1

J. Rajeswari, B. Viswanathan and T. K. VaradarajanJ. Rajeswari, B. Viswanathan and T. K. VaradarajanNational Centre for Catalysis ResearchNational Centre for Catalysis Research

Department of ChemistryDepartment of ChemistryIndian Institute of Technology MadrasIndian Institute of Technology Madras

Chennai – 600 036Chennai – 600 036IndiaIndia

Supercapacitors based on tungsten trioxide nanorodsSupercapacitors based on tungsten trioxide nanorods

2

Introduction Introduction

Synthesis of tungsten trioxide nanorodsSynthesis of tungsten trioxide nanorods

Characterization of tungsten trioxide nanorodsCharacterization of tungsten trioxide nanorods

Electrochemical studies for supercapacitive behaviourElectrochemical studies for supercapacitive behaviour

ConclusionsConclusions

OutlineOutline

3

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

Electrochemical SupercapacitorsElectrochemical Supercapacitors

4

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

Electrochemical SupercapacitorsElectrochemical Supercapacitors

5

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 < x <1) (I)

WO3 + 2yH+ + 2ye- WO3-y + yH2O (0 < y < 1) (II)

Electrochemical behaviour of tungsten trioxidesElectrochemical behaviour of tungsten trioxides

6

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/cm2m 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

Solid state thin film containing W cosputtered RuOSolid state thin film containing W cosputtered RuO22 supercapacitor electrodes supercapacitor electrodes

Tungsten based supercapacitors reported in literatureTungsten based supercapacitors reported in literature

7

Amorphous tungsten oxide – ruthenium oxide composites for Amorphous tungsten oxide – ruthenium oxide composites for Electrochemical capacitorsElectrochemical capacitors

J. Electrochem.Soc., 148, (2001), A189

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

100 % RuO2

~50%WO3 + ~50% RuO2

Tungsten based supercapacitors reported in literatureTungsten based supercapacitors reported in literature

8

Tungsten trioxide systems for supercapacitors studied in the present workTungsten 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

Different synthetic approaches Solvothermal methodSolvothermal method Template directed synthesisTemplate directed synthesis Sonochemiccal synthesisSonochemiccal synthesis Thermal MethodsThermal Methods

DecompositionDecomposition Chemical vapor depositionChemical vapor deposition

Thermal decomposition – simple, easy, inexpensive and contaminants free methodsimple, easy, inexpensive and contaminants free methodOne report for synthesis of WOOne report for synthesis of WO33 nanorods by thermal decomposition method 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

Reported methods for the synthesis of WOReported methods for the synthesis of WO33 nanorods nanorods

10

Preparation of tetrabutylammonium decatungstate ((CPreparation of tetrabutylammonium decatungstate ((C44HH99))44N)N)44WW1010OO3232

Chemseddine et.al, Inorg. Chem. 23 (1984) 2609

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 synthesizethe precursor

Synthesis of the precursorSynthesis of the precursor

11

Recrystallized ((CRecrystallized ((C44HH99))44N)N)44WW1010OO3232

pyrolyzed at 450 pyrolyzed at 450 C, 3h, NC, 3h, N22

(tubular furnace)(tubular furnace)

WOWO3 3 nanorodsnanorods blue powderblue powder

Synthesis of Tungsten trioxide nanorodsSynthesis of Tungsten trioxide nanorods

12

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 economicalSimple, easy and economical

StableStable

Easy storageEasy storageMetal and the cation can be Metal and the cation can be tuned to give a tuned to give a variety of metal oxide variety of metal oxide nanorodsnanorods

Single stepSingle step

Relatively not economical Relatively not economical and also tedious methodand also tedious method

Highly volatile, evaporates to Highly volatile, evaporates to give W(OMe)give W(OMe)66 and and WOWO22(OMe)(OMe)22

Limitation in storageLimitation in storage

No such possibilityNo such possibility

Multiple stepsMultiple steps

Comparison of the features of the synthesis of WOComparison of the features of the synthesis of WO3 3 nanorodsnanorods by our group vs. existing report by our group vs. existing report

13

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

Scheme for the formation of tungsten trioxide (WOScheme for the formation of tungsten trioxide (WO33) nanorods) nanorods

14

Single crystalline monoclinic WOSingle crystalline monoclinic WO33 (JCPDS: 75-2072) (JCPDS: 75-2072)

To study the structure and composition, powder XRD pattern was obtained

X-ray diffraction pattern of tungsten trioxide nanorodsX-ray diffraction pattern of tungsten trioxide nanorods

15

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 WOConfirms the formation of WO33

Raman Spectrum of tungsten trioxide nanorods Raman Spectrum of tungsten trioxide nanorods

16

The synthesized WOThe synthesized WO33 has rod morphology – evident from SEM images has rod morphology – evident from SEM images

Scanning electron microscopic images of tungsten trioxide nanorodsScanning electron microscopic images of tungsten trioxide nanorods

17

Bulk WO3 : No specific morphology – aggregates of particles

Scanning electron microscopic images of bulk tungsten trioxide Scanning electron microscopic images of bulk tungsten trioxide

18

Transmission electron microscopic images of tungsten trioxide nanorods Transmission electron microscopic images of tungsten trioxide nanorods

Dimensions of WODimensions of WO33 nanorods calculated from TEM images : nanorods calculated from TEM images : Length: 130 – 480 nmLength: 130 – 480 nm

Width: 18-56 nmWidth: 18-56 nm

19

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

High resolution transmission electron microscopic image ofHigh resolution transmission electron microscopic image of tungsten trioxide nanorodstungsten trioxide nanorods

20

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

Energy dispersive X-ray analysis of tungsten trioxide nanorods Energy dispersive X-ray analysis of tungsten trioxide nanorods

21

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

Cyclic voltammetry (CV)

Galvanostatic charge–discharge studies (Chronopotentiometry)

The electrochemical properties were studied using

Electrode fabrication

5 mg of WO3 nanorods or bulk WO3 - dispersed in 100L 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

Electrochemical Studies Electrochemical Studies

22

Electrolyte: 1M H2SO4 ; Scan rate : 50 mV/s

Anodic peak (~0.1 V) due to the formation of tungsten bronzes (HxWO3) can be observed

WO3 nanorods Bulk WO3

Cyclic voltammograms of tungsten trioxide nanorods and bulk tungsten trioxideCyclic voltammograms of tungsten trioxide nanorods and bulk tungsten trioxide

23

Anodic peak current density: WOWO33 Nanorods: 24.7 mAcm Nanorods: 24.7 mAcm-2-2

Bulk WOBulk WO3 3 : 3.5 mAcm: 3.5 mAcm-2-2

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

An overlay of cyclic voltammograms of tungsten trioxide An overlay of cyclic voltammograms of tungsten trioxide nanorods and bulk tungsten trioxidenanorods and bulk tungsten trioxide

24

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

WO3 nanorods Bulk WO3

Chronopotentiograms of tungsten trioxide nanorods and bulk tungsten trioxideChronopotentiograms of tungsten trioxide nanorods and bulk tungsten trioxide

25

Specific Capacitance, C(F/g) = it/mV

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 WOSpecific Capacitance for WO33 nanorods : 436 F/g nanorods : 436 F/g Bulk WOBulk WO3 3 : 57 F/g: 57 F/g

Specific CapacitanceSpecific Capacitance

26

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

Factors for the capacitance in WOFactors for the capacitance in WO33 nanorods nanorods

27

Potential vs time at a constant current density of 3 mAcm-2 for 40 cyclesDesired property for devices – stability over long timeWO3 nanorods - Stable over a long period of time – better cycling performance

Cycling performance of tungsten trioxide nanorods electrode Cycling performance of tungsten trioxide nanorods electrode

28

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%

Cycling performance of tungsten trioxide nanorods electrode Cycling performance of tungsten trioxide nanorods electrode

29

Potential vs time at a constant current density of 3 mAcm-2 for 40 cyclesDesired property for devices – stability over long time

Cycling performance of bulk tungsten trioxide electrodeCycling performance of bulk tungsten trioxide electrode

30

0 5 10 15 20 25 30 35 400

10

20

30

40

50

Spec

ific

capa

cita

nce

(Fg-1

)

Cycle number

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%

Cycling performance of bulk tungsten trioxide electrodeCycling performance of bulk tungsten trioxide electrode

31

Tungsten trioxide nanorods have better performance and stability over its counterpartAfter 40 cycles, % loss in specific capacitance for WO3 nanorods: 10%After 40 cycles, % loss in specific capacitance for bulk WO3 : 30%

Overlay of cycling performances of WOOverlay of cycling performances of WO33 nanorods and bulk WO nanorods and bulk WO33

32

MaterialMaterial Specific Specific CapacitanceCapacitance

(F/g)(F/g)

WOWO33 nanorods nanorods 436436

Bulk WOBulk WO33 5757

Tabulation of specific capacitance Tabulation of specific capacitance

33

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

ConclusionsConclusions