International Journal of Hydrogen Energy 32 (2007) 3370 – 3375
www.elsevier.com/locate/ijhydene
Hydrogen release from a mixture of NaBH4 and Mg(OH)2
Vadym Drozd ∗ , Surendra Saxena, Subrahmanyam V. Garimella, Andriy Durygin
Center for the Study of Matter at Extreme Conditions, College of Engineering, Florida International University, VH-140, University Park, Miami, FL 33199, USA
Received 4 January 2007; received in revised form 13 February 2007; accepted 13 February 2007
Available online 18 April 2007
Abstract
Hydrogen generating reaction between sodium borohydride, NaBH4 , and magnesium hydroxide, Mg(OH)2 (brucite), was studied. Reaction
rate was found to depend on the degree of reactants homogenization and/or their particle size. Kinetic of the reaction was studied in isothermal
approach in the temperature range of 240–360 ◦ C. It is shown that the reaction obeys 2D diffusion mechanism and its activation energy
is 155.9 kJ/mol. Powder XRD analysis and Raman spectroscopy reveal that mechanically activated mixture of NaBH4 and Mg(OH)2 reacts
yielding MgO as the only crystalline phase in the temperature range of 240–318 ◦ C. At higher temperatures a new crystalline tetragonal phase
of as yet undetermined composition is developed.
䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrogen generation; Hydride/hydroxide reaction
1. Introduction
Sodium borohydride, NaBH4 has high hydrogen content of
13 wt.% and is considered as a prospective material for hydrogen storage. It is stable compared to other chemical hydrides
and easy to handle. Some applications of NaBH4 in the hydrogen storage and generation systems are summarized in Table 1.
NaBH4 is a thermally stable, white crystalline solid that melts
at 505 ◦ C and decomposes at about 565 ◦ C [1,11]. Its standard
enthalpy of formation is −191.836 kJ/mol [12]. Thus, thermal
decomposition of NaBH4 is irreversible and occurs in the temperature range which is beyond the practical applications for
the hydrogen storage systems. Several attempts were made to
destabilize NaBH4 by reacting it with light elements or with
other hydrides [4,5]. However, a considerable destabilization
effect was not achieved.
To utilize high hydrogen capacity of NaBH4 , its hydrolysis
reaction so far is the most attractive approach [6]. Aqueous
alkaline NaBH4 solutions are non-toxic, non-flammable, stable
and high volumetric and gravimetric energy-dense. They can
∗ Corresponding author. Tel.: +1 305 348 3445; fax: +1 305 348 3070.
E-mail address: drozdv@fiu.edu (V. Drozd).
be used for feeding hydrogen fuel cells [13] or direct borohydride fuel cells [14,15]. The hydrolysis rate is a function of
temperature and pH [6]. It can also be effectively catalyzed by
small quantities of certain metal salts [16,17], Co2 B [18], noble
metals like Ru [19], as well as metals coated on metal oxides
[20].
Recycling of borates to NaBH4 can be achieved in a number
of ways. Several examples are given below as equations of the
chemical reactions [10,21,22]:
8NaH + 3H3 BO3 + 3CO2 → 2NaBH4 + 3Na2 CO3 + 6H2 O.
NaBO2 + 2MgH2 → NaBH4 + 2MgO.
Na2 B4 O7 + 16Na + 7SiO2 + 8H2 → 4NaBH4
+ 7Na2 SiO3 (300.500 ◦ C, 3.5 atm).
Possibility of using methane or coke for NaBO2 reduction
into NaBH4 is suggested by Kojima et al. [21]. Several groups
explore also single step electrolytic conversion of borates into
borohydrides, e.g. [23].
In this paper we report the approach to hydrogen generation
using NaBH4 utilizing the exothermic reaction:
NaBH4 + 2Mg(OH)2 → NaBO2 + 2MgO + 4H2 ,
H300 K = −135.9 kJ/rxn.
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2007.02.007
V. Drozd et al. / International Journal of Hydrogen Energy 32 (2007) 3370 – 3375
3371
Table 1
Application of NaBH4 in the hydrogen generation and storage systems
System
Hydrogen content (wt.%)
Conditions
Ref.
Thermal decomposition
NaBH4 → Na + B + 2H2
10.6
400 ◦ C (vacuum); 565 ◦ C
(1 atm), irreversible
[1,2]
NaBH4 → Na + B + 3/2H2
NaBH4 + Al → 2NaH + AIB2 + 3H2
8
5.8
NaBH4 (20 wt.%) + MgH2 → Mg1-x Nax
+B (MgB2 ) + H2
Hydrolysis (solution)
NaBH4 + 2H2 O → NaBO2 + 4H2
Hydrolysis with a steam
NaBH4 (s) + 2H2 O(g) → NaBO2(s) + 4H2(g)
Alkoholysis
NaBH4 + 2CH2 (OH)CH2 (OH) →
Na[B(OCH2 CH2 O)2 ] + 4H2
Combustion-assisted hydrolysis
Mg + H2 O → MgO + H2
NaBH4 + H2 O → NaBO2 + H2
Solid state reaction
4NaBH4 + (NH4 )2 SO4 → 2NaBO2 + 2BN
+Na2 S + 12H2
7NaBH4 + 2(NH4 )2 Cr 2 O7 → 3NaBO2
+4BN + 2Na2 O + 2Cr 2 O3 + 22H2
188 ◦ C (1 bar H2 );
calculated
330–405 ◦ C
[3]
[4]
[5]
[6]
< 10
Ambient, catalyst
< 10
110 ◦ C, fourfold molar
excess of H2 O
4.9
Fast reaction at room
temperature without
catalyst
7
Self-propagating
reaction after ignition
6.7
650 ◦ C
5.9
240 ◦ C
[7]
[8]
[9]
2. Experimental
NaBH4 and Mg(OH)2 (brucite) were supplied by Alfa Aesar
and Sigma with purity of 95% and 98%, respectively. Reagents
were mixed together in a molar ratio NaBH4 :Mg(OH)2 = 1:2
either by mortar-and-pestle or ball milling method. Mixtures
were pressed into pellets (12 mm diameter) under 200 MPa
pressure. The usual amount of the mixture used for hydrogen
generation experiments was about 0.4 g. All the sample handling and loading were conducted in an Ar-filled glovebox (TerraUniversal). One end sealed quartz loaded with a sample was
put into a tubular furnace. Another end of the quartz tube was
connected to the water filled graduated cylinder. After sample
loading system was evacuated and flushed with Ar gas several
times. Kinetics of hydrogen generation reaction was studied
in isothermal approach by measuring the volume of hydrogen
gas formed in a reaction. The hydrogen gas was collected in
a water-filled graduated cylinder. Partial pressure of water vapor and water column height pressure were extracted from the
total pressure to get hydrogen partial pressure in the cylinder.
Finally, hydrogen volume formed in the reaction was corrected
to standard conditions.
X-ray powder diffraction was done using Bruker GADDS/D8
X-ray system with Apex Smart CCD Detector and direct-drive
rotating anode. The MacSci rotating anode (Molybdenum) operates with a 50 kV generator and 20 mA current. X-ray beam
size can vary from 50 to 300 m. The usual collection time is
1200 s.
Raman spectroscopic measurements were conducted at
room temperature by using Raman spectrometer in the back
[10]
scattering configuration. The 514 nm Ar + laser was operating
at 50 mW. Raman spectra were collected with 10 min exposure
time by using high throughput holographic imaging spectrograph with volume transmission grating, holographic notch
filter and thermoelectrically cooled CCD detector with the
resolution of 4 cm−1 .
3. Results and discussion
Hydrogen generation curves at several temperatures from a
mortar-and-pestle grinded mixture of NaBH4 and Mg(OH)2 are
shown in Fig. 1a. Hydrogen evolution with considerable rate
starts at temperature as high as 290 ◦ C. However, the hydrogen
yield does not increase with further temperature rise. Condensation of water vapor was observed on a cold part of a reactor
indicating that magnesium hydroxide decomposition to magnesium oxide and water occurs without reacting with sodium
borohydride. According to [24] magnesium hydroxide thermal
dehydration takes place at a temperature of 247 or 377 ◦ C in
vacuum and air, respectively.
X-ray diffraction was used to determine the structure of the
remaining species after the reaction of NaBH4 and Mg(OH)2
(Fig. 2a). Unreacted NaBH4 , MgO and sodium metaborate,
NaBO2 , were identified in the remaining mixture produced
at 354 ◦ C. Minor amount of brucite present on the XRD pattern confirms the above-mentioned dehydration reaction of
Mg(OH)2 . Hence, mortar-and-pestle milled mixture of NaBH4
and Mg(OH)2 can react with each other on the initial stage
producing NaBO2 and hydrogen. With the growth of a product
layer on the interface between reacting species the rate of a
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V. Drozd et al. / International Journal of Hydrogen Energy 32 (2007) 3370 – 3375
Fig. 1. Hydrogen generation isotherms from mortar-and-pestle (a) and ball
milled for 30 min (b) mixtures of NaBH4 and Mg(OH)2 in 1:2 molar ratio.
reaction decreases and brucite dehydration starts to predominate. Sodium borohydride reaction with water vapor also can
take place if the insulating layer of the surface of NaBH4 is
permeable to water molecules:
Mg(OH)2 → MgO + H2 O,
NaBH4 + 2H2 O → NaBO2 + 4H2 .
Therefore we can suggest that the hydrogen formation in
this system is due to surface reaction between large particles
of sodium borohydride and brucite and, probably, due to reaction of H2 O molecules formed from brucite decomposition and
NaBH4 .
The suggested mechanism is supported by Aiello et al. [7]
who studied solid–gas hydrolysis reactions of metal hydrides,
including NaBH4 , with a steam for hydrogen generation.
Fig. 2. XRD patterns evolution of mortar-and-pestle (a) and ball milled for
30 min (b) mixtures of NaBH4 and Mg(OH)2 in 1:2 molar ratio with temperature. Peak locations for NaBH4 , Mg(OH)2 , MgO and NaBH4 phases were
obtained from JCPDS # 74-1891, 83-0114, 45-0946, 76-0750, respectively.
A decrease of hydrogen yield was observed with increasing
temperature above 110 ◦ C. Moreover, the reaction rate at all
temperatures was found to depend strongly on the steam flow
rate. It is suggested that high steam flow rates resulted in
the formation of an insoluble layer on the surface of hydride
preventing the steam from further reacting with the compound.
Hydrogen generation curves from NaBH4 and Mg(OH)2
mixture prepared by ball milling of the reactants for 30 min are
shown in Fig. 1b. Compared to the mortar-and-pestle milled
mixture, the ball milled sample shows much improved kinetics. Maximum hydrogen yield, as it is evident from Fig. 1b, is
a function of temperature. While at 240 ◦ C the reaction stops
yielding only 25% of the hydrogen, 87% of the hydrogen is
released at 258 ◦ C and this amount is growing with the temperature. Reaction finishes in less than 1 h at 318 ◦ C producing
V. Drozd et al. / International Journal of Hydrogen Energy 32 (2007) 3370 – 3375
92% of hydrogen. As the reaction studied is exothermic, increasing reaction rate with temperature increases the amount
of heat emitted per period of time accelerating the reaction. At
a certain temperature point (ca. 330 ◦ C as determined by measuring the temperature profile) the reaction is self-accelerating
and finishes in a short time of few minutes.
The kinetics of the reaction was evaluated by using hydrogen
yield as an extent of a reaction and fitting hydrogen generation
curves with different solid state reaction mechanism equations.
Sodium borohydride and magnesium hydroxide reaction obeys
2D solid state reaction model (g() = [(1 − ) ln(1 − )] + )
[25], implying that the reaction limiting stage is a diffusion of
the reactants through a product layer.
An Arrhenius plots of the hydrogen generation rates at different temperatures yield an activation energy for the reaction
of NaBH4 and brucite of 155.9 kJ/mol.
Mikheeva et al. [26] studied a reaction between NaBH4
and NaOH. Several thermal effects were detected on a heating
curves of NaBH4 –NaOH mixtures at 230–270 ◦ C (endothermic
effect), 320–350 ◦ C (exothermic effect) and additional exothermic effect at higher temperatures. Suggested stoichiometry of
the reaction can be expressed as
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Raman spectroscopy proved to be a useful tool for the
study of local atomic arrangement in borate glasses [31,32].
Phase evolution in the NaBH4 –Mg(OH)2 system with temperature studied by Raman spectroscopy is shown in Fig. 4. The
Raman spectra of the samples heated in the temperature range
of 240–318 ◦ C show common feature comprising two intense
vibrations at 1068 and 1079 cm−1 . However, according to the
3NaOH + NaBH4 → Na2 O + NaBO2 + NaH + 3H2 .
Differences in the phase composition of reacted motar-andpestle mixed reagents and ball milled system were found. According to the powder XRD study, ball milled mixture reacts
yielding MgO and amorphous sodium borate phases at the temperatures below 286 ◦ C. Increasing temperature above 286 ◦ C
leads to the formation of unidentified crystalline phase together
with MgO.
Examination of XRD patterns of the ball milled reactants
reveals no reaction between them or borohydride decomposition during ball milling. Sodium borohydride has already been
shown to demonstrate high structural stability under severe conditions of mechanical milling [27]. Thus, stoichiometric mixture of NaBH4 and Mg(OH)2 is stable at room temperature and
in absence of moisture unless it is heated. However, even at
highest temperature studied (360 ◦ C) the hydrogen yield was
93% suggesting that it might not be ruled out that NaBH4 and
Mg(OH)2 might have reacted with each other in a small undetectable by XRD way because of highly energetic conditions
of ball milling. Ball mixed mixture of sodium borohydride and
brucite after heating in the temperature range of 258–318 ◦ C
yield MgO as the only crystalline phase. Na2 O–B2 O3 system
is known to form glassy phase in a wide range of compositions
very easily.
New peaks are developed on XRD patterns of the sample reacted at 360 ◦ C. They do not belong to any phase comprising
sodium, boron, magnesium and oxygen according to JCPDS
database. These additional reflections can be indexed in tetragonal syngony within a space group P4/nmm with lattice parameters a = 3.547(1) Å, c = 4.323(2) Å, V = 54.39(4) Å3 . Results
of Le Beil’s fitting of two-phase XRD pattern are shown in
Fig. 3. This experiment was repeated twice and was found to be
reproducible. Further work is underway to the crystal structure
of a new phase.
Fig. 3. The full profile X-ray diffraction powder pattern of NaBH4 and
Mg(OH)2 mixture after reaction at 360 ◦ C fitting using the Le Bail method
[28] within GSAS [29,30]. The observed (×), simulated (solid line) and the
difference profiles (observed – calculated) are shown. The bars below the
profile indicate the positions of MgO reflections and new tetragonal phase.
Unreacted brucite peak positions are shown by asterisk (*).
Fig. 4. Raman spectra of NaBH4 and Mg(OH)2 reaction products at different
temperatures.
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V. Drozd et al. / International Journal of Hydrogen Energy 32 (2007) 3370 – 3375
reaction rate increases almost twice with increasing ball milling
time from 30 min to 1 h. Further increase of ball milling time
to 120 min gives no gain in the reaction rate as it is shown in
Fig. 5a.
We have tested several potential catalysts for the reaction between sodium borohydrite and brucite (Fig. 5b). The addition
of as much as 2 wt.% of cobalt metal and Co3 O4 to the starting
mixture of reagents and ball milled with them for 60 min were
found to have no effect on the reaction rate at 258 ◦ C. Some
improvement in the hydrogen generation rate was found for
the mixture containing 10 wt.% of amorphous SiO2 (AlfaAesar, 99.8%). It can act as a process control agent upon milling
increasing the specific surface area of the reactants and improving the homogeneity of the mixture. In addition, it can enhance
the diffusion of the ions through the interface between reagents
if it reacted with the products of the reaction and formed a more
permeable for the ions product layer.
4. Conclusions
Reaction of sodium borohydride and magnesium hydroxide
was studied. Reaction rate was found to depend on the degree of
reactants homogenization and/or their particle size. However,
the hydrogen weight content of such a system (5.2 wt.%) is
reduced compared with that one based on the water solution
of sodium borohydride. It might be promising for hydrogen
generation if the operating temperature can be lowered to usable
range.
Acknowledgments
The authors’ work is supported through a grant from National Science Foundation (DMR-0231291) and a grant from
Air Force (212600548).
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Fig. 5. (a) Reactants particle size and (b) catalyst effects on the hydrogen
generation rate studied at 258 ◦ C.
XRD analysis these samples contain unreacted NaBH4 , vibrations of sodium borohydride do not appear on the Raman spectra indicating that only the surface layer of the sample is probed
by the Raman spectrometer. The only borate species that may
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and brucite mixture after reaction at 360 ◦ C is completely different from others. Its characteristic feature is band at 992 cm−1 .
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strongly on the particle size of the reactants. Ball milling of the
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