| The standard equilibrium potential of Mg is-2.37 V vs.standard hydrogen electrode(SHE),and it has high electrochemical activity,so it is often used as the anode material of Magnesium-air batteries.Mg alloys are also promising anodes for Mg-air batteries that normally have a theoretical cell voltage 3.09 V,discharge capacity 2.2 Ah/g,and energy density 6.8 Wh/g.However,the coulombic efficiency and discharge capacity of Mg-based anodes is relatively low,owing to their severe self-corrosion,"negative difference effect(NDE)" and the discharge products adhered on their surfaces.This paper has studied the corrosion behavior and the "negative difference effect"of magnesium.Changing the microstructure of magnesium alloy and adding alloying elements can inhibit the self-corrosion rate of magnesium,and the hydrogen evolution reaction can be reduced or even eliminated during the discharge of magnesium anode.This can improve the discharge performance of magnesium anode and promote its application in practice.The main contents and results are summarized as follows:(1)The corrosion behavior and discharge performance of a die-cast Mg-9Al-1Zn(DC AZ91D)alloy in 3.5 wt.%NaCl solution were investigated by means of immersion and electrochemical measurements.The results indicated that the DC AZ91D had a low corrosion rate(0.10±0.01 mm/y),and superior discharge performance as an anode assembled in Mg-air batteries,especially at low discharge current densities.The anodic efficiency of the DC AZ91D reached 53.47%and 57.72%at 1 and 2.5 mA/cm2,respectively,and the discharge capacity is up to 1206 mAh/g at 1 mA/cm2,which were significantly higher than that of many other Mg-based anodes.This efficiency enhancement could mainly be attributed to the uniform microstructure of the DC AZ91D with a continuous β phase network and fine matrix grains,which could produce a protective corrosion product film.Such a uniform microstructure with a protective product film can suppress self-corrosion and hinder the detachment of the undermined metallic pieces or the second phase particles from the substrate.Therefore,the microstructure of AZ91D prepared by die-casting is changed and its discharge performance is significantly improved.(2)Mg-Zn alloy with high corrosion resistance and no hydrogen evolution was obtained by adding alloying Zn to magnesium by magnetron sputtering non-equilibrium technique.The microstructure of Mg-Zn alloys was transformed from crystalline into amorphous with nano-grains when the Zn content increased from 18 at.%(Mg82Zn18)to 36 at.%(Mg64Zn36).The corrosion rate of Mg64Zn36 is comparable to the ultra-high purity Mg or the most corrosion resistant Mg alloy,and there is not hydrogen evolution reaction during corrosion or anodic dissolution.The corrosion behavior of the amorphous Mg64Zn36 can be mainly attributed to the extended solubility of Zn in Mg,which may lead to a higher equilibrium potential,a high over-potential for the hydrogen evolution reaction,and a protective film is formed on the surface of the alloy.This research will greatly promote the development and application of magnesium alloys,especially as the anodic materials for Mg-air batteries and biodegradable batteries.(3)Mg64Zn36 alloy was used as the anode material for air battery.It was found that Mg64Zn36 alloy has high anode discharge efficiency(94.79%)at low current density(1 mA/cm2),which is the best magnesium anode material in NaCl solution at present.It is attributed to the fact that the Mg64Zn36 completely inhibits the generation of hydrogen at anodic polarization,and the discharge product film has no obstruction to charge conduction and discharge dissolution of active substances,therefore it shows excellent discharge performance. |
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