Microelectrode studies of reversible Mg deposition in THF solutions containing complexes of alkylaluminum chlorides and dialkylmagnesium

A kinetic study of Mg deposition on Pt surfaces has been performed using Pt microelectrodes. The solutions chosen included tetrahydrofuran (THF) and several types of Mg-Al-R-Cl complex electrolytes from which magnesium can be deposited reversibly. The complexes included an all-inorganic, Cl-containing MgCl₂:AlCl₃ (1:1) complex compound, all-organic Bu ₂Mg with different ratios of Bu₂Mg, and various combinations of AlEtCl₂ and AlEt₃. Tafel slopes of the resulting polarization curves obtained both by CV and potential step techniques were carefully analyzed within the framework of a complicated, three-stage electrocrystallization mechanism related to four consecutive and two parallel steps in the first stage (ad-atom formation on the metallic interface) followed by surface diffusion and embedding of Mg-ad-atoms in the growth kink sites. This mechanism not only explains the peculiar kinetic data on Mg deposition from these complex solutions, but also provides a natural route for the comparison of a Mg deposition mechanism with that of gold and Ni depositions from cyanide and AlCl₃-BPC-NiCl₂ room temperature molten salts, respectively. We also discuss the uniqueness of these THF-based complex solutions for metallic Mg deposition, and compare the electrodeposition mechanism in this electrolyte with the electrochemistry of Mg in thionyl chloride solutions.