Cu(II) Aliphatic Diamine Complexes for Both Heterogeneous and Homogeneous Water Oxidation Catalysis in Basic and Neutral Solutions

Abstract

Simply mixing a Cu­(II) salt and 1,2-ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catalysis, depending on pH. In phosphate buffer at pH 12, the Cu­(II) en complex formed in solution is decomposed to give a phosphate-incorporated CuO/Cu­(OH)2 film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm2 was obtained at an overpotential of 540 mV, a significant enhancement compared to other Cu-based surface catalysts. The results of electrolysis studies suggest that the solution en complex decomposes by en oxidation to glyoxal, following Cu­(II) oxidation to Cu­(III). At pH 8, the catalysis shifts from heterogeneous to homogeneous with a single-site mechanism for Cu­(II)/en complexes in solution. A further decrease in pH to 7 leads to electrode passivation via the formation of a Cu­(II) phosphate film during electrolyses. As the pH is decreased, en, with pK b ≈ 6.7, becomes less coordinating and the precipitation of the Cu­(II) film inhibits water oxidation. The Cu­(II)-based reactivity toward water oxidation is shared by Cu­(II) complexation to the analogous 1,3-propylenediamine (pn) ligand over a wide pH range.

Armed by Asp? C-terminal carboxylate in a Dap-branched peptide and consequences in the binding of CuII and electrocatalytic water oxidation

“…While the homogeneous nature of catalysis has been clearly demonstrated in a number of cases, other complexes underwent changes (ligand loss, exchange and/or decomposition) and served as precursors to in situ deposited, stable electrocatalytic thin layers with characteristic activity. 38,[41][42][43][44][45] These phenomena highlight yet another eld of prospective application of metal complexes, namely their use as tuneable pre-catalysts to fabricate highly active catalytic layers on electrodes. Whichever is the case, understanding the role of the pH-dependent behaviour, stability and redox properties of the metal-ligand solutions is inevitable.…”

Analysis of Trends and Emerging Technologies in Water Electrolysis Research Based on a Computational Method: A Comparison with Fuel Cell Research

“…In cluster III-8, different Cu complexes exhibit high OER activity [88,89]. These studies indicate that Cu can be a promising catalyst for HER as it is abundant and low cost, although, for practical use, Cu complexes should be developed as an electrode, or a heterogeneous Cu compound would need to be developed.…”

On the Cu(III)/Cu(II) Redox Chemistry of Cu-Peptide Complexes to Assist Catalyst Design

“…[38][39][40][41][42] Recently, Cu(II) complexes with ligands, including peptides supporting the Cu(II)!Cu(III) redox transition, have found a completely new application in electrocatalytic water oxidation, which represents a rapidly growing research field. [43][44][45][46][47][48][49][50][51][52][53][54][55] The reaction 2H 2 O ! O 2 +4H + + 4e − involves the formation of an O=O bond and the release of four protons.…”

“…O 2 +4H + + 4e − involves the formation of an O=O bond and the release of four protons. The electrocatalytic activity of the copper complexes has been attributed to a proposed [LCu(III)-O • ↔ LCu(IV)=O] active species [43,44,46,55] that is capable of the O-O bond formation via the water nucleophilic attack (WNA) mechanism at high pH, in the presence of phosphate buffer that assists in the neutralization of the released protons. The LCu(III)-O • species is generated by one-electron steps for a number of Cu-ligand systems, starting from LCu(II)-OH 2 via LCu(III)-OH ( Figure 1, proposed mechanism I).…”

“…The LCu(III)-O • species is generated by one-electron steps for a number of Cu-ligand systems, starting from LCu(II)-OH 2 via LCu(III)-OH ( Figure 1, proposed mechanism I). [43,44,46,55] The LCu(II)-OH 2 !LCu(III)-OH transition is associated with a pH-dependent current peak in cyclic voltammetry, indicating the proton coupled electron transfer nature of this process ( Figure 1, PCET 1 ). The water oxidation catalysis is triggered at a higher potential, where the electrocatalytic anodic current has been associated with the proposed LCu(III)-OH!LCu (III)-O • oxidation ( Figure 1, square scheme I, PCET 2 ), followed by WNA and further fast steps yielding LCu(II)-OH 2 and O 2 .…”