# Recent Progress of Metal-Air Batteries--A Mini Review
## 3 Anodes
### Common Challenge of Metal Anode  
The common issues with metallic anodes are corrosion, passivation, and dendrite formation. These mechanisms are displayed in Figure 12.  
Corrosion: Corrosion is one of the major side reactions between metal and electrolyte, and its reaction can be expressed as follows:  
\[\mathrm{M}+(2+\mathrm{x})\mathrm{H}_{2}\mathrm{O}\rightleftharpoons 2\mathrm{M}( \mathrm{OH})+\mathrm{H}_{2} \tag{19}\]  
\[\mathrm{M}+\mathrm{H}_{2}\mathrm{O}\rightleftharpoons\mathrm{MO}_{\mathrm{X}}+ \mathrm{H}_{2} \tag{20}\]  
Equations (19) and (20) evaluates the corrosion rate due to hydrogen evolution reaction (HER). For almost all MABs, the M/MO standard voltage was below that of the hydrogen revolution. Therefore, hydrogen evolution was spontaneously favored. The HER decreased metal anode Coulombic efficiency because it consumed electrons from the metal anode in the charge. Moreover, hydrogen diffusing into electrolyte leads to the increase of internal battery pressure and could result in an explosion. **Hydrogen evolution reaction:** Hydrogen evolution reaction (HER) was a side reaction of metal electrodes during the charge-discharge of batteries. The specific working principle is shown in Equations (19) and (20). Metal releases electrons to the aqueous electrolyte system and hydrogen ions  
Figure 11: Cyclic voltammetry for Fe/C composite electrodes composited carbon nano-fibers (**a**) without and (**b**) with FeS additive [54] (Copyright © 2006, Elsevier). Figure 10: SEM images and EDS maps of (**a**) Al/Cu-CD, (**b**) Al/Cu-ED, (**c**) The cyclic voltammogram in 1 M NaOH of Al (pure), Al/Cu-CD, and Al/Cu-ED, (**d**) EIS measurements of anodes in 1 M NaOH, The Nyquist a Al (pure), Al/Cu-CD, Al/Cu-ED [53] (Copyright © 2018, Springer Nature). replace metal ions obtaining electron reduction in hydrogen. HER in MABs thus influences the rates of metal electrodes. Hydrogen overpotential decreases on the ZnO surface since the self-discharge rate reduced with increasing ZnO on the electrode surface [55]. Increasing overpotential of HER (decreased HER rate) can thus improve the charging efficiency. In addition, the corrosion and oxidation of Al in alkaline electrolytes depend on electrolyte properties, temperature, and purity [56]. Using ionic liquid or solid state electrolyte is an effective solution to reduce the HER rate, which has been confirmed in FABs [57]. An alloy as anodes replacing pure metal also reduced the corrosion rate, and additives, such as bismuth or sulfur, could minimize the corrosion of the iron electrode and the evolution of hydrogen [58]. **Passivation layers**: Passivation used to describe an electrode that could not be further discharged because an insulating film on its surface blocked migration of the discharge product. In MABs, LiOH, ZnO, and Al\({}_{2}\)O\({}_{3}\) were passivation layers for corresponding systems. Soluble species formed at the air cathode will be reduced to a non-conductive layer on the metal surface. This a non-conductive layer increases the internal electrical resistance of the cell and prevents metal dissolution. The efficient method is to use porous electrodes to hinder the formation of passivation layers. **Dendritic formation and deformation**: During the metal electrode cycling in an alkaline electrolyte, the metal anode releases ions during discharge and the metal ions re-deposit on the surface of the anode during charging. As a result, the metal electrode will gradually change shape, and its surface will become roughened with uneven thicknesses. Over several charge and discharge cycles, the uneven shape accumulates to form dendrites, causing the battery system to become unstable or short cut. Different approaches have been attempted to mitigate dendritic formation and deformation, such as coating the zinc metal and using non-reaction additives in the zinc electrode or electrolyte [59]. Lithium alloying with Na [60], Mg [61], Al [62] has been confirmed to effectively suppress the growth of dendritic Li.