The On/Off pH-Dependent Electrocatalytic Activity of the Perfluorinated Iron Phthalocyanine for the Oxygen Reduction Reaction and Electrochemical Hardness as a Reactivity Descriptor: Experimental and Theoretical Study

Abstract

Perfluorinated iron phthalocyanine 16(F)FePc is probably the most active MN₄ molecular catalyst reported to promote the oxygen reduction reaction (ORR) in alkaline media. Its high activity is attributed to the electron-withdrawing properties of the fluoro substituents, which promote a hard-iron active site to interact with a hard-O₂ molecule. However, its activity has been explored shallowly. Here, we modified an edge plane-pyrolytic graphite surface (EPG) with 16(F)FePc to promote ORR in different pH media to build a Pourbaix diagram as an electrocatalytic roadmap for 16(F)FePc. Furthermore, the recently proposed reactivity descriptor for ORR, known as the “electrochemical hardness” (ΔE^h), was determined in the EPG/16(F)FePc system at different pH. It was found that the catalyst’s reactivity is inversely proportional to the ΔE^h values, so small values conduct to high activity. The same behavior was obtained for the oxidation–reduction hardness (η_ox-red) parameter, which was theoretically determined in this work by DFT calculations. The theoretical η_ox-red suggests a decrease of the Fe(II) reactivity with the increase of nitrogen atom protonation in the 16(F)FePc, supporting the pH-dependent ΔE^h values. Moreover, a pH-dependent locked/unlocked mechanical switch behavior for the 16(F)FePc was determined, attributed to the iron center motion above the N₄-plane without a demetalation process. We observed this phenomenon in an acid media using electrochemical techniques coupled with Surface-Enhanced Raman Spectroscopy (EC-SERS), monitoring the Fe(II)/(I), Fe(III)/(II) redox potentials, and the in situ ORR process. The scanning tunneling microscopy-based break junction technique (STM-BJ) revealed this mechanical switch at the single-molecule level. Conversely, the mechanical switch is locked in alkaline media, and the 16(F)FePc is in an on-catalytic state for ORR. Therefore, the unlocked mechanical switch could explain the low ORR catalytic activity of the 16(F)FePc in acidic media (off-catalytic state). These findings are crucial for understanding the catalytic behavior of 16(F)FePc, especially in acid media.