Solid sodium borohydride as a hydrogen source for fuel cells

Article abstract of DOI:10.1016/j.jallcom.2008.01.023

Hydrolysis of sodium borohydride NaBH₄ is a promising method for on-board hydrogen supply for fuel cells. In this study, the hydrolysis reaction was studied by starting with solid NaBH₄ rather than aqueous borohydride solutions, and

Full text of DOI:10.1016/j.jallcom.2008.01.023

Journal of Alloys and Compounds 468 (2009) 493–498
Solid sodium borohydride as a hydrogen source for fuel cells
Bin Hong Liua,∗, Zhou Peng Lia, S. Sudab
a College of Materials and Chemical Engineering, Zhejiang University, Hangzhou 310027, PR China
b Materials & Energy Research Institute Tokyo, Ltd., 1-1 Sawarabi-Daira, Tateshina-Chuokogen, Kitayama, Chino-shi, Nagano 391-0301, Japan
Received 15 December 2007; received in revised form 5 January 2008; accepted 7 January 2008
Available online 4 March 2008
Abstract
Hydrolysis of sodium borohydride NaBH4 is a promising method for on-board hydrogen supply for fuel cells. In this study, the hydrolysis
reaction was studied by starting with solid NaBH4 rather than aqueous borohydride solutions, and adding less amount of water in an attempt to
explore the maximum hydrogen generation capacity. It was found that hydrogen could be liberated at very high rates and over than 90% conversion
rates were achieved when the mole ratio of water to sodium borohydride was not less than 4:1. The final hydrogen generation capacity was achieved
as high as 6.7 wt%. Materials such as CoCl2 or cobalt powder were found to be effective as the catalysts for the hydrolysis reaction at such reaction
conditions. A large heat effect due to the exothermic reaction and a low melting point of NaBO2·4H2O led to the acceleration of hydrogen generation
at the latter part of the hydrolysis process. It is thus promising to utilize these reaction conditions to achieve high hydrogen generation capacity by
proper system design.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Hydrogen generation; Borohydride; Hydrolysis; Catalyst; Kinetics; Capacity
1. Introduction
with CO impurities that will poison electrocatalysts in fuel cells.
Recently, aqueous sodium borohydride solutions are attracting
considerable attentions as hydrogen source for fuel cells due
to high hydrogen contents and the ease of hydrogen generation
by the catalyzed hydrolysis reaction according to the following
equation:
Fuel cells are regarded as one of solutions for solving energy
and environmental problems by generating power with higher
efficiency and zero emission. The proton exchange membrane
fuel cell (PEMFC) is the most promising fuel cell type for
commercialization due to its excellent performance at low
temperatures. However, safe and efficient hydrogen supply to
PEMFCs is still a problem to be solved. There are several
options for hydrogen storage and supply: hydrogen can either
be stored as compressed gas, as a liquid or in hydrogen storage
materials. Hydrogen can also be supplied by reforming hydro-
carbons. However, these methods are inadequate in terms of
their volumetric or gravimetric storage densities. For example,
hydrogen stored in pressurized vessels has low volumetric den-
sity and safety concern. Liquid hydrogen is energy-consuming
for preparation and storage. Hydrogen storage in metal hydrides
is advantageous in its volumetric density and safety but the
gravimetric density needs to be improved. Reformation from
hydrocarbons requires high temperature and produces hydrogen
NaBH4 + 2H2O = NaBO2 + 4H2 + 300 kJ
(1)
There have been quite a lot of studies on hydrogen generation
from hydrolysis of borohydrides [1–17]. It has been found that
Pt [4], Ru [1,9,15], Co2B [17,19], Ni2B [5], Ni [8], Raney Ni
[11,20], Mg2Ni [3] are good catalysts for the hydrolysis reaction.
Some prototype hydrogen generators were built up to power fuel
cells [6,16,17]. On the other hand, it has also been found that the
hydrolysis product sodium meta-borate NaBO2 has a relatively
small solubility in water. Although NaBH4 shows a solubility of
55 g in 100 g water at 25 ◦C, NaBO2 has only 28 g in 100 g water
at 25 ◦C [6]. Correspondingly, it was reported that the optimum
concentration of NaBH4 in starting solution was about 15 wt%
[4,9,12], otherwise NaBO2·4H2O would precipitate out from the
solution and possibly damage the catalyst or clog tubes in the
reaction system. As a result, hydrogen generation capacity in the
system will be limited to 3.2 wt%, quite inadequate considering
∗
Corresponding author. Tel.: +86 571 87951770; fax: +86 571 87951770.
E-mail address: liubh@zju.edu.cn (B.H. Liu).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.01.023

494
B.H. Liu et al. / Journal of Alloys and Compounds 468 (2009) 493–498
the theoretical 10.8 wt% in Eq. (1) and a target of 6.0 wt% set
by Department of Energy (DOE), USA, for hydrogen storage
systems [18].
2.3. Instrumental analyses
The hydrolysis products after the reaction were analyzed by X-ray diffrac-
tion (XRD) on a Rigaku D/MAX-RA using Cu K␣ radiation. Also a commercial
product of NaBO2·4H2O was analyzed by differential scanning calorimetry
(DSC) on Shimadzu DSC60. Alumina crucibles were used in the DSC mea-
surement and ␣-Al2O3 powder was used as the reference. The DSC sample was
heated at a rate of 4.5 ◦C/min.
To make the system more attractive and competitive, it is
highly desirable to increase hydrogen generation capacity for
the borohydride system. However, few investigation results were
reported about hydrogen generation at very high borohydride
concentrations or using solid sodium borohydride as a hydro-
gen source for the PEMFC. The reaction kinetics and hydrogen
yield remain unclear when water is not abundantly supplied. In
this work, we studied characteristics of the hydrolysis reaction
by using solid NaBH4 and adding less amount of water. Hydro-
gen generation rates and conversion rates were investigated by
examining the effects of water amounts as well as catalyst addi-
tions. Reaction mechanisms were discussed on the basis of the
experimental results.
3. Results and discussion
When the initial concentration of sodium borohydride was
more than 10 wt%, it has been found that crystals of the hydrol-
ysis product sodium meta-borate NaBO2·4H2O would appear
after the reaction finished. The presence of hydrated meta-borate
crystals inevitably affects the subsequent reaction, especially
when crystals appear on the surface of the catalyst. In case the
hydrogen generation system is clogged or damaged by crys-
tals, it was well reported that the optimum initial concentration
of sodium borohydride is about 15 wt% [4,9,12]. It inevitably
results in a maximum hydrogen generation capacity of 3.2 wt%,
a value far less than the theoretical 10.8 wt% in Eq. (1). To
explore the possibility of achieving larger hydrogen genera-
tion capacity, it is essential to investigate characteristics of
the hydrolysis reaction when water is not very abundant. It is
expected that solids will be formed during the reaction. We were
interested in studying how hydrogen generation kinetics and the
final conversion rate would become under this kind of reaction
condition.
To make the hydrolysis reaction as deep as possible even after
crystals emerge, it is important to select an active catalyst that
is able to be finely distributed in the reactants. Among possible
catalysts for the borohydride hydrolysis reaction [20], we found
that cobalt chloride CoCl2 could serve the purposes. It has been
reported that the catalytic mechanism for cobalt chloride is first
an acidic catalysis and then a subsequent catalysis from a reduc-
tion product cobalt boride Co2B [20]. The reduction reaction
can be written as follows [21].
2. Experimental
2.1. Reaction mode
In this research, sodium borohydride was not dissolved in water to prepare
a solution prior to the reaction. As shown in Fig. 1, 1 g of sodium borohydride
powder was put at the bottom of a 200 ml three-neck flask. The hydrated cobalt
chloride CoCl2·6H2O was dissolved in a portion of water. Then the solution of
the catalyst was injected into the flask using a syringe to initiate the hydrolysis
reaction. A thermocouple was embedded in borohydride powder to monitor the
temperature during the reaction. No stirring was applied and no water bath was
used during the reaction. The volume of generated hydrogen was measured as a
function of time by passing it through a wet drum gas meter and the volume was
transformed to the standard temperature and pressure (S.T.P) before calculating
hydrogen generation rates. When cobalt powder was used as the catalyst, it was
premixed with sodium borohydride powder in a mortar and put into the flask
before water was injected.
2.2. Materials
Two materials in different forms were tested as the catalysts for the hydrol-
ysis reaction: CoCl2·6H2O and fine cobalt powder. In the case of CoCl2·6H2O,
a certain amount of CoCl2·6H2O was dissolved in different volumes of water
and then the solutions were injected into the flask. A fine cobalt power from
Johnson Mathey Company with a powder size of 1.6 ␮m was also tested as
the catalyst. It was premixed with sodium borohydride powder before the
test. No stabilizer reagent such as sodium hydroxide was employed in this
research.
CoCl2+2NaBH4 + 3H2O = 6.25H2 + 0.5Co2B
+ 2NaCl + 1.5HBO2
(2)
Co2B produced from above reaction was reported as a very
good catalyst for borohydride hydrolysis [19–21]. In this work,
we expected that the formed fine Co2B could be distributed in
the reactants promptly and thoroughly to make the hydrolysis
reaction fast and deep.
Sodium borohydride used in the experiments was commercially available
with a purity of 96%. De-ionized water was used in this research.
Fig. 2 shows hydrogen generation under the catalysis of 0.1 g
CoCl2·6H2O and at a 1:2 weight ratio of sodium borohydride to
water. From Fig. 2, it can be seen that the hydrogen generation
curve shows two steps. The measured temperature is also found
to have the similar shape. During the experiment, it was observed
that the reduction reaction took place very quickly as the slurry
turned into black almost simultaneously when the CoCl2 solu-
tion was injected into the flask. After the black cobalt boride
was formed, it acted as the catalyst of the subsequent hydrol-
ysis reaction. According to the reaction (2), 0.1 g CoCl2·6H2O
would only consume 0.033 g NaBH4 and produce 59 ml H2.
Fig. 1. The experiment setup: 1: three-neck flask, 2: syringe, 3: bottle for gas
washing, 4: gas meter, 5: thermal couple, 6: temperature measurement unit.

B.H. Liu et al. / Journal of Alloys and Compounds 468 (2009) 493–498
495
Fig. 2. Hydrogen generation volume and temperature as a function of time
(NaBH4: 1.0 g, H2O: 2.0 g, CoCl2·6H2O: 0.1 g).
Accordingly, the first step found in the hydrogen generation and
temperature curves in Fig. 2 (about 175 ml of hydrogen and a rise
of temperature to 40 ◦C) was already the result of two catalysis
effects. Therefore, it appears that the reaction (2) only showed
effects at very initial stage of the experiment and the subsequent
hydrolysis was only under the catalysis of the formed Co2B.
Although the mole ratio of H2 to NaBH4 is 3.125:1 in reac-
tion (2), which is smaller than 4:1 in reaction (1), the decrease
of hydrogen volume due to reaction (2) would be only 19 ml
when 0.1 g CoCl2·6H2O was used. This amount is negligible
compared with 2370 ml hydrogen generated from 1 g NaBH4
in reaction (1). Therefore, the influence of CoCl2 reduction on
total released hydrogen volume would be negligible.
Under the catalysis of Co2B, the reaction was then found to
proceed at a constant rate though the temperature was gradually
increased due to the exothermic reaction. However, a surge of
hydrogen generation took place when the conversion reached
about 35%. The burst of hydrogen generation was also accom-
panied by a quick rise of temperature. As can be seen in Fig. 2,
the temperature was elevated to be as high as 110 ◦C. When
the reaction proceeded further and water was almost consumed,
hydrogen generation was found to slow down. The reaction
finally reached a conversion of 92% after 24 h and a hydrogen
weight percentage of 6.1 wt% was achieved on a mass basis of
all reactants and the catalyst.
Fig. 3 shows that hydrogen generation was largely depressed
by decreasing the amount of the catalyst. Although it is definite
that the reaction slows down with the decrease of catalyst, the
large difference in hydrolysis kinetics for two catalyst additions
shown in Fig. 3 is supposed to be mainly caused by the heat
effect. It seems that if the initial reaction was not quick enough
to trigger the heat effect, hydrogen generation would become
rather slow. It would take several hours rather than a few minutes
to reach the same conversion.
Fig. 4(a and b) shows the effects of water amount on the
hydrolysis reaction when the weight of sodium borohydride was
kept at 1 g. As can be seen from Fig. 4(a), hydrogen generation
rate was first decreased and then increased with decreasing water
from 7 to 2 g. On the whole, hydrogen generation was very fast
Fig. 3. Hydrogen generation at two different catalyst additions (NaBH4: 1.0 g,
H2O: 1.5 g).
andfinishedwithinseveralminutes. Theconversioncouldfinally
reachmorethan90%asshownin Fig. 5. Whenwateramountwas
further decreased to be less than 2 g, the conversion rate turned
to be largely decreased. Also the kinetics slowed down with
decreasing water addition. Based on above results, we suppose
that hydrogen generation rates were mainly influenced by two
counter effects: a decrease in water activity and an increase in
heat effect due to a reduction of the total amounts of the reactants
when water was less added.
The experiment results are summarized in Table 1. It shows
that the final hydrogen generation capacity reached 6.7 wt% that
is high enough as a viable hydrogen generation system. Table 1
also shows that there were no apparent decreases of released
hydrogen volume due to the reduction of CoCl2.
Fig. 6 shows the XRD analysis results of the final solid prod-
ucts after hydrogen was released. In these three samples, several
solid phases were detected: NaBO2·2H2O or NaB(OH)4,
NaBO2·4H2O
or
NaB(OH)4·2H2O,
Na2B4O7·5H2O
and Na2CO3·H2O. The presences of NaBO2·2H2O and
NaBO2·4H2O were within expectation but the appearances
of small amount of Na2B4O7·5H2O and Na2CO3·H2O were
unexpected. The formation mechanism of Na2B4O7·5H2O and
Na2CO3·H2O may be originated from the following reaction
[22].
4NaBO2 + CO2 = Na2B4O7 + Na2CO3
(3)
Further investigation is needed to verify the above forma-
tion mechanism for Na2B4O7. Although there existed unreacted
NaBH4 in the remaining samples when water amount was small,
the peaks from NaBH4 were not distinguishable. Also Co2B was
not well detected. The reason may situate in that they were cov-
ered by the products. For the samples produced from NaBH4
and 2.0 g water or 0.8 g water, most NaBO2·4H2O turned into

496
B.H. Liu et al. / Journal of Alloys and Compounds 468 (2009) 493–498
Fig. 6. XRD patterns of three hydrolysis product samples. (a) H2O 2.0 g,
CoCl2·6H2O 0.2 g; (b) H2O 1.5 g, CoCl2·6H2O 0.1 g; (c) H2O 0.8 g,
CoCl2·6H2O 0.2 g.
NaBO2·2H2O. But for the sample produced from NaBH4 and
1.5 g water under the catalysis of 0.1 g CoCl2·6H2O, a con-
siderable portion of NaBO2·4H2O remained. As this sample
underwent a slow kinetics as shown in Fig. 3, it is supposed
that the low reaction temperature resulted in a difficulty in dif-
fusion when NaBO2·4H2O crystals were formed and thus the
reaction was impeded.
Fig. 7 shows the DSC analysis result of NaBO2·4H2O. The
first peak represents the melting of NaBO2·4H2O and dehydra-
tion to NaBO2·2H2O. Further dehydration from NaBO2·2H2O
needs to be higher than 110 ◦C. Combining hydrogen generation
curves with the XRD and DSC analysis results, we suppose that
due to a quick initial kinetics and a large heat effect, the temper-
ature was elevated over the melting point of NaBO2·4H2O and
then water reacted easily with borohydride to achieve a high con-
version rate. If the temperature cannot be raised to the melting
point, reaction kinetics will be much slower due to diffusion dif-
ficulty in solid, and also the conversion rate would be decreased
as a result of slow kinetics. Therefore, it can be concluded that
three factors were responsible for the quick and full conversion
of borohydride to hydrogen with small amounts of water: (1) an
effective catalyst, (2) large heat effect and (3) the low melting
point for NaBO2·4H2O.
Fig. 4. Hydrogen generation characteristics at different water additions. (a)
NaBH4: 1.0 g; H2O: 2.5–7.0 g; CoCl2·6H2O: 0.2 g; (b) NaBH4: 1.0 g; H2O:
2.0–0.8 g; CoCl2·6H2O: 0.2 g.
To investigate effects of catalyst on the reaction charac-
teristics, catalysts other than CoCl2 were also tested. Fig. 8
demonstrates hydrogen generation behavior under the cataly-
sis of cobalt powder. Compared with cobalt chloride, the cobalt
powder has only one catalysis mechanism: a heterogeneous
catalysis. As can be seen from Fig. 8, there show no initial
steps in both the hydrogen generation curve and the temperature
curve. At the initial stage, hydrogen was generated very slowly.
But after several minutes of incubation time, there also showed
a burst in hydrogen generation, similar to that under the catal-
Fig. 5. Decrease of the final conversion rate with increasing of borohydride
concentrations (NaBH4: 1.0 g, CoCl2·6H2O: 0.2 g).

B.H. Liu et al. / Journal of Alloys and Compounds 468 (2009) 493–498
497
Table 1
Hydrogen generation characteristics via hydrolysis of sodium borohydride
Reactants
Catalyst (g)
Theoretical hydrogen
volumea (l)
Released hydrogen
volume (l)
Conversion rate (%)
Hydrogen generation
capacity (wt%)
NaBH4 (g)
H2Ob (g)
NaBH4 (wt%)
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.01
1.03
1.04
1.03
1.03
7.05
5.03
4.00
3.03
2.51
2.00
2.00
1.50
1.50
0.80
1.98
2.07
12.6
16.9
20.3
25.2
28.9
33.8
33.8
40.2
40.7
56.5
34.2
33.2
0.20c
0.20c
0.20c
0.20c
0.20c
0.10c
0.20c
0.10c
0.20c
0.20c
0.5d
2.32
2.32
2.32
2.32
2.32
2.32
2.32
2.30
2.34
2.37
2.34
2.34
2.32
2.32
2.30
2.14
2.10
2.13
2.12
1.58
1.90
0.99
2.21
2.17
100
100
2.57
3.42
4.09
4.72
5.31
6.30
6.27
5.62
6.70
4.80
6.56
6.25
99.1
92.2
90.5
91.8
91.4
68.7
81.2
41.8
94.4
92.7
1.0d
a
According to the hydrolysis reaction (1) and the 96% purity of NaBH4.
b
c
d
Hydrates in CoCl2·6H2O were not included.
CoCl2·6H2O.
Co powder.
ysis of CoCl2·6H2O. When the cobalt powder was decreased
to 0.5 g, hydrogen generation showed some interesting behav-
iors. Within the initial 40 min, hydrogen was liberated at almost
a constant rate though the system temperature was increased
gradually. But there was also a sudden rise in hydrogen release
and system temperature. Combined with the results shown in
Figs. 3 and 4, it was found that the sudden acceleration of the
reaction usually took place around 60 ◦C. It is thus more evident
that this kinetic behavior is highly related with the melting of
NaBO2·4H2O, probably due to alternations in water activity or
thermal property.
Compared with cobalt powder, the cobalt chloride solutions
showed much more effective catalytic effects due to the initial
acidic catalysis and the very fine cobalt boride formed.
From above results, it can be concluded that the following
equation shows the largest hydrogen generation capacity for the
hydrolysis of sodium borohydride, in which water and sodium
borohydride is in a mole ratio of 4:1 or a weight ratio of 2:1:
Fig. 8. Hydrogen generation characteristics under the catalysis of the cobalt
powder (NaBH4: 1.0 g, H2O: 2.0 g).
Considering the possibility of achieving hydrogen genera-
tion capacity as high as 7.3 wt% in above reaction and very fast
hydrogen generation kinetics, it is of interest to develop practical
hydrogen generation systems with such high hydrogen capacity.
NaBH4 + 4H2O = NaBO2·2H2O + 4H2
(4)
4. Conclusions
In this study, the hydrolysis reaction of sodium borohydride
wasstudiedwhensolidNaBH4 wasusedandwaterwasnotabun-
dantly supplied. It was found that the reaction could achieve very
quick kinetics and more than 90% conversion rates if the mole
ratio of water to borohydride was more than 4:1. Three factors
were found to account for this reaction characteristic: the first
was fast initial reaction kinetics under effective catalysis, then
it induced a large heat effect to raise the temperature to higher
than the melting point of NaBO2·4H2O, and finally the melting
of NaBO2·4H2O made the further reaction fast and deep. In this
study, cobalt chloride was found to be more effective than the
cobalt powder due to the initial acidic catalysis mechanism and
fine structure of produced Co2B. In account of the fast kinetics
Fig. 7. DSC analysis result of NaBO2·4H2O.

498
B.H. Liu et al. / Journal of Alloys and Compounds 468 (2009) 493–498
and high conversion rate, it is of interest to develop this reaction
regime into a practical hydrogen generation system.
[8] J.H. Kim, K.T. Kim, Y.M. Kang, H.S. Kim, M.S. Song, Y.J. Lee, P.S. Lee,
J.Y. Lee, J. Alloys Compd. 379 (2004) 222.
[9] Z.T. Xia, S.H. Chan, J. Power Sources 152 (2005) 46.
[10] B.S. Richardson, J.F. Birdwell, F.G. Pin, J.F. Jansen, R.F. Lind, J. Power
Sources 145 (2005) 21.
Acknowledgment
[11] B.H. Liu, Z.P. Li, S. Suda, J. Alloys Compd. 415 (2006) 288.
[12] Q. Zhang, G. Smith, Y. Wu, R. Mohring, Int. J. Hydrogen Energy 31 (2006)
961.
[13] A.M.F.R. Pinto, D.S. Falcao, R.A. Silva, C.M. Rangel, Int. J. Hydrogen
Energy 31 (2006) 1341.
This work is supported by Hi-tech Research and Devel-
opment Program of China (863), under the contract Nos.
2006AA05Z120 and 2007AA05Z144.
[14] R. Pena-Alonso, A. Sicurelli, E. Callone, G. Carturan, R. Raj, J. Power
Sources 165 (2007) 315.
References
[15] J.S. Zhang, W.N. Delgass, T.S. Fisher, J.P. Gore, J Power Sources 164
(2007) 772.
[16] J. Zhang, Y. Zheng, J.P. Gore, T.S. Fisher, J. Power Sources 165 (2007)
844.
[17] S.J. Kim, J.Y. Lee, K.Y. Kong, C.R. Jung, I.G. Min, S.Y. Lee, H.J. Kim,
S.W. Nam, T.H. Lim, J. Power Sources 170 (2007) 412.
[18] S.G. Chalk, J.F. Miller, J. Power Sources 159 (2006) 73.
[19] H.I. Schlesinger, H.C. Brown, A.E. Finholt, J.R. Gilbreath, H.R. Hoekstra,
E.K. Hyde, J. Am. Chem. Soc. 75 (1953) 215.
[1] S.C. Amendola, S.L. Sharp-Goldman, M.S. Janjua, M.T. Kelly, P.J. Petillo,
M. Binder, J. Power Sources 85 (2000) 186.
[2] S.C. Amendola, S.L. Sharp-Goldman, M.S. Janjua, N.C. Spencer, M.T.
Kelly, P.J. Petillo, M. Binder, Int. J. Hydrogen Energy 25 (2000) 969.
[3] S. Suda, Y.M. Sun, B.H. Liu, Y. Zhou, S. Morimitsu, K. Arai, N. Tsukamoto,
M. Uchida, Y. Candra, Z.P. Li, J. Appl. Phys. A 72 (2001) 209.
[4] Y. Kojima, K. Suzuki, K. Fukumoto, M. Sasaki, T. Yamamoto, Y. Kawai,
H. Hayashi, Int. J. Hydrogen Energy 27 (2002) 1029.
[5] H. Dong, H. Yang, X. Ai, C. Cha, Int. J. Hydrogen Energy 28 (2003) 1095.
[6] Y. Kojima, K. Suzuki, K. Fukumoto, Y. Kawai, M. Kimbara, H. NaKanishi,
S. Matsumoto, J. Power Sources 125 (2004) 22.
[7] Y. Kojima, Y. Kawai, H. Nakanishi, S. Matsumoto, J. Power Sources 135
(2004) 36.
[20] C.M. Kaufman, B. Sen, J. Chem. Soc. Dalton Trans. (1985) 307.
[21] A. Levy, J.B. Brown, C.J. Lyons, Ind. Eng. Chem. 52 (1960) 211.
[22] Encyclopedia of Inorganic Chemistry X-2 (in Japanese), Maruzen Co. Ltd.,
1965, p. 157.