In situ Ru K-edge X-ray absorption fine structure studies of electroprecipitated ruthenium dioxide films with relevance to supercapacitor applications

Modifications in electronic and structural aspects of RuO2 films electroprecipitated onto Au electrodes induced by changes in the applied potential have been examined in situ in aqueous 0.50 M H2SO4 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)chi>(*) over bar * (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) Angstrom, respectively, in agreement with results obtained ex situ for RU4+ in hydrous RuO2 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) Angstrom with no evidence for a distant Ru-Ru shell. The long Ru-O distance is in agreement with that reported earlier for the hydrous Ru3+ ion [Ru-(OH2)(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 Ru3+ and Ru4+. On, this basis it has been suggested that films in the reduced state contain Ru3+ sites, consistent with the electrochemical results, in a phase with apparently less order beyond the Ru-O coordination sphere than for hydrous RuO2.