## 2 Spinels as an Electrocatalyst
### NiCo\({}_{2}\)O\({}_{4}\) Spinels  
In the group of transition metals, nickel has emerged as one of the most hopeful electrocatalysts due to its exciting electronic properties and anticipated synergistic effect to alter surface properties of materials to dramatically favor electrocatalysis. The poor conductivity of electrons and noteworthy self-aggregation of Ni particles in the Nickel-based spinels control their application as electrocatalysts [69]. [PERSON] et al. synthesized Ni\({}_{x}\)Co\({}_{(3\text{-}x)}\)O\({}_{4}\) (x = 1.25) on aminated carbon nanotubes (NH\({}_{2}\)-CNTs) in a composition-controlled technique to rectify the above issues. NH\({}_{2}\)-CNTs make the catalyst more active by providing conductivity and more surface area for fast charge transfer, and the easy diffusion of the catalyst. This took place through a hydrothermal reaction in the presence of nickel ions from Co\({}_{3}\)O\({}_{4}\)/NH\({}_{2}\)-CNTs, in which the cation exchanged. Nickel ions amend the metal oxide nucleation and growth due to the hydrolysis difference between cobalt and nickel ions. This exchange does not change the size, crystal structure, and weight of the metal oxide. Nitrogen in NH\({}_{2}\)-CNTs fastens the nanoparticles and, in so doing, improved the electronic coupling along with amine. As usual, the researchers verified ORR in N\({}_{2}\)/O\({}_{2}\)-saturated 0.1 M KOH electrolyte, and the O\({}_{2}\)-saturated material performed better. Though its onset and half-wave potential were close to those of Pt/C, its limiting current density was more than in Pt/C. The researchers also confirmed the electron transfer number that resulted from the KL plot slope and rotating ring disk electrode technique. After ORR, investigators verified OER in O\({}_{2}\)-saturated 0.1 M KOH solution and found that it has a small overpotential and Tafel slope. They analyzed the bifunctionality of Ni\({}_{x}\)Co\({}_{(3\text{-}x)}\)O\({}_{4}\)/NH\({}_{2}\)-CNTs from the \(\Delta\)E, the difference in the potentials between ORR and OER (OER the potential of the current density at 10 mA cm\({}^{-2}\) -E\({}_{10}\)) and ORR current density at 3 mA cm\({}^{-2}\)). \(\Delta\)E of this material was lower than the best bifunctional electrocatalysts reported. There was a nil effect in the methanol tolerance test, as shown in Figure 10. Ni\({}_{x}\)Co\({}_{(3\text{-}x)}\)O\({}_{4}\)/NH\({}_{2}\)-CNTs was applied in a ZAB and exhibited a power density of 168 mW\(\cdot\)cm\({}^{-2}\) and 100 charge-discharge cycles without a change in voltage [67]. Figure 9: (**a**) ORR polarization curves of L-CuCo\({}_{2}\)O\({}_{4}\)@C, CuCo\({}_{2}\)O\({}_{4}\)@C, H-CuCo\({}_{2}\)O\({}_{4}\), and Pt/C samples. (**b**) ORR polarization curves of the CuCo\({}_{2}\)O\({}_{4}\)@C nanotubes at different rotation speeds and the corresponding K\(-\)L plots at different potentials (inset). (**c**) ORR polarization curves of CuCo\({}_{2}\)O\({}_{4}\)@C and Pt/C samples before and after 3000 potential cycles. (**d**) OER polarization curves of L-CuCo\({}_{2}\)O\({}_{4}\)@C, CuCo\({}_{2}\)O\({}_{4}\)@C, H-CuCo\({}_{2}\)O\({}_{4}\), IrO\({}_{2}\), and the overpotential schematic at 10 mA cm\({}^{-2}\) (inset). (**e**) Tafel plots derived from (**d**). (**f**) OER polarization curves of CuCo\({}_{2}\)O\({}_{4}\)@C and IrO\({}_{2}\) during the cycling durability test [66]. Copyright 2017, American Chemical Society. Designing efficient bifunctional electrocatalysts for OER/ORR requires a deep understanding of the mechanisms at the atomic level in the applied material. As such, in 2018, [PERSON] et al. found that pyridinic-N-Co bonding promoted reversible synergistic oxygen electrocatalysis better than pyrrolic-N bonding in the hybrid catalyst, NiCo\({}_{2}\)O\({}_{4}\) nanoparticle/N-doped mesoporous graphene. Using low-intensity pulsed-laser irradiation, the researchers could engineer pyridinic and pyrrolic N contents in NiCo\({}_{2}\)O\({}_{4}\)/N-graphene (NiCo/NG) hybrids by tuning the in-plane mesopores onto graphene oxide (GO) sheets.