Nafion Based Nanocomposite

8/10/2019 Nafion Based Nanocomposite

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Colloids and Surfaces A: Physicochem.Eng. Aspects 461 (2014) 133141

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journal homepage: www.elsevier .com/ locate /colsur fa

Nafion based nanocomposite membranes with improved electricand protonic conduction

Adina Boldeiua, Eugeniu Vasileb, Raluca Gavrilaa, Monica Simion a, Antonio Radoia,Alina Mateia, Iuliana Mihalache a, Razvan Pascua, Mihaela Kuskoa,

a National Institute for Research and Development in Microtechnology (IMT-Bucharest), 126AErou IancuNicolae Str., 077190 Bucharest, Romaniab University Politechnica of Bucharest, Faculty of Applied Chemistry andMaterial Science, Department of Science andEngineering of OxidicMaterials and

Nanomaterials, 011061 Bucharest, Romania

h i g h l i g h t s

A nanocomposite membrane was

obtained based on Nafion with

PtNPGNPDDA/PSS multilayers. Charge transport characteristics (EIS)

were analyzed during the assembling

process. Improved nanocomposite multilayer

structures with simultaneously pro-

ton and electron conduction.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 16 May 2014

Received in revised form 14 July 2014

Accepted 26 July 2014

Available online 4 August 2014

Keywords:

Layer-by-layer

Platinum nanoparticles

Polymer-modified graphene

Nanocomposite material

Charge transport

a b s t r a c t

In this work, a stable aqueous dispersion solution ofpolymer-modified graphene was prepared by in situ

reduction with ethylene glycol from graphene nanosheets (GNs) in the presence ofpoly (diallyldimethy-

lammonium chloride) (PDDA), in order to obtain a (PtNPsGNsPDDA) nanocomposite material, where

Pt nanoparticles (PtNPs) with a narrow size distribution of24 nm uniformly decorates GNs, being both

embedded into the PDDA. The multilayer films consisting ofthis new nanocomposite material and a com-

mon negatively charged polyelectrolyte (PSS) were fabricated on Nafion NRE 212 membrane applying

the versatile layer-by-layer (LbL) self-assembly technique. Both morpho-structural (HR-TEM, FEG-SEM,

AFM, UVvis spectroscopy, ATR-IR spectroscopy, ELS) and charge transport characteristics (EIS) were

analyzed during the assembling process showing that the new nanocomposite membrane presents an

improved electrical conduction without a majoralteration ofthe protonic one. The results may open new

areas ofapplications, from proton exchange membrane fuel cells to biosensors.

2014 Elsevier B.V. All rights reserved.

1. Introduction

The layer-by-layer self-assembly (LbL) based on electrostaticinteractionsrepresentsa simpleandversatilenanofabricationtech-

nique widely used to build three dimensional ultrathin multilayer

Corresponding author. Tel.: +40 21 269 07 68.

E-mail address:[email protected] (M. Kusko).

films [1] or even more complicated structures, such as patternedcolloidal crystals [2]. One of the first applications was in fabrica-tion of directmethanol fuel cells (DMFC), as an efficient solution tomitigate thecommon problemof methanolcrossover.Accordingly,

novel multilayer membranes with low methanol permeabilityand satisfactory proton conductivity were reported, based on sul-fonated polyimide copolymers [3,4], poly(arylene ether sulfone)s[5,6], poly(ether ether ketones) [7] or sulfonated-poly(arylene

etherketone)s[8,9]. Encouraging results related totheuseof Nafion

http://dx.doi.org/10.1016/j.colsurfa.2014.07.041

0927-7757/ 2014 Elsevier B.V. All rightsreserved.
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134 A. Boldeiu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 461 (2014) 133141

and, more recently, multilayer membranes based on Nafion havebeen also reported in biosensor domain. It was shown that usingNafion nanocompositemembraneas outermembrane, theelectro-

chemical biosensor performanceswere improved, in terms of bothprotection and extension of the sensor response linearity [1012].

However, the negative effect of the polyelectrolyte bilayerpresence on the Nafion membrane conductivity [13] remains an

unsolved issue. Different approaches have been proposed, includ-ing addition of metallic nanoparticles (i.e. Pt [14]) and alloys (i.e.PtSn [15], PtRu [16,17]) or multi-walled carbon nanotubes [18]

insideofoneof thepolyelectrolytes.Theplatinumnanoparticlesizecontrol represents a key parameter for the fuel cell electrochem-ical and methanol oxidation [19], which dispersed on the Nafionsurface blocks the methanol cross-over [20].

Graphene is a two dimensional nanomaterialwith a monolayerof carbon atoms packed in a honeycomb lattice. Recently, it hasbecome one of the most studied material due to its noteworthyproperties, such as large surface area, high conductivity, excellent

electrochemical stability and good mechanical stiffness [2325].Graphene has applications in many critical areas, including energyconversion, sensors, solar cells, storage devices [21,22].

During the last years, an intensive research activity has been

done in order to develop uniform and stable functional nanocom-

posites based on graphene nanosheets (GNs). Part of these studiesaimed to find efficient methods to reduce the aggregation phe-nomenon by covalent or non-covalent functionalization with

different molecules [2628]. Taking into consideration that mod-ification of the graphenes carbon hybridization from sp2 tosp3 leads to unwanted alteration or destruction of the basicelectronic properties of graphene [29], the functionalization by

non-covalent interactions with polymers, biomolecules or smallaromatic molecules become preferable [3032].

It has been shown that the presence of noble metal nanoparti-cles and graphene leads to an improved catalytic performance for

methanol electrooxidation in alkaline medium [33], as well as theinterfacial properties between catalyst and electrolyte [34], whichrepresent significant improvements for several electrochemical

applications, and in particular for the fuel cells area. In this regard,PtNPs supported on the GNs revealed to be less susceptible to COpoisoning than those deposited onto traditional carbon supports[35].

However, without a stabilizer, besides the natural tendency to

agglomerate, graphene nanosheets are known to be hydrophobic,and thus, the nanoparticles deposition requires careful choice ofreaction media [36]. A simple approach to solve this issue maybe a preliminary functionalization with polyelectrolytes, which

are adsorbed on the graphenes surface, preventing aggregation[37]. In the last years, it has been demonstrated the beneficial roleof the positively charged polyelectrolyte poly (diallyldimethy-lammonium chloride) PDDA in graphene functionalization,

exhibitinggoodconductivity,biocompatibilityandeven electrocat-

alytic activity for oxygen reduction [38,39]. Based on these provenproperties, in this study, PDDA would be appropriate for function-alization of graphene via interactions, providing in the same

time active sites for reduction of the platinum precursor.Going further toward layer-by-layer self-assembling on Nafion

membrane using Pt modified graphene inside of the polyelec-trolyte based multilayers, the reported studies in thearea of direct

methanol fuel cells as well as biosensors are focused particularlyon the improvements of electrocatalytic activity for methanol oxi-dation [40,41] or biodetection for glucose [42]. In this context, aninsightful analysis of the nanocomposite membrane conductivity

is necessary. Furthermore, the presence of Pt and GN nanofillersprovides the premise [43,44] to investigate the nanocompositecapacity to conduct simultaneously electrons and protons, achiev-

ing in-depth data about their individual contributions.

Herein, we obtained a method to fabricate in one-pot synthesisa nanocomposite material where Pt nanoparticles and graphenenanosheets were embedded into a positively charged polyelec-

trolyte PDDA. Moreover, we showed that this method leads tosignificant improvements regarding morphological and structuralcharacteristics. The obtained PtNPs have a narrow size distribu-tion around 24 nm, fulfilling the most important requirement for

efficient catalysts, and uniformly decorate the PDDA functional-ized graphene nanosheets. The resulting nanocomposite materialwas then used to obtain a multilayered structure on Nafion NRE

212 membrane by applying the versatile LbL self-assembly tech-nique using a common negatively charged polyelectrolyte (PSS).Both morpho-structural and charge transport characteristics wereanalyzed during the assembling process showing that the new

nanocomposite membrane presents an improved electrical con-duction without a major alteration of the protonic one, Thisattribute opens new areas of applications, from proton exchangemembrane fuel cells to biosensors. Therefore, PtNPGNPDDA/PSS

multilayers assembled on Nafion NRE 212 were able to promote,the conduction of both electrons and protons, exhibiting mixedelectronic and ionic conduction in parallel.

2. Materials and methods

2.1. Materials

Hexachloroplatinic acid (H2PtCl6 8wt.% solution in water),ethylene glycol, (EG 99.8%, anhydrous), poly (diallyldimethylam-

monium chloride)(PDDA,200,000350,000MW, 20wt.%in water),poly (sodium styrene sulphonate) (PSS, 200,000 MW 30wt.% inwater), sodium chloride, hydrochloric acid, sodium hydroxideand Nafion NRE 212 membrane, 50m in thickness were from

SigmaAldrich, Germany. Graphene nanosheets solution (50mg/l)was from NanoIntegris, USA (PuresheetsTM Quattro).

2.2. Synthesis procedures

2.2.1. Preparation of the platinum nanoparticles decoratedgraphene nanosheets

A homogeneous dispersion of PDDA functionalized graphenenanosheets(GNs)waspreparedby mixingequalvolumes(10 mL)of

GNs(50mg/l)and PDDA(20mM), andfurthersubjected toanultra-sonicationprocess during 1h. Thepoly(diallyldimethylammoniumchloride)solutionwaspreparedin an increasedionicstrengthenvi-ronment containing 0.5M NaCl, since PDDA has the tendency to

adopt a linear shape when is solubilized in low ionic strengthsolutions, and it is becoming coiled when the ionic strength isincreased [45]. Based on our previous results, we adjusted the pHof this solution to pH= 4.5 [46]. A certain volume (10 mL) of the

prepared dispersion (GNsPDDA) was added to a mixture contain-ing 20mLof ethylene glycol (EG) and 1mLof 0.2M water based

hexachloroplatinic acid solution (H2PtCl6), under continuous stir-ring (600rpm)andheating (40C). After 1h, the pHof solutionwas

adjusted to 11.0 using a 6M NaOH solution and then increasingthe reaction temperature up to 130 C, during 4h, under nitrogenatmosphere. The obtained nanocomposite, PtNPsGNsPDDA, wasallowed tocooloff and thenwas centrifuged at900rpm for 30min.

The obtained precipitate was re-dispersed in 10mLof deionizedwater and centrifuged again in order to eliminate the unreactedprecursors. Finally, the precipitate was re-dispersed in 10mLofdeionized water, sonicated during 30min and then further used.

2.2.2. Preparation of the self-assembled nanocomposite on Nafion

membrane

Nafion contains both hydrophobic and hydrophilic regions,

ascribed to the polyethylene backbone and sulphonic groups,


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A. Boldeiu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 461 (2014) 133141 135

respectively [47]. Methanol cross-over regime is governed by thehydrophilic regions, in Nafion based proton exchange membrane,thus controlling hydrophilic areas in order to stop the methanol

cross-over through the protonated Nafion membrane is a desider-atefor many researchers.The presenceof anionic sulphonicgroupson membranesurface makespossible theelectrostatic interactionsbetween Nafion and a positively charged polyelectrolyte.

The as received Nafion NRE 212 membrane was pre-cleaned in3% (v/v) H

2O

2, during 30min at 80 C, washed in deionized water

(DI), boiled in 5M HCl solution for 30min at 80 C, plenty rinsed

with DI, boiled again in 1M H2SO4 water based solution during30min at 80 C and finally rinsed with DI. The activated Nafionmembrane was immersed in the PtNPsGNsPDDA nanocompos-ite solution during 15min, followed by rinsing with DI and drying

in a nitrogen flow. The positively charged thin film on the Nafionmembrane self-assembledwithits negativelycounterpart, i.e. poly(sodium styrene sulphonate), starting from a 20mM PSS solutionprepared in 0.5 M NaCl solution (pH=8.0) for another 15min. This

procedure was continued alternatively, until the desired numberof bilayers wasobtained.

Additionally, in order to see the influence of the graphenenanosheets decorated with platinum nanoparticles on the Nafion

membraneconductivity,a standardself-assemblednanocomposite

membrane was prepared using Nafion NRE 212 membrane, PDDAandPSSsolutions, except theuseof graphene nanosheets andplat-inum nanoparticles.

2.3. Instrumental analyses

High resolution transmission electron microscopy has been used

to investigate the morphology, geometrical size, shape, and nano-structure of platinum nanoparticles from the PtNPsGNsPDDAstructure. These analyses have been performedusinga TECNAIF30G2S-TWIN microscope operatedat300kVwith EDX andEELSfacil-

ities, very small amounts of samples being deposited on a TEMcoppergrid covered with a thin amorphouscarbonfilmwithholes.Selected area electron diffraction (SAED) analysis was also per-

formed.An atomic force microscope, NTEGRA Aura NT-MDT (NT-MDT

Co., Russia), was also,used toobserve thestructure of thenew plat-inum nanoparticles which decorate the graphene nanosheets. TheAFMsampleswere prepared by immersingclean1cm2 silicon sup-

ports for 5 min into the solution containing the PtNPsGNsPDDAnanocomposite, and dried at room temperature.

Thestabilityof theplatinumbased nanocomposites in very stepof the process was checked using Beckman Coulter DelsaTM Nano

Instrumentutilizing Electrophoretic Light Spectroscopy (ELS). Par-ticles were illuminated by a dual 30mW laser diode, producingtime-dependent fluctuations in the intensity of laser light. Thescattered light is collected at 15 for zeta potential measurements

(diluted concentration samples), and then is measured by a highly

sensitive detector.Field Emission Gun Scanning Electron Microscopy (FEG-SEM) a

Nova NanoSEM 630 (FEI Company, USA) workstation with a res-

olution of 1.8nm @ 3kV was used to analyze the morphologyand the thickness of the resulting multilayer membrane samples.Before measurements, the prepared nanocomposite membraneswere frozen using nitrogen and then cut-off in order to obtain

accurate cross-section images.Thegrowthof theself-assembledmultilayersof polyelectrolytes

on Nafion was studied by room temperature UVvis spectroscopyusing a U-0080D bio-spectrophotometer from Hitachi High Tech-

nologies.Attenuated total reflection infrared absorption spectroscopywas used to investigate the successful preparation of themultilay-ered nanocomposite membrane, recording spectra in the range of

4000 and 520cm1

on Tensor 27 (Bruker Optics, USA).

The charge transport in the nanocomposite structures wasinvestigated using a Princeton Applied Research Model PARSTAT2273 potentiostat/galvanostat/FRA by electrochemical impedancespectroscopy (EIS) method. ZSimpWin 3.21 software package wasused to analyze data. All conductivity values reported in this workcorrespond to the average of two membrane samples.

3. Results and discussion

3.1. Characterization of the graphene nanosheets decorated with

platinum nanoparticles

Fig. 1(a) presents a typical micrograph revealing quite well dis-persedgraphenenanosheetsdecoratedwithspheroidlikeplatinumnanoparticles. The EDX spectrum inset ofFig. 1(a) confirms

Fig. 1. HR-TEM images of the PtNPsGNsPDDAnanocomposite.


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on one hand the presence of the platinum and some oxygen ele-ments. On the other hand, the presence of the Na and Cl ions inEDX pattern is due to the existence of the counter ions from the

PDDA. The HR-TEM image Fig. 1(b) reveals both the nanoparti-clecrystallinityand thedistribution of theirsizes.Furthermore, theselected area electron diffraction (SAED) analysis identified crys-talline planes [(111), (200), (22 0) and (31 1)], all attributed to

face-centered-cubic platinum lattice upper-right corner inset ofFig. 1(b). The diameter distribution of PtNPs is presented in thebottom-right corner inset ofFig. 1(b), and shows a relatively nar-

row size distribution ranging from 2 nm to 5.5nm with an averagediameter of 3.40.3nm.

The AFM images of the graphene sheets before and after deco-ration with PtNPs are presented in Fig. 2. Accordingly, the starting

material contains exfoliated graphene sheetswith the average lat-eral dimension of 517 nmand thickness of1.32nm Fig. 2(a). Afterthe in situ synthesis of the PtNPsGNsPDDA nanocomposite, anincrease of the thickness up to 2.55nm was observed Fig. 2(b).

Going further, the resulting dispersion stability was analyzedusing the electrophoretic light scattering (ELS) method by zetapotential () measurements. Briefly, when an electric field isapplied acrossa samplein a cell, charged particlessuspendedin the

mediumare attractedtoward theelectrode ofoppositecharge. This

movement called electrophoresis produces the light fluctuations,whose frequency is proportional with its velocity. Accordingly, potential can be estimated by measuring the velocity of the par-

ticles using HelmholtzSmoluchowski equation which gives therelationship between the velocity and the amount of the particlessurface charge [48,49]. Therefore, the zeta potential measurementof the GNs suspension in water gives a value of35.92mV, which

indicate a very good stability of the dispersion. The functional-ization of GNs with polyelectrolyte produces a relevant shift inzeta potential, the value becoming positive (+21.00mV), whichcertifies the successful preparation of the GNsPDDA compos-

ite. Thus, the functionalized graphene nanosheets provide a gooddistribution of sites for the negatively charged [PtCl6]

2 ions ofthe Pt precursor. These ions are bound via electrostatic forces to

the functionalized graphene, leading to an improved uniformityof PtNPs distribution. This hypothesis was confirmed by the zetapotential value of +31.10mV indicating that the in situ reductionwas achieved, with a very good stability of the PtNPsGNsPDDAdispersion. In the absence of polyelectrolyte, the measured zeta

potential of PtNPsGNs was15.35mV, demonstrating the impor-tant role of PDDA: facilitates the preparation of a good dispersionof nanoparticles with regular shape anddiameter andstabilizationof the nanocomposite.

3.2. Characterization of the self-assembled nanocomposite on

Nafionmembrane

Assembling of the multilayer nanocomposite structures wasmonitored by microscopic and spectroscopic techniques, such as

scanning electron microscopy (SEM), UVvis spectroscopy andFourier transform infrared spectroscopy (FTIR).

3.2.1. Morphological characterization of the multilayered

nanocompositemembranes

SEM analyses of the membrane cross-section revealthe successfully achievement of the self-assembled layersPtNPsGNsPDDA/PSS on Nafion membrane. The imagesobtained after assembling 4 and 10 bilayers, respectively, are

presented in Fig. 3. As it can be observed, the thickness of theresulting membrane is around 158nm when 4 bilayers weredeposited Fig. 3(a), and become 399nm after 10 bilayers

assemblage Fig. 3(b), leading to the mean value of one bilayer

Fig. 2. AFM images for: (a) graphene nanosheetsGNs; and (b) PtNPsGNsPDDA

nanocomposite.

thickness to be around 40nm, as it results from both sample

analyses.

3.2.2. UV-spectroscopy

TheUVvis spectraofpristineprotonatedNafionmembraneandafter thesuccessive assembling of2, 4, 6, 8, 10bilayersof PDDA/PSS

are presented in Fig. 4(a), which reveals very well the absorbance


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A. Boldeiu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 461 (2014) 133141 137

Fig. 3. Cross-sectional SEM pictures of 4 (a) and 10 (b) bilayers of self-assembled

PtNPsGNsPDDA/PSS.

increasing when more bilayers are added. This behavior is betterobserved in the inset graph from Fig. 4(a), where the absorbancefrom 228 nm, characteristic to the sodium ion from the negative

polyelectrolyte is represented as function of number of assembledbilayers. Thecationic polyelectrolyte is transparent in theUV mea-sured domain. The plot demonstrates that the 228nm absorbanceincreases practically linear, indicating a quite uniform amount of

polyelectrolytes deposited on Nafion membrane [50,51].

In the case of the multilayers membranes wherePtNPsGNsPDDA nanocomposite was used instead of barePDDA, the UV spectra recorded after different bilayer assembling

steps are overlaid in Fig. 4(b), where a similar trend is observed.While the 253nm absorbance, ascribed to PSS, is dominant, theabsorbance from 228nm is perceptible only as a very small shoul-der, becoming more visible as from the8thbilayer. Theabsorbance

from 258nm, which increase more evident starting with the 6thbilayer can be assigned to graphene nanosheets, indicating theirsuccessful embedding into the multilayer films [41]. In order tohave a better image of the multilayered film thickness increment,

the inset graph from Fig. 4(b) depicts the absorbance variationfrom 253nm with the number of bilayers. It is also, presenteda broad spectrum from 280 to approximately 300nm with a

maximum around 288nm, assigned to the electronic conjugation

220 240 260 280 300 3200.00

0.05

0.10

0.15

0.20

0.25

0.30

10 bL

8 bL

6 bL

4 bL

2 bL

258 nm

228 nm

270 nm

288 nm

Absorbance(a.u.)

Wavelength (nm)

253 nm

Naf

Absorbance at 253 nm

R=0.9861SD=0.00637 (b)

220 240 260 280 300 3200,0

0,2

0,4

0,6

0,8

1,0

1,2

1,410 bL

8 bL

6 bL

4 bLAbsorbance(a.u.)

Wavelength (nm)

228 nm

Naf

2 bL

(a)

Fig. 4. UV-absorbance spectra for the self-assembled multilayers membraneswith

210 bilayers of PDDA/PSS (a) and PtNPsGNsPDDA/PSS (b).

fromgraphenenanosheets[52]. Thesmall absorbance peak around270nm can be assigned* transitions of aromatic C C bonds

present in the structure of graphene nanosheets.

3.2.3. Attenuated total reflection infrared spectroscopy (ATR-IR)

Attenuated total reflection infrared absorption spectra of theprepared nanocomposite membranes with 2, 4 and 8 bilayers of

PtNPsGNsPDDA/PSS are presented in Fig. 5. The presence of the

Nafion substrate inside the multilayer structure is confirmed bythe bands from 1230 and 1147cm1, assigned to the CF2 stretch-ing vibration. Also, the peak observed at 980 cm1 is due to the

symmetric stretching of C O C bonds from Nafion membrane,and itsabsorbance intensity increaseswith thenumberof bilayers.A peak at 1640cm1 assigned to stretching double C C from thearomatic ring of theanionicpolyelectrolyte, PSS, canbe observed in

spectra, its intensity increasing with deposited bilayers. The smallpeak from 1320cm1, more intense in the case of the 8th bilayer,may be assigned to the C H bond from CH3group of the polyca-tionicpolymer used to functionalize thegraphene nanosheets. The

decreasing band values in transmittance by increasing thebilayersnumber observed at 1060cm1 are associated with SO3stretch-ing vibration from the negative PSSpolyelectrolyte [53]. The small

bandsfrom810cm1

andthe intenseones from 630 cm1

, assigned


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1800 1600 1400 1200 1000 800 600

0.5

0.6

0.7

0.8

0.9

1.0

630

810

1320

980

1032

1060

12101147

Trans

mitance(%)

Wavenumber (cm-1

)

2 bLs

4 bLs

8 bLs

1640

Fig. 5. ATR-IR pattern of the nanocomposite membrane with 2, 4 and 8 bilayers of

PtNPsGNsPDDA/PSS.

to symmetrical O H bond deformation, are more evidenced as thenumber of bilayers increases [54].

3.2.4. Electrochemical impedance spectroscopy measurements

The influence of graphene nanosheets and the charge kinet-ics in multilayer nanocomposite structures were evaluated bytwo-probe electrochemical impedance spectroscopy, which iswell-established diagnostic and modeling method, widely used in

characterization of protonic membranes. Both, through-plane andin-planemeasurementswererealizedusingcustom-madeconnec-tion systems, schematic presented in Fig. 6. Therefore, while inthe first case Fig. 6(a) the membranes previously equilibrated

with waterwere placedbetween two square(1cm2) copperplatedelectrodes, for the second type of measurements Fig. 6(b) themembranes were placed on two striped copper plated electrodesdistanced at 1cm and on the top a simple plastic sheet is put in

order to uniformly press the entire assembly.The impedancespectroscopy measurementsweredone at25 C,

in the range of 0.1 Hz100kHz, by applying an alternative voltageof10mV,at open circuit potential. The through-plane andin-plane

Nyquist plots of membrane subjected of layer-by-layer assemblyprocess with polyelectrolytes with and without graphene and Ptnanoparticles are presented in Fig. 7.

Both types of measurements, through-plane and in-plane

showed that the charge transfer resistance significantly decreasedwhen the conductive nanoelements (PtNPs and GNs) were added

Fig. 6. Schematic representation of through-plane (a) and in-plane (b) conductivity

measurement systems.

0 50 100 150 200 250

0

10

20

30

40

50

60

70

80

Zimg(k

Zre

(k

Nafion + bilayers (PDDA+Pt+GN)/PSS

10 bLs

8 bLs 6 bLs 4 bLs 2 bLs

Nafion + bilayers PDDA/PSS

10 bLs 8 bLs

6 bLs 4 bLs 2 bLs

through-plane measurements

0 100 200 300 400

0

20

40

60

80

100

120

Nafion + bilayers (PDDA+Pt+GN)/PSS

10 bLs 8 bLs 6 bLs 4 bLs

2 bLsNafion + bilayers PDDA/PSS

10 bLs 8 bLs

6 bLs 4 bLs

2 bLs

Zimg(k

Zre

(k

in-plane measurements

Fig.7. Through-plane(a) andin-plane(b) Nyquistplotsof membraneafter differentnumber ofpolyelectrolytebilayerswith andwithoutgrapheneand Pt nanoparticles.

together with PDDA, suggesting the improvement of charge trans-

port properties of the multilayer membrane. Hence, the injectedcharges can be transferred more rapidly through the graphenenanosheets in the PtNPsGNsPDDA nanocomposite film. More-over, since theimpedance arcs arejust about thereal axis inthelow

frequency regions, an electronic conduction can be identified [55],where the negative influence of assembling dielectric polymericbilayers [39] is reduced, being observed only a two-fold increasingof the charge transfer resistance comparing with five-fold increas-

ingwhen thepolyelectrolytes arenotdoped. TheBode plots,both

absolute value of impedance (|Z|) and phase angle () as a functionof frequency support this behavioraswell, a slightdecreaseof thephase angle accompanied with a significant narrowing and shifts

toward low frequency values being observed, indicating that thenew systems deviate from the case of pure capacitance and havefaster electron transfer rates [33,56].

An equivalent circuit type Rbm(R1Q1) (R2Q2) is proposed to

model the EIS diagrams and to calculate the conductivity of thenanocompositemembranes, taking intoaccountthe presenceof thenew created interfaces[57] andbeing shown inFig. S1 (seeSupple-mentary Information). In theequivalent circuit wehada resistance

in series with parallel combinations of charge transfer resistance(Ri) and the constant phase element (Qi). The constant phase ele-ments represent non-ideal capacitors of capacity Ciand roughness

factor i, where takes values between 0 and 1, an value of 1


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of magnitude lower values after 10 bilayers, from 14.2mS/cm to0.56mS/cmin-planeandfrom3.7S/cmto0.0035S/cmthrough-plane, respectively. On the other hand, the samples containing

bilayers of (PDDA+ Pt+ GN)/PSS show an enhancement of theconductivity, from 14.2mS/cm to 42mS/cm in-plane and from3.7S/cm to 4.4S/cm through-plane, respectively.

The increase in electrical conductivity can be also partially

responsible for improvement of the protonic transport, becausea better electrically conductive structure promotes the motion ofions through the multilayer structure when an electric field is

applied [62].Moreover, the values of the electronic conductivities are still

at the same order of magnitude with protonic conductivities,beingcomparablewiththoseobtainedwhenverticallyaligned car-

bon nanotube columns [63] or reduced graphene oxide [64] weredirectlyembedded ina Nafionmatrixto improve theelectrical con-duction. It is remarkable also that these values of the electronicconductivities obtained for the ((PDDA+ Pt+ GN)/PSS)n/Nafion

membranes where obtained from the measurements made onhydrated membrane, where the electronic conduction is dimin-ished due to the presence of water molecules.

4. Conclusions

In summary, graphene was functionally modified with PDDAand then, decorated with small and uniform platinum nanopar-

ticles of 3.40.4nm, in situ synthesized by hexachloroplatinicacid reduction with ethylene glycol. After that, nanocompos-ite ((PDDA+ Pt+ GN)/PSS)n-Nafion membranes were prepared byLbL self-assembly technique and analyzed by comparison with

corresponding reference ((PDDA)/PSS)n-Nafion membranes usingdifferentmicroscopic andspectroscopic techniques.The UVvis, aswellas ATR-IR spectroscopiesconfirmedthe successful assemblingof thepolyelectrolytes regular or with nanofillers on theNafion

membrane.The impedance spectroscopy measurements revealed interest-

ing conduction properties of the novel membrane obtained usingadditional conductive PtNPsGNs nanofillers. Accordingly, both

through-plane and in-plane analyses were realized to investigatetheir influence on the conduction properties of the resulting mul-tilayer structures. First of all, it was shown that the variation ofthe charge transfer resistances significantly decreased after sim-

ilar steps of the polyelectrolyte assembling process when thePtNPsGNs nanofillers are present, suggesting an improvementof the charge transport properties. Accordingly, on one hand, thedecreasing of protonic conduction slowsdown, achievinga preser-

vation of the conductivity after 5 bilayers. On the other hand,integration of the conducting fillers in the multilayer structuresleads to generation of an equally important secondary conduc-tion path, the electronic one, which becomes non-negligible.

Consequently, it was demonstrated that the nanocomposite

((PDDA+ Pt+ GN)/PSS)n-Nafion membranes are capable of simul-taneously conducting protons and electrons, which represents anadvantage for development of further applications.

Acknowledgement

This work was supported by CNCSUEFISCDI Romania, under

the project number PNII-PCCA1-004/2012.

Appendix A. Supplementary data

Supplementary data associated with this article can befound,in theonline version, athttp://dx.doi.org/10.1016/j.colsurfa.

2014.07.041.

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Nafion Based Nanocomposite

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