## 2 Spinels as an Electrocatalyst
### CuCo\({}_{2}\)O\({}_{4}\) Spinels  
The spinel structures with cobalt in the octahedral sites are more resourceful for the OER and ORR, and copper-based materials have exhibited excellent ORR and OER catalytic activity due to the intense interfacial contact of Cu and its surroundings [64; 65]. In [PERSON] et al. 's work, nitrogen-doped carbon nanotubes were ornamented by spinel CuCo\({}_{2}\)O\({}_{4}\) quantum dots (CuCo\({}_{2}\)O\({}_{4}\)/N-CNTs) with successful Co\({}_{3}\)O\({}_{4}\) doping without changing its crystalline structure. Carbon nanotubes (CNTs) are used to confer high electrical conductivity, large surface area, and intrinsic flexibility. The CNTs are doped to solve the problem of weak catalytic performance. CuCo\({}_{2}\)O\({}_{4}\)/N-CNTs was tested in liquid alkaline and a solid-state electrolyte because of its ORR (onset potential of \(-\)0.04 V, the large diffusion-limited current density of \(-\)5.53 mA\(\cdot\)cm\({}^{-2}\), and low Tafel slope of 76.53 mV\(\cdot\)dec\({}^{-1}\)) and OER (\(-\)0.61 V with a potential of 0.69 V to get 10 mA\(\cdot\)cm\({}^{-2}\) and low Tafel slope of 118.80 mV\(\cdot\)dec\({}^{-1}\) compared with Co\({}_{3}\)O\({}_{4}\)/N-CNTs, CuO/N-CNTs, and CuCo\({}_{2}\)O\({}_{4}\)). The capability of Cu and Co as ORR and OER catalysts gives a \(\Delta E\)  
Figure 5: (**a**) Linear-sweep curves of MnCo\({}_{2}\)O\({}_{4}\)@C, Pt/C, and RuO\({}_{2}\) electrodes in O\({}_{2}\)-saturated 0.1 M KOH solution at a scan rate of 5 mV\(\cdot\)s\({}^{-1}\). (**b**) Schematic illustration of the homemade Zn–air battery (ZAB). (**c**) Open-circuit potential plots. (**d**) Discharge curves of ZAB and the corresponding power density plots. (**e**) Charge and discharge polarization curves for the ZAB. (**f**) Photograph of LEDs powered by two serially connected ZABs with MnCo\({}_{2}\)O\({}_{4}\)@C cathodes. (**g**) Long-term galvanostatic charge–discharge plots at 10 mA\(\cdot\)cm\({}^{-2}\)[54]. Copyright 2019, Elsevier. value of 0.90 V. It is evident from the analysis of the other materials that CuO/N-CNTs outperform the Co\({}_{3}\)O\({}_{4}\)/N-CNTs in ORR, and the reverse was true for OER. To learn the role of the N-CNTs, researchers performed electrochemical impedance spectrum (EIS), electrochemical double-layer capacitance (C\({}_{\text{dl}}\)), and XPS analysis. In the findings, with lower resistance (EIS) and greater capacitance (electrochemically active surface area) along with BET measurement (11.18 m\({}^{2}\) g\({}^{-1}\)), N-CNT provided a large surface area and restricted the aggregation and bonding between N and Cu; additionally, Co offered pyridine nitrogen for good ORR by generating more catalytic sites in CuCo\({}_{2}\)O\({}_{4}\). The catalytic process of CuCo\({}_{2}\)O\({}_{4}\)/N-CNT is illustrated in Figure 7. Further, the authors measured the chronoamperometry and chronopotentiometry to identify the stability and used a rotational ring disk electrode to identify the number of electrons transferred. Methanol tolerance results are demonstrated as a control measure to examine the analyzed material as a stable catalyst for the ORR and ve high durability towards the poisoning of any oxidized products [39, 41, 66, 67, 68]; CuCo\({}_{2}\)O\({}_{4}\)/N-CNTs was found to be a promising oxygen electrocatalyst comparable with Pt/C (ORR) and IrO\({}_{2}\) (OER) with a power density of 83.83 mW cm\({}^{-2}\) and energy density of 653.9 Wh kg\({}^{-1}\). Investigators tested the ZAB stability at 20 mA cm\({}^{-2}\), and after 24 h, both the Pt/IrO\({}_{2}\) electrode and CuCo\({}_{2}\)O\({}_{4}\)/N-CNTs showed some slipping. CuCo\({}_{2}\)O\({}_{4}\)/N-CNTs, with their lower overpotential than that of noble metal electrodes, showed very stable charging and discharging potential even after 48 h [39]. To rectify the drawback of less surface area between the active material and the electrolyte in the carbon matrix, [PERSON] et al. electrospun the CuCo\({}_{2}\)O\({}_{4}\)@C nanotubes via coaxial electrospinning.