Development of a lithium-mediated nitrogen reduction process

Abstract

Ammonia is an important industrial chemical that is used predominantly for producing nitrogen-containing fertilizers as well as nitric acid, polymers, and pharmaceuticals. It is also being considered as an energy-dense, carbon-free energy carrier. Today, ammonia (NH₃) is produced via the Haber-Bosch process, in which air, water, and fossil fuels are used to make nitrogen and hydrogen, which are then reacted at elevated temperatures and pressures to produce NH₃. The use of fossil fuels as a hydrogen and energy source leads to significant CO₂ emissions. In addition, the process is very centralized and capitally intensive, which makes expanding ammonia production capacity, particularly in a distributed manner, difficult. Electrochemical methods for producing ammonia from air, water, and renewable electricity have been proposed as possible solutions to these issues.

In this thesis, we investigated the use of a lithium-mediated electrochemical method for nitrogen reduction to produce NH₃. A setup for reproducibly producing ammonia using the chemistry was developed, and the impacts of changing operating conditions such as electrolyte composition and applied current density were studied. The results of these experiments were used to develop a detailed coupled kinetic-transport model for ammonia production. We found that the rate of diffusion of nitrogen through both the solid-electrolyte interphase (SEI) and the bulk electrolyte generally defines ammonia production rates and selectivities. To overcome diffusion limitations in the bulk electrolyte, solvent-agnostic gas diffusion electrodes were developed. By using these electrodes, a maximum NH₃ formation rate of 30 nmol cm⁻² s⁻1 (8.8 mA cm⁻²) was obtained; the highest Faradaic efficiency for ammonia obtained was 47.5%. Hydrogen oxidation at >25 mA cm⁻² and 100% FE was demonstrated. A technoeconomic model for a general, large-scale electrochemical ammonia production process was developed and was applied to specifically analyze lithium-mediated nitrogen reduction. This work demonstrates that lithium-mediated nitrogen reduction can be used for ammonia production, though significant improvements in operating parameters are necessary for the process to be economically viable in a large-scale process. We believe the tools and findings developed in this work are useful for both electrochemical ammonia synthesis and other synthetically relevant electrochemical reactions.