Hydrogen production catalyzed by earth-abundant metal complexes

Open Access
Author:
Yu, Yinxi
Graduate Program:
Chemistry
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 16, 2013
Committee Members:
  • Sharon Hammes Schiffer, Thesis Advisor
Keywords:
  • hydrogen production
  • catalyst
  • cobalt
  • nickel
  • iron
Abstract:
The design of efficient electrocatalysts based on cheap first-row transition metals for the oxidation and production of H2 is important for the development of renewable energy sources. This work focuses on utilizing computational methods to investigate the effects of ligand modification and ligand protonation on metal oxime H2 evolution electrocatalysts. A series of diimine-dioxime complexes, M(DO)(DOX)pn [(DO)(DOH)pn = N2,N2′-propane-1,3-diylbis(2,3-butanedione-2-imine-3-oxime), (M=Fe, Co, Ni)] and dimethylglyoxime complexes, M(dmg2X1-X2) with proton- or BF2-bridging units X, are studied in acetonitrile and water solvents. Density functional theory is used to calculate MII/I reduction potentials and relative pKa values for ligand protonation. The calculated relative pKa’s allow us to determine the likelihood of protonation and imply that O-H-O bridges but not O-BF2-O bridges can become protonated. The calculated MII/I reduction potentials indicate that the anodic shift due to ligand protonation is greater than the anodic shift due to the replacement of the O-H-O bridge with the O-BF2-O bridge for cobalt and nickel, but not for iron complexes. Even though the cobalt complex with two O-H-O bridges has the most positive MII/I reduction potentials, it would degrade based on experimental data. The overpotential required for H2 evolution often relates to the MII/I reduction potential for these types of electrocatalysts. Therefore, the prediction from this study shows that asymmetric cobalt, nickel, and iron complexes with strongly electron- withdrawing substituents containing only one O-H-O bridge are effective H2 evolution electrocatalysts with relative low overpotential in acetonitrile and water.