Westfälische
Wilhelms-Universität Münster
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Institut für Materialphysik Wilhelm-Klemm-Str. 10 48149 Münster Geschäftsführender Direktor: Prof. Dr. Helmut Mehrer |
Tel. (0251) 83-33571
Fax: (0251) 83-38346 e-mail: mehrer@nwz.uni-muenster.de www: http://www.uni-muenster.de/Physik/MP/ |
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Forschungsschwerpunkte 2001 - 2002 Fachbereich 11 - Physik
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Ionic Transport in Polymer Electrolytes of Variable Composition
Diffusion
experiments with radiotracers allow for the ion-specific determination of transport properties in
polymer electrolytes (PE). Especially for this class of electrolytes there is a strong need to measure
the mobility of cations and anions individually since both species contribute substantially to the total
ion conduction. Moreover, comparing the sum of the cation and anion diffusion coefficient with
conductivity data may reveal the occurrence of mobile ion pairs or other electrically neutral
complexes. However, the few published radiotracer studies on PE materials date back to the 1980s.
We investigated ion motion in an amorphous electrolyte consisting of poly(ethylene oxide) and
sodium iodide by using the radioactive isotopes 22Na and 125I. Radiotracer penetration
profiles were
measured by means of microtome sectioning after isothermal diffusion annealing in a temperature
range from 70 °C to 180 °C. Self-diffusion coefficients determined from the observed
(near-)Gaussian or (near-)erfc-type profile shapes can be described by Arrhenius equations with
activation energies of 0.39 eV for sodium and 0.29 eV for iodine. It is observed that iodine migrates
roughly a factor of 3 faster than sodium which is more accurately expressed by the increase of the
cation transference numbers from 0.16 at 70 °C to 0.32 at 180 °C. This finding agrees with
earlier reports on the mobility of cations and anions in polymer electrolytes.
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