**6. Conclusions**

A rechargeable metal-air battery with the aid of high theoretical energy output will be of prime competence for the carbon-based energy sources that we rely on hugely as of now. Metal-air batteries lack in commercial scale due to the limited kinetics behind OER and HER. The search for bi-functional catalysts that are earth abundant and kinetically facile is important to meet energy crisis. Previously, many breakthrough works have been done with superior activities, and among them, MnO2 and Co3O4 have attracted researchers with their kinetic activities and structure-activity relationships. In this chapter, the important parameters judging the activities of both Co3O4 and MnO2 such as phase and morphology engineering, defects and crystal facets engineering, strains and mixed metals oxide formations have been portrayed with proper investigations along with the fundamentals of water oxidation and reduction reactions. Moreover, the electrochemistry behind MnO2 and Co3O4 has been investigated for the better understanding of catalyst reactions and mechanisms. This will be highly useful for developing and designing mixed oxides for the increased kinetics of rechargeable metal-air batteries with commercial production. This chapter urges the importance of MnO2 and Co3O4 as better bi-functional catalysts with the introduction of fundamental electrochemistry, structure-activity relationship and finally the future directions in the field of metal-air batteries.
