Alternating current voltammetry of high temperature electrolysis reactions

Abstract

A theory of the alternating current voltammetry (ACV) of electrolysis reactions in high temperature ionic melts is developed, providing a rigorous connection to the solution properties of electroactive components in molten electrolytes. The method presented herein addresses key issues in the rational design of electrolytes for extractive metallurgy and other electrolytic processes. The application of ACV for quantitative study of electrode reactions in high temperature molten electrolytes is validated by investigations of the Eu³⁺/²⁺ couple in molten Al₂O₃-CaO-Eu₂O₃ and of the Ir oxidation reaction in molten CaO-MgO-SiO₂. Analytic solutions are derived for the ACV harmonic waveforms of electrodeposition and gas evolution reactions of the form O + ne⁻ <-> R, where the surface activity of the reduced species R is constant. Reversible and quasi-reversible charge transfer kinetics are considered, as well as the effects of ohmic drop and double layer charging. It is shown that ohmic drop produces a characteristic distortion of the waveforms, resulting in the emergence of current-potential extrema in the higher harmonics that are distinct from those of the conventional ACV theory of soluble reduction-oxidation couples. An equation linking the peak potential of the second harmonic current amplitude and the activity coefficient of the solution species O is presented, establishing a voltammetric approach for the study of thermodynamic activities. Confirmation of the validity of the analytic solutions is found by close agreement with measurements of the fundamental, second, and third harmonic waveforms of Pb electrodeposition on liquid Pb and of Cl₂ evolution on graphite in molten PbCl₂-NaCl-KCl at 700 °C.