# Recent Progress of Metal-Air Batteries--A Mini Review
## 2 Cathodes
### Improving ORR and OER  
The appropriate catalyst should be designed and applied to maximize catalytic efficiency. Metal-based catalysts possess high catalytic efficiencies due to different crystal structures. Spinel-type oxide (A\({}_{x}\)B\({}_{3-x}\)O\({}_{4}\)) [28] and perovskite oxides (ABO\({}_{3}\)) [29] are widely used as bifunctional electrocatalysts in alkaline electrolytes. [PERSON] et al. [30] synthesized a spinel-type lithium cobalt oxide LiCoO\({}_{2}\) at  
Figure 5: Various cathodic electrode of MABs, (**A**) metal current collector with a gas diffusion layer and coated catalyst facing electrolyte, (**B**) carbon-base current collector and gas diffusion and catalyst, (**C**) carbon paper-current collector and gas diffusion and catalyst. high-temperature (800 \({}^{\circ}\)C, LiCoO\({}_{2}\)-HT) and low-temperature LiCoO\({}_{2}\)-LT at 400 \({}^{\circ}\)C. LiCoO\({}_{2}\)-LT adopts a lithiated spinel structure [Li\({}_{2}\)]\({}_{16}\)[Co\({}_{2}\)]\({}_{16}\)O\({}_{4}\) in which the Co\({}^{3+}\) ions occupy all the 16d octahedral sites (space group: Pd3m) [31]. As shown in Figure 6a, LiCoO\({}_{2}\)-HT has the \(\alpha\)-NaFeO\({}_{2}\) structure (space group: R3m) arrays. Li\({}^{+}\) ions and Co\({}^{3+}\) ion occupy on alternate (111) NaFeO\({}_{2}\)-style structure arrays, due to the large size and charge differences between the Li\({}^{+}\) and Co\({}^{3+}\) ions [32]. In Figure 6b, it is obvious that HT-LCoO\({}_{2}\) has a better catalytic performance than LT-LiCoO\({}_{2}\) and Co\({}_{3}\)O\({}_{4}\). Catalysts are preferred at nanoscale for better catalytic behavior. By using a vacuum DC arc method, [PERSON] et al. [33] synthesized a novel Mn\({}_{3}\)O\({}_{4}\)/MnO nano spherical transition metal compound. The results showed that the size of Mn\({}_{3}\)O\({}_{4}\)/MnO particles was controlled at a range of 40 to 60 nm. The Mn\({}_{3}\)O\({}_{4}\)/MnO catalyst potential platform reaches to 2.7 V.  
Nanoscale catalysts can also be fabricated with various other morphologies, such as nano-rod LaCoO\({}_{3}\)[34], 3D ordered mesoporous structure Co\({}_{3}\)O\({}_{4}\)[35], hollow cobalt oxide nanoparticles [36]. Different morphologies of catalysts are shown in Figure 7. Many low-cost and efficient catalysts have been developed, including transition metals and nitrogen co-doped carbons (M-N/C, M=Fe or Co) [25], metal oxides [37], transition metal carbides [38], nitrides [39], and metal-free heteroatom-doped carbon-based catalysts [40]. Compared to metal-contained catalysts, the heteroatom-doped carbon-based materials with N, S, B, and P, can promote oxygen adsorption on the carbon nanostructure since these hetero-atoms are more electronegative than carbon, and cause neighboring carbon atoms electron deficiency [41]. Among them, N-doped carbons are extensively studied due to their remarkable ORR catalytic activity. N-doped carbon materials are shown in four ways, graphitic N, Oxidized N, pyrrolic N, and pyridine N in Figure 8.  
Although several materials have shown catalytic activity for oxygen reaction in MABs cathodes, the catalytic efficiency is not ideal when used alone. To improve comprehensive catalyst performance, composition materials have been synthesized and used as catalysts in both ORR and OER. MnO\({}_{2}\) and RuO\({}_{2}\) are single catalysts for ORR and OER, respectively. Combining MnO\({}_{2}\) with RuO\({}_{2}\) is used as a bifunctional catalyst. [PERSON] et al. [42] synthesized RuO\({}_{2}\) nanoparticles (np-RuO\({}_{2}\)/nr-MnO\({}_{2}\)) supported on MnO\({}_{2}\) nanorods by a two-step hydrothermal reaction. Electrochemical characterizations are carried on nanocomposites np-RuO\({}_{2}\)/nr-MnO\({}_{2}\) as catalysts for LABs. Charge-discharge tests showed a reversible discharge capacity of 500 mAh g\({}^{-1}\) for 75 cycles at a current density of 50 mA g\({}^{-1}\). LABs with the RuO\({}_{2}\)/MnO\({}_{2}\) catalyst presented much lower overpotential of 0.58 V at 50 mA g\({}^{-1}\) than that measured with a single catalyst.