1. 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 –
India

2. Outline
Introduction
Synthesis of tungsten trioxide nanorods
Characterization of tungsten trioxide nanorods
Electrochemical studies for supercapacitive behaviour
Conclusions

3. 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

4. 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
Very few reports are available on WO3 as capacitors – as a second component in
RuO2 systems to reduce the loading of Ru

5. 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 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-  HxWO (0 < x <1) (I)
WO3 + 2yH+ + 2ye-  WO3-y + yH2O (0 < y < 1) (II)

6. 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

7. Tungsten based supercapacitors reported in literature
Amorphous tungsten oxide – ruthenium oxide composites for
Electrochemical capacitors
100 % RuO2
~50%WO3 + ~50% RuO2
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

8. 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)

9. 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

10. Synthesis of the precursor
Preparation of tetrabutylammonium decatungstate ((C4H9)4N)4W10O32
Na2WO4.2H2O + 3M HCl
Clear yellow solution
White precipitate
Filtered, washed with boiling water and ethanol
Tetrabutyl ammonium bromide (TBABr)
Recrystallized in hot DMF
Yellow crystals of ((C4H9)4N)4W10O32
Sodium tungstate and tetrabutylammonium bromide – starting materials – to synthesize
the precursor
Chemseddine et.al, Inorg. Chem. 23 (1984) 2609

11. Synthesis of Tungsten trioxide nanorods
Recrystallized ((C4H9)4N)4W10O32
pyrolyzed at 450 C, 3h, N2
(tubular furnace)
WO3 nanorods
blue powder

12. Comparison of the features of the synthesis of WO3 nanorods
by our group vs. existing report
WO3 nanorods prepared by our group
WO3 nanorods from literature
Preparation of precursor
Nature of the precursor
Precursor Tunability
No of steps
Simple, easy and economical
Stable
Easy storage
Metal and the cation can be tuned to give a
variety of metal oxide nanorods
Single step
Relatively not economical and also tedious method
Highly volatile, evaporates to give W(OMe)6 and WO2(OMe)2
Limitation in storage
No such possibility
Multiple steps

13. 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

14. X-ray diffraction pattern of tungsten trioxide nanorods
To study the structure and composition, powder XRD pattern was obtained
Single crystalline monoclinic WO3 (JCPDS: )

15. Confirms the formation of WO3
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

16. Scanning electron microscopic images of tungsten trioxide nanorods
The synthesized WO3 has rod morphology – evident from SEM images

17. Scanning electron microscopic images of bulk tungsten trioxide
Bulk WO3 : No specific morphology – aggregates of particles

18. Transmission electron microscopic images of tungsten trioxide nanorods
Dimensions of WO3 nanorods calculated from TEM images : Length: 130 – 480 nm
Width: nm

19. High resolution transmission electron microscopic image of
tungsten trioxide nanorods
Interplanar spacing, d: nm – corresponds to (020) plane of
monoclinic WO3
This observation agrees with the d value obtained from the XRD

20. 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

21. Electrochemical Studies
The electrochemical properties were studied using
Cyclic voltammetry (CV)
Galvanostatic charge–discharge 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

22. Electrolyte: 1M H2SO4 ; Scan rate : 50 mV/s
Cyclic voltammograms of tungsten trioxide nanorods and bulk tungsten trioxide
WO3 nanorods
Bulk WO3
Electrolyte: 1M H2SO4 ; Scan rate : 50 mV/s
Anodic peak (~0.1 V) due to the formation of tungsten bronzes (HxWO3) can be observed

23. An overlay of cyclic voltammograms of tungsten trioxide
nanorods and bulk tungsten trioxide
Anodic peak current density: WO3 Nanorods: mAcm-2
Bulk WO : 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

24. Chronopotentiograms of tungsten trioxide nanorods and bulk tungsten trioxide
WO3 nanorods
Bulk WO3
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

25. 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 WO : 57 F/g

26. 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 WO nanorods

27. 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

28. 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%

29. 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

30. 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%

31. 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 WO : 30%

32. Tabulation of specific capacitance
Material
Specific Capacitance
(F/g)
WO3 nanorods
436
Bulk WO3 57

33. 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