Poly(ethylene oxide-co-epichlorohydrin)/NaI: a promising polymer electrolyte for photoelectrochemical cells
Introduction
Polymer electrolytes have attracted great attention after the pioneering work of Fenton et al. [1] and the proposal of Armand et al. [2] that these systems could be used in secondary batteries. The increasing interest on polymer electrolytes is related to the possibility of its use in different solid state electrochemical devices, such as: batteries [3], capacitors [4], electrochromic displays [5], [6], light emitting electrochemical cells [7], artificial muscles [8] and photoelectrochemical cells [9]. The major challenge into replacing the liquid electrolyte by a polymer is to keep the high operation efficiency similar to the electrochemical devices based on liquid junctions. Besides of improving the stability of the active interface allowing a long term durability, a polymer electrolyte eliminate problems concerning evaporation or leakage of solvent.
Poly(ethylene oxide) (PEO) is the reference polymer for ionic conduction, since it is the best matrix for alkali salts [2], [10]. The conductivity pathway is assigned to the amorphous phases of the polymeric matrix, assisted by the segmental motion of the polymeric backbone. Due to its high crystallinity, PEO-based electrolytes show a reasonable ionic conductivity only above its melting temperature (ca. 65°C), precluding its application at ambient temperature. To reduce the crystalline phase, increase the amorphous content and the ionic conductivity, several alternatives have been proposed: (i) modification of PEO by formation of cross-linked networks, block or comb branched copolymers or plasticization, (ii) the use of structurally similar polymers such as poly(propylene oxide), and (iii) the use of other amorphous polymers, with lower crystallinity and capable of dissolving salts [11], [12].
Blends obtained by the mixture of PEO with different acrylic polymers showed higher ionic conductivity in comparison to pure PEO–salt complexes [13], [14]. This is assigned to the decrease of PEO crystallinity and the presence of a flexible amorphous phase. Due to the low crystallinity of the copolymers prepared with the comonomers ethylene oxide and epichlorohydrin, Kohijiva et al. [15] investigated their ionic property when filled with LiClO4 obtaining 10−6 S cm−1 at 30°C. The addition of polyethylene glycol to this elastomer complexed with LiBF4 was also reported to produce an ionic conductivity of 10−5 S cm−1 [16].
In previous works, we investigated the thermal behavior and ionic conductivity of complexes of LiClO4 and poly(epichlorohydrin), poly(ethylene oxide-co-epichlorohydrin) (50/50) and the terpolymer composed by ethylene oxide, epichlorohydrin and allyl-glycidylether [17], [18]. The samples presented a good dimensional stability and an ionic conductivity in the range of 10−4–10−5 S cm−1 for the last two copolymer complexes. More recently, a systematic study of the copolymer poly(ethylene oxide-co-epichlorodrin) (comonomers ratio=84/16) was carried out in our laboratory [19]. The copolymer containing 5.5% (w/w) of LiClO4 presented an ionic conductivity of 4.1×10−5 S cm−1 at 30°C ([H2O])<1 ppm).
Attempts have been made to improve the electrochemical parameters of photoelectrochemical devices with solid-state electrolytes, which remain still low in comparison with the cells where the photo and electrochemical effects take place at liquid junctions. Skotheim [20] and Skotheim and Inganäs [21] proposed the application of PEO containing a redox couple I−/I3− deposited onto a silicon substrate as electrolyte into assembling a solid-state photoelectrochemical cell. Rao et al. [22], [23] studied the addition of NaYF4 and KYF4 into PEO and poly(vinylpyrrolidone) (PVP), respectively. They assembled a cell with configuration Na/polymer+salt (I2+Carbon+electrolyte) and studied their discharge characteristics. Mohamed et al. [24] investigated complexes of PEO and NaI with different salt contents. A polymer electrolyte with conductivity of 5.2×10−5 S cm−1 ([Na]/[O]=0.022) was used as electrolyte for the fabrication of an electrochemical cell.
Yohannes et al. [25] and Yohannes and Inganäs [26] constructed a solid-state photoelectrochemical cell based on conducting polymers (poly(3-methylthiophene) and poly(3-octythiophene)), replacing the liquid electrolyte by a polymer electrolyte based on PEO complexed with I3−/I−. However, the performance is still low in comparison with the same cells using a liquid electrolyte.
In this work, we studied the thermal and conductivity properties of the copolymer poly(ethylene oxide-co-epichlorohydrin), Epichlomer-16, produced by Daiso (Japan) with monomer ratio of 84/16, respectively, and complexed with NaI and I2. The latter was added to the samples aiming at assembling electrochemical devices, which require a redox couple.
Section snippets
Experimental
Poly(ethylene oxide-co-epichlorohydrin) with comonomers ratio of 84/16 (Epichlomer-16) was used as received from Daiso. The copolymer molar mass reported by the supplier is 1.3×106 g mol−1. Fig. 1 depicts the structure of this copolymer.
The samples of Epichlomer-16/NaI–I2 complex with different salt concentrations (0–23% (w/w)) were prepared by the dissolution of the copolymer and the salt in a mixture of tetrahydrofurane (THF) and ethanol (1:1). The concentration of iodine was 0.9% (w/w) for
Ionic conductivity measurements
Typical Nyquist plots of polymer electrolytes sandwiched between two blocking electrodes resemble a semicircle, and its diameter supplies a resistance value that is usually used to estimate the ionic conductivity of the polymer film. The ionic conductivities at 26°C for Epichlomer-16/NaI complexes containing different salt concentrations are shown in Fig. 2. The salt concentration is expressed as nEO (ratio between the molar concentration of oxygen and sodium, calculated considering only the
Conclusions
The elastomer poly(ethylene oxide-co-epichlorohydrin), Epichlomer-16 filled with NaI and I2 exhibits an ionic conductivity comparable to other polymer electrolytes and equal to 1.5×10−5 S cm−1 (30°C, humidity<1 ppm) and 2.0×10−4 S cm−1 at 86% relative humidity (22°C).
Temperature-dependence of the ionic conductivity was investigated by fitting to the empirical VTF equation with low deviation in relation to the experimental data. The values obtained for B and σ0 are in agreement with other
Acknowledgements
The authors thank Daiso (Osaka, Japan) for the donation of Epichlomer samples and FAPESP for financial support (Proc. 96/09983-0) and fellowships (AFN Proc. 98/10567-6, EMG Proc. 97/02156-3 and WAG Proc. 97/14132-1). MAP also acknowledges a research fellowship from CNPq and the PRONEX program.
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