Structure and spectroscopy of the supercapacitor material hydrous ruthenium oxide, RuO<sub>2</sub>·<i>x</i>H<sub>2</sub>o, by neutron scattering* ParkerStewart F. RobertsonStephen J. ImbertiSilvia 2019 <p>Hydrous ruthenium dioxide, RuO<sub>2</sub>·<i>x</i>H<sub>2</sub>O, is a material of active investigation as an electrode material for supercapacitors. A combination of elastic and inelastic neutron scattering together with thermal gravimetric studies and DFT calculations have provided new insight into the nature of the surface species present on RuO<sub>2</sub>·<i>x</i>H<sub>2</sub>O. Our results confirm that hydrous ruthenium oxide is a nanocrystalline material consisting of a core of RuO<sub>2</sub>. We show that the surface consists largely of Ru–OH with small amounts of water hydrogen-bonded to the surface. The hydroxyls are stable up to ∼200°C, i.e. over the composition range <i>x</i> = 0.2–2. The optimal supercapacitor material has <i>x</i> = 0.5–0.7, and in this range, the surface is fully hydroxylated. This provides a route for the proton transport: a proton can attach to a surface hydroxyl to generate coordinated water, proton transport then occurs along the hydrogen-bonded chain by a Grotthuss mechanism.</p>