hydrogen production from sodium borohydride in methanol–water mixtures

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Hydrogen production from sodium borohydride inmethanol–water mixtures

V.R. Fernandes a, A.M.F.R. Pinto b, C.M. Rangel a,*a Laboratorio Nacional de Energia e Geologia –LNEG, Fuel Cells and Hydrogen Unit, Paco do Lumiar 22, 1649-038 Lisboa, Portugalb Faculdade de Engenharia da Universidade do Porto, R. Roberto Frias, 4000 Porto, Portugal

a r t i c l e i n f o

Article history:

Received 1 August 2009

Received in revised form

12 November 2009

Accepted 13 November 2009

Available online xxx

Keywords:

Sodium borohydride

Hydrogen generation

Water–methanol mixtures

* Corresponding author. Tel.: þ351 210924657E-mail address: [email protected] (C

Please cite this article in press as: Fernamixtures, International Journal of Hydro

0360-3199/$ – see front matter ª 2009 Publisdoi:10.1016/j.ijhydene.2009.11.064

a b s t r a c t

Hydrogen production systems based on the hydrolysis of sodium borohydride loose effi-

ciency due to the excess water needed to account for the reaction and water capture by the

by-product. Solubility of NaBH4 and sodium borates in water is also a restricting factor

together with the need for stabilizers necessary for reaction control in aqueous medium. In

this work, methanol was used as an alternative to water. Literature data on this subject are

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

low temperatures. The effect of the water fraction on the efficiency of the reaction was

studied at 45 �C. Results indicated increase in the reaction rates with decreasing water

fraction. Sodium tetramethoxyborate was identified as the main by-product in methanol

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 methods are needed for improvement of this option for use in

The transition from energy based on fossil fuels to hydrogen-

based 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 evalua-

tion of storage systems.

Chemical hydrides, particularly borohydrides, are

currently being developed as storage options, since they

exhibit good energy densities, but cost effective recycling

; fax: þ351 217166568..M. Rangel).

ndes VR, et al., Hydroggen Energy (2009), doi:10

hed by Elsevier Ltd on be

selected fuel cell applications.

Sodium borohydride (NaBH4) is currently being studied as

a promising hydrogen storage option due to its high gravi-

metric 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 envi-

ronmentally 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-

en production from sodium borohydride in methanol–water.1016/j.ijhydene.2009.11.064

half of Professor T. Nejat Veziroglu.

mailto:[email protected]

http://www.elsevier.com/locate/he

Fig. 1 – Schematic experimental setup used for kinetics

studies of hydrogen production from sodium borohydride

in water–methanol mixtures.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 0 9 ) 1 – 72

ARTICLE IN PRESS

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. Further-

more, with the by-products species being alkaline, the reac-

tion 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.

NaBH4þ (2þ x)H2O / NaBO2 $ xH2Oþ 4H2[ (�210 kJ mol�1) (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 non-

compliance 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 borohy-

dride for on-board vehicular hydrogen storage, the improve-

ment 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:

NaBH4þ 4CH3OH / NaB(OCH3)4þ 4H2[þheat (2)

2. 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 temper-

ature was controlled using a thermostatic water bath. A sche-

matic 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 producedgases an intermediate water trapreactor

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.

Please cite this article in press as: Fernandes VR, et al., Hydrogmixtures, International Journal of Hydrogen Energy (2009), doi:10

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 boro-

hydride 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

(l¼ 1.54006 A). 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

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 con-

ducted 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.


0

500

1000

1500

2000

2500

3000a

b

0 20 40 60 80t / min

lm /

sa

gV

H2O:CH3OH = 0:10

H2O:CH3OH = 1:9

H2O:CH3OH = 5:5

H2O:CH3OH = 9:1

H2O:CH3OH = 10:0

Expected gas volume

0

0.5

1

1.5

2

2.5

3

0 20 40 60 80 100 120[CH3OH] / vol%

ni

m

/

em

it

n

oi

tc

ud

nI

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.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 0 9 ) 1 – 7 3

ARTICLE IN PRESS

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 meth-

anol, 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 charac-

terized by changes in pH of the solution as the reaction prog-

resses, due to the alkaline character of thereaction by-products.

The observed alkalinisation in the case of the hydrolysis is

found to arrest the reaction rate (Fig. 3a), limiting the effi-

ciency 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.


4

5

6

7

8

9

10

-1 0 1 2

ln [NaBH4]

ln k

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.

0

20

40

60

80

100a

b

0 40 80 120 160t /min

/]

4H

Ba

N[

%

9

9.5

10

10.5

11

11.5

Hp

[NaBH4]pH

0

20

40

60

80

100

0 5 10 15 20

t / min

%/

]4

HB

aN

[

9

9.5

10

10.5

11

Hp

[NaBH4]

pH

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.

0

20

40

60

80

100

120

0 20 40 60 80 100vol % CH

3OH

%

sp

en

t N

aB

H4

3

3.5

4

4.5

5

5.5

6

6.5

7

ΔpH

, p

Hº

spent NaBH4

initial pH

pH variation

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.


ARTICLE IN PRESS

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


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.

3.2. Reaction rates and the effects of sodium borohydrideconcentration 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.


Table 1 – Activation energy values for the production ofhydrogen from sodium borohydride in methanolsolutions. Data for self-hydrolysis and catalysedhydrolysis aqueous solutions are also given.

Solution/catalyst Activation energy(kJ mol�1)

Reference

Methanol with no added

water/no catalyst

13 This work

Pure methanol/no catalyst 52 [13]

Water/no catalyst 87 [4,3]

Water/catalysed hydrolysis

(Ni-based catalyst)/stabilised

with NaOH

68< 35 �C [12]

31> 40 �C

Water/catalysed hydrolysis

(Ruthenium catalyst)

41 [7]

0

500

1000

1500

2000

2500

3000

5 15 25 35 45

2 Theta

yt

is

ne

tn

I

- NaBO2.2H2O

a


ARTICLE IN PRESS

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 mol�1.

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.

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,

x¼ 2 [14].

In methanol solution, with no added water, sodium tetra-

methoxyborate was identified by X-Ray diffraction as the

reaction by-product (see Fig. 7b).

6

7

8

0.0030 0.0032 0.0034 0.0036

1/T/ºK-1

ln k

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.


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 purifi-

cation, 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].

0

100

200

300

400

500

600

700

800

5 10 15 20 25 30 35 40 452 Theta

yt

is

ne

tn

I

- NaB(OCH3)4

b

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.


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).


ARTICLE IN PRESS

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 path-

ways: 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)4þ 2H2O / NaBO2þ 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 associa-

tion 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 min�1). Methanol lowers the freezing temper-

ature of the reactant mixture providing short times for

initiation of the reaction, allowing hydrogen production to

start at low temperatures.



ARTICLE IN PRESS

� 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 detectedas by-product evenat 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.

Acknowledgements

One of the authors, CMR, gratefully acknowledges partial

funding by the European Commission DG Research (Contract

SES6-2006-518271/NESSHY).

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