Effect of Electrolyte Media on the Catalysis of Fe Phthalocyanine toward the Oxygen Reduction Reaction: Ab Initio Molecular Dynamics Simulations and Experimental Analyses

FeN4 macrocycles are among the most promising nonprecious metal catalysts for the oxygen reduction reaction (ORR). Nevertheless, these catalysts perform poorly in acidic media. To understand what impedes the use of these catalysts in acid, graphite electrodes were drop-coated with inks of iron phthalocyanine adsorbed on carbon nanotubes (FePc-CNTs), and the electrocatalytic behavior of the catalyst was studied in four different supporting electrolytes (i.e., HCl, H2SO4, CH3COOH, and NaOH) by means of cyclic voltammetry, polarization curves, and electrochemical impedance spectroscopy. Electrolyte media are theoretically analyzed by ab initio molecular dynamic simulations, with explicit water molecules, to explain and visualize the occurring physical adsorption phenomena. The demetallation of the catalyst could be excluded because more than 3.6 eV would be necessary for this process to occur. In addition, the absorption of the anion of the specific acids at the Fe center was found to compete with the coordination of oxygen and prevent the catalytic process, modifying the ORR rate-determining step and the final product of the reaction. An electrochemical analysis and impedance spectroscopy corroborate this process.

Automated summary

In this study, the electrocatalytic behavior of FeN4 macrocycles in acidic media was investigated through experiments and theoretical simulations. It was found that the adsorption of anions from specific acids at the iron center competes with oxygen coordination, hindering the catalytic process.