A comprehensive investigation of salt concentration effects on electrochemical behavior of magnesium anodes

Aqueous magnesium-air batteries are considered promising for future energy technologies due to their high energy density and environmental friendliness. The hydrogen evolution reaction (HER) plays a crucial role in the dynamic evolution of the magnesium anode/electrolyte interface during electrochemical processes. Therefore, an in-depth analysis of the interfacial HER is key to optimizing the performance of aqueous magnesium-air batteries. In this work, three different electrolytes (MgCl₂, MgSO₄, MgAC₂) were employed, along with quantum mechanics calculations and characterization techniques such as nuclear magnetic resonance and Raman spectroscopy, to investigate the influence mechanism of salt type and concentration on the HER of pure magnesium anode at the microscale. This work revealed that increasing salt concentration in dilute salt solutions accelerates the HER rate, contrary to previous "water in salt" strategies. The active species for the HER are Mg²⁺-solvated H₂O molecules, and the activity of the HER is closely related to the number of solvated H₂O, the structure of the solvation sheath layer, and the hydrogen bonding network. Furthermore, different anions exhibit distinct interaction mechanisms at the anode/electrolyte interface. The Cl^- ions can penetrate the passivation film, promoting electrode reaction kinetics and inducing the HER. In contrast, AC^- ions can adsorb or reduce at the interface, forming a "water-poor" electric double layer that inhibits the HER. Notably, in the 0.5 M MgAC₂ electrolyte, the pure magnesium anode achieved an anode efficiency as high as 85 % at a current density of 5 mA cm^-2. This work provides a novel perspective for the design of high-performance magnesium-air batteries.