# Recent Progress of Metal-Air Batteries--A Mini Review
## 3 Anodes
### Metal Coating and Composite Electrodes  
Different from previous research using Al alloy as anodes, [PERSON] [53] investigated Cu coating on Al and 7075 Al alloy as anodes. Copper was deposited on the Al surface by chemical (Al or Alloy/Cu-CD) and electrochemical (Al or Alloy/Cu-ED) processes, SEM images are shown in the Figure 10a,b. Al has a lower resistance than Al-Cu alloy. EIS measurements showed that copper on the Al surface could decrease anodic potential and improve batteries performance as shown in Figure 10c,d. The solution-electrode interaction resistance (\(Rs\)) was increased by adding copper to aluminum because copper can form a protective layer against the corrosion on the aluminum-solution interface. [PERSON] et al. [54] found that FeS can employ as an additive for the electrode to suppress hydrogen evolution and improve the cyclic performance of the Fe/C composite anode. FeS additive and the carbon component also strongly affected the redox behavior of iron. An electrode with FeS can promote the process (in the Figure 11) of Fe\({}^{0}\) to Fe\({}^{2++}\) and Fe\({}^{2+}\) to Fe\({}^{3+}\) at around \(-\)0.85 V (a\({}_{1}\)), \(-\)0.65 V (a\({}_{2}\)), respectively. Furthermore, there were also additional intermediate species which appeared around \(-\)0.97 V (a\({}_{0}\)). The discharge capacity of the electrode was significantly improved with adding 2 wt.% FeS, which was mainly because the incorporation of FeS in the electrode improved the adsorbed capability of S\({}^{2-}\) on the electrode surface, resulting in the easy breaking of the oxide layer. For HER at the anode in alkaline electrolyte, both molecular recombination and electrochemical desorption can be parallel steps in the overall process. The molecular recombination reaction will appreciably contribute to HER only when the current density is low, and the molecular hydrogen concentration at the liquid boundary layer is near to zero. Under these conditions, it has been indicated that the molecular recombination reaction is affected by S\({}^{2-}\) ion chemisorption.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Grade**} & \multicolumn{2}{c}{\(E_{corr}\) (V vs. Hg/HgO)} & \multicolumn{2}{c}{\(I_{corr}\) (mA cm\({}^{-2}\))} & \multicolumn{2}{c}{\(R_{p}\) (\(\Omega\) cm\({}^{2}\))} \\ \cline{2-7}  & **4 M NaOH** & **4 M KOH** & **4 M NaOH** & **4 M KOH** & **4 M NaOH** & **4 M KOH** \\ \hline
1050 & 1.290 & 1.291 & 186 & 176 & 4.108 & 5.683 \\
2011 & 1.410 & 1.420 & 135 & 142 & 4.124 & 6.674 \\
3003 & 1.315 & 1.340 & 181 & 165 & 4.607 & 5.892 \\
4032 & 1.390 & 1.390 & 145 & 174 & 4.653 & 2.572 \\
5052 & 1.310 & 1.320 & 191 & 157 & 3.766 & 2.539 \\
6061 & 1.370 & 1.380 & 161 & 168 & 4.766 & 2.655 \\
7050 & 1.420 & 1.450 & 189 & 143 & 4.516 & 2.848 \\
8011 & 1.420 & 1.450 & 135 & 144 & 3.628 & 2.670 \\
5N & 1.699 & 1.821 & 24.3 & 4.7 & 9.668 & 17.9 \\ \hline \hline \multicolumn{7}{c}{Grade} \\ \hline Equivalent elements & solution & 1050 & 2011 & 3003 & 4032 & 5052 & 6061 & 7050 & 8011 & 5N \\ \hline \(R_{t}\) (\(\Omega\) cm\({}^{2}\)) & 4 M NaOH & 0.70 & 0.75 & 0.57 & 0.84 & 0.74 & 0.49 & 0.76 & 1.247 & 2.29 \\
4 M KOH & 1.23 & 0.69 & 1.208 & 0.3351 & 0.78 & 0.36 & 0.60 & 1.108 & 15.3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters and electrochemical impedance spectroscopy (EIS) value of different grades of Al anodes [49].