Caltech Young Investigators Lecture

Wednesday February 2, 2022 4:00 PM

Electrifying Nitrogen Transformations: Decarbonizing Ammonia Using Plasma Catalysis and Electrochemical Reduction of Nitrate in Wastewaters

Speaker: Lea Winter, Yale University
Location: Online Event

Abstract: Nitrogen fixation in fertilizers forms the basis of modern agriculture and mediates global food insecurity. However, conventional thermally-activated nitrogen conversion processes consume substantial amounts of fossil fuels as materials and energy inputs, leading to an unsustainable energy and carbon footprint. Furthermore, inefficiencies in reactive nitrogen management lead to energy losses as well as hazardous environmental pollution. Therefore, electron-driven approaches are needed to establish fossil-free nitrogen interconversions. Electrochemical and plasma-activated reactions operate under mild conditions, enabling facile coupling to intermittent renewable energy sources to reduce CO2 emissions and reactive nitrogen pollution, facilitate storage and transportation of renewable energy in nitrogen-based fuels, and improve the global distribution of fertilizer to promote food security.

Emerging plasma-activated catalytic N2 fixation processes may potentially circumvent the limitations of conventional thermocatalytic ammonia synthesis and enable decentralized ammonia production using CO2-free energy and renewable feedstocks. However, fundamental understanding of the reaction mechanisms and plasma-catalyst interactions is lacking. Therefore, an in situ FTIR reactor was employed in combination with steady-state flow reactor experiments and plasma kinetic modeling to elucidate the surface reaction mechanisms. Plasma-catalyst interactions were probed to reveal that elevated temperature and plasma irradiation of the surfaces promoted NH3 desorption. The direct evidence of catalytic surface reactions occurring during a plasma-activated process provides mechanistic insight into plasma-activated ammonia synthesis. In addition, a techno-economic analysis revealed the threshold efficiency required for a plasma process to become environmentally and economically competitive.

Beyond electrification of N2 fixation for ammonia synthesis, existing waste streams containing fixed nitrogen as nitrate may be mined for fertilizer or carbon-free renewable energy storage, via selective nitrate electroreduction to ammonium. The release of nitrate to the environment from wastewater effluent and agricultural runoff contributes to groundwater contamination, harmful algal blooms, and disruption of biogeochemical nitrogen flows. Typical treatment methods are based on nitrate separation, which produces waste streams that are often discharged to the environment. Alternatively, nitrate conversion via electrochemical reduction eliminates the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals to accomplish the reaction. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, catalyst stability, and mass transport of nitrate to electrocatalyst active sites. The use of electrochemical membranes as multifunctional porous flow-through electrodes could potentially address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores. Conductive membranes were fabricated using reduced graphene oxide and functionalized with non-precious transition metal oxynitride electrocatalysts. These multifunctional electrified membranes for reduction of nitrate in wastewaters have the potential to partially displace carbon-intensive industrially synthesized ammonia while simultaneously accomplishing water decontamination. The prospects for reactive nitrogen recovery based on nitrate electrochemical conversion to ammonium are analyzed for various potential source waters.

Bio: Lea R. Winter is a Nanotechnology Enabled Water Treatment (NEWT) Distinguished Postdoctoral Fellow in Chemical and Environmental Engineering at Yale University. Her work in the research group of Prof. Menachem Elimelech focuses on the development of sustainable reactive electrochemical membranes for water decontamination and conversion of nitrate in wastewaters into valuable products. She will be starting as an Assistant Professor in the same department in Summer 2022. Lea received her Ph.D. in Chemical Engineering at Columbia University with Prof. Jingguang Chen. As an NSF Graduate Research Fellow, she researched the conversion of CO2 and N2 to chemicals and fuels using non-precious metal heterogeneous catalysts and non-thermal plasma activation. Lea obtained her B.S. in Chemical Engineering from Yale University, where she researched water treatment and desalination. She also completed fellowships abroad in combustion of nitrogen-based fuels at the Technion Institute; plasma modification of polymer surfaces at the École Nationale Supérieure de Chimie de Paris; and immunogenomics at the Weizmann Institute. Lea founded SciRISE at Columbia, a high school internship program for students who recently immigrated to the U.S. to pursue independent research projects, and she co-founded the Yale Summer Science Research Institute.

This talk is part of the Caltech Young Investigators Lecture Series, sponsored by the Division of Engineering and Applied Science.

Series Caltech Young Investigators Lecture Series

Contact: Bronagh Glaser bglaser@caltech.edu