3D hierarchical CuO/Co3O4 core-shell nanowire array on copper foam for on-demand hydrogen generation from alkaline NaBH4 solution

Article abstract of DOI:10.1039/c6ra17340c

In this work, we demonstrate the first use of a 3D hierarchical CuO/Co₃O₄ core-shell nanowire array in situ grown on copper foam as a robust and highly active catalyst toward the hydrolytic dehydrogenation of NaBH₄ for on-demand hydrogen generation in alkaline solutions. This monolithic catalyst exhibits a maximum hydrogen generation rate of 6162.55 mL min^-1 _g(CuO/Co₃O₄)^-1 under ambient conditions, which is higher than that of most reported Co-based catalysts, with an activation energy of 56.38 kJ mol^-1. It also shows good durability and reusability and can be used as an on/off switch for on-demand hydrogen generation.

Full text of DOI:10.1039/c6ra17340c

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D hierarchical CuO/Co O core-shell nanowires array on copper
3 4
foam for on-demand hydrogen generation from alkaline NaBH4
solution
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
c
a
DOI: 10.1039/x0xx00000x
Lisi Xie, Kunyang Wang, Gu Du, Abdullah M. Asiri and Xuping Sun *
www.rsc.org/
In this work, we demonstrate the first use of 3D hierarchical hinder their widespread uses.
CuO/Co core-shell nanowires array in situ grown on copper In the past decades, Co based materials have been largely
foam as a robust and high-active catalyst toward the hydrolytic investigated as cost-effective catalysts for NaBH hydrolysis
3
O
4
4
2
0,21
22
dehydrogenation of NaBH
4
for on-demand hydrogen generation with favourable activity,
such as Co nanoparticles,
2
3
24
under alkaline solutions. This monolithic catalyst exhibits a Au@Co core-shell nanoparticles, Co
3
O
4
powder,
etc. However, the low surface
mL/min/g(CuO/Co3O4) at ambient conditions, which is higher than area and difficult separation from fuel solutions limit the
that of most reported Co-based catalysts, with an activation application of Co based powder materials in catalysing NaBH
CoO
2
5
26
maximum
hydrogen
generation
rate
of
2
6162.55 nanocrystals, and LiCoO ,
4
energy of 56.38 KJ/mol. It also shows good durability and hydrolysis for hydrogen generation. From the point of
reusability and can be used as an on/off switch for on-demand technology and operation, self-supported catalyst systems
hydrogen generation.
may conquer the above difficulties and have obvious
advantages: free of aggregation, easy and rapid recovery from
the reaction system for reusability and on-demand hydrogen
In present days, hydrogen is considered as a promising
energy carrier with merits of high energy content, renewability
1
and no emission of greenhouse gas. However, one obstacle
2
7
generation. Though several supported Co based catalysts
have been utilized in the hydrolysis of NaBH , it is still a great
challenge to design supported catalysts with larger surface
,2
4
3
,4
for hydrogen economy is the issue of hydrogen storage.
Hydrides are attractive hydrogen-storing materials for portable
1
,28.
area and more active sites.
Hierarchical structure is a
5
-7
applications
and NaBH
4
has attracted particularly
promising method to solve the above problem, as the
supported core-shell hierarchical structure grown directly on
conductive substrate has fascinating synergetic properties and
large surface area with some unique physical and chemical
considerable attention because of its good reducibility of
8
-10
metal precursors and organic compounds,
high hydrogen
1
storage efficiency (10.8 wt.%), economical cost compared
1
1
2
with other chemical hydrides, stability of its solution under
2
9,30
properties in catalyst systems.
Previous report has shown
1
3
14
high pH value, and potentially safe operation. NaBH
hydrolysis is a safe exothermic reaction that produces
4
that copper foam can be oxided into nanostructured arrays
3
1
and applied in catalyst systems. Further, Cu based catalysts
have also been used as catalysts for NaBH hydrolysis due to
1
5
hydrogen according to eq.1:
4
3
2
its cheap and environmental features.
hierarchical nanostructure may have superior catalytic activity
in NaBH hydrolysis reaction due to the synergistic effect
between Cu and Co and the large surface area of hierarchical
Thus, Cu-Co
NaBH (aq) + 2H O → NaBO (aq) + 4H (1)
2 2 2
4
Nevertheless, it is usually kept under alkaline conditions to
prevent its self-hydrolysis and an active catalyst must be
utilized to activate and accelerate its hydrolysis for hydrogen
4
3
3
nanostructure. What’s more, to the best of our knowledge,
such Co-Cu hierarchical structure has not been applied in the
1
6
17
18
19
generation. Noble metals like Pt, Rh, Ru, and Pd are the
most active such catalysts, but the scarcity and high cost
4
hydrolysis of alkaline NaBH solution.
Herein, we demonstrate the development of hierarchical
CuO/Co core-shell nanowires array on copper foam
3
O
4
a
Chemical Synthesis and Pollution Control, Key Laboratory of Sichuan Province,
(CuO/Co O core-shell NA/CF) as a robust and cost-effective
3 4
School of Chemistry and Chemical engineering, China West Normal University,
Nanchong 637002, Sichuan, China. E-mail: sunxp@cwnu.edu.cn
Chengdu institute of Geology and Mineral Resources, Chengdu 610081, Sichuan,
China
Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia
catalyst for on-demand hydrogen generation via the hydrolytic
dehydrogenation of NaBH in alkaline media, with an
activation energy (E ) of 56.38 KJ/mol. This 3D monolithic
b
4
c
a
† Electronic Supplementary Information (ESI) available: Experimental section and
supplementary figures. See DOI:
catalyst offers a maximum hydrogen generation rate (HGR) of
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162.55 mL/min/g(CuO/Co3O4) at ambient conditions. It also spectrum, confirming the presence of only Cu, Co and O. The C
5
3
exhibits good stability and reusability and behaves as an on/off 1s signals arise from exposure to atmosphere. As shown in
switch for on-demand hydrogen generation. These attractive Fig. 2b, the CuO exhibits a Cu 2p3/2 peak and Cu 2p1/2 peak at
3
6,37
features promise its use as an earth-abundant catalyst binding energies (BEs) of 933.8 and 953.8 eV, respectively,
material toward large-scale production of hydrogen fuels for together with two high-intensity shake-up satellites (those
3
located at 942.4, and 963.3 eV). Fig. 2c shows the XPS
8
applications.
CuO/Co
3
O
4
core-shell NA/CF was fabricated by the following spectra of Co. Two prominent peaks at BEs of 795.2 and 779.6
2
+
39
three steps in Fig. 1a (see ESI for preparation details): (1) eV correspond to Co 2p1/2 and 2p3/2, respectively. Two
Cu(OH) nanowires array was grown on commercially available small peaks at 803.5 and 788.9 eV can be attributed to 2p3/2
CF by a wet-chemical route at room temperature; (2) and 2p1/2 of Co , respectively, confirming the existence of
2
3
+
4
.
0,41
CoCO
3
(OH)
2
·nH
2
O core-shell NA/CF was subsequently grown Co
3
O
4
As shown in Fig. 2d, the O 1s core-level spectrum is
on the Cu(OH)
2
backbones via chemical bath deposition (CBD) broad, and two peaks are marked as I and II. Peak I, the lower
3
4
42,43
(
Fig. S2), (3) the resulting Cu(OH)
shell NA/CF as precursor was converted to CuO/Co
shell NA/CF by air annealing. Scanning electron microscopy oxygen ions at the surface.
SEM) analysis suggests that macroporous copper foam (Fig.
b) is completely covered with Cu(OH) nanowires array after
ammonium persulfate treatment, as shown in Fig. 1c and 1d .
Fig. 1e and 1f show the SEM images of CuO/Co core-shell
2
/CoCO
3
(OH)
2
·nH
2
O core- energy of 529.7 eV, is in agreement with lattice O.
The high
3 4
O core- BE of 531.5 eV (peak II), is attributed to the low coordinated
4
0,44
(
1
2
3
O
4
NA/CF, providing one piece of clear evidence to support the
formation of hierarchical core-shell nanowires array on CF. Its
energy-dispersive X-ray (EDX) spectrum confirms the existence
of Cu, Co and O elements (Fig. S3). The SEM and corresponding
EDX elemental mapping images for such nanoarray
demonstrate the uniform distribution of Cu, Co and O
elements (Fig. 1g). The much stronger signals for Co and O also
3 4
imply the shell nature for Co O species.
3 4
Fig. 2. XPS spectra of CuO/Co O core-shell NA/CF. (a) XPS survey spectrum. XPS
spectrum in the (b) Cu 2p, (c) Co 2p, and (d) O 1s regions.
We examined the catalytic activity of the monolithic
CuO/Co
3
O
4
core-shell NA/CF toward NaBH
solution. Co
4
hydrolysis by
putting them in alkaline NaBH
4
3
O
4
NA/CF and CuO
NA/CF were also tested for comparison. As observed, slow
hydrogen release occurs for CuO NA/CF and Co NA/CF (Fig.
S5a and b) while a large amount hydrogen gas appears
vigorously on CuO/Co core-shell NA/CF (Fig. S5c and Movie
S1). Fig. S5d compares the catalytic activities for the NaBH
hydrolysis on CuO NA/CF, Co NA/CF and CuO/Co core-
shell NA/CF. Obviously, CuO/Co core-shell NA/CF shows the
highest catalytic activity as compared to the same amount of
3 4
O
3 4
O
4
3
O
4
3 4
O
3 4
O
Fig. 1. (a) A schematic diagram for illustrating the fabrication procedure of others. This excellent performance may be attributed to the
CuO/Co core-shell NA/CF. SEM images of (b) bare CF, (c, d) Cu(OH) NA/CF
and (e, f) CuO/Co
images of Cu, Co, and O elements for CuO/Co
3
O
4
2
3
O core-shell NA/CF. (g) SEM and EDX elemental mapping following factors: (1) this nanostructured branched metal
4
3 4
O core-shell NA/CF.
oxide has large reactive sites and facilitates the diffusion of
materials; (2) CuO plays the role of substrate and synergistic
Fig. S4 shows the X-ray diffraction (XRD) pattern for
CuO/Co core-shell NA/CF in the diffraction angle range of
0-80°. The three strong peaks at 43.30°, 50.43°, and 74.13°
4
5,46
effect to the hydrolysis reaction of NaBH
The catalytic activity of CuO/Co
further investigated by hydrolyzing alkaline NaBH
under different conditions. Fig. 3a presents the effect of NaBH
concentration on hydrogen generation. The HGR maintains
almost constant with increasing NaBH concentration from 0.2
wt% to 2.0 wt%, suggesting that the catalytic hydrolysis
reaction over CuO/Co core-shell NA/CF catalyst follows
4
.
3
O
4
3
O
4
core-shell NA/CF was
solutions
3
4
can be assigned to the (111), (200), and (220) planes of
metallic Cu substrate (JCPDS No. 04-0836), respectively. Other
peaks at 35.50°, 36.84°, 38.73°, 59.35° and 65.22° arise from
4
4
CuO and Co
suggests the complete conversion of CoCO
CuO/Co after annealing. Fig. 2a shows the XPS survey
3
O
4
. The absence of additional diffraction peaks
3
(OH) ·nH O into
2
2
3 4
O
3
O
4
2
| J. Name., 2012, 00, 1-3
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zeroth order kinetics in this concentration range. The
7
To obtain the E
a 4
for the hydrolysis of NaBH catalyzed by
reaction rate equation based on zero-order kinetics can be CuO/Co
3 4
O
core-shell NA/CF, the hydrolysis reactions were
4
8
described as follow:
4 × d[NaBH
where rH2 is the hydrogen release rate (mL/min).
performed at different temperatures (Fig. 4a). It is quite clear
that the higher reaction temperature leads to larger HGR. The
-1
was determined as 56.38 KJ mol according to Arrhenius
-
4
] / dt = d[H
2
] / dt = rH2 (2)
E
a
5
7
plot of ln (HGR) vs. 1/T for the catalyst (Fig. 4b). A HGR of
162.55 mL/min/g(CuO/Co3O4) was achieved under 298 K and
6
ambient pressure. This value compares favourably to the
behaviours of most reported Co-based catalysts (Table S1).
Such superior catalytic performances can be rationally
attributed to the following two reasons: (1) the nanoarray
3
1
format leads to more exposed active sites for catalysis; (2)
the 3D macroporous configuration of CF and the open spaces
between the nanowires allow for enhanced diffusion of the
5
8
fuel and hydrogen gas generated.
Fig. 3. Effect of concentrations of NaBH
the HGR: (a) and (b) [NaOH] = 1 wt%, CuO/Co
wt%, CuO/Co loading = 0.9 mg. (d) [NaOH] = 1 wt%, [NaBH
hydrolysis experiments were done at 298 K and ambient pressure.
4
and NaOH as well as catalyst loading on
loading = 0.9 mg. (c) [NaBH ] =
] = 1 wt%. All
3
O
4
4
1
3
O
4
4
It has been well understood that the reaction order with
respect to NaBH concentration changes with NaBH
From Fig. 3b, it can be shown that the HGR
increases with further rising NaBH concentration, but this
enhancement is limited to NaBH concentration of 8 wt%. The
HGR decreases when NaBH concentration is further increased
to 10 wt.%. Similar results were also reported by other mL) catalyzed by CuO/Co
4
4
4
9,50
concentration.
4
4
4
Fig. 4. (a) Hydrogen generation kinetics curves for alkaline NaBH
NW/CF using the 1 wt.% NaOH and 1 wt.% NaBH
solution at different solution temperatures. (b) The corresponding Arrhenius
NW/CF in aqueous solution (2 mL) containing
and 1.0 wt% NaOH at 298 K and ambient pressure. (d) On-
NW/CF as an
loading = 0.9 mg, [NaOH] = 1 wt%, [NaBH ] = 1 wt%, T
4
solutions (2
3
O
4
4
5
1,52
researchers.
NaBH
solution; (2) the NaBO
than NaBH and thus could decrease mass transfer rate.
It can be explained as follows: (1) increased plot. (c) Recycling of CuO/Co
3 4
O
1
.0 wt% NaBH
4
4
concentration could increase the viscosity of the fuel demand hydrogen generation in the fuel solution using CuO/Co
3 4
O
on/off switch (CuO/Co
, as a hydrolysate, has lower solubility = 298 K).
3
O
4
4
2
5
3,54
4
High recycle stability is really crucial for every practical
catalyst. Therefore, hydrolysis experiments were performed to
reuse the catalyst several times at room temperature for
durability test. The catalytic activity remains unchanged after
Fig. 3c shows the effect of different NaOH concentrations on
the release rate of hydrogen, suggesting an increase in NaOH
concentration causes decreased HGR. It can be explained as
follows. NaOH is an inhibitor to slow down the self-hydrolysis
1
0 cycles (Fig. 4c). Compared to powdery catalyst systems, the
of NaBH
NaBH decomposition. Increased NaOH concentration also
increases the solution viscosity and reduces the available free
4 2
in solution and thus more NaOH gives less H via
monolithic CuO/Co O core-shell NA/CF can be easily
4
3
4
separated from the fuel solution, promising its use as an
efficient on/off switch for on-demand hydrogen
1
5
2 4
H O to react with NaBH . The hydrogen release rate may be
5
9,28
4
8,55
generation.
hydrogen generation by repeated activation of the catalyst
system. In the absence of CuO/Co NA/CF (OFF), hydrolysis
was terminated without hydrogen release. Hydrogen release
occurs again when immersing CuO/Co core-shell NA/CF into
the fuel solution (ON). XPS analysis for CuO/Co core-shell
Fig. 4d shows the temporal control of
expressed using eq.3:
0
.3
r
H2 = k / (1 + k
1
[NaOH]
)
(3)
3 4
O
where k is rate constant and k
1
the coefficient for the effect of
3 4
O
NaOH concentration. It was also found that the catalyst
loading has great influence on hydrogen generation
performance. As shown in Fig. 3d, an increase of catalyst
loading leads to a higher rH2. Fig. S6 depicts the correlation of
catalyst loading on rH2. The value of rH2 was calculated from
the slope of the linear part of each plot shown in Fig. 3d. The
logarithmic plot of the calculated initial reaction rate vs.
3 4
O
NA/CF suggests the valence state of Co does not change
obviously after reaction (Fig. S7a). But, in contrast, two distinct
peaks at BEs of 932.4 and 952.5 eV can be attributed to
6
0,61
Cu(I),
2
which demonstrate some CuO was reduced to Cu O
during reaction (Fig. S7b).
3 4
In summary, hierarchical CuO/Co O core-shell nanowires
m
CuO/Co3O4 has a slope of 1.03 ≈ 1.0 indicating that the
5
hydrolysis is first order with respect to the catalyst loading.
6
array on copper foam has been proven as an efficient 3D
monolithic catalyst with an E
a
of 56.38 KJ/mol for on-demand
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generation ration from the hydrolysis of alkaline NaBH
4
25 A. Lu, Y. Chen, J. Jin, G. Yue and D. Peng, J. Power Sources,
012, 220, 391-398.
6 O. V. Komova, V. I. Simagina, O. V. Netskina, D. G. Kellerman,
2
solution. It shows a high HGR of 6162.55 mL/min/g(CuO/Co3O4)
with high recyclability and durability. Our study not only
provides us an attractive low-cost catalyst material toward
2
A. V. Ishchenko and N. A. Rudina, Catal. Today, 2008, 138
2
,
60-265.
4
efficient NaBH hydrolysis for mass production of hydrogen 27 S. S. Muir and X. Yao, Int. J. Hydrogen Energy, 2011, 36
,
fuels, but would open new opportunities in designing and
utilizing monolithically integrated nanoarrays as an on/off
switch for on-demand hydrogen generation from borohydrides
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8 L. Cui, X. Sun, Y. Xu, W. Yang and J. Liu, Chem. Eur. J., 2016,
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9 H. Kobayashi, M. Yamauchi, H. Kitagawa, Y. Kubota, K. Kato
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3 4
D hierarchical CuO/Co O core-shell nanowires array works as a robust and high-active catalyst
toward the hydrolytic dehydrogenation of NaBH for on-demand hydrogen generation under
4
alkaline solutions, with a maximum hydrogen generation rate of 6162.55 mL/min/g(CuO/Co3O4) at
ambient conditions and an activation energy of 56.38 KJ/mol.