Linear and Volcano Correlations for the Electrooxidation of Hydrazine on MN4 Catalysts

Abstract

Hydrazine is a powerful reducing agent and has been employed in the anode of fuel-cells. MN4 chelates (which are active for ORR) also show activity for the oxidation of hydrazine and are much more stable compared to their durability in the dioxygen cathode of fuel cells.Correlations of electrocatalytic activity as (log i)E versus the M(II)/(I) formal potential for MN4 catalysts (M=Fe,Co) surface confined on graphite electrodes for hydrazine oxidation give volcano shaped curves, which is typical for electrocatalytic processes (1). However, contrary to previous interpretations of these correlations, the volcano shape does not seem to be related to the Sabatier Principle, i.e. for optimum activity the interaction of hydrazine with the M(II) active site needs to be not too strong, not too weak. If the activity as current density is divided by the real surface concentration of M(II) active sites at the potential of the measurements, then a plot of log(i/ΓM(II)) E versus the M(II)/(I) formal potential gives a linear correlation of slope close to 0.120 V/decade which essentially is similar to a Tafel slope. Further, we have found that CoPc and 16(F)CoPc adsorbed on graphite electrode exhibit voltammograms in alkaline solution (0.2M NaOH) that show the typical redox peaks attributed to the Co(II)/(I) reversible. The peak potential for CoPc is independent of surface concentration of the catalyst. In contrast, for 16(F)CoPc the Co(II)/(I) redox process shifts to more negative potentials when the surface concentration of the catalyst increases. In a volcano correlation of log (i/Γ)E (activity per active site) versus Co(II)/(I) formal potential of catalyst (using several CoN4 chelates) CoPc appears in the ascending portion (activity increases with the Co(II)/(I) redox potential) whereas 16(F)CoPc appears in the region where activity decreases with the redox potential. In a plot of (i/Γ)E versus the Co(II)/(I) formal potential of 16(F)CoPc the declining portion of the volcano is reproduced for one single complex. So 16(F)CoPc at different surface concentrations behaves as Co complexes having different redox potential in the declining portion of the volcano plot, when the activity is normalized for the surface concentration. This is not observed for CoPc. However, if the currents are normalized by the real Co(II) concentrations (which is proportional to the turn over number) a plot of log(i/ΓCo(II)) E gives a straight line, as shown in Figure 2. The slope is close to 0.120 V which essentially is similar to a Tafel slope and the correlation in Figure 1 is a free-energy correlation similar to a ΔG# (log k) E versus ΔGocorrelation and the slope is a “Bronsted slope”. Acknowledgements: This work has been funded by Fondecyt 1100773, Dicyt-USACH and Núcleo Milenio Project. P07-006-F