ESR Investigation of Copper Ion Mobility in PEO:Cu(CF 3 SO 3 ) 2 Complexes
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ESR INVESTIGATION OF COPPER ION MOBILITY IN PEO:Cu(CF 3 SO 3)2 COMPLEXES K.J. ADAMICt, S.G. GREENBAUMt*, S. PANEROQ, P. PROSPERIO and B. SCROSATi: tHunter College of CUNY, Department of Physics, New York, NY 10021 Weizmann Institute of Science, Departmentof Chemical Physics, Rehovot, Israel tUniversita di Roma, Dipartimentodi Chimica, 1-00185 Rome, Italy
ABSTRACT. Polymer electrolytes consisting of poly(ethylene oxide) (PEO) complexed with Cu(CF 3 SO3) 2 have been studied by differential scanning calorimetry (DSC), electron spin resonance (ESR) and ionic conductivity measurements. Although motional effects on the ESR spectra are apparent, the present results cannot distinguish between long range Cu 2+ transport or the motion of copper ions attached to polyether segments. INTRODUCTION Complexes formed between poly(ethylene oxide) (PEO) and alkali metal salts have been studied extensively, with emphasis on properties related to alkali ion mobility which can be exploited in electrochemical applications such as batteries and electrochromic displays [1,2]. Recent work on PEO complexed with divalent metal salts has been driven by general interest in the electrochemistry of divalent ions, although the prospect of applications is somewhat dampened by the relatively low cationic conductivity typical of these materials [3]. For example, cation transference numbers in PEO:Cu 2+-salt complexes range from 0.05 to 0.24, depending on the particular sample and measurement technique employed [4-6]. Although copper ion transport has been demonstrated by Cu plating /stripping cycles on stainless steel electrodes, factors such as interfacial (electrolyte-electrode) reactions often lead to large uncertainties in experimental transference number determinations [6]. Electron spin resonance (ESR) has been employed in attempting to probe the extent of Cu2 + ion mobility in poly(propylene oxide) (PPO):Cu(CF 3SO 3 )2 complexes, by measuring shifts in line positions caused by temperature-dependent motional averaging of the hyperfine interaction [7]. In this paper we report the results of a similar investigation of PEOnCu(CF 3 SO 3) 2 complexes, where n = 20 and 50. Although electrical conductivities are generally an order of magnitude higher in PEO-based complexes compared to their PPO counterparts, they are generally characterized by phase heterogeneity due to the highly crystalline nature of PEO (PPO complexes are amorphous and generally single-phase materials). Other techniques utilized in characterizing these materials include differential scanning calorimetry (DSC), complex impedance-conductivity measurements and cyclic voltammetry.
EXPERIMENTAL Samples were prepared in a dry-box by mixing appropriate ratios of salt and PEO (4 x 106 m.w.) and cast from solution according to the procedure described in Ref. 6. Both acetonitrile (AN) and methyl formate (MF) were utilized as solvents in an attempt to address the concern that trace amounts of solvent may remain associated with Cu2 + ions, for the case of acetonitrile. Loading of the DSC sam
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