Nitrogen-doped carbon nanocones encapsulating with nickel-cobalt mixed phosphides for enhanced hydrogen evolution reaction

Article abstract of DOI:10.1039/c7ta05146h

In this work, nickel-cobalt mixed metal phosphides encapsulated into nitrogen-doped carbon nanocones (Ni₂P/NiCoP@NCCs) have been prepared through a facile ammonia-etching of nickel-cobalt Prussian blue analogs (Ni-Co PBAs) followed by a subsequent phosphidation treatment. During the in situ and confined phosphidation process, very small nickel-cobalt mixed metal phosphide nanocrystallites are uniformly incorporated into the simultaneously produced nitrogen-doped carbon matrix. Due to the unique structure and composition, the resultant Ni₂P/NiCoP@NCCs possess high electrocatalytic activity and excellent durability for the HER in both acidic and alkaline solutions.

Full text of DOI:10.1039/c7ta05146h

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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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Nitrogen-doped Carbon Nanocones Encapsulating with Nickel–
Cobalt Mixed Phosphides for Enhanced Hydrogen Evolution
Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Lei Han, Tongwen Yu, Wen Lei, Wenwen Liu, Kun Feng, Yuanli Ding, Gaopeng Jiang, Pan Xu and
Zhongwei Chen*
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work, nickel-cobalt mixed metal phosphides encapsulated performances.[1-2, 22, 27, 29-30] As reported, the most efficient way
into nitrogen-doped carbon nanocones (Ni2P/NiCoP@NCCs) have to control the morphologies of transition-metal phosphides is
been prepared through a facile ammonia-etching of nickel-cobalt to tune the morphology of the corresponding precursors. For
Prussian blue analogs (Ni-Co PBA) followed by the subsequent instance, nanostructured cobalt oxides with different
phosphidation treatment. During the in-situ and confined morphologies, including nanowires, nanosheets and
phosphidation process, very small nickel-cobalt mixed metal nanoparticles, were firstly prepared and then converted into
phosphides nanocrystallites are uniformly incorporated into the CoP via phosphidation route.[31] In addition, cobalt phosphides
simultaneously produced nitrogen-doped carbon matrix. Due to with hollow and/or porous concave polyhedron structure have
the unique structure and composition, the resultant been successfully prepared by heat treatment of zeolitic
Ni2P/NiCoP@NCCs possesses high electrocatalytic activity and imidazolate framework-67 (ZIF-67) polyhedron under air with
excellent durability for HER in both acidic and alkaline solutions.
the sequential phosphidation treatment with NaH2PO2.[19, 32]
Besides, Zhang et al. also reported the preparation of three-
dimensional (3D) graphene aerogel decorated with cobalt
phosphide nanoparticles through combining self-assembly
with freeze-drying and thermal treatment as well as
phosphidation process.[17] Despite the progress, it still remains
Hydrogen production by electrochemical water splitting is
widely regarded as a promising and attractive way to solve
current energy crisis and environmental issues.[1-8] In order to
make this process efficient and practical, it is very necessary
and important to exploit high active catalysts to accelerate
hydrogen evolution reaction (HER). To date, the state-of-the-
art HER electrocatalysts are mainly Pt-group noble metals and
their alloy due to their high catalytic activity such as low
overpotential and fast reaction kinetic.[9-13] However, their
paucity and high cost largely impede their practical
applications. Fortunately, some transition-metal-based
materials have been demonstrated as promising alternative to
a
challenge for the construction of transition-metal-
phosphides precursors with special complex nanostructured
morphologies such as nanocones because of their extreme
formation condition needed.
In the past few years, chemical etching method has been
reported as an efficient method to prepare some special
33-35]
which can endow some
complex nanostructures,[28,
unique physicochemical properties of materials themselves
resulting from their novel nanostructures. Recently,
Yamauchi’s group and Lou’s group utilized this method to
prepare various Prussian blue analogues (PBAs) and/or PBAs-
derived metal oxides/sulfides with hollow complex
nanostructures like hollow nanocubes, nanocages and
nanoframes.[28, 35-36] Inspired by them, in the present work,
nickel-cobalt Prussian blue analogue (Ni-Co PBA) nanocones
were successfully prepared through a facile ammonia-etching
method (Figure 1). In the preparation procedure, we shorten
the precipitation time of Ni-Co PBA nanocubes, thus achieving
the successful preparation of Ni-Co PBA nanocones after
ammonia-etching (see Supporting Information for
experimental details). Finally, after the phosphidation
treatment of the resultant Ni-Co PBA nanocones in Ar, nickel-
cobalt mixed metal phosphides encapsulated into nitrogen-
noble-metal HER electrocatalysts, including metal sulfides,[14-
15]
metal carbides[9-10]
,
metal nitrides,[16] and metal
phosphides.[17-21] Especially, transition-metal phosphides
receive much more attention owing to their facile preparation
22-27]
The
and low-cost as well as excellent durability.[4,
performances of materials are largely related to their
28]
Therefore, various methods have been
morphologie.[21,
developed to prepare different nanostructured morphologies
of transition-metal phosphides, aiming to achieve optimal
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Figure 1 Schematic of the preparation of Ni2P/NiCoP@N-
doped carbon nanocones (NCCs).
doped carbon nanocones (Ni2P/NiCoP@NCCs) could be
obtained. It should be noted that in our work, Ni-Co PBA is
chosen as precursor mainly based on the following two
reasons. On one hand, as a typical metal-organic framework
(MOF), Ni-Co PBA consists of Ni and Co, which makes it
promising precursor to prepare bimetallic phosphides.
Compared to monometallic phosphides, bimetallic phosphides
provide two electron-donating active sites to facilitate the
proton discharge in HER process, which helps in enhanced HER
activity.[37] On the other hand, active materials encapsulated
into carbon matrix have been demonstrated as promising
route to improve their HER activity because of the increase
electronic conductivity, avoiding the aggregation and leaching
of active materials, and the synergistic interaction between
the encapsulated active materials and carbon matrix.[10, 38-39]
During the phosphidation process in our work, Ni-Co PBA can
be easily converted into nickel-cobalt mixed metal phosphides
encapsulated into nitrogen-doped carbon matrix. These
features endow the high HER activity and excellent stability of
the resultant Ni2P/NiCoP@NCCs.
Figure 2 (a, d-e) SEM images, (b, f) TEM images and (c) Powder
XRD patterns of the resultant NiCo PBA nanocubes (a-c) and
NiCo PBA nanocones (c-f).
depends on the density of defects.[28] In comparison to the flat
planes, the corners of the cubes were reported to expose
more defects. In addition, the formation of Ni-Co PBA
nanocubes is the result of Ostwald ripening, which results in
the interior defect-rich parts along the diagonals of the
cubes.[28] So when reacting with ammonia, the cubes are
initially etched at their eight corners. As the reaction time
prolongs, the diagonal of the cubes begin to be etched, and
finally nanocones are obtained with twelve edges vanish.
The resultant Ni-Co PBA nanocones were further heat-
treated with NaH2PO2 under Ar. The morphology and structure
of the resultant Ni2P/NiCoP@NCCs are characterized by SEM
and TEM, and the results are shown in Figure 3. It can be seen
that the nanocones nanostructures are well maintained in the
resultant Ni2P/NiCoP@NCCs (Figure 3a-c). The high-resolution
TEM images (Figure 3d-e) indicate that very small Ni2P/NiCoP
nanoparticles are successfully uniformly encapsulated into the
produced continuous carbon matrix. The scanning TEM and
EDX mapping were also carried out to unravel the morphology
and element distribution. It can be easily observed that the
four elements including C, Ni, Co, and P are uniformly
distributed in the resultant Ni2P/NiCoP@NCCs (Figure 3f-j),
Scanning electron microscope (SEM) and transmission
electron microscopy (TEM) were firstly conducted to
characterize the morphology and structure of the resultant Ni-
Co PBA. As presented in Figure 2a-b, the resultant Ni-Co PBA
exhibits cube morphology with uniform size of around 400 nm.
The crystal structure of the resultant Ni-Co PBA nanotubes is
investigated by X-ray diffraction (XRD). All XRD peaks (Figure
2c) can match well with phase-pure Ni3[Co(CN)6]2·xH2O (JCPDS
card no. 89-3738). Energy dispersive X-ray (EDX) analysis result
indicates that the composition of the resultant Ni-Co PBA
nanocubes consist of Ni, Co, C, and N (Figure S1a), which is
consistent with the XRD result. After reacting with ammonia
solution for 1 h at room temperature, the resultant Ni-Co PBA
nanocubes are interestingly converted into cone-like
nanoparticles (Figure 2d-f). It can be seen from Figure 2d-e
that the resultant Ni-Co PBA after ammonia-etching exhibits
uniform nanocones morphology. TEM images (Figure 2f) also
confirm the successful formation of cone-like nanostructures.
The composition and crystalline phase of the resultant Ni-Co
PBA nanocones are also confirmed by EDX analysis and XRD. As
seen, the molar ratio of Ni and Co remains the similar as that
of the precursor nanocubes
(Figure S1b), and we also
interestingly find that all diffraction peaks are also the same as
that of the precursor nanocubes (Figure 2c), which are in
accordance with our previous reported results.[28] The
formation of the unique nanostructures could be explained as
follows. As we know, the chemical etching process easily
happens along with the defect-rich region, and its rate largely
Figure 3 (a-b) SEM images, (c-d) TEM images, (e) HRTEM, and
(f) HAADF-STEM and (g-k) elemental mapping images of the
resultant Ni2P/NiCoP@NCCs.
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which is in accordance with EDX result (Figure S1d). XRD
pattern reveals that the resultant product after phosphidation
treatment consists of Ni2P ((JCPDS card no. 89-2742) and
NiCoP (JCPDS card no. 71-2336) (Figure S3), and no peaks
corresponding to NiCo2O4 were observed. Besides, there is a
broad diffraction peak at 25°, which can be assigned to the
(002) plane of the hexagonal graphitic structure of carbon.[40]
The Raman spectra result (Figure S4) also confirms the
existence of carbon, where there are two bands centered at
1340 and 1555 cm-1 related to the disorders of the graphite (D-
band) and the sp2 carbon bonded graphitic structures (G-
band), respectively.[40] As comparison, the resultant Ni-Co PBA
nanocubes were also directly heat-treated with NaH2PO2
under Ar. SEM and TEM images reveal that the resultant
product retains well cube morphology with hollow structures
Figure
4
Electrochemical evaluation of the resultant
Ni2P/NiCoP@NHCCs and Ni2P/NiCoP@NCCs in 1.0 M NaOH (a-
c) and 0.5 M H2SO4 (d-f) with loading amount of 0.5 mg cm-2:
(a-d) Polarization curves with iR-corrected) on the GC
electrode at 5 mV s-1, (b-e) Tafel slopes, and (c-f) Polarization
curves with iR-corrected of Ni2P/NiCoP@NCCs before and after
1000 cycles at a scan rate of 100 mV s-1 between 0.05 V and -
0.233 V.
(Figure S2). Besides, XRD result confirms its composition also
contains Ni2P and NiCoP (Figure S3). Hereafter this product is
labeled as Ni2P/NiCoP@NHCCs for simplicity's sake.
XPS spectra were further carried out to analyze the surface
elemental composition and the valence states in the resultant
Ni2P/NiCoP@NHCCs. The XPS survey spectra in Figure S5a
shows the existence of Co, Ni, P, C, N, and O elements in the
resultant Ni2P/NiCoP@NHCCs. The high-resolution XPS spectra
of Co 2p, Ni 2p, P 2p, C 1s, N 1s, and O 1s are shown in Figure
S5b–g. In Co 2p spectra (Figure S5b), peaks at 778.9 and 781.9
eV are assigned to Co 2p3/2, while the peak located at 786.1 eV
is ascribed to the satellite peak of Co 2p3/2. The Co 2p1/2 region
also shows two main peaks at 793.5 and 798.1 eV and one
satellite peak at 803 eV. For Co 2p3/2, the peak at 778.9 eV
belongs to Co-P. The peak at 781.9 eV could be attributed to a
Co oxidized state, which is related to Co-POx. In Ni 2p spectra
resultant Ni2P/NiCoP@NCCs exhibits the highest HER activity
when the loading amount is 0.5 mg cm-2. Therefore the loading
amount is fixed at 0.5 mg cm-2 in the later experiment. Then
we compared the HER activity of the resultant
Ni2P/NiCoP@NCCs and Ni2P/NiCoP@NHCCs. Figure 4a exhibits
the polarization curves of the resultant Ni2P/NiCoP@NCCs and
Ni2P/NiCoP@NHCCs in 1.0 M KOH. Obviously, the resultant
Ni2P/NiCoP@NCCs shows the lower onset potential and higher
current density towards HER than the resultant
Ni2P/NiCoP@NHCCs. Specially, at a current density of 10 mA
cm-2, the overpotential for the resultant Ni2P/NiCoP@NCCs
(116 mV) is lower as 52 mV than that of the resultant
Ni2P/NiCoP@NHCCs (168 mV). Moreover, the HER
performance is superior to that of other reported bimetallic
(Figure S5c), the peaks at 856.9 eV and 861.6 eV are assigned
to the oxidized Ni species and the satellite of the Ni 2p3/2
,
respectively. The other peaks at 874.8 eV and 880.2 eV
correspond to oxidized Ni species, and the satellite of the Ni
2p1/2, respectively. As shown in Figure S5d, there are two
peaks at 129.3 eV and 134 eV in P 2p spectra, which stand for
reduced phosphorus in metal phosphides and phosphate
species resulting from the oxidation of metal phosphides after
exposure to air, respectively. To further demonstrate the
phosphides such as NiCoP hollow polyhedra and
CuCoP/nitrogen-doped carbon (see Table S1).[18,
20]
At the
same time, the kinetic behaviors of the resultant
Ni2P/NiCoP@NCCs and Ni2P/NiCoP@NHCCs are also further
investigated by Tafel plots derived from Figure 4a according to
the Tafel equation (η = b log j + a, where η is the overpotential,
j is the current density, and b is the Tafel slope). As displayed
in Figure 4b, the Tafel slope of the resultant Ni2P/NiCoP@NCCs
(79 mVꢀdec-1) is slightly smaller than that of the resultant
Ni2P/NiCoP@NHCCs (89 mVꢀdec-1), indicating the improved
HER kinetic over the resultant Ni2P/NiCoP@NCCs. As for
stability, it is also important for practical application. So
recycling CV or LSV experiment was performed to evaluate the
stability of the resultant catalysts. Figure 4c shows the
polarization curves of the resultant Ni2P/NiCoP@NCCs before
and after 1000 continuous cyclic voltammetry (CV) cycles at a
scan rate of 100 mV s-1 between 0.05 V and -0.257 V (vs. RHE).
No obvious change of HER activity is observed, revealing the
good stability of the resultant Ni2P/NiCoP@NCCs. On the other
hand, the HER activity of the resultant catalysts was also
evaluated in 0.5 M H2SO4, and the results are displayed in
oxidized Co/Ni, the O 1s binding energy is shown in Figure S5g
.
As shown, there are also two peaks at 531.5 eV and 533 eV,
which belongs to either MOx or M(OH)x (M = Co, Ni) and
oxidized phosphate species, respectively. Figure S5f shows the
XPS spectra of N 1s, where the peaks at 398.9, 400, and 401 eV
can be attributed to pyridinic-N, pyrrolic-N, and quaternary-N,
respectively. C 1s spectra (Figure S5e) also shows three peaks
at 284.6, 286.2, and 288.9 Ev, which are ascribed to sp2-C, -C-
O-, and -C=O, respectively. All of these XPS data confirm the
successful synthesis of Ni2P/NiCoP@NHCCs.
The HER activities of the resultant products including
Ni2P/NiCoP@NCCs and Ni2P/NiCoP@NHCCs were evaluated in
a three-electrode configuration, where graphite rod and
saturated calomel electrode (SCE) are used as counter
electrode and reference electrode, respectively. The effect of
loading amount of the catalyst on the HER activity was firstly
investigated. It can be clearly noticed from Figure S6 that the
Figure 4d-f
.
The HER activity of the resultant
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Ni2P/NiCoP@NCCs also outperforms obviously that of the Acknowledgement
resultant Ni2P/NiCoP@NHCCs, including lower onset potential This research was supported by the Natural Sciences and
and higher current density (Figure 4d). The overpotential of Engineering Research Council of Canada (NSERC). TEM was
120 mV the resultant Ni2P/NiCoP@NCCs is only required to obtained at the Canadian Center for Electron Microscopy
achieve the current density of 10 mA cm-2, which is lower than (CCEM) located at McMaster University.
that of the resultant Ni2P/NiCoP@NHCCs (136 mV) and
outperforms the reported other metal phosphides such as
Notes and references
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(
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Volmer step (116 mV dec-1), the Heyrovsky step (38 mV dec-1),
or the Tafel step (29 mV dec-1).[19]Therefore, the Tafel slope of
the resultant Ni2P/NiCoP@NCCs indicates that the
corresponding HER follows a Volmer–Heyrovsky mechanism.
However, the large Tafel slope herein may be further
optimized by annealing temperature and the mass ratio of
NiCo PBA and NaH2PO2.
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(
Figure S7). As can be noted, the BET surface area of the
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and
Ni2P/NiCoP@NHCCs
modified
electrodes (Figure S8). It can be seen that the resultant
Ni2P/NiCoP@NCCs shows the smaller electron-transfer
resistance than the resultant Ni2P/NiCoP@NHCCs, which may
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presented in Figure S9 and Figure S10, the resultant
Ni2P/NiCoP@NCCs and Ni2P/NiCoP@NHCCs both have almost
same ECSA. Therefore, the enhanced HER activity could be
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nitrogen-doped carbon matrix. When used as HER
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA05146H
Nickel-cobalt mixed metal phosphides encapsulated into nitrogen-doped carbon
nanocones have been successfully prepared through a facile ammonia-etching method
with the subsequent phosphidation treatment, which exhibit excellent high
electro-catalytic activity and excellent stability toward hydrogen evolution reaction in
both acidic and alkaline solutions.