A Cobalt Porphyrin Doubly Tethered with Piperazine Groups for Highly Selective Electrocatalytic Four-Electron Oxygen Reduction Reaction in Acidic Aqueous Solution

Abstract

Selective electrocatalytic four-electron (4e) oxygen reduction reaction (ORR) is required in many new energy conversion technologies. Although Co porphyrins are active for electrocatalytic ORR, their selectivity for the 4e ORR is usually poor. Herein, we report the design and synthesis of Co porphyrin 1 bearing two tethered piperazine groups and its electrocatalytic performance for the selective 4e ORR in 0.10 M HClO₄ aqueous solutions. By tethering a piperazine group on each side of the porphyrin macrocycle, we introduced four amine units into Co porphyrin 1. These amine units can be protonated in acidic solutions, and the protonated amines can function as both intramolecular proton relays and positively charged groups to improve the four-electron ORR selectivity of Co porphyrins. With this structural modification, 1 electrocatalyzes ORR with a half-wave potential E_1/2 = 0.59 V versus RHE and a transferred electron number n = 3.90 in 0.10 M HClO₄ aqueous solutions. This work is therefore significant to present 1 as a highly selective electrocatalyst for the 4e ORR and demonstrate a new ORR electrocatalyst design strategy by employing multiple amine groups to enhance electrocatalytic 4e ORR in acidic aqueous solutions.