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

Abstract

AbstractReversible Mg deposition/stripping and O2 reduction/evolution on a Pt film electrode in neat and O2‐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 O2, chemical bulk reaction of these components, as well as electro‐oxidation of H2, 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 H2 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.