Facile synthesis of CoSe nanoparticles encapsulated in N-doped carbon nanotubes-grafted N-doped carbon nanosheets for water splitting

Article abstract of DOI:10.1016/j.electacta.2020.135685

Rationally designing a high-efficiency, inexpensive and stable electrocatalyst is important in the renewable energy field. Herein, a one-step strategy is adopted to synthesize 3D hybrid of CoSe nanoparticles encapsulated nitrogen-doped carbon nanotubes graft onto nitrogen-doped carbon nanosheets (denoted as CoSe?NCNT/NCN) with excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity in acidic and alkaline solutions, respectively. Benefiting from the unique morphology, the large surface area and good conductivity, the hybrid exhibits excellent electrocatalytic HER activity in 0.5 M H₂SO₄ solution, which is a low overpotential of 197 mV at a current density of 10 mA cm^?2 with a Tafel slope of 43 mV dec^?1. Meanwhile, it presents a low overpotential of 301 mV at a current density of 10 mA cm^?2 with a Tafel slope of 75 mV dec^?1 in 1 M KOH solution for OER. The good electrocatalytic property is comparable to most of the previously reported Co-based electrocatalysts and the synthetic method is low toxic and simple, which may be expanded to other transition metal dichalcogenides. The strategy provides a possibility for preparation for effective nonprecious high performance HER and/or OER electrocatalysts.

Full text of DOI:10.1016/j.electacta.2020.135685

Journal Pre-proof
Facile synthesis of CoSe nanoparticles encapsulated in N-doped carbon nanotubes-
grafted N-doped carbon nanosheets for water splitting
Ming Yang, Yuanyuan Yang, Kaizhi Wang, Shuwen Li, Fan Feng, Kai Lan, Pengbo
Jiang, Xiaokang Huang, Honglei Yang, Rong Li
PII:
S0013-4686(20)30076-1
DOI:
https://doi.org/10.1016/j.electacta.2020.135685
EA 135685
Reference:
To appear in: Electrochimica Acta
Received Date: 14 March 2019
Revised Date: 18 December 2019
Accepted Date: 9 January 2020
Please cite this article as: M. Yang, Y. Yang, K. Wang, S. Li, F. Feng, K. Lan, P. Jiang, X. Huang, H.
Yang, R. Li, Facile synthesis of CoSe nanoparticles encapsulated in N-doped carbon nanotubes-grafted
N-doped carbon nanosheets for water splitting, Electrochimica Acta (2020), doi: https://doi.org/10.1016/
j.electacta.2020.135685.
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Facile synthesis of CoSe nanoparticles encapsulated in
N-doped carbon nanotubes-grafted N-doped carbon
nanosheets for water splitting
Ming Yang, Yuanyuan Yang, Kaizhi Wang, Shuwen Li, Fan Feng, Kai Lan, Pengbo Jiang, Xiaokang
Huang, Honglei Yang and Rong Li *
State Key Laboratory of Applied Organic Chemistry (SKLAOC), The Key Laboratory of Catalytic Engineering of
Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R.
China. E-mail: liyirong@lzu.edu.cn; Fax: +86-931-891-2582; Tel: +86-18919803060.
ABSTRACT
Rationally designing a high-efficiency, inexpensive and stable electrocatalyst is important in the renewable energy
field. Herein, a one-step strategy is adopted to synthesize 3D hybrid of CoSe nanoparticles encapsulated
nitrogen-doped carbon nanotubes graft onto nitrogen-doped carbon nanosheets (denoted as CoSe@NCNT/NCN)
with excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity in acidic and
alkaline solutions, respectively. Benefiting from the unique morphology, the large surface area and good
conductivity, the hybrid exhibits excellent electrocatalytic HER activity in 0.5 M H2SO4 solution, which is a low
overpotential of 197 mV at a current density of 10 mA cm-2 with a Tafel slope of 43 mV dec-1. Meanwhile, it
presents a low overpotential of 301 mV at a current density of 10 mA cm-2 with a Tafel slope of 75 mV dec-1 in 1
M KOH solution for OER. The good electrocatalytic property is comparable to most of the previously reported
Co-based electrocatalysts and the synthetic method is low toxic and simple, which may be expanded to other
transitional metal dichalcogenides. The strategy provides a possibility for preparation for effective nonprecious

high performance HER and/or OER electrocatalysts.
KEYWORDS
Hydrogen evolution reaction, Oxygen evolution reaction, water splitting, N-doped carbon nanotubes, CoSe
nanoparticles.
1. Introduction
The importance of dealing with the global energy crisis and developing a low-carbon energy resource has given
the motivation to research sustainable clean energy.[1] Recently, the study of electrochemical water splitting has
triggered attention in that it can provide an environmentally-friendly hydrogen production technology and make
full use of other energy (solar and wind energy).[2-4] There are two half reactions during the process of water
splitting: hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode. However,
the sluggish kinetics of OER has limited the entire electrolytic process.[5] To date, precious metals are still the
best catalysts in water splitting. But the scarcity and expensiveness limit the applications in the commerce. In the
past decades, nonprecious metal catalysts (transition metal phosphides,[6-8] sulfides,[9, 10] selenides,[11-13]
borides[14-17] and carbides[18, 19]) grew in various nitrogen-containing carbon supports and metal-free
catalysts[20, 21] have been studied, which possessed excellent activity and stability for HER and/or OER.
Cobalt selenide as a representative highly active transition metal dichalcogenide and earth-abundant catalyst
is an alternative to novel metal catalysts, and it has recently been researched extensively due to its high
electrochemical stability, inexpensiveness and environmental friendliness.[22-24] Many reported cobalt selenide
catalysts for water electrolysis are primarily CoSe2-based electrocatalysts, however, few investigations have been
done on CoSe as electrocatalysts. The CoSe catalyst has also higher electrochemical activity, which is mainly
attributed to moderate bond between Se/Co species.[25] In order to further enhance their electrochemical catalytic
property and the stability in corrosive environment, a nitrogen-doped carbon material was utilized as a supporter,

which could not only prevents the agglomeration of the nanoparticles but also slows down the corrosive speed of
catalysts.[26] In addition, the doped nitrogen functioned as active sites because it could modulate charge
distribution for the HER and OER. Recently, the integration of 1D nitrogen-doped carbon nanotubes (NCNTs) and
2D carbon nanosheets into 3D hierarchical carbon hybrid have triggered attention because they can protect the
metals/metal compounds in corrosive solution and afford an excellent conductive carrier for electronic transfer.
For instance, Chen et al. developed Co nanoparticles embedded in NCNTs-grafted graphene sheets for HER;[27]
Zhou et al. designed CoSe2 nanoparticle embedded in carbon nanotube as effective HER electrocatalyst;[28] Guo
et al. devised Co2P-CoN confined in N-doped carbon nanotube for HER, OER and ORR.[29] Nonetheless, the
fabrication process of these catalysts is still complex and multi-step. Hence, it is vital to develop a facile and
economic way to fabricate a transition metal dichalcogenide, which is encapsulated in nitrogen-doped carbon
nanotubes.
Herein, we rationally design a 3D hierarchical N-doped carbon hybrid in which CoSe nanoparticle was
encapsulated within nitrogen-doped carbon nanotube (NCNT) graft onto nitrogen-doped carbon nanosheet (NCN)
(CoSe@NCNT/NCN) by a facile one-step carbonization-selenylation process using cobalt acetate, selenium
powder, melamine and glucose as precursors under a H2/Ar atmosphere. The as-synthesized catalyst was applied
as an effective electrocatalyst for HER in acidic and OER in basic solutions, exhibiting outstanding
electrochemical performance with the overpotential of 197 mV and 301 mV at current density of 10 mA cm-2 in
acidic and basic solutions, respectively. The excellent electrocatalytic effect was mainly due to the unique
morphology, the lager active surface area and the synergistic effects between CoSe nanoparticles and 3D
conductive N-doped carbon networks.
2. Experimental Section
2.1. Materials

Cobalt ( ) acetate tetrahydrate (Co(CH3COO)2·4H2O), selenium powder, melamine (C3H6N6), glucose and
ethanol were purchased from Sinopharm Chemical Reagents Beijing Co. Lid. Nafion (0.5 wt%), commercial Pt/C
catalyst and iridium (IV) oxide were purchased from Alfa Aesar. DI water (18.2 MΩ cm) was added during the
entire process of experiment. All reagents were applied directly without further purification.
2.2. Preparation of CoSe@NCNT/NCN
The catalyst was obtained through two-step heat up carbonization. In a typical method, melamine (3 g), glucose
(100 mg), cobalt acetate tetrahydrate (500 mg) and excess selenium powder were dissolved in a mixture of 50ml
ethanol and DI water (1:1). The solution was continuously stirred until a homogeneous suspension, which was
totally dried to solid. The solid was finely grinded using a mortar and pestle. The resulting precursor was sent to a
o
o
porcelain boat and was heated at 600 C for 2h and was further heated up to 900 C and lasted for another 2h,
during which the speed of heating was 2 oC per minute. The whole process was protected under a mixture of Ar/H2
(5 %). The collected product was recorded as CoSe@NCNT/NCN. In addition, some other samples were
o
o
o
fabricated by changing the heating temperature during the second step from 600 C to 800 C and 1000 C not
altering the other parameters. The obtained samples were named after CoSe@NCNT/NCN-X (where X refers to
the temperature). The different level of CoSe amounts were performed by altering the content of cobalt acetate
tetrahydrate and selenium powder in precursor.
2.3. Preparation of Co@NCNT/NCN
Co@NCNT/NCN was obtained by the same procedure without adding selenium powder at 900 oC.
2.4. Preparation of bulk CoSe and NCN
The bulk CoSe was gotten with a mixture of cobalt acetate tetrahydrate and selenium powder pyrolysis at 800
o
oC at a heating speed of 5 C min-1 under Ar/H2 (5 %) protection. The nitrogen-doped carbon nanosheets were
acquired with a mixture of melamine (3 g) and glucose (100 mg) heated at 600 oC for 2h and was further heated up

to 900 oC lasting for another 2h with a heating rate of 2 oC min-1 under Ar/H2 (5 %) protection.
2.5. Electrode preparation and electrochemical measurements
By ultrasound, 5 mg as-synthesized catalyst powder was homogeneously dispersed in a mixture of 470 µL
ethanol, 500 µL DI water and 30 µL Nafion solution (0.5 wt%) to obtain a ink. And then, 5µL obtained catalyst
ink was dropped on a glass carbon electrode (GCE, diameter: 3 mm), in which the capacity of loading was 0.357
mg cm-2. The electrochemical measurements were operated with a three-electrode electrochemical workstation
(CHI660E Shanghai Chenhua Equipment, China) under room temperature. A graphite rod is treated as counter
electrode and an Ag/AgCl (3.0 M KCl) as reference electrode in 0.5 M H2SO4 and 1.0 M KOH solution,
respectively. Each of corresponding current density was standardized by the consistent geometrical zone of the
GCE (0.07 cm2).
3. Results and discussion
A facile one-pot pyrolysis to synthesize N-doped CoSe nanoparticles encapsulated CNTs grafted onto NCN from
melamine, glucose, cobalt acetate and selenium powder was illustrated in Scheme 1. A possible growth
mechanism was presented: firstly, cobalt acetate was dissociated to cobalt oxide and gaseous species and then
selenium powder and cobalt oxide underwent a reaction to yield cobalt selenide nanoparticles, meanwhile,
melamine began to undergo thermal decomposition to produce graphic carbon nitride (g-C3N4).[30, 31] During
heating the mixtures under a protective flow of Ar/H2 (5 %), the temporary sandwich-like template (graphic carbon
nitride and carbon nanosheets) formed at about 600 oC, which g-C3N4 was as a sacrificial template and the stacking
thickness of the template was adjusted by changing the quantity of glucose and melamine.[32] Secondly, when the
o
temperature was higher than 700 C, the g-C3N4 template was completely disintegrated to nitrogen and cyano
fragments.[33] Under the catalysis of CoSe, the N-doped CNTs were grown gradually assembling on preformed
CoSe nanoparticle (Figure S1c-h and Figure S2c-f) via a tip-growth mechanism[34, 35] to yield the final N-doped

CoSe nanoparticles encapsulated CNTs grafted onto NCNs.
Scheme 1. Schematic illustration of the synthesis procedure of CoSe@NCNT/NCN
The SEM images (Figure 1a-b) show the morphology and microstructure of the CoSe@NCNT/NCN. As
depicted in Figure S1, the NCN exhibits a crumpled and layered construction (Figure S1a and Figure S2a), the
o
o
CoSe nanoparticles were loaded on carbon nitride at 700 C, until the temperature was above 800 C, the unique
3D hierarchical structure didn’t gradually form (Figure S1c-h). From high magnified SEM images (Figure 1b), the
CoSe nanoparticles were discovered within the tip of NCNTs, further verifying the tip-growth mechanism. TEM
images further demonstrate CoSe nanoparticles were encapsulated in NCNTs (Figure 1c), and the drop-shaped
CoSe particles were observed in the top of the NCNTs, meanwhile, the NCNs were observed around NCNTs
(Figure S2c-f). The carbon layers not only prevent the agglomeration and corrosion of CoSe nanoparticles, but also
enhance their conductivity and catalytic activity.[36] The high resolution transmission electron microscopy
(HRTEM) image (inset of Figure 1c) reveals the clear lattice fringes of CoSe nanocrystal, which can be seen with
a lattice distance of 0.313 nm related to the (100) plane, indicating the high degree of crystallinity. The TEM
elemental maps (Figure 1e-h) of the summit of NCNTs verify the presence of Co, Se, C and N, which is consistent
with XPS analysis results (Figure 3 and Table S3). Clearly, the mapping of N element suggests the N uniformly
disperses on carbon nanotubes and the independent mappings of Co and Se indicate the drop-shaped CoSe particle
was synthesized successfully.

Figure 1. (a, b) SEM images of CoSe@NCNT/NCN and (c) TEM and HRTEM (inset) image, (d) HAADF-STEM
and (e-h) element mapping images of CoSe@NCNT/NCN.
The synthesis procedure can be also revealed by X-ray powder diffracting (XRD) patterns of
CoSe@NCNT/NCN during the pyrolysis process as shown in Figure 2a-b, and the XRD pattern illustrates the
o
o
existence of CoSe2 (JCPDS No.65-3327) at 600 C. When the temperature further rises to 700 C, the peaks of
CoSe2 convert to that of CoSe (JCPDS No.89-2004), which means CoSe2 is reduced to CoSe because of the
existence of abundant carbon resource, and this result is consistent with the HRTEM consequence above. From
700 to 900 oC, the CoSe isn’t reduced until 1000 oC, at which it is partly reduced to Co (JCPDS No.89-7093). The
o
hybrid has the highest electrochemical catalytic activity and the lowest overpotential when it is heated to 900 C
for 2 h. And this result is in accordance with previous reports,[34, 37] which reveals that the activity of catalysts is
influenced by the heating temperature. To demonstrate the important element Se, the Co nanoparticles
encapsulated N-doped CNTs grafted onto NCN (Co@NCNT/NCN) was synthesized by the same procedure
without adding selenium powder. From Figure 2a, the XRD pattern exhibits that Co exists in N-doped carbon
hybrid. The samples with different level of CoSe amounts were prepared by controlling Co2+ and selenium content.
From Figure S3, the SEM images show that CoSe/NCNT@NCN was also well synthesized as we reduced the Co2+

content from 2 mmol to 1 mmol (Figure S3a-b). When the content of Co2+ increased to 3 mmol, more
unencapsulated CoSe particles loaded on the surface of the carrier are revealed in Figure S3c-d clearly, because the
relative amount of melamine decreased and that of cobalt increased. The results indicate that controlling the ratio
of Co to melamine in a certain range is the key to the effective synthesis of the catalyst. To further research the
defects information of as-synthesized carbon materials, the Raman spectra of the four different annealed catalysts
are shown in Figure S4, in which the D-band at around 1335 cm-1 represents the disorder carbon while G-band at
around 1586 cm-1 illustrates the graphitic carbon. The D to G-band (ID/IG) intensity ratio indicates that the rise of
pyrolysis temperature leads to higher graphitization degree of CoSe@NCNT/NCN and lower content of nitrogen
that in accordance with CHN elemental analysis (Table S4). So the appropriately improved temperature to keep a
o
balance (900 C) is beneficial to catalytic activity and conductivity. The superficial area and pore distribution of
as-synthesized sample are analyzed using nitrogen adsorption-desorption isotherm. As shown in Figure 2c,
CoSe@NCNT/NCN presents a typical IV-type isotherm with a typical H3 hysteresis loop at a relative pressures
(0.4–1P/P0), suggesting the mesoporous characteristic of catalyst. The specific surface area of the catalyst is
measured by Brunauer-Emmet-Teller (BET) method, which possesses a high specific surface area of 153.5 m2 g-1
with a Pore Volume of 0.33 cm3 g-1. High surface areas and mesoporous architecture could amplify the
electrochemical active surface area of electrocatalysts and accelerate the reactants and products transport rate,[38]
resulting in higher electrochemical activity. The thermogravimetric analysis (TGA) of CoSe@NCNT/NCN (Figure
2d) suggests that the content of CoSe is about 38 %.

Figure 2. (a) XRD patterns of NCN, Co@NCNT/NCN and CoSe@NCNT/NCN, (b) XRD patterns of
CoSe@NCNT/NCN at different temperature, (c) N2 adsorption–desorption isotherms and the pore size distribution
plot (inset) of Co@NCNT/NCN, (d) TGA and DSC curves of CoSe@NCNT/NCN
To further clarify the chemical constituents and states of the CoSe@NCNT/NCN, X-ray photoelectron
spectroscopy (XPS) measurements were operated. Figure S5 shows the full-scan XPS spectra, the
CoSe@NCNT/NCN surface mainly detects the signals of C, N, Co, Se and O elements. When the temperature
changes from 800 to 1000 oC, the N 1s peak weakens suggesting the content of N reduce with the rise of
temperature which is in accordance with previous research[39]. The existence of the oxygen signal is possibly
because the adsorbed oxygen-species by contacting with air. The electrocatalysts atomic content was analyzed by
XPS data (Table S3), the content of C in the XPS data is much more than that in CHN elemental analysis, which is
a pretty good proof that the CoSe nanoparticles were encapsulated in CNTs, because the detective distance of XPS
measurement is only about 2.0 nm.[40, 41] High resolution XPS spectra of Co 2p1/2 and Co 2p3/2 (Figure 3c) can

be deconvoluted into Co0, Co2+ and satellite peaks, respectively. The peaks located at 778.4 and 794.0 eV are
assigned to reduced Co species, the peaks at 780.8 and 796.9 eV reflect the existence of Co2+, which are
accordance with Co species in CoSe, and the peaks at 785.0 and 803.1 eV are attributed to shake-up satellite peaks
of Co 2p, indicating the antibonding orbital between the Co and Se atom.[42, 43] Figure 3d shows the spectrum of
Se 3d, the peaks at 55.1 and 54.2 eV are owing to Se 3d3/2 and Se 3d5/2, respectively, which is also in conformance
to Co-Se of CoSe. The additional peak at 59.4 eV is ascribed to the surface SeOx, and the another peak at 60.9 eV
is ascribed to Co 3p, consistent with the reported articles.[44] As shown in Figure 3a, the high resolution C 1s XPS
spectrum is fitted into three bonds located at 284.7, 285.3 and 286.3 eV, which is due to C-C, C-N, and C-O bonds,
respectively.[45, 46]The C-N bond indicates the existence of doping N. Furthermore, the high resolution XPS
spectrum of N 1s (Figure 2b) is fitted to four peaks with binding energy at 399.0, 398.4, 401.2, and 406.9 eV, for
the pyrrolic N, pyridinic N, graphitic N and oxygenated N, respectively. From Figure S8, the percentage of
pyridinic N reduced markedly and the graphitic N increased notably with the rise of heating temperature, which is
accordance with previous research.[37, 47] Therefore, the content of N decreased to appropriate level is good for
improving the conductivity and the activity of catalysts.

Figure 3. The high resolution spectra (a) C 1s (b) N 1s (c) Co 2p (d) Se 3d of CoSe@NCNT/NCN
The hierarchical 3D architecture of the as-prepared CoSe@NCNT/NCN presents excellent electrochemical
performance, which is better than other samples. The HER and OER performance of all catalysts are assessed in
N2-saturated 0.5 M H2SO4 solution and O2-saturated 1 M KOH solution, respectively, and the scan rate is 5 mV s-1
with a same loading of approximately 0.357 mg cm-2 at room temperature.
The HER polarization curves acquired by linear sweep voltammetry (LSV) are showed in Figure 4a and b.
CoSe@NCNT/NCN shows more excellent electrocatalysis activity than others, possessing the lowest overpotential
η=-197 mV at 10 mA cm-2 (a metric concerned solar fuel synthesis), which is smaller than
CoSe@NCNT/NCN-800 (η=-202 mV) and CoSe@NCNT/NCN-1000 (η=-247 mV). The HER performance of the
CoSe@NCNT/NCN could mainly ascribe to the charged Se and Co species, as an excellent electrocatalyst should
have weaker hydrogen bonds duly releasing the generated product. Se and Co are principally active sites acting as
proton-acceptor and hydride-acceptor centers for HER, respectively. The isolated metal center with a positive

charge as hydride-acceptor sites weakens the selenium-hydrogen bonds and reduces the energy barrier of the
formation of hydrogen-hydrogen bonds.[48] Meanwhile, the NCNTs combined with NCNs could promote the
transfer of reactants and products and expose more active catalysis sites for HER. In addition, the synergistic effect
of pyrrolic N and CoSe related to improves the HER activity, because that previous studies have shown that lone
pair electrons in pyrrolic N are considered catalytic centers.
The underlying mechanism of HER is revealed via Tafel plots derived from the LSV curves. In general, there
are three theoretical reaction mechanisms of HER in acidic media.[49] Firstly, the Volmer reaction (theoretical
Tafel slope of 118.2 mV dec-1) is primary discharge step, in which the adsorption of hydrogen on the catalyst
active sites switches to intermediate state hydrogen. Secondly, the Heyrovsky reaction (theoretical Tafel slope of
39.4 mV dec-1) is the step of electrochemical desorption and the Tafel reaction (theoretical Tafel slope of 29.6 mV
dec-1) is recombination step. As shown in Figure 4c, the Tafel slope of CoSe@NCNT/NCN (43mV dec-1) is
smaller than that of Co@NCNT/NCN (66 mV dec-1), bulk CoSe (92 mV dec-1) and NCN (159 mV dec-1),
indicating CoSe and N-doped carbon network have a synergistic effect to obtain superior catalytic performance.
The small Tafel slope of as-prepared catalyst is 43 mV dec-1, implying the Volmer-Heyrovsky is the catalytic
mechanism for HER and the desorption of hydrogen is the rate-limiting step.
To further understand the catalytic activity of CoSe@NCNT/NCN electrode, the electrode kinetics is
examined by electrochemical impedance spectroscopy (EIS). The smaller semicircular (represents charge-transfer
resistance Rct) is beneficial to the electrochemical kinetics between the interface of electrolyte and electrocatalyst,
indicating faster charge transfer speed. As shown in Figure S10f, we found that CoSe@NCNT/NCN electrode
owned a smallest Rct than other catalysts, suggesting fast electron transfer speed on the interface between electrode
and electrolyte and good electrical conductivity. In addition, the larger electrochemical surface area (ECSA)
represents more electrochemical active sites for catalysts. Therefore, the Cdl (double-layer capacitance) of the

catalysts were obtained firstly and shown in Figure 4e. The Cdl is calculated via utilizing CV measurements in a
non-faradic region with the potential range of 0.10-0.20 V vs. RHE at different scan rates from 20 to 180 mV s-1
(Figure S10a-e). The Cdl of CoSe@NCNT/NCN is calculated to be 4.70 mF cm-2, which is larger than that of
CoSe@NCNT/NCN-800 (4.51 mF cm-2), CoSe@NCNT/NCN-1000 (3.04 mF cm-2), Co@NCNT/NCN (1.79 mF
cm-2) and bulk CoSe (0.22 mF cm-2), indicating it has more active sites for catalysis. The specific ECSA values of
all catalysts were shown in Table S1, in which the CoSe@NCNT/NCN-900 exhibits the largest ECSA in
consistent with the electrocatalytic performance for HER. Furthermore, to further investigate the intrinsic HER
activity, the turnover frequency (TOF) values for samples were calculated and listed in Table S2. The TOF value
of CoSe@NCNT/NCN-900 is 0.1519 s-1 at the overpotential of 220 mV, which is higher than that of other samples.
The result of TOF values show that CoSe@NCNT/NCN-900 owns more active sites in per cm2 and higher atomic
utilization. The conclusion is consistent with the result of the optimum pyrolysis temperature at 900 °C.
In addition to the activity, the high stability is essential for a electrocatalyst to be used in water splitting. The
CoSe@NCNT/NCN maintains the previous LSV curve with slight decay of the cathodic current after 2000 cycles
of CV (Figure 4f). And after continuous 18h operation it still shows long time durability in acidic solution.

Figure 4. (a) LSV curves of CoSe@NCNT/NCN at different temperature and (b) CoSe@NCNT/NCN,
Co@NCNT/NCN, bulk CoSe, NCN and Pt/C in N2-saturated 0.5 M H2SO4 (sweep rate of 5 mV s-1), (c) the
corresponding Tafel slopes and (d) the overpotentials at 10 mA cm-2, (e) capacitive current densities of the
catalysts with different scan rates, (f) LSV curves of CoSe@NCNT/NCN after 2000 cycles (inset: i-t curve of 18
h).
The OER is a 4-electron transfer process with sluggish kinetics. And sluggish kinetics limits the entire
process of water electrolysis, which has been proved by theories.[50] The electrochemical OER tests of

CoSe@NCNT/NCN are evaluated by the LSV curves in O2-saturate 1.0 M KOH solution at a scan speed of 5 mV
s-1 using a three-electrode configuration. As shown in Figure 5a and b, the activity of CoSe@NCNT/NCN is
evidently better than that of other samples, and it displays a lowest overpotential of η=301 mV at 10 mA cm-2. The
overpotential of CoSe@NCNT/NCN is smaller than that of Co@NCNT/NCN (340 mV) and bulk CoSe (380 mV).
It is nearly close to commercial IrO2 catalyst (304 mV) and some of recently reported OER catalysts (Table S6),
which means the synthesized catalyst clearly showing a superior electrocatalysis performance. Meanwhile, the
Co-Se bond endows the Co element a higher valence state and the NCNTs combined with NCN promote the
transfer of reactants and products. They expose more catalysis active sites for OER.[51]
The Tafel slope of CoSe@NCNT/NCN (75 mV dec-1), Co@NCNT/NCN (69 mV dec-1), bulk CoSe (86 mV
dec-1), NCN (365 mV dec-1) and the commercial IrO2 (63 mV dec-1) are displayed in Figure 5c. The NCN shows
very high overpotential and Tafel slope, but pure CoSe possesses low overpotential and Tafel slope, indicating that
the catalytic activity sites are from CoSe mainly. In contrast, CoSe@NCNT/NCN exhibits superior electrocatalytic
properties better than other recently reported nonprecious-metal OER catalysts. The excellent OER property of
synthesis catalyst primarily ascribes to the synergistic effect of CoSe particles and N-doped carbon material. The
in situ growth NCNTs make the electrochemical active sites of Co more electrophilic by influencing electron
density of Co atoms to enhance the adsorption of OH- groups with CoSe@NCNT/NCN resulting in excellent OER
performance. Besides, the carbon nanotubes can offer ordered channels for charge transfer and the 3D hierarchical
structure enhances the conductivity and specific surface area of the CoSe@NCNT/NCN catalyst, thus resulting in
improved OER activity and stability. Moreover, the synthesis catalyst has a smaller semicircular diameter in EIS
than Co@NCNT/NCN and bulk CoSe (Figure S11f), revealing its small electrochemical impedance and fast
charge transport between electrocatalyst and electrolyte interface. The content of nitrogen can be determined by
CHN elemental analysis, the high N content can act as OER active sites and is beneficial to incorporation metal

species, but improving graphitization degree should be beneficial to electrical conductivity with decreasing the
content of N, so the temperature of 900 oC is the better balance.
The unique morphology of the CoSe@NCNT/NCN not only provides shorter diffusion path for electron
transfer but also affords a large specific surface area to increase catalytic activity sites, which have been calculated
with the electrochemical surface area (ECSA) by measuring the Cdl in non-Faradic reaction region (1.20-1.30 V vs.
RHE) at different scan rates from 20 to 180 mV s-1 (Figure S9a-e). As shown in Figure 5e, the obtained Cdl of
CoSe@NCNT/NCN is (6.88 mF cm-2), higher than that of CoSe@NCNT/NCN-800 (6.75 mF cm-2),
CoSe@NCNT/NCN-1000 (4.25 mF cm-2), Co@NCNT/NCN (3.19 mF cm-2) and bulk CoSe (0.46 mF cm-2). The
ECSA value of sample is calculated using the corresponding Cdl. From Table S1, the CoSe@NCNT/NCN-900
catalyst has the largest ECSA, which means exposing more active sites than other catalysts amplifying the
soild-liquid interface contact of electrolyte and catalytic. Similarly, the CoSe@NCNT/NCN-900 exhibits the
highest TOF value of 0.048s-1 among the other samples, indicating a effective atomic utilization in OER.
In addition to the outstanding catalytic activity, long-term durability of the electrocatalysis represents another
critical parameter for OER. Figure 5f show the CoSe@NCNT/NCN retains the analogous LSV curve after
continuous 2000 cycles. The i-t curve is displayed in the inset of Figure 5f, indicating the CoSe@NCNT/NCN
possesses the long-term durability in 1 M KOH solution in the 20 h continuous testing.
Furthermore, the XRD, TEM and EDX characterization were used to further investigate the catalyst phase,
morphology and compositions of CoSe@NCNT/NCN after long-term test in acidic for HER and in basic for OER.
As shown in Figure S12, the XRD pattern of the post-HER CoSe@NCNT/NCN shows pure CoSe phase,
illustrating it good stability for HER in acidic media. But the XRD pattern of the post-OER sample exhibits the
degradation of the CoSe phase, because small amount of metal oxide is formed. The TEM images in Figure S13a-b
exhibit that the morphology of the sample was well preserved in acidic solution, with N-doped CNTs grafted on

NCNs. The CoSe nanoparticle encapsulated N-doped CNTs most were not damaged in Figure S13c-d, which
means the morphology was well retained. The surface element distribution of catalyst after durability test was
obtained by EDX. From Figure S14-15, the relative oxygen content of the post-OER catalyst was significantly
higher than that of the post-HER sample. The result is in good agreement with the XRD, which is mainly due to
partly oxidized CoSe phase under OER process.
Finally, we expected that the CoSe@NCNT/NCN was used for overall water splitting. Therefore, a
two-electrode cell has been set up in 1.0 M KOH using CoSe@NCNT/NCN-900 as both the anode and cathode
(dropped onto carbon paper 0.4 mg/cm2). As shown in Figure S16, the sample exhibits large implemented potential
of 1.82 V at 10 mA cm-2. Apparently, the electrochemical performance of CoSe@NCNT/NCN-900 for overall
water splitting is inferior to that of catalysts reported previously, mainly due to the poor HER activity of CoSe in
alkaline medium because the mechanism of HER is different in alkaline and acidic solutions. However, the sample
exhibits better HER and OER activity in 0.5 M H2SO4 and 1.0 M KOH, respectively.

Figure 5. (a) LSV curves of CoSe@NCNT/NCN at different temperature and (b) CoSe@NCNT/NCN,
Co@NCNT/NCN, bulk CoSe, NCN and IrO2 in O2-saturated 1 M KOH (sweep rate of 5 mV s-1), (c) the
corresponding Tafel slopes and (d) the overpotentials at 10 mA cm-2, (e) capacitive current densities of the
catalysts with different scan rates, (f) LSV curves of CoSe@NCNT/NCN after 2000 cycles (inset: i-t curve of
20h).
4. Conclusions
In summary, we have developed a simple and effective strategy to fabricate CoSe nanoparticles encapsulated

NCNT grafted onto NCN through a facile one-step pyrolysis method. The unique 3D hierarchical structure endows
the CoSe@NCNT/NCN with large surface area, plentiful porosity and protects CoSe nanoparticles from harsh
corrosive solution. The CoSe@NCNT/NCN exhibits remarkable electrochemical activity and favorable reaction
kinetics for HER and OER, which is in comparison with the commercial Pt/C and IrO2 catalyst, respectively. And
the catalyst is better than most of previous reported carbon-based non-precious metal elecrocatalysts. The obtained
catalyst may be regarded as a succedaneum of scarce noble metal catalysts for energy conversion systems with
inexpensiveness, simple preparation and earth abundant advantages. This present work provides a new path for the
design and synthesis of 3D transition metal dichalcogenide capped in N-doped carbon materials, which have
outstanding catalytic performance and remarkable durability.
Conflicts of interest
The authors declare no conflict of interest
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final
version of the manuscript. †These authors contributed equally.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No. 51602138).
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The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.

The authors declare no conflict of interest