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
1
Outline
 Introduction
 Synthesis of tungsten trioxide nanorods
 Characterization of tungsten trioxide nanorods
 Electrochemical studies for supercapacitive behaviour
 Conclusions
2
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
EDLCs have a lower specific capacitance than an optimal faradaic supercapacitor
A Faradaic supercapacitor is charged by chemisorption of a working cation of the
electrolyte at a reduced complex at the surface of the electrode
Faradaic supercapacitor – electrochemical redox process involving charge transfer by
the electrode material – called as pseudo or redox capacitors
Electrochemical supercapacitance – contributed from redox process and non Faradaic
charging-discharging at the interface
3
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 700 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
 The charge-discharge capacitance behavior of WO3 systems could be attributed4to
the pseudocapacitance mechanism of WO3
Electrochemical behavior 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 occurs at potential more positive than -0.3 V
Second process occur at potential more negative than -0.3 V
WO3 + xH+ + e-  HxWO3
(0 < x <1)
WO3 + 2yH+ + 2ye-  WO3-y + yH2O
(0 < y < 1)
5
Tungsten based supercapacitors reported in literature
Tungsten cosputtered ruthenium oxide electrodes
Specific capacitance per volume after one cycle is 54.2 mF/cm2m for W-RuO2
30.4 mF/cm2 m for RuO2
J. Vac. Sci. Technol. B 21, (2003), 949
6
Tungsten based supercapacitors reported in literature
Amorphous tungsten oxide – ruthenium oxide composites for
Electrochemical capacitors
J. Electrochem.Soc., 148, (2001), A189
7
Tungsten trioxide systems for supercapacitors taken in the present work
Tungsten trioxide nanorods synthesized by our group
Method employed: Thermal decomposition using single precursor compound
Supercapacitive behavior of nanorods have been compared with bulk WO3
Bulk WO3: Commercially obtained from Alfa Aesar ( A Johnson Matthey Company)
8
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]
 A relatively higher temperature
 Multisteps from precursor to product
Pol et.al, Inorg. Chem. 44 (2005) 9938
9
Synthesis of the precursor
Preparation of tetrabutylammonium decatungstate ((C4H9)4N)4W10O32
Na2WO4.2H2O + 3M HCl
Clear yellow solution
TBABr
White precipitate
Filtered, washed with boiling water and ethanol
Recrystallized in hot DMF
Yellow crystals of ((C4H9)4N)4W10O32
Chemseddine et.al, Inorg. Chem. 23 (1984) 2609
10
Synthesis of Tungsten trioxide nanorods
Recrystallized ((C4H9)4N)4W10O32
pyrolyzed under inert atm
WO3 nanorods
blue powder
11
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
Simple, easy and economical
Relatively not economical
and also tedious method
Nature of the precursor
Stable
Highly volatile, evaporates to
give W(OMe)6 and
WO2(OMe)2
Limitation in storage
Precursor Tunability
Easy storage
Metal and the cation can be
tuned to give a
variety of metal oxide
nanorods
No of steps
Single step
No such possibility
Multiple steps
12
Scheme for the formation of tungsten trioxide (WO3) nanorods
13
X-ray diffraction pattern of tungsten trioxide nanorods
(001)
(020)
(200)
1600
1400
(021)
(220)
1200
(132)
(420)
(202)
(400)
(022)
(040)
400
(002)
(121)
600
(221)
(011)
800
(120)
Intensity
1000
200
0
10
20
30
40
50
60
70
80
2
Single crystalline monoclinic WO3 (JCPDS: 75-2072)
14
Raman Spectrum of tungsten trioxide nanorods
440
816
259
420
Intensity
334
703
400
380
360
200
400
600
800
1000
-1
Raman shift (cm )
260 and 334 cm-1 : O-W-O bending modes
703 and 813 cm-1 : O-W-O stretching modes
15
Scanning electron microscopic images of tungsten trioxide nanorods
Morphology of the synthesized WO3: rods in nanometer region
16
Scanning electron microscopic images of bulk tungsten trioxide
No specific morhology – aggregates of particles
17
Transmission electron microscopic images of tungsten trioxide nanorods
Dimensions: Length: 130 – 480 nm
Width: 18-56 nm
18
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
19
Energy dispersive X-ray analysis of tungsten trioxide nanorods
Presence of W and O can be seen
Cu peak – from the grid
20
Electrochemical Studies
The electrochemical properties were studied using

Cyclic voltammetry (CV)

Galvanostatic charge–discharge studies
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
21
35
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
-0.2
4
2
-2
Current Density (mAcm )
-2
Current Density (mAcm )
Cyclic voltammograms of tungsten trioxide nanorods and bulk tungsten trioxide
0
-2
-4
-6
-8
0.0
0.2
0.4
0.6
Potential (V)
0.8
1.0
-10
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
1M H2SO4 at a scan rate of 50 mV/s
Peak due to the formation of tungsten bronzes can be observed
22
An overlay of cyclic voltammograms of tungsten trioxide
nanorods and bulk tungsten trioxide
-2
Current Density (mAcm )
30
WO3 nanorods
bulk WO3
20
10
0
-10
-20
-30
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
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
23
Chronopotentiograms of tungsten trioxide nanorods and bulk tungsten trioxide
0.2
0.1
0.2
0.0
0.1
Potential (V)
Potential (V)
0.3
-0.1
-0.2
0.0
-0.1
-0.2
-0.3
-0.3
-0.4
-0.4
-0.5
-0.5
0
200
400
600
800
Time (s)
1000
1200
-0.6
-20
0
20
40
60
80
100 120 140 160 180
Time (s)
Electrolyte: 1M H2SO4 ; Constant current density: 3 mAcm-2
Symmetric inverted ‘V’ type curves – ideal supercapacitors
WO3 nanorods: exhibit symmetric curve
Bulk WO3: unsymmetry
24
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
Specific Capacitance for WO3 nanorods: 436 F/g
Bulk WO3
: 57 F/g
25
Cycling performance of tungsten trioxide nanorods electrode
0.2
0.1
Potential (V)
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
0
10000
20000
30000
40000
50000
Time (s)
Potential vs time at a constant current denisty of 3 mAcm-2
Stable over a long period of time
26
Cycling performance of tungsten trioxide nanorods electrode
Specific Capacitance (F/g)
500
400
300
200
100
0
0
10
20
30
40
No of Cycles
27
Cycling performance of bulk tungsten trioxide electrode
0.2
Potential (V)
0.1
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
6000
Time (s)
28
Cycling performance of bulk tungsten trioxide electrode
-1
Specific capacitance (Fg )
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
Cycle number
Potential vs time at a constant current denisty of 3 mAcm-2
29
Overlay of cycling performances of WO3 nanorods and bulk WO3
500
-1
Specific capacitance (Fg )
450
400
350
300
250
200
150
100
50
0
0
10
20
30
40
Cycle number
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%
30
Tabulation of specific capacitance
Material
Specific
Capacitance
(F/g)
WO3 nanorods
436
Bulk WO3
57
31
Conclusions
 Tungsten trioxide nanorods by a single step pyrolysis technique has
been manufactured
 The synthesized nanorods have been employed for supercapacitor electrode
applications
 Tungsten trioxide nanorods showed higher performance and stability than
its bulk counterpart
32