*2.1.2.3 Cu non-macrocyclic complexes*

In nature copper-containing laccase is an enzyme that under certain conditions can promote the four-electron reduction of O2 to water without the formation of any peroxide. Laccase only works at a very narrow pH range, and its large molecular size prevents high current densities. The reaction can occur at very low overpotentials, and this is indeed very unusual in ORR catalysts. For this reason, some authors [40–43] have explored the catalytic activity for ORR of simpler Cu complexes. For copper complexes, the active state is Cu(I). We will focus our discussion on the effect of the redox potential. The reactivity trends of metal phthalocyanines and metal porphyrins illustrated in **Figures 5** and **6** show that CuN4 complexes exhibit very low activity for ORR. One of the reason for the low activity is that these Cu complexes are in the oxidation state Cu(II) and cannot be reduced to Cu(I) due to the rigidity of the planar phthalocyanine ligand since the reduction process involves a change in geometry around the Cu centre from planar Cu(II) to tetrahedral (Cu(I) (see **Figure 14**). The other reason is that CuPc has no frontier orbital with *d*-metal character that can bind O2. This was illustrated in

**Figure 6**. Cu phenanthrolines are flexible, and then redox processes can occur on

*Plot of log(i/Γ) at E = 0.0 V vs. NHE versus the Cu(II)/(I) redox potential of the complex for ORR [44]*

*(reproduced by permission of the American Chemical Society).*

*Redox Potentials as Reactivity Descriptors in Electrochemistry*

*DOI: http://dx.doi.org/10.5772/intechopen.89883*

The redox potential of the catalyst plays different roles in electrochemistry. Depending on whether the catalyst is present in the homogeneous phase or anchored or adsorbed on the electrode surface, the correlations between activity and redox potential can be different. For outer-sphere reactions occurring in the solution phase, the catalytic activity as log *k* versus the redox potential of the catalyst *E°* increases linearly with the redox potential, whereas for inner-sphere chemical catalysts, the activities observed are higher than those predicted by the

For reactions promoted by molecular catalysts immobilized on the electrode surface, correlations of (log *i*)*<sup>E</sup>* versus *E*° are observed. Linear and volcano correlations are obtained. When comparing MN4 macrocyclic complexes having a wide range of redox potentials, typical volcano correlations are generally observed in electrocatalysis. In classical volcano correlations, the different electrodes are metals or alloys. In this case the binding energy of key intermediates is used as reactivity descriptor. The activity gradually increases with the binding energy up to a point. Beyond that point the activity decreases with larger binding energies as the surface becomes covered with adsorbed intermediates. Similar correlations are observed with MN4 molecular catalysts. Further, with both metals and molecular catalysts, the strong adsorption region of the volcano involves the four-electron reduction of O2, whereas the weak adsorption region includes the two-electron reduction

The Cu(II)/(I) redox potential of copper phenanthroline is a reactivity descriptor for ORR. **Figure 15** shows a linear correlation between log *i* and *E°* with a slope close to +0.120 V/decade which seems to be an incomplete volcano correlation [44].

the metal centre.

**Figure 15.**

**3. Conclusions**

**69**

redox potential of the catalyst.

**Figure 14.** *Illustration of changes in geometry when Cu(I) is oxidized to Cu(II) in a Cu(phen)2 complex.*

*Redox Potentials as Reactivity Descriptors in Electrochemistry DOI: http://dx.doi.org/10.5772/intechopen.89883*

**Figure 15.**

possibly for other reactions [12, 13, 26]. The literature in this subject is very abundant, and more details and discussion about this pyrolysed MNx catalysts are beyond the scope of this chapter. However, it is important to remark that the redox

In nature copper-containing laccase is an enzyme that under certain conditions can promote the four-electron reduction of O2 to water without the formation of any peroxide. Laccase only works at a very narrow pH range, and its large molecular

overpotentials, and this is indeed very unusual in ORR catalysts. For this reason, some authors [40–43] have explored the catalytic activity for ORR of simpler Cu complexes. For copper complexes, the active state is Cu(I). We will focus our discussion on the effect of the redox potential. The reactivity trends of metal phthalocyanines and metal porphyrins illustrated in **Figures 5** and **6** show that CuN4 complexes exhibit very low activity for ORR. One of the reason for the low activity is that these Cu complexes are in the oxidation state Cu(II) and cannot be reduced to Cu(I) due to the rigidity of the planar phthalocyanine ligand since the reduction process involves a change in geometry around the Cu centre from planar Cu(II) to tetrahedral (Cu(I) (see **Figure 14**). The other reason is that CuPc has no frontier orbital with *d*-metal character that can bind O2. This was illustrated in

potential is a reactivity predictor for this very important family of catalysts.

size prevents high current densities. The reaction can occur at very low

*Illustration of changes in geometry when Cu(I) is oxidized to Cu(II) in a Cu(phen)2 complex.*

*2.1.2.3 Cu non-macrocyclic complexes*

*Redox*

**Figure 14.**

**68**

*Plot of log(i/Γ) at E = 0.0 V vs. NHE versus the Cu(II)/(I) redox potential of the complex for ORR [44] (reproduced by permission of the American Chemical Society).*

**Figure 6**. Cu phenanthrolines are flexible, and then redox processes can occur on the metal centre.

The Cu(II)/(I) redox potential of copper phenanthroline is a reactivity descriptor for ORR. **Figure 15** shows a linear correlation between log *i* and *E°* with a slope close to +0.120 V/decade which seems to be an incomplete volcano correlation [44].

## **3. Conclusions**

The redox potential of the catalyst plays different roles in electrochemistry. Depending on whether the catalyst is present in the homogeneous phase or anchored or adsorbed on the electrode surface, the correlations between activity and redox potential can be different. For outer-sphere reactions occurring in the solution phase, the catalytic activity as log *k* versus the redox potential of the catalyst *E°* increases linearly with the redox potential, whereas for inner-sphere chemical catalysts, the activities observed are higher than those predicted by the redox potential of the catalyst.

For reactions promoted by molecular catalysts immobilized on the electrode surface, correlations of (log *i*)*<sup>E</sup>* versus *E*° are observed. Linear and volcano correlations are obtained. When comparing MN4 macrocyclic complexes having a wide range of redox potentials, typical volcano correlations are generally observed in electrocatalysis. In classical volcano correlations, the different electrodes are metals or alloys. In this case the binding energy of key intermediates is used as reactivity descriptor. The activity gradually increases with the binding energy up to a point. Beyond that point the activity decreases with larger binding energies as the surface becomes covered with adsorbed intermediates. Similar correlations are observed with MN4 molecular catalysts. Further, with both metals and molecular catalysts, the strong adsorption region of the volcano involves the four-electron reduction of O2, whereas the weak adsorption region includes the two-electron reduction

catalysts. The decrease in activity for ORR using MN4 metal complexes does not seem to be related to a gradual occupation of the active site but rather to a gradual decrease in the amount of M(II) active sites. This is observed for those catalysts that have M(III)/(II) redox potentials more negative than the electrode potential chosen for comparing the activities. Cu phenanthroline complexes follow similar correlations. It is observed that the activity increases as the Cu(II)/(I) redox potential increases, showing only a linear correlation.

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A general conclusion from electrocatalytic phenomena is that if volcano correlations are well established, then for a particular reaction the properties of the catalyst can be "tuned" so to improve their activity. The optimal properties can involve many other parameters such as metal-to-metal separation, crystal orientation, stability, alloying, nanostructure and redox potential of catalyst, so this is an open field for both experimentalists and theoreticians to find the ways of improving the catalytic activity of electrode surfaces.

The implications of future development in this area will have a tremendous impact in energy conversion devices, electrosynthesis and electrochemical sensors, just to mention a few.
