Photoelectrocatalysis for Conversion of CO2 to Fuels
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: As the synthesis of fuels and chemical feedstocks from non-fossil sources becomes a necessity, the catalytic reduction of carbon dioxide on synthetic catalysts may surpass nitrogen reduction in global importance. Hydrogenation of carbon dioxide by heterogeneous catalysts has been observed both in the gas phase and in electrochemical systems; however, it is thought that the high-energy chemical intermediates kinetically limit the practical yields. Heterogeneous photoelectrochemistry can use sunlight rather than grid electricity to drive carbon dioxide electroreduction as “artificial photosynthesis”. Surprisingly high yields of methanol and formic acid have been observed in such processes on several semiconductor electrodes. To better understand the potential, and limits, of carbon dioxide electroreduction, a more complete understanding of the chemistry of CO2 on high-surface-area electrode surfaces is necessary. We request funds to achieve the following: (a) synthesis of ultrahigh-surface-area nanostructured photoelectrochemical material systems, (b) morphological, structural, and electronic characterization of those systems, (c) determination of the photoelectrochemical efficiency, (d) the determination of the relationships between the structure and composition of the photoelectrocatalysts and their photoelectrochemical performance for carbon dioxide reduction, and (e) an understanding of some essential aspects of the photoelectrochemical process. In preliminary work we found enormous variability of performance in seemingly identical semiconducting metal oxides. In particular, we observed that iron oxide, Fe2O3, in the hematite structure, which is abundant, inexpensive, and a potential candidate for a widely deployed photocatalyst, shows differences of several orders of magnitude in its photoelectrocatalytic performance. As part of this project we will attempt to understand the origins of this variability in an effort to develop highly efficient photocatalysts. We will also specifically study photoreduction of carbon dioxide on several mixed metal oxides based on Fe and Cu hosts. To these photoelectrochemically active hosts, metal “dopants” up to several percent will be added based on their ability to stabilize M=CO2 adducts, improve electro-optical properties, and maximize the overall material stability in the reactive environment. The surface properties that reduce the formation energy of the radical anion (or possibly utilize a different reactive intermediate) and inhibit formation of less-desirable carbon monoxide or formate will be investigated. Relationships between the energetics and density of the semiconducting surface states and the material composition and processing will be obtained using high-throughput synthesis and screening methods. By evaluating the relationships between compositional variations in the photocatalysts and the photoreduction activity, we will attempt to better understand and predict the properties required for an efficient photoreduction catalyst. Density functional theory will be used to screen a variety of dopants for their ability to bind CO2 to the surface and to weaken the C-O bond and increase its reactivity. One hypothesis to be tested through our research (and from which the inspiration to use nanostructured materials came) is that the electron/hole pair transport distance from photo-creation in the bulk material to the surface reaction sites is a significant parameter determining photoreduction efficiency. Other fundamental questions that will be addressed include: the roles of oxygen vacancies and other oxide surface (donor/acceptor) states in metal-oxide photoreduction; the possibility of shifting the effective absorption band of wide-gap semiconducting oxides into the visible spectrum and the band edge energies adjusted by substitutional doping; and the possible effect of nanostructuring the oxides on simultaneously improving recombination/trapping and absorption, the two limiting factors of photoelectrocatalysts. Density functional theory will be used to investigate how the separation and the localization of the electrons and holes at the surface are affected by the presence of water and ions and to study the reaction of the electrons or holes with ions in solution. In addition, exploratory studies will be made to determine how various surface defects trap electrons or holes.