Structure and spectroscopy of the supercapacitor material hydrous ruthenium oxide, RuO2·xH2o, by neutron scattering*

Hydrous ruthenium dioxide, RuO2·xH2O, 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 RuO2·xH2O. Our results confirm that hydrous ruthenium oxide is a nanocrystalline material consisting of a core of RuO2. 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 x = 0.2–2. The optimal supercapacitor material has x = 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.