Charge storage and transport in thermal ruthenium oxide thin films: In situ detection of local pH changes and asymmetries in charge-discharge cycles at the oxide/aqueous electrolyte interphase

Abstract

The charge-discharge cycle in thermal RuO x electrodes was studied by a combination of voltammetry and a new thin-layer spectroelectrochemical technique developed in this laboratory. The RuO x was deposited pyrolytically on Ti and Au supports for these two techniques, respectively. The oxide anneal temperature and the oxide loading on the substrate were varied to investigate their influence on the local pH changes and the charge consumed in the anodic oxidation and subsequent reduction at the oxide/electrolyte interphase. The potential scan rate was further used as a variable parameter in the voltammetric experiments. The spectroelectrochemical experiments showed conclusively that the voltammetric waves superimposed on a pseudo-capacitive background, which is a signature of RuO x represented proton ejection and reinjection during the anodic and cathodic cycles, respectively. Concomitantly, the integrated current efficiency for proton “collection” in the spectroelectrochemical measurements was much less than unity, signifying the importance of the capacitive components. Finally, the implications of the voltammetric and spectroelectrochemical data for the optimization and use of RuO x in electrochemical capacitors are discussed.