Revelation of multiple underlying RuO2 redox processes associated with pseudocapacitance and electrocatalysis
Advances in basic knowledge relevant to the pseudocapacitive and electrocatalytic properties of RuO2 materials require a detailed understanding of the redox chemistry that occurs at the electrode interface. Although several redox processes have been identified via dc cyclic voltammograms derived from surface-confined RuO2 materials, mechanistic details remain limited because the faradaic signals of interest are heavily masked by the background current. Here, it is shown that the underlying electron transfer reactions associated with the VI to II oxidation states of surface-confined RuO2 materials in acidic medium are far more accessible in the background current free fourth and higher harmonic components available via large-amplitude Fourier transformed ac voltammetry. Enhanced resolution and sensitivity to both electron transfer and protonation processes and discrimination against solvent and background capacitance are achieved so that the Ru(V) to Ru(VI) process can be studied for the first time. Thus, kinetic and thermodynamic information relevant to each ruthenium redox level is readily deduced. The relative rate of electron transfer and the impact of protonation associated with Ru(VI) to Ru(II) redox processes are found to depend on the nature of the RuO2 materials (extent of crystallinity and hydration) and concentration of sulfuric acid electrolyte. In the electrocatalytic oxidation of glucose in alkaline medium, access to the underlying electron transfer processes allows ready detection of the redox couple associated with the catalysis. Thus, application of an advanced ac electroanalytical technique is shown to provide the methodology for enhancing our understanding of the charge transfer processes of RuO2, relevant to pseudocapacitance and electrocatalysis. 2010 American Chemical Society.