Closing the Loop for Hydrogen Storage: Facile Regeneration of NaBH4 from its Hydrolytic Product

Article abstract of DOI:10.1002/anie.201915988

Sodium borohydride (NaBH₄) is among the most studied hydrogen storage materials because it is able to deliver high-purity H₂ at room temperature with controllable kinetics via hydrolysis; however, its regeneration from the hydrolytic product has been challenging. Now, a facile method is reported to regenerate NaBH₄ with high yield and low costs. The hydrolytic product NaBO₂ in aqueous solution reacts with CO₂, forming Na₂B₄O₇?10 H₂O and Na₂CO₃, both of which are ball-milled with Mg under ambient conditions to form NaBH₄ in high yield (close to 80 %). Compared with previous studies, this approach avoids expensive reducing agents such as MgH₂, bypasses the energy-intensive dehydration procedure to remove water from Na₂B₄O₇?10 H₂O, and does not require high-pressure H₂ gas, therefore leading to much reduced costs. This method is expected to effectively close the loop of NaBH₄ regeneration and hydrolysis, enabling a wide deployment of NaBH₄ for hydrogen storage.

Full text of DOI:10.1002/anie.201915988

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
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Accepted Article
Title: Closing the loop for hydrogen storage: Facile regeneration of
NaBH4 from its hydrolytic product
Authors: Yongyang Zhu, Liuzhang Ouyang, Hao Zhong, Jiangwen Liu,
Hui Wang, Huaiyu Shao, Zhenguo Huang, and Min Zhu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915988
Angew. Chem. 10.1002/ange.201915988
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http://dx.doi.org/10.1002/ange.201915988

Angewandte Chemie International Edition
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RESEARCH ARTICLE
Closing the loop for hydrogen storage: Facile regeneration of
NaBH from its hydrolytic product
4
[
a]
[a, b]
[a]
[a]
[a]
[c]
Yongyang Zhu, Liuzhang Ouyang,*
Zhenguo Huang,*[d] and Min Zhu[a]
Hao Zhong, Jiangwen Liu, Hui Wang, Huaiyu Shao,*
[
a]
Y. Zhu, Prof. L. Ouyang, Dr. H. Zhong, Prof. J. Liu, Prof. H. Wang, Prof. M. Zhu
School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials
South China University of Technology
Guangzhou, 510641, People's Republic of China
E-mail: meouyang@scut.edu.cn
[
[
b]
c]
Prof. L. Ouyang
China-Australia Joint Laboratory for Energy & Environmental Materials
Key Laboratory of Fuel Cell Technology of Guangdong Province
Guangzhou, 510641, People's Republic of China
Prof. H. Shao
Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering (IAPME), Department of Physics and Chemistry,
Faculty of Science and Technology
University of Macau
Macau SAR, China
E-mail: hshao@um.edu.mo
Prof. Z. Huang
[
d]
School of Civil and Environmental Engineering
University of Technology Sydney
Sydney, NSW, 2007, Australia
E-mail: zhenguo.huang@uts.edu.au
Supporting information for this article is given via a link at the end of the document.
Abstract: Sodium borohydride (NaBH
4
) is among the most studied
at
received strong attention since it has attractive features such as
higher capacity, better safety, and milder operation conditions.
hydrogen storage materials since it is able to deliver high purity H
2
room temperature with controllable kinetics via hydrolysis, but its
regeneration fromthe hydrolytic producthas been challenging. Herein
Sodium borohydride (NaBH
candidates as a hydrogen storage material. NaBH
4
) is among the most studied
can release
4
we report a facile method to regenerate NaBH
costs. The hydrolytic product NaBO in aqueous solution reacts with
CO forming Na ·10H O and Na CO , both of which are ball
milled with Mg under ambient conditions to form NaBH with a high
yield close to 80 %. Compared with previous studies, this new
approach avoids expensive reducing agent such as MgH , bypasses
the energy-intensive dehydration procedure to remove water from
Na ·10H O, and does notrequire high-pressure H gas, therefore
leading to much reduced costs. This method is expected to effectively
close the loop of NaBH regeneration and hydrolysis, enabling a wide
deployment of NaBH for hydrogen storage.
4
with high yield and low
hydrogen via hydrolysis with good controllability, high hydrogen
purity, high gravimetric hydrogen storage capacity and
2
[2]
2
2B
4
O
7
2
2
3
environmentally benign by-products. The hydrolysis of NaBH
typically expressed by the following reaction:
4
is
4
NaBH
The spent fuel is normally hydrated sodium metaborate
(NaBO ·xH from the hydrolytic
O).[3] The regeneration of NaBH
product so far has featured high costs and low yields.
NaBO in aqueous solution reacts with CO in air forming
·10H O and NaCO according to the following reaction:
4NaBO ·xH O + CO + (10-4x) H O → Na ·10H O +
Na CO (2)
O is the main constituent of naturally abundant
borax mineral. It is therefore highly appealing to develop a simple,
efficient and affordable approach to generate NaBH from
Na ·10H O.
4 2 2 2 2
+ (2+ x)H O → NaBO ·xH O + 4H (1)
2
2
2
4
2
B
4
O7
2
2
2
2
4
Na B
2 4
O
7
2
3
4
2
2
2
2
B O
2 4 7
2
2
3
2 4 7 2
Na B O ·10H
Introduction
4
2 4
B
O
7
2
Hydrogen has been deemed as an ideal energy carrier due
to its high energy density by weight, high abundance, and
environmental friendliness.[1] However, wide utilization of
hydrogen energy has been hampered by a few barriers with one
of them associated with storage. Due to its low energy density by
volume, hydrogen has been conventionally compressed or
liquefied to improve the density. However, these physical
processes cause large energy penalty and special care is always
required when operating under high pressure or cryogenic
conditions. Materials-based hydrogen storage has therefore
Currently, two types of raw materials, H
hydride (H ) have been used as hydrogen sources in the
2
(H°) and metal
¯
(
re)generation of NaBH
synthesized by annealing Na
pressure H
4
.
For example, NaBH
with Na and SiO
4
can be
2
B
4
O
7
2
under high
[4]
2
(over 3 MPa) at elevated temperature (400–500°C).
This method is of high cost because the reaction conditions are
harsh and a considerable amount of sodium metal is needed.
Recently, the preparation of NaBH
annealing the dehydrated borax (Na
magnesium (Mg) at a high temperature (550°C) and high H
4
has been carried out by
2 4 7
B O )
with low-cost
2
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Figure 1. (a) A closed system of NaBH
Na CO in 24.75:1:1 molar ratio for 20 h at 1000 CPM; (c) XRD patterns of standard PDF card of NaBH
SAED pattern of synthesized NaBH ; (e) Hydrolysis curve of the regenerated NaBH in an aqueous solution loaded with 2 wt% CoCl
PDF card of Na ·10H O, raw Na ·10H O, and compounds obtained after hydrolytic aqueous solution naturally dried up in air.
4
hydrolysis and regeneration; (b) XRD pattern of products obtained via ball milling a mixture of Mg, Na
2B4O7·10H2O, and
2
3
4
, commercial and synthesized NaBH ; (d) TEM image and
4
4
4
2; (f) XRD patterns of standard
2B4
O
7
2
2
B
4O
7
2
8b]
pressure (2.5MPa).[5] However, this process is also energy
solution via drying treatment of ˂ 110°C.[ Researchers tried ball
intensive and dangerous. The process can be further optimized
2 2 2
milling NaBO ·xH O (x=2, 4) with Mg-based alloys (e.g. Mg Si,
7
[6]
[ ]
by ball milling Na
MgH ) with a maximum yield of 78% and 76%, respectively. The
use of expensive MgH , however, makes mass production by
these methods less feasible. It should also note that high energy
is required to obtain Na by dehydrating Na ·10H O at
O at temperatures over
can
aqueous
B O
2 4 7
and NaBO
2
with magnesium hydrides
Mg-Al alloy) with/without Mg at room temperature under Argon
atmosphere.[9] The yield can be above 74%, but Mg-containing
alloys are more expensive than Mg and more by-products (stable
metal oxides for example) are formed during ball milling.[9]
Therefore, Mg instead of alloys should be used as a reducing
agent.
(
2
2
2 4
B
O
7
B O
2 4 7
2
approximately 600°C and NaBO
3
2
·xH
2
8
50°C, respectively.[ The hydrolytic by-product of NaBH
]
Herein, we present a new method where H in the coordinate
+
4
exist in the form of NaBO
2
·xH
2
O (x=2, 4), from a NaBO
2
2 4 7 2
water in Na B O ·xH O (x=5, 10) can be directly used as a
2
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
hydrogen source to prepare NaBH
mixture of Na ·xH O (x=5, 10) and Na
exposing NaBO aqueous solution to CO and then drying at a
temperature of < 54°C. Ball milling the mixture with Mg at room
temperature and under atmospheric pressure Argon leads to the
4
. In the new procedure, a
075-1078) powers were obtained, as evidenced by XRD patterns
(Figure 1f) and FTIR spectra (Figure S5). Another set of
2
B
4
O
7
2
2
CO is obtained by
3
2
2
characteristic
Na H(CO ·2H
The transformation of Na
following reaction:
diffraction peaks can be indexed to
O which is composed of Na CO and NaHCO
into NaHCO
3
3
)
2
2
2
3
3
.
2
CO
3
3
occurs in the
formation of NaBH
literature procedures (Table S1) that require dehydrated
Na /NaBO MgH high pressure and/or high
4
with a high yield of 78.9%. Compared with the
Na
2
CO
3
(aq) + CO
2
2 3
+H O → 2NaHCO (3)
2 4
B
O
7
2
,
2
,
H
2
,
NaHCO
hydrolytic aqueous solution to air. This is supported by
experiments that only Na ·10H O and Na CO can be
obtained by tuning the exposure of NaBO aqueous solution to
CO (Figure S6).
We illustrate a pathway to close the cycle of NaBH
3
can be avoided by regulating the exposure of the
temperatures, this new method utilizes low-cost materials and
operates under mild reaction conditions, which allows for facile
B O
2 4 7
2
2
3
scalable manufacturing and ultimately closes the loop of NaBH
regeneration and hydrolysis.
4
2
2
4
hydrolysis and regeneration (Figure 1a). Comparing with the
literature methods, current procedure features low-cost starting
materials, mild reactions at room temperature, and requiring no
Results and Discussion
2
high pressure H . The energy efficiency of regeneration can be
Regeneration and hydrolysis cycle
estimated by systematic modeling, which is beyond the scope of
the current work. One important step in the complete cycle is the
reformation of Mg metal. Although electrochemical process of
extracting Mg is energy intensive, it is widely adopted industrially.
The direct use of hydrated sodium tetraborate avoids an energy
One key challenge in utilizing NaBH
lies in its regeneration. Herein we developed a facile procedure to
regenerate NaBH from its CO treated hydrolytic product (Figure
a). NaBH can be successfully synthesized by ball milling a
mixture of Mg, Na ·10H O, and Na CO at ambient condition.
Figure 1b shows XRD qualitative analysis of the powders after
0h milling of a mixture of Mg, Na ·10H O, and Na CO in a
mole ratio of 24.75:1:1. The diffraction peaks are indexed to
NaBH and MgO.
NaBH was isolated from the as-milled product, and its
identity and morphology were studied using XRD, FTIR, NMR,
SEM, and TEM. XRD pattern of commercial NaBH (Figure 1c(2))
show ten peaks at 25.1°, 28.9°, 41.4°, 49.0°, 51.3°, 60.0°, 66.0°,
8.0°, 75.5°, and 81.0°, corresponding to (111), (200), (220),
311), (222), (400), (331), (420), (422), and (511) of NaBH (ICDD
displays the
(Figure 1c(3)). The
FTIR bands of 2200-2400 and 1125 cm-1 correspond to the B-H
stretching and deformation of pure NaBH
, respectively[10], in
good agreement with the those of commercial NaBH (Figure
4
for hydrogen storage
4
2
1
4
intensive process of drying Na
2
B
4
O
7
·xH
2
O at over 600°C to obtain
2
B
O
4 7
2
2
3
+
Na
Na
2
B
4
O
O
7
. In addition, by directly using the hydrate, H in
B
4
7 2
·xH O
serves as
a
hydrogen source for NaBH
source. This
process therefore reduces costs and energy consumption
associated with hydrogen production. Apart from H , another
commonly used hydrogen source in the literature is MgH
.[6-7] This
hydride is fabricated by a high-temperature (over 300°C) reaction
between Mg and H , and H is produced by a separate process.
Therefore, the current work presents a promising cycle pathway
4
2
2 4
B O
7
2
2
3
2
regeneration, without the need of any external H
2
4
2
4
2
4
2
2
6
(
0
4
for large-scale application of NaBH as a hydrogen carrier.
4
0-009-0386), respectively. The synthesized NaBH
4
same XRD pattern as the commercial NaBH
4
Reaction mechanism
4
4
To understand the reaction, Na
Mg mixtures were ball milled for relatively short periods. The XRD
diffraction peaks of Na ·10H O become invisible after 5 min
milling, along with the appearance of diffraction peaks of
Na ·5H O (Figure 2a), in agreement with FTIR results
(Figure 2b(2) and Figure S7). The actual formula of
2 4 7 2 2 3
B O ·10H O, Na CO , and
S2a). The XRD and FTIR results are also consistent with the
¯
previous studies on NaBH
4
synthesis.[7a, 9c, 10c, 11] The BH
4
anion
B O
2 4 7
2
is further confirmed by NMR analysis (Figure S2b).[12] Similar to
commercial NaBH , the synthesized NaBH exists in cubic
4
4
2 4
B
O
7
2
particles with sizes of several microns (Figure S3). Selected area
electron diffraction (SAED) pattern also indicates the success in
Na B
2 4
7 2 2 4 7 2 2 4 5 4 2
O ·5H O and Na B O ·10H O are Na B O (OH) ·3H O and
obtaining crystalline NaBH
4
(Figure 1d and Figure S4).
Na
2
B
4
O
5
(OH)
4
·8H O, respectively, according to the chemical
2
According to the results obtained from various techniques
XRD, FTIR, NMR, SEM, and TEM), we can conclude that high
structures.[14] Hydrogen can be detected in this period from the
(
MS of the gas atmosphere (Figure S8). After 10 min of ball milling,
quality NaBH
commercial NaBH
milling Na ·10H
temperature. The synthesis conditions are very mild compared
with the previous studies in which NaBH was produced via
NaBO reacting with Mg at 350°C under 7 MPa H
reacting with Mg at 550°C under 2.5 MPa H
.[5]
For closed-loop application, hydrogen evolution
performance of the regenerated NaBH is particularly important.
Figure 1e shows the hydrogen production curve of prepared
NaBH , which showed rapid hydrolytic H evolution that produced
317 mL/g H in 1.8 min. After the hydrolytic aqueous solution
was naturally dried up in air, solid Na ·10H O (ICDD ref. 01-
4
with crystallography and microstructure similar to
was successfully synthesized by directly ball
O and Na CO with Mg in Ar at room
the intensity of diffraction peaks of Na
significantly. As the milling increases to 30 min, XRD diffraction
peaks of MgO become highly visible while those of Na ·5H
and Na CO almost disappear. Although the diffraction peaks of
NaBH
2 4 7 2
B O ·5H O decreases
4
2
B
4
O
7
2
2
3
2 4
B
O
7
2
O
2
3
4
4
are invisible, the B-H group can be detected by FTIR
[
13]
3-
(1500-1300 cm-1)
(OH)
]2-, and
(1450-1400 cm , 877 cm ) originally present in Na CO
2
2
2 4
or Na B O
7
(Figure 2b(4)). The absorption bands of BO
3
5-
-1
2
and BO
4
(1150-950 cm ) originally present in [B
O
4 5
4
2-
-1
-1
a
of CO
3
2
3
4
become very weak and then vanish completely from the spectra
(Figure 2b). As the milling time reaches 5 h, the (200) diffraction
4
2
peak of NaBH
Considering the observation of Na
min, the first step of the reaction is assumed as follows:
4
appears.
2
2
2 4 7 2 2
B O ·5H O and H after 5
B O
2 4 7
2
3
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RESEARCH ARTICLE
Figure 2. (a) XRD patterns and (b) FTIR spectra of commercial Na
in 18:1:1 molar ratio) at 1200 CPM for different durations; (c) Solid-state 11B NMR spectra of commercial Na
for different durations; (d) Proposed reaction mechanism between Mg, Na CO , and Na ·10H O to form NaBH4.
2
B
4
O
7
·10H
2
O and products obtained after ball milling Mg, Na
2B4O7·10H2O, and Na2CO3 mixtures
(
2
B4O
7
·10H O and products obtained after ball milling
2
2
3
2B4O
7
2
Na
2
B
4
O
2
7
·10H
O + Mg → Mg(OH)
Under the experiment conditions, the following reactions will
2
O → Na
2
B
4
O
7
·5H
2
O + 5H
2
O (4)
and Mg.[10b] It should be noted that MgH
between Mg and H in the presence of oxide and hydroxide
species. Unlike dense Al MgO or Mg(OH) layer
is relative loose so H can penetrate through to react with
underlying Mg. In addition, during ball milling, oxide and hydroxide
layers on Mg will be destroyed and fresh Mg surfaces are always
produced. Based upon thermodynamic calculation (Table S2),
2
could form via a reaction
H
2
2
+H
2
(5)
2
2 3
O ,
2
[10b,
subsequently occur as large amounts of magnesium still exist:
2
1
5]
Mg + Mg(OH)
+ Mg → MgH
With increasing milling time, NaBH
Figure S8 and Figure S9) were observed which is associated with
2
→ 2MgO + H
(7)
, MgO, and also CH
2
(6)
H
2
2
MgO is more stable than MgH
ΔGo
of MgH : -35.09 kJ/mol). In this work, MgH
reaction (7) is highly reductive, converting Na to NaBH
forming stable MgO in the end.
The formation of methane is likely due to the reaction
between Na CO andMgH was not in the overall reaction
.[16]
(eqn 9) but was observed by MS, which is due to the nature of
2
(ΔGo
r
of MgO: -565.95 kJ/mol,
formed in
and
4
4
(
r
2
2
this reaction:
Na ·5H
2
B
O
4 7
4
2
B
4
O
7
2
O + Na
2
CO
3
+ 15MgH
10H (8)
The overall reaction equation is therefore can be expressed
in the following equation:
Na ·10H O + Na CO
2 4 4
→ 4NaBH + 15MgO + CH
+
2
2
3
2
H
2
2 4
B
O
7
2
2
3
+ 20Mg → 4NaBH
9)
This reaction is calculated to be favourable in thermodynamics
ΔGo298k= -1326.18 kJ/mol of NaBH
).
In this closed-loop regeneration (Figure 1a), CH
collected and converted to CO , which is then used to produce
Na CO . Mg can be regenerated via the commercial method,
where MgO is first converted to MgCl and Mg is obtained by
electrolysis of MgCl
MgH was not observed in this study, probably because it
was consumed in-situ due to its high activity. This is in agreement
with a previous study where MgH was proposed as an
intermediate during NaBH regeneration via ball milling NaB(OH)
4
+ 20MgO + CH
4
gas-solid reaction between H
100 % conversion.
2
and Mg, where it is hard to achieve
(
To further elucidate the reaction mechanism, the ball milled
products were characterized by solid-state 11B magic-angle
spinning (MAS) NMR (Figure 2c). After 10 min of ball milling, only
[B O (OH)
4 5 4
]2- is detected (Figure 2c(2)). The resonance observed
at ≈-13.4 ppm in Figure 2c(3) after 30 min milling demonstrates
(
4
4
can be
2
2
3
the formation of intermediate
transformation of [B (OH)
Figure 2d. The unit of [B
and two BO triangles as showed in Figure 2d(1). The B-O bond
(average bond length: 1.3683 Å) in BO triangles is stronger
H
2
BOH.[11,
17]
The reaction
2
O
4 5
4
]2- is therefore proposed as shown in
(OH) tetrahedra
]2- contains two BO
2
.
2
4
O
5
4
4
3
2
3
4
4
4
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RESEARCH ARTICLE
Figure 3. (a) XRD patterns and (b) FTIR spectra of the products obtained after ball milling Mg, Na
CPM for different durations; (c) Yields of NaBH
of the products obtained after ball milling Mg and Na
2
B
4
O
7
·10H
2
O, and Na
2
CO
3
mixtures (in 18:1:1 molar ratio) at 1000
4
with reactants in different molar ratios at 1000 CPM for different durations; (d) Yields of NaBH
·10H O in different molar ratios (Na ·10H O and Na CO were fixed at 1:1 molar ratio) for 20h at
000 CPM and 1200 CPM, respectively; (f) XRD patterns of the products obtained via ball milling Mg, Na ·10H O, and Na CO in a molar ratio of 24.75:1:1 at
000 CPM for different durations.
4 and (e) XRD patterns
2B4
O
7
2
2
B4
O
7
2
2
3
1
1
2
B4
O
7
2
2
3
than the one (average bond length: 1.4418 Å) in BO
tetrahedra.[18] Thus, the B-O in BO
tetrahedra preferentially
breaks, and B forms bond with the H in MgH and the O forms
4
bond with Mg. It is reasonable to assume that three intermediates
are formed (Figure 2d(2-4)). Thermodynamically, Mg is more
likely to bond with oxygen to form a more stable compound, MgO
4
2
5
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RESEARCH ARTICLE
o
o
[6-7,
(
ΔG
f
of MgO: -569.3 kJ/mol oxygen; ΔG
f
of B
2
O
3
: -398.1 kJ/mol
amount of Mg used, which is consistent with previous studies.
[19]
19-20]
oxygen).
Therefore, B-O and Mg-H in the B-O-Mg-H
With a high loading of Mg, powders do not stick to the jar and
intermediate break and the B-H and MgO are formed (Figure
balls, resulting in better the ball-milling efficiency. In addition,
more Mg leads to better contact among all the reactants resulting
in favorable yield. Limited by the ball mill machine, only two milling
speeds, 1000 and 1200 CPM, were tried, which proves that ball
mill speed, i.e. energy, does impact the yield. Although the energy
2
2d(2,4)). The breaking of the (B)-O-H (O bonded with sp boron)
in Figure 2d(5) results into formation of intermediate “H
which is detected in NMR spectra (Figure 2c(3)). According to the
literature, [11] B in “H BOH” is Lewis acidic and could accept H¯
2
BOH”,
2
from MgH
2
. As a result, “BH
4
¯” and MgO are generated. In addition, is higher at 1200 CPM, we fail to obtain better yields under any
¯
3
¯
“
OH ” bonded with sp boron (Figure 2d(3,4)) is substituted by H
circumstances, especially for a low molar ratio (Figure 3d). This is
because more Fe peels off the balls and jar for small amount of
in MgH
2
forming “BH
4
¯”, which agrees with previous studies.[10b]
Mg at highspeed and promotes NaBH
We have also found that when Na
·5H O were ball milled simultaneously with Na
Mg in a molar ratio of 0.6:0.4:1:22, NaBH
synthesized (Figure S10). Mg can also be replaced by Al or Ca to
synthesize NaBH (Figure S11). More research is needed to
optimize the yields of NaBH from these reactions.
4
decomposition (Figure 3e).
2 4
B
O
7
·10H
2
O
and
and
Yield of NaBH
4
Na B
2 4
O
7
2
2
CO
3
4
was also successfully
Efforts have been made to optimize the yield of NaBH
milling CO treated hydrolytic products (a mixture of
Na ·10H O and Na CO ) with Mg. Figure 3a shows the XRD
patterns of the products obtained by ball milling Mg,
Na ·10H O, and Na CO in a 18:1:1 molar ratio for different
4
by ball
4
2
2 4
B
O
7
2
2
3
4
2 4
B
O
7
2
2
3
durations. After 2.5 h of ball milling, the diffraction peaks of Conclusion
starting materials disappear, along with the appearance of strong
diffraction peaks of MgO. Although the diffraction peaks of NaBH
are invisible, the typical B-H bands (2200-2400 and 1125 cm )
are detectable by FTIR (Figure 3b(1)), which indicates the
4
In summary, we present a closed pathway for utilizing NaBH
for hydrogen storage. The regeneration of NaBH can be
achieved by ball milling its CO treated hydrolytic product
(Na ·10H O and Na CO ) and Mg under ambient condition,
with the yield being among the highest reported so far. This
process outperforms the literature methods in several key aspects.
First of all, the current yield is the best among the reported
literatures [5-6, 7, 10b, 13, 21] (Table S1). Secondly, the direct use of
4
-
1
4
2
formation of NaBH
increases to 10h, the diffraction peaks assigned to Fe
visible (Figure 3a(3)). With further increase in milling time, the
diffraction peaks of Fe B become stronger while the characteristic
FTIR bands of NaBH become weaker. The FTIR results indicate
that the yield of NaBH firstly increases and then decreases with
the milling time (Figure 3b). Iodometric analysis was carried to
quantify NaBH after its isolation from the ball-milled product. The
relationship between the yield and milling time is consistent
with FTIR results (Figure 3b, c), reflecting the fact that NaBH was
decomposed after long-time ball-milling. This is likely due to the
reaction between NaBH and Fe (peeling off the balls and jar after
long-time ball-milling), as evidenced by the formation of more
Fe B as milling time increases (Figure 3a). The highest yield
4
after 2.5h ball milling. As milling time
B O
2 4 7
2
2
3
2
B become
2
4
Na
required to dehydrate Na
not need expensive MgH
temperature reaction between Mg and H
method with reported process using MgH
Na
,[6a] the cost of the raw materials in this method is reduced
by 24-fold (Table S4). This indicates a promising pathway for
2 4 7 2
B O ·10H
O avoids high temperatures (over 600°C [8a])
·10H O. Thirdly, this process does
, which is usually obtained by high-
. When comparing this
Na CO and
4
2
B
O
4 7
2
4
2
2
4
2
,
2
3
2 4 7
B O
4
4
large-scale application of NaBH as a hydrogen carrier.
2
among the five durations for the mixture with 18:1:1 molar ratio is
about 17.4% after 5 h of ball milling (Figure 3c).
Acknowledgements
The impact of Mg loading on the yield was also studied
(
Figure 3d). For 20h milling, the yield increased when more Mg
was used, and it was ~46.9% for a molar ratio of 20.25:1:1 (Mg:
Na ·10H O: Na CO ), and reached the maximum of 75.7%
This
work
was
financially
supported
by
the
National Key R&D Program of China (No. 2018YFB15-02100),
Foundation for Innovative Research Groups of the National
Natural Science Foundation of China (No. NSFC51621001),
National Natural Science Foundation of China Projects (No.
2 4
B
O
7
2
2
3
for a ratio of 24.75:1:1. The highest yield of 78.9% was obtained
after 30 h of ball milling for the sample with 24.75:1:1 mole ratio
(
2 2
Figure 3c), which is higher than the NaBO ·2H O-Mg system
51771075) and Natural Science Foundation of Guangdong
reported previously (68.55%, Table S1).[10b] There are a few
possible reasons for this improvement in yield. Firstly, from the
calculation, reaction (9) is thermodynamically more favorable than
Province of China (2016A030312011). Z.H. acknowledges
support under the Australian Research Council’s Discovery
Projects funding scheme (project number DP170101773). Shao
acknowledges support from Macau Science and Technology
Development Fund (FDCT) (Project No.: 0062/2018/A2 and
reaction (10) due to its larger ΔGo
value (Table S3). Secondly,
r
Na CO in Na ·10H O-Na CO -Mg system may work as a
dispersant to prevent the materials (especially Mg) sticking onto
the walls of the jar and balls, resulting in better ball-milling
2
3
2
B
4
O
7
2
2
3
118/2016/A3).
efficiency and favorable yield. Thirdly, comparing to NaBO
.[3b] Lastly,
Na has a higher reactivity for recycling to NaBH
the speed of 1000 CPM leads to better yield than 1200 CPM, the
reason of which will be discussed later.[10b] For this particular
batch, the yield after 5 h (38.0%) is already much higher than that
2
,
Keywords: borohydride • hydrolysis • hydrogen • hydrogen
storage • regeneration
2 4
B
O
7
4
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7
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Angewandte Chemie International Edition
10.1002/anie.201915988
RESEARCH ARTICLE
Entry for the Table of Contents
We report a facile method to regenerate NaBH
and low cost from its hydrolytic product NaBO . This method
expects to effectively close the loop of NaBH hydrolysis and
4
with high yield
2
4
regeneration and therefore enables a wide deployment of NaBH
for hydrogen storage.
4
Yongyang Zhu, Liuzhang Ouyang*, Hao Zhong, Jiangwen Liu,
Hui Wang, Huaiyu Shao*, Zhenguo Huang*, and Min Zhu
Closing the loop for hydrogen storage: Facile regeneration
of NaBH from its hydrolytic product
4
8
This article is protected by copyright. All rights reserved.