Novel Dithiolene Nickel Complex Catalysts for Electrochemical Hydrogen Evolution Reaction for Hydrogen Production in Nonaqueous and Aqueous Solutions

Facile synthesis of acridine-based nickel(II) complexes via metal-mediated rearrangement of diphenylamine derivative and application in H2 evolution reaction

In this study, the formation of acridine-based metal complexes from rearrangement of diphenylamine-2,2'-dicarboxaldehyde (2,2'-dpadc) in the presence of transition metal ions was investigated. As a result, two novel isomorphic nickel(II) complexes bearing acridine-based Schiff-base ligand [NiLACR](X)2·CH3CN (X = BF4 (1), ClO4 (2), LACR = (E)-N1-(2-((acridin-4-ylmethylene)amino)ethyl)-N1-(2-aminoethyl)ethane-1,2-diamine) were successfully synthesized via a one-pot condensation of 2,2'-dpadc and tris(2-aminoethyl)amine (TREN) with a satisfactory yield of approximately 60%. These complexes were fully characterized by X-ray crystallography, UV-vis spectroscopy and CHN elemental analysis. Additionally, their thermal stability (thermogravimetric analysis) and electrochemical properties were also determined. A plausible mechanism for the nickel(II)-mediated rearrangement of 2,2'-dpadc to form the acridine-based nickel(II) complex was proposed. To demonstrate their potential applications, complex 1 was explored in the realm of electrocatalysis. It exhibited moderate activity towards hydrogen evolution reaction (HER). During 1-h controlled-potential electrolysis (CPE) experiments, H2 production (16 micromole) was observed with faradaic efficiency of 40% when the reaction was conducted in a TBAPF6/DMF solution at -2.1 V vs. Fc/Fc+ in the presence of acetic acid as a proton source. The facile synthesis of these acridine-based nickel(II) complexes reported herein may stimulate further development of novel acridine-based ligands and their corresponding metal complexes for a wide range of applications.

Azolium-2-dithiocarboxylates as redox active ligands in nickel chemistry

The coordination chemistry of carbene-CS2 adducts of selected NHCs and cAACs and their redox active nature in nickel complexes is reported. These azolium-2-dithiocarboxylate ligands can be considered as 1,1-isomeric dithiolene analogues bearing a 2π electron reservoir. Depending on the co-ligands attached to nickel, square planar mono- or bis-(carbene-CS2) complexes of the types [Ni(IiPr)2(carbene-CS2)] (2a-g) (carbene = cAACMe, IDipp, IMes, BIMe, BIiPr, IiPr, and IiPrMe) and [Ni(carbene-CS2)2] (3a-c) (carbene = cAACMe, IDipp, and IMes) are accessible by alkene substitution using [Ni(IiPr)2(ƞ2-C2H4)] or [Ni(COD)2] as the starting material (cAACMe = 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene, IR = 1,3-diorganylimidazolin-2-ylidene, IRMe = 1,3-diorganyl-4,5-dimethylimidazolin-2-ylidene, and BIR = 1,3-diorganylbenzimidazolin-2-ylidene). In the complexes 2a-g and 3a-c, all carbene-CS2 ligands are coordinated in a κ2-S,S' fashion to nickel(ii) and are ligated either in their formally one electron reduced (3a-c) or two electron reduced (2a-g) redox states. The complexes 3a-c reveal intense NIR absorptions, which shift upon metallic reduction to the nickelate salts of the type [Cat]+[Ni(carbene-CS2)2]˙- (4a-bcat). In these nickelates, an additional electron is shared across a ligand-centered SOMO of π-symmetry which is delocalized over both azolium-2-dithiocarboxylate ligands and results in carbene-CS2 moieties with a formal -1.5 charge per ligand, further demonstrating the flexible redox active nature of these azolium-2-dithiocarboxylate ligands.

Enzymatic and Bioinspired Systems for Hydrogen Production

The extraordinary potential of hydrogen as a clean and sustainable fuel has sparked the interest of the scientific community to find environmentally friendly methods for its production. Biological catalysts are the most attractive solution, as they usually operate under mild conditions and do not produce carbon-containing byproducts. Hydrogenases promote reversible proton reduction to hydrogen in a variety of anoxic bacteria and algae, displaying unparallel catalytic performances. Attempts to use these sophisticated enzymes in scalable hydrogen production have been hampered by limitations associated with their production and stability. Inspired by nature, significant efforts have been made in the development of artificial systems able to promote the hydrogen evolution reaction, via either electrochemical or light-driven catalysis. Starting from small-molecule coordination compounds, peptide- and protein-based architectures have been constructed around the catalytic center with the aim of reproducing hydrogenase function into robust, efficient, and cost-effective catalysts. In this review, we first provide an overview of the structural and functional properties of hydrogenases, along with their integration in devices for hydrogen and energy production. Then, we describe the most recent advances in the development of homogeneous hydrogen evolution catalysts envisioned to mimic hydrogenases.

Non-Covalent Functionalization of Graphene Oxide-Supported 2-Picolyamine-Based Zinc(II) Complexes as Novel Electrocatalysts for Hydrogen Production

Three mononuclear 2-picolylamine-containing zinc(III) complexes viz [(2-PA)2ZnCl]2(ZnCl4)] (Zn1), [(2-PA)2Zn(H2O)](NO3)2] (Zn2) and [Zn(2-PA)2(OH)]NO3] (Zn3) were synthesized and fully characterized. Spectral and X-ray structural characteristics showed that the Zn1 complex has a square-pyramidal coordination environment around a zinc(II) core. The hydroxide complex Zn3 was non-covalently functionalized with few layers of graphene oxide (GO) sheets, formed by exfoliation of GO in water. The resulting Zn3/GO hybrid material was characterized by FT-IR, TGA-DSC, SEM-EDX and X-ray powder diffraction. The way of interaction of Zn3 with GO has been established through density functional theory (DFT) calculations. Both experimental and theoretical findings indicate that, on the surface of GO, the complex Zn3 forms a complete double-sided adsorption layer. Zn3 and its hybrid form Zn3/GO have been individually investigated as electrocatalysts for the hydrogen evolution reaction. The hybrid heterogenized form Zn3/GO was supported on glassy carbon (GC) with variable loading densities of Zn3 (0.2, 0.4 and 0.8 mg cm−2) to form electrodes. These electrodes have been tested as molecular electrocatalysts for the hydrogen evolution reaction (HER) using linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) in 0.1 M KOH. Results showed that both GC-Zn3 and GC-Zn3/GO catalysts for the HER are highly active, and with increase of the catalyst’s loading density, this catalytic activity enhances. The high catalytic activity of HER with a low onset potential of −140 mV vs. RHE and a high exchange current density of 0.22 mA cm−2 is achieved with the highest loading density of Zn3 (0.8 mg cm−2). To achieve a current density of 10 mA cm−2, an overpotential of 240 mV was needed.

Electrocatalytic hydrogen generation using tripod containing pyrazolylborate-based copper(ii), nickel(ii), and iron(iii) complexes loaded on a glassy carbon electrode

Three transition metal complexes (MC) namely, [TpMeMeCuCl(H2O)] (CuC), [TpMeMeNiCl] (NiC), and [TpMeMeFeCl2(H2O)] (FeC) {TpMeMe = tris(3,5-dimethylpyrazolyl)borate} were synthesized and structurally characterized. The three complexes CuC, NiC, and FeC-modified glassy carbon (GC) were examined as molecular electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solution (0.1 M KOH). Various GC-MC electrodes were prepared by loading different amounts (ca. 0.2-0.8 mg cm-2) of each metal complex on GC electrodes. These electrodes were used as cathodes in aqueous alkaline solutions (0.1 M KOH) to efficiently generate H2 employing various electrochemical techniques. The three metal complexes' HER catalytic activity was assessed using cathodic polarization studies. The charge-transfer kinetics of the HER at the (GC-MC)/OH- interface at a given overpotential were also studied using the electrochemical impedance spectroscopy (EIS) technique. The electrocatalyst's stability and long-term durability tests were performed employing cyclic voltammetry (repetitive cycling up to 5000 cycles) and 48 h of chronoamperometry measurements. The catalytic evolution of hydrogen on the three studied MC surfaces was further assessed using density functional theory (DFT) simulations. The GC-CuC catalysts revealed the highest HER electrocatalytic activity, which increased with the catalyst loading density. With a low HER onset potential (E HER) of -25 mV vs. RHE and a high exchange current density of 0.7 mA cm-2, the best performing electrocatalyst, GC-CuC (0.8 mg cm-2), showed significant HER catalytic performance. Furthermore, the best performing electrocatalyst required an overpotential value of 120 mV to generate a current density of 10 mA cm-2 and featured a Tafel slope value of -112 mV dec-1. These HER electrochemical kinetic parameters were comparable to those measured here for the commercial Pt/C under the same operating conditions (-10 mV vs. RHE, 0.88 mA cm-2, 108 mV dec-1, and 110 mV to yield a current density of 10 mA cm-2), as well as the most active molecular electrocatalysts for H2 generation from aqueous alkaline electrolytes. Density functional theory (DFT) simulations were used to investigate the nature of metal complex activities in relation to hydrogen adsorption. The molecular electrostatic surface potential (MESP) of the metal complexes was determined to assess the putative binding sites of the H atoms to the metal complex.