USE OF MICROELECTRODE ARRAYS TO DIRECTLY MEASURE DIFFUSION OF IONS IN SOLID ELECTROLYTES - PHYSICAL DIFFUSION OF AG+ IN A SOLID POLYMER ELECTROLYTE

We report experiments that demonstrate methodology for direct measurement of diffusion of molecular or ionic species in solid electrolyte media. A microelectrode array of closely spaced (1.4-μm) Pt microelectrodes (∼70 μm long × 2.7 μm wide × 0.1 μm high) allows direct measurement of the movement of Ag+ in aqueous electrolyte and in the solid polymer electrolytes, LiCF3SO3/MEEP [MEEP = poly(bis(2-(2-methoxyethoxy)ethoxy)phosphazene)] at 298 K with a molar ratio of LiCF3SO3 to polymer repeat unit of 1 to 4 and LiCF3SO3/PEO [PEO = poly(ethylene oxide)] at 352 K and a molar ratio of LiCF3SO3 to polymer repeat unit of 1 to 8. The crucial experiments involve anodically stripping Ag from a Ag-coated Pt microelectrode (generator) and electrochemically detecting the resulting Ag+ at nearby (1.4-23.4-μm) Pt electrodes (collectors) by reducing the Ag+ back to Ag. The time dependence of the collector current corresponding to Ag+-to-Ag reduction after the generation step allows evaluation of the diffusion coefficient, D, for the Ag+ in the various media and conditions used: D = 5 ± 2 × 10-9 cm2/s at 298 K in LiCF3SO3/MEEP, 2.3 ± 0.3 × 10-5 cm2/s in aqueous 0.1 M LiClO4, and 7 ± 1 × 10-8 cm2/s at 352 K in LiCF3SO3/PEO. For the arrays used, D = 0.22d2/tmt where d is the separation between the generator and the collector and tmt is the time of the maximum collector current. © 1990 American Chemical Society.