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Current Applied Physics 12 (2012) 1235e1238

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Improved dimensional stability of Nafion membrane modified using a layerby layer self-assembly of biophilic polymers

Hye Jin Lee a,**, Eun Ji Nam a, Jung-Je Woo b, Seung-Hyeon Moon b,c, Jaeyoung Lee b,c,*

aDepartment of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu 702-701, Republic of Koreab School of Environmental Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Koreac Ertl center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea

a r t i c l e i n f o

Article history:Received 16 January 2012Accepted 30 January 2012Available online 8 February 2012

Keywords:NafionLayer by layer self-assemblyBiophilic polymerDimensional stabilityFuel cellWettability

* Corresponding author. Ertl center for ElectrochemInstitute of Science and Technology, 123 CheomdanRepublic of Korea.** Corresponding author.

E-mail addresses: hyejinlee@knu.ac.kr (H.J. Lee), ja

1567-1739/$ e see front matter � 2012 Elsevier B.V.doi:10.1016/j.cap.2012.01.015

a b s t r a c t

The modification of perfluorinated proton exchange membranes (e.g. Nafion) utilizing a layer by layer(LbL) electrostatic assembly of oppositely charged biophilic polymers such as poly-L-lysine as positivelayer and dsDNA as a negative layer is developed to protect the interface between the catalyst layer andthe membrane in a low temperature fuel cell. Various physicochemical measurements including wateruptake, proton conductivity and surface tension were investigated for both the as-received Nafion andthe biopolymeric LbL modified Nafion. The smaller water contact angle and the less swelling charac-teristics of the biopolymer modified Nafion membrane was founded compared to that of as-receivedNafion. This clearly indicates that the more hydrophilic surface of biopolymeric layers on Nafion playsan important role in the enhanced dimensional stability. In addition, a potential hypothesis explainingthe higher proton conductivity from the LbL biopolymeric film coated Nafion is suggested.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Of late, chemical energy conversion systems such as fuel cells,photovoltaic cells, and metal air batteries have emerged as poten-tial power sources in the near future. In particular, a fuel cellapplying hydrogen and small organic molecules are the mostpowerful chemical energy conversion systems based on polymerelectrolytemembranes (PEMs), delivering proton from the anode tothe cathode [1e4]. To facilitate a proper transfer of proton throughPEMs, the membrane should be humidified and it swells withexpansion of vertical direction. The control of the humidity, i.e.,keeping proton conductivity and avoiding water flooding/dryingphenomena, is quite a challenge because of the mass-transportlimitation of both fuels and air/oxygen. In a long-term operation,the swelling and drying of the membrane can cause the destroyingof interface between the membrane and the catalyst layer [5e7].

There have been extensive studies reported [8e14] to avoid suchinterfacial problemwhich can be categorized into three approaches;(i) the application of an optimal amount of water, (ii) the betteradhesive and uniform distribution of catalyst nanoparticles onto themembrane and (iii) the coating of additional protecting layer. Recent

istry and Catalysis, Gwangju-gwagiro, Gwangju 500-712,

eyoung@gist.ac.kr (J. Lee).

All rights reserved.

studies of layer by layer (LbL) coating of the polymeric electrolytesonto commercial membrane have mostly focused on the less fuelcrossover in direct methanol fuel cells and the experimental obser-vations represents that the proton conductivity is also decreased[11e15]. For example, Jiang et al. [13] utilized the LbL self-assembly ofpoly(diallyldimethylammonium), polysodium styrene sulfonate,poly(1-4-(3-carboxy-4-hydroxyphenylazo)benzene sulfonamido)-1,2-ethanediyl on a Nafion membrane to create efficient methanolfuel blocking materials. In this work, the LbL self-assembly of oppo-sitely charged biophillic polymers such asDNA and poly-L-lysine ontoNafion is investigated in order to enhance the dimensional stability ofNafion membranes. Nafion 212 film was chosen as a model supportsince it has been the most widely used in polymer electrolytemembrane fuel cells (PEMFCs) applying hydrogen fuel gas. Importantphysiochemical properties ofproton conductivity, surface tensionandhydrophilicity of the biopolymer LbL coated Nafion membranes wasalso studied and compared to that of as-receivedNafionmembrane todesign a novel membrane in a low temperature bioinspired fuel cell.

2. Experimental

2.1. Chemicals

Potassium phosphate dibasic trihydrate (K2HPO4, Sigma), Pota-sium phosphate monobasic (H2KO4P, Sigma), poly-L-lysine hydro-bromide (MW 30,000e70,000, Sigma), deoxyribonucleic acid

H.J. Lee et al. / Current Applied Physics 12 (2012) 1235e12381236

sodium salt from salmon testes (double stranded DNA (dsDNA),w2000 bases, Sigma), sulfuric acid (H2SO4 95%, OCI), hydrogenperoxide (H2O2 30%, Junsei), polymer electrolyte membrane(Nafion 212, Dupont), 5 wt% Nafion ionomer solution (Aldrich),isopropyl alcohol (Junsei), 54.5 wt% Pt/C (Tanaka) catalyst, Carbon10 BC (SGL) were used as received. All aqueous solutions wereprepared using Millipore-filtered water and a pH 8 of phosphatebuffer was used for preparing dsDNA and poly-L-lysine solution.

2.2. LbL assembly of biopolymeric electrolyte membranes ona Nafion 212 films

A simplified layer by layer (LbL) assembly of the oppositelycharged biopolymeric electrolyte layers on Nafion 212 is shown inFig. 1. Nafion 212 film was first treated in 5 wt % H2O2 solution ataround 80 �C for 2 hrs to remove any organic contaminant on thefilm followed by washing with DI water and H2O2. The treatedNafion film was then soaked in 0.5 M H2SO4 solution for 2 h fol-lowed by boiling it for 1 h and cooling it. The sulfuric acid treatedNafion film resulting in the negatively charged surface for theelectrostatic adsorption of positively charged poly-L-lysine wasfinally reserved in DI water at room temperature prior to use. Themultilayers of biopolymeric LbL membranes onto the negativelycharged Nafion was then formed via the electrostatic adsorption ofalternating oppositely charged layers, namely poly-L-lysine asa positive layer and dsDNA from salmon testes as a negative layer ata pH 8 of phosphate buffer. Various thicknesses of biopolymericmultilayers (upto a total of 100 layers) assembled on the Nafionmembranes were investigated for three different measurementsincluding film thickness, proton conductivities and water uptakerates. The surface morphology features of the LbL films werecharacterized using field emission scanning electron microscopy(Hitachi S-4700 SEM). Contact angles (CA) were measured usinga drop shape analysis system DSA100 (Kruss, Germany) in ambientconditions. The average CA value was obtained by measuring fivedifferent positions for each process.

2.3. Proton conductivity measurements of the biopolymeric film

The proton conductivity was measured using the normal fourprobe technique [1]. Prior to conductivity measurement, the

Fig. 1. Schematic showing the formation of biopolymeric LbL electrolyte membranes on NafioL-lysine as a positive layer and DNA from salmon testes as a negative layer.

membrane was transformed into proton form. The measuring cellconsisted of two stainless-steel current-carrying electrodes andtwo platinum wire potential-sensing electrodes (1 cm apart). Afully hydrated membrane (1 cm � 3 cm) was mounted into the cell.The impedance was determined using an Autolab PGSTAT 30 (EcoChemie, the Netherlands) over a frequency range from 1 MHz to50 Hz. The membrane resistance was then obtained from a Nyquistplot. Proton conductivity (k) was calculated according to thefollowing expression:

k ¼ LRWd

(1)

where R is the membrane resistance, L the distance betweenpotential-sensing electrodes, W and d are the width and thicknessof the membrane, respectively.

2.4. Swelling measurements of LbL films

The swelling behavior of the membrane was studied by wateruptake [9]. The membrane in proton form was soaked in distilledwater at room temperature for 24 h. The wet weight of themembrane was measured after removing excessive surface waterwith filter papers. The membrane was then dried at 50 �C undervacuum condition till a constant weight was obtained. The wateruptake was determined as the ratio of the absorbed water to drymembrane weight:

wwet=wdry (2)

in whichwwet andwdry were the wet and dry weight of the sample,respectively.

3. Results and discussions

An appropriate thickness of biophilic polymer for improvedproperties of Nafion was first optimized and a total 50 cycles oflayer by layer assembly of alternating poly-L-lysine and dsDNAlayers onto a negatively charged Nafion 212 shows the mostinteresting experimental observations (the profile of coating cyclesand the thickness onto Nafion are not shown). Fig. 2 showsa representative cross-sectional SEM image of biophilic polymermodified Nafion membrane where the LbL layer thickness is

n 212 film via electrostatically sequential adsorption of oppositely charged layers, poly-

Fig. 2. A representative cross-sectional SEM image of LbL biopolymeric layers onNafion membranes. A total biopolymeric layers of 50 were formed.

Fig. 3. Contact angle changes of (a) unmodified Nafion 212 and (b) LbL coated Nafion212 films. (c) A plot of changes in the contact angle of both membranes as a function ofapplying time after water droplets on the surface. A total biopolymeric layers of 50were self-assembled on the Nafion film.

H.J. Lee et al. / Current Applied Physics 12 (2012) 1235e1238 1237

estimated as w300 nm with an excellent uniformity. It looksalmost a skin like coating of Nafion membrane, but it has prom-ising results in the measurement of wettability, i.e., dimensionalstability.

Physiochemical properties of the skin like coating of the poly-L-lysine layer and dsDNA layer onto Nafion 212 mainly tested whilemaintaining their total thickness constant as about 300 nm (seeTable 1). Note that the modification of N212 with LbL coatingresulted in decreasing water uptake, whereas increasing protonconductivity. As water is essential to a proper transfer of proton,water uptake has been known to be proportional to protonconductivity. The higher conductivity of LbL modified Nafion 212could be originated from the fact that the positively charged poly-L-lysine neutralizes sulfuric acid groups on the surface of Nafionmembrane. The sulfuric acid groups in acidebase interactioncannot adsorb as many water molecules as free sulfuric acid groups[11]. Due to the smaller molecular weight of the lysine than Nafion,poly-L-lysine absorbs into water channel of Nafion, resulting in theneutralization. Therefore, it could be said that the acidebaseinteraction facilitates the transport of protons by the Grotthussmechanism [12].

In addition, the effect of LbL biopolymeric electrolyte layers onthe surface property of a Nafion membrane was investigated. Asshown in Fig. 3, the surface of the LbL-Nafion membrane wasmore hydrophilic than the original Nafion membrane due to thenature of hydrophilic poly-L-lysine and dsDNA. The hydrophilicsurface was reported to facilitate water management of protonexchange membrane fuel cells by enhancing back diffusion of theaccumulated water at the cathode side [10]. Since Nafionmembrane is based on perfluorinated polymer, the surfaceis relatively hydrophobic compared to hydrocarbon electrolytepolymers. Changes in contact angles of both as-received Nafionand LbL coated Nafion membranes become smaller with timebecause of continuous adsorption of water droplet into themembrane.

Table 1A summary of water uptake rate at 80 �C, proton conductivity and thickness of boththe as-received Nafion 212 and the LbL self-assembled poly-Lysine and dsDNA onthe Nafion membrane. H. J. Lee et al.

Membrane Water uptake (%) Proton conductivity (S/cm) Thickness (mm)

N212 21.39 0.086 60 � 2LbL-N212 16.30 0.097 60 � 2

Finally, the swelling degree of as-received and modifiedmembranes was investigated by means of measuring each volumechange before fully wet and after complete dry conditions. In Fig. 4,the LbL-Nafion membrane showed a relatively lower volumechange than that of the Nafion membrane, since LbL coating resultsin less swelling of the Nafion membrane by decreasing the numberof water bonded to sulfuric acid groups, as explained above. Thelower volume difference of modified Nafion membrane betweenwet and dry condition indicates the better dimensional stabilitythat is strongly associated with interfacial resistance betweencatalyst layer and the membrane in fuel cells. As a membrane isrepeatedly hydrated and dried in an operation of fuel cells, thedimensional change of a membrane causes delimitation from theelectrode. Therefore, the modification of Nafion membrane by LbL

Fig. 4. Normalized volume changes in both the as-received Nafion 212 and bio-polymeric LbL coated Nafion 212 with respect to different temperatures. Changingtemperature also modulates humidity level.

H.J. Lee et al. / Current Applied Physics 12 (2012) 1235e12381238

is expected to reduce the loss of energy dissipation of fuel cellperformance.

4. Conclusions

A thin and uniform layer by layer coating of the oppositelycharged biophilic polymer electrolyte layers onto Nafion 212membrane resulted in a better dimensional stability, i.e., lessswelling under different value of humidity in addition to a higherproton conductivity compared to that of as-received Nafionmembranes. We expect that LbL-Nafion in proton exchangemembrane fuel cells result in less problem of interface between thecatalyst layer and the membrane and the better performance ina long-term operation. Furthermore, we envision that the multi-layered biophilic LbL membrane in the absence of Nafion substratecan be used in a biocompatible liquid fuel cell as an alternative PEMin conjunctionwith vitamin-C and lactic acid as a fuel source, whichcan potentially utilized in main power sources of human implant-able medical devices.

Acknowledgment

This workwas supported by the New& Renewable Energy of theKorea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry ofKnowledge Economy (No.20093020030020-11-1-000).

Appendix. Supplementary data

Supplementary data related to this article can be found online atdoi:10.1016/j.cap.2012.01.015.

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