The electrochemical redox reactions in silica sol–gel glass monoliths and films with embedded organic electrolyte

Abstract

The electrochemical redox reaction of ferrocene and Co(II)tris(bipyridine) has been studied in silica sol–gel glass with embedded organic electrolyte. This material has been prepared by acid catalysed hydrolysis of the tetraalkylorthosilicate sol mixed with salt solution in organic polar solvent. Tetramethyl- or tetraethylorthosilicate was used as precursor. Lithium or tetraalkylammonium perchlorate solution in propylene carbonate or sulpholane was used as modifier. Sol–gel glass has been prepared in the form of a monolith or film. The electrochemical behaviour of the redox probe was studied by cyclic voltammetry and chronoamperometry on ultramicroelectrodes in the absence of macroscopic liquid phase. The accessible potential window in solid material is more narrow than in analogous liquid organic electrolyte. The shape of the cyclic voltammograms corresponding to the electrochemical redox reaction in sol–gel monolith is similar to that obtained in liquid electrolyte and does not depend on gel ageing. On the other hand, the current substantially decreases during the first few days after gelation, and later becomes weakly dependent on ageing. The precursor, salt, solvent and redox probe affect the motion rate of the redox probe. The latter is five times slower in electrolyte made of tetraethylorthosilicate in comparison to that obtained using the tetramethyl derivative. The diffusion coefficient of ferrocene is larger in the gel obtained in the presence of the large tetraalkylammonium cations. The electro-oxidation of ferrocene was also studied in films made from tetraethylorthosilicate, cast on the surface of the electrode assembly. The magnitude of the current is similar to that obtained in a glass monolith made of the same material.