Hydrogen Generation from sodium borohydride catalyzed by cobalt(II) acetate

Article abstract of DOI:10.14233/ajchem.2015.17556

Sodium borohydride as a hydrogen storage material for portable application has attracted much attention. A newly designed reactor with a toroidal tube distributor has been developed to generate hydrogen from sodium borohydride accelerated by cobalt(II) salts. The effects of the cobalt solution flow rate, the cobalt(II) acetate concentration, as well as the reaction temperature on hydrogen generation are discussed. The newly designed reactor successful reduced the hydrogen generation startup time and improved the water distribution. The results indicate that the startup time is decreased with an increase in (1) the concentration of the cobalt(II) acetate solution, (2) the reactor temperature and (3) the cobalt salt solution flow rate. The hydrogen yield increases with increase in cobalt(II) acetate concentration and reached a maximum at 1 wt %; further increases in the catalyst loading results in a decrease of the hydrogen yield. Furthermore, the hydrogen yields > 99 % is achieved using the novel reactor.

Full text of DOI:10.14233/ajchem.2015.17556

Asian Journal of Chemistry; Vol. 27, No. 4 (2015), 1279-1282
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.17556
Hydrogen Generation from Sodium Borohydride Catalyzed by Cobalt(II) Acetate
1,2
1,3
1
1
1
CHANGFENG YAN1,2,3,*, ZHAOJUN FANG , CHANGQING GUO , SHILIN HUANG , WEIMIN LUO and WENBO LI
1Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, P.R. China
2University of the Chinese Academy of Sciences, Beijing 100049, P.R. China
3Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, P.R. China
*Corresponding author: Tel/Fax: +86 20 87057729; E-mail: yancf@ms.giec.ac.cn
Received: 5 March 2014;
Accepted: 9 June 2014;
Published online: 4 February 2015;
AJC-16775
Sodium borohydride as a hydrogen storage material for portable application has attracted much attention. A newly designed reactor with
a toroidal tube distributor has been developed to generate hydrogen from sodium borohydride accelerated by cobalt(II) salts. The effects
of the cobalt solution flow rate, the cobalt(II) acetate concentration, as well as the reaction temperature on hydrogen generation are
discussed. The newly designed reactor successful reduced the hydrogen generation startup time and improved the water distribution. The
results indicate that the startup time is decreased with an increase in (1) the concentration of the cobalt(II) acetate solution, (2) the reactor
temperature and (3) the cobalt salt solution flow rate. The hydrogen yield increases with increase in cobalt(II) acetate concentration and
reached a maximum at 1 wt %; further increases in the catalyst loading results in a decrease of the hydrogen yield. Furthermore, the
hydrogen yields > 99 % is achieved using the novel reactor.
Keywords: Hydrogen generator, Hydrogen generation, Cobalt catalyst, Sodium borohydride.
(1) portability of device, (2) controllability of the hydrogen
flow rate, (3) heat released and (4) combined with fuel cells.
Murugesan and Subramanian5 studied flow through type
and diffusion through type hydrogen generators. The maximum
hydrogen yield of 97 % is reached when 3 N HCl was used as
catalyst. Gislon et al.6 designed, built and tested different
reactor layouts and geometries for hydrogen production from
NaBH4. The reactor is capable of producing hydrogen flow
rate in the 5-30 L/h range for several hours.Amendola et al.7,8
have proposed a safe, simple, compact process for generating
pure hydrogen gas on demand from base stabilized, aqueous
solutions of sodium borohydride by using a ruthenium catalyst.
Gervasio et al.9 designed and tested a new generator by cont-
rolling the providing rate of NaBH4 solution to the hydrogen
generation. Pozio et al.10 had designed a device to generate 50
mL/min of hydrogen at atmospheric pressure. They made the
solution of NaBH4 enter the reactor from the bottom. After
reacted over, the byproduct solution had been gone out from
the reactor and then had been recycled to the tank. Wang13
designed a generator using a recycle method. It consists of a
condensation chamber and an evaporation chamber. The
former was out of the generator, while the later was in the
generator. Heat was transferred out of the generator through
changing vapor-liquid phase of water. Combining hydrogen
generation system with fuel cells is one important issue to
INTRODUCTION
Hydrogen as an alternative to fossil fuels is one clean
fuel for proton exchange membrane fuel cell (PEMFC) and
some niche devices. The development of hydrogen energy can
increase the diversity of growing energy needs and construct
the low carbon economy society1. Hydrogen can be stored
using several methods including high pressure cryogenics and
chemical compounds. Sodium borohydride (NaBH4) as a
promising hydrogen storage material has an ideal gravimetric
hydrogen storage capacity of 10.7 wt %. Generating hydrogen
from NaBH4 solution has many advantage i.e., NaBH4 solutions
are non-flammable and rate of H2 generation is easily controlled
in low temperature. The maximum concentration of sodium
borohydride in water is 34.4 wt % at 25 ºC. 1 L solution of
30 % NaBH4 contains 67 g hydrogen2,3. If using sodium
borohydride in solid state, there will be no solubility limitation4.
And it may also decrease excess water utilization to increase
the hydrogen capacity of the hydrogen generation system.
Furthermore the generated hydrogen usually contains certain
of vapor that is benefit for proton exchange membrane fuel
cells (PEMFCs). So, it can be directly provided to PEMFCs
which were called "energy exchange machine".
Design of the reactor is critical to hydrogen generation
from NaBH4. Four points need to be considered including

1280 Yan et al.
Asian J. Chem.
investigate. Galli et al.11 mentioned that a portable energy
generator is able to produce on-demand hydrogen to be
supplied to a small solid polymer fuel cell in their work. The
compact hydrogen generator is designed around a tubular
catalytic reactor, whose catalytic powder is held in place by
magnetic field. The gas is generated by the exothermic hydro-
lysis reaction of an aqueous sodium borohydride solution in
contact with no-noble catalyst particles inside the reactor.
When hydrogen demand stops, the reactor is emptied and the
hydrolysis reaction ceases.
(a)
Hydrogen
outlet
Pressure gauge
Thermal couple
Water
inlet
However, the previous work6-13 had already made a great
of work on the improvement of the experimental apparatus
serving as the hydrogen production of sodium borohydride
hydrolysis. But some problems are still existing and need to
be solved to meet the requirements of reactor design mentioned
above. For example, most of the previous work utilized the
aqueous sodium borohydride as the reactant resulting in
decreasing the portability of device. Thus, this work shows a
reasonable solution to improve the portability of device by
utilizing the solid sodium borohydride powder reacting with
liquid water under catalysts producing hydrogen; meanwhile,
a toroidal tube distributor is brought to the reactor and used to
improve the water distribution and reduce the startup time of
hydrogen production.
Nitrogen
inlet
110
EXPERIMENTAL
H2 storage
tank
Gas
flowmeter
(b)
Sodium borohydride (NaBH4, reagent grade = 98 %, AR)
was used in solid forms as received. Cobalt chloride (CoCl2,
reagent grade = 98 %, AR) and cobalt acetate [Co(CH3COO)2,
reagent grade = 98 %, AR] were tested as catalysts for this
hydrolysis reaction. Pure water (18.25 MΩ cm@25 ºC) was
obtained from EPED's water purification system (GREEN-10T).
Fig. 1 shows the schematic representation of the hydrogen
generator used in this study. The hydrogen generator consists
of a hydrogen generation chamber, a buffer chamber and a
water distributor, which improves the water distribution. The
hydrogen generator is purged by 99.999 % N2 gas at flow rate
of 600 mL/min before the reaction. Hydrogen immediately
produces when catalyst solution contacts with NaBH4 powders.
Generated hydrogen flows into the buffer chamber and releases
from the reactor through the outlet. The gas is then filtered to
remove residual solution drops.
8
6
Nitrogen gas
3
Reactor
2
Shutoff
value
Flowmeter
Dryer
5
7
H2O
1
Flowmeter
display
H2
Syringe pump
Byproduct vessel
4
Fig. 1. Overview of hydrogen generator (a) and schematic representation
of the reactor test (b)
that nickel, cobalt and iron salts were quite effective. Table-1
shows the half-life information of the studied catalysts. Half-
life means the catalytic activity of catalysts. Shorter half-life
means faster hydrolysis rate and higher catalytic activity and
vice versa. The catalytic activity of cobalt chloride is two and
four times higher than nickel chloride and iron chloride. There-
fore, cobalt salt was selected as catalyst in present experiments.
Detection method: The flow rate of dry hydrogen (Q)
was measured by the gas flow meter. The hydrogen yield (%)
is measured by eqn. 1. Hydrogen flow rate can be controlled
by regulating the feed water flow rate. Typically a total weight
about 2 g powders of NaBH4 were placed in the hydrogen
generation chamber.
TABLE-1
INFLUENCE OF HALF-LIFE OF SODIUM
BOROHYDRIDE HYDROLYSIS BY USING
DIFFERENT METAL-BASED CATALYSTS
Hydrogen yield % = Q × t × (Theoretical hydrogen volume)-1 (1)
Metal
Cobalt
Nickel
Iron
Osmium
Iridium
Catalytic compound
T1/2 (min)
here, t is reaction time to reach the maximum volume.
CoCl2
NiCl2
FeCl2
OsO4
IrCl4
9
18
38
19
28
RESULTS AND DISCUSSION
Selection of metal catalyst: Hydrolysis of NaBH4 can
be catalyzed by a large number of salts, such as metal chlorides
or metal acetates. Metal salts have different influences on
half-life of NaBH4 and the hydrogen generation characters.
Schlesinger and Finbolt12 investigated the effects of several
chloride salts on half-life of NaBH4 hydrolysis. They found
Fig. 2 shows the hydrogen generation from NaBH4 using
cobalt chloride and cobalt acetate as catalyst. Two cobalt
catalysts both reached same hydrogen yield of 99 % at the

Vol. 27, No. 4 (2015)
Hydrogen Generation from Sodium Borohydride Catalyzed by Cobalt(II) Acetate 1281
end of reaction. The results indicated that the anion ions (Cl–
and CH3COO–) have no significantly effect on the catalytic
activity for hydrogen production from NaBH4. Similar results
have also been reported by Schlesinger and Finbolt12. Cobalt(II)
chloride as a simple, efficient catalytic which generated almost
100 % of stored hydrogen with an HGR of 86.3 L min-1 g-1
(Co) was reported by Demirci et al.14. Therefore, cobalt acetate
was used as catalyst in the flowing experiment.
5000
(a)
4500
4000
3500
3000
2500
2000
1500
1000
500
0.5 wt %
1.0 wt %
2.0 wt %
5.0 wt %
10.0 wt %
110
100
90
80
0
0
10
20
30
40
50
60
70
80
90
70
Time (min)
60
200
180
160
140
120
100
80
100
(b)
50
40
CoCl2
Co(CH3COO)2
90
80
70
60
50
30
20
10
0
0
10
20
30
40
50
60
Time (min)
60
Fig. 2. Hydrogen generation characteristics of CoCl2 and Co(CHCOO)2
solutions. The experiments were performed at a cobalt solution flow
rate of 0.27 mL/min with 2.0 g of sodium borohydride. T reaction
temperature was kept at 26 oC
40
20
0
2
4
6
8
10
Concentration (wt %)
Effect of catalyst concentration on hydrogen genera-
tion: Fig. 3a shows the effect of cobalt acetate concentration
on the hydrogen generation. The hydrogen generation volume
increases with increase in cobalt(II) acetate concentration and
reached a maximum at 1 wt %; further increases in the catalyst
loading results in a decrease of the hydrogen volume. Fig. 3b
shows the hydrogen yield from hydrolysis reaction of NaBH4.
The hydrogen yield reached 99.3 % using 1 wt % of cobalt
acetate solution. For low catalyst concentration (0.5 wt %),
the number of available catalytic sites is low and was able to
produce small amount of hydrogen. In case of excess of catalyst
(> 1 wt %), significant amount of hydrogen were generated
which prohibits the contact between catalysts and reactants.
Therefore, an optimal value with proper reaction rate was found
to be 1 wt % in this study. The reaction temperature increased
slowly, however, the value of maximum temperature nearly
kept same for all conditions (Fig. 4).
Fig. 3. (a) Hydrogen generation volume at different concentration of
catalytic solution; (b) Yield and hydrogen flow rate at different
concentration of catalytic solution (2.0 g NaBH4, solution flow rate
of 0.3 mL min-1 at 26 oC)
40
35
30
25
0.5 wt %
1.0 wt %
2.0 wt %
20
5.0 wt %
10.0 wt %
15
Effect of solution flow rate on hydrogen generation:
Fig. 5a,b illustrates the hydrogen generated volume at different
solution flow rates. It was found that the hydrogen generated
volume increased with increasing the solution flow rates. How-
ever, the final hydrogen yields of > 98 % were achieved for all
flow rates. It is suggested that we can increase the solution
flow rate to decrease the startup time of generator. Fig. 5c
presents the reaction temperature as a function of solution flow
rate ranging from 0.1-0.5 mL/min. As the solution flow rate
increased, the reaction temperature raised sharply due to the
severe hydrolysis reaction. The maximum temperature also
increased as a result of high flow rate.
10
0
10
20
30
40
50
60
70
80
90
Time (min)
Fig. 4. Reaction temperature at different concentration of catalytic solution
as a function of time (2.0 g NaBH4, solution flow rate of 0.3 mL
min-1, initial T = 26 oC)
Effect of reaction temperature on hydrogen generation:
Fig. 6a. b shows the effect of initial temperature on hydrogen
generated volume and yield. The initial temperatures strongly
affected the reaction rate of the NaBH4 hydrolysis. However,
similar total hydrogen volume and final yield were reached
for all initial temperatures.

1282 Yan et al.
Asian J. Chem.
5000
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
(a)
(a)
4500
4000
3500
3000
2500
2000
1500
1000
500
t = 16 °C
t = 21 °C
t = 26 °C
t = 36 °C
t = 46 °C
0.1 mL/min
0.2 mL/min
0.3 mL/min
0.4 mL/min
0.5 mL/min
0
0
10 20 30 40 50 60 70 80 90 100 110 120
0
0
10
20
30
40
50
60
70
80
90
110
Time (min)
Time (min)
200
180
160
140
120
100
80
110
140
120
100
80
(b)
105
(b)
100
95
90
85
80
75
70
65
60
100
90
80
60
60
70
40
40
10
60
0.1
0.2
0.3
0.4
0.5
20
30
Temperature (°C)
40
50
Solution flow rate (mL min–1)
40
Fig. 6. (a) Hydrogen generation volume at different initial temperatures;
(b) Yield and H2 flow rate (2.0 g NaBH4, 1 wt % solution flow rate
of 0.3 mL/min, at 16-46 oC)
0.1 mL/min
0.2 mL/min
0.3 mL/min
0.4 mL/min
0.5 mL/min
(c)
38
36
34
32
30
28
26
24
ACKNOWLEDGEMENTS
The authors gratefully acknowledged the financial support
of the National Natural Science Foundation of China (No.
21276254); Guangzhou Science and Technology Project (No.
2013J4500027, No. 2012J4300142).
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Conclusion
Cobalt acetate and cobalt chloride were used as catalysts
in this study. Both catalysts showed high catalytic activity for
the hydrogen generation from hydrolysis of NaBH4. The newly
designed reactor with a toroidal tube distributor successful
reduced the hydrogen generation startup time. The hydrogen
yield is nearly 100 % at selected working conditions and the
optimum catalyst concentration is 1 wt %. Our hydrogen gene-
ration system is promising and can be used to supply hydrogen
for fuel cells.