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

Use of solar irradiance to drive CO2 reduction (CO2R) holds great promise for the sustainable generation of energy-dense fuels and chemicals, especially as it relates well to carbon capture technology. Multicarbon (C2+) 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 CO2 to C2+ products with high faradaic efficiency; however, the product distribution of CO2R on Cu is potential and microenvironment dependent. In this talk, we will explore CO2R via different routes using both experiments and simulation. We will focus on obtaining C2+ 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 CO2 concentration, thereby enabling high CO2R to C2+ production. In addition, for direct photoelectrochemical CO2R, 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 CO2R, we will explore the co-design of the photoelectrode bandgaps and device architecture for the generation of C2+ 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 CO2R, we will explore the use of metal-insulator-semiconductor (MIS) structures to obtain high rates of CO2R. 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 CO2R. Overall, the insights between the photoelectrode and device design is critical for the design of monolithic, unassisted PEC CO2R systems that yield high STC2+ 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.