Unusual CoS2 ellipsoids with anisotropic tube-like cavities and their application in supercapacitors

Article abstract of DOI:10.1039/c2cc32750c

Unusual CoS₂ ellipsoids with anisotropic tube-like cavities have been synthesized from the simultaneous thermal decomposition and sulfidation of a preformed cobalt carbonate precursor. The as-prepared CoS₂ ellipsoids show interesting supercapacitive properties with high capacitance and good cycling performance.

Full text of DOI:10.1039/c2cc32750c

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Unusual CoS2 ellipsoids with anisotropic tube-like cavities and their
application in supercapacitorsw
Lei Zhang, Hao Bin Wu and Xiong Wen (David) Lou*
Received 18th April 2012, Accepted 17th May 2012
DOI: 10.1039/c2cc32750c
Unusual CoS2 ellipsoids with anisotropic tube-like cavities have
been synthesized from the simultaneous thermal decomposition
and sulfidation of a preformed cobalt carbonate precursor. The
as-prepared CoS2 ellipsoids show interesting supercapacitive
properties with high capacitance and good cycling performance.
facile sulfidation approach. Many efforts have been made
for the synthesis of diverse transition-metal sulfides, owing
to their unique physicochemical properties and widespread
applications.14–20 However, the lack of well-established
methods makes the synthesis of porous or nanostructured
metal sulfides challenging.21–25 In the synthesis approach
presented here, we use pre-formed CoCO3 ellipsoids as the
precursor, followed by a simultaneous decomposition and
sulfidation process in H2S gas flow. Unlike typical metal oxides
obtained from decomposition of carbonates/hydroxides, in which
the nanopores are uniformly distributed on the surface or
throughout the bulk of the materials,11–13 the as-formed CoS2
ellipsoids possess unique tube-like cavities oriented from the
center to the surface with openings on the two ends of the
ellipsoidal particles. To the best of our knowledge, microparticles
with such anisotropically oriented and distributed cavities have not
been reported before. Further electrochemical evaluations reveal
that these CoS2 ellipsoids manifest attractive supercapacitive
performance, which might be attributed to the intrinsic properties
of cobalt disulfide and the unique porous structure.
Porous functional nanomaterials are of great importance in a
wide range of fields, including catalysis, separation, and
electrochemical devices.1–6 The desirable porous structure
provides high surface area of the solid material, facilitates
diffusion of foreign substances throughout the bulk, and thus
improves the performance in various applications. Many
strategies have been employed to prepare materials with
different porous structures and cavities. For example, organic
surfactants have been extensively used as ‘‘soft templates’’
to generate ordered mesoporous silica materials,4 and such
mesoporous silica materials could be further utilized as ‘‘hard
templates’’ to synthesize other porous functional materials via
nanocasting.6 However, one drawback of hard template assisted
approaches is the tedious procedure that normally leads to
additional cost. Meanwhile, novel strategies for the preparation
of porous materials from pre-formed solid materials have received
much attention in recent years.7–12 Specifically, metal containing
precursors (e.g., metal carbonate/hydroxide/nitrate etc.) are first
prepared and readily transformed to the corresponding metal
oxides with porous structures through procedures such as thermal
decomposition and chemical etching. Very recently, we have
applied manganese carbonate microsized particles to fabricate
complex metal oxide (e.g., LiNi0.5Mn1.5O4) nanostructures
with hollow interiors and porous shells.13 Nevertheless, the
transformation process typically leads to the formation of
disordered but uniformly distributed pores. Therefore, creating
novel porous structures via this simple approach remains largely
unexplored. In addition, most of the studies so far are still
limited to metal oxides as the target products.
To fabricate porous CoS2 ellipsoids, we first synthesized
ellipsoidal CoCO3 microparticles as the precursor using a
one-pot solvothermal method previously developed by us with
minor modifications (detailed experimental process in ESIw).12
The crystallographic phase of the precursor was confirmed by
powder X-ray diffraction (XRD) analysis to be pure CoCO3
(JCPDS no. 11-0692) with high crystallinity (Fig. S1w). The
uniform ellipsoidal shape of the microparticles is revealed by
field-emission scanning electron microscopy (FESEM) with
slightly rough surface (Fig. S2a, bw). The size of the CoCO3
ellipsoids is typically 3–4 mm in length and about 2 mm in
width. Under transmission electron microscopy (TEM) observation
(Fig. S2c, dw), these CoCO3 ellipsoids show dense solid texture
without discernible porosity. Metal carbonates are usually unstable
and easily decomposed into metal oxides at elevated temperatures.
To study the decomposition behavior of the CoCO3 precursor, we
performed thermogravimetric analysis (TGA) in N2 flow (Fig. S3w).
A significant weight loss of about 36 wt% at around 350 1C
corresponds to the thermal decomposition of cobalt carbonate into
cobalt oxide. Hence, the transformation of CoCO3 into cobalt
sulfide was performed at 350 1C, during which the decomposition
and sulfidation processes are expected to take place simultaneously.
Fig. 1 displays the XRD pattern of the as-synthesized
product. All of the peaks in the pattern are sharp and can be
Here, we demonstrate the synthesis of microsized CoS2
ellipsoids with unusual anisotropic tube-like cavities via a
School of Chemical and Biomedical Engineering, Nanyang
Technological University, 70 Nanyang Drive, Singapore 637457.
E-mail: xwlou@ntu.edu.sg; Web: http://www.ntu.edu.sg/home/xwlou
w Electronic supplementary information (ESI) available: Detailed
synthesis conditions, characterization and other SEM/TEM images.
See DOI: 10.1039/c2cc32750c
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non-uniform contrast in the middle region suggests the possible
presence of irregular pores. As a result, the solid particles become
less dense, as also indicated by the improved transparency under
TEM observation. The large particle size and the unique porous
structure lead to a limited BET specific surface area of about
17 m2 gÀ1 (nitrogen adsorption/desorption isotherm shown in
Fig. S4w).
Our previous study shows that direct thermal decomposition of
similar CoCO3 microparticles in air results in the formation of
Co3O4 ellipsoids with homogeneously distributed nanopores,
which is consistent with other reported porous metal oxides from
carbonates/hydroxides.12 Therefore, the apparent difference
between the porous structures of the Co3O4 and CoS2 ellipsoids,
which are from the same precursor particles, suggests that the H2S
gas sulfidation process plays a crucial role in the formation of the
anisotropic tube-like cavities. Although the exact mechanism and
formation process are still under investigation, we postulate that
the H2S gas might have different reactivity on ellipsoid surfaces
with different curvature. Hence, the anisotropic sulfidation on
different parts of the surface leads to the formation of anisotropic
cavities at the two ends of the ellipsoids.
Fig. 1 XRD pattern of CoS2 ellipsoids with anisotropic tube-like
cavities.
unambiguously assigned to CoS2 (JCPDS no. 41-1471),
indicating the high phase purity and crystallinity of the
as-prepared product. The morphology of the as-prepared
CoS2 sample was studied by FESEM as shown in Fig. 2a.
The ellipsoidal shape and size of the precursor microparticles
are perfectly retained after transformation to the CoS2 phase.
A typical magnified FESEM image (Fig. 2b) reveals a very
interesting feature of the CoS2 ellipsoids, which is not found in
the CoCO3 precursor microparticles. It can be clearly observed
that many openings with size of tens of nanometers are located
at the two ends of the CoS2 ellipsoids, resulting in much
rougher and porous surface in regions with high curvature.
To elucidate this unusual structural feature, TEM observation
was performed as shown in Fig. 2c and d. Tube-like cavities
oriented from the center to the surface can be identified in the
ellipsoidal particles, and the openings of the cavities correspond to
the nanopores observed in the FESEM images. In agreement with
the FESEM observation, the tube-like cavities are distributed
around the two ends of the ellipsoidal particles, whereas the
The family of cobalt sulfides, comprising of many compounds
such as CoS, CoS2, Co3S4 and Co1ÀxS, has been studied
as electrode materials for supercapacitors and lithium-ion
batteries.26–32 However, the lack of desirable nanostructures
and their intrinsic instability might undermine their performance.
In this work, we anticipate that the unique nanostructure of our
CoS2 ellipsoids with anisotropic tube-like cavities would benefit
their performance in electrochemical applications. Thus, we
investigated the electrochemical properties of these porous
CoS2 ellipsoids as an electrode for supercapacitors. Typical cyclic
voltammograms (CV) of CoS2 ellipsoids in 2 M KOH aqueous
electrolyte at various scan rates are shown in Fig. 3a. The
shape of the CV curves suggests typical pseudocapacitive
characteristics, in which the redox peaks are in general agreement
with those of reported cobalt sulfides and oxides.12,26,28,29 Thus
the electrochemical capacitance of CoS2 ellipsoids is attributed to
a quasi-reversible electron transfer process that mainly involves
the Co2+/Co3+ redox couple, and probably mediated by OHÀ
ions in the alkaline electrolyte as in the case of CoS.27,28
Compared with the negligible current signal from Ni foam
(Fig. S5w), one can safely confirm that the pseudocapacitance
indeed comes from the porous CoS2 ellipsoids.
To evaluate the specific capacitance and cycling stability, the
CoS2 ellipsoid electrode was galvanostatically charged/discharged
within a voltage window of À0.1 to 0.4 V vs. saturated calomel
electrode (SCE) reference electrode in the same alkaline
electrolyte. Fig. 3b shows representative charge/discharge
curves at various current densities. Consistent with the CV
results, the plateaus in the charge/discharge curves indicate the
existence of Faradaic processes. The specific capacitance is
then simply calculated according to the following equation:
C = I Dt/(mDV),12 where C (F gÀ1) is the specific capacitance,
I (mA) is the discharge current, Dt (s) is the discharge time,
DV (V) is the potential change during discharge, and m (mg) is
the mass of the active material (i.e., CoS2 ellipsoids). On the
basis of the charge/discharge profiles shown in Fig. 3b, the
specific capacitance of the CoS2 sample is determined to be
Fig. 2 (a, b) FESEM and (c, d) TEM images of CoS2 ellipsoids with
anisotropic tube-like cavities.
1040, 980, 965, 750 and 224 F gÀ1 at 0.5, 1, 2.5, 5 and 10 A gÀ1
.
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Fig. 3 (a) Cyclic voltammograms at various scan rates, (b) galvanostatic charge and discharge curves at various current densities, (c) specific
capacitance and coulombic efficiency vs. cycle number at a current density of 5.0 A gÀ1
.
These values of specific capacitance are considered very high for
pseudocapacitive materials, especially in view of the relatively low
BET surface area (B17 m2 gÀ1). The good performance of the
porous CoS2 ellipsoids as an electrode material for supercapacitors
might be attributed to the novel anisotropic cavities that efficiently
facilitate the infiltration of electrolyte and also provide more active
surface for fast redox reactions. The capacitance retention and
Coulombic efficiency upon prolonged cycling are evaluated at
constant current densities of 5 and 2.5 A gÀ1 (Fig. 3c and S6w).
After 1000 cycles, about 66 and 44% of the initial specific
capacitance can be retained, respectively. Meanwhile, a coulombic
efficiency of almost 100% during the whole cycling indicates
the excellent reversibility of pseudocapacitive reactions in the
CoS2 ellipsoids.
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In summary, we report the synthesis of porous CoS2
ellipsoids from the simultaneous thermal decomposition and
sulfidation of preformed CoCO3 ellipsoidal microparticles.
The as-prepared CoS2 ellipsoids possess unusual tube-like
cavities that are oriented from the center to the surface and
the openings are distributed on the two ends of the ellipsoids.
Such anisotropic cavities will not only facilitate the infiltration
of electrolyte but also provide more active surface when
the CoS2 ellipsoids are evaluated as electrode materials for
supercapacitors. Benefiting from the unique porous structure,
the CoS2 ellipsoids show high pseudocapacitance and good
cycling performance. We anticipate that the facile approach
presented in this work could be extended to fabricate other
metal sulfide materials with novel nanostructures.
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