Promotion of Catalytic Oxygen Reduction Reactions: The Utility of Proton Management Substituents on Cobalt Porphyrins

Abstract

Three ABAB-type cobalt meso-tetraarylporphyrins with fluorine (F-CoPor), acetic acid (AC-CoPor), and cyanoacetic acid (CN-CoPor) groups at the para-positions of phenyl rings at the 10,20-positions are synthesized and evaluated as catalysts for oxygen reduction reactions (ORRs). In density functional theory calculations, the frontier molecular orbitals of these complexes were found to be stabilized relative to model complexes with electron-withdrawing atoms or moieties on the meso-aryl rings. Electrochemical measurements suggest that electrodes with CN-CoPor (CN-CoPor/C) exhibit the most positive ORR potential values and the highest limiting current density in both acidic and alkali electrolytes, while the F-CoPor/C electrocatalyst exhibits extremely low ORR performance. The electron transfer numbers for the electrocatalysts are more than 3.0, indicating that a mixture of 2-and 4-electron transfer pathways occurs. The results demonstrate that coupling the hydrogen bonding properties and electron-withdrawing abilities through rational design of the substituent at the meso-position is an efficient way to modify the ORR performance.

Meso‐substituted Metalloporphyrin‐based Composites for Electrocatalytic Oxygen Reduction Reactions

“…[53] For instance, three ABAB-type cobalt porphyrins with different proton management groups at their peripheral exhibited the limiting current densities 1.5 ~2 times of the magnitude in 0.1 M KOH than in 0.5 M H 2 SO 4 , while their electron transfer numbers showed slightly reduced values in base than in acid. [54] The selectivity towards 4-electron reduction of O 2 , however, often shows poorer in alkali than in acid, which possibly relates to the interfacial charge states, the decreased water activity as well as the restricted proton transfer from water in alkali solution. [48,52] Besides, electron transfer number may change with the angular rate of the RRDE, catalyst thickness, loading and geometry, which should be considered.…”

“…The composite catalyst made from compound 55 exhibited the most positive ORR potentials, maximum limiting current density and highest 4-electron selectivity in both acidic and alkaline electrolytes. [54] In many cases of using proton management for altering the electrocatalytic processes, proton-coupled electron transfer (PCET) mechanisms can be adopted for in-depth interpretations. [42,[127][128][129] PCET reactions have been regarded as the engine of energy conversion and storage since their fundamental but importance role in natural and artificial systems.…”

“… [49,52] Caused by the OSET and thereby the substrate reactivity on oxygen reduction under alkaline condition, the given composite catalyst can reach a much higher response current than that in acid [53] . For instance, three ABAB‐type cobalt porphyrins with different proton management groups at their peripheral exhibited the limiting current densities 1.5∼2 times of the magnitude in 0.1 M KOH than in 0.5 M H 2 SO 4 , while their electron transfer numbers showed slightly reduced values in base than in acid [54] . The selectivity towards 4‐electron reduction of O 2 , however, often shows poorer in alkali than in acid, which possibly relates to the interfacial charge states, the decreased water activity as well as the restricted proton transfer from water in alkali solution [48,52] .…”

“…synthesized three ABAB‐type cobalt meso ‐tetraarylporphyrins 53 – 55 containing fluorine, acetic acid and cyanoacetic acid groups, respectively, to investigate the influence of proton management substituents on ORR activity (Figure 11). The composite catalyst made from compound 55 exhibited the most positive ORR potentials, maximum limiting current density and highest 4‐electron selectivity in both acidic and alkaline electrolytes [54] …”

Electrocatalytic oxygen reduction with cobalt corroles bearing cationic substituents

“…4,58 Inspired by CcOs, tremendous efforts have been devoted to tuning the catalytic performance of metal porphyrins through the incorporation of functional groups in the secondcoordination sphere. 28,29,[59][60][61][62][63][64][65][66] For instance, it was demonstrated that introducing hydrogen-bonding interactions, 35,67,68 proton relays 69,70 and electrostatic interactions 71,72 to the molecular backbone of metal porphyrins can dramatically improve the O 2 binding and subsequent conversion to H 2 O. By contrast, metal corrole molecules typically exhibit better catalytic ORR properties than relevant metal porphyrins, 29,31,73 because the trianionic corrole macrocycles are more effective than dianionic porphyrin macrocycles at stabilizing high-valent metal ions, which are intermediates in the ORR process.…”