## 2 Spinels as an Electrocatalyst  
Spinel-based materials exhibit better catalysis because of their controllable composition, structure, valence, and morphology. In particular, transition metal oxides with a spinel structure have received attention as catalysts for ORR and OER because these structures enable good electrical conductivity due to the ready electron hopping between metals at different valence states. The spinel structure also provides surface redox centers for oxygen adsorption and stimulation. The existence of the different transition metals in the interlaced structure of the spinel-based transition-metal oxides is the reason for the high electrocatalytic activity because it is well established that the catalytically energetic element always governs the surface [46]. In addition to spinel, other structural forms can be used as substitute electrocatalysts. However, their usage has been limited because of the high synthesis temperature of perovskite oxides (ABO\({}_{3}\)) [47], the properties of metal-organic frameworks such as electrical insulating and bad stability in electrolytes [48], the difficulty in raising the concentration of active sites in graphene [49], and the poor OER activity of metal-N-decorated nanocarbon materials [17]. Even the noble metals rarely possess bifunctional electrocatalytic activity for both ORR and OER [50; 51].  
Therefore, spinel-based compounds with their diverse compositions, bifunctional catalytic activity, and low cost are suitable oxygen electrocatalysts. Synergism between the elements in the structure, spinel frame, compositing, and the supporting material escalates the electrocatalytic activity. Above  
Figure 4: (**a**) Schematic illustration of fabricating a self-supported Co\({}_{3}\)O\({}_{4}\)/Ni/Gas Diffusion Layer [44]. Copyright 2019, Elsevier (**b**) oxygen reduction reaction (ORR, clockwise), and oxygen evolution reaction (OER, anticlockwise) mechanisms for NiCo/Pyri-NG, where pyridinic N bonds with surface three-fold coordinated Co [45]. Copyright 2018, John Wiley and Sons.  
and beyond, the incorporated cations can form redox couples for the favored catalytic actions. To date, materials with cobalt, manganese, nickel, copper, and iron in spinel structure are widely used as oxygen electrocatalys in ZABs. Since 1980 Co\({}_{3}\)O\({}_{4}\) has been widely investigated as an electrocataly system owing to its chemical and physical properties. The efficiency of electron transport between the Co\({}^{2+}\) and Co\({}^{3+}\) ions promotes catalytic activity in an alkaline medium. The spinel structures with Cobalt in the octahedral sites are more resourceful for the OER and ORR [52; 53]. Co\({}_{3}\)O\({}_{4}\), with the advantages of high activity and stability, has been identified as a catalytically active material for air batteries [54; 55; 56].  
To further improve the catalytic performance, the active sites in the single metal oxide should be increased by adding other metals while synthesizing the spinel structure. For example, MnCo\({}_{2}\)O\({}_{4}\), NiCo\({}_{2}\)O\({}_{4}\), CuCo\({}_{2}\)O\({}_{4}\), and MnFe\({}_{2}\)O\({}_{4}\) have been reported as active catalysts for air batteries [57; 58; 59; 60; 61]. The above-mentioned spinel structures as the electrocatalyst for Zinc-air batteries are discussed in the following sections.