Advances in the development of Pt-group metal-free (PGM-free) catalysts for oxygen reduction reaction in fuel cells produced active catalysts that allow to reduce the performance gap to the incumbent Ptbased materials. However, the utilization of the state-of-the-art PGM-free catalysts in commercial applications is currently impeded by their relatively low durability. Methods designed to study catalyst degradation in operating fuel cells are critical for the understanding and ultimately solving the durability issues. This work is the first report on the use of Fourier-transformed alternating current voltammetry (FTacV) as an electrochemical method for accurately quantifying the electrochemical site density of PGMfree ORR catalysts, and following their degradation during the operation of polymer electrolyte fuel cells. Using this method we were capable of detecting changes in performance of electrochemically active species (electrocatalytic centers in this case), allowing us, for the first time, to calculate the electrochemical active site density (EASD) which is necessary for the elucidation of the degradation mechanisms of PGM-free ORR catalysts in situ fuel cells.

In the search for replacement of the platinum‐based catalysts for fuel cells, MN4 molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum‐group metal‐free (PGM‐free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e− to 4e− pathway in the pH range of 3.5‐6. This phenomenon was shown by density functional theory (DFT) calculations to be related to the protonation of the nitrogen atoms and carboxylic acid groups on the corroles indicated by the pKa values of the protonation sites in the vicinity of the ORR active sites. Control of the electron‐withdrawing nature of these groups characterized by the pKa values could switch the ORR from the H+ to e− rate‐determining step mechanisms and from 2e− to 4e− ORR pathways and also controlled the durability of the corrole catalysts. The results suggest that protonation of the nitrogen atoms plays a vital role in both the ORR activity and durability for these materials and that pKa of the N atoms at the active sites can be used as a descriptor for the design of high‐performance, durable PGM‐free catalysts.

Four β-pyrrole-substituted cobalt(iii) corroles were studied as electrocatalysts for the oxygen reduction reaction. The results disclose high dependence of the corrole's performance on its substituents, but once adsorbed on a high surface area carbon, this effect vanishes, resulting in a better catalytic performance than most well-defined molecular electrocatalysts for this reaction.

Three cobalt(III) complexes of regioisomeric trans-A 2 B-corroles were designed and efficiently synthesized. The corroles were adsorbed on smooth glassy carbon (GC) and black pearls 2000 (BP2000), high-surface-area carbon. Albeit spatially separated from the cobalt reaction center, the position of COOH group has a profound influence on the oxygen reduction reaction electrocatalytic reactivity when on GC, whereas on BP2000, a significant increase in selectivity toward the 4-electron reduction was observed in an alkaline environment. This is attributed to the wetting properties of the hydrophobic pores of BP2000, which considerably lower the dielectric constant in the pore water environment, stabilize the charged OOH − intermediate, and favor the 4-electron reduction pathway with the cobalt-bis-pentafluorophenyl (phenyl-para-carboxylic acid), when compared to analogous corroles with the COOH group at the ortho-and meta-positions.

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