Mechanistic Aspects and Side Reactions during Reversible Mg Deposition and Oxygen Reduction on a Pt Film Electrode in BMP-TFSI-Based Electrolytes: A DEMS Study

Reversible Mg deposition/stripping and O-2 reduction/evolution on a Pt film electrode in neat and O-2-saturated 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP-TFSI) electrolytes, containing Mg(TFSI)(2) and/or Mg(BH4)(2) as Mg source as well as Mg(BH4)(2) and/or the crown ether 18-c-6 as additive, were investigated by online differential electrochemical mass spectrometry (DEMS) and by scanning electron microscopy/energy dispersed X-ray spectroscopy. Combined cyclic voltammetry and DEMS measurements reveal a complex network of partial reactions, including borohydride electro-oxidation by reaction with water or O-2, chemical bulk reaction of these components, as well as electro-oxidation of H-2, and electrolyte decomposition, in addition to the primary reactions Mg deposition/stripping and ORR/OER. They provide detailed insights into the potential dependent reactions occurring under these conditions, demonstrating that also the additive 18-c-6 undergoes decomposition upon reduction of Mg2+. Contributions from chemical bulk reactions are resolved by DEMS measurements in borohydride containing solution without a Pt electrode. Electrocatalytic borohydride oxidation, explored by similar measurements with a Pt electrode, can lead to H-2 or H+ formation. Under open circuit potential conditions, charge compensation by the ORR results in the formation of a mixed potential. Consequences of these findings for applications in Mg-air batteries are discussed.