Metallic Nickel Hydroxide Nanosheets Give Superior Electrocatalytic Oxidation of Urea for Fuel Cells

Article abstract of DOI:10.1002/anie.201606313

The direct urea fuel cell (DUFC) is an important but challenging renewable energy production technology, it offers great promise for energy-sustainable developments and mitigating water contamination. However, DUFCs still suffer from the sluggish kinetics of the urea oxidation reaction (UOR) owing to a 6 e^?transfer process, which poses a severe hindrance to their practical use. Herein, taking β-Ni(OH)₂nanosheets as the proof-of-concept study, we demonstrated a surface-chemistry strategy to achieve metallic Ni(OH)₂nanosheets by engineering their electronic structure, representing a first metallic configuration of transition-metal hydroxides. Surface sulfur incorporation successfully brings synergetic effects of more exposed active sites, good wetting behavior, and effective electron transport, giving rise to greatly enhanced performance for UOR. Metallic nanosheets exhibited a much higher current density, smaller onset potential and stronger durability.

Full text of DOI:10.1002/anie.201606313

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201606313
German Edition: DOI: 10.1002/ange.201606313
Direct Urea Fuel Cells
Metallic Nickel Hydroxide Nanosheets Give Superior Electrocatalytic
Oxidation of Urea for Fuel Cells
Xiaojiao Zhu, Xinyu Dou, Jun Dai, Xingda An, Yuqiao Guo, Lidong Zhang, Shi Tao,
Jiyin Zhao, Wangsheng Chu, Xiao Cheng Zeng, Changzheng Wu,* and Yi Xie
Abstract: The direct urea fuel cell (DUFC) is an important but
challenging renewable energy production technology, it offers
great promise for energy-sustainable developments and miti-
gating water contamination. However, DUFCs still suffer from
the sluggish kinetics of the urea oxidation reaction (UOR)
urea-rich wastewater before urea naturally hydrolyzes in the
environment. Compared to cathodic oxygen reduction reac-
tion (ORR), the anodic urea oxidation reaction (denoted as
ꢀ
ꢀ
UOR, CO(NH ) + 6OH !N + 5H O + CO + 6e ) under-
2
2
2
2
2
ꢀ
goes more sluggish kinetics owing to a 6e transfer process
and requires the use of electrocatalysts for promoting the
reaction rate. Therefore, high-performance UOR catalysts
are required to reduce the overpotential to drive the sluggish
reaction. Although tremendous efforts have been devoted to
pursue efficient electrocatalysts to give superior UOR
ꢀ
owing to a 6 e transfer process, which poses a severe hindrance
to their practical use. Herein, taking b-Ni(OH) nanosheets as
2
the proof-of-concept study, we demonstrated a surface-chemis-
try strategy to achieve metallic Ni(OH)2 nanosheets by
engineering their electronic structure, representing a first
metallic configuration of transition-metal hydroxides. Surface
sulfur incorporation successfully brings synergetic effects of
more exposed active sites, good wetting behavior, and effective
electron transport, giving rise to greatly enhanced performance
for UOR. Metallic nanosheets exhibited a much higher current
density, smaller onset potential and stronger durability.
[7–15]
performance,
they still suffer from inferior electrical
conductivity and “poisoning”, greatly hindering electrochem-
ical efficiency for practical application.
Conductive two-dimensional (2D) nanosheets have been
explored for high-performance electrocatalysts because of
their highly exposed catalytic surface and excellent electron
transportation. Two-dimensional nanomaterials usually pos-
sess exposed surfaces with low-coordinated steps, edges, and
kinks, which provide abundant active sites to mediate the
electrocatalytic process, allowing them to be efficient electro-
D
riven by growing concerns about global warming and the
depletion of fossil fuel, developing renewable energy-pro-
duction and -storage technologies represent an important but
[
1–3]
[16]
challenging issue.
In this regard, direct urea fuel cells offer
catalysts. Owing to the dimensionally reduced structure of
great promise for energy-sustainable developments and also
2D nanomaterials, in which most of atoms are exposed on
surface and thus offer high chemical activity, various strat-
egies directed at the surface atoms, including defect engineer-
[4–6]
mitigating water contamination.
The urea fuel cell is
designed based on 2CO(NH ) + 3O !2N + 4H O + 2CO ,
2
2
2
2
2
2
[17–19]
accomplishing power output and concurrently remedying
ing, surface incorporation and structural distortion,
have
been employed to effectively augment the number of active
sites. Moreover, intrinsic high conductivity is also essential for
improving electrocatalytic performances. A vast array of
highly conductive 2D nanomaterials, spanning from transi-
[
*] X. J. Zhu, X. D. An, Y. Q. Guo, J. Y. Zhao, Prof. C. Z. Wu, Prof. Y. Xie
Hefei National Laboratory for Physical Sciences at the Microscale
iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), CAS Center for Excellence in Nanoscience, and CAS Key
Laboratory of Mechanical Behavior and Design of Materials
University of Science & Technology of China
[
20]
[21]
tion-metal carbides Mo C,
nitrides Ni N,
phosphide
2
3
[22]
Ni P -Ni P
to transition-metal dichalcogenides (TMD)
5
4
2
[23]
1
T-WS2, benefit from abundant active sites and enhanced
conductivity, consequently exhibiting superior electrocata-
lytic performance. However, for the urea-oxidation reaction
Hefei 230026 (P.R. China)
E-mail: czwu@ustc.edu.cn
(
UOR) process, conductive 2D nanomaterials have been
X. Y. Dou
College of Chemistry, Jilin University
[
10,15]
rarely used.
An open question is how to synergistically
2
699 Qianjin Street, Changchun, 130012 (China)
enhance electrical conductivity and regulate the active sites
towards optimizing UOR electrocatalytic performance?
Herein, taking b-Ni(OH)2 nanosheets as a proof-of-
concept study, we demonstrated a surface-chemistry strategy
to achieve a metallic phase of Ni(OH)2 nanosheets by
manipulating their electronic structure, representing the first
metallic case among transition-metal hydroxides. Surface
sulfur incorporation successfully brings synergetic effects of
more exposed active sites by prohibiting the “poisoning”
process, good wetting behaviors, and effective electron trans-
port, giving rise to greatly enhanced performance for UOR.
Metallic nanosheets exhibited a much higher current density,
smaller onset potential, and higher stability. This work shed
J. Dai, Prof. X. C. Zeng
Department of Chemistry, University of Nebraska-Lincoln
Lincoln, NE 68588 (USA)
Prof. L. D. Zhang
National Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei, Anhui 230029 (P.R. China)
S. Tao, Prof. W. S. Chu
National Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei, Anhui 230029 (P.R. China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606313.
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light on designing highly efficient electrocatalysts for energy
conversion.
To enable effective sulfur incorporation, pristine Ni(OH)2
P-Ni(OH) ) nanosheets were treated with an H S flow as the
(
2
2
sulfur source at elevated temperature (Figure 1a,b). Sulfur-
incorporated Ni(OH)2 products with a gradient of sulfur
contents were selectively controlled at different reaction
temperatures of 1008C and 1308C for 1 h (details in the
Supporting Information). In our case, products acquired at
1
008C and 1308C are denoted as S-Ni(OH) and M-Ni(OH) ,
2 2
respectively. The ultrathin nature of the Ni(OH) nanosheets
2
obtained (Figure S2 in the Supporting Information) offers
a structural platform with exposed surface atoms to facilitate
the sulfur incorporation reaction. As a matter of fact,
experimental (Figure S3, Movie S1) and theoretical investi-
gations (Figure 1c and Figure S4) indicate high feasibility for
sulfur incorporation. Characterizations confirm that the
original structure is maintained after surface chemical treat-
ment. The X-ray powder diffraction (XRD) gives evidence
that the long-range order of Ni(OH) is preserved after the
2
sulfur incorporation (Figure S5). X-ray absorption fine struc-
ture (XAFS) measurements, which are sensitive to local
atomic and electronic structures, were performed to probe the
2
local structure of three Ni(OH) samples. The k c(k) oscil-
2
lation curve in Figure 2a for S-Ni(OH)2 and M-Ni(OH)2
nanosheets have a spectral shape similar to that of P-Ni(OH)2
nanosheets, which is accounted for by the preservation of the
Figure 1. Schematic illustration of the sulfur incorporation into
Ni(OH) nanosheets resulting in metallic Ni(OH) nanosheets. a) side
Ni(OH) structure. In Figure 2b, the Fourier transform curve
2
2
2
view of the metallic Ni(OH) nanosheets; b) schematic illustration of
2
of pristine Ni(OH)2 nanosheets displays two peaks at
approximately 1.86 ꢀ and 2.99 ꢀ, which can be attributed
to Ni-O and Ni-Ni correlations, respectively. For S-Ni(OH)2
nanosheets and M-Ni(OH)2 nanosheets, the intensities
of these two peaks are significantly
the reaction between H S and Ni(OH) nanosheets; c) kinetics of the
2
2
sulfur incorporation reaction.
decreased. Furthermore, Ni-O(S) peak for
M-Ni(OH) nanosheets is also accompa-
2
nied with a noticeable shift (amplified in
inset of Figure 2b, and shift trend is
indicated with a pink arrow) in position
of the Ni-O peak towards high-R side,
which can be ascribed to the larger atomic
radius of S over O and in turn confirms
sulfur incorporation into P-Ni(OH) nano-
2
sheets. As described above, in spite of
sulfur incorporation in Ni(OH) structure,
2
the pristine lattice framework was well
maintained, verifying our structural model
of sulfur substitution of oxygen atoms.
Employing element analyses upon iso-
lated nanosheets give consistent results of
sulfur incorporation into pristine Ni(OH)2
framework. Elemental mapping performed
on the M-Ni(OH)2 nanosheet demon-
strates that sulfur is homogeneously dis-
tributed on the nanosheet (Figure S6). In
addition, the corresponding electron
Figure 2. Sulfur incorporation into the pristine Ni(OH) framework (P-Ni(OH) ). a) Ni
2
2
energy-loss spectrum (EELS) which
shows sulfur L-edge from the core in
2
K-edge XAFS oscillation function k c(k) and b) the corresponding Fourier transforms
2
FT(k c(k)), where k=wave vector and c(k)=oscillation as a function of the photoelectron
[24]
1
Figure S7, resembles that of MoS and
2
wavenumber; inset: zoomed-in view of Fourier transforms. c) H solid-state MAS NMR
further confirms sulfur incorporation into spectra of the three Ni(OH) samples. See text for details.
2
2
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the Ni(OH) nanosheets. The introduced -SꢀH group could
observed between the two samples is a shift of absorption
2
be revealed by 1H solid-state MAS NMR spectra. As shown
threshold to lower energy with sulfur incorporation. As
displayed in the inset of Figure 3a, due to smaller electro-
negativity of sulfur compared to that of oxygen, the edge of
M-Ni(OH)2 nanosheets show a downshift, indicating an
increase in the electron concentration in the M-Ni(OH)2
nanosheets. Furthermore, XPS spectra of metallic and pris-
tine nanosheets also verify the increment of the electron
concentration (Figure S8). Given sulfur atoms have a larger
atomic radius and a smaller electronegativity compared to
that of oxygen atoms, sulfur incorporation would lead to
weaker covalent bonding and delocalize immobilized elec-
trons, providing the possibility to change the electronic
structure. The sulfur concentration in metallic nanosheets is
characterized to be approximately 4.44% (Figure S9). Fur-
thermore, density function theory (DFT) helps the under-
standing of the electronic structure evolution after sulfur
incorporation. As shown in Figure 3b, with the sulfur
in Figure 2c, the strongest signals at around d = 7.5 ppm of
the three samples are assigned to -OꢀH group, and downfield
shift of the -OꢀH group signal in the spectra of M-Ni(OH)
2
nanosheets to d = 7 ppm can be attributed to prominent
enhancement of electron density around H atoms triggered
by the sulfur incorporation (the increment of electron
concentration is discussed below). The -SꢀH entity is
confirmed with the signals at around d = 3 ppm in the spectra
[
25]
of S-Ni(OH)2 and M-Ni(OH)2 nanosheets.
The blue
rectangle area in Figure 2c is to highlight the content
evolution of the -HS entity, while no -SꢀH characteristic
peak is observed in the P-Ni(OH)2 nanosheets. Similarly,
signal of -SꢀH in the M-Ni(OH) nanosheets is broadened
2
and even expanded to high field compared to that of
S-Ni(OH) nanosheets, which can also be ascribed to prom-
2
inent enhancement of electron density around H atoms.
Therefore, incorporated sulfur in the form of -SꢀH groups
that are bonded to the Ni atoms in the OꢀNiꢀO structure
incorporation, the wide band gap in pristine Ni(OH) nano-
2
sheets turn into zero band, demonstrating the sulfur incorpo-
scaffold.
ration gives rise to metallic Ni(OH) nanosheets.
2
To further demonstrate whether sulfur incorporation can
effectively change the electronic structure, Ni K-edge
The electrical property measurements revealed metallic
transportation behavior for M-Ni(OH)2 nanosheets. As
XANES spectra of M-Ni(OH) nanosheets and P-Ni(OH)2
shown in Figure 3c, M-Ni(OH) nanosheets showed increase
2
2
nanosheets (Figure 3a) enable the chemical state and geom-
etry structure of the two samples to be discerned. Clearly, the
pre-edge feature A (Figure 3a) didnꢁt change much; and the
spectral features B (Figure 3a) didnꢁt show energy shift,
indicating the pristine Ni(OH)2 framework was well pre-
served after sulfur incorporation. The main change that is
of electrical resistivity as the temperature increased with d1/
dT> 0, which is typical of metallic character. In this case,
ꢀ4
M-Ni(OH) nanosheets had a resistivity of 3.13 ꢂ 10 W·m at
2
room temperature, exhibiting ultrahigh conductivity. The Hall
coefficient (R ) measurements as shown in Figure 3d were
H
performed to further understand the charge-transport proper-
ties in the M-Ni(OH)2 nanosheets. The
negative values of RH at different temper-
atures for M-Ni(OH) nanosheets indicate
2
that the main carriers are electrons, which
is significantly different from the previ-
ously reported transporting model for the
pristine
[
26]
Ni(OH) nanosheets.
Furthermore, the
2
electron concentration of M-Ni(OH)
2
2
0
ꢀ3
nanosheets is approximately 10 cm , ver-
ifying that the sulfur incorporated structure
brings metallicity with ultrahigh conduc-
tivity.
In our case, metallic nanosheets bring
synergistic advantages of active sites
ingrained in Ni(OH) nanosheets as well
2
as enhanced electrical conductivity, hold-
ing great promise for efficient UOR cata-
lyst. To evaluate the UOR performance of
metallic Ni(OH)2 nanosheets, the linear
sweep voltammetry (LSV) of M-Ni(OH)2
catalyst deposited on glassy carbon elec-
trode obtained in the absence and presence
of 0.33m urea in 1m KOH solution at a scan
Figure 3. Electrical behavior regulation by sulfur incorporation in M-Ni(OH) nanosheets.
ꢀ1
2
rate of 50 mVs , is shown in Figure 4a. As
can be clearly seen, in 1m KOH solution, at
a) Ni K-edge XANES of M-Ni(OH) nanosheets and P-Ni(OH) nanosheets; inset: zoomed-in
2
2
view of the Ni K-edge XANES. b) Density of states (DoS) of M-Ni(OH) nanosheets and P-
2
0
.344 V vs. Ag/AgCl, Ni(OH) is oxidized
Ni(OH) nanosheets (the vertical blue bar indicates the decrease in the bandgap after sulfur
2
2
to NiOOH. After the addition of urea,
a significant increase on current density
incorporation); c) Temperature-dependent resistivity of M-Ni(OH) nanosheets; d) carrier
2
concentration and Hall coefficient of M-Ni(OH) nanosheets.
2
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electrocatalysts for UOR. Furthermore, to
demonstrate the superior UOR perfor-
mance of metallic Ni(OH)2 nanosheets,
the comparison of peak current density
among different electrocatalysts (Fig-
ure S11 and Figure S12) is shown in Fig-
ure 4d, demonstrating metallic Ni(OH)2
nanosheets are among the most active
[
4]
electrocatalysts for UOR.
Importantly, aside from the high con-
ductivity, CO2 “poisoning” catalysts also
strongly influence the UOR performance,
which demands effective means to facili-
tate CO desorption from the catalyst
2
8]
[
surface. As is known, UOR on Ni(OH)2
catalysts in alkaline electrolyte involves
[
27–29]
following steps, [Eq. (1)–(3)]
ꢀ
ꢀ
6
NiðOHÞ þ 6 OH Ð 6 NiOOH þ 6 H O þ 6 e
2
2
ð1Þ
6
NiOOH þ COðNH
2
Þ
2
2
þ H O !
2
ð2Þ
Figure 4. UOR catalytic properties of M-Ni(OH) nanosheets. a) LSV plots of M-Ni(OH)
6 NiðOHÞ þ N þ CO
2
2
2
2
electrode in 1m KOH, with and without 0.33m urea, the inset shows the onset potential;
b) LSV plots of the M-Ni(OH) electrode and the P-Ni(OH) electrode; c) Chronoampero-
Net UOR reaction :
2
2
metric response of the P-Ni(OH) and the M-Ni(OH) electrode in 1m KOH+0.33m urea
electrolyte at 0.46 V (vs. Ag/AgCl); d) peak current density comparison among different
electrocatalysts.
ꢀ
ꢀ
2
2
COðNH Þ þ 6 OH ! N þ 5 H O þ CO þ 6 e
2
2
2
2
2
ð3Þ
The rate-determining step in reac-
tion (2) is the adsorbed CO desorption from surface of
2
[30]
with the onset potential at 0.344 V vs. Ag/AgCl was revealed.
By contrast, the onset potential of metallic nanosheets is
active sites. In our case, sulfur incorporation can effectively
lower energy barrier for intermediates formation and thus
effectively facilitate overall reaction rate, as verified by
density function theory calculation. Figure 5 describes kinet-
ics energy barrier profiles of the rate-determining step:
2
8 mV smaller than that of pristine nanosheets (Figure S10).
The onset potential of UOR is consistent with the potential
where the NiOOH is formed, indicating that newly generated
NiOOH species are active sites for urea oxidation, which have
been investigated in detail in most of Ni-
[R·CO2]ads!R + CO . R and R’ stands for respective catalytic
2
[
8–14]
based electrocatalysts.
Compared with pristine Ni(OH) nano-
2
sheets, M-Ni(OH)2 nanosheets deliver
superior UOR performance with a higher
catalytic activity and stability. Specifically,
as shown in Figure 4b, M-Ni(OH) nano-
2
sheets deliver a much higher peak current
density, which is almost all contributed by
oxidation of urea, demonstrating a much
higher UOR catalytic activity. The electro-
oxidation of urea on metallic nanosheets
electrodes were further evaluated with
constant voltage analysis in 1m KOH
solution in the absence and presence of
Figure 5. Kinetics for the CO desorption from electrocatalysts surface. See text for details.
2
0
.33m urea. According to oxidation peak
potential of urea of pristine nanosheets, 0.46 V vs. Ag/AgCl
was chosen as applied potential in the chronoamperometry
measurement. As displayed in the Figure 4c, current density
species in pristine and metallic nanosheets. As can be clearly
seen, energy barrier for the breakage of interaction between
CO and surface active species in [R···CO ] is smaller than
2
2 ads
of M-Ni(OH) nanosheets electrode increased gradually until
that in the P-Ni(OH)2 nanosheets, suggesting itꢁs more
2
it approached a peak value at 15000 s. Clearly, the M-
Ni(OH)2 electrode delivers constant and higher current
density, indicating that M-Ni(OH)2 are stable and active
favorable for CO desorption from the surface of catalytic
2
species in M-Ni(OH) nanosheets, and thus exposing more
2
active sites open for the catalytic procedure. From this
4
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ability to effectively retard the “poisoning” of electrocata-
2
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case of 2D transition-metal hydroxide. The sulfur incorpo-
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[
electron transport, metallic Ni(OH) nanosheets deliver much
2
enhanced UOR performance. Metallic nanosheets have
a much higher current density, smaller onset potential, and
greater stability than pristine Ni(OH) nanosheets. This work
2
provides an effective strategy to regulate the electrocatalytic
performance of Ni-based structures.
[
[
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H32.
Keywords: fuel cells · metallic transition-metal hydroxide ·
nanosheets · sulfur · urea oxidation reaction
30] D. A. Daramola, D. Singh, G. G. Botte, J. Phys. Chem. A 2010,
114, 11513 – 11521.
Received: June 29, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Direct Urea Fuel Cells
X. J. Zhu, X. Y. Dou, J. Dai, X. D. An,
Y. Q. Guo, L. D. Zhang, S. Tao, J. Y. Zhao,
W. S. Chu, X. C. Zeng, C. Z. Wu,*
Y. Xie
&&&& — &&&&
Metallic Nickel Hydroxide Nanosheets
Give Superior Electrocatalytic Oxidation
of Urea for Fuel Cells
Metal as anything: Surface sulfur incor-
more exposed active sites and metallic
electrical conductivity, giving rise to
greatly enhanced performance for the
urea oxidation reaction (UOR) for direct
urea fuel cells.
poration into b-Ni(OH) nanosheets
2
gives the first metallic configuration of
a transition-metal hydroxide. The result-
ing metallic Ni(OH) nanosheets have
2
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!