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 Bu2Mg 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.