Electrochimica
Acta
115 (2014) 358–
363
Contents
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at
ScienceDirect
Electrochimica
Acta
jou
rn
al
hom
ep
age:
www.elsevier.com/locate/elec
tacta
Facile
hydrothermal
synthesis
of
hierarchical
ultrathin
mesoporous
NiMoO
4
nanosheets
for
high
performance
supercapacitors
Daoping
Cai,
Bin
Liu,
Dandan
Wang,
Yuan
Liu,
Lingling
Wang,
Han
Li,
Yanrong
Wang,
Chenxia
Wang,
Qiuhong
Li,
Taihong
Wang
∗
Pen-Tung
Sah
Institute
of
Micro-Nano
Science
and
Technology,
Xiamen
University,
Xiamen
361000,
PR
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
8
September
2013
Received
in
revised
form
21
October
2013
Accepted
22
October
2013
Available online 2 November 2013
Keywords:
Nickel
molybdate
Ultrathin
nanosheet
Mesoporous
Electrode
material
Supercapacitors
a
b
s
t
r
a
c
t
In
this
work,
we
have
successfully
synthesized
hierarchical
ultrathin
mesoporous
NiMoO
4
nanosheets
through
a
simple
hydrothermal
method
for
the
first
time.
The
electrochemical
properties
of
the
as-prepared
NiMoO
4
nanosheets
have
been
investigated
as
electrode
materials
for
supercapacitors.
Remarkably,
the
as-prepared
NiMoO
4
displays
a
high
specific
capacitance
and
excellent
rate
capabil-
ity.
Even
at
a
high
current
density
of
20
A
g
−1
,
it
still
possesses
a
specific
capacitance
up
to
1200.5
F
g
−1
,
and
about
75%
of
the
capacitance
is
retained
as
the
charge–discharge
rate
changes
from
2
to
20
A
g
−1
.
It
is
noted
that
a
high
specific
capacitance
about
1172.4
F
g
−1
is
obtained
after
2000
cycles
at
a
current
density
of
6
A
g
−1
.
These
results
imply
that
the
hierarchical
ultrathin
mesoporous
NiMoO
4
nanosheets
could
be
a
promising
candidate
for
high
performance
supercapacitors.
© 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Compared
with
other
energy
storage
systems
such
as
batter-
ies
and
fuel
cells,
supercapacitors
are
superior
in
the
areas
of
high
power
density,
fast
charge/discharge
process,
long
lifespan,
envi-
ronmental
friendliness,
and
safety
[1,2].
The
US
Department
of
Energy
has
assigned
supercapacitors
to
be
as
important
as
bat-
teries
for
future
energy
storage
systems
[3].
Supercapacitors,
also
called
electrochemical
capacitors,
store
energy
using
either
ion
adsorption
(electrochemical
double
layer
capacitors)
or
fast
sur-
face
redox
reactions
(pseudocapacitors)
[4,5].
In
recent
years,
a
significant
amount
of
effort
has
been
devoted
to
improving
the
elec-
trochemical
performance
of
electrode
materials
in
order
to
build
better
supercapacitors
with
both
high
power
and
energy
densi-
ties,
and
scientists
have
made
a
lot
of
progress
in
the
theoretical
and
practical
research
and
development
of
supercapacitors
[6–11].
It
is
believed
that
progress
in
supercapacitor
technology
benefits
from
the
nanostructured
electrodes
instead
of
traditional
ones
[12].
Nanostructured
materials
have
brought
about
great
advancement
of
new
supercapacitor
technologies
because
of
their
high
surface
area,
short
electron
and
ion
transport
pathways
[13].
It
is
well
accepted
that
pseudocapacitors
store
charge
in
the
first
few
nanometres
from
the
surface,
and
the
ultrathin
nano-
structures
with
sizes
of
less
than
10
nm
would
become
the
goal
∗
Corresponding
author.
Tel.:
+86
0592
2183063;
fax:
+86
0592
2197196.
E-mail
address:
thwang@xmu.edu.cn
(T.
Wang).
for
next
generation
high
performance
supercapacitors
[3,14,15].
Therefore,
an
emerging
new
concept
is
to
develop
ultratin
meso-
porous
nanostructures
because
decreasing
the
particle
size
could
increase
the
usage
of
active
materials
by
providing
a
larger
surface
area.
Ultrathin
and
mesoporous
nanosheets
nanostructures
play
important
roles
in
high
performance
supercapacitors.
Recently,
many
ultrathin
mesoporous
nanostructures
have
been
synthesized
and
displayed
an
enhanced
electrochemical
performance
for
super-
capacitors
[16–20].
For
example,
Yuan
et
al.
developed
a
facile
two-step
method
for
growth
of
ultrathin
mesoporous
NiCo
2
O
4
nanosheets
on
conductive
nickel
foam,
and
the
supercapacitor
dis-
played
an
ultrahigh
specific
capacitance
of
1450
F
g
−1
at
a
very
high
current
density
of
20
A
g
−1
[21].
Jiang
et
al.
have
synthe-
sized
hierarchical
ultrathin
Ni(OH)
2
nanoflakes
with
thickness
of
only
7.4
nm
and
demonstrated
the
electrochemical
performance
depended
on
the
thickness
of
the
nanosheets.
The
hierarchical
ultrathin
Ni(OH)
2
nanoflakes
exhibited
the
best
properties
com-
pared
with
uniformly
stacked
Ni(OH)
2
nanoplatelets
(thickness
of
∼22
nm)
and
randomly
arranged
hexagonal
nanosheets
(thick-
ness
of
∼140
nm)
[19].
Therefore,
it
will
be
of
great
significance
to
develop
simple
hydrothermal
methods
to
synthesize
ultrathin
mesoporous
nanostructures.
However,
the
synthesis
of
ultrathin
mesoporous
nanostructures
still
remains
a
big
challenge.
Recently,
metal
molybdates,
especially
NiMoO
4
has
attracted
great
research
interests
due
to
its
excellent
electrochemical
per-
formance,
low
cost
and
environmental
friendliness
[22–24].
In
the
present
work,
we
report
a
simple
one-pot
hydrothermal
method
to
obtain
hierarchical
ultrathin
mesoporous
NiMoO
4
nanosheets,
0013-4686/$
–
see
front
matter ©
2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.10.154