A dinuclear triazenido–copper complex: A new molecular electro-catalyst for generating hydrogen from acetic acid or water

Magnetic study and DFT analysis of a doubly carboxylato-bridged Co(II) derivative anchored with a 'scorpionate' precursor as a potential electrocatalyst for heterogeneous H2 evolution

A new doubly carboxylato-bridged Co(II) dinuclear complex, [Co(bdtbpza)(NCS)]2 (1), was obtained in a satisfactory yield by employing a 'scorpionate'-type precursor, bdtbpza {bis-(3,5-di-tert-butylpyrazol-1-yl)acetate}, and was then structurally characterized. Single-crystal X-ray diffraction analysis revealed that, in 1, each Co(II) is penta-coordinated, leading to a distorted trigonal-bipyramidal geometry within the coordination environment of N3O2. Weak antiferromagnetic coupling within the Co(II) ions in 1 was found based on the isotropic spin Hamiltonian H = -J(S1·S2) for the Si = 3/2 system. For evaluating the spin density distribution and the mechanism for the magnetic exchange coupling, DFT analysis was performed, with the calculated result agreeing the experimental magnetic data. A study into electrochemical H2 evolution, involving cyclic voltammetry (CV), controlled potential electrolysis (CPE), and gas chromatographic (GC) analyses of the graphite electrode modified with the cobalt complex in a neutral aqueous solution revealed the high catalytic activity of the complex with a low overpotential toward H2O reduction. The faradaic efficiency of the catalyst was found to be 83.7% and the di-cobalt catalyst-modified electrode displayed quite an interesting H2-evolution activity compared with that of bare electrodes. These results are encouraging for the future potential application of 1 in water splitting.

Electrocatalytic hydrogen evolution by a dinuclear copper complex and mechanistic elucidation through DFT studies

A novel dinuclear copper complex, [CuII2(L1)2] (L1 = 2-{[2-(8-hydroxyquinolin-2-yl)-1H-benzimidazol-1-yl]methyl}quinolin-8-ol) was synthesised and characterised through various spectroscopic techniques. This dinuclear complex (as an electrocatalyst) was employed to examine the catalytic ability towards an electrochemical hydrogen evolution reaction (HER). Redox studies in 95/5 (v/v) DMF/H2O with the addition of 30-equivalent AcOH (acid source) led to higher catalytic activities for the HER. The evolved H2, as the resultant product, was detected and confirmed from gas chromatography to afford a faradaic efficiency of 93% at an applied potential of -1.9 V vs. SCE. Based upon measurements of open-circuit potential and electrocatalytic responses, the mechanistic route for the reduction process using [CuII2(L1)2] was elucidated. Density functional theory studies reveal that through a concerted proton-coupled electron transfer (PCET) path, the HER proceeded via the formation of a Cu-H bond with a low activation energy for the dehydrogenation reaction.

Catalytic Mechanism of Competing Proton Transfer Events from Water and Acetic Acid by [CoII(bpbH2)Cl2] for Water Splitting Processes

We performed first principles simulations to explore the water reduction process of the cobalt complex [Co^II(bpbH₂)Cl₂], where bpbH₂ = N,N'-bis(2'-pyridine carboxamide)-1,2-benzene. We considered the sequence steps of electron reduction followed by the proton addition process to observe the hydrogen evolution process. An experimental study of the catalyst showed that the increase in the acetic acid concentration triggers catalytic current and reduction of Co(II) to Co(I), and protonation occurred, yielding a Co(III)-H intermediate. Therefore, we used water and acetic acid as the proton sources. We compare the proton transfer kinetics from both the water and acetic acid. The reduction potentials and proton transfer kinetics from water or acetic acid to the reaction center were studied in a DMF solvent through the implicit solvent model. The first proton transfer from the acetic acid is more favorable, forming a Co^III-H complex and further reducing to Co^II-H. The second proton transfer from water to the Co^II-H moiety requires less free energy than acetic acid and is the rate-limiting step. The nature of the reduction process is also examined through the charge analysis, which reveals that the ligand becomes softer due to the C═O groups.

Copper catalysts for photo- and electro-catalytic hydrogen production

Square planar 1, square pyramidal 2 and trigonal bipyramidal 3 copper complexes are poor catalysts for hydrogen evolution (HER) under photocatalytic conditions, whereas 1 is, or forms, a good and enduring electrocatalyst for HER, but 2 and 3 do not.

A dinuclear porphyrin-macrocycle as efficient catalyst for the hydrogen evolution reaction

We report an unprecedented dinuclear catalyst for the electrochemical hydrogen evolution reaction (HER). A macrocyclic porphyrin complex comprising two nickel centres connected via redox mediating linker molecules gives rise to efficient catalysis, significantly outperforming a mononuclear reference catalyst.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Electrocatalytic reduction of protons to hydrogen by a copper complex of the pentadentate ligand Dmphen-DPA in a nonaqueous electrolyte

An aminopyridine-based copper complex was synthesized and investigated for its electrocatalytic proton reduction activity and the plausible mechanism.

A coordinatively saturated nickel complex supported by triazenido ligands: a new electrocatalyst for hydrogen generation via ligand-centered proton-transfer

The reaction of 1,3-bis[2-aminobenzothiazole]triazene (HL) with Ni(CH₃CO₂)₂ affords a nickel complex, [Ni^II(L)₂] 1, an electrocatalyst for hydrogen generation.

Electrochemical-driven water reduction and oxidation catalyzed by an iron(iii) complex supported by 2,3-bis(2-hydroxybenzylideneimino)-2,3-butenedinitrile

One electrocatalyst, [FeLCl(H₂O)] for both water reduction and oxidation with a TOF of 808.46 moles h⁻¹ and 0.849 s⁻¹, respectively.