Synergistic Effect of Citrate and EDTA Electrolyte Additives Mixture on the Discharge Behavior of AZ31 Anode in Mg–Air Batteries

The performance of aqueous Mg–air batteries is determined by the interplay between magnesium dissolution, self-corrosion, and Mg(OH)2 surface passivation. Electrolyte additives that enhance one of these processes often compromise the others, making it difficult to simultaneously improve discharge voltage and operational stability. The effective regulation of this trade-off was investigated using two Mg-complexing ligands with complementary complexation properties. Citrate (CIT) and ethylenediaminetetraacetic acid (EDTA) were systematically investigated, individually and in combination, in a 3.5 wt% NaCl electrolyte under a discharge current density of 30 mA/cm2. Electrochemical characterization, hydrogen evolution measurements during discharge, discharge testing, post-discharge surface characterization, three-dimensional surface topography, and in situ visualization using the metal-ion indicator Eriochrome Black T (EBT) were employed to reveal the fundamental processes at the Mg anode–electrolyte interface that determine battery discharge behavior. The mixed electrolyte containing 0.15 M CIT and 0.15 M EDTA exhibited superior overall discharge performance, maintaining 804 mV after 35 h of discharge, compared with 584 mV for the EDTA electrolyte under identical conditions, whereas complete discharge occurred after only 16.5 h in the CIT electrolyte. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed enhanced anodic dissolution together with the lowest film and charge-transfer resistances. Surface characterization confirmed increased magnesium dissolution and delayed Mg(OH)2 passivation, while hydrogen evolution measurements demonstrated that the improved discharge performance was accompanied by increased self-corrosion. In situ visualization using the metal-ion indicator EBT further revealed the evolution of Mg–ligand complexation, progressive EDTA depletion, and electrolyte pH during discharge. The combined experimental evidence supports a cooperative complexation mechanism, in which the presence of both CIT and EDTA in the electrolyte modulates the Mg anode–electrolyte interface and facilitates the preferential complexation of dissolved Mg ions in the bulk electrolyte. Their complementary functions coordinate magnesium dissolution with delayed surface passivation, thereby regulating the evolution of the anode/electrolyte interface and maintaining an electrochemically active anode throughout discharge.