“Inner” and “outer” active surface of RuO2 electrodes

The dependence of the voltammetric surface charge q* on solution pH and potential scan rate has been investigated using a set of RuO2 electrodes prepared by thermal decomposition of RuCl3 at temperatures in the range 300–500°C. The systematically higher charge in KOH than in HClO4 in the same potential range (vs rhe) is attributed to the stabilization of higher oxidation states of surface Ru atoms in the alkaline environment. The variation of q* with v, the potential scan rate, is shown to be linearizable as a function of v12. It is thus possible to extrapolate the values of q* to v=0 and v=∞, respectively. The extrapolation enables an “inner” surface to be discriminated from an “outer” surface. The former is pointed out to be composed by the regions of difficult accessibility for the proton-donating species assisting the surface redox reactions. Reasons why the “screened” surface appears to be higher in alkaline than in acid solutions, are discussed. It is stressed that only working with a set of RuO2 electrodes prepared at different temperatures it is possible to discover meaningful correlations.