Metallated azo-naphthalene diimide based redox-active porous organic polymer as an efficient water oxidation electrocatalyst

Abstract

The importance of developing a stable, efficient and low-cost catalyst for the oxygen evolution reaction (OER) is at its pinnacle due to the reaction’s poor reaction kinetics. Porous organic polymers (POPs) have recently emerged as a versatile platform for the development of conventional heterogeneous catalysts, but their role in developing electrocatalysts for water splitting is underexplored. Herein, we report the synthesis and characterization of an azo-functionalized POP (NDI-PG) in aqueous solution, under mild conditions, via the diazo coupling of redox active naphthalene diimide (NDI) based diamine with phloroglucinol. The azo nitrogen from the NDI diamine and one of the adjacent phenolic oxygen atoms in the POP are utilized to coordinate Co^II (Co^II-NDI-PG) and the resulting complex can be exploited as an OER electrocatalyst. Co^II-NDI-PG with a metal loading as low as 8% exhibited the best electrocatalytic activity toward water oxidation in alkaline solution with an overpotential of 340 mV. Additionally, the catalyst showed an improvement in the reaction kinetics with a low Tafel slope of 68 mV per decade. This report also compares the electrocatalytic activity of a Co^II coordinated POP containing benzidine (Co^II-BD-PG) in place of NDI diamine. NDI, being a redox active planar moiety unlike benzidine, exhibits better π-electron delocalization facilitating an uninterrupted charge transfer which is beneficial for electrocatalysis.