P

tAmrtattmtdpsaa

pSdm

0d

Electrochimica Acta 53 (2008) 7769–7774

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

roton-conducting polymer electrolytes based on methacrylatesa,∗,1 b b

ublic

er ele), 2-es of(DMFetho

reachdowgrav

Jakub Reiter , Jana Velicka , Martin Mıkaa Institute of Inorganic Chemistry of the ASCR, v. v. i., 250 68 Rez near Prague, Czech Repb Institute of Chemical Technology Prague, 166 28 Prague 6, Czech Republic

a r t i c l e i n f o

Article history:Received 7 April 2008Received in revised form 27 May 2008Accepted 27 May 2008Available online 3 June 2008

Keywords:Polymer electrolyteProton conductivityPhosphoric acidMethacrylatePropylene carbonateElectrochromic device

a b s t r a c t

Proton-conducting polymmerization of ethyl (EMAwith embedded solutionN,N-dimethylformamideand thermogravimetric m6.7 × 10−5 S cm−1 at 25 ◦Ctrochemical potential winor platinum). The thermo110–130 ◦C.

1. Introduction

Since the introduction of the 1st generation of polymer elec-

rolytes based on poly(ethylene oxide) and lithium perchlorate byrmand [1] and Fenton et al. [2], the field of novel solid poly-er ion-conducting materials have attracted interest of many

esearch teams. In comparison with liquid and ceramic electrolyteshe studied materials can be applied in many areas due to theirdvanced properties. Although the research is mainly aimed athe polymers with entrapped inorganic (mainly lithium) salts orheir solutions in aprotic solvents [3–5], a considerable interest in

embranes exhibiting proton conductivity can be observed due toheir possible application in chemical sensors [6], electrochromicevices [7] or fuel cells [8]. Due to the proposed applications, therepared membranes should meet four basic requirements: rea-onable conductivity (above 10−5 S cm−1), good long-term, thermalnd interfacial stability, stable mechanical properties and elasticity,nd exactly defined and easy way of preparation.

Recently, different materials and approaches in designingolymer electrolytes with proton conductivity have been reported.olutions of phosphoric acid or its organic esters are usually embed-ed in the polymer network of poly(ethylene oxide), poly(methylethacrylate), poly(glycidyl methacrylate), poly(vinylidene

∗ Corresponding author. Tel.: +420 266172198; fax: +420 220941502.E-mail address: reiter@iic.cas.cz (J. Reiter).

1 ISE member.

013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2008.05.066

ctrolytes based on methacrylates were prepared by direct, radical poly-thoxyethyl (EOEMA), and 2-hydroxyethyl methacrylate (HEMA). Samplesphosphoric acid in propylene carbonate (PC), �-butyrolactone (GBL),) and their mixtures were studied using impedance, voltammetricalds. Membranes of long-term stability exhibit ionic conductivity up toed for the sample PEMA–PC–H3PO4 (31:42:27 mol.%). The accessible elec-is 2.2–3 V depending on the working electrode material (glassy carbon

imetric analysis shows that the membranes are thermally stable up to

© 2008 Elsevier Ltd. All rights reserved.

fluoride-hexafluoropropylene), poly-benzimidazoles or theircopolymers [9–12]. Besides H3PO4 and its esters [13], variousheteropolyacids are used such as phosphomolybdic acid [12],phosphotungstic acid [14], silicotungstig acid [15] and methansul-fonic acid [16]. The higher reactivity of strong acids such assulphuric acid or para-toluensulfonic acid is a serious drawbackfor practical applications as decomposition of some solvents was

described [17,18]. Therefore some other composites containinghydrated inorganic oxides (antimonic acid, aluminium oxide,silica) [7,19,26], zirconium phosphate [7] or uranium hydrogenphosphate [20] were suggested. All materials exhibit a reasonableconductivity at room temperature in the range from 10−5 to10−2 S cm−1.

Our recent work is aimed at development of hybridorganic–inorganic membranes based on methacrylates. Thedeveloped method of direct, radical polymerization ensures min-imum contamination of samples with aerial moisture or oxygenand follows our previous work on ternary polymer electrolytesfor modern chemical applications such as lithium-ion batteriesor electrochromic devices [21–24]. Also following our previousresults, a moderately cross-linked polymer was formed due tothe positive effect of the cross-linking agent low concentration(0.3–0.5 mol.% of monomer) on bulk conductivity [22,24]. More-over, polymer cross-linkage improves mechanical properties,mainly long-term elasticity.

In this paper we present results of our investigation of methacry-lates of different polarity and its influence on conductivity. Also,different solvents (propylene carbonate, N,N-dimethylformamide

ica Ac

7770 J. Reiter et al. / Electrochim

and �-butyrolactone) were tested as embedded solvents. Resultsof investigation of particular binary or ternary liquid electrolyteswere used for the polymer electrolyte formation and compositionoptimisation. Polymer gel electrolytes can be described as a sys-tem of immobilised microscopic liquid phase (aprotic solvent) inthe cross-linked polymer network. The qualitative electrochemi-cal behaviour of immobilised inorganic or organic species in thepolymer system corresponds to their behaviour in solutions in par-ticular solvents. The presence of the polymer does not influencesqualitative behaviour of immobilised species, but it strongly affectsthe mobility and therefore conductivity of prepared samples. Inaddition to restricted ion mobility, the conductivity decrease is alsodue to decreased ion concentration.

2. Materials and procedures

Monomers: ethyl, 2-ethoxyethyl and 2-hydroethyl methacrylatewere obtained from Sigma–Aldrich and purified by distilla-tion under reduced pressure. Cross-linking agents, ethylenedimethacrylate (EDMA) and 1,6-hexandiol dimethacrylate (hex-amethylene dimethacrylate, HexadiMA; both Sigma–Aldrich) wereused as received. Polymerization initiator, benzoine ethylether(BEE) was purchased from Fluka and recrystallised from chloro-form. All monomers, cross-linking agents and the initiator werestored at 4 ◦C before use.

Propylene carbonate (PC; Sigma–Aldrich, >99.7%, water con-tent <0.002%), �-butyrolactone (GBL; Sigma–Aldrich, >99%) andN,N-dimethylformamide (DMF; Merck, water content <0.05%) andmixed solvents PC–DMF 2:1 and 4:1 vol. and PC–GBL 1:1 vol. werestored under molecular sieves (3A pellets, Sigma–Aldrich). Phos-phoric acid (anhydrous crystals, >99.99%, Sigma–Aldrich) was usedas received.

2.1. Preparation of the gel electrolyte

The method of electrolyte preparation was described in ourprevious paper [22]. The polymerization was carried out in a cellformed of polypropylene plate, packing distance frame (siliconerubber) and glass plate. The cell was carefully filled with nitrogenand then with the initial liquid mixture containing the monomer,cross-linking agent, solvent, acid and polymerization initiator.

For radical polymerization, benzoine ethylether was used asan initiator active under UV light. The UV light initiated polymer-

ization proceeded for a period of 2 h at room temperature usinga pair of 15 W ReptiGlo 8.0 lamps emitting UV-A and UV-B light(Hagen, Czech Republic). The obtained samples, foils with an area3 cm × 4 cm and thickness corresponding to the thickness of usedsilicone seals (1.00 ± 0.10 mm), were more or less elastic in depen-dence on the solvent content and mostly transparent. This methodof preparation prevents evaporation of the volatile monomer, andthe initial liquid mixture is only in a minimum contact with air. Thecomposition of the gel is expressed in the molar percentage ratiopolymer/solvent(s)/acid. The amount of the cross-linking agent isalways included in the monomer content. Besides the polymer-ization initiated by UV light, one can use the thermally initiatedprocess using, e.g. 2,2′-azobis(isobutyronitrile) or dibenzoylperox-ide [21,25,26].

2.2. Equipment, electrodes and cells

Potentiogalvanostats PGSTAT 10 and 30 (Eco Chemie, TheNetherlands) were used for electrochemical measurementsincluding the FRA-2 module for impedance measurements. Elec-trochemical measurements of both liquid and polymer systems

ta 53 (2008) 7769–7774

were performed with a glassy carbon (2.0 mm diameter) or a plat-inum electrode (1.6 mm diameter, both from Bioanalytical Systems,United Kingdom) as the working, platinum as the counter andPMMA-Cd-Cd2+ as the reference electrode developed in our lab-oratory [27]. All potentials in the paper are related to this Cd/Cd2+

system (E(Cd/Cd2+) = −0.44 vs. SCE, measured in propylene car-bonate). The advantage of the PMMA-Cd-Cd2+ electrode is in itsavailability for both liquid and solid-state electrochemical mea-surements. The surface of the working and the counter electrodewas polished by abrasives (0.3 alumina, Metroohm) and soft clothafter each measurement.

Conductivities of liquid solutions of H3PO4 were measuredat 20 ◦C using a conductivity cell (Jenway, platinum electrodes,cell constant S = 1.00 ± 0.01). Temperature dependent conductiv-ity measurements were performed in the temperature range from−10 to 70 ◦C using thermostated bath (precision of the tempera-ture ±1 ◦C). A similar procedure was used for the measurementof conductivity of solid samples in the temperature range from−25 to 70 ◦C. Here, a two-electrode arrangement was used, when aslice of gel (2 cm × 2 cm, 1 mm thickness) was sandwiched betweentwo parallel stainless steel electrodes. In both cases single poten-tial impedance spectra were measured in the frequency range from100 kHz to 1 Hz. The obtained spectrum was analysed by the Eco-Chemie Autolab software producing the values of the equivalentcircuit elements. The value of ohmic resistance was converted inthe value of specific resistivity or conductivity.

The DSC analysis was performed in the temperature range from−160 to 100 ◦C at the heating rate of 10 ◦C min−1. The simultaneousTGA-DTA measurement was taken in air and argon at the heatingrate of 5 ◦C min−1. Experiments were performed with a Simultane-ous Thermal Analysis Netzsch STA 409 (Germany).

3. Results and discussion

Prepared samples are elastic and homogeneous membranes.They are mostly transparent or slightly yellowish or greenish andare not hygroscopic. Contrary to previously studied electrolyteswith embedded solutions of inorganic salts, samples with H3PO4can contain a higher content of acid solution in propylene carbonate(polymer content 31 mol.%). Although all samples exhibit long-termstability in the air, they are stored in a desiccator. No phase separa-tion or acid crystallisation was observed during 1-month storage.

We attempted to prepare methacrylate-based electrolytes con-

taining silicotungstic and phosphotugstic acid similarly to theternary systems based on organically modified silicates [14,15].Used methacrylates are decomposed (hydrolysed) during the poly-merization process, and the heteropolycids react both with themonomer and also with propylene carbonate. A similar effect wasobserved by Grillone in the case of para-toluensulfonic acid andpropylene carbonate solutions, where ethylene and propylene car-bonate are hydrolysed to carbon dioxide and corresponding glycols[18].

3.1. Impedance and conductivity measurements

The first measurements were done with liquid electrolytesPC–H3PO4 and DMF–H3PO4 in the concentration range from 0 to6 mol dm−3 of H3PO4. Solutions of phosphoric acid in both solventsare very viscous and the conductivity is increasing with increasingacid content up to 5.2 × 10−4 S cm−1 for the 6 mol dm−3 solution inPC and 2.7 × 10−4 S cm−1 for the 6 mol dm−3 solution in DMF. H3PO4solutions in the PC–DMF exhibit a lower conductivity within thewhole concentration range, although DMF-based solutions exhibita lower viscosity. It seems to be due mainly to its lower dielectric

J. Reiter et al. / Electrochimica Ac

H3PO4 are more sensitive to the influence of temperature on con-ductivity ion–ion association and also viscosity. In other aproticsolutions such as tetraalkylammonium perchlorates or tetrafluo-roborates, much lower EA values were found, which indicates thattheir conductivity is less influenced by temperature.

Immobilisation of the PC–H3PO4 solutions in all methacry-lates leads to conductivity decrease. The mobility of both H+

and all phosphate ions is restricted due to the presence of thepolymer network. Our work was aimed at optimisation of theternary system composition to reach reasonable bulk conduc-tivity. Contrary to the previously reported PC–LiClO4 polymerelectrolytes, the physical properties of anhydrous phosphoric acidenabled us to prepare samples with a lower polymer content(down to 31 mol.%) but retaining their good mechanical proper-ties (elasticity and shape stability). Along with sample optimisationwe studied the influence of polymer polarity on the electro-

Fig. 1. Effect of the H3PO4 concentration on ionic conductivity in liquid PC–H3PO4

(full dots) and DMF–H3PO4 (empty dots) electrolytes in the concentration range0.1–6 mol dm−3 H3PO4 measured at 20 ◦C.

constant (38.3) as compared to PC (64.4). Fig. 1 presents the isother-mal relationship between the acid concentration and conductivityfor two liquid electrolytes PC–H3PO4 and DMF–H3PO4. In bothcases the bulk conductivity is increasing in the whole concentra-tion range. The described behaviour differs from the behaviour ofsome other electrolytes in aprotic solvents, e.g. lithium salts, whereion–ion association causes conductivity decrease at higher concen-trations [22,28]. In the case of H3PO4 solutions, the effect of ionpairing at high concentrations is not so strong to cause such effect.

Following the relationship between conductivity and H3PO4concentration, 6 mol dm−3 solutions of H3PO4 were used for fur-ther experiments. Considering the positive effect of the presence ofaprotic protophilic solvents in H3PO4-based systems on their con-ductivity [10,18], we also prepared solutions of phosphoric acid inbinary solvents, PC–DMF 4:1, PC–DMF 2:1 and PC–GBL 1:1. Con-ductivities of prepared binary and ternary liquid electrolytes aresummarised in Table 1. As can be seen, the presence of DMF inPC–H3PO4 results in only a small improvement of conductivity.

Fig. 2 illustrates the relationship between the ionic conductivityof 6 M H3PO4 solutions and temperature. The data are plotted inArrhenius coordinates (specific conductivity is plotted as a decadiclogarithm). PC, PC–DMF and PC–GBL solutions exhibit a similarbehaviour in contrast to the DMF solution where lower conductivi-ties were observed in the whole temperature range correspondingto the values plotted in Fig. 1.

The obtained data can be fitted with the Vogel–Tamman–Fulcher (VTF) equation in the logarithmic form:

�T1/2 = A exp[ −EA

R(T − T0)

](1)

Table 1Conductivities of H3PO4 liquid electrolytes (6 mol dm−3 H3PO4 concentration)

Liquid electrolyte Composition (mol.%) � (20 ◦C) (S cm−1) EA (kJ mol−1)

PC–H3PO4 59:41 5.2 × 10−4 36.9DMF–H3PO4 52:48 2.7 × 10−4 36.2PC–DMF–H3PO4 38:21:41 5.9 × 10−4 35.2PC–DMF–H3PO4 47:12:41 5.6 × 10−4 –PC–GBL–H3PO4 30:30:40 4.9 × 10−4 35.9

ta 53 (2008) 7769–7774 7771

In this particular relationship A is the parameter related to the num-ber of charge carriers, EA is the activation energy for conduction,R is the universal gas constant and T0 the ideal glass transitiontemperature indicating the temperature at which the free volumeextrapolates to zero. The analysis of the experimental conductivitydata in terms of the VTF relationship leads to the determination ofthree empirical parameters: A, EA and T0, when T0 is determined byfitting the experimental data with relationship (1). The conductivityactivation energy corresponds to a slope in the Arrhenius coordi-nates (see Fig. 2) and explains how conductivity is influenced bytemperature.

Activation energy values for liquid systems are given in Table 1,when the EA value describes the dependency of conductivity ontemperature. It can be seen that the EA values for all studied H3PO4solutions do not vary with the type of solvent and are from 35.2 to36.9 kJ mol−1. From this small difference we can conclude that theconduction mechanism is not influenced by change of the solvent,e.g. by presence of protophilic co-solvent (DMF).

Generally, a system with a high concentration of ions such as

chemical properties of prepared electrolytes. Three methacrylateswith increasing polarity were chosen: ethyl, 2-ethoxyethyland 2-hydroxyethyl methacrylate. 2-ethoxyethyl methacrylateis compatible with inorganic salts, e.g. LiClO4, and formsbinary electrolytes PEOEMA–LiClO4, but with low conductivity

Fig. 2. Arrhenius plot of liquid electrolytes containing 6 mol dm−3 H3PO4 in PC, DMF,PC–DMF 2:1 and PC–GBL 1:1 solvents (temperature range from −10 to 70 ◦C).

ica Acta 53 (2008) 7769–7774

7772 J. Reiter et al. / Electrochim

4.2 × 10−7 S cm−1 (cit. [22]). 2-Hydroxyethyl methacrylate is widelyused for its biocompatibility and hydrophilicity in separationtechniques, chemical sensors and biomaterials [29–32]. Our mea-surements showed that the PEMA–PC and the PEOEMA–PC binarysystems without added inorganic salt behave as insulators withconductivity lower than 10−7 S cm−1 at 20 ◦C.

All samples with immobilised PC–H3PO4 solution were moreor less elastic in dependence on the solvent content and mostlytransparent. In the case of equivalent PEMA and PEOEMA samplescontaining DMF, phase-to-phase separation (production of hetero-geneous samples with liquid excluded on its surface) was observed.PEMA is non-compatible with DMF solutions in a wide range ofsample composition. On the contrary, due to its different polarityall samples could be prepared using HEMA with PC–DMF (2:1 and4:1 vol.) and PC–GBL (1:1 vol.) mixed solvents. All attempts to forma polymer electrolyte with DMF only were not successful due tophase to phase separation independently of the polymer type.

Impedance measurements were used for bulk conductivitydetermination. Specific conductivities of some representative gelelectrolytes are summarised in Table 2. The highest conductivitywas found for the PEMA–PC–H3PO4 electrolyte, 6.7 × 10−5 S cm−1,and co-immobilisation of DMF did not lead to increase of conduc-tivity. Co-immobilisation of �-butyrolactone lead to decrease ofconductivity, especially in the case of PEOEMA and PEMA elec-trolytes where the prepared samples exhibited an order lowerconductivity than the corresponding samples with PC and PC–DMFsolvents. A comparison between liquid and polymer electrolytesshows an order lower conductivity values after polymerization,what is corresponding which corresponds to our previous inves-

tigation of PC–LiClO4 electrolytes [21,22].

It is generally accepted that the ionic transport in polymer gelelectrolytes takes place mainly in the solvent phase [33,34]. Thiswas also confirmed in the case of methacrylate-based systems pre-pared and studied in our laboratory [18]. In the series of sampleswith decreasing polymer content we observed increase of conduc-tivity, and the character of the impedance spectrum was changedas well. At low conductivities a wide semicircle representing theparallel connection of a capacitor C and a resistor R was observedcorresponding to the dielectric behaviour of the polymer and thebulk resistivity of the sample, while with increasing conductivitythe semicircle was depressed and a simple serial connection ofresistor R and pseudocapacitor Q (a constant phase element) wasfound. In both cases, similar impedance spectra were obtained inour previous research [22,23].

In methacrylate-based polymer electrolytes with ionic com-pounds the relationship between conductivity and temperatureis more complex due to the presence of polymer. Fig. 3shows the Arrhenius plot for PEMA, PEOEMA and PHEMApolymer electrolytes with PC–H3PO4 solution. These sam-ples were chosen to compare the influence of the polymer

Table 2Specific conductivities (at 20 ◦C) of prepared polymer electrolytes containing H3PO4 sotransition temperatures determined by DSC (Tg) and the ideal glass transition temperatur

Polymer electrolyte Composition (mol.%) � (20 ◦C)

PEMA–PC–H3PO4 31:42:27 6.7 × 10−5

PEMA–PC–DMF–H3PO4 Non-compatible –

PEOEMA–PC–H3PO4 31:42:27 1.5 × 10−5

PEOEMA–PC–DMF–H3PO4 38:28:8:26 1.3 × 10−5

PEOEMA–PC–GBL–H3PO4 41:16:18:24 3.7 × 10−6

PHEMA–PC–H3PO4 34:40:26 2.1 × 10−5

PHEMA–PC–DMF–H3PO4 32:26:14:28 2.7 × 10−5

PHEMA–PC–GBL–H3PO4 35:18:20:26 1.1 × 10−5

Fig. 3. Arrhenius plot of polymer electrolytes PEMA–PC–H3PO4 (31:41:28),PEOEMA–PC–H3PO4 (42:34:24) and PHEMA–PC–H3PO4 (33:39:28 mol.%); temper-ature range from −25 to 70 ◦C, composition expressed in molar percentage.

polarity on the physical–chemical behaviour and ionic equilibrium.As it is often found in the case of systems with a high molar con-centration of ions, e.g. ionic liquids [35] or solutions of inorganicsalts in high concentrations [36]. In the case of PC–H3PO4 poly-

mer electrolytes, three-order difference in conductivity can be seenin the temperature range from −25 to 70 ◦C in Fig. 3. The highestconductivity values were reached for PEMA–PC–H3PO4 electrolyte.These results correspond to those obtained at room temperaturewhile PEOEMA and PHEMA electrolytes are less conductive in thewhole temperature range. The EA values are increasing from 10.7to 32.2 kJ mol−1 in the series PEMA–PEOEMA–PHEMA. Possibleexplanation in increasing polarity of the polymer and consequentincrease of the EA value is however uncertain.

At lower temperatures the presence of polymer, and also theincreasing viscosity of immobilised solutions strongly influencethe dielectric behaviour of all samples. The previously discussedsemicircle appears at temperatures under 10 ◦C, and the parame-ters of the particular equivalent circuit correspond to the dielectricbehaviour of the polymer phase. At higher temperatures, a sim-ple RQ circuit can be found. The conductivity values for 60 ◦Cfor PEMA, PEOEMA and PHEMA–PC–H3PO4 samples are 0.60,0.10 and 0.12 mS cm−1, respectively. In comparison to room tem-perature, one can observe one order increase of conductivity(see Table 2).

lutions together with estimated apparent activation energy values (EA), the glasse (T0)

(S cm−1) EA (kJ mol−1) Tg (◦C) T0 (◦C)

10.7 −89.4 ± 0.8 −122– – –

12.9 −68.9 ± 0.6 −119– – –– – –

32.2 −67.3 ± 0.9 −192– – –– – –

J. Reiter et al. / Electrochimica Acta 53 (2008) 7769–7774 7773

Fig. 5. Cyclic voltammograms of polymer electrolytes PEMA–PC–H3PO4 (31:41:28)and PHEMA–PC–H3PO4 (33:39:28 mol.%) on platinum electrode (1.6 mm diameter,counter electrode glassy carbon, reference PMMA-Cd-Cd2+, 1 mV s−1 scan rate).

solvent oxidation correspond to the values observed in the case ofliquid electrolytes.

3.3. Thermal stability

Concerning applications, thermal stability is another principalparameter to be considered along with electrochemical and long-term stability and reasonable conductivity. Fig. 6 shows TGA anddifferential gravimetric analysis (DTG; 1st derivative of the TGAcurve) of two samples: PEMA–PC and PEMA–PC–H3PO4. The weightloss was found less than 1 wt.% up to 110 ◦C and 8 wt.% under 150 ◦C.These changes can be explained as a partial evaporation of the

Fig. 4. Cyclic voltammograms of 6 mol dm−3 H3PO4 in PC, PC–DMF 2:1 and PC–GBL1:1 solvents on platinum electrode (1.6 mm diameter, counter electrode platinum,reference PMMA-Cd-Cd2+, 10 mV s−1 scan rate).

3.2. Voltammetrical investigation of liquid and polymerelectrolytes

The high electrochemical stability of propylene carbonate iswell known from literature. The PC is widely used in electro-chemical power sources, mainly lithium-ion batteries [37]. Alsothe other solvents (DMF and GBL) are electrochemically reason-ably stable. When propylene carbonate is electrochemically stablefrom −2.5 to 2.5 to 2.8 V vs. Cd/Cd2+, the determining parameterof the cathodic stability of H3PO4 aprotic solutions is the reduc-tion of hydrogen ions. Fig. 4 presents the cyclic voltammetry of6 mol dm−3 H3PO4 in PC, PC–DMF and PC–GBL. A rapid evolutionof hydrogen was observed on the platinum electrode at 0.3 V vs.Cd/Cd2+ independently on of the used solvents. This process pro-ceeds at higher potentials compared to work of Daniele, who foundthe H+ reduction on microelectrodes at lower potentials (−0.6 Vvs. Cd/Cd2+) in comparison with our experiments done in propy-lene carbonate (+0.3 V) [40]. The anodic limit at the potential,

2+

above 2.5 V vs. Cd/Cd can be assigned to the oxidation of solventcorresponding to the previously observed measurements in ourlaboratory and confirmed by other authors [38,39]. In agreementwith literature, a partial oxidation of carbonates at high poten-tials above 2.5 V was observed, close to the anodic limit [39,41].However, the anodic processes in aprotic proton-conductive mem-branes were not studied intensively in literature, and we can expectthat the present phosphoric acid will affect the solvent and par-ticipate in electrode reactions (solvent oxidation). This effect canbe stronger in comparison with polymer electrolytes containinginorganic salts. The possible effect is probably due to H+ cations,while phosphoric anions can be declared as electrochemicallystable.

The substitution of the platinum electrode with glassy carboninfluences only the cathodic limit. Due to the higher overvoltage ofhydrogen on glassy carbon in comparison to platinum, the reduc-tion of hydrogen appears at 0.6 V lower potentials.

Voltammetrical measurements of polymer electrolytes showeda similar behaviour, as it is declared in Fig. 5. The electrochemi-cal stability of used polymers is exceeds 4.8 V on both platinum orglassy carbon electrodes [42]. The potentials of H+ reduction and

Fig. 6. TGA and DTG curves for PEMA–PC (50:50 mol.%) and PEMA–PC–H3PO4

(30:41:29 mol.%) gel electrolytes (5 ◦C min−1 heating rate, temperature range from30 to 380 ◦C; argon atmosphere).

ica Ac

[

[[

[

[[[

7774 J. Reiter et al. / Electrochim

immobilised solvent (PC). However, the change is small and cor-responds to the low vapour pressure of the solvent (pPC = 130 Paat 50 ◦C). Both in air and argon all samples start to decompose attemperatures above 130 ◦C.

It is well known that methacrylates decompose in two exother-mic reaction processes: degradation of the polymer end groups(240–280 ◦C) and total decomposition of the monomer units (above310 ◦C). The presence of immobilised solvent—propylene carbonateis complicating the explanation of the experimental data compli-cates experimental data interpretation. The boiling point of PC is240 ◦C at 101.3 kPa. The PEMA–PC sample is practically (97 wt.%)decomposed at 360 ◦C, similarly to results published by Grillone etal. [18]. Electrolytes containing phosphoric acid behave similarly,when the phosphoric acid is stable within the studied tempera-ture range. The TGA measurement of PEMA–PC–H3PO4 (see Fig. 6)showed that 32 wt.% of the sample is remaining. This value corre-sponds to the H3PO4 content in the sample (27 wt.%).

4. Conclusions

New, proton-conductive membranes based on methacrylateswere prepared using direct, UV initiated radical polymerization.The main advantage of this method is in exact definition of (theprecisely defined) electrolyte composition and minimum contam-ination with water or oxygen during the preparation procedure.Polymerization under the UV light also allows immobilisation ofmore volatile solvents when the procedure is performed at theroom temperature (under 30 ◦C). Prepared samples are optically

transparent, long-term stable and exhibit a suitable conductivity(above 5 × 10−5 S cm−1) together with a reasonable electrochemicaland thermal stability. The perspective application in electrochromicdevices is supported by their material and electrochemical proper-ties summarised in the paper and good adhesion to the oxide thinlayers such as WO3 or V2O5. The devised method of preparation canalso be used to develop technology for manufacturing of large-scaleelectrochromic devices [24].

Acknowledgement

This work was supported by the Czech Science Foundation(grant no. 106/04/1279 and 104/06/1471), by the Grant Agency ofthe Academy of Sciences (grant B400320701), by the Ministry ofEducation (project MSMT LC523) and by the Academy of Sciences(research plan AV0Z40320502).

References

[1] M. Armand, Solid State Ionics 9/10 (1983) 745.[2] D.E. Fenton, J.M. Parker, P.V. Wright, Polymer 14 (1973) 589.

[[[[[

[

[[

[

[

[

[[[

[

[

ta 53 (2008) 7769–7774

[3] W.H. Meyer, Adv. Mater. 10 (1998) 439.[4] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nature 394 (1998) 456.[5] B. Scrosati, Electrochim. Acta 45 (2000) 2461.[6] R. Bouchet, S. Rosini, G. Vitter, E. Siebert, Sens. Actuators B 76 (2001) 610.[7] G. Vaivars, A. Azens, C.G. Granqvist, Solid State Ionics 119 (1999) 269.[8] K. Tadanaga, H. Yoshida, A. Matsuda, T. Minami, M. Tatsumisago, Chem. Mater.

15 (2003) 1910.[9] J. Przyluski, W. Wieczorek, S. Glowinkowski, Electrochim. Acta 37 (1992)

1733.[10] D. Raducha, W. Wieczorek, Z. Florjanczyk, J.R. Stevens, J. Phys. Chem. 100 (1996)

20126.[11] B.K. Choi, S.H. Park, S.W. Joo, M.S. Gong, Electrochim. Acta 50 (2004) 649.12] P. Gomez-Romero, J.A. Asensio, S. Borros, Electrochim. Acta 50 (2005) 4715.

[13] G. Zukowska, W. Wieczorek, M. Kedzierski, Z. Florjanczyk, Solid State Ionics 144(2001) 163.

[14] U. Lavrencic Stangar, N. Groselj, B. Orel, A. Schmitz, Ph. Colomban, Solid StateIonics 145 (2001) 109.

[15] U. Lavrencic Stangar, B. Orel, J. Vince, V. Jovanovski, H. Spreizer, A.S. Vuk, S.Hocevar, J. Solid State Electrochem. 9 (2005) 106.

[16] Y.D. Wang, M.F. Rubner, Synth. Met. 47 (1992) 255.[17] J. Lassegues, in: P. Comoban (Ed.), Proton Conductors: Solids, Membranes and

Gels—Materials and Devices, Cambridge University Press, Cambridge, 1992 (ch.20).

[18] A.M. Grillone, S. Panero, B.A. Retamal, B. Scrosati, J. Electrochem. Soc. 146 (1999)27.

[19] G. Vaivars, J. Kleperis, A. Azens, C.G. Cranqvist, A. Lusis, Solid State Ionics 97(1997).

20] A.T. Howe, M.G. Shilton, J. Solid State Chem. 28 (1979) 345.21] J. Vondrak, J. Reiter, J. Velicka, B. Klapste, M. Sedlarıkova, J. Dvorak, J. Power

Sources 146 (2005) 436.22] J. Reiter, J. Michalek, J. Vondrak, D. Chmelıkova, M. Pradny, Z. Micka, J. Power

Sources 158 (2006) 509.23] J. Reiter, J. Vondrak, J. Michalek, Z. Micka, Electrochim. Acta 52 (2006) 1398.24] J. Reiter, O. Krejza, J. Vondrak, Solar Energy Mater. Solar Cells, in preparation.25] G. Zukowska, V.J. Robertson, M.L. Marcinek, K.R. Jeffrey, J.R. Stevens, J. Phys.

Chem. B 107 (2003) 5797.

26] M. Aparicio, J. Mosa, M. Etienne, A. Duran, J. Power Sources 145 (2005) 231.27] J. Reiter, J. Vondrak, Z. Micka, Solid State Ionics 177 (2007) 3501.28] M.S. Ding, T.R. Jow, J. Electrochem. Soc. 151 (2004) 2007.29] C. Yu, M.H. Davey, F. Svec, J.M.J. Frechet, Anal. Chem. 73 (2001) 5088.30] Y. Sakai, M. Matuguchi, Y. Sadaoka, K. Hirayama, J. Electrochem. Soc. 140 (1993)

432.31] M. Pradny, P. Lesny, J. Fiala, J. Vacık, M. Slouf, J. Michalek, E. Sykova, Coll. Czech.

Chem. Commun. 68 (2003) 812.32] D. Cohn, A.S. Hoffman, B.D. Ratner, J. Appl. Polym. Sci. 29 (1984) 2645.33] M. Caillon-Caravanier, B. Claude-Montigny, D. Lemordant, G. Bosser, Solid State

Ionics 149 (2002) 275.34] M. Caillon-Caravanier, B. Claude-Montigny, D. Lemordant, G. Bosser, Solid State

Ionics 149 (2002) 285.35] A. Chagnes, A. Diaw, B. Carre, P. Willmann, D. Lemordant, J. Power Sources 145

(2005) 82.36] J. Qiao, N. Yoshimoo, M. Ishikawa, M. Morita, Solid State Ionics 156 (2003)

415.37] H.S. Kim, J.H. Shin, S.I. Moon, M.S. Yun, J. Power Sources 119–121 (2003) 482.38] R. Imhof, P. Novak, J. Electrochem. Soc. 146 (1999) 1702.39] D. Aurbach, M. Daroux, P. Faguy, E. Yeager, J. Electroanal. Chem. 297 (1991)

225.40] S. Daniele, I. Lavagnini, M.A. Baldo, F. Magno, J. Electroanal. Chem. 404 (1996)

105.[41] M. Moshkovich, Y. Gofer, D. Aurbach, J. Electrochem. Soc. 148 (2001) 155.42] J. Reiter, O. Krejza, J. Vondrak, J. Michalek, 11th EuroConference on Science and

Technology of Ionics 2007, Batz sur Mer, France, September 9–15, 2007, p. 24(meeting abstracts).