Hydrogen generation from highly activated Al-Ce composite materials in pure water

Article abstract of DOI:10.1111/j.1551-2916.2011.04840.x

The reaction of aluminum with pure water is an eco-friendly approach to generate hydrogen. The main difficulty associated with this approach is that an oxide or hydroxide protective film around aluminum particles prohibit the hydrogen generation. In this

Full text of DOI:10.1111/j.1551-2916.2011.04840.x

J. Am. Ceram. Soc., 94 [11] 3976–3982 (2011)
DOI: 10.1111/j.1551-2916.2011.04840.x
© 2011 The American Ceramic Society
ournal
J
Hydrogen Generation from Highly Activated Al–Ce Composite
Materials in Pure Water
Hui Luo, Jie Liu, Xuxin Pu, Jie Liang, Zhengjun Wang, Feijiu Wang, Kun Zhang,
Yingjie Peng, Bo Xu, Jihong Li, and Xibin Yu†
Department of Chemistry, Shanghai Normal University, Shanghai, 200234, China
The reaction of aluminum with pure water is an eco-friendly
approach to generate hydrogen. The main difficulty associated
with this approach is that an oxide or hydroxide protective film
around aluminum particles prohibit the hydrogen generation. In
this article, a kind of highly activated Al–Ce composite material
has been developed via a convenient ball milling method. Typi-
cally, Al–13 wt% Ce has a total hydrogen production of
1134 mL/g and a conversion yield of 92.42% within 60 min.
The influences of the cerium concentration and ball milling time
on the hydrogen production efficiency are discussed based on
XPS, XRD, FESEM, and BET techniques. In addition, the
hydrogen production efficiency can be improved by adding alkali
chloride (NaCl or KCl) to the Al–Ce composite materials.
removed the inert Al2O3 layer.5–9 Considering that the use of
acid and alkali caused environmental pollution, Deng et al.
reported that metal Al particle surfaces were modified by fine
c-Al2O3 grains.2,10–15 The dense Al2O3 layer was replaced by
the loose and porous c-Al2O3 layer, so the inner Al had a
chance to react with water. The composition Al-70 wt% c-
Al2O3 showed the conversion yield of 88% within 80 h.
Alinejad et al. reported that aluminum was mechanically
milled with fragile salt particles which worked as smaller
millers.16 The salt crystals act to reduce the aluminum parti-
cle sizes, damage the dense oxide layer and to form salt gates
during the milling process. The milled powder can completely
react with pure water at 70°C and quickly release hydrogen
within 40 min. Fan et al. reported that the hydrogen produc-
tion is 970 mL/g in 1 M NaCl solution from Al–16 wt% Bi
alloy after ball milling for 10 h.17–19 The enhanced hydrogen
generation efficiency is explained from the point of micro-
galvanic cell between the anode (Al) and cathode (Bi).
Kravchenko et al. even prepared Al alloy through melting
aluminum with a low melting point metal, such as Ga, Bi,
Pb, Sn, In and so on.20 Typically, Al–5.3 wt% Ga–2.0 wt%
In–5.4 wt% Sn–7.3 wt% Zn alloy produced hydrogen up to
1000 mL/g in 30 min in pure water. Although metal bismuth,
gallium, indium, etc. can activate aluminum, these metals are
expensive, toxic, and corrosive. It is practically necessary to
find a new kind of environment-friendly metal which can
improve the efficiency of hydrogen generation from alumi-
num. Metal cerium is a good candidate. It is abundant in the
earth and has stronger reducibility than Al, so it may dam-
age the dense Al2O3 layer more effectively through redox
reaction. In addition, the toxicity of neutral CeOx and Ce
(OH)x is comparable with NaCl, indicating that they are
much safer to human beings than basic oxides and hydrox-
ides of alkaline metals and alkaline-earth metals.21
I. Introduction
INDING new highly efficient and clean energy sources has
become important to researchers due to the increasing
F
energy demand and environmental pollution. Fuel cell is one
of the most promising candidates for replacing fossil-fuel
power generation. It directly converts chemical energy into
electrical energy effectively and avoids environment-pollution
problems.1 Because it has high energy density and only pro-
duces environmentally benign water after reacting with air,
hydrogen is an ideal fuel for fuel cells. In practical applica-
tions, however, hydrogen storage and distribution is a major
hurdle for its widespread use due to its relatively low volu-
metric energy density.2 Therefore, it is still a great challenge
to develop highly efficient direct hydrogen generation materi-
als which can be used in fuel cells and other capable electri-
cal devices.3,4
Metal aluminum has been considered as an ideal hydrogen
generation material because of its light atomic mass, high
electron density, and abundance in the earth’s crust. Theoret-
ically, at 25°C and 1 atm, the hydrolysis reaction of 1 g Al
can produce 1359 mL of H2. The generated aluminum oxide
hydroxide and/or aluminum hydroxide are eco-friendly and
easily recyclable. In addition, aluminum utilization is a user-
friendly and safe approach for both hydrogen production
and energy storage. However, the reaction of aluminum with
pure water is difficult due to a dense oxide film that covers
the surface of aluminum.5 The oxide film is unstable in
strong acid or basic solutions. Therefore, it was reported that
hydrogen generation took place in basic solutions, which
In this work, environment-friendly and highly activated
Al–Ce composite materials have been prepared by ball mill-
ing method. The effects of Ce content and ball milling time
on the activity of Al–Ce composites are illustrated in detail.
It was thought that the strong reducibility of Ce and unique
porous structure of Al–Ce improved hydrogen generation
efficiency. In addition, the influence of additives of alkali
chlorides on hydrogen production and conversion yield is
also discussed.
II. Experimental Procedure
(1) Preparation and Measurements of Al–Ce Composite
Materials
J. Ferreira—contributing editor
Highly pure Al (99.9% purity, 25 lm) and highly pure Ce
(99.9% purity, 40 lm) were purchased from Aladdin reagent
Co., Ltd. (Shanghai, China) and used without further purifi-
cation. The mixture of Al and Ce was obtained according to
the required weight ratio of Al and Ce. 2.00 g of the mixture
and a small amount of pure ethanol was added into a stain-
less-steel vial and sealed under argon atmosphere. Then ball
Manuscript No. 29499. Received March 23, 2011; approved August 01, 2011.
This work was financially supported by Innovation Program of Shanghai Municipal
Education Commission (10YZ70, 09ZZ136), Science Foundation of Shanghai Normal
University (SK201002), Shanghai Science and Technology Development Fund (no.
09520500500, 11ZR1426500) and the Key Laboratory of Resource Chemistry of Minis-
try of Education of China.
†Author to whom correspondence should be addressed. e-mail: xibinyu@shnu.
edu.cn
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Hydrogen Generation from Al–Ce Composite Materials
3977
Fig. 1. The hydrogen production curves (a) and the reaction rate curves (b) of 8 h ball-milled Al–x wt% Ce composites with pure water.
milling was performed in QM-3SP04 planetary ball miller
(made by the Instrument Factory of University of Nanjing)
with ball-to-powder mass ratio of 30:1 at a rotational speed
of 500 rpm for 1–12 h.
The yield of generated hydrogen was typically measured
as follows. Firstly, in the glove box, Al–Ce composites
(0.10–0.12 g) were put into a 100 mL Pyrex glass reactor
under argon atmosphere. This reactor was placed in a water
bath to keep the environment temperature constant at 70°C.
Then, 70 mL of distilled water at 70°C was added into the
reactor. The hydrogen-generating reaction between Al and
water was performed without stirring. After flowing through
a condenser, the generated gas was collected in a reverted
graduated burette. Its volume can be read directly at any
time. Finally, the volume of generated hydrogen was calcu-
lated at 1 atm and 25°C. The volume of water vapor was
taken into account and removed in the hydrogen yield cal-
culation.
Table I. Hydrogen Generation Rate, Hydrogen Production, Theoretical Hydrogen Production and Conversion Yield at 1 h of
Reaction in 70°C Water
Hydrogen generation rates (mL/min/g)
Theoretical
Hydrogen
production
(mL/g)
hydrogen
production†
(mL/g)
Conversion
yield (%)
Sample
0–5 (min)
5–15 (min)
15–60 (min)
Maximum rate
Al–0 wt% Ce
Al–5 wt% Ce
Al–10 wt% Ce
Al–13 wt% Ce
Al–15 wt% Ce
Al–20 wt% Ce
12.0
68.0
118.8
147.8
158.8
137.8
4.3
0.9
3.2
2.1
2.1
0.9
2.0
39.4
146
873
1000
1134
1098
1036
1359
1308
1258
1227
1207
1157
10.74
66.74
79.49
92.42
90.97
89.54
39.1
24.1
30.1
26.5
26.6
144.8
320.4
454.3
500.4
510.3
†Calculated assuming that cerium completely reacts with water at 25°C and 1 atm.
Fig. 2. (a) XPS survey spectrum and (b) high-resolution Ce 3d spectra of Al–13 wt% Ce.

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Vol. 94, No. 11
Al–x wt% Ce (x = 0, 5, 10, 13, 15, 20) during 1 h of reac-
tion. All the key parameters about the hydrogen generation
property of Al–x wt% Ce series are summarized in Table I.
It is found that cerium has a trade-off effect on the hydrogen
production. Hydrogen production reaches its maximum at
x = 13 and then decreases quickly with the increase of cer-
ium content. The hydrogen generation rate curves of differ-
ent Al–Ce composites with water are shown in Fig. 1(b). It is
obvious that the reaction rate firstly increased and then
decreased, and the maximum reaction rate was achieved at
1 min. The increase of Ce content leads to a considerable
increase of the reaction rate at 1 min from 39.4 mL/min/g
for Al–0 wt% Ce to 510.3 mL/min/g for Al–20 wt% Ce.
These data also show that the existence of cerium has made
aluminum active.
Table II. The Amounts of Different Chemical States
Ce in Al–13 wt% Ce
Chemical states
BE (eV)
Area (%)
Total (at%)
Ce 3d
Ce0
883.7
885.2
898.8
904.4
882.1
887.6
901.7
907.6
916.8
2.58
19.62
9.04
20.53
24.81
6.85
8.13
3.63
4.81
2.58
49.19
Ce3+
Ce4+
48.23
The positive effect of cerium on the hydrogen-generating
reaction is explained in the following two points: Firstly, cer-
ium can damage the dense oxide layer of aluminum during
the ball milling process. Reducibility of metal cerium
(Eo = ꢀ2.336 V) is superior to that of metal aluminum
(Eo = ꢀ1.662 V), so cerium can damage the dense oxide
layer of aluminum through oxidation and reduction reaction,
as shown below:
(2) Characterization
The samples were analyzed by X-ray photoelectron spectros-
copy (XPS; Perkin-Elmer PHI 5000, Wellesley, MA). All the
binding energies were calibrated by using the contaminant
carbon (C1s = 284.6 eV) as a reference. The X-ray powder
diffraction (XRD) pattern was recorded by using a Japan
Regaku (Tokyo, Japan) D/max c¸ A X-ray diffractometer
equipped with graphite monochromatized CuKa radiation
(k = 1.5418 A) irradiated with a scanning rate of 4°/min.
The Field-emission scanning electron microscopic (FESEM)
images were obtained by using a JEOL (Tokyo, Japan) JSM-
7500F microscope operated at an acceleration voltage of
15 kV. The distribution of Ce particles in composite powders
was checked by energy dispersive X-ray (EDX) mapping.
The Brunauer–Emmett–Teller (BET) method was utilized to
calculate the specific surface area by using adsorption data in
the range of the relative pressures (p/p0) from 0.02 to 0.20.
Ce þ x=3 Al2O3 ! 2x=3 Al þ CeOx
(1)
It is recently reported that Al–Sn, Al–Bi and Al–Ga–In
alloy were prepared through mechanical milling.17,22 During
preparation of these materials, the ball milling plays a main
role in ruining the oxide layer. Because their reducibilities are
not strong enough, Sn, Bi, and Ga cannot reduce Al2O3
layer. By comparison, in our ball milling method, both the
reduction of Al2O3 by cerium and the mechanical power of
ball milling contribute to remove the dense Al2O3 layer on
the surface of Al particles. Therefore, the as-prepared Al–Ce
composites exhibit excellent capability of hydrogen genera-
tion as shown in Fig. 1. The XPS investigation of Al–13 wt
% Ce was performed for evaluation of their composition.
Figure 2(a) is a typical XPS survey spectrum of different
atoms. Elements of Al, O, Ce, and carbon contaminant were
detected obviously. To clearly reveal the chemical states of
III. Results and Discussion
(1) Reactivity of Al–Ce Composite Materials in Water
Hydrogen production, hydrogen generation rate and conver-
sion yield are the most important criteria in evaluating the
capability of hydrogen generation materials.5 Figure 1(a)
shows the respective hydrogen production of 8 h ball milled
Fig. 3. FESEM micrographs and XRD patterns of the hydrolysis products from 8 h ball-milled (a) Al and (b) Al–13 wt% Ce composite.

November 2011
Hydrogen Generation from Al–Ce Composite Materials
3979
the Ce species in Al–13 wt% Ce, Ce 3d spectra are shown in
Fig. 2(b). The amounts of Ce were calculated from peaks for
Ce4+ ions (916.8, 907.6, 901.7, 887.6, and 882.1 eV), peaks
for Ce3+ ions (904.4, 898.8, and 885.2 eV) and peaks for Ce0
(883.7 eV). Table II shows that the content of Ce0 was only
2.58 at%.
Secondly, cerium has an important influence on the mor-
phology of the products generated in the reaction of Al–Ce
composites with water. Aluminum without the dense oxide
layer can react with water instantly to generate hydrogen,
but the hydrolysis products cover aluminum particles and
inhibit hydrogen generation.23 In the ball milling process,
cerium elements are dispersed around the surface of alumi-
num particles. When hydrolysis of Al–Ce composites is per-
formed in pure water, Ce may react with water according to
the following reaction24,25
:
Ce þ x H2O ! CeðOHÞx þ x=2 H2
(2)
The CeOx and Ce(OH)x generated in steps (1) and (2) tend
to form loose and porous morphology, moreover, the hydro-
gen generation rate is quick. Therefore, the hydrolysis prod-
ucts fail to enwrap aluminum particles and inner aluminum
particles can continue to react with water. Furthermore, the
products will grow into porous structure due to their pre-
ferred growth direction.26
The FESEM micrograph and XRD patterns of products
from the hydrogen generation reaction of (a) ball milled Al–
0 wt% Ce and (b) Al–13 wt% Ce composites are shown in
Fig. 3. It can be seen in Fig. 3(a1) that the hydrolysis prod-
ucts deposited on the aluminum surface to prevent further
reaction, result in the incomplete reaction of aluminum with
water. Figure 3(b1) shows that products of Al–13 wt% Ce
with water have a flowerlike porous structure. Compared
with the former, the latter products are fluffier and more
porous, making water continuously react with the core of
aluminum particles until the conversion achieves 100%. The
hydrogen production at 1 h is from 146 mL/g for Al–0 wt%
Ce to 1134 mL/g for Al–13 wt% Ce. The XRD pattern of
the ball milling aluminum reacted with pure water is shown
in Fig. 3(a2). It can be observed that the peaks are in funda-
mental agreement with aluminum (No.04-0787), boehmite
(No.21-1307) and bayerite (No.20-0011) crystal in the
JCPDS database. The high intensity of aluminum in the dif-
fraction pattern implied that the majority of aluminum did
not react with water. Figure 3(b2) shows the XRD pattern
of Al–13 wt% Ce after reacting with pure water, which is in
Fig. 4. The hydrogen production curves of Al–13 wt% Ce after
different ball-milling times.
Table III. Hydrogen Production and Conversion Yield of
Al–13 wt% Ce with 70°C Water after Different
Ball-Milling Time
Hydrogen
Ball milling
time (h)
production
(mL/g)
Conversion
yield (%)
1
5
431
778
1134
1143
35.13
63.41
92.42
93.15
8
12
Fig. 5. FESEM micrographs of (a) un-milled Al, Al–13 wt% Ce after ball-milling for (b) 1 h, (c) 8 h, and (d) 12 h.

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Journal of the American Ceramic Society—Luo et al.
Vol. 94, No. 11
(a)
(b)
(c)
(d)
Fig. 6. EDX mapping of Ce in Al–13 wt% Ce composite materials as a function of the milling time: (a) 1 h, (b) 5 h, (c) 8 h, and (d) 10 h.
good agreement with the characteristic peaks of boehmite
crystal according to the JCPDS database. Metal aluminum
diffraction peaks are incognizable in the diffraction pattern.
This implies that reaction has reached the core of aluminum
particles, and almost entire aluminum has turned into
boehmite.
In the ball milling process, the morphology, microstruc-
ture, and composition of metals may change in the repeated
flattening, cold welding, and fracturing. It has been proved
that the deformed aluminum has a higher activity in water
than the strain-free aluminum.27,28 As shown in Fig. 5, the
morphology of the Al–13 wt% Ce composite changed a lot
with the increase of milling time. The unmilled aluminum
powders were quasi-spheres with diameters of 20–40 lm [see
Fig. 5(a)]. When milled for 1 h, the particles of Al–Ce com-
posites are soft, their tendency to flatten, weld together, and
form large particles is high [see Fig. 5(b)]. With continued
deformation, the particles get work hardened and fracture by
the fragmentation of fragile flakes. Fragments generated by
this mechanism may continue to reduce in size in the absence
of strong agglomerating forces. When milled for 8 or 12 h,
the particles of Al–Ce composites became smaller irregular
nuggets [see Figs. 5(c) and 4(d)]. Compared with unmilled Al
particles, the long-time milled Al–13 wt% Ce composite par-
ticles had smaller sizes (~2.5 lm in diameter) and larger spe-
cific surfaces. Meanwhile, ball milling induced pitting
corrosion process by creating numerous defects and fresh
surfaces on metals.17 All these changes are believed to accel-
erate and promote the reaction of aluminum with water.
Besides particle size and microstructures, the long-time
milling also affects the spatial elemental distribution of the
(2) Effect of Ball Milling Time on the Activity of
Al–Ce Composites
Ball milling time is an important parameter in the mechani-
cal alloying process27 and has a vital impact on the activity
of Al–Ce composites. The relationship between hydrogen
production and milling time for the ball milled Al–13 wt%
Ce composite is shown in Fig. 4. With the increase of milling
time, the hydrogen generation rate and the hydrogen produc-
tion of Al–13 wt% Ce composite increased. It was found
that a longer milling time will lead to a higher ability of Al–
Ce composites to produce hydrogen when it reacts with
water (see Table III).
Table IV. Hydrogen Production, Theoretical
Hydrogen Production and Conversion Yield at 1 h of
Reaction in 70°C Water
Theoretical
Hydrogen
production
(mL/g)
hydrogen
production†
(mL/g)
Conversion
yield (%)
Sample
Al–5 wt% Ce
Al–5 wt% Ce–10 wt%
NaCl
873
1085
1308
1173
66.74
92.50
Al–5 wt% Ce–10 wt%
KCl
1101
1173
93.86
†Calculated assuming that cerium completely reacts with water at 25°C and
1 atm.
Fig. 7. The hydrogen production curves of 8 h ball-milled Al–5 wt%
Ce–10 wt% alkali chloride with pure water.

November 2011
Hydrogen Generation from Al–Ce Composite Materials
3981
Fig. 8. FESEM micrographs of 8 h ball-milled (a) Al–5 wt% Ce and (b) Al–5 wt% Ce–10 wt% NaCl.
Al–Ce composites. Figure 6 shows EDX mapping of Ce in
Al–13 wt% Ce composite materials as a function of the mill-
ing time. The cerium particles are distributed unequally when
milled for 1 or 5 h. However, when the milling time was pro-
longed to 8 or 12 h, the particle size of cerium significantly
decreased and they are distributed equally around aluminum
particles. Based on the previous discussion, Ce plays an
important role in destroying the inert Al2O3 film and build-
ing porous structures of the hydrolysis products. It is reason-
ably believed that the improved dispersion of Ce in the Al–
Ce composites is responsible for the enhanced hydrogen gen-
eration effects which depend on the milling time, as shown in
Fig. 4 and Table III.
vents the products from depositing densely on the surface of
composites, so that the composites can continuously react
with water. After reacting with water at 70°C for 60 min, the
8 h milled Al–13 wt% Ce composite produces 1134 mL/g
hydrogen and the conversion yield reaches up to 92.42%.
Furthermore, the effects of ball milling time and alkali chlo-
rides on generating hydrogen were also investigated. Long-
time ball milling and the addition of alkali chlorides can
increase the dispersity of Ce and decrease the particle size of
Al–Ce composites to improve the hydrogen generation capa-
bility.
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