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 k³-weighted extended X-ray absorption fine structure (EXAFS), k³χ(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 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) 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 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.