CAREER:Engineering Interphases for Li-Mediated Nitrogen Reduction at Ambient Conditions

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Chemical manufacturing is a key contributor to the Nation's economy, producing everything from fertilizers and pharmaceuticals to plastics and fuels. Despite the successes of the chemical industry to produce chemicals and materials critical to everyday living, there are issues associated with the carbon-footprint and centralization of chemical manufacturing. Many chemical processes are run in centralized plants at large scale because smaller capacities are not economically viable. An example is the Haber-Bosch process that produces a large portion of the ammonia needed for nitrogen-containing fertilizers. The process is run at high temperatures and pressures and at large scale in a centralized fashion. Furthermore, the Haber-Bosch process contributes to 1 - 2% of global carbon dioxide emissions. Electrochemical synthesis of ammonia could enable competitive manufacture at a lower carbon footprint and with a smaller production capacity. By applying an electrical potential to drive reactions instead of using temperature and pressure, chemical processes can be run at milder conditions, at smaller scales, and closer to the end user, e.g. in a distributed fashion. This CAREER project will focus on fundamental research to study ways to improve selectivity and production rates of an electrochemical process for ammonia production using a lithium based electrochemical system. Methods developed in the project will advance how to selectively synthesize one chemical (ammonia) over another at the highly reactive electrode-electrolyte interface. The research knowledge will be adapted to be used in an outreach program focused on teaching the importance of mass and energy balances to middle school students. Understanding where everyday chemicals and materials come from and how they are produced will allow for critical evaluation of their impact on society and the environment. To date, synthesis methods using electrochemical nitrogen reduction suffer from poor selectivity and low reaction rates in aqueous electrolytes due to the competing hydrogen evolution reaction. In order to improve selectivity for nitrogen reduction, this project will investigate ammonia synthesis in nonaqueous electrolytes, as the proton activity can be well-controlled, with a lithium metal-mediated chemistry, which allows for nitrogen fixation at ambient conditions. The effect of the electrolyte composition on the solid-electrolyte interphase (SEI) species present on the lithium-covered electrode will be studied with both in situ and ex situ spectroscopic methods. The SEI structure and composition is hypothesized to control the selectivity for nitrogen reduction versus competing hydrogen evolution. The project will address the nature of the transport limitations and its impact on the coupled transport-kinetics. As a result of this work, fundamental understanding of the necessary interfacial steps for efficient electrochemical ammonia production in a nonaqueous solvent will be obtained. This understanding will translate to other electrosynthetic reactions in nonaqueous electrolytes that take place at SEIs. The project is structured into three aims. Aim 1 addresses engineering the solid electrolyte interphase to promote desired interfacial reactions. Aim 2 will study the design of omniphobic electrodes for non-aqueous solvents to achieve fast nitrogen transport to the active sites. Finally, Aim 3 will focus on the mass and energy balances for anode and electrolyte design.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.