Electrochemistry in flame plasmas: passive films and impedance analysis

Flame plasmas can serve as unique electrochemical environments, enabling high-temperature electrochemistry in a gaseous medium. In this study, we explore electrochemistry in flame plasmas, focusing on the formation and characterisation of passive oxide layers on metals and alloys, notably platinum and titanium using electrochemical impedance spectroscopy (EIS). Also, we summarise the existing literature on flame plasma electrochemistry and discuss how EIS can probe the evolution of protective oxide films in these extreme conditions. The passivation behaviour of platinum and titanium is compared: platinum tends to form only a thin surface oxide, whereas titanium readily develops a thick TiO₂ scale. The impedance response of these passive layers in a flame reveals distinct characteristics - from high impedance for platinum sparse oxide to low impedance features for TiO₂ films that decrease with thickness. We examine the mass transport conditions in flame plasmas, where low ionic concentrations, high ionic mobility, strong convection, and significant migration effects differentiate flame electrochemistry from conventional liquid electrolytes and key equations are provided to describe the electrochemical processes, and all observations are contextualised with current theory and experimental findings.