Electrocatalytic and photocatalytic water splitting by nickel-based compounds

鎳化合物光, 電催化分解水

Student thesis: Doctoral Thesis

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Author(s)

  • Lingjing CHEN

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date14 Feb 2014

Abstract

Solar energy conversion into chemical energy in the form of hydrogen through water splitting is the most promising way to meet the increasing demand for clean and renewable energy. In order to make the reactions of water splitting at reasonable rates, low overpotentials and with economic feasibility, catalysts made from earth-abundant materials are required. In this thesis, a series of nickel complexes were synthesized and investigated for proton reduction and water oxidation. There are three parts. The first part describes the synthesis of macrocyclic nickel complexes ([Ni(L1)(ClO4)2 to [Ni(L11)](ClO4)2) (L1 = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo[11.3.l]heptadeca-1(17),2,11,13,15-pentaene; L11 = 2,3,11,12-tetramethyl-7-thia-3,11,17-triazabicyclo[11,3,1]heptadeca-1(17),13,15-triene) and their electrocatalytic activities towards hydrogen production. The structures and redox properties of nickel complexes are changed by modification of the ligands. Introducing softer phosphorus or sulfur atoms makes the Ni(II)/Ni(I) couple shift to more positive potential and reduces the overpotential for proton reduction. The amine complex [Ni(L7)](ClO4)2 (L7 = 2,12-dimethyl-7-phenyl-3,11,17-triaza-7-phospha-bicyclo[11,3,1]heptadeca-1(17),13,15-triene) containing phosphorus showed the best electrocatalytic activity at low acid concentration. The current density of bulk electrolysis was high, but the Faradaic efficiency was less than 50% while all the protons were consumed to give reasonable amount of charge, so there should be some other chemical reactions occurring during electrolysis. In the second part, all of the eleven nickel catalysts were further investigated for photocatalytic hydrogen production when [Ir(dF(CF3)ppy)2(dmbpy)]PF6 was used as photosensitizer and TEA (triethylamine) as sacrificial electron donor. The results of H2 evolution are dependent on the solvent, sacrificial donor and the concentration of the photosensitizer and catalyst in the solution. More than 1600 turnovers were obtained after irradiation for 8 h (λ > 420 nm) at optimized conditions. Spectroscopic and mechanistic studies of the system revealed that the iridium sensitizer was first excited and then reductively quenched by TEA. The Ni(II) catalyst was reduced to Ni(I) by the reduced sensitizer. The next step of the process leading to the formation of hydrogen is not very clear at the moment. The third part describes a number of simple nickel salts and complexes for water oxidation. All the simple nickel salts and some rationally designed nickel complexes bearing multidentate N-donor ligands are highly efficient for electrocatalytic and visible light-driven water oxidation with TON > 1200. Under oxidative conditions these nickel salts and complexes are converted to nickel oxide nanoparticles which are the real catalyst for water oxidation. The β-NiOOH particles formed during water oxidation have been isolated and fully characterized, but the real mechanism for O-O bond formation is still unclear in the system.

    Research areas

  • Electrocatalysis, Hydrogen as fuel, Electrolysis, Water, Nickel compounds, Photocatalysis