Experimental Measurement of Overpotential Sources during Anodic Gas Evolution in Aqueous and Molten Salt Systems

Abstract

Current interrupt and galvanostatic EIS techniques were utilized in a complementary fashion to characterize the different sources of overpotential during anodic gas evolution. Room temperature anodic evolution of oxygen at a nickel working electrode in aqueous potassium hydroxide and the high temperature (348°C) anodic evolution of chlorine at a glassy carbon working electrode in molten (LiCl)57.5-(KCl)13.3-(CsCl)29.2 where investigatd. Combining of the two techniques enables to separate the total measured overpotential into its ohmic, charge transfer, and mass transfer components. Potential decay curves indicated that natural convection (due to both bubble evolution and density driven flow) was a major driving force in reestablishing equilibrium conditions at the working electrode surface. During oxygen evolution, charge transfer resistance dominated the total overpotential at low current densities, but as the current density approached ∼100mA/cm2, mass transfer overpotentials and ohmic overpotential became non-negligible. The mass transfer overpotential during chlorine evolution was found to be half that found during oxygen evolution.