In situ surface-enhanced Raman scattering spectroelectrochemistry of oxygen species

Abstract

In situ surface-enhanced Raman scattering (SERS) combined with electrochemical analysis is applied to the determination of oxygen species on silver electrodes in alkaline hydroxide aqueous solution at room temperature and gold electrodes in carbonate melts at high temperature. This technique, referred to as SERS spectroelectrochemistry, reveals Raman spectral lines in the 500–1100 cm⁻¹ range under electrode potential scanning, assignable to superoxide ions (O₂⁻) and peroxide ions (O₂²⁻) on the electrode surface. These lines for oxygen molecule species have potential dependence with changing potential. In the alkaline hydroxide aqueous solution, the Raman peaks due to oxygen molecules are observed at potentials between 0.2 V and −0.8 V (vs. Ag/AgCl) only in the cathodic scan. This irreversible behavior in cyclic voltammograms indicates the existence of an intermediate stage in the oxygen reduction process, in which oxygen is released from the AgO films on the electrode at potentials corresponding to the onset of the last current peak in the voltammogram. This liberated oxygen molecule remains in solution at the interface until hydroxyls or water molecules are formed when the potential reaches the potential zero charge (PZC). In the high-temperature carbonate melts, Raman lines at 1047, 1080, and 800 cm⁻¹ are apparent for the eutectic (62 + 38) mol% (Li + K)CO₃ melt at 923 K, and at 735 cm⁻¹ for the Li₂CO₃ melt at 1123 K. These results suggest that oxygen reduction in the Li₂CO₃ melt involves only peroxide ions, while that in (62 + 38) mol% (Li + K)CO₃ involves both peroxide and superoxide ions at the three-phase boundary interface.