<i>Meso</i>-Formyl Functionalization Enhances Electrocatalytic Hydrogen Evolution Activity of Nickel(II) Octaethylporphyrin

Molecular catalysts optimized with functional groups offer promising alternatives to precious metals for hydrogen evolution reactions (HER). This study investigates the electrocatalytic enhancement achieved by aldehyde functionalization at the <i>meso</i>-carbon position in 5-formyloctaethylporphyrin-nickel(II) ([Ni(OEP-CHO)]). Spectroscopic, cyclic voltammetry (CV), controlled potential electrolysis (CPE), and ultraviolet visible (UV-vis) Spectro electrochemistry reveal that the formyl group modulates the electronic properties of the porphyrin complex. In the absence of a proton source, CV of [Ni(OEP-CHO)] shows anodically shifted redox couples at -1.35 and -1.71 V vs Fc/Fc⁺, indicating redox activity and the electron-withdrawing nature of the aldehyde group. In 5 mM TFA, [Ni(OEP-CHO)] exhibits a lower catalytic onset potential, positively shifted by 0.38 V compared to [Ni(OEP)]. Titration with TFA reveals acid-independent saturation behavior at concentrations above 15 mM, consistent with a proton-shuttling mechanism involving the formyl group. CPE confirms superior HER performance of [Ni(OEP-CHO)], achieving a Faradaic efficiency of (96.5 ± 3.8)% and a turnover frequency (TOF) of 0.54 ± 0.04 h^-1 at an overpotential of 128 mV over 3 h. Notably, its position on Tafel plots reflects high catalytic activity at low overpotentials. These findings present a promising strategy for enhancing porphyrin-based molecular catalysts toward efficient hydrogen fuel production.