Theoretical studies of charge transfer phenomena in molecular systems for renewable energy applications

Abstract

Despite its significant potential to change the energy landscape, artificial photosynthesis faces significant challenges that must be overcome prior to widespread adoption. The issues with the device components typically fall into several categories: thermodynamic efficiency, catalytic rate, catalyst stability, and cost. A successful device must simultaneously optimize all four of these areas. Unfortunately, the vast majority of catalysts developed to date are only able to optimize one of these areas, if any. It is clear that traditional approaches to catalyst development are not sufficient to enable the rapid development of a successful device.

The advent of high-performance computing presents new opportunities for computer-aided rational design of artificial photosynthesis catalysts. Despite the recent availability of petascale supercomputers, adequate software solutions for catalyst design are not yet available. This dissertation reports several new efforts to develop computational methods for the study and design of artificial photosynthesis catalysts.

Chapter 1 provides an overview of the history of artificial photosynthesis and quantum chemistry. Chapter 2 discusses the use of density functional theory (DFT) calculations to study an existing water oxidation catalyst previously reported by our group. Chapter 3 discusses our efforts toward the development a parallel numerical algorithm for efficiently constructing Pourbaix diagrams for proton-coupled electron transfer half-reactions from a minimal set of DFT calculations. Chapter 4 revisits the use of density functional theory for the study of catalyst efficiency and identifies new computational targets for computer-aided catalyst design. Chapter 5 develops a new family of qualitative mathematical approximations termed “fragment-transfer effective Hamiltonian models” for the phenomenological study of oxidative addition and reductive elimination reactions. Chapter 6 develops a fragment-transfer model for the study of the release of oxygen from a dimanganese water oxidation catalyst and applies it successfully to a model water oxidation catalyst. Chapter 7 discusses a combined experimental/theoretical study of a cobalt complex related to a proton reduction catalyst developed by our group, and investigates an interesting ligand-to-metal mixed valence charge transfer using both experimental techniques and density functional theory calculations. Finally, Chapter 8 discusses a number of possible future directions for research in the area of computational catalyst design.

Artificial photosynthesis – the photocatalytic conversion of water and other small molecules into chemical fuels – is a promising technology for the storage of renewable energy. Bioinspired devices for artificial photosynthesis typically consist of a water oxidation catalyst and a reductive fuel-generating catalyst that work in tandem. The water oxidation catalyst splits water into protons and electrons (2 H2O → 4 H+ + 4 e-), while the reductive catalyst recombines these and other molecules into fuel molecules (e.g. 2 H+ + 2 e– → H2).