Robust Molecular Anodes for Electrocatalytic Water Oxidation Based on Electropolymerized Molecular Cu Complexes

Abstract

A multistep synthesis of a new tetra‐amidate macrocyclic ligand functionalized with alkyl‐thiophene moieties, 15,15‐bis(6‐(thiophen‐3‐yl)hexyl)‐8,13‐dihydro‐5H‐dibenzo[b,h][1,4,7,10]tetraazacyclotridecine‐6,7,14,16(15H,17H)‐tetraone, H4L, is reported. The reaction of the deprotonated ligand, L4−, and Cu(II) generates the complex [LCu]2−, that can be further oxidized to Cu(III) with iodine to generate [LCu]−. The H4L ligand and their Cu complexes have been thoroughly characterized by analytic and spectroscopic techniques (including X‐ray Absorption Spectroscopy, XAS). Under oxidative conditions, the thiophene group of [LCu]2‐ complex polymerizes on the surface of graphitic electrodes (glassy carbon disks (GC), glassy carbon plates (GCp), carbon nanotubes (CNT) or graphite felts (GF)) generating highly stable thin films. With CNTs deposited on a GC by drop casting, we obtain hybrid molecular materials labeled as GC/CNT@p‐[LCu]2−. The latter are characterized by electrochemical techniques that show their capacity to electrocatalytically oxidize water to dioxygen at neutral pH. These new molecular anodes achieve current densities in the range of 0.4 mA/cm2 at 1.30 V versus NHE with an onset overpotential at approx. 250 mV. Bulk electrolysis experiments show an excellent stability achieving TONs in the range of 7600 during 24 h with no apparent loss of catalytic activity and maintaining the molecular catalyst integrity, as evidenced by electrochemical techniques and XAS spectroscopy. Further with highly porous graphitic materials such as GF, we obtain TONs in the range of 11,000.This article is protected by copyright. All rights reserved

Polypyrene Cu Amidate Complexes on ITO Electrodes as Molecular Water Oxidation Anodes

“…Electrocatalytic water oxidation is one of the main reactions that needs to be addressed in order to produce efficient PECs. Numerous molecular water oxidation catalysts (WOCs) have been reported during the last decades, reaching turnover frequencies in a range close to the microsecond time scale with high stability reported over millions of tons. − While the best performances are generally achieved using precious metals, there is an increasing interest in moving to more inexpensive and abundant first-row transition metals such as Cu, Fe, Ni, or Co, ,,− although only a few of them have really been demonstrated to be the active species − and a very few of them have been successfully anchored in conductive and/or semiconductive materials. − Within this context, the [(L3)­Cu] 2– complex with an (H 4 L3) tetraprotonated tetra-amido macrocyclic ligand (15,15-dimethyl-8,13-dihydro-5 H -dibenzo­[ b , h ]­[1,4,7,10]­tetraazacyclotridecine-6,7,14,16­(15 H ,17 H )-tetraone; see Chart ) stands out as one the best WOC based on a first-row transition metal complex, given its oxidative robustness and the low operating overpotential. The latter is attributed to the strong σ-donating character and their macrocyclic size and structure that contributes to stabilizing the needed high oxidation states of the metal center. − In addition, its redox noninnocent character plays a key role by contributing to managing the multiple hole access required for the catalytic water oxidation reaction. , …”