## 2 Spinels as an Electrocatalyst
### CuCo\({}_{2}\)O\({}_{4}\) Spinels
The researchers studied the morphology and nanostructure of the obtained cubic spinel-type CuCo\({}_{2}\)O\({}_{4}\) by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images showed that CuCo\({}_{2}\)O\({}_{4}\)@C nanotubes formed with an ample number of pores with a rough texture and unremitting length; this could be the reason for the increased diffusion kinetics of hydroxide and the adsorption-desorption of oxygen. The TEM image showed the outer and inner diameters of the nanotube (130 nm and \(\sim\)80 nm), wall thickness (25 nm), and double the active surface area, which enhanced the contact between hydroxyl, oxygen, and catalyst. The small CuCo\({}_{2}\)O\({}_{4}\) nanoparticles increased the utilization rate. Figure 8 shows the uniform distribution of Cu, Co, O, C, and N with a hollow carbon substrate confirmed by TEM elemental mapping. The carbonization temperature varied from 700 to 900 \({}^{\circ}\)C. Compared with H-CuCo\({}_{2}\)O\({}_{4}\) and L-CuCo\({}_{2}\)O\({}_{4}\)@C (higher and lower oxidation  
Figure 6: (**a**) ORR and (**b**) OER activity on various spinels as a function of e\({}_{g}\) occupancy of the active element at the octahedral site. The black circle represents cubic MnCo\({}_{2}\)O\({}_{4}\) spinels with Mn as the active element. The red circle represents the Mn of tetragonal spinels (Mn\({}_{2}\)CoO\({}_{4}\), Mn\({}_{2}\)Sc\({}_{0}\)O\({}_{4}\), and Mn\({}_{3}\)O\({}_{4}\)). The orange circle represents the Mn of ferrite. The green “up” triangle represents the Mn of cubic Li\({}_{x}\)Mn\({}_{2}\)O\({}_{4}\) spinels. The blue diamond represents the Co of cubic spinels (Co\({}_{3}\)O\({}_{4}\), ZnCo\({}_{2}\)O\({}_{4}\), and CoFe\({}_{2}\)O\({}_{4}\)). The pink square represents the Ni of cubic spinels (NiCo\({}_{2}\)O\({}_{4}\) and NiFe\({}_{2}\)O\({}_{4}\)) [63]. Copyright 2017, John Wiley and Sons. temperature), CuCo\({}_{2}\)O\({}_{4}\)@C exhibited improved OER (low overpotential of 327 mV at 10 mA cm\({}^{-2}\)), ORR (positive half-wave potential of 0.850 V), and electrochemically active surface area; CuCo\({}_{2}\)O\({}_{4}\)@C (313.8 cm\({}^{2}\)), L-CuCo\({}_{2}\)O\({}_{4}\)@C (80.0 cm\({}^{2}\)), and H-CuCo\({}_{2}\)O\({}_{4}\) (61.3 cm\({}^{2}\)) showed better interaction of reactants with the active surface area, as shown in Figure 9. The results showed the excellent catalytic activity of CuCo\({}_{2}\)O\({}_{4}\)@C. In the full cell test, the discharge\(-\)charge voltage gap of 0.79 V at 10 mA cm\({}^{-2}\) cycled up to 160 numbers for 80 h. The methanol tolerance of the material is higher than that of Pt. Additionally, N-doping and interconnected 1D open-ended nanotubes might play roles in oxygen catalysis and conductivity [66]. Figure 8: (**a,b**) Scanning electron microscopy (SEM) and (**c,d**) transmission electron microscopy (TEM) images of CuCo\({}_{2}\)O\({}_{4}\)@C. (**e**) TEM elemental mapping images of CuCo\({}_{2}\)O\({}_{4}\)@C [66]. Copyright 2017, American Chemical Society. Figure 7: The catalytic process of CuCo\({}_{2}\)O\({}_{4}\)/N-CNTs [39]. Copyright 2017, John Wiley and Sons.