Structure of Hydrous Ruthenium Oxides:  Implications for Charge Storage

Hydrous ruthenium oxide (RuO2·xH2O or RuOxHy) is a mixed electron−proton conductor with a specific capacitance as high as 720 F/g/proton, making it a candidate material for energy storage. The correlation between the structure and properties of RuO2·xH2O materials is not well understood due to their amorphous nature and compositional variability. In this study, ruthenium oxides with the compositions RuO2·2.32H2O, RuO2·0.29H2O, and anhydrous RuO2 are characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) and extended X-ray fine structure (EXAFS) analyses. XANES cannot be used to distinguish between Ru(III) and Ru(IV) in the hydrous oxides, but the EXAFS analyses show large differences in the short-range structures of the materials. Whereas anhydrous RuO2 has the rutile structure comprising chains of RuO6 octahedra linked in three dimensions, the structure of RuO2·0.29H2O is rutile-like at the RuO6 core, but less connected and progressively disordered beyond the RuO6 core. The structure of RuO2·2.32H2O is composed of chains of disordered RuO6 octahedra that exhibit no chain-to-chain linking or three-dimensional order. Although the local structures of RuO2·0.29H2O and RuO2·2.32H2O markedly differ, their specific capacitances are large and essentially equivalent, so nonunique local structures can balance effective electron transport (along dioxo bridges) with the effective proton transport (through structural water) necessary for charge storage.