Template-free synthesis of 3D hierarchical nanostructured NiCo2O4 mesoporous ultrathin nanosheet hollow microspheres for excellent methanol electrooxidation and supercapacitors?

Article abstract of DOI:10.1039/c6ra01939k

A facile template-free solvothermal method without any additional alkali was developed to fabricate a 3D hierarchical hollow microsphere precursor, followed by annealing in air, leading to a novel 3D hierarchical NiCo₂O₄ hollow microsphere material, which is composed of mesoporous (16.1 nm) ultrathin nanosheets (11–21 nm) consisting of ultrafine NiCo₂O₄ nanoparticles (11.9 nm). This 3D hierarchical NiCo₂O₄ nanosheet hollow microsphere material possesses a high specific surface area (93.4 m² g¹) and mesoporosity, and thus superior electrochemical performance as an advanced electrode material. For methanol electrooxidation, the 3D hierarchical NiCo₂O₄ nanosheet hollow microsphere displays much higher electrocatalytic activity (95 A g¹, at 0.6 V), lower overpotential (0.27 V, vs. SCE), and higher stability compared with the 3D hierarchical NiCo₂O₄ nanosheet solid microspheres, Co₃O₄ and NiO microspheres. For supercapacitors, the NiCo₂O₄ hollow microsphere exhibits excellent specific capacitance of 1701 F g¹ at 1 A g¹, excellent rate capability (61.5% retention at 15 A g¹), and good electrochemical stability with 78.2% retention after 1000 charge–discharge cycles even at a high current density of 10 A g¹. These findings can be explained by the unique integral characteristics of 3D NiCo₂O₄ hollow spheres with high electron conductivity, large surface area and numerous open spaces between neighboring mesoporous ultrathin nanosheets, which can offer many facile diffusion paths for ion/electrolyte and greatly improve the electron/ion transfer within the electrode and at the electrode–electrolyte interfaces.

Full text of DOI:10.1039/c6ra01939k

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Template-free synthesis of 3D hierarchical nanostructured NiCo2O4 mesoporous ultrathin nanosheets hollow
microspheres for excellent methanol electrooxidation and supercapacitors
Gaowen Chen, Yizhi Gao and Hui Zhang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A facile template-free solvothermal method without any additional alkali was developed to fabricate a 3D
hierarchical hollow microsphere precursor, followed annealing in air leading to a novel 3D hierarchical
NiCo2O4 hollow microspheres material, which is composed of mesoporous (16.1 nm) ultrathin nanosheets
(~11-21 nm) consisting of ultrafine NiCo2O4 nanoparticles (11.9 nm). This 3D hierarchical NiCo2O4
nanosheets hollow microsphere material possesses high specific surface area (93.4 m2/g) and
mesoporosity and thus superior electrochemical performances as advanced electrode materials. For
methanol electrooxidation, the 3D hierarchical NiCo2O4 nanosheets hollow microsphere displays much
higher electrocatalytic activity (95 A/g, at 0.6 V), lower overpotential (0.27 V, vs. SCE), and higher
stability compared with the 3D hierarchical NiCo2O4 nanosheets solid microspheres, Co3O4 and NiO
microspheres. For supercapacitors, the NiCo2O4 hollow microsphere exhibits excellent specific
capacitance of 1701 F g-1 at 1 A g-1, excellent rate capability (61.5% retention at 15 A g-1), and good
electrochemical stability with 78.2% retention after 1000 charge-discharge cycles even at a high current
density of 10 A g-1. These findings can be explained by the unique integral characteristics of 3D NiCo2O4
hollow spheres with high electron conductivity, large surface area and numerous open spaces between
neighboring mesoporous ultrathin nanosheets, which can offer many facile diffusion paths for ion/
electrolyte and greatly improve the electron/ion transfer within the electrode and at the electrode-
electrolyte interfaces.
benign has been widely studied for potential applications in
1. Introduction
lithium-ion batteries,12,13 and optoelectronic devices,14 electro-
catalysis,15,16 and supercapacitors.17,18 More important, upon the
With increasing depletion of traditional fossil fuels, new
much better electrical conductivity of NiCo2O4 than single nickel
renewable energy sources and energy storage materials have been
oxide or cobalt oxide,14,19 many studies have been reported on the
attracting more and more research interest. Three-dimensional
synthesis of NiCo2O4 with varied morphologies for greatly
hierarchical structure, especially hollow nano-/microstructures
improved electrochemical activity compared to corresponding
have become greatly fascinating in different areas including
monometal oxides.20-22 Lou et al.21 reported a simple template-
catalysis,1-3 energy storage systems,2,4 water treatment,5 drug
free solution method combined with a post-annealing for growing
delivery,6 chemical sensors7 owing to their unique structural
interconnected mesoporous NiCo2O4 nanosheets on conductive
features involving low density, hollow interior, large surface area
substrates directly served as the electrodes for supercapacitors
and shell permeability. Generally, hollow micro-/nanostructured
showing greatly improved electrochemical performance. Xiao et
materials are synthesized with the template-directed methods by
al.22 reported a quick microwave-assisted synthesis and post-
using the hard templates8,9 and the soft ones.2,10 However, tedious
calcination to prepare nanosphere-like NiCo2O4 nanostructure
operation steps and long synthetic periods greatly limit their
consisting of porous nanosheets showing high electroactivity and
large-scale applications.4 Recently, template-free routes because
good long-term stability for methanol oxidation.
of the facile and scalable synthesis have been increasingly
Furthermore, 3D hierarchical structures, especially hollow
employed to obtain numerous hollow micro-/nanomaterials with
NiCo2O4 nanostructures possess large surface area, short electron/
various morphologies.4,11 Despite the great effort being made in
ion transport paths, and efficient mass transport channel, and thus
this aspect, it is still a big challenge to create 3D hierarchical
attract increasing research interest. Li et al.23 reported the use of
hollow nano-/microstructure mixed-metal oxides with desirable
carbon spheres template by programmed coating followed
nano-building blocks and secondary architectures for energy field
hydrogenation to prepare double-shelled hollow sphere structure
related applications.
for integrating large surface area and high conductivity to
In the last decades, spinel nickel cobaltite (NiCo2O4) as a
collectively improve the capacitive performance of NiCo2O4
promising mixed-metal oxide due to its intriguing merits
supercapacitors. Huang et al.24 reported a facile template-free
including relatively abundance, low cost, and environmental
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solvothermal method for preparing urchin-like NiCo2O4 hollow
microspheres consisting of nanorods with enhanced electro-
chemical properties in energy and environmental applications.
Lou et al.25 reported self-templated formation of uniform
NiCo2O4 hollow spheres consisting of small nanoparticles with
complex interior structures showing superior electrochemical
performance in both Li-ion batteries and supercapacitors.
However, to our best knowledge, there is no work reported on a
facile template-free solvothermal method to assemble a novel 3D
hierarchical NiCo2O4 hollow microsphere composed of meso-
porous ultrathin nanosheets so far.
Herein, a facile template-free solvothermal method was first
developed to prepare 3D hierarchical nanosheets hollow
microspheres precursors, which was annealed in air to create a
novel 3D NiCo2O4 hollow microspheres material composed of
mesoporous ultrathin nanosheets consisting of ultrafine NiCo2O4
nanoparticles. The typical characteristic of present method is no
need any additive alkali for producing OH- ion, just only use
isopropanol, methanol and water as mixed solvent, nickel and
cobalt nitrate as reagent for the construction of 3D hierarchical
nanosheets hollow microspheres precursor. Moreover, based on
time-dependent morphology monitoring of the precursor, a
multistep-splitting growth mechanism is proposed to understand
the formation of the 3D hierarchical hollow microsphere
precursor’s nanostructure. Remarkably, the as-obtained 3D
hierarchical NiCo2O4 nanosheets hollow microsphere owns high
surface area and rich porosity. When evaluated as an electrode
material, the 3D hierarchical NiCo2O4 nanosheets hollow
microspheres display not only the excellent electrocatalytic
activity and high stability for methanol electrooxidation, but also
the excellent specific capacitance, excellent rate performance, and
good cycling stability for supercapacitors.
solid microsphere precursor (S-NiCo-pre) was obtained and its
calcined product was denoted as S-NiCo2O4. For comparison,
NiO and Co3O4 microspheres were also prepared by the similar
procedure.
2.2 Characterizations
Powder X-ray diffraction (XRD) data were taken on a
Shimadzu XRD-6000 diffractometer using a graphite-filtered
Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 30 mA. The
catalysts, as unoriented powders, were step-scanned in step
of 0.02°(2θ) in the range of 5-70° using a count time of 4 s
per step. The FT-IR spectra were recorded on a Bruker
Vector-22 FT-IR spectrometer in the range of 400-4000 cm-1
with 4 cm-1 resolution and 60 scans by the standard KBr disk
technique (sample/KBr = 1/100) with almost identical mass
of samples. The thermal gravimetric analysis was done on a
thermalanalyzer (Beijing Hengjiu instrument Co. Ltd.) from
25oC to 500oC at a heating rate of 5oC/min in air. The
specific surface areas were determined from low temperature
N2 adsorption-desorption isotherm on a Quantachrome
Autosorb-1C-VP automated gas adsorption system by using
the multi-points Brunauer-Emmett-Teller (BET) method and
the pore size distribution derived from the desorption
isotherm upon Barrett–Joyner–Halenda (BJH) method. The
scanning electron microscopy and energy-dispersive X-ray
spectroscopy (SEM-EDX) analysis was carried out on an
Oxford Instruments INCAx-act EDX detector attached to a
Zeiss Supra 55 field emission scanning electron microscopy
using a 20 kV electron beam and 60 s acquisition time.
Transmission electron microscopy (TEM) was obtained on a
Hitachi H-800 transmission electron microscope with an
accelerating voltage of 200 kV. The high resolution TEM
graphs were recorded on a JEM-2010 transmission electron
microscope at 200 kV. X-ray photoelectron spectroscopy
(XPS) were obtained on a Thermo VG ESCALAB250 X-ray
photoelectron spectrometer at a base pressure of 2×10-9 Pa
using Al Kα radiation (1486.6 eV). The binding energy was
referenced to C1s at 284.9eV of surface adventitious carbon.
2. Experimental section
2.1 Preparation
The 3D hierarchical hollow microspheres precursor was
prepared by a simple template-free solvothermal method without
any additional alkali sources. All chemicals were analytical grade
and used as available without further purification. In a typical
synthesis, 0.009 mol of Co(NO3)2·6H2O, 0.0045 mol of
Ni(NO3)2·6H2O were dissolved in 4 mL water followed ultrasonic
for 5 min to form a transparent pink solution, then 68 mL of
isopropanol (IPA) was added to above solution followed
magnetically stirred for 1 h obtaining a mixed salt solution. Then,
11 mL methanol was added into above mixed solution. And the
resultant solution was magnetically stirred for 30 min, then
transferred to a 100 mL Teflon-lined stainless-steel autoclave and
2.3 Electrochemical measurements
All electrochemical tests were recorded on a CHI660D
computer-controlled potentiostat (Chen Hua Instruments Co.,
Shanghai, China) at 25 oC.
For methanol electrooxidation test, a modified electrode was
prepared as follows. 1) Prior to modification, the bare glassy
carbon electrode (GCE, 3 mm diameter) was sequentially
polished with 1.0 and 0.05 μm alumina powder slurries to a
mirror shiny surface and then successively sonicated with a 1:1
water-HNO3 solution, ethanol, and deionized water for 3 min in
order to remove any adsorbed substances on the electrode surface.
2) 10 mg of the catalyst was dispersed in a mixture of Nafion (0.1
mL) and ethanol (0.9 mL) to give a homogeneous ink. 3) A total
of 10.0 μL homogeneous ink was dripped onto the surface of a
freshly polished GCE and then evaporated at room temperature in
air. Then, cyclic voltammetry (CV), chronoamperometry (CA),
o
kept at 120 C for 24 h. After cooling to room temperature, the
green precipitate was centrifuged, and washed sequentially with
water and ethanol several times and dried at 80oC in vacuum for
24 h giving a green hollow microsphere precursor denoted as
H-NiCo-pre. This precursor was heated to 350 oC at a heating rate
of 1 oC/min in air and kept for 2 h giving a black hollow
microsphere product named as H-NiCo2O4. When the usage
amount of water and methanol is adjusted to 6 and 9 mL,
respectively, and keeping other reaction conditions unaltered, a
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and electrochemical impedance spectroscopy (EIS) measurements
were performed in a traditional three electrode system with a
modified GCE working electrode, a Pt plate counter electrode and
an Hg/Hg2Cl2 (SCE) reference electrode. 1 M KOH with 0.5 M
methanol were used as the electrolyte.
For electrochemical capacitors (ECs) analysis, the working
electrode was prepared as follows. The electroactive material,
acetylene black, and polymer binder (polyvinylidene difluoride)
was mixed in a mass ratio of 70:20:10. And then an appropriate
amount of N-methylpyrrolidinone (NMP) was added to the
mixtures. This slurry was pasted to the Ni foam substrates
(surface, 1 cm×1 cm). After being dried at 110°C overnight under
vacuum, the as-formed electrodes loaded with active material
were then pressed at 10 MPa and further dried at 100 °C for 24 h.
Before and after the sample was dried thoroughly, the Ni foam
was carefully weighed. The mass loading of active materials
(H-NiCo2O4) in a Ni foam electrode was ca. 6 mg. The obtained
H-NiCo2O4 modified electrode was then used as a working
electrode, and the CV and ECs tests were conducted in a KOH
electrolyte (6.0 M) with the same three-electrode cell as described
before. The specific capacitance is calculated by the following
equation:
Fig. 2 (A) IR spectra of H-NiCo-pre (a) and H-NiCo2O4 (b) and (B-D)
XPS spectra of H-NiCo2O4 (Co 2p (B), Ni 2p (C) and O 1s (D)).
The TG-DTG plots of the precursor obtained in static air
from 25 to 500 oC (Fig. 1B) clearly show a weight loss of ~4.9%
below 150 °C, which can be ascribed to the desorption of
physically adsorbed water, interlayer water molecules and
residual solvent in precursor.16 The next sharp mass loss of
~27.6% accompanied by a strong endothermic effect at 170 -
330 °C is related to the decomposition and dehydroxylation of
the Co-Ni nitrate hydroxide precursor. This weight loss is a little
lower than the theoretical value of 28.3% according to Ni-Co
nitrate hydroxide probably due to the presence of trace nitrate as
C = I ·Δt/(m ·ΔV)
(1)
where C (F/g) is specific capacitance, I (A) is the discharge
current, Δt (s) is the discharge time, ΔV (V) is the potential
window of discharge, and m (g) is the mass of the active material.
The EIS tests were performed at open-circuit potential in the
frequency range 100 kHz to 10 mHz with an alternating-current
amplitude of 10 mV.
o
IR indicated (Fig. 2A(b)). Therefore, the 350 C was chosen as
3. Results and discussion
the annealing temperature to obtain high quality NiCo2O4.
After calcination, the green precursor was converted into
black powder of H-NiCo2O4. The XRD pattern of H-NiCo2O4
(Fig. 1A(b)) clearly shows eight diffractions at 18.9o (111), 31.2o
(220), 36.8o (311), 38.6o (222), 44.7o (400), 55.5o (422), 59.1o
(511), and 64.9o (440), coincide with the standard cubic spinel
NiCo2O4 (JCPDS no. 20-0781) perfectly, and no other peaks
detected, indicating the formation of high pure NiCo2O4. The IR
spectrum of H-NiCo2O4 (Fig. 2A(b)) depicts greatly reduced ʋOH
and H2O, and almost disappeared ʋC-H and ʋC-O, implying the
dehydroxylation and decomposition of the hydroxide compound
and the removal of the residual solvent in precursor, while the
sharpened ʋM-O at 655/560 cm-1 clearly confirm the formation of
NiCo2O4,26 well supporting the XRD data. Furthermore, the
lattice parameter ao of H-NiCo2O4 is estimated as 0.8104 nm,
close to the standard 0.8110 nm (JCPDS no. 20-0781),
indicating the presence of well-defined NiCo2O4 crystals. The
particle size upon the Debye–Scherrer formula (D = kλ/( cos),
D is the average crystal size, k is Scherrer constant (0.89), λ is
the wavelength of X-ray (0.1542 nm), β is the full width at half
maximum (in rad.), θ is the Bragg angle) from the sharp (311)
line is ~11.2 nm, implying that H-NiCo2O4 may be composed of
quite small NiCo2O4 crystallites.
3.1 Characterization of H-NiCo-pre and H-NiCo2O4
Fig. 1 (A) XRD patterns of (a) H-NiCo-pre and (b) H-NiCo2O4. (B)
TG-DTA curves of H-NiCo-pre.
The XRD pattern of the green precursor H-NiCo-pre (Fig.
1A(a)) clearly shows typical features of layered nickel-cobalt
nitrate hydroxide, quite similar to those of Ni or Co nitrate
hydroxide (JCPDS no. 22-0752 or 46-0605). The IR spectrum of
the precursor (Fig. 2A(a)) shows a strong broad band at ~3449
cm-1 due to the stretching vibrations of OH groups and the one
at 1618 cm-1 due to the bending mode of physic-adsorbed and
interlayer water molecules (H2O). The strong broad band around
1380 cm-1 is due to interlayer nitrate (ʋ3(NO3)), and those at 660
and 560/525 cm-1 correspond to OH and lattice mode ʋM-O (M =
Co and Ni), respectively. As for the weak bands at 2933 and
2818 as well as 1052 cm-1, which are due to the stretching
vibrations ʋC-H and ʋC-O, respectively, probably from residual
isopropanol and methanol molecules.
To obtain more detailed information about the composition
and the oxidation state of H-NiCo2O4, XPS was performed. The
XPS survey clearly indicates the presence of Co, Ni, and O in
H-NiCo2O4 (Fig. S1). Upon Gaussian fitting method, the Co 2p
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spectra (Fig. 2B) can be fitted with two spin-orbit doublets
characteristic of Co2+ and Co3+ and two weak shakeup satellites.
Particularly, the fitting peaks at 779.6 and 794.6 eV are ascribed
to Co3+, while the peaks at 781.3 and 796.1 eV are assigned to
Co2+.27 In the Ni 2p spectra (Fig. 2C), two kinds of Ni species
(Ni2+ and Ni3+) are detected. The fitting peaks at 854.4 and 871.9
eV are assigned to Ni2+, while those at 856.1 and 873.5 eV are
assigned to Ni3+.27 The two shakeup satellites at 861.5 and 879.8
eV can be clearly seen at the high binding energy side of the Ni
2p3/2 and Ni 2p1/2 edge.28 The O1s spectra (Fig. 2D) depict three
contributions, marked as O1 (529.7 eV), O2 (531.1 eV) and O3
(532.9 eV), associated with typical metal–oxygen bond, high
number of defect sites with low oxygen coordination in the
material with small particle size, and the multiplicity of physic-
and chemisorbed water at and within the surface, respectively.27,29
Based on above observations, the formula for H-NiCo2O4 can be
generally expressed as Co2+1-xCo3+x [Co3+Ni2+xNi3+1-x]O4 (0<x<1),
where the cations in the bracket occupy the octahedral sites and
those outside are in the tetrahedral sites.16,27 The solid redox
couple of Ni2+/Ni3+ and Co2+/Co3+ may endow the present
H-NiCo2O4 enough active sites for electrochemical reaction.
The morphologies of the H-NiCo-pre and the H-NiCo2O4
were carefully studied by SEM. It can be seen that the H-NiCo
-pre (Fig. 3A) shows obvious hierarchical microsphere structure
with an average diameter ~7.2 μm. Some broken spheres (Fig. 3C)
and the TEM image (inset in Fig. 3A) clearly indicate the hollow
interior of the precursor. Markedly, these microspheres are
composed of numerous interconnected ultrathin nanosheets (~27
nm) with smooth surface and numerous open spaces between
neighboring nanosheets (Fig. 3C and E). After calcination, the
hollow interior and nanosheets structure of the precursor is well
preserved in the H-NiCo2O4 (Fig. 3B), which depicts highly
uniform microspheres with slightly reduced size of ~6.4 μm than
Fig. 4 TEM (A, B) and HRTEM (C, D) images of H-NiCo2O4 (Inset in C and D is
particle size distribution and SAED of H-NiCo2O4, respectively).
precursor probably due to the shrinkage during the calcination.
Interestingly, different from the precursor, the H-NiCo2O4
apparently has the even thinner nanosheets (~11-21 nm) with
rough surface consisting of many ultrafine interconnected
nanoparticles. Moreover, there are many nanopores observed in
these nanosheets (Fig. 3F).
In order to obtain the more detailed morphological and
crystallographic properties of the as-prepared H-NiCo2O4, the
TEM, HRTEM and selected area electron diffraction (SAED)
were performed. As shown in Fig. 4A and B, the typical TEM
image of H-NiCo2O4 further confirms the hollow microsphere
structure of H-NiCo2O4 assembled from ultrathin nanosheets with
a length of ~800-1000 nm. And these ultrathin nanosheets with a
transparent feature on their edges (Fig. 4B) shows that the
nanosheets have a porous nature, implying that it could provide
high surface area. As shown in Fig. 4C, the ultrathin nanosheets
are composed by many ultrafine NiCo2O4 nanoparticles of ~11.9
nm (inset in Fig. 4C) and numerous nanopores of ~3-6 nm. These
results are in good agreement with its XRD (Fig. 1A(b)) and
SEM results (Fig. 3B, D and F). The various crystal orientations
of nanoparticles exhibit clearly the lattice fringes (Fig. 4D) with
interplanar distances of 0.467, 0.287, 0.244 and 0.203 nm
corresponding to the (111), (220), (311) and (400) planes of the
cubic NiCo2O4 phase, respectively.23 The SAED pattern (inset in
Fig. 4D) evidently shows the nanocrystal nature of H-NiCo2O4 as
indexed as (111), (220), (311), (222), and (400) diffraction dots or
incontinuous diffraction rings of NiCo2O4. These results are in
good agreement with XRD (Fig. 1A(b)) and HRTEM results (Fig.
4D). Moreover, the EDX spectrum gives the elemental ratio of
Co/Ni as 2.18 further confirming the formation of NiCo2O4
nanocrystals (Fig. S2). It can be assumed that such 3D
hierarchical nanostructured NiCo2O4 hollow microsphere with
numerous open spaces between neighboring ultrathin nanosheets,
mesoporous structure and nanocrystal nature are expected to
possess excellent electrochemical activity.
Fig. 3 SEM images of H-NiCo-pre (A, C, E) and H-NiCo2O4 (B, D, F)
(Bottom inset in A is TEM, upper insets in A and B are particle size distributions).
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Ostwald-ripening process, in which the outer crystalline shells
grew on the solid particle accompanied by continuous dissolution
and recrystallization of the interior structures.13 The growth of
nanosheets on the surface of the initial formed microspheres and
the slight diameter increase of the microspheres with prolonged
solvothermal time also confirms the inside-out Ostwald-ripening
process. Therefore, a plausible formation mechanism of such 3D
hierarchical nanosheets hollow microspheres precursor is
proposed and illustrated in Fig. 6E. Considering neither surfactant
nor additional base used in the reaction system, the presence of
mixed solvent (IPA + methanol + H2O) may play a key role on
the formation of 3D nanosheets hollow microspheres precursor.
Firstly, the creation of hydroxyl groups is greatly dependent on
Fig. 5 N2 adsorption-desorption isotherms of H-NiCo2O4 (inset: pore size
distribution).
-
the amount of methanol. It is accepted that methanol and NO3
The N2 adsorption-desorption analysis of H-NiCo2O4 (Fig. 5)
ions from metal nitrates underwent a redox reaction releasing
o
OH- ions at 120 C (eqn. (2)).30 Then, the released OH- anions
shows a type IV isotherms and a distinct hysteresis loop on the
IUPAC classification, indicating a typical mesoporous structure.
The specific surface area (SSA) upon BET method is 93.4 m2/g,
obviously larger than those previously reported,12,24,25 in line with
its mesoporous ultrathin nanosheets hollow microspheres
structure. The Barrett–Joyner–Halenda pore size distribution
(inset in Fig. 5) data shows a small peak at ~3.9 nm and a wide
larger peak at ~16.1 nm, respectively, further defining the
well-developed mesoporous structure of H-NiCo2O4 along with a
pore volume of 0.37 cm3﹒g-1. The high SSA and mesoporosity
of H-NiCo2O4 with interconnected ultrathin nanosheets and
hollow structure can not only greatly increase the electrode-
electrolyte contact area but also provide abundant active sites and
efficient charge/ion transport channels, which is beneficial for
greatly enhanced electrochemical properties.
-
further reacted with Ni2+, Co2+ and NO3 ions to create Co−Ni
bimetallic nitrate hydroxide nuclei (eqn. (3)).30,31 A large amount
of such nuclei burst and further grew into larger particles, which
readily assembled into primary 2D nanosheets as dominant
morphology upon the proper ratio among water, methanol and
IPA by giving modulations to the homogenization of the reactants
in the medium, the amount of individual Co−Ni nitrate hydroxide
monomer nucleus formed, and amalgamation and orientation of
growing nucleus, considering the main effect of the saturated
vapor pressure and polarity of the solvent on the structure of the
products during the synthesis process.31,32 As shown in Fig. 6E, at
stage I, the primarily formed nanosheets could assemble into
solid microspheres-like architecture (Fig. 6A) owing to
crystal-face attraction, van der Waals force and hydrogen
bond.33,34 These nanosheets further oriented self-assemble into
solid microspheres (stage II). With further prolonged reaction
time, a yolk-shelled structure was formed upon the inside-out
Ostwald-ripening and recrystallization (stage III). With
continuously increased reaction time, a completely hollow
precursor with well-defined 3D nanosheets architecture was
obtained (stage IV). Clearly, in the present template-free and no
additional alkali reaction system, the solvent methanol mainly
contributes to the in situ creation of precipitation agent OH- ions
and the followed formation of primary nanosheets, while the IPA
mainly to the production of hollow microsphere structures. The
similar role of IPA has ever been mentioned in the synthesis of
urchin-like NiCo2O4 hollow sphere with urea as precipitation
agent.24
3.2 Formation mechanism
From above results, it can be seen that upon a simple
template-free solvothermal route with no additional base, we first
obtain
a 3D hierarchical nanosheets hollow microspheres
precursor and thus a novel 3D NiCo2O4 mesoporous ultrathin
nanosheets hollow microspheres material with high surface area
by easy post-annealing in air. Clearly, the formation mechanism
of 3D hierarchical nanosheets hollow microspheres precursor is
quite important and desired. Therefore, a series of time-dependent
morphology evolutions of the precursor have been studied upon
SEM characterization to reveal the growth process of the
precursor.
As shown in Fig. 6A, petal-like nanosheet-assembled solid
microspheres-like product was formed after reaction duration of 6
h. Then for the 8 h reaction sample, nanosheet-assembled solid
microspheres were obtained (Fig. 6B). As for longer reaction time
of 12 h, york-shelled microspheres product was formed (Fig. 6C).
When the reaction time was further prolonged to 24 h, the final
product shows a hierarchical nanosheets microspheres with clear
hollow interior (Fig. 6D). These results reveal that the initially
formed nanosheet solid microspheres followed a dissolution-
recrystallization process occurred at the exterior surface of the
solid microspheres accompanied by consumption of interior core
leading to the growth of york-shelled nanosheets spheres and
finally evolved into the hierarchical nanosheets hollow
microspheres precursor. The transformation from solid particles
to hollow ones is generally based on the so-called inside-out
Fig. 6 (A−D) SEM images of the products obtained after various reaction
durations (A: 6 h, B: 8 h, C: 12 h, and D: 24 h) and (E) schematic
illustration of the formation process of the H-NiCo-pre: (I) initial
nucleation and oriented self-assembly; (II) further oriented self-assembly;
(III) inside-out Ostwald-ripening and recrystallization; (IV) further
dissolution and recrystallization.
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4CH3OH + NO3- → 4HCHO + NH3 + OH- + 2H2O
(2)
-
-
M2+ + (2-x)OH- + xNO3 → M(OH)2-x(NO3 )x (M=Ni, Co) (3)
For comparison, a 3D NiCo2O4 nanosheets solid micro-
spheres denoted as S-NiCo2O4, and monometal oxides Co3O4 and
NiO microspheres were also prepared by similar solvothermal
method without additional alkali followed air-annealing process.
The XRD patterns (Fig. S3A) of the three precursors were similar
to those of bimetal Ni-Co, monometal Co, and Ni nitrate
hydroxide phases, respectively. After annealing, all of these
microsphere precursors were transformed into metal oxide phases
(Fig. S3B). Before and after calcination, the microspheres
morphologies were revealed by the SEM/TEM images (Fig. S4)
and there is a little change in their morphologies. In addition, the
TEM, HRTEM and SAED images reveal that the S-NiCo2O4
microspheres are also assembled by mesoporous nanosheets
consisting of small nanocrystals of ~14.9 nm (Fig. S5). However,
the BET results (Fig. S6) indicated the S-NiCo2O4 possesses a
SSA of 69.4 m2 g−1, an optimal pore size of 5.9 nm and pore
volume of 0.29 cm3 g−1, which were much lower than those of the
hollow sample H-NiCo2O4.
Fig. 7 Cyclic voltammetry curves of H-NiCo2O4, S-NiCo2O4, Co3O4, NiO
modified electrode in (A) 1 M KOH and (B) 1 M KOH + 0.5 M methanol
at a scan rate of 50 mV/s, (C) chronoamperometry curves of H-NiCo2O4,
S-NiCo2O4, Co3O4, and NiO electrodes in 1 M KOH + 0.5 M methanol at
0 V (0–60 s) and 0.6 V (60–1060 s), and (D) CV curves of H-NiCo2O4
electrode in 1 M KOH + 0.5 M methanol at a scan rate of 50 mV/s.
3.3 Electrochemical characterization of the modified electrode
3.3.1 MOR performance of H-NiCo2O4
catalysts,39,40 a common phenomenon for metal oxide catalyst16,
22,41
as the main limitation to their commercialization in DMFCs,
the current density of the present H-NiCo2O4 electrode is highly
improved (Fig. S7). Fig. 7B further depicts a comparison of the
CV curves on H-NiCo2O4, S-NiCo2O4, Co3O4 and NiO electrodes
in the presence of 1M KOH with 0.5 M methanol. The current
density of H-NiCo2O4 at 0.6 V is 95 A/g, which is about two,
fourteen and twelve times those of S-NiCo2O4, Co3O4 and NiO
electrodes, respectively. Remarkably, the activity of H-NiCo2O4
electrode is much better than those of NiCo2O4/SS,16
nanosphere-like NiCo2O4,22 urchin-like NiCo2O4 hollow and
solid microspheres,24 and porous NiCo2O4 nano-structures41
electrodes (Table S1). The mechanism of methanol oxidation on
H-NiCo2O4 electrode is similar to those on NiO and Co3O4
electrodes being simply illustrated as eqns. (4) and (6).36,42
The electroactivity of the as-prepared H-NiCo2O4 is studied
for methanol oxidation compared with S-NiCo2O4, Co3O4 and
NiO. Firstly, the CV curves were obtained in 1 M KOH solution
without methanol and shown in Fig. 7A. Similar to previous
report,35,36 the Co3O4 electrode has redox peaks at ~0.44 and 0.37
V, and the NiO electrode has redox peaks at ~0.38 and 0.28 V
both enclosing a small area, which are related to the redox
couples of Co3O4/CoOOH and NiO/NiOOH. However, the CV
curves of the H-NiCo2O4 exhibit a pair of broader redox peaks
with the highest peak current and the largest enclosed area,
indicating that it has much better electrochemical activity
compared to S-NiCo2O4, Co3O4 and NiO electrodes. It can be
explained that the charge-transfer process of solid-state redox
couples Co2+/Co3+ and Ni2+/Ni3+ may endow the richer redox
peaks of the present spinel H-NiCo2O4.16 The redox reactions in
the alkaline environment are based on eqns. (4) and (5).37,38
2MOOH + 2CH3OH + 2.5O2 
2M(OH)2 + 2CO2 + 3H2O (M=Ni, Co)
(6)
NiCo2O4 + OH- + H2O  NiOOH + 2CoOOH + 2e- (4)
The low onset potential and high current density
demonstrate the high electrocatalytic performance of H-NiCo2O4
for methanol oxidation. This may be explained on the following
reasons. (a) The electronic conductivity of NiCo2O4 is higher than
that of either Co3O4 or NiO, and the high conductivity may
effectively accelerate electron transfer. (b) NiCo2O4 is a mixed
valence oxide, thus the redox couples of Co2+/Co3+ and Ni2+/Ni3+
can afford rich electroactive sites for methanol electrooxidation.
(c) High electrocatalytic oxidation activity may be due to the
H-NiCo2O4 with the large specific surface area and numerous
open spaces between neighboring mesoporous ultrathin
nanosheets consisting of many ultrafine interconnected NiCo2O4
nanocrystals, which can expose more electroactive sites and offer
many facile diffusion paths for ion/electrolyte in the electrode
and at the electrode-electrolyte interfaces.
CoOOH + OH-  CoO2 + H2O + e-
(5)
All the methanol electrooxidation on H-NiCo2O4,
S-NiCo2O4, and NiO electrodes display a strong anodic current in
the presence of 0.5 M methanol compared with those CV curves
only in KOH (Fig. S7), while the Co3O4 electrode show smaller
increment in the oxidation current in the presence of methanol,
implying lower methanol oxidation activity of Co3O4.16,22
Compared with the S-NiCo2O4, Co3O4 and NiO electrodes, the
current density of the H-NiCo2O4 electrode is highly improved. It
is noted that the onset potential of the H-NiCo2O4 electrode is ca.
0.27 V (vs. SCE), quite lower than on S-NiCo2O4 (0.32 V), Co3O4
(0.38 V) and NiO (0.36 V) electrodes. Though the onset potential
of H-NiCo2O4 is still higher than previously reported noble metal
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RSC Advances
respectively. We also investigate the long-term stability of
H-NiCo2O4 electrode by CV. As shown in Fig. 7D, the CV curves
are quite stable and the current density at 0.6 V exhibits 79%
retention after 500 cycles. More importantly, after 500 cycles, the
current density at 0.6 V can be returned to 90% of the original
value by refreshing the electrolyte. This indicates that the current
decay is partly due to the consumption of methanol. This superior
long term stability can be partially explained by the structural
stability of the electrode material originated from the present
NiCo2O4 nanosheets hollow microspheres with larger surafce
area, mesoporosity, and robust 3D hierarchical nanostructures.
These results confirm that the obtained H-NiCo2O4 material could
meet the requirements of both high electro-activity and long-term
stability necessary for using as the anodic materials in DMFCs.
Fig. 8 EIS plots of H-NiCo2O4, S-NiCo2O4, Co3O4, and NiO electrodes in
1M KOH (A) and 1M KOH+0.5M methanol (B).
The charge transfer resistance of electrodes is another key
factor affecting their performance in DMFCs. The EIS analysis
was performed to study charge transfer resistance in methanol
oxidation. As shown in Fig. 8A in 1 M KOH blank, there are two
main parts in the Nyquist diagram. A high frequency semicircle
related to the charge transfer resistance and a low frequency trail
from the well known redox capacitance behavior of nickel and
cobalt oxide.42 As expected, the diameters of the semicircle of
both H-NiCo2O4 and S-NiCo2O4 electrodes are much smaller than
those of Co3O4 and NiO electrodes, confirming much higher
charge transfer rate on the H-NiCo2O4 and S-NiCo2O4 than on
monometal oxides electrode. Importantly, the semicircle of the
H-NiCo2O4 electrode is clearly much smaller than that of
S-NiCo2O4 electrode (Fig. 8A bottom inset), implying that the 3D
3.3.2 Supercapacitors performance of H-NiCo2O4
To evaluate the potential of the prepared H-NiCo2O4 acting
as an electrode material for supercapacitors, the electro-chemical
capacitive behavior was performed. Firstly, Fig. 9A shows the CV
curves of H-NiCo2O4 measured at varied scan rates from 2 to 30
mV s−1 in the potential window of -0.2−0.6 V in 6.0 M KOH. All
the CV curves show a pair of marked broad redox peaks ascribed
to the Faradaic reaction associated with M-O/M-OOH (M = Ni or
Co).37,38 These CV curves are quite varied from the idea
rectangular shape of electric double-layer capacitors, indicating
the pseudocapacitive characteristics of the H-NiCo2O4 material.43
All the CV curves (Fig. 9A) exhibit a similar shape and the
peak current increases with increasing scan rates. The shape of
CVs changes little even at the high scan rate of 30 mV s−1,
demonstrating the good kinetic reversibility of the H-NiCo2O4
electrode. The peaks are located at around 0.06 and 0.25 V when
the scan rate is 2 mV s-1. Then, the peak potential shifts about
hierarchical
NiCo2O4
mesoporous
nanosheets
hollow
microspheres sample H-NiCo2O4 has a much lower interfacial
charge-transfer resistance. The equivalent circuit for fitting the
EIS plot is given as inset in Fig. 8A. In this circuit, Rs, C, and Rct
are the solution resistance, redox capacitance, charge-transfer
resistance in the electrode and at the electrode-electrolyte
interface, respectively. Then, in the presence of 1 M KOH with
0.5 M methanol, the diameter of the semicircle for Co3O4 and
NiO electrodes is obviously increased (Fig. 8B) probably due to
the adsorption of methanol or the oxidation intermediates on the
electrode.16 Then for S-NiCo2O4 electrode, there is a little change
in the diameter of the semicircle with and without methanol
(bottom insets in Fig. 8A and B). However, for the H-NiCo2O4
electrode, nearly no change is detected in the diameter of the
semicircle in the presence and absence of methanol. The fitting
parameters upon proposed equivalent circuit are listed in Table
S2. It can be seen that the Rct value is much lower than those of
Co3O4 and NiO electrodes. Moreover, about 1.35 and 1.65 times
decrease in the Rct value was observed on the H-NiCo2O4
electrode in 1 M KOH and 1 M KOH with 0.5 M methanol,
respectively, compared to S-NiCo2O4 electrode, demonstrating
that the presence of the H-NiCo2O4 on the electrode surface
assists in the faster electron transfer. These results clearly indicate
the high tolerance to intermediate poisoning of H-NiCo2O4. The
low Rct value is in favor of the fast electron or ion transfer in the
electrode and at the electrode- electrolyte interface.
Fig. 9 Electrochemical properties of H-NiCo2O4 electrode in 6 M KOH
solution. (A) CV curves obtained at various scan rates in the voltage
window of -0.2–0.6 V (Inset: the plots of anodic and cathodic peak
current against the square root of the scan rate); (B) Galvanostatic
charge-discharge curves at various current densities; (C) Effects of current
densities on its specific capacitance; and (D) Cycling performance over
1000 cycles at a current density of 10 A g-1 (Inset: charge-discharge curve
at 10 A g-1 ).
Chronoamperometry is a useful tool to study the electro-
chemical stability of an electrocatalyst. The electro-chemical
stability of H-NiCo2O4, S-NiCo2O4, Co3O4 and NiO electrodes
for methanol oxidation at 0.6 V was studied, with the first 60 s
recorded at 0 V. As shown in Fig. 7C, despite a current decay in
the first 50 s, the H-NiCo2O4 electrode shows a high stability in
the next 950 s, which is about two times, thirteen times and
fourteen times those of S-NiCo2O4, Co3O4 and NiO electrodes,
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100 and 200 mV for a 5- and 15-times increase in the scan rate,
respectively, which is due to the polarization effect of the
electrodes.44 The inset in Fig. 9A presents a good linear
relationship of anodic and cathodic peak currents (i.e., current) vs.
the square root of the scan rate, implying the diffusion of OH−
ions as the rate controlling process in the whole process. Fig. 9B
depicts the galvanostatic charge–discharge voltage curves of the
NiCo2O4 electrode in the voltage range of 0-0.35 V at varied
current density from 1 to 15 A g-1. Clearly, there are voltage
plateaus can be seen at ca. 0.2 V, in consistent with the above CV
data. Fig. 9C gives the calculated specific capacitance as a
function of the discharge current density. Notably, the H-NiCo2O4
electrode delivers high specific capacitances of 1701, 1550, 1410,
1216, 1180, 1132, and 1046 F g-1 at current densities of 1, 2, 4, 8,
10, 12, and 15 A g-1, respectively, which may be attributed to the
unique 3D NiCo2O4 mesoporous ultrathin nanosheets hollow
microspheres structure. Compared with the other reported similar
Fig. 10 Nyquist plots of the 1st cycle and 1000th cycle of the H-NiCo2O4
electrode in 6.0 M KOH (Inset: the enlarged high frequency region).
charge-transfer resistance Rct does not change much after the
1000-cycle test. This may benefit from the high conductivity of
the H-NiCo2O4 and its robust 3D porous structure conducive to
the fast ions/electrons transfer within the electrode and at the
electrode/electrolyte interfaces. Meanwhile, both impedance
spectra depict a linear region in the low frequency range
corresponding to the diffusion resistance W. After undergoing the
1000-cycle test, a slightly increased Warburg impedance can be
observed probably due to the formation of a small amount of
inactive sites caused by the partial collapse of the internal 3D
porous structures during the cycle test.45,50 The results are in good
agreement with that from the long-term cycle stability test that
supercapacitance degraded by less than 21.8% after 1000 cycles.
Remarkably, even under the high current density, the H-NiCo2O4
electrode can still exhibit good electrochemical stability in highly
basic environment, which could be readily attributed to the
interconnecting mesoporous ultrathin nanosheets structures in
NiCo2O4 hollow microspheres and their accommodation for the
increased strain resulting from quick insertion and extraction of
OH− ions during Faraday reaction.
spinel
NiCo2O4
electrode
materials
with
varied
morphologies,25,45-49 the capacitance value obtained in our work
is remarkable (Table S3). More importantly, as the current density
increases, the capacitance at 15 A g−1 remains to be 61.5% of that
obtained at the current density of 1 A g−1. These results suggest
that the H-NiCo2O4 electrode has excellent rate capability.
Substantially, the impressive enhancement of capacitive
performance at the quite high current density is attributed to the
better utilization of electroactive surface of the advantageous
hollow microspheres nanostructure. The hollow microspheres
consisting of mesoporous ultrathin nanosheets could provide
numerous electroactive sites for Faradaic reaction. Particularly,
the nanosheets consisting of ultrafine nanocrystallites possess
high porosity, and thus the electrolyte can easily penetrate
through the nanosheets for efficient redox reactions during the
Faradaic charge storage process. On this occasion, the whole
mesoporous nanosheets hollow microspheres can serving as an
“ion-buffering chamber”45-49 facilitating a faster permeation
process of electrolyte into each nanosheets matrix by shortening
the diffusion paths of OH− ions to the electroactive sites and
accommodates the resultant strain originated from high rate
insertion and extraction of OH− ions. Therefore, even under the
high current density, a large proportion of electroactive sites can
still remain, which conduces to improving the rate capability.
The cycling performance of H-NiCo2O4 electrode is tested
by the continuous charge–discharge testing at a current density of
10 A g-1 and the result is shown in Fig. 9D. Clearly, about 78.2%
of the maximum capacitance can still be retained after 1000
continuous charge–discharge cycles, confirming the good long-
term electro-chemical stability of the H-NiCo2O4 even under the
high current density. This good cycling stability is probably
attributed to the unique NiCo2O4 hollow microspheres structure
consisting of mesoporous ultrathin nanosheets that is expected to
improve the structural integrity, which is promising for the
development of high-performance supercapacitors.
Conclusions
In summary, we first synthesize a novel 3D hierarchical
NiCo2O4 hollow microspheres material consisting of mesoporous
(~16.1 nm) ultrathin (~11-21 nm) nanosheets which were
aggregated by ultrafine nanocrystallines upon a template-free
solvothermal method without any additional alkali and
subsequent air-annealing treatment. The growth process of 3D
hierarchical NiCo2O4 hollow microsphere precursors has been
investigated, and a plausible mechanism was proposed. The novel
3D hierarchical NiCo2O4 nanosheets hollow microspheres
materials exhibit typical mesoporous structures with high specific
surface area. As a result, the 3D hierarchical nanostructured
NiCo2O4 nanosheets hollow microspheres manifest superior
electrochemical performance when evaluated as the electrode
material for both electrocatalytic methanol oxidation and
supercapacitors. These findings can be explained by the unique
integral characteristics of 3D hierarchical NiCo2O4 nanosheets
hollow spheres with high electron conductivity, large surface area
and numerous open spaces between neighboring mesoporous
ultrathin nanosheets consisting of many ultrafine interconnected
NiCo2O4 nanocrystals, which can expose more electroactive sites
and offer many facile diffusion paths for ion/electrolyte and
greatly improve the electron/ion transfer within the electrode and
at the electrode-electrolyte interfaces. The present work provides
The EIS analysis was further performed to study the change
of the H-NiCo2O4 from 1st to 1000th charge-discharge cycle as
shown in Fig. 10 along with an equivalent circuit consists of a
bulk solution resistance Rs, a charge-transfer resistance Rct, a
redox capacitance C, and Warburg impedance W (inset in Fig. 10).
In the high frequency region, compared with the 1st cycle, the
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RSC Advances
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This work was supported by National Natural Science
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DOI: 10.1039/C6RPA0a1g93e9K10 of 10
Graphical Abstract
Novel 3D hierarchical NiCo2O4 hollow microspheres assembled by mesoporous ultrathin nanosheets consisting of ultrafine
nanocrystals by a facile template-free solvothermal method followed air-annealing shows excellent methanol electrooxidation
and supercapacitors performance.
10
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Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]