Comparison of Diffusion-Coefficients of Electroactive Species in Aqueous Fluid Electrolytes and Polyacrylate Gels - Step Generation Collection Diffusion Measurements and Operation of Electrochemical Devices

by Tatistcheff, H. B.; Fritschfaules, I.; Wrighton, M. S.

We report electrochemical measurement of diffusion coefficients and operation of electrochemical devices in aqueous polyacrylate gels formed by neutralization of poly(acrylic acid), MW = 4 x 10(6). Upon neutralization, dilute aqueous solutions become gels with viscosities as high as 5 x 10(4) cP at low shear rates. Diffusivity was measured using potential step generation-collection on arrays of closely spaced microelectrodes. The diffusion coefficient, D, of 1,1'-bis(hydroxymethyl)ferrocenium was found to be (5.8 +/- 0.4) x 10(-6) cm/s in the gel electrolyte (aqueous 0.5% poly(acrylic acid) at pH 7) and (8.8 +/- 0.8) x 10(-6) cm2/s in the aqueous fluid electrolyte 0.1 M NaCH3CO2 (pH 7). The small decrease observed in D for the gel vs the fluid with such a large increase in viscosity is contrary to the Stokes-Einstein model for diffusion in viscous solutions but is in accord with the structure of polyacrylate gels which consists of two phases. The continuous, aqueous phase allows for high diffusivity. Electrochemical transistors based on ruthenium oxide exhibit similar operating characteristics in both polyacrylate gel and aqueous fluid electrolytes. Response times of electrochromic polymer films made from N,N'-bis[p-(trimethoxysilyl)benzyl]-4,4'-bipyridinium are slower in gel by about a factor of 10 compared to aqueous fluid electrolytes. The slow electrochromic switching is likely due to sluggish ion motion, owing to the effects of charge compensation by the polyacrylate.

Journal of Physical Chemistry
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