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Materials Science and Engineering B 177 (2012) 488– 493

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering B

j o ur nal homep age: www.elsev ier .com/ locate /mseb

hort communication

hotoluminescent polymer electrolyte based on agar and containing europiumicrate for electrochemical devices

. Limaa, E. Raphaelb, F. Sentaninb, L.C. Rodriguesa, R.A.S. Ferreirac, L.D. Carlosc, M.M. Silvaa,∗, A. Pawlickab

Centro de Química, Universidade do Minho, Gualtar, 4710-057 Braga, PortugalIQSC, Universidade de São Paulo, 13566-590 São Carlos, SP, BrazilDepartamento de Física, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal

r t i c l e i n f o

rticle history:eceived 29 June 2011eceived in revised form6 December 2011ccepted 6 February 2012vailable online 20 February 2012

eywords:atural polymershotoluminescence

a b s t r a c t

Dispersion of photoluminescent rare earth metal complexes in polymer matrices is of great interest due tothe possibility of avoiding the saturation of the photoluminescent signal. The possibility of using a naturalionic conducting polymer matrix was investigated in this study. Samples of agar-based electrolytes con-taining europium picrate were prepared and characterized by physical and chemical analyses. The FTIRspectra indicated strong interaction of agar O H and 3,6-anhydro-galactose C O groups with glyceroland europium picrate. The DSC analyses revealed no glass transition temperature of the samples in the−60 to 250 ◦C range. From the thermogravimetry (TG), a thermal stability of the samples of up to 180 ◦Cwas stated. The membranes were subjected to ionic conductivity measurement, which provided the val-ues of 2.6 × 10−6 S/cm for the samples with acetic acid and 1.6 × 10−5 S/cm for the samples without acetic

anthanide ions acid. Moreover, the temperature-dependent ionic conductivity measurements revealed both Arrheniusand VTF models of the conductivity depending on the sample. Surface visualization through scanningelectron microscopy (SEM) demonstrated good uniformity. The samples were also applied in small elec-trochromic devices and showed good electrochemical stability. The present work confirmed that thesematerials may perform as satisfactory multifunctional component layers in the field of electrochemicaldevices.

. Introduction

The most extensively investigated PEO (poly(ethylene oxide)-ased systems have included lithium salts [1], because of theoreseen applications in advanced primary and secondary batteriesnd electrochromic devices. Although less effort has been devotedo the investigation of polymer electrolytes (PEs) incorporating lan-hanide salts [2–6], these materials are attractive candidates for theabrication of optical devices, such as fiber amplifiers, phosphorsnd solid-state lasers.

The photoluminescent properties of the complexes of Eu3+ haveeen intensively investigated in the past years, principally in theystems where the rare earth metals are dispersed in organicr inorganic matrices [7–9]. The main purpose of the dispersionf europium salts in different matrices is to avoid the satura-

ion of luminescent intensity and also to improve thermal and

echanical stability of the material [7]. Moreover, complexes ofanthanide salts with organic ligands are preferred due to enhanced

∗ Corresponding author. Tel.: +351 253604058; fax: +351 253604382.E-mail address: nini@quimica.uminho.pt (M.M. Silva).

921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2012.02.004

© 2012 Elsevier B.V. All rights reserved.

luminescence as a consequence of ligand to metal energy transfermechanisms [10]. When an excited triplet state of the coordinatingligand overlaps a lanthanide excited electronic level, the lanthanideluminescence is effectively pumped by a large cross-section molec-ular absorbance rather than by the relatively weak lanthanideabsorbance. The luminescent behavior of europium ions in suit-able polymer hosts [11] has suggested that future developmentsmay lead to the multicomponent solid-state displays envisagedby Downing et al. [12], but with the added attraction of pro-cessability which the polymer host can provide. Among differenteuropium–polymer systems, those with poly(methyl methacry-lates) (PMMA) as well as with PEO can be mentioned [7,8,13,14].

The development of polymer electrolytes has a major impact onfuture applications in fields like optics, membranes, batteries andsensors [15,16]. Optical devices and, particularly, electrochromicsmart windows and labels are promising fields for the applica-tion of polymer electrolytes. Electrolyte loss, solvent evaporationand reaction with active electrode material are examples of prob-

lems associated with the use of liquid electrolytes in electrochromicdevices. The assembly of solid-state devices using polymeric elec-trolytes represents an advance in the sense that they improveseveral electrolyte characteristics and may also contribute with

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n adhesive function. Solid-state electrochemical devices presentmportant advantages when compared to conventional devices,uch as enhanced reliability and safety. Accordingly, the devel-pment of materials where luminescent centers are isolated ionsncorporated in either organic or inorganic cage-type hosts is anctive research theme in the area of luminescent materials.

Polymer electrolytes based on PEO and containing europiumons have also been developed [13]. These materials showedtrong luminescence with narrow line widths and surprisinglyong-emission lifetimes of 0.2–0.7 ms [11]. It was also stated thatonic conductivity results of PEO–europium picrate electrolyte [13]re very similar to those observed in the PEO–europium trifluo-omethanesulfonate [14] and PEO–europium perchlorate systems17].

Among different polymer electrolytes, those obtained from nat-ral polymers such as polysaccharides [18–20], proteins [21,22]nd DNA [23,24] have also been developed. Attention has beenocused on the application of natural macromolecules because ofheir biodegrability, low production cost as well as good physicalnd chemical properties. Natural polymers remain attractive pri-arily because they are economical, readily available, capable of

hemical modification and potentially degradable and compatibleue to their origin. It further relates to the use of such polymers iniological implants.

In this work, we report on ionic conducting membranes based ongar and containing europium picrate salt as a new ionic conductingnd photoluminescent system. Agar, similarly to starch, is a hetero-eneous mixture of two polysaccharides: agaropectin and agarose.lthough both polymers share the same galactose-based backbone,garopectin is heavily modified with acidic side-groups such as sul-ate and pyruvate, whereas agarose has a neutral charge. The maindvantage of agar is its capacity to form gels in substitution forelatin.

In the present paper, the influence on the ionic conductivity ofcetic acid content of the agar–europium picrate-based polymerlectrolytes has been studied by AC impedance spectroscopy.

The surface morphology of the films was obtained by SEM, andhe thermal stability and presence of thermal transitions by TG andSC, respectively.

The photoluminescent properties were investigated by pho-oemission spectroscopy. Finally, the best ionic conducting

embranes were applied and tested in small electrochromicevices with glass/ITO/WO3/electrolyte/CeO2–TiO2/ITO/glass con-guration.

. Experimental

Hydrated europium picrate was prepared through the reac-ion of europium carbonate (Aldrich, 99.9%) with picric acid (Fluka99.0%) using the procedure described by Silva and Smith [13]. Theesulting warm solution was filtered, reduced in volume and lefttanding in an ice bath to crystallize. The yellow needles obtainedere recrystallized from water and dried under vacuum over phos-horous pentoxide for 24 h and then stored in a preparative drybox.

Samples of AgarnEu(pic)3 were prepared by dispersion of 0.5 gf agar (Aldrich) in 30 mL of Milli-Q water and heated underagnetic stirring for a few minutes up to 100 ◦C for complete disso-

ution. Next, 0.5 g of glycerol (Himedia, 99.5%) as plasticizer, 0.5 g oformaldehyde (Fluka >98.0%) and 0.1 g of Eu(pic)3 (Sample 1) and.0 g of acetic acid (Sample 2) were added under magnetic stirring.he resulting solutions were then poured on Petri plates and left

o dry up for 48 h at 40 ◦C in order to form transparent membranesith the 150 nm of thickness.

The total ionic conductivity of the AgarnEu(pic)3 wasetermined by introducing an electrolyte disk between two

gineering B 177 (2012) 488– 493 489

10 mm-diameter ion-blocking gold electrodes (Good-fellow, >99.95%) to form a symmetrical cell. Theelectrode/AgarnEu(pic)3/electrode assembly was placed into asuitable constant volume support and installed in a Büchi TO51tube oven with a type K thermocouple located close to theelectrolyte disk in order to measure the sample temperature.Bulk conductivities of electrolyte samples were obtained withSchlumberger Solartron 1250 frequency response analyzer duringheating cycles between 25 and 100 ◦C and at approximately 7 ◦Cintervals.

Infrared spectra were measured by using an ATR-FTIR BOMEMMB 102 spectrophotometer. The films were placed into the holderdirectly in the IR laser beam. Spectra were recorded in a scanningrange from 650 to 4000 cm−1 at a resolution of 4 cm−1 and 16 scans.

Thermal analyses (DSC) were performed under a flowing argonatmosphere (30 mL/min) between −60 and 250 ◦C and at a heat-ing rate of 5 ◦C/min using a Mettler DSC 821e. Samples weretransferred to 40 �L aluminum cans with perforated lids within adry argon-filled glovebox. Thermogravimetric (TG) measurementswere obtained with Rheometric Scientific TG1000 thermobalanceoperating under a flowing argon atmosphere (50 mL/min) between30 and 700 ◦C and at a heating rate of 10 ◦C/min.

The structure of the film was examined by X-ray Rigaku Rotaflexdiffractometer, model RU200B, power of 50 kV/100 mA and CuK�

irradiation, speed of 2◦/min, in an angle range (2�) from 5◦ to 40◦,at room temperature.

SEM micrographs were obtained with LEO model 440.Emission spectra in steady-state mode were recorded at room-

temperature on a Fluorolog-3® 2-Triax, Horiba Scientific, witha modular double grating excitation spectrometer (fitted witha 1200 grooves/mm grating blazed at 330 nm) and a TRIAX 320single-emission monochromator (fitted with a 1200 grooves/mmgrating blazed at 500 nm, reciprocal linear density of 2.6 nm/mm),coupled to a R928 Hamamatsu photomultiplier, using the frontface acquisition mode. The excitation source was a 450 W Xe arclamp. The emission spectra were corrected for detection and opticalspectral response of the spectrofluorimeter.

2 cm2 electrochromic devices having the glass/ITO/WO3/agar–europium-based electrolyte/CeO2–TiO2/ITO/glassconfiguration were obtained by assembling the 2 pieces of coatedglasses. ITO/glass substrates were purchased from Delta Tech-nologies, WO3 [25] and CeO2–TiO2 [26] were obtained by sol–gelprocess and deposited by dip-coating method with the withdrawalrate of 6 cm/min and 20 cm/min, respectively. The WO3 thin filmswere heat-treated at 120 ◦C for 1 h and thin films of CeO2–TiO2were heat-treated at 450 ◦C for 15 min. Agar–europium-basedelectrolytes in the form of membranes were deposited ontoglass/ITO/WO3 coatings and 1 cm free space was left for theelectrical contact. Then the CeO2–TiO2/ITO/glass substrate waspressed onto the membrane in such a way that the two coatingsfaced each other inside the assembled window. A 1 cm wideCu-conducting tape (3 M) was glued to the free edge of eachsubstrate for electrical connection. The mounted cells were finallysealed with an insulator protective tape (3 M).

3. Results and discussion

In order to verify the europium picrate interaction with ionicconducting samples based on agar, FTIR measurements were per-formed. As shown in Fig. 1, the change of sample compositionpromotes a change in the principal bands of the polysaccharide.

Pure agar membrane (graph a in Fig. 1) evidenced a band at1642 cm−1 due to the water absorption and the band at 1370 cm−1

can be associated with ester sulfate. The C O ether bands appearat 1159 cm−1 and the bands at 1070 cm−1 and 930 cm−1 can be

490 E. Lima et al. / Materials Science and Engineering B 177 (2012) 488– 493

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Fig. 2. XDR on silicon wafer of agar–europium picrate with (red line) and without(black line) acetic acid. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

starts at 190 ◦C for Sample 2 and at 200 ◦C for Sample 1 (inset ofFig. 4), thus confirming the DSC measurements. The degradationprocess ends at 380 ◦C for both samples and continues slowly as the

ig. 1. FTIR analysis of agar (a), agar–glycerol (b), agar–glycerol–formaldehyde (c),nd AgarnEu(pic)3 without (d; Sample 1) and with (e; Sample 2) acetic acid mem-ranes.

ttributed to C O of 3,6-anhydro-galactose [27]. Moreover, the FTIRpectra show band at 3408 cm−1 attributed to O H vibration, whicharticipates in intra- and intermolecular bond formation, and theands at 2934 and 2887 cm−1 are characteristic to the asymmet-ic C H vibrations [28]. Glycerol and formaldehyde in the sampleromotes an increase of the polysaccharide alcohol group inter-ctions (graphs b and c in Fig. 1), with a shift to 3375 cm−1, i.e.,he lower wave number is observed for the O H band and alsohe appearance of a new band at 1048 cm−1 attributed to C O cane seen. Addition of europium picrate (graphs d and e in Fig. 1)hifts the O H band again to 3309 cm−1 and C O to 1028 cm−1

nd show a new band at 1260 cm−1 probably due to the acetalrosslinking. The presence of the bands at 1116–1130 cm−1 cane due to the agar–formaldehyde C O C crosslinking formation.imilarly to plasticized starch the shift of agar characteristic bandso lower frequency can be explained in terms of stronger interac-ion of polysaccharide with glycerol and/or europium picrate whenompared to intra- and inter-agar molecules [28,29].

Fig. 2 shows the typical X-ray diffraction patterns obtained forhe agar-based electrolyte measured at room temperature. In thisgure one can observe two broad peaks. The first one at 2� = 12.5◦

s small and the second 2� = 19.6◦ is big and large indicating thathe electrolyte has a semicrystalline structure with predominancef amorphous phase.

DSC traces of the AgarnEu(pic)3 are shown in Fig. 3. The DSCeasurements were performed in the −60 to 250 ◦C temperature

nterval and no glass transition temperature was detected withinhis range. Moreover agar-based samples without (Sample 1) andith (Sample 2) acetic acid show a large endothermic peak cen-

ered at 130 ◦C and 110 ◦C, respectively, which can be attributedo the disruption of acetal crosslinking formed in the reactionith formaldehyde [30]. Furthermore, these measurements reveal

20 ◦C temperature decrease of the agar-based sample after theddition of acetic acid, which support the hypothesis of acetalrosslinking disruption due to the catalytic properties of acetic acidesidues present in the sample. Above 200 ◦C both samples showhe beginning of a second endothermic transition, which can bessociated to the degradation process evidenced by thermogravi-etric analysis, illustrated in Fig. 4.

Fig. 4 shows the results of two agar-based samples with and

ithout acetic acid. From these results, a slight weight loss ofround 5% was recorded in between the room temperature andbout 100 ◦C, explained as the loss of some moisture always present

Fig. 3. DSC analysis of AgarnEu(pic)3-based electrolytes without (Sample 1) andwith (Sample 2) acetic acid.

in the sample [31]. Also in this case, the presence of glycerol,which is a hydrophilic substance, does not promote an increasedwater-absorption capacity of the film as observed for other natu-ral polymer electrolytes-based membranes, when compared withagar powder [19,32]. The sample degradation with 80% weight loss

Fig. 4. TGA of AgarnEu(pic)3-based electrolytes without (Sample 1) and with (Sam-ple 2) acetic acid.

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emperature is increased up to 700 ◦C. This second process is prob-bly due to the degradation of galactose units as already observedy Madera-Santana et al. [31]. The remaining residue was 10% forhe sample without acetic acid (Sample 1) and 15% with acetic acidSample 2), in the form of inorganic trash. These thermal analy-es are also different when compared with plasticized agar-basedlectrolytes and reinforce the supposition that both acetic acid anduropium picrate present in the samples can act as a catalyst in theeactions of decomposition [19].

The ionic conductivity as a function of temperature measure-ents of the AgarnEu(pic)3-based electrolytes are included in Fig. 5.

rom these measurements, it can be observed that the ionic con-uctivity values of the samples with europium picrate and withoutcetic acid (Sample 1) are of 1.6 × 10−5 S/cm, similar to the valuesbserved by Raphael et al. [19] for the samples containing 0.5 g ofcetic acid. At higher temperatures, thermal movement of polymerhain segments and the dissociation of salt would be improved,hich increases ionic conductivity. The increase in the conductiv-

ty with the temperature is interpreted as a hopping mechanismetween coordinating sites, local structural relaxation and seg-ental motions of the polymer salt complexes. The value of the

onductivity as a function of acetic acid concentration increasesecause the amount of ions in the polymer electrolyte increases,nd the energy barrier to the proton transport decreases, leadingo a decrease in the activation energy. However, in the presentase it seems that the addition of acetic acid to the samples pro-otes a decrease of the ionic conductivity values to 2.6 × 10−6 S/cm

Sample 2) at room temperature, which implies that the europiumalt contributes to the ionic movement and, consequently, to ioniconductivity of the sample. According to Majid and Arof [33], theonic conductivity in a polymer is generally linked to the numberf ions and the mobility of conducting species. From the Rice andoth model [34], the values of mobility and diffusion coefficientre related to the acid concentration of the samples. At high acidoncentration, the dipole interaction between the protons in theedium increases, which reduces the ion mobility and thus the

onductivity [35]. The ionic conductivity measurements as a func-ion of temperature were also performed aiming to analyze theossible mechanism of ionic conduction in these systems. Clearlyurther experiments employing other techniques, such as transfer-nce number measurements, are required to probe the transport

rocesses in the picrate systems.

From the obtained results, it can be stated that the sam-le with acetic acid (Sample 2) exhibits almost linear behavior,hich can be fitted with two lines crossing at 60 ◦C. The onset

gineering B 177 (2012) 488– 493 491

temperature of melting transition of the crystalline phase observedin DSC measurements (Fig. 3), may be associated with various pos-sible causes, where a sudden change in behavior was observed (at60 ◦C). Similar results were already reported for the samples of plas-ticized amylopectin rich starch [36]. The ionic conductivity valuesof these samples increase with temperature from 2.6 × 10−6 S/cmat 26 ◦C to 1.2 × 10−4 S/cm at 81 ◦C. The sample without aceticacid (Sample 1) exhibits non-linear variation of conductivity withtemperature, which is typical of polymer electrolytes with pre-dominantly amorphous morphology with lower activation energyfor ion transport. The ionic conductivity values of these samplesincrease from 1.6 × 10−5 S/cm at 21 ◦C to 4.6 × 10−4 S/cm at 100 ◦C.This behavior contrasts with that of semi-crystalline materialsbased on the PEO host matrix that often show two linear seg-ments with a change of gradient close to the melting temperatureof PEO spherulites. As the mechanism of ionic transport is directlydependent of the flexibility of the polymer chain, components thatincrease free volume may be expected to have a beneficial influ-ence on conductivity. A further increase in salt content, beyondthe salt composition associated with the electrolyte with the low-est Tg, results in an increase in the energy required for chainrotation and a decrease in the segment mobility. Restrictions inthe motion of the polymer chain segments, which are responsi-ble for ion transport, explain the sudden decrease in conductivity,a fairly common observation in solvent-free polymer electrolytesystems.

The results have also shown that the agar–europium picrate sys-tem has a similar behavior to that of other trivalent cation-basedsystem [37].

The SEM image of Fig. 6(a) shows that the agar host matrixdisplays a non-porous, tubular texture. The good homogeneitywithout any phase separation and very good surface uniformityof the samples can be observed in the SEM micrograph in Fig. 6 (b).Also, both agar–europium-based SPE samples were slightly yellowbut translucent and showed very good adhesion properties to glassand steel.

To analyze the photoluminescent properties of theagar–europium-based membranes, the emission spectra wereobtained (Fig. 7). The emission spectrum of the AgarnEu(pic)3(Sample 1) displays the Eu3+ intra-4f6 5D0 → 7F0–4 transitions.The high relative intensity of the 5D0 → 7F2 transition indicatesthat the Eu3+ ions occupy a low-symmetry local group withoutan inversion center. Comparison of these emission features withthose of the isolated Eu(pic)3 [38] leads to the conclusion thatthe energy and the full-width at half-maximum (fwhm) of the5D0 → 7F0–4 transitions are strongly altered, indicating an effectiveinteraction between the Eu3+ ions and the polymer host.

Typical cyclic voltammograms of an ECD measured during the500th and 1000th chronoamperometric cycles are shown in Fig. 8.One cathodic peak centered at −1.0 V is observed and is associatedto the color process due to the reduction of WO3 coating. The anodicpeak shape changes upon voltammetric cycling.

With up to 500 cycles, the anodic electrochemical response iscomposed by two peaks, in a way that one is large and its maxi-mum is centered at −0.9 V, whereas the second one is small andcentered at 1.0 V. The presence of this additional oxidation processcan be due to the electrolyte, as already observed for ECDs com-posed of WO3/gelatin–glycerol/CeO2–TiO2 [16] but different fromECD composed of WO3/starch–glycerol/CeO2–TiO2 [39]. The shapeof the waves change under cycling (tested up to 103 chronoamper-ometric cycles) and the cathodic peaks are slightly displaced to lessnegative values. Also from these measurements, a decrease of the

second anodic peak at 1.0 V and a better definition of the peak at0.75 V can be stated.

The charge density response measured by chronoamperometry(−2.5 V/+2.0 V; 15 s/15 s) of ECD containing solid electrolyte based

492 E. Lima et al. / Materials Science and Engineering B 177 (2012) 488– 493

Fig. 6. SEM pictures of the pure agar (a) and AgarnEu(pic)3 (Sample 1) (b) samples.

Fig. 7. Room temperature emission spectra of the AgarnEu(pic)3 (Sample 1) excitedat 330 nm.

Fig. 8. Cyclic voltammetry of electrochromic window with WO3/agar–europium-based electrolyte/CeO2–TiO2 configuration, 500th cycle (—) and 1000th cycle (- - -).

on agar and europium picrate for the cycle 500th and 1000th areshown in Fig. 9. From these measurements, it can be stated that theinsertion (coloration) is fast. For instance, for the 500th cycle, theinserted charge at −2.5 V reaches −2.0 mC in 15 s, and during thenext 500 cycles increases to −3.0 mC. The charge extraction andconsequent bleaching of the device occurs faster and the ECD isalready transparent after applying +2.0 V potential during 2 s. Dur-ing coloration (from light yellow to blue), a charge, in the form ofelectrons and europium picrate ions, is transferred from the ITO(170 nm) and polymer electrolyte layers (150 nm), respectively,to the WO3 layer (160 nm). Under these conditions, WO3 suffersreduction and simultaneous europium ions insertion. Upon revers-ing the polarity of the applied voltage, bleaching results (WO3 isoxidated and europium picrate is extracted) and the device recov-ers its initial state (from blue to light yellow).

This extraction is also faster than that obtained with ECDs con-taining starch-based electrolytes [39] and comparable to the ECDwith gelatin-based electrolyte [22].

In the present case, it seems that the kinetics of inser-tion/extraction processes is slower when compared with the resultsof ECDs containing starch plasticized with EG (Ethylene Glycol),

even though both types of ECDs were made with double-WO3-layerfilms [39].

A new family of environmentally friendly multifunctional mate-rials was prepared in the present work. At room temperature

Fig. 9. Charges densities for ECDs with electrolytes based on agar and europiumpicrate, 500th cycle (—) and cycle 1000th (- - -).

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he materials displayed simultaneously moderated conductivitynd moderated luminescence. The integration of different polymerlectrolyte based on natural macromolecules in a single device isf the utmost interest. This kind of device is expected to have annormous impact on information systems technologies, biomedicalngineering, structural engineering and environment monitoringystems in the near future. Sensitivity, specificity, ease of use, shelf-ife, cost, and recyclability are important requirements for these

aterials, based on natural macromolecules and europium ions40].

. Conclusions

The present work demonstrated that agar–europium-basedonic conducting membranes are very promising materials to bepplied in electrochemical devices. The obtained samples evi-enced strong interaction of agar O H and 3,6-anhydro-galactose

O groups with glycerol and europium picrate, no glass transi-ion temperature in the −60 to 250 ◦C range and thermal stabilityp to 180 ◦C. Moreover, the samples without acetic acid wereore ionic conductive (1.6 × 10−5 S/cm) than the samples with

cetic acid (2.6 × 10−6 S/cm), thus indicating the contribution ofuropium trivalent ions in conductivity properties of this kind ofel electrolyte. Both types of samples also showed an increasef ionic conductivity values as a function of temperature, inhich the value of the sample without acetic acid increased from

.6 × 10−5 S/cm at 21 ◦C to 4.6 × 10−4 S/cm at 100 ◦C and was ingreement with a VTF conducting type model. The emission resultsf the 5D0 → 7F0–4 transitions showed strong alteration, there-ore suggesting an effective interaction between the Eu3+ ions andhe polymer host. The samples applied in small electrochromicevices evidenced the reversible insertion/extraction process dur-

ng 1000 chronoamperometric cycles. The inserted charge was3 mC after 15 s and the extraction occurred in 2 s. Finally, it was

hown that the agar–europium-based electrolytes are promisingaterials to be employed in practical electrochemical devices. Poly-er electrolytes are flexible materials and thus able to provide

ood contact between the electrolytic component and the activelectrochromic layer. In addition, film deposition of large surfacereas is possible, while ensuring that the device is mechanicallyobust.

cknowledgements

The authors are pleased to acknowledge the financial sup-ort provided by the University of Minho and Fundac ão para

Ciência e Tecnologia (contracts PEst-C/QUI/UI0686/2011 andFRH/BD/38616/2007) for laboratory equipment and researchtaff. The authors are also indebted to FAPESP, CNPq andAPES for the financial support given to this research. M. M.ilva gratefully acknowledges the travel support from Fundac ãoriente.

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