Date of Award
Doctor of Philosophy (PhD)
The development of renewable energy resources to replace traditional fossil fuels is among the major challenges facing the world today. One hurdle to fully transitioning to renewable energy sources to meet the world’s energy demand is energy storage. A potential solution to this problem is to store energy generated from renewable resources in the form of chemical bonds, as plants do in the process of photosynthesis. Artificial photosynthetic systems could be used to split water into protons, electrons, and oxygen, and those components could then be used to generate fuel, such as H2, that could be stored until needed. Water oxidation, however, is thermodynamically and kinetically challenging, often requiring large overpotentials in order to drive the reaction. Therefore, water oxidation catalysts must be developed to lower the energy barrier required for the reaction.The first section of this thesis focuses on the development and study of water-oxidation electrocatalysts by Earth-abundant metal systems. Chapter 1 provides an introduction to water oxidation catalysis and proton-coupled electron transfer. Chapter 2 describes the synthesis and characterization of a copper-based water-oxidation electrocatalyst, Cu(pyalk)2 (pyalk = 2-(2′-pyridyl)-2-propanoate). This complex was shown to be a robust and active electrocatalyst for water oxidation under basic conditions. Further characterization of the complex demonstrated that the catalyst operates through a mononuclear mechanism, has a turnover frequency of ~0.7 s-1 at pH 12.5, and is active for over 12 hours and 30 catalytic turnovers. These results demonstrate the ability of the strongly donating pyalk ligand to stabilize Earth-abundant metal electrocatalysts for water oxidation. Chapter 3 presents a mechanistic study of water-oxidation electrocatalysis by Cu(pyalk)2. It is proposed that the catalyst operates through a water-nucleophilic attack mechanism on a copper(III)-oxyl radical species. Experimental and theoretical analysis both support this proposed mechanism, and the measured kinetic isotope effect, turnover frequency, and rate of the first chemical step all showed good agreement with the predicted theoretical values. This work provides one of the first mechanistic studies of a mononuclear copper-based water-oxidation electrocatalyst. Many reactions relevant to energy production and storage include proton-coupled electron transfer steps, in which a proton and electron are transferred in either a stepwise or concerted fashion. The ability to understand and control how protons and electrons move in these systems could therefore prove valuable in designing more efficient catalysts for reactions such as water oxidation. The second section of this dissertation discusses the proton-coupled electron transfer reactivity of two high-valent systems related to the water-oxidation electrocatalyst Cu(pyalk)2. Chapter 4 describes the synthesis, characterization, and PCET reactivity of Ni(pyalk)2+, a square planar nickel(III) species structurally analogous to Cu(pyalk)2. Multiple characterization methods, including X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy, and X-ray crystallography confirm the highly oxidized state of nickel, making it a rare example of nickel in the +3 oxidation state. Ni(pyalk)2 was then shown to undergo proton-coupled electron transfer from a variety of phenolic and hydrocarbon substrates. Analysis of the kinetics of the reaction with various substrates indicates that Ni(pyalk)2+ reacts through concerted proton-electron transfer with these substrates. Further thermodynamic analysis showed that, during PCET, Ni(pyalk)2+ formed an O-H bond with a bond dissociation enthalpy of ~94 kcal/mol. The formation of this fairly strong bond may explain the fast reactivity of this system. Chapter 5 describes the characterization and reactivity of a high-valent copper species, Cu(pyalk)2, the one-electron oxidized form of Cu(pyalk)2. XPS and X-ray crystallography measurements confirm the oxidation of the copper center, demonstrating that the copper of Cu(pyalk)2+ is in the +3 oxidation state, a rare oxidation state for copper. Cu(pyalk)2+ was shown to also react with phenolic and hydrocarbon substrates through a concerted proton-electron transfer mechanism, in a similar manner to nickel. Thermodynamic analysis demonstrated that Cu(pyalk)2+ forms an O-H BDE of 98 kcal/mol, significantly higher than its nickel counterpart, yet it was shown to only react 4-5 times faster than Ni(pyalk)2+ with the same substrates. Calculation of an asynchronicity factor for each compound demonstrated that Ni(pyalk)2+ may react through a more asynchronous mechanism than Cu(pyalk)2+, which may explain the rather small difference in reactivity. These systems are two of only a handful of isolated Ni(III) and Cu(III) species that have been shown to undergo PCET reactivity. These results may provide valuable insight into PCET reactivity of electrocatalytic species, which could be used in designing the next generation of Earth-abundant water oxidation electrocatalysts.
Fisher, Katherine Jennie, "Water-Oxidation Electrocatalysis and Concerted Proton-Electron Transfer by High-Valent Complexes of Copper and Nickel" (2021). Yale Graduate School of Arts and Sciences Dissertations. 45.