# Nanosheets of CuCo\({}_{2}\)O\({}_{4}\) As a High-Performance Electrocatalyst in Urea Oxidation
## 3 Results and Discussion
### Electrocatalytic Activities  
The electrocatalytic activities of the CuCo\({}_{2}\)O\({}_{4}\) electrode were examined using a three-electrode setup in 1 M KOH, without and with 0.33 M urea. As seen in Figure 5a, the anodic current density of CuCo\({}_{2}\)O\({}_{4}\) increased significantly after the addition of urea, confirming a high catalytic activity response of CuCo\({}_{2}\)O\({}_{4}\) for the electro-oxidation of urea. The CuCo\({}_{2}\)O\({}_{4}\) electrode demanded a potential of 1.36 V to deliver a current density of 10 mA/cm\({}^{2}\), which was much lower than that without urea. The potential required to deliver 10 mA/cm\({}^{2}\) using the CuCo\({}_{2}\)O\({}_{4}\) electrode was among the more favorable reported values. For example, the Ni(OH)\({}_{2}\) nanosheet and nanocube required a potential of 1.52 V and 1.55 V, respectively, to produce a current density of 10 mA/cm\({}^{2}\)[14, 22]. Ni\({}_{2}\)P, Rhodium-Ni, and NiMo nanosheets demanded a potential 1.37 V, 1.47 V, and 1.37 V, respectively, to generate a current density of 10 mA/cm\({}^{2}\)[23, 24, 25]. The Tafel slope for the CuCo\({}_{2}\)O\({}_{4}\) electrode was calculated using polarization data, and was observed to be 82 and 46 mV/dec in 1 M KOH without and with 0.33 M urea, respectively (Figure 5b). The lower Tafel slope in the urea solution indicated faster catalytic kinetic energy for the CuCo\({}_{2}\)O\({}_{4}\) electrode. Furthermore, the urea oxidation kinetics were investigated by electrochemical impedance spectroscopy (EIS). The Nyquist plots for the CuCo\({}_{2}\)O\({}_{4}\) electrode at various potentials are given in Figure S1. As seen in the Nyquist plots, CuCo\({}_{2}\)O\({}_{4}\) had a low impedance and, thus, markedly fast kinetics toward urea electro-oxidation. Furthermore, the behavior of the Nyquist plots depended  
Figure 4: XPS spectra of the CuCo\({}_{2}\)O\({}_{4}\): (a) full survey spectra, (b) Cu 2p spectrum, (c) Co 2p spectrum, and (d) O 1s spectrum. Figure 5: (a) Cyclic voltammetry curves in 1 M KOH with and without 0.33 M urea, and (b) corresponding Tafel slopes. on the potential. With the increase in the potential, the diameter of the semicircle was decreased, suggesting a lowering in charge transfer resistance. An important method to evaluate the long-term stability of the obtained electrodes in urea oxidation is the chronoamperometric response test. Figure S2 shows the superior catalytic stability with low degradation in current density after 15 h. From this measurement, it was observed that CuCo\({}_{2}\)O\({}_{4}\) exhibited a stable and durable catalytic performance. The CV curves of the CuCo\({}_{2}\)O\({}_{4}\) electrode in 1 M KOH electrolyte with 0.33 M urea was measured at various scan rates. Figure S3 shows the variation of measured potential versus scan rates determined using CV data at 50 mA/cm\({}^{2}\). As seen in Figure S4, the CuCo\({}_{2}\)O\({}_{4}\) electrode showed almost no change in potential during CV measurements at various scan rates, indicating superior rate capability for the UOR process. Tests performed by [PERSON] et al. demonstrated an increase in the anodic current density for the Ni/Mo/graphene nanocatalysts after addition of urea in the system [26]. This increase was also found by [PERSON] et al. in the urea oxidation test for Ni and Mn nanoparticle-decorated carbon nanofibers [27]. Sodium nickel fluoride nanocubes were studied by [PERSON] et al. in different concentrations of urea for its oxidation [28]. It was found that the peak potentials of urea oxidation increased linearly with increasing urea concentration. [PERSON] et al. studied the effect in NiCo layered double hydroxide for promoted electrocatalytic urea oxidation, which proved to be efficient, showing a low onset potential and a high faraday efficiency [29]. The electrocatalytic performance of the CuCo\({}_{2}\)O\({}_{4}\) electrode for hydrogen evolution reaction (HER) was also studied. Figure 6a shows the polarization curves for a CuCo\({}_{2}\)O\({}_{4}\) electrode in 1 M KOH, without and with 0.33 M urea.