Realizing facile regeneration of spent NaBH4 with Mg-Al alloy

Article abstract of DOI:10.1039/c9ta00769e

The regeneration of sodium borohydride (NaBH₄) is crucial to form a closed cycle after it either supplies hydrogen energy via a hydrolysis process or provides energy through electron transfer at the anode of direct borohydride fuel cells (DBFCs). In both of these cases, the spent fuels are NaB(OH)₄ from NaBO₂ aqueous solution. However, the current regeneration process from (NaB(OH)₄)·xH₂O to form NaBH₄ by reduction reaction and calcination at high temperature with metal hydrides as reducing agents is very expensive. In this work, we developed a simple regeneration process via ball milling with Mg-Al alloys as the reducing agent for NaB(OH)₄ under an argon atmosphere. Under optimized conditions, a high yield of about 72% of NaBH₄ could be obtained. Mechanistic study showed that all the hydrogen atoms from NaB(OH)₄ remain in NaBH₄ and no additional hydrogen sources are needed for the reduction process. The inexpensive Mg-Al alloy works as a reducing agent transforming the H⁺ to H^- in NaBH₄. This approach demonstrates a ～20-fold cost reduction compared with the method using metal hydrides. This opens the door to the commercial implementation of simple ball milling processes for the regeneration of spent NaBH₄ from NaB(OH)₄ with cheap reducing agents.

Full text of DOI:10.1039/c9ta00769e

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0.1039/C9TA00769E.
Volume
4
Number
1
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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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DOI: 10.1039/C9TA00769E
Journal Name
ARTICLE
Realizing Facile Regeneration of spent NaBH with Mg-Al Alloy
4
a
a,b
a
a
a
c
Hao Zhong , Liuzhang Ouyang *, Meiqin Zeng , Jiangwen Liu , Hui Wang , Huaiyu Shao *, Michael
Received 00th January 20xx,
Accepted 00th January 20xx
d,*
a
Felderhoff , Min Zhu
The regeneration of sodium borohydride (NaBH
via hydrolysis process, or provides energy through electron transfer at anode of direct borohydride fuel cells (DBFCs). In
both of these cases, the spent fuels are NaB(OH) from NaBO aqueous solution. However, the current regeneration
process from (NaB(OH) )∙xH O to form NaBH by reduction reaction and calcination at high temperature with metal
hydrides as reducing agents is a very expensive and costly process. In this work we developed a simple regeneration
process via ball milling with Mg-Al alloys as reducing agent for (NaB(OH) ) under argon atmosphere. Under optimized
conditions a high yield of about 72% of NaBH could be obtained. Mechanism study shows that all hydrogen atoms from
NaB(OH) ) remains in NaBH and no additional hydrogen sources are needed for the reduction process. The inexpensive
Mg-Al alloy works as reducing agent transforming the H to H in NaBH
reduction compared with the method using metal hydrides. This opens a door to a commercial implementation of simple
ball-milling processes for the regeneration of spend NaBH from NaBO O with cheap reducing agents.
4
) is crucial to form a closed cycle after it either supplies hydrogen energy
DOI: 10.1039/x0xx00000x
www.rsc.org/
4
2
4
2
4
4
4
(
4
4
+
−
4
. This approach demonstrates a ∼20-fold cost
4
2
H
2
NaBH
4
+ (2 + x)H
2
O → NaBO
2
∙xH
2
O + 4H
2
+ heat
(1)
1
. Introduction
8
where x is the hydration factor. However, it should be noted
Fuel cells provide a promising alternative technology for
electrical power generation from renewable energy carrier, for
instance, hydrogen energy. However, currently fuel supply is
still one of the biggest hindrances for worldwide applications
of mobile fuel cell technologies.2
hydrolysis of sodium borohydride (NaBH
borohydride fuel cells (DBFCs) possess great potentials as
possible solutions. But both of these technologies are facing
the issue of suffering from the high cost of NaBH
the difficulties in the regeneration of the spent fuel
Therefore, a high efficiency and low cost approach for a simple
regeneration process of spent NaBH is highly desirable. This
could be the key step and enabling technology for further
distribution of NaBH powered fuel cell applications.
The spent fuel of NaBH hydrolysis is confirmed by the
following hydrolysis reaction:
that the spent fuel is normally hydrated sodium metaborate
9
(
NaBO
actual formula of NaBO
and NaB(OH) ∙2H O,respectively, according to the chemical
structures. In DBFCs, NaBH is oxidized at the anode
according to the following reaction:
2
) or its aqueous solution after the hydrolysis. And the
1
2
2 2 2 4
∙2H O and NaBO ∙4H O are NaB(OH)
,
3
4
2
Hydrogen supply via
10
4
4
4
)
and direct
5
−
−
−
O + 8e−,
(2)
BH
4
0
+ 8OH → BO
2
+ 6H
2
4
, as well as
E = −1.24 V vs. SHE
6, 7
.
−
The spent fuel of the anode reaction is B(OH)
are generated also as a NaBO
NaBO aqueous solution, NaB(OH)
obtained via drying treatment at temperature of < 54 °C or
4 – 110 °C, respectively; while dehydrated NaBO can be
4
and H
2
, 11
O, which
4
aqueous solution.7
From
2
2
4
∙2H O or NaB(OH) can be
2
4
4
4
5
2
12
formed after treatment at temperature > 350 °C. Over the
past decades, a great deal of efforts has been made toward
dehydrated NaBO
2
reduction. With a calcination treatment at
to NaBH under
5
50 °C, MgH reduced the dehydrated NaBO
2
2
4
a.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
13
hydrogen pressure. Ball milling with metal hydrides at near
room temperature can also achieve the reduction under argon
atmosphere with a NaBH
b.
China-Australia Joint Laboratory for Energy & Environmental Materials, Key
14-16
4
yield of over 70 %.
However, the
Laboratory of Fuel Cell Technology of Guangdong Province, Guangzhou, 510641,
People’s Republic of China.
Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and
Materials Engineering (IAPME), University of Macau, Macau SAR, China.
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: XRD analysis of the raw
materials; XRD analysis of the ball milled products; XPS analysis of the ball milled
synthesis of metal hydrides at high temperature could be one
important factor in energy consuming and costs. Without
usage of metal hydrides, high temperature annealing
c.
d.
†
13, 17
13
treatment under hydrogen pressure for Mg
, Mg and Si or
18, 19
transition metals (Fe, Co or Ni)
mixed with dehydrated
raw material; Cost calculation of raw materials for NaBH
approaches. See DOI: 10.1039/x0xx00000x
4
produced by different
NaBO2 is another reduction technique. But this high
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temperature dehydration process of NaBO
2
is also energy 2.3 Purification and quantification
View Article Online
DOI: 10.1039/C9TA00769E
consuming and additional hydrogen supply is needed, which
increases the costs of the regeneration process (hydrogen
from renewable sources, like water splitting or unsustainably
from fossil fuels). Direct reduction of hydrated NaBO
by annealing at 3 MPa hydrogen pressure may be one of the
4
A 20 mL of ethylenediamine was used to extract NaBH from
the ball milled products. The turbid solution was then filtrated
via a polytetrafluoroethylene filter. The clear NaBH solution
was dried via a freeze dryer (Martin Christ, Alpha 1-2LD Plus,
Germany) to obtain NaBH as white powder and the waste
4
2
with Mg
4
2
0
options, but the yield is only 12.3 % of NaBH
4
. Therefore, an
solvent (ethylenediamine) collected in the cold trap. The
innovative low-cost and high-efficiency approach of NaBH
regeneration is of great importance and it is urgently required.
Herein, instead of only Mg, we introduce aluminum (Al) in
4
purified NaBH
yield of NaBH
equation:
4
was quantified by the iodate method.21 The
4
was calculated according to the following
2
the reduction process of hydrated NaBO because it can offer
ꢅꢆꢇꢈꢁꢉꢂꢄ ꢊꢈꢋꢌ ꢍꢈꢎꢎ
more electrons than Mg but with similar reducibility, which
may further decrease the cost and increase the yield of the
process. In addition, Mg and Al are relatively soft metal making
the ball milling process less efficient. Thus, magnesium
aluminum alloy (Mg17Al12) was chosen as a reducing agent in
4
ꢀꢁꢂꢃꢄ =
× 100%
(3)
ꢇℎꢂꢅꢏꢂꢇꢁꢐꢈꢃ ꢊꢈꢋꢌ4 ꢍꢈꢎꢎ
2
.4 Hydrolysis process
Hydrolysis test was conducted via a hydrolysis apparatus
this work. The alloy was used to react with hydrated NaBO
ball milling under argon atmosphere in order to regenerate
NaBH . During the ball milling process oxide layers on the alloy
2
via
22
introduced here. In each hydrolysis experiment, 0.1 g of
NaBH was used to react with 0.225 mL of a 5 wt% aqueous
solution of CoCl at room temperature and the hydrogen
generation curves were automatically collected.
4
4
will be destroyed and fresh surfaces are always produced
during the milling. This will increase the overall kinetics of the
regeneration process.
2
2
.5 Characterization
Phase composition was measured via X-ray diffractometer
XRD, Rigaku MiniFlex 600) with Cu Kα radiation (λ = 1.5406 Å)
In this process, the Mg17Al12 alloy offers a higher NaBH
4
yield
and a lower cost, while the hydrated NaBO provides a self-
2
sufficient hydrogen source with no need of any additional
hydrogen input. Furthermore, the unnecessity of drying at high
temperature (> 350 °C) may greatly reduce energy
consumption during the regeneration process. Therefore, this
(
at 45 kV and 40 mA. Because both of the raw materials and
milling products are air sensitive, liquid paraffin was used to
protect the XRD samples from air. Chemical bonds of the
products were measured via Fourier-transform infrared
spectroscopy (FTIR, IS50, Nicolet) in transmission mode.
Potassium bromide (KBr) pellets for FTIR measurements were
prepared in the glove box with a sample to KBr ratio of 1:99.
The ball milling products were also characterized by solid-state
4
regeneration approach of NaBH may be a very promising
solution for future energy supply technologies.
2
. Experimental
11
2
.1 Chemicals
Mg17Al12 was purchased from Aike Reagent (China), while
NaB(OH) was obtained by drying (NaB(OH) )∙2H O (> 99 %,
B magic-angle spinning nuclear magnetic resonance (MAS
NMR) spectroscopy (AVANCE III HD 400, Bruker. Scanning
electron microscopy (SEM; Supra-40, Zeiss) was used to
characterize the morphology of NaBH .
4
4
4
2
Sigma-Aldrich) for 12 h. Ethylenediamine (≥ 99 %) was
purchased from Sigma-Aldrich. The chemicals for
quantification, potassium iodate (KIO
NaOH (≥ 99 %), starch indicator (≥ 99 %), and sodium
thiosulfate solution (Na , 1 M, AR grade) were purchased
from Aladdin, and potassium iodide (KI, ≥ 99 %) were
purchased from TCI. The Mg17Al12 and the generated NaB(OH)
were stored and handled in an argon filled glove box
Mikrouna, China). The oxygen and water concentrations in
the glove box were always below 1 ppm.
3 2 4
, AR grade), H SO (98 %),
3
. Results and discussion
3
.1 NaBH
For the NaBH
NaB(OH) in a molar ratio of 4 : 35 was mechanochemically
4
synthesis
2 2 3
S O
4
regeneration, a mixture of Mg17Al12 alloy and
4
4
treated with a ball to powder ratio of 50 : 1 at 1200 cpm under
argon atmosphere. The XRD curves of the raw materials are
shown in Fig. S1. Fig. 1a shows the XRD curves of the
(
generated NaBH
can be seen that the raw materials (Mg17Al12 and (NaB(OH)
lose their intensity gradually with increasing milling time. The
200) diffraction peak of NaBH at around 28.9° in the XRD
pattern confirms the generation of NaBH after two hours of
milling. With the further increase of milling time, diffraction
peak of NaBH becomes stronger after 5 h milling but the
intensity decrease after 10 h milling, which may result from
the combination effect of amorphization and NaBH
4
after milling depending on the milling time. It
))
2
.2 NaBH
For a typical experiment, totally 1 g of Mg17Al12 and
NaBO ∙2H O with different mole ratios and 50 g of steel balls
ball to powder ratio of 50 : 1, 4 steel ball of 10 mm and 68
4
regeneration
4
(
4
2
2
4
(
steel ball of 6 mm) were mixed and loaded in the milling vial in
the glove box. Then, the ball milling reactions were carried out
in a shaker mill (QM-3C, Nanjing, China) at 1200 cycles per min
4
4
(cpm).
generation during ball milling. After 20 h milling, (111) and
2
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220) diffraction peaks of NaBH
ARTICLE
(
4
at around 25.1° and 41.4° commercial NaBH
4
. Therefore, we may conclude that the
View Article Online
DOI: 10.1039/C9TA00769E
appear with the sharper (200) diffraction peak at 28.9° regenerated NaBH
4
with similar crystal structure and bonding
Fig. 1. (a) XRD patterns of the 5, 7.5, 10 and 20 h ball milled products of Mg17Al12 and NaB(OH)
4
mixtures (in a 4 : 35 molar ratio). (b) XRD patterns of the purified product (red
(blue line). (d) SEM images of the commercial NaBH (left)
line) and commercial NaBH (blue line). (c) FTIR spectra of the purified product (red line) and commercial NaBH
4
4
4
and the purified product (right).
4
Fig. 2. (a) Yields of the ball milled products of Mg17Al12 and NaB(OH) mixtures (in 4 : 35 and 4 : 17 molar ratio) for different milling time. (b) Yields of the 10 and 20 h ball
4
milled products of Mg17Al12 and NaB(OH) mixtures with different mole ratios.
because of the crystallization.23
feature as the commercial one was regenerated by the
via ball milling. This
, the ball milling products were method not only avoids high temperature process of 350 °C for
To remove the byproducts from the powder after 5 h milling reaction between Mg17Al12 and NaB(OH)
and obtain high purity NaBH
further purified. Fig. 1b presents the XRD curves for the NaB(OH)
purified NaBH
material. As compared to the one from commercial NaBH
similar diffraction pattern of the purified NaBH and the sharp commercial one. The grain like surface structure in the purified
111), (200), (220), (311), (222), (400), (331) and (420) NaBH is quite similar to the commercial NaBH , which
indicate the successful generation of high indicates that the regenerated NaBH shows almost a similar
4
4
4
reduction, but also realizes the complete H supply
−
4
product and in comparison to the commercial for the regenerated NaBH
4
from the [OH] group of NaB(OH)
4
.
4 4
, the Fig. 1d shows the SEM images of the purified NaBH and the
4
(
4
4
14, 15
diffraction peaks
4
purity NaBH
the XRD pattern. From the FTIR spectra of these two samples
shown in Fig. 1c, the bonds of the purified NaBH were further 3.2 Yield
analyzed. The stretching (2200-2400 cm ) and bending
4 4
phase. No other reflections can be detected from surface morphology to the commercial NaBH .
4
−
1
Fig. 2a presents the yields of high-purity NaBH
the raw materials Mg17Al12 and NaB(OH) in a molar ratio of
: 35 and 4 : 17 depending on the milling time. Quantification
4
prepared from
−
1
(
1125 cm ) vibrations of B-H appear in the spectrum of the
4
14, 16
purified NaBH
4
,
which are similar to the vibrations of
4
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Fig. 3. (a) FTIR spectra of the 1, 2 and 5 h ball milled products of Mg17Al12 and NaB(OH)
4
mixtures (in a molar ratio of 4 : 35). (b) 11B NMR spectra of the 1 and 2 h ball milled
products of Mg17Al12 and NaB(OH)
molar ratio).
4 4
mixtures (in a 4 : 35 molar ratio). (c) XPS spectra of Al 2p of the 1 h ball milled products of Mg17Al12 and NaB(OH) mixtures (in a 4 : 35
of the pure NaBH
4
was done with the iodate method. . For the 3.3 Reaction mechanism
yield after 5 h milling is 20 % and the
4
: 35 ratio, the NaBH
4
To clarify the reaction mechanism between Mg17Al12 alloy and
NaB(OH) , the products of different milling times were also
yields increase with the milling time. After 20 h milling the
yield reaches a value of 37 %. As known from previous studies
the relation of NaB(OH)
4
investigated and characterized with FTIR as shown in Fig. 3a.
According to the XRD patterns in Fig. 1a, the (111) diffraction
peak of Al at 38.4° and the (200) diffraction peak of MgO at
4
to the reduction compound has an
yield.2
4-26
It was further
important influence on the NaBH
4
optimized by varying the molar ratio of Mg17Al12 and NaB(OH)
raw materials ratio). Fig. 2b shows the results of NaBH yields
4
4
2.9° imply the generation of Al and MgO after 1 and 2 h
(
4
milling. After 1 h milling, the formation of NaBH could be
4
depending on the raw materials ratio from 4 : 35 to 4.5 : 17
after 10 and 20 h ball milling. In both of the milling durations,
verified by the appearance of B-H vibrations in the FTIR
spectrum in Fig. 3a and the [BH
−
4
] resonance from Fig. 3b.
4
the NaBH yields first increase then decrease with the
−
According the NMR spectra (Fig. 3b), [B(OH)
reduced to [BH
4
] is gradually
increasing raw materials ratios. However, the highest yield
after 10 h ball milling is 54 % with the raw materials ratio of
]− in this process. Therefore, the first step of
4
the regeneration process can be described by the following
reaction:
3
.5 : 17, while that of 20 h milling is 72 % when the raw
materials ratio is 4 : 17. It should be noted that the highest
NaBH yield in this work of 72 % is higher than the yield of Mg
and NaB(OH)
system in our previous study27 and also
approaches to the ones from others’ work of NaBH
4
Mg17Al12 + NaB(OH)
4
→ MgO + Al + NaBH
4
(4)
4
4
The diffraction peaks of Al then disappear after 5 h milling,
which indicates that Al may become amorphous or work as a
14-16
regeneration via MgH
shorter milling times of the products with raw materials ratio
of 4 : 17 are presented in Fig. 2a. The yields of 5 h (5 %) and
2
and NaBO
2
.
The yields for the
reducing agent and react with NaB(OH)
Because Al was generated after 1 h milling and was possible to
react with NaB(OH) , to further confirm the reaction, the
product was characterized by XPS and the result is shown in
Fig. 3c, which may provide more evidence. The only peak
4
during ball milling.
7
.5 h (9 %) are lower than that with 4 : 35 raw molar ratio, and
the diffraction peaks of NaBH can not be found in the XRD
results (Fig. S2). Diffraction peaks of NaBH appear in the
pattern of the product after 5 h ball milling, while the peak of
NaBH appears in the pattern of the product with 4 : 35 raw
materials ratio only after 2 h ball milling (Fig. 1a). A strong
crystallization of NaBH happens when the ball milling time
increases to 20 h. The diffraction peaks of NaBH in the
4
4
4
3+
which appears at 74.30 eV in the spectrum is indexed to Al ,
while peak belonging to Al in Mg17Al12 can be found in the
4
0
spectra of the Mg17Al12 (Fig. S3) milled with the same
parameters. These evidences indicate that Al reacts with
4
4
NaB(OH)
mechanism, pure Al-metal and NaB(OH)
4 : 9 were ball milled for 5 h with the same other milling
4
in this reaction. To further unveil the reaction
product with raw materials ratio of 4 : 17 become much
sharper (Fig. S2).
4
in a molar ratio of
2
parameters. Only diffraction peaks from Al-metal are found in
4
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the XRD pattern (Fig. S4a) and on the other hand, B-H located at 22.0°, 25.2° and 33.2° are indexed to the (101),
View Article Online
DOI: 10.1039/C9TA00769E
vibrations appear in the FTIR spectrum (Fig. S4b). This (111) and (211) of Na
demonstrates that even Al-metal can react with NaB(OH) as the ones of NaB(OH)
producing NaBH . Considering Mg transfers to MgO in this NaB(OH) is the main phase of the byproduct, which can be
system, it can be assumed that the byproduct is Al but not regenerated by the above method.
Al(OH) , which may be amorphous so that its diffraction peaks
2 4
ClB(OH) , while other peaks are similar
4
4
. We may conclude here that the
4
4
2 3
O
3
can not be observed in the XRD patterns. Therefore, the
reaction of the second step is described as:
Conclusion
In summary, NaB(OH)
Mg17Al12 alloy via ball milling to realize a very easy
4
can be successfully reduced with
Al + NaB(OH)
4
→ Al
2
O
3
+ NaBH
4
(5)
Fig. 4. (a) Hydrolysis curves of the purified product (red line) and commercial NaBH
4
(blue line). (b) XRD pattern of the hydrolysis byproduct of the purified product.
regeneration process of spent NaBH
4
. Using the inexpensive
is directly
Fig. 3b shows the solid-state 11B MAS NMR spectra of boron Mg17Al12 alloy, a H -anion in the regenerated NaBH
−
4
−
−
compounds produced during ball milling and depending on transferred from the [OH] group to H . The yield in NaBH
4
different milling times. When the milling time changes from 1 reaches 72 %, which results from the reducibility Mg and also
−
to 2 h, the intensity of [B(OH)
4
] resonance decreases sharply, Al metal. During the reduction process, firstly the Mg17Al12
−
while the intensity of [BH
4
] resonance increases, indicating alloy reacts with NaB(OH)
4
and generates NaBH
] . Due to the self- metal. Afterwards Al-metal reacts with residual NaB(OH)
supplied H from the [OH]− group in the raw material of produces NaBH
and Al . Since both metals of the cheap
NaB(OH) , and the avoiding of high temperature dehydration Mg17Al12 alloy can act as a reduction agent the commercial
in this system, the cost of the regenerated NaBH is costs of this regeneration method is further reduced by a
significantly reduced, which is benefited from the implication factor of 20 compared to regeneration methods using metal
of the Mg17Al12 alloy. From price calculations of the raw hydrides as reducing agent. This indicates a promising
materials, the expected costs of this process is 20 fold lower commercialization regeneration process and could open a
than the method using MgH and dehydrated NaBO as raw door for broad applications of energy supply from NaBH
4
, MgO and Al-
−
−
the conversion from [B(OH)
4
] to [BH
4
4
and
4
2 3
O
4
4
∼
∼
2
2
4
.
materials (Table S1). Around 25% of raw materials cost drop is
also achieved compared with the commercial method.
Acknowledgement
3
.4 Hydrolysis
This work was supported by the Foundation for Innovative
via Research Groups of the National Natural Science Foundation
The generation of hydrogen from produced NaBH
4
hydrolysis was also examined to confirm its property. Here, a of China (No. NSFC51621001) and National Natural Science
low cost and effective non-noble metal catalyst, cobalt Foundation of China Projects (Nos. 51431001 and 51771075).
28
Author Ouyang also thanks Guangdong Province Universities
According to the hydrogen generation curves in Fig. 4a, the and Colleges Pearl River Scholar Funded Scheme (2014). Shao
regenerated NaBH
shows a fast kinetics of hydrogen acknowledges Macao Science and Technology Development
generation, although with a little lower final hydrogen Fund (FDCT) for project of 118/2016/A3.
generation content than the commercial NaBH one.
2
chloride (CoCl ) , was used in the hydrolysis process.
4
4
−
1
Nevertheless, around 2215 mL g hydrogen can be generated
within 10 min, with a conversion rate of about 86 %. After the
Reference
hydrolysis, the byproduct was collected and placed in 1. U. Eberle, B. Muller and R. von Helmolt, Energy Environ
ambiance condition for 48 h before XRD measurement. In the
Sci., 2012, 5, 8780-8798.
XRD pattern (Fig. 4b), the low intensity diffraction peaks
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