In Situ Ru K-Edge X-Ray Absorption Fine Structure Studies of Electroprecipitated Ruthenium Dioxide Films with Relevance to Supercapacitor Applications

Abstract

Modifications in electronic and structural aspects of RuO 2 films electroprecipitated onto Au electrodes induced by changes in the applied potential have been examined in situ in aqueous 0.50 M H 2 SO 4 by Ru K-edge X-ray absorption spectroscopy (XAS). The Fourier transform of the k 3 -weighted extended X-ray absorption fine structure (EXAFS), k 3 χ( k ), for the film polarized at +1.20V vs RHE is characterized by two shells attributed to Ru−O and Ru−Ru interactions with average distances of 1.94(1) and 3.12(2) Å, respectively, in agreement with results obtained ex situ for Ru 4+ in hydrous RuO 2 by other groups. In contrast, films in the reduced state, i.e., +0.40 V vs RHE, yielded only a single shell ascribed to a Ru−O interaction at 2.02(1) Å with no evidence for a distant Ru−Ru shell. The long Ru−O distance is in agreement with that reported earlier for the hydrous Ru 3+ ion [Ru−(OH 2 ) 6 ] 3+ in the solid state. Moreover, the difference between the average Ru−O bond lengths for the reduced and oxidized films is consistent with the difference in the ionic radii of Ru 3+ and Ru 4+ . On this basis it has been suggested that films in the reduced state contain Ru 3+ sites, consistent with the electrochemical results, in a phase with apparently less order beyond the Ru−O coordination sphere than for hydrous RuO 2 .