Theories of Homogeneous and Electrochemical Electron Transfer in Complex Media

Project: Research project

Project Details

Description

Theories of Homogeneous and Electrochemical Electron Transfer in Complex Media Theories of homogeneous and electrochemical electron transfer in complex media Both the need to curb the level of carbon dioxide in the atmosphere and to produce hydrogen or other fuels contribute to the urgency to design effective schemes for separation of charge with solar energy. This is a global problem, which grows more important on a daily basis and it will soon become the dominant scientic issue. Solar photochemistry research is often driven by empirical approaches searching for optimum combination of parameters, while theories guiding new laboratory approaches are insufficiently developed. The prevailing paradigms suggest that molecules are not the best materials for building photosynthetic conversion systems, and solid semiconductors should be used instead. On the other hand, natural systems based on combinations of molecules with nanoscale interfaces have achieved significant efficiency gains where evolutionary pressure forced them to do so. Natural photosynthesis provides an example of complex interfacial systems where complex dynamics couples with non-trivial energetics affecting electronic states of photosynthetic cofactors. The development of solar photochemistry in recent years seems to re-trace nature's steps by employing increasingly complex solvents and interfaces. While these new systems offer significant advantages for efficient solar energy conversion, they also present challenges for rational design since their properties are not well understood and hard to measure. There is also little from the established general theory that can guide the researcher in what to expect in terms of charge-transfer rates when certain conditions and/or design components change. This proposal aims to address these challenges by developing predictive theoretical formalisms to model rates of charge transfer in non-traditional solvents and at electrodes (electrochemistry). We propose: (i) to develop theories of electron transfer in non-dipolar solvents, (ii) to extend the analysis of cyclic voltammetry to dynamically complex media, including room-temperature ionic liquids, and (iii) to de- velop analytical formalisms for the analysis of charge-transfer band shapes. Our main goal is to arrive at computational algorithms which can be used by practitioners without significant investment in theory. Theories of homogeneous and electrochemical electron transfer in complex media and interfaces The need to curb the level of carbon dioxide in the atmosphere and to produce fuels contribute to the urgency to design eective schemes for separation of charge with solar energy. Solar photochemistry research is often driven by empirical approaches searching for optimum combination of parameters, while theories guiding new practical design are insufficiently developed. The recent focus has been on extending the computational capabilities for material research into the realm of high-performance computing. While significant advances by using parallel computer architectures have in- deed been achieved, there is an urgent need to translate the results of computations into reliable theoretical formalisms cast in terms of measurable material properties which practitioners can use for predictive exploration of the parameters affecting solar energy conversion. This is especially true with the current shift of the field toward the use of complex materials and interfaces. While these new systems offer significant advantages for tunability of material properties, they also present challenges for rational design since their properties are not well understood and hard to measure. The present proposal makes the next step in developing practical theories of charge transfer in complex media and interfaces. We propose to build on our initial success in the previous research cycle and to continue developing models of charge transfer in ionic liquids in the bulk and in contact with electrodes. The development of formal model will be supported by extensive simulations of model ionic liquids to provide the theory with the microscopic structural information. We next propose to take advantage of enormous polarizability of photoexcited quantum dots to use polarizability as a new parameter to control and improve the eciency of charge separation. Both a general theory will be developed and specific calculations performed to connect theory to new experiments recently reported in the literature. Finally, we propose to develop a unified theory of asymmetric voltammetric curves in electrochemistry. Recent advances in the data analysis have shown that such phenomena are much more abundant than previously thought. No consistent theory of volmammetric asymmetry currently exists and we will bridge this gap and connect our results with experiment. Theories of homogeneous and electrochemical electron transfer in complex media Applications of solar photochemistry and design of devices for solar energy conversion increasingly rely on the combination of fast charge separation and the use of complex media. Charge separation in the bulk is not sufficient and one also needs to collect charge carriers at the electrodes. These practical demands drive experimental research in the direction of charge separation under nonequlibrium conditions in media with complex and dispersive dynamics and in interfaces. Such conditions pose significant conceptual challenges to our understanding of how to optimize the rate and efficiency of solar energy conversion. Nonequilibrium condition both open new opportunities for innovative tuning and design and pose questions about limitations of standard theories based on thermodynamics. These fundamental challenges can be resolved only through a close theory-experiment collaboration, and the demand to theory is to develop predictive models cast in terms of measurable material properties which practitioners can use for optimizing parameters affecting solar energy conversion. Developing such theoretical formalisms is the goal of this project. The present proposal makes the next step in developing practical theories of charge transfer in complex media and interfaces. We propose to build on our success in the current research cycle and to continue developing models of charge transfer in ionic liquids in the bulk and in contact with electrodes. The development of formal model will be supported by extensive atomistic simulations, quantum calculations supporting force-field development, and direct measurements of charge-transfer spectra in ionic liquids. Studies in the present cycle have shown an unexpected remarkably high reorganization of charge transfer in nonpolar liquids thus grossly expanding the range of media for solar energy conversion. This problem will be further studies and related to electrochemistry in aromatic hydrocarbons. Finally, we will develop theories of nonequilibrium proton transfer induced by electron transfer to connect to a number of recent experiments where such reactions were observed in photoexcited states. All theory development will be supported by experiments planned in the lab or through external collaborations.
StatusFinished
Effective start/end date6/1/167/31/22

Funding

  • DOE: Office of Science (OS): $506,608.00

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