Electrochemical and X-ray scattering study of well defined RuO2 single crystal surfaces

Electrochemical and synchrotron surface-X-ray scattering measurements were performed on two low index faces, (110) and (100), of RuO2 single crystal electrodes in a variety of solutions. The two crystal faces displayed uniquely different cyclic voltammograms and the structural changes associated with cyclic voltammograms were investigated with synchrotron surface-X-ray scattering measurements. In sulfuric acid solution, the cyclic voltammogram of the (100) surface exhibits a reduction signature near the hydrogen evolution potential. The reduction feature was found to be associated with an expansion of the top ruthenium layer approximately along the (110) direction. The same reduction feature was seen much less clearly at the (110) surface. However, the associated displacement of ruthenium atoms is very similar to that of the (100) surface, leading to the conclusion that the oxygen bonds on the surface are elongated by a chemical reaction of the oxygen atoms with hydronium molecules in solution. In NaOH solution, the cyclic voltammogram of the (110) surface indicates two clearly identifiable oxidation features near the oxygen evolution potential. We find that the oxidation features are associated with the clearly identifiable surface-structure changes and these structure models are presented. The structure models and corresponding charge-transfer amounts were consistent with the pH-dependent cyclic voltammograms and the super-nernstian behavior measured by adding phosphoric acid to the solution.