Difference between surface electrochemistry of ruthenium and RuO2 electrodes

A comparative study of the electrochemical behaviour of Ru and RuO2 electrodes in solutions of 0.5M H2SO4, 1M HCl and 0.1 M KOH has been performed with a view to elucidate in more detail the nature of the processes taking place on the surface of these electrodes within the range of potentials from 0.2 to 1.45 V (reversible hydrogen electrode, RHE). Cyclic voltammetry, chronopotentiometry and spectrophotometric analysis techniques have been employed. In accordance with thermodynamic data, it has been shown that amorphous layers of slightly soluble Ru(OH)(3) and RuO2 center dot nH(2)O form on the surface of an active Ru electrode at 0.2<0.8 V and 0.8<1.1 V respectively, as a result of ruthenium anodic oxidation. Cathodic reduction of these layers proceeds reversibly within the indicated ranges of potential. It has been demonstrated that in the acidic medium, passivation of ruthenium electrode takes place at 1.1<1.35 V due to further oxidation of the ruthenium phase and formation of a more compact layer of anhydrous RuO2. The latter can be reduced only at potentials more negative than 0.2 V. In the case of alkaline medium, no passivation of the Ru electrode is observed within this particular range of potentials. At E>1.35 V, transpassive anodic dissolution of ruthenium electrode takes place in both acid and alkaline electrolytes concomitantly with oxygen evolution. The RuO2 electrode demonstrates capacitor type voltammetric response at 0.4<1.4 V in both acid and alkaline medium. It has been proposed that such behaviour can be accounted for by a non-faradaic double layer process involving reversible adsorption/desorption of OH- ions on the surface of RuO2 electrode as described by the following equation RuO2+H2O <-> RuO2 center dot(OH-)(ad)+H+ where Q(ad) is the charge related to this process.