Mg Anode Corrosion in Aqueous Electrolytes and Implications for Mg-Air Batteries

Abstract

Aqueous Mg-air primary batteries possess many favorable attributes for energy storage because Mg is affordable, abundant, and lightweight. However, parasitic corrosion of Mg in aqueous electrolytes generates H2 and surprisingly increases with increasing current density during battery discharge (Mg oxidation), limiting the faradaic efficiency of aqueous Mg batteries. In this study, differential electrochemical mass spectrometry and H2 pressure rise measurements were used to characterize Mg corrosion in Mg-air batteries employing aqueous electrolytes with salts (NaCl, NaNO3, NaPO4, and a NaCl/NaPO4 mixture) that provide various degrees of Mg passivation. H2 evolution rates were highest in NaCl electrolytes and lowest in NaNO3 electrolytes. However, NaNO3 salts reduced the H2-evolving corrosion rate at the expense of introducing a nitrate to nitrite corrosion reaction into the battery. The combined Mg corrosion rate in the nitrate-based electrolyte was still lowest among those electrolytes studied. The nitrate to nitrite corrosion reaction also lowered the magnitude of the Mg anodic potential and therefore decreased the overall Mg-O2 battery voltage compared to the NaCl electrolyte. Nevertheless, Mg-O2 batteries utilizing a NaNO3 electrolyte allowed for 60% larger discharge capacity and 50% higher Mg oxidation faradaic efficiency compared to a NaCl electrolyte.