Making the Most of a Scarce Platinum-Group Metal: Conductive Ruthenia Nanoskins on Insulating Silica Paper

Subambient thermal decomposition of ruthenium tetroxide from nonaqueous solution onto porous SiO<sub>2</sub> substrates creates 2−3 nm thick coatings of RuO<sub>2</sub> that cover the convex silica walls comprising the open, porous structure. The physical properties of the resultant self-wired nanoscale ruthenia significantly differ depending on the nature of the porous support. Previously reported RuO<sub>2</sub>-modified SiO<sub>2</sub> aerogels display electron conductivity of 5 × 10<sup>−4</sup> S cm<sup>−1</sup> (as normalized to the geometric factor of the insulating substrate, not the conducting ruthenia phase), whereas RuO<sub>2</sub>-modified silica filter paper at ∼5 wt % RuO<sub>2</sub> exhibits ∼0.5 S cm<sup>−1</sup>. Electron conduction through the ruthenia phase as examined from −160 to 260 °C requires minimal activation energy, only 8 meV, from 20 to 260 °C. The RuO<sub>2</sub>(SiO<sub>2</sub>) fiber membranes are electrically addressable, capable of supporting fast electron-transfer reactions, express an electrochemical surface area of ∼90 m<sup>2</sup> g<sup>−1</sup> RuO<sub>2</sub>, and exhibit energy storage in which 90% of the total electron−proton charge is stored at the outer surface of the ruthenia phase. The electrochemical capacitive response indicates that the nanocrystalline RuO<sub>2</sub> coating can be considered to be a single-unit-thick layer of the conductive oxide, as physically stabilized by the supporting silica fiber.