One pot synthesis of Ni12P5 hollow nanocapsules as efficient electrode materials for oxygen evolution reactions and supercapacitor applications

Article abstract of DOI:10.1016/j.electacta.2017.01.169

The Ni₁₂P₅ hollow nanocapsule structure is synthesized adopting a facile one-pot hydrothermal method without any templates. The size of the obtained Ni₁₂P₅ nanocapsules is about 20?nm, and the wall thickness is about 3?nm. The XPS spectra of Ni₁₂P₅ samples were measured to identify the surface chemical composition and valence states. Oxygen evolution reaction results show that the onset overpotential of Ni₁₂P₅ is 290?mV and clearly demonstrates the Tafel slope of 49.8?mV?dec^?1. Meanwhile, the Ni₁₂P₅ nanocapsules show excellent electrochemical properties with specific capacitance of 949?F?g^?1 at 1?A?g^?1 and 81% of the initial value after cycles. Excellent electrochemical performances demonstrate that the nickel phosphide from a facile synthesis strategy could be a candidate for high performance energy storage and conversion.

Full text of DOI:10.1016/j.electacta.2017.01.169

Accepted Manuscript
Title: One pot synthesis of Ni P hollow nanocapsules as
1
2 5
efficient electrode materials for oxygen evolution reactions
and supercapacitor applications
Authors: Houzhao Wan, Lang Li, Yi Chen, Jinli Gong, Maiqi
Duan, Chang Liu, Jun Zhang, Hao Wang
PII:
S0013-4686(17)30208-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.01.169
EA 28836
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
18-11-2016
23-1-2017
25-1-2017
Please cite this article as: Houzhao Wan, Lang Li, Yi Chen, Jinli Gong, Maiqi Duan,
Chang Liu, Jun Zhang, Hao Wang, One pot synthesis of Ni12P5 hollow nanocapsules
as efficient electrode materials for oxygen evolution reactions and supercapacitor
applications, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.169
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

One pot synthesis of Ni12
5
P hollow nanocapsules as efficient electrode
materials for oxygen evolution reactions and supercapacitor
applications
Houzhao Wan, Lang Li, Yi Chen, Jinli Gong, Maiqi Duan, Chang Liu, Jun Zhang,
Hao Wang*
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials,
Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of
Physics and Electronic Science, Hubei University, Wuhan 430062, China
*Corresponding author. Fax: +86 27 88666177.
E-mail address: nanoguy@126.com (H. Wang).
1

Graphical abstract
2

Highlights
1
2
3
. The Ni12
hydrothermal method without any templates.
. The Ni12 nanocapsule shows that the onset overpotential of Ni12
and clearly demonstrates the Tafel slope of 49.8 mV·dec .
. The Ni12
1% of the initial value after cycles.
P hollow nanocapsule structure is synthesized adopting a facile one-pot
5
P
5
P
5
is 290 mV
–
1
−1
−1
P nanocapsules show the specific capacitance of 949 F g at 1 A g and
5
8
Abstract: The Ni12
one-pot hydrothermal method without any templates. The size of the obtained Ni12
nanocapsules is about 20 nm, and the wall thickness is about 3 nm. The XPS spectra
samples were measured to identify the surface chemical composition and
valence states. Oxygen evolution reaction results show that the onset overpotential of
P
5
hollow nanocapsule structure is synthesized adopting a facile
P
5
of Ni12
P
5
–
1
Ni12
P
5
is 290 mV and clearly demonstrates the Tafel slope of 49.8 mV·dec .
Meanwhile, the Ni12 nanocapsules show excellent electrochemical properties with
P
5
−1
−1
specific capacitance of 949 F g at 1 A g and 81% of the initial value after cycles.
Excellent electrochemical performances demonstrate that the nickel phosphide from a
facile synthesis strategy could be a candidate for high performance energy storage and
conversion.
Keywords: Nickel phosphide; Hollow; Nanocapsule; Oxygen evolution reaction;
Supercapacitor
3

1. Introduction
The increasing demand for clean energy and the growing global environmental
concerns have stimulated research activities in the development of effective
conversion and storage systems for sustainable alternative energy sources[1], such as
water splitting [2, 3], metal-air batteries [4, 5], fuel cells [6, 7] and supercapacitors [8,
9]. The oxygen electrode catalysts for oxygen evolution reaction (OER) as key
roles for those systems have attracted particular interests [10-12]. Similarly, it is
urgently needed to develop efficient noble-metal-free catalysts with high catalytic
activity for the OER to substitute high cost and low abundance RuO
2
and IrO [13].
2
Alternatively, supercapacitors have received much research attentions for their
high efficiency energy storage because of their prominent properties such as high
power density and long lifespan.[14-16] Pseudocapacitors can engender high specific
capacitance based on the fast and reversible redox reactions.[17-19] The phase
composition and structural parameters of the efficient pseudocapacitance active
material as research emphasis are the key to determine performance for
supercapacitors. [20, 21] Therefore, one key point of the two energy storage and
conversion systems has been heavily focused on a kind of highly efficient,
inexpensive and environmentally friendly electrode active materials.
Nickel phosphides have received increasing attention owing to their excellent
physicochemical properties and widespread applications in many fields [22], such as
electrocatalysts [23, 24], supercapcitors [25, 26], and lithium-ion batteries[27]. In
4

recent years, among them, nickel phosphides (especially nickel-metal phases, such as
Ni P) are potent catalysts for HER and OER due to their species diversity, efficient
and low-overpotential [23, 28, 29]. For example, the porous nickel phosphides foam
2
-
2
exhibits high catalytic current density of 191.0 mA cm at an overpotential of 0.35 V)
and outstanding long-term stability and durability [30]. Ni P nanoparticles show an
2
-
2
overpotential of only 290 mV at 10 mA cm in 1 M KOH [31]. In addition, nickel
phosphides have begun to attract attention as electrode materials for
supercapacitors.[32] For instance, Au/Ni12
P
5
core/shell nanocrystals [33] show 806.1
F g with a retention of 91.1% after 500 cycles. The Ni P and Ni nanoparticles [34]
exhibit 843.25 F g and 801.5 F g at 1 A g , respectively. The Ni-coated Ni P [35]
−1
2
P
5 4
−1
−1
−1
2
−1
−1
achieves 1115 F g at 2 A g , which can still maintain 92.3% after 3000 cycles. The
amorphous Ni−P [18] exhibits 1597 F g−1 at 0.5 A g−1 and good cycling stability.
However, a large number of work for metal phosphides focused on the preparation of
more complex and high energy consumption, such as solution-phase (using
trioctylphosphine, TOP at about 300 ), gas–solid reaction (calcined over 300 °C
under Ar or N
synthesized via the thermal decomposition approach in TOP at 320 °C for 2 h [36].
Ni P nanoparticles were synthesized in TOP at 320 °C [24]. Ni-P porous nanoplates
were obtained at 300°C for 2 h by thermal decomposition of NaH PO under Ar flow
37]. However, the preparation method and formation mechanism of metal phosphides
with special microstructures have not been worked out well.
2
). For instance, Ni12
P
5
, Ni
2
P and Ni
5
P
4
nanocrystals were
2
2
2
[
Hollow nanostructures are composed of compact walls with a larger surface area
5

than bulk for more abundant active sites, so that hollow structures is an effective
activity to achieve high performance electrochemical process. [38-40] However, there
are few reports on hollow nanostructures by a one-step without using template,
especially for transition metal phosphide. In this work, we report facile one-pot
synthesis of Ni12
method. The size of the obtained Ni12
thickness is about 3 nm. The onset overpotential of Ni12
P
5
hollow nanocapsules without any templates using hydrothermal
nanocapsules is about 20 nm, and the wall
is 290 mV and clearly
demonstrates the Tafel slope of 49.8 mV·dec . Meanwhile, the Ni12 nanocapsules
show excellent electrochemical properties with specific capacitance of 949 F g at 1
P
5
P
5
–
1
P
5
−1
−
1
A g .
2
. Experimental Section
2
.1 Preparation of samples
2.1.1 Preparation of Ni12
P
5
nanocapsules
In a typical experiment, 2 mmol Ni(CH
3
COO)
2
·4H O, 8 mmol red phosphorus,
2
and 1 mmol hexadecyl trimethyl ammonium Bromide (CTAB) were dissolved in
mixed solution of 20 mL deionized water and 40 mL ethanol. Then, the above
solution was transferred into a 100-mL teflon-lined stainless autoclave, and kept at
180 °C for 30 h. Then, the product was filtered washed several times with deionized
water and ethanol. The final product was obtained after drying in vacuum at 60 °C for
12 h.
2
.1.2 Preparation of flower-like Ni(OH)
2
and flower-like NiO.
6

For the synthesis of flower-like Ni(OH)
Ni(NO ·6H O) and 10 mmol hexamethylenetetramine (HMTA) were dissolved in
0 mL deionized (DI) water. The mixture was stirred for 30 minutes, following which
it was transferred into a 100 mL Teflon-lined stainless autoclave and kept at 100 °C
for 10 h. The green Ni(OH) was obtained after washed and dried. For the synthesis
of flower-like NiO, the above green Ni(OH) was put into the sequencing tube furnace
and heated to 400 °C with air drummed into at the rate of 5 °C per minute, and kept at
00 °C for 2 h. Then black NiO was collected after gradually cooled to room
temperature.
2
, 10 mmol Nickel nitrate hexahydrate
(
3
)
2
2
6
2
2
4
2.2 Materials Characterization
X-ray diffraction (XRD, Philips X'Pert PRO; Cu Kα, λ = 0.1542 nm) was used to
analyze the phase of the sample, the micro-structure of sample was examined by FEI
Quanta 200 scanning electron microscope (SEM). Energy dispersive X-ray
spectroscopy (EDS) analyzes the component of the samples during SEM
measurements. The hollow nanostructure was investigated by means of transmission
electron microscopy (TEM, Philips, TecnaiG2 20). The X-ray photoelectron
spectroscopy (XPS, Kratos AXIS Ultra DLD-600W XPS system) spectra was
measured the electronic structure and surface composition with a monochromatic Al
Kα (1486.6 eV) as X-ray source.
2
.3 Electrochemical measurement
For oxygen evolution reaction, the sample as active material and Ketjen black as
conductive additives were mixed in a weight of 5 mg and 1 mg with 32 μL of 5 wt.%
7

Nafion solution in 0.968 mL of 4:1 V/V water/ethanol. And then, 7 μL of the mixed
slurry was loaded onto a glassy carbon electrode of 5 mm in diameter after sonication
for at least 30 min. The OER tests were carried out in a conventional three electrode
electrochemical cell by using CHI660E electrochemical workstation and Pine
Modulated Speed Rotator with a Pt foil electrode as a counter electrode and an
Ag/AgCl electrode as the reference electrode. The reversible hydrogen electrode
(
[
RHE) was used as scale for all potentials measured according to the Nernst equation
41]: ERHE = EAg/AgCl + 0.197 + 0.059*pH (= EAg/AgCl + 1.0006, 1 M KOH PH ≈ 13.62).
Polarization curve measurements for OER were carried out in N -saturated 1 M KOH
2
solution at different scan rates at 1600 rpm to eliminate the side effects of oxygen
bubble adsorbed on the electrode surface. Overpotential was calculated as follows:
η(V)=Evs.RHE−1.23.
For supercapacitor, the sample as active material, poly (tetrafluoroethylene)
(PTFE) suspension (60 wt%) as binder and acetylene black as conductive additives
were mixed in a weight ratio of 8:1:1. The part of mixture were coated on the 1.13
2
cm of foam nickel, and dried in vacuum at 60 °C for 12 h. Then, the electrodes were
−2
pressed by 15 MPa. The mass of the active materials is about 4–6 mg cm on 1.13
2
cm electrodes. Electrochemical properties of as-obtained electrodes were measured
by cyclic voltamogram (CV) and chronopotentiometry via a CHI660E in a
conventional three-electrode system at 6 M KOH solution, in which Pt and Hg/HgO
served as the counter electrode and the reference electrode, respectively.
8

3. Results and discussion
The Ni12
P
5
hollow nanocapsule was obtained with red phosphorus as the
phosphorus source and nickel acetate as the nickel ion precursor by hydrothermal
reaction. Formation mechanism diagram of the Ni12 hollow nanocapsule was shown
P
5
in Figure 1a. At first, the surface of red phosphorus is smooth without any signs of
rough before the reaction (Figure 1b), which reveal that red phosphorus is difficult to
dissolve in deionized water and anhydrous ethanol. With increasing reaction time, red
phosphorus gradually decomposed into phosphorus ions [42] (Figure 1c). In the
following, nickel phosphides formed by the reaction of phosphorus ion and nickel ion
were assembled into a hollow nanostructure, the diameter of hollow nanospheres is
about 20~50 nm as shown in Figure 1d.
The morphology of Ni12
5
P hollow nanocapsule product was investigated by
FE-SEM and TEM images, as presented in Figure 2. From the SEM image from
Figure 2a and 2b, the nano-size of the obtained product is about 20~50 nm. In order to
accurately determine the composition and distribution of nanostructures, the
composition analysis of the product was conducted. The EDS element mapping
confirms that Ni and P homogenously distribute among the nanostructures (Figure
2
d~2f). Moreover, the quantitative analysis of EDS data reveals the average Ni/P ratio
to be approximately 12:5 (Figure 2g). The TEM images in Figure 2h and 2i show that
the edges of the Ni12 spheres are significantly thicker than the center, which further
P
5
confirms that the as-prepared nanospheres are hollow structures, and the wall
thickness is about 4 nm. The HRTEM images (Figure 2j and Figure S1) reveal that the
9

fringe spacings are about 3.07, 1.93, 1.86 Å, corresponding to the (211), (240) and
312) lattice planes of tetragonal Ni12
The crystal of the prepared Ni12
The diffraction peaks are observed at 29.2, 32.7, 35.8, 38.4, 41.2, 41.7, 44.4, 46.9,
8.9, 54, 60.1° correspond to (220), (310), (301), (112), (202), (400), (330), (240),
312), (210), (242). The typical sample can be indexed to the tetragonal phase of
(JCPDS 74-1381) with I4/m space group and a tetragonal unit cell a=b=
.8646 nm and c = 0.5070 nm (the atomic structure inserted in Figure 3a). The XPS
survey scan in Figure 2b shows the sample contain Ni, P, O, and C elements. The high
resolution XPS spectra of Ni and P elements from Ni12 hollow nanocapsules are
shown in Fig. 3c and 3d. The peaks at 853.6, 856.3, and 861.9 eV are assigned to Ni
p3/2 binding energies.[43] The peak at 853.6 eV is close to zero valence Ni (852.8
(
5
P .
5
P was examined by XRD analysis (Figure 3a).
4
(
Ni12
P
5
0
P
5
2
eV). It is therefore suggested that Ni species in Ni12
5
P have a very small positive
δ+
charge (Ni , 0 < δ < 2). The peak at 129.7 eV for P 2p is less than that of elemental P
δ−
(130.2 eV), so P species in Ni12
P
5
has a very small negative charge (P , 0 < δ < 1). In
addition, due to the exposure of the sample to air, the peaks at 856.3 and 861.9 eV in
Ni 2p3/2 and the peak at 133.2 eV in P 2p are likely to be correlated with nickel
phosphate formed on the surface of Ni12P .
5
The OER electrocatalytic performance of the as-prepared samples was carefully
evaluated with a standard three-electrode system using a rotating disk electrode (RDE)
configuration in 1.0 M KOH solution. Figure 4a shows the LSV curves of Ni12
P
5
hollow nanocapsules, NiO flower-like, Ni(OH)
2
flower-like and 20% PtC at 5 mV s-1
1
0

(
the SEM images of NiO and Ni(OH)
2
flower-like were shown in Figure S2, after
active electrode occurs in
.37 V, which attached to Ni oxidized to Ni . The onset overpotentials of four
samples are 290, 380, 370 and 340 mV, respectively. The smallest onset overpotential
indicates the most excellent OER catalytic activity. Furthermore, the
IR-correction). We can see that the oxidation peak of Ni12
P
5
2
+
3+
1
of Ni12
P
5
−2
corresponding overpotentials at j = 10 mA cm , which are commonly used to
evaluate the OER activity, are shown in Fig. 4b, and the lower overpotential further
implies much higher performance toward OER. Further catalytic kinetic evaluation as
an important factor for OER can be observed from the Tafel plot. Tafel slopes of the
samples are based on equation [44]: ŋ = b*log J + a, where ŋ is the overpotential, J is
the current density, a is a constant, and b is the Tafel slope, as shown in Figure 4c. The
Ni12
5
P hollow nanocapsules clearly demonstrates the lowest Tafel slope of 49.8
–
1
–1
mV·dec contrast to those of NiO flower-like (127 mV·dec ), Ni(OH)
2
flower-like
129mV·dec ) and 20% PtC (52 mV·dec ). To make a better comparison between
nanocapsules and other kind of nickel phosphides, the related OER data are
–
1
–1
(
Ni12
P
5
listed in Table 1. From the table we can see that the performances of Ni12
P
5
5
nanocapsules for OER are outstanding. The enhanced OER performance of the Ni12
P
hollow nanocapsules may be ascribed to high activity metal phosphide crystal
structure. Moreover, the special microstructure with hollow nanocapsule can supply
efficient active sites.
The electrochemical impedance spectra (EIS) of the samples were obtained after Tafel
measurements to understand the charge transport process inside the electrodes. The Nyquist plots
1
1

5
of the Ni12P hollow nanocapsule has a smaller diameter of semicircle than those of NiO
2
flower-like and Ni(OH) flower-like (as shown in Figure 4d), suggesting the better conductivity of
5
Ni12P hollow nanocapsule. The equivalent circuits used to fit the EIS data are shown in the insets
s 1 2
of Figure 4d. The impedance R , R and R represent solution resistance of the threeelectrode cell
system, the intrinsic resistance of the catalyst and the resistance of electrocatalytic process,
respectively. The fitting results of impedance data summarized in Tables 2. The resistance of
2
electrocatalytic process on the Ni12
P
5
nanocapsules is 760 Ω cm , which is also smaller than that
2
2
on NiO flower-like (5900 Ω cm ) and Ni(OH)
2
flower-like (1230 Ω cm ). The results explain the
trends shown in OER activity (Figure 4).
In order to investigate the capacitance performance of Ni12
electrode material, the Ni12 as a working electrode was studied by means of CV,
galvanostaticcharge/discharge in 6 M KOH using a three electrode system. Figure
presents the CV curves of Ni12 hollow nanocapsule, NiO flower-like and Ni(OH)
P
5
hollow nanocapsule
P
5
5
P
5
2
−1
flower-like at a scan rate of 5 mV s . All of electrodes have a pair of strong redox
peaks, indicating that the measured capacitance is mainly based on the redox
2
+
3+
2+
mechanism. The oxidation peak near 0.53 V corresponds to the change of Ni /Ni
3
+
valence state. The reduction peak near 0.35 V corresponds to the change of Ni /Ni
valence state. The enclosed area for Ni12
excellent capacitive behavior.
5
P is the largest among them, indicating its
Figure 6a compares the charge/discharge curves of Ni12
P
5
hollow nanocapsules in
-
1
the potential range of 0 ~ 0.5 V (vs. Hg/HgO) at 1 A g . Consistent with the CV
1
2

results, The Ni12
P
5
hollow nanocapsules electrode exhibits a much longer discharge
flower-like,
time and capacitance performance than NiO flower-like and Ni(OH)
2
which is attributed to increased redox reactions caused by the higher conductivity and
activity. Based on the discharge curves, the specific capacitances of the above
electrode materials as a function of current density are shown in Figure 6b. The Ni12
P
5
hollow nanocapsules calculated from each discharge curve are about 949, 882, 835,
-
1
-1
7
90, 756, 723, 701, 675 and 655 F g at 1, 1.5, 2, 3, 4, 5, 6, 8 and 10 A g , which
shows a higher specific capacitance on the whole performance than NiO flower-like
-
1
-1
-1
-1
-1
(
326 F g at 1 A g , and 210 F g at 10 A g ) and Ni(OH) flower-like ( 787 F g at
2
-
1
-1
-1
1
A g and 523 F g at 10 A g ).
Figure 7 shows the cyclic performances of Ni12
P hollow nanocapsules electrode
5
−1
examined by galvanostatic charge–discharge tests for 2000 cycles at 5 A g . It is
found that the specific capacitance of the Ni12 progressively rises to 115% with the
P
5
th
increase of the number of cycles before 300 due to the process of infiltration of
electrolyte and activation of the active materials. And then, the specific capacitance
decreases with the increase of cycle times, which has 81% of the initial value after
st
th
th
cycles. The inset is the galvanostatic charge/discharge curve after 1 , 100 , 1000 and
th
2
000 in accordance with the tendency of cycle life.
4. Conclusions
We firstly synthesized nickel phosphide Ni12
adopting a facile one-pot hydrothermal method. The size of the obtained Ni12
nanocapsules is about 20 nm, and the wall thickness is about 3 nm. Ni and P species
P hollow nanocapsules structure
5
P
5
1
3

δ+
δ−
in the Ni12
P
5
have a very small positive (Ni , 0 < δ < 2) and negative (P , 0 < δ < 1)
is 290 mV and clearly
demonstrates the Tafel slope of 49.8 mV·dec . Meanwhile, the Ni12 nanocapsules
show excellent electrochemical properties with specific capacitance of 949 F g at 1
charge, respectively. The onset overpotential of Ni12
P
5
–
1
P
5
−1
−1
A g . This work provides a facile strategy for the synthesis of nickel phosphide for
high performance energy storage and conversion.
Ackonwledgements
This work is partially supported by the National Natural Science Foundation of China
(NSFC, No. 11574077).
1
4

References
[
[
[
(
[
[
1] J. Zhang, Z. Zhao, Z. Xia, L. Dai, Nat. nanotechnol., 10 (2015) 444-452.
2] Z. Sun, H. Zheng, J. Li, P. Du, Energy Environ. Sci., 8 (2015) 2668-2676.
3] J. Li, J. Li, X. Zhou, Z. Xia, W. Gao, Y. Ma, Y. Qu, ACS appl. mater. inter., 8
2016) 10826-10834.
4] Z. Zhao, M. Li, L. Zhang, L. Dai, Z. Xia, Adv. mater., 27 (2015) 6834-6840.
5] K.N. Jung, S.M. Hwang, M.S. Park, K.J. Kim, J.G. Kim, S.X. Dou, J.H. Kim, J.W.
Lee, Scientific reports, 5 (2015) 7665.
6] S. Sengodan, S. Choi, A. Jun, T.H. Shin, Y.-W. Ju, H.Y. Jeong, J. Shin, J.T.S.
Irvine, G. Kim, Nat. Mater., 14 (2014) 205-209.
[
[7] Y. Hu, D.H. Chua, Scientific reports, 6 (2016) 28088.
[8] B. Xie, Y. Wang, W. Lai, W. Lin, Z. Lin, Z. Zhang, P. Zou, Y. Xu, S. Zhou, C.
Yang, F. Kang, C.-P. Wong, Nano Energy, 26 (2016) 276-285.
9] B. Xie, Y. Chen, M. Yu, T. Sun, L. Lu, T. Xie, Y. Zhang, Y. Wu, Carbon, 99 (2016)
5-42.
10] A. Pendashteh, J. Palma, M. Anderson, R. Marcilla, Applied Catalysis B:
Environmental, 201 (2017) 241-252.
11] L. Yuan, Z. Yan, L. Jiang, E. Wang, S. Wang, G. Sun, J Energy Chem., 25 (2016)
05-810.
[
3
[
[
8
[
(
[
12] L. Wang, C. Lin, D. Huang, F. Zhang, M. Wang, J. Jin, ACS appl. mater. inter., 6
2014) 10172-10180.
13] J. Jiang, S. Lu, H. Gao, X. Zhang, H.-Q. Yu, Nano Energy, 27 (2016) 526-534.
1
5

[
14] X. Peng, H. Liu, Q. Yin, J. Wu, P. Chen, G. Zhang, G. Liu, C. Wu, Y. Xie, Nat.
commun., 7 (2016) 11782.
15] Z. Liu, F. Teng, C. Chang, Y. Teng, S. Wang, W. Gu, Y. Fan, W. Yao, Y. Zhu,
Nano Energy, 27 (2016) 58-67.
[
[16] J. Yan, Q. Wang, T. Wei, Z. Fan, Adv. Energy Mater., 4 (2014) 1300816.
[17] L. Shen, L. Yu, H.B. Wu, X.Y. Yu, X. Zhang, X.W. Lou, Nat. commun., 6 (2015)
6
694.
18] D. Wang, L.-B. Kong, M.-C. Liu, W.-B. Zhang, Y.-C. Luo, L. Kang, J Power
Sources, 274 (2015) 1107-1113.
[
[19] Y.-K. Hsu, Y.-C. Chen, Y.-G. Lin, Electrochim. Acta, 139 (2014) 401-407.
[20] X. Wang, J. Hu, W. Liu, G. Wang, J. An, J. Lian, J. Mater. Chem. A, 3 (2015)
2
3333-23344.
21] M. Qiu, P. Sun, L. Shen, K. Wang, S. Song, X. Yu, S. Tan, C. Zhao, W. Mai, J.
Mater. Chem. A, 4 (2016) 7266-7273.
22] Y. Lu, T. Wang, X. Li, G. Zhang, H. Xue, H. Pang, RSC Adv., 6 (2016)
7188-87212.
23] Y. Shi, Y. Xu, S. Zhuo, J. Zhang, B. Zhang, ACS appl. mater. inter., 7 (2015)
376-2384.
24] E.J. Popczun, J.R. McKone, C.G. Read, A.J. Biacchi, A.M. Wiltrout, N.S. Lewis,
R.E. Schaak, J Am. Chem. Soc., 135 (2013) 9267-9270.
[
[
8
[
2
[
[25] M.-S. Wu, C.-J. Chung, Z.-Z. Ceng, RSC Adv., 5 (2015) 4561-4567.
[26] X. Chen, M. Cheng, D. Chen, R. Wang, ACS appl. mater. inter., 8 (2016)
1
6

3
892-3900.
27] Y. Bai, H. Zhang, X. Li, L. Liu, H. Xu, H. Qiu, Y. Wang, Nanoscale, 7 (2015)
446-1453.
28] X. Wang, Y. V. Kolen’ko, X. Bao, K. Kovnir and L. Liu, Angew. Chem. Int. Ed.,
4 (2015) 8188–8192.
[
1
[
5
[
[
[
[
29] Y. Pan, Y. Liu, C. Liu, J Power Sources, 285 (2015) 169-177.
30] X. Wang, W. Li, D. Xiong, L. Liu, J. Mater. Chem. A, 4 (2016) 5639-5646.
31] L.-A. Stern, L. Feng, F. Song, X. Hu, Energy Environ. Sci., 8 (2015) 2347-2351.
32] C. An, Y. Wang, Y. Wang, G. Liu, L. Li, F. Qiu, Y. Xu, L. Jiao, H. Yuan, RSC Adv.,
3
(2013) 4628.
[
[
(
[
33] S. Duan, R. Wang, NPG Asia Mater., 6 (2014) e122.
34] D. Wang, L. B. Kong, M. C. Liu, Y. C. Luo and L. Kang, Chem.–Eur. J., 21
2015) 17897–17903.
35] Y. Lu, J.-k. Liu, X.-y. Liu, S. Huang, T.-q. Wang, X.-l. Wang, C.-d. Gu, J.-p. Tu,
S.X. Mao, CrystEngComm, 15 (2013) 7071.
36] Y. Pan, Y. Liu, J. Zhao, K. Yang, J. Liang, D. Liu, W. Hu, D. Liu, Y. Liu, C. Liu, J.
Mater. Chem. A, 3 (2015) 1656-1665.
37] X.-Y. Yu, Y. Feng, B. Guan, X.W. Lou, U. Paik, Energy Environ. Sci., 9 (2016)
246-1250.
38] X.-F. Lu, D.-J. Wu, R.-Z. Li, Q. Li, S.-H. Ye, Y.-X. Tong, G.-R. Li, J. Mater.
Chem. A, 2 (2014) 4706–4713.
39] Y. Wang, Y. Sun, X. Zhang, Y.-h. Wen, J. Guo, RSC Adv., 6 (2016) 51710-51715.
[
[
1
[
[
17

[40] Y. Dong, S. Liu, Y. Liu, Y. Tang, T. Yang, X. Wang, Z. Wang, Z. Zhao, J. Qiu, J.
Mater. Chem. A, 4 (2016) 17718-17725.
[
41] S.K. Bikkarolla, P. Papakonstantinou, J Power Sources, 281 (2015) 243-251.
42] Y.-M. Hu, M.-C. Liu, Y.-X. Hu, Q.-Q. Yang, L.-B. Kong, L. Kang, Electrochim.
[
Acta, 215 (2016) 114-125.
[43] Y. Pan, W. Hu, D. Liu, Y. Liu, C. Liu, J. Mater. Chem. A, 3 (2015) 13087-13094.
[44] C. Zhang, J. Xiao, X. Lv, L. Qian, S. Yuan, S. Wang, P. Lei, J. Mater. Chem. A, 4
(2016) 16516-16523.
1
8

FIGURE CAPTIONS
Figure 1 (a) Formation mechanism diagram of the Ni12
5
P hollow nanocapsule. (b)
SEM image of the red phosphorus bulk. (c) SEM image of the sample obtained at 5 h.
d) SEM image of the sample at 30 h.
Figure 2 (a-c) SEM images of Ni12
mapping of the sample. (g) The quantitative analysis of EDS data (h, i) TEM images
nanocapsules. (j) HRTEM image Ni12 nanocapsule.
Figure 3 (a) XRD pattern of Ni12 hollow nanocapsules; XPS spectra of (b) survey
spectrum, (c) Ni 2p, (d) P 2p regions for Ni12 hollow nanocapsules.
Figure 4 (a) LSV curves of Ni12 hollow nanocapsules, Ni flower-like, Ni(OH)
flower-like and 20% PtC at 5 mV s (all the data for OER are iR-correct). (b)
(
5
P hollow nanocapsules. (d-f) The EDS element
of Ni12
P
5
P
5
P
5
P
5
P
5
2
-
1
−2
Overpotentials required to reach j = 10 mA cm of the four samples. (c) Tafel plot
derived from the corresponding polarization curves. (d) Nyquist EIS of the three
samples at 0.5 V in 1 M KOH. Insets: Equivalent circuits for three samples.
Figure 5 CV curves of Ni12
flower-like at 10 mV s-1
Figure 6 (a) the charge/discharge curves of Ni12
flower-like and Ni(OH) flower-like in the potential range of 0 ~ 0.5 V (vs. Hg/HgO)
P
5
hollow nanocapsules, NiO flower-like and Ni(OH)
2
.
5
P hollow nanocapsules, NiO
2
-
1
at 1 A g . (b) The specific capacitances of the three electrode materials as a function
of current density.
Figure 7 Cycle life of Ni12
P
5
hollow nanocapsules electrode by galvanostatic
-
1
charge–discharge tests at 5 A g . The inset is the galvanostatic charge/discharge curve
st
th
th
th
after 1 , 100 , 1000 and 2000 .
1
9

Figure 1
20

Figure 2
2
1

Figure 3
Figure 4
2
2

Figure 5
2
3

Figure 6
2
4

Figure 7
25

Table 1 Comparison of OER activity data for different catalysts.
η (mV)
Tafel slope
E OER (V)
Catalysts
Ref.
at 10 mA cm−2
(mV dec-1)
49.8
127
at 10 mA cm−2
1.52
Ni12
P
5
nanocapsule
NiO flower-like
Ni(OH) flower-like
0% PtC
Ni
P nanowire
C@ Ni-P
290
380
370
340
320
290
300
This work
This work
This work
This work
Ref. [3]
1.61
2
129
1.60
2
52
1.57
x
P
y
72.2
59
1.57
Ni
2
1.52
Ref. [31]
Ref. [37]
64
1.53
26

5
Table 2 EIS fitting results for Ni12P nanocapsules.
Rs
R
1
R
2
-
2
C
2
(F cm-2)
Sample
C
1
(F cm )
(
Ω cm2)
43.8
92
(Ω cm2)
55
(Ω cm2)
760
Ni12
P
5
nanocapsule
NiO flower
Ni(OH) flower
3.42×10-3
2.8×10-3
6.7×10-4
1.6×10-3
9.8×10-4
1.01×10-3
409
5900
1230
2
55
151
27