J' Rajeswari, B' Viswanathan and T' K' Varadarajan

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Supercapacitors based on tungsten trioxide
nanorods
J. Rajeswari, B. Viswanathan and T. K.
Varadarajan National Centre for Catalysis
Research Department of Chemistry Indian Institute
of Technology Madras Chennai 600 036 India
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Outline

• Introduction
• Synthesis of tungsten trioxide nanorods
• Characterization of tungsten trioxide nanorods
• Electrochemical studies for supercapacitive
  behaviour
• Conclusions

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Electrochemical Supercapacitors

• Two types of capacitors - Electric double layer
  capacitors (EDLCs) and
• pseudocapacitors (redox capacitors)
• EDLCs - An electrochemical double layer capacitor
  uses the physical separation of
• electronic charge in the electrode and ions of
  the electrolyte adsorbed at the surface
• A Faradaic supercapacitor is charged by
  chemisorption of a working cation of the
• electrolyte at a reduced complex at the surface
  of the electrode (or)
• Faradaic supercapacitor electrochemical redox
  process involving charge transfer by
• the electrode material called as pseudo or
  redox capacitors
• Electrochemical supercapacitance redox process
  accompanied by the
• non Faradaic charging-discharging at the
  interface
• EDLCs have a lower specific capacitance than an
  optimal faradaic supercapacitor

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Electrochemical Supercapacitors

• Amorphous hydrated ruthenium oxide, RuO2.nH2O,
  in strong acid is capable of
• chemisorbing one proton per Ru atom to give a
  capacity of 720 F/g and
• excellent cyclability
• RuO2.nH2O is too expensive to be commercially
  attractive - search for alternate
• materials
• Small size of proton offers the best chance to
  achieve optimal chemisorption, the
• search has been restricted mostly to
  materials stable in strong acids
• Transition metal oxides such as RuO2, Co3O4,
  MnO2, IrOx etc., have been shown to
• be excellent materials for supercapacitors
• Charge storage property of WO3 has been used
  extensively as electrochromic
• materials
• Very few reports are available on WO3 as
  capacitors as a second component in
• RuO2 systems to reduce the loading of Ru

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Electrochemical behaviour of tungsten trioxides

• Tungsten trioxides form tungsten bronzes
  (MxWO3) M is a metal other than
  tungsten, most commonly an alkali metal or
  hydrogen
• Tungsten bronzes electron and proton
  conductors desired property for a Faradaic
  supercapacitor
• The redox processes that take place in tungsten
  trioxides are as follows
• First process (I) occurs at potential more
  positive than -0.3 V
• Second process (II) occur at potential more
  negative than -0.3 V
• Hence, in WO3, charge separation at the
  electrode-electrolyte interface
• and redox processes due to the formation of
  HxWO3 and WO3-y contribute capacitance to
  the system

WO3 xH e- ? HxWO3 (0 lt x
lt1) (I) WO3 2yH 2ye- ? WO3-y yH2O
(0 lt y lt 1) (II)
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Tungsten based supercapacitors reported in
literature
Solid state thin film containing W cosputtered
RuO2 supercapacitor electrodes

• Under experimental conditions, in W-RuO2 film, W
  is in the form of WO3 revealed from XRD
• Presence of W increased charging and
  discharging time evident from
• chronopotentiometry
• Specific capacitance per volume after one cycle
  is 54.2 mF/cm2?m for W-RuO2
• 
  30.4 mF/cm2 ?m for
  RuO2
• Increased specific capacitance and stability over
  cyle numer in the presence of W

J. Vac. Sci. Technol. B 21, (2003), 949
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Tungsten based supercapacitors reported in
literature
Amorphous tungsten oxide ruthenium oxide
composites for
Electrochemical capacitors

• To reduce the cost of RuO2, WO3 has been added by
  precipitation method
• Results have shown that WO3 can constitute an
  alternate electrode material
• for the high cost RuO2 for supercapacitor
  applications

J. Electrochem.Soc., 148, (2001), A189
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Tungsten trioxide systems for supercapacitors
studied in the present work

• Aim of the present study Supercapacitive
  behaviour of nanorods of WO3
• Tungsten trioxide nanorods - Method employed
  Thermal decomposition using
• tungsten containing precursor
• Supercapacitive behaviour of nanorods have been
  compared with bulk WO3
• Bulk WO3 Commercially obtained from Alfa
  Aesar ( A Johnson Matthey Company)

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Reported methods for the synthesis of WO3 nanorods

• Different synthetic approaches
• Solvothermal method
• Template directed synthesis
• Sonochemiccal synthesis
• Thermal Methods
• Decomposition
• Chemical vapor deposition
• Thermal decomposition simple, easy, inexpensive
  and contaminants free method
• One report for synthesis of WO3 nanorods by
  thermal decomposition method
• Disadvantages of the existing report
• Tedious synthetic method for the precursor
  compound WO(OMe)4
• Highly unstable precursor compound
• A relatively higher pyrolysis temperature
• Multisteps from precursor to product

Pol et.al, Inorg. Chem. 44 (2005) 9938
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Synthesis of the precursor
Preparation of tetrabutylammonium decatungstate
((C4H9)4N)4W10O32
Sodium tungstate and tetrabutylammonium bromide
starting materials to synthesize the precursor
Chemseddine et.al, Inorg. Chem. 23 (1984) 2609
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Synthesis of Tungsten trioxide nanorods
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Comparison of the features of the synthesis of
WO3 nanorods by our group vs. existing report
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Scheme for the formation of tungsten trioxide
(WO3) nanorods

• The WO42- octahedra are surrounded by the
  ((C4H9)4)N groups
• This allows the growth of WO3 in one dimension
• When pyrolysed at 450? C, ((C4H9)4)N group
  decomposes off leaving WO3 nanorods

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X-ray diffraction pattern of tungsten trioxide
nanorods
To study the structure and composition, powder
XRD pattern was obtained
Single crystalline monoclinic WO3 (JCPDS 75-2072)
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Raman Spectrum of tungsten trioxide nanorods
260 and 334 cm-1 O-W-O bending modes of WO3 703
and 813 cm-1 O-W-O stretching modes of WO3
Confirms the formation of WO3
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Scanning electron microscopic images of tungsten
trioxide nanorods
The synthesized WO3 has rod morphology evident
from SEM images
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Scanning electron microscopic images of bulk
tungsten trioxide
Bulk WO3 No specific morphology aggregates of
particles
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Transmission electron microscopic images of
tungsten trioxide nanorods
Dimensions of WO3 nanorods calculated from TEM
images Length 130 480 nm
Width 18-56 nm
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High resolution transmission electron microscopic
image of tungsten trioxide nanorods

• Interplanar spacing, d 0.375 nm corresponds
  to (020) plane of
• monoclinic WO3
• This observation agrees with the d value
  obtained from the XRD

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Energy dispersive X-ray analysis of tungsten
trioxide nanorods

• Presence of constituent elements, W and O is
  confirmed from the
• corresponding EDAX peaks
• Cu peak from the grid

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Electrochemical Studies
The electrochemical properties were studied using

• Cyclic voltammetry (CV)
• Galvanostatic chargedischarge studies
  (Chronopotentiometry)

• Electrochemical measurements were carried out
  using CHI660 electrochemical workstation
• Three-electrode set up
• Pt wire - counter electrode Ag/AgCl/
  (sat KCl) - reference electrode
• Glassy carbon coated with electrode material
  as working electrode
• The electrolyte used was 1 M H2SO4 at room
  temperature and geometrical area of electrode
  0.07cm2

Electrode fabrication

• 5 mg of WO3 nanorods or bulk WO3 - dispersed
  in 100?L H2O by ultrasonication
• 10 ?L of dispersion has been coated on GC and
  dried in an oven at 70? C
• 5 ?L of Nafion (binder) coated and dried at
  room temperature

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Cyclic voltammograms of tungsten trioxide
nanorods and bulk tungsten trioxide
Electrolyte 1M H2SO4 Scan rate 50 mV/s
Anodic peak (0.1 V) due to the formation of
tungsten bronzes (HxWO3) can be observed
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An overlay of cyclic voltammograms of tungsten
trioxide nanorods and bulk tungsten trioxide

• Anodic peak current density WO3 Nanorods
  24.7 mAcm-2
• Bulk WO3 3.5 mAcm-2
• Peak current density of WO3 nanorods is 7
  times higher than the bulk WO3
• Higher redox current for nanorod system shows
  its higher charge storage
• by pseudocapacitance

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Chronopotentiograms of tungsten trioxide nanorods
and bulk tungsten trioxide

• Electrolyte 1M H2SO4 Constant current
  density 3 mAcm-2
• Symmetric inverted V type chronopotentiograms
  will be exhibited by ideal supercapacitors
• For WO3 nanorods, a symmetric curve can be
  observed
• For the bulk WO3, an unsymmetry can be seen
• This shows that WO3 nanorods constitute desired
  ideal supercapacitive behaviour
• Charge- discharge time has increased for
  nanorods several folds than the bulk sytem

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Specific Capacitance
Specific Capacitance, C(F/g) i?t/m?V where i
is the current density used for charge/discharge
3 mA/cm2 ?t is the time elapsed for the
discharge cycle, m is the mass of the active
electrode 7 mg/cm2 ?V is the voltage interval
of the discharge 0.7 V Capacitance values are
calculated from the chronopotentiograms Increased
?t value (evident from chronopotentiogram) for
WO3 nanorods higher capacitance
Specific Capacitance for WO3 nanorods 436
F/g Bulk WO3 57
F/g
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Factors for the capacitance in WO3 nanorods

• Higher capacitance for WO3 nanorods can be
  attributed to the following factors
• Faradaic supercapacitance arise from (1) charge
  accumulation due to chemisorption of
  smaller cations such as H or Li on the redox
  active material and (2) redox process by the
  active material
• Facile formation of HxWO3 acts as driving force
  for the redox process pseudocapacitance
• The chemisorption of H ion on WO3 is more
  facile than on RuO2 as H is an inherent
• part of the HxWO3 system (formed by WO3 in
  acid medium)
• Reduction of particle size to nanoregion
  increased surface to volume ratio- lead to
  signal amplification
• In electrochemical studies the above factor
  contributed to enhanced redox process
• (Faradaic process)
• Increased electrode electrolyte interface (EDLC)
  due to increased number of particles
• Contribution of all these facts has lead to
  enhancement of supercapacitance of WO3
  nanorods

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Cycling performance of tungsten trioxide nanorods
electrode
Potential vs time at a constant current density
of 3 mAcm-2 for 40 cycles Desired property for
devices stability over long time WO3 nanorods -
Stable over a long period of time better
cycling performance
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Cycling performance of tungsten trioxide nanorods
electrode
Inorder to observe the stability, specific
capacitance vs no of cycles has been
plotted Specific capacitance values are taken
from the previous chronopotentiogram at every 10
cycles and has been plotted After 40 cycles,
loss in specific capacitance for WO3 nanorods 10
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Cycling performance of bulk tungsten trioxide
electrode
Potential vs time at a constant current density
of 3 mAcm-2 for 40 cycles Desired property for
devices stability over long time
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Cycling performance of bulk tungsten trioxide
electrode

• Inorder to observe the stability, specific
  capacitance vs no of cycles has been plotted
• Specific capacitance values are taken from the
  previous chronopotentiogram
• at every 10 cycles and has been plotted
• After 40 cycles, loss in specific capacitance
  for bulk WO3 30

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Overlay of cycling performances of WO3 nanorods
and bulk WO3
Tungsten trioxide nanorods have better
performance and stability over its
counterpart After 40 cycles, loss in specific
capacitance for WO3 nanorods 10 After 40
cycles, loss in specific capacitance for bulk
WO3 30
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Tabulation of specific capacitance
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Conclusions

• Tungsten trioxide nanorods by a single step
  pyrolysis technique has
• been prepared
• The synthesized nanorods have been employed for
  supercapacitor electrode applications
• Tungsten trioxide nanorods showed higher
  performance and stability than its bulk
  counterpart
• In terms of the Faradaic capacitance due to the
  chemisorption of H ion on the
  WO3, it appears better than RuO2