Metal Porphyrins as Single Site Catalyst Models Explored by Electrochemical Scanning Tunneling Microscopy: A New Perspective in Electrocatalysis

Abstract M–N–C materials featuring M–N_x single site catalysts, where for example M = Co, Mg, or Fe coordinating x atoms of nitrogen, have been intensely investigated as promising candidates to replace Pt group metals in a variety of electrocatalytic reactions, for example in the O₂ reduction reactions (ORR). However, despite the recent emerging of atomic resolution techniques, there is still a lack of understanding of the precise configuration of the M–N_x single site. For example, metal porphyrins are known to act as catalysts for O₂ and they are indeed good model systems for mimicking M–N₄ sites. In particular, it would be of interest to gain more information on the precise dynamic of the catalytic process at atomic scale, especially in an electrochemical environment, i.e., at electrode/electrolyte interphase. An approach that is becoming more and more consolidated is to combine the information coming from electrochemistry and scanning tunneling microscopy techniques in electrochemical scanning tunneling microscopy (EC‐STM) and the result is that a precise dynamic of the catalytic site at atomic scale can be investigated. This review aims to emphasize the main results and advances of EC‐STM in investigating small molecule electrocatalysis and specifically O₂ reduction at metal porphyrins M–N₄ model system.