Flexible Asymmetric Supercapacitors Based upon Co9S8 Nanorod Co3O4@RuO2 Nanosheet Arrays
in the range of0à1.6V and exhibit superior electrochemical performance aqueous electrolyte and an energy density of1.44mWh/cm3at the power than those reported in early ASC work.Moreover,they present excellent of cycles.The high-performance nanostructured ASCs have signi?cant
sul?de.?exible.RuO2
demands of high-perfor-
storage,the energy density
devoted to the ASCs with higher energy and density by exploring various materials,such as /graphene//graphene,MnO2/graphene//car-bers,MnO2//FeOOH,and so on.15,18,19 weight,practicability,and portability reasons, binder-free electrode materials grown on conductive substrates,acting as new types of more desirable for future electronics.For and co-workers fabricated a solid-state
on H-TiO2@MnO2(positive electrode)and
(negative electrode)grown on carbon Chen et al.reported a transparent and?exible coating active materials on ITO-PET.17However, on cobalt sul?des,a promising,environmen-friendly,and low-cost candidate,grown on a conductive substrate have not been studied one-dimensional(1D)Co3O4nanostruc-
synthesized through a facile hydrothermal
may be a favorable precursor for synthesiz-
sul?des by S2àion exchange.21Such a
overcomes the di?culty of growing cobalt the1D direction.20,22Meanwhile,Co3O4nano-
high speci?c area could also be used as
support the negative electrode materials, carbon materials,FeOOH,RuO,and so on.1,5,19reactions were used as both the precursor nanorod arrays(t-CSC,t represents the
time,in hours)and the sca?olds for RuO2
(t-CRC,t represents the compositing time,
Our optimized ASCs showed a volumetric
as high as3.42F/cm3in aqueous electrolyte
cm3in PVA/KOH electrolyte,both delivering
voltage of1.6V.Expectably,excellent performances with an energy density of1.21mWh/cm3at
density of13.29W/cm3for liquid-state
energy density of1.44mWh/cm3at the power
of0.89W/cm3for solid-state ASC were respectively,which are around10-fold
some reported ASCs.16,23à25
RESULTS AND DISCUSSION
Figure1a shows the X-ray di?raction(XRD)
of the as-prepared samples under di?erent
All peaks of the COC sample excluding the
from the carbon cloth can be indexed to a
phase of Co3O4(JCPDS Card No.42-1467).
pattern of the36-CSC(acicular Co9S8nanorod obtained after36h of sulfuration in Na2S solution),
the peaks of Co9S8can be observed,according JCPDS Card No.86-2273.The broad re?ection
XRD patterns of the as-synthesized COC,20-CRC,and36-CSC and the corresponding standard XRD and Co9S8.XPS patterns of(b)Co2p and(c)S2p of COC and36-CSC.
photoelectron spectroscopy(XPS)analysis was applied verify the surface compositions of the COC
36-CSC samples.High-resolution spectra of 2p are exhibited in Figure1b,c.The XPS
COC for Co2p in Figure1b presents two
with binding energies of780.2and795.2eV
energy of15eV,corresponding to the Co2p3/2
of Co3O4phase,respectively.21In the case of
peaks of Co2p3/2and Co2p1/2are located at
793.5eV,which agrees with the reports very
Figure1c,no S2p peak was found in the
COC,indicating nonexistance of sul?de in Meanwhile,the strong peak of S2p at162.6eV in of36-CSC manifests the formation of Co9S8
electron microscopy(SEM)and transmis-
microscopy(TEM)are employed to in-
morphologies and microstructures of the 36-CSC samples,as shown in Figure2.It can the COC sample composed of1D acicular
with a smooth surface aligned together to form a leaf,arranging randomly on the (Figure2a).TEM image in Figure2b exhibits
Co3O4nanorods possess porous structure crystallinity,as revealed by the SAED pattern. resolution TEM(HRTEM)image at the bottom
?gure reveals the lattice fringe spacings
0.244nm,corresponding well to the(220) crystal planes of the cubic spinel Co3O4phase, tively.After being treated with Na2S for12
the surface of the Co3O4nanorods becomes rough,as shown in Supporting Information
Figure S1c,d shows the surface morphology 24-CSC sample,where many nanoparticles
on the surface of the nanorods and make rougher than the12-CSC sample.By extending sulfuration time to36h(36-CSC),more
are coated on the surface while the1D
and3D leaf morphologies still remain,
Figure2c.Figure2d presents the TEM images 36-CSC sample,from which the porous structure observed clearly.The lattice fringe spacing
SEM images and(b)TEM images of COC with a SAED pattern of the COC sample.(c)SEM images 36-CSC sample with a SAED pattern.(e)Mechanism of the conversion process from COC to CSC.
can be indexed as the(331)lattice plane of Co9S8.SAED pattern at the top right corner indicates the polycrys-talline structure of the sample.In Figure S1e,the energy-dispersive spectrometer(EDS)microanalysis of36-CSC sample reveals that Co and S elements dominate in the compound with the ratio approaching 1:1,further verifying the formation of Co9S8.According to the results above,a possible mechanism is exhibited 200mV/s with a decreased trend dependent on the scan rates,owing to the limited rate of ion di?usion at high scan rates,as shown in Figure3c.The highest capacitance value obtained from36-CSC was2.35F/cm2 (783.3F/g for active materials and113.5F/g for the overall electrode)at5mV/s,which is much better than previously reported data about active materials grown on substrates and some cobalt sul?des.20,29à31Even at
Figure3.(a)CV curves of COC,12-CSC,24-CSC,36-CSC,and the pure carbon cloth electrodes at the scan rate of50mV/s.(b)CV curves of36-CSC at di?erent scan rates.(c)Speci?c areal capacitance of12-CSC,24-CSC,and36-CSC electrodes calculated from the CV curves as a function of scan rates.
the well-crystallized structure of the RuO2nano-
Energy-dispersive X-ray spectroscopy mapping
Figure S3a provides clearer information about element distribution within the hybrid nanostruc-
which further con?rms the formation of the com-Co3O4@RuO2structures.The electrochemical performance of the CRC electrodes was evaluated in three-electrode con?guration in3M KOH.Figure S3b the CV curves of the6-CRC,12-CRC,20-CRC,and
samples at the scan rate of50mV/s in the voltage
à1à0V,indicating that the largest CV inte-
area belongs to the composited Co3O4@RuO2
for20h(20-CRC).CV curves for the20-CRC
at di?erent scan rates from1to200mV were
to calculate the electrochemical capacitance of electrode(Figure4c),which is plotted in Figure4d. Capacitances of1.18F/cm2(590F/g calculated by the mass(2mg/cm2)of the Co3O4and RuO2and59.9 on the overall mass of the electrode)at1mV/s
F/cm2at200mV/s can be achieved.Galvano-chargeàdischarge measurements of the20-CRC were also conducted,as shown in Figure S4a.
capacitances of0.67and0.39F/cm2were
at current densities of10and50mA/cm2, respectively,which are comparable with those calculated curves.Moreover,the capacitance retention of of the material with conductive?bers.Signi?cantly, results are better than those of some other active
used as negative electrodes.16,32,33
Given that the36-CSC and20-CRC electrodes sess stable voltage windows betweenà0.3
and betweenà1and0V,respectively,with
the SCE,it is expected that the operating ASC could achieve1.6V in3M KOH by assembling 36-CSC electrode with the20-CRC electrode.Taking vantage of good?exibility and conductivity of cloth,we fabricated an all solid-state ASC in
gel electrolyte.Figure5a displays the schematic tration of the assembled structure for such ASCs. electrochemical performance of liquid-state and state ASCs were both studied.Figure5b shows curves of the liquid-state ASC(denoted as
di?erent voltage windows.As expected,the electrochemical windows of the LASC can be
to1.6V.In Figure5c,CV curves at di?erent scan were collected from10mV/s,and the typical pseudocapacitive shape was robust enough
the scan rate up to1000mV/s,indicating the stability the LASC at fast chargeàdischarge rates.On hand,the voltage window of solid-state ASC
as SASC)can also achieve1.6V and perform
at the high scan rate of1000mV/s,as shown
(a)SEM images and(b)TEM images of the20-CRC sample with a SAED pattern.(c)CV curves at di?erent c areal capacitance calculated from the CV curves as a function of scan rates.
twisted conditions,revealing its excellent mechan-stability.
further con?rm the superior electrochemical performance of both kinds of ASCs,the galvanostatic discharge measurements were conducted,as
Figure6a,b.A set of current densities of2.5,5,
40,and50mA/cm2were operated.At each current density,both ASCs charge and discharge idly with good electrochemical reversibility stable potential window of0à1.6V.Figure
lates the volumetric capacitances of the ASCs based the chargeàdischarge curves.Signi?cantly,the ?c capacitance of SASC exhibits higher values current densities.While at larger current densities,
Figure5.(a)Schematic illustration of the as-assembled ASC.(b,d)CV curves of the LASC and SASC devices collected in di?erent scan voltage windows at the scan rate of50mV/s.(c,e)CV curves of the LASC and SASC devices collected at di?erent scan rates.(f)CV curves collected at the scan rate of100mV/s for the solid-state ASC device under normal,bent,and twisted conditions.Insets are the device photographs under di?erent test conditions.
(a,b)Galvanostatic chargeàdischarge curves collected at di?erent current density for LASC and SASC devices
window of1.6V.(b)Volumetric capacitance of LASC and SASC devices collected from galvanostatic
curves as a function of current density.(c)Voltage drop associated with the cell internal resistance(IR
current density and corresponding?tted functions.
Figure7.(a)Rate capability of LASC and SASC at di?erent current densities.(b)Cycling performance of LASC SASC devices at the current density of50mA/cm cycles.(c)Ragone plots of LASC and SASC devices.
values reported for other SC devices are added comparison.
cycling performance.During the process,?ve steps chargeàdischarge rates were changed successively from2.5to50mA/cm2.At the?rst400cycles chargeàdischarge current densities of2.5and5mA/cm ASCs show steady volumetric capacitances.In following400cycles at large current densities,the LASC demonstrates a stable performance at each situation while the SASC shows a little decay.When the current turns back to2.5mA/cm2,a fully recovered LASC observed in the following200cycles while the retention capacitance of SASC is calculated to be95.6%.The long-cycling performance of the ASC devices at a large chargeàdischarge rate(50mA/cm2,namely,10 based on the total mass of Co9S8,Co3O4,and RuO also conducted for2000cycles.The LASC exhibits excellent stability with even99.0%retention of the initial capacitance and the SASC remains at90.2%.These retention rates at such a high chargeàdischarge comparable and even better than those reported