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Electrochimica Acta 57 (2011) 112– 122

Contents lists available at ScienceDirect

Electrochimica Acta

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odide-conducting polymer electrolytes based on poly-ethylene glycol and MgI2:ynthesis and structural characterization

ichele Vittadelloa,∗ , David I. Waxmanb , Paul J. Siderisc , Zhehong Gand , Keti Vezzùe , Enrico Negrof ,hmad Safarib, Steve G. Greenbaumc, Vito Di Notof

PECS Department, Medgar Evers College and Graduate Center of the City University of New York, 1638 Bedford Avenue, Brooklyn, NY 11225, USADepartment of Materials Science and Engineering, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, NJ 08854-8065, USADepartment of Physics, Hunter College and Graduate Center of the City University of New York, 695 Park Avenue, New York, NY 10021, USANational High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, FL 32310, USADipartimento di Chimica, Università Ca’ Foscari di Venezia, Dorsoduro, Calle Larga S. Marta 2137, 30123 Venezia, ItalyDipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, 35131 Padova (Pd), Italy

r t i c l e i n f o

rticle history:eceived 17 December 2010eceived in revised form 27 July 2011ccepted 29 July 2011vailable online 5 August 2011

eywords:olymer electrolytesodide-conducting polymers

gI2

SSCslectrical spectroscopy

a b s t r a c t

A major obstacle for a viable technological development of dye sensitized solar cells (DSSCs) is stillthe synthesis of a high performance iodide-conducting polymer electrolyte. Here we present a seriesof eight electrolytic complexes with formula PEG1000/(MgI2)x(I2)y (0.0038 ≤ x ≤ 0.5801, 0 ≤ y ≤ 0.0636).The synthesis involves the preparation of a disordered form of MgI2 by a metallorganic route, whichenables us to dissolve high amounts of salt in the chosen polymer host. The thermal analysis of theresulting polymer electrolytes was performed using modulated differential scanning calorimetry mea-surements. Vibrational studies were carried out using medium FT-IR, far FT-IR and FT-Raman. Thevariation of the CO and OH stretching modes in the medium infrared, as a function of the mole-to-moleratio nMg/nO, was investigated by Gaussian decomposition to provide insight into the polymer–polymerand salt–polymer interactions in these materials. The FT-Raman spectra confirmed and complementedthe vibrational assignment. The conductivity study of these systems was performed by electrical spec-troscopy in the frequency interval 10 mHz–10 MHz. The direct current conductivity (�DC) profiles versusthe reciprocal temperature exhibited a Vögel-Tamman-Fülcher (VTF) behavior. The best �DC at 50 ◦C was

−5 −1

5 × 10 S cm . The overall results indicate the presence of bivalent, monovalent and neutral species,Mg2+, [MgI]+ and MgI2, respectively, which participate in the conduction process. These results are con-sistent with what was previously observed in PEG400-based systems doped with �-MgCl2. The presenceof at least one Mg site containing a distribution in parameters was observed using 25Mg solid state magicangle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The site has been assigned to a Mgcomplex involving the coordination by oxygen atoms of the polymer backbone.

. Introduction

Dye-sensitized solar cells (DSSCs) are photoelectrochemicalevices with the potential of offering a low-cost alternative toilicon-based and thin-film solar cells [1,2]. DSSCS typically consistf nanocrystalline anatase (TiO2) coated with a dye, a rutheniumomplex sensitizer, an electrolyte with I−/I3− redox couple, and

Pt-coated counter-electrode. Conversion efficiencies up to 12%ave been reported [3]. The quest for an iodide-conducting poly-

er electrolyte represents a crucial issue toward a widespread

se of the DSSC technology. It has been shown that polymer elec-rolytes do indeed have the potential to yield efficient solid-state

∗ Corresponding author. Tel.: +1 718 270 6442; fax: +1 718 270 6197.E-mail address: mvittadello@mec.cuny.edu (M. Vittadello).

013-4686/$ – see front matter. Published by Elsevier Ltd.oi:10.1016/j.electacta.2011.07.133

Published by Elsevier Ltd.

dye-sensitized photovoltaic devices [4]. Most of the researchhas been devoted to iodide-conducting gel electrolytes [5]. Gelelectrolytes systems usually afford a higher conductivity, but alsopresent the drawback of potential solvent leakage issues. Forinstance, quasi-solid state dye-sensitized TiO2 solar cells havebeen proposed, based on gel electrolytes containing iodide andiodine impregnated in mesoporous TiO2 [6]. It was shown thatthe formation of less-mobile polyiodide ions in the electrolyteincreased the conductivity in the mesoporous phase. This behaviorwas rationalized as due to the Grotthuss-type electron exchangemechanism caused by packed polyiodide species in the electrolyte.

An alternative strategy to improve the conductivity has

been the synthesis of nanocomposite polymer electrolytes. Forinstance, iodide-conducting poly(ethylene oxide)/titania polymerelectrolytes were synthesized [7]. It was shown that the intro-duction of the titania filler into the poly(ethylene oxide) matrix

chimica Acta 57 (2011) 112– 122 113

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Table 1Composition of the PEG1000/(�-MgI2)x(I2)y polymer electrolytes.

Sample %MgI2 nMg/nOa nI2 /nO

b xa yb

PEG1000 0.0000 0.0000 0.0000 0.0000 0.00007 0.1043 0.0002 0.0027 0.0038 0.06366 0.3020 0.0005 0.0026 0.0111 0.06055 0.5536 0.0009 0.0026 0.0203 0.05954 1.1393 0.0018 0.0023 0.0420 0.05343 2.5874 0.0041 0.0019 0.0966 0.04452 4.3495 0.0071 0.0013 0.1647 0.02951 7.8937 0.0133 0.0003 0.3088 0.00780 13.7025 0.0243 0.0000 0.5668 0.0000

a Determined on the basis of ICP-AES measurements of Mg and the amount of

M. Vittadello et al. / Electro

educes the crystallinity of the polymer and enhances the mobilityf the I−/I3− redox couple. The DSSCs fabricated with these materi-ls, Ru(dcbpy)2(NCS)2 complex as the sensitizer and nanoporousiO2 films as photoanodes, resulted in outstanding overall con-ersion efficiency (4.2% under direct sunlight illumination), one ofhe best efficiencies reported to date for an all solid-state device.nother example is given by alkyloxy-imidazolium iodide ionicolymer/SiO2 nanocomposite electrolyte [8]. By optimizing theontent of I2, 1,2-dimethyl-3-propylimidazolium iodide, and SiO2anoparticles in the electrolyte, a considerable ionic conductivityf 1.51 × 10−3 S cm−1 was achieved due to the formation of high-fficiency electron exchange tunnels.

The influence of different iodide salts on the per-ormance of dye-sensitized solar cells containingon-volatile electrolytes was investigated in phosphazene-ased electrolytic complexes [9]. Electrolytes based onexa[methoxyethoxyethoxycyclotriphosphazene] (MEE trimer)ith dissolved LiI, NaI, NH4I, and 1-methyl-3-propylimidazolium

PMII) and I2 were examined. The anionic conductivitiesere highest in the PMII system and decreased in the order

MII > NH4I > NaI > LiI. The effect of cation size on solid polymerlectrolytes for DSSCs has also been investigated. Solid poly-er electrolyte comprised of a PEO:PEG blend and LiI, NaI, KI,H4I, 1-ethyl-3-methylimidazolium iodide (EMII), and 1-hexyl-3-ethylimidazolium iodide (HMII) with the same ion-to-monomer

atio were investigated [10]. It was shown that the cell perfor-ance of DSSCs fabricated with these systems showed strong

ependence on the cation radii.To date, the research carried out in iodide-conducting polymer

lectrolytes has been limited mostly to monovalent iodide salts.n general, the possibility of preparing PEO-based polymer elec-rolytes with a MX2 salt (M = alkaline earth metal, X = halide) hadeen previously ruled out in the literature. After that, the synthesisf PEG-based MgCl2 electrolytes with high Mg-ion conductivity washown to be possible [11]. The breakthrough was due to the use of

highly disordered special form of MgCl2 (�-MgCl2) [11–13]. It washown that the conduction mechanism of these systems involvesivalent, monovalent and neutral species (Mg2+, [MgCl]+, MgCl2)14].

In this paper we propose the synthesis of a new polymer elec-rolyte family with general formula PEG1000/(MgI2)x(I2)y. These

aterials were characterized by thermal analysis, vibrational spec-roscopy, electrical spectroscopy and solid state NMR.

. Experimental

.1. Reagents

Metallic magnesium, anhydrous 1-iodobutane and PEG1000ere Aldrich reagent grade. PEG1000 was stirred under vacuum for

week at 10−3 mbar at 90 ◦C to ensure dehydration. All the othereagents were used as received. The manipulation and the storagef the samples were carried out under an argon atmosphere.

.2. Synthesis of ı-MgI2

Excess anhydrous 1-iodobutane (∼500 g) was added to 32.5 g ofg metal, and the mixture was refluxed for 72 h. After refluxing,

he residual solvent was removed and the salt was dried at 90 ◦Cnder vacuum for 120 h. The identity of the salt was verified byowder X-ray diffraction measurements (data not shown)..

.3. Synthesis of PEG1000/(ı-MgI2)x(I2)y polymer electrolytes

In a 250 mL Pyrex bottle, 7.9714 g MgI2 was added to 58.8320 gf PEG1000. The mixture was heated to 90 ◦C and stirred until

PEG1000 used in the synthesis.b Determined by difference on the basis of the amount of PEG1000 used in the

synthesis and Mg ICP-AES analysis.

complete dissolution of MgI2. By diluting this mother solution withPEG1000, eight electrolytic complexes with decreasing concentra-tion of MgI2 were obtained. Molecular iodine (I2) was subsequentlyadded to each solution and mixed at 90 ◦C. The resulting solidsolutions were dark red in color. Mg content in the samples wasdetermined by ICP-AES using the method of standard additions andthe emission line at 279.079 nm. The argon reference line was set at404.305 nm. A Spectroflame Modula sequential and simultaneousICP-AES spectrometer equipped with a capillary cross-flow nebu-lizer was used (Spectro Analytical, Kleve, Germany). Mineralizationof PEG1000/(�-MgI2)x(I2)y polymer electrolytes was carried outby treating ca. 15 mg of each sample with a 3 wt.% water solutionof HCl. The composition of each of the PEG1000/(�-MgI2)x(I2)y

polymer electrolytes thus prepared is reported in Table 1.

2.4. Instruments and methods

2.4.1. Thermal analysisModulated differential scanning calorimetry (MDSC) measure-

ments were carried out with a MDSC 2920 Differential ScanningCalorimeter (TA Instruments) equipped with the LNCA low-temperature attachment operating under a helium flux of 30cm3·min-1. Measurements were collected with a heating rate of3 ◦C min−1 in the −60 < T < 60 ◦C temperature range on ca. 7 mg ofsample hermetically sealed in an aluminum pan under a dry Aratmosphere in a glove box.

2.4.2. Infrared spectroscopyFT-IR measurements in both the medium (MIR) and in the far-IR

(FIR) were collected in transmission mode in a Nicolet FT-IR Nexusspectrometer equipped with a triglycine sulphate (TGS) detectorat a resolution of 4 cm−1. MIR measurements were derived byaveraging 500 scans, after sandwiching the samples between twoKBr windows. FIR spectra were derived by averaging 400 scans;measurements were carried out on samples spread as thin filmsbetween two polyethylene windows. The samples for both MIR andFIR measurements were placed in a cell hermetically closed underargon. Baseline correction was performed with Nicolet FT-IR Nexusspectrometer software.

2.4.3. Electrical measurementsThe measurement of complex conductivity spectra was carried

out in the frequency range 10 mHz–10 MHz, using a NovocontrolAlpha-A analyzer. The temperature range from 20 to 90 ◦C wasexplored by using a home-made cryostat operating with a N2 gasjet heating and cooling system. The measurements were carried out

using a closed home-made cell. The geometrical constant of the cellwas determined by measuring the electrode–electrolyte contactsurface and the distance between electrodes with a micrometer. Nocorrections for thermal expansion of the cell were carried out. The

114 M. Vittadello et al. / Electrochimic

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cheme 1. (a) Synthesis of �-MgI2. (b) Synthesis of the PEG1000/(MgI2)x(I2)y poly-er electrolytes.

emperature was measured with an accuracy higher than ±0.05 ◦C.he complex impedance Z*(ω) = Z′(ω) + iZ′′(ω) was converted intohe complex permittivity spectra (ε*(ω) = ε′(ω) − iε′′(ω)), where

= 2�f (f is the frequency in Hz).

.4.4. NMR measurements25Mg MAS NMR experiments were performed at the National

igh Magnetic Field Laboratory at 19.6 T (�Mg0 = 55.99 MHz) using

single channel home-built 4 mm probe. The radiofrequency fieldtrength used for all measurements was approximately 42 kHz.wo samples were run:pure MgI2 (Aldrich 98%) and a polymer elec-rolyte (sample 0, see Table 1). A total of 32 and 819,200 transientsere collected for MgI2 and sample 0, respectively. 25Mg data forgI2 was acquired using a standard single-pulse sequence, a 1 s

ecycle delay, and a spinning speed of 10 kHz. A rotor-synchronizedahn echo sequence was used for sample 0. The polymer elec-

rolyte was spun at 5 kHz. A single rotor period was used forhe delay between the two pulses. The recycle time was 0.5 s.he samples were referenced to an aqueous solution of MgSO4 at

ppm. An exponential smoothing function was applied to the sig-als (LB = 20 Hz and 200 Hz for MgI2 and sample 0, respectively)rior to Fourier Transformation. NMR simulations were performedsing the WinSolids software package. The simulated spectrum wasmoothed using a LB = 350 Hz.

. Results and discussion

.1. Syntheses of ı-MgI2 and PEG1000/(MgI2)x(I2)y polymerlectrolytes

The overarching goal of this paper is to explore the use of alka-ine earth halides in the synthesis of iodide-conducting polymerlectrolytes. A highly amorphous form of MgI2 was synthesizedrom the metal and excess iodobutane (Scheme 1a). This salt haseen called “�-MgI2” by analogy with �-MgCl2, previously synthe-ized from Mg metal and chlorobutane [11,12]. The strategy ofsing �-MgI2 was undertaken because it proved to be successful

n the synthesis of Mg-ion conducting polymer electrolytes suchs PEG400/(MgCl2)x, based on PEG400 and �-MgCl2. While in thease of �-MgCl2, the amorphous nature of this salt was clearlyevealed by broadened peaks in XRD spectra, no clear differenceas observed between the commercial MgI2 and �-MgI2 (data not

hown). However it is also a fact that the dissolution of the salt inEG1000 (Scheme 1b) yields a MgI2 percentage as high as 13.7% ineight. It needs to be recalled that the possibility to dissolve alka-

ine earth halides in high quantities had been ruled out in earlier

iterature [11]. Further studies are necessary to clarify the naturef �-MgI2, which will be carried out in the future.

The choice of PEG1000 as the polymer host was motivated byhe consideration that in a typical DSSC a temperature of at least

a Acta 57 (2011) 112– 122

40 ◦C is expected. In case of a leak a polymer electrolyte that isbased on a polymer which melts at about 40 ◦C would readily solid-ify and minimize toxic effects on the environment. FurthermorePEG1000 is a hydroxyl-terminated polyethylene oxide oligomerendowed with good salt-solvation properties and known vibra-tional spectra characteristics (as shown below). Our original planwas to prepare a series of polymer electrolytes with general formulaPEG1000/(MgI2)x(I2)y, having a decreasing amount of MgI2 and aconstant amount of I2. Unfortunately, the presence of trace quanti-ties of Mg in �-MgI2 produced an undesired variation of moleculariodine. This can be explained by the reaction between Mg tracesand I2 to yield quantitatively MgI2, especially in the samples at thehigher concentration of salt. The final non-constant value of I2 canbe seen in Table 1. We should mention that the compositional anal-ysis presented some challenges. The analysis of the amount of Mgwas straightforward but a direct determination of the iodine con-tent proved to be unreliable. It is well known that iodine is volatileand the loss of iodine from the mineralized samples was unavoid-able. Indirectly, the volatility of iodine affected also the accuracyof the elemental analysis of C and H. Therefore we decided to relyon the ICP analysis of Mg and the PEG1000 experimental quantitiesused in the synthesis, in order to determine the iodine content bydifference (Table 1). As shown below, it turns out that the fact thatI2 is not constant is actually an advantage in the interpretation ofthe equivalent conductivity spectra of the proposed materials (seebelow).

3.2. Thermal analysis of PEG1000/(MgI2)x(I2)y polymerelectrolytes

In Fig. 1 the MDSC profiles of the electrolytesPEG1000/(MgI2)x(I2)y are shown. The main spectral featurethat is visible is the endothermic peak at about 40 ◦C. This peakwas attributed to the melting (Tm) of PEG1000. The overall area ofthe peak was used for calculating the crystallinity of PEG1000 inthe samples. This was possible on the basis of the tabulated valuefor the enthalpy of fusion per number of moles of ethyleneoxydicunits for 100% crystalline PEO (polyethyleneoxide), which is equalto 9.41 kJ [14]. The results of the analysis, carried out as previouslyreported, are presented in Fig. 2. The glass transition temperature(Tg) of the materials could be measured by slowly collecting thespectra below 0 ◦C (data not shown). The Tg of pristine PEG1000was about −44 ◦C. Minor variations of 1–2 ◦C occurred withincreasing salt concentration.

3.3. Vibrational characterization of PEG1000/(MgI2)x(I2)y

polymer electrolytes

The far FT-IR spectra (FIR) of the PEG1000/(MgI2)x(I2)y areshown in Fig. 3. The far FT-IR spectra and the FT-Raman spectra areshown in Figs. 4 and 5, respectively. The full assignment of the MIR,FIR and Raman spectra is given in Table 2. All vibrational spectra arein agreement with those of other PEG-based polymer electrolytespreviously studied in our group [15,16]. The inspection of the MIRand Raman “fingerprint” region (700–1500 cm−1) allowed us tostudy the conformational morphology of the polyether chains inPEG1000. Doping of PEG1000 with I2 and �-MgI2 did not alter thesecondary structure of the polyether chains of PEG1000. The con-formation is of a TGT type (T = trans; G = gauche) and the vibrationalassignment is possible under the group D(4�/7) [15,16]. Main char-acteristic vibrations can be immediately recognized: CO stretchingbands between 800 and 1500 cm−1, CH stretching bands between

2700 and 3000 cm−1 and OH stretching bands between 3000 and3700 cm−1.

Doping of PEG1000 with I2 and �-MgI2 affects the CO andOH stretching regions. The FT-Raman spectra of the electrolytic

M. Vittadello et al. / Electrochimica Acta 57 (2011) 112– 122 115

-3.0

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nMg/nO: 0.0000nI2

/nO: 0.0000

0.00020.0027

0.00050.0026

0.00090.0026

0.00180.0023

0.00420.0019

0.00710.0013

0.01330.0001

0.02430.0000

camFtaa

Fr

4000350030002500200015001000500

Wavenumber / cm-1

nMg/nO: 0.0 000nI2

/nO: 0.0000

0.00020.0027

0.00050.0026

0.00090.0026

0.00180.0023

0.00420.0019

0.00710.0013

0.01330.0003

0.02430.0000

Abs

orbt

ion

/ a.u

.

Temperature / °C

Fig. 1. MDSC profiles of PEG1000/(MgI2)x(I2)y polymer electrolytes.

omplexes, as shown in Fig. 5, confirm the tentative assignmentsnd complete the vibrational analysis. Information about theetal-to-ligand vibrations in the samples was provided by the

IR spectra, as shown in Fig. 4. CCO and COC bending vibra-

ions and CC and CO rotations were detected. Symmetric andnti-symmetric Mg–I stretching vibrations were identifiedlong with Mg–O stretching vibrations. These results reveal the

220

200

180

160

140

120

100

80

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20x10-3151050nMg/nO

100

80

60

40

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G1000 crystallinity (%

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ig. 2. Enthalpy of fusion per gram of PEG1000 and PEG1000 crystallinity withespect to nMg/nO of PEG1000/(MgI2)x(I2)y polymer electrolytes.

Fig. 3. Medium FT-IR absorption spectra of PEG1000/(MgI2)x(I2)y polymer elec-trolytes.

coordination of the possible Mg species, namely Mg2+, [MgI]+,MgI2, by the oxygen of the polyether moieties. This was expectedby analogy with what observed in polyether-based polymersdoped with MgCl2. No clear vibrational proof of the presence of I3−

anionic species was found at 110 cm−1. I3− is most likely presentin these systems but its vibrational signature cannot be resolvedfrom other vibrations.

The salt–polymer interactions were further studied by Gaus-sian decomposition of the MIR CO and OH regions, as shown inFigs. 6 and 7. The changes in the percentage area and wavenum-ber of the individual OH and CO peaks with respect to the molarratio nMg/nO are shown in Figs. 8 and 9. The observed increaseor decrease in wavenumber with nMg/nO can be related to thechange in electronic density among the vibrating atoms caused bythe salt–polymer and polymer–polymer interactions, as previouslydemonstrated. These changes are useful in the vibrational assign-ment as previously reported. The Gaussian analysis confirms theassignment given in Table 2. In the stretching OH region the sym-metric and anti-symmetric OH vibration were detected at 3332and 3491 cm−1, respectively [16]. The vibration at 3121 cm−1 wasattributed to stretching of PEG1000 OH groups which are involvedin hydrogen-bonding cages with other OH groups and PEG1000

OH groups interacting with the anions I− or I3− [�Hy(OH)]. It canbe seen that while both the symmetric and anti-symmetric OHdecrease with increasing nMg/nO ratio, the intensity of �Hy(OH)increases. The crossing occurs at about nMg/nO = 0.0133. In the

116 M. Vittadello et al. / Electrochimica Acta 57 (2011) 112– 122

Table 2FT-IR and Raman band assignments for PEG1000/(�-MgI2)x(I2)y polymer electrolytes.a.

(A) Polyether vib rations

(B) Inorganic vibrati ons

Observed wavenumbers (cm-1)Calculated d Assign ment and pot ential energy distribution d, e Reference s

Speciesb IR c Raman c

3491 (vw) 3444 νa(OH) [11 -18]3332 (m) 3269 νs(OH) [11 -18]3121 (w) 311 4 νHy(OH) [11 -18]

A1 inac tive 2941 (s sh) 2940 νs(CH2) (10 1) [11 -18]2887 (vs) 2874 νs(CH2) (10 1) [11 -18]

inac tive 1482 (m) 1479 sr( CH2) (98) [11 -18]inac tive 1259 (vw) 1252 t(CH2) (81) [11 -18]inac tive 887 (vw) 866 ν(CO) (50 ), r( CH2) (46) [11 -18]inac tive 274 δ(CC O) (24 ), ϕi(CC ) (19), δ(COC) (17) [11 -18]

A2 2871 (s) inac tive 288 3 νs(CH2) (10 0) [11 -18]1454 (w), 1472 (w) inac tive 1470 sr( CH2) (10 0) [11 -18]

1326 (vw sh),1350 (m) inac tive 1344 w(CH2) (10 7) [11 -18]1251 (m) inac tive 1264 t(CH2) (81) [11 -18]1088 (vs) inac tive 1087 ν(CO) (94) [11 -18]996 (vw) inac tive 964 r( CH2) (49), t(CH2) (18) [11 -18]

511 (vw sh), 530 (w) 533 δ(CC O) (89 ), r( CH2) (33) [11 -18]

E1 2931 (s sh) 2941 (s sh) { 2943 νa(CH2) (101 , -16 2°)[11 -18]

2940 νa(CH2) (101 , 18 °)

2832 (w) 2694 (vw), 2796 (vw), 2752 (vw), 2856 (s sh) 2873 νs(CH2) (100 , -5°) [11 -18]

1487 (vw sh) 1448 (w sh) 1471 sr( CH2) (100 , -13 2°) [11 -18]1438 (vw sh) 1396 (vw) 1401 w(CH2) (95 , 53 °) [11 -18]1359 (w sh) 1363 (vw) 1353 w(CH2) (107 , -12 9°) [11 -18]

1285 (w), 1303 (w) 1280 (m) 1286 t(CH2) (73 , -40 °) [11 -18]1237 (w) 1234 (w) 1234 t(CH2) (87 , 146 °) [11 -18]1148 (w) 11 42 (w) 1142 ν(CO) (37 , -27 °), r( CH2) (29 , -66 °) [11 -18]1126 (s) 111 2 ν(CO) (81 , 136 °), ν(CC ) (21) [11 -18]

1036 (m sh) 1065 (w) 1060 ν(CO) (36 , -102 °), r(CH2) (35 , 70 °), ?(CC ) (17) [11 -18]

1063 (vw) ν(C-O-I)935 (m), 953 (m) 935 (vw) 941 r(CH2) (34 , 42 °), ν(CC ) (27 ), ν(CO) (14 , 167 °) [11 -18]

806 (w), 844 (m), 871 (w) 845 (m), 804 (vw) 847 r(CH2) (58 , -149°), ν(CO) (39, -9°) [11 -18]

511 (w) 536 (vw) 524 δ(CC O) (49 , -142 °), δ(COC) (21 ), r(CH2) (17 , 152 °) [11 -18]

301 (vw) 366 δ(CC O) (42 , -73 °), δ(COC) (38) [11 -18]108 (w), 168 (w) 164 ϕi(CC ) (42 ), ϕi(CO) (38 , -161 °), δ(CC O) (18, 16 °) [11 -18]

Observed wavenumbers (cm-1)Assign mente Reference s

IR c Raman c

351 (vw), 391 (vw), 426 (w) ν(Mg-O) [11 -16]301 (w) νa(Mg-I) [11 -16]

137 (vw) νs(Mg-I) [11 -16]

aPEG1000 in the polymer electrolytic complex was assumed under the D(4�/7) symmetry group.bSymmetry species.cRelative intensities of observed bands are reported in parentheses; vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.d m thee nal ro

ssw((ambt

Calculated wavenumber values of the normal coordinate analysis were derived fro�, stretching; ı, bending; w, wagging, t, twisting, r, rocking; sr, scissoring; ϕi , inter

tretching CO region the vibrations that are most affected by thealt–polymer interactions are the bands at 1126 and 1088 cm−1,hich were attributed to [�(CO) (81, 136◦), �(CC) (21)] and [�(CO)

94)], respectively. The former vibration [�(CO) (81, 136◦), �(CC)21)] is diagnostic of the breaking and forming of hydrogen bonds

mong PEG1000 molecules, the latter vibration is indicative of ter-inal CO groups that are influenced by anion coordination. It can

e seen that the crossing between the percentage area of thesewo bands occurs between nMg/nO = 0.0018 and nMg/nO = 0.0041.

references reported in the last column.tations; a, antisymmetric mode; s, symmetric.

The difference in crossing point of the OH and CO vibrations indi-cates that these functional groups have a different role in thedissolution of the salt at different salt concentrations. This lackof syncronism with respect to the metal-to-oxygen molar ratiois in contrast with the synchronism observed in other systems

such as PEG400/(LiCl)x [16], PEG400/(MgCl2)x [11], and PEG400-alt-DEOS/(MgCl2)x [17,18]. Both OH and CO vibrations measuredfor the PEG1000/(MgI2)x(I2)y complexes have a direct correspon-dence with those detected in the polymer electrolytes mentioned

M. Vittadello et al. / Electrochimica Acta 57 (2011) 112– 122 117

Abs

orpt

ion

/ a.u

.

60055050045040035030025020015010050

Wavenumber / cm-1

nMg /nO: 0. 0000nI2

/nO: 0.0000

0.00020.0027

0.00050.0026

0.00090.0026

0.00180.0023

0.00420.0019

0.007 10.001 3

0.013 30.000 3

0.024 30.000 0

Ft

attCtniCc

3e

0tE[

ε

wtrep

Inte

nsity

/ a.

u.

350030002500200015001000500-1

nMg/nO: 0.0000nI2

/nO: 0.0 000

0.00020.0027

0.00050.0026

0.00090.0026

0.00180.0023

0.00420.0019

0.00710.0013

0.01330.0003

0.02430.0000

a

ig. 4. Far FT-IR absorption spectra of the PEG1000/(MgI2)x(I2)y polymer elec-rolytes.

bove. However, there is one exception. It should be mentionedhat the band detected at 1063 cm−1 was attributed to �(C–O–I). Wehink that a small fraction of molecular iodine reacts with terminal–O–H groups forming C–O–I groups. It is interesting to notice thathe percentage area of this vibration decreases slightly with theMg/nO ratio. This decrease in intensity is accompanied by a small

ncrease in wavenumber. This latter observation indicates that the–O–I functional groups are involved in the coordination of theationic species Mg2+, and [MgI]+.

.4. Electrical characterization of PEG1000/(MgI2)x(I2)y polymerlectrolytes

The polymer electrolytes PEG1000/(MgI2)x(I2)y with ≤ x ≤ 0.5801, 0 ≤ y ≤ 0.0636 were analyzed by electric spec-roscopy between 0.1 Hz and 1 MHz, 293.15 K and 363.15 K.lectrical spectra were analyzed by using the following equation19]:

∗(ω) = ε∞ +∑

k

�εk[1 + (iω�k)˛k

]ˇk+

∑n

�el,n

iω

(iω�el,n)n

[1 + (iω�el,n)n

] , (1)

here ε∞ is the permittivity of the material, the second and third

erms in the equation describe the contributions of the dielectricelaxations and of electrodic and interfacial polarization phenom-na, respectively. �εk is the relaxation strength, ˛k and ˇk are shapearameters describing the symmetric and asymmetric broadening

Wavenumber / cm

Fig. 5. FT-Raman spectra of PEG1000/(MgI2)x(I2)y polymer electrolytes.

of the kth relaxation peak, �k = 1/2fk is the dielectric relaxationtime (with fk in Hz the frequency of the kth peak position), �el,nis the conductivity associated to the relaxation �el,n, the relaxationtime associated with the electrode polarization phenomena.

Spectroscopic data were fitted in the following representations:(a) real dielectric spectrum (ε′ versus f) and (b) imaginary dielec-tric spectrum (ε′′ versus f). Selected spectra are shown in Fig. 10.One electrodic polarization term (�el) and two-to-three dielectricrelaxation terms (�k) were used for the spectra measured belowthe melting point (T < Tm) of the polymer electrolytes. Up to 3polarization terms and 2 dielectric relaxation terms were used forT > Tm. The presence of more polarization terms is evident from theimpedance spectra representation, which shows two semicircles(data not shown).

In Fig. 11 the profiles of �el,n (n = 1, 2, 3) for thePEG1000/(MgI2)x(I2)y polymer electrolytes log � versus recip-rocal temperature are shown. Three temperature regions wereidentified: the first (I) at low T (T < Tm), the second (II) for T ∼ Tm,and the third for T > Tm. In the first two regions the fitting wasperformed by using the following equation [20]:

ln(�el,n(T)) = ln(A�el,n) + Ea

RT(2)

while for T > Tm through the Vögel-Tamman-Fülcher (VTF) [21]:[ −E

]

�el,n(T) = A�el,n

T−1/2expR(T − T0)

(3)

where A�el,nis proportional to the number of charge carriers, R

is the ideal gas constant, Ea is the pseudo-activation energy for

118 M. Vittadello et al. / Electrochimica Acta 57 (2011) 112– 122

4x10-3

3

2

1

0

Abs

orpt

ion

/ a.u

.

3800360034003200300028002600

Wavenumber / cm-1

Wavenumber / cm-1

Wavenumber / cm-1

-200

0

200

x10-6

Abs

orpt

ion

/ a.u

.x1

0-6A

bsor

ptio

n / a

.u.

x10-6

5x10-3

4

3

2

1

0

3800360034003200300028002600

-200

0

200

5x10 -3

4

3

2

1

0

3800360034003200300028002600

-200

0

200

a

b

c

F2

ctees

ar

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

Abs

orpt

ion

/ a.u

.

140012001000800600400

Wavenumber / cm-1

Wavenumber / cm-1

Wavenumber / cm-1

-0.02

0.00

0.02

2.0

1.5

1.0

0.5

0.0

-0.5

Abs

orpt

ion

/ a.u

.

140012001000800600400

-0.04

0.00

0.04

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

Abs

orpt

ion

/ a.u

.

140012001000800600400

-0.05

0.00

0.05

a

b

c

ig. 6. Decomposition by Gaussian functions of the FT-IR spectral region450–3800 cm−1 measured on selected PEG1000/(MgI2)x(I2)y polymer electrolytes.

onduction and T0 is the ideal thermodynamic glass transitionemperature. The initial value for T0 was determined from thexperimental value of Tg measured by thermal analysis, using thequation T0 = Tg − 50 ◦C. The Tg of all the materials does not change

ignificantly in the polymer electrolytes and is equal to −44 ± 2 ◦C.

It can be observed that the conductivity increases significantlybove the onset of the melting of the polymer which occurs withinegion II (between 30 and 40 ◦C, see Fig. 2). The highest value among

Fig. 7. Decomposition by Gaussian functions of the FT-IR spectral region400–1540 cm−1 measured on selected PEG1000/(MgI2)x(I2)y polymer electrolytes.

the �el,n is in most cases �el,1. This indicates that �el,1 is mostlyresponsible for the overall direct current conductivity �DC. There-fore, in the following we reasonably assume that �el,1 = �DC. Thepresence of three types of interfacial conductivities �el,n (n = 1, 2,

3) indicates that there are three types of interfaces in the systems.This implies the existence of two types of domains in the materialsunder investigation: domains A and B. The conduction can occurwithin domain A (corresponding to �el,2), within domain B (cor-

M. Vittadello et al. / Electrochimica Acta 57 (2011) 112– 122 119

80 80

60 60

40 40

20 20

0 0

(Ai/Σ

A i)*10

0

20x10-3151050nMg/nO

3121 cm-1

3332 cm-1

3491 cm-1

fit

3500 3500

3400 3400

3300 3300

3200 3200

3100 3100

Wav

enum

ber /

cm

-1

20x10-3151050

nMg/nO

3121 cm-1

3332 cm-1

3491 cm-1

fit

a

b

Fig. 8. Dependence on nMg/nO molar ratio of the fractional percentage (%)i of eachOH band revealed by the OH stretching region (Fig. 6). The semiquantitative valuesof the percentages were determined with the formula (%)i = Ai100/˙, where Ai is theband area of peak i-th, ̇ =

∑n

i=1Ai , and n is the number of bands considered.

80 80

60 60

40 40

20 20

0 0

(Ai/Σ

Ai)*

100

20x10-3151050nMg/nO

1151 cm-1

1126 cm-1

1088 cm-1

1063 cm-1

1036 cm-1

996 cm-1

fit

1140

1120

1100

1080

1060

Wav

enum

ber /

cm

-1

20x10-3151050nMg/nO

1050

1040

1030

1020

1010

1000

990

1151 cm-1

1126 cm-1

1088 cm-1

1063 cm-1

1036 cm-1

996 cm-1

fit

a

b

Fig. 9. Dependence on nMg/nO molar ratio of the fractional percentage (%)i of eachCO band revealed by the CO stretching region (Fig. 7). The semiquantitative valuesof the percentages were determined with the formula (%)i = Ai100/˙, where Ai is theband area of peak i-th, ̇ =

∑n

i=1Ai , and n is the number of bands considered.

Fig. 10. Real and imaginary relative permittivity spectra of selected PEG1000/(MgI2)x(I2)y polymer electrolytes at different temperatures. The molar ratios nMg/nO and nI2 /nO

(underlined) are indicated for each sample.

120 M. Vittadello et al. / Electrochimica Acta 57 (2011) 112– 122

3.43.23.02.8

log(

σ /

S·c

m-1

)

-12

-10

-8

-6

-4

-12

-10

-8

-6

-4

3.4x10-3

3.23.02.8

(1000/T) / K-13.43.23.02.8

-12

-10

-8

-6

-4

0.00000.0000

0.024 30.000 0

0.01330.0003

0.00710.0013

0.004 10.002 0

0.00180.0023

0.000 90.002 6

0.00050.0026

0.00020.0027

Fig. 11. Dependence on the reciprocal absolute temperature of the parameters �el,n

(n = 1, 2, 3) for the PEG1000/(MgI2)x(I2)y polymer electrolytes. �el,1 corresponds tothe overall direct current conductivity �. The fitting was carried out with an Arrhe-nr

r(ratIe

bPnttifAdt

dc[m

2121

0101

88

66

44

22

00

2121

0101

88

66

44

22

00

2121

0101

88

66

44

22

00

Ea

/ kJ

mol

-1

25x10-320151050 nMg / nO

σel,1σel,2σel,3

Fig. 12. Activation energy for the conduction process of the PEG1000/(MgI2)x(I2)y

ius equation in regions I and II, while the VTF was used in region III. The molaratios nMg/nO and nI2 /nO (underlined) are indicated for each sample.

esponding to �el,3) and across domains from A to B or vice versacorresponding to �el,1). This latter phenomenon results in long-ange conductivity. It can be seen in Fig. 11 that for nMg/nO = 0.0005nd nI2 /nO = 0.0026 �el,2 partially prevails over �el,1, indicatinghat a localized conduction phenomenon is taking place in regionII. The conductivity in pristine PEG1000 is accounted for by a singlelectrode polarization phenomenon (a single domain is present).

Further insight about the conductivity mechanism is giveny the activation energy for the conduction process of theEG1000/(MgI2)x(I2)y polymer electrolytes as a function of theMg/nO molar ratio, as shown in Fig. 12. The barrier for conduc-ion of �el,1 is mostly lower than or similar to both �el,2 and �el,3 athe higher concentrations of salt. This finding indicates that whilet is relatively easy for the free ion carriers to move across domainsrom A to B in order to have long-range conductivity (the crossings–B or B–A correspond to �el,1), motion is more difficult withinomain A (which corresponds to �el,2) and B (which correspondso �el,3).

We hypothesize, in analogy with previous studies [14], thatomains A are comprised of one ore more PEG1000 chains

oordinated to bivalent, monovalent and neutral species, Mg2+,MgI]+, and MgI2 via direct Mg–O complexation. These species can

igrate locally by intra-chain hopping. We hypothesize also that

polymer electrolytes as a function of the nMg/nO molar ratio.

domains B would be smaller sites comprised of empty PEG1000chains (with or without I−, I3− or I2) with no MgI2 salt. The bound-ary sites are comprised of A and B sites coordinated to each otherby means of dynamic cross-links via bridging anionic species suchas I−, I3− or via I2. Conducting species can escape both A and Bdomains, giving rise to long range conductivity.

Information about the conducting species in the systems wasgathered by means of equivalent conductivity spectra, as shown inFig. 13. The profiles of � with respect to m1/2

Mg exhibit an overallrapid decrease with increasing salt concentration with the excep-tion of a small local maximum at 0.14 (mol/kg)1/2. The fitting of thedata was conducted by using the following generalized equation[21,22]:

� = H0 + K1exp(−˛1m1/2Mg ) + K2exp(−˛2m1/2

Mg )

+ K3exp(−˛3(m1/2Mg − x0)

2) (4)

This equation reduces to the Onsager equation (� = �0 −Am1/2

Mg ) in the limit of zero concentration, under the conditions that

�0 = H0 + K1 + K2 + K3 and A = K1˛1 + K2˛2 + 2K3˛3:

� = (H0 + K1 + K2 + K3) − (K1˛1 + K2˛2 + 2K3˛3) · m1/2Mg . (5)

M. Vittadello et al. / Electrochimic

800

600

400

200

0

(Λ/1

0^-6

)/S k

g cm

-1 m

ol-1

0.70.60.50.40.30.20.1

c½_Mg/mol ½ kg-½

293 K 298 K 303 308 313 318 323 328 333 338 343 348 353 358 363 fit

Fm

e[c

0(pieIwtw[m

(iPc3sm

FSC

ig. 13. Equivalent conductivity as a function of m1/2Mg of PEG1000/(MgI2)x(I2)y poly-

er electrolytes.

The initial decrement (0 ≤ m1/2Mg < 0.10 mol/kg1/2) can be

xplained in terms of the presence of ionic species such as Mg2+,MgI]+, I− and I3−. In this regime both cationic and anioniconductivity is possible. At the local maximum (0.10 ≤ m1/2

Mg <

.20 mol/kg1/2), which is accounted for by the fourth term in Eq.4), magnesium is present mostly as [MgI]+ and MgI2, opening theossibility of dominant anionic migration. This possible mechanism

s expected on the basis of previous results on PEG-based MgCl2lectrolytes [14]. The formation of triple ions is also a possibility:−· · ·[MgI]+· · ·I− or I3−· · ·[MgI]+· · ·I3−. The formation of triple ions

ill further facilitate anionic conductivity. The equivalent conduc-ivity decreases further (0.20 ≤ m1/2

Mg ≤ 0.70 mol/kg1/2) with theidespread formation of neutral species such as [MgI]+· · ·I− or

MgI]+· · ·I3−. Thus, both cationic and anionic conductivity is dra-atically reduced.Fig. 14 shows the 25Mg MAS NMR of MgI2 and sample 0

see Table 1), along with a preliminary simulation of the dom-nant Mg site in the composite. Sample 0 indicates the systemEG1000/(MgI2)0.5668(I2)0.0000. MgI2 is isostructural with CdI2,rystallizing in the P3̄m1 space group with a Mg atom lying on the

m site. The 25Mg MAS NMR spectrum of MgI2 contains a single,harp resonance at −81.1 ppm. This is consistent with the high sym-etry of the Mg site. The shift of the resonance is highly shielded

ig. 14. 25Mg MAS NMR of MgI2, a polymer composite (see text) and a Win-olids simulation of a Mg site. The simulated parameters are ıiso = 6 ppm, � = 0.5,Q = 3.5 MHz.

a Acta 57 (2011) 112– 122 121

and outside the typical range of octahedrally coordinated Mg sites.This may be attributed to the electron-rich iodide atoms in the firstcoordination shell. Considering the electronegativities of Mg andI, the Mg–I bond is expected to be fairly covalent. Previous workhas shown that the magnitude of the quadrupole coupling con-stant (CQ) increases with an increase in the degree of covalencyof the bonds to the quadrupole nucleus. Surprisingly, the narrowpeak implies a very small quadrupole coupling constant found insymmetric environments.

In contrast to the MgI2 spectrum, PEG1000/(MgI2)0.5668(I2)0.0000displays asymmetrically broad resonances whose intensities liebetween 0 and −125 ppm. The sharp resonance present in MgI2is no longer visible. However, spectral intensity distributed about−80 ppm is present, implying that there may be some MgI2nanoparticles within the polymer matrix. The peak broadening isattributed to the decrease in particle size and an increase in theamount of disorder present in the remaining magnesium iodidecrystallites. A relatively large amount of signal intensity lies atapproximately −25 ppm. This shift is to be expected for Mg atomswhich are no longer part of the MgI2 lattice, but have coordinatedto the oxygen atoms in the polymer backbone. Bonds to oxygenatoms would withdraw electron density at the Mg site and shift theresonance downfield. Inherent distribution in the bonding geom-etry of the Mg atom to the polymer backbone will consequentlylead to a distribution in both the quadrupole coupling constant andasymmetry parameter. These distributions induce a characteristicasymmetric broadening of the second order line shape extendingupfield (lower ppm). The breadth of the line shape was used to esti-mate the NMR parameters of this characteristic site. The simulatedparameters are ıiso = 6 ppm, � = 0.5, CQ = 3.5 MHz.

The 25Mg NMR study has revealed the presence of at least twodifferent Mg sites in the polymer complex. The first site, whoseshift is centered at approximately −80 ppm, has been assigned toresidual MgI2 in the polymer matrix. The second site is presentat relatively higher ppm values and assigned to Mg coordinatedto the oxygen atoms of the PEG backbone. It is characterized bya second-order line shape and distributions in NMR parameters.The site could be adequately simulated with ıiso = 6 ppm, � = 0.5,CQ = 3.5 MHz. The relative intensity of the second site is much largerthan the first site, implying the successful, albeit incomplete, dis-solution of the MgI2 salt into the polymer matrix. Further studies,preferably at different magnetic field strengths, are needed to ver-ify the amount of sites as well as determine the effects of chemicalshift anisotropy on the line shapes. The description emerging fromthe NMR study is compatible with the results obtained from theequivalent conductivity study: the coordination Mg2+, [MgI]+, andMgI2 species by the polymer backbone.

4. Conclusions

A new series of polymer electrolytes with formula PEG1000/(MgI2)x(I2)y was synthesized with a magnesium iodide content ashigh as 13.7% in weight. This achievement was possible on the basisof a highly amorphous form of MgI2 synthesized via a metallorganicroute. Thermal analysis has shown that PEG1000 oligomers par-tially crystallize below Tm = 39–41 ◦C, with a crystallinity varyingparabolically between 46 and 76%. Based on our vibrational inves-tigations we have shown that: (1) full band assignment is possibleby assuming a D(4�/7) symmetry for the PEG1000 polymer host;(2) terminal OH groups dominate the salt–polymer interactionsat low salt concentrations while both OH and CO groups arerelevant at higher concentrations; (3) Mg atoms are coordinated

by oxygen atoms of the polymer host while retaining iodide anionsin their first sphere of coordination. The presence of moleculariodine does not seem to significantly alter the distribution ofionic species for PEG1000/(MgI2)x systems, as expected from our

1 chimic

pTasiptdddbipschcbI

aimM0dc

A

tC

[

[

[[

[[

[[

[

[

[[21] V. Di Noto, S. Lavina, E. Negro, M. Vittadello, in: D. Santos, C. Sequeira (Eds.),

Polymer Electrolytes: Fundamentals and Applications, Woodhead PublishingLtd., Cambridge, UK, 2010, pp. 219–277.

22 M. Vittadello et al. / Electro

revious findings on PEG400/(MgCl2)x electrolytic complexes.his indicates that molecular iodine may be present as I3−, with

substitutional role with respect to iodides. The electric mea-urements highlighted three electrode polarization phenomenan PEG1000/(MgI2)x(I2)y at temperatures higher than the meltingoint of the polymer host. This evidence has been attributed tohe presence of two distinct domains (A and B) characterized byifferent ion concentrations [7,8]. For nMg/nO < 7 × 10−3, �DC isetermined by the mobility of the charge carriers within eachomain; at higher concentrations of the salt, �DC is bottleneckedy the charge transfer between the domains. The overall study

ndicates that at least three types of Mg-containing species areresent: Mg2+, MgI+ and MgI2. Bivalent, monovalent and neutralpecies participate in the conduction process. Residual nanoparti-les of MgI2 may be present. 25Mg MAS NMR measurements on theighest salt concentration sample, PEG1000/(MgI2)0.5668(I2)0.0000,onfirm such a description. Depending on the salt concentration,oth cationic and anionic hopping mechanisms may be possible.

n particular, for 0 ≤ c1/2Mg < 0.10 mol/kg1/2 both cationic and

nionic species (Mg2+, [MgI]+, I− and I3−) are likely to participaten the conduction process, while for 0 ≤ c1/2

Mg < 0.20 mol/kg1/2

ostly iodide hopping is possible among the species [MgI]+ andgI2 and the triple ions I−· · ·[MgI]+· · ·I− or I3−· · ·[MgI]+· · ·I3−. For

.20 ≤ c1/2Mg ≤ 0.70 mol/kg1/2 low residual cationic and anionic con-

uctivity is present due to neutralization caused by extensive ionicoupling.

cknowledgements

The National High Magnetic Field Laboratory is supported byhe National Science Foundation and the State of Florida throughooperative Agreement (DMR-0084173).

[

a Acta 57 (2011) 112– 122

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