The <i>On</i>/<i>Off</i> pH-Dependent\nElectrocatalytic Activity of the Perfluorinated Iron Phthalocyanine\nfor the Oxygen Reduction Reaction and Electrochemical Hardness as\na Reactivity Descriptor: Experimental and Theoretic

Perfluorinated\niron phthalocyanine 16(F)FePc is probably the most\nactive MN<sub>4</sub> molecular catalyst reported to promote the oxygen\nreduction reaction (ORR) in alkaline media. Its high activity is attributed\nto the electron-withdrawing properties of the fluoro substituents,\nwhich promote a <i>hard-iron</i> active site to interact\nwith a <i>hard-</i>O<sub>2</sub> molecule. However, its\nactivity has been explored shallowly. Here, we modified an edge plane-pyrolytic\ngraphite surface (EPG) with 16(F)FePc to promote ORR in different\npH media to build a Pourbaix diagram as an electrocatalytic roadmap\nfor 16(F)FePc. Furthermore, the recently proposed reactivity descriptor\nfor ORR, known as the “electrochemical hardness” (Δ<i>E</i><sup><i>h</i></sup>), was determined in the EPG/16(F)FePc\nsystem at different pH. It was found that the catalyst’s reactivity\nis inversely proportional to the Δ<i>E</i><sup><i>h</i></sup> values, so small values conduct to high activity.\nThe same behavior was obtained for the oxidation–reduction\nhardness (η<sub>ox‑red</sub>) parameter, which was theoretically\ndetermined in this work by DFT calculations. The theoretical η<sub>ox‑red</sub> suggests a decrease of the Fe(II) reactivity\nwith the increase of nitrogen atom protonation in the 16(F)FePc, supporting\nthe pH-dependent Δ<i>E</i><sup><i>h</i></sup> values. Moreover, a pH-dependent <i>locked</i>/<i>unlocked</i> mechanical switch behavior for the 16(F)FePc was\ndetermined, attributed to the iron center motion above the N<sub>4</sub>-plane without a demetalation process. We observed this phenomenon\nin an acid media using electrochemical techniques coupled with Surface-Enhanced\nRaman Spectroscopy (EC-SERS), monitoring the Fe(II)/(I), Fe(III)/(II)\nredox potentials, and the in situ ORR process. The scanning tunneling\nmicroscopy-based break junction technique (STM-BJ) revealed this mechanical\nswitch at the single-molecule level. Conversely, the mechanical switch\nis <i>locked</i> in alkaline media, and the 16(F)FePc is\nin an <i>on-catalytic state</i> for ORR. Therefore, the <i>unlocked</i> mechanical switch could explain the low ORR catalytic\nactivity of the 16(F)FePc in acidic media (<i>off-catalytic state</i>). These findings are crucial for understanding the catalytic behavior\nof 16(F)FePc, especially in acid media.