3D hierarchical Ni(PO3)2 nanosheet arrays with superior electrochemical capacitance behavior

Article abstract of DOI:10.1039/c6ta09528c

A novel electrode material, three-dimensional (3D) hierarchical Ni(PO₃)₂ nanosheet arrays, is explored via a rational topotactic strategy combining hydrothermal growth with subsequent phosphorylation treatment. The as-synthesized Ni(PO₃)₂ nanosheet arrays self-supported on activated carbon cloth (Ni(PO₃)₂@ACC) exhibit superior electrochemical capacitive behaviors to a Ni(OH)₂ analogue with similar structural characteristics. The areal and mass specific capacitances of the arrays are as high as 4.43 F cm^-2 at a current density of 2 mA cm^-2 and 2237 F g^-1 at ～1 A g^-1, respectively, which is comparable to the best reported results for Ni-based and Me-PO_x battery-type electrodes. Furthermore, even at a high rate of 10 mA cm^-2, the arrays maintain ～90% charge retention after 2000 cycles, suggesting their robust stability and their very promising applications for energy storage.

Full text of DOI:10.1039/c6ta09528c

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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
rsc.li/materials-a

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3D Hierarchical Ni(PO3)2 Nanosheet Arrays with Superior
Electrochemical Capacitance Behaviors
Qiao Liu,a Chongyi Chen,b Jinju Zheng,a Lin Wang,a Zuobao Yang,a and Weiyou Yanga,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
to the unfavorable reaction kinetics of their electrode materials. To
achieve superior overall performances, the representative
strategies have been developed, including 1) the fabrication of
composites incorporated conductive carbon materials to improve
electronic conductivity within the electrode, 2) the synthesis of
electrode materials with rich nanostructural features, for instance,
the ultrathin layered materials with most functionalities exposed to
the electrolytes, could create abundant electroactive sites and
thereby have an improved specific capacitance and energy
density,7-9 and 3) the exploration of novel electrode materials for
energy storage. In brief, the development of novel synthetic routes
and/or new class of transition metal-based electrode materials with
excellent electrochemical performances has become a hot spot in
the field of energy storage.
A
novel type of electrode material, three-dimensional (3D)
hierarchical Ni(PO3)2 nanosheet arrays, is expored via a rational
topotactic strategy combining hydrothermal growth with
a
followed phosphorylation treatment. The as-synthesized Ni(PO3)2
nanosheets arrays self-supported on activated carbon cloth
(Ni(PO3)2@ACC) exhibit superior electrochemical capacitive
behaviors than their analogue of Ni(OH)2 with similar structual
characteristics. Their areal and mass specific capacitances could be
up to 4.43 F cm−2 at a current density of 2 mA cm−2 and 2237 F g−1
at ~1 A g−1, respectively, which are comparable to the best ones
among the Ni-based and Me-POx battery-type electrodes ever
reported. Furthermore, even at a high rate of 10 mA cm−2, they
maintains ~90% charge retention after 2000 cycles, suggesting
their robust stability and very promising applications for energy
storage.
Transition metal phosphates (Me-POx) have always drawn
enormous attentions, because of their unique physical/chemical
characteristics and potentially wide applications in energy
conversion and storage.10, 11 To date, a variety of nanostructured
Me-POx materials, such as phosphites of cobalt or nickel
(M11(HPO3)8(OH)6, M=Co, Ni),12-14 phosphates of cobalt
(CoHPO4·3H2O),15 manganese (Mn3(PO4)2)16 or vanadyl (VOPO4),17
pyrophosphate of cobalt (Co2P2O7),18 have emerged as the effective
electrode materials for ultracapacitors. Notably, to the best of our
knowledge, there still has no report on the metaphosphates of
transition metal (Me-PO3) based electrode materials for
electrochemical energy storage.
Several phases of Me-PO3 including NiHPO3·H2O19 and Co(PO3)2,20
have been prepared via hydrothermal synthesis or thermolytic
molecular precursor (TMP) methods. However, these works are
limited by the necessity of long reaction time, the assistance of
coordinating reagents/surfactants, and/or the prefabrication of
complex precursors. Therefore, one of the big obstacles to explore
Me-PO3 based electrode materials is how to realize the facile
growth of Me-PO3 with well-arranged morphology and specified
crystalline, which is still a grand challenge currently.
Introduction
The depletion of fossil fuels and aggravation of environmental crisis
have triggered an urgent demanding for the sustainable and clean
alternative energy technologies. Due to their intrinsic advantages
such as high power-density, fast charging and long lifespan, the
ultracapacitors represent a promising excellent candidate among
various advanced energy storage devices. Especially, the hybrid
ultracapacitors, often employ various battery-type or
pseudocapacitive electrode materials with faradaic reactions
including transition-metal oxides/hydroxides (Me-O/OH),1-3
4
sulphides (Me-S),
nitrides (Me-N)5 and phosphides (Me-P),6
exhibiting higher energy densities than the electrical double-layer
capacitors (EDLC) as well as larger power densities than the
batteries. Nevertheless, the present hybrid devices are still suffered
from poor reversibility and low rate capability which usually relate
a.Institute of Materials, Ningbo University of Technology, Ningbo 315016, P.R.
China. Email: weiyouyang@tsinghua.org.cn
b.School of Materials Science and Chemical Engineering, Ningbo University, Ningbo
315211, P.R. China.
Given the intense interest in the exploration of the facile
synthesis of Me-PO3 with controlled stoichiometry/desired
morphology as well as their potential energy storage properties,
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Fig. 1 Schematic illumination for the growth of 3D hierarchical Ni(PO3)2 nanosheet arrays on ACC.
we herein designed a rational two-step strategy by combining a
hydrothermal fabrication of hierarchical morphologies and
a
followed phosphorylation (i.e., topotactic fabrication). A Ni(PO3)2
nanomaterial with homogeneous crystalline phase and 3D
hierarchical structure was firstly prepared, and the resultant
Ni(PO3)2 nanosheet arrays self-supported on activated carbon cloth
(ACC) (Ni(PO3)2@ACC) exhibit a superior capacitive activity as
compared to the ACC-supported Ni(OH)2 nanosheet arrays
(Ni(OH)2@ACC) that possesses
a similar morphology. The
Ni(PO3)2@ACC hybrid has a high specific capacitance (4.43 F cm−2 at
2 mA cm−2 and 2237 F g-1 at ~1 A g−1 in 3 M KOH), good rate
capability, and excellent cycling stability (maintained 90% of the
capacity at 10 mA cm−2 after 2000 cycles), suggesting that the
nanostructured metaphosphate of transition metal could be a novel
but very promising electrode material for energy storage devices.
Fig. 2 Typical XRD patterns of the Ni precursor (up) and its
corresponding phosphorylated product (down).
obviously higher fibre coverage (see SEM images in Fig. S1g-j,
Supporting Information), which implies that ACC plays a profound
effect on the growth of the Ni(OH)2@CC hybrids with desired 3D
hierarchical nanostructures.
Results and discussion
The fabrication of the self-supported electrode of Ni(PO3)2
nanosheets arrays is designed by
schematically illuminated as Fig. 1. Firstly, a hydrothermal process
was conducted aiming to achieve uniform nanostructured
morphology of Ni2+ precursor, i.e., nanosheets arrays. Then, a heat-
treated phosphorylation was applied to transform Ni2+ precursor
nanosheets arrays into Ni(PO3)2 nanosheets arrays. Carbon cloth
herein was chosen as the conductive support, which could provide a
three-dimensional (3D) conducting network to enhance the
charge/ion transfers within the hybrid electrode.21 Prior to use, the
a
topotactic strategy,
Fig. 2 shows the typical X-ray diffraction (XRD) patterns of the
Ni2+ precursor from the first-step hydrothermal process (the up
pattern) and its phosphorylated product (the down one) grown on
ACC thereby. Besides the detected signals at ~25.8° and ~43.0°
ascribed to the substrate of carbon cloth, the others could be
assigned to the Ni2+ precursor of Ni(OH)2 with an typical
orthorhombic structure (JCPDS Card No. 14-0117). It verifies that,
after phosphorylated treatment, the XRD pattern agrees (the down
pattern) well to those of carbon cloth and monoclinic Ni(PO3)2
(JCPDS Card No. 29-0497), suggesting that the Ni2+ precursor has
been successfully converted to the pure phase of Ni(PO3)2.
a
commercial carbon cloth was pretreated via
a reflux in
concentrated nitric acid to create abundant defect/nucleation sites.
This would result in an improved cohesion between the substrate
and nickel precursor (i.e., a small contact resistance with a high
structural stability), and also allow a uniform distribution even after
sonication treatment. It is also confirmed from the fact that the Ni2+
precursor just grown on pristine carbon cloth (Ni(OH)2@CC) could
be easily detached from the CC substrate (see the digital photo of
Fig. 3 shows the typical SEM and TEM images of the resultant
Ni(OH)2@ACC and Ni(PO3)2@ACC hybrids. It further confirms that
the Ni(OH)2 nanosheet arrays grow uniformly on the ACC fibers,
making the formation of 3D hierarchical nanostructures. The
nanosheets are of typically a few micrometers in width and ~20 nm
in thickness (Fig. 3a and b). After phosphorylation, the obtained
Ni(PO3)2 nanosheets become thinner with rougher surfaces, as
shown in Fig. 3c and d. More SEM images of Ni(PO3)2@ACC are
provided in Fig. S2, Supporting Information. The representative TEM
observation (Fig. 3e) further verifies that the Ni(OH)2 nanosheets
possess a smooth surface with curled tips. However, Ni(PO3)2
Fig. S1b, Supporting Information). Also,
a serious surficial
aggregation of the nanosheets with a low coverage around the
carbon fibers can be observed in the Ni(OH)2@CC hybrid (see SEM
images in Fig. S1c-f and the marked arrow heads in Fig. S1c,
Supporting Information). In comparison, the growth of Ni(OH)2 on
ACC displays a more uniform dispersity of nanosheets with an
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Fig. 3 Typical SEM images of Ni(OH)2@ACC (a-b) and Ni(PO3)2@ACC (c-d). Typical TEM image of Ni(OH)2 (e) and Ni(PO3)2 (f) scratched from
ACC. Typical HRTEM images of Ni(PO3)2 (g-i). The inset in Fig. 3g is the corresponding SAED pattern.
surface region. The survey spectrum (Fig. 4a) reveals the presence
of Ni, P, O and C in Ni(PO3)2@ACC hybrid. The uniform distribution
of Ni, P and O atoms can also be demonstrated by their elemental-
mapping images (Fig. S3, Supporting Information). The multiplet
splittings were carried out for assigning Ni 2p, P 2p and O 1s
spectral profiles and their chemical states. The Ni 2p spectrum can
be deconvoluted into two spin−orbit doublets and two shakeup
satellites (Fig. 4b), indicating the coexist of Ni2+ and Ni3+ species 22-24
on the surface of Ni(PO3)2@ACC hybrid. The presence of Ni3+ ions
suggests a non-reductive process involved in the transformation of
Ni(PO3)2 nanocrystals from Ni(OH)2 precursor. According to the
probed O 1s XPS spectrum (Fig. 4c), it seem like that the oxygen
species exhibit two distinct forms such as P−O−Ni non-bridging
oxygen atoms (533.1 eV) and P−O−P bridging oxygen ones (531.8
eV),25 further confirming the formation of nickel phosphate. The
asymmetric peak envelope from overlapping components in Fig. 4d
can be split into two signals at 133.5 and 134.8 eV, which could be
3− 26
2− 27
assigned to PO4
and PO3
species respectively. The presence
of large amounts of surficial PO43− ions evidences that the formation
Fig. 4 (a) Representative XPS survey spectrum of Ni(PO3)2@ACC. (b-
d) The core-level photoelectron spectra of Ni 2p, O 1s and P 2p,
respectively.
3−
of Ni(PO3)2 nanocrystals involves the transformation from PO4 to
PO32− species.
To further disclose the growth mechanism of the 3D hierarchical
Ni(PO3)2@ACC nanosheet arrays, the phosphorylation temperatures
were tailored at 250~350 °C with an interval of 50 °C. Their XRD
patterns of the corresponding samples are presented in Fig. 5.
Similar to Fig. 2, the diffractions at ~25.8° and ~43.0° relate to the
substrates of ACC. It suggests that the resultant product treated at a
lower temperature of 250 °C retains the original crystalline form of
Ni(OH)2, while the Ni(PO3)2 can be obtained at 300 and 350 °C,
indicating that the phosphorylation reaction is triggered once the
temperature is up to 300 °C. It is quite different from the cases of
previous reports,28, 29 in which PH3 can be generated upon heating
of NiH2PO2 below 250 °C (see Equation 1) and subsequently cause
the phosphorization reaction to form Ni-P with various crystal
phases.
nanosheets exhibit a rough and porous surface (Fig. 3f), disclosing
that they are assembled by the interconnected nanoparticles. This
structural change might arise from the gas release and the
dehydration of Ni(OH)2 during the annealing process. The
corresponding HRTEM image (Fig. 3g) discloses that the
nanoparticles are uniformly sized in ~5 nm. Their typical SAED
pattern (the inset in Fig. 3g) shows the nanoparticles are monoclinic
Ni(PO3)2 (JCPDS Card No. 29-0497) with a highly crystalline nature.
The measured interplane spaces between two neighbored lattice
fringes of 2.35, 2.83, 3.15 and 2.98 Å in Fig. 3h and i respond to the
crystal plane spaces of (-323), (013), (310) and (-222) of monoclinic
Ni(PO3)2, respectively.
X-ray photoelectron spectroscopy (XPS) was further conducted to
investigate the electronic state of the present elements in near
5NaH2PO2=Na4P2O7+NaPO3+2PH3+2H2
(1)
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To clarify this point, the hydrothermally-obtained Ni(OH)2
powder and NaH2PO2 without the use of the ACC substrate are
heated at 300 °C with the same experimental conditions. The XRD
characterization discloses that the obtained product is single phase
of nickel phosphide (Ni2P) with
a high crystallinity (Fig. S4,
Supporting Information), in which PH3 might act as the launcher to
drive the formation of Ni2P via Equation 2 as bellow:
2PH3+2Ni(OH)2=Ni2P+P+4H2O+H2 (≥300 °C)
(2)
To confirm this point, a lower temperature of 250 °C was fixed for
the pyrolysis of the mixture of Ni(OH)2 and NaH2PO2 under the
same other conditions as mentioned above. The XRD pattern of as-
obtained sample (Fig. S5, Supporting Information) suggests the
coexistence of Ni, Ni2P and P, in which the metallic Ni is dominant.
It suggests that the formation of Ni2P might start from the
generation of metallic Ni (shown as Equation 3). This further
confirms that the reaction of the Ni(OH)2 precursor without ACC is
through a PH3-involved reduction-dominant mode.
Fig. 5 Typical XRD patterns of the resultant samples with the
phosphorylation temperatures fixed at 250, 300 and 350 °C.
firstly investigated for the exploration of their potential application
in electrochemical energy storage, via the cyclic voltammogram
(CV) and galvanostatic charge–discharge (GCD) methods in three-
electrode configuration. The annealed product of the
hydrothermally obtained Ni2+ precursor on ACC, identified as
Ni(OH)2@ACC (see XRD pattern in Fig. S6, Supporting information),
was used as a comparison. The representative CV curves for
Ni(PO3)2@ACC and Ni(OH)2@ACC at different scan rates are shown
in Fig. 6a and b. The strong redox peaks for both electrodes indicate
their battery-type electrode characteristics, which are mainly due to
Faradaic redox reactions, and the symmetry of anodic/cathodic
peaks suggests their good reversibility. The shift of the anodic and
cathodic peaks with the increase of the scan rates is attributed to
the internal resistance of the electrode.31 The subtle change in the
CV shape of Ni(PO3)2@ACC with the increase of the scan rates from
2PH3+3Ni(OH)2=3Ni+2P+6H2O (≤250 °C)
(3)
The comparison experiments as mentioned above (see the
summary in Table S1, Supporting information) strongly imply that,
besides the high phosphorylation temperature, the ACC substrates
also plays
a determined role on the formation of Ni(PO3)2
nanocrystals, which is similar to the reported work that the SiO2
substrate is critical to the compositions of final products.30 The
formation of Ni(PO3)2 rather than Ni2P is thus speculated to be
ascribable to the strong interaction between the Ni2+ precursor and
ACC, which could reduce the potential of Ni2+/Ni pair and thus
hinder the reduction of Ni2+→Ni2P/Ni. In this case, the anchored
Ni(OH)2 precursor becomes prior to react with the high-valence
phosphate products (H3PO3 or H3PO4) obtained from the
dismutation reaction of H2PO2−, making the formation of Ni(PO3)2
5
to 50 mV s−1 thereby implies
a smaller equivalent series
−
through the substitution of PO3 for OH− in the Ni-OH lattice.
resistance, as compared to that of Ni(OH)2@ACC electrode.
Interestingly, the Ni(PO3)2@ACC electrode responses an obviously
much larger redox current density than Ni(OH)2@ACC counterpart
at all scan rates, verifying that Ni(PO3)2@ACC is capable of storing
much more electrical energy than Ni(OH)2@ACC. Notably, the CV
profile of Ni(PO3)2@ACC electrode in Fig. 6b shows two sets of
redox activities as indicated by two pairs of anodic/cathodic current
peaks. The first and dominant set of a redox couple, which appears
in the potential range of 0.15~0.45 V vs. Hg/HgO, is associated with
the conversion of Ni2+↔Ni3+. The second set at the right neighbour
pertains is assigned to the Ni3+↔Ni4+ redox couple, which is usually
observed in many nickel-based electrode materials and would
contribute to the overall energy-storage performance,23, 32 while it
is absent in CV curves of Ni(OH)2@ACC electrode (Fig. 6a). The Ni3+
species is known to possess high conductivity, which could facilitate
an increased mobility rate of surface charge and electrolyte-transfer
and thus render an enhanced pseudocapacitive activity to the
electrodes.24, 33 Moreover, A much smaller peak potential difference
(ΔEp=Epa-Epc) between Ni2+↔Ni3+ pairs observed on CV curves of
Ni(PO3)2@ACC in Fig. 6b, indicates a better Ni2+↔Ni3+ reversibility
Accordingly, the decomposition of NaH2PO2 is deduced to release
not only gaseous PH3 (see Equation 2), but also H3PO3 or H3PO4 via
Equation 4 and 5:
6NaH2PO2=3Na2HPO3+H3PO3+2PH3
4H3PO3=PH3+3H3PO4
(4)
(5)
That is to say, the as-prepared Ni(OH)2 nanostructures participate
in two competitive reactions, which favor the formations of the
Ni2P and Ni(PO3)2 respectively. Assisted by the ACC substrate, the
PH3-involved reduction reaction would be suppressed; this thus
allows the H3PO3/H3PO4-involved phosphorylation reaction become
dominant, leading to the formation of the single phase of Ni(PO3)2.
On the above basis, the synthesis mechanism of Ni(PO3)2
nanocrystals follows two possible routes: i) the reaction between
H3PO3 and Ni(OH)2 (Equation 6); ii) the reaction between Ni(OH)2
and (HPO3)4 (Equation 7) converted from H3PO4 via Equation 8.
10H3PO3+3Ni(OH)2=3Ni(PO3)2+4P+18H2O (≥ 300 ) (6)
2Ni(OH)2+(HPO3)4=4H2O+2Ni(PO3)2 (≥ 300 °C)
4H3PO4= (HPO3)4+4H2O
(7)
(8)
Then the electrochemical performance of Ni(PO3)2@ACC was in Ni(PO3)2@ACC electrodes.34
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6c and d further verifies the results from the CV measurements, i.e.,
the battery-type charge storage property. The specific capacitances
of both electrodes at various discharge current densities, calculated
from their GCD curves, are shown in Fig. 6e. It is evident that the
Ni(PO3)2@ACC electrode exhibits very high specific capacitances,
which is two folds higher than the Ni(OH)2@ACC counterpart under
the identical test conditions. In detail, the Ni(PO3)2@ACC electrode
possesses excellent areal capacitances of 4.43, 3.39, 2.95, 2.87 and
2.75 F cm−2 at the current densities fixed at 2, 5, 10, 20 and 30 mA
cm−2, respectively. Meanwhile, its mass specific capacitance can
reach up to 2237 F g−1 at ~1 A g−1 (mass loading: 1.98 mg cm−2). In
comparison, ACC shows a much smaller areal capacitance of 0.28 F
cm−2 at the discharge current of 2 mA cm−2 (see CV and GCD curves
of ACC electrode in Fig. S7, Supporting Information). It implies that
ACC mainly acts as the electronic conductive support for fully
performing the electrochemical activity of Ni(PO3)2 nanostructures
and with a negligible capacitance contribution to the integral
performance of the hybrid material. Furthermore, the capacitance
of the Ni(PO3)2@ACC electrode at 30 mA cm−2 is ca. 62% of that at 2
mA cm−2, revealing its good high-rate capability. This mainly
benefits from the high dispersity nature of Ni(PO3)2 nanosheet
arrays and the constructed 3D hierarchical nanostructure of the
Ni(PO3)2@ACC hybrids, which could profoundly facilitate the
electrolyte transportation. Moreover, it should be pointed out that
the mass specific capacitance (i.e., 2237 F g−1) of our Ni(PO3)2@ACC
electrode is comparable to the best one of Ni-based ultracapacitor
electrodes ever reported (see Table S2 in Supporting Information),
which are often ranged in 500~2600 F g−1. It also could be the
highest one among those of the previously reported Me-POx
pseudocapacitor materials (see Table S3 in Supporting Information).
Meanwhile, its areal capacitance (i.e., 4.43 F cm−2) stands in a high
level in comparison with other self-supported electrode materials
(typically of 2~5 F cm−2) (see Table S4 in Supporting Information).
These further verify that the current 3D hierarchical Ni(PO3)2
nanosheet arrays could be a novel and very promising candidate to
be utilized as the electrode material for the next generation
ultracapacitors.
Fig. 6 Typical CV curves of Ni(OH)2@ACC (a) and Ni(PO3)2@ACC
electrodes (b) with different scan rates. Typical discharge curves of
Ni(OH)2@ACC (c) and Ni(PO3)2@ACC electrodes (d) at various
discharge current densities. (e) The ca. areal specific capacitance of
Ni(OH)2@ACC and Ni(PO3)2@ACC electrodes as
a function of
discharge currents. (f) Typical Nyquist plots of Ni(OH)2@ACC and
Ni(PO3)2@ACC electrodes.
To elucidate the dynamic behaviors of Ni(OH)2@ACC and
Ni(PO3)2@ACC electrodes, the electrochemical impedance spectrum
(EIS) was collected, and their corresponding Nyquist plots are
shown in Fig. 6f. It suggests that the obtained EIS spectra of both
electrodes consist of an arc in the high frequency with a sloped line
in the low frequency. The near-semicircle arises because of the
parallel combination of charge transfer resistance (Rct) and double
layer capacitance (Cdl).35 In general, if the electrochemical kinetics is
governed by a diffusion-controlled process, the linear plot should
be 45° to the real axis. Otherwise, it should be 90° if it is a capacitive
behavior in nature. The linear plot of Ni(PO3)2@ACC electrode
exhibits much steeper than that of Ni(OH)2@ACC counterpart,
suggesting its much more rapid ion diffusion and better capacitor
behavior. The observed smaller semicircle diameter of
Ni(PO3)2@ACC electrode clarifies that it has a much smaller Rct and
a larger Cdl than Ni(OH)2@ACC.
Fig.
7 (a) Typical cycling performance of the Ni(PO3)2@ACC
electrode at a current density of 10 mA cm−2. The inset shows the
GCD comparison of Ni(PO3)2@ACC electrodes at 1th, 800th and
2000th cycles. (b) Typical Nyquist plots of the Ni(PO3)2@ACC
electrode before and after 2000 cycles.
The as-fabricated Ni(PO3)2@ACC and Ni(OH)2@ACC electrodes
were further evaluated by the GCD method, which was carried out
at various current densities ranged from 2 to 30 mA cm−2 under a
potential window of 0−0.48
V and 0−0.40 V (vs. Hg/HgO),
respectively. The nonlinear behavior of the discharge curves in Fig.
The cycling stability of Ni(PO3)2@ACC electrode was also
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evaluated at a current density of 10 mA cm−2 in the potential range available from Shanghai Chemical Reagent Co. Ltd, which were used
of 0 to 0.48 V for 2000 cycles, as shown in Fig. 7a. The capacitance directly without any further purification. Carbon cloth (CC) was
increases at the beginning, which suggests the electrode self- purchased from CeTech, Taiwan. Prior to the synthesis, carbon cloth
activation process to make better electrode/electrolyte contact and was cleaned by acetone, ethanol and deionized water in sequence
more active sites participated in the electrochemical reaction. by sonication, and then activated in 65% HNO3 at 90 °C for 6 h.
Afterwards, the capacitance exhibits a slight decrease at the start of Afterwards, the activated carbon cloth (ACC) was rinsed by
~850th cycling. It still remains ~90.7% after 2,000 cycles as deionized water until pH~7.0, and dried at 60 °C in vacuum
compared to the peak value, and nearly no loss with respect to the overnight.
first cycle. This is accordance with the charge-discharge curves of
the electrodes at 1th, 800th and 2000th cycling (the inset in Fig. 7a). nanosheet arrays were achieved via a two-step process. Firstly, a
Furthermore, the nanostructures almost remain after cycling test hydrothermal process was conducted to achieve uniform
The fabrication of the self-supported electrode of Ni(PO3)2
a
(see SEM images in Fig. S8, Supporting Information), implying the nanostructured Ni2+ precursor, i.e., nanosheet arrays. Secondly, a
satisfied stability of the as-synthesized nanosheet arrays. Moreover, heat-treated phosphorylation was applied to convert the Ni2+
a slight detachment can be observed after the prolonged cycling of precursors to Ni(PO3)2 nanosheet arrays. In details, 2 mmol of
2000 times. Meanwhile, it will contribute to the capacitance NiCl2·6H2O was dissolved in 40 mL of deionized water, followed by
decrease of Ni(PO3)2@ACC electrode with extending cycle time. introducing 6 mmol of NH4F and 10 mmol of urea with continuous
stirring. Then, the ACC (area: 2×3 cm2) was immersed in the above
Overall, the experiments above confirm that the as-synthesized 3D
hierarchical Ni(PO3)2 nanosheet arrays exhibit an excellent cycling
stability at a high current density. The impedance spectra of
Ni(PO3)2@ACC before and after 2000 cycles were conducted, which
are shown in Fig. 7b. It seems that the diffusive resistance of
Ni(PO3)2@ACC becomes larger (from 1.10 to 5.29 Ω) with a higher
Rct after 2000 cycles, which could be possibly ascribed to the
microstructural adjustment of Ni(PO3)2 nanosheets caused by the
repetitive charge/discharge. In addition, the linear part of the
Ni(PO3)2@ACC electrode becomes even closer to 90° after the 2000
cycles, further representing its excellent capacitive behavior and
stability.
solution at 50 °C for 2 h. Subsequently, the solution with the ACC
was transferred into a Teflon-lined stainless autoclave (50 mL). The
autoclave was sealed and maintained at 110 °C for 4 h. After cooling
to room temperature (RT), the obtained hybrid was collected, and
washed by the deionized water for more than three times, and then
dried at 60 °C in vacuum for 8 h. Afterwards, the resultant product
was cut in half. One piece was treated with 5 mmol of NaH2PO2 at
300 °C for
2 h under Ar (heating rate: 4 ) for
°C min−1
phosphorylation treatment, leading to the growth of
Ni(PO3)2@ACC. The other piece was treated under the similar
conditions for the preparation of annealed Ni(OH)2@ACC, which
was used as the comparison sample for the performance
characterizations. Notably, the NaH2PO2 powders were placed at
the upstream side of the crucible and the as-grown hybrids were
located at the opposite end, as shown in Fig. S1a (Supporting
information). To investigate the effect of the reaction parameters
on the formation of nickel salts, the annealing temperatures were
varied during the heat treatment process.
Conclusions
In summary, we have demonstrated the exploration of novel
electrode material of 3D hierarchical Ni(PO3)2 nanosheet arrays,
which is firstly synthesized via a rationally topotactic strategy
combined a hydrothermal growth of nickel precursors with a
followed phosphorylation treatment. The as-synthesized 3D
hierarchical Ni(PO3)2@ACC exhibits a superior capacitive activity as
compared to Ni(OH)2@ACC counterpart. The areal and mass
specific capacitance of Ni(PO3)2@ACC electrode could be up to 4.43
The resultant products were characterized by the X-ray powder
diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα
radiation (λ = 1.5406 Å) and field emission scanning electron
microscopy (FESEM, S-4800, Hitachi, Japan). The nickel
metaphosphates scratched from the ACC were supported on micro-
grid for the observation on high-resolution transmission electron
microscopy (HRTEM, JEM-2010F, JEOL, Japan) and selected area
electron diffraction (SAED) pattern. X-ray photoelectron spectra
(XPS) were determined on an X-ray photoelectron spectrometer
(XPS, AXIS ULTRA DLD, Shimadzu, Japan) with an excitation source
of Mg Kα radiation (1253.6 eV), in which all the binding energies
were referenced by the C1s peak at 284.8 eV.
F cm−2 at a current density of 2 mA cm−2 and 2237 F g−1 at ~1 A g−1
,
respectively, which is twice higher than that of Ni(OH)2@ACC
electrode, and comparable to the best one among the Ni-based and
Me-POx ultracapacitor electrodes ever reported. It can maintain
~90% charge retention after 2000 cycles at a high rate of 10 mA
cm−2, suggesting its robust stability. This work makes an innovation
to the synthesis of Ni(PO3)2 nanostructures with desired structures
as well as their potential applications in advanced electrodes for
ultracapacitors, which would open an exciting new avenue for the
rational design of Me-PO3 materials for energy storage devices with
superior performances.
The electrochemical performances of the ultracapacitor
electrodes based on as-fabricated Ni(OH)2@ACC and Ni(PO3)2@ACC
were evaluated by the cyclic voltammetry (CV), galvanostatic
charge–discharge (GCD), and electrochemical impedance
spectroscopy (EIS) on a CHI 660D electrochemical workstation. EIS
was conducted at the open circuit voltage with a alternate current
amplitude of 5.0 mV in the frequency range of 10 mHz to 10 kHz.
Experimental Methods
All the reagents were of analytical grade, and commercially
6 | J. Mater. Chem. A, 2012, 00, 1-7
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DOI: 10.1039/C6TA09528C
Journal Name
COMMUNICATION
The as-fabricated Ni(PO3)2@ACC and annealed Ni(OH)2@ACC 24 Y. Zhao, Q. Wang, T. Bian, H. Yu, H. Fan, C. Zhou, L.-Z. Wu, C.-H.
(1.0×1.0 cm2) were directly used as the working electrodes, in
Tung, D. O'Hare and T. Zhang, Nanoscale, 2015, 7, 7168-7173.
which the platinum foil (1.5×1.5 cm2), the Hg/HgO electrode and 3
25 J. Tsuchida, J. Schneider, M. T. Rinke and H. Eckert, J. Phys. Chem.
C, 2011, 115, 21927-21941.
M KOH were used as the counter electrode, the reference and the
26 R. Franke, T. Chassé, P. Streubel and A. Meisel, J. Electron
electrolyte, respectively.
Spectrosc. Relat. Phenom., 1991, 56, 381-388.
27 J. A. Rotole and P. M. A. Sherwood, Surf. Sci. Spectra, 1998, 5, 67-
74.
Acknowledgements
28 Q. Guan, W. Li, M. Zhang and K. Tao, J. Catal., 2009, 263, 1-3.
29 N. Yonghong, T. Ali, H. Guangzhi, C. Xiaofeng, W. Xianwen and Y.
Zhousheng, Nanotechnology, 2006, 17, 5013.
30 S. T. Oyama, X. Wang, Y. K. Lee and W. J. Chun, J. Catal., 2004, 221,
263-273.
31 S. Liu, K. S. Hui and K. N. Hui, ACS App. Mater. Interfaces, 2016, 8,
3258-3267.
32 R. Singhal, S. R. Das, O. Oviedo, M. S. Tomar and R. S. Katiyar, J.
Power Sources, 2006, 160, 651-656.
33 X. Li, Q. Li, Y. Wu, M. Rui and H. Zeng, ACS App. Mater. Interfaces,
2015, 7, 19316-19323.
34 L.-F. Chen, Z.-Y. Yu, X. Ma, Z.-Y. Li and S.-H. Yu, Nano Energy, 2014,
9, 345-354.
35 M. Bag, L. A. Renna, R. Y. Adhikari, S. Karak, F. Liu, P. M. Lahti, T. P.
Russell, M. T. Tuominen and D. Venkataraman, J. Am. Chem. Soc.,
2015, 137, 13130-13137.
The work was supported by National Natural Science Foundation of
China (NSFC, No. 51372123 and 21404062), Postdoctoral Science
Foundation of China (No. 2015M581966) and Natural Science
Foundation of Ningbo (No. 2015A610241 and 2016A610109).
References
1 T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nat. Mater.,
2010, 9, 146-151.
2 X. Lang, A. Hirata, T. Fujita and M. Chen, Nat. Nano., 2011, 6, 232-
236.
3 X. Xia, D. Chao, C. F. Ng, J. Lin, Z. Fan, H. Zhang, Z. X. Shen and H. J.
Fan, Mater. Horiz., 2015, 2, 237-244.
4 Y. Liu, Y. Li, H. Kang, T. Jin and L. Jiao, Mater. Horiz., 2016, 3, 402-
421.
5 X. Lu, M. Yu, T. Zhai, G. Wang, S. Xie, T. Liu, C. Liang, Y. Tong and Y.
Li, Nano Lett., 2013, 13, 2628-2633.
6 M. Sun, H. Liu, J. Qu and J. Li, Adv. Energy Mater., 2016, 6, DOI:
10.1002/aenm.201600087.
7 Y. Zhao, Z. Chen, D.-B. Xiong, Y. Qiao, Y. Tang and F. Gao, Sci. Rep.,
2016, 6, 17613.
8 R. A. Patil, C.-P. Chang, R. S. Devan, Y. Liou and Y.-R. Ma, ACS App.
Mater. Interfaces, 2016, 8, 9872-9880.
9 C. Feng, J. Zhang, Y. He, C. Zhong, W. Hu, L. Liu and Y. Deng, ACS
Nano, 2015, 9, 1730-1739.
10 Y. Zhan, M. Lu, S. Yang, C. Xu, Z. Liu and J. Y. Lee, ChemCatChem,
2016, 8, 372-379.
11 D. Yang, Z. Lu, X. Rui, X. Huang, H. Li, J. Zhu, W. Zhang, Y. M. Lam,
H. H. Hng, H. Zhang and Q. Yan, Angew. Chem. Int. Ed., 2014, 126,
9506-9509.
12 H. Pang, C. Wei, Y. Ma, S. Zhao, G. Li, J. Zhang, J. Chen and S. Li,
ChemPlusChem, 2013, 78, 546-553.
13 H. Pang, Y. Liu, J. Li, Y. Ma, G. Li, Y. Ai, J. Chen, J. Zhang and H.
Zheng, Nanoscale, 2013, 5, 503-507.
14 Y. Gao, J. Zhao, Z. Run, G. Zhang and H. Pang, Dalton T., 2014, 43,
17000-17005.
15 H. Pang, S. Wang, W. Shao, S. Zhao, B. Yan, X. Li, S. Li, J. Chen and
W. Du, Nanoscale, 2013, 5, 5752-5757.
16 C. Yang, L. Dong, Z. Chen and H. Lu, J. Phys. Chem. C, 2014, 118,
18884-18891.
17 C. Wu, X. Lu, L. Peng, K. Xu, X. Peng, J. Huang, G. Yu and Y. Xie,
Nat. Commun., 2013, 4, 2431.
18 H. Pang, Z. Yan, Y. Ma, G. Li, J. Chen, J. Zhang, W. Du and S. Li, J.
Solid State Electrochem., 2013, 17, 1383-1391.
19 L. Zhang, Y. Ni, K. Liao and X. Wei, Cryst. Growth Des., 2008, 8,
3636-3640.
20 H. S. Ahn and T. D. Tilley, Adv. Func. Mater., 2013, 23, 227-233.
21 B. C. Kim, J.-Y. Hong, G. G. Wallace and H. S. Park, Adv. Energy
Mater., 2015, 5, DOI: 10.1002/aenm.201500959.
22 A. P. Grosvenor, M. C. Biesinger, R. S. C. Smart and N. S. McIntyre,
Surf. Sci., 2006, 600, 1771-1779.
23 P. Manivasakan, P. Ramasamy and J. Kim, Nanoscale, 2014, 6,
9665-9672.
This journal is © The Royal Society of Chemistry 20xx
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA09528C
3D hierarchical Ni(PO3)2 nanosheet arrays are developed as a novel electrode material with
superior energy storage behaviors.