Surface Engineering of a Nickel Oxide-Nickel Hybrid Nanoarray as a Versatile Catalyst for Both Superior Water and Urea Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.8b00411

Developing efficient and low-cost oxygen evolution reaction (OER) electrodes is a pressing but still challenging task for energy conversion technologies such as water electrolysis, regenerative fuel cells, and rechargeable metal-air batteries. Hence, this study reports that a nickel oxide-nickel hybrid nanoarray on nickel foam (NiO-Ni/NF) could act as a versatile anode for superior water and urea oxidation. Impressively, this anode could attain high current densities of 50 and 100 mA cm^-2 at extremely low overpotentials of 292 and 323 mV for OER, respectively. Besides, this electrode also shows excellent activity for urea oxidation with the need for just 0.28 and 0.36 V (vs SCE) to attain 10 and 100 mA cm^-2 in 1.0 M KOH with 0.33 M urea, respectively. The enhanced oxidation performance should be due to the synergistic effect of NiO and Ni, improved conductivity, and enlarged active surface area.

Full text of DOI:10.1021/acs.inorgchem.8b00411

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Surface Engineering of a Nickel Oxide−Nickel Hybrid Nanoarray as a
Versatile Catalyst for Both Superior Water and Urea Oxidation
Zhihao Yue,† Wenxin Zhu,† Yuanzhen Li,† Ziyi Wei,† Na Hu,‡ Yourui Suo,‡ and Jianlong Wang*,†
†College of Food Science and Engineering, Northwest A&F University, 22 Xinong Road, Yangling 712100, Shaanxi, China
‡Qinghai Key Laboratory of Qinghai−Tibet Plateau Biological Resources, Northwest Institute of Plateau Biology, Chinese Academy
of Sciences, 23 Xinning Road, Xining 810008, Qinghai, China
S
* Supporting Information
ABSTRACT: Developing efficient and low-cost oxygen evolution reaction
(OER) electrodes is a pressing but still challenging task for energy conversion
technologies such as water electrolysis, regenerative fuel cells, and rechargeable
metal−air batteries. Hence, this study reports that a nickel oxide−nickel hybrid
nanoarray on nickel foam (NiO−Ni/NF) could act as a versatile anode for
superior water and urea oxidation. Impressively, this anode could attain high
current densities of 50 and 100 mA cm−2 at extremely low overpotentials of 292
and 323 mV for OER, respectively. Besides, this electrode also shows excellent
activity for urea oxidation with the need for just 0.28 and 0.36 V (vs SCE) to
attain 10 and 100 mA cm−2 in 1.0 M KOH with 0.33 M urea, respectively. The
enhanced oxidation performance should be due to the synergistic effect of NiO
and Ni, improved conductivity, and enlarged active surface area.
layered double hydroxide, and NiCo2O4 phases have been
largely reported to be able to drive the OER catalysis, their
catalytic performances have various restrictions such as skimp
active sites and high series resistance.9,26 Much effort has been
recently devoted to developing Ni-based chalcogenides and
phosphides as precatalysts toward the OER because of their
high conductivity and intrinsic activity. Our group reported a
nickel sulfide microsphere film on Ni foam for enhancing the
OER activities.22 Sun’s group constructed a nickel selenide
nanowire film on Ni foam for superior OER catalysis.23 Hu’s
group further reported that a Ni2P phase could efficiently
catalyze the OER.24 Although the performances reflected from
the above materials are amazing, regrettably, there are still some
flaws with these catalysts; for instance, their fabrication
processes are cumbersome with the release of toxic H2S,
H2Se, or H3P gases. Considering this, researchers shifted their
attention toward how to improve the OER performance of Ni-
based (oxy)hydroxides and mainly proposed two effective
strategies: (i) introducing ion dopants into these (oxy)-
hydroxides phases to improve the native catalytic activity by
altering the electronic structures and providing dual or multiple
active sites;27−29 (ii) surface engineering active substances onto
these (oxy)hydroxides to synergistically improve the overall
catalytic performance.26,30−33 As the most commonly used
surface-engineering approach, electrochemical deposition is
time-saving and nontoxic and could be performed under
ambient temperature, which makes it an effective method to
fabricate hybrid catalysts for water electrolysis.31,34
INTRODUCTION
■
Nowadays, chronically overcommitted fossil energy not only is
leading to an energy shortage but also has brought increasing
environmental damage for excess emissions of carbon.1 Hence,
it is imperative to search for noncarbonaceous and renewable
succedaneums.2 Currently, hydrogen fuel has been regarded as
an immense alternative to fossil fuels for its low carbon content
and optimal energy density.3,4 Water splitting driven by
universal clean energy sources like wind, water, and solar
power has been regarded as the most promising way to obtain
nearly endless pure hydrogen. Water electrolysis is composed
of oxygen evolution reaction (OER) and hydrogen evolution
reaction.5 The voltage required to split water is significantly
higher than the thermodynamic value (∼1.23 V vs RHE)
principally because of the sluggish multistep proton-coupled
electron-transfer process associated with OER.6,7 Efficient
anode catalysts are thereby demanded to facilitate the kinetics
of OER.8,9 To date, the state-of-the-art OER catalysts are
always recognized as ruthenium and iridium and their metal
oxides or complexes.10 However, their poor chemical stability,
scarcity, and high cost badly hinder their extensive practical
applications.11 Thus, developing earth-abundant and non-
noble-metal-based efficient OER anodes is urgently de-
manded.12
Transition-metal compounds, because of their abundant
reserves and low price, have gradually garnered lots of attention
for electrocatalysis.13−21 Among them, nickel (Ni)-based
compounds like (oxy)hydroxides, chalcogenides, and phos-
phides are prominent because of their good catalytic power
toward the OER.22−25 It should be noted that, although Ni-
based (oxy)hydroxides like NiO, Ni(OH)2, NiOOH, NiFe
Received: February 14, 2018
© XXXX American Chemical Society
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In this work, a nickel oxide-nickel hybrid nanoarrays on Ni
foam (NiO−Ni/NF) built by a facile electrodeposition method
could serve as a high-performance 3D OER catalyst is reported.
This electrode shows superior and stable OER catalytic
performance with the need of overpotentials of only 292 and
323 mV (vs RHE) to drive 50 and 100 mA cm−2 in 1.0 M
KOH. Also, considering that another anodic oxidation reaction,
the urea oxidation reaction (UOR), which has been recognized
as a promising alternative to OER for energy-saving hydrogen
evolution.35−37 This electrode was further demonstrated to be
able to drive superior UOR catalysis with the need of just 0.36
V (vs SCE) to drive 100 mA cm−2 in 1.0 M KOH with 0.33 M
urea. The excellent catalytic OER and UOR performances are
mainly due to the synergistic effects and compact contacts
among Ni, NiO, and Ni foam, greatly improved ion and
electronic transfers, as well as enlarged active surface area.
with the standard pattern of Ni [(111), (200), and (220)
planes; JCPDS 04-0850], respectively. The other three weak
peaks at 37.2°, 43.3°, and 62.9° are indexed to the (111), (200),
and (220) planes of NiO (JCPDS 47-1049). By contrast, the
diffraction peak positions of NiO/NF are the same as those for
NiO-Ni/NF, while the peak intensities of Ni are lower than
those for NiO-Ni/NF, indicating that Ni has likely been
deposited on the NiO nanosheets. Moreover, the XRD pattern
of NiO−Ni/CC has the characteristic peaks of both NiO and
Ni phases, while NiO/CC only has the characteristic peaks of
NiO, further confirming the existence of Ni.
Figures S2 and S3 and 1b show the SEM images of bare Ni
foam, Ni(OH)2/NF, and NiO/NF, respectively. As observed,
the NiO nanosheets are thin and smooth (Figure 1b), while the
NiO−Ni nanosheets turn out to be rough and thick (Figure
1c,d). Apparently, the Ni nanoparticle film adheres to the
surface of the NiO nanosheets. Note that the thicknesses of the
hybrid nanosheets will change according to the increase of the
electrodeposition cycles (Figure S4). After 20 cycles, only a
teeny weeny thickness change and infrequent Ni particles could
be observed (Figure S4a). After 40 cycles, the NiO nanosheets
were fully covered by a thick Ni nanoparticle film (Figure S4c).
The distribution of Ni and O elements could be confirmed by
EDX spectroscopy and corresponding elemental mapping
(Figure S5). The TEM image of NiO−Ni nanosheets scratched
from Ni foam further shows a well-defined rough nanosheet
morphology (Figure 1e). High-resolution TEM (HRTEM)
images (Figure 1f and insets) reveal that this configuration has
well-resolved lattice fringes with interplanar distances of about
2.05 and 2.09 Å, which could be, in general, indexed to the
(111) and (200) planes of Ni and NiO, respectively. XPS
measurements were performed to explore the surface chemical
compositions and valence states of NiO−Ni/NF (Figure S6).
The survey spectrum (Figure S6a) shows the Ni and O
elements on the surface of NiO−Ni/NF. The spectrum of
NiO−Ni/NF in the Ni 2p region could be divided into six
peaks at 879.4, 873.1, 871.3, 860.9, 855.6, and 853.8 eV,
respectively (Figure S6b). The peaks centering at 871.3 and
853.8 eV could be assigned to the characteristic feature of Ni0,
and the peaks at 873.1 and 855.6 eV correspond to Ni2+,
indicating the existence of Ni2+ and Ni0 on the surface.38 The
remaining two peaks are the satellites.
RESULTS AND DISCUSSION
■
Intuitively, the changes of the optical characteristics demon-
strate the successful conversion of NiO−Ni/NF (Figure S1).
Then, considering the overlap of the diffraction peak positions
for the electrodeposited Ni and Ni foam, NiO−Ni and NiO
nanosheets on carbon cloth (NiO−Ni/CC and NiO/CC) were
also synthesized with the identical process to verify the
existence of electrodeposited Ni on NiO nanosheets. Figure
1a shows the XRD patterns for (a) NiO−Ni/NF, (b) NiO/NF,
(c) NiO−Ni/CC, and (d) NiO/CC, in which the diffraction
peaks of Ni (electrodeposited Ni or Ni foam), NiO, and carbon
cloth are marked with solid diamonds, solid clubs, and solid
hearts, respectively. As can be seen, for NiO−Ni/NF, three
diffraction peaks located at 44.5°, 51.8°, and 76.4° accord well
An OER test has been performed with a typical three-
electrode cell in 1.0 M KOH. To exclude the distraction of
electrolyte solution resistance and study the inherent electro-
chemical activity of the electrode, ohmic resistance corrections
were applied to the initial data.39,40 Impressively, NiO−Ni/NF
is highly efficient toward the OER, as shown in Figure 2a,b.
Figure 2a shows polarization curves of NiO−Ni/NF, NiO/NF,
and bare Ni foam on the reversible hydrogen electrode (RHE)
scale. The oxidation peak detected at ∼1.38 V (vs RHE) before
the onset potential of the OER could be attributed to the
conversion of low-valence Ni to high-valence Ni species.31,41−43
Obviously, the oxidation peak of NiO−Ni/NF is much higher
than that of NiO/NF (bare Ni foam has no visible oxidation
peak). Also, the polarization curve of NiO−Ni/NF increases
sharply and has a larger gap compared to the contrasts at higher
potentials. In detail, bare Ni foam exhibits inferior OER activity
and NiO/NF has a higher current response with the need of
overpotentials of 415 and 454 mV to drive the current density
of 50 and 100 mA cm−2, respectively. In sharp contrast, NiO−
Ni/NF attains 50 and 100 mA cm−2 at the extremely low
overpotentials of 292 and 323 mV, respectively. It is noted that
Figure 1. (a) XRD patterns. (b) Low- and (inset) high-resolution
SEM images of NiO/NF. (c) Low- and (d) high-resolution SEM
images of NiO−Ni/NF. (e) TEM and (f) HRTEM images for NiO−
Ni nanosheets scratched from Ni foam.
B
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slope compared to other contrasts. The electrochemical
impedance spectroscopy (EIS) measurements of NiO−Ni/
NF and NiO/NF show that NiO−Ni/NF has a smaller
polarization resistance of 18.99 Ω than NiO/NF (348 Ω),
indicating the better electron-transfer rate and OER catalytic
performance of NiO−Ni/NF,26 as shown in Figure 2d.
To more accurately reflect the comparison of the intrinsic
catalytic performance of NiO−Ni/NF and NiO/NF, the mass-
normalized activity and activity based on the real surface area
(RSA) of the electrodes toward the OER were also recorded.
As shown in Figure S9, the mass-normalized OER perform-
ances of NiO−Ni/NF (mass loading is 1.8 mg cm−2) and NiO/
NF (1.7 mg cm−2) need potentials of 391 and 445 mV to attain
50 mA mg−1, respectively. Meanwhile, the RSA of Ni foam was
measured according to the method introduced in the
Supplementary Method. Obviously, a smaller overpotential of
293 mV is needed for NiO−Ni/NF to attain the RSA-based
OER performance of 10 mA cm−2 than that for NiO/NF.
These results indicate that the intrinsic catalytic activity of
NiO−Ni/NF is much higher than that of NiO/NF.
The catalytic stability of water-splitting catalysts is also vital
for application in industry. Figure 2e exhibits a multistep
chronopotentiometric curve obtained in 1.0 M KOH, where the
current density increases from 10 to 100 mA cm−2 with an
increment of 10 mA cm−2 per 500 s. Intuitively, the potential
immediately levels off at 1.475 V (vs SCE) and remains
constant for 500 s at 10 mA cm−2, which is similar to the rest of
the results until 100 mA cm−2 is reached. This result indicates
the excellent mechanical robustness, conductivity, and mass
transportation of NiO−Ni/NF.35 The inset of Figure 2e shows
a short response time within 3 s, wherein the potential has
homologous variation when the current density changes to
another step, further proving the fast mass and electron
transfers.26,37 The chronopotentiometry curve was collected to
verify the long-term OER stability of NiO−Ni/NF (Figure 2f).
Obviously, there is almost no loss in potential (vs RHE) after
20 h of reaction at a static current density of 10 mA cm−2. The
OER polarization curve exhibits only a negligible positive shift
after 20 h electrolysis, revealing the good long-term stability of
NiO−Ni/NF working in 1.0 M KOH. Note that the oxidation
peak of the LSV curve after reaction is higher than that for the
initial one, which could be ascribed to the adequate
transformation from Ni2+ to Ni3+ after long-term OER. The
SEM images (Figure S10a,b) for NiO−Ni/NF after the OER
test show that the morphology gets completely preserved. The
corresponding XPS spectra (Figure S10c,d) show that the
intensities of the characteristic peaks at 871.3 and 853.8 eV for
Ni0 obviously decrease after the OER. Considering the possible
interference of Ni0 in Ni foam on the XPS spectrum of NiO−
Ni/NF after the OER, we further collected the XPS spectra of
NiO−Ni/CC before and after OER electrolysis. Apparently, the
XPS spectrum of NiO−Ni/CC (Figure S11) after the OER also
shows a decrease of the intensities of the characteristic peaks of
Ni0, demonstrating that a portion of the surface Ni might turn
out to be the NiOOH phase that is believed to be part of the
real active site for the OER.37
Figure 2. (a) Polarization curves of NiO−Ni/NF, NiO/NF, and bare
Ni foam at a scan rate of 5 mV s−1. (b) Corresponding Tafel plots. (c)
The histogram of a comparison of the overpotentials and Tafel slopes
between different electrodes. (d) Nyquist plots of NiO−Ni/NF and
NiO/NF in 1.0 M KOH under the influence of an alternating-current
voltage of 5 mV. (e) Multicurrent process of NiO−Ni/NF. The
current density started at 10 mA cm−2 and ended at 100 mA cm−2 with
an increment of 10 mA cm−2 per 500 s (inset: enlarged curves). (f)
Chronopotentiometric curve of NiO−Ni/NF with a constant current
density of 10 mA cm−2. The inset shows polarization curves recorded
for NiO−Ni/NF before and after long-term OER reaction at a scan
rate of 5 mV s−1. All experiments were conducted in 1.0 M KOH.
the OER of NiO−Ni/NF outperforms most of the recently
reported superior OER catalysts (Table S1). Besides, the OER
performances of Ni(OH)2−Ni/NF, Ni/NF, and Ni(OH)2/NF
were also tested (Figure S7). All of these electrodes have
inferior OER catalytic activities compared with NiO−Ni/NF.
Their corresponding cyclic voltammograms were also recorded
in this potential window (Figure S8). The anodic peaks of
NiO−Ni/NF and NiO/NF in the cyclic voltammograms are
similar to that in Figure 2a, and the cathodic peaks could be
attributed to the reduction of high-valence Ni species formed
during the oxidation sweep.19
In addition, the optimal experimental data show that, with
the increase of the deposition cycles, the catalytic activity of
NiO−Ni/NF first goes up and then down, which could be
attributed to the fact that too much Ni hinders access of the
electrolyte to the active sites (Figure S4d). Figure 2b shows the
Tafel plots. The Tafel slope of NiO−Ni/NF attains 101.1 mV
dec−1, which outperforms 110.4 mV dec−1 of NiO/NF and
143.4 mV dec−1 of Ni foam. The smaller Tafel slope of NiO−
The excellent and stable OER catalytic property of NiO−Ni/
NF could be interpreted as the following four reasons. (1) The
growing of high-density and upright NiO−Ni hybrid nano-
sheets on 3D macroporous Ni foam constitutes multistage
topology, which provides better contact between the active sites
and electrolyte, as well as sufficient spaces for better diffusion of
gas bubbles.34,44 (2) The compact contact between Ni foam
Ni/NF implies more rapid OER kinetics for the electrode.45
A
more intuitional comparison of the overpotential and Tafel
slope among NiO−Ni/NF, NiO/NF, and bare Ni foam could
be seen by the form of the histogram (Figure 2c), in which
NiO−Ni/NF has the lowest overpotential and smallest Tafel
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and NiO−Ni hybrid nanosheets ensures good mechanical
adhesion and electron transfer without polymer binders. (3)
The aforementioned EIS results shown in Figure 2d indicate
that NiO−Ni/NF has better conductivity than NiO/NF. (4)
The double-layer capacitances at solid/liquid interfaces of
NiO−Ni/NF and NiO/NF were tested to estimate their
electrocatalytic active surface areas. As shown in Figure S12, the
fitting curves (Figure S12c) are obtained from the cyclic
voltammograms (Figure S12a,b). The results show that NiO−
Ni/NF and NiO/NF have capacitances of 154.5 and 78.2 mF
dec−1, respectively, which implies a larger active surface area
and more active sites of NiO−Ni/NF.26
gain further insight into the intrinsic catalytic activity of NiO−
Ni/NF and NiO/NF for the UOR, the mass-normalized and
RSA-based activity of the electrodes toward the UOR are
compared in Figure S14, proving that there is a higher intrinsic
catalytic activity for NiO−Ni/NF than for NiO/NF. Tafel plots
of the three electrodes are given in the inset of Figure 3b. By
contrast, NiO−Ni/NF has a lower Tafel slope of 55 mV dec−1,
suggesting faster UOR kinetics for NiO−Ni/NF. The UOR
performances of Ni(OH)2−Ni/NF, Ni/NF, and Ni(OH)2/NF
were also tested (Figure S15). Apparently, these electrodes also
have inferior activities toward the UOR compared with NiO−
Ni/NF. Besides, the electrodeposition cycles of Ni were also
optimized to study the effect of different contents of NiO/Ni
on the UOR performance (Figure S16). A total of 30 cycles
were chosen as the optimal electrodeposition condition, which
is consistent with the result in the OER.
Moreover, except for the OER, we found that this NiO−Ni/
NF also has superior UOR activity. The UOR test was
performed in 1.0 M KOH with 0.33 M urea with the same
device as that for the OER. Figure 3a shows a comparison of
Further, Figure 3c shows LSV curves acquired at different
scan rates in alkali with urea. The negligible changes with an
increase of the scan rates from 5 to 50 mV s−1 indicate the
efficient charge and mass transport of NiO−Ni/NF.44,45
Besides, the stable multistep chronopotentiometric curve
(inset in Figure 3d) implies the superior conductivity, mass
transport, and mechanical robustness of NiO−Ni/NF for the
UOR. The long-term UOR catalytic stability of NiO−Ni/NF
was further tested (Figure 3d). Obviously, the potential only
shows negligible degradation after 16 h of reaction under a
constant current density of 10 mA cm−2, indicating that this
electrode has good long-term UOR catalytic stability in alkali
with urea. Therefore, NiO−Ni/NF could be well qualified as an
excellent and stable anode for catalyzing UOR.
The superior urea electrolysis performance of NiO−Ni/NF
also promises its use as a fascinating 3D electrochemical sensor
for urea detection. Figure S17 shows the LSV plots of NiO−
Ni/NF and the corresponding calibration curve, which are used
to study the electrochemical behaviors of NiO−Ni/NF toward
different concentrations of urea.18,46 As observed, the current
density rises with an increase of the concentrations, and there is
a linear relationship between the current density and urea
concentration in the range of 0−8 mM.
Figure 3. UOR catalytic properties. (a) LSV curves of NiO−Ni/NF in
1.0 M KOH with and without 0.33 M urea. (b) Polarization curves of
NiO−Ni/NF, NiO/NF, and bare Ni foam in 1.0 M KOH with 0.33 M
urea (inset: corresponding Tafel plots). (c) Polarization plots for
NiO−Ni/NF at different scan rates. The inset shows the data replotted
as the current density at different scan rates. (d) Chronoamperometric
plot of NiO−Ni/NF at a constant current density of 10 mA cm−2. The
inset shows the multicurrent process at different current densities.
CONCLUSION
■
In summary, NiO−Ni/NF has been successfully developed for
both superior and stable water and urea oxidation via
hydrothermal and subsequent electrochemical deposition
processes. NiO−Ni/NF could attain 50 and 100 mA cm−2 at
the low overpotentials of 292 and 323 mV in 1.0 M KOH for
the OER. This electrode is also efficient for urea electrolysis
and just needs 0.36 V vs SCE to drive 100 mA cm−2. The
superior activity and stability of this electrode, accompanied by
this easy and controllable preparation process, provides us with
a low-cost and high-performance anode alternative for superior
OER and UOR catalysis in future industrial water-splitting
devices.
the LSV plots of NiO−Ni/NF in alkali with and without urea.
It can be observed that NiO−Ni/NF demands a potential of
0.56 V (vs SCE) to attain 100 mA cm−2 in 1.0 M KOH without
urea. In contrast, this electrode exhibits an obvious enhance-
ment in the OER performance, with the need of just 0.36 V to
deliver the same current density in 1.0 M KOH with 0.33 M
urea. This result confirms that the UOR has taken place on
NiO−Ni/NF and is more energy efficient than the OER. Figure
3b shows the UOR activities of NiO−Ni/NF, NiO/NF, and
bare Ni foam, in which Ni foam shows poor activity while NiO/
NF has a higher current response, with the need of 0.39 V to
attain 100 mA cm−2. In contrast, NiO−Ni/NF is much more
efficient for the UOR and attains current densities of 10, 100,
and 200 mA cm−2 at just 0.28, 0.36, and 0.45 V, respectively.
The activity surpasses most of newly reported superior UOR
electrocatalysts (Table S2). As shown in Figure S13, the
increase of the current density of urea oxidation is very rapid
with the increase of the potentials, which fully covers the
oxidation peak of low-valence Ni species. The reason for the
formation of reduction peaks is the same as that in alkali. To
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00411.
Supplementary method, optical photographs, SEM
images, EDX spectrum and elemental mapping, XPS
D
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spectra, CV and LSV plots, electrochemical data, and
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wanglong79@yahoo.com.
ORCID
Jianlong Wang: 0000-0002-2879-9489
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financed by the National Natural Science
Foundation of China (Grant 21675127), the Fundamental
Research Funds for the Northwest A&F University of China
(Grants 2014YB093 and 2452015257), and the Development
Project of Qinghai Key Laboratory (Grant 2017-ZJ-Y10).
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