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.