## 1 Introduction  
The dominance of fossil fuels in energy production has become a massive problem in modern societies because of the resultant global warming and climate change. Because energy obtained from renewable sources faces challenges such as season, climate, and location dependency, the ability to store this energy for uninterrupted use is indispensable if demand is to be met. These circumstances introduce the role of electrochemical energy technology in converting and storing sustainable energy [1; 2; 3; 4; 5]. In the quest to store energy from renewable sources, metal-air batteries have been gaining attention because of their safety; they also use abundantly present oxygen as the cathode source, and they have 2-10 times the higher theoretical energy density than commercial lithium-ion batteries [6; 7; 8; 9]. Compared with other metal-air batteries, zinc-air batteries (ZABs) demonstrate superiority owing to their higher safety, their minimal environmental impact, the abundance of zinc, their cost-effectiveness, and their theoretical specific capacity of 820 Ah kg\({}^{-1}\) and specific energy of 1084 Wh kg\({}^{-1}\)[10; 11; 12; 13]. Generally, the oxygen evolution reaction and oxygen reduction reaction (OER and ORR) at ZAB cathodes are sluggish owing to complex electron transfer [14; 15; 16; 17], and this results in higher overpotential at the cathode than the anode. Because ORR is the surface reaction, O\({}_{2}\) is reduced to OH\({}^{-}\) or H\({}_{2}\)O\({}_{2}\) on the electrode surface after the electron is acquired. The ORR mechanism can happen in two ways: (1) In a two-electron transfer reaction, O\({}_{2}\) reduces to OH\({}^{-}\) in two steps, and (2) in a four-electron transfer reaction, the oxygen reduction happens in one step. The former reaction is more feasible, and the latter is more effective because it avoids the byproduct of H\({}_{2}\)O\({}_{2}\). Practically, the overpotential of OER is higher than that of ORR due to the formation of hydroxide, oxide, and oxy-hydroxide intermediates during the process, and overcoming the overpotential in ORR through four-electron transfer is quite difficult as well. OER is the tailback of the electrochemical reactions because of the four proton-coupled electron transfers and the generation of an O=O bond. Thus, there is a need for a bifunctional electrocataly that catalyzes both the OER and ORR for large-scale applications of these reactions in energy devices [18; 19]. Investigators have been using noble metal-based electrocatalysts such as Pt, RuO\({}_{2}\), and IrO\({}_{2}\) in zinc-air batteries to master the overpotential issues; however, the lower abundance, high cost, and reduced long-term durability of these materials hinder their usage [20]. With a focus on efficient performance, cost-effectiveness, and abundance as the criteria for a better electrocataly, transition-metal oxides are gaining more attention for their reactions in energy storage devices, particularly in metal-air batteries [21]. Spinel-structured transition-metal oxides with the formula AB\({}_{2}\)O\({}_{4}\) (A, B = Fe, Co, Ni, Mn, Zn, Cu, and so on), as shown in Figure 1, can be in A\({}_{3}\)O\({}_{4}\) type single-metal oxide, A\({}_{x}\)B\({}_{3-x}\)O\({}_{4}\) type binary-metal oxide, and A\({}_{x}\)B\({}_{y}\)C\({}_{3-x-y}\)O\({}_{4}\) type ternary-metal oxide forms. Spinel-based compounds consist of oxygen atoms (32) and one or two transition metals (eight in A site and 16 in B site) with a wide range of valence states. At the A site, one metal is surrounded by the four nearest neighboring oxygen atoms, and the B site has a metal that is surrounded by the six nearest neighboring oxygen atoms. In the spinel frame, A is the divalent cation, B is the trivalent cation, and O is an anion; generally, the spinel structure is classified into three types based on the occupancy of cations, whereas the anions are always accommodated by the polyhedral vertexes. The varieties of spinel are normal, inverse, and mixed spinel.