Hydrogen Generation by Hydrolysis of NaBH4 with Efficient Co–La–Mo–B Catalyst for PEM Fuel Cells

Article abstract of DOI:10.1134/S0023158420040047

Abstract: In this study, Co–La–Mo–B catalyst used for hydrogen production by hydrolysis of sodium borohydride (NaBH₄) solution was synthesized by chemical reduction method. Characterization of the synthesized catalyst was carried out with EDX a

Full text of DOI:10.1134/S0023158420040047

ISSN 0023-1584, Kinetics and Catalysis, 2020, Vol. 61, No. 4, pp. 589–594. © Pleiades Publishing, Ltd., 2020.
Hydrogen Generation by Hydrolysis of NaBH with Efficient
4
Co–La–Mo–B Catalyst for PEM Fuel Cells
A. Ekıncı*
Healty of School, Siirt University, Siirt, Turkey
*
e-mail: aekinci@siirt.edu.tr
Received September 12, 2019; revised November 5, 2019; accepted January 15, 2020
Abstract—In this study, Co–La–Mo–B catalyst used for hydrogen production by hydrolysis of sodium boro-
hydride (NaBH ) solution was synthesized by chemical reduction method. Characterization of the synthe-
4
sized catalyst was carried out with EDX and XRD. In hydrolysis experiments, the effect of parameters such
as molybdenum, NaOH and NaBH concentration, catalyst amount and temperature were investigated. The
4
best Mo concentration was found to be 5%. In the presence of the Co–La–Mo–B catalyst in NaBH hydro-
4
–1
–1
lysis, hydrogen initial production rate was found to be 9508 mL g min . The activation energy of NaBH
4
hydrolyses was determined as 39.5 kJ/mol. The effect of hydrogen gas obtained using the Co–La–Mo–B
catalyst in NaBH hydrolysis on fuel cell efficiency was determined by measuring I–V values. Average effi-
4
ciency values according to power and ideal voltage were found as 60 and 80%, respectively. From the results,
it can be said that the Co–La–Mo–B catalyst is an ideal catalyst for PEM fuel cell applications.
Keywords: hydrogen, NaBH , hydrolysis, Co–La–Mo–B catalyst, PEM
4
DOI: 10.1134/S0023158420040047
INTRODUCTION
Co–Cr–B [14], Co–Fe–B [15], Co–Mo–B [16],
Ni–Mo–B [17], Co–La–B [18], La–Ni–Mo–B
Hydrogen has been identified as one of the best
alternative energy carriers to meet the growing
demand for efficient and clean energy supply in the
near future [1]. The use of industrial devices based on
[
19], Co–La–Zr–B [20] are used in the hydrolysis of
NaBH .
4
In this study, the effect of Co/Mo molar ratio on
hydrogen fuel cell technology is preferred. The reason catalytic activity against NaBH hydrolysis were inves-
4
for this is that hydrogen does not require efficient stor-
age and production environment with appropriate
safety measures [2]. Hydrogen is mainly stored in
metal/complex hydrides, chemically absorbed hydro-
gen, carbon materials, inorganic nanostructures and
metal-organic structures [3–9]. Among these, sodium
tigated. An optimal catalyst was used to evaluate the
effect of NaOH and NaBH concentrations on hydro-
4
gen generating performance. Based on the activity at
different temperatures, the activation energy of this
catalytic reaction was calculated by the Arrhenius
equation.
borohydride (NaBH ) has attracted much research
4
interest as a hydrogen storage material since the late
1
990s due to its stability in alkali solutions, non-
EXPERIMENTAL
Catalyst Synthesis
flammability, non-toxic, environmental safety and
high theoretical hydrogen content [10, 11]. NaBH
4
hydrolysis is carried out with the following equation
of reaction:
Co–La–Mo–B catalysts, which were not used
previously in the hydrolysis of NaBH , were synthe-
4
sized by chemical precipitation and reduction method.
(I) Detailed production of the catalysts is given below.
NaBH + 2H O → 4H + NaBO .
4
2
2
2
However, NaOH is added to the NaBH solution to
Cobalt chloride hexahydrate, lanthanum nitrate
4
control this hydrolysis reaction, i.e. to prevent its own and molybdenum nitrate were added to 100 mL of
hydrolysis reaction, and the catalyst is used to acceler- pure water with a Co/La/Mo molar ratio of 7 : 3 : 5.
ate it appropriately. Thus, the catalyst is a key factor The resulting solution was left in an ice bath to ensure
influencing the rate of H production by hydrolysis of that the temperature was in the range of 0–5°C and
2
then added dropwise to a NaBH solution (the molar
NaBH . Catalysts such as Co–B [12], Co–W–B [13],
4
4
ratio of NaBH to total metal salts is 5 in 50 mL of pure
4
Abbreviations: PEM, proton exchange membrane.
water). A black precipitate was obtained. The solution
589

5
90
EKINCI
(
a)
tions was used for application purposes. Hydrogen
8
7
6
5
4
3
2
00
00
00
00
00
00
00
produced from NaBH hydrolysis in the presence of a
4
Co–La–Mo–B catalyst was used in PEM fuel cell
application. The measurements were carried out at
4
0°C with 10 mL of the solution, 25 mg of the catalyst
and a resistance of 10 Ohm.
RESULTS AND DISCUSSION
NaBH Hydrolysis
5% Mo
7% Mo
10% Mo
4
1
3
% Mo
% Mo
100
Effect of molybdenum (Mo) molar fraction. The
experiments in which Mo-metal effect on the Co–
La–B catalysts were investigated were carried out at
0
20
40
60
80
100
4
0°C with 10 mL of solution, 2.5% NaBH concentra-
Time, min
4
tion and 25 mg catalyst amount. The results obtained
(b)
are given in Figs. 1a and 1b.
6
5
4
3
2
500
500
500
500
500
The effect of Mo concentration on catalytic activity
was studied by varying the Mo molar fraction (x% Mo)
in the Co–La–B catalyst from about 1 to 10%. As seen
from Fig. 1a, when Mo concentration increases from 1
to 5%, the initial rate of hydrogen generation increases
and then decreases at higher Mo concentrations. The
maximum H production rate (R ) as a function of
2
max
1
500
00
500
1500
x% Mo is shown in Fig. 1b. The probable cause of this
situation is that the higher Mo content in the Co–La–
Mo–B catalyst may cause decomposition of the
amorphous Co–La–Mo–B followed by its crystalli-
zation as metal and oxide. Another possible cause can
be that the appropriate amount of metal and/or oxide
in the amorphous Co–La–Mo–B catalyst helps to
5
–
–
0
2
4
6
8
10
Mo content, %
increase the activity for NaBH hydrolysis, but too
4
Fig. 1. (a) Hydrogen volume generated as a function of
high metal and/or oxide content may have detrimental
effects [21]. The best x% Mo concentration was found
to be 5%, and studies were continued with this compo-
sition.
reaction time and Mo content upon hydrolysis of NaBH .
4
(
b) H generation rate as a function of % Mo.
2
was washed several times with distilled water and eth-
Effect of NaOH concentration. NaBH is sponta-
4
anol to remove impurities from this precipitate. neously hydrolyzed. At high pH values, NaBH is sta-
4
Finally, the product obtained was dried under nitro- ble. For this reason, it is important to examine the
gen gas at 100°C for 8 h and the catalyst was stored in effect of NaOH concentration on the NaBH hydroly-
4
a closed container for use in the hydrolysis of NaBH .
4
sis. Experiments were performed at 40°C with 10 mL
of solution, 2.5% NaBH concentration and 25 mg
4
catalyst amount. The results obtained are presented in
Figs. 2a and 2b. When the NaOH concentration
Hydrolysis of NaBH4
NaBH hydrolysis experiments were performed in increases from zero to 2.5%, the initial rate of hydro-
4
1
2
0 mL of solution containing 25 mg of the catalyst and gen production first increases and then decreases.
–
.5% of NaBH . The volume of hydrogen obtained in This is probably due to the fact that the excess OH
4
the hydrolysis of NaBH was determined by the cumu- present in the solution has a steric effect on the hydro-
4
lysis of NaBH . However, excessive NaOH concentra-
lative method. The effect of parameters such as Mo
4
metal ratio, NaOH concentration, NaBH concentra- tion results in a reduced solubility of NaBO (a
2
4
tion, catalyst amount and temperature was investi- byproduct of the hydrolysis reaction). The by-product
gated. In addition, the degradation kinetics of NaBH
thus formed may cling to the surface of the catalyst,
thereby preventing the hydrolysis reaction. The effect
of other parameters was investigated in a 2.5% NaOH
medium.
4
was determined.
PEM Fuel Cell Application
Effect of NaBH concentration. Experiments in
4
The single-cell PEM fuel cell system designed which the NaBH effect was examined were carried
4
using a Pt/C catalyst in the anode and cathode por- out at 40°C with 10 mL of solution, 2.5% NaOH and
KINETICS AND CATALYSIS
Vol. 61
No. 4
2020

HYDROGEN GENERATION BY HYDROLYSIS OF NaBH4
a)
591
(
(a)
8
7
6
5
4
3
2
00
00
00
00
00
00
00
2000
800
600
400
200
1000
1
1
1
1
8
6
400
00
00
0
2
5
% NaOH
.5% NaOH
.0% NaOH
1.0% NaBH4
2.5% NaBH4
5.0% NaBH4
100
7.5% NaOH
200
7.5% NaBH4
0
1
2
3
4
5
6
0
5
10
15
20
Time, min
Time, min
(b)
(b)
10000
10000
9
9
8
8
500
000
500
000
9000
8000
7000
6000
5000
7500
7000
6500
6000
–1
0
1
2
3
4
5
6
7
8
0
1
2
3
4
4
5
6
7
8
NaOH concentration, wt %
NaBH concentration, wt %
Fig. 2. (a) Hydrogen volume generated as a function of
Fig. 3. (a) Hydrogen volume generated a function of reac-
tion time and NaBH concentration upon hydrolysis of
reaction time and NaOH concentration upon hydrolysis of
4
NaBH . (b) H generation rate as a function of NaOH
NaBH . (b) H generation rate obtained as a function of
4 2
4
2
concentration.
NaBH concentration.
4
2
5 mg of catalyst. Figure 3a shows the volume of duction did not depend on NaBH concentration.
4
hydrogen produced as a function of time. In Fig. 3b, Jeong et al. [25] and Pinto et al. [26] reported that the
variation of the initial hydrogen generation rate at hydrogen production rate decreased with increasing
these NaBH concentrations is also given. As can be NaBH concentration. However, Pena Alonso et al.
4
4
seen in Fig. 3a, when NaBH concentration increased [27] reported that hydrogen production increased
4
more rapidly with increasing NaBH concentration in
from 1 to 2.5%, hydrogen production rate also
4
increased. Amendola et al. [22] reported that adequate the presence of carbon nanotube supported Pt and Pd
−
4
catalyst. Patel et al. [24] showed that the hydrogen
BH ions in the environment were in contact with the
production rate increased when NaBH concentration
4
active zone on the catalyst surface to produce hydro-
was low, but the hydrogen production rate remained
gen when the concentration of NaBH for the active
4
constant at high NaBH concentration.
4
catalysts increased. Moreover, it was observed that,
when the concentration of NaBH is 7.5%, the hydro-
4
Effect of catalyst amount. The effects of catalyst
amount were investigated at 40°C, with 10 mL of solu-
gen production rate-decreased. The possible cause of
this situation is that the NaBO concentration exceeds
2
tion, 2.5% NaOH and 2.5% NaBH concentration
4
the solubility limit. Thus, NaBO may block the active
2
(Figs. 4a and 4b). As seen from these figures, both
area on the catalyst surface by precipitating [22–24]. hydrogen yield from sodium borohydride and initial
Dependence of hydrogen production rate on alkali production rate increase with increasing catalyst
NaBH solution concentration has been reported in amounts. This result shows that hydrogen production
4
different ways in the literature. For example, Amen- from NaBH in the presence of Co–La–Mo–B cata-
4
dola et al. [2] showed that the rate of Ru catalyzed pro- lyst is catalyst controlled.
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

5
92
EKINCI
(a)
800
800
700
600
500
400
300
200
700
600
500
400
20°C
30°C
40°C
50°C
300
200
100
1
0 mg
25 mg
0 mg
60°C
5
100
70 mg
0
5
10
15
20
25
Time, min
0
2
4
6
8
10 12 14 16
Time, min
Fig. 5. Hydrogen volume generated as a function of reac-
tion time upon hydrolysis of NaBH solution at five differ-
4
(b)
ent solution temperatures.
10000
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
9000
8000
7000
6000
5000
30°C
40°C
50°C
60°C
70°C
0.2
0
.1
10
20 30 40 50 60 70
Amount of catalyst, mg
0
5
10
15
20
Time, min
Fig. 4. (a) Hydrogen volume generated vs. time as a func-
tion of different catalyst amount upon hydrolysis of
NaBH . (b) Hydrogen production rate vs. different cata-
Fig. 6. Graph of 1/C
tures.
vs. time for different tempera-
4
NaBH4
lyst amounts.
Effect of temperature. The experiments were car-
ried out under the conditions of 10 mL of solution,
.5% NaOH, 2.5% NaBH and 25 mg catalyst amount
1
CA
1
CA0
=
(
n −1
)
kt +
.
(2)
n−1
n−1
2
4
(
Fig. 5). The rate of hydrolysis increases with increas-
The graph of 1/C vs. time for different tempera-
tures is shown in Fig. 6. The reaction rate constants (k)
for different temperatures are found from the slope of
the straight lines. As shown in Fig. 6, the n value
selected at all temperatures is consistent with 0.3 and
all plots are linear. The catalytic reaction rate increases
exponentially with increasing reaction temperature
ing temperature. For example, hydrolysis of 2.5%
NaBH is completed at 20°C within 25 min and at
4
6
0°C within 2.5 min. The probable cause of this situa-
−
tion is the increase in BH ion decomposition rates
4
due to the increasing temperature. One of the main
reasons for any reaction to take place at different tem-
peratures is to determine the reaction rate constant
and the activation energy. Therefore, firstly, a reaction
and a maximum value of 9508 mL min–
1
g
–1
is
at the nth degree was used to determine the rate con- obtained at 333 K. The rate of catalytic hydrolysis of
stants at different temperatures. The reaction rate con- the alkali NaBH solution exponentially increases
4
stant for this reaction is determined by the following
equation
with the increase in reaction temperature and a maxi-
mum value is obtained at 60°C. The temperature
dependence of the rate of hydrogen production is usu-
ally expressed by the Arrhenius equation:


1
1
1
CA0

−
 = kt.
(1)
n−1
n−1



n −1 C

A
Ea
RT
lnk = lnk −
,
(3)
0
Equation (1) can be transformed to Eq. (2)
KINETICS AND CATALYSIS
Vol. 61
No. 4
2020

HYDROGEN GENERATION BY HYDROLYSIS OF NaBH4
593
Equatio y = a
Adj. R- 0.993
1.5
.3
1.1
1.6
1.2
0.8
0.4
0
–
0.5
1
Value Standar
B
B
Interc 13.975 0.60847
Slope –4858 189.868
–
1.0
1.5
0.9
0.7
0.5
0.3
–
–
2.0
2.5
0.1
–
0
0.5
1.0
1.5
2.0
2.5
3.0
Current, A
0
.0030 0.0031 0.0032 0.0033 0.0034
–1
1/T, K
Fig. 7. Graph of ln(k) vs. 1/T.
Fig. 8. Voltage–current and power–current curves.
(a)
5
48 K
(100)
(
101)
25
20
2
0
25
30
30
35
35
40
45
45
50
B
La
Pd Co
Mo Ni
Au
Mo Pd
1
5
La
Co Ni
Au
Mo
(b)
300 K
10
5
0
20
25
40
50
0
2
4
6
8
10 12 14 16 18
2θ, deg
Energy, keV
Fig. 9. XRD patterns of the Co–La–Mo–B catalyst at dif-
ferent temperatures, K: 548 (a), 300 (b).
Fig. 10. EDX spectrum of the Co–La–Mo–B catalyst.
–
1
–1
where k is the reaction rate (mL g min ), k is the cell performance values for the catalyst are also indi-
0
–1
–1
reaction constant (mL g min ), E is the activation cated in Fig. 8. The efficiency value according to the
a
–1
–1
energy (J/mol), R is the gas constant (8.314 J mol K ) power is 60%, while the efficiency value according to
and T is the absolute temperature (K). The activation the ideal voltage is 80% [28, 29].
energy from the slope of the straight line obtained
Catalyst characterization. Figure 9 indicates XRD
patterns of the Co–La–Mo–B catalyst. According to
XRD diffraction patterns taken at 548 K, the (100) and
from the graph of ln(k) vs. 1/T shown in Fig. 7 was cal-
culated as 39.5 kJ/mol. Compared to the activation
energies calculated in the previous studies, Co–B
(
101) planes of the hexagonal close packed (HCP) Co
(
68 kJ/mol) [12], Co–La–B (42.4 kJ/mol) [18], Co–
phase were detected, and the sharp peaks explained
that Co electrodeposit has a crystalline structure.
However, an XRD diffraction pattern taken at 300 K
demonstrates that the Co–La–Mo–B catalyst is
amorphous. XRD analysis revealed that the tempera-
ture of XRD plays an important role to change struc-
ture of catalyst from polycrystalline to amorphous.
La–Zr–B (60.06 kJ/mol) [20], it can be said that the
catalyst we synthesized demonstrates higher activity.
PEM fuel cell application of catalyst. Hydrogen
produced from NaBH hydrolysis in the presence of
4
Co–La–Mo–B catalyst was used in PEM fuel cell
application. The values used for the measurement are
temperature—40°C, solution volume—10 mL, catalyst
amount—25 mg, resistance—10 Ohm. Graphical
The chemical composition of the Co–La–Mo–B
analysis of the measurements were performed. The catalyst was determined by EDX. Figure 10 indicates
current–voltage–power curves showing the PEM fuel that Co, La, Mo and B elements can be detected.
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

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EKINCI
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1
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concentrations, NaOH concentrations, catalyst
amounts and reaction temperatures. The volume of
hydrogen produced was found to increase gradually
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4
14. Baytar, O., Acta Chim. Slov., 2018, vol. 65, no. 2, p. 407.
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1
5. Patel, N., Fernandes, R., and Miotello, A., J. Catal.,
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1
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2020