Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Wang, Hailiang

Abstract

The anthropogenic increase in atmospheric CO2 levels has driven a global push toward sustainable energy solutions that mitigate climate change while enabling carbon-neutral fuel production. The photocatalytic and photoelectrochemical reduction of CO2 into value-added fuels such as carbon monoxide (CO) and methanol offers a promising approach to harness solar energy for clean fuel generation. However, achieving efficient, selective, and stable CO2 reduction on a semiconductor surface remains a formidable challenge due to sluggish reaction kinetics, poor charge separation, and competing hydrogen evolution reactions. Recent advancements in molecular catalysts and semiconductor-integrated systems have demonstrated remarkable progress in overcoming these limitations, paving the way for practical solar fuel technologies. This article discusses cutting-edge developments in CO2 reduction catalysis, focusing on hybrid catalyst systems, molecular photocathodes, and innovative electrode architectures that enhance performance and stability.Firstly, CO2-to-CO conversion has been achieved through the development of a hybrid catalyst combining carbon nitride (CNx) and cobalt phthalocyanine tetracarboxylic acid (CoPc-COOH). This system exhibits an impressive reaction rate of 1067 μmol/g·h with over 98% selectivity for CO under simulated solar irradiation. The presence of carboxylic acid substituents on the phthalocyanine ligand plays a crucial role in binding to the amine groups of CNx, facilitating near-monolayer coverage of the molecular cocatalyst on the semiconductor surface. This optimized interface significantly enhances catalytic activity, achieving a reaction rate 16 times higher than a CNx material containing unsubstituted CoPc molecules. Moreover, the activation and deactivation processes of the CNx/CoPc-COOH composite, attributed to the reduction and decomposition of CoPc-COOH, occur at a nearly constant rate, independent of the CO2 reduction reaction rate. This decoupling of charge carrier injection and catalytic function provides valuable mechanistic insights for optimizing future photocatalysts for CO2 reduction. In addition to solid-state hybrid catalysts, molecular catalyst-based photocathodes have emerged as a promising class of materials for aqueous CO2 reduction. A novel photocathode architecture incorporating cobalt phthalocyanine molecules anchored on graphene oxide and interfaced via a (3-aminopropyl)triethoxysilane linker to p-type silicon, protected by a thin titanium dioxide film, demonstrates excellent selectivity for CO and methanol production. This system operates at mild potentials, reducing CO2 to CO at 0 V versus the reversible hydrogen electrode, while methanol formation is observed at an onset potential of -0.36 V vs. RHE. The peak turnover frequency for methanol production reaches 0.18 s-1, marking a significant achievement in molecular catalyst-based photoelectrode performance for multi-electron CO2 reduction pathways. This work not only establishes a strategy for interfacing molecular catalysts with p-type semiconductors but also sets a new benchmark for aqueous-phase CO2-to-fuel conversion. Further innovations in photoelectrochemical CO2 reduction involve the engineering of structured electrode architectures to enhance efficiency and stability. A silicon micropillar array coated with a superhydrophobic fluorinated carbon layer has been designed to optimize the microenvironment for methanol synthesis. The micropillar structure increases the electrode surface area, improves catalyst loading, and ensures better catalyst adhesion without compromising light absorption. Simultaneously, the superhydrophobic coating minimizes parasitic side reactions and enhances the local accumulation of reaction intermediates, facilitating a more effective CO2 reduction process. This approach results in a remarkable Faradaic efficiency of 20% and a partial current density of 3.4 mA cm-2 for methanol, surpassing the last result on planar silicon by a factor of 17. Finally, the quest for unbiased solar fuel production has also led to the development of a monolithic artificial leaf for methanol synthesis directly from CO2 and H2O. This design integrates a new generation of photocathodes based on silicon micropillar arrays with a cobalt tetraaminophthalocyanine molecular catalyst. A crucial addition to this system is the incorporation of a C60 interlayer, which facilitates unidirectional electron transfer through the semiconductor/catalyst interface. This integration results in a photovoltage of 500 mV, the highest recorded for single-junction silicon photoelectrodes in CO2 reduction. The system achieves an unprecedented methanol formation with a Faradaic efficiency of 30% and a partial current density of 6.3 mA cm-2. Furthermore, by coupling the photocathode with a multi-junction perovskite photovoltaic minimodule, a fully integrated monolithic solar fuel system is realized, demonstrating a solar-to-methanol conversion efficiency of 1.0%. This efficiency is 40 times higher than the previous record for solar-to-alcohol conversion using a monolithic artificial leaf, establishing a new standard for solar-driven liquid fuel production.

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