Equivalent circuit modeling of ionomer and ionic polymer conductive network composite actuators containing ionic liquids

SensorsandActuatorsA181 (2012) 70–76

Contents

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available

at

SciVerse

ScienceDirect

Sensors

and

Actuators

A:

Physical

j o u r n a

l

h

o

m e

p a

g e :

w w w . e l s e v i e r . c o m / l o c a t e / s n a

Equivalent

circuit

modeling

of

ionomer

and

ionic

polymer

conductive

network

composite

actuators

containing

ionic

liquids

Yang

Liu

a

_,

_Ran

_Zhao

a

_,

_Mehdi

_Ghaffari

b

_,

_Junhong

_Lin

b

_,

_Sheng

_Liu

a

_,

_Hülya

_Cebeci

c

_,

Roberto

Guzmán

de

Villoria

c

_,

_Reza

_Montazami

d

_,

_Dong

_Wang

e

_,

_Brian

_L.

_Wardle

c

_,

James

R.

Heflin

e

_,

_Q.M.

_Zhang

a

,

b

,

∗

a_{DepartmentofElectricalEngineering,PennsylvaniaStateUniversity,UniversityPark,PA16802,UnitedStates}

b_{DepartmentofMaterialsScienceandEngineering,PennsylvaniaStateUniversity,UniversityPark,PA16802,UnitedStates} c_{DepartmentofAeronauticsandAstronautics,MassachusettsInstituteofTechnology,Cambridge,MA02139,UnitedStates} d_{DepartmentofMechanicalEngineering,IowaStateUniversity,Ames,IA50011,UnitedStates}

e_{DepartmentofPhysics,VirginiaTech,Blacksburg,VA24061,UnitedStates}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received21August2011

Receivedinrevisedform1May2012 Accepted2May2012

Available online 10 May 2012 Keywords:

Ionicelectro-activepolymers(i-EAPs) Ionicpolymeractuators

Equivalentcircuitmodels Warburgdiffusion

a

b

s

t

r

a

c

t

Inthisstudy,wedemonstrateelectricalequivalentcircuitsthatmodelthecomplexfrequency-dependent impedance of1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf) containing electro-activepolymer membranesand ionic polymerconductor networkcomposite(IPCNC)devices. The devicesincludeNafionmembraneactuators,Nafioncoatedwithlayer-by-layer(LbL)Au nanoparti-cle/poly(allylaminehydrochloride)(PAH)compositeactuators,andNafionwithverticallyalignedcarbon nanotube(VA-CNT)/Nafioncompositeactuators.Itisfoundthatthelowfrequencyresponsesofthese devicesindicateWarburgdiffusion.Therefore,Warburgimpedanceisutilizedtomodelthelowfrequency diffusionbehaviorofthedevices,whiletheelectricdoublelayercapacitance(Cdl)representsthestorage

ofdriftingionsunderelectricfieldathighfrequencies.ItisfoundthatCdlforNafionwith40wt%EMI-Tf

is7.5␮F/cm2_{andincreasesto11.4␮F/cm}2_{withincreasingsurfaceareaoftheLbLcompositeelectrode.}

Cdlincreasesfurthertoabove3×103␮F/cm2foranactuatorwith12␮mVA-CNT/Nafioncomposite

elec-trodes,whiletheWarburgcoefficientAWremainsnearlythesameforallthedevices.Asaresult,the

actuationmagnitudeandspeedincreasewithchargesaccumulatedduetohigherCdl,withoutmuch

increaseinthecontributionfromtheslowiondiffusionprocess.

© 2012 Elsevier B.V. All rights reserved.

1.

Introduction

Due

to

their

low

operation

voltages

(

∼

a

few

volts)

and

large

strain

responses

(

∼10%),

ionic

electroactive

polymer

(i-EAP)

actua-tors

such

as

ionic

polymer

metal

composites

(IPMC)

and

conducting

polymer

actuators

are

attractive

for

many

electromechanical

appli-cations

[1–5]

.

Recently,

it

has

been

demonstrated

that

using

ionic

liquids

(ILs)

in

i-EAP

actuators

can

lead

to

improved

device

per-formance

[2,3,6,7]

.

For

example,

the

negligible

vapor

pressure

of

ILs

enables

ionic

devices

to

operate

at

ambient

atmosphere

with

long

life

cycles

(

∼10

5

₎

_[6]

_;

_the

_wide

_{electrochemical}

_window

_allows

higher

voltages

(3–6

V)

than

that

of

using

aqueous

electrolytes

and

hence

improves

strain

magnitude;

and

the

higher

ion

mobility

can

lead

to

faster

device

response.

Since

the

early

2000s,

ionic

poly-mer

conductor

network

composite

(IPCNC)

actuators

with

device

∗ Correspondingauthorat:N219MillenniumScienceComplex,UniversityPark, PA16802,UnitedStates.Tel.:+18148638994;fax:+18148637846.

E-mailaddress:qxz1@psu.edu(Q.M.Zhang).

operation

principle

analogous

to

that

of

IPMC

and

with

ILs

as

the

electrolytes

have

been

investigated.

[6–8]

Fig.

1

(a)

and

(b)

schemat-ically

present

typical

device

configurations

for

electroactive

devices

including

an

IL-containing

membrane

with

planar

electrodes

and

a

5-layer

structure

that

consists

of

an

IL-containing

membrane

and

large

surface

area

composite

electrodes.

While

the

ionomer

mem-brane

actuators

in

Fig.

1

(a)

offer

simple

structure

for

easy

device

fabrication

and

material

analysis,

the

two

nano-composite

elec-trodes

in

the

5-layer

devices

(such

as

IPMC

and

IPCNC)

provide

large

specific

electrode

area

for

ion

storage

under

an

applied

volt-age

which

enhances

the

bending

actuation.

As

shown

in

Fig.

1

(c),

under

electrical

stimulus,

the

excess

ions

drift

and

diffuse

into/out

of

the

two

composite

electrodes

and

therefore

swell/shrink

the

electrode

regions,

consequently

causing

bending

actuation.

On

the

other

hand,

introducing

a

strain

to

these

devices

(such

as

IPMC

and

IPCNC)

can

also

generate

electric

signal

which

allows

them

to

be

used

as

sensors

[1,4,5,9–12]

.

The

actuation

response

of

these

actuators

is

affected

by

the

ion

transport

and

the

elastic

coupling

between

ions

and

ionomers

(in

Fig.

1

(a))

and

the

composite

materials

(in

Fig.

1

(b)).

As

illustrated

0924-4247/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Y.Liuetal./SensorsandActuatorsA181 (2012) 70–76 75

IPCNC

actuators.

We

found

that

the

low

frequency

responses

of

these

devices

indicate

the

presence

of

ionic

diffusion,

which

is

a

slow

ion

transport

process.

For

the

neat

Nafion

membrane

actu-ator,

the

Warburg

impedance

element

dominates

the

impedance

response

at

frequencies

<100

Hz,

consistent

with

the

low

actuation

speed

of

the

membrane

actuators.

It

was

also

found

that

employing

proper

CNCs

in

the

IPCNC

actuators

can

enhance

the

electric

dou-ble

layer

capacitance

while

not

changing

the

diffusion

component

(the

Warburg

element),

which

leads

to

higher

strain

level

and

fast

strain

response.

It

was

further

observed

that

the

equivalent

circuit

for

neat

mem-brane

actuator

can

be

used

equally

well

to

fit

the

electric

impedance

data

if

the

CNC

layer

is

thin,

as

is

the

case

for

the

LbL

IPCNC

actuators.

On

the

other

hand,

when

the

CNC

layer

thickness

␮m,

an

equiv-alent

circuit

model

with

a

transmission

line

should

be

employed

to

describe

the

ion

transport

in

the

composite

electrode.

For

the

IPCNC

actuators

with

VA-CNT

nanocomposite

CNC,

the

significantly

enhanced

electric

double

layer

capacitance

without

incurring

a

large

diffusion

component

(as

indicated

by

the

Warburg

element)

indicates

a

much

large

actuation

strain

generated

within

the

CNC.

These

results

indicate

the

advantages

of

IPCNC

actuators

with

prop-erly

designed

CNC

electrodes

over

the

pure

ionomer

membrane

actuators

in

terms

of

both

the

actuation

level

and

response

speed.

Acknowledgments

This

material

is

based

upon

work

supported

in

part

by

the

U.S.

Army

Research

Office

under

Grant

No.

W911NF-07-1-0452

Ionic

Liquids

in

Electro-Active

Devices

(ILEAD)

MURI

and

by

NSF

under

Grant

No.

CMMI-1130437.

At

MIT

the

work

was

supported

by

Airbus

S.A.S.,

Boeing,

Embraer,

Lockheed

Martin,

Saab

AB,

Spirit

AeroSystems,

Textron

Inc.,

Composite

Systems

Technology,

and

TohoTenax

Inc.

through

MIT’s

Nano-Engineered

Composite

aerospace

Structures

(NECST)

Consortium.

Hülya

Cebeci

acknowl-edges

support

from

Scientific

and

Technical

Research

Council

of

Turkey

(TUBITAK)

for

a

2214-International

Research

Fellowship

Programme.

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Biographies

YangLiureceivedherB.S.degreeinPhysicsfromUniversityofScienceand Tech-nologyofChina,Hefei,China,in2004andreceivedherM.S.degreeinPhysics fromNortheasternUniversity,Boston,MAin2008.In2012,shereceivedherPh.D. fromtheDepartmentofElectricalEngineering,PennsylvaniaStateUniversity.Her researchinterestsincludeelectroactivepolymers&composites,solidstate elec-tromechanicaldevices,andelectricenergystoragesystems.

RanZhaoreceivedtheBachelordegreesinMaterialsScienceandEngineeringfrom NanjingUniversity,Nanjing,China,in2006.SheiscurrentlypursuingMasterdegree inElectricalEngineering,PennsylvaniaStateUniversityPark.Herresearchinterests includehighperformanceactuators,supercapacitors,andbatteries.

MehdiGhaffariiscurrentlypursuinghisPh.D.degreeattheMaterialsScience andEngineeringDepartment,PennsylvaniaStateUniversity.Hisbackgroundisin polymer-baseddevicesforenergyrelatedapplicationsandhisresearchinterest focusesonElectroactiveMaterialsandEnergyHarvestingDevicesincluding actu-atorsandsupercapacitorsbasedonnewgenerationofcarbonmaterialssuchas microporousgrapheneandcarbonnanotubes.

ShengLiureceivedtheB.S.degreeinPhysicsfromNanjingUniversity,Nanjing, China,in2003andMasterdegreeinElectricalEngineeringfromPennState Univer-sityin2008.HereceivedhisPh.D.degreeinElectricalEngineeringfromPennsylvania StateUniversityin2010.Hisresearchinterestsincludehighperformance elec-troactivepolymersandcompositesdevicessuchasactuatorsandtransducers,ion transportinionicpolymerandcompositesdevicessuchassupercapacitorsand batteries.

RezaMontazamiisanAssistantProfessorofMechanicalEngineeringatIowaState University.HereceivedhisB.S.inphysicsandhis M.S.andPh.D.inMaterials ScienceandEngineeringfromVirginiaTech.Montazami’sresearchisfocusedon SmartMaterialsandStructureswithemphasisonpolymer-nanoparticlefunctional thin-filmsforactuationandsensingapplications,torealizenature-inspiredsoft microroboticsandbiomedicaldevices.Hiscurrentandpreviousresearchprojects havebeenincollaborationwithARL,NRL,NIST,UniversityofPennsylvania, Pennsyl-vaniaStateUniversityandDrexelUniversity;andfundedthroughARL,ASEEandNSF. Hiscurrentinterestsareintheareasofbiomaterialsforbiomimeticandbiomedical devices,withemphasisoncytotoxicity,biocompatibilityandbiodegradabilityofthe materials.Montazamihaspublishedmorethan15peer-reviewedjournalarticles andbookchapters,andgiven8invitednationallectures.

76 Y.Liuetal./SensorsandActuatorsA181 (2012) 70–76 JamesR.HeflinisaprofessorofPhysicsatVirginiaTech,wherehehasbeenafaculty

membersince1992.HereceivedhisPh.D.inPhysicsfromUniversityofPennsylvania in1990.HeistheAssociateDirectoroftheCenterforSelf-Assembled Nanostruc-turesandDevicesatVirginiaTech,anAssociateEditoroftheInternationalJournal ofNanoscience,andco-editorthetextbook“IntroductiontoNanoscaleScienceand Technology.”Hisresearchfocusesonself-assemblyoforganicoptoelectronic mate-rialsanddevices.

QimingZhangisadistinguishedprofessorofelectricalengineeringandmaterials scienceandengineeringofPennStateUniversity.Dr.ZhangobtainedPh.D.in1986 fromPennStateUniversity.Theresearchareasinhisgroupincludefundamentals andapplicationsofnovelelectronicandelectroactivematerials.Researchactivities

inhisgroupcoveractuatorsandsensors,transducers,dielectricsandchargestorage devices,polymerthinfilmdevices,polymerMEMS,andelectro-opticandphotonic devices.Hehasover330publicationsand14patents(5pending)intheseareas. Hisgrouphasdiscoveredanddevelopedaferroelectricrelaxorpolymerwhich possessesroomtemperaturedielectricconstanthigherthan50,anelectrostrictive strainhigherthan7%withanelasticenergydensity∼1J/cm3_{.Hisgroupalso}

proposedanddevelopednano-polymercompositesbasedondelocalizedelectron systemstoraisethenano-polymericcompositesdielectricconstantnear1000. Morerecently,hisgroupdemonstratedanewclassofpolar-polymerwithelectric energydensityover25J/cm3_{,fastdischargespeedandlowloss,attractiveforhigh}

efficiencyenergystoragecapacitors.Heistherecipientofthe1999PennState EngineeringSocietyOutstandingResearchAwardandafellowofIEEE.