Substituent effects in iron porphyrin catalysts for the hydrogen evolution reaction

Abstract

For a future hydrogen economy, non precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. MNC catalysts with MN4 active sites show promising performance, but an optimization rooted in structure property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphin substituents on the electronic and electrocatalytic properties of MN4 centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [FeII(P▪)]- radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Protonation of Pendant Pyridine Substituents in an Iron Porphyrin Hangman Complex: Influence on Spectral Visibility and Electrocatalysis

“…7 Apart from interaction with solvents, this can also be used to tune electron density in the porphyrin ring and at the iron center in turn affecting redox potentials and catalytic behavior. 8 The usual insolubility in aqueous media combined with the necessity of an applied external potential quickly led to the immobilization of iron porphyrins on electrode surfaces. This can be done using π−π interactions with the porphyrin ring, side-chain interactions 9 or via the fifth ligand bond on the iron center.…”