Proton-conducting polymer electrolytes based on methacrylates

Electrochimica Acta 53 (2008) 7769–7774

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Proton-conducting polymer electrolytes based on methacrylates Jakub Reiter a,∗,1 , Jana Velicka´ b , Martin M´ıka b a b

ˇ z near Prague, Czech Republic Institute of Inorganic Chemistry of the ASCR, v. v. i., 250 68 Reˇ 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 2008 Received in revised form 27 May 2008 Accepted 27 May 2008 Available online 3 June 2008 Keywords: Polymer electrolyte Proton conductivity Phosphoric acid Methacrylate Propylene carbonate Electrochromic device

a b s t r a c t Proton-conducting polymer electrolytes based on methacrylates were prepared by direct, radical polymerization of ethyl (EMA), 2-ethoxyethyl (EOEMA), and 2-hydroxyethyl methacrylate (HEMA). Samples with embedded solutions of phosphoric acid in propylene carbonate (PC), ␥-butyrolactone (GBL), N,N-dimethylformamide (DMF) and their mixtures were studied using impedance, voltammetrical and thermogravimetric methods. Membranes of long-term stability exhibit ionic conductivity up to 6.7 × 10−5 S cm−1 at 25 ◦ C reached for the sample PEMA–PC–H3 PO4 (31:42:27 mol.%). The accessible electrochemical potential window is 2.2–3 V depending on the working electrode material (glassy carbon or platinum). The thermogravimetric analysis shows that the membranes are thermally stable up to 110–130 ◦ C. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction Since the introduction of the 1st generation of polymer electrolytes based on poly(ethylene oxide) and lithium perchlorate by Armand [1] and Fenton et al. [2], the field of novel solid polymer ion-conducting materials have attracted interest of many research teams. In comparison with liquid and ceramic electrolytes the studied materials can be applied in many areas due to their advanced properties. Although the research is mainly aimed at the polymers with entrapped inorganic (mainly lithium) salts or their solutions in aprotic solvents [3–5], a considerable interest in membranes exhibiting proton conductivity can be observed due to their possible application in chemical sensors [6], electrochromic devices [7] or fuel cells [8]. Due to the proposed applications, the prepared membranes should meet four basic requirements: reasonable conductivity (above 10−5 S cm−1 ), good long-term, thermal and interfacial stability, stable mechanical properties and elasticity, and exactly defined and easy way of preparation. Recently, different materials and approaches in designing polymer electrolytes with proton conductivity have been reported. Solutions of phosphoric acid or its organic esters are usually embedded in the polymer network of poly(ethylene oxide), poly(methyl methacrylate), poly(glycidyl methacrylate), poly(vinylidene

∗ Corresponding author. Tel.: +420 266172198; fax: +420 220941502. E-mail address: [email protected] (J. Reiter). 1 ISE member. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.05.066

fluoride-hexafluoropropylene), poly-benzimidazoles or their copolymers [9–12]. Besides H3 PO4 and its esters [13], various heteropolyacids are used such as phosphomolybdic acid [12], phosphotungstic acid [14], silicotungstig acid [15] and methansulfonic acid [16]. The higher reactivity of strong acids such as sulphuric acid or para-toluensulfonic acid is a serious drawback for practical applications as decomposition of some solvents was described [17,18]. Therefore some other composites containing hydrated inorganic oxides (antimonic acid, aluminium oxide, silica) [7,19,26], zirconium phosphate [7] or uranium hydrogen phosphate [20] were suggested. All materials exhibit a reasonable conductivity at room temperature in the range from 10−5 to 10−2 S cm−1 . Our recent work is aimed at development of hybrid organic–inorganic membranes based on methacrylates. The developed method of direct, radical polymerization ensures minimum contamination of samples with aerial moisture or oxygen and follows our previous work on ternary polymer electrolytes for modern chemical applications such as lithium-ion batteries or electrochromic devices [21–24]. Also following our previous results, a moderately cross-linked polymer was formed due to the positive effect of the cross-linking agent low concentration (0.3–0.5 mol.% of monomer) on bulk conductivity [22,24]. Moreover, polymer cross-linkage improves mechanical properties, mainly long-term elasticity. In this paper we present results of our investigation of methacrylates of different polarity and its influence on conductivity. Also, different solvents (propylene carbonate, N,N-dimethylformamide

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and ␥-butyrolactone) were tested as embedded solvents. Results of investigation of particular binary or ternary liquid electrolytes were used for the polymer electrolyte formation and composition optimisation. Polymer gel electrolytes can be described as a system of immobilised microscopic liquid phase (aprotic solvent) in the cross-linked polymer network. The qualitative electrochemical behaviour of immobilised inorganic or organic species in the polymer system corresponds to their behaviour in solutions in particular solvents. The presence of the polymer does not influences qualitative behaviour of immobilised species, but it strongly affects the mobility and therefore conductivity of prepared samples. In addition to restricted ion mobility, the conductivity decrease is also due to decreased ion concentration. 2. Materials and procedures Monomers: ethyl, 2-ethoxyethyl and 2-hydroethyl methacrylate were obtained from Sigma–Aldrich and purified by distillation under reduced pressure. Cross-linking agents, ethylene dimethacrylate (EDMA) and 1,6-hexandiol dimethacrylate (hexamethylene dimethacrylate, HexadiMA; both Sigma–Aldrich) were used as received. Polymerization initiator, benzoine ethylether (BEE) was purchased from Fluka and recrystallised from chloroform. All monomers, cross-linking agents and the initiator were stored at 4 ◦ C before use. Propylene carbonate (PC; Sigma–Aldrich, >99.7%, water content <0.002%), ␥-butyrolactone (GBL; Sigma–Aldrich, >99%) and N,N-dimethylformamide (DMF; Merck, water content <0.05%) and mixed solvents PC–DMF 2:1 and 4:1 vol. and PC–GBL 1:1 vol. were stored under molecular sieves (3A pellets, Sigma–Aldrich). Phosphoric acid (anhydrous crystals, >99.99%, Sigma–Aldrich) was used as received.

were performed with a glassy carbon (2.0 mm diameter) or a platinum electrode (1.6 mm diameter, both from Bioanalytical Systems, United Kingdom) as the working, platinum as the counter and PMMA-Cd-Cd2+ as the reference electrode developed in our laboratory [27]. All potentials in the paper are related to this Cd/Cd2+ system (E(Cd/Cd2+ ) = −0.44 vs. SCE, measured in propylene carbonate). The advantage of the PMMA-Cd-Cd2+ electrode is in its availability for both liquid and solid-state electrochemical measurements. The surface of the working and the counter electrode was polished by abrasives (0.3 alumina, Metroohm) and soft cloth after each measurement. Conductivities of liquid solutions of H3 PO4 were measured at 20 ◦ C using a conductivity cell (Jenway, platinum electrodes, cell constant S = 1.00 ± 0.01). Temperature dependent conductivity measurements were performed in the temperature range from −10 to 70 ◦ C using thermostated bath (precision of the temperature ±1 ◦ C). A similar procedure was used for the measurement of conductivity of solid samples in the temperature range from −25 to 70 ◦ C. Here, a two-electrode arrangement was used, when a slice of gel (2 cm × 2 cm, 1 mm thickness) was sandwiched between two parallel stainless steel electrodes. In both cases single potential impedance spectra were measured in the frequency range from 100 kHz to 1 Hz. The obtained spectrum was analysed by the EcoChemie Autolab software producing the values of the equivalent circuit elements. The value of ohmic resistance was converted in the 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 simultaneous TGA-DTA measurement was taken in air and argon at the heating rate of 5 ◦ C min−1 . Experiments were performed with a Simultaneous Thermal Analysis Netzsch STA 409 (Germany). 3. Results and discussion

2.1. Preparation of the gel electrolyte The method of electrolyte preparation was described in our previous paper [22]. The polymerization was carried out in a cell formed of polypropylene plate, packing distance frame (silicone rubber) and glass plate. The cell was carefully filled with nitrogen and then with the initial liquid mixture containing the monomer, cross-linking agent, solvent, acid and polymerization initiator. For radical polymerization, benzoine ethylether was used as an initiator active under UV light. The UV light initiated polymerization proceeded for a period of 2 h at room temperature using a pair of 15 W ReptiGlo 8.0 lamps emitting UV-A and UV-B light (Hagen, Czech Republic). The obtained samples, foils with an area 3 cm × 4 cm and thickness corresponding to the thickness of used silicone seals (1.00 ± 0.10 mm), were more or less elastic in dependence on the solvent content and mostly transparent. This method of preparation prevents evaporation of the volatile monomer, and the initial liquid mixture is only in a minimum contact with air. The composition of the gel is expressed in the molar percentage ratio polymer/solvent(s)/acid. The amount of the cross-linking agent is always included in the monomer content. Besides the polymerization initiated by UV light, one can use the thermally initiated process using, e.g. 2,2 -azobis(isobutyronitrile) or dibenzoylperoxide [21,25,26]. 2.2. Equipment, electrodes and cells Potentiogalvanostats PGSTAT 10 and 30 (Eco Chemie, The Netherlands) were used for electrochemical measurements including the FRA-2 module for impedance measurements. Electrochemical measurements of both liquid and polymer systems

Prepared samples are elastic and homogeneous membranes. They are mostly transparent or slightly yellowish or greenish and are not hygroscopic. Contrary to previously studied electrolytes with embedded solutions of inorganic salts, samples with H3 PO4 can contain a higher content of acid solution in propylene carbonate (polymer content 31 mol.%). Although all samples exhibit long-term stability in the air, they are stored in a desiccator. No phase separation or acid crystallisation was observed during 1-month storage. We attempted to prepare methacrylate-based electrolytes containing silicotungstic and phosphotugstic acid similarly to the ternary systems based on organically modified silicates [14,15]. Used methacrylates are decomposed (hydrolysed) during the polymerization process, and the heteropolycids react both with the monomer and also with propylene carbonate. A similar effect was observed by Grillone in the case of para-toluensulfonic acid and propylene carbonate solutions, where ethylene and propylene carbonate are hydrolysed to carbon dioxide and corresponding glycols [18]. 3.1. Impedance and conductivity measurements The first measurements were done with liquid electrolytes PC–H3 PO4 and DMF–H3 PO4 in the concentration range from 0 to 6 mol dm−3 of H3 PO4 . Solutions of phosphoric acid in both solvents are very viscous and the conductivity is increasing with increasing acid content up to 5.2 × 10−4 S cm−1 for the 6 mol dm−3 solution in PC and 2.7 × 10−4 S cm−1 for the 6 mol dm−3 solution in DMF. H3 PO4 solutions in the PC–DMF exhibit a lower conductivity within the whole concentration range, although DMF-based solutions exhibit a lower viscosity. It seems to be due mainly to its lower dielectric

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Fig. 1. Effect of the H3 PO4 concentration on ionic conductivity in liquid PC–H3 PO4 (full dots) and DMF–H3 PO4 (empty dots) electrolytes in the concentration range 0.1–6 mol dm−3 H3 PO4 measured at 20 ◦ C.

constant (38.3) as compared to PC (64.4). Fig. 1 presents the isothermal relationship between the acid concentration and conductivity for two liquid electrolytes PC–H3 PO4 and DMF–H3 PO4 . In both cases the bulk conductivity is increasing in the whole concentration range. The described behaviour differs from the behaviour of some other electrolytes in aprotic solvents, e.g. lithium salts, where ion–ion association causes conductivity decrease at higher concentrations [22,28]. In the case of H3 PO4 solutions, the effect of ion pairing at high concentrations is not so strong to cause such effect. Following the relationship between conductivity and H3 PO4 concentration, 6 mol dm−3 solutions of H3 PO4 were used for further experiments. Considering the positive effect of the presence of aprotic protophilic solvents in H3 PO4 -based systems on their conductivity [10,18], we also prepared solutions of phosphoric acid in binary solvents, PC–DMF 4:1, PC–DMF 2:1 and PC–GBL 1:1. Conductivities of prepared binary and ternary liquid electrolytes are summarised in Table 1. As can be seen, the presence of DMF in PC–H3 PO4 results in only a small improvement of conductivity. Fig. 2 illustrates the relationship between the ionic conductivity of 6 M H3 PO4 solutions and temperature. The data are plotted in Arrhenius coordinates (specific conductivity is plotted as a decadic logarithm). PC, PC–DMF and PC–GBL solutions exhibit a similar behaviour in contrast to the DMF solution where lower conductivities were observed in the whole temperature range corresponding to the values plotted in Fig. 1. The obtained data can be fitted with the Vogel–Tamman– Fulcher (VTF) equation in the logarithmic form: T 1/2 = A exp



−EA R(T − T0 )

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In this particular relationship A is the parameter related to the number of charge carriers, EA is the activation energy for conduction, R is the universal gas constant and T0 the ideal glass transition temperature indicating the temperature at which the free volume extrapolates to zero. The analysis of the experimental conductivity data in terms of the VTF relationship leads to the determination of three empirical parameters: A, EA and T0 , when T0 is determined by fitting the experimental data with relationship (1). The conductivity activation energy corresponds to a slope in the Arrhenius coordinates (see Fig. 2) and explains how conductivity is influenced by temperature. Activation energy values for liquid systems are given in Table 1, when the EA value describes the dependency of conductivity on temperature. It can be seen that the EA values for all studied H3 PO4 solutions do not vary with the type of solvent and are from 35.2 to 36.9 kJ mol−1 . From this small difference we can conclude that the conduction 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 H3 PO4 are more sensitive to the influence of temperature on conductivity ion–ion association and also viscosity. In other aprotic solutions such as tetraalkylammonium perchlorates or tetrafluoroborates, much lower EA values were found, which indicates that their conductivity is less influenced by temperature. Immobilisation of the PC–H3 PO4 solutions in all methacrylates leads to conductivity decrease. The mobility of both H+ and all phosphate ions is restricted due to the presence of the polymer network. Our work was aimed at optimisation of the ternary system composition to reach reasonable bulk conductivity. Contrary to the previously reported PC–LiClO4 polymer electrolytes, the physical properties of anhydrous phosphoric acid enabled us to prepare samples with a lower polymer content (down to 31 mol.%) but retaining their good mechanical properties (elasticity and shape stability). Along with sample optimisation we studied the influence of polymer polarity on the electrochemical properties of prepared electrolytes. Three methacrylates with increasing polarity were chosen: ethyl, 2-ethoxyethyl and 2-hydroxyethyl methacrylate. 2-ethoxyethyl methacrylate is compatible with inorganic salts, e.g. LiClO4 , and forms binary electrolytes PEOEMA–LiClO4 , but with low conductivity



(1)

Table 1 Conductivities of H3 PO4 liquid electrolytes (6 mol dm−3 H3 PO4 concentration) Liquid electrolyte PC–H3 PO4 DMF–H3 PO4 PC–DMF–H3 PO4 PC–DMF–H3 PO4 PC–GBL–H3 PO4

Composition (mol.%) 59:41 52:48 38:21:41 47:12:41 30:30:40

 (20 ◦ C) (S cm−1 ) −4

5.2 × 10 2.7 × 10−4 5.9 × 10−4 5.6 × 10−4 4.9 × 10−4

EA (kJ mol−1 ) 36.9 36.2 35.2 – 35.9

Fig. 2. Arrhenius plot of liquid electrolytes containing 6 mol dm−3 H3 PO4 in PC, DMF, PC–DMF 2:1 and PC–GBL 1:1 solvents (temperature range from −10 to 70 ◦ C).

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4.2 × 10−7 S cm−1 (cit. [22]). 2-Hydroxyethyl methacrylate is widely used for its biocompatibility and hydrophilicity in separation techniques, chemical sensors and biomaterials [29–32]. Our measurements showed that the PEMA–PC and the PEOEMA–PC binary systems without added inorganic salt behave as insulators with conductivity lower than 10−7 S cm−1 at 20 ◦ C. All samples with immobilised PC–H3 PO4 solution were more or less elastic in dependence on the solvent content and mostly transparent. In the case of equivalent PEMA and PEOEMA samples containing DMF, phase-to-phase separation (production of heterogeneous samples with liquid excluded on its surface) was observed. PEMA is non-compatible with DMF solutions in a wide range of sample composition. On the contrary, due to its different polarity all samples could be prepared using HEMA with PC–DMF (2:1 and 4:1 vol.) and PC–GBL (1:1 vol.) mixed solvents. All attempts to form a polymer electrolyte with DMF only were not successful due to phase to phase separation independently of the polymer type. Impedance measurements were used for bulk conductivity determination. Specific conductivities of some representative gel electrolytes are summarised in Table 2. The highest conductivity was found for the PEMA–PC–H3 PO4 electrolyte, 6.7 × 10−5 S cm−1 , and co-immobilisation of DMF did not lead to increase of conductivity. Co-immobilisation of ␥-butyrolactone lead to decrease of conductivity, especially in the case of PEOEMA and PEMA electrolytes where the prepared samples exhibited an order lower conductivity than the corresponding samples with PC and PC–DMF solvents. A comparison between liquid and polymer electrolytes shows an order lower conductivity values after polymerization, what is corresponding which corresponds to our previous investigation of PC–LiClO4 electrolytes [21,22]. It is generally accepted that the ionic transport in polymer gel electrolytes takes place mainly in the solvent phase [33,34]. This was also confirmed in the case of methacrylate-based systems prepared and studied in our laboratory [18]. In the series of samples with decreasing polymer content we observed increase of conductivity, and the character of the impedance spectrum was changed as well. At low conductivities a wide semicircle representing the parallel connection of a capacitor C and a resistor R was observed corresponding to the dielectric behaviour of the polymer and the bulk resistivity of the sample, while with increasing conductivity the semicircle was depressed and a simple serial connection of resistor R and pseudocapacitor Q (a constant phase element) was found. In both cases, similar impedance spectra were obtained in our previous research [22,23]. In methacrylate-based polymer electrolytes with ionic compounds the relationship between conductivity and temperature is more complex due to the presence of polymer. Fig. 3 shows the Arrhenius plot for PEMA, PEOEMA and PHEMA polymer electrolytes with PC–H3 PO4 solution. These samples were chosen to compare the influence of the polymer

Fig. 3. Arrhenius plot of polymer electrolytes PEMA–PC–H3 PO4 (31:41:28), PEOEMA–PC–H3 PO4 (42:34:24) and PHEMA–PC–H3 PO4 (33:39:28 mol.%); temperature 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 concentration of ions, e.g. ionic liquids [35] or solutions of inorganic salts in high concentrations [36]. In the case of PC–H3 PO4 polymer electrolytes, three-order difference in conductivity can be seen in the temperature range from −25 to 70 ◦ C in Fig. 3. The highest conductivity values were reached for PEMA–PC–H3 PO4 electrolyte. These results correspond to those obtained at room temperature while PEOEMA and PHEMA electrolytes are less conductive in the whole temperature range. The EA values are increasing from 10.7 to 32.2 kJ mol−1 in the series PEMA–PEOEMA–PHEMA. Possible explanation in increasing polarity of the polymer and consequent increase of the EA value is however uncertain. At lower temperatures the presence of polymer, and also the increasing viscosity of immobilised solutions strongly influence the dielectric behaviour of all samples. The previously discussed semicircle appears at temperatures under 10 ◦ C, and the parameters of the particular equivalent circuit correspond to the dielectric behaviour of the polymer phase. At higher temperatures, a simple RQ circuit can be found. The conductivity values for 60 ◦ C for PEMA, PEOEMA and PHEMA–PC–H3 PO4 samples are 0.60, 0.10 and 0.12 mS cm−1 , respectively. In comparison to room temperature, one can observe one order increase of conductivity (see Table 2).

Table 2 Specific conductivities (at 20 ◦ C) of prepared polymer electrolytes containing H3 PO4 solutions together with estimated apparent activation energy values (EA ), the glass transition temperatures determined by DSC (Tg ) and the ideal glass transition temperature (T0 ) Polymer electrolyte

Composition (mol.%)

 (20 ◦ C) (S cm−1 )

EA (kJ mol−1 )

Tg (◦ C)

T0 (◦ C)

PEMA–PC–H3 PO4 PEMA–PC–DMF–H3 PO4

31:42:27 Non-compatible

6.7 × 10−5 –

10.7 –

−89.4 ± 0.8 –

−122 –

PEOEMA–PC–H3 PO4 PEOEMA–PC–DMF–H3 PO4 PEOEMA–PC–GBL–H3 PO4

31:42:27 38:28:8:26 41:16:18:24

1.5 × 10−5 1.3 × 10−5 3.7 × 10−6

12.9 – –

−68.9 ± 0.6 – –

−119 – –

PHEMA–PC–H3 PO4 PHEMA–PC–DMF–H3 PO4 PHEMA–PC–GBL–H3 PO4

34:40:26 32:26:14:28 35:18:20:26

2.1 × 10−5 2.7 × 10−5 1.1 × 10−5

32.2 – –

−67.3 ± 0.9 – –

−192 – –

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Fig. 4. Cyclic voltammograms of 6 mol dm−3 H3 PO4 in PC, PC–DMF 2:1 and PC–GBL 1:1 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 polymer electrolytes

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Fig. 5. Cyclic voltammograms of polymer electrolytes PEMA–PC–H3 PO4 (31:41:28) and PHEMA–PC–H3 PO4 (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 of liquid electrolytes. 3.3. Thermal stability

The high electrochemical stability of propylene carbonate is well known from literature. The PC is widely used in electrochemical power sources, mainly lithium-ion batteries [37]. Also the other solvents (DMF and GBL) are electrochemically reasonably stable. When propylene carbonate is electrochemically stable from −2.5 to 2.5 to 2.8 V vs. Cd/Cd2+ , the determining parameter of the cathodic stability of H3 PO4 aprotic solutions is the reduction of hydrogen ions. Fig. 4 presents the cyclic voltammetry of 6 mol dm−3 H3 PO4 in PC, PC–DMF and PC–GBL. A rapid evolution of hydrogen was observed on the platinum electrode at 0.3 V vs. Cd/Cd2+ independently on of the used solvents. This process proceeds at higher potentials compared to work of Daniele, who found the H+ reduction on microelectrodes at lower potentials (−0.6 V vs. Cd/Cd2+ ) in comparison with our experiments done in propylene carbonate (+0.3 V) [40]. The anodic limit at the potential, above 2.5 V vs. Cd/Cd2+ can be assigned to the oxidation of solvent corresponding to the previously observed measurements in our laboratory and confirmed by other authors [38,39]. In agreement with literature, a partial oxidation of carbonates at high potentials above 2.5 V was observed, close to the anodic limit [39,41]. However, the anodic processes in aprotic proton-conductive membranes were not studied intensively in literature, and we can expect that the present phosphoric acid will affect the solvent and participate in electrode reactions (solvent oxidation). This effect can be stronger in comparison with polymer electrolytes containing inorganic salts. The possible effect is probably due to H+ cations, while phosphoric anions can be declared as electrochemically stable. The substitution of the platinum electrode with glassy carbon influences only the cathodic limit. Due to the higher overvoltage of hydrogen on glassy carbon in comparison to platinum, the reduction of hydrogen appears at 0.6 V lower potentials. Voltammetrical measurements of polymer electrolytes showed a similar behaviour, as it is declared in Fig. 5. The electrochemical stability of used polymers is exceeds 4.8 V on both platinum or glassy carbon electrodes [42]. The potentials of H+ reduction and

Concerning applications, thermal stability is another principal parameter to be considered along with electrochemical and longterm stability and reasonable conductivity. Fig. 6 shows TGA and differential gravimetric analysis (DTG; 1st derivative of the TGA curve) of two samples: PEMA–PC and PEMA–PC–H3 PO4 . The weight loss 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. 6. TGA and DTG curves for PEMA–PC (50:50 mol.%) and PEMA–PC–H3 PO4 (30:41:29 mol.%) gel electrolytes (5 ◦ C min−1 heating rate, temperature range from 30 to 380 ◦ C; argon atmosphere).

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immobilised solvent (PC). However, the change is small and corresponds to the low vapour pressure of the solvent (pPC = 130 Pa at 50 ◦ C). Both in air and argon all samples start to decompose at temperatures above 130 ◦ C. It is well known that methacrylates decompose in two exothermic reaction processes: degradation of the polymer end groups (240–280 ◦ C) and total decomposition of the monomer units (above 310 ◦ C). The presence of immobilised solvent—propylene carbonate is complicating the explanation of the experimental data complicates experimental data interpretation. The boiling point of PC is 240 ◦ C at 101.3 kPa. The PEMA–PC sample is practically (97 wt.%) decomposed at 360 ◦ C, similarly to results published by Grillone et al. [18]. Electrolytes containing phosphoric acid behave similarly, when the phosphoric acid is stable within the studied temperature range. The TGA measurement of PEMA–PC–H3 PO4 (see Fig. 6) showed that 32 wt.% of the sample is remaining. This value corresponds to the H3 PO4 content in the sample (27 wt.%). 4. Conclusions

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

[18] [19]

New, proton-conductive membranes based on methacrylates were prepared using direct, UV initiated radical polymerization. The main advantage of this method is in exact definition of (the precisely defined) electrolyte composition and minimum contamination with water or oxygen during the preparation procedure. Polymerization under the UV light also allows immobilisation of more volatile solvents when the procedure is performed at the room 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 electrochemical and thermal stability. The perspective application in electrochromic devices is supported by their material and electrochemical properties summarised in the paper and good adhesion to the oxide thin layers such as WO3 or V2 O5 . The devised method of preparation can also be used to develop technology for manufacturing of large-scale electrochromic devices [24].

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]

Acknowledgement

[35]

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

[36]

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