Structure of hydrous ruthenium oxides: Implications for charge storage

Hydrous ruthenium oxide (RuO2. H2O or RuOxHy) is a mixed electron-proton co nductor with a specific capacitance as high as 720 F/g/proton, making it a candidate material for energy storage. The correlation between the structur e and properties of RuO2. xH(2)O materials is not well understood due to th eir amorphous nature and compositional variability. In this study, rutheniu m oxides with the compositions RuO2. 2.32H(2)O RuO2. 0.29H(2)O, and anhydro us RuO2 are characterized using thermogravimetric analysis (TGA), X-ray dif fraction (XRD), and X-ray absorption near-edge structure (XANES) and extend ed X-ray fine structure (EXGFS) analyses. XANES cannot be used to distingui sh 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. Whe reas anhydrous RuO2 has the rutile structure comprising chains of RuO6 octa hedra linked in three dimensions, the structure of RuO2. 0.29H(2)O is rutil e-like at the RuO6 core, but less connected and progressively disordered be yond the RuO6 core. The structure of RuO2. 2.32H(2)O is composed of chains of disordered RuO6 octahedra that exhibit no chain-to-chain linking or thre e-dimensional order. Although the local structures of RuO2. 0.29H(2)O and R uO2. 2.32H(2)O markedly differ, their specific capacitances are large and e ssentially equivalent, so nonunique local structures can balance effective electron transport (along dioxo bridges) with the effective proton transpor t (through structural water) necessary for charge storage.