Porous NiCoP nanosheets as efficient and stable positive electrodes for advanced asymmetric supercapacitors

Article abstract of DOI:10.1039/c8ta05551c

Materials with good capacitance performance at high loading are desirable for application in supercapacitors at the device level. Here, we have reported the synthesis of sheet-like porous NiCoP (S-NiCoP) by controllable phosphorization of a 2-D NiCo hydroxide precursor. In the synthesis, the NiCo hydroxide sheets were firstly prepared through a solvothermal process in large quantity. Under the phosphorization, the PH₃ released by NaH₂PO₂ could react with NiCo in the precursor to form NiCoP. The release of gas by the decomposition of hydroxide could lead to the formation of pores, thus forming 2-D porous NiCoP with a high specific surface area (216.39 m² g^-1). S-NiCoP has combined the advantages of porous nanosheets (faster ion transport along the pores) and metal phosphides (TMPs) (good conductivity), and thus is promising for application in supercapacitors. S-NiCoP-300 has shown high capacitance (1206 F g^-1 at 1 A g^-1) and good rate performance (612 F g^-1 at 20 A g^-1), much superior to those of the corresponding NiCo hydroxide (566 F g^-1), NiP-300 (436 F g^-1) and CoP-300 (404 F g^-1). Notably, the electrode still exhibits a high capacitance of 1095 F g^-1 at a high loading of 13.5 mg cm^-2, ascribed to the ability of easily transferring electrolyte along the pores. In this case, the area capacitance can reach a high value of 14.78 F cm^-2. An asymmetric supercapacitor composed of activated carbon and S-NiCoP could achieve a high specific capacitance, good cycling stability and high energy density. A white LED (3 V) can be lit for more than 2 h by charging two S-NiCoP-2-300//AC ASC devices for about 170 s. The easy synthesis process and good performance makes 2-D NiCoP promising in portable energy systems.

Full text of DOI:10.1039/c8ta05551c

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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ARTICLE
Porous NiCoP nanosheet as efficient and stable positive
electrode for advanced asymmetric supercapacitors
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaomeng Zhang, Aiping Wu, Xiuwen Wang, Chungui Tian,* Ruiyue An and Honggang Fu*
DOI: 10.1039/x0xx00000x
The materials with good capacitance performance at high loading is desirable for the application of supercapacitors at
device level. Here, we have reported the synthesis of sheet-like porous NiCoP (S-NiCoP) by controllable phosphorization of
2-D NiCo hydroxide precursor. In the synthesis, the NiCo hydroxide sheets were firstly prepared through a solvothermal
process in large quantity. Under the phosphorization, the PH3 released by NaH2PO2 could react with NiCo in precursor to
form NiCoP. The release of gas by decomposition of hydroxide could lead to the formation of pores, thus making the
formation of 2-D porous NiCoP with high specific surface areas (216.39 m2 g-1). The S-NiCoP have combined the
advantages of porous nanosheets (faster ion transport along the pores) and metal phosphides (TMPs) (good conductivity),
thus are promising for supercapacitors. The S-NiCoP-300 have shown the high capacitance (1206 F g−1 at 1A g−1) and good
rate performance (612 F g−1 at 20 A g−1), much superior to corresponding NiCo hydroxide (566 F g-1), NiP-300 (436 F g-1)
and CoP-300 (404 F g-1). Notably, the electrode still exhibit a high capacitances of 1095 F g-1 at high loading of 13.5 mg cm-
2, ascribing to the ability for easily transferring electrolyte along the pores. In this case, the area capacitance can reach high
values of 14.78 F cm-2. A asymmetric supercapacitor composed of active carbon and S-NiCoP could achieve a high specific
capacitance, good cycling stability and high energy density. A white LED (3 V) can be lighted for more than 2 h by
charging two S-NiCoP-2-300//AC ASC devices about 170 s. The easy synthesis process and good performance makes 2-D
NiCoP promising in portal energy systems.
www.rsc.org/
This needs good ability for easily transferring electrolyte along
the vertical direction of electrode. Thus, it is desired to
designed synthesis of materials that can satisfy the above the
1. Introduction
The rapid growth of portable electronic devices has resulted
in an increasing demand for clean and renewable energy
storage devices. Supercapacitors (SCs) have the advantage of
high power density, fast charge/discharge ability,
environmental friendliness and long life span, thus drawing
considerable attention. However, the SCs have relatively low
energy density in comparison with batteries, which limited their
practical application.[1-3] According to the equation E= 1/2 CV2,
the expansion of operating voltage or improvement of
capacitance of materials are two feasible routes to enhance the
energy density of SCs.[4] In this regard, the voltage can be
amplified by matching the potential voltage of two electrodes in
a asymmetric supercapacitor (ASC), thereby increasing the
energy density of devices.[5] The performance can also be
improved by optimizing the capacitance through tuning the
structure of electrode materials.[6,7] In addition, there are usually
need of thick coating of active components (high loading) on
current collector for the practical application at the device level.
demand to promote the practical application of SCs.
Based on the charge storage mechanism, the SCs can be
divided into electrical double-layer capacitors (EDLCs),[8]
which storing energy by ion adsorption on the active materials,
and pseudocapacitors worked through fast surface redox
reactions.[9-11] The pseudocapacitive materials have usually
shown higher capacitance than that of EDLCs, thus receiving
intensive attention. The transition metal-based hydroxides and
oxides are of particular interest owing to their high theoretical
pseudocapacitances and earth abundance.[12] However, they
suffer from a limited kinetics during redox reactions due to its
intrinsic low conductivity, which can result in poor rate
performance and stability. The development of material with
well designed structure and improved conductivity is important
to overcome such drawback. Transitional metallic phosphides
(TMPs) are of promising in catalytic and energy field lithium-
ion battery,[13,14] hydrodesulfurization (HDS)[15] and hydrogen
evolution reaction (HER).[16,17] The TMPs have metalloid
structure and good conductivity compared with corresponding
oxides. The advantages make large potential of TMPs for
capacitors. The phosphide nanoparticle,[18] nanowire[19,20] and
nanosheet have shown good performance as pseudocapacitance
electrode. Typically, the NiCoP@C nanowire arrays can give a
specific capacitance much superior to NiCo-nanowire and
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of
Education of the People's Republic of China, Heilongjiang University, Harbin
150080 (China).
* Email address: chunguitianhq@163.com; fuhg@vip.sina.com
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here. See
DOI: 10.1039/x0xx00000x
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NixCo1−xO@C nanowire.[21] Chen and co-workers have NaCH3COO was supplied by Aladdin Chemical Reagent Co.,
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DOI: 10.1039/C8TA05551C
designed a 3D networked Ni2P nanosheets grown on Ni foam Ltd. Alcohol was purchased from Tianjin Guangfu Fine
with a specific capacitance much larger than those of Ni(OH)2 Chemical Research Institute. Activated carbon (YEC-8) was
NS/NF.[22]
A
3D electrode consisting of self-supported purchased from Fuzhou Yihuan Carbon Co., Ltd. All of the
NiCoP/NiCo-OH/carbon cloth can give a seven fold increase of chemicals in the experiments were used as received without any
capacitance compared to that of NiCo-OH alone.[23]
further purification.
The performance of materials can be tuned by the 2.2. Material synthesis
microstructure besides the components.[24] The 2-D materials
2.2.1 The synthesis of 2-D NiCo oxide sheets
represented by graphene (or reduced graphene oxides)[25] and
some non-graphene 2-D materials (oxides and sulfides)[26, 27]
can be used as effective electrode materials for SCs. Our
previous work have demonstrated the good performance of 2D
nanosheets for electrochemical storage-energy[28-30] and for
HER.[31] Especially, the engineering of pores (or holes) on 2-D
materials can facilitate faster reagent transport into the inner
structure of the electrodes, and shorten ion transport pathway,
which is favorable to improve the application performance of
materials.[32] Especially, the holey (porous) structure can make
the easy transfer of electrolyte, even at thick coating of active
species,[33] which is favourable for practical application at the
device level. Considering the advantage of the porous sheets
and phosphides, the construction of 2-D porous phosphides will
provides an avenue for giving a high-effective electrode
materials. In this study, we have demonstrated that 2-D porous
NiCoP can be used as effective electrode materials for
supercapacitor. The 2-D NiCo hydroxide were selected as
precursor based on 1) the 2-D structure can provide a template
for 2-D NiCoP, and 2) the release of water from hydroxide
under heating can result in the formation of the porous structure.
The synergistic action of two metals in bimetallic compounds
can making the enhanced performance than corresponding
single metal compounds.[34, 35] So, we choose the synthesis of
bimetallic phosphides rather than monometallic phosphides in
this study. The S-NiCoP have shown good electrochemical
performance with high specific capacitance (1206 F g−1 at 1A
g−1) superior to corresponding NiCo hydroxide (566 F g-1),
NiP-300 (436 F g-1) and CoP-300 (404 F g-1). The electrode still
exhibit a high discharge specific capacitances of 1095 F g-1
even at high loading mass of S-NiCoP (13.5 mg cm-2), implying
its potential for practical application. The area capacitance can
reached to 14.78 F cm-2 in this case, much higher than previous
reports. An ASC composed of a active carbon and 2-D porous
sheets could achieve a high specific capacitance, good cycling
stability and high energy density of 36.32 W h kg−1 at an power
density of 375 W kg−1. A white LED (3V) can be lighted for
more than 2h by using two S-NiCoP-2-300//AC ASC devices
charged about 170 s. The ASC devices can also effectively
operate an electrical motor fan. The good performance makes
the 2-D NiCoP promising for portal energy-storage derives for
advanced application.
The 2-D porous NiCoP were obtained by the
phosphorization of 2-D NiCo hydroxide precursor. Firstly, the
2-D NiCo hydroxides were synthesized in a large quantity
based on
Co(NO3)2
a
solvothermal method. In detail, 15 mmol
·
6H2O (4.365g), 15 mmol Ni(NO3)2 6H2O (4.365 g)
·
and 60 mmol of NaCH3COO (4.92 g) were added into 40 mL
polyethylene glycol 200 (PEG-200). The mixture was stirred to
form a homogeneous solution at room temperature. Followed
this, the solution was transferred into a Teflon-lined stainless
autoclave (50 mL volume) and heated at 200 °C for 16 h in an
electric oven. The NiCo-precursor was taken out, washed with
alcohol for several times to remove residues, and finally dried
for 12 h.
2.2.2 The synthesis of 2-D porous NiCoP sheets (S-NiCoP)
To prepare S-NiCoP, 0.25g NiCo-precursor and 1g
NaH2PO2 were putted at two separate porcelain boat and then
placed in a quartz tube (NaH2PO2 at the upstream side of the
furnace). The tube was setted on a tube furnace, in that the
centre of tubular furnace is close to the boat loaded with
precursor. Under N2 atmosphere, the furnace was heated to
300 °C (heating speed of 5 °C min−1) and remained at this
temperature for 2 h. After naturally cooling to ambient
temperature, the solids were washed with deionized water and
then dried in oven. The products were denote as S-NiCoP-2-
300.
The phosphating temperature was tuned as 250 °C and
350 °C to obtain S-NiCoP-2-250 and S-NiCoP-2-350 to study
the effect of calcination temperature on the structure of final
samples. A series of other control samples were also fabricated
by tuned mass ratio of Ni(NO3)2·6H2O and Co(NO3)2·6H2O as
1:0, 1:2, 2:1 and 0:1 with remaining total molar amount of 30
mmol. The phosphorization condition is same to that for S-
NiCoP-2-300. The detail for the preparation of different
samples are listed in Table S1.
2.3 Sample characterization
The morphology and microstructure of the samples were
characterized by scanning electron microscopy (SEM, Hitachi
S-4800, acceleration voltage of 5 kV) and transmission electron
microscopy (TEM: JEM-2100, acceleration voltage of 200 kV).
BET test was performed at 77 K with a Micro-meritics Tristar
II surface area and a porosimetry analyzer. In the test, the
catalyst (more than 100 mg) was degassed under vacuum at 150
°C for 5 h before the measurement. The pore size distribution
and surface area were determined from desorption isotherms by
the BJH and BET methods, respectively. X-ray diffraction
(XRD) patterns were collected on a Bruker D8 diffractometer
2. Experimental section
2.1. Materials and chemicals
Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was purchased
from Shanghai Chemical Technology Co., Ltd. China. Cobalt
nitrate hexahydrate (Co(NO3)2·6H2O), polyethylene glycol 200
(PEG-200), sodium monophosphate (NaH2PO2) and
using Cu Ka (λ
= 1.5406 Å) radiation. XPS (X-ray
photoelectron spectroscopy) analysis was performed on a VG
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ESCALABMK II with an Mg Ka achromatic X-ray source
(1253.6 eV).
where m+, C+, and ∆V+ are represent mass specifiVciecwaAprtaicclietaOnnclinee,
DOI: 10.1039/C8TA05551C
and potential window of positive electrode material, m , C , and ∆V
-
-
-
are represent mass specific capacitance, and potential window of
positive electrode material. The optimized mass ratio of S-NiCoP-2-
300/AC is about 2 based on the equation.
3. Results and discussion
3.1. Morphology and Structure
The preparation procedure for S-NiCoP is illustrated in
Figure 1. Firstly, the 2-D NiCo-precursors were prepared
through reaction of Ni(NO3)2·6H2O and Co(NO3)2·6H2O in the
presence of NaCH3COO in PEG-200. The process is simple and
well-reproductive. The products can be obtained with good
uniformity in large quantity (about 3g for one time synthesis),
which should be favourable for practical application of the
materials. Under heating, the NiCo-precursor can react with
PH3 released from NaH2PO2, thus forming corresponding 2-D
NiCoP. The pore can be formed by the release of water from
the decomposition of hydroxide. The synthetic parameters for
S-NiCoP and other control samples are listed in Table S1 in
detail.
Figure 1. Schematic fabrication process of S-NiCoP-2-300.
2.4 Electrochemical measurements
The electrochemical measurements were carried out using a CHI
760E electrochemical workstation (CH Instruments Inc, China) in a
2 M aqueous potassium hydroxide (KOH) electrolyte with a typical
three electrode confirguration. A Hg/HgO electrode and platinum
electrode were used as the reference electrode and counter electrode,
respectively. The working electrode were prepared by mixing the
samples with 5 wt% polytetrafluoroethylene (PTFE) binder and 5
wt% acetylene black. The mixture is grinded to form a well-
dispersed slurry. The slurry was pasted on a piece of nickel foam
current collector (1.0 ×1.0 cm2) and dried in a vacuum oven. The
foamed nickel with active materials was pressed under 15 Mpa. The
active material loading of the nickel foam was estimated to be about
4 mg cm2 in most case. All capacitance performances were tested at
least for three times, and a intermediate value was used in the paper.
The mass loading of S-NiCoP on Ni foam are changed from 0.32 to
13.5 mg cm−2 to study the effect of loading amount.
2.5 Preparation of asymmetric supercapacitor (ASC) device
A asymmetric supercapacitor was assembled by using a active
carbon as positive electrode and S-NiCoP negative electrode. The
positive electrode was prepared by a route described above (2.4
section). The negative electrode was prepared by mixing the
activated carbon with 5 wt% polytetrafluoroethylene (PTFE) binder
and 5 wt% acetylene black. The slurry was coated onto the nickel
foam current collector and dried overnight at 120 °C in vacuum
oven. All of the fabricated electrode was pressed under 15 Mpa. The
mass loading of active materials on electrode is controlled by
weighting its mass before the dispersion in slurry. The active
materials can combined with foamed nickel firmly with the mass
loading increasing by compression and add of proper amount of
binder. ACS device was assembled with a configuration of 2025-
type coin cell by integrating the as-prepared positive electrode and
negative electrode. A piece of NKK placed was placed between the
two electrodes as separator in 2 M KOH electrolyte. The mass ratio
of the positive and negative electrodes for an ACS full cell can be
calculated based on the principle of charge balance. According to the
equation (1):
Figure 2. (a) SEM images of NiCo-precursor; (b) TEM images of NiCo-precursor, (c)
(d) are the HRTEM images of the selected areas; (e) SEM images of S-NiCoP-2-300; (f)
TEM images of S-NiCoP-2-300, (g) (h)are the HRTEM images of the selected areas; (i)
Dark-field TEM image showing the area for which elemental maps were obtained for S-
NiCoP-2-300 and (j-l)corresponding STEM/EDS elemental mapping.
The formation of 2-D NiCoP sheets were demonstrated by
XRD, TEM and SEM. XRD of the precursor shows the peaks
located at 33.6° and 60.9°, indexing to (110) and (301) planes
of NiCo hydroxide (JCPDS no. 22-0444) (Figure S1). The SEM
image shows the presence of large-scale, uniform nanosheets
with thickness of several tens of nanometers and lateral size
above micrometers (Figure 2a). In TEM image, we can clearly
see the presence of 2-D nanosheet (Figure 2b). An enlarged
TEM image of NiCo-precursor (Figure 2c and d) clarified the
formation of thin sheet structure with many folds. In Figure 2d,
we can see the lattice fringe of 0.26 nm, matching well with
(110) lattice planes of NiCo hydroxide. After phosphorization,
the NiCo hydroxide can be transformed into S-NiCoP. In XRD
patterns (Figure S1), the sharpest diffraction peaks located at
m+/m- = (C- × △V-) / (C+ × △V+)
(1)
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41.0° can be assigned to (111) plane of hexagonal NiCoP
(JCPDS no. 71-2336).[36] Other peaks, corresponding to (201),
(210), (300), (002), and (211) planes, can be observed at 44.9°,
47.6°, 54.4°, 54.7° and 55.3°, respectively. The SEM image
(Figure 2e) shows the presence of sheet-like structure, but with
more rougher surface than NiCo-precursor. The roughness
should be ascribed to the formation of small particles in
phosphorization process. The TEM test can give more
information about the structure of nanosheets. Low resolution
image of S-NiCoP-2-300 shows a sheet-like structure with
wrinkling surface (Figure 2f), being consistent with SEM test.
Notably, an enlarged TEM image of S-NiCoP-2-300 proves the
presence of a large number of pores on sheets (Figure 2g and h).
It is obvious different with NiCo precursor that have a smooth
surface. The formation of pore on sheets should be relative with
the release of water under heating process. Similar
phenomenons were observed in previous reports.[37,38] The
pores can provide suitable avenues for electrolyte diffusion and
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DOI: 10.1039/C8TA05551C
charge transportation, being favorable to improve the
40]
performance for energy storage and catalysis.[39,
Further
details were revealed by high resolution transmission electron
microscopy (HRTEM) (Figure 2h). The interplanar spacing of
0.28 nm is indexed to (111) plane of NiCoP. The EDS
elemental mapping analysis indicates the uniform distributions
of Ni, Co, and P throughout the sheet (Figure 2i-l). The XRD,
SEM and TEM results confirm the formation of 2-D Co-Ni
precursor and its transformation into 2-D porous NiCoP
nanosheets.
Figure 3. (a) XPS full profiles for S-NiCoP-2-300 and NiCo-precursor; High-resolution
XPS spectra of NiCo−precursor for (b) Ni 2p, (c) Co 2pand High-resolution XPS spectra
of S-NiCoP-2-300 for (d) Ni 2p, (e) Co 2p, (f) P 2p.
The transformation of 2-D Co-Ni precursor to 2-D porous
NiCoP nanosheets have also be reflected by XPS method. From
Figure 3a, one can see the presence of Ni, Co, O, C elements in
NiCo-precursor. Further information can be gained by high-
resolution XPS spectra. In the Figure 3b, we can see the
obvious shakeup satellites (shortened as sat.) at 878.9 and 860.9
eV, which is consistent with the Ni 2p1/2 and Ni 2p3/2 of
Ni(OH)2 at 872.6 and 855.0 eV, respectively.[41] The peaks at
797.2 and 781.3 eV were ascribed to Co 2p1/2 and Co 2p3/2 in
Co(OH)2. The binding energies at around 795.4 and 779.6 eV
are indexed to Co3+ (Figure 3c).[42] After phosphorization, the
Ni/Co/P elements can be observed in ternary S-NiCoP-2-300.
The test implies the conversion of NiCo-precursor into NiCoP,
being consistent with XRD results. In high resolution XPS
spectra (Figure 3d-f), the Ni 2p1/2 and Ni 2p3/2 (Figure 3d) can
be observed at 872.2 and 854.5 eV with the energy gap of 17.7
eV. Two sat. peaks of Ni 2p located at 878.5 and 860.5 eV are
also observed. These results are consistent with that reported
for Ni2P.[33,43] The Co 2p spectrum (Figure 3e) shows the peaks
at 778.9, 782.2 eV with one sat. at 786.0 eV, corresponding to
spin-orbit splitting values of Co 2p3/2. The peaks at 798.2 and
794.3 eV with one sat. peak at 802.9 eV can be index to the
spin-orbit splitting value of Co 2p1/2. The results can be
observed for CoP-based materials.[44,45] The weak sat. peak
corresponding to Co3+ should be due to the easy surface
oxidation of phosphide in air.[46-48] The P 2p peaks can be
(Figure 3f). The P 2p1/2 peak at 129.8 eV is lower than that for
elemental P (130.2 eV). This implies that P species in NiCoP
has a negative charge,[49] which is related with the formation of
metal (Co, Ni)-P bond. In addition, the peaks at 133.1 eV show
that the P-metal species (like Ni and Co) is oxidized due to the
exposure of sample in air.[45] Besides the S-NiCoP, other
samples can also be prepared as control. The synthetic
parameters for different samples (NiP-300 and CoP-300 etc.)
are listed in Table S1. The formation of NiP-300 and CoP-300
are also supported by XPS analyses (Figure S2, S3).[50]
For electrochemical capacitance, the ions adsorption and redox
are mainly processed on the surface.[51] Therefore, the high surface
areas and plentiful pores are favourable to improve the capacitance,
which can be verified by N2 isothermal adsorption-desorption
isotherm of different samples (Figure 4 and Figure S4). The specific
surface area (SBET) of NiCo-precursor is about 147.9 m2g-1 with
average pore size of 16.41 nm. After phosphating, the samples with
different SBET can be obtained based on the calcination condition.
Typically, the SBET and pore volume of the S-NiCoP-2-300 are about
216.4 m2g-1 and 0.62 cm3g-1, respectively (Table S2). For S-NiCoP-
2-250 and S-NiCoP-2-350, the SBET is about 148.0 m2g-1 and 104.3
m2g-1. The S-NiCoP-2-300 have highest surface area, implying the
important effect of calcination temperature on the microstructure of
materials. The low calcination temperature of S-NiCoP-2-250 makes
the incomplete conversion of NiCo hydroxide precursor, thus giving
a low SBET values (148.0 m2g-1) close to Ni-Co precursor
divided into two peaks at 129.8 (P 2p1/2) and 129.0 eV (P 2p3/2
)
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Figure 4. Nitrogen adsorptio-desorption isotherms of (a) NiCo-precursor, (b)S-NiCoP-
2-300, (c) NiP-300 and (d) CoP-300 (inset: corresponding pore size distribution).
(147.9 m2g-1). Indeed, the XRD of S-NiCoP-2-250 shows a
similar pattern with that of NiCo-precursor. On the other hand,
the high calcination temperature can make the full
phosphorization, but can also results in growth of particles and
aggregation of nanosheets. So, the S-NiCoP-2-350, composed
of NiCo phosphide, have low specific surface area of 104.3
m2g-1. The S-NiCoP-2-300 from suitable calcination
temperature have given a large specific surface area with
plentiful pores. Further, our results also indicated the ratio of
Co and Ni in samples have obvious effect on the surface area.
The SBET of S-NiCoP-1-300 is about 163.3 m2g-1, and S-NiCoP-
3-300 is about 91.7 m2g-1. In addition, the CoP-300 and NiP-
300 have a low SBET of 55.4 m2g-1 and 88.1 m2g-1, respectively.
Based on above results, we can obviously see that 1) the
bimetallic phosphides have shown the larger surface area than
any corresponding singe metal phosphides. It implies that the
combination of Co and Ni in precursor are favourable for its
decomposition in phosphorization process to give TMPs a
suitable pore structure. [52] 2) the suitable ratio of Co and Ni is
important to give a materials with high SBET. The bi-transition
meta phosphides (BTMPs) from the phosphorization of the
precursor synthesized with 1:1 Co/Ni molar ratio have a higher
specific area than those with 2:1 and 1:2 molar ratio. It is
ascribed to that the precursor synthesized with 1:1 Co/Ni molar
ratio are favourable to give S-CoNiP with little impurity of
single metal phosphide. In contrast, the precursor synthesized
with 2:1 and 1:2 Co/Ni molar ratio is inclined to form mono-
metal Co or Ni species. So, after the phosphorization, the
corresponding mono-transition metal phosphides (MTMPs) can
also be formed besides BTMPs. The presence of MTMPs can
result the decrease of SBET of final materials because its lower
SBET value than BTMPs. In general, the high surface area of the
material is favorable to improve the specific capacitance.[52]
The high SBET of S-NiCoP-2-300 and large pore volume would
be beneficial to the exposure of more active sites and rapid
mass transfer, making its potential useful for supercapacitance.
Figure 5. Electrochemical performance of NiCo-precursor, S-NiCoP-2-300, CoP-300
and NiP-300 for supercapacitors:(a) CV curves at 10mV s-1; (b) Galvanostatic
charge/discharge curves at a current density of 1 A g−1. (c) CV curves of the S-NiCoP-2-
300 at different scan rate; (d) Galvanostatic charge/discharge curves of the S-NiCoP-2-
300 at different current densities; (e) The specific capacitances at different current
densities;(f) The EIS curves. Inset: an expanded high-frequency region of the plots.
The electrochemical performance of the samples was
evaluated in a three-electrode system in 2 M KOH aqueous
electrolyte. Figure 5a shows CV curves of NiCo-precursor, S-
NiCoP-2-300, CoP-300 and NiP-300. All samples show a
couple of the prominent faradaic redox peaks within the
potential window (vs Hg/HgO), implying the presence of a
[23,50]
faradaic behavior during the electrochemical process.
.
Specifically, the emergency of redox peaks around a potential range
of 0.3 to 0.5 V (vs Hg/HgO) is ascribed to the conversion of
[53]
Co(δ+)/Ni(δ+) state in S-NiCoP to Co3+/Ni3+ state.
Also, the
similarity of anodic and cathodic peaks implied the good
reversibility of the system. The redox reaction mechanism of the
NiCoP with respect to the potential involved in a three-electrode
system and the electrochemical reactions corresponding are as
follows[53]
NiCoP + 4(OH-) +x(OH-) ↔ NiP(OH-)2+ CoP(OH-)2 +
:
x(OH-) + 4e-
(2)
The CV curves of S-NiCoP-2-300 electrode exhibit an larger
CV curve area compared to other electrodes. This is a
indicative of the increased area specific capability for S-NiCoP-
2-300. The increased CV area of S-NiCoP-2-300 are related to
their high surface areas and plentiful pore. Galvanostatic
charge/discharge (GCD) curves at 1 A g−1 (Figure 5b) show the
longest discharging time for S-NiCoP-2-300 among all
samples. The capacitance calculated are about 1206 F g-1 for S-
NiCoP-2-300, much higher than those for Ni-Co precursor (566
3.2. The test of electrochemical properties
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F g-1), CoP-300 (404 F g-1) and NiP-300 (436 F g-1). The The mass loading of S-NiCoP-2-300 on Ni foamViaewreArctichleanOgnleinde
advantage of S-NiCoP over CoP and NiP indicate that the from 0.32 to 13.5 mg cm−2 to study the effect (Figure S5). We
DOI: 10.1039/C8TA05551C
synergism of two metals can large promote the performance of can see that the high capacitance can be obtained in the case of
electrode materials; It is further supported by the better low mass loading (2687 Fg-1). However, too little loading
performance of the NiCo hydroxide precursor than the CoP-300 amount of active components are not real for practical
and NiP-300. On the other hand, the improved performance of application. The specific mass capacitance gradually decreased
S-NiCoP-2-300 than NiCo hydroxide precursor demonstrates with the increase mass loading until the mass reached to 2.65
the superiority of the phosphide over corresponding oxides, mg cm-2. Further increase mass loading can not induce obvious
which should be related to the good conductivity of former than change of capacitance value. For example, the value is about
later,[12] as well of specific structure of phosphide.
1240 F g-1 and 1095 F g-1 for the loading mass of 2.65 mg cm-2
Figure 5c shows CV curves of S-NiCoP-2-300 electrode at and 13.5 mg cm-2. Notably, even at high loading mass of 13.5
various scan rates of 5, 10, 20, 50, and 100 mV s-1. With the mg cm-2, the electrode still exhibit a high discharge specific
increase in scan rate, the anodic peak is shifted toward a higher capacitances of 1095 F g-1, being similar to that at 4.06 mg cm-2.
potential and the cathodic peak is shifted toward a lower In this case, the area capacitance can increased from 4.89 F cm-
2
potential due to polarization.[54] The redox peaks have no
to 14.78 F cm-2, which is much higher than previous reports
obvious change in shape, implying high reversibility of (see Table S5). The capacitance behavior is related with the ability
electrode material. Figure 5b shows GCD curves of S-NiCoP-2- of the transmission of ions and the transfer of electrons. The
300 electrode at different current density from 1 to 20 A g−1. excellent capacitance behavior at large mass loading is intimately
All curves have good symmetry, indicating the high coulombic related with the structure of 2-D S-NiCoP materials as follows (1)
efficiency due to the highly reversible redox reactions of S- the S-NiCoP have good conductivity due to its metalloid structure.
NiCoP-2-300 in the electrochemical charging and discharging Also, the S-NiCoP can form dense, stacked structure due to its sheet-
process. The specific capacitances of S-NiCoP-2-300 are 1206 like structure. Therefore, the electrons can easily transfer from the
F g-1, 1080 F g-1, 1050 F g-1, 962 F g-1, 825 F g-1, 709 F g-1 and electrode surface to the current collector at at large mass loading
612 F g-1 at the current densities of 1, 2, 5, 10, 15, and 20 A g-1, (thick coating), which is favorable for improving the capacitance
respectively(Table S3). It implies the good rate performance in behavior; (2) the porous structure of S-NiCoP materials is favourable
fast electrochemical response process (Figure 5e). At various for the transmission of electrolyte , which is positive for giving the
current densities, the S-NiCoP-2-300 shows higher capacitance good performance even at thick coating of active species.
than other samples. The capacitance retention at 20 A g-1 are
The conductivity and ion migration ability have important
50.75% of that at 1A g-1 for S-NiCoP-2-300, which are 17.66%, effect for a electrode materials. The electrochemical impedance
43.26%, 29.33%, 39.83%, 43.24%, 60.40% and 23.17% for Ni- spectroscopy (EIS) of Ni-Co precursor, S-NiCoP-2-300, CoP-
Co precursor, S-NiCoP-1-300, S-NiCoP-3-300, S-NiCoP-2- 300 and NiP-300 are tested to give the information of ion
250, S-NiCoP-2-350, CoP-300 and NiP-300. The samples from migration characteristics and conductivity of the materials
phosphorization shows higher retention than that of NiCo (Figure 5f). The test was carried out in the frequency range of
precursor, whatever they are higher or lower capacitance at 1 A 0.01 Hz to 100 kHz, and the inset diagram is a magnification of
g-1, further demonstrating the good rate performance of the high frequency region. We can see that all of EIS spectra
phosphide. In addition, we can see that CoP-300 have highest were composed of a semicircle in high frequency area and a
retention of 60.40% at 20 A g-1, while NiP-300 have low sloped line in low frequency area. The slope of the straight line
retention of 23.17%. Correspondingly, the capacitance retention for S-NiCoP-2-300 is much larger than that of Ni-Co precursor,
increases with the increase of ratio of Co in the samples. The CoP-300 and NiP-300 at low frequency region, which reveal
results imply the important of Co in designing the high-stable the lower diffusion limitations, and consequently, the efficient
electrode. It is also indicates that one can design suitable bi- electrolyte ions diffusion during redox reaction. The easy ions
metal based electrode materials by tuning the ratio of metal ions diffusion should be related to the proper porous structure.
and kind of counter ions. The specific capacitance for S-NiCoP- Besides, the charge transfer resistance (Rct value) of S-NiCoP-
2-300 is not only significantly higher than those for Ni-Co 2-300 (0.70Ω) is lower than those of Ni-Co precursor (1.88Ω),
precursor and other contrast samples, but also superior to those CoP-300 (1.90Ω) and NiP-300 (0.87Ω), which should be
for metal phosphide, such as Ni2P nanoparticles (668.7 F g−1 at favourable for fast charge transfer during charge/discharge
1 A g−1),[55] Ni2P/rGO nanoparticles (672.4 F g−1 at 1 A g−1),[56] processes. The lower resistance of S-NiCoP than NiCo
Au/Ni12P5 core/shell nanoparticles (571.4 F g−1 at 0.2 A g−1),[57] hydroxide originated good charge and mass transfer ability
Co2P nanoflowers (284 F g−1 at 1 A g−1),[45] Ni2P (843.25 F g−1 conductivity of phosphide. Also, the bi-metal phosphide shows
at 1 A g−1),[58] etc (see Table S4). The 2-D sheet-like porous lower the electronic and ionic resistances which can also
structure, suitable ratio of Co and Ni are responsible for contribute on the good energy-storage ability.[36] The EIS
enhanced redox reaction kinetics of S-NiCoP-2-300.
analytical results further verify that S-NiCoP-2-300 exhibits
There are usually need of thick coating of active favorable reaction kinetics and the internal resistance compared
components (high loading) on current collector in the practical with other samples.
application. Generally, the performance of electrode can be
To evaluate the potential of S-NiCoP-2-300 for the
decreased with the increase of mass loading (especially at high practical application in portable electronic devices, an
loading), which is not favourable for the practical application. asymmetric supercapacitor (ASC) was assembled in a CR2025
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8TA05551C
Figure 7. Electrochemical performances of the S-NiCoP-2-300//AC ASC device: (a)
GCD curves of the ASC device collected at various current densities; (b) The specific
capacitances at different current densities; (c) Cycling performance of the ASC device at
a current density of 5 A g-1; (d) The fully charged ASC operates a white LED and an
electric motor fan, demonstrating its potential suitability for wearable electronic
applications.
Figure 6. Electrochemical performances of the S-NiCoP-2-300//AC ASC device: (a)
Schematic of the assembled structure of energy storage device based on S-NiCoP-2-300
as positive electrode and AC as negative electrode. (b) CV curves of the S-NiCoP-2-300
and AC electrodes at a scan rate of 20 mV s-1 in 2 M KOH electrolyte. (c) CV curves of
the ASC device at 20 mV s-1 with different voltage windows. (d) CV curves of the ASC
device collected at different scan rates.
g-1 at current densities of 0.5, 1, 2, 3, 5 and 10 A g-1,
respectively. Even at high current density, the device still
maintain a high capacitor value, indicating the remarkable rate
capability (Figure 7b). The good rate performance of ASC
device can be attributed to the porous 2D structure and
increased conductibility of S-NiCoP-2-300. A cycling test was
carried out over 8000 cycles at a current density of 5 A g−1 to
evaluate the cycling stability of ASC device. At initial cycles,
the increase of the capacitance is probably due to the activation
process that allows the electrolyte ions to diffuse into the bulk
and increases the number of available electrochemically active
sites. After 8000 cycles, the S-NiCoP-2-300//AC ASC device
can deliver a capacitance retention of 90.6%, indicating an
excellent cycling stability (Figure 7c). All of above tests reveal
that the S-NiCoP-2-300//AC ASC device with favorable
cycling stability and excellent energy density and power energy
has a great potential application in the energy storage field. So
it is suitable for driving realistic low-voltage devices. To
demonstrate the potential, two same ACS device were
connected together to achieve a higher operating voltage. As
shown in Figure 7d, a 3 V white LED can be lighted using the
two charged devices assembled here in series. In the long-term
test, the device can power the white LED efficiently for more
than 2h after charging within 170s (1 A g-1 current density).
Also, one ASC cell can effectively operate an electrical motor
fan (Figure 7d). These results suggest the large potential of S-
NiCoP-2-300 for construction of high-effective ASC devices
for practical applications.
cell by utilizing S-NiCoP-2-300 and AC as the positive and
negative electrodes with a piece of NKK as the separator
(denoted as S-NiCoP-2-300//AC). Figure 6a shows the
assembled structure of ASC device. The CV tests of S-NiCoP-
2-300 and AC electrodes in 2M KOH at 20 mV s-1 was
performed to estimate total voltage of devices (Figure 6b).
According to CV curve, the voltage window of S-NiCoP-2-300
and AC is from 0 V to 0.6 V and -1.0 V to 0 V, respectively.
Figure 6c shows CV curves of ASC device within different
potential windows (from 0-1.3 V to 0-1.7 V) at a scan rate of 20
mV s-1. With the increase of the potential windows from 0-1.3
V to 0-1.7 V, the redox humps can be observed. Obvious
polarization occurred when the voltage was further extended to
1.6 V. Thus, the optimum operating potential window of 0-1.5
V was used to investigate the performance of S-NiCoP-2-
300//AC ASC device. The CV curves at various sweep rates (5
to 50 mV s-1) in a potential window of 0-1.5 V shows a quasi-
rectangular shape coupled with distinguished redox peaks,
which is typical for hybrid capacitive behavior (Figure 6d).
Moreover, the shapes are almost the same even at high scan rate
50 mV s-1, suggesting the good rate capability.
The GCD curves at different current density were
conducted to further explore the capacitive performance of
ASC device. Almost symmetric GCD curves indicate a high
coulombic efficiency and electrochemical reversibility of the
devices (Figure 7a). It also can be seen that the ASC device
exhibits a potential window of 1.5 V, which is much higher
than those of conventional symmetric capacitors of activated
carbon in aqueous solution. The capacitance values calculated
from GCD curves are 116.2, 103.1, 87.8, 78.6, 67.3 and 53.3 F
Figure 8 shows the Ragone plots of the ASC device, energy
density and corresponding power density. The energy density
(E) and the power density (P) are estimated using the following
equations (3) and (4):
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DOI: 10.1039/C8TA05551C
Acknowledgements
We gratefully acknowledge the support of this research by
the National Natural Science Foundation of China (21631004,
21571054), and the special fund of Heilongjiang University for
Fundamental Research Funds of the Central Universitie
(HDRCCX-2016Z12).
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the matter
under discussion, limited experimental and spectral data, and
crystallographic data.
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Journal of Materials Chemistry A
Page 10 of 10
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DOI: 10.1039/C8TA05551C
The porous TMP nanosheets have been prepared by the phosphorization of
sheet-like NiCo hydroxide precursor, and can be used as efficient and stable positive
electrode for advanced asymmetric supercapacitors with high specific capacitance,
good rate performance and long-term stability.