## 2 Spinels as an Electrocatalyst
### Fe-Based Spinels  
Mn-Fe-based spinels are highly electrocatalyically active for the cathode and anode redox reactions on oxygen electrodes. The increment of active sites ensures the further boosted catalytic efficacy [71]. [PERSON] et al. prepared spinel MnFe\({}_{2}\)O\({}_{4}\)/Fe hybrid nanoparticles in nitrogen-doped mesoporous hollow carbon nanospheres (Fe/Mn-N-C) by pyrolysis to increase the active centers with the addition of Fe. Though the spinel-built oxides showed good catalytic efficacy, the conductivity was inefficient. Here, the porous nature of the catalyst augments the OER and ORR activity of a transition metal spinel oxide. Figure 10: (**a**) Illustration of crystal phase preservation during cation exchange. (**b**) Chronoamperometric curves of the Ni\({}_{x}\)Co\({}_{(3x)}\)O\({}_{4}\)/NH\({}_{2}\)-CNTs and Pt/C at 0.7 V. The inset shows the Chronoamperometric curves of the Ni\({}_{x}\)Co\({}_{(3x)}\)O\({}_{4}\)/NH\({}_{2}\)-CNTs and Pt/C with the addition of 1.0 M methanol into O\({}_{2}\)-saturated 0.1 M KOH [67]. Copyright 2013, Royal Society of Chemistry. The ORR and OER evaluation results showed that the material with a 2:1 ratio of Fe\({}^{3+}\) to Mn\({}^{2+}\) was nearly the same as Pt for OER and better than Pt in terms of methanol tolerance, ORR, and ZAB full cell tests. In ORR, Fe/Mn-N-C exhibited slightly more positive onset potential and half-wave potential and, surprisingly, much lower current density than Pt/C. TEM images suggested that the ordered graphitic carbon was responsible for good conductivity and high corrosion resistance and that the disordered carbon accommodated the number of M-Nx active centers. Although metal agglomeration somewhat increased the current density, the onset value was the same with and without its presence, demonstrating efficient methanol tolerance, durability, and cyclability. The power density of Fe/Mn-N-C (37 mW cm\({}^{-2}\)) and Pt/C (28 mW cm\({}^{-2}\)) exhibits its capacity as an excellent oxygen electrocatalyst [41]. [PERSON] et al. engineered a ternary-spinel oxide based on the octahedral site preference energy (OSPE) model; the OSPE is the difference between crystal field stabilization energies at octahedral and tetrahedral sites (CFSE\({}_{\text{oct}}\) and CFSE\({}_{\text{tet}}\)). The authors synthesized ternary-spinel oxides (i.e., M\({}_{0.1}\)Ni\({}_{0.9}\)Co\({}_{2}\)O\({}_{4}\) with M: Mn, Fe, Cu, and Zn) by hydrothermal and annealing processes. The inverse spinel structure of NiCo\({}_{2}\)O\({}_{4}\), in which Ni atoms were in the octahedral sites and Co atoms occupied both octahedral and tetrahedral sites, was confirmed by XRD. Further, the same spinel structures were found in Mn\({}_{0.1}\)Ni\({}_{0.9}\)Co\({}_{2}\)O\({}_{4}\), Fe\({}_{0.1}\)Ni\({}_{0.9}\)Co\({}_{2}\)O\({}_{4}\), Cu\({}_{0.1}\)Ni\({}_{0.9}\)Co\({}_{2}\)O\({}_{4}\), and Zn\({}_{0.1}\)Ni\({}_{0.9}\)Co\({}_{2}\)O\({}_{4}\) (MNCO-01, FNCO-01, CNCO-01, and ZNCO-01), respectively. Due to the cation substitution in Ni and Co, the lattice expanded and contracted. The oxidation state of these doped metal ions can alter the mean oxidation states of Ni and Co; therefore, the site preferences of all metallic ions can vary. The activity of the catalyst is based on the metal atoms' occupancy sites in the spinel structure. The authors  
Figure 11: **(a)** ORR linear sweep voltammetry (LSV) curves of NG, NLG-270, NiCo\({}_{2}\)O\({}_{4}\) NPs, NiCo/NG, NiCo/NLG-270, and Pt/C, in 0.1 M KOH solution (rotation rate: 1600 rpm). **(b)** ORR half-wave potential of the hybrid catalysts with different contents of pyridinic and pyrrol N bonded with Co ions, and Pt/C. **(c)** Comparison of the kinetic currents and the electron transfer number for the ORR process of the control samples, along with the corresponding contents of pyridinic and pyrrol N bonded with Co. **(d)** ORR LSVs of NG, NLG-270, NiCo\({}_{2}\)O\({}_{4}\) NPs, NiCo/NG, NiCo/NLG-270, and Pt/C, in 1 M KOH solution.