(Invited) Exploring Solar-Driven CO₂ Reduction to C₂₊ Products

Use of solar irradiance to drive CO 2 reduction (CO 2 R) holds great promise for the sustainable generation of energy-dense fuels and chemicals, especially as it relates well to carbon capture technology. Multicarbon (C 2+ ) products ( e.g. , ethylene, ethanol, propanol) are particularly attractive because they have a large market size and can be further converted to higher molecular weight hydrocarbons fuels that have high volumetric and mass energy densities. Metallic copper (Cu) has the unique ability to catalyze CO 2 to C 2+ products with high faradaic efficiency; however, the product distribution of CO 2 R on Cu is potential and microenvironment dependent. In this talk, we will explore CO 2 R via different routes using both experiments and simulation. We will focus on obtaining C 2+ products via tailoring the microenvironments including the use of ionomer films that are quite effective at the lower current densities that match against the solar irradiance. We will demonstrate how control of the local environment and dynamic operation provide metastable states of high pH and high CO 2 concentration, thereby enabling high CO 2 R to C 2+ production. In addition, for direct photoelectrochemical CO 2 R, there is an optimum cell design and operating photovoltage that is not at the maximum power point (unlike water splitting) due to the selectivity dependence on potential. Using a continuum model of PEC CO 2 R, we will explore the co-design of the photoelectrode bandgaps and device architecture for the generation of C 2+ products. The simulation results demonstrate the critical importance of simultaneously engineering the photoelectrode and device design and operation in order to ensure the photovoltage and photocurrent from the photoelectrode enables operation at the optimum potential. Finally, for PEC CO 2 R, we will explore the use of metal-insulator-semiconductor (MIS) structures to obtain high rates of CO 2 R. This includes both theoretical and experimental investigations that elucidate the controlling phenomena and when coupled with the ionomer films provide high rates of direct PEC CO 2 R. Overall, the insights between the photoelectrode and device design is critical for the design of monolithic, unassisted PEC CO 2 R systems that yield high STC 2+ efficiency. Acknowledgements This material is based partially on work performed by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266 and a University of California grant.