Electrochemical properties of hydrous ruthenium oxide films formed and measured at different potentials

An attempt is made to describe the electrochemical properties of hydrous ruthenium oxide films in the acid sulphate electrolyte solution over a broad range of polarisation potentials. The influence of the potential/time pre-treatment of the ruthenium substrates and three different potentials of the oxide film formation on the electrochemical behaviour of the oxide films generated are followed by impedance and current transient measurements. It is shown that all oxide films exhibited charge-storage/delivery properties over a certain range of polarisation potentials with nearly equal total (pseudo)capacitance values that indicate no influence of either the formation potential or the ruthenium pre-treatment procedure on the quantity of active metal sites. There is, however, a significant influence of the ruthenium pre-treatment procedure on the content of proton oxygen sites (OH−, H2O species) that is responsible for the degree of kinetic inhibition of the charge-storage/delivery reaction, the fraction of the capacitance that is available rapidly, the stability of the films towards the total film reduction, and the formation of the maximum possible oxidation level of the oxide films. The different contents of the proton oxygen sites are related to the different contents of water molecules inside the differently pre-treated ruthenium substrates prior to anodic oxidation. The potential of formation acts predominantly on the average oxidation state of ruthenium inside the oxide films initially formed. The electrochemical behaviour of the oxide films over a descending range of potentials is determined by the properties of the oxide films initially formed. The electrochemical behaviour of the oxide films over the range of increased potentials is determined by the polarisation potential and the content of available proton oxygen sites, irrespective of the formation potential.