electro active graphene nafion actuators

8/10/2019 Electro Active Graphene Nafion Actuators

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Electro-active grapheneNafion actuators

Jung-Hwan Jung, Jin-Han Jeon, Vadahanambi Sridhar, Il-Kwon Oh *

Structural Dynamics and Smart Systems Laboratory, Division of Ocean Systems Engineering, School of Mechanical,

Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahak-ro, Yuseong-gu,

Daejeon 305-701, Republic of Korea

A R T I C L E I N F O

Article history:

Received 26 September 2010

Accepted 24 November 2010

Available online 29 November 2010

A B S T R A C T

Electro-active actuators based on graphene reinforced Nafion composite electrolytes were

developed and their electro-chemo-mechanical properties and actuation performances

were investigated. The tensile strength of the grapheneNafion ionic membrane was signif-

icantly improved up to 200% within 1.0 wt.% loading, and Youngs modulus was more than

two times with a minute loading of graphene to Nafion electrolyte. The proton conductivity

and the water-uptake ratio were greatly improved, while apparent changes in the ion

exchange capacity were not observed. Morphological tests, chemical techniques, and scat-

tering techniques were used tostudythe interaction mechanismbetween graphene andNaf-

ion, resulting in great improvements of the actuation performances. Present results show

that a minute loading of graphene greatly improves the harmonic responses, the blocking

force and the energy efficiency in Nafion-based ionic polymermetal composite actuators.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Smart materials consisting of polymer composites or gels un-

dergo dramatic and reversible shape deformation in response

to application of external stimuli like electric potential [1],

chemical contact[2], temperature[3], pH[4] and so on. Actu-

ators converting electrical energy into mechanical work find

many applications in medical, mechanical, electrical and

aerospace engineering such as multi-link active catheters,

artificial muscles [5], bio-medical devices [6], micro-pumps,

molecular motors, micro-/nano-robots, micro-manipulators

[7], and biomimetic robots like jelly fish[8].Though many polymers were used [911], Nafion is the

most widely used; however, the degree of actuation in pristine

Nafion is less than desirable. So, in order to improve the actu-

ation and bending functionalities, doped Nafion, Nafion

reinforced with nano-scaled metal-fillers [12] were used

initially. However, metal doping suffers from disadvantages

like high doping levels, as much as 30% which not only

increase the cost of the actuator, but also hinders the

electrolytic mobility of cations, thereby limiting its bending

and actuation.

The recent discovery of nano-scale carbonaceous materi-

als like graphene, carbon nanotubes (CNT), carbon nano-fi-

bers (CNF), and fullerenes and their remarkable mechanical,

electro-chemical, piezo-resistive and other physical proper-

ties has opened the possibility for the development of a new

class of smart nano-materials. Graphene, the basic building

block of most carbonaceous nano-materials, is an intriguing

2D flat material of monolayer carbon atoms whose distinct

properties makes it very promising in applications like field-

effect transistors, lithium ion batteries, hydrogen storage[13], sensors[14], as well as a reinforcing filler in high perfor-

mance polymer composites[1517]. By addition of these car-

bonaceous nano-fillers like CNF[18], CNT[19]and fullerenes

[20] to ionic polymer, one may overcome the severe limita-

tions associated with high metal doping and can enable

new means to generate vivid motion and large bending in

Nafion-based actuators[21]. Though there are some recent re-

ports on the actuation performance of graphite oxideNafion

0008-6223/$ - see front matter

2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2010.11.047

* Corresponding author: Fax: +82 42 350 1510.E-mail address:[email protected](I.-K. Oh).

C A R B O N 4 9 ( 2 0 1 1 ) 1 2 7 91 2 8 9

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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 / c a r b o n
http://dx.doi.org/10.1016/j.carbon.2010.11.047mailto:[email protected]://dx.doi.org/10.1016/j.carbon.2010.11.047http://dx.doi.org/10.1016/j.carbon.2010.11.047http://dx.doi.org/10.1016/j.carbon.2010.11.047http://www.sciencedirect.com/http://www.elsevier.com/locate/carbonhttp://www.elsevier.com/locate/carbonhttp://www.sciencedirect.com/http://dx.doi.org/10.1016/j.carbon.2010.11.047http://dx.doi.org/10.1016/j.carbon.2010.11.047http://dx.doi.org/10.1016/j.carbon.2010.11.047mailto:[email protected]://dx.doi.org/10.1016/j.carbon.2010.11.047


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actuators[22], the grapheneNafion composite actuator was

not reported until now.

In this study, we developed grapheneNafion composite

actuators and systematically investigated its actuation and

electro-active behavior. The interfacial interactions of poly-

mer chains with the surfaces of dispersed rigid filler particles

are critical for understanding and improving the performance

of electro-active polymer composites[23]. Several techniques

like morphological tests, chemical techniques, scattering

techniques are used to elucidate the structural properties

and interaction mechanism between Nafion and graphene.

To investigate the reinforcing effect of graphenes in the poly-

mer matrix, many electro-mechanical tests have been per-

formed including stressstrain measurements, dynamic

mechanical analysis, ionic exchange capacity, proton conduc-

tivity and water-uptake ratio. Also, improvements of the

graphene-based ionic polymer actuators are described in

terms of harmonic responses and blocking forces.

2. Experimental2.1. Fabrication of composite actuators

Graphene was prepared from graphene oxide by using modi-

fied-Hummers method[24]with graphite flakes as the start-

ing material and was then synthesized through pre-reduction

of graphene oxide with sodium borohydride at 80 C for 1 h,

followed by post-reduction with hydrazine at 100 C for 24 h.

Representative morphology of graphene sheets is shown in

Fig. 1a.

The ionic polymer Nafion PFSA (perfluoro sulfonic acid)

were purchased as dispersions containing PFSA/PTFE (poly-

tetrafluoroethylene) copolymer in the acid (H+) form in

alcohol and water. Prior to use, Nafion PFSA polymer disper-

sions were dried in a vacuum oven under a regulated temper-

ature of 80 C for 12 h. Subsequently, the dried Nafion film

was dispersed in Di methyl acetamide (DMAc) solution by stir-

ring for 6 h at a temperature of 80 C. Graphene was dispersed

in ortho-dichlorobenzene at weight percentages of 0.1% and

1.0% and were agitated for 2 h at room temperature to obtain

stable solutions. These two mixtures were subsequently com-

bined and stirred for another 24 h to obtain stable graphene

Nafion dispersions. The stabilized and well-dispersed graph-

eneNafion solution was dried in a casting mold shown in

Fig. 1b at 120 C for 12 h and annealed at 140 C for 2 h.

The thickness of the membranes was measured with a

micrometer at multiple points on the membrane surface,

and the average value of the membrane thickness was about

0.34 mm. The electrodes on both sides of membrane were

made through electroless plating based on the platinum

amine complex salt [25]. Representative morphology of the

actuators is shown inFig. 1c wherein uniform coating of plat-

Fig. 1 SEM micrograph of graphene (a); grapheneNafion composites in molds and their membranes (b); morphology of

actuators after platinum electroless plating with plating thickness of6 lm. Inset in (c) shows a graphene sheet.

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inum layer with the thickness of 6 lm is observed. At the end

of these processes, the polymermetal composite membrane

was placed in the 1.5 N Lithium chloride (LiCl) solution for

more than 1 day for the exchange of Li+ cations with H+ cat-

ions. Specimens of the final electro-active composite actua-

tors were prepared with a length of 25 mm, a width of 5 mm

and a thickness of 0.38 mm under hydrated condition.

2.2. Testing

X-ray diffraction (XRD) of the composites was measured using

DMAX-Ultima III X-ray diffractometer in the range of 290.

SEM (scanning electron microscopy) images were recorded

using a cold field emission scanning electron microscope (S-

4700, Hitachi, Japan). Atomic force microscope (AFM) mea-

surements were made using a commercial XE-100 (Park Sci-

entific Inc.). Tensile testing of fully hydrated and dry

composites was tested using Universal testing machine, PT-

200N, Minebea, Japan as per American society of testing and

materials, ASTM D882 standards. The ionic exchange capacity

was calculated using the titration method with NaOH. Surface

analysis by High resolution X-ray Photoelectron Spectrometer

(XPS) (ESCA Multilab 2000 system) was used to detect the

presence of surface elements. ATR-FTIR (Attenuated total

reflection Fourier transform infra red spectroscopy) of the

membranes was tested using Shimadzu IRPrestige-21 spec-

troscope. The experimental setup for the measurement of

actuation is shown inFig. 2. A charged couple device (CCD)

camera (XC-HR50) and a laser displacement sensor were used

for sensing displacements and motions, and a NI-PXI system

and a current amplifier (UPM1504) were used for signal gener-

ation and actuation of the electro-active artificial muscle.

3. Results and discussion

3.1. Structure and morphology

The effect of graphene addition on the degree of crystallinity

of the composites was investigated using Wide angle X-ray

analysis as shown inFig. 3a. Recast Nafion shows broad dif-

fraction maxima at 2h values of 17.48 corresponding to d-

spacing of 5.5 Awhich can be attributed to the Teflon-like do-

mains of Nafion [26]. In the case of grapheneNafion, there

was a decrease in intensity of both these peaks. The broad

diffraction peaks in the range of 2h= 1220resulted from a

convolution of amorphous (2h= 15) and crystalline (2h=

17.5) scattering from the perfluorocarbon chains of Nafion.

By comparing these deconvoluted diffractograms as shown

inFig. 3b and c, a decrease in the ratio between the area of

the crystalline peak and the amorphous scattering with in-

creased graphene content is apparent from these spectra.

Deconvoluted XRD peaks of grapheneNafion shows the

shifting of crystalline peaks to 14.8. This indicates that a

new type of crystallite was created in the amorphous region

of Nafion, possibly at the interface between graphene and

Nafion due to disruption of the crystalline morphology of Naf-

ion[27]. More evidence of changes in crystallinity can be ob-

served from FTIR spectra. There is a correlation between a

decrease in the crystallinity of Nafion composites and the var-

iation in the intensities and peak position [28]in the region

from 550 to 650 cm1 (Fig. 3d). Our results show a continuous

increase in peak intensities with increasing graphene concen-

trations, which implies progressive loss of crystallinity of the

composites. This is contrary to reported results of graphene-

reinforced poly vinyl alcohol (PVA) composites wherein a pro-

gressive increase in crystallinity of PVA with an increasing

concentration of graphene has been reported[29]. This varia-

tion can be attributed to the unique di-block morphology of

Nafion wherein nano-sized additives such as graphene and

Nafion are embedded in the interstitial regions.

The morphologies of recast Nafion and grapheneNafion

composites were studied by SEM and AFM as shown in

Fig. 4a and c, and b and d. Recast Nafion has typical di-block

morphology, whereas a layered structure in the case of graph-

eneNafion was observed.

The layered structure of graphene layers which are indi-

cated by arrows in the polymer matrix is observed in Fig. 4b

and d, suggesting that during the solvent evaporation of solu-

Fig. 2 Experimental setup for measurement of tip displacement and motion of electro-active grapheneNafion composite

actuators.

C A R B O N 4 9 ( 2 0 1 1 ) 1 2 7 91 2 8 9 1281


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tion-cast films enhanced alignment of graphene layers paral-

lel to the surface of Nafion fibrils. A similar observation in

graphene oxide nano-sheets [30] and nano-clay reinforced

polymer composites has been reported by various researchers

[31,32].

3.2. Tensile properties

The stressstrain curves were obtained from uniaxial tensile

testing of the recast Nafion and grapheneNafion membranes

with 0.1 and 1.0 wt.% in dry and wet conditions as shown in

Fig. 5a and b, respectively. GrapheneNafion membranes

showed significant improvement in mechanical stiffness

and strength more than 100%, although only minute amounts

of graphene, in the weight percentages of 0.1% and 1.0% were

added.

In dry condition, the recast Nafion membrane failed at a

maximum stress of 11.5 MPa, whereas 1.0 wt.% graphene

Nafion membranes failed at the maximum stress of

31.39 MPa, an increase of over 280%. The fact that this signif-

icant increase was achieved at the 1.0 wt.% graphene loading

is very impressive. The magnitude of increase in tensile

strength is much more than that of other graphene-rein-

forced polymer composites [29,33]. Youngs modulus of the

1.0 wt.% grapheneNafion composite membrane was twice

that of the recast Nafion membrane as listed in Table 1. The

intensity of increase in tensile strength is also much more

than that of other plate-like fillers such as nano-clay-rein-

forced Nafion composites [34] or variants of nano-clay like

sulfonated nano-clay reinforced Nafion composites[35].

The mechanical properties of grapheneNafion compos-

ites were also studied in completely water saturated Nafion/

graphene membranes, since wet strength and stiffness are

of paramount importance for actuator applications. The equi-

librium water uptake that infers no further increase in weight

of membranes even after prolonged immersion in water was

determined by a gravimetric method as previously used in our

publication [20]. A minimum of three sets of experiments

were tested and the error was well within the experimental

limits. The water-uptake ratio[38]strongly affects the actua-

tion performance of IPMC which has the actuation mecha-

nism of the ion migration with water molecules in the ionic

exchangeable membrane. The water-uptake ratio of pristine

recast Nafion membrane, 0.1 and 1.0 wt.% graphene-rein-

forced membranes were 16.7%, 10.42% and 6.8%. Water

adsorption reduced by more than 100% with a mere 1.0 wt.%

addition of graphene. This large decrease in water uptake is

consistent with the other layered-filler (nano-clay) reinforced

polymer composites [36]. The stressstrain curves of wet

composite samples are shown inFig. 5b. Irrespective of the

Fig. 3 XRD spectra of recast Nafion and graphene-reinforced composites (a); deconvoluted XRD spectra from peak profile

analysis of recast (b) and 1.0 wt.% grapheneNafion composites (c); FTIR (d) spectra of recast Nafion and graphene reinforced

Nafion composites showing variation in crystallinity.

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graphene content a progressive decrease in the mechanical

properties of wet samples can be observed.

The stressstrain curves also show a substantial increase

in strain with the addition of graphene; this can be explained

on the basis of the deformation behavior of nano-particle

reinforced di-block copolymers. Though many morphological

models of Nafion have been proposed by various researchers,

the general consensus is that Nafion has the morphology of

di-block copolymer. The addition of nano-fillers such as

graphenes into these types of polymers can result in spatial

distribution and a template arrangement of graphene, espe-

cially at low loadings. The presence of graphene alters both

the morphology and the orientation of di-block copolymer

micro-domains [37] as can be observed in SEM and AFM.

When nano-particle reinforced di-block copolymers are sub-

jected to tensile testing, an increase in strain can be expected.

Table 1 Properties of recast Nafion and two graphene-reinforced membranes and actuators.

Sample type Youngs modulus (MPa) Proton conductivity(S/cm)

Wateruptake (%)

IEC (meq/g) Blockingforce

(gf, @ DC 2 V)

Dry Wet

Recast Nafion 11.5 9.85 1.87 102 16.7 0.75 0.1990.1 wt.% GrapheneNafion 17.9 11.60 3.96 102 10.42 0.78 0.3411 wt.% GrapheneNafion 26.86 19.98 11.9 102 6.8 0.95 0.426

Fig. 4 SEM [(a) and (b)] and AFM [(c) and (d)] morphologies of recast Nafion and grapheneNafion composites.

0 5 10 15 200

5

10

15

20

25

30

35

(iii)

(ii)

Stress,

MPa

Strain, %

Recast Nafion (i)0.1 wt% graphene-Nafion (ii)

1 wt% graphene-Nafion (iii)

(i)

a

0 5 10 15 200

5

10

15

20

25

30

35

Stress,

MPa

Strain, %

(iii)

(ii)

(i)

Recast Nafion (i)0.1 wt% graphene-Nafion (ii)

1 wt% graphene-Nafion (iii)

b

Fig. 5 Stressstrain curves of recast and graphene reinforced Nafion in (a) dry and (b) wet conditions.

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Second, the addition of graphenes may also alter the static

(or strained) micro-phase morphology of Nafion, resulting in

improved tensile properties accompanied by an increase in

strain. Semi-crystalline polymers like Nafion are composed

of crystalline lamellae and entangled amorphous chains

show a complex behavior under external tensile strain

wherein the crack propagates in its own plane and the pri-

mary mechanism for arresting crack growth is the fibrils

and lamellar structure; however these fibrils alone are not

capable of arresting the crack growth. Hence, the crack prop-

agates in its own plane in a stable manner and this is evident

from the smooth curves of load versus displacement as

shown inFig. 5a and the fracture morphology is accompanied

by the formation of weld lines as shown in our earlier publi-

cation[20]. This type of crack propagation is irregular and is

evident from the waviness of the load versus displacement

diagrams. Waviness is more pronounced in the stressstrain

curves of 1.0 wt.% grapheneNafion composite, as shown in

Fig. 5a and b and from the unevenness in the fractured spec-

imen. Another explanation for a large increase in strain is the

self-healing nature of layered-filler reinforced polymer com-

posites[31].

3.3. Ion exchange capacity (IEC)

The ionic exchange capacity was calculated using the titra-

tion method with NaOH as used in our previous publication

[20]. The variation in ion exchange capacity (IEC) in Table 1

shows a marginal increase with increasing concentrations

of graphene. Nafion belongs to a class of ion exchange poly-

mers with a perfluoroethylene backbone and a side group

containing sulfuric acid groups. It is widely accepted that a

typical Nafion membrane consists of crystalline and amor-

phous regions[38], and that its hydrated clusters are located

in the amorphous regions as shown in schematicallyFig. 6.

Higher graphene concentration showed a progressive de-

crease in water adsorption, thereby leading to a decrease in

IEC values. The variation of membrane conductance has

two possible origins: inter-particles and intra-particles [39].

The first involves a change in the rate constant of the elemen-

tary ion transfer reaction due to interactions between mobile

ions and fixed sites that depend on the hydration states of in-

ter-particles. The second originates from micro-structural

changes within the membrane (intra-particle). In this case

of grapheneNafion composites, a decrease in the degree

and extent of ion clustering can be expected with the addition

of graphene. This decrease in ion clustering and water

adsorption decreases both the cluster diameter and the ex-

change sites for each cluster, thereby causing marginal varia-

tions in the IEC values.

3.4. Chemical characterization

ATR-FTIR and XPS of composite membrane were used to

determine the extent and nature of chemical interactions be-

tween graphene and Nafion. ATR-FTIR spectra shown in

Fig. 7a, of recast Nafion and grapheneNafion composite show

substantial changes in COC symmetric stretching bands at

968 and 980 cm1, whereas considerable broadening occurred

in SO3 symmetric stretching vibrations bands at 1057 cm1

[40]. These changes were also reflected in asymmetric

stretching of sulfonate groups (SO3) 1260 cm1 peak indicat-

ing good interactions between the sulfonate moieties of Naf-

ion membrane and the dispersed graphene. The doubly-

broadening observed in bending vibrations of OH deforma-

tion and H3O+ peaks respectively observed at 1729 and

1740 cm1 indicate interactions between the dispersed graph-

ene and the hydrate groups of the Nafion polymer matrix. The

insert inFig. 7b shows the changes in the shape and intensity

of the S2p peak of the XPS spectra, which reflects the interac-

Fig. 6 Schematic representation of proton conductivity in hydrated grapheneNafion composites and interactions between

graphene and sulfonated groups of Nafion.

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tions between the sulfonate moieties of Nafion with dis-

persed graphene sheets. The C1s signal of the XPS spectra

of the recast Nafion is shown inFig. 7c. Two prominent peaks

corresponding to CC and CF2were observed. The deconvolu-

tion of the spectra resulted in six peaks. CF2dominates, fol-

lowed by a satellite peak at 281.8 eV. Minor components also

appeared at 294.4 eV and are attributed to terminal CF3groups

[41]. In the case of grapheneNafion composites, substantial

changes in C1s spectra can be observed inFig. 7d. Deconvolu-

tion of the C1s peak revealed the presence of CO (286.34)

and C@O (288.73 eV) in addition to the prominent FCO

(292.84) and CF2 (291.41 eV) peaks of Nafion. The effect of

these interactions on the structure and morphology of graph-

eneNafion composites is shown schematically in Fig. 6. The

variation in FTIR peaks, combined with the decrease in water

uptake and a decrease in crystallinity as previously discussed

in XRD, indicates that substantial changes in the core-chan-

nel morphology of Nafion occurred with the addition of

graphene to Nafion.

3.5. Harmonic actuation performance

A charge-coupled device (CCD) camera (XC-HR50) and a laser

displacement sensor (LK301, KEYENCE) were used to measure

the tip displacements and bending motions of the composite

actuators. A NI-PXI 6252 system and a current amplifier

(UPM1504) were used for signal generation and actuation of

the composite actuators. For simple harmonic responses,

the time histories of the tip displacements for recast Nafion

and graphene-reinforced composite actuators under an elec-

trical sinusoidal input signal of a maximum voltage of 0.5 V

and an excitation frequency of 0.5 Hz are shown in Fig. 8a.

The tip displacement of the 1.0 wt.% graphene-reinforced

actuator is almost two times that of the recast Nafion-based

ionic polymermetal composite (IPMC) actuator.

Fig. 8b and c show the maximum positive values of the

harmonic responses under the sinusoidal electric inputs with

voltage amplitudes of 0.5 and 1.5 V according to the change of

excitation frequencies. The harmonic response of the

graphene-reinforced composite actuators was improved sig-

nificantly as the input voltage increased with 1.0 wt.% graph-

ene-reinforced composite actuators showing almost three

times higher tip displacement than recast Nafion actuator.

This increase in tip displacement is higher than the reported

values in single-walled carbon nano tube (SWCNT)-reinforced

IPMC actuators, even though the electrolyte in this study was

water, whereas Landi et al. [21] used ionic lithium salts as

electrolytes.

However, irrespective of the graphene concentration, there

is a marked reduction in displacement with increasing fre-

quency, thereby indicating an inverse relationship between

displacement and frequency. The reason for this decrease is

due to the reduction in the response time of the actuator with

increasing frequency, at which the potential bias is alternated

[42]. However the intensity of this decrease is less than that

observed in the SWCNT-reinforced IPMC actuators [21]. This

Fig. 7 FTIR [(a) and (b)] and C1s XPS [(c) and (d)] spectra of recast and grapheneNafion composite membranes. The inset in (b)

shows the S1p spectra.

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improvement can be attributed to the increased proton con-

ductivity, ionic exchangeable capacity and exceptional elec-

tron transport behavior of graphenes. Another explanation

for this behavior can be attributed to the platelet morphol-

ogy of graphene. In a typical actuator, when an electric field

is applied across the actuator, fast bending motion toward

one of the electrodes occurs followed by a slow relaxation to-

wards the other electrode. This bending motion is the result

of an electro-osmotic pressure induced by the migration of

hydrated cations and diffusion of water from these cluster re-

gions through the nano-channels [43]. So, reinforcing filler

that can effectively block or delay this water migration will

be advantageous in improving the overall actuator perfor-

mance. Graphene due to its platelet morphology will delay

this water diffusion due to its tortuous path. Overall, our re-

sults indicate that graphene at extremely low concentrations

are advantageous vis-a-vis other carbonaceous nano-fillers

such as SWNCTs or MWNCTs and can greatly improve the

harmonic response and actuation behavior of Nafion-based

IPMC actuators.

3.6. Blocking forces

A low blocking force of the electro-active IPMC actuator is a

critical problem to be improved for the realistic application.

The blocking force is defined as a required force at the tip

of a bending actuator when the tip position is kept fixed,

i.e. identically zero deflection. The blocking force is a mea-

sure of the actuation performance and represents the load

carrying capacity of the electro-active polymers. The blocking

force under DC 2 V was measured by using a load cell (LVS-

5GA, KYOWA). The blocking force of the graphene reinforced

Nafion actuator was about three times as big as that of the re-

cast Nafion-based IPMC actuator as listed inTable 1. Present

results indicate that the graphene reinforcement can in-

crease the blocking force of the IPMC actuator that was a crit-

ical problem of the electro-active polymers for real

applications. In our previous results, the blocking forces of

fullereneNafion and MWCNT-Nafion actuators with

0.1 wt.% loading were 0.128 gf/mm2 [20,44] and 0.186 gf/mm

2

[45], respectively. As reported in Table 1, the blocking force

of the grapheneNafion actuator was 0.341 gf/mm2, showing

nearly two times higher than other nano-composite actua-

tors. This can be attributed to the unique self-assembled

lamellar structure of graphenes in the Nafion matrix. Our re-

sults showed substantial increase in tensile strength and

modulus even with addition of minute concentrations of

graphene in Nafion, indicating considerable increase in stiff-

ness of the membranes. This increase in stiffness of the

membrane results in higher blocking force.

3.7. Currentvoltage tests and efficiency

To consider the effect of graphenes on power consumption

and energy dissipation of the grapheneNafion composite

actuators, the currentvoltage tests were performed. Fig. 9

shows the hysteresis behaviors of a recast Nafion and 0.1

and 1.0 wt.% grapheneNafion composite actuators in the

range of0.5 to +0.5 V at a constant resistance of 1.3 X and

the excitation frequency of 0.5 Hz.

The maximum current density of the composite actuators

increases up to three times in comparison with a recast Naf-

ion-based actuator. Much higher current density is due to

large electro-mechanical deformations of the grapheneNaf-

ion composite actuator reported inFig. 8. The dissipated elec-

trical input energy per the area of the electrode surface during

a time period with an excitation frequency can be calculated

by using a following equation.

Fig. 8 Actuated tip displacements under sinusoidal

electrical inputs of 0.5 * sin(2p* 0.5 * t) (a), tip-displacementhistories in the harmonic responses at the excitation

voltages of 0.5 V (b) and 1.5 V (c) according to applied

frequencies.

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EinputEinput

A R

T

0 itvtdt

A

Z 1=fex0

it

A

Vinput sin2pfextdt 1

whereA, i, v, T, fex, andVinputare the area of electrode, mea-

sured current, input voltage, period, excitation frequency

and voltage amplitude of sinusoidal electrical input. And

the unit kinetic energy, TIPMC, stored in the IPMC actuators

for a cycle can be computed in the following form.

TIPMC TIPMC

A

RT0

RL0

12q

dwx;tdt

2dxdt

A 2

whereq and xx; tare the equivalent density per length and

the transverse displacement, respectively. Here, the separa-

tion of variable by assuming the space-dependent deformedshape can be applied with a mode shape, Ux, of the IPMC

beam and the time-dependent function, ft, as a sinusoidal

oscillation with the excitation frequency[44].

xx; t Wxft WTipmaxUx sin2pfext 3

And then, the electro-mechanical efficiency of the IPMC actu-

ators can be defined as

g TIPMC

Einput

RL0

12qU

2xdx RT

0 2pfex2 cos22pfextdt

W2Tipmax

AEinput

CW2Tipmax

Einput

4

Time and space integral terms in Eq. (4) will be a constant

value, C, for all actuators. The specific electro-mechanical en-

ergy efficiency of the grapheneNafion composite transducers

can be defined in the following form with the reference value

of the recast Nafion-based IPMC transducer[46].

gnano-composite gnano-composite

grecast Nafion

Wnano-compositeTipmax

Wrecast NafionTipmax

!2Erecast Nafioninput

Enano-compositeinput

! 5

Basically, much larger tip displacement means the genera-

tion of much higher kinetic energy in the oscillating cantile-

ver actuators. Higher kinetic energy requires the external

work from the electrical input. While higher current densities

were observed in the V-I diagram, the power consumption re-

quired to generate a unit bending deformation is much smal-

ler in the grapheneNafion composite actuators. The

dissipated electrical input energy densities of the prepared

IPMC actuators that can be calculated by using Eq. (1) will

be the same to the area of the V-I circles. The values of theelectrical input energy per unit area are listed inTable 2.

The input power to activate the 0.1 and 1.0 wt.% composite

actuators increases slightly. But, at this activation, the gener-

ated tip displacements of the 1.0 wt.% composite actuators

increase up to three times as listed in Table 2. Therefore,

the specific electro-mechanical efficiency defined in Eq. (5)

shows two times higher values in comparison with the recast

Nafion-based IPMC actuators. It means that the effect of

graphenes as a reinforcing agent in the Nafion-based IPMC

actuator strongly affects the electro-mechanical energy effi-

ciency, resulting in the reduction of the input power to gener-

ate the unit tip displacement of the IPMC actuators.

4. Conclusions

By incorporation of graphene, significant improvement in the

actuation performance of Nafion-based IPMC actuators was

observed in this study. Morphological studies of graphene

Nafion composites by SEM and AFM show that graphene

platelets are dispersed homogeneously in the Nafion polymer

matrix by solvent recasting method. Tremendous improve-

ments in tensile strength and Youngs modulus were ob-

served even with the addition of minute graphenes. The

proton conductivity increased two times with the addition

of 1.0 wt.% graphenes, whereas a considerable decrease in

water uptake was observed. Chemical studies by XPS andFTIR showed considerable interactions between graphene

platelets and sulfonated moieties of the Nafion polymer ma-

Fig. 9 IV curves of recast Nafion and 0.1 and 1.0 wt.%

grapheneNafion actuators.

Table 2 Specific electro-mechanical energy efficiency of IPMC actuators under the sinusoidal electric input with amplitude of0.5 V and excitation frequency of 0.5 Hz.

IPMC actuators Electrical inputenergy density(mJ/mm2)

Maximum tipdisplacement (mm)

Specific electro-mechanical efficiency

Einput WTipmax gnano-composite

Recast Nafion IPMC 0.046 0.040 1.00.1 wt.% GrapheneNafion IPMC 0.101 0.084 2.01

1.0 wt.% GrapheneNafion IPMC 0.305 0.145 1.98

C A R B O N 4 9 ( 2 0 1 1 ) 1 2 7 91 2 8 9 1287


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trix. Compared to the recast Nafion-based IPMC actuator,

much larger bending deformation and higher blocking forces

were achieved in the graphene-reinforced composite actuator

with minute addition of graphene. Currentvoltage tests and

subsequent efficiency analysis showed that specific electro-

mechanical efficiency of graphene-reinforced actuators is al-

most twice than that of recast Nafion.

Acknowledgements

This work was supported by National Research Foundation of

Korea Grant funded by the Korean Government (2010-0018423)

and (2010-0000300). And this work was supported by a grant

from the Fundamental R&D Programs for Core Technology

of Materials funded by Ministry of Knowledge Economy,

Republic of Korea [K00060-282].

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