## 4 Discussion
### Methanol Electrocatalytic Oxidation at the Modified Electrodes  
Cyclic voltammograms (CVs) were taken in the presence of methanol in alkaline medium in order to study the impacts of the deposition of different metal oxides on methanol oxidation. Figure 6 compares CV responses obtained at CNTs/GCE modified electrodes where (a) CoOx@CNTs/GCE; (b) CuOx@CNTs/GCE; (c) NiOx@CNTs/GCE; and (d) CuCoNiOx@CNTs/GCE in 1 M NaOH containing 1 M methanol at a scan rate of 100 mV/s. Methanol oxidation at CNTs/GCE is not visible which indicates that CNTs/GCE is inert toward methanol oxidation in these conditions of pH and potential (data not shown), while with the modification of CNTs/GCE with any metal oxides, the catalytic activity is enhanced dramatically, which was concluded from the higher oxidation current obtained in the forward scan compared to the unmodified CNTs/GCE. As was expected, CoOx@CNTs/GCE (curve b) has shown the lowest electrocatalytic activity in comparison to the other oxides, and CuOx@CNTs/GCE (curve c) revealed better activity towards methanol oxidation than CoOx@CNTs/GCE, which was shown by higher current values and a negative shift in the onset potential of the methanol oxidation (\(I\) = 1.0 mA and 0.6 mA around 0.6 V for CuOx and CoOx, respectively). As reported in the literature, NiOx is one of the pioneer electrocatalysts for alcohol oxidation in alkaline medium, and that appeared clearly in our work where NiOx@CNTs/GCE (curve d) showed the best catalytic activity for methanol oxidation among the addressed mono-catalysts, where there is a significant negative shift in the onset potential and increase in the oxidation current [9]. Generally, there are several mechanisms that are used to describe the methanol oxidation reactions on the metal oxides in alkaline medium and those reports can be shortened in two ways. The first assumes that the origin of the electrocatalytic activity of the CuOx, NiOx, and CoOx in alkaline medium is due to presence of the catalytic oxidizing agents of Cu(III), Ni(III), and Co(IV), respectively, that interact with methanol to form lower oxides and methanol oxidation products [31; 32]. The second pathway assumes that methanol oxidation does not occur until complete formation of the mentioned higher oxidation states in the course of an anodic potential sweep, to be a support for the reaction and not a catalytic intermediate [33; 34]. Therefore, in summary, we assume that a part of the anodic current is due to methanol oxidation by Cu(III), Ni(III), and Co(IV) and a part of the current is due to methanol oxidation on the surface of the oxide layer by direct electrooxidation. According to [PERSON] et al. the mechanism can be written generally by following [8]. \[\text{Lower oxidation state}\Rightarrow\text{Higher oxidation state}+\text{e}^{-} \tag{6}\]  
And methanol is oxidized by the interaction with X(III) or X(IV) via the following routes:  
\[\text{Lower oxidation state}+\text{methanol}\Rightarrow\text{Higher oxidation state}+\text{intermediate}+\text{e}^{-} \tag{7}\]  
\[\text{Lower oxidation state}+\text{intermediate}\Rightarrow\text{Higher oxidation state}+\text{products}+\text{e}^{-} \tag{8}\]  
In addition, the higher oxidation states are regenerated again by the power source and methanol is oxidized on these sites by direct electrooxidation according to the following:  
\[\text{Lower oxidation state}-\text{methanol}\Rightarrow\text{Higher oxidation state}-\text{intermediate}+\text{e}^{-} \tag{9}\]  
\[\text{Lower oxidation state}-\text{intermediate}\Rightarrow\text{Higher oxidation state}-\text{products}+\text{e}^{-} \tag{10}\]  
Equations (8) and (9) relate to the first pathway mechanism [31; 32], and Equations (10) and (11) relate to the second pathway mechanism [33; 34]. As we mentioned in the introduction, NiOx is the best non-precious metal based electrocataly for alcohol oxidation in alkaline medium, so research attention has been directed to enhance both NiOx stability and activity by alloying with other metal oxides.