Understanding and Controlling the Kinetics of Electron Transfer Events in the Water-Splitting Dye-Sensitized Photoelectrochemical Cell

Open Access
Author:
Mccool, Nicholas S
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 16, 2016
Committee Members:
  • Thomas E Mallouk, Dissertation Advisor
Keywords:
  • Water-splitting
  • Solar energy
  • electron transfer
  • charge recombination
  • sensitized metal oxides
Abstract:
Water-splitting dye-sensitized photoelectrochemical cells utilize a sensitized metal oxide and water-splitting catalyst in order to split water in to oxygen and hydrogen gas as a possible clean, renewable source of molecular hydrogen. Upon light absorption, the excited dye molecule injects an electron into the conduction band of the metal oxide, Once in the electrode, the electron percolates through the material through a complicated network of trapping and dctrapping events to a dark cathode where protons are reduced to hydrogen. The holes on the dye molecules diffuse across the electrode surface to catalytic water oxidation particles. These particles oxidize water to protons and oxygen, thereby using water as a clean, renewable source of protons and electrons. Despite a growing amount of research in molecular sensitizers, electrode material and catalysts, power conversion efficiency remains very low. In this dissertation, we focus on understanding and controlling the electron transfer events that dictate the efficiency of these devices. Chapter 1 presents an overview of the development of water oxidation systems, originating with simple UV-driven water oxidation at bare metal oxide surfaces and moving in to the more complex molecular sensitizer, metal oxide support and water splitting catalyst triad largely used today. We also discuss the corresponding light-driven cathode system and the potential for developing a 2-photon 1-electron Z-scheme through utilizing both systems in conjunction. Chapter 2 discusses the electron injection and recombination dynamics in various model metal oxide systems that represent the systems most commonly used for water splitting and analyzes the impact of a core/shell structure on charge injection. In Chapter 3, we revisit proton intercalation in order to demonstrate how proton-induced trap states impact overall charge mobility and stability as well as how these impact overall device performance. Chapter 4 extends on the previous study to determine the effect of shell thickness on charge injection dynamics in an energy-cascade core/shell structure. Chapter 5 expands on this by utilizing a wide band-gap semiconductor to control injection and recombination kinetics. Finally, Chapter 6 examines a monomeric iridium molecule, which has previously been shown to negatively impact device performance, as a precursor for an active, single-site water oxidation catalyst, as well as discusses the recent observations of adventitious catalysis.