Effect of Stacking Interactions on the Translation of Structurally Related Bis(thiosemicarbazonato)nickel(II) HER Catalysts to Modified Electrode Surfaces

Designing a Phenalenyl-Based Dinuclear Ni(II) Complex: An Electrocatalyst with Two Single Ni Sites for the Oxygen Evolution Reaction (OER)

A new dinuclear Ni(II) complex 1, [Ni2II(dtbh-PLY)2], is synthesized from 9-(2-(3,6-di-tert-butyl-2-hydroxybenzylidene)hydrazineyl)-1H-phenalen-1-one, dtbh-PLYH2 ligand, and structurally characterized by various analytical tools including the single-crystal X-ray diffraction (SCXRD) technique. In the solid state, both Ni(II) metal centers in complex 1 exist in a distorted square planar geometry and display the presence of rare Ni···H-C anagostic interactions to form a one-dimensional (1-D) linear motif in the supramolecular array. Complex 1 is further stabilized in the solid state by π-π-stacking interactions between the highly delocalized phenalenyl rings. The redox features of complex 1 have been analyzed by the cyclic voltammetry (CV) technique in solution as well as in the solid state, revealing the crucial involvement of both the Ni(II) metal centers for undergoing quasi-reversible oxidation reactions on the application of an anodic sweep. A complex 1-modified glassy carbon electrode, GC-1, is employed as an electrocatalyst for oxygen evolution reaction (OER) in 1.0 M KOH, giving an OER onset at 1.45 V, and very low OER overpotential, 300 mV vs the reversible hydrogen electrode (RHE) to reach 10 mA cm-2 current density. Furthermore, GC-1 displayed fast OER kinetics with a Tafel slope of 40 mV dec-1, a significantly lower Tafel slope value than those of previously reported molecular Ni(II) catalysts. In situ electrochemical experiments and postoperational UV-vis, Fourier transform infrared (FT-IR), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) studies were performed to analyze the stability of the molecular nature of complex 1 and to gain reasonable insights into the true OER catalyst.

Ligand-Centered Hydrogen Evolution with Ni(II) and Pd(II)DMTH

In this study, we report a pair of electrocatalysts for the hydrogen evolution reaction (HER) based on the noninnocent ligand diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-pyridinehydrazone) (H₂DMTH, H₂L¹). The neutral complexes NiL¹ and PdL¹ were synthesized and characterized by spectroscopic and electrochemical methods. The complexes contain a non-coordinating, basic hydrazino nitrogen that is protonated during the HER. The pK_a of this nitrogen was determined by spectrophotometric titration in acetonitrile to be 12.71 for NiL¹ and 13.03 for PdL¹. Cyclic voltammograms of both NiL¹ and PdL¹ in acetonitrile exhibit diffusion-controlled, reversible ligand-centered events at -1.83 and -1.79 V (vs ferrocenium/ferrocene) for NiL¹ and PdL¹, respectively. A quasi-reversible, ligand-centered event is observed at -2.43 and -2.34 V for NiL¹ and PdL¹, respectively. The HER activity in acetonitrile was evaluated using a series of neutral and cationic acids for each catalyst. Kinetic isotope effect (KIE) studies suggest that the precatalytic event observed is associated with a proton-coupled electron transfer step. The highest turnover frequency values observed were 6150 s^-1 at an overpotential of 0.74 V for NiL¹ and 8280 s^-1 at an overpotential of 0.44 V for PdL¹. Density functional theory (DFT) computations suggest both complexes follow a ligand-centered HER mechanism where the metals remain in the +2 oxidation state.

Unravelling the potential of disposable and modifiable pencils as catalyst supports for hydrogen evolution reaction

Pencil substrates are promising ideal durable carbon support also amenable for post characterization for evaluation of hydrogen evolution reaction catalysts.

Utilizing Charge Effects and Minimizing Intramolecular Proton Rearrangement to Improve the Overpotential of a Thiosemicarbazonato Zinc HER Catalyst

The zinc(II) complex of diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazonepyridine), ZnL¹ (1), was prepared and evaluated as a precatalyst for the hydrogen evolution reaction (HER) under homogeneous conditions in acetonitrile. Complex 1 is protonated on the noncoordinating nitrogen of the hydrazonepyridine moiety to yield the active catalyst Zn(HL¹)OAc (2) upon addition of acetic acid. Addition of methyl iodide to 1 yields the corresponding methylated derivative ZnL²I (3). In solution, partial dissociation of the coordinated iodide yields the cationic derivative 3'. Complexes 1-3 were characterized by ¹H NMR, FT-IR, and UV-visible spectroscopies. The solid-state structures of 2 and 3 were determined by single crystal X-ray diffraction. HER studies conducted in acetonitrile with acetic acid as the proton source yield a turnover frequency (TOF) of 7700 s^-1 for solutions of 1 at an overpotential of 1.27 V and a TOF of 6700 s^-1 for solutions of 3 at an overpotential of 0.56 V. For both complexes, the required potential for catalysis, E_cat/2, is larger than the thermodynamic reduction potential, E_1/2, indicative of a kinetic barrier attributed to intramolecular proton rearrangement. The effect is larger for solutions of 1 (+440 mV) than for solutions of 3 (+160 mV). Controlled potential coulometry studies were used to determine faradaic efficiencies of 71 and 89% for solutions of 1 and 3, respectively. For both catalysts, extensive cycling of potential under catalytic conditions results in the deposition of a film on the glassy carbon electrode surface that is active as an HER catalyst. Analysis of the film of 3 by X-ray photoelectron spectroscopy indicates the complex remains intact upon deposition. A proposed ligand-centered HER mechanism with 1 as a precatalyst to 2 is supported computationally using density functional theory (DFT). All catalytic intermediates in the mechanism were structurally and energetically characterized with the DFT/B3LYP/6-311g(d,p) in solution phase using a polarizable continuum model (PCM). The thermodynamic feasibility of the mechanism is supported by calculation of equilibrium constants or reduction potentials for each proposed step.