Copper-incorporated hierarchical wire-on-sheet α-Ni(OH)2 nanoarrays as robust trifunctional catalysts for synergistic hydrogen generation and urea oxidation

Article abstract of DOI:10.1039/c9ta02891a

Copper-incorporated α-Ni(OH)₂ nanoarrays with a unique hierarchical wire-on-sheet structure and local Ni³⁺ species are explored as efficient trifunctional electrocatalysts for urea oxidation and water electrolysis. The unique hierarchical morphology can not only provide abundant reactive sites for the in situ generation of catalytically active species for electrooxidation reactions, but also guarantee a robust and conductive skeleton for facile charge transport and offer effective buffering space for the volume change during the Ni²⁺-to-Ni³⁺ pre-oxidation process. Besides, the presence of local Ni³⁺ ions and the optimal Cu concentration further enhance the electrooxidation behaviors such as the UOR and OER, and the role of the Cu dopant in boosting the HER activity toward long-term operation is illustrated as the generation of low-valence Cu that could effectively facilitate the electron transfer process. As a result, remarkable UOR, HER and OER performance can be achieved with the synergistic benefits of large surface area, enhanced charge transfer and robust structural stability, which can be beneficial for electrolytic cells for efficient coupled urea electrolysis and overall water splitting.

Full text of DOI:10.1039/c9ta02891a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C9TA02891A
PAPER
Copper-Incorporated α-Ni(OH)2 Hierarchical Wire-on-Sheet
Nanoarrays as Robust Trifunctional Catalysts for Synergistic
Hydrogen Generation and Urea Oxidation
Received 00th January 20xx,
Accepted 00th January 20xx
Junfeng Xie†,*, Li Gao†, Shanshan Cao, Weiwei Liu, Fengcai Lei, Pin Hao, Xinyuan Xia and Bo Tang*
DOI: 10.1039/x0xx00000x
Copper-incorporated α-Ni(OH)2 nanoarrays with unique hierarchical wire-on-sheet structure and local Ni3+ species are
explored as efficient trifunctional electrocatalysts for urea oxidation and water electrolysis. The unique hierarchical
morphology can not only provide abundant reactive sites for the in-situ generation of catalytically active species for
electrooxidation reactions, but also guarantee robust and conductive skeleton for facile charge transport and offer
effective buffering space for the volume change during the Ni2+-to-Ni3+ pre-oxidation process. Besides, the presence of
local Ni3+ ions and the optimal Cu concentration further enhance the electrooxidation behaviors such as UOR and OER, and
the role of Cu dopant in boosting the HER activity toward long-term operation is illustrated as the generation of low-
valence Cu that could effectively facilitate the electron transfer process. As a result, remarkable UOR, HER and OER
performance can be achieved with the synergistic benefits of large surface area, promoted charge transfer and robust
structural stability, which can be coupled as electrolytic cells for efficient urea electrolysis and overall water splitting.
into electrolytic cells for synergistic pollution treatment and
clean energy production. Recently, nickel-based catalysts have
been proved to be active for HER, OER and UOR in alkaline
Introduction
Water electrolysis is deemed to be a promising route for
converting electricity to clean chemical energy in the form of
hydrogen owing to its high efficiency and environmentally
benign quality, which requires appropriate catalysts to lower
down the overpotential for the half reactions, namely, the
cathodic hydrogen evolution reaction (HER, equation 1) and
the anodic oxygen evolution reaction (OER, equation 2).1, 2
However, the equilibrium potential of OER is as high as 1.23 V
vs. RHE, therefore raising the cost of hydrogen production via
overall water splitting. In this situation, urea (CO(NH2)2), a
highly energetic compound which is a common waste in
industrial and sanitary wastewater, is considered to be a
promising chemical to undergo the anodic urea oxidation
reaction (UOR, equation 3) and couple with HER more
efficiently.3-7
conditions, respectively. However, the HER activity is severely
limited by the insufficient active sites and the poor electron
transfer ability, while the oxidation reactions (i.e. OER and
UOR) require a facile pre-oxidation process to generate
catalytically active high-valence species (equation 4). Not only
that, the UOR reaction mechanism involves harsh 6-electron
transfer pathway,7 therefore efficient UOR catalysts with HER
and OER activity are rarely explored up to date.
α(β)-Ni(OH)2 + OH− ↔ γ(β)-NiOOH + H2O + e−
(4)
In order to enhance the exposure of active sites (for HER) or
facilitate the in-situ generation of active species (for OER and
UOR), engineering hierarchical nanostructures is considered to
be effective in enriching the highly reactive metal ions with
unsaturated coordination environment.6-14 Sun et al. proposed
that β-Ni(OH)2-Ni3N nanoarray heterostructure grown on
titanium mesh could realize efficient alkaline HER activity.15
Gao and colleagues highlighted α-Ni(OH)2 hollow spheres
could display higher OER activity than β phase.16 Recently, our
group reported that introducing nanopores in β-Ni(OH)2 or
NiFe layered double hydroxide (LDH) nanosheets can
2H2O + 2e− → H2 + 2OH−
4OH− → O2 + 2H2O + 4e−
CO(NH2)2 + 6OH− → N2 + 5H2O + CO2 + 6e−
Eθ = 0 V vs. RHE
Eθ = 1.23 V vs. RHE (2)
(1)
Eθ = 0.37 V vs. RHE (3)
Considering the fluctuating concentration of urea in variable
pH environments of the wastewater, exploring advanced
electrocatalysts with triple functions of HER, OER and UOR
activity is urgently demanded, which can be directly assembled
significantly boost the generation of active high-valence
species that benefit OER catalysis.17,
18
Apart from the
structural engineering, optimizing the electron structure via
defect engineering or elemental incorporation is also of great
importance in boosting the electron transfer processes and
optimizing the adsorption energy of precursors during the
electrocatalytic reactions. Aimed on this strategy, Tong et al.
indicated that the porous NiMoO4 nanosheets with abundant
oxygen vacancies could enrich the active sites with optimized
urea adsorption energy, therefore realizing efficient UOR
College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory
of Molecular and Nano Probes (Ministry of Education), Collaborative Innovation
Center of Functionalized Probes for Chemical Imaging in Universities of Shandong,
Institute of Molecular and Nano Science, Shandong Normal University, Jinan,
Shandong, 250014, P. R. China. E-mail: xiejf@sdnu.edu.cn; tangb@sdnu.edu.cn.
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Additional physical
characterizations of the catalysts and supplementary electrochemical data. See
DOI: 10.1039/x0xx00000x
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performance.19 Zhu et al. synthesized metallic β-Ni(OH)2
nanosheets via surface sulfur incorporation, displaying
enhanced UOR activity benefitting from the optimized charge
transfer behavior.20 However, bifunctional or trifunctional
catalysts that can realize efficient coupled reactions for urea
electrolysis and water splitting are still in demand until now.
scanning transmission electron microscopy (HAADF-STEM)
DOI: 10.1039/C9TA02891A
image and the corresponding elemental mapping analyses
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were carried out on an FEI Tecnai G2 F20 TEM. X-ray
photoelectron spectroscopy (XPS) analyses were performed on
a VGESCALAB MKII X-ray photoelectron spectrometer with an
excitation source of Mg Kα = 1253.6 eV, and the resolution
level was lower than 1 atom%.
Electrocatalytic study
Experimental section
All the electrochemical measurements were performed in a
three-electrode system linked with an electrochemical
workstation (CHI660E) at room temperature. All potentials
were calibrated to a reversible hydrogen electrode (RHE) and
the data are presented without iR correction. The cyclic
voltammetry (CV) and linear sweep voltammetry (LSV) with a
scan rate of 5 mV s-1 were conducted in the electrolyte
containing 1 M KOH and 0.33 M urea. A Hg/HgO electrode was
used as the reference electrode, a platinum gauze electrode (2
cm × 2 cm, 60 mesh) was used as the counter electrode, and
the nanoarray catalyst was served as the working electrodes
which was fixed with an electrode holder connected by a
glassy carbon plate. The electrochemical impedance
spectroscopy (EIS) measurements were operated at variable
potentials from 10-2-105 Hz.
Materials
The chemicals were purchased from Sinopharm Chemical
Reagent Co., Ltd.. All chemicals were used as received without
further purification.
Synthesis of Cu:α-Ni(OH)2/NF
The Cu:α-Ni(OH)2/NF catalysts with variable Cu concentration
were synthesized via a one-step hydrothermal route. Typically,
NiCl2·6H2O and CuCl2·2H2O with total metal amount of 0.4
mmol were dissolved in 40 mL of distilled water containing 3.2
mmol urea and stirred for 30 min to form a transparent
solution. The nickel foam (1cm × 4cm × 1 mm, 100 mesh) was
carefully cleaned with acetone by ultrasonication for 5 min to
remove the surface organics, rinsed in 1% HCl solution to
remove the surface oxide layer, and washed by deionized
water. After that, the nickel foam was placed in a 50 mL
Teflon-lined stainless-steel autoclave and the solution was
added, and the autoclave was sealed and maintained at 120 ºC
for 12h, and then allowed to cool to room temperature
naturally. The as-obtained foam electrode was rinsed with
distilled water and ethanol for several times and finally dried
under vacuum. By changing the Ni:Cu ratios of the added salt
precursors, pristine, 1%, 2% and 5% Cu-incorporated α-Ni(OH)2
hierarchical nanosheet arrays grown on Ni foam were
fabricated.
Results and discussion
In this work, copper-incorporated α-Ni(OH)2 nanoarrays with
1D-2D-3D hierarchical nanostructure grown on nickel foam
(Cu:α-Ni(OH)2/NF) were fabricated by hydrothermally reacting
soluble Cu/Ni salts with urea in the presence of NF (see
Experimental Section in Supporting Information). The phase
purity of the samples with variable Cu concentrations was
identified by X-ray diffraction (XRD) technique (Fig. 1A), where
the patterns are well consistent with the standard card of α-
Ni(OH)2 (i.e., Ni(OH)2·0.75H2O, JCPDS Card No. 38-0715).
Detailed analysis indicates that the interlayer spacing is slightly
decreased, which may stem from the partial substitution of the
intercalated water by carbonate ions derived from the
decomposition of urea. Low-resolution scanning electron
microscopy (SEM) images in Fig. S3 indicates the 3D conductive
skeleton of nickel foam, which could guarantee facile electron
transport and offer permeable micropores for ion diffusion
and gas release. Magnified SEM images of the pristine and
copper-incorporated α-Ni(OH)2 nanoarrays in Fig. 1B and Fig.
S2-5 further reveal the 2D nanosheet array morphology with
1D nanowires on the surface, therefore confirming the unique
1D-2D-3D hierarchical structure. Transmission electron
microscopy (TEM) was used to provide more detailed
morphological information of the 1%Cu:α-Ni(OH)2 hierarchical
nanosheet. As can be seen from Fig. 1C and Fig. S6, the
nanowires are roughly 10 nm in width and interconnected with
each other, and of note, most of the nanowires are aligned in
intersection angles of 60°, suggesting that the formation of the
surface nanowires may be directed by the hexagonal lattice of
Structural characterizations
X-ray diffraction (XRD) was performed on a Philips X’Pert Pro
Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). The
scanning electron microscopy (SEM) images were taken on a
JEOL JSM-6700F SEM. The transmission electron microscopy
(TEM) was carried out on a JEM-2100F field emission electron
microscope at an acceleration voltage of 200 kV. The high-
resolution TEM (HRTEM) was performed on a JEOL-2010
transmission electron microscope at an acceleration voltage of
200 kV. The atomic force microscopy (AFM) was conducted on
a Cypher VRS AFM equipment (Oxford Instruments) in tapping
mode by using an AC240TS probe at a scan rate of 1 Hz. The
nitrogen adsorption-desorption isotherms were carried out by
using a Micromeritics ASAP 2000 system, and all the gas
adsorption experiments were performed at liquid-nitrogen
temperature (77 K) after degassed at 200 °C for 6 h. A Perkin
Elmer Optima 7300DV ICP emission spectroscope was used for
the inductively coupled plasma optical emission spectrometry
(ICP-OES) analyses to identify the concentration of copper
dopant as Cu/(Cu+Ni)%. The high-angle annular dark-field
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the 2D α-Ni(OH)2 matrix. The selected area electron diffraction
(SAED) pattern of the dashed region (inset of Fig. 1C) clearly
identifies the single-crystalline nature of the Cu:α-Ni(OH)2
hierarchical nanosheet with hexagonal symmetry, therefore
suggesting the crystallographic attachment between the
nanowires and the nanosheet. The atomic force microscopy
(AFM) image and the corresponding height profile (Fig. 1D-E)
further confirm the hierarchical feature of the nanosheet with
uniformly distributed nanowires on the surface. Considering
the amplification effect of the AFM tip, the width of the
nanowires is slightly larger than 10 nm as determined by TEM,
while the height of the nanowires can be precisely identified to
be 2~4 nm, corresponding to 2~5 Ni(OH)2 layers, and the
thickness of the nanosheet is measured to be ~10 nm. Of note,
owing to the tip magnification effect in the AFM operation, the
width of the nanowires is slightly larger than the results
derived from TEM analysis. Besides, the crystallographic
continuity as well as the phase purity can be further verified by
the high-resolution TEM (HRTEM) image in Fig. 1F. As indicated,
the interplanar spacing of 0.26 nm can be indexed as (012)
facets of α-Ni(OH)2, and the lattice is continuous at the
interface between the nanowire and the nanosheet, further
confirming the single crystallinity of the hierarchical
nanosheets. Interestingly, the angles between the orientation
of the nanowires and the (012) facets are also 60°, further
proving the continuous lattice of the Cu:α-Ni(OH)2 hierarchical
nanosheets. To further investigate the structural benefits of
the hierarchical wire-on-sheet nanoarrays, the Brunauer-
Emmett-Teller (BET) specific surface areas of the pristine and
Cu-incorporated samples were identified from the nitrogen
adsorption-desorption isotherms (Fig. S8). As listed in Table S1,
the Cu-incorporated samples exhibit similar N2 adsorption-
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DOI: 10.1039/C9TA02891A
Fig. 1 (A) XRD patterns of the pristine and copper-incorporated
α-Ni(OH)2/NF. (B) SEM image of 1%Cu:α-Ni(OH)2/NF. (C) TEM
image of the 1%Cu:α-Ni(OH)2 nanosheet and the
corresponding SAED pattern of the dashed region reveal the
desorption behavior and comparable BET specific surface single-crystalline feature with 1D-2D wire-on-sheet
areas in the range of 5.319~6.542 m2 g-1, which can be
morphology. The main directions of the nanowires are labeled
attributed to the hierarchical wire-on-sheet morphology with
large surface area. In addition, the Barrett-Joyner-Halenda profile of an individual nanosheet. (F) HRTEM image of the
(BJH) pore size distribution data derived from the N2
desorption branch reveal the presence of multiple nanopores directions of two interconnected nanowires are labeled by
by black arrows. (D-E) AFM image and corresponding height
edge area of a hierarchical nanosheet where the growth
with diameter less than 15 nm, which is consistent with the
results from morphological analysis. The BJH pore volume
derived from the N2 desorption branch indicates that all the
tested samples possess large pore volume of 1.568×10-2 cm3 g-
dashed lines. (G-H) XPS spectra of Ni and Cu.
The inductively coupled plasma optical emission
spectrometry (ICP-OES) was performed to survey the
1, confirming the high porosity of the hierarchical wire-on- compositional information of the as-obtained nanoarray
catalysts. As can be seen from Fig. S9, the concentration of Cu
dopant is slightly lower than the added amount, and for
1%Cu:α-Ni(OH)2/NF, 0.65% Cu can be identified, which may
arise from the coordination between Cu2+ ions and NH3
derived from the decomposition of urea. X-ray photoelectron
spectroscopy (XPS) was conducted to further investigate the
valence information of the 1%Cu:α-Ni(OH)2 hierarchical
nanoarray catalyst. As can be seen from Fig. 1G, the binding
energy of Ni 2p3/2 and Ni 2p1/2 regions can be deconvoluted
into two peaks, respectively. The peaks centered at 855.9 eV
and 873.7 eV can be indexed to the Ni2+ ions, while the peaks
located at 857.1 eV and 874.9 eV correspond to the local Ni3+
sheet nanoarray catalysts, which is beneficial to the
electrocatalysis. The hierarchical nanoarray structure can not
only benefit the diffusion of ions in the electrolyte and the
release of gas bubbles during the electrochemical processes,
but also provide more Ni ions with unsaturated coordination
for HER catalysis and abundant reactive sites for the pre-
oxidation of Ni2+ ions to generate catalytically active high-
valence species for promoted UOR and OER.17, 21, 22
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species.23, 24 Besides, two broad peaks at 862.3 eV and 880.4 solution, while a wide range of KOH concentration with 0.33 M
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eV can be assigned to the satellite peaks, respectively. Hence, urea can result in efficient UOR activity. IDnOoIr: d10e.r10t3o9/mC9aTkAe0a28f9a1iAr
the presence of local Ni3+ ions is confirmed, which may comparison with previous advances, the electrolyte with 1 M
originate from the partial oxidation of Ni in the α-Ni(OH)2 as KOH and 0.33 M urea was selected for evaluating the UOR
well as the highly exposed edges with unsaturated activity. As demonstrated in the linear sweep voltammetry
coordination environment of Ni ions.25 Besides, the typical (LSV) curves in Fig. 2A, the α-Ni(OH)2/NF catalysts with 1% and
band at 933.8 eV of the Cu 2p spectrum matches well with the 2% Cu incorporation exhibit a low onset potential of 1.308 V vs.
Cu2+ ions, suggesting the atomic substitution of Cu2+ in the Ni RHE. This value is superior to the NiFe LDH nanosheet arrays
sites of the α-Ni(OH)2 lattice (Fig. 1H).26 Besides, the elemental with partial amorphization,24 the porous NiMoO4 nanosheets
mapping analyses equipped on SEM and TEM further identify with oxygen vacancies19 and the metallic β-Ni(OH)2
the homogeneous distribution of Cu on both the hierarchical nanosheets,20 indicating the efficient UOR activity of the Cu-
nanosheets and the nanoarrays (Fig. S10-11). Therefore, the incorporated α-Ni(OH)2 nanoarrays. Of note, the early UOR
Cu2+-incorporated α-Ni(OH)2 hierarchical nanoarray catalyst onset may arise from the local Ni3+ species that can oxidize the
with local Ni3+ species is identified, which may promote the urea molecules before the pre-oxidation process.24
electrocatalytic processes with the merits of the enriched Furthermore, the UOR activity of the 1%Cu:α-Ni(OH)2/NF
active sites as well as the optimized electron structure.
catalyst surpasses the pristine and Cu-incorporated
counterparts with higher doping concentrations. As listed in
Table 1, the 1%Cu:α-Ni(OH)2/NF requires only 1.405 V vs. RHE
to achieve a large UOR current of 100 mA cm-2, which is 22~54
mV lower than the other UOR catalysts, including the sub-
nanometer MnO2 nanocrystals,14 the NiMoO4 nanoarrays with
oxygen vacancies,19 the sulfur-incorporated metallic Ni(OH)2,20
and the partially amorphous NiFe LDH nanosheet arrays.24
Besides, the anodic current of the 1%Cu:α-Ni(OH)2/NF at 1.6 V
vs. RHE is as high as 355.8 mA cm-2, displaying 1.4~4.2 folds
enhancement than the others, therefore suggesting that 1%
Cu incorporation is optimal for UOR. Corresponding Tafel plots
in Fig. 2B reveal that all of the α-Ni(OH)2 nanoarray based
catalysts possess small Tafel slopes of 35.3~42.2 mV decade-1,
indicating the similar UOR reaction kinetics guaranteed by the
facile charge transport and enriched active sites. The
electrochemical impedance spectroscopy (EIS) data measured
at 1.3 V vs. RHE further confirm the significant role of Cu
incorporation in reducing the charge transfer resistance (Rct)
during UOR (Fig. 2C), and in optimal, 1%Cu:α-Ni(OH)2/NF
exhibits the smallest Rct value of 96 Ω, which may be
responsible for its enhanced UOR activity. In order to further
eliminate the influence of the surface area on the UOR
behavior, the electrochemical double-layer capacitance (Cdl), a
reliable criterion for estimating the electrochemical surface
area (ECSA), was investigated (Fig. 2D).27-31 The pristine and
Cu-incorporated α-Ni(OH)2 nanoarrays possess similarly high
Cdl values in the range of 1.18~1.48 mF cm-2, which can be
attributed to their unique 1D-2D-3D hierarchical morphology.
Subsequently, the UOR curves were normalized by the Cdl
Fig. 2 (A) Comparison of LSV curves of the pristine and Cu-
incorporated α-Ni(OH)2/NF. (B) Corresponding Tafel plots. (C)
EIS spectra measured at 1.3 V vs. RHE. (D) The estimation of Cdl.
(E) UOR curves normalized by Cdl. (F) Chronoamperometry
data of the 1%Cu:α-Ni(OH)2/NF at a constant potential of 1.6 V
vs. RHE. Inset: SEM image after UOR stability test. All the data
are acquired in the electrolyte with 1 M KOH and 0.33 M urea.
values to give a fair comparison on the intrinsic UOR activity
27-29
without the influence of ECSA.
As can be observed in Fig.
2E and Table 1, the 1%Cu:α-Ni(OH)2/NF catalyst holds a
highest Cdl-normalized current (jCdl) of 490.2 A F-1 at an applied
voltage of 1.8 V vs. RHE, which is 1.34~3.41 times higher than
its counterparts, therefore confirming the optimal Cu
concentration of 1% for UOR catalysis. The faradaic efficiencies
of UOR were calculated by measuring the amount of nitrogen
via GC-MS since another gaseous product (CO2) will partially
dissolve in the alkaline electrolyte. As shown in Fig. S16A, high
faradaic efficiencies over 98% can be resulted at lower
potentials (1.4-1.6 V vs. RHE). While at higher potentials, slight
A typical three-electrode setup was built to survey the
electrocatalytic performance of the hierarchical nanoarray
catalysts. As shown in Fig. S13, the optimal concentration of
urea for UOR was determined to be 0.33 M in 1 M KOH
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decrement (96.3% at 1.7 V vs. RHE and 92.1% at 1.8 V vs. RHE) 40h UOR operation, and a considerable enrichment of Ni3+ can
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can be observed. The slightly decreased faradaic efficiency at be identified from the XPS analyses, whDicOhI:i1s0.c10o3n9s/iCst9eTnA0t2w89it1hA
higher potential range may arise from the competitive OER the activation process of the catalyst and responsible for the
process, while even so, the faradaic efficiency still maintains enhanced UOR activity (Fig. S19A, S20A). Besides, CV stability
over 92% for UOR catalysis, demonstrating the high efficiency test is also satisfied after 2000 cycles (Fig. S21), further
of the hierarchical nanoarray catalyst. Fig. 2F displays the confirming the superior UOR stability. The good
chronoamperometry data measured under
a constant electrochemical stability may arise from the unique 1D-2D-3D
potential of 1.6 V vs. RHE. As can be seen, the UOR current hierarchical morphology which could provide enough room for
density shows a considerable degradation of 12% during the buffering the volume change during the electrochemical redox
first 20 hours, while after replacing the electrolyte with fresh reactions between Ni2+ and Ni3+/4+ 17, 18
and the corresponding
,
solution, the anodic current shows remarkable recovery and SEM image after 40h continuous UOR operation (inset of Fig.
results in only 2.6% degradation after 20h continuous UOR 2F) further confirms the negligible morphology change toward
operation. Of note, the Cu dopants are mainly reserved after long-term catalysis.
Table 1 Comparison table of the UOR performance.
Required potential
jgeo @1.6 V vs. RHE
[mA cm-2]
Tafel slope
[mV decade-1]
jCdl @1.8 V vs. RHE
[A F-1]
for 100 mA cm-2
[V vs. RHE]
1.644
1.453
1.405
1.427
1.446
-
bare NF
84.0
215.6
355.8
259.0
231.6
19
45.7
35.3
37.8
38.2
42.2
28
143.6
239.6
490.2
366.9
292.4
-
α-Ni(OH)2
1%Cu:α-Ni(OH)2/NF
2%Cu:α-Ni(OH)2/NF
5%Cu:α-Ni(OH)2/NF
7
Ni(OH)2
14
sub-nanometer MnO2
19
NiMoO4-VO
1.450
1.430
-
225
75
-
248
32.5
-
-
20
metallic Ni(OH)2
partially amorphous NiFe LDH24
23
-
1.459
146.6
33
221.2
Besides UOR, efficient HER and OER behaviors are also effectively boost the electron transfer and subsequently
important for assembling coupled electrolytic cells via HER- accelerate the HER process. As shown in the EIS spectra
UOR or HER-OER routes in the electrolyte with fluctuating urea measured at -0.2 V vs. RHE in Fig. 3D, the fitted Rct value
concentration and pH values. As can be seen from Fig. 3A, reduces from 105 Ω to 36 Ω after the stability test, thereby
after growing the nanoarrays on NF, the HER activity shows clearly confirming the promoted electron transfer by means of
considerable enhancement, and the HER performance exhibits the CV cycling. Besides, low-valence Cu0/+ species can be
negligible differences in 1 M KOH electrolyte with or without detected by XPS analysis for the catalyst after 2000 cycles (Fig.
the addition of urea (Fig. S23), guaranteeing its potential 3E), further proving the important role of Cu incorporation in
application in both aforementioned coupled routes. enhancing the HER activity toward long-term operation. The
Furthermore, Cu incorporation shows negligible influence on faradaic efficiencies of HER are also measured at different
the HER activity. All of the α-Ni(OH)2 nanoarray based catalysts potentials. As shown in Fig. S16B, the faradaic efficiencies
display considerable HER activity with required overpotentials maintain over 99% at a wide potential range (-0.2 to -0.5 V vs.
of roughly 210 mV for 10 mA cm-2, and the Tafel slopes for RHE), therefore indicating the high efficiency for HER catalysis
alkaline HER are in the range of 135.8~142.2 mV decade-1 (Fig. of the 1%Cu:α-Ni(OH)2/NF catalyst.
3B). CV cycling was conducted to further evaluate the
By using the O2-saturated 1 M KOH solution as the
performance variation of the 1%Cu:α-Ni(OH)2/NF catalyst electrolyte, OER performance was also investigated for the
toward long-term operation. As shown in Fig. 3C, the HER nanoarray based catalysts. As shown in Fig. 3F, all of the α-
activity increases drastically after 1000 cycles and maintains Ni(OH)2 hierarchical nanoarrays exhibit a broad peak centered
highly active even after 2000 cycles. In detail, the required in the range of 1.50~1.53 V vs. RHE, which corresponds to the
potentials for 10 and 100 mA cm-2 exhibit remarkable positive electrooxidation of α-Ni(OH)2 to generate the catalytically
shifts of 105 and 114 mV after 1000 cycles and the HER activity active γ-NiOOH phase. The significantly enhanced
appears no degradation after 2000 cycles. Furthermore, the electrooxidation behavior of the nanoarrays can be attributed
corresponding Tafel slope also shows obvious decrement from to the unique hierarchical nanostructure which could expose
136 mV decade-2 for the fresh catalyst to 112 mV decade-2 abundant low-coordinated edge sites for the Ni2+-to-Ni3+
after 2000 cycles (Fig. 3B), demonstrating the promoted HER reaction.17, 18 And further to be noted, the 1%Cu:α-Ni(OH)2/NF
rate from the kinetic viewpoint. We attribute the obvious HER catalyst exhibits the largest peak area with a high peak current
enhancement during CV operation to the partial reduction of of 202.4 mA cm-2, which may stem from the optimal
Cu2+ to generate highly conductive Cu0 species which could concentration of Cu dopant in tuning the electron structure of
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α-Ni(OH)2. The remarkable Ni2+-to-Ni3+ oxidation peak
Fig. 3 (A) LSV curves of HER measured in H2-saturated 1 M KOH
View Article Online
DOI: 10.1039/C9TA02891A
solution. (B) Corresponding Tafel plots. (C) HER stability test of
indicates the fast and substantial generation of the high-
valence species for promoted OER.17,
18
For instance, the
the 1%Cu:α-Ni(OH)2/NF. (D) EIS spectra before and after 40h
HER operation. (E) XPS spectrum of Cu after 2000 CV cycles
indicates the presence of low-valence Cu species. (F) LSV
curves for the estimation of OER activity obtained in O2-purged
1M KOH solution. (G) Chronoamperometry data of the
1%Cu:α-Ni(OH)2/NF under a fixed potential of 1.7 V vs. RHE.
The inset shows the stability test by CV cycling.
anodic current density of the 1%Cu:α-Ni(OH)2/NF catalyst
reaches 283 mA cm-2 at 1.8 V vs. RHE, which is higher than the
pristine and Cu-incorporated counterparts with higher Cu
concentration (191~263 mA cm-2). The faradaic efficiencies for
OER catalysis are readily affected by the Ni2+-to-Ni3+ oxidation
reaction at lower potentials (1.5-1.6 V vs. RHE), while at higher
potentials (1.7-1.8 V vs. RHE) beyond the pre-oxidation process,
the faradaic efficiency increases to over 99%, demonstrating
the high efficiency in oxygen evolution (Fig. S16C). The
electrochemical stability of OER was further evaluated by
means of chronoamperometry and CV measurements. As
shown in Fig. 3G, the current density displays 88% retention
after 40h continuous OER operation at 1.7 V vs. RHE. Besides,
the OER activity exhibits remarkable enhancement after 1000
cycles and remains highly active even after 2000 cycles (inset
of Fig. 3G), which could be attributed to the activation effect of
Ni-based OER catalysts.17, 18 This activation process can be
further proved by the XPS analysis of the catalyst after long-
term OER operation where a remarkable enrichment of Ni3+
ions can be identified (Fig. S19C). Therefore, the trifunctional
electrocatalyst with efficient UOR, HER and OER activity is
identified, which provides the opportunity for us to realize
coupled urea electrolysis and overall water splitting.
In order to verify our hypothesis on the coupled urea
electrolysis and overall water splitting, two-electrode setup
was constructed by using the same nanoarray catalysts as the
electrodes. Fig. 4A reveals the polarization curves of coupled
HER-UOR measured in the electrolyte containing 1 M KOH and
0.33 M urea, from which an early onset voltage of 1.380 V can
be identified for the 1%Cu:α-Ni(OH)2/NF catalyst. Moreover,
the current density at 1.8 V reaches 101 mA cm-2 for the
1%Cu:α-Ni(OH)2/NF, which is 1.4~1.8 times larger than the
other nanoarray catalysts. The enlarged plot of the onset
region in the inset of Fig. 4A clearly demonstrates that the
1%Cu:α-Ni(OH)2/NF catalyst requires an ultralow voltage of
1.49 V for a current density of 10 mA cm-2. Similarly, the
1%Cu:α-Ni(OH)2/NF catalyst is also prominent for overall water
splitting in O2-purged 1 M KOH electrolyte. As shown in Fig. 4B,
the cell voltage for a 50 mA cm-2 current is determined to be
1.675 V for the 1%Cu:α-Ni(OH)2/NF, which is 63~268 mV lower
than the pristine and Cu-incorporated α-Ni(OH)2 nanoarray
catalysts with higher Cu concentration, indicating its high
activity for overall water splitting. It is noteworthy that
although the performance of overall water splitting is good,
the coupled HER-UOR activity is more prominent owing to the
more efficient route for hydrogen generation via coupled UOR
process in principle.3-5 Besides, CV test was performed for the
1%Cu:α-Ni(OH)2/NF catalyst in the electrolyte with 1 M KOH
and 0.33 M urea to further evaluate the operational durability
for the coupled HER-UOR electrolytic cell. As shown in Fig. 4C,
the current density at 1.8 V exhibits considerable increment
during the first 1000 cycles (~5% increment) and results in
slight degradation of only 3% after 2000 cycles, indicating the
superior stability of the 1%Cu:α-Ni(OH)2/NF catalyst for urea
electrolysis. Besides, the current variation at a constant cell
voltage of 1.6 V also reveals the stable hydrogen generation
via the coupled HER-UOR route for over 40 hours (Fig. 4D). The
efficient and stable electrocatalytic behavior of the 1%Cu:α-
Ni(OH)2/NF catalyst may arise from its unique 1D-2D-3D
hierarchical structure with optimal Cu incorporation which
could not only provide abundant reactive sites for the
generation of active species, but also endow it with robust
skeleton for avoiding the structural collapse during the
electrooxidation reactions. As can be seen from the SEM
images in the insets of Fig. 4D, the unique hierarchical
morphology can still be revealed after 40h continuous overall
urea electrolysis, making the self-supported nanoarray catalyst
a promising candidate for the industrial wastewater treatment
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ARTICLE
and clean energy production via coupled urea/water production and pollution treatment from inVdieuwstArrtiiaclle Oannlinde
DOI: 10.1039/C9TA02891A
electrolysis.
sanitary wastewater.
Conclusions
In summary, hierarchical copper-incorporated α-Ni(OH)2
nanoarray catalyst was explored which exhibits robust
electrocatalytic performance towards urea oxidation,
hydrogen evolution and oxygen evolution. The unique 1D-2D-
3D hierarchical structure not only provides abundant reactive
sites for the generation of catalytically active species, but also
guarantees robust and conductive skeleton for facile charge
transport and offers effective buffering space for the Ni2+-to-
Ni3+ pre-oxidation process. Besides, the presence of local Ni3+
ions and the optimal Cu incorporation further promote the
electrooxidation behaviors such as UOR and OER, and the in-
situ reduction of Cu2+ during HER operation effectively boosts
the electron transfer and leads to the enhanced HER activity,
therefore synergistically resulting in efficient and stable
trifunctional electrocatalysis. With the combined benefits of
the large surface area, promoted charge transfer, robust
structural stability and multi-functionality, the 1%Cu:α-
Ni(OH)2/NF hierarchical nanoarray catalyst displays excellent
performance in coupled urea electrolysis and overall water
splitting, which provides a promising material candidate for
industrial wastewater treatment and clean energy production.
Fig. 4 (A-B) Polarization curves of the coupled HER-UOR and
HER-OER, respectively. (C) Polarization curves of coupled HER-
UOR obtained after specific CV cycles. The inset is the as-
assembled urea electrolytic cell. (D) Chronoamperometry
curve of the 1%Cu:α-Ni(OH)2/NF in 1 M KOH and 0.33 M urea
at a bias of 1.6 V. The insets are SEM images of the two
electrodes after stability test.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Natural Science Foundation of
Shandong Province (ZR2018JL009 and ZR2016EMQ02), the
National Natural Science Foundation of China (21501112,
21535004 and 91753111) and the Key Research and
Development Program of Shandong Province (2018YFJH0502).
Notes and references
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