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Electrochemical Characterization of Bis‐Cobalt Hexaphyrin: A Selective Electrocatalyst for the Two‐Electron Reduction of Oxygen in Acid Media
Author(s) -
Ye Lina,
Fang Yuanyuan,
Ou Zhongping,
Feng Ru,
Chen Minyuan,
Guo Rui,
Kadish Karl M.
Publication year - 2021
Publication title -
chemelectrochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.182
H-Index - 59
ISSN - 2196-0216
DOI - 10.1002/celc.202100063
Subject(s) - cobalt , chemistry , pyridine , electrochemistry , catalysis , electrocatalyst , cyclic voltammetry , redox , inorganic chemistry , medicinal chemistry , organic chemistry , electrode
A bis‐cobalt(II) hexaphyrin was synthesized and characterized as to its electrochemical and spectroelectrochemical properties in nonaqueous media. The examined compound is represented as HexaPyCo 2 where HexaPy represents an expanded macrocycle containing six pyrroles and six meso ‐2,6‐difluorophenyl substituents. Three or four reductions can be observed for the bis‐cobalt hexaphyrin depending upon the solvent. The first two reductions are reversible and well separated in PhCN ( E 1/2 =−0.50 and −0.90 V) or CH 2 Cl 2 ( E 1/2 =−0.52 and −0.87 V) containing 0.1 M TBAP. This contrasts with what is observed in pyridine where the first two one‐electron additions are overlapped in potential and occur at an apparent E 1/2 =−0.73 V vs SCE. The third and fourth reductions of the bis‐cobalt(II) hexaphyrin are irreversible and coupled to chemical reactions in PhCN or CH 2 Cl 2 but well‐defined one‐electron transfers are seen in pyridine where the 250 mV difference between reversible E 1/2 values (−1.28 and −1.53 V) matches almost perfectly with a 240 mV separation in potentials between the third and fourth reductions of a related bis‐Cu(II) hexaphyrin under the same solution conditions. The oxygen reduction reaction (ORR) was also investigated using HexaPyCo 2 as a catalyst in 1.0 M HClO 4 . The results of rotating‐disk and rotating‐ring‐disk voltammetry show that the bis‐cobalt hexaphyrin is a selective catalyst for the 2‐electron ORR and gives 95 % H 2 O 2 as the product in acid media.