Corrosion resistance and discharge performance of Mg-Gd binary alloys with different grain sizes as the anodes for Mg-air batteries

When considering their application as anodes in Mg-air batteries, the corrosion resistance and discharge performance of three distinct Mg-Gd binary alloys with different grain sizes (designated as A1, A2, and A3) are investigated. Microstructural analysis revealed that the extruded A1 alloy possesses significantly finer and more uniform grains compared to the annealed A2 and A3 alloys. The findings reveal that grain boundaries, as crystal defects with Gd segregation, possess a Volta potential approximately 30-36 mV lower than adjacent grain interiors. This potential difference makes grain boundaries more prone to electrochemical dissolution compared to grain interiors, thus resulting in their role as preferential attack sites during corrosion and discharge processes. A decrease in the grain size of Mg-Gd alloys leads to reduced corrosion resistance, as the adverse impact of an increased number of grain boundaries on corrosion resistance surpasses the beneficial effect exerted by the protective corrosion product film. The fine-grained extruded A1 alloy demonstrates superior discharge performance relative to the coarse-grained annealed A2 and A3 alloys. At all tested current densities, A1 maintains the highest average discharge voltage, reaching 1.5513 V at 2.5 mA cm -2 , while A3 only achieves 1.5302 V. At 10 mA cm -2 , A1 alloy exhibits the maximum capacity density (1232.1 mAh g -1 ) and specific energy (1759.8 mWh g -1 ). Overall, tailoring the microstructure via grain refinement coupled with improved size uniformity, which provides more uniform active reaction sites, is an effective approach to significantly improve the discharge performance of Mg-Gd alloys and advance their application in Mg-air batteries.