Penta-coordinated transition metal macrocycles as electrocatalysts for the oxygen reduction reaction

Abstract

The oxygen reduction reaction (ORR) is a highly important reaction in electrochemistry. The following short review details recent advances in novel non-precious metal catalysts containing transition metal macrocycles for use in the ORR. Unbound, many of these electrodes were found to generate high levels of side products such as O 2 − and H 2 O 2 via 2-electron processes, and for this reason, it is aimed to create systems which would favor a 4-electron process which would completely convert oxygen to H 2 O. The 4electron reduction of O 2 releases the most energy in a fuel cell. Novel catalytic materials containing metal macrocycles were created mimicking the structure of enzyme metal centers, the metal in the macrocycle bound to a fifth axial ligand. These structures were observed to exhibit improved catalytic activity, and in the case of cobalt, phthalocyanine systems were observed to move away from the inefficient 2-electron process towards the more complete 4-electron process in alkaline media. Both experimental results (XPS, EPR, cyclic voltammetry and polarization curves) and theoretical models were gathered for various pentacoordinate systems and various electronic effects of the axial ligand on metal center were proposed. Penta-coordinate macrocycles are an important tool for further manipulating and tuning the electronic behavior of transition metal centers for catalysis of the ORR.

Evidence of carbon-supported porphyrins pyrolyzed for the oxygen reduction reaction keeping integrity

“…This also relates with the differences in the M–N bond length in the catalyst structure after pyrolysis. Finally, if we consider the catalytic activity as log j k at constant potential ( E : 0.8 V vs. RHE) versus binding energy M-O 2 and also versus the formal potential, as proposed by several authors 10 , 16 , 61 , 97 – 99 , we obtain the figure reported as graphical abstract, where the MN4 structure is maintained, and the catalysts follow the trend of the correlations.…”

Theoretical study of the effect of coordination environment on the activity of metal macrocyclic complexes as electrocatalysts for oxygen reduction

“…Previously, Tasca et al. ( Cao et al., 2013 ; Riquelme et al., 2018 ; Viera et al., 2020 ; Loyola et al., 2021a , 2021b ; Govan et al., 2021 ; Oyarzun et al., 2021 ) systematically studied similar Fe-centered and Co-centered complexes with axial ligands by combining experiments and theoretical calculation. They also demonstrated that the additional coordination promotes the overall ORR performance.…”

Molecular Control of Carbon‐Based Oxygen Reduction Electrocatalysts through Metal Macrocyclic Complexes Functionalization

“…Since cobalt phthalocyanine (CoPc) was first used in fuel cells in 1964, many N 4 ‐chelated MMC have been studied as excellent alternatives to Pt‐based ORR catalysts in fuel cells and metal–air batteries. [ 38–46 ] Such complexes are comprised of different central metal ions and macrocyclic ligands exhibiting flexible and adjustable substituents and forming a unique M N 4 structure. Furthermore, the square planar geometry of the central metal ion is conducive to the binding and activation of ORR‐related intermediates.…”

“…For example, O 2 molecules can axially combine with metal ions and are transformed stepwise into different reaction intermediates through the transfer of electrons and protons. [ 39,47,48 ]…”