Phosphorized CoNi2S4 Yolk-Shell Spheres for Highly Efficient Hydrogen Production via Water and Urea Electrolysis

Article abstract of DOI:10.1002/anie.202108563

Exploring earth-abundant electrocatalysts with excellent activity, robust stability, and multiple functions is crucial for electrolytic hydrogen generation. Porous phosphorized CoNi₂S₄ yolk-shell spheres (P-CoNi₂S₄ YSSs) were rationally designed and synthesized by a combined hydrothermal sulfidation and gas-phase phosphorization strategy. Benefiting from the strengthened Ni³⁺/Ni²⁺ couple, enhanced electronic conductivity, and hollow structure, the P-CoNi₂S₄ YSSs exhibit excellent activity and durability towards hydrogen/oxygen evolution and urea oxidation reactions in alkaline solution, affording low potentials of ?0.135 V, 1.512 V, and 1.306 V (versus reversible hydrogen electrode) at 10 mA cm^?2, respectively. Remarkably, when used as the anode and cathode simultaneously, the P-CoNi₂S₄ catalyst merely requires a cell voltage of 1.544 V in water splitting and 1.402 V in urea electrolysis to attain 10 mA cm^?2 with excellent durability for 100 h, outperforming most of the reported nickel-based sulfides and even noble-metal-based electrocatalysts. This work promotes the application of sulfides in electrochemical hydrogen production and provides a feasible approach for urea-rich wastewater treatment.

Full text of DOI:10.1002/anie.202108563

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Accepted Article
Title: Phosphorized CoNi2S4 Yolk-Shell Spheres for Highly Efficient
Hydrogen Production via Water and Urea Electrolysis
Authors: Xiong-Wen (David) Lou
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10.1002/anie.202108563
Angewandte Chemie International Edition
Phosphorized CoNi2S4 Yolk-Shell Spheres for Highly Efficient Hydrogen
Production via Water and Urea Electrolysis
Xue Feng Lu, Song Lin Zhang, Wei Lok Sim, Shuyan Gao,* and Xiong Wen (David) Lou*
[*] Prof. S. Y. Gao
School of Materials Science and Engineering, Henan Normal University, Xinxiang, Henan 453007,
P.R. China. Email: shuyangao@htu.cn
Dr. X. F. Lu, S. L. Zhang, W. L. Sim, Prof. X. W. Lou
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang
Drive, Singapore, 637459, Singapore
Email: xwlou@ntu.edu.sg; davidlou88@gmail.com Webpage: https://personal.ntu.edu.sg/xwlou/
Abstract
Exploring earth-abundant electrocatalysts with excellent activity, robust stability, and multiple
functions is crucial for electrolytic hydrogen generation. Herein, porous phosphorized CoNi2S4 yolk-
shell spheres (P-CoNi2S4 YSSs) are rationally designed and synthesized by a combined hydrothermal
sulfidation and gas-phase phosphorization strategy. Benefiting from the strengthened Ni3+/Ni2+
couple, enhanced electric conductivity, and hollow structure, the P-CoNi2S4 YSSs exhibit excellent
activity and durability towards hydrogen/oxygen evolution and urea oxidation reactions in alkaline
solution, affording low potentials of -0.135 V, 1.512 V, and 1.306 V (versus reversible hydrogen
electrode) at 10 mA cm-2, respectively. Remarkably, when used as the anode and cathode
simultaneously, the P-CoNi2S4 catalyst merely requires a cell voltage of 1.544 V in water splitting
and 1.402 V in urea electrolysis to attain 10 mA cm-2 with excellent durability for 100 h,
outperforming most of the reported nickel-based sulfides and even noble-metal-based electrocatalysts.
This work therefore not only promotes the application of sulfides in electrochemical hydrogen
production but also provides a feasible approach for urea-rich wastewater treatment.
Keywords: CoNi2S4, phosphorization, hydrogen, urea, electrolysis
1
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Renewable energy-driven electrolytic hydrogen generation has been recognized as an eco-friendly
manner to alleviate the energy crisis and environmental issues faced by the international
community.[1-3] However, the unfavorable thermodynamics and sluggish kinetics of both cathodic
reduction and anodic oxidation reactions severely restrict the hydrogen production rate and energy
efficiency.[3-7] Noble-metal-based electrocatalysts are known as the most active catalysts for
electrolytic hydrogen production, but their large-scale application is largely hindered by the
prohibitive costs, low reserves, and unsatisfactory stability.[8-10] Recent years have witnessed the rapid
development of earth-abundant electrocatalysts, especially towards hydrogen/oxygen evolution
reactions (HER/OER) for alkaline water splitting.[11,12] But most of them do not possess excellent
performance for both HER and OER in the same electrolyte due to their incompatible activity over
different pH ranges.[10] In addition, the high thermodynamic equilibrium potential (1.23 V versus
reversible hydrogen electrode (vs. RHE)) of OER undoubtedly increases the energy consumption of
hydrogen production.[7,9] Therefore, to reduce the overall cell voltage and manufacturing cost,
exploring more efficient low-cost multifunctional electrocatalysts and hybrid water electrolysis
systems coupled with some diligent anodic oxidations is highly desired.
Urea oxidation reaction (UOR) has been reported as an ideal alternative to OER due to its much
lower thermodynamic equilibrium potential (0.37 V vs. RHE), enabling the urea electrolysis a
promising electrochemical approach for energy-saving and high-efficiency hydrogen production.[13-
16] Moreover, urea-rich wastewater is generated from industrial synthesis and sanitary sewage, thus
making urea electrolysis attractive for mitigating the problem of urea-rich water pollution.[15]
However, the complicated six-electron transfer process of UOR (CO(NH2)2 + 6OH− → N2 + 5H2O +
CO2 + 6e−) leads to sluggish kinetics and higher practical cell voltage of urea electrolysis.[16] Among
all the transition metal-based electrocatalysts, nickel (Ni) is the most widely used element for energy
and environmental applications.[17] For example, Raney Ni and Ni alloys have been widely used in
conventional alkaline water electrolyzers due to their low cost and high efficiency.[18] Recently, Ni
2
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and Ni-based compounds have received great attention for the UOR to accelerate the kinetics, during
which they undergo an oxidation process to generate active Ni3+ (NiOOH) for catalyzing UOR.[13,15,19]
Nevertheless, there are few reports on trifunctional Ni-based electrocatalysts for highly efficient
hydrogen production via both water and urea electrolysis.
Among all the Ni-based electrocatalysts, sulfides have shown promising trifunctional
electrocatalytic performance towards HER, OER, and UOR, and various strategies have been applied
to improve their performance. One appealing idea is composition manipulation by heteroatom doping
or substitution, which can modulate the adsorption energies of intermediates by optimizing the
electronic structure of catalysts. For example, Yin et al. prepared different metal-doped NiS2 (M-NiS2,
M= Fe, Co, Cu) nanosheets, in which Co-NiS2 achieved prominent HER performance in alkaline
1
media due to the optimal eg electron configuration.[20] Besides, cation doping can also significantly
increase the content of high-valence Ni3+ (NiOOH) species, which have been identified as the
catalytic active sites for anodic OER and UOR.[14,19,21,22] Recently, phosphorization has also been
widely reported to improve the HER intrinsic activity and conductivity by tuning the electronic
structure and distorting the lattices of the parent sulfides.[23-26] Morphological and structural
engineering is another efficient approach for enhancing the catalytic performance through exposing
more active sites. In particular, porous yolk-shell structures are favored because of their high specific
surface area, large void volume, and reduced ion-diffusion path.[27-29]
Inspired by these advancements in multi-element Ni-based sulfide electrocatalysts, we rationally
design and synthesize porous phosphorus substituted CoNi2S4 yolk-shell spheres (P-CoNi2S4 YSSs)
via a facial hydrothermal sulfidation and subsequent gas-phase phosphorization strategy (Figure 1).
Benefiting from the advantageous features of bimetallic elements, high electronic conductivity and
enriched Ni3+ content induced by phosphorization, as well as unique hollow structure, the obtained
P-CoNi2S4 YSSs electrocatalyst exhibits superior activity and stability towards HER, OER, and UOR
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in alkaline media, yielding low potentials of -0.135 V, 1.512 V, and 1.306 V vs. RHE at 10 mA cm-
2, respectively. Consequently, the P-CoNi2S4||P-CoNi2S4 catalyst couple for water electrolysis only
needs a low cell voltage of 1.544 V to steadily drive a benchmark current density of 10 mA cm-2 for
100 h, much better than that of commercial Pt/C||RuO2 couple. Remarkably, the cell voltage can be
further reduced to 1.402 V when 0.5 M urea is added, almost rivaling the state-of-the-art urea
electrolysis. This work highlights the effectiveness of Ni-based bimetallic sulfides for water and urea
electrolysis and strengthens the significance of the nickel valence for performance improvement,
hence enabling cost-effective and energy-saving electrochemical hydrogen production.
Uniform CoNi-glycerate solid spheres (CoNi-G SSs) are prepared by a previously reported
method with slight modification (see the experimental details in Supporting Information (SI)).[28]
Field-emission scanning electrode microscopy (FESEM) and transmission electron microscopy
(TEM) images demonstrate the smooth surface and solid nature of CoNi-G SSs with an average
diameter of 490 nm (Figure 2a,d; Figure S1a,b, SI). X-ray diffraction (XRD) pattern (Figure S1c)
and energy-dispersive X-ray (EDX) spectrum (Figure S1d) indicate the amorphous phase of metal
alkoxides and the chemical composition of a Co/Ni atomic ratio around 1/2, respectively.[30] CoNi2S4
YSSs are obtained via the chemical etching/anion exchange reactions between CoNi-G SSs and
thioacetamide solution under hydrothermal conditions (see the experimental details in SI). FESEM
and TEM images show the rough surface and void space between the interior solid core and the outer
porous shell of CoNi2S4 YSSs with an average diameter of 570 nm (Figure 2b,e; Figure S2a, SI).
Subsequently, P-CoNi2S4 YSSs are achieved by a gas-phase phosphorization of CoNi2S4 YSSs (see
the experimental details in SI). The porous yolk-shell structure is well preserved with a slightly
reduced diameter of 520 nm (Figure 1c,f; Figure S2b). A typical TEM image of an individual P-
CoNi2S4 YSS further demonstrates that the yolk-shell structure with a porous thin shell (about 35 nm
in thickness) is assembled by ultrafine nanoparticles (Figure 2g), which is beneficial for the exposure
of active sites. The high-resolution TEM (HRTEM) and inverse fast Fourier transform (IFFT) images
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(Figure 2h) clearly show two sets of distinct lattice fringes with interplanar spacings of 0.24 and 0.28
nm, which are readily assigned to the (400) and (311) planes of CoNi2S4 (JCPDS card No. 24-0334),
respectively. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-
STEM) and corresponding elemental mapping images (Figure 2i) show the homogenous distribution
of Co, Ni, S, and P elements in P-CoNi2S4 YSSs.
XRD analysis (Figure 3a) indicates the similar diffraction patterns in CoNi2S4 YSSs and P-
CoNi2S4 YSSs, which correspond well with the standard JCPDS card of No. 24-0334, manifesting
that phosphorization would not destroy the crystal structure, which may be due to the similar atomic
radius of S and P.[31,32] The average atomic ratio of P/S is determined to be about 0.9 from the EDX
spectra (Figure 3b), giving about 47% substitution of the S sites. Further insights into the changes of
surface chemical states after P doping could be acquired from the X-ray photoelectron spectroscopy
(XPS) spectra. The peak-fitting analysis of Co 2p spectra (Figure 3c) suggests that two chemical
states exist in both CoNi2S4 and P-CoNi2S4, corresponding to the spin-orbital characteristics of Co3+
(778.56 eV for 2p3/2 and 793.55 eV for 2p1/2) and Co2+ (781.50 eV for 2p3/2 and 798.01 eV for 2p1/2),
accompanied with a pair of broad satellite peaks.[33,34] Similarly, the fitting peaks of Ni 2p spectra
(Figure 3d) show two pairs of main peaks (853.27 eV for Ni2+ 2p3/2 and 870.43 eV for Ni2+ 2p1/2;
856.74 eV for Ni3+ 2p3/2 and 874.64 eV for Ni3+ 2p1/2) and two broad satellites.[34,35] Notably, the
higher ratios of both Co2+/Co3+ and Ni3+/Ni2+ calculated by integrating the peak areas indicate much
higher crystalline purity of CoNi2S4 in P-CoNi2S4, thereby being conducive to stronger redox reaction
and enhancing the catalytic performance.[14,34,35] For the S 2p spectra in Figure 3e, both of them can
be fitted with three distinct doublets of 2p3/2 and 2p1/2 with an area ratio of 2:1, including the first one
(161.48 eV, 162.59 eV) that arises from a sulfide species, the second one (163.50 eV, 164.66 eV)
indicative of sulfur in a thiolate-type environment, and the third one (168.81 eV, 169.97 eV) ascribed
to surface oxidation.[31,32,36] Furthermore, the reduction in the area ratio of the fitting peaks belonging
to oxides and sulfides in P-CoNi2S4 suggests phosphorization could suppress the oxidation of sulfides
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in air.[24,32,34] The P 2p spectrum of P-CoNi2S4 (Figure 3f) only displays one doublet (133.52 eV,
134.39 eV) assigned to the P bonded with Co/Ni, no oxidized P species are observed.[31,37] This fully
confirms the substitution characteristics of P, which will induce more electronic interactions for
enriched Ni3+ content, thereby promoting electrocatalytic performance.
The electrocatalytic performance of P-CoNi2S4 YSSs for water splitting was first evaluated using
a three-electrode configuration cell in alkaline media (1.0 M KOH, pH=14) at room temperature.
Figure 4a shows the polarization curves of CoNi2S4 and P-CoNi2S4 towards HER, which are
corrected for ohmic potential drop (iR) loss. The P-CoNi2S4 catalyst exhibits a smaller onset
overpotential (20 mV, Figure S3, SI) than CoNi2S4, beyond which the cathodic current density rises
rapidly under more negative potentials. Specifically, the P-CoNi2S4 catalyst requires a small
overpotential of 135 mV to reach the current density of 10 mA cm-2, which is a common criterion for
evaluating the activity of water splitting.[38,39] Moreover, the P-CoNi2S4 catalyst displays a rapid rise
in current density and constantly higher current density throughout the applied potential range, which
is further evidenced by the analysis of Tafel slopes (Figure 4b). The reduction in Tafel slope from 81
mV dec-1 for CoNi2S4 to 65 mV dec-1 for P-CoNi2S4, together with the smaller overpotential of P-
CoNi2S4 at the same current density, fully confirms the promotional effect of P substitution on the
activity and kinetics of alkaline HER. This enhancement may be due to the electronic modulation
effect of P on metal and/or S sites to facilitate the water dissociation, and in addition the P sites could
also serve as the active sites for HER.[10,40-42] Moreover, the P-CoNi2S4 catalyst shows superior HER
activity and kinetics when compared to some previously reported sulfide-based electrocatalysts
(Table S1, SI). The 40-h chronopotentiometry test (Figure S4, SI) also shows the superior catalytic
stability of P-CoNi2S4.
The iR-compensated cyclic voltammetry (CV) curves of these two samples (Figure S5, SI) both
show the pre-oxidation process to generate catalytically active high-valence species.[22,43-45] To avoid
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the effect of oxidation peak, we use the reverse sweep branch of the CV curve to evaluate the OER
activity and kinetics. As shown in Figure 4c,d, the OER activity and kinetics of CoNi2S4 have been
greatly improved after P substitution, suggesting positive effects of P on OER. To reach a current
density of 10 and 100 mA cm-2, the P-CoNi2S4 catalyst requires overpotentials of 282 and 327 mV,
respectively, which are lower than that of CoNi2S4 (288 and 349 mV). The smaller Tafel slope of 40
mV dec-1 for P-CoNi2S4 than that of CoNi2S4 (53 mV dec-1) shows the faster OER reaction kinetics
after P substitution. Such superior activity and kinetics of P-CoNi2S4 compare favorably with that of
many sulfide-based OER electrocatalysts (Table S2, SI). In addition, the 60-h chronopotentiometry
test (Figure S6, SI) also shows the superior catalytic stability of P-CoNi2S4 towards OER.
CV curves at different scan rates (Figure S7, SI) are measured to evaluate the electrochemically
active surface area (ECSA).[9,38] As shown in Figure 4e, the P-CoNi2S4 catalyst shows a much higher
value (4.7 mF cm-2) than CoNi2S4 (2.6 mF cm-2), indicating more exposed active sites in the former.
The increased ECSA may be attributed to the formation of additional active sites that originated from
the incorporation of P into CoNi2S4 by adjusting the catalytic properties of adjacent metal sites in the
CoNi2S4,[46,47] which can be seen from the difference in the CV curves (Figure S5, SI). In addition,
the incorporated P sites may serve as new active sites for water dissociation.[40,41] Electrochemical
impedance spectroscopy (EIS) measurements are also performed to evaluate the charge-transfer
resistance (Rct), which is associated with electrocatalytic kinetics.[9,48,49] The smaller diameter of the
semicircle in the high frequency region for P-CoNi2S4 suggests faster charge transfer and
electrocatalytic kinetics (Figure 4f; Figure S8, SI), which may be due to the increase in electron
delocalization induced by lone-pair electrons in 3p orbitals and empty 3d orbitals of phosphorus.[50,51]
Previously reported density functional theory calculation results also confirm that P-doped sulfides
display a much lower band gap than undoped sulfides due to the generation of new electronic states
in the conduction band by P doping.[46,52] This effect of P substitution on increasing ECSA and
decreasing Rct is expected from previous studies, in which heteroatom doping or substitution has been
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reported to be a versatile strategy for modulating the electronic and/or surface structures of the host
materials.[26,31]
In view of the distinguished electrocatalytic performance of the as-prepared P-CoNi2S4 catalyst
towards both HER and OER, a two-electrode electrolyzer was fabricated with P-CoNi2S4 as both the
cathodic and anodic catalysts and 1.0 M KOH solution as the medium. The polarization curve for
water electrolysis (Figure 4g) indicates the superior overall water splitting performance of the P-
CoNi2S4||P-CoNi2S4 cell, which requires a low cell voltage of 1.544 V to afford a current density of
10 mA cm-2 compared to 1.533 V for a Pt/C||RuO2 cell using commercial catalysts. More importantly,
the advantage of this P-CoNi2S4||P-CoNi2S4 cell is much more significant at higher current densities.
One can see that this low cell voltage of 1.544 V for P-CoNi2S4 is comparable or superior to that of
the most efficient bifunctional electrocatalysts reported (Table S3, SI), including some recently
reported values, such as 1.46 V for Mo-doped Ni3S2/NixPy hollow nanorods,[53] 1.58 V for
CoFeO@black phosphorus,[54] 1.59 V for Fe2CoPS3,[55] and 1.62 V for Fe-Ni2P@P-doped
carbon/CuxS arrays.[56] Besides, the chronopotentiometry curves recorded at 10 mA cm-2 (Figure 4h)
show that the P-CoNi2S4 catalyst exhibits outstanding overall water splitting durability over 100 h,
much better than that of the Pt/C||RuO2 cell. These results suggest that the as-prepared P-CoNi2S4
sample is a promising catalyst to replace noble metal catalysts for efficient and long-lasting hydrogen
production in the practical water electrolysis device.
Although the P-CoNi2S4 catalyst exhibits excellent performance in overall water splitting, the
energy efficiency for hydrogen production still needs to be further improved from the industrial
application perspective. As mentioned earlier, UOR is an amiable anodic reaction to efficiently reduce
the energy input for hydrogen generation.[13-16] The UOR performance of P-CoNi2S4 is evaluated
through a three-electrode configuration in 1.0 M KOH solution containing 0.5 M urea (the
concentration screening is shown in Figure S9, SI). To illustrate the electrocatalytic mechanism of
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UOR, CV curves of P-CoNi2S4 in 1.0 M KOH solution with and without urea are plotted in Figure
5a for comparison. In the bare KOH solution, a typical pre-oxidation process occurs before OER,
while when urea is added, the current density in the positive sweep dramatically increases. The
potential to reach current densities of 10 and 100 mA cm-2 is drastically reduced to 1.306 and 1.367
V in 1.0 M KOH containing 0.5 M urea. A small Tafel slope of 55 mV dec-1 (Figure S10, SI) also
well supports the observed rapid increment in the current density, indicating the fast kinetics towards
UOR. Such superior UOR performance endows the as-prepared P-CoNi2S4 as one of the state-of-the-
art Ni-based electrocatalysts (Table S4, SI). Moreover, the negligible anodic current for OER
compared with that for UOR demonstrates the superior selectivity of P-CoNi2S4 towards UOR at low
potential (<1.5 V vs. RHE). The smaller reduction peak in the reverse sweep branch of CV curves
also indicates that metal species are mostly involved directly in UOR rather than pre-oxidized to
catalyze OER.[57] Both branches of the CV curves confirm the low onset potential for UOR, which is
close to the onset potential of pre-oxidation, proving that the high-valence metal species promotes
the catalysis of urea.[14,16,57] The UOR rate capability of P-CoNi2S4 is studied by tuning the scan rate
from 2 to 100 mV s-1 (Figure 5b). The current density and square root of the scan rate maintain a good
linear relationship at 1.40 V vs. RHE (inset in Figure 5b), which indicates that UOR is a surface
controlled process with highly efficient charge and mass transfer.[16,58]
To verify the feasibility of UOR as an alternative anodic reaction, the HER activity of P-CoNi2S4
is also evaluated in a 1.0 M KOH solution containing 0.5 M urea. The almost overlapped LSV curves
(Figure S11, SI) indicate the negligible influence of urea on the HER activity, showing the
bifunctional activity of P-CoNi2S4 towards both HER and UOR. Subsequently, a two-electrode
configuration in 1.0 M KOH solution containing 0.5 M urea is fabricated using P-CoNi2S4 as both
the anode and cathode to perform urea-assisted electrochemical hydrogen production. As shown in
Figure 5c, the cell voltage to achieve the current density of 10 mAꢀcm-2 is 1.402ꢀV, which is much
lower than that required for water splitting (1.544ꢀV), proving the assistance of urea oxidation can
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lower the energy consumption of hydrogen production. The performance of P-CoNi2S4 in overall urea
electrolysis is also superior to that of many other electrocatalysts (Table S5, SI). Furthermore, the
chronopotentiometry curve recorded at 10 mA cm-2 (Figure 5d) shows good durability with only a 26
mV increment in the cell voltage after a continuous operation of 100 h. The excellent catalytic activity
and long-term durability indicate that the synthesized P-CoNi2S4 YSSs might serve as a low-cost and
high-efficiency bifunctional electrocatalyst for feasible urea-assisted electrochemical hydrogen
production.
In summary, we have rationally designed and synthesized phosphorus-substituted bimetallic
cobalt-nickel sulfide yolk-shell spheres (P-CoNi2S4 YSSs) via composition manipulation and nano-
structure engineering. Thanks to the desired composition and structure, the obtained P-CoNi2S4 YSSs
exhibit excellent electrocatalytic performance for HER and OER, giving rise to reduced cell voltage
in overall water splitting. Moreover, the enriched Ni3+ content induced by P substitution endows the
P-CoNi2S4 YSSs one of the state-of-the-art UOR catalysts with a low potential of 1.306 V vs. RHE
at 10 mA cm-2. When replacing OER with more amiable UOR, the urea-mediated electrolysis cell
requires a very low potential of 1.402 V to reach 10 mA cm-2, which is 142 mV less than that required
for water splitting. This work provides a feasible and efficient strategy for the design and synthesis
of high-performance electrocatalysts based on multifunctional nickel-based sulfides, which might
open an avenue towards cost-effective and energy-saving electrochemical hydrogen production as
well as promote the advancement of sulfides in related energy and environmental fields.
Acknowledgements
X.W.L. acknowledges the funding support from the Ministry of Education of Singapore through
Academic Research Fund (AcRF) Tier-1 funding (RG116/18).
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Figures and captions
Figure 1. Schematic illustration of the preparation of P-CoNi2S4 YSSs.
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Figure 2. (a-c) FESEM and (d-g) TEM images of CoNi-G SSs (a,d), CoNi2S4 YSSs (b,e), and P-
CoNi2S4 YSSs (c,f,g). (h) HRTEM image, (h1,h2) the inverse fast Fourier transformation (IFFT)
images, and (i) elemental mapping images of P-CoNi2S4 YSSs.
16
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Figure 3. (a) XRD patterns, (b) EDX spectra, and (c-f) high-resolution XPS spectra of Co 2p (c), Ni
2p (d), S 2p (e), and P 2p (f) of CoNi2S4 YSSs and P-CoNi2S4 YSSs.
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Figure 4. (a) LSV curves and (b) corresponding Tafel slopes of CoNi2S4 YSSs and P-CoNi2S4 YSSs
towards HER. (c) CV curves in the reverse sweep and (d) corresponding Tafel slopes of CoNi2S4
YSSs and P-CoNi2S4 YSSs towards OER. (e) Half of the capacitive current density difference at 0.97
V vs. RHE as a function of the scan rate and (f) EIS spectra of CoNi2S4 YSSs sand P-CoNi2S4 YSSs.
(g) LSV curves and (h) chronopotentiometry curves at 10 mA cm-2 of P-CoNi2S4||P-CoNi2S4 and
Pt/C||RuO2 couples towards overall water splitting.
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Figure 5. (a) CV curves of P-CoNi2S4 YSSs towards OER and UOR. (b) LSV curves of P-CoNi2S4
YSSs for UOR at scan rates from 2 to 100ꢀmVꢀs-1 (Inset is the relationship between the current density
at 1.4ꢀV vs. RHE and square root of the scan rate). (c) Polarization curves of P-CoNi2S4 YSSs towards
water splitting and urea electrolysis. (d) Chronopotentiometry curve at 10 mA cm-2 of bifunctional P-
CoNi2S4 YSSs towards urea electrolysis.
19
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for Table of Content Entry
Phosphorus substituted CoNi2S4 yolk-shell spheres (P-CoNi2S4 YSSs) have been rationally
designed and synthesized by a facile hydrothermal sulfidation and subsequent gas-phase
phosphorization strategy. The desired hollow structure and multi-element composition with abundant
Ni3+ active sites endow P-CoNi2S4 YSSs with high electrocatalytic activity and robust stability
towards electrochemical hydrogen production via water splitting and urea electrolysis.
20
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