## 2 Spinels as an Electrocatalyst
### MnCo\({}_{2}\)O\({}_{4}\) Spinels  
To date, many cationic distributions of MnCo\({}_{2}\)O\({}_{4}\), such as Co\({}^{2+}\)[Co\({}^{2+}\)Mn\({}^{4+}\)]O\({}_{4}\) and Co\({}^{3+}\)[Mn\({}^{2+}\) Co\({}^{3+}\)]O\({}_{4}\), have been proposed. Through different synthetic methods, these cations can be tuned to enhance the physical, physicochemical, and interfacial properties. Besides the porosity and surface area, they offer a fast pathway for electrolyte/electron diffusion and more electroactive sites [62]. [PERSON] et al. studied mesoporous MnCo\({}_{2}\)O\({}_{4}\) spinel oxide because it consisted of two transition metals in a lattice; thus, they expected high ORR/OER activity because the catalytically active element always dominates on the surface. The authors prepared mesoporous MnCo\({}_{2}\)O\({}_{4}\) by a hard template method with mesoporous silica as an inorganic template. This process formed material with more surface area, an average pore size of 2 nm, and lower overpotential in ORR/OER. [PERSON] et al. then evaluated the material's catalyzing property in a ZAB with 4 M KOH aqueous electrolyte and Hg/HgO as a standard electrode, and it showed a capacity of about 700 mAh/g-Zn, discharge potential of 1.05 V, and could withstand more than 200 charge-discharge cycles [46]. [PERSON] et al. coated carbon on MnCo\({}_{2}\)O\({}_{4}\) nanowire by self-templating from a NiCo\({}_{2}\)O\({}_{4}\)-nitrogen triacetic acid (MnCo\({}_{2}\)-NTA) nanowire precursor. The resulting material, MnCo\({}_{2}\)O\({}_{4}\)@C, showed better ORR than MnCo\({}_{2}\)O\({}_{4}\). The authors proposed that the improved ORR was attributable to the conductivity of MnCo\({}_{2}\)O\({}_{4}\)@C nanowire, where the carbon layer helped in the electron movement of MnCo\({}_{2}\)O\({}_{4}\) nanoparticles, thereby decreasing the charge transfer resistance. Additionally, the porous texture and large surface promoted the diffusion of mass transfer and enlarged the triple-phase contact area, and the rigid carbon shell provided mechanical strength to gain progressed stability. The evaluated bifunctionality \(\Delta\)E, which is a small difference, is the reason for excellent reversibility in oxygen electrocatalysis. The authors obtained galvanostatic charge-discharge plots at 10 mA cm\({}^{-2}\) and power density status that revealed admirable stability of 70 h in a homemade ZAB (Figure 5) [54]. A combination of spinel compounds and N-doped carbon enriches the electrocatalytic activity of ORR and OER. [PERSON] et al. adopted this strategy and achieved good OER activity of MnCo\({}_{2}\)O\({}_{4}\) because OER and ORR are the core components of ZABs. Therefore, the authors decorated MnCo\({}_{2}\)O\({}_{4}\) on the N-doped macroporous carbon nanofiber arrays (MnCo\({}_{2}\)O\({}_{4}\)/NMCNA). The researchers synthesized the macroporous carbon nanofiber array (MCNA) using the surfactant-templating self-assembly of organic resol and a natural crab shell comprising a well-aligned porous macrostructure as a hard template; they attained an interpenetrating ordered array. The pores were larger in the pristine form than in the MnCo\({}_{2}\)O\({}_{4}\)/NMCNA; the authors analyzed the porosity by N\({}_{2}\) adsorption-desorption and confirmed microporous structure formation. [PERSON] et al. stated that the e\({}_{\text{g}}\) occupancy of the active cation in the octahedral site is the activity descriptor for the ORR/OER of spinels, consolidating the role of electron orbital filling in metal oxide catalysis. The spinel structure affords multiple sites for transition-metal cations and an extensive range of valence states to give rise to a large number of oxides. In this work, the researchers varied the temperature to tune the electronic structure of MnCo\({}_{2}\)O\({}_{4}\) cubic spinels, which they prepared by solid-state chemistry. Activity on spinel as a function of the Mn valence state of the active element at the octahedral site resulted in a volcano shape, as displayed in Figure 6a,b.