Portland State University. Department of Chemistry
Term of Graduation
Date of Publication
Doctor of Philosophy (Ph.D.) in Chemistry
carbon dioxide reduction, electrocatalyst, electrochemistry, hydrogen evolution reaction, nickel
1 online resource (xvii, 128 pages)
In the research presented, we use electrochemical techniques, such as cyclic voltammetry and controlled potential electrolysis, to study in situ formation of hydrogen evolution catalysts which also reduce carbon dioxide. Using pyridinethiol-based ligands and a nickel (II) precursor, both homo- and heteroleptic complexes were investigated as catalysts to produce alternative fuels while mitigating greenhouse gas emissions. Existing synthetic procedures to obtain these Ni(II) catalysts lead to low yields of the complexes and difficulties in crystallizing samples for further analysis, limiting sample size and restricting the number of studies. While current research is heavily focused on photochemically-driven experiments, given that solar radiation is highly inconsistent across the world, a major shift must be made to understand electrocatalysts and how they yield pertinent information about solar fuel production.
Cyclic voltammetry was used to investigate production of hydrogen gas from the in situ formation of nickel(II) tris-(pyridinethiolate), 3P+Ni. Comparing a sample of the isolated versus the in situ catalyst, the voltammograms suggested that the complex not only self-assembled in solution but also performed proton reduction. Four separate homoleptic catalysts with electron donating and electron withdrawing groups were employed to explore how ligand modifications could influence the reduction potential needed to produce hydrogen gas. Although 3P+Ni did not have ligand substituents, it afforded the most positive reduction potential (-1.41 V v. SCE) and concurrently a low rate-constant (376 mM-1·s-1) for hydrogen production. 3(3-F)+Ni, in situ nickel(II) tris (3 (trifluoromethyl)pyridine-2-thiolate), exhibited the most negative reduction potential (-1.68 V v. SCE) and the highest rate constant (951 mM-1·s-1) for proton reduction.
Using these findings, heteroleptic catalysts were proposed to develop catalysts with tunable reduction potentials. By using a 1:2:1 stoichiometric ratio (metal precursor, major ligand, and minor ligand), heteroleptic catalytic solutions were developed in situ and tested for hydrogen production. Reduction potentials for all the heteroleptic pairs were found to be more positive than their ligand modified homoleptic parent complex, i.e., heteroleptic 2P+1(3 F)+Ni (-1.53 V v. SCE) and 2(3-F)+1P+Ni (-1.50 V v. SCE) have lower overpotentials when compared to 3(3-F)+Ni (-1.68 V v. SCE). When comparing voltammograms for the heteroleptic pairs, similarities in the traces suggested that one hydrogen evolution electrocatalyst is formed for both ratio pairs. Despite the differences in the stochiometric ligand ratios, the electrochemical studies and reduction potentials are consistent with this theory. With respect to the rate constants, most of the catalytic solutions were also found to work better than the ligand modified parent analogues; therefore, the studies imply that in situ heteroleptic catalysts offer improvements without rigorous synthetic techniques.
Expanding on the versatility of these in situ nickel(II) catalysts, preliminary experiments using controlled potential electrolysis and quantitative gas analysis of the headspace tested carbon dioxide reduction. Exploring homoleptic 3P+Ni and 3(3-F)+Ni, both electrocatalysts were found to produce a maximum partial pressure change of ~14-15% CH4, ~15-16% HCOOH, and ~21% CO. However, they were found to produce the highest pressure changes from H2 evolution (~49%) which identified 3P+Ni and 3(3-F)+Ni as non-selective for CO2 reduction over proton reduction catalysts. Three heteroleptic catalysts, namely 2P+1(3-F)+Ni, 2(3 M)+1P+Ni, and 2P+1(3-M)+Ni, were investigated similarly and where found to show a significant improvement in the pressure changes related to HCOOH (+ ~4-5%). Although these in situ heteroleptic catalysts were also deemed better proton reduction catalysts, they afforded selectivity for HCOOH formation that was not expected. Through this research, the benefits of in situ studies in yielding measurable hydrogen gas production through electrochemical studies is shown, as well as interesting results that showcase the further development of these Ni(II) catalysts for carbon dioxide reduction and formic acid formation.
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Brown, Dayalis S.V., "Electrochemical Hydrogen Gas Production and Carbon Dioxide Sequestration via In Situ Formation of Nickel (II) Tris-(Pyridinethiolate) Derivatives" (2023). Dissertations and Theses. Paper 6508.