## 3 Conclusions and Perspectives  
In this review, we have summarized some strategies from the literature on applying spinel-based compounds as the bifunctional oxygen electrocatalysts in the air cathodes of ZABs. The authors of all the studies we reviewed had aimed at improving battery performance. Among the post-Li-ion battery energy storage devices, ZABs, which operate in alkaline electrolytes, have the most advantages required for addressing the energy, safety, and environmental problems. The spinel-structured materials catalyze the lethargic ORR and OER activity in ZABs through their better intrinsic properties. ORR and OER can be improved with the engineering of structures, active sites, morphologies, interfaces, surface areas, porosities, etc. by compositing, using conductive substrates, creating defects, modifying the synthesis procedure, and hybridizing with other materials. Thus, the spinel-based materials contribute to electrocatalysts that surpass the noble electrocatalysts. However, the lack of analysis of intrinsic specific activity, synergistic effects between the compounds in spinel and their hybrids, and atomic-level data calls for more detailed investigation of spinels with the help of theoretical inquiries and in operando analysis. In addition, spinels are used to increase the discharge capacity and decrease the discharge-charge gap in both aqueous and aprotic electrolytes. The most promising spinels are transition-metal oxides used as bifunctional electrocatalysts because of their low price and high catalytic efficiency [43; 46]. Therefore, the analysis needs to be extended to spinel-sulfides, selenides, and nitrides in acidic and nonaqueous electrolytes. The distribution of cations in spinels also warrants exploration, using factors such as cation radius, Coulombic interactions between the cations, and crystal field effects of the octahedral site preference energy of cations.  
Overall, several research efforts have been devoted to investigating ZABs as well as designing bifunctional ZAB electrocatalysts. On considering the practical usage of ZABs, excellent performance and long-term stability are the key properties that promote their adoption. Consequently, the catalyst needs to be designed with the maximum exposure of active sites. Critical anode behavior, cathode architecture, three-phase interfacial reactions at the gas diffusion layer, ZAB fabrication procedure, and electrolyte effects in ZABs should also be considered during catalyst design. With the development of theory, techniques of characterization, innovative synthesis methods, catalyst design, and in  
Figure 13: Power density of ZABs with various electrocatalysts in the spinel structure [39; 40; 41; 45; 71; 72; 74; 75; 76; 78; 82; 85; 88; 90; 92].  
operando analysis, the mechanistic features of spinel-based electrocatalysts in ZABs can be unleashed, and practical applications of ZABs will become a reality. In short, the challenge remains to design well-performing spinel-based electrocatalysts along with the commercialization of sustainable clean energy techniques, such as metal-air and metal-ion batteries, water splitting, supercapacitors, and CO\({}_{2}\), CO, and NO\({}_{x}\) reduction.  
This research received no external funding.  
The authors acknowledge this work supported by the National Research Foundation of Korea grant funded by the Ministry of Science, ICT & Future Planning, Republic of Korea (2018R1A5A1025224) and the Human Resources Development program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade migrant number 20194030202470.  
The authors declare no conflict of interest.