Solid state voltammetric characterization of iron hexacyanoferrate encapsulated in silica

We describe a sol-gel approach by which iron hexacyanoferrate is immobilize d in silica in a manner suited to investigation by electrochemistry in the absence of a contacting liquid phase. Such physicochemical parameters as co ncentration of redox sites (C-o) and apparent (effective) diffusion coeffic ient (D-app) are estimated by performing cyclic voltammetric and potential step experiments in two time regimes, which are characterized by linear and spherical diffusional patterns, respectively. Values of D-app nd C-o there by obtained are 2.0 x 10(-6) cm(2) s(-1) and 1.4 x 10(-2) mol dm(-3). The D -app value is larger than expected for a typical solid redox-conducting mat erial. Analogous measurements done in iron(III) hexacyanoferrate(III)(III) solutions of comparable concentrations, 1.0 x 10(-2) and 5.0 x 10(-3) mol d m(-3), yield D-app on the level of 5-6 x 10(-6) cm(2) s(-1) Thus, the dynam ics of charge propagation in this sol-gel material is almost as high as in the liquid phase. The residual water in the silica, along with the pore str ucture, are important to the overall mechanism of charge transport, which a pparently is limited by physical diffusion rather than electron self-exchan ge. Under conditions of a solid state voltammetric experiment which utilize s an ultramicroelectrode, encapsulated iron hexacyanoferrate redox centers seem to be in the dispersed colloidal state rather than in a form of the ri gid polymeric film.