ARTICLE IN PRESS
international journal of hydrogen energy xxx (2009) 1–7
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Hydrogen production from sodium borohydride in
methanol–water mixtures
V.R. Fernandes a, A.M.F.R. Pinto b, C.M. Rangel a,*
a
b
Laboratório Nacional de Energia e Geologia –LNEG, Fuel Cells and Hydrogen Unit, Paço do Lumiar 22, 1649-038 Lisboa, Portugal
Faculdade de Engenharia da Universidade do Porto, R. Roberto Frias, 4000 Porto, Portugal
article info
abstract
Article history:
Hydrogen production systems based on the hydrolysis of sodium borohydride loose effi-
Received 1 August 2009
ciency due to the excess water needed to account for the reaction and water capture by the
Received in revised form
by-product. Solubility of NaBH4 and sodium borates in water is also a restricting factor
12 November 2009
together with the need for stabilizers necessary for reaction control in aqueous medium. In
Accepted 13 November 2009
this work, methanol was used as an alternative to water. Literature data on this subject are
Available online xxx
scarce. Methanol lowers the freezing temperature of the reactant mixture with the
advantage of providing short times for the initiation of the reaction and possibility of use at
Keywords:
low temperatures. The effect of the water fraction on the efficiency of the reaction was
Sodium borohydride
studied at 45 C. Results indicated increase in the reaction rates with decreasing water
Hydrogen generation
fraction. Sodium tetramethoxyborate was identified as the main by-product in methanol
Water–methanol mixtures
with no added water. When using methanol with no added water the reaction follows
a first order rate kinetics with respect to sodium borohydride. Activation energy is reduced
by a factor of 5 in the presence of methanol with no added water, when compared to values
found in 100% water solutions. Methanol can be recovered by reaction of the by-product
with water, offering increased storage and energy density to the system.
ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.
1.
Introduction
The transition from energy based on fossil fuels to hydrogenbased systems, involves overcoming a number of significant
scientific, technological and socio-economic barriers.
Regarding hydrogen and fuel cells implementation, four main
obstacles are put forward: hydrogen production, storage, and
distribution, as well as the high costs of fuel cells.
Energy densities, cost, safety and ease of manufacture are
amongst the factors to be taken into account for the evaluation of storage systems.
Chemical hydrides, particularly borohydrides, are
currently being developed as storage options, since they
exhibit good energy densities, but cost effective recycling
methods are needed for improvement of this option for use in
selected fuel cell applications.
Sodium borohydride (NaBH4) is currently being studied as
a promising hydrogen storage option due to its high gravimetric capacity (10.73 wt%), well within DOE targets for 2015.
Its good stability in alkaline solution, easy control of hydrogen
generation rate, moderate operation temperatures and environmentally benign hydrolysis product has prompted
numerous research works contemplating catalysed hydrolysis
as a means to produce meaningful reaction rates [1–12].
It is to be noticed that the chemical hydride system, based
on the hydrolysis of sodium borohydride, looses efficiency of
storage due to the fact that the reaction needs excess water
to account for the solubility of NaBH4 and the borate by-
* Corresponding author. Tel.: þ351 210924657; fax: þ351 217166568.
E-mail address: [email protected] (C.M. Rangel).
0360-3199/$ – see front matter ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.
doi:10.1016/j.ijhydene.2009.11.064
Please cite this article in press as: Fernandes VR, et al., Hydrogen production from sodium borohydride in methanol–water
mixtures, International Journal of Hydrogen Energy (2009), doi:10.1016/j.ijhydene.2009.11.064
ARTICLE IN PRESS
2
international journal of hydrogen energy xxx (2009) 1–7
products; furthermore the latter capture water reducing even
further the efficiency of the reaction, equation (1).
This has become a critical issue in the developing of an
efficient generator based on sodium borohydride. Furthermore, with the by-products species being alkaline, the reaction medium promotes a low yield of the hydrolysis making it
necessary to use a catalyst to take the reaction to its full
extent. As recently found [14,15], four moles of water are
necessary for full hydrolysis of 1 mol of borohydride, equation
(1). It is to be noticed that gravimetric capacity (materials
based only) decreases down to 7.34 wt% for a hydration factor
of 2. Reducing the value of x is a key factor to have more
available specific energy and energy density.
NaBH4 þ (2 þ x)H2O / NaBO2 $ xH2O þ 4H2[ (210 kJ mol1) (1)
Because of the low solubility of sodium borohydride and
the by-products in water and furthermore, the necessary
addition of stabilizer (generally NaOH), storage efficiency of
sodium borohydride hydrolysis reduces considerably. A
degree of hydration as high as 19.37 for a 8.5 wt% NaBH4 and
5 wt% NaOH solution is been put forward by Shang and Chen
[16] corresponding to a storage capacity of only 1.81 wt%.
These findings have implications regarding the noncompliance with DOE targets published for FreedomCAR
specifications for 2015 regarding automotive hydrogen storage
systems. Taking into account allowed reactor mass to mass of
reactant ratio, an x value of 0.84 has been estimated [15].
In spite of the no-go recommendation of sodium borohydride for on-board vehicular hydrogen storage, the improvement of the system is sought for portable applications [17].
In this work, results are presented of a study of the effect of
methanol–water mixtures on the kinetics of hydrolysis of
sodium borohydride. Methanol, known to be reactive in
sodium borohydride, is used as reactant as an alternative to
water. Literature data on this subject are scarce. The overall
reaction can be described as follows:
NaBH4 þ 4CH3OH / NaB(OCH3)4 þ 4H2[ þ heat
2.
(2)
Fig. 1 – Schematic experimental setup used for kinetics
studies of hydrogen production from sodium borohydride
in water–methanol mixtures.
Experiments were performed to study the effect of
different methanol/water ratios on the hydrolysis reaction
rate, in the absence of stabilizers and catalysts. Reaction rates
were determined in a range of temperatures from 5 to 55 C.
One gram of sodium borohydride was used throughout,
except when studying the effect of hydride concentration
when it was varied from to 0.54 to 5.47 M.
All the experiments were performed using sodium borohydride from ROHM and HAAS and methanol (99,8%) from
Fluka.
A pH/ion meter, model 25 from Denver Instruments was
used for pH measurements. The change in pH during the
reaction was studied for solutions containing 100% water and
50% and 100% methanol without added water.
The by-products of reaction were analysed on a ‘‘Rigaku
Geigerflex’’ X-Ray diffractometer, employing Cu-Ka radiation
(l ¼ 1.54006 Å). The diffracted radiations were measured in
a range of 2q from 3 to 123 , operating at 45 kW/20 mA.
Analysis of some of the by-products was also carried out
using an SEM (Scanning Electron Microscope from Philips,
Model XL 30 FEG), coupled to EDAX (Energy-dispersive X-Ray
spectroscopy).
3.
Results and discussion
3.1.
Reaction rates and water:methanol ratios
Experimental
A study of the hydrolysis reaction of sodium borohydride was
conducted at ambient pressure, in a tubular glass reactor which
was modified to accommodate a thermocouple, pH sensor and
an opening for the injection of methanol. The reactor temperature was controlled using a thermostatic water bath. A schematic drawing of the experimental setup is shown in Fig. 1.
The volume of generated gas was measured by a water
displacement method rendering values at standard pressure
and temperature. The produced gas volumes were measured as
a function of time at controlled temperature, till complete
exhaustion of the reactant. To avoid the presence of methanol
vapour in the produced gases an intermediate water trap reactor
was implemented and located before the volumetric measuring
device, see Fig. 1. The total reaction time was measured starting
from the instant of methanol injection into the reactor.
Fig. 2a shows the volume of gas collected as a function of time
for the conversion of 1 g of sodium borohydride into hydrogen,
in solutions with different water:methanol ratios, at
a controlled temperature of 45 C. Experiments were conducted in the absence of stabilizer or catalyst.
Data show that hydrogen production rate increased with
increasing methanol concentration. Full expected gas volume
for the total conversion of sodium borohydride is indicated in
Fig. 2a by a horizontal broken line drawn across the graph.
The lowest conversion rate was obtained for the case of
100% water. Furthermore, for methanol with no added water,
full conversion of all the available hydrogen contained in the
added sodium borohydride was attained at relatively short
times.
Please cite this article in press as: Fernandes VR, et al., Hydrogen production from sodium borohydride in methanol–water
mixtures, International Journal of Hydrogen Energy (2009), doi:10.1016/j.ijhydene.2009.11.064
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international journal of hydrogen energy xxx (2009) 1–7
a
3000
2500
H2O:CH3OH = 0:10
H2O:CH3OH = 1:9
2000
H2O:CH3OH = 5:5
V g a s / ml
H2O:CH3OH = 9:1
1500
H2O:CH3OH = 10:0
Expected gas volume
1000
500
0
0
20
40
60
80
b
3
Induction ti m e / m in
t / min
2.5
2
1.5
1
0.5
0
0
20
40
60
80
[CH3OH] / vol%
100
120
Fig. 2 – Volume of gas generated as a function of time for solutions containing 1 g of NaBH4, for different water:methanol
ratios at 45 8C.
It was noticed that there is a measurable lag time for the
initiation of the reaction. In Fig. 2b it is shown the remarkable
decrease of the reaction induction time with the increase in
methanol concentration.
Changes in solution pH are found to be closely related to
rate of hydrogen production in both cases, water and methanol, increasing with the amount of borohydride converted to
hydrogen. Fig. 3 shows the variation of solution pH for the case
of methanol with no added water and also for the case of 100%
water as a function of reaction time. The amount of
borohydride spent as a function of time was simultaneously
recorded.
Self-hydrolysis of sodium borohydride in water is characterized by changes in pH of the solution as the reaction progresses, due to the alkaline character of the reaction by-products.
The observed alkalinisation in the case of the hydrolysis is
found to arrest the reaction rate (Fig. 3a), limiting the efficiency of the reaction. Only 40% of the borohydride present in
the aqueous solution is found to have reacted after 2 h, for an
attained pH > 11 at 45 C.
Please cite this article in press as: Fernandes VR, et al., Hydrogen production from sodium borohydride in methanol–water
mixtures, International Journal of Hydrogen Energy (2009), doi:10.1016/j.ijhydene.2009.11.064
ARTICLE IN PRESS
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international journal of hydrogen energy xxx (2009) 1–7
120
11.5
100
7
6.5
100
6
10.5
60
pH
[ NaBH4] /%
80
10
[NaBH4]
40
% spent NaBH 4
11
80
5.5
60
5
4.5
40
spent NaBH4
pH
0
3
0
0
40
80
120
60
80
100
10.5
[NaBH4]
pH
pH
10
40
9.5
20
0
methanol concentrations. This could bear implications in the
high observed reaction rates and observed solution pH values.
This point will be also discussed in Section 3.3, in relation to
the identified reaction by-products.
9
0
5
10
15
20
t / min
Fig. 3 – Sodium borohydride conversion (%) as a function of
reaction time for a NaBH4 solution with (a) 100% water; (b)
methanol with no add water; at 45 8C. Simultaneous
changes in solution pH are also shown.
In the case of methanol with no added water, pH variations
were also registered and found to be closely related to the
amount of borohydride left in the solution as a function of
reaction time. Furthermore, for the same experimental
conditions as for the case of 100% water, w90% of the
hydrogen present in the borohydride is delivered in the first
5 min of the reaction (see Fig. 3b).
A general idea of what may happen when the amount of
water is decreased in the mixture is given in Fig. 4. The initial
pH, pH0, was measured before addition of sodium borohydride
to the reaction mixture. The spent borohydride as a function
of the amount of methanol in the mixture is depicted, whilst
at the same time the initial solution pH (pH0) and the pH
variation expressed in terms of DpH are registered. Water
addition is shown to lower the reaction rate in a significantly
way. An exponential increase is noted in the amount of spent
borohydride with an increase in the % of methanol in the
mixture, but significant variations in pH are only registered for
more than 70 vol% of methanol.
Methanol displays hydrophilic characteristics associated
to its solubility in water, but is also known by its hydrophobic
character; while its –OH radical can readily bond with
surrounding water molecules, the methyl radicals, which
cannot bond, give methanol its hydrophobic character.
Breakdown of hydrogen-bonded chains, characteristics of
pure methanol, is expected in water mixtures with high
3.2.
Reaction rates and the effects of sodium borohydride
concentration and solution temperature
The effect of sodium borohydride concentration on the
hydrogen producing reaction rate was studied at 45 C, by
performing a series of experiments with varying initial
concentrations of NaBH4 from 0.54 to 5.47 M in a methanol
solution with no added water. The hydrogen generation rate
was determined from the linear portion of the plot of the
produced gas vs. time, for each NaBH4 concentration.
Fig. 5 shows the ln–ln plot of the hydrogen generation rate
vs. sodium borohydride concentration. A straight line with
a slope of 1.05 was obtained. This result indicates that the
reaction in methanol with no added water is pseudo-first
order with respect to the concentration of sodium
borohydride.
10
9
8
ln k
[NaBH4] / %
11
80
40
Fig. 4 – Hydrogen production from sodium borohydride in
various water/methanol mixtures at 45 8C, expressed in
terms of the % of spent NaBH4 variation with vol%
methanol. Variations of pH are also registered.
b
100
20
vol % CH3OH
9
160
t /min
60
3.5
pH variation
9.5
20
0
4
initial pH
20
ΔpH, pHº
a
7
6
5
4
-1
0
1
2
ln [NaBH4]
Fig. 5 – Plot of hydrogen generation rate versus sodium
borohydride concentration, both on logarithmic scales, for
hydrolysis in methanol solution with no added water, at
45 8C, for NaBH4 concentrations from 0.54 to 5.47 M.
Please cite this article in press as: Fernandes VR, et al., Hydrogen production from sodium borohydride in methanol–water
mixtures, International Journal of Hydrogen Energy (2009), doi:10.1016/j.ijhydene.2009.11.064
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international journal of hydrogen energy xxx (2009) 1–7
3.3.
Reaction by-products
Fig. 7 shows the X-ray diffraction patterns for by-products of
the hydrogen production from NaBH4 in methanol solutions
with no added water, and for a 50% water/methanol mixture.
The data analysis indicated the presence of dihydrated
sodium metaborate as the main product of the reaction in
methanol/water 50% mixtures (see Fig. 7a). The same
compound was also found when studying the catalysed
hydrolysis of NaBH4 using a Ni-based catalyst, confirming the
capture of 2 water molecules with the precipitation of NaBO2,
x ¼ 2 [14].
In methanol solution, with no added water, sodium tetramethoxyborate was identified by X-Ray diffraction as the
reaction by-product (see Fig. 7b).
Table 1 – Activation energy values for the production of
hydrogen from sodium borohydride in methanol
solutions. Data for self-hydrolysis and catalysed
hydrolysis aqueous solutions are also given.
Methanol with no added
water/no catalyst
Pure methanol/no catalyst
Water/no catalyst
13
This work
52
87
[13]
[4,3]
Water/catalysed hydrolysis
(Ni-based catalyst)/stabilised
with NaOH
Water/catalysed hydrolysis
(Ruthenium catalyst)
68 < 35 C
31 > 40 C
[12]
41
[7]
These findings suggest that the hydrogen generation
reaction occurs preferentially via a pathway associated with
methanolysis of NaBH4, and that the amount of water, present
in methanol when used at 99.8% and without further purification, has no significant effect on the reaction rate. The
reaction proposed for this process is reaction (2).
In methanol/water mixtures, the XRD analysis showed
dihydrated sodium metaborate as the main by-product. No
traces of sodium tetramethoxyborate were detected.
This evidence may suggest, in a first approach, that in
presence of methanol/water mixtures, the reaction occurs
preferentially by a pathway of hydrolysis instead of the
methanolysis [13].
a
3000
2500
- NaBO2.2H2O
2000
1500
1000
500
0
5
15
25
45
800
700
- NaB(OCH3)4
Intensity
600
ln k
35
2 Theta
b
8
Activation energy Reference
(kJ mol1)
Solution/catalyst
Intensity
The activation energy of sodium borohydride reaction in
methanol, without added water, was estimated in the
temperature range between 5 and 55 C, using 1 g of NaBH4.
The values of the rate constant k, determined for six different
temperatures were used to create the Arrhenius plot shown in
Fig. 6.
It is possible to confirm that in the presence of methanol
with no added water, the conversion reaction rate of NaBH4 is
higher than in self-hydrolysis [4,13]. The reaction exhibits
rapid kinetics at low temperatures. Methanol lowers
the freezing temperature of the reactant mixture with the
advantage of providing short times for the initiation of
the reaction and possibility of use at sub-zero temperatures
where water is solid.
In the presence of methanol mixtures with added water,
the temperature has a significant effect on the hydrogen
production rate, and only at temperatures >45 C it is
possible to obtain significant reaction rates, as evident in
Fig. 2a.
The activation energy (Ea) was found to be 13 kJ mol1.
This value is smaller than the activation energy found in our
previous study about sodium borohydride self-hydrolysis
and also of catalysed hydrolysis in water stabilized with
10 wt% NaOH, see Table 1. Results from other authors [13]
are shown for comparison. It is to be noticed that in the
latter case anhydrous methanol with a purity of 99.995% was
used.
7
500
400
300
200
100
0
6
0.0030
5
0.0032
0.0034
0.0036
1/T/ºK-1
Fig. 6 – Arrhenius plot for the hydrogen production from
NaBH4 in methanol solutions with no added water, in the
temperature range of 5–55 8C, using 1 g NaBH4.
10
15
20
25
30
35
40
45
2 Theta
Fig. 7 – X-ray diffraction patterns of by-products of the
reaction of production of hydrogen in 50% mixture of
water/methanol solution (a); methanol with no add water
(b), at a temperature of 45 8C.
Please cite this article in press as: Fernandes VR, et al., Hydrogen production from sodium borohydride in methanol–water
mixtures, International Journal of Hydrogen Energy (2009), doi:10.1016/j.ijhydene.2009.11.064
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international journal of hydrogen energy xxx (2009) 1–7
Fig. 8 – EDAX analysis of by-product of the reaction of the production of hydrogen in methanol with no add water at
a temperature of 45 8C (a); comparison with spectra of the by-product after reaction with water (b).
However, it must be noted that in the study of the influence
of the methanol/water ratios, results indicated an increase in
the hydrogen production reaction rate with an increase in the
share of methanol (see Fig. 2).
In order to explain the faster rates obtained with increasing
methanol concentration measured in methanol/water
mixtures, it is proposed that:
- sodium borohydride in contact with methanol/water
mixtures, reacts following both of the mentioned pathways: by methanolysis – faster hydrogen production,
reaction (2) and by hydrolysis – slow hydrogen production,
reaction (1),
- the by-products are a mixture of dihydrated sodium
metaborate and sodium tetrametoxyborate, even though
only the first is detected by X-Ray diffraction,
- the reason by which only dihydrated sodium metaborate is
present in the by-products might be explained by the fact
that sodium tetramethoxyborate reacts, in presence of
water, to form sodium metaborate and methanol, according
to reaction (3), allowing regeneration of the reactant, adding
storage and energy density to the system.
NaB(OCH3)4 þ 2H2O / NaBO2 þ 4CH3OH
(3)
The by-product obtained from the reaction with methanol
without added water was collected and dried before samples
were taken for X-Ray Diffraction. Afterwards the by-products
were allowed to react with water in a small reactor at room
temperature. After removal of the excess water, the elemental
composition of the resulting powder was analysed using
EDAX. Comparison of the spectrum is made with that
obtained for by-product prior to reaction with water, which
indicates that the carbon from the tetrametoxyborate is
remarkably diminished suggesting its conversion to borate
and methanol, see Fig. 8a and b.
Conversion of sodium tetramethoxyborate could prove
interesting when using methanol without added water,
allowing regeneration of the reactant, offering increased
storage and energy density of the system. In case of association of this method of hydrogen production/storage with
a fuel cell, the water produced provides a further advantage.
In spite of the theoretical storage density of the reaction of
methanolysis of sodium borohydride amounting only to
4.9 wt% it is advantageous regarding self-hydrolysis at 45 C
which only reaches 2.9 wt%, taking into account Fig. 2,
furthermore, it offers a ready start of the reaction and the
possibility to generate hydrogen at sub-zero temperatures.
4.
Conclusions
In the present work the use of methanol as an alternative to
water in the production of hydrogen from sodium borohydride
was studied. The following conclusions can be drawn:
When using methanol with no added water, the reaction
follows a first order rate kinetics with respect to sodium
borohydride concentration.
The activation energy is reduced by a factor of 5 in presence
of methanol when compared with values found in 100%
water solutions.
In methanol solutions with no added water, the by-product
of the hydrogen producing reaction was sodium
tetramethoxyborate.
Using sodium borohydride in methanol solutions allows
high rates of hydrogen production at low temperatures. In
this work at 5 C, the hydrogen generation is considerably
high (1200 L min1). Methanol lowers the freezing temperature of the reactant mixture providing short times for
initiation of the reaction, allowing hydrogen production to
start at low temperatures.
Please cite this article in press as: Fernandes VR, et al., Hydrogen production from sodium borohydride in methanol–water
mixtures, International Journal of Hydrogen Energy (2009), doi:10.1016/j.ijhydene.2009.11.064
ARTICLE IN PRESS
international journal of hydrogen energy xxx (2009) 1–7
In the presence of methanol and water mixtures, sodium
borohydride reacts following two pathways: methanolysis –
faster reaction (2) and by hydrolysis – slow reaction (1),
producing different reaction by-products.
Possible conversion of sodium tetramethoxyborate back to
producing methanol is suggested, since tetramethoxyborate
was not detected as by-product even at 50:50 water:methanol
mixtures and furthermore, rates were observed to increase
with the increase in the methanol share in solution.
Conversion of sodium tetramethoxyborate, allowing
regeneration of the reactant, offers increased storage and
energy density of the system.
[7]
[8]
[9]
[10]
Acknowledgements
[11]
One of the authors, CMR, gratefully acknowledges partial
funding by the European Commission DG Research (Contract
SES6-2006-518271/NESSHY).
[12]
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Please cite this article in press as: Fernandes VR, et al., Hydrogen production from sodium borohydride in methanol–water
mixtures, International Journal of Hydrogen Energy (2009), doi:10.1016/j.ijhydene.2009.11.064