Nickel incorporated Co9S8 nanosheet arrays on carbon cloth boosting overall urea electrolysis

Article abstract of DOI:10.1016/j.electacta.2020.135883

Hydrogen production via water electrolysis is deemed to be a viable strategy to exploit renewable energy and relieve the crisis of energy and environment. Compared with water electrolysis, the introduction of urea not only improves the energy conversion efficiency of urea electrolysis but also makes this process apply in the treatment of sewage treatment. Herein, we highlight a novel bifunctional electrocatalyst of Co₉S₈ nanosheet arrays grown on carbon cloth with optimal nickel incorporation. Benefited from the incorporation of Ni, the electronic structure of Co₉S₈ is optimized, resulting in the enhanced electrical conductivity and charge transfer efficiency. The structure of nanosheets endows the 2D charge transfer pathway and exposes more active sites to achieve the robust electrocatalytic activity for overall urea electrolysis simultaneously. The optimal nickel incorporated Co₉S₈/carbon cloth catalysts display remarkable catalytic activity. It delivers the current density of 10 mA cm^?2 with the potential of 1.28 V vs. RHE for urea oxidation reaction (UOR), almost rivalling the state-of-the-art UOR catalysts. Overall urea splitting based on Ni–Co₉S₈/carbon cloth as bifunctional catalyst is also carried out and only a potential of 1.52 V is required to achieve 10 mA cm^?2.

Full text of DOI:10.1016/j.electacta.2020.135883

Journal Pre-proof
Nickel incorporated Co S nanosheet arrays on carbon cloth boosting overall urea
9 8
electrolysis
Pin Hao, Wenqian Zhu, Liyi Li, Jian Tian, Junfeng Xie, Fengcai Lei, Guanwei Cui,
Yongqi Zhang, Bo Tang
PII:
S0013-4686(20)30275-9
DOI:
https://doi.org/10.1016/j.electacta.2020.135883
EA 135883
Reference:
To appear in: Electrochimica Acta
Received Date: 26 April 2019
Revised Date: 24 December 2019
Accepted Date: 10 February 2020
Please cite this article as: P. Hao, W. Zhu, L. Li, J. Tian, J. Xie, F. Lei, G. Cui, Y. Zhang, B. Tang, Nickel
incorporated Co S nanosheet arrays on carbon cloth boosting overall urea electrolysis, Electrochimica
9 8
Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135883.
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Nickel incorporated Co9S8 nanosheet arrays on carbon cloth boosting
overall urea electrolysis
Pin Hao*‡a, Wenqian Zhu‡a, Liyi Lib, Jian Tianc, Junfeng Xiea, Fengcai Leia, Guanwei Cuia,
Yongqi Zhang*d and Bo Tang*a
a
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of
Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and
Nano Probes, Ministry of Education, Shandong Provincial Key Laboratory of Clean Production of Fine
Chemicals, Shandong Normal University, Jinan 250014, P. R. China.
b Intel Corporation, Hillsboro, OR 97124, United State.
c
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao
266590, P. R. China.
d
Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China,
Chengdu 611731, P. R. China.
‡ These authors contributed equally to this work
*Corresponding Author
E-mail: tangb@sdnu.edu.cn; haopin@sdnu.edu.cn; yqzhang@uestc.edu.cn
ABSTRACT: Hydrogen production via water electrolysis is deemed to be a viable strategy
to exploit renewable energy and relieve the crisis of energy and environment. Compared with
water electrolysis, the introduction of urea not only improves the energy conversion
efficiency of urea electrolysis but also makes this process apply in the treatment of sewage
treatment. Herein, we highlight a novel bifunctional electrocatalyst of Co9S8 nanosheet arrays
1

grown on carbon cloth with optimal nickel incorporation. Benefited from the incorporation of
Ni, the electronic structure of Co9S8 is optimized, resulting in the enhanced electrical
conductivity and charge transfer efficiency. The structure of nanosheets endows the 2D
charge transfer pathway and exposes more active sites to achieve the robust electrocatalytic
activity for overall urea electrolysis simultaneously. The optimal nickel incorporated
Co9S8/carbon cloth catalysts display remarkable catalytic activity. It delivers the current
density of 10 mA cm-2 with the potential of 1.28 V vs. RHE for urea oxidation reaction
(UOR), almost rivalling the state-of-the-art UOR catalysts. Overall urea splitting based on
Ni-Co9S8/carbon cloth as bifunctional catalyst is also carried out and only a potential of 1.52
V is required to achieve 10 mA cm-2.
KEYWORDS: Bifunctional electrocatalyst; Urea electrolysis; cobalt sulfide; Ni
incorporation; electronic structure modulation.
1. Introduction
Energy crisis and environmental pollution are two major issues that the international
community has to face. Hydrogen is considered as an ideal energy carrier to exploit and
utilize renewable energy via water electrolysis due to its high energy density.[1-3] However,
during the process of water splitting, a considerable high cell potential is indispensable to
overcome the large energy barriers and slow kinetics of both the half reactions.[4-8]
Fortunately, the high overpotential could be reduced via replacing H2O with more easily
oxidized small molecules, such as urea and methyl alcohol. It is noted that the replacement of
2

anodic reaction from oxygen evolution reaction (OER) to urea oxidation reaction (UOR) in
water electrolysis can shift the thermodynamic equilibrium potential from 1.23 to 0.37 V vs.
revisable hydrogen electrode (RHE).[4] Hence, UOR is a more efficient process, in theory,
than OER to realize water electrolysis for the production of hydrogen. In addition, it will kill
two birds with one stone to apply urea for hydrogen generation since urea is a common
pollutant in industrial and sanitary wastewater.[9, 10] During this process, urea is
decomposed into N2, CO2 and H2O at anode, the hydrogen are generated at cathode through
hydrogen evolution reaction (HER), simultaneously realizing hydrogen production and
urea-rich wastewater purification.[4, 10-12] Unfortunately, the sluggish six electrons transfer
process (CO(NH2)2+6OH-→N2+5H2O+CO2+6e-) at anode leads to a high overpotential to
promote the UOR. To improve the reaction rate and minimize the high overpotential, efficient
catalysts are in dire needed. [13, 14] In addition, catalysts are also necessary to accelerate
HER at cathode. Although Pt-based compounds are the superior electrocatalysts to reduce the
overpotential of HER, their expensive price and rarity badly limit the widespread application
in hydrogen generation.[15] Thus, it is imperative to exploit highly active electrocatalysts
with low-cost for UOR and HER. Moreover, integrating HER and UOR catalysts into a single
material as bifunctional catalysts is rather attractive to facilitate the design of electrolyzer and
decrease the energy consumption.
Transition metals, and their compounds (carbides, nitrides, oxides, sulfides, phosphides,
etc.) have been regarded as a promising candidate for water electrolysis.[16-19] In particular,
metal sulfides have been extensively studied owing their noble-metal-like catalytic
properties.[20-23] For example, cobalt sulfide-based materials, including CoS2 and Co3S4, et
3

al, have been applied in water splitting as efficient HER catalysts.[24-28] However, it is
reluctant for single metal-based materials to meet the requirement as adequate bifunctional
catalysts for UOR and HER. Recently, bi-, tri-metallic compounds have been demonstrated
obvious improved bifunctional catalytic activity compared with corresponding single metal
materials owing to the optimizing electronic structure. For example, NiCo2O4 [29], NiCoP
[30] and NiFeN [31, 32], have been systematically studied as highly active electrocatalysts
for both HER and OER. Ternary NiCoFe-based materials as bifunctional catalysts for OER
and ORR also have been studied.[33] Till now, bifunctional catalysts for UOR and HER
based on the bi-, tri- metallic compounds, especially sulfides, have rarely been reported.
Herein, for the first time, we fabricated a series of hierarchical nickel-incorporated Co9S8
nanosheet arrays on modified carbon cloth (CC) as bifunctional catalysts for electrocatalytic
overall urea electrolysis through a carefully controlled electro-deposition method and
followed by a sulfuration process (Figure 1). The vertically aligned nanosheet arrays grown
on the conductive CC directly can expose more active sites and facilitate the transfer of row
materials and final products. With the merits of the incorporation of Ni, the UOR and HER
catalytic activity of Co9S8 are both improved because of the modulated electronic structure
and the synergism of Ni and Co. It only takes 1.52 V to reach 10 mA cm-2 for the optimized
Ni incorporated Co9S8 as bifunctional catalysts during overall urea electrolysis.
2. Experimental Section
2.1 Modification of the carbon cloth
In a typical procedure, the pristine carbon cloths (1×4 cm2) were immersed in the
concentrated HNO3 for 30 min, and washed with 20 min ultrasonics in DI water. The above
4

carbon cloths were immersed into a 40 mL Ni(NO3)2·6H2O (0.25 M) and hexamine (0.5 M)
aqueous solution and then poured into the Teflon-lined stainless steel autoclave (50 ml), and
o
heated at 120 C for 10 h. Subsequently, carbon cloths were rinsed with DI water. After
o
drying, they were calcined at 900 C for 1.5 h in the atmosphere of N2. At last, the obtained
carbon cloths were immersed into the concentrated HCl for 12 h, and rinsed in DI water until
the pH value was equal to 7 to obtain the etched porous carbon cloths (represented as CC).
2.2 Synthesis of Ni-Co precursor on CC (Ni-Co/CC)
The Ni-Co/CC precursors were synthesized through electro-deposition process in a 100 ml
aqueous electrolyte involving 3 mM NiCl2·6H2O and 6 mM CoCl2·6H2O, which was
conducted at -1.0 V (vs SCE) for 10 min using the chronoamperometry method in a
three-electrode set-up on the CHI660E electrochemical workstation. The reference and the
counter electrode was calomel electrode and platinum plate, respectively. CC (1×4 cm2) was
employed as the working electrode with the surface area of 2 cm2 (1×2 cm2) in the
electrolyte. Then the Ni-Co/CC precursors were rinsed in distilled water several times.
2.3 Synthesis of Ni incorporated Co9S8 nanosheets on CC (NCS/CC).
One piece of Ni-Co/CC precursor was immersed into a 40 mL thioacetamide (TAA) (0.2
M) aqueous solution, and then poured into the Teflon-lined stainless-steel autoclave (50 ml)
o
and kept at 130 C for 5 h. Followed washing and drying, the obtained sample of Ni
incorporated Co9S8 nanosheets on CC was labeled as NCS/CC 1:2. To systematically research
the effect of Ni incorporation on the electrocatalytic behaviors, different molar ratios of Ni
and Co (3 mM NiCl2·6H2O and 3 mM CoCl2·6H2O, 3 mM NiCl2·6H2O and 9 mM
CoCl2·6H2O) were also studied, named as NCS/CC 1:1 and NCS/CC 1:3, respectively. For
5

comparison, pure Co9S8 nanosheets grown on CC were synthesized through the same method
without the addition of NiCl2·6H2O, denoted as CS/CC.
2.4 Structural characterization
The microstructure of catalysts was observed by Field-emission scanning electron
microscope (FE-SEM, Hitachi SU8010A) and high-resolution transmission electron
microscopy (HRTEM, JEOL JEM2100F). Electron microprobe analysis (EPMA-1720H,
Shimadzu) and energy dispersive X-ray spectroscopy (EDX) were employed to test the
element distribution. The compositional information of the samples was recorded by X-ray
diffraction (XRD) using a Bruker D8 Advance powder X-ray diffractometer and X-ray
photoelectron spectroscopy (XPS, Thermo Scientific ESCLAB250Xi). The molar ratios of Ni
and Co were confirmed by a Perkin Elmer Optima 7300DV inductively coupled plasma (ICP)
emission spectroscope. The atomic force microscopy (AFM) was conducted on a Cypher
VRS AFM equipment (Oxford Instruments) to measure the thickness of nanosheets.
2.5 Electrochemical measurements
The three-electrode configuration was assembled in electrochemical measurements on a
CHI660E electrochemical workstation. The obtained potentials were converted to RHE using
the equation: Evs RHE = Evs Hg/HgO + Eo
+ 0.059pH. Without further specification, all
Hg/HgO
potentials are referred to RHE. For HER test, the as-prepared catalysts were employed as the
working electrode directly without binder, a Hg/HgO electrode was used as the reference
electrode and a graphite rod was the counter electrode. The test device for UOR and OER
was similar to that of HER, except for a platinum gauze (2×2 cm2, 60 mesh) using as the
counter electrode.
6

The aqueous solution of 1 M KOH (pH=13.6) was employed as the electrolyte for the
HER, OER and water splitting. The mixed aqueous solution of 1 M KOH with 0.33 M urea
was employed for the UOR and urea electrolysis. The overall urea and water electrolysis
based on the as-prepared catalysts were tested using the two-electrode system. The distance
between anode and cathode was kept at 2 cm. Electrochemical impedance spectroscopy (EIS)
curves were tested at 1.4 V under different frequencies from 0.01 Hz to 100 kHz. All linear
sweep voltammetry (LSV) curves were obtained under the scan rate of 2 mV s-1. The stability
was recorded through the chronopotentiometric method and long-term cyclic voltammetry
test.
3. Results and Discussion
For preparing Ni incorporated Co9S8 nanosheets, carbon cloths were etched in advance.
SEM images of carbon cloths in Figure S1a and b show that the glossily surface of carbon
fiber became coarse and porous after etching, which can offer larger area for
electrodepositing metal precursors to supply more active sites and store more electrolyte.
XPS results demonstrate that Ni has been completely removed (Table S1). The Ni-Co
precursors are then electrochemical deposited on CC. After the deposition, the coarse
surface of etched carbon fibers are covered uniformly by vertical aligned Ni-Co precursor
nanosheets (Figure S1c-f). Following the hydrothermal sulfuration process, Ni-Co
precursors transform into Co9S8 nanosheets with the incorporation of Ni into the Co9S8 as
the host. After the sulfuration, the nanosheet morphology is preserved very well as shown in
Figure 2a-c and Figure S2, excepting an increasement of the thickness of nanosheets from
30 to 60 nm because of the Kirkendall effect.[34] In the process of sulfuration, the diffusion
7

rate between S2− and metal ions is different. Due to the larger size of sulfide ions, the
outward diffusion rate of metal ions is faster, resulting in the larger thickness than the
precursor. It is also found in the counterparts (Figure S3). Figure S4 is the CV curves of
ferri/ferrocyanide solution for CC and NCS/CC 1:2. We can see that the potential difference
of Fe2+/Fe3+ redox peaks for CC is 150 mV, while this value for NCS/CC 1:2 is only 80 mV
with the increased current density, indicating the good permeability of the prepared catalyst.
The space between the nanosheets can facilitate the transfer of raw materials (H2O, Urea)
and the final gas products (H2, O2, CO2 and N2), and the vertical aligned nanosheets can
expose more active sites for the catalytic reaction, further enhancing the catalytic behavior.
Detailed structure of NCS/CC 1:2 was characterized by HRTEM. As shown in Figure 2d, we
can obviously find the lattice fringes, indicating the high crystallinity degree. The lattice
distance of the nanosheets is 0.35 nm, consistent with the (220) facet of Co9S8 (cubic,
a=b=c=0.99 nm). The derived Fourier transform (FFT) pattern inset in Figure 2d further
displays the diffraction points of (220) in Co9S8. The distribution of Ni and Co9S8 can be
confirmed by the EDX analysis, and the corresponding element mappings are shown in
Figure 2e. The hot spots of Ni scatter on the nanosheet uniformly, indicating the successful
incorporation of Ni in Co9S8 nanosheets. In addition, the EPMA element mappings were
measured to further identify the element distribution of this sample (Figure S5). The dense
hot spots of Ni on the CC also reveal the successfully incorporation of Ni in Co9S8
backbones.
XRD patterns were employed to confirm the phase of the as-prepared catalysts. The
diffraction peaks of samples with and without Ni incorporation are well indexed to the phase
8

of Co9S8 (Figure 3a, JCPDS Card No. 02-1459) and carbon (JCPDS Card No. 41-1478). No
peaks of NiS or Ni3S2 is observed in the patterns, indicating that Ni substitutes partial Co
atoms in Co9S8 crystal structure to form Ni incorporated Co9S8. In the XPS spectra of Co
(Figure 3b), the peaks centered around 778.9 and 793.9 eV are ascribed to the Co 2p3/2 and
Co 2p1/2 derived from cobalt sulfides.[35] The binding energies of 782.1 and 798.2 eV are
originated from the Co-O bond, resulted from the oxidation of the sample in air.[35] The
broad shoulders located at 785.8 and 802.9 eV are the shakeup satellites of Co2+.[36] In
comparison with CS/CC, the peaks of Co 2p3/2 and Co 2p1/2 in NSC/CC 1:2 at around 779.4
and 794.4 eV display a positive shift, due to the change in electronic structure caused by the
implantation of Ni into Co9S8. The two binding energies at 163.2 and 164.5 eV in the XPS
spectrum of S can be assigned to S2− and the sulfur ions in low coordination, respectively
(Figure 3c).[37] The existence of Ni could be confirmed in Figure 3d. Two spin-orbit
doublets can be fitted in the spectrum of Ni (Ni2p1/2 and Ni2p3/2), characteristic of Ni in di-
and trivalence, and two shakeup satellites.[4] The Ni in trivalence in samples may be due to
the higher oxidation level of Ni ions caused by the higher synthesis temperature.[4] The
molar ratios of Ni and Co in the catalysts derived from XPS analysis are illustrated in Table
S2. The actual molar ratio of Ni and Co in NCS/CC 1:2 is about 1:2.56, which is also
confirmed by ICP (Table S3).
In order to confirm the effect of Ni incorporation on their catalytic activiy, all samples are
investigated using the three-elecrode set-up in 1 M KOH with 0.33 M urea. LSV curves for
UOR and OER are displayed and compared in Figure 4a and Figure S6. With urea in the
KOH eletrolyte, the current density increased dramatically, indicating a superior catalytic
9

behavior towards urea oxidation. Moreover, NCS/CC 1:2 displays the highest current density
at all range of working potential among all catalysts, suggesting a more sensitive response to
UOR. NCS/CC 1:2 achieves the high current of 100 mA cm-2 at 1.43 V vs. RHE, almost
about 9-folds higher than that of CS/CC, revealing the synergistic effect of Ni and Co (Figure
4a). More importantly, the potential at 10 mA cm-2 for NCS/CC 1:2 is 1.28 V, which is
remarkably lower than that of pristine CS/CC (1.42 V) and other previously reported catalysts
(1.33-1.45 V).[4, 19, 38] As we all know, electrochemical surface area (ECSA) is a
significant factor that can affect the overall catalytic performance. Since electrochemical
double-layer capacitance (Cdl) is proportional to ECSA, Cdl values were measured by CV
curves vs. the scan rate at non-redox region (Figure S7). As shown in Figure 4b, the catalysts
without and with various Ni/Co ratios display different values of ECSA (36.3-49.6 mF cm-2),
indicating that different amounts of active sites exposed in the electrolyte. Especially for
NCS/CC 1:2, the Cdl value is 49.6 mF cm-2, indicating that it has the highest electrochemical
surface area and can expose more active sites for UOR, further contributing to the superior
catalytic activity to other counterparts. To study the intrinsic catalytic activity of all samples,
the LSV curves in Figure 4a are normalized by corresponding Cdl, and shown in Figure 4c.
NCS/CC 1:2 still exhibits superior catalytic activity to other samples despite of the effect of
ECSA, revealing the active role of Ni on the electrocatalytic properties. The Cdl values of Ni
incorporated Co9S8 nanosheets grown on the etched carbon cloth and the pristine carbon cloth
are also compared. From Figure S8, we can see Ni incorporated Co9S8 nanosheets grown on
the etched carbon cloth display a higher Cdl value (49.6 mF cm-2) than that grown on the
pristine carbon cloth (40.4 mF cm-2), indicating that the carbon cloth after etching treatment
10

can offer larger specific surface area for electrodepositing metal precursors. And the enlarged
specific surface area boosts the overall catalytic activity of samples as a result of the better
exposed active sites.
Table 1. The values of Tafel slope, Rs and Rct of samples.
Tafel slope /
mV per decade
Rs / ꢀ
Rct / ꢀ
CS/CC
185
4.57
4.92
2.85
3.01
17.2
4.98
2.86
4.54
NCS/CC 1:1
NCS/CC 1:2
NCS/CC 1:3
99
56
98
The kinetics during UOR is further studied through evaluating the Tafel plots, which are
shown in Figure 4d and Table 1. The Tafel slope of NCS/CC 1:2 is 56 mV per decade, which
is much lower than the value without urea (115 mV per decade). In addition, this Tafel slope
of NCS/CC 1:2 is obvious smaller than other counterparts, especially to the pristine sample of
CS/CC (185 mV per decade), indicating the fast reaction kinetics for urea oxidation.
Electrochemical impedance spectroscopy (EIS) was conducted to get further comprehension
for the UOR dynamics at the interface between electrode and electrolyte (Figure 4e). The
intersection point of the Nyquist plots and the X-axis represents the series resistance (Rs)
which includes not only the solution resistance, but also the collector resistance, the
packaging contact resistance as well as the resistance of the active materials. And the
diameter of the semicircle in plots stands for the charge transfer resistance (Rct).[39-41] The
Nyquist plots can reflect the changes in electronic structure of Co9S8. After Ni incorporation,
the diameter of the semicircle changed obviously. The fitting Rct values of Co9S8 with Ni
11

incorporation are much smaller than that of pristine Co9S8, suggesting an improved electrical
conductivity and an efficient charge transfer during UOR, which further reveals that the
electronic structure of Co9S8 was modulated after Ni atoms substituting for partial Co atoms
in the crystal structure. The difference of Ni/Co concentration will result in the different
amounts of Ni into the crystal structure of Co9S8. In this case, the degree of modulation on
the electronic structure is different. Thus, the catalysts with different Ni/Co concentrations
display varied Rct values, contributing to the different Tafel slopes, current densities and
voltages, and so on. NCS/CC 1:2 displays the smallest value of Rct, which is fitted to be 2.86
ꢀ, contributing to the superior catalytic activity to other counterparts. The UOR activity of
NCS/CC 1:2 are compared with reported UOR catalysts and shown in Figure 4f.[4, 15, 19,
21, 22, 38,42-48] Clearly, the onset potential of NCS/CC 1:2 surpasses that of the best
reported catalysts. The UOR performance of NCS/CC 1:2 in 1 M KOH with different urea
concentrations and in 0.33 M urea solution with different concentrations of KOH is studied
and demonstrated in Figure S9. 1 M KOH and 0.33 M urea can be determined as the optimal
concentration in the electrolyte to accomplish the superior catalytic activity.
Stability is another important parameter to evaluate a catalyst. Chronoamperometry test,
performed at 1.43 V, shows the excellent durability of NCS/CC 1:2 with a slight decay of
3.7% as well as no obvious collapse of the structure after 20 h chronoamperometry test
(Figure 5a). Long-term CV curves in Figure 5b display an obvious increment of peak current
density from 181.4 to 292.4 mA cm-2 from the 1st to 5000th cycle, indicating an apparent
activation process in the initial 5000 cycles. At the beginning of the test, the oxidation peak of
urea was not obvious because the carbon cloth cannot be entirely wetted in the electrolyte,
12

resulting in only a bulge on the LSV curve. With the increase of the CV cycle number, more
and more urea and electrolyte ions participate in the electrocatalytic urea oxidation reaction,
resulting in the obvious oxidation peak and higher peak current density with the synergy of
growing number of catalytic active Ni3+. Only a weak attenuation of current density can be
observed after 6000 cycles due to the aging of the material itself and the consumption of urea
and electrolyte ions, further confirming the superior catalytic behavior and excellent
durability for constant urea oxidation test.
As a bifunctional catalyst, the superior HER performance is also important. As depicted in
Figure 6a, the NCS/CC 1:2 catalyst presents excellent catalytic performance for HER
compared with the other counterparts, confirming the HER activity of Co9S8 can be greatly
enhanced by the incorporation of Ni. Only an overpotential of 295 mV is required to achieve
the HER current of 100 mA cm-2. More importantly, there is only a slight positive shift of the
polarization curve (20 mV at 100 mA cm-2) after introducing 0.33 M urea, indicating the
negligible influence of urea on the HER activity (Figure 6b). The normalized current
densities of the samples are shown in Figure S10. Similar with the results of UOR, the
catalyst of NCS/CC 1:2 displays the highest intrinsic catalytic behavior, further indicating the
significant role of Ni on the electrocatalytic performances. By introducing Ni, the local
electronic structure of Co9S8 can be optimized effectively, contributing to the enhanced
electrical conductivity and efficient charge transfer based on the evidence of EIS results.
Moreover, the synergistic effect of Ni and Co can also boost the catalytic behaviors of HER
and UOR. In addition, the high Cdl values of samples reveal that the catalysts with two
dimensional (2D) nanosheets structure can ensure good exposure of active sites and promote
13

the transfer of raw materials and final gas products, synergistically contributing to the robust
electrocatalytic behavior.
Based on the above results, the overall urea electrolysis was conducted in a two-electrode
electrolytic cell using NCS/CC 1:2 as both the anode and the cathode. From Figure 6c we can
see that the NCS/CC 1:2//NCS/CC 1:2 system could deliver a current of 10 mA·cm-2 at 1.52
V in 1.0 M KOH with 0.33 M urea with robust gas bubbles at both electrodes. In contrast, it
takes 1.81 V to reach 10 mA·cm-2 in 1 M KOH, demonstrating a superior catalytic efficiency
of the urea electrolyzer system. This small potential of 1.52 V at 10 mA·cm-2 surpasses other
reported urea electrolyzers, confirming the robust catalytic activity of NCS/CC 1:2 for urea
electrolysis (Figure 6d).[9, 47, 49-51] Other urea electrolyzers based on pristine Co9S8 and
Co9S8 with different Ni:Co ratios were also established. As shown in Figure 6e, the NCS/CC
1:2//NCS/CC 1:2 system exhibits the superior current response to other electrolyzers,
suggesting that the optimal additive molar ratio of Ni2+ and Co2+ is 1:2 for urea electrolysis.
Stability is also important in the real application. A high current retention of 93.6% can be
reached after 20 h continuous catalytic operation (Figure 6f). The excellent catalytic activity
as well as the long-term durability indicates that the Ni incorporated cobalt sulfide is an
effective and low-cost bifunctional catalyst for feasible urea electrolysis and hydrogen
generation.
4. Conclusions
In summary, we have prepared a new type of bifunctional electrocatalysts based on Ni
incorporated Co9S8/CC through a carefully controlled electro-deposition method and
followed by a sulfuration process for the overall urea electrolysis. The Ni incorporated
14

nanosheet arrays not only endow the enhanced electrical conductivity and charge transfer
efficiency, but also represent the synergism of metal cations and guarantee the 2D charge
transfer pathway and the more exposed active sites to enhance the electrocatalytic
performance. The optimal nickel incorporated catalyst (NCS/CC 1:2) displays outstanding
electrocatalytic behavior and stability for the urea oxidation and hydrogen generation. Only
1.52 V is needed to achieve the current of 10 mA cm-2 for overall urea electrolysis. Our work
may provide a new strategy to design bifunctional electrocatalysts for electrooxidation of
small molecules to facilitate the hydrogen generation, and offer a promising candidate for
urea-rich wastewater treatment and urea-assisted hydrogen generation.
Electronic Supplementary Information (ESI). SEM images of the pristine carbon cloth,
CC, Ni-Co/CC, CS/CC, NCS/CC 1:1, and NCS/CC 1:3. AFM images of Ni-Co/CC and
NCS/CC 1:2. EPMA characterization of NCS/CC 1:2. Tables of molar ratios from XPS and
ICP. CV curves of ferri/ferrocyanide solution for CC and NCS/CC 1:2. OER performance of
all samples (LSV and Tafel plots). CV curves of samples. LSV curves of Co9S8 nanosheets
grown on the carbon cloth with and without etching. LSV curves of NCS/CC 1:2 in 1 M
KOH solution with different urea concentrations and in 0.33 M urea solution with different
concentrations of KOH. Normalized LSV curves of samples for HER.
Notes
The authors declare no competing financial interest.
Acknowledgments
15

This work was supported by National Natural Science Foundation of China (51602182,
21535004, 91753111, 51372142, 21390411) and the Natural Science Foundation of
Shandong Province, China (ZR2016EMQ02).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org.
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Figures
Figure 1. Schematic illustration of the formation process of Ni-Co9S8/CC.
24

Figure 2. Morphological information of NCS/CC 1:2. (a, b) SEM, (c) TEM, (d) HRTEM and
(e) EDX element mapping. The inset in (d) is the corresponding FFT pattern.
25

Figure 3. Compositional information of the samples: (a) XRD, (b-d) high-resolution XPS
spectra of Co, S and Ni, respectively.
26

Figure 4. UOR performances of catalysts. (a) LSV curves without IR correction at 2 mV s-1,
(b) current density variation at 1.05 V vs. RHE as a function of scan rates, (c) LSV curves
normalized by Cdl, (d) the corresponding Tafel plots, (e) Nyquist plots of catalysts at 1.4 V vs.
RHE, the inset is the equivalent circuit, (f) comparison of UOR performance of catalysts.
27

Figure 5. The stability of NCS/CC 1:2 for UOR. (a) chronoamperometry test at 1.43 V vs.
RHE and (b) CV cycling of NCS/CC 1:2. The inset in (a) is the SEM image after
chronoamperometry test.
28