|M.Sc Student||Engelberg Lee|
|Subject||Peptoid-Co(II) Complexes as Water Oxidation Electrocatalysts|
|Department||Department of Energy||Supervisor||Professor Galia Maayan|
|Full Thesis text|
Clean and sustainable energy production is a goal of mankind. A promising approach to this problem is an artificial photosynthetic system that would split water into oxygen and hydrogen. Hydrogen can be used directly as a sustainable fuel. Water-splitting requires the coupling of two half-reactions: water oxidation and proton reduction; water oxidation requires transfer of 4 electrons and is the more challenging reaction of the process. In nature, water spitting is catalyzed by the Oxygen-Evolving Center of PSII, producing O2, protons and electrons. A sustainable approach to an artificial water-oxidation catalytic system is harnessing earth-abundant 1st row transition-metals as molecular catalysts.
Cobalt-based complexes, which has rich redox chemistry and is abundant and cheap, have drawn attention as WOCs, as they can operate in ambient conditions. A challenge in designing cobalt-based molecular WOCs is kinetic lability of the cobalt-coordination center; in oxidative environments the metal-ligand bonds can decompose to yield oxides, which act as heterogenous, difficult to detect WOCs. Oxidatively-stable ligand environment is a way to maintain homogeneity; such ligands can be in the form of peptidomimetic oligomers named peptoids. Peptoids, N-substituted glycine oligomers, enable incorporation of various side chains: catalytic groups, metal-binding ligands and stabilizing groups, which can be rationally designed for different aims.
In this work, previously reported rationally-designed peptoid scaffold (BPT) was used to synthesize cobalt complexes, aimed to act as molecular WOCs. The initial cobalt complex was found to be a precursor of heterogenous catalysis; minor changes were applied to the initially designed sequence to achieve homogeneity. The new cobalt-peptoid complex TPT-Co(II) [(terpy-ethanol-benzyl)Co]2(ClO4)2 was characterized and investigated as a WOC.
TPT-Co(II) is composed of a trimer peptoid backbone, TPT. The side chains were rationally designed for different functions: terpyridine as metal binding group, OH group as co-catalyst and benzyl group for structural support. The terpyridine tridentate ligand was meant to accommodate Co2’s preferred coordination and maintain stability throughout the catalysis.
The water oxidation electrocatalytic activity of TPT-Co(II) and homogeneity were explored: CV of the complex showed catalytic activity, indicated by catalytic peak and O2 reduction. Mechanism testing by different methods suggested homogeneity. Catalytic activity was quantified by bulk electrolysis which included direct oxygen measurements, at pH 7, 633 mV overpotential, which is low compared to Co-based WOCs. Catalytic parameters over 90 minutes were TON of 7.85, TOF of 1.45x10-3 s-1 and remarkable FE of 98.3%. The kobs? of the catalytic process was found using FOWA methodology, which gave the value of 0.356 s-1.
This work presents a homogenous Co-peptoid WOC, operating in ambient conditions in relatively low overpotential, demonstrating the major significance ligand environment has on catalytic character of complexes.