Electrochemistry Communications 6 (2004) 432–434 www.elsevier.com/locate/elecom
Ambient temperature proton conducting plastic crystal electrolytes Y. Abu-Lebdeh, A. Abouimrane *, P.-J. Alarco, A. Hammami, L. Ionescu-Vasii, M. Armand Department of Chemistry, International Laboratory on Electro-active materials, University of Montreal, C.P. 6128, Succ Centre-Ville, Montreal, Canada H3C 3J7 Received 6 February 2004; received in revised form 24 February 2004; accepted 24 February 2004 Published online:
Abstract A non-aqueous ionic liquid based on imidazole was prepared and showed high proton conductivity. Its incorporation into the plastic crystal phase of succinonitrile at low molar proportions resulted in a solid material with high conductivity throughout the plastic crystal range and of particular interest at 25 °C (1.5 103 S/cm). The addition of a compatible polymer have resulted in a remarkable increase in the mechanical properties but diminished the conductivity by a one-and-a half order of magnitude. Ó 2004 Published by Elsevier B.V. Keywords: Plastic crystals; Protonic conductor; Fuel cell; Solid electrolytes
1. Introduction Most proton conducting membranes suﬀer from the problem of the dependence of their conductivities on changes in the humidity and temperature of the environment because of the requirement of structural water in conductivity mechanisms involving H3 Oþ [1,2]. This imposes diﬃculties when they are used in devices like fuel cells, sensors, batteries, and displays . In order to resolve the problem, non-aqueous heterocyclic compounds such as imidazole have been studied as nonaqueous proton acceptors to support ionic conduction by a Grotthus-type mechanism . Previous work on the subject has been based on immobilizing the heterocyclic compound into polymers to be used as membranes for fuel cells at intermediate temperatures [5,6]. However, useful conductivities were only obtained at high temperatures. Using a diﬀerent approach, high conductivities were obtained near room temperature from the ionic liquid Im2 I obtained by combining bis-triﬂuoromethane sulphonimide and imidazole in a 1:2 molar ratio [7–9]. The equal concentrations of imidazole (I) and imidazolium imide salt (II) resulted in a composition close *
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1388-2481/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.elecom.2004.02.015
to the eutectic point and a maximum theoretical conductivity according to a simple Grotthus mechanism  (Scheme 1). Interest in plastic crystals has been revived recently after the recognition of their potential use as a new class of solid electrolytes [11–14]. These are characterized by the rotational and/or orientational disorder of individual species (molecules, atoms, ions) whilst keeping the long-range translational order within a crystal lattice. A consequence of this type of disorder is a high diﬀusivity and plasticity [15,16]. Previous examples of plastic crystals with proton conductivity have been found, based on the CsHSO4 family . However, in that case, the plastic phase and the associated high conductivities were only encountered at high temperatures. Despite the preliminary good performance in fuel cell testing, technological and other problems have still to be overcome. It has been recently reported that the plastic crystalline phase of succinonitrile (SC) (III) can be doped with various salts to give ﬂexible solid materials with high conductivities over a wide temperature range [18,19]. An attempt by MacFarlane and co-workers  to dissolve an acid into SC unfortunately resulted very low conductivities, probably due to poor dissociation given the low aﬃnity of the nitrile group to protons. The alternative approach we are taking herein is to dope with
Y. Abu-Lebdeh et al. / Electrochemistry Communications 6 (2004) 432–434
Scheme 1. Structures of Electrolyte components.
Im2 I liquid which, by contrast with a pure acid, is already dissociated and therefore does not require basic properties in the host matrix.
2. Experimental Imidazole (Aldrich) was added to bis(triﬂuoromethylsulfonyl)imide (Fluka) in a 2:1 molar ratio. The exothermic reaction of the two solids gave the lightyellow liquid Im2 I, which was dried in vacuo overnight to remove any water introduced with the reactants. Im2 I–SC samples were made by adding the required amount of Im2 I to molten SC, Aldrich. Poly(acrylonitrile), Aldrich, was added in small proportions to the molten Im2I–SC sample. Conductivity measurements were performed using the impedance spectroscopy technique. The sample was sandwiched between two stainless steel electrodes with a 1 cm diameter. The frequency was swept between 5 Hz and 13 MHz using an HP frequency analyzer. The resistance of the sample was taken as the value at which the low-frequency end of the semi-circle crosses the x-axis of the complex impedance plot. The temperature was varied between )10 and 80 °C with 5 °C steps, allowing 20 min for thermal equilibration at each temperature. Diﬀerential Scanning Calorimetric analysis (DSC) was performed using a Perkin–Elmer Pyris 1. All the samples were sealed in aluminium pans in a dry box and then scanned from )150 to 160 °C at 20 °C/min. All manipulations were performed in a He-ﬁlled glove box.
3. Results and discussion Addition of 2.5 mole% or less Im2 I to SC resulted in solid materials. Above that, as in the case of 5 mole%, gel-like materials were obtained. The conductivity of the 2.5 mole% sample was found to be 1.5 103 S/cm at 25 °C, approaching that of the neat Im2 I, which is 5 103 S/cm. The surprisingly high conductivity value is unprecedented for such a low concentration of a conducting species in an immobilizing solid matrix. (Previous work on incorporating the Im2 I into various polymeric hosts, e.g., PVdF, have resulted in conductivities reaching only 104 S/cm, at the
most, and only at much higher concentrations [9,20].) It can also be observed that the conductivity increased as a function of temperature with low apparent activation energy of conduction within the plastic crystal range. The conduction mechanisms in the molten Im2I diﬀer considerably from that in the plastic crystalline state, and therefore diﬀerent temperature-conductivity dependence are observed. Their apparent activation energies are Ea ¼ 0.9 kJ/mole (calculated from VTF)  and Ea ¼ 17.1 kJ/mole (calculated from Arrhenius) for the Im2 I and the 2.5 mole% Im2 I–SC, respectively, representing the comparable ease of ionic conduction of the plastic crystalline phase, with regard to that of the ionic liquid. Succinonitrile is highly plastic and, like other plastic crystals, deforms easily without fracture under low stress . However, its poor mechanical strength is observed as it snaps easily on little manual stretching. In order to overcome the latter a compatible polymer poly(acrylonitrile), PAN, was added in low weight percentages to improve the bulk mechanical properties. We found that a maximum of 3 wt% PAN can be added to give a very ﬂexible homogeneous material that can be shaped into thin ﬁlms. The conductivity of the 2.5 mole% Im2 I–SC with 3 wt% PAN is shown as a function of temperature in Fig. 1. It can be seen that the addition of PAN decreased the conductivity by one-and-a half orders of magnitude from 1.5 103 to 3.8 105 S/cm at 25 °C. Similar behaviour was reported by MacFarlane and coworkers  in the case of organic plastic crystal electrolytes and amorphous poly(ethyleneoxide). The DSC scans of the neat SC, 2.5 mole% Im2 I–SC and 2.5 mole% Im2 I–SC containing 3 wt% PAN were performed in order to check for the integrity of the plastic crystal phase, Fig. 2. SC exhibits a plastic crystal phase between )35 and 65 °C. Upon the addition of the
-1 2.5 mole% Im2I in SC
Log (conductivity/ S/cm)
25 mole% Im2I in SC containing 3 wt.% PAN.
T / ˚C
Fig. 1. Conductivity as a function of temperature of 2.5 mole% Im2 I in SC and 2.5 mole% Im2 I–SC containing 3 wt% PAN.
Y. Abu-Lebdeh et al. / Electrochemistry Communications 6 (2004) 432–434
were successful but this was unfortunately accompanied by large decrease in conductivity. Neat SC Endotherm
Acknowledgements 2.5 mole% Im2I in SC
The authors thank Dr. J. Owen for useful discussions and proof-reading the manuscript.
2.5 mole% Im2I in SC containing 3 wt.% PAN
T / ˚C
Fig. 2. DSC scans of neat SC, 2.5 mole% Im2 I–SC and 2.5 mole% Im2 M in SC containing 3 wt% PAN.
Im2 I the melting point was depressed to 45 °C while Tpc (the normal crystal to plastic crystal transition temperature) remained unaﬀected. A small shoulder can be observed on the transition peak. (This is possibly due to the melting of an eutectic formed between SC and Im2 I; although no bulk liquid was observed, it is possible that liquid regions could be housed within the extensive dislocation networks known to exist in the plastic crystal phase.) When PAN was added a similar decrease in melting point was observed, but this time to 55 °C. The DSC scan also indicates an increase of Tpc to )25 °C and a small broad additional peak with a maximum at 20 °C. Both can be interpreted as evidence of interaction between the polymer chains and the plastic crystal phase, thus explaining the large decrease in the conductivity of the 2.5 mole% Im2 I–SC upon the addition of the PAN.
4. Conclusions A proton conducting solid electrolyte with conductivities reaching above 103 S/cm was prepared by incorporating a non-aqueous liquid electrolyte into the plastic crystalline phase of succinonitrile. The solid can support conductivities in dry atmosphere due to the existence of Grotthus mechanism between the imidazole and imidazolium species. Attempts to improve the mechanical properties of the solid by the addition of PAN
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