Probing the Chemical Stability & Adsorption Properties of Molecule|Metal Oxide Architectures under Conditions of Photoelectrochemical Water Oxidation

Abstract

Photoelectrochemical (PEC) water splitting is one viable method for harvesting and storing solar energy, serving as a benign alternative to fossil fuels as a way of powering society sustainably. It involves the conversion of water to O2 and H2 gas to be used as a portable fuel. However, many cheap, binary metal oxides like TiO2, WO3, and a-Fe2O3 don’t have the ideal properties needed for the first step of oxidizing water, such as high Faradaic efficiency, absorb visible light, high stability at a wide range of pH, or high rates of reaction and high external quantum efficiencies. On the other hand, homogeneous water oxidation catalysts (WOCs) are much better suited to oxidize water selectively at low overpotential compared to metal oxide semiconductors. They are easily tunable based on the ligand design and can achieve high turnover numbers. However, they are not capable of oxidizing water without the use of an external energy source. In this way, they can serve as a coating on top of a light-absorbing semiconductor that suffers from low Faradaic efficiency. In addition, molecular dyes such as [Ru(bpy)3]2+ can serve to increase visible light absorption on metal oxide semiconductors that suffer from low visible light absorption without significantly hindering their catalytic activity for water oxidation. Many researchers have designed devices that functionalize metal oxide photoanodes with WOCs or dyes, but they also suffer from instability of the molecular|solid-state interaction due to either anchoring group desorption or instability, and/or catalyst/chromophore decomposition during water oxidation in a PEC cell. Few have probed the inherent stability of the WOC that they anchor to the surface toward the conditions in which it experiences in a PEC cell, such as illumination in the presence of oxygen, and acidic or basic conditions. This thesis will quantify these phenomenon as a means of getting to the heart of the problem, in order to determine ways to improve the system. First, I explore the binding constants and adsorption/desorption kinetics associated with phosphonic acid and hydroxamic acid on two common photoanodes for water oxidation, TiO2 anatase and WO3. In this section, I discuss hydroxamic acid as a more suitable anchor for TiO2 anatase under neutral and basic conditions, while showing that phosphonic acid is primarily only suitable for highly acidic conditions on TiO2 and WO3. I will then dive into a deep discussion on the inherent chemical stability of a known WOC, Fe(bpmcn)Cl2 toward typical water oxidation conditions in a PEC cell – visible light in the presence of O2, and acidic and basic conditions. In all three cases, the non-heme iron complex shows reactivity with different rate constants associated with each phenomenon. In acid and base, the Fe(bpmcn)Cl2 dissociates into [Fe(H2O)6]2+ in acidic conditions, or becomes oxidized in air or electrochemically in basic conditions to become a-FeOOH(s). Here, I present the case that the basic precipitation of Fe(bpmcn)Cl2 to form a-FeOOH allows for a controlled, anisotropic morphology that leads to an electrochemically active powder for water oxidation. Finally, I discuss at the end of the thesis potential areas in which you could move forward with this information. This discussion includes the pursuit of new anchoring groups better suited for the neutral to basic regime other than hydroxamic acid.