Na2SnO3 functions as outstanding magnesium alloy passivator by synergistic effect with trace carboxymethyl chitosan for Mg–air batteries for standby protection

Abstract

Different concentrations of sodium stannate (Na₂SnO₃) (0.2, 0.5, 2.0, and 5.0 mM) as well as their mixture with trace amount of carboxymethyl chitosan (CMCS) (0.1 mM) are selected as the electrolyte additive for the AZ61 alloy in 3.5 wt% NaCl electrolyte in this study. The corrosion inhibition and oxide film activation (during discharging) effect on the AZ61 alloy are evaluated by weight loss measurement, hydrogen evolution test, potentiodynamic polarization measurement, electrochemical impedance spectroscopy measurement, and half-cell investigations. These tests show that Na₂SnO₃ acts as a passivation inhibitor, CMCS acts as a cathodic type inhibitor, the maximum value of IE% can be easily boosted to higher levels by adding the mixed inhibitors, as compared to that by adding single Na₂SnO₃. Also, the addition of the mixed inhibitors subject the AZ61 anode to relatively high utilization efficiency, specific capacity, and energy density at the constant current density (10 and 40 mA cm⁻²), hardly affecting the discharge potential in 3.5 wt% NaCl solution. Their possible inhibition/activation mechanism are discussed by a series of surface analysis methods. These analyses suggest that the added Na₂SnO₃ can promote the deposition of metal hydroxides over the whole surface of the product film including breaking areas, and the formation of the Mg(OH)₂ product film rich in MgSn(OH)₆. For the mixed inhibitors, CMCS molecules are physically adsorbed on the alloy surface as well as the subsequent initial product film; then, part of the physiosorbed CMCS molecules are changed to the chemisorbed CMCS molecules, forming the Mg–CMCS interface film, as well as Mg(OH)₂–CMCS and MgSn(OH)₆–CMCS composite product film, whereas most of the physiosorbed CMCS molecules may also play the role of a protective shield role due to its planar structure. During the discharge process with the addition of Na₂SnO₃, the inability to form stable attachment of the discharge product layer due to the chunk effect, the lack of Mg(OH)₂ structural basis for product layer, and friction action between the MgSnO₃/MgSn(OH)₆ particles carried by flowing liquid and the anode surface. Overall, the mixed inhibitors as additives in the NaCl electrolyte (3.5 wt%) of the Mg–air batteries using the AZ61 anode can play a role in standby protection and improving the utilization efficiency during the no-discharge and discharge periods.