Fe-B catalyst fabricated by hybrid capacitive adsorption-chemical reduction method and its application for hydrogen production from NaBH4 solution

Article abstract of DOI:10.1016/j.cattod.2013.05.024

Sodium borohydride (NaBH₄) in the presence of sodium hydroxide as a stabilizer was a hydrogen generation source with high hydrogen storage efficiency and stability. The catalyst has a key role in H₂ generation reaction from NaBH₄. We reported that a structured Fe-B catalyst prepared by capacitive adsorption of Fe ions followed by alkaline chemical reduction step. A porous carbon clothes could hold Fe ions with capacitive current and it was reduced by NaBH₄ hydrolysis. The loading amount and the density of Fe-B electrodes were strongly dependent of the experiment conditions of an applied potential at the first step and the reduction time at the second step. We observed that Fe-B showed a stable hydrogen production from NaBH₄ without fast degradation phenomena; even if the production rate was slower than that using Co-B or Ni-B. In addition, H₂ generation rate using Fe-B/C cloth electrode can achieve 813 mL min^-1 g^-1 catalyst at room temperature.

Full text of DOI:10.1016/j.cattod.2013.05.024

Catalysis Today 216 (2013) 240–245
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Fe–B catalyst fabricated by hybrid capacitive adsorption–chemical
reduction method and its application for hydrogen production from
NaBH4 solution
Trinh Ngoc Tuana, Youngmi Yia, Jae Kwang Leeb, Jaeyoung Leea,b,∗
a Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Technology, Gwangju 500-712, South Korea
b Ertl Center for Electrochemistry and Catalysis, RISE, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Sodium borohydride (NaBH4) in the presence of sodium hydroxide as a stabilizer was a hydrogen genera-
tion source with high hydrogen storage efficiency and stability. The catalyst has a key role in H2 generation
reaction from NaBH4. We reported that a structured Fe–B catalyst prepared by capacitive adsorption of
Fe ions followed by alkaline chemical reduction step. A porous carbon clothes could hold Fe ions with
capacitive current and it was reduced by NaBH4 hydrolysis. The loading amount and the density of Fe–B
electrodes were strongly dependent of the experiment conditions of an applied potential at the first step
and the reduction time at the second step. We observed that Fe–B showed a stable hydrogen production
from NaBH4 without fast degradation phenomena; even if the production rate was slower than that using
Co–B or Ni–B. In addition, H2 generation rate using Fe–B/C cloth electrode can achieve 813 mL min−1 g−1
catalyst at room temperature.
Received 4 March 2013
Received in revised form 20 May 2013
Accepted 22 May 2013
Available online 6 July 2013
Keywords:
Capacitive deionization
Hydrogen generation
NaBH4
Fe–B catalyst
© 2013 Elsevier B.V. All rights reserved.
Carbon cloth
1. Introduction
Among the chemical hydrides, sodium borohydride (NaBH4)
has been intensively studied as a H2 storage material because of
Along with increasing demand in using environmental friendly
and efficient energy sources for transportation and domestic elec-
tronic applications, fuel cells are attracting current global attention
due to their many large advantages. Among the various types of
fuel cell, the proton-exchange membrane fuel cells (PEMFC) have
been widely researched in recent years [1–4]. Hydrogen (H2) can
be mainly produced from chemical reactions [5], from the elec-
trolysis of water [6], or the reforming and thermal decomposition
of hydrocarbons [7]. H2 has been stored in tanks in the form of
compressed or liquefied H2, in H2-storing alloys, and on activated
carbon or nanoscale materials [8]. However, none of these meth-
ods are suitable for portable applications due to the low volumetric
and gravimetric efficiency as well as their associated safety issues.
Alternative storage in forms of liquid fuels (methanol, ethanol,
gasoline, etc.) and chemical hydrides (NaBH4, KBH4, LiH, NaH, etc.)
could be employed as H2 sources for portable PEMFC. In the case
of the liquid fuels, high-temperature reforming processes are too
complex to satisfy the requirements of portable PEMFC operation.
its advantages of: nonflammability and stability in air, easily con-
trolled H2 generation rate and side product recyclability, [8,9] and
its capability of storing 10.8 wt% of H2 [1]. The reaction product
(borax) obtained after removal of H2 from NaBH4, is environmen-
tally clean and can be recycled to generate the reactant [9]. H2 gas
can be generated by the hydrolysis of borohydride in the presence
of specific catalyst at room temperature [3,10]:
NaBH4 + 2H2O
NaBO2 + 4H2 ꢀH = −248.9 kJ mol−1
Catalyst
−−−−−→
The catalyst has a key role in H2 generation reaction from NaBH4.
The catalysts based on noble metals such as Ru and Pt have been
experimented for hydrolysis of NaBH4 [11–15], but they are expen-
sive, causing a limitation for their use. There are numerous papers
reported using non-noble metal salt for the fabrication of cata-
lyst to replace the use of noble metals. Eom et al. studied the
cobalt–phosphorous (Co–P) catalysts by electroless deposition. It
was found that Co–P catalysts with finer crystalline Co exhibited
a higher H2 generation rate of 3300 mL min−1 g−1catalyst in 1 wt%
NaOH and 10 wt% NaBH4 solutions at 30 ◦C [16]. The Co and Co–P
catalysts can be prepared by electroplating on Cu in sulphate-based
−
solution with or without an addition of H2PO2 ions [17].
∗
Corresponding author at: Electrochemical Reaction and Technology Laboratory
Lee studied the effect of a structured Co–B catalyst on H2 genera-
tion from an alkaline NaBH4 solution [8]. The catalyst was prepared
by chemical reduction of Co precursors coated on a Ni foam
(ERTL), School of Environmental Science and Technology, Gwangju 500-712, South
Korea. Tel.: +82 62 715 2440; fax: +82 62 715 2434.
E-mail address: jaeyoung@gist.ac.kr (J. Lee).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.05.024

T.N. Tuan et al. / Catalysis Today 216 (2013) 240–245
241
support. He found that the optimum H2 generation was obtained at
a Co/B molar ratio of 1.5/2.8, with a maximum activity at tempera-
ture of 250 ◦C. Kim et al. [10] reported the operating conditions in
a flow reactor for continuous generation of H2: optimum flow rate
of NaBH4 aqueous solution at 17.5 mL min−1; appropriate concen-
trations of NaBH4 and NaOH in the feed solution at 20 and 1 wt%,
respectively for the generation of H2. The system can produce more
than 6 L min−1 of H2.
Fernandes et al. reported the Co/(Co + Ni) molar ratio in Co–Ni–B
catalyst of 0.85 having much superior activity with the highest
H2 generation rate of 1175 mL min−1 g−1 catalyst [9]. Synergetic
effect of the Co and Ni atoms in Co–Ni–B catalyst is able to lower
the activation energy up to 34 kJ mol−1 as compared to 45 kJ mol−1
obtained with Co–B powder. Dong et al. studied the effect of heat
treatment of nickel boride catalyst (NixB) on the catalysis of NaBH4
hydrolysis reaction. It was found that the NixB catalyst showed
great improvements in catalytic activity and operational stability
after heat treatment at 150 ◦C in vacuum [18].
Using iron for fabrication of catalyst was studied by several
reports. C. Wu et al. firstly produced ferric catalyst from reac-
tions between pre-infused ferric salts and simultaneous feeding of
NaBH4 solution into a H2 generating reactor [19]. It was found that
H2 generation route using FeCl3 show excellent activity; result-
ing in a high H2 generation efficiency (over 94%) and an average
H2 generation rate of 1.08 L min−1. Besides, Fe also was added to
Co–B catalyst by chemical reduction of the corresponding its salts
[20,21]. Fe–Co–B catalyst showed significantly promoting ability of
H
2 generation rate of 22 mL min−1 g−1 in a 15 wt% NaBH4 and 5 wt%
NaOH solutions at 30 ◦C, and the apparent activation energy of the
hydrolysis reaction is determined to be 27 kJ mol−1. The advantages
of these catalyst preparation methods are cheap, simple, and better
catalyst activity but difficult to control the weight of catalyst and
reduction reaction rate because cannot correct chemical flow rate
during reaction process.
Fig. 1. Illustration of capacitive deionization process: (a) capacitive deionization
process and (b) reduction reaction process.
2.2. Catalyst characterization
In this study, we prepared a structure Fe–B catalyst on the car-
bon cloth electrodes by electrochemical adsorption techniques,
capacitive deionization (CDI). CDI system is very simple and can
operate continuously, thus it is easy to control the weight and
structure of catalyst loading on electrodes by changing flow rate
of reactants, applied voltage and time of reduction reaction. Fur-
thermore, CDI system is versatile by used for H2 production process
with applying aqueous solution of NaBH4 and NaOH following Fe–B
catalyst preparation step. With this new method, the effects of the
preparation conditions on the resulting catalysts’ activities and H2
production efficiency were discussed.
The morphology, physical properties, and phase structure of
the prepared catalysts were characterized by several analyti-
cal methods. The surface morphology and elemental analysis of
the prepared catalysts were examined by a scanning electron
microscope (SEM, JEOL JSM 5200) and energy dispersive X-ray
spectroscopy (EDX). The phase structures of the prepared catalyst
were characterized by X-ray diffraction (XRD, Rigaku D/MAXIIIA).
Surface composition and electronic state were analyzed using X-ray
photoelectron spectrum (XPS, VG Multilab 2000).
2.3. Hydrogen generation test
2. Experimental
Activities of the synthesized catalysts for H2 production were
examined in a continuous reactor which was initially used for
reduction reaction of fabrication of Fe–B catalyst. The catalyst plate
with the size of 9 cm2 (3 cm × 3 cm) was loaded into the reactor
and the H2 production test was started by introducing the aqueous
solution containing 20 wt% NaBH4 and 1 wt% NaOH into the reactor.
The temperature of solution reactor was measured by temperature
meter. The products of reaction which consists of H2 gas and water
vapor is then flowed into separating instrument which contains
silica gel where the water vapor is held and H2 gas flows out and
measured by the mass flow meter (Fig. 2).
2.1. Catalyst preparation
The carbon cloth 3 cm × 3 cm at two sides of electrodes (pos-
itive and negative) was prepared as an electrode. The structured
metal-B catalysts were fabricated by chemical reaction of iron salts
with borohydride solution. The FeCl3 (Sigma Aldrich) and NaBH4
(DAEJUNG Chemicals, South Korea) were used as a metal precur-
sor and a reducing agent, respectively. Metal precursor solution
flows into CDI system, where the voltage has been applied. Metal
ions are adsorbed on positive and negative electrodes as a result
of electric field created by the applied voltage. Then, CDI system
is washed with dilute water in condition of applied voltage and
the reducing agent, NaBH4 solution, flows into CDI system for
chemical reduction until iron ion is entirely converted into Fe–B
catalyst (Fig. 1). The metal-B catalyst coated on carbon material by
capacitive deionization was then dried at 60 ◦C in 6 h to remove
water.
3. Results and discussion
3.1. Effect of applied voltage on catalyst preparation
At this experiment, FeCl3 concentration is fixed at 10 wt%, with
a flow rate through the CDI system of 3 mL min−1 and a flow time

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T.N. Tuan et al. / Catalysis Today 216 (2013) 240–245
Fig. 2. Schematic diagram of H2 generation test.
Fig. 4. Effect of applied voltage on the H2 generation rate.
order to detail understand about adsorption mechanism of Fe3+
on electrodes (anode and cathode), we conduct reduction reaction
experiment with FeCl2 and FeCl3 solutions and explain clearly at
following parts of this study.
H2 generation rate shown in Fig. 4 increases with increasing of
the applied voltage from 0.5 to 1.1 V and decreasing at 1.2 V which
is almost similar to the trend observed in Fig. 3. The H2 generation
rate decreases in 10 min of reaction time since the part of catalyst
is detached from the electrode surface. H2 generation rate is stable
after about 10 min of the reaction process.
Fig. 3. Effect of applied voltage on the catalyst loading in the carbon cloth.
of 20 min. The effect was first studied by applying 0.5; 1.0; 1.1 and
1.2 V, respectively. The NaBH4 and NaOH concentrations flowing
into CDI system for reduction reaction are 1 and 0.1 wt%, with flow
rate of 2 mL min−1 and the reaction time of 12 min. The coated cata-
lyst is then dried at 60 ◦C for 6 h to remove humidity. The weight of
catalysts is shown in Fig. 3. We found that the weight of catalysts is
directly proportional to increasing of applied voltage on electrodes
from 0.5 to 1.1 V and decreases slightly at 1.2 V.
Ion adsorption characteristic on electrodes also is shown in
Fig. 3. Ions in feed solution are adsorbed on the surface of elec-
trodes when a voltage is applied to the cell. It was found out that
amount of catalyst at anode is larger than that at the cathode. In
3.2. Characterization of Fe–B catalyst
Fig. 5 shows the SEM of the precipitate at the surface of car-
bon cloth electrodes. The precipitates are formed via the reduction
reaction between Fe3+ ion adsorbed on electrodes and 1 wt% NaBH4
reduction solution. The voltage applying in this case is 1.1 V. The
catalysts with different morphology and size show various com-
ponents. Catalysts found both anode and cathode also prove the
adsorption ability of carbon cloth with ions and compounds during
reduction reaction. Particles fully covering on the support’s surface
indicate good interfacial contact between the support and catalyst
Fig. 5. SEM and EDX images of the catalyst loading on the carbon cloth: (a) anode and (b) cathode.

T.N. Tuan et al. / Catalysis Today 216 (2013) 240–245
243
equations of Fe3+ hydrolysis reactions in water condition can be
displayed as follow [22]:
Fe3+ + H2O → FeOH2+ + H+ K1
Fe3+ + 2H2O → Fe(OH)+ + 2H+ K2
2
Fe3+ + 3H2O → Fe(OH)3 + 3H+ K3
−
Fe3+ + 4H2O → Fe(OH)4 + 4H+ K4
2Fe3+ + 2H2O → Fe2(OH)24+ + 2H+ K5
3Fe3+ + 4H2O → Fe3(OH)45+ + 4H+ K6
Fig. 6. XRD images of the catalyst loading on the carbon cloth: (a) anode and (b)
cathode.
With K: equilibrium constant, pH = 1.24, [Fe]T = 0.36 M
From K1 − K6, pH and [Fe]T, we can calculate [ions]:
Fe3+ = 0.323 M
particle that is needed to sustain high catalytic activity, especially
during the vigorous initial hydrogen generation.
EDX analysis confirmed that the principal components of the
materials are boron, carbon, oxygen, and iron, as indicated in Fig. 5.
The weight ratio of Fe/B at anode and cathode are 3.4:73.9 and
1.4:78.6, respectively showing easy adsorption ability of Boron on
electrodes and the Fe3+ weight adsorbed on anode are higher than
cathode.
The XRD patterns of the precipitates formed through reduction
reaction process are shown in Fig. 6. The broad diffraction peaks
around 23.5◦ (0 0 2) and 43.8◦ (1 0 1) are assigned to the diffraction
of carbon cloth electrode [23]. The diffraction peak around 34.5◦
(0 0 1) is assigned to the diffraction of Fe–B, and peaks around 36.7◦
(3 0 1) and 61.2◦ (0 0 2) are assigned to the diffraction of FeOOH.
Fe(OH)2+ = 3.6 × 10−2 M
Fe(OH)2+ = 2 × 10−4 M
Fe(OH)3 = 2.4 × 10−6 M
Fe(OH)4− = 7.4 × 10−18 M
Fe2(OH)24+ = 3.5 × 10−2 M
Fe3(OH)45+ = 1.5 × 10−3 M
3.3. Differences between Fe3+ and Fe2+
Clearly, FeCl3 solution consists of main cations, very few anions,
so that they will be absorbed mainly on cathode when voltage is
applied. Thus, Fe3+ exists mainly positive charges in aqueous solu-
tion and be adsorbed on cathode during time of applied voltage.
Besides, when applying solution of NaBH4 and NaOH in reduction
reaction of Fe–B catalyst preparation, part of these positive charges
Adsorption of Fe2+ and Fe3+ on electrodes is investigated by
reduction reaction of FeCl2 and FeCl3 solutions with aqueous solu-
tion of NaBH4 and NaOH. Experiment condition of Fe2+ and Fe3+ is
the same with 10 wt% FeCl3, 10 wt% FeCl2, 1 wt% NaBH4 and 0.1 wt%
NaOH. Applied voltage is fixed of 1.1 V. As shown in Fig. 7, amount of
catalyst loaded on anode is larger than cathode with FeCl3 solution
and this is contrast to the case of FeCl2 solution. It can be explained
by processes Fe3+ is hydrolyzed in water and basic condition. The
immediately combines OH− to become negative charge Fe(OH)4
+
and be adsorbed continuously on anode to contribute increasing
catalyst loading on anode:
−
Fe3+ + OH− → Fe(OH)4 → FeO(OH)2− · H2O
Negative charge phenomenon of Fe3+ in basic condition is not
indicated with Fe2+ which is positive charge and to be adsorbed
on cathode. Fig. 7 also shows total amount of catalyst loading on
electrodes using FeCl2 solution is larger than using FeCl3 solution,
2.88 and 2.47 mg cm−2, respectively. Clearly, adsorption ability of
Fe2+ on electrode with applied voltage is better than Fe3+. However,
Fig. 8 shows that H2 generation rate using Fe2+–B catalyst is smaller
than Fe3+–B catalyst during experiment time. The less active ability
of Fe2+–B catalyst comparing to Fe3+–B catalyst is explained for this
result.
To determine the presence and oxidation state of iron in catalyst,
we conduct X-ray photoelectron spectroscopy (XPS) measurement
and the results are shown in Fig. 9. The O 1s spectra are composed
of four peaks, three peaks at more 530 eV show binding state Fe2O3
and Fe3O4 and another component at approximately 531.9 eV cor-
responds to FeOO*H [24]. The B 1s spectra (about 192 eV) show
3−
mainly binding state of B2O3 and BO6 [24]. The elemental Fe in
both catalysts (using FeCl2 and FeCl3 solution) with binding ener-
gies of approximately 724 and 710 eV indicated that Fe can mainly
exist at Fe2O3, Fe3O4, and FeOOH states during reduction reactions
of FeCl2 and FeCl3 solutions [24]. This also can be suggested that
Fig. 7. Comparison of catalyst loaded on electrode using FeCl3 and FeCl2 solutions.

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T.N. Tuan et al. / Catalysis Today 216 (2013) 240–245
dependant on the NaBH4 solution flow rate at the room tempera-
ture.
The H2 generation efficiency H is calculated via the following
equation [10]:
real H2 generation
theorectical H2 generation
V
H =
=
u × x/38 × 4 × 22.4
where V is the H2 generation rate (L min−1), u the solution flow
rate (mL min−1 ≈ g min−1) and x is the NaBH4 concentration. In this
equation, the values 38, 4 and 22.4 represent the NaBH4 molecular
weight, moles of H2 generation per a mole of NaBH4 and gas volume
per mole, respectively.
Fig. 10 shows the H2 generation rate and efficiency against
the flow rate. The H2 generation efficiency increases from NaBH4
flow rate of 1–2 mL min−1 and decrease at higher flow rates but
H2 generation rate increases linearly with increasing flow rates.
Considering both H2 generation rate and efficiency, 5 mL min−1 was
chosen as the solution flow rate for our system.
Fig. 8. H2 generation rate using FeCl3 and FeCl2 solutions.
4. Conclusions
In this report, we designed experiment to study the catalyst
adsorption mechanism and H2 generation process. The voltage
from power supply is determined of 1.1 V and at this voltage; the
H2 generation rate is the maximum. Adsorption of Fe3+ and Fe2+
on electrodes is entirely different. In water and slightly basic con-
dition and applied voltage, Fe3+ will react with H2O and OH− to
form negative charge FeO(OH)2−H2O and then adsorbed on anode.
This is not found with Fe2+ since Fe2+ is adsorbed on cathode. The
H2 generation test also shows the less active ability of Fe2+–B cat-
alyst comparing to Fe3+–B. In addition, H2 generation rate using
Fe–B/C cloth electrode can achieve 813 mL min−1 g−1 catalyst at
room temperature.
Fig. 9. X-ray photoelectron spectra of: (a) anode of Fe3+–B catalyst, (b) cathode of
Fe3+–B catalyst, (c) anode of Fe2+–B catalyst and (d) cathode of Fe2+–B catalyst.
Fe2+ and Fe3+ ions (of FeCl2 and FeCl3 solution) change to similarly
oxidized states after reduction process of catalyst fabrication.
Acknowledgments
This work was supported by the New & Renewable Energy
Development Program of the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) grant funded by the Korea Gov-
ernment Ministry of Knowledge Economy (No. 20113020030010).
3.4. Effect of flow rate of NaBH4 solution on H2 generation rate
The variation of H2 generation rate with the flow rate of NaBH4
solution is given in Fig. 10. The flow rate of the NaBH4 (20 wt%)
and NaOH (1 wt%) solution used was from 1 to 18 mL min−1 gave
corresponding H2 generate from 59 to 813 mL min−1 g−1 catalyst.
These data show that a faster flow rate gives rise to increased H2
generation. It is concluded that the H2 generation rate is strongly
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