Electrocatalytic water oxidation by a water-soluble copper complex with a pentadentate amine-pyridine ligand

The excellent performance of oxygen evolution reaction on stainless steel electrodes by halogen oxyacid salts etching

Development of low-cost, efficient, and stable electrocatalysts for oxygen evolution reaction (OER) is the key issue for a large-scale hydrogen production. Recently, in-situ corrosion of stainless steel seems to be a feasible technique to obtain an efficient OER electrode, while a wide variety of corrosive agents often lead to significant difference in catalytic performance. Herein, we synthesized Ni-Fe based nanomaterials with OER activity through a facile one-step hydrothermal etching method of stainless steel mesh, and investigated the influence of three halogen oxyacid salts (KClO3, KBrO3, KIO3) on water oxidation performance. It was found that the reduction product of oxyacid salts has the pitting effect on the stainless steel, which plays an important role in regulating the morphology and composition of the nanomaterials. The KBrO3-derived electrode shows optimal OER performance, giving the small overpotential of 228 and 270 mV at 10 and 100 mA cm-2 respectively, a low Tafel slope of 36.2 mV dec-1, as well as durable stability in the long-time electrolysis. This work builds an internal relationship between the corrosive agents and the OER performance of the as-prepared electrodes, providing promising strategies and research foundations for further improving the OER performance and optimizing the structure of stainless steel electrodes.

Improvement of electrocatalytic water oxidation activity of novel copper complex by modulating the axial coordination of phosphate on metal center

The structure of organic ligand scaffolds of copper complexes critically affects their electrocatalytic properties toward water oxidation, which is widely regarded as the bottleneck of overall water splitting. Herein, two novel mononuclear Cu complexes, [Cu(dmabpy)](ClO4)2 (1, dmabpy = 6,6'-bis(dimethylaminomethyl)-2,2'-bipyridine) and [Cu(mabpy)](ClO4)2 (2, mabpy = 6,6'-bis(methylaminomethyl)-2,2'-bipyridine), with four-coordinated distorted planar quadrilateral geometry were synthesized and explored as efficient catalysts for electrochemical oxygen evolution in phosphate buffer solution. Interestingly, complex 1 with a tertiary amine group catalyzes water oxidation with lower onset overpotential and better catalytic performance, while complex 2 containing a secondary amine fragment displays much lower catalytic activity under identical conditions. The water oxidation catalytic mechanism of the two complexes is proposed based on the electrochemical test results. Experimental methods indicate that phosphate coordinated on the Cu center of the two complexes inhibits their reaction with substrate water molecules, resulting in lower activity toward water oxidation. Electrochemical tests reveal that the structure of the coordinated nitrogen atom improves the catalytic performance of the Cu complexes by modulating the coordination of phosphate on the Cu center, indicating that a minor alteration of the coordinating nitrogen atom of the ligand has a detrimental effect on the catalytic performance of electrochemical WOCs based on transition metal complexes.

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

The present work describes electrocatalytic water oxidation of three monomeric copper complexes [CuII(L1)] (1), [CuII(L2)(H2O)] (2), and [CuII(L3)] (3) with bis-amide tetradentate ligands: L1 = N,N'-(1,2-phenylene)dipicolinamide, L2 = N,N'-(4,5-dimethyl-1,2-phenylene)bis(pyrazine-2-carboxamide), L3 = N,N'-(1,2-phenylene)bis(pyrazine-2-carboxamide), 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.

Impact of the hybridization form of the coordinated nitrogen atom on the electrocatalytic water oxidation performance of copper complexes with pentadentate amine-pyridine ligands

The field of molecular catalysts places a strong emphasis on the connection between the ligand structure and its catalytic performance. Herein, we changed the type of coordinated nitrogen atom in pentadentate amine-pyridine ligands to explore the impact of its hybridization form on the water oxidation performance of copper complexes. In the electrochemical tests, the copper complex bearing dipyridine-triamine displayed an apparently higher rate constant of 4.97 s-1, while the copper complex with tripyridine-diamine demonstrated overpotential reduction by 56 mV and better long-term electrolytic stability.

Unusual Water Oxidation Mechanism via a Redox-Active Copper Polypyridyl Complex

To improve Cu-based water oxidation (WO) catalysts, a proper mechanistic understanding of these systems is required. In contrast to other metals, high-oxidation-state metal-oxo species are unlikely intermediates in Cu-catalyzed WO because π donation from the oxo ligand to the Cu center is difficult due to the high number of d electrons of CuII and CuIII. As a consequence, an alternative WO mechanism must take place instead of the typical water nucleophilic attack and the inter- or intramolecular radical-oxo coupling pathways, which were previously proposed for Ru-based catalysts. [CuII(HL)(OTf)2] [HL = Hbbpya = N,N-bis(2,2'-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active HL ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [CuII(L-)(H2O)]+ is the predominant species in solution. The overall WO mechanism was found to be initiated by two proton-coupled electron-transfer steps. Kinetically, a first-order dependence in the catalyst, a zeroth-order dependence in the phosphate buffer, a kinetic isotope effect of 1.0, a ΔH⧧ value of 4.49 kcal·mol-1, a ΔS⧧ value of -42.6 cal·mol-1·K-1, and a ΔG⧧ value of 17.2 kcal·mol-1 were found. A computational study supported the formation of a Cu-oxyl intermediate, [CuII(L•)(O•)(H2O)]+. From this intermediate onward, formation of the O-O bond proceeds via a single-electron transfer from an approaching hydroxide ion to the ligand. Throughout the mechanism, the CuII center is proposed to be redox-inactive.

Theoretical study on the mechanism of water oxidation catalyzed by a mononuclear copper complex: important roles of a redox non-innocent ligand and HPO4 2- anion

The water oxidation reaction is the bottleneck problem of the artificial photosynthetic system. In this work, the mechanism of water oxidation catalyzed by a mononuclear copper complex in alkaline conditions was studied by density functional calculations. Firstly, a water molecule coordinating with the copper center of the complex (Cuii, 1) generates Cuii-H2O (2). 2 undergoes two proton-coupled electron transfer processes to produce intermediate (4). The oxidation process occurs mainly on the ligand moiety, and 4 (˙L-Cuii-O˙) can be described as a Cuii center interacting with a ligand radical antiferromagnetically and an oxyl radical ferromagnetically. 4 is the active species that can trigger O-O bond formation via the water nucleophilic attack mechanism. This process occurs in a step-wise manner. The attacking water transfers one of the protons to the HPO4 2- coupled with an electron transfer to the ligand radical, which generates a transient OH˙ interacting with the oxyl radical and H2PO4 -. Then the O-O bond is formed through the direct coupling of the oxo radical and the OH radical. The triplet di-oxygen could be released after two oxidation processes. According to the Gibbs free energy diagram, the O-O bond formation was suggested to be the rate-limiting step with a calculated total barrier of 19.5 kcal mol-1. More importantly, the copper complex catalyzing water oxidation with the help of a redox non-innocent ligand and HPO4 2- was emphasized.

Modulating the electrocatalytic activity of mononuclear nickel complexes toward water oxidation by tertiary amine group

Water oxidation is the bottleneck of water splitting, which is a promising strategy for hydrogen production. Therefore, it is significant to develop efficient water oxidation catalysts. Herein, electrochemical water oxidation catalyzed by three nickel complexes, namely [Ni(bptn)(H₂O)](ClO₄)₂ (1), [Ni(mbptn)(CH₃CN)](ClO₄)₂ (2), and [Ni(tmbptn)(H₂O)](ClO₄)₂ (3) (bptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane, mbptn = 5-methyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane, and tmbptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane), is studied under near-neutral condition (pH 9.0). Meanwhile, the homogeneous catalytic behaviors of the three mononuclear nickel complexes were investigated and confirmed by scanning electron microscopy, energy dispersive spectrometry, X-ray photoelectron spectroscopy and electrochemical method. Complex 1 stabilized by a pentadentate ligand with three N-H fragments homogeneously catalyzes water oxidation to oxygen with the lowest onset overpotential. Complex 2 stabilized by a similar ligand with two N-H groups and one N-CH₃ group exhibits relatively higher onset overpotential but higher catalytic current and turnover frequency. However, complex 3 with three N-CH₃ coordination environment shows the highest onset overpotential and the highest catalytic current at higher potential. Comparison of catalytic behaviors and ligand structure of the three complexes reveals that the methyl group on the polypyridine amine ligand affects the water oxidation activity of the complexes obviously. The electronic effect of N-CH₃ coordination environment leads to higher redox potential of the metal center and potential demand for water oxidation, while it leads to higher reaction activity of high-valent intermediates, which account for higher catalytic current and efficiency of water oxidation. This work reveals that electrocatalytic water oxidation performance of nickel complexes can be finely modulated by constructing suitable N-CH₃ coordination.

A low-cost commercial Cu(ii)–EDTA complex for electrocatalytic water oxidation in neutral aqueous solution

A low-cost commercial Cu complex [Cu(EDTA)(H2O)] is developed as a molecular catalyst for OER with high efficiency and durable stability.

Efficient homogeneous electrochemical water oxidation by a copper(ii) complex with a hexaaza macrotricyclic ligand

A copper complex [CuII(L)](ClO4)2 with a hexaaza macrotricyclic ligand is found to be an efficient homogeneous electrocatalyst for water oxidation with onset overpotential of 480 mV and a turnover frequency of 3.65 s−1.