH sample, the onset
hydrogen desorption temperature is about 140 °C lower than that of the pure LiBH . With increasing
loading of the Pt/G catalysts in LiBH samples, the onset dehydrogenation temperature and the two
main desorption peaks from LiBH were found to decrease while the hydrogen release amount increased.
About 17.8 wt% can be released from the 50 wt% Pt/G doped LiBH sample below 500 °C. Moreover,
(
4
4
4
4
4
4
variation of the equilibrium pressure (350–450 °C) indicates that the dehydrogenation enthalpy is
reduced from 74 kJ mol−
1
H
2
for the pure LiBH
to ca. 48 kJ mol−1
H
for the 10 wt% Pt/G doped LiBH
4
2
4
,
showing improved thermodynamic properties. More importantly, a reversible capacity of ca. 8.1 wt% in
the 30th de-/rehydrogenation cycle was achieved under 3 MPa H at 400 °C for 10 h, indicating that the
Pt/G catalysts play a crucial role in the improvement of the hydrogen uptake reversibility of LiBH at
Received 9th April 2013,
Accepted 2nd May 2013
2
4
DOI: 10.1039/c3dt50933h
4 2 10 10
lower temperature and pressure conditions. Especially, LiBH was reformed and a new product, Li B H ,
www.rsc.org/dalton
was detected after the rehydrogenation process.
rehydrogenation reaction of LiBH4 more reversible and
capable of operation under mild conditions.
Various additives have been utilized to thermodynamically
1
. Introduction
1
3
One key issue for the development of sustainable hydrogen
economy is to explore efficient, reversible, safe and cheap destabilize LiBH4, reduce the desorption enthalpy and
1
hydrogen storage materials. In particular, complex hydrides enhance de-/rehydrogenation kinetics. Züttel et al. reported
2
–5
(
(
LiBH
4
,
NaBH
4
,
Al
3
Li
4
(BH
4
)
13
,
NaNH
2
,
etc.),
alloys that the initial hydrogen desorption temperature was
6
–8
La MgNi , Mg MnNi , Mg0.9Ti0.1Ni, Mg0.9Ti0.1NiAl0.05, etc.),
metal hydrides (MgH , Mg NiH , Mg CoH , Mg MnH , etc.)
decreased from 400 to 240 °C and about 9 wt% of hydrogen
2
9
3
2
9
,10
can be released below 400 °C in the case that LiBH was ball-
2
2
4
2
5
3
7
4
1
4
and carbon materials (graphene, carbon nanotube, meso- milled with SiO
2
micro particles with a mass ratio of 1 : 3.
have attracted considerable attention Pan et al. reported that ca. 6.3 wt% hydrogen can be released
due to their high storage capacities. from the LiBH –0.25TiF composite below 150 °C with dra-
LiBH is presently considered as one of the most promising matically improved kinetics. It was found that the synergetic
hydrogen storage materials for hydrogen-powered vehicles as it effect of LaH3 and MgH on LiBH4 is more effective than
1
1,12
porous carbon, etc.)
4
4
1
5
4
2
1
6
offers a high theoretical hydrogen storage density (18.5 wt% MgH or LaH alone, and ca. 8.0 wt% hydrogen can be rever-
2
3
H
2
). However, the pure LiBH
4 4 2 2
is so unfavorably stable that only sibly stored in the 6LiBH –CaH –3MgH composite below
1
7
a limited degree of hydrogen uptake was acquired even at 400 °C. As reported by Shim et al., the dehydrogenation reac-
2
6
00 °C and 35 MPa H , which limits its practical applications tion enthalpy of the LiBH –YH composite was reduced to
2 4 3
for on-board hydrogen storage. Therefore, considerable strat- 51 KJ mol−
1
H
.
18
Moreover, the catalytic effect of transition
2
1
9–23
egies such as the destabilization with various effective cata- metals (Ni, Co, Ru, Mg, Fe, etc.),
lysts, confinement in nanoporous materials, and partial MnCl , CuCl , CaHCl, NbF , etc.),
cation substitution have been adopted to make the de-/ CeO
metal halides (CdCl2,
and metal oxides (TiO2,
on the hydrogen storage pro-
2
4–29
2
2
5
3
0,31
2
, SiO
2
, Fe
2
O
3
, Al
2
O
3
, etc.)
perties of LiBH have been widely investigated. Although these
4
catalyst additives can lower the dehydrogenation temperature,
increase the dehydrogenation capacity, and improve the rever-
sibility, the requisite weight percent of most catalysts is so
high that usually lead to an unsatisfactory weight ratio of
a
Jiangsu Key Laboratory for Solar Cell Materials and Technology, School of
Petrochemical Engineering, Changzhou University, Changzhou 213164, China.
E-mail: zdchen.lab@gmail.com; Fax: +86-0519-86330239; Tel: +86-0519-86330239
b
Qualtec Co. Ltd., Changzhou 213164, China
1
2926 | Dalton Trans., 2013, 42, 12926–12933
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hydrogen. Thus, one essential research object is the quest for Sinopharm Chemical Reagent Co. Ltd, China), KMnO
the efficient and low levels of catalysts. purity, Sinopharm Chemical Reagent Co. Ltd, China), H SO
On the other hand, improved thermal decomposition beha- (96% purity, Sinopharm Chemical Reagent Co. Ltd, China),
viors were successfully achieved by confining LiBH in porous (30% aqueous solution, Sinopharm Chemical Reagent
4
(99%
2
4
4
2 2
H O
carbon materials to avoid the agglomeration effect. As con- Co. Ltd, China), carbon supported Pt nanoparticles (Pt/C, with
firmed by Yu et al., the initial hydrogen desorption temp- a Pt loading of 60 wt%, HispecTM 9000, Johnson Matthey Inc.,
2
−1
erature was lowered to 250 °C and about 3.8 wt% hydrogen was UK), platinum black (Pt, with surface area 28 m g , Johnson
released from the ball-milled LiBH and multiwalled carbon Matthey Inc., UK), and polyvinyl pyrrolidone (PVP, Sinopharm
4
3
2
nanotube (MWCNT) composite. Agresti et al. also reported Chemical Reagent Co. Ltd, China) were used as received.
that the dehydrogenation temperature of LiBH was decreased
by more than 60 °C when LiBH was dispersed on MWCNT by
4
2.2. Preparation of graphene oxide (GO)
4
3
3
a solvent infiltration method. About 7.5 wt% hydrogen was GO used in present study was prepared by a complete oxi-
3
4
47
generated by the hydrolysis of the MWCNT doped LiBH
4
.
In dation and microwave irradiation method. First, 1 g natural
the meantime, other carbon catalysts like single-walled carbon graphite powder was mixed with 1 g NaNO in a reaction vessel
3
3
5
36–38
nanotubes, mesoporous carbon,
highly ordered hexa- that was preliminarily immersed in an ice bath and then,
SO was slowly added under violent stirring. In order
2 4
fibers and fullerenes (C60) were respectively used as sub- to fully oxidize graphite powder to GO, 5 g KMnO was gradu-
3
9
40,41
gonal nanoporous hard carbon, carbon aerogel,
carbon 35 mL H
4
2
43
4
4
strates for LiBH , and various destabilized hydrogen storage ally added into the above mixture over about 1 h at room temp-
systems were generated. Specifically, we found that the gra- erature. Subsequently, the GO sample was added into a
phene catalysts doped LiBH4 showed superior dehydrogena- 100 mL 5 wt% H SO aqueous solution over about 2 h with
2
4
tion and rehydrogenation performances to Vulcan XC-72, gentle stirring, and then 100 mL 30 wt% H
carbon nanotube and BP2000 doped LiBH .
4
2
O
2
was added.
Nevertheless, After centrifugation and washing with a mixed aqueous solu-
carbon catalysts with a high doping amount often results in tion of 3 wt% H SO and 0.5 wt% H O , the GO powder
4
4
2
4
2 2
the formation of inert fractions in the dehydrogenated sample. obtained was irradiated for 1 min in a domestic microwave
Nearly or even more than 50 wt% capacity loss in the first de-/ oven (1100 W) and then was filtrated, washed and dried at
rehydrogenation cycle was repeatedly observed in the high 65 °C for 24 h.
3
2,33,36,37,44
levels of carbon doped LiBH
4
samples.
2.3. Synthesis of Pt/G catalysts
Recently, it was found that the dehydrogenation perform-
ance of LiBH4 can be greatly improved by introducing both Pt/G catalysts were prepared by an ethylene glycol (EG) co-
metallic catalysts and porous carbon materials. For example, reduction method under flowing argon. First of all, 200 mg GO
Jongh et al. reported that the dehydrogenation temperature was dispersed in a 400 mL 0.5 wt% PVP solution and exfoliated
4
5
was decreased to ca. 350 °C, and about 14 wt% of hydrogen into single-layered GO sheets by ultrasonication (JAC 4020,
1
9
can be absorbed at 320 °C under 4 MPa H
2
because of the 400 W, Sonic) for 100 min. After being filtered, rinsed with EG
synergetic effects of Ni addition and nanoconfinement of and dried at 50 °C, 100 mg GO was uniformly dispersed in a
nanoporous carbon. Mao et al. also reported that MWCNTs 100 mL EG solution. Meanwhile, 1.2 mL 0.0962 M Na PtCl ,
2
6
supported Ru nanoparticles can reduce the onset dehydro- 2.1 mL 0.1 M EDTA, and 8 mL water were blended together,
genation temperature of the mixture of LiBH and MgH to and kept at 60 °C for 40 min under vigorous stirring to form a
44 °C. In our previous investigation we also found that the bright yellow Pt-EDTA complex. After cooling the Pt-EDTA solu-
4
2
2
1
3
doped with commer- tion to room temperature, the above 1 mg mL− GO suspen-
1
hydrogen release properties of the LiBH
cial carbon-supported Pt nanoparticles (Pt/C) can be improved sion was dropwise added under uniformly stirring for 4 h.
4
4
6
significantly. Despite these progresses, a huge challenge still Subsequently, a solution of 3.0 M NaOH in EG was added to
remains to achieve reversible hydrogen uptake at moderate adjust the pH value to above 13, and then the mixture was
conditions and high hydrogen capacity simultaneously. Herein heated at 120 °C for 8 h to reduce the Pt and GO completely.
we investigate the hydrogen release and uptake processes of After filtration, washing and drying under vacuum at 70 °C for
the as-milled graphene-supported Pt nanoparticles (Pt/G) 8 h, the Pt/G catalyst with a 60 wt% Pt loading was obtained.
doped LiBH
4
sample, as graphene possess huge surface area to The filtrated solvent was colorless and the weight calculation
uniformly disperse Pt NPs, and suitable pore size to confine based on thermogravimetric analysis showed that the Pt
LiBH
4
system.
loading amount in the Pt/G catalyst is nearly 60 wt% during
the deposition process.
2
.4. Doping LiBH with Pt/G catalysts
4
2
. Experimental section
4
The mixture of LiBH and Pt/G catalyst was prepared through a
ball milling method: 1 g LiBH and Pt/G mixture with various
2
.1. Materials
4
Commercial LiBH
4
(95% purity, J&K Chemical Ltd, Sweden), mass ratios was mechanically milled for 1.5 h (Planetary
natural graphite (99.9995% purity, Alfa Aesar Ltd, UK), QM-1SP2) under argon atmosphere at room temperature. The
Na PtCl ·6H O (Alfa Aesar Ltd, UK), NaNO (99% purity, ball-to-power weight ratio was 30 : 1 at 580 rpm using stainless
2
6
2
3
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steel balls with 10 mm diameter. According to the same prepa-
ration process, pure graphene or Pt nanoparticles doped LiBH4
was obtained. All the sample preparation was performed in the
3
. Results and discussion
3.1. Microstructure characterization of the Pt/G catalysts
glove box under argon atmosphere to minimize H
contamination.
2
O and O
2
Fig. 1 presents TEM image of the Pt/G catalyst as well as its
corresponding particle size distribution histogram. As
observed, highly dispersed Pt nanoparticles with a relatively
narrow particle size distribution are uniformly attached to
large, disordered and over-lapped multilayer graphene sheets
by a simple and cost-effective procedure. No aggregated Pt
nanoparticles were observed, showing that graphene can
stabilize the Pt nanoparticles and prevent them from aggrega-
tion. The mean size of the Pt nanoparticles on graphene
support is ca. 4.1 nm, which was obtained by measuring more
than 200 nanoparticles randomly.
2
.5. Hydrogen releasing property measurements
4
In order to compare with the neat LiBH system, the hydrogen
capacity of the Pt/G doped LiBH sample is calculated based
4
only on the mass of pure LiBH4 if it was not specifically
pointed out, and the amount of impurities and added catalysts
has been deducted.
Hydrogen desorption performance was investigated using a
Netzsch STA449C TG-DSC thermoanalyzer coupled with a
Balzers Thermostar Quadrupole Mass Spectrometer. Samples
−1
were heated at a rate of 10 °C min under argon flowing at a 3.2. Dehydrogenation properties of the Pt/G catalyst doped
3
−1
purging rate of 20 cm min
.
LiBH4
Cyclic hydrogen releasing properties were examined by volu-
metric method using a carefully calibrated Sievert’s type appar-
atus. Typically, the 10 wt% Pt/G doped LBH sample was
4
repeatedly dehydrogenated at 550 °C for 5 h, and rehydroge-
nated at 400 °C with an initial 3 MPa hydrogen pressure for
Fig. 2 presents temperature programmed hydrogen release pro-
files and corresponding thermogravimetric (TG) curves of
LiBH
amounts, while the detailed data are listed in Table 1. The
pure LiBH requires a temperature of ca. 420 °C to desorb
4
doped with the Pt/G catalyst at different doped
4
1
0 h. As it is difficult to accurately characterize the rehydro-
main hydrogen and reaches two peaks at 485 and 610 °C. The
total weight loss is 10.7 wt% (curve a), indicating that only half
genation process under high hydrogen pressure, the restored
hydrogen amounts were precisely determined in the sub-
sequent dehydrogenation half-cycle.
of the hydrogen was released from the pure LiBH
even by adding 5 wt% of the Pt/G catalyst to the LiBH
4
. However,
com-
4
pound, the onset hydrogen desorption temperature of LiBH4
still shifts to 280 °C, about 140 °C lower than that of the pure
2
.6. Structure and morphology characterization
4
The morphology of the Pt/G catalysts was analyzed by trans- LiBH milled for an identical period, and the other main dehy-
mission electron microscopy (TEM) using a Technai G2 20s- drogenation peaks of the Pt/G doped LiBH locates at 340 and
4
Twin Microscope (FEP Inc., USA). Samples were prepared by 525 °C, with an increased total weight loss of 12.6 wt% (curve
dispersing the dry Pt/G powder in ethanol to form a homo- b). With increase in the Pt/G content, the onset dehydrogena-
geneous suspension, and then dropped on a 300-mesh copper tion temperature and two main desorption peaks from the
grid for observation.
4
mixtures of Pt/G and LiBH (up to 50 wt%), are gradually
X-ray diffraction (XRD) measurements were carried out to decreased while the total hydrogen release amount was found
investigate the structure change and infer the possible reaction to increase. When LiBH is doped with 50 wt% Pt/G, the onset
4
mechanism during the de-/rehydrogenation process. Diffrac- dehydrogenation temperature approaches 150 °C, and the two
tion patterns were collected on a Rigaku D/MAX-2500 diffracto- major desorption peaks occur at 200 and 330 °C (curve e in
meter with Cu Kα radiation at a scanning rate of 2° min−
and with a step of 0.02°.
1
Fig. 2(A)). Importantly, the dehydrogenation capacity of the
50 wt% Pt/G catalyst doped LiBH is 17.8 wt% (curve e in Fig. 2
4
Fig. 1 TEM image (a) and corresponding particle size distribution histogram (b) of the Pt/G catalyst.
1
2928 | Dalton Trans., 2013, 42, 12926–12933
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inset of Fig. 2(B). Compared with the weight loss of 10.7 wt%
for the pure LiBH (curve a), the real dehydrogenation capacity
4
4
of the 5 wt%, 10 wt% and 30 wt% Pt/G doped LiBH sample
reaches 12.1, 12.5 and 11.5 wt% (curve b–d), respectively. On
the other hand, the real hydrogen release amount from the
5
0 wt% Pt/G doped LiBH
4
sample is only 8.9 wt% (curve e),
, indicating that the overhigh
less than that of the pure LiBH
4
doping amount can lead to an unsatisfactory weight ratio of
hydrogen.
2 6 4
Fig. 3 shows B H release of the pure LiBH , 10 wt% gra-
phene and Pt/G doped LiBH . Compared with the pure LiBH ,
4
4
no B
graphene doped LiBH
50 °C, which may come from the micro-mesopores structure
of graphene, in good agreement with the data of LiBH doped
These results
clearly demonstrate that the total weight loss of the Pt/G
doped LiBH is solely attributed to the hydrogen release.
2
H
6
gas was detected from the MS profile of the Pt/G or
4
during the heating process up to
5
4
4
8,49
by titanate nanotube and carbon templates.
4
In order to elucidate the possible roles of graphene and Pt
nanoparticles in the catalytical dehydrogenation of LiBH , a
4
comparison of dehydrogenation behaviors of LiBH
with graphene, pure Pt nanoparticles and Pt/G catalysts at
0 wt% doping amount is determined in Fig. 4. In the case of
LiBH doped with the pure graphene catalyst, the first dehy-
drogenation temperature for the LiBH sample starts at
4
doped
5
4
4
ca. 210 °C, and the temperatures for two major desorption peaks
are 290 and 465 °C, with a hydrogen released amount of
12.8 wt% (curve a), demonstrating better hydrogen release pro-
perties than the pure LiBH . For the LiBH doped with pure Pt
4
4
nanoparticles, the first dehydrogenation temperature locates at
ca. 305 °C, two main hydrogen desorption peaks occur at 375
Fig. 2 Temperature programmed hydrogen release profiles (A) and TG curves and 590 °C, and 15.4 wt% hydrogen can be released (curve b).
(
B) based on pure LiBH for the evolution of hydrogen from the mixture of Pt/G
4
By adding the Pt/G catalyst to LiBH
4
, the initial dehydrogena-
and LiBH
and LiBH
4
. The weight percentages of as-prepared Pt/G in the mixture of Pt/G
are 0 (a), 5 wt% (b), 10 wt% (c), 30 wt% (d) and 50 wt% (e), respect-
tion temperature of LiBH transfers to 150 °C, and the main
4
4
dehydrogenation peaks of LiBH shift to 200 and 330 °C, with
ively. The inset shows the real H
lysts and LiBH
2
wt% based on the total amount of Pt/G cata-
4
.
an greatly increased weight loss of 17.8 wt% (curve c). In
4
Table 1 Hydrogen release properties of the mixtures of Pt/G-60 and LiBH
4
with various mass ratios
Pt/G weight Onset
Main
Tdesorb (°C) (wt%)
H
2
desorbed
a
a
Sample
LiBH
Doped LiBH
Doped LiBH
Doped LiBH
Doped LiBH
(wt%)
T
desorb (°C)
4
0
5
10
30
50
420
280
230
190
150
485; 610
340; 525
290; 480
235; 440
200; 330
10.7
12.6
13.7
16.4
17.8
4
4
4
4
a
T
desorb represents the desorption temperature.
(
B)), which is very close to the theoretical value of ca. 18.5 wt%.
The onset dehydrogenation temperature is greatly decreased
and the quantity of hydrogen desorbed from LiBH is highly
4
increased even using the Pt/G catalyst with a very low content.
The real hydrogen release amount based on the total
Fig. 3
B
2
H
6
release with increasing temperature for the pure LiBH
4
(a), gra-
amount of both the Pt/G catalysts and LiBH
4
is shown in the phene (b) and Pt/G (c) doped LiBH
4
. The loading amount of catalysts is 10 wt%.
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Fig. 5 Desorption PCT curves for pure LiBH
LiBH
Hoff plot for the dehydrogenated Pt/G doped LiBH
4
at 400 °C (a), 10 wt% Pt/G doped
4
at 350 (b), 400 (c), and 450 °C (d), respectively. The inset shows the van’t
sample.
4
Fig. 5. No plateaus can be observed for the pure LiBH4
sample measured at 400 °C (curve a), while the dehydrogena-
tion isotherms of the 10 wt% Pt/G doped LiBH show obvious
4
plateaus (curves b–d). As for the dehydrogenation at 450 °C,
the dehydrogenation plateau was kept from 4.7 to 9.3 wt%
and the total hydrogen capacity is approximately 10.3 wt%
(
curve d).
The van’t Hoff plot for the dehydrogenation reaction of the
0 wt% Pt/G doped LiBH4 system (logarithm of the equili-
1
brium pressure versus the inverse of the absolute temperature),
using the medium equilibrium pressures, is shown in the
inset of Fig. 5. According to van’t Hoff equation and the deter-
mined thermodynamic parameters, the resultant van’t Hoff
Fig. 4 Temperature programmed hydrogen release profiles (A) and TG curves
(B) of the pure graphene (a), pure Pt nanoparticles (b) and Pt/G (c) catalysts
doped LiBH . The doping amount of various catalysts is 50 wt%.
4
4
equation of the 10 wt% Pt/G doped LiBH sample can be
numerically expressed as ln(peq./p ) = −5730/T + 10.34, and the
0
comparison to the pure LiBH , the dehydrogenation of the gra-
phene or Pt nanoparticles doped LiBH shifts to lower temp-
4
erature and larger amount of hydrogen can be released,
suggesting that both graphene and Pt nanoparticles can dra-
4
dehydrogenation reaction enthalpy changes (ΔH) are calcu-
−
1
lated as ca. 48 kJ mol
H
2
, which is far less than that of pure
−
1
33,50
LiBH (ca. 74 kJ mol H2),
demonstrating that the dehy-
4
4
drogenation thermodynamics of LiBH is largely improved
through the doping of Pt/G catalysts. It may due to the nano-
matically destabilize LiBH
present significantly higher catalytic effect on LiBH
phene and Pt nanoparticles alone, proving that the observed
catalytic enhancement of LiBH is the combined effect of gra-
4
. Furthermore, the Pt/G catalysts
4
than gra-
confinement effect of LiBH within porous graphene scaffold
4
5
1–53
hosts.
4
phene and Pt nanoparticles. The remarkable improvement in
dehydrogenation properties is likely attributable to increased
3.3. Rehydrogenation properties of the Pt/G doped LiBH4
4
contact area between Pt/G catalyst and LiBH , newly produced The theoretical reversible hydrogen storage of the pure LiBH4
structure defects during the ball-milling process for forming below 600 °C is governed by the equilibrium, LiBH ⇌ LiH +
4
the composite material, and nanoconfinement effect of LiBH4 B + 1.5H , which accounts for a reversible hydrogen capacity
2
in graphene.
of 13.9 wt%. However, the rehydrogenation condition is
After the hydrogen desorption performances of the Pt/G rather demanding due to slow kinetics, which leads to
doped LiBH sample have been ascertained, its dehydrogena- serious cyclic capacity degradation. In order to develop a
4
tion pressure-composition isotherms were collected to further reversible H2 storage and release system for practical appli-
investigate its thermodynamic properties and dehydrogenation cations, the 10 wt% Pt/G doped LiBH sample that had been
4
behavior. Pressure–composition–temperature (PCT) measure- dehydrogenated for 5 h under a vacuum up to 550 °C
ments on the ball-milled pure LiBH
4
and 10 wt% Pt/G doped were rehydrogenated at 400 °C for 10 h under 3 MPa
LiBH were conducted in the range of 350–450 °C, as shown in hydrogen pressure. Fig. 6 presents a comparison on cycling
4
1
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Fig. 7 Comparison on cycle life of the 10 wt% Pt/G (a) and graphene (b)
4
doped LiBH .
somewhat lower than the 2015 hydrogen storage target set by
the DOE system: 9 wt% H , the reversible hydrogen amount
is much higher than those reported in literatures,
2
1
8,46,54,55
and substantially lower temperature and pressure conditions
were applied.
In order to clarify the possible role of graphene substrate in
the rehydrogenation process, TG curves of the 10 wt% Pt/G or
commercial Pt/C doped LiBH for the first and fourth dehydro-
4
genation are determined in Fig. 8. The dehydrogenation
capacities of the Pt/G or Pt/C doped LiBH at the first dehydro-
4
genation are 13.7 wt% and 13.1 wt%, respectively. For the
fourth dehydrogenation, the weight loss corresponds to
Fig. 6 Comparison on the dehydrogenation cycle profiles between the pure 11.2 wt% for the Pt/G doped LiBH (curve c), but only 5.3 wt%
4
LBH
sample can be dehydrogenated (at 550 °C for 5 h) and rehydrogenated (at 3
MPa H , 400 °C for 10 h) repeatedly and measured gravimetrically.
4 4 4
(A) and 10 wt% Pt/G doped LBH (B). The Pt/G catalyst doped LBH
for the commercial Pt/C doped LiBH
proved that the cyclic reversibility of LiBH
when the graphene substrate exists, which maybe due to that
4
(curve d). These results
was much better
4
2
dehydrogenation properties between the neat LiBH
4
and
1
0 wt% Pt/G doped LiBH after recombination with H .
4
2
Clearly, for the pure LiBH4 sample, only 2.1 wt% hydrogen
could be restored within 10 h at 400 °C with an initial hydro-
gen pressure of 3 MPa after the first rehydrogenation (Fig. 6
(
A)). In contrast, the total H released amount of the 10 wt%
2
4
Pt/G doped LiBH sample decreases from the initial 13.7 wt%
to the 12.2 wt% in the second cycle, and further to 11.0 wt% in
the fifth cycle and to 9.9 wt% in the tenth cycle (Fig. 6(B)).
These results indicate that the cyclic capacity and capacity
retention of the 10 wt% Pt/G doped LiBH
higher than that of pure LiBH4.
Fig. 7 shows cyclic capacity life of the 10 wt% Pt/G and
graphene doped LiBH , respectively. The capacity of the Pt/G
doped LiBH4 sample at the 30th cycles is still kept at
ca. 8.1 wt%, while that of the graphene doped LiBH sample
at the tenth dehydrogenation is only 3.2 wt% H , indicating
that Pt nanoparticles can greatly improve the hydrogen
release and uptake reversibility of LiBH . Although it is still
4
sample is much
4
4
2
Fig. 8 TG profiles for the 10 wt% Pt/G (a, c) and commercial Pt/C (b, d) doped
LiBH
4
at the first (a, b) and the fourth dehydrogenation (c, d) after recharging
4
2
H .
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4. Conclusions
The de-/rehydrogenation performances, reversibility and kine-
tics of LiBH were markedly enhanced upon mechanically
4
milling with the Pt/G catalysts even at extremely low catalyst
content due to the combined effect of graphene and Pt nano-
particles, and the graphene supported Pt nanoparticles pre-
sents significantly higher catalytic effect on LiBH than pure
4
graphene or Pt nanoparticles alone. Moreover, the dehydro-
−
1
genation reaction enthalpy is calculated as ca. 48 kJ mol
2
H ,
−
1
which is far less than that of the pure LiBH (ca. 74 kJ mol
4
H
2
), demonstrating largely improved dehydrogenation thermo-
dynamics of LiBH through the doping of the Pt/G catalysts.
4
Importantly, it should be noted that Pt nanoparticles sup-
ported on graphene play a crucial role in improving the hydro-
gen desorption and uptake reversibility of LiBH
4
at reduced
4
Fig. 9 XRD patterns for the ball-milled pure LiBH (a), 10 wt% Pt/G doped
temperature and pressure conditions. LiBH is reformed and
4
4
LiBH before (b) and after (c) the first dehydrogenation at 550 °C for 5 h, and
Li
0 h under 3 MPa hydrogen pressure, with a capacity of
approximately 8.1 wt% in the 30th cycle. Thus, destabilizing
LiBH using the Pt/G catalysts is a new and promising way for
2 10
B H10 is detected after being rehydrogenated at 400 °C for
after the tenth rehydrogenation (d) at 400 °C for 10 h with an initial hydrogen
pressure of 3 MPa.
1
4
graphene possess huge surface area to uniformly disperse Pt
nanoparticles.
4
the catalytic dehydrogenation and rehydrogenation of LiBH .
3
.4. Dehydrogenation/rehydrogenation reaction mechanism
Acknowledgements
To elucidate the possible reaction mechanism during the
re-/dehydrogenation process and investigate the kinetic enhance-
ment arising upon adding the Pt/G catalysts, Fig. 9 shows XRD
The authors gratefully acknowledge financial support from the
National Science Foundation of China (21003015 and
patterns of the ball-milled pure LiBH , 10 wt% Pt/G doped
4
21103014), the Science Foundation of Jiangsu Province
4
LiBH in the as-milled status, initial dehydrogenation condition
(
BK2012591, 12KJA150003, BE201113 and 2011Z0062), the
at 550 °C for 5 h, and the tenth rehydrogenation state at 400 °C
for 10 h. For the ball-milled pure LiBH , only LiBH and LiH
Science Foundation of Changzhou (CJ20115020), the Foun-
dation of Jiangsu Key Laboratory for Solar Cell Materials and
Technology (201106), Jiangsu Key Laboratory for Green Cata-
lytic Materials and Technology, Graduate Students Cultivation
and Innovation project of Jiangsu Province (CXZZ12-0734),
Qing Lan project of Jiangsu Province and the Priority Academic
Program Development (PAPD) of Jiangsu Higher Education
Institutions for support of this work.
4
4
diffraction peaks are observed (curve a); the minor appearance
of the LiH peaks may result from slight decomposition of LiBH
4
during the ball milling process. After the doping of Pt/G cata-
lysts, new Pt and LiC diffraction peaks appears besides the
4
LiBH and LiH (curve b), indicating that the Pt/G catalyst can
react with LiBH to form LiC. The decomposition product of B
4
1
1
from LiBH
results in literatures,
XRD. After the first dehydrogenation at 550 °C for 5 h, the
LiBH phase disappears, and Pt, LiC, LiH and Li PtH phases
4
is suggested to be amorphous state by B NMR
55,56
and therefore, cannot be detected by
4
5
3
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