**1. Introduction**

As far the energy consumption is concerned in the current energy scenario, the role of fossil fuels is exceptional by delivering a rich source in comparison with other sources [1, 2]. The catastrophic changes to the earth's climate are unavoidable with these carbon-emitting energy supplies. As the depletion of fossil fuels alarms the scientific community to move forward for future fuels, the search is triggered with the application of renewable energy sources like solar, tidal and wind-assisted energy devices. Considering the electrochemical energy conversion devices such as batteries, alkaline fuel cells and solar water-splitting devices [3], these are the technological functions based on the electrochemical reactions such as oxygen evolution reaction (OER), oxygen reduction reaction (ORR) [4, 5], hydrogen evolution reaction (HER) [6] and hydrogen oxidation reaction (HOR) [7]. The chemistry behind the hydrogenbased oxidation and reduction is facile compared with the oxygen-based oxidation and reduction reactions. The OER and ORR reactions are uphill processes and thus drag more overpotentials, and hence, the voltage applied also increases [8]. In metalair batteries (MABs), the activity of the device predominantly depends on OER and ORR taking place during the charge and discharge process, respectively. The most active catalysts are the expensive Pt (HER, ORR), IrO2 and RuO2 (OER), and the search for inexpensive catalysts is unavoidable for the commercial usage of MABs. Moreover, the use of two different catalysts for ORR and OER in MAB makes the device more complicated and expensive. Besides, the ORR being the most active catalyst, Pt is not a suitable candidate for OER owing to the formation of the oxide over the surface of the catalyst, which gives the catalytic ability of OER. Similarly, IrO2 is a good candidate for OER, due to its high conductivity and stability, and is not preferable for ORR [9]. Therefore, the development of non-precious transition metalbased bifunctional catalysts, which can simultaneously catalyze both OER and ORR, becomes the utmost important parameter to enhance the efficiency of these electrode reactions. Even though there are catalysts with increased efficiency for the selective ORR-like carbon-derived metal catalysts and their oxides, the OER becomes a matter of concern as the conversion of carbon to their oxides occurs before the thermodynamic potential of OER. For OER, the chalcogenides and oxides of transition metals are found to be more active and stable catalysts in basic environment. Therefore, identifying electrocatalysts with significant activity in both OER and ORR is more important for applying them in the energy conversion MAB devices [10].

In addition to the bifunctional activity, the stability of the electrocatalyst in the electrolyte medium is also vital for commercializing the technology. The corrosive nature of acids during long-term usage restricts their commercial usage, and in such cases, alkaline electrolytes become preferable, which facilitates the sluggish oxygen electrode reactions (ORR/OER) with ease and thus provides relatively less corrosive to the transition metal-based materials. Additionally, transition metal-based catalysts have more advantages such as the ability to exist in multiple (two or more) and mixed oxidation states, different coordination environments, and flexibility to replace one transition metal by another that is highly desirable to further promote the bifunctional activity. Electrocatalysts derived from transition metals like transition metal oxides (TMOs), sulfides and hydroxides have been demonstrated as efficient electrocatalysts in oxygen electrode reactions [11]. Among all TMOs, particularly cobalt oxide (Co3O4, spinel oxide) and manganese oxides are found to be the superior bifunctional catalysts for oxygen electrode reactions [10].

Manganese oxides, with their highly affordable nature and handling, could also deliver enhanced ORR activity and moderate OER activity through the metal and lattice oxygen that act as active sites. The enhanced activity from manganese oxides is ascribed to the ease of conversion of Mn to higher oxidation states and its preferable four-electron transfer reduction pathway in ORR [12]. However, in spinel oxides like

### *A Perspective on the Recent Amelioration of Co3O4 and MnO2 Bifunctional… DOI: http://dx.doi.org/10.5772/intechopen.109922*

Co3O4, Co2+ and Co3+, ions assist the conversion of reversible adsorption-desorption of oxygen to facilitate the oxygen electrode reactions [9, 13]. Therefore, much effort has been dedicated based on these TMOs for the efficient conversion of both OER and ORR.

After evaluating the need for new and advanced energy conversion devices in brief, in this chapter, we have first introduced fundamental electrochemical parameters and requirements for the efficient ORR/OER electrocatalyst in basic electrolytes. The bifunctional behavior of the superior non-precious TMOs such as MnO2 and Co3O4 is explained in more detail and its recent developments from the literature are reviewed in extreme depth [13]. The fundamental factors that can be used to enhance the bifunctional activity are summarized, which include morphology, phase, crystal facets, defects, mixed metal oxides and doping of metals on the metal oxide surface. From this chapter, one can easily find the fundamentals behind the formation of highly active, earth abundant transition metal-based catalysts for both OER and ORR processes with increased efficiency. In addition, the exact catalytic behavior of the bifunctional catalyst is not fully understood, which varies from material to material. To pinpoint the active sites and discover the mechanism of the particular catalyst, density field theory (DFT) calculation and in situ study by using specific analytical techniques such as Raman spectroscopy, Mossbauer spectroscopy, X-ray adsorption/ diffraction and Fourier transform infrared spectroscopy are highly recommended. Theoretical studies combined with these in situ experiments are able to provide a deeper insight into structures and processes at the atomic level, which together with laboratory experiments could lead to a better understanding of the mechanistic steps involved in the reactions, and these can pave the way for the future directions of OER and ORR for commercial application. Finally, the existing challenges and muchneeded effort for improving the catalytic activity of bifunctional catalysts are discussed as the future effort directions.
