Electrocatalytic Water Oxidation by a Trinuclear Copper(II) Complex

Abstract

We report a trinuclear copper­(II) complex, [(DAM)­Cu3(μ3-O)]­[Cl]4 (1, DAM = dodecaaza macrotetracycle), as a homogeneous electrocatalyst for water oxidation to dioxygen in phosphate-buffered solutions at pH 7.0, 8.1, and 11.5. Electrocatalytic water oxidation at pH 7 occurs at an overpotential of 550 mV with a turnover frequency of ∼19 s–1 at 1.5 V vs NHE. Controlled potential electrolysis (CPE) experiments at pH 11.5 over 3 h at 1.2 V and at pH 8.1 for 40 min at 1.37 V vs NHE confirm the evolution of dioxygen with Faradaic efficiencies of 81% and 45%, respectively. Rinse tests conducted after CPE studies provide evidence for the homogeneous nature of the catalysis. The linear dependence of the current density on the catalyst concentration indicates a likely first-order dependence on the Cu precatalyst 1, while kinetic isotope studies (H2O versus D2O) point to involvement of a proton in or preceding the rate-determining step. Rotating ring-disk electrode measurements at pH 8.1 and 11.2 show no evidence of H2O2 formation and support selectivity to form dioxygen. Freeze-quench electron paramagnetic resonance studies during electrolysis provide evidence for the formation of a molecular copper intermediate. Experimental and computational studies support a key role of the phosphate as an acceptor base. Moreover, density functional theory calculations highlight the importance of second-sphere interactions and the role of the nitrogen-based ligands to facilitate proton transfer processes.

Electrocatalytic water oxidation by copper(ii) complexes with a pentadentate amine-pyridine ligand

“…In terms of overpotential, complex 2 performs better than complex 1 as well as some reported Cu-based WOCs under identical conditions. 36–39…”

Immobilization of “Capping Arene” Cobalt(II) Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

“…We utilized DFT quantum mechanics calculations to probe plausible pathways for electrocatalytic water oxidation by complexes 1 and 2 . This methodology has been previously validated to determine mechanisms and kinetics for numerous electrocatalytic processes including trinuclear Cu, Co-doped TiO 2 , IrO 2 , and Fe-doped NiOOH . Our computational studies aimed to determine the reaction pathway for electrocatalytic water oxidation that provides the most facile mechanism for O–O bond formation.…”

“…Artificial photosynthesis for water splitting using renewable energy-based alternatives to fossil fuels offers potential to implement clean energy processes. − Improvement of the efficiency and stability of the electrocatalysts for the anodic reaction, the oxygen evolution reaction (OER), is imperative for the development of large-scale water splitting. , Homogeneous molecular electrocatalysts for water oxidation have been extensively investigated, and the tunable structures and well-defined active sites have led to detailed mechanistic understanding as well as the advancement of more active and longer-lived catalysts. − There has been interest in first-row transition metal complexes due to the advantages of earth abundance and low cost compared to their noble metal counterparts. − …”

Bioinspired Trinuclear Copper Catalyst for Water Oxidation with a Turnover Frequency up to 20000 s–1

“…In contrast, the mechanism of dioxygen activation and the associated O–O bond cleavage is well documented, especially for copper enzymes. − Inspired by this Cu/O 2 chemistry, Mayer and his co-workers reported the first homogeneous copper catalyst, (bpy)­Cu­(OH) 2 , for electrocatalyzed water oxidation in an alkaline solution . Subsequently, copper-catalyzed water oxidation was considered to have great potential, and many molecular catalysts emerged. − Most of the reported molecular copper-based catalysts, except for several multinuclear catalysts, − can be roughly divided into two categories according to the proposed O–O bond formation pathways (molecular copper-based catalysts are summarized in Table S5): the first category is mononuclear complexes − ,,,,− with a mononuclear end-on peroxo-Cu II (Cu II –OOH) species as the proposed intermediate in the O–O bond formation step, and this proposed key intermediate is analogous to mononuclear copper enzymes and noncoupled binuclear copper enzymes (Figure a, Cu–O 2 ) . The second category is binuclear catalysts ,, inspired by coupled binuclear copper enzymes, such as hemocyanin and tyrosinase, with a binuclear end-on bridged peroxo-Cu 2 II (Cu II –O–O–Cu II ) species as the proposed intermediate (Figure a, Cu 2 –O 2 ) .…”