Charge transfer mechanisms underlying Contact Glow Discharge Electrolysis

Abstract

Understanding charge transfer at strongly polarised gas-evolving electrodes is crucial for the development and control of Contact Glow Discharge Electrolysis (CGDE), a method of interest in a wide range of applications spanning from biochemistry to materials engineering. Since the pioneering work of Kellogg in 1950, the transition from conventional electrolysis to the CGDE regime has been generally associated with excessive ohmic heating at the working electrode, disregarding basic principles of electrochemical kinetics. Here we attempt to address this controversy by investigating of a CGDE process at a Fe-13%Cr anode in aqueous solutions of (NH4)2SO4 using an original method of in-situ impedance spectroscopy combined with voltammetric, gravimetric, chemical analysis and optical emission spectroscopy techniques. The study reveals three characteristic relaxation processes representing charge transfer through the surface oxide-hydroxide layer, the presence of which is confirmed by Glow Discharge Optical Emission and Raman Spectroscopies, and adsorption-desorption behaviour which we identify with oxo- and peroxo-intermediates of the oxygen evolution reaction (OER) coupled with anodic metal dissolution. We explain the transition to CGDE by the OER pathway change from direct recombination of two terminally coordinated oxides to a single-site route involving early peroxide formation. This change triggers electrochemical luminescence which proceeds via a co-reactant mechanism involving fragments of metastable peroxide structures and metal cations at the active sites on the anode surface that produce electrically excited intermediates prone to radiative relaxation. Our results undermine existing CGDE theories whilst emphasising common bases in behaviour of conventional and plasma-assisted electrochemical systems, thus providing far-reaching implications for understanding, development and practical use of electrolytic plasma processes.