Local atomic structure and conduction mechanism of nanocrystalline hydrous RuO2 from X-ray scattering

Hydrous ruthenium oxide (RuO2.xH(2)O or RuOxHy) is a mixed proton-electron conductor which could be used in fuel cells and ultracapacitors. Its charge-storage (pseudocapacitance) and electrocatalytic properties vary with water content and are maximized near, the composition RuO2.0.5 mol % H2O. We studied the atomic structure of RuO2.xH(2)O as a function of water content from x = 0.84 to 0.02 using X-ray diffraction and atomic pair density function (PDF). Even though the diffraction patterns of samples containing 0.84 to 0.35 mole of water are suggestive of "amorphous" structures, the PDF analysis clearly shows that up to 0.7 nm, the short-range atomic structure of all of these RuO2.xH(2)O samples resembles that of the anhydrous rutile RuO2 structure. We conclude that RuO2.xH(2)O is a composite of anhydrous rutile-like RuO2 nanocrystals dispersed by boundaries of structural water associated with Ru-O. Metallic conduction is supported by the rutile-like nanocrystals, while proton conduction is facilitated by the structural water along the grain boundaries. This structural picture explains the charge-storage and electrocatalytic properties of RuO2.xH(2)O in terms of competing percolation networks of metallic and protonic conduction pathways, that vary in volume as a function of the water content of the RuO2.xH(2)O. The control and optimization of electron and proton conducting volumes and pathways will lead to improved performance and guide the design of new materials.