Electrocatalytic Water Oxidation by Mononuclear Copper Complexes of Bis-amide Ligands with N4 Donor: Experimental and Theoretical Investigation

Abstract

The present work describes electrocatalytic water oxidation of three monomeric copper complexes [Cu II (L1)] (1), [Cu II (L2)(H 2 O)] (2), and [Cu II (L3)] (3) with bis-amide tetradentate ligands:for the production of molecular oxygen by the oxidation of water at pH 13.0. Ligands and all complexes have been synthesized and characterized by single crystal XRD, analytical, and spectroscopic techniques. X-ray crystallographic data show that the ligand coordinates to copper in a dianionic fashion through deprotonation of two −NH protons. Cyclic voltammetry study shows a reversible copper-centered redox couple with one ligand-based oxidation event. The electrocatalytic water oxidation occurs at an onset potential of 1.16 (overpotential, η ≈ 697 mV), 1.2 (η ≈ 737 mV), and 1.23 V (η ≈ 767 mV) for 1, 2, and 3 respectively. A systematic variation of the ligand scaffold has been found to display a profound effect on the rate of electrocatalytic oxygen evolution. The results of the theoretical (density functional theory) studies show the stepwise ligand-centered oxidation process and the formation of the O−O bond during water oxidation passes through the water nucleophilic attack for all the copper complexes. At pH = 13, the turnover frequencies have been experimentally obtained as 88, 1462, and 10 s −1 (peak current measurements) for complexes 1, 2, and 3, respectively. Production of oxygen gas during controlled potential electrolysis was detected by gas chromatography.

Enhanced Electrocatalytic Oxygen Evolution by In Situ Growth of Tetrametallic Metal–Organic Framework Electrocatalyst FeCoNiMn-MOF on Nickel Foam

“…Among the various electrocatalysts, metal–organic frameworks (MOFs) are organic–inorganic hybrid crystalline porous materials that are formed through coordination between central metal ions and organic ligands . With diverse porous channel structures of a rapid mass transport, a large specific surface area, and various active sites on the MOF surfaces, they have been considered as promising electrocatalysts for utilization in energy conversion and storage during OER. − Furthermore, mixed-metal ions could exist within the porous channel structure of MOFs as the redox-active metal sites, endowing the material with multifunctionality and adaptability. However, these mixed-metal ions might be intertwined by the organic framework, impeding electron transport, and the poor electroconductivity of pristine MOFs hinders the practical application as OER electrocatalysts. − Therefore, to eliminate these drawbacks of pristine MOFs, much attention has been focused on the preparation of convenient and efficient three-dimensional (3D) electrodes to enhance interface effects and electron conduction.…”